yb-fiber-laser-pumped continuous-wave frequency conversion sources from the mid-infrared to the...

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014 0902823

Yb-Fiber-Laser-Pumped Continuous-WaveFrequency Conversion Sources from the

Mid-Infrared to the UltravioletMajid Ebrahim-Zadeh, Member, IEEE, Suddapalli Chaitanya Kumar, and Kavita Devi

(Invited Paper)

Abstract—We describe a wide range of versatile, practical andhigh-power sources of coherent continuous-wave (cw) radiationfrom the near- to mid-infrared (mid-IR) to the visible and ultravi-olet (UV) developed in our laboratory by exploiting second-ordernonlinear frequency conversion in novel quasi-phase-matched(QPM) and birefringent materials pumped by the Yb-fiber laserat 1064 nm. By exploiting optical parametric oscillators in combi-nation with internal and external single-pass harmonic generationin QPM nonlinear materials of MgO:PPLN and MgO:sPPLT, andbirefriengent crystal of BiB3 O6 , we have generated stable, high-power cw radiation covering broad spectral regions from ∼4 μmin the mid-IR, to 532 nm in the visible, down to 355 nm in theUV. The developed sources can deliver total output powers of up to17.5 W in the near- to mid-IR, 13 W in the visible, and 68 mWin the UV, at exceptional efficiencies, with excellent passive stabil-ity, high spectral and spatial beam quality, in compact, portableand practical design. We also demonstrate successful realizationof cw fiber-laser-pumped Ti:sapphire lasers enabled by the devel-opment of simplified high-power cw green sources at 532 nm. Thedescribed techniques represent a highly versatile, practical, and ef-fective approach for the extension of fiber laser technology acrossthe entire UV to mid-IR spectrum, and ultimately the THz spectralrange, also offering the potential for further power scaling with theincrease in the fiber laser pump power.

Index Terms—Nonlinear optical devices, optical parametric os-cillators, frequency conversion, nonlinear materials, second har-monic generation, sum frequency generation, continuous wavesources, thermal effects.

Manuscript received December 14, 2013; revised February 10, 2014;accepted February 11, 2014. This work was supported in part by the Ministry ofScience and Innovation, Spain, under Projects OPTEX (TEC2012–37853) andNovalight (TEC2009–07991), and in part by the Consolider program SAUUL(CSD2007–00013). This work was also supported by the European Office ofAerospace Research and Development (EOARD) under Grant FA8655-12-1-2128 and the Catalan Agencia de Gestio d’Ajuts Universitaris i de Recerca(AGAUR) under Grant SGR 2009-2013.

M. Ebrahim-Zadeh is with the ICFO-Institut de Ciencies Fotoniques,Barcelona 08860, Spain, and also with the Institucio Catalana de Recerca iEstudis Avancats, Barcelona 08010, Spain (e-mail: [email protected]).

S. Chaitanya Kumar and K. Devi are with the ICFO-The Institute of Pho-tonic Sciences, Barcelona 08860, Spain (e-mail: [email protected];[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2014.2307299

I. INTRODUCTION

THE development of coherent light sources in spectral re-gions inaccessible to lasers has been a cornerstone of pho-

tonics science and technology for more than five decades. Thepotential of nonlinear optical techniques for the generation ofcoherent radiation at new wavelengths was recognized soonafter the invention of the laser, when the first observation ofsecond harmonic generation of ruby laser was reported in thecrystal of quartz in 1961 [1]. Shortly after, the fundamentalprinciples of nonlinear frequency conversion in transparent di-electric media were formulated [2], laying the foundation ofnonlinear optics, and paving the way for the exploitation of var-ious nonlinear processes including second- and third-harmonic-generation (SHG/THG), sum- and difference-frequency gener-ation (SFG/DFG), and optical parametric generation and ampli-fication (OPG/OPA). In particular, the importance of OPG/OPAas a powerful technique for the generation of widely tunableradiation across broad wavelength regions was recognized, inorder to expand the spectral reach of lasers to new limits [3]–[5].In 1965, the OPG/OPA concept was extended to a resonantcavity, as in a laser, when the first optical parametric oscilla-tor (OPO) was demonstrated using LiNbO3 as the nonlinearcrystal [6]. In the intervening five decades, there have beentremendous advances in nonlinear frequency conversion in dif-ferent spectral regions, driven by the steady progress in materialscience and pump laser technology.In particular, the develop-ment of new birefringent crystals such as β-BaB2O4 , LiB3O5 ,and BiB3O6 , and the more recent introduction of ferroelec-tric quasi-phase-matched (QPM) nonlinear materials, especiallyperiodically-poled LiNbO3 (PPLN) and LiTaO3 (PPLT), com-bined with the progress in crystalline solid-state, semiconduc-tor and fiber pump lasers, have led to numerous advances inthis field over the recent years, transforming nonlinear fre-quency conversion systems from laboratory concepts to vi-able sources of tunable coherent radiation for many practicalapplications. Operation of frequency conversion sources, onceconfined to restricted spectral and temporal domains, now ex-tends from the ultraviolet (UV) and visible to the mid-infrared(mid-IR) and terahertz (THz) spectrum, and encompasses alltime-scales from the continuous-wave (cw) and pulsed nanosec-ond to the ultrafast picosecond and femtosecond regime. Inno-vative device architectures based on refined design concepts andnovel pumping schemes have led to the realization of versatiletunable sources with previously unattainable performance

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0902823 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014

Fig. 1. Schematic of various Yb-fiber-laser-based cw frequency conversion sources developed in our laboratory during the past five years.

characteristics, while the availability of high-power pump laserswith enhanced spectral and spatial coherence has enabled thegeneration of unprecedented power levels.

In this paper, we describe the latest advances in cw nonlin-ear frequency conversion sources developed in our laboratoryduring the past five years, by exploiting the rapidly advanc-ing fiber laser technology as the pump source, in combinationwith the new generation of QPM and birefringent nonlinearmaterials as the gain medium. The emergence of QPM non-linear crystals has had a tremendous impact on cw frequencyconversion sources, a traditionally more challenging operat-ing regime due to the low nonlinear gain. The flexibility of-fered by grating-engineered QPM materials, allowing access tothe highest nonlinear tensor coefficients, combined with non-critical phase-matching (NCPM) over long interaction lengths(30–80 mm), has enabled the low available nonlinear gains tobe overcome, permitting the development of practical cw fre-quency conversion sources in various configurations and in dif-ferent spectral regions. Combined with the progress in fiber lasertechnology over the past decade, these developments have led tothe practical realization of a new class of cw light sources withpreviously unattainable performance capabilities with regard towavelength coverage, output power and efficiency, frequencyand power stability, spectral and spatial coherence, and finefrequency tuning.

The exploitation of fiber lasers offers a particularly attractiveroute to the development of next-generation frequency conver-sion sources by providing a more compact, robust, portable,and simplified pumping architecture compared to the relativelycomplex and costly solid-state lasers, with the additional ad-vantages of air-cooling and power scaling. Moreover, with theability of fiber lasers to now deliver multiwatt output powers incw as well as mode-locked operation, the development of high-power fiber-pumped wavelength conversion sources in differenttime-scales has become a practical reality. Here we demonstratehow the unique features of fiber lasers can be exploited to de-velop a wide range of versatile and practical tunable sourcesin the cw regime, covering spectral regions from the near- andmid-IR to the visible and UV, and delivering multiwatt outputpowers at exceptional efficiencies with excellent output char-acteristics. By deploying high-power cw Yb-fiber pump laser

at 1064 nm in combination with QPM nonlinear materials ofMgO-doped PPLN (MgO:PPLN) and MgO-doped stoichiomet-ric PPLT (MgO:sPPLT), and birefringent crystal of BiB3O6(BIBO), we have developed powerful and widely tunable cwlight sources, covering spectral regions from above ∼4 μm inthe mid-IR down to ∼355 nm in the UV. The main focus ofour strategy has been the development OPO sources for fre-quency down-conversion of the Yb-fiber pump laser to accessbroad wavelength regions in the near- and mid-IR, while ad-ditional frequency up-conversion techniques based on externaland internal SHG/THG/SFG are deployed to reach the visibleand UV spectral range. Using the various frequency conver-sion processes, we have generated cw output powers of up to17.5 W in the near- and mid-IR, 13 W in the green, and 68 mWin the UV, with excellent output characteristics, making thedeveloped sources of great practical utility for many applica-tions. A summary of the different Yb-fiber-laser-pumped cwfrequency conversion sources developed in our laboratory dur-ing the past five years, together with their respective tuningcoverage, is presented in Fig. 1.

II. OPTICAL PARAMETRIC OSCILLATORS

For the development of Yb-fiber-laser-pumped cw OPOs de-scribed in this study, we deploy the singly-resonant oscillator(SRO) configuration, with only the signal wave resonant in theoptical cavity, and the non-resonant idler extracted in a singlepass through the resonator. While characterized by the highestpump power threshold, the SRO is the optimum configurationfor generating maximum output power from cw OPOs [7], andthus optimally exploits the potential of fiber lasers in deliver-ing multiwatt pump powers, with the promise of power scaling.The advent of QPM materials, particularly MgO:PPLN withhigh effective nonlinearity (deff ∼ 17 pm/V) and long interac-tion lengths (up to 80 mm) under type 0 (e→ee) NCPM, hashad an unprecedented impact on cw SROs [8]–[13]. With thedevelopment of MgO:sPPLT, the spectral range of cw SROshas been further extended to shorter wavelengths in the near-IR and visible by deploying green pumping at 532 nm [14],[15]. Over the past few years, cw SROs pumped by solid-statelasers at ∼1064 nm have been established as viable sources of

EBRAHIM-ZADEH et al.: YB-FIBER-LASER-PUMPED CONTINUOUS-WAVE FREQUENCY CONVERSION SOURCES 0902823

Fig. 2. Experimental setup for Yb-fiber-laser-pumped, high-power cw SRObased on MgO:PPLN as the nonlinear crystal. FI—Faraday isolator; λ/2—half-wave plate; PBS—polarizing beam-splitter; M—dichroic mirror.

high-power and widely tunable radiation for the near- to mid-IR, addressing a variety of applications including spectroscopyand trace gas sensing [9], [10]. When combined with high-power fiber lasers as the pump source [11]–[13], they offer aviable route to the realization of compact, portable, practicaland cost-effective sources of coherent radiation across broadspectral regions, capable of delivering multiwatt output powersat exceptional efficiencies, with high spatial and spectral beamquality and robust output stability.

A. Near- to Mid-IR OPOs

1) SRO Based on MgO:PPLN: We describe here the de-velopment of a stable high-power cw SRO for the near- tomid-IR based on MgO:PPLN, pumped by a cw Yb-fiber laserat 1064 nm [13]. In such SROs, the attainment of high opticaloutput powers is an experimentally challenging task, essentiallydue to heavy thermal loading of the nonlinear crystal resultingfrom the high intracavity signal power at increased pump pow-ers. This can lead to effects such as thermally induced opticalbistability, saturation, and subsequently a substantial drop inextraction efficiency, thus limiting the available output power.In order to overcome thermal effects, various schemes such assignal output coupling [12], [14] and intracavity frequency dou-bling [15] can be employed to maintain high output power. In thisstudy, we have deployed optimized output coupling of the reso-nant signal wave in the cw SRO cavity to maximize extractionefficiency, output power, and usable signal tuning range [13].Optimization of signal output coupling is also a critical factorfor power scaling of cw SROs with increasing fiber laser pumppower, while maintaining output stability and extraction effi-ciency. Under optimized signal output coupling, the describedcw SRO can provide as much as 17.5 W of total power (9.8 Wsignal, 7.7 W idler) at an overall extraction efficiency of 61%,with peak-to-peak idler power stability of 5% over 14 hours in aTEM00 spatial profile. We also compare the performance of thedevice with the conventional cw SRO in the absence of signaloutput coupling.

A schematic of the experimental setup for the fiber-laser-pumped MgO:PPLN cw SRO is shown in Fig. 2. The Yb-fiberpump laser (IPG Photonics, YLR-30–1064-LP-SF) delivers acw single-frequency output power of up to 30 W at 1064 nmin a linearly polarized beam of 4-mm diameter in TEM00 spa-tial mode (M 2 < 1.01), with a nominal linewidth of 89 kHz.

Fig. 3. Variation of extracted signal (1627 nm) and idler (3070 nm) powerwith OC transmission. Inset: Variation of threshold pump power with OC trans-mission [13].

To maintain stable output characteristics, the laser is operated atmaximum power and a combination of a half-wave plate and po-larizing beam-splitter is used as a variable attenuator. A secondhalf-wave plate controls the pump polarization for type 0 (e→ee)phase-matching in the crystal, which is a 50-mm-long, 1-mm-thick, 5 mol.% MgO:PPLN with five grating periods (Λ = 29.5to 31.5 μm, in steps of 0.5 μm). The end-faces of the crystal areantireflection (AR)-coated for the pump (R < 3% at 1064 nm),signal (R < 1% over 1450–1750 nm), and idler (R < 3% over2500–3700 nm). The crystal is housed in an oven with a stabilityof ±0.1 ◦C, which can be adjusted from room temperature to200 ◦C. The SRO cavity is a symmetric ring, comprising twoconcave mirrors, M1 and M2 (r = 150 mm), and two plane mir-rors, M3 and M4 . All mirrors have high reflectivity (R > 99.5%)over 1.3–2.0 μm and high transmission at the pump (T > 90%)and idler (T > 85%) over 2.2–4 μm, ensuring singly-resonantsignal oscillation. Output-coupled SRO (OC-SRO) operation isachieved by replacing mirror, M4 , by a suitable output cou-pler (OC) across 1.6–1.7 μm. The pump beam is confocallyfocused to a beam waist radius, w0p = 63 μm, at the center ofthe MgO:PPLN crystal, corresponding to a focusing parameter,ξ = l/b ∼ 1 [16], where l is the crystal length and b is the con-focal parameter of the pump beam. The cavity design ensuresoptimum overlap of pump and resonant signal at the center ofthe crystal (bp ∼ bs), with a signal waist radius of w0s = 76 μm.A dichroic mirror, M, separates the non-resonant idler outputfrom the transmitted pump beam. The total optical length ofthe cavity including the crystal is 941 mm, corresponding to afree-spectral-range (FSR) of ∼319 MHz.

The output power characteristics of this SRO were studiedby employing different OCs with signal transmissions fromT ∼ 3% to∼6%. Fig. 3 shows the simultaneously extracted idlerand signal power from the OC-SRO for different OC values at amaximum available pump power of 28.6 W. The measurementswere performed at the center of the tuning range (Λ = 31 μm,100 ◦C). As evident from the plot, an increase in OC transmis-sion from 3% to 3.8% results in a rise in signal power from7.8 W to 9.8 W, without significant compromise in idler power.Further increase in OC transmission result in the drop in signaland idler power to 7.7 W and 6 W, respectively, at ∼6% outputcoupling. As a consequence of increased output coupling, theOC-SRO threshold increases from 7.2 W at 3% to 15.4 W at 6%(inset, Fig. 3), and the intracavity signal power estimated from


Fig. 4. (a) Total extracted output power as a function of pump power from SROand OC-SRO. (b) Power scaling of signal and idler in OC-SRO at a temperatureof 100 ◦C, Λ = 31 μm [13].

the OC transmission varies from 259 W (T ∼ 3%) to 129 W(T ∼ 6%). For optimal OC ∼ 3.8%, a maximum signal powerof 9.8 W (at 1627 nm) together with 7.7 W of idler (at 3070 nm)is extracted for 28.6 W of input power at a pump depletion of69.4%. Under this condition, the OC-SRO threshold is 10.5 W.So, the available pump power enables OC-SRO operation at∼2.74 times threshold where a maximum pump depletion istheoretically predicted, confirming that this is the optimum out-put coupling for this OPO. We investigated power scaling ofthe optimal OC-SRO (T ∼ 3.8%) compared with the SRO, forthe same grating period (Λ = 31 μm, 100 ◦C), with the resultsshown in Fig. 4(a). For the SRO, an idler power up to 8.6 W wasobtained at 3061 nm for 28.6 W of pump at 30% extraction effi-ciency, with a threshold of 3.6 W and a pump depletion of 79%.No saturation of idler power was observed at this pumping level.On the other hand, with the optimal OC-SRO, we were able togenerate a total power of 17.5 W (9.8 W of signal at 1627 nm,7.7 W of idler at 3070 nm) at an overall extraction efficiency of61%. Fig. 4(b) shows the simultaneous power scaling of signaland idler in the OC-SRO. Given a pump depletion of 69.4%,this means that as much as 88% of the down-converted pump issuccessfully extracted as OC-SRO output.

The generated output power from the cw OPO across thefull tuning range, in both SRO and OC-SRO configuration, ob-tained by varying the MgO:PPLN crystal temperature, is shownin Fig. 5. In the SRO, we generated idler wavelengths across3147–2787 nm by changing the temperature from 55 ◦C to200 ◦C, providing a total tuning of 360 nm, as shown in Fig. 5(a).For a pump power of 28.6 W, idler powers >7 W and pumpdepletions >65% were obtained almost over the entire tuningrange, except for a drop in the idler power near 2.8 μm, corre-

Fig. 5. Output power and temperature tuning range of (a) SRO idler,(b) OC-SRO idler, and (c) OC-SRO signal with ∼3–5% output coupling [13].

sponding to the OH− absorption in MgO:PPLN. However, wewere not able to tune the device down to room temperature, be-cause of heavy thermal loading of the crystal. In the OC-SRO,by employing an OC of T ∼ 3–5% in 1.6–1.7 μm range, idlertuning over 3196–2803 nm together with signal tuning across1594–1714 nm was obtained, enabling room-temperature oper-ation of OC-SRO at 31 ◦C, providing a total power of 15.7 W(9.1 W of signal at 1594 nm, 6.6 W of idler at 3196 nm), as shownin Fig. 5(b). Thus, in addition to a usable signal tuning rangeand room temperature operation, output coupling extended theidler wavelength range by 33 nm compared to the SRO. With theOC-SRO, we were also able to extract signal powers from 9.1 Wat 1594 nm (31 ◦C) to 7.3 W at 1714 nm (200 ◦C), and corre-sponding idler power varying from 6.6 W at 3196 nm to 5.6 W at2803 nm. The OC values were not optimized at each wavelength,so still higher powers across the tuning range are attainable. Thedrop in signal and idler power at higher temperatures is due tothe lower OC transmission for the corresponding wavelengths,and the dip around 2.8 μm is again due to the OH− absorptionin MgO:PPLN. Hence, output coupling not only increases theoverall extraction efficiency and helps manage thermal effects,but also extends the total tuning coverage of the cw OPO. Wehave also observed Raman generation and spectral shift in thesignal and idler wavelengths due to high circulating intensitywhen operating the cw OPO as a SRO with no output coupling,while no such effects were observed in the OC-SRO [13], thusconfirming efficient management of thermal effects by optimaloutput coupling in the cw OC-SRO.

We investigated the cw OPO power stability in SRO andOC-SRO configurations at the maximum output idler power.For the SRO, at a temperature of 100 ◦C (idler at 3061 nm),


Fig. 6. Long-term power stability and far-field spatial profile of the idlerbeam from cw OC-SRO at a crystal temperature of 44 ◦C, corresponding to awavelength of 3174 nm [13].

we measured a peak-to-peak stability of 17.4% over 1 hour.For the OC-SRO, at a temperature 100 ◦C (idler at 3070 nm),we recorded a stability of 13.6% over 1 hour. However, in theOC-SRO, at a temperature of 44 ◦C (idler at 3174 nm), wemeasured a long-term stability of 5% over 14 hours at 7 W ofidler power, as shown in Fig. 6. The corresponding peak-to-peakpower stability of the fiber pump laser was 0.5% over 10 hours.The improved stability of the OC-SRO at 44 ◦C is attributed tobetter temperature control of the MgO:PPLN crystal near roomtemperature, demonstrating another important advantage of theOC-SRO. Similar improvement in the power stability is also tobe expected by employing a temperature controller with stabilitybetter than ±0.1 ◦C, or by using a thermo-electric cooler. In theSRO, the minimum operating temperature could not be reducedbelow ∼55 ◦C due to the heavy thermal loading of the crystalin the absence of signal output coupling.

We measured the quality factors of the output beams from theOC-SRO, where we obtained M 2

x ∼ 1.28 and M 2y ∼ 1.22 for

the idler (at 3070 nm) and M 2x ∼ 1.29 and M 2

y ∼ 1.37 forthe signal (at 1627 nm). For the SRO, we obtained M 2

x ∼ 1.3and M 2

y ∼ 1.22 for idler (at 3061 nm). We attribute the largerM 2 value for the signal in OC-SRO to the stronger impact ofthe thermal lens on signal beam quality than on the idler. Thethermal lens itself is caused primarily by the absorption of thehigh circulating signal power, although absorption at pump andidler wavelengths cannot be completely ruled out. Since thesignal is a resonant mode of the SRO cavity, it is expected tobe highly sensitive to small changes in mode-matching with thepump, as well as changes in the cavity stability conditions causedby the intracavity thermal lens formed within the crystal. On theother hand, the idler is generated in a single pass through thecrystal as a result of mixing between the single-pass pump andthe resonant signal beam. Therefore, the effect of the thermallens is expected to be less detrimental to the idler beam qualitythan the signal, and so the impact on the M 2 value of idler is notas severe as on the M 2 value of the signal beam. Also shownin the inset of Fig. 6 is the idler spatial beam profile from theOC-SRO at 44 ◦C and at 7 W, measured at a distance of 1 mfrom the OPO output, confirming TEM00 spatial mode.

2) Broadband SRO Based on MgO:PPLN: The generationof coherent broadband radiation in the mid-IR is of interestfor a variety of applications including quantum optics, multi-

component trace gas detection and analysis, single-shot spec-troscopy of large molecules, speckle-free mid-IR imaging, andas potential seeds for high-energy ultrafast lasers and ampli-fiers in the mid-IR. The most widely established technique forbroadband generation in mid-IR is parametric down-conversionof femtosecond laser sources in suitable nonlinear crystals. Byexploiting the broad bandwidth and high intensity of femtosec-ond pulses, and using short interaction lengths to provide largephase-matching bandwidths, efficient generation of broadbandradiation can be achieved at high efficiency using OPG/OPAtechniques or synchronously-pumped OPOs. In the cw regime,however, broadband parametric down-conversion is generallydifficult, since the low intensities necessitate long interactionlengths for non-negligible nonlinear gain. This limits the avail-able phase-matching bandwidth for parametric interaction, thushampering the use of broadband pump radiation. The low non-linear gains also strictly preclude the exploitation of single-passOPG/OPA schemes, making the deployment of an OPO in aresonant cavity imperative. As such, successful operation of cwOPOs generally relies on single-frequency or narrow-linewidthpump sources in combination with long interaction lengths innonlinear crystals, resulting in single-frequency or narrowbandoutput generation [8]–[15].

Despite these limitations, we have extended the operationof fiber-laser-pumped cw SROs to provide broadband mid-IRradiation at high efficiency and multiwatt power level by de-ploying a broadband cw Yb-fiber pump laser at ∼1060 nm andtaking advantage of the extended phase-matching bandwidth inMgO:PPLN [17]. This is made possible since in a cw SRO thesignal frequency is constrained to one (or a limited number) ofaxial modes in the cavity, so that any frequency spread in thepump is directly transferred to the non-resonant idler in orderto maintain energy conservation. Hence, the use of broadbandpump leads to broadband idler generation in the cw SRO, witha narrowband signal output.

The generation of broadband radiation in a SRO, under condi-tions of energy conservation, thus also requires phase-matchingof individual spectral components of a broadband pump to thecorresponding component in the idler spectrum for the resonantsingle-frequency signal. This would be satisfied if the phase-mismatch, Δk = kp − ks − ki , where kp , ks , ki are wave vec-tors of the pump, signal and idler, respectively, exhibits a neg-ligible variation with pump wavelength. In MgO:PPLN, it isalso possible to identify such a condition [17]. In Fig. 7(a), weshow the variation of phase-mismatch, Δk, in MgO:PPLN withpump wavelength near 1064 nm. The calculation is performedby evaluating Δk in terms of pump wavelength for a fixed sig-nal frequency and varying pump and idler, under type 0 (e→ee)parametric interaction at 100 ◦C, using the Sellmeier equationsfor the material [18]. It is evident that close to a pump wave-length of 1059 nm, MgO:PPLN exhibits a negligible variationin Δk with wavelength. Also shown in Fig. 7(a) is the corre-sponding poling period (Λ = 2π/Δk), from which the requiredgrating period to achieve Δk = 0 and ∂(Δk)/∂λ = 0 is foundto be Λ ∼ 30 μm. Thus, if we use a 50-mm-long MgO:PPLNcrystal with Λ = 30 μm at 100 ◦C, the parametric gain curvecentered at a pump wavelength of 1059 nm, calculated from the


Fig. 7. (a) Variation of phase-mismatch and corresponding grating periodrequired as a function of pump wavelength at 100 ◦C. (b) Normalized parametricgain as a function of pump wavelength for a 50-mm-long MgO:PPLN crystal(Λ = 30 μm) [17].

variation of Δk in Fig. 7(a), results in a pump acceptance band-width (for low pump powers) of 11.7 nm (FWHM), as shown inFig. 7(b).

For the implementation of broadband cw SRO, we deploy anexperimental setup and OPO cavity configuration identical tothat shown in Fig. 2, except for the use of a broadband Yb-fiberlaser as the pump source. The laser in this case delivers a max-imum cw output power of 28 W in a linearly polarized beamwith good spatial quality (M 2 < 1.12) at a center wavelengthof ∼1060 nm with a FWHM spectral bandwidth of 8.3 nm. Toexploit the extended phase-matching bandwidth of MgO:PPLN,we use a 50-mm-long crystal with a grating period of Λ = 30 μmat a temperature of 100 ± 0.1 ◦C. In Fig. 8(a), we show the mea-sured spectrum of the broadband Yb-fiber pump laser at themaximum available power relative to the parametric gain curvefor the 50-mm-long MgO:PPLN crystal at 100 ◦C (Λ = 30 μm).As can be seen, a large fraction of the ∼8.3 nm laser band-width lies within the pump acceptance bandwidth of the crystal.Therefore, a substantial portion of the pump spectrum could beutilized for efficient parametric conversion to broadband idleroutput in the mid-IR. This is confirmed by the measured spec-trum of the transmitted pump, shown in Fig. 8(b), where it is seenthat ∼7 nm of the input pump spectrum undergoes discernabledepletion by parametric conversion in the cw SRO. It should benoted that the resonant signal (λs ∼ 1523 nm) exhibits a narrowlinewidth due to high finesse of the SRO cavity (Δν ∼ 6 MHz).Using the depleted pump spectrum, Fig. 8(a), and a single-frequency signal, the reconstructed idler spectrum exhibits abandwidth of 76 nm (FWHM) centered at λi ∼ 3460 nm, asshown in Fig. 8(c).

Fig. 9 shows the measured broadband idler output power asa function of the Yb-fiber pump power. We found the thresh-old for the broadband cw SRO to be ∼5.8 W, and for a pumppower of 25.5 W at the input to the crystal, as much as 5.3 W ofbroadband mid-IR power was generated at an extraction effi-ciency of 20.8%. The pump depletion was measured to be 80.6%at the maximum input fiber power. In order to maximize the to-tal power extraction and reduce thermal load in the MgO:PPLN

Fig. 8. Recorded (a) Pump spectrum (red solid line) and the calculated pumpacceptance bandwidth (dark-blue solid line) for 50-mm-long MgO:PPLN crys-tal, (b) Input (red) and depleted (blue) pump spectrum for the SRO, and (c)reconstructed idler spectrum of the cw SRO [17].

Fig. 9. Variation of total extracted power as a function of pump power forMgO:PPLN SRO and OC-SRO. Inset: Spatial beam profile of the extractedsignal and idler for the OC-SRO [17].

crystal, we also employed partial output coupling of the res-onant signal wave by replacing mirror, M4 , in Fig. 2 with anoutput coupler (OC) with ∼3.5% transmission (un-optimized)at the signal wavelength, λs ∼ 1523 nm. From Fig. 9, it is ev-ident that although the OC-SRO threshold increases to 12.1 Wcompared to the SRO, the maximum total (signal plus idler)power extracted from the device is now 11.2 W (7.2 W signaland 4.0 W idler) for a maximum available fiber pump power of25.5 W at an extraction efficiency of 44%. Under this condition,we recorded a pump depletion of 73.3% for the cw OC-SRO.

The far-field energy distribution of the signal and idler beamsat the maximum input pump power, measured at a distance >1 mfrom the cw OC-SRO output, is also shown in the inset of Fig. 9.Although the data appear to confirm a Gaussian distribution, wealso measured M 2 factor of the signal and idler beams at the


highest available pump power, where we obtained M 2x = 1.16

and M 2y = 1.10 for the signal and M 2

x = 1.50 and M 2y = 1.47

for the idler, confirming Gaussian nature of the output beams.3) SRO Based on MgO: SPPLT: While practical develop-

ment of high-power cw SROs for the near- to mid-IR has re-lied primarily on the widely established QPM nonlinear mate-rial, MgO:PPLN, overcoming detrimental thermal effects in thiscrystal due to the resonant signal, as well as the non-resonantidler at mid-IR wavelengths, still remains challenging [12],[13], presenting a potential barrier to future power scaling offiber-laser-pumped cw SROs based on MgO:PPLN. The emer-gence of alternative QPM ferroelectric materials, particularlyMgO:sPPLT, with improved optical and thermal properties [19],has lead to important new advances in nonlinear frequency con-version techniques, including single-pass SHG of cw Yb-fiberlasers into the green at unprecedented efficiencies [19]–[21], asdescribed in Sections III-A-1 and III-A-2. In spite of a lowereffective nonlinear coefficient (deff ∼ 9 pm/V) than MgO:PPLN(deff ∼ 16 pm/V), increased resistance to photorefractive dam-age and higher thermal conductivity, together with increasedoptical damage threshold [22], make MgO:sPPLT an attrac-tive alternative material to overcome performance limitations ofMgO:PPLN cw SROs due to thermal effects. Progress in polingtechnology has also enabled the fabrication of bulk MgO:sPPLTcrystals with improved optical quality, long interaction lengths(30–40 mm) and wide apertures [23], paving the way for thedevelopment of practical high-power cw SROs from the visi-ble to the mid-IR [15], [24]. Moreover, LiTaO3 is consideredto exhibit lower absorption in the 3–4 μm spectral range thanLiNbO3 , making it a potential candidate for high-power mid-IRgeneration [22]. It is thus important to explore the feasibility ofexploiting MgO:sPPLT in fiber-laser-pumped cw SROs for thegeneration of high-power mid-IR radiation. In this section, wedescribe a cw SRO based on MgO:sPPLT and pumped by anYb-fiber laser at 1064 nm, providing high optical powers withwide tuning in the near- and mid-IR. The cw SRO is tunableover 430 nm with an idler power >4 W over more than 60% ofthe tuning range and a peak-to-peak power stability of 12.8%over 5 hours at 3299 nm. We also investigate mid-IR powerscaling of this device and compare its performance with theYb-fiber-laser-pumped MgO:PPLN cw SRO [13] described inSection II-A-1.

The MgO:sPPLT cw SRO employs the same cw Yb-fiberpump laser as described in Section II-A-1 and similar cavity con-figuration to that in Fig. 2. The crystal is a 30-mm-long, 1-mm-thick, 1 mol.% MgO:sPPLT with six grating periods, rangingfrom Λ = 29.15 μm to Λ = 30.65 μm, and is housed in an ovenwith a temperature stability of ±0.1 ◦C. The crystal faces areAR-coated for the pump (R < 1% at 1064 nm), signal (R < 1%over 1400–1700 nm), and idler (R < 10% over 2900–4000 nm).The OPO cavity is a symmetric ring formed by two concave(r = 100 mm) and two plane mirrors. All mirrors have highreflectivity (R > 99%) over 1.3–1.9 μm and high transmission(T > 90%) over 2.2–4 μm, ensuring SRO operation with nosignal output coupling. For frequency control, a 500-μm-thickuncoated fused silica etalon (FSR∼ 205 GHz) is used at the sec-ond cavity waist between the plane mirrors. The pump beam is

Fig. 10. (a) Idler power, and (b) Pump depletion across the tuning range ofMgO:sPPLT cw SRO [25].

confocally focused to a beam waist radius of w0p = 48 μm (ξ =l/b ∼ 1) at the center of the crystal. The cavity design ensuresoptimum overlap of pump and resonant signal at the center ofthe crystal (bp ∼ bs), with a signal waist radius of w0s = 58 μm.A dichroic mirror separates the generated output idler from thedepleted pump beam. The total optical length of the cavity in-cluding the crystal and the etalon is 575 mm, corresponding to aFSR ∼ 522 MHz.

The characterization of the cw SRO with regard to tuningcoverage and output power was performed by varying the tem-perature of the MgO:sPPLT crystal from 40 ◦C to 200 ◦C [25].Fig. 10(a) shows the idler power extracted from the MgO:sPPLTcw SRO across the mid-IR tuning range. The data was obtainedfor a maximum available fiber pump power of 28.5 W at theinput to the MgO:sPPLT crystal. Using the two grating periods(Λ = 30.65, 30.15 μm), temperature tuning the cw SRO from∼35 ◦C to 200 ◦C resulted in an idler coverage from 3032 to3462 nm, corresponding to a total tuning range of 430 nm, witha maximum idler power of 5.5 W at 3221 nm, and >4 W overmore than 60% of the tuning range. The corresponding pumpdepletion across the tuning range is shown in Fig. 10(b), whereit can be seen that a pump depletion of >50% is achieved overmore than 40% of the tuning range. It is interesting to note thatalthough the thermal load in the nonlinear crystal due to pump,idler, and the high intracavity signal cannot be completely ig-nored, high-power operation of the MgO:sPPLT cw SRO can beeasily achieved at lower temperatures down to∼35 ◦C. This is incontrast to the MgO:PPLN cw SRO described in Section II-A-1,where high power operation is not attainable below ∼55 ◦C dueto increased thermal effects [13]. The reduced thermal effects inMgO:sPPLT cw SRO are attributed to intrinsic material proper-ties, including higher thermal conductivity (8.4 W/m-K), bettertransmission, and reduced circulating intracavity signal powerdue to lower deff as compared to MgO:PPLN. Also, the shorterinteraction length of the nonlinear crystal (30 mm) results inreduced absorption at pump, signal, and particularly the idler,which becomes significant at longer wavelengths.

Measurements of idler power scaling at different wavelengthsacross the mid-IR tuning range of the MgO:sPPLT cw SROare shown in Fig. 11(a). The results were obtained at a crystaltemperature of 40 ◦C for a grating period of Λ = 30.65 μm(idler wavelength of 3291 nm). Also included for comparison


Fig. 11. (a) Idler power scaling comparison of MgO:PPLN and MgO:sPPLTcw SROs at ∼3.3 μm, and (b) Idler power scaling of MgO:sPPLT cw SRO at3.21 μm and 3.4 μm [25].

is the power scaling results for the MgO:PPLN cw SRO [13],described in Section II-A-1, at a similar wavelength and oper-ating under the same conditions. Owing to the lower deff andshorter crystal length (30 mm), the threshold pump power ofthe MgO:sPPLT cw SRO is 17.5 W, while that of the 50-mm-long MgO:PPLN cw SRO is only 5.6 W. However, a maximumidler power of 5.2 W is generated for a fiber pump power of28.1 W at an idler efficiency of 18.5%, with no saturation in theidler power. The corresponding maximum idler power gener-ated from the MgO:PPLN cw SRO is 7.6 W for 26.6 W of pumppower at an idler efficiency of 28.6%. On the other hand, operat-ing the MgO:sPPLT cw SRO at a higher temperature of 100 ◦Cusing the same grating period, corresponding to an idler wave-length of 3212 nm, resulted in a reduced pump power thresholdof 13.2 W, with as much as 5 W of idler generated for 29.8 W ofpump power, as shown in Fig. 11(b). Also shown in Fig. 11(b) isthe power scaling at an idler wavelength of 3403 nm, providing3.8 W of idler for a pump power of 29 W, at temperature of40 ◦C, using a grating period of Λ = 30.15 μm. These mea-surements confirm that despite the lower deff resulting in higherpump threshold, multiwatt idler output powers in the mid-IR canbe generated in cw SROs using MgO:sPPLT as the nonlinearmaterial. The high output power, low thermal effects, and near-room-temperature operation thus validate that MgO:sPPLT is anattractive material for multiwatt cw mid-IR generation, with thepotential for further power scaling with increased fiber pumppower.

We measured the idler output power stability of theMgO:sPPLT cw SRO close to room temperature at ∼35 ◦C at awavelength of 3299 nm. We obtained a peak-to-peak power sta-bility better than 12.8% over 5 hours at an idler power >4.5 W,as shown in Fig. 12. The corresponding signal spectrum at1570 nm, obtained with the intracavity etalon, and recordedusing a scanning Fabry-Perot interferometer (FSR = 1 GHz,finesse = 400), is shown in the inset of Fig. 12, confirmingsingle-frequency operation with an instantaneous linewidth of∼21 MHz. Under similar conditions, we also recorded the idlerwavelength stability using a wavemeter with an absolute accu-racy of 1 ppm and a measurement rate of∼0.7 Hz. The frequencystability of the idler output recorded over a period of 1 hour, isshown in Fig. 13, confirming a peak-to-peak stability of∼1 GHzwithout stabilization. With better thermal isolation and active

Fig. 12. Idler power stability over 5 hours at room temperature. Inset: Corre-sponding signal single-frequency spectrum [25].

Fig. 13. Idler frequency stability over 1 hour at room temperature. Inset:Corresponding idler spectrum [25].

control, further improvement in frequency stability is expected.Also shown in the inset of Fig. 13 is the measured idler spectrumcentered at 3299 nm with a FWHM linewidth of ∼0.2 nm, lim-ited by the resolution of the wavemeter. Similar linewidths havebeen measured at other signal and idler wavelengths across thetuning range. Given the single-frequency nature of the Yb-fiberpump laser with a typical linewidth of ∼89 kHz, the generatedidler wave from the SRO is also expected to be in a single axialmode.

B. Near-IR to Visible OPOs

1) Green-Pumped SRO Based on MgO:sPPLT: As descri-bed in Sections II-A-1 to II-A-3, the advent of QPM non-linear materials, particularly MgO:PPLN, and more recentlyMgO:sPPLT, has enabled the development of practical fiber-laser-pumped cw SROs delivering stable multiwatt output pow-ers in the mid-IR with excellent spatial, spectral and frequencycharacteristics. By deploying the Yb-fiber laser at 1064 nm asthe pump source, such SROs can now provide spectral coveragefrom ∼1.4 μm to above ∼4 μm [8]–[13], [25]. For the extensionof the tuning range of cw SROs to wavelengths below ∼1.4 μm,the most direct approach is to deploy laser pump sources in thegreen. However, the potential of MgO:PPLN for visible pump-ing is constrained by photorefractive damage, making stableoperation of cw SROs increasingly problematic at higher opti-cal powers and shorter wavelengths. An alternative QPM mate-rial for green-pumped cw SROs is periodically-poled KTiOPO4(PPKTP), offering moderate nonlinearity (deff ∼ 10 pm/V), but


despite its wide temperature acceptance bandwidth, low ther-mal conductivity, higher absorption in the green, together withgrey tracking, make this crystal more sensitive to thermal ef-fects, limiting its use at higher powers. On the other hand,MgO:sPPLT is an attractive candidate for cw SROs pumpedin the green. Its increased resistance to photorefractive dam-age, moderate effective nonlinear coefficient (deff ∼ 9 pm/V),high thermal conductivity, low coercive field for poling, andavailability in short grating periods and long interaction lengths(30–40 mm) under type 0 (e→ee) NCPM, make it a promis-ing material for the development of green-pumped cw SROsoperating below ∼1.4 μm. In earlier experiments, we demon-strated this potential by deploying frequency-doubled solid-stateor semiconductor green lasers at 532 nm to pump cw SROs basedon MgO:sPPLT, providing watt-level output power and tuningcoverage from ∼850 nm to 1.4 μm [14], [24]. We also achievedwavelength extension of MgO:sPPLT cw SROs into the visi-ble [15] and UV [26] using additional frequency up-conversionschemes within the OPO cavity. The pump source deployed inthese experiments is based on a high-power diode-pumped cwsolid-state laser at 1064 nm with relatively complex intracavityfrequency doubling schemes involving intricate cavity designs,careful management of thermal effects, and active stabilization,to provide stable high-power, single-frequency, cw green powerto drive the cw SRO. On the other hand, the exploitation ofmore simplified pumping platforms based on Yb-fiber laserscan be very effective in drastically reducing system complexityand cost associated with such cw solid-state green lasers, whilemaintaining or enhancing device performance with regard toall important operating parameters, including the potential forpower scaling with increasing power of fiber lasers. However,extension of cw Yb-fiber laser technology to the green is in itselfchallenging, because of the difficulty in achieving sufficient cwgreen power with the required characteristics in a simplified de-sign. One approach would be resonant frequency doubling of cwfiber lasers in external enhancement cavities, but this techniquealso suffers from the drawback of elaborate resonator design,requirement for a single-frequency input laser, and active stabi-lization. We have shown that it is possible to extend the operationof green-pumped cw SROs to fiber pump lasers by single-passSHG of a cw Yb-fiber laser in the QPM nonlinear material,MgO:sPPLT, providing high-power, cw, single-frequency greenradiation at high efficiency and in a very simplified and practicaldesign [19]–[21]. Here, we present the results on an Yb-fiber-laser-based green-pumped cw SRO based on MgO:sPPLT [27].A detailed description of the fiber-laser-based green source ispresented in Section III-A-1.

The schematic of the experimental setup is shown in Fig. 14.The fundamental pump source is the same cw Yb-fiber laser usedin Sections II-A-1 and II-A-3. It delivers up to 30 W of single-frequency output at 1064 nm with a linewidth of ∼89 kHz, anda frequency stability <120 MHz over 1 hour and <50 MHz over30 minutes. The laser is frequency-doubled in a single-pass ina 30-mm-long MgO:sPPLT crystal containing a single grating(Λ = 7.97 μm). A dichroic mirror, M, coated for high reflectiv-ity (R > 99%) at 1064 nm and high transmission (T > 94%)at 532 nm, separates the generated green from the fundamental.At the highest input fundamental power of 29.5 W, we generate

Fig. 14. Schematic of experimental design for the fiber-laser-green-pumpedMgO:sPPLT cw SRO. λ/2—half-wave plate; PBS—polarizing beam-splitter;M—mirror; L—lens.

a cw, single-frequency output of 9.64 W at 532 nm at 32.7%efficiency, in a TEM00 spatial mode, with a peak-to-peak powerstability of 9% over 13 hours and frequency stability <32 MHzover 30 minutes [20], see Section III-A-1. In order to controlthe pump power, the fiber laser is operated at the maximumoutput, and the input green power to the SRO is varied using ahalf-wave-plate and a polarizing beam-splitter. A second half-wave plate is used to control the pump polarization for type 0(e→ee) phase-matching in the crystal. The MgO:sPPLT crystalused for the cw SRO is identical to the SHG sample (30-mm-long, Λ = 7.97 μm). The crystal temperature is controlled usingan oven with a stability of ±0.1 ◦C. The crystal coatings, SROring cavity, and mirror coatings are all identical to our earlierwork [15], [24]. However, to maximize total power extractionand useful tuning range, we operate the oscillator as an OC-SRO [13], [14], with mirror M4 providing varying transmission(T = 0.71%–1.1%) across the signal wavelength range. A planemirror, M′′, identical to the cavity mirrors, but with high re-flectivity (R > 99%) at 532 nm, is used as a cutoff filter forthe signal and residual pump, to enable measurements of theidler output. The pump waist radius at the center of the crys-tal is w0p ∼ 30 μm, corresponding to a focusing parameter,ξ = l/b ∼ 1. The SRO cavity provides a signal waist radius ofw0s ∼ 40 μm at 900 nm, resulting in optimum spatial overlapwith the pump inside the crystal (bs ∼ bp). For frequency se-lection, a 500-μm-thick uncoated fused silica etalon (FSR =196 GHz, finesse ∼ 0.6) is used at the second cavity waist. Thetotal optical length of the cavity is 711 mm (FSR ∼ 422 MHz).

The extracted signal and idler output powers across the tuningrange of the fiber-green-pumped cw OC-SRO, obtained by vary-ing the crystal temperature from 59 ◦C to 236 ◦C, are shown inFig. 15. The SRO can simultaneously provide useful signal andidler power over a total tuning range of ∼550 nm. Although theSHG source generates a green power of up to 9.64 W, the pumppower available at the input to the OC-SRO crystal is 7.3 W,due to un-optimized coatings of the transmission optics. Theoutput-coupled signal power, Fig. 15(a), varies from 725 mW at1000 nm (59 ◦C) to 277 mW at the extreme of the tuning rangeat 855 nm (236 ◦C), with a maximum of 800 mW at 927 nm(122 ◦C) at the highest OC ∼ 1.04% [13]. The correspondingidler power, see Fig. 15(b), varies from 1.9 W at 1136 nm (59 ◦C)to 745 mW at 1408 nm (236 ◦C), with a maximum power of


Fig. 15. Simultaneously extracted (a) Signal power. (b) Idler power, acrossthe cw OC-SRO tuning range. Green pump power is 7.3 W [27].

Fig. 16. Power stability of (a) Out-coupled signal. (b) Idler over 40 minutes.The crystal temperature is 80 ◦C [27].

2.1 W at 1168 nm (74 ◦C). As evident in Fig. 15(b), the idlerpower is nearly constant at ∼1.8 W in the range 1136–1252 nm,which is >40% of the total idler tuning range, and gradually de-creases towards the extreme of the tuning range due to differentfactors, including thermal lensing at higher crystal temperaturesand higher pump powers, as described previously [14], [24].However, unlike our earlier report [14], here we generate morethan 745 mW of idler power across the full tuning range of theOC-SRO, which is 1.5 times more than our previous results [14],and simultaneously generate over 300 mW of out-coupled signalpower. We attribute the increase in idler power to the reductionin pump and intracavity signal power density due to the looserfocusing of the pump, resulting in reduced thermal effects. Thepump depletion of the OC-SRO remains close to ∼55% overmost of the tuning range, before dropping to ∼34% at the ex-treme of the tuning range.

We recorded the power stability of the output-coupled signalat 971 nm, and the corresponding idler at 1176 nm, for a crystaltemperature of 80 ◦C. The results are shown in Fig. 16(a) and(b), respectively. The signal power exhibits slightly higher peak-to-peak stability (10.7%) than the idler power (11.7%) over40 minutes. The power instability of the signal and idler can beattributed to the green pump power fluctuation, measured to be∼8% peak-to-peak [20], and also to the residual thermal effectsin the cw OC-SRO crystal.

Fig. 17. Long-term frequency stability of signal at wavelength 971.14928 nmfor crystal temperature of 80 ◦C over 15 minutes. Inset: Short-term frequencystability over 10 seconds [27].

The single-frequency nature of the generated signal and idlerwas confirmed using a confocal interferometer (FSR = 1 GHz,finesse ∼ 400). The frequency stability of the output-coupledsignal at 971 nm (80 ◦C), measured at 600 mW using a waveme-ter, is shown in Fig. 17. The signal frequency exhibits a passivepeak-to-peak stability <75 MHz over 15 minutes, with a nearlyperiodic variation in frequency deviation also evident with time.The inset of Fig. 17 shows the short-term frequency stability,where we can observe stable signal frequency to <10 MHz(limited by the relative accuracy of the wavemeter) over 10 sec-onds, before shifting to another frequency. We observe similarfrequency stability across the signal tuning range. The observedfrequency instabilities are attributed to pump frequency jitter,pump and intracavity signal induced thermal noise, mechanicalvibration of the experimental setup, and jitter of the wavemetermeasurement instrument. We expect that the frequency stabil-ity of the system can be enhanced with active stabilization andimproved temperature stability of both crystals below ±0.1 ◦C.This should also minimize the long-term drift in signal fre-quency, also evident in Fig. 17. Due to the limited spectralresponse of the wavemeter, we were not able to record the fre-quency stability of the non-resonant idler in this experiment,but we expect the idler will not exhibit higher stability than thesignal due to the frequency fluctuations of the green pump [20].

The far-field energy distribution of the output signal beamat 971 nm, measured at a distance >2 m from the OC-SRO,together with the intensity profiles and the Gaussian fits alongthe two orthogonal axes, are shown Fig. 18. Although the dataappear to confirm a Gaussian distribution, we measured M 2

factor of the signal and idler beams at the highest availablepump power for five different crystal temperatures from 80 ◦Cto 200 ◦C, in 30 ◦C intervals, using a f = 25 cm focusinglens and scanning beam profiler. The M 2 values for the signalwere found to increase from M 2

x ∼ 1.13, M 2y ∼ 1.13 at 80 ◦C

(971 nm) to M 2x ∼ 1.52, M 2

y ∼ 1.47 at 200 ◦C (874 nm). Forthe corresponding idler, the M 2 values were comparable to thepump (M 2

x ∼ 1.28, M 2y ∼ 1.24), with a small variation in M 2

x

from 1.26 to 1.18 and M 2y from 1.26 to 1.12 across the tuning

range. The increase in M 2 value of the signal beam with crystaltemperature signifies thermal lensing effects in the cw OC-SROat higher crystal temperatures [24].

2) Frequency-Doubled SRO Based on MgO: PPLN: An al-ternative approach to the development of fiber-laser-pumped cw


Fig. 18. Far-field TEM00 energy distribution and intensity profiles of thegenerated signal beam at crystal temperature of 80 ◦C (971 nm) recorded at adistance 2 m away from OC-SRO output [27].

OPOs for wavelengths below ∼1 μm is intracavity SHG of theresonant signal wave in cw SROs pumped directly by Yb-fiberlaser at ∼1064 nm. In cw SROs based on QPM nonlinear mate-rials such as multigrating MgO:PPLN [13], offering wide signaltuning range, as described in Section II-A-1, the high circulat-ing signal powers can be exploited for internal SHG to achievewavelength extension into the visible and near-IR at high effi-ciency and over a broad spectral range. In such an approach,MgO:sPPLT also offers an excellent material choice, becauseof its favorable properties highlighted earlier, in particular itshigh resistance to photorefractive damage and availability inlong interaction lengths (30–40 mm) with fanout grating de-sign, providing additional advantage of continuous tuning at afixed temperature. Thus, the combination of MgO:sPPLT to-gether with latest generation of cw Yb-fiber lasers can lead tothe realization of efficient, compact and high-power sourcesof tunable cw radiation for the 700–1000 nm spectral range,which could also become a potential future alternative to theTi:sapphire laser. We have demonstrated this potential by de-veloping a high-power, single-frequency, all-periodically-poled,near-infrared source tunable across 775–807 nm [28], with thewavelength coverage limited by the available grating periods ofthe QPM crystals. The source is based on intracavity SHG of aMgO:PPLN cw SRO pumped by a Yb-fiber laser at 1064 nm,with MgO:sPPLT used as the frequency-doubling crystal.

The schematic of the experimental setup is shown in Fig. 19.The pump source is the same cw Yb-fiber laser used in SectionsII-A-1 and II-A-3. It delivers up to 30 W of output power at1064 nm in a single-frequency, linearly-polarized beam withM 2 ∼ 1.01 and a linewidth of ∼89 kHz. The nonlinear crys-tal for the cw SRO is 48-mm-long, 6.2-mm-wide, 1-mm-thick,multigrating MgO:PPLN with five gratings ranging in periodfrom Λ = 29.5 to 31.5 μm, in steps of 0.5 μm, with the end-faces AR-coated over 1450–1750 nm (R < 1%), 2500–3700 nm(R < 3%) and 1064 nm (R < 3%). For intracavity SHG, we usea 30-mm-long, 16-mm-wide, 0.5-mm-thick MgO:sPPLT crys-tal with a fan-out grating, ΛQPM = 19.6–22.3 μm, across the16-mm width. The crystal end-faces are AR-coated over 1500–1600 nm (R < 1%) and 750–800 nm (R < 1%). The crystals are

Fig. 19. Schematic of the intracavity frequency-doubled cw OPO. FI—Faraday isolator; λ/2—half-wave-plate; PBS—polarizing beam-splitter, L—lens; M1−6 —cavity mirrors; M′ and M′ ′—dichroic mirrors.

housed in two separate ovens with stability of ±0.1 ◦C, whichcan be controlled from room temperature to 200 ◦C. The OPOcavity is a folded ring, comprising four concave mirrors, M1−4(r = 150 mm) and two plane mirrors, M5,6 . All mirrors arehighly reflecting (R > 99.6%) for the signal (1250–1800 nm)and transmitting (T > 90%) for the idler (2200–4000 nm), en-suring SRO operation with no signal output coupling. Mirror,M4 , is also highly transmitting (T > 97%) over 600–800 nm.A lens of focal length, f = 200 mm, is used to focus the pumpbeam to a waist radius of w0p = 66 μm, corresponding to con-focal focusing parameter of ξp = l/b ∼ 0.87, while the designof the SRO cavity results in a primary signal waist radius ofw0s1 = 80 μm at the centre of the MgO:PPLN crystal (bp ∼ bs)between mirrors M1 and M2 , and secondary signal radius ofw0s2 = 117 μm (ξs2 ∼ 0.26) at the centre of the MgO:sPPLTdoubling crystal, between mirrors M3 and M4 . Dichroic mirrors,M′ and M′′, are used to separate the pump from the generatedidler beam through M2 , and the generated SHG beam from theleaked-out signal through M4 , respectively. In this study, weused the Λ = 30.5 μm grating period in the MgO:PPLN crystal,providing a signal tuning range of ∼1544–1618 nm and an idlerrange of ∼3423–3107 nm, for a change in the crystal tempera-ture from ∼25 ◦C to 200 ◦C. We initially varied the temperatureof the MgO:sPPLT crystal (TSHG) keeping the crystal fixed atthe longer grating periods, while maintaining the temperature ofthe MgO:PPLN crystal (TOPO) at 177 ◦C (λsignal = 1602 nm).At a fixed pump power of 27.4 W, we measured the QPM tem-perature for SHG to be 129 ◦C, confirming the longer gratingperiod under use to be ΛSHG = 22.12 μm.

The SHG tuning in this cw SRO was achieved by varyingthe grating period or temperature of the MgO:sPPLT crystal,while keeping either parameter fixed. Initially, SHG coverageacross 775–801 nm was obtained by simultaneously varying thetemperature of MgO:PPLN crystal, TOPO , over 57–177 ◦C, andthe grating period of MgO:sPPLT, ΛSHG , at a fixed MgO:sPPLTtemperature of TSHG = 129 ◦C. The SHG tuning range was thenfurther extended up to 807 nm by increasing TOPO to 195 ◦C,for a fixed ΛSHG = 22.12 μm, with corresponding increase inTSHG from 129 ◦C to 200 ◦C. This resulted in an overall idlertuning over 271 nm, across 3125–3396 nm, corresponding to a


Fig. 20. (a) Variation of SHG power as a function of SHG wavelength. Inset:Theoretical SHG (i) grating tuning range, and (ii) temperature tuning range inpresent setup. (b) Variation of idler power and corresponding pump depletionacross the idler tuning range [28].

Fig. 21. Variation of SHG power, idler output power, and pump depletion asa function of pump power. Inset: Signal power scaling [28].

resonant signal wavelength range of 1550–1613 nm, and tunableSHG coverage over 775–807 nm. Fig. 20(a) shows the measuredSHG power across the tuning range for a fixed input pump powerof 27.4 W. As evident, the SHG output is continuously tunableover the entire range, providing >3 W of output power over56% of the tuning range, with a maximum of 3.7 W at λSHG =793 nm. Also shown in the inset of Fig. 20(a) are the calculatedSHG tuning curves, (i) and (ii), using the relevant Sellmeierequations [29], where good agreement with the experimentaldata in Fig. 20(a) is confirmed. The simultaneously measuredidler power across the tuning range is shown in Fig. 20(b).We achieved >3.8 W of output power over 77% of the idlertuning range with a maximum of 4.3 W at λidler = 3133 nm.The pump depletion remains above 61% over 67% of the idlertuning range, and follows the similar behavior to the idler powerover the entire tuning range, as evident in Fig. 20(b).

We performed simultaneous power scaling measurements ofSHG output and the corresponding idler at the SHG wavelengthof 801 nm. The results are shown in Fig. 21. The SHG powershows quadratic dependence up to ∼17 W of pump power, be-yond which it grows linearly, with a maximum of 3.2 W gener-ated for 27.4 W of pump power. The corresponding maximumidler power obtained is 4.1 W at λidler = 3168 nm, repre-senting a total extraction efficiency of 26.6%. We obtained the

Fig. 22. Simultaneously recorded passive power stability of (a) SHG, and(b) Idler output from the SRO. (c) Single-frequency spectrum, and (d) (coloronline) Far-field energy distribution of generated SHG at 801 nm for a pumppower of 27.4 W [28].

maximum overall extraction efficiency of 28% at a SHG wave-length of 793 nm, which corresponds to a SHG power of 3.7 Wtogether with an idler power of 4.0 W at 3232 nm. The pumppower threshold for the internally frequency-doubled cw SRO is7.5 W. The corresponding pump depletion reaches a maximumof ∼65% at 24.6 W of pump power, as shown in Fig. 21. Alsoshown in the inset of Fig. 21 is the variation of the signal powerleaked-out through mirror, M4 , as a function of pump power.The signal power has a linear dependence on pump power upto ∼17 W, beyond which there is evidence of saturation. Theeffect of saturation at higher pump powers could be attributedto non-optimal mode-matching between the pump and signal inthe MgO:PPLN crystal, since the cavity is optimized for maxi-mum SHG output, and not maximum signal power. Consideringthe signal leakage through all OPO cavity mirrors, a total sig-nal output power of ∼370 mW is extracted at maximum pumppower. Using the leaked-out signal power and transmission ofmirror, M4 , at 1602 nm, we calculated the intracavity signalpower at maximum pump power to be 23.5 W, resulting in a cwsingle-pass SHG conversion efficiency of 13.6%.

We also recorded the passive power stability of the generatedSHG at 801 nm and the corresponding idler, simultaneously, forthe maximum input fiber pump power of 27.4 W. The resultsare shown in Fig. 22(a) and (b), where the SHG power exhibitsa passive rms stability better than 3.5%, while the idler powerstability is better than 1.3% over more than one minute. The out-put power stability could be improved with active stabilization,and further thermal and mechanical vibration isolation of the cwSRO. We also analyzed the output spectrum of the SHG outputusing a scanning confocal Fabry-Perot interferometer (FSR =1 GHz, finesse = 400). The measurement was performed at801 nm and at maximum SHG power. The transmission spec-trum confirms single-frequency operation with an instantaneouslinewidth of 8.5 MHz, as shown in Fig. 22(c). Similar behav-ior was observed across the full SHG tuning range. Fig. 22(d)shows the recorded far-field energy distribution of the SHG out-put at 801 nm, together with the orthogonal intensity profiles,at maximum fiber pump power. The data confirm Gaussian dis-tribution with a beam circularity >90%. For different pumpinglevels and across the tuning range, we observed similar behav-ior. We measured the M 2 factor of the SHG beam at maximum


input power and obtained M 2x < 1.4 and M 2

y < 1.4, confirmingTEM00 spatial mode. The SHG tuning range of the frequency-doubled cw SRO can be further extended to cover almost theentire Ti:sapphire wavelength range (∼740–972 nm) using dif-ferent grating periods of the employed MgO:PPLN crystal andsimultaneously varying the grating of the MgO:sPPLT doublingcrystal. Given the power scaling capability of fiber lasers, thedescribed approach also offers clear potential for even higherSHG output powers with increased pump powers.

III. HARMONIC GENERATORS

For the extension of spectral coverage of cw fiber-pumpedfrequency conversion sources to shorter wavelengths in the vis-ible and UV, we can deploy nonlinear up-conversion techniquesbased on SHG and THG/SFG. The high cw optical powersnow available from cw Yb-fiber lasers in excellent spatial andspectral beam quality can be exploited to develop high-powercw sources in the visible and UV in more simplified and cost-effective formats. A particularly effective approach is direct ex-ternal single-pass SHG (SP-SHG) of the cw Yb-fiber laser intothe green by deploying QPM materials, where the low nonlineargains under cw pumping can be overcome by exploiting opti-mal focusing over long interaction lengths under type 0 (e→ee)NCPM in the absence of spatial walkoff. The SP-SHG is alsoattractive approach for high-power cw generation in the green,not only because of a compact and practical architecture, butalso due to the power scaling potential of the fiber laser, as wellas the narrow linewidth and high spatial beam quality that areinherently transferable from the fiber pump laser to the greenoutput. The green power so generated can be further mixed withthe fundamental in a simple single-pass configuration by ex-ploiting THG/SFG in a suitable nonlinear crystal to generatecw UV radiation. By deploying a high-power cw Yb-fiber laserat 1064 nm as the fundamental source, the QPM nonlinear ma-terial of MgO:sPPLT for SP-SHG and birefringent crystal ofBIBO for SP-THG, we have generated multiwatt output powersat exceptional efficiencies in the green and practical powers inthe UV, with high spatial and spectral beam quality and goodoutput stability, in a highly compact, simplified and practicaldesign.

A. Green Sources at 532 nm

1) Single-Pass SHG Based on MgO:sPPLT: As highlightedin Section II-B-1, the development of high-power fiber-laser-based cw green sources is of considerable interest for a varietyof applications, including pumping of cw SROs operating inthe visible and near-IR [27]. Such sources also offer promiseas pumps for future generation of cw and ultrafast Ti:sapphirelasers in compact, practical and portable design [30], with thepotential for power scaling. Traditionally, the development ofhigh-power cw green sources has relied almost exclusively oninternal SHG of cw solid-state lasers at ∼1064 nm, or externalSHG using resonant enhancement cavities, both involving elab-orate systems designs, careful management of thermal effects,and active cavity stabilization for stable single-frequency gen-eration, culminating in relatively high complexity and cost. On

Fig. 23. Schematic of the experimental design for SP-SHG of cw Yb-fiberlaser in MgO:sPPLT. λ/2—-half-wave plate; PBS—polarizing beam-splitter;L—lens; M—dichroic mirror.

the other hand, SP-SHG of cw Yb-fiber lasers at ∼1064 nm innew QPM materials can offer a highly effective approach forhigh-power green generation in a uniquely simplified and cost-effective design. Of the different QPM materials developed todate, MgO:sPPLT is a particularly attractive candidate for SP-SHG into the green because of its resistance to photorefractivedamage, high thermal conductivity, and low transmission loss,compared to MgO:PPLN and PPKTP [19]–[21]. It is also avail-able in long interaction lengths (30–40 mm) with sufficientlyshort grating period (Λ<8 μm) for 1st-order QPM SHG intothe green. By exploiting these favorable properties, we havedeveloped a high-power cw green source in a simple designbased on external SP-SHG of a cw Yb-fiber laser at 1064 nmin MgO:sPPLT. The source can deliver 9.64 W of cw, single-frequency green power at 532 nm at a conversion efficiencyas high as 32.7% in a TEM00 spatial mode, with peak-to-peakpower stability of 9% over 13 hours and frequency stability<32 MHz over 30 minutes. With its high passive stability, mul-tiwatt power, and single-frequency performance, the fiber-laser-based green source can be readily deployed for pumping cwSROs, as already demonstrated in Section II-B-1, and for thedevelopment of the first cw fiber-laser-pumped Ti:sapphire laser,as described in Section IV.

The configuration of the SP-SHG source is shown in Fig. 23.The fundamental laser is the same cw Yb-fiber at 1064 nm usedpreviously, delivering ∼30 W of single-frequency output powerin a linearly polarized beam with M 2 < 1.01 and a linewidth of∼89 kHz. Using a wavemeter, the frequency stability of the laseris measured to be <120 MHz over 1 hour and <50 MHz over30 minutes. The nonlinear crystal is a 30-mm-long MgO:sPPLTcontaining a single grating (Λ = 7.97 μm), and is housed inan oven with a temperature stability of ±0.1 ◦C. The crystalfaces have low reflectivity (R < 0.5%) at 1064 nm and hightransmission (T > 99%) at 532 nm. A combination of a half-wave plate and a polarizing beam-splitter is used to attenuatethe pump power, and a second half-wave plate controls thepolarization of the fundamental for type 0 (ee→e) SHG in theMgO:sPPLT crystal. A dichroic mirror (R > 99% at 1064 nm;T > 94% at 532 nm) separates the generated green from theunconverted fundamental.

In order to optimize SHG conversion efficiency and outputpower, we used several focusing conditions corresponding todifferent values of the focusing parameter, ξ = l/b [16]. Herel is the crystal length and b = kw2

0p is the confocal parame-ter of the pump, with k = 2πnp /λp , where np , λp , and w0p

are the refractive index, wavelength, and waist radius of thefundamental pump beam inside the crystal, respectively. Wemeasured maximum generated SHG powers and correspond-ing optimum phase-matching temperatures using seven different


Fig. 24. Maximum SH power and corresponding phase-matching temperatureas a function of the focusing parameter, l/b. The vertical dashed line correspondsto the optimal focusing condition (l/b ∼ 2.84) [20].

focusing conditions, ξ = 0.32, 0.81, 1.23, 1.74, 2.48, 4.50, and6.60, at a fixed fundamental power of 29.5 W at the input tothe crystal. The results are shown in Fig. 24. For weak focusing(ξ < 2.48), the maximum SHG power increases with higher ξ,whereas for tight focusing (ξ > 2.48), it decreases with increas-ing ξ. Interestingly, the extrapolated power curve has a clear peaknear ξ ∼ 2.84, corresponding to the theoretical prediction foroptimum SHG in the cw (or long-pulse) limit [16]. We obtaineda maximum SHG power of 9.64 W at ξ = 2.48 (w0p ∼ 31 μm),corresponding to a single-pass conversion efficiency of 32.7%.It is also clear in Fig. 24 that the phase-matching temperature de-creases with tighter focusing. This is to be expected, since crystalheating effects due to various absorption mechanisms includinggreen-induced IR absorption (GRIIRA) of fundamental, two-photon absorption (TPA) of fundamental and green, and linearabsorption at both wavelengths, are stronger under tight focus-ing, leading to a greater optically induced temperature rise inthe crystal, and therefore, necessitating lower externally appliedheat to the sample. However, GRIIRA is not expected to makea significant contribution to crystal heating at these power den-sities, because of its suppression due to MgO doping [31]. Ourstudies have confirmed that thermal effects in the MgO:sPPLTcrystal are neither due to GRIIRA nor TPA, but a result of in-trinsic linear absorption in the IR and green [20]. It has beensuggested that the maximum available SHG power is limitedby the thermal effects resulting either from only the absorptionof fundamental radiation [32], or only the SHG power [33].However, our investigations [20], together with our earlier re-port [14], confirm that the thermal effects are due to the ab-sorption of both fundamental and SHG power, with the majorcontribution from the green. From measurement of generatedgreen power in Fig. 24, the normalized conversion efficiencywas calculated to vary from 0.42%/W at ξ = 0.32 to 1.26%/Wat ξ = 6.60, with a maximum of 1.70%/W at ξ = 2.48. Thenormalized efficiency is not limited by fundamental linewidth,since the FWHM spectral acceptance of the 30-mm-long crystalcalculated from Sellmeier equations [29] is 0.082 nm, far widerthan the fiber laser linewidth of ∼89 kHz.

To characterize the power scalability of SHG, we recorded theSHG power and efficiency at ξ = 2.48 (w0p ∼ 31 μm) up to themaximum available fiber laser power, as shown in Fig. 25. Weobtained 9.64 W of green power at the full fundamental power

Fig. 25. Dependence of the measured cw SHG power and the correspondingconversion efficiency on the incident fundamental power. Inset: Variation in theSHG power as a function of square of the fundamental power [20].

Fig. 26. Frequency stability of green beam at 9.64 W over 90 minutes. Inset:Corresponding single-mode spectrum [20].

of 29.5 W at a single-pass efficiency of 32.7%. The quadraticincrease in SHG power and the corresponding linear variationin efficiency are maintained up to a fundamental power of 22 W,after which saturation sets in. The saturation effect is also evi-dent from the deviation of the linearity of SHG power with thesquare of the fundamental power, in the inset of Fig. 25, andis attributed to pump depletion, back-conversion and thermalphase-mismatch effects in the MgO:sPPLT crystal. Further in-creases in SHG power beyond 9.64 W will be possible using apump beam waist of w0p ∼ 29 μm (ξ = 2.84), and improvedthermal management to overcome saturation.

The frequency stability of the generated green, measured at9.64 W using a wavemeter, is shown in Fig. 26. Under pas-sive conditions and without thermal isolation, the green out-put exhibits a peak-to-peak frequency fluctuation <115 MHzover 90 minutes, with a short-term stability <32 MHz over30 minutes. The transmission spectrum of the generated green,monitored through a confocal scanning interferometer (FSR =1 GHz, finesse = 400), is also shown in the inset of Fig. 26,confirming single-frequency operation with an instantaneouslinewidth of ∼6.5 MHz. Similar behavior was observed for allpumping levels with the same instantaneous linewidth, confirm-ing robust single-mode operation at all fundamental powers.

The power stability near the maximum green power of 9.64 Wis shown in Fig. 27, demonstrating a peak-to-peak fluctuationof 7.6% over the first 8 hours and 9% over 13 hours. The powerfluctuation is attributed mainly to the change in laboratory en-vironment, so further improvements in power stability, below3%, are expected through thermal isolation of the system andbetter temperature control. The far-field energy distribution of


Fig. 27. Green output power stability at 9.64 W over 13 hours. Inset: Far-field TEM00 energy distribution and intensity profiles of the generated greenbeam [20].

the green beam at 9.64 W, together with the intensity profile andthe Gaussian fits along the two orthogonal axes, are shown ininset of Fig. 27. Using a focusing lens (f = 25 cm) and scanningbeam profiler, we measured M 2 values of the green beam to beM 2

x ∼ 1.29 and M 2y ∼ 1.33 with a circularity of ∼0.96, confirm-

ing TEM00 spatial mode. Similar M 2 values were measured atdifferent input power levels, showing a small variation in M 2

x

from 1.11 to 1.29 and M 2y from 1.17 to 1.33. We observed no de-

terioration in the green power and beam quality during repeatedmeasurements and with continuous operation over many hoursand days, confirming the absence of photorefractive damage.We have operated the source reliably and regularly on a dailybasis for several years, without any degradation in output poweror stability. These characteristics make the fiber-laser-pumpedSP-SHG source a simple, compact and viable alternative to con-ventional cw solid-state green lasers for numerous applicationsincluding as pump for cw SROs, as already demonstrated inSection II-B-1. The source has also been successfully used todevelop the first cw fiber-laser-pumped Ti:sapphire laser, as de-scribed in Section IV.

2) Multicrystal Single-Pass SHG Based on MgO:sPPLT:With the demonstrated merits of the SP-SHG technique inMgO:sPPLT, including simplified design, high conversion effi-ciency and output power, passive power and frequency stability,and high spectral and spatial beam quality, we have extendedthe approach to a cascaded multicrystal scheme, which can pro-vide the highest conversion efficiency at any given fundamentalpower [21]. By using the same cw Yb-fiber laser at 1064 nmand identical MgO:sPPLT crystals, we have achieved cw SP-SHG efficiencies of up to 56% at low to moderate (<10 W)as well as high (>10 W) fundamental power by exploiting asuitable number of crystals in the cascade. We obtain a nearlyfour-fold increase in cw SP-SHG efficiency at low pump powersand a two-fold increase at high pump powers compared to thesingle-crystal SP-SHG approach, without compromising beamquality and output stability. The technique has particularly im-portant implications for SP-SHG of more commonly availablecw lasers offering <10 W of pump power, where the attain-ment of highest efficiency is mandatory to achieve significantcw SHG power.

The schematic of the experimental setup is shown in Fig. 28.The cw Yb-fiber laser delivers 30 W of single-mode output at

Fig. 28. Experimental design for MC cw SP-SHG. λ/2—half-wave plate;PBS—polarizing beam-splitter; L—lens; M—mirrors. X1 –X3 —MgO:sPPLTcrystal in oven.

1064 nm in a linearly polarized beam with M 2 < 1.01, witha linewidth of ∼89 kHz, and power stability of 0.5% over10 hours. The SP-SHG arrangement consists of three identi-cal MgO:sPPLT crystals, each 30-mm-long (temperature ac-ceptance bandwidth ∼1.3 ◦C [19]), with a single grating (Λ =7.97 μm), and housed in three separate ovens with tempera-ture stability of ±0.1 ◦C. All crystals end-faces are AR-coated(R < 0.5%) at 1064 nm and have high transmission (T > 99%)at 532 nm. A combination of two half-wave plates and a polar-izing beam-splitter is used for power attenuation and polariza-tion control of the fundamental for type 0 (ee→e) SHG in theMgO:sPPLT crystals. The design consists of three separate SP-SHG stages in series. In the first stage, the fundamental beam isfocused at the center of the first crystal (X1) to a beam radius,w0p ∼ 31 μm, close to the optimum focusing for SHG [20].In the second stage, both the fundamental and SHG beams arecollimated using mirror, M1 (r = 150 mm), and focused atthe centre of the second crystal (X2) using mirror, M2 (r =250 mm), with w0p ∼ 46 μm. In the third stage, both beamsare again collimated using mirror, M3 (r = 150 mm), and arefocused at the centre of the third crystal (X3) using mirror, M4(r = 200 mm), with w0p ∼ 40 μm. The fundamental beamwaists inside X2 and X3 were larger than the optimum focusingto avoid possible damage to the crystals. In order to circumventpotential phase shift between the fundamental and SHG in pass-ing through refractive lenses, we employ concave mirrors forrefocusing into consecutive crystals. With this arrangement, weare also able to maintain perfect mode-matching of the funda-mental and the SHG beam at the center of crystals, X2 and X3 .Considering the phase shift between the fundamental and SHGin air as 27.4 ◦/cm [34], due to the mechanical constraint ofthe experimental setup, we maintained the separation betweenX1 and X2 at 37 cm and between X2 and X3 at 86.5 cm [35].These separations, however, did not necessarily correspond tooptimum phase retardation between the fundamental and SHGat the input to X2 and X3 , because of the exact content of lab-oratory air. Nevertheless, we did not observe any variation inthe green power with small changes in these separations over


Fig. 29. Maximum cw SP-SHG output efficiency obtained in the SC, DC, andMC schemes versus fundamental power [21].

several millimeters, although substantial changes are expectedwith variations in the separations over centimeters [36], [37].A dichroic mirror, M (R > 99% @ 532 nm; T > 94% @1064 nm), separates the generated green after the final stagefrom the unconverted fundamental.

We studied SHG efficiency as a function of fundamentalpower in the single-crystal (SC), double-crystal (DC), and mul-ticrystal (MC) schemes, with the results shown in Fig. 29. Thefundamental power was measured before X1 and the green out-put power was recorded after the mirror, M. In the SC scheme,we obtained a maximum conversion efficiency of ∼33% at thehighest input power of 30 W. The efficiency increases linearly upto ∼21 W of pump, after which saturation occurs, as confirmedby the quadratic variation of SC green power, also observed inour earlier experiment in Section III-A-1. This behavior arisesfrom the crystal heating effects, resulting in a maximum effi-ciency of ∼33% with a SHG power of 9.6 W. Although thisis a moderately high cw SP-SHG efficiency for 30 W of fun-damental, for lower input powers (10–20 W) the efficiency andpower are significantly lower. To increase the efficiency at lowerpowers, we deployed the DC scheme, where we effectively in-creased the crystal length to 60 mm from the SC scheme. TheSP-SHG efficiency now increases linearly at low pump powersup ∼10 W, rising to as much as 54.8% at ∼21 W of input fun-damental. Further increasing the fundamental power to 25.1 Wleads to a roll-off in efficiency from 54.8% to 52%, correspond-ing to a green power of 13 W. This can be attributed to thermaldephasing in the second crystal due to the high green power(13 W), pump depletion and back-conversion. To ascertain themaximum attainable cw SP-SHG efficiency, we used the MCscheme to obtain higher efficiency at lower pump power un-der reduced thermal dephasing effects. In the MC scheme, weused a total effective crystal length of 90 mm. The SP-SHGefficiency in this case increases linearly for low fundamentalpowers, reaching as much as 56% at ∼10 W, providing 5.6 Wof green power. Further increase in input power results in aroll-off in efficiency. It is to be noted that all data presentedhere are not corrected for the absorption losses of the crystals(0.1%/cm at 1064 nm and 1.5%/cm at 532 nm [19]) and thereflection losses at the crystal surfaces. We have experimentallyconfirmed that the roll-off in SHG efficiency in the MC schemeis due to pump depletion and back-conversion, and not thermaldephasing effects in the MgO:sPPLT crystals [21].

Fig. 30. Efficiency enhancement factor (ratio of the green power after X3 tothat after X1 ) in the MC scheme versus pump power. Solid lines are guides tothe eye. Inset: Table shows comparison of maximum SH power and efficiencyin SC, DC, and MC schemes at 10 W of pump power [21].

In order to optimize the MC scheme, we adjusted individualcrystal temperatures to manipulate pump depletion such that thetotal output green power is always at its maximum for each fun-damental power. Higher depletion of the fundamental power inone stage reduces the fundamental power to be depleted in thesubsequent stages, hence lowering the overall efficiency. How-ever, we have the flexibility to control individual crystal tem-peratures separately. We determined the efficiency enhancementfactor in the MC scheme by comparing the green power afterX1 and X3 as function of pump power. The results are shownin Fig. 30. Without any correction for losses, we obtained anenhancement of ∼8 times up to 10 W of input fundamentalpower. This confirms that the MC scheme behaves as an opti-mized SC scheme with an equivalent crystal length of 90 mm,where the 3 times longer crystal length results in a theoreticalenhancement factor of 9. For higher input powers, we observe areduction in the enhancement factor, which we attribute to theroll-off in efficiency after reaching the highest value of ∼56%due to pump depletion and back-conversion, as shown in Fig. 29.We also compared the SHG efficiency and power in SC, DC,and MC schemes based on the data in Fig. 29 at a fixed fun-damental power of 10 W, with the results tabulated in the insetof Fig. 30. We obtained a maximum green power of 1.5 W and3.7 W in the SC and DC schemes, respectively, whereas in MCscheme we obtained 5.6 W of green power, representing >373%and >151% power enhancement with respect to the SC and DCschemes, respectively.

To verify the long-term reliability of the MC scheme, wemeasured the green power stability near the highest conversionefficiency of 56%. The result is shown in Fig. 31, where a peak-to-peak fluctuation of 10.5% over 10 hours is obtained, whichcan be further improved through thermal isolation of the sys-tem. The far-field energy distribution of the green beam at 56%efficiency, together with the intensity profile and the Gaussianfits along the two orthogonal axes, are shown in the inset ofFig. 31. Using a focusing lens (f = 20 cm) and scanning beamprofiler, we measured M 2 values of the green beam to be M 2

x <1.60 and M 2

y < 1.34, confirming the TEM00 spatial mode. Thegreen beam has a circularity of 0.80, which can be improved byreducing the angle of incidence on the concave mirrors usingimproved mechanical design. The performance of the MC SP-SHG scheme demonstrates the flexibility and viability of this


Fig. 31. Green output power stability over 10 hours using MC scheme. Inset:Far-field TEM00 energy distribution and intensity profiles of the generated greenbeam [21].

technique in attaining the highest SHG efficiency and power fora wide range of pump powers available with cw fiber lasers. Thetechnique permits independent focusing, mode-matching, andtemperature control in each stage to provide maximum overallSP-SHG efficiency and output power, while also allowing con-trol of thermal effects and risk of optical damage in each crystal.The technique is generic and can be deployed for efficient SP-SHG of cw laser sources in other spectral regions and usingalternative QPM materials.

B. UV Source at 355 nm

1) Single-Pass THG Based on BiB3O6: We have further ad-vanced the spectral reach of fiber-laser-based frequency con-version sources to the UV by extending the SP-SHG schemeto THG. By deploying the cw Yb-fiber laser in combinationwith MgO:sPPLT for SP-SHG, and the birefringent crystal ofBiB3O6 (BIBO) for SP-THG, we have generated 68 mW ofcw single-frequency radiation at 355 nm in a simple designwith high frequency and power stability and in TEM00 spa-tial profile [38]. We use BIBO for THG because of its uniquelinear and nonlinear optical properties for frequency conver-sion into the UV [39], [40]. These include a relatively highoptical nonlinearity (deff ∼ 3.9 pm/V) and bulk UV damagethreshold (50 MW/cm2) [41], and low UV absorption coeffi-cient (αUV < 0.02 cm−1) [39]. The nonlinear figure of merit(FOM = deff

2 /n1n2n3) of BIBO is >20 times higher than that ofLBO, and it offers low spatial walkoff under type I (ee→o) sum-frequency mixing (SFM) between 1064 and 532 nm radiationinto the UV, which is particularly important for the attainmentof maximum efficiency under cw and low-intensity pumpingwith tight focusing. Since both input beams are extraordinarypolarized, they experience a spatial walk-off angle of 64.8 mradand 68 mrad, respectively, resulting in a small relative walk-off angle, Δρ ∼ 3.2 mrad, thus maximizing interaction length.These characteristics make BIBO a highly attractive birefringentcrystal for cw UV generation using SFM of 1064 and 532 nmradiation.

The schematic of experimental setup is shown in Fig. 32. Thecw Yb-fiber pump laser delivers up to 30 W of single-frequencyradiation at 1064 nm with a linewidth of ∼89 kHz in linearpolarization. An isolator at the output end of the fiber laser pro-tects it from back-reflections. A combination of two half-waveplates and a polarizing beam-splitter is used for power attenu-

Fig. 32. Schematic of experimental setup for the Yb-fiber-laser-based single-pass cw UV source. FI—Faraday isolator; λ/2—half-wave plate; PBS—- polar-izing beam-splitter; L—lens; M—mirror; F—filter.

ation and polarization control of the pump for phase-matchingin the nonlinear crystals. The setup essentially consists of a SP-SHG stage followed by a SP-THG stage. We used a 30-mm-longMgO:sPPLT crystal with a single grating period of Λ = 7.97 μmfor first-order QPM SHG in the first stage [20]. The crystal ishoused in an oven with a temperature stability of ±0.1 ◦C. TheTHG stage uses a 10-mm-long BIBO crystal, cut at θ = 146.3◦

(φ = 90◦) for type-I (ee→o) SFM of 1064 nm and 532 nmradiation to generate UV output at 354.7 nm. The SHG stageis temperature phase-matched, while the THG stage is phase-matched by rotation of BIBO crystal at room temperature. Theend-faces of both crystals are AR-coated (R < 1%) at 1064 nmand 532 nm, while the BIBO faces are also AR-coated for hightransmission at 354.7 nm. Using a f = 125 mm focal lengthlens, the fundamental beam at 1064 nm is focused to a waistradius of wF 1 ∼ 30 μm at the center of the MgO:sPPLT crystalfor SHG to 532 nm. The generated green and the undepletedfundamental are again collimated and refocused by using M1and M2 in the THG stage [21]. The angle of incidence on thesemirrors is limited by the mechanical constrains and kept as smallas possible. All plano-concave mirrors are coated for high re-flectivity (R > 99%) at 1064 nm and 532 nm, and are mountedon translation stages so as to adjust the inter-crystal spacing.The radius of curvature of M1 and M2 are r = 150 mm and r =200 mm, respectively, resulting in estimated fundamental andSHG beam waist radii of wF 2 ∼ 43 μm and wSH 2 ∼ 30 μmat the center of the BIBO crystal. It is well-known that for themost effective interaction, the two beams must have the sameconfocal parameter [16]. In practice, optimum THG efficiencyis achieved when the two interacting beam optimally overlapthroughout the length of the nonlinear crystal. For the focusedwaist radii used, the resulting confocal parameter for both beamsis bF 2 ∼ bSH 2 > 40 mm, well above the crystal length of 10 mm,hence ensuring maximum interaction throughout the full crystallength. The generated UV radiation is separated using dichroicmirrors, M, and a piece of Schott glass (UG1) is used to filterout any residual fundamental and SHG light while recording theUV power.

We performed power scaling measurements of the generatedUV radiation at the output of the THG stage. Fig. 33 shows thevariation of single-pass cw UV output from the BIBO crystal as


Fig. 33. Variation of the cw UV power as a function of fundamental powerat the output of the THG stage. Inset: Dependence of the measured cw SHGpower and the corresponding conversion efficiency on the incident fundamentalpower at the output of SHG stage. The solid curves in both plots represent thepredicted quadratic fits [38].

a function of fundamental power at the input to the SHG stage.It can be seen that up to 68 mW of output power at 354.7 nmis generated for a fundamental power of 27.8 W at 1064 nm.Also shown in the inset of Fig. 33 is the SP-SHG efficiency andpower scaling at 532 nm in the MgO:sPPLT crystal as functionof fundamental power at 1064 nm. While generating the max-imum UV power, the SHG power was recorded to be 8.2 Wat the input to the THG stage. At higher fundamental powersbeyond 20 W, the SHG efficiency saturates at a maximum valueof 29%, as evident in the inset of Fig. 33. The ratio of SHG tofundamental power varied from 0 to 0.3 during the UV powerscaling measurements. During the measurements, as the funda-mental power was increased, the phase-matching temperature ofMgO:sPPLT crystal was always adjusted to achieve maximumSHG power.

To study the effect of SHG-to-fundamental power ratio onthe generated UV power, we changed the temperature of theMgO:sPPLT crystal away from optimum phase-matching con-dition, thereby reducing the SHG power and increasing the un-depleted fundamental, while pumping at the maximum availablefiber laser power of 27.8 W. However, we found that the UVpower is always maximized when the SHG power is maximum,implying that the generated UV power is limited by the availableSHG power. Further, by fixing the phase-matching temperatureof MgO:sPPLT crystal at its optimum value to generate max-imum SHG power, and pumping above 20 W of input power,we found that the generated UV power is strongly affected bydepletion of the fundamental power, resulting in >5% drop inthe UV power associated with fundamental beam quality degra-dation, which soon reached a stable, steady-state condition withno further reduction in the UV output power. Under certain con-ditions, we were able to generate >70 mW of UV power, butit was not stable. After continuous operation of the UV sourceover >10 hours at maximum fundamental power, we observeda ∼30% drop in the UV output power, which could be retrievedby translating the BIBO crystal to use a new portion of the crys-tal. This drop in the output UV power is attributed to the ARcoating damage on the exit face of the BIBO crystal, which wasalso previously observed in well-established materials such asLBO [42]. Further studies are necessary to understand the origin

Fig. 34. UV output power stability at 50 mW over 2 hours. Inset: UV spectrummeasured at full output power [38].

Fig. 35. Long-term frequency stability of the generated UV radiation at354.7945 nm, over a period of 2.5 hours [38].

of this damage. In order to avoid any damage to the BIBO crys-tal, we limited the UV power to ∼50 mW during the remainderof our measurements.

We also recorded the long-term power stability of the gener-ated UV radiation at ∼50 mW of output power. The results arepresented in Fig. 34, where it can be seen that the UV powerexhibits a passive power stability better than 3.2% rms over2 hours and 1.5% rms over 50 minutes. Also shown in the insetof Fig. 34 is the UV output spectrum, centered at 354.7 nm,measured using a spectrometer with a resolution of 0.27 nm.As the fundamental source has a single-frequency linewidth of∼89 kHz, the green and UV output are also expected to be single-frequency. To study the frequency deviation of the single-passUV source, we recorded the output wavelength as a function oftime. The frequency stability of the UV radiation, measured at50 mW of output power using a wavemeter, is shown in Fig. 35.Under passive conditions and in the absence of thermal isola-tion, the UV output exhibits a peak-to-peak frequency deviationof 436 MHz over 2.5 hours, measured at a central wavelengthof 354.7945 nm, as compared to 156 MHz over a similar timescale for the Yb-fiber laser at 1064.3812 nm. The additionalfrequency fluctuations in the UV could be attributed to environ-mental noise such as mechanical vibrations, air currents, andtemperature fluctuations in the setup.

The far-field energy distribution of the UV output beam at354.7 nm, together with the intensity profiles and the Gaussianfits along the two orthogonal axes, measured at the output ofthe THG stage, are shown in Fig. 36. The UV beam exhibitsa circularity of 66% due to the spatial walk-off between the


Fig. 36. Far-field energy distribution and intensity profiles of the generatedUV beam at full output power [38].

fundamental, green and UV beams, but can be further circu-larized using suitable cylindrical optics. In order to confirm aGaussian distribution, we measured the M 2 factor of the UVbeam at the highest available fundamental power, using a f =10 cm focusing lens and scanning beam profiler, resulting inM 2

x < 1.6 and M 2y < 1.8, confirming a TEM00 spatial mode.

IV. FIBER-LASER-PUMPED TI:SAPPHIRE LASER

The development of viable multiwatt cw green sources inhighly simplified design based on SP-SHG of high-power cwYb-fiber laser in MgO:sPPLT has also paved the way for theadvancement of the future generation of fiber-laser-pumpedTi:sapphire lasers. In addition to reduced system complexityand cost, and a more compact, practical and portable architec-ture, the transition from solid-state to fiber pump sources offersthe advantages of air-cooling and power scaling available tofiber lasers. We have demonstrated this potential by developinga high-power cw Ti:sapphire laser using the SP-SHG cw fibergreen source described in Section III-A-1. This laser has beenshown to deliver an output power of >2.7 W for ∼11 W of fiber-pumped green power, with tunability across 743–970 nm, lim-ited by the available mirrors, and in a TEM00 spatial profile [30].We have further extended the operation of this fiber-pumped cwTi:sapphire laser to a ring geometry and achieved high-powersingle-frequency performance with as much as 2.3 W of outputpower for an incident green pump power of 11.3 W [43].

The schematic of the experimental setup is shown in Fig. 37.The Yb-fiber pump laser and the SP-SHG scheme are as de-scribed in Section III-A-1. At a maximum Yb-fiber power of∼33 W, a cw green power of 11.3 W in a TEM00 spatial pro-file (M 2 < 1.3) is available for pumping the Ti:sapphire ringlaser. The cw green source has a frequency stability better than32 MHz and a peak-to-peak power stability better than 3.3% over1 hour, and is so robust that it takes only 30 minutes to reach sta-ble output power during the Yb-fiber laser warm-up time [20].To maintain stable output characteristics, the green source isoperated at maximum power, and a combination of a half-waveplate and a polarizing beam-splitter is used to vary the inputpower to the Ti:sapphire laser. A second half-wave plate is used

Fig. 37. Schematic of the fiber-laser-based green-pumped cw Ti:sapphirering laser. FI—Faraday isolator; λ/2—half-wave plate; PBS—polarizing beam-splitter; L—lenses; M—mirrors; OC—output coupler; PZT—piezoelectrictransducer; BRF—birefringent filter; Det—detector [43].

to provide the correct pump polarization relative to the crystalorientation. The green pump beam is focused to different waistradii at the center of the 10-mm-long, Brewster-cut Ti:sapphirecrystal (0.15 wt.% doping, FOM > 270). The crystal is mountedon a brass slab, and water-cooled on the lower side. The greenbeam is polarized along the c-axis of the crystal in order to max-imize absorption [44], measured to be >80%. The laser cavityis an astigmatic-compensated four-mirror ring comprising twoplano-concave mirrors, M1 and M2 (r = 10 cm), a plane mirror,M3 , mounted on a piezoelectric transducer, and a plane outputcoupler (OC). All mirrors are broadband AR-coated with hightransmission (T > 97%) at 532 nm and high reflectivity (R >99.5%) across 760–840 nm. The OC transmission varies from24% at 773 nm to 18% at 822 nm. The total optical length of thecavity is 109 cm (FSR ∼ 275 MHz). A birefringent filter (BRF)is used to control and tune the laser wavelength. An intracav-ity optical diode comprising a Faraday rotator in combinationwith a half-wave plate ensures unidirectional operation of thering laser, and a 500-μm-thick uncoated intracavity fused silicaetalon (FSR ∼ 206 MHz, finesse ∼ 0.6) provides frequencyselection and single-mode operation. A home-made ultrastableFabry-Perot interferometer made of super-Invar is used as anexternal reference to lock the laser for long-term frequency sta-bility. The reference can be easily tuned, allowing continuoustuning of the stabilized laser.

The power scaling results for the Ti:sapphire ring laser, underfree-running condition, with and without intracavity elementssuch as BRF, optical diode, and etalon, are shown in Fig. 38.The plots correspond to an optimum green pump waist radius ofw0p ∼ 24 μm and an OC ∼ 20%. In free-running condition, thebidirectional ring laser operated at a threshold of 3.6 W, gener-ating a total output power of 2.6 W for a pump power of 11.3 W,with a slope efficiency of 32.7%. Unidirectional operation wasachieved by introducing an optical diode into the ring laser. Theinclusion of all the intracavity elements led to an increase in thethreshold to 4.5 W and ∼16% reduction in the output poweras compared to the free-running condition, generating a max-imum single-frequency output as high as 2.3 W for the sameincident pump power of 11.3 W at a slope efficiency of 33.7%.This nominal increase in slope efficiency could be attributedto the alleviation of the spatial hole burning effect in unidirec-tional ring laser operation. The corresponding optical-to-optical


Fig. 38. Power scaling characteristics of Ti:sapphire ring laser in free-runningoperation, as well as with the intracavity elements (BRF, optical diode, andetalon) [43].

Fig. 39. Frequency stability of the Ti:sapphire ring laser with stabilization tothe home-made super-Invar Fabry–Perot cavity [43].

efficiencies in the two cases with respect to the incident andabsorbed pump power were thus 20% and 25%, respectively,with no sign of saturation when pumping up to the maximumavailable green power.

We achieved coarse wavelength tuning of the Ti:sapphire ringlaser over 774–821 nm using the BRF. The tuning range waslimited by the reflectivity of the available cavity mirrors, andso could be readily extended to cover the full gain bandwidthof Ti:sapphire across 680–1100 nm with the available greenpower by using more suitable mirrors. The spectral characteris-tics of the Ti:sapphire laser were studied using a scanning Fabry-Perot interferometer (FSR = 1 GHz, finesse = 400), wheresingle-frequency output with a typical instantaneous linewidthof 5.4 MHz at the maximum output power was measured, whichis comparable to commercially available Ti:sapphire lasers. Fur-ther reductions in linewidth can be achieved by deploying anetalon with higher finesse. The single-frequency operation ofthe laser was maintained at different wavelengths across theentire tuning range.

In order to improve the performance of the Ti:sapphire ringlaser in terms of frequency stability, we set up a stabilizationsystem using an external home-made super-Invar Fabry-Perotreference cavity (FSR = 750 MHz), carefully designed for max-imum stability. After locking the laser cavity to the interferom-eter, the output wavelength of the stabilized Ti:sapphire laserwas recorded using a high-resolution wavemeter. The observedstability was better than 12 MHz over 10 minutes at an outputpower of 2.2 W around 817 nm, as shown in Fig. 39. We ob-tained a similar result at other wavelengths, with no evidence

Fig. 40. Power stability of the Ti:sapphire output at 2.25 W recorded over aperiod of 70 minutes. Inset: Far-field energy distribution of the Ti:sapphire ringlaser output beam at 812 nm [43].

of mode-hopping. We have also demonstrated rapid continuousmode-hop-free tuning of the Ti:sapphire laser over 181 MHz in5 seconds, while maintaining frequency-stable operation [43].Similar frequency tuning was observed across the wavelengthrange of the Ti:sapphire ring laser. The fine-tuning range of181 MHz can also be further extended, for example with morestringent isolation of the laser to preserve the lock for a longertime.

We investigated the output power stability of the Ti:sapphirering laser by recording the variation of output power with timeat 814 nm, and at the maximum output power of 2.2 W. Theresults are shown in Fig. 40, where a peak-to-peak power sta-bility of 5.4% over more than 1 hour is recorded, comparablewith the fiber-based green pump source (3.3% over 1 hour). Thepower stability can be further improved by providing proper iso-lation from the mechanical vibration and air turbulence in thelaboratory environment. Also shown in the inset of Fig. 40 isthe far-field energy distribution of the Ti:sapphire output beamat 812 nm, recorded at a distance of >2 m, together with theintensity profiles and the Gaussian fits at the maximum outputpower. Using a scanning beam profiler and a focusing lens, wemeasured the M 2 quality factor of the output beam, resulting inM2

x ∼ 1.22 and M2y ∼ 1.34, confirming Gaussian spatial distri-

bution. These characteristics demonstrate the competitive per-formance of the developed fiber-based green source comparedto the well-established internally doubled solid-state lasers at532 nm as pump for Tisapphire lasers, affirming the viability ofthe SP-SHG technique as a robust, reliable, power-scalable, andeffective approach for high-power cw green generation.

V. SUMMARY AND OUTLOOK

In summary, we have described a new class of cw non-linear frequency conversion sources developed in our labora-tory during the past five years, capable of delivering stable,high-power and widely tunable coherent radiation, in com-pact, portable, simplified and practical formats, using cw Yb-fiber lasers at 1064 nm as the pump source. By exploitingMgO:PPLN and MgO:sPPLT as the nonlinear gain mediumfor OPOs, and deploying additional frequency up-conversionschemes based internal and external SP-SHG in MgO:sPPLT,and external SP-THG (SFG) in BiB3O6 , we have achieved tun-able generation across broad spectral regions form the near- and


TABLE IPERFORMANCE CHARACTERISTICS OF YB-FIBER-LASER-PUMPED CW NONLINEAR FREQUENCY

CONVERSION SOURCES IN OUR LABORATORY DURING

THE PAST FIVE YEARS

mid-IR to the visible and UV, with the tuning coverage lim-ited only by the available optical coatings and grating peri-ods of QPM crystals deployed. We have also demonstrated thepromise of the Yb-fiber-laser-pumped green sources as pump forcw Ti:sapphire lasers, demonstrating comparable performancewith well-established solid-state green pump lasers. With highoutput power, good passive stability, and excellent beam quality,the developed cw green source also pave the way for the real-ization of compact, fiber-laser-pumped Kerr-lens-mode-lockedultrafast Ti:sapphire lasers.

Table I summarizes the performance characteristics of the Yb-fiber-laser-pumped cw nonlinear frequency conversion sourcesdeveloped during this study. With additional grating periodsand suitable crystal and mirror coatings, expansion of the wave-length coverage throughout the 600–4500 nm spectral range willbe attainable. By deploying intracavity SP-SFM schemes [26],tunable cw generation in the UV is also a clear possibility,potentially enabling access to cw wavelength range of 300–600 nm. Moreover, the extension of fiber laser technology tohigh-power Tm-fiber pump lasers at ∼2 μm will pave theway for the development of practical cw nonlinear frequencyconversion sources deeper into the mid-IR, potentially across4–12 μm, by exploiting QPM nonlinear material of OP-GaAsor birefringent crystals such as ZnGeP2 . In a preliminary ef-fort, we have already demonstrated the potential of cw Tm-fiber lasers for nonlinear frequency conversion in a simple SP-SHG experiment in PPLN, where we have generated >13 W

of output at 970 nm at ∼33% efficiency [45]. By exploitingadditional techniques such as interferometric output couplingand dual-crystal OPO schemes [46], [47], further improvementsin the performance and versatility of fiber-laser-pumped cwfrequency conversion sources can be brought about with re-gard to output power, extraction efficiency, wavelength flex-ibility, and spectral generation into THz range. With powerscaling of Yb-fiber lasers beyond 30 W, and the availabil-ity of MgO:PPLN and MgO:sPPLT crystals of larger aperture(>2-mm-thick), the generation of cw output powers >20 W inthe near- to mid-IR using cw SROs will become increasinglyattainable. In this case, a key parameter in achieving practicaloperation and stable output will be the management of ther-mal issues through optimized output coupling, focusing, andmode-matching by using suitable cw SRO cavity designs. Theattainment of higher watt-level cw powers in the visible and UVusing the described SHG and THG/SFG techniques will alsobecome feasible with increasing Yb-fiber laser power. In thiscase, optical damage to the nonlinear crystal and coatings willbe the main limiting factor in the attainment of maximum power,efficiency, and stability, particularly in the UV, but this can beavoided using suitable focusing and mode-matching strategies,as well as thermal and environmental isolation of the system.

With the rapid advances of fiber laser technology to the ultra-fast time-scales, we have also extended the operation of fiber-laser-pumped frequency conversion sources to the picosecondtime-scales by deploying mode-locked Yb-fiber lasers as pump


source, and by exploiting the same generic approaches basedon OPOs in combination with additional internal and exter-nal SP-SHG schemes in MgO:PPLN, MgO:sPPLT and BiB3O6as nonlinear gain material [48]–[54]. We have generated aver-age powers of as much as 11.7 W in the near- and mid-IR, inpulses of 15–20 ps duration at repetition-rates of 80 MHz to7 GHz. Wavelength coverage from ∼870 nm to above ∼4 μmhas been achieved, limited by the available QPM grating periodsand optical coatings. More recently, operation of a femtosecondYb-fiber-laser-pumped OPO at wavelength beyond 6 μm hasalso been reported by exploiting the new mid-IR birefringentcrystal, CdSiP2 [55]. These developments further point to therapid penetration of fiber-laser-pumped frequency conversiontechnology to all time-scales, providing a new generation ofcompact, versatile and high-power coherent sources coveringbroad wavelength regions from the UV to mid-IR, and ulti-mately the THz spectrum, and in all temporal domains from cwto ultrafast femtosecond time-scales.

REFERENCES

[1] P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, “Generationof optical harmonics,” Phys. Rev. Lett., vol. 7, no. 4, pp. 118–119, Aug.1961.

[2] J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Inter-action between light waves in a nonlinear dielectric,” Phys. Rev., vol. 127,no. 6, pp. 1918–1939, Sep. 1962.

[3] R. H. Kingston, “Parametric amplification and oscillation at optical fre-quencies,” in Proc. Inst. Radio Eng., 1962, vol. 50, pp. 472–473.

[4] N. M. Kroll, “Parametric amplification in spatially extended media andapplication to the design of tuneable oscillators at optical frequencies,”Phys. Rev., vol. 127, no. 4, pp. 1207–1211, Aug. 1962.

[5] S. A. Akhmanov and R. V. Khokhlov, “Concerning one possibility ofamplification of light waves,” Sov. J. Exp. Theor. Phys, vol. 16, pp. 252–257, 1963.

[6] J. A. Giordmaine and R. C. Miller, “Tunable coherent parametric oscilla-tion in LiNbO3 at optical frequencies,” Phys. Rev. Lett., vol. 14, no. 24,pp. 973–976, Jun. 1965.

[7] M. Ebrahim-Zadeh and M. H. Dunn, “Optical parametric oscillators,” inHandbook of Optics, vol. IV. New York, NY, USA: McGraw-Hill, 2000,pp. 2201–2272.

[8] W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, andR. L. Byer, “93% pump depletion, 3.5 w continuous-wave, singly resonantoptical parametric oscillator,” Opt. Lett., vol. 21, no. 17, pp. 1336–1338,Sep. 1996.

[9] M. Ebrahim-Zadeh, “Continuous-wave optical parametric oscillators,” inHandbook of Optics, vol. IV. New York, NY, USA: McGraw-Hill, 2010,ch. 17.

[10] D. D. Arslanov, M. Spunei, J. Mandon, S. M. Cristescu, S. T. Persijn,and F. J. M. Harren, “Continuous-wave optical parametric oscillator basedinfrared spectroscopy for sensitive molecular gas sensing,” Laser Photon.Rev., vol. 7, no. 2, pp. 188–206, Mar. 2013.

[11] P. Gross, M. E. Klein, T. Walde, K.-J. Boller, M. Auerbach, P. Wessels,and C. Fallnich, “Fiber-laser-pumped continuous-wave singly resonantoptical parametric oscillator,” Opt. Lett., vol. 27, no. 6, pp. 418–420, Mar.2002.

[12] A. Henderson and R. Stafford, “Spectral broadening and stimulated ramanconversion in a continuous-wave optical parametric oscillator,” Opt. Lett.,vol. 32, no. 10, pp. 1281–1283, May 2007.

[13] S. Chaitanya Kumar, R. Das, G. K. Samanta, and M. Ebrahim-Zadeh,“Optimally-output-coupled, 17.5 W, fiber-laser-pumped continuous-waveoptical parametric oscillator,” Appl. Phys. B, Lasers Opt., vol. 102, no. 1,pp. 31–35, Jan. 2010.

[14] G. K. Samanta and M. Ebrahim-Zadeh, “Continuous-wave singly-resonant optical parametric oscillator with resonant wave coupling,” Opt.Exp., vol. 16, no. 10, pp. 6883–6888, Apr. 2008.

[15] G. K. Samanta and M. Ebrahim-Zadeh, “Continuous-wave, single-frequency, solid-state blue source for the 425–489 nm spectral range,”Opt. Lett., vol. 33, no. 11, pp. 1228–1230, May 2008.

[16] G. D. Boyd and D. A. Kleinman, “Parametric interaction of focused Gaus-sian light beams,” J. Appl. Phys., vol. 39, no. 8, pp. 3597–3639, Jul.1968.

[17] R. Das, S. Chaitanya Kumar, G. K. Samanta, and M. Ebrahim-Zadeh,“Broadband, high-power, continuous-wave, mid-infrared source using ex-tended phase-matching bandwidth in MgO:PPLN,” Opt. Lett., vol. 34,no. 24, pp. 3836–3838, Dec. 2009.

[18] O. Paul, A. Quosig, T. Bauer, M. Nittmann, J. Bartschke, G. Anstett,and J. A. L’Huillier, “Temperature-dependent sellmeier equation in theMIR for the extraordinary refractive index of 5% MgO doped congruentLiNbO3 ,” Appl. Phys. B, Lasers Opt., vol. 86, no. 1, pp. 111–115, Jan.2007.

[19] S. Chaitanya Kumar, G. K. Samanta, and M. Ebrahim-Zadeh, “High-power, single-frequency, continuous-wave second-harmonic-generationof ytterbium fiber laser in PPKTP and MgO:sPPLT,” Opt. Exp., vol. 17,no. 16, pp. 13711–13726, Jul. 2009.

[20] G. K. Samanta, S. Chaitanya Kumar, and M. Ebrahim-Zadeh, “Sta-ble, 9.6 W, continuous-wave, single-frequency, fiber-based green sourceat 532 nm,” Opt. Lett., vol. 34, no. 10, pp. 1561–1563, May2009.

[21] G. K. Samanta, S. Chaitanya Kumar, K. Devi, and M. Ebrahim-Zadeh,“Multicrystal, continuous-wave, single-pass second-harmonic generationwith 56% efficiency,” Opt. Lett., vol. 35, no. 20, pp. 3513–3515, Oct.2010.

[22] N. E. Yu, S. Kurimura, Y. Nomura, M. Nakamura, K. Kitamura, J. Sakuma,Y. Otani, and A. Shiratori, “Periodically poled near-stoichiometric lithiumtantalate for optical parametric oscillation,” Appl. Phys. Lett., vol. 84,no. 10, pp. 1662–1664, Mar. 2004.

[23] H. Ishizuki and T. Taira, “High energy quasi-phase matchedoptical para-metric oscillationusing Mg-doped congruent LiTaO3 crystal,” Opt. Exp.,vol. 18, no. 1, pp. 253–258, Jan. 2010.

[24] G. K. Samanta, G. R. Fayaz, and M. Ebrahim-Zadeh, “1.59 W,single-frequency, continuous-wave optical parametric oscillator basedon MgO:sPPLT,” Opt. Lett., vol. 32, no. 17, pp. 2623–2625, Sep.2007.

[25] S. Chaitanya Kumar and M. Ebrahim-Zadeh, “High-power, continuous-wave, mid-infrared optical parametric oscillator based on MgO:sPPLT,”Opt. Lett., vol. 36, no. 13, pp. 2578–2580, Jul. 2011.

[26] K. Devi, S. Chaitanya Kumar, and M. Ebrahim-Zadeh, “Tun-able, continuous-wave, ultraviolet source based on intracavitysum-frequency-generation in an optical parametric oscillator usingBiB3 O6 ,” Opt. Exp., vol. 21, no. 21, pp. 24829–24836, Oct.2013.

[27] G. K. Samanta, S. Chaitanya Kumar, R. Das, and M. Ebrahim-Zadeh,“Continuous-wave optical parametric oscillator pumped by a fiber lasergreen source at 532 nm,” Opt. Lett., vol. 34, no. 15, pp. 2255–2257, Aug.2009.

[28] K. Devi, S. Chaitanya Kumar, and M. Ebrahim-Zadeh, “High-power,continuous-wave, single-frequency, all-periodically-poled, near-infraredsource,” Opt. Lett., vol. 37, no. 24, pp. 5049–5051, Dec. 2012.

[29] A. Bruner, D. Eger, M. B. Oron, P. Blau, M. Katz, and S. Ruschin,“Temperature-dependent Sellmeier equation for the refractive index ofstoichiometric lithium tantalate,” Opt. Lett., vol. 28, no. 3, pp. 194–196,Feb. 2003.

[30] G. K. Samanta, S. Chaitanya Kumar, K. Devi, and M. Ebrahim-Zadeh,“High-power, continuous-wave ti:sapphire laser pumped by Fiber-lasergreen source at 532 nm,” Opt. Lasers Eng., vol. 50, no. 2, pp. 215–219,Feb. 2012.

[31] K. Kitamura, Y. Furukawa, S. Takekawa, T. Hatanaka, H. Ito, andV. Gopalan, “Non-stoichiometric control of LiNbO3 and LiTaO3 in ferro-electric domain engineering for optical devices,” Ferroelectrics, vol. 257,no. 1, pp. 235–243, Mar. 2001.

[32] D. S. Hum, R. K. Route, G. D. Miller, V. Kondilenko, A. Alexandrovski,J. Huang, K. Urbanek, R. L. Byer, and M. M. Fejer, “Optical proper-ties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium tantalate for frequency conversion,” J. Appl. Phys.,vol. 101, no. 9, pp. 093108-1–093108-12, May 2007.

[33] S. V. Tovstonog, S. Kurimura, I. Suzuki, K. Takeno, S. Moriwaki,N. Ohmae, N. Mio, and T. Katagai, “Thermal effects in high-power CW second harmonic generation in Mg-doped stoichiometriclithium tantalite,” Opt. Exp., vol. 16, no. 15, pp. 11294–11299, Jul.2008.

[34] J. M. Yarborough, J. Falk, and C. B. Hitz, “Enhancement of optical secondharmonic generation by utilizing the dispersion of air,” Appl. Phys. Lett.,vol. 18, no. 3, pp. 70–73, Oct. 1971.


[35] S. Chaitanya Kumar, G. K. Samanta, K. Devi, and M. Ebrahim-Zadeh,“High-efficiency, multicrystal, single-pass, continuous-wave secondharmonic generation,” Opt. Exp., vol. 19, no. 12, pp. 11152–11169, May2011.

[36] G. C. Bhar, U. Chatterjee, and P. Datta, “Enhancement of second harmonicgeneration by double-pass configuration in barium borate,” Appl. Phys. B,Lasers Opt., vol. 51, no. 5, pp. 317–319, Nov. 1990.

[37] T. Kojima and K. Yasui, “High-power green-beam generation incontinuous-wave modes by use of long KTiOPO4 crystals with a Nd:YAGlaser,” Opt. Lett., vol. 19, no. 10, pp. 713–715, May 1994.

[38] S. Chaitanya Kumar, K. Devi, and M. Ebrahim-Zadeh, “Stable,continuous-wave, ytterbium-fiber-based single-pass ultraviolet sourceusing BiB3 O6 ,” Opt. Lett., vol. 38, no. 23, pp. 5114–5117, Dec.2013.

[39] M. Ebrahim-Zadeh, “Efficient ultrafast frequency conversion sources forthe visible and ultraviolet based on BiB3 O6 ,” IEEE J. Sel. Topics QuantumElectron., vol. 13, no. 3, pp. 679–691, Jun. 2007.

[40] V. Petrov, M. Ghotbi, O. Kokabee, A. Esteban-Martin, F. Noack,A. Gaydardzhiev, I. Nikolov, P. Tzankov, I. Buchvarov, K. Miyata,A. Majchrowski, I. Kityk, F. Rotermund, E. Michalski, and M. Ebrahim-Zadeh, “Femtosecond nonlinear frequency conversion based on BiB3 O6 ,”Laser Photon. Rev., vol. 4, no. 1, pp. 53–98, Jan. 2010.

[41] M. Peltz, J. Bartschke, A. Borsutzky, R. Wallenstein, S. Vernay,T. Salva, and D. Rytz, “Harmonic generation in bismuth triborate(BiB3 O6 ),” Appl. Phys. B, Lasers Opt., vol. 81, no. 4, pp. 487–495, Aug.2005.

[42] S. Moller, A. Andresen, C. Merschjann, B. Zimmermann, M. Prinz, andM. Imlau, “Insight to UV-induced formation of laser damage on LiB3 O5optical surfaces during long-term sum-frequency generation,” Opt. Exp.,vol. 15, no. 12, pp. 7351–7356, May. 2007.

[43] S. Chaitanya Kumar, G. K. Samanta, K. Devi, S. Sanguinetti, andM. Ebrahim-Zadeh, “Single-frequency, high-power, continuous-wavefiber-laser-pumped Ti:sapphire laser,” Appl. Opt., vol. 51, no. 1, pp. 15–20,Jan. 2012.

[44] P. A. Schulz, “Single-frequency Ti:Al2 O3 ring laser,” IEEE J. QuantumElectron., vol. 24, no. 6, pp. 1039–1044, Jun. 1988.

[45] K. Devi, S. Chaitanya Kumar, and M. Ebrahim-Zadeh, “13.1 W,high-beam-quality, narrow-linewidth continuous-wave fiber-based sourceat 970 nm,” Opt. Exp., vol. 19, no. 12, pp. 11631–11637, Jun.2011.

[46] K. Devi, S. Chaitanya Kumar, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Antiresonant ring output-coupled continuous-wave optical para-metric oscillator,” Opt. Exp., vol. 20, no. 17, pp. 19313–19321, Aug.2012.

[47] G. K. Samanta and M. Ebrahim-Zadeh, “Dual-wavelength, two-crystal,continuous-wave optical parametric oscillator,” Opt. Lett., vol. 36, no. 16,pp. 3033–3035, Aug. 2011.

[48] O. Kokabee, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Efficient, high-power, ytterbium-fiber-laser-pumped picosecond optical parametric oscil-lator,” Opt. Lett., vol. 35, no. 19, pp. 3210–3212, Sep. 2010.

[49] S. Chaitanya Kumar, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Inter-ferometric output coupling of ring optical oscillators,” Opt. Lett., vol. 36,no. 7, pp. 1068–1070, Mar. 2011.

[50] S. Chaitanya Kumar and M. Ebrahim-Zadeh, “High-power, fiber-laser-pumped, picosecond optical parametric oscillator based on MgO:sPPLT,”Opt. Exp., vol. 19, no. 27, pp. 26660–26665, Dec. 2011.

[51] S. Chaitanya Kumar, O. Kimmelma, and M. Ebrahim-Zadeh, “High-power, Yb-fiber-laser-pumped, picosecond parametric source tunableacross 752–860 nm,” Opt. Lett., vol. 37, no. 9, pp. 1577–1579, May2012.

[52] O. Kimmelma, S. Chaitanya Kumar, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Multi-gigahertz picosecond optical parametric oscillator pumpedby 80-MHz Yb-fiber laser,” Opt. Lett., vol. 38, no. 22, pp. 4550–4553,Nov. 2013.

[53] S. Chaitanya Kumar and M. Ebrahim-Zadeh, “Fiber-laser-based green-pumped picosecond MgO:sPPLT optical parametric oscillator,” Opt. Lett.,vol. 38, no. 24, pp. 5349–5352, Dec. 2013.

[54] S. Chaitanya Kumar and M. Ebrahim-Zadeh, Laser Phys., vol. 24, no. 2,pp. 025401-1–025401-5, Feb. 2014.

[55] Z. Zhang, D. T. Reid, S. Chaitanya Kumar, M. Ebrahim-Zadeh,P. G. Schunemann, K. T. Zawilski, and C. R. Howle, “Femtosecond-laserpumped CdSiP2 optical parametric oscillator producing 100 MHz pulsescentered at 6.2 μm,” Opt. Lett., vol. 38, no. 23, pp. 5110–5113, Nov.2013.

Majid Ebrahim-Zadeh received the Ph.D. degreefrom St Andrews University, UK, in 1989. He is an In-stitucio Catalana de Recerca i Estudis Avancats Pro-fessor at ICFO-The Institute of Photonic Sciences,Barcelona, Spain. He was a Royal Society of LondonResearch Fellow at St Andrews from 1993 to 2001,and a Reader from 1997 to 2003. He has been activein the advancement of nonlinear optics and paramet-ric sources for more than 20 years. His research hasled the realization of new generations of innovativelight sources from the UV to mid-IR and from the

cw to femtosecond time-scales. He has published more than 460 journal andpeer-reviewed papers, including 75 invited and 12 post-deadline papers at ma-jor international conferences, has edited 2 books and authored 12 major invitedbook chapters and reviews in volumes such as Science, OSA Handbook ofOptics, Springer, Handbook of Laser Technology and Applications, and Laserand Photonics Reviews. He has been a regular instructor of the short course onOPOs at CLEO/USA since 1997 and at CLEO/Europe since 2007, has servedmore than 40 times on the technical, organizing, and steering committees ofmajor conferences worldwide, and has chaired 3 international conferences. Heis an Associate Editor of IEEE PHOTONICS JOURNAL and has served as advisoryand topical editor of Optics Letters, and guest editor of Journal Optical SocietyAmerica B. He is the Co-founder, President, and Chief Scientist of Radiantis,a company created from his research laboratory in 2005. He received severalhonors and awards, including Royal Society of London university fellowshipand merit awards (UK: 1993–2001), Innova Prize for technology innovation andenterprise (Spain: 2004), and Berthold Leibinger Innovation Prize (Germany:2010). Dr. Ebrahim-Zadeh is a Fellow of OSA and SPIE.

Suddapalli Chaitanya Kumar received the B.Sc.degree in mathematics, physics, and electronics fromAcharya Nagarjuna University, India, in 2003, and theM.Sc. degree in physics from the Indian Institute ofTechnology Guwahati, India, in 2006. He received thePh.D. degree in photonics with “Excellent Cum Laudewith Honours” from ICFO-The Institute of PhotonicSciences, Barcelona, Spain, in 2012, for his thesison high-power, fiber-laser-pumped optical paramet-ric oscillators from the visible to mid-infrared. Heis currently a Postdoctoral Researcher at ICFO. His

research interests include fiber-based optical frequency conversion sources, andcontinuous-wave and ultrafast optical parametric oscillators (OPOs) from theultraviolet to mid-infrared. He has published more than 30 peer-reviewed andinvited papers in leading international journals in photonics, with more than300 citations, and has presented more than 50 contributed, post deadline, andinvited papers at major international conferences such as CLEO-USA and CLEO-Europe. Throughout his career, he has worked on several funded projects withactive international collaborations, and some of his research works have beenhighlighted in Laser Focus World and Nature Photonics. He received the ICFOPh.D. thesis award in 2013 for his outstanding contributions to applied researchduring his doctoral studies. He is a professional member of the Optical So-ciety of America and The International Society for Optics and Photonics. Healso serves as an International Outreach Project Manager at the Knowledge andTechnology Transfer office of ICFO.

Kavita Devi was born in Delhi, India, in 1984. Shereceived the M.Sc. degree in physics from the IndianInstitute of Technology, Guwahati, India, in 2006,and the Ph.D. degree in photonics (with ExcellentCum Laude with Hons.) from ICFO-The Institute ofPhotonic sciences, Barcelona, Spain, in 2013. She hasalso worked in the area of plasma physics as a juniorresearch fellow, at the Institute of Advanced Study inScience and Technology, Guwahati, India. In 2009,she joined ICFO, where she worked mainly in thedevelopment of optical frequency conversion systems

from the ultraviolet to the mid-infrared. Some of her research work has also beenhighlighted in Laser Focus world and Virtual Journal of Biomedical Optics. Sheis a professional member of Optical Society of America, and The InternationalSociety for Optics and Photonics. She is currently a Postdoctoral Researcherat ICFO. She has authored and coauthored more than 45 papers, presented inpeer-reviewed scientific journals and conferences that include CLEO-USA andCLEO-Europe.

yb-fiber-laser-pumped continuous-wave frequency conversion sources from the mid-infrared to the...

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