Three optical cycle mid-IR Kerr-lens mode-lockedpolycrystalline Cr2+:ZnS laserSERGEY VASILYEV,1,* IGOR MOSKALEV,1 MIKE MIROV,1 SERGEY MIROV,1,3 AND VALENTIN GAPONTSEV21IPG Photonics-Mid-Infrared Lasers, 1500 1st Ave. N, Birmingham, Alabama 35203, USA2IPG Photonics Corporation, 50 Old Webster Rd, Oxford, Massachusetts 01540, USA3Center for Optical Sensors and Spectroscopies, University of Alabama at Birmingham, 1530 3rd Avenue South,Birmingham, Alabama 35294, USA*Corresponding author: svasilyev@ipgphotonics.com

Received 18 August 2015; revised 24 September 2015; accepted 1 October 2015; posted 1 October 2015 (Doc. ID 248013);published 28 October 2015

We report Kerr-lens mode-locked polycrystalline Cr2�:ZnS lasers at 2.4 μm central wavelength optimized for shortpulse duration. By control of the second- and third-orderdispersion within 500 nm bandwidth we obtained pulsesof three optical cycles (

the overall chromatic dispersion in the resonator of a mode-locked laser is an important prerequisite for generation offew-optical-cycle pulses. The de facto standard method fordispersion compensation in mode-locked Cr2�:ZnS∕ZnSelasers is the use of plane-parallel plates, made of materials withnegative group delay dispersion (GDD) [5]. However, platecompensators introduce significant amounts of third-orderdispersion (TOD) into the resonator. Properties of optical ma-terials in the 2–3 μm range are such that the use of prism com-pensators does not bring much improvement in terms of TODcontrol [8]. Therefore, we avoided the use of prisms and platesfor dispersion compensation and instead relied on dielectricmirrors with tailored GDD spectra.

The experimental setup is similar to the one described in [1].The X-folded standing-wave resonator consists of two concavemirrors, plane end-mirrors, and plane folding mirrors, as shownin Fig. 1. The resonator’s legs are unequal with a 2∶5 ratio. Thelaser is optically pumped at 1567 nm by a linearly polarizedEr-doped fiber laser (EDFL). The pump beam (∅3 mm) isfocused in the gain element by a lens (L) and coupled in theresonator through a curved mirror with high reflectivity in the2200–2700 nm range and high transmission at the pump wave-length. The output coupler (OC) has ∼90% reflectivity inthe mid-IR range. A dichroic mirror (DM) with high reflectiv-ity in the mid-IR range and high transmission in 1100–1350 nm range is used as an output coupler for SHG.

An AR-coated polycrystalline Cr2�:ZnS gain element ismounted in the resonator at normal incidence between twocurved mirrors. The gain element is 5 mm long with 11%

low-signal transmission at pump wavelength and is cooled withroom-temperature water. The angle of incidence at the curvedmirrors is minimized to 3.5°–4° to reduce the astigmatism ofthe resonator. In our opinion, the normal incidence mountingof the gain element has a number of advantages: (i) better man-agement of the thermal optical effects due to circularity of thepump and laser beams; (ii) significant increase of pump andlaser intensity inside the gain element (if compared with thestandard Brewster mounting). Our experiments show thatsome astigmatism of the resonator associated with this schemeis not an impediment for the KLM laser regime.

Dispersion of the resonator was controlled within the 2200–2700 nm range. Dispersion spectra of the gain element and ofthe high reflectors (HRs) are illustrated in Fig. 2. Dispersion ofthe gain element was calculated using the standard Sellmeierequation for undoped ZnS; the theoretical GDD spectra for themirrors were provided by the coaters. TOD introduced aftertwo passages through the gain element (�3100 fs3) wascompensated by a single reflection from the mirror HR*(−3000 fs3). Net GDD of the resonator was adjusted in discretesteps by changing a type and number of installed mirrorsHR�1�and HR�2� with negative GDD and low TOD. The OC, theDM, and the AR coatings of the gain element have low GDD.

The experiments were carried out at two distinctly differentrepetition rates of 100 and 300 MHz (resonator’s length of 1.5and 0.5 m, respectively). At 100 MHz repetition rate [Fig. 1(a)]we used the curved mirrors with 100 mm radii. At 300 MHzrepetition rate [Fig. 1(b)] the mirrors with 50 mm radii wereinstalled, and the dichroic DMwas replaced by curved dispersivemirror HR�1� with ∼50% transmission at SHG wavelength.

The lasers were optimized for maximum cw output power.The distance between the curved mirrors was then fine-adjustedin order to enable the KLM regime (initiated byOC translation).The polarization of the laser emissionwas linear and vertical (nor-mal to the plane of Fig. 1) in cw as well as in mode-locked re-gimes, while the polarization of the pump beam was horizontal.

Spectral and temporal parameters of the mode-locked laserswere characterized using a 0.15 m dual grating monochromator(Princeton Instruments) and an interferometric autocorrelator(A·P·E GmbH). The laser emission spectra in mid-IR, near-IR,and visible bands were acquired using a lock-in amplifier andPbSe, extended InGaAs, and Si detectors (Thorlabs PDA20H,PDA10D, and PDA8A, respectively). We relied on the auto-correlator’s control software and used sech2 fit of the autocor-relation functions for evaluations of the pulse duration. Theexperiments were carried out in a standard lab environment.Presented results correspond to long-term-stable single-pulseoscillations in the KLM regime.

The first experiment was set up at 300 MHz repetitionrate, see [Fig. 1(b)]. Optimization of the laser for short pulseduration is illustrated in Fig. 3:

Table 1. Output Parameters of Polycrystalline Cr2�:ZnSKLM Lasersa

P, W Δν, THz Δτ, fs

f , MHz Pump MIR SHG MIR SHG MIR

100 4.6 0.44 0.055 12.3 22

(i) Initially, TOD compensator HR* was replaced by themirrorHR�1�. Net GDD of the resonator was overcompensatedto about −600 fs2. The KLM regime with 4.3 THz broad spec-trum was obtained at 4.5 W pump power using pump focusinglens L with the focal length f � 80 mm.(ii) Introduction of TOD compensator HR* and an increase

of the resonator’s net GDD to about −400 fs2 resulted inbroadening of the emission spectrum to 5.3 THz.(iii) Further increase of net GDD to about −250 fs2 resultedin 7 THz broad spectrum.(iv) Finally, we fine-tuned the pump power (was increased to5.1 W) and the pump focusing (focal length of the lens L wasdecreased to f � 40 mm, pump waist ∅30 μm).

As result of the optimizations, we obtained 165 nm(8.52 THz) broad spectrum (full width at half-maximum,FWHM). The spectrum was centered at 2410 nm. The averagepower of this mode-locked laser was 0.7 W.

Autocorrelations of the optimized KLM laser at 300 MHzrepetition rate are shown in Fig. 4. First autocorrelation wasmeasured directly behind the OC [Fig. 4(a)]. The pulse dura-tion of 60 fs, which was derived from the autocorrelation, doesnot match with broad and smooth spectrum of the pulses (iv inFig. 3). Apparently, the pulse is broadened during propagationthrough a 3.2 mm thick ZnSe substrate of the OC (GDD ��710 fs2 at 2400 nm). We used 5 mm thick YAG plate(GDD � −740 fs2) in combination with the HR* mirror tocompensate for the OC substrate’s dispersion, as shown in[Fig. 1(c)]. The obtained autocorrelation [Fig. 4(b)] reveals trans-form-limited 38 fs pulses with 0.32 time-bandwidth product.

In the next experiment, the resonator was reconfiguredfor 100 MHz repetition rate [Fig. 1(a)]. We performed severalrounds of optimization of the resonator’s dispersion and of thepumping conditions, as described above. The parameters of theoptimized KLM laser at 100 MHz repetition rate are summa-rized in Fig. 5. The broadest spectrum of pulses [shown inFig. 5(c)] was obtained at 4.6 W pump power using the lensL with f � 60 mm (pump waist∅40 μm). The average powerof the mode-locked laser in the mid-IR band was 0.44 W.FWHM of the central peak (located at 2415 nm) can be con-servatively estimated as 240 nm (12.3 THz). The spectral span

is as broad as 950 nm (1900–2850 nm) at −30 dB level, whichsignificantly exceeds the bandwidth of the resonator’s mirrors(500 nm). The spectrum is distorted due to leakage through themirrors (on the left) and water vapor absorption (on the right).

The autocorrelations of mid-IR pulses, obtained withoutand with compensation for the OC substrate’s dispersion areshown in Figs. 6(a) and 6(b), respectively. Asymmetry of thetraces is due to the unbalance of the autocorrelator’s beam split-ter assembly: two replicas of the input pulse acquire differentchirps and temporal broadenings during propagation throughthe beam splitter. The autocorrelator’s unbalance becomesmore evident with the reduction of the pulse duration; for in-stance, the autocorrelations of the transform-limited 38 fspulses in Fig. 4 are almost symmetric while the autocorrelationsin Fig. 6 are asymmetric, which suggests considerably shorterpulse. The width of the cross correlation of the two unequalreplicas is defined by the longer one. Therefore, the 29 fs pulseduration obtained by sech2 fit of the trace in [Fig. 6(b)] can beconsidered as an upper limit [18]. Most likely the actual pulse isshorter, e.g., one can estimate 26 fs pulse duration taking intoaccount only the central peak of the spectrum (12.3 THzbroad) and assuming 0.32 time-bandwidth product.

The resonators optimized at 100 and 300 MHz repetitionrates have the same set of dispersive mirrors and hence approx-imately the same net GDD.However, the spectra of the pulses atthe two repetition rates are distinctly different. We attribute thespectral broadening at 100MHz to strong self-phasemodulationin Cr2�:ZnS. Indeed, one can estimate >1.3 MW peak powerinside the resonator, which is 3 times higher than the criticalpower for self-focusing in ZnS (∼420 kW) [19]. On the otherhand, peak power inside the resonator of the laser at 300MHz isabout 530 kW, which approximately equals the critical power.

Fig. 3. Measured spectra of pulses at 300 MHz repetition rate (seemain text). Δν is full width at half-maximum of the spectra. The insetshows the beam profile of themode-locked laser in final configuration iv.

Fig. 4. Autocorrelations of optimized KLM laser at 300 MHz rep-etition rate: (a) measurement without compensation, (b) measurementwith compensation. Δτ is the pulse duration (sech2) fit.

Fig. 5. Parameters of the KLM laser at 100 MHz repetition rate:(a) detected pulse train; (b) net GDD of the resonator; (c) measuredspectrum of pulses (solid line), transmission of standard air in 1.5 mlong resonator (gray background), net reflectivity of the resonator’soptics (dashed line). The inset shows the beam profile of the mode-locked laser.

Fig. 6. Autocorrelations of optimized KLM laser at 100 MHz rep-etition rate: (a) measured without compensation, (b) measured withcompensation. Δτ is the pulse duration (sech2) fit.

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RQPM in the polycrystalline Cr2�:ZnS gain element of themode-locked laser results in a number of three-wave mixing ef-fects. Figure 7 shows the laser emission spectra in different spec-tral bands. Apart from strong SHG emission (spectra ii), wedetect signals at third and fourth optical harmonics (spectraiii and iv) as well as signals that correspond to the sum frequencygeneration (SFG) between fs mid-IR pulses and cw pump radi-ation at 1567 nm (spectra v). A photo of the polycrystalline gainelement of the mode-locked laser is shown in [Fig. 7(c)]. Thefourth-harmonic emission at about 600 nm is clearly visibleto the naked eye. That allows us to visually detect the mode-locked regime of the laser without instrumental aid and makeeducated guesses about the central wavelength of the laser.

Output parameters of the optimized KLM are summarizedin Table 1. The SHG signal was measured in one direction(shown by arrows in Fig. 1) and was separated from residualpump radiation by a dichroic mirror. A similar SHG signalcan be expected in the opposite direction. SHG emission spec-tra are twice as broad as laser emission at fundamental mid-IRwavelength. Therefore, proper de-chirping of SHG output maypotentially lead to rather short 15–20 fs pulses in the near-IRwavelength range.

In conclusion, we demonstrate a flexible KLM laser designthat provides for few optical cycle mid-IR pulses with subwattaverage power in a broad range of repetition rates. Pulses of threeoptical cycles and 950 nm spectral span (3/5 of an octave) wereobtained by dispersion control within 500 nm bandwidth (1/3of an octave). This result leads us to expect that further improve-ments of the optical coatings will result in octave-spanning pol-ycrystalline Cr2�:ZnS∕ZnSe lasers. A significant fraction ofmid-IR laser emission is converted to the second harmonic di-rectly in the polycrystalline gain element of a mode-locked laser.The observed laser power at SHG wavelength (50–160 mW) isalready high enough for some real-world applications.

We also detect third and fourth optical harmonics as wellas SFG signal. There is little doubt that the effects of terahertzgeneration (via optical rectification of femtosecond laser pulses)and difference-frequency mixing (between fs pulses andpump-laser emission) can be revealed by proper detection.Synchronous pumping of polycrystalline Cr2�:ZnS∕ZnSe by

available 1.5 μm picosecond and femtosecond fiber laser radi-ations may radically increase the yield of sum- and difference-frequency mixing by several orders of magnitude.

Availability of high-power KLM lasers in the 2–3 μm spec-tral range opens several avenues for extension of ultrafast laseroscillations to 3–10 μm range. A mode-locked Cr2�:ZnS laseris a convenient pump source for synchronously pumped syn-chronously pumped optical parametric oscillators (OPOs) [20].The megawatt-level optical power inside the resonator of amode-locked laser allows us to consider a synchronouslypumped OPO based on RQPM in polycrystallineCr2�:ZnS∕ZnSe, which may lead to ultrafast oscillators withexceptionally broad spectral coverage spanning from 2 to 6 μm.

Acknowledgment. Sergey Mirov declares competingfinancial interests.

REFERENCES

1. S. Mirov, V. Fedorov, D. Martyshkin, I. Moskalev, M. Mirov, and S.Vasilyev, IEEE J. Sel. Top. Quantum Electron. 21, 292 (2015).

2. V. I. Kozlovsky, V. A. Akimov, M. P. Frolov, Yu. V. Korestelin, A. I.Landman, V. P. Martovitsky, V. V. Mislavskii, Yu. P. Podmar’kov,Ya. K. Skasyrsky, and A. A. Voronov, Phys. Status Solidi B 247,1553 (2010).

3. S. Mirov, V. Fedorov, D. Martyshkin, I. Moskalev, M. Mirov, and S.Vasilyev, in Advanced Solid-State Lasers, OSA Technical Digest(online) (Optical Society of America, 2015), paper AW4A.1.

4. E. Sorokin, I. T. Sorokina, M. S. Mirov, V. V. Fedorov, I. S. Moskalev,and S. B. Mirov, in Lasers, Sources and Related Photonic Devices,OSA Technical Digest Series (CD) (Optical Society of America, 2010),paper AMC2.

5. I. T. Sorokina and E. Sorokin, IEEE J. Sel. Top. Quantum Electron. 21,273 (2015).

6. I. T. Sorokina, E. Sorokin, and T. Carrig, Conference on Lasers andElectro-Optics/Quantum Electronics and Laser Science Conferenceand Photonic Applications Systems Technologies, Technical Digest(CD) (Optical Society of America, 2006), paper CMQ2.

7. M. N. Cizmeciyan, H. Cankaya, A. Kurt, and A. Sennaroglu, Opt. Lett.34, 3056 (2009).

8. M. N. Cizmeciyan, H. Cankaya, A. Kurt, and A. Sennaroglu, Appl.Phys. B 106, 887 (2012).

9. E. Sorokin, N. Tolstik, and I. T. Sorokina, Proc. SPIE 8599, 859916(2013).

10. M. N. Cizmeciyan, J. W. Kim, S. Bae, B. H. Hong, F. Rotermund, andA. Sennaroglu, Opt. Lett. 38, 341 (2013).

11. N. Tolstik, E. Sorokin, and I. T. Sorokina, in Advanced Solid-StateLasers, OSA Technical Digest (online) (Optical Society of America,2014), paper AM3A.1.

12. N. Tolstik, E. Sorokin, and I. T. Sorokina, Opt. Express 22, 5564 (2014).13. S.Vasilyev,M.Mirov, andV.Gapontsev,Opt.Express22, 5118 (2014).14. S. Vasilyev, M. Mirov, and V. Gapontsev, in Advanced Solid State

Lasers, OSA Technical Digest (online) (Optical Society of America,2014), paper AM3A.3.

15. S. Vasilyev, M. Mirov, and V. Gapontsev, in Advanced Solid StateLasers, OSA Technical Digest (online) (Optical Society of America,2015), paper AW4A.3.

16. E. Yu. Morozov, A. A. Kaminskii, A. S. Chirkin, and D. B. Yusupov,J. Exp. Theor. Phys. Lett. 73, 647 (2001).

17. M. Baudrier-Raybaut, R. Haïdar, Ph. Kupecek, Ph. Lemasson, and E.Rosencher, Nature 432, 374 (2004).

18. L. Keller, A·P·E-Applied Physics & Electronics, Inc. 45401 ResearchAve. Fremont, California, 94539 (personal communication, 2015).

19. H. Liang, P. Krogen, R. Grynko, O. Novak, C.-L. Chang, G. J. Stein,D. Weerawarne, B. Shim, F. X. Kärtner, and K.-H. Hong, Opt. Lett.40, 1069 (2015).

20. V. O. Smolski, S. Vasilyev, P. G. Schunemann, S. B. Mirov, and K. L.Vodopyanov, Opt. Lett. 40, 2906 (2015).

Fig. 7. Emission spectra of optimized KLM lasers presented in log-arithmic scale at (a) 300MHz and (b) 100MHz repetition rates: (i) fun-damental mid-IR band, (ii) SHG band, (iii) third optical harmonic,(iv) fourth optical harmonic, (v) sum frequency generation between fsmid IR pulses and cw pump radiation, (vi) residual pump at 1567 nm.The mid-IR spectrum is acquired by a PbSe detector and normalized tounity. The SHG spectrum is acquired by an extended InGaAs detectorand normalized to optical power. Spectra iii, iv, and v are acquired by theSi detector. The insert (c) shows the gain element of a mode-lockedmid-IR laser. The visible emission is generated in RQPM polycrystallineCr2�:ZnS as the fourth harmonic of mid-IR femtosecond pulses.

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