high power narrow-linewidth nanosecond all-fiber lasers and their actively coherent beam combination

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014 0903913

High Power Narrow-Linewidth NanosecondAll-Fiber Lasers and their Actively

Coherent Beam CombinationRongtao Su, Pu Zhou, Xiaolin Wang, Yanxing Ma, Hu Xiao, Pengfei Ma, Xiaojun Xu, and Zejin Liu

(Invited Paper)

Abstract—We present theoretical and experimental research onhigh power narrow-linewidth nanosecond all-fiber lasers and theiractively coherent beam combination (CBC). Nonlinear effects lim-itations of nanosecond pulses, such as stimulated Brillion scatter-ing (SBS), stimulated Raman scattering (SRS), are discussed. SRSlimited narrow-linewidth nanosecond all-fiber laser with averagepower of 913 W is obtained. A novel double-frequency nanosecondSRS nonlinear amplifier is proposed, which can be used to furtherscale the output power. CBC system for nanosecond fiber lasers istheoretically analyzed in detail. Effects of aberrations on CBC ofnanosecond fiber lasers are theoretically discussed. We also experi-mentally researched CBC of nanosecond lasers with low repetitionrate and high average power, respectively. Combined pulses withpeak power of 21.5 kW are obtained.

Index Terms—Optical pulses, optical fiber amplifiers, nonlinearoptics, coherent beam combining.

I. INTRODUCTION

ARANGE of applications such as material processing, non-linear frequency generation and LIDAR desire high power

nanosecond laser with good beam quality [1]–[8]. For example,Liu et al. obtained 60 W green output by frequency doubling of5 ns pulsed fiber laser with M2 of 1.1 and average/peak powerof 110 W/2.4 kW [2], and pulsed fiber laser with pulse widthof 80 ns, pulse energy of 0.5 mJ and M2 of 1.4 was employedin a coherent Doppler wind LIDAR by Zhu et al.. [5]. Pulsedlaser systems based on all-fiber master oscillator power amplifier(MOPA) structure have been widely researched recently becausethey can offer compact, robust and maintenance-free operationto those applications [9]. The MOPA structure consists of alow-power pulsed seed followed by cascaded high-power fiber

Manuscript received November 28, 2013; revised February 28, 2014 andMarch 11, 2014; accepted March 11, 2014. This work was supported by theNational Natural Science Foundation of China under Grants 11274386 and61322505 and Innovation Foundation for graduates of the National Universityof Defense Technology under Grant B120703. The authors R. Su and P. Zhoucontributed equally to this paper.

The authors are with the College of Optoelectronic Science and Engineering,National University of Defense Technology, Changsha, Hunan, China (e-mail:[email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [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.2014.2312927

amplifiers, since it is difficult to obtain stable pulses with enoughpeak power from an oscillator, especially for single-frequencypulses. However, power scaling of fiber amplifiers faces limita-tions of nonlinear effects such as stimulated Brillion scattering(SBS) and stimulated Raman scattering (SRS), especially forpulsed lasers with high peak power [10], [11]. Therefore, non-linear effects should be carefully considered in nanosecond fiberamplifiers. In order to suppress the nonlinear effects and obtainhigher output power, large mode area (LMA) fibers and rod-typephotonic crystal fibers (PCF) could be considered in fiber am-plifiers [12]–[17]. However, further power scaling is very chal-lenging because of limitations such as nonlinear effects, facetdamage, thermal management, the brightness of diode pumppowers and mode instability [10], [18].

Although the output power from a single fiber is limited,further scaling the overall output power while simultaneouslymaintaining good beam quality can be realized by coherent beamcombining (CBC) of several fiber amplifiers. There is a contin-uing desire to scale output power of fiber lasers via CBC, anda book focusing on this topic has been published this year [19].In this book, CBC was classified into two categories: passivephase locking [20]–[28] and active phase control [29]–[33].Passive phase locking is simple in design and operation becauseit does not need the complicated active phase control system.Several passive CBC approaches have been proposed, such asself-imaging resonator [21]–[23], self-Fourier cavity [24], Self-organization [25]–[28], all-optical feedback loop [34]. However,it is found that the maximum possible brightness gain of the pas-sive CBC system saturates near 10 for large arrays [35], so thetotal brightness increase is limited. In active CBC, the phasesof lasers can be effectively locked because a phase control loopis employed. It had been proved that active phasing can be usedto combine laser arrays with kilowatts output power [36] andtens of amplifiers [37] in continuous wave (CW) regime. Inrecent years, CBC of pulsed lasers have been focused by re-searchers because of the intensive desire of high power pulsesin practical and fundamental research applications [38]–[46].For example, in order to provide large-scale coherently com-bined pulsed fiber lasers with high energy to compact accelera-tors, the International Coherent Amplification Network (ICAN)project was proposed [47] and CBC of ultrashort pulsed lasershave been intensively reported [40]–[44]. In order to providecompact high power nanosecond lasers for applications suchas material processing, nonlinear frequency generation and

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0903913 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014

Fig. 1. Schematic of interaction length for laser pulses in optical fiber when(a) L < ctp /2n, (b) ctp /2n < L ≤ cT/2n and (c) L > cT/2n.

LIDAR, we have done substantial works on high power narrow-linewidth nanosecond all-fiber lasers and their actively coherentbeam combination in recent years. In this paper, we will reportthese researches in detail.

This paper is organized as follows. We begin in Section II byintroducing the SBS threshold for nanosecond laser in opticalfiber. A SRS limited narrow-linewidth nanosecond all-fiber laserwith high-average power of 913 W is obtained, and a double-frequency nanosecond laser system is proposed for overcomingthe SRS limitation in pulsed fiber amplifiers. In order to furtherscale the overall output power, Section III focus on the CBC sys-tem for nanosecond lasers. We analyze phase locking of pulsedlasers and impact of temporal and spectral aberrations on CBCof nanosecond lasers. Section IV demonstrates the experimentsof CBC of nanosecond amplifiers. Section V concludes thispaper with a brief discussion and gives an outlook.

II. POWER SCALING OF NANOSECOND FIBER AMPLIFIERS

A. SBS Threshold of Nanosecond Laser in Optical Fiber

Power scaling of narrow-linewidth fiber laser is mainly lim-ited by undesirable SBS, which is characterized by efficientenergy conversion from the pump laser to the potentially de-structive backscattered Stokes wave. An important feature ofSBS is that it occurs only when the pump power exceeds acertain threshold level. SBS threshold (Pth) is always definedas the pump power at which the Stokes wave increases rapidlyand comparable with some fraction μ of the maximum pumppower [48].

The process of SBS can be classically described as a nonlinearinteraction between the pump laser and Stokes wave throughan acoustic wave [48]. The interaction of the pump wave andStokes wave is the key in SBS. We define the interaction length(Loverlap) as the length through which the pump laser and Stokeswave can interact with each other. For CW laser, Loverlap equalsthe fiber length (L) and Pth ∝ 1/Loverlap if loss of the fiberis neglected [11]. However, it is more complicated for pulsedlasers, as shown in Fig. 1. When L≤ ctp /2n,Loverlap equals Lbecause the backward Stokes wave [A as shown in Fig. 1(a)]generated by the forepart of the pump pulse exit end of the fiberwill transmit with a time of tp /2 before it encounter the back-end of the pump pulse, where c is the velocity of the light, tp ispulse width of the laser pulses and n is the refractive index in

the fiber. It is to be noted that we are using the same refractiveindex for pump and Stokes beam because the Stokes shift isvery small for SBS acoustic phonon frequencies. As shown inFig. 1(b), Loverlap equals ctp /2n when ctp /2n < L ≤ cT/2n,where T = 1/fRR is the pulse period, fRR is repetition rate ofthe pulses. However, when T ≥ 2 nL/c, Stokes pulse 1 generatedby Pump pulse 1 at the exit end of the fiber will interact withnot only pump pulse 1 but also other pump pulses arrive later,because the Stokes pulses and the pump pulses transmit forwardand backward, respectively, as shown in Fig. 1(c). Neglectingthe influence of the pulse shape and the loss in the fiber, theLoverlap of the pulses laser can be expressed as

Loverlap

≈

⎧⎨

⎩

min(L,

ctp

2n

), L ≤ Tc/2n

(⟨ 2nLcT

⟩−1

)× ctp

2n +min{

ctp

2n , L− 12

(⟨ 2nLcT

⟩−1

) (T cn

)},

L > Tc/2n

(1)

where 〈〉 represents the calculation of ceil. For example,〈3.4〉 = 4. It is used to calculate the number of pump pulsesthe Stokes pulse 1 can interact with.

It is well known that the SBS threshold of CW fiber laser isin inverse proportion to L(=Loverlap). We have experimentallyproved that SBS threshold peak power for nanosecond pulses isalso in inverse proportion to Loverlap [49]. That means in orderto obtain narrow linewidth pulses with higher output power, theLoverlap of the amplifier system should be designed to be asshort as possible.

B. SRS Limited 913 W Average Power Narrow-LinewidthNanosecond Amplifier

SBS has the lowest threshold for narrow-linewidth CW lasers,however, it is more complicated in pulsed lasers [49]. Acousticalphonons participate in SBS while optical phonons are involvedin the case of SRS. The response time of SBS is governed by theacoustical phonon lifetime of∼10 ns whereas that of SRS is lessthan 100 fs. The peak power for SBS threshold of nanosecondfiber lasers can be given by Pth-SBS = 21Aeff /gB Loverlap , wheregB is the peak value of the Brillouin gain, Aeff is the effectivefiber area of fiber core [49]. While the peak power for SRSthreshold of nanosecond pulses equals to that of CW laser, whichis given by Pth-SRS = 16Aeff /gRLeff , where gR is the peak valueof the Raman gain, and Leff is the effective length [11]. In theshort pulse fiber amplifier, Loverlap could be very short and thusSBS is suppressed, however, Leff is relatively long and so SRSmay be the limitation of this kind of fiber laser [10].

Nanosecond fiber lasers with high repetition rate and highaverage power can be widely employed in material process-ing field such as laser drilling and laser welding. They canalso offer high conversion efficiency for nonlinear frequencyconversion such as optical parametric oscillator (OPO) andsecond-harmonic generation (SHG). According to the principlesdiscussed in Section II-A, we have carried out substantial ex-perimental works [50], [51], here we demonstrate a recently

SU et al.: HIGH POWER NARROW-LINEWIDTH NANOSECOND ALL-FIBER LASERS AND THEIR ACTIVELY COHERENT BEAM COMBINATION 0903913

Fig. 2. Experiment setup of the nanosecond all-fiber MOPA laser system.

developed nanosecond all-fiber MOPA laser system with highaverage power of 913 W.

Fig. 2 shows the experiment setup of the nanosecond all-fiber MOPA laser system, which consists of a seed generatingsystem and a two stage LMA Yb-doped fiber (YDF) based am-plifier system. The single-frequency CW laser has a linewidthof 20 kHz at ∼1064 nm [52]. It is directly modulated by usingan electro-optic intensity modulator (EOIM). An acousto-opticmodulator (AOM) with optical rise-time of 30 ns is used to re-move the leakage between the pulses and increase the extinctionratio. Both the EOIM and AOM are synchronously driven byan arbitrary function generator (AFG). The temporal full widthat half maximum (tFWHM) and fRR of the optical pulses areset to be ∼3 ns and 10 MHz. Two single-mode YDF basedpre-amplifiers (SMF-PA) are located after the EOIM and AOM,respectively, and the average power of these pulses is boost to∼100 mW. In order to provide pulsed seed with enough powerfor the LMA YDF based amplifiers, the average power is am-plified to be ∼3 W by another double-cladding YDF basedpre-amplifier (DCF-PA). A tapper is set after the DCF-PA tomonitor the pulsed seed and isolators (ISO) are used to protectthe components by preventing backward light such as amplifiedspontaneous emission (ASE) and SBS induced Stokes wave.

Then the pulsed seed, whose linewidth is measured to be∼220 MHz from the tapper by using a Fabry–Perot interferom-eter, is amplified to average power of 21 and 913 W by twoLMA YDF based amplifiers, respectively. The 1st stage ampli-fier is based on the YDF with core/inner-cladding diameter of20/125 μm. The YDF is cladding pumped by two 976 nm laserdiodes (LD) via a (2 + 1) × 1 signal/pump combiner. Anothertapper connected after the first stage amplifier is used to moni-tor the backward power of the main amplifier. Devices may bedestroyed if nonlinear increasing of the backward power occursbecause of the undesired SBS. A mode-field adaptor (MFA) isspliced after the tapper to improve the coupling of the first stageamplifier with the main amplifier based on LMA YDF withcore/inner-cladding diameter of 30/400 μm. The signal/pumpcombiner of the main amplifier has six pump arms. Each armis spliced with a 7 × 1 pump power combiner, which coupleseven LDs with output power of 25∼32 W into one fiber andcan provide enough pump power of 180∼200 W for the main

Fig. 3. Characteristics of the pulses output from the main amplifier. (a) outputpower, (b) typical pulse shape and (c) spectrums.

amplifier. A ∼8◦ angle is polished at the output port of the fiberto avoid signal feedback and prevent end facet damage.

Fig. 3(a) shows the amplified output power as a function ofthe pump power. The maximum average output power of 913 W(corresponding to a pulse energy of 91.3 μJ) is obtained at thepump power of 1085 W. The overall amplifier slope efficiencyis calculated to be 82%. The nonlinear increase of the backwardpower is not observed in the main amplifier, SBS is effectivelysuppressed for short Loverlap of ∼0.31 m in the main amplifier.Fig. 3(b) shows the pulse shapes of the output pulses. The am-plified pulses are slightly compressed and the tFWHM is ∼3 ns.The maximum peak power can be approximately calculated tobe 28.6 kW by using the formula of Gaussian-like pulses givenby Ppeak = 2(ln2/π)1/2Pave /(tFWHMfRR) [53], where Pave isthe average power. The spectrums of the pulses from the mainamplifier are measured by using an optical spectrum analyzerwith spectral resolution of 0.06 nm. Due to the high peak powerof 28.6 kW, SRS is observed at the average power of 913 W,as shown in Fig. 3(c). That means the output power is mainlylimited by SRS for the moment. The beam quality of the pulsedlaser is measured by using M2 factor measurement equipment(M-200 s-FW), and the vertical and horizontal directions havea value of ∼1.72 and 1.64.

C. Double-Frequency Nanosecond SRS Nonlinear Amplifier

As mentioned above, SBS can be suppressed in fiber am-plifiers with short Loverlap , but the output power is limited bySRS. In order to further scale the output power from a single


Fig. 4. Schematic of the double-frequency nanosecond SRS nonlinearamplifier.

fiber, we propose a novel double-frequency nanosecond SRSnonlinear amplifier. As shown in Fig. 4, two single-frequency(SF) CW lasers at 1064 nm and 1120 nm are modulated by twoEOIMs. The nanosecond pulses from the EOIMs are amplifiedby pre-amplifiers (PA) at 1064 and 1120 nm, respectively. It isto be noted that the 1120 nm PA can be YDF amplifier as wellas the well known Raman amplifier [54]. The amplified pulsedlasers at 1064 and 1120 nm are coupled via a wavelength di-vision multiplexer (WDM). The temporal overlap of the 1064and 1120 nm pulses can be realized by tuning the delay of thedriven signals of the EOIMs generated by the AFG. Then thedouble-frequency nanosecond pulses are used as the laser seedand followed by cascaded amplifiers pumped by 976 nm LDs.

In the cascaded amplifiers, the 1064 nm pulses are firstlyamplified by the 976 nm pump power. SBS should be suppressedby optimizing the tp and fRR of the pulses and the fiber lengthof amplifiers. When the 1064 nm pulses are amplified to SRSthreshold, the 1120 nm pulses are amplified by absorbing theenergy of 1064 nm pulses through the SRS process. Finally,the pulses at both 1064 and 1120 nm can be export from thesystem. It is to be noted that such double-frequency lasers canbe used in the CBC system for further scaling the overall outputpower [55].

III. ANALYSIS OF CBC SYSTEM FOR

NANOSECOND FIBER LASERS

Some preconditions have to be fulfilled for actively CBC ofpulsed amplifiers. Firstly, the operation of the actively phasecontrol needs a feedback signal independent of the pulse it-self. In CW laser CBC systems, the feedback signal could beextracted from the intensity varieties of the combined beaminduced by phase differences between each laser channel. How-ever, in pulsed laser CBC systems, the intensity of the combinedbeam is also influenced by the temporal character of the pulsechain, which will be discussed in Section III-A. Secondly, com-bining efficiency suffers from kinds of aberrations in the CBCsystem. Factors such as asymmetric intensity distribution andphase error have been studied drastically based on CBC of CWfiber lasers [56], [57]. However, more effects such as temporalaberrations and spectral phase mismatch should be take intoconsideration in the pulsed laser CBC systems [58].

For nanosecond fiber lasers, dispersion can be neglected, butself-phase modulation (SPM), which leads to nonlinear phaseshift and may impact the process of CBC, must be paid attentionto. There always have optical path difference (OPD) in the CBCsystem, which will induce the spectral phase shift and reducetemporal overlap of the pulses. Neglecting the influences of spa-

tial distribution which have been discussed in-deep, we considera pulsed fiber laser array of N nanosecond laser, where the fieldof nth beam is

En (ν, t) = E0nAn (t)Sn (ν) exp [−i (2πνt + φn (ν, t))] (2)

where E0n is maximum amplitude, An (t) is the normalizedamplitude, Sn (ν) is the normalized spectral, and φn (ν, t) is thephase of the field and can be expressed as

φn (ν, t)=ϕchirp(ν)+ϕdelay−n (ν)+ϕN L−n (t)+ϕother−n (t)(3)

where ϕchirp−n (ν) is the chirp phase, ϕdelay−n (ν) is the OPDinduced spectral phase shift, ϕNL−n (t) is the SPM inducednonlinear phase shift, ϕother−n (t) consists of the piston phasenoise in the amplifier and the phase correction implemented byphase control system. The intensity of the combined beam canbe expressed as

I(ν, t) =

(n=N∑

n=1

En (ν, t)

) (n=N∑

n=1

En (ν, t)

)∗

(4)

I(t) =∫

I(ν, t)dν. (5)

A. Phase Locking of Pulsed Laser

Firstly, we consider the influence of the temporal character ofthe pulses on phase locking of pulsed lasers. Assuming that allthe beams are temporal overlapped and have the same normal-ized amplitudes A(t), formula (5) equals

I(t) = A(t)2∫

dν

[n=N∑

n=1

I0n (ν)

+n,m=N∑

n �=m

(E0nSn (ν)E∗m S∗

m (ν) exp (φn (ν, t) − φm (ν, t)))

]

(6)

where I0n (ν) = |E0nSn (ν)|2 . In this formula, A(t)2 is a pe-riodic function with period of 1/fRR . By employing Fourierexpansion, A(t)2 can be expressed as

A(t)2 =a0

2+

∞∑

n=1

an cos (2πnfRR t). (7)

It is not hard to obtain the frequency spectrum (F1) of theA(t)2 , which is approximatively shown in Fig. 5(a) and can beexpressed as

F1 =a0

2δ(0) +

12

∞∑

k=1

ak [δ(f − kfRR) + δ(f + kfRR)].

(8)

Formula (6) contains the information of phase noises andphase modulations/perturbations induced by the phase controlalgorithm. The frequency spectrum (F2) of the integral in for-mula (6) is approximatively shown in Fig. 5(b), where fM isthe highest frequency of the phase noises and phase modula-tions/perturbations induced intensity fluctuation. The frequency


Fig. 5. Frequency spectrum characteristic of (a) A(t)2 , (b) intensity fluctua-tion induced by phase noises and phase modulations/perturbations and (c)–(d)intensity fluctuation of the combined beam.

spectrum (F ) of I(t) can be obtained by

F = F1 ∗ F2 =a0

2F2 +

12

∞∑

k=1

ak

[F2(f − kfRR)

+ F2(f + kfRR)]. (9)

Figs. 5(c) and (d) shows the frequency spectrums of the in-tensity fluctuation of the combined beam. The frequency ofphase noises induced by amplifiers, turbulence and mechanicalquivering are always lower than several kHz, and the frequencyof phase modulations/perturbations should be high enough forphase control, so fM is always tens of kHz to hundreds of kHz.As shown in Fig. 5(c), when fRR is higher than fM , a low passfilter (LPF) with cut-off frequency of between fM and fRR canbe used to eliminate the undesired frequency spectrum and onlyremain the frequency spectrum induced by the phase noises andphase modulations/perturbations. The phase locking can be re-alized the same as the CBC system of CW lasers, excepting thata appropriate LPF should be employed. When fRR is lower thanfM , phase locking is more complicated. However, CW signalleak between the pulses can be used for phase locking, and anovel method will be demonstrated Section IV-A.

B. Impact of Temporal and Spectral Aberrations

In order to simplify the discussion, we only considering thesituation of CBC of two beams using a partially reflectivesurface. Supposing that the pulses have the same chirp phase(ϕchirp−1(ν) = ϕchirp−2(ν)), equation (5) can be simplified tobe

I(t) =I1(t)

2+

I2(t)2

+∫

√I1(ν, t)I2(ν, t) cos(Δφ(ν, t))dν

(10)

Δφ(ν, t) = Δϕdelay (ν) + ΔϕNL(t) + Δϕother(t) (11)

where In (t) =∫

(E0n An (t)Sn (ν))2dν, n = 1, 2. The combina-tion efficiency (η) can be expressed as

η =∫

I(t)dt∫

I1(t) + I2(t)dt

=12

+∫ ∫ √

I1(ν, t)I2(ν, t) cos (Δφ(ν, t)) dνdt∫

[I1(t) + I2(t)] dt. (12)

We consider CBC of pulsed lasers with Gaussian pulses,whose normalized amplitude and spectral distribution is de-fined as: An (t) = exp[− 2ln2(t − t0)2 /t2FWHM ] and Sn (ν) =exp[−2ln2(ν−ν0)2 /ν2

FWHM ], respectively, where νFWHM is thespectral full width at half maximum.

When SPM can be neglected, the combined pulses are mainlyinfluenced by OPD (ΔL). On the one hand, it reduces temporaloverlap of the pulses, where the time difference is ΔL/c. On theother hand, it induces the spectral phase shift ϕdelay−n (ν) =2πΔLν/c. Assuming that the phase difference for the centralfrequency is locked to be zero, equations (10) and (12) can besimplified to be

I(t) =νFWHM

√π

4√

ln(2)

[

E201 exp

(

−4 ln(2)t2

t2FWHM

)

+ E202 exp

(

−4 ln(2) (t − ΔL/c)2

t2FWHM

)

+ 2E01E02 exp

(

−2 ln(2)t2

t2FWHM− 2 ln(2) (t − ΔL/c)2

t2FWHM

− (πΔLνFWHM)2

4 ln(2)c2

)]

.

(13)

η =12

+E01E02

E201 +E2

02exp

(

− ln(2)ΔL2

c2t2FWHM− (πΔLνFWHM)2

4 ln(2)c2

)

.

(14)

It is to be noted that (t − ΔL/c)2 in equation (13) and ΔL2

in equation (14) are induced by temporal overlap of the pulses,and (πΔLνFWHM )2 in equation (13) and equation (14) resultsfrom the spectral phase shift.

Supposing tFWHM is 3 ns, and E01 equals E02 , the typicalpulse shapes of the combined pulses are shown in Fig. 6. Asshown in Fig. 6(a), if the νFWHM is 0.16 GHz (νFWHM tFWHM =0.48), the pulse shape almost does not change even when OPDis 0.1 m. However, much shorter OPD will drastically decreasethe peak power (Ppeak) when νFWHM is broader. For example,when νFWHM is 5 GHz and OPD is 0.025 m, the Ppeak of thecombined beam decreases to less than 77% compared with thatwhen OPD is zero. It means that the pulse shapes are influencedmainly by reduction of temporal overlap of the pulses when CBCof lasers with very narrow linewidth, while mainly by spectralphase shift when the linewidth of lasers is broader. It can beconcluded that for pulses with tFWHM of 3 ns, the limitation isnot temporal overlap but spectral phase when νFWHM is broader


Fig. 6. Typical shapes of the combined pulses when CBC of two pulsed laserswith νFW HM of 0.16 GHz (a), 1 GHz (b) and 5 GHz.

Fig. 7. The maximum acceptable OPD when the normalized Pp eak and η ismore than 95%.

1 GHz. The maximum acceptable OPDs when the normalizedPpeak and η is more than 95% are shown in Fig. 7.

In pulsed fiber lasers, SPM induced nonlinear phase shift isϕNL(t) = BA(t)2 , where B = γPpeakLeff is the B-integral ofthe amplifier, Leff is the effective length, and γ is nonlinearparameter. The SPM induced phase difference of the two laserscan be expressed as ΔϕNL(t) = B1A1(t)2 − B2A2(t)2 . As-suming Δϕother = Δϕo1 + Δϕo2 and Δϕdelay (ν0) + Δϕo1 =

0, equation (10) and (12) can be simplified to be

I(t) =νFWHM

√π

4√

ln(2)

[

E201A

21(t)

+ E202A

22(t) + 2E01E02A1(t)A2(t)

cos (ΔϕSPM(t)

−Δϕo2) exp

(

− (πΔLνFWHM)2

4 ln(2)c2

) ]

(15)

η =12

+2√

ln(2)E01E02√π(E2

01 + E202)tFWHM

×∫ [

A1(t)A2(t) cos(ΔϕSPM(t)

− Δϕo2) exp(

− (πΔLνFWHM)2

4 ln(2)c2

) ]

dt (16)

where A1(t) = exp[−2ln2t2 /t2FWHM ] and A2(t) =exp[−2ln2(t − ΔL/c)2 /t2FWHM ]. It is to be noted that thecos(ΔϕSPM(t)−Δϕo 2) in equation (15) and (16) results fromSPM. When Δϕo 2 is locked to be ΔϕSPM(t0) +kπ, k = 0, 1,. . ., the Ppeak at the time of t0 is highest. While the maximumη can be obtained when Δϕo 2 is locked to beΔϕo2 =

arctan

⎡

⎢⎢⎣

∫sin (ΔϕSPM (t)) exp

(

A1 (t)−A2 (t)− (π Δ LνF W H M )2

4 ln(2)c 2

)

dt

∫cos (ΔϕSPM (t)) exp

(

A1 (t)−A2 (t)− (π Δ LνF W H M)2

4 ln(2)c 2

)

dt

⎤

⎥⎥⎦ .

(17)

SPM will not impact the CBC of nanosecond lasers if the am-plifiers have the same B-integral (B1 =B2) and OPD is zero.However, both the influences of nonlinear phase shift ΔϕNL(t)and spectral phase shift Δϕdelay (ν) should be taken into con-sideration when OPD is not zero. The typical pulse shapes ofthe combined pulses are shown in Fig. 8, where tFWHM is3 ns, B1 equals B2 and Δϕo 2 is locked to be zero. The pulseshapes almost do not change when OPD is 0.01 m. However,when OPD is 0.1 m, the pulse shapes drastically changed: whenνFWHM is 0.16 GHz, ΔϕNL(t) is the primary factor, the pulseshapes distort because of SPM; while when νFWHM is 5 GHz,Δϕdelay (ν) is the primary factor, it is close to incoherent beamcombination. Fig. 9 shows the maximum acceptable OPD whenthe η is more than 95%. It can be found that as the increasingof νFWHM , spectral phase shift becomes the primary factor andthe combination efficiency is mainly depended on the νFWHM .

The B-integral differences of the amplifiers, which are in-duced by the differences of amplifiers, such as output powerand fiber length, should also be paid attention to. Assumingthat the OPD is zero, and pulses from each channel have thesame pulse shape. The maximum η can be obtained is shown inFig. 10. We can find that the pulse energy is mainly dependenton the ΔB, and the influence of the differences of output powersfrom laser channels is slight. The ΔB must be less than 0.52 πif we want to obtain a η of more than 95%.


Fig. 8. Typical shapes of the combined pulses when CBC of pulsed laserswith the same B-integral and with νFW HM of (a) 0.16 GHz, (b) 1 GHz, and(c) 5 GHz.

Fig. 9. The maximum acceptable OPD when the pulse energy is more than95%.

IV. EXPERIMENT FOR CBC OF NANOSECOND PULSES

Nanosecond pulses with different repetition rates are re-quired in specified application fields. For example, LIDARsneed pulsed lasers with repletion rate of less than several kilo-hertz [6], while nonlinear frequency conversion employ pulseswith high repetition rate for high average power output [2]. Inthis section, we will demonstrate our experimental researches

Fig. 10. The maximal combination efficiency as a function of B-integraldifference.

Fig. 11. Experimental setup for CBC of two nanosecond lasers with lowrepetition rate.

on CBC of nanosecond lasers with low repetition rate and highaverage power.

A. CBC of Two Nanosecond Lasers With Low Repetition Rate

Narrow-linewidth pulsed lasers with repetition rate of severalhertz to several kilohertz are required in some applications.For example, the pulsed laser of a temperature LIDAR wasworking at 1083 nm at repetition rate of 1 kHz [3]. Output powerof single-frequency long pulses is limited by SBS, however,CBC can be employed to increase the overall output power. ForCBC of pulses with high repetition rate, the feedback of thephase control algorithm occurs lower than the laser repetitionrate and LPF can be employed to eliminate the impact of laserpulses. However, the feedback rate cannot compensate the phasevariation when the repetition rate of the laser is low. In orderto solve this problem, a novel structure is employed to activelyCBC of pulsed lasers with low repletion rate [59], where CWsignal leak between the pulses is used to lock the phases.

The experimental setup is shown in Fig. 11. A single-frequency CW laser with output power of∼60 mW at∼1083 nmis split into two arms by a coupler (Coupler1). The arm withpower of ∼99% is spliced with a AOM (AOM1: extinction ra-tio >50 dB, rise-time is 30 ns) and a single-mode YDF basedpre-amplifier (PA). The AOM is used to modulate the CW laser,and the PA is employed to boost the peak power of the pulses.Then the pulsed laser and the CW laser are launched into the


Fig. 12. Measured cost function J in open loop and close loop.

input ports of a 2 × 2 single-mode fiber coupler (Coupler2)with coupling ratio of 50:50. The lasers from the output ports ofcoupler 2 are pulsed lasers with signal leak between the pulses.Then one output port of the coupler is spliced with a LiNbO3electro-optic phase modulator (EOPM), and the other port iscoupled into a single-mode passive fiber, which is used to con-trol the OPD of the two laser channels. Two all-fiber amplifiersare employed to boost the average powers of the two channels tobe 16 and 17 mW. The signal leak power should be enough forphase locking. But it must be kept as low as possible, becausetoo much signal leak degrades the performances of the pulsesand the SBS threshold. As shown in Fig. 11, the extinction ratioof the pulses can be controlled by the coupling ratio of coupler1and the gain of the PA. Then the laser beams are sent out intofree space via collimators (CO) and geometrically combined ona 50:50 bulk beam splitter (BS) in a Mach–Zehnder structure.The spacial overlap of the beams along their propagation axis isrealized by carefully tuning the COs. Samples of the combinedbeam are extracted by samplers (S1 and S2) for phase lockingand intensity profile observing. A free-space AOM (AOM2) isinserted before the photoelectric detector (PD) to suppress thepulses and extract the weak CW signal leak. The intensity ofthe CW signal leak detected by PD is defined as cost functionJ and used in the SPGD algorithm for phase locking [29], [30].The cost function J is in proportion to the intensity of the com-bined beam and influenced by the phase differences betweenthe lasers. Cost function J is a function of the control voltagesof the EOPMs and it keeps to its extremum when iteration forupdating the control voltages is realized by SPGD algorithm.The operations of the AOM1, AOM2 and phase controller areharmonized by an AFG. The implement of SPGD algorithm onthe phase controller is only enabled when CW signal leak isdetected by PD.

When the phase is not locked and the whole system is inthe open loop, the output power from the BS change all thetime and the cost function J fluctuated randomly, as shown inFig. 12(a). The observed intensity profiles change in the openloop, as shown in Figs. 13(c)–(f). When the two laser channelsare phase locked and the whole system is in the close loop,the cost function J is locked to be maximum in most the timeas shown in Fig. 12(b), and the output power from the BS is∼28.7 mW stably. The observed intensity profile is steady as

Fig. 13. Measured intensity profiles of (a)-(b)the individual beams and(c)-(f) the combined beam.

Fig. 14. Experimental setup of CBC of five pulsed fiber amplifiers.

shown in Fig. 13(f). Combining efficiency is calculated to be89% by using η = Pout /(P1 + P2), where Pout is the combinedoutput power, P1 and P2 are the output power of the amplifiersbefore the BS.

It is testified that signal leakage between the pulses can beemployed for phase control. This method can be used to CBC ofpulsed lasers with lower repetition rate, such as pulsed-pumpedlasers.

B. CBC of Five Nanosecond Lasers With High Average Power

Fig. 14 shows the experimental setup for CBC of five nanosec-ond lasers [60]. In the pulsed MOPA, the master oscillator (MO)is obtained by modulating a single frequency 1064 nm CW laserby using an EOIM. The seed pulses have a fRR of 10 MHz anda tFWHM of ∼3.5 ns by setting the driven signal generated bythe AFG. A single mode YDF based pre-amplifier (Pre-AMP)is developed to boost the average power of the seed pulses to∼100 mW. Then the pulsed laser is split into five channels us-ing a beam slipper. The pulsed laser in each channel is coupledinto a LiNbO3 EOPM and amplified by a three-stage all-fiberamplifier. The laser pulses are amplified to an average powerof ∼6 W by the first-stage and second-stage amplifiers, andlaunched into the main amplifiers, where ∼4 m LMA YDFswith core/inner-cladding diameter of 30/250 μm are employedand pump power is coupled into each YDF through (6 + 1) ×1 signal/pump combiner. The pump diodes used in the thirdchannel have a maximal output power of 45 W, while the pumpdiodes for the other four channels have a maximal output powerof 32 W. The output power of the main amplifiers are about


Fig. 15. Pulse shape of a single channel and combined beam.

148, 153, 202, 151 and 146 W, for an overall output power of800 W. The pulses from the main amplifiers are sent into free-space via five collimators (CO). The laser beams with diameterof ∼6 mm are tiled side by side into a crisscross array by using abeam combiner (BC), which consisted of an aluminous pedestaland several mirrors, as shown in Fig. 14. The laser channel withhigher average power is located at the center of the crisscross.The combined beam has a fill factor of 50% in the near field,where fill factor is defined as the diameter of the beam dividedby the distance of the adjacent beamlets.

The combined beam is then sampled by using a mirror withreflectivity of more than 99%. The reflected light achieves thepower meter for the total power test. The other beam with lowerpower propagates ∼12 m and reaches at a CCD. In order totestify that our system can compensate not only the phase fluc-tuations inside the amplifiers but also the turbulence inducedpiston type aberrations in the turbulent atmosphere. We simu-late the turbulence by employing fans as shown in Fig. 14. Asampler with 50% reflectivity is inserted before the CCD. Thecentral lobe of the sampled beam passes through a pinhole withdiameter of ∼300 μm and is then detected by a PD. A LPFwith cut-off frequency of ∼3 MHz is employed to eliminatethe fluctuation caused by pulses with fRR of 10 MHz. The out-put signal of the LPF is defined as cost function J and usedin SPGD algorithm as well as in the CW system. In order toensure that the laser beams temporally overlap with each other,the OPD is carefully controlled as follows: firstly, the delay timeΔt between each pulse are measured by a high speed PD, andpassive fiber with length of L= cΔt/n is spliced in the channelwith shorter optical path, where n is the refractive index of thecore of the fiber; secondly, OPD is further minished by carefullyadjusting the positions of COs. Because of the OPD control, thepulse shape of the combined beam is almost the same as that ofa single channel, as shown in Fig. 15. The tFWHM of the pulseswas ∼3.5 ns. The peak power can be approximately calculatedto be 21.5 kW using the formula of Gaussian-like pulses.

Figs. 16 and 17 show the normalized cost function and theintensity pattern, respectively. When the phase fluctuation in-duced by amplifiers and simulated turbulence is not controlledand the whole system is in the open loop. The cost functionJ fluctuates randomly and intensity pattern keeps shifting andis blurred. When SPGD algorithm is implemented to lock the

Fig. 16. Normalized cost function in open loop and close loop.

Fig. 17. Normalized far-field long-exposure intensity pattern of the combinedbeam in (a) open loop and (b) close loop, and (c) far-field intensity pattern of asingle amplifier.

phase and the whole system is in the close loop. The normal-ized cost function J is more than 0.9 most of the time and theobserved intensity pattern is clear and steady.

The intensity of the central lobe had been remarkably in-creased because of phase locking, as shown in Fig. 17(a) and (b).The fringe contrast of the long-exposure intensity pattern is in-creased from about zero in the open loop to 91.6%, where fringecontrast is defined by (Imax − Imin)/(Imax + Imin), Imax andImin are the maximum optical intensity and the adjacent mini-mum optical intensity on the intensity pattern. It is to be notedthat the intensity in the central lobe of combined beam is sig-nificantly higher than that achieved from a single amplifier withthe same output power, as shown in Fig. 17(c).


Fig. 18. Experimental setup of CPBC of four nanosecond lasers.

C. Coherent Polarization Beam Combining of FourNanosecond Lasers

In a CBC system with multiple tiled collimators, a portion ofpower is encircled into the side-lobes in the far-field pattern, itdegrades the beam quality. In order to improve the beam qual-ity, one method is tiling the collimators as tightly as possible.However, there are better solutions based on filled aperture de-signs, where the multiple outputs overlap in both the near andfar field. Coherent polarization beam combining (CPBC) pro-vides a filled aperture solutions for CBC of lasers with highcombining efficiency and excellent beam quality [61]. In thissubsection, we will present a proof-of-concept test of CPBC offour nanosecond lasers [62].

Fig. 18 shows the experimental setup of CPBC of fournanosecond lasers. The master oscillator is the same as we de-picted in Fig. 13. The pulses from master oscillator, which havea fRR 4 MHz and a tFWHM of ∼70 ns, are split into four chan-nels. Pulses in each channel are coupled to a EOPM followedby an all-fiber amplifier based on PM YDF with core diameterof 6 μm, and then sent out into free space via a CO. The averagepowers from the COs are 89.2, 88.5, 95.8, and 93.6 mW, respec-tively. The four laser beams are combined by the polarizationbeam combiners (PBCs) by adjusting the half wavelength plates(HWPs). In our experiment, M1 is an all-reflectance mirror andM2 is a high-transmittance mirror. It is to be noted that a polar-izer (P) located behind PBC3 is used to stabilize the polarizationof the combined beam from PBC3, and relative phases betweenthe beams can be extracted for phase control. A little part of thebeam is sampled by M3 and sent to an CCD, and another part ofthe beam passes a pinhole with a 50 μm radius and arrives thePD located behind the pinhole. The voltage signal transformedby the PD is used as phase control signal of the phase controllerbased on single-frequency dithering algorithm. It is to be notedthat PD has a cut-off frequency of ∼2 MHz and the fluctuationcaused by pulses with fRR of 4 MHz is eliminated. It is to benoted that the OPDs among the channels are less than 1 cm byusing the same amplifiers and adjusting the positions of COs.

In the experiment, we use a 1 MHz sine wave as the phasemodulation signal. When the phase is unlocked and the wholesystem is in the open loop, the polarization state distributionsof the beams from the PBC1 and PBC2 are uncertain, and partof the power is leaked through PBC3 and the combined output

Fig. 19. Experiment results of CPBC of four nanosecond lasers. (a) normalizedpower and in open loop and closed loop, (b) the far-field intensity profile fromsingle amplifier and (c) the combined beam in closed loop.

Fig. 20. Phase locking system for active CBC of large number fiber lasers.

power is unsteady because the phase difference among eachbeam is random. The output signal monitored the PD fluctuatesrandomly as shown in Fig. 19(a), and the intensity profile at theCCD is fluctuant. When output signal from the PD is locked tobe maximum by the single-frequency dithering algorithm [63],the polarization states of the combined beams from the PBCsare still linear-polarized, the intensity profile is steady as shownin Fig. 19(a). The measured combined power at the output portof M2 is stably at 338 mW and the combining efficiency iscalculated to be 92%. The intensity profile of the single beam andthe coherently combined beam are shown in Figs. 19(b) and (c).The coherently combined beam quality is similar to one of thefour amplified single mode beams and CPBC does not degradethe beam quality. Experimental results show that CPBC can bestraightly employed for nanosecond fiber lasers.


V. CONCLUSION AND OUTLOOK

We have presented nanosecond all-fiber lasers and their ac-tively coherent beam combination from both theoretical andexperimental aspects. In order to analysis the SBS thresholdof nanosecond laser in optical fiber, a concept of interactionlength was introduced, and it was testified from the experimentthat SBST peak power is in inverse proportion to the interac-tion length. A SRS limited all-fiber nanosecond laser with highaverage power of 913 W was built, and a double-frequencynanosecond SRS nonlinear amplifier was proposed. We theoret-ically studied the influence of temporal character of the pulseson phase locking of pulsed lasers. If the repetition rate of thepulsed laser is high enough, the phase locking can be realizedthe same as the CBC system of CW lasers, excepting a appro-priate LPF should be employed. While CW signal leak betweenthe pulses can be used to phase locking of pulses with lowrepetition rate. Impact of temporal and spectral aberrations onthe CBC of nanosecond lasers has been demonstrated, it canbe concluded that SPM will not impact the CBC of nanosecondlasers if the amplifiers have the same B-integral and OPD is zero.Experimental investigations on CBC of nanosecond lasers werepresented. A novel structure was employed to actively CBC oftwo nanosecond amplifiers with repetition rate of 5 kHz andpulse width of ∼500 ns. Actively phase locking of five nanosec-ond amplifiers with average/peak power of 0.8/21.5 kW wasdemonstrated. CPBC of four nanosecond laser was experimen-tally verified.

Based on the research results demonstrated in this paper,active CBC of nanosecond lasers can provide high average/peakpower with almost arbitrary repetition rate for applications suchas welding, cutting, nonlinear frequency generation and LIDAR.However, for applications such as laser peening and inertialconfinement fusion (ICF) which need nanosecond pulses withenergy of ∼100 J and ∼MJ, respectively, and a large numberof laser channels are required, in this case, CBC of nanosecondpulsed lasers faces challenges as the increasing of the numberof the lasers. In fact, ICAN, who believes massive arrays ofthousands of fiber lasers could provide the driving force ofthe next-generation particle accelerators [47], faces the samechallenges.

First, phase locking of large number of fiber lasers is lim-ited by the performance of the phase controller. Although activephase locking of 64 fiber lasers has been achieved [37], it isobjective that the control bandwidth decreases as the increas-ing of the number of laser channels [30]. This problem may besolved by employing a structure as shown in Fig. 20. The masteroscillator (MO) is first split into N channels, and each channelis then split into M sub-channels. So the laser array consists ofN sub-arrays, i.e., M ·N lasers. Each sub-array is phased lockedby using a phase controller. Then the N combined beams arephase locked by “phase controller N + 1” (as shown in Fig. 20).In order to avoid the influences between the phase controllers,“phase controller N + 1” and other phase controllers can im-plement alternately. If single frequency dithering technique isemployed [63], the influence between the phase controllers canalso be eliminated if “phase controller N + 1” and other phase

controllers use modulation signals with different frequencies.Phase locking of 1,000 lasers may be feasible based on a 33 ×33 laser array.

Another challenge is combination of massive laser arrays.Beam combination can be classified into tiled aperture andfilled aperture (as discussed in Section IV-C). Tiled aperturestructure has potential of all-electrical fast beam steering andatmospheric turbulence compensation [29], [30]. However, aportion of power is encircled into the side-lobes in the far-field pattern and the beam quality degrades badly. The powerin the side-lobes is relative to the fill factor. So the laser beamsshould be tiled as tightly as possible using beam combiners suchas lenslet array. Filled aperture structures, such as CPBC anddiffractive optical element (DOE), deposit all the power into asingle beam. We have experimentally testified CPBC of eightfiber lasers [64] and CBC of five fiber amplifiers with more than90% efficiency using a DOE was reported [65]. It is still a openquestion that combining efficiency degrades into less than 80%when the power of the amplifiers is more than 100 W [66], [67].The degrading of combining efficiency is mainly induced byloss of beam quality and thermal accumulation of the combin-ing element. Generally speaking, actively CBC of nanosecondall-fiber lasers opened a new roadmap towards high power fiberlaser system, however, there are many difficulties and challengeshave to be overcome.

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Rongtao Su received the B.S. and M.S. degrees in optical engineering fromNational University of Defense Technology, Chang Sha, China, in 2008 and2010, respectively. He is currently working toward the Ph.D. degree at the Col-lege of Optoelectronic Science and Engineering, National University of DefenseTechnology. His current research include high power fiber laser technology, andcoherent beam combination.

Pu Zhou received the B.S. degree and Doctor’s degree in optical engineeringfrom National University of Defense Technology, Chang Sha, China, in 2005and 2009, respectively. He is currently a Professor at the College of Optoelec-tronic Science and Engineering, National University of Defense Technology.His current research interests include fiber laser/amplifier technology, coherentcombining of fiber lasers/amplifiers, and adaptive optics.

Xiaolin Wang received the B.S. degree in optical engineering from the Uni-versity of Electronic Science and Technology, Cheng Du, China, in 2006, andthe Doctor’s degree in optical engineering from National University of De-fense Technology, Chang Sha, China, in 2011. He is currently a Instructor atthe College of Optoelectronic Science and Engineering, National University ofDefense Technology. His current research interests include fiber laser/amplifiertechnology, and coherent combining of fiber lasers/amplifiers.

Yanxing Ma received the B.S. degree in optical engineering from Shanxi Uni-versity, Tai Yuan, China, in 2006, and the Doctor’s degree in optical engineeringfrom National University of Defense Technology, Chang Sha, China, in 2012.He is currently an Instructor at the College of Optoelectronic Science and En-gineering, National University of Defense Technology. His current researchinterests include coherent beam combination and adaptive optics.

Hu Xiao received the B.S. degree in optical engineering from Sichuan Univer-sity, Cheng Du, China, in 2007, and the Doctor’s degree in optical engineeringfrom National University of Defense Technology, Chang Sha, China, in 2013.He is currently an Instructor at the College of Optoelectronic Science and En-gineering, National University of Defense Technology. His current researchinterests include fiber lasers.

Pengfei Ma received the B.S. degree in optical engineering from ShandongUniversity, Ji Nan, China, in 2010, and the M.S. degree in optical engineeringfrom National University of Defense Technology, Chang Sha, China, in 2012.He is currently working toward the Ph.D. degree at the College of Optoelec-tronic Science and Engineering, National University of Defense Technology.His current research include high power fiber laser technology, and coherentbeam combination.

Xiaojun Xu received the Doctor’s degree in optical engineering from NationalUniversity of Defense Technology, Chang Sha, China, in 2000. He is currentlya Professor at the College of Optoelectronic Science and Engineering, NationalUniversity of Defense Technology, Changsha, China. His current research in-terests include solid state laser, aero-optics, and adaptive optics.

Zejin Liu received the Doctor’s degree in optical engineering from NationalUniversity of Defense Technology, Chang Sha, China, in 1997. He is currentlya Professor at the College of Optoelectronic Science and Engineering, NationalUniversity of Defense Technology, Changsha, China. His current research in-terests include high-power laser technology.

high power narrow-linewidth nanosecond all-fiber lasers and their actively coherent beam combination

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