Simultaneous spectrum and coherent combiningby active phasing dual

two-tone all-fiber MOPA chainsXiaolinWang, Jingyong Leng, Hu Xiao, YanxingMa, Pu Zhou,Wenbo Du, Xiaojun Xu, Zejin Liu,* and Yijun ZhaoCollege of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha, 410073, China

*Corresponding author: zejinliu@sina.vip.com

Received February 10, 2011; revised March 14, 2011; accepted March 14, 2011;posted March 16, 2011 (Doc. ID 141998); published April 6, 2011

We present a new approach for simultaneous spectral and coherent combining in amaster oscillator power amplifier(MOPA) configuration with active phasing. Spectrally combined single-frequency seed lasers are employed as amaster oscillator for the all-fiber power amplifier chain, which provides robust performance suppressing of thestimulated Brillouin scattering effect, while coherent combining of spectrally combined amplifiers with activephase control provides stable in-phase combining, despite the strong phase fluctuation. In experiments, two spec-trally combined seed MOPA chains are coherently combined with a total output power of 390W. The power ofthe main lobe in the closed loop is two times of that value in the open loop, and visibility of more than 75%of the long-exposure interference pattern at the receiving plane is obtained. © 2011 Optical Society of AmericaOCIS codes: 140.3280, 140.3290, 140.3510, 140.3580.

High-power single-frequency/narrow-linewidth fiber la-sers are widely used in materials processing, gravita-tional wave sensors, nonlinear frequency conversion,harmonic generation, and remote sensing. Nevertheless,restricted in terms of thermal load, fiber damage, andnonlinear effects, the ultimate output power of the sin-gle-frequency fiber amplifier has been limited to the sev-eral-hundred-watts level [1,2]. It is believed thatstimulated Brillouin scattering (SBS) is the major obsta-cle toward high-power narrow-linewidth fiber amplifiers[3]. In order to increase output power, two solutions aresuggested: (i) mitigate or suppress SBS in fibers [4] and(ii) beam combining [5–8]. Various techniques for SBSsuppression, i.e., large-mode area (LMA) fibers, stress,thermal gradients [4], multitone-driven amplifiers [3,9]have been proposed to suppress the SBS effect. The re-cently suggested multitone-driven amplifier may be oneof the most attractive techniques for robust performancefor the suppression of SBS effects [9]. Beam combining ofa fiber laser/amplifier array is another feasible way to in-crease output laser power from a single gain mediumwhile simultaneously maintaining good beam quality.Of the various kinds of beam-combining techniques, in-cluding polarization [10], spectral [11], incoherent [12],and coherent combining [13], spectral and beam combin-ing are two of the most promising ways for high-powerdemonstrations. Up to now, spectral combining has beenscaled to more than 2 kW [11], while coherent combininghas been scaled to be as high as 100 kW [13]. By spec-trally combining two pairs of coherently added lasers,the simultaneous coherent and spectral addition of fiberlasers was presented [14] with a spatial configuration. Inthis configuration, coherent addition is implemented byan interferometric combiner, and spectral combination isperformed by a grating. Scaling this configuration to highpower is a great challenge for the power limitations of theinterferometric combiner and grating. Meanwhile, a mas-ter oscillator power amplifier (MOPA) with active phasecontrol is one of the most effective ways for coherentcombining [14], and the highest power demonstration

of coherent combining has involved active phasing ina MOPA configuration [13].

In this Letter, we will present what we believe to be anew approach for simultaneous spectral and coherentcombining in a MOPA configuration with active phasing.Spectrally combined single-frequency seed lasers are em-ployed as the master oscillator (MO) in MOPA chains,with which configuration the SBS can be suppressed ef-fectively. Amplified multitone lasers are then coherentcombined spatially with active phase control. In thisway, the combined advantage of spectral and coherentcombining provides robust performance both in the sup-pressing of SBS effects and stable in-phase coherentbeam combining.

The experimental setup is shown in Fig. 1. Two single-frequency seed lasers are spectrally combined with awideband polarization maintained coupler (C) and usedas an MO. Seed 1 is commercial linearly polarized single-frequency laser (RLFM-25-1-1064-1, NP Photonics) gener-ating 40mWof power at a wavelength of 1064:4nmwith alinewidth of 87 kHz. Seed 2 is a homemade (by South Chi-na University of Technology [15]), single-frequency lasergenerating 40mW of power at a wavelength of 1063:8 nmwith a linewidth of 250 kHz. The spectrally combined MO

Fig. 1. (Color online) Experimental setup of simultaneousspectral and coherent combining in MOPA configuration: S1,S2, seed laser; A0, preamplifier; PM, phase modulator; AI,AII, AIII, amplifiers in different stages; BS1, BS2, beam splitter;YDF, Yb3þ doped fiber; PD, PD1, PD2, photodetector; SMFC,single mode filter coupler; DF, delivery fiber, CO, collimator.

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is firstly scaled up to 300mW by the preamplifier (A0),and then split into two channels by a 1 × 2 splitter. Eachchannel is first coupled into a phase modulator (PM) andthen sent into a three-stage all-fiber power amplifierchain. Each three-stage power amplifier chain can scalethe power from 15mW more than 250W (15mW to150mW in AI, 150mW to 9W in AII, 9W to more than250W in AIII). After the output port of the second ampli-fier (AII.1), single-mode filter couplers [(SMFCs), withwhich a signal-to-power ratio of about 0.1% is coupledto the tap], and two photodetectors (PD1, PD2) are usedto monitor the output power of AII.1 and the backwardsignal in the main amplifier (AIII.1). When the power de-tected by PD1 is more than 0:15mW (which means thetotal power of the SBS is ∼150mW), the power supplyof the pumps in the main amplifier is turned off by thecontrol circuit, which can avoid damage caused by theSBS and protect the amplifier chains.The main power amplifier is based on a ∼3:5m long

LMA ytterbium-doped fiber (YDF, LMA-YDF-30/250, Nu-fern, Inc.) for high-power operation. The LMA is pumpedby six 60W level 974 nm laser diodes through a ð6þ 1Þ ×1 combiner. The output end of the delivery fiber is anglecleaved at 8° (to suppress spurious lasing as a result ofFresnel reflections) and inserted into a homemade high-power collimator. The diameter of the laser beam outputfrom the collimator is ∼5mm. The collimated beam arrayis sent to the free space and sampled by a beam splitter(BS1). The transmitted beam, which contains 99% of theincident power, is sent to a powermeter. The reflectedbeam that contains 1% of the incident power is usedfor beam profile diagnosis and active phase control. Alens is used to simulate the far-field pattern, and thena sampler (BS2) is located behind the lens. The reflectedbeam from the sampler is sent into an IR camera(SP620U, Ophir-Spiricon, Inc.) to diagnose the profileof the combined beam. The transmission beam is sent toa homemade pinhole, and a photodetector (PD) is lo-cated immediately behind the pinhole. The optical powerdetected by the PD is defined as metric function J andwill be used for active phase control using the stochasticparallel gradient descent (SPGD) algorithm. In our ex-periment, the SPGD algorithm is performed on a digitalsignal processor (DSP)-based SPGD controller. The up-date rate of the SPGD controller is more than 100,000iterations per second [16].In the experiment, we first investigate the power char-

acteristics and SBS suppression in one of the two ampli-fier chains. The experimental results are shown in Fig. 2.In Fig. 2(a), limited by SBS, the ultimate power of the

amplifier is 120 and 168W with a power conversion effi-ciency of 64.9% and 74.6%, when driven by seed 1 andseed 2, respectively. When the amplifier is seeded bythe spectrally combined MO with a power ratio about1:1.6, the highest output power of the main amplifier is275W with a power conversion efficiency of 78.9% atthe limited pump power of 348:4W. The power of themonitored backward signal is less than 50mW at thehighest power. The output power is limited by the avail-able pump power in this experiment, not by SBS, whichpredicts that by adding pump power, the output power ofthe amplifier can still be further increased. The spectrumof the main power amplifier is shown in Fig. 2(b). Whenthe output power is 275W, the amplified spontaneousemission is suppressed by ∼30 dB. The results show thatin two-tone driven amplifier, the output power are about2.29 times and 1.63 times that of the value when driven byseed 1 and seed 2, respectively. The output power of an-other amplifier reached 115W with a power conversionefficiency of 77.7% at the pump-limited power of 150W.

Coherent combining of the spectrally combined ampli-fier chains were then studied. When the whole systemwas in open loop at the absence of active phase control,the power encircled in the target pinhole (the units ofwhich were transformed into voltage by using the PD)fluctuated, and the intensity pattern at the observingplane kept shifting owing to phase fluctuations in eachfiber channel. The metric function fluctuated from 0 to0:37V, and the mean value is 0:165V, as presentedin the last 22:5 s in Fig. 3(a). The corresponding long-exposure interference pattern is an incoherent one[Fig. 3(b)]. When the SPGD algorithm was implementedand the whole system was in closed loop, the main lobeof the interference pattern was locked to the center de-spite of the phase fluctuations. The dependence of J ontime is larger than 0:3V for most of the time. The meanvalue is 0:34V, which is two times of the value when thesystem is in open loop, as presented in the last 22:5 s inFig. 3(a). The corresponding long-exposure interferencepattern is a coherent one, with a visibility as high as 75%,as shown in Fig. 3(c).

The fidelity of coherent combining and the phase fluc-tuations suppressing result can be further studied usingthe time standard deviation of the metric function. Thecalculated standard deviation before and after phase

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locking is 0.125 and 0.0286, respectively. This means thatthe noisy phase fluctuations are well suppressed whenactive phase locking is implemented. Results show thatrobust, stable, and in-phase coherent combining of twospectrally combined amplifier chains has been success-fully implemented despite the phase fluctuation. Whenthe total combined power is 390W, the visibility in-creases from 0% to 75% as the system evolves from opento closed loop.Scaling the configuration to higher power can be ef-

fected by three methods. The first is to increase theSBS-limited ultimate output power in the multitone-driven amplifier by increasing the number of single-frequency lasers in spectrum combining. In theory, thenumber of the spectrally combined lasers is limited bythe control accuracy of the optical path difference in co-herent combining. In two-channel coherent combining, ifthe optical path difference control accuracy can be atleast 10 μm, then more than 80 lasers with an equal powerlevel can be spectrally combined and used in coherentcombining. The second method is to increase the powerper amplifier chain by increasing the pump power in theamplifiers. The third way is to increase the number ofamplifier chains for coherent combining. In the activephasing technique using SPGD algorithm, the iterationnumber k is about k ¼ 5N − 10N for a system metricfunction that evolves from the initialization to the extre-mum [17]. The number of amplifier channels can be ex-tended more than N ¼ F=10f N , according to the iterationrate F of the SPGD controller and the phase fluctuationfrequency f N . For an SPGD controller with an iterationrate of 10,0000 and a kilowatt-level fiber amplifier witha phase fluctuation frequency of less than 1000Hz [2],the limited channel number is N ¼ 10. By increasingthe iteration rate of the SPGD controller and reducingthe phase fluctuation frequency, the number will befurther increased.In summary, we have presented what we believe to be

a new approach for simultaneous spectral and coherentcombining in a MOPA configuration with active phasing.Spectral combining using a wideband coupler gets rid ofspatial gratings, and the all-fiber based configurationwithout alignment of spatial optics makes the systemcompact and stable. Robust phase locking is realized,despite the strong phase fluctuation, by using SPGDalgorithm. Simultaneous spectral and coherent combin-

ing was implemented with a total power of 390W anda visibility of more than 75% for the long-exposure inter-ference pattern. Scaling the configuration to higherpower can be expected by increasing the number of spec-trally combined lasers, the power per fiber chain, and thenumber of laser channels.

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