Coherent beam combining of two all‑PM thulium‑doped fiber chirped pulse amplifiers

In this paper,we report a coherent beam combining(CBC)system that involves two thulium-doped all-polarization maintaining(PM)fiber chirped pulse amplifiers.Through phase-locking the two channels via a fiber stretcher by using the stochastic parallel gradient descent(SPGD)algorithm,a maximum average power of 265 W is obtained,with a CBC efficiency of 81%and a residual phase error of λ/17.After de-chirping by a pair of diffraction gratings,the duration of the combined laser pulse is compressed to 690 fs.Taking into account the compression efficiency of 90%and the main peak energy proportion of 91%,the corresponding peak power is calculated to be 4 MW.The laser noise characteristics before and after CBC are examined,and the results indicate that the CBC would degrade the low frequency relative intensity noise(RIN),of which the integration is 1.74%in[100 Hz,2 MHz]at the maximum combined output power.In addition,the effects of the nonlinear spectrum broadening during chirped pulse amplification on the CBC efficiency are also investigated,showing that a higher extent of pulse stretching is effective in alleviating the spectrum broadening and realizing a higher output power with decent combining efficiency.

High-power mode-programmable orbital angular momentum beam emitter with an internally sensed optical phased array

The high-power mode-programmable orbital angular momentum(OAM)beam has attracted significant attention in a wide range of applications,such as long-distance optical communication,nonlinear frequency conversion,and beam shaping.Coherent beam combining(CBC)of an optical phased array(OPA)can offer a promising solution for both generating the high-power OAM beam and rapidly switching the OAM modes.However,achieving real-time phase noise locking and formation of desired phase structures in a high-power CBC system faces significant challenges.Here,an internal phase-sensing technique was utilized to generate the high-power OAM beam,which effectively mitigated thermal effects and eliminated the need for large optical devices.An OPA with six elements was employed for experimental demonstration.The first effective generation of over 1.5 kW mode-programmable OAM beam in a continuous-wave domain was presented.Moreover,the results demonstrated that the generated OAM beam could be modulated with multiple dimensions.The topological charge can be switched in real time from-1 to-2.Notably,this OAM beam emitter could function as an OAM beam copier by easily transforming a single OAM beam into an OAM beam array.More importantly,a comprehensive analysis was conducted on power scaling,mode switching speed,and expansion of OAM modes.Additionally,the system’s compact design enabled it to function as a packageable OAM beam emitter.Owing to the advantages of having high power and programmable modes with multiple dimension modulation in phase structures and intensity distribution,this work can pave the way for producing high-power structured light beams and advancing their applications.

High power all-fiberized and narrow-bandwidth MOPA system by tandem pumping strategy for thermally induced mode instability suppression

An all-fiberized and narrow-bandwidth master oscillator power amplification(MOPA) system with record output power of 4 kW level and slope efficiency of 78% is demonstrated. Tandem pumping strategy is tentatively introduced into the narrow-bandwidth MOPA system for thermally induced mode instability(TMI) suppression. The stimulated Brillouin scattering(SBS) effect is balanced by simply using one-stage phase modulation technique. With different phase modulation signals, SBS limited output powers of 336 W, 1.2 kW and 3.94 kW are respectively achieved with spectral bandwidths accounting for 90% power of ～0.025, 0.17 and ～0.89 nm. Compared with our previous 976 nm pumping system, TMI threshold is overall boosted to be >5 times in which tandem pumping increases the TMI threshold of >3times. The beam quality(M～2 factor) of the output laser is well within 1.5 below the TMI threshold while it is ultimately saturated to be 1.86 with the influence of TMI at maximal output power. Except for SBS and TMI, stimulated Raman scattering(SRS) effect will be another challenge for further power scaling. In such a high power MOPA system, multidetrimental effects(SBS, SRS and TMI) will coexist and may be mutual-coupled, which could provide a well platform for further comprehensively investigating and optimizing the high power, narrow-bandwidth fiber amplifiers.

High-power broadband supercontinuum generation through a simple narrow-bandwidth FBGs-based fiber laser cavity

The generation of supercontinuum(SC) often requires ultrashort pulsed lasers with high peak power and gain media with large nonlinear coefficients,such as a long piece of fiber or photonic crystal fiber.In this Letter,we propose and demonstrate that high-power SC can be generated through a simple narrow-bandwidth fiber Bragg gratings(FBGs)-based laser cavity without any modulation,based on the mechanism of intense nonlinear effects induced by the inherent self-pulsation generated inside the cavity.In the experiment,an ~80 W SC laser with the spectrum range from <600 nm to 1600 nm was achieved.To the best of our knowledge,this is the first report about SC generation through a simple fiber laser cavity.This work enriches the research content of SC and provides a cost-effective method for high-power SC lasers.

Coherent combining of a fiber laser array via cascaded internal phase control technique

We experimentally demonstrated a cascaded internal phase control technique.A laser array with 12 channels was divided into three sub-arrays and a stage array,and phases of the sub-arrays and the stage array were locked by four phase controllers based on the stochastic parallel gradient descent(SPGD)algorithm,respectively.In this way,the phases of the whole array were locked,and the visibility of the interference pattern of the whole emitted laser array in the far field was∼93%.In addition,the technique has the advantage of element expanding and can be further used in the high-power coherent beam combination(CBC)system due to its compact spatial structure.

High-power single-frequency fiber amplifiers:progress and challenge[Invited]

Unlike conventional continuous-wave lasers with wide spectra,the amplification of single-frequency lasers in optical fibers is much more difficult owing to the ultra-high power spectral density induced nonlinear stimulated Brillouin scattering effect.Nevertheless,over the past two decades much effort has been devoted to improving the power scaling and performance of high-power single-frequency fiber amplifiers.These amplifiers are mostly driven by applications,such as high precision detection and metrology,and have benefited from the long coherence length,low noise,and excellent beam quality of this type of laser source.In this paper,we review the overall development of high-power single-frequency fiber amplifiers by focusing on its progress and challenges,specifically,the strategies for circumventing the stimulated Brillouin scattering and transverse mode instability effects that,at present,are the major limiting factors of the power scaling of the single-frequency fiber amplifiers.These factors are also thoroughly discussed in terms of free-space and all-fiber coupled architecture.In addition,we also examine the noise properties of single-frequency fiber amplifiers,along with corresponding noise reducing schemes.Finally,we briefly envision the future development of high-power single-frequency fiber amplifiers.

First experimental demonstration of coherent beam combining of more than 100 beams

Coherent beam combining of 107 beams has been demonstrated for the first time to the best of our knowledge.When the system was in closed loop,the pattern in far-field was stable and the fringe contrast was>96%.The impact of the dynamic tilt error,the piston error,and power inconsistency was theoretically analyzed.Meanwhile,the distribution law of dynamic tilt error was estimated and the correlation of the tilt dithering of different axis was analyzed statistically.The ratio of power in the central lobe was^22.5%.The phase residue error in the closed loop was~λ∕22,which was evaluated by the root-mean-square error of the signal generated from the photoelectric detector.

Deep-learning-based phase control method for tiled aperture coherent beam combining systems

We incorporate deep learning(DL)into tiled aperture coherent beam combining(CBC)systems for the first time,to the best of our knowledge.By using a well-trained convolutional neural network DL model,which has been constructed at a non-focal-plane to avoid the data collision problem,the relative phase of each beamlet could be accurately estimated,and then the phase error in the CBC system could be compensated directly by a servo phase control system.The feasibility and extensibility of the phase control method have been demonstrated by simulating the coherent combining of different hexagonal arrays.This DL-based phase control method offers a new way of eliminating dynamic phase noise in tiled aperture CBC systems,and it could provide a valuable reference on alleviating the long-standing problem that the phase control bandwidth decreases as the number of array elements increases.

Deep-learning-assisted, two-stage phase control method for high-power mode-programmable orbital angular momentum beam generation

High-power mode-programmable orbital angular momentum(OAM)beams have received substantial attention in recent years.They are widely used in optical communication,nonlinear frequency conversion,and laser processing.To overcome the power limitation of a single beam,coherent beam combining(CBC)of laser arrays is used.However,in specific CBC systems used to generate structured light with a complex wavefront,eliminating phase noise and realizing flexible phase modulation proved to be difficult challenges.In this paper,we propose and demonstrate a two-stage phase control method that can generate OAM beams with different topological charges from a CBC system.During the phase control process,the phase errors are preliminarily compensated by a deep-learning(DL)network,and further eliminated by an optimization algorithm.Moreover,by modulating the expected relative phase vector and cost function,all-electronic flexible programmable switching of the OAM mode is realized.Results indicate that the proposed method combines the characteristics of DL for undesired convergent phase avoidance and the advantages of the optimization algorithm for accuracy improvement,thereby ensuring the high mode purity of the generated OAM beams.This work could provide a valuable reference for future implementation of high-power,fast switchable structured light generation and manipulation.

High-peak-power temporally shaped nanosecond fiber laser immune to SPM-induced spectral broadening

High-peak-power transform-limited narrow-linewidth nanosecond all-fiber lasers are desired in a range of applications.However,their linewidths will be broadened by self-phase modulation(SPM).We propose a novel concept that generates transform-limited laser pulses by temporally shaping the pulse seed.The impact of the pulse shape on SPM-induced spectral broadening was studied numerically and experimentally.It was found theoretically that the square-shape pulsed laser is immune to SPM-induced spectral broadening.Based on this principle,we built a high-peak-power,linearly polarized,square-shape nanosecond all-fiber laser in a master oscillator power amplifier(MOPA)configuration.Stimulated Brillouin scattering(SBS)limited peak powers of 4.02 kW,5.06 kW,6.52 kW and 9.30 kW were obtained at pulse widths of 8 ns,7 ns,6 ns and 5 ns.Thanks to the square-shape pulsed seed,the linewidths at maximum peak power remained at 129.5 MHz,137.6 MHz,156.2 MHz and 200.1 MHz,respectively,close to the transform-limited values of110.8 MHz,126.6 MHz,147.7 MHz and 177.3 MHz.

Efficient phase-locking of 60 fiber lasers by stochastic parallel gradient descent algorithm

Coherent beam combining of 60 fiber lasers by using the stochastic parallel gradient descent algorithm has been demonstrated. The functions of pinhole(s) on the power distributions in the far-field have been systematically simulated on both in-phase and out-of-phase modes. Only one photoelectric detector was used to detect the combined power in the far-field central lobe of the in-phase mode state. When the phase controller was in a closed loop, the contrast of the far-field intensity pattern was as high as ~97% with residual phase error of ~λ/30, and ~34.7% of the total power was contained in the central lobe.

Monolithic high-average-power linearly polarized nanosecond pulsed fiber laser with near-diffraction-limited beam quality

An all-fiberized high-average-power narrow linewidth ns pulsed laser with linear polarization is demonstrated. The laser system utilizes a typical master oscillator power amplifier(MOPA) configuration. The stimulated Brillouin scattering(SBS) is effectively suppressed due to the short fiber length and large mode area in the main amplifier, combined with the narrow pulse duration smaller than the phonon lifetime of SBS effect. A maximal output power of 466 W is obtained with a narrow linewidth of ～203.6 MHz, and the corresponding slope efficiency is ～80.3%. The pulse duration is condensed to be ～4 ns after the amplification, corresponding to the peak power of 8.8 kW and the pulse energy of 46.6 μJ. Neardiffraction-limited beam quality with an M2 factor of 1.32 is obtained at the output power of 442 W and the mode instability(MI) is observed at the maximal output power. To the best of our knowledge, this is the highest average output power of the all-fiberized narrow linewidth ns pulsed fiber laser with linear polarization and high beam quality, which is a promising source for the nonlinear frequency conversion, laser lidar, and so on.

Kilowatt high average power narrow-linewidth nanosecond all-fiber laser

A high power narrow-linewidth nanosecond all-fiber laser based on the master oscillator power amplifier(MOPA)configuration is demonstrated. A pulsed seed with high repetition rate of 10 MHz was generated by modulating a continuous-wave(CW) single-frequency fiber laser at ～1064 nm by using an electro-optic intensity modulator(EOIM).After multi-stage cascaded power amplification, the average power was boosted to be kilowatt level. The pulses from the main amplifier had a pulse width of ～3 ns and an average/peak power of 913 W/28.6 kW. Further power scaling of the pulses was limited by stimulated Raman scattering(SRS) for the moment, method for SRS suppression and further power scaling was briefly discussed.