Coherent Beam Combination Research Articles

A high-speed, high-efficiency phase controller for coherent beam combining based on SPGD algorithm

A phase controller for coherent beam combining (CBC) of fibre lasers has been designed and manufactured based on a stochastic parallel gradient descent (SPGD) algorithm and a field programmable gate array (FPGA). The theoretical analysis shows that the iteration rate is higher than , and the average compensation bandwidth of CBC for 5 or 20 channels is or , respectively. The tests show that the phase controller ensures reliable phase locking of lasers: When the phases of five lasers are locked by the improved control strategy with a variable gain, the energy encircled in the target is increased by than that in the single output, the phase control accuracy is better than , and the combining efficiency is 92 %.

Orthogonal coding methods for increasing the number of multiplexed channels in coherent beam combining

We introduce orthogonally coded modulations for active coherent beam combining (CBC), and show how using Walsh codes enables the increase of the number of combined channels. Analytical, numerical, and experimental results are presented and compared to the standard multifrequency dithering technique, demonstrating that at least 5 times more fibers can be combined using the same modulation frequency. The modulation format excluded, no particular changes in the system need to be introduced and the benefits of active coherent beam combing, such as atmospheric turbulence compensation capability, are kept unchanged. Finally, we point out that this approach can be applied in partially CBC.

Coherent polarization beam combination of four mode-locked fiber MOPAs in picosecond regime

In this manuscript, we report on coherent polarization beam combination (CPBC) of a four-channel pulsed amplifier array in the picosecond regime by using single frequency dithering technique. By employing a photo-detector with low-pass bandwidth (8.5 MHz at 10 dB gain) to filter the intensity fluctuation and obtain phase errors for feedback, a combined laser pulse with~480 ps pulse width at~60 MHz repetition rate is obtained with an average power of 88 W. By adjusting the optical path differences (OPDs) and controlling the pump power to ensure the synchronizations and alleviate the influence of nonlinear phases among each channel, more than 90% combining efficiency is achieved with excellent beam quality (M(2)~1.1). Finally, the efficiency loss of the system along with the power scaling process is discussed.

Stimulated Brillouin scattering, phase noise, and dynamical evolution in a passive fiber laser coherent beam combination

A theoretical model is proposed to study a passive fiber laser coherent beam combination (CBC) with an all-optical ring-cavity feedback loop. The dynamic evolution of a frequency filter is carefully studied and is shown to be sensitive to stimulated Brillouin scattering (SBS) and phase noise. SBS plays an important role in the process of mode selection or CBC because the linewidth of the longitudinal mode is narrowed continuously in the ring-cavity feedback. Numerical studies show that the SBS process is significantly influenced by phase noise, optical path length difference, and four-wave mixing. Our results indicate that SBS would be a remarkable problem in this type of CBC unless the phase noise causes a rapid mode jump in the frequency domain. Since the mode jump worsens the efficiency of the coherent combination or beam quality, a trade-off consideration is necessary for the SBS, phase noise, and beam quality. The maximal SBS-allowed CBC efficiency is obtained numerically for output laser power of 700 W–3.5 kW.

An active phase locking of multiple fiber channels via square wave dithering algorithm

A novel active phase locking technique base on single detector is proposed and demonstrated experimentally. Using dual-level rectangular-wave phase modulation and time division multiplexing, the proposed multi-level phase dithering technique can achieve the same phase error signal as single/multi-frequency dithering technique, but without coherent demodulation circuit. Experimental investigation on coherent beam combination of 10 fiber laser beams is successfully demonstrated, with imposing an additional phase disturbances to mimic the phase noises in the high power fiber amplifiers. The combining efficiency with modulated phase noise is achieved above 97.55% and the accuracy of phase control is improved to λ/42rms. One of the application fields, i.e., beam steering based on coherent beam combining using square wave dithering algorithm are proposed.

Dammann-grating-based passive phase locking by an all-optical feedback loop

A Dammann grating is used as a spatial filter for a passive coherent beam combination (CBC) of three Yb-doped fiber amplifiers with an all-optical feedback loop. Using this diffractive-optics-based spatial filtering technique, we demonstrate CBC with 20 W output power, and the visibility of the far-field interference pattern is up to 88.7%. Measurements suggest that this approach is robust with respect to laboratory environment perturbations, and it can scale to high powers and large arrays.

倾斜相差对激光相干合成影响的仿真分析

倾斜相差对激光相干合成影响的仿真分析

相干合成中基于SPGD算法的平移误差和倾斜误差控制

相干合成中基于SPGD算法的平移误差和倾斜误差控制

Influence of filling factor on far-field intensity distribution in coherent beam combination

Theoretical analysis on the far-field diffraction of the coherent beam combination (CBC) is presented. An introduction is given to the definitions of filling factor and evaluation parameter of CBC. Based on the theoretical mode and definition, the influence of filling factor on far-field intensity distribution and the relationship between evaluation parameter and filling factor are theoretically described in detail. It is found that the filling factor influences far-field intensity distribution through space modulation factor. Results also indicate that SR is a constant which has not connection with the filling factor, and CR is of a linear relation with filling factor, and NACL has little connections with filling factor, and RCL varies approximately linearly with the square of filling factor, and PIB decreases when filling factor increases in general. If we make the filling factor larger than √2/2, more than half of the best values of RCL and PIB may be obtained.

两路120 W固体激光高光束质量相干合成

两路120 W固体激光高光束质量相干合成

基于液晶相控阵阵列的激光相干合成方法

基于液晶相控阵阵列的激光相干合成方法

相干合成子光束一致性检测研究

相干合成子光束一致性检测研究

Passive Coherent Polarization Beam Combination of a Four-Fiber Amplifier Array

We present what we believe to be a new architecture for coherent beam combination of a fiber amplifier array. Four polarization-maintained fiber amplifiers are passively coherent polarization beam combined by using an optical feedback in a ring cavity. Experimental results verify that the architecture is robust and effective. The combining efficiency of the whole system can be as high as 91%, and the phase noise can be suppressed effectively. We believe that the configuration presented in this paper is a promising approach for high-power laser combination without degrading the beam quality of the coherently combined beam.

Mutual injection phase locking coherent combination of solid-state lasers based on corner cube

Coherent beam combination is an effective way to develop high-power lasers with high beam quality and high brightness. Coherent combination of six solid-state lasers based on the technique of mutual injection phase locking by using the natural coherent combination property of corner cube is first investigated. The coherent combination with 15.3 J of output energy, 1.7 mrad of divergent angle is obtained, and the combining efficiency is as high as 95.6% at 10 Hz and 85 A. The far-field profile is flattened protuberance.

Coherent beam combination of fiber lasers with a strongly confined tapered self-imaging waveguide: theoretical modeling and simulation

Coherent beam combination (CBC) of fiber lasers based on self-imaging properties of a strongly confined tapered waveguide (SCTW) is studied systematically. Analytical formulas are derived for the positions, amplitudes, and phases of the N self-images at the output of a SCTW, which are important for quantitative analysis of waveguide-based CBC. The formulas are verified with numerical examples by a finite difference beam propagation method (FDBPM) and the errors of the analytical expressions are studied. This study shows that the analytical formulas agree well with the FDBPM simulation results when the taper angle is less than 1.4° and the phase distortion is less than λ/10. The relative errors increase as the taper angle increases. Based on the theoretical model and the FDBPM, we simulated the CBC of fiber laser array and compared the CBC based on the tapered waveguide with that based on the nontapered one. The effects of input beam number, aperture fill factor, and taper angle on the combination performance have been studied. The study reveals that a beam which has near-diffraction limited beam quality (M2≤1.41) and a single beam without side lobe in the far field can be achieved with tapered-waveguide-based CBC. It is shown that beam quality depends on input beam number, aperture fill factor, and taper angle. There exists a best fill factor which will increase as input beam number increases. The tolerance of the system on the fill factor and the taper angle is studied, which is 0.45<t<0.67 and θ<0.8°, respectively. The results may be useful for practical, high-power fiber laser systems.

Experimental demonstration of using divergence cost-function in SPGD algorithm for coherent beam combining with tip/tilt control

A novel approach of tip/tilt control by using divergence cost function in stochastic parallel gradient descent (SPGD) algorithm for coherent beam combining (CBC) is proposed and demonstrated experimentally in a seven-channel 2-W fiber amplifier array with both phase-locking and tip/tilt control, for the first time to our best knowledge. Compared with the conventional power-in-the-bucket (PIB) cost function for SPGD optimization, the tip/tilt control using divergence cost function ensures wider correction range, automatic switching control of program, and freedom of camera's intensity-saturation. Homemade piezoelectric-ring phase-modulator (PZT PM) and adaptive fiber-optics collimator (AFOC) are developed to correct piston- and tip/tilt-type aberrations, respectively. The PIB cost function is employed for phase-locking via maximization of SPGD optimization, while the divergence cost function is used for tip/tilt control via minimization. An average of 432-μrad of divergence metrics in open loop has decreased to 89-μrad when tip/tilt control implemented. In CBC, the power in the full width at half maximum (FWHM) of the main lobe increases by 32 times, and the phase residual error is less than λ/15.

Actively Coherent Beam Combining of Two Single-Frequency 1083 nm Nanosecond Fiber Amplifiers in Low-Repetition-Rate

We report on an experiment of active coherent beam combining (CBC) of two single-frequency nanosecond fiber amplifiers with repetition rate of several kilohertz using stochastic parallel gradient descent algorithm. Pulsed lasers with pulse width of ~500 ns and repetition rate of 5 kHz are phase locked with a combining efficiency of 89% by employing the signal leakage between the pulses for phase locking. This method can be used to CBC of pulsed-pumped laser with lower repetition rate.

0.4 J/10 ns/10 kHz‐4 kW coherent beam combined laser using stimulated Brillouin scattering phase conjugation mirrors for industrial applications

AbstractA laser system that can produce 0.4 J/10 ns/10 kHz‐4 kW using a coherent beam combination with stimulated Brillouin scattering phase conjugate mirrors (SBS‐PCMs) is proposed. The 4 kW coherent beam combination laser system consists of a front‐end system, a pre‐amplifier module, a coherent beam divider/combiner, and 4 main amplifier modules. The front‐end system produces 1064 nm @ 10 kHz laser pulses, and it is amplified by the pre‐amplifier. After the pre‐amplifier module, the laser beam is divided into 4 sub‐beams. Each sub‐beam is amplified by the main amplifier module in a double‐pass configuration and reflected by SBS‐PCMs. Sub‐beams are combined by a coherent beam combiner, and the output of the total system is designed to be 4 kW. With this 4 kW laser module, high‐speed 2D holographic laser processing will be utilized. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interferometric phase shift compensation technique for high-power, tiled-aperture coherent beam combination

We propose a simple coherent beam combining technique for applications in high-power multichannel laser amplifier systems with tiled aperture design. Using a photodiode pair coupled with piezo-actuator mirrors, we demonstrated robust beam combining bandwidth (~1 KHz) and root mean-square deviation (~λ/25) for two beam channels. We estimate that the performance of this technique can be further enhanced in terms of operational bandwidth and phase locking accuracy. It is not limited by single beam power or channel number restrictions, does not require optical phase retrieval algorithms, or calibrations, and can be integrated into various master oscillator power amplifier architectures.

Theory of phase locking of Gaussian beam amplifier array based on multidithering technique

The theory of phase locking of a Gaussian beam amplifier array based on the multidithering technique is presented and developed in detail. The role and importance of the pinhole in the coherent beam combination (CBC) system, which had been unsolved for a long time, have been studied in detail. The effects of the size and position of the pinhole on CBC are analyzed based on this theory.