Objective Coherent beam combining (CBC) can improve laser brightness while maintaining beam quality, which is one of the most promising techniques to break through the power limitation of a single fiber laser. CBC requires high-purity linearly-polarized lasers, which are usually obtained from all-polarization-maintained fiber lasers. Unfortunately, nonlinear effect thresholds of all-polarization-maintained fiber lasers are lower than that of non-polarization-maintained ones, and the system cost is higher. Non-polarization-maintained fiber laser system with active polarization control technology is one of the alternatives for achieving linearly-polarized laser sources. However, there is little research on the theory of active polarization-phase control, and no comprehensive analysis of different control schemes has been reported. In this paper, active polarization-phase control of multi-channel CBC system with different control schemes is modeled. The convergence and feasibility of the control schemes are verified, and the channel scalabilities as well as the effect of extra residual phase and dynamic noise on combining efficiency have been studied.

Methods Based on the structure of filled-aperture CBC, active polarization-phase control model is constructed, where phase control adopts multi-dithering method while the polarization control adopts SPGD algorithm to ensure fair comparison. According to the sampling scheme of laser beams, the polarization-phase control schemes is divided into individual sampling and simultaneous sampling categories. The latter ones are further divided into SPGD-based schemes and multi-dithering-based schemes. CBC performance based on the above three kinds of control schemes has been simulated with their convergence and feasibility being verified. The channel scalabilities of different schemes are compared, and the effect of the extra residual phase on combining efficiency of simultaneous sampling schemes is analyzed. Moreover, dynamic noise is modeled with emphasis on either low-frequency or high-frequency contributions, and the impact of both dynamic polarization and phase noise is simulated and analyzed.

Results and Discussions The control scheme of individual sampling had good channel scalability, and could converge in an average of 9 steps as shown in Fig.4. The control scheme of simultaneous sampling of beams based on SPGD algorithm was relatively insensitive to extra residual phase, where efficiency loss caused by extra residual phase of 0.1 rad was about 7% as illustrated in Fig.9(b), but the number of convergent steps was about 4.5 times of the number of channels as listed in Tab.2. The control scheme of simultaneous sampling of beams based on multi-dithering method had the fastest convergence speed (less than 7 steps for different channel numbers), but its combining performance was greatly affected by residual phase as shown in Fig.9(a). Fortunately, efficiency loss caused by extra residual phase could be reduced by increasing the amplitude of reference laser, and combining efficiency loss of < 5% could be achieved when the amplitude of reference laser was increased to be greater than 10 times of the sampled combined beam as suggested by Fig.9(c). The behaviors In dynamic perturbations were exhibited in Fig.12, indicating that the scheme of individual sampling of beams performed best, while the control scheme of simultaneous sampling of beams based on multi-dithering method showed advantage in the case of slow-varying polarization noise, but it performed worse than others when high frequency perturbations were inevitable.

Conclusions The convergence speed, channel scalability and sensitivity to residual phase of different active polarization-phase control schemes are investigated numerically. When the number of channels is small and the system complexity is low, the control scheme of individual sampling of beams can be adopted. When the requirement of polarization bandwidth is low, the control scheme of simultaneous sampling of beams based on SPGD algorithm is more suitable, which has the simplest structure. For large-scale CBC systems, the control scheme of simultaneous sampling of beams based on multi-dithering method has great potential, but the accuracy of polarization error signal should be improved to be robust with dynamic perturbations, which deserves further research.