Influence Factor of Coherent Beam Combining Based on Single-Detector Electronic-Frequency Tagging

Objective Coherent Doppler wind radars are widely used in aviation safety, climate modeling, and wind-farm-project optimization. As the demand for the radar detection range increases, the requirement for higher single-pulse energy in single-frequency lasers increases accordingly. However, in single-frequency fiber-pulsed amplifiers, energy improvement is significantly hindered by thermal effects and nonlinear effects in the fiber, such as stimulated Brillouin scattering. Using coherent beam-combining (CBC) technology, several fiber lasers can be combined to increase the output energy exponentially while maintaining the line width, beam quality, and polarization degree, as well as overcome the limited output energy of single-frequency fiber amplifiers. In CBC systems, achieving multibeam phase locking requires high-speed and precise phase control. In an active CBC system, the output-light phase of the combined beam is detected and a closed-loop feedback forms to correct the phase error, thus achieving phase locking for each sub- Single frequency phase technology orthogonal signal. The error signal, which is proportional to the phase difference between the measured beam and other beams, provides excellent phase-error correction for multibeam and high-power coherent combination systems. For the locking of optical coherence by single-detector electronic-frequency tagging (LOCSET) CBC system, parameter optimization is crucial for enhancing active phase control. Methods In this study, the principle of a single-detector electronic-frequency algorithm for achieving CBC was analyzed; subsequently, two optical-fiber CBC systems were constructed. The selection criteria for the single-detector electronic-frequency algorithm parameters were investigated experimentally. In the experiments, five parameters-integration time, modulating signal amplitude, modulating signal frequency, feedback coefficient, and control loop delay-were varied. The criteria for selecting the parameters of the single-detector electronic-frequency algorithm were summarized. Results and Discussions The longer the integration time r, the smaller is the error caused by the non-integral modulation signal period. To minimize the effect of non-integral integration time on the synthesis effect, the integration time should be more than 10 times the modulation signal period T (Fig. 3). Increasing the modulating signal amplitude 9 can reduce the error caused by r being a non-integer multiple of T. However, higher 9 values introduce phase noise, which worsens the phase-locking effect. To satisfy the coherent synthesis output target, 9 should be set within a specific range in the LOCSET system (Fig. 4). The phase error triangle 5 between the demodulated and marked signals must be less than 90 degrees owing to the delay in the control loop. As the modulation frequency increases, greater precision is required for loop-delay error compensation (Fig. 6). The control bandwidth increases with the modulation frequency. Under the system inherent delay of 5 ms, a 100 kHz modulation frequency minimally affects the iteration time and slightly changes the control bandwidth. In practical applications, optimizing the modulation frequency while considering the phase noise, inherent delay, and control circuit costs can improve both the control bandwidth and iteration rate of the coherent synthesis system (Table 1). As the feedback coefficient increases, the effective control bandwidth increases. An appropriate feedback coefficient should be selected that balances between the control bandwidth and phase control accuracy. For the system, the optimal value is A/20 (Figs. 5 and 9). Conclusions In this study, two all-fiber CBC systems were constructed based on the LOCSET algorithm. Additionally, the effects of integration time, modulating signal amplitude, modulating signal frequency, feedback coefficient, and control loop delay on active phase control were investigated experimentally. The experimental results show that to mitigate the effects of the non-integral integration time on the combining effect, the integration time should be approximately 10 times the modulation signal period. The modulation signal amplitude should be within a specific range to achieve the target combining efficiency; as the integration time increases, the lower limit of the required amplitude decreases. Higher modulation signal frequencies require greater precision in loop- delay error compensation and a broader system control bandwidth. The feedback coefficient should be selected based on the array size and phase noise level to balance between the control bandwidth and phase-control accuracy. Additionally, the delay between the quadrature demodulation signal and modulated signal should be less than 90 degrees to prevent lock loss in the control system. This study serves as a basis for parameter selection in the LOCSET algorithm and provides a direction for optimizing CBC technology to enhance the energy of coherent-laser wind radar light sources.