Coherent polarization beam combining （CPBC） is an effective method to obtain a high-brightness laser with linear polarization state. In this paper, we propose a method to apply optical heterodyne detection to CPBC. An optical phase modulation is implemented to transform the phase difference of two beams to amplitude modulation for polarization state detection. The CPBC of two lasers with the same frequency is realized by linear proportional-integral-derivative (PID) phase-locked control. Based on this principle, a three-channel CPBC system is realized to obtain a linearly polarized output beam with a high PER. The PER obtained in the experiment reached 17.44 dB with the output power of 431 mW, while the control bandwidth is 66.1 kHz. The residual phase noise of the first-stage CPBC system achieved to 1×10-4 rad/ √Hz at 1 Hz and 4×10-6 rad/ √Hz for more than 100 Hz.Compared with other CPBC phase-locking methods, this method can effectively suppress phase noise and has a significant improvement in control bandwidth and polarization extinction ratio.
In this study, the total deviation of central diffracted beams caused by the periodic projection effect and refraction of zero-order diffracted beam of diffractive optical element was theoretically investigated. The theoretical model of the relationship between the total deviation of central diffracted beams and the combining angle of the ith order incident beam was developed. Inversely, the total deviation of central diffracted beams can be used to calibrate the actual combining angle of the ith order incident beam, further guide the correction of the actual combining angle in coherent beam combining.
Aimed to maintain excellent beam quality, the influence of pointing deviation on the beam quality is theoretically studied in the dual-grating spectral beam combination (SBC). The incident light field of the fiber laser array with the pointing deviation is built by the transformation of coordinates, and the variation rule of the combined beam quality with random perturbations is discussed by the principle of beam diffraction and the statistical analysis. As a result, the degradation of beam quality for the pointing deviation is respectively 0.31(±0.13) and 3.06(±1.27) for the standard deviation of 0.1 mrad and 0.5 mrad, spreading as a Normal distribution. It can be concluded that the pointing deviation of laser array will destroy the condition of the SBC of the common aperture output, resulting in the continuous growth of the M2 factor. These analyses provide a valid basis for setting up the experimental system of dual-grating SBC.