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.
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.
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.
We have constructed an all-fiber single-frequency amplifier system and an additional all-fiber amplified spontaneous emission (ASE) source system to simulate the effect of generated backward ASE on stimulated Brillouin scattering (SBS) in master oscillator power amplification (MOPA) system. By injecting the artificial ASE source backward into the Yb-doped MOPA system directly and measuring the evolutions of SBS threshold power with the increasing ASE power level, we can get the changes of SBS process during the amplification. Effects of counterpropagating ASE on SBS threshold decreasing were obvious: When there was no counter-propagating ASE being injected, the backscattered power started to rise very slowly. However, with the added artificial ASE increasing, the backscattered power departed from linear quite early. And the SBS threshold power was reduced to different levels with different power level of additional ASE. As the artificial counter-propagating ASE power increased from 0 to 275.9mW, the SBS threshold power was correspondingly dropped down from 4.75W to 3.35W. The 30% reduction of the SBS threshold can be a siginificant influence in a single-frequency fiber amplifier.