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A mathematical model considering the transmission of a partially coherent Gaussian Schell-model (GSM) beam in slant turbulence atmosphere of heterodyne detection was established. A closed-form expression of the weighting factor for the partially GSM beam at the receiving end was derived. The effect of the beam mode on the performance of the proposed detection system was theoretically investigated. The results show that the proportion of the fundamental mode and heterodyne efficiency can be optimized by controlling the waist radius of the signal and local oscillator beams. The inner scale of turbulence significantly affects the heterodyne efficiency and normalized M 2 . With a larger mode order, the proportion of the fundamental mode and heterodyne efficiency are lowered.

Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degrading beam quality and restricting the enhancement of power output for diffraction-limited high power fiber laser. The experimental study on MI in all-fiberized master oscillator power amplifer (MOPA) is presented in detail for the first time. It is revealed that MI in all-fiberized MOPA results in reducing the slope efficiency. The theoretical threshold power of MI for 20/400 step-index large-mode-area double-cladding Yb-doped fiber is about 1 kW and MI is rooted mainly from the thermal effect.

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The effect of atmosphere turbulence on phase modulation signal in coherent beam combination with multi-dithering technique is studied. Analytical formulas for the phase modulation signal of beam array propagating in a turbulent atmosphere are derived based on extended Huygens-Fresnel integral. Then the effects of propagation distance, turbulence intensity, filling factor and size of beam array on phase modulation signal are numerically calculated. The results show that with transmission distance increasing, there occurs a maximum of the amplitude of phase modulation signals. With the intensity of atmosphere turbulence increasing, the maximum of amplitude of phase modulation signals will appear in a shorter distance. When fill factor does not change, the distance between the laser source and the maximum of amplitude of phase modulation signals will be increased with of the dimension of laser beam increasing. When the dimension of laser beam does not change, the distance between the laser source and the maximum of amplitude of phase modulation signals will be increased with fill factor decreasing. Finally, the effects of parallel beam array and conformal focused beam array on phase modulation signal are compared, finding that the distance between the laser source and the maximum of amplitude of phase modulation signals can be changed by changing the focusing distance of conformal focused beam array.