Based on the randomness of spontaneous emission, the statistical characteristics of phase noise are discussed. A theoretical analysis model, focusing on the amplitude randomness of spontaneous emission, is established to calculate laser phase noise. Then, the coherence of a laser before and after phase-locked control is calculated when an ideal laser and nonideal laser are used as a reference light in an optical phase-locked loop (OPLL). The effects of the amplitude randomness of spontaneous emission on phase-locked laser coherence are analyzed in detail. The results show that phase noise randomness increases with the increase of the expectation or variance of spontaneous emission amplitude, which represents amplitude randomness. When an ideal light is used as reference light, if the expectation and the variance of spontaneous emission amplitude are about 10 and 100, respectively, the time constant of an OPLL should be set as 1 ns, while a favorable noise suppression result can be achieved. However, to achieve a favorable noise suppression result, the time constant of an OPLL should be set as 0.1 ns when a nonideal laser is used as reference light.
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