Theory of noise in a kilo-Hz cascaded high-energy Yb-doped nanosecond pulsed fiber amplifier

A theoretical analysis of noise in a high-power cascaded fiber amplifier is presented. Unlike the noise theory in low power communication, the noise of a high power system is redefined as the leaked output energy between pulses with coherent beat noise uncounted. This definition is more appropriate for high power usage in which the pulse energy receives more attention than the pulse shape integrity. Then the low power pre-amplifying stages are considered as linear amplification and analyzed by linear theory. In the high-power amplification stages, the inversion is assumed to recover linearly in the time interval between pulses. The time shape of the output pulse is different from that of the input signal because of different gains at the front and back ends of the pulse. Then, a criterion is provided to distinguish the nonlinear and linear amplifications based on the signal-to-noise ratio （SNR） analysis. Then, an experiment that shows that the output SNR actually drops off in nonlinear amplification is performed. The change in the noise factor can be well evaluated by pulse shape distortion.     

高功率脉冲光隔离器非线性及热效应分析

从理论上分析了脉冲情况下,磁光晶体横向温度梯度引起的光弹效应、消光比以及克尔效应对光隔离器隔离度的影响,并与连续入射情况进行了对比。结果表明:在平均功率相同的情况下,脉冲光束入射比连续光束入射情况隔离度更低;当平均功率较小时,消光比对隔离度的影响较大;当平均功率较大时,消光比对隔离度的影响有限,主要表现出光弹效应对隔离度的影响;当峰值功率密度较大时,克尔效应将显著降低隔离度,并且峰值功率密度越大,对隔离度的降低程度越大。