阶跃光纤低阶线偏振模的M2因子分析

针对阶跃光纤输出低阶线偏振模（Linear Polarization,LP）,利用二阶距定束宽,多点拟合法计算了几个低阶LP mn模相干和非相干叠加时的M2因子.结果表明,对同一LP mn模式,光纤的归一化频率参量V减小到截止频率时,M2因子出现峰值；当V远离截止频率时M2因子趋于一定值.低阶LP mn模非相干叠加时,叠加后的较高阶模成分越大,合成光束的光束质量越差,光束在一定的条件下可能比基模更接近于理想高斯光束；而相干叠加时,合成后的M2因子可以大于参与叠加的较高阶模的光束质量因子.

M2 Factor of Linear Polarization Modes in a Step-index Fiber

Using the second order moment method to determine the beamwidth and multipoint fitting methor,the M2 factor in the case of incoherent and coherent superposition of several low order LPmn modes,which are outputed from the step-index fiber,have been calculated.The results indicate that,for an individual LPmn mode,the value of the M2 factor takes its maximum value when the normalized frequency parameter V of the fiber approaches to the cut-off frequency and it gradually goes to a constant value when V apparted from its cut-off frequency.In the case of incoherent superposition,under certain circumstance,the beam that comprises few lowest order LP modes may become even smaller and closer to the ideal Gaussian beam than that of the fundamental mode.However,in the case of the coherent superposition,the value of the M2 factor may be greater than that of the higher order constituent mode.The results reported here could be helpful for the application of the high power fiber systems.