M2 Factor of Linear Polarization Modes in a Step-index Fiber
阶跃光纤低阶线偏振模的M2因子分析

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.