Scattering and Absorption Losses of Multimode Optical Fibers and Fiber Lasers

Abstract

We present an approximate theory of loss coefficients for modes of step-index fibers with various types of distortions and for fibers with lossy claddings. The fiber irregularities are assumed to be sinusoidal and random variations of the core-cladding interface. Formulas for the loss coefficients are presented and plotted for different values of the compound mode number M. For fiber lasers, we plot the loss coefficients as functions of the mirror tilt angles. We consider as an example a Nd-YAG fiber laser with refractive index n 1 = 1.8 and a core radius of a = 40 µm operating at a wavelength of λ = 1.06 µm. For this example, we find that radiation losses are caused by Fourier components of fiber irregularities in the spatial wavelength range between 0.4 and 1.3 µm. Intrinsic losses may be as low as 2α = 10−3 cm−1. It is thus desirable to limit scattering losses to values below 10−3 cm−1. This requirement imposes tolerance restrictions of 0.01 µm on the permissible core radius fluctuations. For core radius fluctuations of this order of magnitude, mirror tilts should not exceed approximately 5 degrees. Cladding losses are not critical, but their influence on laser losses depends on the refractive index ratio of the core and cladding materials. Tolerable cladding losses may range from 10 to 300 cm−1.