Measurement of mode coupling distribution along few mode fiber based on two photonic lanterns

In the process of mode coupling measurement in FMF, the reflection from the front facet of the fiber generates a dead zone, which seriously affects the accuracy of the measurement. This paper analyzed mode coupling over FMF based on Rayleigh backscattering and built an experimental system using two photonic lanterns as the mode converter and demultiplexer respectively. And then we successfully measured the mode coupling distribution along a 4-km six-mode fiber. The values of the mode coupling coefficients obtained with the experimental system agreed well with those obtained with the conventional transmission method, and the deviations were less than 0.66 dB/km while the deviations were more than 1.31 dB/km using other experimental system which cannot eliminate the Fresnel reflection when a 300 ns optical pulse was coupled into the LP01 mode. The experimental system based on two photonic lanterns can eliminate the Fresnel reflection and the associated dead zone from the front facet of the fiber.