Determination of the mode composition of optical radiation from the reflection spectrum of multimode fiber Bragg grating

A novel approach for the reconstruction of the mode composition of optical radiation in multimode fiber is presented. It is based on the analysis of the reflection spectrum from the multimode fiber Bragg grating. The mode composition of radiation in the multimode fiber with 40 µm core diameter, sustaining 17 transverse LP modes, was experimentally studied. The experimentally obtained spectral reflection peaks from the multimode fiber Bragg grating coincide very well with the theoretically calculated ones. At the same time, additional spectral peaks corresponding to the cross-reflection of some modes into others were discovered. The introduced method also made it possible to determine the parameters of the studied multimode fiber Bragg grating. Furthermore, the presented method allows the determination of the bending losses for different transverse modes of radiation. The obtained results can help to characterize the operation of few-mode or multimode laser systems.