Simultaneous mode-by-mode impulse response measurement of multi-mode optical systems based on linear optical sampling

Linear optical sampling is presented as a useful method to characterize mode-by-mode impulse responses or the spectral transfer matrix of multimode optical systems. In a two non-degenerate linear-polarization (LP) mode transmission line, which actually supported six independent spatial modes, 6 × 6 spectral transfer functions were simultaneously measured using multiplexed mode-by-mode impulse responses in series. According to the singular value analysis of the matrix, the mode-dependent loss spectrum was analyzed over a frequency bandwidth of several hundred GHz, which was determined by the spectra of the probe and reference pulses. The technique would be useful for characterizing various multi-spatial-mode fiber systems that are intensively being developed to overcome the limit in capacity of single-mode optical fibers.