Theoretical time-domain study of self-imaging properties in a multimode interference coupler

We report a theoretical and numerical study of self-imaging properties, including time domain and pulse spreading, caused by modal group-delay dispersion in generalized N x N multimode interference devices achieved by using a mode-propagation analysis and finite-difference time-domain method. It was found that the spatial self-imaging condition does not realize temporal self-imaging but lets waveforms separate whose shape depends on input position and input field distribution. Pulse spreading, which is sensitive to beam diameter, has a very large variation (420 fs) among input positions as well as rising to a very high 900 fs in response to a 21 fs and spatially Gaussian pulse for the conveniently smallest size with 10 channels.