Signal bandwidth of general N x N multimode interference couplers

Analytical expression of signal bandwidth of general straight and tapered N x N multimode interference (MMI) couplers is presented. The signal bandwid th is characterized as a function of mode relative energy, mode propagation delay time, and mode pulse broadening in the multimode section of MMI coup ler. The model is used to evaluate the signal bandwidth of specific coupler s. Results indicate that the signal bandwidth decreases seriously with the increase of channel number and channel guide space. Compared with the strai ght MMI coupler, the tapered MMI coupler has an improved signal bandwidth.