Optical bandwidth and fabrication tolerances of multimode interference couplers

Analytical expressions are derived which relate the optical bandwidth and the fabrication tolerances of multimode interference (MMI) couplers to their most important design parameters. These expressions compare adequately with mode analysis simulations. For strong guided structures, the optical bandwidth is shown to he inversely proportional to the number of input and output ports N and to the length of the device. The fabrication tolerances are independent of N and proportional to the output channel separation D. Measurements on MMI couplers in InP/InGaAsP corroborate the theoretical prediction