New 2/spl times/2 and 1/spl times/3 multimode interference couplers with free selection of power splitting ratios

We report on the concept and realization of new 2/spl times/2 and 1/spl times/3 multimode interference (MMI) couplers that offer the possibility of a free choice of the power splitting ratio. These MMI devices, using a new butterfly geometrical design, are compact, polarization-insensitive, and tolerant to fabrication parameters. Realized in InGaAsP/InP with large fabrication tolerances, they permit the control of the output powers within a few percentage points for both polarizations. The measurement results fit well the theoretical predictions. Extensions of the design are presented that assure a fully symmetrical geometry and therefore an optimal homogeneity of the device characteristics. Based on the same idea, a new compact mode converter-combiner is proposed.