Self-imaging properties of generalized N × N multimode interference couplers are derived. Positions, amplitudes, and phases of the self-images are directly related to the lengths and widths of the coupler by solving the eigenmode superposition equation analytically for any arbitrary length. Devices of length (M/N) 3L(c), where M is the multiple occurrence of the N self-images, are analyzed in detail. The general formalism is applied to practical N × N couplers used in integrated optics, and simple phase relations are obtained.

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We report on the concept and realization of new 2x2 and 1 x 3 multimode interference (MMI) couplers that offer the possibility of a free choice of the power splitting ratio. These MMI devices, using a new buaterfly geometrical design, are compact, polarization-inscnsitive, and tolerant to fabrication parameters. Realized in InGaAsPhP 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.

Overlapping-image multimode interference (MMI) couplers, a new class of devices, permit uniform and nonuniform power splitting. A theoretical description directly relates coupler geometry to image intensities, positions, and phases. Among many possibilities of nonuniform power splitting, examples of 1 × 2 couplers with ratios of 15:85 and 28:72 are given. An analysis of uniform power splitters includes the well-known 2 × N and 1 × N MMI couplers. Applications of MMI couplers include mode filters, mode splitters-combiners, and mode converters.

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