Abstract
We report a simple analytical calculation method for evaluating relative phase relations of tapered 2×4 multimode interference (MMI) couplers in a novel optical 90° hybrid. By using the proposed calculation model, the tapered 2×4 MMI coupler can easily be optimized to achieve phase matching with a 2×2 MMI coupler in the novel optical 90° hybrid. It was theoretically clarified that for several kinds of tapered MMI shapes, the proposed calculation model provides accurate solutions as long as the contribution of mode interference is much greater than that of mode conversion in the tapered MMI region. Based on the calculation model, we fabricated the optical 90° hybrid consisting of the linearly tapered 2×4 MMI coupler and the 2×2 MMI coupler. The extinction ratios of the measured transmission spectra via a delayed interferometer were comparable for in-phase and quadrature channels. This indicates that the phase matching was achieved by the tapered 2×4 MMI coupler and verifies the validity of the proposed calculation model.
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