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Two-step lateral taper spot-size converter for efficient fiber coupling to InP-based photonic integrated circuits

Spot-size converters (SSC) are an important building block of InP-based photonic integrated circuits since it allows a standard single-mode fiber with a large and symmetric mode spot to be efficiently coupled with large displacement tolerance to a semiconductor waveguide with a small and asymmetric mode spot. Having an on-chip SSC is practically advantageous since such an element greatly simplifies the packaging process while increasing its reliability. In this paper, a SSC utilizing two-step lateral tapering is proposed for converting the semiconductor waveguide device mode into that suitable for fiber coupling without compromising the designs of the device and coupling waveguides. This is achieved by inserting a transient taper between the device and coupling waveguide as an impedance matcher. This paper describes the design principles and characterization results for such a two-step SSC, compatible with earlier reported InP-based photonic integrated circuits for WDM. Transmission and photoresponsivity measurements (the last - by using an on-chip waveguide photodetector monolithically integrated with the SSC) show excellent performance of the two-step SSC. In good agreement with simulations, it was experimentally demonstrated that using this integrated component for fiber coupling can reduce the coupling loss to below 1dB and the alignment tolerances are improved by two and three times on the horizontal and vertical directions, respectively.