Optimized Design and Tolerance Evaluation of an InP Spot-Size Converter for Enhanced Coupling Efficiency

Abstract InP-based spot size converters (SSCs) are essential components in integrated photonics, addressing the coupling loss issue arising from the mode field mismatch between the InP chip optical waveguide and the optical fiber. During the fabrication of InP-based photonic chips, variations in the structural size of the spot size converter, due to lithography and etching errors, can dimin-ish coupling efficiency. Researchers have consistently focused on developing spot size converters with high coupling efficiency and robust fabrication process tolerance. This paper presents an InP-based mode field converter that effectively transitions from a rectangular waveguide with a mode field size of 2 μm×1.7 μm to a diluted waveguide with a mode field size of 10.8 μm×7.8 μm through a lateral tapered structure. For coupling applications involving single-mode fiber and InP-based optical waveguides, the optical fiber's mode field size is a circular spot with a 9.5 μm diameter, while the InP-based optical waveguide's mode field size is an elliptical spot of 2 μm×1.7 μm. Utilizing the spot size converter increases the coupling efficiency from 7.6% to 78.8%. To address coupling loss due to lithography and etching errors in chip manufacturing, the coupling efficiency remains above 60% when the waveguide pattern linewidth and overlay align-ment accuracy vary by ±1 μm, and it stays above 66.4% when the tip width changes from 100 nm to 500 nm. The spot size con-verter enhances the chip manufacturing process tolerance. The lateral tapered multi-step spot size converter proposed in this paper offers high-efficiency optical coupling and a high-tolerance fabrication process, showing potential for applications in inte-grated optoelectronic chips.