Effects of thermal treatment on spot size converters (SSC): analyzing germanium distribution and cross-sectional size changes

Abstract SSC had been extensively employed in the realm of optical coupling between optical fibers and planar silicon waveguide circuits. Investigating the optical coupling loss of SSC at various heat treatment temperatures (ranging from 25 to 100 ℃) has been essential for enhancing the design principles of SSC in extreme environments. In this study, the performance of SSC fabricated under four distinct heat treatment conditions (25, 50, 75, and 100 °C) was evaluated. The experimental results showed that the SSC exhibited an optical coupling loss of 1.8 dB at 50 °C, which gradually decreased with increasing temperature and reached −1.5 dB at 100 °C. The shape, band structure, and refractive index of the waveguide were analyzed in relation to temperature and doping concentration through SEM characterization and first-principles calculations. The results indicated that the abnormal loss resulting from heat treatment was primarily attributed to the uneven distribution of Ge within the material. Furthermore, excessive accumulation of Ge had led to lattice distortion in the waveguides, which in turn caused microscopic deformations.