Inversely Designed Compact 12-Channel Mode Decomposition Spectrometer for On-Chip Photonics

Abstract

Through inverse design, we present and demonstrate a compact and high-resolution spectrometer for on-chip photonics based on mode decomposition. By employing the effective index method, we significantly reduce the computational resources required for optimization. In 3-dimensional simulations, the decomposition efficiency reaches up to 0.95. The spectrometer operates through mode mixing at varying ratios along wavelengths in the mode mixing region. The spectrum is reconstructed via the inverse calculation of the intensity ratios of the decomposed modes within the structure. Experimental validation is performed by fabricating the designed structure on a silicon-on-insulator platform by using electron beam lithography. The reconstructed spectrum achieves a resolution of 0.1 nm with high accuracy, evidenced by a normalized cross-correlation of 0.99. The entire structure is compact, measuring 10 × 34 μm². This proposed design is advantageous due to its compactness, reduced computational cost, and straightforward fabrication process.