Stray-Light-Free Monolithically Integrated Silicon Photonic Spectrometer

Achieving stray-light-free on-chip spectral detection with high resolution across a broad wavelength range is of paramount importance for practical applications, as stray light in spectral measurements often introduces spurious signals that obscure genuine spectral detection. To address this challenge, we propose a novel architecture that combines narrow-band filtering using a tunable high-Q microring resonator with wide-passband filtering through cascaded multimode-waveguide grating (MWG) filters. The high-Q microring, realized with broadened waveguides, generates multiple high-fineness transmission peaks spaced by the free spectral range (FSR) across the entire wavelength range. These peaks are selectively isolated one by one using corresponding wide-passband MWG filters with precisely tailored central wavelengths and bandwidths, effectively overcoming the FSR limitation of the microring. A key point lies in the design of the MWG filters, which exhibit a single passband over an exceptionally large wavelength range of 1200–1700 nm, ensuring the complete suppression of out-of-band stray light. Furthermore, the monolithic integration of on-chip Ge/Si photodetector arrays minimizes the disturbance of environmental stray light. Experimentally, we demonstrate a fully packaged silicon photonic spectrometer system featuring a high spectral resolution of 0.15 nm across the wavelength range of 1263–1355 nm, with a minimum detectable optical power of 1 nW. The system successfully identifies the absorption peaks of hydrogen fluoride gas, demonstrating its capability for practical applications such as smartphone-based sensing modules, gas monitoring, and optical coherence tomography (OCT). This work represents a significant advancement in on-chip spectrometers, offering a stray-light-free, high-performance, and compact solution for practical applications.