Report on high-performance silica-on-silicon waveguide spectral lens spectrometer

Spectroscopy is fundamental for astronomical observations. However, conventional bulk-optic spectrographs face critical challenges in size, weight, and environmental stability. To overcome these limitations, integrated photonic spectrometers offer a transformative path toward miniaturization and robustness. While integrated photonics spectrometers based on arrayed waveguide gratings or planar etched diffraction gratings have been extensively studied, they typically remain only as the dispersive element, and still require external free-space optics such as lenses and mirrors for beam relaying and shaping to match the detector. Though the chip is compact, these free-space optical elements dominate the system size and complicate its assembly. To address this problem, a novel design called waveguide spectral lens (WSL) is developed to simultaneously integrates the critical functions of spectral dispersion and spatial focusing entirely within the planar waveguide array, eliminating the need for traditional free-space optical elements after the chip. Lately, the design and fabrication of WSL have been transferred from a lab-based polymer waveguide platform to a commercial silica-on-silicon foundry. With different designs, WSLs have reached resolving powers ranging from 150 to 21,000 experimentally over a broad wavelength range from 700 nm to 1600 nm. The fabricated devices also exhibit extremely low polarization dependence, with the polarization effect barely detectable at 21,000 resolving power. Thanks to the low-loss characteristics of the silica waveguide, complemented by the emitter array design similar to a diffraction optimized blazed grating, the overall throughput exceeds 60%. The device design and performance test will be reported. This WSL technology provides a compact, stable, and high-performance spectroscopic core for future ground-based and space-borne instruments.