An integrated miniature spectrograph based on waveguide spectral lens

Abstract

The scaling of next-generation ground- and space-based telescopes presents a critical challenge, as traditional diffraction-grating spectrographs scale in volume and mass roughly with the square of the telescope diameter, leading to prohibitive costs and stability issues. Photonic technology offers a promising solution to these barriers, yet traditional arrayed waveguide gratings (AWGs) suffer from discrete wavelength channels and internal focusing. Often auxiliary free-space optics is needed after the chip, resulting again a bulky system. Here, we report the development and initial on-sky validation of a compact spectrograph enabled by the novel waveguide spectral lens (WSL) technology. The WSL incorporates specific optical path differences to achieve simultaneous spectral dispersion and focusing, thereby eliminating the need for the output slab waveguide required in standard AWG and simplifying the free-space optical path. It also features an emitter array similar to a blazed grating, concentrating light on the selected diffraction order and improving the throughput. The instrument is housed in a compact chassis measuring approximately 43×16×13 cm3. It incorporates two distinct WSL spectrographs providing two different spectral resolutions, with resolving power R around 200 and 2000, respectively, across the 700–900 nm band. To achieve miniaturization and optical simplicity, a novel "cylindrical grism lens" is integrated. This component performs vertical focusing and cross-dispersion simultaneously, reducing the entire free-space optical train to a single element. The instrument has been deployed on the Lijiang 1.8-meter telescope with a high-performance adaptive optics (AO) system for an initial first-light experiment. We present the design methodology of the WSL, the optomechanical architecture of the dual-channel spectrograph, and the initial results of the astronomical observation campaign. By integrating dispersion and focusing functions into the chip and simplifying cross-dispersion, this technology reduces instrument size by orders of magnitude, paving the way for cost-effective, high-stability instrumentation for Extremely Large Telescopes with adaptive optics and future space-borne missions.