Emitter design for efficient waveguide spectral lens

Abstract

Abstract As an alternative to arrayed waveguide gratings (AWGs), the waveguide spectral lens (WSL) stands out with the ability to focus light in free space, thereby eliminating the need for relay optics between the chip and the camera. This becomes convenient in constructing a truly compact instrument for astronomical spectroscopic analysis. Besides dispersion and focusing, WSL offers another important function: the envelope of the diffraction orders can be manipulated via the output emitter, i.e., the waveguide array at the facet. Through careful emitter design, the diffraction efficiency can be largely improved because the side orders are well suppressed, and light is concentrated to the selected order. This feature, though particularly important for the photon-hungry astronomical application, has not been well explored in the previous works. Here, we come up with four emitter designs and evaluate their performance, including linear taper, parabolic taper, multimode interference (MMI), and slot MMI. A figure of merit (FoM) considering both diffraction efficiency and uniformity is introduced to identify the optimal structure. Experimental results agree well with the simulation and confirmed that the optimal parabolic taper can achieve a diffraction efficiency of 90.9%, making it the most attractive design. This work highlights the potential of WSLs for astronomical spectroscopy with an efficiency that rivals conventional blazed gratings. It may also inspire emitter designs for side-lobe suppression in optical phase array applications.