Adaptive optics in waveguide: conversion from arbitrary multimode to single-mode

Abstract

Astrophotonics is an emerging field that exploits photonic technologies to enable the miniaturization and integration of astronomical instrumentation. However, the fiber-coupling issue is arguably the most heated debate to determine whether photonics devices could become a practical solution for the development of next-generation astronomical instruments. In many observations, only multimode fibers (MMFs) are adopted to transfer the light from the focal plane to the spectroscopy instruments. On the contrary, the reported integrated photonics spectroscopy (IPS) so far only works under single-mode regime. To address this “size-mismatch issue”, we proposed an adaptive optical device named the mode detangler (MD), which can transform arbitrary multimode light distribution into a single-mode Gaussian-like spot. As demonstration, we have fabricated the device with a waveguide core of 65-μm to adapt with a standard 50-μm MMF and embedded three layers of microheaters (32 in total) to shape the beam. A feedback optimization algorithm is employed to rapidly identify the optimal electrode settings for a given input multimode condition. Experimental results show that the mode detangler yields a transmission improvement exceeding 15 dB compared to the direct multimode-to-single- mode fiber connector coupling. Furthermore, the device exhibits a high throughput across a broad spectral range, spanning from the visible band (635 nm) to the near-infrared band (1270 nm and 1550 nm). We believe this active, compact waveguide device, analogous to adaptive optics (AO) in free-space optics, will provide a solution to the “size mismatch” problem in astro-photonics and facilitate the integration of photonics technologies into the next-generation astronomical instruments.