Micro-lens array as tip-tilt sensor for single-mode fiber coupling

TL;DR

This paper presents a micro-lens array tip-tilt sensor integrated with a fiber bundle for precise vibration detection in large telescopes, improving coupling efficiency and enabling effective beam position sensing for adaptive optics correction.

Contribution

It introduces an improved micro-lens array design for tip-tilt sensing with enhanced throughput and linear response, along with a new testbed for vibration simulation and fiber coupling testing.

Findings

Achieves 69% coupling efficiency into the single-mode fiber under perfect alignment.

Demonstrates linear sensor response to tip-tilt movements.

Extends a testbed to simulate telescope vibrations and test fiber coupling.

Abstract

We introduce a design for a tip-tilt sensor with integrated single-mode fiber coupling for use with the front-end prototype of the iLocater spectrograph at the Large Binocular Telescope to detect vibrations that occur within the optical train. This sensor is made up of a micro-lens array printed on top of a fiber bundle consisting of a central single-mode fiber and six surrounding multi-mode fibers. The design in based on a previous prototype that utilized a multi-core fiber with seven single-mode fibers. With this updated design, we are able to achieve a better sensing throughput. We report on the modeled performance: if the beam is perfectly aligned, 69% light is coupled into the central single-mode fiber feeding the scientific instrument. When the beam is not aligned, some of the light will be coupled into the outer sensing fibers, providing the position of the beam for tip-tilt correction. For this design we show that there is a linear response in the sensing fibers when the beam is subject to tip-tilt movement. Furthermore we introduce an adaptive optics testbed, which we call the Koenigstuhl Observatory Opto-mechatronics Laboratory (KOOL), this testbed currently simulates vibrations at the Large Binocular Telescope, and in collaboration we have extended it to allow single-mode fiber coupling tests.