Characterization of Single-Mode Fiber Coupling at the Large Binocular Telescope

TL;DR

This study measures and analyzes the efficiency of single-mode fiber coupling at the Large Binocular Telescope, identifying limiting factors and their effects on radial velocity precision for astrophotonic applications.

Contribution

It provides the first on-sky measurements of SMF coupling efficiency at LBT and quantifies the impact of various optical effects on injection performance.

Findings

Maximum fiber coupling efficiency limited by telescope pupil geometry (0.78).

AO correction, tip-tilt residuals, and static aberrations significantly affect coupling.

Average SMF efficiency achieved was 0.18, impacting RV precision.

Abstract

Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF injection system at the Large Binocular Telescope (LBT) in April 2016. The fiber injection unit was built as part of the de-risking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of rho_tel=0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (non-common-path) aberrations contribute coupling coefficients of rho_Strehl=0.33, rho_tip/tilt=0.84, and rho_ncpa=0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity (RV) precision as a function of stellar apparent magnitude.