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

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 ρ 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 ρ Strehl ≈ 0.33, ρ tip/tilt ≈ 0.84, and ρ 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.

Potential of commercial SiN MPW platforms for developing mid/high-resolution integrated photonic spectrographs for astronomy

“…However, this option requires using multiple diffraction-limited spectrographs for each SMF [8,9]. With the advent of "extreme AO" systems [10][11][12], the Strehl ratio has finally reached 90%+ in the near IR (NIR) in better than median seeing conditions on bright stars, which means efficient direct coupling to an SMF (without a photonic lantern) is finally a reality [13,14].…”

3D-M3: high-spatial-resolution spectroscopy with extreme AO and 3D-printed micro-lenslets

“…Various efforts have been undertaken to increase the coupling efficiency of SMFs, with great success in recent years (e.g., Refs. [17,[19][20][21][22][23][24][25][26][27]). This is in large part due to adaptive optics (AO) systems, which reduce the point spread function (PSF) size and wavefront deformation to acceptable levels.…”

On-sky results for the integrated microlens ring tip-tilt sensor

“…As these variations can have both large amplitude and high frequencies, the adaptive optics (AO) system may not be able to compensate for them sufficiently and, if they can occur outside the path to the wavefront sensor (WFS), they will not be sensed. These variations can effect the performance significantly [21] and turn out to be a limiting factor when coupling into SMFs, with coupling efficiency being degraded by as much as a factor of two [22]. Numerically calculated theoretical normalized coupling efficiency assuming an optimally coupled diffractionlimited PSF with additional residual tip-tilt variation, plotted in units of λ/D.…”

“…The SR in Tab. 1 was estimated by fitting a Gaussian to the centroid corrected PSF and taking the ratio between the normalized central intensities of this fit and the expected telescope PSF as described in Ref 22…”