Modal noise in an integrated photonic lantern fed diffraction-limited spectrograph

TL;DR

This paper presents a novel integrated photonic spectrograph for astrophotonics, revealing a new wavelength-dependent noise source caused by modal mismatch, which impacts spectral calibration and can be mitigated by direct injection.

Contribution

The study introduces an all-photonic device combining a photonic lantern and arrayed waveguide grating for astrophotonics, and identifies a previously unreported modal noise source affecting spectral accuracy.

Findings

Discovered wavelength-dependent loss mechanism causes spectral noise.

Modeling confirms the noise source and its impact on spectral output.

Mitigation strategy involves direct injection into the photonic lantern.

Abstract

In an attempt to develop a streamlined astrophotonic instrument, we demonstrate the realization of an all-photonic device capable of both multimode to single mode conversion and spectral dispersion on an 8-m class telescope with efficient coupling. The device was a monolithic photonic spectrograph which combined an integrated photonic lantern, and an efficient arrayed waveguide grating device. During on-sky testing, we discovered a previously unreported type of noise that made spectral extraction and calibration extremely difficult. The source of the noise was traced to a wavelength-dependent loss mechanism between the feed fiber's multimode near-field pattern, and the modal acceptance profile of the integrated photonic lantern. Extensive modeling of the photonic components replicates the wavelength-dependent loss, and demonstrates an identical effect on the final spectral output. We outline that this could be mitigated by directly injecting into the integrated photonic lantern.