# Photonic ring resonator filters for astronomical OH suppression

## TL;DR

This paper explores the design, fabrication, and testing of photonic ring resonators as notch filters to suppress atmospheric OH emission lines in astronomical spectra, aiming to improve observational accuracy.

## Contribution

It provides a theoretical framework, simulation results, and initial fabrication progress for small-radius Si and Si3N4 ring resonators tailored for OH suppression in astronomy.

## Key findings

- Small rings (<10 microns) are necessary for adequate free spectral range.
- High self-coupling coefficients (>0.9) achieved in preliminary devices.
- Current devices have Q ≈ 4000 with ~10 dB suppression.

## Abstract

Ring resonators provide a means of filtering specific wavelengths from a waveguide, and optionally dropping the filtered wavelengths into a second waveguide. Both of these features are potentially useful for astronomical instruments. In this paper we focus on their use as notch filters to remove the signal from atmospheric OH emission lines from astronomical spectra, however we also briefly discuss their use as frequency combs for wavelength calibration and as drop filters for Doppler planet searches. We derive the design requirements for ring resonators for OH suppression from theory and finite difference time domain simulations. We find that rings with small radii (<10 microns) are required to provide an adequate free spectral range, leading to high index contrast materials such as Si and Si$_{3}$N$_{4}$. Critically coupled rings with high self-coupling coefficients should provide the necessary Q factors, suppression depth, and throughput for efficient OH suppression. We report on our progress in fabricating both Si and Si$_{3}$N$_{4}$ rings for OH suppression, and give results from preliminary laboratory tests. Our early devices show good control over the free spectral range and wavelength separation of multi-ring devices. The self-coupling coefficients are high (>0.9), but further optimisation is required to achieve higher Q and deeper notches, with current devices having $Q \approx 4000$ and $\approx 10$ dB suppression. The overall prospects for the use of ring resonators in astronomical instruments is promising, provided efficient fibre-chip coupling can be achieved.