Astrophotonics: the application of photonic technology to astronomy

Integrated optics has the potential to play a transformative role in astronomical instrumentation. It has already made a significant impact in the field of optical interferometry, through the use of planar waveguide arrays for beam combination and phase-shifting. Additionally, the potential benefits of micro-spectrographs based on array waveguide gratings have also been demonstrated.

Here we examine a new application of integrated optics, using ring resonators as notch filters to remove the signal from atmospheric OH emission lines from astronomical spectra. We also briefly discuss their use as frequency combs for wavelength calibration and as drop filters for Doppler planet searches. We discuss the theoretical requirements for ring resonators for OH suppression. We find that small radius (<10 μm), high index contrast (Si or Si₃N₄) rings are necessary to provide an adequate free spectral range. The suppression depth, resolving power, and throughput for efficient OH suppression can be realised with critically coupled rings with high self-coupling coefficients.

We report on preliminary laboratory tests of our Si and Si₃N₄ rings and give details of their fabrication. We demonstrate high self-coupling coefficients (>0.9) and good control over the free spectral range and wavelength separation of multi-ring devices. Current devices have Q ≈ 4000 and ≈ 10 dB suppression, which should be improved through further optimisation of the coupling coefficients. The overall prospects for the use of ring resonators in astronomical instruments is promising, provided efficient fibre-chip coupling can be achieved.