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Hector: a new multi-object integral field spectrograph instrument for the Anglo-Australian Telescope
To make the maximal use of astrophotonic integration such as coupling the AWGs with multiple single-mode fibers coming from photonic lanterns or fiber Bragg gratings (FBGs), we require a multi-input AWG design. In a multi-input AWG, the output spectrum due to each individual input channel overlaps to produce a combined spectrum from all inputs. This on-chip combination of light effectively improves the signal-to-noise ratio as compared to spreading the photons to several AWGs with single inputs. In this paper, we present the design and simulation results of an AWG in the H band with three input waveguides (channels). The resolving power of individual input channels is ~1500, while the overall resolving power with three inputs together is ~500, 600, 750 in three different configurations simulated here. The device footprint is only 16 mm x 7 mm. The free spectral range of the device is ~9.5 nm around a central wavelength of 1600 nm. For the standard multi-input AWG, the relative shift between the output spectra due to adjacent input channels is about 1.6 nm, which roughly equals one spectral channel spacing. In this paper, we discuss ways to increase the resolving power and the number of inputs without compromising the free spectral range or throughput.
We investigate the origin of these core-to-core variations using finite difference time domain and finite element simulations, combined with analysis of fabricated multicore fibre. We find that the ellipticity of the core, the size of the core, and the coupling between cores all affect the propagation constants. However, the dependence on ellipticity is very weak, and cores would have to be highly deformed in the manufacturing process for this to be a concern. A variation in radius of ~ 2:5% could account for the observed variation in propagation constants. However, the measured variation in the fabricated MCF is too small and does not display any radial trend. The coupling between cores is too small to change the propagation constants significantly, but even if it were significant any effect would be expected increase the Bragg wavelengths of the central cores, the opposite of what is observed.
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.
Optical-mechanical operation of the F2T2 filter: a tunable filter designed to search for First Light
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