High-resolution integrated photonic micro-spectrographs for radial velocity exoplanet astronomy

Summary form only given. The Integrated Photonic Spectrograph (IPS) is a complete spectrograph within a single silica photonic chip that has no moving parts and is far smaller than existing bulk-optic spectrographs [1,2]. The typical IPS format employs a lithographically created Arrayed Waveguide Grating (AWG) structure in a silica-on-silicon chip, which inputs light from a standard optical fibre and outputs a spectrum that is dispersed over several millimetres, rather than the hundreds of centimetres with conventional spectrographs [3].One of the key benefits of this integrated photonic approach is a high resistance to stress and temperature induced flexure and misalignment [1]. This makes them ideal candidates for use in certain areas of astronomy, in particular for radial velocity measurements in exoplanet science, as well as stellar seismology and binary system interactions. One technique astronomers use to constrain parameters about exoplanets such as the planet´s mass and its period, involves measuring the amplitude of periodic wavelength shifts in the absorption lines of a star. These radial velocity shifts are due to a movement in the star brought on by the gravitational pull of an unresolvable planetary companion. For detecting small Earth-like planets, this Doppler effect is maximised in a particular class of stars called M-Dwarfs, who´s spectral features are found predominantly in NIR wavelengths. The feasibility of photonic spectrographs for astronomical use has already been demonstrated with a successful on-sky demonstration at the Anglo-Australian Telescope of a working AWG-based IPS prototype, with successful observations of stellar absorption features in the NIR [4]. However, because AWG designs were initially developed for telecommunication, they have therefore been optimized for such applications. For example, the central operating wavelength and free spectral range are often tailored to match C-band NIR optical communication wavelength range (1- 30 - 1565 nm), commonly used in fibre-optic networks. Other telecommunication requirements, for instance the minimization of cross-talk between neighbouring channels, are very different for astronomical applications and hence some parameters of the device´s traditional design are over-engineered [5]. This gives rise to the particular set of AWG parameters that are not well suited to highprecision radial velocity measurements required for Earth-like exoplanet detection. To address this limitation, we present a comprehensive redesign of AWG architecture to improve specific performance parameters of interest to high-resolution exoplanet astronomy. These include a vastly increased resolving power (R = λ/Δλ= 60,000, an order of magnitude improvement compared to previous prototypes), changes to the free spectral range to obtain reasonable wavelength coverage when re-imaging onto a low noise S-CMOS detector, and a broadening of the device´s operational wavelength. We provide detailed simulations using beam propagation algorithms of both expected throughputs and mode profiles, and present the overall layout design for the AWG chips. We discuss the challenges involved in fabrication of the new designs, and their benefits and capabilities in an astronomical context. This unique approach to AWG design enables the construction of integrated photonic spectrographs far more powerful and better tailored to astronomy then achieved previously.