Integrated photonics can be used for stable, cost-effective and precision instruments in astronomy. We present our development and testing of a silicon ring resonator as a tunable correlation filter, facilitating real-time gas contrast for specific molecules with low cross-sensitivity. Ring resonators for various gases in H-band, polarization-selective filters, and fiber-coupled prototypes are described. We present the first on-sky demonstration of silicon-on-insulator astrophotonics, and telluric CO2 absorption feature detection as a proof-of-concept using the 1.2m DAO telescope and REVOLT adaptive optics instrument. Comparisons with traditional spectrographs inform discussions on improving performance and extensions towards an observatory-class instrument for exoplanet biosignature detection.
Bulk optical astronomical instruments face significant cost, complexity, flexure and alignment challenges with increasing next generation telescope sizes. Astrophotonics can mitigate these issues by using compact optical fiber or chip-based instruments. Here we present the design and development of a single-mode fiber coupled optical telescope system (ARTEMIS) designed for the demonstration of novel integrated astrophotonic instrumentation. Using a 4 cm fiber collimator as a telescope, we show on-sky measurements from an integrated astrophotonic chip. We have demonstrated the ability to detect <0.002% absorption depth changes of telluric CO2 lines using a sub-centimeter scale astrophotonic correlation spectroscopy chip with the sun as a background light source. These results provide a route towards demonstrating astrophotonic instrumentation on the larger 35 cm ARTEMIS telescope for the atmospheric characterization of smaller, fainter targets such as planets.
High sensitivity spectroscopy of astronomical targets is used for determining stellar radial velocities, exoplanet detection, and even exoplanet atmosphere sensing. However, high resolution spectrographs are bulky, highly complex and expensive instruments. While this bulk optical approach is versatile, fiber optic photonic instruments can be lower cost, more compact, and simpler to parallelize for multiple targets. Here we present a low-cost fiber-based correlation spectroscopy technique which can be used for simultaneously measuring radial velocity and molecular/atomic composition of astronomical targets. The correlation is achieved using a commercial, piezoelectrically tunable fiber Fabry-Pérot (FFP) filter that can be tuned from 1520 to 1620 nm. The output of the filter is measured using a single channel photodetector and processed using a lock-in amplifier. By adjusting the bias and modulation amplitude of the transmission spectrum of the FFP filter, the device can be optimized for maximum sensitivity to a certain absorption/emission line. We perform an on-sky demonstration using a 4.25 cm telescope to detect telluric CO2 with the sun as a background light source.
Using photonic devices, we developed a new approach to traditional spectroscopy where the spectral cross-correlation with a template spectrum can be done entirely on-device. By creating photonic devices with a carefully designed, modulated transmission spectrum, the cross-correlation can be carried out optically without requiring any dispersion, vastly simplifying the instrument and reducing its cost. The measured correlation lag can be used for detecting atomic/molecular species within and determining the radial velocity of a particular astrophysical object. We present an overview of two design approaches that are currently being developed that use different photonic platforms: silicon and fibre-based photonics. The silicon photonic approach utilizes ring resonators that can be thermo-optically modulated to carry out the cross-correlation. The fibre approach uses customized fibre Bragg gratings (FBGs) with transmission spectra that can be strain-modulated. Both approaches have been able to detect molecular gas in a lab setting, and we are now in the process of on-sky testing. Lastly, we discuss the future for these types of devices as their simplicity opens up the possibility of developing low-cost, purpose-built multi-object or integral field spectroscopic instruments that could make significant contributions to scientific programs requiring stellar RV measurements and exoplanet detections.
Determining the radial velocity and atmospheric composition of exoplanets is typically performed using dispersive spectroscopy. However, while this approach is versatile, spectrometers for such applications are complex, expensive and are bulky instruments. In contrast, tunable fiber-based filters are commercially available and can be used for low cost, passive remote gas sensing. In this work, we experimentally demonstrate Fabry-Pérot based correlation spectroscopy in a simple, low-cost, compact, and stable instrument package for astrophotonic gas sensing. We also show via simulation that exoplanet radial velocities can be determined simultaneously.
Exoplanetary biosignatures, molecular compounds which indicate a likelihood of extraterrestrial life, can be detected by highly sensitive spectroscopy of starlight which passes through the atmospheres of exoplanets towards the Earth. Such sensitive measurements can only be accomplished with the next generation of telescopes, leading to a corresponding increase in cost and complexity spectrometers. Integrated astrophotonic instruments are well-suited to address these challenges through their low-cost fabrication and compact geometries. We propose and characterize an integrated photonic gas sensor which detects the correlation between the near-infrared quasi-periodic vibronic absorption line spectrum of a gas and a silicon waveguide ring resonator transmittance comb. This technique enables lock-in amplification detection for real-time detection of faint biosignatures for reduced observation timescales and rapid exoplanetary atmosphere surveys using highly compact instrumentation.
The Multi-site All-sky CAmeRA MASCARA is an instrument concept consisting of several stations across the globe,
with each station containing a battery of low-cost cameras to monitor the near-entire sky at each location. Once all
stations have been installed, MASCARA will be able to provide a nearly 24-hr coverage of the complete dark sky, down
to magnitude 8, at sub-minute cadence. Its purpose is to find the brightest transiting exoplanet systems, expected in the
V=4-8 magnitude range - currently not probed by space- or ground-based surveys. The bright/nearby transiting planet
systems, which MASCARA will discover, will be the key targets for detailed planet atmosphere observations. We
present studies on the initial design of a MASCARA station, including the camera housing, domes, and computer
equipment, and on the photometric stability of low-cost cameras showing that a precision of 0.3-1% per hour can be
readily achieved. We plan to roll out the first MASCARA station before the end of 2013. A 5-station MASCARA can
within two years discover up to a dozen of the brightest transiting planet systems in the sky.
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