Applications of Astrophotonics

Historically, photonics' primary application has been in the telecommunications industry. Astrophotonics takes photonics such as as optical fibres and planar waveguides and uses them for astronomical purposes. Many of the technologies that have been developed by astrophotonics researchers are a direct result of a problem faced by the astronomical and astrophysics community. As a result, the fields are intimately intertwined.

The key two drivers behind photonics technologies in Astronomy are the increasing cost associated with increasing telescope size; and implementing photonic functions to new instrument concepts. To detect fainter or more distant targets at ever-higher spatial and spectral resolution, telescopes are built to be larger, requiring larger, more expensive optics and components. Astrophotonics can break the cost cycle by miniaturising instruments, enabling multiplexing on a whole new scale, and at the same time enabling technological advances and photonic functions not previously possible in astronomical instrumentation.

Integral Field Spectroscopy

We are now moving into an era where multi-object wide-field surveys, which traditionally use single fibres to observe many targets simultaneously, can exploit compact integral field units in place of single fibres. Current multi-object integral field instruments such as SAMI have driven the development of new imaging fibre bundles (hexabundles) for multi-object spectrographs.

Hexabundles essentially remove the need for microlens arrays in integral field spectroscopy, and can be integrated into existing systems using a conventional, low tension fibre positioner. They are ideal for wide-field ELT science. They will allow much more detailed investigation of the galaxy environments.

SAMI was built for the AAT as a demonstrator for 61-fibre-core hexabundles, and now is undertaking the [[http://sami-survey.org||SAMI Galaxy Survey[[ – the largest IFU survey in the world of nearby (z<0.12) galaxies. The SAMI Galaxy Survey targets key galaxy evolution questions, including how mass and angular momentum build up in galaxies, feedback mechanisms including quenching of star formation by outflows/winds or AGN activity, and the role of the environment in galaxy evolution through the morphology-density and SFR-density relations. The success of SAMI has lead to duplication of this idea by the USA in an instrument called MANGA. Furthermore, the next generation version of SAMI, called HECTOR, will be realised within the next ~5 years. It will incorporate new small replicable spectrographs (possibly photonic spectrographs) with new hexabundle designs and the latest in fibre positioning technology, to create an IFU with a multiplex of up to 100 hexabundles, leading to potentially a 100,000 galaxy survey – an order of magnitude larger than any IFU survey that will have been done.

OH Background Suppression

The hydroxyl radical is a molecule that exists in the Earths atmosphere. However, hydroxyl emission lines account for a vast majority (approximately 98%) of the near IR (NIR) background in the night sky over the wavelength range 0.9-2.0 mm: right across the J and H infrared bands where many optical and infrared telescopes operate. The hydroxyl is produced from a reaction between ozone and hydrogen at high altitudes (~87 kilometres). The emission results from the vibrational decay of the excited OH molecule.

Detection of emission lines (such as H-alpha for measurements of star formation rates) in high redshift galaxies is limited in the NIR by the OH emission - 1000 times brighter than in the optical bands. Solving the NIR sky background problem is a critical challenge for high redshift astronomy. The background cannot be subtracted as it is highly variable and the contaminating lines scatter in the spectrograph, adding contamination between the OH lines, and therefore need to be blocked before entering the spectrograph.

A novel astrophotonic solution came from combining fibre Bragg gratings and photonic lanterns into the GNOSIS instrument – an H-band feed for the NIR IRIS2 instrument on the AAT. GNOSIS has the advantage of photonically-filtering OH sky lines, but still maintaining the light-collecting advantage of multi-mode fibres. A successor to GNOSIS on the AAT, called PRAXIS, will extend this technology to include the J-band. This will leave a background continuum in the H and J-bands that is close to that seen in the optical, which allows the detection of emission lines from faint high redshift galaxies, opening up a host of galaxy evolution studies not previously possible from the ground.

In addition, within the next decade, observations will be obtained through the TAIPAN survey of galaxies out to z ~1, enabled by the TAIPAN instrument (populated with hundreds of Starbugs) on the UKST. These observations will yield new insights into galaxy formation and evolution, as well as cosmology (through constraints on H0).

Interferometry

Interferometry is the technique of using an array of telescopes in conjunction to observe and determine the location of an object. Better optical fibres and optical circuitry will enable a greater efficiency in interpolating data collected.

Space Applications

Some astrophotonics technology is being incorporated in micro-satellites.