Abstract
Research activities during the last decade have shown the strong potential of photonic devices to greatly simplify ground based and space borne astronomical instruments and to improve their performance. We focus specifically on the mid-infrared wavelength regime (about 5-20 microm), a spectral range offering access to warm objects (about 300 K) and to spectral features that can be interpreted as signatures for biological activity (e.g. water, ozone, carbon dioxide). We review the relevant research activities aiming at the development of single-mode guided optics and the corresponding manufacturing technologies. We evaluate the experimentally achieved performance and compare it with the performance requirements for applications in various fields of astronomy. Our goal is to show a perspective for future astronomical instruments based on mid-infrared photonic devices.
Highlights
Photonics components can be placed in direct contact with a cryostat window or a detector [10]; They allow flux transportation in a much easier way than long optical trains; They permit faster and simpler instrument upgrade when dealing with optical chips; single-mode fibers and integrated optics allow very accurate beam cleaning, thanks to spatial and modal filtering, which is an essential aspect of aperture synthesis and nulling interferometers [12, 13]; When the waveguide is placed in a chilled enclosure, its limited numerical aperture acts as a field stop, avoiding straylight entering the system from outside the acceptance cone
Alternatives to planar dielectric integrated optics exist to produce functions: a possibility is to emulate the near-infrared fiber couplers as this was done by Eyal et al (1994) who mechanically joined uncladded 900-μm core silver-halide fibers to produce a Y-coupler for the mid-infrared [43]
For crystalline Photonic crystal fibers (PCF) the preform cannot be obtained by drilling holes in the preform as these would be destroyed during the extrusion process
Summary
Any celestial object with a non-zero temperature emits infrared radiation (i.e. heat). Photonics components can be placed in direct contact with a cryostat window or a detector [10]; They allow flux transportation in a much easier way than long optical trains; They permit faster and simpler instrument upgrade when dealing with optical chips; single-mode fibers and integrated optics allow very accurate beam cleaning, thanks to spatial and modal filtering, which is an essential aspect of aperture synthesis and nulling interferometers [12, 13]; When the waveguide is placed in a chilled enclosure, its limited numerical aperture acts as a field stop, avoiding straylight entering the system from outside the acceptance cone This is an advantage when operating in the infrared, where thermal background from nearby warm optics is very high. The increasing use of photonic devices for astronomical instrumentation has led to the neologism astrophotonics
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