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The power of the next generation of telescopes that will rely largely on the combination of light-collecting area with excellent (ideally: diffraction limited) image quality. Therefore, the focus will heavily lean on adaptive optics and the near infrared wavelength regime. A severe limiting factor is the presence and strength of atmospheric OH emission lines in the NIR. OH suppression techniques involving fiber Bragg gratings (FBG) have been proposed, however as yet not fully demonstrated on sky. We are involved in the first generation FBG prototype development with partners in Australia, including the GNOSIS and PRAXIS on-sky experiments.
Since the supply of suitable multi-notch filters is no longer available from industry, we have made an effort at innoFSPEC Potsdam to build a specialized laboratory for the development and manufacture of 2nd generation FBGs for OH suppression.
Suppression of the strong NIR OH emission lines requires a single grating that reflects multiple wavelengths, spaced at non-periodic intervals, with flat-top profile and high suppression ratio. It has been shown that aperiodic fiber Bragg gratings (AFBGs) can provide such functions. However, the fabrication technology requires accurate optical alignment of several degrees of freedom as well as complex control of modulated beams to form a varying interference pattern. In our work, an algorithm is developed from the index profile of a multi-notch AFBG to the design of a complex phase-mask that can generate a matching UV diffraction pattern, which will in turn inscribe an single-mode fiber into the chosen AFBG. With such a phase mask, the fabrication of the AFBGs will be reduced to a simple UV-exposure process, i.e., the complex alignment and control processes of the interference pattern from modulated beams are avoided altogether. The resulting reliable and reproducible fabrication process will dramatically reduce of the cost of such filters. Packaging aspects for a complete sky emission filter system will also be discussed.
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