Compact yet highly functional optical components are desired in modern astronomical instruments targeted at low system cost and reduced maintenance complexity. Integrated photonic spectrometers based on planar lightwave circuits are attractive as the planar miniature device can provide high spectral resolution but also great robustness and flexibility in the design of spectrograph systems. Arrayed waveguide gratings (AWGs) have the potential to be adapted and optimized to function as compact spectrometers in astronomical spectrographs. In this work high-resolution AWGs based on low-loss silica waveguides have been designed, fabricated and characterized. The measured spectral resolution exceeds 104 with Δλ = 150 pm at 1548 nm. The insertion loss (including two times fiber-chip coupling) is merely 2.07 dB, amounting to a peak throughput of 62%. Adiabatic fiber taper is developed to bring down the mode field diameter of a standard single mode fiber to match the mode size of the designed waveguide, resulting in almost lossless coupling from the fiber to the waveguide. The free-spectrum range is 48 nm and the side-band suppression is 22 dB. The AWG is also polarization-insensitive. Rotating the linearly polarized input light by 180° results in a slight shift of the central wavelength ~ 30 pm. The excellent overall performance makes this AWG an ideal candidate as the key building block for the development of an integrated astronomical spectrograph module.
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.
Based on the macro-bend induced losses in an optical fiber a linear displacement sensor is developed. The oscillatory
bend radius dependent loss is suppressed by coating the bent section with an absorption layer. The sensor is designed as
an open-ended fiber probe for easy practical application, where the Fresnel reflection at the cleaved end is utilized.
Mechanical modelling is given to optimize the fiber lead length to avoid buckling of the fiber. Resolutions of 30 μm and
0.1 μm, with coefficient of determination of 0.97 and 0.96, respectively over a total measurable displacement of 0 to 30
mm are achieved by utilizing a ratio-metric measurement of the bend loss. Finally a sensor with a smaller footprint is
also demonstrated by utilizing a reduced clad fiber with a cladding diameter of 80 μm.