Abstract
At near infrared wavelengths, the night sky background seen from the Earth's surface is almost completely dominated by bright spectral lines due to hydroxyl in the upper atmosphere. In the wavelength range 1- 2microm, more than 100 intrinsically narrow spectral lines account for about 98% of the sky background. Now that the performance of infrared detectors is comparable to CCDs at optical wavelengths, the bright infrared sky is the last remaining hurdle to ground-based infrared telescopes reaching sensitivity levels associated with optical telescopes. We demonstrate an aperiodic fibre Bragg grating (AFBG) which performs 94% suppression of OH emission in the 1.50-1.57microm window at a resolving power of R=10,000. This is a working prototype for a device which will allow comparable levels of OH suppression at R=50,000 across the entire near infrared (1.0-2.0microm) spectrum.
Highlights
The near infrared (1-2.5μm) region of the electromagnetic spectrum has become a key frontier of observational cosmology
In grating mask spectrographs like Cambridge OH Suppression Instrument (COHSI) at the University of Cambridge, the observed spectrum is dispersed at R ≈ 5000 onto a photoetched mask where the desired spectral regions are reflected towards the camera and the unwanted lines are absorbed [18]
We manufactured two gratings (AFBG#1, aperiodic fibre Bragg grating (AFBG)#2) in order to test the repeatability of the printing process
Summary
The near infrared (1-2.5μm) region of the electromagnetic spectrum has become a key frontier of observational cosmology. Astronomers have long searched for a technology to suppress this background signal from OH emission lines (see Fig. 1) but with limited success. This effort intensified when it was realized that the night-glow continuum between the OH lines is very faint [14]. In grating mask spectrographs like Cambridge OH Suppression Instrument (COHSI) at the University of Cambridge, the observed spectrum is dispersed at R ≈ 5000 onto a photoetched mask where the desired spectral regions are reflected towards the camera and the unwanted lines are absorbed [18]. The mechanical tolerances of an OH suppression spectrograph are very tight and the overall efficiency of the system tends to be low (
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