First demonstration of OH suppression in a high-efficiency near-infrared spectrograph

Abstract

Ground-based near-infrared astronomy is severely hampered by the forest of atmospheric emission lines resulting from the rovibrational decay of OH molecules in the upper atmosphere. The extreme brightness of these lines, as well as their spatial and temporal variability, makes accurate sky subtraction difficult. Selectively filtering these lines with OH suppression instruments has been a long standing goal for near-infrared spectroscopy. We have shown previously the efficacy of fibre Bragg gratings combined with photonic lanterns for achieving OH suppression. Here we report on PRAXIS, a unique near-infrared spectrograph that is optimised for OH suppression with fibre Bragg gratings. We show for the first time that OH suppression (of any kind) is possible with high overall throughput (18 per cent end-to-end), and provide examples of the relative benefits of OH suppression.

Integrated Arbitrary Filter With Spiral Gratings: Design and Characterization

“…Suffering from the additional loss caused by cladding mode coupling at shorter wavelengths, the overall throughput from 1450 to 1640 nm decreases to 35%. This is still close to the 39% throughput achieved in the most recent OH suppression filter fabricated with FBG [12]. Compared with the typical FBG notch depth 30 dB and notch FWHM 0.2 nm, our integrated device's performance is also on the same level.…”

“…For the design we use here, the length of each piece is Δ = 4 µm and the 50-mm-long grating corresponds to N = 12500 pieces. They are all positioned according to the starting phase θ i and the slope m i , with i = 1, 2, 3 … 12500. m i is calculated from θ i by (12), and θ i is determined by θ i = θ i-1 + φ i , where φ i is the angular coordinate change brought by the i-th grating piece.…”

“…Over the past decades, a lot of works in these areas have been done with fiber-optic technology, most prominently the fiber Bragg grating (FBG). With the development of fiber grating inscription methods, such as the phase mask and the direct writing technique, now people can fabricate ultra-long, complicated and accurate FBGs [10]- [12].…”

Potential of commercial SiN MPW platforms for developing mid/high-resolution integrated photonic spectrographs for astronomy

“…The additional benefit of using these devices is an even more compact instrument than can be achieved with bulk optics, reducing volume and mass, and increasing thermal and mechanical stability. At the same time, the single-mode regime offers immense flexibility to manipulate the light (before the spectroscopy step) such as adding in-line notch filters to specifically suppress atmospheric emission lines using waveguide/fiber Bragg gratings [16,17], precision wavelength calibrators [18], interferometers [19,20], and photonic nullers to suppress unwanted starlight in a binary or a star-planet system [21]. Thus, the benefits of miniaturization and additional light-manipulation capabilities promote applications for both ground-based and space-borne instruments.…”

Development of an integrated near-IR astrophotonic spectrograph

“…The AO system is used to reduce the degrees of freedom of the focused light to create a focused spot which is closer to the diffraction limit (few-moded light). The light can be directly focused on a single-mode fiber (in case of high Strehl ratio extreme-AO systems, as previously demonstrated 13 ) or can be focused on a few-mode photonic lantern 14,15 which then adiabatically guides the light into several single-mode fibers (as previously demonstrated 16 ). The AWG chip acts as a primary disperser and disperses the light along the focal line as shown in Fig.…”