PRAXIS: low thermal emission high efficiency OH suppressed fibre spectrograph

Abstract

PRAXIS is a second generation instrument that follows on from GNOSIS, which was the first instrument using fibre Bragg gratings for OH suppression to be deployed on a telescope. The Bragg gratings reflect the NIR OH lines while being transparent to the light between the lines. This gives in principle a much higher signal-noise ratio at low resolution spectroscopy but also at higher resolutions by removing the scattered wings of the OH lines. The specifications call for high throughput and very low thermal and detector noise so that PRAXIS will remain sky noise limited even with the low sky background levels remaining after OH suppression. The optical and mechanical designs are presented. The optical train starts with fore-optics that image the telescope focal plane on an IFU which has 19 hexagonal microlenses each feeding a multi-mode fibre. Seven of these fibres are attached to a fibre Bragg grating OH suppression system while the others are reference/acquisition fibres. The light from each of the seven OH suppression fibres is then split by a photonic lantern into many single mode fibres where the Bragg gratings are imprinted. Another lantern recombines the light from the single mode fibres into a multi-mode fibre. A trade-off was made in the design of the IFU between field of view and transmission to maximize the signal-noise ratio for observations of faint, compact objects under typical seeing. GNOSIS used the pre-existing IRIS2 spectrograph while PRAXIS will use a new spectrograph specifically designed for the fibre Bragg grating OH suppression and optimised for 1.47 µm to 1.7 µm (it can also be used in the 1.09 µm to 1.26 µm band by changing the grating and refocussing). This results in a significantly higher transmission due to high efficiency coatings, a VPH grating at low incident angle and optimized for our small bandwidth, and low absorption glasses. The detector noise will also be lower thanks to the use of a current generation HAWAII-2RG detector. Throughout the PRAXIS design, from the fore-optics to the detector enclosure, special care was taken at every step along the optical path to reduce thermal emission or stop it leaking into the system. The spectrograph design itself was particularly challenging in this aspect because practical constraints required that the detector and the spectrograph enclosures be physically separate with air at ambient temperature between them. At present, the instrument uses the GNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OH suppression unit based on multicore fibre Bragg gratings which will allow an increased field of view per fibre. Theoretical calculations show that the gain in interline sky background signal-noise ratio over GNOSIS may very well be as high as 9 with the GNOSIS OH suppression unit and 17 with the multicore fibre OH suppression unit.

The photonic lantern

“…Although the type #2 lanterns described in the previous section have been successful, they are "hand crafted" components that are poorly scalable to large N [66]. The fabrication of a single pair of lanterns requires N SMFs to be prepared, assembled, threaded inside a tight-fitting capillary and then tapered.…”

“…28. The PIMMS schemes combine such a lantern filter with the formation of a pseudo-slit (Section 4.3) input to a spectrograph or an arrayed waveguide grating to produce powerful but compact and economical spectroscopic instrumentation [10,23,49,61], and these ideas are being tested on-sky [21,26,28,31,34,39,66]. This is an important application in the field of astrophotonics, which seeks to take advantage of photonics technology (as developed for telecommunication for example) to solve problems in observational astronomy [23,146].…”

“…Thus photonic lanterns have applications wherever there is a need to convert light between single-mode and multimode systems, especially if the distribution of light between output states (cores or modes) is unimportant. One example is in astronomical instrumentation, where the multimode light delivered by a telescope needs to be spectrally-filtered by single-mode fibre Bragg gratings [1,3,[10][11][12]18,21,23,25,26,28,31,37,39,41,49,54,64,66]. Another is in optical telecommunications, where individual channels of information delivered by single-mode fibres need to be multiplexed via orthogonal combinations of modes in a common few-mode fibre [20,33,35,36,40,42,43,[46][47][48]52,53,57,58,67,[69][70][71][72][73][74][75][76][77][78][79].…”

Photonic lantern behaviour and implications for instrument design

“…PRAXIS is a near infrared (NIR) spectrograph intended to serve as a testbed for FBG OH suppression. 17,18 It is a successor to the GNOSIS instrument which successfully demonstrated the suppression of OH lines by FBGs but was unable to produce a robust measurement of the resulting interline sky background due to low throughput and relatively high instrumental noise levels. 14,15 PRAXIS will improve upon the sensitivity of GNOSIS and confirm whether the reduction in interline background expected from FBG OH suppression 2 occurs, and to what extent.…”

Writing Bragg Gratings in Multicore Fibers

“…The next generation OH suppression instrument PRAXIS 7 will make use of multi-core fibers (MCFs). These fibers contain any number of singlemoded cores embedded in a single cladding.…”