Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

Abstract

We describe the design of the fiber-optic coupling and light transfer system of the WISDOM (WIYN Spectrograph for DOppler Monitoring) instrument. As a next-generation Precision Radial Velocity (PRV) spectrometer, WISDOM incorporates lessons learned from HARPS about thermal, pressure, and gravity control, but also takes new measures to stabilize the spectrograph illumination, a subject that has been overlooked until recently. While fiber optic links provide more even illumination than a conventional slit, careful engineering of the interface is required to realize their full potential. Conventional round fiber core geometries have been used successfully in conjunction with optical double scramblers, but such systems still retain a memory of the input illumination that is visible in systems seeking sub-m/s PRV precision. Noncircular fibers, along with advanced optical scramblers, and careful optimization of the spectrograph optical system itself are therefore necessary to study Earth-sized planets. For WISDOM, we have developed such a state-of-the-art fiber link concept. Its design is driven primarily by PRV requirements, but it also manages to preserve high overall throughput.Light from the telescope is coupled into a set of six, 32 µm diameter octagonal core fibers, as high resolution is achieved via pupil slicing. The low-OH, step index, fused silica, FBPI-type fibers are custom designed for their numerical aperture that matches the convergence of the feeding beam and thus minimizes focal ratio degradation at the output. Given the demanding environment at the telescope the fiber end tips are mounted in a custom fused silica holder, providing a perfect thermal match. We used a novel process, chemically assisted photo etching, to manufacture this glass fiber holder.A single ball-lens scrambler is inserted into the 25m long fibers. Employing an anti-reflection (AR) coated, high index, cubic-zirconia ball lens the alignment of the scrambler components are straightforward, as the fiber end tips (also AR coated) by design touch the ball lens and thus eliminate spacing tolerances. A clever and simple opto-mechanical design and assembly process assures micron-level self-alignment, yielding a ~87% throughput and a scrambling gain of >20,000.To mitigate modal noise the individual fibers then subsequently combined into a pair of rectangular fibers, providing a much larger modal area thanks to the 34x106 micron diameter. To minimize slit height, and thus better utilize detector area, the octagonal cores are brought very close together in this transition. The two outer fibers are side polished at one side, into a D-shaped cladding, while the central fiber has a dual side polish. These tapered, side-flattening operations are executed with precise alignment to the octagonal core. Thus the cores of the 3 fibers are brought together and aligned within few microns of each other before spliced onto the rectangular fiber.Overall throughput kept high and FRD at bay by careful management of fiber mounting, vacuum feed-throug...

Pupil slicer design for the NASA-NSF extreme precision Doppler spectrograph concept WISDOM

“…A more complete description of the overall instrument and fiber link design can be found in the references. 1,2 Figure 1. WISDOM front end attached to the WIYN telescope mirror cell (left); front end design overview (right).…”

Optical design of the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

“…For the WIYN telescope instrument, this issue is addressed by the design of the pupil slicer and scrambling fibre optic train, details of which can be found elsewhere in these proceedings. 2 Nevertheless, the possibility still remains that some residual illumination variability will remain, and that this will impart a systematic offset in any one radial velocity observation. In order to examine and address this concern, a detailed study of the spectrographs performance was made in the presence of a range of pupil illumination variations.…”

WISDOM: the WIYN spectrograph for Doppler monitoring: a NASA-NSF concept for an extreme precision radial velocity instrument in support of TESS

“…Project costs scale strongly with beam size and such a design would be difficult to fit within the space constraints of the designated spectrograph room at WIYN. More on the implementation of the pupil slicer can be found in [7] and [10], here we just point out that with a 6-way slicing of the pupil we managed to reach the R=110k target resolution for the 1.2″ fiber size with a 125mm beam. (HARPS has a 200mm beam for similar R and Dtel, while for a smaller aperture of φ=1.0″.)…”

“…Both of these are typically induced by changes (guide error, atmospheric effects) at the coupling of telescope light into the fiber, and result in spurious instrumental RV signals, that can be as severe as tens of m/s. As described in the WISDOM fiber link paper 7 we did follow best practices in our design to mitigate such error sources, and while the NASA AO call did not require the proposing teams to develop the telescope interface, we felt that it is still a critical and integral part of any PRV instrument aiming subm/s precision. And so we took responsibility for both the optical and opto-mechanical design concept, except for the guide camera optics.…”

“…This optical selector stage can inject any single input (or combination of them) into: a) the science fibers, via a fiber optic link that directly runs up to the pupil slicer (at the telescope Front End) from the calibration system; b) into a dedicated simultaneous calibration fiber that runs directly into the spectrograph, positioned in between the two 3:1 rectangular fibers of the slit assembly 7 . Part of the optical selector/multiplexer is a pair of neutral density filter wheels that allow for SNR matching between the light of the science and simultaneous calibration fibers.…”