Efficient photonic reformatting of celestial light for diffraction-limited spectroscopy

Abstract

The spectral resolution of a dispersive astronomical spectrograph is limited by the trade-off between throughput and the width of the entrance slit. Photonic guided-wave transitions have been proposed as a route to bypass this trade-off, by enabling the efficient reformatting of incoherent seeing-limited light collected by the telescope into a linear array of single modes: a pseudo-slit which is highly multimode in one axis but diffraction-limited in the dispersion axis of the spectrograph. It is anticipated that the size of a single-object spectrograph fed with light in this manner would be essentially independent of the telescope aperture size. A further anticipated benefit is that such spectrographs would be free of 'modal noise', a phenomenon that occurs in high-resolution multimode fibre-fed spectrographs due to the coherent nature of the telescope point-spread-function (PSF). We address these aspects by integrating a multicore fibre photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect to spatially reformat the modes within the PSF into a diffraction-limited pseudo-slit. Using the CANARY adaptive optics (AO) demonstrator on the William Herschel Telescope, and 1530 ± 80 nm stellar light, the device is found to exhibit a transmission of 47 -53 % depending upon the mode of AO correction applied. We also show the advantage of using AO to couple light into such a device by sampling only the core of the CANARY PSF. This result underscores the possibility that a fully-optimised guided-wave device can be used with AO to provide efficient spectroscopy at high spectral resolution.

Simulation and optimisation of an astrophotonic reformatter

“…This relation can also be thought of as the number of spatial modes that form a telescope PSF, which scales with the square of the telescope aperture D T divided by the Fried seeing parameter r 0 (Harris & Allington-Smith 2013;Spaleniak et al 2013;MacLachlan et al 2017).…”

Astronomical Applications of Multi-Core Fiber Technology

“…This promising method enabled PL devices to be consolidated to a single component greatly reducing the difficulty of handling the device and hence the risk of breakage during manufacturing. A MCF based PL was tested in conjunction with a device that remapped the 2D output of the MCF to a linear pseudo-slit (known as fan out, discussed below), on-sky behind the Canary adaptive optics system at the William Herschel Telescope [23]. The total throughput of the reformatting device including coupling was > 50%.…”

Optimizing astrophotonic spatial reformatters using simulated on-sky performance

“…Also, a value of 0.15 m for the seeing parameter (Fried parameter-r 0 ) was selected, representative of the aforementioned on-sky experiments. 11 Soapy produced 200 near infrared (NIR) data frames, each with a 6 ms exposure time as an input to Beam-Prop. Each Soapy frame covers a 3.0 arcseconds squared field on-sky, slightly more than three times the HR entrance angular size.…”

“…The design structure of the device was followed as described in Ref. 11. The HR is composed of a PL section followed by a ULI reformatter section, ending in a slit profile output.…”

“…8 This dependence imposes a correlation of the number of the spatial modes forming the telescope's PSF being analogous to the ratio of square of the telescope's diameter D T over the Fried seeing parameter r 0 . [9][10][11] Currently the largest ground-based telescopes are 8-10 m in diameter requiring large spectrographs, to couple the light efficiently while also achieving high resolving power (e.g., Ref. [12][13][14].…”