Development of photonic technologies for astronomical instruments using ultrafast laser inscription
Abstract
Recently there has been a desire to apply photonic concepts and technologies to
astronomical applications, with the aim of replacing traditional bulk optic instruments.
This astrophotonic approach is envisioned to produce compact devices that have the
potential to provide the unprecedented precision and stability required for current
astronomical goals, such as the detection of Earth-like exoplanets capable of supporting
life.
The work in this thesis covers the investigation of the technique of Ultrafast Laser
Inscription (ULI) to create the building blocks that may lead to a fully integrated compact
spectrograph for astronomy. Unlike conventional fabrication technologies, ULI allows
custom three-dimensional optical devices to be directly inscribed within a bulk substrate.
Volume gratings with high first order diffraction efficiencies optimised for a variety of
wavelengths are demonstrated, with a view to providing efficient gratings for the midinfrared
wavelength range. Initially the mid infrared transmitting material GLS was used
to create gratings with a first order efficiency of 63 % up to a wavelength of 1.35 μm.
Anti-reflection coatings were applied to GLS and gratings with an efficiency of 95 % at
1.02 μm were produced.
A second material, IG2 was used and diffraction gratings with a first order efficiency of
63 % were produced, which were efficient up to a wavelength of 2.5 μm, with thicker
gratings produced which have yet to be characterised in a mid-infrared setup. These
developments show that practical mid-infrared volume gratings can be produced by the
process of ULI.
Photonic reformatters have also been developed to reshape a multimode telescope point
spread function into a pseudo-slit, suitable as an input for a diffraction-limited
spectrograph. Two device designs were investigated. The first was a fully integrated ULI
component which, tested in the laboratory reformatted a multimode input at 1550 nm into
a slit, single mode in one axis and highly multimode in the orthogonal axis with an
efficiency of 66 %. The device was tested on-sky at the William Herschel Telescope and
performed with an efficiency of 19.5 % over the wavelength range 1450 to 1610 nm.
The second, improved device combined a ULI component with a multicore fibre
component, and performed with a similar performance in the laboratory demonstrating
an efficiency of 69 %, but a much improved on sky efficiency of 53 % showing a potential
for such devices to be used as an input for a diffraction limited spectrograph.