Ultrafast laser inscription (ULI) is a rapidly maturing technique which usesfocused ultrashort laser pulses to locally modify the refractive index ofdielectric materials in three-dimensions (3D). Recently, ULI has been appliedto the fabrication of astrophotonic devices such as integrated beam combiners,3D integrated waveguide fan-outs and multimode-to-single mode convertors(photonic lanterns). Here, we outline our work on applying ULI to thefabrication of volume phase gratings (VPGs) in fused silica and galliumlanthanum sulphide (GLS) glasses. The VPGs we fabricated had a spatialfrequency of 333 lines/mm. The optimum fused silica grating was found toexhibit a first order diffraction efficiency of 40 % at 633 nm, but exhibitedapproximately 40 % integrated scattered light. The optimum GLS grating wasfound to exhibit a first order diffraction efficiency of 71 % at 633 nm andless than 5 % integrated scattered light. Importantly for future astronomyapplications, both gratings survived cooling to 20 K. This paper summarises thegrating design and ULI manufacturing process, and provides details of thediffraction efficiency performance and blaze curves for the VPGs. In contrastto conventional fabrication technologies, ULI can be used to fabricate VPGs inalmost any dielectric material, including mid-IR transmitting materials such asthe GLS glass used here. Furthermore, ULI potentially provides the freedom toproduce complex groove patterns or blazed gratings. For these reasons, webelieve that ULI opens the way towards the development of novel VPGs for futureastronomy related applications.
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