3D free-form optical lens — miniaturised fibre couplers for astrophotonics

Abstract

In astronomy, multi-object spectrographs employ fibre positioning robots to couple the light from multiple astronomical sources (stars or galaxies) into multiple multi-mode fibres, which are distributed across the focal plane of the telescope. These fibres transport the celestial light to the entrance slit of a spectrograph (or bank of spectrographs) for analysis. For any multi-object system, mm-scale opto-mechanical solutions are required to couple the telescope light efficiently into the fibre. We demonstrate a unique micro( μ )-optics solution to replace current optical fibre couplers. Specifically, we target technology on board the Keck telescope’s FOBOS - Fibre-Optic Broadband Optical Spectrograph — which operates at UV to IR spectral ranges. For spectrally broadband UV-IR band operation, we use glass and crystals: fused silica, crystalline quartz (transparency 0 . 16 − 2 μ m ), sapphire Al 2 O 3 ( 0 . 2 − 5 μ m ), CaF 2 ( 0 . 2 − 7 μ m ), and BaF 2 ( 0 . 2 − 10 μ m ). The miniaturised μ -coupler is monolithic, with the entire light path contained within glass or crystal, seamlessly extending to the fibre entrance, which is laser-machined and precisely aligned with the optical axis.