Demonstration and design of a compact diffraction limited spectrograph

PIMMS IR is a prototype high resolution diraction limited spectrograph operating in the near infrared. Its current conguration has a bandwidth of 8nm centred on 1550nm with a resolving power, λ/Δλ, of 31000 with the option to increase this to ~60000 using a dual grating system. Remarkably, this is 85% of the theoretical limit for Gaussian illumination of a diraction grating. It is based upon the PIMMS#0 (photonic integrated multi-mode micro-spectrograph), a design that utilises the multi-mode to single-mode conversion of the photonic lantern. By feeding the spectrograph with the single-mode bres we are able to design and build a spectrograph whose performance is diraction limited and independent of the input source (i.e. a telescope) it is attached to. The spectrograph has with a throughput of ~70% (that is the light from the single-mode entrance slit that lands on the detector). The spectrograph is also extremely compact with a footprint of just 450mm x 190mm. Here we present the design of PIMMS IR and its performance characteristics determined from ray tracing, physical optics simulations and experimental measurements.Δ

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