Fibre modal power distributions in astronomy and their application to OH-suppression fibres

Knowledge of the modal power distribution (MPD) in multimode optical fibre is crucial for the design of OH-suppressing fibre systems based on single-mode fibre arrays. MPDs allow us to estimate the minimum number of modes required to provide good data quality for astronomy so that we can reduce the size and cost of the single-mode array. This paper shows how the MPD of multimode fibres changes when fed with the telescope point spread function and when illuminated with an image of the telescope exit pupil via a field lenslet under conditions of ideal, natural and partially corrected seeing. We show that OH-suppressing fibres operating at 800 nm require arrays of single-mode fibres that will be challenging to manufacture. The complete J and H bands, however, require as few as 40–80 modes.