Fibre modal noise occurs in high spectral resolution, high signal-to-noise applications.
It imposes fundamental limits upon the photometric accuracy of state-of-the-art fibre-spectrograph
systems. In order to maximize the performance of current and future instruments
it is therefore essential to predict fibre modal noise. Explicitly theoretical approaches
are often restricted to specific cases, therefore this paper focuses on the conditions relevant
to astronomy. The goal of this work is to derive a reliable model which can be used for the
optimization of future spectrograph designs. We give a description of experimental investigations
undertaken in Durham, displaying preliminary results. The first laboratory tests
of a square fibre have shown modal noise characteristics that are interestingly similar to
standard circular fibre.
Fibre modal noise is a photometric uncertainty in a spectrographic resolution element found in high dispersion, high signal to noise spectra. This paper describes how the noise manifests itself and the theory used to describe it. Using a rigorous modal coupling analysis we then show the theories used to date must be called into question due to the changes in the statistics of modal noise with changes in the modal power distribution which occur with arbitrary illumination of the fibre.