Enhancement of fringe visibility in a fibre interferometer

A simple method for increasing fringe visibility of an interference pattern in a Mach-Zehnder type fibre interferometer is presented. In order to obtain high visibility, two elliptically polarized waves emitted from fibres with birefringence are transformed into two linearly polarized waves of equal amplitude and with parallel planes of polarization by using ordinary optical components. Instability of fringe patterns due to temperature fluctuation of the air surrounding the fibres is also discussed.