Some Aspects of Fringe Counting in Laser Interferometers

A number of arrangements are possible for deriving signals in phase quadrature from interferometers, so that bi-directional counting of the interference fringe movements can be carried out. Arrangements which avoid splitting the image field are preferred for laser light sources, and can give a better signal-to-noise ratio. In environments where the interferometer is subjected to vibration, the fringe counting system may be required to handle rapid reversals in the direction of movement. In these circumstances the rate of production of rapid counting signals of alternate direction may be considerably reduced by using an appropriate logic system between the interferometer and the counter.