Error Analysis of a Laser Strainmeter

Abstract

A servo-controlled LASER strainmeter has been designed based on an optical interferometer. The design goal is a strain sensitivity of 10^-10 using a small interferometer — of the order of 1 m. To achieve this sensitivity with such a small instrument, it is necessary to pay careful attention to possible noise or error sources.

The major error source has been found to be due to temperature fluctuations. The next most important error source is the electrical noise in the instrument circuits. The possible error sources due to frequency and amplitude instabilities of the LASER and index of refraction changes can be made small and negligible by proper design. To make these effects negligible, the arm lengths of the interferometer are made equal, the interference or fringe pattern is modulated at an audio frequency and the light paths of the interferometer are kept in a partial vacuum.

To achieve the necessary temperature stability, the instrument must be placed in a constant temperature environment such as found in deep mines or tunnels. However, it is shown that the stringent temperature requirements are not unique to small strainmeters.