Measurement of length shifts down to 10-5 A with a three-mode laser

The frequency of one axial mode of a three-mode He/Ne laser can be tuned independently from the other modes if a third mirror is added to the usual two laser mirrors. Provided the adjustment is correct, the frequency shift is proportional to a change of the optical path length between the third mirror and the neighboring laser mirror. By observing the beat frequency of the laser, measurements of optical path variations down to 10-5 A are possible. (U.S.)