Abstract: An interferometric gauge for high precision absolute and incremental measurements of distances of up to at least 100 meters. An accuracy of 0.02 Angstrom has been demonstrated, free of the .about.10 Angstrom polarization-leakage errors of conventional gauges. The absolute distance measurement requires no a priori information, and multiple synthetic wavelengths need not be used. The gauge can be used as the sensor for a null servo. There are reduced-cost versions of lower accuracy. The gauge can employ a resonant cavity for greater sensitivity, and can be built with no transmissive elements in the measured path. Without additional hardware, the gauge readout can be in the form of a frequency or an analog electrical signal, in addition to a digital count of an integral number of wavelengths. The gauge employs a high-sensitivity null sensor. Feedback to an optical frequency shifter maintains the null condition; the frequency shift is measured electronically, providing a high sensitivity distance readout.