The use of wavelength-shifting interferometry with a He-Ne laser source is presented. The approach is based on the thermal expansion of the laser cavity during the warm-up time, in conjunction with an unequal path interferometric configuration. The optics principles are reviewed, and a demonstrative measurement is presented. The technique extends the capability of phase shift acquisition and processing to cases where conventional methods with piezoelectric transducers cannot be used, and particularly to external measurement cavities of large diameter.