The frequency of one axial mode of a two-mode He-Ne laser is tuned by mirror displacement independently from the other mode if a third mirror is added to the usual two laser mirrors. The frequency shift is proportional to the mirror displacement and can be detected with high sensitivity by observing the beat frequency with the other axial mode. The sensitivity of such a two-mode laser with a coupled resonator for measurements of small length shifts has been tested.
H. Boersch, H. Eichler, and W. Wiesemann, "Measurement of Length Shifts down to 10-3 Å with a Two-Mode Laser," Appl. Opt. 9, 645-648 (1970)