Frequency stability of spatial mode interference (tilt) locking

This paper describes the investigation of the frequency stability of tilt locking, a technique which derives an error signal for locking a laser to an optical cavity by use of spatial-mode interference. By independently locking two lasers to the same optical cavity and measuring the beat frequency, a direct measurement of the stability of tilt locking was obtained. The beatnote exhibited a normal Allan deviation of 3.3 Hz over a time interval of 100 ms, yielding a relative frequency stability of 1.3 /spl times/ 10/sup -14/. This result implies at least a fractional locking stability of 1.3 /spl times/ 10/sup -14/. Moreover, this stability was limited by fluctuations in the cavity spacer length and not the tilt-locking technique.