the two resonant polarizations in the crds resonator and maintained equal separation of the two laser frequencies this technique can use sideband locking by generating one sideband for each of the two required laser polarizations hansch couillaud locking see e g u s pat no 4 451 923 uses polarization rotation within the optical resonator to generate an error signal throughout the instant specification and in the appended claims the term u201c technique of hansch couillaud u201d shall be understood to mean the technique described in t w hansch and b couillaud u201c laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity u201d optics communications 35 441 444 1980 incorporated by reference spatial mode interference tilt locking involves use of a transverse mode of the optical resonator other than the frequency locked mode to generate an optical phase reference in the beam reflected from the resonator input instead of the pdh sidebands the error signal is the difference in optical power measured by two adjacent photo detectors e g a split detector observing the interference pattern of the spatial modes reflected from the resonator throughout the instant specification and in the appended claims the term u201c technique of spatial mode interference u201d shall be understood to mean the technique described in d a shaddock m b gray and d e mcclelland u201c frequency locking a laser to an optical cavity by use of spatial mode interference u201d optics letters 24 1499 1501 1999 incorporated by reference additional advantages and modifications will readily occur.