Surface-stabilized ferroelectric liquid crystals (FLC's) are promising materials for semiconductor integrated-circuit-based spatial light modulators. For coherent optical processing applications, phase stability upon repeated switching is critically important. The phase characteristics of an FLC device were measured at switching rates of up to 1 kHz and found to be very stable. The change in the total optical path length through the cell was found to be < 0.0025λ at a wavelength of 632.8 nm. The static optical characteristics were measured for a range of temperatures at and above room temperature in order to be able to identify any temperature-induced phase changes upon switching. The temperature of the FLC cell was externally varied, and changes in the birefringent optical path difference, the optical path length, and the optic axis tilt angle were measured. However, because of the observed phase stability of the FLC, the change of temperature caused by switching was determined to be < 0.046°C. It is clearly shown that FLC's can exhibit the stability needed for critical coherent and incoherent optical data-processing applications.