A fiber optic sensor includes at least two Fabry-Perot (FP cavities)
defined by at least three partially reflecting surfaces which
individually and together are capable of generating different
interference spectra which are affected by temperature. One of the FP
cavities is formed at an end of the sensor and includes a surface which
is capable of supporting a thin film, the optical thickness of which is
to be measured. The other FP cavity between the lead-in fiber and the
first FP cavity thus does not include the film and can thus independently
provide highly accurate temperature information for calibrating the
optical length of the second FP cavity and compensation for temperature
effects on measurement of the thin film supported thereon, preferably by
subtraction of a calibrated temperature-dependent change in optical
length of the second FP cavity from the measurement made.