A method and
system for stabilizing and demodulating an interferometer at quadrature are described. In response to
receipt of a
signal indicative of
optical power of the interferometer, an interferometer
control system determines an
optical path length correction required to stabilize the interferometer at quadrature utilizing
signal amplitudes appearing at multiple
harmonics of the
signal. In a particularly preferred embodiment, the signal amplitudes are calculated utilizing the
Goertzel algorithm, a computationally efficient
discrete Fourier transform. The interferometer
control system then outputs an
error signal indicative of the
optical path length correction. In a preferred embodiment, the
error signal forms the DC component of a composite stabilization signal, whose AC component is the reference modulation signal utilized to excite a
transducer to modulate the
optical path length of the interferometer. With the interferometer stabilized at quadrature, the interferometer
control system determines a first signal amplitude of a particular
harmonic without target oscillation and a second signal amplitude of the same
harmonic with target oscillation. Based upon the relative magnitudes of the first and second signal amplitudes, the interferometer control
system determines a solution set for the displacement amplitude of the interferometer target. Any
ambiguity in the displacement amplitude can then be eliminated utilizing additional information gained from the signal amplitude present at a
harmonic of a target oscillation frequency.