  We present a novel remote gas detection and identification technique based on
correlation spectroscopy with a piezoelectric tunable fibre-optic Fabry-P\'erot
filter. We show that the spectral correlation amplitude between the filter
transmission window and gas absorption features is related to the gas
absorption optical depth, and that different gases can be distinguished from
one another using their correlation signal phase. Using an observed
telluric-corrected, high-resolution near-infrared spectrum of Venus, we show
via simulation that the Doppler shift of gases lines can be extracted from the
phase of the lock-in signal using low-cost, compact, and lightweight
fibre-optic components with lock-in amplification to improve the
signal-to-noise ratio. This correlation spectroscopy technique has applications
in the detection and radial velocity determination of faint spectral features
in astronomy and remote sensing. We experimentally demonstrate remote CO2
detection system using a lock-in amplifier, fibre-optic Fabry-P\'erot filter,
and single channel photodiode.
