  Optical spectroscopic sensors are powerful tools for analysing gas mixtures
in industrial and scientific applications. Whilst highly sensitive
spectrometers tend to have a large footprint, miniaturized optical devices
usually lack sensitivity or wideband spectroscopic coverage. By employing a
widely tunable, passively stable fiber Fabry-Perot cavity (FFPC), we
demonstrate an absorption spectroscopic device that continuously samples over
several tens of terahertz. Both broadband scans using cavity mode width
spectroscopy to identify the spectral fingerprints of analytes and a fast,
low-noise scan method for single absorption features to determine
concentrations are exemplary demonstrated for the oxygen A-band. The novel scan
method uses an injected modulation signal in a Pound-Drever-Hall feedback loop
together with a lock-in measurement to reject noise at other frequencies. The
FFPC-based approach provides a directly fiber coupled, extremely miniaturized,
light-weight and robust platform for analyzing small analyte volumes that can
straightforwardly be extended to sensing at different wavelength ranges, liquid
analytes and other spectroscopic techniques with only little adjustments of the
device platform.
