  Traditional absorption spectroscopy has fundamental difficulty in resolving
small absorbance from strong background due to the instability of laser
sources. Existing background-free methods in broadband vibrational spectroscopy
help to alleviate this problem but face challenges in realizing either low
extinction ratios or time-resolved field measurements. Here, we introduce
optical-parametric-amplification-enhanced background-free spectroscopy, in
which the excitation background is first suppressed by an interferometer and
then the free-induction decay that carries molecular signatures is selectively
amplified. We show that this method can further improve the limit of detection
in linear interferometry by order(s) of magnitude without requiring lower
extinction ratios or time-resolved measurement, which can benefit sensing
applications in detecting trace species.
