Real-time ultrasensitive detection of ammonia gas using a compact CRDS spectrometer

Trace-level ammonia gas in air was analyzed using a fiber-based compact cavity ring-down spectrometer (CRDS). For the compact spectrometer, a 20 cm linear cavity with two high reflectivity (>99.999%) mirrors was employed. The laser frequency was locked to the cavity resonance by using frequency shifted (160 MHz) optical feedback. For N H ₃ detection, a strong absorption band at 1513.98 nm with an absorption cross section of 3.3⋅10^-21 c m/m o l e c u l e was used. As a result, a detection sensitivity of ∼0.1p p b (3-σ) was achieved for N H ₃ within 10 s.