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 detection, a strong absorption band at 1513.98 nm with an absorption cross section of 3.3⋅10 / was used. As a result, a detection sensitivity of ∼0.1 (3- ) was achieved for within 10 s.