A DFG-based cavity ring-down spectrometer for trace gas sensing in the mid-infrared

Continuing studies into an all-diode laser-based 3.3 μm difference frequency generation cavity ring-down spectroscopy system are presented. Light from a 1,560 nm diode laser, amplified by an erbium-doped fibre amplifier, was mixed with 1,064 nm diode laser radiation in a bulk periodically poled lithium niobate crystal to generate 16 μW of mid-IR light at 3,346 nm with a conversion efficiency of $0.05,%,{text{W}}^{-1},{text{cm}}^{-1}$ . This radiation was coupled into a 77 cm long linear cavity with average mirror reflectivities of 0.9996, and a measured baseline ring-down time of $6.07pm 0.03,upmu{rm s}$ . The potential of such a spectrometer was illustrated by investigating the $P(3)$ transition in the fundamental $nu_{3}(F_{2})$ band of ${text{CH}}_4$ both in a 7.5 ppmv calibrated mixture of ${text{CH}}_4$ in air and in breath samples from methane and non-methane producers under conditions where the minimum detectable absorption coefficient ( $alpha_{rm min}$ ) was $2.8 times 10^{-8},{rm cm}^{-1}$ over 6 s using a ring-down time acquisition rate of 20 Hz. Allan variance measurements indicated an optimum $alpha_{rm min}$ of $2.9times 10^{-9},{rm cm}^{-1}$ over 44 s.