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

摘要

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 uW of mid-IR light at 3,346 nm with a conversion efficiency of 0.05 % W~(-1) 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 6.07 ± 0.03 μs. The potential of such a spectrometer was illustrated by investigating the P(3) transition in the fundamental v_3(F_2) band of CH4 both in a 7.5 ppmv calibrated mixture of CH4 in air and in breath samples from methane and non-methane producers under conditions where the minimum detectable absorption coefficient (αmin) was 2.8 × 10~(-8) cm~(-1) over 6 s using a ring-down time acquisition rate of 20 Hz. Allan variance measurements indicated an optimum αmin of 2.9 × 10~(-9)cm~(-1)over 44 s.