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

K. E. Whittaker, L. Ciaffoni, G. Hancock, R. Peverall and G. A. D. Ritchie

Abstract

Continuing studies into an all-diode laser-based 3.3µm 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 16µW of mid-IR light at 3,346 nm with a conversion efficiency of 0.05%W-1cm-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±0.03µs. The potential of such a spectrometer was illustrated by investigating the P(3) transition in the fundamental 3F2 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 x 10-8cm-1 over 6 s using a ring-down time acquisition rate of 20 Hz. Allan variance measurements indicated an optimum αmin of 2.9 x 10-9cm-1 over 44 s.