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

Abstract

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 $$6.07\pm 0.03\,\upmu{\rm s}$$ . 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 $$2.9\times 10^{-9}\,{\rm cm}^{-1}$$ over 44 s.