An efficient and compact difference-frequency-generation spectrometer and its application to ¹²CH₃D/¹²CH₄ isotope ratio measurements

We have developed an efficient and compact 3.4 μm difference-frequency-generation spectrometer using a 1.55 μm distributed feedback (DFB) laser diode, a 1.06 μm DFB laser diode, and a ridge-waveguide periodically poled lithium niobate. It is continuously tunable in the 30 cm-1 span and is applied to ¹²CH₃D¹²CH₄ isotope ratio measurements. The suitable pair of ¹²CH₃Dv4 pP(7,6) and ¹²CH₄ ν2+ν₄ R(6) F₁(1) lines enabled us to determine their isotope ratio with a precision repeatability of 0.8‰ using a sample and a working standard of pure methane with an effective signal averaging time of 100 ms.