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 12CH3D/12CH4 isotope ratio measurements. The suitable pair of 12CH3D n4 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.
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