4.6 °m-band difference frequency generation and CO isotopologue detection

There are many strong absorption lines of environmental gases in the mid-infrared region in atmospheric air, including those of greenhouse effect gases. Reported is difference frequency generation in a direct-bonded periodically poled LiNbO₃ ridge waveguide in the 4.6 m band, which is the most appropriate wavelength range for observing carbon monoxide absorption spectra. It is also shown that isotopologue absorption lines can be detected using the output light generated in a waveguide with mature telecommunication band laser diode light sources. The direct-bonded ridge waveguide consisted of a 50 mm-long chip and its wavelength conversion efficiency was approximately 4% /W.