Experimental study demonstrates 35-ppb ambient methane detection via difference frequency generation, indicating potential for highly sensitive optical greenhouse gas monitoring.
Methane in ambient air is detected by difference frequency generation in a direct-bonded quasi-phase-matched LiNbO3 ridge waveguide using near infrared communication band laser diodes as pump and signal sources. Absorption lines for the Q branch in the 3.3μm ν3 band are measured by using a Hanst configuration multipass cell with a 107m path length. The authors detect 35ppb CH4 in 2kPa ambient air with a single wavelength scan of a signal external cavity laser diode by suppressing various spectroscopic fringes and stabilizing the pump laser diode oscillation mode with a fiber Bragg grating.
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Yanagawa et al. (2006) studied this question.
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