Abstract

A thorough analysis of the shape and strength of Doppler-broadened wavelength-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals is presented, and their dependence on modulation frequency, modulation amplitude, and detection phase is investigated in detail. The conditions that maximize the on-resonance signal are identified. The analysis is based on standard frequency modulation spectroscopy formalism and the Fourier description of wavelength modulation spectroscopy and is verified by fits to experimental signals from ${\mathrm{C}}_2{\mathrm{H}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ measured at $1531\mathrm{nm}$. In addition, the line strengths of two $\mathrm{C}{\mathrm{O}}_2$ transitions in the ${\nu }_2\to 3{\nu }_1+{\nu }_2+{\nu }_3$ hot band [${\mathrm{P}}_{\mathrm{e}}\left(7\right)$ and ${\mathrm{P}}_{\mathrm{e}}\left(9\right)$] were found to differ by $\sim 20\%$ from those given in the HITRAN database.

© 2009 Optical Society of America

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