Detection of harmonics and recovery of the absorption line profile using logarithmic-transformed wavelength modulation spectroscopy

Menglong Cong; Dandan Sun

doi:10.1117/1.OE.55.7.076119

28 July 2016 Detection of harmonics and recovery of the absorption line profile using logarithmic-transformed wavelength modulation spectroscopy

Menglong Cong, Dandan Sun

Author Affiliations +

Funded by: Fundamental Research Funds for the Central Universities

Optical Engineering, Vol. 55, Issue 7, 076119 (July 2016). https://doi.org/10.1117/1.OE.55.7.076119

Abstract

A versatile signal processing strategy for eliminating the residual amplitude modulation (RAM) and distortion in tunable diode laser wavelength modulation spectroscopy is theoretically demonstrated and experimentally validated. The strategy involves logarithmic transformation and differential detection, which are achieved using a homemade circuit. Through the logarithmic transformation, the optical intensity modulation of the laser, which performs as the source of RAM and distortion, is separated from the absorption-induced power attenuation and further balanced during the differential detection. The first harmonic, which is proportional to the first-order derivative of the absorption line profile in the case of a small modulation index, is extracted along with the second harmonic and is integrated for the recovery of the absorption line profile. The experiments are carried out for CH₄ at its R(3) absorption line of the 2ν3 overtone for validation of the system, and the derived results are found to be in good agreement with the theoretical simulations. These promising results indicate the high potential of the strategy for absorption spectrum-based determination of gas properties.

Citation Download Citation

Menglong Cong and Dandan Sun "Detection of harmonics and recovery of the absorption line profile using logarithmic-transformed wavelength modulation spectroscopy," Optical Engineering 55(7), 076119 (28 July 2016). https://doi.org/10.1117/1.OE.55.7.076119

Published: 28 July 2016

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