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Modulation Index Adjustment for Recovery of Pure Wavelength Modulation Spectroscopy Second Harmonic Signal Waveforms

School of Information Science and Engineering and Shandong Provincial Key Laboratory of Laser Technology and Application, Shandong University, Jinan 250100, China
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Academic Editor: Vittorio M. N. Passaro
Sensors 2017, 17(1), 163; https://doi.org/10.3390/s17010163
Received: 22 November 2016 / Revised: 26 December 2016 / Accepted: 3 January 2017 / Published: 15 January 2017
(This article belongs to the Section Physical Sensors)
A new technique of modulation index adjustment for pure wavelength modulation spectroscopy second harmonic signal waveforms recovery is presented. As the modulation index is a key parameter in determining the exact form of the signals generated by the technique of wavelength modulation spectroscopy, the method of modulation index adjustment is applied to recover the second harmonic signal with wavelength modulation spectroscopy. By comparing the measured profile with the theoretical profile by calculation, the relationship between the modulation index and average quantities of the scanning wavelength can be obtained. Furthermore, when the relationship is applied in the experimental setup by point-by-point modulation index modification for gas detection, the results show good agreement with the theoretical profile and signal waveform distortion (such as the amplitude modulation effect caused by diode laser) can be suppressed. Besides, the method of modulation index adjustment can be used in many other aspects which involve profile improvement. In practical applications, when the amplitude modulation effect can be neglected and the stability of the detection system is limited by the sampling rate of analog-to-digital, modulation index adjustment can be used to improve detection into softer inflection points and solve the insufficient sampling problem. As a result, measurement stability is improved by 40%. View Full-Text
Keywords: water; fiber optics sensors; diode lasers; wavelength modulation spectroscopy; modulation index adjustment water; fiber optics sensors; diode lasers; wavelength modulation spectroscopy; modulation index adjustment
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Wei, W.; Chang, J.; Wang, Q.; Qin, Z. Modulation Index Adjustment for Recovery of Pure Wavelength Modulation Spectroscopy Second Harmonic Signal Waveforms. Sensors 2017, 17, 163.

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