Abstract
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%.
Highlights
Tunable diode laser spectroscopy (TDLS) has become the primary technology for most kinds of gas detection, for trace gas concentration detection [1,2,3,4,5]
A new method of modulation index adjustment is studied in TDLS with wavelength modulation spectroscopy (WMS) to recover pure
A new method of modulation index adjustment is studied in TDLS with WMS to recover pure harmonic signal waveforms and eliminate the effect brought by amplitude modulation (AM) and residual amplitude modulation (RAM)
Summary
Tunable diode laser spectroscopy (TDLS) has become the primary technology for most kinds of gas detection, for trace gas concentration detection [1,2,3,4,5]. Other than wavelength modulation for tunable diode lasers, the drive current signal produces amplitude modulation (AM) at the same time This interaction between modulation of the transmitted laser power and absorption of target gas produces residual amplitude modulation (RAM) signals at the fundamental sinusoid frequency and its higher harmonics. Application of first harmonic detection in TDLS with WM techniques is mainly limited by the presence of a large unwanted background signal, caused by the AM of the laser. This high background signal limits amplification of signal-processing modules and deteriorates sensitivity of the detection system. The modulation index adjustment method can be used in other aspects which involve profile improvement and another example is presented as well
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