Recovery of pure wavelength modulation second harmonic signal waveforms in distributed feedback diode laser-based photoacoustic spectroscopy

• Problem of second harmonic waveform distortion in DFB DL-based photoacoustic spectroscopy was successfully solved using this technique.
• Both residual amplitude modulation and varying laser power were considered in analysis of disturbance factors.
• The experiment result was compared with a calculated pure-WM second harmonic waveform with good agreement, regression coefficient, R2 = 0.9948.
• Limitation analysis demonstrated the good applicability of this technique in gas sensing.

Application of second harmonic signal waveforms in distributed feedback diode laser (DFB DL)-based wavelength modulation (WM) spectroscopy is limited by the waveform distortions arising from modulation characteristics of DFB DL. The principle and implementation of a technique for the recovery of pure-WM second harmonic signal waveforms are proposed and particular attention is devoted to the problem in DFB DL-based photoacoustic spectroscopy (PAS). The disturbance factors responsible for the distorted second harmonic signal are analyzed, and their characteristics are utilized to eliminate the distortion. A simple and practical prototype for water vapor detection is assembled to demonstrate and validate this technique. The waveform of the recovered second harmonic signal in WM-PAS using the technique matches that of a calculated pure-WM second harmonic signal using a Fourier decomposition of the Lorentzian profile well with a high regression coefficient, R2 = 0.9948. This offers the advantage of simplifying the spectral analysis, which, otherwise, would require a numerous calculation if the distortions were considered.