A high-accurate and universal method to characterize the relative wavelength response (RWR) in wavelength modulation spectroscopy (WMS)

Abstract

Considering the importance of the laser wavelength response and the difficulty in its real-scenario measurement in WMS, a high-accuracy and universal method was developed to characterize the relative wavelength response (RWR) by analyzing the laser current response. A coupling term that depends on both the current scan and the modulation characteristic was introduced to describe the coupling effect between the wavelength scan and modulation. The accuracy of the proposed method was verified with different laser working conditions and scan waveforms. All fitting residuals of the RWR result from the proposed method are smaller than 0.1% of the total scan range and the fitting residual of the ramp scanned WMS is twice smaller than the minimum value from literature. The better calibration-free 2f/1f fitting and more accurate CO2 concentration results also suggest the high accuracy and superiority of the proposed method. Finally, based on the precise prediction of RWR with small scan and modulation indices, the spectral parameters, including line strength and self-collisional broadening coefficient, of CO2 transition at 6976.2026 cm-1 were successfully measured using WMS.