Time-to-frequency conversion method for tunable diode laser absorption spectrum

The dynamic wavelength tuning characteristics of the laser is one of the most important factors affecting the measurement accuracy of Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology. We propose a time-frequency conversion method to obtain accurate measurement spectrum, which uses experimentally measured direct absorption signal (time domain) of standard methane gases to compare with simulated spectrum (frequency domain) from the HITRAN database. Then, a polynomial model is used to establish the relationship between time domain (sample points) and frequency domain (wavenumber). To verify the feasibility and accuracy of the method, the methane absorption signals of three standard concentrations (5%, 10% and 15%) were time-to-frequency converted by our method and etalon, and the methane concentrations were calculated and compared. Comparing methane concentration results, the maximum relative error between our method and etalon is less than 0.5%. The results showed that it is viable to use this method to achieve dynamic wavelength measurement in fast scanning of distributed feedback lasers (DFB), and the restricted polynomial model can properly describe the dynamic wavelength tuning characteristics of lasers. Furthermore, the method has the advantages of high accuracy and low cost, which is important for improving the accuracy of TDLAS technology applications.