Study On Simulation Of Tunable Diode Laser Wavelength Modulation Spectroscopy

As one of the best potential technologies of real-time online gas monitory, tunable diode laser wavelength modulation spectroscopy (TDLWMS) has the advantages of high spectroscopy resolution, low detection limitation, fast response and good sensitivity. Benefiting from the rapid development of computer technology, we can make use of the virtual instrument platform to simulate the function of various instruments and finally carry out the TDLWMS simulation. The simulation of gas absorption signals and harmonic signals varying with the changed parameters such as gas concentration, absorption path length, temperature, pressure, modulation index and so on is helpful for understanding the physical progress and evaluating the feasibility of the TDLWMS. Moreover, it can offer us some suggestions on how to set parameters in actual experiments, especially in the wave range which has not been covered by the commercialized TDL where TDLWMS is applied. In this paper we performed the simulation of TDLWMS based on the Lab VIEW platform and lay a theoretical foundation for the application of TDLWMS in gas monitory.By simulating gas direct absorption signal and wavelength modulation spectroscopy, we achieved the simulation of TDLWMS. Based on the Beer-Lambert's Law, we performed the simulation of gas absorption signal by using Lab VIEW, where the necessary spectrum information of gas absorption line came form HITRAN database. To verify the correction of the simulation results, we compared the calculation results of different lineshapes with the Hitran-PC, which is the most popular commercial spectroscopy software currently. The discrepancy of absorption signals of CO2 in the wavenumber range of 4815cm-1 and 4835cm-1 between the simulation and Hitran-PC is less than 1%. We discussed the effect of temperature and pressure to the absorption signal. Take the absorption of 330ppm CO2 at line 4828.4cm-1 for example, a temperature-modify curve was given between 300K and 560K which the pressure was set to 0.1atm and the absorption pathlength was 1000 meters, and we also simulated a gas absorption signal changing with pressure using same absorption line of CO2 at 296K.Based on the fundamental principle of TDLWMS we derived the harmonic components after the absorption signal simulation has been performed and discussed the second harmonic signal distortion caused by intensity modulation. Subsequently we used Lab VIEW program to simulate the modulation and demodulation of absorption signal. We measured the basic tuning index of TDL using etalon so that derived the parameters of TDL for the TDLWMS simulation. Based on the current condition of the lab, we simulated the second harmonic signal of CO2 absorption around 1.57μm, and got the relationship between gas absorption and the amplitude of second harmonic signal The influence of temperature and pressure to second harmonic signal has also been discussed. At last, we checked the correction of TDLWMS simulation by investigating the variation of the amplitude of second harmonic signals according to the modulation index theoretically and experimentally.