Narrow-linewidth External-cavity Diode Research Articles

Revising the H2 16O line-shape parameters around 1.1 μm based on the speed-dependent Nelkin–Ghatak profile andthe Hartmann–Tran profile

Accurate spectroscopic data for H2 16O in the 1.1 μm region are particularly important for the study of Earth’s atmosphere. The pure water vapor molecular spectra were measured based on direct laser absorption spectroscopy using a narrow line-width external cavity diode laser combined with a high-precision Fabry–Pérot etalon. A total of 31 H2 16O transitions were studied for the first time by using the speed-dependent Nelkin–Ghatak profile and the Hartmann–Tran profile. From an accurate line-shape analysis, we obtained the line intensities and the self-broadening coefficients, and they are compared with the available data reported in the HITRAN 2016 database and the HITRAN 2020 database. Finally, we obtained information on the influence of Dicke narrowing, as well as the correlations between Dicke narrowing and speed dependence, and of speed-dependent effects.

CO<sub>2</sub>-broadened coefficients of water vapor molecule in 1.1 μm band

The absorption spectral parameters of water vapor molecules are the key basic scientific data for the remote sensing detection and the planetary observation applications. Based on a narrow line-width external cavity diode laser and a long-path absorption cell, 18 absorption spectral lines of CO<sub>2</sub>-broadened water vapor molecules in a 9332–9722 cm<sup>–1</sup> range at room temperature are measured. To obtain the CO<sub>2</sub>-broadened water vapor molecule coefficients, the Voigt profile and the quadratic speed-dependent Voigt profile are used to fit the absorption spectrum data. The quadratic speed-dependent Voigt profile shows better fitting capability. Comparing with the air-broadened coefficients of the corresponding region from the HITRAN2020 database, the mean ratios of the CO<sub>2</sub>-broadened coefficients of water vapor molecules and the air-broadened coefficients obtained from the two models of the line shape are 1.327 and 1.454, respectively, which verifies that the method of estimating the CO<sub>2</sub>-broadened coefficient by the air-broadened coefficient of water vapor molecules has certain reliability. This study can provide reference data of measured spectral parameters for the detection technology and related research of atmospheric structures of Mars and Venus in the near-infrared region.

Study on dual-optical paths for simultaneous measurement method of water vapor absorption spectrum in 1 μm band

Based on the gas multi-pass absorption cell with dual-optical paths (long optical path: 72.46 m; short optical path: 36.23 m), a measurement method of simultaneously detecting water vapor absorption spectra is advanced. Combining with a narrow line-width external cavity diode laser and a high-precision Fabry-Perot etalon, a high-resolution simultaneous measurement device with dual-optical paths for water vapor absorption spectra in 1 μm band is developed. Since the external cavity diode laser has excellent polarization characteristics which could be combined with a half-wave plate and a polarization beam splitter to implement the laser transmissions in dual-optical paths simultaneously. Both the multi-pass absorption cell and the Fabry-Perot etalon in the measurement device have pressure and temperature control units, which are utilized for achieving ambient stability. The free spectral range of Fabry-Perot etalon is accurately measured by the method of optical comb frequency. Corresponding free spectral range with a deviation of only 0.02 % from the theoretical value is obtained to be a value of 749.52 MHz, and the influence of temperature on the frequency shift of etalon is less than 1 % of the measured value. The stability of the pressure and the temperature in the dual-optical path gas multi-pass absorption cell in the system are evaluated in detail, and the calculated relative errors are not more than 0.03 % and 0.02 %, respectively. At a temperature of 300 K, the system is used to measure the absorption spectra of water vapor at 9152.53 cm<sup>–1</sup> from 400 Pa to 2000 Pa on dual-optical paths, then the integrated absorbance and Lorentzian line-width of water vapor for long optical path and short optical path are inverted by fitting absorption spectra with Voigt profile respectively. The absorption line intensities and self-broadening coefficients are acquired by performing linear fitting to the integrated absorbance and Lorentzian line-width under different pressures. And the relative deviations of the average values of the dual-optical path absorption line intensities (converted to the reference temperature of 296 K) and the self-broadening coefficients and the corresponding data of the HITRAN2016 database are 0.78 % and 3.8 %, respectively. Consequently, the feasibility of the dual-optical path simultaneous measurement method and the reliability of the measurement device are demonstrated by the results.