Atmospheric and Oceanic Optics|3 Article(s)
Sensitivity of on-line wavelength during retrieval of atmospheric CO2 vertical profile
Wei Gong, Ailin Liang, Ge Han, Xin Ma, and Chengzhi Xiang
Accurately measuring the differential molecular absorption cross section is the key to obtaining a high-precision concentration of atmospheric trace gases in a differential absorption lidar (DIAL) system. However, the CO2 absorption line is meticulous at 1.6 μm, easily translating and broadening because of the change of temperature and pressure. Hence, measuring the vertical profile of atmospheric temperature and pressure to calculate the vertical profile of the CO2 weight parameter is necessary. In general, measuring atmospheric temperature and pressure has a certain amount of uncertainty. Therefore, this study proposes the concept of a balanced on-line wavelength, where the differential molecular absorption cross section is larger and the CO2 weight parameter is insensitive to the uncertainty of atmospheric temperature and pressure. In this study, we analyzed the influence of uncertainty on the CO2 weight parameter at every preselected wavelength, as well as determined an appropriate wavelength near one of the absorption peaks. Our result shows that 1572.023 nm should be one of the appropriate balanced on-line wavelengths. The measurement errors of the mixing ratio of CO2 molecule in this wavelength are only 0.23% and 0.25% and are caused by 1 K temperature error and 1 hPa pressure error, respectively. This achievement of a balanced on-line wavelength will not only depress the requirement of the laser’s frequency stabilization but also the demand for measurement precision of the atmospheric temperature and pressure profile. Furthermore, this study can achieve the exact measurement of the vertical profile of atmospheric CO2 based on an independent differential absorption laser.
Photonics Research
  • Publication Date: Jun. 11, 2015
  • Vol.3 Issue, 4 04000146 (2015)
Bessel–Gauss photon beams with fractional order vortex propagation in weak non-Kolmogorov turbulence
Jie Gao, Yu Zhu, Donglin Wang, Yixin Zhang, Zhengda Hu, and Mingjian Cheng
We model the effects of weak fluctuations on the probability densities and normalized powers of vortex models for the Bessel–Gauss photon beam with fractional topological charge in the paraxial non-Kolmogorov turbulence channel. We find that probability density of signal vortex models is a function of deviation from the center of the photon beam, and the farther away from the beam center it is, the smaller the probability density is. For fractional topological charge, the average probability densities of signal/crosstalk vortex modes oscillate along the beam radius except the half-integer order. As the beam waist of the photon source grows, the average probability density of signal and crosstalk vortex modes grow together. Moreover, the peak of the average probability density of crosstalk vortex modes shifts outward from the beam center as the beam waist gets larger. The results also show that the smaller index of non-Kolmogorov turbulence and the smaller generalized refractive-index structure parameter may lead to the higher average probability densities of signal vortex modes and lower average probability densities of crosstalk vortex modes. Lower-coherence radius or beam waist can give rise to less reduction of the normalized powers of the signal vortex modes, which is opposite to the normalized powers of crosstalk vortex modes.
Photonics Research
  • Publication Date: Feb. 22, 2016
  • Vol.4 Issue, 2 02000030 (2016)
Wavelet modulus maxima method for on-line wavelength location of pulsed lidar in CO2 differential absorption lidar detection
Wei Gong, Chengzhi Xiang, Feiyue Mao, Xin Ma, and Ailin Liang
Differential absorption lidar (DIAL) is an excellent technology for atmospheric CO2 detection. However, the accuracy and stability of a transmitted on-line wavelength are strictly required in a DIAL system. The fluctuation of a tunable pulsed laser system is relatively more serious than that of other laser sources, and this condition leads to a large measurement error for the lidar signal. These concerns pose a significant challenge in on-line wavelength calibration. This study proposes an alternative method based on wavelet modulus maxima for the accurate on-line wavelength calibration of a pulsed laser. Because of the different propagation characteristics of the wavelet transform modulus maxima between signal and noise, the singularities of a signal can be obtained by detection of the local modulus maxima in the wavelet transform maximum at fine scales. Simulated analysis shows that the method is more accurate than the general method such as quintic polynomial fitting and can steadily maintain high calibration precision at different signal-to-noise ratios (SNRs). Last, 16 groups of real experiments were conducted to verify the simulated analysis, which shows that the proposed method is an alternative for accurately calibrating an on-line wavelength. In addition, the proposed method is able to suppress noises in the process of wavelength calibration, which gives it an advantage in accurate on-line wavelength calibration with a low SNR.
Photonics Research
  • Publication Date: Mar. 22, 2016
  • Vol.4 Issue, 2 02000074 (2016)