In the laser absorption spectrometry (LAS) system with the open atmosphere, scintillation caused by the atmospheric turbulence is the principal interference for the system accuracy. However, the scintillation could be used for monitoring of the gas flux because it includes the information of the flow of the atmosphere. Therefore we developed a LAS system and attempted to monitor both the water vapor density and the scintillation simultaneously and individually. In the system, the scan time for the absorption lines is set shorter than the principal fluctuation period of the scintillation for reducing the influence of the scintillation. Based on the actual data about temporal sequence of transmittance for the laser beam through the target air, it was found that the scintillation was almost frozen if the time for spectral scanning was done within about 0.1 second. With this scan time, some indoor experiments were carried out and their data were split into the absolute absorption of the water vapor and the scintillation with numerical filters. As the results, actual fluctuation data of the water vapor density buried under the scintillation noise appeared, and the correlation between the fluctuation of the water vapor and the scintillation was able to be estimated.
A Mie lidar system was built at Okayama University in 1998. The system is featured by the ability of discriminating depolarization of received signals with a spatial resolution of 15m. Examinations were conducted on the received rangenormalized signal and the depolarization ratio from Asian dust. Intense Asian dust occurred in springs of 1998-2000. Vertical distribution of Asian dust was found to be inhomogeneous contrary to the homogeneity assumption employed in preceding works. Multiple scattering in the layers of Asian dust also observed. Complicated structure of Asian dust wrapped by liquid or water droplets were also conjectured from the sudden changes of depolarization ratio in Asian dust layers.
This paper describes a velocimetry system using scintillation of a laser-beam with spatial filters based on sensor arrays for a laser- based gas flux monitor. In the eddy correlation method, gas flux is obtained by mutual relation between the gas density and the flow velocity. The velocimetry system is developed to support the flow velocity monitor portion of the laser-based gas flux monitor with a long span for measurement. In order to sense not only the flow velocity but also the flow direction, two photo diode arrays are arranged with difference of a quarter period of the weighting function between them; the two output signals from the sensor arrays have phase difference of either (pi) /2 or -(pi) /2 depending on the sense of flow direction. In order to obtain the flow velocity and the flow direction instantly, an electronic apparatus built by the authors extracts frequency and phase from crude outputs of the pair of sensors. A feasibility of the velocimetry was confirmed indoors by measurement of the flow- velocity vector of the convection. Measured flow-velocity vector of the upward flow agreed comparatively with results of an ultrasonic anemometer.
Fluctuation for the laser beams of wavelengths on and off absorption lines were recorded, and the power spectral density function for each record was calculated, and the co- spectrum of the two records as well. A system with a 7 micrometers band lead-salt semiconductor laser that can be changed its wavelength were made up for examining the correlation between the fluctuation of the laser beam of 7859.50 nm and of 7860.27 nm. The former hit two absorption lines of water vapor located at 7859.49 nm and 7859.51 nm and the later were out of them. This experiment was carried out on the in- door corridor using a round-trip propagation path. Three electrical heaters and a humidity source set on the floor under the propagation path, which disturbed the temperature and humidity of the environment. In addition a fan agitated the air. As the results, the power spectral density function for the on-line data had a peak in the lower frequency region at which a contribution of the absorption is strong. The peak frequency agree with a predicted frequency by the integral length scale L<SUB>0</SUB>. Moreover, the co-spectrum increased in the frequency region less than 0.1 Hz. On the contrary, it decreased dramatically in the frequency region higher than 10 Hz. The results tell us the lower frequency region reflects the transportation of water vapor in the air.
Infrared laser beam as suffer from absorption form various gas species not to say of water vapor or carbon dioxide. If the tunable diode laser is employed as source, we can obtain more information from the fluctuating absorption signal than ever by virtue of TDL's spectral purity and its quick tunability. By scanning an absorption spectrum around an absorption line, not sticking to the center of a line, spectral interference from an adjacent line or scintillation can be eliminated with a real-time digital signal processing. Based on the actual data about temporal sequence of transmittance for TDL beam through open atmosphere, it is found that the scintillation is frozen if the spectral scanning is done within 0.1 second, and the more the less. In order to facilitate this principle, received electronic signal is treated by a high speed digital signal processing system. A special integrated circuit device, DSP, is exploited of its high performance in numerical calculation speed. Elimination of spectral interference is performed with a personal computer based on an algorithm named as 'adjoint spectrum' invented by the authors. This is also implemented by DSP. Function of the test equipment the authors have built is shown.
Results of the field measurement of scintillation for a 7 micrometers band semiconductor laser beam are described. To investigate the contribution of absorption, wavelength of the IR laser was tuned to a H<SUB>2</SUB>O absorption line center. The scintillation for a 0.67 micrometers visible laser was also monitored to compare with the IR's scintillation. This experiment was carried out in the daytime in the winter as well as in the summer of 1993. The power spectral density functions (PSDFs) were calculated from the experimental results of each season. In winter, the level of the PSDFs for the visible light was found about 10 times bigger than that for IR light. The PSDFs agreed with the theory reported by Filho. In summer, the PSDFs levels of the IR light, however, rise up to those of the visible one. The measured PSDFs for the TDL disagreed with the theoretical PSDFs. The scintillation for the IR lights is weaker than that for the visible lights in the theory only based on the temperature fluctuation. However, our experimental results show that the PSDFs for IR light is as strong as that for the visible lights in the condition of the high temperature and humidity.