Zongjia Qiu

A new method for obtaining aerosol backscattering ratio with partial pure rotational Raman spectrum

The error of retrieving the aerosol backscattering ratio with partial Raman spectrum is calculated in present paper. A new method is presented for reducing the error. One low-level and one high-level spectrum line are chose to make up a new pseudo-line independent on the temperature. Finally, aerosol backscattering coefficient ratio profiles of the atmosphere were obtained from the signals of a rotational Raman Lidar in our lab. The results show that Tropospheric aerosol backscattering coefficient ratio can be retrieved by using the new rotational Raman pseudo-line without considering the relation between aerosol extinction and backscattering.

SNR analysis according to the practical factors for Mie-scattering lidar

The effectiveness of the lidar scattering signal can be intuitively reflected by SNR. Thus the effective detection range can be objectively evaluated. Usually by theoretical analyzing the devices of lidar signal detection system, an ideal SNR model of scattering signal can be built. Under these circumstances the theoretical SNR might be an inflated value. For instance, when the detection distance is 20Km the SNR can be up to 20dB. However, in the practical experiment the 10Kms SNR is merely 3dB. From the results, it can be deduced that the effective distance range is far less than the simulation value. Consequently, other practical factors must be considered in SNR analysis for Mie-scattering lidar. In the paper the results of Mie scattering lidar experiment are shown firstly. Then the SNR is calculated through deducting the background noise. The background radiation specific gravity is also presented in this paper. The photomultiplier tubes dark-current is detected. And the effect of all noise components to the SNR is estimated. Meanwhile, some improve measurements to reduce the interference noise are mentioned.

The influence factors and optimization methods of the local oscillator power for coherent doppler lidar

Coherent doppler lidar for wind is based on the heterodyne detection between local oscillator signal and the echo signal. Optimum optical local oscillator power is an important factor of the signal to noise ratio. The dynamic range of echo signal, the relative intensity noise and the photoelectric detector saturation effect are studied comprehensively as the local oscillator power influence factors for the first time. The expression of the local oscillator power and SNR is derived. Using the computer simulation, the conclusion that the dynamic range of the echo signal will not affect the optimum of the local oscillator is obtained. Selecting different values of the relative intensity noise, the curves of the SNR versus local oscillator are given. Comparing the SNR formula in the quantum limit with the derived SNR formula, the saturation effect of the photoelectric devices is studied.

Comparing methods for retrieving aerosol extinction coefficient with U.S. Standard Atmospheric Model and Temperature Gradient

Aerosol extinction coefficient in troposphere is retrieved from Raman-Mie lidar returns. Both the basic principle and the two retrieving methods are introduced in this paper. Calculated by U.S. Standard Atmospheric Model and Temperature Gradient, the latter of which means temperature value may decrease with the rising of the altitude, their results are compared so that the differences between the two can be clearly seen. A more suitable condition, which matches the U.S. Standard Atmospheric Model more and which matches Temperature Gradient, is presented.

Longitudinal resolution improving of 3D range imaging lidar through redundant detection and intensity distribution analysis

3D detection is an important application of Lidar. A 3D range imaging Lidar system is presented in this paper. The longitudinal resolution of 3D range imaging Lidar is poor because of the length of pulse width and gate time of ICCD, which together determine the detected longitudinal range from a single laser pulse shot. To improve the longitudinal resolution, power distribution received by one pixel of ICCD is analyzed, and a method is put forward. In this method, by setting the gate time and step interval of delay time to the value of pulse width, one object will be detected in two neighboring images, and it can be precisely located through analysis of the pixel values in the two images. The locating precision of this method is verified by experiments, and results show that the longitudinal resolution is improved by ten times, that is from 1.65m to about 0.15m. Meanwhile, the detection efficiency is reduced only a little.

Research on the application of lidar extinction coefficient inversion method

As the extinction coefficient is related to directly many atmosphere character, it is important to get extinction coefficient of atmosphere. Lidar has been proven to be a kind of useful method for remotely sensing the optical properties of atmospheric aerosols. The relationship between the Mie backscattering signal intensity and the detection range can be accurately achieved by Mie Lidar, and the atmospheric aerosol extinction coefficient can be retrieved from the backscattering signal. In this paper, a practical 532nm system of Lidar on the basis of MIE and Raman scattering, which consists of laser transmitter, signal receiver and data processing system, is presented and some experimental results have been gathered. In last some years, there are many data processing methods in inversion of aerosol extinction coefficient. Among these algorithms, Fernald inversion method is adopted abroad to obtain optical parameters in inhomogeneous atmosphere. This paper discusses the data processing method gives the extinction coefficient profile. At last, the effect of some estimated parameters is discussed.

Weak signal detection system and noise analysis for aerosol detection lidar

The information of location and scattering intensity of the target can be detected by lidar system. The physical characteristics of the target can be retrieved from the scattering intensity. Since the corresponding relationship between the echo signal intensity and the detection range requires to be measured accurately, a weak signal detection system of lidar is needed, with strong electromagnetic immunity, high sensitivity and wide dynamic range. In this paper, firstly, various characteristics of echo signals obtained by aerosol detection lidar, including the signal intensity and frequency spectrum are analyzed. Secondly, the influencing factors associated with those signal characteristics, such as photoelectric detector response time and the dynamic range, are also described. At last, a signal-to-noise ratio model for lidar is established. The influencing factors of the photomultiplier tube shot noise, dark current noise, and background radiation noise to the final SNR can be assessed. Meanwhile, some effective ways to reduce the interference noise are discussed.

Noise filtering of Rotational Raman Lidar using threshold amendment for atmospheric temperature measurement

Pure Rotational Raman Lidar is a new technique for detecting vertical troposphere temperature profile. Since the backscattering Raman signals are so weak that the photon-counting method is applied by using a multiscaler. By setting the threshold of the multiscaler, most noises mixed with the signals can be filtered. However, difference exists between the set threshold and the actual threshold of the two count channels, and also exists between the set thresholds of the two count channels. This will affect quick-setting of the appropriate thresholds. In this paper, a rule is put forward and the measuring system is established to confirm the set threshold value corresponding to an actual one, making use of the amplitude fluctuation of square wave. By this system, pairs of set threshold and actual threshold are obtained, and the curve of the set threshold against the actual threshold of a single count channel, as well as the curve of thresholds of the two count channels, is fitted. The theoretical analysis is well agreed with the experimental results, and it indicates that the appropriate threshold can be set more quickly by using the curve.

Application of digital pulse delay device in range-gated control for range-gated imaging lidar

Digital pulse delay device is one of the key techniques of range-gated imaging lidar. At present, Digital method and analog method are the two main implementations of pulse delay device. Digital method is mainly achieved by counter or FIFO memory. With the development of Complex Programmable Logic Device (CPLD), the digital delay device can be achieved with a single chip of CPLD. With this method, the digital delay device enjoys the advantages of high integration, high reliability and strong ability of anti-electromagnetic interference. However, since the maximum clock frequency of CPLD is limited, the improvement of temporal resolution is restricted. Analog method is mainly realized by the delay-line, which is one of the dedicated integrated circuit. Using this method, a higher time resolution can be arrived. In this paper, the timing characteristics of the delay signal are analyzed. Three design options are presented and the advantages and deficiencies are discussed. Based on the theoretical analysis and numerical simulation, the digital delay device combined with the delay chip AD9501 and Field Programmable Gate Array (FPGA) is chosen because of its large dynamic range and high accuracy. Besides, the output pulse width can be adjusted conveniently. The digital delay device is simulated and the result shows that the delay control for range-gated imaging lidar is feasible.