Demonstration of a spatial-spectral holographic LIDAR range-Doppler processor

We present a new approach to laser interferometric Doppler and ranging (LIDAR) processing using spatial-spectral holography (SSH). In this approach, broadband optical signals from a random noise or frequency-modulated laser are transmitted and reflected off remote targets. The return signals interfere spatially and spectrally with a local copy of the original transmit signal in an SSH medium, resulting in spectral gratings that have a spectral period inversely proportional to the LIDAR target's range and a position proportional to the target's Doppler (or velocity). These gratings are subsequently read out by a slowly chirped source onto a parallel detector array, and the velocity and range of the targets are inferred. We present the theoretical framework that describes the function of the LIDAR processor, as well as proof-of-concept experimental results.     

Description of a Doppler rayleigh LIDAR for measuring winds in the middle atmosphere

The possibility to measure winds in the middle atmosphere with a Doppler LIDAR was demonstrated in 1989. It has been used since then to study the wave-mean flow interaction, in association with the Rayleigh LIDAR providing density and temperature and their fluctuations. The Doppler LIDAR relies on Rayleigh scattering from air molecules and was originally designed to cover the height range 25–60 km, a region where radars cannot operate. The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of two narrow band-passes of a single dual Fabry-Perot interferometer tuned to either side of the emitted laser line. Its extension to lower altitudes where Mie scattering is present is under study.     

A stochastic model and a variance-reduction Monte-Carlo method for the calculation of light transport

We compare the transport theoretic and a stochastic approach of modeling light propagation in the atmosphere. Computations of LIDAR return signals using algorithms based on the two different approaches show very good agreement of the numerical data. Furthermore, a deeper analysis of the formulas for the LIDAR return signal obtained from two kinds of modeling shows that they are equivalent. Combining these approaches and introducing splitting techniques, variance reduction Monte Carlo methods and codes are designed which are adapted to the configuration of the LIDAR. These codes allow the calculation of multiple scattering effects with high accuracy. Finally, we point out the importance and some perspectives of ultilization of multiple scattering in laser remote sensing of clouds.     

A method based on iterative morphological filtering and multiple scattering for detecting layer boundaries and extinction coefficients with LIDAR

Layer boundaries detection with LIDAR is of great significance for the meteorological and environmental research. Apart from the background noise, multiple scattering can also seriously affect the detection results in LIDAR signal processing. To alleviate these issues, a novel approach was proposed based upon morphological filtering and multiple scattering correction with multiple iterations, which essentially acts as a weighted algorithm with multiple scattering factors in different filtering scales, and applies integral extinction coefficients as media to perform correction. Simulations on artificial signals and real LIDAR signals support this approach.     

A LIDAR technique to measure the filament length generated by a high-peak power femtosecond laser pulse in air

We demonstrate a new method for measuring the length of a femtosecond laser pulse induced filament in air using a LIDAR (LIght Detection And Ranging) technique. The LIDAR involves a detector with a fast response time. The back-scattered multiphoton induced fluorescence from nitrogen molecules excited inside the filament is measured, from which the length of the filament can be determined. We find good agreement between the measured filament length and the length estimated from burn patterns on paper. In addition, good qualitative agreement between the experimental measurement and numerical simulations is obtained for the signal features of the filament. We propose that this new method can be used to quantitatively determine filamentation at longer distances. Received: 11 June 2002 / Revised version: 21 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-418/656-2623, E-mail: aiwasaki@phy.ulaval.ca     

A four-wavelength depolarization backscattering LIDAR for polar stratospheric cloud monitoring

A four wavelength backscattering depolarization LIDAR designed for polar stratospheric cloud and stratospheric aerosol measurement is described. The system uses the following wavelengths: 355 nm, 532 nm, 750 nm, and 850 nm. These wavelengths, obtained by means of the third- and second-harmonic of a Nd: YAG laser and by means of a tunable Ti: Sapphire laser, are chosen in a way to better characterize the particel size of such stratospheric aerosols. They are not emitted simultaneously as the LIDAR system is designed with only two detection channels permitting to detect, in the analog and in the photon counting mode, both the direct and the depolarized backscattered signal. The system has been operational in northern Finland since the end of November 1991.     

Dynamic range in terahertz time-domain transmission and reflection spectroscopy

We present a quantitative method for identification of the dynamic range of the detectable absorption coefficient in the analysis of transmission terahertz (THz) time-domain spectroscopy data. In transmission measurements the largest detectable absorption coefficient is determined by the dynamic range of the THz signals, whereas in reflection measurements the largest detectable absorption coefficient is determined by the scan-to-scan reproducibility of the signal.     

Promoting the Range and Range Resolution of a LIDAR (DIAL) System Using a Suitable Pinhole Plasma Shutter

Pulsed transversely exited atmospheric (TEA) CO₂ lasers, employed extensively in various applications such as light detection and ranging (LIDAR), have a pulse duration of about a microsecond due totheir nitrogen tail. In order to promote the measurement accuracy and the mean power of the laser pulse, the pulse duration should be shortened. In this research, we present the details of making a passive pinhole plasma shutter for a LIDAR (DIAL) system, which shortens the pulse duration of CO₂ lasers from 1.5 μs to 25 ns in air at atmospheric pressure. This instrument increases the range resolution of the LIDAR system from 225 to 3.75 m. Also we show the results of investigation of the clipped pulse duration of the microsecond CO₂ laser pulse using aluminum and copper pinhole metal targets with different pinhole diameters (1.5 and 1.8 mm) and at various laser output energies (338 and 309 mJ). Our experimental results show that the aluminum pinhole is more suitable than the copper pinhole for shortening the nitrogen tail of the CO₂ laser pulse with a larger output average power. Thus, the range of the LIDAR system, which is proportional to the logarithm of the output pulse power, is increased.     

Extended-range optical time domain-reflectometry system at 1.65 microm based on delayed Raman amplification

We describe both theoretical and experimental results obtained in an investigation of a new technique for increasing the dynamic range of 1.65-microm optical time domain-reflectometry (OTDR) systems. The technique utilizes delayed Raman amplification of a 1.65-microm signal pulse by a 1.53-microm pump pulse. Amplification occurs when the two pulses overlap, and this position is determined by the initial delay between the pulses and the fiber dispersion. An increase in dynamic range of 17.5 dB has been observed, and the OTDR backscattered Rayleigh signal was detected up to 100 km. No significant noise penalty is introduced as a result of the directionality of the Raman gain.     

360°连续扫描的便携式三维激光雷达设计与开发

针对三维激光雷达在国内有巨大的应用市场,而国外商业三维激光雷达十分昂贵的现状,开发了一种360°连续扫描的便携式三维激光雷达系统。采用一个二维激光扫描仪与高精度转台连接,通过步进电机控制形成三维激光扫描。在分析系统误差来源的基础上,提出了系统误差校正方法,给出了三维坐标精确计算公式。实验结果表明,系统测量精度高,数据质量好。系统作用距离80 m,测距精度可达6 mm,测量速度每秒7256点,可满足室内室外大规模场景三维数据快速获取的需求,而成本仅是国外同类商业三维激光雷达价格的四分之一左右,且重量轻体积小,携带方便。     

A simple method to significantly increase filaments’ length and?ionization density

A simple method to produce longer filaments with higher ionization density in air by controlling the diameter of an aperture in the laser beam path is studied via an analysis of the backscattered N₂ fluorescence collected by LIDAR. Significant increase in the fluorescence signal (approximately by a factor of five depending on the conditions) and an increased filament length was observed at an optimum diameter. 3D + time stochastic numerical simulations have shown that the optimum aperture size corresponds to the case of multiple filament ‘squeezing’ around the propagation axis forming the regularized elongated structure with higher overall amount of plasma. The optimum range of aperture sizes is the same for the initial transverse perturbation scale variation at least within a factor of three.     

Current state-of-the-art of LIDAR inversion methods for atmospheres of arbitrary optical density

It is the intention of this paper to report on the currently used methods to solve the different LIDAR signal inversion problems for molecular atmospheres, aerosols and clouds. Apart from more traditional approaches, we shall present a recent one using multiple scattering effects rather than avoiding them, which is useful especially for dense clouds.     

False alarm and detection probability factors and their implementation on LIDAR design

A statistical model has been developed for the analysis and design of Laser Radar systems (LIDAR).A set of equations, developed for conventional Radar systems, was reconfigured and adapted to the optical case.The analysis and the related developed set of equations were dedicated to the case of detecting co-operative retro reflector carrying targets against background which is a composition of clutter, noise and backscatter sources.The equations developed were evaluated against realisitic system and atmospheric conditions presenting the relations between detection and false alarms probabilities and effective range. This method is very useful in evaluation and design of LIDAR Systems.     

线阵CCD用于实时动态测量技术研究

本文研究的是在连续光条件下线阵ＣＣＤ实时动态测量技术,除了考虑ＣＣＤ像元响应非均匀性、非线性等影响因素外,还应认真考虑采样频率、测量精度、动态范围、快速电路等问题。本文还详细讨论了测量精度和动态范围问题,推导了测量精度公式与动态范围公式。为了减小测量误差和提高被测信号的频率,必须相应减小积分时间。精度公式是进行线阵ＣＣＤ电路设计的基本依据公式。动态范围与频率无关,只与结构参数有关,为了充分利用ＣＣＤ的动态范围,激光功率要可调。     

Monte-Carlo calculations of cloud returns for ground-based and space-based LIDARS

A Monte-Carlo model is described which has been developed for calculation of multiply scattered LIDAR returns. Results are shown for the common problem selected by the MUSCLE (MUltiple SCattering LIDAR Experiments) group for intercomparison, which represents a typical ground-based cloud-sensing scenario. This is contrasted with returns from the same cloud sensed by a space-based LIDAR, where multiple-scattering effects are much greater. The magnitude of multiple-scattering effects is seen to be largely determined by the optical depth across the receiver field of view at the cloud.     

Deconvolution filtering of ground-based LIDAR returns from tropospheric aerosols

A deconvolution filtering model of multiple scattering in ground-based single field of view (SFOV) LIDAR returns is described. It is based on time series deconvolution techniques. The contribution of multiply scattered photons in SFOV LIDAR returns can be numerically modeled by processing LIDAR signals without additional information about aerosol properties and measurement geometry. Deconvolution results are in good agreement with those performed by Monte Carlo calculations, showing that the significance of multiply scattered photons is strongly correlated with aerosol concentration. It is found that, for ground-based LIDAR, the contribution of multiply scattered photons to LIDAR signals is typically below 5% in a clear urban atmosphere, and up to 14% in a very dirty urban atmosphere in Hong Kong during winter seasons. Received: 8 October 2002 / Revised version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-8229, E-mail: gao@proteo.gr.jp RID="**" ID="**"Present address: Preteomics Lab., Amakubo 1-16-1, Tsukuba 305-0005, Japan     

Analytical solution to LIDAR return signals from clouds with regard to multiple scattering

An analytical approach to LIDAR return signal calculation with regard to multiple scattering is suggested. Two versions of the method are developed. The first one is completely analytical and undispensible for qualitative studies. The second semianalytical approach provides a sufficient accuracy up to a sounding optical depth about 5. Being somewhat more tedious than the proposed analytical solution, the second approach appears to be time saving in comparison with known methods.     

Use of the application of heterodyne laser interferometer in power ultrasonics

We are particularly interested in the measurement of nonlinear acoustic waves in air, within the range of the Raman-Nath diffraction regime. We used a heterodyne laser interferometric system with a Bragg cell frequency of 80 MHz. A harmonic power acoustic wave with a frequency in the range of 20 kHz was generated, but higher harmonics appear when nonlinear propagation occurs in restricted volumes or waveguides. This topic raises a number of interesting questions. The most complex problem arises when processing an extremely wide-band output signal from a photo-detector, where the signal contains a huge number of high amplitude harmonics in a measured acoustic signal. The wide-band output signal from nonlinear wave measurement was simulated by means of similar wide-band output signals obtained by measuring known dynamic mechanical displacements. Because the frequency band of common analog circuits used for signal processing is too narrow, the output signal from the interferometer was digitalized and processed. This paper discusses the results of these tests and provides estimates of the frequency restrictions of signal processing.