**LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers**

Dimitar Stoyanov, Ivan Grigorov, Georgi Kolarov, Zahary Peshev and Tanja Dreischuh *Institute of Electronics, Bulgarian Academy of Sciences, Sofia Bulgaria* 

## **1. Introduction**

344 Advanced Photonic Sciences

[55] E. Devaux, T. W. Ebbesen, J.-C. Weeber and A. Dereux, *Appl. Phys. Lett.*, 83, 4936

[56] T. J. Wang, J. Y. Chu, Y. C. Chang, M. W. Lin, J. T. Yeh, and J. K. Wang, unpublished.

(2003).

LIDAR systems have widely been used for remote investigation of atmospheric parameters (Measures, 1984; Kovalev & Eichinger, 2004; Weitkamp, 2005). They are based on the socalled LIDAR (LIght Detection And Ranging) principle which consists in sending a laser pulse to the atmosphere and subsequent detecting of the radiation backscattered (at angle *π*) by atmospheric constituents or pollutants. As LIDAR is a time-of-flight technique, the return signal profile detected in the time domain contains range-resolved information about the atmospheric characteristics along the line of laser beam propagation. Advantages of the lidar measurement approaches, as compared to other available active techniques (e.g. radars), are the high spatial and temporal resolution, higher sensitivity and accuracy in sensing atmospheric particles, covering large observation areas, etc. These features make lidar systems powerful instruments for environmental measurements. At present, lidars find a variety of applications in different fields of the human activity. Along with the meteorology, atmospheric physics, and ecological monitoring, lidars are extensively used for volcanic and fire alerting, laser ranging, altimetry and bathymetry, lidar mapping and forestry, coastal morphology and hazards assessment in geology, as well as for many other applications in physics and astronomy, nuclear fusion, military, aviation, robotics, transportation, etc. There exists a variety of ground-based, air-borne and space-borne lidar systems distinguished by their types, schematics, regimes of operation, monitored parameters, constructions, etc. (Kovalev & Eichinger, 2004; Weitkamp, 2005). Among the most widely used systems are the one- or multi-wavelength aerosol lidars exploiting elastic scattering of light.

The present chapter describes the capabilities of LIDAR sensing techniques for atmospheric aerosol profiling by using elastic-scatter lidars based on metal vapor (MV) lasers, as well as on Nd:YAG lasers. First, a brief overview of the basic principles of the LIDAR remote sensing of the atmosphere is given. The single-scattering equations connecting return signal profiles, parameters of the experimental system and characteristics of the probed aerosols along the laser beam are presented in Sec.2, as well as some theoretical approaches for solving the lidar equations in the case of non-absorbing atmosphere (aerosol and molecular). General lidar schematics and methods for detection of lidar signals are also discussed. Special attention is paid to metal vapor and Nd:YAG lasers (Sec.3), and to advantages of

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers 347

point of the telescope, provides angular spatial filtering of the backscattered radiation, forming by this manner the telescope's field of view in conformity with the laser beam divergence. An important part of the multiwavelength lidar receivers is the optical module for wavelength- and/or polarization separation and discrimination. It represents an optical assembly containing dichroic beam-splitters forming the lidar spectral channels for initial wavelength separation of the backscattered laser radiation. Narrow bandpass (1-3 nm FWHM) interference filters are usually placed in each of the spectral channels, providing the main fine spectral selection of the corresponding wavelengths and suppressing the solar radiation background, especially in day-time measurements. Residuals from the other laser wavelengths are further suppressed by using additional spectrally-selective optical elements (e.g. edgepass filters), leading to final transmission of less than 10-4% in the blocking spectral regions. As a result, a good enough signal-to-noise ratio is normally achieved, allowing reliable detection of weak lidar signals and, hence, reaching high altitudes of lidar sounding. Lidar spectral channels can be equipped for measurement of the depolarization ratio of aerosol backscatter on defined wavelengths as it

Spectrally selected optical lidar signals are detected and converted to electrical ones by using highly-sensitive photodetectors such as photomultipliers, avalanche photodiodes, and CCD-cameras. Further, the received signals enter the acquisition system which provides sampling and pre-processing of raw signals to standard lidar data. The acquisition system is designed to operate in either analog or photon-counting modes, depending on the lidar type, laser pulse energy, measurement tasks, etc. The acquired lidar profiles are stored and

is important for aerosol particle shape characterization.

Fig. 1. General block diagram of a lidar system.

using them in the lidar remote sensing of the atmosphere (Sec.4). Then, in Sec.5, a number of experimental results on lidar atmospheric sensing, obtained with these systems are presented. Reported are measurements focused on evaluations and range resolved profiling of a defined set of important optical characteristics of the atmospheric aerosol, such as backscattering and extinction coefficients, Ångström exponent, etc., as averaged over time so in their temporal evolution. This is in view of the close relation of these characteristics to the spatial distribution, concentration, size parameters, and dynamics of the atmospheric aerosol content. Also, the use of lidars for ecological measurements and detection of hidden aerosol pollution transport by lidar mapping, including the trans-border pollution transport over the Danube River, is shown. Special consideration is given to applications concerning remote sensing of different atmospheric phenomena such as volcanic ash and Saharan dust, to parallel observations with space-borne lidars, etc. In Sec.6, the role of multi-wavelength aerosol lidar probing in the mid-visible and near infrared (IR) ranges is underlined, as a powerful and reliable approach for atmospheric observations providing accurate rangeresolved profiling of valuable atmospheric parameters with high spatial and temporal resolution.
