**3. Metal vapor and Nd:YAG lasers: Basic parameters**

The requirements for successful lidar atmospheric sensing impose strong limitations on the laser parameters as the pulse width, pulsed and mean powers, repetition frequency, operational wavelengths, stability, etc. That is why the number of laser types applied in lidars is limited. The Nd:YAG lasers are widely used in the most of lidar systems (Measures, 1984; Weitkamp, 2005) providing simultaneous sensing in analog and photon counting modes at typically 4 to 6 wavelengths (using harmonic generation techniques) in the IR, visible and near UV ranges (including Raman channels). Lasers emitting a set of basic wavelengths of proper parameters, say approximately equal output powers, are also of great importance for multiwave lidar atmospheric sensing. The use of such lasers can simplify the opto-mechanical lidar design.

The MV lasers eligible for lidar probing (in the sense of above requirements) are mainly lasing on two active media, namely copper (Cu) and gold (Au) vapors. They offer unique output parameters (Astadjov et al., 1988; Kim, 1991; Stoilov et al., 2000) attractive for development of lidars in the mid-visible range, capable to probe simultaneously the troposphere and stratosphere. These lasers emit pulses with mean power of up to 2 kW at relatively high repetition frequencies, normally ranging from 2 KHz to 100 KHz, depending on the laser type. The pulsed energy is substantially low (~0.1 mJ at 5-10 ns pulse duration). The combination of low pulse energy, high mean power, high repetition frequency and multiwavelength performance of MV lasers are their key advantages for application in lidar

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers 355

the visible range. The simultaneous use of laser radiations covering the green-to-red part of visible range can provide better description of the backscattering wavelength dependence on the aerosol particle sizes, especially in multimode size distributions. Efficient sources of radiation in this spectral domain are the mentioned above Cu- and Au- vapor lasers. The combination of lidars based on these lasers with Nd:YAG lidars operated at 1064 nm and 532 nm offers additional advantages to cover practically the mid-visible and near IR ranges,

In Arshinov et al., (1983) the application of Cu-vapor lidar for measuring profiles of the atmospheric humidity and temperature is described. The Cu-vapor lidar system is used for detection of stratospheric aerosol layers at heights of up to 28 km (Kolarov et al., 1988). The scattering ratios (aerosol and molecular to molecular) exceeding 1.5 within the height range 22-28 km are measured at the wavelength of 510.6 nm (1-3 W mean power, 5 KHz repetition frequency, 19cm telescope diameter, and 10-100 s accumulation times) with a photon counting system. A complex system, containing Cu-vapor (510.6 nm) and Au-vapor (627.8 nm) lasers, combined with MW and IR radiometers and MW radar is described in Stoyanov et al., (1988). The two lasers are synchronized in time. The output powers are of 1 -3 W at 510.6 nm and 0.5 W at 627.8 nm, at pulse repetition frequencies 5-15 KHz. The experimentally demonstrated operational heights are 30 km for 510.6 nm and 22 km for 627.8 nm at 100 s accumulation times. The short accumulation times provided opportunities for studying some dynamic processes in the lower stratosphere. One of the most important results of these experiments is the demonstration of good lidar performance in the PC mode within the ranges from the planetary boundary layer (PBL) heights (~750 m) up to 30 km in the stratosphere. The first lidar and MW radiometers remote sensing experiment is performed using Cu-vapor laser of parameters as given above (Gagarin et al., 1987). The operational wavelengths of MW radiometers are 0.8 cm and 1.35 cm to be sensitive to the free water in cloud droplets and the water vapor. The measurement accuracy for the brightness temperature is below 0.1 K. The accuracy of determining cloud water content and water vapor is ~ 0.5% and 1%, respectively. In this schematic, the lack of range resolution of the radiometers is compensated by the good lidar resolution. In series of experiments the links between the time variations of the backscattered time-resolved lidar signals and the brightness temperatures at the radiometer wavelength are demonstrated and analyzed.

The aerosol lidar with CuBr-vapor laser developed at Laser Radars Lab of the Institute of Electronics (LRL-IE), Bulgarian Academy of Sciences, is shown in Fig.3a (Grigorov et al., 2010). The CuBr-laser generates pulses at high-repetition frequency of 13 kHz (10 kHz in the upgraded version), with duration of 10 ns at 510.6 nm and 578.2 nm. Laser beam is directed vertically upward. Two Cassegrain telescopes with 15 cm aperture and 2.25 m focal length receive the backscattered radiation at the two wavelengths. A registration in PC mode is applied. Received backscattered lidar signals are stored in the computer by means of a photon counting board providing spatial resolution of 15-30 m, in 1024 samples and averaging time of 1 min. The maximum height is 15 km, limited by the laser pulse repetition frequency. Under daytime conditions, the sounding height decreases to about 4-5 km, due to intensive sky illumination, reducing the signal-to-noise ratio (SNR). Each lidar measurement lasts about 3-4 hours and more. The lidar profiles, integrated over accumulation time of 1 min, are additionally averaged by summation of data from 30 profiles. Thus, the measurement time for each profile amounts to 30 min. The stored data are subsequently processed by Fernald's algorithm, using a program in MATLAB

where the absorption effects can be neglected as a rule.

environment, developed in LRL-IE.

remote sensing. Typically, output powers of 1-3 W are sufficient for probing simultaneously the troposphere and stratosphere.

Most of the available Cu-vapor lasers operate using one of the two active substances - pure Cu or CuBr. They emit two basic spectral lines (at 510.6 nm and 578.2 nm) of close mean output powers. The beam and temporal characteristics at both lines are practically equal. These lasers provide Gaussian beams of low intensity fluctuations and simultaneous emission of pulses at both wavelengths. The CuBr lasers are more preferable for lidars, because of their very low readiness time (~5-15 min), due mainly to the quite lower working temperature of about 450 0C as compared to 1500 0C -1700 0C for lasers on pure Cu.

Lasers on Au-vapor emit radiation basically at a wavelength of 627.8 nm with temporal and beam parameters quite similar to these of pure Cu-vapor lasers. The combination of Cu- and Au-vapor lasers in a single laser tube is very attractive for lidar applications. Mean output powers of ~0.5 W for each of the three lines 510.6 nm, 578.2 nm and 627.8 nm will be sufficient to cover heights above 15 km in PC mode.

Conventional flashlamp-pumped Nd:YAG lasers, operating in Q-switching mode, provide pulses of 5-10 ns FWHM, at 10-100 Hz typical repetition rates, with extremely high pulse energy reaching more than 1 J at the fundamental wavelength (1064 nm) and up to hundreds of millijoules at the second (532 nm), third (355 nm), and fourth (266 nm) harmonics. These lasers can be designed for simultaneous operation at all the available wavelengths or at optional combinations of them, being by this manner powerful multiwavelength sources of UV, visible, and IR light. They exhibit perfect shot-to-shot energy stability (instabilities of ≤±2%), long-term power drift of less than ±3%, and temporal jitter of ≤1 ns. In addition, Nd:YAG lasers possess excellent beam-quality characteristics such as Gaussian spatial profile, beam divergence down to 0.5 mrad, pointing stability of < 50 μrad, and polarization ratio > 80%. Diode-pumped Nd:YAG lasers, using bars of powerful laser diodes instead of flash-lamps, are also commercially available. They combine the features quoted above with compactness and high averaged power at kHz pulse frequencies.
