Satellite lidars are now beginning to provide new capabilities for global atmospheric sensing from space. Following the Lidar In-space Technology Experiment (LITE), which flew on the Space Shuttle in 1994, and the Geoscience Laser Altimeter System (GLAS), which launched in 2003, the CALIPSO satellite was launched on April 28, 2006. Carrying a two-wavelength polarization lidar along with two passive imagers, CALIPSO is now providing unique measurements to improve our understanding of the role of aerosols and clouds in the Earth's climate system. The primary instrument on CALIPSO is CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization), a two-wavelength polarization lidar. Using a linearly polarized laser and a polarization-sensitive receiver, the instrument allows the discrimination of cloud ice/water phase and the identification of non-spherical aerosols. First light was achieved in June, 2006 and five months of nearly continuous observations have now been acquired. Initial performance assessments and calibration activities have been performed and instrument performance appears to be excellent. CALIPSO was developed within the framework of a collaboration between NASA and CNES.

Ground based lidars are widely used all over for the study of physical and optical properties of aerosols and clouds in the atmosphere. The observed parameters on aerosols and clouds and their dependence on various meteorological parameters are being studied using the ground based lidars at different laboratories. But the results obtained are mostly applicable to local / regional particular to the lidar observation site. Space borne lidar is a unique system for observing the global distribution of aerosols and clouds. It provides vertical profiles of the physical properties of the clouds and aerosols with global coverage. Such data is useful for the validation of climate models and for process studies related to the climate change and also for studies on transport of aerosols and pollutants. Retrieval of optical properties of clouds and aerosols from the data obtained by the space borne lidar is very complex. Currently we are developing algorithms to produce calibrated data products for space borne and ground based lidars. A software to produce simulated lidar backscatter profiles applicable to space borne and ground based lidars has been developed, which generates data that matches the expected performance of the lidars under varying conditions. Output simulated data includes 1064 nm total backscatter profiles and 532 nm profiles for both the parallel and perpendicular polarization states. This paper describes the methodology used for inverting the ground based lidar data and the strategy for validating the data which will be obtained from the proposed space borne lidar to be launched by ISRO.

The space-time variability of aerosol inhomogeneities provides unique information on atmospheric behavior needed for climate and environmental research and operational programs. An additional indirect forcing from aerosols results from their involvement in nucleation and growth of cloud droplets, reducing droplet size and thereby potentially influencing cloud albedo. These studies have particular significance over tropics where the convective and dynamical processes associated with high-altitude thunderstorms greatly affect the vertical distributions of aerosols and pre-cursor gases. As the anthropogenic share of the total aerosol loading is quite substantial over many parts of the world, it is essential to monitor the aerosol features systematically over longer time scales. Such observations are very important for understanding the coupling processes that exist between physico-chemical, radiative, dynamical and biological phenomena in the Earth's environment, and provide valuable input information for modeling and simulation studies of climate and air quality. The multi-year aerosol number density data acquired during October 1986-September 2000 with a computer-controlled lidar at the Indian Institute of Tropical Meteorology (IITM), Pune, an urban station in India have been utilized to investigate (i) climate variability, (ii) cloud macro-physical parameters and (iii) environmental pollution. The results reveal a long-term trend in aerosol loading, single and multiple layer clouds with low cloud-base during the south-west monsoon months, and high pollution potential during winter late evenings. The trends in aerosol loading and air quality are found to be changing from year to year depending upon meteorological parameters (precipitation in particular). Some of these parameters have also been compared with co-located complementary facilities such as solar radiometers. In order to enlarge the scope of these studies, a dual polarization micro pulse lidar (DPMPL) has been installed at IITM recently to investigate the cloud composition, and aerosol-cloud-climate interactions. The initial results obtained from this state-of-the-art lidar system showed interesting features in the time evolution of nocturnal (stable) boundary layer which have strong bearing on air pollution potential over the experimental station. The complete details of the lidar systems used in the above studies together with discussion of salient results are presented in this paper.

Boundary layer height is determined by analyzing the BLL data using different methods. The methods are briefly discussed and results presented.

Comparative study is performed for a mid-infrared differential absorption lidar system with different wavelengths around 1.6 μm and 2 μm for remote sensing of green-house effect gas CO₂. Simulation based on lidar equations is performed for estimating detectable range and detectable concentration per unit volume. Different approaches for developing the coherent light sources, such as a KTiOPO₄ nonlinear optical crystal based optical parametric oscillator, widely tunable Ti³⁺:sapphire laser based Raman shifter are also considered and the effect of strong and weak absorption lines, spectral width and output energy of the coherent light source on the detection sensitivity is investigated at different wavelengths of interests. Simulation results show that DIAL system around 1.6 μm can provide a long range measurement with a narrow spectral width (0.03 cm^-1) of the light source. However, with a wider spectral width (0.3 cm^-1) the detectable range is comparable to that of the DIAL system around 2 μm wavelength. The detection sensitivity is also limited by the available output energy from the light source.

A new concept of DIAL (DIfferential Absorption Lidar) system for global CO₂ monitoring using microwave modulation is introduced. This system uses quasi-CW lights which are intensity modulated in microwave region and receives a backscattered light from the ground. In this system, ON/OFF wavelength laser lights are modulated with microwave frequencies, and received lights of two wavelengths are able to be discriminated by modulation frequencies in electrical signal domain. Higher sensitivity optical detection can be realized compared with the conventional microwave modulation lidar by using direct down conversion of modulation frequency. The system also has the function of ranging by using pseudo-random coding in modulation. Fiber-based optical circuit using wavelength region of 1.6 micron is a candidate for the system configuration. After the explanation of this configuration, feasibility study of this system on the application to global CO₂ monitoring is introduced.

Computer simulations have been carried out to optimize the IR Differential Absorption Lidar (DIAL) system in order to measure the gaseous pollutants released by the industries. The concentration of the gaseous pollutants due to elevated sources is computed using the Gaussian dispersion model. For given atmospheric conditions and stack physical parameters, the downwind distance (x) at which the SO₂ reaches the safe limit of its toxicity has been computed at given other two coordinates (y, z) with respect to chimney. The gaseous pollutants released by the industries will be effectively monitored by the proposed DIAL system, which will be placed at New Delhi (28.35 degrees N, 77.12 degrees E), India. The performance of the Lidar has been optimized based on the various system parameters incorporating the atmospheric conditions and stack physical parameters. Further, the backscattered return powers at on- & -off line wavelengths, the required energy to be transmitted and the position at which the lidar system should be posted have been computed in order to monitor SO₂.

Simulation studies have been carried out to analyze the performance of a Differential Absorption Lidar (DIAL) system for the remote detection of a large variety of toxic agents in the 2-5 μm and 9-11 μm spectral bands. Stand-alone Graphical User Interface (GUI) software has been developed in the MATLAB platform to perform the simulation operations. It takes various system inputs from the user and computes the required laser energy to be transmitted, backscattered signal strengths, signal-to-noise ratio and minimum detectable concentrations for various agents from different ranges for the given system parameters. It has the flexibility of varying any of the system parameters for computation in order to provide inputs for the required design of proposed DIAL system. This software has the advantage of optimizing system parameters in the design of Lidar system. As a case study, the DIAL system with specified pulse energy of OPO based laser transmitter (2-5 μm) and a TEA CO₂ laser transmitter (9-11μm) has been considered. The proposed system further consists of a 500-mm diameter Newtonian telescope, 0.5-mm diameter detector and 10-MHz digitizer. A toxic agent cloud with given thickness and concentration has been assumed to be detected in the ambient atmospheric conditions at various ranges between 0.2 and 5 km. For a given set of system parameters, the required energy of laser transmitter, power levels of the return signals, signal-to-noise ratio and minimum detectable concentrations from different ranges have been calculated for each of these toxic agents.

The strength of backscattered signals for a Differential Absorption Lidar (DIAL) system operating in the mid infrared band is estimated for the cases of (i) back scattering from topographic targets and (ii) Mie scattering from the aerosols. The estimation is based on the following input parameters : (a) the energy of the lidar transmitted pulses is 20 mJ with pulse duration of 10 nanoseconds and (b) the receiver system consists of a telescope with 500 mm aperture with a field of view of 0.33 mrad. The inherent noise of the detector, background noise due to solar radiation and the noise due to thermal radiation from the objects within the field of view of the receiver, are separately estimated to obtain the total noise in the detection setup. For the case of topographic targets, the reflectivity is taken as 10%. It is shown that atmospheric trace gases like methane with (path integrated) concentration upto 0.5 ppm-km, can be detected without difficulty upto ranges of 5 km. For the Mie scattering from the aerosols, typical meteorological visibility of 10 km is assumed. The case for methane detection is studied. It is shown that during day time, range resolved measurements of trace chemicals can made upto 3 km range, which can be extended to 5 km during night time.

The Atmospheric Dynamics Mission ADM-Aeolus will be the wind lidar in orbit. The aim is to provide global observations of wind profiles with a vertical resolution that will satisfy the requirements of the World Meteorological Organization. ADM-Aeolus will carry just one large instrument-the Atmospheric Laser Doppler Lidar Instrument (ALADIN). This is a direct detection lidar operating in the ultra-violet spectral region (355 nm), using a frequency-tripled Nd:YAG laser as transmitter. The 1.5-m-telescope in ALADIN collects the backscattered light and directs it to an optical receiver, which measures the Doppler shift of the received light. Wind profiles will then be derived showing the relative strength and direction of winds at different altitudes. Aeolus will determine the wind velocity component normal to the satellite velocity vector. These wind profile measurements will be assimilated into numerical forecasting models to improve the quality of the global three-dimensional wind fields. EADS-Astrium (UK and France) and their subcontractors develop Aeolus and ALADIN. The structure models of satellite and instrument have been successfully tested in Summer 2005. The structure model of the satellite has been fully qualified, and the transmitter laser is under development at Galileo Avionica (Italy). Still, many challenges still have to be faced to demonstrate the reliable long lifetime operation of this laser before the launch in late 2008.

This study presents the coherent Doppler lidar (CDL) observations and numerical simulations of local strong easterly wind Kiyokawa-dashi, which is most famous gap wind in Japan. Main goal of this study is to clarify the three dimensional structure and dynamics of Kiyokawa-dashi under the different synoptic situations. Observations were conducted in August 29 and 30, 2004. A 2μm eye-safe airborne CDL developed by the National Institute of Information and Communications Technology (NICT) established at the exit of the narrow valley of Mogami-River. The vertical scanning of CDL with the velocity-azimuth display technique shows that the easterly wind was confined below 600 m, which is almost same or slightly lower than the crest, and accelerated at the down stream side. The upper layer above the easterly wind was weak westerly and these vertical structures were common to the all of the events. Horizontal scanning sounds the line-of-sight (LOS) wind velocity up to 6 km of downstream and presents detailed temporal wind shift within 5-10 minutes. Numerical simulations have been performed with non hydrostatic atmospheric model (MRINPD NHM) with horizontal grid spacing down to 1 km. Many of the observed structures were realistically simulated, but it still has a bias that the detailed temporal evolution wasn't shown in the numerical simulation. The CDL captured the fine structure and temporal variations of Kiyokawa-dashi. It is a powerful and useful system for studying the wind field.

One of the main functions of a Doppler Lidar system is to measure the atmospheric wind speed and direction. It is done by measuring the Doppler shift in frequency of the backscattered laser beam. A frequency stabilized Nd: YAG laser source operating at its fundamental wavelength of 1064 nm will be used in the proposed incoherent detection system. Edge technique is employed along with a high-resolution optical filter in this system to achieve high accuracy in the measurement of wind velocity. The performance of Doppler lidar system is estimated with realistic parameters. Analysis of the uncertainty in wind measurement is made by considering factors like lidar return signal levels, laser spectral width, etalon filter pass band (FWHM) etc. An accuracy of 0.25 m/s has been achieved in the wind speed up to an altitude of 5 km with 15-m range resolution in the proposed ground based Doppler Lidar system.

We have studied a 2-micron airborne coherent Doppler lidar to observe wind profile downward from flying object. We investigated the algorithms required to extract the Doppler-shifted frequency compensating for a speed of the flying object. The airborne experiments were conducted to demonstrate the feasibility of the airborne coherent Doppler lidar from a flying object in 2002, 2004 and 2006. We extracted the Doppler-shifted frequency corresponding to aircraft speed with developed algorithms and obtained wind profiles through airborne experiment. To examine wind profiles measured by the airborne coherent Doppler lidar, we compared those profiles with profiles measured by a GPS-dropsonde and a windprofiler. Although the volume measured by the airborne coherent Doppler lidar system differed spatially and temporally from those by other instruments, the wind profiles observed by the airborne coherent Doppler lidar agreed well with those observed by other instruments.

We have developed a combined remote telescopic Raman and laser-induced native fluorescence (LINF) spectrograph with 532 nm pulsed laser excitation and a gated CCD detector. With this system, we have measured time-resolved Raman and LINF spectral measurements at 9 m with 10-ns time resolution. A comparison of Raman spectra of calcite crystal and that of chicken eggshell show that the CaCO₃ in the chicken eggshell is arranged in a calcite structure. The strong LINF band in the spectrum of the calcite crystal has lifetime longer than 1 μs, whereas the lifetime of LINF bands of the eggshell are in 10's of nano-sec (ns). The time-resolved Raman spectra of tomato and poinsettia (Euphorbia pulcherrimum) green leaves show resonance Raman features of carotenes. The time-resolved remote LINF spectrum of ruby crystals, and LINF spectra of tomato and poinsettia green leaves yield information that the LINF lifetime of ruby lines is much longer (in milliseconds (ms)) as compared with the fluorescence lifetime of the tomato and the poinsettia leaves (in 10s of ns). These results show that it will be possible to discriminate between inorganic and biogenic materials on the basis of LINF lifetimes even with 8 nano-sec laser pulses and gated detection.

The design and development of the new Raman lidar of the Space Physics Laboratory, Vikram Sarabhai Space Centre is presented here. This station is located at 8 degrees 33 minutes N, 77 degrees E in India. This lidar can monitor atmospheric temperature (using Pure Rotational Raman Spectrum), aerosol extinction coefficient, water vapor profile and clouds. Advantages of Pure Rotational Raman method over Vibrational Raman method are presented with the result obtained using Vibrational Raman lidar. Optical layout of the lidar system, PRRS method and aerosol extinction measurements are described briefly.

This work reports the development and preliminary results of the Vibrational Raman lidar at a coastal station, Trivandrum (8°33'N, 77°E). A Raman lidar technique for measuring atmospheric temperature and water vapor using vibrational Raman spectra of N2 and H₂O are discussed in detail. Interference filters at 607 and 660nm of 1nm band- width are used in the Raman lidar channel. Nighttime temperature and water vapor profiles are obtained from 1-5km in the lower atmosphere. Lidar water vapor profiles are in good agreement with the Regional Model data. The variation in the temperature profiles may be due to the indirect aerosol effect in the lower atmosphere.

Altitude profiles of middle atmospheric temperature data using Rayleigh Lidar at Gadanki have been utilized to study the gravity wave characteristics. The wave activity during the period November 2002-April 2005 is investigated. The vertical propagation characteristics show waves with maximum amplitude of ~5-8 K and vertical wavelength of ~10 km. Potential energy density of these two bands of periodicities in the altitude regions 30-60 km is estimated for different seasons. Equinoctial enhancement in the wave activity is observed. Momentum fluxes of these two bands of periodicities of gravity waves also exhibit seasonal variation with maximum around equinox and minimum in solstial months. A high correlation exists between the gravity wave activity and one of its major sources namely convection.

Continued hardware upgrades have permitted extension of the Colorado State University (CSU) sodium fluorescence doppler temperature/wind lidar to two-beam operation overall full diurnal cycles. The lidar utilizes the abundance of neutral sodium atoms in the mesopause region to probe the sodium D_2A transition at 3 pre-selected frequencies. Range-gated ratios of the returns at the 3 frequencies are then used to deduce temperature and horizontal wind profiles with different time resolutions, dependent on signal-to-noise ratio of the returns. From these measurements, atmospheric convective and dynamic stability can be assessed via the Brunt-Vaisala frequency and gradient Richardson number. The mesopause region is a crucial coupling region between the lower and upper atmosphere. The coupling occurs via waves which consist of perturbations to the temperature and wind fields. Metal fluorescence lidars are the only ground-based instrument able to simultaneously measure vertical profiles of mesopause temperatures and winds, thereby maximizing geophysical information obtained. This paper outlines major hardware upgrades leading to the current lidar capabilities, in addition to measurement methodology. Sample seasonal temperature and wind statistics, taken from April 2002-May 2003, will then be provided, followed by samples from a 12-month seasonal statistical study of derived parameters such as wind shears, Brunt-Vaisala frequency, and Richardson number.

We present in this paper sporadic sodium layers (SSLs), which we observe with a Resonance Lidar at Gadanki, India (13.5°N, 79.2°E). The SSLs were observed on a total of 63 occasions during 464 hours of Na lidar observations from January 2005 to February 2006. The SSL occurrence rate of 1 event/7 h at Gadanki was obtained. These results show that the rate of occurrence of SSLs fall between 20°N and 2°S. The most prominent sporadic layer, which formed on the night of Feb'12, 2005 exhibited a peak density of 60722 c.m^-3 near 92 km. At our Gadanki site, SSLs have the following properties (1) they develop between 88 and 98 km with average height of 94 km (2) they develop maximum in the early morning between 0200 and 0500 LT (3) The ratio of the maximum peak density to the average Na density is normally 3 to 5, but values as high as 11 have been observed in the most outstanding cases (4) The events last from a few minutes to several hours. Most of the Sporadic Sodium layer events were downward phase.

Using the altitude profiles of temperature fluctuations obtained from Rayleigh Lidar observations at Gadanki (13.5°N, 79.2°E), India, the characteristics of gravity waves were studied. For this study the temperature profiles on 30 nights during January to March 1999 and 21 nights during February 2000 in the 27-65 km altitude region are used. The gravity wave perturbations showed periodicities in the 0.5 - 3 hours range and attain large amplitudes ~8 K in the mesosphere. The phase profiles shows downward propagation indicating the upward wave energy propagation with a vertical wavelength of 5-7 km. The vertical profile of potential energy per unit mass are computed from the temperature fluctuations. The observed energy growth is less than the theoretically expected energy growth, which indicates that the wave is getting damped. The percentage of dissipation of prominent periodicities of gravity waves is calculated at various height levels. It is observed that gravity waves are dissipated more in mesosphere than in stratosphere. The day to day variability of prominent periodicities are studied.

AGLITE is a multi-wavelength lidar developed for the Agricultural Research Service (ARS), United States Department of Agriculture (USDA) and its program on particle emissions from animal production facilities. The lidar transmitter is a 10 kHz pulsed NdYAG laser at 355, 532 and 1064 nm. We analyze lidar backscatter and extinction to extract aerosol physical properties. All-reflective optics and dichroic and interferometric filters permit all wavelengths to be measured simultaneously, day or night, using photon counting by MTs, an APD, and fast data acquisition. The lidar housing is a transportable trailer suitable for all-weather operation at any accessible site. We direct the laser and telescope FOVs to targets of interest in both azimuth and elevation. The lidar has been applied in atmospheric studies at a swine production farm in Iowa and a dairy in Utah. Prominent aerosol plumes emitted from the swine facility were measured as functions of temperature, turbulence, stability and the animal feed cycle. Particle samplers and turbulence detectors were used by colleagues specializing in those fields. Lidar measurements also focused on air motion as seen by scans of the farm volume. The value of multi-wavelength, eye-safe lidars for agricultural aerosol measurements has been confirmed by the successful operation of AGLITE.

This paper discusses the development plan of multiwave Lidar at Laser Science and Technology Centre (LASTEC) at Delhi. The Lidar is designed mainly for the detection of very low concentrations of Chemicals and pollutant of the order of few ppm level at a remote distance of 5 kilometres. This Lidar system not only detects the pollutant it also identifies and quantifies the pollutant. The Lidar is supported by live software and library to display the required information online.

Lidar techniques are based on the interaction of the laser beam with various constituents of the atmosphere like aerosols, gas molecules etc. Various atmospheric conditions like temperature turbulence, refractive index variation, fog, rain etc. really influence the transmission properties of the laser beam. An Imaging Lidar provides a 3-D Image of the targets like clouds when used vertically up in the atmosphere or any terrestrial object on the ground when used horizontally. Various image processing techniques are used to improve the image quality by using various mathematical models related to atmospheric conditions. A portable IR Imaging lidar system has been designed and developed for imaging the terrestrial targets during nighttime in complete dark conditions. The system is also being used for study of the structure of clouds in the troposphere. The system mainly consists of a CW laser source operating in the IR region and a CCD array-imaging device with zooming capability to cover the long range. The CCIR standard video output available from the CCD camera is monitored by a high resolution monochrome monitor. The video output is digitized using a frame grabber board. The digitized image is subjected to online and offline processing methods. The image signal depends on the integral response of the laser source, reflection/scattering properties of the objects, atmospheric effects etc. Based on the image processing methods needed to improve the quality of image under different atmospheric conditions, known a priori, an empirical model is developed. This paper describes the imaging lidar system developed and the image processing.

This study examines the feasibility of optical remote measurement of the electromagnetic field or the electron density distribution in thundercloud. We considered the contribution of Faraday effect as the magneto-optical effect to the change in polarization of the backscattered light, assuming a polarizing lidar configuration. We estimate that, if the lidar can detect the polarizing rotation angle in the plane perpendicular to the propagating beam with a dynamic range of more than 30dB, the lidar can be used to predict lightning strikes.

This paper discusses the various requirements of data acquisition and processing for Space Borne Lidar (Light Detection and Ranging) system being developed in Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum for the study of aerosols and clouds in the troposphere and lower stratosphere (0-40 km). The lidar system will be housed in a polar orbiting satellite at an altitude of 600 km with a period of approximately 90 minutes providing global coverage. The lidar operates by transmitting a laser pulse down (nadir looking) and receiving the backscatter returns from the atmosphere. The laser source operates at dual wavelengths of 1064 and 532 nm with a pulse repetition rate of 5/10 Hz with energy of 100 mJ. The receiving system consists of a 265 mm Fresnel lens telescope followed by backend optics and detector systems. The data acquisition system uses three channels with two types of photo detectors, namely photo multiplier tube and avalanche photo diode and operate either in analog (current) mode or discrete pulse (photon counting) mode. The data acquisition system has to handle signals of wide dynamic range (4-5 decades) and acquire the backscattered signal intensity with good spatial resolution. The analog channel will receive and digitize the 1064 nm signal with 16 bit resolution and the photon counting channels will count the 532 nm signal upto 200 MHz rate. The data backed up onboard is telemetered down to ground station during periods of visibility of satellite.

LIDAR operates by transmitting light pulses of few nanoseconds width into the atmosphere and receiving signals backscattered from different layers of aerosols and clouds from the atmosphere to derive vertical profiles of the physical and optical properties with good spatial resolution. The Data Acquisition System (DAS) of the LIDAR has to handle signals of wide dynamic range (of the order of 5 to 6 decades), and the data have to be sampled at high speeds (more than 10 MSPS) to get spatial resolution of few metre. This results in large amount of data to be collected in a short duration. The ground based Multiwavelength LIDAR built in Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum is capable of operating at four wavelengths namely 1064, 532, 355 and 266 nm with a PRF of 1 to 20 Hz. The LIDAR has been equipped with a computer controlled DAS. An Avalanche Photo Diode (APD) detector is used for the detection of return signal from different layers of atmosphere in 1064 nm channel. The signal is continuous in nature and is sampled and digitized at the required spatial resolution in the data acquisition window corresponding to the height region of 0 to 45 km. The return signal which is having wide dynamic range is handled by two fast, 12 bit A/D converters set to different full scale voltage ranges, and sampling upto 40 MSPS (corresponding to the range resolution of few metre). The other channels, namely 532, 355 and 266 nm are detected by Photo Multiplier Tubes (PMT), which have higher quantum efficiency at these wavelengths. The PMT output can be either continuous or discrete pulses depending upon the region of probing. Thick layers like clouds and dust generate continuous signal whereas molecular scattering from the higher altitude regions result in discrete signal pulses. The return signals are digitized using fast A/D converters (upto 40 MSPS) as well as counted using fast photon counters. The photon counting channels are capable of counting upto 200 MHz with a spatial resolution of few metres. The LIDAR data generated comes in burst mode and gets transferred to computer system. Pulse to pulse averaging is done rangebinwise for SNR improvement. The range normalized signal power is computed and the vertical profiles of backscatter and extinction coefficients are derived. This paper describes the intricacies in the design of the high resolution DAS developed in-house to obtain the scientific data. The optimization methodology used for handling the data is also described.

Differential Absorption Lidar (DIAL) Systems are advantageously used to detect and measure very small concentrations of trace gases in the atmosphere. There is a requirement to interrogate and search for the presence of one or more of toxic agents out of a large number (about 20 or so) of possible agents at distances up to several kilometers with the help of a ground-based multi-wavelength DIAL system employing pulsed, tunable laser sources in the wavelength bands of 2-5 micron and 9.2-10.8 micron. The Laser beams from the two sources are directed in the atmosphere with a predefined divergence to scan the atmosphere. Two methodologies can be implemented to provide the beam steering, one is to mount the entire telescope of transmitting and receiving channel on to a motorized gimbal platform and second is to keep the optical telescope stationary and use a slewing mirror to steer the beam in required direction. The first scheme is named as mass control and second scheme is called mirror control. Both the schemes have relative advantages and disadvantages and in the present DIAL application second scheme is being adopted. The present opto-mechanical configuration of DIAL system employs a 700 x 500 mm² (Elliptical) steering mirror for transmitting the collimated beams in a required direction and receiving the reflected beam as well. In the receiving channel a Telescope is used which collects the return beam and focuses the same on to a detector. The slewing mirror is housed in a gimbal mount having a sufficient FOR (Field of Regard) in Azimuth and elevation plane. The paper describes the modeling and simulation of Opto-mechanical and servo-mechanical subsystems of precision gimbaled mirror and also discusses the issues related to design of control system. The requirement specifications in regard to field of regard, slew rates 5°/s, scanning rates 1°/s are to be met with stringent beam pointing and scanning accuracies. The design of this system is categorized as multidisciplinary problem. The design parameters obtained from opto-mechanical analysis forms the input for control system design. The design of control system is carried out using conventional design methodologies.

Present day remote sensing satellites orbiting in low earth orbit (LEO) have increasingly sophisticated and high resolution onboard sensors. Their frequency and area of observation is also increasing. This generates large volume of data which needs to be communicated. However their visibility to ground station is limited. Free space optical communication between remote sensing satellite in LEO and communication satellite in geostationary earth orbit (GEO) can be favorable approach. Subsequently GEO satellite relays the data to ground station. To demonstrate this, a concept model operating at data rates greater than 1 Gbps is under development at LEOS. The system consisting of laser transmitter with 20cm diameter telescope and receiver with 30cm telescope is planned. It uses commercially available optical and optoelectronic components. This concept model will demonstrate and verify link margins available as against expected. Subsequent to this, it is planned to concentrate on design and other issues involved in acquisition, tracking and pointing (ATP) due to highly narrow laser beam.

We developed a diode-side-pumped Tm:GdVO₄ laser with a conductively cooled pump head as a pump source of a Ho laser. The laser produced an output energy of 32 mJ at a pulse repetition frequency of 5 Hz. In addition, continuous-wave operation was demonstrated in a Tm,Ho:GdVO₄ microchip laser. An output power of 0.43 W and a slope efficiency of 33% were achieved near room temperature.

In the review article we explain the recent investigations on rare-earth doped glass and optical fibres for designing lasers which may be suitable for remote sensing and LIDAR applications. The paper explains the importance of engineering efficient lasing transitions in visible (480-650 nm) for generating UV lasers via one-stage harmonic generation. Besides visible transitions, we also demonstrate the transitions in near- and mid-IR via near-IR pumping scheme.

The design of a compact coherent laser radar transmitter for tropospheric wind sensing is presented. This system is hardened for ground and airborne applications. As a transmitter for a coherent wind Lidar, this laser has stringent spectral line width and beam quality requirements. Although the absolute wavelength is not fixed, the output wavelength should avoid atmospheric CO₂ and H₂O absorption lines. The design architecture includes a seed laser, a power oscillator and a single amplifier. The laser material used for this application is a Ho:Tm:LuLF crystal. The 3-meter long folded ring resonator produces 100-mJ with a temporal pulse length around 185 ns. A final output of 300 mJ at a repetition rate of 10 Hz is achieved by using an amplifier in a double pass format. The operating temperature is set around 15°C for the pump diode lasers and 5°C for the rod. Since the laser design has to meet high-energy as well as high beam quality requirements, close attention is paid to the laser head design to avoid thermal distortion in the rod. A side-pumped configuration is used and heat is removed uniformly by passing coolant through a tube slightly larger than the rod to reduce thermal gradient. This paper also discusses issues related to beam distortion due to high repetition rate. In addition, energy, seeding technique, and beam quality evaluation of the engineering verification laser will be presented.

The design of an efficient optical parametric oscillator (OPO) tunable in the range of 2.3 to 4.5 micron, as LIDAR source, for sensitive, selective and rapid remote detection and measurement of atmospheric trace gases at ranges upto 5 km is described. Potassium Titanyl Arsenate (KTA) a nonlinear crystal, having good transmission from 350 to 5000 nm is proved to be most suitable for this application. Tuning is achieved by angle tuning in critical phase matching of type II in X-Z plane, by changing the propagation direction from about 41 to 49 degrees with the Z-axis, when pumped with Nd:YAG laser at 1064nm. The expected linewidth of KTA OPO in the absence of any wavelength selective device is found to vary from 10 to 110 cm^-1 which reduces to 0.1 cm^-1 on introduction of an intracavity grating at grazing incidence. Using a 600 l/mm grating, with the tuning mirror fixed on a commercially available fast rotation stage, random tuning to any desired wavelength can be achieved in 40 ms. Threshold of optical parametric oscillation is found to vary from 2 to 3 J/cm² as the idler wavelength varies from 2.3 to 4.5 micron. With this limitation, the pump energy requirement varies from 450 mJ to 600 mJ for 20 mJ output energy at different wavelengths in singly resonant oscillator configuration. Tuning arrangement for rapid tuning of output to lambda_on and lambda_off wavelengths of different chemical species for DIAL detection is described.

Significant advancements in the 2-micron laser development have been made recently. Solid-state 2-micron laser is a key subsystem for a coherent Doppler lidar that measures the horizontal and vertical wind velocities with high precision and resolution. The same laser, after a few modifications, can also be used in a Diffrencial Absorption Lidar (DIAL) system for measuring atmospheric CO₂ concentration profiles. The world record 2-micron laser energy is demonstrated with an oscillator and two amplifiers system. It generates more than one joule per pulse energy with excellent beam quality. Based on the successful demonstration of a fully conductive cooled oscillator by using heat pipe technology, an improved fully conductively cooled 2-micron amplifier was designed, manufactured and integrated. It virtually eliminates the running coolant to increase the overall system efficiency and reliability. In addition to technology development and demonstration, a compact and engineering hardened 2-micron laser is under development. It is capable of producing 250 mJ at 10 Hz by an oscillator and one amplifier. This compact laser is expected to be integrated to a lidar system and take field measurements. The recent achievements push forward the readiness of such a laser system for space lidar applications. This paper will review the developments of the state-of-the-art solid-state 2-micron laser.

Laser Science and Technology Center (LASTEC), Delhi, is developing a space qualified diode pumped Nd: YAG laser transmitter capable of generating 10 ns pulses of 30 mJ energy @ 10 pps. This paper presents the results of experiments for comparative studies between electro-optic and passively Q-switched Nd: YAG laser in a crossed porro prism based laser resonator. Experimental studies have been performed by developing an economical bench model of flash lamp pumped Nd: YAG laser (rod dimension, &nullset; 3 X 50 mm). Electro-optic (EO) and Passive Q-switching were performed employing LiNbO₃ crystal (9 x 9 x 25 mm) and Cr: YAG (7 &nullset; 10 mm) saturable absorber respectively. Laser output of 30 mJ was achieved in EO Q-switching mode by optimizing Pockels cell operation. More than 80 % Q-switching efficiency was achieved. However, at the same input level in passive Q-switching mode at optimized initial transmission of Cr: YAG, only 36% efficiency could be achieved. Comparative studies were made for output pulse energy at different input levels. In passive Q-switched mode, deviation from the optimum flash lamp input either stops the lasing action or leads to multiple pulsing. Thus in view of the very stringent requirements of reliability and efficiency of space-based system, the electro-optical method of Q-switching has been adopted in the design.

Development of a laser transmitter for space applications is a highly challenging task. The laser must be rugged, reliable, lightweight, compact and energy efficient. Most of these features are inherently achieved by diode pumping of solid state lasers. Overall system reliability can further be improved by appropriate optical design of the laser resonator besides selection of suitable electro-optical and opto-mechanical components. This paper presents the design details and the theoretically estimated performance of a crossed-porro prism based, folded Z-shaped laser resonator. A symmetrically pumped Nd: YAG laser rod of 3 mm diameter and 60 mm length is placed in the gain arm with total input peak power of 1800 W from laser diode arrays. Electro-optical Q-switching is achieved through a combination of a polarizer, a fractional waveplate and LiNbO₃ Q-switch crystal (9 x 9 x 25 mm) placed in the feedback arm. Polarization coupled output is obtained by optimizing azimuth angle of quarter wave plate placed in the gain arm. Theoretical estimation of laser output energy and pulse width has been carried out by varying input power levels and resonator length to analyse the performance tolerances. The designed system is capable of meeting the objective of generating laser pulses of 10 ns duration and 30 mJ energy @ 10 Hz.

LASTEC Delhi in a joint collaborative activity with LEOS, Bangalore is developing a space qualified diode array pumped Nd:YAG laser transmitter delivering 30 mJ @ 10 pps of 10 ns duration. For space applications laser diodes are preferred because of their excellent reliability with lifetimes exceeding 100,000 hours. However, they are extremely sensitive to electro-static discharge, excessive current levels, and current spikes and transients. Small variations in bias voltage may produce large fluctuations in the current causing instability and damage to the device. Hence instead of the traditional power supplies a current controlled laser diode driver is required. This paper presents the design of a Q-CW laser diode driver based on closed loop current regulator, capable of driving 24 QCW laser diode bars each with 75W peak power at 70 A. The driver can generate up to 100 Amp peak current and 200μsec pulse width operating at 10 Hz. The current source design includes special circuits for low noise operation, slow turn-on and turn-off, circuits for over voltage and transient current protection; and good regulation. Space qualified and radiation hardened components are required to be used to sustain stringent space environment requirements during mission life of two years.

We report an all-solid-state coherent 589 nm light source in single-pass sum-frequency generation (SFG) with actively mode-locked Nd:YAG lasers for the realization of sodium lidar and laser guide star adaptive optics. The Nd:YAG lasers are constructed as a LD-side-pumped configuration and are operated at 1064 and 1319 nm for 589 nm light generation in SFG. Output powers of 16.5 and 5.3 W at 1064 and 1319 nm are obtained with two pumping chambers. Each chamber consisted of three 80-W-LD arrays. Single transverse mode TEM₀₀; M² ~1.1 is achieved with adjustment of cavity length considering thermal lens effect with increase of input LD power. The cavity length is set to approximately 1 m. Accordingly the mode-locked lasers are operated at a repetition rate of approximately 150 MHz. Synchronization of two pulse trains at 1064 and 1319 nm is accomplished by control of phase difference between two radio frequencies input in acousto-optic mode-lockers. Then temporal delay is controlled with a resolution of 37 ps/degree. Pump beams are mixed in periodically poled stoichiometric lithium tantalate (PPSLT) without an antireflection coating. The effective aperture and length of the crystal are 0.5 × 2 mm² and 15 mm. When input intensity is set at 5.6 MW/cm , an average output power of 4.6 W is obtained at 589.159 nm. Precise tuning to the sodium D₂ line is accomplished by thermal control of etalons set in the Nd:YAG lasers. The output power at 589.159 nm is stably maintained within ±1.2% for 8 hours.

Advances in Laser Technology and nonlinear Optical techniques can be effectively utilized for LIDAR applications in space and atmospheric sciences to achieve better flexibility and control of the available optical power. Using such devices, one can achieve highly accurate and resolved, measurement of the distribution for atmospheric scattering layers. In the present investigation a diode double end pumped high repetition rate, multi wavelength Nd:YAG laser is designed, fabricated and various laser beam parameters have been characterized for LIDAR applications. Nonlinear optical techniques have been employed to generate higher harmonics like 532nm, 355nm and 266nm for various spectral studies. The experimental setup mainly consists of two Fiber coupled pump laser diodes (Model FAP- 81-30C-800B, Coherent Inc, USA) with a maximum output power of 30Watt at a wavelength of 807-810nm at 30°C set temperature. A second harmonic LBO crystal cut for critical phase matching placed within the laser resonator is provided for converting a fraction of the fundamental beam to a second harmonic beam. A type II frequency tripling LBO nonlinear crystal (cut for critical phase matching) is also located inside the laser resonator. The third harmonic beam and the unconverted fundamental beam are then directed across a type I fourth harmonic LBO crystal cut for critical phase matching where a portion of the fundamental beam and a portion of the third harmonic beam are converted to a fourth harmonic frequency when both fundamental and third harmonic beams propagate through the frequency quadrupling crystal. The resulting beams which are the fundamental (1064nm), second harmonic (532nm), third harmonic (355nm) and fourth harmonic (266nm) are then directed to a fourth harmonic separator in which the fourth harmonic beam is separated from the fundamental beam. A maximum average power of 12W at 1064nm, 8W at 532nm, 5W at 355nm and 3W at 266nm have been measured at a repetition rate of 10KHz. A minimum pulse width of 25ns have been observed.

Thulium and holmium-doped yttrium aluminum garnet (YAG) ceramic materials are investigated. Compared with YAG crystal, there is hardly any spectroscopic difference between ceramic and crystal at the same doping level. Laser oscillation was successfully carried out under quasicontinuous- wave diode pumping. Optical-to-optical efficiency higher than 5% was achieved. It was calculated that total absorbed power hardly changes around 783 nm with a bandwidth of about 7 nm using a diode array of 3.5-nm linewidth as a pump source.

Tropospheric aerosol play an important role in regional meteorology and energy balance of radiation. Specially in huge urban areas like New Delhi, India a large amount of aerosols from anthropogenic origins is continuously produced and released in the atmospheric boundary layer. The effect of aerosols on atmospheric energy balance is a key global change problem. Aerosol vertical distribution monitoring can be significantly improved using active remote sensing by Lidar. Micro-pulse lidar proved to be an important state of art tool providing a detailed picture of the vertical structure of boundary layer and elevated dust or tiny aerosol. Aerosols are spatially and temporarily varied in short period. The movement of the pollutants can be tracked or mapped out as a function of time by the help of Lidar which is very important to understand the dynamics of particulate matters. The in-situ measurements of aerosol at ground will not be a true representation of total aerosol and its vertical distribution in the atmosphere, therefore the monitoring of vertical profiles of aerosol is very important and timely which is not possible by conventional methods. In view of the above a micro pulse lidar is being setup at NPL, New Delhi to get vertical profiles of aerosol to study the radiative forcing and characterization of aerosols using depolarization ratio. In the present communication details of the system and some preliminary results will be presented.

The cirrus clouds which are global in nature have been identified as one of the important constituents if the atmosphere. They play a dual role in the earth radiation budget increasing the Earth's albedo while simultaneously decreasing the emission of Infrared radiation to space. Tropical cirrus clouds come in a variety of forms ranging from optically thick anvil cirrus closely associated with deep convection to optically thin cirrus layers frequently observed near the tropopause. For better understanding of the formation, subsistence and dissipation of cirrus clouds extended studies are necessary. From earlier investigations it is realized that the climatology of cirrus clouds is distinctly different at the low latitude coastal station at the west coast of India. Some of the important characteristics of the cirrus clouds like time history of formation and dissipation, geometrical and optical properties during the winter time have been investigated using the ground based Mutiwavelength Lidar system designed and developed in house at the Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India. The lidar provides a vertical resolution of 3.75m by making use of the modified receiver electronics of the MWL system. The high resolution measurements have facilitated the study of the fine internal structure, optical depth extinction coefficient and other parameters of importance of cirrus clouds. The present paper describes lidar system and the results obtained over a period of one year covering all the seasons and the peculiar characteristics of the cirrus during winter time at this coastal station.

Dust profiles have been observed by a laser Ceilometer (MEISEI CT25K) at Shapatou, China which is located at the edge of the Tengger Desert. The observation was conducted throughout one year of 2004 successfully and showed the behaviors of the atmospheric dust profile from near the surface to about the 1000-m height. The results of the observation were compared with several other meteorological data, such as surface, radio sonde and sky radiometer measurements. In particular, the dust profiles observed on calm and fair weather days were analyzed and compared with other meteorological data. The dust profile at the calm and fare weather day in the desert area is mainly influenced by thermal convection due to strong surface heating in the warm season. The dust amount lower than the 500-m height decreases in the daytime and recovers in the nighttime. The data of the sky radiometer shows that the total amount of the dust at the same calm and fair-weather days is almost constant during the daytime. The observational evidence was explained in the paper.

Recently a low cost portable lidar system has been developed at the National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E), India for profiling the atmospheric aerosol and clouds. The lidar system utilizes state-of-the-art technology with a high repetition rate, low pulse energy diode pumped Nd:YAG laser and photon-counting detection, which considered to be first of its kind in India. The lidar system measures the laser backscattered radiation from aerosols and molecules at 532 nm that provide information on the height profiles of aerosols, boundary layer mixing height, cloud base height and cloud vertical extent. The system is capable of continuous, autonomous data acquisition in both daytime and nighttime. The lidar range go up to 3-4 km during daytime, where as its visibility extends to the end of troposphere during nighttimes.

Laser Radar-Lidar has been established as a promising tool in the remote sensing of aerosols and cloud layers in the atmosphere and in obtaining the altitude profiles of aerosol extinction coefficient. A variety of inversion methods have been used to obtain the altitude profiles of extinction / backscattering coefficients of the aerosols from the Mie lidar signals. Fernald's method which offers a general solution for the two component atmosphere involving aerosols and molecules is widely used to obtain the altitude profile of aerosol extinction coefficient and backscattering coefficient. This solution is most sensitive to the Boundary value at the calibration level and the aerosol extinction to backscattering ratio. In this paper the sensitivity of the above mentioned parameters on the live lidar data obtained from the tropical coastal station Trivandrum is investigated. In the following some numerical calculations are also carried out confining the situation to lidar measurements in the horizontal direction in order to investigate the significance of extinction to backscatter ratio and boundary value term in the solution for the two component lidar equation. This analysis is carried out at various altitude regions under different turbidity conditions in order to obtain a profile for the aerosol extinction to backscatter ratio for which the solution is less sensitive. Hence a new inversion method is proposed in the following using this variable lidar ratio at each altitude while inverting the lidar signal so that the possible error can be minimized.

Lidar observations had been conducted to study the long-range transport of aerosol and their effect at tropical station, Trivandrum during the period of 2001-2003. The presence of aerosol layers was observed on many days below about 5 km during the above period. The monthly values of aerosol extinction coefficient profile showed the presence of aerosol layer in the height region up to about 5 km during the summer monsoon periods. However, during the Asian winter monsoon period the aerosol layers were observed in the altitude region between 0.6 and 3 km. The extinction values were high in the winter season and were typically found to be 3.4×10-4 m-1. The aerosol optical depth was calculated by integrating the extinction values in the aerosol layer region and it was found to be between 0.2 and 0.35. The plausible reasons for the formation of these layers were explained using the wind circulation pattern and air back trajectories.