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A bathymetric, polarization lidar system transmitting at 532 nm and using a single photomultiplier tube is employed for applications of shallow water depth measurement. The technique exploits polarization attributes of the probed water body to isolate surface and floor returns, enabling constant fraction detection schemes to determine depth. The minimum resolvable water depth is no longer dictated by the system's laser or detector pulse width and can achieve better than 1 order of magnitude improvement over current water depth determination techniques. In laboratory tests, an Nd:YAG microchip laser coupled with polarization optics, a photomultiplier tube, a constant fraction discriminator, and a time-to-digital converter are used to target various water depths with an ice floor to simulate a glacial meltpond. Measurement of 1 cm water depths with an uncertainty of [+ or -] 3 mm are demonstrated using the technique. This novel approach enables new approaches to designing laser bathymetry systems for shallow depth determination from remote platforms while not compromising deep water depth measurement. [c] 2010 Optical Society of America OCIS codes: 010.3640, 120.0280, 280.1355, 280.3640.

An incoherent Doppler wind lidar based on iodine edge filters has been developed at the Ocean University of China for remote measurements of atmospheric wind fields. The lidar is compact enough to fit in a minivan for mobile deployment. With its sophisticated and user-friendly data acquisition and analysis system (DAAS), this lidar has made a variety of line-of-sight (LOS) wind measurements in different operational modes. Through carefully developed data retrieval procedures, various wind products are provided by the lidar, including wind profile, LOS wind velocities in plan position indicator (PPI) and range height indicator (RHI) modes, and sea surface wind. Data are processed and displayed in real time, and continuous wind measurements have been demonstrated for as many as 16 days. Full-azimuth-scanned wind measurements in PPI mode and full-elevation-scanned wind measurements in RHI mode have been achieved with this lidar. The detection range of LOS wind velocity PPI and RHI reaches 8-10 km at night and 6-8 km during daytime with range resolution of 10 m and temporal resolution of 3 min. In this paper, we introduce the DAAS architecture and describe the data retrieval methods for various operation modes. We present the measurement procedures and results of LOS wind velocities in PPI and RHI scans along with wind profiles obtained by Doppler beam swing. The sea surface wind measured for the sailing competition during the 2008 Beijing Olympics is also presented. The precision and accuracy of wind measurements are estimated through analysis of the random errors associated with photon noise and the systematic errors introduced by the assumptions made in data retrieval. The three assumptions of horizontal homogeneity of atmosphere, close-to-zero vertical wind, and uniform sensitivity are made in order to experimentally determine the zero wind ratio and the measurement sensitivity, which are important factors in LOS wind retrieval. Deviations may occur under certain meteorological conditions, leading to bias in these situations. Based on the error analyses and measurement results, we point out the application ranges of this Doppler lidar and propose several paths for future improvement. [c] 2010 Optical Society of America OCIS codes: 280.3640, 170.3340, 300.6320.

The 1.56 [micro]m eye-safe aerosol lidar at the Institut for Meteorologie und Klimaforschung was upgraded, which resulted in a doubled laser pulse energy of 0.28 J and an improved performance of the detection electronics. The high pulse energy, achieved by stimulated Raman backscattering of 0.9 J from a Nd:YAG laser in deuterium (33% conversion efficiency when neglecting the residual optical losses), was mainly achieved by improving the beam quality of the pump beam. The Stokes pulses showed a subnanosecond peak with some underlying 10 ns component. Because of the substantial increase in pulse energy, this reliable and simple method considerably gains in value. Examples are presented, demonstrating an operating range between 0.4 and more than 10 km for quasi-horizontal measurements. [c] 2010 Optical Society of America OCIS codes: 280.0280, 280.1100, 280.3640.

Government and commercial airborne light detection and ranging (lidar) systems have enabled extensive measurements of the Earth's surface and land cover over the past decade. There is much interest, however, in employing smaller lidar systems that require less power to enable sensing from small unmanned aerial vehicles or satellites. Technological advances in the performance of small microlasers and photodetector sensitivity have recently enabled the development of experimental airborne lidar systems with low signal-to-noise ratios (LSNRs). Recent government and academic prototypes have indicated that LSNR airborne lidars could significantly increase the fidelity of terrain reconstruction over what is possible with existing conventional lidars. Thus, there is a need to build up a modeling capability for such systems in order to aid in future system and mission design. A numerical sensor simulator has been developed to model the expected returns from LSNR microlaser altimeter systems and predict their performance. Both optical and signal processing system components are considered, along with other factors, including atmospheric effects and surface conditions. Topographic (solid Earth) and bathymetric (littoral zone) measurement scenarios are considered. The analysis of topographic simulation data focuses on the effect of solar noise on SNR and elevation accuracy while bathymetric performance is evaluated with regard to water depth and scan angle for different water clarities. The mission conditions chiefly responsible for limiting the performance of LSNR lidar are discussed in detail, along with suggestions for further algorithm development and system performance evaluation. Index Terms--Bathymetry, light detection and ranging (lidar), photonics, simulation.

Holographic aperture ladar is a variant of synthetic aperture ladar that seeks to increase cross-range scene resolution by synthesizing a large effective aperture through the motion of a smaller receiver and through the subsequent proper phasing and correlation of the detected signals in postprocessing. Unlike in conventional synthetic aperture ladar, however, holographic aperture ladar makes use of a two-dimensional translating sensor array, not simply a translating point detector. Also unlike in conventional synthetic aperture ladar, holographic aperture images will be formed in the two orthogonal cross-range dimensions parallel and perpendicular to the sensor platform's direction of motion. The central focus is on the development of the stripmap and spotlight holographic aperture transformations. These transformations will allow sequentially collected pupil plane field segments to be coherently stitched together in order to synthesize complex pupil plane fields with larger spatial extent. The challenge in this process is in accounting for the practical fact that both the receiver aperture and the transmitter will be in motion in real-world airborne applications. However, we demonstrate that, owing to the synchronous motion of the transmitter and receiver, resolution enhancements of more than two (stripmap case) or three (spotlight case) times the ratio of the synthetic aperture to the real receiver aperture diameter can be realized. We also demonstrate that in practical applications the holographic aperture ladar image formation process is relatively insensitive to scene depth if a good estimate of nominal scene range is available. OCIS codes: 120.0280, 280.3640, 280.4788, 280.6730.

We have developed a 1.6 [micro]m carbon dioxide (C[O.sub.2]) differential absorption lidar utilizing a quasi-phase-matching optical parametric oscillator (OPO) and a photon-counting detector. The operating wavelengths were chosen based on their low interference from water vapor and low temperature sensitivity. The online wavelength was in the (30012[left arrow]0001) band of C[O.sub.2], which was insensitive to atmospheric temperature. The established OPO laser achieved a 10mJ, 200 Hz repetition rate at the online and offline wavelengths. Our observations confirmed the statistical error of 2% with 5 h of accumulation for the C[O.sub.2] density profile less than 5.2km. Also, the statistical error of l% at an altitude of 2km was demonstrated. The results of the vertical C[O.sub.2] concentrations acquired using a 1.6[micro]m wavelength are presented. OCIS codes: 000.0280, 010.3640, 280.1910, 190.4970.

A CW tunable 10.6 [micro]m C[O.sub.2] laser differential absorption lidar (DIAL) system has been developed, for the first time to our knowledge, for the remote sensing of triacetone triperoxide (TATP) gas vapors, which have strong absorption lines at several wavelengths, including 3.3, 8.3, and 10.6 [micro]m. The DIAL laser beam was transmitted through an enclosed absorption cell containing TATP or [SF.sub.6], and backscattered returns were measured from a retroreflector array target at ranges of 5-100 m. DIAL sensitivity for the detection of TATP was about 0.5ng/[micro]l [52 parts in [10.sup.6] (ppm)] for a 0.3 m path. OCIS codes: 010.280, 010.3640, 280.1910.

In the present work, a picosecond lidar system aiming at single-ended combustion diagnostics in full-scale combustion devices with limited optical access, such as power plants, is described. The highest overall range resolution of the system was found to be OCIS codes: 120.1740, 280.3640, 280.1910, 280.6780.

We present a new approach for radiometric calibration of light detection and ranging (LIDAR) intensity data and demonstrate an application of this method to natural targets. The method is based on 1) using commercially available sand and gravel as reference targets and 2) the calibration of these reference targets in the laboratory conditions to know their backscatter properties. We have investigated the target properties crucial for accurate and consistent reflectance calibration and present a set of ideal targets easily available for calibration purposes. The first results from LIDAR-based brightness measurement of grass and sand show that the gravel-based calibration approach works in practice, is cost effective, and produces statistically meaningful results: Comparison of results from two separate airborne laser scanning campaigns shows that the relative calibration produces repeatable reflectance values. Index Terms--Calibration, laser measurements, laser radar, laser radiation effects, remote sensing.

The Ozone Profiling Atmospheric Lidar is a scanning four-wavelength ultraviolet differential absorption lidar that measures tropospheric ozone and aerosols. Derived profiles from the lidar data include ozone concentration, aerosol extinction, and calibrated aerosol backscatter. Aerosol calibrations assume a clear air region aloft. Other products include cloud base heights, aerosol layer heights, and scans of particulate plumes from aircraft. The aerosol data range from 280 m to 12 km with 5 m range resolution, while the ozone data ranges from 280 m to about 12 km with 100 m resolution. In horizontally homogeneous atmospheres, data from multiple-elevation angles is combined to reduce the minimum altitude of the aerosol and ozone profiles to about 20 m. The lidar design, the characterization of the photomultiplier tubes, ozone and aerosol analysis techniques, and sample data are described. Also discussed is a double-gating technique to shorten the gated turn-on time of the photomultiplier tubes, and thereby reduce the detection of background light and the outgoing laser pulse. OCIS codes: 010.3640, 010.4950, 040.5250, 280.1100, 280.1120, 280.1910.

When lidar pulses travel through a short path that includes a relatively high concentration of aerosols, scattering phenomena can alter the power and temporal properties of the pulses significantly, causing undesirable effects in the received pulse. In many applications the design of the lidar transmitter and receiver must consider adverse environmental aerosol conditions to ensure the desired performance. We present an analytical model of lidar system operation when the optical path includes aerosols for use in support of instrument design, simulations, and system evaluation. The model considers an optical path terminated with a solid object, although it can also be applied, with minor modifications, to cases where the expected backscatter occurs from nonsolid objects. The optical path aerosols are characterized by their attenuation and backscatter coefficients derived by the Mie theory from the concentration and particle size distribution of the aerosol. Other inputs include the lidar system parameters and instrument response function, and the model output is the time-resolved received pulse. The model is demonstrated and experimentally validated with military fog oil smoke for short ranges (several meters). The results are obtained with a lidar system operating at a wavelength of 0.905/[micro]m within and outside the aerosol. The model goodness of fit is evaluated using the statistical coefficient of determination whose value ranged from 0.88 to 0.99 in this study. OCIS codes: 280.3400, 280.3640, 290.1090, 290.1350.

A new energy minimization approach based on curve evolution and optimal estimation techniques is presented for processing laser radar range images. Findings reveal the preservation of fine target features and better anomaly suppressing performance.

An optimal algorithm for detecting a target using a ladar system employing Geiger-mode avalanche photodiodes (GAPDs) is presented. The algorithm applies to any scenario where a ranging direct detection ladar is used to determine the presence of a target against a sky background within a specified range window. A complete statistical model of the detection process for GAPDs is presented, including GAPDs that are inactive for a fixed period of time each time they fire. The model is used to develop a constant false alarm rate detection algorithm that minimizes acquisition time. Numerical performance predictions, simulation results, and experimental results are presented. OCIS codes: 040.1345, 280.3400, 040.3780, 280.3640.

The aim of this paper is to investigate the optimal polarizations (i.e., emitted and received polarizations for which the received power is maximum or minimum) in the general case of random mechanisms observed by a bistatic full-polarimetric radar. We are particularly interested in the methods leading to analytical solution as far as this is possible. It is the reason why the Kennaugh matrix is taken as the starting point of the optimizations. The optimal polarizations corresponding to the monostatic and bistatic deterministic cases are found again as particular cases. Index Terms--Bistatic scattering, optimal polarizations, radar polarimetry, random scattering.

Similar in principle to recent implementations of a lidar system at 355 nm [Opt. Lett. 25, 1231 (2000), Appl. Opt. 44, 6023 (2005)], an incoherent-detection Mie Doppler wind lidar at 1064 nm was developed and deployed in 2005 [Opt. Rev. 12, 409 (2005)] for wind measurements in the low troposphere, taking advantage of aerosol scattering for signal enhancement. We present a number of improvements made to the original 1064 nm system to increase its robustness for long-period operation. These include a multimode fiber for receiving the reference signal, a mode scrambler to allow uniform illumination over the Fabry-Perot interferometer, and a fast scannable Fabry-Perot interferometer for calibration and for the determination of outgoing laser frequency during the wind observation. With these improvements in stability, the standard deviation of peak transmission and FWHM of the Fabry--Perot interferometer was determined to be 0.49% and 0.36%, respectively. The lidar wind measurements were validated within a dynamic range of [+ or -] 40 m/s. Comparison experiments with both wind profiler radar and Vaisala wiresonde show good agreement with expected observation error. An example of 24 h continuous observations of wind field and aerosol backscatter coefficients in the boundary layer with 1 min and 30 m temporal and spatial resolution and 3 m/s tolerated wind velocity error is presented and fully demonstrates the stability and robustness of this lidar. OCIS codes: 010.0010, 010.3640, 280.0280, 280.3340, 280.1100.

Polarization radar offers the promise of much more accurate rainfall-rate R estimates than are possible from radar reflectivity factor Z alone, not only by better characterization of the drop size distribution, but also by more reliable correction for attenuation and the identification of hail. However, practical attempts to implement retrieval algorithms have been hampered by the difficulty in coping with the inherent noise in the polarization parameters. In this paper, a variational retrieval scheme is described that overcomes these problems by employing a forward model for differential reflectivity [Z.sub.dr] and differential phase shift [[phi].sub.dp] and iteratively refining the coefficient a in the relationship Z = a[R.sup.b] such that the difference between the forward model and the measurements is minimized in a least squares sense. Two methods are used to ensure that a varies smoothly in both range and azimuth. In range, a is represented by a set of cubic-spline basis functions; in azimuth, the retrieval at one ray is used as a constraint on the next. The result of this smoothing is that the retrieval is tolerant of random errors in [Z.sub.dr] of up to 1 dB and in [[phi].sub.dp] of up to 5[degrees]. Correction for attenuation is achieved simply and effectively by including its effects in the forward model. If hail is present then the forward model is unable to match the observations of [Z.sub.dr] and chap simultaneously. This enables a first pass of the retrieval to be used to identify the radar pixels that contain hail, followed by a second pass in which the fraction of the Z in those gates that is due to hail is retrieved, this time with the scheme being able to forward-model both [Z.sub.dr] and [[phi].sub.dp] accurately. The scheme is tested on S-band radar data from southern England in cases of rain, spherical hail, oblate hail, and mixtures of rain and hail. It is found to be robust and stable, even in the presence of differential phase shift on backscatter.

Circular arrays of plasma filaments induced by femtosecond laser pulses in atmospheric air are shown to support guided modes of electromagnetic radiation in the centimeter and millimeter wavelength range. With the refractive index of laser-induced filaments being lower than the refractive index of nonionized air, arrays of such filaments can serve as a structured waveguide cladding, providing an index guiding of radar signals in a nonionized gas region. In spite of attenuation of radar radiation induced by plasma absorption, filament-array waveguides are shown to enhance radar signal transmission relative to freely propagating radar beams.

A regularized algorithm that has the potential to improve the quality of Raman lidar data processing is presented. Compared to the conventional scheme, the proposed algorithm has the advantage, which results from the fact that it is based on a well-posed procedure. That is, the profile of the aerosol backscatter coefficient is computed directly, using the explicit relationships, without numerical differentiation. Thereafter, the profile of the lidar ratio is retrieved as a regularized solution of a first-kind Volterra integral equation. Once these two steps have been completed, the profile of the aerosol extinction coefficient is computed by a straightforward multiplication. The numerical simulations demonstrated that the proposed algorithm provides good accuracy and resolution of aerosol profile retrievals. The error analysis showed that the retrieved profiles are continuous functions of the measurement errors and of the a priori information uncertainties. OCIS codes: 010.3640, 280.1100, 280.3640, 290.5860, 290.1350, 290.2200.

Motion models for a simple car model are formulated in order to compensate for the negative impact. Thus, the accuracy and the control mode can be improved, which is particularly important for safety-critical applications like autonomous vehicles.

A UV ozone differential-absorption lidar (DIAL) utilizing a Nd:YAG laser and a single Raman cell filled with carbon dioxide (C[O.sub.2]) is designed, developed, and evaluated. The generated wavelengths are 276, 287, and 299 nm, comprising the first to third Stokes lines of the stimulated Raman scattering technique. The correction terms originated from the aerosol extinction, the backscatter, and the absorption by other gases are estimated using a model atmosphere. The experimental results demonstrate that the emitted output energies were 13 mJ/pulse at 276 nm and 287 nm and 5 mJ/pulse at 299 nm, with pump energy of 91 mJ/pulse and a C[O.sub.2] pressure of 0.7 MPa. The three Stokes lines account for 44.0% of the available energy. The use of argon or helium as a buffer gas in the Raman cell was also investigated, but this leads to a dramatic decrease in the third Stokes line, which makes this wavelength practically unusable. Our observations confirmed that 30 min of integration were sufficient to observe ozone concentration profiles up to 10 km. Aerosol extinction and backscatter correction are estimated and applied. The aerosol backscatter correction profile using 287 and 299 nm as reference wavelengths is compared with that using 355 nm. The estimated statistical error is less than 5% at 1.5 km and 10% at 2.6 km. Comparisons with the operational carbon-iodine type chemical ozonesondes demonstrate 20% overestimation of the ozone profiles by the DIAL technique. OCIS codes: 010.3640, 010.4950.

An iterative method for determining slope in noisy lidar data is considered based on the use of a corrected ('shaped') inverted function and an assumed behavior of the unknown function of interest (an 'image function'). The method is utilized for extracting extinction-coefficient profiles from data of multiangle measurements. The sequence and specifics of the retrieval procedure, results of simulations, and essentials of the practical retrieval of particulate extinction-coefficient profiles from signals of the elastic scanning lidar are considered. The methodology may be applicable when extracting the extinction-coefficient profiles from an elastic lidar operating in a multiangle scanning mode, a combined Raman elastic-backscatter lidar, or a high spectral resolution lidar operating in a fixed angular position. OCIS codes: 280.3640, 280.1310.

We present a lidar concept for wind-speed measurements, in which a pulsed laser is used as the source for measurement and reference beams. A fraction of the transmitted pulse is stored in a fiber-optic ring resonator with a path length longer than the pulse. The output of the resonator is a pulse train that is used as the reference beam and can be mixed with the Doppler-shifted measurement signal. Because this reference has traveled a distance equivalent to the measurement beam's path length, low-coherence sources can be used. Inserting an erbium-doped fiber amplifier into the resonator ensures that the stored pulses do not decay in amplitude. Experiments prove that 16 reference pulses of sufficiently constant amplitude and stability can be generated. This would correspond to a measurement range of 240 m in free air over which the returned signal is sampled at equal intervals. Velocity measurements of a hard target have been carried out in the range of 1-10 m/s. The Doppler-measured velocities agree with tachometer reference measurements within [+ or -] 0.09 m/s. OCIS codes: 280.3640, 140.4780, 060.2320.

Recently, a data processing and retrieval algorithm (version 2) for ozone, aerosol, and temperature lidar measurements was developed for an ozone lidar system at the National Institute for Environmental Studies (NIES) in Tsukuba (36 [degrees]N,140 [degrees]E), Japan. A method for obtaining the aerosol boundary altitude and the aerosol extinction-to-backscatter ratio in the version 2 algorithm enables a more accurate determination of the vertical profiles of aerosols and a more accurate correction of the systematic errors caused by aerosols in the vertical profile of ozone. Improvements in signal processing are incorporated for the correction of systematic errors such as the signal-induced noise and the dead-time effect. The mean vertical ozone profiles of the NIES ozone lidar were compared with those of the Stratospheric Aerosol and Gas Experiment II (SAGE II); they agreed well within a 5% relative difference in the 20-40 km altitude range and within 10% up to 45 km. The long-term variations in the NIES ozone lidar also showed good coincidence with the ozonesonde and SAGE II at 20, 25, 30, and 35 km. The temperatures retrieved from the NIES ozone lidar and those given by the National Center for Environmental Prediction agreed within 7 K in the 35-50 km range. OCIS codes: 280.3640, 280.1910.

We introduce a new design concept of laser radar systems that combines both phase comparison and time-of-flight methods. We show from signal-to-noise ratio considerations that there is a fundamental limit to the overall resolution in three-dimensional imaging range laser radar (ladar). We introduce a new metric, volume of resolution, and we show from quantum noise considerations that there is a maximum resolution volume that can be achieved for a given set of system parameters. Consequently, there is a direct trade-off between range resolution and spatial resolution. Thus, in a ladar system, range resolution may be maximized at the expense of spatial image resolution and vice versa. We introduce resolution efficiency [[eta].sub.r] as a new figure of merit for ladar that describes system resolution under the constraints of a specific design, compared with its optimal resolution performance derived from quantum noise considerations. We analyze how the resolution efficiency could be utilized to improve the resolution performance of a ladar system. Our analysis could be extended to all ladars, regardless of whether they are imaging or scanning laser systems. OCIS codes: 010.0010, 010.3640.

An in-line type compact micropulse lidar (MPL) with an annular beam was developed for low-altitude cloud measurement. An optical circulator and a couple of axicon prisms for an annular beam were installed on the lidar optics. The advantage of using the in-line MPL is its ability to obtain a near-range measurement with a narrow field of view of 0.1 mrad and to obtain a depolarization measurement of the orthogonally polarized echoes caused by ice crystals of a low-altitude cloud. The total insertion loss of the lidar optics was 3 dB. Detectors such as avalance photodiode detectors can be operated in an analog mode near the breakdown voltage because of the high isolation of the optical circulator. The ideal lidar echo variation from the nearest distance was verified by measuring the mountain echoes at various distances. The depolarization measurement of a low-altitude ice cloud was also demonstrated. OCIS codes: 280.3640, 120.4820, 010.2940.

An in-line type micropulse lidar (MPL) with an annular beam was designed and the transmitting and receiving characteristics were analyzed. Because the in-line MPL utilizes a common telescope for a transmitter and a receiver and the annular beam always overlaps with the receiver's field of view (FOV), it can measure near-range lidar echoes with a narrow FOV. The transmitting annular beam changes its shape to a nearly nondiffractive beam through propagation. It improves the spatial resolution of the lidar observation. The receiving characteristics showed the ideal lidar echo variation, which was inversely proportional to the square of the distance the beam propagated, even if it was in the near range. OCIS codes: 000.4430, 010.3310, 220.4830, 280.3640.

A compact frequency-modulated, continuous wave (FM-cw) lidar system for measurement of distributed aerosol plumes and hard targets is presented. The system is based on intensity modulation of a laser diode and quadrature detection of the return signals. The advantages of using laser diode amplitude modulation and quadrature detection is a large reduction in the hardware required for processing and storing return signals as well as the availability of off-the-shelf integrated electronic components from the wireless and telecommunication communities. Equations to invert the quadrature signal components and determine spatial distributions of multiple targets are derived. Spatial scattering intensities are used to extract aerosol backscatter coefficients, which can then be directly compared to microphysics aerosol models for environmental measurements. Finally, results from laboratory measurements with a monostatic FM-cw lidar system with both hard targets and aerosols are discussed. OCIS codes: 010.3640, 280.1100, 290.5820, 290.5850.

Field trials carried out at Tarbes airfield in the summer of 2002 offered the unique opportunity to compare the results of simultaneous wake-vortex measurements by the 2-[micro]m pulsed Doppler lidar from DLR, German Aerospace Research Center, and the 10-[micro]m continuous wave (cw) Doppler lidars from ONERA and QinetiQ. The discrepancies in vortex core position obtained from the data of the pulsed lidar and the cw lidars are 9 m for the vertical and 13 m for the horizontal co-ordinates. The accuracies of the vortex circulation measurements with the DLR and ONERA lidars are almost the same and equal 13 [m.sup.2]/s. This accuracy and the long-range capability of the pulsed lidar allows precise measurements over long periods from the moment of wake generation to a progressed state of vortex decay. Moreover, the influence of different atmospheric turbulence conditions and aircraft configurations on the wake-vortex circulation can be analyzed. This has been demonstrated out of ground effect under conditions of weak to moderate levels of turbulence.

Improvements to the tropospheric ozone lidar at the Jet Propulsion Laboratory Table Mountain Facility for measurements of ozone profiles in the troposphere and lower stratosphere, between approximately 5and 20-km altitude, are described. The changes were primarily related to the receiver optical subsystems and the data-acquisition system. The original 40-cm Cassegrain telescope was replaced with a faster (f/3) 91-cm Newtonian mirror. In the focal plane of this mirror, the lidar signal is divided into two parts by use of two separate optical fibers as field stops corresponding to different but neighboring 0.6-mrad fields of view. We then separate the two received wavelengths by aligning each transmitted beam to one of the fibers. In addition, two 50-mm telescopes are used for the collection of near-range returns. The four optical signals are brought to a chopper wheel for independent signal selection in the time and range domain. For each channel, an interference filter is used for skylight rejection and additional cross-talk prevention. The signals are detected with miniature photomultiplier tubes and input to a fast photon-counting system. The goals of these modifications were to increase the spatial and temporal resolution of the lidar, to extend the altitude range covered, to improve the quality of the raw data, and to enable regular and routine operation of the system for long-term measurements. OCIS codes: 010.0010, 010.4950, 010.7030, 010.3640, 280.1910.

Results of lidar modeling based on spatial-angular filtering efficiency criteria are presented. Their analysis shows that the low spatial-angular filtering efficiency of traditional visible and near-infrared systems is an important cause of low signal/background-radiation ratio (SBR) at the photodetector input. The low SBR may be responsible for considerable measurement errors and ensuing the low accuracy of the retrieval of atmospheric optical parameters. As shown, the most effective protection against sky background radiation for groundbased biaxial lidars is the modifying of their angular field according to a spatial-angular filtering efficiency criterion. Some effective approaches to achieve a high filtering efficiency for the receiving system optimization are discussed. OCIS codes: 010.3640, 080.2720, 330.6110, 280.3640.

A new principle of lidar--radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar--radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical preamplification is demonstrated at 1.06 [micro]m. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.

A method of lidar data collection by simultaneous registration of signals from the anode and several dynodes of the photomultiplier is suggested. The dynamic range of the receiver has been extended as many as 5 orders of magnitude in the case of cloud sensing. The stable operation under strong background illumination is possible without losses in fine signal structure. OCIS codes: 040.5250, 120.0280, 230.5160, 280.3640.

The design, development, and first measurements of a novel mesospheric temperature lidar are described. The lidar technique employs mesospheric Fe as a fluorescence tracer and relies on the temperature dependence of the population difference of two closely spaced Fe transitions. The principal advantage of this technique is that robust solid-state broadband laser source(s) can be used that enables the lidar to be deployed at remote locations and aboard research aircraft. We describe the system design and present a detailed analysis of the measurement errors. Correlative temperature observations, made with the Colorado State University Na lidar at Fort Collins, Colorado, are also discussed. Last, we present the initial range-resolved temperature measurements in the mesosphere and lower thermosphere over both the North and the South Poles obtained with this system. OCIS codes: 280.0280, 280.3640, 120.0120, 140.0140.

We have designed a novel rotational Raman and Rayleigh lidar system that incorporates a simple holographic optical element. The hologram simultaneously disperses and focuses the backscattered signal light so that narrow spectral features can be isolated and detected with high efficiency. By measuring the relative strength of several nitrogen rotational Raman lines, we can obtain an accurate temperature of the atmosphere at a given altitude without the need for external calibration. Simultaneous photon counting of the Rayleigh backscatter signal permits temperature measurements at much higher altitudes. OCIS codes: 280.0280, 280.3640, 090.2890, 290.5860, 290.5870.

We describe the design of a small Rayleigh scattering lidar for launch on a sounding rocket as well as the first, to our knowledge, in situ measurements of neutral number density performed with a rocketborne lidar in the mesosphere. The aim of the experiment is to study the dynamics of the neutral atmosphere with emphasis on turbulent structures and gravity waves. The altitude resolution of the density profile is better than 10 m. The uncertainty is 0.3% below 55 km and better than 1% to an altitude of 65 km. The lidar technique meets the requirement of measurement of total molecular density outside the shock front surrounding the supersonic payload, which is necessary for precision measurements of neutral atmospheric density. We have compared different component technologies and design approaches and show performance calculations for two electro-optical systems. The first system has laser and detector components that were available in 1993, the second has new solutions that became available in 1995. The second system has a signal-to-noise ratio that is five times higher than the first and employs a pulsed high-power laser diode array as the transmitter and a large-area avalanche photodiode as the receiver.

We describe a high-performance Raman lidar system with combined day and night capability for tropospheric water-vapor profile measurements. The system incorporates high-performance UV interference filters and a narrow-band, dual-field-of-view receiver for rejection of background sunlight. Daytime performance has been demonstrated up to 5 km with 150-m vertical and 5-min temporal averaging. The nighttime performance is significantly better with measurements routinely extending from 10 to 12 km with 75-m range resolution and a 5-min temporal average. We describe design issues for daytime operation and a novel daytime calibration technique.

The accuracy and the resolution of water-vapor measurements by use of the ground-based differential absorption lidar (DIAL) system of the Max-Planck-Institute (MPI) are determined. A theoretical analysis, intercomparisons with radiosondes, and measurements in high-altitude clouds allow the conclusion that, with the MPI DIAL system, water-vapor measurements with a systematic error of < 5% in the whole troposphere can be performed. Special emphasis is laid on the outstanding daytime and nighttime performance of the DIAL system in the lower troposphere. With a time resolution of 1 min the statistical error varies between 0.05 g/[m.sup.3] in the near range using 75 m and - depending on the meteorological conditions - approximately 0.25 g/[m.sup.3] at 2 km using 150-m vertical resolution. When the eddy correlation method is applied, this accuracy and resolution are sufficient to determine water-vapor flux profiles in the convective boundary layer with a statistical error of < 10% in each data point to approximately 1700 m. The results have contributed to the fact that the DIAL method has finally won recognition as an excellent tool for tropospheric research, in particular for boundary layer research and as a calibration standard for radiosondes and satellites.

An all-solid-state laser transmitter for a water-vapor and temperature differential absorption lidar (DIAL) system in the near infrared is introduced. The laser system is based on a master-slave configuration. As the slave laser a Q-switched unidirectional alexandrite ring laser is used, which is injection seeded by the master laser, a cw Ti:sapphire ring laser. It is demonstrated that this laser system has, what is to my knowledge, the highest frequency stability (15 MHz rms), narrowest bandwidth (< 40 MHz), and highest spectral purity (> 99.99%) of all the laser transmitters developed to date in the near infrared. These specifications fulfill the requirements for water-vapor measurements with an error caused by laser properties of < 5% and temperature measurements with an error caused by laser properties of < 1 K in the whole troposphere. The specifications are maintained during long-term operation in the field. The single-mode operation of this laser system makes the narrow-band detection of the DIAL backscatter signal possible. Thus the system has the potential to be used for accurate temperature measurements and for simultaneous DIAL and Doppler wind measurements.

Design considerations for a coaxial lidar receiver are examined, including details of coupling to an optical fiber for transfer of return light to a remote detector box. Attention is concentrated on the influence of fiber position on return-light capture efficiency and dynamic range of the return signal. The effect of a central obstruction on short-range signals is included. The analysis is augmented with simulations of lidar receiver performance. Key words: Lidar, optical instruments, simulation, remote sensing, atmospheric optics.

Two continuous-wave (CW) focused C[O.sub.2] Doppler lidars (9.1 and 10.6 [[micro]meter]) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is [approximately]3 x [10.sup.-12][m.sup.-1][sr.sup.-1] at 9.1 [[micro]meter] and [approximately]9 x [10.sup.-12][m.sup.-1][sr.sup.-1] at 10.6 [[micro]meter]. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-[[micro]meter] lidar intercomparisons were made. Measurements at 9.1 [[micro]meter], a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope 12C 18[O.sub.2] gas.

An airborne differential absorption lidar (DIAL) system has been developed at the NASA Langley Research Center for remote measurements of atmospheric water vapor ([H.sub.2]O) and aerosols. Asolid-state alexandrite laser with a 1-pm linewidth and >99.85% spectral purity was used as the on-line transmitter. Solid-state avalanche photodiode detector technology has replaced photomultiplier tubes in the receiver system, providing an average increase by a factor of 1.5-2.5 in the signal-to-noise ratio of the [H.sub.2]O measurement. By incorporating advanced diagnostic and data-acquisition instrumentation into other subsystems, we achieved additional improvements in system operational reliability and measurement accuracy. Laboratory spectroscopic measurements of [H.sub.2]O absorption-line parameters were performed to reduce the uncertainties in our knowledge of the absorption cross sections. Line-center [H.sub.2]O absorption cross sections were determined, with errors of 3-6%, for more than 120 lines in the 720-nm region. Flight tests of the system were conducted during 1989-1991 on the NASA Wallops Flight Facility Electra aircraft, and extensive intercomparison measurements were performed with dew-point hygrometers and [H.sub.2]O radiosondes. The [H.sub.2]O distributions measured with the DIAL system differed by [less than or equal to] 10% from the profiles determined with the in situ probes in a variety of atmospheric conditions. Key words: Differential absorption lidar, alexandrite laser, water vapor, aerosols.

A key issue in designing laser radar devices for short-range applications is the ability to estimate accurately the power seen by the receiver as a function of the measurement distance. To obtain a reasonable approximation of this power, the irradiance distribution over the sensor as well as the target surface, which is highly dependent on the type of the detector used, must be analyzed in detail. The calculation of signal power function by means of radiometry is discussed. A software package developed for simulating power transfer as a function of various optical parameters is presented. It can be applied to various types of laser sources, including high-power laser diodes (wide-stripe or stacked) and pigtailed laser diodes.

The need for an instrument capable of measuring water-vapor fluxes over mixed canopy and large areas has long been recognized. Such a device would greatly enhance the study of evapotranspiration processes and has great practical value for water management. To address this problem, a scanning water Raman lidar has been designed and constructed. Analytical methods have also been developed to take advantage of the type of information that this lidar can generate. The lidar is able to measure the absolute water content and calculate the evaporative flux quickly over relatively large areas. This capability provides new opportunities for the study of microscale atmospheric processes. The variogram data indicate that the spatial sampling size must be of the order of 10 m if fluxes and scalars are to be properly represented. Examples of data are presented.

We describe the instrument structure and the operation of a Doppler Rayleigh lidar located at the Arecibo Observatory in Puerto Rico. The device consists of an injection seeded, frequency doubled, Nd:YAG laser coupled with a small Newtonian telescope and a single etalon, Fabry-Perot interferometer receiver. With this combination of transmitter and receiver, observations of the Doppler spectral components of the Rayleigh backscattered signal from the atmosphere are possible. With it we have obtained the neutral winds and temperatures up to mesospheric heights. We also discuss the improvements to the instrument that are underway, enabling us to increase the temporal and spectral sampling of the winds.

An eye safe, compact, solid state lidar for profiling atmospheric cloud and aerosol scattering has been demonstrated. The transmitter of the micro pulse lidar is a diode pumped microjoule pulse energy, high repetition rate Nd:YLF laser. Eye safety is obtained through beam expansion. The receiver employs a photon counting solid state Geiger mode avalanche photodiode detector. Data acquisition is by a single card multichannel scaler. Daytime background induced quantum noise is controlled by a narrow receiver field-of-view (FOV) and a narrow bandwidth temperature controlled interference filter. Dynamic range of the signal is limited by optical geometric signal compression. Signal simulations and initial atmospheric measurements indicate that systems built on the micro pulse lidar concept are capable of detecting and profiling all significant cloud and aerosol scattering through the troposphere and into the stratosphere. The intended applications are scientific studies and environmental monitoring which require full time, unattended measurements of the cloud and aerosol height structure.

Coherent laser radar systems using 2 micrometer Tm- and Tm, Ho-doped solid-state lasers are useful for the remote range-resolved measurement of atmospheric winds, aerosol backscatter, and DIAL measurements of atmospheric water vapor and CO2 concentrations. Recent measurements made with a 2 micrometer coherent laser radar system, advances in the laser technology, and atmospheric propagation effects on 2 micrometer coherent lidar performance are described.

We have developed a Fabry - Perot interferometer and image-plane detector system to be used as a receiver for a Doppler lidar. This system incorporates the latest technology in multichannel detectors, and it is an important step toward the development of operational wind profiler systems for the atmosphere (troposphere, stratosphere, and lower mesosphere). The instrumentation includes a stable high-resolution optically contacted plane etalon and a multiring anode detector to scan the image plane of the Fabry - Perot interferometer spatially. The high wavelength resolution provided by the interferometer permits the aerosol and molecular components of the backscattered signal to be distinguished, and the Doppler shift of either component can then be used to determine the wind altitude profile. The receiver performance has been tested by measuring the wind profile in the boundary layer. The Fabry - Perot interferometer and image-plane detector characteristics are described and sample measurements are presented. The potential of the system as a wind profiler in the troposphere, the stratosphere, and the mesosphere is also considered.

We have developed a method for giving direction and/or position information in an optical guidance system based on the pulse position modulation property of a reflected light pulse train from a revolving retroreflector. The principle and theoretical analysis are described. Experimental verification of the one-axis, closed-loop tracking system is presented, which resulted in a static angular resolution as low as 22 [mu]rad. This method suggests some potential applications to coordinate determinations or line-of-sight guidance systems. Key words: Optical guidance, reticle, beam riding.