Coherent Lidar System Research Articles

Computer simulation of coherent Doppler lidar measurement of wind velocity and retrieval of turbulent wind statistics

We describe the algorithms for the simulation of cw and pulsed coherent Doppler wind lidar operation in a turbulent atmosphere and the methods to estimate the mean wind and the parameters of small-scale wind turbulence from lidar data. The algorithms are realized as LabView computer codes, which include parts simulating the atmo- spheric turbulent wind fields based on the Karman model, lidar signal, and data processing. The errors for lidar measurement of turbulent pa- rameters and the retrieval of the wind vertical profiles are estimated based on a computer simulation of the coherent Doppler lidar system

Wind Profiling with an Eye-Safe Coherent Doppler Lidar System: Comparison with Radiosondes and VHF Radar

An eye-safe airborne coherent Doppler lidar system has been developed at the National Institute of Information and Communications Technology (NICT). This system can also be used at ground level and ground-based experiments were made to evaluate the system performance at the NICT Wakkanai radio observatory. The obtained wind profiles were compared with those measured by radiosondes launched at the Wakkanai local meteorological observatory, and by a VHFR system installed by NICT near the Wakkanai airport. Although the volume measured by the coherent Doppler lidar system differed spatially and temporally from those by the radiosonde and the NICT VHF radar system, the wind profiles observed by the coherent Doppler lidar system agreed well with those observed by other instruments. The wind velocities obtained through the full correlation analysis of the VHF radar system were a little smaller than those from the coherent Doppler lidar system, which is consistent with previous statistical results for the VHF radar system. The ground-based results showed that the standard deviations of wind velocity estimates in the laser pulse accumulation seemed to approach a constant value under the real atmospheric conditions, and that the lower limit of the signal-to-noise ratio for the reliable estimate of the wind velocity decreases with the laser pulse accumulation. The ground-based experiments showed that the coherent Doppler lidar system is a reliable system for wind-related research.

Lidar Measurement of Ammonia Concentrations and Fluxes in a Plume from a Point Source

A field experiment was performed that demonstrated the ability of a scanning carbon dioxide (CO2) coherent lidar system to measure the concentration distribution of ammonia in a plume from a point source. This application of the differential absorption lidar (DIAL) method used wavelengths of CO2 lasers at 10.288 μm (online) and 10.274 μm (offline). The addition of wind information, which was obtained from Doppler measurements with the same lidar, permitted ammonia flux measurements through a vertical plane. The measurements were performed in the early morning when the atmosphere was stable, cool, and dry. Ammonia fluxes calculated from the lidar data showed satisfactory agreement with the ammonia release rates measured by a flowmeter. Modifications are planned to improve sensitivity and to enable measurement of ambient ammonia concentrations in polluted regions.

Amplitude and frequency stabilisation of a Tm–Ho:YAG laser for coherent lidar applications at 2.1 μm

We developed a low-noise, single-frequency Tm–Ho:YAG laser tunable in the wavelength interval between 2087 and 2099 nm. To suppress both amplitude and frequency fluctuations the laser has been stabilised by two different control loops. Intensity noise has been effectively reduced using a feedback loop acting on the pump diode current, based on a biquadratic bandpass filter, which provides up to 17 dB suppression at the relaxation oscillation peak. Absolute frequency stabilisation has been achieved by locking the oscillator to the P(12) absorption line of the HBr molecule at 2097.2 nm using the fringe side locking technique, obtaining a long-term frequency stability better than 32 kHz over an observation time of 60 min. This stabilised source is aimed to injection seeding of a coherent lidar system for high precision measurements of wind velocity.

2-µm Doppler lidar transmitter with high frequency stability and low chirp

A coherent Doppler lidar system was frequency stabilized in a master-slave configuration by a phase-modulation technique. The short-term frequency stability, ~0.2 MHz rms, was maintained in a vibrational environment on a ship during a field campaign in the tropical Pacific Ocean. The long-term frequency stability was <2.6 kHz/h. Thus, in many applications, shot-to-shot frequency correction can be disregarded, which will result in increased speed and simplicity of the data-acquisition system. A frequency chirp could not be detected. These properties permit Doppler wind measurements with high efficiency and duty cycles to be made, even on airborne and spaceborne platforms.

Line-pair selections for remote sensing of atmospheric ammonia by use of a coherent CO_2 differential absorption lidar system

Research on wavelength selection of CO2 laser lines for range-resolved remote sensing of atmospheric ammonia by use of a coherent differential absorption lidar system is described. Four laser line pairs are suggested for different levels of ammonia concentrations from approximately a few parts per billion to 1 part per million in a polluted atmosphere. The most suitable line for measuring ambient ammonia concentrations is 9R(30), because it has the highest absorption coefficient. 10R(14) has the lowest absorption coefficient, making it suitable for strong source mapping. 10R(8) and 10P(32) are best for intermediate levels of ammonia concentration. Absorption coefficients of ammonia calculated from the HITRAN96 database are in good agreement (mostly within +/-10% )with other experimental results.Sensitivity of measurement, interference from water-vapor lines with typical humidity in the summer,and sensitivity of ammonia absorption cross section to temperature and pressure are analyzed and calculated for the four wavelength pairs. The results show that the interference from water-vapor lines is easily correctable to a negligible amount, and errors caused by uncertainties in temperature and pressure are insignificant.

108 コヒーレント・ドップラー・ライダーによる風観測の可視化データ解析

Coherent Doppler lidar provide important data for investigating turbulent transport and exchange processes in the lower troposphere with high vertical and time resolution.. TIT (Tohoku Institute of Technology) and CRL(Communication Research Laboratories) have been developing the 2-μm in coherent lidar system. In this paper, we discuss the conceptual design of lidar system and also the algorithm for data analysis in order to visualize wind data. current status of our coherent Doppler lidar.

Solid-state coherent laser radar wind field measurement systems

Diode-pumped solid-state pulsed coherent laser radar systems have recently been developed at Coherent Technologies, Inc., for the remote measurement of atmospheric wind fields. Flashlamp-pumped systems have been utilized since 1990 for obtaining wind field measurements. These flashlamp-pumped lidar systems have been applied to wind profiling, aircraft-wake vortex measurements, airport wind shear and gust front monitoring, military cargo air drops and many other applications. The diode-pumped coherent lidar systems currently available are capable of near turnkey operation. The Tm:YAG laser transceivers operate at with output pulse energies of 1-10 mJ with PRFs of 1000 to 100 Hz, respectively. Range resolutions of 30-75 m are typical. A real-time lidar signal processor has also been developed for collecting and analysing laser radar (lidar) data. The signal processor is based on a commercial PC architecture and offers a real-time data acquisition, analysis, display, recording and playback environment. Wind measurements and overall system performance results will be presented. Wind measurement performance, for a variety of applications, is presented using the flashlamp- and diode-pumped coherent lidars including measured wind profiles from ground and on aircraft, wake vortex tracking results, and example flows over mountain terrain.

Atmospheric refractive turbulence effect on diffraction-limited infrared coherent lidar

This paper, based on the Kavaya-Suni format, discusses the signal-to-noise ratio equation of the diffraction-limited coherent CO2 lidar in detail, which is applied to atmospheric turbulence. The cumulative SNR and relative SNR, which are all affected by the nonlinear effects of the diffraction-limited Gaussian beam, atmospheric molecule and atmospheric turbulence, are simulated by microcomputer. Six instructions for the optimal design of IR CO2 Coherent Lidar System, are provided.

Modeling heterodyne efficiency for coherent laser radar in the presence of aberrations

Heterodyne efficiency of a coherent lidar system reflects the matching of phase and amplitude between a local oscillator (LO) beam and received signal beam and is, therefore, an indicator of system performance. One aspect of a lidar system that affects heterodyne efficiency is aberrations present in optical components. A method for including aberrations in the determination of heterodyne efficiency is presented. The effect of aberrations on heterodyne efficiency is demonstrated by including Seidel aberrations in the mixing of two perfectly matched gaussian beams. Results for this case are presented as animations that illustrate the behavior of the mixing as a function of time. Extension of this method to propagation through lidar optical systems is discussed.

Frequency Spectral Measurements of a Pulsed, TE CO2

Abstract Experimental measurements of the intra-pulse chirp and temporal coherence from a Joule class TE CO2 laser incorporating LAWS transmitter design features are presented. Digitized quadrature data (I and Q) from our ground-based coherent Doppler lidar system utilizing return signals off a hard target in the telescope far field are processed using fast Fourier transform and pulse pair techniques to obtain laser pulse frequency spectral components (offset frequency and spectral width) and high-resolution (∼ 50 ns/sample) frequency chirp profiles. Less than 300 kHz of frequency chirp is observed in the first 3·5 μs of the laser pulse which contains approximately 90% of the pulse energy. Spectral width of the laser pulse, including both chirp and transform limited components, are measured to be less than 300 kHz full width at half maximum.

Coherent CO&lt;inline-formula&gt;&lt;sup&gt;&lt;roman&gt;2&lt;/roman&gt;&lt;/sup&gt;&lt;/inline-formula&gt; lidars for measuring wind velocity and atmospheric turbulence

We have developed two coherent lidar systems based on cw and TEA CO₂ lasers. The lidars have been tested and applied for the atmospheric wind velocity measurements. We have elaborated and experimentally verified a new method of nondestructive long-range measurement of the parameters of the atmospheric turbulence with a cw Doppler lidar (DL). The method is based on certain processing of the heterodyne Doppler spectra. In addition to the wind velocity magnitude, the method yields the value of the velocity structure constant. The method has been experimentally verified. Some results of applications of the DL for detecting artificially generated vortexes (aircraft traveling vortexes) and distinguishing them from natural winds and turbulence based on a comparison of the magnitudes of the velocity structure constant are presented.

Coherent Doppler lidar signal covariance including wind shear and wind turbulence

The performance of a coherent Doppler lidar is determined by the statistics of the coherent Doppler signal. The derivation and calculation of the covariance of the Doppler lidar signal for random atmospheric wind fields and wind shear are presented. The signal parameters are defined for a general coherent Doppler lidar system in terms of the atmospheric parameters. There are two distinct physical regimes: one in which the transmitted pulse determines the signal statistics and the other in which the wind field and the atmospheric parameters dominate the signal statistics. When the wind fields dominate the signal statistics, Doppler lidar data are nonstationary and the signal correlation time is proportional to the operating wavelength of the lidar. The signal covariance is derived for signal-shot and multiple-shot conditions. For a single shot, the parameters of the signal covariance depend on the random, instantaneous atmospheric parameters. For multiple shots, various levels of ensemble averaging over the t emporal scales of the atmospheric processes are required. The wind turbulence is described by a Kolmogorov spectrum with an outer scale of turbulence. The effects of the wind turbulence are demonstrated with calculations for a horizontal propagation path in the atmospheric surface layer.

Receiving efficiency of monostatic pulsed coherent lidars 2: Applications

Using the theory developed in Part 1, the receiving efficiency as a function of range, eta(z), is calculated under different conditions for the NOAA/ERL/Wave Propagation Laboratory CO(2) Doppler lidar. Theoretical analyses, numerical calculations, and experimental measurements are carried out to quantify the sensitivity of eta(z) to transmitted laser beam quality, telescope focal setting, telescope power, scanner astigmatism, LO beam divergence, and system misalignment. These results bring insight to the design of practical coherent lidar systems.

Remote wind profiling with a solid-state Nd:YAG coherent lidar system

A solid-state coherent lidar system employing Nd:YAG lasers and heterodyne detection has been successfully developed, and range-resolved aerosol backscatter and wind-velocity measurements have been demonstrated.

Rapid-tuning device for CO2 heterodyne detection lidar

A device for rapid-tuning cw, Q-switched lasers for a CO2 heterodyne detection lidar is presented. It is shown that it is possible to utilize galvanometer-driven mirrors to rapidly switch wavelengths over randomly selected lasing transitions in the 9–11 μm portion of the spectrum. Both a transmitter and a local oscillator are simultaneously switched between transitions while still achieving the frequency stability typically required for a coherent lidar system.

Aerosol speckle effects on atmospheric pulsed lidar backscattered signals

The effects of refractive turbulence along the path on the aerosol speckle field propagation and on the decorrelation time are studied for coherent pulsed lidar systems.

Optimum truncation of a lidar transmitted beam

The problem of optical beam truncation through a transmitter aperture to maximize signal returns from a coherent lidar system is considered. For common transmitter and receiver apertures of the same size, the optimum ratio of receiver diameter to beam diameter is ~1.75 when viewing a far-field diffuse target. The beam diameter is defined as twice the radius at which the beam intensity is exp(-1) of the peak intensity.

Conceptual test of frequency diversity lidar

The present consideration of the estimation accuracy of lidar-backscattered signals in atmospheric remote sensing proposes a method that can furnish additional uncorrelated data within the atmospheric decorrelation time, with enhanced energy efficiency. This frequency-diversity method recognizes that additional uncorrelated data from a given illuminated region is obtainable by changing the frequency of the incoming radiation. The mathematical analysis of the frequency-diversity concept by Wallace (1953) confirms the physical scenario employed. A simple version of a coherent lidar system incorporating the frequency-diversity concept is included to illustrate the design principles involved.

Investigation of tunable single frequency diode lasers for sensor applications

The coherence and tuning properties of tunable single-frequencies lasers used in coherent detection systems have been investigated. The use of a fixed external cavity was found to increase significantly the coherence length of such systems but simultaneously reduce the tunability. An essentially all-electronic phase-modulated external cavity was used to restore the tunability, and a tuning range coherence length product of approximately 500 GHz-m was achieved. The modulated external activity laser was used in a coherent lidar system that achieved a maximum range of 150 m and a fractional resolution of 0.5%. The system is also shown to be a powerful tool for mapping out the tuning and coherence properties of tunable single-frequency lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>