Low-altitude atmospheric wind measurement from the combined Mie and Rayleigh backscattering by Doppler lidar with an iodine filter

This paper briefly discusses the mobile ground-based incoherent Doppler wind lidar system, with iodine filters as receiving frequency discriminators, developed by the Ocean Remote Sensing Laboratory, Ocean University of Qingdao, China. The presented result of wind profiles in October and November 2000, retrieved from the combined Mie and Rayleigh backscattering, is the first report to our knowledge of wind measurements in the troposphere by such a system, where the required independent measurement of aerosol-scattering ratio can also be performed. A second iodine vapor filter was used to lock the laser to absolute frequency reference for both wind and aerosol-scattering ratio measurements. Intercomparison experiments of the lidar wind profile measurements were performed with pilot balloons. Results showed that the standard deviation of wind speed and wind direction, for the 2-4 km altitude range, were 0.985 m/s and 17.9 degrees, respectively.     

A newly designed high-spectral-resolution Rayleigh temperature lidar based on two-stage Fabry–Perot interferometer

A two-stage Fabry–Perot interferometer (FPI)-based high-spectral-resolution (HSR) Rayleigh temperature lidar technology is proposed that is capable of simultaneously detecting tropospheric temperature and aerosol optical properties with high-precision. The system structure is designed and the measurement principle is analysed. A two-channel integrated FPI used forming a two-stage FPI ensures the relative stability of the two FPI spectrums. The first-stage FPI with high spectral resolution can effectively separate Mie and Rayleigh signals to derive the signal components. Two adjacent-order transmission spectrums of the second-stage FPI are just located in the two wings of Rayleigh–Brillouin (R–B) scattering spectrum to measure temperature. Two multimode polarization insensitive optical circulators used in receiver system can achieve high-efficiency utilization of signals. A narrow linewidth semiconductor laser at 852 nm is used as light source. Using the selected and optimized system parameters, the lidar performance simulation results show that in the sunny weather conditions for 0.15WSr^–1 m^–2 nm^–1 sky brightness, with 0.3 W laser power, a 30 cm diameter telescope, 60 m range resolution and 30 min observation time, the temperature measurement errors are below 0.4 K in night-time and below 1.6 K in daytime; the relative measurement errors of backscatter ratio are below 0.04% in night-time and below 0.13% in daytime respectively up to 6 km height. Compared with the traditional FPI-based HSR technique, the technique we proposed can improve the detection accuracy of temperature by 2.5 times and can also significantly improve the detection accuracy of backscatter ratio.     

Fabry-Perot interferometer based Mie Doppler lidar for low tropospheric wind observation

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 +/-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.     

Comment on "Theory of the double-edge molecular technique for Doppler lidar wind measurement"

A theory of the double-edge technique for lidar measurement of wind speed Doppler shifts was recently presented by Flesia and Korb [Appl. Opt. 38, 432 (1999)]. It is shown here that the technique proposed by Flesia and Korb to achieve equal responsivity to aerosol and Rayleigh backscatter signals was previously conceived and demonstrated by another group.     

Ultraviolet Rayleigh-Mie lidar with Mie-scattering correction by Fabry-Perot etalons for temperature profiling of the troposphere

A Rayleigh-Mie-scattering lidar system at an eye-safe 355-nm ultraviolet wavelength that is based on a high-spectral-resolution lidar technique is demonstrated for measuring the vertical temperature profile of the troposphere. Two Rayleigh signals, which determine the atmospheric temperature, are filtered with two Fabry-Perot etalon filters. The filters are located on the same side of the wings of the Rayleigh-scattering spectrum and are optically constructed with a dual-pass optical layout. This configuration achieves a high rejection rate for Mie scattering and reasonable transmission for Rayleigh scattering. The Mie signal is detected with a third Fabry-Perot etalon filter, which is centered at the laser frequency. The filter parameters were optimized by numerical calculation; the results showed a Mie rejection of approximately -45 dB, and Rayleigh transmittance greater than 1% could be achieved for the two Rayleigh channels. A Mie correction method is demonstrated that uses an independent measure of the aerosol scattering to correct the temperature measurements that have been influenced by the aerosols and clouds. Simulations and preliminary experiments have demonstrated that the performance of the dual-pass etalon and Mie correction method is highly effective in practical applications. Simulation results have shown that the temperature errors that are due to noise are less than 1 K up to a height of 4 km for daytime measurement for 300 W m(-2) sr(-1) microm(-1) sky brightness with a lidar system that uses 200 mJ of laser energy, a 3.5-min integration time, and a 25-cm telescope.     

Modeling of direct detection Doppler wind lidar. I. The edge technique

Analytic models, based on a convolution of a Fabry-Perot etalon transfer function with a Gaussian spectral source, are developed for the shot-noise-limited measurement precision of Doppler wind lidars based on the edge filter technique by use of either molecular or aerosol atmospheric backscatter. The Rayleigh backscatter formulation yields a map of theoretical sensitivity versus etalon parameters, permitting design optimization and showing that the optimal system will have a Doppler measurement uncertainty no better than approximately 2.4 times that of a perfect, lossless receiver. An extension of the models to include the effect of limited etalon aperture leads to a condition for the minimum aperture required to match light collection optics. It is shown that, depending on the choice of operating point, the etalon aperture finesse must be 4-15 to avoid degradation of measurement precision. A convenient, closed-form expression for the measurement precision is obtained for spectrally narrow backscatter and is shown to be useful for backscatter that is spectrally broad as well. The models are extended to include extrinsic noise, such as solar background or the Rayleigh background on an aerosol Doppler lidar. A comparison of the model predictions with experiment has not yet been possible, but a comparison with detailed instrument modeling by McGill and Spinhirne shows satisfactory agreement. The models derived here will be more conveniently implemented than McGill and Spinhirne's and more readily permit physical insights to the optimization and limitations of the double-edge technique.     

Rayleigh-Mie Doppler wind lidar for atmospheric measurements. II. Mie scattering effect, theory, and calibration

A theoretical and experimental study is conducted for the direct-detection Doppler Lidar developed by the Service d'Aéronomie du Centre National de la Recherche Scientifique. Thanks to a specific design, the double-edge technique that applies primarily to Rayleigh scattering can also be employed in presence of aerosols backscatter. We focus on a careful estimate of the particle-induced error on the wind measurements. With a theoretical model for the Fabry-Perot interferometer and two sets of calibration measurements, the true spectral properties of the interferometer and the calibration curves are recovered. Furthermore, the particle-induced error is estimated for varying values of the scattering ratio at 532 nm. When applied to real atmospheric signals, this error is shown to be negligible. A comparison between ancillary data and the wind and backscatter ratio as retrieved from the Doppler lidar signals confirms our estimate.     

Ultraviolet Rayleigh-Mie lidar for daytime-temperature profiling of the troposphere

A UV Rayleigh-Mie scattering lidar has been developed for daytime measurement of temperature and aerosol optical properties in the troposphere. The transmitter is a narrowband, injection-seeded, pulsed, third-harmonic Nd:YAG laser at an eye-safe wavelength of 355 nm. Two Fabry-Perot etalons (FPEs) with a dual-pass optical layout filter the molecular Rayleigh scattering components spectrally for retrieval of the temperature and provide a high rejection rate for aerosol Mie scattering in excess of 43 dB. The Mie signal is filtered with a third FPE filter for direct profiling of aerosol optical properties. The Mie scattering component in the Rayleigh signals, which will have influence on temperature measurements, is corrected by using a measure of aerosol scattering because of the relative insufficiency of Mie rejection of Rayleigh filters in the presence of dense aerosols or clouds, and the Mie rejection capability of system is thus improved. A narrowband interference filter is incorporated with the FPEs to block solar radiation. Also, the small field of view (0.1 mrad) of the receiver and the UV wavelength used enhance the ability of the lidar to suppress the solar background signal in daytime measurement. The system is relatively compact, with a power-aperture product of 0.18 W m(-2), and has a high sensitivity to temperature change (0.62%/K). Lidar measurements taken under different weather conditions (winter and summer) are demonstrated. Good agreement between the lidar and the radiosonde measurements was obtained in terms of lapse rates and inversions. Statistical temperature errors of less than 1 K up to a height of 2 km are obtainable, with an averaging time of approximately 12 min for daytime measurements.     

Modeling the performance of direct-detection Doppler lidar systems including cloud and solar background variability

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar system: the double-edge and the multichannel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only approximately 10-20% compared with nighttime performance, provided that a proper solar filter is included in the instrument design.     

Lidar calibration technique using laboratory-generated aerosols

A new calibration technique for continuous-wave Doppler lidars that uses an aerosol scattering target has been developed. Calibrations with both single- and many-particle scattering were performed at the same lidar operating conditions as in atmospheric measurements. The calibrating targets, simulating atmospheric aerosols, were laboratory-generated spherical silicone oil droplets with known complex refractive indices and sizes, hence with known single-particle backscatter cross sections as obtained from Mie theory. Measurements of lidar efficiency with the conventional hard target calibration method were consistently higher by a factor of ~2 than measurements with the aerosol calibration technique. This result may have important implications for lidar backscatter estimates both for aerosol modeling efforts and for optimal design of future lidar systems. The aerosol calibration method provides a validation of basic lidar theory for particle scattering for coherent detection.     

High spectral resolution lidar to measure optical scattering properties of atmospheric aerosols. 1: theory and instrumentation

A high spectral resolution lidar technique to measure optical scattering properties of atmospheric aerosols is described. Light backscattered by the atmosphere from a narrowband optically pumped oscillator-amplifier dye laser is separated into its Doppler broadened molecular and elastically scattered aerosol components by a two-channel Fabry-Perot polyetalon interferometer. Aerosol optical properties, such as the backscatter ratio, optical depth, extinction cross section, scattering cross section, and the backscatter phase function, are derived from the two-channel measurements.     

Modeling of Direct Detection Doppler Wind Lidar. II. The Fringe Imaging Technique

A simple analytic model is developed for the shot-noise-limited measurement precision of Doppler wind lidars based on the fringe imaging technique by use of either molecular or aerosol atmospheric backscatter. The model leads to etalon design parameters for an instrument optimized for precision. The ultimate measurement precision possible is two to four times the limit for a perfect, lossless receiver. The corresponding result for the double-edge Doppler analyzer was a ratio of 2.5, showing that the two methods are little different in this respect. For aerosol backscatter instruments, the wind speed dynamic range of the fringe imager is substantially greater than that for the edge detector. The etalon aperture needed to meet system etendue requirements is derived and shown to be approximately half that of each of the two etalons required by the double-edge technique. A comparison with more detailed modeling of fringe imaging Doppler-shift analyzers shows good agreement for the Rayleigh model and fair for the aerosol version, confirming the validity of this simpler technique for analyzer design and performance prediction.     

Measurement of the lidar ratio for atmospheric aerosols with a 180 degrees backscatter nephelometer

Laser radar (lidar) can be used to estimate atmospheric extinction coefficients that are due to aerosols if the ratio between optical extinction and 180 degrees backscatter (the lidar ratio) at the laser wavelength is known or if Raman or high spectral resolution data are available. Most lidar instruments, however, do not have Raman or high spectral resolution capability, which makes knowledge of the lidar ratio essential. We have modified an integrating nephelometer, which measures the scattering component of light extinction, by addition of a backward-pointing laser light source such that the detected light corresponds to integrated scattering over 176-178 degrees at a common lidar wavelength of 532 nm. Mie calculations indicate that the detected quantity is an excellent proxy for 180 degrees backscatter. When combined with existing techniques for measuring total scattering and absorption by particles, the new device permits a direct determination of the lidar ratio. A four-point calibration, run by filling the enclosed sample volume with particle-free gases of a known scattering coefficient, indicates a linear response and calibration reproducibility to within 4%. The instrument has a detection limit of 1.5 x 10(-7) m(-1) sr(-1) (~10% of Rayleigh scattering by air at STP) for a 5-min average and is suitable for ground and mobile/airborne surveys. Initial field measurements yielded a lidar ratio of ~20 for marine aerosols and ~60-70 for continental aerosols, with an uncertainty of ~20%.     

Special relativity corrections for space-based lidars

The theory of special relativity is used to analyze some of the physical phenomena associated with space-based coherent Doppler lidars aimed at Earth and the atmosphere. Two important cases of diffuse scattering and retroreflection by lidar targets are treated. For the case of diffuse scattering, we show that for a coaligned transmitter and receiver on the moving satellite, there is no angle between transmitted and returned radiation. However, the ray that enters the receiver does not correspond to a retroreflected ray by the target. For the retroreflection case there is misalignment between the transmitted ray and the received ray. In addition, the Doppler shift in the frequency and the amount of tip for the receiver aperture when needed are calculated. The error in estimating wind because of the Doppler shift in the frequency due to special relativity effects is examined. The results are then applied to a proposed space-based pulsed coherent Doppler lidar at NASA's Marshall Space Flight Center for wind and aerosol backscatter measurements. The lidar uses an orbiting spacecraft with a pulsed laser source and measures the Doppler shift between the transmitted and the received frequencies to determine the atmospheric wind velocities. We show that the special relativity effects are small for the proposed system.     

High-pulse-repetition-freqyency lidar system using a single telescope for transmission and reception

The design, construction, and operation of a stratospheric Rayleigh lidar system is outlined. The lidar system was designed to operate as a Doppler lidar; however, for the first stage of the project it was set up to operate in a manner similar to a more conventional stratospheric Rayleigh lidar. This system includes a number of unique design features, including a high-pulse-repetition-frequency laser and the use of a single 1-m-diameter telescope for transmission of the laser pulse and reception of the backscattered light. An associated high-speed rotating shutter system switches the optical system from the transmission to the reception mode. The system was operated at Adelaide, Australia (35° S, 138° E). Scattering ratio and temperature profiles are calculated for data collected during the period from 10 March 1992 to 11 May 1993. The scattering ratio profiles clearly show the reduction in the scattering from the stratospheric aerosol layer. This is due to the removal of the aerosol injected by the eruption of Mt. Pinatubo. The measured relative density profiles show very good agreement with the Cospar international reference atmosphere model densities, as do the temperature profiles calculated from these.     

Two-channel direct-detection Doppler lidar employing a charge-coupled device as a detector

A direct-detection Doppler lidar system is demonstrated that uses a CCD as a detector for the first time to our knowledge. The ability to use this linear device with the circular output from a Fabry-Perot etalon comes from use of a circle-to-line converter [Appl. Opt. 29, 1482 (1990)]. In addition to the gains in quantum efficiency obtained through use of this detector, the lidar system described in this paper also has the capability to measure winds from aerosol and molecular backscatter simultaneously in two separate channels by directing the light reflected from one channel into the other. Early measurements with this system are presented; it is shown that, although accurate aerosol wind measurements are easily obtained, molecular measurements require a carefully calibrated inverse model and special hardware to derive accurate wind measurements with this channel.     

Lidar aerosol backscatter cross sections in the 2-νm near-infrared wavelength region

Lidar backscatter cross-sectional measurements at 1.064, 0.532, and 1.54 μm were acquired during November 1989 and May-June 1990 around the Pacific region by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The primary motivation for the Global Backscatter Experiment was the study of lidar backscatter cross sections for the development of a spaceborne wind-sensing lidar. Direct backscatter measurements obtained by the NASA Goddard Space Flight Center visible and infrared lidar are compared with backscatter cross sections calculated from aerosol size distributions obtained by particle counters. Results for one flight with pronounced aerosol layers in the upper troposphere southeast of Japan are presented. Because 2-μm region wavelengths are possible candidates for a spaceborne wind-sensing lidar, the visible and infrared lidar backscatter cross sections at 1.064, 0.532, and 1.54 μm are extrapolated to the 2-μm region. The extrapolated 2-μm cross sections are compared with lidar measurements at 9 μm. A significant range in the ratio of 2-9-μm backscatter cross sections is found, but a large number of points concentrate near ratios of three to ten. A large number of 1.064- and 1.54-μm cross sections were binned to provide an estimate of backscatter for various percentiles for the flight. The ratio of the 50-percentile backscatter values at 1.064 and 1.54 μm suggest a λ(-1.9) to λ(-3.0) wavelength dependence of aerosol backscatter cross section in the near infrared for the observational case.     

Theory of the double-edge technique for Doppler lidar wind measurement

The theory of the double-edge technique is described by a generalized formulation that substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared with the single-edge technique. Use of two high-resolution edge filters reduces the effects of Rayleigh scattering on the measurement by as much as an order of magnitude and allows the signal-to-noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined. The effects of Rayleigh scattering are then subtracted from the measurement, and we show that the correction process does not significantly increase the measurement noise for Rayleigh-to-aerosol ratios as high as 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photons, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh-to-aerosol ratio of 5. Methods for increasing the dynamic range to more than +/-100 m/s are given.