Coherent DIAL measurement of range-resolved water vapor concentration

Three-dimensional detection of CO2 and wind using a 1.57 µm coherent differential absorption lidar

A 1.57-µm coherent differential absorption lidar is demonstrated for measuring three-dimensional CO2 and wind fields simultaneously. The maximum detection range of CO2 is up to 6 km with a range resolution of 120 m and a time resolution of 1 min. A preliminary assessment of instrument performance is made with a 1-week continuous observation. The CO2 concentration over a column from 1920 to 2040 m is compared with the one measured by an optical cavity ring-down spectrometer placed on a 2 km-away meteorological tower. The concentration is strongly correlated with the in-situ spectrometer with a correlation coefficient and RMSE of 0.91 and 5.24 ppm. The measurement accuracy of CO2 is specified with a mean and standard deviation of 2.05 ppm and 7.18 ppm, respectively. The regional CO2 concentration and the three-dimensional wind fields are obtained through different scanning modes. Further analysis is conducted on vertical mixing and horizontal transport of CO2 by combining with the measured wind fields.

Evaluation of a coherent 2-µm differential absorption lidar for water vapor and radial wind velocity measurements

The performance of a coherent 2-µm differential absorption lidar (DIAL) for simultaneously measuring water vapor (H₂O) and radial wind velocity was evaluated. For measuring H₂O, a wavelength locking technique was applied to the H₂O-DIAL system. The H₂O-DIAL system was evaluated under summer daytime conditions in Tokyo, Japan. H₂O-DIAL measurements were compared with measurements from radiosondes. The H₂O-DIAL-derived volumetric humidity values agreed with the radiosonde-derived values over the range from 11 to 20 g/m³ with a correlation coefficient of 0.81 and a root-mean-square difference of 1.46 g/m³. Comparisons between the H₂O-DIAL and the in-situ surface meteorological sensors demonstrated the simultaneous measurement of H₂O and radial wind velocity.

Dual-wavelength locking technique for coherent 2-µm differential absorption lidar applications

A dual-wavelength locking technique for coherent 2-µm differential absorption lidar (DIAL) applications has been developed for simultaneously measuring water vapor (${{\rm{H}}_2}{\rm{O}}$) and radial wind velocity profiles. The two wavelengths for DIAL measurement were stabilized by sidebands of an electro-optic modulated laser, which was locked to the ${{\rm{CO}}_2}$ R30 absorption line. We selected wavelengths of 2050.550 and 2051.103 nm to minimize the effects of height-dependent temperature and pressure variations. The long-term wavelength stability of the two locked lasers of ${\lt}{0.2}\;{\rm{pm}}$, which corresponds to 14 MHz, was achieved. The DIAL instrument using the developed technique meets the requirement of measuring the ${{\rm{H}}_2}{\rm{O}}$ concentration with a systematic error ${\lt} {{5}}\%$ below an altitude of 5 km.

Demonstration of the 1.53-µm coherent DIAL for simultaneous profiling of water vapor density and wind speed

The 1.53-µm coherent differential absorption lidar (DIAL) is demonstrated for the simultaneous profiling of water vapor (H₂O) density and wind speed. The optical setup is fiber-based. The wavelength locking circuit can achieve precise locking of 13.0 MHz by the combination of the line center locking to the hydrogen cyanide (HCN) absorption line and offset locking to the H₂O absorption wavelength. The measurable range for the simultaneous profiling is up to 1.2 km. The DIAL-measured H₂O density is compared with the one measured by an in-situ sensor. Qualitative good agreement is shown with the random error of 0.56 g/m³.

Performance assessment of a coherent DIAL-Doppler fiber lidar at 1645 nm for remote sensing of methane and wind

We report on the performances of a coherent DIAL/Doppler fiber lidar called VEGA, allowing for simultaneous measurements of methane and wind atmospheric profiles. It features a 10µJ, 200 ns, 20 kHz fiber pulsed laser emitter at 1645 nm, and it has been designed to monitor industrial methane leaks and fugitive emissions in the environment. The system performance has been assessed for range-resolved (RR) and integrated-path (IP) methane measurements in natural background conditions (i.e. ambient methane level). For RR measurements, the measured Allan deviation at τ=10 s is in the range of 3-20 ppm, depending of the aerosol load, at a distance of 150 m, with 30 m range resolution, and a beam focused around 150-200 m. For IP measurements, using a natural target at 2.2 km of distance, the Allan deviation at τ=10 s is in the range of 100-200 ppb. In both cases, deviation curves decrease as τ^-1/2, up to 1000 seconds for the longest averaging time. Finally, the lidar ability to monitor an industrial methane leak is demonstrated during a field test.

Wavelength selection and measurement error theoretical analysis on ground-based coherent differential absorption lidar using 1.53 µm wavelength for simultaneous vertical profiling of water vapor density and wind speed

A feasibility study of coherent differential absorption lidar is conducted using a 1.53-µm wavelength for simultaneously retrieving the water vapor density and wind speed profiles. We selected the ON and OFF wavelengths to be 1531.383 and 1531.555 nm, respectively, for minimizing the effect of the temperature change in the atmosphere. The systematic measurement error can be reduced to below 5% by stabilizing the ON wavelength from ${-64}$-64 to 102 MHz around the center of the water vapor absorption line. Analysis of the speckle and photon statistics errors reveal that the relative error of the water vapor density is less than 10% at the altitude from 0.1 to 1.7 km with the 100 m range resolution with 10 min data accumulation time. The simultaneous measurement of wind speed and direction can also be achieved by employing a conical scan mechanism.

Modeling the performance of a diode laser-based (DLB) micro-pulse differential absorption lidar (MPD) for temperature profiling in the lower troposphere

Ground-based, network-deployable remote sensing instruments for thermodynamic profiling in the lower troposphere are needed by the atmospheric science research community. The recent development of a low-cost diode-laser-based (DLB) micro-pulse differential absorption lidar (DIAL) has begun to address the need for ground-based remote sensing instruments for water vapor profiling in the lower troposphere. Now, taking advantage of the broad spectral coverage of the DLB architecture, an enhancement to the water vapor micro-pulse DIAL (MPD) instrument is proposed to enable atmospheric temperature profiling. The new instrument is based on measuring a temperature-dependent oxygen (O₂) absorption coefficient and using this to retrieve the range-resolved temperature profile. In this paper, a retrieval method is proposed based on the recently developed perturbative solution to the DIAL equation that takes into account the Doppler broadening of the molecularly backscattered signal. This perturbative solution relies on an ancillary high spectral resolution lidar (HSRL) measurement of the backscatter ratio. Data from an operational water vapor MPD combined with a DLB-HSRL were used to create an atmosphere model, from which return signals for the O₂-MPD were generated. The perturbative retrieval was then applied to these data and a comparison of the retrieved temperature and the model temperature profile allowed the efficacy of retrieval to be evaluated. The results indicate that the temperature profile may be retrieved from a theoretical O₂-MPD instrument with a ±1 K accuracy up to 2.5 km and ±3 K accuracy up to 4.5 km with a 150 m range resolution and 30-minute averaging time. Using data from a recently developed O₂-MPD in combination with a WV-MPD, and a DLB-HSRL, an initial temperature retrieval is demonstrated. The results of this initial demonstration are consistent with the performance modeling.

Multi-Channel Optical Receiver for Ground-Based Topographic Hyperspectral Remote Sensing

Receiver design is integral to the development of a new remote sensor. An effective receiver delivers backscattered light to the detector while optimizing the signal-to-noise ratio at the desired wavelengths. Towards the goal of effective receiver design, a multi-channel optical receiver was developed to collect range-resolved, backscattered energy for simultaneous hyperspectral and differential absorption spectrometry (LAS) measurements. The receiver is part of a new, ground-based, multi-mode lidar instrument for remote characterization of soil properties. The instrument, referred to as the soil observation laser absorption spectrometer (SOLAS), was described previously in the literature. A detailed description of the multi-channel receiver of the SOLAS is presented herein. The hyperspectral channel receives light across the visible near-infrared (VNIR) to shortwave infrared (SWIR) spectrum (350–2500 nm), while the LAS channel was optimized for detection in a narrower portion of the near-infrared range (820–850 nm). The range-dependent field of view for each channel is presented and compared with the beam evolution of the SOLAS instrument transmitter. Laboratory-based testing of each of the receiver channels was performed to determine the effectiveness of the receiver. Based on reflectance spectra collected for four soil types, at distances of 20, 35, and 60 m from the receiver, reliable hyperspectral measurements were gathered, independent of the range to the target. Increased levels of noise were observed at the edges of the VNIR and SWIR detector ranges, which were attributed to the lack of sensitivity of the instrument in these regions. The suitability of the receiver design, for the collection of both hyperspectral and LAS measurements at close-ranges, is documented herein. Future development of the instrument will enable the combination of long-range, ground-based hyperspectral measurements with the LAS measurements to correct for absorption, due to atmospheric water vapor. The envisioned application for the instrument includes the rapid characterization of bare or vegetated soils and minerals, such as are present in mine faces and tailings, or unstable slopes.

Development of a Multimode Field Deployable Lidar Instrument for Topographic Measurements of Unsaturated Soil Properties: Instrument Description

The hydrological and mechanical behavior of soil is determined by the moisture content, soil water (matric) potential, fines content, and plasticity. However, these parameters are often difficult or impractical to determine in the field. Remote characterization of soil parameters is a non-destructive data collection process well suited to large or otherwise inaccessible areas. A ground-based, field-deployable remote sensor, called the soil observation laser absorption spectrometer (SOLAS), was developed to collect measurements from the surface of bare soils and to assess the in-situ condition and essential parameters of the soil. The SOLAS instrument transmits coherent light at two wavelengths using two, continuous-wave, near-infrared diode lasers and the instrument receives backscattered light through a co-axial 203-mm diameter telescope aperture. The received light is split into a hyperspectral sensing channel and a laser absorption spectrometry (LAS) channel via a multi-channel optical receiver. The hyperspectral channel detects light in the visible to shortwave infrared wavelengths, while the LAS channel filters and directs near-infrared light into a pair of photodetectors. Atmospheric water vapor is inferred using the differential absorption of the on- and off-line laser wavelengths (823.20 nm and 847.00 nm, respectively). Range measurement is determined using a frequency-modulated, self-chirped, coherent, homodyne detection scheme. The development of the instrument (transmitter, receiver, data acquisition components) is described herein. The potential for rapid characterization of physical and hydro-mechanical soil properties, including volumetric water content, matric potential, fines content, and plasticity, using the SOLAS remote sensor is discussed. The envisioned applications for the instrument include assessing soils on unstable slopes, such as wildfire burn sites, or stacked mine tailings. Through the combination of spectroradiometry, differential absorption, and range altimetry methodologies, the SOLAS instrument is a novel approach to ground-based remote sensing of the natural environment.

Development of wavelength locking circuit for 1.53 micron water vapor monitoring coherent differential absorption LIDAR

We have studied a ground based coherent differential absorption LIDAR (DIAL) for vertical profiling of water vapor density using a 1.5μm laser wavelength. A coherent LIDAR has an advantage in daytime measurement compared with incoherent LIDAR because the influence of background light is greatly suppressed. In addition, the LIDAR can simultaneously measure wind speed and water vapor density.

We had developed a wavelength locking circuit using the phase modulation technique and offset locking technique, and wavelength stabilities of 0.123 pm which corresponds to 16 MHz are realized. In this paper, we report the wavelength locking circuits for the 1.5 um wavelength.

Turbulence-induced measurement errors in coherent differential absorption lidar ground systems

The presence of atmospheric refractive turbulence makes it necessary to use simulations of beam propagation to examine the uncertainty added to the differential absorption lidar (DIAL) measurement process of a practical heterodyne lidar. The inherent statistic uncertainty of coherent return fluctuations in ground lidar systems profiling the atmosphere along slant paths with large elevation angles translates into a lessening of accuracy and sensitivity of any practical DIAL measurement. This technique opens the door to consider realistic, nonuniform atmospheric conditions for any DIAL instrument configuration.

Coherent differential absorption lidar measurements of CO2

A differential absorption lidar has been built to measure CO2 concentration in the atmosphere. The transmitter is a pulsed single-frequency Ho:Tm:YLF laser at a 2.05-microm wavelength. A coherent heterodyne receiver was used to achieve sensitive detection, with the additional capability for wind profiling by a Doppler technique. Signal processing includes an algorithm for power measurement of a heterodyne signal. Results show a precision of the CO2 concentration measurement of 1%-2% 1sigma standard deviation over column lengths ranging from 1.2 to 2.8 km by an average of 1000 pulse pairs. A preliminary assessment of instrument sensitivity was made with an 8-h-long measurement set, along with correlative measurements with an in situ sensor, to determine that a CO2 trend could be detected.

Future performance of ground-based and airborne water-vapor differential absorption lidar. I. Overview and theory

The performance of a future advanced water-vapor differential absorption lidar (DIAL) system is discussed. It is shown that the system has to be a direct-detection system operating in the rhovarsigmatau band of water vapor in the 940-nm wavelength region. The most important features of the DIAL technique are introduced: its clear-air measurement capability, its flexibility, and its simultaneous high resolution and accuracy. It is demonstrated that such a DIAL system can contribute to atmospheric sciences over a large range of scales and over a large variety of humidity conditions. An extended error analysis is performed, and errors (e.g., speckle noise) are included that previously were not been discussed in detail and that become important for certain system designs and measurement conditions. The applicability of the derived equation is investigated by comparisons with real data. Excellent agreement is found.

Improved speckle statistics in coherent differential absorption lidar with in-fiber wavelength multiplexing

Remote detection of gaseous pollutants and other atmospheric constituents can be achieved with differential absorption lidar (DIAL) methods. The technique relies on the transmission of two or more laser wavelengths and exploits absorption features in the target gas by measuring the ratio of their detected powers to determine gas concentration. A common mode of operation is when the transmitter and receiver are collocated, and the absorption is measured over a return trip by a randomly scattering topographic target. Hence, in coherent DIAL, speckle fluctuation leads to a large uncertainty in the detected powers unless the signal is averaged over multiple correlation times, i.e., over many independent speckles. We examine a continuous-wave coherent DIAL system in which the laser wavelengths are transmitted and received by the same single-mode optical fibers. This ensures that the two wavelengths share a common spatial mode, which, for certain transmitter and target parameters, enables highly correlated speckle fluctuations to be readily achieved in practice. For a DIAL system, this gives the potential for improved accuracy in a given observation time. A theoretical analysis quantifies this benefit as a function of the degree of correlation between the two time series (which depends on wavelength separation and target depth). The results are compared with both a numerical simulation and a laboratory-based experiment.

Ground-based differential absorption lidar for water-vapor and temperature profiling: development and specifications of a high-performance laser transmitter

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.

Pulsed heterodyne CO(2) laser rangefinder and velocimeter with chirp correction

A pulsed hybrid CO(2) transversely excited atmosphere (TEA) laser has been used in a bistatic laser rangefinder-velocimeter system with heterodyne detection. Several techniques have been applied to improve the performance of the system. These include the stabilization of the hybrid CO(2) TEA-laser and the stabilization of the frequency offset of the local oscillator (better than ±74 kHz peak to peak), phase-front matching at the detector surface resulting in a heterodyne beat efficiency of 0.4-0.6, and chirp correction. With this system, targets at distances of up to 25 km can be detected with an accuracy of 15 m. The velocity of the targets can be estimated with an accuracy of approximately ±0.5 m/s.

Airborne remote sensing of tropospheric water vapor with a near-infrared differential absorption lidar system

A near-infrared airborne differential absorption lidar (DIAL) system has become operational. Horizontal and vertical water vapor profiles of the troposphere during summer (nighttime) conditions extending from the top of the planetary boundary layer (PBL) up to near the tropopause are investigated. These measurements have been performed in Southern Bavaria, Germany. The system design, the frequency control units, and an estimation of the laser line profile of the narrow-band dye laser are discussed. Effective absorption cross sections in terms of altitude are calculated. Statistical and systematic errors of the water vapor measurements are evaluated as a function of altitude. The effect of a systematic range-dependent error caused by molecular absorption is investigated by comparing the DIAL data with in situ measurements. Typical horizontal resolutions range from 4 km in the lower troposphere to 11 km in the upper troposphere, with vertical resolutions varying from 0.3 to 1 km, respectively. The lower limit of the sensitivity of the water vapor mixing ratio is calculated to be 0.01 g/kg. The total errors of these measurements range between 8% and 25%. A sine-shaped wave structure with a wavelength of 14 km and an amplitude of 20% of its mean value, detected in the lower troposphere, indicates an atmospheric gravity wave field.

High pulse repetition frequency, multiple wavelength, pulsed CO(2) lidar system for atmospheric transmission and target reflectance measurements

A multiple wavelength, pulsed CO(2) lidar system operating at a pulse repetition frequency of 200 Hz and permitting the random selection of CO(2) laser wavelengths for each laser pulse is presented. This system was employed to measure target reflectance and atmospheric transmission by using laser pulse bursts consisting of groups with as many as 16 different wavelengths at a repetition rate of 12 Hz. The wavelength tuning mechanism of the transversely excited atmospheric laser consists of a stationary grating and a flat mirror controlled by a galvanometer. Multiple wavelength, differential absorption lidar (DIAL) measurements reduce the effects of differential target reflectance and molecular absorption interference. Examples of multiwavelength DIAL detection for ammonia and water vapor show the dynamic interaction between these two trace gases. Target reflectance measurements for maple trees in winter and autumn are presented.

Tunable 2.1-,microm Ho lidar for simultaneous range-resolved measurements of atmospheric water vapor and aerosol backscatter profiles

An eye-safe, tunable differential-absorption lidar system has been developed for the range-resolved measurement of aerosol backscatter and water vapor in the atmosphere. The lidar uses a flash-lamp-pumped, qswitched, 10-mJ solid-state Ho:YSGG laser that is continuously tunable over a 20cm(-1) wavelength range near 2.084 microm. Both path-averaged and range-resolved measurements were performed with the Ho differential-absorption lidar system. Preliminary measurements have been made of the temporal variation of atmospheric aerosol backscatter and water-vapor profiles at ranges out to 1 km. These results indicate that the Ho lidar has the potential for the eye-safe remote sensing of atmospheric water vapor and backscatter profiles at longer ranges if suitably enhanced in laser power and laser linewidth.

Heterodyne Doppler 1-microm lidar measurement of reduced effective telescope aperture due to atmospheric turbulence

We performed an experimental study on the effect of atmospheric turbulence on heterodyne and direct detection lidar at 1 microm, employing a pulsed Nd:YAG bistatic focused beam lidar that permitted simultaneous heterodyne and direct detection of the same lidar returns. The average carrier-to-noise ratio and statistical fluctuation level in the lidar return signals were measured in various experimental and atmospheric conditions. The results showed that atmospheric turbulence could reduce the effective receiver telescope diameter of the l-microm heterodyne lidar to <5cm at a relatively short range of approximately 450 m near the ground. The observed effective telescope aperture and heterodyne detection efficiency varied during the day as the atmospheric turbulence level changed. At this time, we are not able to compare our experimental lidar data to a rigorous atmospheric turbulence and lidar detection theory which includes independently variable transmitter, receiver, and detector geometry. It is interesting to note, however, that the observed limitation of the effective receiver aperture was similar in functional form with those predictions based on the heterodyne wavefront detection theory by D. L. Fried [Proc. IEEE 55, 57-67 (1967)] and the heterodyne lidar detection theory for a fixed monostatic system by S. F. Clifford and S. Wandzura [Appl. Opt. 20, 514-516 (1981)]. We have also applied such an effective receiver aperture limitation to predict the system performance for a heterodyne Ho lidar operating at 2 microm.

Monte Carlo computer simulations of ground-based and space-based coherent DIAL water vapor profiling

Ground-based and space-based coherent DIAL water vapor measurement performance at the 2.1-microm Ho:YAG wavelength is presented using a Monte Carlo computer simulation. The stochastic simulation allowed improved modeling of lidar system, platform, atmospheric, and data processing parameter effects on performance and better understanding of their interrelationships. Results indicate that accurate water vapor measurements in the lower troposphere are potentially achievable from both ground- and space-based platforms.

Measurements of the urban ozone vertical profile with an airborne CO(2) DIAL

An airborne DIAL system with two color pulsed CO(2) lasers has been developed for remote measurements of ozone distributions in an urban area. The airborne CO(2) DIAL is a nadir-directed and direct range-resolved system, since the system was designed to use aerosols between an aircraft and the ground surface as a distributed target. Flight tests were successfully conducted in September 1986 over Tokyo, Japan, at a flight altitude of 5000 ft (1.7 km). The airborne CO(2) DIAL instrument and its first measurements of the vertical profile of urban ozone are described.

Technique for correcting effects of long CO(2) laser pulses in aerosol-backscattered coherent lidar returns

The convolution effect in aerosol backscattered CO(2) coherent lidar returns caused by the long tail of the laser pulse is analyzed by modifying the original lidar equation and introducing a correction function C(r)(R). The characteristics of the correction function and its effect on differential absorption lidar water vapor measurements are investigated for coherent lidars. A deconvolution technique is developed consisting of a leastsquares fitting and iteration procedure for retrieval of the mean value of the atmospheric backscattering coefficient beta and a reverse filtering procedure for estimating the fluctuation components of the beta profile. Data obtained with the WPL coherent CO(2) lidar are reanalyzed using the above method, giving improved estimates of the structures and the mean water vapor content.

CO(2) DIAL measurements of water vapor

CO(2) lidars have heretofore been used to measure water vapor concentrations primarily using the 10R(20) line at 10.247 microm, which has a strong overlap with a water vapor absorption line. This paper discusses the use of that line as well as other CO(2) laser lines for which the absorption coefficients are weaker. The literature on measurement of water vapor absorption coefficients using CO(2) lasers is reviewed, and the results from four laboratories are shown to be generally consistent with each other after they are normalized to the same partial pressure, temperature, and ethylene absorption coefficient for the 1P(14) CO(2) laser line; however, the agreement with the Air Force Geophysics Laboratory's HITRAN and FASCOD 2 spectral data tapes is not good either for the water vapor absorption lines or for the water vapor continuum. Demonstration measurements of atmospheric water vapor have been conducted using the Mobile Atmospheric Pollutant Mapping System, a dual CO(2) lidar system using heterodyne detection. Results are discussed for measurements using three sets of laser line pairs covering a wide range of water vapor partial pressures.

Measurement of the integrated water vapor content in the atmosphere by a radiometric method

An improved version of a two-wavelength radiometric method is described by which the total water vapor amount along an optical path may be determined by the use of a radiometer and a source at the two ends of the optical path. The method requires two transmission measurements: one at 1.14 microm (at the center of an absorption band) and another at 1.06 microm (an atmospheric window). The spectral transmittance is calculated using the FASCODE computer code, convolved with the source, filter, and detector response curves of the transmissometer. Good agreement (1-7%) is obtained with experimental observations of this quantity as a function of total water vapor amount. The method was verified for horizontal paths of up to 10 km.

Target and atmospheric influence on coherent CO2 laser radar performance

Experimentally verified signal amplitude distributions from a coherent CO2 laser radar have been used to derive radar performance for atmospheric remote sensing and hard target detection. Different target types include man-made diffuse, semirough, and glint targets as well as terrain backgrounds. The results, given as gas concentration accuracy and probability of detection, respectively, show the importance of including beam wandering especially for glint targets. It is shown how Doppler sensing and range gating improve target detection against terrain background.