Coherent lidar airborne windshear sensor: performance evaluation

Detection of molecular backscattering with a tapered fiber amplifier based coherent heterodyne lidar

Fiber based coherent heterodyne lidars are highly valued and robust tools especially in sensing of wind speed and turbulence in the atmosphere. The magnitude of aerosol backscattering is also possible to be analysed from the data. However, the aerosol backscattering values cannot be calibrated without the data of molecular backscattering reference, which has not been available earlier due to power and bandwidth limitations. We present the detection of aerosol and molecular backscattering simultaneously with a fiber based coherent lidar instrument utilising a tapered fiber amplifier that yields to a pulse peak power of 1.9 kW at the wavelength of 1053 nm. Further, our receiver bandwidth of 1.5 GHz enables the spectral analysis of aerosol and molecular scattering spectra, which are recorded and analysed for multiple altitudes up to 1 km. The results demonstrate the potential of coherent heterodyne lidars to extend their capabilities toward backscattering and extinction analysis.

CTH:YAG : from laser medium to luminescent concentrator

This work presents what we believe is a new way to use a CTH:YAG crystal for spontaneous emission instead of laser emission. The spontaneous emission is collected in one main direction thanks to a luminescent concentrator configuration. The CTH:YAG is indirectly LED-pumped by a Ce:YAG delivering 3.5 ms pulses at 10 Hz with an energy of 2 J in the visible (550-650 nm). In a configuration optimized for light extraction, the CTH:YAG luminescent concentrator provides a broadband emission between 1.8 µm and 2.1 µm with a unique combination of power (1 W) and brightness (21.2 W/cm2/sr) that could be useful for short-wave infrared (SWIR) lighting applications.

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.

Wind lidar signal denoising method based on singular value decomposition and variational mode decomposition

A denoising method based on singular value decomposition (SVD) and variational mode decomposition (VMD) is proposed for wind lidar. Utilizing the covariance matrix based lidar signal simulation model, the performance of VMD, SVD, and VMD-SVD is evaluated. The results show that the VMD-SVD method is of better performance, and the output signal-to-noise ratio (SNR) is about 12 dB at the input SNR of -9dB. The actual lidar signals processing is performed with this combined denoising method, and the detection range and wind speed at pulse accumulation numbers of 50,100, and 300 are compared. We set the wind speed resulting from noisy signal with pulse accumulation number of 300 as the reference wind speed, and the mean value and standard deviation of wind differences are analyzed. The results show that the denoising method can not only increase the detection range while ensuring the accuracy of wind speed estimation but also achieve the same detection distance with fewer pulse accumulations, thereby improving the temporal resolution. For the pulse accumulation number of 50, the detection range is extended to 24 km from 18.45 km, and the standard deviation of speed difference is 0.88 m/s; for the same detection range, the temporal resolution is increased by about 6 times.

1645 nm coherent Doppler wind lidar with a single-frequency Er:YAG laser

Solid-state single-frequency lasers around 1.6 µm are ideal sources for coherent Doppler wind lidars (CDWLs). A CDWL system with 1645 nm sing-frequency, injection-seeded Er:YAG ceramic laser is demonstrated. The Er:YAG laser based on an "M-shaped" ring resonator operates at pulse repetition frequencies (PRFs) of 300-1000 Hz at room temperature. The maximum single-frequency output energy is 10.1 mJ with a pulse width of 179 ns at 300 Hz. The 1645 nm Er:YAG laser is first used in a long-range CDWL system, and a line of sight (LOS) wind velocity up to 25 km is detected with 90 m range resolution in 0.5 s observation. To verify the reliability of the measurement results, the relationship between detection range, pulse energy, and accumulated numbers is also demonstrated.

Onboard wake vortex localization with a coherent 1.5 µm Doppler LIDAR for aircraft in formation flight configuration

An onboard LIght Detection And Ranging (LIDAR) sensor designed to track wake vortex created by aircraft in formation flight is presented. It uses short pulses (75 ns) to obtain a spatial resolution of ∼22.5 m required to resolve small-scale structures of vortices and a blind zone of 17.5 m to locate vortices next to the wing tip. Monte Carlo simulations show that vortex centers could be located within ±0.5 m. Flight tests were performed with two aircraft in formation flight configuration. The LIDAR, installed in the following aircraft, was able to measure, in real time (every 6 s), the air flow velocities induced by the vortices created by the leading aircraft. The software was used to determine the vortex centers. These measurements were coupled to global positioning system (GPS) measurements of the two aircraft positions to determine the falling velocity of the vortices and infer their circulations.

Characterisation of Small-Scale Atmospheric Wind-Field Structures Using Coherent Wind Lidar With Short Pulses

A lidar design has been developed at ONERA that uses short square pulses (75 ns) to have a small spatial resolution (22.5 m) and be able to measure small-scale atmospheric wind-field structures. Results show that the system is able to resolve the small-scale structures of vortices and to measure wind field structures of a turbulent wind field down to ~20 m.

Wind ranging and velocimetry with low peak power and long-duration modulated laser

A novel approach to simultaneous wind ranging and velocimetry using low peak power, long-duration modulated laser pulse transmissions is proposed. Received signals backscattered by aerosol particles are processed by a multi-reference matched-filter (MRMF) which performs matched filter processing between the received signal and several reference signals in parallel and outputs a range-velocity profile of received power. Ranging and velocimetry are performed simultaneously by estimating received power, radial velocity, and velocity dispersion from a velocity profile at an arbitrary range in the range-velocity profile. The accuracies of the three estimates improve in proportion to the square root of pulse duration; that is, a 100 times longer pulse is equivalent to a 10-dB amplification.

Experimental investigation into infrasonic emissions from atmospheric turbulence

Clear air turbulence (CAT) is the leading cause of in-flight injuries and in severe cases can result in fatalities. The purpose of this work is to design and develop an infrasonic array network for early warning of clear air turbulence. The infrasonic system consists of an infrasonic three-microphone array, compact windscreens, and data management system. Past experimental efforts to detect acoustic emissions from CAT have been limited. An array of three infrasonic microphones, operating in the field at NASA Langley Research Center, on several occasions received signals interpreted as infrasonic emissions from CAT. Following comparison with current lidar and other past methods, the principle of operation, the experimental methods, and experimental data are presented for case studies and confirmed by pilot reports. The power spectral density of the received signals was found to fit a power law having an exponent of -6 to -7, which is found to be characteristics of infrasonic emissions from CAT, in contrast to findings of the past.

Performance evaluation of a dual fringe-imaging Michelson interferometer for air parameter measurements with a 355 nm Rayleigh-Mie lidar

A new concept of spectrum analyzer is proposed for short-range lidar measurements in airborne applications. It implements a combination of two fringe-imaging Michelson interferometers to analyze the Rayleigh-Mie spectrum backscattered by molecules and particles at 355 nm. The objective is to perform simultaneous measurements of four variables: the air speed, the air temperature and density, and the particle scattering ratio. The Cramer-Rao bounds are calculated to evaluate the best expectable measurement accuracies. The performance optimization shows that a Michelson interferometer with a path difference of 3 cm is optimal for air speed measurements in clear air. To optimize density, temperature, and scattering ratio measurements, the second interferometer should be set to a path difference of 10 cm at least; 20 cm would be better to be less sensitive to the actual Rayleigh-Brillouin line shape.

Compact all-fiber pulsed coherent Doppler lidar system for wind sensing

A compact 1.5 microm all-fiber pulsed coherent Doppler lidar system for wind sensing, which includes the functions of variable pulse width and automatic polarization control has been developed. The system configuration is introduced and key components used in the system are explained. Theoretical performances of the system in wind sensing are estimated and compared with experimental results. The measurable range corresponding to the detection probability of >80% is approximately 1 km or more in the case of 150 m range resolution under the normal atmospheric conditions.

Amplified reference pulse storage for low-coherence pulsed Doppler lidar

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 +/-0.09 m/s.

Bistatic coherent laser radar signal-to-noise ratio

We investigate the signal-to-noise ratio (SNR) for a bistatic coherent laser radar (CLR) system. With a bistatic configuration, the spatial resolution is determined by the overlap of the transmit beam and the virtual backpropagated local oscillator beam. This eliminates the trade-off between range resolution and the bandwidth of the transmitted pulse inherent in monostatic systems. The presented analysis is completely general in that the expressions can be applied to both monostatic and bistatic CLR systems. The heterodyne SNR is computed under the assumption of untruncated Gaussian optics and untruncated Gaussian beam profiles. The analysis also includes the effects of refractive turbulence. The results show that, for maximum SNR, small transmit and local oscillator beam profiles (e-1 intensity radius) are desired.

11-mJ, 15-Hz single-frequency diode-pumped Q-switched Er, Yb:phosphate glass laser

A single-frequency diode-pumped Q-switched Er, Yb:phosphate glass laser that oscillates at an eye-safe 1.54etam wavelength has been developed for use in coherent Doppler lidar. A maximum TEM(00)-mode Q-switched output energy of 10.9 mJ and a relatively long pulse width of 228 ns were obtained at a repetition rate of 15 Hz by use of a modified 2-m-long telescopic cavity. Frequency stability of as high as +/-1.9-MHz standard deviation and a side-mode suppression ratio of more than 30 dB were also achieved.

Autonomous beam alignment for coherent Doppler lidar with multielement detectors

Autonomous beam alignment for coherent Doppler lidar requires accurate information about optical misalignment and optical aberrations. A multielement heterodyne detector provides the required information without a loss in overall system performance. The effects of statistical variations from the random backscattered field (speckle field) are determined with computer simulations for both ground-based operation with a fixed calibration target and for space-based operation with random target backscatter.

Extracting vertical winds from simulated clouds with ground-based coherent Doppler lidar

The performance of mean velocity estimators is determined by computer simulations for solid-state coherent Doppler lidar measurements of wind fields at a cloud interface with deterministic profiles of velocity and aerosol backscatter. Performance of the velocity estimates is characterized by the standard deviation about the estimated mean and the bias referenced to the input velocity. A new class of estimators are required for cloud conditions, as traditional techniques result in biased estimates. We consider data with high signal energy that produces negligible random outliers.

Evaluation of coherent Doppler lidar velocity estimators in nonstationary regimes

We evaluate the mean velocity estimator performance for coherent Doppler lidar measurements of wind fields with wind shear and nonuniform system response as a function of target range. Performance of the velocity estimates is characterized by the bias and standard deviation that are determined by computer simulations. Results are for solid-state lasers with a Gaussian transmitted pulse. We consider data with high signal energy that produces negligible random outliers.

Coherent-burst laser ranging: decoupling resolution and unambiguous range

A novel signal encoding and decoding method for laser ranging is proposed and experimentally demonstrated. The coherent-burst method uses a quasi-continuous interrupted burst of modulated laser radiation to achieve high-precision ranging at all distances. Both time of flight and phase shift are determined based on an amplitude-modulation pattern that has electronic coherence with a master oscillator. High-frequency modulation provides high absolute precision, while measured temporal delay from transmission to reception eliminates potential uncertainty from aliasing. The method is demonstrated with low-power diode lasers that can be modulated at frequencies in excess of 250 MHz.

Coherent lidar airborne wind sensor II: flight-test results at 2 and 10 νm

The use of airborne laser radar (lidar) to measure wind velocities and to detect turbulence in front of an aircraft in real time can significantly increase fuel efficiency, flight safety, and terminal area capacity. We describe the flight-test results for two coherent lidar airborne shear sensor (CLASS) systems and discuss their agreement with our theoretical simulations. The 10.6-μm CO(2) system (CLASS-10) is a flying brassboard; the 2.02-μm Tm:YAG solid-state system (CLASS-2) is configured in a rugged, light-weight, high-performance package. Both lidars have shown a wind measurement accuracy of better than 1 m/s.

Intrapulse temporal and wavelength shifts of a high-power 2.1-µm Ho:YAG laser and their potential influence on atmospheric lidar measurements

A high-power, flash-lamp-pumped, Q-switched Ho:YAG laser has been developed to produce up to 150 mJ in a 100-ns Q-switched pulse. The Ho laser was initially used in a direct detection lidar-differential absorption lidar (DIAL) system to measure vertical density profiles of aerosols and water vapor in the atmosphere. It was found, however, that the Ho laser operated simultaneously on two closely spaced spectral emission wavelengths (2.090 and 2.097 µm) and that the distribution of energy between the two wavelengths could change significantly on time scales of several seconds to minutes. Such intrapulse temporal and wavelength shifts were found to alter the atmospheric lidar return significantly because one of the laser lines coincided with a water vapor absorption line in the atmosphere. This laser spectral output problem was overcome by the use of intracavity étalons that controlled the laser spectral-temporal characteristics but reduced the laser output energy to approximately 75 mJ/pulse in a 100-ns pulse length. These results are important as they serve to point out the difficulties of developing and using a high-power 2.1- µm Ho laser for atmospheric lidar when high-resolution spectral and temporal characteristics can significantly influence the lidar return and be misinterpreted as resulting from atmospheric signals.

Laser Doppler optical air-data system: feasibility demonstration and systems specifications

A compact laser Doppler velocimeter with diode-pumped solid-state laser technology at 1.06 µm and a novel solid-block beam-splitting interferometer was demonstrated. This system allows for accurate Doppler velocity measurements even in a vibration environment. A comparison of the backscatter measurements with a backscatter sonde from the University of Wyoming showed excellent agreement. Based on demonstrated performance and technology projection, the system specifications of an optical air-data system at 2.015 µm are determined, and a detailed design concept is presented. In addition, the potential for a multifunctional sensor that can determine air data and detect wind shear and wake vortices is addressed.

Recent climatological trends in atmospheric aerosol backscatter derived from the Jet Propulsion Laboratory multiyear backscatter profile database

An update is provided on the Jet Propulsion Laboratory aerosol backscatter climatology database, with emphasis on the impact of the June 1991 eruption of Mt. Pinatubo. The data set is acquired at thermal infrared wavelengths with a range-gated coherent CO(2) lidar system, which has been in regular operation since 1984. A number of analyses have been carried out to assess long-term trends in the tropospheric and lower stratospheric aerosol backscatter, as observed from the lidar site at Pasadena, California.

Coherent launch-site atmospheric wind sounder: theory and experiment

The coherent launch-site atmospheric wind sounder (CLAWS) is a lidar atmospheric wind sensor designed to measure the winds above space launch facilities to an altitude of 20 km. In our development studies, lidar sensor requirements are defined, a system to meet those requirements is defined and built, and the concept is evaluated, with recommendations for the most feasible and cost-effective lidar system for use as an input to a guidance and control system for missile or spacecraft launches. The ability of CLAWS to meet NASA goals for increased safety and launch/mission flexibility is evaluated in a field test program at Kennedy Space Center (KSC) in which we investigate maximum detection range, refractive turbulence, and aerosol backscattering efficiency. The Nd:YAG coherent lidar operating at 1.06 µm with 1-J energy per pulse is able to make real-time measurements of the three-dimensional wind field at KSC to an altitude of 26 km, in good agreement with our performance simulations. It also shows the height and thickness of the volcanic layer caused by the volcanic eruption of Mount Pinatubo in the Philippines.

Investigation of 2.1-microm lasing properties of Ho:Tm:Cr:YAG crystals under flash-lamp pumping at various operating conditions

Flash-lamp-pumped normal-mode and Q-switched 2.1-microm laser operations of Ho:Tm:Cr:YAG crystals have been evaluated under a wide variety of experimental conditions in order to determine an optimum lasing condition and to characterize the laser outputs. Q-switched laser-output energies equal to or in some cases more than the normal-mode laser energies were obtained in the form of a strong single spike by optimizing the opening time of a lithium niobate Q switch. The increase of the normal-mode laser slope efficiency was observed with the increase of the Tm concentration from 2.5 to 4.5 at. % at operating temperatures from 120 K to near room temperature. Laser transitions were observed only at 2.098 and 2.091 microm under various conditions. The 2.091-microm laser transition appeared to be dominant at high-temperature operations with low-reflective-output couplers and to have an energy-level assignment from 5313 cm(-1) to 534 cm(-1) or (and) from 5313 cm(-1) to 536 cm(-1).

Coherent laser radar performance for general atmospheric refractive turbulence

The signal-to-noise ratio (SNR) and heterodyne efficiency are investigated for coherent (heterodyne detection) laser radar under the Fresnel approximation and general conditions. This generality includes spatially random fields, refractive turbulence, monostatic and bistatic configurations, detector geometry, and targets. For the first time to our knowledge, the effects of atmospheric refractive turbulence are included by using the path-integral formulation. For general conditions the SNR can be expressed in terms of the direct detection power and a heterodyne efficiency that can be estimated from the laser radar signal. For weak refractive turbulence (small irradiance fluctuations at the target) and under the Markov approximation, it is shown that the assumption of statistically independent paths is valid, even for the monostatic configuration. In the limit of large path-integrated refractive turbulence the SNR can become twice the statistically independent-path result. The effects of the main components of a coherent laser radar are demonstrated by assuming untruncated Gaussians for the transmitter, receiver, and local oscillator. The physical mechanisms that reduce heterodyne efficiency are identified by performing the calculations in the receiver plane. The physical interpretations of these results are compared with those obtained from calculations performed in the target plane.