Jack A. McKay

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 Spinhirnes and more readily permit physical insights to the optimization and limitations of the double-edge technique.

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.

Single and tandem Fabry–Perot etalons as solar background filters for lidar

Atmospheric lidar is difficult in daylight because of sunlight scattered into the receiver field of view. In this research methods for the design and performance analysis of Fabry-Perot etalons as solar background filters are presented. The factor by which the signal to background ratio is enhanced is defined as a measure of the performance of the etalon as a filter. Equations for evaluating this parameter are presented for single-, double-, and triple-etalon filter systems. The role of reflective coupling between etalons is examined and shown to substantially reduce the contributions of the second and third etalons to the filter performance. Attenuators placed between the etalons can improve the filter performance, at modest cost to the signal transmittance. The principal parameter governing the performance of the etalon filters is the etalon defect finesse. Practical limitations on etalon plate smoothness and parallelism cause the defect finesse to be relatively low, especially in the ultraviolet, and this sets upper limits to the capability of tandem etalon filters to suppress the solar background at tolerable cost to the signal.

Fabry–Perot etalon aperture requirements for direct detection Doppler wind lidar from Earth orbit

The design of Fabry-Perot etalons for direct detection Doppler wind lidar from a satellite is considered for two direct detection methods, fringe imaging (multichannel) and double edge. The area solid-angle product of the etalon for each technique is derived and shown to be inherently larger, for a given etalon aperture, for the fringe imager than for the double-edge Doppler analyzer. Modeling of the Doppler measurement accuracy of a spaceflight direct detection wind lidar shows that a very large optical aperture, 2 m or more, is necessary. Optical throughput matching to a 2-m collector requires, for the fringe-imaging Doppler analyzer, an etalon with 60 mm aperture, whereas the double-edge technique would require two etalons of 200 mm aperture, or a split-aperture etalon of 400 mm working aperture. Because the two direct detection methods have been shown to have practically identical intrinsic sensitivities (measurement accuracies per unit signal), this difference in etalon dimensions may be a significant selection consideration.

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.

Direct detection wind speed Doppler lidar systems

Direct detection Doppler lidar systems for tropospheric wind speed profiling are considered, concentrating on the spaceflight application. A precisely developed model for the fringe imaging technique is extended to the case of the aerosol signal with a significant Rayleigh background. An analytic model for the Doppler precision of an edge technique analyzer with dual filters, offset in frequency, is developed, also with provision for the Rayleigh background signal. The two models are compared to recent modeling results from McGill at NASA Goddard Space Flight Center, and good agreement found. The less rigorous analytic models developed here offer insights into the design and limitations of fringe imaging and edge detection Doppler analyzers. In particular, a requirement here offer insights into the design and limitations of fringe imaging and edge detection Doppler analyzers. In particular, a requirement for wide wind speed dynamic range implies low sensitivity for he edge detection technique, but not for the fringe imaging technique. The fringe imaging and edge detection techniques are compare for relative precision, for lidar signals of specified amplitude. A detailed conceptual design of a spaceflight Doppler wind lidar system, employing a very large optical collector in order to obtain adequate backscatter signals, shows that the fringe imaging analyzer is a factor 10 superior to the dual-etalon edge detector.

UV laser approach to Doppler tropospheric wind sounding from a satellite

The possibility of direct detection of tropospheric wind speed Doppler shift with an ultraviolet laser is considered. The use of the UV eliminates all practical concerns of eye safety, permits the use of uncooled detectors, and yields enhanced aerosol and Rayleigh backscatter signals. The Rayleigh signal, which in the free troposphere can exceed the aerosol signal by three orders of magnitude, is itself a candidate for wind speed measurement, despite the Doppler broadening of this signal. The basis of this approach is a diode-pumped, frequency-doubled alexandrite laser, which offers very high electrical to optical energy efficiency, an estimated 9%, in generating UV output. Efficiency is critical for a satellite based lidar system due to the size, cost, and mass of solar power generation and waste heat disposal subsystems. Pumping of alexandrite with 680 nm laser diodes has been demonstrated. Narrow linewidth, high spectral purity, and high frequency stability have been obtained with laser diode injection seeding of a ring alexandrite laser. The tunable diode laser control allows tuning of the laser for spacecraft velocity compensation. The potential performance of a wind sounding lidar scaled to match the 300 W power capability of a mid-sized satellite is evaluated for the extremely weak aerosol conditions of the southern hemisphere oceans. A 20 W output laser system, with 1 m aperture telescope, at 350 km altitude, may yield measurement precisions better than plus or minus 3 m/s through most of the troposphere, deteriorating to plus or minus 10 m/s under extreme conditions. A Rayleigh backscatter system will yield plus or minus 3 m/s precision to 8 km altitude, plus or minus 5 m/s at 15 km, even with zero aerosol content.

Implications of New Alexandrite Ring Laser Technology for Spaceborne Lidar (Aerosol Backscatter, Water Vapor, Doppler Winds)

Recent developments in the technology of the alexandrite laser warrant a reexamination of this laser as a candidate for satellite lidar applications, particularly in view of the strong emphasis now being placed on cost-effective, free-flyer satellites that will provide relatively limited electrical power to payload instruments. The lidar observations discussed are aerosols (750 and 375 nm wavelengths), water vapor DIAL (tunable in the 720 nm band), and Doppler winds (375 nm). The laser technology includes the diode-laser injection-seeded alexandrite ring laser, diode pumping of the alexandrite, and active stabilization of the diode injector frequency. The linewidths, spectral purity, frequency stability, wallplug efficiencies, and eye safety parameters for the requisite fundamental and frequency-doubled outputs of alexandrite are reviewed. This laser is seen to have advantages over other lasers for the important lidar applications considered. Further development of the diode pumping technology for alexandrite is important for the early realization of this potential. A spaceflight lidar system based on the alexandrite laser will be capable of useful global measurements of aerosols, water vapor, and Doppler winds, while consuming less than 400 W of spacecraft power and minimizing the associated heat disposal burden.

Diode-pumped alexandrite laser for DIAL and Doppler lidar

The problem of laser selection for spaceflight DIAL or Doppler lidar is considered. Spaceflight lidar requires tens of watts of laser output, and the low efficiency of lasers imposes costly burdens on the spacecraft platform. DIAL requires a tunable laser, and Doppler an ultraviolet laser, so the high efficiency of the Nd:YAG laser is compromised. The alexandrite laser can in principle provide higher systems efficiency for DIAL or Doppler than the Nd:YAG, being intrinsically tunable, and capable of reaching the ultraviolet with frequency doubling. High power 680 nm laser diodes are now available with sufficient power to pump alexandrite to the necessary power levels. A Q-switched laser configuration is modeled to obtain a projection laser efficiency of 13 percent. A more conservative estimate is 3.5 percent, well below the 9 percent achieved with Nd:YAG. Considering the energy savings through intrinsic tunability, frequency doubling to the ultraviolet, and extremely narrow spectral linewidth, a Doppler wind lidar system based on the alexandrite laser would have four to nine times the efficiency of the Nd:YAG alternative.

Laser source for space-flight elastic backscatter, differential absorption, and wind speed Doppler lidar

Active atmospheric sounding with lidar in principle offers great advantages over passive sounding, including higher spatial resolution and better species selectivity. These improved capabilities have been unavailable in practice due to the great spacecraft burdens of conventional lasers. The long range and high ground speed of spaceflight operation lead to high laser output power requirements. The low efficiency of most lasers leads in turn to exorbitant electrical power and heat removal requirements. Space qualification of Nd:YAG will provide a high efficiency laser suitable for elastic backscatter measurements, but his laser will not be capable of DIAL operation, nor is it practical for an eye-safe wind speed Doppler lidar. Alexandrite is a laser source that is being proven in certain demanding lidar applications, such as resonant backscatter from mesospheric metals. This laser has the great practical advantage of tunability, permitting its use for differential absorption lidar. Laser diode pumping of alexandrite has been demonstrated, using the recently developed short wavelength, high power laser diodes. Laser diode injection seeding of a ring laser yields tunability and extremely narrow linewidth, under 20 MHz. Spaceflight applications of alexandrite are considered, including two- wavelength measurements of aerosols, differential absorption measurements of atmospheric molecules, and Doppler measurement of tropospheric and stratospheric wind speeds. The lidar support requirements are compared to the capabilities of relatively small spacecraft for low cost missions.