In this paper, we describe the development of a three-beam elastic lidar that utilizes aerosol backscatter correlation to measure three-component wind profiles for detecting and tracking aircraft wake vortices; turbulence intensity and wind shear profiles. High-resolution time-resolved wind information can currently be obtained with ultrasonic or hot-wire anemometers suitable for local point measurements, or with Doppler wind lidars that only measure line-of-sight wind speeds and have to be scanned over large measurement cone angles for obtaining three-component winds. By tracking the motion of aerosol structures along and between three near-parallel laser beams, our lidar obtains three-component wind speed profiles along the field of view (FOV) of the lidar beams. Our prototype lidar wind profiler (LWP) has three 8-inch transceiver modules placed in a near-parallel configuration on a two-axis pan-tilt scanner to measure winds up to 2km away. Passively q-switched near-infrared (1030nm) Yb:YAG lasers generate 12 - 18ns wide pulses at high repetition rate (about 10KHz) that are expanded and attenuated to eye-safe levels. Sensitive low noise detection is achieved even in daytime using a narrow FOV receiver, together with narrowband interference filters and single photoncounting Geiger-mode Si detectors. A multi-channel scaler retrieves the lidar return with 7.8ns bins (∼1.2m spatial resolution) and stores accumulated counts once every 50ms (20 profiles/sec). We adapted optical flow algorithms to obtain the movement of aerosol structures between the beams. The performance of our prototype LWP was validated using sonic anemometer measurements.
In this paper, we describe the development of a prototype differential absorption fluorescence lidar for nighttime tropospheric formaldehyde (H2CO) concentration profiling. H2CO has a strong absorption band in the 352-357nm region and fluoresces strongly in the 390-500nm region. Here, we obtain high sensitivity (∼0.1ppb) measurements of H2CO profiles from differential fluorescence signals obtained by injection seeding a Nd:YVO laser and tuning its wavelength on and off the peak of a strong absorption line. The fluorescence signal strength is further improved by using a multi-line bandpass filter whose pass-bands are aligned to multiple fluorescence peaks of H2CO. A H2CO filled photo-acoustic absorption cell is utilized for tuning the seed laser wavelength to the center of absorption line.
An inexpensive aerosol lidar is proposped by combining inexpensive fiber-coupled diode laser and photon counting detection technology for air quality monitoring in metropolitan area. A prototype lidar system was built with a 100 μm fiber-coupled diode laser as the laser source and a 200 mm Schmidt-Cassegrain telescope as the receiver. The transmitted beam diveregence from a 50 mm diameter Galilean transmitter was 433 μradian and the receiver field of view was 133 μradian in full angle. With a repetition rate of 5 kHz of 100 ns pulse, a signal-to-noise of 2 was obtained up to 2 km in a clear day nighttime at the averaging time of 60 s.
Remote detection of biological warfare agents (BWA) is crucial for providing early warning to ensure maximum survivability of personnel in the battlefield and other sensitive areas. Although the current generation of stand- off aerosol and fluorescence lidars have demonstrated stand- off detection and identification of BWA, their large size and cost make them difficult for field use. We have introduced a new eye-safe portable digital lidar (PDL) technique based on digital detection that achieves orders of magnitude reduction in the size, cost and complexity over the conventional lidar, while providing an equal or better sensitivity and range. Excellent performance has been obtained with two of our PDL sensors during two bio-aerosol measurement campaigns carried out at Dugway Proving Grounds. In the JFT 4.5 (Oct 98) tests, high aerosol sensitivity of 300 ppl of biosimulant particles at up to 3 km was demonstrated with an eye-safe wavelength (523nm) aerosol micro PDL that utilized a 8 inch telescope, <10(mu) J/pulse energy at 2.5kHz, photon counting digital detection and 2 sec averaging. For the JBREWS DFT (June 99) tests an eye-safe two wavelengths (523nm and 1.047mum) horizontally scanned, aerosol micro PDL with the same 8 inch telescope was utilized. With this lidar, high sensitivity, preliminary differentiation between natural and unusual clouds, and the ability to track the aerosol cloud location, their wind speed and direction were also demonstrated. Lidar simulations of both PDL and conventional analog detection have been performed. Based on these model calculations and experimental results an analysis and comparison of the inherent capabilities of two types of systems is given.
By combining the capability of a differential absorption lidar (DIAL) and the excellent characteristics of a micro pulse lidar (MPL) we have designed and tested a micro pulse DIAL system, which could be operated from the ground or airborne platform, to monitor the atmospheric water vapor mixing ratio. To maintain the compact and rugged optical frame work of an MPL it employs a diode pumped tunable Cr:LiSAF laser operating at 825 - 840 nm range, a fiber optic beam delivery system, and an APD photon counting detector. The system parameters were optimized through extensive DIAL simulations, and the design concept was tested by building a breadboard lidar system. Based on the results of the simulations and the performance of the breadboard lidar the Micro Pulse DIAL system design has been refined to (1) minimize scattered laser light -- the major source of signal induced bias, (2) permit near field measurements from less than 400 m, (3) produce a compact, rugged, eye-safe instrument with a day and night operating capability. The lidar system is expected to provide 150 m vertical resolution, high accuracy (approximately 5%), and 3 km range looking up from the ground.
A lidar system employing a diode laser pumped Nd:YLF laser and photon counting technique is described for use in automated cloud and aerosol measurements. A Nd:YLF laser provides 523 nm 10 (mu) J/pulse energy at 2500 Hz repetition rate. A coaxial configuration is used for transmitting laser pulse and receiving the signal with a 0.2 m Schmidt-Cassegrain telescope. An avalanche photodiode is used for back scattered photon counting in Geiger mode. This micro pulse lidar (MPL) is capable to detect subvisible cirrus and boundary layer within 10 second averaging time. Also the MPL takes back scattered signal at four different spatial resolutions of 30 m, 75 m, 150 m, and 300 m to meet various user requirements. The detected signal is processed and displayed on a personal computer. The 32 bit data processing software is running on the Window 95 platform.