Non-Doppler laser sensor measures wind speeds

A prototype of a non-Doppler, single-ended laser wind sensor has been developed by Mikhail Benlen`kii and Gary Gimmestad at the Georgia Institute of Technology (Atlanta, GA). The sensor, based on a laser-beam degradation phenomenon known as the residual turbulent scintillation effect, measures crosswind speeds over long distances by sending a laser beam through open air to a reflective target, thereby creating a moving laser speckle, or fringe, pattern.

The fringe contrast--shadowy waves moving across the laser beam--is strictly related to the strength of turbulence on the propagation path. The fringe motion is related to the air motion, or path-integrated crosswind. Scientists observe the moving speckle pattern with a receiving telescope to determine the path-integrated wind speed by a cross-correlation between the signals from two horizontally separated detectors in the telescope`s focal plane (see figure on p. 48).

The prototype wind sensor uses a 4-mW HeNe laser and a 0.4-m-diameter Newtonian telescope aimed at a retro reflective sheet-material target, the type commonly used on highway signs. The telescope collects light reflected from the target and focuses it onto a 0.6-mm-diameter field-stop aperture placed at the target image. The field stop reduces the background light level incident on the detectors. A 10X microscope objective forms a magnified image of the field stop on a dual diaphragm with two 0.1-mm apertures, spaced 2.5 mm apart. The dual-aperture diaphragm isolates the two small areas on the target where the fringes are measured.

Light is collimated by a lens, passed through a narrowband filter, and focused onto a photovoltaic silicon quadrant detector by a second lens. "We used a quadrant detector and just looked at elements in a horizontal path--from left to right," says Gimmestad. "We can look from top to bottom and record vertical measurements and can put two targets at right angles and get a three-dimensional plot of wind speed." The path length measured is limited only by the power of the laser and the size of optical system, says Gimmestad. The receiving telescope "can be as big or little as you want."

System compact and easy to use

Some conventional laser wind-speed measurement methods require electronic equipment at both the laser and target areas. This optical sensor system is more compact, with all the equipment at one end. "This helps keep everything in a nice environment," says Gimmestad, "free from dust and rain."

Some other wind-speed measurement methods exploit the Doppler effect of light and can be very expensive. "We wanted to be able to measure a long path where we didn`t exploit change in wavelength, frequency shift, or the Doppler effect," says Gimmestad. This helps keep down system cost. The system is also easier to use than Doppler systems, says Benlen`kii. And because it measures wind across the beam of light instead of along the beam, the optical sensor can measure air turbulence.

Gimmestad says that their wind sensor is more sensitive than the conventional anemometer. During a laboratory test, the anemometer stopped registering as soon as the fan creating the artificial air movement was shut off. The optical sensor continued recording, however, at a wind speed of about 0.25 m/s.

Although the system works well in sunlight and darkness, it does not operate well in rain or fog, which obscure the target. And the sensor can only measure the component of wind that crosses the laser beam at right angles.

The researchers suggest a number of applications, such as monitoring winds and pollutant concentrations along chemical plant boundaries, as well as source characterization. The sensor also can be used in meteorology to measure intense turbulence, such as that created by the downdraft of a microburst. Future work includes testing the sensor with devices that measure airborne-pollutant concentrations at a refinery plant.