LIDAR Frequently Asked Questions (FAQ)

What is LIDAR?
What is the Doppler Effect, and how does it apply to LIDAR?
How do Molecules and Aerosols differ? How does this apply to LIDAR?
What is a Fabry-Perot Etalon?
How do you determine the air Velocity, Temperature and Density?
What does MAC use for a detector subsystem?
Why is MAC's LIDAR superior to many other LIDAR systems?

What is LIDAR?

LIDAR is actually an acronym for "LIght Detection And Ranging". It can be thought of as a laser radar. Radar utilizes radio waves to make its measurements, as opposed to LIDAR which uses laser light. A basic LIDAR instrument is made up of a laser source, telescope, and detection system. A laser pulse is emitted into the atmosphere, scatters from the air molecules and aerosols (particulate in the air such as dust) and returns through the telescope system. The detection system collects the light, and analyzes the data to determine the wind velocity, air temperature, and air density (air pressure is derived from the temperature and density).

What is the Doppler Effect, and how does it apply to LIDAR?

The Doppler effect was discovered in 1842 by an Austrian Physicist named Christian Doppler. This effect is characterized by a change in wavelength (or frequency) of a wave due to the motion of either the source or receiver of the waves. In a LIDAR system, the laser light which is characterized by a light wavelength, experiences this wavelength shift as the light scatters off of the moving air molecules or aerosols in the atmosphere. By comparing the incoming and outgoing laser pulses, one can determine the magnitude of the Doppler shift, and convert this into a velocity number. The diagram below shows both the source light (black), and the reflected light (red) separated by a small wavelength due to the Doppler effect.

How do Molecules and Aerosols differ? How does this apply to LIDAR?

Aerosols consist of particulate matter suspended in the atmosphere (ie. dust, water, etc), whereas air molecules are the building blocks that make up the atmosphere. Molecules are very small compared to aerosols. One can see in the Doppler effect diagram above that aerosols result in a much narrower, larger amplitude return signal, compared to air molecules. The aerosol backscatter return can improve the accuracy of velocity measurements, but cannot be used in density and temperature measurements. In addition, aerosols cannot be relied upon for velocity measurement since many areas of the globe (especially the oceans) are devoid of aerosols.

What is a Fabry-Perot Etalon?

A Fabry-Perot etalon is an optical filtering device. Only a very narrow bandwidth of light is allowed to pass through the etalon, while the remaining light is reflected back. The Fabry-Perot etalon physically consists of two extremely flat quartz surfaces. Both surfaces are covered with a uniform reflective coating, while the reflectance of each surface plane is of equal magnitude. These two surfaces can be separated by an air gap, a solid quartz gap, or by other means. However, the two surfaces must be kept parallel with a high degree of precision. If an air gap is used, then an active control system may be used to ensure parallelism.

Pictured on the right is a solid model of an air-gapped Fabry-Perot etalon. The inner surfaces of the two quartz plates are covered with a reflective coating (dark blue region), and are separated by three posts to maintain parallelism.

If the light source entering the etalon is collimated, then the light is reflected back and forth within the etalon, creating an interference pattern. The

transmitted light, formed by the interference pattern, produces concentric circular rings called "fringes". The light contained within these fringes is extremely narrow in bandwidth. Pictured below on the left is the basic Fabry-Perot etalon optical system. In addition, an actual set of fringes is displayed on the right.

The fringes are periodic by nature. By analyzing the fringe characteristics, we can gain understanding about the particles or molecules that this light has scattered off of in the atmosphere.

How do you determine the Velocity, Temperature, and Density?

By analyzing the Fabry-Perot etalon fringes, we can extract velocity, temperature, and density data. Pictured below is a simulated data example from one of these fringes.

Velocity: Air velocity is directly proportional to the magnitude of the Doppler shift; the greater the velocity, the greater the Doppler shift. There would be no Doppler shift in wavelength if air molecules or particles were at rest. Since aerosols travel at the same speed as air molecules, scattering from aerosols can also be used in velocity measurement. The backscatter from aerosols is narrower and larger in amplitude than molecular backscatter, resulting in greater measurement accuracy. Michigan Aerospace systems utilize both molecular and aerosol backscatter returns to provide the highest possible accuracy and to ensure that measurements can be made even in clear air.

Temperature: Absolute temperature is determined by the width of the molecular signal return. As the air temperature rises, the molecules vibrate faster. This is realized in the return signal by a broader wavelength region. The aerosol component of the return signal cannot be used when measuring temperature, since aerosols do not vibrate at the same rate as air molecules.

Density: Air density is determined by the integral magnitude of the molecular signal return. As the air becomes more dense, there will be more molecules to scatter the light back to the LIDAR system. As with temperature, the aerosol component of the return signal cannot be used when measuring density, since aerosol scattering is not representative of air density.

What does MAC use for a detector sub-system?

We normally use a CCD as the detector. CCD's have a high quantum efficiency, which makes them a good choice compared to photo-diode arrays. We use specially designed binning and streaking routines which allow us to incorporate ranging into our measurements if desired. The binning routines allow for a quicker chip readout. The streaking routines allow for a time history of the light return, enabling us to record data at many ranges or altitudes before reading the data off of the CCD chip.

Why is MAC's LIDAR superior to many other LIDAR systems?

Michigan Aerospace's Advantages:

Michigan Aerospace LIDAR systems measure scattered light from both molecules and aerosols, whereas many other systems rely purely on aerosols. This gives our systems a significant advantage, especially over the oceans and other continental regions where large portions of the atmosphere are devoid of aerosols.
Temperature, density, and other derived measurements can only be determined from molecular returns, providing Michigan Aerospace systems with an advantage over aerosol-only systems and enabling measurement of all air data products with a single instrument.
Many other LIDAR systems require difficult and expensive diffraction-limited optics.
Michigan Aerospace has patented technology which allows for unprecedented performance and efficiency. This has resulted in hardened systems with the size, weight, and power characteristics that enable operation on demanding aircraft, spacecraft and ground platforms.
Michigan Aerospace LIDAR technology has heritage on aircraft and spacecraft platforms, as well as ground stations. In 2006 this experience base will be expanded to include high-altitude balloon platforms.