NASA - National Aeronautics and Space Administration

Remote Sensing and Lasers
FS-1998-03-35-LaRC
March 1998

Remote sensing is any technique for measuring, observing or monitoring a process or object without physically touching the object under observation. Optical and radio elescopes, cameras, even eyesight, are types of remote sensing with which you are probably familiar.

Because the remote sensing instrumentation is not in contact with the object being observed, remote sensing allows the monitor to:

• Avoid hazardous or difficult to reach regions, such as inside nuclear or chemical reactors, in biological hot spots, behind obstacles, inside smoke stacks, on the freeway, in the ocean depths, on mountain tops, in polar regions, on other planets, or on the sun.
• Measure a process without disturbance, such as monitoring flow around an aircraft model in a wind tunnel or measuring temperature during an experiment.
• Probe large volumes economically and quickly, such as providing global measurements of aerosols, air pollution, agriculture, human impact on the environment, ocean surface roughness, and large scale geographic features.
• Smooth local fluctuations by averaging over a large volume.

Active vs. Passive Remote Sensors

There are two classes of remote sensors: passive remote sensors and active remote sensors. Passive remote sensors do not include the energy source on which the measurement is based. The eye and optical telescopes are passive remote sensors: they rely on an external light source. You cannot see at night if the room lights are not turned on.

Active remote sensing instrumentation includes the energy source on which the measurement is based. RADAR is a widely known form of active remote sensing. In radar, the instrument emits a radio wave and senses the returned energy which is reflected from the target. Since the speed of radio waves and the time delay between emission and return are known, the distance to the target can be determined. Lidar (Light Detection and Ranging) is the optical analogue of radar. Lidars emit a concentrated light beam onto the target and measure the energy reflected back to the lidar receiver.

Graphic above is pictorial of remote-sensing.

The intensity or amount of reflected energy measured by the receiver provides the needed information about the target. With lidars, the light source is not a radio wave, but rather it is in the visible and adjacent (ultraviolet and infrared) regions of the electro-magnetic spectrum. The light source is generally a laser.

Most of the remote sensors in use today are passive sensors: they are easier, and therefore, cheaper to build. Unfortunately, passive remote sensors deployed on satellites only work when the sun is shining on the area under the satellite.

Lidars

Lidar comes in three major varieties: range finders, dial (Differential Absorption Lidar) and Doppler lidar.
Range finders, like radar, emit a pulse of light and measure the time interval between when the pulse is emitted and when the reflected pulse is detected. Like radar, they are used to determine the distance to an object. Because of its much shorter wavelength, laser range finders are effective for much smaller targets. Indeed, by scanning the surface of the target, advanced range finders can determine the target's shape and size in addition to its distance.

Dial lidar can be used to measure the temperature, density, and pressure of trace gases and aerosols in the atmosphere. Two probe laser beams are used, one with a wavelength that corresponds to a target gas absorption peak (on-line) and the other that falls between the target gas's absorption features (off-line). The ratio of the on-line to off-line returned signal strength is related to the absorption characteristics (which in turn depend on the temperature, density, and pressure) of the target gas.

Doppler lidars use the Doppler shift, the small change in wavelength due to motion of the surface, to measure the target's velocity. You may already be familiar with the Doppler shift: it causes the change in pitch of a train whistle or a car horn as the vehicle first approaches you then recedes from you. Using a technique called heterodyning, the return signal is combined with another laser beam so the two interfere slightly, yielding a more easily measured radio wave in place of the two infrared light waves. The frequency of the radio wave will match the difference between the outgoing and incoming signals.

A common application of Doppler radar in atmospheric remote sensing is to measure wind velocity, i.e. wind speed and direction. Wind velocity measurements have numerous military, civilian government, and commercial applications which include:

• monitoring of local winds during shuttle launches and landings and during rocket launches,
• measuring wind turbulence, wind shear, and wake vortices near airports and during flight, giving airline pilots advanced warning of dangerous situations,
• improving weather forecasts,
• analyzing the effect of potential new construction on existing wind fields.

By providing data for computer models, wind measurements allow better weather forecasting, including more accurate predictions of the paths of tropical storms and hurricanes.

Current Projects

Because of NASA's unique applications, one-of-a-kind lasers are often required. For all active atmospheric remote sensors, eye-safety is an absolute requirement. If visible wavelength laser energy is focused on the retina by the eye, concentrating the energy can lead to eye damage. In the infrared, energy is absorbed by the volume of the eye with the consequence that IR wavelength lasers that are as powerful as visible wavelength lasers do much less, if any, damage to the eye. For eye safety, lasers that operate outside the visible region of the electromagnetic spectrum are strongly preferred.

Further, to limit the returned signal from reflection by the target gas and to simplify data analysis, lasers used in atmospheric remote sensors have a line width that is small in comparison to the target gas's absorption, a narrow line width indeed. For relativity long lifetimes and sufficient ruggedness to withstand the rigors of the space environment, diode pumped solid state lasers are preferred for space-based platforms.

Image to left is the LASE logo.

DIAL lidars such as NASA's LASE (Lidar Atmospheric Sensing Experiment), require two nearly simultaneous laser pulses of slightly different wavelengths.
In LASE, a Ti:sapphire laser is pumped by a novel double pulsed Nd:YAG laser, the "on" and "off" wavelengths are seperated by less than 70 picometers, a distance about 1,000,000 times smaller than a human hair. The frequency of the Ti:sapphire laser is controlled by injection seeding using a diode laser that is frequency locked to a water vapor line in the 815-nm region. The laser pulse pairs at a 5 Hz repetition rate are sequentially transmitted with about 400 miscroseconds separation. The LASE instrument is used to measure water vapor, the most important radiative gas in the troposhere effecting the earth's radiation budget.

The goals of the LITE mission were to validate key lidar technologies for spaceborne applications, to explore the applications of space lidar, to provide the first lidar science measurements of clouds and aerosols from space, and to gain operational experience which will benefit the development of future systems on free-flying satellite platforms.

Image to right is the LITE logo.

LITE's scientific data provide the first highly detailed global view of the vertical structure of cloud and aerosol from the Earth's surface through the middle stratosphere.

The LITE laser transmitter, the most powerful civilian laser ever flown in space, consists of two identical flashlamp-pumped, q-switched Nd:YAG lasers operating at a wavelength of 1.06 um and a pulse width of 30 nanoseconds with each pulsecapable of generating greater than 1.0 J per pulse at a rate of 10Hz. For experimental reasons, both lasers are not operated simultaneously. Doubling and tripling crystals provide simultaneous pulses at 0.46 J at 532 nm and 0.20 J at 355 nm.

Graphic to left is ORACLE.

NASA and the Canadian Space Agency are jointly developing ORACLE (Ozone Research with Advanced Cooperative Lidar Experiments), and autonomously operated, compact DIAL instrument to be placed in orbit using a Pegasus class launch vehicle.

ORACLE will provide real time, global remote sensing of stratospheric ozone which protects life on earth from harmful UV radiation from the sun. For the first time ever, Ti:sapphire and Nd:YLF will generate narrowband UV energy. To date, maximum output energy for a solid-state UV laser is 25 mJ, the ORACLE laser will produce 480 mJ. Further, although typical existing UV lasers have less than 0.5% electrical-to-optical efficiency, the ORACLE laser will have an efficiency of about 2%.

More accurate and precise measurements of tropospheric winds, an important element in NASA's Earth Science Program, would provide a greater impact on numerical weather prediction models than any other space-based observation.

Image to right is SPARCLE logo.

The Doppler Lidar experiment, SPARCLE (Space Readiness Coherent Lidar Experiment), will reside in two pressurized Shuttle Hitchhiker modules the size of large wastepaper baskets. SPARCLE will aim pulses of eye-safe laser light into the atmosphere and measure the light which is reflected back to it by dust and aerosols in the atmosphere. Using the optical heterodyne detection technique to measure the Doppler shifts in the return signals, wind velocities will be determined.

Heterodyne detection involves the optical mixing of the lidar return with another laser operating at or near the lidar transmitter wavelength, with detection of the difference or beat frequency of the mixed signal.

RSTB has designed and developed a two-micron diode-pumped Ho:Tm:YLF pulsed laser transmitter for SPARCLE shuttle mission, enablingthe first ever wind lidar system to be flown in space in Year 2001. The laser transmitter will deliver 100 mJ pulses at 6 Hz pulse repetition frequency. The researchers at RSTB have recently demonstrated an all solid-state, room-temperature, two-micron laser system producing laser pulses of 600 mJ at 10 Hz, an order of magnitude improvement over the previously developed solid-state 2 micron lasers. Using an ultra-stable micro-chip laser, as an injection seed for wavelength control,the output energy of the laser oscillator was mplified by adding four gain stages. The complete laser system was diode pumped. The output beam from the last amplifier stage had a pulse length of about 250 nsec and a line width less than 1.0 MHz.

Graphic to right wake vortex Lidar graphic.

Aircraft vortices present a hazard to other aircraft. Given adequate time, a vortex will dissipate and another aircraft can safely travel through that airspace. Vortices do, however, limit how closely one aircraft can safely follow another, and hence, how much time must elapse between successive aircraft using a given airspace and runway. NASA's Aircraft Vortex Spacing System (AVOSS), part of a future air traffic control system, wil enable planes to take-off and land more safely while decreasing the separation between aircraft.

NASA LaRC is developing and demonstrating eyesafe pulsed coherent lidar systems for ground-based measurement of wake vortices in the airport terminal area. One system, based on a 2 micron wavelength, 100 Hz pulse repetition frequency laser, has been deployed to three different airports resulting in a database of approximately 1000 aircraft landings. Vortex position and strength are displayed and sent to the AVOSS in real time.

Spin Offs

Numerous commercial applications for various laser materials and lasers developed by NASA exist. Potential commercial uses of these lasers include chemical sensing, llumination, surgical tools, dermatology, ophthamology, dentistry, and probes.

Ti:sapphire was primarily a research laser material before NASA selected it as the gain material for the LASE laser. Ti:sapphire was not commercially available. By funding materials growth programs to significantly improve the quality of Ti:sapphire and by demonstrating that Ti:sapphire has therequisite characteristics to make relatively high-power, long-lived lasers, NASA made significant contributions toward bringing this material out of the lab and into commercial products.

NASA also improved that old work-horse of the laser industry, Nd:YAG. Through SBIR grants, NASA sponsored research developed techniques for growing Nd:YAG with 1/5 the loss of older Lasers are and will remain important components of NASA's Earth Science Strategic Enterprise and of NASA's technology development for a safer, faster, more fuel-efficient air-traveling future. Past laser research enabled NASA to develop lasers for the current generation of active remote sensors. Today's research will enable development of lasers for tomorrow's active remote sensors, yielding a safer, more productive world.

For more details, visit the (RSTB) website.