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.
Graphic above is pictorial of remote-sensing.
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.
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.