If you know what atmospheric constituent you are looking for, a lidar system tuned to a specific wavelength will do nicely. However, trying to discern the nature and components of unknown constituents is another story. To solve this problem, ITT Industries Inc. teamed with Laser Energetics Inc. to develop a broadly tunable remote sensing system capable of detecting chemical agents and a variety of atmospheric constituents. The system would rely on Laser Energetics' alexandrite technology. "If you're trying to measure chemical agents and you don't know what you're going to see, you need to back out the absorption feature," said Scott Higdon, a program manager for ITT's Advanced Engineering and Sciences Div. "That means you need to send a number of wavelengths out there." The tunability of alexandrite lasers makes them adaptable sources for lidar systems that must detect a broad range of atmospheric constituents. Courtesy of Laser Energetics Inc. Most lidar systems performing molecular detection employ differential absorption techniques in which two or more wavelengths probe the atmosphere and a sensor measures comparative backscatter. In these systems, one wavelength is absorbed more than the other. If the wavelengths are close together, the only difference in backscatter should be due to absorption by the molecule of interest. Because chemical agents and atmospheric molecular constituents can have absorption features ranging from the UV into the long-wave infrared, ITT's proposed system required a laser source tunable over as broad a range as possible. A lidar system performs only as well as the laser providing its source of photons, Higdon said. ITT researchers have worked with several laser technologies in the past, including optical parametric oscillators, excimers and Ti:sapphire lasers. Most often, however, the company has relied on Nd:YAG lasers operating at their fundamental and harmonic wavelengths or pumping nonlinear optical crystals. While a mature technology, Nd:YAG lasers can vary in the average power their beams demonstrate when transmitted through nonlinear crystals. This was a concern for ITT because the laser's average power determines range. "Normally, with lidar you want the longest range possible. The more photons you can get out there in less time, the more information you can collect," Higdon said. Alexandrite operates in the near-infrared, but it retains its tunability around doubled and tripled wavelengths. Laser Energetics' alexandrite technology and its intellectual property portfolio are compelling, he said, adding that its conductively air-cooled alexandrite systems can run hotter, allowing an additional 50 nm of tunability at the top end of the laser's range. That means its fundamental operation extends between 720 and 860 nm, while its second- and thirdharmonic generation range from 360 to 430 nm and 240 to 280 nm, respectively.