Laser Development at Q-Peak for Remote Sensing
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Laser Development at Q-Peak for Remote Sensing Peter F. Moulton Q-Peak, Inc. 135 South Road Bedford, MA 01730, U.S.A.
INTRODUCTION AND MATERIALS BACKGROUND Q-Peak has been developing solid state laser and nonlinear optical systems applied to remote sensing for nearly 20 years. Our first devices were based on the use of gas-discharge lamps to optically pump the laser crystals, a standard technology that traces back to 1960 with the first laser demonstration, using ruby (Cr3+:Al2O3) as the laser material. In later and most current systems we have employed the newer and more efficient use of semiconductor diode lasers as pump sources, which has allowed operation at higher levels of performance and makes practical the deployment of satellite-based, active remote-sensing systems. Since our work is based on the use of advanced materials in the laser medium, nonlinear crystals and pump sources, it is important to draw the connection between materials research and the advances that others and we have made in sources for remote sensing. Thus, before we begin specific discussion of our efforts, we highlight key materials that have enabled our work. We consider specifically tunable solid state laser materials and nonlinear crystals. The development of high-power III-V compound semiconductor lasers is also important, but a subject for a separate paper. Tunable solid state lasers Detection of specific atmospheric species with the capability of mapping in three dimensions can be done by the Differential Absorption Lidar (DIAL) technique, in which at least two different pulsed-laser wavelengths are transmitted into the region of interest, and the light backscattered by molecules and aerosols is detected as a function of time for the different wavelengths used. If one of the wavelengths (“on”) is partially absorbed by the atmospheric species of interest and the other (“off”) is not, the signal resulting from subtraction of one return from another will indicate the density of the species. One can determine, by converting time to range, the spatial distribution of the species along the path of the beam. Scanning of the beam in different directions allows development of a three-dimensional map of the species concentration. In order to carry out DIAL, the laser system used must be capable of being tuned to wavelengths that are either absorbed or not absorbed by the species. While all lasers are tunable, most are limited to a small fraction of the center wavelength. To carry out DIAL measurements without a broadly tunable laser, one must rely on a fortuitous overlap between the laser line and an absorption line of the species of interest, but the particular line may not be the most optimal for the sensing application. The first broadly tunable lasers, demonstrated starting in 1964 by L.F. Johnson and associates at Bell Laboratories [1], were based on divalent-transition-metaldoped crystals such as Ni2+- and Co2+- doped MgF2. However, due to the fundamental nature of the electronic transitions of the divalent-metals, the laser
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