No abstract

The Differential Absorption Lidar (DIAL) in its several implementations has found applications in many aspects of optical remote sensing of the atmosphere, including pollution monitoring and environmental studies. Most standard formulations describing performance, however, do not allow ready evaluation of the interactions of lidar parameters, the ambient atmosphere, and the characteristics of the gas that is to be sensed. As a result, a preferred wavelength for detection of a candidate molecule, and the laser transmitter that might be most suitable for interrogation in the required atmosphere, are not always clear.

The Differential Absorption Lidar (DIAL) method is reviewed and performance evaluations are presented for a NO2 detection system. Discussions on the preferred wavelengths for candidate gases, and a review of a new laser radar DIAL performance equation are presented. Applications in the areas of drug interdiction, forest fire detection and combustion research, and military uses are discussed.The DIAL measures the absorption of a suspect molecule by transmitting two pulses of slightly different frequencies and with minimal time separation. The transmitter may be two different lasers or a single oscillator/amplifier with frequency diversity. The essential techniques are illustrated in Figure 2-1 . One of the laser wavelengths, Xon, commonly called lambdaon, is set near the maximum of the absorption profile of the candidate molecule, while the second, 2off' (lambdaoff), is placed in the "wings" of the curve. The half-width defines the spread of the ideal pressure-broadened (Lorentzian) absorption profile, and thus how far out the wings are, but most molecules are considerably more complex than shown. 2 ISPIE Vol. 2271 O-8194-1595-2/94/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

The Raman technique, while a valuable tool in chemical and combustion research, is limited in many remote sensing applications because of the low Raman scattering cross-section, which may be three to five orders of magnitude below the Rayleigh (elastic) values.Two concepts for increasing the signal level are discussed. First, use a range-gated Fourier transform spectrometer to increase the system throughput and allow multiplexing advantages. The spectrum is obtained by performing a FFT on the resulting interferogram. Second, since the cross section goes as the fourth power of the optical frequency, use ultra-violet laser illumination, and separate the resulting florescence radiation by placing a known dispersion on the transmitted waveform. The techniques for achieving this function, and the mathematical formulation for the phase-modulated auto-correlation which result, are not evaluated in this paper. However, the approach does not appreciably lower the available resolution because the limits are imposed by the sampling function inherent to the finite-duration Michelson minor scan.A conceptual design using a long-pulse, flashlamp-pumped dye laser is shown, and typical performance equations in the detection of Freon 12, CC12F2, are presented. For a one joule laser and a thirty (30) cm aperture operating in darkness, a concentration of 1023 molecules/rn3 can be detected in a 60 km visibility at a range of 3.4 km.

No abstract