The weaponization and dissemination of biological warfare agents (BWA) constitute a high threat to civilians and
military personnel. An aerosol release, disseminated from a single point, can directly affect large areas and many people
in a short time. Because of this threat real-time standoff detection of BWAs is a key requirement for national and
military security. BWAs are a general class of material that can refer to spores, bacteria, toxins, or viruses. These
bioaerosols have a tremendous size, shape, and chemical diversity that, at present, are not well characterized .
Lockheed Martin Coherent Technologies (LMCT) has developed a standoff lidar sensor with high sensitivity and robust
discrimination capabilities with a size and ruggedness that is appropriate for military use. This technology utilizes multiwavelength
backscatter polarization diversity to discriminate between biological threats and naturally occurring
interferents such as dust, smoke, and pollen. The optical design and hardware selection of the system has been driven by
performance modeling leading to an understanding of measured system sensitivity. Here we briefly discuss the
challenges of standoff bioaerosol discrimination and the approach used by LMCT to overcome these challenges. We
review the radiometric calculations involved in modeling
direct-detection of a distributed aerosol target and methods for
accurately estimating wavelength dependent plume backscatter coefficients. Key model parameters and their validation
are discussed and outlined. Metrics for sensor sensitivity are defined, modeled, and compared directly to data taken at
Dugway Proving Ground, UT in 2008. Sensor sensitivity is modeled to predict performance changes between day and
night operation and in various challenging environmental conditions.
Tunable single-frequency sources in the 2-4 micron wavelength region are useful for remote DIAL measurements of chemicals and pollutants. We are developing tunable single-frequency transmitters and receivers for both direct and coherent detection lidar measurement applications. We have demonstrated a direct-diode-pumped PPLN-based OPO that operates single frequency, produces greater than 10 mW cw and is tunable over the 2.5 —3.9 micron wavelength region. This laser has been used to injection seed a pulsed PPLN OPO, pumped by a 1.064 micron Nd:YAG laser, producing 50-100 microJoule single-frequency pulses at 100 Hz PRF near 3.6 micron wavelength. In addition, we have demonstrated a cw Cr:ZnSe laser that is tunable over the 2.1 —2.8 micron wavelength region. This laser is pumped by a cw diode-pumped Tm:YALO laser and has produced over 1.8 W cw. Tm- and Tm,Ho-doped single-frequency solid-state lasers that produce over 50 mW cw and are tunable over approximately 10 nm in the 2 —2.1 micron band with fast PZT tuning have also been demonstrated. A fast PZT-tunable Tm,Ho:YLF laser was used for a direct-detection column content DIAL measurement of atmospheric CO2. Modeling shows that that all these cw and pulsed sources are useful for column-content coherent DIAL measurements at several km range using topographic targets.
A standard grating-tuned extended-cavity diode laser is used for injection seeding of a tapered semiconductor laser/amplifier. With sufficient injection power the output of the amplifier takes on the spectral characteristics of the master laser. We have constructed master-oscillator power-amplifier systems that operator near 657 nm, 675 nm, 795 nm, and 850 nm. Although the characteristics vary from system to system, we have demonstrated output powers of greater than 700 mW in a single spatial mode, linewidths less than 1 kHz, coarse tuning greater than 20 nm, and continuous single-frequency scanning greater than 150 GHz. We discuss the spectroscopic applications of these high power, highly coherent, tunable diode lasers as applied to Ca, Hg+, I2, and two-photon transitions in Cs.
Linewidth reduction of an extended cavity diode laser at 657 nm was accomplished by negative feedback to an intra-cavity ADP crystal. High resolution (170 kHz wide) saturated absorption signals were recorded of the calcium intercombination line which is of interest for a frequency standard. The spectrum of the red 62p3/2 - 92s1/2 cesium line in a magneto-optical cell trap was also investigated.