Radiant energy – Luminophor irradiation
Reexamination Certificate
2001-05-11
2003-07-15
Porta, David (Department: 2878)
Radiant energy
Luminophor irradiation
C250S341100
Reexamination Certificate
active
06593582
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a portable light detecting and ranging (LIDAR) system for long-range aerosol detection of biological weapon gas clouds. As such, the system can be used to provide early warning for field personnel, providing the necessary time for personnel to prepare for the arriving gas cloud.
2. Discussion of the Background
Remote stand-off detection of chemical/biological (chem/bio) agents is considered to be a critical necessity in early warning systems enabling maximum survivability of personnel in the battlefield and other sensitive areas. Pulsed elastic backscatter lidar operating in the visible, as described by Lee, et al, “Micro Pulse Lidar for Aerosol & Cloud Measurement”,
Advances in Atmospheric Remote Sensing with Lidar,
pp 7-10, A. Ansmann, Ed., Springer Verlag, Berlin, 1997, the entire contents of which are incorporated herein by reference, and near IR, as described by Condatore, et al, “U.S. Army Soldier and Biological Chemical Command Counter Proliferation Long Range—Biological Standoff Detection System (CP LR BSDS)”,
Proceedings of SPIE,
Vol. 3707, 1999, the entire contents of which are incorporated herein by reference, have demonstrated the high sensitivity and long-range (up to 50 km) capability to detect aerosol clouds. Consequently, aerosol lidar is a chosen technique for long-range detection of bio-warfare aerosols. However, single wavelength aerosol lidars, as currently employed, do not provide discrimination between biological weapon (BW) agent aerosols and other natural or interferent aerosol clouds. The capability to differentiate can be augmented by using multiple wavelength and/or multiple polarization elastic scattering signatures. However, the elastic scattering technique lacks the required specificity for deterministic application of the data in the battle field.
Aerosol lidar is an ideal complement to uv fluorescence lidar, as demonstrated by Wilson, et al, “Development of IR and UV Lidar systems for standoff detection of airborne biological materials” Final Report, Contract DAAA15-91-C-0138, STC Technical Report, 1993, the entire contents of which are incorporated herein by reference, which discloses a UV laser that excites fluorescence from the biological constituents of the aerosol and measures the fluorescence signature of the biological constituents to provide specificity for discrimination between bio-aerosols and other naturally occurring or interfering aerosols. Since atmospheric absorption at UV wavelengths is high and fluorescence cross-section of the target particles is small, even the use of a high energy laser source with a large aperture telescope only enables conventional fluorescence lidar to achieve a range coverage of three to four kilometers. Jezek and Cannaliato, “Biological Standoff Detection”, Joint Workshop on Standoff Detection for Chemical and Biological defense, pp. 26-30, October 1998, the entire contents of which are incorporated herein by reference, have been actively developing both long range and short range sensor systems. Long-range biological standoff detection system LR BSDS, as described in Condatore, et al, “U.S. Army Soldier and Biological Chemical Command Counter Proliferation Long Range—Biological Standoff Detection System (CP LR BSDS)”,
Proceedings of SPIE,
Vol. 3707, 1999, the entire contents of which are incorporated herein by reference, is based on elastic scatter aerosol lidar. Short range biological standoff detection system SR BSDS, as described in Suliga, et al, “Short Range Biological Standoff Detection System (SR-BSDS)”,
Fourth Joint Workshop on Standoff Detection for Chemical and Biological Defense,
Sep. 15, 1998, the entire contents of which are incorporated herein by reference, is based on fluorescence and aerosol lidar.
Current chem/bio defense detection systems can provide a rapid indication of a possible BW attack by utilizing multiple independent technologies to provide separate lines of data, which are less likely to be wrong at the same time, thus reducing false alarms. However, present technologies, owing to the complexity and laser power levels required for fluorescence and aerosol lidar, are limited in range and not well suited for an in-the-field, portable early warning detection system.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide an early warning detection system which can provide accurate detection of biological weapons at sufficient distance to provide an adequate response time.
Another object of the present invention is to integrate multiple analysis techniques into a detection system to reduce a probability of false alarms.
Still a further object is to provide a self-aligned detection system which improves reliability of the detection system in the field.
A further object of the present invention is to provide a digital detection system which can by multiplexing improve the signal-to-noise ratios and the detected signals.
These and other objects are achieved in a lidar system including a laser which provides laser pulses of at least two wavelengths, a transmitter which transmits the laser pulses, a receiver which receives both elastically backscattered signals from airborne agents and fluorescence signals from the airborne agents, a common telescope which both focuses a laser beam transmission of the laser pulse from the transmitter to a far field and receives the elastically backscattered signals and the fluorescence signals from the far field, a digital detection system having at least one of a backscatter optical detector which detects the elastically backscattered signals and a fluorescence optical detector which detects the fluorescence signals from the airborne agents.
Indeed, the lidar system of the present invention maximizes the signal-to-noise ratio (SNR), thus maximizing the range capability for a given SNR. An acceptable criterion for the confident detection of BW agent aerosol in the atmosphere is for the signal-to-noise ratio of the lidar signal to be about four. In addition to photon shot noise generated by the laser scattered light falling on the detector, a detection system itself can contribute to the noise. In a conventional lidar system, where analog detection technique is used, the noise depends on the detector dark noise together with the signal shot noise, i.e., the noise in the associated amplifier and the detection bandwidth. The bandwidth of a lidar system is determined by the desired spatial resolution of the lidar measurement. For example, a 15 m spatial resolution requires at least 5 MHz bandwidth. The minimum signal required for an analog lidar system to successfully measure an aerosol is determined by the detector dark current and the bandwidth. Thus, in the analog lidar design approach of the present invention, increasing the measurement range requires increasing the signal, which normally implies a high-energy laser and a large telescope for collecting the signal. For the BSDS system previously discussed, a laser energy>100 mJ, and a telescope receiver size of 65 cm dia. is utilized.
In contrast, the portable digital lidar (PDL) system of the present invention is based on a different approach, i.e., digital detection, where the detection noise is minimized so that a much lower signal level is adequate to yield the required SNR. Digital detection utilizes photon-counting which generates digital pulses for every photon that is detected and is not affected by the bandwidth or the amplifier noise. In one embodiment of the present invention, a Geiger mode avalanche photodiode (APD) detector is utilized with low signal induced noise. Other than the photon shot noise, the only noise source in digital detector is the detector dark count noise, which is about three orders of magnitude smaller than the dark current noise in an analog detector.
Hence, digital detection system is capable of detecting signals nearly a thousand times smaller than analog detection. Thus, the laser energy for the fluorescence excitation can be reduced to
Hwang In Heon
Lee Hyo Sang
Prasad Coorg R.
Moran Timothy J.
Porta David
Science & Engineering Services Inc.
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