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Advanced Wavelength Tuners for Chem-Bio Detection Lasers


OBJECTIVE: We are seeking advanced, robust wavelength tuners for laser transmitters operating in the 3-5 um and 8-12 um bands for application to point and standoff detection of chemical and biological agents. DESCRIPTION: A variety of wavelength agile laser transmitters are contemplated for advanced point and standoff sensors to probe for chemical and biological agents. These include most notably Quantum Cascade Lasers (QCLs), CO2 TEA (Transverse Electric Atmospheric) lasers, CO2 waveguide lasers, and solid state lasers with optical parametric oscillators (OPOs). The CO2 types operate at moderate to high peak or average power and typically use precision rotating gratings to achieve wavelength selection. They can also be wavelength shifted by OPO, but in that case output power is limited by optical damage on the nonlinear crystal surfaces and shifter resonator optics. QCLs can be wavelength shifted by thermal means, but such lasers operating in Fabry-Perot resonators with angle tuned gratings can offer broader band coverage at high speed. Solid state lasers have proven to be most effective in the MWIR, although attempts have been made to extend their reach to the LWIR. In both these bands, angle tuned OPO shifters have been effective. In all these cases, one of the critical elements of the wavelength shifter is the precision angle tuning mechanism which must satisfy the combined requirements of speed, position accuracy and repeatability, compactness, and robustness. One example is the CO2 TEA laser now incorporated in the FAL (Frequency Agile Laser) which operates at a pulse repetition frequency (PRF) of 200 Hz. The FAL grating angle tuner requires a 10 rad angular resolution over a 5 degree angle with a settling time of 5 msec and an angular repeatability of 10 rad over temperature ranges of at least 0-40oC. The present FAL tuner achieves only a 2.5 degree of angle in 5 msec. Therefore, a two-fold increase in speed is required to solve the present requirement; and a speed increase of three to four times over the present device will be required to satisfy the advanced requirement for a higher data rate FAL system. Proposed improvements to the FAL laser transmitter to make more laser lines available by using isotope gas mixtures would require a two-fold improvement in the angular resolution and repeatability due to the increased selectivity required. The CO2 waveguide laser typically operates at tens of kHz PRF; however in that case, high number multiple pulse averaging is typical, and broadband wavelength shift rates on the order of 200-500 Hz are still applicable. The QCL type offers the advantage of small size even after the necessary thermal controllers and power supplies are added to the transmitter volume. In order to realize the small size potential of these relatively compact transmitters, an equivalently small tuner would be very desirable. Importantly, a small tuner would allow for efficient close coupling to the external Fabry-Perot resonator. Present tuners have volumes typically on the order of 600 cc, not including power supplies and ancillary electronics. What is needed is a reduction in tuner volume by at least 50% while achieving the speed and position accuracy required for an external resonator QCL. It is also very desirable to develop a tuner that is lightweight and electrically efficient so that the overall package size and weight can be kept below 2 liters and 1 kg, respectively. Wavelength tuning speed and precision are key to acquiring high fidelity target data upon which advanced detection algorithms can operate. For example, in the case of LWIR standoff detection, atmospheric and target evolution effects can induce significant data noise which can be alleviated to some extent by high laser PRF and wavelength tuning speed. High data speeds provide for the possibility of fast and effective pulse averaging leading to fast algorithm throughputs and reduced time to alarm. PHASE I: Perform analysis and feasibility studies. Develop conceptual designs for the tuner including a means to test and verify the performance. Provide a detailed development plan for design, fabrication, and testing of the wavelength tuner to be carried out in the Phase II program. PHASE II: Use the results of Phase I to design, fabricate, test and deliver a tuner prototype for demonstration with a government-furnished laser. Provide a roadmap for the integration of the laser and tuner combination with a government-furnished sensor. PHASE III DUAL USE APPLICATIONS: The result of Phase II will be demonstration of a tuner that can be used for rapidly interrogating chem-bio agents. In addition to immediate military applications, the tuner will find widespread use in important civilian applications, including WMD (weapons of mass destruction) detection, pollution monitoring, commercial manufacturing process monitoring, and commercial equipment that is necessary for scientific and aerospace corporations. REFERENCES: (1.) D. Cohn, W. Griffin, L. Klaras, E. Griffin, H. Marciniak, J. Fox, C. Swim,"WILDCAT sensor", SPIE Proceedings 4036, 210, Orlando, Florida, 24-25 (April 2000); (2.) D. Cohn, L. Klaras, J. Fox, C. Swim,"WILDCAT sensor design", Proceedings Fourth Joint Workshop on Standoff Detection for Chemical and Biological Defense, Williamsburg, VA, 305, 26-30 (Oct 1998); (3.) D. Cohn, J. Fox and C. Swim,"Frequency agile CO2 laser for chemical sensing", SPIE Proceedings, Los Angeles, California, vol. 2118, p 72, (Jan 1994).
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