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Coherent Imaging Laser Source


OBJECTIVE: Develop an innovative, flight compatible, high-energy, pulsed laser system for use with coherent imaging systems. DESCRIPTION: Coherent imaging systems have the capability to provide high-resolution 3D imagery for target identification at stand-off ranges. Coherent imaging methods which combine image synthesis and multi-wavelength 3D imaging offer exciting possibilities for future imaging systems [1-3]. These systems theoretically provide arbitrarily high target depth precision without the need for high-bandwidth detectors and modulators. The holographic imaging techniques may also lead to low volume, conformal active imaging systems. Unfortunately, these systems require relatively short, transform-limited linewidth pulsed lasers with high per-pulse energy to illuminate the target of interest. Current state of the art narrow linewidth (<100kHz) pulsed fiber laser systems around 1550 nanometer wavelength region are limited to ~100 microjoules for the pulse durations desired. While state of the art Q-switched lasers do have sufficient pulse energies they do not have sufficiently narrow linewidths and/or access to a continuous wave (CW) seed laser with the same wavelength as the pulsed output. An innovative and novel solution is required in order to design a laser system capable of delivering high energy pulses for standoff coherent imaging applications. Systems designed to utilize coherent imaging techniques would require high-energy, transform-limited pulses to ensure adequate Signal-to-Noise Ratio and to maintain coherence throughout the target volume. High pulse energies allow larger areas to be imaged; pulse energies of greater than 10 millijoules may be required to image extended targets of interest at range. Pulse lengths between 30-300 nanoseconds should strike a balance between freezing target motion and providing coherent return across a relatively deep target. The laser wavelength chosen should have high atmospheric transmission and be between 1.4 and 1.7 microns to mitigate eye-safety risks and allow the surrounding imaging system to utilize commercial-off-the-shelf fiber components and detector arrays. Longer wavelengths may be considered as long as suitable detector arrays and additional electro-optic components are identified that enable the short pulse, shot noise limited, digital holography techniques desired. While a Master Oscillator Power Amplifier architecture is not necessarily required, access to a CW"seed"output is required to provide a local oscillator for the imaging sensor. The system must have an adequate gain bandwidth over 10"s of gigahertz to support multi-wavelength imaging techniques. In summary, the proposed system should have limited pulses with lengths between 30-300 nanoseconds, pulse energies greater 10 millijoules, while providing a CW seed source for coherent mixing with the return beam. Pulse repetition rates should be scalable to 1 kHz or greater. The proposed system should operate at an eye-safe wavelength with high atmospheric transmission. The ultimate goal is to develop a laser system that is flight compatible with minimal size, weight and power (SWAP) [i.e. high wall-plug efficiency]. These considerations should drive any design solution. PHASE I: The effectiveness of various candidate concepts will be evaluated. Preliminary designs will be modeled and fabrication feasibility of those designs will be evaluated. Shortcomings in fabrication and critical technology that would require additional development during Phase II will also be identified. PHASE II: A prototype laser which demonstrates key design principles will be developed and tested. A path towards a producible, low-SWaP laser system will be described. PHASE III: A flight compatible laser system will be developed as part of a holographic ladar sensor and integrated into an electro-optic sensing pod for performing identification at tactical standoff ranges. REFERENCES: 1. J. Marron and K. Schroeder,"Holographic laser radar,"Opt. Lett. 18, 385-387 (1993). 2. T. Hft, R. Kendrick, J. Marron, and N. Seldomridge,"Two-Wavelength Digital Holography,"in Adaptive Optics: Analysis and Methods/Computational Optical Sensing and Imaging/Information Photonics/Signal Recovery and Synthesis Topical Meetings on CD-ROM, OSA Technical Digest (CD) (Optical Society of America, 2007), paper DTuD1. 3. David J. Rabb, Douglas F. Jameson, Jason W. Stafford, and Andrew J. Stokes,"Multi-transmitter aperture synthesis,"Optics Express Vol. 18, pp. 24937-24945 (2010).
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