N131-017 TITLE: Polarization insensitive diffraction grating for Navy tactical airborne high energy lasers (HEL) applications

TECHNOLOGY AREAS: Air Platform, Weapons

ACQUISITION PROGRAM: PMA-265

RESTRICTION ON PERFORMANCE BY FOREIGN CITIZENS (i.e., those holding non-U.S. Passports): This topic is "ITAR Restricted". The information and materials provided pursuant to or resulting from this topic are restricted under the International Traffic in Arms Regulations (ITAR), 22 CFR Parts 120 - 130, which control the export of defense-related material and services, including the export of sensitive technical data. Foreign Citizens may perform work under an award resulting from this topic only if they hold the "Permanent Resident Card", or are designated as "Protected Individuals" as defined by 8 U.S.C. 1324b(a)(3). If a proposal for this topic contains participation by a foreign citizen who is not in one of the above two categories, the proposal will be rejected.

OBJECTIVE: Develop and fabricate a polarization insensitive diffraction grating for High Energy (HEL) Laser Weapon systems on tactical air platforms.

DESCRIPTION: Recent developments in subsystem performance have made realistic the potential for the incorporation of high energy lasers (HEL) on tactical aircraft. High power fiber lasers have shown the potential for achieving 1 kW or more per fiber. By combining multiple fiber lasers via spectral beam combination (SBC) it is possible to achieve power levels suitable for the demonstration of a Laser Weapon System (LWS). In SBC, each individual fiber laser operates at a different wavelength and when the output of each individual fiber is incident on a diffractive element at the appropriate angle, all of the outputs can be spatially coaligned and directed at the target by the beam director system. The volume and weight restrictions of air platforms as well as performance requirements have placed serious constraints on the overall system. Improvements in system efficiency lead to reduced requirements for prime power and thermal management and therefore an overall reduction in weight and volume. A key component for achieving SBC is the diffractive grating. The SBC grating determines the combining efficiency of the system and must be able to withstand the full output of all of the individual fiber lasers. The focus of this topic is for a high power SBC grating that can address the requirements of multiple platform-based high energy laser systems, such as the F/A-18 and MH-60S.

These requirements include:
� High power handling capability: The grating should withstand a minimum of 20 kW/cm2 (goal of > 30 kW/cm2).
� High efficiency: The grating should have a minimum diffraction efficiency of 98% near the grating Littrow angle over a 40 nm bandwidth near 1.06 ?m (goal of > 99% efficiency) independent of polarization.
� Polarization insensitivity: The high efficiency should be maintained regardless of the polarization state of the input laser beams.
� Absorption: The diffractive grating should have an absorption less than 1 percent (goal < 0.1%).
� Clear aperture: The diffractive grating should have uniform performance (ie, meets minimum requirements above) with a usable area in excess of 79 cm.

Innovative ways to improve the performance of diffractive gratings for SBC are sought. Although specifically targeted for implementation in future high energy laser systems for tactical air platforms, the same technology would undoubtedly provide benefits to ground and sea based high energy lasers as well. The target of this program will be to demonstrate a polarization insensitive, highly efficient grating with high power handling capability. Operation in a military environment will be essential for future applications; therefore the grating must be robust to the shock, vibration, and temperature environments of a deployed device. Finally, the resultant diffractive element must be designed in a way as to be consistent with current state-of-the-art high volume manufacturing practices in the industry. The goal for the end of the program would be to achieve a per unit cost of less than $30,000.

PHASE I: Develop a conceptual design for a polarization insensitive, high efficiency diffraction grating that meets Navy requirements. Include methodology and prototype performance that will demonstrate the proposed concept at the performance above.

PHASE II: Develop detailed designs for the Phase I diffraction grating and fabricate a limited number of gratings suitable for proof of concept testing. Conduct preliminary testing in a laboratory and in Government-sponsored facilities, including appropriate environmental considerations and report the results of this preliminary testing to the Government. Provide sample(s) to the Navy for evaluation.

PHASE III: Scale up for mass production of the grating. This grating, upon meeting Navy requirements, will be transitioned into various laser weapon programs on multiple air platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Potential commercial use is in for lasers systems that operate in a rugged environment where efficiency is critical to system weight and volume requirements.

REFERENCES:
1. Thomas H. Loftus, et al.(2007). 522 W average power, spectrally beam-combined fiber laser with near-diffraction-limited beam quality. Optics Letters 32, 349-51

2. S. J. Augst, et al. (2003). Wavelength beam combining of ytterbium fiber lasers. Optics Letters 28, 331-4

KEYWORDS: Precision Optical Systems; Diffraction gratings; High Energy Laser Systems; Spectral Beam Combination (SBC)

** TOPIC AUTHOR (TPOC) **

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