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High Power Fiber Laser [HPFL]

High Power Fiber Lasers (HPFL) have the potential to become one of the principal technologies for defense against the rapidly proliferating unmanned airborne threats. The High Power Fiber Lasers program will develop and demonstrate single mode fiber lasers with output powers of nearly one kilowatt from a single aperture. Tens of kilowatts output power and capability to scale to greater than 100s of kilowatts output power and beyond will be demonstrated through coherent combining of the output power from multiple fiber lasers.

The High Power Fiber Lasers program will develop and demonstrate single mode, single polarization fiber lasers with output powers greater than one kilowatt from a single aperture. Tens of kilowatts output power and capability to scale to greater than hundreds of kilowatts output power and beyond will be demonstrated through coherent combining of the output power from multiple fiber lasers. High power fiber lasers will provide a quantum leap in defense capabilities by simplifying the logistic train and providing a deep magazine, limited only by electric power, in a compact footprint. For theater/area defense and self-protection of combat platforms, they will provide speed of light engagement and flexible response against cruise missiles, reconnaissance unmanned air vehicles (UAVs), and rockets.

This program has two separate tasks. In Task 1, single mode fiber lasers with output powers of one kilowatt or greater from a single aperture shall be developed and demonstrated. In Task 2, tens of kilowatts output power and capability to scale to greater than 100s of kilowatts output power and beyond shall be demonstrated through coherent combining of the output power from multiple single mode fiber lasers.

The program plans to demonstrate greater than 100-watt single mode polarized output power from a single large mode-field area fiber. It will demonstrate greater than 1 kilowatt single mode single polarization output power from a single large mode-field area fiber. It will also demonstrate 1 kw single mode output power from coherently combining the out-power from greater than ten fiber lasers. Finally, it will demonstrate tens of kilowatt output power and capability to scale to greater than hundreds of kilowatts output power.

High power fiber arrays have significant potential for HEL systems by providing beam steering capability as a conformal antenna, light weight system for HEL beam steering, greater efficiencies than slab lasers thus reducing the thermal management requirements and the potential to compensate for system and atmospheric aberrations.

The High Energy Laser Joint Technology Office (HEL-JTO) executes basic research, applied research and advance technology in HEL for the Department of Defense's Deputy Under Secretary of Science and Technology. The HEL-JTO was established in June of 2000 to advocate, coordinate and execute military investments in HEL research and advanced technology.

The Tactical High Energy Laser (THEL), Advanced Tactical Laser (ATL) and the Airborne Laser (ABL) emphasize the potential of HEL weapons on the modern battlefield. To make these types of weapons more viable to the military, significant advances in laser technology are needed to reduce the size and weight and increase the efficiency of laser sources and beam control technologies. In addition, to make these weapons more deployable, eye safer wavelengths are of increasing interest.

Recent advances in power scaling of fiber lasers have made this technology ripe for investment. Fiber laser has the potential for all electric operation eliminating the logistical issues for the chemicals required for the systems mentioned above, and also are showing great promise in terms of size and weight efficiencies.

The goal of this effort is to achieve a nearly diffraction limited beam with output powers of 10-kW in the near term and 100's-kW in the long term that is combined from multiple fiber laser apertures. The system must have high wall plug efficiency, be lightweight, have a compact size and be robust enough to operate with high reliability in a fielded environment.

The important features of this goal are:

  • Fiber laser component development with an emphasis on single aperture scaling and amplifier development.
  • Fiber beam combining solutions such as spectral or tiled combining to produce near diffraction limited beams. Beam control solutions that include target loop or a relay mirror uplink loop are also of interest.
  • Fiber laser research at eye-safer wavelengths and within atmospheric propagation windows. The wavelengths of interest are 1.5-1.8 microns. This could be either lasers emitting directly in that range, or techniques and means for frequency shifting the output of more conventional 1-m high power systems. In all cases, scalability to multiple kW average power must be addressed.

During 2003 DARPA researchers set a world record by demonstrating one-kilowatt continuous wave output power from a single fiber. This successful test demonstrated the viability of the fiber laser concept and is an initial step in the ultimate goal of portable, and affordable, laser-based platform self-protection.

Enabling technologies sought include: 1) 1xN and NxN fiber optic couplers (multi-mode and single-mode core fibers with dual-clad designs), fiber bundles, tapers, optical isolators (both polarization dependent and polarization independent), circulators and power splitters; 2) Fiber Bragg gratings for high reflectance, nonlinear optical effects mitigation, and pump-coupling enhancement to fiber lasers and amplifiers; 3) Phase modulators and other integrated photonic devices aimed at high-power manipulation and power scaling.

While similar devices are being produced commercially for low power telecommunications applications (milliwatts to several watts), at 1550 nanometers they are not compatible with high power optical amplifiers and fiber laser systems being developed. Fiber optic components for operation at 1100 nanometers, 10-1000 watts (cw) are not mature and require further research and development.

The materials used in commercial telecom fiber devices may not be suitable for operation at high optical power levels. In particular, non-linear optical effects induced at high optical powers limit the performance of silica based fiber systems. Research and development in the selection and optimization of materials and processing is needed to scale fiber lasers and amplifiers to kilowatt levels. There is an immediate need for high-power components at the Yb-doped fiber laser pump and emission wavelengths (915nm, 975nm and 1100nm respectively). It is imperative that new designs be compatible with high-power fiber laser operation and dual-clad fiber designs.



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