Army

Fiber laser collimating for directed energy systems

Eliminates the parasitic radiation of divergent Gaussian beams in a compact fiber optic collimator

Photonics

High energy laser (HEL) transmitters using electrical power have application in directed energy owing to a) extremely precise target selection without collateral damage; b) delivery of energy at the speed of light; c) easy scalability using a relatively small laser energy focused into very small spot; d) the virtually unlimited pulses available. There is an applicability of HEL directed energy systems in shooting at light targets such as UAVs, mortars, small boats, and missiles. However, current systems are bulky and heavy and require a heavy-weight carrier, such as a truck or ship.

The collimator 50 comprises a redirecting interior lens 12. The interior lens has a hole 20 with a shape exactly matching the shape of the output collimating lens 25 having the restricted aperture according to the requirements of compactness and the performance of the array. The output lens 25 is part of two optical systems: 1) a collimating system providing the focusing of a high-power fraction onto the focal plane of the target 30; and 2) the disposing system which is re-directing the much smaller power of the dispersed beam forward into space. The area 5 may comprise a Fresnel lens.

Fiber lasers are considered the most advanced laser sources because of their high wall-plug efficiency reaching 40% and almost ideal beam quality. However, the increase of power of fiber laser beam with conservation of high quality is restricted with non-linear effects in single-mode fibers. The use of multi-mode fibers for increasing power above 10 kW leads to lower beam quality. This also requires increasing the size of focusing mirrors, the addition of adaptive optics with an increase of SWaP, and with slowing down the speed of targeting.

The practical way to scale the power to 100 kW or above is combining multiple fiber lasers each with modest power and high beam quality. A weaponry level can be achieved by combining tens of fiber lasers with power 1-2 kW each. Different methods of laser combination have been explored by conventional solutions. The array of fiber laser collimators is among the most suitable for mobile applications. Unfortunately, collimators suffer from heating issues as a result of reflected energy of divergent beams keeping the laser from operating at high power without sophisticated cooling systems.

Army researchers have developed a fiber laser collimator and an array of seven such collimators configured to transmit approximately 100% of full power from the fiber tip despite the restricted output lens aperture. In this collimator and array, the truncated beam tails with parasitic radiation are not intercepted and dissipated within the array, but instead, are re-directed into the output lenslet. No cooling of the array is necessary if the total emitted power will exceed 99.9% and only less than 0.1% of full power will remain inside of the array.  The collimator emits a high-power beam (1-3 kW level) and the array may transmit 7-21 kW into free-space.

The high compactness and small weight of the collimator allows one to develop a portable array of densely packed collimators with the maximum efficiency of coherent beam combining (CBC) on the remote target or receiver for directed energy applications, free-space optical communication, and laser machining.

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