Wave front shaping, the ability to manipulate light fields both spatially and temporally, in complex media is an emerging field with many applications. This review summarizes how insights from mesoscopic scattering theory have direct relevance for optical wave control experiments and vice versa. The results are expected to have an impact on a number of fields ranging from biomedical imaging to nanophotonics, quantum information, and communication technology.

Light fields in complex media: Mesoscopic scattering meets wave control

Commercial 0.5 kW Yb-doped fiber amplifiers have been characterized and found to be suitable for coherent beam combining. Eight such fiber amplifiers have been coherently combined in a tiled-aperture configuration with 78% combining efficiency and total output power of 4 kW. The power-in-the-bucket vertical beam quality of the combined output is 1.25 times diffraction limited at full power. The beam-combining performance is independent of output power.

Coherent combining of a 4 kW, eight-element fiber amplifier array.

The first rare-earth-doped fiber lasers were operated in the early 1960s, and produced a few milliwatts at a wavelength around 1 mum. Since the beginning of the decade, an enormous increase of fiber laser output power has been reported, the realm of kilowatt power has been entered, and power levels as high as 100 kW are envisaged. Apart from the power, fiber laser systems are renowned for their inherent compactness, monolithic architecture, and a power-independent beam quality. This paper reviews the challenges, achievements, and perspectives of high-power continuous-wave (CW) laser generation and amplification in fibers.

The Rising Power of Fiber Lasers and Amplifiers

We present theoretical and experimental research on coherent beam combining of fiber amplifiers using stochastic parallel gradient descent (SPGD) algorithm. The feasibility of coherent beam combining using SPGD algorithm is detailed analytically. Numerical simulation is accomplished to explore the scaling potential to higher number of laser beams. Experimental investigation on coherent beam combining of two and three W-level fiber amplifiers is demonstrated. Several application fields, i.e., atmosphere distortion compensating, beam steering, and beam shaping based on coherent beam combining using SPGD algorithm are proposed.

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

We discuss development and integration of a coherent fiber array system composed of densely packed fiber collimators with built-in capabilities for adaptive wavefront phase piston and tilt control at each fiber collimator. In this system, multi-channel fiber-integrated phase shifters are used for phase locking of seven fiber collimators and the precompensation of laboratory-generated turbulence-induced phase aberrations. Controllable x and y displacements of the fiber tips in the fiber collimator array provide additional adaptive compensation of the tip and tilt phase aberration components. An additional control system is utilized for equalization of the intensity of each of the fiber collimator beams. All three control systems are based on the stochastic parallel gradient descent optimization technique. The paper presents the first experimental results of adaptive dynamic phase distortion compensation with an adaptive phase-locked fiber collimator array system.

Adaptive Array of Phase-Locked Fiber Collimators: Analysis and Experimental Demonstration

Coherent combining of a record 48 PM fibres in a phased array configuration is reported. The resulting Strehl ratio degrades by 1000 fibres.

Coherent beam combining of large number of PM fibres in 2-D fibre array

We demonstrate, for the first time to our knowledge, successful beam control of a fiber optic phased array containing a large number of polarization maintaining fibers. As many as forty-eight fibers have been coherently combined via individual all-fiber phase modulators. The residual phase error is less than 1/30th of a wave. Results with both near-field interferometric control and target-in-the-loop control have been obtained. Experimental results are compared with numerical simulations and excellent agreement has been achieved. We investigated propagation of this phased array output through a turbulent atmosphere, and used the all-fiber phase modulators for the compensation of turbulence effects on the array output. This work paves the way towards scaling such fiber optic phased arrays to very high fiber count. Eventually thousand of fibers can be controlled via such a scheme.

Beam control of a 2D polarization maintaining fiber optic phased array with high-fiber count

We have conducted active-tracking experiments in support of the Air Force's Airborne Laser program. These tests were conducted using the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. Target illumination was accomplished using a multibeam illuminator laser provided by Lockheed Martin Missiles & Space; the tracking was performed using an imaging tracker provided by the Air Force Research Laboratory. Experiments were conducted using a number of different illuminator configurations, and with a variety of track algorithms. Results from these tests have quantified the performance benefits of multibeam illumination.

Active tracking using multibeam illumination

We have conducted atmospheric compensation experiments using active illumination for both adaptive-optics and tracking. Tests were performed in support of the Air Force's Airborne Laser program. The tests utilized the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. The adaptive-optics beacon was provided by actively illuminating the target with a multibeam laser illuminator. A second multibeam laser illuminator was used to provide a beacon for an imaging tracker. Experiments were conducted using two different adaptive-optics illuminator configurations, as well as with point-source beacons. Data were collected over a range of atmospheric conditions. Results from these tests have helped to provide a performance benchmark for the Airborne Laser program.

Adaptive optics compensation using active illumination

■ The U.S. Air Force is developing the airborne laser (ABL), whose mission is to engage and destroy theater ballistic missiles such as the SCUD while these missiles are in their boost phase. This mission capability requires high-energy laser propagation over long horizontal paths (200 to 300 km) through the upper atmosphere. To be effective in the presence of atmospheric turbulence, the ABL must utilize precision tracking and adaptive-optics compensation. Although the strength of atmospheric turbulence at ABL altitudes (35,000 to 45,000 feet) is relatively weak compared to sea level, the long horizontal laser-propagation paths create severe challenges for the adaptive-optics and tracking systems. An equally difficult challenge is created because the missile provides no beacon for the adaptive-optics and tracking systems. The target missile must be actively illuminated so that backscatter from the missile body can be used to form an image for the tracking system and provide a beacon for the adaptive-optics system. To understand this problem better and to improve system performance, we conducted propagation experiments at the Firepond telescope facility on Millstone Hill in Westford, Massachusetts. These tests utilized a 5.4-km horizontal propagation range between Millstone Hill and a fire tower in the town of Groton, Massachusetts. These experiments, which demonstrated for the first time active tracking and adaptive compensation under ABL conditions, suggest that the ABL can meet its mission goals and perform at levels required for effective theater missile defense.

Charles Higgs

Papers

Coherent beam combining of large number of PM fibres in 2-D fibre array

Beam control of a 2D polarization maintaining fiber optic phased array with high-fiber count

Active tracking using multibeam illumination

Adaptive optics compensation using active illumination

Atmospheric Compensation and Tracking Using Active Illumination