Intelligent Fiber-Array Laser Beam Transmitters

In response to increasing demand for laser transmitter systems (laser beam-forming telescopes or beam directors) for various applications, II-VI's team developed the Intelligent Fiber Array (INFA) product line – first commercially available fiber-array-based adaptive laser beam transmitter systems. The INFA laser beam transmitter is composed of an array of densely-packed fiber collimators with capabilities for coherent and incoherent beam combining. The fiber arrays operate in CW or pulse regime with capability for transmitting up to kW optical power through a single fiber collimator (subaperture). Among key advantages of these systems are compactness, lightweight and scalability in respect to the output power and the transmitter aperture size. The INFA systems have build-in capabilities for electronic precision pointing and re-focusing of the outgoing combined beam, compensation of mechanical and acoustic jitter, and adaptive mitigation of atmospheric-turbulence-induced and/or thermal-convection-induced phase aberrations. The INFA systems have numerous civilian and defense applications and are considered as an attractive alternative to the existing laser beam transmitter telescope systems based on monolithic-aperture optics.

II-VI’s INFA coherent laser beam transmitter systems can be composed of different number of fiber collimators with built-in capabilities for adaptive wavefront phase piston and tip and tilt control at each fiber-collimator subaperture. In these systems, multi-channel fiber-integrated phase shifters are used for the target-plane phase locking (coherent combining) of the transmitted laser beams as well as for the pre-compensation of atmospheric-turbulence-induced piston-type phase aberration components. In addition, fast, controllable x and y displacements of the fiber tips in the fiber-collimator array provide unique capabilities for adaptive compensation of tip and tilt phase aberration components at each subaperture. Custom configurations of the INFA systems can also include an additional control system for equalization or control of the power transmitted by each fiber collimator. All three control systems are based on the advanced stochastic parallel gradient descent (SPGD+) optimization technique.

Among the key advantages of the INFA are:

• Compact, lightweight and robust.
• Scalable design in both output power and the transmitter aperture size enabled by modular construction.
• Built-in capabilities for electronic precision pointing and re-focusing of the outgoing combined beam.
• Compensation of mechanical and acoustic jitter.
• Adaptive mitigation of atmospheric-turbulence-induced and/or thermal-convection-induced phase aberrations.

The INFA system provides capabilities for precision pointing of the outgoing combined beam, for compensation of mechanical and acoustic jitter, and for adaptive mitigation of turbulence-induced phase aberrations.

Multiple applications

II-VI A&D’s fiber-array laser beam transmitters vs. conventional beam director systems

1. II-VI's fiber-array laser beam directors of various aperture sizes are assembled using identical building blocks – INFOCO transmitters. This eliminates the need for designing and manufacturing a new beam-forming telescope for every application. A new laser beam transmitter can be assembled simply by reusing the existing fiber-collimator components.
2. Being composed of relatively small-sized fiber collimators, a large-aperture INFA laser beam transmitter can be significantly lighter than the equivalent conventional system based on the beam-forming telescope with monolithic optical elements. Consequently, the INFA systems can be integrated on a lighter and more precise gimbals for beam pointing and tracking.
3. In conventional beam-forming telescopes, outgoing beam focusing (projection) is performed using mechanical displacement of an optical element (commonly the telescope’s secondary mirror) along the optical axis. This results in relatively slow (from hundreds of milliseconds up to several seconds, depending on the telescope size) transmitter system refocusing. In the fiber-array systems, outgoing beam refocusing is performed by tilting wavefront phase of the outgoing beams (beamlets) at the fiber-collimator system pupil. This allows on the orders of magnitude faster re-focusing of the outgoing beam.
4. Fast (> 5 kHz) control of wavefront phase tip and tilts allows efficient suppression of the outgoing beam jitter resulting from mechanical vibrations – a well-recognized shortcoming of conventional beam directors.
5. II-VI's fiber-array systems are robust to element failure which is especially important for system operation in hazardous environments.
6. With II-VI’s fiber-array laser transmitter systems, atmospheric-turbulence-induced phase distortions can be efficiently pre-compensated using adaptive optics (AO) control of piston and tip and tilt phase components at each fiber collimator. In the INFA systems, the piston phase control is based on super-fast fiber-integrated phase shifters. This allows achieving > 15 kHz closed-loop bandwidth – the performance that cannot be achieved in the laser beam director systems based on monolithic optics and conventional adaptive optics hardware (deformable mirrors).

To learn more, see special issue on Laser beam combining and fiber laser systems,IEEE J. Sel. Top. Quantum Electron., vol. 15, no. 2, pp. 237–470, 2009.