Advances Toward Affordable High-Energy Laser Modules

There are numerous potential military, industrial, and scientific applications.

A multidisciplinary research project entitled “Affordable High-Energy Lasers” has made numerous contributions to the development of several types of advanced laser modules, including not only lasers but also coupling optics and integral laser/ coupling-optic combinations. There are numerous potential applications for such modules, including weaponry, lidar, high-data-rate optical communications, interferometry, spectroscopy, remote sensing, and processing of materials. The devices developed in this project include novel fiber lasers, novel vertical-external-cavity surface emitting lasers (VECSELs), and a radially emitting photonic-bandgap (PBG) polymer fiber laser. Somewhat more specifically, the contributions are summarized as follows:

• Fiber Lasers
  

  This Phosphate Glass Optical Fiber containing micron-sized longitudinal holes was fabricated by a novel stack-and-draw technique.
  The fiber laser developed in this project include compact (chip-scale) devices based on phosphate glass optical fibers heavily doped with Er and Yb ions. These fiber lasers function as high- power (as much as several watts continuous), low-noise, single-wavelength (1.55-μm), single-mode oscillators and are expected to be useful as building blocks of laser systems that can be scaled up to power levels of multiple kilowatts. The project included integration of fabrication, testing, and optimization of all materials, components, and techniques necessary for manufacturing fiber laser modules. This integration effort yielded advances in highly doped specialty glasses, fiber preforms, fiber-drawing techniques, fiber Bragg gratings, fiber facet coatings, and fusion splicing of fiber components. A novel stack-and-draw technique was used to produce microstructured single- and multiple-core optical fibers (see figure). Other accomplishments involving laser modules made from these fibers include generation of ultra-short (durations of the order of picoseconds) pulses and distortion-free amplification of these pulses to peak power levels of tens of kilowatts.



• VECSELs

  An overriding theme of the project was the development and use of sophisticated theoretical and computationalsimulation capabilities to drive the design and testing of the various novel devices. The effect of this theme was greatest in the VECSEL subproject, in which a first-principles quantum-based approach was followed in designing the successions of epitaxial semiconductor layers to obtain quantum-well structures needed for lasing, effectively accelerating the optimization of VECSEL suboptical-cavity structures by obviating what would otherwise be a costly, time-consuming iterative process of fabrication, testing, and design modification. The VECSEL subproject yielded high-power, high-brightness, tunable VECSELs that, like the aforementioned fiber lasers, can be utilized as building blocks of scalable multi-kilowatt laser systems. Like the fiber-laser subproject, the VECSEL subproject included integration of fabrication, testing, and optimization of all components. Special attention was given to designing semiconductor quantum wells to afford gain, designing distributed Bragg reflectors, designing microcavity resonators, designing external optical cavities, fabricating VECSEL chips, low-reflection coating of the chips, and dissipation of heat from the chips.

• Radially Emitting PBG Polymer Fiber Laser

  This fiber laser consists of a core containing a gain medium (an organic dye) surrounded by a concentric multilayer PBG resonator structure, that, in turn, is surrounded by a cladding layer. All fiber lasers developed prior to the conception of this fiber laser emit radiation only along their fiber axes. In this fiber laser, interactions of a linearly polarized axial pump beam with the PBG resonator and the gain medium cause radiation to be emitted radially outward and to be azimuthally isotropic. Low-threshold lasing was demonstrated at nine different wavelengths in the visible and near-infrared spectrum. Concomitantly, dispersion characteristics of two-dimensional dielectric PBG structures were calculated.

This work was done by Jerome V. Moloney, Axel Schülzgen, Nasser Peyghambarian, Pavel Polynkin, Masud Mansuripur, and Mahmoud Fallahi of the University of Arizona, and Yoel Fink and Chiping Chen of Massachusetts Institute of Technology for the Air Force Research Laboratory.
AFRL-0064

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