Single-crystal Fibers


Single-crystal Fiber Lasers & Amplifiers

Principal Investigators:


Dr. James A. Harrington, Rutgers University
Dr. Stephen C. Rand, University of Michigan
Dr. Eric G. Johnson, Clemson University

Abstract


The proposed research is designed to model, fabricate, and characterize single-crystal (SC) fibers for both scalable laser power generation (when doped with rare earths) as well as for the delivery of high laser power. SC fibers grown from crystalline oxides, which today make up some of the most powerful bulk laser materials known, have optical and thermal properties that exceed the common glass fiber lasers in use today. The intent of the program is to explore the solution space of SC fibers by exploiting novel growth techniques based on laser heated pedestal growth and to provide experimental evidence as to the superior properties of SC fiber for High Energy Laser applications.

Project Summary


Essentially all fiber lasers in use today are made of glass. These glass structures normally involve a double-clad structure in which the core glass has been doped with a variety of rare-earth ions notably ytterbium-doped fiber lasers (YDFL). Yet there are some limitations to power scaling of glass fiber lasers which result from laser induced damage to the small cores, nonlinear effects, and thermal loading. The goal of the proposed research is to develop a new and novel class of high power fiber lasers based on crystalline materials rather than the conventional glass fiber structure. The basic premise of this work is the rather straightforward idea that crystalline materials such as YAG and other garnets are known to deliver extremely high powers. The technology of Nd:YAG lasers employing conventional rods and disks is well established and reliable. The intention of the proposed work is to draw from the broad knowledge base for solid-state lasers, eg. Nd:YAG, to extend this technology to fabricate single-crystal (SC) fiber lasers. SC fiber lasers would be scalable to much higher power levels compared to their glass counterpart largely because SC fiber lasers have a significantly higher thermal conductivity and also reduced nonlinear effects (i.e., SBS, SRS). The impact of SC fiber laser with the potential to perform in a manner analogous to the proven capabilities for the bulk crystalline oxide lasers would be a significant improvement in the current DoD arsenal of glass fiber lasers.

The approach used to fabricate SC fibers will be Laser Heated Pedestal Growth. In this well-established technique, the tip of a doped-oxide host preform, for example, Nd3+ in YAG, is melted with a CO2 laser and an SC fiber is pulled from the molten oxide. The SC fibers will be clad either by starting with a core/clad preform to produce a graded-index cladding or in a post-cladding method in which a cladding will be added to a small diameter core. The SC fibers will be studied using optical spectroscopy while the structure of the fibers will be investigated using electron microscopy. Modeling of the SC fiber refractive index profiles, fiber structure, and thermal properties as well as lasing parameters will be done to help guide the selection of materials needed to make a true fiber laser. Finally, the lasing properties of the SC fibers will be measured.

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