Single-Crystal Fiber Optics

CO2 Laser synthesis in apparatus in MSE laboratories grows Nd:YAG crystals at high temperatures in localized positions to produce the precursors for specialty laser fibers.

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. 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 current 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 arsenal of glass fiber lasers.

The approach used to fabricate SC fibers is 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. SC fibers are as yet unclad but the intention is to clad the SC fibers 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. SC fibers are studied using optical spectroscopy, electron microscopy, and fluorescence spectroscopy.

Rutgers Materials Scientist and engineers collaborate with colleagues at the University of Michigan and Clemson University. The Rutgers effort involves growth of the SC fibers from rare-earth doped YAG crystals and their optical and mechanical characterization. There are perhaps only about five LHPG apparatus in the world today and Rutgers is one of the leading sources of this technology. SC fibers may be used for passive applications such as laser power delivery and fiber sensors while the active, rare-earth doped fibers are relegated to fiber laser and amplifier uses.

The lowest losses achieved to date are for the SC sapphire fibers with losses of about 0.3 dB/m or about 5%/m. Most activity today is devoted to making a good clad active SC fiber. Harrington believes that if a low-loss, rare-earth-doped YAG fiber can be fabricated then a SC fiber laser could delivery over 6 kW of laser power per fiber compared to the approximately 1.5 kW per fiber now available from glass fiber lasers.