Heat and temperature distribution in a cladding-pumped, Er: Yb co-doped phosphate fiber

High-gain-per-unit-length, Er: Yb co-doped, phosphate glass fibers are a new class of active photonic materials. Due to their high concentration of active ions (typically 2-6 × 10 ²⁰ cm ^-3), the generation of heat in these materials is rather severe. To facilitate the design of cladding-pumped, high-power lasers and amplifiers using these materials, we introduce two diagnostic techniques for measuring the total heat and the profile of temperature distribution along the length of an active fiber. Thermal experiments on a 6.0-cm-long piece of cladding-pumped phosphate fiber with Er: Yb doping (3:16 wt %) are conducted, and the results are compared with indirect estimates of total heat by scattered light measurements using a power-balance argument. The difference between the two methods is about 8.0%. Even at low pump powers, the temperature of the core (without heat-sinking) is found to be a large fraction of the glass transition temperature. The temperature distribution along the length of the fiber is found to be relatively flat compared with the absorption profile. Our thermal diagnostic tools yield valuable information that can be used to optimize the design of fiber lasers and amplifiers.