Impact of angular pump power distribution on double-clad fiber temperature and incidence on high-power fiber laser reliability

Double-clad fibers (DCF) found in kilowatt-class fiber lasers typically have a second cladding made of fluoroacrylate. At high power, thermal damage or accelerated aging of this material becomes an issue. The operating temperature of the fluoroacrylate coating is found to be highly sensitive to the numerical aperture (NA) distribution of the pump light flowing through the fiber. Characterization of this effect with an optical loss measurement is impractical as this loss remains typically very low. Measurement of the coating temperature for a given input power and far-field distribution is much more sensitive. Furthermore, it directly gives the parameters that are key to the design of a high-power fiber laser. A system for the measurement of the thermal slope of DCF fibers and high-power fiber components has been built and tested. This system allows varying the input power and the source NA under high power with a unique splice to the device being tested. To achieve this, different types of fiber-coupled pump diodes are spliced to the inputs of a pump combiner. Fiber tapers are used to fine tune the sources’ NA. By turning on different diodes, the NA of the injected pump light can be varied. The thermal slope for a given NA can then be measured with a thermal camera and a power meter. Measurements show differing thermal slopes of DCF measured before and after a damp heat tests. These thermal slope variations are stronger when operating at a high numerical aperture.

E. Brown-Dussault, S. Boudreau, P. M. Belzile, D. Faucher, and M. Faucher "Impact of angular pump power distribution on double-clad fiber temperature and incidence on high-power fiber laser reliability", Proc. SPIE 10899, Components and Packaging for Laser Systems V, 108990Q (4 March 2019); https://doi.org/10.1117/12.2510374