Abstract
Background absorption and the quantum defect loss result in heat load and thereby in an increase of the fiber and the coating temperature in an optical fiber amplifier [1]. As a consequence, high power continuous wave double clad fiber amplifiers often are limited by thermal degradation of polymer coatings and splices. Therefore, thermal management and temperature control plays an important role in the power scaling of active ion doped fiber lasers [2]. In terms of the beam quality, the output power of fiber amplifiers is furthermore limited by mode instabilities. As a thermo-optical effect, the dependence of the threshold on the thermal heat load is significant and therefore essential to develop [3]. In this contribution we report on a setup for in situ temperature measurement in high power amplifier fibers. The used measurement method is based on an Optical Frequency Domain Reflectometer (OFDR), whereby the spectral shift of the OFDR measured Rayleigh scatter is scaled to a temperature shift of the fiber core and enables to compute the longitudinal temperature distribution of the inner fiber [4]. This method allows extracting much more comprehensive information than a single spot measurement that is provided by a fiber bragg grating (FBG) based temperature measurement [5]. The measurement results allow comparing and evaluating different cooling concepts and cooling mounts. Furthermore, since heat storage correlates with scattering inhomogeneity and defects in the fiber core, the temperature information allows also evaluating the fiber behaviour in terms of high power applications.
© 2015 IEEE
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