On the Use of Brillouin Scattering to Evaluate Quantum Conversion Efficiency in Yb-doped Optical Fibers

A method, based on Brillouin scattering, to evaluate the quantum conversion efficiency (QCE) in Yb-doped optical fibers is designed and tested. An adapted version of the heterodyne approach to measuring the Brillouin spectrum is first used to precisely quantify the temperature change of the fiber upon pumping. Next, a theoretical model based on the thermal conduction equation is constructed in order to simulate the temperature change assuming a QCE of one. Lastly, the QCE value of the fiber can be determined by comparing the experimental and simulation results. Importantly, the analysis can provide insight into the origin of thermal ene9rgy in a fiber. Serving in the proof-of-concept measurement, a commercial, lightly Yb-doped fiber is determined to have a QCE of 99.07%. Finally, this result is compared with slope efficiency measurements from a linear cavity laser using the same fiber as the active medium. Excellent agreement between the two measurements demonstrates the feasibility of the approach, with greater accuracy afforded by the Brillouin-based measurement. The proposed method is particularly useful for optical fiber optimization in high-power fiber laser and amplifier applications. It can also be valuable as a sensitive temperature probe in the study of anti-Stokes fluorescence cooling in optical fibers.