A non-destructive method for measuring the absorption coefficient of a doped optical fiber

To study the cooling efficiency and radiation-balanced condition in optical fiber lasers, it is essential to measure the absorption coefficient over the appropriate spectral range accurately. The most common technique to measure the absorption coefficient in rare-earth doped optical fibers is the cut-back method. Unfortunately, the cut-back method is destructive and requires mechanical movement and optical realignment, which are troublesome for fragile fibers like ZBLAN fibers or tapered optical fibers. Moreover, the presence of the cladding modes is a source of inaccuracy in the final result, and it is challenging to properly remove the cladding modes in highly doped fibers to achieve high accuracy in the measurement of the absorption coefficient. We introduce a non-destructive method based on analyzing the side-light along the doped fiber. The method is presented and explained in detail, and it is successfully used for measuring the resonance absorption of the Yb-doped silica and ZBLAN fiber in single-mode and multi-mode rare-earth doped fibers.