Measuring the spatial distribution of rare-earth dopants in high-power optical fibers

For the first time, a non-destructive technique for spatially resolving the location and relative concentration of rare-earth dopants in an optical fiber is demonstrated. This novel technique is based on computerized tomographic detection of spontaneous emission and achieves micron-scale spatial resolution with the aid of oil-immersion imaging. In addition to elucidating interactions between the signal, pump, and dopant distributions, the measurement described here can reveal shortcomings in fiber manufacturing. Since the technique is non-destructive and can be scanned along the fiber length, it can map the full 3-dimensional distribution of complex rare-earth-doped fiber structures including gratings, physical tapers, fusion splices, and even couplers. Experimental data obtained from commercially available Yb-doped silica optical fibers is presented, contrasted, and compared to refractive index profile data. In principle the technique can also be applied to Er-, Bi-, or Tm-doped silica or non-silica optical fibers.