Fictive-Temperature Mapping in Highly Ge-Doped Multimode Optical Fibers

In this paper, a Fourier transform infrared focal-plane-array detector was used to image the "bond-stretching" vibration mode observed near sigma=1120 cm^-1 of highly Ge-doped graded-index multimode optical fibers (GI-MMFs). We show that the distribution of the peak wavenumber sigma is nonuniform across the core and cladding of the MMF, i.e., sigma is smaller in the core due to the Ge-doping. Next, as calibration curves between sigma and the fictive temperature T_f are not available in the literature for highly Ge-doped glasses (above 7w%), we have determined calibration curves from 1w% to 30w% in Ge. Then, we have applied these corrections in order to estimate, for the first time to our knowledge, the fictive-temperature distribution within multimode-fiber cross section. Using these curves, we show that T_f is higher at the fiber edges, presumably due to faster cooling. Furthermore, there is also a T_f increase in the center of the core (i.e., higher Ge content)