Fig. 1. Loss (red line) and emission (blue line) spectra of the pristine FUT; inset, cross section photo of the FUT.

Fig. 2. Experimental setup of (a) loss and (b) emission measurements; (c) experimental process of repeated pumping.

Fig. 3. (a) Emission spectra of the FUT under 830 nm excitation at 0, 5, 10, 20, 30, and 60 min during the first pumping process; (b) normalized luminescence intensities of BAC-Si (blue points) and BAC-Al (green points), and SEF fitting curve (red solid lines) with pumping time.

Fig. 4. (a) The change of insertion loss of the FUT during the recovery relaxation after the pump is off for five experiments; (b) enlarged parts of the band from 780 to 850 nm. irrad., irradiation.

Fig. 5. (a) Photobleaching process and recovery relaxation using an electron transferring theory and distribution function concept of activation energy; (b) irreversible and reversible PB percentages after 73 h of recovery relaxation under repeated pumping.