Fig. 1. Principle and results of PD testing method based on distributed optical fiber sensing technique[31]. (a) Principle diagram of pulsed fiber amplifier; (b) cross section of three-fiber GTWave; (c) output power versus time; (d) PD-induced OTDR loss measurement for counter-pumped fiber amplifier with pulsed operation; (e) time-averaged Yb3+ population inversion

Fig. 2. Experimental setup and measurement results of indirect PD diagnosis induced by fiber core pump[23]. (a) Measurement methods forPD characterization; (b) PD-induced loss spectrum; (c) relationship between excess losses at 633 nm and at signal wavelength of 1040-1070 nm

Fig. 3. Experimental setup and measurement results of indirect PD diagnosis induced by cladding pump[27]. (a) Test facility of photodarkening by cladding pump; (b) PD and PB of fiber sample under same pump power after darkening with high pump power; (c) transmission measured at fiber sample (L=5 cm) with different initial pump powers; (d)(e) core excess loss of fiber sample (L=1 cm) versus time

Fig. 4. Experimental setup and measurement results of PB under 793 nm pump[20]. (a) Schematic of experimental setup; (b) PD at 915 nm pump and PB at 793 nm pump versus time; (c) PD and PB under different 793 nm pump powers versus time; (d) bleaching loss versus 793 nm LD pump power

Fig. 5. Steady PD loss versus mass fraction of Yb3+43 (solid black line indicating square fitting with exponent of 2.08)

Fig. 6. Effect of Ce-doping on PD and PB processes of Yb/Al co-doped fibers[15]. (a) PD-induced excess loss measured at 702 nm versus time; (b) PB at 702 nm under 793 nm LD pump versus time; (c) PD-induced excess loss value after 16 hours of pumping versus time

Fig. 7. Effect of Yb3+ inversion on PD performances of Yb-doped fiber[44]. (a) PD loss curves measured at fiber samples with different Yb3+ population inversions; (b) self-similarity of PD loss curves; (c) PD rate versus Yb3+ population inversion; (d) steady PD loss versus Yb3+ population inversion

Fig. 8. Effect of temperature on PD performances of Yb-doped fiber[55]. (a) PD losses under different temperatures during pump versus time; (b) maximum PD loss and PD rate versus temperature