Fig. 1. Spectral figure of typical spontaneous scattering

Fig. 2. Working modes of two kinds of APD^［15］

Fig. 3. Histogram of photon counting and time

Fig. 4. Distributed temperature sensing system based on detection of PMT^［29］.

Fig. 5. Set-up of the single photon detection system based on PPLN waveguide^［33］

Fig. 6. Setup of photon counting OTDR with sensing fiber of 217 km^［34］

Fig. 7. Temperature measurement of distributed sensing fiber with 12 mm spatial resolution^［35］

Fig. 8. （a） System noise equivalent （NEP） as a function of bias current in units of critical current （b） Backscattered signal from the FUT at around 111 km as detected with the single photon detector^［12］

Fig. 9. Comparison between level trigger （left） and CFD operation （right） ^［40］

Fig. 10. Operation principle of a TAC^［40］

Fig. 11. （a）Schematic diagram of externally time-gated PC-OTDR（b） Gating signals provided to optical switch（c） Simulated OTDR traces of 60 m fiber link^［46］

Fig. 12. （a） Optoelectronic circuit of the photon-counting OTDR （b） PON configuration for photon counting OTDR testing （c） Zoom OTDR trace to the end of the PON link^［56］