Fig. 1. Schematic diagram of BOTDR

Fig. 2. Schematic diagram of BOTDR basic structure. (a) Direct detection; (b) local heterodyne detection; (c) microwave heterodyne detection; (d) self-heterodyne detection^[16]

Fig. 3. Method for improving spatial resolution. (a) Double-pulse BOTDR^[23]; (b) synthetic spectrum BOTDR^[24]; (c) phase shift pulse BOTDR^[25]; (d) differential pulse BOTDR^[27]; (e) iterative subdivision method

Fig. 4. Device improvement of BOTDR system. (a) Optical source; (b) modulator^[39-40]; (c) frequency shifter; (d) depolarizer^[41]; (e) detector

Fig. 5. Schematic diagram of BOTDR with in-line Raman amplification^[52]

Fig. 6. Pulse coding. (a) Linear combination codes; (b) correlation codes

Fig. 7. Schematic diagram of hybrid Raman/Brillouin optical time domain reflectometer^[81]