Fig. 1. Principle of single-pulse OTDR system^[70]. Np is sampling points of single-pulse signal pn; Ns is sampling points of rsmn, rsn, and esn

Fig. 2. Principle of optical pulse coding^[82]

Fig. 3. Coding principle of conventional aperiodic codes and periodic codes

Fig. 4. 7-bit Cyclic code and its coded response^［83］

Fig. 5. Coding gains of GO code and Simplex code (left-hand side vertical axis) and their difference (right-hand side vertical axis) as a function of energy enhancement factor FE for m=3^[70]

Fig. 6. Schematic diagram of colored codes. Colors represent optical pulses of different frequencies, while white color is for absence of intensity pulse^[82]. (a) 3-bit BMCB-based time-frequency code; (b) 7-bit colored Simplex codes

Fig. 7. Schematic diagram of bipolar coding in BOTDA system^[77]. (a) BOTDA principle based on bipolar coding; (b) time domain distribution of bipolar coding

Fig. 8. Optimization block diagram of pulse-coded BOTDA system

Fig. 9. Noise behavior varying with FE^[70]. (a)σs-ASE2+σPD2, σs-SpBS2+σpol2, and their summation at fiber far end as a function of FE; (b) ratio between σtotal2 and σsingle2 as a function of FE

Fig. 10. Pulse coding sequence^[82]. (a) NRZ format; (b) RZ format

Fig. 11. Overall power attenuation trend of 127-bit pulse code sequence after EDFA amplification^[39]

Fig. 12. Power performance of optical pulses of different wavelengths^[41]. (a) Before amplified by EDFA; (b) after amplified by EDFA

Fig. 13. Schematic illustration of signal processing in pulse-coded BOTDA^[87]

Fig. 14. Normalized power of 512-bit Golay coded optical pulse sequence^[87]. (a) At EDFA input; (b) at EDFA output

Fig. 15. Comparison between optimized single-pulse BOTDA and coded BOTDA^[88]. Noise STD of time domain curve obtained by 20 consecutive measurements without averaging for (a) 63 bits Simplex coded system and (b) 67 bits Cyclic coded system. SNR profiles at Brillouin resonance peak for (c) 63 bits Simplex coded system and (d) 67 bits Cyclic coded system