Fig. 1. Schematic diagram of the pulse transmission and characteristics of the superimposed Raman signals.

Fig. 2. Numerical simulation model.

Fig. 3. Superimposed Raman OTDR traces under different pulse widths. (a) Whole superimposed Raman OTDR traces. Superimposed Raman OTDR traces based on (b) 10 ns pulse width, (c) 20 ns pulse width, and (d) 30 ns pulse width.

Fig. 4. Schematic diagram based on the slope-assisted demodulation scheme (LFUT is the true length of the FUT). (a) Superimposed Raman OTDR traces at different FUT temperatures. (b) Schematic diagram of pulse transmission in the slope-assisted area when pulse width is greater than the length of the FUT.

Fig. 5. Experimental setup based on slope-assisted demodulation sensing.

Fig. 6. Distribution of superimposed Raman OTDR traces in the FUT region under the pulse width of (a1) 30.30 ns, (a2) 45.45 ns, and (a3) 53.03 ns. Distribution of superimposed Raman OTDR traces in the slope-assisted region under the pulse width of (b1) 30.30 ns, (b2) 45.45 ns, and (b3) 53.03 ns.

Fig. 7. Relationship between the pulse width and FUT length as measured by Raman signal.

Fig. 8. Relationship between the slope-assisted coefficients and the FUT temperature under the pulse width of (a) 30.30 ns, (b) 45.45 ns, (c) 53.03 ns, and (d) 55.56 ns.

Fig. 9. Distribution of superimposed Raman OTDR traces in the FUT region under the FUT with length of (a1) 6.0 m, (b1) 4.8 m, and (c1) 5.6 m. Distribution of superimposed Raman OTDR traces in the slope-assisted region under the FUT with length of (a2) 6.0 m, (b2) 4.8 m, and (c2) 5.6 m.

Fig. 10. Relationship between the slope-assisted coefficients and the FUT temperature under FUT lengths of (a) 2.0 m, (b) 3.0 m, and (c) 4.0 m. (d) Comparative results based on the different FUT lengths.

Fig. 11. Temperature measurement results at a centimeter-level spatial scale detected using a conventional demodulation method and the slope-assisted demodulation method. (a) Results measured using the conventional demodulation method. (b) Distribution of superimposed Raman OTDR trace after attenuation compensation. (c) Results measured using the slope-assisted coefficients.