Fig. 1. (a) Schematic diagram of bidirectional EDFA relay module. (b) Schematic diagram of bidirectional EDFA relay-based DAS system.

Fig. 2. Simulation results. (a) Signal and light leakage power in receiving end with different ER. (b) Power ratio of signal from span end and ASE noise of EDFA with different span length. (c) Maximum span length and maximum sensing distance with different span numbers.

Fig. 3. Configuration of multispan relay amplification based DAS system.

Fig. 4. Temporal and spatial distribution patterns of the phase noise. (a) Cascaded AOMs DAS; (b) single AOM DAS. The probability distribution of the phase noise. (c) Cascaded AOMs DAS; (d) single AOM DAS. The RMS means root mean square.

Fig. 5. Experiment results. (a) Backscattering signal of a probe pulse. Time domain signals at (b) 100.3 km, (c) 200.5 km, and (d) 300.2 km. (e) PSD signal at each span end.

Fig. 6. Signals at 300.2 km. (a) The amplitudes of sinusoidal signals with different drive voltages. (b) Frequency domain of sinusoidal signals with different frequencies.

Fig. 7. Noise RMS possibility distribution at the first span end with span numbers from 1 to 6.

Fig. 8. (a) ER measurement system. Optical spectra of (b) AOM1 and (c) AOM2 when AOMs are set as pulse mode and off mode.