**4. The application in phase-sensitive OTDR**

The experimental setup is shown in **Figure 10** [25]. The light source is a DFB laser with an output power of 20 mW and a wavelength of 1551.72 nm, which is injected into the acousticoptic modulator (AOM) to generate the pulses with a width of 40 ns and a repetition rate of 100 kHz. Before being injected into the fibre under test (FUT), the pulses would be amplified by the Erbium-doped fiber amplifier 1 (EDFA1). The backward Rayleigh scattering is amplified by the EDFA2 and then launched into the circulator2, and ASE noise of EDFA2 has been filtered by the fibre Bragg grating (FBG). Then the amplified scattering with better signal-to-noise ratio (SNR) is injected into one port of the 3 × 3 coupler through circulator3. There are two ports on the other side of the 3 × 3 coupler connected to FRMs and the other one port made reflectionfree. The interferometer with 4 m delay is housed in a sealed box for thermal and acoustic isolation, avoiding disturbance from the environment. The fiber length between PDs and each coupler port is set as equal to guarantee the same optical path. A trichannel digital acquisition (DAQ) card is used to acquire the voltage signal, and a radio frequency driver (RFD) is used to trigger the AOM and DAQ card simultaneously for synchronization. The collected trichannel signals are processed by the software program to demodulate phase information by using Eq. (16).

**Figure 10.** Experimental setup used to demodulate the distributed phase in ϕ-OTDR [25].

Phase sensitive optical time domain reflectometer (*ϕ*-OTDR) based on 120° phase difference Michelson interferometer is a new demodulation scheme used to demodulate the distributed phase. The 120° phase difference interferometer *ϕ*-OTDR can detect the phase along a 3 km fiber and the acoustic signal within the whole human hearing range from 20 Hz to 20 kHz is reproduced accurately and fast. The results show that the scheme has high accuracy and realtime response. The acoustic vibration system can be used in audio monitoring, and the health and state monitoring of railway or other structures [25].
