**Author details**

optic 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

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

and state monitoring of railway or other structures [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

A laser phase and frequency noise measurement technique based on a 120° phase difference unbalanced Michelson interferometer composed of a 3 × 3 optical fiber coupler and two Faraday rotator mirrors is proposed. In the method, the laser differential phase fluctuation

Eq. (16).

246 Optical Interferometry

**5. Conclusion**

Yang Fei1\*, Xu Dan1,2, Cai Haiwen1 and Qu Ronghui1

\*Address all correspondence to: fyang@siom.ac.cn

1 Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, China

2 LNE-SYRTE, Observatoire de Paris, Paris, France
