**3. A brief review of previous works**

*Fiber Optic Sensing - Principle, Measurement and Applications*

the detector sensing the signal output. The type of fiber sensing may be intrinsic sensing or extrinsic sensing. The parameters such as intensity, phase, polarization, wavelength, and other measurands can be detected and sensed when the light source passes through the monitoring zone where these parameters have direct or

*An example of BOTDA setup: DFB-LD, distributed feedback laser diode; SG, signal generator; EDFA, erbiumdoped fiber amplifier; PC, polarization controller; EOM, electrooptical modulator; ISO, optical isolator; AWG, arbitrary waveform generator; PPG, pulse pattern generator; MBC, modulator bias controller; MBC, modulator bias controller; PD, photodetector; RTO, real-time oscilloscopes; OC, optical circulator; FUT, fiber under test.*

indirect effect on the propagating light source.

*A standard fiber-optic sensing system [6].*

**4**

**Figure 4.**

**Figure 3.**

In this section, some previous works of sensing are introduced and addressed. In [7], the authors proposed a FBG liquid level sensor based on the Archimedes' law of buoyancy [8]. They experimentally demonstrated the capability of the proposed device to perform the measurement of water level. It is quite simple to design the device for specific applications without changing the complex cantilever structure. In [9], a D-shaped fiber structure combined with a FBG for refractive index (RI) and temperature sensing is experimentally investigated. The possibility of simultaneous measurement of the RI and temperature relies on monitoring the resonance dip of the D-shaped fiber modal interferometer and the Bragg wavelength of the FBG. An online monitor of moisture concentration in transformer oil that permits the control of moisture buildup is proposed in [10]. The authors presented a methodology for measurement of moisture concentration in transformer oil using a poly(methyl methacrylate) (PMMA)-based optical FBG. In [11], the authors demonstrated a Brillouin optical correlation domain analysis (BOCDA) system with high-speed random access measurement and temporal gating scheme to extend the range of measurement. Dynamic strain applied at two points was selected arbitrarily along the fiber, and it was measured simultaneously [11]. Other Brillouin scattering effect based fiber sensing has also been addressed in [12]. In summary, fiber sensing is more and more important, and quite a few applications could be found in daily lives.
