**2.2 Types of fiber-optic sensing**

Fiber-optic sensors can be split into two big categories: point-by-point sensors and distributed sensors. On the one hand, the point-to-point sensors are usually

#### **Figure 1.**

*(a) The intrinsic sensing, the optical fiber plays as both the sensing element and the transmission media; and (b) the extrinsic sensing, the optical fiber just plays as the transmission media.*

**3**

**Figure 2.**

*analyzer; FBG, fiber Bragg grating.*

*Introductory Chapter: An Overview the Methodologies and Applications of Fiber Optic Sensing*

based on FBG. They can measure parameters at a particular location where there is a FBG with high resolution and sensitivity. A standard FBG-based sensing system is shown in **Figure 2** with most required components. Using an optical switch (OSW), a broadband light source may transmit to bridge 1, bridge 2, or bridge 3, respectively, for strain, temperature, and/or stress sensing. Several FBGs are used to monitor multiple parameters/points at the same time. The optical switch is used to share the cost. FBG here is not only the sensing element but also the cavity end of fiber laser. The reflected signals are detected by an optical spectrum analyzer

On the other hand, the detectable range of the distributed sensors is based on the Brillouin scattering effect with moderate resolution and limited distances. Nevertheless, a sensor head like a FBG is not required, so distributed sensors are more cost-effective than numerous FBG sensors in long-range sensing distance. A standard Brillouin optical time-domain analysis (BOTDA) sensing system is shown in **Figure 3** with most required components. Firstly, a highly coherent DFB laser source is split into pump source and probe source by using a 50/50 fiber coupler. An erbium-doped fiber amplifier (EDFA1) is used to boost the laser power. A pulse pattern generator is used to drive the electrooptic modulator (EOM1). Then a microwave sine wave around the optical fiber Brillouin frequency of ∼11 GHz) is fed into the probe source. The signals are scrambled by polarization controllers before they arrive at the Mach-Zehnder modulators (EOMs). The pumped light at the lefthand side is further amplified by an EDFA2 and launched into the fiber under test (FUT) region. The reflected pumped light and probe light comes from other side travel through the optical circulator (OC) and then to EDFA3. A tunable filter or its equivalent is used to filter out pump backscattering and the upper sideband signal. Then the residual signal is monitored and analyzed by a real-time oscilloscope. In general, point-by-point sensing is practical for short distance and remote monitoring up to 100 km. Distributed sensing based on the Brillouin scattering

**Figure 4** represents a standard fiber-optic sensing system [6]. There is a light source (laser or LED) launching into the optical fiber, and at the right-hand side is

*An example of FBG-based sensing system: OC, optical circulator; OSW, optical switch; OSA, optical spectrum* 

(OSA). Then the data may be in time analyzed by a data logger.

effect is used to detect strain and temperature for up to 10 km.

**2.3 Fiber-optic sensing system**

*DOI: http://dx.doi.org/10.5772/intechopen.86525*

### *Introductory Chapter: An Overview the Methodologies and Applications of Fiber Optic Sensing DOI: http://dx.doi.org/10.5772/intechopen.86525*

based on FBG. They can measure parameters at a particular location where there is a FBG with high resolution and sensitivity. A standard FBG-based sensing system is shown in **Figure 2** with most required components. Using an optical switch (OSW), a broadband light source may transmit to bridge 1, bridge 2, or bridge 3, respectively, for strain, temperature, and/or stress sensing. Several FBGs are used to monitor multiple parameters/points at the same time. The optical switch is used to share the cost. FBG here is not only the sensing element but also the cavity end of fiber laser. The reflected signals are detected by an optical spectrum analyzer (OSA). Then the data may be in time analyzed by a data logger.

On the other hand, the detectable range of the distributed sensors is based on the Brillouin scattering effect with moderate resolution and limited distances. Nevertheless, a sensor head like a FBG is not required, so distributed sensors are more cost-effective than numerous FBG sensors in long-range sensing distance. A standard Brillouin optical time-domain analysis (BOTDA) sensing system is shown in **Figure 3** with most required components. Firstly, a highly coherent DFB laser source is split into pump source and probe source by using a 50/50 fiber coupler. An erbium-doped fiber amplifier (EDFA1) is used to boost the laser power. A pulse pattern generator is used to drive the electrooptic modulator (EOM1). Then a microwave sine wave around the optical fiber Brillouin frequency of ∼11 GHz) is fed into the probe source. The signals are scrambled by polarization controllers before they arrive at the Mach-Zehnder modulators (EOMs). The pumped light at the lefthand side is further amplified by an EDFA2 and launched into the fiber under test (FUT) region. The reflected pumped light and probe light comes from other side travel through the optical circulator (OC) and then to EDFA3. A tunable filter or its equivalent is used to filter out pump backscattering and the upper sideband signal. Then the residual signal is monitored and analyzed by a real-time oscilloscope.

In general, point-by-point sensing is practical for short distance and remote monitoring up to 100 km. Distributed sensing based on the Brillouin scattering effect is used to detect strain and temperature for up to 10 km.

#### **2.3 Fiber-optic sensing system**

**Figure 4** represents a standard fiber-optic sensing system [6]. There is a light source (laser or LED) launching into the optical fiber, and at the right-hand side is

#### **Figure 2.**

*An example of FBG-based sensing system: OC, optical circulator; OSW, optical switch; OSA, optical spectrum analyzer; FBG, fiber Bragg grating.*

*Fiber Optic Sensing - Principle, Measurement and Applications*

ity, acceleration, torque, and temperature [5].

**2.2 Types of fiber-optic sensing**

the extrinsic sensing is that the optical fiber just plays as the transmission media, as is shown in **Figure 1(b)**. Both of them are important and are frequently used in temperature sensing, strain sensing, or pressure sensing, depending on which parameters we want to measure. Specifically, intrinsic fiber-optic sensors provide distributed sensing over a long-distance zone [4], and extrinsic sensors can help us reach inaccessible places, for example, the measurement of temperature inside aircraft jet engines and the measurement of the high temperature inside the electrical transformer. Extrinsic fiber-optic sensors provide excellent protection of measurement signals against noise corruption, and they can be used to measure vibration, rotation, displacement, veloc-

Fiber-optic sensors can be split into two big categories: point-by-point sensors and distributed sensors. On the one hand, the point-to-point sensors are usually

*(a) The intrinsic sensing, the optical fiber plays as both the sensing element and the transmission media; and* 

*(b) the extrinsic sensing, the optical fiber just plays as the transmission media.*

**2**

**Figure 1.**

#### **Figure 3.**

*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.*

**Figure 4.** *A standard fiber-optic sensing system [6].*

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 indirect effect on the propagating light source.

**5**

*Introductory Chapter: An Overview the Methodologies and Applications of Fiber Optic Sensing*

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

*DOI: http://dx.doi.org/10.5772/intechopen.86525*

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

found in daily lives.

**Author details**

Shien-Kuei Liaw

National Taiwan University of Science and Technology, Taiwan (R.O.C.)

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: peterskliaw@gmail.com

provided the original work is properly cited.

*Introductory Chapter: An Overview the Methodologies and Applications of Fiber Optic Sensing DOI: http://dx.doi.org/10.5772/intechopen.86525*
