Preface

In recent years, much effort has been devoted to the study, development, and application of point-to-point fiber sensing for various parameter sensing. Fiber Bragg gratings (FBGs) are key components in the endeavor, usually fabricated using UV laser sources and a phase mask or interferometric techniques. An FBG can be used as a band reject filter; to detect strain, pressure, and temperature; and in telecommunication systems for wavelength selection, among other uses. On the other hand, distributed fiber sensing can monitor the environment along the fiber change based on the Brillouin scattering effect. Distributed Brillouin sensing technique has developed rapidly over the last thirty years. Quite a few investigations on the performance enhancement of Brillouin sensors have been conducted on sensing distance and spatial resolution, paving the way to industrial and commercial applications.

This book presents recent advances in fiber sensing technologies, both in theoretical and real applications, reflecting the cutting-edge technologies and research achievements within these research fields. After a rigorous review process, the editors selected five outstanding chapters from among the submissions for inclusion in this contributor volume. Of these, four are focused on the subject of point-to-point fiber sensing, and the fifth covers distributed fiber sensing. The authors work in academia and industry in Austria, United States, Korea, and Taiwan.

The book consists of the following chapters:

In Chapter 1, "Introductory Chapter: An Overview of the Methodologies and Applications of Fiber Optic Sensing," the editors briefly address the importance of fiber optic sensing, which may be applied in various fields where optical fiber is used either as a transmission medium or as a sensing head. Point-to-point fiber sensing using fiber Bragg gratings (FBGs) and distributed fiber sensing based on Brillouin scattering effect will be introduced. Some prior works based on either of these fiber sensing methodologies are introduced.

In Chapter 2, Fathy Mohamed Mustafa and Mofreh Toba introduce the "Theoretic Study of Cascaded Fiber Bragg Grating." They simulate and analyze the spectral characteristics of the fiber Bragg grating to obtain narrow bandwidth and minimization side lobes in reflectivity. Model equations of cascaded uniform fiber Bragg grating and different cascaded apodization functions are numerically handled and processed via specially cast software to achieve maximum reflectivity, narrow bandwidth without side lobes. For better performance, the proper values for grating length and refractive index modulation must be chosen to achieve maximum reflectivity and narrow bandwidth.

In Chapter 3, "Femtosecond Transient Bragg Gratings" are investigated by Avishay Shamir et al. The authors briefly review the advantages of femtosecond fabrication of fiber Bragg gratings. Then they focus on transient FBGs for optical switching. An experimental result is achieved on generation and characterization of the transient FBGs. A possible mechanism to realize high-power femtosecond laser is introduced. The immunization technique presented here can be used to implement transient

thermal gratings in transparent materials and may serve as a diagnostic tool for dielectric materials with different compositions and doping.

In Chapter 4, the "Vital Sign Measurement Using FBG Sensor for New Wearable Sensor," by Shouhei Koyama et al., the authors measured the vital signs from a living body by installing the FBG sensor at a pulsation point. The method of calculating each vital sign from the FBG sensor signal was described. The FBG sensor signal was found to correspond to changes in diameter of the artery caused by the pressure of the blood flow. Vital signs such as pulse rate, respiratory rate, stress load, and blood pressure could be calculated by the FBG sensor head. All the vital signs were calculated with high accuracy. The study helps establish that these vital signs can be calculated continuously and simultaneously.

In Chapter 5, Cheng Feng et al. study "The State-of-the-Art of Brillouin Distributed Fiber Sensing." This chapter provides an overview of different Brillouin sensing techniques and mainly focuses on the most widely used one, the Brillouin optical time domain analysis (BOTDA). The history and development of Brillouin sensing regarding the performance enhancement in various methods and their records will be reviewed, commented on, and compared with one other. In addition, related sensing errors and limitations will be discussed together with corresponding strategies to avoid them.

As editor, I would like to take the opportunity to express my sincere gratitude to all the authors and coauthors who contributed manuscripts to this edited book. We also thank Ms. Jane Liao and Ms. Minglun Tsai for their kind help in typo and format checking.

> **Dr. Shien-Kuei Liaw** National Taiwan University of Science and Technology, Taipei City, Taiwan

> > **1**

**2. Types of fiber-optic sensing**

**2.1 Intrinsic sensing and extrinsic sensing**

**Chapter 1**

Sensing

**1. Introduction**

*Shien-Kuei Liaw*

Introductory Chapter: An

Overview the Methodologies

and Applications of Fiber Optic

Fiber sensors have several advantages compared to some conventional sensors. They are lightweight and have a small size, high resolution, and good stability; fiber sensors not only are insensitive to electromagnetic interference but also can withstand high temperature and radiation. A variety of linear and nonlinear optical transduction mechanisms have been studied in the last 30–40 years, dealing with the conversion from all kinds of measurands to local measurable optical effects in the fiber. There are previous works, for instance, that designed temperaturecompensated fiber Bragg grating (FBG) sensor for monitoring the stress [1], FBG-integrated spherical-shape structure for refractive index sensing [2], and D-shaped fiber combined with a FBG for refractive index and temperature sensing [3]. Fiber sensor can measure and/or monitor many parameters such as strain, weight, temperature, speed, pressure, and so on. Moreover, fiber sensor can also measure the variation of light intensity, wavelength, frequency, phase, and polarization by combining other detectors with optical fiber. Firstly, optical fiber sensors for temperature and pressure have been developed for measurement in oil wells. For example, a precise and real-time ammonia monitoring technique is important especially for gas sensing [3]. Once the gas leakage happens, an immediate alarm is helpful to prevent danger. Secondly, fiber sensing is also used to make a hydrogen sensor. Temperature can be measured by using a fiber that has evanescent loss with various temperature ranges or by analyzing the Brillouin scattering in the optical fiber. Thirdly, angle measurement sensors can be designed based on the Sagnac effect. In recent years, various sensing materials are available for biosensor fabrication, so various fiber-optic biosensors have been proposed and demonstrated. Finally, optical fiber sensors have been developed to simultaneous measurement of temperature and strain with very high accuracy by using fiber Bragg gratings.

According to the role optical fiber plays, fiber sensing can be divided into intrinsic sensing and extrinsic sensing. The intrinsic sensing is that the optical fiber itself plays as both the sensing element and the transmission media, as is shown in **Figure 1(a)**;
