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Spie Press Book

Fiber Optic Sensors: Fundamentals and Applications, Fourth Edition
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Book Description

This fourth edition of Fiber Optic Sensors is revised and updated to include the new sensing technologies emerging in broad commercial use, with a focus on scattering-based distributed sensing systems. In addition, a chapter was added to describe biophotonic sensing systems and their applications.

This book covers a broad range of point sensors and distributed sensor technologies and their applications in a multiplicity of markets including energy, biomedical, smart structures, security, military, and process control. It illustrates how this portfolio of technologies has addressed many sensing problems that are difficult for conventional approaches and often require survival in extremely harsh conditions.

With the addition of two authors who bring 75 years of combined experience in fiber optic sensor technology, this edition is a significant update and an excellent resource for any engineer who has an interest in advanced sensing systems.

Book Details

Date Published: 7 January 2015
Pages: 332
ISBN: 9781628411805
Volume: PM247

Table of Contents
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Table of Contents


1 Fiber Optic Fundamentals
1.1 Refraction and Total Internal Reflection
1.2 Meridional Rays
1.3 Skew Rays
1.4 Bent Fibers
1.5 Mechanisms of Attenuation
1.6 Waveguide Propagation
1.7 Evanescent Wave
1.8 Cross Coupling
1.9 Scattering
1.10 Mode Patterns
1.11 Fiber Types
1.12 Polarization-Maintaining Fibers

2 Fiber Optic Sensor Fundamentals
2.1 Background
2.2 Sensor Categories
2.3 Distributed Fiber Optic Systems

3 Intensity-Modulated Sensor
3.1 Introduction
3.2 Transmissive Concept
3.3 Reflective Concept
3.4 Microbending Concept
3.5 Intrinsic Concept
3.6 Transmission and Reflection with Other Optical Effects
3.7 Speckle Pattern
3.8 Sources of Error and Compensation Schemes

4 Phase-Modulated Sensors
4.1 Introduction
4.2 Interferometers
      4.2.1 Mach-Zehnder
      4.2.2 Michelson
      4.2.3 Fabry-Perot
      4.2.4 Sagnac
4.3 Phase Detection
4.4 Detection Schemes
4.5 Practical Considerations

5 Wavelength-Modulated Sensors
5.1 Introduction
5.2 Bragg Grating Concept
5.3 Bragg Grating Sensors
5.4 Distributed Sensing
5.5 Wavelength Detection Schemes
5.7 Harsh Environments

6 Scattering-Based Sensors
6.1 Absorption and Transmission Loss in Optical Fibers
6.2 Optical Time-Domain Reflectometry (OTDR)
6.3 Light-Scattering Mechanisms
      6.3.1 Elastic versus inelastic scattering
      6.3.2 Stokes and anti-Stokes scattering components
      6.3.3 Scattering emission spectrum
6.4 Rayleigh Scattering
6.5 Raman Scattering
6.6 Brillouin Scattering
      6.6.1 Stimulated Brillouin scattering
6.7 Distributed Fiber Sensing and Scattering Effects

7 Polarization Based Sensors
7.1 Introduction
7.2 Analysis of Birefringent Optical Systems
7.3 Birefringent Effects in Bragg Gratings

8 Digital Switches and Counters
8.1 Introduction
8.2 Scan Modes
8.3 Excess Gain
8.4 Contrast
8.5 Beam Diameter
8.6 Electro-Optic Interface
8.7 Applications

9 Displacement Sensors
9.1 Introduction
9.2 Reflective Technology
9.3 Microbending Technology
9.4 Modulating Technology
9.5 Fabry-Perot Technology
9.6 Bragg Grating Technologies
9.7 Applications

10 Strain Sensors
10.1 Strain Sensors
10.2 Interferometric Strain Sensors
10.3 Applications

11 Temperature Sensors
11.1 Introduction
11.2 Reflectance and Absorbance Sensors
11.3 Fluorescence Sensors
11.4 Microbending Sensors
11.5 Black-body Radiation
11.7 Interferometric Sensors
11.8 Fiber Bragg Grating Sensors
11.9 Distributed Temperature Sensing (DTS)
11.10 Applications

12 Pressure Sensors
12.1 Introduction
12.2 Conventional and Specialized
12.3 FBG-based Optical Sensors
12.4 Fabry-Perot-based Optical Sensors
12.5 Packaging
12.6 Field Installation

13 Flow Sensors
13.1 Introduction
13.2 Turbine Flowmeters
13.3 Cantilevered-Beam Flow Sensors
13.4 Differential-Pressure Flow Sensor
13.5 Vortex-Shedding Flow Sensor
13.6 Laser Doppler Velocity Sensors
13.7 Indirect Flow Monitoring
13.8 Applications

14 Magnetic and Electric Field Sensors
14.1 Introduction
14.2 Magnetic Field
      14.2.1 Faraday Rotation-based Sensors
      14.2.2 Phase modulation
14.3 Electric Field
      14.3.1 Polarization Modulation
      14.3.2 Phase modulation

15 Chemical Analysis
15.1 Introduction
15.2 Fluorescence
15.3 Absorption
15.4 Scattering
15.5 Refractive Index Change
15.6 Color Changes
15.7 Interferometry
15.8 Distributed Fiber Optic Chemical Sensors
15.9 Fiber-Optics-Enabled Spectroscopy
15.10 Applications

16 Biophotonic Sensors
16.1 Introduction
16.2 Intrinsic Biophotonic Sensors
      16.2.1 Intrinsic biophotonic sensors: evanescent wave interaction
      16.2.2 Intrinsic biophotonic sensors: using photonic crystal fibers
      16.2.3 Intrinsic biophotonic sensors: fluorescent microsphere array sensors
      16.2.4 Intrinsic biophotonic sensors: distributed sensor concepts
      16.2.5 Intrinsic biophotonic sensors: surface plasmon resonance
16.3 Extrinsic Biophotonic Sensors

17 Rotation Rate Sensors
17.1 Introduction
17.2 Sensor Mechanism
17.3 Reciprocity
17.4 Noise Limitations
17.5 Resonators
17.6 Comparison of Resonator (RFOG) and Interferometer (IFOG) Gyroscopes

18 Distributed Sensing Systems
18.1 Introduction
18.2 Applications
18.3 Distributed Temperature Sensing Applications in the Oil and Gas Industry

19 Market Opportunities
19.1 Introduction
19.2 Barriers to Market Growth
19.3 Summary and Conclusions



Fiber optic sensor technology is not new, but is continuing to evolve after over 60 years of development and commercialization. The sensing designs are not based on a single concept but on a variety of optical phenomena that can be used to measure a broad range of physical and chemical parameters.

In early industrial applications, single point fiber optic sensors were used as an alarm to indicate the absence or presence of an object. As the technology evolved, the functionality increased to accurately determine the position of an object. Many of the sensing concepts that will be discussed throughout this book will be for single point sensors which operate by detecting changes in the intensity of light (see Chapters 3, 8, and 9). They operate by altering the transmitted or reflected light intensity in a manner proportional to the parameter being sensed such as temperature, strain, or displacement (position). The sensing functionality can be expanded to monitor electric and magnetic field measurements using polarization concepts. As an example, certain materials exhibit Faraday rotation, which alters the plane of polarization and the resulting transmitted light intensity in the presence of a magnetic field. Polarization-based sensors are discussed in Chapter 7.

Interferometric sensors compare the phase of light in a sensing fiber to a reference fiber. Small phase shifts can be detected with extreme accuracy. The phase shifts are generated by changes in strain and/or temperature in the sensing fiber. This family of sensors has been especially useful in monitoring dynamic strain (vibration) (see Chapter 4). Also, a Sagnac interferometer is an interferometric sensor configured to be sensitive to rotation (Chapter 17). Two examples of successful commercialization of interfermetric-based sensors are hydrophones for submarine detection and fiber optic gyroscopes for advanced navigation systems. Both are for military applications primarily and have performed well for over 30 years with thousands of systems deployed.

A wavelength or spectral shift is another sensing approach. By introducing coatings on the fiber or a target that fluoresces, under certain conditions (usually related to chemical interaction or temperature fluctuation), a chemical reagent can be detected or temperature can be monitored. A more widely used spectral shift approach uses Bragg gratings, which are reviewed in detail in Chapter 5. A Bragg grating is characterized by having a resonant wavelength that is reflected as light is transmitted through the grating. The reflected light is very sensitive to the grating spacing and the index of refraction of the grating material. Temperature and strain alter both of these parameters. As a result, Bragg gratings can function as temperature or strain sensors. While Bragg gratings have been used as single point sensors, they have had great utility as quasi-distributed sensors in which multiple sensors are located along a single fiber.

Bragg gratings, which have been in development for over 25 years, are in wide commercial use. They have been especially effective in enabling smart civil structures and as pressure and temperature sensors in smart oil wells. Bragg gratings have evolved to handle the very harsh environment associated with energy applications.

Light-scattering phenomena have emerged in the last 10 years to be a key family of technologies to enable fully distributed fiber optic sensing systems. Distributed sensing systems allow any point along a fiber to function as a sensor, with virtually thousands of sensing points along a single fiber that may exceed 30 km in length. The basic sensing mechanisms are Raman scattering, Rayleigh scattering, and Brilloiun scattering. Detailed descriptions of how these sensors work are given in Chapters 6 and 18. Briefly, Raman scattering is sensitive to temperature but not strain, and makes an excellent distributed temperature sensor referred to as DTS. Brillioun scattering is sensitive to temperature and strain and is the basis for distributed temperature and strain sensors referred to as DTSS. Rayleigh scattering is sensitive to acoustic vibrations and is used a distributed acoustic sensor referred to as DAS. DTS and DAS approaches have been especially effective for oil and gas applications.

A very important point that is understated is that fiber optic sensing systems have enabled smart oil and gas wells that are allowing North America to gain energy independence. Fiber optic sensor technology has a long history of development and commercialization successes. The technology has not yet reached maturity and will likely expand and create many new applications and commercialization opportunities.

David A. Krohn
August 2014

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