An Introduction to Distributed Optical Fibre Sensors

Arthur H. Hartog

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9780367849740
• eBook ISBN: 9781351645300
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

This book explains physical principles, unique benefits, broad categories, implementation aspects, and performance criteria of distributed optical fiber sensors (DOFS). For each kind of sensor, the book highlights industrial applications, which range from oil and gas production to power line monitoring, plant and process engineering, environmental monitoring, industrial fire and leakage detection, and so on. The text also includes a discussion of such key areas as backscattering, launched power limitations, and receiver sensitivity, as well as a concise historical account of the field's development.

OneFundamentals

1Introduction

Sensors are the means by which inputs about the real world are provided to electronic systems that archive and process the data collected; often, these systems also make decisions based upon that information. Sensors convert measurands,1 such as temperature, flow, pressure or strain, from the physical world into signals that can be read in electronic form.

A stylised illustration of the industrial use of sensors is shown in Figure 1.1, where a petro-chemical process taking place in the bulbous vessel (which is typically a refractory-lined pressure housing, shown to the right) is monitored by a number of sensors that penetrate the housing through apertures that preserve the pressure seal and report their readings to the control room. Here, the data are not only normalised and archived but also passed to a control computer that monitors the process and issues signals to control valves and actuators that keep the process within intended parameters.

Figure 1.1Simplified representation of a chemical process plant, its sensor systems and process control.

1.1The Concept of Distributed Optical Fibre Sensors

1.1.1Optical Fibre Sensors

Optical fibre sensors are devices that use light to convey the information which they sense. A typical optical fibre sensor system (Figure 1.2) consists of the sensor itself that is probed by the input light which it modulates in accordance with the value of the measurand. The system also includes an optical fibre within a transit cable that conveys the probe light to the sensor and returns the modulated light from the sensor to the interrogator through a patch panel that houses the connectors or splices. The interrogator is the opto-electronic system that emits the probe light and converts the returned light into an electrical signal which is processed to create the output of the system. The sensor is designed to respond to the intended measurand and ideally not at all to any other external influence. Likewise, the transit cable, that carries the probe and the returning optical signal, should protect the fibres it contains from external influence and from damage caused by the environment through which it passes.

Figure 1.2General arrangement of an optical fibre sensor system.

This book is about a particular family of sensors, distributed optical fibre sensors (DOFSs), that determine the spatial distribution of a measurand along a section of fibre, often many kilometres long, rather than measuring the physical parameter of interest in one location only. DOFSs use optical fibres both as the sensing element and as the means of carrying the optical signals used for this purpose.

In the early 1970s, rapid advances were made in the technology of optical fibres for telecommunications, including low-loss fibres [1], reliable laser diodes operating at room temperature [2] and an understanding of the design of transmitting and receiving electronics. This progress led to pioneering thoughts about applying this same technology to physical [3–6] and even chemical [7] sensing. A number of concepts appeared in the scientific literature at the time, demonstrating that optical fibres could be used as sensors for a wide variety of measurands. The literature also showed that many attributes of the light travelling in the fibre could be applied to the sensing task, including its intensity [8], polarisation [9], phase [10], propagation time [6], optical spectrum [11] and coherence [12].

Thus, a highly versatile sensing technology emerged, able to provide compact, lightweight sensors that could be shaped to match a wide variety of space, power and environmental constraints. In particular, the ability of optical sensors to convey the information in optical form from a hostile environment to the interrogating electronics located in a more benign site enabled the development of sensors operating in very high-temperature conditions, for example, in borehole applications.

Intrinsic vs. extrinsic optical fibre sensors

At this point, we should make the distinction (illustrated in Figure 1.3) between intrinsic (Figure 1.3a) and extrinsic (Figure 1.3b) optical fibre sensors. In intrinsic sensors, the light stays within an optical fibre throughout the system and is modulated within the fibre, for example, by induced loss, changes to its spectrum or polarisation and so on. In contrast, in extrinsic sensors, the sensor is a bulk-optic device, such as an electro-opti...

Table of contents

• Cover
• Half Title Page
• Series Page
• Title Page
• Copyright Page
• Dedication
• Contents
• Series Preface
• Foreword
• Preface
• Author
• List of Symbols
• List of Abbreviations
• Part One Fundamentals
• Part Two Distributed Sensing Technology
• Part Three Applications of Distributed Sensors
• Index