Fiber Optic Sensors
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Fiber Optic Sensors

An Introduction for Engineers and Scientists

Eric Udd, William B. Spillman, Eric Udd, William B. Spillman

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eBook - ePub

Fiber Optic Sensors

An Introduction for Engineers and Scientists

Eric Udd, William B. Spillman, Eric Udd, William B. Spillman

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About This Book

Since the technology has moved strongly into a number of different areas a textbook of this sort could be used by a wide variety of academic departments including physics, electrical engineering, mechanical engineering, civil engineering, aerospace engineering and bioengineering. To make the second edition as widely appealing as possible a series of significant upgrades were made.

1. The book is structured to support a variety of academic programs and it can also be used as a general reference by practicing engineers and scientists.

2. The introductory chapter has been revised to outline the new content of the second edition and provide a overview of the current status of fiber optic sensor technology.

3. A new, extensive chapter has been added covering fiber optic grating sensor technology and its application to aerospace, civil structures, oil and gas and power generating applications.

4. A second new chapter has been added on the emerging field of biomedical fiber optic sensors. This is one of the most rapidly growing fields of use for fiber optic sensors and with rising health costs and medical advances promises to be an important area for many years to come.

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Chapter 1
The Emergence of Fiber Optic Sensor Technology
Eric Udd
Columbia Gorge Research, Fairview, Oregon
Telecommunications have been revolutionized by fiber optic technology. The revolution began with limited system applications needing superior performance provided by fiber optics. The revolution became a rout as mass production techniques coupled with technical improvements resulted in superior performance at lower cost than those of alternative approaches. Simultaneous improvements and cost reductions in optoelectronic components in combination with mass commercial production led to similar displacements and the emergence of new product areas, including compact disc players, personal copiers, and laser printers. A third revolution is emerging as designers combine the product outgrowths of fiber optic telecommunications with optoelectronic devices to create fiber optic sensors.
The areas of opportunity are staggering and include the potential of replacing the many of the environmental sensors in existence today as well as opening up entirely new markets where sensors with comparable capability do not exist. Figures 1.11.3 provide an overview of the types of fiber optic sensors that are being developed and the environmental parameters that are most often associated with each type of sensor. The chapters of this book that correspond to each of the sensors are also indicated. Figure 1.1 lays out the various types of extrinsic or hybrid fiber optic sensors. Extrinsic fiber optic sensors are distinguished by the characteristic that sensing takes place in a region outside the fiber. Hybrid fiber optic sensors are similar and can be thought of as a “black box” sensor for which fibers are used to carry light to the box and data back. For most cases, the two terms can be applied interchangeably. A major distinction arises for the case of power by light sensors when a light beam is used to power an electronic sensor and data are carried back via a fiber optic data link. In this case, the hybrid designation would appear to be more appropriate.
Figure 1.1 Extrinsic or hybrid fiber optic sensors: light transmits into and out of the fiber to reach the sensing region.
Figure 1.2 Intrinsic or all-fiber fiber optic sensors: the environmental effect is converted to a light signal within the fiber.
Figure 1.3 Interferometeric fiber sensors.
Figure 1.2 shows a diagram illustrating many of the intrinsic or all-fiber-optic sensors. “Intrinsic” and “all-fiber” indicate that the sensing takes place within the fiber itself. In this case, the two designations can be and are commonly used interchangeably. A large and important subclass of intrinsic or all-fiber sensors is the interferometric sensors of shown in Fig. 1.3. Many of the highest-performance sensors fall into this group. The fiber sensors of Figs. 1.11.3 have been grouped into categories that are representative of their most common current state of development. Crossovers may occur; perhaps the most important example is the case of the interferometric sensors, many of which have been or are still being built in extrinsic or hybrid form.
From Figs. 1.11.3, it is apparent that virtually any environmental effect that can be conceived of can be converted to an optical signal to be interpreted. The usual case is that each environmental effect may be measured by dozens of fiber optic sensor approaches. The key is often to design the sensor so that only the desired environmental effect is measured.
Initial penetration of fiber optic sensors into markets has been driven by performance advantages. Some of these advantages are compared to conventional electronic sensors in Table 1.1. Fiber optic sensors offer an all-passive dielectric approach that is often crucial to successful applications, including electrical isolation of patients in medicine, elimination of conductive paths in high-voltage environments, and compatibility with placement in materials. The light weight and small size of these devices are critical to such areas as aerospace and provide substantial advantages to many products. Coupled to the issue of size and weight is immunity to electromagnetic interference. Conventional electrical sensors often require heavy shielding, significantly increasing cost, size, and weight. Environmental ruggedness provides key opportunities for fiber optic sensors, including high-temperature operation and all-solid-state configurations capable of withstanding extreme vibration and shock levels. Complementing these attributes are high sensitivity and bandwidth of fiber optic sensors. When multiplexed in arrays of sensors, the large bandwidth of the optical fibers offers distinct advantages in their ability to transport the resultant data.
Table 1.1 Advantages of Fiber Optic Sensors.
Passive (all-dielectric)
Light weight
Small size
Immunity to electromagnetic interference
High-temperature performance
Large bandwidth
Environmental ruggedness to vibration and shock
High sensitivity
Electrical and optical multiplexing
Component costs driven by large commercial telecommunication and optoelectronic market
Early work on fiber optic sensors generally fell into two distinct categories. Relatively simple fiber optic sensors were developed rapidly into commercial products, often by small firms, to perform measurements in specialized markets. An early example was the measurement of temperature in high-voltage environments. More complex fiber sensors such as fiber optic gyroscopes and acoustic hydrophones arrays were pursued by large industrial firms, complemented by government programs in an effort to access potentially large, high-payoff markets. Initial penetration into markets directly competing with conventional sensor technology from 1980 to 2000 was slow, due largely to the high cost of a limited number of suitable components. This situation has started to change, and trends for the future are extremely positive. As illustrated in Fig. 1.4, the cost of key optical elements has been falling rapidly, while the number and variety of choices has been increasing. These factors have been complemented by increasing reliability and quality of components.
Figure 1.4 Trends for fiber optic sensors.
The net result has been a rapidly expanding array of fiber optic sensor products and the start of significant market penetration. Table 1.2 illustrates the dramatic trends for a few key elements. Single-mode laser diodes in the late 1970s were a few thousand dollars each and often had lifetimes on the order of a few hours. By 2000, these elements were being used by the millions for compact disc players and laser printers, with costs dropping to a few dollars per unit and lifetimes measured in tens of thousands of hours. Single-mode fiber costs in the late 1970s were on the order of $10 per meter and it was difficult to obtain. In the 1990s, it became the standard of the telecommunication industry with millions of kilometers of fiber being laid each year and costs were less than $0.10 per meter. Both these developments represented improvements in cost of about three orders of magnitude, and the improvements in quality and uniformity were equally dramatic.
Table 1.2 Critical Components for Fiber Optic Sensors are Increasing in Performance While being Lower in Price.
Following behind these basic elements were more complex devices, such as integrated optic modulators, which were laboratory curiosities in the late 1970s and far from being a product. In 2000, these elements were available in small quantities for several hundred dollars each. By the year 2020, one could fairly safely project that their cost would fall at least to about the $50 level, based on the emergence of future products, including the fiber optic gyro. This sensor, which itself was a rare laboratory device in the last 1970s, relies on integrated optic phase modulators as a key element. In 2000, these devices were being marketed widely with prices from $500 for a low-end unit supporting a delivery truck to $5000 for aircraft navigation. To further penetrate the mass market, the cost must fall to about $500 –$1000 each for modest-performance devices. This in turn will drive the necessary cost of the phase modulator down to about $50 each.
With each new successful product, the cost of existing and newly introduced components continues to drop, opening the door for new waves of fiber optic sensor products. This situation is illustrated by in Figs. 1.5 and 1.6. In 1980, there were very few components available and their cost was relatively high, leading to the application of fiber optic sensors to small-niche markets often involving only a few items. By 1990, the prices of components of fiber and light sources and fiber beam splitters fell dramatically, and multiplexing elements were readily available at moderate cost. Several new elements, including integrated optic devices, were also being introduced at relatively high prices. These developments allowed the introduction of fiber optic sensors in instrumentation to support manufacturing, the introduction of sensors for the electric power industry, and the introduction of a limited numbers of sophisticated prototype fiber optic sensors such as the fiber optic gyro. By the year 2000, the number of devices available at low cost grew substantially, allowing fiber optic designers to produce a wide variety of devices that offered superior performance at lower cost than the existing technology while enabling the use of sensors in totally new areas of endeavor. These developments would include replacement of conventional spinning mass inertial sensors with fiber optic gyros, widespread use of fiber optic sensors in process control and manufacturing, and initial demonstration of fiber optic health monitoring systems in the aerospace and construction industries. By 2010, fiber gratings were used to support health monitoring functions on bridges and dams. The fiber gyro was standard equipment on the Boeing 777 and Dornier commuter aircraft and after years still served to navigate the Spirit and Opportunity rovers on Mars. Oil and gas service companies made major investments in fiber optic sensor technology to support exploration and extraction of oil and gas and newer applications such as robotic surgery began to be seriously considered. In the future, the breadth and scope of fiber optic sensor applications can be expected to con...

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