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Application of Fiber Optic Sensing

Today, fiber optic (FO) sensors are used to monitor large composite aircraft structures, concrete constructions, and to measure currents in high-voltage equipment. They are also applied in electrical power industry to measure electric fields as an alternate to current and voltage transformers. They are also finding many applications in the field of medicine, chemical sensing, as well as to monitor temperatures around large vessels in the oil and gas processing industries. There have been recent attempts in Japan to monitor the wings of a fighter aircraft to monitor dynamic strain and temperatures when the aircraft is taking off and landing to better understand the load and temperature-induced stresses and how these affect fatigue performance. Considerable work is underway to map strain in large composite and concrete structures.

Research on the application of FO sensors (FOSs) has been conducted over many years. They were first demonstrated in the early 1970s (Culshaw and Dakin 1996; Grattan and Meggitt 1998) and are the subject of considerable research since 1980s. Early applications were focused on military and aerospace uses. FO gyroscopes and acoustic sensors are examples, and they are widely used today. With the increase in the popularity of FOSs in the 1980s, a great deal of effort was made toward their commercialization with an emphasis on the intensity-based sensors. In the 1990s, new technologies emerged, such as in-fiber Bragg grating (FBG) sensors (Morey et al. 1989; Rao 1997), low-coherence interferometric devices (Grattan and Meggitt 1995; Rao and Jackson 1996), and Brillouin scattering distributed sensors (Bao et al. 1995). Dramatic advances in the field of FO sensors have been made as a result of the emergence of these new technologies leading to a significant proliferation of their use.

FO sensors offer significant advantages over conventional measuring devices, most important of which are: electromagnetic immunity (EMI), small size, good corrosion resistance, and ultimate long-term reliability. For example, FBG sensors offer a number of distinguishing advantages including direct absolute measurement, low cost, and unique wavelength-multiplexing capability. These new measuring technologies have formed an entirely new generation of sensors offering many important measurement opportunities and great potential for diverse applications.

This book is devoted to the application of the FO technology in electric power cables. However, before we tackle this subject, we would like to offer a brief review of the application of this technology in other fields. In the following sections, an attempt is made to provide an overview of the types of FO sensors and we will list some of the industries where FOSs have been applied. These include:

• Oil and gas industry
• Fire detection and integrating them with the firefighting equipment
• Large composite structure such as bridges and dams
• Mines
• Aircraft industry
• Medicine
• Power industry

1.1 Types of Available FO Sensors

One of the first applications of FOSs involved the, so-called, limit sensors with an ability to detect motion beyond a certain limit and initiate action once this limit is exceeded. These types of sensors can be used for monitoring linear or rotational motion. Another type of an early application is a level sensor when a solid or a liquid rises or falls beyond a set point. Proximity sensors use infrared emission, reflection, or pressure change to perform such detection without the need for any physical contact. Another example involves a beam of light crossing a doorway. Beam interruption can be detected by a photo sensor and trigger an alarm. This application is typically used in the process industry for counting or having access control.

Another class of sensors uses FOs for linear or angular position control. One may have an array of optical fibers that are placed in parallel. The object to be detected when passing this array of sensors may alter the transmission or reflection of light. The sensor processing electronics can then infer the proper position of each object within the sensing region. In this case, the resolution of the detected positions will depend on the spacing between the sensing points. This idea has been extended by placing fibers in an angular fashion and has been used to detect or establish the angular position of a gear on drive systems.

FO sensors have also been used to measure linear as well angular speed or velocity of shafts (tachometers). Some of them use Doppler phase shift methods for such measurements.

A FO gyroscope is another type of sensor. It consists of a coil, either polarization preserving or not, of optical fibers in which the light is simultaneously propagating in the clockwise and counterclockwise directions. The SAGNAC effect induces a differential phase shift between the clockwise and counterclockwise guided waves in the rotating media. The phase difference in the detected signal is converted into a rate or angle of rotation. Examples of this application are the Brillouin or resonant FO gyroscopes.

Optical fibers have also been used for temperature sensing. They may be broadly classified as FBG devices. Phosphor coated, Fabry-Perot cavity terminated, or thermo-chromatic terminated optical fibers are some examples of such point sensors. In the electric power industry, these devices have been used to measure transformer winding temperatures.

Another class of temperature sensors are the distributed FO temperature devices. These are broadly classified as based either on the Raman scattering or Brillouin principles. Laser light injected into a fiber is continuously scattered. This backscattered light is used for calculating temperature profiles along the fiber. Brillouin scattering exhibits sensitivity to temperature as well as strain, so care has to be exercised when interpreting the temperature data. Devices based on Raman scattering fall into two principal categories: those that rely on the optical time domain reflectometry (OTDR) while the others use the optical frequency domain reflectometry (OFDR). It is the latter that provides the highest spatial resolution along a fiber.

While discrete monitoring based on FBG FO system has been and is continued to be used, it is expected that the distributed FOSs will play a more important role in health monitoring of large structures, such as dams, because the use of a single fiber makes installation easy with the ability to measure over long distances. While considerable advances have been made in measuring strain, the effects of temperature-induced strain appear elusive especially when one needs to improve spatial resolution and accuracy. Because of its complexity, simultaneous measurement of multi-axis strain and temperature in composites remains a major challenge for the optical fiber sensors community.

Because of a spate of explosive in-service failures of the 525 and 800 kV oil-filled current transformers in the late 1990s, the industry was looking for alternate devices that could provide accurate current measurements with a dynamic rating ranging from several 1000s to several 100 000s amperes. This led to the development of FO electric current sensors based on the Faraday effect. After more than two decades of development, they have successfully entered the market. At the University of British Columbia in Canada, Dr. Jaeger and his research team developed an electric field or voltage sensors and had conducted several field tests to prove the concept and develop a practical system. These systems were placed in the local utility's 500 kV substation on a trial basis (Jaeger et al. 1998). Further collaborative efforts led to the development of integrated electro-optic voltage and current sensors. Initially, the challenge with these systems appeared to be their inability to match the needs of the conventional protection equipment that required 120 V and 5 A input signals. In parallel, the modernization of the electric power grid and changes that occurred with the development for solid-state relays, which now require lower voltage (~5 V) and current signals (100 mA), the devices developed at the university became more appealing to the industry. Moreover, these electro-optical devices provided higher sensitivity, improved frequency bandwidth, with significantly improved EMI. This led to general acceptance of electro-optical devices for current and voltage measurements in the power industry.

For medical applications, extrinsic FOSs based on multimode fiber transmission have matured. A number of FBG temperature and ultrasound sensor systems have been developed. With further engineering, it is anticipated that these systems could be used for in-vivo measurement of temperature or/and ultrasound as well as blood pressure in the heart and other organs. In chemical sensing, FOSs based on evanescent wave coupling is still under development.

The following sections describe in more detail the application of the FO sensors in various industries.

1.2 Fiber Optic Applications for Monitoring of Concrete Structures

When compared with traditional electrical gauges used for strain monitoring of large composite or concrete structures, FOSs have several distinguishing advantages, including:

• A longer lifetime, which could probably be used throughout...