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Appendix E Fiber-Optic Sensor Performance This appendix describes the properties and performance of representative fiber sensors, as well as uses and potential markets for these sensors. HIGH-PERFORMANCE DEVICES In their 8 years of development, the fiber-optic hydrophore, gyro, and magnetometer have attracted much attention and are arguably the most advanced of the high-performance devices. Performance parameters of these sensors are shown in Table 5.1. In the case of the hydrophore, various configurations of packaged interferometer hydrophores have routinely achieved detection levels equal to or better than conventional piezoelectric devices. These sensors have also demonstrated a flat frequency response in the band of interest as well as insensitivity to environmental parameters such as pressure and temperature. The technology is that of the f~ber-optic interferometer (typically a Mach-Zender), in which the incident acoustic field, either through a compliant mandrel design or by a compressible coating, strains the optical fiber. The reference fiber either is isolated or has a coating that is relatively insensitive to pressure. The resulting phase shift in the interferometer is then demodulated (or converted to an amplitude modulation) to give a stable output. The fiber gyroscope is a single fiber arrangement for a fiber interferometer. The output of the optical source is split and launched as two counter-rotating beams in a fiber coil of length L and radius R. Under ideal conditions the two counter-propagating beams should see the same optical path (i.e., reciprocity. 95
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96 APPENDIX E However, when the fiber coil is rotated, there is an optical phase shift (known as the Sagnac effect) between the beams traveling in opposite directions. This phase shift is proportional to the product of the angular rotation rate, the total length of fiber in the loop, and the radius of the fiber loop. For a fiber length of 500 m -wound on a 10-cm radius coil, a rotation rate of 0.1 ° /h corresponds to a Sagnac phase shift of 1 microrad, which is readily measurable. To obtain stability over the long fiber lengths involved, polarization-preserving fiber is used. This approach has led to the development of a number of in-line fiber- optic components such as polarizers, depolarizers, and polarization-preserving couplers. Measurements have shown that polarization-preserving fiber systems can achieve a sensitivity of 10-3 o /h and a random drift of 5 x 10 - o /h (see Table 5.1) for other performance parameters). These and other experiments have shown, at least in a laboratory environment, that the fiber-optic gyro can operate at the level required for navigation-quality, strapdown inertial systems. CONTROL SENSORS Fiber-optic pressure sensors (see Table 5.1) have been developed to a level almost equivalent to that of position sensors. This lag in development is due to the fact that conventional pressure sensors, which offer low cost and adequate resolution, are readily available for most commercial and military applications; furthermore, the quantity of pressure sensors required is surprisingly small compared to other sensor types such as those used for temperature, liquid level, and flow. In spite of an excellent research base, the commercialization of f~ber- optic pressure sensors requires the identification of unique applications where their total dielectric nature provides a significant advantage over conventional technology. Pressure-sensor development vim result from either hazardous/ explosive environment applications or from a gradual evolution towards all- f~ber sensor systems following the development of other more frequently used sensor types. Similarly, the development of f~ber-optic vibration sensors depends on specific applications where their dielectric nature has distinct advantages. Two important vibration-measurement applications have such requirements. The first application, vibration monitoring in generators, could provide assurance against costly equipment damage. Generator high-voltage levels produce an EMI environment that in many cases prevents use of conventional piezoelectric accelerometers. The second application involves measuring acceleration in the presence of explosive gases, such as those in an offshore oil platform and in mines, where electrically based measurements are hazardous. Temperature sensors have been commercially available for many years. The most common sensor types use fiber to transmit infrared radiation from a high-temperature process and are limited on the lower end to a range of about
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APPENDIX E 97 300 O C to 2000 O C. More recently, sensors based on fluorescence of materials placed at the end of a fiber-optic~probe and fiber-optic-based temperature switches have become available. In addition, laboratory demonstration of temperature sensing has been completed utilizing a great variety of techniques. Recently the development of fire-resistant fiber cable capable of operation to at least 650 ° C has been reported. Since it does not incorporate any polymeric materials, it does not give off toxic fumes or smoke at high temperatures. Temperature sensors are also used to monitor excessive temperature increases. Fiber temperature sensors, because of their EMI immunity and small size, may be placed directly within the windings of electrical equipment or other electri- cally noisy, confined areas. As an additional example, f~ber-optic temperature sensors have been successfully used in chlorine production plants. In this application, galvanic cells are used to separate chlorine from sodium chloride and require 62 kiloamperes per cell. The processing environment is electrical- ly noisy and contains explosive hydrogen and corrosive gases and liquids. For optimum efficiency the baths are temperature controlled to a few degrees Cel- sius. The optical temperature sensors provide the performance and reliability required, and expanded use is envisioned. Several types of fiber-optic flow meters have been developed for control applications. Most are simple adaptations of conventional technology. In general, the following four approaches are used: (1) orifice plates (differential pressure), (2) turbines (frequency), (3) vortice shedding (frequency), and (4) laser doppler velocimetry-direct optical interrogation. Laser doppler velo- cimetry may be considered the most advanced technique; it is offered as a commercial product by several vendors. The advantages of this technique include an all-optical sensing head as well as wide dynamic range. The device works as a conventional laser doppler velocimeter, which measures particle velocities; however, fibers are used to transmit the light to and from the flow region. Work on the other three approaches has been limited to laboratory demonstrations, and the performance of these approaches is typical of the conventional technique from which they are derived. Liquid-level sensors based on several different principles have been demonstrated. Sensors based on refractive index variations have received the most attention to date. These devices typically fall into two categories. Either fiber termination senses the liquid level through a coupling of guided optical modes to radiated optical modes in a region where the core of the fiber is exposed to the liquid, or the fiber is terminated in a prism where total internal reflection is used to couple the guided energy into a return fiber. In the presence of the liquid, the internal reflection in the prism is frustrated, resulting in optical power coupling into the liquid. Such devices are available commer- cially. Another type of sensor is based on measuring the differential absorption between two wavelengths. The absorption type sensor, like sensors based on an exposed fiber core, can in principle be extended to continuous level sensing.
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