iaetsd fiber optics in sensing and measurement

Fiber Optics in Sensing and Measurement

ABSTRACT—We are living in a world now which happened to be a so lucid system that no one ever has anticipated things are becoming easier day in day out.Optical techniques for measurement—interferometry, spectrometry and polarimetry—have long been used in materials measurement and environmental evaluation. The optical fiber lends yet more flexibility in the implementation of these basic concepts. Fiber-optic technology has, for over 30 years, made important contributions to the science of measurement. This paper presents a perspective on these contributions which while far from exhaustive highlights the important conceptual advances made in the early days of optical fiber technology and the breadth of application which has emerged. There are also apparent opportunities for yet more imaginative research in applying guided-wave optics to emerging and challenging measurement requirements ranging from microsystems characterization to cellular biochemistry to art restoration. 1. INTRODUCTION FIBER OPTICS has undoubtedly had a profound impact on the communications industry [1]. This can be traced back to the seminal papers of Kao and Hockham, and Simon and Spitz [2], [3], who basically appreciated in the early to mid 1960s that optical signals could be transmitted along glass or silica fibers with a loss potentially below that experienced in coaxial copper cables. Further, unlike copper where skin effect increases loss with baseband modulation frequency, the loss in optical fibers could be maintained for all conceivable modulation frequencies. A little while later, Dyott [4] observed that there were regions of zero dispersion in the transmission characteristic of silica. The rest is well-documented history brilliantly encapsulated in Hecht’s book [1]. But fiber optics was not new in 1965. Glass fibers had alreadyappeared in ornamental lamps and the basic ideas of the dielectricwaveguide were well established. Using optical fibers toguide light to and from a place at which a measurementwas to bemade had already emerged and had gone from concept to practice.The first patents had been filed on fiber-optic sensing andhad emerged as a tentative product [5] described in the literature.At around the same time, Eli Snitzer’s ever-creative intellectwas proposing using fiber optics to transmit

phase-modulated signals[6]—the basis of another important class of fiber-optic sensors. So this brief paper will present a predominantly historical perspective on the evolution of research, development, and application in fiber-optic sensor technology. From the early beginnings almost 35 years ago, the optical fiber sensor (OFS) community became infected with communications euphoria, and by the mid 1970s to the early 1980s felt OFS technology was the solution to everything. Realism percolated, though perhaps a little slowly, and now we know that there are areas of real application, but that there are still interesting and relevant problems left to excite the research community. II. SENSOR TECHNOLOGY Sensing and measurement is a specialized art. Sensor technologies are applications specific. The thermometer switch which controls your central heating is totally unsuited to control a cooling system in your automobile. Sensors all operate in niche markets. The sensing mechanisms are based on literally dozens of physical and chemical phenomena interfaced to electronic signal conditioning through dozens more custom designed protocols. The industry is consequently highly fragmented— a source simultaneously of frustration, challenge, and satisfaction to those within it. Fiber-optic sensing is therefore conceptually orthogonal to fiber-optic communications which involves very large numbers of essentially the same system configuration and components. There is very little spin-off in practice from the communications industry into sensor technologies. The fiber itself, some guided-wave components, some, but by no means all, connectors and some, but by no means all, sources and detectors are the common elements. Conceiving and realizing the necessary mechanical and electronic infrastructure around the guided-wave components is the major portion of optical fiber sensor technology. In particular, packaging is an immense challenge—indeed the case with all sensing techniques. Sensing techniques have another important generic feature.Virtually all physical and chemical phenomena which are usedin the transduction process are temperature sensitive. Most measurementsare not concerned with temperature.

Deepak Kumar Mohapatra Electrical and Electronics Engineering

Gandhi Institute for Education and Technology, Bhubaneswar [email protected]

Proceedings of International Conference on Advances in Engineering and Technology

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International Association of Engineering and Technology for Skill Development27

Consequently,most sensing and measurement systems must first of all correctfor temperature variations. This is a perennial issue which hasmost certainly marked the process of fiber-optic sensor technology,producing solutions and approaches with varying degreesof elegance and ingenuity. Consequently, sensors are never big business, e.g., the automobile industry is often perceived as the enormous market. The worldwide volume is about 50 million vehicles per annum. None of the mass market sensors in automobiles is worth more than a few dollars and there are numerous different suppliers worldwide. Consequently, even in the automobile industry, small companies or small sub divisions of major corporations provide the sensors. There is no homogenous multibillion dollar market for anything in sensing and measurement technology. If we examine fiber sensors specifically, the niches are currently very narrow, so the few companies which are operating in this domain, while profitable and interesting, employ typically 10–100 people. They are also patient. Above all else, sensors have to work, often in difficult environments ranging from the physical extremes of aerospace to the chemical extremes of biomedicine or process technologies. Sensors take time. A decade from concept to product is normal. Precision gyroscopes, for example, can take twice this! III. FIBER SENSOR TECHNIQUES: THE BASICS The fiber sensor is illustrated diagrammatically in Fig. 1. The basic components are simple. Light is taken to a modulation region using an optical fiber and modulated therein by physical, chemical, or biological phenomena, and the modulated light is transmitted back to a receiver, detected, and demodulated. Hopefully, there is a one-to-one correlation between the phenomenon of interest and the demodulated signal. There are two substantial issues in realizing a viable optical fiber sensor technology: 1) to ensure the one-to-one relationship between the parameter to be measured and the demodulated signal; 2) to match the technology to the application in terms of both performance and cost. The first of these is the simpler one despite the fact that theimpact of the fibers to and from the modulation region, variationsin source and detector characteristics with temperature andtime and the influence of temperature on the modulation processare all important. The second of these must recognize the presenceof established techniques and in particular must identifyotherwise insoluble problems which are important but for technicalreasons have not been

satisfactorily resolved. There aresufficient examples of such problems to continue to stimulate the fiber-optic sensor community [7], [21]–[24]. IV. THE FOTONIC SENSOR: INTENSITY MODULATED SYSTEMS The Fotonic sensor is described exhaustively in [5] and was patented somewhat earlier. It is based (Fig. 2) on bifurcated fiber bundles.

Fig. 1.Basic functions of the optical fiber sensor.

Fig. 2.“Fotonic” sensor measuring the position of a reflector relative to a fiber(bundle) end.

The basic concept is simple. The fraction of light transmitted between the receive elements in the bundle and the transmit elements in the bundles depends on the separation between the reflector and the bundle itself and is, to a first approximation, independent of small rotational angles of the target object, particularly for the high numerical aperture fibers uniformly illuminated with light from an incandescent bulb for which the sensor was originally conceived. Of course, if the reflectivity of the target changes, then the transmission ratio also changes. However, the inventors had foreseen this eventuality and designed compensation schemes to overcome the problem. These involved use of two receiving bundles, one a known distance from the other.. It was capable ofresolutions in the micrometer range,


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made no physical contactwith the target, and had the usual optical advantages concernedwith the electromagnetic interference and pickup. This basic sensor has been reinvented in numerous formats with some regularity ever since. The fiber bundle concept has been replaced by the use of individual fibers and the light source has been increased in intensity by using light-emitting diodes and superradiant systems. There have been slight modifications to the intensity fluctuation compensation scheme. Sometimes separate transmit and receive fiber locations have been used. The application context has been changed.

Fig. 3. Dual-beam all-fiber interferometers used in sensing normally the interferometer is biased, dynamically or statistically, to the quadrature optimum sensitivity point X.

The Fotonic sensor exemplifies much of optical fiber sensor technology. It was conceived and demonstrated 35 years ago. It has been frequently reinvented. It has occasionally been used in earnest in applications which were not dreamed of when the idea was originally put forward. It uses simple intensity modulation and it has within it the capability if needed for referencing to remove the impact of fluctuations in reflectivity, fiber loss, and source output. V. PHYSICAL SENSING: MODULATING LIGHT OUTSIDE THEFIBER The Fotonic sensor which started the process was based upon what now would be termed an extrinsic modulation scheme—in other words, the fiber is used to link the optical signal to the sensing point. In addition to the many variations on the basic Fotonic sensor, where the signal clearly depends upon attenuation, a number of other architectures have been investigated and some exploited using attenuation independent modulation onto the optical carrier.

Three basic approaches have been examined: 1)Colour modulation, for example, using spectral

slicing of the radiation from a light-emitting diode;

2)Interferometric measurements of the distance between the end of the optical fiber and the reflective surface;

3) Configuring the parameter to be measured into a modulation scheme, which introduces an amplitude modulation at a frequency dependent upon the measurand?

The basic principles of these approaches are illustrated in Fig. 3, and again the initial demonstrations of these principles date back to the period between 1975 and 1985 [17]–[19]. Digital shaft encoding or longitudinal displacement measurement operating on a spectral slicing principle proved to be robust and capable of up to 12-bit accuracy. Mechanical resonators—variations on the vibrating wire gauge—are well established as precision measurement concepts. A mechanical strain applied along the axis of the wire changes the resonant frequency and through measuring this frequency, the strain may be inferred with the usual proviso concerning temperature sensitivity. The optical fiber versions of these resonators use thermo-optic excitation and with careful design can operate through fiber links several kilometres in length.

(a)

(b)

(c)

Fig. 4. Three intensity insensitive modulation schemes for optical fiber sensors. (a) Spectral slicing. (b) Interferometric filtering. (c) Amplitude


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modulation at a measurand-dependant resonant frequency. The mechanical engineering of the sensor head is demanding but these sensorshave now found their place in very demanding applications,notably the measurement of pressure “down hole” in oil wells.Interferometric measurement systems for diaphragm displacementhave been based upon both using the space between thediaphragm and the fiber end as a Fabry–Perot cavity and measuringthe spectral transmission characteristic, and on exploitingwhite-light interferometry to replicate the displacement of thediaphragm within the receiver. In white-light interferometry, anincoherent source is used to illuminate the measurement interferometerand fringes are detected at the receiver only when thepath difference of a reference interferometer exactly matchesthat of the measurement. The first demonstration of this principle[20] highlighted the potential precision with which thismeasurement could be made in sensing, and subsequently thebasic principle has been extensively exploited. VI. FIBER BRAGG GRATING The photorefractive effect in optical fibers was first reported during the same very productive era in the mid 1970s [21].

(a)

(b)

Fig. 5. Bragg grating geometry and functions. The reflected spectrum closely approximates to the Fourier transform of the grating. (a) Short broadband reflector/mode coupler. (b) Long

narrowband wavelength selective coupler functions performed by fiber Bragg gratings.

The photorefractive effect can be used to make periodic structures along the core in optical fiber, and these periodic structures (Fig. 4) act as highly selective optical filters. The reflection wavelength depends on the period of the grating, which in turn could be modified through temperature and phase excursions which change the optical path length within the grating structure. Fiber Bragg gratings is an extensive subject predominantly with applications in the communications area for elements such as wavelength tuning and stabilization structures and optical filters. Initially, the gratings were written by sending counter-propagating beams along the fiber, and it was a decade or thereabouts later that side illumination techniques using ultraviolet lasers introduced the necessary flexibility in defining the periodic structure [22]. In the sensor context, fiber Bragg gratings can be written sequentially at predetermined points along a single fiber and can also be made to reflect light in different wavelength bands. Consequently, a linear array of, say, 16 gratings can provide 16 separate sensing points, each individually identified through a spectral slicing technique. Precision measurement of wavelength in the reflected spectrum, usually involving a stabilized reference, can then yield the period of each grating

Fig. 6. All fiber current monitor and crystalline current (magnetic field) or voltage (electric field) fiber sensor architecture. VII. MEASURING ELECTROMAGNETIC FIELDS Faraday rotation (the dependence of circular birefringence on magnetic fields) and the electro optic effect (dependence of—usually—linear birefringence on applied electric fields) are well-known phenomena in the world of optical physics. With the advent of optical fiber transmission, it was natural then that the fiber should be used to link an optical signal to a suitable crystalline material to


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measure electromagnetic fields. The great benefit of the fiber link lies potentially in sensing at high voltage, since current transformers are expensive, unwieldy, and have very limited frequency response. It seems that fiber optics could provide an insulating link to high-voltage areas and make the measurement using a very broadband detector with a rapid response interaction mechanism. Additionally, theFaraday Effect occurs in all materials, particular the silica ofthe optical fiber, so perhaps looping the optical fiber along themagnetic field (Fig. 8) could also produce Faraday rotation. Measuring electromagnetic fields has developed into an importantniche for fiber-optic sensing and again the evolutionfollows the now familiar pattern. The original references dateback more than 20 years [23], [24] with subsequent engineering and refinement dealing with the practicalities of vibration sensitivity,temperature sensitivity, and packaging to protect fromenvironmental interference. VIII. CHEMISTRY AND BIOMEDICINE: OPTRODES Colour and chemistry are inherently compatible. Materials are frequently characterized through their absorption spectra, much of which lies in the optical region, and visual indicators exemplified by litmus paper are well-characterized means of interrogating materials, especially liquids, for their content.

Fig. 7. Basic optrode which features in many, perhaps most, optical fiber chemical sensor architectures. Optical fibers have a natural part to play and their potential for chemical and biochemical measurements became recognized within the same familiar timescale [21], [24]. The term “optrode” (Fig. 7) emerged later to describe a device which uses colour-sensitive intermediate chemistry to monitor the almost invariably liquid immediate surroundings. In optrode-based devices, the optics is usually very simple, and updated versions of the filter wheel first described in [24] are still used (though typically using two colours of LED). Most of the progress in optrode- based devices lies in the

complex area of immobilization chemistry and in protective packaging especially for use in biological systems. Applications ranging from monitoring of bile in the digestive tract to blood oxygen and blood pH to pollutant monitoring in water supplies have now all been realized. For biomedical sensing, the usual approach is to incorporate a disposal sensing head, though for water supply measurement systems, long-term stability of the sensor chemistry remains problematic. The intermediate chemistry in optrodes has the benefit that the sensor can be made highly specific to a particular species and the signal from that species can be made to occur within a wavelength range compatible with optical fiber technology. Stability in packaging remains an issue, and more direct measurement systems eliminating the intermediate chemistry are in principle more attractive. Direct spectroscopy therefore has an important role to play in optical fiber chemical measurement, though this emerged as a potentially viable technique a decade or so later than the optrode was first mooted [23]. Gas spectroscopy remains the most interesting potential application though the strongest absorption lines for most species of interest lie outside the transmission window of optical fiber waveguides. There are, however, frequently overtones in the near infrared, and these have been used with some success in the detection of methane, acetylene, hydrogen sulphide, and several other species [24]. Once more, while there are niche applications, the peculiar properties of optical systems must be recognized. In this context, the very specific wavelengths required to address the absorption bands are invariably outside the communications spectrum, so optical sources with the appropriate power level and spectral purity remain relatively special items and are therefore expensive. However, a sufficiently large array of sensors can make a system cost competitive, and its selectivity compared to electrical pellistor technology (which simply detects flammable gases) can be beneficial. The low attenuation of optical fibers also facilitates measurements over a very wide area inaccessible to more established approaches. IX. USING OPTICAL FIBERS SENSORS: THE APPLICATIONS Sensing and measurement are idiosyncratic niche-oriented activities, and the thought that optical fiber sensing can solve all measurement problems has long since disappeared. Numerous niches have emerged. These range from monitoring pressure transients in diesel engines, to using white-light interferometry to measure strain in long-gauge-length (to 50 m) sensors for civil engineering, to intravenous pressure sensing, to landfill monitoring for combustible gases.


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The Bragg grating strain gauge is arguably the most frequently quoted example in the literature, finding applications to measure strain and/or temperature in bridges, in composite materials for marine and aerospace engineering and in down-hole pressure and temperature monitoring. Numerous experimental systems based on Bragg grating technology havebeen demonstrated, and while these do illustrate the principles,the grating does remain very expensive equivalent to a straingauge and true commercial activity remains sparse. In contrast, the fiber-optic gyroscope is now a modestly high-volume production subsystem. The gyroscope first appeared in commercial aircraft when the Boeing 777 was introduced, where it was used as an aid to vehicle stabilization. It has also been used in automobiles as a navigation aid, in missiles and munitions for guidance systems, and even in prototype remotely powered lawnmowers for ultrastraight stripes on football pitches. The hydrophone too has begun to make its mark where Mach–Zehnder interferometer configuration can be made more sensitive with better directional control and in more flexible packaging configurations than the piezoceramic equivalent. Distributed sensing also has identified some of its niches. Distributed Raman backscatter has found a place in the Channel Tunnel as a fire-alarm system, in large process ovens for monitoring temperature profiles, and in numerous other very specialized applications where temperature field measurements are important. Brillouin scatter too has been used to measure temperature, watching the concrete set in a major dam project in Switzerland. It has also been used in earnest to measure strain paradoxically in telecommunication cables especially in regions where landslip could cause local stresses which could compromise cable integrity. The Brillouin system also shows promise in the oil industry to monitor the integrity of safety-critical and very expensive anchors and tethers. To attempt an exhaustive list of applications is impractical as well as uninformative. The general observation that the principles first elucidated a quarter of a century ago are now emerging in practice can be applied for many of the techniques which have been briefly mentioned herein. Additionally, every application is manifestly unique and requires quite specific engineering to translate the concept into reality. Fiber sensing has emerged as a true parallel to everything else in the sensing and measurement industry with relatively small and specialized market opportunities each with its own specific challenges. X. WHAT OF THE FUTURE Guided-wave optics, particularly fiber systems, continues to offer unique possibilities in a measurement context. Where this will lead depends

particularly on the initiatives of the research community. There remains considerable activity in chemical sensing with distributed measurement as one of many major interests. Tapered fibers have emerged as means to monitor intracellular chemistry with significantly submicron resolution. The same resolution too could be applied to measuring material properties in microengineered structures, though these have yet to be demonstrated. There are also prospects for the very large and gravitational telescopes, based on an enormous Sagnac interferometer, have been proposed though their realization is probably unlikely. So most of the work in fiber-optic sensors is now focused on developmental opportunities emerging from the very productive era from the mid 1970s to the mid 1980s. But the more speculative research will continue, and at least some will lead into demanding, exotic, innovative measurements.

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Univ. Press, 1999. [2] C. K. Kao and G. Hockham, “Dielectric fiber

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[3] J. C. Simon and E. Spitz, “Propagation guidée de lumière coherente,”Commun. à la SocietéFrançaise de Physique, vol. 24, no. 2, pp.149–169, 1963.

[4] R. B. Dyott and J. R. Stern, “Group delay in glass fiber waveguides,” inIEE Conf. Trunk Telecommunications by Guided Waves, London, U.K.,Sept.-Oct. 1970, pp. 176–181.

[5] C. Menadier, C. Kissinger, and H. Adkins, “The fotonic sensor,” Instrumentsand Control Systems, vol. 40, p. 114, 1967.

[6] E. Snitzer, “Apparatus for controlling the propagation characteristics ofcoherent light within an optical fiber,” U.S. Patent 3 625 589, Dec. 7,1971.

[7] B. Culshaw, Optical Fiber Sensing and Signal Processing. Stevenage,U.K.: Peregrinus, 1984.

[8] B. Culshaw, D. E. N. Davies, and S. A. Kingsley, “Acoustic sensitivityof optical fiber waveguides,” Electron. Lett., vol. 13, pp. 760–761, 1977.

[9] J. A. Bucaro, H. D. Dardy, and E. F. Carome, “Fiber optic hydrophone,”J. Acoust. Soc. Amer., vol. 52, p. 1302, 1977.

[10] G. Sagnac, “L’étherlumineuxdémontre par l’effet du vent relatifd’étherdans un interférometer en rotation uniform,” C. Royal Acad. Sci., vol. 95,pp. 708–710, 1913.


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[11] V. Vali and R. W. Shorthill, “Fiber ring interferometer,” Appl. Opt., vol.15, pp. 1099–1100, 1976.

[12] A. J. Rogers, “POTDR: A technique for the measurement of field distributions,”Appl. Opt., vol. 20, pp. 1060–1074, 1981.

[13] M. C. Farries and A. J. Rogers, “Distributed sending using stimulatedRaman interaction in a monomode optical fiber,” in Proc. 1st OpticalFiber Sensors Conf., London, U.K., 1983, pp. 121–133.

[14] D. Culverhouse, F. Farahi, C. N. Pannell, and D. A. Jackson, “Exploitationof stimulated Brillouin scattering as a sensing mechanism for distributedtemperature sensors,” in Optical Fiber Sensors, Arditty, Dakin,and Kersten, Eds. New York: Springer Verlag, 1989, pp. 552–559.

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[16] B. Culshaw, G. Stewart, C. Tandy, and D. Moodie, “Fiber optic techniquesfor methane gas detection-from detection concept to system realization,”Sensors and Actuators B Chemical, vol. 51, pp. 25–37, 1998.

[17] J. P. Dakin, “Analogue and digital extrinsic optical fiber sensors basedon spectral filtering techniques,” in Proc. Fiber Optics London, 1984,pp. 219–226.

[18] S. Venkatesh and B. Culshaw, “Optical activated vibration in a micromachinedsilica structure,” Electron. Lett., vol. 21, pp.

[19] J. I. Peterson, S. R. Goldstein, and R. V. Fitzgerald, “Fiber optic pHprobe for physiological use,” Anal. Chem., vol. 52, p. 864, 1980.

[20] A. M. Scheggi, “Optical fiber sensors in medicine,” in Proc. 2nd OpticalFiber Sensors Conf., 1984, pp. 91–102.

[21] A. Mohebati and T. A. King, “Remote detection of gases by diode laserspectroscopy,” J. Modern Opt., vol. 38, pp. 319–324, 1988.

[22] L. S. Rothman et al., “The HITRAN molecule database editions of 1991and 1992,” J. Quantum Spectroscopic Radiation Transfer, vol. 48, pp.469–507, 1991.

[23]B. Culshaw and J. P. Dakin, Eds., Optical Fiber Sensors. Norwood, MA: Artech, 1988, 1989, 1996, 1997, vol. 1–4.

[24] E. Udd, Fiber Optic Sensors. New York: Wiley, 1990.


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