of benefits in detecting stimuli ranging from pressure to magnetism. After eight ... A fiber sensor is simply a length of optical fiber that modu.. computer overa fiber ...
Optical-fiber sensors challenge the competition Resistance to corrosion and immunity to interference head the list of benefits in detecting stimuli ranging from pressure to magnetism After eight years of development, optical-fiber sen-
sors are no longer brash upstarts, newcomers in the neighborhood. Through alteration of light by external stimuli, these sensors can now detect virtually any stimulus that other sensors can-from pressure and magnetism to acidity and acceleration-and
often more sensitively and accurately, over a wider dynamic range, Optical-fiber sensors are more rugged and more resistant to corrosion than other sensors; immune a-...:~a-.;lr..to electromagnetic interference, and, of course, compatible with optical-fiber telemetry.Several types of fiber sensors are in commercial production, others are being field-tested, and still others are in advanced development. Whereasdevelopersused to concentrate on demonstrating the sensors' superior sensitivity, they now emphasize improved packaging, cost, and reliabilit)'-signs of a maturing technology.
Commercial
IISes
are growing
Opucal-fibersensorsare making their firstcommercialappear-
ances in measuring instruments and controls where such attributes as sensitivity, resistance to hostile environments, and compactness are essential. Optical-fiber sensors for measurements of temperature. pressure, acceleration, flow of liquids and gases, and the level of a liquid in a container are commercially available. Several machinery manufacturers, for example, offer fiber sensors as options in control systems. Another important application is medical probes, where fiber sensors may bethe technologyof the future. They are small, compatible with livingtissue,and require no electric wiring (see "New sensors for medical researchers," p, 48J. Eventually invasive medical probes with fiber sensors may be as commonplace as endoscopes in clinics and laboratories. The next big application is likely to be all-fiber control systerns in whichmany, varied fiber sensors feed signals to a central computer overa fiber network. In these "passive" systems, there will be no intervening conversion of light signals to electronic signals. Aircraft, ships, power plants, and manufacturing processes are prime candidates for all-fiber control systems.
High cost, drift still Q problem Relative to some other sensors, however, optical-fiber sensors still have limitations. Several fiber sensors are more costly today than the technology they are expected to replace, while others suffer from drift or linearity problems, as the fiber gyro does. Many sensor development programs are under way in Japan, Europe. and North America to address these problems. The IngallsShip Building Co.•in Pascagoula, Miss.• is developing a suite of fiber sensors for measuring temperature, smoke. displacement,
ThonJQS G. Giallorenzi, Joseph A. Bucaro,
Anthony Dandridge, and James H. Cole U.S. Naval Research Laboratory
44
torque, and other variables for machine control and damage control systems in ships. In Great Britain, Standard Telecommunication Laboratories in Harlow, Essex, is working on fiber sensors for flow, pressure, machine vibration, and marine oil pollutionaThe BoeingMilitary Airplane Co, in Huntsville, Ala., the GeneralElectric Co. 's Aircraft Engine Business Group in Cincinnati, Ohio, and the Hamilton Standard Div,of United 'Iechnologies in Farmington, ~...a...;..,..:.....;............ Conn., are developing fiber sensors for night and engine control in Navy and Army aircraft.
New weapon against oil pollution
Fiber sensors for monitoring oil spills have proved more accuratethan other devices, and they can distinguish betweenmarine oil pollutants and solid pollutants. Such devices from Standard
Telecommunication laboratories are sailing on 1000 vessels. The sensors measure the light scattered by oil in water. Multiplewavelength sensors to identify different solid pollutants are now under development. Further in the future are military surveillance and navigation systemsbased on highly sensitive fiber devices, like hydrophones, magnetometers (Fig. lA and IB), and gyroscopes. Although such experimental devices have performed according to expectations in laboratory demonstrations. it will take time to adapt them to military specifications and to manufacture them, build up inventories, and phase out older sensor systems-perhaps as long as 1S years. Further development is required for passive optical telemetry, which would allow a single system to handle navigation, fire and damage control, pollution monitoring, and engine and climate control (Fig. 2).
Sensing by modulation
A fiber sensor is simply a length of optical fiber that modu.. lates the light passing through it in some way. The sensor con-
Defining terms Cantilever (.ensor): employing the mechanical advantage of a lever. Hydrophone: underwater acoustic microphone. Interferometric (sensor): based on interference between two light beams. LaMr Doppler veloclmetry: measurement of a fluid velocity by the detection of a Doppler shift of laser light scattered by the fluid. Magnetaatractlve (material): characterized by changes In dimensions when exposed to a magnetic field. sensing head: the portion of a sensor that is affected by the sensed stimulus. Single-mode fiber: an optical fiber that supports only a single mode of light propagation through It.
OO18·923SI86/0900-0044S1.00@1986 IEEE
IEEE SPECTRUM
SEPTEMBER 1986
[I] Thetubularelectronicsptlckagefor an optical-fiber hydrophone (A) contains a laser, laserdrivecircuitry, photodetectors, and a preamplifier. The package is connected to two optical-fibersensors. The large red ovalsensor hasa directional or omnidirectional response depending on whether thesensor'sdimensions areclose to orsignificantly smallerthan the acoustic wavelength. The smallerroundsensor is a high-frequency device, measuring up to I MHz. The components of the fiberoptic gradiometer (B) aresimilar, except that the elementsaremagnetically sensitive instead of acoustically sensitive.
Toform magneticelements, fiber is bonded to a
strip of magnetostrictive metallic glass (visible in cutawayright-hand coil). A magnetic field creates strain in the metallic glass, which transfers it to thefiber. The two wire coils on thegradiometer generate Q high-frequency magneticfield. which allows measurement of slowly varying fields. sists of a core of glass and a cladding of similar material having a slightly different index of refraction, so light is guided along the core. A light-emitting diode or laser at one end of the fiber provides light, and a photodetector at the other end collects it. Most fiber sensors in production are based on intensity modulation of light, either outside the fiber or within it. At present fibers in which the modulation is done externally [Fig. 3A] predominate commercially. A device distinct from the fiber-a material or mechanical structure that is sensitive to some external stimulus -modulates light transmitted by the fiber. Sensitive microphones and accelerometers with external elastic sensing elements have been built. 'Iemperature, current, magnetism, water pollution, displacement, torque, position, and radiation are other examples of phenomena measured with externalmodulation sensors. Three classes of fiber sensors are based on internal modulation. The phenomenon to be measured modulates the light as it travels through the fiber itself. The simplest of these classes functions by intensity modulation [Fig. 3D]. Radiation sensors were among the earliest of this class. A radiation dosimeter was the first fiber-optic sensor incorporated in a space system-the Navigation Technology Satellite 2, flown in 1978. Developed at the U.S. Naval Research Laboratory in Washington, D.C., it consisted of a glass fiber that darkened in a known way when irradiated. Other sensors based on internal intensity modulation include temperature, liquid-level, and pollution monitors. In one such sensor, the cladding is stripped from a portion of the fiber so light can pass out of the core. The amount that leaves depends on the refractive index of the medium surrounding the stripped fiber. Battelle Research Institute in Geneva, Switzerland, has demonstrated an inexpensive battery charge indicator in which the refractive index of the electrolyte solution-and the optical coupling between the electrolyte and the fiber-varies with the charge.
The silver lining oj microbends A variation on internal-modulation sensors is the microbendloss sensor. Microbends are small, lateral deformations in fibers
Giallorenzi, Bucaro, Dandridge. Cole-Optical fiber sensors challenge the competition
that cause some radiation of light from the fiber. As a result, microbends are a problem in fiber-optic telecommunicationas few as 100 of them only micrometers long in 1 kilometer of fiber can attenuate the light signal by as much as 20 decibels. In sensors, mechanical deformers are used to create microbends intentionally. Depending on the deformer design and material, it is possible to make acoustic, magnetic, electric, temperature, acceleration, and displacement sensors. The Naval Research laboratory, Optech Inc. in Herndon, Va., and others have built sensors in which displacements as small as 10- 6 micrometers can be detected-a high enough sensitivity for measuring most phenomena. Another internal-modulation class is the interferometric fiber sensor, which operates on the principle that a perturbation shifts the phase of light propagating in a single-mode fiber. In the twofiber, or Mach-Zehnder, version [Fig. 3Q, the phase in one fiberthe sensing fiber-is compared with that in a reference fiber to determine the change in phase and therefore the magnitude of the perturbation. The selection of an appropriate material and thickness for a jacket around the sensing fiber is the key here. The jacket responds to a perturbation by expanding or contracting, thereby altering the length of the fiber and inducing strain in it. This strain changes the index of refraction. The changes in length and index of refraction stimulate the jacket's response. Researchers are studying hundreds of plastic, rubber, epoxy, and metal materials that can be extruded or dip-coated on a fiber to form the jacket. They have come up with formulations tailored to specific stimuli and combinations of stimuli. For example, they have developed elastic jacket materials that are relatively insensitive to temperature and pressure-characteristics well suited to acoustic sensors that must operate over a wide range of conditions. The researchers have also identified polymers with elastic properties that make the polymers useful as high- and low-pass acoustic filters. 45
Interferometric two-fibersensors for chemicals have been made with a jacket that shrinks or expands when it absorbs a specific chemical. Researchers have built one such sensor, for the detection of hydrogen. by coating the fiber with palladium. Optical-fi. ber sensors for flammablechemicals are especiallysafe.since they do not generate sparks. Yetanother major class of fiber sensors is single-fiber interferometers, 'lWocounterrotating beams propagate through a fiber loop [Fig. 3DJ. If the loop is rotated, the beams experiencedifferent phase shifts. which can be measured as an indication of the
of their immunity to electrical noise and their small size. they could be beneficial in the windings of motors to monitor temperature. At the chlorine production plants of Norsk Hydro in Sweden, fiber-optic sensors reliably monitor the temperature ofelectrochemical cells in an environment of electrical noise. explosive hydrogen. and corrosive gases and liquids. They provide input to a controller that maintains the cells within a few degrees of
the optimum value. Experimental temperature sensors of the two-fiber interferometricclass are incredibly sensitive, detecting variations as small as a millionth of a degree. They can also be designed to respond to temperature fluctuations many times higher in frequency than those measurable with other technologies. A jacket material with a high thermal expansion and high conductivity is coated over the fiber. Fiber-optic pressure sensors are available commercially from companies like EOTec.. However, they have not been widely used for two reasons: inexpensive conventional pressure sensors with adequate resolution are readily available, and the number of pres.. sure sensors needed for instruments and controls is surprisingly small compared with the number of sensors for temperature, liquid level, and flow.
loop's angular velocity, as in the fiber-optic gyroscope.The gyro
is inexpensive and rugged and has the potential for substantially higher performance than a ring laser gyro. In general, sensorsemplO)ing multimode fibers and light-emitting diodes [Fig. 3" typesA and B)are in the most advancedstages of development. Sensors employing single-mode fibers and laser sources (types C and DJ have largely been limited to laboratory and field demonstrations, because they are still costly and hard to make. However, the price of 3-.dD couplers, a key component of single-mode sensors, recently dropped below $75 a unit. The predicted 530 coupler is not far off.
The basics: temperature and pressure
Acceleration and vibration monitoring likely
Fiber temperature sensors have been available commercially for many years. The early type carries infrared energy radiatec into it by a high-temperatureprocess to a detector. It can measure
Fiber-optic acceleration sensors may have two important uses. both exploiting the nonelectrical nature of these devices. As vibration monitors in generators, fiber sensors guard against costly equipment damage, and they are immune to the high electrical noise that plagues piezoelectric vibration sensors. And in explosive atmospheres in mines or on offshore oil drilling platforms, fiber sensors could safely monitor equipment vibration. American High Voltage lest Systems Inc, in Accident, Md., markets an acceleration sensor based on reflection from a tiny cantilevered beam at the end of a fiber. The photoluminescence of neodymium glass in the fiber produces a compensation signal that is processed to eliminate the effects of losses in the fiber and connectors. Laboratory prototypes of acceleration sensors based on interferometry have greater sensitivity and bandwidth than those of the cantilevered beam type.
over a range of 300 to 2000°C, although the ambient temperalure of the sensor must be lower. Hughes Research Laboratoryin Malibu, Calif., recently demonstrated an optical fiber that can withstand ambient temperatures of 650°C or more. The Hughes fiber contains no polymers, does not give off smoke or toxic fumes, and is ideal for high-tempera-
ture sensing. More recently the LuxtronCorp., of Mountain View, Calif., introduced its Fluoroptic thermometer, based on the temperaturedependent fluorescenceof materials at the end of a fiber-optic probe. Other sensors usetemperature.. dependent effects in semiconductors, liquid crystals, or refractive polymers. These externalmodulation sensorsare accurate within ± 0.2 OC and havea range of several hundred degrees. The EOlec Corp., in West Haven, Conn." offers a fiber-optic switch based on a bimetallic element
Measuring flow and liquid level
Four characteristics are used to measure flow with fiber sensors: • Differential pressure across orifice plates.
that acts as a shutter.
Innovative usesof fiber temperature sensors abound. Because
/2/ A ship 'with an integra/eli all-opticalsensor and telemetry system shows the wide range of stimuli that can be measured for control and monitoring. Not just sensors. but multiplexing techniques, will need development b(fOTP such Q systemcon be built, but researchers believe it is feasible. Three ofthe many types ofsensors Off' highlighted. In a liquid-level sensor; light propagates along the fiber when the fiber is surrounded by air and escapes through the cladding when the fiber is immersed in the liquid. Light from the input fiber through an aperlUI? to the smoke detector is scattered by particulates. collected by Q micro/ens. and injected into the return fiber: In the pressure sensor; light from the inputfiber is reflected from Q suvered diaphragm. The fraction of the reflected light collected by the exitfiber depends on the diaphragm position. which in turn depends on the ambient pressure.
Smoke detector
Pressure sensor
,1
:,J'tet,~thefluorescenceofllvtngtlssuelnstudles
'ofG2ddattorH'eduetlon reactions.
r:c.r~turemeaaurements. fiber .mlcroprobes
"areW'PrJed with traneduceraat thelrtlpa.Ina sensor frOmu.a.tron -,...01
.1
~ Moun"'n View, Callt, the tumlnes-
aCtyataI at the mtcropfotMia t.p
temperature-
,~Llhe t8mperaturerangeoftttese thetm0m8--.·""5Oto +200OC. and they are accurate within :*O:.~.8Ut the'reostremalns blgh.
,',-: '~Ictd sensors arealso being usedto monitorIntra· ~'~lOvaScularo
urethral, and rectal presaure.
',encased ina catheterandoperates .by.dlsplacement of a membrane or other part of the t'fl' ~ A ",madeby LaddAeaearchLaboratories of 8ur~ Ilnotot\.,Vt..for use-In the skuUemplGY8 a cantilevered mlrroiattacbadto. Membrane·tnthe tip; with deflec~:.flber
tlon,~f themembtant\
••0"'.
the mirror reflects light from a
.CtW\l.... flber.toflbera.urroundlng It. Differences In the
In the collecting fiber8 Indicate the magnitude of,U.deflectlon and the pressure.Theworldng range 110to 300millimeters ofmeteury, accuratewlthln:tO_5 percent.. 'n. variationolthis device-light ia reflected ~tytromth8 membrane rather than the cantilevered mirror~
~ •.1 8ftIOf8 forblomedtcal use.,. usualty con-
atNoted with a permeable membrane on oneendof the flbWprobe;lhemembraneeontalna a f8Yersiblelndi-
catorJ,",t,responds to.chem'cal stimulua by.changIna tt.~ton or luminescence. Forexample, a pH ~changesfrom9~nglnabasetobtu.
abIOfbtng Inanackt4hcanmeuurepH over the .bJoIogleat""'(7 to 7.4)towltNn :to.ootStnsonsfor oxygen and()8rbon dioxide are stillexperimentaJ, but they are
.prom'.lng .formonitoring metabolism and resplraUon. Inone'~ygen
·aensor.the fluorescence of a dye is
~. by the presence Ofthe commondiatomicoxy-
,.' 08ft~ItI. nomore than 500micrometers Indiameter. A , .~·forcarbon dloxkle combines an lndlcator dye .",Ith",inaolubl.matrlx Inside a g. .permeabl.. lo~ Impermeable slUconemembrane. , . -7:0.G.• J.A.&. A.I1.J.H.c.
48
the McDonnell Douglas Astronautics Co. in Huntington Beach. Calif., are addressing multiplexed sensors, but so far only small arrays with a few sensors have been built. Fiber devices based on the Faraday effect are useful for sensing large magnetic fields-those with flux densities greater than about 10- 4 testa. In these devices, the fiber rotates the polarization of light in proportion to the field. For magnetic flux densities down to 0.1 nanotesla,two-fiber interferometric sensors, with magnetostrietive jackets, are used. Strain induced in the jacket by a magnetic field is transferred to the fiber. Fibers coated with or bonded to magnetostrictive materiallike iron, cobalt, nickel, or metallic glass, which change dimensions in a magnetic field-make sensitive magnetic field and fieldgradient sensors. Gradient sensors are used-typically in pairs-to cancel out background geomagnetic fields which vary slowly with distance between the fibers. The magnetostrictive effect is nonlinear. To get closer to a linear device, researchers biased the magnetometers so that the jackets operated in the most linear (and most sensitive) part of the response. Demodulation of light in these sensors allowed the researchers to measure accurately magnetic fields varying at rates from several kilohertz down to about 10 hertz. At lower frequencies, thermal drift in the interferometer introduced error. However, in many cases magnetic measurements are needed mostly for fields varying at a slow rate-about 1 Hz. The Naval Research Laboratory therefore developed a heterodyning technique, in which an external high-frequency magnetic field is superimposed on the slowlyvarying field. This technique utilizes the nonlinear response of the material. The resulting fiber strain is proportional to the product of the two fields. As in radio heterodyning, the slow field is lifted out of the noisy, low-frequency band. A drawback to the technique is that the high-frequency field must beapplied to the magnetostrictive material, requiring electric current close to the sensing head. With feedback to stabilize the interferometer and the magnetic circuit, the laboratory sensor is highly stable. Variations amount to less than 1 percent over many days. Steady flux densities as small as 10-'10 tesla and varying fields as small as 10- 12 T have been measured. Eventually measurements of fields as small as 10 -14 T may be achieved at room temperature. Unlike the acoustic sensor, only a few packaged versions of the magnetic sensor have yet been tested in the field . The fiber gyro is still in an earlier state of development than the technologies with which it must compete. Dynamic range has to be increased, and drift has to be reduced. But it is reasonable to expect that the fiber gyro will be a commercial product in five to seven years. Research at the Massachusetts Institute of Technology, the Naval Research Laboratory, Stanford University in California, Thomson CSF in France, and other organizations is aimed not only at improving sensitivity but also at achieving a stable, lowdrift performance by eliminating noise mechanisms. As in the hydrophone and the magnetometer, the output has to be demodulated to linearize it and to maximize the sensitivity. A variety of demodulation schemes are being evaluated, including Bragg cells, in which an acoustic wave modulates the frequency of light passing through the cell; time-varying phase shifts by an integrated optical modulator; phase nullers; and electronic signal processors. Nearly all fiber gyro applications pose difficult packaging problems. The gyros have to fit into volumes ranging from about 0.03 cubic meter in airplanes to a few cubic centimeters in missiles. An all-fiber system created at Stanford University eases package design. In this system, light never leaves the fibers; alignment of optical components is unnecessary, and deterioration in performance because of vibration or thermal cycling is eliminated. The McDonnell Douglas Corp. has developed a rugged all-fiber gyro for logging data at oil wells. The gyro can operate to depths of 600 meters (2000 feet) at temperatures ranging from 0 to 125°C. It can withstand shocks of 100 g for 0.5 millisecond and vibraIEEE SPECTRUM SEPTEMBER 1986
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Sensing element
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