Fiber Optic Sensors

Fiber Optic Sensors

Fiber Optic Sensors

edited by

Francis To So Yu

Shizhuo Yin

The Pennsylvania State University

Un ive rsity Park, Pen ns y Ivan ia

ISBN: 0-8247-0732-X

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OPTICAL ENGINEERING

Founding Editor

Brian J Thompson

University of Rochester Rochester, New York

Editorial Board

Toshimitsu Asakura

Hokkai-Gakuen UniversitySapporo, Hokkaido, Japan

David S Weiss

Heidelberg Digital L.L.C.Rochester, New York

1 Electron and Ion Microscopy and Microanalysis: Principles and Applications,

Lawrence E Murr

2 Acousto-Optic Signal Processing: Theory and Implementation, edited by Nor

man J Berg and John N Lee

3 Electro-Optic and Acousto-Optic Scanning and Deflection, Milton Gottlieb, Clive

L M Ireland, and John Martin Ley

4 Single-Mode Fiber Optics: Principles and Applications, Luc B Jeunhomme

5 Pulse Code Formats for Fiber Optical Data Communication: Basic Principles and

Applications, David J Morris

6 Optical Materials: An Introduction to Selection and Application, Solomon

Musikant

7 Infrared Methods for Gaseous Measurements: Theory and Practice, edited by

Joda Wormhoudt

8 Laser Beam Scanning: Opto-Mechanical Devices, Systems, and Data Storage

Optics, edited by Gerald F Marshall

9 Opto-Mechanical Systems Design, Paul R Yoder, Jr.

10 Optical Fiber Splices and Connectors: Theory and Methods, Calvin M Miller with

Stephen C Mettler and Ian A White

11 Laser Spectroscopy and Its Applications, edited by Leon J Radziemski, Richard

W Solarz, and Jeffrey A Paisner

12 Infrared Optoelectronics: Devices and Applications, William Nunley and J Scott

Bechtel

13 Integrated Optical Circuits and Components: Design and Applications, edited by

Lynn D Hutcheson

14 Handbook of Molecular Lasers, edited by Peter K Cheo

15 Handbook of Optical Fibers and Cables, Hiroshi Murata

16 Acousto-Optics, Adrian Korpel

17 Procedures in Applied Optics, John Strong

18 Handbook of Solid-State Lasers, edited by Peter K Cheo

19 Optical Computing: Digital and Symbolic, edited by Raymond Arrathoon

20 Laser Applications in Physical Chemistry, edited by D K Evans

21 Laser-Induced Plasmas and Applications, edited by Leon J Radziemski and

David A Cremers

22 Infrared Technology Fundamentals, Irving J Spiro and Monroe Schlessinger

23 Single-Mode Fiber Optics: Principles and Applications, Second Edition, Re vised

and Expanded, Luc B Jeunhomme

24 Image Analysis Applications, edited by Rangachar Kasturi and Mohan M Trivedi

25 Photoconductivity: Art, Science, and Technology, N V Joshi

26 Principles of Optical Circuit Engineering, Mark A Mentzer

27 Lens Design, Milton Laikin

28 Optical Components, Systems, and Measurement Techniques, Rajpal S Sirohi

and M P Kothiyal

29 Electron and Ion Microscopy and Microanalysis: Principles and Applications,

Second Edition, Revised and Expanded, Lawrence E Murr

30 Handbook of Infrared Optical Materials, edited by Paul Klocek

31 Optical Scanning, edited by Gerald F Marshall

32 Polymers for Lightwave and Integrated Optics: Technology and Applications,

edited by Lawrence A Hornak

33 Electro-Optical Displays, edited by Mohammad A Karim

34 Mathematical Morphology in Image Processing, edited by Edward R Dougherty

35 Opto-Mechanical Systems Design: Second Edition, Revised and Expanded, Paul

R Yoder, Jr.

36 Polarized Light: Fundamentals and Applications, Edward Collett

37 Rare Earth Doped Fiber Lasers and Amplifiers, edited by Michel J F Digonnet

38 Speckle Metrology, edited by Rajpal S Sirohi

39 Organic Photoreceptors for Imaging Systems, Paul M Borsenberger and David

S Weiss

40 Photonic Switching and Interconnects, edited by Abdellatif Marrakchi

41 Design and Fabrication of Acousto-Optic Devices, edited by Akis P Goutzoulis

and Dennis R Pape

42 Digital Image Processing Methods, edited by Edward R Dougherty

43 Visual Science and Engineering: Models and Applications, edited by D H Kelly

44 Handbook of Lens Design, Daniel Malacara and Zacarias Malacara

45 Photonic Devices and Systems, edited by Robert G Hunsberger

46 Infrared Technology Fundamentals: Second Edition, Revised and Expanded,

edited by Monroe Schlessinger

47 Spatial Light Modulator Technology: Materials, Devices, and Applications, edited

by Uzi Efron

48 Lens Design: Second Edition, Revised and Expanded, Milton Laikin

49 Thin Films for Optical Systems, edited by Francoise R Flory

50 Tunable Laser Applications, edited by F J Duarte

51 Acousto-Optic Signal Processing: Theory and Implementation, Second Edition,

edited by Norman J Berg and John M Pellegrino

52 Handbook of Nonlinear Optics, Richard L Sutherland

53 Handbook of Optical Fibers and Cables: Second Edition, Hiroshi Murata

54 Optical Storage and Retrieval: Memory, Neural Networks, and Fractals, edited by

Francis T S Yu and Suganda Jutamulia

55 Devices for Optoelectronics, Wallace B Leigh

56 Practical Design and Production of Optical Thin Films, Ronald R Willey

57 Acousto-Optics: Second Edition, Adrian Korpel

58 Diffraction Gratings and Applications, Erwin G Loewen and Evgeny Popov

59 Organic Photoreceptors for Xerography, Paul M Borsenberger and David S.

Weiss

60 Characterization Techniques and Tabulations for Organic Nonlinear Optical

Materials, edited by Mark G Kuzyk and Carl W Dirk

61 Interferogram Analysis for Optical Testing, Daniel Malacara, Manuel Servin, and

Zacarias Malacara

62 Computational Modeling of Vision: The Role of Combination, William R Uttal,

Ramakrishna Kakarala, Spiram Dayanand, Thomas Shepherd, Jagadeesh Kalki, Charles F Lunskis, Jr., and Ning Liu

63 Microoptics Technology: Fabrication and Applications of Lens Arrays and

De-vices, Nicholas Borrelli

64 Visual Information Representation, Communication, and Image Processing,

edited by Chang Wen Chen and Ya-Qin Zhang

65 Optical Methods of Measurement, Rajpal S Sirohi and F S Chau

66 Integrated Optical Circuits and Components: Design and Applications, edited by

Edmond J Murphy

67 Adaptive Optics Engineering Handbook, edited by Robert K Tyson

68 Entropy and Information Optics, Francis T S Yu

69 Computational Methods for Electromagnetic and Optical Systems, John M.

Jarem and Partha P Banerjee

70 Laser Beam Shaping, Fred M Dickey and Scott C Holswade

71 Rare-Earth-Doped Fiber Lasers and Amplifiers: Second Edition, Revised and

Expanded, edited by Michel J F Digonnet

72 Lens Design: Third Edition, Revised and Expanded, Milton Laikin

73 Handbook of Optical Engineering, edited by Daniel Malacara and Brian J.

Thompson

74 Handbook of Imaging Materials: Second Edition, Revised and Expanded, edited

by Arthur S Diamond and David S Weiss

75 Handbook of Image Quality: Characterization and Prediction, Brian W Keelan

76 Fiber Optic Sensors, edited by Francis T S Yu and Shizhuo Yin

77 Optical Switching/Networking and Computing for Multimedia Systems, edited by

Mohsen Guizani and Abdella Battou

78 Image Recognition and Classification: Algorithms, Systems, and Applications,

edited by Bahram Javidi

79 Practical Design and Production of Optical Thin Films: Second Edition, Revised

and Expanded, Ronald R Willey

80 Ultrafast Lasers: Technology and Applications, edited by Martin E Fermann,

Almantas Galvanauskas, and Gregg Sucha

81 Light Propagation in Periodic Media: Differential Theory and Design, Michel

82 Handbook of Nonlinear Optics, Second Edition, Revised and Expanded,

Richard L Sutherland

Additional Volumes in Preparation

Optical Remote Sensing: Science and Technology, Walter Egan

In the past two decades, the fiber optic sensor has developed from the perimental stage to practical applications For instance, distributed fiber opticsensors have been installed in dams and bridges to monitor the performance

ex-of these facilities With the rapid advent ex-of optical networks, the cost ex-of fiberoptic sensors has substantially dropped because of the commercially viablekey components in fiber optic communications such as light sources andphotodetectors We anticipate that fiber optic sensors will become a wide-spread application in sensing technology

This text covers a wide range of current research in fiber optic sensors,although it is by no means complete Each of the 10 chapters is written by anauthority in the field.Chapter 1gives an overview of fiber optic sensors thatincludes the basic concepts, historical development, and some of the classicapplications This overview provides essential documentation to facilitate theobjectives of later chapters

Chapter 2deals with fiber optic sensors based on Fabry–Perot ferometers The major merits of this type of sensor include high sensitivity,compact size, and no need for fiber couplers Its high sensitivity and multi-plexing capability make this type of fiber optic sensor particularly suitable forsmart structure monitoring applications

inter-Chapter 3introduces a polarimetric fiber optic sensor With tion, a guided lightwave of a particular fiber can be changed through externalperturbation, which can be used for fiber sensing Thus, by using a polar-ization-maintaining fiber, polarization affecting the fiber can be exploited forsensing applications One of the major features of this type of sensor is that itoffers an excellent trade-off between sensitivity and robustness

polariza-Chapter 4reviews fiber-grating-based fiber optic sensors Fiber gratingtechnology (Bragg and long-period gratings) is one of the most importantachievements in recent optic history It provides a powerful new component in

a variety of applications including dispersion compensations and spectralgain control (used in optics communications) In terms of fiber optic sensorapplications, in-fiber gratings not only have a very high sensitivity but alsoprovide distributed sensing capability due to the easy implementation ofwavelength division multiplexing

Chapter 5introduces distributed fiber optic sensors One of the uniquefeatures of fiber optic sensors is the distributed sensing capability, whichmeans that multiple points can be sensed simultaneously by a single fiber Thiscapability not only reduces the cost but also makes the sensor very compact.Thus, many important applications such as structure fatigue monitoring (e.g.,monitoring the performances of dams and bridges) can be implemented in aneffective way Both continuous and quasi-distributed sensors are discussed.The continuous type of distributed sensor is based on the intrinsic effectexisting in optic fibers (such as Rayleigh scattering) The most widely usedtype is optical time domain reflectrometry (OTDR), which has become anindispensable tool for checking the connections of optics networks

Chapter 6 discusses fiber specklegram sensors Fiber specklegram isformed by the interference between different modes propagated in the multi-mode optics fibers Since this interference is common-mode interference, it notonly has a very high sensitivity for certain environmental perturbations (such asbending) but also has less sensitivity to certain environmental factors (such astemperature fluctuations) Thus, this is a very unique type of fiber optic sensor

Chapter 7introduces interrogation techniques for fiber optic sensors.This chapter emphasizes the physical effects in optic fibers when the fiber issubjected to external perturbations

Chapter 8focuses on fiber gyroscope sensors First, the basic conceptsare introduced The fiber gyroscope sensor is based on the interferencebetween two light beams propagated in opposite directions in a fiber loop.Since a large number of turns can be used, a very high sensitivity can berealized Second, more practical issues related to fiber optic gyroscopes such

as modulation and winding techniques are reviewed It is believed that fiberoptic gyroscopes will be used more and more in many guiding applications(such as flight by light) due to the consistent reductions in their cost

Chapter 9 introduces fiber optic hydrophone systems This chapterfocuses on key issues such as interferometer configurations, inter-rogation=demodulation schemes, multiplexing architecture, polarizationfading mitigation, and system integration Some new developments includefiber optic amplifiers, wavelength division multiplexing components, opticalisolators, and circulators

The last chapter discusses the major applications of fiber optic sensors.

Chapter 10 covers a variety of applications used in different areas such asstructure fatigue monitoring, the electrical power industry, medical andchemical sensing, and the gas and oil industry Although many types ofsensors are mentioned in the chapter, the focus is on applications of fiberBragg grating sensors

This text will be a useful reference for researchers and technical staffengaged in the field of fiber optic sensing The book can also serve as a viablereference text for engineering students and professors who are interested infiber optic sensors

Francis T S YuShizhuo Yin

Fabry–Perot Interferometer

Henry F TaylorChapter 3 Polarimetric Optical Fiber Sensors

Craig MichieChapter 4 In-Fiber Grating Optic Sensors

Lin Zhang, W Zhang, and I BennionChapter 5 Distributed Fiber Optic Sensors

Shizhuo YinChapter 6 Fiber Specklegram Sensors

Francis T S YuChapter 7 Interrogation Techniques for Fiber Grating

Sensors and the Theory of Fiber Gratings

Byoungho Lee and Yoonchan Jeong

Chapter 8 Fiber Gyroscope Sensors

Paul B RuffinChapter 9 Optical Fiber Hydrophone Systems

G D Peng and P L ChuChapter 10 Applications of Fiber Optic Sensors

Y J Rao and Shanglian Huang

Shanglian Huang Chongqing University, Chongqing, China

Yoonchan Jeong Seoul National University, Seoul, Korea

Byoungho Lee Seoul National University, Seoul, Korea

Craig Michie University of Strathclyde, Glasgow, Scotland

G D Peng The University of New South Wales, Sydney, Australia

Arsenal, AlabamaHenry F Taylor Texas A&M University, College Station, Texas

Shizhuo Yin The Pennsylvania State University, University Park,

Pennsylvania

Francis T S Yu The Pennsylvania State University, University Park,

Pennsylvania

In parallel with these developments, fiber optic sensor [1–6] technologyhas been a major user of technology associated with the optoelectronic andfiber optic communications industry Many of the components associatedwith these industries were often developed for fiber optic sensor applications.Fiber optic sensor technology, in turn, has often been driven by the devel-opment and subsequent mass production of components to support theseindustries As component prices have fallen and quality improvements havebeen made, the ability of fiber optic sensors to displace traditional sensors forrotation, acceleration, electric and magnetic field measurement, temperature,pressure, acoustics, vibration, linear and angular position, strain, humidity,viscosity, chemical measurements, and a host of other sensor applications hasbeen enhanced In the early days of fiber optic sensor technology, mostcommercially successful fiber optic sensors were squarely targeted at marketswhere existing sensor technology was marginal or in many cases nonexistent.The inherent advantages of fiber optic sensors, which include (1) their ability

to be lightweight, of very small size, passive, low-power, resistant to

electromagnetic interference, (2) their high sensitivity, (3) their bandwidth,and (4) their environmental ruggedness, were heavily used to offset theirmajor disadvantages of high cost and end-user unfamiliarity.

The situation is changing Laser diodes that cost $3000 in 1979 withlifetimes measured in hours now sell for a few dollars in small quantities, havereliability of tens of thousands of hours, and are widely used in compact discplayers, laser printers, laser pointers, and bar code readers Single-modeoptical fiber that cost $20=m in 1979 now costs less than $0.10=m, with vastlyimproved optical and mechanical properties Integrated optical devices thatwere not available in usable form at that time are now commonly used tosupport production models of fiber optic gyros Also, they could drop in pricedramatically in the future while offering ever more sophisticated optical cir-cuits As these trends continue, the opportunities for fiber optic sensordesigners to product competitive products will increase and the technologycan be expected to assume an ever more prominent position in the sensormarketplace In the following sections the basic types of fiber optic sensorsbeing developed are briefly reviewed followed by a discussion of how thesesensors are and will be applied

FIBER OPTIC SENSORSFiber optic sensors are often loosely grouped into two basic classes referred to

as extrinsic, or hybrid, fiber optic sensors and intrinsic, or all-fiber, sensors.Figure 1 illustrates the case of an extrinsic, or hybrid, fiber optic sensor

Figure 1 Extrinsic fiber optic sensors consist of optical fibers that lead up to andout of a ‘‘black box’’ that modulates the light beam passing through it in response to

an environmental effect

In this case an optical fiber leads up to a ‘‘black box’’ that impressesinformation onto the light beam in response to an environmental effect Theinformation could be impressed in terms of intensity, phase, frequency,polarization, spectral content, or other methods An optical fiber then carriesthe light with the environmentally impressed information back to an opticaland=or electronic processor In some cases the input optical fiber also acts asthe output fiber The intrinsic or all-fiber sensor shown in Fig 2 uses anoptical fiber to carry the light beam, and the environmental effect impressesinformation onto the light beam while it is in the fiber Each of these classes offibers in turn has many subclasses with, in some cases, sub-subclasses [1] thatconsist of large numbers of fiber sensors.

In some respects the simplest type of fiber optic sensor is the hybrid typethat is based on intensity modulation [7,8] Figure 3 shows a simple closure orvibration sensor that consists of two optical fibers held in close proximity toeach other Light is injected into one of the optical fibers; when it exits, thelight expands into a cone of light whose angle depends on the difference

Figure 2 Intrinsic fiber optic sensors rely on the light beam propagating throughthe optical fiber being modulated by the environmental effect either directly orthrough environmentally induced optical path length changes in the fiber itself

Figure 3 Closure and vibration fiber optic sensors based on numerical aperture can

be used to support door closure indicators and measure levels of vibration inmachinery

between the index of refraction of the core and cladding of the optical fiber.The amount of light captured by the second optical fiber depends on itsacceptance angle and the distance d between the optical fibers When thedistance d is modulated, it in turn results in an intensity modulation of thelight captured.

A variation on this type of sensor is shown in Fig 4 Here a mirror isused that is flexibly mounted to respond to an external effect such as pressure

As the mirror position shifts, the effective separation between the opticalfibers shift with a resultant intensity modulation These types of sensors areuseful for such applications as door closures where a reflective strip, incombination with an optical fiber acting to input and catch the outputreflected light, can be used

With two optical fibers arranged in a line, a simple translation sensorcan be configured as in Fig 5 The output from the two detectors can beproportioned to determine the translational position of the input fiber.Several companies have developed rotary and linear fiber optic positionsensors to support applications such as fly-by-light [9] These sensors attempt

Figure 4 A numerical aperture fiber sensor based on a flexible mirror can be used

to measure small vibrations and displacements

Figure 5 A fiber optic translation sensor based on numerical aperture uses theratio of the output on the detectors to determine the position of the input fiber

(1) to eliminate electromagnetic interference susceptibility to improve safetyand (2) to lower shielding needs to reduce weight Figure 6 shows a rotaryposition sensor [10] that consists of a code plate with variable reflectancepatches placed so that each position has a unique code A series of opticalfibers is used to determine the presence or absence of a patch.

An example of a linear position sensor using wavelength division tiplexing [11] is illustrated by Fig 7 Here a broadband light source, whichmight be a light-emitting diode, is used to couple light into the system A singleoptical fiber is used to carry the light beam up to a wavelength divisionmultiplexing (WDM) element that splits the light into separate fibers that areused to interrogate the encoder card and determine linear position The boxes

mul-on the card of Fig 7 represent highly reflective patches, while the rest of the

Figure 6 Fiber optic rotary position sensor based on reflectance used to measurethe rotational position of the shaft via the amount of light reflected from dark andlight patches

Figure 7 A linear position sensor using wavelength division multiplexing decodesposition by measuring the presence or absence of a reflective patch at each fiberposition as the card slides by via independent wavelength separated detectors

card has low reflectance The reflected signals are then recombined andseparated by a second wavelength division multiplexing element so that eachinterrogating fiber signal is read out by a separate detector.

A second common method of interrogating a position sensor using asingle optical fiber is to use time division multiplexing methods [12] In Fig 8 alight source is pulsed The light pulse then propagates down the optical fiberand is split into multiple interrogating fibers Each of these fibers is arranged

so that the fibers have delay lines that separate the return signal from theencoder plate by a time that is longer than the pulse duration When thereturned signals are recombined onto the detector, the net result is an encodedsignal burst corresponding to the position of the encoded card

These sensors have been used to support tests on military and mercial aircraft that have demonstrated performance comparable to con-ventional electrical position sensors used for rudder, flap, and throttleposition [9] The principal advantages of the fiber position sensors areimmunity to electromagnetic interference and overall weight savings.Another class of intensity-based fiber optic sensors is based on theprinciple of total internal reflection In the case of the sensor inFig 9,lightpropagates down the fiber core and hits the angled end of the fiber If themedium into which the angled end of the fiber is placed has a low enoughindex of refraction, then virtually all the light is reflected when it hits themirrored surface and returns via the fiber If, however, the medium’s index ofrefraction starts to approach that of the glass, some of the light propagatesout of the optical fiber and is lost, resulting in an intensity modulation.This type of sensor can be used for low-resolution measurement ofpressure or index of refraction changes in a liquid or gel with 1% to 10%

com-Figure 8 A linear position sensor using time division multiplexing measure decodescard position via a digital stream of ons and offs dictated by the presence or absence

of a reflective patch

accuracy Variations on this method have also been used to measure liquidlevel [13], as shown by the probe configuration of Fig 10 When the liquidlevel hits the reflecting prism, the light leaks into the liquid, greatly attenu-ating the signal.

Confinement of a propagating light beam to the region of the fiber coresand power transfer from two closely placed fiber cores can be used to produce

a series of fiber sensors based on evanescence [14–16].Figure 11illustrates twofiber cores that have been placed in close proximity to one another For single-mode optical fiber [17], this distance is on the order of 10 to 20 microns.When single-mode fiber is used, there is considerable leakage of thepropagating light beam mode beyond the core region into the cladding ormedium around it If a second fiber core is placed nearby, this evanescent tail

Figure 9 Fiber sensor using critical angle properties of a fiber for pressure=index ofrefraction measurement via measurements of the light reflected back into the fiber

Figure 10 A liquid-level sensor based on the total internal reflection detects thepresence or absence of liquid by the presence or absence of a return light signal

will tend to couple to the adjacent fiber core The amount of coupling depends on a number of parameters, including the wavelength oflight, the relative index of refraction of the medium in which the fiber cores areplaced, the distance between the cores, and the interaction length This type offiber sensor can be used for the measurement of wavelength, spectral filtering,index of refraction, and environmental effects acting on the medium sur-rounding the cores (temperature, pressure, and strain) The difficulty with thissensor that is common to many fiber sensors is optimizing the design so thatonly the desired parameters are sensed.

cross-Another way that light may be lost from an optical fiber is when thebend radius of the fiber exceeds the critical angle necessary to confine the light

to the core area and there is leakage into the cladding Local microbending ofthe fiber can cause this to occur, with resultant intensity modulation of lightpropagating through an optical fiber A series of microbend-based fibersensors has been built to sense vibration, pressure, and other environmentaleffects [18–20].Figure 12shows a typical layout of this type of device con-sisting of a light source, a section of optical fiber positioned in a microbendtransducer designed to intensity-modulate light in response to an environ-mental effect, and a detector In some cases the microbend transducer can beimplemented by using special fiber cabling or optical fiber that is simplyoptimized to be sensitive to microbending loss

One last example of an intensity-based sensor is the grating-based device[21] shown inFig 13.Here an input optical light beam is collimated by a lensand passes through a dual grating system One of the gratings is fixed while theother moves With acceleration the relative position of the gratings changes,resulting in an intensity-modulated signal on the output optical fiber

Figure 11 Evanescence-based fiber optic sensors rely on the cross-coupling of lightbetween two closely spaced fiber optic cores Variations in this distance due totemperature, pressure, or strain offer environmental sensing capabilities

One of the limitations of this type of device is that as the gratings movefrom a totally transparent to a totally opaque position, the relative sensitivity

of the sensor changes, asFig 14shows For optimum sensitivity the gratingsshould be in the half-open=half-closed position Increasing sensitivity meansfiner and finer grating spacings, which in turn limit dynamic range

To increase sensitivity without limiting dynamic range, use part gratings that are offset by 90
, as shown inFig 15.If two outputs arespaced in this manner, the resulting outputs are in quadrature, as shown in

When one output is at optimal sensitivity, the other is at its lowestsensitivity, and vice versa By using both outputs for tracking, one can scanthrough multiple grating lines, enhancing dynamic range and avoiding thesignal fadeout associated with positions of minimal sensitivity.

Intensity-based fiber optic sensors have a series of limitations imposed

by variable losses in the system that are not related to the environmental effect

to be measured Potential error sources include variable losses due to nectors and splices, microbending loss, macrobending loss, and mechanicalcreep and misalignment of light sources and detectors To circumvent theseproblems, many of the successful higher-performance, intensity-based fibersensors employ dual wavelengths One of the wavelengths is used to calibrateout all of the errors due to undesired intensity variations by bypassing the

con-Figure 14 Dynamic range limitations of the grating-based sensor ofFig 13are due

to smaller grating spacing increasing sensitivity at the expense of range

Figure 15 Dual grating mask with regions 90
out of phase to support quadraturedetection, which allows grating-based sensors to track through multiple lines

sensing region An alternative approach is to use fiber optic sensors that areinherently resistant to errors induced by intensity variations The next sectiondiscusses a series of spectrally based fiber sensors that have this characteristic.

Spectrally based fiber optic sensors depend on a light beam modulated inwavelength by an environmental effect Examples of these types of fibersensors include those based on blackbody radiation, absorption, fluorescence,etalons, and dispersive gratings

One of the simplest of these sensor types is the backbody sensor ofFig 17 A blackbody cavity is placed at the end of an optical fiber When thecavity rises in temperature, it starts to glow and act as a light source.Detectors in combination with narrow band filters are then used todetermine the profile of the blackbody curve and, in turn, the temperature, as

Figure 16 Diagram of a quadrature detection method that allows one area ofmaximum sensitivity while the other reaches a minimum, and vice versa, allowinguniform sensitivity over a wide dynamic range

Figure 17 Blackbody fiber optic sensors allow the measurement of temperature at

a hot spot and are most effective at temperatures of higher than 300
C

in Fig 18 This type of sensor has been successfully commercialized and used

to measure temperature to within a few degrees C under intense RF fields Theperformance and accuracy of this sensor are better at higher temperatures andfall off at temperatures on the order of 200
C because of low signal-to-noiseratios Care must be taken to ensure that the hottest spot is the blackbodycavity and not on the optical fiber lead itself, as this can corrupt the integrity

of the signal

Another type of spectrally based temperature sensor, shown in Fig 19,

is based on absorption [22] In this case a gallium arsenide (GaAs) sensorprobe is used in combination with a broadband light source and input=output

Figure 18 Blackbody radiation curves provide unique signatures for each perature

tem-Figure 19 Fiber optic sensor based on variable absorption of materials such asGaAs allow the measurement of temperature and pressure

optical fibers The absorption profile of the probe is temperature-dependentand may be used to determine temperature.

Fluorescent-based fiber sensors [23–24] are widely used for medicalapplications and chemical sensing and can also be used for physical parametermeasurements such as temperature, viscosity, and humidity There are anumber of configurations for these sensors, Fig 20 illustrates two of the mostcommon In the case of the end-tip sensor, light propagates down the fiber to aprobe of fluorescent material The resultant fluorescent signal is captured bythe same fiber and directed back to an output demodulator The light sourcescan be pulsed, and probes have been made that depend on the time rate ofdecay of the light pulse

In the continuous mode, parameters such as viscosity, water vaporcontent, and degree of cure in carbon fiber reinforced epoxy and thermo-plastic composite materials can be monitored

An alternative is to use the evanescent properties of the fiber and etchregions of the cladding away and refill them with fluorescent material Bysending a light pulse down the fiber and looking at the resulting fluorescence,

a series of sensing regions may be time division multiplexed

It is also possible to introduce fluorescent dopants into the optical fiberitself This approach causes the entire optically activated fiber to fluoresce Byusing time division multiplexing, various regions of the fiber can be used tomake a distributed measurement along the fiber length

In many cases users of fiber sensors would like to have the fiber opticanalog of conventional electronic sensors An example is the electrical strain

Figure 20 Fluorescent fiber optic sensor probe configurations can be used tosupport the measurement of physical parameters as well as the presence or absence ofchemical species These probes may be configured to be single-ended or multipoint

by using side etch techniques and attaching the fluorescent material to the fiber

gauge widely used by structural engineers Fiber grating sensors [25–28] can

be configured to have gauge lengths from 1 mm to approximately 1 cm, withsensitivity comparable to conventional strain gauges

This sensor is fabricated by ‘‘writing’’ a fiber grating into the core of agermanium-doped optical fiber This can be done in a number of ways Onemethod, illustrated by Fig 21, uses two short-wavelength laser beams that areangled to form an interference pattern through the side of the optical fiber.The interference pattern consists of bright and dark bands that represent localchanges in the index of refraction in the core region of the fiber Exposure timefor making these gratings varies from minutes to hours, depending on thedopant concentration in the fiber, the wavelengths used, the optical powerlevel, and the imaging optics

Other methods that have been used include the use of phase masks aswell as interference patterns induced by short, high-energy laser pulses Theshort duration pulses have the potential to be used to write fiber gratings intothe fiber as it is being drawn

Substantial efforts are being made by laboratories around the world toimprove the manufacturability of fiber gratings because they have thepotential to be used to support optical communication as well as sensingtechnology

Once the fiber grating has been fabricated, the next major issue is how toextract information When used as a strain sensor, the fiber grating is typicallyattached to, or embedded in, a structure As the fiber grating is expanded or

Figure 21 Fabrication of a fiber grating sensor can be accomplished by imaging toshort-wavelength laser beams through the side of the optical fiber to form an in-terference pattern The bright and dark fringes imaged on the core of the optical fiberinduce an index of refraction variation resulting in a grating along the fiber core

compressed, the grating period expands or contracts, changing the grating’sspectral response.

For a grating operating at 1300 nm, the change in wavelength is about

103 nm per microstrain This type of resolution requires the use of spectraldemodulation techniques that are much better than those associated withconventional spectrometers Several demodulation methods have been sug-gested using fiber gratings, etalons, and interferometers [29,30] Figure 22illustrates a system that uses a reference fiber grating The reference fibergrating acts as a modulator filter By using similar gratings for the reference andsignal gratings and adjusting the reference grating to line up with the activegrating, one may implement an accurate closed-loop demodulation system

An alternative demodulation system would use fiber etalons such asthose shown inFig 23.One fiber can be mounted on a piezoelectric and theother moved relative to a second fiber end The spacing of the fiber ends aswell as their reflectivity in turn determine the spectral filtering action of thefiber etalon, illustrated byFig 24

The fiber etalons in Fig 23 can also be used as sensors [31–33] formeasuring strain, as the distance between mirrors in the fiber determines theirtransmission characteristics The mirrors can be fabricated directly into thefiber by cleaving the fiber, coating the end with titanium dioxide, and thenresplicing An alternative approach is to cleave the fiber ends and insert theminto a capillary tube with an air gap Both of these approaches are beinginvestigated for applications where multiple in-line fiber sensors are required.For many applications a single point sensor is adequate In thesesituations an etalon can be fabricated independently and attached to the end

Figure 22 Fiber grating demodulation systems require very high-resolution tral measurements One way to accomplish this is to beat the spectrum of light re-flected by the fiber grating against the light transmission characteristics of a referencegrating

spec-of the fiber.Figure 25shows a series of etalons that have been configured tomeasure pressure, temperature, and refractive index, respectively.

In the case of pressure, the diaphragm has been designed to deflect.Pressure ranges of 15 to 2000 psi can be accommodated by changing thediaphragm thickness with an accuracy of about 0.1% full scale [34] Fortemperature the etalon has been formed by silicon–silicon dioxide interfaces.Temperature ranges of 70
to 500
K can be selected, and for a range of about

100
K a resolution of about 0.1
K is achievable [34] For refractive index ofliquids, a hole has been formed to allow the flow of the liquid to be measured

Figure 23 Intrinsic fiber etalons are formed by in-line reflective mirrors that can beembedded into the optical fiber Extrinsic fiber etalons are formed by two mirroredfiber ends in a capillary tube A fiber etalon-based spectral filter or demodulator isformed by two reflective fiber ends that have a variable spacing

Figure 24 The transmission characteristics of a fiber etalon as a function of finesse,which increases with mirror reflectivity

without the diaphragm deflecting These devices have been commercializedand are sold with instrument packages [34].

One of the areas of greatest interest has been in the development of performance interferometric fiber optic sensors Substantial efforts have beenundertaken on Sagnac interferometers, ring resonators, Mach–Zehnder andMichelson interferometers, as well as dual-mode, polarimetric, grating, andetalon-based interferometers This section briefly reviews the Sagnac, Mach–Zehnder, and Michelson interferometers

high-1.4.1 The Sagnac Interferometer

The Sagnac interferometer has been principally used to measure rotation[35–38] and is a replacement for ring laser gyros and mechanical gyros Itmay also be employed to measure time-varying effects such as acoustics,vibration, and slowly varying phenomena such as strain By using multipleinterferometer configurations, it is possible to employ the Sagnac inter-ferometer as a distributed sensor capable of measuring the amplitute andlocation of a disturbance

The single most important application of fiber optic sensors in terms ofcommercial value is the fiber optic gyro It was recognized very early that the

Figure 25 Hybrid etalon-based fiber optic sensors often consist of micromachinedcavities that are placed on the end of optical fibers and can be configured so thatsensitivity to one environmental effect is optimized

fiber optic gyro offered the prospect of an all solid-state inertial sensor with nomoving parts, unprecedented reliability, and a potential of very low cost.The potential of the fiber optic gyro is being realized as several manu-facturers worldwide are producing them in large quantities to support auto-mobile navigation systems, pointing and tracking of satellite antennas,inertial measurement systems for commuter aircraft and missiles, and as thebackup guidance system for the Boeing 777 They are also being baselined forsuch future programs as the Comanche helicopter and are being developed tosupport long-duration space flights.

Other applications using fiber optic gyros include mining operations,tunneling, attitude control for a radio-controlled helicopter, cleaning robots,antenna pointing and tracking, and guidance for unmanned trucks andcarriers

Two types of fiber optic gyros are being developed The first type is anopen-loop fiber optic gyro with a dynamic range on the order of 1000 to 5000(dynamic range is unitless), with a scale factor accuracy of about 0.5% (thisaccuracy number includes nonlinearity and hysterisis effects) and sensitivitiesthat vary from less than 0.01
=hr to 100
=hr and higher [38] These fiber gyrosare generally used for low-cost applications where dynamic range and line-arity are not the crucial issues The second type is the closed-loop fiber opticgyro that may have a dynamic range of 106and scale factor linearity of 10 ppm

or better [38] These types of fiber optic gyros are primarily targeted atmedium- to high-accuracy navigation applications that have high turningrates and require high linearity and large dynamic ranges

The basic open-loop fiber optic gyro is illustrated by Fig 26 Abroadband light source such as a light-emitting diode is used to couple lightinto an input=output fiber coupler The input light beam passes through apolarizer that is used to ensure the reciprocity of the counterpropagating light

Figure 26 Open-loop fiber optic gyros are the simplest and lowest-cost rotationsensors They are widely used in commercial applications where their dynamic rangeand linearity limitations are not constraining

beams through the fiber coil The second central coupler splits the two lightbeams into the fiber optic coil, where they pass through a modulator used togenerate a time-varying output signal indicative of rotation The modulator isoffset from the center of the coil to impress a relative phase difference betweenthe counterpropagating light beams After passing through the fiber coil, thetwo light beams recombine the pass back though the polarizer and aredirected onto the output detector.

When the fiber gyro is rotated clockwise, the entire coil is displaced,slightly increasing the time it takes light to traverse the fiber optic coil.(Remember that the speed of light is invariant with respect to the frame ofreference; thus, coil rotation increases path length when viewed from outsidethe fiber.) Thus, the clockwise propagating light beam has to go through aslightly longer optical pathlength than the counterclockwise beam, which ismoving in a direction opposite to the motion of the fiber coil The net phasedifference between the two beams is proportional to the rotation rate

By including a phase modulator loop offset from the fiber coil, a timedifference in the arrival of the two light beams is introduced, and an optimizeddemodulation signal can be realized The right side of Fig 27 shows this Inthe absence of the loops the two light beams traverse the same optical pathand are in phase with each other, shown on the left-hand curve of Fig 27.The result is that the first or a higher-order odd harmonic can be used as

a rotation rate output, resulting in improved dynamic range and linearity

Figure 27 An open-loop fiber optic gyro has predominantly even-order harmonics

in the absence of rotation Upon rotation, the open-loop fiber optic gyro has an oddharmonic output whose amplitude indicates the magnitude of the rotation rate andwhose phase indicates direction

Further improvements in dynamic range and linearity can be realized byusing a ‘‘closed-loop’’ configuration where the phase shift induced by rotation

is compensated by an equal and opposite artificially imposed phase shift Oneway to accomplish this is to introduce a frequency shifter into the loop, shown

in Fig 29

The relative frequency difference of the light beams propagating in thefiber loop can be controlled, resulting in a net phase difference proportional tothe length of the fiber coil and the frequency shift In Fig 29, this is done byusing a modulator in the fiber optic coil to generate a phase shift at a rate o

Figure 29 Closed-loop fiber optic gyros use an artificially induced nonreciprocalphase between counterpropagating light beams to counterbalance rotationally in-duced phase shifts These fiber gyros have the wide dynamic range and high linearityneeded to support stringent navigation requirements

Figure 28 A typical open-loop fiber optic gyro output, obtained by measuring one

of the odd harmonic output components amplitude and phase, results in a sinusoidaloutput that has a region of good linearity centered about the zero rotation point

When the coil is rotated, a first harmonic signal at w is induced with phase thatdepends on rotation rate in a manner similar to that described above withrespect to open-loop fiber gyros By using the rotationally induced first har-monic as an error signal, one can adjust the frequency shift by using a syn-chronous demodulator behind the detector to integrate the first harmonicsignal into a corresponding voltage This voltage is applied to a voltage-controlled oscillator whose output frequency is applied to the frequencyshifter in the loop so that the phase relationship between the counter-propagating light beams is locked to a single value.

It is possible to use the Sagnac interferometer for other sensing andmeasurement tasks Examples include slowly varying measurements of strainwith 100-micron resolution over distances of about 1 km [39], spectroscopicmeasurements of wavelength of about 2 nm [40], and optical fiber char-acterization such as thermal expansion to accuracies of about 10 ppm [40] Ineach of these applications frequency shifters are used in the Sagnac loop toobtain controllable frequency offsets between the counterpropagating lightbeams

Another class of fiber optic sensors, based on the Sagnac interferometer,can be used to measure rapidly varying environmental signals such as sound[41,42] Figure 30 illustrates two interconnected Sagnac loops [42] that can beused as a distributed acoustic sensor The WDM (wavelength division mul-tiplexer) in the figure is a device that either couples two wavelengths l1and l2

in this case) together or separates them

The sensitivity of this Sagnac acoustic sensor depends on the signal’slocation If the signal is in the center of the loop, the amplification is zero

Figure 30 A distributed fiber optic acoustic sensor based on interlaced Sagnacloops allows the detection of the location and the measurement of the amplitudealong a length of optical fiber that may be many kilometers long

because both counterpropagating light beams arrive at the center of the loop

at the same time As the signal moves away from the center, the outputincreases When two Sagnac loops are superposed, as in Fig 30,the twooutputs may be summed to give an indication of the amplitude of the signaland ratioed to determine position

Several other combinations of interferometers have been tried forposition and amplitude determinations, and the first reported success con-sisted of a combination of the Mach–Zehnder and Sagnac interferometers[41]

One of the great advantages of all fiber interferometers, such as Mach–Zehnder and Michelson interferometers [43] in particular, is that they haveextremely flexible geometries and a high sensitivity that allow the possibility

of a wide variety of high-performance elements and arrays, as shown inFig 31

Figure 32shows the basic elements of a Mach–Zehnder interferometer,which are a light source=coupler module, a transducer, and a homodynedemodulator The light source module usually consists of a long coherencelength isolated laser diode, a beamsplitter to produce two light beams, and ameans of coupling the beams to the two legs of the transducer The transducer

is configured to sense an environmental effect by isolating one light beam fromthe environmental effect; using the action of the environmental effect on thetransducer induces an optical path length difference between the two light

Figure 31 Flexible geometries of interferometeric fiber optic sensors’ transducersare one of the features of fiber sensors attractive to designers configuring special-purpose sensors

beams Typically, a homodyne demodulator is used to detect the difference inoptical path length (various heterodyne schemes have also been used) [43].One of the basis issues with the Mach–Zehnder interferometer is that thesensitivity varies as a function of the relative phase of the light beams in thetwo legs of the interferometer, as shown in Fig 33 One way to solve the signalfading problem is to introduce a piezoelectric fiber stretcher into one of thelegs and adjust the relative path length of the two legs for optimum sensitivity.

Figure 33 In the absence of compensating demodulation methods, the sensitivity

of the Mach–Zehnder varies with the relative phase between the two light beams Itfalls to low levels when the light beams are completely in or out of phase

Figure 32 The basic elements of the fiber optic Mach–Zehnder interferometer are alight source module to split a light beam into two paths, a transducer used to cause

an environmentally dependent differential optical path length between the two lightbeams, and a demodulator that measures the resulting path length difference betweenthe two light beams

Another approach has the same quadrature solution as the grating-basedfiber sensors discussed earlier.

Figure 34 illustrates a homodyne demodulator The demodulator sists of two parallel optical fibers that feed the light beams from the transducerinto a graded index (GRIN) lens The output from the graded index lens is aninterference pattern that ‘‘rolls’’ with the relative phase of the two input lightbeams If a split detector is used with a photomask arranged so that theopaque and transparent line pairs on the mask in front of the split detectormatch the interference pattern periodicity and are 90
out of phase on thedetector faces, sine and cosine outputs result

con-These outputs may be processed using quadrature demodulation tronics, as shown in Fig 35 The result is a direct measure of the phasedifference

elec-Further improvements on these techniques have been made; notably thephase-generated carrier approach shown inFig 36.A laser diode is current-modulated, resulting in the output frequency of the laser diode being fre-quency-modulated as well If a Mach–Zehnder interferometer is arranged sothat its reference and signal leg differ in length by an amountðL1 L2Þ, thenthe net phase difference between the two light beams is 2pFðL1 L2Þn=c,where n is in index of refraction of the optical fiber and c is the speed of light invacuum If the current modulation is at a rate o, then relative phase differ-ences are modulated at this rate and the output on the detector will be odd andeven harmonics of it The signals riding on the carrier harmonics of o and 2oare in quadrature with respect to each other and can be processed usingelectronics similar to those of Fig 35

Figure 34 Quadrature demodulation avoids signal fading problems The methodshown here expands the two beams into an interference pattern that is imaged onto asplit detector

The Michelson interferometer inFig 37is in many respects similar tothe Mach–Zehnder The major difference is that mirrors have been put on theends of the interferometer legs This results in very high levels of backreflection into the light source, greatly degrading the performance of earlysystems Using improved diode pumped YAG (Yttrium Aluminum Garnet)

Figure 36 The phase-generated carrier technique allows quadrature detection viamonitoring even and odd harmonics induced by a sinusoidally frequency-modulatedlight source used in combination with a length offset Mach–Zehnder interferometer

to generate a modulated phase output whose first and second harmonics correspond

to sine and cosine outputs

Figure 35 Quadrature demodulation electronics take the sinusoidal outputs fromthe split detector and convert them via cross-multiplication and differentiation into

an output that can be integrated to form the direct phase difference

ring lasers as light sources largely overcame these problems In combinationwith the recent introduction of phase conjugate mirrors to eliminate polari-zation fading, the Michelson is becoming an alternative for systems that cantolerate the relatively high present cost of these components.

In order to implement an effective Mach–Zehnder or Michelson-basedfiber sensor, it is necessary to construct an appropriate transducer This caninvolve a fiber coating that could be optimized for acoustic, electric, ormagnetic field response Figure 38 illustrates a two-part coating that consists

of a primary and secondary layer These layers are designed for optimal

Figure 38 Coatings can be used to optimize the sensitivity of fiber sensors Anexample would be to use soft and hard coatings over an optical fiber to minimize theacoustic mismatch between acoustic pressure waves in water and the glass opticalfiber

Figure 37 The fiber optic Michelson interferometer consists of two mirrored fiberends and can utilize many of the demodulation methods and techniques associatedwith the Mach–Zehnder

response to pressure waves and for minimal acoustic mismatches between themedium in which the pressure waves propagate and the optical fiber.These coated fibers are often used in combination with compliantmandrills or strips of material as in Fig 39 that act to amplify the envir-onmentally induced optical path length difference.

In many cases the mechanical details of the transducer design are critical

to good performance such as the seismic=vibration sensor of Fig 40 erally, the Mach–Zehnder and Michelson interferometers can be configuredwith sensitivities that are better than 106radians per square root Hertz Foroptical receivers, the noise level decreases as a function of frequency Thisphenomenon results in specifications in radians per square root Hertz As anexample, a sensitivity of 106radians per square root Hertz at 1 Hertz means asensitivity of 106radians, while at 100 Hertz, the sensitivity is 107radians

Gen-As an example, a sensitivity of 106radian per square root Hertz means thatfor a 1-meter-long transducer, less than 1=6 micron of length change can be

Figure 39 Optical fiber bonded to hollow mandrills and strips of environmentallysensitive material are common methods used to mechanically amplify environmentalsignals for detection by fiber sensors

Figure 40 Differential methods are used to amplify environmental signals In thiscase a seismic=vibration sensor consists of a mass placed between two fiber coils andencased in a fixed housing