Photonics FOS

8/3/2019 Photonics FOS

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TECHNICAL UNIVERSITY OF KOSICE

Faculty of Electrical Engineering and Informatics

Department of Electronics and Multimedia

Communications

Fiber Optic Sensor

Professor Student

Jn Turn Fabio Cavaliere


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Sommario

INTRODUCTION ............................................................................................................................. 4ADVANTAGES AND DISADVANTAGES IN FOS ........................................................................ 5ROLE OF FIBER OPTIC IN FOS ....................................................................................................... 5

A brief introduction to fiber optic .................................................................................................... 8FOS CLASSIFICATION AND THEIR FIELDS APPLICATION ..................................................... 9

How to perform the detection ........................................................................................................ 10The detection point......................................................................................................................... 10The role of fiber optics in the process of detection ........................................................................ 10

FOSs COMPONENTS ..................................................................................................................... 10Source............................................................................................................................................. 10

Led ............................................................................................................................................. 11Laser ........................................................................................................................................... 12

Detectors ........................................................................................................................................ 13AMPLITUDE FOS ............................................................................................................................ 14

Microbend Sensor .......................................................................................................................... 15Reflective sensors .......................................................................................................................... 16

PHASE (INTERFEROMETRIC) FOS .............................................................................................. 16Classical fiber interferometers ....................................................................................................... 16

Michelson interferometer ........................................................................................................... 17Mach-Zehnder interferometer .................................................................................................... 18Fabry-Perot interferometer ......................................................................................................... 19Sagnac interferometer ................................................................................................................ 20

POLARIMETRIC FOS ...................................................................................................................... 21Single-Mode operation ................................................................................................................... 21Two-Mode operation...................................................................................................................... 22Polarimetric pressure sensor .......................................................................................................... 23Polarimetric current sensor ............................................................................................................ 24


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DISTRIBUTED SENSORS ............................................................................................................... 25OTDR ......................................................................................................................................... 26

FOS SYSTEMS ................................................................................................................................. 27Basic Multiplexing Concepts ......................................................................................................... 28Examples of discrete sensor multiplexing techniques ................................................................... 29

Space Division Multiplexing ..................................................................................................... 29Wavelength Division Multiplexing ............................................................................................ 29Time Division Multiplexing....................................................................................................... 30Frequency Division Multiplexing .............................................................................................. 31Coherence Division Multiplexing .............................................................................................. 31

APPLICATIONS OF FOS ................................................................................................................. 32


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INTRODUCTION

Sensors are optical devices that exploit the variation of the parameters that characterize an

electromagnetic wave such as light intensity or phase. They can be used in various applications and

are an essential element for control and diagnostics in areas such as housing, car, medical, industrial

automation, telecommunications, environment, agriculture. Moreover, acceleration of the

development of sensors is the evolution of data processing, also the development of

microprocessors and integrated circuits specifically designed for application development has made

the low-cost, accurate and reliable, thus increasing the intelligence system. This new scenario has

stimulated research in the area of sensors and therefore the development of related technologies and

new devices. New products are born from this effort, with added value in terms of ease of use,

energy saving, security and intelligence.

Among the different categories of sensors, in recent years has had considerable importnace the

research and development of optical and electrooptical sensors. This interest is linked to some of

their important characteristics, in particular the ability to take measurements at a distance (without

contact), the remarkable versatility in the measurement of physical and chemical properties not

detectable with other techniques, flexibility in terms of dynamics and sensitivity. However, there

was great difficulty in optical sensors due to interference between multiple effects such as a

temperature variation and external disturbance, or more generally any disruption of the initial

conditions of use.


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ADVANTAGES AND DISADVANTAGES IN FOS

Exploiting the capacity of optical fibers to send and receive signals over long distances, a current

trend is to create networks of sensors or arrays of optical sensors. This avoids having to perform

conversions between electronics and photonics separately at each detection site, thereby reducing

costs and increasing flexibility. A difficulty for all sensors, either optical or not, is the interference

from multiple effects. Sensors to measures of tension or pressure may be, for example, very

sensitive to temperature. In recent years considerable progress has been made in this area aimed at

limiting the effects of disturbance using more accurate technology.

In the following list, are reported the main advantages and disadvantages in FOS:

Small size and lightweight;

High sensitivity;

Multifunctional (great variety in the measurable parameters);

Insensitive to EM fields;

Durability and reliability in demanding environments;

No need for electrical power;

Long-distance remote monitoring; Insensitive to vibration;

Little or no impact to physical structures;

Safe (operate around explosive and flammable materials);

In contrast with all these advantages, one of the main disadvantages is the fact that intervention

characteristics are strongly linked to the type of surface of the object to be measured.

ROLE OF FIBER OPTIC IN FOS

Since a large percentage of optical sensors involves the use of fiber optics, it is appropriate to

discuss in more detail to them. In most applications, designers of sensor glass fibers that are easily

commercially available and relatively inexpensive because of their huge use in the

telecommunications sector. The interferometer sensors require single-mode fiber glass, while, for

example, the intensity sensors use multimode fiber typically required for achieving a more light-gathering. High Fiber numerical aperture plastic are used for some types of sensors in intensity but


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the properties of transmission and fluorescence of the plastic complicate the spectral response of the

fiber and glass fiber are preferable in applications where it is important the frequency response of

fiber.

The polarization of light transmission is very important for a large number of sensors, and many

fibers are designed to preserve this property of light along the entire length of the fiber and also in

the presence of micro and macro bending.

Formany chemical sensors is important that the light wave interacts with the material surrounding

the sensor, so the fibers are constructed so that the core is close to the interface between the coating

and the environment outside. In figure is shown the section of the different types of optical fiber

currently produced and used.

Singl mode fiber: is designed such that all the higher order waveguide modes are cut-off by a

proper choice of the waveguide parameters as

given below:

V2

an12 (n1 n2/n1)

where, is the wavelength and a is the core radius and Vis called normalized frequency (V).When V < 2,405 single mode condition is ensured. SM fiber is an essential requirement for

interferometric sensors. Due to the small core size (~4m) alignment becomes a critical factor.

However, in real case, SM fiber mentioned above is affected by degenerate polarization

states that can lead to signal interference and noise in the measurement.


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Multimode fiber:

Variation of refraction index n to vary the distance, for step index and graded index respectively

Graded index: The light beams for longer paths (those in the vicinity of the cladding) moving in

material with refractive index smaller and then at higher speed, while the beams below shorter paths

(those near the center of the nucleus) propagate in the middle with higher refractive index and

therefore more slowly. Thus, the components of the pulse coming fromdifferent paths to reach the

receiver with time lags smaller than in the case of fiber step - index and the signal appears

less deformed. Due to the gradual variation of refractive index, the bundles do not describe light

straight paths, but their trajectory assumes a helical pattern almost.

Step index: the various modes propagate with same speed describing different paths.

This lead a time lag of the several beams, with consequent distortion of the light signal being

received.


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D-fiber

Elliptic core

A brief introduction to fiber optic

We can consider an optical fiber as a waveguide to a cylindrical shape, made with a dielectric

material, usually silicon oxide and characterized by the phenomenon of propagation inside the light

radiation. This phenomenon is based on the variation of refractive index within the dielectric

material. The index of refraction is defined as the ratio between the speed of propagation of the

light in a vacuum (c = 3 * 10 8 m / s) and the speed of propagation in a medium different from

vacuum:

n=

where v depends, of course, from the characteristics and physical properties of the medium itself, if

the medium is homogeneous and isotropic, then n indicates a number greater than one and in the

dielectric constant.

A beamthat affects an area of interface between two

media of different indices (n1> n2) is partly reflectedand partly refracted or transmitted, in accordance

with the law of Snell note:

n1*sen1 = n2*sen2

where 1 is the angle of incidence of the normal

range compared to the surface at the point of incidence and 2 is the angle that the radius refracted

form with the same standard in the second half. Since n2


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we are in the phenomenon of total reflection, where the angle of incidence beyond which there is no

refraction is c = arcsin (n2/n1), generally referred to as critic angle.In first approximation we can

imagine that the rays on a plane comprising the axis of optical fiber, which affect the interface glass

/ air with an angle greater than c are totally reflected and thus remain confined within the fiber

indefinitely. The principle just described is the basis of

the functioning of all types of fiber. The result is the

basic structure of the realization of a fiberconsisting of

a cylinder inside, as the nucleus or core, and an outer

shell, indicated as cladding. Both are formed by the

glassy material, but the two indexes of refraction are

varied and

precisely controlled during manufacture of the fiber

through the addition of external doping (oxides of

germanium, aluminum). The fiber thus produced

would be mechanically fragile, and it is then necessary

to strengthen by further plastic coating.

FOS CLASSIFICATION AND THEIR FIELDS APPLICATION

At this stage of the development of optical sensing technology, it can measure almost all the

physical and chemical properties, some are listed below:

Stress;

Displacements;

Force;

Temperature;

Chemical comcentration;

Acceleration;

Rotation;

Currents, electric fields, ecc;

The main techniques by which the measurements are being taken can be grouped into three

categories.


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How to perform the detection

According to this classification sensors measure a change in the intensity or phase of one or more

beams of light as a result of the interaction or interference with the wave of reference, is referred to

as sensors or sensors intensity interferometer. The techniques used in the case of intensity sensors

include light scattering (both Rayleigh and Raman), changes in the transmission spectrum (i.e.

simple attenuation of the transmitted light due to absorption), or microbending losses of radiation

change in reflectance and modal changes in the distribution of the fiber.

The detection point

Sensors can be classified into "point sensors" in which the transduction is done at the end of optical

fiber that has the sole purpose of bringing a beam of light to and from the transducer and sensors

distribuited "in which the snapshot is taken along entire length of the fiber. Examples of this kind of

sensors are Bragg gratings that are used for measurement of voltage and temperature.

The role of fiber optics in the process of detection

A furtherdistinction can be made according to the use of fiber optics: you define sensors estrinsic in

which the role of fiber is reduced to that of driving the signal, while talk of intrinsic sensors where

the role of fiber is crucial to as the measure for the spectroscopic measures.

FOSs COMPONENTS

A fiber optic sensor in general consist of a light source, a length of sensing (and transmission) fiber,

a photodetector, demodulation, processing and display optics and the required electronics. Fiber

optics are already treated in the previous chapter, so we will focalize the attention in the next

sections on sources and detectors, which will be described more detailed.

Source

Optical sources active devices that emit electromagnetic radiation at optic frequencies.

LED LASER


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LED: Light Emitting Diode LASER: Light Amplification by

very large band optical emission (tens of nm) Stimulated Emission of Radiation

narrow band optical emission

Led

This is a diode with an energy gap that generate electromagnetic radiation at destre frequencies. It

works thanks to the phenomenon of spontaneous emission. Photons are

generated random in all directions and on a broad range of wavelengths.

For each carrier flowing into the junction, can be a photon emitted by

spontaneous emission with a certain probability. A fraction of the

emitted photons are coupled in output with the fiber. In first

approximation, the optical power output is proportional to the current

injected.

Pout (t) kI(t)

In practice, this report is true till a certain frequency. More precisely, we must take into account the

effectof electrical filtering.

Pout (t) khLED(t)*I(t)

Other LED characteristics:

o Typical wavelength: 830 nm(GaAsLED), 1300 nm(InGaAsPLED);

o Low output power: from -10dBm to -20dBm;

o Low modulation rate;


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Laser

The operation of a semiconductor laser is based on:

Generation of photons in an appropriate directly

polarized pn junction (in similar manner to the

LEDs);

Optical "Feedback": by the equivalent filter partially

reflective on both sides of the structure, the emitted

photons transit many times within the same structure;

while transit, the photons are amplified due to stimulated emission.

Here are reported the main characteristics in lasers:

Directionality

In contrast to traditional sources, laser can emit radiation in one direction. More precisely,

the solid angle subtended by a laser beam is extremely small. This feature is exploited in

several areas, for example, allows you to treat the surfaces in an extremely accurate manner

(lithography, CD, etc.). In spectroscopy it is possible to substantially increase the optical

path and thus the sensitivity using a laser source through the sample with a zig-zag path

through a system of mirrors. Monocromatic

The enlargement of the band emission is due to the natural width and by Doppler effect

(which can be eliminated or contained a lot). In spectroscopy this feature is exploited to

obtain high resolution spectra. It would be very difficult to obtain Raman spectra without

this characteristic of the laser.

Brilliance

Laser in the amount of energy emitted per unit solid angle is incomparably higher thantraditional sources. In particular, the high number of photons per unit of frequency. This

feature is a direct consequence of the two previously mentioned. With this feature you have

the possibility to observe phenomena, such as for example the many-photon absorption. The

high intensity was also found different technological applications, for example in metal

cutting.

Coherence

While in spontaneous emission photons are emitted random one with respect to others, in

stimulated emission photons have the same phase of the photon that induced the emission.

The phase is maintained over time and space.


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Ultra-short pulses

By various techniques it is possible to build lasers that emit very narrow packets of

waves in the time domain, now it has come the development of the order of femtosecond

pulses. These lasers have found applications in various fields of research, for example,have allowed the birth of a new discipline, which has been called femtochemistry.

Detectors

Photodiodes are components that allow to convert an optical input power into an electric

current. These components are based on semiconductor pn junctions. The main effect in this

case is the absorption. Each photon that passes along the p-n junction can be used, with a

certain probability, to generate a carrier at conduction level. It then generates a current

proportional to optical power.

PIN photodiodes (P-Intrinsic-N) and Avalanche Photodiodes (APDs) are the most suitable

detectors in FOS. APD can sense low light levels due to the inherent gain because of avalanche

multiplication, but need large supply voltage typically about 100 V.

The output current is:

Io= d

q

Where d is the probability that a photon is absorbed and create a carrier (quantum efficiency), and

Popt/hf is the number of photons per unit time. In addition to the current generation, the process of

photodetecting generates a certain amount of noise. It is possible write:

i(t)= RPopt(r) + inoise(t)


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Where R is the photodiode responsivity. inoise(t) is a random process that takes into account the

noise produced during photodetecting.

For APD (Avalanche PhotoDiode), for each incoming photon can be generated more than one

carrier, so the process photodetecting act with a mechanism of "gain".

i(t)= R GAPD Popt(r) + iAPD

noise (t)

With the same power, APD results in a current output significantly higher than the PIN. However, it

result (also) in a larger amount of noise compared to the PIN. It is obtained that is a signal-noise

ratio will be worse than the PIN.

PIN APD

AMPLITUDE FOS

In this case, the signal to be measured (the measurable), modulates the intensity (amplitude) of the

light carried by an optical fiber. For this class of sensors a normalized modulation index m

can be defined as:

m=I

where I is the change in optical power as a result of modulation by the measurable; I0 is optical

power reaching the detector when there is no modulation; and P is the perturbation (measurable).

The sensor response expressed as a differential voltage (S) per unit change in perturbation is

given by:


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S= qI0 Rm

where, q is detector responsivity; R is the load resistance.

The limiting performance is reached when the signal voltage (power) is equal to the noise voltage

(power). There are many noise sources within the detector as also in the processing circuits. For

purposes of estimating the sensor performance, usually detection limit (i.e. the fluctuations in the

photon field that could be detected is the ultimate limit) is considered. The minimum measurable

quantity in the shot noise limit is given by:

Pmin=2

where e is electronic charge and B is the detection bandwidth.

Microbend Sensor

Microbend sensors are based on coupling and leakage of modes that are propagating in a deformed

fiber. Usually one achieves this deformation by employing corrugated plates that deforms the fiber

into a series of sharp bend with small bending radii. The modulation due to a measurable could be

considered as a form of a microbend loss modulation, for example moving the fiber. The microbend

sensor is an example of an intensity modulation sensor. It is designed using Multimode fiber of a

few meters in length which is placed between two rigid plates having an optimum corrugation

profile such that the fiber experiences multiple bends. Due to the microbending induced losses, the

lower order guided modes are converted to higher order modes and are eventually lost resulting in a

reduction of the optical intensity coming out of the fiber. A displacement of the plates (due to

pressure for example) causes a change in the amplitude of the bends and consequently an intensity

modulation of light emerges from the fiber core. Microbend sensors have been used in some smart

structure applications.


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Reflective sensors

Reflective sensors incorporate an infrared emitter and photo detector adjacent one to the other as

shown in the figure behind. When an object is in the sensing area, the emitted light is reflected back

towards the photo detector, the amount of light energy reaching the detector increases. This change

in light energy or photo current is similarly used an input signal in the application. The reading area

is a radius of tens mm.

PHASE (INTERFEROMETRIC) FOS

Phase sensors compare the phase of light in a sensing fiber to a reference fiber in a device called

interferometer. This type of sensor is more complex in design, but present better sensitivity and

better resolution.

Fiber-optic interferometer sensors are usually designed following classical configuration of optical

interferometer. In these devices, a range of physical measurands can induce modulation of phase in

a sensing signal light beam, while the reference light beam remain unchanged. This phase change

then has to be electronically processed, to produce a useful intensity-type output signal from the

interferometer proportional to the measurand. Alhtough this technique offers very high sensitivity, it

is extremely difficult to use outside, due to the unavoidable interference caused by environmental

perturbation. One notable exception is low-coherence (or white light) interferometry.

Classical fiber interferometers

The three best-known configurations of two-beam FOS interferometers:

The reflective all-fiber Michelson interferometer;

The all-fiber Mach-Zehnder interferometer;

The remote Fabry-Perot interferometer;


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Michelson interferometer

The all-fiber Michelson interferometer is based on a bidirectional single-mode fiber coupler that

divides the input light beam from the laser source into two components one propagated by the

sensing arm, the other by the reference arm of the interferometer- and then combines the two

reflected beams so that their interference can be registered by the photodetector. Assuming for

simplicity that the polarization effect can be ignored, the electric field propagating in the signal arm

and in the reference arm can be treated as scalars and described as:

In this equation, ES0 and ER0 denote the amplitudes, is the wavelength of the light, and is its

angular frequency. The phase difference will then be proportional to the path difference L and can

be expressed as follows:

It may finally be shown that the irradiance at the detector is proportional to the absolute value of the

squared sum of the superimposed electrical fields. The resulting formula is given by:


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Where V is the visibility and depending on both the reflectivity of the fiber ends and the coherence

properties of the light source, but independent of the splitting ratio of the coupler.

1 Michelson interferometer set-up

Mach-Zehnder interferometer

The mach-Zehnder interferometer is based on two bidirectional fiber couplers, the first to divide the

light beam into two components and the second to recombine the two beams exiting from the

sensing arm and from the reference arm of the system. The sensitivity of this interferometer is only

half that of the Michelson interferometer, as light propagates in each arm only once, and the phase

difference is consequently described by:

The Mach-Zehnder configuration has, however, two significant advantages that more than

compensate for the lower sensitivity. Two antiphase output signals from two photodetectors:

can conveniently be used to provide a feedback loop for assuring operation at maximum sensitivity.

This configuration is also characterized by much lower backreflection into the laser diode, which

assures the higher wavelength and power stability of the system


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2 Mach-Zehnder interferometer set-up

Fabry-Perot interferometer

FFPI are an example of multiple-beam technique. A resonant cavity of this device may be contained

within the fiber, with semireflective spliced, cleaved or mirrored end faces, or Bragg gratings

serving as reflective surfaces. This cavity may also be external to the fiber, taking the form of an air

gap between two cleaved fiber end faces, or between a fiber end face and a thin moving or

deformable diaphragm. The transfer function of an FFPI for the transmitted signal can be expressed

by:

Where F is a parameter describing the phase resolution and known as the finesse of the

interferometer, and is the phase retardance after the light has passed through the cavity twice.

When attenuation is disregarded, F may be described in terms of the mirror reflectance R:

In the case of frequently used low-finesse interferometer, the reflective surfaces may simply be the

normally cleaved uncoated fiber ends, for which R=0,04. The FFPI is then operated in a reflective

configuration with visibility approaching 1 as the reflectivity is decreased. For R


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Sagnac interferometer

The Sagnac effect manifests itself in a setup called ring interferometry. A beam of light is split and

the two beams are made to follow a trajectory in opposite directions. To act as a ring the trajectory

must enclose an area. On return to the point of entry the light is allowed to exit the apparatus in

such a way that an interference pattern is obtained. The position of the interference fringes is

dependent on the angular velocity of the setup. This arrangement is also called a Sagnac

interferometer.

Usually several mirrors are used, so that the light beams follow a triangular or square

trajectory.Fiber opticscan also be employed to guide the light. The ring interferometer is located on

a platform that can rotate. When the platform is rotating the lines of the interference pattern are

displaced as compared to the position of the interference pattern when the platform is not rotating.

The amount of displacement is proportional to the angular velocity of the rotating platform. The

axis of rotation does not have to be inside the enclosed area.

When the platform is rotating, the point of entry/exit moves during the transit time of the light. So

one beam has covered less distance than the other beam. This creates the shift in the interference

pattern. Therefore, the interference pattern obtained at each angular velocity of the platform features

a different phase-shift particular to that angular velocity.

In the above discussion, the rotation mentioned is rotation with respect to an inertial reference

frame. Since this experiment does not involve a relativistic velocity, the same wording is valid both

in the context ofclassical electrodynamics andspecial relativity.

The Sagnac effect is the electromagnetic counterpart of the mechanics of rotation. A gimbal

mounted gyroscope remains pointing in the same direction after spinning up, and thus can be used

as the reference for aninertial guidance system. A Sagnac interferometer measures its own angular
http://en.wikipedia.org/wiki/Interference_patternhttp://en.wikipedia.org/wiki/Angular_velocityhttp://en.wikipedia.org/wiki/Angular_velocityhttp://en.wikipedia.org/wiki/Interference_pattern


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velocity with respect to the local inertial frame; hence just as a gyroscope it can provide the

reference for an inertial guidance system.

3 Sagnac interferometer

POLARIMETRIC FOS

Polarimetric sensors may function in a single-mode or in a few-mode regime of operation.

Single-Mode operation

If quasimonochromatic light linearly polarized at an angle with respect to the fibers x axis is

launched into the fiber and an analyzer turned to an angle is placed at the output of the fiber, then

the optical intensity detected will be:

Where 0= 01 L is the phase. When external perturbations are introduced, they cause changes in

the phase 0. This will lead to a cosine variation of the observed intensity Imeasured after the

analyzer, a variation that is in fact a polarization interference. The setup is then a polarimetric

sensor. With we represent the correlation function between the polarization modes. This is a

function of the product of the length L of the fiber, its polarization dispersion , and the spectral

half.width of the source. The visibility V of the observed polarimetric response is:


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An input polarizer (if the light isnt linearly polarized) acts as a splitter, and the analyzer acts as a

recombiner. If we define

k1= sin2 ; k2= sin2

as the power coupling coefficients of the splitter and the recombiner, then the expression for

visibility becomes:

This expression is analogous to the formula for visibility in classical two beam interferometry.

Maximum visibility is obtained for

= = 45. In this case theintensity becomes:

Evidently, if a monochromatic source is used, =0 and then the visibility will be =1.

4 Basic configuration of a single-mode or bimodal polarimetric pressure sensor

Two-Mode operation

For a two-mode regime of operation, a bimodal sensing fiber must be used. By letting x= xL and

y= yL, the intensity observed at the output of the fiber excited with x- or y- polarized

quasimonochromatic light can be obtained as:

In the equation above, 0 and 1 are the relative optical power carried by the spatial modes.

Depending on the detection setup, different expression for the visibility can be obtained.


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Polarimetric pressure sensor

We will describe in this section the high-hydrostatic-pressure polarimetric pressure sensor. If we

consider it with a predesigned sensitivity expressed by the parameter Ti,p, then we have this

expression:

Ti,pL= Ci

Where Ci is a constant. Thus, a longer fiber will exhibit greater sensitivity and vice versa. If the

source isnt monochromatic, a longer fiber will produce a lower value of the correlation function

. Therefore, proper choice of the laser source is essential. For pressure measurement

(considering temperature a disturbing parameter), temperature- and pressure-induced phase shifts

s(p,t) will then be transformed into intensity changesIs (p,t) I0 in accordance with the equation:

In the next figure is depicted a topology of a polarimetric pressure sensor (PPS) in both reflection

and transmission configurations. The sensing (L2) and compensating (L1) parts of the sensor are

assumed to be equal.

5 Temperature-compensated polarimetric FOS sensor in (a) transmission and (b) reflection configuration

The advantage of the reflection configuration is that only one fiber leadthrough is required to

connect the sensor to the laser source and to the detection electronics. The advantage of the

transmission version lies in the much higher level of optical signal, allowing for longer transmission

distances and/or multiplexing of several sensing devices. The polarization axes of the sensing and

the compensating fiber elements are rotated by 90, while the input and output fibers are rotated by45 relative to the sensing and compensating parts, respectively. Ideally, if equal sensing and

compensating elements remain at the same temperature, their temperature-induced phase retardation


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will cancel out, and if they are placed under different pressure, the pressure-modulated output signal

will be immune to temperature changes. In reality, however, if such an arrangement is to satisfy the

requirements for an accurate pressure-measuring instrument, several important first- and second-

order coefficients have to be considered to allow for simultaneous pressure and temperature effects

on fiber birefringence. In a polarimetric cross-spliced sensor, the resulting unwanted sensitivity to

temperature will still be present and can be described by differentiation of the phase retardation

with respect to temperature:

Where L = L1 L2, Kt is the first order phase sensitivity to temperature, and Ktp is the

temperature-pressure cross-sensitivity coefficient. This simple but informative equation provides

important insight into designing a sensor with minimum temperature error. The first term in it can

be minimized by choosing a small L. The second term depends on fiber properties but will always

be nonzero, and can only be minimized by carefully adjusting the fibers technological and

construction parameters.

Polarimetric current sensor

The need for fiber-optic sensing technology for high magnetic field and large current monitoring is

well known. Conventional magnetic field and current sensor system suffer from high susceptibility

to electromagnetic interference, may lack the necessary bandwidth, are difficult to miniaturize, and

cannot accommodate large numbers of measuring points at remote locations. Such monitoring can

be especially valuable for protection, control, and fault detection in power plants, high-power

transmission lines, distribution lines, where the high intrinsic electrical insulation of optical fibers is

a significant advantage. There are several techniques for fiber-optic magnetic field sensing, but only

two approaches appear to be really viable. The first is based on detection of a magnetic field by

magnetostrictive effects, involving measuring the longitudinal strain produced in the optical fiber to

which a magnetostrictive material has been bonded. The performance of such sensors is limited by

the coupling efficiency of the magnetostrictive material and the optical fiber. Altough various

bonding and coating techniques have been explored, all usually lead to substantial hysteresis,

temperature drift, and changes of fiber birefringence. The second approach is based on the Faraday

effect, consisting of a nonreciprocal circular birefringence induced in the propagation medium by amagnetic field. The most convenient detection approach in this case is polarimetric sensing.


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The Faraday effect may occur directly in standard or specifically doped optical fibers, but as V

(Verdet constant) in silica fiber is very small, this type of sensor needs to be very long and as such

will be prone to a variety of instabilities. For N turns of fiber around a conductor with a current I,

the Faraday rotation is given by:

WhereLFis the propagation path andHthe magnetic field. The material-dependent Verdet constant

V(,T) is dispersive and often varies strongly as a function of temperature. To assure successful

operation of a sensor based on the fiber-sensitive element, it is extremely important to avoid

intrinsic birefringence induced by core ellipticity or stress in the core-cladding area, and extrinsic

birefringence induced by packaging and mounting. This parasitic effect can be alleviated to some

extent by annealing the fibers at an elevated temperature.

6 Polarimetric current FOS

DISTRIBUTED SENSORS

Due to unidimensional structure of an optical fiber, fiber-optic sensing technology offers the unique

possibility to simultaneously measure not only a given parameter but also its real-time spatial

distribution. Distributed fiber sensor are intrinsic FOS that allow sensing of the variation of the

measured quantity along the fiber as a continous function of distance. They rely in principle on

optical time-domain reflectometry.


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OTDR

Is the acronymous of Optical Time-Domain Reflectometer. The principle of operation is shown in

the next figure.

7 Principle of an OTDR-based distributed sensor

A high-power light pulse is launched into the sensing fiber and propagates with the group velocity

vg. The light is reflected at discontinuities and is scattered mainly in the elastic process of Rayleigh

scattering, which is caused by the microscopic fluctuations of the refractive index in the fiber. The

detector measures the time dependence of the backscattered light signal, and the time of the

measurement tdetermines the distancez = tvg/2 at which the light pulse was backscattered.

In the figure below there is a sample of the return signal measured by the OTDR.

8 Sample of the detected signal from a distributed fiber-optic sensor

Since the light is attenuated in accordance with an exponenetial law, the backscattered power P

measured by the detector is calculated as:


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Where is an attenuation coefficient, P0 the initial pulse power, D the length of the pulse, R the

backscattering reflection coefficient per unit length, and the coupling ratio in the directional

coupler. The slope of the logarithm of the detected power at constant reflection R is proportional to

the attenuation:

Thus those parts of the fiber with higher losses are recognized as regions where the detected

characteristic has a larger slope, and those parts with higher backscattering are recognized as

regions where there is a higher value of detected power. Any discontinuities (as splices or a fiber

end) produce high reflections and jumps in the characteristic of the detected signal.

The OTDR sensors detect the changes of the backscattered reflection R or changes of the losses

induced by the measured quantity. Measuring the losses in OTDRs with specially prepared fibers

makes it possible to detect the temperature, pressure, liquid leakage, displacement, and other

mechanical and chemical measurand. Another possibility is measuring the state of polarization of

the backscattered light. Such polarization OTDRs measure changes in birefringence of the fiber,

which is sensitive to strain, pressure, magnetic field, electric field and temperature.

The OTDR allows measurements of changes in the measurand with a spatial resolution

approximately equal to 1m. Another technique, optical-frequency domain reflectometry (OFDR),

offers better resolution. It is based on a chirped radio-frequency-modulated light source and on

determining the position of light backscattering via the measured frequency.

FOS SYSTEMS

In a simple one-sensor, one fiber arrangement of a discrete FOS, the optical fiber is largely

underutilized as a transmission medium, mostly because the capacity of one information channel is

much greater than the information generated by a typical sensor output. Important gains can

therefore be made by multiplexing the fiber link by tapping several sensing devices into one passive

fiber highway to increase the maximal number of sensors and to establish data telemetry channels.

Such a configuration will obviously decrease the installation costs per sensor and at the same time

increase the attractiveness of fiber-optic sensing technology for many potential users. Furthermore,

industry has needed to install increasing numbers of sensors in automation systems in factories,

chemical plants, spacecraft, etc. Another driving force behind development of multiplexed FOSsystem is their close relation to fiber-based LANs; the fact that they utilize the same or similar


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components will keep their prices low even if the market for industrial FOS systems is only in the

early stages.

Basic Multiplexing Concepts

There are two principal multiplexing concepts that can be applied in designing a sensor system: in

one class, a given number of sensors, having only one and the same property modulated by the

external measurand field, can be incorporated into a single passive highway loop. Then some kind

of multiplexing, addressing and decoding is added to the system to allow monitoring, detection and

interpretation of a returning output signal from a given sensor at a particular location. A second

class involves the so-called orthogonal modulation scheme, where one measurand modulates

different optical properties of the sensor. Such a system is capable of simultaneously performing

measurements of several different physical parameters.

With the variety of optical fiber sensor, it does not seem possible yet to characterize the multiplexed

system most likely to emerge as the standard, although its topology and parameters will obviously

depend upon the type of sensor used, and in almost any multiplexed system at least four basic

functions will have to be performed: powering the system at a level adequate for the predesigned

power budget of the network, addressing and interrogating (identifying) a sensing device at a

chosen discrete location, detecting the measurand-modulated signal of a given sensor, and

eventually evaluating and calibrating the acquired individual sensor signal. The topologicalarrangement of a network, will largely depend on the scheme chosen for sensor addressing and

demodulation.

9 Generalized FOS network


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Examples of discrete sensor multiplexing techniques

Although many different multiplexing schemes, often very sophisticated and costly, have been

reported, industrial applications are usually looking for simpler, and cheaper ideas. In the next

section will be described some multiplexing configuration.

Space Division Multiplexing

Space Division Multiplexing (SDM), utilizing separate fiber paths and separate detector-source

arrangements for individual sensors, is the easiest method to use and has been already implemented.

Thanks to the rapidly decreasing of optoelectronic components, this technique has been reevaluated.

The power budget is excellent, crosstalk is nonexistent, and the failure of one channel can usually

be tolerated. The method can also be combined with a TDM or WDM scheme. Several other

possible topologies of the SDM method involve common light source with multiple detector array,

multiple sources with common detector, single source and single detector with one-to-NandN-to-

one switching, and synchronous switching.

Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) has the advantage that it still permit an excellent power

budget for the system, as in the SDM, while affording savings in the form of a smaller number of

necessary fiber links and connections. The light signal sent to each sensor is directed through the

appropriate WDM coupling components, which are broadly similar to those designed for fiber

communication systems. The most important problem related to this method is obtaining sufficient

selectivity of the utilized wavelength division filters. To further increase the number of

multiplexable devices, the technique may obviously be used not only with single, but also with

dual- or multiple- fiber optical links.


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Time Division Multiplexing

Time Division Multiplexed (TDM) systems usually require fast and costly electronics and because

of this are less attractive for many industrial applications where the cost of installed HW becomes a

dominating factor. In a TDM system, each individual sensor signal can be identified and decoded at

the detection end of the setup by arranging different propagation delays for the light signals

returning from sensors at different locations. This method can also be coupled with WDMtechnique in multiwavelength monitoring system, with one wavelength affected by the measurand

and another used for the reference. The method has several important advantages, including the

large number of channels, the one-source, one-detector configuration, and equal applicability to

both coherent and noncoherent system. However, usually small optical path differences between the

sensors require nontrivial processing. To this end, many complex topologies have already been

proposed. One possible form is a TDM optical passive sensor highway incorporating a

commercially available electromechanical switch. Such a system has practically no crosstalk, but a

stringent requirement to contain the time-sharing switching sequence within a very short period

must be satisfied.


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Frequency Division Multiplexing

The general approach to FDM of fiber-optic sensors is to send an amplitude or frequency-

modulated output signal from every sensor in a given network through an assigned frequency

channel. This method consists of modulating several light sources by signals of different frequency,

or modulating one light source with several frequencies and then combining and separating signals

at the detection end of the system employing a multichannel phase-sensitive detection scheme. One

solution is a so-called matrix array method of FD multiplexing of intensity sensors. It has much

simpler and slower electronics than typical TDM systems, and has a good potential for industrial

intensity-modulated sensor multiplexing.

Coherence Division Multiplexing

Light sources can have widely varying coherence lengths depending on their spectrum. By using

light sources that have coherence lengths that are short compared to offsets between the reference

and signal legs in Mach-Zehnder interferometers and between successive sensors, a coherencemultiplexed system may be set up. The signal is extracted by putting a rebalancing interferometer in

front of each detector so that the sensor signals may be processed. Coherence multiplexing is not as

commonly used as time, frequency and wavelength division multiplexing because of optical power

budgets and the additional complexities in setting up the optics properly. It is still a potentially

powerful technique and may become more widely used as optical component performance and

availability continue to improve, especially in the area of integrated optic chips where control of

optical pathlength differences is relatively straightforward.


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APPLICATIONS OF FOS

Fiber optic sensors are being developed and used in two major ways. The first is as a direct

replacement for existing sensors where the fiber sensor offers significantly improved performance,

reliability, safety and/or cost advantages to the end user. The second area is the development and

deployment of fiber optic sensors in new market areas.

For the case of direct replacement, the inherent value of the fiber sensor, to the customer, has to be

sufficiently high to displace older technology. Because this often involves replacing technology the

customer is familiar with, the improvements must be substantial.

The most obvious example of a fiber optic sensor succeeding in this arena is the fiber optic gyro,

which is displacing both mechanical and ring laser gyros for medium accuracy devices. As this

technology matures it can be expected that the fiber gyro will dominate large segments of this

market. Significant development efforts are underway in the United States in the area of fly-by-light

where conventional electronic sensor technology are targeted to be replaced by equivalent fiber

optic sensor technology that offers sensors with relative immunity to electromagnetic interference,

significant weight savings and safety improvements. In manufacturing, fiber sensors are being

developed to support process control. Oftentimes the selling points for these sensors are

improvements in environmental ruggedness and safety, especially in areas where electrical

discharges could be hazardous. One other area where fiber optic sensors are being mass-produced is

the field of medicine, where they are being used to measure blood gas parameters and dosage levels.

Because these sensors are completely passive they pose no electrical shock threat to the patient and

their inherent safety has lead to a relatively rapid introduction.


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The automotive industry, construction industry and other traditional users of sensors remain

relatively untouched by fiber sensors, mainly because of cost considerations. This can be expected

to change as the improvements in optoelectronics and fiber optic communications continue to

expand along with the continuing emergence of new fiber optic sensors.

New market areas present opportunities where equivalent sensors do not exist. New sensors, once

developed, will most likely have a large impact in these areas. A prime example of this is in the area

of fiber optic smart structures. Fiber optic sensors are being embedded into or attached to materials

during the manufacturing process to enhance process control systems, to augment nondestructive

evaluation once parts have been made, to form health and damage assessment systems once parts

have been assembled into structures and to enhance control systems. A basic fiber optic smart

structure system is shown in the below figure.

10 Fiber optic smart structure systems

Fiber optic sensors can be embedded in a panel and multiplexed to minimize the number of leads.

The signals from the panel are fed back to an optical/electronic processor for decoding. Theinformation is formatted and transmitted to a control system which could be augmenting

performance or assessing health. The control system would then act, via a fiber optic link, to modify

the structure in response to the environmental effect.

In the next figure, is shown how the system might be used in manufacturing. Here fiber sensors are

attached to a part to be processed in an autoclave. Sensors could be used to monitor internal

temperature, strain, and degree of cure. These measurements could be used to control the

autoclaving process, improving yield and the quality of the parts.


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11 Smart manufacturing system

Interesting areas for health and damage assessment systems are on large structures such as

buildings, bridges, dams, aircraft and spacecraft. In order to support these types of structures it will

be necessary to have very large numbers of sensors that are rapidly reconfigurable and redundant. It

will also be absolutely necessary to demonstrate the value and cost effectiveness of these systems to

the end users. One approach to this problem is to use fiber sensors that have the potential to be

manufactured cheaply in very large quantities while offering superior performance characteristics.

Two candidates that are under investigation are the fiber gratings and etalons. Both offer the

advantages of spectrally based sensors and have the prospect of rapid in line manufacture. In the

case of the fiber grating, the early demonstration of fiber being written into it as it is being pulled

has been especially impressive.

These fiber sensors could be folded into the wavelength and time division multiplexed modular

architecture. Here sensors are multiplexed along fiber strings and an optical switch is used tosupport the many strings. Potentially the fiber strings could have tens or hundreds of sensors and the

optical switches could support a like number of strings. To avoid overloading the system, the output

from the sensors could be slowly scanned to determine status in a continuously updated manner.


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12 A modular architecture for a large smart structure system for a vehicle health management bus.

When an event occurred that required a more detailed assessment the appropriate strings and the

sensors in them could be monitored in a high performance mode. The information from these

sensors would then be formatted and transmitted via a fiber optic link to a subsystem signal

processor before introduction onto a health management bus. In the case of avionics the system

architecture might look as shown in the above figure. The information from the health management

bus could be processed and distributed to the pilot or more likely, could reduce his direct workload

leaving more time for the necessary control functions.

13 A typical vehicle health management bus for an avionics system

As fiber to the curb and fiber to the home moves closer to reality there is the prospect of merging

fiber optic sensor and communication systems into very large systems capable of monitoring the

status of buildings, bridges, highways and factories over widely dispersed areas. Functions such as

fire, police, maintenance scheduling and emergency response to earthquakes, hurricanes and


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tornadoes could be readily integrated into very wide area networks of sensors as shown in the next

figure.

14 Fiber optic sensor networks to monitor the status of widely dispersed assets as buildings and bridges

It is also possible to use fiber optic sensors in combination with fiber optic communication links to

monitor stress build up in critical fault locations and dome build up of volcanoes. These widely

dispersed fiber networks may offer the first real means of gathering information necessary to form

prediction models for these natural hazards.

Photonics FOS

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