1.1 Historical Developments of Fiber Optic...

Introduction to Fiber Optic Sensors and Fiber Gratings

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1.1 Historical Developments of Fiber Optic Sensors

Through a long process of evolution, the development of human society has now

entered a highly information-oriented era. The information technology has become

the major trend of contemporary development. With more and more high technology

products going into peoples’ lives, there is more demand for high-speed and large-

capacity information carriers. These have been the targets for many researchers. In

past forty years, revolution is brought out in information technology due to

developments in optoelectronics and telecommunication industries. Improvements in

quality and cost reduction in optoelectronic components led the industry to bring out

high performing devices such as compact disc, players, bar code scanner, high end

lasers, laser printers, CCD camera, LCD projector, stable photodiodes, high sensitive

photodetectors, phototransistors, etc. Ultra fast development in telecommunication

was due to developments in fiber optics communication industry by providing high

performance and more reliable telecommunication links, larger bandwidth for

information carriage at low cost.

The invention of lasers in 1960’s opened new window for researchers to study the

optical fibers for data communication, sensing and other applications in the next

decade. Researchers conducted experiments by transmitting the laser beam in

different waveguides. In the beginning large losses in optical fibers prevented the

replacement of co-axial cables by optical fibers. Early fibers had losses of

1000dB/km, means only 1% of light transmitted in 20m fiber, making them

impractical in communication use. Charles Kao who won Nobel Prize in 2009 for his

contributions along with his co-worker G. A. Hockham investigated fundamental

properties of optical fibers in detail way back in 1966 for optical communication and

came to conclusion that – the losses in dielectric media were mostly caused by


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absorption and scattering. The latter was predominantly caused by impurities, in

particular iron ions present. Fibers with glass of higher purity could be a good

candidate for optical communication. As coated by C. Kao - “Compared with existing

coaxial cable and radio systems, this form of waveguide has a larger information

capacity and possible advantages in basic material cost. The realization of a

successful fiber waveguide depends, at present, on the availability of suitable low-loss

dielectric material. The crucial material problem appears to be one which is difficult

but not impossible. Certainly, the required loss figure of around 20dB/km is much

higher than the lower limit of loss figure imposed by the fundamental mechanisms ,”

[1]. Later in 1969 C. Kao along with other co-workers, showed that fused silica (SiO2)

had purity for good optical communication. An intense worldwide research with aim

to produce low loss optical fibers began. In 1970, research team from the Corning

Glass Works in the United States consisting of F. P. Kapron, D. B. Keck, P. C.

Schultz, F. Zimar, under the leadership of R. D. Maurer, succeeded in making glass

fibers of fused silica with low losses as C. Kao had envisioned. The team fabricated

fiber by chemical method called chemical vapor deposition (CVD). To make a core

and cladding with very close refractive indices, they doped titanium in the fused silica

core and used pure fused silica in the cladding. A few years later (in 1974), loss even

reached 4dB/km at 850nm using germanium instead of titanium. Several other

technologies were developed in Japan, USA and UK. In that direction J. B.

MacChesney and co-workers at Bell Laboratories developed a modified CVD

technique, allowing efficient production of optical fibers. Within a few years,

attenuation less than 1dB/km was achieved, which was even far below the target set

by C. Kao. Today, attenuation of light around 1.55µm wavelength in fibers is below

0.2dB/km [2]. Modern optical fibers are extraordinarily transparent media, with more


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than 95% light transmitted after 1km propagation [3]. Later advances in fiber optic

technology have significantly changed the telecommunication industry. The ability to

carry gigabits of information at the speed of light increased the research potential in

optical fibers. Improvements and cost reductions in optoelectronic components led to

emergence of new product area.

With improved technologies of manufacturing, optical fiber material loss almost

disappeared and the sensitivity for detection of the losses increased. With

development in low power measurement detectors, one could even sense small

changes in phase, intensity and wavelength of a light carried by an optical fiber due to

outside perturbations on the fiber. Optical fiber being physical medium is subjected to

perturbation of one or the other kind. Therefore it experiences geometrical (size,

shape, strain) or optical (RI, mode conversion) changes depending upon the nature

and magnitude of the perturbation. In telecommunication applications, one tries to

minimize such effects so that signal transmission and reception is reliable. On the

other hand, in fiber optic sensor (FOS) field, the response to external perturbation is

deliberately enhanced, so that the resulting change in optical radiation can be used as

measure of external perturbation. In FOS, the fiber acts as modulator. It also serves as

transducer and converts measurement data like temperature, stress, strain, rotation or

electrical and magnetic currents into corresponding change in optical radiation. This

sub branch of optical fiber technology soon saw an intense R&D activities around the

world, which led to the emergence of the new field called ‘Fiber Optic Sensors’.

Development of optical fiber sensors started in 1977 even though some isolated

demonstrations were made in earlier days. Many laboratories entered into the field

resulting into rapid progress. In the beginning fiber optics sensors were developed for

sensing sound [4-6], pressure [7-9], temperature [10], magnetic field [11, 12], rotation


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[13, 14], current [15, 16], acceleration, fluid level, torque, photo acoustics, current,

displacement etc [17].

Light is characterized by phase, polarization, frequency, wavelength and intensity

(amplitude). Any one or more of these parameters may undergo a change due to

external perturbation. Our ability to measure and quantify the change reliably and

accurately is state of art of sensor design. In fiber optic sensors, information is

conveyed by change either in phase, polarization, frequency, wavelength, intensity or

combination of above properties of optical fiber. But the potodetector, being

semiconductor device only senses the intensity of light at the detector surface. Hence,

the art of sensing with phase, frequency or polarization modulation involves

interferometric or grating based signal processing optical circuits.

1.2 Advantages of Fiber Optic Sensors

The main drive of research in FOS area today is to produce a range of optical-

fiber based techniques which can be used to measure different physical parameters,

providing a foundation for an effective measurement technology, strengthen the

technology that can compete with conventional methods and tackling difficult

measurement situations where conventional sensors are not well suited to use in a

particular environment. The resulting sensors have a series of characteristics those are

significantly advantageous compared to conventional electrical sensors. Following are

some advantages of FOS compared to conventional electrical sensors. Following are

some advantages of FOS compared to conventional electrical sensors [18].

1. Sensed signal is immune to electromagnetic interference (EMF) and radio

frequency interference (RFI).

2. Intrinsically safe in explosive environments.


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3. Highly reliable and secure with no risk of fire /spark.

4. High voltage insulation and absence of ground loops and hence avoid any

necessity of isolation devices like optocouplers.

5. Low volume and light weight (1kilometer of 200µm silica fiber weighs only

70grams and occupies volume of nearly 30cm3).

6. As point sensors, they can be used to sense parameters in inaccessible regions

without perturbation of the transmitted signals.

7. They be easily interfaced with low-loss optical fiber telemetry and hence

affords remote sensing by locating the control electronics for LED/lasers and

detectors far away from the sensor head.

8. Large bandwidth and hence offer possible multiplexing a large number of

individually addressed point sensors in a fiber network or distributed sensing.

9. Chemically inert and they can be readily employed in chemical processes and

biomedical instrumentation due to their smaller size and mechanical

flexibility.

10. Multifunctional sensing capabilities such as strain, pressure, corrosion,

temperature and acoustic signals.

11. Robust, more resistant to harsh environments.

These advantages were sufficient to attract intensive research and development

activities around the world to develop new class of sensors based on fiber optics.

This has eventually led to the emergence of variety of fiber optic sensors for accurate

sensing and measurement of physical parameters [18].

Some of the disadvantages of FOS were also noticed during different sensor

developments. One is the poor elastic property that makes optical fiber extremely

brittle. This is not particularly desired for sensing applications. Consequently, the


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handling, treatment and operation of FOS require extreme care. In addition, each

process requires technical skills. With the current technology, in situ assembly of the

optical system is a cumbersome and exhaustive process, since the measurement setup

of the optical fiber sensor system is comprised of different modules (for example,

light source, coupler and receiver), difficult splicing process and the constant need

for careful treatment. Although the method of manufacturing fiber gratings has

improved considerably, current technology only manages to produce one at one time

and most of the process still requires manual handling. In addition, the cost of

running fiber grating writing facilities is high due to expensive laser, masks (both

phase mask and amplitude masks) and the requirement for a highly skilled operator.

The lifespan of optical fiber is considered long for telecommunications, between 20

to 25 years, it may be not enough for some of sensing applications. Typical civil

engineering structures such as buildings, bridges and dams typically have lifespan of

more than 70 years where FOS is used for structural health monitoring. Therefore,

packaging FOS is next big task after a sensor design [19-23].

1.3 Basic Principle of Fiber Optic Sensor

Although light is trapped within the dielectric medium of the optical waveguide,

the radiation that propagates inside the waveguide can be perturbed by the external

environment and this perturbation can be used to draw useful information for sensing

purposes. In fact, the interaction of the physical parameter of interest that is measured

with the waveguide produces a modulation in the propagation constants of the guided

light beam. This modulation is a sensitive function of the measurand of interest.

Following are basic elements constituting a fiber optic sensor [19-21].


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1. Optical sources: Commonly used light sources in visible or infra red range

are applicable. E.g. Laser, LED, Laser diode etc

2. Optical fiber: A sensing or modulator element (which transduces the

measurand to an optical signal). Varieties of optical fibers - both multimode

and single mode fibers - are used.

3. Optical detector and processing electronics: High resolution detector

compatible to detect light from above mentioned light sources are used.

E.g. oscilloscope, optical spectrum analyzer etc).

The general structure of an optical fiber sensor system is shown in Fig.1.1.

Figure 1.1: Basic components of optical fiber sensing system.

1.4 Design of FOS

A fiber optic sensor can be designed in different ways based on the requirement.

A schematic of the three most common sensing designs - point, distributed and quasi-

distributed sensing is illustrated in Fig.1.2 (a-c).

1.4.1 Point Sensor

The measurement of any particular measurand at a particular location is basic

need of a sensor system. This is achieved with a point sensor. Here, tip of the fiber is

allowed to sense the change (Fig. 1.2 (a)). Most of fiber optic sensors such as those

Detector

Light

source

Interaction

with

measurand

Optical fiber


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used in the monitoring of temperature, acceleration, pressure or chemical parameters

use this design [22].

1.4.2 Distributed Sensor

With some sensors, the whole length of the fiber can be used as a sensing

element. The measurand action is sensed along the length of the fiber itself and

process is termed distributed sensing, illustrated in Fig. 1.2(b). This principle has been

employed widely in the measurement of temperature using non-linear effects in fibers,

such as Brillouin or Raman scattering or in some types of strain sensing. This yields

either an enhanced sensitivity if the fiber is wrapped up to form a single sensing point

or it can be used for spatial averaging of the signal [22].

1.4.3 Quasi Distributed Sensor

A style of sensor that is somewhat in between these two types of sensors is

termed quasi-distributed. As shown schematically in Fig. 1.2(c), the measurand

information is obtained at different pre-determined points along the length of a fiber

network. Here, the fiber has been sensitized or special materials have been introduced

into the fiber loop to allow the measurement to be taken and this technique has been

applied to temperature and chemical sensing, e.g., using different fiber types [22]

Figure 1.2: Scheme of three most common fiber optic sensor designs: (a) Point sensor,

(b) Distributed Sensor, (c) Quasi distributed sensor.

Light source,

detection &

signal

processing

Light

sourceDetection &

signal

processing

Light

source

Detection &

signal

processing

(a) (b) (c)


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1.5 Types of Fiber Optic Sensors

Varieties of fiber optic sensors were developed in the direction to full fill the

needs of human society. Optical fiber sensors are classified into different classes

based on different categories: the sensing location, the operating principle, the

applications etc [18, 20].

1.5.1 Based on Sensing Location

In FOS, the light may be modulated either inside or outside the optical fiber i.e.

sensing location may be inside or outside the fiber. Hence, based on sensing location

FOSs are classified broadly as intrinsic and extrinsic sensors. This is the simplest

classification of fiber optic sensors.

1.5.1.2 Intrinsic Sensor

In intrinsic fiber optic sensor, the interaction occurs within an element of optical

fiber itself and light never leaves the waveguide. External environment acts on the

fiber and the fiber in turn changes some characteristic of the light inside the fiber that

is measured using the detector. One or more of the physical properties of the guided

light, e.g., intensity, phase, polarization or wavelength is modulated by the

measurand. A schematic illustration of intrinsic sensor can be seen in Fig. 1.3.

Examples for intrinsic sensors are pressure sensor, temperature sensor, strain sensor,

etc. [18, 20, 23, 25].

Fiber Optic Sensor

Intrinsic Sensor Extrinsic Sensor


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Figure 1.3: Intrinsic sensor illustration.

1.5.1.2 Extrinsic Sensor

In extrinsic fiber optic sensor, the optical fiber is used to couple light, usually to

and from the region where the light beam is influenced by the measurand (or external

environment). In this case, the fiber just acts as a means of getting the light to the

sensing location and to detector as shown in Fig 1.4. Common examples of this type

of sensors are optical fiber endoscopy, displacement sensor and fiber-optic

fluorescence sensor. In fiber-optic fluorescence sensor the light is coupled out of the

fiber and excites the analyte. The light emitted by fluorescence of the analyte is

collected by the second fiber and guided to the detector [18-21].

Figure 1.4: Extrinsic sensor illustration.

1.5.2 Based on Modulation Technique

Based on the operating principle or modulation and demodulation process, a fiber

optic sensor can be classified into 4 groups

1. Intensity modulated sensor

2. Wavelength modulated sensor

Light

Source Detector

Feed

Fiber

Receiver

fiber

Action


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3. Polarization modulated sensor

4. Phase modulated sensor

All these parameters may be subject to change due to external perturbations. Thus, by

detecting these parameters and their changes, the external perturbations can be sensed

1.5.2.1 Intensity Modulated Sensor

Intensity modulation is simplest and cheapest method of detecting different

parameters using optical fiber. Intensity-based fiber optic sensors are based on

intensity undergoing change. In intensity modulated fiber optic sensors, measurand

modulates the intensity of light transmitted through the optical fiber and variations in

the intensity are measured at the output end of the optical fiber using a detector.

Intensity modulated sensors are analogue in nature and have significant usage in

digital applications for switches and counters [23]. These require more light and

therefore usually use multimode optical fibers. Intensity modulated FOS can be found

in variety of intrinsic and extrinsic configurations. The intensity modulation can be

achieved through variety of methods or mechanisms such as transmissive, reflective,

microbending, attenuation and evanescent fields that can produce a measurand

induced change in the optical intensity propagated by an optical fiber [23].

The advantages of these sensors are: Simplicity of implementation, low cost,

possibility of being multiplexed and ability to perform as real distributed sensors. The

drawbacks are: Relative measurements and variations in the intensity of the light

source may lead to false readings, unless a reference system is used [20].

1.5.2.2 Wavelength Modulated Sensors

Wavelength modulated fiber optic sensors are based on changes in the wavelength of

a light for detection. Fluorescence sensors [24, 25], black body sensors [26] and the

fiber grating sensor [27-29] are examples of wavelength-modulated sensors. Fiber


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grating sensors will be discussed in next chapters in detail. Fluorescent based fiber

sensors are being widely used for medical applications, chemical sensing and physical

parameter measurements such as temperature, viscosity and humidity. Different

configurations are used for these sensors. One of the simplest wavelength based

sensor is the blackbody sensor as shown in Fig.1.5. A blackbody cavity is placed at

the end of an optical fiber. When the cavity rises in temperature it starts to glow and

act as a light source. Detectors in combination with narrow band filters are then used

to determine the profile of the blackbody curve. This type of sensor has been

successfully commercialized and has been used to measure temperature to within a

few degrees centigrade under intense RF fields. Because of better signal to noise ratio,

this sensor performance is good at higher temperatures – above 2000C and falls off

below 2000C [20, 30].

Figure 1.5: Blackbody Fiber Optic Sensor.

1.5.2.3 Phase Modulated Fiber Optic Sensors

Phase modulated sensors use modulation in phase of the light being transmitted

in an optical fiber for detection. The optical phase of the light passing through the

fiber is changed by the physical quantity to be measured. Phase modulated FOS are

highly sensitive to measurand. Hence they are employed when extreme sensitivity is

required. Most of phase modulated FOS use single mode fiber and are of intrinsic


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type. The phase angle (ф) for lightwave traveling in the fiber of length L is defined as

[23]

ф =2πn L/ λ (1.1)

where ‘n’ is refractive index of the core and λ is the wavelength of light. A change in

length or refractive index under the influence of external physical parameter will

cause a phase change as defined by the equation

∆ ф =2π/ λ (n ∆L +∆n L) (1.2)

The expression n∆L+∆nL, is called optical path difference (OPD). The optical

intensity at the output of a interferometer is function of OPD. Very small change in

length produces large phase difference. Similarly, very small changes of refractive

index at longer sections of fiber produce large phase differences. When phase shift is

integral multiple of wavelength, lights from the two arms of the interferometer are in

phase providing constructive interference and maximum intensity at the output. If the

phase shift is integral multiple of half of the wavelength, lights from the two arms of

the interferometer are out of phase providing destructive interference and minimum

intensity [23].

In phase modulated sensors, this sensitive phase change of the light is encashed

to design sensor. But the difficulty is optical phase change cannot be directly detected

(optical waves have frequencies in the range of few hundred THz). The optical phase

change of the light is detected by comparing the phase with reference fiber which is

identical to measuring fiber but unperturbed by the measurand. In order to detect

phase difference, it is necessary to convert phase difference to optical intensity change

which can be measured by a detector. This is achieved by combining two optical

signals – one from sensing fiber and other from reference fiber. In an interferometer,

the light is split into two beams, where one beam is exposed to the sensing


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environment and undergoes a phase shift. The other is isolated from the sensing

environment, which is used as a reference. The whole system is called interferometer.

This phase modulation is then detected interferometerically, by comparing the phase

of the light in the signal fiber to that in a reference fiber. Once the beams are

recombined, they interfere with each other. There are four most commonly used

interferometric configurations. They are - Michelson, Mach-Zehnder, Fabry-Perot and

Sagnac interferometers [22, 23, 32-35].

Michelson Interferometer: Experimental configuration of Michelson interferometer

is shown in the Fig.1.6. One 2X2 coupler (3dB coupler) that splits input light into

exactly two parts is used in the Michelson interferometer. The coherent laser light

launched in one arm of the coupler is split and injected into two arms on the other side

of the coupler. One of these two arms is allowed to be perturbed by a physical

parameter to be measured and reference arm is protected. These two arms (sensing

and reference) have mirrored ends to reflect the beam back through the same fibers

and coupler to a detector. Any perturbation due to physical change at sensing arm

varies phase of light in that arm.

Figure.1.6: Michelson interferometer configuration.

The phase shift is then detected. Because the light passes both through the sensing and

reference fibers twice, the optical phase shift per unit length of fiber is doubled.

Michelson interferometer can intrinsically have better sensitivity. Another clear


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advantage of the Michelson interferometer is that the sensor can be designed with a

single coupler between the source-detector module. However, good-quality reflection

mirrors are required. The disadvantage of Michelson interferometer is that the coupler

feeds light into both the detector and laser. Feedback into the laser is source of noise,

especially in high performance systems [23].

Mach-Zehnder Interferometer: Mach-Zehnder interferometer uses two identical

2X2 couplers. The experimental configuration is shown in the Fig.1.7. Light launched

into one of the arm of the 1st coupler is split into two identical beams and launched

into other two arms considered one as sensing and other as reference arm. These

beams are combined using another coupler and phase shift is measured. The phase

shift results from changes in the length or refractive index of the sensing fiber. If the

path lengths of the sensing and reference fiber are same or differ by an integral

multiple of wavelengths, the combined beams are exactly in phase and the beam

intensity is maximum. However, if the two beams are out of phase by λ/2, the

recombined beam is at its minimum value of intensity. A modulation of 100% occurs

over λ/2 change in fiber length. With this sensitivity, movements as small as 10-3

meters can be detected [23].

Figure 1.7: Mach-Zehnder interferometer configuration.

Fabry-Perot Interferometer: Fabry-Perot interferometer is type of multi beam

interferometer. It is again classified into intrinsic Fabry-Perot interferometer (IFPI)


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and extrinsic Fabry-Perot interferometer (EFPI). The experimental configuration of

IFPI shown in the Fig. 1.8. It consists of two partial reflecting fiber mirrors. The

injected coherent beam is partially reflected back and partially transmitted into

interferometer. At second partial reflecting mirror, again the beam is partially

reflected and partially transmitted. In this type of interferometers, the light bounces

back and forth many times in the cavity, increasing the phase delay many times. This

transmitted light is detected through the detector at the other end. Successive

reflection sequences will reduce the detection beam. The multiple passage of light

along the fiber magnifies the phase difference resulting into highly sensitive sensor

[23, 36].

Figure 1.8: Fabry-Perot Interferometer configuration.

Fiber Optic Gyroscopes: The principle of fiber optic gyroscope is based on Sagnac

interferometry. Hence, they are also called as Sagnac interferometers. Gyroscopes are

principally used to measure rotation and are replacement for ring laser gyros and

mechanical gyros. Fiber optic gyroscopes are also used to measure time varying

effects such as angular velocity, acoustics, vibrations and slowly varying strain. Fiber

optic gyroscopes are most developed FOS. Several manufacturers worldwide are

producing them in large quantities to support automobile navigation systems, pointing

Source

Partial reflecting

mirrors

Optical fiber

Detector


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and tracking of satellite antennas, inertial measurement systems for commuter aircraft

and missiles and as the backup guidance system for the Boeing 777. [20, 23, 26].

Figure 1.9: Configuration of Fiber optic gyroscope.

The basic configuration of fiber optic gyroscope is shown in Fig. 1.9. The fiber

optic gyroscope consists of loop of fiber. Light from the laser is split into two beams

and launched simultaneously into both end of the fiber loop. Both beams travel in

counter propagating directions around the loop of fiber and recombined to analyze at

a detector. In non-rotating loop, clockwise and anticlock wise beams arrive at same

time in phase and form constructive interference. If the loop is rotated in clockwise

direction, the entire coil is displaced slightly increasing the time it takes light to

traverse the fiber optic coil. Thus, the clockwise propagating light beam has to go

through a slightly longer optical pathlength than the counter-clockwise beam, which is

moving in a direction opposite to the motion of the fiber coil. Two beams reach the

detector at different times and phase difference is introduced. These differences in

arrival time and phase difference are directly proportional to the rotation rate and can

be conveniently measured as phase differences with great sensitivity and accuracy

[37-39]. Fiber optic gyroscopes are used in two configurations - open loop fiber optic

gyros and closed loop fiber optic gyros.

1.5.2.4 Polarization Modulated Fiber Optic Sensors

Standard single mode fibers transmit light without regards to polarization. But


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some special fibers known as polarization mainlined fibers maintain input

polarization or transmit only one polarization state. The refractive index of a fiber

changes when fiber undergoes stress or strain depending on the direction. The

refractive index undergoing change due to stress or strain is called induced refractive

index. Because of induced refractive index, there is an induced phase difference

between different polarization directions. This phenomenon is called photoelastic

effect. The induced refractive index changes with the direction of applied stress or

strain. Thus, there is an induced phase difference between different polarization

directions. Therefore, by detecting the change in the output polarization state, the

external perturbation can be sensed. Fig. 1.10 shows the optical setup for the

polarization based fiber optic sensor. It is formed by polarizing the light from a source

via a polarizer. The polarized light is launched into polarization maintained fiber. This

section of fiber is served as sensing fiber. Under external perturbation such as stress

or strain, the phase difference between two polarization states is changed. Then, the

output polarization state is changed according to the perturbation. Hence, by

analyzing the output polarization state at the exit end of the fiber, the external

perturbation can be detected [20].

Figure 1.10: Polarization based sensor.


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1.6 Fiber Gratings

Even though the improvements in optical fiber manufacturing and advancements

in the field were reality in late 1970’s; integrating optical components such as mirrors,

wavelength filters and partial reflectors to optical fibers for various applications was a

challenging job. However, all these hurdles were cleared with the discovery of fiber

gratings [40] and further refinement of fiber gating inscription method externally, that

was demonstrated by Meltz G., et. al. [41]. Fiber gratings were discovered by Hill et.

al. in 1978 [40] where the team successfully altered the core refractive index of a

single-mode optical fiber by optical absorption of UV light.

The change in core refractive index can be explained in terms of photosensitivity

of optical fibers, that allows the fabrication of phase structures in the core of fibers.

These phase structures or phase gratings are obtained by permanently changing the

refractive index in a periodic pattern along the core of the fiber. The grating period

and length together with the strength of the modulation of the refractive index

determine whether the grating has a high or low reflectivity over a wide or narrow

range of wavelengths. Therefore, these parameters determine whether the grating acts

as a wavelength division multiplexer in telecommunications, a narrow-band high-

reflectance mirror in laser or sensor applications or a wavelength-selective filter

removing unwanted laser frequencies [42].

Fiber grating is periodic perturbation along the optical path. Fiber grating is an

optical device with periodic perturbation along short section of the optical fiber core.

This perturbation in the fiber may be either in the form of refractive index, thickness

or density of glass. Most of the fiber gratings have periodic refractive index

modulation in the core that poses perturbation to the optical path. When light

propagates through periodically alternating regions of higher and lower refractive


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index, it is partially reflected and partially refracted at each interface between those

regions. This periodic perturbation in the optical path directs coupling of power from

one mode to another and pitch of the perturbation selects coupling modes. [43, 44].

Fiber grating technology is widely applicable in optical communication systems and

sensing field.

Fiber grating acts as common diffraction grating of period Λ. A light wave incident

on the grating at an angle θ1 is diffracted out with an angle θ2 (Fig. 1.11). The grating

equation is given as [45],

Λ

+=λ

θθ mnn 12 sinsin (1.3)

‘m’ is diffraction order. Now considering that incident and diffracted rays

correspond to bounded modes of optical fiber, the propagation constant β can be

defined as

effnλ

πβ

2= (1.4)

with neff = nco sinθ, where neff represents effective refractive index of the mode and nco

represents refractive index of the core [46].

Figure 1.11: Diffraction by grating.

Now Eq. 1.3 can be written as

Λ

−=π

ββ2

12 (1.5)


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From this equation one can predict for which grating period the coupling occurs

between two modes of propagation constant β1 and β2 at the wavelength λ.

In uniform gratings, grating planes are perpendicular to the fiber axis and are of a

constant period. These grating periods are considered as fundamental building blocks

of fiber gratings. In general fiber gratings consists of the periodic refractive index

modulation in the optical fiber core. The pitch is called period of the grating (Λ) as

shown in Fig.1.12.

Figure 1.12: Fiber grating.

Fiber gratings are broadly divided into two types based on the grating period and

light coupling scheme.

– fiber Bragg gratings also called short period gratings or reflecting gratings

– long period gratings also called transmission gratings

The relative grating period of both types of fiber gratings can be realized by observing

Fig.1.13 where there is large difference in grating period.

Figure 1.13: (a) Fiber Bragg grating (b) Long period grating.


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1.6.1 Fiber Bragg Grating (FBG)

Fiber Bragg gratings consists of periodic modulation of refractive index in the

core of single mode fiber with grating period less than 100µm, practically submicron

period. FBGs act as narrow band reflection filters. FBG couples forward coupling

core mode to backward coupling core modes those scattered by grating planes,

reflecting back a small band of source spectrum [42, 47]. Reflected spectrum can be

observed in optical spectrum analyzer using 2x2 coupler or circulators. In transmitted

spectrum of FBG, a narrow band will be missing as it is reflected back. Input and

reflected spectra of typical FBG are shown in Fig.1.14. Advantages of FBGs is that-

in FBGs information is encoded in absolute parameter i.e. wavelength, they can act as

point sources, any number of FBGs can be multiplexed in single line. More

discussion on FBG will be continued in next the chapter.

Figure 1.14: FBG action.

1.6.2 Long Period Grating (LPG)

Long period gratings have grating period ranging between 100µm to 1mm. LPGs

promotes coupling between forward coupling core mode and co propagating cladding

modes. Hence, the transmitted spectrum of LPG consists of series of attenuation dips

centered at discrete wavelengths. Each attenuation dip corresponds to each cladding

mode [48]. These modes decay rapidly as they propagate along the fiber axis because


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of scattering. LPG acts as band rejection filter [49]. For practical applications,

transmitted spectrum of LPG is observed. Typical input and transmitted spectrum is

shown in the Fig. 1.15.

Figure 1.15: Input and transmitted spectrum of LPG.

1.7 Lightwave Applications of FBG

An enthusiastic scenario where FBG’s are liberally applied to enhance the

lightwave network all the way from the central office to the subscriber’s premise.

Despite competitive technologies, many of these grating applications have matured,

others are nearing commercialization and, as lightwave systems evolve to optical

networks and fiber moves toward the home, the number of uses will increase. Some

light wave applications of FBG are briefly discussed below.

Laser Stabilization: Fiber Bragg grating reflectors are used as feedback mirrors in

wavelength-stabilized semiconductor lasers. Wavelength-stabilized 980nm pump

lasers [50, 51] for erbium-doped fiber amplifiers are commonly deployed today,

benefiting both pump laser yield and amplifier reliability. Grating stabilization of

1480nm pump lasers has also attracted attention [51]. Stabilization is achieved using

weak, narrow-band FBG reflectors in the fiber pigtail to couple light back into a

Fabry–Perot pump laser, creating an external laser cavity. Grating reflectivities of 1–

10% are chosen to achieve robust stabilization and maximize the laser output. Having


24

a narrow reflection bandwidth of 0.2–3nm allows the pump wavelength to be

accurately placed for optimum pumping of the fiber amplifier. This is particularly

important with 980 nm pumping as the erbium absorption bandwidth is only 7–10 nm

[52]; pump wavelength fluctuations from temperature, injection current and aging are

then significant risks to optimum amplifier performance. The use of a single FBG

reflector to simultaneously stabilize three pump lasers for use in multistage optical

amplifiers has been demonstrated. This reduces the number of gratings and provides

pump redundancy to increase the amplifier’s reliability [53, 54].

Fiber Lasers: Fiber lasers can be constructed using FBGs as wavelength-selective

resonator mirrors and erbium-doped fiber as the gain medium. These fiber lasers are

optically pumped, often directly without the use of pump wavelength division

multiplexing (WDM) couplers. Narrow line width fiber lasers suitable as externally-

modulated continuous wave sources have been demonstrated in gigabit/s transmission

experiments [55] and short distributed-feedback fiber lasers having Er–Yb-doped gain

sections have also exhibited robust single-frequency operation [56]. Low-cost

configurations have been proposed using 650 nm lasers to pump the erbium doped

gain region [53]. The output behavior is described by a simple above-threshold, two-

mirror model of the erbium fiber laser derived from rate equations. The laser

threshold and slope efficiency are easily calculated, whether pumped at 650, 980 or

1480 nm wavelength [57].

Reflectors in Fiber Amplifiers: Numerous fiber amplifier configurations have been

proposed to utilize reflectors or filters to enhance performance. Reflecting only the

pump light may increase the amplifier saturated output power in those cases of

amplifiers having marginal pump power. FBG’s can be used as efficient wavelength-

selective reflectors that discriminate between pump and signal light. A simple


25

analytical of the erbium-doped fiber amplifier [58] can be adapted to the idealized

case of negligible amplified spontaneous emission and unity reflectance of the FBG

reflector. Typically, reflecting the signal doubles the small-signal gain while

reflecting the pump may yield a 1–3dB improvement in small-signal gain. Pump

reflectors have been used to enhance performance of remotely pumped amplifiers in

repeaterless systems. Span lengths were extended by placing remote sections of

erbium doped fiber and pumping them at 1485 nm with light generated from high-

power lasers located at a terminal station. Demonstrations of greater than 500 km

repeaterless transmission at 2.5 Gb/s [59-60] and 352 km of 8 10 Gb/s [61] have been

reported. In the 352 km transmission experiment, a preamplifier 123 km from the

terminal station was remotely pumped with light from a 1.3-W source and residual

pump light was reflected back into the amplifier with a FBG reflector [54].

Raman-Shifted Lasers and Raman Amplifiers: Raman-shifted lasers and Raman

amplifiers enable efficient conversion of short-wavelength light into longer

wavelengths suitable for long-distance fiber transmission. Raman gain is obtained

through energy transfer from the pump light to the laser output or amplified signal as

mediated by molecular vibrations in the silica fiber [62]. A high-power Raman-shifted

pump laser was used to generate the 1485nm light used in the repeaterless

experiments mentioned in the previous section. In those experiments FBG resonator

mirrors enhanced the Raman amplification to convert 6 W of 1117nm light from a

diode pumped Yb3+

cladding-pumped laser into more than 1.5W of 1485 nm. The

1117nm light was down converted through five stages of Raman gain. Having so

many gratings and conversion stages requires that the grating reflectors exhibit low

insertion loss (0.2dB) and low fiber loss for the Raman shifting process to be efficient.


26

Dispersion Compensators: Chromatic dispersion in transmission fiber can cause

significant distortion of optical pulses, leading to system penalties. Upgrading existing

lightwave systems to 10Gb/s channels usually requires dispersion compensators often

using long lengths of dispersion-compensating fiber [63]. In the absence of optical

nonlinearities, compensation is achieved by passing the distorted signal through a

device whose dispersion is equal to that of the transmission fiber and of opposite sign.

This compensating all-pass filter must also have sufficient optical bandwidth to accept

the signal spectrum. The FBG is one such proposed filter [64]. In an idealized model,

chirp in FBG dispersion compensators should be close to linear, resulting in a

differential delay of reflected light. Disregarding optical nonlinearities and higher-

order dispersion, the chirp is selected so that grating dispersion upon reflection

cancels that of the fiber. The grating must also be long enough to ensure that the

entire signal spectrum is accommodated.

Gain Equalizers: The useful optical bandwidth of amplified lightwave systems is

limited because of gain-narrowing through concatenated optical amplifiers. Erbium-

doped silica fiber amplifiers in particular show gain peaking at 1530 and 1560nm and

the useful gain bandwidth may be reduced to less than 10nm. Pre-emphasis of the

WDM channels at the transmitter increases the available bandwidth by equalizing the

signal-to noise ratio of the received WDM channels, but is only suited for point-to-

point systems [65]. The loss spectrum of the gain-equalizing filter must match the

erbium-gain spectrum at the nominal operating condition of the amplifier. Placing a

gain-equalizing filter inside the optical amplifier flattens the gain-spectrum and with

appropriate design, may have minimal effect on the amplifier noise figure or saturated

output power. A blazed Bragg grating has been used as a gain equalizing element, but

has not led to widespread use [66].


27

1.8 Lightwave Applications of LPG

Long period grating which acts as wavelength dependent loss elements has many

applications in both communication and sensors. Even though, LPG applications in

various fields are under research, lot of R&D work is under the progress. Researchers

demonstrated many applications of LPG. Following are some of LPG applications in

lightwave networks already demonstrated.

Optical filters: An LPG band pass filter has been demonstrated by darkening the

fiber core in the middle section of an LPG with a focused UV beam [67] or by

insertion of a section of a hollow-core fiber between two identical LPGs [68].

Introducing π-phase shifts along an LPG also results in special band pass filters [69],

which have found applications in actively mode-locked fiber ring lasers. LPGs can

also form multi-port couplers. It has been demonstrated both theoretically and

experimentally that the light energy coupled to the cladding mode can be collected by

using two parallel LPGs [70-73]. The transmission spectra from the two parallel

gratings are complementary to each other, one showing band rejection characteristics

and the other showing band pass characteristics. The structure of two parallel LPGs

thus operates as an all-fiber broadband add/drop multiplexer and has potential

applications in WDM systems. Recently, a broad-band optical coupler based on three

parallel identical LPGs has been demonstrated [74]. A total power transfer efficiency

of 85% at the resonance wavelength has been achieved. Other implementations

include placing a tapered fiber in parallel to an LPG [75] and writing an LPG in a

two-core fiber with slightly different cores [76]. The structure of two cascaded LPGs

has been demonstrated as a multi-channel filter for multi wavelength signal

generation in WDM systems [77]. The transmission spectrum of the filter has a


28

sinusoidal fringe pattern with an envelope governed by the shape of the rejection band

of the individual grating and the channel spacing can be controlled by changing the

physical separation of the two gratings [78]. A technique of generating high-

repetition-rate pulses based on cascaded LPGs has also been demonstrated [79]. When

an LPG is written in a birefringent fiber, two resonance dips appear in the

transmission spectrum, which correspond respectively to the two principal

polarizations of the fiber. Such a grating has been used to realize a wavelength-

selective fiber polarizer [80]. An extinction ratio >30dB and an insertion loss < 0.5dB

have been achieved. LPG polarizers have also been implemented with few-mode

fibers [81]. Using the polarization-dependent spectrum of an LPG, a wavelength-

switchable erbium-doped fiber ring laser has been demonstrated, where dual-

wavelength operation is accomplished by rotating the polarization plane of the fiber

laser cavity [82].

Gain flatteners for optical amplifiers: LPG filters can be considered as wavelength-

selective attenuators and therefore used as gain flatteners for erbium-doped fiber

amplifiers (EDFAs) [83, 84]. By careful control of the filter spectrum and fiber

length, the EDFA gain has been flattened to within 1dB over 40nm while producing a

noise figure below 4.0dB and an output power of nearly +15dBm [84]. Another

method of achieving broad-band gain flattening is to use a phase shifted LPG [85]. To

flatten a gain spectrum that contains several peaks, a compact module based on using

different cladding modes of a number of concatenating LPGs has been proposed [86].

More recently, an EDFA gain spectrum flattened to within 0.35dB over 30nm has

been realized by using an optimized design of step changed LPGs [87]. Other gain-

flattening schemes include etching [88] and UV trimming [89] of LPGs. Dynamic


29

gain flattening has been demonstrated with bending of two cascaded gratings [90] or

twisting of a grating [91, 92].

Fiber coupling: Fiber-to-fiber coupling via the cladding mode of the fiber can relax

the alignment tolerances substantially [93, 94]. A lens-free fiber-to-fiber connector

that has a long working distance and a wide alignment tolerance has been

implemented by using two matched LPGs written in a double-cladding fiber [95].

Lateral alignment tolerances of ~450µm and ~3mm for coupling losses less than 1dB

and 3dB respectively have been achieved. Laser-to-fiber coupling based on an LPG

and a lens has also been demonstrated. A working distance longer than 100µm and a

lateral tolerance of 2.5µm have been obtained [94]. Fiber-to-waveguide coupling has

also been demonstrated with a CO2-laser-induced LPG. [95, 96].

Dispersion compensation: LPGs in dispersion-tailored few-mode fibers have been

employed in dispersion compensation. Higher-order mode (HOM) dispersion

compensation using the LP02 mode can offer dispersion-slope matching to practically

any transmission fibers [97]. An HOM fiber having a propagation loss of 0.44dB/km

and a dispersion coefficient of −210ps/nm/km has been fabricated [98]. Using the

HOM dispersion-compensation scheme, a 40Gb/s hybrid Raman/erbium-amplified

system with a transmission distance of 1700km has been demonstrated [99]. A

tunable dispersion compensator that utilizes an HOM fiber and switchable LPGs has

also been made, which provides a dispersion tuning range of 435ps/nm over a

bandwidth of 30nm [100].


30

1.9 Potable Water

Water is most fundamental building block of life. Most of us are fortunate to be

living in a place where water is easily available. Most of the earth surface is covered

with water, there is much more water than there is land. About 70% of the earth's

surface is covered with water. Water also exists in the air as vapour and in aquifers in

the soil, as groundwater. Oceans store most of the earth's water, this is apparently

97% of the total amount of water on earth, 2% of which is frozen. Of all the water on

earth only 2.59% is freshwater. Of this, about 2% is trapped in ice caps and glaciers.

The rest of the freshwater is either groundwater (0.592%) or readily accessible water

in lakes, streams, rivers, etc. (0.014%). From the above statistics, one can conclude

that less than 1% of water on earth can be used as drinking water. This includes both

surface and ground water [101, 102].

Over large parts of the world, humans have inadequate access to potable water

and use sources contaminated with disease vectors, pathogens or unacceptable levels

of toxins or suspended solids. Drinking or using such water in food preparation leads

to widespread acute and chronic illnesses and is a major cause of death and misery in

many countries. Reduction of waterborne diseases is a major public health goal in

developing countries.

With increasing world population and run for the high profile life, it is becoming

difficult to supply safe drinking water. Industries are growing to supply the demands

of population which is also the reason for the growing water difficulty. Industries

demand more water and pollution will increase if the industrial effluents are not

managed properly. Other factors such as fertilizer runoff and leaking from septic

tanks can further contaminate water. Just because one has water source (lake, river,

well or bore well) that yields plenty of water doesn't mean you can go ahead and just


31

take a drink. Because water is such an excellent solvent that it can contain lots of

dissolved chemicals. Since, ground water moves through rocks and subsurface soil, it

has a lot of opportunity to dissolve substances as it moves. For that reason, ground

water will often have more dissolved substances than surface water. Even though the

ground has an excellent mechanism for filtering out matter such as leaves, soil, bugs,

dissolved chemicals and gases can still occur in large enough concentrations in

ground water to cause problems. As ground water flows through sediments, metals

such as iron, manganese, potassium, calcium, manganese, lead, cadmium, mercury,

organic compounds etc are dissolved and may later be found in high concentrations in

the water. If water is badly polluted (like raw sewage) it might be obvious from its

appearance or odor. It might be colored or turbid (cloudy) or have solids, oil or foam

floating on it. Even though water appears clean, clear and odor less it may not be fit to

drink. Many harmful and beneficial materials in water are invisible and odorless.

Higher concentration of these dissolved chemicals in water may cause serious health

problems in human by prolonged intake. A number of chemical contaminants have

been shown to cause adverse health effects in humans as a consequence of prolonged

exposure through drinking-water. In water, many radicals such as iron, manganese,

copper, cadmium, mercury, arsenic, lead, zinc, cyanides, fluorides etc are present in

ppm / ppb level. Slight increase in the concentration of these chemicals in water may

cause significant problems; even crises can occur on prolonged use of such water.

Many research and analysis were taken up in the past to recommend the

minimum level of these dissolved chemicals in the drinkable water and these

recommendations are known as standards of drinking water or guidelines for drinking

water. Every country has its own drinking water standards based on geographical

condition. World Health Organization (WHO) is universal body that regularly


32

releases guidelines for drinking water. The WHO Guidelines for Drinking-water

Quality (GDWQ) provide a point of reference for drinking water quality regulations

and standards all over the world.

Hence, regular monitoring of the water quality- level of dissolved chemicals

present in the water is very important. There are many methods of analysis such as

colorimetric, gravimetric, electrochemical, chromatography, mass spectrometry, etc.

But to determine metal radicals present in very low concentrations (ppm level),

specialized equipments are required such as atomic absorption spectrometer (AAS),

inductively coupled plasma (ICP) spectrometer, flow injection method. These all

instruments are laboratory based (not portable), expensive and require special

technical person to handle the instrument. In AAS every elemental analysis requires

respective light source (E.g. Iron source for Fe analysis). ICP spectrometer

experiences lot of interference of other elements while analyzing particular one [103].

Flow injection method requires preprocessing of a test sample. To avoid all these

methods, in the present work we present very simple, low cost method based on fiber

grating technology. Here, we present highly sensitive fiber grating sensor to

determine the concentration of manganese and nitrate present in water at ppm level.

Our fiber grating sensors are sensitive, robust and reliable. Results obtained by grating

sensors are confirmed by AAS and ICP methods. Later sensitivity of the sensor is

enhanced by coating sensor region with gold nanoparticles synthesized in the

laboratory.

1.10 Optical Fibers in Nuclear Environment

Since optical fibers and optical fiber components have excellent features such as

low-loss large information capacity, compactness, low insertion loss and little


33

influence by electromagnetic induction compared with conventional copper cable

systems, those have been applied to not only in public telecommunication, but also

space applications [104], in nuclear industry [105] and in high energy physics

experiments [106]. Hence, it has become essential to investigate the influence of

ionizing radiation on the characteristics of optical fibers and many fiber-optic

components. Radiation effects in bulk optical glasses and in optical fibers have been

extensively studied in the last decades. In bulk silica glass, different effects have been

seen such as radiation-induced attenuation, luminescence and refractive index changes

[107-111]. Transmission loss is considered to be the most affected parameter of the

optical fiber. Radiation induced changes in transmission characteristics of optical

fibers depend upon type of radiation, intensity (flux), rate of flux, total exposure,

optical fiber composition, wavelength of transmission etc. Radiation resistivity of

various types of optical fibers was therefore noted. Numerous investigations about the

resistivity and sensitivity of optical fiber are being undertaken all over the world

[112]. We studied the optical properties of plastic optical fiber and silica fiber under

nuclear radiation of gamma radiation emitted by Amerisium-241 and FBG

characteristics under radiation emitted by Na22

and Tl204

.


34

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