TERMS DEFINITION

Fiber Optics A branch of optics that deals with communication by transmission of light through ultrapure fibers
of glass or plastic.

Fiber Optic System Is a communications system that carries information through a guided fiber cable.

Optical Fiber Dielectric waveguide used for the propagation of electromagnetic energy at optical frequencies.

It matches impedance and limits input signal amplitude.

Voltage to Current Converter

It serves as an electrical interface between input circuitry and light source.

Light Source IT either an LED (light Emitting Diode) or ILD (Injected Laser Diode); amount of light emitted is
proportional to the amount of drive current.

Source to Fiber Interface

A mechanical interface; couples light into the optic fiber cable.

Fiber-to-Light Detector

A mechanical coupler; couples as much light as possible from the fiber cable into the light detector.

Light Detector It’s either a PIN diode or an APD (Avalanche Photodiode); both convert light energy to current.

Current to Voltage Converter

It transforms changes in detector current to changes in output signal voltage.

Analog or digital

Electronic communications system is directly proportional to bandwidth.

Bandwidth It is common to express the bandwidth of an analog communications system

ilxxiii
Utilization Ratio as a percentage of its carrier frequency.

Photophone

Device constructed from mirrors and selenium detectors that transmitted sound waves over a beam of light.

Alexander Graham Bell

He experimented with an apparatus called photophone.

Flexible fiberscope Used extensively in medical field.

Laser (light amplification by stimulated emission

of radiation)

Invented in 1960, which relatively high output power, high frequency of

operation, and capability of carrying an extremely wide bandwidth signal make it ideally suited for high-capacity
communications system.

Karpon, Keck and Maurer

In 1970, they developed an optical fiber with losses less than 2dB/km.

Three Regions of Optical Fiber

1. Core – a transmission area of the fiber; typical diameters: 50 to 500 µm.

2. Cladding – surrounds the core, has a different index of refraction; defines the optical boundary of the core
and makes sure that the total internal reflection occurs at the core outer skin.

3. Coating – specially formulated plastic coating that provides a first level shock and abrasion resistance for
the fiber; typical thickness is 250µm to 1000µm.

Types of Optical Fiber

1. Plastic Core and Cladding

-Flexibility and ruggedness

-Easy to install

-Less Weight

-More economical

-Can withstand stress

-High attenuation

-Inefficient

-Limited for short distance applications

2. Glass core with plastic cladding (PCS; Plastic Clad Silica)

-Less affected by radiation

-Attractive to military applications

3. Glass core and glass cladding (SCS fiber; Silica -Clad Silica

Susceptible to increase in attenuation when exposed to radiation

-Least rugged

-Easier to terminate

Advantages of Optical Fiber cables.

1. wider bandwidth and greater information capacity

2.immunity to crosstalk

3. immunity to statistic interference

4. environmental immunity

5. safety and convenience

6. lower transmission loss

7. security

8. durability and reliability

9. economics

Disadvantages of Optical Fiber cables.

1. interfacing cost 2. strength

3. remote electrical power

4. optical fiber cables are more susceptible to losses introduced by bending the cable 5. specialized tools,
equipment, and training

Stress corrosion Reduces the possibility of the occurrence of a detrimental phenomenon.

Buffer jacket

Protective coating and which provides the additional protection against abrasion and shocks.

Strength member Increases the tensile strength of the overall cable assembly.

Plank’s Law

It described the photoelectric effect, which state, “When visible lights or high frequency electromagnetic
radiation illuminates a metallic surface, electrons are emitted”.

mode (path) of propagation

A stable propagation state in an optical fiber. The number of modes a fiber can transmit depend on its
numerical aperture as well as the wavelength.

Index Profile A graphical representation of the refractive index of the core.

Classifications of Optical Fibers

1. Single Mode (Monomode)

-Only one path for light to take down the cable.

-Core diameter of a single mode fiber is extremely small (2 to 4um diameter).

-Extremely wide bandwidths and low losses. This gives the highest and longest distances but it requires
powerful and precisely aligned light sources.

2. Multimode Fibers

-The light wave rays take many paths between the source and the far end of the fiber.

-The fiber core diameter must be large enough to permit propagation in several modes.

-Since some paths are longer than the others, the original sharp pulse at the source is spread out in time at the
receiver. This pulse spreading reduces the bandwidth and the maximum data rate of the system.

Two Types:

a. Step Index

-The core has a uniform index of refraction providing an abrupt change in refraction index at the core-cladding
interface.

-These types of fibers have relatively high dispersion making it useful only at lower rates and shorter distance.

cladding core
cladding core

b. Graded Index

-The core has index of refraction that changes continuously from the center to the outside.

-This core is made out of many thin layers, each with lower index of refraction than the adjacent inner one. The
effect of this grading is that light waves are propagated by refraction so they are bent in a sinusoid like curve
about the fiber.

core cladding

Spontaneous decay or spontaneous

emission

Process of decaying from one energy level to another energy level.

Absorption The process of moving from one energy to another energy level is called?

Photometry Science of measuring only light waves that are visible to human eye.

Light Intensity

It is a complex concept that can be expressed in either photometric or radiometric terms.

Optical power Measures the rate at which electromagnetic waves transfer light energy.

Radian Flux

It is the equivalent to joule per second and is the power that is measured electrically or thermally in watts.

Prismatic Refraction

This phenomenon that cause rainbows, where water droplets in the

atmosphere act as a small prism that split the white sunlight into the various wavelength, creating a visible
spectrum of color.

Refractive index

Simply the ratio of velocity of propagation of a light ray in free space to the velocity of propagation of a light ray
in a given material.

The refractive index (η)

MEDIUM η

Vacuum 1.0

Air 1.00029

Water 1.33

Ethyl Alcohol 1.36

Fused Quartz 1.46

transmissive materials that have different indices of refraction using the law which states that:

-When a light travels from a less dense material into a more dense material, the wave is refracted towards the
normal.

-When a light ray enters a less dense material, the ray bends away from the normal.

Normal It is simply a line drawn perpendicular to the interface at the point where the incident ray strikes the
interface.

Angle of incidence Angle at which the propagating ray strikes the interface with respect to the normal.

Angle of refraction Angle formed between the propagating ray and the normal after the ray has entered the
second medium.

Critical Angle

Acceptance Angle or Acceptance Cone Half Angle

(in)

Maximum angle in which external light rays may strike the air/fiber interface and still propagate down fiber.

A figure of merit used to measure the light gathering or light collecting ability of the optical fiber.

; fractional change in the index of refraction of multimode-mode step index

; fractional change in the index of refraction of multimode-mode graded index a = radius of the core of the
fiber

λ = w avelength

η1= index of refraction of core η2 = index of refraction of cladding g = gradient of graded fiber

= 2; parabolic profile process. Cooling process of glass irregularities, imperfections such as tiny bubbles, and
imperfections in the internal uniformity causing light rays to be diffracted (dispersed) or to escape.

Linear scattering The power transferred from a wave is proportional to the power of the wave.

Rayleigh Scattering The light interacting with inhomogeneities in the medium that are much smaller than the
wavelength of the light.

Mie Scattering Inhomogeneities that is comparable in size to a wavelength.

Nonlinear Scattering

Significant power to be scattered in the forward, backward, or sideways directions, depending on the nature of
the interaction.

Brillouin Scattering

Modulation of the light by the thermal energy in the material. The incident photon of light undergoes the
nonlinear interaction to produce vibrational energy (phonons) in the glass as well as scattered light (as
photons).
Raman Scattering The nonlinear interaction produces a high-frequency phonon and a scattered photon.

Absorption Losses Impurities in the fiber absorb the light and convert it to heat.

Types Of Absorption Loss

Ultraviolet Absorption – light ionizes the valence electrons into conduction; ionization is equivalent to a loss in
the light field.

Infrared Absorption – photons of light absorbed by atoms of the glass core are converted to mechanical
vibrations typical of heating.

Ion Resonance absorption – caused by OH- ions from water molecules trapped in the glass during
manufacturing process.

Dispersion

Spreading of pulse out in the time domain, changing its shape so that it may merge into the previous and
succeeding pulses. The pulses maybe separated by spacing them out at the transmitter but this means
reducing the maximum bit rate.

Types of Dispersion

Material or Chromatic or Wavelength Dispersion – light sources emit light that contains a combination of
wavelengths. Therefore, different wavelengths do not arrive at the same.

Waveguide Dispersion – attributed to the dependence of the phase and group velocities on the geometric
characteristic of the waveguide.

Modal Dispersion – it is caused by the different path lengths associated with each of the modes of a fiber.
Caused by the difference in the propagation times of light rays that take different path down a fiber; occur only
in multimode fibers. A portion of the light energy traveling in the cladding.

Coupling Losses 1. Lateral Misalignment

2. Gap Misalignment or Longitudinal Displacement

3. Angular Misalignment

4. Imperfect Surface Finish

Light Sources A light source is any material or device which gives off energy with a wavelength from about 300
microns down to 0.5 microns.

Requirement of Light Source

-Their light must be as nearly monochromatic (single frequency) as possible.

-Capable of being easily modulated; PCM for better noise immunity

-High intensity light output so that sufficient energy is transmitted to overcome the losses encountered during
transmission down the fiber.
-Devices should be small, compact, and easily couple to the fibers so that excessive coupling losses do not
occur.

-Must be inexpensive to manufacture

Light Emitting Diodes (LED)

-Non-coherent injection light sources which are low-cost, low heat light sources and are the most promising
light sources for optical

transmission.

-LEDS have the necessary reliability, life expectancy, and simplicity for optical transmission purposes.

-It emits light over a rather large angle thus coupling less power into the fiber.

-LEDS are able to couple about 100uW of power with a coupling efficiency of 2%.

Injection Laser Diodes (ILD)

ILDs make the most efficient use of electric energy. This coherent light source can couple a few milliwatts of
light power into a fiber since it has a more direct radiation pattern.

Advantages of ILDs over LED

-Reduced coupling losses

-Greater radiant output power

-Can be used at higher bit rates

-Reduced wavelength dispersion; monochromatic

Disadvantages of ILD’s

-Expensive

-Shorter lifetime

-Temperature dependence

-Requires automatic level control circuit to protect the device from power Supply transients:

Homojunction

LED’s A p-n junction made from two different mixtures of the same types of atoms is called?

Epitaxially grown Generally constructed of silicon-doped gallium-arsenide.

Planar diffused Homojunction LED’s output approximately 500 µ at a wavelength of 900 nm.

Heterojunction LED’s
This are made from p-type semiconductor material of one set of atom and an n-type semiconductor material
from another set.

Edge Emitters The light emitted from the edge of the material is called?

Planar Heterojunction LED

It is quite similar to the epitaxially grown LED except that the geometry is designed such that the forward
current is concentrated to a very small area of the layer.

Advantages of heterojunction devices over homojunction devices

-Increase in current density generates a more brilliant light spot.

-Smaller emitting area makes it easier to couple its emitted light into a fiber.

-Small effective area has a smaller capacitance, which allows the planar heterojunction LED to be used at a
higher speed.

PIN diode Depletion-layer photo diode and is probably the most common device used as a light detector in fiber
optic communications system.

Light Detectors

-PIN (Positive Intrinsic Negative) diodes - When photons are absorbed by intrinsic layer’s electrons in the
valence band, they add sufficient energy to generate carriers in the depletion region and allow current to flow
thru the device.

-APD (Avalanche Photo Diode) - Light enters diode and is absorbed by the thin, heavily doped n-layer causing a
high electric field intensity to be across i-p-n junction thus ionization occur and continues like avalanche.

Advantages of APD over PIN

-APD's give better sensitivity over PIN

-APD's provides larger amplification

Disadvantages of APD

-High bias requirement

-Temperature dependence

-Long transit time

Characteristic of a light detector.

-Responsivity

-Dark currents
-Transit time

-Spectral response

-light sensitivity

Laser Technology deals with the connection of light into a very small, powerful beam.

Types of Laser

Gas Laser – use a mixture of helium and neon enclosed in a glass tube.

Liquid Laser – use organic dyes enclosed in a glass tube for an active medium.

Solid Laser – use a solid cylindrical crystal, such as ruby, for the an active medium.

Semiconductor Laser – are made from semiconductor p-n junction and are commonly called ILDs.

Optical Fiber System Link Budget

Cable Losses – it depends on the cable length, material and material impurity.

Connector Losses – if mechanical connection is not perfect, light energy can escape, resulting in a reduction of
optical power.

Source to Cable Interface Loss – the mechanical interface used to house the light source and attach it to the
cable is seldom perfect.

Cable to Light Detector Interface Loss - the mechanical interface used to house the light source and attach it to
the cable is also not perfect and therefore, prevent a small percentage of power leaving the cable from the
entering the light detector.

Splicing Loss – if more than one continuous section of cable is required, cable section can be fused together.

Cable Bend – when an optical cable is bend at too large an angle, the characteristics of the cable can change
dramatically.

CHAPTER 13 OPTICAL FIBER TRANSMISSION MEDIA

Item Definitions Terms

s

1 Uses light as a carrier of information. Optical

communicatio
ns

System
2 Electronic communications system is directly proportional to Information-ca
bandwidth. rrying

capacity

3 Bandwidth of an analog communications system as a bandwidth

percentage of its carrier frequency. utilization
ratio

4 Device constructed from mirrors and selenium detectors that Photophone

transmitted sound waves over a beam of light.

5 Experimented with an apparatus called photophone. Alexander

Graham Bell

6 Used extensively in medical field. Flexible

fiberscope

7 Invented in 1960. Laser (light

amplification
by stimulated
emission of
radiation)
8 Advantages of Optical Fiber cables. 1. wider
bandwidth
and greater
information
Capacity

2.immunity to
crosstalk

3. immunity to
statistic
Interference

4.
environmental

Immunity

5. safety and
convenience

6. lower
transmission
loss

7. security

8. durability
and reliability

9. economics
9 Disadvantages of Optical Fiber cables. 1. interfacing
cost

2. strength

3. remote
electrical
Power

4. optical fiber
cables are
more
susceptible to
losses
introduced by
bending the
cable

5. specialized
tools,
equipment,
and training

10 Reduces the possibility of the occurrence of a detrimental Stress

phenomenon. corrosion

11 One wire is at the ground potential, whereas the other is at Single-ended

signal potential. or unbalanced

12 Protective coating. Buffer jacket

13 Increases the tensile strength of the overall cable assembly. Strength

member

14 Process of decaying from one energy level to another energy Spontaneous

level. decay or
spontaneous
emission

15 Science of measuring only light waves that are visible to human Photometry
eye.

16 Measures the rate at which electromagnetic waves transfer light Optical power
energy.

17 Spectral separation of white light. Prismatic

refraction
18 Simply the ratio of velocity of propagation of a light ray in free Refractive
space to the velocity of propagation of a light ray in a given index
material.

19 Angle at which the propagating ray strikes the interface with Angle of
respect to the normal. incidence

20 Angle formed between the propagating ray and the normal after Angle of
the ray has entered the second medium. refraction

21 Closely related to acceptance angle and is the figure of merit Numerical

commonly used to measure the magnitude of the acceptance aperture
angle.

22 Simply means path. Mode

23 More than one path. Multimode

24 Probably the most important characteristic of the cable. Power loss

25 Three factors that contribute to the absorption losses in optical 1. ultra violet
fibers: absorption

2. infrared
absorption

3. ion
resonance
absorption

26 Caused mainly by small bends and kinks in the fiber. Radiation

losses

27 Caused by excessive pressure and tension and generally occur Contants-radi

when fibers are bent during handling or installation. us bends

28 Caused by the difference in the propagation times of light rays Modal

that take different path down a fiber. dispersion

29 A p-n junction made from two different mixtures of the same Mojunction
types of atom. structure

30 Generally constructed of silicon-doped gallium-arsenide. Epitaxially

grown

31 Homojunction LED’s output approximately 500 µ at a wavelength Planar

of 900 nm. diffused
32 Advantages of heterojunction devices over homojunction 1. increase in
devices; current
density
generates a
more brilliant
light spot.

2. smaller
emitting area
makes it
easier to
couple its
emitted light
into a fiber.

3. small
effective area
has a smaller
capacitance,
which allows
the planar
heterojunction
LED to be
used at a
higher speed.

33 Depletion-layer photo diode and is probably the most common PIN diode
device used as a light detector in fiber optic communications
system.

34 Characteristic of a light detector. 1.

responsivity

2. dark
currents

3. transit time

spectral
response

5. light
sensitivity