Optical Fiber Communication

Light Basics

Dr. Abid Karimakarimpk@iqra.edu.pk

Course ContentsWeek Topic to be Covered

1. Historical Background, The Nature of Light, Basic Laws of Light, Interaction of Light with Materials. Historical Development, Electromagnetic Spectrum

2 Advantages of Optical Communication Systems, Light Propagation in Optical Fiber, Optical Fiber Losses

3 Dispersion and Fiber Bandwidth, Types of Optical Fibbers4 Optical Fiber Components, Fiber Couplers, Optical Switching5 Introduction to Lasers, Amplification in Two-Energy Level System and

Einstein Relations, Population Inversion, Optical Feedback, Lasing Threshold, Lasers Modes and Gain Condition

6 Optical Absorption and Gain in Semiconductor Materials, Types of Semiconductor Lasers and their Structure, Practical Laser Characteristics

7 Single Mode Semiconductor Lasers and its requirement in Optical Communication

8 Light sensitive Material, Principle of Photodetection, Semiconductor photodetectorsMid Term Examinations

Course ContentsWeek Topic to be Covered

9 Types of Photodiodes, Responsitivity and Quantum Efficiency of a Photodiode

10 pn-Photodiode, pin-Photodiodes, Avalanche Photodiodes Photodiode, Biasing Techniques, Noise Consideration of Photodetector, Phototransistor and Optocoupler

11 Light Amplifiers, Types of Amplifiers

12 Erbium Doped Optical Amplifiers

13 Modulation and Multiplexing, Systems Design Considerations 

14 Wavelength Division Multiplexing and Dense Wavelength Division Multiplexing

15 Optical Networks

16 Revision and group discussion

Final Examinations

Quizzes Due on– 3, 7, 11, 15th Week Assignment Due on – 5, 10, 16th Week

Recommended Books

• Fibre Optic: Communication and other Applications, By Henry Zanger & Cynthia Zanger

• Optical Fibre Communication: Practice and Principles, By John. M. Senior

• Optical Technology, Compiled by Abid Karim• Fiber-Optic Communications Technology, By D.

K. Mynbaev & Lowell L. Scheiner

Marks Distribution

• Assignments + Class Quizzes +Project(s) + Presentation(s) 25%

• Midterm Examination 25%

• Final Examination 50%

Assignments• Assignments would be assigned at least one

week before the due date • Must be submitted on or before due date• No late assignment will be accepted• Total of 3 assignments would be assigned

during the semester. • Handwritten• Avoid plagerism• Do not try copy

Quizzes

• To check the class performance, sudden death test or class quizzes

• At least 4 quizzes• Quizzes have to be solved in the class • There would be no LATE submission or

MAKEUP for quizzes.

Increase in Bit Rate-Distance product

Why Light?

Why Light?

Progress In Lightwave Communication Technology

The Nature of Light

• Wave Theory – Light travels as a transverse electromagnetic wave

• Quantum or Particle Theory – Light consists of small particles (photons)

• Ray Theory – Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength

Light ray model• Particle-like view• Photons travel in straight

lines• Applications

– Mirrors– Prisms– Lenses

Wave model• Traces motions of wave

fronts• Best explains

– Interference– Diffraction– Polarization

The Nature of Light

• Light is the part of Electromagnetic Radiation Spectrum. Speed of Light can be calculated by

• Electromagnetic energy is radiant energy that travels at 300,000km/s or 186,000 miles/s.

oo

c

1

Light – Wave Nature

• Hence Light is an electromagnetic wave.

• Other electromagnetic waves:– Radio Waves– Radar– X-Rays– Gama Rays– Cosmic Rays

Light – Wave Nature

• Einstein lead to concept of packet of energy (Photons) – Based on Plank’s work on emission of light from hot bodies

• Plank’s observation – Light emits in multiple of certain minimum energy unit.

• The size of the unit (quantum) depends on the wavelength (λ) and given by.

Light – Particle Nature

hvE

• Wave theory of light explains most phenomena involving light: – propagation in straight line – reflection– refraction – superposition, interference, diffraction – polarization – Doppler effect

• Wave theory does not explain: – frequency dependence of thermal radiation – photoelectric effect

Nature of Light: Waves and Particles

Nature of Light: Waves and Particles

• Light exhibits properties of waves and particles – Wave-Particle Duality (by Louis de Broglie 1924)

where h is the Plank’s constant and p is the momentum.

• Complimenting each other rather than opposing each others

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Nature of Light: Waves and Particles

• The duality of light will be used in understanding the propagation of light in particular medium.– Wave Nature – When photons are

moving – Particle Nature – When light is detected

or generated

Electromagnetic Wave• Consists of a oscillating electric and

magnetic fields at right angles to each other

• Direction of propagation perpendicular to both field

• Frequency (): Number of cycles/second• Wavelength (): Distance between the

same 2 points.

Electromagnetic Wave

Frequency and Wavelength

• Relationship of frequency and wavelength:wavelength = velocity/frequency

• In free space or air velocity is the speed of light.

• The higher the frequency the shorter the wavelength.

c

Wavelength Examples• 50 Hz power has a wavelength of 3100

miles. That is, the wave will have traveled 6000 miles before the wave begins a new cycle.

• A 55.25 MHz signal (TV Channel 2) has a wavelength of 5.42 m.

• Deep red has a frequency of 430THz and wavelength of 700nm (billionths of a meter).

Electromagnetic Spectrum• The electromagnetic spectrum is a continuous

spectrum of energy from subsonic to RF to microwaves to visible light and beyond.

• Visible light has wave lengths from 400nm (violet) to 700nm (red).

• Ultraviolet light has a shorter wavelength and infrared has a longer wavelength.

• Fiber commonly uses infrared (890nm – 1600nm) due to different reasons.

power, radio frequencies, "microwaves", millimeter waves, IR, visible, UV, X-rays , -rays, and cosmic rays.

long wave Cosmic Rays

RaysXRays

UV

Visible Spectrum~0.7µm-~0.4µm

Optical Fiber Communications ~1.7µm - 0.8µm

Far Infrared

Infrared

mm wave

Microwave

UHFVHF

Short Wave

Standard Broadcast

Electromagnetic Spectrum

Electromagnetic Radiation Spectrum

Wavelength Ranges

Nuclear Decay

Electrons in Atoms(High Energy)

Electrons in Atoms(Low Energy)

Thermal Vibrations of Molecules

Microwave Oven

FM Radio

AM Radio

Typical Source

Electromagnetic Radiation Spectrum

Photons• A photon has zero rest mass (unlike an electron).

i.e. If it is not in motion, it does not exist!• It has no charge • Energy of a Photon would not change – Colour

would be the same• Energy possessed by a photon is proportional to its

frequency.

where h is Plank's constant which is equal to 6.63x10-34 J-s

(eV) m)(

24.1

hchvE

Interaction of Light with Materials• The 'Speed of Light' is simply the velocity of

an electromagnetic wave in a vacuum.• Light travels slower in materials.• As light passes from one material to another,

its direction changes – Reflected or Refracted • Different wavelengths of light travel at

different speeds in the same material.

• Interaction begins at surface and depends on– Smoothness of surface– Nature of the material– Angle of incidence

• Possible interactions– Absorption and

transmission– Reflection– Refraction

Interaction of Light with Materials

Law of Reflection

• With reflection, the angle of reflection is equal to the angle of incidence.

n1 > n2

Speed of Light in a MediumAs a monochromatic wave propagates through media of different refractive indices, its frequency remains same, but its velocity and wavelength are altered.

Index of Refraction• The Index of Refraction is a unit representing

the ratio of the velocity of light in a vacuum to the velocity of light in a material

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Index of Refraction

• As the index of refraction increases, the slower the wave will travel and the greater it will 'bend' when entering from a material with a lower index.

Definitions for Refraction • Normal: Imaginary line

perpendicular to the interface between 2 materials.

• Angle of incident: Angle between the incident ray and the normal.

• Angle of Refraction: Angle between the normal and the refracted ray. n1 < n2

Refraction for n1>n2

• With n1 > n2, as the incidence angle increases, the refractive angle increases.

• At the critical angle, the refractive angle is 90 degrees.

• Above the critical angle, the incident ray is totally reflected.

n1 > n2

n1 > n2

Law of Refraction: Snell's Law

• The relationship between the incident ray and refracted ray is:n1sin1 = n2 sin2

• For reflection to occur, angle of incidence must exceed the critical angle - crt. The

critical angle 2 may be found by:

crt = arcsin(n2/n1)

A Practical Example

• Assuming there are 2 layers of glass with indices of 1.48 (n1) and 1.42 (n2)

crt = sin-1(1.46/1.48) = 80.6

Fresnel Reflections

• Even when refraction occurs and light enters a material, a small amount is reflected back – Fresnel Reflection ().

• The greater the index of refraction, the greater the amount of losses.

material.for refraction ofindex isn material andair between boundry For

11 2

nn

Fresnel Reflections

• Fresnel losses occur when:– Light from source enters fiber– Between connected fibers.– Losses are the same regardless of the

order of materials (from air to glass or from glass to air).

– Loss (dB) = 10 log(1- ρ)

Total Reflection

• With the angle of incidence greater than the critical angle, total reflection occurs.

Total Internal Reflection

• With material with indices on both sides (cladding), the light will be continually reflected and follow the core.

Electromagnetic Wave

Polarization

(TransverseDirection)

Polarization

Light is polarized when its electric fields oscillate in a single plane, rather than in any direction perpendicular to the direction of propagation

A phenomenon that occurs in transverse waves only

Polarization

• These waves are plane or linearly polarized

• All the motion is confined to a plane

Diffraction

• Light beam cannot bend itself• Light beam can be bent by reflection,

refraction and Diffraction• The amount of bending that occurs

depends on the relative sizes of the object and the wavelength of the wave

• Longer wavelengths bend easier than short ones

Diffraction

InterferenceWhen the waves are hitting the edges of something, the new bending waves tend to interfere with each other and we get some new patterns Aperture

Screen

Intensity of diffracted light from the bottom slit.

Intensity of diffracted light from the top slit.

E1

E2

E1

E2

Etotal = 0Etotal

Etotal = E1 + E2 EtotalConstructiveInterference

DestructiveInterference

InterferenceInterference is the superposition of two or more waves resulting in a new wave

Constructive and destructive interference requires that the interfering waves have the same frequency (wavelength) and polarization