Optical fibers communicatiobyom

Prof.Byomakesh MahapatraAssistant ProfessorUniversity of Bombay

Prof. Byomakesh Mahapatra

Holger Møller Hansen in his workshop.Courtesy Holger Møller Hansen

Heinrich Lamm as a Germanmedical student in 1929, about the timehe made the first bundle of fibers totransmit an image. Courtesy MichaelLamm

People behind OFC Development


Brian O’Brien, who suggestedthat cladding would guide light along fiber.

Charles K. Kao making optical measurements at StandardTelecommunications Laboratories


General and Optical Communication systems


Need for Fiber Optical Communication

Increase of the bandwidth and decreases of the cost per transmitted bit for optical communication systems during the 1990‘s.


Different frequency range:-


Different frequency range:-

EHz:-Exahertz

PHz :- Petahertz


The Electromagnetic SpectrumShortest wavelengths(Most energetic photons)

Shortest wavelengths(Most energetic photons)

Longest wavelengths(Least energetic photons)

Longest wavelengths(Least energetic photons)

E = hν = hc/λE = hν = hc/λ

h = 6.6x10-34 [J*sec](Planck’s constant)


Properties of light:-

There are three theories are used to explain light:

Ray theory

Wave theory

Quantum theory


Properties of light/ important terms:- RefractionReflectionRefractive indexTotal internal reflection Geometrical opticsAbsorptionDispersion scatteringOptical wave guide Wave front Isotropic mediumPlane wavepolarization

Ray theory

Related to optical fiber propagation loss

Wave propagation


Ray theory:-It state that light is a stream minute particles and travel in a straight line .

A beam of light is a group of rays called as ray congruence.

A ray is the is the thinnest pencil of light with a well defined boundaries with

out diffraction.

aperture

Light patch


Wave theory of light :- Light is considered as an electromagnetic radiation. It

consists of two components i.e., the electric component and

the magnetic component which oscillate perpendicular to

each other as well as to the direction of path of radiation.

It propagates in a particular direction according to this E&H

components known as polarization.Prof. Byomakesh Mahapatra

Quantum theory:-Max Planck proposed Quantum theory by considering light to possess particle nature.

1) Energy is emitted due to vibrations of charged particles in the black body.

2) The radiation of energy is emitted or absorbed discontinuously in the

form of small discrete energy packets called quanta.

3) Each quantum is associated with definite amount of energy which is given by the equation E=hν.Where h = planck's constant = 6.625 x 10-34 J. sec = 6.625 x10-27 erg. sec

ν= frequency of radiation


Quantum theory:-4) The total energy of radiation is quantized i.e., the total energy is

an integral multiple of hν. It can only have the values of 1 hν or 2

hν or 3 hν. It cannot be the fractional multiple of hν.

5) Energy is emitted and absorbed in the form of quanta but

propagated in the form of waves.

Photo electric effect


There was a problem with wave theory

In the early 20th century, several effects were observed which could not be understood using the wave theory of light.

Two of the more influential observations were:

1) The Photo-Electric Effect

2) The Compton Effect


Photoelectric Effect (I)

No electrons were emitted until the frequency of the light exceeded a critical frequency, at which point electrons were emitted from the surface! (Recall: small λ large ν)

electrons emitted ?

No

Yes, withlow KE

Yes, withhigh KE

Increase energy by increasing amplitude

“Classical” Method

electrons emitted ?

No

No

No

No


Reflection:-

Normal

Angle of incidence

Angle of reflection

Mirror

Incident ray Reflected rays

Mirror


The Law of Reflection:-

Angle of incidence = Angle of reflectionAngle of incidence = Angle of reflection

In other words, light gets reflected from a surface at ____ _____ angle it hits it.

The same !!!

same


Refraction:-

Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks bent from the normal position.

In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______.

Words – speed up, medium, bent, water, air,


Refraction:-


Ex:-

Light banding due to refractionProf. Byomakesh Mahapatra

Snell’s Laws Snell's law gives the relationship between angles of incidence and refraction for a wave impinging on an interface between two media with different indices of refraction. The law follows from the boundary condition that a wave be continuous across a boundary, which requires that the phase of the wave be constant on any given plane.


Refractive index :-The ratio of speed of light in vacuum to that in matter is the index of refraction (n)

n = c/vwhere c = speed of light in vacuum and v = speed of light in the medium

Refractive index of air: 1.0003 water: 1.33 glass: 1.4 – 1.7 silicon : 3.96 GaP : 3.5 Gas :3.9 diamond :2.41 LiNbO3 :2.21


Total Internal Reflection:-

When there is a change the incident angle of the light on the block, the reflected light varies. At a certain angle, called the critical angle all the light is reflected, and none is refracted. When this happens we have total internal reflection.

TIR in optical fiber


Geometrical optics Geometrical optics, or ray optics, describes light propagation in terms of "rays". It include all the ray properties of the light like reflection ,refraction and corresponding optics lenses polarizer, mirror etc

The "ray" in geometric optics is an abstraction, or "instrument", which can be used to approximately model how light will propagate.


Geometrical optics


Absorption:-When light enter some of its energy is dissipated as heat energy and loose some of its intensity.

When the absorption of energy occur selectively for a certain wave length the wave length only reflected which are not absorbed.


Dispersion:- When the light ray passes through a prism it is separate in to a ray of constituent color this phenomenon is known as dispersion.

It can also be define as the broadening of the light plus in an optical wave propagation in an optical fiber there are different kind of dispersion is phenomenon is found

Intermodal or chromatic dispersion

Polarization mode dispersion

Material dispersion

Wave guide dispersion

Material dispersion:-It

occur due to the varying material properties which provide different velocity to the light in an optical fiber.


Fiber Dispersion

Polarization Mode DispersionPolarization mode dispersion (PMD) occurs when different

planes of light inside a fiber travel at slightly different speeds, making it impossible to transmit data reliably at high speeds(multimode fiber)

Waveguide dispersion- Waveguide dispersion occurs because the mode propagation

constant is a function of the size of the fiber's core relative to the wavelength of operation.

– Waveguide dispersion also occurs because light propagates differently in the core than in the cladding.

Total Dispersion– Total dispersion is due to all types of dispersion

Δt tot=Δt2modΔt

2chromΔt2pol


Modal dispersion


Scattering:-•Scattering can be broadly defined as the redirection of radiation out of the original direction of propagation, usually due to interactions with molecules and particles

• Reflection, refraction, diffraction etc. are actually all just forms of scattering

• Matter is composed of discrete electrical charges (atoms and molecules – dipoles)

• Light is an oscillating EM field – excites charges, which radiate EM waves

• These radiated EM waves are scattered waves, excited by a source external to the scattered

• The superposition of incident and scattered EM waves is what is observed

rainbow


Scattering geometry

Forward scatteringBackward scattering

(backscattering)


What is Rayleigh scattering?(or why is the sky blue)

The shorter the wavelength, the more light is scattered blue is scattered more than red. this is why the sky is blue and

sunsets are red.

Think of white light from sun as a mixture of R, G and B

Blue is scattered the most so sky looks blue when we look away from the sun

For same reason sun looks yellow (red + green)More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks redProf. Byomakesh Mahapatra

Optical wave guideAn optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and rectangular waveguides.

Optical waveguides are used as components in integrated optical circuits or as the transmission medium in local and long haul optical communication systems

Rectangular waveguidesDifferent type of waveguides


Wave front:-

a wavefront is the locus of points having the same phase a line or curve in 2d, or a surface for a wave propagating in 3d

In an optical system the wave front can be describe by simple maxuall equation

The general wave front of any light source is the spherical wave front, but when the radius of this spherical is very large then this can be evaluated by the simplest form of a wavefront is known as the plane wavefront, where the rays are parallel to one another


Wave polarization:-Polarization is a property of waves that can oscillate with more than one orientation

The polarization of light is described by specifying the orientation of the wave's electric field at a point in space over one period of the oscillation. When light travels in free space, in most cases it propagates as a transverse wave—the polarization is perpendicular to the wave's direction of travel. In this case, the electric field may be oriented in a single direction (linear polarization), or it may rotate as the wave travels (circular or elliptical polarization). In the latter case, the field may rotate in either direction


x

Looking at ray "head-on" see green arrows up & down

y

z

x

y

z

Light is polarized if the waveform and electric force field arrows remains in the same plane

The electric force arrows must always be perpendicular to the ray

This is a light ray traveling in the z-direction and polarized in the y-direction

Here is a light ray traveling in the same direction but polarized in the x-direction

We will visualize the polarization in the x-y plane, looking at rays head-on

The white force arrows point up and down or left and right, stacked up behind one-another.

Here is the convention for visualizing vertical and horizontal polarization

y

x

What is polarized light?


Engineering

Optical fibers communicatiobyom