OFC - Lecture 1 - Introduction

8/7/2019 OFC - Lecture 1 - Introduction

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Optical Fiber Communications

Introduction & Basics


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The basic optical fiber system

Optical

sourceModulator

Optical

receiverElectronics.

Optical

fiber

Optical

amplifier

Electronics.


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The basic optical fiber system

Source produces the optical signal

Modulator turns electronic data into an optical signal

Optical fiber carries the optical signal over long distances.

Optical amplifier boosts the signal as it travels

Optical receiver turns the optical signal back to an electronic

data signal


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What is light?

In one description light is a form of electromagneticwave (EM) radiation very similar to radio waves thedifference being the frequency (f) of the radiation.Th

ese are basically sine (or cosine) waves th

at moveaway (propagate) from a source.

It can also be expressed in terms of a parameter calledwavelength (P). This actually describes what it lookslike with respect to distance.

If the wave propagates at a speed (c) and has afrequency f then the wavelength is given by P = c/f


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Electromagnetic waves -frequency

Time

T - period

Set the distance to a fixed value and look at the

wave - movie


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Electromagnetic waves -wavelength

LightSource

Distance

P

Set the time to a fixed value and look at the wave -

photograph


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Basics of quantum physics

A picture of lights sources can be built up by

considering the concept of energy levels

within an atomic system.

This describes an atomic system in terms of

energy levels where electrons reside.

Here a two level system is shown level E1

and

level E2 where E2>E1.


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Spontaneous emission

E2

E1

E2

E1

Energyinputtosystem.

Releaseof energyasspontaneousemission.

Inputenergy

Photonoutput


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Stimulated emission

E2

E1

InputphotonCoherentlightoutput


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Spontaneous vs. Stimulated

Spontaneous Emission

Photon emission is entirelyrandom.

Each photon emitted hasdifferent energy hfanddifferent polarization.

Hence, large number ofelectron transitions

produces incoherentradiation.

Used in LED.

Stimulated Emission

Photon emission is not

random.

Each photon emitted hasidentical energy, phase and

polarization.

Hence, produced radiation

is highly coherent. Used in Laser.


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Boltzmann Distribution

E2 Excited State

E1 Ground State

N1 atoms

N2 atoms


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Example

T = 300 K

f= 5 x 1014 Hz

h = 4.135 x 10-15 eV.s

k= 8.617 x 10-5 eVK-1

E = hf= 2.068 eV

kT = 0.026 eV


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Band Gap

Unfilled Bands

Band Gap

Filled Bands

Valence Band

Conduction Band

Free Electron Energy

Energy


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Population Inversion

When the system is in thermal equilibrium,the lower energy state is more populated thanhigher energy state.

Population inversion is having more membersin the higher energy state than the lowerenergy state.

So, in order to achieve Population Inversion,we need to push the system into a non-equilibrated state.


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Population Inversion Cont.

To achieve populationinversion atoms shouldbe excited to the upper

energy level using anexternal energy source.

This is called Pumping

However, two level

systems do not providesuitable populationinversion.

E2 Excited State

E1 Ground State

LasingPumping


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Population Inversion Cont.

Three Level System Four Level System

E1

E0

LasingPumping

E2

E1

E0

Lasing

E2Rapid

Decay

E3

Ruby (Crystal) Laser He-Ne (gas) Laser


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Semiconductor Laser Materials

An optical source can be formed from a semiconductor P-Njunction or a diode.

Here population inversion is created by the injection ofcurrent into a pn junction.

The p and n junction form energy levels, with a gapbetween them Eg (the bandgap). The continual flow ofcurrent causes these levels to be dynamically populatedwith electrons.

In an electronic pn junction this is the current flow.

However if an optical device is needed then thesepopulated electrons should return to the lower level viaphoton emission rather than participate in current flow. Ifthis happens then photons may be released.


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Semiconductor Laser Materials

If the material is efficient at releasing photons then it is anelectroluminescent material, recombination of many electronsacross the bandgap occurs, with the release of a photon of energy(Eg)and frequency (f) where

Now it is quite possible that some electron transitions may notproduce a photon and may release the energy in another form,

called a phonon. Non radiative recombination occurs as a result ofenergy released possibly as lattice vibrations in the form ofheat.

Many optical sources are built from semiconductor p-n junctions.

E hf g =


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LED - Optical power Vs current.

Current.

Power

These use spontaneous emission


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Laser - Optical gain.

Now in a material with population inversion it

is common to speak of the optical power gain.

It describesh

ow th

e optical signal increases inpower i.e. due to stimulated emission, as it

propagates through a material with

population inversion.

Optical gain is possible in a device with

population inversion.


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Interaction Of Photons And Carriers In

Semiconductor Laser

I

n

Spontaneous

emission.

Stimulated

emission.

S

Loss Output

power

Current

Population

inversion.

Photons - optical

power


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Laser oscillation

To achieve laser oscillation the semiconductor p-n

junction is placed between end reflectors called

facets and current is then supplied.

The structure is attributed to scientists whodeveloped this and is often called a Fabry Perot

etalon.

Basically it is a cavity with end mirrors that allows the

signal to repeatedly reflect.


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Typical laser cavity

Mirror. Mirror.

OutputAmplifying medium


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Light Amplification in Laser

If a photon colliding with an atom causes

stimulated emission, it will emit two photons.

If th

ose ph

otons release two more andcontinuation of this process releases more and

more photons causing an avalanche

multiplication It is light amplification.

To do that a suitable medium should be

present to amplify the emission of photons.


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Laser Optical power Vs current.

Current.

Power.Stimulated

Emission Region

Spontaneous

Emission Region


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The lineshape range of wavelengths actually

emitted by a source

To get a stable output, optical gain should be

matched by the losses incurred in the

amplifying medium.

Major losses result from

Scattering (in medium, at mirrors)

Absorption

Diffraction at mirrors


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emitted by a source

Oscillations occur in the laser cavity over a

small range of frequencies, where the cavity

gain is sufficient to overcome the loss.

Hence, the device is not perfectly

monochromatic but emits over a narrow

spectral band.


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emitted by a source

P

RelativeAmplification

Frequency


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Lineshape and modes of lasers.

If oscillation builds up in a laser device laser action may takeplace. The propagating waves in the laser will also establishstanding waves between the mirrors and these standingwaves only exist at certain wavelengths within the laser such

th

at th

at th

e expression 2LN/P

is equal to a positive integer k.In relation to the length of the cavity L the mode wavelengthsare:

P = 2LN/k

where k is a positive integer.

P is the wavelength of the mode in a vaccumN the medium (the material the laser is made from) refractiveindex

L the distance between the mirrors.


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Laser modes the discrete wavelengths emitted

by a laser.

Gainofmodes/arb.units

ModesPp

(P

GT


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Laser modes

Adjacent modes are given by:

and

the mode spacing is given by:

Thus a number of modes can exist within a laser cavity and it will be the

modes that have sufficient gain to over come the losses, which are emitted.

k

LN2

1

2

k

LN

LN2

2P

P !(

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OFC - Lecture 1 - Introduction

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