eecb 483 optoelectronics & fiber opticslecture5v2

8/19/2019 EECB 483 Optoelectronics & Fiber Opticslecture5v2

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EECB 483 Optoelectronics &Fiber Optics

Lecture 5:

Optical sources and Amplifiers



Optical sources

optical sources generates light signal as transmission medium for

optical communication system

most typical source

light emitting diode (LED):

incoherent output ;

no need optical cavityThe output radiation has broad spectral width, since the

emitted photons energies range over the energy distribution

of the recombining electrons and holes

Emitted into hemisphere according to a cosine powerdistribution and has large beam divergence



Laser diode (LD):coherent output;

the optical energy is produced in an

optical resonant cavity.

energy released is spatial and temporal

coherence

highly monochromatic and

Output beam is very directional



Choosing an optical Source Compatibility with Optical

Waveguide:

The following characteristics need to be consider:

i. Its geometry

ii. Its attenuation as function of wavelength

iii. Its group delay distortion (bandwidth)

iv. Its modal

The interplay of these factors with optical source power, spectral width,

radiation pattern and modulation capability

The spatially directed coherent optical output of Laser Diode can be coupledinto either single mode or multimode fibers.

The LED are used in multimode, because the incoherent output power can be

coupled in sufficient quantities.



Light emitting diode (LED)

a pn-junction semiconductor that emit light when forward

biased

two energy bands separated by a gap energy, Wg

upper band is conduction band has free electrons

lower band is valance band has free holes

free electron can recombine with free holes to return

to neutral state and release energy in the process



/Wg



http://upload.wikimedia.org/wikipedia/commons/7/7c/PnJunction-LED-E.PNG



Material Symbol Band gap (eV) @ 300K

Silicon Si 1.11 [1]

Germanium Ge 0.67 [1]

Silicon carbide SiC 2.86 [1]

Aluminum phosphide AlP 2.45 [1]

Aluminum arsenide AlAs 2.16 [1]

Aluminium antimonide AlSb 1.6 [1]

Gallium(III) phosphide GaP 2.26 [1]

Gallium(III) arsenide GaAs 1.43 [1]

Gallium(III) nitride GaN 3.4 [1]

Gallium(II) sulphide GaS 2.5 (@ 295 K)

Gallium antimonide GaSb 0.7 [1]

Indium(III) phosphide InP 1.35 [1]

Indium(III) arsenide InAs 0.36 [1]

List of band gaps

http://www.answers.com/topic/electronvolt

http://www.answers.com/topic/river-kelvin

http://www.answers.com/topic/silicon

http://www.answers.com/topic/band-gap

http://www.answers.com/topic/germanium


http://www.answers.com/topic/silicon-carbide


http://www.answers.com/topic/aluminium-phosphide


http://www.answers.com/topic/aluminium-arsenide


http://www.answers.com/topic/aluminium-antimonide


http://www.answers.com/topic/gallium-iii-phosphide


http://www.answers.com/topic/gallium-iii-arsenide


http://www.answers.com/topic/gallium-iii-nitride


http://www.answers.com/topic/gallium-antimonide


http://www.answers.com/topic/indium-iii-phosphide


http://www.answers.com/topic/indium-iii-arsenide



http://www.answers.com/topic/indium-iii-arsenide


http://www.answers.com/topic/indium-iii-phosphide


http://www.answers.com/topic/gallium-antimonide


http://www.answers.com/topic/gallium-iii-nitride


http://www.answers.com/topic/gallium-iii-arsenide


http://www.answers.com/topic/gallium-iii-phosphide


http://www.answers.com/topic/aluminium-antimonide


http://www.answers.com/topic/aluminium-arsenide


http://www.answers.com/topic/aluminium-phosphide


http://www.answers.com/topic/silicon-carbide


http://www.answers.com/topic/germanium


http://www.answers.com/topic/silicon

http://www.answers.com/topic/river-kelvin

http://www.answers.com/topic/electronvolt



LED

when p-and n-doped material put close together,

energy band gap is produced without any applied voltage

free electron and free hole cannot jump the barrier

due to insufficient energy

when forward biased about the same as Wg, freeelectron and free hole has the sufficient energy to move

into the junction



LED

electron can fall into valance band andrecombine with holes to release energy in the

form of photon

the wavelength of photon released depends onenergy band gap, Wg given by this formula

The photon energy and frequency are related

by



LED

homojunction – a pn-junction formed from single

semiconductor. photon emitted over an extensive region

and diverge

heterojuction – pn-junction formed by dissimilar

material. two material with different band gap energies

and different refractive indices. recombination occurs

only in small well-define active layer and differentrefractive index formed waveguide.



LED

output power of LED is linearly proportional to forwarddriving current

the current i is the injected charge per second

# of charge/second, ; e is the magnitude of charge

on each electron

if fraction of recombine charge and produce photon, η , the output

power will be P = ηNWg = (η¡Wg)/e this will be joules

In electron volt: P= η¡Wg



LASER

Individual lower energy electrons absorb light wave energy and change theirspatial electric charge configuration (they “move” to a higher energy level) atrandom time intervals

Individual higher energy level electrons radiate light wave energy at random

time intervals. When light waves are present at a frequency corresponding tothe normal radiation frequency (f =∆E/h) for that downward energy change, thenumber of electrons that randomly radiate per unit time is increased. Thisincrease is called “stimulated emission.”

To continue amplifying, there must be a ready supply of high energy electrons.Electrons are continuously “pumped” up from a still lower energy level to thehigh energy level by constant irradiation using a much higher frequency,

shorter wavelength optical source. Arthur L. Schawlow (1921-1999) and Charles H. Townes first built an amplifier

using stimulated emission for amplifying microwaves. Gordon Gould is alsocredited with theoretical invention of the LASER in the patent office. Samemethod later applied to visible and infra-red light. Schawlow and Townesreceived Nobel prize with others.

Terminology:

MASER (Microwave Amplification via Stimulated Emission of Radiation)

LASER (Light Amplification via Stimulated Emission of Radiation)



The electrons are generally found in the ground state or the lowest energy level.They can occupy higher energy levels leaving lower levels vacant. They change

from one level to another by (1) absorption or (2) emission of energy. This

changing of energy levels is called radiative transition. There are three types of

radiative transition.

A. Stimulated emission

B. Spontaneous emissionC. Absorption

E1(ground state)

E3 (Excited State)

Spontaneous

Absorption Stimulated



Laser principle

atom/ion/molecule of the active material will

absorb energy supplied to raise from ground state toexcited state

once in excited state, the atom/ion/molecule will release

its energy to return back to ground state in two situation:

first process is called spontaneous emission (without external energy), theoutput are isotropic and of random phase, appear as narrowband Gaussian

output

Second process is called Stimulated emission, the electron is induced to

downward transition by an external stimulation; that is a photon energy (hv)

impinges the electron while in excited state to immediately stimulated to drop

to the ground state . The emitted photon is in phase with the incident photon.

the energy release is in the form of photon or light

energy



Laser Principle

in order to keep the stimulated emission high, number ofatom/ion/molecule in excited state must be more than number of

atom/ion/molecule in ground state

this is to make sure more emission than absorption

this condition is called population inversion

In a Semiconductor laser, population inversion is accomplished by

injecting electrons into the material as the device contacts to fill the

lower energy states of the conduction band.



a laser come from oscillation of photon

generation within a resonant cavity

the mirrors at both end provide reflection so

that light will be amplified as it pass throughamplifying medium back and forth

the traveling light will interfere and createstanding wave

LASER



Laser

to produce standing wave, cavity length must be and

integral number of half wavelength long, L = mλ/2

resonant frequencies are found from f = mc/2nL, where

n is the refractive index of the material within cavity

various resonant frequency is called longitudinal modes

∆fc = c / 2Ln



Laser

relation with wavelength

∆fc/f = ∆λc/λo

λo is free space mean wavelength

f is mean frequency

since f = c/λo, ∆λc = (λo2/c)∆fc



Laser

we will discuss four types of laser

semiconductor laser

gas laser

solid state laserfiber laser

all use the same principle, oscillation in a cavity



Principal components:1. Gain medium

2. Laser pumping energy

3. High reflector

4. Output coupler

5. Laser beam

http://en.wikipedia.org/wiki/Laser_beam

LASER Amplification

http://en.wikipedia.org/wiki/Output_coupler


http://upload.wikimedia.org/wikipedia/commons/1/1f/Laser.svg



A laser consists of a gain medium inside a highly reflective optical cavity,

also use to supply energy to the gain medium.

cavity consists of two mirrors arranged such that light bounces back and

forth, each time passing through the gain medium.

The gain medium is a material with properties that allow it to amplify

light by stimulated emission.

The output coupler, is partially transparent. The output laser beam is

emitted through this mirror.

LASER Amplification cont…

http://en.wikipedia.org/wiki/Active_laser_medium

http://en.wikipedia.org/wiki/Optical_cavity



http://en.wikipedia.org/wiki/Optical_cavity

http://en.wikipedia.org/wiki/Active_laser_medium



Light of a specific wavelength that passes through the gain medium is

amplified (increases in power);

the surrounding mirrors ensure that most of the light makes many passes

through the gain medium, being amplified repeatedly. Part of the light

that is between the mirrors (that is, within the cavity) passes through the

partially transparent mirror and escapes as a beam of light.

The process of supplying the energy required for the amplification is

called pumping.

The energy is typically supplied as an electrical current or as light at a

different wavelength. Such light may be provided by a flash lamp orperhaps another laser.

Most practical lasers contain additional elements that affect properties

such as the wavelength of the emitted light and the shape of the beam.

LASER Amplification cont…

http://en.wikipedia.org/wiki/Optical_amplifier

http://en.wikipedia.org/wiki/Laser_pumping

http://en.wikipedia.org/wiki/Flash_lamp

http://en.wikipedia.org/wiki/Flash_lamp

http://en.wikipedia.org/wiki/Laser_pumping

http://en.wikipedia.org/wiki/Optical_amplifier



Laser type

Gas laser

active medium : Helium & Neon gas

wavelength : 633nm

Semiconductor laser

Active medium : AlGaAs, InGaAsP

Wavelength : 800-900nm, 1000-1700nm

Solid State laser

Active medium : Neodymium yttrium-aluminium-garnet (Nd:YAG)

Wavelength : 1060nm, 1350nm

Fiber laser

Active medium : Erbium ions

Wavelength : around 1550nm



Optical Repeaters

In general, optical repeaters perform functions similar toelectrical digital repeaters: the “Three Rs”:◦ Regenerate (amplify, compensate for power loss)

◦ Reshape (correct pulse wave-shape for distortions due to time dispersion)

◦ Retime (correct for jitter)

OEO Repeaters◦

Historical optical repeaters use an OE detector, electrical amplification andpulse shaping, and a EO LED or LD to transmit the repeated pulse streaminto the next span.

◦ Only one wavelength can be processed by a single OEO repeater.

◦ Many optical and electronic components and some manual adjustment atinstallation time are required. This is a relatively complicated and costlydevice.

◦

A simpler type of all-optical repeater, particularly one that amplifies all theinfrared wavelengths that are present, is desirable



EDFA: Direct Optical Amplification

Erbium-doped fiber amplifier (EDFA) is an Infra-Red LASER(Light Amplification by Stimulated Emission of Radiation)

which converts shorter wavelength IR source (“pump”)power into greater power at signal wavelength(s).

Advantages:

◦ Simpler, uses less components than electro-optics,particularly for multiple wavelengths on same fiber

(WDM)◦ Amplifies many different optical wavelength signals

present in WDM (present and future as well)

◦ Compensates for optical losses due to filters and opticalcombiners used in WDM

But… optical amplification does not correct timing or waveshape (two of the “3 Rs”)



Why Erbium?

Erbium atoms have three important energy levels. The top twolevels differ by an energy difference E= E3-E2 corresponding to~1300 nm wavelength, the desired wavelength of the amplifiedsignal. The lowest of the three energy levels differs from the topenergy level (E3-E1 ) by an amount corresponding to the “pump”signal wavelength.

A section of glass fiber made with Erbium doping is spliced intothe signal-carrying fiber. This Erbium section is continuouslyilluminated with a “pump” optical signal of wavelength

corresponding to E3-E1

Radiative energy level transitions occur from level E3 down to E2in proportion to the incoming (signal) light power level. Theoutgoing light power level is stronger.

◦ Electrons eventually fall in energy from level E2 backto E1 as well, but produce light of a different

wavelength than the 1300 nm wavelength used foroptical signals. These other wavelengths areeventually absorbed by “colored” filters that onlysubstantially pass 1300 nm infra red light.

E1

E2

E3

Pump

Action

(sche-

Matic)

Radiation



E1(ground state)

E2

E3 (Excited State)

Pump

Action(schematic)

Radiation



Optical amplifier

amplifier is needed to boost amplitude of signal after long

transmission

three types of optical amplifier

semiconductor amplifier

erbium-doped amplifier

raman amplifier



Semiconductor amplifier

it is essentially a semiconductor laser without the oscillation

SOA has one major disadvantages which is too much noise and

gain of polarization dependent

SOA is usually used in application using 1300nm wavelength

range



Raman amplifier

use raman scattering phenomena

raman scattering is the scattering of a photon by and optical

phonon which is the vibration of a crystal or molecule

the phonon will cause the downshift in frequency called Stoke

shift

usually operating in S-band (1460 -1530nm)



Erbium-doped amplifier

two types

fiber amplifier (EDFA)

waveguide amplifier (EDWA)

different between these two is just the medium hosting

the Erbium ions.

EDFA uses silica fiber while EDWA uses silica waveguide



EDFA

uses laser diode as source of energy (called pump). twowavelength : 980nm & 1480nm

pump energy is absorbs by Er ions and input signal (information

carrying signal) stimulated the energy release

Er will release energy (photon) which is exactly similar in

wavelength and phase of the photon that stimulates to release



Noise figure

measure of noise characteristic of EDFA

it is the ratio of input SNR to output SNR

it give an indication of signal degradation due to amplification

since noise is amplified with the signal

the lesser the value the better

NF is measured in unit dB



EDF laser

if the EDFA is closed-loop, it can become a laser, EDFL

two setups

ring laser – where the oscillation is in circular motion

linear laser – where the oscillation is linear

similar principle as other lasers

eecb 483 optoelectronics & fiber opticslecture5v2

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