Lecture 12 - Fiber optic communication- Semiconductor Laser.pdf

Fiber Optic CommunicationFiber Optic CommunicationSemiconductor Laser

Lecture 12Lecture 12

Khosrow GhadiriElectrical Engineering Department Electrical Engineering Department

San Jose State University

Principle of LASER diodeLASER is an acronym for Light Amplification by Stimulated EmissionLASER is an acronym for Light Amplification by Stimulated Emissionof Radiation.Laser diodes are, like LEDs, direct bandgap pn-junction used underforward bias.Laser diodes are devices emitting coherent light produced in thestimulated emission process whereas LEDs under injectionexcitement emit light produced in spontaneous emission process.Laser lasing wavelength ranges from the visible to the infraredLaser lasing wavelength ranges from the visible to the infraredwavelength depending on the material of the active layer.In the 850 nm-band AlGaAs/GaAs and in the 1300/1550 nm-bandsInGaAsP/InP material system are commonly used in fiber opticcommunication. AlGaAs/AlGaAs, and group-III nitride are used inthe visible wavelength.Laser diodes composed of III-V compound semiconductor and aswell as some II-VI compound are used in visible wavelength.

© Khosrow Ghadiri Fiber Optics Communications EE Dept. SJSU

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well as some II VI compound are used in visible wavelength.

HistoryThe first semiconductor laser were reported by R N Hall et al fromThe first semiconductor laser were reported by R.N. Hall et al. fromGeneral Electric research laboratory and Marshal I. Nathan et al.from IBM, Thomas J. Watson Research Center based on GaAs pnjunction in 1962.

( ) l h dContinuous-wave (CW) lasing at room temperature was achieved inan AlGaAs/GaAs double-heterostructure laser in 1970.I. Hayashi et al. Junction lasers which operates continuously at roomtemperature, Appl. Phys. Lett., 17, 109, 1970.temperature, Appl. Phys. Lett., 17, 109, 1970.Zh. I. Alferov, et al. Investigation of the influence of the AlAs-GaAsheterostructure parameters on the laser threshold current and therealization of continuous emission at room temperature, Sov. Phys,S i d 4 1573 1971Semicond. 4, 1573, 1971.


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Laser safetyLaser safetyLaser safety


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Laser safety ANSI standardsAmerican National Standard Institute Laser safety classification:American National Standard Institute Laser safety classification:Class 1:

Inherently safe.No viewing hazard during normal use or maintenanceNo control or label requirements (typically 0 4 microwatts or lessNo control or label requirements (typically 0.4 microwatts or lessoutput power)

Class 1M:Inherently safe if not view through collecting optics.Designed to allow for arrays of optical sources which may beDesigned to allow for arrays of optical sources which may beviewed at the same time.

Class 2:Normal human eye blink response or aversion response issufficient to protect the user.sufficient to protect the user.Low power visible laser (<1 mW continuous operation).In pulsed operation, waning labels are required if power levelsexceed the class 1 acceptable exposure duration, but do notexceed class 1 limits for a 0.25 second exposure.


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p

Laser safety ANSI standardsAmerican National Standard Institute Laser safety classification:American National Standard Institute Laser safety classification:Class 2A:

Warning labels are required.Low power visible laser that do not exceed class 1 acceptablelimits for 1000 seconds or less and system which are notlimits for 1000 seconds or less and system which are notdesigned for intentional viewing of the beam.

Class 3A:Normal human eye blink response or aversion response issufficient to protect the user, unless laser is viewed throughp , gcollecting optics.Requires warning labels, enclosure/interlocks on the system,and warning signs at room entrance where the laser is housed.Typically 1-5 mW power.

Class 3B:Direct viewing is hazard, and specular reflections may pose ahazard.Same warning label requirements as class 3A plus power

t t d i li ht h l i i ti


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actuated warning light when laser is in operation.

Laser safety ANSI standardsAmerican National Standard Institute Laser safety classification:American National Standard Institute Laser safety classification:Class 3B: continuation

Typically 5-500 mW continuous output power, <10 joules persquare centimeter pulsed operation for <0.25second.

Class 4:Class 4:Direct viewing is hazard, and specular or diffuse reflections maypose a hazard.Skin protection and fire protection are concerns.Same warning label requirements as class 3B plus a locked door,Same warning label requirements as class 3B plus a locked door,door actuated power kill switch or door actuated optical filter,Shutter, or equivalent.Typically >500 mW continuous output power, >10 joules persquare centimeter pulsed operation.


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Safety goggleSafety goggle is categorized by wavelength and maximum powerSafety goggle is categorized by wavelength and maximum power


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Periodic table of elements.


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Simplified orbital model of many electron atoms

Bohr’s modelBohr s model


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Carrier concentration and bandgapBandgap width 2Bandgap width

Intrinsic carrier concentration

2

g goTE E

Tαβ

= ++

3 2 2gE

kTi ion n T e−=

( ) 15 37 3 10n Si cm−= ×( ) 7.3 10ion Si cm= ×

Semiconductor

Germanium 0.7437 0.66 235

@ 0 ( )gE K eV @300 ( )gE K eV ( / )eV Kα ( )Kβ44 .7 7 4 1 0 −×

Silicon 1.170 1.12 636

GaAs 1.519 1.42 204

44.73 10−×

45.405 10 −×


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Comparison of Three Statistical Distributionr Maxwell- Bose-Einstein Fermi-DiracMaxwell

BoltzmannBose Einstein Fermi Dirac

Particle system Identical but distinguishable particles

Identical indistinguishable particles with integer

Identical indistinguishable particles with half-integer spin

particles particles with integer spin

Particles Classical Bosons Fermions

Examples Ideal gas at thermal equilibrium

Photons in a cavity.Photons in a solid

Electrons in a metal. Conduction in semiconductors. thermal equilibrium Photons in a solid semiconductors.

Distribution function F(ε) = (Aeε/kT) -1 F(ε) = ( Aeε/kT -1) -1 F(ε) = ( Aeε/kT +1) -1

Obeying Pauli Exclusion principal No No Yes

Properties of distribution

No limit to how many particles can occupy a given

No limit to how many particles can occupy a given quantum state

Never be more than one particle in

any quantum state


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occupy a given quantum state

given quantum state any quantum state

LaserGas die and semiconductor lasersGas, die, and semiconductor lasers


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Laser wavelength Laser wavelength range in visible and infrared regionsLaser wavelength range in visible and infrared regions

Loss

0.85μ 1.30μ 1.55μdB

Loss

( )


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( )mλ μ

LaserPhoton energy:Photon energy:

The radiated wavelength:photon gE E hf= =

1.24hcλ

The optic power output when gap energy is in joule:g gE E

λ = =

gEP NE i

ηη

Where N is the number of charge per second:

ggP NE i

qη= =

iN =

The optic power output when gap energy is in eV:

Nq

=

P iEη=


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gP iEη

Bandgap engineeringTernary alloys: for example GaAs laser doped with Al cover theTernary alloys: for example GaAs laser doped with Al cover thewavelength range from 850 nm to 780 nm with increasing x

AlX Ga 1-X As

Cd Mn TeCd 1-X MnXTe

GaAS 1-X PX

GaXIn 1-X As

Quaternary alloys: GaAs doped with In and P cover the wavelengthrange from 900 nm to 1670 nm with increasing x and y

X 1-X

Hg 1-X CdXTe

range from 900 nm to 1670 nm with increasing x and yGaX In1-X As1-Y PY

AlX Ga1-X AsY Sb1-Y


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X 1 X Y 1 Y

Laser power rangeLaser power ranges:Laser power ranges:Continuous wave power:

~1 mW for communications, data storage, laser pointersto ~100kW for machiningto 100kW for machiningto ~5MW for military

Pulsed power:to ~ W1510

Pulse length as short as ~5 fsecCavity length ~1 for VCSEL to 6.5 kmmμ


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Principle of laser diodeSemiconductor laser commonly used in fiber optic Semiconductor laser commonly used in fiber optic communication because:

Long lifeHigh reliabilityHigh reliabilityRuggednessCompactnessLight weightLight weightHigh efficiency of electrooptric conversionLow applied voltageSpectral purity compared to non laser sourceSpectral purity compared to non laser source.Direct modulation capability up to tens of gigahertz.


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The gas laserThe gas laserThe gas laser


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The gas laserThe gas laserThe gas laser


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Laser bench Laser benchLaser bench


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Laser diodesLaser diodesLaser diodes


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Principle of LASER diodeIn degenerately doped pn junction The Fermi levelIn degenerately doped pn junction, The Fermi level


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pn Junction diode

W

p n p n

FV

p n

RV−

p n

+ − + −

E E EE

cpE

FpE cnE0qV( )0 Fq V V−

( )0 Rq V V+

cpE

FpEE

cnEFnE

cpE

E cnEE

cpE

EFp

vpE FnE

vnE

( )0 Rq V V+

V( )0 FV V−

vpEvnE FVFpE

vpE FnE

vnE cnEFnE

vnE

FpEvpE

RV

0V

pV

nV

( )0 RV V+

Equilibrium Forward biasedBefore Contact Reverse biased


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Equilibrium Forward biasedBefore Contact Reverse biased

Degenerate pn Junction diode

W

p n p n

FV+ −

W

p+ n+ p+ n+

FV+ −

E E

cpE

FE cnE0qV( )0 Fq V V−

cpE

E cnEE

E E

cpE

EvpE

E0qV

( )0 Fq V V−cpE

E cnEE

++++−−−−

FpEvpE

cn

FnE

vnE

V( )0 FV V−

FVFpEvpE FnE

vnE

FpEcnEFnE

vnE

V( )0 FV V−

FVFpEvpE FnE

vnE

−−−−++++

VV

0V

pV

nV

Equilibrium Forward biased

0V

pV

nV

Equilibrium Forward biased

nV

pVnV

pV


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Equilibrium Forward biased Equilibrium Forward biased

Degenerately doped pn junctionIn degenerately doped pn junction The Fermi level in the p-sideEIn degenerately doped pn junction, The Fermi level in the p sideis in the valence band and that in the n-side is in the conductionband.All the energy levels up to The degenerately direct bandgap

d h h b d d

fpEfnE

semiconductor pn junction such as GaAs has band diagram:

hf −FreeElectron

Conduction Band

V

si

i

outV ElectronE

hf

hf 8W

hf ElectronCreated

+ +

FV+ −

DCI

Energyhf +FreeHoleCreated

JunctionValence Band

p+ n+

E


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Digital modulation of laser diodeIn the forward biased p-n junction laser diode The potentialIn the forward biased p n junction laser diode, The potentialenergy barrier between the p and n regions decreases , so electronand hole flow, hence current flow.

Incident photon after passing through p-region will be absorbed in the depletion layer.


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InGaAsP laser diode structureInGaAsP laser diode structureInGaAsP laser diode structure.


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LED and LDCurrent flow in LED and LD Current flow in LED and LD

Light power Laser diode 3030 0.30 15

A)Plaser mode

LEDmode

25 1300C nm°

)Ω

LED

5 mW

10 mW

2020G

E, V

(V)

ower

, P (m

W)

0.20 10

/ fac

et, d

P/d

l (W

/A

sist

ance

, dV/

dl

V

P

/ ( )s stdP dl η −=

(Ω

Current0

100 mA50 mA

1010VO

LTA

G

Ligh

t out

put p

o

0.10 5

Slo

pe e

ffeci

ency

Diff

eren

tial r

es

/ ( )dV dl Rs=

ypical optical power output vs. forward currentr a LED and a laser diode.999 S.O. Kasap, Optoelectronics (Prentice Hall)

C t l ( A)

00 0 050 100 1500

threshold current


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999 S.O. s p, Optoelect onics ( e ce )Current, l (mA)

RecombinationRecombination is the electron transition from the conduction bandRecombination is the electron transition from the conduction bandto fill a hole in the valence band.Recombination can be viewed as the annihilation of a negativecarrier (conduction electron) and a positive carrier (hole in thevalence band) and energy is released either as radiated photon orvalence band), and energy is released either as radiated photon orphonon (heat in the lattice of crystal).The electron density which is the number of electrons per unitvolume in energy levels between and in the conductionband is given by:

( )2n E2E 2 2E dE+

band, is given by:

where the is the density of states per unit energy per unitl i th d ti b d t l l

( ) ( ) ( )2 2 2 2 2c cn E dE g E f E dE=

( )2cg EEvolume in the conduction band at energy level

is the effective mass of electron

( )*3 2

2 23 2

2 ec c

mg E E Eπ

= −

2E

*m


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is the effective mass of electronem

Fermi-Dirac distributionThe Fermi-Dirac distribution function specifies the probability( )2f EThe Fermi Dirac distribution function specifies the probabilitythat the energy level in the conduction band is occupied by anelectron.

2E( )2cf E

( ) ( )22

1fn

c E Ef E

−=

1kTe +

The is the quasi-Fermi level of electrons in the conduction bandand is the bottom edge of the conduction band.E

fE


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and is the bottom edge of the conduction band.cE

Hole densitySimilarly the hole density which is the number of holes per unit( )1p ESimilarly the hole density which is the number of holes per unitvolume in energy levels between and in the conductionband, is given by:

( ) ( ) ( )1 1 1 1 11p E dE g E f E dE= −⎡ ⎤⎣ ⎦

( )1p E1E 1 1E dE+

where the is the density of states per unit energy per unitvolume in the conduction band at energy level

( ) ( ) ( )1 1 1 1 11v vp E dE g E f E dE⎡ ⎤⎣ ⎦

*3 22

( )1vg E1E

is the effective mass of holeThe Fermi-Dirac distribution function specifies the probability

( )*3 2

1 13 2

2 hc v

mg E E Eπ

= −*hm

( )f EThe Fermi-Dirac distribution function specifies the probabilitythat the energy level in the valence band is occupied by anelectron

1E( )1vf E

( ) ( )11

1fp

v E Ef E

−=


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1kTe +

Hole densityThe probability of the absence of electron or the presence of theThe probability of the absence of electron or the presence of thehole at in the valence band1E 1 1E dE+

( ) ( )11

11 1fp

v E Ef E

−− = −

The potential energy of bias voltage is:FeV

eV E E= −

1kTe +

FV

Gain is the difference between stimulated emission and absorption.Stimulated emission in semiconductor occurs when an incidenth t i d l t t k th t iti f d ti

F fn fpeV E E=

photon induces an electron to make the transition from conductionband to the valence band and is proportional to productwhich is the density of electrons in the conduction band and thedensity of holes in the valence band.


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Photon densityAbsorption is transition in the reverse direction and is proportionalAbsorption is transition in the reverse direction and is proportionalto the product of density of electrons in the valence bandexpressed by:

( ) ( ) ( )1 1 1 1 1v vn E dE g E f E dE=The density of vacant states in the conduction band expressed by:

( ) ( ) ( )2 2 1 2 21c cp E dE g E f E dE= −⎡ ⎤⎣ ⎦The net increase in the photon density S (number of photons perunit volume) by simulated emission is:

( ) ( ) ( ) ( )2 1 1 2 1 2dS W n E p E n E p E dE dEdt

= −⎡ ⎤⎣ ⎦∫∫Where the stimulated transition probability is given by

B is Einstein's B coefficient and is the energy density of the light

dt ⎣ ⎦∫∫

dW BE=dE


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B is Einstein s B coefficient and is the energy density of the lightthat is stimulating transition.

dE

Photon densityThe energy density of the light that simulating the transition isThe energy density of the light that simulating the transition isgiven by:

The increase in the energy density of the light is:dE hfS=

g g

is related to the light intensity by:

( ) ( ) ( ) ( )2 1 1 2 1 2d

ddE BhfE n E p E n E p E dE dEdt

= −⎡ ⎤⎣ ⎦∫∫dE SI

Where is the velocity of light:

sd

IEv

=v

( ) ( ) ( ) ( )2 1 1 2 1 2d s s

stim stim stim

dE dI dI Bhf n E p E n E p E dE dEdt vdt dz v

⎛ ⎞ ⎛ ⎞ ⎛ ⎞= = = −⎡ ⎤⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎣ ⎦⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ∫∫


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Light intensityThe net increase in the photon density S (number of photons perThe net increase in the photon density S (number of photons perunit volume) by spontaneous emission is:

( ) ( )2 1 1 2spon

dS An E p E dE dEdt

⎛ ⎞ =⎜ ⎟⎝ ⎠ ∫∫

A is Einstein's A coefficient.The change in light intensity variation due to spontaneous emissionis:

spon⎝ ⎠

( ) ( )dI hfA d d⎛ ⎞∫∫

The total resultant light intensity variation is

( ) ( )2 1 1 2s

spon

dI hfA n E p E dE dEdt v

⎛ ⎞ =⎜ ⎟⎝ ⎠ ∫∫

s s s

stim stim

dI dI dIdz dz dz

⎛ ⎞ ⎛ ⎞ ⎛ ⎞= +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠


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GainThe total resultant light intensity variation is:The total resultant light intensity variation is:

( ) ( )s s ss

stim stim

dI dI dI g hf I h hfdz dz dz

⎛ ⎞ ⎛ ⎞ ⎛ ⎞= + = +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

Where:

( ) ( ) ( ) ( ) ( )2 1 1 2 1 2s

stim

dI hfg hf B n E p E n E p E dE dEdz v

⎛ ⎞= = −⎡ ⎤⎜ ⎟ ⎣ ⎦⎝ ⎠ ∫∫

or

stim⎝ ⎠

( ) ( ) ( )2 1 1 2s

spon

dI hfh hf A n E p E dE dEdt v

⎛ ⎞= =⎜ ⎟⎝ ⎠ ∫∫

( ) ( ) ( ) ( ) ( )2 1 2 1 1 2s

c v c vstim

dI hfg hf B g E g E f E f E dE dEdz v

⎛ ⎞= = −⎡ ⎤⎜ ⎟ ⎣ ⎦⎝ ⎠ ∫∫


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The condition for positive gain is obtained if 2 1fn fpE E E E− > −

Light intensityThe total resultant light intensity variation is:The total resultant light intensity variation is:

( ) ( )s s ss

stim stim

dI dI dI g hf I h hfdz dz dz

⎛ ⎞ ⎛ ⎞ ⎛ ⎞= + = +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

Where:

( ) ( ) ( ) ( ) ( )2 1 1 2 1 2s

stim

dI hfg hf B n E p E n E p E dE dEdz v

⎛ ⎞= = −⎡ ⎤⎜ ⎟ ⎣ ⎦⎝ ⎠ ∫∫

or

stim⎝ ⎠

( ) ( ) ( )2 1 1 2s

spon

dI hfh hf A n E p E dE dEdt v

⎛ ⎞= =⎜ ⎟⎝ ⎠ ∫∫

( ) ( ) ( ) ( ) ( )2 1 2 1 1 2s

c v c vstim

dI hfg hf B g E g E f E f E dE dEdz v

⎛ ⎞= = −⎡ ⎤⎜ ⎟ ⎣ ⎦⎝ ⎠ ∫∫


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The condition for positive gain is obtained if 2 1fn fpE E E E− > −

Optical cavityOptical cavity: Optical cavity:

lP1R 2R

( )1 ( )R P iL

exp( )lP iLα−

1 2 exp( 2 )lR R P iLα−( )1 21 exp( 2 )lR R P iLα− −

( )21 exp( )lR P iLα− −

P exp( )P gL

exp(2 )lP gL

lP exp( )lP gL

exp(2 )lP gL

L


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Amplitude condition for laser oscillation

Power amplification with distance: Power amplification with distance:

Amplitude amplification for one round trip:

gzoP P e=

Amplitude amplification for one round trip:

( )2 22e

g Lj L

oE REα

β−

+=

is amplitude of the light before the trip. L is the length ofactive region, is the power attenuation coefficient, ispropagation constant, R is the amplitude reflection coefficients of

oEα β

p p g , pthe mirror whenTwo conditions should be met:

1 2andR R 1 2R R R=

( ) 1 1Re 1 ln and largeg L g Rα α− ≥ ⇒ ≥ +


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Re 1 ln and largeg RL R

α≥ ⇒ ≥ +

Analog modulation

Analog modulation: Analog modulation:

20

(mW

)

10t out

put p

ower

, P (

Time

0

Ligh

t

Current, l (mA)50 1000si


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Time

Digital modulationDigital modulation: Digital modulation:

20P

(mW

)

10

ight

out

put p

ower

, P

0

L

Time50 1000

Current, l (mA)

si


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Time

Temperature dependent of laser diode

Temperature dependent of laser diode : Temperature dependent of laser diode :

8w

er, P

(mW

)

60 C°70 C°80 C°6

2

ght o

utpu

t pow

30 C°40 C°50 C°4

0

Lig

50 2000 100 150


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Current, l (mA)

Edge emission laser

Asymmetric radiation from edge emitting LDAsymmetric radiation from edge emitting LD

120°Beam Intensity

30°Parallel Plane

045− °90− ° 45° 90°Beam Angle

Perpencicular Plane


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Beam Angle

Edge emission laser

Asymmetric radiation from edge emitting LDAsymmetric radiation from edge emitting LD

0


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ReferencesS O Kasap Optoelectronics and photonics principle and practiceS. O. Kasap, Optoelectronics and photonics principle and practicePearson Education, 2001.Titsuo Fukuda, Optical semiconductor devices, Wiley series inmicrowave and optical engineering, Wiley interscience, 1999.Gerd Keiser, Optical Fiber Communication, McGraw-hill, third edition2000.


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Lecture 12 - Fiber optic communication- Semiconductor Laser.pdf

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