3rd Week Seminar - Spring 2011 - Revision3

8/4/2019 3rd Week Seminar - Spring 2011 - Revision3

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YONSEIUNIVERSITY

Optical Sources Iwa Kartiwa

Broadband Transmission Network Lab.

Department of Electrical & Electronic Engineering



Photonics Conference 2010

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Contents

Semiconductor Physics.2.

Light-Emitting Diodes.

3.

Laser Diodes.

4.




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Review of Semiconductor Physics

a) Energy level diagrams showing the excitation of an electron from the valence band to the conduction band.

The resultant free electron can freely move under the application of electric field.

b) Equal electron & hole concentrations in an intrinsic semiconductor created by the thermal excitation of

electrons across the band gap

-123 JK1038.1 Bk

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000




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n-Type Semiconductor

a) Donor level in an n-type semiconductor.

b) The ionization of donor impurities creates an increased electron concentration distribution.



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p-Type Semiconductor

a) Acceptor level in an p-type semiconductor.

b) The ionization of acceptor impurities creates an increased hole concentration distribution




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Intrinsic & Extrinsic Materials

Intrinsic material: A perfect material with no impurities.

Extrinsic material: donor or acceptor type semiconductors.

Majority carriers: electrons in n-type or holes in p-type.

Minority carriers: holes in n-type or electrons in p-type.

The operation of semiconductor devices is essentially based on the injection

and extraction of minority carriers.

)2

exp(T k

E n pn B

g

i

ly.respectiveionsconcentratintrinsic&holeelectron,theare&&in pn

e.Temperaturisenergy,gaptheis T E g

2

in pn

[4-1]

[4-2]



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The pn Junction


Electron diffusion across a pn junctioncreates a barrier potential (electric field)

in the depletion region.



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Reverse-biased pn Junction


A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.



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Forward-biased pn Junction


Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.



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Direct Band Gap Semiconductors



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Indirect Band Gap Semiconductors

E

CB

k –

k

Direct Bandgap

(a) GaAs

E

CB

VB

Indirect Bandgap, E g

k –

k

k cb

(b) Si

E

k –

k

Phonon

(c) Si with a recombination center

E g

E c

E v

E c

E v

k vb VB

CB

E r

E c

E v

Photon

VB

(a) In GaAs the minimum of the CB is directly above the maximum of the VB. GaAs istherefore a direct bandgap semiconductor. (b) In Si, the minimum of the CB is displaced fromthe maximum of the VB and Si is an indirect bandgap semiconductor. (c) Recombination of an electron and a hole in Si involves a recombination center .

© 1999 S.O. Kasap,Optoelectronics(Prentice Hall)



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Light-Emitting Diodes (LEDs)

For photonic communications requiring data rate 100-200 Mb/s with

multimode fiber with tens of microwatts, LEDs are usually the bestchoice.

LED configurations being used in photonic communications:

1- Surface Emitters (Front Emitters)

2- Edge Emitters



YONSEIUNIVERSITY

13 / 13Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

a)Cross-section drawing of a typical

GaAlAs double heterostructure light

emitter. In this structure, x>y to provide

for both carrier confinement and optical

guiding.

b) Energy-band diagram showing the

active region, the electron & hole

barriers which confine the charge carriers

to the active layer.

c) Variations in the refractive index; the

lower refractive index of the material in

regions 1 and 5 creates an optical barrier

around the waveguide because of the higher

band-gap energy of this material.

)eV(

240.1m)(

g E [4-3]

Heterojunction Structures




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Surface-Emitting LED


Schematic of high-radiance surface-emitting LED. The active region is limitted

to a circular cross section that has an area compatible with the fiber-core end face.




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Edge-Emitting LED

Schematic of an edge-emitting double heterojunction LED. The output beam is

lambertian in the plane of junction and highly directional perpendicular to pn junction.

They have high quantum efficiency & fast response.





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Spectral width of LED types


R t ti Q t Effi i & P f




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Rate equations, Quantum Efficiency & Power of

LEDs

When there is no external carrier injection, the excess density decays

exponentially due to electron-hole recombination.

n : is the excess carrier density,

Bulk recombination rate R :

Bulk recombination rate (R )=Radiative recombination rate +nonradiative recombination rate

/

0)( t ent n

lifetime.carrier:

densityelectronexcessinjectedinitial:0

n

n

dt

dn R




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)1rate(ionrecombinatvenonradiati)1(rateionrecombinatradiative

)1(rateionrecombinatbulk

r nr nr r / τ R / τ R

/ τ R

With an external supplied current density of J the rate equation for the electron-hole

recombination is:

regionionrecombinatof thickness:electron;theof charge:

)(

d q

n

qd

J

dt

t dn

[4-6]

In equilibrium condition: dn/dt=0

qd

J n

[4-7]

Rate equations, Quantum Efficiency & Power of LEDs




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r nr r

nr

nr r

r

R R

R

int

Internal Quantum Efficiency & Optical Power

[4-8]

regionactivein theefficiencyquantuminternal:int

Optical power generated internally in the active region in the LED is:

q

hcI h

q

I P intintint [4-9]

regionactivecurrent toInjected:

power,opticalInternal:int

I

P




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External Quantum Eficiency

In order to calculate the external quantum efficiency, we need toconsider the reflection effects at the surface of the LED. If we considerthe LED structure as a simple 2D slab waveguide, only light fallingwithin a cone defined by critical angle will be emitted from an LED.

photonsgeneratedinternallyLEDof #

LEDfromemittedphotonsof #ext [4-10]




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d T c

)sin2()(4

1

0

ext [4-11]

2

21

21

)(

4)0(tCoefficienonTransmissiFresnel:)(

nn

nnT T

[4-12]

2

11

ext2)1(

11If

nn

n [4-13]

2

11

intintext

)1( power,opticalemittedLED

nn

PPP

[4-14]

External Quantum Eficiency

M d l i f LED




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Modulation of LED

The frequency response of an LED depends on:

1- Doping level in the active region

2- Injected carrier lifetime in the recombination region, .

3- Parasitic capacitance of the LED

If the drive current of an LED is modulated at a frequency of the output

optical power of the device will vary as:

Electrical current is directly proportional to the optical power, thus we can

define electrical bandwidth and optical bandwidth, separately.

2

0

)(1)(

i

PP

[4-15]

i

currentelectrical: power,electrical:

)0(log20

)0(10logBWElectrical

I p

I ) I(

p) p(

[4-16]




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)0(

)(log10)0(

)(log10BWOptical I

I

P

P [4-17]


Modulation of LED

LASER




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LASER (Light Amplification by the Stimulated Emission of Radiation)

Laser is an optical oscillator. It comprises a resonant optical amplifier

whose output is fed back into its input with matching phase. Anyoscillator contains:

1- An amplifier with a gain-saturated mechanism

2- A feedback system

3- A frequency selection mechanism

4- An output coupling scheme In laser the amplifier is the pumped active medium, such as biased

semiconductor region, feedback can be obtained by placing activemedium in an optical resonator, such as Fabry-Perot structure, twomirrors separated by a prescribed distance. Frequency selection isachieved by resonant amplifier and by the resonators, which admits

certain modes. Output coupling is accomplished by making one of theresonator mirrors partially transmitting.




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Pumped Active Medium

Three main process for laser action:

1- Photon absorption

2- Spontaneous emission

3- Stimulated emission





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Lasing in a pumped active medium

In thermal equilibrium the stimulated emission is essentially negligible, since

the density of electrons in the excited state is very small, and optical emission

is mainly because of the spontaneous emission. Stimulated emission will

exceed absorption only if the population of the excited states is greater than

that of the ground state. This condition is known as Population Inversion.

Population inversion is achieved by various pumping techniques.

In a semiconductor laser, population inversion is accomplished by injecting

electrons into the material to fill the lower energy states of the conduction

band.




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Fabry-Perot Resonator

A

B

L

M 1 M 2 m = 1

m = 2

m = 8

Relative intensity

m

m m + 1 m - 1

(a) (b) (c)

R ~

0.4

R ~ 0.81 f

Schematic illustration of the Fabry-Perot optical cavity and its properties. (a) Reflectedwaves interfere. (b) Only standing EM waves, modes, of certain wavelengths are allowed

in the cavity. (c) Intensity vs. frequency for various modes. R is mirror reflectance andlower R means higher loss from the cavity.

© 1999 S.O. Kasap, Optoelectronics(Prentice Hall)

)(sin4)1(

)1(22

2

kL R R

R I I inctrans

[4-18]

R: reflectance of the optical intensity, k : optical wavenumber

1,2,3,.. :modesResonant mmkL




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Laser Diode

Laser diode is an improved LED, in the sense that uses stimulated emissionin semiconductor from optical transitions between distribution energy states of

the valence and conduction bands with optical resonator structure such asFabry-Perot resonator with both optical and carrier confinements.





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Laser Diode Characteristics

Nanosecond & even picosecond response time (GHz BW)

Spectral width of the order of nm or less

High output power (tens of mW)

Narrow beam (good coupling to single mode fibers)

Laser diodes have three distinct radiation modes namely, longitudinal,lateral and transverse modes.

In laser diodes, end mirrors provide strong optical feedback in

longitudinal direction, so by roughening the edges and cleaving thefacets, the radiation can be achieved in longitudinal direction ratherthan lateral direction.

DFB(Di t ib t d F dB k) L




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DFB(Distributed FeedBack) Lasers

In DFB lasers, the optical resonator structure is due to the incorporation ofBragg grating or periodic variations of the refractive index into multilayer

structure along the length of the diode.


L O i L i C di i




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Laser Operation & Lasing Condition

To determine the lasing condition and resonant frequencies, we should focus

on the optical wave propagation along the longitudinal direction, z-axis. The

optical field intensity, I , can be written as:

Lasing is the condition at which light amplification becomes possible by

virtue of population inversion. Then, stimulated emission rate into a given

EM mode is proportional to the intensity of the optical radiation in that mode.In this case, the loss and gain of the optical field in the optical path determine

the lasing condition. The radiation intensity of a photon at energy varies

exponentially with a distance z amplified by factor g, and attenuated by

factor according to the following relationship:

)()(),( zt je z I t z E [4-19]

h

O C




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zhhg I z I )()(exp)0()( [4-20]

1 R 2 R

Z=0 Z=L

)2()()(exp)0()2(

21

Lhhg R R I L I [4-21]

2

21

21 t,coefficienabsorptioneffective:

tcoefficiengain:gfactor,tconfinemenOptical:

nn

nn Rα

1n

2n

Lasing Conditions:

1)2exp(

)0()2(

L j

I L I

[4-22]

Laser Operation & Lasing Condition




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Threshold gain & current density

21

1ln21

R R Lgth

thgg :iff lase""tostartsLaser

[4-23]

For laser structure with strong carrier confinement, the threshold currentDensity for stimulated emission can be well approximated by:

thth J g

[4-24]

onconstructidevicespecificondependsconstant:




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Optical output vs. drive current


S i d t l t ti




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Semiconductor laser rate equations

Rate equations relate the optical output power, or # of photons per unit volume, ,

to the diode drive current or # of injected electrons per unit volume, n. For active

(carrier confinement) region of depth d , the rate equations are:

emissionstimulatedionrecombinatsspontaneouinjectionrateelectron

lossphotonemissionsspontaneouemissionstimulatedratePhoton

Cn

n

qd

J

dt

dn

RCndt

d

sp

ph

sp

[4-25]

densitycurrentInjection:

timelifephoton:

modelasingtheintoemissionsspontaneouof rate:

processabsorption&emissionopticaltheof intensitytheexpressingtCoefficien:

J

R

C

ph

sp

Threshold current Density & excess electron density




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Threshold current Density & excess electron density

At the threshold of lasing:

The threshold current needed to maintain a steady state threshold concentration of the

excess electron, is found from electron rate equation under steady state condition

dn/dt=0 when the laser is just about to lase:

0,0 / ,0 sp Rdt d

th

ph

ph nC

nCn

1

0 / 25]-[4eq.from [4-26]

sp

thth

sp

thth n

qd J

n

qd

J

0 [4-27]

L i b d h h h ld




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Laser operation beyond the threshold

The solution of the rate equations [4-25] gives the steady state photondensity, resulting from stimulated emission and spontaneous emissionas follows:

th J J

sp phth

ph

s R J J qd

)( [4-28]

External quantum efficiency




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External quantum efficiency Number of photons emitted per radiative electron-hole pair

recombination above threshold, gives us the external quantum

efficiency.

Note that:

)mA()mW(]m[8065.0

)(

dI dP

dI dP

E q

g

g

g

th

thiex t

[4-29]

%40%15 %;70%60 ex t i

L R t F i




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Laser Resonant Frequencies

Lasing condition, namely eq. [4-22]:

Assuming the resonant frequency of the m th mode is:

,...3,2,1 ,2L2 1)2exp( mm L j

n2

1,2,3,.. 2

m Ln

mcm

Ln Lnc

mm22

2

1

[4-30]

[4-31]

S t f l Di d




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Spectrum from a laser Diode

widthspectral: 2

)(exp)0()(

2

0

gg [4-32]

L Di d St t & R di ti P tt




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Laser Diode Structure & Radiation Pattern

Efficient operation of a laser diode requires reducing the # of lateral

modes, stabilizing the gain for lateral modes as well as lowering thethreshold current. These are met by structures that confine the opticalwave, carrier concentration and current flow in the lateral direction.The important types of laser diodes are: gain-induced, positiveindex guided, and negative index guided.

(a) gain-induced guide (b) positive-index waveguide (c) negative-index waveguide

Laser Diode with buried heterostructure (BH)




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Laser Diode with buried heterostructure (BH)

Single Mode Laser




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Single Mode Laser

Single mode laser is mostly based on the index-guidedstructure that supports only the fundamental transversemode and the fundamental longitudinal mode. In order tomake single mode laser we have four options:

1- Reducing the length of the cavity to the point where thefrequency separation given in eq[4-31] of the adjacent

modes is larger than the laser transition line width. This ishard to handle for fabrication and results in low outputpower.

2- Vertical-Cavity Surface Emitting laser (VCSEL)

3- Structures with built-in frequency selective grating

4- tunable laser diodes

.

VCSEL




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Frequency-Selective laser Diodes: Distrib ted Feedback (DFB) laser




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Distributed Feedback (DFB) laser

k

ne B

2

[4-33]

Frequency-Selective laser Diodes: Distributed Feedback (DFB) laser




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Output spectrum symmetrically distributed around Bragg wavelength in an idealized DFB laser diode

)2

1(

2

2

m Ln ee

B B

[4-35]

Distributed Feedback (DFB) laser

Frequency-Selective laser Diodes: Distributed Feedback Reflector (DBR) laser




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Distributed Feedback Reflector (DBR) laser

Frequency-Selective laser Diodes:

Distributed Reflector (DR) laser




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Distributed Reflector (DR) laser

Modulation of Laser Diodes




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Modulation of Laser Diodes

Internal Modulation: Simple but suffers from non-linear effects.

External Modulation: for rates greater than 2 Gb/s, more complex,higher performance.

Most fundamental limit for the modulation rate is set by the photon lifetime in the laser cavity:

Another fundamental limit on modulation frequency is the relaxationoscillation frequency given by:

th

phgn

c

R R Ln

c

21

1ln2

11

[4-36]

2 / 1

11

2

1

th phsp

I

I f

[4-37]

Pulse Modulated laser




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In a pulse modulated laser, if the laser is completely turned off aftereach pulse, after onset of the current pulse, a time delay, givenby:

d t

)(

ln

th B p

p

d

I I I

I t

currentBias:

amplitudepulseCurrent: timelifecarrier:

B

p

I

I

[4-38]

Temperature variation of the threshold current




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current

0 / )(

T T

zthe I T I

Linearity of Laser




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y

Information carrying

electrical signal s(t)

LED or Laser diode

modulator

Optical output power:

P(t)=P[1+ms(t)]

Bias Point and Amplitude Range




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Nonlinearity




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y

...2coscos)(

cos)(

210

t At A At y

t At x

x(t) Nonlinear function y=f(x) y(t)

Nth order harmonic distortion:

1

log20

A

An

Intermodulation Distortion




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nm

mn m,nt nm Bt y

t At At x

,

21

2211

2,...1,0, )cos()(

coscos)(

Harmonics:

21, mn

Intermodulated Terms:

,...2,2,212121

Laser Noise




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Modal (speckel) Noise: Fluctuations in the distribution of energy among

various modes.

Mode partition Noise: Intensity fluctuations in the longitudinal modes of alaser diode, main source of noise in single mode fiber systems.

Reflection Noise: Light output gets reflected back from the fiber joints into

the laser, couples with lasing modes, changing their phase, and generate

noise peaks. Isolators & index matching fluids can eliminate these

reflections.



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Thank you for attention

Q & A

Broadband Transmission Network Lab.

Department of Electrical & Electronic Engineering

3rd Week Seminar - Spring 2011 - Revision3

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