Chapter 3 Optical Transmitters (10!12!12)1

7/27/2019 Chapter 3 Optical Transmitters (10!12!12)1

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Chapter 3

OPTICAL TRANSMITTERS

Fiber-Optic Communications Systems, Third Edition.

Govind P. Agrawal


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Describe the basic concepts of Emission and Absorption

Processes, pn Junctions,

Discuss the operation principle of Light-Emitting Diodes

Describe the PowerCurrent Characteristics

Discuss the details of LED Spectrum, LED Structures.

Do the Lab 3Determining the critical parameters of LED

CONTENTS


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Laser Diodes: Optical Gain, Feedback and Laser

Threshold, Laser Structures.

Control of Longitudinal Modes: Distributed Feedback

Lasers (DFB), Coupled-Cavity Semiconductor Lasers,

Tunable Semiconductor Lasers.

CONTENTS


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OPTICAL TRANSMITTER

The role of the optical transmitter is to convert the

electrical input signal into the optical output one and then

launch it into the optical fiber working as a

communication channel.

The major component of optical transmitters is an

optical source.


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Emission and Absorption Processes

(a) absorption; (b) spontaneous emission c) stimulated emission.

- The energy levels E1 the ground state and E2 the excitedstate of atoms.

- Ifh = Eg = E2 E1, the photon is absorbed by the atom, which

ends up in the excited state.

The absorption and emission processes of the two energy states of an atom


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-The excited atoms eventually return to their normal ground state

and emit light in the process.

-Light emission can occur through two fundamental processes

known as spontaneous emission and stimulated emission.

- Light wavelength emitted:

PRINCIPLE OF LIGHT EMISSION

where:

h=6.625.10-34 js (Planck Constant)

c=3.108 m/s (the speed of light)


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Spontaneous emission: photons are emitted in random directions

with no phase relationship among them.(Incoherent light)

Stimulated emission: In contrast, is initiated by an existing photon.

The emitted photon matches the original photon not only in phase but

also in its other characteristics, such as the direction of propagation,

same phase. (Coherent light)

Spontaneous emission

Stimulated emission

LED

LASER

PRINCIPLE OF LIGHT EMISSION (cont)


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FEATURES OF LED AND LASER

LED

1. Spontaneous emission

2. The incoherent light

3. Double-heterostructure to

confine the carriers in resonant

cavity.

4. Without reflector5. Larger spectrum: several nm to

tens of nm

Laser

1. Stimulated emission.

2. The coherent light

3. Double-heterostructure to confine

the carriers in resonant cavity.

4. With 2 reflectors + injected

mechanism to confine and amplify

photon for generating coherentlight.

5. Narrower spectrum: 0.05 nm to

several nm


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Spectral Width of LED and LASER


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Direct and Indirect bandgap

Direct bandgap GaAs (a) Indirect bandgap of Si (b )


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The Double Heterostructure confines electrons and holes to active

layer where light is generated as a result of electron hole

recombination.

Confinement carriers Increasing light power

A heterostructure=junction of two materials with different energy bandgap.

HETEROSTRUCTURE


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HETEROSTRUCTURE


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The active layer also has a slightly larger refractive index than that of

the surrounding p-type and n-type cladding layers .

Larger n1 of active layer same structure of fiber Total internal

reflection lawconfinement light Increasing light power

HETEROSTRUCTURE


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LED (Light-Emitting Diodes)

Wide spectral width of LED:

Emits light through spontaneous emission, a phenomenon referred

to electroluminescence.

Radiative recombination of electronhole pairs in the depletion

region generates light; some of it escapes from the device.

The emitted light is incoherent with a relatively wide spectral width

(3060 nm) and a relatively large angular spread.

LED is used in system with bit rate under 200 Mbps.


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TYPES OF LED

Surface LED

EDGE LED

The surface-emitting or edge-emitting, depending on whether the

LED emits light from a surface that is parallel to the junction plane or

from the edge of the junction region.

Both types can be made using a heterostructure design in which the

active region is surrounded by p- and n-type cladding layers.


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Schematic of a Surface-Emitting LED

Surface Light Emitting Diode: SLED

light collected from the one surface, other attached to a

heat sink

easyto couple light with multimode fibers


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Schematic of a Surface-Emitting LED


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Schematic of a Edge-Emitting LED

Edge Light Emitting Diode: ELED

like stripe geometry lasers but no optical reflectors

easy make coupling light with multimode and single mode

fibers


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Schematic of a Edge-Emitting LED


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LIGHT-EMITTING DIODES

(a) Powercurrent curves at several temperatures; (b) spectrum of the

emitted light for a typical 1.3-m LED. The dashed curve shows the

theoretically calculated spectrum.

Power-Current Characteristics and LED Spectrum


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LASER DIODE

LASER = Light Amplification by Stimulated Emission of

Radiation. Laser light is monochromatic, coherent, and moves in the same

direction.

A semiconductor Laser is a Laser in which a semiconductor

serves as a photon source.

The most common semiconductor material that has been used in

lasers is Gallium Arsenide.


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A broad-area semiconductor laser. The active layer is sandwiched between

p-type and n-type cladding layers of a higher-bandgap material

Laser Structures


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Nonradiative

Recombination

Spontaneous

emssion

Stimulated

emssion

Stimulated

photon

Absorption

Conduction Band

Valence Band

Ec

Ev

External

electron

Injectiom

External hole

Injectiom

B21B12 A21


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PopulationInverse

At equilibrium, the carriers have the same speed

between the two states. According to Einstein

expression, we have:

(3.5)

S: photon energy density [J/m3.Hz],

N1 andN2[1/m3]: densities at E1 and E2 respectively.

B12: stimulated absorption rate coefficient [1/sec],

B21: stimulated emission rate coefficient [1/sec],

A21: spontaneous emission rate coefficient [1/sec].

Rp: External Pump Rate [1/m3.sec]

PRSNBSNBNA 112221221


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(3.5)

The left side of the equation (3.5) show full speed from

E2 to E1, and the right side is full speed from E1 to E2.

PRSNBSNBNA 112221221

PopulationInverse


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Therefore, to increase the light intensity, we must have:

(3.7)

This condition is called the population inversion.

When B21 = B12N2 > N1

To achieve the population inversion external pumping

speed RPshould be > spontaneous emission rate. This

can be represented by altering the expression (3.5) as

follows:

(3.8)

112221 NBNB

112221

221

NBNB

NARS

P

PopulationInverse


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Since S> 0RP>A21N2 when

112221

221

NBNB

NARS

P

112221 NBNB

PopulationInverse


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SEMICONDUCTOR LASERS (SL)

SLs emit light through stimulated emission; high powers (~10mW),

the coherent light, high coupling efficiency (~50%) into SMF.

A narrow spectral width (0.05-0.1)nm Rb (~10 Gb/s)

Most FOCS use SLs

Optical Gain

Stimulated emission can dominate only if the condition of

population inversion is satisfied.

When the injected carrier density in the active layer exceeds a

certain value, known as the transparency value, population

inversion is realized and the active region exhibits optical gain.


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Optical Gain

(a) Gain spectrum of a 1.3-m InGaAsP laser at several carrier densitiesN.

(b) Variation of peak gaingp withN. The dashed line shows the quality of a

linear fit in the highgain region.

SEMICONDUCTOR LASERS


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The optical gain alone is not enough for laser operation.

Optical feedbackis neededconverts an amplifier into an

oscillator In most lasersFabryPerot(FP) cavity formed

by using two mirrors.

Laser thresholdcondition: compensatesa certain fraction of

photons generated by stimulated emission is lost because of

cavity losses and needs to be replenished on a continuous

basis.

Feedback and Laser Threshold


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If the optical gain is not large enough to compensate for the

cavity losses, the photon population cannot build up a

minimum amount of gain is necessary for the operation of a

laser.

A simple way to obtain the threshold condition is to study

how the amplitude of a plane wave changes during one round

trip.



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Structure of a semiconductor laser and the FabryPerot cavity

associated with it. The cleaved facets act as partially reflecting mirrors



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Condition: nL=m/2

Geometry of Laser Cavity

m

Ln

m

2


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Gain and loss profiles in semiconductor lasers. Vertical barsshow the location of longitudinal modes.

The laser threshold is reached when the gain of the longitudinal

mode closest to the gain peak equals loss.



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Fabry-Perot Spectrum


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m

Ln

m

2

LnmLn

mLn

m

mmL

2

12}

1

1

m

1{2

2

21


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Laser cavity InGaAsP has characteristics: length: 500

m, refractive index: 3.63 and wavelength range

[1540 nm-1560 nm]. Find space between two sequentlongitudinal modes and number of modes that it can

have. Calculate this space in [GHz].

Example


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CONTROL OF LONGITUDINAL MODES

Distr ibuted Feedback(DFB) and Bragg ref lector (DBR) laser structures.

The shaded area shows the active region and the wavy line indicates the

presence of a Bragg grating.


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C l d C it S i d t L


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Coupled-cavity laser structures: (a) external-cavity laser; (b)cleaved-coupledcavity laser; (c) multisection DBR laser

Coupled-Cavity Semiconductor Lasers

T bl S i d t L


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Modern WDM lightwave systems require single-mode, narrow-

linewidth lasers whose wavelength remains fixed over time.

DFB lasers satisfy this requirement but their wavelength stability

comes at the expense of tunability.

Multisection DBR laser: three sections: active, phase-control,

Bragg; independent bias by injecting different amounts of

currents.

The current injected into the Bragg section: change B = 2n

through carrier-induced changes in the refractive index n.

The current injected into the phase-control section: change the

phase of the feedback from the DBR through carrier-induced

index changes in that section. the range of change: 1015 nm

Tunable Semiconductor Lasers


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Chapter 3 Optical Transmitters (10!12!12)1

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