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Sources and detectors of light

1) Revision: semiconductors

2) Light emitting diodes (LED)

3) Lasers

4) Photodiodes

for integrated optics and optical communications

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1) Revision: Energy band diagrams for metal…

Metals characteristically have partially filled energy bands.

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… and semiconductor

Electron energy, E

Conduction Band (CB)Empty of electrons at 0 K.

Valence Band (VB)Full of electrons at 0 K.

Ec

Ev

0

Ec+

(b)

Band gap = Eg

(a)

Covalent bondSi ion core (+4e)

(a) A simplified two dimensional view of a region of the Si crystalshowing covalent bonds. (b) The energy band diagram of electrons in theSi crystal at absolute zero of temperature.

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

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Generation of electron-hole pair

e–hole

CB

VB

Ec

Ev

0

Ec+

Eg

Free e–h > Eg

Hole h+

Electron energy, E

(a) A photon with an energy greater than Eg can excite an electron from the VB to the CB.(b) Each line between Si-Si atoms is a valence electron in a bond. When a photon breaks aSi-Si bond, a free electron and a hole in the Si-Si bond is created.

h

(a) (b)

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

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Semiconductor statistics

DOS FD

area =

area = p

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n-type semiconductor

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p-type semiconductor

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Intrinsic, n-type, and p-type semiconductor

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Degenerate semiconductor

heavily doped n-type semiconductor

cNn vNp

donors form a band that overlaps the CB

heavily doped p-type semiconductor

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E-k diagram

band diagramE-k diagram

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Direct and indirect bandgap

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pn junction

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pn junction

open circuit forward bias V0

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2) Light emitting diodes (LED)

h Eg

Eg (b)

V

(a)

p n+

Eg

eVo

EF

p n+

Electron in CBHole in VB

Ec

Ev

Ec

Ev

EF

eVo

Electron energy

Distance into device

(a) The energy band diagram of a p-n+ (heavily n-type doped) junction without any bias.Built-in potential Vo prevents electrons from diffusing from n+ to p side. (b) The appliedbias reduces Vo and thereby allows electrons to diffuse, be injected, into the p-side.Recombination around the junction and within the diffusion length of the electrons in thep-side leads to photon emission.

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

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Surface emitting LED

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How to get the light out?

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External and internal quantum efficiency

IV

Poutext

injected pairs hole-electron ofnumber

generated photons ofnumber int

injected pairs hole-electron ofnumber

direction desiredin emitted photons ofnumber ext

Task: Estimate the external quantum efficiency of GaAs LED.

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Double heterostructure LED

two junctions between materials with different bangaps

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LED materials

direct bangap

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LED materials

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LED materials

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LED characteristics

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LED characteristics

hc

TkB32

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surface emitting LED(SLED)

Edge-emitting LED(ELED)

Double-heterosturuture

light

light

- LEDs are preferred for short haul applications

- more economical

- wider output spectrum, i.e. not suitable for wide bandwidth systems

LEDs for optical fiber communications

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Coupling the radiation from a SLED into an fiber

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DH ELED

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Coupling the radiation from a ELED into an fiber

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3) Lasers

- population inversion

- optical feedback (optical cavity)

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Stimulated emission

absorption spontaneous emission Stimulated emission

The emitted photon has the same energy, polarization, direction and phase as the incoming photon.

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Population inversion

1. (optical) pumping

2. rapid decay to the long-lived (metastable) state

3. population inversion4. Stimulated emission

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Stimulated emission: Optical amplifiers

E.g.: Erbium doped fiber amplifier (EDFA)

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Erbium doped fiber amplifier (EDFA)

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Semiconductor optical amplifier (SOA)

Fabry-Perot

Traveling wave

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population inversion

Laser diode (LD)

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DOS for electrons a holes in SCL under forward bias

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cleaved surface = mirror

Optical feedback

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Laser oscillation conditions

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Laser oscillation conditions

FP cavity

must be the same in the steady state

phase condition

amplitude condition = = threshold condition

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stimulated emission

spontaneous emission

Optical power

diode current

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Principle of a double heterostructure LD

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Example: DH stripe contact LD

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Example: A buried DH LD

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LD characteristics

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LDs for optical fiber communications

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DBR Laser

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Distributed feedback (DFB) laser

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Distributed feedback (DFB) laser

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Distributed feedback (DFB) laser

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Cleaved-coupled-cavity (C3) laser

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Tunable lasers

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Quantum well devices

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Single quantum well (SQW)

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Multiple quantum well (MQW)

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Vertical cavity surface emitting laser (VCSEL)

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Vertical cavity surface emitting laser (VCSEL)

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Vertical cavity surface emitting laser (VCSEL)

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4) Photodiodes

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Absorption in direct and indirect semiconductor

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Responsivity

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p-i-n photodiode

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Avalanche photodiode

electrode

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