Transcript 4. Sources and detectors of light for integrated optics

Sources and detectors of light
for integrated optics and optical communications
1)
2)
3)
4)
Revision: semiconductors
Light emitting diodes (LED)
Lasers
Photodiodes
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1) Revision: Energy band diagrams for metal…
Metals characteristically have partially filled energy bands.
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… and semiconductor
Covalent bond
Si ion core (+4e)
Electron energy, E
Ec+
Conduction Band (CB)
Empty of electrons at 0 K.
Ec
Band gap = Eg
Ev
Valence Band (VB)
Full of electrons at 0 K.
0
(a)
(b)
(a) A simplified two dimensional view of a region of the Si crystal
showing covalent bonds. (b) The energy band diagram of electrons in the
Si crystal at absolute zero of temperature.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Generation of electron-hole pair
Electron energy, E
Ec+
CB
h > Eg
Ec
Free e–
h
Eg
Ev
Hole h+
hole
e–
VB
0
(a)
(b)
(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 a
Si-Si bond, a free electron and a hole in the Si-Si bond is created.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Semiconductor statistics
area =
area = p
DOS
FD
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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
heavily doped p-type
semiconductor
donors form a band that
overlaps the CB
n  Nc
p  Nv
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E-k diagram
E-k diagram
band 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)
Electron energy
n+
p
Ec
eVo
(a)
EF
Ec
EF
Eg
n+
p
Eg
(b)
h- Eg
Ev
eVo
Distance into device
Ev
V
Electron in CB
Hole in VB
(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 applied
bias 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 the
p-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
 ext 
number of photons emitted in desired direction
P
 ext  out
number of electron - hole pairs injected
IV
 int 
number of photons generated
number of electron - hole pairs injected
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
3k BT
  
hc
2
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LEDs for optical fiber communications
- LEDs are preferred for short haul applications
- more economical
- wider output spectrum, i.e. not suitable for wide bandwidth systems
light
Doubleheterosturuture
light
surface emitting LED
(SLED)
Edge-emitting LED
(ELED)
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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
4. Stimulated emission
2. rapid decay to the
long-lived
(metastable) state
3. population inversion
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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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Laser diode (LD)
population inversion
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DOS for electrons a holes in
SCL under forward bias
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Optical feedback
cleaved surface = mirror
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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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Optical
power
stimulated
emission
spontaneous
emission
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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