Staff Site Universitas Negeri Yogyakarta
Introduction to Optoelectronics
Optical communication (2)
Prof. Katsuaki Sato
Lasers
• Spontaneous emission and stimulated emissio
n
• Application of Lasers
• Classification of lasers according to the way of
pumping
• Laser diodes
– What is semiconductor?
– p/n junction diode
– Light emitting diode and laser diode
What is Laser?
• Spontaneous and stimulated emission
• Different pumping methods
• Characteristics of laser light
Spontaneous and stimulated emission
• Spontaneous emission ： Light emission by
relaxation from the excited state to the ground
state
• stimulated emission ： Light emission due to
optical transition forced by optical stimulation;
• This phenomenon is the laser=light
amplification by stimulated emission of
radiation
Optical transition
2 • Transition occurs from
Energy
p12
Optical
absorption
Spontaneous
emission
the ground state 1 to
the excited state 2
with the probability of
P
12 by the perturbation
1
of the electric field of
light: This is an optical
2
absorption.
• The excited state 2
relaxes to the ground
state 1 spontaneously
with a light emission to
1
achieve thermal
equilibrium
Energy
Stimulated emission
2 • Transition from the
E
p12
Stimulated
emission
p21
excited state 2 to the
ground state 1 occurs
Stimulated emission
by the stimulation of the
electric field of incident
1
light with the transition
probability of P21(=P12),
leading to emission of a
2
photon. This process is
called stimulated
emission.
• The number of photons
1
is doubled since first
photon is not absorbed.
Emission is masked by absorption
under normal condition
N2
Stimulated
emission
p21
N1
N2
p12
N1
• Under normal condition
2
stimulated emission cannot
be observed since
absorption occurs at the
same probability as
1 emission (P12=P21), and the
2 population N1 at 1
dominates N2 at 2 due to
Maxwell-Boltzmann
Optical
distribution.
Therefore,
absorption
N2P21N1P12),
N2, the population of the state 2 should exceed
N1, the population of the state 1.
• This is called population inversion, or negative
temperature, since the distribution feature
behaves as if the temperature were negative.
Energy
Distribution function
2
0
1
E
exp(E/kT)
1
Characteristics of laser
• Oscillator and amplifier of light wave
• Wave-packets share the same phase leading to
Coherence: two different lasers can make interference fringes
Directivity: laser beam can go straight for a long distance
Monochromaticity: laser wavelength is “pure” with narrow width
High energy density: laser can heat a substance by focusing
Ultra short pulse: laser pulse duration can be reduced as short a
s femtosecond (10-15 s)
• Bose condensation quantum state appearing macros
copically
Application of lasers
•
•
•
•
•
•
Optical Communications
Optical Storages
Laser Printers
Diplays
Laser Processing
Medical Treatments
Optical fiber communication
Optical fiber
communication system
Multi-pl
exer
Electrooptical
conversion
Amplifier
Opto-electro
nic Conversi Demulti
on
-plexer
Optical fiber
Laser diode
Photodiode
Optical Storages
• CD 、 DVD 、 BD
• MD 、 MO
Laser Printers
photosensitive drum
Computer
BD lens
controller
optical fiber
BD signal
BD signal
DC controller
toric lens
spherical lens
polygon mirror
horizontal sync
mirror
opt. box
cylindrical lens
laser diode/
laser driver
http://web.canon.jp/technology/detail/lbp/laser_unit/index.html
video signal
scanner motor/
motor driver
Laser Show
• Polygon mirror
Laser Processing
Web site of Fujitsu
Medical Treatment
• CO2 laser
Classification of lasers
according to the way of pumping
• Gas lasers ：
eg., He-Ne, He-Cd, Ar+, CO2,
pump an excited state in the electronic structure of gas ions o
r molecules by discharge
• Solid state lasers
eg., YAG:Nd, Al2O3:Ti, Al2O3:Cr(ruby) ：
pump an excited state of luminescent center (impurity atom)
by optical excitation
• Laser diodes (Semiconductor lasers)
eg., GaAlAs, InGaN
high density injection of electrons and holes to active layer of
semiconductor through pn-junction
Gas laser
HeNe laser
Showa Optronics Ltd.
http://www.soc-ltd.co.jp/index.html
HeNe laser, how it works
•He atoms become excited by an im
pact excitation through collision
•The ground state is 1S (1s2; L=0, S=
0) and the excited states are 1S (1s1
2s1 ; L=0, S=0) and 3S (1s12s1 ; L=
0, S=1)
•The energy is transferred to Ne atom
s through collision.
•Ne has ten electrons in the ground
state 1S0 with 1s2 2s2 2p4 configurati
on, and possesses a lot of complex e
xcited states
http://www.mgkk.com/products/pdf/02_4_HeNe/024_213.pdf
He
2S
3
21S
1
S
Ne
HeNe laser: different wavelengths
•
•
•
•
•
•
3.391 m mid IR
1.523 m near IR
632.8 nm red 赤
612 nm orange 色
594 nm yellow 黄色
543.5 nm green グ
リーン
He
23S
21S
1
S
Ne
Gas laser
Ar+-ion laser
• Blue458nm
• Blue488nm
• Blue-Green 514nm
Application of gas laser
Ar ion laser
• Illumination (Laser show)
• Photoluminescence
Excitation Source
Gas laser
CO2 laser
• 10.6m
• Purpose
– manufacturing
– Medical surgery
– Remote sensing
Solid state laser
YAG laser YVO4laser
•
•
•
•
YAG:Nd
1.06m
Micro fabrication
Pumping source for SH
G
http://www.fesys.co.jp/sougou/seihi
n/fa/laser/fal3000.html
Solid state laser
Titanium sapphire laser
• Al2O3:Ti3+ (tunable ）
Ti-sapphire laser in Sato lab.
Solid state laser
Ruby laser
•
•
•
•
•
Al2O3:Cr3+
Synthetic ruby single crystal
Pumped by strong Xe lamp
Emission wavelengths; 694.3nm
Ethalon is used to select a wavel
ength of interest
Ruby laser
Ruby rod
LD (laser diode)
• Laser diode is a
semiconductor device
which undergoes
stimulated emission by
recombination of injected
carriers (electrons and
holes), the concentration
being far greater than that
in the thermal equilibrium.
What is semiconductor?
• Semiconductors possess electrical conductivity
between metals and insulators
insulator
diamond
semiconductor
metal
Resistivity (cm)
Energy band gap (eV)
Energy band gap (eV)
Conductivity (S/cm)
Electric resisitivity of K
Temperature (K)
Electric resitivity (cm) log scale
Electric resitivity (cm)
Temperature dependence of electrical
conductivity in metals and semiconductors
Temperature (K)
• Resistivity of metals increases with temperature due to
electron scattering by phonon
• Resistivity of semiconductors decreases drastically with
temperature due to increase in carrier concentration
Conductivity, carrier concentration, mobility
• Relation between conductivity and carrier c
oncentration n and mobility
= ne
• Resistivity and conductivity is related by
=1/
• Mobility is average velocity v[cm/s] introduced
by electric field E[V/cm] , expressed by equati
on v= E
Periodic table and semiconductors
IIB
IIIB
IV
V
VI
B
C
N
O
Al
Si
P
S
Zn
Ga
Ge
As
Se
Cd
In
Sn
Sb
Te
Hg
Tl
Pb
Bi
Po
IV (Si, Ge)
I-VII (CuCl, CuI)
III-V (GaAs, GaN, InP, InSb) I-III-VI2 (CuAlS2 ， CuInSe2)
II-VI (CdS, CdTe, ZnS, ZnSe) II-IV-V2 (CdGeAs2, ZnSiP2)
Crystal structures of semiconductors
• Si. Ge: diamond structure
• III-V, II-VI: zincblende structure
• I-III-VI2, II-IV-V2: chalcopyrite structure
Diamond structure
Energy band structure for explanation of
metals, semiconductors and insulators
Fermi level
3s,3p
Conduc
tion
band
3s,3p
Conduc
tion
band
3s
band
3s,3p
Valence
band
3s,3p
Valence
band
2p
shell
2p
shell
2s
shell
2s
shell
1s
shell
1s
shell
Metals
intrinsic
extrinsi
c
Semiconductors
Insulators
and semiconductors
at 0K
Difference of metals, semiconductors and insulators
Concept of Energy Band
Two approaches
• Approximation from free electron
– Hartree-Fock approximation
– Electron is treated as plane waves with wavenumb
er k
– Energy E=(k)2/2m (parabolic band)
• Approximation from isolated atoms
– Heitler-London approximation
– Linear combination of s, p, d wavefunctions
isolated
atom
Band gap of silicon
covalent
bonding
conducti
on band
Antionding orbitals
Bonding orbitals
Energy
3p
3s
Energy
gap
valence
band
lattice
constant of Si
Si-Si distance
Schematic illustration of variation of
electronic states in silicon with Si-Si
distance
Band gap and optical absorption spectrum
Direct gap
InSb, InP, GaAs
Indirect gap
Ge, Si, GaP
Band gap and optical absorption edge
・When photon energy E=h is less than
Eg, valence electrons cannot reach
conduction band and light is transmited.
・When photon energy E=h reaches
Eg, optical absorption starts.
conduction band
1240 / h
h
h>Eg
valence band
Eg
Color of transmitted light and band gap
ZnS Eg=3.5eV
CdS
黄
Eg=2.6eV
GaP
橙
Eg=2.2eV
赤
Eg=2eV
HgS
Eg=1.5eV
GaAs
黒
800nm
300nm
4eV
白
transparent region
3.5eV
3eV
2.5eV
2eV
1.5eV
Semiconductor ｐｎ junction
Energy
N type
P type
space
charge
potential
Carrier diffusion takes place when p
and n semiconductors are contacted
-
+
+
+
+
space charge potential
+
LED, how it works?
hole
•
•
•
•
Forward bias to pn junction diode
electron is injected to p-type region
hole is injected to n-type region
Electrons and holes recombine at th
e boundary region
• Energy difference is converted to ph
oton energy
hc
E h
E (eV)
recombination
p -
+
+
+ n
+
electron
electron
Space charge layer
+
1239.8
(nm)
-
electron drift
energy gap
or
band gap
recombination
light emission
hole drift
Semiconductors for LD
• Optical communication ： 1.5m; GaInAsSb, In
GaAsP
• CD ： 780nm GaAs
• DVD ： 650nm GaAlAs MQW
• DVR ： 405nm InGaN MQW
Double hetero
structure
• Electrons, holes an
d photons are confi
ned in thin active la
yer by using the he
tro-junction structur
e
http://www.ece.concordia.ca/
~i_statei/vlsi-opt/
Invention of DH structure (1)
• Herbert Kroemer and Zhores Alferov suggested in 19
63 that the concentration of electrons, holes and phot
ons would become much higher if they were confined
to a thin semiconductor layer between two others - a
double heterojunction.
• Despite a lack of the most advanced equipment, Alfer
ov and his co-workers in Leningrad (now St. Petersb
urg) managed to produce a laser that effectively oper
ated continuously and that did not require troublesom
e cooling.
• This was in May 1970, a few weeks earlier than their
American competitors.
• from Nobel Prize Presentation Speech in Physics 2000
Invention of DH structure (2)
• In 1970, Hayashi and Panish at Bell Labs and Alferov in Russi
a obtained continuous operation at room temperature using d
ouble heterojunction lasers consisting of a thin layer of GaAs
sandwiched between two layers of AlxGa1-xAs. This design a
chieved better performance by confining both the injected carr
iers (by the band-gap discontinuity) and emitted photons (by t
he refractive-index discontinuity).
• The double-heterojunction concept has been modified and im
proved over the years, but the central idea of confining both th
e carriers and photons by heterojunctions is the fundamental
philosophy used in all semiconductor lasers.
from Physics and the communications industry W. F. Brinkm
an and D. V. Lang Bell Laboratories, Lucent Technologies, Mu
rray Hill, New Jersey 07974
http://www.bellsystemmemorial.com/pdf/physics_com.pdf
Optical communication (2)
Prof. Katsuaki Sato
Lasers
• Spontaneous emission and stimulated emissio
n
• Application of Lasers
• Classification of lasers according to the way of
pumping
• Laser diodes
– What is semiconductor?
– p/n junction diode
– Light emitting diode and laser diode
What is Laser?
• Spontaneous and stimulated emission
• Different pumping methods
• Characteristics of laser light
Spontaneous and stimulated emission
• Spontaneous emission ： Light emission by
relaxation from the excited state to the ground
state
• stimulated emission ： Light emission due to
optical transition forced by optical stimulation;
• This phenomenon is the laser=light
amplification by stimulated emission of
radiation
Optical transition
2 • Transition occurs from
Energy
p12
Optical
absorption
Spontaneous
emission
the ground state 1 to
the excited state 2
with the probability of
P
12 by the perturbation
1
of the electric field of
light: This is an optical
2
absorption.
• The excited state 2
relaxes to the ground
state 1 spontaneously
with a light emission to
1
achieve thermal
equilibrium
Energy
Stimulated emission
2 • Transition from the
E
p12
Stimulated
emission
p21
excited state 2 to the
ground state 1 occurs
Stimulated emission
by the stimulation of the
electric field of incident
1
light with the transition
probability of P21(=P12),
leading to emission of a
2
photon. This process is
called stimulated
emission.
• The number of photons
1
is doubled since first
photon is not absorbed.
Emission is masked by absorption
under normal condition
N2
Stimulated
emission
p21
N1
N2
p12
N1
• Under normal condition
2
stimulated emission cannot
be observed since
absorption occurs at the
same probability as
1 emission (P12=P21), and the
2 population N1 at 1
dominates N2 at 2 due to
Maxwell-Boltzmann
Optical
distribution.
Therefore,
absorption
N2P21N1P12),
N2, the population of the state 2 should exceed
N1, the population of the state 1.
• This is called population inversion, or negative
temperature, since the distribution feature
behaves as if the temperature were negative.
Energy
Distribution function
2
0
1
E
exp(E/kT)
1
Characteristics of laser
• Oscillator and amplifier of light wave
• Wave-packets share the same phase leading to
Coherence: two different lasers can make interference fringes
Directivity: laser beam can go straight for a long distance
Monochromaticity: laser wavelength is “pure” with narrow width
High energy density: laser can heat a substance by focusing
Ultra short pulse: laser pulse duration can be reduced as short a
s femtosecond (10-15 s)
• Bose condensation quantum state appearing macros
copically
Application of lasers
•
•
•
•
•
•
Optical Communications
Optical Storages
Laser Printers
Diplays
Laser Processing
Medical Treatments
Optical fiber communication
Optical fiber
communication system
Multi-pl
exer
Electrooptical
conversion
Amplifier
Opto-electro
nic Conversi Demulti
on
-plexer
Optical fiber
Laser diode
Photodiode
Optical Storages
• CD 、 DVD 、 BD
• MD 、 MO
Laser Printers
photosensitive drum
Computer
BD lens
controller
optical fiber
BD signal
BD signal
DC controller
toric lens
spherical lens
polygon mirror
horizontal sync
mirror
opt. box
cylindrical lens
laser diode/
laser driver
http://web.canon.jp/technology/detail/lbp/laser_unit/index.html
video signal
scanner motor/
motor driver
Laser Show
• Polygon mirror
Laser Processing
Web site of Fujitsu
Medical Treatment
• CO2 laser
Classification of lasers
according to the way of pumping
• Gas lasers ：
eg., He-Ne, He-Cd, Ar+, CO2,
pump an excited state in the electronic structure of gas ions o
r molecules by discharge
• Solid state lasers
eg., YAG:Nd, Al2O3:Ti, Al2O3:Cr(ruby) ：
pump an excited state of luminescent center (impurity atom)
by optical excitation
• Laser diodes (Semiconductor lasers)
eg., GaAlAs, InGaN
high density injection of electrons and holes to active layer of
semiconductor through pn-junction
Gas laser
HeNe laser
Showa Optronics Ltd.
http://www.soc-ltd.co.jp/index.html
HeNe laser, how it works
•He atoms become excited by an im
pact excitation through collision
•The ground state is 1S (1s2; L=0, S=
0) and the excited states are 1S (1s1
2s1 ; L=0, S=0) and 3S (1s12s1 ; L=
0, S=1)
•The energy is transferred to Ne atom
s through collision.
•Ne has ten electrons in the ground
state 1S0 with 1s2 2s2 2p4 configurati
on, and possesses a lot of complex e
xcited states
http://www.mgkk.com/products/pdf/02_4_HeNe/024_213.pdf
He
2S
3
21S
1
S
Ne
HeNe laser: different wavelengths
•
•
•
•
•
•
3.391 m mid IR
1.523 m near IR
632.8 nm red 赤
612 nm orange 色
594 nm yellow 黄色
543.5 nm green グ
リーン
He
23S
21S
1
S
Ne
Gas laser
Ar+-ion laser
• Blue458nm
• Blue488nm
• Blue-Green 514nm
Application of gas laser
Ar ion laser
• Illumination (Laser show)
• Photoluminescence
Excitation Source
Gas laser
CO2 laser
• 10.6m
• Purpose
– manufacturing
– Medical surgery
– Remote sensing
Solid state laser
YAG laser YVO4laser
•
•
•
•
YAG:Nd
1.06m
Micro fabrication
Pumping source for SH
G
http://www.fesys.co.jp/sougou/seihi
n/fa/laser/fal3000.html
Solid state laser
Titanium sapphire laser
• Al2O3:Ti3+ (tunable ）
Ti-sapphire laser in Sato lab.
Solid state laser
Ruby laser
•
•
•
•
•
Al2O3:Cr3+
Synthetic ruby single crystal
Pumped by strong Xe lamp
Emission wavelengths; 694.3nm
Ethalon is used to select a wavel
ength of interest
Ruby laser
Ruby rod
LD (laser diode)
• Laser diode is a
semiconductor device
which undergoes
stimulated emission by
recombination of injected
carriers (electrons and
holes), the concentration
being far greater than that
in the thermal equilibrium.
What is semiconductor?
• Semiconductors possess electrical conductivity
between metals and insulators
insulator
diamond
semiconductor
metal
Resistivity (cm)
Energy band gap (eV)
Energy band gap (eV)
Conductivity (S/cm)
Electric resisitivity of K
Temperature (K)
Electric resitivity (cm) log scale
Electric resitivity (cm)
Temperature dependence of electrical
conductivity in metals and semiconductors
Temperature (K)
• Resistivity of metals increases with temperature due to
electron scattering by phonon
• Resistivity of semiconductors decreases drastically with
temperature due to increase in carrier concentration
Conductivity, carrier concentration, mobility
• Relation between conductivity and carrier c
oncentration n and mobility
= ne
• Resistivity and conductivity is related by
=1/
• Mobility is average velocity v[cm/s] introduced
by electric field E[V/cm] , expressed by equati
on v= E
Periodic table and semiconductors
IIB
IIIB
IV
V
VI
B
C
N
O
Al
Si
P
S
Zn
Ga
Ge
As
Se
Cd
In
Sn
Sb
Te
Hg
Tl
Pb
Bi
Po
IV (Si, Ge)
I-VII (CuCl, CuI)
III-V (GaAs, GaN, InP, InSb) I-III-VI2 (CuAlS2 ， CuInSe2)
II-VI (CdS, CdTe, ZnS, ZnSe) II-IV-V2 (CdGeAs2, ZnSiP2)
Crystal structures of semiconductors
• Si. Ge: diamond structure
• III-V, II-VI: zincblende structure
• I-III-VI2, II-IV-V2: chalcopyrite structure
Diamond structure
Energy band structure for explanation of
metals, semiconductors and insulators
Fermi level
3s,3p
Conduc
tion
band
3s,3p
Conduc
tion
band
3s
band
3s,3p
Valence
band
3s,3p
Valence
band
2p
shell
2p
shell
2s
shell
2s
shell
1s
shell
1s
shell
Metals
intrinsic
extrinsi
c
Semiconductors
Insulators
and semiconductors
at 0K
Difference of metals, semiconductors and insulators
Concept of Energy Band
Two approaches
• Approximation from free electron
– Hartree-Fock approximation
– Electron is treated as plane waves with wavenumb
er k
– Energy E=(k)2/2m (parabolic band)
• Approximation from isolated atoms
– Heitler-London approximation
– Linear combination of s, p, d wavefunctions
isolated
atom
Band gap of silicon
covalent
bonding
conducti
on band
Antionding orbitals
Bonding orbitals
Energy
3p
3s
Energy
gap
valence
band
lattice
constant of Si
Si-Si distance
Schematic illustration of variation of
electronic states in silicon with Si-Si
distance
Band gap and optical absorption spectrum
Direct gap
InSb, InP, GaAs
Indirect gap
Ge, Si, GaP
Band gap and optical absorption edge
・When photon energy E=h is less than
Eg, valence electrons cannot reach
conduction band and light is transmited.
・When photon energy E=h reaches
Eg, optical absorption starts.
conduction band
1240 / h
h
h>Eg
valence band
Eg
Color of transmitted light and band gap
ZnS Eg=3.5eV
CdS
黄
Eg=2.6eV
GaP
橙
Eg=2.2eV
赤
Eg=2eV
HgS
Eg=1.5eV
GaAs
黒
800nm
300nm
4eV
白
transparent region
3.5eV
3eV
2.5eV
2eV
1.5eV
Semiconductor ｐｎ junction
Energy
N type
P type
space
charge
potential
Carrier diffusion takes place when p
and n semiconductors are contacted
-
+
+
+
+
space charge potential
+
LED, how it works?
hole
•
•
•
•
Forward bias to pn junction diode
electron is injected to p-type region
hole is injected to n-type region
Electrons and holes recombine at th
e boundary region
• Energy difference is converted to ph
oton energy
hc
E h
E (eV)
recombination
p -
+
+
+ n
+
electron
electron
Space charge layer
+
1239.8
(nm)
-
electron drift
energy gap
or
band gap
recombination
light emission
hole drift
Semiconductors for LD
• Optical communication ： 1.5m; GaInAsSb, In
GaAsP
• CD ： 780nm GaAs
• DVD ： 650nm GaAlAs MQW
• DVR ： 405nm InGaN MQW
Double hetero
structure
• Electrons, holes an
d photons are confi
ned in thin active la
yer by using the he
tro-junction structur
e
http://www.ece.concordia.ca/
~i_statei/vlsi-opt/
Invention of DH structure (1)
• Herbert Kroemer and Zhores Alferov suggested in 19
63 that the concentration of electrons, holes and phot
ons would become much higher if they were confined
to a thin semiconductor layer between two others - a
double heterojunction.
• Despite a lack of the most advanced equipment, Alfer
ov and his co-workers in Leningrad (now St. Petersb
urg) managed to produce a laser that effectively oper
ated continuously and that did not require troublesom
e cooling.
• This was in May 1970, a few weeks earlier than their
American competitors.
• from Nobel Prize Presentation Speech in Physics 2000
Invention of DH structure (2)
• In 1970, Hayashi and Panish at Bell Labs and Alferov in Russi
a obtained continuous operation at room temperature using d
ouble heterojunction lasers consisting of a thin layer of GaAs
sandwiched between two layers of AlxGa1-xAs. This design a
chieved better performance by confining both the injected carr
iers (by the band-gap discontinuity) and emitted photons (by t
he refractive-index discontinuity).
• The double-heterojunction concept has been modified and im
proved over the years, but the central idea of confining both th
e carriers and photons by heterojunctions is the fundamental
philosophy used in all semiconductor lasers.
from Physics and the communications industry W. F. Brinkm
an and D. V. Lang Bell Laboratories, Lucent Technologies, Mu
rray Hill, New Jersey 07974
http://www.bellsystemmemorial.com/pdf/physics_com.pdf