Introduction to Optoelectronics
Optical communication (2)
Prof. Katsuaki Sato
Lasers
• Spontaneous emission and stimulated
emission
• 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
P12 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
is doubled since first
1
photon is not absorbed.
Emission is masked by absorption
under normal condition
N2
Stimulated
emission
p21
N1
N2
p12
N1
Optical
absorption
• Under normal condition
2
stimulated emission cannot
be observed since
absorption occurs at the
same probability as
1
emission (P12=P21), and the
population
N
1 at 1
2
dominates N2 at 2 due to
Maxwell-Boltzmann
distribution. Therefore,
N2P21<N1P12
1
Maxwell-Boltzmann distribution
Energy
• The population at the excited state 2
located at E above the ground state 1
is expressed by a formula exp(-E/kT)
2
exp(-E/kT)
E
1
0
1
Distribution function
population inversion for lasing
• In order to obtain net emission (N2P21>N1P12),
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
1
exp(E/kT)
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
as femtosecond (10-15 s)
• Bose condensation  quantum state appearing
macroscopically
Application of lasers
•
•
•
•
•
•
Optical Communications
Optical Storages
Laser Printers
Diplays
Laser Processing
Medical Treatments
Optical fiber communication
Optical fiber
communication system
Multiplexer
Electrooptical
conversion
Amplifier
Optoelectronic
Conversion
Demulti
-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
or 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
impact 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
(1s12s1 ; L=0, S=1)
•The energy is transferred to Ne
atoms through collision.
•Ne has ten electrons in the
ground state 1S0 with 1s2 2s2 2p4
configuration, and possesses a
lot of complex excited states
http://www.mgkk.com/products/pdf/02_4_HeNe/024_213.pdf
He
23S
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
1S
Ne
Gas laser
+
Ar -ion
• Blue458nm
• Blue488nm
• Blue-Green 514nm
laser
Application of gas laser
Ar ion laser
• Illumination (Laser show)
• Photoluminescence
Excitation Source
Gas laser
CO2 laser
• 10.6m
• Purpose
– manufacturing
– Medical surgery
– Remote sensing
Solid state laser
YAG laser YVO4laser
•
•
•
•
YAG:Nd
1.06m
Micro fabrication
Pumping source for
SHG
http://www.fesys.co.jp/sougou/sei
hin/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
wavelength 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
concentration 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
equation 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
Insulators
Semiconductors 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
wavenumber k
– Energy E=(k)2/2m (parabolic band)
• Approximation from isolated atoms
– Heitler-London approximation
– Linear combination of s, p, d wavefunctions
isolated
atom
covalent
bonding
conducti
on band
Band gap of silicon
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
Eg=2eV
HgS
赤
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
the boundary region
• Energy difference is converted to
photon energy
E  h 
hc

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：1.5m; GaInAsSb,
InGaAsP
• CD：780nm GaAs
• DVD：650nm GaAlAs MQW
• DVR：405nm InGaN MQW
Double hetero
structure
• Electrons, holes
and photons are
confined in thin
active layer by
using the hetrojunction structure
http://www.ece.concordia.ca/
~i_statei/vlsi-opt/
Invention of DH structure (1)
• Herbert Kroemer and Zhores Alferov suggested in
1963 that the concentration of electrons, holes and
photons 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,
Alferov and his co-workers in Leningrad (now St.
Petersburg) managed to produce a laser that
effectively operated continuously and that did not
require troublesome 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
Russia obtained continuous operation at room temperature
using double heterojunction lasers consisting of a thin layer of
GaAs sandwiched between two layers of AlxGa1-xAs. This
design achieved better performance by confining both the
injected carriers (by the band-gap discontinuity) and emitted
photons (by the refractive-index discontinuity).
• The double-heterojunction concept has been modified and
improved over the years, but the central idea of confining both
the carriers and photons by heterojunctions is the
fundamental philosophy used in all semiconductor lasers.
from Physics and the communications industry W. F.
Brinkman and D. V. Lang Bell Laboratories, Lucent
Technologies, Murray Hill, New Jersey 07974
http://www.bellsystemmemorial.com/pdf/physics_com.pdf