Introduction to Optoelectronics Optical communication (2)

1
Introduction to OptoelectronicsOptical
communication (2)

• Prof. Katsuaki Sato

2
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

3
What is Laser?

• Spontaneous and stimulated emission
• Different pumping methods
• Characteristics of laser light

4
Spontaneous and stimulated emission

• Spontaneous emissionLight emission by relaxation
  from the excited state to the ground state
• stimulated emissionLight emission due to optical
  transition forced by optical stimulation
• This phenomenon is the laserlight amplification
  by stimulated emission of radiation

5
Optical transition

• Transition occurs from the ground state ?1? to
  the excited state ?2? with the probability of P12
  by the perturbation of the electric field of
  light This is an optical absorption.
• The excited state ?2? relaxes to the ground state
  ?1? spontaneously with a light emission to
  achieve thermal equilibrium

Energy
?2?
Spontaneous emission
?1?
6
Stimulated emission
Energy
?2?

• Transition from the excited state ?2? to the
  ground state ?1? occurs by the stimulation of the
  electric field of incident light with the
  transition probability of P21(P12), leading to
  emission of a photon. This process is called
  stimulated emission.
• The number of photons is doubled since first
  photon is not absorbed.

E
p12
Stimulated emission
?1?
7
Emission is masked by absorption under normal
condition

• Under normal condition stimulated emission cannot
  be observed since absorption occurs at the same
  probability as emission (P12P21), and the
  population N1 at ?1? dominates N2 at ?2? due to
  Maxwell-Boltzmann distribution. Therefore,
  N2P21ltN1P12

N2
?2?
p21
Stimulated emission
?1?
N1
N2
?2?
p12
Optical absorption
?1?
N1
8
Maxwell-Boltzmann distribution

• The population at the excited state ?2? located
  at ?E above the ground state ?1? is expressed by
  a formula exp(-?E/kT)

9
population inversion for lasing

• In order to obtain net emission (N2P21gtN1P12),
  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.

10
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

11
Application of lasers

• Optical Communications
• Optical Storages
• Laser Printers
• Diplays
• Laser Processing
• Medical Treatments

12
Optical fiber communication
13
Optical Storages

• CD?DVD?BD
• MD?MO

14
Laser Printers
http//web.canon.jp/technology/detail/lbp/laser_un
it/index.html
15
Laser Show

• Polygon mirror

16
Laser Processing
Web site of Fujitsu
17
Medical Treatment

• CO2 laser

18
Classification of lasersaccording 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., YAGNd, Al2O3Ti, Al2O3Cr(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

19
Gas laserHeNe laser
Showa Optronics Ltd. http//www.soc-ltd.co.jp/inde
x.html
20
HeNe laser, how it works

• He atoms become excited by an impact excitation
  through collision
• The ground state is 1S (1s2 L0, S0) and the
  excited states are 1S (1s1?2s1 ? L0, S0) and
  3S (1s1?2s1 ? L0, S1)
• 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
21
HeNe laser different wavelengths
He

• 3.391 ?m mid IR
• 1.523 ?m near IR
• 632.8 nm red ?
• 612 nm orange?
• 594 nm yellow??
• 543.5 nm green ????

Ne
23S
21S
1S
22
Gas laserAr-ion laser

• Blue458nm
• Blue488nm
• Blue-Green 514nm

23
Application of gas laserAr ion laser

• Illumination (Laser show)
• Photoluminescence Excitation Source

24
Gas laserCO2 laser

• 10.6?m
• Purpose
• manufacturing
• Medical surgery
• Remote sensing

25
Solid state laserYAG laser YVO4laser

• YAGNd
• 1.06?m
• Micro fabrication
• Pumping source for SHG

http//www.fesys.co.jp/sougou/seihin/fa/laser/fal3
000.html
26
Solid state laserTitanium sapphire laser

• Al2O3Ti3 (tunable)

Ti-sapphire laser in Sato lab.
27
Solid state laserRuby laser

• Al2O3Cr3
• 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
28
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.

29
What is semiconductor?

• Semiconductors possess electrical conductivity
  between metals and insulators

30
Temperature dependence of electrical conductivity
in metals and semiconductors

• 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

31
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 vcm/s introduced
  by electric field EV/cm , expressed by equation
  v? E

32
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) III-V (GaAs, GaN, InP, InSb) II-VI
(CdS, CdTe, ZnS, ZnSe)
I-VII (CuCl, CuI) I-III-VI2 (CuAlS2,CuInSe2) II-IV
-V2 (CdGeAs2, ZnSiP2)
33
Crystal structures of semiconductors

• Si. Ge diamond structure
• III-V, II-VI zincblende structure
• I-III-VI2, II-IV-V2 chalcopyrite structure

Diamond structure
34
Energy band structure for explanation of metals,
semiconductors and insulators
35
Concept of Energy BandTwo 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

36
Band gap of silicon
covalent bonding
isolated atom
conduction band
3p
Energy gap
Energy
Antionding orbitals
3s
valence band
Bonding orbitals
lattice constant of Si
Si-Si distance
Schematic illustration of variation of electronic
states in silicon with Si-Si distance
37
Band gap and optical absorption spectrum
Direct gap InSb, InP, GaAs
Indirect gap Ge, Si, GaP
38
Band gap and optical absorption edge

• When photon energy Eh? is less than Eg, valence
  electrons cannot reach conduction band and light
  is transmited.
• When photon energy Eh? reaches Eg, optical
  absorption starts.

conduction band
Eg
h?gtEg
h?
valence band
39
Color of transmitted light and band gap
40
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
41
LED, how it works?
hole
electron

• 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

electron

-
electron drift
energy gap or band gap
recombination
light emission
hole drift
42
Semiconductors for LD

• Optical communication1.5?m GaInAsSb, InGaAsP
• CD780nm GaAs
• DVD650nm GaAlAs MQW
• DVR405nm InGaN MQW

43
Double hetero structure

• Electrons, holes and photons are confined in thin
  active layer by using the hetro-junction structure

http//www.ece.concordia.ca/i_statei/vlsi-opt/
44
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

45
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