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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

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lasers
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
What is Laser?
  • Spontaneous and stimulated emission
  • Different pumping methods
  • Characteristics of laser light
spontaneous and stimulated emission
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

p12

Optical

absorption

1

Optical transition

Energy

  • 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 opticalabsorption.
  • The excited state 2 relaxes to the ground state 1 spontaneously with a light emission to achieve thermal equilibrium

2

Spontaneous

emission

1

stimulated emission

2

p21

Stimulated emission

1

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

emission is masked by absorption under normal condition
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 (P12=P21), and the population N1 at 1 dominates N2 at 2 due to Maxwell-Boltzmann distribution. Therefore, N2P21<N1P12

N2

2

p21

Stimulated emission

1

N1

N2

2

p12

Optical

absorption

1

N1

maxwell boltzmann distribution

2

exp(-E/kT)

Energy

E

1

1

0

Distribution function

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)
population inversion for lasing

Distribution function

1

0

2

Energy

E

exp(E/kT)

1

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.
characteristics of laser
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
Application of lasers
  • Optical Communications
  • Optical Storages
  • Laser Printers
  • Diplays
  • Laser Processing
  • Medical Treatments
optical fiber communication

Optical fiber communication system

Electro-optical conversion

Opto-electronic Conversion

Demulti-plexer

Multi-plexer

Amplifier

Optical fiber

Laser diode

Photodiode

Optical fiber communication
optical storages
Optical Storages
  • CD、DVD、BD
  • MD、MO
laser printers

Computer

BD lens

photosensitive drum

controller

optical fiber

BD signal

video signal

BD signal

DC controller

toric lens

spherical lens

polygon mirror

horizontal sync

mirror

scanner motor/ motor driver

opt. box

cylindrical lens

laser diode/ laser driver

Laser Printers

http://web.canon.jp/technology/detail/lbp/laser_unit/index.html

laser show
Laser Show
  • Polygon mirror
laser processing
Laser Processing

Web site of Fujitsu

classification of lasers according to the way of pumping
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., 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
Gas laserHeNe laser

Showa Optronics Ltd.

http://www.soc-ltd.co.jp/index.html

hene laser how it works

He

Ne

23S

21S

1S

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

hene laser different wavelengths
HeNe laser: different wavelengths

He

  • 3.391 m mid IR
  • 1.523 m near IR
  • 632.8 nm red赤
  • 612 nm orange色
  • 594 nmyellow黄色
  • 543.5 nm green グリーン

Ne

23S

21S

1S

gas laser ar ion laser
Gas laserAr+-ion laser
  • Blue458nm
  • Blue488nm
  • Blue-Green 514nm
application of gas laser ar ion laser
Application of gas laserAr ion laser
  • Illumination (Laser show)
  • Photoluminescence Excitation Source
gas laser co 2 laser
Gas laserCO2 laser
  • 10.6m
  • Purpose
    • manufacturing
    • Medical surgery
    • Remote sensing
solid state laser yag laser yvo 4 laser
Solid state laserYAG laser YVO4laser
  • YAG:Nd
  • 1.06m
  • Micro fabrication
  • Pumping source for SHG

http://www.fesys.co.jp/sougou/seihin/fa/laser/fal3000.html

solid state laser titanium sapphire laser
Solid state laserTitanium sapphire laser
  • Al2O3:Ti3+ (tunable)

Ti-sapphire laser in Sato lab.

solid state laser ruby laser
Solid state laserRuby 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
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

Conductivity (S/cm)

insulator

diamond

semiconductor

Energy band gap (eV)

Energy band gap (eV)

metal

Resistivity (cm)

What is semiconductor?
  • Semiconductors possess electrical conductivity between metals and insulators
temperature dependence of electrical conductivity in metals and semiconductors

Electric resisitivity of K

Electric resitivity (cm) log scale

Electric resitivity (cm)

Temperature (K)

Temperature (K)

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
conductivity carrier concentration mobility
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 equationv= E
periodic table and semiconductors
Periodic tableand semiconductors

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)

crystal structures of semiconductors
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

3s,3p

Conduction

band

3s,3p

Conduction

band

Fermi level

3s

band

3s,3p

Valence

band

3s,3p

Valence

band

2p

shell

2p

shell

2s

shell

2s

shell

1s

shell

1s

shell

intrinsic

Insulators

and semiconductors at 0K

extrinsic

Metals

Semiconductors

Difference of metals, semiconductors and insulators

Energy band structure for explanation of metals, semiconductors and insulators
concept of energy band two approaches
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
band gap of silicon
Band gap of silicon

covalent bonding

isolated atom

conduction band

3p

Antionding orbitals

Energy

Energy gap

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

band gap and optical absorption spectrum
Band gap and optical absorption spectrum

Direct gap

InSb, InP, GaAs

Indirect gap

Ge, Si, GaP

band gap and optical absorption edge
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

Eg

h>Eg

h

valence band

color of transmitted light and band gap

ZnS

Eg=3.5eV

transparent region

CdS

Eg=2.6eV

GaP

Eg=2.2eV

HgS

Eg=2eV

GaAs

Eg=1.5eV

800nm

300nm

4eV

3eV

2.5eV

2eV

3.5eV

1.5eV

Color of transmitted light and band gap
semiconductor junction

-

+

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

recombination

-

-

-

-

+

+

+

+

p

n

Space charge layer

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

semiconductors for ld
Semiconductors for LD
  • Optical communication:1.5m; GaInAsSb, InGaAsP
  • CD:780nmGaAs
  • DVD:650nm GaAlAs MQW
  • DVR:405nm InGaN MQW
double hetero structure
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/

invention of dh structure 1
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
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

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