Introduction to OptoelectronicsOptical 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• 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

1

2

p12 Optical absorption

Energy

1

2

Spontaneous emission

Stimulated emission• 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.

1

2

p21Stimulated emission

1

2

p12 Stimulated emission

Energy

E

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

1

2

p21Stimulated emission

1

2

p12 Optical absorption

N2

N2

N1

N1

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)

1

2

Distribution function

Energ

y1

E

exp(-E/kT)

0

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.

1

2

Distribution function

Energ

y 1

E

exp(E/kT)

0

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

Multi-plexer

Electro-optical conversion

Laser diode

Amplifier

Photodiode

Opto-electronic Conversion

Demulti-plexer

Optical fiber

Optical Storages

• CD 、 DVD 、 BD• MD 、 MO

Laser Printers

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

scanner motor/ motor driver

laser diode/ laser driver

cylindrical lensopt. box

horizontal syncmirror

polygon mirror

spherical lens

toric lens

BD lensphotosensitive drumComputer

optical fiber

DC controllerBD signal BD signal video signal

controller

Laser Show

• Polygon mirror

Laser Processing

Web site of Fujitsu

Medical Treatment

• CO2 laser

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 laserseg., 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, InGaNhigh 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

http://www.mgkk.com/products/pdf/02_4_HeNe/024_213.pdf

•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 (1s12s1 ; L=0, S=0) and 3S (1s12s1 ; 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

He Ne

1S

21S

23S

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 Ne

1S

21S

23S

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/seihin/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 rod

Ruby laser

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

Resistivity (cm)

Conductivity (S/cm)

Ene

rgy

band

gap

(eV

)

Ene

rgy

band

gap

(eV

)

semiconductor

metal

insulator

diamond

Electric resisitivity of K

Temperature (K) Temperature (K)

Ele

ctric

res

itivi

ty (

cm)

Ele

ctric

res

itivi

ty (

cm)

log

scal

e

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

• 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)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

• 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,3pConduc

tionband

3s,3pValence

band

3sband

2pshell

2sshell

1sshell

2pshell

2sshell

1sshell

3s,3pConduc

tionband

3s,3pValence

band

intrinsic extrinsic

Metals SemiconductorsInsulatorsand semiconductors at 0K

Difference of metals, semiconductors and insulators

Concept of Energy BandTwo 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

Band gap of silicon

Si-Si distanceSchematic illustration of variation of electronic states in silicon with Si-Si distance

valence band

conduction band

lattice constant of Si

covalent bondingisolated

atom

Energy gap

3p

3s

Ene

rgy

Bonding orbitals

Antionding orbitals

Band gap and optical absorption spectrum

Indirect gapGe, Si, GaP

Direct gap InSb, InP, GaAs

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.

h/1240

valence band

h>Egh Eg

conduction band

Color of transmitted light and band gap

1.5eV

CdS

GaP

HgS

GaAs

3eV 2.5eV 2eV

800nm300nm

　　 ZnS

Eg=2eV

Eg=2.2eV

Eg=2.6eV

Eg=3.5eV

Eg=1.5eV

白

黄

橙

赤

黒

3.5eV4eV

transparent region

Semiconductor ｐｎ junction

N typeP type

++++

----

Carrier diffusion takes place when p and n semiconductors are contacted

space charge potential

Energy

space charge potential

+

-

LED, how it works?

• 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 t

he boundary region• Energy difference is converted to p

hoton energy

p n

recombination

Space charge layer

++++

----

electron

+ -

electron drift

hole drift

recombination

light emission

electronhole

energy gapor

band gap

hc

hE (nm)

8.1239(eV)

E

Semiconductors for LD

• Optical communication ： 1.5m; 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 hetro-junction structure

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 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