Staff Site Universitas Negeri Yogyakarta

Introduction to Optoelectronics

Optical communication (2)

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

What is Laser?

• Spontaneous and stimulated emission

• Different pumping methods

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

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

p_{12 Optical}

absorption

Energy

1 2

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

p₂₁

Stimulated emission

1 2

p_{12 Stimulated emission}

Energy

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

p₂₁

Stimulated emission

1 2

p_{12 Optical}

absorption N2

N2 N1

Maxwell-Boltzmann distribution

• The population at the excited state



2



l

ocated at



E above the ground state



1



is expressed by a formula exp(-



E/kT)

1 2

Distribution function

E

n

e

rg

y

1

E

exp(-E/kT)

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

E

n

e

rg

y ₁

E

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 a s femtosecond (10-15 s)

Application of lasers

• Optical Communications

• Optical Storages

• Laser Printers

• Diplays

Optical fiber communication

Optical fiber

communication system

Multi-pl exer

Electro-optical conversion

Laser diode

Amplifier

Photodiode

Opto-electro nic Conversi

on Demulti_-plexer

Optical Storages

• CD 、 DVD 、 BD

Laser Printers

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

scanner motor/ motor driver

laser diode/ laser driver

cylindrical lens opt. box

horizontal sync mirror

polygon mirror spherical lens toric lens BD lens

photosensitive drum Computer

optical fiber

DC controller

BD signal BD signal video signal

Laser Show

Laser Processing

Medical Treatment

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

Gas laser

HeNe laser

Showa Optronics Ltd.

HeNe laser, how it works

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

•He atoms become excited by an im pact excitation through collision

•The ground state is 1_{S (1s}2_{; 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 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

He _Ne

1_S 21_S

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

1_S 21_S

Gas laser

Ar

+

-ion laser

• Blue458nm • Blue488nm

Application of gas laser

Ar ion laser

• Illumination (Laser show) • Photoluminescence

Gas laser

CO

₂

laser

• 10.6m • Purpose

Solid state laser

YAG laser YVO

₄

laser

• YAG:Nd • 1.06m

• Micro fabrication

• Pumping source for SH G

Solid state laser

Titanium sapphire laser

• Al2O3:Ti3+ (tunable ）

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

LD (laser diode)

• Laser diode is a

semiconductor device

which undergoes

stimulated emission by

recombination of injected

carriers

(electrons and

What is semiconductor?

• Semiconductors possess electrical conductivity between metals and insulators

Resistivity (cm)

Electric resisitivity of K

Temperature (K) _{Temperature (K)}

E le ct ric r es iti vi ty (  cm ) E le ct ric r es iti vi ty (  cm ) lo g sc al e

Temperature dependence of electrical

conductivity in metals and semiconductors

• Resistivity of metals increases with temperature due to electron scattering by phonon

Conductivity, carrier concentration, mobility

• Relation between conductivity



and carrier c

oncentration

n

and mobility



　



=

ne



• Resistivity



and conductivity



is related by



=

1

/



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)

Crystal structures of semiconductors

• Si. Ge: diamond structure

• III-V, II-VI: zincblende structure

• I-III-VI2, II-IV-V2: chalcopyrite structure

Energy band structure for explanation of

metals, semiconductors and insulators

Fermi level 3s,3p Conduc tion band 3s,3p Valence_band 3s band 2p shell 2s shell 1s shell 2p shell 2s shell 1s shell 3s,3p Conduc tion band 3s,3p Valence band intrinsic extrinsi c

Metals _{Semiconductors} Insulatorsand semiconductors at 0K

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

Band gap of silicon

Si-Si distance

Schematic illustration of variation of electronic states in silicon with Si-Si distance valence band conducti on band lattice

Band gap and optical absorption spectrum

Indirect gap Ge, Si, GaP Direct 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.







1240

/

h

valence band

h>Eg

h Eg

Color of transmitted light and band gap

1.5eV CdS GaP HgS GaAs

3eV 2.5eV 2eV

Semiconductor

ｐｎ

junction

N type P 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 th e boundary region

• Energy difference is converted to ph oton energy

p n

recombination

Space charge layer + + + + -electron + -electron drift hole drift recombination light emission electron hole energy gap or band gap   hc h

E  

Semiconductors for LD

• Optical communication

：

1.5



m; GaInAsSb, In

GaAsP

• CD

：

780nm

　

GaAs

Double hetero

structure

• Electrons, holes an

d photons are confi

ned in thin active la

yer by using the he

tro-junction structur

e

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.

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