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 Transition occurs from the ground state  1  to the excited state  2  with the probability of P 12 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1 22 p 12 Optical absorption Energy 11 2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 P 21 (=P 12 ), 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1 22 p 21 Stimulated emission 11 22 p 12 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 (P 12 =P 21 ), and the population N 1 at  1  dominates N 2 at  2  due to Maxwell-Boltzmann distribution. Therefore, N 2 P 21 <N 1 P 12 11 22 p 21 Stimulated emission 11 22 p 12 Optical absorption N2N2 N2N2 N1N1 N1N1

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1 22 Distribution function Energy 1 EE exp(-E/kT) 0

population inversion for lasing In order to obtain net emission (N 2 P 21 >N 1 P 12 ), N 2, the population of the state  2  should exceed N 1, 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1 22 Distribution function Energy 1 E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 ( 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 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 video signal controller

Laser Show Polygon mirror

Laser Processing Web site of Fujitsu

Medical Treatment CO 2 laser

Classification of lasers according to the way of pumping Gas lasers ： eg., He-Ne, He-Cd, Ar +, CO 2, pump an excited state in the electronic structure of gas ions or molecules by discharge Solid state lasers eg., YAG:Nd, Al 2 O 3 :Ti, Al 2 O 3 :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.

HeNe laser, how it works He atoms become excited by an impact excitation through collision The ground state is 1 S (1s 2 ; L=0, S=0) and the excited states are 1 S (1s 1 2s 1 ; L=0, S=0) and 3 S ( 1s 1  2s 1  ; L=0, S=1) The energy is transferred to Ne atoms through collision. Ne has ten electrons in the ground state 1 S 0 with 1s 2 2s 2 2p 4 configuration, and possesses a lot of complex excited states He Ne 1S1S 21S21S 23S23S

HeNe laser: different wavelengths  m mid IR  m near IR nm red 赤 612 nm orange 色 594 nm yellow 黄色 nm green グ リーン He Ne 1S1S 21S21S 23S23S

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 CO 2 laser 10.6  m Purpose –manufacturing –Medical surgery –Remote sensing

Solid state laser YAG laser YVO 4 laser YAG:Nd 1.06  m Micro fabrication Pumping source for SHG hin/fa/laser/fal3000.html

Solid state laser Titanium sapphire laser Al 2 O 3 :Ti 3+ (tunable ） Ti-sapphire laser in Sato lab.

Solid state laser Ruby laser Al 2 O 3 :Cr 3+ Synthetic ruby single crystal Pumped by strong Xe lamp Emission wavelengths; 694.3nm Ethalon is used to select a wavelength 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) Energy band gap (eV) semiconductor metal insulator diamond

Electric resisitivity of K Temperature (K) Electric resitivity (  cm) Electric resitivity (  cm) log scale 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 IIBIIIBIVVVI BCNO AlSiPS ZnGaGeAsSe CdInSnSbTe HgTlPbBiPo IV (Si, Ge) III-V (GaAs, GaN, InP, InSb) II-VI (CdS, CdTe, ZnS, ZnSe) I-VII (CuCl, CuI) I-III-VI 2 (CuAlS 2 ， CuInSe 2 ) II-IV-V 2 (CdGeAs 2, ZnSiP 2 )

Crystal structures of semiconductors Si. Ge: diamond structure III-V, II-VI: zincblende structure I-III-VI 2, II-IV-V 2 : chalcopyrite structure Diamond 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 Insulators 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

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 constant of Si covalent bonding isolated atom Energy gap 3p 3s Energy Bonding orbitals Antionding orbitals

Band gap and optical absorption spectrum Indirect gap Ge, 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. valence band h > Eg h Eg conduction band

Color of transmitted light and band gap 1.5eV CdS GaP HgS GaAs 3eV2.5eV2eV 800nm 300nm ZnS Eg=2eV Eg=2.2eV Eg=2.6eV Eg=3.5eV Eg=1.5eV 白 黄 橙 赤 黒 3.5eV 4eV transparent region

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 the boundary region Energy difference is converted to photon energy pn recombination Space charge layer electron + - electron drift hole drift recombination light emission electron hole energy gap or band gap

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 hetro- junction structure ~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