1	Introduction to Optoelectronics Optical 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 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
5	Optical transition Transition occurs from the ground state \|1ñ to the excited state \|2ñ with the probability of P12 by the perturbation of the electric field of light: This is an optical absorption. The excited state \|2ñ relaxes to the ground state \|1ñ spontaneously with a light emission to achieve thermal equilibrium
6	Stimulated emission Transition from the excited state \|2ñ to the ground state \|1ñ 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.
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 (P12=P21), and the population N1 at \|1ñ dominates N2 at \|2ñ due to Maxwell-Boltzmann distribution. Therefore, N2P21<N1P12
8	Maxwell-Boltzmann distribution The population at the excited state \|2ñ located at DE above the ground state \|1ñ is expressed by a formula exp(-DE/kT)
9	population inversion for lasing In order to obtain net emission (N2P21>N1P12), N2, the population of the state \|2 ñ should exceed N1, the population of the state \|1ñ. 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 Û 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
15	Laser Show Polygon mirror
16	Laser Processing
17	Medical Treatment CO₂ laser
18	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 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
19	Gas laser HeNe laser
20	HeNe laser, how it works
21	HeNe laser: different wavelengths 3.391 mm mid IR 1.523 mm near IR 632.8 nm red　赤 612 nm orange色 594 nm yellow黄色 543.5 nm green グリーン
22	Gas laser Ar⁺-ion laser Blue458nm Blue488nm Blue-Green 514nm
23	Application of gas laser Ar ion laser Illumination (Laser show) Photoluminescence Excitation Source
24	Gas laser CO₂ laser 10.6mm Purpose manufacturing Medical surgery Remote sensing
25	Solid state laser YAG laser YVO₄laser YAG:Nd 1.06mm Micro fabrication Pumping source for SHG
26	Solid state laser Titanium sapphire laser Al₂O₃:Ti³⁺ (tunable）
27	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 wavelength of interest
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 s and carrier concentration n and mobility m 　 s = nem Resistivityr and conductivitys is related by r=1/s Mobility is average velocity v[cm/s] introduced by electric field E[V/cm] , expressed by equation v=m E
32	Periodic table and semiconductors
33	Crystal structures of semiconductors Si. Ge: diamond structure III-V, II-VI: zincblende structure I-III-VI₂, II-IV-V₂: chalcopyrite structure
34	Energy band structure for explanation of metals, semiconductors and insulators
35	Concept of Energy Band Two approaches Approximation from free electron Hartree-Fock approximation Electron is treated as plane waves with wavenumber k Energy E=(hk)²/2m (parabolic band) Approximation from isolated atoms Heitler-London approximation Linear combination of s, p, d wavefunctions
36	Band gap of silicon
37	Band gap and optical absorption spectrum
38	Band gap and optical absorption edge
39	Color of transmitted light and band gap
40	Semiconductor ｐｎ junction
41	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
42	Semiconductors for LD Optical communication：1.5mm; GaInAsSb, InGaAsP CD：780nm　GaAs DVD：650nm GaAlAs MQW DVR：405nm InGaN MQW
43	Double hetero structure Electrons, holes and photons are confined in thin active layer by using the hetro-junction structure
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