Introduction to Optoelectronics Optical communication (2)

1. Introduction to OptoelectronicsOptical 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. 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

6. 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

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 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

8. 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)

9. 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.

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  quantum state appearing macroscopically

11. Application of lasers • Optical Communications • Optical Storages • Laser Printers • Diplays • Laser Processing • Medical Treatments

12. Optical fiber communication system Electro-optical conversion Opto-electronic Conversion Demulti-plexer Multi-plexer Amplifier Optical fiber Laser diode Photodiode Optical fiber communication

13. Optical Storages • CD、DVD、BD • MD、MO

14. 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

15. Laser Show • Polygon mirror

16. Laser Processing Web site of Fujitsu

17. Medical Treatment • CO2 laser

18. 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

19. Gas laserHeNe laser Showa Optronics Ltd. http://www.soc-ltd.co.jp/index.html

20. 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

21. 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

22. Gas laserAr+-ion laser • Blue458nm • Blue488nm • Blue-Green 514nm

23. Application of gas laserAr ion laser • Illumination (Laser show) • Photoluminescence Excitation Source

24. Gas laserCO2 laser • 10.6m • Purpose • manufacturing • Medical surgery • Remote sensing

25. 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

26. Solid state laserTitanium sapphire laser • Al2O3:Ti3+ (tunable） Ti-sapphire laser in Sato lab.

27. 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

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. 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

30. 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

31. 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

32. 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)

33. Crystal structures of semiconductors • Si. Ge: diamond structure • III-V, II-VI: zincblende structure • I-III-VI2, II-IV-V2: chalcopyrite structure Diamond structure

34. 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

35. 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

36. 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

37. Band gap and optical absorption spectrum Direct gap InSb, InP, GaAs Indirect gap Ge, Si, GaP

38. 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

39. ZnS 白 Eg=3.5eV transparent region CdS 黄 Eg=2.6eV GaP 橙 Eg=2.2eV HgS Eg=2eV 赤 GaAs Eg=1.5eV 黒 800nm 300nm 4eV 3eV 2.5eV 2eV 3.5eV 1.5eV Color of transmitted light and band gap

40. - + 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

41. 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

42. Semiconductors for LD • Optical communication：1.5m; GaInAsSb, InGaAsP • CD：780nmGaAs • 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 http://www.ece.concordia.ca/~i_statei/vlsi-opt/

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 http://www.bellsystemmemorial.com/pdf/physics_com.pdf