Semiconductor Lasers for Optical Communication

Transcription

1 Semiconductor Lasers for Optical Communication Claudio Coriasso Manager Turin Technology Centre 10Gb/s DFB Laser MQW 1

2 Outline 1) Background and Motivation Communication Traffic Growth Why Photonics? Photonics evolution 2) Semiconductor Laser Basics Active material Optical Feedback 2

3 Communication Growth Communication has always been one of the main driving force for the development of new technologies: Telegraph, Telephone, Fiber Optic, Laser, Tim Berners-Lee computer at CERN: World s first Web Server 1991 (34.3% world population) The Internet Worldwide communication traffic is doubling every 18 months (2dB/year) 3

4 Traffic structure and Energy Consumption Data Centers and the Internet consume about 4% of electricity ( kwhr/year including PCs) By 2018 the energy utilized by IP traffic will exceed 10% of the total electrical power generation in developed countries (1) Most of the traffic is between machines and much of the information created today cannot even be stored (2) FACEBOOK (3) : 1.01 billion active users worldwide (23M in Italy, 39.5% population) 584 million active users every day (1) L.Kimerling, MIT (2) R.Tkatch, Alcatel Lucent (3) D. Lee, Facebook European Conference on Optical Communication billion minute/day 4

5 Why Photonics? Short monochromatic optical pulses are easily produced with semiconductor lasers (ps range Gb/s to Tb/s) Photons do not interact each other Photons can be propagated in optical fiber with very low loss (0.2dB/km) Several data streams at different wavelength can be combined, propagated together in optical fibers and then split (high channel capacity)

6 Photonics in Optical Communication Photonics Electronics Today Photonic Network: Wide Area Network Storage Area Network Local Area Network Metro Area Network Traffic volume 10m 100m 1km 10km 100km 1000km 6

7 Photonics Photonics: Science and technology of light Emission, Transmission, Processing (modulation, switching, amplification, ) Detection Began with Laser (1960) and Fiber Optic (1966) inventions. These inventions formed the basis for the telecommunications revolution of the late 20th century and provided the infrastructure for the internet. 1 st Laser demonstration : T. Maiman st Low-Loss Fiber Optic Proposal: C. Kao

8 Fiber Optics 1966: First proposal of fiber optic for telecom. Basic design [Kao STC, Nobel Prize 2009] 1970: Production of first fiber optic [Corning] : First fiber optic networks 1988: First transoceanic fiber-optic cable (3148 miles, simultaeous telephone calls) First operative optical cable in urban areas Torino 1976 First optical system carrying live traffic over public network Chicago 1977 C. K. Kao receiving his Nobel Prize Stockholm

9 Semiconductor Laser Basics 9

10 Semiconductor LASER history 1962: First Realization of Semiconductor Laser = o C) [GEC, IBM, MIT] 1963: Proposal of Heterostructure Semiconductor Laser (H. Kroemer, Z. Alferov: Nobel Prize 2000) 1970: First Realization of Heterostructure Semiconductor Laser (Z. Alferov) 1972: Proposal of Distributed Feedback Laser (DFB) 1970: Room Temperature CW operation of 1.5m Laser 1977: Proposal of Vertical Cavity Surface Emitting Laser (VCSEL) 1984: First Realization of Strained MQW in semiconductor laser 1988: First Realization of VCSEL 2000: First Uncooled Telecom Lasers Z. Alferov receiving his Nobel Prize Stockholm

11 Laser requirements for optical communication Electrodes (current injection) Grating (optical feedback) High bandwidth ( 10Gb/s) Single mode operation Low consumption Uncooled operation (up to 80 o C) 25Gb/s State of Art of uncooled high-speed lasers Waveguide (optical confinement) Quantum wells (optical gain) DFB MQW Laser 11

12 High-speed laser key factors Laser rate equations: d dt High carrier density and high photon density in an active material within a small-volume optical resonator 4 1. Electrical confinement (p-n heterostructure) 2. Optical confinement (single-mode waveguide) 3. Gain (quantum-confined material) 4. Feedback (distributed Bragg grating) 12

13 Semiconductor material basic requirements for photonic devices Optical gain, light emission (direct band gap)... e h photon emission through e-h recombination... at wavelength of interest: = 1,3 m e 1,55 m compatibility with semiconductor substrates: Si, GaAs, InP 13

14 III-V semiconductor materials III V There are no single elements or binary compounds compatible with commercial substrates and emitting light at 1.3 m e 1.55 m. Semiconductor alloys of III-V elements are the best materials for photonic devices. 14

15 Quaternary alloy InGaAsP T. P. Pearsall,, Wiley (1982) In 1-x Ga x As y P 1-y alloy cover all the spectral range required for optical telecom High quality material Established growth techniques and material processing InP InGaAs ( g =1.65m) ( g =0.92m) Suited for active devices (lasers, amplifiers, modulators,...) and passive structures (waveguides, couplers,...) In 1-x Ga x As y P 1-y lattice matched to InP ( g = m) 15

16 Further alloy systems 1 In 1-x Ga x As y P 1-y on InP 2 Al 1-x-y Ga x In y As on InP 3 3 Al 1-x Ga x As on GaAs

17 (Basic) Optical properties of semiconductors E e JDOS hh lh E g k a Three bands are involved in optical transitions: - Electrons Conduction Band - Heavy holes - Light holes Valence Band m m * hh * lh 9m m * e * e E g JDOS E g Joint density of states (available for optical transitions) is a square root function of the energy in excess of the energy gap 17

18 Semiconductor Heterostructures p n Double Heterostructure (DH) A B B A A (Substrate) ph > e p n Separate Confinement Heterostructure (SCH) A C B B C A A (Substrate) refractive index Y photon E g (A)> E g (B) e-h confinement E g (A)> E g (C) )> E g (B), n(a)< n(c) < n(b) Photon confinement Combination of layers of different crystalline semiconductors. H. Kroemer, Varian associates 1963 (Nobel Prize in Physics, 2000) The idea was experimentally demonstrated using the Liquid Phase Epitaxy (LPE) 18

19 Quantum Wells Quantum-Size Double Heterostructure (Quantum Well) is a planar waveguide for electrons C. H. Henry, Bell Labs 1972 The idea was experimentally demonstrated in 1974 using the newly developed Molecular Beam Epitaxy (MBE). B A B x A BULK d = z E g B E g A E g QW y QUANTUM WELL 1 2 B A B d = 2 L w ~ e L w QW is now a widely spread quantum product based in atomic-scale technology Transmission Electron Microscope view of QW 19

20 Photon wave eqn. vs. Electron wave eqn. Helmholtz equation (photon) Y ph (z) 2 d k0 n ( z) ( z) n ( z) 2 eff dz Schroedinger equation (electron) 2 2m d dz 2 2 V ( z) ( z) E ( z) Y e (z) n eff 2 n 2 (z) V(z) E z n 2 ( z) V( z) cladding barrier core well refractive index ridges confine photons (optical waveguides) potential wells confine electrons (quantum wells) 20

21 Eigenfunction/Eigenvalues Calculation Optical Waveguide Quantum Well Y TE0 n TE0 Y 1 Y TE1 Y E n 0 0 TE1 E 1 n TE0 E 1 n TE1 E 0 Y TE ( z Y z TE ) Y ( z TE ) ( z ) Y z TE ( z ) Y( z 1 m( z ) Y( z Y( z ) z ) ) 1 m( z ) z Y( z ) 21

22 Reduced dimensionality structures z A BULK d = 3 x y Q.M. E.M. B A B QUANTUM WELL d = 2 L z ~ e PLANAR WAVEGUIDE 1975: R.Dingle and C.Henry USA Patent Application B A B QUANTUM WIRE d = 1 L z,l y ~ e CHANNEL WAVEGUIDE B A B QUANTUM DOT d = 0 L z,l y,l x ~ e OPTICAL RESONATOR 22

23 CONDUCTION BAND VALENCE BAND MQW heterostructures Multi Quantum Wells are stacks of decoupled QWs (with sufficiently thick barriers): enhancement of single QW effects Z X Z Y ENERGY WELLS SUBSTRATE BARRIERS 23

24 QW band structure E e 2 e 1 One step for every allowed transition JDOS bulk E E g TM lh 1 hh 1 hh 2 TE lh 2 z E g bulk e 1 -hh 1 e 1 -lh 1 e 2 -hh 2 2D JDOS related features: Sharp absorption edge (2D JDOS) High differential gain Wide gain bandwith High electroabsorption efficiency (QCSE) Strong optical nonlinearities... JDOS E g QW i j i j 2 ( E ) i j 24

25 Excitons h h e e - h semi-bound states (t ~ 100 fs) which produce sharp absorption peaks detuned from the transition energies (absorption steps) by their binding energy a E b (~8 mev per InGaAsP) E 25

26 a( ) JDOS exciton QW optical properties state n( ) c 1 0 a( ') d' 2 2 ' nˆ( ) a( ) c n( ) i 2 a TE n TE a TM n TM e 1 -hh 1 e 1 -lh 1 e 1 -hh 1 e 1 -lh 1 Strong dichroism Polarisation selection rules: TE: 3/4 hh, 1/4 lh TM: 0 hh, 1 lh 26

27 Strain(1) The epitaxial layer can be grown with a lattice parameter slightly different from the substrate lattice parameter (lattice mismatch). m a a L a S S R, S a lattice of epitaxial layer T L = lattice parameter of the epitaxial layer a S = lattice parameter of of substrate the substrate a L a L a S a S compressive strain m>0 tensile strain m<0 27

28 Strain(2) T. P. Pearsall,, Wiley (1982) InP InP ±1% InGaAs InGaAs 28

29 compressive strain E Strain effect on band structure: unstrained E E tensile strain e e e m>0 lh hh k m=0 lh hh k m<0 lh hh k e 2 e 2 e 2 e 1 e 1 e TE hh 1 lh 1 TM Low escape time High T 0 (low thermal dependence) High-speed uncooled Lasers TE TM hh 1 lh 1 hh 1 lh 1 TE, TM Low dichroism Polarization-independent devices 29

30 Optical gain in MQW Rel. Amplitude (db) dg db ( I I th ) dn [GHz] high differential gain (dg/dn) wider spectral bandwidth step-like shape of density of states deep penetration of Fermi function 30

31 Distributed Feedback DF /4 a + a - a + a - B da dz da dz i a ika ika i a B Single- reflectivity: Single mode laser T R cm 65 cm 0.24 m0.24m ( i) T R 31

32 Quantum Wells: Atomic-Controlled Artificial Structures SEMICONDUCTOR LASER ACTIVE LAYER MQW BARRIER QUANTUM WELL ATOMS 2 2 d n(, N, F) i k(, N, F) V ( z) ( z) E ( z) 2 2m dz Control of Optical Properties through atomic-scale technology Quantum Well requires sub-monolayer manufacturing control achievable with Molecular Beam Epitaxy or Metal Organic Chemical Vapor Deposition. 32

33 Thanks for your attention! 33