Semiconductor Lasers EECE 484 Winter 2013 Dr. Lukas Chrostowski 1
484 - Course Information Web Page: http://siepic.ubc.ca/eece484 (password DBR, check for updates) + Piazza, https://piazza.com/ubc.ca/winterterm22013/eece484 Marking: Projects 40% Midterm 15% Homework 10% Instructor: Text: Midterm / Exam: Homework: Project: Lab Report: Final Exam 20% Lab Report 15% Dr. Lukas Chrostowski (office: Kaiser 4039, contact via Piazza) Photonics: Optical Electronics in Modern Communications by A. Yariv and P. Yeh, 6th Ed, 2007 Lecture notes 1 Midterm: in-class Exam: exam period ~ 6 homework assignments Laser Cavity Design & Fabrication Model a Semiconductor Laser 1 experimental (DFB laser characterization) 2
484 - Course Information Suggested additional texts: Teaching Assistant S.O. Kasap, Principles of Electronic Materials and Devices, 2005 Silicon Photonics Design, Lukas Chrostowski, Michael Hochberg, Book draft, 2012 Xu Wang 3
Material to be Covered Lasers & applications Optical communication Electromagnetics review Laser cavities Design Optical gain Semiconductor Lasers Distributed Feedback Lasers Vertical Cavity Lasers Tunable Lasers Fabrication of semiconductor lasers Semiconductor theory band diagrams hetero-junctions semiconductor engineering (e.g. quantum wells) 4
Course Outline Maxwells Equations Light confinement Optical Modes Fabry-Perot Resonators Design, Foundry Fabrication, Test Compact models: Laser: Rate Equations Semiconductor Optoelectronic Devices Lasers, Detectors, Amplifiers Semiconductor Theory Band Diagrams Carrier Density Distributions Quasi-Fermi Levels Light-Matter Interaction Optical Transitions (Emission, Absorption) Semiconductor Optical Gain Pumping (Current Injection) Applications Optical / Lightwave Communication Systems Biomedical 5
Learning Objectives At the end of this course, the student will be able to: Predict laser characteristics quantitatively and qualitatively Perform analytic calculations predicting the optical properties of laser cavities Design and test a laser cavity. Perform simulations of the laser rate equations to predict laser characteristics, including the impact of laser parameters on fiber propagation Experimentally characterize a laser 6
Laser Cavity Design & Fabrication Project Design a laser resonator cavity Objective: Design the highest possible Q factor cavity class competition Parts Waveguide and cavity modelling and design Peer feedback Mask layout Fabrication done by outside e-beam lithography facility February 1 st, 2013. Measurements done by TA Report 7
Waveguide Bragg Gratings Xu Wang Bragg gratings strip, rib waveguides 15 Single-mode BW: 0.5 35 nm Highest ER: 30 db 20 Transmission (db) 25 30 35 40 1460 1480 1500 1520 1540 1560 1580 Wavelength (nm) Phase shifted gratings Transmission 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 ~10 nm 0 1470 1475 1480 1485 Wavelength (nm) 8
Model a Semiconductor Laser Project In Matlab, develop a rate equation model for a laser Use the model to predict the performance of the optical communication link e.g., Fibre to the Home at 10 Gb/s Central Office Digital Data Source 10 Gb/s Down-stream data link for Fiber to the Home (upstream is similar) Home Data Appliances (Internet, Telephone, Video on Demand) Laser Current Driver Optical Fiber Optical Fiber Receiver Electronics Semiconductor Laser Optical Splitter 1:32 Optical Detector 9
What s a laser? LASER = Light Amplification by Stimulated Emission of Radiation. A laser is an oscillator that operates at very high (optical) frequencies (usually in the range 10 13-10 15 Hz, e.g. 192 THz). In common with an electronic circuit oscillator, a laser is constructed from an amplifier with positive feedback. Lasers are constructed using three essential elements: CAVITY positive feedback PUMP energy source GAIN ELEMENT optical frequency amplifier...output light beam... 100% reflective mirror partially reflective mirror 10
Laser Properties Laser light is monochromatic i.e. single colour In contrast to rainbows, white light, etc Laser light is in the form a beam Laser Light 11
Optical Spectrum FM Radio/TV Shortwave Radio AM Broadcast Ultrasonic Sonic Visible Light Infrared Light Radar Ultraviolet X-Rays Frequency 1 khz 1 MHz 1 GHz 1 THz 1 YHz 1 ZHz Wavelength 1 Mm 1 km 1 m 1 mm 1 nm 1 pm µm nm Frequency Wavelength (vacuum) 1.8 1.6 1.4 1.2 193 229 353 461 THz Near Infrared UV 1.0 0.8 0.6 0.4 0.2 µm Longhaul Telecom Regional Telecom Local Area Networks 1550 nm 1310 nm 850 nm HeNe Lasers 633 nm CD Players 780 nm 12
Semiconductor Laser Applications Internet Optical communications Telecom Datacom (millions) Computercom (billions) Other CD players Entertainment Machining Bio-Med Applications l Surgery l Disease detection l Environmental sensing l Drug discovery Q 13
Optical Telecommunications Lucent China Sumitomo Electric Industries, Ltd 14
History 1916 Albert Einstein Foundations for laser - spontaneous and stimulated emission 1953 Charles Townes 1st MASER demonstrated 1957 Gordon Gould (graduate student) Schalow & Townes 1964 Charles Townes, Nikolay Basov Aleksandr Prokhorov Optical wavelength LASER name, theory Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle." 1960 Theodore H. Maiman 1st laser. Used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light 1960 Ali Javan, et. al. 1st gas laser - HeNe. continuous operation. 1962 Basov, Javan Robert Hall Nick Holonyak, Jr semiconductor laser diode proposed 1st NIR GaAs laser 1st visible laser 1970 Zhores Alfrerov heterojunction structure - semiconductor laser Room Temperature operation http://en.wikipedia.org/wiki/laser#history 15
The Ruby Laser First man made laser (built by Theodore Maiman in 1960). Optical pumping usually achieved with a xenon flashlamp (pulsed operation). energy fast transition (non-radiative) pump 29 cm -1 split metastable Theodore Maiman lived in level Vancouver in the last part of R 1 = 694.3nm his life, and died in 2007. R 2 = 692.7nm ground state 2010 was the 50 th anniversary of the laser 16
Recent History Laser designs: DFB laser Theory, Kogelnik and Shank (1972) Demonstration, A. Yariv et al. (1973) VCSEL (Surface emitting Laser) invention, K. Iga (1977) Room temperature operation (1988) Mass production (started in 1999) Materials Heterostructures, Quantum Wells, Quantum Dots Growth uniformity, composition, doping Work towards higher efficiency, higher power, higher speed, many wavelengths, etc. 17
850 nm Vertical Cavity Laser (VCSEL) http://www.mina.ubc.ca/txvcsel Lasers fabricated at UBC in the AMPEL Nanofab 18
Atom Models Classical (Billiard ball) Bohr debroglie (shell model) Schroedinger PhET.colorado.edu hydrogen-atom 19
Hydrogen Atom Electron energy in the hydrogen atom is quantized. n is a quantum number: 1, 2, 3, 4 Each defined state has a wave-function From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) 20
Hydrogen Atom Electronic transitions Transitions from one energy level to another occur via energy loss or gain (e.g., via photons) From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) 21
Hydrogen Atom An atom can become excited by a collision with another atom. When it returns to its ground energy state, the atom emits a photon. From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) 22
Concept of Spontaneous Emission N 2 = population density of energy level 2. N 2 E 2 (i.e. # of electrons per cm 3 ) N 1 E 1 (h = 6.626 x 10-34 J.s) An electron can spontaneously fall from energy level E 2 to E 1. A photon of wavelength λ photon is emitted in the process. The photon is emitted in a random direction. The probability of a spontaneous jump is given quantitively by the so-called Einstein A coefficient. A 21 = probability per second of a spontaneous jump from level 2 to level 1. 23
Process of Stimulated Emission Electron transitions can be stimulated by the action of an external radiation field. N 2 external field ρ(v) hv 21 hv 21 N 1 ρ(v) = energy density of the applied radiation field at frequency v. (energy per unit volume per unit frequency interval: J.m -3.Hz -1 ). E 2 hv 21 E 1 output photons have: same direction same frequency same phase 24
Process of Stimulated Absorption Electrons can also make stimulated transitions in the upward direction between energy levels of an atom by absorbing energy from ρ(v) : N 2 E 2 external field ρ(v) hv 21 N 1 E 1 25
Summary: three types of transitions E 2 E 1 spontaneous emission stimulated absorption stimulated emission contributes to noise inside a laser loss mechanism amplification mechanism All three processes occur simultaneously inside a laser. What about LEDs? Detectors? Optical Amplifiers? 26
Optical Amplification PUMPING MECHANISM I(0) amplifying laser medium I(L) z=0 z=l z Can we calculate the output intensity using Stimulated emission provides optical amplification. We can calculate the intensity at position L, given the gain function γ 0 : 27
Condition for Lasing Gain = Loss For self-sustaining oscillations, the optical power lost through the mirrors must be replenished by the gain medium. Gain Loss e.g., Electronic Oscillator 28
Course Outline Maxwells Equations Light confinement Optical Modes Fabry-Perot Resonators Design, Foundry Fabrication, Test Compact models: Laser: Rate Equations Semiconductor Optoelectronic Devices Lasers, Detectors, Amplifiers Semiconductor Theory Band Diagrams Carrier Density Distributions Quasi-Fermi Levels Light-Matter Interaction Optical Transitions (Emission, Absorption) Semiconductor Optical Gain Pumping (Current Injection) Applications Optical / Lightwave Communication Systems Biomedical 29
Semiconductor Laser 1) Optics light propagation, reflections, waveguides, optical modes, resonator (Ch. 1-4, 12) 2) Optical Gain in a 2 level atomic system (Ch. 5) 3) Laser Theory Fabry-Perot Laser (Ch. 6) 4) Semiconductor Lasers Semiconductor theory (parts of Ch. 15, 16) Real devices (DFBs, VCSELs), performance Design, Fabrication 1) Optical resonator 3) Light out Semiconductor: 4) Active layer 2) Optical gain 30
Laser Candle Analogy Legend: 1 student with arms waving = 1 electron in the excited state with arms down = 1 electron in the ground state with a flame = 1 photon Processes: Spontaneous emission = student lights a candle (with lighter) Stimulated emission = student A s candle lights student B s candle Absorption = student s candle is extinguished student becomes an excited electron Photons have: Direction, Polarization, Wavelength/frequency Cavity: two walls, one partially reflective 31