Semiconductor Lasers

EECE 484

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Winter 2013

Dr. Lukas Chrostowski

UBC EECE 484 – 2013

484 - Course Information

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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%Final Exam 20% Lab Report 15%

Instructor: Dr. Lukas Chrostowski (office: Kaiser 4039, contact via Piazza)Text: Photonics: Optical Electronics in Modern Communications

by A. Yariv and P. Yeh, 6th Ed, 2007Lecture notes

Midterm / Exam:

1 Midterm: in-classExam: exam period

Homework: ~ 6 homework assignmentsProject: “Laser Cavity Design & Fabrication”

“Model a Semiconductor Laser”Lab Report: 1 experimental (DFB laser characterization)

UBC EECE 484 – 2013

484 - Course Information

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Suggested additional texts:

S.O. Kasap, “Principles of Electronic Materials and Devices”, 2005

“Silicon Photonics Design”, Lukas Chrostowski, Michael Hochberg, Book draft, 2012

Teaching Assistant

Xu Wang

UBC EECE 484 – 2013

Material to be Covered

§ Lasers & applications§ Optical communication

§ Electromagnetics review§ Laser cavities

§ Design

§ Optical gain § Semiconductor theory

§ band diagrams§ hetero-junctions§ semiconductor engineering

(e.g. quantum wells)

§ Semiconductor Lasers § Distributed Feedback Lasers§ Vertical Cavity Lasers§ Tunable Lasers

§ Fabrication of semiconductor lasers

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UBC EECE 484 – 2013

Course Outline

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Semiconductor Theory

Band DiagramsCarrier Density DistributionsQuasi-Fermi Levels

Light-Matter Interaction

Optical Transitions (Emission, Absorption)Semiconductor Optical GainPumping (Current Injection)

Semiconductor Optoelectronic Devices

Lasers, Detectors, Amplifiers

Maxwells Equations

Light confinementOptical ModesFabry-Perot Resonators

Applications

Optical / Lightwave Communication SystemsBiomedical

Design, Foundry Fabrication,

Test

Compact models:• Laser: Rate Equations

UBC EECE 484 – 2013

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

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UBC EECE 484 – 2013

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 1st, 2013.§ Measurements – done by TA§ Report

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UBC EECE 484 – 2013

Waveguide Bragg Gratings – Xu Wang

§ Bragg gratings – strip, rib waveguides

§ Phase shifted gratings

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1470 1475 1480 14850

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Wavelength (nm)Tr

ansm

issi

on

~10 nm

1460 1480 1500 1520 1540 1560 1580−40

−35

−30

−25

−20

−15

Wavelength (nm)

Tran

smis

sion

(dB)

1500 1505 1510 1515 15200

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Ref

lect

ivity

0.68 nm

Single-modeBW: 0.5 – 35 nmHighest ER: 30 dB

UBC EECE 484 – 2013

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

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Central Office

Semiconductor Laser

Digital Data Source

Laser Current Driver

Optical Splitter

1:32

Home

Optical Detector

Data Appliances (Internet, Telephone, Video on Demand)

Receiver ElectronicsOptical Fiber Optical Fiber

10 Gb/s Down-stream data link for Fiber to the Home

(upstream is similar)

UBC EECE 484 – 2013

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 1013 - 1015 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:

PUMP GAIN ELEMENTCAVITY

...output light beam...

positive feedback optical frequency amplifier

100% reflective mirror partially reflective mirror

energysource

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UBC EECE 484 – 2013 11

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

LW Technology (Cover, Appendix).PPT - 23© Copyright 1999, Agilent Technologies

Revision 1.1December 11, 2000

E lectromagnetic Spectrum

Frequency

Sonic

Ultrasonic

AM Broadcast

Shortwave Radio

FM Radio/TVRadar

Infrared Light

Visible Light

UltravioletX-Rays

Wavelength 1 Mm 1 km 1 m 1 mm 1 pm1 nm

1 kHz 1 MHz 1 GHz 1 THz 1 ZHz1 YHz

c = f • ! • nc: Speed of light ( 2.9979 m/µs )f: Frequency!: Wavelengthn: Refractive index

(vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to 1.48)

LW Technology (Cover, Appendix).PPT - 24© Copyright 1999, Agilent Technologies

Revision 1.1December 11, 2000

LW Transmission Bands

Near InfraredF requency

Wavelength1.6

229

1.0 0.8 µm0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks850 nm

1550 nm

1310 nmCD Players780 nm

HeNe Lasers633 nm

UBC EECE 484 – 2013

Optical Spectrum

12

µm nm

UBC EECE 484 – 2013 13

§ CD players§ Entertainment§ Machining§ …

Bio-Med Applications

l Surgeryl Disease detectionl Environmental sensingl Drug discovery

Internet• Optical communications• Telecom• Datacom (millions)• Computercom (billions)

Q

Other

Semiconductor Laser Applications

UBC EECE 484 – 2013 14

Sumitomo Electric Industries, Ltd

Optical Telecommunications

Lucent

China

UBC EECE 484 – 2013

History

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1916 Albert Einstein Foundations for laser - spontaneous and stimulated emission

1953 Charles Townes 1st MASER demonstrated1957 Gordon Gould

(graduate student)Schalow & Townes

Optical wavelength LASER name, theory

1964 Charles Townes, Nikolay Basov Aleksandr Prokhorov

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 HallNick Holonyak, Jr

semiconductor laser diode proposed1st NIR GaAs laser1st visible laser

1970 Zhores Alfrerov heterojunction structure - semiconductor laser Room Temperature operation

http://en.wikipedia.org/wiki/Laser#History

UBC EECE 484 – 2013

The Ruby Laser§ First man made laser (built by

Theodore Maiman in 1960).§ Optical pumping usually

achieved with a xenon flashlamp (pulsed operation).

ground state

R2 = 692.7nm

R1 = 694.3nm

splitmetastable

level

fast transition(non-radiative)

ener

gy

pump

29 cm-1

• Theodore Maiman lived in Vancouver in the last part of his life, and died in 2007.

• 2010 was the 50th anniversary of the laser

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UBC EECE 484 – 2013

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.

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UBC EECE 484 – 2013

850 nm Vertical Cavity Laser (VCSEL)§ http://www.mina.ubc.ca/txvcsel

§ Lasers fabricated at UBC in the AMPEL Nanofab

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UBC EECE 484 – 2013

Atom Models§ Classical (Billiard ball) Bohr

§ deBroglie (shell model) Schroedinger

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PhET.colorado.edu “hydrogen-atom”

UBC EECE 484 – 2013

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

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From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

UBC EECE 484 – 2013

Hydrogen Atom – Electronic transitions§ Transitions from one energy level to another occur

via energy loss or gain (e.g., via photons)

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From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

UBC EECE 484 – 2013

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.

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From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

UBC EECE 484 – 2013

Concept of Spontaneous Emission

§ An electron can spontaneously fall from energy level E2 to E1. A photon of wavelength λphoton 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.§ A21 = probability per second of a spontaneous jump

from level 2 to level 1.

E1

E2N2

N1

N2 = population density of energy level 2.

(i.e. # of electrons per cm3)(h = 6.626 x 10-34 J.s)

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UBC EECE 484 – 2013

Process of Stimulated Emission

§ Electron transitions can be stimulated by the action of an external radiation field.

• 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).

E1

E2

N1

N2

external field ρ(v) hv21hv21

hv21

output photons have: • same direction• same frequency• same phase

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UBC EECE 484 – 2013

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) :

E1

E2

N1

N2

external field ρ(v) hv21

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UBC EECE 484 – 2013

Summary: three types of transitions

E1

E2

spontaneousemission

stimulatedabsorption

stimulatedemission

contributes to noiseinside a laser

amplification mechanism

loss mechanism

• All three processes occur simultaneously inside a laser.• What about LEDs? Detectors? Optical Amplifiers?

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§ Stimulated emission provides optical amplification.§ We can calculate the intensity at position L, given

the gain function γ0:

UBC EECE 484 – 2013

Optical Amplification

amplifying laser mediumI(0) I(L)

z=0 z=Lz

• Can we calculate the output intensity using

PUMPING MECHANISM

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UBC EECE 484 – 2013

Condition for Lasing

§Gain = Loss§For self-sustaining oscillations, the optical power lost through the mirrors must be replenished by the gain medium.

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Loss

Gain

e.g., Electronic Oscillator

UBC EECE 484 – 2013

Course Outline

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Semiconductor Theory

Band DiagramsCarrier Density DistributionsQuasi-Fermi Levels

Light-Matter Interaction

Optical Transitions (Emission, Absorption)Semiconductor Optical GainPumping (Current Injection)

Semiconductor Optoelectronic Devices

Lasers, Detectors, Amplifiers

Maxwells Equations

Light confinementOptical ModesFabry-Perot Resonators

Applications

Optical / Lightwave Communication SystemsBiomedical

Design, Foundry Fabrication,

Test

Compact models:• Laser: Rate Equations

UBC EECE 484 – 2013

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

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Semiconductor: 4) Active layer2) Optical gain

1) Optical resonator

3) Light out

UBC EECE 484 – 2013

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

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