Introduction to Photonics

Lecture 1: IntroductionSeptember 3, 2014

Course syllabus Introduction to photonics Optics for communications Ray optics

Syllabus

Course: ENG EC 560, Introduction to Photonics (4 credits) Lectures: Mon/Wed 2-4pm, PHO 211

Instructor: Jonathan Klamkin, klamkin@bu.edu, PHO 828 Office Hours: Tue 10-11am, Wed 4-5pm, or by appointment

Textbook: Fundamentals of Photonics, Saleh & Teich Supplemental Books: Optics, Hecht; Photonics: Optical

Electronics in Modern Communications, Yariv and Yeh; Integrated Photonics, Pollock and Lipson; Diode Lasers and Photonic Integrated Circuits, Coldren, Corzine and Masanovic

Grading policy: Homework (30%), Exam 1 (35%), Exam 2 (35%),

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Our BookFundamentals of Photonics

Second Edition

B. E. A. Saleh, M. C. Teich, John Wiley & Sons Inc., NY (2007)

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Course Objectives

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Introduce physical principles and engineering applications of optical fields and their interactions with materials

Learn design principles governing the behavior of optical components and photonic devices

Basic theories and key concepts Basic optical components and devices Ray Optics Wave Optics Fourier Optics Electromagnetic Optics Polarization Optics

Photonic Crystal Optics Guided Wave Optics Fiber Optics Optical Interconnects and Switches

Integration and Systems Optical Fiber Communications Integrated Photonics

Electromagnetic Spectrum

c=

wavelength

frequencyspeed

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Optical Spectrum

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Optics versus Photonics

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Optics = Photonics

The Pervasiveness of Optics

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Communications Computing Medicine / Biology Defense applications Navigation systems Data storage Imaging NanotechnologyNew industries emerge!

Optics is everywhere!

Photonics: Technology of Light

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Generate, encode, transmit and detect information with optical carrier signals; optical waves carry information with enormous data rates (~Tb/s)

Example: nearly 1 million simultaneous TV channels (~6 MHz bandwidth per channel) can be transmitted using only 1% of a typical laser frequency (1014 Hz) bandwidth.

K.C. Kao, inventor of optical fiberNobel Prize Winner, 2009 PROG. IEE, vol. 113, No. 7, July 1966

Fiber Optics

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Extremely low loss near 1.55 m ~15 THz of bandwidth available in low-loss region Telecommunications bands: O-band: 1260 1360 nm; S-band: 1492 1530 nm;

C-band: 1530 1570 nm; L-band: 1570 1612 nm

Loss spectrum for silica single mode fiber

Components to generate, manipulate and detect light near 1.3 and 1.55 m

Fiber Optics

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Wavelength Division Multiplexing (WDM) Spectrum

WDM increases fiber utilization 40 channels 100 GHz spacing 40 Gb/s per channel 1.6 Tb/s capacity

Optical Fiber Link

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Coaxial Cable Loss

Optical fiber

Miniaturization Higher performance Greater functionality Lower energy

consumption Lower cost

WDM fiber-optic networksCo-axial cable lines

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Long Distance Communications

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Photonics: A Disruptive Technology

A hard disk

A letter

A word

A library

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Revolution in Long-Haul Communications

Over 420,000 km of fiber in over 100 undersea fiber optic systems are deployed

Source: J. X. Cai, Tyco Telecommunications

Short-Reach Optical Interconnects

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Data center to data center

Within data center

Figure of merit: DistanceData Rate

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Computer to computer

Board to board

Chip to chip

On chip

Short-Reach Optical Interconnects

Moores Law (1967)

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The number of transistors on a chip will double every two years.

Metal Interconnects

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Many levels of metal interconnects for densely integrated circuits

Multi-tier metal interconnects

Microelectronics E(in)volution

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Technology cut-off 0.18m

Things get smaller, problems get biggerRC time constantsR = L/AC = kA/d

Local wire interconnection limits the device performance below the 90 nm

The Interconnect Bottleneck

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Interconnects limit device performance

Power dissipation heating Delay latency Spacing crosstalk Cost per interconnect

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Power Consumption and Dissipation

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Integrated Optics/Photonics

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Short-medium reach interconnects

Long-haul telecommunications

On-chip interconnects

Research Level: Nanophotonics

Quantum dots Metal nanostructures Carbon nanotubes Molecular sensors Photonic crystals Micro resonators Microcavities Plasmonic elements Random lasers

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Hierarchy of Optics Theories

Rays

Waves

EM waves

Photons

Rayoptics

Start from the center

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Narcissus, by Michelangelo Caravaggio,ca. 1598.

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Ray Optics

Ray optics: geometrical theory concerned with determination of the path of light rays as they reflect from mirrors of various shapes and traverse boundaries between media of various n

n1

n2

n1 n2

Mirrors

Boundariesbetweentransparent(homogeneous)media

n2n1 n1n1 n1n2

Graded-indexmedia

Planar Concave Convex

Scope of Ray Optics

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Light travels in the form of rays emitted by light sources and observed by opticaldetectors

A transparent medium is characterized by its refractive index nc = speed of light in the medium = co/nco = speed of light in free space

Time taken by light to travel a distance d is d/c = nd/co optical path length nd.

In an inhomogeneous medium the optical path length along a path =

Fermat's Principle: rays traveling between two points follow a path such that time of travel (or optical path length) is an extremum relative to neighboring paths:

The extremum is usually a minimum: rays travel along the path of least time.If the minimum time is shared by more than one path, all paths are followedsimultaneously by the rays.

Postulates of Ray Optics

Sir Isaac Newton

Pierre de Fermat

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Rays A ray is a line drawn is space corresponding to the

direction of flow of radiant energy Rays are mathematical constructs, not physical

entities In uniform media rays are straight For isotropic media (the same in all directions) rays

are perpendicular to the wavefronts (will be clear in wave optics)

Rays are a useful concept if we can assume: 0

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Homogenous Media

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n = constant

Hero's principle: path of minimum distance (= path of shortest time)

Path of minimum distance between two points is a straight line

Shadows

Fermat's Principle

Light chooses to travel only along paths of minimum time minimize Optical Path

Length (OPL)

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Straight ray path in homogeneous medium

Fermats Principle

++==B

A

dzdydxzyxnndsOPL 222),,(

Light rays follow optical path extrema

Light bends in nonhomogeneous media

The optical path length (OPL) is measured in terms of the refractive index n integrated along the trajectory

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Observation

The answer is beyond ray optics need wave optics

Wave optics resolves this problem because wave emitted at A propagates in all directions (with greatly varying amplitude)

The path of optical length extremum is place where constructive interference occurs between all possible paths

Preview

Fermat's principle shows how the ray's destiny is fulfilled (arrives at point B using certain path) but does not explain why the light ray arrives at B instead

of some other point

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Law of Reflection

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ProofAB + BC must be a min.Occurs when AB + BC is a min. when B=B

=

Applies to reflection from mirror or a boundary between two different media

Snells Law

Proof

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d1

d2

d

A

C

B

Apply Fermats principle: minimize optical path length

n1d1sec1 + n2d2sec 2

subject to condition

d1tan1 + d2tan2 = d

Law of Refraction

2211 sinsin nn =

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n1 n2

MiragesRays always take a route that minimizes the OPL

Refractive index decreases with temperature

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Why does image on road appear to wobble?Why does mirage disappear as you approach?

Mirrors

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Planar: creation of a virtual source Paraboloidal: parallel rays focus onto a point

Elliptical: point to point imaging

Image

Spherical: no focusing; parallel rays close to axis approximately focused

Concave R negative

Convex R positive

Focus

Applications

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What type of mirror is used here?

Paraxial Optics

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Paraxial approximation: consider only rays at small inclination angle to the optical axis

sin tan

Paraxial optics = first-order optics = Gaussian optics

Paraxial Optics

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For paraxial rays, the spherical mirror approximatesparaboloidal mirror and therefore focuses parallel rays into a single point

For paraxial rays, the spherical mirror approximateselliptical mirror and therefore focuses rays from a single point into another single point

In the paraxial approximation ( = only paraxial rays considered), a spherical mirror has a focusing property like that of the paraboloidal

mirror and an imaging property like that of the elliptical mirror.