Fiber Optical Communication - ttu.ee 1-2016.pdf• Lab exercises • Home assignments ... • For...

Fiber Optical Communication Lecture 1, Slide 1

Fiber Optical Communication

Examiner/lectures:

Prof. Peter Andrekson

[email protected], +372 – 5558 7388; +46 – 70 3088 606

Laboratory tutorials and exercises:

Egon Astra

[email protected], +372 – 5560 2230

mailto:[email protected]

mailto:[email protected]


Lecture 1

• Course information

• Introduction to photonics and fiber optics

– Applications of photonics

– Telecommunication history

– System overview and terminology

– Fiber basics

• Geometric description (ray optics) of optical fibers


• The book that is used is very well known…

• …and in the fourth edition

• Govind P. Agrawal: Fiber-Optic Communication Systems, ISBN: 9780470505113

Course outline• Lectures:

- 7 times in total

• Lab exercises

• Home assignments

• Exam on xxx


Course objectivesAfter completion of this course, the student should be able to

• Describe the fundamental properties and limitations of fiber-optic systems

• Describe and analyze the most important system components and their limitations: transmitters, fibers, receivers, optical amplifiers

• Evaluate a proposed system design and understand the trade-offs

• For optical transmitters: Understand different implementations, primarily using external modulators

• For optical fibers: Quantify dispersion, attenuation, and to some extent nonlinearities as well as their impact on signal transmission

• For optical receivers: Analyze receivers with and without optical pre-amplifier, know relevant noise mechanisms, evaluate the bit error rate

• For optical amplifiers: Understand the fundamental properties of erbium-doped fiber amplifiers and quantify the impact of using them

• For systems: Evaluate different implementations such as wavelength division multiplexing and time division multiplexing.


Plan for lectures• Information and introduction

• General concepts, modulation and detection

• Modes in optical fibers

• Dispersion and losses

• Nonlinear effects in optical fibers

• Optical transmitters

• Optical receivers

• Error probability, power penalties

• Multichannel systems

• Optical amplification, gain

• Optical amplification, noise

• Dispersion management

• Advanced lightwave systems

Please note:

• Some of the contents of the lecture notes are not available in the book

• The opposite is of course also very true!

• Some sections of the book are outside the scope of the course

• The section corresponding to a slide has been indicated (when possible)


Information about the homepage

• There is a course homepage http://lr.ttu.ee/irm0120/

• Will eventually contain

– News

– Lecture notes

– Home assignments

– Lab instructions

http://lr.ttu.ee/irm0120/


Some photonic applications

LED-TV

LED light bulb

Blu-Ray disc

Solar cellOptical communication

Fiber-optic gyroscope

Environmental monitoring


Some photonic applications• Telecommunication

– Lasers, modulators, fibers, detectors for communication systems

– Free-space optical links

• Information and Communication Technology

– CCD and CMOS sensors for imaging

– Data storage and retrieval (CD, DVD, BluRay)

– Optical interconnects (mainly in high performance computing context today)

• Sensors and spectroscopy

– ”Smart cameras” for image processing/machine vision

– Many, many applications, including sensors for measuring:

• Position, distance, thickness etc.

• Angular rate (ring laser/fiber gyroscopes)

• Gas concentration (using absorption)


Some photonic applications• Security

– Intrusion detection

– Laser radar (LIDAR)

• Lighting

– LEDs for indoor lighting

– LEDs and Lasers for artistic lighting

• Energy

– Solar cells

• Biophotonics

– Optical tweezers, optical scalpels

– Optical tomography

• Military

– Surveillance

– Weapon guidance

– Countermeasures and laser guns


The motivation: the Internet• Figure shows number of hosts connected to the Internet

– Around 109 hosts...

• Traffic grows quickly

– How to keep up with the increasing demand?


Optical fibers vs wireless communication

Fiber systems:

• High data rates

• Long distances

• One “ether” per system

• Static links

• Expensive installation

Wireless systems:

• “Low” data rates

• Short distances

• Shared “ether”

– Limiting regulations

– Cross-talk problems

• Enables mobility

• Easy and flexible installation

Wireless and optical fiber communication are complementary rather than competing technologies


Transatlantic cables

Fibers are used for high speed, long haul communication, but also for:

• Intercity connects and city networks

• Providing high speed connections to terminals providing wireless services

• High speed (100 Mbit/s and above) services FTTx, where x = home etc.

A submarine cable1. Polyethylene2. Mylar tape3. Stranded steel wires4. Aluminum water barrier5. Polycarbonate6. Copper or aluminum tube7. Petroleum jelly8. Optical fibers

The continents are today connected by fiber-optical communication links


The electromagnetic spectrum

Frequency: ν ≈ 200 THz

Wavelength: λ, typ. 1.55 μm

Light velocity in vacuum:

c ≈ 2.998×108 m/s

The carrier frequency is much higher in lightwave systems than in microwave systems

Lightwave systems typically use infrared light

Frequency Wavelength

1018 Hz

1 THz

1 GHz

1 MHz

1 µm

1 nm

1 mm

1 m

1 km

Photon energy

1 eV

1 keV

1 meV

10-6 eV

10-9 eV

ultra-violet

infrared

x-ray

mm-waves

microwaves

radio waves

1015 Hz visible


Properties of optical fibersAdvantages:

• Low attenuation (0.2 dB/km)

• Large bandwidth (1.55 μm–1.3 μm = 250 nm > 30 THz)

• Low weight, compact, flexible

• Isolated from the environment

– No crosstalk from other fibers or microwave sources

• Low sensitivity to environmental conditions

– Can operate on the ocean floor

– Immune to electromagnetic interference

• Provides electrical isolation between terminals

– No ground loops, damage cannot cause sparking

Disadvantages:

• Not wireless, installation is costly and slow

• Hardware is expensive compared to mass-produced electronics


Early communication history (1.1)• Ancient time: Fire beacons, smoke signals, etc.

• ”Optical telegraph”, 1794

– Coded signals using semaphore signaling

– Used light: Otherwise you could not see the signals!

• Telegraphy, 1830s

– Morse code

– 10 bit/s, 1000 km

– Trans-Atlantic cable

• Telephone, 1876

– Analog modulation

• Communication systems, 1940–1980

– Coaxial-cable systems

– Microwave systems


Optical Communication History

1962 First semiconductor laser (GE, IBM, Lincoln Lab)

1966 First optical fiber, loss: 1000 dB/km (Corning Glass)

1970 Fiber with an optical attenuation of 20 dB/km (Corning Glass)

1970 AlGaAs-lasers operating at room temperature

1982 0.16 dB/km (theoretical limit) single-mode fiber

1986 First erbium-doped fiber optical amplifier

1988 Trans-Atlantic and trans-Pacific cable systems (565 Mbit/s)

1995 Repeaterless trans-oceanic systems (5 Gbit/s)

1997 Commercial WDM systems

2004 Multiple band transmission (S + C + L)

2007 “Advanced” nonbinary formats; 40 Gbit/s systems


Optical Communication History• First generation, 1980

– GaAs lasers, 0.8 μm, 45 Mbit/s, using electrical repeaters

• Second generation

– InGaAsP lasers, 1.3 μm (minimum dispersion), 100 Mbit/s

– Single-mode fibers, 2 Gbit/s over 44 km in 1981

• Third generation, 1.55 μm (minimum loss)

– Dispersion-shifted fibers

– Single longitudinal mode lasers

– Still using electrical repeaters

• Fourth generation

– Optical amplification

– Wavelength-division multiplexing (WDM)

• Fifth generation

– Increased spectral range

– Increased spectral efficiency


Progress in lightwave communication

The bit rate-distance product has increased eight orders of magnitude


Progress in lightwave communication

Efforts have been made to:

• Increase the data rate per channel

• Increase the number of channels

• Increase the distance between repeaters

– Or at least keep it from shrinking when increasing other parameters

Rapid progress has been made, laboratory systems show the way forward


A fiber-optic communication link

optical pre- amplifier

photo- detector

semiconductor laser

optical modulator

optical fiber

electrical signal

optical signal

optical receiver

electronicsoptical

transmitter

optical amplifier

optical fiber

optical fiber

optical transmitter

optical receiver

repeater

information receiver

receiver electronics

information source

drive electronics

optical pre- amplifier

photo- detector

semiconductor laser

optical modulator

optical fiber

electrical signal

optical signal

optical receiver

electronicsoptical

transmitter

optical amplifier

optical fiber

optical fiber

optical transmitter

optical receiver

repeater

information receiver

receiver electronics

information source

drive electronics


Channel multiplexing (1.2.2)Wavelength-division multiplexing (WDM)

Time-division multiplexing (TDM)

Optical fiber

Pulse source

O-MUX1,2...N

Data encoders

x Gbit/s Timing control clock1,2...N

receivers

x Gbit/s

O-DEMUX

N x Gbit/s

transmission

Multiplexing in wavelength (frequency)

Interleaving in time


Coherent fiber systems

• Were used before optical amplifiers (EDFA) for increased sensitivity

– Very hard to obtain stable phase-locked operation

• Recently re-gained popularity

– Allows increased spectral efficiency (phase modulation, dual-polarization)


The dB units (1.4.2)• Decibel (dB) expresses a power ratio according to

• The photo current is proportional to the optical power

– Idet Popt ⇒ Pel Popt2

– dBopt ≠ dBel (3 dB optical power diff. ⇒ 6 dB electrical power diff.)

• dBm expresses the absolute power on a log scale relative to 1 mW

• Examples:

– 1 mW = 0 dBm, 2 mW = 3 dBm, 4 mW = 6 dBm, 8 mW = 9 dBm

– 0.5 mW = –3 dBm, 1 μW = –30 dBm

– 100 mW = 20 dBm, 400 mW = 26 dBm

2

110log10

P

P

mW1

W][log10 10dBm

PP


Optical fibers (2.1)Typical core sizes:

• Single-mode fibers: d ≈ 5–10 µm

• Multi-mode fibers: d ≈ 50–200 µm

• Typical attenuation:

– 0.2 dB/[email protected] µm

– 4% power loss per [email protected] µm

• Available bandwidth:

– >30 THz in modern fibers

1.3 1.55Wavelength (µm)

Att

enuat

ion (

dB

/km

)0.2

15 THz 20 THz

n1 > n2

cladding, n2

core, n1d


Fiber basics• Wave-guiding: n1 > n2

• A finite number of modes can propagate in the fiber

• Modes are solutions to Maxwell's equations + boundary conditions

– One mode: single-mode fiber

– Several modes: multi-mode fiber

• Most commonly used fiber material is silica (SiO2)

• To change index of refraction dopants are added

– Dopants can increase or decrease the index of refraction

– Can dope either the core or the cladding

core cladding protective coating

2a2b

n1 n2

refr

active

ind

ex

dopant addition [mol %]

1.44

1.46

1.48

5 10 15 200

F

GeO2

B2O3


Geometrical-optics description• The fractional index change is

– Δ ≈ 1–3% for MM fibers

– Δ ≈ 0.1–1% for SM fibers

– n2 = n1(1 – Δ)

• Apply Snell's law at input

• Minimum critical angle φc for total internal reflection

• Relate to maximum entrance angle

cladding, n2

core, n1

unguided ray

ir guided ray

n0(normally = 1)

1/)( 121 nnn

ri nn sinsin 10

1221 /sin)2/sin(sin nnnn cc

2

2

2

1

2

111max,1max,0 sin1cos)2/sin(sinsin nnnnnnn cccri


Numerical aperture• The numerical aperture (NA) is a measure of the light-gathering power of

an optical system

– The term originates from microscopy

• For fibers, we have

• Clearly, a higher NA is always better!?!

– No, we get problems with dispersion

cladding, n2

core, n1

unguided ray

ir guided ray

n0(normally = 1)

2sinNA 1

2

2

2

1max,0 nnnn i


• Compare the propagation times along the slowest and the fastest path

• For this approximate model, use the condition TB = 1/B > ΔT

– Means: Propagation time difference < bit slot

• We get a limit on the bit rate-distance product

– Do not confuse ΔT with Δ

Modal dispersion (multi-mode fiber)

cladding, n2

core, n1

i,max

c

fastest ray path

slowest ray path

2

2

11

2

11

1

fastslow 1sin/ n

n

c

L

c

n

n

nL

c

nL

L

nc

LLT

c

c

n

nBL

2

1

2


Modal dispersion, example

Modal dispersion is a severe limitation

Use single-mode fibers when possible

Remember that the geometrical-optics description has limited accuracy

• (Hypothetical) fiber without cladding:

– n1 = 1.5, n2 = 1

– BL < 0.4 (Mbit/s) × km

– Large index-step:

• Wide “acceptance cone”

– Easy to get the light into the fiber

• Very small bandwidth!

• Typical communication fiber: Δ = 0.002 ⇒ BL < 100 (Mbit/s) × km

– 1 Gbit/s over 100 m

– Estimate is a bit too conservative


Basic fiber typesSingle-mode step-index:

• No intermodal dispersion gives highest bandwidth

• Small core radius, difficult to launch light

Multi-mode step-index:

• Large core radius, easy to launch light

• Intermodal dispersion reduces the bandwidth

Multi-mode graded-index:

• Reduced intermodal dispersion increases bandwidth

n

2a: 5-12 m

2b: 125 m

n

a b

n2

n1

2a: 50-200 m

2b: 125-400 m

n

2a: 50-100 m

2b: 125-140 m


Other types of fiberPolarization preserving fibers

• Using birefringence

Photonic crystal fibers

• Air capillaries inside fiber


Fiber fabrication (2.7)• Preform is made from glass (2–20 cm thick cylinder)

• Heated and pulled to 125 µm diameter

• Adding coating for mechanical protection


A 2 inch wafer with optical components


Optical modulator (LiNbO3)


A complete transponder

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