Generalitati Fibre

Chalmers University of Technology / Photonics Lab

Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20031

Fiber Optic CommunicationsQuarter IV, march-may 2003

web page: http://www.elm.chalmers.se/fotonik/fiber/

Lecturers: Magnus Karlsson, Per-Olof HedekvistPhotonics Lab, Dept of [email protected], poh @elm.chalmers.se Dan Anderson, Mietek LisakDept of [email protected], [email protected]



Course info Fiber optic introduction

fiber basics history modulation formats digital/analog modulation ray optics description of fibers

Relevant chapters in the book:1-2.1

Lecture 1 - outline



Course outline (1)

Undergraduate course in Fiber Optic Communication LPIV 2003

Lecture Topic Lecturer1 18/3 Introduction, Optical fibers - geometrical description Magnus2 21/3 Optical fibers - waveguiding, Maxwells equations Magnus3 25/3 Optical fibers - dispersion, pulsebroadening, attenuation Magnus4 28/3 Optical fibers - nonlinearities Dan/Mietek5 1/4 Solitons, nonlinear phenomena Dan/Mietek6 4/4 Light emitting diodes, semiconductor lasers Magnus7 11/4 Photodetectors, receivers Magnus8 29/4 Optical amplifiers P-O9 6/5 Optical amplifiers P-O10 9/5 Receiver performance Magnus *11 13/5 System design P-O12 16/5 Dispersion compensation P-O13 20/5 Multi-channel systems, WDM / OTDM P-O14 23/5 Coherent systems, Microwave Photonics P-O

* On 9/5 the ecture is held in Kollektorn, floor 4, MC2.



Course outline (2)

Home assignments: Responsible: Thomas Torounidis, 1609 4 assignments One assignment will appear on exam Solved x assignments=potential upgrade

to grade x+1

Lab exercises (week 5-8): Lab 1: Dispersion/Amplifiers Lab 2: System Characterization



Introductury lecture

Contents: History Fiber basics Analog/digital communications Modulation formats Ray description of light propagation



A definition of Fiber Optics

Utilization of electromagnetic waves in dielectric, circular waveguides combined with optoelectronic devices (LEDs, lasers, photodiodes, amplifiers, etc.)

Applications of fiber optics:

communication medical applications optical sensing power distribution (e.g. in "nasty" environments) welding, drilling...



The electromagnetic spectrumFrequency Wavelength

1018 Hz

1 THz

1 GHz

1 MHz

1 m

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 visiblen = frequency of light

( 200 THz in fiber optics)l =wavelengthc = light velocity in vacuum (3108 m/s)



Fiber Optic Commnication Link

100 million km optical fiber employed world wide !!!

optical pre-amplifier

photo-detector

semiconductorlaser

opticalmodulator

opticalfiber

electrical signaloptical signal

opticalreceiverelectronics

opticaltransmitter

opticalamplifier

opticalfiber

opticalfiber

optical transmitter

optical receiver

repeater

informationreceiver

receiverelectronics

informationsource

driveelectronics



Optical Fibers

n1 > n2

cladding, n2core, n1 d

Single mode fibers: d 5 - 10 mMulti mode fibers: d 50 - 200 m

Core: GeO2-doped SiO2Cladding: SiO2

1.3 1.55Wavelength (m)

Atte

nuati

on (d

B/km

)

0.2

15 THz 20 THzAttenuation characteristics

- Minimum attenuation = 0.2 dB/km at 1.55 m -> 4% lost after 1 km !!!

- High carrier frequency 200 THz ->Available bandwidth 35 THz !!!

(equivalent to 3.5 million HDTV-cannels, in one single optical fiber !!!)



Fiber manufacture



Low attenuation (0.2 dB/km) Large bandwidth (35 THz) Wavelength independent attenuation in the transmission window The enormous capacity of an installed fiber can be utilized

in the future as the demand increases Small geometry and low weight Flexible Easy to install Low sensitivity to moisture The fiber endpoints handle large differences in voltage Immune to electromagnetic interference No crosstalk between fibers Damage can not cause sparking Potentially low cost Well suited for future broadband services

Fiber advantages



Optical communication history1854 Water jet as an optical waveguide (John Tyndall)1880 The photo phone (Alexander Graham Bell)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 temperature1976 First semiconductor lasers at 1.3 and 1.55 m1977 First generation commercial systems (0.85 m)1980 Second generation commercial systems (1.3 m)1982 0.16 dB/km ( theoretical limit) singelmode fiber1983 420 Mbit/s over 119 km fiber without repeaters (Bell Labs.)1984 Third generation commercial systems (1.55 m)1985 1.37 Tbitkm/s WDM system;10 channels @ 2 Gbit/s (Bell Labs.)1986 Semiconductor laser with 20 GHz bandwidth (Bell Labs.,GTE)1986 First erbium-doped fiber optical amplifier1988 Trans-Atlantic and trans-Pacific cable systems (565 Mbit/s)1989 Coherent semiconductor laser with sub-MHz spectral linewidth1990 2.5 Gbit/s repeaterless soliton transmission over 13 Mm (Bell Labs.)1992 Fourth-generation commercial systems (amplifiers+WDM)1995 Repeaterless (fiber amplifiers) trans-oceanic cable systems (5 Gbit/s)1997 Commercial WDM systems2001 1Tb/s OTDM transmision over 70 km (NTT)2003 10 Tb/s over 10 Mm



Progress in Lightwave communication (1)



A multi-disciplinary technology

Drive circuits

Laser

Optical fiber Amplifier

Detector

Electromagnetic field theoryWave propagation

Semiconductor physicsQuantum electronics

Laser technology

Semiconductor physicsQuantum electronics

ElectronicsCircuit theory

ElectronicsCircuit theory

Communication theory, modulation theory



Undersea systems



Undersea systems (2)



WDM-OTDM

1,2...Npulse source

(x GHz)

data encoders(x Gbit/s each) timing control

O-DEMUX

clock

O-MUX

x Gbit/s

1,2...N

Wavelength-division-multiplexing(WDM)

Optical time-division-multiplexing (OTDM)

Optical fiber

DEMUX

MUX

Laser 1

Laser 2

Laser 3

Laser 4

Laser N

1

2

3

4

0

N

Detector 1

Detector 2

Detector 3

Detector 4

Detector N

1

2

3

4

N

1 2 3 4 N...

receiversNx Gbit/stransmission



Progress in Lightwave communication (2)optical channels

repeaterlessdistance

bitrate

256 carriers

1024 Gbit/s

research

in use

millionsof km's

10 Gbit/s

10.000 km

100 carriers



Direct detection digital and analog systems

Laser

Optical fiber

Detector

Laser

Optical fiber

Detector

Digital

Analog



Coherent fiber systems

Optical fiber

MUX

Laser 1

Laser 2

Laser 3

Laser 4

Laser N

f1

f2f3f4

fN

Demodulator

f1 f2 f3 f4 fN...

fLO

Detector

Amplitude, frequency, orphase modulation

Loca

l osc

illato

rla

ser

(fk - fLO)



Analog and digital signalsConversion techniques:

pulse-position modulation pulse-duration modulation pulse-code modulation (PCM) (absence/

presence of pulse)Binary PCM is, by far, the most used technique

Required bit-rate:

Df = analog signal bandwidth, M = number of quantized levelsB >> Df may seem as a disadvantage, [example: telephone Df = 3.1 kHz, B = 64 kbit/s]

BUT: SNR required in digital system ~ 25 dB (analog ~ 50 dB)

Transmitters/fibers more suitable for digital format (distortion, dispersion)

B (2f) log2(M)



Digital on-off keying

time

non-

retu

rn-to

-zer

o(N

RZ)-s

igna

l

0 1 10 1 1 0

time

retu

rn-to

-zer

o(R

Z)-s

igna

l

Ttpulse duration bit period

(bit-rate, B = 1/T)

NRZ (t = T): smaller bandwidth, clock extraction complicated RZ (t < T): used in some advanced systems

(solitons, all-optical time-division multiplexing)

NRZ

RZ (here = 0.5T)

0 0.5B B 1.5B 2B

0 0.5B B 1.5B 2B

frequency

frequency

Spectrum



Modulation formats

Optical carrier wave:

complex notation:

often simply written as:

OK for linear operation but not e.g. products: Re[X]. Re[Y] Re[X.Y]

EEE(t) = eA cos(0t + )

EEE(t) = eRe[Aej(0t+)]

EEE(t) = eAej(0t+)

Amplitude modulation (AM)Frequency modulation (FM)Phase modulation (PM)

amplitude-shift keying (ASK)frequency-shift keying (FSK)phase-shift keying (PSK)

Analog:

Digital:

Simplest technique: intensity-shift keying (or on-off keying) (OOK)



decibel (dB) expresses power ratios as Optical power generates a photo-current in a detector

(idet ~ Popt -> Pel ~ P2opt)

Therefore: dBopt dBel(3 dB optical power difference 6 dB electrical power difference)

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

1 mW=0 dBm, 2 mW= 3 dBm, 4mW=6 dBm, 8mW=9 dBm 10 mW= 10 dBm, 20 mW=13 dBm 100 mW=20dBm, 400 mW=26

The dB units

10 log10(P1P2

)

PdBm = 10 log10(PmW)



Our text-book uses the following definitions:

This means that various Fourier transformation rules may be different from other books, e.g.:

derivative:

frequency translation:

As a consequence of this, a travelling wave (in the positive z-direction) is described by:

where b is a propagation constant.

The Fourier transform

t jw

e jWt f(t) F[w + W]

E(z,t) = Re [E0e j(bz wt)]

E() =

+

E(t)ejtdtE(t) =1

2pi

+

E()ejtd



Fiber basics

core cladding protective coating

2a2b

n1 n2

Condition for waveguiding: n1 > n2A finite number of modes can propagate in the fiber.Modes are solutions to Maxwell's equations + boundary conditions.

One mode ^ single-mode fiberSeveral modes ^ multi-mode fiber

Most commonly used fiber material is silica (SiO2).

To change index of refraction dopants are added:

refra

ctiv

e in

dex

dopant addition [mol %]

1.44

1.46

1.48

5 10 15 200

F

GeO2

B2O3

Examples: GeO2 - SiO2 core / SiO2 claddingSiO2 core / B2O3 - SiO2 cladding

10 mm 125 mm



Fiber typesMulti-mode step-index fibers: Large core radius ^ Easy

to launch power, LEDs can be used

Intermodal dispersion reduces the fiber bandwidth

Multi-mode graded-index fibers:

Reduced intermodal dispersion gives higher bandwidth

Single-mode step-index fibers:

No intermodal dispersion gives highest bandwidth

Small core radius ^ difficult to launch power, lasers are used

n

r

2a: 5-12 mm2b: 125 mm

n

ra b

n2n1

2a: 50-200 mm2b: 125-400 mm

n

r2a: 50-100 mm2b: 125-140 mm



Ray-optics description of step-index fiber (1)cladding, n2

core, n1

unguided ray

qiqr guided ray

n0(normally = 1)

f

Apply Snell's law at the input interface:n0 sin(qi) = n1 sin(qr)

For total internal reflection at the core/cladding interface we havea critical, minimum, angle:

n1 sin(fc) = n2 sin(90) ^ sin(fc) = n2/n1Relate to maximum entrance angle:

n0 sin(qi,max) = n1 sin(qr,max) = n1 sin(90-fc) =

n1 cos(fc) = n1[1 - sin2(fc)] = (n1

2 - n22)

n2 = n1(1-D) where D is the index difference = (n1- n2)/n1



The numerical aperture, NA, is a measure of the light gathering power of an optical system, originating from microscopy.

For fibers it is defined as

Numerical apereture

NA = n0 sin i,max =

n21 n22 n1

2



Pulse broadening from intermodal dispersioncladding, n2

core, n1qi,max

fcfastest ray path

slowest ray path

t

d(t)

t

DT

DT = n1/c [Lslow - Lfast] = n1/c [L/sin(fc) - L] = L n1/c [n1/n2 - 1] = L n12/(n2c)D

If we assume that the maximum bit-rate (B) is limited by a maximum allowed pulse broadening equal to the bit-period : TB = 1/B >DT

we find: B L < (n2c)/(n12D)



Pulse broadening from intermodal dispersion, cntd.

Example:

Silica core without cladding (air): n1 = 1.5, n2= 1^ BL < 4.108 [bits/(s m)] = 0.4 Mbit/(s km)

A large index-step gives small bandwidth !!!

Typical communication fiber: D 0.5% ^ BL < 40 Mbit/s.km

These are, however, conservative estimates since all rays are treated equally!

A wave-optics treatment will give better performance.

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