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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]
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20032
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20033
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.
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20034
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20035
Introductury lecture
Contents: History Fiber basics Analog/digital communications Modulation formats Ray description of light propagation
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20036
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...
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20037
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20038
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 20039
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 !!!)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200310
Fiber manufacture
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200311
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200312
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200313
Progress in Lightwave communication (1)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200314
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200315
Undersea systems
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200316
Undersea systems (2)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200317
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200318
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200319
Direct detection digital and analog systems
Laser
Optical fiber
Detector
Laser
Optical fiber
Detector
Digital
Analog
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200320
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200321
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200322
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200323
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200324
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200325
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200326
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200327
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200328
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200329
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
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200330
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)
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Chalmers University of Technology / Photonics Lab
Fiber Optic Communication E4/F4 Lecture 1: Introduction, Ray Description, p. M. Karlsson, 18/3 200331
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.