Transcript Lectures 12&13

Telecommunications Networking
I
Lectures 12&13
Fiber Optics
Copyright 1998, S.D. Personick. All Rights Reserved.
Fiber Optics: Overview
• 1966 C. Kao et. al, propose that strands of
glass can be produced, which can carry light
over long distances (>2 km)
• 1970 First demonstration of a fiber with less
than 20dB/km of loss (Maurer, et.al., at
Corning)
• 1975-77 Experiments and field trials
• 1979 Real systems are placed in service
Copyright 1998, S.D. Personick. All Rights Reserved.
Basic Fiber Optic System
Digital pulses
(On/Off)
Light Source
Detector/Receiver
Glass fiber
Optical Transmitter
Digital pulses
Copyright 1998, S.D. Personick. All Rights Reserved.
The Radio Spectrum
•
•
•
•
•
•
•
AM Radio ~ 1 MHz (300 meter wavelength)
Television ~ 50-500 MHz
Digital cordless phone ~ 900 MHz
Wireless LAN ~ 2.5 - 5 GHz
DBS ~ 10 GHz (0.3 meter wavelength)
Visible light ~ 4-7.5 x 10**14 Hz (0.8-0.4 um)
Fiber optics ~ 0.9 - 1.55 um (not visible)
Copyright 1998, S.D. Personick. All Rights Reserved.
Optical Transmitter: example
~20 mA peak
2V
Light output
50 ohm resistor
Peak
Light Emitting Diode
Copyright 1998, S.D. Personick. All Rights Reserved.
Optical Transmitter Example
driver
bias
Laser
Light output
Data in
1V peak
current
Light output
Copyright 1998, S.D. Personick. All Rights Reserved.
current
Optical Transmitter: example
• Current = 20mA = .020A
• # electrons per second =
.020 A/[1.6 x 10**-19] Coulombs per electron = n
• # photons produced/second =n x [Quantum Efficiency]
• optical power out = n x QE x [~1.5 x 10**-19 Joules
per photon] ~ .020 QE x [1.5/1.6] (W) ~20 x [1.5/1.6]
x QE (mW)
• If QE~ 20%, then power out ~3.75 milliwatts
Copyright 1998, S.D. Personick. All Rights Reserved.
Optical Fiber
Optical Pulses
Optical Pulses
Fiber
Optical output pulses are attenuated and spread in time
compared to optical input pulses
Copyright 1998, S.D. Personick. All Rights Reserved.
Causes of Attenuation
• Light is absorbed by fiber impurities and the
principal fiber material itself
• Light is “scattered” out of the fiber because of the
inherently random density fluctuations of any
“glass” (Rayleigh scattering) as well as by more
macroscopic density fluctuations
• Typical long distance fiber: <0.5 dB per km
Typical plastic fiber: >100 dB per km
Copyright 1998, S.D. Personick. All Rights Reserved.
Causes of Pulse Spreading:
Modal Delay Spread
T (min) = nL/c, where c/n = speed of light in fiber
T (max) = T(min) x [1/cos(max angle that is captured)]
core
cladding
“Multimode” Fiber
c =300,000,000 m/s, n~1.5... n/c ~ 5ns/m
Copyright 1998, S.D. Personick. All Rights Reserved.
Modal Delay Spread
•The rays in the previous slide represent the
solutions of Maxwell’s equations…each of
which is called a “mode”
•If one actually solves Maxwell’s equations,
one finds a discrete set of modes, each correspond
-ing to a ray at a different angle relative to the axis
•The spacing between these allowed rays is  D
•If  D is large enough (e.g., 0.2 radians
corresponding to ~11.4 degrees, then only the axial
ray is below the critical angle.
Copyright 1998, S.D. Personick. All Rights Reserved.
Modal Delay Spread
• If only the axial ray is below the critical
angle, then there is only one solution to
Maxwell’s equations (one mode) which is
guided by the fiber.
• Such a fiber is called a single mode fiber
• With only one ray (mode) there is no modal
pulse spreading!
Copyright 1998, S.D. Personick. All Rights Reserved.
Dispersion: ps/km-nm
Causes of Pulse Spreading
Dispersion: a change in the delay
down the fiber as the wavelength
changes- ps/[nm-km]
Zero dispersion at ~1.3 um
Wavelength
Copyright 1998, S.D. Personick. All Rights Reserved.
Causes of Pulse Spreading
• Pulse spreading can be caused by the variation of
delay vs angle in multimode fibers (delay
spreading). Typical plastic multimode fiber: >100
ns/km
• Pulse spreading can also be caused by the
variation of delay with wavelength (“dispersion”).
Typical glass fiber with 900 nm LED source ~5
ns/km; with 1550 nm laser source < 0.1 ns/km
Copyright 1998, S.D. Personick. All Rights Reserved.
Pulse Spreading: Examples
• Multimode fiber: Maximum angle captured
in fiber is 0.2 radians (for example)~11.5
degrees. 1/cos(0.2 rad) = 1.020. Delay
spreading = 5 ns/m x 0.02 = 0.1 ns/m =
100 ns/km
• Single mode fiber +900 nm LED source:
Dispersion at 900 nm wavelength ~100
ps/nm-km. LED sprectral width ~50 nm.
Dispersion ~ 100 x 50 = 5000ps/km
=5ns/km Copyright 1998, S.D. Personick. All Rights Reserved.
Optical Receiver
Amplifier +
Regenerator
Output pulses
Photodiode
Copyright 1998, S.D. Personick. All Rights Reserved.
Optical Receiver
• The detector converts photons to electrons
~ 0.5 mA/mW (output current/input power)
• The amplifier amplifies the weak current
that is produced by the detector in a typical
optical fiber application
• The regenerator produces a new electrical
pulse stream (clock recovery, comparitor,
D-flip flop)
Copyright 1998, S.D. Personick. All Rights Reserved.
Causes of Errors in Optical Fiber
Systems
• Noise produced by the amplifier in the receiver
• “Quantum” noise associated with the detection
process
• Intersymbol interference due to pulse spreading
• Bottom line: In a typical fiber optic system, we
require ~20,000 received photons per pulse to
produce an error rate of 10**-9; assuming that
we don’t have a significant amount of
intersymbol interference (pulse spreading <0.5
pulse spacing)
Copyright 1998, S.D. Personick. All Rights Reserved.
Fiber Optic System: example
Digital pulses
(On/Off)
LightSource
source
Light
Detector/Receiver
Glass fiber
Optical Transmitter
Assume: Bit rate = 100Mbps; Optical transmitter
output = 1 mW; Coupling loss into fiber = 3dB;
Pulse spreading<0.1 ns/km; Fiber loss = 0.5 db/km;
Required optical energy per received pulse:
20,000 photons x 1.5 x 10**-19 J/photon
Copyright 1998, S.D. Personick. All Rights Reserved.
Digital pulses
Optical Fiber System: example
• Receiver requires 20,000 photons per
received optical pulse = 20,000 x 1.5 x
10**-19 J per pulse = 3 x 10**-15 J/pulse
• Bit (pulse) rate is 100Mbps; therefore the
average received power level must be
greater than 0.5 x (3 x 10**-15) x (10**8)=
1.5 x 10**-7 watts = 1.5 x 10**-4 mW~
-38.2 dBm
Copyright 1998, S.D. Personick. All Rights Reserved.
Fiber Optic System: example
• Transmitter average power into the fiber is: 1 mW
x 0.5 (coupling loss) x 0.5 (duty cycle) = 0.25 mW
• Allowable loss = 0.25/0.00015 = 1.66 x 10**3 ~
32.2 dB
• At 0.5 dB/km loss, we can allow ~64 km of fiber
• Checking the pulse spreading; we get: 64 km x 0.1
ns/km = 6.4 ns (0.64 x pulse spacing)
• Bottom line, if we allow up to 50 km of fiber, we
will be within the pulse spreading limit, and we
will have about 14 x 0.5 = 7 dB of margin w.r.t.
noise limited operation
Copyright 1998, S.D. Personick. All Rights Reserved.