Download Optical Fiber Basics

Optical Fiber Basics-Part 2
Prof. Manoj Kumar
Dept. of Electronics and Communication
Engineering
DAVIET Jalandhar
24.01.2006
Lecture 3
1
Single-Mode Step Index
Fiber
The Core diameter is 8 to 9mm
All the multiple-mode or multimode
effects are eliminated
However, pulse spreading remains
Bandwidth range 100GHz-Km
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Typical Core and Cladding
Diameters (mm)
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Multiple OFC
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Standard Optical Core
Size
•The standard telecommunications core sizes in
use today are:
8.3 µm (single-mode),
50-62.5 µm (multimode)
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How a light ray enters an
optical fiber
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Numerical Aperture (NA)
The numerical aperture (NA) is a
measurement of the ability of an optical
fiber to capture light. The NA is also
used to define the acceptance cone of
an optical fiber. OR Numerical aperture
(NA) determines the light accepting
ability of a fiber
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Light Guidance in Optical
Fiber
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Low-order and high-order
modes
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PROPERTIES OF OPTICAL
FIBER TRANSMISSION
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Fiber Loss & Dispersion
Fiber Loss
- 0.35 dB/Km at 1.3mm
- 0.2 dB/Km at 1.5mm
- Minimum Reduction Expected in future is
0.01dB/Km
Fiber Dispersion
-Material dispersion
- Waveguide Dispersion
- Multimode group Delay Dispersion
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What is Group Velocity ?
Group Velocity (Vg) is Considered as the
velocity of energy propagating in the
direction of the axis of the guide fiber.
In order to convey intelligence;
Modulation is done. When is done, there
are group velocities those must be
propagating along the fiber.
The waves of different frequencies in the
group will be transmitted with slightly
different velocities. Vg = dw/db.
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Cause of Fiber Dispersion
Types of Dispersion
Multimode
Dispersion
Material
Dispersion
Waveguide
Dispersion
• -
Multimode group delay/dispersion is the variation in group velocity among the
propagation modes at a single frequency
• - Material Dispersion is due to variation in the refractive index of the core material as
a function of wavelength.
• - Waveguide dispersion depends upon the fiber design. The propagation constant
which is the function of the ratio of fiber dimension (i.e. core radius) to the wavelength.
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Dispersion Curves
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Dispersion in Optical Fibers
There are two main types of dispersion that cause
pulse spreading in a fiber:
- Chromatic dispersion
- Inter-modal dispersion
Dispersion is typically measured as a time spread per
distance traveled (s/km)
Single-mode fiber has only one mode, so inter-modal
dispersion is not an issue
In multimode fiber, inter-modal dispersion is the
dominant cause of dispersion, but chromatic
dispersion can be important at 850 nm
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Chromatic Dispersion
The speed of light is dependent on the
refractive index
c = c0/ n
where c0 is the speed of light in a vacuum
The index of refraction, n, varies with the
light transmission wavelength
All light sources (LEDs and LDs) have some
coloration, or variation, in wavelength
output
The low wavelength portion of the pulse
travels slower than the high wavelength one
– creating pulse spreading
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Chromatic Dispersion
(continued)
Chromatic dispersion is measured in units of
time divided by distance and Tx source
spectral width (ps/nm-km)
It is zero near 1310 nm in silica optical fibers
It is zero near 1550 nm in Dispersion Shifted
optical fibers
Even at the dispersion zero, there is some
pulse spreading due to the spectral width of
the light source
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Pulse Spreading due to
Dispersion
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Pulse Spreading
T
Pulse from zero-order mode
T
T
Pulses from other modes
T
Pulse from highest-order mode
T
Resulting pulse
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time
21
Calculation of Pulse Spread
y/2
y/2


C
C
x
x  y cosC
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Dispersion Management: Problem
Chromatic Dispersion (CD)
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
Bit 1
Bit 2
The optical pulse tend to spread as it propagates down the
fiber generating Inter-Symbol-Interference (ISI) and
therefore limiting either the bit rate or the maximum
achievable distance at a specific bit rate
Physics behind the effect
The refractive index has a wavelength dependent factor, so the
different frequency-components of the optical pulses are traveling at
different speeds
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Pulse Spreading due to
Dispersion
z=0
z=L
Dispersion
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Dispersion Curves
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Dispersion Management: Problem
Fiber Dispersion Characteristic
Dispersion Coefficient ps/nm-km
Normal Single Mode Fiber
(SMF) >95% of Deployed Plant
17
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l
0
1310 nm
1550nm
Dispersion Shifted Fiber (DSF)
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Dispersion Management: Problem
Increasing the Bit Rate
Higher Bit Rates experience higher signal
degradation due to Chromatic Dispersion:
1)
Time Slot
2.5Gb/s
Dispersion
Dispersion
16 Times Greater
10Gb/s
OA
OA
Dispersion Scales as (Bit Rate)2
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Dispersion Management: Solution
Direct vs. External Modulation
Direct Modulation
Iin
External Modulation
DC Iin
Electrical
Signal in
Electrical
Signal in
Mod.
Unmodulated
Optical
Optical Signal
External Signal
Modulator
Optical
Signal out
Laser diode’s bias current
is modulated with signal
input to produce
modulated optical output
Approach is
straightforward and low
cost, but is susceptible to
chirp (spectral broadening)
thus exposing the signal to
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higher
The laser diode’s bias current
is stable
Approach yields low chirp and
better dispersion performance,
but it is a more expensive
approach
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Dispersion Management: Limitation
Chromatic Dispersion
CD places a limit on the maximum distance a signal
can be transmitted without electrical regeneration:
•For directly modulated (high chirp laser)
•
LD = 1/ B Dl (1)
•D dispersion coefficient (ps/km-nm): 17ps/nm*km @1.55μm
•l source line width or optical bandwidth (nm): 0.5nm
•B bit rate (1/T where T is the bit period): 2.5Gb/s
•
LD ~ 47 km (*)
-For externally modulated (very low chirp laser f ~ 1.2B )
LD ~ 1000 km @ 2.5Gb/s (*)
-LD ~ 61 km @ 10Gb/s (*)
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@1.55μm and 17ps/nm*km
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Dispersive properties
Anomalous dispersion: b2 < 0 or D > 0
— short wavelength components (blue) travel faster than long
wavelength components (red)
Normal dispersion: b2 > 0 or D < 0
— long wavelength components (red) travel faster than short
wavelength components (blue)
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Dispersion Management: Solution
Dispersion Compensation
Note: f = c/l
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Chromatic Dispersion in
Optical Fiber
A high-speed pulse contains a spectrum of l components
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Explaining Material Dispersion
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Chromatic Dispersion
Definitions
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Dispersion Management: Solution
Dispersion Compensation (Cont.)
Dispersion
Compensating Fiber:
By joining fibers with CD of
opposite signs and suitable
lengths an average dispersion
close to zero can be obtained;
the compensating fiber can be
several kilometers and the reel
can be inserted at any point in
the link, at the receiver or at the
transmitter
Note: Although the Total Dispersion Is Close to Zero, This Technique
Can Also Be Employed to Manage FWM and CPM Since at Every
Point We Have Dispersion Which Translates in Decoupling the
Different Channels Limiting the Mutual Interaction
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Why Require Dispersion
Compensation ?
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Dispersion Compensating
Fiber (DCF) Application
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Thanks
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