1. Optical Fiber Basics-Part 2
Prof. Manoj Kumar
Dept. of Electronics and Communication Engineering
DAVIET Jalandhar
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2. 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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3. Typical Core and Cladding Diameters (mm)
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4. Multiple OFC
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5. Standard Optical Core Size
The standard telecommunications core sizes in use today are:
8.3 µm (single-mode),
µm (multimode)
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6. How a light ray enters an optical fiber
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7. 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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8. Light Guidance in Optical Fiber
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9. Low-order and high-order modes
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10. PROPERTIES OF OPTICAL FIBER TRANSMISSION
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11. Fiber Loss & Dispersion
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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12. 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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13. 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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14. Dispersion Curves
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15. Lecture 3

16. Lecture 3

17. 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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18. 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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19. 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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20. Pulse Spreading due to Dispersion
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21. Pulse Spreading T Pulse from zero-order mode T T
Pulses from other modes T
Pulse from highest-order mode T
Resulting pulse
time
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22. Calculation of Pulse Spread
y/2
y/2 C C x
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23. 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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basica di prova - autore Andreoli 1

24. Pulse Spreading due to Dispersion
z=0
z=L
Dispersion
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25. Dispersion Curves
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26. Dispersion Management: Problem Fiber Dispersion Characteristic
Normal Single Mode Fiber
(SMF) >95% of Deployed Plant 17
Dispersion Shifted Fiber (DSF)
Dispersion Coefficient ps/nm-km l
1310 nm
1550nm
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27. Dispersion Management: Problem Increasing the Bit Rate
Higher Bit Rates experience higher signal degradation due to Chromatic Dispersion:
Time Slot OA
2.5Gb/s
Dispersion 1) OA
10Gb/s
Dispersion
16 Times Greater
Dispersion Scales as (Bit Rate)2
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28. Dispersion Management: Solution Direct vs. External Modulation
Direct Modulation
External Modulation
Electrical Signal in
Iin
DC Iin
Electrical Signal in
Unmodulated Optical Signal
Mod. Optical Signal
Optical Signal out
External Modulator
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 higher dispersion
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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29. 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 (1)
D dispersion coefficient (ps/km-nm):
 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 ~ Gb/s (*)
LD ~ 61 10Gb/s (*)
@1.55μm and 17ps/nm*km
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30. 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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31. Dispersion Management: Solution Dispersion Compensation
Note: f = c/
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32. Chromatic Dispersion in Optical Fiber
A high-speed pulse contains a spectrum of l components
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33. Explaining Material Dispersion
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34. Chromatic Dispersion Definitions
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35. 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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36. Why Require Dispersion Compensation ?
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37. Dispersion Compensating Fiber (DCF) Application
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38. Thanks
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