24.01.2006 lecture 3 1 optical fiber basics-part 2 prof. manoj kumar dept. of electronics and...
TRANSCRIPT
24.01.2006 Lecture 31
Optical Fiber Basics-Part 2
Prof. Manoj Kumar
Dept. of Electronics and Communication Engineering
DAVIET Jalandhar
24.01.2006 Lecture 32
Single-Mode Step Index Fiber
The Core diameter is 8 to 9m
All the multiple-mode or multimode effects are eliminated
However, pulse spreading remains
Bandwidth range 100GHz-Km
24.01.2006 Lecture 33
Typical Core and Cladding Diameters (m)
24.01.2006 Lecture 34
Multiple OFC
24.01.2006 Lecture 35
Standard Optical Core Size
•The standard telecommunications core sizes in use today are:8.3 µm (single-mode), 50-62.5 µm (multimode)
24.01.2006 Lecture 36
How a light ray enters an optical fiber
24.01.2006 Lecture 37
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
24.01.2006 Lecture 38
Light Guidance in Optical Fiber
24.01.2006 Lecture 39
Low-order and high-order modes
24.01.2006 Lecture 310
PROPERTIES OF OPTICAL FIBER TRANSMISSION
24.01.2006 Lecture 311
Fiber Loss & Dispersion
Fiber Loss
- 0.35 dB/Km at 1.3m
- 0.2 dB/Km at 1.5m
- Minimum Reduction Expected in future is 0.01dB/Km
Fiber Dispersion
-Material dispersion
- Waveguide Dispersion
- Multimode group Delay Dispersion
24.01.2006 Lecture 312
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 = d/d
24.01.2006 Lecture 313
Cause of Fiber Dispersion
Material Dispersion
Types of Dispersion
Multimode Dispersion
WaveguideDispersion
•- 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.
24.01.2006 Lecture 314
Dispersion Curves
24.01.2006 Lecture 315
24.01.2006 Lecture 316
24.01.2006 Lecture 317
Dispersion in Optical FibersThere 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
24.01.2006 Lecture 318
Chromatic DispersionThe 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
24.01.2006 Lecture 319
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
24.01.2006 Lecture 320
Pulse Spreading due to Dispersion
24.01.2006 Lecture 321
Pulse Spreading
time
Pulse from zero-order mode
Pulse from highest-order mode
Pulses from other modes
Resulting pulse
T
T
T
T
T
24.01.2006 Lecture 322
Calculation of Pulse Spread
C C
x
y/2 y/2
Cyx cos
24.01.2006 Lecture 323
Dispersion Management: ProblemChromatic Dispersion (CD)
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 ratePhysics 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
Bit 1 Bit 2 Bit 1 Bit 2Bit 1 Bit 2Bit 1 Bit 2 Bit 1 Bit 2
24.01.2006 Lecture 324
Pulse Spreading due to Dispersion
z=0 z=L
Dispersion
24.01.2006 Lecture 325
Dispersion Curves
24.01.2006 Lecture 326
Dispersion Management: Problem Fiber Dispersion Characteristic
Dis
per
sio
n C
oef
fici
ent
ps/
nm
-km
17
0
1310 nm 1550nm
Normal Single Mode Fiber (SMF) >95% of Deployed Plant
Dispersion Shifted Fiber (DSF)
24.01.2006 Lecture 327
Dispersion Management: Problem
Increasing the Bit Rate
Higher Bit Rates experience higher signal degradation due to Chromatic Dispersion:
OA10Gb/s Dispersion
16 Times GreaterDispersion
16 Times Greater
Dispersion Scales as (Bit Rate)2
Time Slot
OA2.5Gb/s DispersionDispersion
1)
24.01.2006 Lecture 328
Dispersion Management: SolutionDirect vs. External Modulation
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
Electrical Signal in
Direct Modulation External Modulation
Iin
Optical Signal out
Electrical Signal inDC Iin
Mod. Optical Signal
Unmodulated Optical Signal
External Modulator
24.01.2006 Lecture 329
Dispersion Management: LimitationChromatic 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): 17ps/nm*km @1.55μm
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 (*)
@1.55μm and 17ps/nm*km
24.01.2006 Lecture 330
Dispersive propertiesAnomalous dispersion: 2 < 0 or D > 0
— short wavelength components (blue) travel faster than long wavelength components (red)
Normal dispersion: 2 > 0 or D < 0
— long wavelength components (red) travel faster than short wavelength components (blue)
24.01.2006 Lecture 331
Dispersion Management: Solution Dispersion Compensation
Note: f = c/
24.01.2006 Lecture 332
Chromatic Dispersion in Optical Fiber
A high-speed pulse contains a spectrum of l components
24.01.2006 Lecture 333
Explaining Material Dispersion
24.01.2006 Lecture 334
Chromatic Dispersion Definitions
24.01.2006 Lecture 335
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
24.01.2006 Lecture 336
Why Require Dispersion
Compensation ?
24.01.2006 Lecture 337
Dispersion Compensating Fiber (DCF) Application
24.01.2006 Lecture 338
Thanks