OFC Basics 2

7/29/2019 OFC Basics 2

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24.01.2006 Lecture 31

Optical Fiber Basics-Part 2

Prof. Manoj Kumar

Dept. of Electronics and Communication

Engineering

DAVIET Jalandhar


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24.01.2006 Lecture 32

Single-Mode Step Index

Fiber

The Core diameter is 8 to 9mm

All the multiple-mode or multimode

effects are eliminatedHowever, pulse spreading remains

Bandwidth range 100GHz-Km


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24.01.2006 Lecture 33

Typical Core and Cladding

Diameters (mm)


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24.01.2006 Lecture 34

Multiple OFC


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24.01.2006 Lecture 35

Standard Optical Core

Size

The standard telecommunications core sizes in

use today are:

8.3 m (single-mode),

50-62.5 m (multimode)


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24.01.2006 Lecture 36

How a light ray enters an

optical fiber


7/3824.01.2006 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


8/3824.01.2006 Lecture 38

Light Guidance in Optical

Fiber


9/3824.01.2006 Lecture 39

Low-order and high-order

modes


10/3824.01.2006 Lecture 310

PROPERTIES OF OPTICAL

FIBER TRANSMISSION


11/3824.01.2006 Lecture 311

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


12/3824.01.2006 Lecture 312

What is Group Velocity ?

Group Velocity (Vg) is Considered as thevelocity of energy propagating in thedirection of the axis of the guide fiber.

In order to convey intelligence;Modulation is done. When is done, thereare group velocities those must be

propagating along the fiber.The waves of different frequencies in thegroup will be transmitted with slightlydifferent velocities. Vg = dw/db.


13/3824.01.2006 Lecture 313

Cause of Fiber Dispersion

Material

Dispersion

Types of Dispersion

Multimode

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.


14/3824.01.2006 Lecture 314

Dispersion Curves


15/3824.01.2006 Lecture 315


16/3824.01.2006 Lecture 316


17/3824.01.2006 Lecture 317

Dispersion in Optical Fibers

There are two main types of dispersion that cause

pulse spreading in a fiber:

- Chromatic dispersion

- Inter-modal dispersionDispersion 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


18/3824.01.2006 Lecture 3

18

Chromatic Dispersion

The speed of light is dependent on therefractive index

c = c0/ n

where c0

is the speed of light in a vacuum

The index of refraction, n, varies with thelight transmission wavelength

All light sources (LEDs and LDs) have some

coloration, or variation, in wavelengthoutput

The low wavelength portion of the pulsetravels slower than the high wavelength one

creating pulse spreading


19/3824.01.2006 Lecture 3

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(continued)

Chromatic dispersion is measured in units oftime divided by distance and Tx sourcespectral width (ps/nm-km)

It is zero near 1310 nm in silica optical fibersIt is zero near 1550 nm in Dispersion Shiftedoptical fibers

Even at the dispersion zero, there is some

pulse spreading due to the spectral width ofthe light source


20/3824.01.2006 Lecture 3

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Pulse Spreading due to

Dispersion


21/3824.01.2006 Lecture 3

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Pulse Spreading

time

Pulse from zero-order mode

Pulse from highest-order mode

Pulses from other modes

Resulting pulse

T

T

T

T

T


22/3824.01.2006 Lecture 3

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Calculation of Pulse Spread

C

C

x

y/2 y/2

Cyx cos


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24.01.2006 Lecture 323

Dispersion Management: Problem

Chromatic Dispersion (CD)

The optical pulse tend to spread as it propagates down thefiber generating Inter-Symbol-Interference (ISI) andtherefore limiting either thebit rate or the maximum

achievable distance at a specific bit ratePhysics behind the effect

The refractive index has a wavelength dependent factor, so thedifferent frequency-components of the optical pulses are traveling atdifferent speeds

Bit 1 Bit 2Bit 1 Bit 2Bit 1 Bit 2

Bit 1 Bit 2Bit 1 Bit 2


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24.01.2006 Lecture 324

Pulse Spreading due to

Dispersion

z=0 z=L

Dispersion


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24.01.2006 Lecture 325

Dispersion Curves


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24.01.2006 Lecture 326


Fiber Dispersion Characteristic

l

Dispersio

nCoefficientps/nm-km

17

0

1310 nm 1550nm

Normal Single Mode Fiber

(SMF) >95% of Deployed Plant

Dispersion Shifted Fiber (DSF)


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24.01.2006 Lecture 327


Increasing the Bit Rate

Higher Bit Rates experience higher signal

degradation due to Chromatic Dispersion:

OA10Gb/s Dispersion

16 Times Greater

Dispersion Scales as (Bit Rate)2

Time Slot

OA2.5Gb/s Dispersion

1)


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24.01.2006 Lecture 328

Dispersion Management: Solution

Direct vs. External Modulation

Laser diodes bias current

is modulated with signal

input to producemodulated optical output

Approach isstraightforward and lowcost, but is susceptible tochirp (spectral broadening)

thus exposing the signal tohigher dispersion

The laser diodes bias current

is stable

Approach yields low chirp andbetter dispersion performance,but it is a more expensiveapproach

Electrical

Signal in

Direct Modulation External ModulationIin

Optical

Signal out

Electrical

Signal inDC Iin

Mod.

Optical

Signal

Unmodulated

Optical Signal

External

Modulator


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24.01.2006 Lecture 329

Dispersion Management: Limitation


CD places a limit on the maximum distance a signalcan be transmitted without electrical regeneration:

Fordirectly modulated (high chirp laser)

LD = 1/ B Dl (1)

D dispersion coefficient (ps/km-nm): 17ps/nm*km @1.55ml 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 (*)

-Forexternally modulated (very low chirp laserf~ 1.2B)

LD ~ 1000 km @ 2.5Gb/s (*)

-LD ~ 61 km @ 10Gb/s (*)@1.55m and 17ps/nm*km


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24.01.2006 Lecture 330

Dispersive propertiesAnomalous dispersion: b2 < 0 orD > 0

short wavelength components (blue) travel faster than longwavelength components (red)

Normal dispersion: b2 > 0 orD < 0 long wavelength components (red) travel faster than short

wavelength components (blue)



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24.01.2006 Lecture 331


Dispersion Compensation

Note: f = c/l


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24.01.2006 Lecture 332

Chromatic Dispersion in

Optical FiberA high-speed pulse contains a spectrum of l components


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24.01.2006 Lecture 333

Explaining Material Dispersion


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24.01.2006 Lecture 334


Definitions

Di i M t S l ti


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24.01.2006 Lecture 335


Dispersion Compensation (Cont.)

DispersionCompensating Fiber:

By joining fibers with CD ofopposite signs and suitablelengths an average dispersion

close to zero can be obtained;the compensating fiber can beseveral kilometers and the reelcan be inserted at any point inthe link, at the receiver or at thetransmitter

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

Wh R i Di i


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24.01.2006 Lecture 336

Why Require Dispersion

Compensation ?

Di i C ti


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24.01.2006 Lecture 337

Dispersion Compensating

Fiber (DCF) Application


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OFC Basics 2