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