WHAT IS Dispersion

• Dispersion is a phenomenon related to the variation in velocity of different frequencies (wavelengths) or different modes.

• The velocity of different frequencies can be different due to intrinsic properties of the medium or due to dispersive nature of the bound structure like the optical fiber.

• Dispersion is measured in ps/km.

CAUSES OF DISPERSION

• IT’S DEPENDANCE OF THE PHASE VELOCITY IN A MEDIUM ON THE OPTICAL FREQUENCY OR THE PROPAGATION MODE. IT CAN BE….

– CHROMATIC DISPERSION (CD)

– POLARIZATION MODE DISPERSION (PMD)

CHROMATIC DISPERSION

CD IS THE PHENOMENON OF DIFFERENT

WAVELENGTHS INSIDE AN OPTICAL SIGNAL

TRAVELLING AT DIFFERENT VELOCITIES ALONG A

FIBER AND ARRIVING AT RECIEVER DIFFERENT TIME

IN THE RECEIVER.

CD EFFECTS AND SOLUTIONS

EFFECTS • DECREASE OF PEAK POWER

• PULSE BROADENING

• BIT ERRORS

SOLUTIONS• USE OF DCM (DISPERSION COMPENSATION

MODULE)

• USE OF DCF (DISPERSION COMPENSATION FIBER)

WHICH

HAS NEGATIVE DISPERSION COEFFICIENT,IT CAN

COUNTERACT POSITIVE DISPERSION IN

TRANSMISSION.

CD COEFFICIENTS FOR VARIOUS FIBER TYPES

POLARIZATION MODE DISPERSION (PMD)

THE EFFECT OF DIFFERENT POLARIZATION MODES

(VERTICAL AND HORIZONTAL) OF A SIGNAL

STATISTICALLY TRAVELLING AT DIFFERENT VELOCITIES

DUE TO FIBER IMPERFACTIONS.

PMD EFFECTS AND SOLUTIONS

EFFECTS • DECREASE OF PEAK POWER

• PULSE BROADENING

• BIT ERRORS

SOLUTIONS• CAREFUL FIBER LAYING ( NO STRESS )

• USE OF NEW FIBERS WITH LOW PMD VALUES

• EXACT FIBER GEOMETRY

Consequences of Dispersion

• Due to dispersion when the optical pulses travel along the fiber they broaden as shown in Fig.

• Due to broadening, slowly they start overlapping with each other.

• Consequently, the AC content of the signal reduces and the DC content of the signal increases.

• After certain distance the pulses barely remain distinguishable. If the signal propagates further the pulses lose their identity and the data is lost.

Consequences of Dispersion

 

                      

                                                                                                                                                                                                                                                                                                                                                         

Dispersation VS Wavelength

WHERE Dmat is Material Dspersation Dwg is Waveguide Dispersation

What is Dispersion Compensation

• The control of the overall chromatic dispersion of a system by adding optical elements with a suitable amount of dispersion is called Dispersion Compensation.

• The module used for Dispersion Compensation is called DCM

PRINCIPLE OF DCM

• It uses dispersion-compensating fiber, which has negative chromatic dispersion several times larger than the positive chromatic dispersion of single-mode transmission fiber.

• Adding a length of dispersion compensating fiber offsets the transmission fiber's dispersion, giving low overall dispersion.

DISPERSION COMPENSATION

Types OF DCMDCM's are usually one of two types.

• DCF or Dispersion Compensating Fiber This is simply a spool of a special type of fiber that has

very large negative dispersion. Typically DCF dispersion can be in the range of -80 ps/(nm∙km), so a 20 km length of DCF can compensate for the dispersion in a 100 km length of NDSF.

• FBG (Fiber Bragg Grating) type Here, a series of FBG's or one very long FBG is written

into a tens of meter length of fiber to perform the dispersion compensation.

Both of these types of DCM's have relatively high insertion loss. A 60

km compensator may exhibit 6 dB of loss or more. Because of this, DCM's are usually co-located with EDFA's.

DCF Vs FBG

Technology Advantage Disadvantage

DCF 1) Simple construction, highly reliable.

2) Provides continuous compensation over a wide range of optical wavelengths (i.e. Does not require precise laser wavelengths.)

1) Usually DCF has a small core size which may make it prone to certain types of nonlinearities

FBG 1) Potentially lower insertion loss. 1) Less prone to nonlinear effects

DISPERSION COMPENSATION MODULE(DCM)

DCM IN

DCM OUT

DISPERSION COMPENSATION MODULE(DCM)

Type Band Application for… Distance

DCM-A C Band G.652 fiber 20Km

DCM-B C Band G.652 fiber 40Km

DCM-C C Band G.652 fiber 60Km

DCM-D C Band G.652 fiber 80Km

Types Of Dispersion Compensation Module (DCM)

DCM-B DCM-B

DCM-B

DCM-B

DCM-A

DCM-D

DCM-CDCM

-D

DCM-C

DCM-C17.5dB

38km

24.5dB20.4dB16.0dB 24.8dB

66km49.4km32.0km 67km

G652 G652G652G652G652

A DWDM LINK WITH APPROPRIATE DISPERSION COMPENSATION