dispersion compensation module
DESCRIPTION
It describes DCM phenomenonTRANSCRIPT
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