Dispersion compensation module
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Transcript of Dispersion compensation module
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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.
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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)
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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.
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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.
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CD COEFFICIENTS FOR VARIOUS FIBER TYPES
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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.
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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
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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.
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Consequences of Dispersion
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Dispersation VS Wavelength
WHERE Dmat is Material Dspersation Dwg is Waveguide Dispersation
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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
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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.
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DISPERSION COMPENSATION
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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.
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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
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DISPERSION COMPENSATION MODULE(DCM)
DCM IN
DCM OUT
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DISPERSION COMPENSATION MODULE(DCM)
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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)
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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