•DWDM is a core technology in an optical transport network.The essential components can be classified by their place in the system as follows:
•
-Transmit Side - a.Lasers with stable,precise wavelengths
-Link Side - a. Optical Fiber with high transmission and low loss
performance
b. Optical amplifiers to boost signal on longer spans
-Receive Side - a. Photo detectors and optical demultiplexers using thin film filters or diffractive elements.
b. Optical add/drop Multiplexers and
optical cross- connect components.
Light Emitting Devices
I. Light Emitting Devices:
Light emitters are transmit side devices that convert electrical signals to pulse signals
1.Light Emitting Diodes
-Used in Multimode fiber communications
-Suitable for use of speeds less than 1Gbps
-Exhibit a relatively wide spectrum width
-Transmit light in a wide cone
2.Laser Diodes/Semiconductor Lasers
-Used in Single Mode fiber applications
-Have precise wavelengths
-Narrow spectrum width
-Sufficient power and control of chirp(change in frequency of a signal over time)
-2 types widely in use:
a.Monolithic Fabry –Perot Lasers
b.Distributed Feedback Lasers
Light Detectors
II .Light Detectors:
They are receive side opto electronic devices that convert light pulses into electrical signals
Two types of photo detectors are widely deployed:
1.Positive Intrinsic Negative(PIN):
-Light is absorbed and photons are converted to electrons in a 1-1 relationship.
2.Avalanche Photodiode(APD):
-One photon acting on the gain provided by amplification process releases many electrons
APD’s have higher receive sensitivity and accuracy
PIN are less expensive and more reliable
Optical Amplifiers
III.Optical Amplifiers :
Due to attenuation ,there are limits to how long a fiber segment can propagate a signal with integrity before it has to be regenerated.Before the arrival of OA’s,there had to be a repeater for every signal transmitted.
•Also used to boost signal power after multiplexing or before demultiplexing,both of which introduce loss into the system
•Performs 3R on the signals-reshape,retime,retransmit
•Using OA’s signals can travel upto 120Km (74mi) before regeneration.
•They are available in the C-band(1310nm) and L-band(1625nm)
•Signal to noise ratio is the ultimate limiting factor in the number of amplifiers that can be concatenated .Hence,low noise is a requirement as noise along with signal is amplified
•
•Erbium Doped Fiber Amplifier(EDFA)
-is a key enabling technology as it makes possible to carry the large loads that DWDM is capable of transmitting over long distances
•OA is a optical link that amplifies all the wavelengths at once and without optical-electrical-optical (OEO) conversion
-Capable of gains of 30dB or more and output power of +17dB or more
Multiplexers & Demultiplexers
IV a) Multiplexers:DWDM systems send signals from several sources over a single fiber.The multiplexer combines these incoming signals from the multiple fibers and converges them into one beam.
b) Demultiplexers:At the receiving end ,the system must be able to separate out the components of light so they can be discreetly detected.The demultiplexer performs this function by separating the received beam into it’s wavelength components and coupling them to individual fibers.
Unidirectional system Bi-directional system
Optical Add/Drop Multiplexers
V. OADM:
Between multiplexing and demultiplexing points in a DWDM system,there is an area in which multiple wavelengths exist.To remove or insert one or more wavelengths at some point along this span,OADM are used.
Selectively Removing and Adding Wavelengths
Types of OADM:
a.Thin Film Filters:This is a first generation fixed device that is physically configured to drop specific predetermined wavelengths while adding others
It is used in current metropolitan DWDM systems because of their low cost and stability
b.Tunable Fiber Gratings: This is a second generation device which is reconfigurable and capable of dynamically selecting which wavelengths are added and dropped
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