A DWDM system generally consists of five components: Optical Transmitters/Receivers, DWDM Mux/DeMux Filters, Optical Add/Drop Multiplexers (OADMs), Optical Amplifiers, Transponders (Wavelength Converters).
Optical Transmitters/Receivers
Transmitters are described as DWDM components since they provide the source signals which are then multiplexed. The characteristics of optical transmitters used in DWDM systems is highly important to system design. Multiple optical transmitters are used as the light sources in a DWDM system. Incoming electrical data bits (0 or 1) trigger the modulation of a light stream (e.g., a flash of light = 1, the absence of light = 0). Lasers create pulses of light. Each light pulse has an exact wavelength (lambda) expressed in nanometers (nm). In an optical-carrier-based system, a stream of digital information is sent to a physical layer device, whose output is a light source (an LED or a laser) that interfaces a fiber optic cable. This device converts the incoming digital signal from electrical (electrons) to optical (photons) form (electrical to optical conversion, E-O). Electrical ones and zeroes trigger a light source that flashes (e.g., light = 1, little or no light =0) light into the core of an optical fiber. E-O conversion is non-traffic affecting. The format of the underlying digital signal is unchanged. Pulses of light propagate across the optical fiber by way of total internal reflection. At the receiving end, another optical sensor (photodiode) detects light pulses and converts the incoming optical signal back to electrical form. A pair of fibers usually connects any two devices (one transmit fiber, one receive fiber).
DWDM systems require very precise wavelengths of light to operate without interchannel distortion or crosstalk. Several individual lasers are typically used to create the individual channels of a DWDM system. Each laser operates at a slightly different wavelength. Modern systems operate with 200, 100, and 50-GHz spacing. Newer systems support 25-GHz spacing and 12.5-GHz spacing is being investigated. Generally, DWDM transceivers (DWDM SFP, DWDM SFP+, DWDM XFP, etc.) operating at 100 and 50 GHz can be found on the market nowadays.
DWDM Mux/DeMux Filters
Multiple wavelengths (all within the 1550 nm band) created by multiple transmitters and operating on different fibers are combined onto one fiber by way of an optical filter (Mux filter). The output signal of an optical multiplexer is referred to as a composite signal. At the receiving end, an optical drop filter (DeMux filter) separates all of the individual wavelengths of the composite signal out to individual fibers. The individual fibers pass the demultiplexed wavelengths to as many optical receivers. Typically, Mux and DeMux (transmit and receive) components are contained in a single enclosure. Optical Mux/DeMux devices can be passive. Component signals are multiplexed and demultiplexed optically, not electronically, therefore no external power source is required. The figure below is bidirectional DWDM operation. N light pulses of N different wavelengths carried by N different fibers are combined by a DWDM Mux. The N signals are multiplexed onto a pair of optical fiber. A DWDM DeMux receives the composite signal and separates each of the N component signals and passes each to a fiber. The transmitted and receive signal arrows represent client-side equipment. This requires the use of a pair of optical fibers; one for transmit, one for receive.
Bi-Directional DWDM Mux/DeMux Operation
Optical Add/Drop Multiplexers
Optical add/drop multiplexers (i.e. OADMs) have a different function of “Add/Drop”, compared with Mux/DeMuxfilters. Here is a figure that shows the operation of a 1-channel OADM. This OADM is designed to only add or drop optical signals with a particular wavelength. From left to right, an incoming composite signal is broken into two components, drop and pass-through. The OADM drops only the red optical signal stream. The dropped signal stream is passed to the receiver of a client device. The remaining optical signals that pass through the OADM are multiplexed with a new add signal stream. The OADM adds a new red optical signal stream, which operates at the same wavelength as the dropped signal. The new optical signal stream is combined with the pass-through signals to form a new composite signal.
1-Channel DWDM OADM Operation
OADM designed for operating at DWDM wavelengths are called DWDM OADM, while operating at CWDM wavelengths are called CWDM OADM. Both of them can be found on the market now.
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