Brief introduction of DWDM Technology and DWDM System Components

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Telecommunications
makes wide use of optical techniques in which the carrier wave belongs to the
classical optical domain. The wave modulation allows transmission of analog or
digital signals up to a few gigahertz (GHz) or gigabits per second (Gbps) on a
carrier of very high frequency, typically 186 to 196 THz. In fact, the bitrate
can be increased further, using several carrier waves that are propagating
without significant interaction on a single fiber. It is obvious that each
frequency corresponds to a different wavelength. Dense Wavelength Division
Multiplexing (DWDM) is reserved for very close frequency spacing. This blog
covers an introduction to DWDM technology and DWDM system components. The
operation of each component is discussed individually and the whole structure
of a fundamental DWDM system is shown at the end of this blog.

 

Introduction
to DWDM Technology

DWDM
technology is an extension of optical networking. DWDM devices (multiplexer, or
Mux for short) combine the output from several optical transmitters for
transmission across a single optical fiber. At the receiving end, another DWDM
device (demultiplexer, or Demux for short) separates the combined optical
signals and passes each channel to an optical receiver. Only one optical fiber
is used between DWDM devices (per transmission direction). Instead of requiring
one optical fiber per transmitter and receiver pair, DWDM allows several
optical channels to occupy a single fiber optic cable. As shown below, by
adopting high-quality AAWG Gaussian technology, FS DWDM Mux/Demux provides low
insertion loss (3.5dB typical), and high reliability. With the upgraded structure,
these DWDM
multiplexers and demultiplexers can offer easier installation.

 

A
key advantage of DWDM is that it’s protocol and bitrate independent. DWDM-based
networks can transmit data in IP, ATM, SONET, SDH and Ethernet. Therefore,
DWDM-based networks can carry different types of traffic at different speeds
over an optical channel. Voice transmission, email, video and multimedia data
are just some examples of services that can be simultaneously transmitted in
DWDM systems. DWDM systems have channels at wavelengths spaced with 0.4nm or
0.8nm spacing.

 

DWDM
is a type of Frequency Division Multiplexing (FDM). A fundamental property of
light states that individual light waves of different wavelengths may coexist
independently within a medium. Lasers are capable of creating pulses of light
with a very precise wavelength. Each individual wavelength of light can
represent a different channel of information. By combining light pulses of
different wavelengths, many channels can be transmitted across a single fiber
simultaneously. Fiber optic systems use light signals within the infrared band
(1mm to 750nm wavelength) of the electromagnetic spectrum. Frequencies of light
in the optical range of the electromagnetic spectrum are usually identified by
their wavelength, although frequency (distance between lambdas) provides a more
specific identification.

 

DWDM
System Components

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
connect 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 that support 25-GHz spacing and 12.5-GHz spacing
are 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.