An Overview of DWDM Technology and DWDM System Components

Updated on Jul 23, 2021 by

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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 and is designed to maximize the capacity and efficiency of fiber-optic networks. It achieves this by allowing multiple data streams to be transmitted simultaneously over a single optical fiber using different wavelengths of light. DWDM is a subset of wavelength-division multiplexing (WDM) that typically uses the spectrum band within 1530nm and 1625nm, or more commonly the C-band and L-band, to input 40, 88, 96, or even 160 wavelengths, or channels, onto a single strand of fiber optic cable.

DWDM got its name from using tighter wavelength spacing (dense) to fit more channels, with each channel being only about 0.8nm wide. This is opposed to its WDM sibling, CWDM, which uses a wider range of frequencies, with each channel spread farther apart. With a CWDM channel being 20nm wide, the high number of channels available to DWDM means it can cram much more data into the 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: DWDM Mux/DeMux, DWDM Transceivers, Optical Add/Drop Multiplexers (OADMs), Optical Amplifiers, and Transponders (Wavelength Converters).

DWDM Mux/Demux

DWDM Mux/Demux, integral to Dense Wavelength Division Multiplexing (DWDM) systems, serves as a critical passive optical component facilitating the simultaneous transmission of multiple data channels over a single fiber. The Mux combines diverse wavelengths onto the fiber, adhering to the ITU grid for standardized channel spacing. At the receiving end, the Demux separates these wavelengths, enabling the independent processing of individual data streams. Its passive nature ensures low insertion loss and advanced configurations may include add-drop functionality for increased network flexibility. 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 a 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 fibers. A DWDM Demux receives the composite signal ~~and~~ separates each of the N component signals and passes each to a fiber. The transmitted and received signal arrows represent client-side equipment. This requires the use of a pair of optical fibers; one for transmission, one for receive.

Bi-Directional-DWDM-Operation

DWDM Transceivers

A DWDM (Dense Wavelength Division Multiplexing) transceiver is a crucial component in fiber-optic communication systems, enabling the simultaneous transmission and reception of multiple data signals at specific wavelengths over a single optical fiber. It comprises a transmitter converting electrical signals to designated wavelengths and a receiver detecting incoming signals at those wavelengths. DWDM transceiver leverages DWDM technology to multiplex different wavelengths or several optical signals onto a single fiber to save valuable fiber resources. The collaborative operation of DWDM transceivers and associated optical networking infrastructure enhances network efficiency, making it a key component for achieving high-capacity and bandwidth optimization in fiber-optic communication networks.

There are various types of DWDM transceivers supporting transmission rates from 1 Gbit/s to 400 Gbit/s. Categorized by data rate and form factor, there are DWDM SFP transceivers, DWDM SFP+ transceivers, DWDM SFP28 transceivers, DWDM QSFP28 transceivers, and so on.

Optical Add/Drop Multiplexers

Optical add/drop multiplexers (i.e. OADMs) have a different function of "Add/Drop", compared with Mux/Demux filters. Here is a figure that shows the operation of a 1-channel DWDM 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.

OADM designed for operating at DWDM wavelengths is called DWDM OADM while operating at CWDM wavelengths are called CWDM OADM. Both of them can be found on the market now.

Optical Amplifiers

Optical amplifiers boost the amplitude or add gain to optical signals passing on a fiber by directly stimulating the photons of the signal with extra energy. They are "in-fiber" devices. Optical amplifiers amplify optical signals across a broad range of wavelengths. This is very important for DWDM system applications. Erbium-Doped Fiber Amplifiers (EDFAs) are the most commonly used type of in-fiber optical amplifiers. EDFAs used in DWDM systems are sometimes called DWDM EDFA, compared to those used in CATV or SDH systems. To extend the transmission distance of your DWDM system, you can choose from different types of optical amplifiers, including DWDM EDFA, CATV EDFA, SDH EDFA, EYDFA, and Raman Amplifier etc. Here is a figure that shows the operation of a DWDM EDFA.

Transponders (Wavelengths Converters)/OEO

Transponders convert optical signals from one incoming wavelength to another outgoing wavelength suitable for DWDM applications. Transponders are Optical-Electrical-Optical (O-E-O) wavelength converters. A transponder performs an O-E-O operation to convert wavelengths of light, thus some people call them "OEO" for short. Within the DWDM system, a transponder converts the client optical signal back to an electrical signal (O-E) and then performs either 2R (Reamplify, Reshape) or 3R (Reamplify, Reshape, and Retime) functions. The figure below shows bi-directional transponder operation. A WDM transponder is located between a client device and a DWDM system. From left to right, the transponder receives an optical bit stream operating at one particular wavelength (1310 nm). The transponder converts the operating wavelength of the incoming bitstream to an ITU-compliant wavelength. It transmits its output into a DWDM system. On the receiving side (right to left), the process is reversed. The transponder receives an ITU-compliant bitstream and converts the signals back to the wavelength used by the client device.

O-E-O-Operation

Transponders are generally used in WDM systems (10 to 400 Gbps), including not only DWDM systems but also CWDM systems. And WDM transponders (OEO converters) can come with different module ports (SFP to SFP, SFP+ to SFP+, SFP28 to SFP28, QSFP28 to QSFP28, etc.).

How DWDM System Components Work Together with DWDM Technology

As a DWDM system is composed of these five components, how do they work together? The following steps give out the answer (also you can see the whole structure of a fundamental DWDM system in the figure below):

1. The data stream comes in and is input into the transponder.
2. The transponder maps the signal to a DWDM wavelength and sends it to the multiplexer (Mux) to consolidate the optical signal.
3. As the signal leaves the multiplexer, optical amplifiers boost the signal to allow the signal to travel over longer distances.
4. Along the way, optical add/drop multiplexers (OADM) can add and remove bitstreams of a specific wavelength. Also, additional amplifiers can be used to further boost the signal’s distance.
5. The signal arrives and gets de-multiplexed (DeMux) into individual DWDM wavelengths, which are then passed through the transponder to be converted into the corresponding signals to be routed to their final destination.

DWDM-System-Structure

Using DWDM technology, DWDM systems provide the bandwidth for large amounts of data. In fact, the capacity of DWDM systems is growing as technologies advance that allow closer spacing, and therefore higher numbers, of wavelengths. But DWDM is also moving beyond transport to become the basis of all-optical networking with wavelength provisioning and mesh-based protection. Switching at the photonic layer will enable this evolution, as will the routing protocols that allow light paths to traverse the network in much the same way as virtual circuits do today. With the development of technologies, DWDM systems may need more advanced components to exert greater advantages.