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.
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.
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).
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.
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.
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.
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 called 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 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 receive 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.
Transponders are generally used in WDM systems (2.5 to 40 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+, XFP to XFP, etc.).
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 transponder accepts input in the form of a standard single-mode or multimode laser pulse. The input can come from different physical media and different protocols and traffic types.
2. The wavelength of the transponder input signal is mapped to a DWDM wavelength.
3. DWDM wavelengths from the transponder are multiplexed with signals from the direct interface to form a composite optical signal which is launched into the fiber.
4. A post-amplifier (booster amplifier) boosts the strength of the optical signal as it leaves the multiplexer.
5. An OADM is used at a remote location to drop and add bitstreams of a specific wavelength.
6. Additional optical amplifiers can be used along the fiber span (in-line amplifier) as needed.
7. A pre-amplifier boosts the signal before it enters the demuliplexer.
8. The incoming signal is demultiplexed into individual DWDM wavelengths.
9. The individual DWDM lambdas are either mapped to the required output type through the transponder or they are passed directly to client-side equipment.
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.