The WDM System

In fiber optic communications, Wavelength Division Multiplexing (WDM) is a technology that multiplexes a number of optical carrier signals into a single optical fiber by using different wavelengths of laser light. This technique allows bi-directional communications on a fiber strand, as well as capacity multiplication. A WDM system (Figure 1) uses a multiplexer at the transmitter to link the signals and a demultiplexer at the receiver to separate them. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an add-and-drop optical multiplexer. The concept was first published in 1978, and in 1980 WDM systems were being made in the laboratory. As a system concept, WDM forms include coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM).

The CWDM system
In simple terms, the CWDM team performs two functions: segregate light to ensure that only the desired combination of wavelengths are used, multiplexing and demultiplexing the signal over a single fiber link.

CWDM solutions typically provide the capacity of 8 wavelengths, separated by 20 nm, from 1470 nm to 1610 nm, allowing the transport of 8 client interfaces through the same fiber, as shown in the Figure 2. Additionally, CWDM has the ability to transport up to 16 channels (wavelengths) on the spectrum grid from 1270nm to 1610nm with a channel spacing of 20nm. Each channel can operate at 2.5, 4, or 10 Gbit / s. CWDM cannot be amplified since most channels are outside the operating window of the erbium doped fiber amplifier (EDFA) used in dense wavelength division multiplexing (DWDM) systems. This results in a shorter global range of approximately 100 kilometers. However, due to the larger channel spacing in CWDM, cheaper uncooled lasers are used, giving a cost advantage over DWDM systems.

CWDM proves to be the initial entry point for many organizations due to its lower cost. Each CWDM wavelength generally supports up to 2.5 Gbps and can be expanded to 10 Gbps. This transfer rate is sufficient to support GbE, Fast Ethernet or 1/2/4/8 / 10GFC, STM-1 / STM-4 / STM-16 / OC3 / OC12 / OC48, as well as other protocols.

CWDM is the technology of choice to cost-effectively transport large amounts of data traffic in telecommunications or business networks. Optical networks and especially the use of CWDM technology have proven to be the most cost-effective way to address this requirement.

In CWDM applications, a fiber pair (separate transmit and receive) is typically used to serve multiple users by assigning a specific wavelength to each subscriber. The process begins at the headend (HE) or center, or central office (CO), where individual signals at discrete wavelengths are multiplexed or combined into one fiber for downstream transmission. The multiplexing function is performed by means of a passive CWDM (Mux) multiplexer module that uses a sequence of specific wavelength filters. Filters are connected in series to combine the various specific wavelengths into a single fiber for transmission to the field. On the outside floor, a CWDM demultiplexer module (Demux), essentially a Mux mirror, is employed to extract each specific wavelength of the feeder fiber for distribution to individual FTTX applications.

CWDM is suitable for use in metropolitan applications, it is also used in cable television networks, where different wavelengths are used for downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example, the downstream signal could be at 1310 nm while the upstream signal is at 1550 nm. CWDM can also be used in conjunction with a fiber switch and a network interface device to combine multiple fiber lines from the switch through one fiber. CWDM is optimized for cost-conscious budgets in mind, with low-cost, low-power laser transmitters that enable deployments to closely match guaranteed revenue streams.

The DWDM system
DWDM stands for Dense Wavelength Division Multiplexing. Here “dense” means that the wavelength channels are very narrow and close to each other. DWDM uses the same transmission window but with a denser channel space. Channel plans vary, but a typical system would use 40 channels with a 100 GHz separation or 80 channels with a 50 GHz separation.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber, as shown in Figure 3. In effect, one fiber is transformed into multiple virtual fibers. So if you were to multiplex eight OC -48 signals into one fiber, you would increase the load capacity of that fiber from 2.5 Gb / s to 20 Gb / s. Currently, due to DWDM, individual fibers have been able to transmit data at rates of up to 400 Gb / s.

A basic DWDM system contains five main components: a DWDM terminal multiplexer, an intermediate line repeater, an optical add-and-drop multiplexer (OADM), a DWDM terminal demultiplexer, and an optical monitoring channel (OSC). A DWDM terminal multiplexer contains a wavelength conversion transponder for each data signal, an optical multiplexer, and an optical amplifier (EDFA). An intermediate line repeater is placed approximately every 80–100 km to compensate for the loss of optical power as the signal travels along the fiber. A drop-and-drop optical multiplexer is a remote amplification site that amplifies the multiple wavelength signal that may have traveled up to 140 km or more before reaching the remote site. A DWDM terminal demultiplexer consisting of an optical demultiplexer and one or more wavelength conversion transponders separates the multiple wavelength optical signal into individual data signals and outputs them on separate fibers for client layer systems (such as SONET / SDH). An Optical Monitoring Channel (OSC) is a data channel that uses an additional wavelength, usually outside the EDFA amplification band (at 1,510nm, 1,620nm, 1,310nm, or other proprietary wavelength).

DWDM is designed for long distance transmission where the wavelengths are close together and do not suffer from scattering and attenuation. When erbium doped fiber amplifiers (EDFAs), a kind of performance enhancer for high-speed communications, these systems can operate for thousands of kilometers. DWDM is widely used for the 1550nm band to take advantage of EDFA’s capabilities. EDFAs are commonly used for 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L band).

A key advantage of DWDM is its protocol and bit rate independence. DWDM-based networks can transmit data over IP, ATM, SONET / SDH and Ethernet, and handle bit rates between 100Mb / s and 2.5Gb / s. Therefore, DWDM-based networks can carry different types of traffic at different speeds through an optical channel. From a QOS standpoint, DWDM-based networks create a lower cost way to quickly respond to bandwidth demands and protocol changes from clients.

DWDM DeMultiplexer. Transmitters. 200 km. Each wavelength behaves as if it has it own virtual fibre Optical amplifiers needed to overcome losses in mux/demux and long fibre spans.

WDM, as multiplexing technology in the optical field, can form an optical layer network called a “fully optical network”, which will be the most advanced level of optical communications. It will be the future trend of optical communications to build an optical network layer based on WDM and OXC to eliminate the bottleneck of photoelectric conversion with a pure all-optical network. As the first and most important step in all-optical network communications, the application and practice of WDM is highly advantageous for developing the all-optical network and driving optical communications.

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