Know More About WDM System

A WDM system uses a multiplexer at the transmitter to join the several signals together, and a de-multiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer.

Followings are some required equipment to build a WDM system.


Multiplexer is a device that multiplex a number of optical signals at different wavelengths onto a single optical fiber.


Contrary to multiplexer, de-multiplexer is used to de-multiplex or split hybrid optical signals at different wavelengths and makes them transmitted at their own wavelengths on different optical fibers.


OADM stands for Optical Add-Drop Multiplexer which is a device used in WDM system. Its function is to selectively transmit and receive wavelengths on some channels without affecting the transmission on other channels.


FWDM stands for Filter-Based Wavelength Division Multiplexer which is a kind of multiplexer based on the Thin Film Filter (TFF) technology. FWDM can combine or separate light at different wavelengths in a wide wavelength range. It is commonly used in Erbium-Doped Fiber Amplifier, Romam amplifier and WDM optical network.

Compact WDM

As the name suggests, it is a kind of multi-channel WDM product. It is relatively small in size, so it is able to provide more channels in the application of FOSC (Fiber Optic Splice Closure), splice tray or splice holder. Compact WDM adopts free-space multiple bounce technology which refers to light is reflected directly from one filter element to the next filter, instead of parallel into the optical fiber. In addition, using bend insensitive fiber can combine individual TFFs with a multi-channel product.

Banded Skip Filter

Banded skip filter is applied to BWDM (Bandpass Wavelength Division Multiplexer). This type filter has wide pass band which contains multiple channels. For example, DWDM Red/ Blue C-band Filter is used to combine or separate Red and Blue band wavelength signals in C-band DWDM system and high-power amplification system. It works like ordinary FWDA, with the only difference that the wavelengths are combined in WDM system while separated in Red/Blue Filter.

Ports on WDM Equipment

ports on wdm equipment

Common Port

Common port is the connection point of multiple channels in WDM device. For multiplex product, multiple channels are transmitted from the common port; while for de-multiplex product, multiple channels are received at the common port.

Express or Upgrade Port

For CWDM product, there usually is either express port or upgrade port, but both of them will not exit on the same one WDM product. The express port or upgrade port on CWDM multiplexer or de-multiplexer is used to add, drop or pass through new channels. Those new channels can cascade two CWDM multiplexers or de-multiplexers, so the channel capacity is doubled in the original fiber optic link.

For DWDM product, the function of express port is to add, drop or pass through C-band DWDM channels which are not used, referring to channels whose band is between 1530 nm and 1565 nm. If the DWDM product has an upgrade port, then the express port is usually used for new channels outside C-band, such as most CWDM channels.

1310 nm Port

1310 nm port is a kind of port with wide band which is used for adding other special CWDM wavelengths. For example, when the eight-channel channel uses wavelengths between 1470 nm to 1610 nm, it may need the 1310 nm port at the same time. The 1310 nm port is usually used in some traditional networks and sometimes as a return path. If an existing traditional network is using 1310 nm port, all the optical fibers have already been fully occupied, and ways to increase network capacity are wanted, then other CWDM wavelengths can be added into the fibers while allowing the use of 1310 nm. What’s more, 1310 nm port can support LR optics, LX optics.

1550 nm Port

It is similar to 1310 nm port and can transmit traditional 1550 nm optical signals. Also, it can support ER optics, ZR optics, LX optics, ZX optics and so on.

Monitor Port

Monitor port is used to monitor or test the power signal which is multiplexed by CWDM but not de-multiplexed. Monitor port is usually connected with testing or monitoring device, such as power meters or network analyzers. If the signal changes or fails to to be transmitted, the network administrator can use monitor port to detect fault without interrupting the existing network.


The traffic volume of telecommunication networks is rapidly increasing, and this trend will clearly continue into the next century. Therefore, it is important to build network systems which can be easily upgraded to cope with increases in traffic volume. Because WDM system has the merits of high-capacity, high-speed, low-cost and good-upgradability, it will be a fundamental system in future networks.

How Much Do You Know About WDM Technology?

WDM stands for Wavelength Division Multiplexing. It is a technology which combines two or more kinds of optical signals at different wavelengths and transmits them on one optical fiber. It is well known that white light consists of multiple colors of light, so a beam of white light passing through a prism creates a rainbow. WDM technology is much like this. It separates light with all the colors in the spectrum and transmits them on one optical fiber. Light at different wavelengths carries different signals and does not interfere with each other during the transmission. The following figure may help to learn about WDM.


Two Types of WDM


CWDM stands for Coarse Wavelength Division Multiplexing. It is a special technology defined by the ITU (International Telecommunication Union) in ITU-T G.694.2 spectral grids. It uses the wavelengths from 1270 nm to 1610 nm with a channel spacing of 20 nm. CWDM is suitable for use in metropolitan applications and cable television networks.


DWDM stands for Dense Wavelength Division Multiplexing. It is also a special technology defined by the ITU in ITU-T G.694.1 frequency grid. DWDM has a reference frequency fixed at 193.10 THz, with channel spacing varied from 12.5 GHz to 200 GHz, and a channel spacing of 100 GHz is common. In practical application, DWDM frequency is usually converted to wavelength. DWDM can transmit at most 80 channels (wavelengths) in the Conventional band (C-band) spectrum, and all 80 channels can transmit at the wavelength of 1550 nm at the same time.

There is a figure showing the comparison between CWDM and DWDM.


Fiber Optic Transmission in WDM System

Single Fiber Bidirectional Transmission

Single fiber bidirectional transmission refers to bidirectional communication on one strand of fiber. It utilizes two sets of same wavelengths for bidirectional transmission on one optical fiber. In single fiber bidirectional transmission system, each channel can realize bidirectional transmission.

Dual Fiber One-direction Transmission

Dual fiber refers to communication on two fiber. One fiber is used for communication on transmitted direction, and the other fiber is used for communication on received direction. In dual fiber one-direction transmission system, the same wavelength is usually used for both transmitted direction and received direction. In redundant system, the second fiber can be used as a backup fiber or it can provide an optical path in the opposite direction.


Upstream & Downstream

The transmission direction of signals can be expressed in upstream or downstream. The upstream direction refers to that the communication is sent from the service user to the service supplier, while the downstream direction is in the opposite direction.

The Topology Used in WDM system

Network Topology

By using multiple channels on optical fiber, the products of WDM bring higher efficiency to fiber optical network. An entire network usually consists of several different kinds of sub-network topologies. The network is invisible, but it can be identified by fiber cabling or topology. Sometimes, particular WDM products will be used in some networks with topology, such as Mesh, Ring, P2P (Point-to-Point), and P2MP (Point-to-Multipoint). Therefore, it is necessary to know about the type of network when choosing WDM product.

Ring Topology

In metropolitan area networks, the infrastructures are usually structured with ring topology. The network with ring topology is a closed loop consisting of a series of optical fiber spans. And those spans are terminated at the nodes in the loop. Each node is merely connected with two adjacent nodes through the optical fiber span. Ring network usually adopts dual fiber bidirectional system.



In network topology, node is the termination of single branch or multiple branches of the network. WDM network is composed of a set of nodes which are connected through optical fiber (physical topology). After establishing the light link between the nodes, a logical topology cover the entire network. Using WDM technology in optical fiber can make one node become several serving area which can expand the customer base and available bandwidth.


WDM Technology brings unprecedented increasing of bandwidth capacity and it becomes an ideal solution for more bandwidth and lower cost in modern telecommunication network. Therefore, some basic terminologies above are useful.