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.
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.
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 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.