With the evolution of single wavelength point-to-point transmission lines to wavelength division multiplexed optical networks, wavelength selective OADMs have been introduced. OADM is the acronym of optical add-drop multiplexer considered to be a specific type of optical cross-connect, and widely used in wavelength-division multiplexing (WDM) systems for multiplexing and routing different channels of light into or out of a single-mode fiber. It is a type of optical node, which is generally used for the formation and the construction of optical telecommunications networks. “Add” refers to the capability of the device to add one or more new wavelength channels to an existing multi-wavelength WDM signals, while “drop” refers to drop one or more channels, passing those signals to another network path. This article will make an overview of OADM to help you understand it better.
A traditional OADM consists of MUX (multiplexer), DEMUX (demultiplexer) and between them a method of reconfiguring the paths between the MUX, DEMUX and a set of ports for adding and dropping signals. The MUX multiplexes the wavelength channels that are to continue on from DEMUX ports with those from the add ports, onto a single output fiber, while the DEMUX separates wavelengths in an input fiber onto ports as shown in the following picture. The reconfiguration can be achieved by a fiber patch panel or by optical switches which direct the wavelengths to the MUX or to drop ports.
There are main two types of OADMs that can be used in WDM networks—fixed optical add-drop multiplexer (FOADM) and reconfigurable optical add-drop multiplexer (ROADM). The former one is used to drop or add data signals on dedicated WDM channels, while the latter one is used to electronically alter the selected channel routing through the optical network.
The FOADM is a traditional wavelength arrangement scheme that can only input or output a single wavelength via the fixed port. It uses a filter to select a dropping wavelength and a multiplexer to add a new channel at the same wavelength. Normally FOADM is built with thin-film filters (TFFs), fiber bragg grating (FBG), and integrated planar arrayed waveguide grating (AWG). The operational cost is high for FOADM because of the stationary nature of the arrangements and requirement of manual changes.
The ROADM is a dynamic wavelength arrangement scheme that allows for dynamic wavelength arrangement scheme using a wavelength selective switch (WSS). The WSS provides an eight-dimensional cross-connect and enables quick services start-up, remote cross-connect and WDM mesh networking. Different from FOADM, ROADM provides flexibility in rerouting optical streams, bypassing faulty connections, allowing minimal service disruption and the ability to adapt or upgrade the optical network to different WDM technologies. ROADM functionality originally appeared in long-haul DWDM equipment, but by 2005, it began to appear in metro optical system because of the need to build out major metropolitan networks in order to deal with the traffic driven by the increasing demand for packet-based services.
Dual single-channel CWDM OADM (shown in the picture below) allows you to add/drop two channels of the same wavelength into the two direction of an optical ring, while the rest wavelengths are passed through the OADM. Dual fiber is used for both the network and the CWDM GBIC connections. Eight versions of this OADM are available, one for each wavelength of light. The dual single-channel OADMs are color coded and match the color coding of the CWDM.
Dual four-channel DWDM OADM (see in the following image) multiplexes up to four channels on fiber pair. It is available in standalone 19” Rack Mount, LGX module and field module packing. The four pair of add/drop channels with 100 GHz channel spacing into two-fiber paths, going in opposite directions into a network (east/west).
OADM is an essential element compatible with both LAN (local area network) and long haul networks. It enables carriers to exploit fiber bandwidth, improves network reliability and reduces the cost of providing broadband services. A further development of OADM is absolutely the future trend in fiber optic communications.