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Optical add-drop multiplexer

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Not to be confused with add-drop multiplexer.
Optical add-drop multiplexer, using a fiber Bragg grating and two circulators.

An optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of a single mode fiber (SMF). This is a type of optical node, which is generally used for the construction of optical telecommunications networks. "Add" and "drop" here refer to the capability of the device to add one or more new wavelength channels to an existing multi-wavelength WDM signal, and/or to drop (remove) one or more channels, passing those signals to another network path. An OADM may be considered to be a specific type of optical cross-connect.

A traditional OADM consists of three stages: an optical demultiplexer, an optical multiplexer, and between them a method of reconfiguring the paths between the demultiplexer, the multiplexer and a set of ports for adding and dropping signals. The demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a fiber patch panel or by optical switches which direct the wavelengths to the multiplexer or to drop ports. The multiplexer multiplexes the wavelength channels that are to continue on from demultiplexer ports with those from the add ports, onto a single output fiber.

All the light paths that directly pass an OADM are termed cut-through lightpaths, while those that are added or dropped at the OADM node are termed added/dropped lightpaths. An OADM with remotely reconfigurable optical switches (for example 1×2) in the middle stage is called a reconfigurable OADM (ROADM). Ones without this feature are known as fixed OADMs. While the term OADM applies to both types, it is often used interchangeably with ROADM.

Physically, there are several ways to realize an OADM. There are a variety of demultiplexer and multiplexer technologies including thin film filters, fiber Bragg gratings with optical circulators, free space grating devices and integrated planar arrayed waveguide gratings. The switching or reconfiguration functions range from the manual fiber patch panel to a variety of switching technologies including microelectromechanical systems (MEMS), liquid crystal and thermo-optic switches in planar waveguide circuits.

Although both have add/drop functionality, OADMs are distinct from add-drop multiplexers. The former function in the photonic domain under wavelength-division multiplexing, while the latter are implicitly considered to function in the traditional SONET/SDH networks.

Types of OADM

Fixed Optical Add-Drop Multiplexers (FOADM) Fixed Optical Add-Drop Multiplexers (FOADMs) were originally developed to improve the delivery of "express" traffic through networks, without requiring expensive OEO regeneration. FOADMs use fixed filters that add/drop a selected wavelength "band" and pass the rest of the wavelengths through the node. Static wavelength-filtering technology eliminates the cost and attenuation to demultiplex all DWDM signals in a signal path. The solution is called FOADM because the wavelength(s) added and dropped are fixed at the time of add/drop filter installation on the optical path through a node. No additional filters can be added without interrupting express wavelengths traveling through the node. Reconfigurable Optical Add-Drop Multiplexers (ROADM) Reconfigurable Optical Add Drop Multiplexers (ROADMs) were developed to provide 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.

The ROADMs reconfiguration functions are achieved using various switching technologies including liquid crystal, thermo optic , micro electromechanical systems (MEMS)and tunable optical filter technology.ROADM were implemented in many ways such as Fixed point ROADM. It is similar to fixed point OADMs though and has tunable filters. Fixed point ROADMs employ wavelength blocker (WB) devices with high insertion loss and integrated photonic light wave circuit (PLC). Wavelength selective ROADMs use flexible filters, wavelength selective switches (WSS) and optical cross-connects. Study illustrated that ROADM are a significant network element in the next generation all optical networks for the applications in both long-haul and metro systems. It further enhances the communication networks efficiency as well as dynamic connectivity in the optical networks with added network survivability. It uses field-proven technology and components, contributing a reliable and low cost solution due to switching.

See also

References

  • "OADM". Optical Network. Retrieved 2006-08-07.
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