OADM (Optical Add Drop Multiplexer)

What Is OADM (Optical Add Drop Multiplexer)?

The full form of OADM is Optical Add-Drop Multiplexer. OADM is a crucial component in wavelength-division multiplexing (WDM) systems, responsible for routing different light channels into or out of a single-mode fiber (SMF). An OADM provides the capability to add or remove one or multiple wavelength channels from an existing multi-wavelength WDM signal. Here, "Add" refers to the ability of the device to introduce one or multiple wavelength channels to an existing WDM signal, while "Drop" refers to removing or dropping one or multiple channels and routing those signals to a different network path.

Different Types of OADM

OADMs are classified as FOADM (Fixed Optical Add-Drop Multiplexer) and ROADM (Reconfigurable Optical Add-Drop Multiplexer). In fixed-wavelength OADM, the wavelength has been selected and remains the same until human intervention changes it. In reconfigurable wavelength OADM, the wavelengths between the optical demultiplexer/multiplexer may be dynamically directed from the outputs of the demultiplexer to any of the inputs of the multiplexer.

Fixed Optical Add Drop Multiplexer

FOADM was initially designed to enhance the transmission of "express" traffic in networks by avoiding the expensive Optical-Electrical-Optical (OEO) regeneration process. FOADM uses fixed filters to add or drop selected wavelength bands while allowing the remaining wavelength signals to pass through the network node without being demultiplexed. This static wavelength filtering technique reduces the cost and signal attenuation associated with demultiplexing all Dense Wavelength Division Multiplexing (DWDM) signals. The term FOADM originates from its fixed wavelength add/drop mechanism, where the wavelengths involved are predetermined and cannot be dynamically adjusted. Once the filters are configured, adding additional filters is not possible without interrupting the "express" wavelengths passing through the node. Therefore, FOADM is best suited for networks with stable wavelength requirements where frequent reconfiguration is unnecessary.

Reconfigurable Optical Add Drop Multiplexer

ROADM was developed to provide flexibility in rerouting optical signals, bypassing faulty connections, minimizing service interruptions, and enabling the adjustment or upgrade of optical networks to different Wavelength Division Multiplexing (WDM) technologies. ROADM systems use Wavelength Selective Switches (WSS), offering 8-dimensional cross-connect functionality, which allows for rapid service provisioning, remote cross-connections, and support for WDM mesh network architectures. Unlike Fixed OADM, ROADM can add or drop individual wavelengths or wavelength groups through fixed ports without converting optical signals to electrical signals and back. It reconfigures signals directly at the optical layer without impacting other traffic. This flexibility makes ROADM well-suited for dynamic, high-reliability optical networks, particularly those requiring frequent wavelength adjustments, upgrades, or expansions.

Application of OADM

In conventional long-haul transmission systems, the primary focus has been on maximizing capacity and extending transmission distances. However, in metropolitan and access networks, the emphasis shifts towards minimizing costs and enhancing system flexibility. An Optical Add-Drop Multiplexer (OADM) addresses these demands effectively, supporting standard network topologies such as point-to-point and ring configurations. OADMs facilitate the selective insertion, removal, or routing of specific channels along the optical link, significantly enhancing network flexibility.

The primary application of OADMs is in Metropolitan Area Networks (MANs), where they provide high flexibility, easy upgrades, and signal amplification. As an ideal multi-service transport platform, OADMs support the multiplexing of wavelengths at different locations. In Wavelength-Division Multiplexing (WDM) systems, OADMs enable the demultiplexing (dropping) and remultiplexing (adding) of specific wavelengths on a fiber while allowing other wavelengths to pass unaffected. Additionally, OADMs are utilized in Optical Cross-Connects (OXCs), allowing different networks to connect to dynamic, on-demand wavelength resources and facilitating a broader range of network interconnections. OADMs and OXCs only need to download the information at the nodes to send to the person handling the equipment, including the ATM switchboard, SDH switchboard, IP router, etc., which greatly improves the efficiency of the node in processing information.