A WDM signal contains channels of multiple wavelengths, and these channels are multiplexed and transmitted in an optical fiber, and each wavelength channel carries an independent service flow.

Assuming that only one wavelength of optical signal is transmitted in an optical fiber, the cost is expensive relative to the cost of the optical fiber. If optical signals of multiple wavelengths are transmitted in one optical fiber, and these optical signals do not interfere with each other, this will not only save costs but also increase the transmission bandwidth. At present, WDM technology transmits optical signals of multiple wavelengths in one optical fiber. The optical signal of each wavelength represents a channel, and multiple channels transmit data simultaneously in an optical fiber, which is multiplexing of an optical fiber.

OADM realizes the functions of traditional electrical SDH add-drop multiplexing in the time domain in the optical domain, and it is transparent and can handle signals of any format and rate, which is better than the electrical ADM (add-drop multiplexing) used in SDH networks. device) is superior.

The interpretation of sub-insertion here means top and bottom.

Adding a channel means adding a channel of a new wavelength to the optical signal entering the optical add-drop multiplexer, and multiplexing it into the optical fiber together with other channels.
The drop means that in the optical signal entering the optical add-drop multiplexer, the channel of one wavelength is removed, and other irrelevant channels directly pass through the optical add-drop multiplexer. Note: The downlink channel is directly transferred to the device for business processing, which does not mean truncation.

The main function of OADM is to drop or insert one or more wavelengths from multi-wavelength channels, and there are two types: fixed type and reconfigurable type.

The fixed type can only add and drop one or more fixed wavelengths, and the routing of nodes is determined; it lacks flexibility, but has reliable performance and low delay.
The reconfigurable type can dynamically adjust the wavelength of the upper and lower channels of the OADM node, and can realize the dynamic reconfiguration of the optical network, so that the wavelength resources of the network can be well allocated. The OADM of the whole fiber optic structure can also be constructed with the fiber optic technology.
But no matter what kind of structure OADM adopts, the basic requirements are the same (insertion loss should be small, isolation between channels should be high, insensitive to ambient temperature changes and polarization, and can tolerate wavelength drift and signal source within a certain range. jitter). The OADM must be able to ensure that the power between the transmitted channels is basically consistent during the process of adding and dropping calls. The operation of OADM should strive to be simple and convenient, and can achieve a high performance-price ratio.

An optical add-drop multiplexer is a communication device in the network, and a communication device also represents a network node. Some of the services transmitted from the network need to "get off" at the network node and enter the network node for subsequent processing, and some directly pass through the network node, and there are also services in the network node that also need to "get on the vehicle" to be sent to the network here go. Therefore, the role of the optical add-drop multiplexer is to separate the channels that need to be dropped, add the channels that need to be added, and pass the unrelated channels directly.