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A Brief Introduction to ROADM Technology

Date Submission: October 14, 2016 By Date Submission

It’s known to all that optical multiplexing is the key function of a WDM (wavelength division multiplexing) network and reliable way for data transport networks. Configurations in WDM networks, such as optical add-drop multiplexers (OADM), also provide flexibility and simplicity for data transmission. As one kind of OADM, ROADM (Reconfigurable Optical Add-Drop Multiplexers) is produced to achieve huge bandwidth for data transport at minimum expense. Now this article is to give a short overview of ROADM technology.
 

Basics of ROADM

As we all know, OADM is a device used in WDM systems for adding one or more wavelengths to exist signal, and dropping one or more wavelengths, passing signals to another network path. ROADM (reconfigurable optical add-drop multiplexer), one kind of OADM, is a device that can add, block, pass or redirect modulated infrared (IR) and visible light beams of various wavelengths in a fiber optic network. ROADM is also used in WDM systems. A laser tuned to a particular wavelength is able to electronically change the selected channel routing through the optical network. It can provide flexibility in rerouting optical streams, bypassing faulty connections, decreasing service disruption and adapting or upgrading the network to different WDM technologies.

Fixed OADM is based on simple optical principles. It’s simple to install and requires no power or maintenance to operate. The defect is that once it’s installed, it can’t be easily changed without disrupting the entire network. Compared with fixed OADM, ROADM has more flexibilities. ROADM can connect several locations flexibly and create a network that is easily modified as network requirements change. Without any doubt, ROADM is more expensive due to its advantages. But it requires power and additional space to operate.
 

Two Important Technologies Deployed in ROADM
Wavelength Selective Switches (WSS)

Wavelength selective switches (WSS) are the heart of the ROADM. The WSS is an active component that performs the actual wavelength switching and monitoring. It can dynamically route, block and attenuate wavelengths within a network node. And the output patterns can be changed or reassigned to several output fibers through electrical interface. In order to vary the fiber connectivity between the transceiver and networks access ports for a given direction, directionless extra WSSs were deployed.

wavelength-selective-switches

Optical Cross-Connect

An optical cross-connect (OXC) is a network device used to switch high-speed optical signals in a fiber optic network, such as an optical mesh network. It is one of the key network elements in the optical layer and is often employed for wavelength cross-connect switching and important for the mesh networks. Generally, OXC is able to operate without having to convert to electrical and back again. It is usually used in long-distance networks.

optical-cross-connect

The Evolution of ROADM Technology
First Generation

The first generation ROADM employed a demux-switch-mux approach to add/drop/pass-through, a forced approach compared to today's ROADMs. The original ROADMs were complex, large and expensive. The main purpose is to solve fiber exhaust problems, which were caused by inflexibility issues in long haul networks and metro. All wavelengths are demultiplexed from the input fiber. And then each is passed to the destination, while added clients are switched onto outgoing destination wavelengths. Every add/drop wavelength must be hardwired to specific client ports. The first generation ROADMs have high loss and polarization dependent loss. This causes spectral narrowing and requires high quality DWDM transceivers.

first-generation

Second Generation

The difference between the first and second generation is that second one applies wavelength blocker technology, which makes it better than the first generation. A tap (a splitter and filter tray) is used to drop any number of selected wavelengths. All wavelengths enter the blocker and the pass-through wavelengths are not blocked. At last, a combiner is used to add wavelengths into its tap to enter the output fiber.

2nd-generation

A wavelength blocker ROADM can equalize, attenuate, add/or block any or all wavelengths. Compared with the first generation, this ROADM is more efficient since it has low dispersion and low power dissipation. Pass-through wavelengths do not need regeneration. Wavelength blockers are primarily used in long haul, regional, and metro core WDM networks.

Third Generation

The basic element of the third generation ROADM is the wavelength selective switch (WSS). As mentioned above, the WSS is used to switch and monitor the wavelength. With WSS technology, users can dynamically drop any wavelength to any port and then change drop capacity as needed. The advanced WSS component provides a flexible channel plan together with dynamic per-channel power equalization, which is the best wavelength solution for core metro and edge networks.

3rd-generation

Summary

As the bandwidth requirements grow rapidly, optical network managers face continued challenges of modifying the current optical networks to meet this growing demand. But the ROADM offers a easy and flexible solution for them, which makes ROADM popular in a WDM network.

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