Diagram of WDM wavesEarly fiber-optic  transmission systems put information onto strands of glass through simple  pulses of light. A light was flashed on and off to represent digital ones and  zeros. The actual light could be of almost any wavelength—from roughly 670  nanometers to 1550 nanometers. Wavelength Division Multiplexing, or WDM, is a technique in  fiber-optic transmission that uses multiple light wavelengths to send data over  the same medium.

During the 1980s, fiber-optic data communications modems  used low-cost LEDs to put near-infrared pulses onto low-cost fiber. As the need  for information increased, so did the need for bandwidth. Early SONET systems  used 1310 nanometer lasers to deliver 155 Mb/s data streams over very long  distances.

But this capacity was  quickly exhausted. Over time, advances in optoelectronic components allowed the  design of systems that simultaneously transmitted multiple wavelengths of light  over a single fiber, significantly increasing fiber capacity. Thus, WDM was  born. Multiple high-bit-rate data streams of 10 Gb/s, 40 Gb/s, 100 Gb/s, 200  Gb/s and more recently, 400 Gb/s and 800 Gb/s, each carrying distinct throughputs,  can be multiplexed over a single fiber.

Diagram of the flow of data in an optical coupler

There are two types of WDM today:

  • Coarse       WDM (CWDM):  CWDM is defined by WDM systems with fewer than eight       active wavelengths per fiber. CWDM is used for short-range communications,       so it employs wide-range frequencies with wavelengths that are spread far       apart. Standardized channel spacing permits room for wavelength drift as       lasers heat up and cool down during operation. CWDM is a compact and       cost-effective option when spectral efficiency is not an important       requirement.

With DWDM, vendors have found various techniques for  cramming 40, 88, or 96 wavelengths of fixed spacing into the C-band spectrum of  a fiber. Traditional DWDM line systems use Wavelength Selective Switches (WSS)  designed with fixed 50GHz or 100GHz filters. These fixed-grid line systems can accommodate  channels from early generations of coherent transponders whose wavelengths require  less than 50GHz or 100GHz of spectrum (depending on the filter used). Today, networks  with high- bandwidth applications and sustained bandwidth growth that are  quickly facing capacity exhaustion are turning to C+L-band  solutions, which also leverage the L-band spectrum of a fiber to  potentially double the fiber capacity.

Diagram of an electromagnetic spectrum

As optical networks evolve to meet today’s ever-increasing  bandwidth demands, so has the dependence on next-generation programmable coherent  technology to maximize fiber capacity and lower the cost per bit of transport.  To fully take advantage of these benefits requires a flexible-grid line system  that can accommodate these higher-baud channels, such as an 800G wavelength,  that require more than 100GHz of spectrum.

WDM is a technique in fiber optic transmission for using multiple light wavelengths to send data over the same medium.

In fact, today’s next-generation coherent modems are so  intelligent and programmable that the modem considers a greater variety of  constellation and baud options, enabling extremely granular tunability. Today,  flexible channel plans are possible, enabling anything from 64 x 75GHz channels  or 40-45 channels for higher, 800G line rates—leveraging a flexible grid (or  gridless) architecture that supports channels with a minimum size of 37.5GHz, with  adjustable increments of 6.25GHz—to accommodate any channel available today or  in the future.

Diagram of flex grid network showing channel spacing

When boosted by Erbium Doped-Fiber Amplifiers (EDFAs) and  Raman amplification—two performance-enhancing technologies for high-speed  communications—the reach of these DWDM systems can be extended to work over  thousands of kilometers. For robust operation of a system with densely packed  channels, high-precision filters are required to peel away a specific  wavelength without interfering with neighboring wavelengths. DWDM systems must  also use precision lasers that operate at a constant temperature to keep  channels on target.

One of the best features of deploying DWDM over a flexible  grid photonic line system is signal independence—the ability to support  multiple generations of transponders independent of format, bit rate, symbol  rate, etc. As such, many networks designed for 10 and 40 Gb/s are now carrying  200 Gb/s channels, and many that were deployed with flexible grid capability  are now carrying 400 Gb/s and even 800 Gb/s signals!

Ciena offers the full breadth DWDM solutions to address  customer requirements, from the edge to the core, over a flexible range of platforms.  Ciena’s 6500 Family, Waveserver Family, and Routing and Switching portfolio of 51xx and 81xx platforms leverage programmable WaveLogic  coherent technology across integrated hardware modules, as well as  pluggable coherent optics.

As an example, Ciena’s popular 6500 Packet-Optical  Platform leverages the latest technology innovation to deliver new levels  of scale, flexibility, and programmability across three comprehensive  networking layers for customizable service delivery over any distance. Built  for efficient network scaling from the access to the backbone core, the 6500  provides technology-leading programmable infrastructure that enables the  software control, automation, and intelligence required for a more adaptive  network. It offers the full gamut of CWDM and DWDM solutions across a fully agile,  instrumented photonic system, including support for flexible grid CDC ROADMs, with  DWDM solutions ranging from 10 Gb/s to 800 Gb/s.

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Helen Xenos talking
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