What are coherent optics?

It’s a web-scale world. On-demand content, bandwidth-hungry mobile apps, high-definition video streaming, and new cloud-based IT applications are driving massive scale and unpredictable traffic patterns. Network capacities are increasing by 25 to 50 percent every year, and systems running at 10 Gb/s just cannot keep up with this kind of rapid scalability.

Coherent optics solves the capacity problems network providers are facing. It takes the typical ones and zeroes in a digital signal—the blinking on and off of the light in the fiber—and uses sophisticated technology to modulate the amplitude and phase of that light and send the signal across each of two polarizations. This, in turn, imparts considerably more information onto the light speeding through a fiber optic cable.

Coherent optics provides the performance and flexibility to transport significantly more information on the same fiber.

Coherent optical technology forms the foundation of the industry’s drive to achieve transport speeds of 100G and beyond, delivering Terabits of information across a single fiber pair. Digital signal processors electronically compensate for Chromatic and Polarization Mode Dispersion (CD and PMD) to enable robust performance over old and new fibers alike, and eliminate the need for dispersion-sloped compensating modules from the photonic line. Coherent optics enables greater network flexibility and programmability by supporting different baud rates and modulation formats. This results in greater flexibility in line rates, with scalability from 100G to 400G and beyond per single signal carrier, delivering increased data throughput at a lower cost per bit.

Advanced coherent optical technology has a number of key attributes, including:

• High-gain soft-decision Forward Error Correction (FEC), which enables signals to traverse longer distances while requiring fewer regenerator points. It provides more margin, allowing higher bit-rate signals to traverse farther distances. This results in simpler photonic lines, less equipment, and lower costs—while, of course, increasing bandwidth significantly.
• Spectral shaping, which provides greater capacity across cascaded Reconfigurable Optical Add-Drop Multiplexers (ROADMs), resulting in increased spectral efficiency for super channels. Spectral shaping is critical in flexible grid systems because it allows carriers to be squeezed closer together to maximize capacity.
• Programmability, which means the technology can be tailored for a wide variety of networks and applications and the same card can support multiple modulation formats and/or different baud rates, enabling operators to choose from a variety of line rates. Fully programmable coherent transceivers provide a wide range of tunability options with fine granularity between incremental capacities, enabling network operators to make use of all available capacity and convert excess margin into revenue-generating services.
• Strong mitigation to dispersion, which offers better optical performance at higher bit-rates. Coherent processors must account for dispersion effects after the signal has been transmitted across the fiber, including compensating for CD and PMD. The advanced digital signal processors in coherent optics take away the headaches of planning dispersion maps and budgeting for PMD by mitigating these effects. Additionally, coherent processors improve tolerances for Polarization-Dependent Loss (PDL) and must rapidly track the State of Polarization (SOP) to avoid bit-errors due to cycle slips that would otherwise affect optical performance. As a result, operators can deploy line rates up to 400G per carrier across longer distances than ever; high bit-rate signals can even be deployed on old fiber that previously couldn’t support 10G.

Ciena pioneered coherent optics in 2008 and continues to push technology boundaries with its WaveLogic family of coherent technology platforms. The foundation of hundreds of networks today, WaveLogic 3 solutions enable higher fiber capacities via 150G and 200G wavelengths that fit within standard 50GHz spacing—effectively increasing line capacity and spectral efficiency up to a factor of two times. Recently announced, WaveLogic Ai delivers unprecedented levels of automation and intelligence to operators providing access to real-time link monitoring, and capacity tunability from 100G to 400G in 50G increments. WaveLogic Ai also builds on the best-in-class performance of its predecessor and delivers twice the capacity per channel with single-carrier 400G, three times the distance at equivalent capacity, and four times the service density, all at less than half the power.

High-bandwidth web-scale applications are a challenge and an opportunity for network providers. Solutions for coherent optics, like those from Ciena, provide new levels of automation, intelligence, and scale that help providers meet or exceed their business goals in the new web-scale world.