Coherent Optics and 400G Applications

In today’s high-tech and data-driven environment, network operators face an increasing demand to support the ever-rising data traffic while keeping capital and operation expenditures in check. Incremental advancements in bandwidth component technology, coherent detection, and optical networking have seen the rise of coherent interfaces that allows for efficient control, lower cost, power, and footprint.

Below, we have discussed more about 400G, coherent optics, and how the two are transforming data communication and network infrastructures in a way that’s beneficial for clients and network service providers.

What is 400G?

400G is the latest generation of cloud infrastructure, which represents a fourfold increase in the maximum data-transfer speed over the current maximum standard of 100G. Besides being faster, 400G has more fiber lanes, which allows for better throughput (the quantity of data handled at a go). Therefore, data centers are shifting to 400G infrastructure to bring new user experiences with innovative services such as augmented reality, virtual gaming, VR, etc.

Simply put, data centers are like an expressway interchange that receives and directs information to various destinations, and 400G is an advancement to the interchange that adds more lanes and a higher speed limit. This not only makes 400G the go-to cloud infrastructure but also the next big thing in optical networks.


What is Coherent Optics?

Coherent optical transmission or coherent optics is a technique that uses a variation of the amplitude and phase or segment of light and transmission across two polarizations to transport significantly more information through a fiber optic cable. Coherent optics also provides faster bit rates, greater flexibility, modest photonic line systems, and advanced optical performance.

This technology forms the basis of the industry’s drive to embrace the network transfer speed of 100G and beyond while delivering terabits of data across one fiber pair. When appropriately implemented, coherent optics solve the capacity issues that network providers are experiencing. It also allows for increased scalability from 100 to 400G and beyond for every signal carrier. This delivers more data throughput at a relatively lower cost per bit.


Fundamentals of Coherent Optics Communication

Before we look at the main properties of coherent optics communication, let’s first understand the brief development of this data transmission technique. Ideally, fiber-optic systems came to market in the mid-1970s, and enormous progress has been realized since then. Subsequent technologies that followed sought to solve some of the major communication problems witnessed at the time, such as dispersion issues and high optical fiber losses.

And though coherent optical communication using heterodyne detection was proposed in 1970, it did not become popular because the IMDD scheme dominated the optical fiber communication systems. Fast-forward to the early 2000s, and the fifth-generation optical systems entered the market with one major focus – to make the WDM system spectrally efficient. This saw further advances through 2005, bringing to light digital-coherent technology & space-division multiplexing.

Now that you know a bit about the development of coherent optical technology, here are some of the critical attributes of this data transmission technology.

  • High-grain soft-decision FEC (forward error correction):This enables data/signals to traverse longer distances without the need for several subsequent regenerator points. The results are more margin, less equipment, simpler photonic lines, and reduced costs.
  • Strong mitigation to dispersion: Coherent processors accounts for dispersion effects once the signals have been transmitted across the fiber. The advanced digital signal processors also help avoid the headaches of planning dispersion maps & budgeting for polarization mode dispersion (PMD).
  • Programmability: This means the technology can be adjusted to suit a wide range of networks and applications. It also implies that one card can support different baud rates or multiple modulation formats, allowing operators to choose from various line rates.

The Rise of High-Performance 400G Coherent Pluggables

With 400G applications, two streams of pluggable coherent optics are emerging. The first is a CFP2-based solution with 1000+km reach capability, while the second is a QSFP DD ZR solution for Ethernet and DCI applications. These two streams come with measurement and test challenges in meeting rigorous technical specifications and guaranteeing painless integration and placement in an open network ecosystem.

When testing these 400G coherent optical transceivers and their sub-components, there’s a need to use test equipment capable of producing clean signals and analyzing them. The test equipment’s measurement bandwidth should also be more than 40-GHz. For dual-polarization in-phase and quadrature (IQ) signals, the stimulus and analysis sides need varying pulse shapes and modulation schemes on the four synchronized channels. This is achieved using instruments that are based on high-speed DAC (digital to analog converters) and ADC (analog to digital converters). Increasing test efficiency requires modern tools that provide an inclusive set of procedures, including interfaces that can work with automated algorithms.

Coherent Optics Interfaces and 400G Architectures

Supporting transport optics in form factors similar to client optics is crucial for network operators because it allows for simpler and cost-effective architectures. The recent industry trends toward open line systems also mean these transport optics can be plugged directly into the router without requiring an external transmission system.

Some network operators are also adopting 400G architectures, and with standardized, interoperable coherent interfaces, more deployments and use cases are coming to light. Beyond DCI, several application standards, such as Open ROADM and OpenZR+, now offer network operators increased performance and functionality without sacrificing interoperability between modules.

Article Source:Coherent Optics and 400G Applications

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WDM Technology

After languishing for many years as an interesting technology without a cost-effective application, wavelength-division multiplexing started playing a major role in telecommunications networks in the early 1990s, This resulted from the surge in demand for high-capacity links and the limitation of the installed fiber plant in handling high-rate optical signals over any substantial distance.

This limitation led to a rapid capacity exhaustion of long-haul fiber networks.
While installing an optical fiber cable plant is both expensive and extremely time consuming, expanding the capacity of an installed network is economically attractive. Tradition carries upgraded their link capacity by increasing the transmission rate. This worked well initially, with speeds eventually reaching 2.5 Gb/s. However, when going to the next multiplexing level of 10Gb/s, people starts to encounter the effects that can seriously degrade WDM network performance such as the dispersion, reflections, scattering, etc.

New fiber designs, special dispersion-compensation techniques, and optical isolators can mitigate these limitations, and newly installed links are operating very well as 10Gb/s per wavelength.

However, a large portion of the older installed fiber base is limited to OC-48 rates (2.5Gb/s) at a given wavelength. Thus, a great interest has been established in using WDM, not only for older links but also to have a very high capacity new links.

For a typical WDM link. At the transmitting end, there are several independently modulated light sources, each emitting signals at a unique wavelength. Here a multiplexer is needed to combine these optical outputs into a continuous spectrum of signals and couple them onto a single fiber. At the receiving end, a demultiplexer is required to separate the optical signals into appropriate detection channels for signal processing. At the transmitter, the basic design challenge is to have the multiplexer provide a low-loss path from each optical source to the multiplexer output. Since the optical signals that are combined generally do not emit any significant amount of optical power outside of the designated channel spectral width, interchannel cross-talk factors are relatively unimportant at the transmitting end.

WDM Multiplexers
Wavelength multiplexers are specialized devices that combine a number of optical streams at distinct wavelengths and launch all their powers in parallel into a single fiber channel. This
combination need not be uniform for all wavelengths; that is. One may want to combine 50% of the power from on wavelength, 75% from another source, and 100% from other wavelengths. However, for WDM applications it is usually desirable that the multiplexers combine the optical powers from multiple wavelengths onto a single fiber with little loss. Wavelength demultiplexers divide a composite multichannel optical signal into different output fibers according to wavelength without splitting loss. This section describes a phased-array-based WDM multiplexer and a fiber-grating multiplexer as examples of such components.