Need for Raman amplification in long-haul optical networking

By Bertrand Clesca, head of global marketing, Xtera Communications

The quest for paramount bandwidth in optical backbone networks seems never ending.  To address this capacity need, new modulation formats and high channel rates (100G and beyond) have been proposed.  In real network conditions (i.e. aged fibre plants, non-uniform span lengths between intermediate sites, presence of optical connectors at the ODF level - and, possibly, of lumped losses along the optical path - attenuation margins on a per span basis for coping with future repairs following fibre cuts, etc.), however, 100G transmission using only erbium-doped fibre amplifiers (EDFAs) is limited to regional applications, while 16QAM/200G/400G/1T equipment is restricted to metro applications.  16QAM modulation format with EDFA amplification will work only for a limited number of applications in the real world, being understood that backbone network operators will not go to the expense of putting in intermediate signal regeneration to employ this technology over long distances.

From an optical reach/capacity performance perspective, Raman optical amplifiers are superior to EDFAs for three fundamental reasons:

  • Their superior noise performance leads to a higher optical signal-to-noise ratio (OSNR) at the output end of the optical path;
  • Raman optical amplifiers create distributed optical amplification inside the line fibre, mitigating the nonlinear effects experienced by the optical WDM channels; and
  • Raman amplifiers offer broader spectrum than EDFA amplifiers (100 nm Raman amplifiers were deployed in 2004 while the typical EDFA spectrum is about 36 nm).

Reach-wise, Raman amplification enables 16QAM modulation format and channel rates exceeding 100G on long distances over the existing fibre plants, as validated by numerous commercial deployments and also by extensive lab and field trials.  Raman-based line equipment can currently transport 100G and 400G channels over more than 4,500 and 2,000 km, respectively, on aged fibre plants in real network environments.

Operators' requirements and traditional response from the industry

At a time when Raman optical amplification is crucial for long-haul 100G and 100G+ optical networking, operators' requirements for Raman amplifiers go beyond the simple level of optical transmission performances.  Like with all new technologies introduced into backbone networks supporting a huge amount of traffic, operators' requirements include:

  • High levels of integration, power efficiency and reliability;
  • High robustness toward imperfections in the fibre plant (high fibre attenuation, lumped fibre losses, bad splices, bad connectors);
  • Simple operation (turn-up, adding new wavelengths, adding new network elements or degrees, etc.);
  • Fast response to transient conditions (e.g. sudden partial channel loss, switching to protection route); and
  • Automatic laser shutdown (ALS) mechanism working smoothly when it comes to turn off quickly and then power up the Raman amplifiers after a fibre cut.

Most Raman solutions offered in the market today still rely on EDFA-based core amplifiers that were not originally designed for a hybrid Raman/EDFA configuration.  The approach generally followed by vendors was to optimise their products around EDFA amplifiers first.  When met with the technical challenges of 100G, these vendors then added add-on Raman boxes, often originating from third-party suppliers. Such an approach results in suboptimal hybrid Raman/EDFA implementation with poor levels of integration, uneasy operations (with, e.g., ALS not working properly or lack of flexibility when new wavelengths need to be added), limited reach extension capabilities, and no spectral enhancement.

Raman technology: myths and reality

Because of a lack of experience and poor implementation of the technology in non-optimal configurations, Raman amplification has previously suffered from multiple unfair criticisms that turned out to be pure myth.  Among them are:

  • Raman technology is complex - A proper design based on Raman amplification from the ground up results in equipment as simple, if not simpler, to operate than an EDFA-based product.  One key point to be aware of is that the design of an EDFA controller cannot be used as such for a Raman amplifier controller;
  • Raman requires 'ideal' fibre plant - Although most of the Raman amplification implementations require ultra-low loss fibre, perfect splices and zero connectors in the first tens of kilometers at the end or beginning of the spans, well-designed Raman amplifiers can be deployed on a variety of fibre types, vintages and qualities in multiple environments (including cascades of ODFs and aged fibres with lumped losses); and
  • Raman technology is unsafe - With a proposer design, Raman amplifiers can be classified Class 1M; for added safety, the input/output connectors can be equipped with optical shutters.

Raman amplification - as simple as it can be!

Unlike most offerings, where integrating optical Raman amplifier with EDFA is an afterthought, optical networking platforms designed from the ground up to combine different optical amplification flavours, ranging from simple EDFA to all-distributed Raman amplification, lead to efficient and effective Raman solutions meeting operators' requirements for both high-performance and seamless introduction into their networks.

For more than 10 years, well designed Raman amplifiers have been demonstrating flawless operation, high efficiency, excellent reliability and unparalleled reach-capacity performances in field conditions due to the high level of integration of the different optical amplification technologies and development of powerful controllers.

Properly designed Raman solutions are transparent to operators in their networks and facilitate operation in the various daily tasks. With software embedded at different levels within the product - from hardware to management system - automation and control loops can be built in the equipment to facilitate installation, turning up, testing, commissioning and operation, including optimisation of span and link performances, ALS and power restart, addition of new channels, and more. The control algorithm can work continuously to adjust optical amplifiers setting points and optimise performance for changing line conditions, including slowly increasing fibre attenuation or transients like sudden cable cuts.

To illustrate the robustness of properly designed Raman systems with respect to non-ideal fibre plants, it is worth noting that 684 optical connectors were passed through by 40G optical wavelengths along a 3,040 km link made of Raman amplifiers and aged fibre in a field trial conducted in October 2005.  In a more recent field trial (September 2013), 150 waves at 100G were transmitted along a link made of Raman amplifiers, a string of 114 optical connectors and 1,504 km of aged fiber (with lumped losses up to 2 dB).

Robust Raman implementation for long-haul optical networking

Raman amplification is recognised by the industry as the key future-proof technology to efficiently support 100G and 100G+ long-haul optical networking.

Based on a unique R&D expertise, backed by an unrivalled 10 years of operational experience, built on field-proven photonic design, and supported by robust control algorithms and automation, Wise Raman is Xtera's field-proven, future-proof solution for introducing Raman optical amplification seamlessly into optical networks, with a specific focus on combining high-end optical reach and capacity performances with operational excellence and simplicity. An advanced and robust Raman solution is not only about the photonic and hardware design of the Raman optical amplifier module, but is also about its integration and operation in both the equipment and the network.

As demonstrated by Xtera with flawless operation and seamless integration of its Wise Raman solution into existing networks in the past decade, issues commonly blamed on Raman technology are the results of poor implementation by other vendors, not of the technology itself.  Xtera achieved the challenge of making Raman amplification a new high-performance network technology without imposing additional constraints or new practices to operations teams.

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