From a Necessity to a Business Enabler

By Geoff Bennett, Infinera Director of Solutions & Technology

Submarine cable systems can involve some of the most demanding examples of optical transmission in the world.  For example, a single fiber pair on the MAREA trans-Atlantic cable can carry 24 Tb/s of traffic, and, since the next generation of long-distance submarine cables is focused on total cable capacity as opposed to individual fiber pair capacity (an architecture called space-division multiplexing), this may well remain the record for some time.

One of the critical elements in achieving this kind of capacity is the amplifier chain – in this case, amplifier units at 55 km intervals along the 6,600 km length of the cable. Designing the operating conditions for these amplifiers is an art, aiming to add the lowest possible amount of noise to the data wavelengths as well as for very long lifespans to avoid failure of the submerged equipment. As a result, such amplifier chains have strict requirements for the power management of the launched signals, which submarine cable operators have come to accept as a necessity for the stable running of the cable. Therefore, submarine line terminating equipment suppliers have developed active power management approaches that balance the data wavelengths with loading signals, which do not carry any information but are used to balance the spectrum and ensure target operating conditions in the subsea amplifier chain.  Recent developments in open cables and spectrum sharing have transformed these approaches from a necessity into a business enabler – creating opportunities to deploy cost-effective submarine network capacity quickly and with outstanding granularity.

Submarine Amplifiers: Constant Power vs. Constant Gain
A key thing to understand about submarine amplifiers is that, unlike typical terrestrial amplifiers, they operate at constant optical power, rather than constant per-wavelength gain. Figure 1 shows one of the major consequences of this. On the left, the figure shows a set of eight wavelengths that have suffered attenuation and are at low optical power levels. After they pass through the amplifier, they are all boosted, ready to continue the journey. Each wavelength will have experienced a certain level of gain (the ratio of the output power to the input power) from the amplifier. The dotted line represents the nonlinear threshold for this fiber – this means that if we increase the gain of any wavelength, or multiple wavelengths, above this threshold, it will experience nonlinear penalties and reduced optical performance. In Figure 2 we see the

same amplifier, but we now have only five wavelengths on the fiber because three wavelengths have been dropped unexpectedly. The amplifier will continue to deliver the same power, but now it is spread over fewer wavelengths, which causes the data wavelength power levels to exceed the nonlinear threshold.

Subsea Graphics Figure01

Figure 1. Source: Infinera.

Subsea Graphics Figure02

Figure 2. Source: Infinera.

So, what could cause a submarine cable to “lose” wavelengths as we see in Figure 1?  Figure 2 shows one possible cause in the case of a submarine cable branching unit (BU), which splits the cable path between two or more landing stations.  Modern submarine cables can be configured with wavelength selective switch (WSS) devices in the BU to manage the distribution of channels between the different routes.  Traffic flow in Figure 2 is from left to right and it shows three traffic wavelengths in the upper branch and five traffic wavelengths in the lower branch, which combine in the BU to deliver eight traffic wavelengths heading toward the cable landing station (CLS).  A break in the upper branch causes the three wavelengths it is carrying to be dropped, and the five remaining wavelengths after the BU experience excessive gain, which results in them exceeding the nonlinear threshold for this fiber.

A similar scenario arises if, instead of digitally terminating the optical signal in an OTN switch at the CLS, the cable operator decides to continue the analog optical path of the traffic through a ROADM to an inland point of presence or data center. This is referred to as “glassing through” at the CLS. There is now a terrestrial section in the optical path and, if this is lost due to a terrestrial cable break, some or all the wavelengths on the submarine section will also be affected.

Both scenarios – branching units and glassing through – help submarine cable operators to optimize their business models.  In the case of branching units, most of the cable can be laid toward a given CLS, but close to the shore, the cable can be branched to deliver capacity to another key location because there is a demand there, or to connect into a larger-scale resilience topology.  Glassing through at the landing station can avoid the cost of installing an OTN switch and corresponding OEO regeneration, which can be significant as we approach capacity levels in the tens of terabits.  Digitally terminated traffic in landing stations can be extremely useful, and easy to manage, but it probably represents a small fraction of the traffic on some cables.

Note that, in terrestrial networks, an automatic control protocol is used in modern amplifiers to prevent this kind of gain surge in the event of loss of data wavelengths.  This is impractical in a submarine line system because many more amplifiers are in the chain (around 120 amplifiers in the MAREA cable), and the physical distance across the cable can introduce excessive control loop delays.

So, in either of these scenarios, it is possible that some number of the data wavelengths on the submarine section of the cable could experience gain fluctuations because submarine amplifiers tend to operate in constant power mode.  So, what can be done about it?

Idlers and ASE Noise
The title of this article implies that submarine optical power management has been going on for some time, and this is true. From the dawn of submarine optical transmission, a typical submarine fiber pair has been filled with traffic over time, rather than all at once. In order to ensure that the submarine amplifier chain can maintain gain stability as the cable is gradually filled up, it is common to install devices known as idlers.

There are two types of idlers. The most widely used type of idlers uses a specially modified erbium-doped fiber amplifier (EDFA) to generate optical power across a wide range of wavelengths. A combination of tuneable filters and gain flattening elements can then be used to shape this optical power as needed. An effect called amplifier spontaneous emission (ASE) is used to create the optical power, and so these devices are known as ASE noise or power generators. ASE idlers are used to fill wide parts of the fiber spectrum.

A second type of idler is called continuous wave or CW idler. These are narrow line width, tuneable, high-power lasers that run in continuous wave mode. They are used if it is necessary to provide high optical power in a small part of the spectrum.

Figure 3 shows how ASE generators or idlers might be installed on a cable. As new transponders are added to the fiber pair, the idler on that wavelength is simply switched off. If wavelengths are dropped, it is possible to turn on idlers and rapidly tune them to the absent parts of the spectrum to rebalance the amplifier chain gain levels. Historically this was done manually, but today it is possible to automate this operation using an intelligent power management system that has active control of the ROADM and idlers. A system like this can effectively manage optical power across the entire fiber spectrum, and thereby maintain service stability under a variety of challenging conditions. These power management solutions are a necessity, and they are deployed on nearly every submarine fiber pair running today. Recent developments have extended this necessity into a business enabler.

Subsea Graphics Figure03

Figure 3. Source: Infinera.

Spectrum Sharing
As I stated earlier, submarine cable services can be terminated using OTN switches, and one of the functions these perform is that of bandwidth management – slicing up the capacity on the fiber pair into more economically attractive chunks for resale. But the trend on modern cable deployments is to connect transponders or muxponders directly into the open line system, so fiber capacity must be managed in other ways. The AAE-1 cable, for example, is made up of five fiber pairs, and its total cable length is around 25,000 km, making it the world’s largest submarine cable to be constructed in almost 15 years. One obvious bandwidth management option is for different fiber pairs to be sold or leased to given network operators. But the AAE-1 website lists 19 network operators participating in the cable consortium, so how do so many operators gain access to the capacity they need in an economically attractive way? A particular challenge for such a huge cable system is that AAE-1 terminates at two points of presence in Singapore and is the only next-generation submarine cable that continues further into Asia through diverse terrestrial routes across Thailand and providing connectivity to Vietnam, Cambodia, and Hong Kong. While this diversity allows AAE-1 to offer one of the lowest-latency routes between Hong Kong, India, the Middle East, and Europe, it is essential to maintain amplifier stability across each contiguous all-optical section of the cable.

A recent approach is to allow fiber pair spectrum to be partitioned and sold to different network operators – known as spectrum sharing. While this can provide an extremely cost-effective way to slice and dice fiber pair capacity, it also suffers from challenges around power management – especially with respect to terrestrial backhaul topologies.

In addition to the usual job of providing stable operating conditions for the submerged amplifier chain, the intelligent power management functions for a spectrum sharing solution must also monitor and police the spectrum allocated to different tenants sharing the spectrum of the complete fiber pair, and take action in case of any violation of launch conditions.

Summary
The constant power characteristics of submarine amplifiers have meant that, in the past, tools such as ASE generators and idlers needed to be manually controlled to maintain cable stability. Today, a modern, open submarine cable system can bring together these intelligent power management capabilities, automate them, and allow next-generation submarine transponders to operate at the highest levels of stability, efficiency, and performance.