ASN Submarine Blog

SDM: What does this (R)Evolution mean for Submarine Transmission?

October 13, 2020

In 2016, the first 16 Fibre Pair (FP) cable was announced by ASN for the first time during Suboptic’16. Why propose 16FP while the average number of FP sold in a cable was 6? Indeed in 2016, the objectives were to optimize the capacity of the 6FP (maximum 8 FP) in the cable using more power in one single fibre and an even greater effective area (110µm² ,130µm²,150µm² and 150µm²).

The 16 FP cable was the first element of a global solution called SDM for Spatial Division Multiplexing that was launched by ASN at the end of 2017. The goal was quite simple and a bit radical since the objectives were to keep on increasing the cable capacity (up to 70% w.r.t. traditional cable) of the cable and at the same time decrease the cost per bit.

In order to reach the target, 3 innovative and disruptive ideas are proposed:

  • • A greater number of FP in the cable to increase the capacity;
  • • The use of lower effective area fibre to optimize the cost;
  • • The implementation of a new “pump farming” architecture in the repeater allowing pump laser power to be shared between multiple FPs while at a minimum, maintaining, or even increasing the amplifiers reliability.

SDM allows an increase of the capacity transmitted between city A and city B using parallelism (multiple paths to transmit the signals) within the constraint of the maximum 18kW constant power provided by the Power Feed Equipment

What is the basic concept of SDM?

SDM can transmit higher capacity with the same power, because it is more efficient. All the losses of the optical transmission are studied and, where possible eliminated, with the complete system optimised for capacity. In the case of SDM, nonlinearities are carefully managed with the optical power launched in the FP being reduced to limit them. The resulting saving in overall power requirements may then be used by additional FP (used for the parallelism).

Is SDM using multicore or multimode fibres?

SDM is a solution that multiplies the number of paths to transmit a given capacity. Currently 3 solutions exist to multiply the number of paths:

  • • Multiplying the number of conventional fibres (one single core and one single mode)
  • • Multiplying the number of cores in the fibre (MCF: Multi-Cores Fibre)
  • • Multiplying the number of modes in the fibre (MMF: Multi-Modes Fibre)
Mutiple Fibres

From an industrial point of view, MCF and MMF were not mature enough in 2016/2017 when the SDM submarine solution was built. The expectation is that MCF could be mature in the medium term.

It is for this reason that the first SDM solution proposed by ASN uses multiple conventional fibres.

How can we compare SDM with other solutions?

The major competitor to SDM is an increase to the transmission bandwidth. This is where instead of only using the C-band to transmit the signal, the bandwidth is extended by approximately a factor of 2, by using both C+L bands.

Put simply, with SDM the number of fibres are doubled whilst using C band only, while with C+L the fibre bandwidth is doubled and the number of FP is maintained.

However, the C+L transmission is less efficient since C+L has to be separated and recombined (mux/dmux) in the repeater for each span, leading to an extra loss per span of around 1dB which is contrary to the SDM concept of optimising efficiency.

  • • Lower capacity per FP - FP becomes consequently the new granularity
  • • FP switching used to drop a whole FP in a branch
  • • Easier to sell FP
  • • Easier to swap FP
  • • Limits the number of FP in the cable
  • • No need to develop big cable to contain all the FPs
  • • Higher quantity of FP used
  • • Bigger cable needed to contain all the FPs
  • • Less efficient as MUX/DMUX should be used (~1dB losses)
  • • Attenuation is higher in L band
  • • Interaction between C and L bands
  • • Spectrum sharing needed to sell/swap a portion of a FP
  • • Higher cost for BU with WSS to manage spectrum sharing
  • • L band amplifier needed

I heard that fibre is the new granularity. What does it mean?

With the introduction of the SDM solution, the potential capacity per FP decreased. While in the past, 200 Tb/s could be transmitted using 8FPs at a speed of 25 Tb/s/fp. Now that same 200 Tb/s are transmitted using 12FP, or even 16FP at 12.5 Tb per FP. As a consequence, for a customer that previously bought half a 25Tb FP (12.5 Tb), it is now possible to buy a single FP with SDM, without the complication of spectrum sharing. Hence people are saying that the FP is the new granularity.

What is the impact of this new FP granularity?

From an economical point of view, it is easier to sell, or to swap, a FP than a portion of a FP. From a technical point view, new BUs (Branching Units) are used in order to drop a full FP in a branch instead of a quarter of the bandwidth for example. We are speaking in this case of FP switching.

ROADM (Reconfigurable Optical Add-Drop Multiplexer) WSS (Wavelength Selective Switch) has just been proposed to the market, does it mean they are already obsolete?

No, both technologies can co-exist on the same cable. For branches that will have a significant capacity with 1 or 2 FPs among the 16FP of the cable could be fully dropped, while for branches where lower capacity is needed, ROADM WSS can be used and consequently it will be possible to drop only 25% of the spectrum. The portion of the spectrum can be changed with time since WSS are active components.

Is SDM valid only for cable with high capacity?

No. As SDM concept is based on efficiency, it can be applied to all types of cable. For example, during a tender we proposed to replace a 3FP cable where around 50Tb/s were needed by a 4FP cable with 50Tb/s but at a lower cost or, for the same cost, a 6FP cable with an increase of capacity of 40%. 6 is a multiple of 3 so it was easier to manage for the 3 customers: 2 FP per customer instead of 1FP.

Will I need to change my transponders with SDM?

No. SDM is compatible with all current transponders. With these transponders it is possible to vary the bitrates (100G to 400G for example) and so to adapt it to the capacity of the fibre. The main difference with SDM is that, since the capacity per FP is reduced, the more efficient modulation formats can be used (QPSK-like with ~2-3 b/s/Hz).

What is the market share of SDM in 2020?

While in 2018, SDM solutions in tenders were proposed as an option, in 2019 and 2020 it’s now the “must have” solution. Almost all submarine cables are now sold using an SDM solution.

What is the future of SDM?

The current solution available on the market is the first step, proposing up to 16 FP in the cable. The industry is currently working on an evolution of this solution in order to go beyond 16FP in answer to the Petabit cable demand. We can expect within 5 years to also have on the market a solution using MCF.

Read more about ASN SDM1 solution here.