Amazing and Bright future for fiber optical networks for the digital world

In the last 30 years or so, the rate at which data can be sent down core fibre has increased by an amazing 10million times. And the UK has a long and enviable track record in this remarkable progress, with research groups and companies continuing to influence and drive the industry.

One company leading the charge is BT, whose head of optical research, Dr Andrew Lord, said there is much more to come, adding that should quieten critics who suggest the industry is facing a ‘capacity crunch’ as demand for IP traffic grows exponentially.

“In any case, that is such an emotive term; it is much more subtle than that. What we and the equipment suppliers need to ensure – and the industry will achieve through a host of promising and, in many cases, collaborative innovations – is increasing capacity in a space, power and cost efficient way,” Dr Lord told New Electronics.

Dr Lord is in an ideal position to influence some of the exciting developments under way, having served as technical chairman of the prestigious Optical Fibre Communications Conference in 2015 and will serve as general chairman for this year’s event.

In what could be described as the first wavelength division multiplexing (WDM) era, a mixture of quadrature coding and coherent detection – coupled with the ability to transmit using two different polarisations of light; one a QSPK modulation format, the other a DSP – was the norm. This began at about the turn of the century, and lasted perhaps until a couple of years ago.

With the latest transmitter and receiver systems, operators can transmit data at 100Gbit/s over long distance using a single optical channel in their core network. But that fibre was designed to carry only 10Gbit/s.

Because a typical fibre can accommodate perhaps 100 channels, the total capacity of the fibre can approach 10Tbit/s.

In the next phase, the aim is to increase data rates to 200Gbit/s and then 400Gbit/s using a combination of higher order QAM quadrature amplitude modulation (QAM) and multiple carriers. BT is already trialling 200Gbit/s wavelength links deploying polarisation multiplexing 16QAM (PM-16QAM).

For the longer term, Dr Lord suggests the industry has a whole array of tools ‘to take us to the next level and thus meet the demands of increasingly information hungry businesses and entertainment hungry consumers’.

When it comes to scaling optical channels towards Tbit/s rates and higher capacities – in what some describe as the second coherent era – researchers like to talk about three degrees of freedom. The first relates to increasing the baud (or symbol) rate of individual wavelengths; the second focuses on the deployment of higher order modulation formats; and the third is the move to so-called super-channels.

In addition to this three-pronged approach, the industry will need to consider the extension of the optical spectrum beyond the standard C-band (1530 to 1565nm) to, for, example, combined C- and L-band (1565 to 1625nm) amplification.

Ultimately, more work will be necessary to devise novel fibre structures optimised to support higher speed transmission, such as fibres with lower loss and larger cores.

Each of these has its trade-offs, Dr Lord cautions. For example, while higher order modulation increases capacity and spectral efficiency, there can also be significant loss in performance over longer distances. But for a geographically small country like the UK, 16QAM rates can suffice.

For longer-haul fibre networks, engineers have devised a promising method to squeeze channels closer together in a such a way that signals can be encoded, but not interfere with each other. This relates to the third degree of freedom and is necessary because existing dense wavelength division multiplexing (DWDM) systems rely on fixed grid filters to allocate the available spectral bandwidth, typically 50GHz segments.

These fixed chunks of spectrum are extremely limiting in terms of supporting higher speed channel bandwidths, as well as restricting how closely optical channels can be spaced and thus limiting the number of optical channels a single fibre can carry.

With the new flexible grid designs, an optical channel is no longer thought of as a single wavelength, but more like a single capacity entity comprising multiple sub-channels – commonly referred to as a super-channel – that can be configured and managed across the entire network.

Figure 1: According to some industry experts, so called super channels could boost network capacity by 30%

These super-channels can be implemented in several ways, depending on rate and reach requirements. For example, a 400G version could either be four 100Gbit/s sub-channels for long reach; two 200Gbit/s sub-channels for medium reach; or one 400Gbit/s short reach channel, which would be the most spectrally efficient. There is a consensus within the industry that super-channels could boost capacity by up to 30%.