Fujitsu Labs has developed a technique that compensates for non-linear effects in a coherent receiver. The technique promises to boost the reach of 100 Gigabit-per-second (Gbps) and future higher-speed optical transmission systems.
“That is one of the virtues of the technology; it is not dependent on the modulation format or the bit rate”
Takeshi Hoshida, Fujitsu Labs
Why is it important?
Much progress has been made in developing digital signal processing techniques for 100Gbps coherent receivers to compensate for undesirable fibre transmission effects such as polarisation mode dispersion and chromatic dispersion (See Performance of Dual-Polarization QPSK for Optical Transport Systems). Both dispersions are linear in nature and are compensated for using linear digital filtering. What Fujitsu Labs has announced is the next step: a digital filter design that compensates for non-linear effects.
A key challenge facing optical-transmission designers is extending the reach of 100Gbps transmissions to match that of 10Gbps systems. In the simplest sense, reach falls with increased transmission speed because the shorter-pulsed signals contain less photons. Channel impairments also become more prominent the higher the transmission speed.
Engineers can increase system reach by boosting the optical signal-to-noise ratio but this gives rise to non-linear effects in the fibre. “When the signal power is higher, the refractive index of the fibre changes and that distorts the phase of the optical signal,” says Takeshi Hoshida, a senior researcher at Fujitsu Labs.
The non-linear effect, combined with polarisation mode dispersion and chromatic dispersion, interact with the signal in a complicated way. “The linear and non-linear effects combine to result in a very complex distortion of the received signal,” says Hoshida.
Fujitsu has developed a non-linear distortion compensation technique that recovers 2dB of the transmitted optical signal. Moreover, the compensation technique will equally benefit 400 Gigabit or 1 Terabit channels, says Hoshida: “That is one of the virtues of the technology; it is not dependent on the modulation format or the bit rate.”
Fujitsu plans to extend the reach of its long-haul optical transmission systems using the technique. The 2dB equates to a 1.6x distance improvement. But, as Hoshida points out, this is the theoretical benefit. In practice, the benefit is less since a greater transmission distance means the signal passes through more amplifier and optical add-drop stages that introduce their own signal impairments.
Method used
Fujitsu Labs has implemented a two-stage filtering block. The first filter stage is linear and compensates for chromatic dispersion, while the second unit counteracts the fibre's non-linear effect on the optical signal. To achieve the required compensation, Fujitsu Labs uses multiple filter-stage blocks in cascade.
According to Hoshida, optical phase is rotated according to the optical power: “If the power is higher, the more phase rotation occurs – that is the non-linear effect in the fibre.” The effect is distributed, occurring along the length of the fibre, and is also coupled with chromatic dispersion. “Chromatic dispersion changes the optical intensity waveform, and that intensity waveform induces the non-linear effect,” says Hoshida. “Those two problems are coupled to each other so you have to solve both.”
Fujitsu tackles the problem by applying a filter stage to compensate for each optical span – the fibre segment between repeaters. For a terrestrial transmission system there can be as many as 20 or 30 such spans. “But [using a filter stage per span] is rather inefficient,” says Hoshida. By inserting a weighted-average technique, Fujitsu has reduced by a factor of four the filter stages needed.
Weighted-averaging is a filtering operation that smoothes the signal in the time domain. “It is not necessary to change the weights [of the filter] symbol-by-symbol; it is almost static,” says Hoshida. Changes do occur but infrequently, depending on the fibre’s condition such as changes in temperature, for example.
Fujitsu has been surprised that the weighted-averaging technique is so effective. The technique’s use and the subsequent 4x reduction in filter stages reduce by 70% the hardware needed to implement the compensation. The reason it is not the full 75% is that extra hardware for the weighted averaging must be added to each stage.
What next?
Fujitsu has demonstrated that the technique is technically feasible but practical issues remain such as power consumption. According to Hoshida, the power consumption is too high even using an advanced 40nm CMOS process, and will likely require a 28nm process. Fujitsu thus expects the technique to be deployed in commercial systems by 2015 at the latest.
There are also further optical performance improvements to be claimed, says Hoshida, by addressing cross-phase modulation. This is another non-linear effect where one lightpath affects the phase of another.
Fujitsu Labs has developed two algorithms to address cross-phase modulation which is a more challenging problem since it is modulation-dependent.
For a copy of Fujitsu’s ECOC 2010 slides, please click here.