The stages of coherent transmissions development

Currently, for most companies, a transmission speed of 10 of 40 Gbit/s is too slow. In fact, they would need faster speeds of 100G and 400G in their core networks. Traditional optical transmission is based on on-off keying with a limit of the keying device speed, which makes it impossible to reach faster transmission. Similarly, distance is an important consideration. For a speed of 10Gbit, the limit is 80 kilometres due to its chromatic dispersion influence.

How has coherent transmission developed?

A new modulation was introduced which supports a higher efficiency of transmission. The usage of a few signal parameters — phase of the light wave, amplitude, frequency and polarity — create the coherent modulation where pictures, called constellations, are being transferred. Modulation QPSK (QAM) can transfer up to two bites for one symbol. There are also more complicated modulations of QAM, for example: QAM8 - 3 bit/symbol, QAM16 - 4 bit/symbol or QAM64 - 5 bits/symbol.

For this type of modulation, it is necessary to use Digital Signal Processing (DSP) because creating the constellations and decoding the data requires significant computing power. DSP supports the creation of the constellations and reconstruction of original signals received by detectors via the fibre optic lines. Complex technology and usage of costly components makes the development of coherent optical transmission quite expensive in comparison to other solutions used in “grey” optics.

When coherent optical transmission was first invented – around 2010 - this technology was used only in powerful, long haul transmission systems built as a cards for modular transmission systems with embedded optics. This path of development is called performance path. After about five years, coherent transceivers appeared as a compact coherent solutions. At that stage of development, baud rate 30 Gbaud/s was used and named Gen30. The transmission speed of such products was 100-200 Gbit/s. The next generation – Gen60 – with 60 Gbaud/s and a transmission speed of 400-600 Gbit/s, appeared around 2017 in performance path and in 2020 in compact path. Thus, the time to adapt the performance solutions to the compact path is shorter with each following generation. Gen120 is under development and should be available soon – we expect it in 2023.

Standardisation

Compact solutions available in Gen30 – 100-200 Gbit/s – were built as transceivers with CFP or CFP2 interface. However, there was no clear standardisation for such products and many manufacturers provided their own solutions, which were not compatible with each other. On top of that, two versions have been created of the compact coherent optics in Gen30:Analog Coherent Optics (ACO) and Digital Coherent Optics (DCO). In the ACO version, the coherent optical transceiver was coupled with DSP bounded on the network device card, while the communication between the transmitter and the receiver used analog signals. In DCO — the more popular version — DSP was built-in to the transceiver. However, due to the aforementioned lack of standardisation, there was often no way of building interconnection between coherent systems from different vendors. The appendix for coherent transceivers published to MSA CFP/CFP2 standard was also not precise enough.

The Gen60 of the coherent solutions involves the transmission speed of 400-600 Gbit/s with 60 Gbaud/s. The compact version of the coherent optics in this generation uses a QSFP-DD interface and two standards have been developed. The first, OIF 400ZR, is dedicated to solutions used mainly in Data Centre Interconnect (DCI). Coherent optical transceivers compatible with OIF 400ZR support only Ethernet protocol and use QAM16 modulation and C-FEC (Forward Error Correction) algorithm for error correction. The transmission distance is up to 120km.

The second standard, MSA OpenZR+, is dedicated to telco transmissions. It supports not only Ethernet transmission, but also the OTN transport protocol. Transceivers compatible with OpenZR+ can work in many modes, where different transmission speeds, modulation or error correction methods are used. With a transmission speed of 100Gbit/s, this kind of transceiver could work even with OSNR on the level of 10.5dB, reaching hundreds of kilometres of transmission range.

Exemplary list of the modes supported with OpenZR+ transceiver:
400G – 16QAM – oFEC – OSNR 21.7dB, CD 26.000ps/nm
400G – 16QAM – CFEC – OSNR 24.0dB, CD 2.400ps/nm
300G – 8QAM – oFEC – OSNR 18.3dB, CD 50.000ps/nm
200G – QPSK - oFEC – OSNR 14.0dB, CD 50.000ps/nm
100G – QPSK – oFEC – OSRN 10.5dB, CD 80.000ps/nm

Both QSFP-DD coherent transceivers are available in the GBS Photonics offer.

Expanding capacity

Coherent optical transceivers are used in DWDM systems, which allows the transmission of many optical streams using one black fibre optic. It is worth mentioning that coherent solutions allow users to reach very high spectral efficiency due to the fact that many transmission channels could be multiplexed and transported through one pair of fibre optics.

It is also possible to establish high-speed transmissions on the same infrastructure used for 10G DWDM transmissions. Coherent modulations offer one more valuable advantage:  there is no need to use chromatic dispersion compensators in the transmission lines (which requires additional attenuation), because the chromatic dispersion influence is compensated by the methodology of the constellation creation and decoding.

Coherent solutions in the compact form of QSFP-DD modules can be installed directly in the switches or routers ports. Thanks to that, there is no need to use transponders and grey optics for interconnection anymore. Standardisation of the coherent QSFP-DD transceivers allows users to build interconnections between devices of different brands.

Market forecasts for the ZR/ZR+ coherent solutions predict that the quantity of such transceivers could rise more than ten times in the next five years.

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