Optoelectronics
Speed up networks to 100G with O-band technology
Migration to 400Gb/s and 800Gb/s is one of the hottest topics in telecoms at present. However, most operators are still largely powered by 10Gb/s or 25Gb/s technology, especially in access network and LTE/5G base station uplink. To ensure networks are prepared for the next wave of transmission, operators need to build wave multiplexing systems that will allow connections to migrate to 100Gb/s. Here, Marcin Bala, CEO of telecommunications network specialist Salumanus, explains what devices to use to run N x 100Gb/s Ethernet in the urban or access infrastructure using O-band transmission
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00Gb Ethernet transmissions are becoming increasingly popular in applications such as 5G networks and data centres. One way of ensuring operators can successfully migrate to 100Gb Ethernet is by using O-band transmission. O-band, or original band, was the main band used in telecommunications, due to its zero chromatic dispersion. With its spectrum width between 1260 nm to 1360 nm, O-band was the basis for creating lasers and detectors.
Over time, C-band became the preferred choice for operators due to the high attenuation rate of O-band in long distance applications. However, the increasing bit rates, forced further changes. The 100G transmission in the C-band could work only for 2-3 km distances (for NRZ/PAM4 modulation). To send the data farther, operators need to compensate for the chromatic dispersion or use more expensive coherent optics.
How to run 100G?
There are several ways to run 100Gb/s links. The most conventional 100Gb/s transmission solution is using grey LR4 or ER4 modules. The limitation of this technology is the number of parallel transmissions that can be run. We can run a maximum of one 100Gb/s transmission on one fibre.
The second option is to run N x 100Gb/s using a DWDM system based on transceivers that use PAM4 technology. Because of the way the modules work, the DWDM solution requires, apart from multiplexers, the use of chromatic dispersion compensators and optical amplifiers, which effectively increases capital expenditure (CAPEX).
The third method is the use of coherent modulation, which allows us to implement
60 June 2023
connections without the need to use compensators. Due to the power consumption of currently available coherent modules, this solution requires the use of classical architecture with transponders, because 100G coherence modules are in the form of CFP/ CFP2 interfaces.
GBC Photonics offers another solution that allows operators to run Nx100Gb/s. This solution is based on a 200GHz grid in the O-band and allows users to work at a distance of up to 30 km. Operations in the O-band enable the elimination of chromatic dispersion compensators. According to the chart of chromatic dispersion (Figure 1), for the most popular fibre (G.652) the dispersion is almost equal to 0 at around 1300 nm. Thanks to the use of a 200GHz grid, we are able to create up to 16 independent transmission channels.
PAM4 and Direct Detect
One of the greatest advantages of O-band solutions is the use of PAM4 and Direct Detect modulation, which allows the use of GBC Photonics modules for transmission on one and two fibres. The patented nCP4 processor based on the PH18 Silicon Photonics Tower Semiconductor platform was used to implement the correct PAM4 modulation. nCP4 allows operators to convert N electrical lines with a 56baud stream into N optical lines at a speed of 100-800Gbit/s. The integration of several optoelectronic elements offers better parameters compared to conventional bonding of discrete elements. The PH18 Silicon Photonics Tower Semiconductor solution is a parallel technology development trend to the indium phosphide technology. Additionally, the improvement of the receiving sensitivity was obtained by using the APD receiving diode. As a result, the main advantage
Components in Electronics
Figure 1
of combining PAM4 and Direct Detect modulation is the ability to implement modules in both single- and double-fibre applications.
O-band multiplexers
To run several transmissions on the same pair of optical fibres, network operators need to use multiplexers. Using O-band in this case, only the distance between the channels and their number changes. O-band multiplexers from GBC Photonics allow operators to run 16 channels with a channel spacing equal to 200GHz. Each port carries one specific channel, the width of which is ± 0.12 nm from the central wavelength.
GBC Photonics has also developed a slightly cheaper, 8-channel version. It uses the same modules, while the multiplexer itself has half the number of channels with a spacing of 400GHz, thus reducing the cost.
Passive O-band track
The multiplexers themselves are 100 per cent passive devices, requiring no power
or software connection. By linking two multiplexers, thanks to a special cascade of filters on each channel, we have the same attenuation, which is about 4dB. In the case of O-band WDM technologies, this attenuation is the most important parameter as it limits the distance at which we can run the transmission.
Each optical module has its own power budget, which is the difference between the power of the transmitted signal and the sensitivity of the receiving diode. The O-band modules have a power budget of 15dB. Based on this, we can calculate that the module itself can provide services up to 30 km. However, when building a wave multiplication system, in the calculations we must take into account the attenuation of all passive elements, i.e. the fibre optic line and multiplexers. In this case, we can run services at a distance of up to 25 km.
Simplex and duplex solutions Wavelength division multiplexing systems can realise transmissions using either one or a pair of fibre connecting edge
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