Based on the booming growth of network services, the demand for bandwidth in data centres is getting higher and higher. Originally, the demand could be met by bundling multiple links, but nowadays cloud computing, online gaming and online HD video all require a large amount of network bandwidth, which cannot be met by simply adding more links. For example, the basic 10G interconnect ports on the uplink of a traditional data centre are bundled to 8, which means 80G bandwidth. If the cost of adding more links is too high, many network devices cannot support more port bundles, so they can only demand port devices with higher forwarding rates, and 40G/100G is created in the context of such a huge demand. Now 40G/100G has been scaled to fly into ordinary data centres, becoming a must-have option for large data centres, with several 40G or 100G interconnect ports being deployed at data centre interconnection outlets, boosting the data centre's external access bandwidth to over 100G, or even up to 1T. Fortunately, the technical difficulty of 100G optical modules has been overcome for people, but there are still many problems with this part of the technology, which is still under continuous development, so let's talk about the technical progress in this area.
High-speed optical modules generally refer to optical modules with 40G/100G transmission or above, which are technically difficult to achieve, especially in terms of transmission distance, and it is difficult for 100G optical modules to reach a transmission distance of more than 10KM. This has slowed down the popularity of 100G optical modules in data centre applications. However, this development trend is irreversible, just like our computers and mobile phones, which are running faster and faster, as long as the technology is improving, the speed will continue to increase. High-speed optical module technology is also constantly evolving. The most mature ones are currently PLC technology, as well as InP-based integration technology and silicon photonics-based integration technology.
PLC (Planar Lightwave Circuit) is called platform optical waveguide technology, refers to the optical waveguide is located in a plane, its production process is compatible with the traditional semiconductor production process, and cheaper than the traditional optical assembly process, packaging technology is good. PLC has two basic structures: one is a rectangular optical waveguide, the optical core layer is columnar; one is a ridge-shaped optical waveguide, the optical core layer is a rectangle on top of a ridge. PLC technology is the core of the integrated optical process according to the functional requirements made of a variety of flat optical waveguides, some also have to deposit electrodes at certain locations, and then optical waveguides and then coupled with optical fibres or fibre arrays, using a highly integrated preparation technology, the number of taps up to 128. The use of photolithography, growth and dry etching processes to form buried optical waveguides on a quartz substrate for optical power distribution is the best technology for optical splitter production. PLC can be achieved using different media, such as lithium niobate titanium plated optical waveguides, silicon-based deposited silicon dioxide optical waveguides, InGaAs/InP optical waveguides and polymer optical waveguides, etc. These different There are some differences in cost and transmission efficiency of these different materials, and the advantages and disadvantages of each, so I will not go into details here. In short, PLC technology is not a completely new technology, but still borrows many of the original optical technology, with the help of advanced production process, to achieve the purpose of improving the transmission bandwidth of individual optical modules.
When the speed of the optical module is increased from 10G to 40G or 100G, it can still be satisfied by using PLC technology, but if it has to be increased to 400G or even 1T, this technology is somewhat overwhelmed. Current technological processes do not yet have the means to achieve such bandwidth densities, and if this is achieved by making optical modules larger, it is clearly not a good solution, and PLC increases significantly with the complexity of the manufacturing process, which also makes the price of PLC-type optical modules remain high and cannot be reduced, so silicon photonics technology emerged. This is a low-cost, high-speed optical communication technology based on silicon photonics, which uses laser beams to transmit data instead of electronic signals. This low-cost technology not only dramatically reduces the cost of data centre expansion, but also breaks Moore's Law lifetime in terms of rate (if Moore's Law is followed, it is impossible for Ethernet transmission rates to reach 1T), making it possible to break through 1T of bandwidth on a single port, which is a new data centre technology that has received a lot of attention since 2016. However, there are still technical challenges in coupling silicon photonics with optical fibres, and there are challenges in aligning 10 micron core fibres with waveguides of only 0.35 to 0.5 micron size for wafer-level inspection. Happily there are still some manufacturers who have broken through these technical difficulties and have produced some silicon photonic optical modules for official sale, which overcome the problem of the short transmission distance of 100G high-speed optical modules. Although these optical modules are not yet able to provide 200G and higher rates, it is believed that with the continuous improvement of the technology, it will certainly be possible in the future. The fact that the Ethernet standards organisation has now started to develop a 400G transmission standard shows that this is theoretically possible, otherwise it would not be possible to develop such a transmission standard.
Photonic integration is also a technology that may be chosen for future high-speed optical modules. An optical waveguide-based integrated circuit with a dielectric waveguide as the centrepiece integrates optical devices, i.e. a number of optical devices are integrated on a substrate to form a whole, and the devices are connected to each other with a semiconductor optical waveguide to form a high-speed forwarding optical module. Photonic integration is the most cutting-edge and promising area of fibre-optic communications, and it is one of the best ways to meet the bandwidth requirements of future networks. Of course, the manufacture of photonic integrated optical modules is not an easy task. Photonic devices have a three-dimensional structure and require repetitive deposition and etching on multiple thin film dielectric layers of different materials to produce, and this type of complex technology is only expected to be seen at 400G.
Data centre high speed optical module technology is still evolving and once there is a breakthrough it will be very beneficial for data centres to increase their network bandwidth. To a large extent, the technology of high-speed optical modules has prevented data centres from moving to higher network bandwidths. From the past network bandwidth enhancement process, once higher speed optical modules are designed and implemented and commercially available, they will soon set off a wave of replacement in the actual network, and all its supporting network equipment, optical fibres, network chips, etc. will soon be matched with support, so the development level of optical module technology determines the overall bandwidth level of the data centre and is the It is the most crucial part of the data centre to improve the network bandwidth.
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