1.Introduction of optical transceiver

Optical module is a device for converting optical and electrical signals in optical communication system. Its basic structure includes: laser (TOSA) + driver circuit, detector (ROSA) + receiver circuit, multiplexer (MUX), demultiplexer (DEMUX), interface, auxiliary circuit and housing, etc. According to the application of different scenarios, different kinds of optical modules are derived. The specific scenarios are: inside data center, data center interconnection, metro network, core network, 5G Fronthaul, etc. Further classification can be obtained by elements such as package form, transmission rate, single-mode and multi-mode, plugging mode, operating temperature, and whether digital diagnostics are available.

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Structure diagram of 100G optical module

Driven by technology upgrade, cost reduction and other factors, optical modules continue to be "high-speed, miniaturized and integrated". QSFP (Four Channel SFP Interface) series is increasingly popular in the market because of its high-speed, high-density, hot-pluggable and other characteristics. It is the main packaging form of data communication optical module. For 100G optical modules, CFP vs QSFP28, SR/LR mainly adopts QSFP28 package, while ER mostly adopts CFP4 package.

2.The standards of optical transceiver

The transmission distance of optical module is divided into short range (2km and below), medium range (2 - 40km), and long range (40km and above); at present, the short and medium range occupies the main market share in data communication optical module. There are two key standard organizations for optical modules, IEEE and MSA (Multi Source Agreement), which complement and learn from each other. The standards beginning with 100G are proposed by IEEE802.3. The following are the naming rules:

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As shown in the figure, for example, in the name of 100GBASE-LR4, “LR” means long rage, i.e. 10Km, and “4” means four channels, i.e. 4*25G, combined together as a 100G optical module that can transmit 10Km. Since the introduction of 100G networks, IEEE, the Multi-Source Agreement (MSA) industry consortium and other organizations have developed several standards for 100G optical modules. Taking 100G optical modules as an example, IEEE has defined standards such as 100GBASE-SR4, 100GBASE-LR4, 100GBASE-SR10, etc. Multi-Source Protocol (MSA) has 100G PSM4, 100G QSFP28 ER4, 100G CLR4, 100G SWDM4, etc. Among the many standards, the PSM4 and CWDM4 standards developed by the Multi-Source Protocol (MSA) industry group are more suitable for the mainstream 100G QSFP28 optical modules in the market today.

3.IDC market and 100G optical module demand

Like a raging fire in the past few years, the concept of Internet + , Big Data and Cloud Computing has been booming. The IDC business revenue has been increasing continuously. At present, the optical module market is developing rapidly, and 100G optical modules applied in data centers are in short supply. 100G SR4 and 100G LR4 are the most commonly used 100G interface specifications defined by IEEE. However, for the internal interconnection scenario of large data centers, the distance supported by 100G SR4 is too short to meet all interconnection requirements, and the cost of 100G LR4 is too high. MSA has brought medium distance interconnection solutions to the market. PSM4 and CWDM4 are the products of this revolution.

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DCN Connectivity Evolution example of large Internet company

CWDM4 vs. LR4

The 100G CWDM4 standard is designed for data center deployment of 2km 100G links, and the 100G CWDM4 optical module interface conforms to the duplex single-mode 2km 100G optical interface specification and can reach a transmission distance of 2km. 100G QSFP28 series optical modules are currently the most widely used in data centers. The 100GBASE LR4 capability fully covers CWDM4, but in the 2km transmission scenario, the CWDM4 solution is lower cost and more competitive. LR4 and CWDM4 are similar in principle in that they both multiplex four parallel 25G channels onto a single 100G fiber link via the optical device MUX as well as DEMUX. However, there are several differences between the two:

  • LR4 uses more expensive optical MUX/DEMUX devices;
  • LR4 uses a more expensive laser and consumes more power;
  • LR4 requires an additional TEC (semiconductor thermoelectric cooler).

Based on the above three points, the optical module cost of 100G BASE LR4 standard is higher, so the 100G CWDM4 standard proposed by MSA is a good supplement to the gap caused by the high cost of 100G BASE LR4 within 2km.

PSM4 vs. CWDM4

In addition to CWDM4, PSM4 is also a medium distance transmission solution, so what are the advantages and disadvantages of PSM4 compared to CWDM4? The 100G PSM4 optical module is a single-mode parallel four-channel optical module, mainly suitable for 500m applications. The 100G PSM4 specification defines a point-to-point 100 Gbps link with eight single-mode fibers (four transmit and four receive), with each channel transmitting at 25 Gbps. Four identical wavelength and independent channels are used in each signal direction. Therefore, these two types of transceivers typically communicate over an 8-fiber MTP/MPO single-mode patch cable, and the QSFP28 PSM4 transmission distance is a maximum of 500m.

4.Summary

With the rapid development of data communication and telecommunication transmission technology, the information capacity of optical network increases sharply. The optical module with high bandwidth is a research and production hotspot in the field of optical communication. QSFP28 100G modules are not enough for meeting the current requirements. At present, China is in the historical opportunity period from 100G to 400G optical network. The advent of 5G era will bring tens of millions of optical modules. The optical module market will have a market share of more than 30 billion in the next few years.

At present, the application of data center is in the period of rapid development of 100G optical module. In 2010, 100G optical communication technology standard has been adopted in ITU-T (ITU), IEEE and OIF. The three standards organizations have successively stipulated 100G technical standards for 100G line side, user side and interworking. Subsequently, with the wide attention and promotion of global operators, all links of the industrial chain (optical modules, optical transmission equipment, router equipment and test instruments) have rapidly promoted the commercialization process of 100G.