The optical module is developing towards high speed and integration. Telecom and data center are two main application scenarios. Optical modules can be classified into 10Gb / s, 25Gb / s, 50GB / s, 100GB / s, 400Gb / s, etc. according to the speed, they can be divided into SFP +, SFP28, QSFP 28, CFP2, QSFP-DD, OSFP, etc. according to the transmission distance, they can be divided into 100m, 500m, 2km, 10km, 40km, 80km, etc.; according to the modulation format, it can be divided into NRZ intensity modulation, pam4 intensity modulation, DP-QPSK / NQAM phase modulation According to whether WDM is supported, it can be divided into gray light and color light; according to different working temperature, it can be divided into commercial grade and industrial grade.
Telecom and data center are two major application scenarios of optical module, which promote the development of optical module technology to higher speed and higher integration. In mobile communication forward network, optical module is evolving from 10Gb / s supporting 4G CPRI interface to 25Gb / s supporting 5G ecpri interface; optical module in medium back transmission network is evolving from 50GB / s, 100GB / s to 200GB / s and 400Gb / s. In the traditional three-layer switching architecture of data center, the optical module rate is evolving to 25 / 50 / 100 / 200 / 400Gbps; in the new two-layer switching architecture, the optical module rate is evolving to 800Gb / s. In recent years, with the rapid development of bandwidth requirements and optical module technology, the standardization of optical module photoelectric interface and packaging form at home and abroad has been promoted simultaneously.
The 800Gbase-r specification defines the basic layer of 800G network, which is expected to double the performance again to meet the requirements of data center. In aDDition to the basic layer of the network, the 800G pluggable MSA team released the 800 Gbps Ethernet transmission specification based on pam-4 modulation technology for data center applications in September 2019.
800G SR8: a cost-effective 8x100G module for short-range transmission applications that can transmit 60-100 meters over single-mode fiber.
800G FR4: 4x200G optical module, need to support new FEC function.
In addition, pluggable optical modules, QSFP112-DD and OSFP 32 packaging types may also be adopted in the specification, and the pluggable optical modules are still the ideal choice for 800G data centers and operators.
The first 800G QDFP dual density pluggable optical module standard defines the size of 800G optical module - QSFP-DD800. It is reported that the optical module can support eight high-speed channels, each channel speed is 100G / s, and can be compatible with QSFP-DD, QSFP +, QSFP28, QSFP56 and other optical modules, providing network operators with great convenience and advantages in 800G Ethernet deployment.
8 × 100G solution
800G SR scenario requirements
For short distance application scenarios, cost is the focus of attention, and this kind of module is mainly selected by ultra large scale data center operators. The goal of MSA is to develop a low-cost 8 × 100G module for SR scenarios, covering the best points of 60-100m. MSA will define a low-cost PMD specification for single-mode fiber interconnection based on 100G pam4. In addition, for SR Application with low delay, KP4 FEC will be used in 800G MSA optical module to achieve error correction. Other DSP algorithms also include simple clock recovery and equalization. Finally, MSA will assign a connector to the psm8 module, which can fan out to 8x100G.
8 × 100G technical feasibility plan
In the evolution from 400G-sr8 to 800G-SR8, a single channel 100G rate may limit solutions based on multimode fiber (MMF). According to the theoretical model used in IEEE, we can estimate that when the baud rate is as high as 50G, the transmission distance that MMF can support is no more than 50m. The limiting factors are the modulation bandwidth of VCSEL and the inter mode dispersion of MMF. By optimizing the device, fiber media and enhanced DSP algorithm, 100m transmission can be realized at the cost of higher cost, higher delay and greater power consumption. Therefore, MSA pluggable 800G optical module working group recommends using single-mode transmission technology to realize 800G-SR8 scene interconnection.
4 × 200G FR solution
800G FR demand analysis
The single channel 200G pam4 technology is the next major technical step of optical intensity modulation and direct detection interconnection. It will become the foundation of 4-channel 800G connection and the basic component of 1.6tb/s interconnection in the future. MSA will define a complete PMD and some PMA layers, including a new low-power, low delay FEC scheme as a package over the KP4 FEC of 112G electrical input signals to improve the net coding gain (NCG) modem. One of the main objectives of the industry alliance is to develop new broadband electrical and optical analog components for transmitter and receiver components, including digital to analog converters and analog to digital (AD / DA) converters. In order to meet the strict power consumption requirements of pluggable modules, a DSP chip with lower nm number will be designed in CMOS technology, and low-power signal processing algorithm will be used to achieve channel equalization
Technical feasibility of 4 × 200G scheme
Considering that the temperature control of Tec is needed in LAN WDM, and no temperature control measures are expected in single channel 200G scheme, the power consumption will be analyzed based on cwdm4. Link insertion loss, multipath interference (MPI), differential group delay (DGD) and transmitter dispersion loss (TDP) contribute to link power consumption. According to the model published by IEEE standard, the calculation of MPI and DGD loss is shown in Table 4. Considering the increase of baud rate of 200G in single channel, the dispersion loss is expected to be larger than that of 100G. The reasonable suggestion of transmitter dispersion loss (TDP) is 3.9 db. Therefore, considering the aging of the receiver and the residual coupling loss, as well as the typical transmitting optical power value of the transmitter, we think that the receiver sensitivity should be about - 5 DBM.
Possible solutions for 800G DR
In an 800 DR scenario, there are four possible paths. First, the 800G SR8 solution defined in the 800G MSA can be defined to extend coverage to 500m. Since parallel fiber solutions require more fiber channel, the main consideration in this case is the cost of up to 500m of fiber. Second, the 2x400G cwdm4 solution doubles the transmitter and receiver pairs using the available FR4 solutions. The solution seems to be a balance between fiber resources and technology maturity. However, power consumption and module cost are the main limitations. Third, the next generation of single channel 200G solutions may cover this situation. The solution is considered to be only 4 pairs of transmitters and receivers with the lowest cost and power consumption. As for the available time of the solution, it still needs feasibility study and industrial maturity consideration. In summary, several solutions are discussed for DR use cases. MSA will track the development of technology and make recommendations for this application in the future.
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