Optical module is the key hardware equipment to realize optical network interconnection in data center. With the increase of port number and density, the cost of optical module will account for nearly half of the cost of optical network in data center.At present, 100G interconnection technology has been widely used in the newly built data centers of major Internet companies, and 400G interconnection technology will be widely used in the next 1-2 years.Therefore, the implementation technology of 400G optical module has become the focus of the industry.
In terms of light wave length, 400G light module can be divided into multi-mode (MM) and single-mode (SM).In terms of signal modulation mode, it can be divided into NRZ and PAM4 modulation (PAM4 is the main one at present).In terms of transmission distance, 400G optical module can be divided into SR, DR, FR and LR. From the packaging form, 400G optical module can be divided into CDFP, CFP8, OSFP, qsfp-dd and so on. Below is the technical classification of the 400G optical module.
The early 400G optical module used 16-channel 25Gbps NRZ implementation (such as 400g-sr16), using CDFP or CFP8 package. The advantage of NRZ is that it can be borrowed from the mature 25G NRZ technology on 100G optical module, but the disadvantage is that it needs 16 channels of signal for parallel transmission, with relatively large power consumption and volume, which is not suitable for the application of data center.
In the current 400G optical module, 8-channel 53GbpsPAM4 (400g-sr8,FR8,LR8) or 4-channel 106GbpsPAM4 (400g-dr4,FR4,LR4) is mainly used to achieve 400G signal transmission in the optical port side, and 8-channel 53GbpsPAM4 electrical signal is used in the electrical port side, adopting the packaging form of OSFP or qsfp-dd. Both OSFP and QSFP-DD packages provide 8 – channel electrical signal interfaces.In comparison, qsfp-dd package size is smaller (similar to the traditional 100G optical module QSFP28 package), which is more suitable for data center applications. OSFP package size is slightly larger, because can provide more power consumption, so more suitable for telecommunications applications.
The following figure shows the implementation of 400g-fr8 /LR8 and 400g-fr4 respectively
（Reference： OSFPMSA Specification for OSFP OCTAL SMALLFORM FACTOR PLUGGABLE MODULE.）It can be seen that the interface side is 8-channel 53Gbps PAM4 signal. For 400g-sr8 /FR8/LR8 and other modules,
CDR (clock restoration) and electrical/optical or optical/electrical conversion are only performed inside the optical module. Therefore, the optical port side is also an 8-channel 53Gbps PAM4 signal, just like the electrical port measurement.For 400g-dr4 /FR4/LR4 and other modules, there are Gearbox chips inside the optical module that multiplexes the input of the two ports into one signal and modulates it to the light, so the speed of the optical port side is twice that of the port side, namely, 106GbpsPAM4 signal.
