With the widespread application of cloud computing and big data, large-scale data centers are the most important infrastructure for the digital rejuvenation industry in the future, and the number of constructions is increasing. The computing power and internal data exchange capabilities of data centers are also showing exponential growth. develop.
For data centers, optical modules are generally used for interconnection between internal servers and switches, and high-speed optical modules, such as 100G QSFP28 and 400G QSFP-DD, have become an industry development trend. The quality of high-speed optical module products determines the quality level of the entire communication system operation.
Therefore, how to better design higher-rate optical module products while ensuring the high performance and stability of the optical module is very critical. This article briefly introduces the overall design of 400G optical modules.
The 400G optical transceiver module product has the largest demand among 400G products. It adopts the transmission mode of 8 channels for transmission and reception, in which the transmission rate of each channel is 50 Gb/s, and the signal system is pulse amplitude modulation (PAM4). line simultaneously to meet the rate transmission of 400 Gb/s.
The overall structural block diagram of the optical module product is shown in Figure 1. Among them, electrical chips mainly include digital signal processing (DSP) chips, transimpedance amplifier (TIA) chips at the receiving end, modulation driver (Driver) chips at the transmitting end, etc.; optical chips and optical passive devices mainly include vertical cavity surface emission Laser (VCSEL) chips, photodiode detector (PD) chips, optical lenses, etc.
The electrical signal at the transmitting end enters the PAM4 DSP chip through the electrical interface of the connector for electrical signal shaping processing. The processed high-frequency signal is divided into two groups of 4-channel signals and enters Driver1 and Driver2 respectively.
Finally, the electrical conversion is realized through the driving effect of the Driver on the VCSEL. light process. The optical signal at the receiving end enters the PD chip through the optical interface of the MPO16, and the PD chip generates an induced photocurrent. The photocurrent is amplified by the TIA and then enters the DSP chip. The high-frequency signal is shaped and output in the DSP chip, thereby completing Light/electricity conversion.
The PCB of the entire module adopts a 10-layer board structure, of which 4 layers are used for high-frequency differential lines, and the other 6 layers are used for reference layers and DC layers.
This paper uses impedance calculation software (Polar SI9000) and 3D signal simulation software (HFSS) to calculate the impedance of high-frequency differential lines (the results are shown in Figure 3, under the outer differential structure, the 100Ω impedance differential line width is 4 mil line wide, the spacing is 8 mil) and simulated simulation test experiments.
In the PCB manufacturing process, high-frequency differential signal layer boards must choose high-speed special boards with low dielectric constants, and the more commonly used ones are Rogers or Panasonic M6.
In addition, in order to ensure the physical symmetry of each stack and avoid the deformation of the PCB due to uneven heating, it is necessary to use symmetrical materials for the board. The conventional reference layer and the DC wiring layer are made of ordinary FR4 materials.