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Analysis of the reasons for using active optic cable for Data Center integrated cabling

June 27, 2023

Compared to 40G DAC copper cables, 40G AOC(Active Optical Cable) has incomparable advantages in data transmission environments above 7 meters, including smaller size, lighter weight, easier bending, easier  management, longer signal transmission distance, etc. The following is the structure of full-duplex AOC components.

In the data center, the same objectives can be obtained with 40G SR4 optical modules.

The following is an analysis of the differences between the 40G AOC and SR4 modules as follows.

1. insertion and return loss analysis: for the same transmission distance, When different fiber patch cords are plugged into the SR4 optical modules, there will be varying levels of insertion and return loss, this situation is basically a common problem. However, as long as the  changes remain within a certain range, the tested eye diagram and other related indicators will not exhibit obvious changes. And this aspect of a well-made AOC is basically capable of maintaining stability, with better swing performance. the repeatability test of SR4 is shown in the table below, showing significant volatility.

2. Four quadrant test: the input voltage and signal amplitude are measured in four combinations to ensure that the performance is maintained at the lowest and highest voltage and temperature. The AOC needs to pass the four quadrant test over a wide temperature range to ensure that the AOC’s MPO interface and fiber optic cable will not melt at high temperatures. The MPO interface itself can meet the 0.3dB variation in high and low-temperature performance, and the materials used can withstand PEI material above 120 degrees. As an all-in-one molded product, comprehensive testing guarantees product performance, which may appear more robust than the MPO jumper that uses SR4 modules with optional uncertain performance. Compared to SR4, which confirms the quality through photo-eye diagrams, AOC mainly judges the superiority and inferiority of indicators through electrical eye diagrams.

    100G QSFP28 AOC

    3. DDM monitoring: The DDM monitoring function of QSFP+ SR4 Optical module is used to monitor the ADC value at the receiving end in real time to determine the optimal coupling. Therefore, in terms of receiving sensitivity, the SR4 appears to be superior to the AOC. However, both SR4 optical modules and 40G AOCs are currently unable to achieve real time power monitoring.

    4. Transmission distance: There is little differentiation between the two on OM3 fiber. However, 40G QSFP+ SR4 optical module can control the performance better. For longer distance (>300m) transmission, the QSFP+ SR4 is recommended.

    40G QSFP+ SR4 850nm 100m MMF Transceivers

    Based on the data connectivity within the data center, it is recommended to use active optical cable (AOC) . The reason is that this AOC provides better cabling consistency and repeatability, and the closed ends are more easily protected from environmental factors and vibrations. In case of a failure, it can be directly replaced without the need for on site termination and plugging tests of 40G QSFP+SR4 optical modules and MPO patch cords , which is more cost-effective.