Nowadays, data center begun to use high speed high bandwidth 40G QSFP AOC cable all over the world. Compared with 40G DAC cable, AOC 40G are smaller size, lighter weight, easier to bend, easier to manage, and longer signal transmission distance, so it is widely used in longer distance transmission (from 7 meters to hundreds of meters).
The following is the full-duplex AOC cabling structure:
However, in data center, if use 40G SR4 optical modules can also achieve the above goals. So what is the difference between 40G QSFP+ AOC cables and 40G QSFP+ SR4 optical modules?
Read more:
What are the obvious advantages of AOC cable compared with DAC cables?
Why should data centers choose AOC active cables?
What is AOC Active Optical Cable?
1. Insertion loss and return loss analysis
For the same transmission distance, according to the current process and design, the repeatability and interchangeability of the SR4 optical module interface are inferior to those of the AOC QSFP. Meanwhile, different optical patch cords insert into the same SR optical module, we will get different insertion loss and return loss. But as long as the loss change is within the qualified range, the eye diagram and other related indicators of the test will not change significantly.
While a well-made AOC fibre can basically remain stable in this regard, and the swing performance is also better. The repeatability test of the SR4 optical transceiver is shown in the following table, showing that a large fluctuation has occurred.
| Channel | CH1（dBm） | CH2（dBm） | CH3（dBm） | CH4（dBm） |
| 1 | -0.3 | -0.34 | -3.7 | -0.03 |
| 2 | -0.35 | -0.33 | -3.8 | -0.1 |
| 3 | -0.38 | -0.34 | -3.83 | -0.14 |
| 4 | -0.23 | -0.37 | -3.6 | -0.02 |
| 5 | -0.39 | -0.18 | -3.8 | -0.13 |
| 6 | -0.13 | -0.37 | -3.59 | -0.04 |
| Biggest changes | 0.26 | 0.19 | 0.29 | 0.1 |
2. Four-quadrant test
The four-quadrant test is to measure the input voltage and signal amplitude under four combinations to ensure that the performance can be maintained at the lowest and highest voltages and temperatures. All AOC cable assembly need to pass the four-quadrant test with a wide temperature range, so that the MPO interface and optical fiber cable of the AOC will not melt at high temperatures. The change in the high and low temperature performance of the MPO interface itself can meet the change of 0.3dB, and the materials used can also withstand PEI materials above 120 degrees. As an integrated molding product, the integrated test guarantees the product performance, which may be more stable than the MPO patch cord with uncertain performance selected by the SR4 module. Compared with the SR4 optical module, which confirms the quality through the optical eye diagram, the AOC cable mainly judges the quality of the indicators through the electrical eye diagram.
3. DDM monitoring
SR4 optical module has DDM monitoring function. When receiving coupling, it monitors the ADC value of the receiving end in real time to determine the optimal coupling. Therefore, SR4 is superior to AOC in receiving sensitivity. However, SR4 optical module and 40G AOC cannot realize real-time optical power monitoring at present.
4. Transmission distance
There is not much difference between AOC cable and SR4 optical module on OM3 fiber. However, SR4 optical module can better control performance. If you want to achieve longer distance transmission (>300 meters), it is recommended to use SR4 optical module.
Conclusion
Based on the data connection within the data center, it is recommended to use active optical cables (AOC). The reason is that AOC has better wiring consistency and repeatability, and the closed ends are more easily protected from the impact of the environment and vibration. If a fault occurs, it can be directly replaced without the need to perform a series of termination plug-in tests on the SR4 optical module and MPO patch cord on site, saving costs. AOC is less likely to be damaged during use and has a longer lifespan.
