What Makes 40G Active Optical Cable So Popular in Data Center?

Posted on by FS.COM

40 GbE (Gigabit Ethernet) era is already upon us and makes a contribution to satisfying the increasing demands of higher throughputs and bandwidth. 40G Active Optical Cables (AOCs), a great way to match 40 GbE, were introduced into the field of data center for interconnection applications. As we know, 40G copper direct attach cables (Copper DAC) and 40GBASE-SR4 QSFP+ optics are also used in data center to achieve 40G interconnection. Compared with them, what makes 40G AOC so popular?

About the basics of AOCs, we have written a blog titled as “Active Optical Cable (AOC) – Rising Star of Telecommunications & Datacom Transceiver Markets” a few months ago. Thus, we skip the introduction of AOC here. 40G AOC, is a type of active optical cable for 40GbE applications that is terminated with 40GBASE-QSFP+ on one end, while on the other end, in addition to QSFP+ connector, it can be terminated with SFP+ connector, LC, SC, FC, and ST connector etc. 40G AOCs have great advantages over 40G copper DACs especially when transmission distance reaches above 7 meters. In addition to achieving longer reach, the lower weight and tighter bend radius of AOCs enable simpler cable management and the thinner cables allows better airflow for cooling.


In fact, the above advantages can also be achieved by using 40GBASE-SR4 QSFP+ optics. So, why don’t we use SR4 modules but highly recommend 40G AOCs? The differences between 40G AOC and QSFP+ optics will be analyzed in the following:


Cost: 40G AOCs cost lower than SR4 modules and do not need to use with extra fiber patch cables. In particular, 40G breakout AOCs, such as 40GBASE QSFP+ to 4 x SFP+ or 40GBASE QSFP+ to 8 x LC AOCs are cost-effective solutions to achieve 40G migration. Additionally, using AOCs, there are no cleanliness issues in optical connector and there is no need to do termination plug and test when troubleshooting, which can help user save more time and money.

Insertion Loss & Return Loss: Under the same case of transmission distance, the repeatability and interchangeability performances of SR4 module interface are not good as 40G AOC. What’s more, when different fiber optic patch cables plug into the module, it will have the different insertion loss and return loss. Even for the same module, this issue is existed. Of course, the related metrics such as the testing eye pattern will have no significant changes so long as the variation in and conformed to the scope. In contrast, an AOC with good performance is more stable and has better swing performance than SR4 modules in this situation. The following table shows the result of the repeatability test of SR4 module. From the data, it is clear to see that the repeatability performance of SR4 module is not stable.

Test Result of 40GBASE-SR4 QSFP+ Repeatability
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
Max variation 0.26 0.19 0.29 0.1


Four-Quadrant Test: The so called four-quadrant test is a testing under four combinations of input voltage and signal amplitude which are used to ensure the product to keep better performance even under the lowest and highest voltage and temperature situation. Four-quadrant test in wide temperature range is used to test the MTP/MPO interface and optical cable of AOC in order to ensure them not to be melted at a high temperature. Generally, the current products of AOC can all satisfy this demand. In addition, as an integration product, the performance of AOCs is more stable than SR4 modules which should be used with indeterminacy-performance MTP/MPO connectors. Unlike SR4 module, the quality index of AOCs is judged by electric eye pattern but not by light eye pattern.

Digital Diagnostic Monitoring (DDM): DDM can help end users to monitor real-time parameters of the modules. Such parameters include optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage etc. 40GBASE-SR4 QSFP+ modules with DDM function can ensure it’s optimal coupling by the ADC (analog to digital converters) value of real-time monitoring receiver when receive coupling. Thus, SR4 modules have better receiving sensitivity than AOC. However, at present, both SR4 module and AOC can not reach the function of real-time power monitoring.

Transmission Distance: When transmitting over OM3 fiber, there is no significant difference between SR4 module and 40G AOC. But SR4 modules can control the performance better than AOCs. Moreover, proposals for transmission distance that is longer than 300 m will be SR4 module in order to ensure a good performance.

As was mentioned above, AOCs have better consistency and repeatability cabling performance. With the integration and sealed design, AOCs can avoid the influence of environment and vibration. Additionally, for troubleshooting, AOCs are more easier to manage. Because users do not need to do a seires of termination plug and test on-site as they do when using SR4 modules and patch cables. Thus, AOCs are highly recommended to use in data center interconnection. Fiberstore offers high-performance active optical cable assemblies which is the ideal high-speed interconnect solution for data center. Welcome to visit www.fs.com and get more information.

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