Active Optical Cables Go the Distance
One solution to the signal integrity and reach issues in copper cabling is the use of active copper cable assemblies. Bob Hult takes a look at today’s technology and where it could lead the industry.
The long predicted demise of high-speed copper cables continues to be delayed/frustrated by continuing improvements in the technology of data transmission. A combination of well-documented characteristics, proven performance, simple repair, and, ultimately, economics makes copper cables the choice in most high-speed links.
That’s not to say copper has no shortcomings. Issues include:
- Bandwidth limitations – Data rates continue to increase, requiring more costly signal conditioning support as well as increased power and PCB space consumption.
- Bulk and weight – The thousands of copper cables flowing over cable trays in data centers make a rat’s nest of assemblies.
- Limited reach – As data rates increase, the effective length of copper cables decrease due to attenuation and signal distortion.
One solution to the signal integrity and reach problem has been the use of active copper cable assemblies. These cables feature integrated electronic devices that amplify and de-clutter high-speed signals to enable longer cable lengths. The Thunderbolt cable that has become ubiquitous on Apple devices is an example of an active copper cable assembly.
In many applications, active optical cables (AOCs) offer a better solution.
AOCs consist of a standard copper connector at both ends, but active components within the connector strain relief convert the electrical signals into optic pulses which are coupled into permanently attached optical fiber. The reverse conversion occurs at the other end of the assembly. The result is a plug-and-play, full-duplex, high-speed link with greatly extended range.
The beauty of this approach is the fact that AOCs plug directly into a legacy copper interface. In addition to extending reach and signal fidelity, AOCs introduce application flexibility that simplifies the process when equipment must be reconfigured. From a user’s perspective, an AOC interface looks identical to the standard copper connector, while the signals are transmitted optically via low-loss, small-diameter fiber. Field installers do not have to deal with a precision fiber termination process, interface contamination, or exposure to potentially harmful laser radiation. If a piece of equipment must be located beyond the reach certified by the specification of the copper interface, a compatible AOC assembly simply replaces it with no other changes. Data security is also improved as optic signals are extremely hard to tap and are immune to ambient electromagnetic interference.
Contrary to the widely held belief that they would be a short-term niche product to ease the transition between copper and fiber optic interconnects, AOCs have blossomed into a broad interface category.
The first AOCs to enter the market were centered on InfiniBand applications that use the 4X SFF 8470 CX4 interface.
Since then, a host of additional standardized high-speed I/O interfaces have been paired with active optical cable equivalents. The list now includes:
- USB 3.0
Leading connector suppliers including Amphenol, FCI Electronics, Molex, Samtec, and TE Connectivity have entered this market along with optical cable suppliers such as Corning. Traditional cable assembly suppliers such as Siemon and optical component manufacturers including Avago Technologies and Finisar now offer AOCs. Asian manufacturers including Hitachi, Fujitsu, Sumitomo, and Fujikura are also building AOC cable assemblies.
Not that AOCs are perfect for every application: Users must purchase assemblies in specified lengths as opposed to assembling cables in the field to the exact length. The electro-optic conversion components located under the cable strain reliefs require power to operate. That is not a problem with standard interfaces that include power circuits. The conversion components draw power from these lines. Suppliers promote low current consumption as a competitive advantage of their AOCs. There is some heat generated by these components, but since they are located external to the device they do not add to the equipment thermal budget. Other standard interfaces such as HDMI must add an external power supply module.
Engineers are taking advantage of AOCs in high-performance computing, mass storage, server, and router applications that range in lengths of one to 100 meters. Cable assemblies may have the same or different interface at opposite ends, or may consist of a fan-out cable with a QSFP+ at one end and four SFP connectors at the far ends. Assemblies can support a variety of signaling protocols including InfiniBand SDR, DDR, QDR, FDR, SAS 3.0, 10/40/100 Gigabit Ethernet, and PCIe. A new CDFP AOC from TE Connectivity provides 16 duplex channels operating at 25Gb/s each to deliver an aggregate of 400Gb/s bandwidth.
Active optical cable assemblies continue to grow in new markets and applications. Zephyr Photonics offers a ruggedized AOC designed for harsh environments. Based on D-Subminiature and MIL-DTL 38999 circular military interfaces, these assemblies will find applications in the industrial and even military sectors.
What started as a relatively small niche product category has already grown to nearly $500 million with plenty of new applications lined up to take advantage of these optical assemblies. According to a recent Bishop & Associates market research report, “Multi-Gigabit Datacom Connector and Cable Assemblies,” published in November of 2014, the values of QSFP+ AOCs for 40 and 100 Gigabit Ethernet are expected to reach $55 million by 2019.
Medium-to-large data centers consume the greatest number of AOCs today, but many more applications are expected to benefit from the advantages offered by AOCs. The entire global AOC market could hit $1 billion by 2020.
Design engineers are always anxious to find ways to extend performance, without major changes to existing products. Active optical cables offer an attractive option.