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Test active optical cables during design, production

Posted: 20 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:copper cable? fibre-optic? USB? Thunderbolt? Active optical cables?

As communication data rates increase, maximum propagation distances over copper cable decrease. This trend is driving the use of fibre-optic links to shorter distances. Already well established in the telecom and datacom markets, optical fibre is now poised to find applications in the consumer and industrial markets.

Consumer protocols such as USB and Thunderbolt are achieving data rates of 10 Gbit/s today. Thus, the reach of traditional copper interconnects has become limited to a few meters. Optics can remove the distance limitations and enable longer-reach applications with a thinner and lighter cable. These data rates are more than adequate for most applications in the consumer and industrial space because their reach ranges from a few meters to tens of meters. Active optical cables (AOCs), which maintain electrical connections, are ideal to address the higher-speed longer-reach portion of the consumer and industrial market.

To be successful in the consumer or industrial market, optical cables need to be robust!both optically and mechanically!and low cost. The various elements of the optical link!including the fibre, the cable, the coupling optics, and the optoelectronics!can be engineered to work together at a system level to meet those requirements. Testing and analysis done at critical junctures during the design process can further reduce costs by optimising the design for high yield given specified manufacturing tolerances. Additionally, testing during manufacturing can catch any defective parts early to reduce cost of materials and assembly.

At Corning, we developed the ClearCurve VSDN fibre and we perform optical testing in-house. Optical systems (the optoelectronics, coupling optics, and fibre) at minimum have to be sufficient to meet the electrical standards imposed by consumer specifications. We test for optical power, modes, and noise and correlated those results to tests of the electrical signal. Therefore, we can impose a minimum standard for power and noise on the optical path despite the overall system (e.g. USB and Thunderbolt) recognising only electrical signals. The optical parameters are also tested throughout manufacturing and production. Key tests catch any issues on the manufacturing line while also minimising overall costs.

While the data transmission performance of optical links is still important in consumer and industrial applications, the prerequisites for the market!size, robustness and low cost!require certain trade-offs during design. Testing during the manufacturing process and during system design further ensures each element of the design performs to spec despite manufacturing variability, which reduces waste and cost.

The fibre
To be widely accepted in the consumer market, optical solutions have to be small, rugged, and low-cost while still maintaining a minimum optical power throughput or maximum optical loss (i.e. the "optical link budget"). Mechanical robustness of an optical fibre can be improved by reducing the overall fibre diameter as it reduces bend-induced stress. The fibre diameter is made up of an inner core and an outer cladding. Reducing the fibre diameter effectively improves the mechanical robustness without having a strong impact on the optical throughput.

Similarly, the core diameter can be reduced to slightly improve the optical bend loss by increasing confinement of the optical signal, but at the severe penalty of reduced ease of optical coupling. As an extreme example, SMF (single-mode fibre) with typically an 8?m core would need 10〜 or better alignment accuracies to maintain the same optical coupling compared to MMF (multi-mode fibre) with typically a 50?m core. SMF can go very long distances with nearly zero optical loss while MMF is limited to a few hundred meters for the same data rates. Consequently, there is a clear trade-off between optical propagation distance and ease of optical coupling for a given data rate in the fibre's design. Tailoring the fibre design helps meet the application needs for cost, bend sensitivity, bandwidth, and distance.

Because consumer and industrial applications cover distances of less than a few hundred meters, MMF!with better optical coupling and shorter propagation distance!is the better choice. To improve the bend performance and optical coupling for these applications, Corning developed the ClearCurve VSDN fibre. This fibre has a reduced diameter from the typical 125?m to 100?m to mechanically provide a bend radius as low as 1.5 mm.

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