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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?

For the end-to-end electronic signal, large signal analysis using a bit-error rate tester and a real-time oscilloscope was used to assess how these trade-offs affected the quality of the electrical eye diagram and error-free transmission. These measurements were performed with a variation of input jitter to understand the sensitivity of the system as well. In this manner, the transceiver parameters were selected to meet the electrical specification while also determining the optical throughput requirements for testing during manufacturing (figure 10).

Through the manufacturing process
Once the design and parameters were specified, the AOC was ready for manufacturing. It is, however, important to test throughout the manufacturing process, which lets us scrap defective parts early. With this in mind, after assembly of the board, which includes the PDs, VCSELs, and coupling optics (module with the TIR, lenses and v-grooves), the board is tested to meet optical performance metrics using a golden cable assembly. Once two known good boards are verified using the golden assembly, they are assembled together to form the AOC. Prior to placing the mechanical strain relief and overmold that form the end connectors, we test the end-to-end cable again to ensure it was built within manufacturing tolerances. To do this test efficiently, we designed a test box and developed test code to run the cable through a series of electrical and optical tests. Through these tests, we can verify that all lanes operate with correct optical throughput, an open electrical eye, zero bit errors, as well as many other tests specific to the protocol. If the cable passes this series of tests, the overmolds are placed on the end connectors and the final cable assembly steps are completed. The same tests are performed again prior to shipping the cable to the end customer.

Figure 10: The test setup for large signal analysis uses a bit-error-rate tester with a real-time oscilloscope.

As data rates continue to rise, electrical signal transmission over shorter and shorter distances becomes increasingly impractical. As a result, optical fibre is now poised to find applications in the consumer and industrial markets. Protocols such as USB and Thunderbolt, which can achieve data rates of 10 Gbit/s extend the reach of traditional copper interconnects.

Optics can reduce these distance limitations. Although solutions in the consumer and industrial market need to be both small, low-cost, and robust, advancements in fibre optic systems are bringing them closer to cost parity with electrical systems. The entire optical link can be engineered with cost in mind. Testing and analysis done at crucial junctures in the design and manufacturing process can further reduce costs by creating a design that accommodates larger manufacturing variances, making active optical cables a compelling solution for the rapidly-expanding consumer and industrial markets.

About the author
Rebecca K. Schaevitz received her B.S. in Electrical Engineering, Summa Cum Laude, from Tufts University in 2005 and her M.S. and Ph.D in Electrical Engineering from Stanford University in 2008 and 2011, respectively. Since joining Corning in 2011, Rebecca has contributed to the Thunderbolt program and aided in releasing the first consumer product for Corning in decades. Her initial focus was on the prototype build development, working with a number of contractors to ensure a seamless process flow. After prototyping, Rebecca focused on understanding and testing towards Thunderbolt certification standards by working with Intel and Apple. She helped ensure certification of all cable lengths now available to consumers on and other online retailers.

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