Page 92 - 2020 Interconnect Innovations eBook

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lower numerical aperture, called the cladding, that keeps the light signal within the core. It is important to note that the core and cladding cannot be separated, as they form a single piece of glass. The diameter of the entire fiber is 125μm, which is roughly the diameter of a human hair. The core area that carries the signal is a fraction of that size. In multimode fiber, the core accounts for 40–50% of the overall fiber; in single-mode fiber, which has greater bandwidth and distance capabilities, the core only accounts for 6.5% of the overall diameter. With that in mind, it becomes very obvious that airborne particulate, dander, lint, fingerprints, and even contact wear from mating surfaces can degrade signal performance. Advanced dry-contact cleaning tools help remove contaminate, but they are not always 100% effective, especially on array-type connectors, where the debris can be moved from one fiber to another.
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» Various technological approaches to dust-insensitive fiber connectivity. The legacy ball-lens expanded-beam contact technology has formed the groundwork for newer designs that not only improve insertion loss, return loss, and repeatability but also enable dust-insensitive multifiber connectors, which has been an ongoing challenge.
Ball-lens expanded-beam (EB) connectivity was the first successful approach to dust-insensitive connectivity and is still used today in harsh-environment military and aerospace applications. It could theoretically be used in enterprise and data center applications as well, but since it is designed for a single ferrule, it tends to increase the footprint and raise costs. The idea behind EB optical connectors is that the first ball lens expands the light many times to overcome the impact of dust, and then the second ball lens re-collimates the light back into the core of the receiving fiber. So, there is a loss penalty, but it is an accepted tradeoff.
When it comes to multifiber or array connectivity, the individual ball lenses are not practical. Over the years, advancements in EB technology have helped adapt to array connectivity. One example is coplanar EB technology, which features MT-sized, monolithically molded ferrules with lens features molded into place for each fiber in the array. Because the lenses are part of the ferrules, polishing is not required, but a precision cleave is. Similar to single-fiber, ball-lens, EB contact technology, coplanar EB contacts expand the light multiple times and bridge a gap between the lenses before being re-collimated by the receiving lens. Like their ball-lens brethren, these types of EB connectors are easy to clean and dust-insensitive. As such, they are very effective for multimode connectors and on-board optics applications; however, they have difficulties achieving required performance loss limitations in single-mode connectors.
To provide a solution for single-mode arrays, air-gap connector technology was developed. This technology utilizes a traditional MT-type ferrule but incorporates special features that create a precision-engineered and aligned air gap between end-face mating fibers with various geometries. This air gap aims to ease cleaning processes and reduce mechanical force on the fibers and prevent embedded debris that could cause permanent damage. These types of connectors mate with both other air-gap fiber optic connectors and standard, physical-contact fiber optic connectors. Like coplanar EB technology, air-gap connector technology is finding various opportunities in plug-to-plug and on- board optics applications.