Mil-Aero Fiber-optics Apps Spur Connector Innovation
Commercial aerospace termini team up with rugged-application circular connector shells.
Let’s begin with two facts. First, today’s aerospace applications need more bandwidth to accommodate the ever more sophisticated sensors and controls. Second, space and weight are critical issues and anything that can reduce weight or take up less space is a good thing. Saving weight in an unmanned aerial vehicle, for example, directly relates to longer flight times and heavier payloads. Bandwidth in copper cable is limited by distance: the higher the speed, the shorter the transmission distance. Fiber bandwidth is independent of distance, so you can transmit the highest speeds the full length and width of an aircraft. Add in EMI immunity for noise-free signal integrity, and you can see why the use of fiber is growing.
As the use of fiber-optics in military and aerospace applications has increased, so has the variety of fiber-optic connectors. The military imperative to adopt commercial off-the-shelf (COTS) products in ruggedized formats is causing more products to come on line that help field current technology sooner. New solutions are also reducing costs by using higher-volume commercial products of a proven pedigree as qualified to ARINC standards.
TE Connectivity (TE), for example, recently introduced the MC801 connector (Figure 1), which combines the most popular optical termini from commercial aerospace and the most widely used connector shell in military applications: the ARINC 801 termini and 38999-style shell with a purpose-built for fiber-optic insert set.
While MIL-PRF-29504/4 and /5 termini for 38999 Series III connectors already offer comparable performance, one problem with these termini is the method for aligning the two ceramic ferrules. All ferrules need an alignment sleeve to align them precisely for low insertion loss and high return loss. MIL-PRF-29504/4 and /5 termini use a pin and socket configuration, where the alignment sleeve is built into one of the mating socket terminus. The drawback is the maintenance and cleaning. Access is limited to the ferrule’s tip, located deep within the sleeve, making inspection and cleaning inconvenient.
Genderless Termination Increases Ease and Simplicity
Ceramic ferrules are a well-established technology that has long dominated network and telecommunication applications. The ARINC 801 standard brought this technology to the more challenging commercial aerospace environment. ARINC 801, like most connectors based on ceramic ferrules, employs a genderless approach (Figure 2). The alignment sleeve “sandwich” is a single, removable element that aligns all the ferrules and makes them easily accessible for cleaning. Because only one type of termini is needed for each connector, the parts count remains low, and assembly is straightforward. One other benefit of genderless terminations is there is only one terminus part number. M29504 termini require stocking a separate pin and socket.
The ARINC 801 contact also has a size advantage. Across industries, the standard ferrule diameters are 2.5 mm and 1.25 mm. The 2.5-mm ferrule is the older standard, found in such connectors as the SC. The smaller 1.25-mm ferrule was made popular by the LC connector. The ARINC 801 ferrule is an industry standard 1.25 mm. M29504 termini were originally designed to fit Size 16 cavities and with several ferrule diameters to accommodate a range of fiber sizes.
For example, when originally designed, the 100/140-μm multimode fiber was a contender for widespread use. It has fallen by the wayside, and the 50/125-μm fiber is the de facto standard. While the ability to be used in a standard 38999 connector and be mixed with electrical contacts is a typical application, M29504 termini, being a Size 16 product, limit the contact density possible. Take a Size 19 shell for example. The standard 38999 provides 11 Size 16 cavities. The MC801 accommodates 12 fibers.
Using 38999 Series III style connectors provides scoop proof mating and access to the wide range of backshells and other hardware.
The MC801 connector is available in eight shell sizes to accommodate 2 to 32 fibers. Shells are available in several materials. Lightweight aluminium is the traditional material. Composites are growing in popularity because of the significant weight savings. Marine bronze is available for the best salt spray resistance in naval applications as well as stainless steel. Shell finishes include olive drab cadmium, nickel, and zinc nickel.
Expanded Beam Options for 38999 Connectors
While the ceramic ferrule remains the most widely used fiber-optic connector technology, expanded beam lensed technology finds use where vibration is exceptionally high, dirt and contamination are issues, or the fiber/termini endfaces must be protected.
The termini’s ball lens physically expands and collimates the optical signal into an optical beam well beyond its original size to help provide easier optical alignment, low sensitivity to contamination, and consistent performance over thermal changes. The beam is then refocused back down onto the core of the receiving fiber. The non-contacting interface means less wear and tear overall, especially in high-mating cycle or high-vibration applications.
While expanded beam technology is not new, it was not easily adapted to high-density applications. The first generation termini were relatively large—at best, you could fit 8 termini in a size 25 shell. More recently, the technology has evolved to allow it to fit into a standard size 16 cavity. The termini are typically positioned as an alternative to MIL-PRF-29504 termini, but much easier to clean and maintain.
Consider the Entire Link
As with any communication link, you must consider the whole link from beginning to end. Does the link deliver sufficient power for the optical receiver’s sensitivity? Within the link’s power budget, connectors play a significant role in ensuring the budget is met. Connectors are typically the greatest source of power loss in a link. Depending on the connector/termini type, the fiber and the light coming through it, and other factors, a connector’s insertion loss can range from a modest 0.1 dB to over 1.5 dB. In military and aerospace applications, connectors must be purpose built to withstand the mechanical and environmental stresses while still offering low insertion loss. Once the budget is satisfied, users look to ease of use and economies of scale that can derive from COTS-pedigree products. A connector that brings together tried and true military and COTS technology as well as high optical performance and ease of use is recommended.
Earle Olson is Business Development Manager Global Aerospace Defense and Marine, at TE Connectivity. With 35 years of experience in the electronics and fiber-optic interconnect industry, Earle has a wide range of experience in Commercial and Mil Aerospace applications as well as innovations such as the first video phone, early versions of the PC and servers to early In-Flight Entertainment systems and C4ISR.