VITA 66 expands VPX with fiber-optic connectivity

A trinity of fiber-optic VITA 66 module varieties takes VPX lighter, faster, farther.

As rugged embedded computing systems strive for higher operating speeds and densities, adding fiber-optic connectivity to the proliferating VITA 46 (VPX) market space brings benefits in higher bandwidth, longer transmission distances, and lower weight. VITA 66 defines fiber-optic connector modules for VPX using well-established and proven optical termini. Each style of termini offers different benefits in terms of density, ruggedness, repairability, and other characteristics.

The thirst for higher bandwidth and faster processing speeds in military and aerospace applications seems unquenchable. (), with a backplane connector system supporting 6.25 Gbps in a switched-fabric architecture, is the latest generation of VMEbus and offers new levels of performance for embedded computer systems. VPX systems are designed for the flexible application of demanding high-speed protocols – such as , , and – in ground, aerospace, and marine applications. While the basic architecture of VPX is defined in VITA 46 and 65 (), ongoing efforts by the VITA Standards Organization () to enhance VPX capabilities include – “Optical Interconnect on VPX” – which adds fiber-optic interfaces to the connector platform.

The issue at hand is one of elegant, high-volume data transport. As operating speeds increase, copper cables become increasingly limited in transmission distance, besides becoming heavier and more expensive to deploy. For example, Category 6 twisted-pair cable runs 100 m at 1 Gbps. As the I/O speed increases to 10 Gbps, the recommended cabling distance reduces to 55 m in a benign electrical environment. Where high levels of alien crosstalk may exist – such as with high densities of closely packed cables – the distance is reduced to 37 m. Military and aerospace applications also can involve substantial levels of EMI, elevating the need for cable shielding, which increases the size, weight, and complexity of the interconnection system.

Fiber-based I/O does not offer the same trade-offs in bandwidth and distance. Among the well-known advantages of fiber-optic cables are: lighter weight, higher bandwidth, and longer transmission distances. For instance, with single-mode fibers multikilometer transmission distances at multigigabit speeds are possible. In addition, fiber-optic cables are inherently EMI resistant and require no electromagnetic shielding.

The installed advantages are clear, too. In cutting-edge composite vehicles, fiber-optic lines minimize shielding and bonding challenges. Problematic installation of heavy metallic components is eliminated, saving weight and time and reducing risk. Vehicle designers achieve location-independent architecture. Boxes that are meters or kilometers apart communicate as though colocated. VITA 66, by defining optical I/O capabilities, additionally provides improved density, ruggedness, and repairability via three termini styles.

Flexible options in optical connections

The VITA 66.0 base specification defines the common mounting interface requirements for the various fiber-optic interconnects within 3U and 6U VPX applications. This includes definition of the mounting provisions, permitted locations, and range of engagement. Additional “dot” specifications define requirements for the specific optic module interfaces. A fundamental goal of VITA 66 is to offer designers multiple existing and fielded mil/aero termini technologies, allowing them to quickly and confidently implement the best solution for specific applications. The three module varieties are based upon proven optical termini:

  • MT ferrule (VITA 66.1)
  • (VITA 66.2)
  • Expanded Beam (EB) insert (VITA 66.3)

Each style of termini offers different benefits in terms of density, ruggedness, repairability, and other characteristics. In addition, the modules are designed to meet the requirements of (Environments, Design and Construction, Safety, and Quality for Plug-In Units), which covers environmental and mechanical ruggedness for VPX systems, including temperature cycling, vibration, shock, altitude, and more.

MT ferrule for high fiber counts

VITA 66.1 modules use the MT ferrule, configured to enable up to 24 optical fibers per the standard. With two ferrules per module, this equates to 48 fibers in a 3U system and up to 240 fibers in a 6U system. Of all industry-standard optical connectors, the MT ferrule provides the highest-density interconnections for both multimode and single-mode fibers in a ferrule with an end-face only 6.4 mm by 2.4 mm. The MT, with its history in rugged box-level applications, is ideal for switches and concentrators.

Application considerations for the MT ferrule are the inability to perform field terminations and to replace individual fibers. In addition, the MT is a physical contact style termini, meaning the glass end-faces are in direct contact. While this style of termini initially can provide very low loss, polish degradation via end-face abrasion is possible.

The interconnect for MT ferrules is the first “dot” specification to be published, and will be VITA 66.1. The intention is to have it serve as a template, allowing the additional “dot” specifications to be created and ratified quickly. It is anticipated that VITA 66.0 and 66.1 will be submitted to ANSI in mid-2011, with VITA 66.2 and 66.3 following later in the year.

ARINC 801 termini for highest optical performance

ARINC 801 termini, as used in VITA 66.2 modules, are based on industry-standard 1.25 mm ceramic ferrules, bringing all the advantages of discrete ceramic ferrule connectivity. The ceramic ferrules offer high-performance features that include physical contact technology for very low insertion loss, angled polishes for minimal reflection loss, and keyed orientation for optimal single-mode performance.

In contrast to the MT ferrule, ARINC 801 termini allow individual optical fibers to be installed or removed. The contacts are a standard technology within commercial aviation, but they are also finding wider use in military/aerospace, marine, and ground applications. Like MT ferrules, the ARINC 801 termini are a physical contact technology and may experience higher loss over time due to end-face abrasion.

Expanded Beam (EB) termini for extreme environments

EB connectors are widely known for being extremely robust.

The EB insert, defined in VITA 66.3 and based on ball lens technology in MIL-DTL- 83526/20 and 21, is the most rugged interface and supports up to four fibers per module. The ball lens technology expands the effective diameter of the channeled light and collimates the beam, projecting it across an air gap to the receiving ball lens, which reverses process and reintroduces the light to the awaiting fiber core. Where a speck of dust might completely block a 50 micron or smaller fiber core, the expanded beam minimizes the impact of dirt and debris on insertion loss. A small dirt particle on the glass face of the expanded beam connector does not dramatically block the light as it can on the end-face of a typical ceramic-based ferrule.

The non-contacting EB interface eliminates potential abrasion induced by shock, vibration, or repeated mating/unmating. The fiber ends are basically protected behind safety glass, allowing very easy cleaning and inspection. The EB interface makes them suited to two-level maintenance or applications calling for frequent insertion/extraction, such as a secure storage device requiring removal after each flight operation.

Insertion losses—the loss of optical power as light passes through the connectors—for EB connectors are higher initially. But unlike MT and ARINC 801 termini, the losses remain constant over the life of the connector without degradation because its non-contacting optic interface does not become damaged. The EB interface is a better solution for applications with higher levels of vibration, dirt, and mating cycles. Where multiple mating cycles are expected, it only takes one mating of an MT or 801 terminus without adequate cleaning to permanently damage the fiber endface. Table 1 summarizes the performance characteristics of the different optical styles.

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Table 1: Performance characteristics of VITA 66 optical termini. (*proposed)
(Click graphic to zoom by 1.9x)

While VITA 46.0 defines P5/J5 and P6/J6 as user defined locations in the 6U implementation, and P2/J2 in the 3U, the VITA 66 fiber-optic modules have been designed for installation in P2/J2 through P6/J6. The integrator could even mix the ferrule technologies along the same card edge if the situation warrants doing so. Figure 1 illustrates the three module types on a notional 6U VPX card edge.

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Figure 1: VITA 66 modules applied
(Click graphic to zoom by 1.9x)

VITA 66: Flexibility in VPX optical interconnects

VITA 66 is an important addition to the VPX family and further empowers the rugged embedded computer architect. It supports a wide range of fiber-optic interconnection needs, from intrasystem board-to-board links to intersystem networks spanning many kilometers. Because application requirements can vary greatly between airborne, shipboard, and ground defense needs, the standard offers a range of high-integrity choices. By using well-established termini, VITA 66 will also speed adoption of optical interconnections in VPX systems. Distance, weight, EMI, and bandwidth limitations are no longer insurmountable challenges for data transport in rugged embedded computing. CS

Gregory Powers serves as Market Development Manager for the Electronic Systems and Space segments within the Global Aerospace, Defense & Marine business unit of . Mr. Powers received a BMSE from Syracuse University, has completed numerous graduate-level studies, and holds two patents relative to optic datacom devices. He can be contacted at gregory.powers@te.com.

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