Interconnect Product Showcase
Gregory B. Noll
MIL-C 38999 Series III connectors have been utilized for avionics electrical connection systems for years. From cockpit electronics to weapon systems, the MIL-C-38999 connector has provided a rugged fault-free electrical transmission junction. With the advent of optical fiber communication technologies, the standard MIL-C-38999 electrical connector was pressed into service as a fiber optic interconnect. While the connector has been used successfully in a wide range of fiber optic applications, the goal of improving data loss values to an ambitious .3dB has driven standards organizations to develop a new specification and qualification model for a next generation fiber optic connector. The necessary evolution in connector (and termini) dimensional, environmental and optical specifications to achieve the target .3dB data loss value is the subject of this white paper.
After discussing the design characteristics for the new advanced MIL-C-38999 Series III style connector specified by the SAE AE8-C1 group, and presenting complementary termini design issues, the author concludes that the primary design goal of .3dB nominal data loss is achievable for multimode fibers and possible for future high bandwidth applications using singlemode fiber.
Fiber optic systems are used to transmit analog and digital data signals in lightweight applications where RFI/EMI/EMP immunity is required. Such fiber optic systems generally consist of multiple channel optical fibers or cables which are terminated to standard electrical and specialized optical connectors. Termination of fiber optic cables requires precise axial alignment of the optical fibers to avoid reflection of the transmitted light in order to prevent errors and distortions of the output signal.
The output signal emerging from the end of the fiber at the detector is always less than that entering due to optical losses. Losses occur in an optical link in one of two ways: The first is caused by backscattering or absorption in the core. This fiber loss is dependent on the length of the fiber in the system and results in an exponential proportional decay or decrease in optical power. In short length avionic systems this does not usually contribute to a noticeable power budget concern. The more critical element is connector related losses caused by core to core misalignment which results in imperfect reflection at the optical interface. To minimize optical loss in a butt-joint connector, axial and angular fiber alignment is critical, otherwise insertion loss at the connector interface will reach unacceptable levels. In applications where glass to glass contact has been eliminated to prevent possible damage of the optical end-face, additional insertion loss values are introduced due to the change in refractive index from glass to air and back to glass. Such fresnel reflection loss can be mitigated with effective physical contact polishing techniques or through the use of pre-radiused terminus ferrules.
The use of fiber optics in avionics is not new. In 1976 the U.S. Air force replaced a wiring harness of an A-7 aircraft with an all optical data link in its airborne light technology program (ALOFT): 302 electrical cables, over 1,200 meters in length and weighing over 40,000 grams were replaced with 12 fibers, 76 meters in length, weighing less than 1,700 grams.
Regardless of the cable media (copper or fiber), the MIL-C-38999 connector is currently the most commonly specified multi-pin cylindrical interconnect for avionics. Given its wide use, performance specifications for MIL-C-38999 connectors have been continuously revised over the years to accommodate new coupling styles, materials specifications and so on.
Various connector manufacturers have been qualified to supply this connector series to meet the harsh avionics requirements contained within the current MIL-C-38999 specification. With the growth of fiber optics in avionics, several qualified connector manufactures developed their own commercial fiber optic termini compatible for use within the standard electrical Series III connector shell. These companies marketed their termini to users of Series III connectors as a practical approach to retrofitting this popular electrical connector for optical fiber. The terminus was designed around the standard size #16 AWG electrical contact and thus could be installed into any connector with a size #16 AWG cavity. In this way, the MIL-C-38999 Series III connector was effectively evolved to accommodate both fiber and electrical contacts.
The first military qualified fiber optic terminus was developed for shipboard use with a MIL-C-28876 style circular connector shell. These front release, rear insertable termini eventually were incorporated into a (then) new MIL-T-29504 termini specification. The MIL-C-28876 dedicated fiber optic connector specification was established to mirror the MIL-DTL-28840 electrical connector environmental requirements. With MIL-C-28876 being a strictly fiber optic connector, only optical test criteria was specified. Termini were qualified to a seperate MIL-T-29504/1 and /2 document. In this manner, the complete fiber optic connector system was qualified.
This connector however was not designed to meet MIL-C-38999 avionics requirements. The MIL-T-29504 specification was eventually revised to support a rear release, rear insertable avionics fiber optic terminus. The MIL-T-29504 specification does call for a number of environmental tests such as temperature life, thermal shock, mating durability, physical shock, and vibration. All of these tests are applied to the terminus when used with a qualified MIL-C-38999 Series III connector.
Numerous fiber optic avionics programs incorporated systems using standard MIL-C-38999 Series III connectors with MIL-T-29504/4 (pin) and /5 (socket) termini with good success - achieving loss values under .5dB. There is considerable agreement however that since the MIL-C-38999 connector is by design an electrical connector, not originally developed to align optical fibers, additional improvements in data loss values are unlikely to be achieved - at least not without a substantive revision of the MIL-C-38999 specification. Many fiber optic engineers are also in agreement on the need for further evolution of combined connector-termini qualification specifications, addressing both optical, dimensional and environmental variables, in order to meet improved nominal insertion loss values.
The F-22 fighter and RAH-66 helicopter fiber optics working group (FOWG) began the process of evaluating and testing composite MIL-C38999 Series III connectors and termini a number of years ago. The group was established to create standards for common fiber optic components between both avionics platforms.
In an effort to develop a high reliability, low insertion loss multichannel circular connector, the FOWG established a list of preferred design goals for a new fiber optic connector for avionics. This effort is now being coordinated by the Society of Automotive Engineers (SAE) under the SAE AE8-C1 task group. This group is also developing new design characteristics for this advanced avionics connector which will mirror the harsh environmental requirements of MIL-C-38999 Series III. The group's recently published recommendations for the design of the dedicated MIL-C-38999 series III fiber optic connector include a number of design characteristics which will enhance the ability of the new connector to achieve the .3dB nominal insertion loss for 100 micron graded index fiber. Of the some 15 recommendations, the following are considered critical:
- The connector shall have close tolerance design features to insure precise optical alignment. The connector polarization keys and keyways, for example, will be controlled with tighter tolerances than MIL-C-38999 to reduce radial misalignment of the optical termini.
- The connector shells must have a positive stop or bottoming surface to insure shell to shell bottoming. This will insure that the linear dimensional relationship of the termini are the same after each connector mating. The connector effectively "seats" at a predetermined location each and every time. This location or datum surface provides a reference location back to the terminus retention clip area. The pin and socket location can thus be dimensioned from this stable bottoming surface to achieve a repeatable and reliable connection.
- The connector coupling thread, backshell attachment configuration and mounting footprint shall comply with the existing MIL-C-38999 Series III connector specification. This allows for possible retrofitting of new advanced fiber optic connectors on existing programs without changing equipment or panels.
- Molded, plated composite thermoplastic construction is a desired method to cut off EMI/RFI penetration through the connector into the electronics equipment area. Although optical fiber is immune to EMI/RFI, plated composite construction would prevent any possible penetration through the connector into the electronics area. Molded composite materials will also provide substantial weight savings, corrosion resistance and result in an extremely tight tolerance, repeatable design. A molded part, once tooled, offers closely controlled dimensions with little variability from one part to the next. Once the insert is tooled, the cavity locations of the optical termini will be more reliably and repeatedly located than with a machined part.
- It is the primary design goal of the SAE working group to incorporate these and other key recommendations into a new design for a next generation MIL-C-38999 fiber optic connector. Complimentary termini design criteria affecting nominal loss are equally critical.
- The fiber optic contact or terminus is the primary alignment mechanism for connecting two optic fibers. Over the past two decades there have been dramatic tolerance improvements in terminus design to insure precise, repeatable, axial and angular alignment between pin and socket termini within the connector shell. The terminus ferrule which connects the optical fiber within the terminus has seen the most substantial tolerance improvement.
- Ferrule designs have been fabricated out of stainless steel with a precision micro-drilled hole. Jewel inserts and centering rods have also been utilized within stainless steel ferrules. These alignment methods worked well with large core multimode fiber, but with the advent of high bandwidth multimode and single mode fiber (core sizes from 100 to 10 microns in diameter) additional improvements in fabrication tolerance control was required.
- Precision ceramic ferrules with concentricity and diametric tolerances controlled within a micron (.00004 of an inch) were developed for application in high bandwidth systems. These ferrules are now approximately 10 times more accurate than previous designs. This improvement resulted in insertion loss values from 1.5dB to less than .5dB. Additional ferrule tolerance control can be achieved using specialized active alignment termination techniques to center the fiber. Although this method achieves extremely low insertion loss values, special equipment is required to perform this type of termination, and is not necessarily a recommendation of SAE AE8-C1.
- The SAE group has called for termini insert density equal to or better than the current MIL-C-38999 Series III, as well as compliance with MIL-T-29504/4 and /5 rear release and rear insertable specifications to insure no special tools are required beyond standard electrical contact tools.
The following design features of qualified termini are considered critical if target insertion loss levels are to be achieved:
- A resilient spring characteristic to secure ferrules within the socket. The spring must provide a sufficient load bias to hold the two termini glass faces together through all environmental tests such as vibration and shock.
- The alignment sleeves provide the mechanical means necessary for bringing the two terminus ferrules in precise axial and angular alignment. The sleeves are slotted to offer a resilient encasement of the terminus ferrule. Sleeve material selection is critical in the terminus design. The two most widely used materials are stainless steel and ceramic. Stainless steel sleeves work well and are extremely durable, however, they do not offer the surface finish quality, hardness, or dimensional stability of ceramics. When mated with hard ceramic ferrules, the softer stainless alignment sleeves can exhibit wear characteristics which may result in unwanted contamination. Ceramic sleeves can be manufactured to tightly controlled tolerances but are more fragile than stainless. This design concern has been over-come with the use of metallic protective alignment sleeve covers.
- Ferrule designs must conform to two critical dimensional features: the outer and inner diameter of the ferrule and the concentricity of these two diameters. The end-face profile of the terminus ferrule must also be configured or polished to eliminate unwanted back reflections in single mode fibers. This can be achieved in two ways: by polishing the fiber end-face and ferrule on a rubber pad, or by using pre-radiused ceramic ferrules. Such convex polishing techniques reduce fresnel air-gap reflections to achieve loss values as low as .2dB for multimode fiber. Physical Contact (PC) and concave polishing methods are both considered acceptable, depending on the application and ability to clean the terminus after each insertion. However, PC polish must be used to achieve the .3dB design goal.
- Termini must be able to provide a mechanism to provide strain relief to the fiber optic cable for captivation. Three methods are considered suitable: crimp sleeve, shrink sleeve, and epoxy.
If the design goals defined by SAE AE8-C1 are reached for both connector and termini, future high bandwidth applications will achieve a nominal insertion loss value of .3dB or less for 100 micron graded index fiber. The resultant design would then be suitable for developing singlemode applications. This conclusion is based on the clear advantages the specified tight tolerance fiber optic connector and termini will provide over the current MIL-C-38999 series III electrical connector configuration.
Even with the anticipated improvements, fiber optic engineers may still be faced with a cumbersome qualification process. Granted, the new SAE connector specification will be an important improvement to the qualification process, as the new document will specify all optical test measurements within the connector specification as well as environmental variables such as vibration, shock, altitude, thermal shock and mating durability.
But all discussion of the advanced MIL-C-38999 series III connector and its new robust qualification specifications aside, an important additional step which should be advanced in the fiber optic community is the development of a recognized qualification specification covering optical, dimensional and environmental variables for both connectors and termini. In the end, we must have two parallel efforts in place: a design that provides precise optical alignment, and a qualification specification that addresses both optical and mechanical requirements. Such a step will facilitate additional advances beyond the .3dB nominal loss threshold.
Gregory B. Noll is the fiber optics product manager for Glenair, Inc. He received his bachelor's degree in Industrial Technology Engineering and holds a number of patents relating to fiber optic interconnect systems. Prior to joining Glenair he was a senior design engineer for Hughes Connecting Devices.