 Figure 1. A MIL-C-83723 connector with fiber optic pin contacts. |
By George Schaeffer
Recent developments in fiber optic technology have led to cost reductions for circular, multipin connectors.
For the past two decades, there has been relatively little innovation in military fiber optic connectors for multimode cable. During this period, the price of these connectors has remained in the stratosphere. Now, recent developments in fiber optic technology have led to dramatic cost reductions for circular, multipin connectors (see Figure 1). At the same time, new fiber polishing techniques for "butt-joint" contacts have produced superior optical performances in vibration and shock environments.
Single-body Contact Construction
The first cost-saving innovation is eliminating the expensive ceramic ferrule component. Almost all military fiber optic contacts have a ceramic ferrule that is pressfit or bonded into a stainless-steel body. The main purpose of this ferrule is to accurately align opposing fibers in the mated condition. Thus far, the connector industry has managed to convince its customers that ceramic is the only material that can be used to align fibers. The absolute insistence on ceramic has prevailed despite the negatives, such as:
- Frequent industry-wide shortages of ceramic ferrules
- Labor involved in assembly of the ferrule to the contact body
- Additional tight tolerances on the components for purposes of assembly
- Different coefficients of expansion between ceramic and stainless steel, leading to occasional fracturing of the components during thermal cycling.
This "old-style" attitude about ceramic has been challenged by state-of-the-art screw-machining equipment. The equipment can fabricate "all-metal," single-body contacts with tolerances equivalent to ceramic parts for multimode applications. Figure 2 shows a one-piece MIL-T-29504/10-style pin contact and MIL-T-29504/11-style socket contact as an example.
The critical tolerances that are currently being maintained on a production basis for stainless-steel contacts are 3 mm (approximately one ten thousandths of an inch). That is 3 mm for the fiber hole diameter and 3 mm for concentricity of the hole relative to the outer contact diameter. On a production basis, the tolerances are monitored by both physical and optical measurements.
Zirconia ceramic with a smaller microstructure or particle size than stainless steel is preferable for singlemode fiber. But for multimode applications, the two materials are now basically equivalent with respect to the required tolerances. And for singlemode applications, considerable research is going into the development of corrosion-resistant metals with finer microstructures. These new alloys will be machinable to singlemode tolerances.
Some connector designers have stated in the past that the hardness of a ceramic ferrule is an advantage. They contend the ceramic's hardness resists abrasion during the polishing of the fiber by hand-held pucks. However, since the advent of spring-loaded, constant-pressure polishing tools, the hardness of the ferrule has become an obsolete issue.
Another benefit of steel over ceramic is the versatility of in-house machining of stainless steel. Holes for nonstandard-size fiber are accomplished with a simple tooling change. This translates to dramatically shortened delivery schedules for all-metal contacts that can be controlled directly by the connector manufacturer. Special hole sizes for ceramic ferrules require a complex series of grinding and polishing operations that can only be accomplished by a very limited number of ceramic suppliers.
 Figure 2. A MIL-T-29504/10 pin contact and MIL-T-29504/11 socket contact. |
To further reduce cost, an inexpensive, solid-state preform is factory-installed in the contact body. This allows the end user to forgo mixing and injecting two-part liquid epoxy during cable termination. The epoxy preform can be used for one-piece metal contacts or with ceramic ferrule contacts.
The socket contact (see Figure 2) consists of a machined body and sleeve along with a helical coil spring and an epoxy preform. The socket contact is designed for use with standard military circular connector housings. The envelope dimensions for the one-piece pin contact are basically equivalent to the standard MIL-Spec copper wire contact.
Connector Housing Myth
Almost all military fiber optic contacts are completely self-aligning and individually spring-loaded (see Figure 3). The contacts do not depend on the connector housing in any manner for alignment of the fibers. The basic purpose of the plug and receptacle is to protect and seal the contacts from the environment. In this sense, the connector is the "low-tech" portion of the interconnect.
 Figure 3. An all-metal fiber optic pin-and-socket contact. |
The "low-tech" label for military fiber optic connectors is true in every sense of the phrase. Most of the connectors now used for fiber optics are the same as those manufactured back in the 1950s. In the last 40 years, little has changed. The design, construction and materials have remained the same. The seals are still molded out of silicone rubber or similar elastomer. The insulators are molded out of the same thermoset plastics. The metal shells are still made of plated aluminum. When these connectors are sold for use with copper wire contacts, the price is less than $100.00 per mated pair. When the same connectors are sold for use with fiber optic contacts, the price is often more than $500.00 per mated pair.
What allows this pricing policy to persist is lack of competition in the military fiber optic connector industry and intentional confusion about the "low-tech" role of connector housings. Fortunately, word is slowly filtering out to end users.
Military fiber optic connectors do not have to be expensive; ceramic ferrules are not needed for multimode fiber; and "low-tech" connector housings do not impact the alignment of fibers.
Standard MIL-Spec Connectors
Another cost saving is the utilization of standard MIL-Spec connectors, such as MIL-C-83723 and MIL-C-38999. "Off-the-shelf" connectors can be employed for fiber optics with slight tolerance adjustments to the components. A few axial dimensions of the plug and receptacle in the mated condition should be controlled. This primarily entails the distance between the pin-and-socket contact retention clips. Also, metal-to-metal bottoming by connector halves should occur for continuous optical performance in vibration and shock environments. In many cases, standard connectors straight out of the manufacturer's warehouse meet all the requirements for high-quality fiber optic interconnects.
Optical Performance
Next to fiber alignment, the most important factor in an optical connection is the polishing of the fiber endfaces. Typically, the endfaces are polished in a convex shape, referred to as PC for physical contact. This interface works well in a benign environment but for equipment exposed to vibration and shock, the glass interface becomes scratched, which decreases optical performance.
 Figure 4. Concave polish of fiber endfaces. |
A new endface configuration that withstands the rigors of constant, long-term vibration in military applications has been developed, with the cooperation of an aerospace corporation. The ends of the fiber are polished in a concave shape to produce a carefully controlled air gap between opposing fibers (see Figure 4). This proprietary polishing procedure creates a stable optical connection that maintains insertion values below 1.2 dB over the life of the connector.
The requirements for the topography of the fiber endface (see Figure 5) were determined first by finite element analysis of the contact/epoxy/fiber structure using a target contact spring force. Then, empirical measurements were made to verify the elastic deformation within the structure. Finally, prototypes were tested at vibration levels exceeding MIL-Spec standards for circular connectors.
High-force Contacts
 Figure 5. Interferometer plot of concave endface polish. Courtesy of The Boeing Co., Space Station Div. |
All "butt-joint"-type fiber optic contacts require a constant spring force to press the mating contacts together. This force prevents the fiber endfaces from temporarily separating because of external disturbances or vibration. For traditional PC polished fibers, the minimum force to withstand normal usage and maintain adequate optical performance is about 1 lb. Above 4 or 5 lb, the spring force produces Hertzian stresses in the glass interface of PC polished fibers that takes the glass beyond the elastic range to ultimate fracture.
One advantage of concave polishing on fiber endfaces is that almost all the compressive spring force at the optical interface is absorbed by the steel contact. The relatively soft glass is isolated from external forces. A contact design can be employed utilizing much higher spring forces because fiber fracture is not a concern. These high-force contacts perform much better at extreme vibration levels.
Testing has shown a direct correlation between the spring force preventing separation of mated fiber optic contacts and optical performance in severe vibration and shock. The main factor preventing light loss spikes resulting in signal deterioration is the ability of the optical interface to remain intact.
The high-force contact allows for use of fiber optics in severe environments from high-performance aircraft to constantly vibrating heavy machinery. The elimination of fiber endface scratching can provide a long-term, maintenance-free connector for many new areas of broadband communication.
Making the Case
Many in the military and aerospace recognize the benefits of broadband communication through fiber optics. But in various instances, the cost and lack of ruggedness exclude fiber and fiber connection from the realm of feasibility. Now, developments that combine lower connector cost with more robust performance turn optical communication into a possibility. In the near future, any place that copper can reach, affordable fiber optics will soon follow.
GEORGE SCHAEFFER is R&D project engineer for fiber optics, J-Tech/Conesys Fiber Systems, 14251 Franklin Ave., Tustin, CA 92780-7008; (714) 665-2080; Fax: (714) 665-2099; E-mail: gschaeffer@conesys.com; Web site: www.j-tech.com.
SPEC SHEET
End Applications: Military and aerospace broadband equipment
Related Products: Fiber optic contacts, circular, multipin fiber optic connectors
Main Point: Drawbacks of fiber optic connectors for military use are the cost and the lack of resistance to continual vibration and shock. New developments in alignment and polishing, however, have reduced the cost and improved the ruggedness of the optical interface. Now, developments that combine lower connector cost with more robust performance turn optical communication into a possibility.
