Multifiber-ferrule ribbon cable connector shrinks installation costs

Oct. 1, 1996

Multifiber-ferrule ribbon cable connector shrinks installation costs

Replicating the form factor of the SC single-fiber connector, a novel multifiber ribbon cable connector uses two plastic ferrule halves to achieve higher packing density, save panel space and lower termination time and expense

Roger E. Weiss

prp systems inc.

Fiber-optic connector designs that allow higher fiber density and functionality are expected to play a major role in promoting the installation of optical-fiber premises networks. By employing multiple-fiber bus-type components, such as multifiber transceivers, multifiber ribbon cables and multifiber connectors, new and upgraded on-site fiber-optic networks can reduce the per-fiber interconnection cost and provide a higher connection density.

Most separable optical connections are presently achieved with the use of single-fiber-ferrule-type connectors. These devices require considerable labor to install and occupy valuable real estate, both of which impede the growth of fiber-optic premises networks. One corrective measure is to diminish the size of the interconnec - tion network by use of a multifiber- ferrule connector system.

These connector systems greatly increase the interconnection density via smaller size and offer a reduction in per-fiber interconnection cost by providing 2 to 12 fibers per connector. Over the years, the major telephone companies have installed large numbers of multifiber connectors (or separable splices) to terminate multifiber ribbon cable in their networks. Although these older multifiber connector designs are effective, they are not cost-competitive with the price reduction provided with present interconnection methods.

With improving and cost-saving connector designs, network planners and providers have been able to steadily lower the cost of fiber-optic cable installation. In the outside plant, fiber-optic cable is methodically making its way closer to homes and businesses. In the inside plant, all-fiber premises wiring methods have been directed to reducing the installed cost per unit bandwidth. Economic factors that are encouraging the widespread implementation of fiber-based premises wiring networks include:

Decreasing component price

Decreasing component size

Standardizing on a fixed set of subunits

Using pre-assembled and pre-tested subassemblies

Incorporating parts that facilitate system growth.

To achieve fiber-optic cabling and communications all the way to the home or to the desktop, there needs to be a dramatic change in fiber-optic connectors` product design. Reduced costs must be formulated through advanced con nector de sign, changes in materials, and manufacturing developments. To become an industry standard, transferable connector manufacturing methods must be used and that capability made available to the industry. Multiple independent sources would ensure connector supply and price competition.

Numerous multifiber ferrule designs are available in the connector market. However, an MP multifiber connector system recently developed by Methode Electronics Inc. provides both functional and economic advantages. Lower installed costs are achieved by the careful design of all connector mate rials, piece parts and assemblies.

Based on these guidelines, the MP connector system comes in a 12-fiber connector format with the same form factor as the SC single-fiber connector. This format allows the placement of 12 fibers in the same panel opening as provided by the SC single-fiber connector. In this manner, the MP connector achieves multiple size and bus advantages and lower installed cost when used in cable management systems.

The most costly connector part is the ferrule, and this is due mainly to the limited availability of suppliers. To boost availability, the MP connector has markedly changed the design, material and manufacturability of the ferrule. In general, fiber-optic connectors incorporate ceramic, silicon or expensive molded materials for ferrules and their housings. To date, only one U.S. manufacturer makes commonly used ferrules.

To achieve lower costs, the MP ferrule is composed of a stable, injection-molded plastic. The plastic ferrule houses 2 to 12 fibers, which are held in precise relative alignment by molded grooves. To that end, alignment surfaces are molded into the outer edges of two mating ferrule halves. The structure of the identical ferrule halves thus translates into a single-surface design (see Fig. 1).

The ferrule halves contact only the fibers and are mutually aligned by them. When the two halves are mounted on the fibers, the outer edges form a precision alignment groove. Full connector alignment is achieved by pressing two round pins into the grooves. Ferrule accuracy is also aided by using precision-molded inserts and tightly controlled molding processes. Consequently, the MP ferrule with its alignment sleeve system permit integration of this device into numerous connector configurations.

Common standards

The three-piece MP connector consists of two herm aphroditic plugs and a coupling (see Fig. 2). This format emulates the common optical connector standards used for the SC, ST, duplex SC and other widely accepted fiber-optic connector systems. The key design advantage of the MP connector is that all cable terminations are the same.

To achieve a precise alignment system, the MP connector employs a spring-loaded alignment sleeve inside the coupling to hold the two round alignment pins. As the connector is inserted into the coupling, the two pins press into the alignment grooves at the ferrule edges. The mating connector, inserted into the opposite end of the coupling, has the same two alignment pins pressed into its ferrule`s alignment grooves. As the ferrules come together, an axial load forces them into close contact. The shared alignment pins coupled with the applied axial force result in the accurate alignment of the 12 fibers.

With this pin-in-groove method, the number of constraints on the alignment system is minimized. The only requirement is that the pin be round and smooth. Another advantage of this approach is that as the ferrule slides into the coupling, the alignment pin automatically wipes any debris from its path, thereby ensuring precise alignment. Other mechanical designs require that a close-tolerance pin be placed into a close-tolerance hole to achieve alignment. In addition to higher cost, this tight fit can trap debris between the pin and the ferrule, leading to misalignment. The alignment system mounted inside the coupling comes as a pre-assembled unit; thus the cost of assembling the alignment hardware is avoided.

Improved plug housing

The MP plug housing assembly adds functionality at low cost. A standard crimp sleeve ties the cable sheath and the support members to the plug body, thereby providing strain relief for the fibers. A bend-radius-limiting boot protects the fiber from excessive bending. Using a snap-fit design allows easy assembly of all the plastic parts. Positive latching action secures the plug to the latch assembly in the coupling. To remove the plug, the latch is slid forward, rather than pulled back, which reduces the possibility of a mistaken release--a common occurrence with SC designs.

The MP connector design promotes easy field installation similar to that used for the SC and ST connector styles. A standard crimp sleeve allows the use of available crimp tools. Standard tools can also be used for ferrule termination and preparation. The present MP ferrule embodiment accommodates as many as 12 fibers on 250-micron centers.

Tools are available for simultaneously preparing, stripping and terminating the 12 fibers of a ribbon cable. To perform this task, the first ferrule half is placed in a fixture. Then the ribbon cable, after stripping, is mounted in the ferrule half with the fibers lying in the V-grooves. Next, epoxy is applied and the second ferrule half is placed on the fiber array, where it is aligned by the fixture and the fibers. The assembly is then clamped in position.

After the epoxy is cured, a hand polishing puck is used to finish the face of the connector so that it is perpendicular to the alignment grooves, as is done with conventional single-fiber connectors. u

Roger E. Weiss is president and founder of PRP Systems Inc., Foxborough, MA.

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