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CONNECTOR CHARACTERISTICS AND THEIRIMPACT ON SYSTEM PERFORMANCE &

RELIABILITY

Telect, Inc.2111 N. Molter Road, Liberty Lake, WA 99019

Tel 509-926-6000, 1-800-551-4567Copyright 2000, Telect, Inc.

There are a large number of issues that affect the performance of fiber optic connectors intoday’s networks. These factors are increasingly important as data rates, the number ofwavelengths, and transmission distances continue to escalate. The old analogy of asystem only being as strong as its weakest link is especially relevant today, and whenconsidering the amount of revenue being carried on each fiber an investment in a qualityconnection is easily justified. This paper will discuss the issues that affect the fiberconnection’s reliability and performance. It begins with the impact of three keyconnector components on a termination, followed by the importance of three measures offerrule geometry on short and long term performance, and finishes with the impact ofcontaminants and defects on reliability.

Background

Fig. 1: Cross section of an interconnection

While there are several new technologies on the market for fiber optic connectors, thevast majority of today’s connectors utilize a cylindrical ferrule to capture the fiber. Eachferrule is then aligned to another ferrule with a precision sleeve, as illustrated in Figure 1.In order to ensure good contact, these ferrules are typically pressed together by means of

Spring: forces ends of thefiber ferrule together

Ferrule ends make contactand deform under load.

Cylindrical sleeve alignsthe mating ferrules.

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a spring housed inside the connector. The force from these springs is approximately 0.9kg. However, because this force is applied over a very small area it causes deformation atthe end of the ferrule, even when rigid materials such as zirconia are used. Thisdeformation has the positive effect of compensating for imperfections in the shape of theferrule end. The result is that even if a termination contains some imperfections, a goodconnection can still be achieved. More discussion of this phenomenon follows.

A. Component Impact on Fiber Connections

Ferrule

Ferrule Hole

Core

Fiber

Fig. 2: Effects of Component Eccentricity (Not to Scale)

1. Fiber and Ferrule

The main function of any connector/adapter is to align two ferrules, which in turn alignsthe two mating fibers. No matter how well this is accomplished there are other factorsworking against good alignment, particularly eccentricities of the various components(see Fig 2). Lack of concentricity in the ferrule is considered to be the leadingcontributor to insertion loss, and the concentricity of the core to cladding of the fiber canadd to misalignment of the fiber cores. Because the core diameter is on the order of 8microns, the effects of the ferrule hole or the fiber core being off center can have a largeeffect on optical performance.

Performance issues are exacerbated if the hole diameter of the ferrule is larger than theoutside diameter of the fiber, a situation which causes the fiber to “sit” off to one side ofthe hole. Nominal outside diameter for fiber is 125 um, but many cable assemblies aremanufactured using ferrules with 126 um or larger holes in order to ease the

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manufacturing process. This creates a crescent moon shaped gap on one side, and forcesthe core away from center.

To minimize the effects of eccentricities, it is advisable to use reputable fibermanufacturers and specify the use of ferrules with hole diameters of 125 microns.

2. Epoxy

The primary purpose of epoxy is to secure the fiber within the ferrule during polishingand subsequently throughout the service life of the connector. The methods of curingvarious types of epoxies are well known, but even after curing and polishing it is verycommon for the fiber to move relative to the ferrule. (3) This is known as fiber pushback,or pistoning of the fiber, and is caused by shear force from two main factors. The firstcause is normal force on the ferrule and fiber ends from the connector springs. Thesecond contributor is the inconsistent coefficient of thermal expansion between ferrulesand fiber.

Because ferrules are convex on the mating end face, the force from the spring is actingonly on the center portion of the ferrule, an area of approximately 225 microns afterdeformation. The resulting normal forces in this area can approach 2,260 kg/cm^2. Thisforce manifests itself as shear force at the ferrule to fiber interface that the epoxy isattempting to secure.

To combat this effect, recent studies have shown that the most important parameter of theepoxy is its glass transition temperature, which correlates to its shear strength overtemperature. Studies conducted at Telect have shown that epoxies with a lower Tg mayexperience fiber pistoning soon after termination when placed back in the curing oven fora short time, even less than one minute. While this is an accelerated test, other studieshave shown that fiber pistoning or “pushback” occurs quite often in the first few hours ofdeployment. (3)

Another important facet of epoxy is the presence of air bubbles introduced during mixingand processing. When these bubbles are present the ferrule cavity only partially fills.This can lead to inconsistent pressures being placed on the fiber that effect birefringence,or possibly fracture the glass and cause failures. (1) For this reason it is important toplace the epoxy in a vacuum chamber prior to use so that these voids can be removed.This process is called outgassing the epoxy and should be required of manufacturerswhose product is used in high reliability networks.

B. Polishing and Processing for Reliable Connections

The methods and results of polishing the fiber are really the measures that differentiateone termination from the next. It was recognized some time ago that ferrules needed tohave a convex surface with the fiber at the apex to guarantee consistent results when

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mating fibers. The purpose of a convex surface is to ensure that when the ferrules cametogether they will have glass to glass contact and avoid an air-gap (which causes higherloss and higher backreflection). These are called Physical Contact or PC connectors, andfor these reasons essentially every ferrule today comes pre-radiused. There are variancesin the geometrical dimensions of these ferrules depending on vendor, but how oneaccounts for, enhances, or overcomes these built in qualities determines the quality of thefiber termination.

1. Endface Geometry

Telcordia (formerly Bellcore), as well as the EIA/TIA have published specifications onwhat the end results should look like. These parameters are referred to as the EndfaceGeometry of the connector. Numerous papers have been published that detail themethods by which these specifications were arrived (2), as well as the impact of notmeeting them (typically involving Insertion Loss and Return Loss). As mentioned above,it is imperative to maintain fiber to fiber contact if there is to be a reliable connection.Following is a brief explanation of each parameter and it’s impact on the connection

RadiusR

Fig. 3: Radius of Curvature

Radius of Curvature

Radius of curvature specifies the magnitude of curvature on the end of the ferrule, asmeasured by the radius of the arc describing the surface. For smaller values of R therewill be a smaller contact area, which concentrates the spring force of the connector into asmaller area of the end face, resulting in increased deformation of the glass and ferrule.This results in more stress on the fiber to ferrule interface (epoxy), increasing thelikelihood of fiber pushback. However, more deformation can also compensate for thefiber being below the surface of the ferrule, called undercut (see Fiber Height below).Conversely, larger values of R result in the end face being flatter, causing the contact areaof the ferrule to be larger and resulting in less deformation. The GR-326 Issue 3recommendation is a Radius between 7 and 25 mm.

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Some studies have shown that the deformation can become plastic or permanent overtime when subjected to environmental stresses such as temperature and humidity (4).Permanent deformation can result in inconsistent performance on subsequent matings,but specifying zirconia ferrules from reputable manufacturers minimizes these effects.

Apex Offset

Fig. 4: Apex Offset

Apex Offset

Apex offset is a measure of how far off center the highest point of the convex end of theferrule is. It is important to minimize the offset so that the glass truly is at the highestpoint of the ferrule end. Furthermore, because the offset can be in any direction, twomated connectors can have offsets that are additive. Obviously, if the offset is too great,core to core contact will not be achieved. The GR-326 Issue 3 recommendation is anapex offset of less than 50 µm.

5(a) 5(b)

Fig. 5(a) Undercut, and 5(b) Protrusion

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Fiber Height

In some ways the most important parameter of a fiber termination, the Fiber Heightspecifies the position of the fiber surface relative to the surface of the ferrule. The ferrulesurface is measured one of two ways. 1) Spherical height - as if the surface were tocontinue across the fiber hole in an arc of the same radius as the rest of the end face. 2)Planar height - described by a straight line from one edge of the ferrule hole to the edgeon the opposite side. The former is more common.

Positive height is commonly termed protrusion, and a degree of protrusion typically aidsgood fiber to fiber contact resulting in improved insertion loss performance. Excessiveprotrusion can cause increased normal force to be exerted on the fiber, reducing thedurability of the connector in terms of mating cycles. In extreme cases excess fiberheight can cause a material fracture in the glass and a catastrophic failure to the link.

Negative height is typically termed undercut, and surprisingly a degree of undercut canlead to improved return loss performance. It also improves connector performance interms of mating cycles and durability, although a small performance penalty is realized ina higher insertion loss. When manufactured appropriately the deformation of the ferrulecompensates for the undercut position of the fiber so that fiber to fiber contact is stillachieved. Issue 2 of GR-326 mandated a fiber height of + /- 50 nm, but Issue 3 hasrelaxed this requirement. The Issue 3 assumption is that a smaller radius will result inincreased deformation of the ferrule and compensate for higher degrees of undercut.Accordingly the fiber height specification is now a function of the radius of the ferrule,and allows as much as –125nm of undercut. One risk to this approach is that a reducedspring force due to friction in the adapter sleeve can result in less normal force at theferrule interface. Additionally, connectors with large amounts of undercut are moresensitive to the effects of fiber pushback. Studies have shown that excessive undercutcan cause return loss failures when subjected to temperature fluctuations. (#1) Because aFiber Height from –50 to +50 nm can be achieved in a controlled process, it is ourrecommendation that this stricter specification is maintained.

2. Defects and Contaminants on the Endface

Specifications on defects and contaminants on the interface of a connection have been illdefined and subjective at best. Typically, after the polishing process, a fiber is inspectedvisually to check that no pits, scratches or blemishes appear in the core. Knowledgeablemanufacturers also check the inner portions of the cladding because a smaller amount oflight is also being transmitted in the inner portions of cladding. This combined area isknown as the Mode Field Diameter of the fiber and is a function of the fibermanufacturing process.

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In a recent study, 60 cables from various manufacturers who are recognized leaderswithin the industry were inspected. The results showed inconsistency in the monitoringof surface defects, with some cables exhibiting no defects while others had defects incritical areas. Examples of these are shown in Figures 6(a) and 6(b).

Fig 6(a) Fig 6(b)

Figs. 6(a) and 6(b) showing defects common in the industry.

Glass is a very brittle substance, and for this reason it’s strength is limited mainly bymaterial defects, be they internal or on the surface. Furthermore, defects in glass canpropagate. We all have experienced this when witnessing a crack in a vehicle windshieldincrease in length until it has covered a great distance. An additional concern is thatunder stress, silica bonds weaken in the presence of moisture (#1). To assure a qualityconnection, it is important to eliminate defects and contaminants (especially moisture)from the critical areas on the fiber endface. For example, Figure 7 below is a SEM photoof a chip in a polished fiber endface that was small enough to be barely visible at 200xmagnification. One can see the potential for propagation of these defects over time,especially when subjected to mechanical stress from the connector spring force,temperature extremes, and in the presence of humidity.

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Fig. 7: SEM photo of a surface defect

It has been noted that ferrules deform when subjected to loads in the mated condition.This results in contact being made over a fairly large region, approximately 225 µm. Forthese reasons it is wise to specify that no defects be present within this “contact zone.”Not only could defects in the entire surface area of the fiber propagate over time due toenvironmental stresses into the core of the fiber, but defects in the contact zone of theferrule can also act as reservoirs for contaminants. Likewise, a long scratch in this zonecan act as a capillary channel through which contaminants and moisture can migrate.Two smooth surfaces brought in contact with each other will act as a barrier tocontaminants.

C. Conclusion

There is clearly a wide range of issues that effect the performance of a fiber patchcord orpigtail termination. Unfortunately, typical end users do not have the time, equipment, orexpertise to inspect the cable assemblies that they purchase. Since no one has perfectedthe fiber termination process, but instead must rely on yielded processes, the customersare left with only two choices. Either make a large investment in time and equipment todo the inspections themselves, or demand that their suppliers provide this service anddocument the results. Nothing less than 100% inspection of all the critical terminationparameters can guarantee a reliable connection.

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References

1. William Wood et al., Bellcore; “Reliability of Interconnection Devices,” Proc. 9th

Annual NFOEC, pgs. 209-221, 1993.

2. L.A. Reith, P.B. Grimado, J. Brickel, Bellcore; “Effect of Ferrule-Endface GeometryOn Connector Intermateability,” Proc. 11th Annual NFOEC, pgs. 635-646, 1995.

3. L.A. Reith et al.; “Connector Materials Reliability In Hot, Humid Environments,”Bellcore, 1997 NFOEC Proceedings.

4. E. Makrides-Saravanos et al.; “Creep Deformation of Zirconia Ceramic: Effect ofComposition and Sintering Temperature,” 1998 NFOEC Proceedings.