Optical Connector Characteristics and Their Impact on System Performance and Reliability There are a large number of issues that affect the performance of fiber optic connectors in today s networks. These factors are increasingly important as data rates, the number of wavelengths and transmission distances continue to escalate. The old analogy of a system only being as strong as its weakest link is especially relevant today, and when considering the amount of revenue being carried on each fiber, an investment in a quality connection is easily justified. This paper will discuss the issues that affect the fiber connection s reliability and performance. It begins with the impact of three key connector components on a termination, followed by the importance of three measures of ferrule geometry on short and long term performance, and finishes with the impact of contaminants and defects on reliability. Spring: forces ends of the fiber ferrule together Ferrule ends make contact and deform under load Cylindrical sleeve aligns the mating ferrules Fig. 1: Cross-section of an interconnection While there are several technologies on the market for fiber optic connectors, the vast majority of connectors utilize a cylindrical ferrule to capture the fiber. Each ferrule 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 a spring housed inside the connector. The force from these springs is approximately 0.9 kg. However, because this force is applied over a very small area it causes deformation at the end of the ferrule, even when rigid materials such as zirconia are used. This deformation has the positive effect of compensating for imperfections in the shape of the ferrule end. The result is that even if a termination contains some imperfections, a good connection can still be achieved.
Optical Connector Characteristics, Page 2 of 6 A. Component Impact on Fiber Connections Ferrule Ferrule Hole Fiber Core 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 aligns the two mating fibers. No matter how well this is accomplished there are other factors working against good alignment, particularly eccentricities of the various components (see Fig 2). Lack of concentricity in the ferrule is considered to be the leading contributor to insertion loss, and the concentricity of the core to cladding of the fiber can add to misalignment of the fiber cores. Because the core diameter is on the order of 8 microns, the effects of the ferrule hole or the fiber core being off center can have a large effect on optical performance. Performance issues are exacerbated if the hole diameter of the ferrule is larger than the outside diameter of the fiber, a situation which causes the fiber to sit off to one side of the hole. Nominal outside diameter for fiber is 125 µm, but many cable assemblies are manufactured using ferrules with 126 µm or larger holes in order to ease the manufacturing process. This creates a crescent moon shaped gap on one side and forces the core away from center. To minimize the effects of eccentricities, it is advisable to use reputable fiber manufacturers 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 polishing and subsequently throughout the service life of the connector. The methods of curing various types of epoxies are well known, but even after curing and polishing it is very common for the fiber to move relative to the ferrule. 1 This is known as fiber pushback, or pistoning of the fiber and is caused by shear force from two main factors. The first cause is normal force on the ferrule and fiber ends from the connector springs. The second contributor is the inconsistent coefficient of thermal expansion between ferrules and fiber. Because ferrules are convex on the mating end face, the force from the spring is acting only on the center portion of the ferrule, an area of approximately 225 microns after deformation. The resulting normal forces in this area can approach 2,260 kg/cm^2. This force manifests itself as shear force at the ferrule to fiber interface that the epoxy is attempting to secure.
Optical Connector Characteristics, Page 3 of 6 To combat this effect, recent studies have shown that the most important parameter of the epoxy is its glass transition temperature, which correlates to its shear strength over temperature. Studies conducted at Telect have shown that epoxies with a lower Tg may experience fiber pistoning soon after termination when placed back in the curing oven for a short time, even less than one minute. While this is an accelerated test, other studies have shown that fiber pistoning or pushback occurs quite often in the first few hours of deployment. 2 Another important facet of epoxy is the presence of air bubbles introduced during mixing and 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. 3 For this reason it is important to place 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 manufacturers whose 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 differentiate one termination from the next. It was recognized some time ago that ferrules needed to have a convex surface with the fiber at the apex to guarantee consistent results when mating fibers. The purpose of a convex surface is to ensure that when the ferrules came together they will have glass to glass contact and avoid an air-gap (which causes higher loss and higher back reflection). These are called Physical Contact or PC connectors and for these reasons essentially every ferrule today comes pre-radiused. There are variances in the geometrical dimensions of these ferrules depending on vendor, but how one accounts for, enhances, or overcomes these built in qualities determines the quality of the fiber termination. 1. Endface Geometry Telcordia (formerly Bellcore), as well as the EIA/TIA have published specifications on what the end results should look like. These parameters are referred to as the Endface Geometry of the connector. Numerous papers have been published that detail the methods by which these specifications were derived 4, as well as the impact of not meeting 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. Radius of Curvature: Radius of curvature specifies the magnitude of curvature on the end of the ferrule, as measured by the radius of the arc describing the surface. For smaller values of R there will be a smaller contact area, which concentrates the spring force of the connector into a smaller 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 the likelihood of fiber pushback. However, more deformation can also compensate for the fiber 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 area of the ferrule to be larger and resulting in less deformation. The GR-326 Issue 3 recommendation is a radius between 7 and 25 mm. Fig. 3: Radius of Curvature
Optical Connector Characteristics, Page 4 of 6 Apex Offset: Apex offset is a measure of how far off center the highest point of the convex end of the ferrule is. It is important to minimize the offset so that the glass truly is at the highest point of the ferrule end. Furthermore, because the offset can be in any direction, two mated 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 an apex offset of less than 50 mm. Fig. 4: Appex Offset Fiber Height: In some ways the most important parameter of a fiber termination, the Fiber Height specifies the position of the fiber surface relative to the surface of the ferrule. The ferrule surface is measured one of two ways. 1) Spherical height - as if the surface were to continue 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 edge on the opposite side. The former is more common. Positive height is commonly termed protrusion, and a degree of protrusion typically aids good fiber to fiber contact resulting in improved insertion loss performance. Excessive protrusion can cause increased normal force to be exerted on the fiber, reducing the durability of the connector in terms of mating cycles. In extreme cases excess fiber height 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 can lead to improved return loss performance. It also improves connector performance in terms of mating cycles and durability, although a small performance penalty is realized in a higher insertion loss. When manufactured appropriately the deformation of the ferrule compensates for the undercut position of the fiber so that fiber to fiber contact is still achieved. Issue 2 of GR-326 mandated a fiber height of + /- 50 nm, but Issue 3 has relaxed this requirement. The Issue 3 assumption is that a smaller radius will result in increased 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 125 nm of undercut. One risk to this approach is that a reduced spring force due to friction in the adapter sleeve can result in less normal force at the ferrule interface. Additionally, connectors with large amounts of undercut are more sensitive to the effects of fiber pushback. Studies have shown that excessive undercut can cause return loss failures when subjected to temperature fluctuations. (#1) Because a Fiber Height from 50 to +50 nm can be achieved in a controlled process, it is our recommendation that this stricter specification is maintained. Fig 5(a) Fig 5(b) Fig. 5(a) Undercut and 5(b) Protrusion
Optical Connector Characteristics, Page 5 of 6 2. Defects and Contaminants on the Endface Specifications on defects and contaminants on the interface of a connection have been ill defined and subjective at best. Typically, after the polishing process, a fiber is inspected visually to check that no pits, scratches or blemishes appear in the core. Knowledgeable manufacturers also check the inner portions of the cladding because a smaller amount of light is also being transmitted in the inner portions of cladding. This combined area is known as the Mode Field Diameter of the fiber and is a function of the fiber manufacturing process. In a recent study, 60 cables from various manufacturers who are recognized leaders within the industry were inspected. The results showed inconsistency in the monitoring of surface defects, with some cables exhibiting no defects while others had defects in critical areas. Examples of these are shown in Figures 6(a) and 6(b). Fig 6(a) Fig 6(b) Fig. 6(a) and 6(b) showing defects in the industry Glass is a very brittle substance, and for this reason its strength is limited mainly by material defects, be they internal or on the surface. Furthermore, defects in glass can propagate. We all have experienced this when witnessing a crack in a vehicle windshield increase in length until it has covered a great distance. An additional concern is that under stress, silica bonds weaken in the presence of moisture (#1). To assure a quality connection, 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 photo of a chip in a polished fiber endface that was small enough to be barely visible at 200x magnification. 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. Fig. 7: SEM photo of a surface defect
Optical Connector Characteristics, Page 6 of 6 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 mm. For these 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 to environmental stresses into the core of the fiber, but defects in the contact zone of the ferrule can also act as reservoirs for contaminants. Likewise, a long scratch in this zone can 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 to contaminants. C. Conclusion There clearly is a wide range of issues that affect the performance of a fiber patch cord or pigtail termination. Unfortunately, typical end users do not have the time, equipment, or expertise to inspect the cable assemblies that they purchase. Since no one has perfected the fiber termination process, but instead must rely on yielded processes, the customers are left with only two choices. Either make a large investment in time and equipment to do the inspections themselves, or demand that their suppliers provide this service and document the results. Nothing less than 100% inspection of all the critical termination parameters can guarantee a reliable connection. Notes 1. L.A. Reith et al.; Connector Materials Reliability In Hot, Humid Environments, Bellcore, 1997 NFOEC Proceedings. 2. Reith, et al., ibid. 3. William Wood et al., Bellcore; Reliability of Interconnection Devices, Proc. 9 th Annual NFOEC, pgs. 209-221, 1993. 4. L.A. Reith, P.B. Grimado, J. Brickel, Bellcore; Effect of Ferrule-Endface Geometry On Connector Intermateability, Proc. 11th Annual NFOEC, pgs. 635-646, 1995. Additional Resources 1. E. Makrides-Saravanos et al.; Creep Deformation of Zirconia Ceramic: Effect of Composition and Sintering Temperature, 1998 NFOEC Proceedings.