Endface Geometry and Connector Reliability in the Outside Plant

PON Deployment of Angle Polished Connectors: Endface Geometry and Connector Reliability in the Outside Plant PON Deployment of Angle Polished Connectors: Endface Geometry and Connector Reliability in the Outside Plant By Steven C. Zimmel Abstract: Recent wide-scale deployment of APC connectors in the outside plant for FTTH initiatives has forced connector manufacturers to consider reliability issues of these connectors. Angled connectors exhibit two phenomena not found in non-angled connectors: First, the lower ferrule endface radius of angled connectors makes the connector more susceptible to permanent fiber withdrawal at elevated temperatures. Second, any rotation of the ferrule in angled connectors will increase the apex offset of the connector. These phenomena may result in loss of physical contact between the fibers of a mated connector pair. This air gap may increase reflectance and insertion loss, reducing system reliability. The air gap may also allow contaminants to migrate on the fibers. Ferrule rotation and permanent fiber withdrawal need to be minimized for acceptable connector performance in the outside plant. ©2005 Optical Society of America OCIS codes: (060.2340) Fiber optic components Introduction Angled physical contact (APC) fiber optic connectors with zirconia ferrules have become the de facto standard in PON networks currently being deployed for FTTH initiatives. The angle on the fiber in APC connectors produces low back reflections when the connector is not mated to another connector: Reflected light is directed into the fiber cladding because the endface angle is greater than the acceptance angle of the fiber. The low reflection of an unmated APC connector provides an advantage for PON networks: A connector in the PON will usually remain unmated until service is required from that circuit. This results in unused connectors emitting their signal into air, which can produce Frensel reflections as high as –14dB for connectors without an angle on the endface. Service providers need to minimize system reflections so it won’t interfere with OLT card operation. One solution is to use non-angled connectors and mate all unused connectors to a terminator. This solution adds cost because terminators are an additional component. Using APC connectors provides a cost-effective method to greatly reduce reflection issues caused by unmated connectors in the PON. Using APC connectors eliminates the need for terminators because the reflectance of unmated APC connectors is less than -50dB. Endface Geometry and Connector Reliability in the Outside Plant FTTH deployments are expected to install millions of APC connectors into outside plant environments over the next several years. Up to this point, APC connectors have been primarily used for central office and testing applications. The upcoming large-scale deployment of APC connectors in the outside plant forces us to think about some of the issues that are unique to APC connectors in extreme environmental conditions. This paper will consider two of these issues. First, it is necessary to understand the importance of connector ferrule endface geometry. The parameters used to describe the endface of an APC connector are radius, apex offset, fiber height, and endface angle. These parameters are defined in the left side of Figure 1. Fiber height is the distance the fiber protrudes or recesses from the ferrule. If the fiber is recessed too far into the ferrule, obvious air gaps will occur. Apex offset of an APC connector is the distance from the apex of the polished ferrule radius to the center of the fiber core when the endface is viewed at and an angle of 8° (viewed perpendicular to the plane of the Apex of the Radius when viewed at 8˚ Optical Fiber Apex offset Endface Radius Fiber Height Air Gap Fig 1. Endface Geometry of an APC Connector and of Two Mated APC Connectors APC Endface Geometry and Outside Plant Reliability APC connectors provide two benefits. First, APC connectors produce fiber-to-fiber contact in mated connector pairs. Physical contact between fibers in mated connectors is needed because it prevents air gaps from forming between optical fibers. Air gaps cause increased reflectance and inconsistent insertion losses that will reduce system performance and reliability. In addition, physical contact prevents contamination from migrating onto the fiber cores. The second benefit of APC connectors is that they minimize reflectance. If the radius is polished at an angle such that the angle of the fiber is greater than the fiber’s angle of acceptance, reflected light will be directed into the cladding. Using this method, one can produce connectors with reflectance in the –50dB range when unmated and as low as -80dB or better when mated. Physical contact is realized by polishing a radius onto the ferrule containing the potted optical fiber and by polishing the fiber such that it is close to level with the ferrule surface. The key for physical contact to occur is to center the apex of the radius over the optical fiber. If the apex is approximately centered on the fiber, physical contact is guaranteed because the fibers will be the first objects to contact each other when connectors are mated. angled surface). If the apex offset becomes too large, an air gap will develop because the high point of the ferrule on the radiused endface will be too far away from the fiber core per the right side of Figure 1. The endface radius is applied to the ferrule by polishing the connector on a compliant surface. If the radius becomes too large, the ferrule will become effectively flat. This situation could cause local concave features that will introduce a gap between the fiber. The radius is usually applied at an 8° angle. The angle is in a plane that is 8° from perpendicular to a plane that intersects the connector key. 9° connectors are used, but are rare. APC endface geometry parameters are specified such that physical contact will be maintained over a large range of operating conditions. For example, at high temperatures, pressure on ferrule endfaces caused by the spring in the connector may cause the fibers to permanently withdraw into the ferrule if the epoxy holding the fibers creep. If the initial fiber height is too recessed into the ferrule, the fiber may creep too far back into the ferrule causing loss of physical contact. Proper specification of these endface parameters is necessary to guarantee physical contact in the outside plant. Page 3 Endface Geometry and Connector Reliability in the Outside Plant Several industry standards exist that address APC connector endface geometry. The purpose of these specifications is to guarantee that physical contact between different vendor’s connectors will not be lost. The most used standards are IEC-60874-14-6 and Telcordia GR-326, Issue 3. These documents require apex offset to be less than 50 microns, radius to be from 5mm to 12mm, and fiber height to be ±100nm. These three requirements will allow physical contact to be maintained at temperatures as high as 85°C with proper epoxy selection. The IEC is in process of updating endface requirements for APC connectors. The new requirements will reflect current ferrule material properties and will tie all three properties together into one function. The new requirements are expected to be published in 2005. The preceding discussion shows that proper ferrule endface geometry is required for good system performance. Properly specified endface geometries guarantee physical contact between fibers in mated connectors. Physical contact is needed so air gaps don’t form between fibers in connectors, which will cause increases in reflectance and insertion loss, reducing system performance. Now that endface geometry and the importance of physical contact have been defined, we can discuss issues concerning these properties in APC connectors in the outside plant. The effort to maintain physical contact in APC connectors deployed in the outside plant has two unique challenges not present in non-angled connectors: First, APC connectors are much more prone to large permanent fiber withdrawals when exposed to high temperatures. If the fiber withdraws too much, physical contact can be lost. Second, ferrule rotation can create air gaps because of the angle of the endface. These phenomena must be accounted for to guarantee proper connector performance and reliability in the outside plant. Permanent Fiber Withdrawal, Radius, and Ferrule Cleanliness: One manner that fiber height can change, causing fibers to lose physical contact, is if the fiber permanently withdrawals into the ferrule. This phenomenon occurs when mated connectors are exposed to elevated temperatures. The force on mated ferrule endfaces transmits pressure to the fiber that is held in place with an epoxy. This pressure may cause the epoxy and fiber to creep back into the ferrule at elevated temperatures. If the pressure is greater than the epoxy bond strength, the creep will not recover and the fiber/epoxy will permanently withdraw into the ferrule inside diameter. Figure 2 shows an electron microscope scan of an connector before (left) and after (right) GR-326 environmental testing. The region on the left is the optical fiber, the region on the right is the ferrule, and the thin section in the middle is the epoxy. Notice how the fiber is lower than the ferrule in the right photo. This is the phenomenon of permanent fiber withdrawal. It has been observed that permanent fiber withdrawal is significantly larger in APC connectors than non-angled connectors that are subjected to GR-326, Issue 3 environmental testing. We routinely observed APC connectors start a GR-326 test with a protruding fiber and finish with fibers recessed to a point that it no longer meets the –100nm requirement. However, we rarely see this occur in non-angled connectors. The question we must ask ourselves is why do APC connectors permanently withdraw so much more than non-angled connectors? Before we answer that question, we need to determine which of the GR-326 tests causes the withdrawal. An experiment was performed to determine which GR-326 environment causes the most permanent fiber withdrawal. The experiment consisted of six groups of 12 mated connector pairs (24 APC/SC connectors mated in 12 receptacles). Each group was subjected to either one week of one of the four GR-326 environmental tests, a –40°C cold age test, or an ambient age as a control group. The connectors were measured for endface geometry using an interferometer, mated together in an adapter, and subjected to one week of one of the tests listed above. After 1 week the connectors were removed from the chambers and allowed to rest at room temperature for one day. Next the connectors were uncoupled and the endface geometry was measured. The average, maximum, and minimum permanent changes in fiber height are shown in Table 1. Page 4 Endface Geometry and Connector Reliability in the Outside Plant Table 1: Permanent Fiber Height Withdrawal for Various GR-326 Environments Average Maximum Minimum Group 1: Thermal Age -109.9 nm -61.2 nm -104.9 nm Group 2: Thermal Cycle -71.0 nm -14.6 nm -15.7 nm Group 3: Humidity Age -164.3 nm -108.1 nm -291.3 nm Group 4: Condensation Cycle -129.8 nm -31.7 nm -23.6 nm Group 5: Cold Temp Age -42.8 nm -34.7 nm -62.6 nm Group 6: Ambient (Control) -0.2 nm -3.7 nm -4.5 nm Table 1 shows that thermal age and humidity age induced the most average withdrawal and produced the highest extremes. Interestingly, the sample that went through thermal cycle saw less change than thermal age. Even more interesting is that –40° age showed very little change. Prolonged exposure to elevated temperatures induces the most fiber height change. This is evident from the fact that the tests that spend the most time at high temperatures (thermal and humidity age) exhibit the most change. Thermal and condensation cycles are exposed to high temperatures, but only for 1/8 of the test cycle. However, even though the cycling tests show far less change, there is still a significant amount of withdrawal. This indicates that even short exposures to elevated temperatures may cause the fiber to withdraw in APC connectors. Experiments were then conducted to determine what specifically about APC connectors induces more permanent fiber withdrawal than non-angled connectors. Non-angled (UPC) and angled connectors (APC) differ in two major ways: First, APC connectors have an endface radius of 5mm to 12 mm and UPC connectors have an endface radius of 10mm to 25mm. These endface radii are defined in IEC and Telcordia standards. The smaller radius in APC connectors will cause the pressure on the fiber to be higher because the spring force is spread out over a smaller contact area. This higher pressure may cause the fiber to creep more during GR-326 testing. Second, many APC connector manufacturers perform a secondary grinding operation of the ferrule to change the chamfer to geometries that aid in achieving low apex offsets. This operation is done before the ferrule is potted and may introduce contaminants to the ferrule I.D. The experiment looked at two factors: Radius and ferrule I.D. cleanliness. APC connectors were made with radii of 5-7 mm, 9-11mm, and 18-22 mm. Ferrule inside diameters were either used as provided or cleaned with a steam bath and acetone. The sample allocations for the experiment are shown in Table 2. Groups are not equally sized because the data represents a summation of three separate experiments evaluating the same two parameters. Fig 2. Above: SEM Scan of a Non-Withdrawn APC Fiber Below: SEM Scan of a Permanently Withdrawn APC Fiber Page 5 ... - tailieumienphi.vn