WHITE PAPER Singlemode Fiber Optic Connector Endface Workmanship Performance Effects of the Singlemode Fiber Optic Connector Endface Workmanship Introduction In order to assess endface quality, manufacturers and users of singlemode fiber optic connectors use magnification to visually examine fiber optic connector endfaces. Workmanship defects such as scratches, pits, and cracks, when located in the wrong area of the endface and when severe enough, can cause high insertion loss and/or low return loss. Locations and severity of defects, as well as the degree of magnification used in inspection, must be defined to ensure consistent connector performance. A survey of fiber optic connector manufacturers and end users produces a variety of endface workmanship requirements and procedures. This paper covers the following topics: • Fiber optic connector manufacturing process • Important measurement system considerations • Various workmanship parameters • Performance problems caused by defects • Current industry standard activities • ADC's endface workmanship specification Fiber Optic Connector Manufacturing Process Fiber optic connectors with a ceramic ferrule are manufactured by filling the inside diameter hole of the ferrule with a heat-cure epoxy, then inserting an optical fiber. The epoxy is cured so that the fiber is permanently bonded into the ferrule. Next the fiber is cleaved near the surface of the ferrule and the fiber and ceramic are subjected to various polishing processes with grit that becomes increasingly fine until the fiber and ferrule endface is smooth and defect free. At this point in the process the endface workmanship quality is inspected and, if acceptable, insertion and return loss are measured. Singlemode Fiber Optic Connector Endface Workmanship Page 3 Optical Viewing System Parameters Optical viewing systems used for evaluation of connector endface workmanship have a large impact upon the results obtained. The parameters that influence how well an optical viewing system works include: Magnification: Describes the extent to which an optical system enlarges an image. The lower the magnification, the fewer defects will be found. Magnification must be set so that performance affecting defects can be observed. Illumination: Lighting used to illuminate the endface being inspected for workmanship. Connector endfaces are particularly difficult to inspect because of the difference in reflection from the glass fiber and the ceramic ferrule. The illumination that is best for inspection of the ceramic causes the glass fiber to appear black. There are different illumination methods such as brightfield, darkfield, differential interference contrast (Nemarski), Koehler, and polarized, which can make defect identification easier. Illumination for cameras must meet a minimum required intensity; for microscopes, intensity must be adjusted to allow detection of defects. Resolution: The smallest distinguishable difference between two features. The Abbe limit restricts conventional optical microscopy to a resolution that is, at best, about half the wavelength of the light used. For the visible light used in inspecting connector endfaces, the best resolution is approximately 0.25 microns. Resolution depends on numerical aperture and light’s wavelength, not on magnification. Defined by the microscope objective, resolution is not enhanced by a microscope eyepiece or by magnification of an image by a camera or video monitor. Resolution can be decreased, however, by using a camera that does not have enough pixels to display features that can be resolved by the microscope objective. Defect Recognition: A manual endface workmanship inspection system done by a worker looking at the image of the endface. The worker must evaluate all defects for size and location and make a decision about the acceptability of the endface. Differences in worker eyesight, judgment, attitude, etc., can cause variation in results. Automated systems for computer controlled defect recognition are available and improve the repeatability of endface inspection. Other parameters include monitor age and number of pixels, cabling resistance, microscope age and resolution, perpen- dicularity of the endface to the microscope, and the illumination angle of the connector endface. Singlemode Fiber Optic Connector Endface Workmanship Page 4 Endface Workmanship Parameters Fiber optic connector endface workmanship is assessed by visually examining the endface under magnification for defects. Defects are defined and placed into the following categories: Scratches: Permanent linear features in either the glass fiber or ceramic ferrule. They are regions where the material has been removed to a certain depth and width. Scratches are usually caused by cleaning or during the polishing process. Pits, Chips and Other Defects: Permanent defects that are not scratches, including pits, chips, cracks, voids, and fixed contamination. Pits are irregularly shaped areas in the glass fiber or ceramic ferrule where material has been removed due to handling or polishing. Voids are relatively large areas within the ferrule that were not filled with ceramic. Chips are areas of the glass fiber that have broken off and are most often caused during the cleaving processes. Fixed contamination is surface material that cannot be cleaned off; examples include cured epoxy, stains in the ferrule material, and embedded metallic particles. Cracks: Permanent features in either the glass fiber or ceramic ferrule. Cracks are fractures that appear as lines on the surface. Whereas scratches are surface defects, a crack can go deep into the fiber. Cracks are never acceptable regardless of the size or location. Cracks can propagate over time and eventually cause a catastrophic failure of the connector. Loose Contamination: Debris on the surface such as dirt, dust, streaks, oil, grease, and metallic particles that are not permanent and can be removed with proper cleaning. The effect of loose contaminates varies depending on type, size, and location. Oil contaminations on the mode field diameter cause no insertion loss increase, but will cause a large decrease in return loss 1 . Loose particles on the mode field diameter generally cause very large changes in insertion loss and return loss. In addition, large loose particles that are not on the fiber core can prevent two mated connectors from achieving physical contact. Loose contamination is never desirable regardless of size or location because it can migrate to the mode field diameter during connector mating and unmating. 1 IPC-8497-1: Cleaning Methods and Contamination Assessment for Optical Assembly, December 2005 Singlemode Fiber Optic Connector Endface Workmanship Page 5 Performance Effects of Workmanship Defects Defects are further evaluated by their location on the endface. A defect’s location is important because certain areas of the endface are more critical due to the increased risk of performance problems. One such area is the fiber core, because it guides the light in the fiber. Defects in this area can directly affect the amount of light transmitted and reflected. The figures below show typical endface inspection zones. Zone A Zone C Zone B Zone D Epoxy Ring Fiber Fiber Core Ceramic Ferrule Figure 1 – Endface Zone Definition Defects are also assessed by size and quantity. Assessment of defects’ size and quantity is performed for each defined endface location, with allowable limits of size and quantity specified for each location and defect type. Zone A: Measures 25 microns in diameter around the center of the connector ferrule. This diameter encompasses the optical fiber’s core and a small portion of the cladding. Defects such as pits, chips, fixed contaminates, or voids that interfere with the mode field diameter will absorb or scatter the optical signal, causing an increase in insertion and return losses. Scratches in the mode field diameter less than 2 microns in width and depth have been shown to have no effect on insertion loss. However, scratches in the mode field diameter cause an approximately 4 dB drop in return loss 2 . Therefore, no defects of any type should be allowed in Zone A. Zone B: From 25 microns to 120 microns around the center of the connector ferrule, encompassing most of the fiber cladding. Research performed by iNEMI has shown that scratches and other small defects in Zone B do not have an impact on insertion or return loss 3 . Endface workmanship criteria in Zone B should be defined so that consistent and reliable manufacturing processes are used 4 . In other words, a polishing process that yields no defects in Zone A should yield few defects in Zone B. Any defect criteria for Zone B can be limited to a reasonably small size or low number. Zone C: As in Zone B, small defects in Zone C do not cause optical degradation of connectors and are an indication of manufacturing consistency. Workmanship criteria in Zone C should also provide a reasonable limit to the size and number of defects to validate the manufacturing process. The size of defects in Zone C can be larger than Zone B because most of Zone C is outside of the ferrule contact region. 2 IPC-8497-1: Cleaning Methods and Contamination Assessment for Optical Assembly, December 2005 3 Tatiana Berdinskikh, et al., “At the Core: How Scratches, Dust, and Fingerprints Affect Optical Connector Performance”, Connector Specifier Jan 2004 4 Tatiana Berdinskikh, et al., “Keeping it Clean: A Cleanliness Specification for Singlemode Connectors”, Connector Specifier Aug 2005 Singlemode Fiber Optic Connector Endface Workmanship Page 6 Industry Standards Before 2005, there were very few industry standards covering workmanship criteria for singlemode connector endfaces. In 2004, iNEMI began research to develop the framework in which endface parameters could be defined. The work performed by iNEMI and the subsequent standard written by IPC are the only useful published industry standards on ferrule workmanship. The following summarizes endface standards by the major bodies governing singlemode connectors. iNEMI: In 2004, iNEMI began the Fiber Optic Signal Performance Project to investigate the effect of contamination on connector performance. Based on these studies, iNEMI proposed a set of inspection criteria for singlemode connectors (see Table 1 5 ). Table 1 Zone A Zone B Epoxy Ring Zone C Defect type 0μm - 25μm 25μm to 120μm 120μm to 130μm 130μm to 250μm Scratches None > 3μm None > 3μm Any # Any # Pits/Chips/ Other defects None Any # < 2μm (5) from 2μm to 5μm None > 5μm Any # None > 10μm Table 1: iNEMI Endface “Suggested” Criteria iNEMI is currently collaborating with IEC and TIA to develop endface workmanship or cleaning standards based on these criteria. iNEMI plans to submit proposals to update IEC 61300-3-35 (Fiber optic cylindrical connector endface visual and automated inspection). IPC: iNEMI collaborated with IPC to develop endface workmanship standards. In July 2005, iNEMI submitted a draft standard to IPC, which IPC published in December 2005, as IPC-8497-1 (Cleaning Methods and Contamination Assessment of Optical Assembly). Table 2 below shows the workmanship criterion presented in IPC-8497-1: Table 2 Zone A Zone B Epoxy Ring Zone C Defect type 0μm - 25μm 25μm to 120μm 120μm to 130μm 130μm to 250μm Scratches None None > 3μm Any # Any # Pits/Chips/ Other defects None Any # < 2μm (5) from 2μm to 5μm None > 5μm Any # None > 10μm Table 2: IPC-8497-1 Criteria IPC-8497-1 is similar to the original criteria proposed by iNEMI, except no scratches are allowed in Zone A. IEC: IEC has no current standard with normative requirements for endface workmanship. IEC 61300-3-35 (Fiber optic cylindrical connector endface visual and automated inspection) contains an informative annex with suggested endface workmanship criteria. iNEMI has approached IEC about collaborating on a new IEC standard based on iNEMI research. Table 3 Zone A Zone B Epoxy Ring Zone C Defect type 0μm - 25μm 25μm to 120μm 120μm to 130μm 130μm to 250μm Scratches None (5) > 3μm Any # Any # Pits/Chips/ Other defects None Any # < 5μm Any # Any # Table 3: IEC Informative Endface Criteria TIA: The Telecommunications Industry Association (TIA) has no published endface workmanship standards. iNEMI has approached TIA committee FO 4.3.2 about publishing an endface standard. The proposed standard will be a connector cleaning standard based on the work used to develop IPC 8497-1. 5 Tatiana Berdinskikh, et al., “Keeping it Clean: A Cleanliness Specification for Singlemode Connectors”, Connector Specifier Aug 2005 Singlemode Fiber Optic Connector Endface Workmanship Page 7 ADC Endface Workmanship Specification ADC’s endface inspection criteria are based primarily on the work done by iNEMI and IPC and are shown in Table 4. To ensure that we maintain a robust and consistent manufacturing process, ADC has chosen to limit the scratches in Zone C to a maximum of 3 microns rather than allowing an unlimited number of scratches. ADC inspects at a magnification of 400x. Table 4 Zone A Zone B Epoxy Ring Zone C Defect type 0μm - 25μm 25μm to 120μm 120μm to 130μm 130μm to 250μm Scratches None None > 3μm Any # None > 3μm Pits/Chips/ Other defects None Any # < 2μm (5) from 2μm to 5μm None > 5μm Any # None > 10μm Table 4: ADC Endface Criteria ADC connectors manufactured with the above endface standards have been subjected to three GR-326 test programs that were witnessed or performed by an independent third party (most recently in the fall of 2006). These tests were all done under the Verizon FOC fiber optic component qualification program. All three test programs included GR-326 service life testing (five weeks of environmental testing) and reliability (2,000 hours of environmental testing). There were no insertion loss or return loss change failures that could have been attributed to defects on the ferrule endface, showing that connectors manufactured per Table 4 have no long-term reliability performance issues. Summary ADC’s endface workmanship standards are based on the latest industry knowledge regarding how defects affect singlemode connector performance, as defined in IPC 8497-1. We do not allow any defects in Zone A because of the potential of defects in this zone to effect optical performance. ADC limits defects in Zone B and Zone C. Although small defects in these zones do not affect optical performance, we limit the number of defects here in support of our consistent manufacturing processes. Extensive GR-326 FOC service life and reliability testing shows that ADC’s endface workmanship criteria change supports long-term performance of singlemode connectors. 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WHITE PAPER Singlemode Fiber Optic Connector Endface Workmanship Performance Effects of the Singlemode Fiber Optic Connector Endface Workmanship. illumination angle of the connector endface. Singlemode Fiber Optic Connector Endface Workmanship Page 4 Endface Workmanship Parameters Fiber optic connector endface