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MECHANICAL FIBER POLISHINGTechniques for Achieving&Exceeding Connector Polishing RequirementsBackgroundWithin the communications field,high standards of connector performance are needed for telephonetransmission and even higher standards for CATVPolished connectors are quality tested to ensure they allow maximum light transmission with minimal interferenceIn this presentation we discuss the various important polishing performance criteria and suggest methods to ensure connectors are polished right first time,every timeEnsuring PerformanceThe key performance criteria for polished connectors are:?Backreflection?Insertion Loss?Apex Offset?Fiber Undercut/Protrusion?Visual Quality BackreflectionBackreflection(or“return loss”)refers to light reflected back through the fiber toward the source.It must be minimized(to less than 55dB)to insure speed and clarity of information transfer.Backreflection is tested as shown below:Backreflection can be improved(minimized)by control of the following factorsImproving BackreflectionPolishing Method Use of a mechanical polisher ensures an even polish of all sides of the connector endface This leads to reproducible finishes and connectors that mate perfectly improving backreflection Polishing Technique Recipes exist to improve clarity of the polish and so improve backreflectionCleaving A bad cleave before polishing will affect all subsequent polishing steps and overall performanceCleanliness This cannot be stressed enough.Thorough cleaning of connectors between each step,along with the use of rapid change polishing plates reduces the possibility for cross-contamination helps ensure backreflection resultsAsk ULTRA TEC for more detailsInsertion LossInsertion loss is the amount of optical power lost at the interface of two connectors.Poor insertion loss readings are generally a result of fiber misalignment,separation between connections and/or the quality of the finish on the end of the connector.Improving Insertion LossInsertion loss can be improved(minimized)by?CAREFUL SELECTION OF MACHINERY-A machine that produces poor end-face geometry will almost always generate unacceptable levels of loss.The currently stated standard for insertion loss is.3db,but the commonly expected level,has become.2db or less.In addition to the above performance characteristics,there is a specified product geometry-specified to assure reliability and ongoing proper connector performance under adverse conditions such as vibration and temperature cycling.These characteristics are are improved by the high level of control that a repeatable mechanical polisher provides.?POLISHING TECHNIQUE recipes exist to help you minimize loss.Ask ULTRA TEC for more detailsApex OffsetThe term Apex defines the highest point on the spherical surface at the end-face of the connector.Apex Offset is the measured distance between the center of the fiber and the actual high point of a polished connector.Improving Apex OffsetAlthough Apex Offset describes a physical condition of the polished fiber,rather than a performance parameter,it is considered an acceptance criterion in itself.An excessive Apex Offset contributes to a high Insertion Loss and high Backreflection reading.Key to optimizing(minimizing)apex offset is the use of a mechanical polisher that polishes all sides of the connector equally.Ask ULTRA TEC for more detailsRadius of CurvatureRadius of curvature is the measurement of a connectors end-face spherical condition.The radius generated on a connector end-face affects connector performance,and so is specified-the industry specification is 10-25mm.Improving Radius of CurvatureIn general,ferrules are pre-radiused.This radius is maintained during polishing by applying pressure between the connector and a resilient polishing surface,by applying weight or by setting a compression dimensionally.Although increasingly rare,the forming of a flat-ended ferrule into a PC end can be achieved through the same basic technique of applying pressure against a resilient surface.The harder the resilient polishing surface the larger will be the resultant connector radius(more flat).Conversely,the softer the polishing surface the smaller the connector radius.A proper radius,in conjunction with fiber undercut,allows for correct fiber-to-connector compression.Ask ULTRA TEC for more detailsFiber Undercut/ProtrusionWhen a fiber is recessed inside a connector ferrule the term used is Fiber Undercut.When a fiber protrudes above the ferrule it iscalled Fiber Protrusion.Measurement of this characteristic ismade on an interferometer observing the offset of the interference lines that pass over the fiber.Excessive Fiber Undercut or Protrusion is usually specified as more than+/-50nm.Improving Fiber Undercut/ProtrusionUndercut is more common than protrusion as a result of the polishing process.Most polishing sequences begin with aggressive materials,silicon carbide to remove epoxy and diamond films for beginning and intermediate polishing.These remove both the ferrule and fiber at the same rate.During the last polishing step,however,a less aggressive material,usually aluminum oxide,or silicon dioxide,is used because it attacks only the fiber can lead to undercut.Recipes exist to minimize undercut for use with mechanical polishersWhen connectors are mated,the ferrule material surrounding the fiber compresses,which optimally allows fibers with an acceptable undercut to make contact.Fibers that do not make intimate contact have an air gap.An air gap-and consequently Backreflection and Insertion Loss-may worsen as a result of temperature cycling which would cause the fibers to recess further into the ferrule over time.Close control of the polishing process is therefore key to producing optimum results.Ask ULTRA TEC for more detailsVisual QualityVisual inspection will always play an important role in evaluating the quality of a polished connector.A general guide detailing criteria for acceptance,re-polish requirement or rejection is shown on the right.Additional Tips A1.Polishing Films-Films are the most significant factor in polishing.Quality and gradations vary from supplier to supplier.When a polishing technique is developed the film type,make and particle size must be chosen carefully.Excessively aggressive films can destroy the end-face geometry of a connector.Also,of critical importance to real cost,is not only the the initial cost of the polishing film but also how it relates to the cycle life that the films provide-this can vary significantlyTip-Clean each piece of polishing film before and after each use.Cleanliness will increase the film life and so decrease thecost per connector.2.Epoxy-different types of epoxies can be removed more easily with specific grades of silicon carbide polishing films.The films to use on this step depend on the epoxy type and the size of the epoxy bead mounted on the connector end-face.Different epoxies have varying levels of hardness-some are tacky,and some are firm-Hard epoxies are removed easily with coarser particle sized films(20 um,30 um,etc.),while softer epoxies are better suited to smaller particle sized films,i.e.9 um,5 um,etc.Tip-The epoxy bead that remains on the connector before polishing should be minimized(the size of a pinhead).This willextend the life of all the polishing films.Also,try different gradations of silicon carbide until you find the epoxy removal film that works best for your needs.Additional Tips BSummaryMechanical Polishing offers improved connector performance and reproducibility(as well as cost-effectiveness)Best results are achieved by using proven polishing recipes with premium lapping filmsCareful control of all processes and material handling operations before and after polishing,contributes to improved connector performance Applying these principles will help insure your polishing results are right first time,every timeAbout UsULTRA TEC is a leading manufacturer and supplier of fiber optic connector polishing machinery,accessories and consumables.We also offer solutions for preparing bare fibers,capillaries,waveguides,wafers and related optical components for the communications industries.Our equipment is manufactured in Southern California,for sale both direct and through a worldwide network of agents and distributors.To contact ULTRA TECCALL TOLL FREE(USA):1 877 542 0609 OVERSEAS:1 714 542 0608FAX:[phone]VISITMECHANICAL FIBER POLISHINGTechniques for Achieving&Exceeding Connector Polishing Requirements