How to properly polish fiber-optic connectors
A thorough understanding of the reasons for and methods of polishing a fiber-optic connector helps ensure consistent, high-quality terminations
Fred A. fons
Polishing the fiber/ferrule endfaces of a fiber-optic connector critically influences optical performance and is highly susceptible to error. Yet the polishing process is neither difficult nor mysterious. Other steps in the connector termination procedure, such as crimping, involve mechanically securing the fiber in the connector. As the final step, polishing prepares the fiber optically to ensure that defects and nonuniformities in the fiber/ferrule endfaces or geometry do not degrade the passage of light across the connector joint.
A fiber-optic connector has the fiber placed within a precision ferrule, which is made of ceramic, stainless steel, or polymer. Polishing removes any excess epoxy or fiber stub left after cleaving, shapes the ferrule, and removes scratches in the glass, enabling an end finish that passes optical signals with minimum loss. It affects two types of optical loss--insertion loss and return loss.
Low insertion loss is the basic optical performance requirement of a fiber-optic connector. Connectors currently on the market offer insertion losses of 0.2 to 0.3 dB. Factors that affect insertion loss include
variations in the ferrule that move the bore (and the fiber) from a dead-center position in the ferrule,
variations in the fiber, particularly ellipticity or lack of concentricity, (These parameters refer to departures from a perfectly round core exactly centered within a perfectly round cladding. The core might be slightly off center or slightly elliptical in shape.)
an incorrectly formed ferrule/fiber endface that prevents fibers from touching,
poor fiber end finish.
The first two factors are determined during connector and fiber manufacture and are beyond control during connector termination. The last two factors are directly influenced by polishing. End finish is important because scratches, fractures, and other imperfections prevent the maximum transfer of optical power between the two joining fibers.
A few years ago, conventional termination practice suggested that the best mating between two fibers required a small air gap between them. The reasoning was that mated fibers could be damaged if they were allowed to touch, especially if a fiber extended beyond the end of the ferrule. Recent studies show that the physical contact of two fibers is now preferred for best optical performance.
Return loss is a measure of reflected light. Whenever light encoun ters a change in the refractive index of the material through which it is propagating, a portion of the light is reflected back toward the source. Some of this reflected light can interfere with the proper operation of light sources, especially lasers. Because of its small core size, a singlemode fiber connection requires a high return loss to limit the amount of reflected energy.
The main causes of reflections are an air gap between fibers (which causes changes in refractive indices) and the presence of an altered layer in the fiber endfaces. Return reflections are minimized in a physical contact end finish, which has a radius ferrule/fiber endface that brings mated fibers into contact with one another. Because of its large core size, a multimode fiber connection can tolerate some return reflections; this type of loss proves to be a minor issue that is easily controlled.
For example, in a premises cabling application, the tia/eia-568a standard for fiber-optic connectors recommends a return loss of -20 dB for multimode connectors and -26 dB for singlemode connectors. The more-rigorous Bell Communications Research requirements (GR-326-core, which covers generic requirements for singlemode connectors) specify return losses as high as -65 dB.
From a polishing viewpoint, achieving high return loss is more difficult than achieving low insertion loss. A connector can be polished to achieve low insertion loss, while it still yields low return loss. For technicians making field or factory terminations, visual inspection with a handheld microscope usually provides an accurate indication of the optical polish regarding insertion loss. Visual inspection, however, offers few, if any, clues to return-loss performance. Therefore, good polishing procedures that remove the altered layer are critical for singlemode connectors. In addition, following the manufacturers` recommended procedures is a good way to develop quality polishing techniques. In fact, many manufacturers and independent organizations offer hands-on training in connector termination and polishing. (See Calendar section, page 112, for training sessions--Ed.)
To determine acceptable and nonacceptable ferrule polishing, consider that, in the ideal connector, a ferrule meets the following conditions (see Fig. 1):
The endface forms a smooth radius, with the center of the radius coinciding with the centerline of the fiber/ ferrule.
The high point of the radius coincides with the axis of the fiber and the ferrule.
The end of the fiber is flush with the end of the ferrule.
The fiber is finished to a scratch-free and defect-free finish.
The radius of curvature of the ferrule end is 10 to 15 mm.
A nonacceptable ferrule endface differs from an acceptable one because of one or more of the following conditions:
The radius is not centered on the ferrule centerline.
The dimension of the radius according to the specification is incorrect.
The high point of the radius is off-center. This condition is called an apex offset. Any offset should be less than 50 microns.
The fiber endface is not flush with the ferrule endface; it either protrudes or recedes in relation to the ferrule endface. This condition is termed undercut and labeled as being either positive (protruding) or negative (receding). A small undercut or protrusion (50 nm for ceramic-ferrule connectors) does not degrade performance.
The fiber endface finish has significant scratches or other defects.
Singlemode fiber polishing is a multistage process that begins with a quick, coarse polish and ends with a final polish in a slurry. Different polishing materials are involved in each step. In most cases, a singlemode fiber connector uses epoxy to hold the fiber within the ferrule. A six-step process ensures the proper techniques for polishing a singlemode fiber-optic connector (see Fig. 2).
The first step consists of a quick hand polish--lasting perhaps 5 seconds, with medium pressure applied--using a coarse 5- to 15-micron film. This step removes the fiber stub and levels the protruding material (including epoxy) close to the ferrule. Typically, a technician is able to feel the epoxy/fiber before polishing, but not after this step.
The second step requires a 5-micron aluminum-oxide film. Hand-applying the film gently across the fiber endface removes the epoxy down flush with a ceramic ferrule. However, this action could remove some ceramic material as well.
In the third step, a medium-grit (3- to 6-micron) diamond film is used to begin hand-shaping the fiber/ferrule endfaces. Unlike aluminum oxide, diamond film treats ceramic and glass materials similarly. This hand-shaping minimizes undercut, so that the fiber recedes only slightly into the ferrule. Because the main purpose of this step is shaping, it usually leaves visible scratches on the fiber endface.
To remove scratches and achieve a smooth surface, a fine-grit (1-micron) diamond film is used next in step four. In terms of insertion loss, the endface finish should be acceptable at this point.
However, to obtain a high-return loss of at least -45 dB, a fifth or finish step is required because of the altered-index layer on the fiber endface. During abrasive polishing, a small layer on the end of the fiber becomes altered so that its refractive index changes. This change increases reflections (in other words, produces a lower return loss), and the fifth step of slurry polishing is needed to remove the altered layer.
The fifth step uses a special film (termed HX), which contains a top layer of polishing slurry. The slurry has both a lubricating and a chemical effect on the fiber during hand application to change the refractive index back to its original value.
The last step involves inspection. The use of a hand microscope determines polishing acceptance or nonacceptance.
Multimode and epoxyless connectors call for less-intense polishing requirements than singlemode, epoxy connectors. Multimode connectors do not have the rigorous return-loss requirements of singlemode connectors so that shaping the fiber/ferrule endface is less critical. Polishing multimode connectors is a shorter process than that for singlemode, and polishing epoxyless multimode connectors is even shorter.
To achieve consistent, high-quality fiber/ferrule finishes, the connector manufacturer`s procedures must be followed explicitly. The procedures usually differ, however, with the type of connectors used. For instance, the polishing procedures for FC and SC connectors could be different. Also, the procedure for polishing ceramic ST connectors and polymer ST connectors could differ. Although the basic polishing principles apply, important minor differences generally exist.
Cleanliness is critical
Maintaining a clean work area is mandatory. Although this is easy in the laboratory, cleanliness is equally important in the field and in indoor and outdoor installations. Keep all films and polishing pads clean; clean films outperform dirty ones. However, diamond films can be used to process several polishes, but aluminum-oxide films must be changed for every polish.
Likewise, clean the tips of connectors with alcohol and a wipe between each polishing step. Some technicians prefer to clean a connector before and after each step. This cleaning removes any grit from the fiber/ferrule endfaces.
Clean all polishing pads regularly. Grit on a pad adversely changes its polishing properties.
Inspect connector holders to make sure that their bores remain grit-free. The buildup of dirt prevents the ferrule from protruding through the holder at the proper distance. A wrong distance translates into incorrect polishing.
Visual inspection with a microscope is the most valuable method to evaluate the fiber/ferrule endface. Observe that all remaining scratches are small and no fractures or major flaws are detected. If scratches remain at this step, the technician must perform additional fine polishing.
For unacceptable end finishes, some deficiencies can be corrected by additional polishing (see Fig. 3). Other deficiencies mean that the connector should be rejected. If the connector is rejected, the technician cuts the connector from the cable and begins the polishing process anew.
Machine versus hand polishing
Polishing machines are available for either factory or field polishing (see photo). These machines typically can batch-process 6 or 12 connectors at a time.
Machine polishing has two main advantages: consistent end finishes and lower termination costs in high-volume production. Consistency in polishing is especially important in singlemode connector applications, where high-return loss is required. Even in multimode connector applications, the need for consistency might make the use of machines imperative.
Because polishing machines are a major investment (several thousand dollars), they find widest use in high-volume applications, such as factory production of cable assemblies. In such applications, machine polishing is more economical than hand polishing (see Fig. 4).
Studies show that hand polishing costs approximately $0.50 per connector, based on a $15-per-hour labor rate and a 2-minute polishing time. A 12-connector polishing machine can finish approximately 144 connectors per hour. When the cost of the machine is amortized over one year, the termination cost runs from $0.48 per connector if the machine is used one hour per day, to only $0.19 if the machine is used six hours a day. Even at modest usage rates, the costs of machine polishing are equivalent to those for hand polishing and yield the added benefits of consistency.
In evaluating polishing machines, look for a machine that minimizes any variations in the polishing process. Variations lead to changes in vertex offset, radius of curvature, polishing angle, undercut and end finish. Commercial polishing machines are designed to virtually eliminate variations in polishing and to achieve high-quality, consistent polishes. They include the following features:
Orbital polishing motion. An orbital polishing motion, in which the polishing disk rotates in one direction while eccentrically moving in the other direction, creates a counter-rotating polishing pattern. This motion distributes the polishing movement evenly and across a larger area of polishing film than does a simple circular motion.
Four-corner holddowns. Holddown fasteners in all four corners of the connector holder evenly distribute firm pressure to minimize vertex offset. A popular alternative--using center pressure from above--allows the possibility of wiggling or vibrating the connector holder. Such motion increases the vertex offset and leads to inconsistent finishes.
Bottom pressure. The polishing film rests on a rubber polishing pad. These pads, in conjunction with the four-corner holddowns, distribute pressure evenly across the polishing area. Because the pads are resilient, they also help to control the radius of curvature because the ferrule is pressed into the pad during polishing. Different pads (and companion spacers) are available for different styles of connectors. It is important to use the proper pad.
Precision connector holders. Connector holder devices are machined to exacting tolerances so that ferrules are precisely positioned for polishing.
The choice of hand polishing versus machine polishing depends on both the application requirements and on the connector volume. Hand polishing can achieve acceptable results in many applications, particularly those that do not require high return losses. A technician can be trained in a few hours to make acceptable hand polishes. However, consistency is still not as proficient as with a machine polish, although hand-polished connectors perform well within required specifications. For example, a premises-cabling or a fiber-to-the-desk application has generous requirements in relation to typical connector performance. A fiber-optic connector typically presents an insertion loss of 0.3 dB. The tia/eia-568a specification allows 0.75 dB per connection. Technicians easily achieve hand polishes that fall well under the 0.75-dB requirement. u
Fred A. Fons is a product manager for the global optical interconnection systems business unit at AMP Inc., in Harrisburg, PA.