超精磨削

STR/03/024/MTUltra Precision Grinding of Fibre Optic ConnectorsH. Huang, W. K. Chen, L. Yin, Z. J. Xiong, Y. C. Liu and P. K. TeoAbstract – The present paper reports on the development of an ultra precision grinding tech-nology using inclined resin bond diamond cupwheels for machining spherical end faces of fi-bre optic connectors. The ground spherical endfaces obtained using the wheel of grit size of 4 µm have an average roughness (R a) value of ~7 nm in the fibre area and a profile error of below 0.4 µm. The corresponding average return andinsertion losses are ~55 dB and 0.1 dB respec-tively. The results are competitive to those ob-tained from the polishing process that is cur-rently used in industries. Furthermore, this studyhas revealed the relationships between theground surface quality characterised by rough-ness and profile accuracy and the optic per-formance evaluated by insertion and returnlosses.Keywords: Cup wheel grinding, Fibre connector, Surface quality, Tool wear, Insertion loss, Return loss1 BACKGROUNDFibre optic connectors are one of the basic components in optic communication [1]. There exist many types of connectors. The commonly used connector consists of a zirconia ferrule, a glass fibre and a connector body. The fibre is centred inside the ferrule which has a spherical face at its open end. After assembly, the fibre protrudes from the spherical end face of the fer-rule. The spherical end face, together with the fibre, has to be polished to achieve an extremely even and smooth surface. However, there are some problems encountered in the current pol-ishing processes. Typically, it is difficult to con-trol end face profile accuracy during polishing because of the use of flat polishing pads.Ultra-precision grinding is a proven technology for high dimensional accuracy and mirror sur-face finish [2-7]. This technology has been ex-tensively used as a final finishing process for numerous optic products [4-5]. It is thus possible to use the “one-step” ultra precision grinding to replace the current polishing for machining spherical fibre connector end faces. If the “one-step” grinding technology is feasible, the cycle time for machining fibre end face could be short-ened. Moreover, grinding has an advantage over polishing for maintaining the profile accuracy of racy of end faces. The development of ultra pre-cision grinding technology for machining fibre end faces was started in the late of 1980’s. Kariki-Doy et al. [8] developed an automatic ma-chine for grinding fibre connector end faces. It was reported that the machining time for each end face was 2 to 5 min and the achieved inser-tion and return losses were 0.1 dB and 40 dB, respectively. Kanda et al. [9] also studied sur-face grinding technology for fabrication of fibre end faces. They have achieved a surface roughness of 60 nm in R max and thus a better average return loss of 47 dB. Their results need further improvement for the high quality prod-ucts, such as those used for long distance communications. In addition, these earlier stud-ies focused more on the development of grind-ing technology and lacked systematic investiga-tion of effect of surface quality on the return and insertion losses.2 OBJECTIVEIn this project, an ultra precision grinding tech-nology will be developed using inclined resin bond diamond cup wheels for machining the spherical end faces of fibre connectors. In the first part, the effects of grinding conditions on ground surface finish and profile accuracy were systematically investigated. The tool wear and its effect on grinding quality were also dis-cussed. In the second part, the relationship be-tween the ground surface quality, characterised by surface roughness, profile radius and form error, and the optic performance, evaluated by insertion and return losses, was established.3 METHODOLOGY3.1 Fibre connector and its machiningrequirementsAs illustrated in Fig. 1, the fibre optic connectors to be machined in this study consist of a ferrule with a diameter of 2.5 mm and a fibre of 125 µm in diameter. The materials for ferrule and fibre are partially stabilised zirconia and silica, respectively. The ferrules were fabricated by powder injection moulding.Fig. 1. Illustration of a fibre optic connector.3.2 Grinding experimentsGrinding of fibre connector end faces was com-pleted on a CNC grinding machine (Loh Sphe-romatic 25-2SL, Germany). A cup wheel grind-ing method [10-11] was used to generate the spherical profile of end faces. In this method, the cup wheel was tilted and the top part of the tilted wheel contacted the workpiece during grinding, as shown in Fig. 2(a). The radius of the generated convex spherical surface, R, can be expressed as [11],ααsin 2sin 2r D R m −= (1) where r is the lip radius of the cup wheel illus-trated in Fig. 2(b), α the tilt angle and 2/)(i o m D D D += (2)where D i and D o are the cup wheel inner andouter rim’s diameters, respectively.Fig. 2. (a) Schematic illustration of convex spherical surface generation using a cup wheel (b) Detailedwheel lip.During grinding coolant was supplied to the grinding area. Various wheel speed and feedrate were used to examine their effects on ground surface quality. The total feed was fixedat 30 µm. Under each grinding condition three connectors were ground. Spark-out was carriedout at the end of each grinding cycle at the samegrinding speed for ten seconds. Detailed ma-chining conditions are listed in Table 1.As shown in Table 1, the resin bond diamond wheels of different grit sizes were used in the experiments. A medium grade of wheel hard-ness (Grade N) was selected in order to obtain a reasonably good self-dressing. This was taken the consideration of that the combination mate-rials of zirconia (relatively ductile) and silica (brit-tle) need to be ground simultaneously. Prior to grinding, the resin bond diamond wheels were trued and dressed using a cast iron lapping plate. After truing, an alumina paste of grit size of 3 µm was used to dress the wheels until a satisfactory wheel topography was achieved. Oil was used as a lubricant for dressing to reduce abrasive pull-outs. It takes a few minutes for the whole dressing process.Table 1. Grinding conditions.Parameter ValueGrinding wheel Diamond abrasives,resin bond, concentra-tion of 25%Grit size (µm)4, 20Wheel speed (m/s) 2, 6, 10, 14, 18, 22, 26Work speed (m/s) = 0.023 times wheel speedFeed rate (µm/min.) 10, 50 Total feed (µm) 30 3.3 Measurement and characterisation Surface roughness of the ground end faces was measured using a WYKO White Light Interfer-ometer. Radius and form errors of spherical end faces were measured using a Form Talysurf Profilometer (Taylor Hobson). Grinding wheel topography was examined using a Leica Cam-bridge scanning electron microscope (SEM). Insertion and return losses of the machined connectors were measured using a loss tester (EXFO LTS-3900). 4RESULTS & DISCUSSION 4.1 Grindabilitystudy 4.1.1 Effect of wheel grit size Resin bond diamond wheels were selected for the work because this type of wheels has been proven to be the best choice for mirror surface ConnectorαD iR D o(a)r (b)finish of optic components [4-5]. To select a suitable grit size, two wheels of different grit sizes of 4 and 20 µm were tried. Fig. 3 shows the WYKO images of ground surfaces obtained using the two different wheels, indicating the influence of wheel grit size on surface finish. Apparently, the surface generated using the wheel with a grit size of 4 µm. Fig. 3(a) is much smoother than that with a grit size of 20 µm Fig. 3(b).The profile accuracy of the ground end faces was measured using the Talysurf Profilometer. Using the least square arc fitting method, the radius and form error can be obtained from the traced profile. The average profile errors (peak to valley value) for the wheels of grit sizes of 4 and 20 µm are 0.22 and 0.25 µm, respectively. This indicates that the grit size has little influ-ence on the profile accuracy.Fig. 3. WYKO 3D images of specimens ground using wheels with grit sizes of (a) 4 µm and (b) 20 µm. Grinding speed and feed rate are 26 m/s and 10 µm/min, respectively.A more systematic experiment was completed to examine the effect of grit size on the ground sur-face finish. For each grinding speed, three con-nectors were ground. The average values of the three ground specimens are shown in Fig. 4. The lower and upper error bars represent the minimum and maximum values among the three data, respectively. This rule is also applied to Figs. 5-7. It can be seen that for various grinding speeds, the smaller grit size wheel has pro-duced a much better surface finish, which met the required roughness requirement of < 50 nm in R a. This wheel was thus selected for further parametric studies.Fig. 4. Effect of wheel grit size on surface roughness of (a) fibre and (b) ferrule.4.1.2 Effect of grinding speedFigs. 5 and 6 show the effects of grinding speed on the surface roughness, profile radius and form error for the wheel with grit size of 4 µm. Under the grinding conditions selected in this study, the surface finish in the fibre area is much better than that in the ferrule area, as shown in Fig. 5. The machining responses to the selected grinding conditions are apparently in favor of meeting the final quality requirements as only the surface finish of the fibre area will signifi-cantly affect the return and insertion losses. In general, the wheel speed shows little influence on the roughness for either fibre or ferrule. Nev-ertheless, an increase in wheel speed leads to a slight decrease in surface roughness of the fibre region.Fibre: R a = 6.81 nmFerrule: R a = 30.4 nm (a) Fibre: R a = 30.88 nmFerrule: R a = 69.09 nm (b)102030400510********Sur faceRoug hness,Ra(nm)Grinding Speed (m/s)204060800510******** Sur faceRoug hness,Ra(nm)Grinding Speed (m/s)102030405060051015202530Su r f a c e R o u g h n e s s , R a (n m )Grinding Speed (m/s)1617181920212200.20.40.60.81051015202530R a di u s o f G ro u n d P r o f i l e (m m )P VV a l u e o f G r o un d P r o f i l e (µm )Grinding Speed (m/s)10203040051015202530S u r f a c e R o u g h n e s s , R a (n m )Grinding Speed (m/s)Fig. 5. Effect of grinding speed on surface rough-ness.The effects of grinding speed on the profile ra-dius and the form accuracy are shown in Fig. 6. It is also observed that the speed has little influ-ences on either profile radius or form error in terms of PV (peak to valley) values. The form errors achieved for the 4 µm grit size wheel are all below 0.6 µm, giving an average value of about 0.3 µm. The fitted radius of the spherical end faces ranges from 19.5 to 20.3 mm.Fig. 6. Effect of grinding speed on fitted radius and form error of ground profiles.4.1.3 Effect of feed rateFig. 7 shows the effect of feed rate on surface roughness in the fibre area. It is seen that smaller feed rate results in a better surface fin-ish. There exist some exceptions in Fig. 7. At the two highest grinding speeds of 22 and 26 m/s, the surface roughness values for the feed rate of 50 µm/min are even slightly better than those obtained for the feed rate of 10 µm/min. How-ever, the respective roughness values in the ferrule area are near the critical value of 50 nm. It is impractical to further reduce the feed ratebecause the feed rate directly influences the cycle time. In this study, we also aim at a ma-chining time of about 3 minutes for one end face. This requires a feed rate to be at least 10 µm/min. The effects of feed rate on the profile errors and radius are not significant and thus not demonstrated here.Fig. 7. Effect of feed rate on fibre surface roughness.4.2 Repeatability and tool wear studyTo examine the repeatability of grinding results and the effect of tool wear on surface quality, a set of experiments was carried out. As dis-cussed earlier, the wheel of a grit size of 4 µm gives a much better surface quality than that of 20 µm. Thus, this wheel was selected for re-peatability study. The grinding speed and the feed rate were 26 m/s and 10 µm/min., respec-tively. During the experiment, the wheel was trued and dressed prior to grinding and was then used for continuous grinding until obvious wear evidence was observed. The profile radius and the form error of the ground end faces are plot-ted against the connector number machined in Fig. 8. It can be seen that as the machining pro-gresses, the profile radius is slightly increased at a rate of 0.029 mm per machined part (from part no. 1 to 18), as shown in Fig. 8(a). There is a sharp increase when the machined connector number reaches 19. After that the increasing rate (of 0.074 mm per part) in radius is also greater. In accordance with the sharp increase in radius, the PV value of form error also has a much faster increase starting at the number of 20, as shown in Fig. 8(b). Before that, the profile error remains constant. However, it was found that the wheel wear has little influence on the surface roughness. Apparently, the wheel wear becomes significant and loses its shape after grinding 18 end faces. Further continuation of grinding results in a significant increase in radius and form error. It is thus recommended that theoptimal connector number before redressing the wheel is 18.Fig. 8. Effect of wheel wear on profile radius and form error of the end faces.The effect of wheel wear on the profile radius can be explained using the geometrical illustra-tion shown in Fig. 2. It is assumed that there is a wheel wear of ∆r along the lip radius, which re-sults in a reduction in lip radius. From Eq. 1, it is derived that the lip radius reduction (r-r ∆) re-sults in an increase in profile radius. The new profile radius, R', can thus be approximately de-scribed as follows,r R r r D R m ∆+=∆−−=ααsin 2sin )(2' (3)From Eq. 3, it is clearly seen that the wheel wear directly affects the radius of the generated pro-file, which is in good agreement with the result in Fig. 8(a).Fig. 9 shows the wheel surface characteristics of grit size of 4 µm before and after grinding. It is observed in Fig. 9(a) that the abrasive grains before grinding are sharp. The flatted areas were built on the grains after a certain time of grinding, as shown in Fig. 9(b). Pores and loos-ened abrasive grains are also observed, asshown in Fig. 9(b), indicating that pull-outs oc-curred or would occur in the subsequent grind-ing.Fig. 9. Topography of Wheel with grit size of 4 µm (a) before and (b) after grinding.To prolong the wheel life, in our grinding ex-periments the wheels were trued to have profiles with a larger lip radius shown in Fig. 10, com-pared with the conventional one shown in Fig. 2(b). For wheels with the same manufacturing dimensions, a larger lip radius has increased the wheel functional area, thus reduced the wear along the radius direction. The results shown in Fig. 8 were obtained using this method.Fig. 10. Schematic illustrations of the large lip radiuscup wheel used.192021220510********R a d i u s o f F i br e E n d F a c e (m m )Fibre End Faces Machined00.20.60.8051015202530PVV al u eo f E n d F a c e P r o f i l e (µm )Fibre End Faces Machined (a)(b) rαtdR4.3Effect of ground surface quality onreturn and insertion lossesFibre optic connectors are used for interconnec-tion of various components and devices in anoptic fiber system, which play a vital role on sys-tem performance. Two parameters are oftenused to evaluate the quality of the machined connectors, i.e. insertion loss and return loss.While insertion loss predicts directly the powerloss caused by a connector, return loss charac-terises its influence on the system performancesuch as noise and interference. Insertion loss,IL, defines the system’s power budget and thesystem margin, which can be written as:)(log 10IL 21P P10= (4)where P 2 is the initial power and P 1 the powerafter the connector is applied. IL < 0.3 dB is usually required for long-haul telecommunica-tions and < 0.75 dB for local area networks. Re-turn loss, RL, measures the total fractional power that is reflected from a test device, which is defined as)(log 10RL back in P P 10=where P in is the input power and P back the re-flected power. The return loss is normally ~55dB for an ultra polished physical contact (UPC)connector.There are many factors that cause losses in fi-bre optic communications, including end surface Fresnel reflection, separation between two con-nectors, lateral misalignment, angular misalign-ment, mismatch in diameters of core and clad-ding, refractive index profile difference and fiber end preparation. Apparently, the surface finish and the profile accuracy of the end faces are key parameters to evaluate the fibre end prepara-tion.To establish the relationships between the sur-face roughness and the two loss parameters, a set of experiments was completed to obtain the end faces with various surface roughness val-ues. The end faces with large roughness values shown in Fig. 11 were intentionally generated to examine the effect of surface roughness. Under normal grinding conditions with the wheel of a grit size of 4 µm, the ground surface finish could not be so rough. The influences of surface roughness of the end faces in the fibre area on insertion and return losses are shown in Fig. 11(a) and Fig. 11(b), respectively. In Fig. 11(a), it is seen that when the surface roughness val-ues are below ~60 nm, all the insertion losses are below 0.3 dB. The average value for the data below 0.3 dB is 0.1 dB. There is a poor cor-relation between the insertion loss and the roughness for those data. However, when the roughness values are above ~60 nm, an in-crease in roughness value results in a greater insertion loss. Surface roughness has a much more influence on return loss than on insertion loss. As can be seen in Fig. 11(b), an increase in surface rough-ness leads to a significant decrease in return loss, when the surface roughness is below 50 nm. Above this value, the influence becomes much less, implying that the surface Fresnel reflection becomes dominant. It is also seen that the surface roughness required for meeting the return loss target is below 20 nm in R a , much lower than the R a value for insertion loss (~60 nm). Fig. 11. Relationship between surface roughness and (a) insertion loss and (b) return loss.The data in Fig. 8 was also used to examine the effects of profile radius and form error on inser-tion and return losses. It was found that there is a poor correlation between profile radius and either insertion or return loss. Though the re-10203040506070050100150200250Re t u r n L o s s (d B )Surface Roughness, Ra (nm)0.00.501.01.5050100150200250I n s e rt i o n L o s s (d B )Surface Roughness, Ra (nm)quired nominal radius is 20 mm, a variation of radius from 19.5 to 21 mm has little influence on the insertion or return loss. Similarly, it is difficult to relate the profile accuracy of ground end faces to the insertion or return loss. This is probably because the profile error variation in Fig. 8 is too small to see its influence on inser-tion and return losses. The above results sug-gest that the influence of profile radius and error is not significant on the optic performance of fibre connectors.4.4 Comparison between grinding andpolishingTwo typical microscopic images of the ground and polished end faces are shown in Fig. 12. The surface finishes in the ground and polished fibre areas looks similar. The average value of surface roughness (R a) in the fibre area for the ground end faces is 6.9 nm, which is close to the roughness of 4.8 nm from polishing. However, the surface finishes in the ferrule area for the two processing methods are quite different. The ground ferrule surface (R a = ~30 nm) is much rougher than the polished one (R a = ~10 nm). Nevertheless, the ground connectors performed better than those polished in terms of insertion and return losses. This should be attributed to the much better profile accuracy obtained from the grinding process. The result also indicates that the surface finish in the fibre area has a dominant influence on the two optic parameters. Fig. 12. Comparison of (a) ground and (b) polished end faces. The yield obtained from the grinding process was 100%, in contrast to the 80% from the pol-ishing process. Apparently, the grinding process can produce a controlled end face profile, and thus a consistent quality in terms of the optic performance. However, during polishing the end face profile is difficult to be controlled. This re-sults in an inconsistent quality and thus a lower yield than grinding. It is worthy to point out that the cycle time calculated for polishing is based on the fact that only one connector was ma-chined each cycle. In fact, the modern polishing machines can process at least 12 connectors simultaneously. Therefore, the machining cycle time is more than 12 times shortened. However, this low efficiency problem can be solved by de-veloping a specially designed machine for grind-ing multiple connectors, as indicated in [8].5 CONCLUSIONUltra precision grinding using resin bond dia-mond cup wheels has been successfully devel-oped for machining spherical end faces of fibre connectors. The ground spherical end faces ob-tained using the wheel of grit size of 4 µm have an average value of R a of ~ 6.9 nm in the fibre area and a profile error of below 0.4 µm for the entire surface. These correspond to an average return loss of ~ 55 dB and insertion loss of 0.1 dB. The comparison between grinding and pol-ishing has demonstrated that both methods can achieve similar optic performance in processing fibre optic connectors, and the former is more consistent.It is found that the influence of grinding speed on surface finish and profile accuracy is not signifi-cant. However, the feed rate or the wheel grit size significantly affects the ground surface roughness, but not the profile accuracy. There exist relationships between ground surface roughness and insertion or return losses. A rougher end face certainly results in larger inser-tion loss and smaller return loss, which would worsen optic system performance. The industrial requirement on insertion loss can be easily achieved with the roughness below 60 nm, while that on return loss requires a roughness of be-low 20 nm.6 INDUSTRIALSIGNIFICANCEThe optics/photonics industry has a huge global market and is expected to surge to US$700 billion in 2025. Singapore is now tapping about S$2 billion in optics-related business. USA and Europe are the major players, followed by markets in China, Japan and Taiwan. Singapore50 µm Ferrule(a)(b)is one of the first countries in Southeast Asia to see its growth potential and is quickly adopting new technologies and creating new platforms for training and R&D to spearhead the growth. Local companies, have approached SIMTech for new technologies on fibre/ferrule connector machining. The technologies developed in this project should be transferred into industries in the near future when the photoics business picks up, making a direct impact on Singapore economy.REFERENCES[1] D. 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Syoji and K.Tanaka, “Study on ultra-precision grinding ofmicro aspherical surface (3rd report) – minia-turization of aspherical surface in inclined ro-tational grinding”, Bulletin of Japan Societyof Precision Engineering, Vol. 64, pp. 1350-1354, (1998).[11] G.E II Storz and T.A. Dow, “Cup wheelgrinding geometry”, Proceedings of the 9thASPE Annual Meeting, Ohio, 2-7 October 1994, pp. 105-108.。