polishing of fibre optic connectors · 2015-07-29 · str/03/027/mt 1 polishing of fibre optic...

STR/03/027/MT

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Polishing of Fibre Optic Connectors

L. Yin, H. Huang, W. K. Chen, Z. Xiong, Y. C. Liu and P. L. Teo

Abstract - This study reports the development of high efficiency polishing protocols of fibre connectors, by replacing the presents three step polishing with two step polishing protocols, and by using various abrasives as polishing medi-ums, to achieve necessary geometrical and op-tical qualities for physical contact transmission using a commercially available connector pol-isher. The study focuses on surface integrity of the fibre and ferrule as well as other geometry of the polished connector end faces covering the fibre height or undercut, the apex offset, and the radius of curvature. Optical performances of the polished connectors were evaluated by return and insertion losses. Finally, relationship be-tween geometrical qualities and optical perform-ances was established. This study shows that the efficiency for polishing fibre connectors can be raised by at least 30% by selection of suit-able abrasives and grit sizes. Keywords: Fibre optic connectors, Polishing, Surface roughness, Fibre height/undercut, Apex offsets, Radius of curvature, Return loss, Inser-tion loss 1 INTRODUCTION In the applications of fibre optic technology, the fibre connectors have been playing a substantial role to bring together and hold the two cores of the glass fibre ends for communications. Among the many types of connectors, the physical con-tact (PC) type has emerged as the most popular connector, capturing the vast majority of instru-mentation applications [1]. The physical contact connectors are designed to have a spherical end face with the fibre at the highest point. On connection, this allows the fibres to come into intimate optical contact and compress slightly for the optimum system performance over time, temperature, and vibration. The quality of the end face geometry determines the fibre-fibre interface where a perfect contact without air gap is expected over a long term or when intermated with another connector. Therefore, it is critical for finishing of a connector end face to obtain not only surface integrity for the fibre and ferrule and a reasonable radius of curvature of the end face, but also a minimum fibre height or under-cut over the ferrule and a small apex offset be-tween the fibre and ferrule. Ideally, a polished

connector is expected to achieve a maximum light transmission with a minimum insertion loss. Practically, it is required to achieve of an inser-tion loss of < 0.3 dB and a return loss of < -45 dB for the physical contact connectors [1]. Currently, mechanical polishing is used to pro-duce large volumes of connectors meeting or exceeding good optical and geometrical quali-ties as well as keeping high level of consistency from batch to batch. The polishing process often involves rough polishing of the cleaved and ep-oxy removed fibre connectors, intermediate pol-ishing, and final polishing, taking at least 1.5 ~ 3 minutes for finishing such a process circle [2, 3]. This is expensive in consideration of the vast needs of connectors in fibre communications. Therefore, it is essential to develop the high effi-ciency polishing processes for connectors. Since fibre connector polishing involved abra-sive machining of two brittle materials, the glass fibre and the ceramic ferrule simultaneously, it is likely to have different material responses to polishing in respects to surface roughness, ma-terial removal and form accuracy [4]. Further-more, the lack of understanding of the relation-ship between the geometrical and optical quali-ties for connectors and the internationally stan-dardized technical requirements for assessment of these qualities [5-6] has led to confusion in manufacturing fibre connectors. 2 OBJECTIVE The primary objective of this study is to develop high efficiency polishing protocols of fibre con-nectors to achieve necessary geometrical and optical qualities for physical contact transmis-sion, using a commercially available connector polisher. Considering the abrasive machining-induced damage in machining brittle materials, the present study focuses on surface integrity of the fibre and ferrule as well as other geometry of the polished connector end faces covering the fibre height or undercut, the apex offset, and the radius of curvature. For optical performance, the polished connectors are investigated in terms of the return and insertion losses with an optic loss tester. Finally, we attempt to establish the rela-tionship between the geometrical quality and the optical performance.


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3 METHODOLOGY 3.1 Physical contact fibre ends The samples used in this investigation were the commercially available, unpolished single mode fibre connectors. The 125 µm diameter glass fibre was centered in the yettria partially stabi-lized zirconia ferrule and exited at its endface. The 2.5 mm diameter zirconia ferrule was keyed in a mechanical assembly to hold the cable rigid and aligned. The end faces were pre-convex, spherically curved with the radius of curvature of about 20 mm. The fibres were cleaved and the epoxy was removed. 3.2 Apparatus for fibre connector pol-

ishing The polishing experiments were conducted us-ing a commercially available polishing machine (8671X-6100 Series, Molex), as shown in Fig. 1. The polishing machine had a timer ranging from 0 to 60 s, a constant pressure for polishing, and a universal connector holder for 12 connectors. An air cushion metal pad with the 12 circle or-bits, and a flat metal pad and a flat rubber pad were applied for polishing. The machine pro-vided an orbital polishing motion, which was a circular orbital oscillation. During polishing, each connector rotated independently along its circu-lar orbit of a 17.5 mm diameter at a speed of 15 rpm; meanwhile it spun itself at a speed of 285 rpm. Disposable, self-adhesive polishing films of SiC with grit sizes of 3 µm and 5 µm, alumina with grits of 0.05 µm and 0.5 µm, and diamond with grits of 0.1 µm and 0.5 µm were selected. Typical sizes and distributions of the polishing films are shown in Fig. 2. The larger sizes of SiC grits are clearly identified in Fig. 2. The sub-micro alumina and diamond abrasives are shown in grit clusters in Fig. 2.

Fig. 1. The polishing machine.

5 µm SiC 3 µm SiC

0.5 µm Alumina 0.5 µm Diamond

0.1 µm Diamond 0.05 µm Alumina

Fig. 2. SEM micrographs of the polishing films. 3.3 Polishing procedures Usually, polishing of the single mode PC con-nectors involves three steps including rough polishing, intermediate polishing and final pol-ishing for 30 s in each step, or 90 s in total for a polishing circle [2]. To reduce the polishing cycle time and to increase the polishing efficiency, in this investigation a two-step processes were proposed and conducted for a total circle time of 60 s, maintaining 30 s for each step. Before each polishing step, a pure grade of iso-propyl alcohol was applied on the top of the pol-ishing film as a lubricant. In each process, six randomly loaded connectors were polished to evaluate the repeatability of the results. The de-tailed polishing procedures for the industry-applied three-step processes and the two-step processes proposed are listed in Table 1. Be-fore and after each step, the fibre connector end faces were carefully cleaned with alcohol and optical tissues for further polishing or analyses. 3.4 Characterization methods Before and after each polishing step, the con-nector end faces were evaluated using a fibre connector microscope and an optical interfer-ence profiler (WYKO 3300, Veeco) to investi-gate the polishing scratches, the surface rough-ness for the fibre and ferrule, and the relative fibre height or undercut. The fibre and ferrule


Table 1. The detailed description of the polishing processes

surface roughness in terms of the arithmetic average roughness (Ra) was measured using a vertical scanning interferometry (VSI) in a measured area of 185 × 243 µm2 covering the fibre region of the end face. The relative fibre height or undercut is evaluated using the vertical distance of the highest point of a fibre end face to the epoxy between the fibre and ferrule. The mean values and the standard deviations of these measurements were determined from the six connector end faces studied in each step polishing. The Wyko optical interferometer lacks the func-tions for fibre array and precision alignment and for multiple parameter measurements acquired in geometrical quality assessment. Therefore, the final geometry quality for the polished con-nector end faces was further and thoroughly evaluated using an automated non-contact inter-ferometer system for array-type fibre optic con-nectors (AC-3005, Norland). The geometrical parameters included the fibre and ferrule sur-face roughness, the fibre height or undercut, the apex offset, and the radius curvature. The fibre height or undercut is defined as the distance between the fibre end face and the best fit of the spherical surface of the average ferrule end face [7]. When a fibre is recessed inside a ferrule, it is named a fibre undercut.

When a fibre protrudes above the ferule it is called a fibre height. The radius of curvature is defined as the radius of the best fit of the spherical ferrule end face [7]. The apex offset is the linear distance from the center of the fibre to the apex or highest point on the best fit of the spherical end face [7]. This can also be ex-pressed as angle between these two points, namely the angular offset [7]. The former is des-ignated as the linear apex offset [7]. The optical performance in term of the return and insertion losses was measured using a loss test set (LTS-3900, EXFO), which combines a stable optical source, an optical power meter and an optical return loss meter. The return loss designates the total fractional power that is re-flected from a test unit, which is defined as

Return Loss = -10 log10 (Pin / Pback),

where Pin is the input power and Pback the re-flected power. The insertion loss is the amount of optical power lost at the interface of two con-nectors, which can be written as

Insertion Loss = 10 log10 (Pi /P0),

where Pi is the initial power and P0 the power after the connector is applied.

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Process Step 1

Step 2

Step 3

A

3 µm SiC film Metal air cushion pad Alcohol 30 second polishing

0.5 µm alumina film Metal flat pad Alcohol 30 second polishing

0.05 µm alumina film Rubber flat pad Alcohol 30 second polishing

B


0.1 µm diamond film Metal flat pad Alcohol 30 second polishing

0.05 µm alumina film Rubber flat pad Alcohol 30 second polishing

C


0.1 µm diamond film Metal flat pad Alcohol 30 second polishing

D


0.1 µm diamond film Rubber flat pad Alcohol 30 second polishing

E 3 µm SiC film Metal air cushion pad Alcohol

0.5 µm diamond film Metal flat pad Alcohol


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4 RESULTS 4.1 Effects of polishing processes on

surface integrity of connector end faces

The effects of polishing on the fibre surface roughness Ra are summarized in Fig. 3. After the first 30 s of polishing, the surface roughness values for the cleaved fibres of about 125 ~ 350 nm Ra dropped to about 50 ~ 150 nm Ra. It is noticed that Processes A, B and E using 3 µm SiC films generated the better fibre surfaces than those using 5 µm SiC in the first 30 s pol-ishing. After the second 30 s polishing, the fibre surface roughness values were reduced to less than 50 nm Ra in all the processes. The coarser fibre surfaces were generated in Process A with the average value of 50 nm Ra; the finer were made in Processes B, C, D, and E with the val-ues of 15 ~ 30 nm Ra. For the third 30 second polishing involved only in Processes A & B, the roughness did make a significant progress from 50 nm Ra to about 15 nm Ra in the former; how-ever, it improved very slightly obtaining about 15 nm Ra in the latter. The results indicate that the two-step polishing in Processes C, D, and E is capable of achieving as a good fibre surface finish as that obtained in the three-step polishing processes A and B. It is therefore possible to use the two-step polishing to replace the three-step processes for improvement in the fibre sur-face roughness, shortening the polishing circle time from 90 s to 60 s. The effects of polishing on the ferrule surface roughness Ra values are plotted in Fig. 4. The initial ferule surface roughness obtained in the cleaving and epoxy removal process can be very rough with the maximum roughness value of about 1900 nm Ra. After the first 30 s polish-ing, the ferrule surface roughness values were tremendously diminished to < 75 nm Ra for all the processes. After the second 30 s polishing, the ferrule roughness values dropped to < 20 µm Ra in Processes C, D and E, and to about 30 nm Ra in Processes A and B. After the third 30 s polishing in Processes A and B, it is observed that the ferrule surface roughness slimly im-proved to < 25 nm Ra. It is thus concluded that the two-step polishing of Processes C, D and E is also more effective than the three-step polish-ing of Processes A and B for finishing the ferrule surfaces. The effects of polishing on the relative fibre heights or undercuts is shown in Fig. 5. The ini-tial relative fibre heights after cleaving and ep-oxy removing could be as high as about 12 µm. After the first 30 s polishing, the relative fibre

heights were reduced below 2 µm in all the processes. In the second step polishing, the relative fibre heights in Processes C, D and E dropped rapidly to about 0.05 µm; however, in Processes A and B, they just reduced to 0.2 µm and 0.15 µm respectively. Finally, after 90 s pol-ishing in Processes A and B, the relative fibre heights diminished to about 0.05 µm. The three-step polishing processes A and B appear to have disadvantages in reducing the fibre heights. Fig. 6 shows the optical views of the fibre end face taken before and after each polishing step in Process C. Fig. 6(a) is a typical image for a cleaved and epoxy-removed fibre connector end face. Fig. 6(b) is the image taken after 30 s pol-ishing, in which many polishing marks and scratches can be observed. Fig. 6(c) is image taken after the second-step polishing, where no visible scratches and damage can be seen. Fig. 7 shows the two-dimensional and three-dimensional optical interference images corre-sponding to the optical images in Figs. 6(a), 6(b) and 6(c), in Process C. Fig. 7(a) shows the es-timation of the relative fibre height above the epoxy of 5.8 µm for the cleaved and epoxy-removed connector end face. Fig. 7(b) exhibits the estimation of the relative fibre height of about 2 µm and the polishing scratches after 30 s polishing. Fig. 7(c) shows the estimation of the relative fibre height of about 50 nm and the damage free end face after the second-step pol-ishing.

Polishing Time (s) -30 0 30 60 90 120

Fibe

r Rou

ghne

ss in

Ra

(nm

)

0

50

100

150

200

250

300

350

400Process AProcess BProcess CProcess DProcess E

Polishing Time (s) 30 60 90 120

Fibe

r Rou

ghne

ss in

Ra

(nm

)

0

25

50

75

100

125

150

175

200Process AProcess BProcess CProcess DProcess E

Fig. 3. Influence of polishing on fibre surface rough-ness.


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Polishing Time (s) -30 0 30 60 90 120

Ferr

ule

Rou

ghne

ss in

Ra

(nm

)

0

250

500

750

1000

1250

1500

1750

2000Process AProcess BProcess C Process DProcess E

Polishing Time (s) 30 60 90 120

Ferr

ule

Rou

ghne

ss in

Ra

(nm

)

0

25

50

75

100Process A

Process BProcess C Process DProcess E

Fig. 4. Influence of polishing on ferrule surface roughness.

Polishing Time (s)-30 0 30 60 90 120

Rel

ativ

e Fi

ber H

eigh

t (µµ µµm

)

0

2

4

6

8

10

12

14Process AProcess BProcess C Process D Process E

Polishing Time (s)60 90 120

Rel

ativ

e Fi

ber H

eigh

t (µµ µµm

)

0.00

0.05

0.10

0.15

0.20

0.25Process AProcess BProcess C Process D Process E

Fig. 5. Influence of polishing on the relative fibre heights.

50 µm

50 µm

50 µm

(a)

(b)

(c) Fig. 6. Optical images of a connector end face pol-ished in Process C. (a) Before polishing, (b) After the first-step polishing, and (c) After the second-step pol-ishing.


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Fig. 7. Corresponding optical interference images of the connector end face shown in Fig. 6, demonstrat-ing the two- and three-dimensional views, and the relative fibre height assessment. (a) Before polishing.

Fig. 7(b). The first-step polished connector end face.

Fig. 7(c). The second-step polished connector end face. 4.2 Geometrical quality assessment for

polished fibre connectors The fibre and ferrule surface roughness Ra val-ues of the polished fibre connectors were also assessed using an automated non-contact inter-ferometer system for array-type fibre optic con-nectors. The fibre surface roughness values were smaller than 20 nm Ra in Processes A, B, C, and E and smaller than 30 nm in Process D. The ferrule surface roughness values are smaller than 20 nm Ra in either the two-step or three-step polishing processes. It indicates that suitable selection of polishing films and grit sizes enables the achievement of a good sur-face finish for both fibres and ferrules and the reduction of polishing time. The fibre heights/undercuts for the polished fibre connectors measured using the automated non-contact interferometer system are summarized in Fig. 8. In Processes A, B, and C, the fibres are protruded above the ferrules with the fibre heights of smaller than 75 nm. In Processes D and E, the fibres are either recessed below the ferrules or protruded above the ferules within a range of ±25 nm for either case. In terms of the fibre heights or undercuts, it is also comparable for the two-step polishing in Processes C, D and


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E with the three-step polishing in Processes A and B.

Fibe

r Hei

ght/U

nder

cut (

nm)

-100

-50

0

50

100

A B C D EProcess

Fig. 8. Fibre heights/undercuts of the polished con-nectors measured with an automatic non-contact in-terferometer system. The radius of curvature of the polished fibre connectors measured using the automated non-contact interferometer system is shown in Fig. 9. The ranges of the radius of curvature generated in Processes A, B, and C are about 20 ~ 75 mm. In Process D, the radius of curvature is about 20 mm with a very small variation. However, in Process E, the radii of curvature oddly ranged from about 50 ~ 250 mm, which were about 4 times larger than those obtained in the three-step polishing in Processes A and B. These in-dicate that the two-step polishing in Processes C and D, excepting Process E, are comparable in regards to the radii of curvature of the pol-ished connector end faces.

Process

Rad

ius

of C

urva

ture

(mm

)

0

50

100

150

200

250

A B C D E

Fig. 9. Radii of curvature of the polished connectors measured with an automatic non-contact interferome-ter system The linear and angular apex offset values for the polished connectors measured using the auto-mated non-contact interferometer system are plotted in Figs. 10(a) and 10(b), respectively. The linear apex offsets of smaller than 150 µm and the angular apex offsets of smaller 0.2 de-

gree are generated in Processes A, B, and C. In Process D, the linear and angular offsets are smallest, 30 µm and 0.15 degree, respectively. In Process E, however, both the linear and an-gular apex offsets increased significantly to 600 µm and 0.45 degree, respectively. These indi-cate that the different two-step polishing proc-esses could generate either better or worse apex offsets than those obtained in the three-step polishing processes.

Process

Line

ar A

pex

Offs

et ( µµ µµ

m)) ))

0

100

200

300

400

500

600

700

A B C D E

Process

Ang

ular

Ape

x O

ffset

(deg

ree)

0.0

0.1

0.2

0.3

0.4

0.5

A B C D E

(a)

(b)

Fig. 10. (a) Linear and (b) angular apex offsets of the polished connectors assessed with an automatic non-contact interferometer system. 4.3 Optical quality assessment for pol-

ished connectors The corresponding return losses for the polished connectors are given in Fig. 11(a). For the con-nectors polished in Processes A, B, C and D, the return loss values are all smaller than the critical value of –45 dB. In Process E, however, the return loss values are larger than –45 dB and even the two are greater than -20 dB, indi-cating that a high apex offset could cause more return losses for the polished connectors. The corresponding insertion losses for the polished connectors are plotted in Fig. 11(b), suggesting


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that the insertion losses achieved using in all the polishing processes are smaller than the re-quired value of 0.3 dB.

Process

Inse

rtio

n Lo

ss (d

B)

0.0

0.3

0.6

A B C D E

(a)

(b)

Processes

Bac

kref

lect

ion

(dB

)

-60

-45

-30

-15

0

A B C D E

Fig. 11. Optical performance of the polished connec-tors. (a) Return loss and (b) Insertion loss.

5 CONCLUSIONS The following conclusions are drawn from the present investigation on the polishing processes and the assessment of geometrical quality and optical performance of the polished fibre optic connectors. • The efficiency for polishing the physical con-

tact connectors having the return loss of < -45 dB and the insertion loss < 0.3 dB can be raised by at least 30%, involving two-step polishing and consuming 1 minute circle time, by selection of suitable abrasives and grit sizes.

• The high linear and angular apex offsets of

the polished connector end faces could sig-

nificantly deteriorate the optical quality, es-pecially the return loss.

• The relationship between the geometrical

quality and optical quality for the polished connectors has been established. The fibre and ferrule surface roughness Ra values of < 30 nm, the fibre height or undercut of –25 ~ 75 nm, the radius of curvature of 10 – 75 mm, the linear apex offset of < 150 µm and angular apex offset of < 0.2 degree appear to be acceptable for the physical contact fi-bre connectors.

6 INDUSTRIAL SIGNIFICANCE The current research has developed a high effi-ciency polishing processes for single mode physical contact connectors. The polishing pro-tocols can be applied in optic fibre industry for mass production of connectors. REFERENCES [1] D. Derickson, Fibre Optic Test and Meas-

urement, Prentice-Hall, New Jersey, pp. 621-638, (1998).

[2] Fibre Optic Polishing Machine 8671X-6100 Series, Molex, Illinois, (2001).

[3] OFL-12 Series OFL-126001 & PFL0127001 Mass Production Polisher Instruction Man-ual, Seiko Instruments Inc., Chiba, (1995).

[4] S. Jahanmir, H.K. Xu and L.K. Ives, “Mechanisms of materials removal in abra-sive machining of ceramics”, in Machining of Ceramics and Composites, S. Jahanmir, M. Ramulu and P. Koshy (Ed.), Marcel Dekker, New York, pp. 11-84, (1999).

[5] T. Kanda, M. Misuhashi, T. Ueda, A. Toyo-hara and K. Yamamoto, “New micro-finish surface technology for the fabrication of op-tical device endfaces”, in Proceedings of the International Conference on Optical Fabrica-tion and Testing, T. Kasai (Ed.), Vol. 2576, pp. 84-91, (1995).

[6] T. Karaki-Doy, T. Satoh, J. Watanabe and K. Matsunaga, “Development of a new automatic processing machine for optic-fibre connector ends”, Bulletin of Japan Society for Precision Engineering, Vol. 22(3), pp. 216-222, (1988).

[7] TIA Standard, Telecommunications Industry Association, Arlington, VA, USA, (2002).

polishing of fibre optic connectors · 2015-07-29 · str/03/027/mt 1 polishing of fibre optic...

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