Development of a Novel Vibration Polishing Method
for Fiber Protrusion in Multi-fiber Connectors †by Jie Xu*, Akihiro Murakami * and Takashi Shigematsu*

A novel ultrasonic vibration polishing method for multi-fiber connector with highprecision has been developed by introducing a supplementary polishing ferrule
between the fiber-ends and the ultrasonic vibrator. This method enables us to obtain effectively
the desired fiber protrusion regardless of the materials used for the connector. Good and stablephysical contact (PC) connection was achieved even in single-mode 12-fiber MPO connectors,and return loss and insertion loss remained stable during 500 cycles of reconnection. Theseresults demonstrate that this polishing method is effective in achieving both sufficient fiber
protrusion and good PC in multi-fiber connectors. The method can be applied in high-precision
processing.

ABSTRACT1.

INTRODUCTION

Mechanically transferable (MT) and multi-fiber pushon (MPO) connectors were developed to meet the needfor rapid transmission of large volumes of data 1), 2), andboth types are widely applied due to their excellent
characteristics. In these connectors, index-matchingmaterial is usually employed to reduce Fresnel reflection
caused by the presence of air between the fiber endfaces.
However a physical contact (PC) connection without
index-matching material is preferable in terms of long-term
stability and good optical characteristics.
PC connection requires that all the fiber endfaces areeven and protrude beyond the ferrule end, preferably,
3 μm or more. Thus, it is difficult to achieve good PC
connection by the conventional polishing method, whichtakes advantage of the difference in hardness between theplastic ferrule and the fiber (silica) to obtain fiber protrusion.
It generally takes about 5 minutes to produce 1 μm of fiber
protrusion.
Unfortunately, owing to the limitation of the differencein hardness between the two materials, it is almost
impossible to obtain a fiber protrusion of above 3 μm
even when polishing is continued for a long time. It is alsodifficult to achieve both even endfaces and sufficient fiber
protrusion for all the fibers, especially in connectors withfiber counts of 12 or more.

†*

Compiled from Trans. JSME, Category-C, Vol.67, No.662,
pp3317~3322 (2001)
Production Engineering Development Center, Plant &Facilities Div.

Here we describe a novel vibration polishing method for
multi-fiber connectors by which, sufficient fiber protrusion
is easily obtained in a short time regardless of the material
used for ferrule and fiber. Good PC connection was alsoachieved. This method can be applied to angle-polished12-fiber MPO connector and good optical characteristics
are obtained.

2.

NOVEL POLISHING METHOD

2.1

Fiber Endface Conditions Required for PCConnection
Figure 1 shows the structure of an 8-fiber MPO connector.
To decrease reflection, fiber endfaces in this kind ofconnector are polished at a gradient of 8° to the fibercross section. The connector endfaces are held together
by the engagement of the guide pins in guide-pin holes.
To achieve PC connection in such a connector, it isnecessary that the process of polishing achieve thefollowing three main elements:
1. sufficient fiber protrusion,
2. flat endfaces, and3. angular precision between the fiber endface andthe connection endface.
The novel polishing method enables us to achieve
PC connection since all three of these conditions
are achieved by polishing connector and fiber ends
separately, guaranteeing the flatness of fiber endfaces.
2.2 Principle of the Novel Polishing Method
Figure 2 illustrates the novel method of fiber protrusion
polishing by means of an ultrasonic vibration polishingmachine. First, the MT type ferrule is lapped by a special

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Development of a Novel Vibration Polishing Method for Fiber Protrusion in Multi-fiber Connectors

3.

MPO connector

8°HoleFiber endFigure 1Ferrule end

8°MPO optical fiber connector.

lapping wheel to align the fiber ends with a high degreeof flatness. The MT ferrule with highly flat endfaces isthen aligned by the guide pin with the assisting ferrule,in which there are empty fiber holes, and an abrasive isadded between the ferrule ends. The assisting ferruleis then vibrated by the vibrator attached to it, and only
the MT ferrule endface is polished leaving the fiberends untouched because they are free from contact withthe assisting ferrule. This polishing method leaves thefiber endfaces unpolished without failure. The desired
fiber protrusion can be obtained easily in a short time bysuitably selecting the vibration frequency and the abrasive
particle size.
In this method, fiber protrusion and fiber-end flatness
are achieved by completely separate polishing processes.
Sufficient fiber protrusion can thus be obtained while
keeping the fiber endfaces precisely even, thereby
realizing good PC contact.

EXPERIMENTAL FIBER PROTRUSIONPOLISHING

3.1 Apparatus and Method
Figure 3 is a photograph showing an external view of theexperiment apparatus. A small commercially available
ultrasonic polishing machine was used as the vibrator.
The position of the connector and the assisting ferruleare settled by two guide pins set horizontally, and the two
endfaces are pressed together by a compressed springwith abrasive between them.
The polishing load can be established quantitatively
using a micrometer and load cell. During the polishingprocess, however the load is kept virtually constant, since
the amplitude of vibration and real protrusion are so smallthat the displacement caused by the compressor spring ispractically negligible.
In this experiment, a 12-fiber MPO connector with thepre-polished endfaces was used, and the effect of loadon the protrusion and eccentricity of the fiber protrusion
center was investigated. The load was set at 2 N, 3 N,
and 4 N, and fiber protrusion was measured every 10
seconds using a two-dimensional roughness detector. The
frequency of the vibrator was set at 20 kHz. To improve

Auxiliary ferrule
MT ferrule
Hone

Spring

100 mmFigure 3Spring

MTferruleAbrasiveHoleMicrometer

Load cell

Guide pinFiber end

External view of experimental equipment.

Auxiliary ferrule

Ultrasonic honeVibration direction

Ultrasonictransducer

Linear slide
HolderFigure 2

Principle of the novel vibration polishing method.

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200335Development of a Novel Vibration Polishing Method for Fiber Protrusion in Multi-fiber Connectors

3.2 Experimental Results and Discussion
By means of this novel polishing, good fiber protrusionand endfaces were obtained. The characteristics ofprocessed fiber endfaces were investigated.

1412

Eccentricity ( µ m)

polishing efficiency and to decrease peripheral damage,
10 wt% diamond abrasive with a particle diameter of 3 μmwas used.

10864203.2.1 Relationship between Fiber Protrusion and Load
As Figure 4 shows, fiber protrusion is proportional to thepolishing time at any given load, and the slope of thecurve increases as load is increased. From Figure 4 it canalso be demonstrated that polishing efficiency is muchgreater with this novel method than with the conventional
method, in which as much as 5 minutes was required toproduce a fiber protrusion of no more than 1 μm.
During the initial 20 seconds of polishing, the increasein protrusion was a little slow. We attribute this to thedifference in the original flatness between the connector
and ferrule endfaces, as well as the property of theabrasive which make it is easily ejected from the gap.
It is known that the type and grade of abrasive have aneffect on the efficiency of polishing. In our experiment,
polishing efficiency was found to decrease about 10
percent when alumina abrasive was used instead of
diamond abrasive of the same grade.
3.2.2 Eccentricity of Fiber Protrusion Center
As shown in Figure 2, in this novel method, it is essential
to accurately insert the fibers, with a diameter of 125 μm,into the corresponding holes with a diameter of 126 μm, inthe assisting ferrule, because this positioning determines
whether the protruded fiber end is in the center of the core(diameter 10~62.5 μm) which forms the optical pathway.
Especially in the case of fibers with a diameter of 62.5 μm,
if the eccentricity exceeds 20 μm, it is possible that somepart of the core end will be removed by vibration polishing.
In practice it is desirable that the variation in eccentricity
be kept below 10 μm.
We next investigated the relationship between variationin fiber protrusion center and load. In three independentexperiments, eccentricity in the fiber protrusion center,using our novel polishing method, were both below 5 μm

8Fiber protrusion ( µ m)

72N

63N

54N

43
2
100

102030

40

50

6070

Time (sec)Figure 4012

3

5

4Load (N)Figure 5

Relationship between fiber protrusion eccentricity
and load.

( µ m)

6.844
5.000
3.000
1.000
-1.000-3.000-4.973

Figure 6

Fiber protrusion in 12-fiber MPO connector.

under the loads of 2 N and 3 N (Figure 5). These results
indicate that the fiber protrusion center was accurately
kept unchanged by this novel method. Our experiments
also showed a tendency for the eccentricity in the fiber
protrusion center to increase with increasing load.
The reason, we think, is that the horizontal component
produced by the load exerted at the angle of inclination of8°, caused the vibration in the horizontal plane.
3.2.3 Status of Protruding Endface of Processed Fiber
Figure 6 is an example of the result of measuring
processed fiber protrusion endfaces in a 12-fiber MPO
connector examined by a surveying instrument from WKYO
Co., Ltd. All the fiber endfaces are smooth with 5 μm ofprotrusion from the MT ferrule.
Figure 7 shows photographs of a fiber endface before
and after vibration polishing. It is clear that the endface offiber was kept untouched without damage after polishing.
In addition, the breach damage on the periphery of thefiber endface was very slight so that roundness was wellcontrolled within a few micrometers.
Also, the effect of diamond particle size on the status of
the fiber endface was investigated. Although the efficiencyof polishing was proportional to the size of diamondparticle abrasive for 3 and 6 μm diameters, breachdamage exceeded 10 μm in the case of 6 μm diameter.
Thus a diamond particle diameter of about 3 μm is mostsuitable.

Relationship between fiber protrusion and load.

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Development of a Novel Vibration Polishing Method for Fiber Protrusion in Multi-fiber Connectors

0.7
N = 48 × 2

Li = Ln

Li (dB) with matching material

0.6(a) Before vibration polishing

Novel method

0.5

Conventional method
0.4

0.3

0.2

0.1

30 µ m

000.1

0.2

0.3

0.4

0.5

0.6

0.7

Ln (dB) without matching material

Figure 8(b) After vibration polishing

Figure 74.

30 µ mFiber and ferrule surfaces before andafter vibration polishing.

EVALUATION OF OPTICAL
CHARACTERISTICS

PC connection was evaluated in single-mode 12-fiber
MPO connectors with 5 μm of fiber protrusion processedby the novel vibration polishing method. Stability test forconnection-reconnection 500 cycles was also conducted
to determine insertion loss and return loss in thisconnector.
4.1 PC Connection Test
In order to evaluate whether good PC connection isachieved, we measured insertion loss with and without
index-matching material in single-mode 12-fiber MPO
connectors processed by the novel vibration polishingmethod.
Without index-matching material, if the fiber endfaces
do not achieve PC connection, a layer of air will appear
between endfaces, causing a large insertion loss. It is
known that with single-mode fibers, a layer of air 0.3 μm
thick can cause insertion losses of above 0.6 dB. In general,
if the difference in the insertion loss with and without
index-matching material is above 0.05 dB, good PC
connection cannot be said to be achieved.
Figure 8 shows the insertion loss with and without
index-matching material detected in single-mode 12-fiber
MPO connectors processed by the novel vibration
polishing method. If there is no difference between

Relationship between insertion losses with andwithout index-matching material in single-mode
12-fiber MPO connectors polished by the novel
vibration and conventional polishing methods.

insertion loss with index-matching material (Li) andwithout (Ln) the line will be straight, representing an ideal
PC connection. In the sample polishing by the novel
method, virtually no difference was found between Li andLn. The maximum difference in Li and Ln, insertion loss
was less than 0.04 dB (n=48). These results indicatethat excellent PC connection was achieved by the novel
polishing method.
4.2 Stability Test
The novel method presented here enables us to obtainsufficient protrusion regardless of the materials usedin the connector, but doubts persist that such a longprotrusion might increase the risk of fiber end damage.
To evaluate the stability of fiber protrusion in practical
performance, the changes in insertion loss and return loss
were evaluated in a 500 cycles connection-reconnection
test of the single-mode 12-fiber MPO connectors,
conducted without index-matching material. Fiber
protrusion was 5 μm, and the ferrules were connectedunder compression force of 9.8 N. The insertion loss andreturn loss at 1310 nm wavelength were measured for
all the fibers every 25 cycles. For the sake of simplicity,
Figure 9 and Figure 10 show only the results of the first,sixth, seventh and twelfth fibers measured every 100
cycles. The results demonstrate that there is virtually nochange in optical characteristics in these reconnection
tests, indicating that the performance of connectors, evenwith a fiber protrusion of 5 μm, is very stable.

5.

CONCLUSION

A novel vibration polishing method for multi-fiberconnectors has been developed to achieve physicalcontact (PC). By polishing 12-MPO connectors by meansof this method, we drew the following conclusions:

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Development of a Novel Vibration Polishing Method for Fiber Protrusion in Multi-fiber Connectors

1.00
Fiber number

Insertion loss (dB)

0.80

0.60

167

12

0.40

0.20

0.00

0100

200

300

400

500

Reconnection cycles

Figure 9

Change in insertion loss during 500
reconnection cycles.

701) Sufficient fiber protrusion with flat endface can beobtained easily without restrictions on the ferrulematerial as a result of using a supplementary ferrule.
2) Fiber protrusion is proportional to the load employedin polishing.
3) Eccentricity of the fiber protrusion center can be kept
to less than 10 μm under the proper load.
4) Excellent PC is achieved.
5) Insertion loss and return loss remain stable during
the 500 cycles of reconnection when fiber protrusion
is 5 μm.
6) Process efficiency is 10 fold higher than with theconventional method.
These results demonstrate that this novel polishingmethod can be employed effectively even in highprecision processing.

Return loss (dB)Fiber number6516712REFERENCES605501002003004005001) T. Satake, et al. J. Light. Tech., 4 (No.8), 1232 (1986)
2) S. Nagasawa, et al. Tech. Dig. IOOC’89, 21C2-1, 48 (1989)
3) S. Nagasawa, et al. Photon Tech. Lett., 3 (No.10), 937 (1991)
4) W.C. Young, et al. Photon Tech. Lett., 1 (No.12), 461 (1989)
5) K. Okude, Ultrasonic Physics, Corona Publisher (1995) (inJapanese)

Reconnection cycles

Figure 10 Change in return loss during 500
reconnection cycles.

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