Ultrasonic splicing of polymer optical fibres
silicone sleeve
PVDC film

Y. Mizuno, S. Ohara, N. Hayashi and K. Nakamura
POF 2POF 1

A new method of splicing polymer optical ﬁbres by irradiation ofmegahertz ultrasonic waves is demonstrated. The lowest connection
loss of 1.5 dB is obtained at 2 MHz, and a high mechanical strengthof 59 N (close to the strength of the base polymer) is achieved at
1 MHz. It is shown that the loss and strength are in a trade-off relationship, and that the duration of ultrasonic irradiation has a certain optimalvalue of ∼1 s.

degassed water
acrylic container
ultrasonic wave
transducer
stage

Introduction: Polymer optical ﬁbres (POFs) have many advantages
such as extremely high ﬂexibility, low-cost installation and highsafety [1, 2]. Therefore, although their loss is higher than that of standard silica glass ﬁbres, POFs have been used both in medium-range communication applications such as home networks and automobiles [3] andin high-strain monitoring applications including Brillouin-based distributed strain/temperature sensing [4, 5]. One of the most important
requirements for furthering these studies is a POF splicing technique.
POF splicing based on thermal fusion has already been demonstrated[6, 7]. However, this method requires a large apparatus for moving
the heat source away from the POF as well as accurate core alignmentof the POFs. As no other POF splicing techniques (such as an arc-fusion
splicer for glass ﬁbres) have been reported, they need to be connectedusing proper adapters (e.g. SC, LC), which results in inconvenient
on-site handling as well as insufﬁcient mechanical strength for some
applications where a large strain is applied. However, ultrasonicwelding is known as a technique for fusing and connecting polymer
materials [8, 9]. With this technique, rapid well-sealed connections
can be achieved without any consumable tools (including glue andsolvent) because of the high repeated stress of the ultrasonic waves.
Although a POF coupler has been fabricated using ultrasonic irradiation
at low frequency (40 kHz) [10], POF splicing has not been reported;
high-frequency (megahertz-range) ultrasonic waves with a small oscillation amplitude [11, 12] should be used for accurate splicing of POFs
with small core and outer diameters.
In this Letter, we demonstrate for the ﬁrst time the splicing of POFs
using ultrasonic irradiation at several megahertz. Even without polishing
the ﬁbre ends, we successfully spliced POFs with a low connection loss
without using any consumable tools. The lowest connection loss of
1.5 dB is obtained at 2 MHz. A high mechanical strength of 59 N,
which is comparable to the strength of the base polymer material, isalso achieved at 1 MHz. In addition, we show that the connection loss
and mechanical strength are in a trade-off relationship, and that theduration of ultrasonic irradiation should be ∼1 s to minimise theconnection loss.
Experiments: The experimental setup for ultrasonic splicing of POFs isschematically shown in Fig. 1. A concave transducer (C203; Fuji
Ceramics Co.; 30 mm diameter; 60 mm curvature radius) was placedon a stage at the bottom of a cylindrical acrylic container ﬁlled withdegassed water. The top surface of the container was covered with an
8 μm-thick polyvinylidene chloride (PVDC) ﬁlm. The height of thetransducer (∼60 mm) was adjusted so that the ultrasonic waves werefocused immediately above the ﬁlm. Step-index POFs composed ofpoly(methyl methacrylate) (PMMA) (Super Eska; Mitsubishi RayonCo.; 980 μm core diameter; 1000 μm cladding diameter; 190 dB/km
propagation loss at 650 nm; 70°C heatproof temperature) were buttcoupled at the ultrasonic focal point. A silicone rubber sleeve (5 mmheight; 5 mm depth; and 10 mm length) with a hole diameter of500 μm was used to align the POFs, and the compressive preload waskept at ∼0.5 N in this experiment. Note that the acoustic impedancesof water, PVDC, PMMA and silicone rubber are ∼1.5 × 106, 3.2 ×
106, 3.4 × 106 and 1.1 × 106 Ns/m3, respectively [13]; they are so close
to one another (note that the acoustic impedance of air is 4.1 × 102 Ns/m3[13]) that the ultrasonic waves can be propagated through each boundaryof the water/ﬁlm/sleeve/POF structure with low reﬂection. A laser diodewith a 1 mW output power at 641 nm (red) was used to evaluate the connection loss at the spliced part. The exact amount of the loss wasmeasured using an optical powermeter attached to the other end of thesliced POF. To investigate the temporal response of the loss, the powermeter was replaced by a photodiode (200 kHz bandwidth), the electrical
output of which was monitored using an oscilloscope.

Fig. 1 Schematic of experimental setup

Figs. 2a and b show photographs of the butt-coupled part of the POFs
in the sleeve before and after 1 MHz ultrasonic irradiation, respectively.
The ends of the POFs to be connected were left unpolished. Before ultrasonic splicing, scattered red light was clearly observed. In contrast, after
ultrasonic irradiation, the scattered light was no longer observed, whichindicates that ultrasonic splicing can reduce the connection loss. Fig. 2cshows a microscopic image of the spliced POFs.

abc

Fig. 2 Butt-coupled region of POFs
a Photograph before ultrasonic irradiationb Photograph after ultrasonic irradiationc Microscopic image of ultrasonically spliced POFs

Fig. 3a shows the temporal response of the transmitted light power
after 1 MHz ultrasonic irradiation at t = 0. The transmitted power
started to increase dramatically at t = ∼0.8 s and reached its maximum
at t = ∼1.2 s. Then the power decreased irregularly. This result implies
that the duration of ultrasonic irradiation has a certain optimal value,
which in this case is ∼1.2 s, after which it should be stopped. The temporal responses of the transmitted light power for 1, 2 and 5 MHz ultrasonic irradiation are shown in Fig. 3b. The ultrasonic irradiation wasstarted at t = 0 and stopped at the optimal timing (1 MHz, 1.2 s;
2 MHz, 1.4 s; and 5 MHz, 1.6 s). After the irradiation was stopped,
the transmitted power was kept almost constant. The highest transmitted
power was obtained when the ultrasonic frequency was 2 MHz.
The measured connection loss was plotted against the ultrasonic frequency, as shown in Fig. 4a. The averages of 10 values were used, andthe error bars represent the standard deviation. The obtained averaged
losses for 1, 2 and 5 MHz ultrasonic irradiation were comparable tothat of manual coupling using a V-shaped groove (∼2.9 dB). The
lowest loss achieved among all the samples was 1.5 dB at 2 MHz, as
predicted in the temporal response measurement. The mechanicalstrength of the ultrasonically spliced parts was measured using atension tester (AG500N; Shimadzu Co.), as shown in Fig. 4b. The
tension rate was ﬁxed at 100 mm/min. Three typical samples splicedat each frequency were used, but the samples spliced at 5 MHz wereso fragile that their data were not plotted. One sample spliced at
1 MHz had the highest mechanical strength of 59 N, which is extremely
high considering that the intrinsic strength of PMMA is 70 N. The connection loss and mechanical strength seem to be in a trade-off relationship, the details of which need to be further studied.

ELECTRONICS LETTERS 11th September 2014 Vol. 50 No. 19 pp. 1384–1386

set to ∼1 s to minimise the loss. We believe that this technique will
remarkably improve the convenience of POF-to-POF connection indeveloping future POF-based devices and systems, including highly
ﬂexible Brillouin sensors.

normalised power, a.u.

1.0
0.8
0.6

Acknowledgments: The authors are grateful to H. Hosoda of the Tokyo
Institute of Technology, Japan, for lending the tension tester. This work
was partially supported by a Grant-in-Aid for Young Scientists (A) (no.
25709032) from the Japan Society for the Promotion of Science (JSPS),
and by research grants from the General Sekiyu Foundation, the Iwatani
Naoji Foundation and the SCAT Foundation. N. Hayashi acknowledges
a Grant-in-Aid for JSPS Fellows (no. 25007652).

0.4
0.2
0012time, sa34

© The Institution of Engineering and Technology 201424 April 2014
doi: 10.1049/el.2014.1224
One or more of the Figures in this Letter are available in colour online.

normalised power, a.u.

1.4 s
1.1 s

1.0

Y. Mizuno, S. Ohara, N. Hayashi and K. Nakamura (Precision andIntelligence Laboratory, Tokyo Institute of Technology, 4259
Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan)

1.6 s

E-mail: ymizuno@sonic.pi.titech.ac.jp

1 MHz0.55 MHzReferences
2 MHz000.5

1.0time, sb

1.5

2.0Fig. 3 Temporal responses of transmitted light powera Continuous ultrasonic irradiation at 1 MHzb Irradiation at 1, 2 and 5 MHz for 1.2, 1.4 and 1.6 s, respectively

connection loss, dB

connection loss, dB

4432
101 MHz

322 MHz100

24frequency, MHz
a

6020
4060
mechanical strength, NbFig. 4 Measured connection lossa Against ultrasonic frequencyb Against mechanical strength for ultrasonic frequency of 1 and 2 MHzConclusion: The splicing of POFs using megahertz ultrasonicirradiation has been demonstrated for the ﬁrst time. Even though thePOF ends were unpolished, the POFs were successfully spliced with alow connection loss with no use of consumable tools. The averaged
losses obtained after 1, 2 and 5 MHz ultrasonic irradiation were comparable to that of manual coupling aligned with a V-shaped groove. The
lowest connection loss of 1.5 dB was obtained at 2 MHz, whereas amechanical strength as high as 59 N was achieved at 1 MHz. We also
showed that the connection loss and mechanical strength are in a tradeoff relationship, and that the duration of ultrasonic irradiation should be

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