Applied Physics Express 6 (2013) 076601
http://dx.doi.org/10.7567/APEX.6.076601

Fast Flaw Detection in Polymer Optical Fibers with Infrared Thermometer
Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received May 17, 2013; accepted June 11, 2013; published online June 27, 2013
We demonstrate a fast and cost-effective method of detecting flaws in polymer optical fibers (POFs) using an infrared thermometer. The opticalloss dependence of the measured temperature at the flaw is found to be linear with a proportionality constant of approximately 0.74  C/dB when
the propagating light is 24.5 dBm (282 mW) at 1.55 m. The propagating optical power dependence of the measured temperature at the flaw with afixed loss also shows a linear behavior, which predicts that a high optical input power is preferable to precise estimation of the loss.
# 2013 The Japan Society of Applied Physics

he fabrication technology of widely used silica
single-mode optical ﬁbers (SMFs) has matured
suﬃciently to keep their quality extremely high andstable; and as a result, km-order-long silica SMFs with apropagation loss of as low as 0.2 dB/km at 1.55 m arecommon.1–4) In contrast, the fabrication technology ofpolymer optical ﬁbers (POFs) is relatively new. Currently,
poly(methyl methacrylate) (PMMA)-based POFs can befabricated with a stable propagation loss owing to their large
core diameters of up to 980 m.5–7) However, the propagation loss of perﬂuorinated graded-index (PFGI-) POFs withcore diameters of as small as 50 m, which have becomecommercially available only recently, is largely inﬂuenced
by the ﬂaw in the core induced during the fabrication process, resulting in the diﬃculty in producing long PFGI-POFs
with stable quality.8–10) Such PFGI-POFs with a length of
one hundred meters or longer are now highly demanded
not only in POF-based high-capacity transmission systems11)
but also in next-generation distributed sensors exploiting
nonlinear eﬀects in POFs.12–14) Thus, to develop an eﬀective
method of detecting ﬂaws in PFGI-POFs is of substantial
signiﬁcance.
Conventional methods for detecting ﬂaws in optical ﬁbers
include optical time-domain reﬂectometry (OTDR),15,16)
optical low-coherence reﬂectometry (OLCR),17,18) and optical frequency-domain reﬂectometry (OFDR).19,20) They canmeasure the accurate loss distribution along the POFs with ahigh spatial resolution, but suﬀer from some drawbacks, suchas high installation cost required for the devices (photodetectors, specially conﬁgured lasers, etc.), long measurement time for signal processing, and the inability to resolve
the ﬂaw location at a glance in long POFs.
In this paper, a fast and cost-eﬀective method of detecting
ﬂaws in POFs using an infrared thermometer (IRT) isdemonstrated, which can overcome the disadvantages of theconventional techniques. Since an optical loss at a ﬂaw in a
POF induces the rise in temperature, the loss distribution
along a POF can be detected with the IRT as a temperature
distribution. First, we show that the locations of the ﬂaws
in POFs can be detected, whether the POFs are jacketedor bare. Then, we ﬁnd the optical loss dependence ofthe measured temperature at the ﬂaw to be linear with aproportionality constant of 0:74  C/dB with the propagating light of 24.5 dBm (282 mW) at 1.55 m. We also show
that, when the loss is ﬁxed, the propagating optical power
dependence of the measured temperature at the ﬂaw islinear, predicting that high-power optical power is desirable
to estimate the loss precisely.

TA blackbody, i.e., a perfect absorber of any radiation
incident on it, is capable of emitting radiation.21) Whenheated to a uniform temperature, it generates blackbody
radiation, the characteristics of which are determined solely
by the temperature. The total radiant emittance W of anobject (both blackbody and non-blackbody) is known to begiven by Stefan-Boltzmann’s law asW ¼ "T 4 ;ð1Þ

where " is the emissivity of the object (1 for a blackbody), is the proportionality constant, and T is the surface
temperature of the object.22) The surface temperature ofan object including a POF can thus be measured with anIRT. The IRT-based method for detecting a ﬂaw, which
exploits the rise in temperature induced by an optical lossat the ﬂaw, has such advantages as (1) cost eﬃciency, (2)real-time measurement, and (3) visual display of the ﬂaw
location.
We employed two PFGI-POFs as ﬁbers under test (FUTs),
which had a numerical aperture of 0.185, a core diameter of
50 m, a cladding diameter of 100 m, a core refractive
index of 1:35, and a propagation loss of 250 dB/km at
1.55 m. One FUT, denoted by FUT 1, was a long jacketed
PFGI-POF with an inner jacket diameter of 750 m andan outer jacket diameter of 2.8 mm, in which a ﬂaw had
been caused probably during the fabrication process. Both ofthe jackets were composed of poly(vinyl chloride) (PVC).
The other FUT, denoted by FUT 2, was a 60-cm-longbare PFGI-POF, at the middle point of which a ﬂaw wasartiﬁcially induced by repeating bending and releasing. The
loss at the ﬂaw was moderately variable from 2.5 to 20 dB.
An IRT (FLIR System; i7) was used to detect the ﬂaws,
which had a spectral range from 7.5 to 13 m, and an image
frequency of 9 Hz. The emissivity " was set as follows:
for the jacketed POF, " ¼ 0:98 (value for PVC23)); for thebare POF, " ¼ 0:95 (value for PMMA24,25)). The roomtemperature was 24  C. Note that the optical power and/or
the loss at the ﬂaw were estimated considering the opticalloss caused during the light propagation from one end of thePOF to the ﬂaw.
First, we detected the ﬂaw in the FUT 1 wound on a reelof 20 cm diameter. The incident optical power was 24.5 dBm(282 mW). Figure 1(a) shows its photograph, from which noinformation on the ﬂaw was derived. In its IR image shownin Fig. 1(b), however, the location of the ﬂaw was roughly
but clearly detected despite the thick jackets, which is not
feasible only by detecting the scattered visible light with ournaked eyes. The maximum temperature was 24.4  C, and the076601-1# 2013 The Japan Society of Applied Physics

Appl. Phys. Express 6 (2013) 076601

N. Hayashi et al.

Fig. 1. (a) Photograph and (b) IR image of the jacketed POF with a ﬂaw
wound on a reel.

Fig. 3.

Measured temperature dependence on optical loss at the ﬂaw.

Fig. 2. (a) Photograph of the bare POF with a ﬂaw, and its IR images withlight propagating from (b) right to left, and (c) left to right.

high-temperature region had a tail extending toward theleft. To clarify this tailing eﬀect, we injected 21.3 dBm(135 mW) light into both ends of the FUT 2, one by one, as
shown in Figs. 2(a)–2(c). The optical loss of the ﬂaw at
the midpoint of the FUT 2 was ﬁxed at 4.9 dB in thismeasurement. When the light was injected from the righthand side, the high-temperature region had a tail extending
for approximately 5 cm toward the left, as shown in
Fig. 2(b), and vice versa, as shown in Fig. 2(c). This phenomenon can be exploited to identify the unknown directionof the propagating light through the POF. Since the measured maximum temperature of 28  C was almost the same
in both conﬁgurations, the light injection only from one endof the POF appears to be suﬃcient for rough estimation of
the optical loss at the ﬂaw.
Next, we measured the dependence of temperature on
the optical loss at the ﬂaw when the incident optical power
was ﬁxed at 24.5 dBm (282 mW), as shown in Fig. 3. The
error bars calculated from the signal ﬂuctuations for 5 min
were 0:8  C. As the loss was raised, the temperature wasincreased linearly with a slope of 0:74  C/dB, which isof practical use for the rough estimation of the loss using
the measured temperature. We also measured the dependence of temperature on the optical power of the propagating
light when the optical loss at the ﬂaw was ﬁxed at 20 dB,
as shown in Fig. 4. As the incident power was raised, thetemperature was increased almost linearly, which indicates
that, by employing high-power light, the measured temperature for a certain loss can be increased. This leads to theenhancement of the proportionality constant between themeasured temperature and the loss (see Fig. 3), resulting
in a more precise estimation of the loss. The slight decreasein the slope observed in the range of the optical power

Fig. 4. Measured temperature dependence on optical power of thepropagating light at the ﬂaw.

of >200 mW seems to be caused by the heat dissipationeﬀect.
In conclusion, we demonstrated a fast and low-costdetection of the ﬂaws in POFs using an IRT, which canovercome the drawbacks of the conventional techniques.
The locations of the ﬂaws were successfully detected,
even when the POF was protected by thick jackets. The
optical loss dependence of the temperature at the ﬂaw was
linear with a coeﬃcient of 0:74  C/dB when the opticalpower was 24.5 dBm (282 mW) at 1.55 m. The opticalpower dependence of the measured temperature at the ﬂaw
with a ﬁxed loss was also linear, which predicts that a higher
optical power is desirable for the loss estimation with moreprecision. We hope that this method will be of common
use, with its fast measurement capability, ease of handling,
and cost eﬃciency, in examining the quality of POFs at theﬁnal stage of their fabrication process in the near future.
Acknowledgments This work was partially supported by a Grant-in-Aidfor Young Scientists (A) (No. 25709032) from the Japan Society for thePromotion of Science (JSPS), and by research grants from the General Sekiyu
Foundation, the Iwatani Naoji Foundation, and the SCAT Foundation. N.H.
acknowledges a Grant-in-Aid for JSPS Fellows (No. 25007652).

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