Applied Physics Express 4 (2011) 012501
DOI: 10.1143/APEX.4.012501

Fresnel Reflection Spectra at Multimode Optical Fiber Ends with Heterodyne Detection
Yosuke Mizuno, Weiwen Zou1 , and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
1
State Key Laboratory of Advanced Optical Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University,
800 Dongchuan, Minhang, Shanghai 200240, P. R. China
Received October 22, 2010; accepted December 14, 2010; published online January 7, 2011
The Fresnel reflection (FR) spectra at the ends of a step-index (SI-) multimode fiber (MMF) and a graded-index (GI-) MMF were investigated withheterodyne detection, and compared with that of a standard single-mode fiber (SMF). The FR spectra of the MMFs were found to spread broader
than that of the SMF, and the FR spectrum of the SI-MMF was even broader than that of the GI-MMF. It was also demonstrated that theobservation of the Brillouin gain spectrum in a polymer optical fiber (POF) is severely influenced by the FR spectrum. We believe that thisinformation will provide a guideline for the design of robust Brillouin sensing systems using MMFs including POFs.
# 2011 The Japan Society of Applied Physics

ptical ﬁber sensors based on Brillouin scattering
have been widely studied as a promising technology for smart materials and structures, due tothe capability of distributed measurement of strain and/ortemperature along an optical ﬁber. They are classiﬁed into
two types: ‘‘analysis systems’’,1–5) in which two light beams
need to be injected into both ends of the ﬁber under test(FUT), and ‘‘reﬂectometers’’,6–10) in which one light beam
is injected into only one end of the FUT. Several kindsof analysis systems have been proposed so far, such as
Brillouin optical time-domain analysis (BOTDA)1–3) andBrillouin optical correlation-domain analysis (BOCDA).4,5)
Since these methods are based on stimulated Brillouinscattering (SBS), we can obtain relatively large signals.
However, they need two-end access, which is often not
feasible in long-range applications, and cannot work
properly when the FUT has even one breakage point.
Therefore, one-end-access reﬂectometers, such as Brillouin
optical time-domain reﬂectometry (BOTDR)6,7) andBrillouin
opticalcorrelation-domain
reﬂectometry(BOCDR) systems,8–11) are more favorable in this sense,
even though the signals are weaker because they are basedon spontaneous Brillouin scattering.
Recently, in BOTDR employing a single-mode ﬁber(SMF) as the FUT, it has been reported that the measurement
sensitivity is deteriorated by the Fresnel reﬂection (FR)spectra at the end of the FUT.12) Meanwhile, ﬁber-optic
sensors using multimode ﬁbers (MMFs),13–16) including
polymer optical ﬁbers (POFs),17–20) have also been extensively studied, because their fracture toughness andﬂexibility can make the systems much simpler to handle inﬁeld applications. However, there have been no reports on
the observation of the FR spectra at the end of MMFs.
In this work, we investigate the FR spectra at the ends of astep-index (SI-) MMF and a graded-index (GI-) MMF, andcompare them with that of an SMF. It is experimentallyshown that the FR spectra of the MMFs spread broader thanthat of the SMF, and that the FR spectrum of the SI-MMF
spreads broader than that of the GI-MMF. We also show thatthe FR spectrum in a POF has a great inﬂuence on theobservation of the Brillouin signal.
An SI-MMF and a GI-MMF, manufactured by Newport
Corporation, and a standard SMF were used as the FUTs.
Their physical properties are summarized in Table I. The
experimental setup is shown in Fig. 1, which is based on
self-heterodyne detection and has been used to study theOTable I. Physical properties of the SI-MMF, GI-MMF, and SMF. d, corediameter; neff , core eﬀective refractive index; NA, numerical aperture; ,propagation loss; L, ﬁber length.

Fiberd(m)

neff

NA


(dB/km)L(m)SI-MMF50

1:460.2

1:0

100

GI-MMF50

1:460.2

1:0100

SMF

9

1:47

0.13

0:5

1Fig. 1. Experimental setup for characterizing the Fresnel reﬂection
spectra. DAQ, data acquisition; DFB-LD, distributed-feedback laser diode;
EDFA, erbium-doped ﬁber ampliﬁer; ESA, electrical spectrum analyzer;
FUT, ﬁber under test; PC, polarization controller; PD, photodetector.

Brillouin scattering in a POF.21,22) A distributed-feedback
laser diode (DFB-LD) at 1552 nm with a linewidth of about
10 MHz was used as a light source. A 1-km-long delay ﬁber
was inserted in the Brillouin pump path, which ensuresthat this measurement is well beyond the coherence length(several tens to 100 m) regardless of the FUT length. The
power of the incident light at the end of each FUT wasﬁxed at 17 dBm, and the polarization state was adjusted tominimize the FR using a polarization controller (PC).
The spectra of the light beams reﬂected at the ends of theSI-MMF, GI-MMF, and SMF within the range from DC to3 GHz are shown in Figs. 2–4, respectively. The two curvesin each ﬁgure represent the spectra without and with indexmatching oil (n ¼ 1:46) applied to the ﬁber ends, and thespectra were drastically decreased by applying matching oil
for all three ﬁbers. After applying matching oil, the spectrumin Fig. 2 was almost the same as that in Fig. 3. Since thespectra with matching oil applied were in good agreementwith those when a large bending loss was applied near theﬁber ends, it is reasonable to think that the FR can be almost
completely suppressed by applying matching oil and that thereﬂected spectra with matching oil applied correspond to012501-1# 2011 The Japan Society of Applied Physics

Appl. Phys. Express 4 (2011) 012501

Y. Mizuno et al.

Fig. 2.

Fig. 4. Spectra of the light reﬂected from the SMF with and without
matching oil applied.

Fig. 3. Spectra of the light reﬂected from the GI-MMF with and without
matching oil applied.

Fig. 5. Fresnel reﬂection spectra at the ends of the SI-MMF, GI-MMF,and SMF.

Rayleigh scattering and 1=f noise. Therefore, the net FR
spectrum excluding Rayleigh scattering and 1=f noise can be
simply calculated as the diﬀerence between the spectra withand without matching oil applied.
The net FR spectra calculated from Figs. 2–4 are shownin Fig. 5. It is known that the FR spectrum depends on
the phase noise of the light source,12) and that its 3-dB
bandwidth with heterodyne detection is twice as broad as
the linewidth of the light source.12,23) Therefore, the shapesof the FR spectra are the same in principle regardless ofthe ﬁber type. Considering that the noise ﬂoor in Fig. 5 isroughly estimated to be about 90 dBm, the three spectra
have similar shapes but with quite large diﬀerences inmagnitude. The FR spectra of both the MMFs are over 20 dB
higher and spread much broader than that of the SMF. The
physical reason can be understood from the approximate
expression of the FR coeﬃcient FðÞ on the ﬁber-endsurface:24)

GI-MMF is about 15 dB lower than that of the SI-MMF.
This is because the light beams in the GI-MMF are moreconcentrated inside the center of the core and reach the ﬁber
end at deeper incidence angles (i.e., GI-MMF < SI-MMF ).
Finally, we demonstrate that, when a POF with low
Brillouin frequency shift (BFS) is employed as the FUT, theobservation of the Brillouin gain spectrum (BGS) is severely
inﬂuenced by the FR spectrum. Since the coupling loss
between the POF and the SMF devices (such as the opticalisolator) is over 10 dB, and the optical propagation loss ofthe POF is much higher than that of silica-based ﬁbers, it
is diﬃcult to fairly estimate the net FR spectrum of thelight reﬂected at the end of the POF. However, we canexperimentally show the inﬂuence of the FR spectrum, withRayleigh scattering and 1=f noise included, on the BGS
observation in the POF. The POF used in the experiment was
a 5-m perﬂuorinated graded-index (PFGI-) POF with 0.185
in numerical aperture (NA), 120 m in core diameter, 1:35
in core refractive index, and 150 dB/km in propagation
loss at 1.55-m wavelength. The experimental setup was thesame as that depicted in Fig. 1, except that a 23-dB electricalpre-ampliﬁer was inserted before the electrical spectrum
analyzer (ESA) to enhance the weak BGS of the POF. The
spectra of the light beams reﬂected at the end of the PFGIPOF within the range from DC to 3 GHz are shown in Fig. 6.
The incident power was set to 19 dBm, and the measurement

Spectra of the light reﬂected from the SI-MMF with and without
index-matching oil applied to the ﬁber end.

FðÞ ¼ Fð0Þ þ ð1  Fð0ÞÞð1  cos Þ5 ;ð1Þ

where  is the incidence angle on the ﬁber-end surface, andFð0Þ is the FR coeﬃcient when  ¼ 0, i.e., when an SMF isemployed. Since the light beams in the MMFs reach the ﬁberends not only vertically but also at diﬀerent angles due totheir multimode nature, the FR magnitude of MMFs is larger
than that of SMF. On the other hand, the FR spectrum of the012501-2# 2011 The Japan Society of Applied Physics

Appl. Phys. Express 4 (2011) 012501

Y. Mizuno et al.
Acknowledgments The authors wish to thank Professor Kazuo Hotate of
the University of Tokyo, Japan, for his helpful comments on an early draft of thispaper. This work was partially supported by Research Fellowships for Young
Scientists from the Japan Society for the Promotion of Science (JSPS). W. Zou
was in part supported by the National Natural Science Foundation of China(Grant No. 61007052) and the State Key Lab Project of Shanghai Jiao Tong
University (GKZD030021).

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012501-3# 2011 The Japan Society of Applied Physics

3)
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Fig. 6. Spectra of the light reﬂected from the PFGI-POF with twopolarization states.

7)

was performed with two polarization states, where theFR spectral power became minimal and maximal. When theFR power was minimal, the BGS was clearly observed at
2.83 GHz with no overlap of the FR spectrum. In contrast,
when the FR power was maximal, the BGS was completely
overlapped by the FR spectrum and was not observed. Thus,
it was shown that the BGS observation in the PFGI-POF isgreatly inﬂuenced by the FR spectra.
In conclusion, we investigated the net FR spectra at
the ends of an SI-MMF, a GI-MMF, and an SMF withheterodyne detection. The FR spectra of the MMFs spread
broader than that of the SMF, and the FR spectrum of the SIMMF was even broader than that of the GI-MMF. We also
showed an example where the Brillouin measurement basedon MMFs, especially POFs, is more severely inﬂuenced by
the FR spectra than that based on SMFs. One of the methods
to suppress this inﬂuence is to taper the open ends of theMMFs. We believe this paper is of great assistance indeveloping robust one-end-access Brillouin sensing systems
based on MMFs including POFs.

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