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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 17, SEPTEMBER 1, 2011

Core Alignment of Butt Coupling Between
Single-Mode and Multimode Optical Fibers by
Monitoring Brillouin Scattering Signal
Yosuke Mizuno and Kentaro Nakamura, Member, IEEE

Abstract—We propose a new method to optimize the corealignment of the butt coupling between a single-mode optical fiber(SMF) and a graded-index multimode optical fiber (GI-MMF) bymonitoring the spectrum of Brillouin scattered light. This method
is stable, free from costly or elaborate equipments, applicableto the coupling of optical fibers with different core diameters ormaterials, and available even when the power of transmitted lightcannot be measured. We experimentally show the usefulness of
this method in the following three cases: where a silica-based SMF
is butt-coupled to 1) a silica-based GI-MMF with 50- m core diameter; 2) a perfluorinated GI polymer optical fiber (PFGI-POF)
with 62.5- m core diameter; and 3) a PFGI-POF with 120- m
core diameter. Depending on the relative core position, not only
the Stokes power but also the Brillouin frequency shift changed,
which probably originates from the excitation of higher ordermodes.
Index Terms—Brillouin frequency shift (BFS), Brillouin scattering, butt coupling, core alignment, graded-index multimodeoptical fiber (GI-MMF), nonlinear optics, polymer optical fiber.

I. INTRODUCTIONFIBER-OPTIC sensors have a number of advantages
compared to other sensor elements, such as flexibility,
light weight, tolerance of electromagnetic interference, etc.,
and have been widely studied as a promising technology forachieving smart materials and structures [1]–[7]. Recently,
graded-index multimode optical fibers (GI-MMFs) [8]–[10]including graded-index polymer optical fibers (GI-POFs)[11]–[16] have attracted considerable attention as sensing
heads, because their fracture toughness and flexibility canmake the systems much easier to handle in field applications.
It is generally difficult to implement all the optical parts of thesensing systems using GI-MMFs only, because most of the optical devices, such as lasers, isolators, couplers, circulators, andamplifiers, are composed of single-mode optical fibers (SMFs).
Manuscript received April 03, 2011; revised June 08, 2011; accepted July 05,
2011. The work of Y. Mizuno was supported by the Research Fellowships for
Young Scientists from the Japan Society for the Promotion of Science.
The authors are with Precision and Intelligence Laboratory, Tokyo Instituteof Technology, Kanagawa 226-8503, Japan (e-mail: ymizuno@sonic.pi.
titech.ac.jp, knakamur@sonic.pi.titech.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2011.2161754

Therefore, it is extremely important to optically couple an SMF
and a GI-MMF with high efficiency.
If connectors (FC, SC, LC, etc.) are attached to both ends ofthe fibers to be coupled, we can easily align the relative core position by employing a proper adapter. However, when such connectors are not available, we must align the relative core position
to achieve highly efficient butt coupling with one of the fibers
fixed on a multiaxis positioning stage. In general, the alignmentis optimized by monitoring and maximizing the power of transmitted light at the output end of the GI-MMF, but in some cases
it is difficult to measure the power. For example: 1) when theGI-MMF is embedded in structures as a sensing head; 2) when
the output end of the GI-MMF is located quite far away; and3) when the light propagation loss in the GI-MMF is extremely
high. In these cases, the butt-coupling efficiency needs to be optimized only from the incident side. In commercially available
fiber splicers, charge-coupled device cameras are often used forthis purpose, but they are expensive and not easy to handle.
Another approach is to make use of reflected light, withwhich more accurate core alignment is potentially feasible,
because the light beams propagating in the GI-MMF areconcentrated in the core center. However, the light reflectedfrom the GI-MMF, mostly due to Rayleigh scattering, is mixed
with unstable Fresnel-reflected light generated at the couplingboundaries. Therefore, the power of the reflected light becomesso unstable that it is difficult to simply use it for accurate corealignment. One way to separate the Fresnel-reflected light is toutilize the light reflected from the GI-MMF due to nonlinear
scattering with frequency shift, such as Brillouin scatteringand Raman scattering. Brillouin scattering seems to be themore suitable of the two, because it is known to have lower
frequency shift, narrower bandwidth, and lower threshold, i.e.,higher Stokes power than backward Raman scattering [17].
In this paper, we experimentally demonstrate that the buttcoupling efficiency between an SMF and GI-MMFs can be optimized by monitoring and maximizing the power of the Brillouin-scattered light. Three different GI-MMFs are employed:1) a silica-based GI-MMF with 50- m core diameter; 2) a perfluorinated GI-POF (PFGI-POF) with 62.5- m core diameter;and 3) a PFGI-POF with 120- m core diameter, and these experimental results well explain the usefulness of this method.
Besides, we show that not only the Stokes power but also theBrillouin frequency shift (BFS) changes depending on the relative core position. This is probably because of the excitation ofhigher order modes with different BFS.

0733-8724/$26.00 © 2011 IEEEMIZUNO AND NAKAMURA: CORE ALIGNMENT OF BUTT COUPLING

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TABLE IPHYSICAL PROPERTIES OF THE SILICA SMF, SILICA GI-MMF, AND PFGI-POFS.
, CORE DIAMETER;, EFFECTIVE CORE REFRACTIVE INDEX;,NUMERICAL APERTURE; , PROPAGATION LOSS; , FIBER LENGTH

Fig. 2. Experimental setup of the core alignment system. DFB-LD, distributed-feedback laser diode; EDFA, erbium-doped fiber amplifier; ESA,electrical spectrum analyzer; FS, frequency shifter; PC, polarization controller;
PD, photodetector.

Fig. 1. Schematic of core alignment, where an SMF is butt-coupled to aplane is
GI-MMF. The axis is defined as the fiber axis direction, and thedefined as the plane perpendicular to the -axis. Both and are defined to be
zero when the core alignment is optimal.

II. EXPERIMENTAL SETUP
We used a standard silica-based SMF and three kinds of
GI-MMFs: one silica-based GI-MMF and two PFGI-POFs.
Their physical properties are summarized in Table I.
The position of the SMF was fixed, and the GI-MMFs weremounted on a three axis positioning stage. Here, we define eachaxis as shown in Fig. 1. The alignment procedure was as follows.
First, the relative angle of the fibers to be coupled was roughly
adjusted. Next, index-matching oil
was placed on
one of the fiber ends to fill the air gap and suppress the Fresnel
reflection [18]. Then, the axis was adjusted so that the distance
between the two fiber ends became nearly zero. In this step,
angular misalignment was automatically compensated. Finally,- and -axes were adjusted using the Brillouin-based method.
The whole experimental setup for detecting the Brillouin signal is depicted in Fig. 2. The output light of a
distributed-feedback laser diode (DFB-LD) at 1552 nm
was divided into pump light and reference light. The pumplight was amplified with a moderate-power erbium-doped
fiber amplifier (EDFA) and injected into GI-MMFs via butt
coupling. A polarization controller (PC) was used to suppress polarization-dependent fluctuations of the signal. The
Brillouin-scattered Stokes light was heterodyned with the polarization-optimized reference light, and the optical beat signal
was converted to an electrical signal with a photodetector (PD).
The signal was preamplified by 23 dB, and monitored with anelectrical spectrum analyzer (ESA). Here, a 9-GHz frequency
shifter (FS) was used to monitor the Brillouin signal of a silica
GI-MMF (typical BFS 11 GHz), because the bandwidths ofthe preamplifier and the ESA were 0.9–4.5 GHz and 9 kHz–8.1
GHz, respectively. When the Brillouin signal is sufficiently
large, the EDFA, PCs, and preamplifier need not be employed.

Fig. 3. BGSs of the silica GI-MMF when (relative distance from the optimal, andm with fixed at 0 m.
position) wasIII. RESULTS AND DISCUSSIONS
Experimental results and discussions are presented in thissection, which are classified according to the types of the GI
fibers used (See Table I).
A. Silica GI-MMF
Fig. 3 shows the dependence of the Brillouin gain spectrum(BGS) on with fixed at 0 m, i.e., on the relative distancefrom the optimal core position. The optical power at the endof the SMF
was 15 dBm. Two peaks were observed; onearound 10.5 GHz corresponds to the Brillouin Stokes signal ofthe silica GI-MMF, and the other at 10.92 GHz to that of theSMF. As the absolute value of, increased, the Stokes peakpower of the GI-MMF drastically decreased. The peak power ofthe SMF also slightly decreased due to the polarization-dependent fluctuations, which are so unstable that it is difficult to use
this effect to align the core.
Fig. 4 shows the -dependence of the Stokes peak powerof the GI-MMF. The -dependence of the transmission power
measured with an optical power meter is also provided, which isnormalized so that its peak power is 0 dB. The Stokes power wasmaximum whenm, and monotonically decreased asincreased, just in the same way as the transmission power. Outside the core region of the GI-MMF, i.e., whenm,the Brillouin signal was completely buried by the noise floor.
This behavior was well fitted by a Gaussian curve. Thus, it wasshown that efficient core alignment is feasible by monitoring

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Fig. 4. Stokes power and normalized transmission power of the silica GI-MMFas functions of relative distance . The solid line shows a Gaussian fit, and thedotted vertical lines indicate the core boundaries.

Fig. 6. BGS of the PFGI-POF (A) whenFig. 5. BFS of the silica GI-MMF versus relative distance . The solid line
shows a second-order polynomial fit.

and maximizing the Brillouin Stokes power of the GI-MMF.
Even when the BGS of the GI-MMF happens to overlap thatof the SMF, this method is available unless the BGS of theGI-MMF is extremely small.
In Fig. 3, we notice that not only the Stokes power but also thefrequency shift, called BFS, of the GI-MMF is also dependenton . Fig. 5 shows the BFS as a function of , where the BFS
changed in proportion to the square of . This indicates thatthe core alignment can be performed not only by maximizing theStokes power but also by minimizing the BFS. This effect seems
to originate from the excitation of many higher order modes
with different Brillouin frequencies, which is also the reason forthe broadening of the Brillouin bandwidth with the increasing, seen in Fig. 3.
B. PFGI-POF (A)The -dependence of the BGS in the PFGI-POF with 62.5m core diameter is shown in Fig. 6. The optical input power
was fixed at 12 dBm. Since the BFS of PFGI-POFs is approximately four times lower than that of silica-based fibers [19], only
one Brillouin peak of the PFGI-POF was observed in the rangeof 2.5–3.0 GHz. The -dependences of the Stokes power and thenormalized transmission power shown in Fig. 7 were similar to
those of the silica GI-MMF, which confirms that this alignment
method can be applied also to the coupling of the fibers composed of different materials.

was, andm.

Fig. 7. Stokes power and normalized transmission power of the PFGI-POF (A)as functions of . The solid line shows a Gaussian fit, and the dotted verticallines indicate the core boundaries.

Compared to the case of silica GI-MMF, the -dependenceof the BFS was extremely small, and the sign of its coefficient
was opposite, as seen in Fig. 6. In this case, we cannot align thecore accurately by maximizing the BFS.
C. PFGI-POF (B)The -dependences of the BGS in the PFGI-POF with 120are shown in Figs. 8 andm core diameter whenand9, respectively. The input power was fixed at 15 dBm. In Fig. 8,
the peak power and the BFS behaved in the same way as thosein Fig. 6. In contrast, in Fig. 9, the peak power hardly changed
tom. We confirmed
while was in the range ofusing a fiber microscope that this irregular behavior was due tothe relatively rough surface of the PFGI-POF end. In general,
it is difficult to make a highly smooth surface by polishing theend of large-core PFGI-POFs, because they are much softer andmore easily damaged than silica fibers.
The dependence of the Stokes power on is shown in Fig. 10.
As shown in the figure, even when there are some irregularpoints originating from the rough surface, they have little influence on core alignment itself. Instead, we can roughly estimate
the smoothness of the fiber end surface by detecting such irregular points.
IV. CONCLUSION
We demonstrated that highly efficient butt coupling between
an SMF and GI-MMFs is easily achievable by monitoring andMIZUNO AND NAKAMURA: CORE ALIGNMENT OF BUTT COUPLING2619

Prof. T. Ishigure at Keio University for his helpful discussions
and comments. In addition, they would like to thank an anonymous reviewer of an earlier version of this paper for his/her constructive and insightful comments and suggestions.
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Fig. 8. BGS of the PFGI-POF (B) when

Fig. 9. BGS of the PFGI-POF (B) whenm.

was 0, 20, 40, and 60 m.

was, andFig. 10. Stokes power of the PFGI-POF (B) as a function of . The solid line
m, and theshows a Gaussian fit without using the irregular data at
dotted lines indicate the core boundaries.

maximizing the power of the Brillouin-scattered light. The usefulness of this method was confirmed by the experimental results with three different GI-MMFs. Furthermore, we found thatthe BFS also depends on the relative core position due to theexcitation of many higher order modes. We believe this Brillouin-based core alignment method will greatly help GI-MMFs
including GI-POFs to be utilized as sensing heads in fiber-optic
sensing systems with ease of handling, fracture toughness, andhigh flexibility.
ACKNOWLEDGMENTThe authors are grateful to Sekisui Chemical Co., Ltd., forproviding the PFGI-POF samples. They are also thankful to[1] J. Dakin and B. Culshaw, Optical Fiber Sensors: Principles and Components. Norwood, MA: Artech House, 1988.
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Yosuke Mizuno was born in Hyogo, Japan, on October 13, 1982. He received
the B.E., M.E., and Dr. Eng. degrees in electronic engineering from the University of Tokyo, Tokyo, Japan, in 2005, 2007, and 2010, respectively.
In 2004, he was involved in optical information processing at the Universityof Tokyo. From 2005 to 2007, he was involved in spin electronics, and studied
spin hot-carrier transistors for his master degree. From 2007 to 2010, he wasinvolved in Brillouin optical correlation-domain reflectometry for his Dr. Eng.
degree at the University of Tokyo. In 2010, he became a Postdoctoral Research
Fellow of Tokyo Institute of Technology, Kanagawa, Japan. His research interests include fiber-optic sensors, polymer optics, and ultrasonics.
Dr. Mizuno is a member of the Japanese Society of Applied Physics, and theInstitute of Electronics, Information, and Communication Engineers of Japan.

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From 2007 to 2010, he was a Research Fellow, DC1, of the Japan Society for thePromotion of Science (JSPS). He is currently a Research Fellow, PD, of JSPS.

cation of ultrasonics and the measurement of vibration and sound using opticalmethods as well as fiber-optic sensors.
Dr. Nakamura received the Awaya Kiyoshi Award from the Acoustical Society of Japan (ASJ) in 1996. He received the best paper awards from the Institute of Electronics, Information and Communication Engineers in 1998 andfrom Ultrasonic Electronics in 2007, respectively. He received the Japanese
Journal of Applied Physics Editorial Contribution Award from the Japan Society of Applied Physics in 2007. Since 2009, he has been the Vice President ofASJ.

Kentaro Nakamura (M’00) was born in Tokyo, Japan, on July 3, 1963. Hereceived the B.E., M.E., and Dr. Eng. degrees from the Tokyo Institute of Technology, Kanagawa, Japan, in 1987, 1989, and 1992, respectively.
Since 2010, he has been a Professor at the Precision and Intelligence Laboratory, Tokyo Institute of Technology. His research interests include the appli-