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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 20, OCTOBER 15, 2011

Refractive Index Sensor for Liquids and Solids
Using Dielectric Multilayer Films Deposited
on Optical Fiber End Surface
Kyung-Su Kim, Yosuke Mizuno, Masayuki Nakano, Seiichi Onoda, and Kentaro Nakamura, Member, IEEE

Abstract—We propose and demonstrate a novel fiber-optic
refractive index (RI) sensor using dielectric multilayer thin films(DMFs) deposited on an optical fiber end surface, which rejects
the incident light with particular wavelength. We experimentallyshow that the center frequency of the reflected light spectrum
changes in proportion to the RI of six kinds of liquids attachedto the fiber end. Its coefficient of 0.61 nm/RIU (refractive indexunit) agrees well with our simulation results based on characteristic-matrix technique. We also show that this sensor is applicableto the RI measurement of solids.
Index Terms—Bandpass filter, dielectric multilayer film, opticalfiber sensor, refractive index sensor.

I. INTRODUCTIONR

EFRACTIVE-INDEX (RI) sensors are of great significance for wide applications in biochemistry, chemicaland environmental analysis, etc [1]–[4]. Many kinds of RI sensors using optical fibers have been developed so far due to theiradvantages such as compact size, cost efficiency, flexibility indesign, remote-sensing capability, multiplexibility, and immunity to electromagnetic interference. Since the simplest fiberoptic RI sensors exploit the power of Fresnel reflection, it isdifficult to correctly perform the RI measurement when considerable loss is induced in the fiber. Therefore, various kinds offrequency-domain techniques have been proposed so far. Fourof their typical examples are: RI sensors based on (1) fiber Bragg
gratings (FBGs), (2) long-period gratings (LPGs), (3) interferometers, and (4) tapered fibers. RI sensors using FBGs [5]–[7]including tilted FBGs (TFBGs) [8]–[10] have high sensitivity,
but suffer from low spatial resolution due to their relatively large

sensing areas. Besides, we need to etch the cladding to gain
access to the evanescent field of the guided mode. LPG-based
RI sensors [11]–[13] with high sensitivity have also been developed. Although we need not etch the cladding, they havethe same drawbacks as FBG-based RI sensors. On the other
hand, many types of RI sensors based on interferometers, including Mach–Zehnder type [14], [15], Michelson type [16],Fabry–Perot type [17], [18], etc. have been implemented, someof which have such advantages as high sensitivity, high spatial
resolution, cost efficiency, and simple setup. Furthermore, using
tapered fiber ends, Y.-H. Tai et al. [19] have recently developed
RI sensors with high sensitivity and high spatial resolution. All
the fiber-optic RI sensors previously developed, however, havebeen applied only to the RI measurement of liquids and gases,
partly because of their fragility of the sensor heads.
In the meantime, we have recently developed a method to deposit dielectric multilayer thin films (DMFs) on an optical fiber
end as an optical bandpass filter (BPF). This structure, calleda BPF on a fiber end (BOF) [20], eliminates the incident lightwith particular wavelength. Since this wavelength depends on
temperature and pressure, BOFs have been utilized to develop
temperature/pressure sensors with robustness and high spatial
resolution [20], [21].
In this letter, we demonstrate that a BOF can be applied toan RI sensor with applicability not only to gases or liquids but
also to solids. The center frequency of the reflected light spectrum was found to shift in proportion to the RI of the materials
attached to the BOF. Its coefficient was 0.61 nm/RIU, whichis in good agreement with our simulation results using characteristic-matrix technique.
II. PRINCIPLE

Manuscript received May 23, 2011; revised June 30, 2011; accepted July 16,
2011. Date of publication August 15, 2011; date of current version September
23, 2011. This work was supported in part by the “Global Center of Excellence(G-COE) Program” from the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan. The work of Y. Mizuno was supported by
the Research Fellowships for Young Scientists from the Japan Society for the
Promotion of Science (JSPS).
K.-S. Kim, Y. Mizuno, and K. Nakamura are with Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503,
Japan (e-mail: kskim@sonic.pi.titech.ac.jp; ymizuno@sonic.pi.titech.ac.jp;
knakamur@sonic.pi.titech.ac.jp).
M. Nakano and S. Onoda are with the R&D Division, Watanabe,Inc., Saitama 338-0835, Japan (e-mail: nakano@watanabe-mj.co.jp;
onoda@watanabe-mj.co.jp).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2011.2163063

A BOF is composed of DMFs deposited on the end faceof an optical fiber [20]. The DMFs, typically comprising SiO
and TiO layers accumulated by ion-assisted evaporation, form
an optical cavity, which serves as a BPF. Thus, the incident
light with particular wavelength is selectively eliminated. Fig. 1
shows a scanning electron microscope (SEM) image of the BOF
layer structure used in the experiment. It has a Fabry-Perot-type
cavity composed of a 925-nm-thick SiO layer. The reflective
layers surrounding the cavity, which serve as distributed Braggreflectors (DBRs), comprise several pairs of SiO
layers with the same physical thicknesses
and TiOof 215 nm (Note that, in general, not physical but optical thickness is designed to be the same). The TiO top layer protecting

1041-1135/$26.00 © 2011 IEEE

KIM et al.: RI SENSOR FOR LIQUIDS AND SOLIDS USING DIELECTRIC MULTILAYER FILMS

Fig. 1. Layer structure of BOF (SEM image).

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Fig. 3. (a) Simulated RI dependence of the reflected spectrum. The verticalaxis is in arbitrary units (b) Center wavelength versus RI.

Fig. 4. Experimental setup for the RI measurement using a BOF. ASE” amplified spontaneous emission; DAQ: data acquisition; OSA: optical spectrum
analyzer; PC: personal computer.

Fig. 2. Simulation model of the BOF structure. The inset table shows the thicknesses of each layer.

the whole structure is 450-nm-thick, which slightly broadens the
linewidth of the reflection spectra.
BOFs have been exploited as key components of optical fiber
sensors, since its reflection wavelength almost linearly depends
on temperature [20] and pressure [21]. This is because the thickness of each layer changes according to temperature and pressure. In this letter, we show that a BOF can be applied also to
RI sensing. When materials with different RIs are attached to
the BOF, the center wavelength
shifts depending on the RI.
Since
dependence on RI is almost linear, we can easily derive RI by measuring . This dependence appears to be causedby the change of Fresnel reflectivity at the interface between the
top layer and the materials attached, this behavior should be further clarified in the simulation.
III. SIMULATION
In order to clarify the RI dependence of the center wavelength, we performed a numerical simulation based on so-called
characteristic-matrix technique [22], [23]. This technique isoften used to characterize the optical properties of DMFs,
because we need not directly consider the effects of standing
waves or integrate multiple reflected signals.
Fig. 2 depicts the BOF model used in the simulation, which
was produced based on the SEM image shown in Fig. 1. It includes several pairs of SiO TiO layers and one cavity composed of SiO . The materials to be measured are located on the
top layer. The thicknesses of each layer are summarized in theinset table in Fig. 2.
Fig. 3(a) represents the simulated RI dependence of the reflection spectra around the center wavelength . The valuesof RI for the materials were set to 1.0, 1.3, 1.6, and 1.9. From
this figure, we can derive the RI dependence of , as shownin Fig. 3(b). Its slope was almost linear with a coefficient of
0.6893 nm/RIU. Thus, we confirmed in the simulation that a
BOF can be applied to RI sensing.
IV. EXPERIMENTS
The experimental setup for measuring RI using a BOF isschematically shown in Fig. 4. Amplified spontaneous emission (ASE) of an erbium-doped fiber (EDF) was used as a lightsource, and its output was guided to the BOF through an opticalcirculator. The reflected light from the BOF (reflectivity 8 dB)
was observed with an optical spectrum analyzer (OSA), and thespectral data was transferred to a personal computer (PC) for
further processing. We employed air, six kinds of liquids, andtwo kinds of solids as materials to be measured. The liquids
were water, ethanol, salt water (30%), index-matching oil (I), matching oil (II)and matching oil (III); and the solids
were rubber
and plastic.
Fig. 5(a) shows the measured spectra when air and six kindsof liquids were attached to the BOF. It is clear that the spectrachange according to the materials with different RI. From
Fig. 5(a), the RI dependence of the center wavelength
canbe derived as shown in Fig. 5(b). The measured coefficient of
0.6115 nm/RIU was in good agreement with the simulated
value of 0.6893 nm/RIU. The difference in power between
Figs. 3(a) and 5(a) and the discrepancy of the coefficientsprobably originate from the simulation method where lightattenuation in each layer is not taken into consideration.
Then, the experiments were performed in the same manner,when the materials to be measured were solids. Fig. 6 shows themeasured spectra when air, rubber, and plastic were attached to
the BOF end by hand. The center wavelength
in the case

1474IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 20, OCTOBER 15, 2011

Fig. 5. (a) Reflection spectra with air and six kinds of liquids employed asmaterials to be measured. (b) Dependence of the center wavelength on RI.

Fig. 6. Reflection spectra with air and two kinds of solids employed as materials to be measured.

of air was longer than that in Fig. 5(a), because the room temperature was slightly higher in Fig. 6 than in Fig. 5(a). From
the figure, the RIs of the rubber and the plastic were calculatedback to be 1.49 and 1.65, respectively, which agree well with
the RIs described in their specification sheets. Thus, we clarified that BOF-based RI sensors can be applied not only to gases
or liquids but also to solids. Here, we must bear in mind that
it is difficult to apply the BOF sensors to solids with extremelysmooth surfaces, such as metals, because induced multiple reflection significantly distorts the reflected spectra.
V. CONCLUSION
We demonstrated that a BOF sensor with high spatial resolution and robustness is applicable not only to gases or liquids but
also to solids. First, its operation was clarified in the simulation
using the characteristic-matrix technique. Then, in the experiments with air and six different kinds of liquids, a coefficient of0.61 nm/RIU was obtained, which agreed well with the simulation result. Furthermore, the RIs of two kinds of solids werealso successfully measured.
In order to practically use this RI sensor, a method to compensate the temperature effect must be developed by, for example, fabricating the BOF onto an FBG [24]. The sensitivity
also must be improved by employing so-called push-pull technique [20] and using BOFs with different structures. We believe
this RI sensor will be of great use in wide range of biochemical,
chemical, and engineering applications in future.
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