Research ArticleHigh Sensitivity Refractive Index Sensor byD-Shaped Fibers and Titanium Dioxide Nanofilm

Chuen-Lin Tien ,1,2 Hong-Yi Lin,2 and Shu-Hui Su2

1Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan2Ph. D. Program of Electrical and Communications Engineering, Feng Chia University, Taichung 40724, Taiwan

Correspondence should be addressed to Chuen-Lin Tien; cltien@fcu.edu.tw

Received 17 November 2017; Accepted 7 December 2017; Published 5 February 2018

Academic Editor: Shuan-Yu Huang

Copyright © 2018 Chuen-Lin Tien et al.This is an open access article distributed under the Creative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper presents a high sensitivity liquid refractive index (RI) sensor based on lossy mode resonance (LMR) effect. The D-shaped fibers coated with nanosized titanium dioxide (TiO2) thin film as a sensing head were submerged into different refractiveindex solutions. The variations in the optical spectrum of the proposed RI sensor with different refractive index solutions weremeasured. The LMR resonance peaks were used to determine the wavelength shifts with different refractive index solutions. Theresults show that the optical spectrum peaks shifted towards the longer wavelength side with increasing the refractive index. For theproposed fiber sensing head with a polishing residual thickness of 72𝜇m, the maximum shift of the absorption peak was 264 nm.The sensitivity of the proposed RI sensor was 4122 nm/RIU for the refractive index range from 1.333 to 1.398.

1. Introduction

Many fiber-optic sensors for refractive index (RI) sensinghave been developed due to some advantages such as smallsize, high sensitivity, light weight, and immunity to externalelectromagnetic interference. Various fabrication methodsfor fiber-based RI sensors have been reported in the literature[1], including surface plasmon resonance (SPR) [2, 3], evanes-cent field [4], fiber gratings [5, 6], and optical fiber interfer-ometry [7]. However, the sensitivity of the traditional sensingtechniques needs to be improved in practical applications.Iadicicco et al. [5] reported an etched fiber Bragg grating RImeasurement, while etching can also be applied to a longperiod of grating to increase sensitivity to external refractiveindices [6]. Optical fiber sensors based on evanescent wavegenerate a strong interaction between the guided wave andthe surrounding materials. However, most of the reportedsensors showed a low-sensitivity or low-detection range. Toovercome this problem, we proposed a D-shaped opticalfiber coated with a high refractive index titanium dioxide(TiO2) nanofilm to enhance the sensitivity and to extend thedetection range of the sensor.

In the last decades, thin film coated onto a fiber-optic hasbecome a widely explored technique in the field of sensors.

The effects of depositing a thin, highly refractive index (RI)layer onto the cladding over the grating region have beenreported [8, 9]. Since titanium dioxide (TiO2) is one of thehighly refractive index materials, TiO2 is known to be a goodphotocatalytic material under UV radiation. In this study,we present the fabrication of high sensitivity refractive indexfiber-optic sensors based on D-shaped fibers and nanosizedTiO2 coatings. From the optical point of view, TiO2 nanofilmdeposited on D-shaped fibers can generate lossy mode reso-nance (LMR) effect under particular conditions [10]. In thiswork, we fabricate a new liquid refractive index fiber-opticsensor called a D-LMR (D-shaped fiber with lossy moderesonance) sensor that combines with the D-shaped fibersand nanosized TiO2 coatings to achieve a high sensitivitysensor with a wide range of liquid refractive indices. Wealso utilized a DCmagnetron sputtering technique to depositTiO2 nanofilms on the polished surface ofD-shaped fibers forthe fabrication of liquid refractive index fiber sensors. Theproposed D-shaped fiber sensor structure includes a lowindex upper cladding (about 3–5𝜇m thick) between the fibercore and the TiO2 layer that is different from an uncladdedfiber structure in the literature [10–12]. Our fiber structuredesignwill help in giving theD-shaped fiber goodmechanicalsupport after side-polishing. This paper demonstrates the

HindawiAdvances in Condensed Matter PhysicsVolume 2018, Article ID 2303740, 6 pageshttps://doi.org/10.1155/2018/2303740

2 Advances in Condensed Matter Physics

D-LMR type RI sensors with a high sensitivity for liquidrefractive index measurements. The characteristics of theresonance-based fiber-optic sensors could be extensivelyused as refractometers to detect the refractive index changesin liquids.

2. Method

If a side-polished optical fiber is coated with a thin film,the propagation of the guided wave is affected. The thinfilm coatings combined with side-polished optical fiberscan produce two different resonance types: one is surfaceplasmon resonances (SPRs) [8] and the other is lossy moderesonances (LMRs) [9–11]. The surface plasmon resonance(SPR) occurs when the real part of the thin film permittivityis negative and higher in magnitude than both its own imag-inary part and the permittivity of the material surroundingthe thin films. The LMR effect can be generated by the metaloxide films whose real part of the permittivity is positiveand higher in magnitude than both the imaginary part ofthe permittivity and the permittivity of the surroundingmaterial. The LMR effect is achieved for metal oxide filmswith low imaginary part of the complex refractive index. Ingeneral, a specified thin film coated onto a D-shaped fibercan generate LMR effect based on a modified Kretschmannconfiguration [10]. LMR phenomenon has been observedwith metal oxides, such as indium tin oxide (ITO) [12],titanium oxide [13], and indium oxide [14]. Apart from this,LMR effect appears for both TM and TE polarized light, andthe generation of multiple resonances without modifying theoptical fiber geometry is also possible [15]. According to themode coupling theory, an LMR is the result of light couplingbetween evanescent waves and lossy modes guided in theabsorbing thin films.Thus, the analysis of the evolution of themodal effective refractive index is helpful in understandingthe generation of the resonance.

Since all of thematerials are characterized by their relativepermittivity, titanium dioxide thin films can be characterizedby a complex form of relative permittivity. The oscillatorymodel represents the dielectric constant of TiO2 thin filmsand it can be expressed as follows [16]:

𝜀 (𝜔) = 𝜀∞ −𝜔2𝑝

𝜔2 + 𝑖 (𝜔/𝜏), (1)

where 𝜀∞ is the dielectric constant of the high electrondensity region in the film, 𝜏 is the Drude damping time forfree electrons, 𝜔 is the incident light frequency, and 𝜔𝑝 isthe plasma frequency. It should be noted that the dielectricconstant was predicted by the Drude model [17]. The plasmafrequency is determined by the following formula:

𝜔2𝑝 =𝑁𝑒2

𝜀0𝑚∗𝑚, (2)

where𝑁 is the electron density, 𝑒 is the electron charge, 𝜀0 isthe dielectric constant, 𝑚 is the electron mass, and 𝑚∗ is theoptical effective mass for carriers [18].

As mentioned above, the generation of LMR is basedon D-shaped fibers coated with absorbing thin films which

modified the well-known Kretschmann configuration. IfTiO2 films are deposited on the polishing surface ofD-shapedfibers, then the real part of the refractive index of TiO2 filmswill always be higher than that of the D-shaped fiber inthe wavelength region of our study. Besides, when the indexof the surrounding medium above the TiO2 nanolayer is1.333, the structure will support guided modes. These modeswill be lossy because of the imaginary part of the index ofTiO2 films. Thus, the dispersion curves for the proposedsensor structure (D-shaped fiber/TiO2 layer/surroundingmedium) will be similar to the dielectric waveguides [19].The number of modes supported by this waveguide increaseswith increasing the film thickness. Batchman and McWright[20] studied the light propagation through semiconductorcladded waveguides. The attenuation maxima of the lightpropagating through the optical waveguide can be obtainedfor specific thicknesses and at certain wavelengths.This effectcan be explained as a periodic coupling between the guidedmodes of the lossless structure and the lossymodes supportedby the high refractive indexmaterial.Therefore, themode lossis maximal if the thickness of the coating layer correspondsto the cut-off thickness for that particular mode. Carson andBatchman [21] reported that a thin film is deposited on thesingle mode waveguide; the mode losses for either TE or TMbecomemaximal at its particular thickness.This happens dueto the lossy nature of the thin film and the phase matchingbetween the guided mode supported by a lossless dielectricwaveguide and the lossy mode supported by the thin film.

In this work, the proposed D-LMR fiber sensor can befabricated by using the D-shaped fiber and thin film coatingtechnique. It is a challenging work to keep a uniform thick-ness of the TiO2 nanolayer coated on a side-polished surfaceof the single mode fibers. A DC magnetron sputtering tech-nique is an efficient approach to obtain a uniform nanolayercoating of TiO2 films with a low surface roughness. Theperformance of the proposed RI sensor is evaluated in termsof wavelength sensitivity and linearity.

3. Experimental

Using a side-polishing technique to make D-shaped fiber, wemodified themotor-driven polishingmethod [22] to suspendthe optical fibers and to control the polishing surface depth.To ensure correct polishing depth, a PC-based measuringsystem with a photometer was used to monitor and controlthe polishing process in real time. A large increase of powerloss during polishing indicates that the cladding is thinenough for the light to radiate from the fiber core and thatthe polishing should be stopped.

A high sensitivity fiber-optic sensor is reached by thin-ning the optical fiber enough to obtain an evanescent waveinteraction of the guided light with the surrounding medi-ums. For D-shaped optical fibers, the core is very close tothe flat surface of the structure. As a direct consequence, anoptical fiber thinning is enough to allow evanescent waveinteraction of the guided light with the surrounding medi-ums.The D-shaped fiber section is sensitive to the surround-ing medium refractive index allowing the measurement ofdifferent refractive indices in liquids.

Advances in Condensed Matter Physics 3

Power meter

Optical output

DFB-LD 1550 nm

Optical fiber

Figure 1: Schematic diagram of optical fiber polishing arrangement.

A single mode fiber (SMF-130V) was used to fabricate thesensing devices. Before the thin film deposition, the claddingwas partially removed, and the resultant portion was ultra-sonically cleaned. After this cleaning process, the fabricationof the sensors involved two different steps. First, we used thesingle mode fiber, ground/polished by using a home-madeside-polishing machine. During the process of polishing thefiber, we need to monitor the polishing depth by checkingthe transmission light power levels, as shown in Figure 1.This measurement can confirm when the polished surfacereaches the near fiber core region. The polished length wasaltered by changing the contact angle of the optical fiber onthe wheel or by using a polishing wheel of different diameter.Under the optical fiber polishing conditions, where the wheelrotated at 15 rpm, the tension force on the optical fiber wasapproximately 0.2–0.3N and the wheel was lubricated withliquid paraffin; the radial force on the wheel from the opticalfiber was essentially uniform over the contact length andthe optical fiber was polished uniformly over that length.The polished fiber diameter was also further measured byan optical microscope to precisely obtain the polished depth.The thickness and polishing length of the side-polished fiberswere also measured by using an optical microscope. Figure 2shows the microscopic image of the D-shaped fiber. Theresidual thickness of the side-polished fiber was about 72 𝜇m(including a 5 𝜇m-thick cladding layer), and the polishinglength of theD-shapedfiberwas 30mm.Then, TiO2 nanofilmwas deposited on the D-shaped fiber surface by using aDC magnetron sputtering system. TiO2 has the advantagesof nontoxicity, environmental compatibility, high refractiveindex, and low price; therefore, we choose TiO2 as the coatingmaterial.

Prior to the deposition, the coating substrates (includingsilicon wafers and the D-shaped fibers) had been ultrason-ically cleaned in acetone and ethanol and then had beendried by a dryer. In a high-vacuum sputtering chamber,D-shaped fibers and silicon wafer were mounted onto asubstrate holder 200mm in diameter that rotated at a speedof 30 rpm. The flat side of D-shaped optical fibers offersa unique substrate on which thin films can be deposited.The D-shaped optical fibers were embedded in V-shaped

10.0 mTilt angle: 0

Lens: Z100:×1000

Figure 2: Microscopic image of the D-shaped fiber.

groove of a special fiber holder and the flat side of D-shapedfibers was parallel mounted to the surface of the substrateholder to keep the coating thickness uniform. For the DCsputtering depositions, titanium targets with purity of 99.99%and diameter of 76.2mm were used. The target to substratedistance was 120mm, the total pressure being set at 1.5 ×10−3 Torr. The reactive gas partial pressure was kept constantat 4.5 × 10−4 Torr during the deposition. For the preparationof the nanosized TiO2 films, a mixture of argon and oxygengases was used. The DC power and heating temperaturewere controlled to be 100W and 80∘C, respectively. Thin filmwas simultaneously deposited on the silicon wafers and theside-polished surfaces of the single mode fibers (SMF). TheTiO2 film’s thickness of 84 nm was measured by using anellipsometer.

To characterize the optical response of the D-LMR sens-ing device, a typical transmission setupwas used. A portion ofthe coated optical fiberwas cleaved at both ends and spliced tooptical fiber patch cords. The spectral response of the sensorwas obtained by using a typical optical transmission setup.Figure 3 shows that this setup consisted of a halogen whitelight source (Ocean Optics, HL-2000) connected at the inputof the optical fiber, and the other end was attached to a near-infrared spectrometer (Ocean Optics, NIR 512). Thus, thelight coupled to the side-polished fiber passed through thecladding removed sensitive region, which is located in theoptical transmission pathway.

4. Results and Discussion

The absorption peaks generated by the LMR phenomenoncan be observed in optical transmission spectrum. Thewavelength interrogation method along with D-LMR typeoptical fiber sensors was considered in our analysis. TheLMR wavelengths were determined from the normalizedtransmittance versus wavelength plot for different sensingliquid refractive indices. The data analysis for the proposedsensor was done by using MATLAB� software. The differentsets of sensing medium (NaCl solutions) refractive indices

4 Advances in Condensed Matter Physics

NIR 512

Transmission signalInput signal

Halogen light source

D-shaped fiber

Sensitive regionTiO2

Figure 3: Schematic of the proposed RI sensor and experimental setup.

0

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsity

(cou

nts)

17001600150014001300120011001000900Wavelength (nm)

1.3331.3381.356

1.3661.3821.398

Figure 4: LMR spectra of the D-shaped fibers coated with TiO2nanofilm for different liquid refractive indices.

used for the analysis were 1.333, 1.338, 1.356, 1.366, 1.382, and1.398. Figure 4 shows the LMR transmission spectra of D-shaped fibers coated with nanosized TiO2 film for differentliquid refractive indices. The sensing sensitivity is definedas the shift of resonance wavelength versus the change insurrounding refractive index. If the refractive index valueis increased by higher concentration of NaCl solutions, theLMR peak shifts to higher wavelengths. From the trans-mittance spectra, when the refractive index of the externalmedium is 1.333, the absorption peak is located at 1307 nm.When the refractive index of the external medium is 1.338,the absorption peak is located at 1335 nm. When the RI is1.382, the peak is at 1520 nm, and when its value is 1.398the peak reaches 1571 nm. Figure 5 illustrates the normalizedtransmittance of D-LMR optical fiber sensor for different

17001600150014001300120011001000900Wavelength (nm)

1.3331.3381.356

1.3661.3821.398

0.00

0.05

0.10

0.15

0.20

0.25

0.30Tr

ansm

ittan

ce (%

)

Figure 5: The transmittance spectra of different refractive indexsolutions by D-LMR optical fiber sensor.

refractive index solutions. Here the transmittance is definedas the ratio of the transmitted light intensity to the incidentlight intensity. It is important to note that the wavelength’sshift of the resonance peaks is a function of the refractiveindex of NaCl solutions, as shown in Figure 6. The sensingsensitivity of the D-LMR sensor was 4122 nm/RIU; RIU isrefractive index units, for the fiber residual thickness of 72 𝜇mand the polishing length of 30mm. Figure 6 also shows thelinear fitting of the resonant wavelength as a function of theliquid refractive index. The linear fitting curve demonstrates𝑅2 value of 0.980. In our previous publication [23], a liquidrefractive index sensor using double-sided polishing longperiod fiber grating (DSP-LPFG) was reported. A sensingsensitivity of 143.4 nm/RIU for the DSP-LPFG sensor wasobtained from the RI range of 1.333–1.375. In this work, the

Advances in Condensed Matter Physics 5

1200

1250

1300

1350

1400

1450

1500

1550

1600

1650

Wav

eleng

th (n

m)

1.34 1.35 1.36 1.37 1.38 1.39 1.401.33Refractive index unit

D-LMR sensorSlop = 4122 nm/RIU

R-square = 0.980

Figure 6: LMR wavelength as a function of the refractive index fordifferent NaCl solutions.

experimental results show that the sensing sensitivity of theD-LMR sensor is 28 times higher than that of the DSP-LPFGsensor. These results also show that the sensing sensitivity ofthe D-LMR sensor is about 12 times higher than that of atilted-LPFG sensor for liquid refractive index measurements[6]. The detection range of the proposed RI sensor is alsoextended.

5. Conclusion

We have proposed and demonstrated a novel D-LMR fibersensor by coating TiO2 nanofilms on D-shaped fibers. TheD-LMR sensor response to the liquid refractive index changeshas been carried out bymeasuring the resonancewavelength’sshift. The results show that LMR peaks shift to higherwavelengths with increasing the liquid refractive index. Thegreatest sensitivity of the RI sensor that has been experimen-tally demonstrated is 4122 nm/RIU for theRI range from 1.333to 1.398. Compared with other sensors, we propose that asimple structure based on a D-shaped fiber and nanosizedTiO2 coating layer will be more sensitive for the liquidrefractive index measurements with the advantages of beingmore compact, of low cost, and easily portable. The generalidea of proposed D-LMR sensor can also be applied to othersensing applications.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Ministry of Science andTechnology (MOST) of Taiwan under Grants MOST 103-2221-E-035-032 and 104-2221-E-035-058-MY2. The authorsalso appreciate the Precision Instrument Support Center

of Feng Chia University for providing the measurementfacilities.

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