Fiber-Optic Probe Based on a Bifunctional Lensed ... - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 11, NO. 5, MAY 2011

Fiber-Optic Probe Based on a Bifunctional Lensed Photonic Crystal Fiber for Refractive Index Measurements of Liquids Gopinath Mudhana, Kwan Seob Park, Seon Young Ryu, and Byeong Ha Lee

Abstract—We report a fiber-optic sensor for measuring the refractive index (RI) of a liquid sample. The sensing probe is constructed from an extrinsic cavity formed by a micromirror and a lensed photonic crystal fiber (PCF) tip. We show that PCF lens with an unconventionally large radius of curvature is bifunctional; an effective reflector as well as a collimator and thus suitable for realizing long single-arm interferometers, with a cavity length of the order of few mm. A sensing head suitable for measuring the refractive index (RI) of liquids is constructed by encapsulating the bifunctional lens tip and a micromirror in a glass tube. A hole is made in the glass-tube to allow free-flow of the liquid sample in-and-out of the cavity. The cavity length was about 1 mm. The group index of the liquid samples is obtained from the Fourier peak position of the interference signal measured in wavelength with optical spectrometer. The RIs of distilled water, acetone and ethanol (at 829 nm, 20 C) were measured to be 1.32822, 1.35416, and 1.35715, respectively. The resolution of the sensor was analytically found to be 2.6 10 5 and the response was experimentally shown to be linear. Index Terms—Fiber-optic sensor, interferometric measurement, lensed-fiber, photonic crystal fiber (PCF), refractive index (RI) measurement, spectral domain interferometry.

I. INTRODUCTION

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HE refractive index (RI) is one of the important properties of a substance and knowing the RI of liquid is becoming important in various fields including biomedicine. The refractometers widely used in laboratories and industry are usually based on the Abbe refractometer [1], named after Ernst Abbe who developed the first refractometer in 1869. The prism coupler [2], ellipsometer [3], and Pulfrich refractometer [4] methods employ prisms. They measure the change in the critical angle of the prism due to the presence of liquid on the faces of the prism. Thus, knowledge of the refractive index of Manuscript received July 10, 2010; revised September 28, 2010; accepted October 01, 2010. Date of publication October 14, 2010; date of current version March 16, 2011. This work is supported in part by the Korea Science and Engineering Foundation (KOSEF) grants funded by the Korean government: (MEST) (No. R01-2007-000-20821-0) and NCRC (MEST) (No. R152008-006-02002-0). The associate editor coordinating the review of this paper and approving it for publication was Dr. M. Abedin. G. Mudhana and K. S. Park are with the Gwangju Institute of Science and Technology, Gwangju 500712, Korea. S. Y. Ryu is with the Division of Instrument Development, Korea Basic Science Institute, Yusung-gu, Daejeon 305-333, Korea. B. H. Lee is with the Department of Information and Communications, Gwangju Institute of Science and Technology, Gwangju 500712, Korea (e-mail: [email protected]). 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/JSEN.2010.2087323

the prism is required. Further, since the temperature dependencies of the indices of the prism (glass) and the liquid under test, in general, are quite different, the measurement has to be done with temperature regulation in many cases. Although bulkiness is addressed by the compact handheld refractometers, all the forementioned limitations of the prism-based refractometers still remain along with the need for calibration and careful alignment. Fiber-optic solutions are undoubtedly most sought-after due to their innumerable advantages such as compactness, cost-effectiveness, flexibility to reach samples, remote operation, and multiplexing capability. The fiber-optic techniques can be broadly classified into reflective, refractive and interferometric techniques. In reflective and refractive techniques, RI is measured by the change in the reflected or transmitted power, respectively, when the tip or the cladding surface of the fiber is in contact with the liquid sample. The interferometric techniques on the other hand use the interference between two beams; one passes through a sample and the other comes from the so-called reference arm. The interferometric techniques obtain RI by measuring the change in the optical path difference (OPD) induced by the sample. The reflective methods on the other hand, rely on the change in the reflectivity at the surface of contact making the measurements susceptible to fluctuations in the source power and the quality of the surface. Moreover, these reflective methods cannot be used for the liquid whose RI exceeds the fiber core index [5]. Despite these disadvantages, most of the reported fiber-optic refractometers or sensors are reflective or refractive types [5]–[8]. Some of the interferometric sensors reported are not strictly interferometric [9], as they still rely on the Fresnel reflection at the interface of fiber and the liquid. Fabry–Perot type interferometric sensors in which the sample cavity is formed by laser-machining [10] and microchanneling [11] through the fiber core have been reported. These had the advantage of being single-arm interferometers. However, they require expensive micromachining laser systems and the micro machined fibers are definitely fragile. Further, the cavity length was very small to be practical with liquids. When the cavity length is small, some liquids do not flow easily into the cavity by themselves without the aid of a microneedle or air-blow due to the property of surface tension of the liquids. Here, we report a common-path interferometer having a single arm and also a long cavity length of the order of . The need of using two arms in a conventional in1000 terferometer is overcome by devising a special bifunctional photonic crystal fiber (PCF) lens. The specially designed PCF lens plays the roles of the reflector of the reference arm as well

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MUDHANA et al.: FIBER-OPTIC PROBE BASED ON A BIFUNCTIONAL LENSED PCF FOR RI MEASUREMENTS OF LIQUIDS

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Fig. 1. Schematic of the refractive index measurement system based on a PCF lens. Fig. 3. The coupling efficiency of the SMF and the PCF probes having cleaved or lensed tips. The optical intensity of the beam recoupled to the fiber after being reflected from a mirror was measured with respect to the displacement distance of the mirror. The inset shows the microscope images of the probe tips. LPCF, lensed PCF; PCF, cleaved photonic crystal fiber; LSMF, lensed SMF; SMF, single-mode fiber.

Fig. 2. (a) Schematic of the sensor-head for refractometer; MT- metal tube, GT- glass tube, M-Mirror. (b) Photograph of the sensing probe (sensor-head spliced to a patch cord). (c) Close-up of the sensor-head.

as the collimator of the sample arm at the same time. While, the reflective property of the bifunctional lens helps in realizing single-arm interferometer, the collimation property aids the realization of cavity length of the order of millimeters. II. THEORY A. Sensor Head Design The schematic of the refractive index measurement system and sensor head is shown in Figs. 1 and 2, respectively. The sensor-head is composed of an extrinsic fiber optic cavity formed by a lensed fiber tip made from a piece of PCF and a micromirror. The lensed PCF fiber-tip and the micromirror are encapsulated in a glass tube, (GT) with a hole to allow the liquid sample freely move in-and-out of the cavity. Two metal tubes (MT) of different diameters are used to hold the fiber tightly in the GT. In this section, we explain the advantages of using PCF and the reason for choosing the unusually large radius of curvature for the lensed PCF fiber-tip. In order to realize long cavity length, optical beam collimation is required. Optical beam collimation is a two step process where, first the divergent beam exiting from the fiber tip is expanded followed by a focusing element. The beam expansion is commonly achieved by arc-splicing or laser-fusing the fiber-tip with a silica rod, while the focusing element is realized by forming a lens surface at the far-end of the beam-expanding fiber either by etching or laser machining or by aligning a

bulk-optic micro-lens. The need for handling and splicing short length fiber for beam expansion along with the expensive and time-consuming lens formation technology involved increases the manufacture cost of the fiber optic collimators. In this section, we explain the principle of forming single-step monolithic fiber-optic collimators based on PCF. The advantage of using PCF for realizing fiber-optic collimator is twofold; PCF being single mode over infinitely large bandwidth [12] and also the possibility of making collimator in a simple single-step process. When an electric arc is applied at the PCF tip, in addition to the lens formation on the tip, a uniform silica region is formed over a short length behind the lens due to the collapsing of air-hole structure [13], [14]. The uniform silica region at the end of the PCF is no longer a waveguide but just acts as a free space; the beam coming out of the PCF core expands (or diffracts) freely in this region. Then, with the help of the lens at the end of it, the beam can be collimated or focused. The air-hole collapsed region will be referred hence forth as beam-expansion region. In brief, when a PCF fiber-tip is used, two-step process of making fiber collimators can be done in a single step by just placing an electric arc near the tip. The fiber lenses are usually characterized by measuring the variation of the optical power recoupled to the fiber after being reflected by the mirror that is moving away from the lensed-tip [15]. Fig. 3 shows the recoupled power for SMF and PCF with and without the lensed tip. Comparing the cleaved fibers without the lens tip, PCF has moderately larger working distance than SMF, shown by slower falloff of the recoupled power. On the other hand, lensed PCF demonstrates distinctly large increase in the working distance. The distinctly large working distance of the lensed PCF is due to the beam-expansion region formed behind the lens surface. In other words, lensed PCF is not a simple lens but acts as a collimator. Fiber lenses have been developed for coupling light between the lasers and fiber pigtails. The coupling efficiency, in general, is dependent on the radius of curvature as well as the shape of the lens. The coupling efficiency can reach up to 90% by proper choice of the lens radius and shape to mode-match the field in

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the laser and the fiber [16]. Often tapered fibers are used for making lenses with small radius of curvature for better coupling efficiency. Hemispherical shaped lenses [17], [18] are known to have the best coupling efficiency. In laser coupling applications, increasing the coupling efficiency is the only objective and any reflection from the lens surface contributes to the loss. However, in our application, we require large enough reflection at the lens surface to form a single-arm interferometer. This can be achieved by forming lens with large radius of curvature. The large radius of curvature of the lensed PCF has twofold advantages; increased reflection along with increased coupling efficiency. In contrast to the lensed SMF, lensed PCF has increased coupling efficiency for the case of large radius of curvature. This difference in the behavior of the lensed PCF can be explained from the presence of beam-expansion region behind the lens surface. In the case of lensed PCF, since the presence of beam-expansion region pushes the fiber core far-behind the lens surface, lens with larger radius of curvature is required to focus the light incident on the lens surface onto the core of the PCF. Thus, PCF lens with large radius of curvature, which we refer as bifunctional lens act as an effective reflector as well as a collimator. In Section III, we will show that the bifunctional lens can be fabricated by simply choosing low arc power while forming the lens with the conventional fused-arc method.

Fig. 4. The variation of radius of curvature and length of air-hole collapsed region with splicing parameters. For the left column, the arc duration was kept constant and the arc power varied, and for the right column, the arc duration varied with a fixed arc power.

8.5 in a wavelength range of 635–980 nm. The splicing loss of the PCF with the conventional SMF was measured to be 0.2 dB. In this section, first we will describe the characterization of the PCF lens and the choice of the parameters for making the PCF lens, followed by the construction of the sensor-head and the measurement of RI with the sensor head.

B. Refractive Index Measurement

A. Characterization of Bifunctional Lens

As shown in Fig. 2(a), the interference occurs between the beam reflected at the lens surface and the one reflected at the mirror , where is the optical path-length difference (OPD) between the lens surface and the mirror. The interference is analyzed using the Fourier transform spectral white-light interferometry [19]. The interference in the spectral domain can be written as

In the arc-fusion method of making fiber lenses, lenses are formed by placing an electric arc near the fiber-tip. In general, the radius of curvature of a fiber lens can be adjusted by controlling the power or/and duration of electric arc. However, in the case of PCF lens, in addition to the radius of curvature of the lens, the length of the beam-expansion region is affected by the arc parameters. Fig. 4 shows the microscope images of eight PCF lenses fabricated with different arc parameters. The lenses of the left and right columns were made with different arc powers at constant arc duration and vice versa, respectively. The collapsed region of the PCF can be easily identified in the image by the absence of hairy structure (caused by the diffraction at the air-holes). The left and the right columns indicate that the radius of curvature and the length of the beam expansion region can be independently controlled with the arc power and the arc duration, respectively. In order to show that a larger radius of curvature is more suited for the sensor-head, three samples were fabricated with arc powers of 120, 130, and 140 mW, which will be referred as samples A, B, and C, respectively. Sample A with a lower arc power represents the lens with a larger radius curvature and hence is expected to have better performance as the sensor-head. The schematic of the setup used for characterizing the working distance of PCF lenses is shown in Fig. 5(a). For the case of conventional fiber lenses, it is enough to measure the variation of the power recoupled to the fiber after being reflected by the mirror. The reflection from the lens surface is neglected. However, in the case of the proposed bifunctional PCF lens, the reflection from the lens surface plays an important role, as the reference beam. Therefore, we need to consider the three beams depicted in Fig. 5(b). The beam reflected by the lens surface and the beam recoupled to the PCF after being reflected by the mirror will be hence forth referred as reflected

(1) If we measure as a function of , by applying inverse Fourier transform (IFFT) with respect to , we obtain

(2) where the first two terms are the autocorrelations of the fields and related to the Fourier transform of the source spectrum. The last two terms are cross correlations and give two symmetric peaks in the Fourier domain. The measurement of RI involves two steps. First, the reflection spectrum is measured without liquid, and then the second measurement is followed with the liquid under test. Since the Fourier cross-correlation peaks of the two measurements are loand , respectively, the RI is cated at between them. We note obtained by taking the ratio, that, the obtained RI in this scheme is the group refractive index. III. EXPERIMENT AND RESULTS The large mode area PCF (LMA-10), supplied by Crystal Fiber™, was used for making the PCF lens. The core diameter was about 11 and the mode-field diameter was about


Fig. 5. (a) The schematic of the setup to characterize the PCF lens. (b) Nomenclature of the beams involved in interference. SLD, superluminescent diode; PM, power meter; C, circulator; FH, fiber holder; M, mirror mounted on a translational stage.

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Fig. 6. The powers of the reflected beams, shown as dotted horizontal lines, and the recoupled beams, the solid lines with symbols, measured in terms of represents the relative the mirror distance for the three lenses, A, B, and C. difference between the power levels of the two beams for each lens.

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and recoupled beams, respectively, while the combined beam will be called as the composite beam. An SLD source operating at 1310 nm with a 3 dB bandwidth of 50 nm was used as the light source. First, the power of the reflected beam from the PCF lens was measured, and then the PCF lens was positioned in front of a mirror, as shown in Fig. 5(a). After aligning the fiber lens and the mirror with each other, the power variation of the composite beam with the mirror distance in was measured, while the mirror was moved up to 4000 . The power of the recoupled beam was obtained steps of 50 indirectly from the two measurements, by subtracting the power of reflected beam from that of the composite beam. Fig. 6 shows the power variation of the reflected and the recoupled beams with the mirror distance for the three lenses. The horizontal dotted lines represent the power levels of the reflected beam, measured without the mirror. The relative power level of the reflected and recoupled beams, which decides the performance or the contrast of interference, is indicated with in the figure. Neglecting the difference in the alignment of the three samples, comparison of the variation of three solid curves will suggest the preferable condition for better performance. Fig. 6 clearly suggests that Sample A is preferred due to the longer working distance and better fringe contrast indicated between the by slower roll-off and smaller difference, relative powers of the interfering beams, respectively. The performance of the lensed PCFs was confirmed by measuring the interference fringes with an optical spectrum analyzer (OSA-Agilent 86142B). The spectra in Fig. 7 were measured for the three lensed-PCF samples at two mirror distances, 1000 and 2000 . From Fig. 6, it can be seen that, the power of the recoupled beam for lens C was greater than B and even A (until, ). However, in Fig. 7, we see that the contrast of interference for lens A is the best. Therefore, increasing the power of the reflected beam is more important than increasing the coupling efficiency of the recoupled beam for the RI measurement. For increasing the coupling efficiency of the recoupled beam, both the radius of curvature or/and the shape of the lens should be controlled. However, for increasing the power of the reflected beam, which is more important, simply low arc , power is asked. The reduction of fringe contrast at 2000

Fig. 7. Normalized interference spectra for the three PCF lenses (A, B, and C) measured at mirror distances, 1000 and 2000 .

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for all the three cases is due to the well-known alignment sensitivity of hemispherical optical cavities. By careful alignment or replacing the flat-surfaced micromirror with a spherical one, it . is possible to obtain cavity length of even larger than 2000 B. Measuring Refractive Index of Liquid Specimen An SLD source with a center wavelength 829 nm and 3-dB bandwidth of 50 nm was used for measuring the RI of distilled water, acetone and ethanol (Merck). To improve the measurement speed, a CCD-based spectrometer (Ocean Optics™) was used. The resolution and the number of CCD pixels (number of sampling points) were 0.05 nm and 3648, respectively. The measurement range was from 748.74 to 931.2 nm. As described in Section II, the interference spectrum was measured with and without the liquid under test in the cavity. The spectrum with liquid was obtained by simply dipping the sensing probe in the beaker containing the specimen liquid. After making the measurement with each specimen, the probe was rinsed with water and acetone, and dried with air-blow for other measurements.

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Fig. 9. The RIs measured for 20 labeled index matching oils, which ranged from 1.400 to 1.438 with an index step of 2 10 .

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the well-known temperature dependency of the refractive index of liquids [22], [23]. Fig. 8. The Fourier peaks of air (black line at left) and liquid samples (colored lines at right). The peak locations are marked.

IV. ANALYSIS TABLE I COMPARISON OF REFRACTIVE INDICES

From the uncertainty principle, the resolution in the is related with the one in the spatial dowavenumber as , where is the number of data main points. Therefore, using , the resolution of locating the peak in the spatial domain can be written in terms of the resolution in the wavelength domain as (4)

The spectrum measured in the wavelength domain was converted into the wavenumber domain with interpolation, then inverse Fourier-transform (IFFT) was applied on the windowed interference signal. Fig. 8 shows the Fourier peaks of the air and liquid specimens. The peak locations are marked and specified. As described in Section II, the RI was obtained by taking the ratio of peak positions. The RIs (group indices) of water, acetone and ethanol, at 829 nm (20 C), were obtained as 1.34198, 1.36361 and 1.36619, respectively. Although refractive group index is measured in some applications [20], most RI measurement systems focus on measuring refractive phase index. In order to compare the results of our measurements with the reported phase index, we have used the relationship of (3) The derivative of the phase index on the right-hand side of the equation is obtained from the Sellmeier coefficients reported in literatures [1], [21]. The phase indices thus obtained from the measured group indices and the corresponding phase indices reported in the literature are tabulated in Table I. Considering the sensitivity of RI to contaminants and temperature, matching to the third decimal place can be considered accurate. An uncertainty of is observed from the repeated measurements. This uncertainty is not from the measurement process but

where and are the center-wavelength and the wavelength spacing of the measured interference spectrum, respectively. , However, since the peak position is related to OPD, the resolution of the RI measurement can be written as (5) Therefore, with a cavity length of , , spectral resolution of center wavelength of , and the number of data points in spatial domain , the resolution (total number after zero-padding), . The high resolution of the sensor is calculated as 2.6 of the sensor can be verified by measuring the index change of solutions such as salt water with varying concentration. However, such an experiment should be performed in thermally stabilized environment as the RI of liquids including water is sensitive to temperature. To demonstrate the performance of the sensor, we have measured the RIs of 20 labeled index matching oils (Cargille 18061). The RIs of the samples were from 1.400 to 1.438 (at 589 nm) . Fig. 9 shows the measured RIs with an index step of 2 for the 20 samples plotted with respect to the labeled RIs. The successful measurement of RIs verifies the performance of the system as well as demonstrates the linearity, which is a desirable property of a good sensor. Although, the sensor head was rinsed after making each measurement, much of the deviations observed for some data points might be due to unthorough cleaning and contamination of the sample.


V. CONCLUSION We have proposed a simple fiber-optic sensor system based on a bifunctional lensed PCF probe. The bifunctional lens is formed by using unconventionally large radius of curvature to produce considerable reflection at the lens surface to realize common-path interferometer. Lens formed with an arc power of 120 mW and the arc duration of 500 ms gave very high contrast interference fringes. The resolution of the system is analytically . Since, the refractive index of liquids is found to be 2.6 sensitive to temperature, in order to verify the high resolution of , the measurement should be performed in a thermally isolated chamber. With the proposed probe, the RIs of acetone, ethanol and distilled water were measured. The linear response of the sensor is demonstrated with RI measurements of 20 liquid standard samples with index steps of 0.002. The main merit of the proposed method is the simplicity in configuration, fabrication and measurement. The diameter of the sensor-head can be reduced by replacing the micromirror with a reflection coated fiber. Therefore, with small improvements in packaging, we believe the proposed sensor system and the probe will be useful in endoscopic applications as well. REFERENCES [1] J. Rheims, J. Köser, and T. Wriedt, “Refractive-index measurements in the near-IR using an Abbe refractometer,” Meas. Sci. Technol., vol. 8, pp. 601–605, 1997. [2] R. Ulrich and R. Torge, “Measurement of thin film parameters with a prism coupler,” Appl. Opt., vol. 12, no. 12, pp. 2901–2908, 1973. [3] D. E. Aspnes and A. A. Studna, “High precision scanning ellipsometer,” Appl. Opt., vol. 14, no. 1, pp. 220–228, 1975. [4] E. Moreels, C. de Greef, and R. Finsy, “Laser light refractometer,” Appl. Opt., vol. 23, no. 17, pp. 3010–3013, 1984. [5] P. Nath, H. K. Singh, P. Datta, and K. C. Sarma, “All-fiber optic sensor for measurement of liquid refractive index,” Sens. Actuators A, vol. 148, pp. 16–18, 2008. [6] C.-B. Kim and C. B. Su, “Measurement of refractive index of liquids at 1.3 and 1.5 micron using a fiber optic Fresnel ratio meter,” Meas. Sci. Technol., vol. 15, pp. 1683–1686, 2004. [7] J. Turan, E. F. Carome, and L. Ovsenik, “Fiber optic refractometer for liquid index of refraction measurements,” TELSIKS, pp. 489–492, 2001. [8] A. Banerjee et al., “Fiber optic sensing of liquid refractive index,” Sens. Actuators B, vol. 123, pp. 594–605, 2007. [9] S. F. O. Silva et al., “Optical fiber refractometer based on a Fabry-Pérot interferometer,” Opt. Eng., vol. 47, p. 054403, 2008. [10] T. Wei, Y. Han, Y. Li, H.-L. Tsai, and H. Xiao, “Temperature-insensitive miniaturized fiber inline Fabry-Perot interferometer for highly sensitive refractive index measurement,” Opt. Express, vol. 16, no. 8, pp. 5764–5769, 2008. [11] Z. Ran, Y. Rao, J. Zhang, Z. Liu, and B. Xu, “A miniature fiber-optic refractive-index sensor based on laser-machined Fabry-Perot interferometer tip,” IEEE J. Lightw. Technol., vol. 27, no. 23, pp. 5426–5429, Dec. 2009. [12] T. A. Birks, J. C. Knight, and P. S. J. Russell, “Endlessly single-mode photonic crystal fiber,” Opt. Lett., vol. 22, no. 13, pp. 961–963, 1997. [13] G.-J. Kong, J. Kim, H.-Y. Choi, J. E. Im, B.-H. Park, U. C. Paek, and B. H. Lee, “Lensed photonic crystal fiber obtained by use of an arc discharge,” Opt. Lett., vol. 31, no. 7, pp. 894–896, 2006. [14] H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express, vol. 15, no. 9, pp. 5712–5720, 2007. [15] E. Li, “Characterization of a fiber lens,” Opt. Lett., vol. 31, no. 2, pp. 169–171, 2006. [16] C. A. Edwards, H. M. Presby, and C. Dragone, “Ideal microlenses for laser to fiber coupling,” J. Lightw. Technol., vol. 11, no. 2, pp. 252–257, Feb. 1993.

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[17] A. Hokkanen and S. Tammela, “Hemispherically ended optical fiber lenses,” Physica Scripta., vol. T69, pp. 159–162, 1997. [18] H.-M. Yang, “A novel scheme of hyperbolic-end microlens using the arc technology,” Optik., vol. 120, pp. 905–910, 2008. [19] C. Dorrer, N. Belabas, J.-P. Likforman, and M. Joffre, “Spectral resolution and sampling issues in Fourier-transform spectral interferometry,” J. Opt. Soc. Amer. B, vol. 17, no. 10, pp. 1795–1802, 2000. [20] I. Z. Kozma, P. Krok, and E. Riedle, “Direct measurement of the groupvelocity mismatch and derivation of the refractive-index dispersion for a variety of solvents in the ultraviolet,” J. Opt. Soc. Amer. B, vol. 22, no. 7, pp. 1479–1485, 2005. [21] M. Daimon and A. Masumura, “Measurement of the refractive index of distilled water from the near-infrared region to the ultraviolet region,” Appl. Opt., vol. 46, no. 18, pp. 3811–3820, 2007. [22] K. M. Aly and E. Esmail, “Refractive index of salt water: effect of temperature,” Opt. Mat., vol. 2, pp. 195–199, 1993. [23] W. Lu and W. M. Worek, “Two-wavelength interferometric technique for measuring the refractive index of salt-water solutions,” Appl. Opt., vol. 32, no. 21, pp. 3992–4002, 1993. Gopinath Mudhana was born in Chilakaluripet, India, in 1977. He received the B.S. (Hons.) and M.S. degrees in physics from Sri Sathya Sai University, Puttaparthi, India, in 1997 and 1999, respectively. He is currently working towards the Ph.D. degree at the Gwangju Institute of Science and Technology, Gwangju, Korea. He worked as a Project Assistant from 2000–2002 at the Indian Institute of Science, Bangalore, India, His work is related to characterization of optical fibers and fiber devices, using novel signal processing techniques. His research interests include fiber optic sensors and fiber Bragg gratings.

Kwan Seob Park received the B.S. degree in physics from the Kwangwoon University, Seoul, Korea, in 2007, and the M.S. degree in information and communication from the Gwangju Institute and Science and Technology, Gwangju, Korea, in 2009. Currently, he is working towards the Ph.D. degree at the Department of Information and communications, Gwangju Institute and Science and Technology. His research interests include integrated fiber optic sensors and devices.

Seon Young Ryu received the B.S. and M.S. degrees in physics from Chungbuk National University, Chungbuk, Korea, in 1999 and 2002, respectively, another M.S. degree and Ph.D. degree from the Gwangju Institute of Science and Technology, Gwangju, Korea, in 2005 and 2010, respectively. Currently, she is working in the Division of Instrument Development, Korea Basic Science Institute, Korea. Her research interests are developing optical imaging systems for biomedical applications.

Byeong Ha Lee received the B.S. and M.S. degrees from the Department of Physics, Seoul National University, Seoul, Korea, in 1984 and 1989, respectively, and the Ph.D. degree in physics from the University of Colorado, Boulder. After working as a STA Fellow at the Osaka National Research Institute of Japan from 1997 to 1999, he joined Gwangju Institute of Science and Technology, Gwangju, Korea, where he is currently serving as a Full-Time Professor. He specialized in the areas related to fiber-optic sensors and optical imaging for biomedicine including optical coherence tomography and fluorescence spectroscopy.