IEEE SENSORS JOURNAL, VOL. 13, NO. 5, MAY 2013
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Simple Fiber-Optic Refractive Index Sensor Based On Fresnel Reflection and Optical Switch Wei Xu, Xu Guang Huang, and Jing Shun Pan
Abstract— In this paper, a simple fiber optical sensor system used to measure the refractive index (RI) is proposed. It only consists of one optical source, one 2 × 2 optical coupler with two fiber sensing ends, and one mechanical 1 × 2 optical switch that is controlled by the drive voltage applied on it. The operation principle is based on relative Fresnel reflective intensity. The refractive indices of chemical species including benzene, ethanol, methanol, acetone, and glycerol are measured at 1550 nm wavelength. The dependence of the RI of tap water on the temperature is also addressed. A short-time measurement precision of 8 × 10−6 and a long-time measurement precision of 5 × 10−5 were obtained. Applying the relative technique, the errors resulted from the fluctuation of light source and the influences of environment are effectively eliminated. The sensor system is of low cost, simple operation, high sensitivity, and precision. Index Terms— Fiber-optic sensor, Fresnel reflection, optical switch, refractive index.
I. I NTRODUCTION
F
IBER-OPTIC refractive-index (RI) sensors have attracted considerable interest in recent years. It is because fiber-optic-based sensors have many excellent characteristics, including corrosion resistance, immunity to electromagnetic interference, high precision, etc. [1] These advantages are important for applications in areas such as biomedical measurement and environmental protection. Various parameters, including temperature, concentration, magnetic field, electric field, potential of hydrogen (PH), etc. [2]–[6], influence the RI. It naturally encourages us to do research in order to measure the RI faster, and even higher resolution with a low cost. So far, the means to measure the RI have already been proposed a lot, including critical angle refractometer [7], surface plasma resonance (SPR) [8], [9], grating-based RI sensors [10]–[13], etc. However, there are still some drawbacks that need to be overcome, including high cost, complex fabrication, sensitive to temperature variation or cumbersome, etc. Commercial Abbe refractometers/modified Abbe refractometers are instruments for measuring the refractive index of a
Manuscript received November 24, 2012, accepted December 21, 2012. Date of publication December 28, 2012; date of current version March 27, 2013. The associate editor coordinating the review of this paper and approving it for publication was Prof. Massood Atashbar. This work was supported in part by the National Key Technologies R&D Program of China under Grant 2011BAF06B02 and the Science and Technology Plan of Guangzhou under Grant 11C42090778. (Corresponding author: X. G. Huang.) The authors are with the Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China (e-mail:
[email protected];
[email protected];
[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.2012.2236751
Fig. 1. Schematic diagram of RI sensor system based on the 1 × 2 optical switch.
specimen using these methods. The refractive index resolution of these instruments roughly ranges from 1×10−4 to 1×10−5 depending on their complexity and cost. SPR RI sensors can offer advantages of high accuracy (generally 10−4 ∼ 10−7 ) and real-time responses, but they are relatively expensive to implement. On the other hand, grating-based fiber-optic RI sensors can operate in the 1550 nm band with 10−5 ∼ 10−6 resolution, but the sensing heads of them are sensitive to the temperature variation. Chang-Bong Kim [14], et al. proposed a double-pulse calibration method to obtain the RI. An expensive pulse laser and a long fiber about two hundred meters are used in the system, which is cumbersome and can’t be simply extended to multi-point measurements. In this paper, we use a different structure of a Fresnel reflection-based fiber optic technique to measure the RI of a liquid. The reflected signal from the liquid-fiber interface and the reflected signal from the air-fiber interface are respectively measured with only one photo-detector (PD) through an optical switch. Undesirable effects due to possible source power drifts and detector response changes are eliminated. The measurement principle of the system and the precision and the stability of RI measurement are discussed. II. P RINCIPLE OF O PERATION To measure the refractive index, a simplified but promoted fiber-optic sensor based on relative Fresnel reflection technique is presented. The experiment setup for the fiber-optic sensor is sketched in Fig. 1. It consists of a broad band source (BBS, purchased from LightComm Techn., Ltd.), one single-mode fused coupler (T&S Communications) (or one circulator), one PD (LightComm Techn., Ltd.), one optical switch (Advanced
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Fiber Resource), two fiber sensing ends. The light source operates in the wavelength around 1550 nm in the experiment. Split ratio of the coupler is 50:50. The fiber sensing ends, which are placed into standard telecommunication ferrules with a diameter of 2.5 mm for protection and ease of cleaning, are vertically planar surface. All fibers used are SMF-28 (Single Mode Fiber) fibers. The light sent from BBS is first split into two beams (Beam 1 and Beam 2, shown in Fig. 1) by the optical coupler equally. Beam 1 propagates towards the optical switch, while Beam 2 transmitting into the twisted fiber vanishes to avoid harmful reflection. Then Beam 1 will pass into air (Beam 3) or liquid under probe (Beam 4), which depends on the drive voltage sequences applied on the optical switch. The drive voltage and PD was controlled by the MCU (Micro Controller Unit). The light reflected from the fiber-air interface is considered as reference beam, and the light from the fiber-liquid interface considered as probe beam. The reflected lights of Beam 3 and Beam 4 are respectively detected by the same PD after across the optical coupler, as shown in Fig. 1. When the optical switch connects to the air side, the reflected intensity I3 detected by PD can be obtained by Fresnel formula [4] n f − n air 2 I3 = I0 α1 K 1 K 1 K 3 . (1) n f + n air Likewise, the reflected intensity I4 is given by n f − nx 2 I4 = I0 α2 K 1 K 1 K 4 . (2) n f + nx Here I0 and I0 are the output intensities of the light source at different moments. Compared to the slow-variation of the fluctuation of the light source, the time interval of the switching is short enough if the intensity of the light source can be well considered to be constant. Thus, the I0 equals to I0 . n f is the effective index of the single-mode fiber. n air and n x are respectively the indices of the air and the liquid. K 1 and K 1 are the transmittances of the bar-state or the crossstate of the coupler. If the insertion loss of the coupler can be neglected, the transmittances should satisfy the following formula: K 1 + K 1 = 1. K 3 and K 4 are respectively the transmittances of the switch to the air side and to the liquid side. α1 and α2 are respectively the additional attenuation, including insertion losses in different optical paths towards the air and the liquid. From Eq. (1) and (2), the relative intensity R can be obtained as follows: n f − n x n f + n air 2 α1 K 3 I4 = · · . (3) R= I3 α2 K 4 n f + n x n f − n air Eq. (3) can be rewritten as follows: n f − n x n f + n air 2 R=K· · , (4) n f + n x n f − n air 3 where K = αα21 K K 4 . Apparently, the value of K is a constant which is only determined by the attenuations of the optical paths and the transmittances of the optical switch. When the probe end is also exposed to the air environment, nx is equal to nair . From Eq. (4), one obtains
K = I4air /I3air .
(5)
Fig. 2. Wave form of the voltage applied on the optical switch between poles A and B.
By putting both sensing ends in air environment, the reflective intensities from the probe end and the reference end are respectively measured to be −8.196 dBm and −8.736 dBm. Therefore, the value of K equals to 1.132400. Thus, the value of K can be measured directly and conveniently in the setup, which avoids the difficulties of transmittances and attenuation measures and calculations. From Eq. (3) and (4), one obtains 1−η . (6) nx = n f · 1+η n f − n air R η= · 0.5. (7) n f + n air K The refractive index of air nair is 1.0003 [14] at the wavelength of 1550 nm. The effective index nf of the fiber mode can be calculated from the known fiber group index ng and the dispersion relation. The given value for nf is 1.44961 at λ = 1550 nm [14]. From Eqs (6) and (7), one could calculate the RI. III. E XPERIMENT When the optical switch is applied with a voltage whose magnitude varies from 5 Volt to −5 Volt between pole A and pole B with a period of T at least 20 ms and a duty cycle of 50% shown in Fig. 2, the optical switch alternately switches between the reference side and probe side. The period T is determined by the switch’s switching time of 10 ms. Therefore, reflected intensities from the fiber-air interface and the fiber-liquid interface can be detected by the PD within each period T. When the period T is much shorter than the variation time of the light power due to thermal effect, the fluctuation of the light source can be ignored. Analytical reagent grade chemicals, including ethanol, glycerol, methanol, acetone and benzene, and tap water were used. All chemicals from Tianjin Damao chemical reagent factory were used as received without further purification. Furthermore, the tap water’s refractive indices were measured with an increasing temperature from 40 Celsius degree to 75 Celsius degree. IV. R ESULTS AND D ISCUSSION The RI of tap water versus temperature which ranges from 40 Celsius degree to 75 Celsius degree was measured and shown in the Fig. 3. One can see that the RI decreases with the increase of the temperature, which is accordant with the
XU et al.: SIMPLE FIBER-OPTIC RI SENSOR BASED ON FRESNEL REFLECTION AND OPTICAL SWITCH
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(a) Fig. 3. RI of tap water versus the temperature at the wavelength of 1550 nm. TABLE I M EASURED RI AT THE WAVELENGTH OF 1550 nm AND A DMITTED RI AT THE
WAVELENGTH OF 589 nm OF E ACH C HEMICAL , I NCLUDING B ENZENE , E THANOL , M ETHANOL , AND A CETONE Chemical Benzene Ethanol Methanol Acetone
Measured RI (1550nm) 1.4157 1.3417 1.3194 1.3515
Admitted RI (589nm) [15] 1.5011 1.3611 1.3288 1.3588
trend in Reference 15. Numerical fitting shows that the data measured can be fitted very well to linear equation with the value of R^2 over 0.995. Refractive indices of Ethanol, methanol, acetone and benzene were measured. Fig. 4(a)–(d) show the relative powers of Fresnel reflections from the probe and the reference versus the time within one and a half hours, while the insets in those figures show the calculated refractive indices versus the time. One can see that the RI standard deviations of benzene ethanol, methanol, and acetone are respectively 3.4 × 10−5 , 5.1 × 10−5 , 5.4 × 10−5 , and 5.6 × 10−5 , which reveals that the sensor system has a good long-term stability with high precision of ∼ 5 × 10−5 . Table I shows the measured RI at the wavelength of 1550 nm and the admitted RI [15] at the wavelength of 589 nm (no admitted data at 1550 nm in the reference) of each chemical, including benzene, ethanol, methanol and acetone. The reason for the difference between the measured refractive indices at 1550 nm wavelength and the ones at 589 nm wavelength can be mainly attributed to the chromatic dispersion of a material. Based on the above stability experiments, the proposed system can be used for real-time and long-term monitoring. To learn the possible maximum precision of the system, the short-term stability of the system was also evaluated. Fig. 5 shows the RI of the glycerol versus the time within three minutes. The standard deviation 8.4 × 10−6 reveals that the precision of the system is fairly high. It is because that the errors resulted from the fluctuation of light source, electrical noises of detector and the influences of environment are effectively eliminated by using the relative technique of two-channels detected with only one photo-detector.
(b)
(c)
(d) Fig. 4. (a)–(d) Power of Fresnel reflection versus the time for chemicals, including benzene, ethanol, methanol and acetone, and air, respectively. The insets show the RI of benzene, ethanol, methanol and acetone at the wavelength of 1550 nm, respectively.
It could be predicted that our method can be easily extended to multi-point or RI-based multi-parameter sensing applications by using a 1-by-N optical switch and N sens-
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Fig. 5.
RI of the glycerol versus the time at the wavelength of 1550 nm.
Fig. 6.
Schematic of the multipoint or RI-based multiparameter system.
ing heads. As is shown in Fig. 6, a 1 × N optical switch replaces the 1 × 2 optical switch to achieve the multi-point or multi-parameter measurements with a compact structure and low cost. The RI-based multi-parameter could be concentration [3], or temperature by coating a thermal-sensitive film [2], etc. V. C ONCLUSION In this paper, a simple fiber optical sensor system used to measure the refractive indexis presented. The refractive indices of tap water, ethanol, methanol, acetone, benzene and glycerol were measured. A short term measurement precision of 8.4 × 10−6 and a long term measurement precision of the order of magnitude ∼ 10−5 were achieved. Furthermore, a multi-point or RI-based multi-parameter system with a 1 × N optical switch is proposed.
[3] D. D. Sell, H. C. Casey, and K. W. Wecht, “Concentration dependence of the refractive index for n-and p-type GaAs between 1.2 and 1.8 eV,” J. Appl. Phys., vol. 45, no. 6, pp. 2650–2657, Jun. 1974. [4] L. X. Chen, X. G. Huang, J. H. Zhu, G. C. Li, and S. Lan, “Fiber magnetic-field sensor based on nanoparticle magnetic fluid and Fresnel reflection,” Opt. Lett., vol. 36, no. 15, pp. 2761–2763, 2011. [5] J. H. Zhu, X. G. Huang, W. Xu, and L. X. Chen, “Plasmonic optical switches based on Mach-Zender interferometer,” Phys. Plasmas, vol. 18, no. 7, pp. 72112–72116, 2011. [6] B. Gu, M. J. Yin, A. P. Zhang, J. W. Qian, and S. He, “Lowcost high-performance fiber-optic pH sensor based on thin-core fiber modal interferometer,” Opt. Exp., vol. 17, no. 25, pp. 22296–22302, 2009. [7] G. H. Meeten and A. N. North, “Refractive index measurement of turbid colloidal fluids by transmission near the critical angle,” Meas. Sci. Technol., vol. 2, no. 5, pp. 441–447, May 1991. [8] B. Rothenhausler and W. Knoll, “Surface-plasmon microscopy,” Nature, vol. 332, pp. 615–617, Apr. 1988. [9] D. Monzón-Hernández and J. Villatoro, “High-resolution refractive index sensing by means of a multiple-peak surface plasmon resonance optical fiber sensor,” Sens. Actuators B, vol. 115, no. 1, pp. 227–231, May 2006. [10] K. S. Chiang, Y. Liu, M. N. Ng, and X. Dong, “Analysis of etched long-period fiber grating and its response to external refractive index,” Electron. Lett., vol. 36, no. 11, pp. 966–967, May 2000. [11] X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of longperiod fiber gratings,” J. Lightw. Technol., vol. 20, no. 2, pp. 255–266, Feb. 2002. [12] J. F. Ding, A. P. Zhang, L. Y. Shao, J. H. Yan, and S. L. He, “Fiber-taper seeded long-period grating pair as a highly sensitive refractive-index sensor,” IEEE Photon. Technol. Lett., vol. 17, no. 6, pp. 1247–1249, Jun. 2005. [13] W. Liang, Y. Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive indexsensors,” Appl. Phys. Lett., vol. 86, no. 15, pp. 151122-1–151122-3, Apr. 2005. [14] C.-B. Kim and C. B. Su, “Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter,” Meas. Sci. Technol., vol. 15, no. 9, pp. 1683–1686, May 1991. [15] W. M. Haynes, CRC Handbook of Chemistry and Physics, 93rd ed. Boca Raton, FL: CRC Press, 2012.
Wei Xu was born in Jiangsu, China, on November 13, 1986. He received the Bachelor’s degree from the College of Physical Science and Technology, Soochow University, Suzhou, China, in 2010. He is currently pursuing the Postgraduate degree with the Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, China. His current research interests include fiber-optic sensors and fiber-optic communications.
Xu Guang Huang received the Ph.D. degree in optics from Sun Yat-sen University, Guangzhou, China, in 1992. He was a Post-Doctoral Research Associate with the University of Miami and Rensselaer Polytechnic Institute, from 1996 to 2000. He was a Senior Product Engineer with two Canada and U.S. fiber-optic technology companies from 2000 to 2003. He has been a Professor with the Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, since 2004. He has authored or co-authored more than 80 peerreviewed papers in international academic journals and holds two patents. His current research interests include integrated photonics, fiber-optic communications, and fiber-optic sensors.
R EFERENCES [1] X. Wang, J. Xu, Y. Zhu, K. L. Cooper, and A. Wang, “All-fused-silica miniature optical fiber tip pressure sensor,” Opt. Lett., vol. 31, no. 7, pp. 885–887, 2006. [2] K. Y. Lam and M. A. Afromowitz, “Fiber-optic epoxy composite cure sensor. I. Dependence of refractive index of an autocatalytic reaction epoxy system at 850 nm on temperature and extent of cure,” Appl. Opt., vol. 34, no. 25, pp. 5635–5638, 1995.
Jing Shun Pan was born in Guangdong, China. He received the Bachelor’s degree from South China Normal University, Guangzhou, China, in 2010, where he received the Master’s degree from the Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering. His current research interests include fiber-optic sensors.
