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Miniature optical fiber refractometer using cladded multimode tapered fiber tips
Sensors and Actuators B 110 (2005) 36–40
Miniature optical fiber refractometer using cladded multimode tapered fiber tips David Monz´on-Hern´andez, Joel Villatoro ∗ , Donato Luna-Moreno Centro de Investigaciones en Optica A. C., Loma del Bosque 115, Leon GTO. 37150, Mexico Received 20 May 2004; received in revised form 14 December 2004; accepted 10 January 2005 Available online 10 February 2005
Abstract Cladded multimode tapered fiber tips with mirrored end are proposed as refractometers. The working mechanism of such devices is based on radiation of the modes guided by the cladding, which is induced by the refractive index of the sample medium. Using conventional gradedindex multimode fiber, a low-power 850 nm LED, and a simple detection scheme some inexpensive, robust, and versatile refractometers are demonstrated. The length of the tips is typically 2 mm, while their diameter is less than 80 m. The refractometers can cover a refractive index range between 1.36 and 1.46 with resolution up to 3 × 10−5 . © 2005 Elsevier B.V. All rights reserved. Keywords: Refractive index sensing; Optical fiber sensors; Evanescent field sensors
1. Introduction The refractive index (RI) is a fundamental material property. It may vary with different parameters, such as, temperature, concentration, pressure, etc. That is why, measurements of RI are very important. In a number of chemical, food, beverage, or medical analysis conventional bulk refractometers are employed. However, bulk refractometers are inappropriate for remote RI measurements, or in applications where size and weight are a concern. Bulk refractometers are not convenient for in situ RI measurements. Neither for measurements in very small spaces (volumes on the order of microliters), nor in applications where a very high resolution is required. In bio-sensing, for example, there is a need to monitor RI changes on the order of 10−4 to 10−7 which may be caused by molecular binding, chemical or biochemical reactions, or by changes suffered by a thin biolayer [1–3]. Optical fiber refractometers (OFRs) are an alternative to conventional bulk refractometers. OFRs have some advantages over their bulk counterparts, such as high reso∗
Corresponding author. Tel.: +52 477 773 1017; fax: +52 477 717 5000. E-mail address: [email protected] (J. Villatoro).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.01.011
lution, low cost, light weight, and multiplexing possibilities. Moreover, with OFRs in situ and remote RI measurements can be carried out. Several OFRs based on side-polished single-mode fiber, core-exposed or tapered multimode fibers, fiber Bragg gratings (FBGs), or long period gratings (LPGs) have been reported so far [4–18]. The resolution, i.e., the minimum RI change that can be measured, of the majority of the fiber-based refractometers reported so far is typically between 10−3 and 10−5 [4–18]. Their dynamic range, i.e., the range of RI values that can be measured, is typically from 1.33 to 1.46. Both, the dynamic range and resolution of OFRs are good enough for the most popular industrial applications. OFRs based on the phenomenon of surface plasmon resonance (SPR) can offer higher resolution (from 10−4 to 10−7 ), however, their dynamic range is very limited [19–24]. Moreover, a thin metallic film needed in SPR-based OFRs may suffer changes (in thickness or optical properties) due to reactions with the liquid sample. The simplest and, therefore, the lowest-cost OFRs are those based on multimode fibers since they require conventional fiber, a low-power LED, and a simple detection scheme [4,6,7]. In addition, their resolution and dynamic range may be adjusted in a simple manner [4,6,7]. The interaction length,
D. Monz´on-Hern´andez et al. / Sensors and Actuators B 110 (2005) 36–40
i.e., the length of fiber exposed to the sample medium, of OFRs based on multimode fibers is typically of a few centimeters. Refractometers based on LPGs or FBGs have also an interaction length of a few centimeters. The interaction length of OFRs based on multimode fibers or Bragg gratings is the thinnest section of the fiber. This may be a drawback because it makes the refractometer fragile and sensitive to bending. To overcome such an inconvenience we propose here the use of cladded multimode tapered fiber tips with mirrored end for RI measurements. This approach allows the development of robust and compact OFRs. Moreover, in situ and remote RI measurements can be carried out with the tips. In addition, very small amounts of liquids, typically a few 100 l is needed to carry out the measurements. The maximum resolution achievable with the tips is around 3 × 10−5 . The fabrication of the tips is simple since it consists of stretching the fiber while it is being heated with a flame. The mirror can be made of a stack of thin films.
2. Operating principle of the fiber refractometer When an optical fiber is stretched while it is being heated with an oscillating flame torch a taper is formed. The tapered fiber has a zone of uniform diameter surrounded by a contracting and an expanding zone [25,26]. If the tapered fiber is cleaved in the middle to identical tips as that schematically shown in Fig. 1(a) are obtained. If the cladding is not removed then the tapering causes that both the cladding and the core diameters are reduced in size. The cladding diameter ρ (units m) is predicted to vary with the elongation distance z (units mm) as [25,26] −z ρ = ρ0 exp (1) 2L where ρ0 (units m) is the initial fiber diameter, typically 125 m, and L (units mm) is the length of oscillation of the flame torch, and also the length of the taper waist, see Fig. 1 (2L0 = L). With our tapering station, z and L (or L0 ) can be
Fig. 1. (a) Illustration of a cladded multimode tapered fiber tip with mirrored end (shadow area). ρ0 , ρ, and L0 are, respectively, the initial fiber diameter, the tip diameter, and the length of the uniform section. (b) Cross-section of the tapered fiber tip. n1 , n2 , n3 , are respectively, the RI of the fiber core, fiber cladding, and external (sample) medium.
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precisely controlled, therefore, tapers with any waist diameter and length can be fabricated [26]. The reduction of the core diameter in a multimode optical fiber makes that some of the guided modes that do not satisfy the condition of total internal reflection radiate from the core. The presence of the cladding, however, makes that the radiated modes from the core can still be guided by the fiber. To do so the RI of the cladding, n2 , has to be higher than that of the external medium, n3 , see Fig. 1. When n3 > n2 all the modes guided by the cladding are leaked but the remaining modes in the core are not. Therefore, the transmission of a cladded multimode tapered fiber (MMTF) is not zero when n3 > n2 . The tapering in a cladded multimode fiber introduces losses, but they are minimal, typically below 0.8 dB. Conversely, radiation losses can be increased as n3 approaches n2 , i.e., when the RI of the sample medium approaches that of the fiber cladding. Thus, a cladded MMTF can be used as a refractometer. In a recent publication we reported some experimental RI measurements with a cladded MMTF [7]. We found that the resolution and dynamic range can be adjusted with the taper waist diameter. Now, we propose the use of tapered fiber tips with mirrored end for RI measurements in very small amounts of volumes. The theoretical transmission of a cladded MMTF as a function of the external RI, n3 , was calculated by Gou and Albin using a simple ray-optic approach [27]. For the calculation, the authors considered a step-index multimode fiber and a parabolic taper; but the RI profile of the fiber core was not taken into account. As a matter of fact, the cladding of an optical fiber is made of a homogeneous material, i.e., with a step-index RI profile. Thus, the model of Gou and Albin can provide good qualitative information on the performance of an OFR based on a cladded MMTF. In Fig. 2 we show the theoretical reflection of cladded MMTF tips with mirrored end with ρ = 37.5, 50, 62.5, and 87.5 m, curves from left to right, respectively, versus n3 .
Fig. 2. Theoretical reflection of a cladded MMTF tip with mirrored end vs. n3 . The curves shown from left to right correspond, respectively, to tips with ρ = 37.5, 50, 62.5, and 87.5 m. A step-index multimode fiber with core and cladding diameters, respectively, of 50 and 125 m is considered. The following values were assumed: L0 = 2 mm, n1 = 1.469, and n2 = 1.459.
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For the simulation the analysis reported in Ref. [27] was considered and the following values were assumed: L0 = 2 mm, n1 = 1.469, n2 = 1.459, and ρ0 = 125 m (see Fig. 1). The reflection of the tapered fiber tips was normalized to the reflection of the tips when n3 = 1. For the results shown in Fig. 2 the contracting zone of the taper was taken into account and the mirror was assumed to be 100% reflecting. Note from Fig. 2 that the slope of the curves is more pronounced for thicker tips than that for thinner tips. On the other hand, the range of measurable indices is narrow for thicker tips than that of thinner tips. Note also that when n3 > n2 the reflection of the tips reaches a stable value since the light guided by the core never radiates. Thus, it is clear that the diameter of the tips can be used to adjust the dynamic range and resolution (sensitivity) of our refractometers. The tip diameter in turn can be precisely controlled during the tapering process [26]. It is important to point out that the dynamic range and resolution of refractometers based on FBGs or LPGs cannot be adjusted in a simple manner. Moreover, they require a highresolution spectrum analyzer which increases the cost of the device. Thus, the refractometer proposed here can be better in terms of cost, simplicity, and performance than those based on FBGs or LPGs. We next show some experimental results.
diameter. An output arm of the coupler delivered light to a photodetector (OZ Optics model SDH-IR) used as reference and the other output arm delivered light to the tip. The reflected light from the tip passed again by the coupler and was monitored in the other input arm of the coupler with another photodetector (OZ Optics model SDH-IR). The signal was taken as the ratio of the reflected power from the tip to the reference power. In this way, fluctuations of the optical source were cancelled. When the external medium was air the signal was assumed to be one. In Fig. 3(a) we show experimental results obtained with two tapered fiber tips with mirrored end. The curve with squares was obtained with a sample with ρ = 80 m, and the one with dots with a sample of ρ = 60 m. In both cases L0 was 2 mm. Only the uniform section of the tips (of length L0 ) was immersed into the Cargille oils. Between consecutive measurements the tips were cleaned with acetone and dried with air to avoid contamination of the oils. The measurements were carried out at room temperature and the volumes of the oils were typically a few 100 l. Note that the experimental results shown in Fig. 3(a) are not in total accordance with the theoretical results shown in Fig. 2. The discrepancy is because in the theoretical results all the tapered section of the fiber is taken into account. Also no losses are assumed
3. Results and discussion The fabrication of uniform-waist tapered fibers has been reported by the authors in a previous publication [26]. Basically, the technique consists of stretching a cladded fiber while it is heated with an oscillating flame torch. Once the uniform-waist tapered fibers are fabricated they are cleaved in the middle. In this way, two identical tips like that shown schematically in Fig. 1(a) are obtained. The tips are then protected with plastics, or polymers, except the cleaved end. In an evaporation chamber the mirrored end was achieved by depositing first a 3 nm thick chromium layer, then a 100 nm thick gold layer, and finally a 60 nm thick SiO2 layer. After evaporation the protection of the tips was removed and the mirrors were tested when the external medium was air. Only tips with highly reflecting mirrors were used for RI measurements. Note that a number of samples can be fabricated in a single process since the tips occupy a small space inside an evaporation chamber. The fiber employed to fabricate our samples was conventional graded-index multimode optical fiber with core and cladding diameters, respectively, of 50 and 125 m. The external (sample) medium was Cargille oils with calibrated RI (calibrated at a wavelength of 519 nm and 25 ◦ C). The light source employed for the measurements was an LED, with peak emission at 850 nm, and 20 W of optical power. The LED source was packaged in an ST receptacle, which ensured us a high coupling efficiency and lunching stability. Light from the LED was injected into one input arm of a 3 dB 2 × 2 coupler by using a commercial ST connector. The coupler was made also of graded-index fiber with 50 m core
Fig. 3. (a) Normalized reflection of two cladded MMTF tips of ρ = 80 m (dashed line), and 60 m (solid line) vs. the external refractive index. (b) Refractive index change of 4 × 10−4 monitored with a 60 m-thick tip.
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and a step-index multimode fiber is considered for the simulation. During the evaporation some residual material might be deposited over the sensing area, which can affect the performance of our OFRs. Also the mirrors were not 100% reflecting owing to some technical limitations. The results of Fig. 3(a) do confirm that when n3 > n2 the reflectance has a constant value and is not zero. Fig. 3(b) shows a RI change (n) of 4 × 10−4 obtained with a tip of ρ = 60 m and L0 = 2 mm. The reflected power as a function of time was recorded when the tip was immersed into 400 l of an oil with RI = 1.454. Approximately 200 s later 100 l of another oil with RI = 1.452 was added to the tip. According to the well-known Lorentz–Lorenz formula the mixture had a RI = 1.4536, i.e., n = 4 × 10−4 . We chose the above indexes because they are in the zone of maximum slope of the curve shown in Fig. 3(a). A decrement of 4 × 10−4 in the RI caused an increment in the reflected power of 1.5%, clearly shown in Fig. 3(b). The fluctuations of the LED employed for the measurements were around 0.1%. This means that in the linear range the minimum RI change that can be detected with our 60 m-thick OFR is about 3 × 10−5 . With thicker tips probably smaller RI changes can be detected. Finally, in Fig. 4 we show the transmission output as a function of n3 for three cladded MMTFs of different waist diameter. The curves shown in the figure form left to right were obtained, respectively, with samples of ρ = 40, 60, and 80 m. All the tapered fibers had an L = 4 mm and the whole tapered section (from 6 to 13 mm) was immersed into the Cargille oils. The fiber employed was also graded-index fiber with core diameter of 50 m but from a different manufacturer. Note that the maximum slope is around n3 = 1.44 and not around 1.45 as in Fig. 3(a). The discrepancy is caused by the cladding RI. Thus, the cladding RI can be used to tune the range of maximum resolution of an OFR. The experimental results shown in Fig. 4 show that with thinner tips the range of measurable indices is broader than
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that of thicker tips. Also that thicker tips may offer higher resolution than thinner tips. These two features of refractometers based on cladded MMTF tips are in agreement with the theoretical results shown in Fig. 2. This was the main reason why we carried out measurements with our tapers in the transmission mode. Therefore, from theoretical and experimental results we can conclude that the diameter of the tips, ρ, can be an adjusting parameter of the resolution and dynamic range of our refractometers. To end this section we would like to point out that the refractometers proposed here can be used to develop different sensors. Recall that the RI may change with different parameters, such as temperature, pressure, concentration, and so forth. Therefore, our tips coated with a thin film made of a variable-index material may be used as a miniature temperature sensor, concentration sensor, etc. Improvements on the fabrication of the mirrors and applications of our tips as sensors are in a planning stage. 4. Conclusions Cladded multimode tapered fiber tips are proposed for refractive index measurements. The fabrication of such tips is simple and repetible. It consists of stretching the optical fiber while it is being heated with an oscillating flame torch. Then the tapered fibers are cleaved in the middle and a mirror is deposited on the end of the tips. The length of the tips is typically 2 mm and its thickness is less than 80 m. With conventional graded-index multimode fiber, an 850 nm LED, and a simple detection scheme we have demonstrated some simple and versatile refractometers. Experimentally, it was demonstrated that a refractive index change of 4 × 10−4 can be detected with no especial effort with a 60 m-thick fiber tip, even though the mirrors had not the desired quality. The maximum resolution, estimate to be of 3 × 10−5 , is achieved for indices that are close to that of the fiber cladding. The diameter of the tips and the refractive index of the fiber cladding can be used to adjust the dynamic range and the region of maximum resolution of the proposed refractometers. The cost of the refractometers proposed here is very low since all the components are very inexpensive and widely available. Acknowledgements J. Villatoro is grateful to Concejo Estatal de Ciencia y Tecnolog´ıa de Guanajuato for financial support under project 04-04-K117-011. The authors acknowledge the assistance of Carlos Ju´arez in sample fabrication.
Fig. 4. Experimental transmission of three cladded MMTFs as a function of the external refractive index. Squares, triangles, and dots are experimental points obtained with samples of ρ = 40, 60, and 80 m, respectively. The continuous lines are fitting lines. The fiber employed was graded-index multimode fiber with core diameter of 50 m.
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[2] M.D. Marazuela, M.C. Moreno-Bondi, Fiber optic biosensors –an overview, Anal. Bioanal. Chem. 372 (2002) 664–682. [3] F.S. Ligler, C.A.R. Taitt (Eds.), Optical Biosensors: Present and Future, Elsevier, Amsterdam, The Netherlands, 2002. [4] T. Takeo, H. Hattori, Silica glass fiber photorefractometer, Appl. Opt. 31 (1992) 44–50. [5] W. Johnstone, G. Thusrby, D. Moodie, K. McCallion, Fiber-optic refractometer that utilizes multimode overlay devices, Opt. Lett. 17 (1992) 1538–1540. [6] J. Zubia, G. Garitaonaindia, J. Arrue, Passive device based on plastic optical fibers to determine the indices of refraction of liquids, Appl. Opt. 39 (2000) 941–946. [7] J. Villatoro, D. Monzon-Hernandez, D. Talavera, High resolution refractive index sensing with cladded multimode tapered optical fibre, Electron. Lett. 40 (2004) 106–107. [8] A. Asseh, S. Sandgren, H. Ahlfeldt, B. Sahlgren, R. Stubbe, G. Edwall, Fiber optical Bragg grating refractometer, Fiber Int. Opt. 17 (1998) 51–62. [9] K. Schroeder, W. Ecke, R. Mueller, R. Willsch, A. Andreev, A fibre Bragg grating refractometer, Meas. Sci. Technol. 12 (2001) 757–764. [10] X. Shu, B.A.L. Gwandu, L. Zhang, I. Bennion, Sampled fibre Bragg grating for simultaneous refractive-index and temperature measurement, Opt. Lett. 26 (2001) 774–776. [11] G. Laggont, P. Ferdinand, Tilted short-period fibre-Bragg-gratinginduced coupling to cladding modes for accurate refractometry, Meas. Sci. Technol. 12 (2001) 765–770. [12] S. Keren, M. Horowitz, Distributed three-dimensional fiber Bragg refractometer for biochemical sensing, Opt. Lett. 28 (2003) 2037–2039. [13] V. Bathia, A.M. Vengsarkar, Optical fiber long period grating sensors, Opt. Lett. 21 (1996) 692–694. [14] B.H. Lee, Y. Liu, S.B. Lee, S.S. Choi, J.N. Jang, Displacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive index, Opt. Lett. 22 (1997) 1769–1771. [15] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period gratings to external index of refraction, J. Lightwave Technol. 16 (1998) 1606–1612. [16] X. Shu, X. Zhu, S. Jiang, W. Shi, D. Huang, High sensitivity of dual resonance peaks of long-period fibre grating to surrounding refractive index changes, Electron. Lett. 35 (1999) 1580–1581. [17] V. Bhatia, Applications of long-period gratings to single and multiparameter sensing, Opt. Express 4 (1999) 457–466. [18] R. Falciai, A.G. Mignani, A. Vannini, Long period gratings as solution concentration sensors, Sens. Actuators B 74 (2001) 74–77. [19] R. Alonso, F. Villuendas, J. Tornos, J. Pelayo, New in-line optical fiber sensors based on surface plasmon excitation, Sens. Actuators A 37/38 (1993) 187–192. [20] L. De Maria, M.G. Martenelli, G. Vegetti, Fibre optic sensor based on surface plasmon interrogation, Sens. Actuators B 12 (1993) 221–223. [21] A.J.C. Tubb, F.P. Payne, R.B. Millington, C.R. Lowe, Single-mode optical fibre surface plasma wave chemical sensor, Sens. Actuators B 41 (1997) 71–79. [22] R. Slavik, J. Homola, J. Ctyroky, Single-mode optical fiber surface plasmon resonance sensor, Sens. Actuators B 54 (1999) 74–79.
[23] A. D´ıez, M.V. Andr´es, J.L. Cruz, Hybrid surface plasma modes in circular metal-coated tapered fibers, J. Opt. Soc. Am. A 16 (1999) 2978–2982. [24] D. Monzon-Hernandez, J. Villatoro, D. Talavera, D. Luna-Moreno, Optical fiber surface plasmon resonance sensor with multiple resonance peaks, Appl. Opt. 43 (2004) 1216–1220. [25] T.A. Birks, Y.W. Li, The shape of fiber tapers, IEEE J. Lightwave Technol. 10 (1992) 432–438. [26] J. Villatoro, D. Monzon-Hernandez, E. Mejia, Fabrication and modeling of uniform-waist singlemode tapered optical fiber sensors, Appl. Opt. 42 (2003) 2278–2283. [27] S. Gou, S. Albin, Transmission property and evanescent wave absorption of cladded multimode fiber tapers, Opt. Express 11 (2003) 215–223.
Biographies David Monz´on-Hern´andez received the Engineering degree in Communications and Electronics, in 1993, from the University of Guanajuato, Guanajuato, Mexico, the M.Sc. and the Ph.D. degrees in optics from the Centro de Investigaciones en Optica A. C., Leon, Mexico, in 1995 and 1999, respectively. In the year 2000, he joined the Applied Physics Department, University of Valencia, Valencia, Spain, as a Posdoctoral Fellow. Since March 2002 he has been a Research Associate with the Fiber Optics Department at the Centro de Investigaciones en Optica A. C. His main research interests are optical sensors based on fiber Bragg gratings and evanescent field interactions. Joel Villatoro received the B.Sc. degree in physics from the Autonomous University of Puebla, Puebla, Mexico, in 1992, and the M.Sc. and Ph.D. degrees in optics from the National Institute for Astrophysics, Optics, and Electronics, Puebla, Mexico, in 1995 and 1999, respectively. He was a Visiting Scholar for two-and-a-half years at the Centro de Instrumentos of the National Autonomous University of Mexico, Mexico City. In 1999, he joined Case Western Reserve University, Cleveland OH, as a Research Associate to work on a project on optical fiber hydrogen sensors supported by the Boeing Corporation, Seattle, WA. One year later, he joined the Applied Physics Department, University of Valencia, Valencia, Spain, as a Postdoctoral Fellow. Since November 2001, he has been a Research Scientist with the Fiber Optics Department at the Centro de Investigaciones en Optica A. C., Leon, Mexico. He teaches Fiber Optics, Optical Fiber Sensors, and Optoelectronics at undergraduated and graduated levels. His main research interests are optical fiber sensors and devices and optical sensing techniques. J. Villatoro is a member of SPIE. Donato Luna-Moreno received the B.Sc. degree in physics from the Universidad de Guadalajara, Guadalajara, Mexico, in 1988, the M.Sc. degree in optics from the Centro de Investigaciones en Optica, A. C., Leon, Mexico, in 1991, and the Ph.D. degree in optics from the Instituto Nacional de Astrof´ısica, Optica y Electr´onica, Puebla, M´exico, in 1997. Since May 1998, he has been a Research Scientist with the Photonics Department at the Centro de Investigaciones en Optica A. C. His research interests include thin films, holographic materials, and optical sensors.
Miniature optical fiber refractometer using cladded multimode tapered fiber tips David Monz´on-Hern´andez, Joel Villatoro ∗ , Donato Luna-Moreno Centro de Investigaciones en Optica A. C., Loma del Bosque 115, Leon GTO. 37150, Mexico Received 20 May 2004; received in revised form 14 December 2004; accepted 10 January 2005 Available online 10 February 2005
Abstract Cladded multimode tapered fiber tips with mirrored end are proposed as refractometers. The working mechanism of such devices is based on radiation of the modes guided by the cladding, which is induced by the refractive index of the sample medium. Using conventional gradedindex multimode fiber, a low-power 850 nm LED, and a simple detection scheme some inexpensive, robust, and versatile refractometers are demonstrated. The length of the tips is typically 2 mm, while their diameter is less than 80 m. The refractometers can cover a refractive index range between 1.36 and 1.46 with resolution up to 3 × 10−5 . © 2005 Elsevier B.V. All rights reserved. Keywords: Refractive index sensing; Optical fiber sensors; Evanescent field sensors
1. Introduction The refractive index (RI) is a fundamental material property. It may vary with different parameters, such as, temperature, concentration, pressure, etc. That is why, measurements of RI are very important. In a number of chemical, food, beverage, or medical analysis conventional bulk refractometers are employed. However, bulk refractometers are inappropriate for remote RI measurements, or in applications where size and weight are a concern. Bulk refractometers are not convenient for in situ RI measurements. Neither for measurements in very small spaces (volumes on the order of microliters), nor in applications where a very high resolution is required. In bio-sensing, for example, there is a need to monitor RI changes on the order of 10−4 to 10−7 which may be caused by molecular binding, chemical or biochemical reactions, or by changes suffered by a thin biolayer [1–3]. Optical fiber refractometers (OFRs) are an alternative to conventional bulk refractometers. OFRs have some advantages over their bulk counterparts, such as high reso∗
Corresponding author. Tel.: +52 477 773 1017; fax: +52 477 717 5000. E-mail address: [email protected] (J. Villatoro).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.01.011
lution, low cost, light weight, and multiplexing possibilities. Moreover, with OFRs in situ and remote RI measurements can be carried out. Several OFRs based on side-polished single-mode fiber, core-exposed or tapered multimode fibers, fiber Bragg gratings (FBGs), or long period gratings (LPGs) have been reported so far [4–18]. The resolution, i.e., the minimum RI change that can be measured, of the majority of the fiber-based refractometers reported so far is typically between 10−3 and 10−5 [4–18]. Their dynamic range, i.e., the range of RI values that can be measured, is typically from 1.33 to 1.46. Both, the dynamic range and resolution of OFRs are good enough for the most popular industrial applications. OFRs based on the phenomenon of surface plasmon resonance (SPR) can offer higher resolution (from 10−4 to 10−7 ), however, their dynamic range is very limited [19–24]. Moreover, a thin metallic film needed in SPR-based OFRs may suffer changes (in thickness or optical properties) due to reactions with the liquid sample. The simplest and, therefore, the lowest-cost OFRs are those based on multimode fibers since they require conventional fiber, a low-power LED, and a simple detection scheme [4,6,7]. In addition, their resolution and dynamic range may be adjusted in a simple manner [4,6,7]. The interaction length,
D. Monz´on-Hern´andez et al. / Sensors and Actuators B 110 (2005) 36–40
i.e., the length of fiber exposed to the sample medium, of OFRs based on multimode fibers is typically of a few centimeters. Refractometers based on LPGs or FBGs have also an interaction length of a few centimeters. The interaction length of OFRs based on multimode fibers or Bragg gratings is the thinnest section of the fiber. This may be a drawback because it makes the refractometer fragile and sensitive to bending. To overcome such an inconvenience we propose here the use of cladded multimode tapered fiber tips with mirrored end for RI measurements. This approach allows the development of robust and compact OFRs. Moreover, in situ and remote RI measurements can be carried out with the tips. In addition, very small amounts of liquids, typically a few 100 l is needed to carry out the measurements. The maximum resolution achievable with the tips is around 3 × 10−5 . The fabrication of the tips is simple since it consists of stretching the fiber while it is being heated with a flame. The mirror can be made of a stack of thin films.
2. Operating principle of the fiber refractometer When an optical fiber is stretched while it is being heated with an oscillating flame torch a taper is formed. The tapered fiber has a zone of uniform diameter surrounded by a contracting and an expanding zone [25,26]. If the tapered fiber is cleaved in the middle to identical tips as that schematically shown in Fig. 1(a) are obtained. If the cladding is not removed then the tapering causes that both the cladding and the core diameters are reduced in size. The cladding diameter ρ (units m) is predicted to vary with the elongation distance z (units mm) as [25,26] −z ρ = ρ0 exp (1) 2L where ρ0 (units m) is the initial fiber diameter, typically 125 m, and L (units mm) is the length of oscillation of the flame torch, and also the length of the taper waist, see Fig. 1 (2L0 = L). With our tapering station, z and L (or L0 ) can be
Fig. 1. (a) Illustration of a cladded multimode tapered fiber tip with mirrored end (shadow area). ρ0 , ρ, and L0 are, respectively, the initial fiber diameter, the tip diameter, and the length of the uniform section. (b) Cross-section of the tapered fiber tip. n1 , n2 , n3 , are respectively, the RI of the fiber core, fiber cladding, and external (sample) medium.
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precisely controlled, therefore, tapers with any waist diameter and length can be fabricated [26]. The reduction of the core diameter in a multimode optical fiber makes that some of the guided modes that do not satisfy the condition of total internal reflection radiate from the core. The presence of the cladding, however, makes that the radiated modes from the core can still be guided by the fiber. To do so the RI of the cladding, n2 , has to be higher than that of the external medium, n3 , see Fig. 1. When n3 > n2 all the modes guided by the cladding are leaked but the remaining modes in the core are not. Therefore, the transmission of a cladded multimode tapered fiber (MMTF) is not zero when n3 > n2 . The tapering in a cladded multimode fiber introduces losses, but they are minimal, typically below 0.8 dB. Conversely, radiation losses can be increased as n3 approaches n2 , i.e., when the RI of the sample medium approaches that of the fiber cladding. Thus, a cladded MMTF can be used as a refractometer. In a recent publication we reported some experimental RI measurements with a cladded MMTF [7]. We found that the resolution and dynamic range can be adjusted with the taper waist diameter. Now, we propose the use of tapered fiber tips with mirrored end for RI measurements in very small amounts of volumes. The theoretical transmission of a cladded MMTF as a function of the external RI, n3 , was calculated by Gou and Albin using a simple ray-optic approach [27]. For the calculation, the authors considered a step-index multimode fiber and a parabolic taper; but the RI profile of the fiber core was not taken into account. As a matter of fact, the cladding of an optical fiber is made of a homogeneous material, i.e., with a step-index RI profile. Thus, the model of Gou and Albin can provide good qualitative information on the performance of an OFR based on a cladded MMTF. In Fig. 2 we show the theoretical reflection of cladded MMTF tips with mirrored end with ρ = 37.5, 50, 62.5, and 87.5 m, curves from left to right, respectively, versus n3 .
Fig. 2. Theoretical reflection of a cladded MMTF tip with mirrored end vs. n3 . The curves shown from left to right correspond, respectively, to tips with ρ = 37.5, 50, 62.5, and 87.5 m. A step-index multimode fiber with core and cladding diameters, respectively, of 50 and 125 m is considered. The following values were assumed: L0 = 2 mm, n1 = 1.469, and n2 = 1.459.
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For the simulation the analysis reported in Ref. [27] was considered and the following values were assumed: L0 = 2 mm, n1 = 1.469, n2 = 1.459, and ρ0 = 125 m (see Fig. 1). The reflection of the tapered fiber tips was normalized to the reflection of the tips when n3 = 1. For the results shown in Fig. 2 the contracting zone of the taper was taken into account and the mirror was assumed to be 100% reflecting. Note from Fig. 2 that the slope of the curves is more pronounced for thicker tips than that for thinner tips. On the other hand, the range of measurable indices is narrow for thicker tips than that of thinner tips. Note also that when n3 > n2 the reflection of the tips reaches a stable value since the light guided by the core never radiates. Thus, it is clear that the diameter of the tips can be used to adjust the dynamic range and resolution (sensitivity) of our refractometers. The tip diameter in turn can be precisely controlled during the tapering process [26]. It is important to point out that the dynamic range and resolution of refractometers based on FBGs or LPGs cannot be adjusted in a simple manner. Moreover, they require a highresolution spectrum analyzer which increases the cost of the device. Thus, the refractometer proposed here can be better in terms of cost, simplicity, and performance than those based on FBGs or LPGs. We next show some experimental results.
diameter. An output arm of the coupler delivered light to a photodetector (OZ Optics model SDH-IR) used as reference and the other output arm delivered light to the tip. The reflected light from the tip passed again by the coupler and was monitored in the other input arm of the coupler with another photodetector (OZ Optics model SDH-IR). The signal was taken as the ratio of the reflected power from the tip to the reference power. In this way, fluctuations of the optical source were cancelled. When the external medium was air the signal was assumed to be one. In Fig. 3(a) we show experimental results obtained with two tapered fiber tips with mirrored end. The curve with squares was obtained with a sample with ρ = 80 m, and the one with dots with a sample of ρ = 60 m. In both cases L0 was 2 mm. Only the uniform section of the tips (of length L0 ) was immersed into the Cargille oils. Between consecutive measurements the tips were cleaned with acetone and dried with air to avoid contamination of the oils. The measurements were carried out at room temperature and the volumes of the oils were typically a few 100 l. Note that the experimental results shown in Fig. 3(a) are not in total accordance with the theoretical results shown in Fig. 2. The discrepancy is because in the theoretical results all the tapered section of the fiber is taken into account. Also no losses are assumed
3. Results and discussion The fabrication of uniform-waist tapered fibers has been reported by the authors in a previous publication [26]. Basically, the technique consists of stretching a cladded fiber while it is heated with an oscillating flame torch. Once the uniform-waist tapered fibers are fabricated they are cleaved in the middle. In this way, two identical tips like that shown schematically in Fig. 1(a) are obtained. The tips are then protected with plastics, or polymers, except the cleaved end. In an evaporation chamber the mirrored end was achieved by depositing first a 3 nm thick chromium layer, then a 100 nm thick gold layer, and finally a 60 nm thick SiO2 layer. After evaporation the protection of the tips was removed and the mirrors were tested when the external medium was air. Only tips with highly reflecting mirrors were used for RI measurements. Note that a number of samples can be fabricated in a single process since the tips occupy a small space inside an evaporation chamber. The fiber employed to fabricate our samples was conventional graded-index multimode optical fiber with core and cladding diameters, respectively, of 50 and 125 m. The external (sample) medium was Cargille oils with calibrated RI (calibrated at a wavelength of 519 nm and 25 ◦ C). The light source employed for the measurements was an LED, with peak emission at 850 nm, and 20 W of optical power. The LED source was packaged in an ST receptacle, which ensured us a high coupling efficiency and lunching stability. Light from the LED was injected into one input arm of a 3 dB 2 × 2 coupler by using a commercial ST connector. The coupler was made also of graded-index fiber with 50 m core
Fig. 3. (a) Normalized reflection of two cladded MMTF tips of ρ = 80 m (dashed line), and 60 m (solid line) vs. the external refractive index. (b) Refractive index change of 4 × 10−4 monitored with a 60 m-thick tip.
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and a step-index multimode fiber is considered for the simulation. During the evaporation some residual material might be deposited over the sensing area, which can affect the performance of our OFRs. Also the mirrors were not 100% reflecting owing to some technical limitations. The results of Fig. 3(a) do confirm that when n3 > n2 the reflectance has a constant value and is not zero. Fig. 3(b) shows a RI change (n) of 4 × 10−4 obtained with a tip of ρ = 60 m and L0 = 2 mm. The reflected power as a function of time was recorded when the tip was immersed into 400 l of an oil with RI = 1.454. Approximately 200 s later 100 l of another oil with RI = 1.452 was added to the tip. According to the well-known Lorentz–Lorenz formula the mixture had a RI = 1.4536, i.e., n = 4 × 10−4 . We chose the above indexes because they are in the zone of maximum slope of the curve shown in Fig. 3(a). A decrement of 4 × 10−4 in the RI caused an increment in the reflected power of 1.5%, clearly shown in Fig. 3(b). The fluctuations of the LED employed for the measurements were around 0.1%. This means that in the linear range the minimum RI change that can be detected with our 60 m-thick OFR is about 3 × 10−5 . With thicker tips probably smaller RI changes can be detected. Finally, in Fig. 4 we show the transmission output as a function of n3 for three cladded MMTFs of different waist diameter. The curves shown in the figure form left to right were obtained, respectively, with samples of ρ = 40, 60, and 80 m. All the tapered fibers had an L = 4 mm and the whole tapered section (from 6 to 13 mm) was immersed into the Cargille oils. The fiber employed was also graded-index fiber with core diameter of 50 m but from a different manufacturer. Note that the maximum slope is around n3 = 1.44 and not around 1.45 as in Fig. 3(a). The discrepancy is caused by the cladding RI. Thus, the cladding RI can be used to tune the range of maximum resolution of an OFR. The experimental results shown in Fig. 4 show that with thinner tips the range of measurable indices is broader than
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that of thicker tips. Also that thicker tips may offer higher resolution than thinner tips. These two features of refractometers based on cladded MMTF tips are in agreement with the theoretical results shown in Fig. 2. This was the main reason why we carried out measurements with our tapers in the transmission mode. Therefore, from theoretical and experimental results we can conclude that the diameter of the tips, ρ, can be an adjusting parameter of the resolution and dynamic range of our refractometers. To end this section we would like to point out that the refractometers proposed here can be used to develop different sensors. Recall that the RI may change with different parameters, such as temperature, pressure, concentration, and so forth. Therefore, our tips coated with a thin film made of a variable-index material may be used as a miniature temperature sensor, concentration sensor, etc. Improvements on the fabrication of the mirrors and applications of our tips as sensors are in a planning stage. 4. Conclusions Cladded multimode tapered fiber tips are proposed for refractive index measurements. The fabrication of such tips is simple and repetible. It consists of stretching the optical fiber while it is being heated with an oscillating flame torch. Then the tapered fibers are cleaved in the middle and a mirror is deposited on the end of the tips. The length of the tips is typically 2 mm and its thickness is less than 80 m. With conventional graded-index multimode fiber, an 850 nm LED, and a simple detection scheme we have demonstrated some simple and versatile refractometers. Experimentally, it was demonstrated that a refractive index change of 4 × 10−4 can be detected with no especial effort with a 60 m-thick fiber tip, even though the mirrors had not the desired quality. The maximum resolution, estimate to be of 3 × 10−5 , is achieved for indices that are close to that of the fiber cladding. The diameter of the tips and the refractive index of the fiber cladding can be used to adjust the dynamic range and the region of maximum resolution of the proposed refractometers. The cost of the refractometers proposed here is very low since all the components are very inexpensive and widely available. Acknowledgements J. Villatoro is grateful to Concejo Estatal de Ciencia y Tecnolog´ıa de Guanajuato for financial support under project 04-04-K117-011. The authors acknowledge the assistance of Carlos Ju´arez in sample fabrication.
Fig. 4. Experimental transmission of three cladded MMTFs as a function of the external refractive index. Squares, triangles, and dots are experimental points obtained with samples of ρ = 40, 60, and 80 m, respectively. The continuous lines are fitting lines. The fiber employed was graded-index multimode fiber with core diameter of 50 m.
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Biographies David Monz´on-Hern´andez received the Engineering degree in Communications and Electronics, in 1993, from the University of Guanajuato, Guanajuato, Mexico, the M.Sc. and the Ph.D. degrees in optics from the Centro de Investigaciones en Optica A. C., Leon, Mexico, in 1995 and 1999, respectively. In the year 2000, he joined the Applied Physics Department, University of Valencia, Valencia, Spain, as a Posdoctoral Fellow. Since March 2002 he has been a Research Associate with the Fiber Optics Department at the Centro de Investigaciones en Optica A. C. His main research interests are optical sensors based on fiber Bragg gratings and evanescent field interactions. Joel Villatoro received the B.Sc. degree in physics from the Autonomous University of Puebla, Puebla, Mexico, in 1992, and the M.Sc. and Ph.D. degrees in optics from the National Institute for Astrophysics, Optics, and Electronics, Puebla, Mexico, in 1995 and 1999, respectively. He was a Visiting Scholar for two-and-a-half years at the Centro de Instrumentos of the National Autonomous University of Mexico, Mexico City. In 1999, he joined Case Western Reserve University, Cleveland OH, as a Research Associate to work on a project on optical fiber hydrogen sensors supported by the Boeing Corporation, Seattle, WA. One year later, he joined the Applied Physics Department, University of Valencia, Valencia, Spain, as a Postdoctoral Fellow. Since November 2001, he has been a Research Scientist with the Fiber Optics Department at the Centro de Investigaciones en Optica A. C., Leon, Mexico. He teaches Fiber Optics, Optical Fiber Sensors, and Optoelectronics at undergraduated and graduated levels. His main research interests are optical fiber sensors and devices and optical sensing techniques. J. Villatoro is a member of SPIE. Donato Luna-Moreno received the B.Sc. degree in physics from the Universidad de Guadalajara, Guadalajara, Mexico, in 1988, the M.Sc. degree in optics from the Centro de Investigaciones en Optica, A. C., Leon, Mexico, in 1991, and the Ph.D. degree in optics from the Instituto Nacional de Astrof´ısica, Optica y Electr´onica, Puebla, M´exico, in 1997. Since May 1998, he has been a Research Scientist with the Photonics Department at the Centro de Investigaciones en Optica A. C. His research interests include thin films, holographic materials, and optical sensors.