Improvement of the sectorial fiber for evanescent-wave sensing

Improvement of the sectorial fiber for evanescent-wave sensing
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  ELSEVIER Sensors nd Actuators 3 38-39 (1997) 334-338 B CHEMICAL Improvement of the sectorial fiber for evanescent-wave sensing Vlastimil MaEjec *, Miroslav ChomBt, Milo8 Hayer, Daniela BerkovB, Marie PospGilov6, Ivan Kas’ilc Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Chaberska 57, CZI82 Sl Prague 8, Czech Republic Abstract Several problems associated with a fiber of sectorial cross section for evanescent-wave sensing, the s-fiber, have been addressed and the results achieved are reported. The approximate theoretical analysis of the s-fiber sensitivity has been extended from single-mode to low- multimode fibers with the aim of determining the influence of the fiber shape and Iaunching conditions. The technological research has been aimed at fabricating preforms of the s-fibers with a structure as close as possible to the theoretical one. The drawing temperature has been optimized to 1900°C. A novel s-fiber structure making possible its excitation by a low-multimode fiber joined to the s-fiber incorporated in a matched-size circular capillary, the capillary s-fiber, has been developed. The evanescent-wave sensitivity of the fibers has been determined by immersing the fiber core in aqueous solutions of methylene blue. Keywords: Evanescent-wave ensing; Sectorial ibers; Capillary sectorial ibers 1. Introduction Several single-mode and multimode silica fiber-optic structures providing improved access o the evanescent ield or increasing the fraction of optical power in it have been investigated for evanescent-wave sensing. These structures include tapered single-mode and multimode fibers [ 11, D- shaped ibers 121, sectorial fibers f3], etc. Much effort has been devoted to and promising results achieved with single- mode D-shaped fibers. A fraction of power transmitted in the evanescent ield equal to about 0.2 has been reported [2]. Recently, the theoretical analysis, fabrication and proper- ties of single-mode and few-mode sectorial fibers, s-fibers, have been published [ 31. The theoretical analysis of single- mode sectorial fibers has shown that, in comparison with D- fibers, these ibers provide improved access o the evanescent field. Advantages and drawbacks of few-mode s-fibers have been discussed and preliminary results on the fabrication and study of properties of few-mode s-fibers have been given. In this paper the results of the approximate theoretical analysis of low-multimode s-fibers are reported as well as he results of improving the technology of fabrication of sectorial fibers. A novel sensing structure, a few-mode capillary sec- torial fiber and the results of immersing the fiber cores in aqueous solutions of methylene blue are reported, too. * Corresponding uthor. Phone: +420 2 344 105. Fax: + 420 2 688 0222. E-mail: 0925-4005/97/ 17.00 6 1997 Elsevier Science S.A. All rights reserved PIIs0925-4005(97)00035-x 2. Theoretical analysis The power attenuation coefficient of the s-fiber, yS, was determined on the basis of the ray optics. Details of the d.eter- mination and results are published elsewhere [4]. In this paper starting assumptions and the resulting equation are shown. With reference o Figs. 1 and 2 the assumptions can be summarized as follows: l weakly guiding, step-index, circular-core multimode fiber (I- 1) 0 weak optical absorption of the sensed medium (k < 1) l refractive-index matching ( n3 = nz) sens \ “lay Fig. 1. The structure of the s-fiber. ig. 1. The structure of the s-fiber.  V. MatCjec et al. /Sensors and Actrtators B 38-39 (1997) 334-338 335 ../ et ......,............. . ._........... ,sz Fig. 2. Directions of light rays in the core of the s-fiber. only meridional rays considered polarization dependencies neglected angle of the bound rays inside the fiber, 0,, not very close to the complementary critical angle 19, < 1 exponential decrease of the evanescent ield with distance in the sensed medium a beam of rays with a very small angular width launched into the fiber (6< 1, coupling of a tilted narrow illumi- nation cone [ 51) . Starting from these assumptions and expressions given by Snyder and Love [6] and Katz et al. [5] Eq. ( 1) has been derived: Its graphical representation is shown in Fig. 3. In Figs. 1-3 and Eq. (I), IY is the power absorption coefficient of the sensed medium ( LY 4&/h), V is the normalized frequency of the s-fiber, R is the angle of opening of the s-fiber (a= V- p), p is the vertex angle of the s-fiber, a is the radius of the fiber core, n1 is the core refractive index, n2 is the cladding refractive index, n3 + jk is the complex refractive index of the sensed medium and h( rp) is the thickness of the fiber cladding at an angle rp. For the estimate of the output power P,,, from an s-fiber, an angular distribution of the input power PO launched into the fiber in a cone with vertex angle 20, which is a linear function of @< 0, is assumed. Using this assumption and the equation 1 Pm= PO I exp( - rsl) d(eJ&) (2) 0 Fig. 3. Dependence of the attenuation coefficient of the s-fiber ‘yS on the angle of bound rays 0, for two values of the angle of opening LI (value a= 0 belongs to the D-fiber). changes of PO,, which are induced by immersing a segment of the fiber with the length 1 n a medium with the bulk power absorption coefficient cy can be determined [7]. 3. Experiments and results 3.1. Improvement of the shape of a sectorial preform With the aim of decreasing the thickness of the optical cladding in the vertex region of the s-fiber, a technique has been developed based on very accurate grinding and polish- ing of a standard circular preform to the sectorial shape. For this purpose, a circular preform with core refractive index n1 = 1.4635 ( SiOJGe02) and cladding refractive index n2 = 1.4565 ( Si02/P20JF) was used. The cross section of a sectorial preform fabricated by this technique is shown in Fig. 4. Fibers with a core diameter of about 30 pm were drawn from these preforms at a temperature of 1900°C and coated with the polysiloxane polymer Sylgard 184. Fibers with the vertex angle /3 equal to about 60” were drawn. On the basis of the refractive indices and the core diameter, value of V= 20 can be determined at 660 nm, which means hat this fiber supports propagation of about 200 guided modes f73. 3.2. Development of the capillary sectorialfiber Due to the sectorial shape of the s-fiber, special techniques for exciting the s-fibers, providing them with optical con- nectors or coupling them to standard optical fibers should be adopted. n order to overcome possible imitations following from the use of these techniques, a novel sectorial fiber, the Fig. 4. Photograph of the cross section of the preform for drawing the s-fiber.  336 V. MatZjec et al. /Sensors and Actuators B 38-39 (1997) 334-338 Fig. 7. Attenuation spectra of methylene blue (MB) with concentrations of 4.4 and 88 ppm wt. measured with the s-liber immersed in the solutions. c MB =4.4 porn Active length 6 cm ; JX,,.,] Fig. 5. Schematic structure of the capillary s-fiber. Fig. 6. Cross section of the fabricated capillary s-fiber taken in an optical microscope on a segment with a length of about 6 mm (the real outer diameter of the glass capillary is equal to about 230 pm). capillary s-fiber, has been developed. n this fiber, the s-fiber is located in a circular capillary so that the center of the core of the s-fiber coincides with the center of the capillary. The fiber with the structure schematically depicted in Fig. 5 has been abricated by the ‘rod in the tube’ method rom a preform of sectorial shape placed inside a circular tube. Fibers with outer diameters from 125 to 400 p,rn have been drawn and were coated with a UV acrylate. The cross section of a fab- ricated capillary s-fiber with a UV acrylate coating is shown in Fig. 6. 3.3. Immersion experiments With the aim of determining the fraction of power in the evanescent ield of the prepared s-fibers, the cores of these fibers were immersed n aqueous solutions of methylene blue In immersion experiments the output intensity was meas- ured using a fiber segment with a length of about 0.5 m and approximately homogeneous excitation of all modes (with numerical aperture of 0.18). The s-fiber was immersed in water or in the solution of methylene blue n a cell or a column of the solution was drawn into the capillary s-fiber and the output intensity was determined. The experimental condi- tions have been reported elsewhere [ 31, The results of the immersion experiments are shown inFigs. 7 and 8 with water as a reference. Using the values of the bulk power absorption coefficients from Table 1, changes of the fiber attenuation Table I Results of theoretical calculations and experiments on the effect of methylene blue on the s-fiber attenuation ~~~~~~~ Wavelength [nm] 725 000 Fig. 8. Results of immersion experiments with the capillary s-fiber. with concentrations of 4.4 and 88 ppm by weight. The values of the bulk power absorption coefficient a of the methylene blue solutions at 610 and 660 nm were determined by meas- uring the bulk attenuation spectra of the solutions with water attenuation as a reference value. These values are shown in the second and third columns of Table 1. C LY dB ppm-t cm-‘] 10 W&J~,,) Id31 Attenuation [dB] hem wt.1 610 nm 660 nm 610 nm 660 nm 610nm 660 nm 4.4 1.04 1.69 0.36 OS6 0.49 0.49 88 0.95 0.91 2.86 2.18 1. 0.94  V. MatEjec et al. /Sensors attd Actuators B 38-39 (1997) 334-338 337 induced by immersing the fiber have been estimated on the basis of Eqs. (1) and (2). These values are shown in the fourth and fifth columns of Table 1. In the sixth and seventh columns, the values measured with the sectorial fiber (Fig. 7) are given. 4. Discussion The curves depicted n Fig. 3 show that a fraction of power in the evanescent ield equal to 0.05-0.07 can be achieved by using the selective excitation of a low-multimode s-fiber if the angle of the bound rays 0, is close to the complementary critical angle 0,. This fraction is about five times higher than that for the D-fiber with the same value of V [4]. It can still be increased by increasing the angle a. To approach the predicted values of the s-fiber sensitivity, an s-fiber with a nearly zero thickness of the optical cladding over the angle 0 should be fabricated. As can be seen rom Fig. 4, this can be achieved by accurate grinding and polish- ing of a standard circular preform. Due to the tendency of the sector-id preform to take on an outer circular shape at drawing temperatures, the circular core of the preform is to some extent deformed during the fiber drawing even in the case of the capillary s-fiber (see Fig. 6), This effect can be decreased by decreasing the drawing temperature. However, there is a relation between the decrease of the drawing temperature below 1900°C and the decrease of the mechanical strength of the fiber. Therefore, the fibers were drawn at a temperature of 1900°C. As one can see from Fig. 6, the capillary s-fiber has an outer circular shape and a coaxial core. By changing the core diameter, low-multimode fibers which support propagation of 100-1000 guided modes or multimode fibers have been fabricated. The dimensions of the core and capillary hole can easily be controlled. These fibers exhibit good mechanical properties and can be coupled to standard circular few-mode silica fibers. The immersion experiments with the s-fiber or capillary s- fiber have shown a nonlinear relationship between the eva- nescent-wave attenuation spectra of the methylene blue and its concentration. There are two reasons for this effect. The first is the nonlinear dependence of the bulk power absorption coefficient of methylene blue on its concentration (Table 1, second and third columns). The second eason results from the nonlinearity caused by the multimode character of the light propagation in the fiber. Payne and HaIe [7] have proved the existence of such a nonlinearity in the case of multimode circular fibers. A similar approach has been employed for the s-fibers (Eqs. (1) and (2)). The compar- ison of the calculated values (Table 1, fourth and fifth col- umns) and measured values (Table 1, sixth and seventh columns) shows a similar agreement between the experiment and theory as in the paper of Payne and Hale [7] . The dif- ference between the calculation and measurements can be explained by differences in the excitation of modes or by the effect of the refractive index of the solutions. As the refractive index of the immersing solution n3 is lower than that of the optical cladding ( 1.32 and 1.4565, respectively), the power in the direct evanescent ield is lower than estimated by Eq. ( 1). As one can deduce rom Eq. ( 1) , the sensitivity of the sectorial fiber can be increased by launching light into the fiber core at an angle close to the critical angle. 5. Conclusions Few-mode and low-multimode sectorial and capihary sec- torial fibers represent fiber-optic structures suitable for eva- nescent-wave sensing. Satisfactory mechanical strength, easy excitation and the possibility of connecting with standard optical connectors are some of the advantages of the capillary s-fibers. The fiber sensitivity can be increased by selective excitation using a light beam with a small angular width. Acknowledgements This work was financially supported by the Grant Agency of the Czech Republic under contracts nos. 10219610939 nd 102/95/0871 References [l] B.D. Gupta, A. Sharma and CD. Singh, Fiber-optic evanescent wave absorption sensors based on uniform aud tapered fibers, Abstracts, EUROPT(R)ODE Ii, Florence. Italy, 19-21 April, 1994, Poster P3.11, p. 189. [2] G. Stewart, W. Jin and B. Culshaw, Evanescent wave gas sensors: prospects and possibilities, Abstracts, EUROPT{R)ODE III> Zurich, Switzerland, 31 March-3 April, 2994, Invited Paper 1.2.1, p. 17. [3] V. Margjec, M. Chomrit, M. PospBiov6, M. Hayerand I. KaXk, Optical fiber with novel geometry for evanescent-wave sensing, Sensors and Actuators B,29 (1995) 4161122. [4] M. Chom6t and V. Matejec, Approximate theoretical analysis of the sensitivity of few-mode sectorial fiber for evanescent-wave sensing, prepared for publication in Opt. Eng. [5] M. Katz, A. Katzir, I. Schnitzer and A. Bornstein, Quantitative evaluation of chalcogenide glass fiber evanescent wave spectroscopy, Appl. Opt., 33 (1994) 5888-5894. [6] A.W. Snyder and J.D. Love, Optical Waveguide Theory, Chapman and Hall, London, 1983. 173 F.P. Payne and Z.M. Hale, Deviation from Beer’s law in multimode optical tibre evanescent field sensors, ht. J. Optoelectron., 8 (1993) 743-748. Biographies Vlastimil MatZjec received his Ph.D. in chemistry from the Institute of Chemical Technology, Prague, in 1982. He is now a research scientist and the head of the Laboratory of Optical Fibers at the Institute of Radio Engineering and Elec- tronics, ASCR, Prague. His research interest is in the tech-  338 V. MutZjec et al. /Sensors and Actuators B 38-39 (1997) 334-338 nology of preparation of special optical fibers by MCVD and the sol-gel method. Mirodau Choma’t received his Ph.D. in technical sciences from the Institute of Radio Engineering and Electronics, ASCR, Prague, n 1963. He is now a senior research scientist at this institute. His current research nterest is in fiber-optic structures for evanescent-wave sensing. MiZos’Hayer received his Ph.D. in technical sciences rom the University of Mining and Metallurgy, Ostrava, in 1990 (awarding the degree withheld since 1971). He is now a research scientist at the Institute of Radio Engineering and Electronics, ASCR, Prague. His research nterest is in meth- ods for fabricating special fibers for fiber-optic sensors. Daniela Berkovci received her Ph.D. degree n chemistry from the Institute of Chemical Technology, Prague, n 1990. She s now with the Institute of Radio Engineering and Elec- tronics, ASCR, Prague, where she s investigating the fabri- cation of thin layers by the sol-gel method and their properties. Marie PospiEEovd received her Ph.D. in mathematics and physics from the Czech Technical University of Prague in 1979. She s now a research scientist at the Institute of IRadio Engineering and Electronics, ASCR, Prague. She has been working on measurements of optical-fiber properties and fiber-optic sensing structures. Ivan Kri&‘k received his Ph.D. degree n chemistry from the Institute of Chemical Technology, Prague, n 1995. He is now a research scientist at the Institute of Radio Engineering and Electronics, ASCR, Prague, and s dealing with the nves- tigation of fabrication techniques for special-shaped silica optical fibers.
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