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Optical fiber with novel geometry for evanescent-wave sensing

First results in the preparation and analysis of an optical fiber with a novel geometry which facilitates the access of chemical species to the evanescent field for sensing purposes are presented. This ‘s-fiber’ is of approximately sectorial cross
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  cHEBc L E L S E V I E R Sensors and Actuators B 29 (1995) 416-422 Optical fiber with novel geometry for evanescent wave sensing Vlastimil Matejec Miroslav Chomfit Marie Pospi~ilovfi Milo~ Hayer Ivan Ka~ik Institute of Radio Engineering and Electronics, Academy of Scienees of the Czech Republic, Chabersk4 57, 182 51 Prague, C=ech Republic Abstract First results in the preparation and analysis of an optical fiber with a novel geometry which facilitates the access of chemical species to the evanescent field for sensing purposes are presented. This 's-fiber' is of approximately sectorial cross section with the core located in the carefully rounded vertex of the sector. Using a perturbation method, the dependence of the attenuation coefficient of the fundamental mode in a weakly-guiding, step-index s-fiber on the fiber normalized frequency, vertex angle and cladding thickness are determined. Attenuation coefficients several times higher than in D-fibers are theoretically attainable. Preforms for drawing s-fibers are prepared from standard MCVD preforms by accurate grinding and polishing the preforms to a desired sectorial shape. Multimode s-fibers with core dimension of about 30 pm and cladding size of about 170 pm and exhibiting satisfactory strength have been drawn. Resulting shapes of the fiber and core depend on the shape, structure and composition of the preform, drawing temperature and drawing velocity. Results have proved the feasibility of the chosen approach to the laboratory preparation of s-fibers. In preliminary experiments the sensing ability of the drawn fibers has been examined. Keywords: Optical fibers; Novel geometry; Evanescent-wave sensing 1 Introduction For evanescent-wave chemical sensing a variety of optical fibers differing greatly in geometry and structure have been investigated and tested. They include multi- mode PCS fibers with removed or modified cladding and various types of tapered, etched, side-polished, eccentrically clad and D-shaped single-mode or few- mode fibers [1-3]. In order to achieve high detection sensitivity of evanescent-wave fiber-optic chemical sensors the frac- tion of total optical power that is carried by the evanes- cent wave should be large, there should be good access of the detected chemical species to the evanescent field of the fiber, and long fiber interaction lengths should be used. For most practical applications sufficient strength, ruggedness and durability of the sensing fiber are also required. Commercially available and inexpensive multimode PCS fibers with removed or newly formed claddings and with core diameters ranging from 100 to 600 pm as well as tapered fibers of this type still seem to be the most widely used type of sensing fibers. Their evanes- cent field is easily accessible in the whole area where it exists, they possess good mechanical properties and long interaction lengths are easily achievable with them. However, the fraction of the total optical power that is 0925-4005/95/$09.50 1995 --Elsevier Science S.A. All rights reserved SSD 0925-4005(95)0 717-A carried by the evanescent wave, which is under certain assumptions inversely proportional to the normalized frequency of the fiber V [4], is rather low, being usually of the order of 10 4-10 3. It results in relatively low detection sensitivity of the sensor. Of the other types of sensing fibers, single-mode or few-mode D-fibers are very promising for future evanescent-wave sensing [3]. The main reasons for this n3-jk , , ~v ROUNDED COATING, ~ VERTEX • ikx CORE /// i'- ro ' , CLADDING Fig. 1. Geometry and structure of the analyzed s-fiber in an ab- sorbing coating.  V. MatOjec et al. / Sensors and Actuators B 29 1995) 416 422 417 are an easy access to the evanescent field in the whole area over the fiber flat, fiber ruggedness, the possibility of achieving long fiber interaction lengths and, finally, their commercial availability. However, the optical power of the evanescent wave which may interact with the chemical species is, due to the D-fiber geometry, only a small part of the total optical power of the evanescent wave that could theoretically be utilized for this purpose. We propose a sensing fiber with novel geometry which brings the possibility of enhancing the evanescent interaction for single-mode as well as for multimode evanescent-wave sensing [5]. Further we report results that have been obtained. 2. Basic features of the s fiber The proposed novel fiber is of approximately secto- rial cross section with rounded corners as sketched in Fig. 1. The core of this 's-fiber' is located on the sector axis very close to the rounded vertex of the sensor. The lateral flats of the s-fiber form an angle fl which is equal to or less than 180 °, which was chosen as the limit angle for the s-fiber. The height H of the sectorial cross section of an s-fiber is chosen to be in the range 100 600 ~tm, which is comparable with diameters of most conventional sensing fibers. The thickness of the fiber cladding over a certain area in the rounded vertex is either very small or the cladding may even be completely missing in this area. Depending on the shape of the core in the pre- form and on conditions under which the s-fiber is drawn, the fiber core is either circular or takes other forms. The envisaged advantage of single-mode or few- mode s-fibers, with a suitably chosen vertex angle, over single-mode or few-mode D-fibers with the same core dimensions and corresponding refractive-index profiles is the possibility of achieving stronger evanescent inter- action which follows from improved access to the evanescent field in s-fibers. The idea of the s-fiber may also be applied to multimode sensing. The evanescent interaction in a multimode circular-core fiber can be made stronger by decreasing the fiber normalized frequency V which can be achieved by decreasing the core diameter. Of course, there are certain practical limits for diminishing the fiber core. In most sensing applications it would be rather difficult to handle and employ long lengths of a declad or thinly coated PCS fiber with a relatively small core diameter, e.g. in the range 20-40 ~tm. On the other hand, handling and employing a multimode s-fiber with core dimensions in the mentioned range and with outer dimensions ranging from 100 to 600 gm should not present a serious problem. Although the beneficial effect of decreasing the core dimensions will be partly reduced due to the impossibility of accessing the whole evanescent field of the s-fiber it can be expected that the resulting net enhancement of the evanescent interaction may be significant. 3. Theoretical determination of attenuation of the fundamental mode of the s fiber We determine the attenuation of the fundamental mode in a weakly-guiding step-index circular-core s- fiber coated with a weakly-absorbing coating with the aim of assessing the improvement that may be brought by the novel fiber geometry. The light-absorbing prop- erty of the coating is caused by the penentration of a chemical species into the coating. We consider an s-fiber with a core of radius a and refractive index n] and with a cladding of refractive index n whose outer radius ro and thickness are con- stant over a certain area in the vertex region (Fig. 1). The complex refractive index of the coating is n3-jk where we assume that k << n 3 and n 3 ~ n2. The possibil- ity of fulfilling the last assumption is in using, for example, a suitable porous glass coating prepared by the sol gel process [4]. Generally, a chemical species that penetrates into the coating influences ,not only the imaginary part k of the refractive index, but, to some degree, also its real part n 3. Induced variations of n 3 cause changes of the evanescent field which also influ- ence the attenuation of the guided mode. Here this effect is not taken into account. For the sake of simplic- ity of calculations we further assume that the coating is uniform and infinite and that n 3 =/'/2 holds. Under these assumptions the coated s-fiber may be treated as a slight perturbation of an unperturbed fiber with the same core and non-absorbing infinite cladding of refractive index n: for which the fundamental mode solution is known and perturbation methods may be applied [6]. The area S in which the refractive index of the cladding is perturbed includes the whole region outside the cross section of the s-fiber. The power attenuation coefficient ), of the guided mode can be expressed as [6] 7 = '%q (1) where ~0 = kko is the power absorption coefficient of the coating (for bulk absorption), k0 = 2~r/2o and q is the fraction of the total power that propagates through the area S [7] f Lel dA s (2) ~7 .Z  418 V. Mat~jec et al. / Sensors and Actuators B 29 (1995) 416 422 In Eq. (2), E is the electric field of the fundamental mode of the unperturbed fiber. The determination of r/ is based on the relation for the fraction r/I of the total power that propagates in the circular area of the cladding of the unperturbed fiber with inner radius r I i> ro and outer radius r 2 -+ o0, which is for the radially symmetric modal field of the funda- mental mode given by U2RI 2 r/l -- V2K~--W) [K12(WRI) - K02(WR1) ] (3) In Eq. (3), U, W and V are given by U = koax/~21 2 -- n~ 2 W = koajne 2 - n2 2 V= v/~+ W 2 (4) where ne is the effective refractive index of the mode, R1 = q/a and K 0, K~ are the modified Bessel functions. Eq. (3) can easily be obtained using the expressions for the fraction of the power in the core given in Table 14-3 in Ref. [6] and the electric field of the mode in the cladding of the unperturbed fiber. On the basis of Eq. (3) and Fig. 1, we can express Eq. (2) as 2= if I=~ rh [Rl(~p)] d( p (5) o The function &(~o) in Eq. (5) is given by RI = Ro, Ro =ro/a in the interval of q) in which the thickness of the cladding is constant, and by R1 = Ro/cOs (p for q) in the intervals (0, 6) and (rc+fl-a, ~+fl), where the meaning of the angle 5 is evident from Fig. 1. Neglect- ing the contribution of ~/ from the angular interval (5, ~ + fl - 6), where the relative thickness of the clad- ding is large and the value of */i is relatively small, we then get for 7 of the fundamental mode O{ U2Ro 2 { ~ 1rV2K12(WRo) (it - fl)[K]2(WRo) - Ko2(WRo)] c~ f 6) a~lim ~ L \cos~o/ \cos~o/j 2 0 The integral term in Eq. (6), which is independent of the vertex angle fl, represents the contribution to the attenuation srcinating from two identical parts of an s-fiber for which ~o is in the intervals (0, 5) and (~ + fl - 6, zc + fl) (see Fig. 1). These fiber parts may be seen as two separated halves of the corresponding D-fiber. The first term in Eq. (6), which linearly de- pends on fl, represents the added contribution to the attenuation appearing owing to the geometry of the s-fiber. For a D-fiber it is fl = ~ and this term vanishes. I I R : I. a) Fiber with --- f~ - 180 ° 0.4 ~ r/emoved cladding e oooe ~ - 150 o "~ ====:- t20 ° % A ~-a-~ 90 ° o0'3 ~~... ..... 360o60 i~'-0.2 V 0.5 Ro = 1.5 b) 0.4 0.3' ..... = o -~ ~ ~ibceirrc Wlot r cladding °eee~ 8 i~: 0'2 ~ & b &-~ i 90 0 ....... 60 o 0.1 ~. .... V Fig. 2. Dependence of the ratio of the attenuation coefficient 7 and the absorption coefficient % on the normalized frequency of the fiber V for several values of the vertex angle fl and the case of a fiber with a circular cladding and for R0 = 1.0 (a), 1.5 (b). Using numerical evaluation of Eq. (6) important depen- dencies of the attenuation coefficient 7 on various fiber parameters have been determined. Fig. 2(a) and (b) shows that rano %/% as a function of the fiber normalized frequency V for several values of the angle fl and two values of R 0. Also shown in Fig. 2 are the curves for a fiber with removed cladding (R0 = 1.0) and a fiber with a circular cladding. In Fig. 3(a) and (b) the ratio 7/7D, where 7D is the attenuation coefficient for the corresponding D-fiber, is plotted as a function of the angle fl for several values of R 0 and two values of V. 4. Experiments and results Preforms for drawing s-fibers were prepared in two steps. In the first step a preform with a cylindrically symmetric glass core GeO2-SiO2 and glass optical clad- ding F-P2Os-SiO2 was prepared by the MCVD method (device Special Gas Company, UK). A typical refractive-index profile of the preform, measured by using refractive-index profiler P101 (York Technology, UK) is shown in Fig. 4. In the second step accurate grinding and polishing of this preform was used to prepare a preform of sectorial cross section with carefully rounded vertex region. The core and cladding structure can be seen in a photo of the prepared sectorial preform with a completely re-  V. Mat jet et al. ,' Sensors and Actuators B 29 (1995) 416-422 419 ~ V = 1.6 (a) 2.8 ~'~'~ . Ro= 1.0 - ~.-..'c->.. :::=: Ro=1 1 ~2.2 ~ ~ Z'>.. ..... Ro= 1.,3 1 6 x~ 1.0 20 SO 1 O0 180 B °] ~~ o . V=2 0 2.8 ~i~ b),~ Ro =1 .? ~;~2.2 R°=l Ro=l ,3 Ro=l .5 1.6 1.0 ................... , .................. , .................. , .................. ., 20 60 1 O0 14Q 180 [°] Fig. 3. Dependence of the ratio of the attenuation coefficient ;, for the s-fiber and ?'D for the corresponding D-fiber on the vertex angle fl for several values of the relative radius R~ of the cladding and for V- 1 6 (a), 2.0 (b). moved part of the cladding in the vertex region, shown in Fig. 5. The vertex angle fl of this preform is equal to 70 °. The tomographic refractive-index profile of this preform measured in immersion is shown in Fig. 6. The fibers were drawn from the preforms using a graphite resistance furnace (Centor, USA). Drawing temperatures in the range from 1850 to 1950 °C were investigated. A multimode fiber with core size of about 30 Hm. outer size (height H) of about 170 lain, and coated with a layer of silicone polymer Sylgard 184 (Dow Coming, USA) which was approximately 50 ~tm thick was prepared. A drawing velocity of about 15 m/ min was used. The endface of a broken s-fiber is shown in Fig. 7. 0006 o 004 .c_ 0002 I oooo c]f -0.002 ................... , .................... . .................. , -600 -200 2 O0 60~ Radius [ram] Fig. 5. Photo of a prepared sectorial preform. Preliminary experiments were performed in order to examine the sensing ability of the prepared s-fibers. Solutions of methylene blue in glycerol and ethanol with concentrations of 277 ppm (wt.) and 27 ppm (wt.) were used in the experiments. Their refractive indices were both 1.439. The silicone claddings were removed from the fibers in lengths of about 5 cm. This fiber section was placed into a measuring cell (no solution circulation was used) and its spectral attenuation was measured using a labo- ratory device with a lock-in amplifier. The diagram of the optical measurement system is shown in Fig. 8. A dynamic range of up to 30dB and an accuracy of attenuation measurement of about 0.1 dB can be achieved with this device. In order to verify the detection ability of the optical measurement system for evanescent-wave sensing, ex- periments with commonly used PCS fibers were also carried out. In these experiments PCS fibers with core diameter of 200 ~tm and coated with a Sylgard 184 layer of approximately the same thickness as in the case of the s-fiber were used. The PCS fibers were excited with an input numerical aperture of about 0.19 using the optical system of the device. The s-fibers were excited 0.0(3, t Fig. 6. Tomographic refractive-index profile of a sectorial preform Fig. 4. Refractive-index profile of the MCVD preform. (measured in immersion, n = 1.4565 at 580 nm).  42 V. MatO/ec et al. / Sensors and Actuators B 29 1995) 416- 422 4] METHYLENE LUE 27 p~m (wt.) 37 LENGTH OF OPTICAL PATH 5.5 cm -~g ............ gg6 ........... ggg ........... 4~0 ........... gd5 Wavelength [nm] Fig. 9. Spectral dependence of the attenuation of the s-fiber and PCS fiber in solution of methylene blue. Fig. 7. Endface of the broken s-fiber (excited with numerical aperture 0.19 at 580 nm). with a few-mode fiber with a core of diameter of 10 gm and numerical aperture of 0.125. In the experiments the spectral dependence of the output intensity Io of the fibers in the empty cell in air was measured at first. Then the cell was filled with the solution of methylene blue and the intensity I~ was determined. The attenuation 7. was calculated from the equation [Io\ 7, = 10 log~l) (7) from which ;'] = 4.342;,L, where L is the length of the fiber section without silicon polymer. The results of these measurements for the methylene blue concentra- tion of 27 ppm are shown in Fig. 9. For the concentra- tion of 277 ppm the attenuation at 660 nm was found to be 11.8dB for the s-fiber and 4.3 dB for the PCS fiber. 5 Discussion In the preceding sections theoretical and experimen- tal results on s-fibers have been presented. The aim of ,~_~~cH~LE x,y,z x,y ~ J LEGEND L LA~P 5OW, MC-MONOCNROMATOR LE-LENS, CH-CHOPPER, SO S'¢NCHR DETECTOR 0 DETECTOR, PA PP-4EAMPUFIER, -MOTOR, LR-UNE REFER3JJCE, PLL-PHASE LOCK LOOP, PS-PHA.SE SHIFF~R, $1-SAMPL I ,rT'EGRATOR, A-OIP AMPUFIER. ADC A/D CONVERTER, C-COMPUTER Fig. 8. Diagram of the optical measurement system. the theoretical analysis was to provide basic informa- tion on the sensing ability of the s-fiber in single-mode operation. The analysis confirmed the expected rapid increase of the attenuation coefficient ), of the funda- mental mode as the fiber normalized frequency V de- creases and the mode is less confined. Light launching problems and microbending effects, however, would prevent the use of s-fibers with low V values. A reason- able compromise might be the choice of V in the range 1.6-2.0. The results also reveal the beneficial effect of decreasing the vertex angle on the enhancement of the attenuation coefficient. The experiments have proved the feasibility of the chosen approach for laboratory preparation of s-fibers. Attention was given mainly to the preparation of multi- mode s-fibers with small-diameter cores (20 30 ~tm) which are expected to comprise the advantages of high fractional power in the evanescent field and the strength and ruggedness of D-fibers. A good strength and ruggedness of the s-fiber is expected on the basis of similar methods of preparation of s-fibers and D-fibers utilizing grinding and polishing the preforms which should induce similar types of surface flaws in both cases. The drawn fibers exhibit satisfactory mechanical strength and have not shown any obvious signs of degradation even in the parts from which the polymer cladding had been removed. No serious problems have been met when handling the s-fibers. For these reasons they are seen as very promising for applications. Comparing the shape of the drawn s-fiber in Fig. 7 and the shape of the starting preform in Fig. 5 one can conclude that the s-fiber maintained the srcinal shape of the preform only roughly. For example, the vertex angle fl measured in the vertex region of the fiber is approximately 100 ° in contrast to the value of fi equal to 70 ° at the preform. The resulting shape of the s-fiber is in a complex way determined by the shape of the starting preform, material viscosities, surface tension, drawing temperature and drawing velocity. Establishing the relationship between these parameters will require further extensive research. Nevertheless, from Fig. 7, one can draw a qualitative conclusion that the access to
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