Chemical tapering of polymer optical fibre

Sensors and Actuators 76 Ž1999. 365–371 www.elsevier.nlrlocatersna

Chemical tapering of polymer optical fibre D.F. Merchant a

a,)

, P.J. Scully a , N.F. Schmitt

b

School of Engineering, LiÕerpool John Moores UniÕersity, OFSRG, Room 518, Byrom Street, LiÕerpool L3 3AF, UK b Hewlett–Packard FCO, White House Road, Ipswich IP5 1PB, UK Accepted 13 November 1998

Abstract We introduce a novel method of chemically removing the cladding of PMMA based polymer optical fibre ŽPOF. using organic solvents which can also be used to create etched tapers of any profile within lengths of POF or at fibre ends. The process is simple, inexpensive, low in chemical hazard and operator skill and has application to both improve the performance of numerous POF devices and allow conversion of silica devices to polymer. We give details of the etching processes involved and optical properties of the devices produced. We believe that this is the first application of this chemical process to the tapering of POF cores and suggest possible future applications of the technique. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polymer optical fibre; PMMA; Cytop; Tapering; Acetone; MIBK

1. Introduction Creating tapers in silica glass optical fibres is a wellknown process both for sensing and communications applications w1x. A taper is a localised diameter reduction. They may have any profile from linear and gradual changes to abrupt steps, with the point of smallest diameter referred to as the waist ŽFig. 1.. They offer unique optical properties that have application to delivery systems Žsuch as the production of couplers or fibre type matchers. and sensors. The change in diameter can redistribute the modes within the core or remove selected modes. The penetration depth of the evanescent field and the proportion of power within this field increases in the tapered section. The change in diameter is itself of use for coupling fibres of different types or interfacing fibres to devices. The methods for creating these tapers in glass optical fibre are well-established though only applicable to glass. Polymer optical fibre ŽPOF. is becoming widely used in such areas and there would be a significant enhancement of POF devices if tapering techniques could be applied to them. In addition, existing silica taper devices could be converted to POF for cost savings or operational reasons.

We have developed a method of etching tapers in step-index POF that offers this prospect.

2. Existing tapering techniques for glass optical fibre Tapering has been applied to silica fibre for many years and has become common practice on a commercial scale for producing couplers, sensors and integrated devices. Silica fibre tapering in the most part relies on two basic methods: flame drawing and acid etching. 2.1. Flame-drawing In the most common method, a section of the fibre is heated in a clean gas flame or laser source w2x close to the glass transition temperature then pulled using a micrometer translation stage before cooling to create a smooth tapered region w3,4x. It is a relatively simple process to automate, however, the polymers used in POF are not suitable for this technique, as their low ductility and tendency for

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Corresponding author. Tel.: q44-151-231-2379; Fax: q44-151-2982624; E-mail: [email protected] 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 0 0 8 - 4

Fig. 1. Schematic of a typical fibre taper.

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uneven melting lead to breakages, uneven profiles and in many cases a total melting of the fibre. 2.2. Acid etching To produce silica tapers with a more controllable profile, an etching process can be performed. Concentrated hydrofluoric acid must be used to give realistic process times w5x. Etching cannot taper fibres and maintain an intact cladding. It also requires great care due to the highly dangerous nature of the reagents used, including specialised containment and handling systems. It does however allow tapers of any profile to be made—linear, stepped or curved—and requires no specialised gas flames or drawing tools. The simple process control and flexibility of final profile possible with etching leads to the method becoming a prime contender for application to POF. However, the polymers must be matched to a suitable solvent to ensure the etching is a surface effect and does not dissolve the material in bulk. 3. Etching of polymer optical fibre (POF) All of the methods we describe below apply to step index POF with a pure polymethylmethacrylate ŽPMMA. core and a doped Cytop cladding only. We have used Raytelae POF from Toray Industries ŽPFU-FB1000. which has a 980 mm diameter core and 20 mm thick cladding. There are several other POF materials in commercial use and these are discussed in Section 5. The cladding of the Toray Raytelae POF is based upon Cytop, a cyclic polymer of ‘BVE’ ŽCF2 s CFOCF2 CF2 CF s CF2 . registered commercially by Ashai Glass, with several dopants to alter the refractive index. The nature of these dopants has not been released. We cannot therefore model the chemical properties of the cladding and the selection of etching chemicals we describe here are based on empirical results. PMMA is a thermoplastic with a softening temperature of 70–808C. Despite the fact that the initial POF production process is a hot drawing method, if heat drawing is attempted on finished POF, the fibre breaks before any elongation occurs. Some small scale heat deformation processes have been applied w6x though these are a long way from true drawing. We believe that the polymer resists drawing partly due to the latent stresses within the structure from production and the differing physical properties of the core and cladding. Etching is therefore the only realistic option. PMMA is dissolved by concentrated inorganic acids w6x though the effects are to produce a swollen region of hydrolysed porous and opaque material rather than removing the polymer in concentric layers as we require. It can however be dissolved by organic solvents such as acetone and methyl isobutyl ketone ŽMIBK.. The action of these solvents on samples of bulk polymer material leads to a crazed surface w7x which, if formed

on an optical fibre, would seriously impair the waveguiding process. For this reason, it has been assumed until recently that such solvents were unsuitable for POF etching and so the methods we describe here were not previously investigated. 3.1. POF cladding remoÕal by organic solÕent etching It has been previously reported w8x that the doped Cytop cladding of POF can be chemically removed using a solution of acetone in water. This research concentrated solely on removing the cladding layer. The details of this work were presented orally at a conference though not included in the publication. We have extended the method to core tapering by modifying the solvent composition, exposure process and immersion times. As a result of this, we believe we have also created a more reliable process for cladding removal than has been previously reported. Our research has shown that pure acetone, without the dilution in water used by previous published work, can be used to efficiently remove the cladding using an improved approach. The method involves a process during which the etched section becomes susceptible to brittle stress fracture for a short while. It is, therefore, critical to success that the section to be stripped is supported between clamps, with no tension applied. We support the fibre in the stress-free state: a fibre taken from a spool should be supported in a curve and a de-stressed fibre supported in a straight line. A lint-free lens tissue Žapprox. 30 mm square. is folded in two and allowed to hang on the fibre. Two to four drops of 100% acetone are applied to the tissue, enough to saturate it. The tissue is immediately rotated and slid along the treatment region to rapidly expose the cladding to the solvent. The process must be done with great care as the fibre becomes extremely brittle. After 8–10 s of careful movement, three drops of distilled water are applied to the tissue, resulting in an approximately 1:1 aqueous–acetone mixture. This concentration halts the solution process while keeping the cladding soft enough to remove. The tissue is then immediately gripped between the fingers and briskly rubbed along the treatment region to remove the now semi-soft cladding. It is possible to detect the exposed core areas as the fingers are moved along the fibre as a decrease in surface friction. The exposed region is then washed in water to fully neutralise the solvent, then in isopropyl alcohol to leave a clean, grease-free exposed core. Any areas of cladding remaining in place can be seen optically under a magnifying glass or low-power microscope. If cladding remains on the stripped section it can be removed by repeating the entire process. The core will be unaffected by further exposure to the solvent provided that the exposure times are limited as described. Once the region has been washed it returns to the original physical and chemical properties of PMMA. Our experiments have shown it is possible to produce single


stripped regions between 5 mm and 750 mm in length, though there is no upper limit as long regions can be created by sequentially stripping many small lengths. The process as described here is repeatable and requires low operator skill. We have trained competent students to produce etched POF with high success rates and surface quality using the information presented here. There are several important points to note. As we have indicated during the initial solution phase using 100% acetone, the exposed region is extremely susceptible to breakage. This is the result of preferential attack of stressinduced microfissures. At 2 s after the 50% neutralisation has been applied, the fibre loses this fragility and can withstand the stresses of finger pressure. Secondly, it is essential to ensure the fibre is thoroughly washed after etching. Given the high water absorptivity, it is possible that the aqueous solvent can be carried into the surface. If this is not flushed out by washing, it can create long-term damage as is detailed in Section 4.1. 3.2. POF core tapering The cladding removal process is prevented from affecting the core by time constraints. When immersed in a solution containing a suitable mixture of organic solvents for longer times, the fibre is slowly dissolved. Firstly, the cladding is removed by chemical action Žwithout the need to apply the physical processes described above.. Thereafter, the exposed PMMA core is etched in a uniform manner resulting in an even reduction of the core diameter. Unlike with bulk samples the core surface exposed by this etching remains smooth. We use a solution of 60% acetone, 20% MIBK and 20% distilled water. The ratios are variable—the primary etchant is acetone though as the acetone concentration increases, the likelihood of brittle fracture increases. We have experimented with 100% acetone and could only achieve a 10% success rate. With 3:1:1 proportions 90% success rates are possible. Our group has produced a large number of tapers and can achieve a 1 in 10 breakage rate without difficulty. MIBK also etches the fibre though at a much slower rate Ž100% MIBK will etch PMMA at about 0.5 mm per week.. In this application, it serves to linearise the etching process with small solvent volumes and helps to keep the dissolved material in solution. At room temperatures the acetone evaporates, a useful property as we shall describe below, however, as this happens the PMMArdoped Cytop mixture resolidifies in a layer on the liquid surface. This can be a problem if the fibre must be lifted from the solvent through this ‘skin’ of polymer. Adding MIBK, which is far less volatile, keeps this effect to acceptable levels. The time required to etch a taper ranges from 1 h to several days depending on the final waist diameter required and the solvent ratios used. The fibre region that remains below the solvent surface during the procedure will be uniformly etched producing a

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linear waist region. To create tapered regions the exposure time must be varied. A smooth diameter reduction will be produced over a region of the fibre if it is slowly removed from the solvent during the etching, as the fibre is immersed in the solvent for differing times depending on the initial depth below the surface. Thus, by control of the immersion time tapers ranging from slow smooth profiles to distinct steps can be created. The diameter profile of the taper can be controlled by two methods described below. Ž1. The change in solvent surface level by simple evaporation losses can be used. By varying the surface area of the solventrair boundary, evaporation speed can be controlled. Allowance must be made for sufficient residue of solvent to hold the dissolved polymer in solution. Ž2. The fibre can be slowly moved out of the solvent by a motor system. This has the advantage of creating any immersion time profile required—so a non-linear taper can be made. To remove evaporation effects the surface area of solvent exposed to the air should be a minimum. In both cases, the solvent should, if possible, be gently stirred every 15 min to reduce the formation of the polymer ‘skin’. Given a low acetone ratio Ž- 75%., the fibre is strong enough to resist careful stirring. If evaporation is not being used as the profile control method, then a large reservoir of solvent can be used to continually refresh the volume in contact with the fibre by drip-feeding new fluid into an overflowing container to maintain a fixed surface height. This prevents formation of polymer residues by maintaining a low concentration in the solvent. The methods can be used on fibre ends and in the middle of a length by immersing an arc of a fibre loop in the solvent. Fibre end tapers can be as short as 1 mm in length, though inline tapers are limited to a minimum length of 2–3 cm due to the difficulty in immersing small lengths in an open solvent bath. The profile of the tapers can be varied from smooth to step profiles by controlling immersion. Fibres of 1 mm diameter have been etched to less than 100 mm over 30 mm lengths. Fig. 2 shows an end-fibre taper created in 1 mm POF with a linear reduction to 300 mm followed by a semicircular end-face. It is possible to replace a cladding layer over the taper by recoating in a suitable polymer as we have described for stripped-cladding POF. 3.3. Reproducibility The etching process is gradual, and it is a simple process to monitor the waist diameter physically or optically. It is possible to monitor the tapering as it occurs by measuring either the transmitted power w9x or the effective numerical aperture. A reflectance measurement for endfibre tapers can be used. If a small amount of unreactive dye were to be added to the solvent, a marked absorbance change would be seen as the taper progresses. The taper surface finish is consistent across samples provided fresh solvent is used for each process. The profile

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Fig. 2. Photograph of end-fibre taper produced in 1 mm diameter POF.

of the taper is created by the operator by controlling immersion and it is not difficult to create several nominally identical tapers with care. Existing tapering methods for silica fibre have difficulty producing large numbers of identical components without exact monitoring of the processes. We believe the method we describe here is ideally suited to research requiring small numbers of devices with specific profiles.

4. Properties of POF tapers

ever, if solvent is absorbed into the PMMA during etching and not removed it will slowly damage the microstructure of the PMMA leading to a milky semi-opaque appearance. The optical quality of the material is thus dramatically reduced. This effect can ‘cloud’ etched regions over several weeks or months. We recommend soaking the etched fibre in distilled water for several hours to ensure the solvent is removed. Given this washing stage, we have seen a relatively low ratio of clouding, of the order of 10%. We are, at present, researching the exact processes at play so that we may devise a method of preventing the effect from occurring.

4.1. Physical properties Unlike the effects on bulk material samples, the etching process of POF gives an optically smooth surface of similar quality to the original core surface. We believe this is due to the superior polymer quality of POF though work to establish the process is planned. The only factor likely to effect the surface finish is the presence of polymer that has fallen out of solution, for example, if the taper is withdrawn through a skin of polymer on the solvent surface. Once removed these extra deposits rapidly harden and adhere to the fibre creating a non-uniform surface that is difficult to repair. The tapered region, if washed correctly, is simply a pure PMMA core material section and so retains the properties of PMMA in terms of flexibility, chemical resistance, etc. In particular, the bend radius is reduced in proportion to the diameter so sensor systems requiring a small curved POF segment w8x could be significantly improved by using the method to either allow smaller bend radii or larger coupling surfaces at the fibre ends. How-

4.2. Optical properties The tapered regions created by this method have the same optical properties as any other tapers in multimode fibres w10x. However, given the large diameter of POF several effects are of particular note. 4.2.1. Effect on the number of modes In a large core multimode optical fibre, such as we are using here, the number of guided modes M can be approximated by the following equation w11x: 2

ž (

M f Ž krc N . s krc n 20 y n12

2

/

Where k is the fibre propagation constant, rc is the radius of the core and N the numerical aperture. n 0 and n1 are the refractive indices of the core and cladding, respectively. Tapering the fibre results in a reduction of the diameter which filters out high-order modes within the fibre, creating an effective reduction in NA w11x which is


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Fig. 3 shows the far field emission profile from the polished end-face of a 750 mm length of 1 mm diameter POF with a centrally placed 5 cm long 520 mm waist diameter taper overlaid on an untapered sample of the same fibre. The untreated fibre shows a far field pattern where 90% of the flux occurs within 308 of the axis, giving an effective NA of sinŽ308. s 0.50. The taper limits the angle to 108—an effective NA of only 0.17. The fine structure of the profiles is due to the launching conditions rather than the effect of the tapering. This effect is of significant benefit when used with end-face diffracting elements as developed by our group w13x as the smaller emission angle gives correspondingly higher resolution to the diffracted spectrum.

Fig. 3. Emission profiles of normal and tapered POF.

approximately proportional to the square of the taper diameter. This is of benefit for sensors where a measurand replaces the cladding as tapering can be used to correct for the mismatch in effective NA between the fibre and the sensor w12x. Conventional mode filtering in large diameter POF relies on external masks and therefore, needs alignment and space. Using a POF taper requires no alignment and has a constant attenuation of low-order modes. The modal redistribution length of POF is a few hundred metres and so the effect of tapers is local to these distances.

4.2.2. EÕanescent field effects The fraction of transmitted power outside the core of a fibre Ždue to the evanescent field. can be considered inversely proportional to the V-number w14x. If a bare core is surrounded by an absorbing liquid, the effective absorption coefficient of the evanescent field is inversely proportional to the radius w14x. A POF taper is therefore more effective for creating an evanescent field sensor than a simple bare section of core w15x. Fig. 4 shows the transmission spectrum of a tungsten halogen source through three 750 mm lengths of Toray 1 mm POF. The centre 50 mm of each fibre is immersed in a 1 ppm solution of methylene blue in water. The normal trace is an untreated sample and shows the baseline trans-

Fig. 4. Transmission spectra of normal, declad and tapered POF in methylene blue solution.

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mission Žunaffected by the presence of the dye.. The declad fibre has the cladding removed by the method we have described over the immersed 50 mm. The tapered sample has been etched to form a 50 mm long 800 mm waist diameter bi-conical taper in the immersed region. As can be seen, even this slight reduction in diameter gives more than twice the absorption change than simply removing the cladding. Tapered POF is therefore ideally suited to sensors operating using this principle and should convey an instant gain in signal size Žhence, an increase in sensitivity or a reduction in the costs of the electronics..

5. Other POF types Several manufacturers produce step-index POF using other materials, for example polystyrene and pure Cytop w16x are used for both core and cladding. Undoped Cytop is not expected to react to our current solvent mix in the same way as the cladding layer we have detailed in this paper. Polystyrene dissolves as a bulk sample, turning a fibre into a liquid gel rather than tapering it. Graded-index POF has recently been developed w16x using a Cytop core with a radially variable doping. This is not yet available for tests though the developers indicate pure Cytop is soluble in perfluoro-alkane solvents and we believe the methods described can be applied using these compounds in place of the current mix.

6. Conclusions A method has been developed to permit the etching of tapers in the cores of polymer optical fibres either within a length or at a fibre tip. We present technical information on the process and the effects it has on the fibre as information to others working with POF devices. Any diameter profile is possible except re-entrant designs, with lengths of 2 cm to many metres. The process is simple, low cost, requires little or no specialist equipment and is safer than processes involving highly concentrated acids. It is applicable to all diameters of POF having a PMMA core and doped Cytop cladding and can produce waist diameters as low as 100 mm. The time required to etch each taper is long Žseveral hours. and so the process can be monitored manually and errors in timing or solvent mix are not critical. Optical methods can be used to measure the progress of etching. Both predicted and measured optical and physical properties of the tapers offer significant advantages either for existing POF device improvement or conversion of silica devices w17x to POF. We believe the technique is of interest to all currently working with POF sensors and couplers as a laboratory tool for improving device performance. The technique is intended to permit modification of POF on a laboratory scale using no specialist equipment

and is suited to research and prototyping. Operation on a commercial scale must be matched against the ability to request uncladded POF from manufacturers, though this option is open to only the largest end users. We are currently investigating the application of these devices to sensor systems for measuring biofouling, humidity and water quality. Work is planned to apply the methods to graded-index POF when it becomes available.

References w1x R.A. Lieberman, Recent progress in intrinsic fiberoptic chemical sensing II, Sensors and Actuators B 11 Ž1993. 43–55. w2x A.J.C. Grellier, N.K. Zayer, C.N. Pannell, Heat transfer modelling in CO2 laser processing of optical fibres, Optics Communications 152 Ž4–6. Ž1998. 324–328. w3x S.R. Choudhury, Y. Jaluria, Practical aspects in the drawing of an optical fiber, J. Mat. Res. 13 Ž2. Ž1998. 483–493. w4x M. GarciaParajo, T. Tate, Y. Chen, Gold-coated parabolic tapers for scanning near-field optical microscopy: fabrication and optimisation, Ultramicroscopy 61 Ž1–4. Ž1995. 155–163. w5x B. Schoch, B.E. Jones, A. Franks, A simple technique for the manufacture of optical probes for scanning near-field optical microscopes, Meas. Sci. Tech. 5 Ž6. Ž1994. 663–666. w6x S. Yamakawa, Plastic optical fiber chemical sensor with pensil shaped distal tip fluorescence probe. Proceedings of POF 97 Session G, 1997, pp. 109–110. w7x L. Elgel, Atlas of polymer damage surface examination by scanning electron microscope, Wolfe, New York, 1981. w8x J. Bayle, J. Mateo, Plastic optical fibre sensor of refractive index, based on evanescent field. Proceedings of POF 96 Session 9: Sensors 1, 1996, pp. 220–227. w9x M.R. Lovely, P. Radhakrishnan, V.P.N. Nampoori, C.P.G. Vallabhan, Evanescent wave sensor to monitor the etching rate of an optical fibre, Indian J. Pure Appl. Phys. 34 Ž1. Ž1996. 34–35. w10x A.W. Snyder, J.D. Love, Optical Waveguide Theory, Chapman and Hall, 1983. w11x J.M. Senior, Optical Fibre Communications Principles and Practice. Prentice–Hall, London, 1985. w12x J. Golden, S. Rabbany, G. Anderson, Proceedings of the SPIE 1796 Ž1992. 9–13. w13x N.F. Schmitt, E. Lewis, P.J. Scully, UV photo induced grating structures on plastic optical fibres. Proceedings of POF 96 Session 4: Waveguides, 1996, pp. 120–127. w14x V. Ruddy, An effective attenuation coefficient for evanescent wave spectroscopy using multimode fiber, Fiber and Integrated Optics 9 Ž2. Ž1990. 143–151. w15x B.D. Gupta, C.D. Singh, A. Sharma, Fiber optic evanescent field absorption sensor—effect of launching condition and the geometry of the sensing region, Opt. Eng. 33 Ž6. Ž1994. 1864–1868. w16x H. Murofushi, Low loss perfluorinated POF. Proceedings of POF 96 Plenary Session 1, 1996, pp. 17–23. w17x O.S. Wolfbeis, Fiberoptic sensors in biomedical sciences, Pure Appl. Chem. 59 Ž1987. 663–672. David Merchant is a postgraduate researcher with the Optical Fibre Sensors Research Group at Liverpool JMU, specialising in polymer optical fibre devices. A physics graduate of St. Peter’s College, Oxford University and Liverpool JMU, he joined the group in 1996 to develop sensors for measuring fluorescence in fluids using fibre technology for his PhD thesis and currently specialises in polymer fibre devices. He has also worked with the temperature standards section at the National Physical Laboratory, Teddington, UK, developing laser-based gas transfer standards and remote-surface thermometry techniques.

D.F. Merchant et al.r Sensors and Actuators 76 (1999) 365–371 Patricia Scully is a Senior Lecturer in Applied Physics at Liverpool JMU, joining in 1990 and helping to form the Optical Fibre Sensors Research Group in 1992. She graduated from the University of Manchester with a BScŽHons. in Physics in 1985, and obtained an MSc in Instrumentation and Analytical Science from UMIST in 1986. She studied for her PhD in Optical Sensors for Physiological Monitoring at the University of Liverpool from 1987–1990. Her current research interests include silica and polymer optical fibre sensors for chemical, biological and environmental monitoring.

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Nicolas Schmitt is currently employed by Hewlett-Packard working on high speed telecommunication systems. In 1993, he graduated from Liverpool JMU with a BScŽHons. in Applied Physics. He joined the Optical Fibre Sensors Research Group in 1994 for his PhD to investigate and develop the photo-induction of grating structures in polymer optical fibre.