Recent progress in microstructured polymer optical fibre fabrication and characterisation

Optical Fiber Technology 9 (2003) 199 209 www.elsevier.com/locate/yofte Recent progress in microstructured polymer optical fibre fabrication and characterisation Martijn A. van Eijkelenborg, a, Alexander Argyros, a,b Geoff Barton, c Ian M. Bassett, a Matthew Fellew, a,b Geoffrey Henry, a Nader A. Issa, a,b Maryanne C.J. Large, a Steven Manos, a,b Whayne Padden, a Leon Poladian, a and Joseph Zagari a,c a Optical Fibre Technology Centre, Australian Photonics Cooperative Research Centre, University of Sydney, 206 National Innovation Centre, Australian Technology Park, Eveleigh, NSW 1430, Sydney, Australia b School of Physics, University of Sydney, NSW 2006, Australia c Department of Chemical Engineering, University of Sydney, NSW 2006, Australia Received 18 December 2002; revised 16 April 2003 Abstract Recent progress in microstructured polymer optical fibre fabrication and characterisation will be presented. A wide range of different optical functionalities can now be obtained by modifications of the microstructure, as is demonstrated by the fibres presented here. Microstructured fibres that are single-mode, highly birefringent or show twin-core coupling are described, in addition to gradedindex microstructured fibres and hollow core fibres, the latter case being where light is guided in an air core. Microstructured polymer optical fibres are an exciting new development, offering opportunities to develop fibres for a wide range of applications in telecommunications and optical sensing. 2003 Elsevier Inc. All rights reserved. Keywords: Polymer optical fibres; Microstructure; Photonic crystal fibres; Photonic band gap fibre; Local area networks; Optical sensing 1. Introduction A new class of polymer optical fibres was reported recently [1 5], in which the guiding of light is achieved by the introduction of a pattern of microscopic air holes that run * Corresponding author. E-mail address: m.eijkelenborg@oftc.usyd.edu.au (M.A. van Eijkelenborg). 1068-5200/$ see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/s1068-5200(03)00045-2

200 M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 down the entire length of the fibre. This eliminates the need for chemical modifications to the polymer material as is required for many conventional polymer optical fibres. Microstructured polymer optical fibres (MPOFs) offer new opportunities and can significantly enhance the functionality of polymer optical fibres (POFs). The idea of guiding light using microstructure has previously been explored in silica fibres, in so-called photonic crystal fibre (PCF), microstructured fibre (MOF) or holey fibre (HF) [6,7]. It has been shown that single-mode guidance in a relatively large core is possible [8], that non-linear processes can be enhanced by orders of magnitude [9,10], and that guidance in an air core can be achieved when the microstructure is arranged to create a photonic band gap [6]. MPOFs, however, offer many advantages over their glass counterparts. To name a few, the fabrication of MPOF is much easier due to the much more favourable balance between surface tension and viscosity at the draw temperature which reduces the chance of holes collapsing; the MPOF microstructure is not restricted to close-packed arrangements of circular holes, as is the case for glass PCF fabricated by the capillary stacking technique; more material modifications are possible, owing to the much lower processing temperatures and the intrinsic tailorability of polymers; and the base materials and the fibre processing is cheaper, while the fibres remain flexible even at large diameters. The combination of low-cost fibre fabrication and large-spot single-mode or multimode guidance provides potential advantages for MPOF in applications such as local-area networks (LANs) or fibre-to-the-home (FTTH). In addition, polymers with high glass transition temperatures can be used for MPOF, and problems associated with dopant diffusion are eliminated, offering advantages for high temperature applications (for example in the automotive industry). An overarching advantage is that quite different MPOFs can be manufactured within the same fabrication framework, as simple changes in the microstructure can provide fibres with distinctly different functionalities. This is demonstrated in this paper by presenting an overview of recent MPOF work, including single-mode, highly-birefringent, twin-core, graded-index, and hollow-core MPOFs. 2. Fabrication and materials A range of different materials and fabrication methods can be used to make MPOF preforms. In addition to the capillary stacking technique, as is traditionally used for glass PCF, polymer preforms can be made using techniques such as extrusion, polymerisation in a mould, drilling, or injection moulding. With such techniques available, it becomes straightforward to obtain different cross-sections in the preform, with holes of arbitrary shapes and sizes in any desired arrangement. This is a major advantage over silica-glass based PCF, where the hole structure is mostly restricted to hexagonal or square closepacked structures due to the commonly used capillary stacking technique. In the present study, commercial extruded polymethylmethacrylate (PMMA) preforms with a glass transition temperature (Tg) of 115 C were used for proof of concept exploratory MPOF experiments. This material is of relatively poor optical quality; the scattering losses from impurities being around 3 db/m. After a proof of concept of a particular

M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 201 Fig. 1. Example of a single-mode microstructured polymer optical fibre (MPOF) with 1.9 µm diameter air holes spaced at 3.5 µm. MPOF, the material is replaced with purified or fluorinated PMMA to obtain low-loss properties. MPOF is drawn at a rate of several m/min at a constant tension of 60 to 100 g and a hot-zone temperature of 160 C. The resulting MPOF structures, such as that shown in Fig. 1, are maintained over lengths in excess of 100 m. Fibres are generally drawn to an external diameter of 200 µm, with a fibre diameter uniformity of ±1 µm achieved by utilisation of a well-tuned feedback control loop between the capstan speed and the fibre diameter monitor. In drawing a microstructured fibre, there exists a balance between the surface tension and the viscosity of the material at the draw temperature [11]. Clearly, a high draw temperature (leading to a lower viscosity), will result in surface tension effects dominating, leading to possible distortion of the hole structure, such as partial or complete collapse of the holes. A small reduction in the size of the air holes relative to the hole spacing is often observed in drawn fibres. However, this can be minimised by lowering the draw temperature, thus increasing the viscosity and maximising the fibre draw tension, although this approach is limited by the risk of fibre breakage. At the drawing temperature, the surface tension of PMMA is 0.032 N/m [12] and the viscosity was measured to be 5 10 6 Pa s. This compares favourably with the equivalent values for silica glass, though for many glasses accurate values for surface tension and its variation with temperature are unavailable. Vitreous silica, which is commonly used to fabricate PCF, has a surface tension of 0.30 N/m and a viscosity of 3.6 10 6 Pa s at the draw temperature [11,13]. Clearly, for PMMA the balance between surface tension and viscosity shows a ten times more favourable relationship compared to glass, making

202 M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 MPOF drawing with PMMA an inherently more robust process than glass PCF drawing. In addition, by avoiding the capillary stacking technique, the monolithic nature of the MPOF preform provides further robustness against structural deformation. 3. Single-mode MPOF Using MPOF technology we can readily produce single-mode polymer fibres a feat that is difficult with conventional POF technology due to the small core size and refractive index contrast required. Microstructured fibres such as shown in Fig. 1 have been shown to be single moded at a wavelength of 633 nm, by demonstrating that the near and far field patterns are insensitive to the launching conditions and bending of the fibre, and that an interference experiment with a standard glass single-mode fibre showed a clear interference pattern [1]. The chromatic dispersion of the single-mode MPOF reported in [1] was measured to be 100 ps/nm km at a wavelength of 855 nm, with a zero-dispersion wavelength of 1.35 µm [14]. Note that, as with conventional glass PCF, ultra-high dispersion microstructured fibres can be fabricated for applications in dispersion compensation. In addition, dispersion flattened MPOF can be fabricated, where the dispersion is small and constant over a broad wavelength range, or dispersion shifted fibres can be made, where the dispersion zero is shifted towards (or into) the visible range of the spectrum for non-linear applications. As part of our fabrication procedure, we have developed a polymer preform sleeving technique, which involves jacketing a microstructured cane of 3 mm diameter with a sleeve of 12 mm outer diameter and 6 mm inner diameter. The space between the cane and the sleeve is filled with capillaries to achieve a tight fit. After fusing and annealing of the sleeved preform, it is redrawn to obtain an MPOF with a much smaller inter-hole spacing and hence a smaller core size. An electron microscope image of one such fibres is shown in Fig. 2a with a hole spacing of 1.3 µm, along with a 0.5 µm hole diameter. The strong confinement achieved by the size-reduction of the hole structure increases effects such as waveguide dispersion, birefringence and non-linearity, each with their own potential applications. Figure 2b shows a contour plot of the measured near-field guided-mode intensity profile of this fibre. 4. Graded-index multi-mode MPOF Multi-mode graded-index POF (GI-POF) has been previously developed for data communication applications, providing thick flexible fibres with high bandwidth and large spot sizes for easy installation in LANs. However, the technology behind conventional GI-POF is very complex and costly, since a polymerisation process that produces a near-perfect parabolic index profile is required for efficient compensation of the modal dispersion [15]. This is seen as an opportunity for employment of MPOF technology, by allowing a costeffective alternative. It has been shown that for small enough hole structures, the microstructure in the fibre remains unresolved by the light, and the whole structure can be approximated by an effective refractive index profile. This has been explored in a previ-

M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 203 Fig. 2. (a) Electron microscope image of a single-mode microstructured polymer optical fibre with inter hole spacing of 1.3 and 0.5 µm diameter holes. The core diameter is 2.1 µm. (Image courtesy of the Electron Microscope Unit, Sydney University.) (b) Contour plot of the measured near-field intensity profile of the guided mode in the core of the fibre. Contours trace the 90, 70, 30, and 5% intensity levels.

204 M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 Fig. 3. Graded-index microstructured polymer optical fibre (GIMPOF). ous paper, where MPOFs with multiple depressed-index rings were presented [2]. Using the average-index effect, we have fabricated a large-core multi-mode graded-index MPOF (GIMPOF) by using a graded hole structure as shown in Fig. 3. Here, the hole diameter increases with distance from the centre, so that the azimuthal average provides an approximation to the ideal near parabolic graded-index profile to compensate for modal dispersion [4]. The GIMPOF in Fig. 3 was drawn from a preform with 216 holes of varying diameter (1.2, 1.5, 2.0, 2.5, 4.0 mm). It has an outer diameter of 220 µm and a core region of 65 µm diameter. Large-spot multi-mode guidance is observed in these fibres, but more work is required to establish the potential transmission bandwidth. Experiments are planned with low-loss materials to further investigate this application. 5. Highly birefringent MPOF Conventional circularly symmetric optical fibres do not maintain the polarisation state of the guided mode along their length. Bends and stresses in the fibre introduce regions of birefringence with varying orientation, leading to changes in the polarisation of the guided light. The use of highly birefringent (HiBi) fibres can reduce such environmental influences. In conventional fibres, form birefringence (as induced by a non-circular core shape) leads to relatively small amounts of birefringence. Stress-induced birefringence, achieved through introducing stress elements next to the core (as e.g., in bow-tie or panda fibres) is generally much stronger. In microstructured fibres this situation is different, and form

M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 205 (a) (b) Fig. 4. Example of a single-mode highly birefringent MPOF: (a) preform and (b) close up of the fibre structure. birefringence can in fact be the dominant birefringence mechanism due to the large index contrast between the air holes and the host material. HiBi microstructured fibres can be fabricated by introducing an asymmetry in the waveguide structure, e.g., by using arrangements of different hole sizes and spacing in order to break the symmetry, or alternatively by using non-circular shaped holes. A HiBi microstructured fibre with a beat length below 1 mm arising from an arrangement of two different hole sizes has previously been demonstrated in silica [16]. Compared to conventional HiBi fibres, birefringence arising from microstructure asymmetries is relatively temperature insensitive, which is an important benefit for sensing applications. We have developed two techniques to fabricate MPOFs such as shown in Fig. 4. This fibre was designed to exhibit high levels of form birefringence due to the elliptically shaped holes around the core and the asymmetrical shape of the core. The fabrication of the preform with elliptical holes (Fig. 4a) was achieved by a combination of squashing and annealing a preform that initially had circular holes. This structure is then maintained in the fibre draw [M. Fellew et al., unpublished]. The birefringence of a fibre such as that shown in Fig. 4b was measured using the technique reported in [16]. White light is launched into the fibre polarised at 45 to the birefringence axis, while the output of the fibre is collected with an optical spectrum analyser after a polariser orientated at 90 to the launch polarisation. The features of the measured spectrum allow a determination of the birefringence as well as the variation of birefringence versus wavelength. Polarisation beat lengths of 8 mm at 800 nm were measured. Much smaller beat lengths are expected by reducing the dimensions of the structure and by optimising the asymmetry of the microstructure design. By further exploring the effects

206 M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 of non-circular holes, we expect that highly birefringent MPOF with beat lengths below 1 mm (birefringence larger than 10 3 ) can be fabricated in the near future. 6. Twin-core MPOF A twin-core MPOF such as that shown in Fig. 5 has been fabricated by the omission of two (rather than one hole) to create the cores. The fabrication of twin-core MPOF is much less involved than the fabrication of conventional twin-core silica fibre, which requires chemical vapour deposition to make two preforms, which are subsequently sliced in half (slightly off centre), polished and fused to form a single preform with two cores. Fabrication of silica fibre with more than two cores is even more complicated still. In contrast, multiple-core MPOF is fabricated in a single stage, using exactly the same techniques as used in the single-core case. Multi-core fibres are attractive for applications in telecommunications and optical fibre sensing. One example is in the use of a twin-core MOF for strain measurement, specifically for measuring curvature in engineering structures by interrogating the two cores interferometrically [17]. As the two cores are embedded in the same cladding structure, common mode rejection is very effective at preventing unwanted sensitivity to external disturbances. In addition, the exit face of a twin-core fibre, with two single-mode localised spots, acts as an ideal fringe projector. The periodicity in the twin-core fibre in Fig. 5 is 4.8 µm, leading to a spacing of 9.6 µm between the centres of the two cores. The coupling length of this fibre was measured by launching white light into one core and collecting light from the other end with a spectrum analyser. From the observed periodicity in the measured spectrum, a coupling length of 6.8 mm at 650 nm could be inferred. Calculations were performed using the ABC FDM method [18], taking into account the actual structure of the fibre, including slight structural asymmetries. One of the holes next to the core was found to be about 25% larger than the others, and as a result, a remarkable Fig. 5. Twin core MPOF with two cores separated by 9.6 µm.

M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 207 reduction in the coupling length arose. Good agreement between experiment and theory was obtained [W. Padden et al., unpublished]. 7. Photonic band gap structures In a photonic band gap fibre, light is guided due to the photonic band gap (PBG) effect, similar to an electronic band gap for electrons in semiconductors. In this case, the microstructure prohibits propagation of certain wavelengths through the cladding. This effect can be used to confine light to a fibre core. Since PBG guidance does not rely on total internal reflection, the core can either be a solid, or, more interestingly, an air core. This effect has been previously demonstrated in silica PCF with a large and very regular hexagonal pattern of holes [6,10]. The creation of an air-core guiding PBG MPOF could reduce the effects of material absorption, and thus provide a possible new route to achieve further reduction of the losses of POFs. Air-cored PBG polymer fibres have been fabricated, an example is shown in Fig. 6. Evidence of photonic band gap guiding through short lengths of fibre with a structure as shown in Fig. 6b has been observed [19]; a brightly coloured orange mode was transmitted through the air core when white light was launched into the fibre, a signature of PBG guiding. Unfortunately, these initial results have proved difficult to reproduce, a fact that has been attributed to non-uniformities in the fibre structure, both in the transverse and longitudinal direction, which lead to a closing of the band gap. On-going work is focusing on the fabrication of PBG MPOF with a larger air fraction, a larger core and smaller hole spacing, in order to achieve a larger bandwidth for the guided light. In addition, MPOFs in which the air holes are positioned on concentric rings are being investigated in the context of multi-layer Bragg fibres [2]. As with PBG guiding, Bragg (a) (b) Fig. 6. Examples of photonic band gap MPOF: (a) an air-core polymer PBG fibre of 220 µm external diameter and (b) a close-up of the microstructure with a 5 µm hole spacing.

208 M.A. van Eijkelenborg et al. / Optical Fiber Technology 9 (2003) 199 209 guidance also facilitates guidance in an air core, and it has been predicted that pure singlemode guiding of a polarisation non-degenerate mode is possible with Bragg fibres [20,21]. 8. Conclusions Microstructured polymer optical fibres (MPOFs) offer intriguing possibilities for developing fibres with new functionalities. In this paper, we have presented an overview of our recently fabricated MPOFs, including single-mode, highly birefringent, graded-index, and air-core MPOF. This new class of polymer fibres clearly has a bright future ahead of it, and we have only just begun to realise the true potential of these fibres. Acknowledgments We acknowledge the Australian Photonics Cooperative Research Centre and the Australian Research Council for partial funding of this work. We would also like to thank Barry Reed for the preparation of the preforms and the Electron Microscope Unit at the University of Sydney for the electron microscope image. References [1] M.A. van Eijkelenborg, M.C.J. Large, A. Argyros, J. Zagari, S. Manos, N.A. Issa, I. Bassett, S. Fleming, R.C. McPhedran, C.M. de Sterke, N.A.P. Nicorovici, Microstructured polymer optical fibre, Opt. Express 9 (7) (2001) 319 327. [2] A. Argyros, I.M. Bassett, M.A. van Eijkelenborg, M.C.J. Large, J. Zagari, N.A.P. Nicorovici, R.C. McPhedran, C.M. de Sterke, Ring structures in microstructured polymer optical fibres, Opt. Express 9 (13) (2001) 813 820. [3] M.C.J. Large, M.A. van Eijkelenborg, A. Argyros, J. Zagari, S. Manos, N.A. Issa, I. Bassett, S. Fleming, R.C. McPhedran, C.M. de Sterke, N.A.P. Nicorovici, Microstructured polymer optical fibres: a new approach to POFs, in: POF 2001 Conference, Amsterdam, The Netherlands, September, 2001, post deadline paper. [4] M.C.J. Large, M.A. van Eijkelenborg, A. Argyros, J. Zagari, S. Manos, N.A. Issa, I. Bassett, S. Fleming, R.C. McPhedran, C.M. de Sterke, N.A.P. Nicorovici, Microstructured polymer optical fibres: progress and promise, in: Proceedings of SPIE, Vol. 4616, San Jose, CA, January, 2002, paper 33. [5] M.C.J. Large, M.A. van Eijkelenborg, A. Argyros, J. Zagari, S. Manos, N.A. Issa, I. Bassett, S. Fleming, R.C. McPhedran, C.M. de Sterke, N.A.P. Nicorovici, Single-mode microstructured polymer optical fibre, in: Optical Fibre Communication Conference (OFC 2002), March, 2002, paper ThS. [6] R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.St.J. Russell, P.J. Roberts, D.C. Allan, Single mode photonic band gap guidance of light in air, Science 285 (1999) 1537 1539. [7] T.M. Monro, W. Belardi, K. Furusawa, J.C. Baggett, N.G.R. Broderick, D.J. Richardson, Sensing with microstructured optical fibres, Meas. Sci. Technol. 12 (2001) 854 858. [8] J.C. Knight, T.A. Birks, R.F. Cregan, P.St.J. Russell, J.-P. de Sandro, Large mode area photonic crystal fibre, Electron. Lett. 34 (1999) 1347. [9] T.M. Monro, K.M. Kiang, J.H. Lee, K. Frampton, Z. Yuso, R. Moore, J. Tucknott, D.W. Hewak, H.N. Rutt, D.J. Richardson, High nonlinearity extruded single-mode holey optical fibers, in: Optical Fiber Communications Conference, 2002, PD-FA1. [10] F. Benabid, J.C. Knight, G. Antonopoulos, P.St.J. Russell, Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber, Science 298 (2002) 375 399.

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