1. Introduction

Polymer optical fibre (POF) technology has advanced rapidly in recent years, with the expectation that POF will form an integral part of datacom networks. POF may be one technology that will provide the backbone of the critical last mile of the telecommunications networks, offering a high-bandwidth, easy to install, fibre-optic replacement for copper cables. In contrast to a glass optical fibre, a thick polymer fibre will remain mechanically flexible, which, in combination with a large core, allows easy and inexpensive connectivity of the fibres during installation. Data transmission rates of 11 Gbits/sec over 100 meters at 1300 nm in POF have been demonstrated, and a POF-based local-area network has been installed at Keio University in Japan [1].

Despite these achievements, however, POF has not yet achieved widespread deployment in either datacom applications or telecommunications networks. The technical reasons behind this are clear; conventional POF has a number of disadvantages. Traditional large-core multimode step-index POF suffers from very large modal dispersion. The fabrication of single-mode POF, ideal for telecommunication purposes, has proved to be challenging, and the associated small mode area limits the applications. The technology used to manufacture the current state-of-the-art POF, the large mode-area graded-index multi-mode POF (GI POF), relies on a complex polymerization process to obtain a particular graded refractive-index profile across the diameter of the fibre. This process involves free radical polymerization and the selective addition of a low molecular weight dopant to the polymer, which requires the use of a polymer with a relatively high glass transition temperature to prevent diffusion of the dopant at normal operating temperatures. The choice of monomers that will form amorphous glassy polymers with high glass-transition temperatures by free-radical polymerization is extremely limited. A further reduction of the number of useful polymers is related to the absorption losses in the material, which has led to the use of fluorinated polymers. However, even in the case of these fully fluorinated materials, the use of conventional POF is limited to a few hundred metres.

In this paper, we report the fabrication of the first microstructured polymer optical fibre (MPOF). This new type of POF is likely to be of great importance, because all of the disadvantages of conventional POF are potentially addressed by the use of MPOF. The light guiding mechanism in MPOF is fundamentally different from conventional POF. It arises from a pattern of microscopic air channels that run along the length of the fibre, rather than from variations in the refractive index of the fibre material. This mechanism was demonstrated in recent years in microstructured silica optical fibres, also called photonic crystal fibres (PCFs) [2,3]. The application of this guiding mechanism to POF offers a range of unique new properties and benefits. These include the possibility to combine single-mode behavior with large mode areas [4], which is the ideal combination for easy-to-install transmission fibre. In addition, the recently developed photonic band gap fibres [5] offer the promise of guidance in air rather than in the fibre material, allowing the problem of absorptive losses of polymers to be addressed. The potential benefits of MPOF will be discussed further in section 4, after the presentation of the experimental results in section 2 and the theoretical results in section 3.

2. The single-mode microstructured polymer optical fibre

MPOF preforms were fabricated using commercially available extruded polymethyl methacrylate, PMMA. The details of the fabrication methods will be described in a subsequent paper. The PMMA was of poor optical quality, and attenuation tests on an unstructured fibre drawn from this material indicated an absorption loss of about 32 dB/m at a wavelength of 632.8 nm. MPOF was drawn on a polymer fibre draw tower at a rate of 10 m/min at a temperature of approximately 175°C and to an outer diameter of 250 µm.

 

Fig.1. Electron micrographs of the microstructured polymer optical fibre (MPOF).

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Electron microscope images of the fibre are shown in Figure 1. It can be seen that the air hole microstructure consists of four rings of holes in a hexagonal pattern, embedded in an outer sleeve. Small deformations are visible, such as in the hole diameters and shapes. Compared to the preform that the fibre was drawn from, the hole structure in the fibre has a slightly reduced hole diameter to hole spacing ratio, d/Λ=0.46, whereas in the preform d/Λ=0.67. The optical experiments reported below were performed on a piece of this fibre, with an average hole diameter d=1.3 µm and an average hole spacing Λ=2.8 µm, which defines a core size of 4.3 µm.

To demonstrate that the MPOF effectively guides only a single mode, three independent experiments were conducted, identical to those reported to confirm the single-mode operation of glass microstructured fibres [4,6,7]. In the first experiment, HeNe laser light of 632.8 nm was launched into a 1-m length of MPOF by butt-coupling to a multi-mode silica fibre. The fibre output was observed, both in the near field and far field. It was found that both patterns were independent of the launching conditions from the multimode fibre (both angle and position) and independent of any bends or twists in the fibre. This is strong evidence of single-mode guiding [4].

In the second experiment, HeNe laser light was launched directly into the MPOF with a 10x microscope objective and the cladding modes were stripped off with index matching fluid. The near field and far field intensity patterns were recorded with a digital camera, and the results are shown in Fig. 2 a), b) and c). These images are very similar to those obtained for single-mode glass PCF [6]. The hexagonal symmetry is clear, though some distortions are visible, both in the fibre and the mode profiles. These are attributed to the hot-knife method that was used to cleave the fibre. This cleaving method results in a very short, tightly bent section of fibre just before the exit face, which causes the light to emerge from the fibre with a slight transverse offset, distorting the overall hexagonal symmetry. As in the previous experiment, the patterns were found to be independent of the launching conditions and of fibre bending, again confirming single-mode guidance [6].

In the third experiment, a spatial interference measurement was performed, between the collimated output mode of the MPOF and the output of a standard single-mode glass fibre. The resulting stable interference pattern is shown in Fig. 2 d). A translation of the fringes could be observed due to thermal and mechanical disturbances, and as a function of fibre bending. The clear fringe pattern with a visibility close to unity indicates that only a single mode was guided in the microstructured fibre [7].

 

Fig.2. Optical testing of the single mode guiding of the microstructured polymer optical fibre (MPOF). a) the mode pattern in the near field, b) a contour plot of the near field pattern, c) the far field mode pattern d) the interference pattern between a standard single-mode fibre and the MPOF. The white patches in the images a) and c) are due to overexposure of the camera.

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As verified experimentally in three different ways, the MPOF consistently behaves as a single-mode guiding structure at a wavelength of 632.8 nm. The near field and far field mode patterns, though distorted, showed no evidence of the structure associated with higher-order modes (see also section 3). It is important to mention that in addition to the single-mode fibre, we also fabricated a multi-mode MPOF. The different behavior of this multi-mode fibre in the experiments described above is striking. The details of the multi-mode MPOF will be reported elsewhere.

Bending of both the single-mode and multi-mode MPOF to a radius of about 3 mm showed no significant change in transmitted power, indicating a low critical bending radius. Unfortunately the high losses of the PMMA made it difficult to perform further optical tests such as a determination of the bending losses and dispersion measurements. In future experiments, the poor quality PMMA will be replaced with a low-loss optical-quality PMMA and other low-loss polymers.

3. Modeling of the structure

We modeled the MPOF using a recently developed multipole method [8,9]. Compared with other modeling techniques, it has advantages in terms of speed, accuracy, and the ability to calculate confinement losses [8], although at present it is restricted to holes of circular cross-section. Using this method, we modeled regular structures composed of varying numbers of rings of holes of diameter d=1.3 µm and spacing Λ=2.8 µm, inserted in a lossless matrix of index n=1.4897 for PMMA at 632.8 nm. The dimensions correspond to the fibre parameters reported in section 2. We found the structure to have three bound modes, two of which are doubly degenerate, together with a nondegenerate mode located in close proximity to the second degenerate mode. In terms of the effective indices n_eff=β/k₀, where β is the propagation constant and k₀ is the free space wave number, we find Re(n_eff)=1.48655 for the fundamental mode, independent of the number of rings. For both higher order modes Re(n_eff)=1.48240 for a four-ringed structure, with a weak dependence on the number of rings. The beat length between the fundamental and higher order modes is only 150 µm at 632.8 nm, essentially precluding internal mode coupling due to random perturbations inside the fiber. The energy flow in the fiber direction for the two degenerate modes in a two-ringed structure is shown in Fig. 3.

It is important to note that the modes’ confinement losses, which are associated with the finite number of rings [8], are vastly different. The reduction of confinement loss with an increasing number of rings is quite different for the fundamental mode and the first two higher-order modes. For two, three and four rings, the losses for the fundamental mode are respectively 2.4, 5×10^-3 and 3×10^-6 dB/m. The losses of all the higher-order modes are roughly equal, and they decrease much more slowly: 2×10³, 5×10² and 2×10² dB/m respectively. In the four-ring case (which corresponds to the MPOF in section 2) the fundamental mode thus exhibits a loss of 3×10^-6 dB/m, which is much smaller than the absorption loss of 32 dB/m, and is therefore essentially negligible. In contrast, the higher-order modes have a loss around 200 dB/m. Not only is this confinement loss much larger than the absorption loss, it also precludes any light that was originally in these modes being transmitted through the 1-m length of fiber. The MPOF structure therefore behaves as if it is single moded at λ=632.8 nm for 2, 3, 4 and probably more rings. These theoretical results are entirely consistent with the experimental demonstration of single-mode behavior.

 

Fig.3. Axial component of the Poynting vector for the first two degenerate modes of a two-ring MOF (d=1.3 µm, Λ=2.8 µm, matrix index n=1.4897, λ=632.8 nm). The holes of the fibre are located at the positions where indents are observed in the mode profile of the second mode.

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4. Advantages of microstructured polymer optical fibre

Microstructured optical fibres have, to date, only been fabricated in glass, which usually entails the stacking of circular glass capillaries to form a preform with a close packed hexagonal arrangement of air holes. With this technique, it is difficult to obtain alternative geometries or hole shapes. In this respect, MPOF has a number of advantages. The much lower processing temperatures of polymers and the controllability of the polymerisation process allow a variety of ways to produce the polymer preforms. In addition to the capillary stacking technique, polymer performs can be made using techniques such as extrusion, polymer casting, polymerization in a mould and 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.

There are, in fact, many applications for which it is desirable to have holes of specific shapes in the fibre, such as for polarization-maintaining microstructured fibre [10,11]. Creating holes of specific shapes in a glass microstructured fibre is problematic because the balance between viscosity and surface tension tends to modify the hole structure during the draw process, and often the structure in the fibre is significantly different from that in the preform [12,13]. In addition, the drawing temperature is a crucial parameter when drawing glass microstructured fibre; temperature changes of a few percent can lead to very significant changes in the fibre microstructure. This does not seem to be the case for MPOF, which can be drawn over a large temperature range. For the PMMA example in this paper, the drawing temperature could be varied from 150 °C to 200 °C without significant change to the fibre structure. There seems to be a much closer relationship between the preform structure and the final structure in the fibre. The reason for this is well understood in polymer processing. The drawing process aligns the polymer chains, so that the final material is anisotropic along the fibre direction, and has enhanced strength properties [14]. Annealing after the draw can be used to relieve the internal stresses while maintaining the microstructure in the fibre.

In glass fibres, the possibilities for modifying the properties by doping are limited both by the high processing temperatures, which cause many materials to decompose, and the need to avoid phase separation. By contrast, polymers are intrinsically modifiable; it is possible to design and manufacture polymers that include any atomic species, molecular components, dispersed molecules and dispersed phases. For example, the use of techniques such as grafting of functional groups onto the polymer and co-polymerisation make it possible to obtain much higher concentrations of the desired additive than is possible by doping. In addition, by use of surfactant or block co-polymer techniques, it is possible to obtain substantial quantities of inclusions. Examples of the types of materials that could be used in MPOF are: polymers with enhanced non-linearities, electro or magneto-optic effects, metallic or rare-earth inclusions, birefringent materials such as liquid crystals, photorefractive and photochromic materials, dyes, polymers used in the detection of particular compounds and porous materials. The polymers can be specifically designed to allow the fabrication of particular fibre-optic components based on MPOF.

MPOF also has a number of very important advantages over conventional POF. MPOF can be fabricated from a single polymer, without the use of dopants. This eliminates the restrictions related to the high glass-transition temperature and the use of free radical polymerization as required for conventional GI POF. As a result, a much larger range of polymers is available for MPOF, including condensation polymers, catalytically formed polymers, biopolymers, sol-gel polymers and chain addition polymers. This provides great potential for reducing the absorption losses in the fibre material, thereby extending the useful range of POF beyond 200 m, and for finding a replacement for the expensive fluoropolymers. In addition, there is potentially an exceptionally low cost associated with the production of MPOF. Since the fibres are fabricated from a single polymeric material, no complex chemistry is required, allowing low-cost large-volume production, possibly involving extrusion.

5. Conclusions

Optical guiding in the first polymer microstructured optical fibre (MPOF) has been demonstrated. Experimental evidence has been presented to establish that the MPOF is effectively single moded at optical wavelengths. This is in agreement with our theoretical calculations, which confirm that the MPOF structure effectively guides only a single-mode for two, three and four rings of air holes.

MPOFs have strong potential for applications in telecommunication networks, both as low-cost, low-loss transmission fibre as well as for fibre-optic components. Compared to GI POF, MPOF has important advantages, such as the lifting of a number of the material restrictions. In addition, MPOF can in principle be fabricated at a much lower cost. Compared to glass microstructured fibres, MPOFs effectively have an additional two degrees of freedom: the variety of fibre cross-sections that can be produced is much less restricted, and the choice of available material properties is much greater.

The fabrication of a range of more complex MPOF has been initiated, incorporating the fabrication of single-mode large mode-area MPOF, effective graded-index multi-mode MPOF and polymer photonic band gap fibre. In addition, MPOF will be fabricated with new materials, exploring the possibilities for low-loss transmission and novel functionalities.