1. Field of the Invention
This invention relates to a process for manufacturing a micro-structured optical fibre and to a method for producing a structured intermediate preform to be used in such a process.
2. Description of the Related Art
Optical fibres are used for transmitting light from one place to another. Normally, optical fibres are made of more than one material. A first material is used to form a central light-carrying part of the fibre known as the core, while a second material surrounds the first material and forms a part of the fibre known as the cladding. Light can become trapped within the core by total internal reflection at the core/cladding interface.
A more recent type of optical fibre waveguide, having a significantly different structure from that of conventional optical fibres, is the micro-structured fibre (also known as “photonic crystal fibre” or “holey fibre”). A micro-structured optical fibre is a fibre made of a same homogeneous material (typically silica), having inside a micro-structure (i.e. a structure on the scale of the optical wavelength) defined by micro-structural elements extending longitudinally along the fibre and having a predetermined distribution. As a micro-structural element it is possible to identify a micro-hole or a filiform element of a different material than the bulk.
The most common type of micro-structured optical fibre has a cladding region showing a plurality of equally-spaced tiny holes, surrounding a homogeneous and uniform central (core) region. A fibre of this type is described, for example, in international patent application WO 99/00685. In a different embodiment, the central region of the fibre may have a central hole, as described, for example, in international patent application WO 00/60388.
These two types of fibres convey light in the core according to different optical phenomena.
In the absence of a central hole, propagation of light in the cladding region is forbidden due to the presence of a lowering of the average refractive index with respect to the core region. Such a structure forms a low-loss all-silica optical waveguide that, for appropriate parameters, remains single-mode for all wavelengths within the transmission window of the silica. The waveguiding mechanism in that case is closely related to that in conventional optical fibres and is a form of total internal reflection between two materials (air and silica) having different refractive indexes.
In order to achieve light propagation in a central hole, the “photonic band-gap” effect, as induced by the presence of a periodic array of holes in the cladding region, is exploited. The “photonic band-gap” phenomenon, which is analogous to the “electronic band-gap” known in solid-state physics, avoids light of certain frequencies to propagate in the zone occupied by the array of holes, this light being therefore confined in the core region. Propagation of light in fibres showing a photonic band gap is described, for example, in already cited WO 00/60388.
Optical characteristics of the above-described micro-structured fibres depend on the number of holes, the holes diameter, the distance between adjacent holes and the hole geometrical pattern. Since each of these parameters can broadly vary, fibres of very different characteristics can be designed.
It has been shown that, by properly adjusting the ratio between hole diameter d and hole-to-hole distance Λ, single mode or few mode behaviour over a wide wavelength range can be obtained.
Micro-structured optical fibres are typically manufactured by the so-called “stack-and-draw” method, wherein a number of solid and hollow rods are stacked inside a hollow glass cylinder, so as to constitute an array with the same structure as that of the final fibre. The stacked rods are then welded together, and the preform so obtained is drawn by conventional methods, producing the fibre.
In U.S. Pat. No. 5,802,236A for example, a core element (e.g., a silica rod) and a multiplicity of capillary tubes (e.g., silica tubes) are provided, and the capillary tubes are arranged as a bundle, with the core element typically in the centre of the bundle. The bundle is held together by one or more overclad tubes that are collapsed onto the bundle. The fibre is then drawn from the thus prepared preform.
The Applicant has noted that the stack-and-draw manufacturing method has several drawbacks.
Assembling a large number of very thin canes (defined by rods or tubes) is an awkward operation. Also it is likely to generate interstitial cavities due to stacking and drawing of cylindrical canes. This may affect dramatically the fibre attenuation by introducing impurities, undesired interfaces and inducing a reshaping or deformation of the starting holes. Other problems of the stack-and-draw method may be represented by the low purity of the tubes and/or rods materials and by the difficulties in producing tubes and/or rods of the required shapes (in particular, in the case of hexagonal bodies) and dimensions and in obtaining the required pattern of holes (due for example to the difficulty in realizing geometries different from triangular when positioning rods and tubes in close-packed arrangement). Moreover, the relatively low productivity and high cost make this method less than optimal for industrial production.
The Applicant further observes that the stack-and-draw method for fabricating a holey optical fibre has the drawback that the holes and the core dimensions in the final fibre are limited by the inner and outer diameter of the tubes and rod employed in the assembly.
Polymeric optical fibres (POFs) are known as a promising alternative to glass fibres, in particular for short distance applications, in view of their low cost, ease of connectorization, and flexibility. Ordinary polymer fibres have a relatively large core. This causes the fibres to guide a very high number of transverse modes, with significant limitations to the available bandwidth.
Recently, a number of papers concerning microstructured polymeric fibres have been presented.
M. A. van Eijkelenborg et al., Microstructured Polymer Optical Fibre, Optics Express, Vol. 9, No. 7, 24 Sep. 2001, pp. 319–327, disclose a polymer-based microstructured fibre that is single moded at optical wavelengths. According to the authors, polymer preforms, in addition to the capillary stacking technique, can be made using techniques such as extrusion, polymer casting, polymerisation in a mould and injection moulding. Different cross sections of the preform can be obtained, with holes of arbitrary shapes and sizes in any desired arrangement. A large range of polymers is available for microstructured polymer optical fibres, including condensation polymers, catalytically formed polymers, biopolymers, sol-gel polymers and chain addition polymers.
The paper does not disclose details of the preform production process. The authors, during a post-deadline oral presentation at the POF2001 conference, Amsterdam, NL, Sep. 27–30, 2001, mentioned the technique of casting around plastic capillaries that exploits the low processing temperatures of polymers. As disclosed in J. Choi, D. Y. Kim, U. C. Paek, Fabrication and Properties of Polymer Photonic Crystal Fibers, Proceedings POF2001, Amsterdam, NL, Sep. 27–30, 2001, pp. 355–360, polymer photonic crystal fibres have been drawn from a polymethylmethacrylate (PMMA) preform. The fibre consisted of a pure PMMA core surrounded by a photonic crystal pattern with air holes of a hexagonal symmetry running along the length of the fibre. For fabrication of the preforms, thermal polymerisation of methylmethacrylate (MMA) is performed. Once the polymerisation is completed, an aging process is conducted in a vacuum oven with reduced pressure for 12 hours, then the temperature is slowly reduced to room temperature. A fibre is then drawn in a drawing tower while setting the furnace at a temperature of 270° C. A resulting polymer photonic crystal fibre is shown that has a diameter of 190 μM with air hole diameter of about 11 μm and core diameter of 25 μm.
This paper does not disclose details of how the hexagonal hole pattern is impressed in the fibre preform.
JP 6-67040, in the name of The Furukawa Electric Co., discloses the production of a hollow-core plastic optical fibre for endoscopic applications. A hollow preform for drawing such fibre is produced by filling a thermoplastic resin into a cylindrical mould provided with a base and with a cylindrical core protruding coaxially from the mould base. In a different embodiment, the cylindrical core is manufactured as a separate body from the cylindrical mould and is detachably mounted to the mould base. In the reported examples, the cylindrical core had a diameter between 9.5 and 10 mm and was coated with a polytetrafluoroethylene tube, so as to result in a hole diameter between 11 and 13 mm for the preforins. In the drawn hollow core fibres the diameter of the hollow core was between 470 and 530 μm.
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