Method for the fabrication of polarisation-maintaining optical fibres

Abstract: To manufacture polarisation-maintaining optical fibres, a fibre (8) is directly and continuously irradiated with at least one beam (9) of UV radiation during drawing and before application of external coatings.

Description: The invention described herein relates to physical carriers for optical communications and in particular its object is to provide a method for the fabrication of monomode polarisation-maintaining optical fibres. In many applications of monomode optical fibres - including coherent communication systems - it is important for the optical signal which propagates along a fibre to maintain its polarisation characteristics constant and stable also with respect to a reference rigidly connected to the fibre. A way to obtain this condition is to use, as a transmission medium, a so-called polarisation-maintaining fibre. A polarisation-maintaining fibre is essentially a birefringent element, i.e. an element with different refractive indices in two directions orthogonal with respect to each other and to the axis of the fibre. In the production of these fibres, in particular for use in telecommunications, two techniques are generally employed to obtain birefringence in a naturally isotropic material, such as glass. The first technique entails the production of fibres whose core does not have axial symmetry, e.g. it is elliptical or rectangular or is associated to external elements which after the distribution of the guided electromagnetic field, and the second technique entails the fabrication of fibres whose core is kept in a condition of transverse mechanical stress. The main drawback of these techniques is that they require dedicated manufacturing processes, which cannot be used to manufacture conventional fibres. It would, instead, be desirable to use a technique allowing to induce birefringence in the course of a manufacturing process which can also be used for conventional fibres. The paper "Single Pulse Bragg Gratings Written during Fibre Drawing" by L. Dong et al., Electronics Letters, 19 August 1993, Vol. 29, No. 17, pp. 1577-1578, describes a method to fabricate gratings with periodic variation of the refractive index inside the core of a fibre, while the fibre is being drawn. The method produces refractive index alterations in the germania doped silica, utilising a UV beam, of such energy as to break the Ge-Ge bridges present inside the doped silica matrix. In particular according to the cited paper, the fibre is irradiated, in an area immediately upstream of the devices applying the coating, with a wave front resulting from the interference of two parts of a same pulse which have been sent along the two branches of an interferometer and which recombine in correspondence with the core of the fibre: a periodic modulation of the refractive index of the core is thus obtained, which brings about the periodic variation in reflectivity necessary to fabricate a grating. Instead of the interferometer a phase mask can be used, as described for instance by K. O. Hill et al. in "Bragg gratings fabricated in monomode photosensitive optical fibre by UV exposure through a phase mask", Applied Physics Letters, Vol. 62, No. 10, 8 March 1993. Although the method described in the paper by L. Dong is oriented to the fabrication of a sequence of discrete components, the same technique could be used, in theory, to realise a grating distributed along a whole fibre, which would thus be a polarisation-maintaining fibre, since the profile of the refractive index is different along different axes of a same section of the fibre. However such a solution is arduous to realise in practice: since the grating is "written" into the fibre in a sequence of steps, the grating could present discontinuities. Moreover, the fabrication of a grating presents problems as to the correct alignment of the means which bring about the modulation, especially if it is desired to after the profile of the refractive index of the original matrix according to multiple, different axes; furthermore, a grating is an intrinsically wavelength-selective structure, which limits the flexibility of employment of the fibres thus obtained. EP-A-0045345 concerns the production of polarisation-maintaining optical glass fibres by radiation-induced modifications of the core, and more particularly obtaining a strain region by an ionic bombardment. According to the invention, a method is instead provided which allows to produce the desired profile, even according to multiple axes, and to obtain non-selective structures. This is obtained in that the beam irradiates the fibre directly and continuously, and it is sent towards the fibre with such an aperture that, by effect of the refractive index variations at the interface between the outside environment and the cladding of the fibre and between cladding and core, the beam going through the core is a collimated beam. The invention shall be better understood from the following description, with reference to the enclosed drawings, where: Figure 1 is a schematic representation of an optical fibre drawing apparatus, to which the invention is applied; and Figures 2 and 3 are schematic representations, in enlarged scale, of the irradiated area of the fibre. Figure 1 shows a conventional drawing tower for optical fibres, indicated as a whole by 100, comprising a chuck 1 supporting preform 2, a furnace 3, means 4 for monitoring the diameter of fibre 8 being drawn, a coating cup 5 to apply the external polymeric coating to the fibre, coating curing UV lamp 6, and take-up capstan 7 for drawing fibre 8. Before reaching the coating cup, the fibre is irradiated continuously and directly with a UV radiation beam 9 emitted by a source 10, e.g. a KrF excimer laser, with emission wavelength λ = 248 nm. The UV beam is sent towards the fibre by means of a suitable optical system, represented schematically by lens 11. The optical system makes beam 9 arrive on the surface of fibre 8 with such an aperture that, taking into account the refractive index steps at the interfaces between air and fibre cladding 12 and between cladding 12 and core 13, the beam is essentially collimated in correspondence with core 13, as shown in Figure 2. Irradiation with a UV beam, as stated above, causes an increase in the refractive index of the silica glass along the axis corresponding with the direction of propagation of the beam, to an extent linked to the power of the beam and to the characteristics of the matrix. For example, in the case of a SiO 2 - GeO 2 matrix, an increase in refractive index equal to 3·10 -4 can be obtained by irradiating with a source whose power is 300 mJ/cm 2 . Consequently, the irradiated core behaves like a core having different dimensions along two perpendicular axes and therefore the fibre is actually a polarisation-maintaining fibre. Obviously, the fibre can be irradiated with multiple beams sent along different directions (e.g. two beams 9,9' crossing the core in perpendicular directions, as shown in Figure 3) and with different powers, in order to maintain asymmetry in the refractive index profile in different directions. Thanks to direct irradiation, the alignment between the different beams presents no problems.

Claims: Method for the fabrication of polarisation-maintaining silica optical fibres, in which a preform (2) is drawn into a fibre (8) comprising a core (13) wherein at least one dopant suitable to be broken in its molecular bridges by a UV radiation is present, and a cladding (12), and the fibre (8) is irradiated, during drawing and before the application of external coatings, with at least one UV radiation beam (9), characterised in that said beam (9) irradiates the fibre (8) directly and continuously, and it is sent towards the fibre with an aperture such that, by effect of the variations in refractive index at the interfaces between the outside environment and the cladding (12) of the fibre and between the cladding (12) and the core (13), the beam (9) which goes through the core (13) is a collimated beam. Method as claimed in claim 1, characterised in that the fibre (8) is irradiated with multiple UV radiation beams (9, 9'), having different powers, sent along different directions.

Inventors: COCITO GIUSEPPE, GREGO GIORGIO, COCITO, GIUSEPPE, GREGO, GIORGIO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1996
Main classification: C03B37
IPC codes: C03B37, G02B6