CIRCULARLY BIREFRINGENT DIELECTRIC WAVEGUIDE

Abstract: A round birefraction dielectric wave-guide has in section a central round zone with a constant, or substantially con­ stant, refractive index n1, an outer zone preferably with a con­ stant refractive index n2 and an intermediate zone divided into 8 (or multiples of 8) angular sectors. Respective alternate sectors have refractive indices n3 and n4. Refractive index n3 may be constant but index n4 varies in inverse proportion to the square of the distance from the axis of the waveguide. N1 is greater than n2, n1 is greater than n3 and n4 lies between n1 and n2. The wave-guide is subjected to torsion around the longitudinal axis of symmetry thereof and/or is spray wound around a cylinder.

Description: For some years there has been increasing technical interest in dielectric waveguides (or optical fibres, if the term is preferred) made of glass silica with very low attenuation, characterised by marked and deliberately anisotropic behaviour. For reasons connected with the principal uses of such means of transmission, this interest has been concentrated on the said guides (with a slight inaccuracy now generally accepted in current use) having a single mode of transmission (also termed monomodal or unimodal guides), characterised in reality by the fact that they can guide rays in two different states of polarisation which, in general and strictly, are elliptical only. Nearly all present or foreseeable applications are concentrated on two extreme cases: one in which such polarisation ellipses are equivalent to two orthogonal lines, and one in which they are equivalent to two circumferences, travelled in opposite directions. They are then referred to as linearly birefringent guides and circularly birefringent guides respectively. There have been more experiments and there is greater present knowledge on the former than the latter, but more recently the latter have also been the subject of numerous studies. Present technical applications of circularly birefringent dielectric waveguides mainly concern sensors and transducers of physical magnitudes; in particular, they are useful in the construction of measuring instruments for static or slowly variable magnetic fields. But it is not difficult to foresee their use also in the manufacture of components for application at the ends, or in particular sections, of telecommunication systems that employ isotropic or linearly birefringent fibres as means of transmission. Both in this case, and in the field of sensors and transducers, it is very important to give consideration to the problem of connectability between isotropic guides and circularly birefringent guides. To obtain circularly birefringent guide in a simple way, it is necessary to have a material characterised by a dielectric permittivity dyad (or tensor, if the term is preferred) which, in a Cartesian orthogonal scheme, has two out-of-diagonal elements different from zero (in particular, imaginary and of opposite signs), and the elements of the main diagonal equal to each other. With glass silica these conditions cannot be obtained on a microscopic scale, i.e. by acting on the local chemical or physical properties of the material. The techniques through which circularly birefringent guides can be obtained involve mechanical operations which ensure that, on a macroscopic scale (i.e. on distances appreciably greater than the wavelength of the guide rays), the average properties of the guide are equivalent to those of a means having the above-stated characteristics. The mechanical operations can be reduced in substance to the following two steps: a structure is obtained which, unlike an isotropic optic guide, is not of a symmetrical nature revolving round the longitudinal axis or direction of propagation, which axis will hereinafter be termed axis z; the said structure is subjected to torsion round axis z and/or spirally wound on a cylinder whose generatrices are parallel to axis z. For a better understanding of the drawbacks of the circularly birefringent optical guides hitherto produced - drawbacks that this invention intends to overcome - a fuller description of the guides in question is now given with reference to Figures 1a and 1b of the accompanying drawings which are diagrammatic end views of respective known circularly birefringent dielectric waveguides. They can be grouped into two classes: a) guides in which the section (of the central core, or the entire guide) is divided into m angular sectors (normally m=8 or a multiple of 8, in order to avoid linear birefringence superimposed on birefringence) having alternate refraction indices n1 and n2, with n1 > n2. Torsion takes place around the axis on which the sectors in question converge (see Fig. 1.a); b) guides in which the core section is electrically homogeneous, but in which the said core is then spirally wound around an axis other than that of symmetry of the core itself (see Fig. 1.b). In type "a" guides, the guided electromagnetic energy is distributed over the section in a highly different way from that found in an isotropic monomodal optic fibre. The light inlet and outlet operations at the two ends of a length of type "a" guide are therefore accompanied by large losses; in theory these can be reduced in part, but at the price of interposing guide lengths with slowly varying characteristics, i.e. a technologically very high and hitherto practically never obtained price. This drawback does not arise in type "b" guides; but this occurs at the cost of an appreciable reduction in maximum circular birefringence compatible with maximum torsion through which a fibre is still capable of guiding light. If, in fact, there is a drop below a certain minimum pitch in the winding, an optical fibre radiates the light instead of guiding it. Therefore, given parity of external overall performances, a device made with a type "b" guide requires a much greater guide length than in the case of type "a", with consequent increase in losses linked to length of the radiation course. The two drawbacks just described are mutually exclusive of each other, but the present state of technology compels selection of one or the other of them, without possibility of compromise. The invention, on the other hand, allows the two drawbacks to "offset" each other, in controllable quantity, so as to allow the optimisation of the performances of each device of given nominal characteristics. The present invention provides a circularly birefringent dielectric waveguide which is subjected to torsion around its longitudinal axis of symmetry and/or is spirally wound around a cylinder, the cross-section of said waveguide having a region divided into eight or multiples of eight angular sections, respective alternate sections having different refractive indices n3 and n4 characterised in that said region is an intermediate region lying between a central circular zone and an outer zone surrounding said intermediate region; said central circular zone has a constant, or substantially constant refractive index n1; said outer zone has a refractive index n2; n3 is constant or varies in inverse proportion to the square of the distance from the axis of the waveguide; n4 varies in inverse proportion to the square of the distance from the axis of the waveguide; n1 is greater than n2 and also greater than n3; and n4 lies between n1 and n2. The central circular zone may have its radius as small as desired. In the intermediate region two adjacent sections may have angular widths whose sum is equal to 4π divided by the total number of sections. The refractive index n2 of the outer zone may be constant, alternatively it may vary in the radial direction. In the intermediate region the refractive indices n3 and n4 of the sections may vary in inverse proportion to the square of the distance from the axis of the waveguide with n3 being smaller than n4 for equal distances from said axis. The guide may show a gradual variation of the refractive indices, as well as of the dimensions of the regions and of the sections in the longitudinal direction. A circularly birefringent dielectric waveguide may also be provided consisting of a cascaded sequence of a number of waveguides as defined above. An embodiment of the invention, which is given by way of example only, is shown in Figure 2 of the accompanying drawing. The embodiment consists in an optical guide in which the refractive index, in the cross-section, has the distribution described below. The said guide must be subjected to torsion (for example, on being spun from a preform) around the axis of symmetry of the said section. The section is made up as follows: there is a circular central zone, of outside radius a, in which the refractive index is constant (or roughly constant); it is indicated as n1; there is an outer zone, delimited internally by a circumference of radius b, in which the refractive index is constant; it is indicated as n2; in addition n2 < n1; the intermediate region (a > r > b) is divided into m angular sections (m = 8 or a multiple of 8, for the reasons already stated above); the angular sections of even position (starting from a suitable angular reference) have constant refractive index, n3 (possibly coinciding with n2 and in any event smaller than n1); the angular sections of odd position have a refractive index n4 gradually decreasing with increase in the radial coordinate r, starting from value n1 (or a value slightly lower than this) for r=a, down to value n2 (or a slightly higher value) for r=b; the said index n4 varies according to a law of proportionality to 1/r²; the angular width of the even (or odd) sections is a design parameter of the structure. Referring, for the sake of simplicity, to the case m=8 and using shading for graphic representation of the refractive index (n MAX = dark, n min = white), the foregoing is represented by the sketch in Fig. 2, in which the guide cross-section has a shape which may be described as that of a "war cross". A fundamental point of the invention is the selection of the law h/r² (h = proportionality constant) for the variation in the refractive index in the odd sections. It can in fact be demonstrated by means of mathematical developments starting from Maxwell's equations that this selection allows very precise control of the law by which guided electromagnetic energy is distributed in the guide cross-section both in radial direction and in azimuthal direction. It follows from this that it is possible, within arbitrarily pre-established limits, to obtain adjustment of the "war cross" guide field to that of immissary and emissary isotropic guides, thus minimising losses at the terminal connections of the circularly birefringent guide. More precisely, by acting on the design parameters, which are: indices n1, n2, n3; radii a, b; the proportionality constant h; and the angular width ⊘ of the odd sections, it is possible to obtain any compromise between the above adjustment (and hence the reduction of losses at the ends of the anisotropic guide) and the maximum circular birefringence compatible with the condition that the structure should guide energy without radiating it (and hence the extent of the losses distributed along the guide). In fact, if for the sake of simplicity of description it is assumed that b is equal to the radius of the core of the immissary guide, the core and cladding which, still for the sake of simplicity, are assumed to have indices equal to n1 and n2,it can be seen that: the obtainable circular birefringence increases with decrease of a, decrease of ⊘, and increase of h; the adjustment improves if the inverse operations are carried out. If necessary, the cross-section parameters (refractive indices and geometric dimensions) can be made to vary gradually along the longitudinal axis, for example in order to obtain an initial zone with excellent adjustment followed by another zone with higher circular birefringence.

Claims: A circularly birefringent dielectric waveguide which is subjected to torsion around its longitudinal axis of symmetry and/or is spirally wound around a cylinder, the cross-section of said waveguide having a region divided into eight or multiples of eight angular sections, respective alternate sections having different refractive indices n3 and n4 characterised in that: said region is an intermediate region lying between a central circular zone and an outer zone surrounding said intermediate region; said central circular zone has a constant, or substantially constant refractive index n1; said outer zone has a refractive index n2; n3 is constant or varies in inverse proportion to the square of the distance from the axis of the waveguide; n4 varies in inverse proportion to the square of the distance from the axis of the waveguide; n1 is greater than n2 and also greater than n3; and n4 lies between n1 and n2. A waveguide as claimed in claim 1, wherein the central circular zone has its radius as small as desired. A waveguide as claimed in claim 1, wherein in the intermediate region two adjacent sections have angular widths whose sum is equal to 4π divided by the total number of sections. A waveguide as claimed in claim 1, wherein the refractive index n2 of the outer zone is constant. A waveguide as claimed in claim 1, wherein the refractive index n 2 of the outer zone varies in the radial direction. A waveguide as claimed in claim 1, wherein in the intermediate region the refractive indexes n3 and n4 of the sections vary in inverse proportion to the square of the distance from the axis of the wave-guide with n3 being smaller than n4 for equal distances from said axis. A wave-guide as claimed in claim 1, wherein the waveguide shows a gradual variation of the refractive indices, as well as of the dimensions of the regions and of the sections in the longitudinal direction. A circularly birefringent dielectric waveguide comprising a cascaded sequence of a number of waveguides, each of which is formed according to claim 1.

Inventors: SOMEDA, CARLO GIACOMO
Applicants: PIRELLI GENERAL PLC
Application year: 1986
Main classification: G02B6
IPC codes: G02B6, C03B37