GB2214326A - Optical waveguide for visible light - Google Patents

Description

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DESCRIPTION

2 4 % i2M ODtical waveQuide for visible light The present invention relates to an optical waveguide for visible light of the sheathed core type for transmitting light and particularly for transmitting continuous light beams over a distance.

It has already been disclosed, particularly by the Japanese Patent Application 23165/78, how to produce an optical waveguide made from thermoplastic materials comprising an assembly of optical fibres having non-circular cross-section and being resistant to mechanical stresses. In order to make assembling of the fibres easier, these are preferably rectangular or hexagonal in section, the dimensions of the section being of the order of a few millimetres. Nevertheless, this assembly of optical fibres has three disadvantages when it comes to forming an optical waveguide: the transmitted light is in the form of discrete illuminated points, the space occupied is significant and joining of the ends of the optical fibres causes problems. Thus, an optical waveguide of this type is not suitable, for example, for producing easily read lettering.

The present invention has the object therefore of providing an optical waveguide for visible light of the sheathed core type having an elongated constant cross-section, and being suitable for transmitting light in continuous luminous beams, without having the disadvantages mentioned above.

The optical waveguide according to the invention can moreoever be used as a traffic sign or advertisement or even as a decorative element by removal in places of the sheath which covers the core so as to cause texts or designs to appear which are made luminous from the lateral illumination of the optical waveguide for visible light.

To this end, the present invention relates to is an optical waveguide for visible light of the sheathed core type, comprising transparent polymers having different refractive indices, such that the refractive index of the sheath is less than the refractive index of the core, in which the core has a constant elongated cross-section, the ratio of the greatest dimension to the smallest dimension of this section being within the range 5 to 1000 and, preferably, in the range 10 to 200.

In the optical waveguide according to the invention, the smaller dimension of the cross-section is preferably in the range 0.5 to 5 mm and the section, which may take any shape, is preferably rectangular. The optical waveguide according to the invention therefore exhibits the shape of a strap, a flat band or a sheet.

It has been shown that an optical waveguide of this type allows light to be transmitted very satisfactorily over an appreciable distance, this being adequate particularly for producing letterings, scoreboards, advertising material etc. of enhanced quality.

The principal property required in materials to be used for producing the core of the optical waveguide is transparency. Thermoplastic materials to be used for the core must moreover be completely pure and free from foreign matter or volatile constituents. Further, these thermoplastic materials must have good thermal stability so that they may be used in the molten state particularly by extrusion. Among very transparent thermoplastic materials which may be used to produce the core of the optical wavg.quide, may be mentioned homopolymers and copolymers of methyl methacrylate, polystyrene, polycarbonate. These thermoplastic materials are normally prepared according to a specific polymerization process which is carried out continuously and in bulk, yielding a very high degree of conversion while using a low catalyst concentration, as described particularly in the Patent Application t.

is FR-2,,252,586 (MITSUBISHI RAYON Co., Ltd.). Other techniques may be used however insofar as they provide a highly pure polymer which is free from microscopic contamination which could affect the optical quality. Among the thermoplastic materials mentioned above, methyl methacrylate homopolymer has the best performance from the point of view of transparency.

The propagation of a ray of light within an optical waveguide for visible light comes about essentially by the principle of total reflection of the waves on the interface formed between the two media, the core and the sheath, both having different refractive indices, of which the reflective medium (the sheath) has the lower index. Where the sheath is concerned, apart from having the appropriate refractive index, the thermoplastic material must have high transparency and good adhesion to the thermoplastic material used for the core, so as to avoid any break in the optical path caused by even a very partial delamination.

The combination of these requirements limits the pairs of thermoplastic materials which are suitable for producing the core and the sheath. When homopolymers or copolymers of methyl methacrylate, having a refractive index of the order of 1.49 are selected for the core., the choice of thermoplastic materials for the sheath is limited to fluorinated thermoplastic materials or to mixtures of these with polymers of methyl methacrylate insofar as they are compatible, as for example the homopolymers and copolymers of vinyl fluoride, tetafluoroethylene, hexafluoropropylene and fluorinated esters of acrylic acid or methacrylic acid. The range of fluorinated polymers which may be used is limited by the requirements of transparency and extrusion temperature. Mixtures of methyl methacrylate polymer with a copolymer of vinylidene fluoride and hexafluoropropylene are particularly suitable.

If the core of the optical waveguide for visible light is produced from polycarbonate, having a refractive index of 1.59, the choice of thermoplastic materials which may be used for the sheath becomes wider, it being possible to use for these for example fluorinated thermoplastic materials, polymers of methyl methacrylate, polymethylpentene, etc.

It has also been observed that when the thermoplastic material which forms the core contains a fluorescent pigment, the luminous effect of the optical waveguide according to the invention is enhanced. Fluorescent pigments which may be used to this end are described particularly in the Patent Applications EP 4,655, EP 33,079, EP 41,274, EP 46,164, EP 46,861, EP 73,007, EP 77,496 the technical content of which is incorporated in the present invention, by reference.

According to the invention, the elongated cross-section of the optical waveguide for visible light can have any shape, so long as the ratio of the greatest dimension to the smallest dimension is from 5 to 1000. The preferred shape, from the point of view of ease of application, being the rectangular shape.

The optical waveguides for visible light of the sheathed core type according to the invention may be produced by any technique of sequenced addition of the various constitutive layers. The manner of production which proves most advantageous in terms of economics is the coextrusion technique. In this preferred manner of producing the optical waveguide for visible light according to the invention, the normal te(hniques for coextrusion through a die may be applied, the thickness of the respective constitutive polymeric layers and the total thickness of the optical waveguide not being critical. In general, however, it is preferable, as previously mentioned, that the thickness of the core should be in the range 0.5 to 5 mm. The thickness of the sheath is generally within the range 0.010 to 0.5 millimetre and preferably in the range 0.020 to 0.250 millimetre.

The temperature at which the core and the j c ? 1 sheath of the optical waveguide for visible light are extruded through the die varies according to the particular thermoplastic materials used for producing it and it is generally in the range of 180 to 3000C,, preferably of 210 to 2800C. The viscosities of the materials forming the sheath and the core in the molten state must be as similar as possible, so as to obtain a stable multilayer flow and an extrudate having a geometry which is as perfect as possible.. Preferably the viscosity in the molten state of the sheath polymer should be less than or, at most, equal to that of the core polymer, at the operating temperature and under the velocity gradients which exist within the head and die. If necessary it is possible to modify the viscosity in the molten state by making the appropriate choice of molecular weight of the materials forming the sheath or the core.

Moreover the molecules of the coextruded optical waveguide for visible light may be advantageously oriented to increase its mechanical properties and in particular its flexibility. This orientation may be carried out by axially stretching the coextrudate, generally by from 1.3 to 2.5 times its original length, in a post-extrusion stage and at a temperature such that forces of orientation are induced.

For certain applications, the optical waveguide for visible light according to the invention may be encased in a third thermoplastic material to provide it with mechanical or chemical protection, and to avoid damage to the surface of the sheath during manipulations and use. The fitting of this third layer may be carried out by coextrusion, at the same time as the core and the sheath are produced, or may be produced in sequence by a third extruder which ensures sheathing by this third material after stretching of the optical waveguide-for visible light.

As previously mentioned, it is possible with the optical waveguide according to the invention to 1 is produce decorative panels or other panels by removing in places the sheath covering the core so as to cause any desired design to appear, this being made luminous by means of lateral illumination. The desired design can particularly be produced by abrasive engraving. It is obviously advisable that this engraving should be carried out to a depth adequate to remove the whole thickness of the sheath. In order to be sure of achieving this result, the engraving may be.carried out to a depth slightly greater than the thickness of the sheath, the thickness of the layer of core which is removed not being greater than 0.1 mm. To augment the luminosity of the engraved design, it is desirable to make the end of the optical waveguide opposite to that receiving the light source opaque. This luminosity may be still further augmented by incorporating a fluorescent pigment in the material forming the core of the optical waveguide. Alternatively, it is also possible to coat the engraved design with a layer of a transparent varnish, based on polymethyl methacrylate, containing fluorescent pigments.

The attenuation, expressed in db/km of length, which is the usual standard for evaluating light transmission, is determined according to the following method. A diode, emitting monochromatic light of wavelength 565 nm, is used as a fixed light source. This wavelength was selected from the relevant part of the light transmission spectrum which is located in the range 500 to 680 nm for waveguides based on methyl methacrylate polymer. The intensity of the transmitted light is measured using a photodiode, te maximum light sensitivity of which corresponds to a wavelength of about 565 nm. For a sample of optical waveguide for visible light having any length greater than 5 metres, the voltage value VO is noted by reading the voltmeter which is connected to the output terminal of the measuring amplifier of the photodiode. The length of the optical waveguide is then reduced by 1 metre and the transmitted light is measured again. By repeating 7 1 t T 1, ? this operation of reducing the length of the optical waveguide, voltage readings VO are obtained corresponding to optical waveguide segments whose length L differs on each occasion from the preceding one by 1 metre.

A graph of the logarithm of the voltage VO as a function of the length L then gives a straight line the slope of which determines a coef f icient b which is a function of the attenuation according to the formula:

Vo = aebL in which a is a constant which depends on the photodiode, e is the base of natural logarithms and L is the length of the waveguide in metres. The attenuation, expressed in db/km of length, is equal to 10,000 b/2.3 or 4348 b.

The invention is further described in the example of the practical application given below.

Example

A 20 mm diameter WTTFERT extruder having a compression ratio of 4, is fed with vacuum-dried granules of methyl methacrylate polymer (PMMA having the trade mark PLEX 8699F marketed by R8hm). The temperature of the molten material leaving the extruder is maintained at 2350C and the output is maintained at 980 g/hour.

A 20 mm diameter COLLIN extruder having a compression ratio of 4, is fed with vacuum-dried granules of a mixture containing 65 percent by weight of methyl methacrylate polymer (PMMA having the trade mark Diakon MG102 marketed by Imperial Chemical Industries) and 35 percent by weight ok a copolymer of vinylidene fluoride and hexafluoropropylene (PVW having the trade mark SOLEF 21010 marketed by the Applicant). The temperature 'of the molten material leaving the extruder is maintained at 2350C and the output is maintained at 220 g/hour.

A coextrusion head 12 mm in diameter serves as the junction between the two extruders.

Using a flat die, of rectangular cross-section 8 - is having a width of 15 mm and an aperture of 2 mm, heated to 2350C, a strap is coextruded having a width of 11.5 mm and a thickness of 1.7 mm. The thickness of the core is about 1.4 mm and that of the sheath is about 0. 15 mm. The linear output speed of the strap downstream of the take-off rolls is equal to 52 m/hour.

The attenuation of light, measured at a wavelength of 565 nm and following the method described above is equal to 3000 dblkm.

Uniaxial stretching to a value of 125% at a temperature of about 1300C is carried out by incorporating a second pair of take-off rolls, between the die and the take-off rolls for the strap. After stretching, the strap has a rectangular cross-section 10 mm wide and 1.6 mm thick and has enhanced mechanical properties, particularly flexibility. The attenuation of light, measured at a wavelength of 565 m still has the value 3000 dblkm.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

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Claims (7)

C L A I M S

1. Optical waveguide for visible light of the sheathed core type, comprising transparent polymers having different refractive indices, such that the refractive index of the sheath is less than the refractive index of the core, characterized in that the core has a constant elongated cross-section. the ratio of the greatest dimension to the smallest dimension of the section being within the range 5 to 1000.

2. optical waveguide for visible light according to Claim 1. characterized in that the smallest dimension is in the range 0.5 to 5 mm.

3. Optical waveguide for visible light according to Claim 1 or 2, characterized in that the crosssection is of rectangular shape.

4. Optical waveguide for visible light according to any one of Claims 1 to 3, characterized in that the sheath which covers the core is interrupted in places, in such a way as to form a text or design which shows up luminously during lateral illumination of the optical waveguide.

5. Optical waveguide for visible light according to any one of Claims 1 to 4, characterized in that the core contains a fluorescent pigment.

6. Optical waveguide for visible light according to any one of Claims 1 to 5, characterized in that it is produced by the coextrusion process.

7. optical wave guide for visible light according to claim 1, produced by a procedure substantially as described in the foregoing Example.

-1, Published 1989 at The Patent office, State House, 66'71 High Holborn, London WCIR4TP. Further copies maybe obtained from The Patent Offtce Scae.- Bran-'2.a, St ldaa7! 0,'011196011. Ifeil, LITO" -=O)"rinted by Multiplex techniques ltd, St Mw7 Cray, Kent, Con- 1/87