NO853303L - PREFORM WITH GRADED REFRIGERATION INDEX AND PROCEDURE FOR PREPARING SAME. - Google Patents

Description

The present invention relates to a preform with a graded refractive index and a method for producing preform materials for use in drawing or extruding optical fibers.

When electromagnetic waves are propagated in a material, the propagation speed will depend on the material's refractive index. If you use electromagnetic waves with the same frequency as visible light, you can say for simplicity that the electromagnetic wave propagates in the material at the speed of light. If one now imagines that the light is to be propagated in a material with a higher refractive index than e.g. air, the speed of light will be lower in this material than in air. This can be derived from Snell's law.

As a consequence of this, it can be said that light that follows the fiber axis in an optical fiber will propagate at a specific speed depending on the fiber's refractive index.

Light that does not follow the fiber axis but is propagated outside of it, will either be reflected from the fiber's outer limit, i.e. the transition between the fiber's material and the material that is outside the fiber, or it will disappear out of the fiber.

This will mean that the light sent through the fiber will lose some of its intensity on the way from the sending point to the receiving point. It is said that the light is dimmed. Moreover, the reflected light will arrive with a certain phase shift compared to the light that follows the fiber axis due to the longer path the reflected light has to travel through.

In practical terms, one can say that the aforementioned phenomenon is present in the so-called step-graded fibers, i.e. optical fibers where the fiber material has a uniform refractive index from the axis to the next change in the refractive index, which is usually in the transition between the fiber's inner core and its outer sheath.

In other words, information sent through such fibers in the form of light pulses is blurred as the light pulses propagate through the fiber.

If, on the other hand, you use optical fibers that are optically in-homogeneous (graded index profile), i.e. that the refractive index changes continuously from a higher refractive index in the fiber axis to a lower refractive index in the fiber periphery, the light sent through the fiber will not be reflected as in step-graded fibers, but be deflected continuously towards the axis.

Information in the form of light pulses sent through such fibers will arrive at the receiving location at approximately the same time, and the signals will therefore be easily readable.

This property of graded index fibers has been known for some time and has been used successfully in the transmission of information in glass and quartz optical fibers.

However, optical fibers made of plastic which are based on this principle have not been described. From the literature, only optically homogeneous fibers based on the principle of fibers with a graded index are known.

U.S. patents 4,138,194 and 4,161,500 describe a method for producing preforms where the material is optically homogeneous. A copolymer based on methyl methacrylate and a small addition of other acrylates and methacrylates is used. The produced preform is then used later for the extrusion of optical fibres.

West German specification no. 14 97 545 also describes the production of graded fibers where the core of the fiber is based on polystyrene and where the fiber sheath is based on polymethyl methacrylate. The production thus takes place by filling the core material into a tube of polymethyl methacrylate so that a preform with a polystyrene core and polymethyl methacrylate jacket is produced. The preform is then heated until it becomes plastic, after which the optical fiber is drawn.

Optical fibers produced from preforms of the aforementioned kind will be optically homogeneous and thereby be less suitable for transmitting information over longer distances.

Now it turns out in a surprising way that it is also possible to produce optical fibers from plastic with a graded index, where the refractive index is continuously reduced from the fiber axis out towards the fiber periphery. This is done according to the invention by bringing monomers with approximately the same reaction parameters and solubility parameters but with different refractive indices together while continuously changing the molar ratio between the monomers.

The monomers are then deposited according to the invention on a rotating wall of glass, quartz and/or another suitable material, e.g. a tubular polymeric material that can form the sheath.

The monomer layer is brought to polymerization in a manner known for the material, e.g. by using peroxide-based initiators or by irradiation, e.g. by using UV radiation or by using e.g. IR radiation for heating or e.g. by a combination of two or more of the aforementioned polymerisation ion forms.

If the polymerization takes place in connection with the use of glass as the outer form and UV radiation is used in connection with the polymerization, the UV radiation should have a wavelength that coincides with the smallest UV absorption in the glass.

For a better understanding of the invention, reference is made to Fig. 1, which shows an apparatus for the production of preforms seen from the side.

The apparatus, shown in Fig. 1, consists of an outer tube (1) of glass, quartz or plastic. Here, glass is understood to mean glass of all qualities. Plastic refers to both water-soluble and non-water-soluble plastics of a synthetic and natural nature.

The pipe (1) is sealed with the end pieces (2' and 2") which have several functions. Firstly, the end pieces (2<1> and 2") will be the supporting body for the pipe (1), and secondly the body with around which the tube (1) can be rotated.

The device itself for rotation of the tube (1) can be of a known type, e.g. in that the end pieces (2' and 2") are equipped with axle pins or that the rotation is carried out directly over the end pieces by attaching them to a rotating bearing. The device for turning the pipe does not form part of the invention and is thus not shown in Fig. 1.

The end pieces (2<1>and 2") are attached to the pipe (1) in a known manner, e.g. via a screw connection, via sealing with O-rings of an inert material or with another form of frictional connection between the end pieces and the pipe.

The end pieces can be made of metal and/or plastic. It would be an advantage if the material in the end pieces is of such a nature that they are resistant to attack from the monomers and that they withstand heat, i.e. do not deform under the influence of heat. The end piece (2") is shown with a grommet (3) and a needle (4) which can be moved back and forth in the tube (1) through the end piece (2"). The reciprocating movement is controlled by means of a motor (5), and is indicated by a double arrow above the needle (4).

To ensure that oxygen and dust particles do not penetrate into the pipe (1) between the end pieces (2' and 2"), a supply (6) can be arranged for eg nitrogen, such as inert gas (I), which leads into the pipe (1) along the passage (3). Through the cannula (4) the reagents, which in this case are made up of the monomers and/or Vi^ either as pure monomers or as a combination of them, are led. There is no requirement that you use only one or two monomers, since one can imagine monomer combinations with both two and more monomers. The invention therefore places no limitation on how many monomers are to be used or which ones are to be used. The decisive factor in the choice of monomers is the optical properties and the other physical data for the finished product.

The cannula (4) is connected to the tube for the supply of monomer (M1 and M2), UV initiator, chain transfer and possibly other aids (H) that are suitable for the purpose.

The apparatus described above has two movements. Firstly, the tube (1) with the end pieces (2<1>and 2") will rotate. Secondly, the cannula (4) will be moved back and forth inside the tube (1) so that after a time the inner surface of the tube will be covered with material from the needle (4).

The preform device is rotated at such a high speed that the materials are evenly distributed. At the moment the materials start to sag, the rotation speed is too low. Practice shows that you can choose rotation speeds between 500 and 2,000 rpm, preferably approx. 1000rpm.

It is mentioned above that the cannula (4) will have separate supplies of the monomers and I^, but it may also be advantageous to have a third supply (H) for e.g. chain transfer and/or UV initiators.

You can also use other methods for the initiation, such as e.g. use of peroxides and azo compounds for the formation of free radicals for the initiation.

It is an advantage for the later use that the manufactured preform is as optically clean as possible, that is to say that it contains as little as possible of substances that can interfere with the optical signals and that will have a dampening effect on them. It is therefore an advantage to initiate the polymerization and to carry it out using UV irradiation.

It should be stated that it is difficult to conduct away the heat of polymerization in the preform if you use systems that are initiated by e.g. peroxides. One would therefore rather prefer the simpler and cleaner method of polymerization using UV irradiation.

If UV radiation is used, the tube (1) must be permeable to this. In practice, this means that you choose UV rays with a wavelength in the range of 300 to 400 nm.

In fig. 1 shows a device (7) for irradiation with UV rays. As the tube (1) rotates at a relatively high speed, the irradiation can be considered uniform over the entire inner part of the tube (1).

It is not desirable for the molecular weight to be too high, as this will later affect the drawing speed and the flowability of the polymer. A chain transfer agent is therefore added to keep the molecular weight in the correct molecular weight range. For example, it can be mentioned that polyacrylates will have suitable molecular weights in the range of 30,000 to 200,000, while polystyrene will be best suited with molecular weights in the range of 100,000 to 500,000.

The purpose of the above-mentioned apparatus is to be able to produce a preform with a decreasing refractive index from the axis of rotation of the preform outwards towards the periphery.

This is done by adding the monomer or monomers with the lowest refractive index at the start and then increasing the refractive index by continuously changing the molar amounts in the mixture of the monomers and . Finally, a small cavity will remain around the cannula (4) which is polymer-free. When this cavity is as small as possible and before it grows back with the cannula, the cannula must be pulled out and the cavity filled with the last monomer mixture, i.e. the monomer or monomer mixture that has the highest refractive index. The preform is then post-polymerized until it is uniformly polymerized.

Above where it is assumed that the refractive index profile in the graded index fiber must be uniformly decreasing from the axis of the fiber outwards towards its outer limit, i.e. the boundary towards the fiber's sheath.

It should also be stated that according to the present method, any optical fiber can be produced according to the graded index fiber principle if the refractive index profile is uniformly decreasing or parabolically decreasing or decreasing in some other way. It is also possible to produce stepwise graded fibers according to the aforementioned method in that the polymer layer inside the tube (1) is built up by polymer layer by layer with increasing refractive index from the tube (1) and towards the axis of the preform.

UV irradiation of monomer can take place at any temperature, but a temperature is chosen that does not cause temperature build-up in the preform. Practice shows that room temperatures (approx. 20 °C) or lower can be used if the polymerization is carried out with UV irradiation.

For example, a typical composition of the added materials for the production of the preform can be given.

Those which preferably have a reaction parameter equal to 1.0 are chosen as monomer and iv^, i.e. r^= r2 — 1.0, but the reaction parameters can also be different from 1.0.

At the same time, the monomers and 's solubility parameters & i'°^^2 should preferably have a greater difference than aS= 1.0, but this is not decisive as there are monomer systems where said difference is greater but where the monomers yet are soluble in each other.

Claims (5)

1. Preform with graduated refractive index, characterized in that the preform constitutes a system of polymeric compounds 20 with decreasing refractive index, nD , from the axis of rotation of the preform towards the periphery of the preform and where the polymers in the preform have been produced by a continuous or discontinuous addition of monomers and/or monomer mixtures with increasing refractive index and added the necessary additives and that said monomers are polymerized by means of irradiation and/ or by radical polymerization. 2. Preform according to claim 1, characterized in that ultraviolet irradiation (UV radiation) is used for polymerization of monomers and that the wavelength is from 200 to 400 nm, preferably from 300 to 400 nm. 3. Preform according to claims 1 and 2, characterized in that a UV initiator and/or chain transfer agent is used during the polymerization. 4. Preform according to claim 1, characterized in that organic peroxides and/or azo compounds are used in the radical polymerization. 5. Method for producing preform with graded refractive index according to claims 1 to 4, characterized in that in a device consisting of an outer tube (1), which is finished with the end pieces (2' and 2") and where the tube and the end pieces are brought to rotation about the longitudinal axis, after which monomeric material with necessary additives is supplied to the tube (1) via a reciprocating cannula (4) in such a way that the inner surface of the tube (1) is covered with a layer of monomer which is brought to polymerisation using of irradiation and/or by radical polymerization and that the further build-up of the monomer layers with their subsequent polymerization takes place continuously or discontinuously so that the formed preform is composed of polymeric compounds with an increasing refractive index from the periphery of the preform towards the axis of rotation of the preform and that the cannula ( 4) is finally pulled out of the preform before it is completely filled with polymer and that the resulting cavity in the preform is filled with monomer with the highest refractive index which is then polymerized.