GB2046239A

GB2046239A - Optical fibres

Info

Publication number: GB2046239A
Application number: GB7912762A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1979-04-11
Filing date: 1979-04-11
Publication date: 1980-11-12

Abstract

Monomide optical fibres wherein both the core and cladding are of doped vitreous silica are characterized in that the cladding dopant is a phosphorus oxide plus either fluorine or boric oxide. The core dopant is preferably a phosphorus oxide plus germania. The cladding is preferably surrounded by an outer layer of undoped vitreous silica. Manufacture is by the "modified chemical vapour deposition process". An optical fibre can be made using this method in which the core is formed by evaporation of dopants during the production of the preform.

Description

SPECIFICATION Monomode optical fibres The present invention relates to monomode optical fibres made from doped vitreous silica.

In a monomode optical fibre, the radiation propagates in a single mode, and for this reason the problems of group delay dispersion are not encountered. This enables very high bandwidth to be obtained. However in this type of optical fibre, the core diameter is usually very small, frequently less than 8 microns in diameter. Furthermore a large proportion of the energy associated with the guided wave propogates in the cladding. This means that the cladding glass, unlike the situation encountered in multi mode optical fibres having core diameters of the order of 50 microns or more, must have low optical loss. In addition it is essential that the cladding structure should not support secondary modes, since this would defeat the prime object of using a monomode fibre.

Because of the need to use high quality cladding glass and eliminate the possibiiity of secondary guiding structures, attempts at manufacturing monomode fibres have in the past emphasised the need to use pure vitreous silica as the cladding material. To understand the significance of this, it is necessary to consider the way in which vitreous silica optical fibres are made. In general the modified chemical vapour deposition process is employed. in this the cladding and core are built up in turn on the inside of a pure vitreous support tube. This tube after the deposition of cladding and core layers is then collapsed to form a preform which is subsequently drawn into optical fibre.Since the material used for the support tube is not usually of high optical quality, in the sense that it tends to contain water which increases the loss of the silica, it is necessary to form a relatively large diameter (5 times the core diameter) cladding on the inside of the support tube, which is of high quality, so that radiation does not propogate to a significant extent through the silica of the support tube.

When optical fibre preforms of this type are made using pure vitreous silica, it is necessary to carry out a high temperature sintering operation to convert the deposited silica into a transparant glass. Typically the sintering operation must be carried out at 1 ,700'C, this temperature, is refered to in this specification as the sintering temperature. This creates a problem, since at high temperatures there is a tendancey for the whole preform to distort during manufacture. This in turn imposes a limit on the maximum length of preform that can be produced. Furthermore the high temperatures required for the deposition of pure vitreous silica can cause a premature partial collapse of the silica support tube.Details of fabrication of monomode optical fibres can be found in the following two papers:-- "Fabrication of low loss single mode fibre" A Kawana et al Electronics Letters Volume 1 3 No. 7 page 138; "Low Loss Single Mode Fibres at the Material Dispersion-free wavelength of 1.27 microns" M Kawachi et all Electronics Letters Volume 13. No. 1 5 Page 442.

In order to overcome the problem caused by the high melting point of silica without creating further problems by creating a secondary waveguiding structure in the optical fibre, the present invention proposes to use as a cladding, silica doped with two dopants, one of which raises the refractive index and the other of which lowers the refractive index. In general the addition of any dopant except fluorine will lower the sintering temperature i.e. the temperature at which the deposited layer will just sinter to form a glass, so that by using two dopants in this way it should be possible to lower the sintering temperature of the silica without significantly increasing it's refractive index. (It may in fact be possible to obtain a cladding with a refractive index lower than that of pure vitreous silica by this technique).To implement this invention in an efficient manner, it is necessary to choose dopants which have the maximum effect on the sintering temperature, and the minimum effect on the refractive index. In this way it is possible to introduce more dopant hence lowering the sintering temperature by a greater amount, without affecting the refractive index. The dopant selected for this purpose must produce a doped vitreous silica, which is compatible with the general requirements for low loss optical fibres, i.e.

the dopant must not introduce absorption or have other undesirable properties affecting the optical loss of the resultant fibre. There are only two known dopants available which will lower the refractive index of vitreous silica, these are boric oxide and fluorine. Two of the most commonly use dopants which raise the refractive index of vitreous silica are oxides of phosphorous believed to be in the form of phosphorus pentoxide and germania. For the purposes of this invention, oxides of phosphorus are markedly better dopants to use than germania. This is because, very small quantities of germania have a strong effect of the refractive index, and for additions of germania to vitreous silica on a mole for mole comparison the sintering temperature is not effected to the same extent as with phosphorus pentoxide.

According to a first aspect of the present invention there is provided a monomode optical fibre for use with radiation having a wavelength greater than 1.1 microns comprising a core of doped vitreous silica, a cladding of vitreous silica doped with fluorine or boric oxide, and an oxide of phosphorus having a refractive index substantially equal to or less than the refractive index of pure vitreous silica.

Said optical fibre may have an outer layer of pure vitreous silica.

Said doped vitreous silica used for said core may be doped with germania and an oxide of phosphorus.

Preferably said core has a diameter of less than 1 5 microns and said cladding has a thickness greater than 1 5 microns and such that the cladding diameter is at least 4 times, preferably 5 times, the core diameter.

According to a second aspect of the present invention there is provided a monomode optical fibre for use with electromagnetic radiation having a wavelength greater than 1.1 microns comprising a cladding of vitreous silica doped with fluorine, or boric oxide, and an oxide of phosphorus, having a refractive index substantially less than the refractive index of pure vitreous silica, and a core having a refractive index greater than the refractive index of said cladding wherein said core is formed during the production of a fibre preform from which said optical fibre is drawn, by evaporation of dopents used in said cladding.

According to a third aspect of the present invention there is provided a method of making an optical fibre preform comprising the steps of: (a) depositing a plurality of layers of vitreous silica doped with fluorine, or boric oxide and an oxide of phosphorus, on the inner wall of a pure vitreous silica support tube, and (b) heat treating the resultant structure of (a) to cause evaporation of said dopents from the inner most of said deposited layers, and (c) collapsing said silica support tube and said plurality of layers to form an optical fibre preform.

According to a fourth aspect of the present invention there is provided an optical fibre preform comprising a core of doped vitreous silica, a cladding of vitreous silica doped with fluorine or boric oxide, and an oxide of phosphorus having a refractive index substantially equal to or less than the refractive index of pure vitreous silica.

Said optical fibre preform may have an outer layer of pure vitreous silica.

Optical fibres, and optical fibre preforms according to the present invention, do not have germania added to the cladding as a deliberate dopant.

For the avoidance of doubt this invention does not seek to cover within its ambit vitreous silica when used either for core or cladding doped with fluorine and boric oxide in combination.

The use of vitreous silica doped with germania, phosphorus, and boric oxide for cladding glasses, in such proportions that the refractive index of the cladding is the same as pure vitreous silica has been previously reported, see: "Optimum Profile Perimeter on Graded-lndex Optical Fibre at 1.27 micron wavelengths" Electronics Letters Volume 14 No. 24 page 764. However in this paper the use of both germania and phosphorus as the refractive index increasing dopants is described. No mention is made of the use of phosphorus as the sole refractive index raising dopant. Furthermore no explanation is given for the selection of dopants used, or the reason for choosing the doped vitreous silica to have the same refractive index as pure vitreous silica. It should be noted that this paper relates to multi-mode dielectric optical waveguides.It will thus be seen that the present invention differs from the invention disclosed in this document in two key respects, it is concerned with multi-mode optical fibres, and secondly it does not disclose phosphorus as the role refractive index increasing dopant.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a section through an optical fibre preform before collapsing.

Figure 2 is a section through an optical fibre pre form after collapsing.

Figure 3 is a section through an optical fibre according to the present invention.

Figure 4 is a graph showing the variation of optical loss with wavelengths for a typical optical fibre having a cladding doped with fluorine and a phosphorus oxide.

Figure 5 is a graph showing a comparison between the variation of optical loss with wavelength for a fibre having a cladding doped with a mixture of germania and a phosphorus oxide, and a cladding doped with a phosphorus oxide alone.

Figure 6 Shows the form of refractive index profile of a fibre according to the present invention.

Figure 7 shows the variation of total optical loss with wavelength for a fibre according to the present invention.

Figure 8 shows a refractive index profile for an optical fibre having a cladding formed for vitreous silica doped with fluorine and a phosphorus oxide.

Figure 9 shows some possible refractive index profiles which can be attained by using the doping technique disclosed in this specification.

Figure 10a shows the refractive index of a conventional doped vitreous silica optical fibre according to the present invention.

Figure lOb shows the refractive index of a new type of optical fibre.

The present invention is concerned with monomode optical fibres made from doped vitreous silicas. It should be noted that whether or not an optical fibre behaves as a monomode fibre depends on both the geometry of the fibre, and the wavelength at which it is operated. Thus a fibre which behaves as a monomode fibre at a wavelength of 1.5 microns, can behave as a multimode fibre at a wavelength of .8 microns. The technique used for making fibres of this type is well known, and is usually referred to as the modified chemical vapour deposition method. In this technique a vitreous silica support tube is mounted in a glass blowers lathe and silicon tetrachloride vapour together with the vapour of appropriate volatile compounds containing the dopant, in a stream of oxygen are passed along the inside of the silica support tube.The silica support tube is rotated in the glass blowers lathe and an oxygen gas burner is traversed backwards and forwards along the length of the support tube. This causes thermal decomposition of the silicon tetrachloride, and the dopant containing compound, which are deposited in the form of an oxide soot on the inner wall of the silica support tube. Each traverse of the burner causes a distinct layer of material to be built up. The composition of the deposited layers can be varied by varying the proportions of the compounds in the gas flowing through the silica support tube. When sufficient layers have been deposited, the temperature of the oxygen gas burner is raised, and the silica support tube is collapsed in two to four traverses of the burner to produce a solid preform.An optical fibre can be made from the preform by heating it in a furnace and pulling the fibre from the softened preform.

The temperature at which the oxides are deposited on the inner wall of the silica support tube is dictated by the temperature required to sinter the soot into a clear glass. In considering the importance of deposition temperatures and sintering temperatures in relation to core and cladding glasses it should be remembered that for monomode optical fibres a relatively thick cladding is required which may well require more than 50 traverses of the oxy-gas burner, on the other hand the core is relatively small and can usually be built up with as few as two or three traverses of the burner.

Optical fibres according to the present invention are particularly useful for use at long wavelengths e.g. 1.2-1.3 microns and 1.5-1.7 microns. They will in general start behaving as monomode fibres from wavelengths of 1.1 microns out to 2 microns. Taking as an example optical fibre for use at 1.27 microns, a reasonable core size is 8 microns diameter. There is a general requirement that the cladding diameter of a monomode optical fibre should be at least four times the core diameter, and preferably five times the core diameter. Given a core diameter of 8 microns, it is necessary to have a refractive index difference between core and cladding of approx. 0.0046. For a monomode fibre having the above core parameters, the cladding should have a diameter of approximately 40 microns i.e. five times the core diameter.As has already been explained, monomode optical fibres based on vitreous silica have previously been made using a pure vitreous silica cladding. so that no secondary guiding effect is created between the cladding and the pure vitreous silica of the support tube. It is worth noting that for an optical fibre having the dimensions referred to in the previous paragraph, a refractive index difference of .00015 between the cladding refractive index and pure silica, would give rise to a structure capable of supporting a single mode in the cladding in addition to the primary mode supported by the core. This is only 3% of the refractive index difference between core and cladding. It is this problem which has led designers of optical fibres to use pure vitreous silica for the cladding in the past.

In order to avoid stability problems in the maufacture of the preform, it is desirable that the deposition, and sintering of the deposited layer to form a clear glass (these steps are frequently carried out as a single operation) should be performed at a temperature well below the sintering temperature of silica. However if a pure vitreous silica cladding is used, it is not possible to achieve this. It should be noted in passing that the silica support tube can not be used as the cladding since commercially available silica tubes tend to have a high OH ion content, and this gives rise to absorption of electromagnetic radiation. This is of course a problem in monomode optical fibres, since the radiation field of the guided wave penetrates into the cladding.

The present invention proposes to overcome this problem by using a cladding of a doped vitreous silica having a composition so selected that it's refractive index is the same as that of pure silica, or lower than that of pure vitreous silica. This means that there can be no separate guiding action generated between the cladding and the silica support tube. To achieve this objective it is necessary to have 9 dopant which lowers the refractive index of vitreous silica. If it is intended to lower the sintering temperature of the vitreous silica as far as possible, it is advantageous to add a second dopant which will raise the refractive index of the vitreous silica, and provide further lowering of the sintering temperature.These two dopants should then be added in such proporations that there is either little net change in the refractive index, or the resultant refractive index of the doped vitreous silica is lower than that of pure vitreous silica.

There are many dopants which can be added to vitreous silica in order to raise it's refractive index, however the two most commonly used dopants at the present time are phosphorus pentoxide, and germania. Two dopants, which may be used to lower the refractive index of vitreous silica are fluorine, and boric oxide. The use of germania as one of the dopants is to be avoided if possible, since germania has a strong effect on the refractive index of the resultant vitreous silica, i.e. small quantities of germania make a relatively large difference to the refractive index. Again relative to phosphorus additions of germania to vitreous silica, have little effect on the sintering temperature of the vitreous silica.For this reason the dopants selected for use in the present invention are a phosphorus oxide, to raise the refractive index of the vitreous silica, and either boric oxide or fluorine to lower the refractive index. An additional advantage in the use of a phosphorus oxide for the cladding is that it alleviates the problem created by water impurities in the silica support tube. It is obviously important to prevent water diffusing from the silica support tube deep into the cladding or into the core where the propagating radiation field is high. Phosphorus oxide tends to trap diffused water in the outer layers of the cladding where it can do relatively little harm. Boric oxide has similar properties to phosphorus in this respect.

For use at fairly long wavelengths, i.e. 1.5-1.7 microns, boric oxide suffers from absorption losses caused by infra red tail effects. This is clearly illustrated in Fig. 5, which shows the much greater increase in absorption loss at wavelengths beyond 1.5 microns for an optical fibre doped with a mixture of phosphorus oxide and boric oxide as compared with a fibre having a cladding doped with phosphorus oxide alone. Thus for long wavelength work it is essential to use fluorine as the dopant rather than boric oxide.

It should be noted that in the past attempts to dope vitreous silicas with fluorine have not been too successful. However it has proved possible to obtain vitreous silicas doped with fluorine by using volatile fluorides e.g. freon and other carbon halides containing fluorine.

Vitreous silicas doped with fluorine, and methods of achieving such doping have been reported at the Paris Conference in September 1 976 on Optical Communications, in a paper entitled "Fluorine doped Silica for Optical Waveguides" given by K Abe. However attempts to obtain fluorine doped vitreous silica by MCVD technique, using freon, CCl2 F2 as the source of dopant, causes the reaction rate of silicon tetrachloride with oxygen to be reduced.However The addition of phosphorus, in the form of phosphorus oxychloride (POCI2) along with fluorine dramatically increases the reaction rate, allowing an easily sinterable layer (sintering temperature 1,570"C) to be deposited having a refractive index less than or equal to that of silica, with low losses at waveguides out of 1.8 microns except for the OH absorption peak at 1.24 and 1.4 microns. It should be noted that the addition of fluorine to vitreous silica has no or little effect on the sintering temperature of the resultant doped vitreous silica. The lowering of the sintering temperature is created by the phosphorus oxide. The use of this technique thus allows relatively large amounts or fluorine to be taken up in the doped vitreous silica using the relatively simple modified chemical vapour deposition technique.A typical spectrum of optical loss against wavelength for an optical fibre having a cladding doped with a mixture of fluorine and phosphorus pentoxide is shown in Fig. 4. The peaks in the absorption spectra are due to a number of causes. Peak 21 and 23 are due to mode cutoffs in the fibre employed, peak 22 is the second overtone absorption peak for OH, peak 25 is the first absorption overtone of OH.

Peak 24 is an OH combination absorption band. From the drawing it can be clearly seen that absorption losses below 1dB per Km can be obtained both in the region of 1.2 microns and in the region between 1.5 and 1.7 microns.

The effect of phosphorus on the take up of fluorine in a doped vitreous silica is clearly illustrated in Table 1. This shows the dramatic reduction in deposition temperature than can be otained when phosphorus in the form of POCK, is present in the gas stream in addition to freon.

The effect on refractive index in the resultant vitreous silica is also shown.

The ability to produce doped vitreous silica having refractive indices substantially less than that of pure vitreous silics enables novel refractive profiles for optical fibres to be envisaged.

Some examples of refractive index profiles which might be produced by the present technique are shown in Fig. 9. Fig. 9a shows a refractive index profile for an optical fibre in which there is a cladding layer 31 having a refractive index substantially equal to, but slightly lower than the refractive index of vitreous silica, and a graded index core 32. The cladding is surrounded by a layer of pure vitreous silica 33, which corresponds to the support tube employed in the production process. Fig. 9b shows a refractive index profile for a graded index optical fibre in which the maximum core refractive index is equal to that for pure vitreous silica, and the cladding is formed from vitreous silica doped with fluorine and phosphorus having a refractive index substantially less than that of pure vitreous silica. Fig. 9c shows a refractive index profile for a monomode step index optical waveguide in which the core is formed from pure vitreous silica, and the cladding is again formed from a doped vitreous silica having a refractive index substantially less than that of pure vitreous silica. Finally Fig. 9d shows the refractive index profile for an optical fibre having a doped vitreous silica cladding with a refractive index equal to pure vitreous silica. The two arrows show the boundary between the cladding formed of doped vitreous silica, and a pure vitreous silica layer corresponding to the support tube. Throughout Figs. 9a-9d like parts of the optical fibre structure are designated by like reference numerals.

The refractive index profile obtained in a practical example of a dielectric optical waveguide prepared by the use of the process of the present invention, is shown in Fig. 6. The overall cladding diameter is 101 microns, and the refractive index difference between core and cladding is 0.0102, the cladding has a refractive index of .0030 less than that of pure vitreous silica.

The core diameter was 9.3 microns, the numerical aperture 0.173, the cladding diameter was 30 microns. This particular fibre structure only became single mode in operation at wavelengths beyond 2.1 microns. i.e. in terms of the wavelength regions of interest in the present specification, the fibre behaved as a multimode optical fibre. It should be noted that the dip in the centre of the core is a result of the method of making optical fibres using the modified chemical vapour deposition process, is not placed there by design, and does not appear to exert an adverse effect on the operation of the optical fibre as a waveguide. The core for the fibre of Fig. 6 was vitreous silica doped with a mixture of germania and phosphorus, and the cladding was made of a vitreous silica doped with fluorine and phosphorus.

Example 1 An optical fibre having a cladding comprising vitreous silica doped with fluorine and phosphorus was prepared using the modified chemical vapour deposition process. The support tube consisted of a 16mm outside diameter Heralux WG tube. The cladding was formed by making 50 passes of an oxy-gas burner with a gas stream containing phosphorus oxychloride, freon, silicon tetrachloride, and oxygen passing through the tube. The deposition of the cladding layers occured at a temperature of approximately 1,570"C, and the resultant cladding contained 1 mol % phosphorus oxide calculated as P205 mol % fluorine.A core of vitreous silica doped with 2 mol % germania, and 1 mol % phosphorous oxide calculated as phosphorus pentoxide, was then formed in the usual way by two passes of the oxy-gas torch, generating a temperature of 1,600 C. The preform was then collapsed in the usual way, using 3 passes of the oxy-gas torch, at a temperature of approximately 1,750"C, and the resultant preform was sheathed in 16mm Heralux WG tube. The resultant preform was then drawn to yield 1 kilometre of single mode fibre. Fig. 8 shows the measured refractive index profile for the fibre of this example the core diameter is 7.2 microns and there is no measurable difference between the cladding refractive index and that of pure vitreous silica.The fibre operates as a single mode optical fibre for wavelengths greater than 1.1 microns, and the refractive index of the core is 0.1 24 greater than that of pure vitreous silica. The total loss spectrum of this fibre is shown in Fig. 7, and it can be seen that at a wavelength of 1.6 microns the loss is 0.8 dB per kilometre.

Example 2 This example is concerned with the production of an optical fibre having a cladding of vitreous silica doped with boric oxide and phosphorus oxide. A fibre preform was prepared by taking a support tube of 16mm outside diamter Heralux WG tubing, and forming the cladding using the modified chemical vapour deposition process. The clad was formed by a total of 55 passes of an oxy-gas burner, deposition occurring at a temperature of 1,570"C. The resultant cladding contained approximately .5 mol % boric oxide, and approximately 1 mol % phosphorous oxide calculated as phosphorus pentoxide. The core was formed by two passes of the oxy-gas burner deposition occuring at 1,600 C. The core contained a vitreous silica doped with approximately 2 mol % germania, and 1 mol % phosphoruous oxide calculated as phosphorus pentoxide.The tube was then collapsed in the normal way by three passes of the burner to produce a preform 560mm long and 9.6mm in diameter. This was sheathed with an 18mm diameter Heralux WG tube and the whole preform was drawn down into fibre in a carbon furnace, yielding approximately 10 kilometres of fibre.

It should be emphasised that one of the advantages of producing as low a sintering temperature as possible for the cladding, especially in relation to monomode fibres, is that deposition of the cladding requires many more passes of the oxy-gas burner. This means that the temperature of deposition and sintering of the cladding determines to a very large extent the probability of distortion in the preform through excessively high temperatures in the production process. It can thus be seen that the lower the sintering temperature for the cladding, the more passes of the oxy-gas torch that can be achieved without distortion, therefore enabling bigger preforms to be manufactured, which in turn enables longer lengths of fibre to be prepared per preform.It can thus be seen that there are tremendous practical advantages in preparing monomode fibre with as low a cladding deposition and sintering temperature as possible.

Fig. 1 of the drawings shows a section through a fibre preform before it has been collapsed. 1 shows the annular hole through the centre of the uncollapsed preform. The preform itself consists of a pure vitreous silica support tube 14 on which has been deposited a layer of vitreous silica 13, doped with 2% by weight of boric oxide, and 1 % by weight of phosphorus pentoxide. Up to 5% be weight of boric oxide, and 2.5% by weight of phosphorus pentoxide, could be used. Layer 12, deposited on top of the cladding is a vitreous silica doped with germania and phosphorus to form the core of the optical fibre after collapse and drawing.

After collapsing the preform appears as shown in Fig. 2, in which the germania doped core is shown at 15, the cladding doped with boric oxide and phosphorus pentoxide is shown at 16, and 1 7 is an outer layer of pure vitreous silica which corresponds to the silica support tube of Fig. 1. This outer layer and its equivalent in the optical fibre is, in this specification, referred to as the support tube although it has undergone subsequent processing.

After drawing, the resultant optical fibre appears in sections as shown in Fig. 3, and has the refractive index profile shown at the bottom of the figure. The optical fibre has 3 regions, the core 1, the cladding 2, and support tube 3. As will be seen from the drawing there is a stepped refractive index difference between the material of the core and that of the cladding, but there is little difference in refractive index between the cladding and the support tube.

It is relatively easy to dope vitreous silica with phosphorus, germania and boric oxide. The literature abounds with references to the use of these substances, in modified chemical deposition. Furthermore the use of germania doped vitreous silica as the core material for monomode fibres is well known. Doping vitreous silica with fluorine is another matter, and the problem of doping vitreous silica with fluorine and its solution has been discussed earlier in this specification.

A novel form of waveguide structure, which can be produced using fluorine and phosphorus doped claddings is worth particular mention. Referring to Fig. 1 Oa of the drawings where there is shown a conventional refractive index profile for a monomode optical fibre having a vitreous silica cladding doped with phosphorous and fluorine, and a core doped with germania and phosphorus, a number of features can be identified. 50 shows the position of the vitreous silica support tube 51 shows the cladding having a refractive index less than that of pure vitreous silica 52 shows the core. A feature which is invariably found on the refractive index profile of such fibres is the dip in refractive index at the centre of he core. In terms of the operation of such optical fibres, this dip has no apparent adverse affect on the guiding properties of the optical fibre.The dip is caused by evaporation of dopents from the inner layers of the preform during manufacture. This phenomena can be used to produce a monomode optical fibre in which no core layers are deposited. This is achieved by using a doped vitreous silica, doped with fluorine and phosphorus oxide, having a refractive index substantially less than that of pure vitreous silica. This structure with no core layers is then collapsed. The evaporation phenomena occurs as it does for conventional waveguides. However since in this particular structure the dopents are depressing the refractive index of the vitreous silica, loss of dopent by evaporation causes instead of a dip, a hump in the refractive index profile. This hump acts as the core of the monomode fibre.If a preform of this type is processed in an identical manner to that in which conventional optical fibre preforms are prepared, the central "core" has a diameter of approximately 3 microns, which is rather too low to be useful. However by subjecting the preform structure to a prolonged heat treatment at a temperature between the deposition temperature of the doped vitreous silica and the collapse temperature for the tube the diameter of the core can be increased to a useful size. The refractive index profile of a fibre produced by this process is shown in Fig. 1 0b in which 50 represents the fluorine and phosphorus oxide doped cladding and 53 represents the region of doped vitreous silica in which dopent has been lost be evaporation.

The advantage of this technique is that it enables a monomode dielectric optical waveguide to be produced with out the need for depositing pure vitreous silica. The problems associated with the deposition of pure vitreous silica have already been extensively discussed in this specification namely high temperature for deposition and sintering, limiting the preform size and causing problems with distortion of the preform.

Table I Flourine Doped SiO2 Flow of CF2 C12 in ml/min 10 3 3 0 10 Mol % P205 1.0 1.0 0 0 0 Layer thickness per traverse in microns 6 8 8 8 8 Deposition temperature "C 1630 1570 1750 1750 1750 Refractive index difference from SiO2 -0.003 - 0.0003 -0.001 0.000 - 0.003

Claims

1. A Monomode optical fibre for use with electromagentic radiation having a wavelength greater than 1.1 microns, comprising a core of doped vitreous silica, and a cladding of vitreous silica doped with fluorine, or boric oxide, and an oxide of phosphorus, having a refractive index substantially equal to or less than the refractive index of pure vitreous silica.

2. A Monomode optical fibre as claimed in claim 1 wherein said cladding is surrounded by a circumjacent layer of un-doped vitreous silica.

3. A Monomode optical fibre as claimed in claim 1 or 2 wherein said core comprises vitreous silica doped with germania and an oxide of phosphorus.

4. A Monomode optical fibre as claimed in any previous claim wherein said core has a diameter of less than 1 5 microns and said cladding has a thickness greater than 1 5 microns such that the cladding diameter is at least 4 times the core diameter.

5. A Monomode optical fibre as claimed in any previous claim having a cladding with a composition as set forth in either example 1 or 2.

6. An optical fibre preform for making a monomode optical fibre as claimed in any previous claim.

7. A monomode optical fibre as claimed in any previous claim prepared by the modified chemical vapour deposition process in which vitreous silica is deposited on the inside wall of a vitreous silica support tube by thermal decomposition of POCI3, CCI2 F2, SiCI4 in appropriate proportions, carried in a stream of oxygen.

8. A monomode optical fibre for use with electromagnetic radiation having a wavelength greater than 1.1 microns comprising a cladding of vitreous silica doped with fluorine, or boric oxide, and an oxide of phosphorus, having a refractive index substantially less than the refractive index of pure vitreous silica, and a core having a refractive index greater than the refractive index of said cladding, wherein said core is formed during the production of a fibre preform from which said optical fibre is drawn, by evaporation of dopants used in said cladding.

9. A method of making an optical fibre preform comprising the steps of: (a) depositing a plurality of layers of vitreous silica doped with fluorine, or boric oxide, and an oxide of phosphorus, on the inner wall of a pure vitreous silica support tube, and b) heat treating the resultant structure of a) to cause evaporation of said dopants from the inner most of said deposited layers, and c) collapsing said silica support tube and said plurality of layers to form an optical fibre preform.

1 0. A method as claimed in claim 9 wherein the initial concentration of dopants in said plurality of layers is substantially constant throughout all said layers prior to said evaporation step.

11. An optical fibre preform made by the method of claims 9 or 10.

1 2. A method of making an optical fibre substantially as herein before described with reference to Fig. 1 Ob of the accompanying drawings.

1 3. An optical fibre substantially as herein before described with reference to Fig. 1 0b of the accompanying drawings.