AU4295393A - Light transmitting device and method for the manufacture thereof - Google Patents

Description

LIGHT TRANSMITTING DEVICE AND METHOD FOR THE

MANUFACTURE THEREOF TECHNICAL FIELD

The present invention relates to light transmitting devices and to methods for manufacturing light transmitting devices. In the context of the present invention, the expression "light transmitting device" is to be understood to include planar optical structures, optical fibres, and preforms for optical fibres.

BACKGROUND ART

The optical properties of a glass are strongly affected by the composition of the glass and various techniques have been proposed for manufacturing lightguides including OVPO (outside vapour phase oxidation), MCVD (modified chemical vapour deposition), the double crucible technique etc, and solution doping techniques.

In the MCVD technique, gaseous reagents are oxidised and deposited at high temperature on the inside of a rotating glass tube. The tube is typically heated externally using an oxyhydrogen torch and the reagents are entrained in controlled amounts in a gas stream by either passing carrier gases such as O₂, Ar or He through liquid dopant sources, or by using gaseous dopants. Due to their reasonably high vapour pressures at room temperatures, halides such as SiCl₄, SiF₄, GeCl₄, BCl₃, BBr₃, PCl₃, POCl₃, SF₆, CF₄ and CCl₂F₂ are typically used.

The gaseous reagents are of high purity and react when heated, typically by an oxyhydrogen torch, with oxygen to form glassy particles which are deposited, in the case of the MCVD process, as a glassy film on the inside of the glass tube. Typically, the deposition temperature is sufficient to sinter the deposited material without causing distortions of the substrate tube. Repeated passes of the oxyhydrogen torch results in the build up of layers of glass material; the composition of which is dependent upon the type and quantity of reagents used. Typically, a number of cladding layers are deposited followed by a layer or layers which ultimately form the core of an optical fibre. Following deposition, the tube and deposit are collapsed, by increasing the temperature of the oxyhydrogen torch, to form a solid preform. The optical fibre is then drawn from the preform.

In solution-doping techniques, the core layers of an optical fibre are deposited in a manner similar to that used in MCVD but at a reduced temperature with the result that a frit (an unsintered porous soot) is deposited rather than a sintered glass. A frit has a significantly greater surface area than a sintered glass.

Electronic Letters Vol 23, No. 7 pages 329-331 discloses a solution-doping technique for fabrication of rare-earth doped optical fibres. The paper discloses immersion of the unsintered porous soot in an aqueous solution of the required dopant precursor at a concentration in the order of 0.1M for periods up to 1 hour. Following immersion, the unsintered soot was rinsed with acetone to remove excess water and then heated to approximately 600°C in the presence of a flow of chlorine gas. The dried unsintered soot was then sintered and an optical fibre preform formed by collapse in the conventional manner. Doping with Nd³⁺, Ho³⁺, Eu³⁺, Er³⁺, Yb³⁺ and Dy³⁺ was reported.

ECOC, 1988 pages 433-436 discloses fabrication of Al₂O₃ co-doped optical fibres by a solution doping technique. AlCl₃ was dissolved in water to prepare aqueous solutions up to concentrations of 3M into which frits were immersed. After soaking in the aqueous solutions, the tube was removed and dried in a flow of a mixture of oxygen and chlorine gas as rinsing with acetone was found to remove the AlCl₃. Al₂O₃ dopant levels of up to 8.5 molar% were reported.

European patent application no. 89122561.7 (publication no. 0372550) discloses a method of fabricating an optical fibre using a solution doping technique in which a dopant precursor is dissolved in an anhydrous organic solvent. The application exemplifies the use of AlCl₃ and ErCl₃ dissolved in ethanol.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method for manufacturing a light transmitting device comprising the steps of

(a) depositing a porous frit on a glass substrate,

(b) wetting the frit with an acidic glass network modifier precursor,

(c) subjecting the acidic glass network modifier precursor to a condensation reaction, and

(d) sintering the frit.

Various oxides (including SiO₂, GeO₂, B₂O₃ and P₂O₅) are capable of forming amorphous networks which are generically referred to as glasses. Such oxides are collectively referred to as glass formers. Additionally, various combinations of oxides are capable of forming glasses (for example - germanosilicate glasses) in which the amorphous network of a primary oxide (SiO₂ in the case of germanosilicate glasses) is modified by a secondary oxide (GeO₂ in the case of germanosilicate glasses). When secondary oxides alter the structural properties of primary oxides they are referred to as glass network modifiers. All glass formers are glass network modifiers but some oxides are capable of modifying a glass network even though they are incapable of forming a "pure" glass. Glass network modifiers include P₂O₅, B₂O₃, SnO₂, As₂O₃, SeO₂, GeO₂, SiO₂, Al₂O₃, MgO and PbO. In addition to glass formers and glass network modifiers, some metal oxides give rise to fluorescent properties in a glass and such metal oxides are referred to as dopant metal oxides in this specification. Dopant metal oxides include oxides of transition metals such as chromium, titanium and copper; and rare earth metals such as praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium.

Various acids containing a source of oxygen undergo condensation reactions in which water is eliminated and a glass network modifying oxide, which is capable of co-ordinating with an amorphous glass network, is produced. Such acids are referred to in this specification as acidic glass network modifier precursors. Acidic glass network modifier precursors include phosphorous acid, phosphoric acid, boric acid, stannic acid, arsenic acid, selenic acid, selenious acid, germanic acid, silicic acid and mixtures thereof.

Preferably, the acidic glass network modifier precursor consists of or contains o-phosphoric acid. Phosphoric acid is commercially available in a number of forms and it is desirable to use a form that contains a minimal level of substances which are contaminants in a light transmitting device. A preferred form of phosphoric acid is an 85 wt % solution in water of 99.99% o-phosphoric acid.

Unlike the MCVD technique in which glass network modifying oxides are derived exclusively from gaseous halides, at least some of the glass network modifiers of the present invention are derived from liquids. In contrast to solution doping techniques in which the dopant is derived from a metal halide dissolved in aqueous or anhydrous organic solvents, the present invention utilizes an oxygen containing acid which is subjected to a condensation reaction.

Some glass network modifier precursors such as phosphoric acid are quite viscous at ambient temperature. For example, 99% phosphoric acid can be semi- crystalline at ambient temperature. To assist in wetting the frit, it may be desirable to reduce the viscosity of the acidic glass network modifier precursor by heating or mechanical agitation.

To induce the condensation reaction it is preferred that the frit wetted with the acidic glass network modifier precursor is flash heated with a low temperature flame (typically less than 1000°C). Water eliminated in the condensation reaction is boiled off as a consequence of the heating.

Silicate and germanosilicate glasses are most commonly used in optical fibres and the optical properties of such fibres can be substantially altered by doping the glass with dopant metal oxides. However, the solubility of dopant metal ions in glass is relatively low with the solubility level in germanosilicate glasses being in the order of 100 ppm. Exceeding the solubility level generally gives rise to clustering of the doped metal ions which results in reduced efficiency of the optical fibre due to rapid ion-ion interactions. The solubility of dopant metal ions in glasses can generally be increased by incorporation of a glass network modifier in the glass.

Preferably, the frit of the present invention is wetted with a source of dopant metal oxide(s) in addition to the acidic glass network modifier precursor. Doping with a source of dopant metal oxides by solution doping techniques has previously been reported. However, the reported techniques have not coupled the doping with increasing the level of glass network modifier by subjecting an acidic glass network modifier precursor to a condensation reaction.

Preferably, the source of dopant metal oxide(s) is the corresponding metal chloride(s). Preferably, the metal chloride(s) is/are dissolved in the acidic glass network modifier precursor to form a solution which is used to wet the frit. Wetting the frit with such a solution has the advantage that a homogenous distribution of acidic glass network modifier precursor and source of dopant metal oxide(s) can be achieved in the frit. In one embodiment of the present invention, the solution takes the form of a melt prepared by adding the metal chloride(s) to heated acidic glass network modifier precursor(s). In addition to, or in lieu of, the source of dopant metal oxide(s), the frit can be wetted with a source of glass network modifier(s). That is to say that the present invention embraces the introduction of glass network modifier to the frit in addition to that which is derived from the acidic glass network modifier precursor. For example, P₂O₅ and Al₂O₃ can be introduced by an acidic glass network modifier precursor and a source of glass network modifier respectively. As with the source of dopant metal oxide (s), it is preferred that the source of glass network modifier (s) is the corresponding chloride (s) which is/are preferably dissolved in the acidic glass network modifier precursor to form a solution which is used to wet the frit. Preferred sources of glass network modifier are AlCl₃, MgCl₂ and PbCl₂.

In a second aspect, the present invention provides a light transmitting device manufactured by a method according to the first aspect of the present invention.

The method according to the first aspect of the present invention is applicable to the manufacture of light transmitting devices derived from both flat and tubular glass substrates. In the case of a tubular substrate, the frit will typically be deposited on the inner surface of the tube. After sintering, the tube and sintered frit would typically be collapsed in a conventional manner to form a solid preform from which an optical fibre could be drawn. In such a case, the sintered frit would form the core of the resultant optical fibre. Alternatively, after sintering and prior to collapsing, a glass layer could be deposited on the sintered frit. Again, the tube, sintered frit and deposited glass layer would then be collapsed to form a solid preform from which an optical fibre could be drawn. In such a case, the sintered frit would form a cladding layer or outer core layer of the resultant optical fibre with the deposited glass layer forming the core or an inner cladding layer of the fibre. Accordingly, the method according to the first aspect of the present invention facilitates modification of the amorphous network of the core of an optical fibre, a cladding layer of an optical fibre, both the core and a cladding layer of an optical fibre, or part of both or either the core and a cladding layer of an optical fibre. In the case of a flat glass substrate, the sintered frit will typically form a layer, or part of a layer, of a planar optical structure.

As previously mentioned, the solubility of dopant metal ions in glasses can be increased by incorporating a glass network modifier in the glass and phosphorous pentoxide (P₂O₅) is suitable for this purpose. Quite apart from increasing the solubility of dopant metal ions, incorporating phosphate glass in an optical fibre can significantly alter the optical properties of the optical fibre. For example, the incorporation of phosphate glass can change the shape and/or lifetime of dopant metal ion fluorescence bands. Accordingly, it is desirable to produce an optical fibre containing a significant proportion of phosphate glass as measured by the mole percentage of P₂O₅.

Attempts to dope the cores of optical fibres with phosphorus have relied upon chemical vapour deposition techniques in which PCl₃ and/or POCl₃ are vaporised and injected into a rotating tube mounted on a glass working lathe. The tube is typically traversed by an oxyhydrogen torch in the same direction as the gas flow. The vapours react at high temperature forming phosphate glass (P₂O₅) particles which are deposited on the surface of the tube. The heat fuses the material to form a transparent phosphate glass film. The tube bearing the film is subsequently collapsed to form an optical fibre preform from which the optical fibre is drawn. Temperatures in the order of 2000°C are reached during collapse of the tube to form the preform. P₂O₅ sublimes at approximately 300°C and hence the amount of phosphate glass that can be incorporated in this manner is limited by the volatility of P₂O₅ .

The method according to the first aspect of the present invention has been found to be particularly useful in the manufacture of optical fibres which contain a significant proportion of phosphate glass. In a particularly advantageous embodiment according to the second aspect of the present invention, an optical fibre is provided which has a core containing greater than 8.5 mole % P₂O₅. Preferably, the phosphate glass containing core is surrounded by a high silica content cladding region. More preferably, the cladding region contains greater than 95 mole % SiO₂.

In a further advantageous embodiment according to the second aspect of the present invention, an optical fibre is provided which has a phosphate glass containing cladding region surrounding a core. Preferably, the P₂O₅ content of the cladding region is greater than 8.5 mole %.

The incorporation of phosphate glass in another optical fibre forming glass alters the refractive index of the glass. In manufacturing optical fibres, it is fundamental that the refractive indices of the materials forming the core and the surrounding cladding region differ. Accordingly, although the present invention embraces an optical fibre in which both the core and the surrounding cladding region contain phosphate glass, it is preferred that one of the components (either the core or the surrounding cladding region) contains a significantly greater proportion of phosphate glass than the other component. Preferably, the P₂O₅ content of either the core or the surrounding cladding region is greater than 10 mole % and more preferably, it is greater than 15 mole %.

Advantageously, optical fibres according to the second aspect of the present invention can also be doped with dopant metal oxides. In the case of optical fibres having' a high P₂O₅ content, in either the core or the surrounding cladding region, the present invention allows a dopant metal oxide content of in excess of 2 mole % with respect to the high P₂O₅ content core or cladding region.

In a third aspect, the present invention provides an optical fibre having a phosphate glass containing core surrounded by high silica content cladding region, wherein the core contains greater than 8.5 mol % P₂O₅, preferably greater than 10 mol % P₂O₅, and more preferably greater than 15 mol % P₂O₅. Preferably, the high silica content cladding region contains greater than 95 mol % SiO₂. Preferably, the phosphate glass containing core has a dopant metal oxide (s) content of greater than 2 mol % . In a preferred embodiment according to the third aspect of the present invention, an optical fibre has a core comprising 80-90 mol % SiO₂, 10-20 mol % P₂O₅, 0-3 mol % Yb₂O₃, 0-0.3 mol % Er₂O₃, 0-0.3 mol % Nd₂O₃ and 0-0.3 mol % Tb₂O₃.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a refractive index profile of the preform manufactured according to Example 1,

Figure 2 is an absorption spectrum of the optical fibre manufactured according to Example 1,

Figures 3 and 4 are fluorescence spectra of the optical fibre manufactured according to Example 1,

Figure 5 is fluorescence spectrum of the optical fibre manufactured according to Example 4,

Figure 6 is an oxide distribution profile within the preform manufactured according to Example 12, and

Figure 7 is an absorption spectrum of the optical fibre manufactured according to Example 13.

BEST METHOD OF PERFORMING INVENTION

The ensuing examples are illustrative of preferred embodiments of the present invention and should not be construed as limiting the scope of the invention in any way.

Example 1

Two 100cm lengths of OPTIQ-100 (trade mark) fused quartz tubing having outside diameters and inside diameters of 20mm and 16mm respectively were joined together and soaked overnight in a PYRONEG (trade mark) solution. The tube was thoroughly rinsed with demineralised water and acetone and was then dried with a flow of nitrogen. The cleaned and dried tube was placed in a glass working lathe and the exterior of the tube was wiped with acetone.

The tube was rotated in the lathe and warmed with a natural gas torch which passed along the exterior of the tube. The interior of the tube was etched with a flow of O₂ saturated with SF₆. The O₂ flow was maintained throughout the ensuing depositions. The temperature of the torch was adjusted to approximately 1700°C as measured by a pyrometer and a SiO₂/P₂O₅/F cladding region was deposited using a conventional MCVD technique with the sources of SiO₂, P₂O₅ and F being SiCl₄, POCl₃ and SF₆ respectively. The cladding region was deposited over fifteen passes of the torch.

Introduction of SiCl₄, POCl₃ and SF₆ was ceased and the torch temperature increased to approximately 2000°C using a hydrogen flame. This resulted in partial collapse of the tube with consequential reduction in the diameter of the tube. The torch temperature was reduced to approximately 1200°C and SiCl₄ and POCl₃ re-introduced to deposit a SiO₂/P₂O₅ frit. The frit was sintered by increasing the torch temperature to approximately 1700°C with the resulting layer forming a barrier between the previously deposited cladding layer and the yet to be deposited core frit.

The torch temperature was reduced to approximately 1400°C and a silica core frit deposited by re- introducing SiCl₄. One end of the tube was sealed by melting and the tube removed from the lathe.

A pre-prepared solution of neodymium chloride (NdCl₃) dissolved in warmed 99% o -phosphoric acid (H₃PO₄) was heated to approximately 150°C and poured into the tube. The tube was placed in an oven for approximately 1 hour to allow the frit to be wet by the solution. The solution was prepared by warming 80ml of the acid to approximately 150°C and slowly dissolving 1.0g of NdCl₃.6H₂O with stirring.

A condensation reaction was induced by heating the exterior of the tube with heated air. Water eliminated in the condensation reaction was boiled off and the frit was observed to change from opaque to transparent. The tube was removed from the oven and excess solution drained from the tube. The sealed end of the tube was removed by cutting the tube, the exterior of the tube was cleaned with acetone and the cleaned tube was re-mounted in the lathe.

The condensation reaction was continued by heating the tube in the lathe at a temperature below 1000°C, with a flow of O₂ and POCl₃ carrying away eliminated water. The flow of O₂ was then stopped and the torch temperature increased to approximately 2000°C to sinter the frit and collapse the preform.

The preform was removed from the lathe and its refractive index was measured. The refractive index profile of the preform is illustrated in Figure 1. It is to be noted that the horizontal axis of Figure 1 corresponds to the refractive index of SiO₂ and that each of the broader horizontal dashes on the vertical axis represents a refractive index increment of 0.001. Accordingly, the peaks at approximately 0.5 mm either side of centre have a refractive index approximately 0.0195 greater than SiO₂.

A slice of the core of the preform was analysed by electron microscopy and the concentrations of SiO₂, P₂O₅ and Nd₂O₃ averaged over a number of samples are listed in Table 1.

An optical fibre was drawn from the preform in a conventional manner and spectral properties of the fibre were measured. An absorption spectrum of the fibre is illustrated in Figure 2 and fluorescence spectra are illustrated in Figures 3 and 4. Narrow fluorescent linewidths are evident from Figures 3 and 4, as is the shift in the peak of the 4F_3/2 → ⁴I_11/2 fluorescence to

1051 nm which suggests a glass network structure similar to that reported in Journal of Applied Physics, 1975, 46, pp 1191-1196 for neodymium penta-phosphate glasses.

Example 2

Example 1 was repeated with slightly different processing parameters to produce an optical fibre having a silicate glass core modified by P₂O₅ and doped with Nd₂O₃. The resultant core contained less P₂O₅ and more SiO₂ than Example 1. Electron microscopy results are listed in Table 1.

Example 3

Example 1 was repeated except that in preparing the pre-prepared solution, 2.0g of YbCl₃.6H₂O and 0.1g of ErCl₃.6H₂O were dissolved in 70ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Example 4

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving a further 6.5g of YbCl₃.6H₂O in 60ml of the solution pre-prepared for Example 3. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

A fluorescence spectrum of the optical fibre is illustrated in Figure 5 from which the very narrow fluorescence linewidth is apparent.

Example 5

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 11.5g of YbCl₃.6H₂O and 0.5g of ErCl₃.6H₂O in 90ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1. Example 6

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 10. Og of YbCl₃.6H₂O and 1.0g of TbCl₃.6H₂O in 90ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Tb₂O₃. Electron microscopy results are listed in Table 1.

Example 7

Example 1 was repeated except that both the frit and the pre-prepared solution were different from that prepared in Example 1.

In addition to SiCl₄, a flow of GeCl₄ was introduced to the tube during deposition of the frit. The pre-prepared solution was prepared by dissolving 2.0g of YbCl₃.6H₂O, 2.0g of AlCl₃.6H₂O and 0.1g of ErCl₃.6H₂O in 70ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅, GeO₂ and Al₂O₃ and doped with Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Example 8

Example 1 was repeated except that the pre-prepared solution was prepared by saturating 80ml of H₃PO₄ with Pb(NO₃)₂ (approx 5g) . The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with PbO. Electron microscopy results are listed in Table 1.

Example 9

Example 1 was repeated except that the pre-prepared solution was prepared by saturating approximately 280ml of water with approximately 50.0g of H₃BO₃. The core of the preform cannot be analysed by electron microscopy as boron is too light to be detected; however, the core of the resultant optical fibre is believed to contain silicate glass modified by B₂O₃.

Example 10

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 11.0g of YbCl₃.6H₂O and 1.0 g of ErCl₃.6H₂O in 85 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Optical fibre amplifiers constructed from the fibre have shown saturated output levels of +21.2 dBm with 475 mW of absorbed pump power at 1053nm.

Example 11

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 3.0 g of NdCl₃.6H₂O in 80 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with Nd₂O₃. Electron microscopy results are listed in Table 1.

Example 12

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 7.0 g of YbCl₃.6H₂O and 0.65 g of ErCl₃.6H₂O in 50 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

The oxide distribution within the preform was measured by Electron Microprobe and is illustrated in Figure 6 from which it is apparent that the distribution of the dopant metals (ytterbium and erbium) reflects that of the glass network modifier (P₂O₅). It is to be noted that two different scales of oxide concentrations are represented on the vertical axes of Figure 6 with the 0-20 mol % scale relevant to P₂O₅ and the 0-5 mol % scale relevant to Yb₂O₃ and Er₂O₃.

Example 13

Example 1 was repeated except that the pre-prepared solution was prepared by diluting 10 ml of the preprepared solution of Example 12 in 50 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1 and an absorption spectrum is illustrated in Figure 7. Example 14

Example 1 was repeated except that the pre-prepared solution was prepared by dissolving 5.50 g of YbCl₃.6H₂O and 0.50 g of ErCl₃.6H₂O in 45 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Example 15

Example 1 was repeated except that both the pre-prepared solution and a step in the procedure differed from Example 1.

The pre-prepared solution was prepared by dissolving 4.2 g of YbCl₃.6H₂O and 0.42 g of ErCl₃.6H₂O in 50 ml of the acid. The procedure differed from Example 1 in that the water eliminated during the course of the condensation reaction in the glass working lathe was carried away by a flow of O₂, POCl₃ and Cl₂ rather than O₂ and POCl₃. The use of Cl₂ was found to improve removal of the water and this is believed to be a consequence of the formation of HCl from reaction of Cl₂ and water. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Example 16

Example 15 was repeated except that the pre-prepared solution was prepared by dissolving 10.0g of YbCl₃.6H₂O and 1.0 g of ErCl₃.6H₂O in 100 ml of the acid. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

Example 17

Example 15 was repeated except that the pre-prepared solution differed from that of Example 15.

Rather than using warmed 99% o-phosphoric acid (H₃PO₄), an 85 wt % solution in water of 99.99% o-phosphoric acid (H₃PO₄) was used into which was dissolved 3.0 g of YbCl₃.6H₂O and 0.27 g of ErCl₃.6H₂O.

The use of the diluted acid obviated the need to warm the pre-prepared solution during dissolution of the YbCl₃.6H₂O and ErCl₃.6H₂O. The core of the resultant optical fibre contained silicate glass modified by P₂O₅ and doped with both Yb₂O₃ and Er₂O₃. Electron microscopy results are listed in Table 1.

No phase separation or devitrification on cooling was observed in any of the optical fibres prepared in the Examples. A homogenous composition has been observed throughout the length of optical fibres prepared in accordance with the present invention and reproducibility of results has been achieved for repeated optical fibre preparations.

Claims (28)

CLAIMS :

1. A method for manufacturing a light transmitting device comprising the steps of

(a) depositing a porous frit on a glass substrate,

(b) wetting the frit with an acidic glass network modifier precursor,

(c) subjecting the acidic glass network modifier precursor to a condensation reaction, and

(d) sintering the frit.

2. A method as claimed in claim 1 wherein the acidic glass network modifier precursor is selected from the group comprising phosphorous acid, phosphoric acid, boric acid, stannic acid, arsenic acid, selenic acid, selenious acid, germanic acid, silicic acid, and mixtures thereof.

3. A method as claimed in claim 1 wherein the acidic glass network modifier precursor either consists of or comprises phosphoric acid.

4. A method as claimed in any one of the preceding claims wherein the frit is wet by immersion in the acidic glass network modifier precursor.

5. A method as claimed in any one of the preceding claims wherein water produced during the condensation reaction is caused to form steam.

6. A method as claimed in claim 5 wherein the steam is removed from the vicinity of the frit by a flow of gas.

7. A method as claimed in claim 6 wherein the gas is a mixture of O₂, POCl₃ and Cl₂.

8. A method as claimed in any one of the preceding claims wherein the condensation reaction is induced by elevating the temperature of the wetted frit.

9. A method as claimed in claim 8 wherein the temperature is elevated by application of a low temperature flame.

10. A method as claimed in any one of the preceding claims wherein a source of dopant metal oxide (s) and/or a source of glass network modifier (s) is/are dissolved in the acidic glass network modifier precursor prior to wetting of the frit.

11. A method as claimed in claim 10 wherein the dopant metal oxides are selected from rare earth metal oxides, transition metal oxides and mixtures thereof.

12. A method as claimed in claim 11 wherein the rare earth metal oxides are selected from oxides of praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium.

13. A method as claimed in claim 11 wherein the transition metal oxides are selected from oxides of chromium, titanium and copper.

14. A method as claimed in any one of claims 10-13 wherein the source of dopant metal oxide(s) is the corresponding metal chloride(s).

15. A method as claimed in claim 10 wherein the source of glass network modifier(s) is selected from AlCl₃, MgCl₂, PbCl₂ and mixtures thereof.

16. A method as claimed in any one of the preceding claims wherein the substrate is a flat glass substrate and the light transmitting device is a planar optical device.

17. A method as claimed in any one of claims 1-15 wherein the substrate is a tubular glass substrate and the light transmitting device is an optical fibre or a preform for an optical fibre.

18. A light transmitting device manufactured by a method as claimed in any one of the preceding claims.

19. A light transmitting device as claimed in claim 18 wherein the light transmitting device is an optical fibre having a phosphate glass containing cladding region surrounding a core.

20. A light transmitting device as claimed in claim 18 wherein the light transmitting device is an optical fibre having a phosphate glass containing core.

21. An optical fibre having a phosphate glass containing core surrounded by a high silica content cladding region, wherein the core contains greater than 8.5 mol % P₂O₅.

22. An optical fibre as claimed in claim 21 wherein the core contains greater than 10 mol % P₂O₅.

23. An optical fibre as claimed in claim 22 wherein the core contains greater than 15 mol % P₂O₅.

24. An optical fibre as claimed in any one of claims 21-23 wherein the cladding region contains greater than 95 mol % SiO₂.

25. An optical fibre as claimed in any one of claims 21-24 wherein the core contains dopant metal oxide(s).

26. An optical fibre as claimed in claim 25 wherein the core has a dopant metal oxide(s) content of greater than 2 mol %.

27. An optical fibre as claimed in claim 21 wherein the core comprises 80-90 mol % SiO₂, 10-20 mol % P₂O₅, 0-3 mol

% Yb₂O₃, 0-0.3 mol % Er₂O₃, 0-0.3 mol % Nd₂O₃ and 0-0.3 mol % Tb₂O₃.

28. An optical fibre as claimed in any one of claims 21-27 when manufactured by a method as claimed in claim 1.