GB2033372A - A Method of Producing an Optical Waveguide - Google Patents

Abstract

A transparent glass rod which has been prepared by collapsing a porous glass rod, the pores of which have been doped with a dopant deposited on the internal wall thereof to enhance the index of refraction of the rod, is drawn and the viscosity and coefficient of thermal expansion of the central portion of the glass rod are matched with the viscosity and coefficient of thermal expansion of the portions of the glass rod which have a refractive index lower than that of the central portion. An optical waveguide produced by the method of the invention has a high mechanical strength and a low absorption loss. <IMAGE>

Description

SPECIFICATION A Method of Producing an Optical Waveguide The present invention relates to a method of producing an optical waveguide. A low transmission loss of light, a proper refractive-index distribution over the cross section of the fiber and a high mechanical strength are qualities desired of optical waveguides. A number of manufacturing processes of such waveguides have been proposed; e.g. CVD (chemical vapor deposition) processes and improved CVD processes have been proposed for producing silicate-type glass waveguides and double-crucible processes for producing multi-component glass waveguides. Also M-CVD processes, O-CVD processes and VAD processes are well known as methods for producing preforms, a starting material for the formation of glass fibers.Apart from these methods, a method known as the molecular doping method has been proposed as a more advanced method by which the mass production of economical preforms is made possible.

The present invention relates to-an improved process in which the above-mentioned molecular doping method is used.

Detailed information concerning the molecular doping method may be found in laid open Japanese Patents Nos. 50-28339, 51-13591 9, 51-126207 and 53-102324. In such processes, a porous glass rod, which consists of SiO2 and several per cents of B203 and which has been produced by phase-separation, is used as the starting material. The above patents refer only to porous glass made by phase-separation: however, the same methods are also applicable to porous glasses produced by half-sintering CVD glass powders or by half-sintering fine glass fibers.

In the prior art such as is disclosed in Japanese Patent No.51-126207, a porous silicate glass including a small amount of B203 is doped with a dopant material in a way such that the dopant distribution produces a desired refractive-index distribution in the porous glass rod. In order to dope the rod with the dopant in the desired manner, the porous glass is immersed in a solution containing a compound which is later converted into the dopant, and the special distribution of the dopant to be deposited on the surface of the micropores is controlled by using the fact that the solubility of the compound varies according to temperature and the type of solvent used. The method is described in detail as follows.

A porous glass rod is immersed in an aqueous solution of a compound which will later be converted to an oxide dopant to enhance the refractive index of the glass (the solution is herein referred to as the "doping agent"). For example, an aqueous solution of CsN03, which decomposes at high temperature to Cs2O, is doped into the pores of the rod at 1 00 C, and the rod is then immersed in a cooler liquid, e.g., water at 0 to 40C, in order to reduce the solubility of CsN03, whereby an amount of CsN03 is deposited on the surface of the micro pores. The glass rod is subsequently immersed in a fresh comparatively poor solvent such as water, a water-alcohol system or other alcoholic solution, so that the deposited material is gradually removed from the periphery of the rod by dissolution.The concentration gradient of the solute along the radius is controlled by varying the temperature of dissolution. If the temperature of the dissolution agent is low enough, the radial distribution of the dopant will become sharp and abrupt due to the reduced diffusion rate.

A less sharp distribution will be obtained if the dissolution process is carried out at a higher temperature. Furthermore, the radial distribution of the dopant (the sum of CON03 deposited on the surface of the micro pores and CON03 still dissolved in the solution) is easily controlled to generate; for example, a stepwise or parabolic distribution by properly choosing the sequences of temperatures of dissolution. The glass rod having a suitable radial distribution of CON03 (both deposited and dissolved) is then immersed in a fresh very poor solvent in order that any CsNO3 remaining undissolved in the solution deposits on the surface of the micropores. The rod is then dried under vacuum and the solvent or water absorbed of the surface of the micro-pores is subsequently removed by heating.As the temperature is raised, the compound CsNO3 is decomposed to the dopant Cs2O according to the reaction: 2CSNO2oCs20+N20s The rod is then heated still further in a suitable atmosphere until the pores collapse, and a transparent glass rod doped with Cs2O in a desired radial distribution is obtained.

Finally, an optical waveguide is formed by melt-drawing the preform thus prepared until the cross-sectional area is reduced to the desired dimensions.

However, the conventional practices, above, have the following disadvantages.

(1 ) the dopant concentration cannot be reduced to zero at the periphery of the preform; accordingly, a certain amount of dopant remains in the outer portion, resulting in a preform having a higher refractive index (N1460) in the peripheral portion than in the portion of non-doped silicate glass containing a small amount of B203 (N1.458).

(2) The refractive index at the periphery can exceed the refractive index of the cladding because the compound once deposited on the surface of the micro pores dissolves through the outer surface of the rod during the dissolution process. The dopant distribution thus obtained is always lower at the periphery than at the central portion of the rod as is shown in Figure 1 where, (a) is a stepwise distribution with a core (11) and a cladding (12) and (b) is a graded distribution (13). In other words, a refractive index distribution such as are shown in Figure 2 can never be obtained by the conventional practice.

These fibres may be used as optical waveguides after covering them with a plastic resin for reinforcing the mechanical strength of the fiber. In this case, however, if the refractive index of the plastic resin is lower than the refractive indices of the portions (12) or (13), the light energy would travel along both the portions (11) and (12) or the whole of (13) and would not be confined to the core; moreover, if the refractive index of the plastic resin is higher than that of (12) or (13), the light energy would leak into the resin, resulting in an unfavourable absorption loss and narrower bandwidth.

(3) a portion of the cladding layer (12) of the optical waveguides shown in Figure 1 works only as a mechanical support and need not always be made of an expensive low-loss material; hence a fiber structure with more inexpensive support portions cladded around low-loss portions would be desirable.

However, in the case of the prior art technique, it is impossible to produce very long fibers when the cross-sectional area of the fiber falls below a certain limit, in spite of the fact that the manufacture of porous glass rod with the required dopant distribution is easy using this technique.

(4) The fibers produced in accordance with conventional practices cannot be melt-drawn at temperatures high enough to obtain high mechanical strength after being quenched, because the viscosity of the core glass is lower than that of the surroundings at the drawing temperature due to the high concentration of dopant, and the core portion starts bubbling before the more viscous surrounding glass reaches a high enough temperature for drawing.

(5) The viscosity and the coefficient of expansion of the high refractive-index portion and the low refractive-index portion differ so much that an elastic strain occurs between these portions, resulting in a high transmission loss even when the melt-drawing is carried out at a suitable temperature for drawing the high viscosity portion.

One method for solving the above-mentioned problems of the conventional molecular-doping process may be to provide at least one transparent and corrosion resistant layer at the exterior of the rod which has been doped with a suitable material in the desired distribution. However, the idea of providing a transparent jacket around a rod requires detailed consideration on the physical properties of the glasses which are to be fused to each other.

The formation of an optical waveguide includes the process of melt-drawing a preform having a core portion, a cladding portion and a jacket portion; and the physical properties of the preform such as viscosity and coefficient of thermal expansion, strongly influence this process. For example, the preform might suffer from cracks or bubbles during melt-drawing if there were great differences in the physical properties of the said portions of the preform.

We have now developed a method for economically producing an optical waveguide having a high mechanical strength and a low absorption loss.

Accordingly, the present invention provides a method of producing an optical waveguide by drawing a transparent glass rod which has been prepared by collapsing a porous glass rod, the pores of which have been doped with a dopant deposited on the internal wall thereof to enhance the index of refraction of the rod, which method comprises matching the viscosity and coefficient of thermal expansion of the central portion of the glass rod with the viscosity and coefficient of thermal expansion of the other portions of the glass rod which have a refractive index lower than that of the central portion.

The optical waveguide produced according to the invention has a high mechanical strength produced by matching the viscosity and coefficient of thermal expansion of the glass of lower refractive index with those characteristics of the doped glass of higher refractive index.

The method of the invention may employ alumino-silicate glass or borosilicate glass as a tube material, whereby the viscosity and the coefficient of expansion of the central glass of higher refractive index and those characteristics of the surrounding glass having a lower refractive index, are matched with each other.

The present invention is further described in the accompanying description with reference to the accompanying drawings, in which: Figure 1 is an illustration of the structure and the refractive-index distribution diagram of an optical waveguide produced by a conventional molecular-doping process.

Figure 2 is an illustration of the structure of the refractive index distribution diagram of optical waveguides produced in accordance with the method of the present invention, Figure 3 is an illustration of the drawing process used in the present invention, Figure 4a is an illustration of steps for collapsing the tube, when a rod-in-tube method is employed in the present invention, Figure 4b is an illustration of the process of drawing the preform prepared by the rod-in-tube method.

Figure 5(a) is a cross-sectional view of the optical waveguide produced according to the present invention, Figure 5(b) is an illustration of an optical interference pattern of the optical waveguide produced according to the present invention, Figure 5(c) is a refractive-index distribution diagram of the optical waveguide produced according to the present invention, Figure 5(d) is a diagram showing the transmission loss characteristics of the optical waveguide produced according to the present invention, Figure 6 is a diagram showing the dependence of refractive index of SiO2-glass on the amount of alkali metal oxide dopant, Figure 7 is a diagram showing the dependence of viscosity of S,02-glass on the amount of alkali metal oxide dopant.

The present invention relates to a method of producing an optical waveguide by melt-drawing a transparent glass rod which has been produced by collapsing a porous glass rod which has been doped with a material by depositing the material on the surface of the micro pores in order to enhance the refractive index of the glass rod. The method of the present invention makes it possible to draw the glass rods at a high temperature, very close to the melting point of the outermost glass layer, without causing any over-melting.

The optical waveguide produced according to the invention has a high mechanical strength and high durability against water, and also low transmission loss characteristics.

The method of providing a preselected dopant distribution in a porous glass rod will be described, by way of example, with respect to a doping agent CsNO3 and the dopant Cos20.

A rod of porous glass such as Vycor, which is a silicate glass including a small amount of B203, is immersed in a hot aqueous solution OSLO3 which will later be deposited in a form of a dopant Cs2O on the internal walls of the micropores. The rod is subsequently immersed in pure cold water so that the solution in the pores saturates and excess CsNO3 deposits on the surface of the pores. Since the concentration of the OSLO3 at the outside of the glass rod is substantially zero, the CsNO3 in the rod comes out of the pores by diffusion, and the dissolution of OSLO3 once deposited on the surface of the micro pores starts first at the periphery and then in the inner portions of the rod.The solubility of this compound is 66.8 g CsN03/1 00 g solution at 1 00C and 8.54 g CsN0$100 g solution at OOC. The CsNO3 distribution over the cross section of the rod thus obtained is higher at the central portion and lower at the peripheral portions. The glass rod is subsequently immersed in a poorer solvent such as methanol (CH3OH), so that the residual solute still dissolved in the pores is further deposited.The rod is then dried in a vacuum and heated slowly until the solute decomposes into Cso2 by the reaction: 2CsNO3eCs20+N205 The rod heat-treated in this way has a predetermined distribution of dopant Cs2O deposited on the pore walls, and as the temperature is raised still further, the viscosity of the glass drops until the pores of the glass finally collapse and the porous rod turns to a transparent glass rod. The optical fiber drawn from this rod is shown in Figure 1 in which (1 2) denotes the portion almost free from the dopant and (11) the portion still containing the dopant.The amount of dopant in portion (11) results from the differences in the solubility of CsN03 in hot water and in cold methanol, and the amount of dopant in portion (12) results from the difference in solubility in cold water and in cold methanol. It is to be noted that the examples described in the above are only illustrative of the method and are not meant to limit the scope of the present invention.

The fibres obtained from such a glass rod do not have a desirable long-term reliability when used in severe circumstances, because a small amount of alkali remains at the outer surface thereof. It might seem that this difficulty could be overcome by providing, at the periphery another layer without any Cs2O content, and a SiO2 glass with a small B203 content made from a non-doped porous glass (such as Vycor) might seem preferable for such a purpose, because the coefficient of thermal expansion of this glass is similar to that of the glass rod itself.

However, the above-mentioned glass is actually not suitable for the purpose intended, since the refractive index of this glass is around 1.458 which is lower than the refractive index of the cladding glass having a small Cs20 content. In this case, the light energy to be transmitted is not confined to the core portion and leaks into the cladding portion, resulting in a large transmission loss and a narrow bandwidth. Furthermore, the drawing temperature of such a glass is considerably higher than that of the core glass, because it has a higher viscosity at a high temperature, and if the whole rod were drawn at this high temperature, bubbling might result in the core.

The applicants have found that alumino-silicate and borosilicate glasses are preferred for use because they have a higher refractive index than the cladding glass (the refractive index of which is around 1.460), and a higher resistance to humidity and the corrosive effects of water.

It is generally known in the art that the differences in the coefficient of expansion and the softening temperature of the core and cladding portions should be within 25x 1 0-7/ C and 5000 C, respectively, when an integrated assembly of rod and tube is to be melt drawn. However, it is desirable to keep the differences in the coefficient of expansion and the softening temperature as low as possible, preferably, within 10x10-7/OC and 2000C respectively, in order to minimize the residual strain in the longitudinal direction at the surface of the fiber.

Table 1 Core Core Jacket Jacket CsO(10%) CsO(3%) Clad Clad Boro- Almino -B2O3(4%) -B2O3(4%) B2O3(4%) B2O3(7%) silicate silicate Glass -SiO2 -SiO2 -SiO2 -SiO2 (No. 7740) (No. 1720) Coefficient of thermal expansion (0-300 C, x10-7/ C) 18 26 10 20 32.5 42 Viscosity ( C) (i) at the softening temp. 1200 920 1400 1200 820 915 (ii) at the working temp. 1450 1340 1650 1450 1240 1200 Durability against water and acid less less better better better better Refractive index 1.475 1.483 1.458 1.455 1.474 1.530 As shown in Table 1 , the refractive indices of alumino-silicate glass and borosilicate glass are 1.53 to 1.55 and 1.47, respectively, which are higher than the refractive index of the cladding glass (*1.460). These glasses are also easy to fuse to the rod for forming an integrated assembly of rod and tube, because the working temperatures of aluminosilicate glass and borosilicate glass are 1 50 to 1 2000C and 1 200 to 1 2500C, respectively, which are close to that of the Si02-B203 glass doped with Cs2O.

Since these glasses are inexpensive and commercially available, the combination of glass material disclosed in this invention enables more economic and easy production of optical waveguides. Atypical composition of borosilicate glass is SiO2:8 1 %, B203:12.7%, Aí203:2.3% and Na3O(+K20):4%, and a typical composition of alumina-silicate is glass is; S102:6%.

B2Os:5%, Na2O(+K2O):1.1%. Al203:18.5%, MgO:7.9% and C,O:7.3%.

The optical waveguide in accordance with the present invention has a refractive-index distribution shown in Figure 2, i.e. the optical fiber has, at the exterior of the cores (21), (21'), (24), (24') and the claddings (22), (22'), (25!, (25'), the jacket layers (23), (23'), (26), (26') which have a refractive index higher than the cladding layers.

Figures 3 and 4 show the process of inserting the glass rod into the tube and melt-drawing the combination into a fiber. Referring now to Figure 3, a glass rod (31) doped with a suitable dopant is inserted into a glass tube made of one of the aforementioned materials (such as Pyrex glass) (38), and the rod and the tube are sealed at one end by fusing them to each other. The combination of the rod and pipe is then heated by a heater (34) and drawn to into a fiber, while evacuating the gap between the rod and the pipe.

Figure 4 shows another example in which a glass rod (41) doped with a suitable dopant and a glass tube made of one of the aforementioned glass materials is heated, while rotating, by an oxyhydrogen flame.

The pipe is then fused and collapsed to form a new single glass rod (45) as shown in Figure 4 (a).

It may be necessary to cool the rod slowly enough to reduce the thermal strain which may have been introduced during assembly of the rod and tube. The rod is subsequently melt-drawn to a fiber as shown in Figure 4(b).

The method in accordance with the present invention has the following advantages: (1) The mechanical strength and the long-term reliability are much improved since the glass provided at the exterior of the fiber exhibit a high resistance to humidity and water. Particularly, the combination of glasses in accordance with the present invention provides fibers of high mechanical strength, because the melting point of the jacket glass is lower than that of the core glass and cladding glass and so when the preform consisting of these glasses is drawn, the surface of the jacket glass becomes softer and smoother than that of the other glasses and, after cooling, the solidity of the jacket glass gives the fiber high strength.

(2) A long fiber can be easily produced from a small glass rod doped with a suitable material.

(3) The fiber is economical, because low-priced glass such as Pyrex may be used as the surrounding material.

(4) Because the index of refraction is greater in the outermost jacket glass than in the cladding glass and the transmission loss in the jacket glass is much higher than in the core/cladding glasses, the light energy which is to be transmitted through the fiber is confined completely to the core and the higher modes are absorbed by the jacket glass so that the transmission loss and the signal distortion characteristics of the optical waveguide are maintained at a very low and stable level while a wide band-width is retained.

(5) The soft glass jacket layer around the rod permits the working of the preform at a temperature suitable for drawing high-refractive Cs2O-B2O3-SiO2 glass, and this allows the production of optical waveguides free from structural imperfections which cause transmission losses. An example of the rod-in-tube process will be described later in a preferred embodiment.

The present invention provides a method of improving the disadvantages of the conventional methods.

Among the qualities desired of an optical waveguide mechanical strength, low transmission loss and low signal distortion are especially required. On account of this, it is a general practice to apply a primary coating of thermal hardened resin such as a silicon resin or an epoxy-resin immediately after drawing to maintain the initial mechanical strength of the fiber. In order to produce a mechanically strong fiber, the surface of the preform should be clean and smooth, and the preform should be placed in a clean atmosphere when heated in a furnace. Apart from these conditions it is also required that the fiber is quenched rapidly immediately after the surface has been smooth by heat.Such drawing conditions are realized by making the longitudinal temperature gradient abrupt (a large reduction ratio, i.e., a decrease of cross-sectional area per unit length, is required for high temperature drawing, because the whole glass rod softens in this case). A CO2-laser, flame or Joule or induction electric heater having a small diameter and height are examples of means for achieving these drawing conditions.

As described with respect to conventional practices, the core of a preform prepared in accordance with the conventional method consists of a Cs2O-B203-SiO2 glass containing a large amount of Cs2O and having a comparatively low melting point, and the cladding of such a preform generally consists of B,O--SiO, glass which has a high melting point as is the case with the commercially available "Vycor" which is used as a economic substitute for silicate glasses.

The drawing temperature of the preforms should be matched to the softening temperature of the hard cladding glass when the preform is drawn alone or being inserted in a tube glass which has lower softening temperature than the cladding glass. However, the core glass softens so much at such a high temperature that bubbling takes place in the core and the resulting fiber has large diameter variations due to the bubbles, which may cause a blockage or fracture of the dies for coating the fiber.

Accordingly, it is impossible using the conventional method to produce optical waveguides of high mechanical strength with an outer surface which is quenched rapidly from a high temperature.

The present invention provides a method of matching the viscosity and coefficient of thermal expansion of the central high refractive-index glass and the surrounding low refractive-index glass in order to produce an optical fiber which is free from the above-mentioned disadvantages. It has already been mentioned, by way of example, that the present invention may employ a borosilicate or alumino silicate glass as the tube material when the rod-in-tube method is employed. In the following, a method will be described in which the viscosity and the coefficient of expansion are positively matched by doping materials for adjusting the viscosity of the glass. Therefore, another aspect of the present invention is to provide a method for preparing a preform which has a soft cladding and has a uniform high-temperature viscosity throughout its cross-section.The method consists of matching the viscosity of the cladding glass with that of the core glass and the jacket glass by lowering the high-temperature viscosity of the cladding glass by doping the cladding glass with a material which softens the glass. The dopants for viscosity adjustment should not react with the dopants, such as Cs20, for controlling the refractive index, and also they should not have any significant influence on the refractive index of the glass. Compounds such as P205 or B203 are suitable for use as a dopant for viscosity control, because they are stable oxides having low melting points and they are obtainable by oxidising compounds which are soluble in water or alcohol for stuffing and deposition.Compounds such as Li20, Na20, H 20, MgO or CaO, which have a tendency to enhance the refractive index slightly but have a much greater tendency to reduce the viscosity, can also be used as this kind of dopant.

The present invention will be described in more detail below. An S,02-porous glass (containing several per cents of B203) prepared by phase-separation and an elution process or a porous glass prepared by sintering S,02-glass powder obtained by flame hydrolysis is immersed in a doping agent for controlling the refractive index. The glass is subsequently immersed in a poorer solvent in order for the doping agent to deposit on the pore wall. The glass is then immersed in a liquid such as water, alcohol or a water-alcohol system, whose temperature has been adjusted so that the compound has a predetermined solubility in the solvent.

The stuffing compound still dissolved in solution in the pores or deposited on the pore wall starts dissolving out of the rod during this process.

If this dissolution liquid contains a proper amount of stuffing material for viscosity adjustment, the dissolution process removes, by diffusion, only the doping agent for refractive-index adjustment, and the doping agent for viscosity control remains in the pores. After having obtained the required distribution of the doping material, the porous glass is immersed in a poor solvent for the doping material for refractive-index adjustment or a poor solvent for both the doping material for refractiveindex adjustment and the doping material for viscosity adjustment, in order to make the material still dissolved deposit on the pore wall.After this, the rod is subjected to the conventional sequence of processes, vacuum drying, heating, decomposition of the compound and collapsing of the pores to produce a transparent preform, the cladding portion of which has a low viscosity at the drawing temperature.

The doping material used as a dopant for refractive-index adjustment may be, for example, CsNO3 which is described above. The doping materials for adjusting the viscosity may be boron compounds such as H3BO3, NH4HB402 (NH4)2B409, Na2B40,, K2B407 or Li2B40, or phosphorus compounds such as H3P04, (NH4)3PO4, (NH4)3HPO4, (NH4)H2PO4, K3HPO4, KH2PO4, Na2PO4, NaHP04, NaH3PO4, LiH2PO4, Li2PO4 or Li2PO4 which all dissolve in water; alcohol or a water-alcohol system. The doping material should be chosen so that the solubility in the dissolution solvent used is suitable.For example, if a concentrated aqueous solution of C2H50H is to be used as the dissolution solvent, suitable doping compounds may be H3BO2, or H3PO4, and a suitable solvent for final deposition of these compounds, in this case, may be organic liquids such as C2H5OH, C3H8OH, alcohol or acetone.

Another method for doping a material for viscosity adjustment is to add the compound to the solution in which the porous glass is immersed for final deposition after the material for refractiveindex adjustment has been impregnated, deposited, and dissolved. For example, if C2H50H is used for the final solution, the doping material to be used in this method will be limited only to a few materials such as H3PO4 and H3BO3. The rod is then subjected to the sequence of conventional processes, vacuum drying, heating, decomposition and collapsing of the pores and a transparent preform is obtained.

If necessary, a method may be employed in which the material for the refractive-index adjustment is deposited by a doping solution for viscosity control and then the rod is immersed in a poor solvent for the viscosity agent in order to deposit the agent. This method has an advantage that the material for viscosity control is fixed on the pore walls and does not move afterwards in the vacuum drying process during which the solution moves outwardly, and a uniform distribution of the viscosity dopant is obtained.

A further method for stuffing the dopant for viscosity control is as follows.

The glass is subjected to the conventional sequence of doping, first deposition, dissolution and second deposition of the refractive index material. Then, the glass rod is immersed in a solution of a viscosity-control agent which has no solvent power over the compound for refractive-index control. (An example of such a solvent is C2H5OH). After the viscosity-control agent has been impregnated, the glass rod is immersed in a poor solvent for this agent, such as acetone, for deposition, and then subjected to vacuum drying, heating, decomposition and collapsing processes.

A still further method is one in which the glass rod is doped at the same time, with two agents for adjusting the viscosity and refractive index, and then the material for adjusting the refractive index is dissolved by a solvent which is not a solvent for the viscosity agent.

The methods described above can be repeated several times to obtain a preselected refractiveindex distribution, taking into account the solubility and diffusion rate of the doping agents. Examples of the above-mentioned methods are described in the Examples 2 and 3.

Compounds Li, Na, K, Mg, and Ca such as LiNO3, NaN03, KNO3, MgSO4 and Ca(NO3)2 dissolve in pure water, a water-methanol system or pure methanol and can be used as doping agents for controlling the viscosity of glasses. Examples of the refractive index and viscosity of silicate glasses doped with Li20, Na2O and K20 are shown in Figures 6 and 7.

According to Figure 6, the refractive-index increments in silicate glasses doped with, respectively, 2.5 wt% Li2O, 5 wt% Na2O and 5 wt% K20 are less than 0.010, 0.0075 and 0.0075.

Figure 7 shows the viscosity of silicate glasses doped with 20 wt% of dopant. From this diagram, one may safely infer that the glasses become soft enough as well when the dopant concentration is no more than 5 wt%. An example of such a stuffing agent is shown in Example 4.

In this way, the present invention allows the preform to be melt-drawn through a sharp temperature gradient at a temperature at which no bubbling takes place in the core, by reducing the high temperature viscosity of the cladding glass down to the viscosity of the core and jacket glass portions. Hence, the outermost glass layer (cladding layer in case of direct drawing and jacket layer in case of rod-intube drawing) is maintained clean and smooth during the drawing, and optical waveguides having an extremely high mechanical strength are obtained by applying a primary coating immediately after the drawing and before the fiber surface is exposed to any contamination.

Apart from the rod-in-tube method, the preform may be drawn after an integrated combination of rod and tube has been assembled, by collapsing the tube in which the rod is inserted. This is possible because the viscosity of the cladding layer has been brought to near that of the jacket layer.

Since all the portions of the preform in accordance with the present invention have a similar viscosity and coefficient of thermal expansion, the preform can be drawn very smoothly to form an optical fiber free from structural imperfections and having a low transmission loss.

The present invention will be further described with reference to the following Examples.

Example 1 A rod of 10 mm in diameter was prepared by drawing a glass consisting of 3.5% K20, 3.5% Na20, 33% B203 and 60% 8it2, and subsequently cooling it at a rate which does not influence the phase separation. The rod is then heat-treated at 5500C for 1 5 hours, leached in 30 NHCI at 950C for 48 hours and rinsed in water at 950C in order to obtain a porous glass rod. The porous glass rod is stuffed in CsNO3 100 g/H20 100 g solution for 4 hours, immersed in pure water at a temperature of 0-40C for 4 hours for depositing and unstuffing CsNO3 and then immersed in CH30H at a temperature of 0- 40C for 4 hours for further depositing CsNO3. The rod is then dried in a vacuum and removed of water by heating.The stuffing compound decomposes, by further heating, into C520 by the reaction: 2C5NO3Q2O+N2O5.

The rod is then held at 650"C in an atmosphere of oxygen gas and subsequently heated further up to 850 C in the oxygen atmosphere under 60 mm Hg of pressure until the porous rod is collapsed into a transparent glass rod of 8 mm in diameter.

The preforms thus prepared were drawn in two ways: the first group was drawn without inserting into jacket pipes and the second group was drawn at 1 2500C and under 30 mmHg of atmosphere while being inserted into Pyrex jacket pipes as shown in Figure 3. The fibers produced by the first method were considerably short and the transmission loss was as large as 30 dB/km for the wavelength A=0.83 Mm. The fibers produced by the second method were longer and mechanically stronger than the fibers made by the first method, and the absorption loss was 1 8 dB/km for the wavelength A=0.85 ,um which was considerably smaller than that of the fibers made by the first method.

Figure 5a shows a cross-sectional view of such fiber, Figure 5b the interference pattern of the fiber, Figure Sc the refractive-index distribution derived from the pattern shown in Figure 5b and Figure 5d the transmission loss characteristics of the fiber.

Example 2 An Na2O-K2O-B2O3-SiO2 glass was prepared by fusing the powders of 40% SiO2, 48% H3BO3, 5% K2CO2 and 7% Na2CO3 and formed into a glass rod of 10 mm in diameter, The rod was heat-treated at 540 C for 15 hours, leached at 100 C in 3HNCl solution and rinsed with pure water.

The porous glass thus obtained had a poroslty of approximately 50%. The rod was then stuffed in 67% CsNO3 aqueous solution at 100 C, deposited of CsNO3 in pure water at a temperature of 20 C, unstuffed in C2H5OH 80%-H2O 20% solution for 24 hours and immersed in a saturated acetic acid solution of H3BO3 for deposition of CsNO3 and, at the same time, stuffing of H3BO3 in the pores. The rod is finally immersed in (CH3)2CO for 12 hours for depositing H3BO3 as well as C5NO3. The rod is, subsequently, dried up slowly in a vacuum at a temperature of 0-20 C and heated to decompose H3BO3 and CsNO3 by the reactions: 2H2BO3oB203+2H20 and 2CsNO3#Cs2O+N2O5.

After that, the rod is further heated until the pores collapse and the rod turns to a transparent preform A with a diameter of about 8 mm. A preform 8 was also prepared exactly in the same way as Preform A except that the CsNO3 was deposited in a pure C2H5OH which contains no H3BO3.

These preforms were cleaned on the surface with 5% HF aqueous solution, melt-drawn by heating with a carbon heater 20 mm high and 20 mm in inner diameter and coated with a primary coating on the surface thereof. The fibers thus obtained had a diameter of 1 50 ssm. The drawing temperature of the preform B was over 1 5000C and the fiber sometimes fractured during drawing owing to spikes, and the minimum strength of the fiber thus produced was 1.0 kg per fiber. The drawing temperature of perform A was under 1 3000C and the minimum strength of the fiber was 3.5 kg per fiber which was considerably higher than that of the previous case.

Example 3 A porous glass rod, prepared and stuffed with CsNO3 in the same way as in Example 1, was deposited of CsNO3 in pure water at 4 C, unstuffed in an aqueous solution of H3PO4 at 4 C for 4 hours and then immersed in C3H8OH at a temperature of 20 C for depositing both CsNO3 and H3PO4 still remaining dissolved in the solution. The rod was subsquently subjected to the processes of vacuum drying, heating, decomposition, and collapsing. The preform thus obtained will be denoted as preform C.

Preform D was prepared exactly in the same way as preform C except that the unstuffing was performed in pure water which does not contain any H3P03. These preforms were heat-treated in the same way as in Example 1 and inserted in a Pyrex pipe 8 mm in inner diameter and 12 mm in outer diameter which has been rinsed with 5% HF aqueous solution after an optical polishment. The combination of rod and tube was melt-drawn under a reduced pressure and the fibers drawn was immediately coated with a primary coating on the surface. The drawing temperature of the preform D was over 1 5O00C. The fibers obtained sometimes fractured during drawing owing to bubbles, and the minimum strength of the fiber was found to be 1.5 kg per fiber.

The drawing temperature of the preform C was below 1 3000C and the minimum mechanical strength of the fiber obtained was 4.0 kg per fiber which was considerably higher than the previous case.

Example 4 An Na2O-K20-B203 glass was prepared by mixing and fusing powders of 40% 5102, 48% H3BO3, 5% K2CO3 and 7% Na2CO3, and a rod of 10 mm in diameter was formed out of the glass. The glass rod was heat-treated at 540 C for 15 hours, leached in 3 NHCl aqueous solution at 100 C and rinsed with pure water. The porous glass rod thus prepared had a poroslty of 50%. The rod was subsequently immersed for stuffing in a 87% CsNO3 aqueous solution at 100 C and then deposited of CsNO3 in pure water at a temperature of 20 C.Five sorts of preforms A to E were prepared by carrying out the unstuffing process for five hours in five different solutions; 48.5 g LiNO3/100 cc water (preform A), 28.9 g NaN03/100 cc water (preform B), 22.6 g KN03/100 cc water (preform C), 93.6 g Ca(NO3)24H2O/100 cc water (preform D) and 64.4 g MgSO4 7H2O/100 cc water (preform E), which were all kept at 20 C during the unstuffing, Each rod was subsequently immersed in C2H5OH which was kept at 20 C so that the stuffing compounds for viscosity control, i.e., LiNO3, NaNO3, KNO3, Ca(NO3)2 and MgSO4, were deposited on the pore wall as well as refractive-index agent CsNO3.

Transparent preforms A to D which are 8 mm in diameter were obtained after the conventional treatments vacuum drying at 20 C, decomposition by heating, and further heating up to 850 C for collapsing the pores. Two additional preforms F and G were also prepared. The preform F was unstuffed by treating the rod doped with CsNO3 in pure water at 200C for 2 hours and the preform G was unstuffed by treating said rod in a mixture of C3H50H 80%+H20 20% at 2O0C for24 hours.

One group of these preforms were rinsed on the surface with 5% HF aqueous solution and then directly drawn by heating with a carbon heater 20 mm in inner diameter and 20 mm high. The fiber was coated with a primary coating immediately after drawing and then nylon coating was formed by extrusion.

Another group of preforms A to G were inserted in Pyrex pipe 8 mum in I.D. and 12 mm in O.D.

after being rinsed with a HF-H2SO4 mixture, and the preforms were drawn under a reduced pressure, coated with a primary coating and then coated by nylon. The mechanical strength of said fibers stuffed with different viscosity agents are compared in Table 2.

Preforms F and G, particularly preform G, fractured during drawing owing to the bubbles; however, preforms A to E could be drawn at much lower temperatures so that there was no bubbling during drawing.

The transmission losses of two kinds of fiber, one is directly drawn and the other rod-in-tube drawn, were compared, by way of example, with respect to the fibers drawn from preforms C and F: the transmission losses in the fibers directly drawn and rod-in-tube drawn from the preform C were 1 5 dB/km and 14 dB/km respectively for a wavelength A=0.85 ssm and 6 dB/km and 3 dB/km respectively for a wavelength A=1.05 ym.

With respect to the preform F, the fiber directly drawn and rod-in-tube drawn had absorption losses of 30 dB/km and 19 dB/km respectively for a wavelength A=0.85 um and 21 dB/km and 10 dB/km for a wavelength A=1.05 ,um.

It is seen from Table 2 that the fibers produced in accordance with the present invention, by both direct drawing and rod-in-tube drawing, have more mechanical strength than the fibers produced by the conventional methods.

Table 2 Minimum Mechanical Strength of Coated Fibers (Kgw/fiber) Preform A B C D E F G Directly drawn 3.3 3.5 3.6 3.2 3,0 1.5 1.0 Rod-in-tube drawn 3.0 4.0 3.8 3.5 3.4 2.3 1.5

Claims (16)

Claims

1. A method for producing an optical waveguide by drawing a transparent glass rod which has been prepared by collapsing a porous glass rod, the pores of which have been doped with a dopant deposited on the internal wall thereof to enhance the index of refraction of the rod, which method comprises matching the viscosity and coefficient of thermal expansion of the central portion of the glass rod with the viscosity and coefficient of thermal expansion of the portions of the glass rod which have a refractive index lower than that of the central portion.

2. A method as claimed in claim 1 which comprises providing the glass rod with a core portion of high refractive-index and a cladding portion of low refractive-index by means of doping and dissolution processes in which a doping agent for controlling the refractive index of the glass is, respectively, deposited on or removed from the inner wall of the pores of the porous glass rod.

3. A method as claimed in claim 1 wherein the transparent glass rod is surrounded by a borosilicate or aluminosilicate glass layer.

4. A method as claimed in claim 3 wherein the transparent glass rod is inserted in a transparent glass tube and the combination of the rod and the tube is drawn while the gap between the rod and the tube is abolished by evacuating the gap.

5. A method as claimed in claim 3 wherein the transparent glass rod is inserted in a transparent glass tube and the combination of the rod and the tube is melt-drawn after they have been fused to each other and assembled into an integrated glass rod by abolishing the gap between the rod and the tube.

6. A method as claimed in any one of claims 3 to 5 wherein borosilicate or aluminosilicate glass has a viscosity and coefficient of thermal expansion substantially the same as those of the core glass at the drawing temperature, and melt-drawing the combination at a temperature which is very close to the melting points of the core portion and the surrounding borosilicate or aluminosilicate glass layer.

7. A method as claimed in claim 2 or any one of claims 3 to 6 when appendent to claim 2 which comprises matching the viscosity and the coefficient of thermal expansion of the portions of low refractive index with those of the central portion of high refractive index by doping the porous glass rod with a dopant for controlling the glass viscosity as well as the dopant for enhancing the index of refraction of the glass, the dopant for controlling the viscosity having little effect on the index of refraction, and the two dopants being applied to the glass either simultaneously or separately.

8. A method as claimed in claim 7 wherein the dopant for controlling the viscosity is a boron compound which dissolves in water, alcohol or a water-alcohol system or a phosphorus compound which dissolve in water or a water-alchol alcohol system.

9. A method as claimed in claim 8 wherein the dopant for controlling the viscosity is H3BO3, NH4BH402 (NH4)2B409, Na2B40,, K2B40, or Li2B40, or phosphorus compounds such as H3P04, (NH4)3PO4, (NH4)3HPO4, (NH4)H2PO4, K3PO4, K3HPO4, KH2PO4, Na2PO4, NaHPO4, NaH3PO4, LiH2PO4, LiHPO4 or Li2PO4.

10. A method as claimed in claim 7 wherein the dopant for controlling the viscosity of glass is a salt of an alkali metal or an alkaline earth metal.

11. A method as claimed-in any one of claims 7 to 10 which comprises doping a porous glass rod with a hot aqueous solution of a dopant for controling the refractive index of the glass, depositing the dopant on the pore walls by immersing the rod in pure water, a water-alcohol mixture or pure alcohol which is at ambient temperature near room temperature, dissolving the compound with pure water, a water-alcohol mixture of pure alcohol containing a dopant for controlling the viscosity of the glass, and depositing this dopant with a liquid.

12. A method as claimed in any one of claims 7 to 10 which comprises doping a porous glass rod with a hot aqueous solution of a dopant for controlling the refractive index of the glass, depositing and dissolving the dopant by immersing the rod in pure water, a water-alcohol mixture or pure alcohol, which is at ambient temperature and depositing the dopant for controlling the viscosity of the glass by immersing the rod in pure water, a water-alcohol system, or pure alcohol having a dopant for viscosity control added thereto.

13. A method as claimed in any one of claims 7 to 10, which comprises doping a porous glass rod with a hot aqueous solution of a dopant for controlling the index of refraction in a depositing, dissolving and once more depositing the dopant by immersing the glass rod in a solution of pure-water, a water-alcohol mixture or pure alcohol which is at ambient temperature and doping the rod with a dopant for controlling the viscosity of the glass and depositing the dopant on the walls of the pores.

14. A method as claimed in claim 2, or in any one of claims 3 to 13 when appendant to claim 2, wherein the doping agent for controlling the refractive index is CsNO3.

1 5. A method as claimed in claim 1 substantially as hereinbefore described.

16. A method as claimed in claim 1 substantially as hereinbefore described with reference to any one of the Examples.

1 7. An optical waveguide whenever prepared by a method as claimed in any one of the preceding claims.