Joint Doping of Porous Glasses to Produce Materials With High Modifier Concentrations

The use of porous glasses as substrates for the molecular deposition of selected materials has shown great promise in the production of materials with selected physico-chemical properties and selected property variations. This process, called "Molecular Stuffing" or doping has been described in detail in U.S. Pat. No. 3,938,974 and U.S. patent Nos. 4,110,093 and 4,110,096. In addition to such porous glasses, the present invention is applicable to porous glass produced by such other methods as chemical vapor deposition. (See U.S. Pat. No. 3,859,093).

By the molecuar stuffing process, selected solutions containing materials which alter the physico-chemical properties of high silica glasses are diffused into the pores of a high silica base glass preform to achieve a homogeneous concentration of the modifying agent or agents (solute). For step concentration profiles these modifiers are subsequently precipitated and a cladding region is formed by their removal from the outer regions of the preform by a suitable solvent before drying and sintering of the preform assembly. For graded concentration profiles, the concentration of selected modifiers is altered to a desired variation by a second soaking of the preform in selected solvent solutions containing selected concentrations of modifiers. This is followed by precipitation of the modifiers and subsequent drying and sintering of the preform assembly. (See U.S. Pat. No. 4,110,093)

The change in physical property achieved by the addition of dopants is a function of the dopant concentration. Therefore the addition of high dopant concentrations generally induces a large change.

Several products benefit from large variations in physical properties and therefore large variations in dopant concentration. For example, in fiber optics, a large change in index of refraction between the core and cladding regions of a fiber yields a high numerical aperture, while in strengthening brittle materials, a large change in thermal expansion coefficient and/or in glass transition temperature between the surface and the interior of an article allows the formation of large surface compressions (prestressing) and thus the achievement of correspondingly increased strengthening.

The numerical aperture, NA, of a light transmitting device is a measure of its acceptance angle. In optical waveguides the numerical aperture is related to the difference in refractive index, n, between the axis or center of the waveguides and the off axis elements. An increase in numerical aperture is obtained by increases in the index difference between these elements (for example, in waveguides with step index profiles, the difference is between the refractive index of the core, n.sub.1, and the clad, n.sub.2, regions; thus NA=.sqroot.n.sub.1.sup.2 -n.sub.2.sup.2).

Since numerical aperture is related to the angle of acceptance of the incident light beam, high numerical apertures are desirable since this allows transmission of relatively more energy from a given light source. High numerical apertures are also desirable from the standpoint of reducing microbending losses in optical waveguide fibers, and for the preparation of lens elements and other optical elements.

The process described in U.S. Pat. No. 3,938,974 and U.S. Pat. No. 4,110,093, the disclosures of which are incorporated herein by reference, demonstrates how molecular stuffing of porous glasses may be conducted using a series of dopants both individually and in groups to develop integrated optics components with tailored refractive index distributions and strengthened articles with tailored thermal expansion coefficient and glass transition temperature distributions.

This invention employs glass compositions and dopants similar to those in U.S. Pat. Nos. 4,110,093 and 4,110,096. However, we have found that certain dopants, such as lead, induce noticeable scattering in the glass when large dopant concentrations are used. Though scattering has little effect on most uses such as integrated optics components and strengthened members, some uses such as the transmission of very high quality images (e.g. in cystoscopes) or long, extremely low loss, (e.g. below 20 dB/km) optical fibers with a high numerical aperture (e.g. N.A. greater than 0.35) may be limited by the amount of light scattering present. For example, when lead is used as a dopant oxide at doping concentrations above 40 grs. Pb(NO.sub.3).sub.2 /100 cm.sup.3 of water, scattering is observed in the final glass. This is an important limitation because lead has a high atomic polarizability and is useful to obtain high index glasses. It would be of considerable advantage if glasses could be made with large concentrations of Pb as a dopant in those situations where a combination of high NA and very low scattering loss were desired.

We have discovered that when the dopant in molecular stuffing is composed of certain combinations of lead and/or bismuth with cesium, rubidium, and/or potassium, then a remarkable and unexpected decrease in the scattering loss occurs. Further, we have discovered that certain of these combinations which give low scattering losses may be used to obtain high numerical apertures and/or high pre-stressing levels as well.

The desired combination of dopants leads to glass articles with the following final composition. The composition of these glasses consists of at least 85 mole percent SiO.sub.2 with improvements which comprise at least 7 wt. percent of at least one member selected from Group (I) consisting of PbO and Bi.sub.2 O.sub.3 and at least 1.5 mole percent of at least one member selected from Group (II) consisting of K.sub.2 O, Rb.sub.2 O, and Cs.sub.2 O.

Even though the maximum dopant concentration is limited by the concentration of SiO.sub.2, the broad limits are a maximum of 25 wt. percent for Group (I) and a maximum of 9 mole percent for Group (II). Our preferred range covers at least 2 but not more than 9.5 mole percent B.sub.2 O.sub.3 and at least 7 but not more than 20 wt. percent of Group (I) and at least 1.5 but not more than 7 mole percent of Group (II).

In another embodiment of this invention, we have discovered that a method which comprises adding a dopant to a porous matrix with interconnective pores, immersing the porous matrix in a solution of a dopant causing the dopant to precipitate in the matrix, removing solvent and where necessary, decomposition products, from the porous matrix and collapsing the porous matrix to a solid form, can be used to produce high silica glasses. These glasses can be produced with the following mixed dopant compositions: Group I being Pb and/or Bi and Group II being K, Rb, and/or Cs in the form of nitrates, carbonates, acetates, borates, phosphates, arsenates and/or silicates in either hydrated or unhydrated form or mixtures therefrom used to produce a glass having a composition of 7 to 25 wt percent (preferred range 7 to 20 wt. percent) of the oxide equivalent of at least one member selected from Group I consisting of Pb and Bi and 1.5 to 9 mole percent (the preferred range being 1.5 to 7 mole percent) of the oxide equivalent of at least one member selected from Group II consisting of K, Rb, and Cs.

Finally in another embodiment of this invention, letting I represent [Pb(NO.sub.3).sub.2 ] and [Bi(NO.sub.3).sub.2 ], and II represent the alkali nitrates of Cs, Rb and K, taken either singly or in combination, the ranges of dopant which yield glasses with high numerical aperture and low scattering loss are:

where the weight represents the weight of at least one member of the group in the form of a nitrate salt. The solutions may be water, optionally with small amounts of low molecular weight alcohols such as methanol. The solvents used in precipitating the dopants may be low molecular weight alcohols such as methanol and ethanol.

Porous glasses are used as substrates for the deposition of the selected dopant combinations. Any porous glass preform is satisfactory. In this example we prefer to describe a specific method for forming the porous glass substrate by phase separation, although other processes are just as useful. (See for example Schultz, U.S. Pat. No. 3,859,073.)

An alkali borosilicate glass of composition 57% SiO.sub.2, 36% B.sub.2 O.sub.3, 4% Na.sub.2 O and 3% K.sub.2 O is melted in a Pt crucible in an electric furnace at temperatures between 1300.degree. and 1450.degree. C. The melt is homogenized by stirring with a Pt stirrer, and is then pulled in the form of rods 5/16" diameter by 4' length. These rods are then cut into rods 4" in length which are heat-treated at 550.degree. C. for 11/2 hours to induce phase separation and subsequently leached in a 3 N HCl acid solution. During the phase separation heat-treatment, the homogeneous glass decomposes into two phases, one with high silica content and one with high boron and alkali content and lower silica content. These phases are interconneced sufficiently that exposure to the leaching solution completely removes the alkali rich phase leaving behind a high silica porous glass substrate. The porous glass is washed with deionized water.

The porous glass substrate is immersed in a solution containing the desired concentrations of dopants (see Table 1) for 3 hours or longer to allow the solution to fill the pores completely. The dopant compounds are then precipitated from solution.

In following this process we have found it desirable to achieve precipitation of the dopants by thermal means; that is, lowering the temperature of preform and solvent to a point where the solution within the pores becomes supersaturated with the dopant causing the dopant to precipitate in the pores. The sample is then transferred to an unstuffing solution of solvent whereby some of the dopant is allowed to diffuse out of the pores yielding a sample with graded dopant concentration. This step is necessary in both fiber optics to achieve high numerical apertures and in strengthening to achieve high surface compressive stresses. When graded properly, the dopant concentration is nil near the surface of the object thus yielding a low refractive index and/or a low thermal expansion coefficient in the cladding region. The unstuffing step is often conducted sequentially in two different solutions to insure maximum dopant removal from regions near the surface of the object (see Table 1) steps (a) and (b). The sample is kept at 0.degree. C. where it is exposed to vacuum for 24 hours and then heated at 15.degree. C./hour up to 625.degree. C. under vacuum and sintered between 830.degree. and 850.degree. C.

Table I reports details (concentration of solution, temperature and times) of the preparation and measurements of refractive index in the core (central) and cladding (outer) regions of the objects made by using lead nitrate and cesium nitrate as dopants. Corresponding numerical apertures are also listed. Table II lists the compositions at the center of the final glass articles. A fiber was pulled from Sample II and scattering losses were found to be less than 20 dB/km in each case.

A porous glass preform prepared as described in Examples I to III is soaked in a dopant solution as described in Table III at a temperature specified in Table III for 16 hours. This temperature is chosen to be at or above the solubility temperature of the designated dopant concentrations. This allows the dopant solution to fill the pores of the preform completely and uniformly. The preform is then removed from the dopant solution and is soaked in a single solvent as specified in Table III for three hours in order to precipitate the dopant within the pores. No dopant removal was attempted since these glasses were made only to observe index change with concentration. The sample is then kept at 0.degree. C. where it is exposed to vacuum for 24 hours and warmed up at 15.degree. C. per hour up to 625.degree. C. also under vacuum. It is then heated to between 830.degree. and 850.degree. C. where sintering occurs. Refractive index measurements conducted on these samples are reported in Table III. Table IV details the compositions of the final glass articles listed for Examples VI and VII.