CA1116448A - Optical fibers formed of aluminum borophosphate glass compositions - Google Patents

Abstract

OPTICAL FIBERS FORMED OF
ALUMINUM BOROPHOSPHATE
GLASS COMPOSITIONS
ABSTRACT OF THE DISCLOSURE
Novel optical fibers are formed from glass composi-tions enclosed by the iso-composition lines, 2 and 18 mole per-cent P2O5, 30 and 70 mole percent A12O3 and 20 and 60 mole per-cent B2O3 or glass compositions bounded by lines connecting the compositions 28 m% A12O3 - 72 m% P2O5 - 0 m% B2O3; 5 m%
Al2O3 - 45 m% P2O5 - 50 m% B2O3; 10 m% A12O3 - 30 m% P2O5-60 m% B2O3 and 42 m% A12O3 - 58 m% P2O5 - O m% B2O3. The glass compositions forming the optical fibers can contain the usual oxide modifiers.

Description

~1164~8 A-229/DIV OPTI AL FI~ERS FORMED OF
ALUMINUM BOROPHOSPHAq'E G~ASS COMPOSITIONS

This application is a division of Canadian application Serial No. 297,994 filed March 1, 1378.
This invention relates to novel optical fibers formed of aluminum borophosphate glass compositions. Optical fibers currently are undergoing intensive development as the trans-mission link in optical communication systems. Among the properties required of successful fibers are low optical attenuation, low optical dispersion, large numerical aperture and long service life. Present technology utilizes two types of glass for optical fibers, simple silicates and complex silicates.
Simple two-component silicate glasses are made by vapor depositing highly purified raw materials onto mandrels which are subsequently heat treated to give fully densified preforms.
Fibers are then pulled from the preforms at high temperatures.
Refractive index profiles, either step or graded are incorporated into the preforms by varying the composition of the gas mixture during vapor deposition. Glass compositions most commonly used are germanosilicate core/silica cladding or silica core/borosilicate cladding. These high silica fibers possess favorable properties, including low attenuation (due both to the high purity of the st~rting materials and to the deep W cutoff of silica), satisfactory dispersion characteristics and good solarization resistance. However, the high melting temperature of silica is a disadvantage of these materials. ~'emperatures on the order of ~000~C would lead to unacceptable impurity levels in the glass caused by .~.

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excessive corrosion of the crucible materials. The careful control of temperature and deposition rates of materials is a second production difficulty of the high silica fibers.
The complex nature of the co-deposition process limits the number of components which can be included in the glass com-position, and in all practical cases to date, the limit has been three separate oxide components. This constraint affords little flexibility in adjusting the relevant physical proper-ties of the glass, primarily refractive index, thermal expan-sion coefficient and viscosity-temperature relation. Since the refractive index profile is of greatest importance, the glass composition is generally adjusted to optimize that para-meter and this precludes any substantial control of the other physical properties of the glass.
Optical fibers can also be manufactured from complex silicates. The processing involves preparing batch quantities of two g~asses of distinct composition by standard glass melt-ing methods, taking care to suppress the level of transition metal impurities. The glasses are then remelted in a concen-tric, platinum double crucible and fibers are drawn directly from the melts through a bottom orifice. The melt from the central crucible gives rise to the core of -the fiber while that in the annular crucible provides the cladding. Two vari-ations are possible: if the fiber is cooled quickly, a step index fiber results; whereas if the fiber is maintained at a sufficiently elevated temperature, interdiffusion between the core and the cladding occurs and a graded index fiber is pro-duced. The complex silicate glasses largely avoid the diffi-culties associated with high silica fibers. Specifically, ~116d,4!~ 1 they can be melted at temperatures low enough (approximately 1500C) so that platinum crucibles can be used without intro-ducing excessive impurity concentrations, and the multi-component nature of the glass composition provides adequate flexibility for independently adjusting the glass properties by altering the relative concentrations of the various com-ponents. However, complex silicates also have disadvantages for optical fiber applications. First, such glasses are known to be subject to solarization effects which could limit the operational lifetime of the fibers. Second, the relative-ly shallow UV cutoff of these glasses has two deleterious effects on their optical properties: it gives a relatively high residual (i.e., non-impurity related) absorption coeffi-cient, and it leads to a relatively high optical dispersion, which is particularly undesirable for communications systems i using broad-band emitters such as light emitting diodes for light sources.
It would be desirable to provide optical fibers formed of glass compositions having relatively low melting points so that the composition of the fiber can be controlled relatively easily. In addition, it would be desirable to provide optical fibers having good solarization resistance, low optical dispersion and good transparency to ultraviolet light. Furthermore, it would be desirable to provide op-tical fibers from glass compositions which have satisfactory index - profiles while maintaining low internal mechanical stress.
. ~
Accordingly, the present invention provides an opti-cal fiber comprising a core having a refractive index of between 1.47 and 1.5~ and formed from a glass composition ~116~

selected from the group consisting of: (a) a high phosphate glass of a composition lying within the region of a ternary A12O3-P2O5-B2O3 composition diagram wherein said region is bounded by lines connecting the compositions 28 mole percent A12O3-72 mole percent P2O5-0 mole percent B2O3; 5 mole percent A12O3-45 mole percent P2O5- 50 mole percent B2O3; 10 mole per-cent A12O3-30 mole percent P2O5-60 mole percent B2O3; and 42 mole percent A12O3-58 mole percent P2O5-0 mole percent B2O3;
and (b) a low phosphate glass of a composition lying within the region of a ternary A12O3-P2O5-B2O3 composition diagram wherein said region is bounded by the isocomposition lines 2 and 18 mole percent P2O5, 30 and 70 mole percent A12O3, and 20 and 60 mole percent B2O3; and a cladding formed from a material selected from the group consisting of a mixture of said high phosphate glass and an oxide modifier, a mixture of said low phosphate glass and an oxide modifier, said oxide modifier selected from the group consisting of SiO2, MgO, CaO, TiO2, ZrO2, Na2O, CdO, ZnO, SnO2 and CeO2, present in an amount less than 40 mole percent.
The glasses used to form the optical fibers can be modified, if desired, by the addition of a wide variety of -` oxides with the modifier levels allowable being dependent upon the specific modifier com~osition being added. Glasses in the aluminum borophosphate system are found to have good ;'; solarization resistance, low optical dispersion and good trans-parency for ultraviolet wavelengths (indicative of both low dispersion and low intrinsic absorption at optical wavelengths).
These aluminum borophosphate glasses offer the advantage that fibers can be prepared by standard techniques, such as the .Y~

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double crucible method at relatively ]ow temperatures. More-over, these gI2sses offer the flexibility of independent vari-ation of important material properties such as refractive index and thermal expansion coefficient. Finally, although other techniques may be used, certain of the aluminum boro-- phosphate glasses seem particularly suited to the production of graded index fibers by a novel technique involving the preferential vaporization of volatile species from the glass surface disclosed in U.S. Patent No. 4,110,090 to Richard M.
Xlein entitled "~ethod of Forming Optical Fibers".
The low phosphate aluminum borophosphate glasses utilized to form the optical fibers of this invention are formed in a manner so that vaporization of the glass forming constituents, particularly P2O5 is minimized and controlled.
Materials which are sources of A12O3, B2O3 and P2O5 are blended, calcined, if desired, and preferably melted in a . ., crucible which does not dissolve in the glass composition or the precursor of the glass composition. Normal melting tem-peratures range between about 1450C and 1800C, usually between about 1500C and 1650C, depending upon the specific glass composition being formed. Normal seed-free firing time is between about 0.25 and 4.0 hours, preferabl~ between about 1.0 and 2.0 hours. It is preferred to maintain minimum fir-; ing times in order to reduce vaporization of the glass-forming constituents. After substantially complete reaction of the - reactants is obtained in the melt, it is cooled in any conven-tional manner.
Since P2O5 is the most volatile glass-forming consti-tuent, it can be added in concentrations in excess of the desired final concentration in the glass composition.

.

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Alternatively and preferably, the P205 is added to the crucible in the form of a refractory compound such as AlP04, BP04, AlP309 or refractory modifying cation phosphates such as Mg2P207, Ca3(P04), ZrP207 or the like~ By adding P205 as a refractory compound, improved control of the final glass com-position is attained.
As set forth above, the low phosphate glass composi-tions contain from 2 to 18 mole percent P205, from 30 to 70 mole percent A1203 and from 20 to 60 mole percent B203. In addition, the ternary glass composition can contain oxide mod-ifiers including SiO2, MgO, CaO, TiO2, ZrO2, ~a20, CdO, ZnO, SnO2, CeO2 and the like. Silica in concentrations up to about 40 mole percent provides increased resistance to devitrifica-tion of the aluminum borophosphate glasses while MgO expands the glass-forming region. In a similar way other specific modifiers or combinations of modifiers can be used to effect other changes in glass properties.
Representative sources of P205 are AlP04, BP04, ~lP309 or refractory modifying cation sources such as Mg~P207, Ca3(P04~2 or ZrP207. Representative sources of A1203 are A1203, AlP04, AlP309 and modifying cation aluminates such as MgA1204. Representative sources of B203 are BP04, B203 or modifying cation borates such as Mg3B206.
The low phosphate glass compositions utilized in the optical fibers of this invention are stable against degrada-tion by water as are the conventional multicomponent soda-lime silicate glasses even though both B203 and P205 in their pure state are strongly deliquescent. In addition to those proper-ties of the low phosphate aluminum borophosphate glasses which are broadly similar to conventional silicate materials, there ~116~

are other properties of the new glasses which are unusual.
Their optical properties are particularly significant. Refrac-tive index ranges between 1.47 and 1.58, with optical disper-sion, given as the v-value, between 48 and 71 ~where a high ~-value represents a low optical dispersion). Because most silicates have higher refractive indices and smaller v-value (i.e. higher dispersions), the ranges for these two parameters place the new glasses in an advantageous position fox optical applications. Provided that special preparation techniques are used to ensure that low impurity levels are maintained, the ultraviolet cutoff of certain compositions in the aluminum borophosphate system occurs at about 190 nm, which approaches that of fused silica. Thus, the low phosphate glasses are more transparent at ultraviolet wavelengths than any conven-tional glasses except fused silica. This feature, coupled with their good solarization resistance, makes them particu-larly favorable for optical fiber applications. The low phos-phate aluminum borophosph~te glasses also show unusual capa-bilities for independent property control. For instance, the iso-property contour lines for refractive index and thermal expansion coefficient are not parallel in a large area of the low phosphate glass-forming region. This means, for example, that refractive index can be adjusted while retaining a con-stant thermal expansion coefficient, which is a particularly favorable feature for applications such as optical fibers~
The low phosphate glasses utilized in this invention have refractive indices between 1.47 and 1.58, densities between 2.1 and 2.7 gm/cm3 and thermal expansion coefficients between about 26 and 55 x 10 7/oC.

11~6441~

The high phosphate aluminum borophosphate glasses used to form the optical fiber of this invention also can be prepared in the manner set forth above for the low phosphate glasses. In addition, the high phosphate aluminum borophos-phate glasses can contain the same oxide modifiers as set forth above for the low phosphate glasses. Most high phos-phate glasses also are stable against degradation by water.
Refractive index ranges between 1.49 and 1.53, decreasing with decreasing P2O5 content. Optical dispersion, given as v-value ranges between 50 and 74. The ultraviolet cutoff occurs at below about 200 nm and the glasses can exhibit good solariza-tion resistance. In addition, like the low phosphate glasses, extensive portions of the glass-forming region in the high phosphate aluminum borophosphate system offer flexibility for independentl~ adjusting the important material properties.
For example, the contours corresponding to constant values of the thermal expansion coefficient and refractive index are not parallel in large par-ts of the high phosphate glass-forming region. Hence, fibers produced from glasses in this zone of compositions can have a gradient in refractive index across their diameters while retaining the favorable mechanical properties inherent in a constant thermal expansion coefficient.
Both the low and high phosphate aluminum borophosphate glasses described above are utilized to form optical fibers having a composition on and near its-surface with a refractive index lower than the refractive index of the glass composition forming the interior of the fiber. The difference between the refractive index of the cladding and the core is at least about 0.007, preferably greater than 0.015. Generally, the optical fibers have a core diameter between about 45 and 85 ~m 11~6448 or can be larger or smaller. The cladding has a thickness be-tween about 30 and 60 ~m although this can also vary. The optical fibers can be formed from the low phosphate glass, the high phosphate glass or a combination of the low and high phos-phate glasses wherein the cladding or core can be either the high phosphate glass or the low phosphate glass.
The optical fibers of this in~ention can be produced by conventional opitcal fiber-making techniques such as vapor deposition techniques and by the double crucible method; in addition, fibers from the high phosphate glasses can be pre-pared by the selective vaporization method described and claim-ed in U.S. Patent No. 4,110,090 to Rich,ard M. Klein entitled "Method of Forming Optical Fibers." In vapor deposition tech-nique a vapor phase reaction is used to deposit materials o-f varying refractive index onto a mandrel. During deposition, the concentrations of the individual materials are varied so as to produce the composition gradient required for the desired index profile. After deposition, the preform produced is - thermally sintered and collapsed, and then drawn into a fiber.The sintering, collapsing and drawing are controlled so that no unanticipated changes occur in the gradient. Since the deposited materials are co-deposited, they must be compatible with a single set of deposition condition so that the deposi-tion technique is limited to two-component systems.
In the double crucible method, bulk glasses first are prepared which glasses have compositions suitable for use as the core/cladding end members. These glasses then are remelted and fibers are pulled from concentric double platinum crucibles.
The combined glass stream commonly is maintained at a high tem-perature to permit interdiffusion between the core and cladding 4f~

in order to provide a graded refractive index profile. Since the index gradient is formed during fiber pulling, simultaneous control is maintained for both the fiber drawing process and the interdiffusion process.
In United States Patent number 4,110,090, there are described processes involving the selective vaporization of a composition which forms the surface of the optical fiber;
this method is applicable to high phosphate glasses. The cladding is formed by heating the glass composition to selec-tively vaporize volatile P2O5, thereby to reduce the refrac-tive index at the fiber surface. In general, the temperature is chosen high enough (usually above the glass transition temperature) so that the vaporization process is not unduly long. However, the temperature and time of heat treatment must be chosen such that crystallization and phase separation, which may occur for specific compositions, are avoided. More-over, for those embodiments which involve heating preforms ; or fibers, the time and temperature utilized should not cause undesirable deformation due to viscous flow. Typical vapor-ization temperatures when treating the fibers or preforms are between about 500C and 800C. The vaporization step can be conducted in a vacuum in order to increase the rate of P2O5 vaporization. In one embodiment, the optical fiber is made by first forming a preform of the optical fiber, then heat treating the preform to establish the refractive index gra-dient and drawing the preform to make the fiber. The preform can be made by casting or pulling or other suitable techniques.
After the preform is heat treated, the fiber is drawn at high temperature, under conditions to assure that the portion of ;448 the preform deficient of P2O5 forms the cladding of the opti-cal fiber and the poxtion of the preform which retains the P2O5 after heat treatment forms the core of the optical fiber.
In another aspect of the process for forming the optical fibers, the components of the glass composition are melted to form the glass. The fiber then is drawn from the melt and while being drawn, is heat treated to effect selec-tive vaporization of P2O5 under the conditions set forth above.
This technique also can be utilized by first forming a pre-form, drawing the optical fiber from the preform and heat treating the fiber while it is being drawn. An alternative process comprises forming a fiber by drawing it from the glass melt or preform. Thereafter, the fiber is heat treated to vaporize P2O5 from its surface under the conditions set forth above. In another aspect of this invention, the surface of the melt can be heated to effect selective vaporization of P2O5 at the melt surface while the composition in the body of the melt remains intact. The fiber then is drawn from the rnelt so that the cladding of the fiber is formed from the composition at the melt surface and the core of -the fiber is formed from the composition in the body of the melt.
The procedures given below were used to prepare glasses of improved purity for UV absorption measurements.
Although these procedures were not generally used to prepare glasses, they do not constitute a substantive change in pre-paration techni~ue since in each case, prior to melting, one has an intimate mixture of the preferred starting oxides.

EXAMPLE I
55.2 ml of 1.245 M Al(NO3)3 solution (made from Al(NO3)3 ~ X H2O with less than 10 ppm cation (impurities) was mixed with 13.0 ml of 15.05 M H3PO4 (with less than 50 ppm cation impurities). The mixture was heated to about g0C
while stirring. After about 3.5 hours, the remaining solution was transferred to an A12O3 crucible and then heated slowly to 700C (25 C/h to about 200C, 60 C/h to ~40C and 120C/h to 700C) and maintained at that temperature ~or 80 min.

The resultant mixture was fired in the A12O3 crucible at 1425C for 1 hour. The melt was then poured into a pre-heated graphite mold and annealed using standard techniques.

EXAMPLE II
15.5 ml of 15.05 M H3PO4 was diluted with about , 150 ml of H2O. The solution was heated to about 100C while-stirring and 6.147 g Al(OH)3 and 3.549 g H3BO3 (less than 5 ppm cation impurities) was added. After about 4 hours, the suspen-sion was transferred to an A12O3 crucible which was then heated slowly to 700 C. (See preceding example for heating schedule).

The resultant mixture was fired at 1500C for 2 hours.
The melt was quenched between steel plates, than annealed us-ing standard techniques.

EXAMPLE III
115.0 ml of 1.245 M Al(NO3)3 solution was heated slowly to about 400 C to drive off H2O and NO2. After cooling to room temperature, about 150 ml of H2O and 4.0 ml of 15.05 M H3PO4 were added. The suspension was heated to about 90C
while stirring and 9.056 g Ei3BO3 was added. After ~ hours, ll~Lfi4~4t~

the suspension was transferred to an A12O3 crucible which was then heated slowly to 700 C using the previously described schedule. After cooling to room temperature, the resultant material was removed from the crucible, mixed and then re-turned to the A12O3 crucible, The resultant mixture was fired at 1620C for 2 hours. The melt was quenched b~tween steel plates and then annealed using standard techni~ues.
- The properties of the compositions produced by the processes described in the examples which are relevant herein are given below:

Example I Example II Example III

Wavelength (nm) at which a 0.5 mm thickness would transmit 50% of the inci- 188 194 210 dent radia-tion (neglect-ing reflection) Refractive Index (nd) 1.522 1.507 1.526 Optical Dispersion 64 67 58 (v-number) Thermal Expansion Coefficient 60 52 34 (10-7/C) i

Claims

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED IS DEFINED AS FOLLOWS:

1. An optical fiber comprising:
a core having a refractive index of between 1.47 and 1.58 and formed from a glass composition selected from the group consisting:
a. a high phosphate glass of a composition lying within the region of a ternary A12O3-P2O5-B2O3 composition diagram wherein said region is bounded by lines connecting the compositions 28 mole percent A12O3-72 mole percent P2O5-O
mole percent B2O3; 5 mole percent A12O3-45 mole percent P2O5- 50 mole percent B2O3; 10 mole percent A12O3-30 mole percent P2O5-60 mole percent B2O3; and 42 mole percent A12O3-58 mole percent P2O5-0 mole percent B2O3; and b. a low phosphate glass of a composition lying within the region of a ternary A12O3-P2O5-B2O3 composition diagram wherein said region is bounded by the isocomposition lines 2 and 18 mole percent P2O5, 30 and 70 mole percent A12O3, and 20 and 60 mole percent B2O3; and a cladding formed from a material selected from the group consisting. of a mixture of said high phosphate glass and an oxide modifier a mixture of said low phosphate glass and an oxide modifier, said oxide modifier selected from the group consisting of SiO2, MgO, CaO, TiO2, ZrO2, Na2O, CdO, ZnO, SnO2 and CeO2, present in an amount less than 40 mole percent.