Method of making optical fiber with low melting glass core

The disclosed method of making a mixed glass optical fiber exemplarily comprises providing a high-silica tube, and causing molten non-high silica glass to flow into the bore of the tube by application of a pressure differential. In order to prevent cracking, the tube desirably has an outer diameter/inner diameter ratio of at least 5, preferably about 10 or even more, and an inner diameter of at most 1 min. In a preferred embodiment, a conventional SiO.sub.2 tube is partially collapsed to an inner diameter less than 1 mm, a quantity of a non-high-silica glass is placed in a neck of the partially collapsed tube and heated such that molten glass communicates with the reduced-diameter portion of the bore and can be drawn into the reduced-diameter portion by means of a vacuum. The resulting mixed glass body is then further stretched to result in a core rod of core diameter at most 0.3 min. After overcladding the core rod with SiO.sub.2, fiber is drawn from the thus produced preform. A thus produced fiber with SiO.sub.2 cladding and SiO.sub.2 -Al.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Er.sub.2 O.sub.3 core was used as an optical fiber amplifier and provided high gain.

FIELD OF THE INVENTION 
This invention pertains to methods of making optical fiber. 
BACKGROUND OF THE INVENTION 
Silica-based optical fibers are in widespread use and perform admirably in 
most respects. However, for some applications it would be desirable to 
have available a mixed-glass fiber, i.e., a fiber with high-silica (i.e. 
greater than 85 mole %, frequently greater than 95% silica) cladding and a 
non-high-silica (.ltoreq.85% silica) core. 
For example, to date attempts to dope the core of high-silica optical 
fibers with very high levels (e.g., several mole percent) of rare earth 
(atomic numbers 57-71) ions have been unsuccessful. Yet such highly doped 
fibers would be useful for very short fiber lasers, amplifiers and optical 
isolators. On the other hand, it is known that, for instance, sodium 
silicates and aluminosilicates allow very high (up to about 10 mole %) 
rare earth doping, and chalcogenide glasses are known to be excellent 
hosts for praseodymium for optical amplifiers. 
Those skilled in the art will recognize that it would at best be 
impractical to combine non-silica-based fibers with conventional 
silica-based fibers. For instance, it would be difficult to effect a 
fusion splice between such dissimilar fibers. Thus, it would be desirable 
to have available a technique for making mixed glass fibers, i.e., fibers 
with non-high-silica (NHS) core and high-silica (HS) cladding. 
Several techniques of making mixed material fibers have been disclosed. For 
instance, UK patent GB 2,020,057 (T. Kobayashi et al.) discloses providing 
a preform consisting of a drawable glass cladding tube and molten core 
material in the tube, and drawing fiber from the preform. The core 
material is an inorganic crystalline material (e.g., LiF) when solid. In a 
second embodiment the UK patent discloses a double crucible technique. 
U.S. Pat. No. 5,160,521 discloses making a mixed glass preform by heating 
a quantity of core glass to the core glass softening temperature and 
forcing a cladding tube into the softened core glass such that the core 
glass fills the cladding tube without bubbles or other defects. Fiber is 
drawn from the thus produced preform. U.S. Pat. No. 5,106,400 discloses a 
method and apparatus for forming a glass preform from core and cladding 
glasses having low liquidus viscosities and narrow working ranges. 
Despite the prior art efforts, there is still need for a simple, reliable 
method of making glass fibers with HS cladding and NHS core. This 
application discloses such a method. The method can be used to make fibers 
that combine glasses of widely dissimilar thermal properties and 
composition, which frequently cannot be made by prior art methods. 
SUMMARY OF THE INVENTION 
The invention is embodied in a process of making optical fibers comprising 
a first glass (typically non-high-silica glass) core and a second glass 
(typically high-silica glass) cladding. An important aspect of the 
inventive method is a difference in viscosity between the first and second 
glasses at an appropriate working temperature, with the first glass having 
a softening temperature lower than the softening temperature of the second 
glass. Typically the difference in viscosity is such that the first glass 
has relatively low viscosity (is "molten") at the appropriate working 
temperature, whereas the second glass is relatively rigid at that 
temperature. 
More specifically, the inventive method comprises making a preform that 
comprises a first glass core and a second glass cladding surrounding the 
core, and drawing optical fiber from the preform. Significantly, making 
the preform comprises providing a second glass tubular body having a bore, 
providing a quantity of the first glass, heating the quantity such that at 
least a portion thereof is at or above the first glass softening 
temperature and is in communication with the bore, and creating in the 
bore of the tubular body a pressure differential that is effective for 
causing at least some of the first glass to flow into the bore, such that 
a mixed glass body results. 
Typically the relevant part of the second glass tubular body has an 
outside/inside diameter ratio of at least 5 (preferably about 10 or even 
more), and an inside diameter of no more than 1 mm, both in order to avoid 
cracking of the mixed glass body upon cooling. 
Exemplarily, the second glass tubular body is made by partially collapsing 
at least a portion of a second glass starting tube such that a tubular 
body with a reduced-diameter section results. However, the second glass 
tubular body can be an appropriately dimensioned uniform tube, provided 
care is taken to eliminate flaws at the surface of the bore, and 
provisions are made for confining the molten first glass prior to its 
introduction into the bore. 
In currently preferred embodiments the thus produced mixed glass body is 
stretched such that the first glass core is further reduced in diameter, 
exemplarily to a diameter of 0.3 mm or less. This reduction can further 
contribute to the prevention of cracking. The resulting preform rod is 
typically overclad in known manner with second glass to get the desired 
clad/core ratio, and fiber is drawn from the thus produced preform.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows a conventional silica tube 10. Exemplarily the tube has 25 mm 
outer diameter and 19 mm inner diameter. 
FIG. 2 shows the intermediate tube 11 that results from partial collapse of 
the starting tube. Typically the starting tube is mounted in a glass 
blower's lathe and the center portion of the tube is heated in 
conventional fashion. Exemplarily the outside diameter of the partially 
collapsed center portion of the tube is about 15.5 mm and the bore (12) 
diameter thereof is less than about 1 mm. 
The core glass is produced by a conventional technique, e.g., melting of 
the starting materials in a platinum crucible. It can be any NHS glass 
that meets the above-discussed thermal requirements. Exemplarily the first 
glass is a chalcogenide glass or a sodium or alumino silicate. First 
glasses typically "melt" at temperatures below about 1500.degree. C., and 
thus have sufficiently low viscosity at about 1500.degree. C. to readily 
flow under a moderate applied force. 
Since low viscosity and high thermal expansion generally go hand in hand in 
glasses, the first glasses of interest herein typically have relatively 
high thermal expansion coefficients, frequently more than 10 times that of 
silica. This mismatch of thermal properties between typical HS glasses and 
typical NHS glasses has made it difficult to produce mixed glass optical 
fibers by prior art processes. For instance, the high thermal expansion of 
typical core glasses frequently has led to cracking of preforms, and the 
high processing temperature of silica (typically &gt;2000.degree. C.) has 
caused boiling of the core glasses. The instant method can substantially 
overcome these difficulties, and is not limited to use with HS second 
glasses. 
An appropriate quantity (30) of the first glass is placed into the neck of 
the intermediate tube, as shown in FIG. 3. The intermediate tube is then 
heated (typically in a glass blower's lathe) to a temperature (at or above 
the softening temperature of the first glass but below the softening 
temperature of the second glass) at which the first glass has sufficiently 
low viscosity such that all or part of the first glass can be caused to 
flow into the reduced diameter bore (12) of the intermediate tube by a 
pressure differential across the intermediate tube. The pressure 
differential exemplarily is created by means of a vacuum pump connected to 
end 40 of the intermediate tube, as schematically indicated in FIG. 4. 
Other means for creating the pressure differential are also contemplated. 
For instance, pressure can be applied (by means of, e.g., pressurized air) 
at end 41 of the intermediate tube, or vacuum and pressure can be applied 
simultaneously. 
FIG. 4 shows that first glass 42 has been caused to move into the 
reduced-diameter portion of the bore. Since the partial collapse of the 
starting tube by necessity is carried out at relatively high temperature, 
the surface of the reduced-diameter portion typically is substantially 
free of defects. Consequently the low viscosity first glass can form a 
substantially defect free interface with the intermediate tube. 
In order to prevent core cracking during cool-down, it is generally 
desirable to stretch the central portion of the thus produced body to a 
smaller diameter, as shown schematically in FIG. 5. The stretching can be 
accomplished in conventional fashion, utilizing a heat source 52. 
Exemplarily, the outer diameter of the central portion of the body is 
reduced from about 15.5 mm to about 4 mm. Since the core material 
typically is substantially free of defects, bubbles typically do not form 
or grow during stretching of the body, and the body may be heated far 
above the vaporization temperature of the first glass. 
After removal of the end pieces (e.g., 50) and/or cutting of preform rod 51 
to the required length, the preform rod typically is inserted into an 
overclad tube 60 in conventional fashion, or is overclad by any other 
appropriate technique. The radial dimensions of the overclad are selected 
to yield, after drawing of fiber from the thus produced preform 70, fiber 
71 having the desired core diameter and cladding/core ratio. 
EXAMPLES 
A quantity of NHS glass of molar composition 65% SiO.sub.2 -25% Al.sub.2 
O.sub.3 -9.6La.sub.2 O.sub.3 -0.4Er.sub.2 O.sub.3 was prepared in 
conventional fashion by melting of appropriate quantities of the starting 
oxides in a crucible. This composition is a member of the class of 
compositions disclosed in the concurrently filed co-assigned patent 
application by A. J. Bruce et al., entitled "Optical Device and Process of 
Making the Device". The central portion of a 19.times.25 mm diameter 
silica starting tube was partially collapsed to 15.5 mm outer diameter, 
leaving a bore of 0.7 mm diameter. The partial collapse was carried out in 
standard fashion on a glass blower's lathe. The thus produced intermediate 
tube was removed from the lathe, and about 0.5 g of the prepared NHS glass 
was placed into one of the neck regions of the tube and heated to about 
1300.degree. C. 
The intermediate tube was remounted on the lathe, and a conventional vacuum 
pump was connected to the other neck region of the intermediate tube. Next 
the tube was heated such that the reduced-diameter central part of the 
tube and the one neck region were at about 1400.degree. C. The other neck 
region was then evacuated such that a pressure differential existed in the 
tube, and the "molten" non-high-silica glass was drawn into the bore of 
the reduced diameter section to a length of about 12 cm. Next, while 
maintaining the unstretched central portion of the resulting body at about 
800.degree. C. to prevent cracking, the central portion was stretched in 
conventional fashion to 4.5 mm outside diameter, resulting in a core 
diameter of 0.23 mm. Cracking was observed for core diameters greater than 
about 0.3 mm, but it may be possible to find conditions which permit 
larger core sizes. After completion of stretching the end portions of the 
body were removed such that a core rod of substantially uniform diameter 
resulted. This core rod was then overclad with SiO.sub.2 in conventional 
fashion to yield a 14 mm outer diameter preform, and optical fiber was 
drawn from the preform in conventional manner. The core/cladding index 
difference of the fiber was about 4.2%, and the core diameter was about 
2.1 .mu.m. 
A 23 cm length of the fiber was configured as an optical amplifier in known 
manner. When pumped with 250 mW of 980 nm light from a Ti:sapphire laser, 
the amplifier provided 23 dB gain at 1533 nm. This is, to the best of our 
knowledge, the highest gain per unit fiber length reported to date. 
By a method substantially as described, an optical fiber having an 
alkalisilicate core (K.sub.2 O-4SiO.sub.2 -0.05Er.sub.2 O.sub.3 
-0.1GeO.sub.2) and silica cladding was produced, as was a preform having a 
chalcogenide glass core and an aluminosilicate cladding. 
As is known, the addition of one or more alkalis to a glass (e.g., to a 
glass with high rare earth content) can result in reduced viscosity. For 
this reason it may at times be advantageous to add an alkali (or alkalis) 
to a core glass, since the resulting reduced viscosity can facilitate 
drawing the molten core glass into the bore of the intermediate tube.