Disclosed is BeF.sub.2 -based optical fiber. Such fiber can have, in addition to low loss, other advantageous properties. For instance, BeF.sub.2 -based dispersion shifted single mode fiber can have lower core-cladding index difference and larger core diameter than the corresponding SiO.sub.2 -based fiber, and BeF.sub.2 -based graded index multimode fiber can have larger bandwidth than the corresponding SiO.sub.2 -based fiber. The inventive fibers have a core and a cladding containing at least 30 mol % BeF.sub.2, and may contain up to 40 mol % of AlF.sub.3, and one or more members of the group consisting of NaF, KF, MgF.sub.2, CaF.sub.2, PbF.sub.2, PF.sub.5, and SiF.sub.4. An exemplary and currently preferred glass has nominal composition (in mol percent) 30KF-(15-x)CaF.sub.2 -xPbF.sub.2 -10AlF.sub.3 -45BeF.sub.2, with x.ltoreq.15. Single mode fibers according to the invention have minimum total dispersion in the range 1.5-2.0 .mu.m, and typically have 0.25%.ltoreq..DELTA..sub.esi .ltoreq.0.6%, and 2.5 .mu.m.ltoreq.a.sub.esi .ltoreq.3.4 .mu.m, where .DELTA..sub.esi and a.sub.esi are the equivalent step index core-cladding index difference and core radius, respectively. Rules and techniques that can advantageously be used to design BeF.sub.2 -based single mode fibers are also disclosed.

FIELD OF THE INVENTION 
This invention pertains to optical fibers for telecommunications. 
BACKGROUND OF THE INVENTION 
Optical fiber is rapidly becoming the transmission medium of choice in many 
areas of telecommunications, including, for instance, long haul data and 
voice transmission. Essentially without exception, communications-grade 
optical fiber currently is silica-based fiber. As is well known, such 
fiber has a relative loss minimum at about 1.3 .mu.m, and an absolute 
minimum at about 1.55 .mu.m, and these are the wavelengths of currently 
greatest interest for optical communications purposes. 
Even though silica-based optical fiber has reached a high degree of 
perfection, such that routinely achieved loss figures are close to the 
expected intrinsic loss of the material, a significant economic incentive 
still exists for development of lower loss optical fiber. For instance, 
fiber communications systems that are currently being installed typically 
have a repeater spacing of the order 20 to 40 km, which can in some cases 
be as high as 65 km, e.g., in an intercontinental submarine cable system 
that is soon to be installed. However, many applications exist where the 
distance between transmitter and receiver is of the order of 100 km to 
several hundred kilometers, and it would be highly desirable to have 
available low loss optical fiber which would permit repeaterless 
transmission over such distances. 
Many glass systems have been identified which have substantially lower 
intrinsic loss than silica. Many of these contain heavy metals and have a 
refractive index greater than that of silica, but there also exist glasses 
that have lower intrinsic loss and lower refractive index than silica. 
Prominent among the latter are the fluoride glasses, and this application 
is principally concerned with optical fibers based on fluoride glasses 
containing BeF.sub.2 and having a refractive index that is smaller than 
that of SiO.sub.2. 
The properties of fluoride glasses have been investigated in the past, 
primarily to determine their suitability for use in high power laser 
systems. From such investigations, it is known that the fluoride glasses 
tend to have relatively small linear and non-linear refractive indices, 
and frequently are transparent over a wide spectral region in the near to 
mid-infrared (see, for instance, K. H. Sun, Glass Technology, Volume 
20(1), February 1979, pages 36-40). 
Fluoride glasses are also known to have properties which make them 
potentially attractive for optical fibers. In particular, the intrinsic 
loss of these glasses is projected to be significantly less than that of 
silica, with minimum intrinsic losses of the order of 10.sup.-3 dB/km 
predicted from theory. Recently, results of research into possible designs 
of heavy metal fluoride glass optical fibers have been published. See, for 
instance, K. C. Byron, Electronics Letters, Vol. 18(15), pages 673-674 
(1982), wherein it is reported that in such fibers the wavelength of zero 
total dispersion can be shifted over a relatively wide frequency range, 
that such fibers can have a relatively low slope of total dispersion as a 
function of wavelength, and that the total dispersion can be kept to a 
very small value over a relatively wide spectral region. 
Attempts have also been made at the fabrication of heavy metal fluoride 
glass single mode optical fibers. For instance, Y. Ohishi et al, Journal 
of Lightwave Technology, Vol. LT-2(5), pages 593-596 (1984), report on the 
manufacture of single mode optical fiber using ZrF.sub.4 -based on 
fluoride glasses. 
Heavy metal fluoride glasses typically have refractive indices greater than 
the refractive index of silica, and typically have minimum intrinsic loss 
at a wavelength larger than about 2 microns. Telecommunication systems 
using optical fiber made from heavy metal fluoride glass thus are expected 
to operate at wavelengths greater than about 2 microns, for which 
appropriate sources and detectors do not yet exist. On the other hand, 
BeF.sub.2 -based fluoride glass is predicted to have minimum loss at about 
2 microns, and lower intrinsic loss than silica over the range 1.5-2 
.mu.m. Since sources and detectors for these wavelengths either exist or 
can be produced by adapting existing technologies, communications systems 
that use BeF.sub.2 -based fluoride glass fiber as the transmission medium 
and that operate in the 1.5-2 .mu.m wavelength region offer advantages 
over both prior art SiO.sub.2 -based systems and proposed systems that use 
heavy metal fluoride glass fibers. However, such fibers can be expected to 
have different parameters (e.g., core radius a and core-cladding index 
difference .DELTA.) than prior art SiO.sub.2 -based fibers. It would thus 
be advantageous to have available design criteria and a process for 
designing BeF.sub.2 -based optical fibers having a predetermined 
wavelength of minimum dispersion .lambda..sub.o in the range 1.5-2.0 
.mu.m. This application discloses such criteria and such a process. It 
also discloses BeF.sub.2 -based optical fibers that embody the criteria.

THE INVENTION 
BeF.sub.2, which has been extensively investigated for laser application, 
has apparently not been considered previously for use in optical fiber. 
One reason for this neglect probably is the fact that BeF.sub.2 is 
hygroscopic. It is known, however, that addition of other fluorides (e.g., 
CaF.sub.2, MgF.sub.2) to BeF.sub.2 increases the resistance to moisture 
attack, even though such addition increases the tendency of the glass for 
devitrification. See, for instance, K. H. Sun (op. cit.). It has 
previously been reported that in binary and ternary fluoride glass systems 
containing BeF.sub.2 at least 40 mol% of BeF.sub.2 has to be present in 
order to form a glass. Such glasses are known not to be resistant to 
moisture attack. On the other hand, K. H. Sun (ibid) reports the 
fabrication of a moisture resistant fluoride glass containing AlF.sub.3 
and as little as 20% BeF.sub.2. All compositional percentages herein are 
mol%, unless noted otherwise. 
We have found BeF.sub.2 -based glasses to have optical properties that, we 
believe, make such glasses useful materials for optical fiber, in 
particular, for optical fiber systems operating at a wavelength (or 
wavelengths) in the spectral region between about 1.5 and about 2 .mu.m. 
Among these optical properties are low intrinsic loss, low refractive 
index, and low refractive index dispersion, all relative to SiO.sub.2. 
Furthermore, we have found that such materials can have acceptable 
resistance to atmospheric corrosion, and are excellent glass formers. 
Glasses of concern in this application contain at least about 30% 
BeF.sub.2. They may also contain up to about 40% AlF.sub.3, and, 
optionally, one or more members of the class consisting of NaF, KF, 
MgF.sub.2, CaF.sub.2, PbF.sub.2, PF.sub.5, and SiF.sub.4. Typically, the 
aforementioned constituents make up at least 95% of the glass. 
Some preferred glasses according to the invention have compositions 
described by the expression: wX-(30-x)CaF.sub.2 -xPbF.sub.2 -yAlF.sub.3 
-zBeF.sub.2, wherein X is at least one member of the class consisting of 
KF, MgF.sub.2 and NaF, and wherein O&lt;w.ltoreq.40% , O.ltoreq.x.ltoreq.30%, 
O&lt;y.ltoreq.20%, and 30.ltoreq.z.ltoreq.60%. The glasses provide a range of 
optical properties by compositional variation within the indicated ranges, 
especially by Pb-Ca substitution, and are resistant to environmental 
corrosion. 
Particularly preferred glasses have composition w'KF-(20-x')CaF.sub.2 
-x'PbF.sub.2 -y'AlF.sub.3 -z'BeF.sub.2, with 20.ltoreq.w'.ltoreq.40%, 
O.ltoreq.x'.ltoreq.20%, 5.ltoreq.y'.ltoreq.15%, and 
40.ltoreq.z'.ltoreq.50%. Exemplary preferred glasses have the nominal 
composition 30KF-(15-x")CaF.sub.2 -x"PbF.sub.2 -10AlF.sub.3 -45BeF.sub.2, 
with O.ltoreq.x".ltoreq.15%. 
Inventive glasses that contain more than about 60% BeF.sub.2 may require 
protection from the environment, due to their hygroscopic nature. Such 
protection can be provided, for instance, by deposition of such glass on 
the inside of an environmentally stable glass tube, and collapse of the 
composite tube. 
Glass according to the invention not only has a relatively small refractive 
index but also a relatively small value of dD.sub.m (.lambda.)/d.lambda., 
where D.sub.m (.lambda.) is the material dispersion of the glass, and 
.lambda. is the wavelength. These advantageous properties lead to certain 
fiber design consequences which will be discussed below. 
In one aspect, this invention is concerned with single mode optical fiber 
that comprises a core of radius a that is contactingly surrounded by a 
cladding, with both the core and the cladding consisting of BeF.sub.2 
-based glass according to the invention. The cladding has refractive index 
n.sub.2, the core has a maximum refractive index n.sub.1, with n.sub.2 
&lt;n.sub.1 &lt;n.sup.s, where n.sup.s is the refractive index of fused silica. 
It is to be understood that refractive indices are always to be compared 
at the same wavelength. The core has a nominal refractive index profile, 
which can be represented, for instance, by the well-known expression 
EQU n(r)=n.sub.1 [1-2.DELTA.(r/a).sup..alpha. ].sup.1/2, 
with r.ltoreq.a, O&lt;.alpha..ltoreq..infin., preferably .alpha..gtoreq.1, and 
with the core-cladding refractive index difference .DELTA. being 
approximately equal to (n.sub.1 -n.sub.2)/n.sub.1. As is well known, 
.alpha.=.infin. corresponds to a step profile, .alpha.=2 to a parabolic 
profile, and .alpha.=1 to a triangular profile. It will be appreciably 
that an actual fiber profile unavoidably differs to some extent from the 
corresponding nominal profile, and that the transmission characteristics 
of optical fiber can always be determined numerically, regardless of 
profile shape. 
Associated with the inventive single mode fiber is a total dispersion 
D.sub.t (.lambda.), a material dispersion D.sub.m (.lambda.), and a 
material dispersion slope dD.sub.m (.lambda.)/d.lambda.. The absolute 
value of the total dispersion and of the material dispersion has a minimum 
at wavelength .lambda..sub.o and at .lambda..sub.m, respectively, with 
.lambda..sub.m &lt;.lambda..sub.o. Such fiber is known as dispersion shifted 
single mode fiber, and utilizes the fact that fiber can be designed such 
that the waveguide dispersion shifts the minimum of the total dispersion 
towards longer wavelengths by a predetermined amount. An important fiber 
parameter is the normalized frequency or V-number, which can be defined as 
follows: 
EQU V=(2.pi.an.sub.2 /.lambda.)(2.DELTA.).sup.1/2. 
As is well known, a step index fiber will support only one guided mode for 
V.ltoreq.2.405, and more than one guided mode for V&gt;2.405. The wavelength 
corresponding to V=2.405 is referred to as the cut-off wavelength, and the 
V-number corresponding to cut-off is designated herein as V.sub.o. For 
profiles with .alpha..noteq..infin., V.sub.o =2.405(1+2/.alpha.).sup.1/2. 
The V.sub.o for other than power law profiles can be determined by 
numerical computation. 
It is also well known that for any single mode fiber having an index 
profile such that .alpha..noteq..infin., it is possible to determine an 
equivalent step index profile having a core-cladding index difference 
.DELTA..sub.esi that differs from the core-cladding index difference of 
the actual fiber. The equivalent step index profile has index difference 
##EQU1## 
and core radius 
EQU a.sub.esi =a(2.405/V.sub.o)(.DELTA./.DELTA..sub.esi).sup.1/2, 
where r=r/a. For example, when .alpha.=1, .DELTA..sub.esi =2.DELTA./3, and 
a.sub.esi =a/.sqroot.2. Because an equivalent step index profile can 
always be determined for any single mode fiber, and because the inverse 
process can also be carried out, no loss of generality results from a 
restriction of the discussion herein to the design of step index single 
mode fiber. 
As was pointed out above, the wavelength of minimum dispersion 
.lambda..sub.o of an optical fiber is an important fiber parameter, since, 
in order to achieve high bandwidth and long repeater spacings, the 
operating wavelength of high capacity long haul systems advantageously is 
chosen at or near .lambda..sub.o. 
We have made the unexpected discovery that curves of .lambda..sub.o, for 
different values of index difference .DELTA., assume a "universal" shape 
when plotted as a function of the normalized V-number (V/V.sub.o). This is 
illustrated in FIGS. 1A) and B), which show .lambda..sub.o as a function 
of V/V.sub.o for exemplary BeF.sub.2 -based step index fiber and for 
SiO.sub.2 step index fiber, respectively. As can be seen, the curves for 
both materials are of similar shape, in all cases having their maximum at 
V/V.sub.o .about.0.48. We have found such curves of .lambda..sub.o vs. 
V/V.sub.o to be useful tools in the design of optical fibers based on new 
glass systems, and, in particular, for the BeF.sub.2 -containing optical 
fibers according to the invention. 
An advantageous general approach to the design of fibers according to the 
invention that does not require extensive computation is to first select 
the desired wavelength of minimum total dispersion .lambda..sub.o, then to 
determine (from relationships provided herein) the appropriate index 
difference and core diameter of a (step index) silica-based reference 
fiber having the desired value of .lambda..sub.o, and, finally, with the 
aid of relationships that are also provided herein, to determine the 
parameters of the BeF.sub.2 -based inventive fiber from those of the 
reference fiber. The approach thus uses well-established methods and 
criteria for designing silica-based fiber in the design of the inventive 
fiber. By "silica-based" fiber we mean herein a fiber having an 
essentially undoped SiO.sub.2 cladding and an appropriately up-doped core. 
We will now demonstrate the design procedure for an exemplary BeF.sub.2 
-based single mode fiber having .lambda..sub.o =1.55 .mu.m. It will be 
appreciated that the same approach can be used for fiber having any other 
.lambda..sub.o in the range under consideration herein. 
FIG. 1B shows that the SiO.sub.2 -based reference fiber must have 
.DELTA..sup.s .gtoreq.0.54%, in order to have .lambda..sub.o =1.55 .mu.m 
(the superscript s herein always refers to parameters that pertain to the 
silica-based reference fiber). The reference fiber core radius 
corresponding to .DELTA..sub.min.sup.s, the minimum allowed .DELTA..sup.s, 
can be determined from FIG. 5, or from the expression 
EQU a.sub.m.sup.s =0.58.lambda..sub.o /.pi.n.sup.s 
(2.DELTA..sub.min.sup.s).sup.1/2, 
with n.sub.s=the refractive index of SiO.sub.2 at .lambda..sub.o. 
Choosing .DELTA..sup.s =.DELTA..sub.min.sup.s maximizes the dimensional 
tolerances of the fiber, thus increasing the manufacturability of the 
design, since small variations in core radius have negligible effect on 
.lambda..sub.o. However, this choice may result in a fiber having 
relatively high bending loss, and therefore .DELTA..sup.s typically is 
chosen to be greater than .DELTA..sub.min.sup.s. 
In order for fiber to have low bending loss the effective core/cladding 
index difference .DELTA..sub.e typically should be at least 0.05%, 
preferably .gtoreq.0.1%, and FIG. 2 shows curves that relate the actual 
and the effective core/cladding index difference for SiO.sub.2 -based step 
index fiber. For instance, in order for .DELTA..sub.e.sup.s to be 0.1% it 
is necessary that .DELTA..sup.s .about.0.57% (for .lambda..sub.o =1.55 
.mu.m). Referring to FIG. 1B one sees that V/V.sub.o .about.0.56. Using 
FIG. 5, or definition of V, one finds that a.sup.s =2.15 .mu.m for this 
choice of .DELTA..sub.e.sup.s. In general, it is desirable to choose the 
largest possible core radius, consistent with a core-cladding index 
difference that is manufacturable and that results in acceptable bending 
losses in the fiber. 
This concludes the first part of the exemplary design procedure, namely, 
the design of the silica-based reference fiber. The second part of the 
design procedure involves the determination of the parameters of the 
BeF.sub.2 -based fiber (in particular, core radius a.sub.esi and 
equivalent step index core/cladding difference .DELTA..sub.esi) from the 
reference fiber parameters. 
In general, we have found that single mode Be.sub.F.sub.2 -containing 
optical fibers according to the invention typically have a core-cladding 
refractive difference and a core radius such that 
EQU 0.5.DELTA..sup.s .ltoreq..DELTA..sub.esi &lt;.DELTA..sup.s, 
and 
EQU a.sub.esi &gt;(a.sup.s n.sub.2.sup.s /n.sub.2). 
Furthermore, our work indicates that, for fibers according to the 
invention, it is typically advantageous to choose .DELTA..sub.esi and 
a.sub.esi in the ranges 0.25-0.6% and 2.5-3.4 .mu.m, respectively. 
SiO.sub.2 -based reference fibers typically have .DELTA..sup.s &gt;0.5%, and 
1.5&lt;a.sup.s &lt;2.2 .mu.m. Thus it is evident that the inventive BeF.sub.2 
-based fiber can have substantially lower core-cladding difference and 
substantially larger core radius than SiO.sub.2 -based fiber. As will be 
appreciated by those skilled in the art, a smaller index difference is 
advantageous because it implies a lower doping level, and therefore lower 
Rayleigh scattering loss, and a larger core diameter is advantageous 
because, inter alia, it makes it easier to achieve low loss fiber splices. 
As is well known to those skilled in the art, the effective index 
difference of the fundamental mode in optical fiber is defined by 
.DELTA..sub.e =.beta./k.sub.o, where .beta. and k.sub.o are the 
fundamental mode propagation constants in the lightguide and in free 
space, respectively. It is a weighted average between the core and 
cladding indices, and can always be determined numerically. FIG. 3 shows 
exemplary curves that relate .DELTA..sub.e and .lambda..sub.o, for various 
values of .DELTA., for BeF.sub.2 -containing step index fibers according 
to the invention. Entering FIG. 3 with the previously selected values of 
.lambda..sub.o and .DELTA..sub.e permits determination of .DELTA..sub.esi 
for the inventive fiber that is to be designed. For instance, if 
.lambda..sub.o =1.55 .mu.m, and .DELTA..sub.e =0.1%, then .DELTA..sub.esi 
=0.38%. 
With the core/cladding index difference of the equivalent step index 
BeF.sub.2 -containing fiber thus determined, the core radius can be found 
with the aid of FIG. 4, which contains curves relating core radius and 
.lambda..sub.o, for various values of .DELTA..sub.esi. For instance, 
.lambda..sub.o =1.55 .mu.m and .DELTA..sub.esi =0.38% requires that the 
core radius a.sub.esi be about 3.3 .mu.m. This concludes the determination 
of the parameters of the equivalent step index profile of the single mode 
fiber according to the invention by the novel design procedure. 
If a profile other than a step index profile is to be used, then the 
parameters of the actual profile can easily be derived from 
.DELTA..sub.esi and a.sub.esi. For instance, fiber having a triangular 
core profile (.alpha.=1) has .DELTA.=3.DELTA..sub.esi /2, and 
a=(.sqroot.2)a.sub.esi. 
Given the desired .lambda..sub.o and .DELTA..sub.esi, as well as the 
refractive index of the glass to be used for a fiber according to the 
invention, together with .DELTA..sub.esi.sup.s and a.sup.s, it is also 
possible to determine the appropriate radius a.sub.esi directly by means 
of the following simple relationship, discovered by us during the course 
of our work: 
EQU a.sub.esi =(a.sup.s n.sup.s /n.sub.2)(.DELTA..sup.s 
/.DELTA..sub.esi).sup.1/2 
Not only does the use of BeF.sub.2 -containing glass according to the 
invention make possible the design of single mode, dispersion shifted 
optical fibers having, inter alia, relatively large core and relatively 
low refractive index difference, but it also can result in graded index 
multimode fibers having high bandwidth over a relatively large range of 
wavelengths. 
As is well known, in multimode fibers intermodal dispersion can severely 
limit the bandwidth. Grading of the core profile can substantially reduce 
intermodal dispersion. However, the bandwidth B (which is inversely 
proportional to pulse dispersion) typically sharply peaks at some 
wavelength .lambda..sub.m at which the value of the profile shape 
parameter .alpha.=.alpha..sub.m, the optimal profile shape parameter for 
that wavelength. See, for instance, Optical Either Telecommunications, S. 
E. Miller and A. G. Chynoweth, editors, Academic Press (1979), pp. 
255-257). The value of .alpha..sub.m is a function of wavelength. This 
effect is referred to as profile dispersion. See, for instance, R. 
Olshansky et al, Applied Optics, Vol. 15(2), pp. 483-491 (1976). In 
typical prior art multimode optical fibers, profile dispersion causes 
.alpha..sub.m to differ by more than 5% from the .alpha. of the fiber if 
.delta..lambda.=.vertline..lambda.-.lambda..sub.m .vertline..gtoreq.0.2 
.mu.m. This difference between .alpha. and .alpha..sub.m causes the fiber 
bandwidth to decrease to about 1.7.DELTA. GHz.multidot.km (for 
.delta..lambda.=0.2 .mu.m), with .DELTA. being the core-cladding index 
difference in percent. 
We have discovered that BeF.sub.2 -based multimode fiber can have much 
smaller profile dispersion than prior art fiber having the same .alpha. 
and .DELTA., making possible the design of graded index multimode fibers 
of greatly increased bandwidth. Thus, B&gt;1.7.DELTA. GHz.multidot.km for 
.delta..lambda..ltoreq.0.2 .mu.m, preferaby B&gt;2.DELTA. GHz km for 
.delta..lambda..ltoreq.0.2 .mu.m. 
The refractive index of BeF.sub.2 -containing glass according to the 
invention depends on the chemical composition of the glass, and thus the 
desired refractive index profile can be produced by appropriate variation 
of the chemical composition. A particularly advantageous approach involves 
substitution of Pb for Ca, and FIG. 6 shows the core-cladding index 
difference .DELTA. in step index fiber of composition 30KF-(15-x)CaF.sub.2 
-xPbF.sub.2 -10AlF.sub.3 -45BeF.sub.2 where x indicates the mol% of 
PbF.sub.2 in the fiber core, and x=0 in the cladding. 
BeF.sub.2 -based fiber according to the invention can be fabricated by any 
appropriate process, including the double crucible (see, for instance, H. 
Tokiwa et al, Electronics Letters, Vol. 21(24), pp. 1131-1132 (1985)) and 
the rod-in-tube process, as well as by vapor phase processes (e.g., MCVD) 
especially vapor phase processes that use organometallic precursors such 
as ketonates or alkoxides. 
When using processes such as the rod-in-tube or the double crucible 
process, diffusion (in-diffusion or out-diffusion) of a selected element 
can be used to obtain a graded core index profile. For instance, Pb/Ca ion 
exchange could be used to selectively remove Pb from a PbF.sub.2 
containing core rod prior to its insertion into the PbF.sub.2 -free 
cladding tube. Fusing such a rod concentrically within the tube results in 
a preform having a graded index core. The rod as well as the tube can be 
produced by known methods, e.g., by casting or extrusion. 
When using an inside vapor deposition process such as MCVD, it is typically 
necessary to use a substrate tube made of a material that is chemically 
stable (i.e., not subject to environmental corrosion) and whose 
thermophysical properties (e.g., coefficient of thermal expansion) 
sufficiently match those of the BeF.sub.2 -containing glass to be 
deposited thereon. Multicomponent silica glass (e.g., sodium borosilicate 
glass) meets these requirements and is thus an exemplary substrate 
material. 
Example 
Step index optical fiber, with .DELTA..about.0.4%, is prepared as follows: 
KF, CaF.sub.2, PbF.sub.2, Al.sub.2 F.sub.6.(H.sub.2 O).sub.x and 
(NH.sub.4).sub.2 BeF.sub.4 powders (&lt;325 mesh) of 6N purity, in mol ratio 
30:13.8:1.2:10:45, are introduced into a silica beaker and wet mixed, 
using anhydrous alcohol. The resulting mixture is dried, thereby producing 
a powder. The powder is placed into a fused silica crucible, which in turn 
is put into a cooling assembly within an RF coil. The charge in the 
crucible is initially heated by means of a graphite susceptor within a 
fused silica protective tube. After coupling of RF to the charge is 
achieved, the charge is RF heated to 1100.degree. C. for 20 minutes. The 
crucible is then removed from the heating assembly and the still very 
fluid charge poured into a heated (200.degree. C.) graphite cylindrical 
mold (8 mm inner diameter) and allowed to solidify, resulting in a core 
rod of about 8 mm diameter and refractive index of about 1.337. A 
cladding tube is produced by substantially the same procedure, except that 
the powder mixture contains 15 mol% CaF.sub.2 and no PbF.sub.2, and that 
the charge is poured into a cylindrical mold of 125 mm inner diameter, 
with a 8.5 mm diameter concentric cylindrical inset. The cladding tube 
refractive index is about 1.332. The core rod is inserted into the 
cladding tube, and fiber is drawn from the assembly in the conventional 
manner.