Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ ;3352 ~ :
This invention relates to novel glass compositions and
to glass articles particularly suitable for forming or being con-
verted to articles or devices for use in the guided transmission
of light both for imaging and communication purposes. It is par-
ticularly suitable for ~orming glass articles which are subse-
quently drawn into glass fibers ~or use as optical fibers used
to transmit and guide electromagnetic energy above 300 GHz; or
for forming glass articles to be used to couple said electroma-
gnetic energy between two or more optical fibers (or bundles of
optical fibers), or couple said electromagnetic energy between
optical sources or detectors of said electromagnetic energy, or
for forming glass articles to be used as a part of an integrated
optical device; or or forming a glass article to be used as an
active element in a laser or optical amplifier; or for forming a
glass article ~o be used as a lens; or for forming a glass arti-
cle to be used to relay images. In this disclosure the term glass
waveguide will be used to embody the above and other similar uses. ';
Such waveguides can be made with care with any desired attenua-
tion from the order of 2-5 dB/km to 100 and upwards. For inte-
grated optics applications, values of attenuation as high as 1 i ~
dB/cm are acceptable, though for purposes such as telecommunica- ~j ;
tions, a level o less than 5 dB/km is necessary. The term
"glass article" as used in these specifications includes artlcles
which are to some extent crystalline.
... ... .
In our previous U.S. Patent No. 3,938,974 issued Febru-
. ~''.
~ 33SZ
ary 17, 1976 we have described a process in which a phase-sepa~
rable glass is converted to a porous form. This porous form is
substantially formed of silica and can then be converted to a
solid glass article with either a uni~orm or non-uniform refrac-
tive index profile across at least one cross-sectional axis, by
adding refractive index modifying components to the porous mate-
rial, and collapsing the article thus formed into a solid glass
article. We have called the process of adding such refractive
index modifying components "molecular stuffing."
I~e have found that such a process is not only applica-
ble to the stuffing of porous matrices produced ~y the leaching
of phase-separated glasses, but is also applicable to other in-
ter-connective porous structures having a matrix eonstituted of
at least one glass network forming material. One well-known
process for forming interconnective porous structures other than
the phase separation route is by chemical vapor deposition. A
convenient description of such a process is contained in U.S.
Patent 2,272,342, issued to J.F. Hyde and U.S. Patent 2,326,059 `
issued to R.E. Nordberg. More partieularl~, U.S. Patent
3,859,074 issued to P.C. Sehultz deseribes the formation of a
porous body and its subse~uent impregnation with a dopant. Such
impregnation is concerned with the deposition of small quantities
of materials from relativel~ dilute solutions in the pores of
the porous body.
In or~er to achieve a particular refractive index chan-
~ ~;3352 ~:
ge and a particular refractive index profile, it is necessary todeposit relatively large ~uantities of material in the porous
matrix. We have found that in order to obtain greater control
of the desired refractive index profile it is essential to carry
out certain steps of the process in a particular way and in a
particular sequence not previously disclosed in order to achieve
greater control of the desired profile in the finished article.
As indicated above, our prime purpose in adding a ma-
teriaL to the pores is to obtain a particular refractive index
or refractive index variation in the glass article. By this
means one can produce an article suitable for use, e.g., as an
optical waveguide. If the refractive index is constant through-
out the article, the article is suitable for use as a eore to be ;
surrounded by a material of diferent lower refractive index as , ;
,., ' . .:
a cladding. The same effect can be produced in cross-section ;
w1thout a cladding by producing a stepped profile in an arti-
cle. A "parabollc" profile is a term used to describe the situa-
tion where there is a reraetive index gradient in any transver-
se eross-seetion sueh that the index deereases "pro~res5ively"
or "continuously" from the central axis toward the periphery of `
`, : .: . .
the artie~e. Various~desirable profiles produced by other means
have been deseribed, e.g., in U.S. Patent 3,830,640 issued to
Nippon Selfoc KK. In any process or forming the glass articles
of this invention, the formation of the porous matrix constitu- ` -
tes a value added step and losses (for example, breakage) after
- 3
.:
3352 , ~
this stage may decrease the economic yield of a ~ull~scale com- --
mercial process. We have found it difficult to determine the ~ -
factors causing losses either through breakage, due to cracking,
or the occurence of light scattering centers such as includions
or hubbles in the final collapsed article. ,-
To obtain both increased yields and more control over
refractive index profile conslstently and satisfactorily, we ~ -
have now identified and improved those stages of the process whe-
re it is necessary to operate in a particular manner, and in a
particular sequence not previously disclosed.
With regard to yield, as in the normal manufacture of `~
any glass article modifications to the process do not necessari-
ly result in every article cast or formed being fault free and
ensure that such articles will all survive the su~sequent pro-
cessing steps., By an increased yield we mean that we have found
how to improve the statistical chances of a rod or other article
surviving; the processing steps. This is,
'
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. ~ .
~3352
!i'
howe~er, on a statistical basis and one cannot guarantee that
even when all the es=Pntial steps of our process are used~ an
acceptable product will always be obtained. As indicated pre-
viously, the improvement p~oduced is not only in yield but in
insuring that a desired refractive index profile is obtained
consistently and satisfactorily.
This is primarily based on our discovery that for best
results it is essential that the step of depositing the solid
:: :1;: ,: ,
materiaI in tl~e pores be~carr ed out by a process which aoes ~'`
not involve evaporation~of solvent, and that the subse~uent i -
:. ~ ~,.-, ,~ .
heating step to raise the temperature of the article so as
to remove the solvent from the pores, and, where necessary,
decomposition products should ~e regulAted so as to achieve ~ `
retentLon or production~of a desired reraotive index pxofile.
We have also found~that while achieving a satisfactory
artic~le, certain~dopants~give particularly~ advantageous re-
sults because cf their physlcal characteristics.
Wa also prefer that,~ if the porous article to be stuffed
has been ~ade by phase separation~fro~ a glass followed by
leachin~, certain precautions be taken.to reduce losses during
the prccessing of the rcd. We~have found that craoking in
this form of oux process can be oaused by problems arising
f rom one or more o the following~
(a) Incorrect glass composltion~; -
.. .
b~ Incorrect heat treatment conditions for phase-
separation; and ~
:
(c) Incorrect leaching procedure. ~
,. ~ ,.
~ - 5 - ~
1~i335Z
Gui~ance is given below as to how to choose glass compo~i~ion
and processing conditions so as to reduce Loss due to cracking
;.n subsequent processing both during the ormation of the -~
porous matrix and the subsequent stuffing and drying. r~
The present invention~is concerned with a method of
,
producing a desired refractive index distribution in a glass
article as a function of its dimensions by the addition of `~-
a refractive index modifying component` (hereinafter referred
to as a aopant) to a porous~matrix with interconnective pores
whose walls are formed from at~least one ~Iass networ]c forming
component and, where desi~ed, glass network modifying compo-
nents. The method comprises the steps of immersing the po~ous `
matrix in a solutLon of a dopant, causing the dopant t~ sepa-
rate in the matrix, removing solvent and, whexe necessary, de-
.. . . .
composition products; from the porous matrix and coLlapsing the
porous matrix to a~solid form,~characterized in that part or
all of the dopant is caused to~be ;precipitated by a method
Which does not involve evaporation of solvent, the xemovaL
of solvent is not commenced until a substantial part of the
precipitation has taken plaae and the rate at which heat is ~"
applied to remove solvent, and where necessary, decomposition ,!' '` '
products is regulated so as tc achieve and/or retain the de~
sired refractive index distribution profile within the glass
article.
..
The steps and the sequence of steps which we have found
suitable to produce a~partlcular profiLe are outlined below,
.~ ~
all starting wi~h a porous article having interconnected pores. -
1,;
~ , ,
- 6 -
~ j33SZ
The steps of our invention comprise the following:
(1) The dopant is precipitated in the pores by non-evaporative
steps. These include (a) thermal precipitation in which by
lowering the temperature of the object, the solubility of the ~ -
dopant or dopant compound in the solvent is decreased sufficiently
..
to cause precipitation o~ the dopant or dopant compound and r`~
(b~ chemica~ precipitation such as alteration o~ solution pH ` ~ `
to a point of precipitation, replacement of the original ;~
solvent by a solvent in which the dopant or dopant compound is
:~:: , . .
. .. ..
less soluble or introduction of a chemical into the solution '
which reacts with original dopant or dopant compound to form
a less soluble dopant cpecies. Hereinafter the term solvent
is used to de~cribe the chemical species which at some stage
: : :
is the Iiquid ~illing the pores.
(2) Removal of the final solvent is commenced only after pre-
oipitation is substantlally complete.
(3)~ The rate at which~heat is applied to remo~e solvent a~d ~`
where~necessary, decomposition products, .i~ re~ulated so as
to achieve and/or retain the desired re~racti~ve index profile
within the glass article.
The steps and the sequence of steps which we have found
suitable to produce particular profiles are outlined belowr
starting with a porous article and using thermal precipitation `
.. .
of dopant or dopant compound. ;
Flat profile
(a) Immerse the pOL-OUS matrix in a solution
. .
of dopant or dopant compound. ;
~b) Precipitate the dopant by a temperature
.
;~ ~ 7 ~ '
-:
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-` ` . 1~i335Z
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drop. ~-
(c) ~vapora~e any solvent present.
(d) Heat to collapsing temper~ture.
Stepped proflle ~1) (a) Immerse the porous matrix in a
solution of dopant or dopant com~
pound. ~-
~(b) Precipitate the dopant by dropping
, . . .
the tempe~ature.
: ~ J
(c) Immerse in a solvent for the do-
: " .
p~nt and allow the dopant to par- -
tially redissolve and diffuse out
. ~ ,.. ..
of the matrix. Only the dopant
precipitated near the outer sur-
face of the article is removed in
!
this step.
(d) Evaporate any solvent.
: . .,
(e) Heat to collapsing temperature. "~ ,
(2) ~Alternatively:
(a) Immerse the porous matrix in a
solution o~ dopant or dopant aom-
pound.
b) Precipitate the dopant by dropping
,. . .
the temperature.
~c) Partially dry the porous rod.
(d) Immerse in a solvent for the dopant ;~
. . . . :~
and allow the dopant to partially
redissolve and diffuse out of the
matrix.
. . ~.
~ ;335;~
Only the dopant precipitated near the ,
outer surface of the article is removed ' "'~
in this step. t '~;" '
(e) Evaporate any solvent. ,~;~ '''
~f) Heat to collapsing temperature.
Parabolic profile (a) Immerse the porous matrix in a solution '~
of dopant or dopant compound. ,'-
. . .: . . .
(b) Immerse in a solvent for the dopant at ' ,~
substantially the same temperature as that
at which stuffing took place. The artic~e ,'~ ,
remains in the solvent for a sufficient ' '
time to produce a diffusion created profi- ,
le of dopant in the article such that the ! ,
dopant concentration decreases as a func~
tlon of radial distance from the central
axis.
c) Precipitate dopant in pores b:y dropping '~ -
temperature. ~,','
~d) Evaporate~ the solvent. `
e) Heat to collapsing temperature.
.. . .
As indicated above, we prefer to use as a dopant a ma- ;'~ '
terial whose solubili~ty characteristics are such that we can ~,'' '
achieve the desired concentration of the dopant'in ~he pores, by ~' '
!~
dif~using a solution of the dopant into the pores at one tempera- ;
, ture and then cause its precipitation by a simple drop in tempe- ,' ,'~
,.
rature. W,e refer to such a process as thermal precipitation. x,,
While we prefer to use this process, other routes are feasible. : ~
Our invention therefore includes a process for the production of ~ - ,'
a~glass article with ~a desired reractive index distribution ' '
using a suitable porous matrix as a starting material in which '
,., ~ .
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1~;3352
a refractive index modifying component i~ cau~ed to separate
.. . .
out of solution by lowering the temperature of the solution.
, ~ .
Amongst other routes we find we can precipitate the solute
by chemical means rather than by temperature drop. The common -~
ion effect has been used to reduce solubilities and cause pre-
cipitation of the solute (e.g., the solubility of CSN03 in
water is reduced in the presence of lN HN03). The exchange
... . . .
of solvents has also been used to reduce solubilities and thus
precipitation by means not invoLving evaporation of solvent can
be used. These include the a~dition of a suitable precipitant
which reacts with the dopant or ca~s a suitable change in pH.
We have also used a combination of steps consisting of both
thermal and chemical precipitation means. ~his is particular-
ly useful in cases ~n whioh more than one dopant or dopant ~ ;
,..
compound is being introduced into the pores. We avoid any pre-
cipitation methods involving evaporation of solvent as the
sole means of preciptation, since~we have been unable to ob~
tain consistent results~using such methods.~ We believe this
is due to the foll~owing factors.
In the direct evaporative process the solution evaporates
r~m the surfaae of the article causingtransport of the dopant
from the interior to the~surface. There is also a vertical
transport process~due to gravity which causes accumulation of
the dopant at the bottom of the article. Together these effects ~ .-
~ ~ ,
~ tend to produce undesirable profiles.
. :
, :
It is essential to regulate the rate of heating so as toavoid destroying either~the incipient refractive index profile,
or damaging the inter~onnectiVe pore structure. It is possible
:
- 1 0 -
:
,335z :-
.... .
by allowing the evolution of vapor or gas in an uncontrolled !,,, ~ ,
':.',,', .,' "',
manner to produce a pressure sufficient to destroy the integrity ;~
,.. ..... .
of the structure. We prefer therefore to avoid allawing the sol~
vent to reach its boiling point at a point when large volumes of
vapor are liable to be produced. Various heating regimes are
described below, and show how to regulate the heati~g to achieve
a desired end.
.
The regulation of the solvent removal step is based on ~
.:-
the need to avoid destruction of the integrity of the porous ;
: .' .
structure, and upsettlng the distribution of the dopant in the
pores. The precautions we take are to commence solvent removal
at room temperature or below by a non-boiling method, and avoid
.~ . . . .
any violent change in temperature which would cause an excessive- ~ ~
,: , .
ly fast evolution of solvent vapor in a confined space. Conve-
nient methods of commencing solvent removal include placing the
:: '
article, where the solvent is water, in a dessicator at about
22C for about 24 hours, or in a vacuum at temperatures slightly
above 0C (i.e., 4C) for about 4 hours, and then to proceed to
,.
raise the temperature. We have also found that it is necessary
in some cases to hold or~reduce the heating rate to a very low
rate so that the article stays in a particular temperature range
for a time sufficient to ensure particular events have occurred s ,
.
beore heating is continued. At other points we believe it pre-
ferable to move rapidly rom one temperature to another, e.g.,
when solvent removal has been completed to the temperature of
.
collapse. Later in this speci~ication we give some guidance in
terms of an aqueous system, but the warnings given can be seen to
apply equally to the system where organic
i :.
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.. .....
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335Z
solvents or other non-aqueous systems or mixtures of such
systems are used.
The following criteria can be used to select a suitable
dopant from among the larger group of refractive index modi-
fying components.
(a) It must be soluble in suitable concentrations in a
. ~ , .
solvent which does not interfere with subsequent processing `
after stuffing. -
(b) It must be~ able to be incorporated into the matrix
either as deposited or after thermally induced decomposition.
(c) The dopant or mixture of dopants used must be
capable of being incorporated into the glassy structure at or
.
below the highest temperature at which the article is sub-
sequently processed.
(d) The dopant must not change its physical or chemical
state in such a way before colla~se as to be lost from the
matrlx.
(e) For low optical loss items the following added con-
ditions apply: ~;
,.,
~ 1) the dopant when used must be sufficiently freeof iron, copper, and other undesirable transition metal elements.
(2) when the porous matrix is collapsed, the im-
miscibility temperature for the composition then formed must
be below the temperature of any subsequent forming or shaping
pxocess needed to bonvert the article to any other shape or
form.
", ~
- 12 - ~
: , .
63352 ~ ~
Well-known compounds modifying the refractive index of ~ :
glasses include those of Ge, Pb, Al, P, B, the alkali metals, the
alkaline earths and the rare earths in the form of oxides, nitra-
tes, carbonates, acetates, phosphates, borates, arsenates, sili- `
cates and other suitable salts in either hydrated or unhydrated ~ ;
form. Of these we prefer to use compounds of Cs, Rb, Pb, Ba, Al,
Na, Nd, B and K. Other dopants and mixtures of dopants can be u- -
1. . - .
sed as long as the above criteria are satisfied. It is impossi~
ble to list all the~potential combinations of dopant elements but
it is believed that based on the guidance given, such selection
of useful combinations is within the competence of those practi- i~
~ ... .
ced in the art.
The concentration of the dopant or dopant mixtures in
the finished waveguide will usually vary with position. However,
it is highest at the optical axis and it should be in the range i ``
1-20 mole percent of the oxide of the dopant or dopant mixtures
in the total glass composition, the preferred range being 2-lS ~` :
and~the most preerred,~5-10~mo~le~percent. As a result the sili- ;
ca content of the glass will be greater than 75 mole percent,
preferred greater than 80~mole percent, most preferred greater
than 90 mcle percent, the diff~erence between the silica and do-
, . .
pant concentration usually being made up by B203.
In the selection of solvents the ollowing considera- ~
tions are important. The solvent selected `" ;~,
(a) should not damage the porous matrix;
(b) should be capable o being purified to low concen-
trations o undesirable impurities; ,
;~ (c) should be one that can be substantially removed by ~ -
either exchange with~anothe~r solvent, evaporation, or thermal de-
composition followed by~oxldation (or~high temperature reaction
with chemically active atmospheres);
(d) should have sufficient solubility`for the dopant s
13 - ~ ;
; ;` 1~ti3352
compound or combination of dopant compounds to allow the desired
dopant levels within the pores to be achieved by molecular stuf- `
fing.
,; ,. . .
(e) should be such that, i used in thermal precipita-
tion process, any dopant solutions in the solvent will have suf-
ficiently high temperature dependence of solubility to deposit
dopant within the pores when cooled;
(f) should be such that, when used to precipitate by
a solvent exchange process, will have the specific solubility
properties as needed by the process;
(g) should be, for economic considerations, low cost
and capable of high speed of drying.
It is impossible to test all possible combinations o
solvents; however we have found that water, alcohols, ketones,
ethers, mixtures of these and salt solutions in these solvents
can be used satisfactorily applying the above criteria to the
selection of a particular solvent.
In general, we prefer to use thermal precipitation be-
cause of its ease and convenience, and because we prefer to carry
out the first stage~of the~subsequent solvent removal step after
doping at room temperature or below, and hence it is usually ne-
cessary to cool the stuffed article.
The dopants used are pre~erably water soluble and have
a steep solubility coef~icient, that is, that the material is
very soluble at temperatures of the order of 100C, and on coo-
ling to room temperature or below, a substantial amount of mate-
rial separates, thus making them suitable for thermal precipita-
.
tion. The dopant should also be easy to purify, i.e., to reduce
the iron and transition metal content to negligible proportions.
:
,. .
','.
~l :
1~;3352 . - `
,,,: - . .
F~rther de~iled guidance concerning the choice of sol-
vent is given by reference to Table I below in which the
solubility of var.ious dopants in solvents at different tempera-
tures are illustrated.
As already inaicated above, in choosing a particular
route to a desired end product, a number o guidelines need to
be considered, and these can be illustrated by reference to
Table I.
?~
irst, in order to obtain the desired concentration of ;;
dopant in the article to yield a significant change in inde~, a
~. .
solution havinq a suEficiently high concentration of dopant must
, :
~; be found by suitable choice of dopant compound, solvent and tem-
perature. In order to precipltate the dopant or dopant com-
. pound, the use of solvents w~th sufiaiently low solubility
is necessary. Often there is a need to remove substantial ~
amounts o dopant rom designated areas o the article, such ` -
:: .~-, .
~ ~ as in the cladding region of a fiber, in which case solvents
..
~ with~intermediate solubilities are useful. Such removal of
, .
~ dopant is referred to as unstu~ing, as opposed to molecular
,:
stuf~ing. Suitable~ control of solubilities for proper pre-
cipitation of the dopant or dopant compound can be achieved by
a number of methods.
. . .
.
~ 1) Thermal precipitation is most suitable for solvents
whose~solubility for the dopant or dopant compound is strongly
temperature dependent. ~Thermal precipitation has the added
advantage of being able to arrest diffusion in the shortest
time, thus enabling us to freeze-in a desired coneentration
profile with high accuracy.
, . .
- 15 -
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~j335Z
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.
> ~( ~ ~ N ~ N N N N O N C~ ~ N ~ (!>~
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'oP~ o~o~ oP '~ o~O~o~, ~o~~ ~o',O ~ '`
O ~ ~ o o U~ ~ ~In C~ o Ul ;
N~ b ~~ o o Ln o
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t,`l t~l ln t~ O t,~t O C~
tl) o t~l t~ o~ t,`l O t~
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0~0 ~ 0~0 0\0 Op 0\0 0\0 ':i
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N O O O O O O , ... .
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t,~l t~
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This is illustrated for CsN03 dopant in Table I whereby the
solubility changes from a desirable stuffing level at 95C to
a desirab~e unstuffing level at 4C.
(2) Precipitation by co~mon ion effect and thermal pre-
cipitation. Precipitation can be produced or further enhanced
by the common ion effect. For example when the dopant is a
nitrate the concentration of nitrate ions in solution is in-
creased by adding another source of nitrate ions to the solvent
(i.e., HN03 acid). T~is reduces the solubility of the nitrate
dopant (see CsN03, Table 1) -
(3) Precipitatlon by solvent exchange. Precipitation is
induced by substituting a low solubility solvent for a higher
solubility solvent. The high solubility of nitrates in water
has allowed us to use water as solvent for the stu~finy process.
Exchange of water with either alcohols, ketones or ethers or
combinations has induced precipitation of the dopants. Typical
solubilities are illustrated in Table I.
(4)~ Variation in dopant compound. The range o~ solubility
of the dopant compounds may be altered by choosing a different
anion such as replacing CsN03 by Cs2~C03)2 to increase solubility
in water ~see Table I, line 7).
~ '
' ~ .
-.
- 18 -
. .
~i3352
I.EG~NDS . :
FIGURE 1. Plot of the fractional weight gained by a porous .
rod immersed in a solution of CSN03 at 100C.
"`
FIGURE 2. Plot of the fractional weight loss by a porous rod u
,~ ~
after being stuffed with CsNO3 at 100C, which is .
now immersed in H2O at 100C.
FIGURE 3. Plot of the~fractional welght 105s by a porous rod .. :~
after beLng stuffed with CsNO3~at 100C, which is
: now immersed~in H2O at 4C. ~; :
:~ ~ ,.,.: . .
i.,.
. . , . ,.:
F~GURE 4. Plot of the index of refraction proile o~ rods in , `:
, , ,: . .
~ Example V. Curves 1 and 2 indicate the rods un-
,,,~ , .
stuffed for 11 and 20 minutes respectively.
FIGURE S ~Plot of the;index~profile~stuffed according to rod
l3,Example;Vr,~Table V. ~he rods were rate heated
~ according to Example VII where curves 1, 2 and 3
; repxesent heating rates o~ 15, 30 and 50C/hX. ,
;~ respectively. . ~.
: : , : , ~ ,
~: ,
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1 9 - ' ' I
. ~ . ,
: .
~ 335Z '' . '
As indicated above, when operating the pr~cess of the
present invention with a porous interconnective structure
which has been produced by the phase separation of a suitable
glass followed by a leaching step, it is necessary to optimize
the various stages of the process in order to achieve aonsis-
tently and satisfaotorlly a saleable end product in good
i economic yields, and to interrelate the various parameters
involved.
The factors on whic~ guidance is required by the man
, ~ . . .
practiced in the art are:
(1) selection of g~ass composition and heat treatment
~ to obtain suitable phase separation;
: ~ ~; : :
(2J leaching and washing;
~3) stuffing; ~'
t4) unstuf~ing where~needed; and
: :, , ,
~ 5) drying and consolidation.
The guidance given ln steps (3-5) above applies to all
matrices, not just those produced from a phase separated glass.
1. Glass composition, time and temperature of heat treatme~t.
In order to aahieve a satisfactory product it is necessary
~: to choose;a phase-separable~composition, which on heat treat~ ;
ment at a particular temperature separates into approximately
':
equal volume fractions, and when held at that temperature, ;;
..... .
develops an interconnective structure with a deslrsble pore
size. A number of guidelines can be given to the man practiced ~:;
,, .
~ in the art. We find it convenient to choose compos~ions from ` ;~
.: . . , :.
::
.
- 20 -
~:,,
335Z
the aXe~ o~ al~ali ~et~ bo~os~cate ~l~sses, and ~urther
guidance is given below as to suitable compositions.
Many compositions have been reported as suitable for use
in the production of Porous glasses for diverse purposes (see
U,S. Patents 2,106,744; and 2~221~709
~,, ~ . . .
usually not for opticaI use by a route based on phase separa-
tion and leac~iing of the soluble~phase. We have discovered
that for optical waveguide manufactur~e, only small regions
; ~ . . .
within prior art compositions ranges are suitable. U.S. Patent
3,843,341 is one representative disclosure of such compositions
: .
which for the most part are not sati~factory in a process in
which the articles produced are usually rods or okher elanga-
ted shapes with the smallest dimension above 4 mm. For example,
a number of glasses from wlthin the preferred region of U.S.
Patent 3,843,341 and from previous disclosures of Corning (see
Table II below) were vertically drawn into 8 mm rods at a rate ~-
o~ one inch per minute. These were phase separated as dis-
closed herein, but all the Table II compositio~s cracked
during leaching.
TABLE II Prior Art Compositions Which Crac]; Upon Leaching*
2 B203~SiO2 ~1~03
8 30 62 0
I
8 35~ ~ S7 0
8 40 52 0
0
.
0 i
0
59
54
~0 49
, .............................. ....... ................................. .... . .
* All concentrations are in units of mol percent.
21
: . . . .
.
1~3352
Moxe specifically~,w~ haye'~oun~ ~h~t ,`'
(1) All of the compositions in the' range'of the sodium
borosilicate system disclosed in U.S. ~atent 3,843,341 and
drawn into rods as described abové,,cracked upon leaching. `~ '
`: :
This includes the region denoted as the preferred range in
said patent. '"
(2) Many of the compositions in the range of the sodium '
alumina borosilicate disclosed in U.S. Patent 3,843,341
crarked when treated as described above. This was true even
for compos-tions in the preferred range. ''
(3) Many of the preferred compositions disclosed in the ,' ;
U.S. Patent 2,221,709 cracked when treated as described above. ,'
~ lthough we find most of the previously disclosed boro~
silicate range of composition unsatis~actory because of
the requirements we need to insure a satisfactox~ ,ield' of end
product, we have discovered certain specific compositions
in~this broad range which~are useful and which have not been ~ '
previously~described.~In addition we have discovered a set '' ,
"
of criteria which can~be applied to identiy other specific
limited areas o phase-separable glass compositions which , .;
would give a satisfactory yield of product. '~:
:: : ,
. :
~;: : .
: : :; . ...
. ... .
,- ,
::.....
,::,.. :.
... .
., ,. . ':
,
~ - 22 - ' '"
' '' ~'`
3352
, .~ '~, :- .~ ..
From ~ commercial point of vi.e~, and bec~use o~ the ]ar~e ;~
region of phase separation which they show it is most con-
venient to work with alkali borosilicate glasses though
i. ::
almost all silicate glassy systems exhibit composition re-
gions of phase separation.
, .
In order to achieve a satisfactory product it is necessary ¦
to choose a composition:
(1) which on suitable heat treatment separates into
~two phases, one silica-richj the other silica-poor. The
latter is preferentially soluble in a suitable solv~nt.
I ~ ~ (2) which on heat ~reatment at a particular temperature `~
separates into phases of approximately equal volume fractions
:
and when held at that temperature develops interconnected mi-
crostructure.
:~
(3) which is easy to melt and is easy to refine using
: :: : .,.
conventional techniques.
(4~ which~ is relatively easy to form in the shape of a
rod or a shaped artiale with minimum dimensions of > 4 mm
; (e~g.r thickness, diameter,~etc.) and does not phase sepa-
rate 9ignificantly during~the ~orming sta~es. `
.
;~,: . 1,',
The followin7 provides a systematic p~ocedure for :
selecting a suitable composition.
(1) Almost all silicate glassy systems exhibit composition
~ . .
,
~; regions of phase separation. However, of commercial interest
are the alkali borosilicate glasses which show a large region
~; of phase separation. The~ silica-poor phase o these glasses 1"
i "
` can be readily dissolued by simple acidic sol~tions. Frequently
i .' . .
.. . .
- 23 - j
~ . . . .
~, .
,~. .
1~&335Z
i~ is necessary to add other components SUC}l a~ aluinin~lrll
oxide to modi~ certain properties of these glasses. ~low-
ever, some oxides are not
desirable because on phase separation they end up in the ~;
silica-poor phase and make it difficult to dissolve by simple
acidic solutions. Thus only those other components axe suitable
which do not diminish the solubility of silica-poor phase
significantly. `
(2~ After deciding on the glassy syste~l (along with
dopants~, one should next det~rmine the immiscible regions
of composition, C, and their coexistence temperat:ures, T~.
Tp~C) is the temperature above which a glass (C) i9 hOnlO-
geneous. Techniques for determining Tp are well described
in literature (see for example W. Haller, D~H. Blackburn,
F.E. Wagstaff and R.J. Charles, "Metastable immiscibility
surface in the system Na2O-B2O3-SiO2" J. Amer. Ceram. Soc.
53 (1), 34-9 (1970)).
(3) We have dlscovered that one should determine those
compositions,Cl, which exhibit an e~uilibrium volume fraction `
of about 50~ at least at some heat treatment temperature. We
i~` .,
shall denote this temperature by To~Cl). The method ~or l~
determination of this temperature is as follows: ` ;
~a) Select three ~or more) temperatures, say Tl,
T2 and T3 (about 50 apart from each other) such that ~ ~-
1. . ..
T3<T2<Tl<TP (Cl)
Carry out long heat treatments on glass samples until theyl;l
:.'- .
turn white at temperatures Tl, T2 and T3. ;
"' "` '~.
- 24 - ;
.; . .:
1~i335Z ~:
(b) by electron microscopy of these heat-treated
samples measure volume fractions, V(T), of one of the phases
~the same phase should be selected for all samples) for each
of the samples.
(c) Make a plot of volume fraction V(T) against
heat treatment temperature, T. By interpolation (or extra-
polation) determine the temperature for which the volume
fraction will be 50 (~5)~ (i.e., temperature To(Cl)).
(4) Knowing Tp(C) and To(Cl), one should determine the
composition range (C2) within the range Cl such that
(a) 575C ~ To (C2) ~ 500C and
(b) 750C ~ Tp (C2) ~ 600C.
These temperatures are selected so as to give not over long
heat treatment times; other ranges can of course be selected
if one is willing to accept long heat treatment times of a
week or more. `
(5) The composition range, C2, is further narrowed
by the requirement that, during melting, it should be easily
refinable. This demands that the high temperature viscosity
of the melt should be sufficiently low. We shall call this
sub-range of C2, C3 ~i.e., all glasses belonging to C~ which,
in addition, refine properly), We have found for example
that one ~onvenient feature identifying some of the refinable
glasses is that those conta~ing at least 2~% B2O3 refine
satisfactorily.
~ 6) Not all compositions of C3 are desirable even though
they will refine easily, and will phase separate with 50~
volume fraction. An additional requirement is that the de- -
,~ .,,
- 25 -
...
1~i3352
sired composition should not phase separate significantly du-
ring forming operation. The degree of phase separation in the
forming process is influenced by the viscosity characterics of
the glass in the region at and below the co-existence temperatu-
re, and also the dimensions o the article being formed, and ra- -
te at which it can be cooled.
In order to determine those compositions, C4, which
do not appreciably phase separate upon cooling through and be~
low the co-existence temperature, articles with dimensions of
the order of the size of preforms useful for further drawing
into optical waveguides are cooled at rates sufficiently low to
prevent the build-up of large thermal stresses. The degree of
phase separation occurring within these articles can then be de-
termined. Those compositions within the area C3 which do not `
phase separate in this forming process can then be grouped in
the further restricted area C4. The compositions fulilling
this condition preEerably have a Tp between 710 and 600C, most
preferably between 695 and 640C.
(7) The final criterion we applv insures that thereis sufficient composition difference between the two phases when
separated that leaching will take place effectively. For this
purpose we select onlv those compositions C which within the C4
range satisfy the condition that `
* *
Tp(C ) - TotC ) > 75 C
Havin~ found the desired composition range C , any composition,
Co, can be selected within it. The suitable heat treatment tem-
perature and time for this particular composition Co can then be
found by the following procedure:
,
- 26 -
1~i3352
(2) The heat treatment temperature of Co is set
equal to To(Co), i.e., the temperature at which the volume
fraction of the two phases will be equal. If C* has been
properly chosen according to the above criteria, this tempera-
ture will not be so low that the time needed to obtain a ~
suitable microstructure for leaching would be too long and -
uneconomic. Similarly, it will not be too high otherwise
[~] distortion of the ~lass may occur during
.....
heat treatment; ~
,,,
~2] if the temperature of heat treatment is
well above say, over 160 centigrade degrees above the glass ~ -
"
transition temperature, phase separation tends to be rapid
which reduces the degree of control on phase sepaxation.
. :.
These requirements limit our preferred heat treatment tempera-
ture, TH, to the following range
575C ~ TH(Co) - To(Co) ~ 500C
(b) Having found TH(Co), the heat treatment i5 de-
termined by the c~ndition that A microstructure state suitable
for leachlng is developed.
Heat treatments at different times are carried out say
tl~l hour) ~ t2 ~2 hours) ~ t3 ~3 hours)..... By electron
microscopy, it is possible to determine the timè, tmax, beyond
which the interconnectivity of the structure begins to break
down. The size of the leachable phase is measured from micro-
graphs, and the preferred heat treatment times are those which
are less than tmaX but for which the microstructure size is
at least 150 A, and preferably less than 300 A.
- 27 -
,~ I .
~335Z
We have applied the above criteria within the alkali
borosilicate system and have identified certaln characteris-
tics of the composition ranges which contribute to good yields :~
of the fiber optic preforms which are at least 2 mm in diameter,
avoiding the problems arising from, e.g., phase separation .~ -
.during forming, or insufficient .. -
phase separation when the phase separation stage is being ~.:
carried out. Phase separation during forming of the glass ~;
article from the melt, and insuficient.phase separa'cion or
breakdown of the interconnectOive structure during the phase
separation heat treatment can both cause or contribute to .
: .
cracking during either or both of the following steps, leach~
ing of the phase separated glass, and drying of the leached ..
. ~ and stuffed glass.
It has become clear to us that the compositions as- 1
sociated with the best yields are those contained within the .. :
; . . :
following broad composition area (all percentages being in . ; :.
mol percent): :.
Broad Preerred
Si02 48-64 49.5 - 59 . .
B203 28-42 33-37
2 6.5 - 8
A123 0~3 0-2.0 ~ .
p 0-1.0 0.20-0.8 :
a 0-3 0-2.4 .
A . 0-0-5 ~ :
x 0.1 - 1.0 0.2 - 0.8 ~
;,
- 28 - .
. :.
".
3352
~he~e a i5 ~he A~03 concentration in mole percen~,
x = p ~1/3~_~ and p is defined as the ratio A20~R20 for A20
and R20 in mole percent, A20 is the sum of the concentrations ....
in mole percent of K20, Rb20 and Cs20; and R20 is the sum of .
the concentrations in mole percent of Li20, Na20, K20, ~b20,
and Cs20; and A is the ratio Li20/R20.
Because of the presence of A1203 in the glass significantl~ . .
affects the results, we will first discuss glasses which have
: no A1203 content. Under these conditions r the ranges listed .. ::
above are appropriate with A1203 content o:E zero, with R20 .-.
the sum of all the alkali metal oxides Li20, Na20, R20, Rb20
.
Cs20 and the broad range for p limited between o.l and 1Ø
If the concentration of K20 is zero, then the upper limit of
the range for p should be 0.8.
Lithium glasses tend to..devi.t~ and therefore it .is
often preferable not to use that chemical. In this case, R20
becomes the sum of the concentrations of Na20, K20, Rb20 and
Cs 0. All limits and conditions above are maintained.
: 2
.
Rubidium and cesium glasses are more expensive than those .
made with sodium and potassium. ~hey can be left out ~or
economLc reasons. Then R20 becomes the sum of Na20 and K20.
All limits and conditions above are maintained.~
When more than 0.5 mole percent A1203 is present in the
glass, the broad range of R20 is taken between 6 and 9 mole
percent.
The most economically favorable compositions with A1203
consist of R20 having Na20 and K20 only, or R20 can also con-
sist of Na20 only.
~ - 29 -
.; .
``:` ~ i
i63352 1 '':
The glasses ~elo~ in Tahle III ~re ~lasses ~hich ~e have ~ ' :
identified using the above criteri~ and found satisfactory fori'
1. .
use in the molecular stu~ing process of the present invention ~ .
as we achieve a satisfactory contxol of phase separation and
pore structure after leachin~ using these compositions, and a i`~
good overall yield of finished product of the invention. : -
'~'.~' .:
"' ' ' ~ '
' . :,~ '
.' ' ' ':.
.
~ ' ' ''
'~'' ,``
- 30 -
f ~ .
. ,. . ~ .
1063352
. .
:
. .:
~I '~' '
Q~ d' I 1-- ~1 oo oo I 1-- ~1 oo . :
~C I ~ a~ O ~, " "
o O o O O ~ O O O
OO O OO O '.
~ o '.' .'
OOoOOOOO~100
~ o c~ n ~P ~ o
,Ioooooooooo. o ..
a o I O
o
~.v ~1ooooooo~oooo ",,
E~ ~cl ~ I o o, o o o o ~ o o o o o
:
.
o ~ ~ ~ O~ ~ , ;
O ~ ~ ~ ~ O O C~ ~ O
Z ~ I ~ ~1 U)d~ U~ ~ ~ CO
o I ' , . ~`, .
0 5~ ) ~ d'
~ .
O ~ l O ~ O`~
.~ ~D ~ W ~ U) Il') LnLl~ 11'1 LO 11^~ Ul
U~
H H H p p H H H X X H H
H t-l H . P H H H ~ H
p .
-- 31 --
- . . .. . : ~ . ~ ~ ....... . .
;: ~
1~;3352 :::
i '; '
~. . .
,`:
:~ ` .; .: .
.
Another aspect of our invention invol~es leaching of ~`
~; borosilicate phase separable glasses. We pref~ar be~ore leach-
ing the glass to etch the article to be leached with dilute
hydrofluoric acid for about 10 seconds to remove any surface
oontamination, or surfaae layer of glass having a slightly
different~composition from the interior due to vol~a~i~lzation
of components such ~as B2O3or Na2O during ~ormation.
~; The concentration of the acid solution, amount of leach-
ing solution and temperature o~ leaching have a direct bear-
ing on the progress of leaching. It is essential to insure ` ~`
that a sufficient quantity of the leachin~ solution is brought
into contact with the article to dissolve the soluble phase.
The rate of leaching~may be conveniently controlled by ad-
~usting the temperature. The glass should be above 80C,
preferably above 90C. As has previously been described in
, . .
U.K. 442,526, it is desirable
' :
- 32 -
., : ' . '
1~3352
to use an acid solution which has been sat~rated witll NH4Cl
or other equivalent compounds capable of reducing the concen-
tration of water in the acid leaching solution. This assists
in controlling any swelling of the treated layer and reduces
considerably the chances of loss due to cracking of the article,
as the inner untreated layer goes into tension because of the
thickness of the swollen outer layer. `
We have found that the rate of leaching, and the re-
deposition of borates in the pores of the glass during leach-
ing can be controlled by controlling the concentration of
borate salts in the acid leaching solution.
We have measured leaching rates at 95C for glass rods
(length lO cm, diameter 8 mm, and compOsition 57% SiO2, 35%
B2O3, 4% Na2O and 4% K2O) heat treated for 1 1/2 hours at
55QC with leaching solutions containing ~27.3 gm of NH4Cl,
33.6 ml of HCl per liter of water and varying amounts of
B203. We ound that leaching time increased witll increasing
B2O3 content in the leaching solution. The results are
summarized below:
TABLE ~V
Amount o~ Boric Acid ~g/llter) Leaching Time(minutes)
O
4~.~ 625 -
61.5 642 + 50
84.7 725 ~ 50
106.1 1670 + 50
We believe redeposition of borates in the pore5 also contri-
butes to rod breakage. This can be avoided by e.g.~eplacing
~ - 33 -
1~i335Z `: ~:
the leachin~ s~l~tion ~s t~e concen~tion of bo~ate builds ~ l
up. But this re~uires lar~e quantities o~ leachin~ solution.
For example, in order fo~ leachin~ time to be no more than
660 minutes, the volume of leaching solution per 100 ml of
~la~-~ will be on the order of 1550 ml. This, however, can
increase costs and provide a possible source of contamination.
We find it more convenient to provide a cold trap so that
excess material is continuously removed from tlle solution as
it comes into solution from the article being leached. The
cold trap is effective in speeding the process even if it is
only a few degrees below the temperature of the glass articIe.
Preferably it should be 20C below the temperature of the
glass article. We fin~ it convenient when NH4Cl is present to
choose a temperature for the cold trap at which the acid solu-
tion remains saturated with NH4Cl. It is possible to operate
with a low level of rod breakage without NH4C1 or other equi- ; ;
valent compounds present in the leaching solution. In general
we pre~er to use at least 10 weight percent NH4Cl, preferably l~
2Q weiqht percent as we find that on a statistical basis there 1 -
is an even lower level of rod breakage when NH4Cl is present.
The most convenient way to determine a suitable leach- ~;
': "
ing time is to take an article and subject it to th~ leaching
treatment measuring the mass of the article at intervals of ~`
time until little or no further weight loss is observed.
.
The article, once leached, is conveniently washed with
deionized water, With certain compDsitions there can be de-
position o~ silica gel in the pores, and we find this can be
removed by washing with NaOH. We have ~ound it possible by
I
selection of compositions to minimiæe this deposition. The
compositions shown in Table III alleviate this problem,
especially those with minimum silica. - ~-
,`::
'''' ':
. '; :":','
- 34 -
''
335Z
Once the porous matrix has been produced, elther from a
phase-separable glass as outlined above, or by, e.g., a
chemical vapor deposition technique, the selection of suitable
conditions for stuffing and unstuffing we have found can be
made by foll~ing the guidelines given below. - ,
Using well-known formulas for optical waveguides, the
desired ph~sical propert~es of the waveguide (such as size,
numerical aperture, band pass, etc.) ~can be related to a varia-
tion of index as a function of its dimensions. The depen-
.
dence of refractive index on dopant and dopant Compound concentration
can be determined by literature search or by suitable experi- ~`
ments. From these, the maximum concentration of dopant or
dopant compound needed in the article is determined. Sufficient
dopant or dopand compound must then be dissolved in the stuf- ;
fing solution so that the desired concentration is reached at
a particular stuffing temperature and time of stuffing. The
following procedure enables these parameters to be determined:
tl~ Determination of Stuffing Temperature of Porous Rod
(a~ Determine the dependence of the 501ubility of the do-
pant or dopant compound in the appropriate solvent on tem-
perature.
(b) The stuffing temperature range lies between the ~em-
perature at which the desired concentration of dopant or
dopant compound is saturated in solution from (a) and the
boiling temperature of the solution.
~2) Determination of Stuffing Time of Porous Rods
The stuffing time depends not only on the concentration,
temperature, and the composition of dopant solution, but also
.
~ - 35 -
.
i335~
on the microstructure si~e in the porous rod. The l?rocedure
given here is for a given set of dopant solution, tempera-
ture and microstructure of porous rods. For a change in -
any of these variables, the procedure should be repeated,
or suitably modified according to our guidelines.
~ a) Measure the diameter (aO) of a porous rod and im-
merse it in the dopant solution.
(b) Monitor its weight as a function of time.
(c) Determine the time, to, beyond whicll the weight
does not increase significantly by plotting the ractional
weight change, y~t) = [M(t) - M(o)]/~M(~) - M(o)} versus
time, t, where M(o~, M(t), and M(~) are the respective weights
.1 , .:
initially, at time t and at infinity (very long times).
(d) Time required to stuff, t, another p~rous rod of !~ :
diameter a, with the same dopant solution at the same tempera-
ture is
` t = to [ aa ]
; Example
' : . ' `'' '
We stuffed a porous rod with a concentrated solution of
CSNO3 in water (120 g CsNO3 per 100 ml of solution) at 100C.
~he radius of the rod was 0.42 cm. We measured the wei~ht
,. .
gain as a function of time. The results are shown in Fig. 1.
It can be;seen that after about 200 minutes the weight of the
rod does not increase significantly. Thus, the proper stuf-
~: .. .:
fing time for this rod is about four hours. ~`
......:, ',:
- .
.: .
. ';' .
- 36 -
-, . ... .. .. . . . i , .. i, .. . ...
1~3352 ~ ` ~
~3) Determination of Unstuffin~ Time to Produce a Parabolic
Profile in a Poxous Rod by Thermal Precipitation
~ o produce a parabolic profile, the stuffed rod, pro-
duced as (2) above, is partially unstuffed by immersing it in
the solvent. This should be done at a temperature where the
dopant does not precipitate. The unstufing time depends on ~-
the temperature as well as on the microstructure of the
porous rod. The procedure described here is for a given tem~
perature of stuffing and microstructure.
(a) Carry out an unstuffing study at the temperature at
which the rod was stuffed by monitoring the wei~ht change
~; as a function of time while the rod is immersed in solvent.
(b) Plot the fractional change y(t)
y(t) = M(()) ((o) (1)
against time t.
(c) The time of unstuff~ng, to, depends on the desired
profile; often it is
: ~ ~ 1/3~ y(tO) ~ 2/3
Example
We chose a porous rod stuffed with concentrated CsN03
solution tL20 g CsNO3 per 100 ml solution) at 100C as described
above. We then unstuffed the rod in water at 100C monitor-
ing its weight loss as a function of time. The results are
shown in Fig. 2. The range of unstuffing times can be calcu-
lated from the graph.
.
' ;~ ~ 1
- 37 -
10~;335;Z` :
(~) Determination of Unstuffing TempeJ.ature and Time to ~ `
Produce a Step Profile by Thermal PrPcipitation
The temperature for unstuffing for a step index-type `~ -
fiber depends on the numerical aperture desired in the fiber
and on the dopant solution. Since one would like to have ;
as low a refractive index in the clad as possible, the un-
stuffing temperature is typically a few degrees above the
freezing point of the dopant solution,
The time required for unstuffing depends on the desired s
clad thickness, as well as onosuch parameters as temperature
of unstuffing, the concentration of the stuffing solution
pxeviously used and the size of microstructure in the porous `;
rod. The procedure described here is for a given set of
these variables. In case of a change in the values of any
of these parameters, the entirQ procedure should be re-
peated, and adjusted accordiny to the guideli.nes herein.
Suppose the desired clad thickness is "d" and the radius
of the stuffed rod is a~d). Let
; y - 2 d - (a)
Knowing Y, it is possib'le to determine tlle proper unstuffing
time by following the procedure described here.
(a) Carry out an unstuffing study in the desired solution
at the desired temperature by monitoring the fractional weight
change ytt) as a function of time.
~ b~ Plot the fractional weight change against time,
~see eq. 1) as shown in Fig. 3.
~ c) Find time to for which
y~tO) = Y, ~ ~
- 38 -
1~i335~ ` :
from the above plot. This is the desired unstuffing time.
The practical application of the use of the unstuffing
proeedure is in most cases to reduce the concentration of ;
dopant in the outer layers of the article so as to attain a
desired refractive index profile.
This can be done as indicated above by, e.g., when the
actual stuffing has been completed with a saturated solution ! ;~ -
of a dopant at ~5C, replacing the dopant sol~tion by the
solvent used free of dopant at the same temperature, or whel-e
the system is aqueous, water or dilute nitric acid~ The do-
pant then tends to diffuse outwards, thus varying the concen-
tration through the cross-sectional area of the porous matrix.
The time required for this"u~stuffing"is of course dependent upon
the volume being treated,but a rod of diameter 8 mm requixes
about 20 to 30 minutes. We prefer to stop the unstuffing by
replacing the liquid surrounding the rod with~cold solvent,
or in the case of an aqueous system, water at a temperature
approaching freezing point or ice-cold nitric acid. In the
case of an aqueous system we have also found it possible to
cor.trol the end point bv measuring the change in conductivi~y
o the water being used to unstuf.
(5) Drying, i.e., Remova~ of Solvent
Two problems occur in drying which affect the economics
of the process and the quality of the product. These are
cracking of the porous glass structure and changes in the
dopant concentration profile. Cracking is a statistical pro-
blem and it is possible to have samples survi~e the process
regardless of the drying procedure. However, in order to
:.
~ ; .
i335Z ::
, ~ ..
oL~e~ate on a commercial scale, it is necessary to adopt
a procedure which minimizes cracking and thus improves the
economics of the process. Such a procedure should also pre-
ferably avoid profiles being ~ltered in such a way that do-
pant is transferred from the interior of the article toward
the surface as this is not usually a desirable profile. This
results in a depression of in~ex in the center and an increase
near the edges as illustrated in Example VII. As indicated
above, the profile obtained is dependent on the unstuffing.
Having achieved a suitable pro~ile with solvent still present
in the porous structure, it is essential to dry, i.e., re-
move sQlvent in a way which will not alter that profile t~ an
undesirable state.
In an analysis of the drying process we have found that
a number of events affect the process. These are:
,: .
(a) Gas evolution. The sources of gases can be the
solvent, dopant decomposition products and dissolved gases.
If the gas evolution is too fast because of rapid heating or
insuffioient gas removal the r~sulting differential pxes-
sures in the pores can break the glass and/ox ca.rry dopant
from inside of the article.
(b~ Size change. As the bulk of the solvent is re-
moved from the porous glass, the surface layer may remain
chemically bonded to the glass. We have observed this effect
with water and found the layer to persist up to high tempera-
tures. This may also occur with other solvents. As this
layer is removed, the sample shrinks. With sufficient
shrinkage difference across the porous structure stresses
. . .
~ ~ '`\ :
~;3352
can be developed to the point of cracking. -
(c) Dopant compound decomposition.The dopant as available
in solution is generally a compound wh~ch thermally decomposes.
We have chosen dopant compounds which decompose before the
collapsing temperature. This decomposition step is generally
accompanied by a large evolution of gases. It is generally
desirable to control the heating rate while going through ~he
,
temperature range where decomposition occurs in order to pre-
vent cracking and transport of dopant.
(d) Mass transport can occur at se~eral points in the
drying process. When the article is dried initially, dopant
which remains in solution can be transported to the surface and
deposited there as the solvent is evaporated. If the solvent
evaporates violently or boils even precipitated dopant can be
displaced. After decomposition, i~ the dopant crystals are
small, they may be carried through the gas phase. If the do- ,
pant has a significant vapor pressure, dopant redistribution
through~the vapor phase may occur. If dopant becomes a liquid
it may redistribute according to gravity. `,
(e) Hydroxyl ion removal. Hydroxyl ions form absorption
bands in the near infrared which oten are detrimental to use
as a waveguide. I one w~nts to remove the hydroxyl ion
because of this or any other reason, the following complications
arise. A significant amount of hydroxyl ions are found to be
entrapped in the glass and can only be removed during prolonged
heat treatments at high temperatures. However, in the same
temperature range collapse begins to occur trapping hydroxyl
ions in the glass.
- 41 ~
: ,: '
:... .:
1~;33S2
Outlined b~low is a pre~erred process for the suitable
solution of these problems. The initial bul~ removal of the
solvent has to be performed by the use of conditions where ;~
boiling does not occur; in the case ofaqueous solutions, we
have used two procedures. One consists o initially drying
the porous glass articles with precipitated dopant in a des-
sicator (at atmospheric pressure) or 24 hours at 22C and
then placing them in a drying o~en. The second consists of
placing the article under vacuum at temperatures below 10C -
and above the freezing point of the solution. We have found
4C for 24 hours to 1~e convenient when using CsNO3 in aqueous --
solution as a dopant. In order to minimize the chances of
cracking even further, we find that when using aqueous sol-
vents, it is convenient to subject the article to a final wash
with a non-aqueous solvent which is non-reactive with the glass.
We believe this can assist in removing hydroxyl ions from the
structure. An example of a suitable solvent is methyl alcohol.
We have ound it preferable to warm the articles wllich
have been maintained below 10C under vacuum slowly to room
temperature and to maintain under vacuum at room temperature
;
conveniently for about 24 hours before the articles are trans~-
ferred to a drying oven.
In the case o non-aqueous solutions of dopants, we have
ound it suitable to place the articles under vacuum at room
temperature for 24 hours and then transfer to a drying oven.
~his significantly speeds up the proaess as compared to an
aqueous process.
In the drying oven, we have found it desirable to heat
the samples to the upper drying temperature under vacuum at
- 42 -
~;3352
a rate below 30 C/hour, Pre~erabl~ 15C/hour, since a slow hea-
ting rate significantly lowers the cracking probabilit~ and a-
voids dopant redistribution.
The selection o~ a suitable slow heating rate will be
dependent on the economics of the process. It may in some cir-
cumstances be cost effective to accept a higher breakage rate in
order to increase the rate of throughput of articles through a
processing system. However, an~ increase in heating rate must
also be balanced against the increased risk o~ destroying the
desired refractive index profile in the articles which are not
cracked. Example VII shows that at least with the dopant used
in that example, this problem occurs at a heating rate o 50C/
hour.
The upper drving temperature depends upon the porous
glass matrix. A suitable value can be found by first collapsing
an undoped article and measuring its glass transition temperatu-
re, Tg. The upper drying temperature is then pre~erably chosen
to be in the range between 50 and 150C belo~ the glass transi-
tion temperature. We prefer to use a narrower range of 75 to
1~5C.
The next stage o~ drying consists o~ holding the glass
at or about this upper drying temperature ~or periods of 5 to
200 hrs., pre~erably 40 - 125 hours. In this period, the glass
may be held under vacuum or under a selected gas at atmospheric
pressure. We have found it desirable to pass gas around the ar- f.~"' ,''"
ticle since this helps the drying process. It should be noted
that whatever the choice o~ drying conditions during the holding ;
time, it is desirable to expose the sample to oxidizing condi- ~
tions ~ ;
' ~-.".'':;''
; :'' ",'
- 43 - ~
I', ', ', ,, , .. ,.,.. . " ., I' , ,,'. .' ., ' . ",, . . ,:, ",. .~, . . , . " , ,. .'":, ., .:, . . ' ,.. ' " . "' . '
', , ' . . ' . , ' ', , . . ', "', ., ., .", , ,,.,, . " 1 , .,, ,,', ,' ., . . ,"~ ,. ", ~ ", ,," '. , . ' ,.,~,. , ;"
~ ~ ~ 6 33 5 2 ~'
i~ one wants to l~ cr the nea~ infra~rea absorpt:ion and there
are residual iron impurities in tl~e glass. This oxidizing -:
stage reduces the Fe /Fe ratio in the glass,thus lower-
ing the absorption by Fe ions. In ou~ preferred procedure,
we heat treat a porous ~lass article having a T of 725C at
625C (100C below the glass transition) for 96 hours while ~ ;
passing dry oxygen gas around the sample.
(6) Consolidation
Once the above drying process is complete, the article
is now ready to be collapsed. The article is raised rapidly
in temperature to the point where consolidation occurs. Once
the pores are collapsed,
consolidation is complete and the article may be cooled back
to room temperature. The consolidation step must be conduc-
ted at atmospheric pressure or below if the article is to be
further worked by reheating above the consolidation tempera-
ture otherwise some gas evolution is likely to occur in re-
heating and bubbles are formed. '`
We have found it desirable where the matrix is produced
from a phase-separable glass to heat the porous glass samples
.. , j.
under a reduced pressure of oxygen (approximately l~S bar)
up to 825C where consolidation oacurs. ~ !
The following examples illustrate the molecular stuffing
a~spect of the invention but do not limit the invention. ~ ~
., . :.
",'
'~ :
- 44 -
~ i3352
Examples I to V illustrate'the'use of v~rious concentrations
of do~ant in aqueous solutions in treating a porous matrix '
which result in a glass on consolidation with differing ,
overall concentrations of dopant. The general procedure used
for producing the porous matrix from a phase-separable glass
and the subsequent treatment are shown in the paragraphs below '
and the actual numbered examples i1lustrate the use of dif- '~
'ferent dopants at a range of concentration, collapsing tem- '
peratures and final averall glass composition.
Melting and Forming
A glass having the composition in mol%; 4 NaO~, " ,
4 K2O, 36 B2O3, 56 SiO2 was melted and stirred to produce a
homogeneous melt from which rods were drawn having a diameter
in the range 0.7 to''0.8 cm. -
Heat Treatment using a Cooling Coil
The drawn rods were heat treated at 550C for two hours
to oause phase-separation. `,
' Etchin~ before Leaching
Each rod was etched for 1~ seconds in 5% IIF followed by ';
a 30 second wash in water. ,''
Leaching , .. .
, The rods were leached at 95C with 3N HCl containing 20~ ''
. :: . .:
NH4Cl by weight, the time being chosen on the basis of pre- '
,.:
vious trials so as to reach a stage where the rate of weight ,', ' '
: -: .:: : : .
loss has dr,opped to almost nil. The leaching time of the rods ~ '
used ln these examples was chosen to be in excess oE, 30 hours. , , "
During leaching,by providing a cold spot at 40C, the boric
acid concentration in the leaching agent was kept below , '
,:
- 45 -
. ~ . ~ , .
,.
:
.
3352
50 g/liter, this speeding up le~ching ànd avoiding possible `
re-deposition of boron compounds in the pores of the matrix.
i... ..
40C is chosen so that there is no precipitation of ~H4Cl from
the leaching solu~ion as this temperature is above the sa-
turation temperature of the NH4Cl present to maintain a suit- i~
able amount in solution to reduce breakage drastically.
Washing
The leached materlal is washed with de-ionized water. ~`
The washing cycle is conveniently controlled by determining `~
the concentration of iron in the ef~luent. Washing is con-
veniently carried out at room temperature using 10 volumes
of water to 1 volume of glass. We prefer in a non-continuous
process to change the water about 6 to 8 times, giviny a
washing time of about 3 days; at each change the iron concen- ~
tration is reduced to l/lOth of its concentration at the ~`
time of addition of ~resh washing water, and in this way one
can assume without measuring the iron content that a sufficient-
ly low level has been achieved during washing.
Stu ~ing
With aqueous solutions of dopants (see ~xamples I to VI
below) w~ prefer to move smoothly from the last washing stage
to ctuffing by simply replaaing the water by the stuf~ing
solution. This is done by draining the water from the last
wash and filling the tube containing the porous rod with
stuffing solution.
In Examples I through IV, the samples were removed ~rom
the stuffing solution and cooled to 22C where the dopant
precipitated partially with an amount equivalent to its
4 6 ~ r
~. ~
~0~33S2
aqueous solubility at 22 C remaining in solution in the liquid
filling the pores. (For exalnple, l0 g ~a(NO312~l00 ml
solution rernained dissolved in water in the pores a~ter thermal
precipitation t 22C. Similarly, 6 g H3BO3/l00 ml solution --
remained dissolved in water in the pores.) The remainder of
the dopant was precipitated during the drying procedure which
was commenced by placing the porous article in an atmospheric `
pressure dessicator for 24 hours at 22C. Drying was then
continued under vacuu.n in a furnace whose temperature was
raised at 15C/hour to the upper drying temperature (as
defined above~. This is determined in the manner described
above and for the samples used in Examples I to VIII was 625C.
',.: '.
Hold Time
~ . .
The rods were then held at atemperature of 625C ~or ;
96 hrs. while passing dry oxygen gas around the rods.
. ',: '
Consolidation
,
On completion of the hold time, the article is ready
to be collapsed and is raised rapidly to a temperature where
collapsing takes place and a consolidated rod is produced.
This step is carried out under a reduced pressure of oxygen
~approximately l/5 bar) and the ~inal temperature is given ;
in each example.
:
,~' '
- 47 -
: ~ :
.. .. : . ( . . . ~
` 1(7~3352 :;
EXAMPLE I ;
Mol'ec'u'l'ar 'Stu-f~'in-g wi'th BaO
."
Dopant Ba(N03)2 in Water ,
Dopant Stuf'fi'ng ' Details C'on'soli- C'omposition Wt % j~ '
gms/lOOcc Time: Temp.UC dationMole % ~-
Rod of H20 Hours Temp.UC
1 12 4 85 820 6.0 B203 6.86
93.11 5iO2 91.10 '-
.82 BaO 2.04
2 18 4 85 820 6.0 B203 6.79 "'
92.73 SiO2 90.18
1.22 BaO 3.03
3 24 4 85 830 6.00 B203 6.72 ; ~ '
92.35 SiO2 89.28 ,
1.62 BaO 4.01 ' ,
'.''',".
EXAMPLE II
.,: , . .
, MoIecular Stuffing with B20
~; ~ D,opant H3B03 in Water '
!
. "'
' Do~ Stufing Details Consoli- Composition Wt %
Rod gms/lOOcc ~lme Temp.UC datlon Mole %
of H20 Hou'rs Temp.C ;'
Compa- -
rison
Rod O - - 820
93.88 SiO2 ~3
.
4 12 4 85 815 7.73 B203 8.83 ,,,~,
92.27 SiO2 91.17 ',' `',,
18 4 85 815 8.51 B203 9.71 , ,,
91.49 SiO2 90.29 ~, '
6 24 4 85 810 9.28 B203 10-58 ,'
90.72 SiO2 89.42 '''
,
..
- 48 - '
1~3352
~XA~PI!E III
Molecular ~tuffing ~i~h PbO ~ B2O3
Dopant Pb (NO3) 2 and H3BO3 in Water
- , ~
Rod ~ 7 Doped with 40 gms Pb(NO3)2 and 10 gms
H3BO3 per 100 cc of H2O at 85C for t',''' ' ':
1~ hours. Collapsed at 825C.
EXAMPLE IV
:-... ...
Molecular Stuffing with BaO ~ B2O3 :
Dopant Ba (NO3) 2 and H3BO3 in Water
: ,.
Rod ~8 Doped with 12 gms Ba(NO3)2 ; i
and 6 gms~H3BO3per 100 cc of H2O at 85C
for 4 hrs. Co}lapsed at 830C. :
EXAMPLE V
The above examples all xelate to uniform doping of the
rod. As described above, it is possible once a rod has been
left for suf~icient tlme to diffuse the dopant solution into
the pores' to then reduce the concentration in the outermost
part of the rod so as to achieve a reractive index profile
in the collapsed rod. æwO porous rods were produced by
the pxocedure outLined above, and were immersed for more than
four hours at 95C in an aqueous solution of CsNO3 with a
.
concentration of 120 g CsNO3/100 ml solution. The rods were
then transierred to water at 95C, and left in the water for
~ - 49 -
,~-,. .. . .
' ' ' .
~ i3352
pe:r7 ods OI 11, and 2 0 minutes respective].y . ~ach rod at the
expiration of .~he time in water was i~ersed in watel- at 0C
for 10 minutes to cause thermal precipitation of CSNO3. The
rods were then treated to remove solvent and collapsed in the
.. . . . ;~ , ,
manner' descr'ibed above:' The'ref'ractlve'index profiles ob-
tained are shown in Fig. 4.
EXAMPLE VI
Several porous rods produced by the method described above ~ '
were immersed in a series of solutions of CsNO3 and Cs2CO3 as
i per Table V below for more than four hours at ~5C. These were
then unstuffed to produce a step profile. (The profile for
rod #13 is sho~m in Fig. 5.) The time required for unstuffin~
was determined using Fig. 3 (i.e., for rod ~13 the time for which
y~tO~ = 0;50 is 300 min.)'. The stuffed rods were unstuffecl by ''
immersion in ice water for 300 min. and the water was removed
and rods were colIapsed as described above. The resulting in- ~ '
dex of refraction in the center of the rod i~ listed in Table V.
~,, .
TABLE V Index
Rod Number Stuffing Solution at center of rod '
9 20 grs Cs2CO3/lOOml H2O 1.462
30 grs Cs2CO3/l~Q~1 ~12O - 1.475
11 75 grs Cs2CO3/lOOml H2O 1.~48g
; 1260 grs CsNO3/lOOml aqueous solution 1.475
; 13120 grs CsNO3/lOOml aqueous solution 1.~8
' EXAMPLE VII
;:, ' .
As discussed previously, we emphasized the importanae of `'
drying at a slow rate between temperatures near 0C and 600C ''
when using rods stuffed with dopants. Here we show an example
of d~fferences in index distributions due to different heating
rates. Several stufed porous rods as used in Example V were
unstued to produce stepped'profiles as in Example VI. After
thermal precipitation, the rods were dried under vacuum at 4C
for 24 hours. They were then heated under vacuum at rates of
50,30 and 15C~hr respectively. The resulting index profiles -
are shown in Fig. 5. Heating at rates above 100C/hr caused
appreci'able breakage in rods. Thus, the preferred heating rate
is below 20C/hrl when one does not want to alter the precipi-
.,. . ~ ~ . . ..
- 50 -
. . . .
.
~ i33~;2
tated profile.
The rod heated at 15C/hr was selected for fiber pul- ~
.,: - ' .
ling. Tha rod preform is fed into the opposing flames of two gas
oxygen torches and the melted tip of the preform is drawn manual-
ly into a fiber whose end is attached to a revolving drum which
draws the fiber down to a 185 ~ diameter. The drawn fiber is il-
luminated with a white light source with a spectral region selec-
ted by a 100 A wide interference filter with transmission cente-
red at 0.85 ~. An absorbent material is placed in contact with
the fiber clad along a sufficient length to remove cladding mo-
des. The transmission intensity I(l) is measured on a long fiber ;
segment. All but 1 meter of the fiber is then removed and the
transmission is again measured. The loss in dB/km is given by i
Loss (dB/km) ~ 1 log I~ -2r
where 1 2 12 - ll and 12 and ll are the lengths in km of the long
and short fiber segments respectively. It was found to be 25 ` ;
dB/km, which is less than 100 dB/km required in many communica~
tion applications.
EXAMPLE VIII
After leaching and washing as described above in the
introduction to Examples I-IV, a stuffed porous rod is immersed `
for four hours in a solution aontaining 120 g of Cs(N03)~100 cc
of solution at 95C. The sample is cooled to 22C where the do-
pant i~ partially precipi~ated. It is then dried in a dessicator
for 24 hours at room temperature. The sample is uniformly stuf-
fed and in order to introduce a profile, it is then washed at 4C
in water for two hours and then in 3N HN03 for 30 minutes. This
is followed by drying in a vacuum at the same temperature. Once
the bulk of the water is removed, it is slowly dried by progres~
sively raising the temperature as described in our preferred pro-
cedure. At intermediate temperatures, the CsN03 decomposes into
- 51 -
i33S2
Cs20 and various nitrogen oxides. When the sample changes from
white to clear, the consolidation is complete and the sample is
removed from the furnace. The composition of the final article
is 90.6% SiO2, 3.4% s203 and 6.0~ Cs20 by mole, and has less than
10 p.p.m. of transition metal impurities.
EX~MPLE IX
: :,
As indicated above, it is possible to vary widely the
choice of dopant, solvent and conditions of operation during
. .
stuffing and unstuffing and the combinations and permutations of
these parameters in order to achieve a desired end result, or
modification of processing conditions. We have given guidance
. .
to the man practiced in the art; this example illustrates some of ;
the permutations and combinations we have found satisfactory.
The porous rods used were all produced by the general procedure
described above and solvent removal and heating carried out under
our preferred conditions.
The results obtained are given below in Table VI. The
columns in this table give the following information:
Column 1: ~opant used.
~ .. .
Column 2: Concentration of dopant/100 ml solution.
Column 3: Solvent, i.e., solvent used for initial stuffing
C71urn 4 ~ Temperature in C, and time taken for initial stuf- ;
fing. `~
Column 5: Solvent A--this is the solvent used to reduce the
concentration and produce refractive index varia-
tion, and also to cause precipitation of the do-
pant.
.~, . .
Column 6: Temperature in C and time taken for precipitation
and variation in refractive index profile.
Column 7: Solvent B is used where appropriate to adjust dopant
r 2
. .
~;33S2
distribution in matrix. By causing further preci- `
pitation before solvent removal begins so as to j ;
enable a more thorough decrease in dopant concen-
tration near glass surface.
Column 8: Temperakure in C and time taken for adiusting do-
pant distribution by further solvent treatment.
Column 9: Indicates temperature at which drying commenced in
C, and by "V" or l'A'I whether drying in vacuum (V)
or in dessicator at atmospheric pressure (A) for ~ -~
the first staye.
Column 10: Gives the index of refraction where measured. ;~
: .
In the table,
Line 1: is the same as Rod #13 in Example VI above and is
, . .:
included for comparison with line 2, where by in-
cluding a further treatment with methanol and water,
while using the same stuffing solution, the diffe- ` ;
rence in index is increased.
Line 3: demonstrates how by replacing one compound by ano-
. .
ther, in this case CsN03 by Cs2C03 because of higher ;
solubility, stuffing can be carried out at room
temperature.
Lines 4-9: illustrate the use of different dopan~ and solvent
combinatlons.
Lines 10 & 11: show the use of a mixture of dopants.
Line 12: illustrates the use of neodymium nitrate as a do-
pant, and of the use of an organic solvent.
i .
" ''.
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i3352
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335Z
..
Although presently preferred embodiments o~ the in~
vention have been shown and described with particularity, it . .
would be appreciated that various changes and modifications may
suggest themselves to those of ordinary skill in the art upon ,~
being apprised of the present invention. It is intended to en-
compass all such changes and modifications as fall within the .~ : .
'.~ ' '
scope ~nd spirit or the appended clairs. ~ `
.. ~ ~; ....
,
-'
;
`
,
- 56 -
