CA1150737A - Optical waveguide and method and compositions for producing same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
The present invention relates to a glass composition for forming glass articles suitable for forming or being conver-ted to articles or devices for use in the guided transmission of light, for both imaging and communications purposes. Base glass compositions are purified by phase separation, leached to remove impurities, and dopants are then deposited into interconnected pores of porous glass in such a fashion that properties of the glass are varied, e.g., for optical purposes, the index of re-fraction is controlled.

Description

3L~5~73~

This application is a division of Application No.
345,952 filed February 19, 1980.
This inventionrelates to novel glasscomposition and to glass articles particularly suitable for forming or being conver-ted to articles or devices for use in the guided transmission of light both ~or imaging and communication purposes. It is particu-larly suitable for forming glass articles which are subsequently drawn into glass fibers for use as optical fibers used to trans-mit and guide electromagnetic energy above 300 GHz; or for forming glass articles to be used to couple said electroma~netic energy between two or more optical fibers (or bundles o~ optical fibers), or couple said electromagnetic energy between optical sources or detectors of said electromagnetic energy; or for forming glass ar-ticles to be used as a part of an integrated optical device; or for forming a glass article to be used as an acti~e element in a laser or optical amplifier; or for forming a glass article to be used as a lens; or for forming a glass article 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 attenuation from the order of

2-5 dB/km to 100 and u~wards; Forintegrated optics applications, values of attenuation as high as 1 dB/cm are accep~able, though for purposes such as telecommunications, a levelof less than 5 dB/km is necessary. The term "glass article" as used in these specifica-tions includes articles which are to some extent crystalline.
In our previous U.S. Patent No. 3,938,974 issued Febru-~k ~lSl~737 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 uniform 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."
r~e have found that such a process is not only applica-ble to the stuffing of porous matric~s produced by the leaching o phase-separated glasses, but is also applicable to other in-ter-connective porous structures having a matrix constituted 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 dep~sition. 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 particularly, U.S. Patent

3,859,074 issued to P.C. Schultz describes the formation of a porous body and its subsequent impregnation with a dopant. Such impregnation is concerned with the deposition of small quantities of materials from relatively dilute solutions in the pores of the porous bodY~
In order to achieve a particular refractive index chan-~50737 ge and a particular refractive index profile, it is necessary to deposit relatively large quantities 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 se~uence 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 reractive index variation in the glass article. By this means one can produce an article su:itable for use, e.g., as an optical waveguide. If the refractive index is constant through-out the article, the article is suil:able for use as a core to be surrounded by a material of different lower refractive index as a cl~adding. The same effect can be produced in cross-section without a cladding by producing a stepped profile in an arti-cle. A "parabolic" proile is a term used to describe the situa tion where there is a refractive index gradient in any transver-se cross-section such that the index decreases "progressively"
or "continuously" from the central axis toward the periphery of the article. Various desirable profiles produced by other means have been described, e.g., in U.S. Patent 3,830,640 issued to Nippon Selfoc KX. In any process for 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 - ~lS(~ 7 this stage may decrease the economic yield of a full-scale com-mercial process. We have found it difficult to determine thè
factors causing losses either through breakage, due to cracking, or the occurence of light scattering centers such as includions or bubbles in the final collapsed article.
To obtain both increased yields and more control over refractive index profile consistently 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 subseauent pro-cessin~ 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, 1~5(~73~
. .

howevex, on a statistical basis and one cannot guarantee that even when all the essential steps of our process are used, an acceptable product will always be obtained. As indicated pre~
~iously, the improvement produced 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 discover~r that for best results it is essential that tlle step o~ depositing 'che solid material in tlle pores be carried out by a process which does no~ involve evaporation of solvent, and tha_ the su~sequen~
heating step to raise the temperature of the artiale so as to remove the solvent from the porecs~ an~ here necessary, decom~osition produc~s ~llould be res~ulated sc as to achieve retention or prodùction of a dbsired refracti~e index profile.
h~e have also found that whil~ achievillg a satisfac~ory article, certain dopants give particularly advantageous re-sults because of their physical characteristics.
~ We al-~o prefer that, if the porous article to be st.urfed has been m~de by pllase separation from a glass followed b~
leaching, certain precautions be taken to reduce losses during the processing of the rod. We have ~ound that cracking in this form of our process can be caused by problems arising from one or more of the following:
~a) Incorrect glass co~position;
(b) Incorrect heat trea~nent conditions for phase-separation; and (c) Incorrect leaching procedure.

iCI7;37 .

Guidance is given below as to how to choose glass compo~i~ion and processin~ conditions so as to reduce loss due to cracking in subsequent processing both during the formation o the porous matrix and the subsequent stuffing and drying.
The present invention is concerned with a method oE
producin~ a desired ~efractive index distribution in a glass axticle as a function of its dimensions by the additicn of a refractive index modifying component (hexeinafter refe red to as a dopant) to a porous matrix with interconnective pores whose walls are formed from at least one glass network formlng component and/ where desired, glass networ~ rl~odifyiIlg compo-nents. The method comprises the steps o~ immersing the po~ous matxix in a solution of a dopant, causing the dopant to sepa-rate in the matrix, removing solvent ~nd, where necessary, de-composition products from the porous matrix and collapsing the porous matrix to a solid form, characterlzed in that part or all of the dopant is caused to be precipitated by a method Which does not involve evaporation of solven~, ~he xemoval of solvent is not commenced until a substantial part of the precipitation has taken place and the rate at which heat is applied to remove solvent, and where necessary, decomposition products is regulated so as to achieve and/or xetain the de-sired refractive index distribution profile within the glass article.
The steps and the sequence o~ steps which we have found suitable to produce a particular proile are outlined below, all starting wi~h a porous article having interconnected pores.

` ~LlS~7'3~7 Th.e steps of our invention comprise the following:
(1) The dopatlt is precipitated in the pores by non-evaporative steps. These include (a) thermal precipitation in ~hich by lowering the temperature of the object, the solubility of the dopant or dopant cornpound in the solvent is decreasad sufficiently to cause precipitation of the dopant or dopant compou~d and (b) chemical precipitation such as alteration of solution pM
to a point of precipitation, repl~cement o~ the oricJinal solvent by a solvent in which the dopant or dopant compound i.s less soluble or introduction of a chemical into the solution which reacts with original dopant or dopant compQund to form a less soluble dopant species. ~ereinaXter the term solvent is used to de~cribe the chemical species ~hich at sorlle stage is the liqui.d illing the pores.
(2) Removal o~ the ~inal solvent is co~nenced only after pre-cipilation is substantially complete.
(3) The rate at whic~ heat is applied to remo~-e solvent a~d wllere necessary, decomposition products, .is re~ulated so as to achieve and/or retain the desired refractive index profile within the glass article~
The steps and the sequence of steps which we ha~e found suitable to produce particular profiles are outlined below~
starting with a porous article and using thermal precipitation of dopant or dopant compound.
Flat Frofile (a) Immarse the pOQ'OUS matrix in a solution of dopant or dopant compound.
(b) Precipitate the dopant by a temperature - 7 - :

~:~LS(;~737 drop.
tc) Evaporate any solvent present.
(d) ~Ieat to collapsing temperature.

E~ ofile (1) (a) Immerse the porous matrix in a solution of dopant or dopant com-pound.
(b) Precipitate the dopant by droppin~
the temperatuxe.
(c) Immerse in a solvent for the do-pant and allow the dopant to par-tially redissolve and diffuse out of the matrix. Only the ~opant precipita~ed near the outer sur-~ace o~ the article is removed in this step.
(d) Evaporate any solv~nt.
(e) Heat to collapsing temperature.
(2) Alternatively:
(a) Immerse the porous matrix in a ~olution or dopant or dopant com-pound.
~b) Pre~cipitate the dopant by dropping the temperature.
(c) Partially dry the porous rod.
~d) Immerse in a solvent ~or the dopant and allow the dopant to partially redissolve and diffuse out of the matrix.

~L5~37 Only the dopant precipitated near the outer surface of the article is removed in this step.
(e) Evaporate any solvent.
tf) 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 article 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-tion of radial distance from the central axls.
(c) Precipitate dopant in pores b~ 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 solubility characteristics are such that we can -~ achieve the desired concentration of the dopant in the pores, by diffusing a solution of the dopant into the pores at one tempera-tura and then cause its precipitation b~ a simple drop in tempe-` rature. We refer to such a process as thermal precipitation.
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 refractive index distribution using a suitable porous matrix as a starting material in which '.

_ 9 _ .

3L~L5~737 a refractive index modifying component is caused 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. ~he cor~mon ion effect has been used to reduce solubilities and cause pre-cipitatlon of the solute (e.g., the sol~ility of CsN03 in water is reduced in the presence of lN HN03~. The exchange of solvents has also been used to re~uce solubilities and thu5 precipitation by means not involving evaporation of solvent can be used. These include the addition of a suitable precipitant which reacts ~ith t~e dopant or ca~s a suitable change in pH.
We have also used a combination of steps consistin~ of both thermal and chemical precipitation means. This is particular-ly useful in cases in which more than one dopant or dopant compound is being introduced into the pores. We avoid any pre-cipitation methods involving evaporation o~ solvent as the sole means o~ preciptation, since we have been unable to ob-tain consistent results using such ~ethods. We believe thls is due to the following factors.
In the direct evaporative process the solution evapora-tes from the surface 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 axticle. Together these effects tend to produce undesixable profiles.
It is essential to regulate the rate of heating so as to avoid destroying either the incipient refractive index profile, or damaging the interconnective pore structure. It is possible ~L~50737 by allowing the evolution of vapor or gas in an uncontrolled manner to produce a pressure sufficient to destroy the integ`rity of the structure. We prefer therefore to avoid allowing 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 heating 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 upsetting 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 i.n 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 ha~e occurred before heating is continued. At other points we believe it pre-ferable to move rapidly from one temperature to another, e.g., when solvent removal has been completed to the temperature of collapse. Later in this specification 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 ~15~3t7 solvents or other ~on-aqueous systems or mixtures of such systems are used.
The following criteria can be used to select a suitable dopant from among the larger ~roup 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 mat i~
either as deposited or after thermally induced decomposition.
(c) The dopant or mixture o~ dopants used must be capable of being incorporated into tlle glassy ~trtlcture at or below the highest temperature at whicll the article is sub-sequently processed.
~ d~ Th~ dopant must not change its physic~l or chemical state in such a way before colla~se as to be l~st from the matrix.
(e) For low optical loss items the ol]owing added con-ditions apply:
(1) the dopant when used must be sufficiently free of 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 o~ any subsequent ~orming or shaping process needed to convèrt the article to any other shape or form.

~5q~37 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-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-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 1-20 mole percent of the oxide o~ the dopant or dopant mixtures in the total glass composition, the preferred range being 2-15 and the most pre~erred, 5-10 mole 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 mole percent, the difference between the silica and do-pant concentration usually being made up by B203.
In the selection of solvents the following considera-tions are important. The solvent selected (a) should not damage the porous matrix;
~ b~ should be capable of being purified to low concen trations of undesirable impurities;
(c) should be one that can be substantially removed by either e~change with another solvent, evaporation~ or thermal de-composition followed by oxidation (or high temperature reaction with chemically active atmospheres);
(d) should have sufficient solubility for the dopant ~15~737 compound or combination of dopant compounds to allow the desired dopant levels within the pores to be achieved by molecular stuf-ing.
(e) should be such that, if 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 oE high speed of drying.
It is impossible to test all possible combinations of solvents; however we have found thal: 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 ther~al precipitation be-cause of its ease and convenience, and because we pre~er 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 preferably water soluble and have a steep solubility coefficient, 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.

~L150~37 Further de~iled guidance concerning the choice of sol-. ~
vent is given by reference to Table I below in which the solubility of various dopants in solv~n~s at different tempera-~ures are illllstrated.
As already indicated a~ove, in choosing a particular route to a desired end product, a number of guidelines nced to be considered, and these can be illustrated by ref~rence to Table I.
First, in order to obtain the desired concentxation of dopant in the article to yield a significant chan~e in inde~, a solution having a sufficiently high concentration of dopant must be found by cuitabl.e choice of dopant compound, solvent and tem-perature. In order to precipitate the dopant or dopant com-pound, the use of solvents w.ith sufficielltly low solubility is necessaxy. Often there is a need to remove sul~tantial amounts of dopant from designated areas of the article, such as in the cladding region o~ a fiber, in which case solvenks with intermediate solubilities are useful. Such removal of ': ' dopant is referred to as unstuffing, as opposed to molecular stufing. Suitable controI of sol~bilities for proper pre-cipitation of the dopant or dopant compound can be achieved by a number of methods.
(l) 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 o~ being able to arrest diffusion in the shortest time, thus enabling us to freeze-in a desired concentration profile ~ith high accuracy.

~5~737 N N N N N O N N N N

~1 o~ I z :
~r a~
d o~o O .-1 h O
: H~ (d o\ o~O
~ ~1 ~ U~ ~o , ~ ~' ~0 o~ o~o , ~
.. ~
' ~ UP~ ~ O'U~

N ¦ o o ~ O It~ o o o ~ d :

o O O ~P _l O -I O ~ -1 0 0 -1 0 '. U~
' u~l U~ Z =~ æ z U') ~0 ~ co 0~ O ~i N

- 16, ~5~)~7~37 ~, _ "~ ~ o ~ o o ~ N tS~ N N O

':
0~ ~ ' ~;

a O
~ IJ a~ a~

1:~ t~l dP d~
O .~ ~-1 c~
U ~ O
._ ~ ~
.' . ' .
`~ ~ ~ S ~:

~ ~ O ¦ O O dP o'P o dP
$~1 ,, ~ o o c~ o ,~
Uo~

~1 o o _I ~ o ~n t~l ~ ~1 ~9 N
U~ Ll C~
._ . ~

~ ~ z - - Z m C~ ~ _ O ~i N ~`1 t~l N t~l N

L5~37 This is illustrated for CsN03 dopant in Table I whereby the solubility changes from a desirable stuffing level at 95C to a desirable unstuffing level at 4C.
(2) Precipitation by common ion effect and thermal pre-cipitation. Precipitation can be produc2d or further enhanced by the common ion effect. For e~ample wilen the dopant is a nitrate the concentration oE nitrate ions in solution is in-creased by adding another source o~ nitrate ions to the solvent (i.e., MN03 acid). This ~reduc*s -the solubility o~ ~he nitxate dopant (see CsN03, Table 1) (3) Precipita~ion by solvent e~change. Precipitation is induced by substituting a low solubility solvent ~or a higher ~olubility solvent. The high solubility of nitrates in water has allowed U5 to use water as solvent for the stu~fir.g 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 of 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).

~L~50737 LEGENDS

FIGURE 1. Plot of the fractional weight gained by a porous ___ rod immersed in a solution oE CSNO3 at 100C.

IGU~E 2. Plot of the fractional weigllt loss by a po~ous rod after being stuffed with CSNO3 at 100C, which is now immersed in H2O at lOO~C.

IGURE 3. Plot of the fractional weight loss bv a porous rod after being stuffed wi.th CS~O3 at 100C, which is now immersed in EI20 at ~C.

,.
F~GURE 4. Plot of the index of refrac~ion profile o~ rods in Example V. Curves 1 and 2 indicate the rods un-stuffed for 11 and 20 minu~es respe~ Jely.

IGURE S Plo~ of the index profile ~tuffed according ~ ro~l X13,Example VI, Table V. ~he rods were rate heated according to Example VII where curves 1, 2 and 3 represent heating ra~es of 15, 30 and 50C/hr.
respec~ively.

1~5(~737 As indicated above, when operating the process of the presen~ invention with a porous interconnective struc~uxe which has been produced by the phase separation ~f a s~litable glass followed by a leaching step, it is necessc.ry to op~imize the various stages of the process in ord.er to achieve consis-tently and satisfactorily a saleable end product in good economic yields, and to interrelate the various parc..metexs involved.
The factoxs on whic~ guidance is required by the man practiced in the art are:
~1) selection o~ g~ass compositioll and h~at treatment to o~tain suitable phase separation;
(2) leaching and washing;
(3) stu~fing;

(4) unstu~ing where needed; and

(5) dr~i.ng and consolidation.
The guidance given in 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 treatment.
In order to achieve 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 desirable pore size. A number o~ guidelines can be given to the man practiced in the art. We find it convenient to choose compos~ions from ~5~73t7 the aXe~ o~ al~ali ~et~ bo~o$;i~ica~e ~lasses., ~nd ~urt~er guidance is given below as to suitable composi~ions.
Many compositions have heen reported as suitable for use in the production of porous glasses for diverse puL-poses (see U.S. Paten~s 2,106,744; and 2,221~709 usually not for op~ical use by a route based on phase separa-tion and leac1iing of the soluble ph~se. We have discovered that for optical waveguide manufacture, only small regions within prior art compcsltions ranges are sui ~ble. U.S. Pa~ent 3,843,341 is one representa~i~;e disclosure o~ such compositions which Cor the most part are not sat:Ls~astory in a proce~s i.n which the articles produced are usually rod~ or ot.her elonga-ted shapes with the smallest dimens:ion ~bo~e ~ n~. For exa.mple, a number of glasses ~rom within tlle pre~er~ed re~ion o~ U.S.
Patent 3,843,341 and from previous dlsclosures of Corniny (see Table II below) were vertically drawn into 8 mm rods at a rate o~ one inch per minute. These were phase separated as dls-closed herein, but all the Table II compositions cracked during leaching.
TABLE II Prior Art Compositions wllich Crack upon Leaching*
Na2O B2O3 SiO2 A1~03 .
8 30 6~ 0 8 35 57 o ~ 40 52 0 3~ 60 0 4~ ~9 * All concentrations are in units of mol percent.

~IS~;37 .
More specifically, ~e ha~e ~oun~ ~hat (1) A11 of the compositions in the range of the sodiu~
borosilicate system disclosed in U~S. Patent 3~843,341 and drawn into rods as described above, cracked upon leaching.
This includes the region denoted as the preferred range in sai~ patent.
t23 Many of the compositions in the range of the sodium alumina borosilicate disclosed in U.S. Patent 3,843,341 cra~ked when treated as described above. This was true even ~or compositions in the preferred range.
t3) Many o~ the preferred compositions disclosed in the U.S. Patent 2,221,709 cracked when treated as desc~^ibed above.
Although we ind most of the previously disclosed boro-silicate range of composition unsatisfactory because of the requirements we need to insure a satis~actory yield o en~
- product, we have discovered certain specific comp.os1tions in this broad range which are useful and which ha~e not been previously described. In addition we have discovered a set of criteria which can be applied to identiry other specific ~limited areas of phase-separable glass compositions which would give a satisfactory yield of product.

~ ~LSt~7'37 `~

From a commercial point of view, and bec~use of the large region of phase separation which they show it is most con~
venient to work with alkali borosilicate glasses though 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 in~o two phases, one silica-rich, the other silica-poor. The latter is preferentially soluble in a sui~able solvent.
(2) which on heat treatment at a particular temperature separates into phases o approximately equal volume fractions and when held at that temperature clevelops interconnected mi-crostructure.
(3) which is easy to melt and i5 easy to refine using conventional techniques.
~ 4) which is relatively easy to form in the shape o a rod or a shaped article with minimum dimensio~s of > 4 mm ~e.g., thickness, diameter, etc.) and does not phase sepa-rate siynificantly ~uring the forming stages.

The following provides a systematic procedure for selecting a suitable composition.
(1) Almost all silicate glassy systems exhibit composition regio~s of phase separation. However, of commercial interest are the alkali borosilicate glasses which show a large region of phase separation. The silica-poor phase of these glasses can be readily dissolved by simple acidic sol~tions. Frequently ~L11 50~37 it is necessary to add other components such as aluminum oxide to modify certain properties of these glasses. How-ever, some oxides are not desirable because on phase separation they end up in the silica-poor phase and make it difficu]t to dissolve by simple acidic solutions. Thus only those o~her components are sui~able which do not diminish the solubility of si].ica~poor phase signi~icantly.
(2) After deciding on the gla6sy sysLem (along with dopants), one should next determine the immiscible regions of composition, C, and their coexistence terapera~ures, T~.
Tp(C) is the temperature above which a glass (C) is llomo-geneous. Techniques for determinin~ Tp are well descri~ed in literature (see for example W. Haller, D.H. Blackburn, F.E. Wagstaf~ and R.J. Charles, "Metastable ir,mliscibility surface in the system Na2O-B2O3-SiO2" J. Amer. Ceram. Soc.
53 (l), 34-9 (1970)~. `
~ 3) We have discovered that one should determine those compositions t Cl, which exhibit an equilibrium volume fraction of about 50% at least at some heat treatm~nt temperaturs. We shall denote this temperature by To(Cl). The msthod for determination of this temperature is as follows:
(a) Select three (or more) temperatures, say Tl, T~ and T3 (about 50 apart from each other) such that 3 2 l P( l) Carry out long heat treatments on glass samples until they turn white at temperatures Tl, T2 and T3.

llS0737 (b) by electron microscopy of these heat-tr~ated 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 eY.tra-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 com~osition range (C2) within the range Cl such that (à) 575C ~ To (C2) ~; 500C and (h) 750C ~ Tp (C2) ~ 600C.
These temperatur~s 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 o~ 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 C2 which, in addition, refine properly). We have found for example that one convenient feature identifying some of the refinable glasses is that those contai~ing at least 28~ B2O3 refine satisfactorily.
~ 6) Not all compositions of C3 are desirable even .hough they will refine easily, and will phase separate with 50%
volume fraction. An additional requirement is that the de-~15073~7 sired composition should not phase separate significantly du-ring forming operation. The degree o~ ~hase 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 o~
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 ~e de-termined. Those compositions within the area C3 which do not phase sepa~ate in this ~orming process can then be grouped in the further restricted area C4. The compositions fulilling this condition preferably have a Tp ~etween 710 and 6~0C, most preferably between 695 and 640C.

(7) The final criterion we appl~ insures that there is sufficient composition difference between the two phases when separated that leaching will take Place effectively. For this pur~ose we select onlv those compositions C which within the C~
range satisfy the condition that Tp(C ) - To(C ) > 75 C

Having 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 b~ the following procedure:

~lS0737 (a) 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 properl~ chosen accord-ng to the above criteria, this tenpera-ture will not be so low that the time needed to obtaln a suitable microstructure ~or leaching would be too long and uneconomic. Similarly, it will not be too high otherwise [l] distortion of the glass may occur during heat treatrnent;
[2] if the temperature of heat treatment is well above say, over 160 centigrade deyrees above the glass transition temperature, phase separation tends to be rapid whicll reduces the degree of control on phase separation.
These requirements limit our preferred heat ~rea~men~ tempera-ture, TH, to the following ran~e ; 575~C S TH(Co) - To(Co),~ 500C
(b) Having found TH~Co), the heat treatment i5 de-termined by the condition that a microstruc~ure state suitable for leaching is developed.
Heat treatments at different times are carried out say tl(l hour) ~ t2 (2 hours) C t3 (3 hours)......... By electron microscopy, it is possible to determine the time, tmaX, beyond which the interconnectivity of the structure begins to break down. The size o the leachable phase is measured from micro~
graphs, and the prefexred heat treatment times are those which are less than t but for which the microstructure si~e is max O O
at least 150 A, and preferably less than 300 A.

115~37 We have applied the above criteria within the alkali boxosilicate sy~tem and have identified certain characteris-tics of the composition ranges which contribute to good yields o~ the ~iber optic pre~orms whicll are at least 2 mm in diameter, avoiding the problems arising ~rom, e.g., phase separation during forming, or insufficient phase separation when the phase separa~ion stage is being carried out. Phase separation durin~ form~ng ol the glass article from the melt, and insufficient pha~e separation or breakdown of the interconnective structure during the phase separation heat treatment can both cause or contribute to cracking during either or both of ~lle ~ollowin~ steps, le~ch-ing o~ the phase separated glass, and drying oE tlie leached and stuf~ed glass.
It has become clear to us that: the compositions as~
sociated with the best v~elds are those contained within the ~ollowing broad composi~ion area (all percentage3 being in mol percent):

Broad Preferred `:
SiO 48-64 49.5 - 59 R2O 4~9 6.5 - 8 2 3 0-2.0 p 0-1.0 0.20-0.8 a 0-3 0-2.4 A 0-0.5 0 x 0.1 - 1.0 0.~ - 0.~

~L5073~7 where ~ is the A1203 concentration in mole percent, 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 percellt o~ K20, Rb20 and Cs20; and R20 is the sum of the concentrations in mole percent of Li20, Na20, K20, r~2~
and Cs20; and ~ is the ratio Li20/R20.
Because of the presence of A1203 in the glass significantly a fects i-he results, we will first discuss glasses which have no A1203 content. Under these conditions, the ranges listed above are appropriate with A1203 content oE zero, with R20 the sum of all the alkali metal oxides L.i~O, Na~O, IC20, 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.
I,ithium glasses tend to devi~ y and therefore it is often preferable not to use that chemical. In this case, P~20 becomes the sum of the concentrations o Na20, K20, Rb20 and Cs20. All limits and conditions above are maintained.
Rubidium and cesium glasses are more expensive than those made with sodium and potassium. They can be left out for economic 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 R20 only, or R~O can also con-siæt of Na20 only.

1~L5(1 73'~

The glasses below in TabLe III are glasses ~7hich we have identified using the above criteri~ and found satisfactory for use in the molecular stu~fing process of the present invention as we achieve a satisfactory control of phas~ separation and pore structure after leaching using these compositions, and a good overall yield of finished product of the invention.

1:1507~7 .

~ ' o o I I ~I ~D ~D W ~D

N ~I Lr)Lr~ l . . L~
O O O O O ' O O O
~:
. OOC~OOOOOOOOC~
2; 1 ` ~D o O O O O O O O ~i ~i ~ O ~ ~ ~D Ln ~ o N ,~ LnLn N 1` N N Ln 11) Q
~ OOOOOOOOOO~O
U) -1 ~ I O O O O O O O ~ ,i O O ~i H O
H ~ ¦
~3'~ ~IOOOOOOO~`IOOO

¦ O O O O N O O O O O

~ ` ~

: `: O ~ 00 ~ C~ ~
~ : ` -K ~ o N N1--l ';PO O O ~ N O

1~
~Zi In ~ N N Ln _I Ln ~ Ln ~r ~9 CO
.
N OS~ D 013 I` Nt`I` ~ ~~1 1 ~ ~ . . . . .
O ~ ~ ~ o cn ~ QD ~ r~
~-1 ~ ~ DLDLn Lr)LnLf~ Ln LO Ln Lfl U~ ~ .

H H H ~::~ H H1-1 ~C X 1-1 H
H H H . ~ H H H ~ H
H ~ ~ X

... .

~507~7 Another aspect o~ our invention invol~es leaching of borosilicate phase separahle glasses. We pre~er before leach-ing the ~lass to etch the article to be leached with dilute hydrofluoric acid for about 10 seconds to remove any surface ~contamination, or surface layer of glass having a slightly -different composition from the interior due to volatilization of components such as B203or Na20 during formation.
The concentration of tha acid solution, amount of leach-ing solution and temperature of leaching have a direct bear-ing on the progress of leaching. It is essential to insure that a suffi~ient quantity of the leachin~ solution is brought into contact with the`article to dissolve the sol~le phase.
The rate of leaching may be conveniently controlled by ad-justing the temperatuxe. The glass should be above 80C, prefexably above 90C. As has previously been described in U.K. 442,526, it is desirable 1~L5(~737 to use an acid solution whic~ has been sat~rated with NH4Cl or other equivalent compounds capable of reducin~ the concen-tration of water in the acid leaching solution. This assists in contxolling any swelling of the treated layer and reduc2s considerably the chances of loss due to crac3ing of the article t as the inner untreated layer yoes into tension because of the thickness o~ the swollen outer layer.
We have ound that the rate of leach:ing, and the re-deposition of borates in the pores of the g~ass during leach-ing can be controlled by controlling the ~..oncen~ration of borate salts in the acid leaching solution.
We have measured leaching rates at 95C ~or glass rods (length 10 cm, diameter 8 mm, and compOsition 57% SiO2, 35 B2O3, ~ Na2O and 4~ K2O) heat treated for 1 1/~ hours at 550C wi~h leachiny solutions containing 327.3 ~m of NH4Cl, 33.6 ml of HCl per liter of water and varying amo-~ts of B203. We found that leaching time increased with increasing B2O3 content in the le~ching solution. The resul~s are summarized below:

TABLE IV
Amount of Bori-c Acid tg/liter?Leaching Time(minutes) 0 425 $ 50 41.2 625 + 50 61.5 642 + 50 8~.7 725 + 50 106.1 1670 + 50 We believe redeposition o~ borates in the pores also contri-butes to rod breakage. This can be avoided by e.g.~eplacing ~L507~

the leaching solution ~s the concentr~tion of bo~ate builds up. But t~is requires lar~e quantities o~ leachin~ solution.
For example, in order for leaching time to be no more than 660 minutes, the volurne of leaching solution per 100 ml of qlasc will be on the order o~ 1550 ml. This, however, can increase costs and provide a possible source of contamination.
We ~ind it more convenient to provide a cold trap so that excess material is continuously removed from t~e solution as it comes into solution from the ~rticle being leached. The cold trap is effec~ive in speeding -~he process even if it is only a few degrees helow the temperature of the glass articleO
Preferably it should be 20C below the temperature of the glass article. We find it convenient when NH4Cl is present to choose a tempexature or the cold trap at which the acid solu-tion remains saturated with NH~Cl. It is possible to operate with a low level of rod breakage witho~t NrI4Cl or other equi-valent compounds present in the leaching solution. In general we prefer to use at least 10 weight percent NH4Cl, preferably 2~ weiqht percent as we find that on a statistical basis tl~ere is an even lower level of rod breakage when MH4Cl is present.
The most convenient way to determine a suitable leach-ing time is to take an article and subject it to the 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 watert With certain compPSitions there can be de-position of silica gel in the pores, and we find this can be removed by washing with NaOH. We have ~ound it possible by selection of compositions to minimize this depo~ition. The compositions shown in Table III alleviate this problem, especially those with minimum silica.

~ s~

Once the porous matrix has been produced, either from a phase-separable glass as outlined above, or by, e.g., a chemical vapor deposition technique, the selection of ~uitable conditions for stuffing and unstulfing we have found can be made by foll~t~ing the guidelines given below.
Using well-]cnown fo.rmulas for optical waveguides, the desired physical propert~es of the waveguide (such as size, numerical aperture, band pass, etc.) can be related to a varia-tion o~ index as a function of its dimenslons~ The depen-dence of refractive index on dopant ~ld dopant Co~pound concen~ration can be determined by literature search or by sui~able exp~ri-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 di.ssolved in the stuf-fing solution so that the desired concentra~ion is reached at a particular stuffing temperature and time of stuffing. The following procedure enables these parameters to be determined:
(1~ Determination of Stuffing Temperature of Porous Rod (a~ Determine the dependence of the solubility of th~ do-~ pant or dopant compound in the appropriate solvent on tem-perature.
(b) The stuffing temperature range lies between the tem-perature at which the desired concentration of dopant or dopant compound is saturated in solution from (a) and the boiling tempexature of the solution.
t2) 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 ~L~511~37 on the microstructure si~e in the porous rod. The procedure 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 whicl~ the weighc does not increase significantly by plotting the fracti.onal weight change, y(t) = ~M(t) - M(o)]/~M(~) - M(ol] versus time, t, where M(o), M(t), and M(~) are the respective weights initially, at time t and at infinity (very long times).
(d) Time required to stuff, t, another parous rod of diameter a, with the same dopant solution at the same tempera-ure ist = t [ a ]

Example We stuffed a porous rod with a concentrated solution of C~NO3 in water (120 g CsNO3 per 100 ml of solution) at 100C.
The radius of the rod was 0~42 cm. We measured the weight 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 in~rease significantly. Thus, the proper stuf-fing time ~or this rod is about four hours.

~ ~iO~737 (3) Determination of Unstuf~ g Time to Produce a Parabolic Profile in a Porous Rod by Thermal Precipitation ~ o produce a parabolic profile, the stuffed rod, pro-duced as (2) above, is partially ~mstuffed by i~mersing it in the solvent. This should be done at a temperature where the dopant does not precipitate. The unstuffing 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 ~ractional change y(t) ~tt) - M(t) - M(o) (1) M (~) - M(o) against time t.
(`c) The time o~ unstuff-ng, t~, depends on tlle d~sired profile; often it is 1/3 ~ y(tO) ~ 2/3 Example We chose a porous rod stuf fed with concentrated CsNO3 solution (120 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.

iO~37 (4) Determination of Unstuffing Tempe~ature and Time to Produce a Step Profile by Thermal Precipitation The temperature for unstuffing for a step index-type fiber depends on the numerical aperture desired in the iber and on the dopant solution. Since one would like to have as low a refractive index in the clad as possible, the un-stu~fing ternperature is typically a few degrees above the freezing point of the dopant solution.
The time required for unstuffing depends on the desired clad thickness, as well as on such parameters as temperature of unstuffin~, the conc~ntration of the stu~fing solution previously 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 chanqe in the values of any o~ these parameters, the entirs pxocedure should be re-peated, and adjusted accordiny to the guidelines herein.
Suppose the desired clad thickness is "d" and the radius of the stuffed rod is a(~d). Let 2 a (a) Knowing Y, it is possible to determine the 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 y(t) as a functlon of time.
(b3 Plot the fractional weight change against time, (see eq. 1) as shown in Fig. 3.
(c) Find time to or which y(tO3 = Y' 7~

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 95C, replacing the dopant solntion by the solvent used free of dopant at the sar.~e temperature, or where the system is aqueous, water or dilute nitric acid. The do-pant then tends to di~fuse outwards, thus varying the concen-tration through the cross-sectiona] area of the porous matrix.
The time required for this"~nstuff~lg"is o~ course dependent upGn the volume being treated,but a rod o~ diameter 8 mm re~uires about 20 to 30 minutes. We pre~er to stop the unstuf~ing by replacing the liquid surrounding the rod w,ith 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 cortrol the end point by measuring the change in conductivi~y of the water being used to unstuff.
(5) Drying, i~e., Removal of Solvent Two problems occur in drying which affect the ~conomics of the process and the quality of the product. These are cracking of the porou.s 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 o~ the drying procedure. However, in order to ~50~37 operate on a comrnercial scale, it is necessaxy to adopt a procedure wnich minimizes cracking and thus improves the economics of the process. Such a procedure should also pre-ferably avoid profiles beiny altered in such a way that do-pant is transferred from the interior of the article toward the surace as this is not usually a desirable profile. This results in a depression of index in the center and an increase near ,he edges as illustrated in Example ~II. As indicated above, the profile obtained is dependent on the unstuffing.
Having achieved a suitable profile with solvent stiIl present in. the porous structure, it is essential to dry, i.e., re-move sQlvent in a way which will not alter that profile to-an undesirable state~
In an analysis of the drying ~rocess we have found that a-n~mber of events affect the process. These are:
(a) Gas evolution. The sources of gases can ~e the solvent, dopant decompositioll products and dissolved gases.
If the gas evolution is too fast because of rapid heating ox insufficient gas removal the rasulting differential pres sures in the pores can break the glass and/or caxry 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 tempaxa-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 iO737 can be developed to the point of cracking.
(c) Dopant compound decomposition.The dopant as availàble in solution is generally a compourld wh~lch thermally decomposes.
We have chosen dopan~ 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 the temperature range where decomposition occurs in order to pre-vent cracking and transport of dopant.
(d) Mass transport can occur at several points in the drying process. When the article is dried initi.ally, 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 precipi~ated dopant can be displaced. After decomposition, if 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) Hydro~y1 ion removal. Hydroxyl ions form absorption bands in the near infrared which often are detrimental to use as a waveguide. If one wants to remove the hydroxyl ion because of this or any other reason, the following complications arise. A signi~icant 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.

~1i 5~737 Outlined below is a preferred process for the suitable solution of these problems. The initial bulk removal of the solvent has to be performed by the use of conditions where boiling does not occur; in the case ofaqueous solut ons, we have used two procedures. One consists of initially drying . .
the porous glass articles with precipitated dopant in a des-sicator (at atmospheric pressuxe) for 24 hours at 22C and then placing them in a drying oven. 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 ~e convenient wllen using CSNO3 in aqueous so].ution as a dopant. In order to minimiæe the chances of crac~ing even further, we find that when using aqueous 501-vents, it is convenient to subject the article to a final wach with a non-aqueous solvent which is non-reactive with the glas.s.
We believe this can assist in removing hydroxyl ions from the structure. An example of a suitable solvent i~: methyl alcohol.
~ e have found it preferabLe to warm the artlcles which have been maintained below 10~C under vacuum slowly to room temperature and to maintain under vacuum at room tempexature conveniently for about 24 hours before the articles are trans--ferred to a drying oven.
In the case of non-aqueous solutions of dopants, we have found it suitable to place the articles under vacuum at room temperature for 24 hours and then transfer to a drying oven.
This significantly speeds up the process as compared to an aqueous process.
In the drying oven, we have found it desirable to heat the samples to the upper drying ~`emperature under vacuum at ~1 50737 a rate below 30 C/hour, preerably 15C/hour, since a slow hea-ting rate significantly lowers the cracking probability and`a-voids dopant redistribution.
The selection of 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,any increase in heating rate must also be balanced against the increased risk of 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 of 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 preferably chosen to be in the range betwe~n 50 and 150C below the glass transi-tion temperature. We prefer to use a narrower range of 75 to The next stage of drying consists of holding the glass at or about this upper drying temperature for periods of S to 200 hrs., preferably 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-ticle since this helps the drying process. It should be noted that whatever the choice of drying conditions during the holding time, it is desirable to expose the sample to oxidizing condi-tions ~:L5~;)737 if one wants to lower the ne~r infr~red absorption and there are residual iron impurities in tlle glass. This oxidizing stage reduces ~he Fe /Fe ratio in the glass,thus lower-ing the absorption by Fe ions~ In our preferred procedure, we heat treat a porous glass 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 xapidly in temperature to the point where consolida~ion occurs. Once the pores are collapsed, consolidation is complete and the article may be cooled back to room temperature. The consolidat:ion step mus~ 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 ~as evolution is likely to occur in re-heating and bubbles are formed.
We have found it d~sirable where the matrix is produced fxom a phase-separable glass to heat the porous glass samples under a reduced pressure of oxygen (approximately 1/5 bar) up to 825C ~7here consolidation occurs.
The following examples illustrate the molecular s~uffing a~pect of ~he invention but do not limit the invention.

~:~LSq~3~7 Examples I to V illustrate the use of v~rious concentrations of dopant 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 ~rom a phase-separable glass and the subsequent treatment are sh~ in the paragraphs below -and the actual numbered examples illustrate the use o dif-ferent dopants at a range of concentration, collapsing tem-peratures and final overall glass composition.
Melting and Forming A glass having the composition in mol~O; 4 NaO~, 4 K2O, 36 B2O3, 56 SiOz was melted and stirred to produce a homogeneous melt from which rods we~e drawn having a diameter in the range 0.7 to 0.8 cm.
He.at Treatmsnt usin~ a Coolin/~ Coi.l The drawn rods were heat trea~ed at 550C fox two hours to cause phase-sep~ration.
Etching before Leaching Each rod was etched for l~ seconds in 5~ HF followed by a 30 second wash in water.
Leaching The rods were l~ached at 95C with 3N HCl containing 20 NH4Cl by weight, the time being chosen on the basis of pre-~ious trials so as to reach a stage where the rate of weight loss has dropped to almost nil. The leaching time of the rods used in these examples was chosen to be in excess of 30 hour~.
During leaching,by providing a cold spot at 40C, the boric acid concentration in the leaching agent was kept below 115iD73~7 50 g~liter, this speeding up leaching and avoiding possible re-deposition of boron compounds in the pores of the matrix~
40C is chosen so that there is no precipitation of NH4Cl from the leaching solution as this temperature is above the sa-turation temperature of t~le NE~4Cl present to maintain a suit-able amount in solution to reduce breakage drastieally.Washing The leached materlal is washed with de-ionized water.
The washing cycle is conveniently controlled by determining the concentration of iron in the effluent. Washing is con-veniently carried out at room temperature using 10 volurnes of water to 1 volume of glass. We prefer in a non-continuous process to change the water about 6 to 8 times, giving a washing time of about 3 days; at each change the iron concen-tration is reduced to l/lOth of it~ concentration at the time of addition of fresh washing water, and in this way one can as~sume without measuring ~he iron content that a su~ficiPnt-ly low level has been achieved during washing.
Stu~fin~
- .
With aqueous soLutions of dopants (see ~xamples I to VI
below) we prefex to move smoothly rom the last washing stage to stuffing by simply replacing the water by the stuffing solùtion. This is done by draining the water from the last wash and filling the tube containing the porous rod with stu~fing solution.

In Examples I through IV, ~he samples were removed from the stuffing solution and cooled to 22C where the dopant precipitated partially with an amount equivalent to its ~15~73'7 aqueous solubility at 22 C remaining in solution in the liquid filling the pores. (For example, lO g ~a(~03~2/lOO ml solution remained dissolved in water in the pores after thermal precipitation t 22C. Similarly, 6 g H3B03/100 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 vacuum 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 l'ima The rods were then held at atemperature o~ 6~5C for 96 hrs. while passing dry oxygen gas around the rods~
Consoli~ation On completion of the hold time, the article is ready to be collapsed and is raised rapidly to a temperat~lre where collapsing takes place and a consolidated rod is produced.
This step is carried out under a reduced pressure of oxygen (approximately 1/5 bar) and the final temperature is given in each example.

~LlS~37 .

EXAMPLE I
Molecu'lar ~S:tuffing: with BaO
Dopant B.a(N03)2 in Water Dopant Stuffing Deta:ils Consoli- Composition Wt %
: qms/lOOcc Time- Temp.~C dation- Mole %
_ H20 Eours ~emp.UC
1 12 4 85 820 6.0 B203 6.86 93.11 SiO2 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
.. . . __ :: Molecular Stuffing with B203 .
Dopant H3B.03. in Water ` Eop'ant S'tuffin~ D~etails Consoli- Com~osition Wt %
Rod gm_/lOOcc Time Temp.C dation Mole % .' of H20 Hours Temp.C
Compa-rison Rod ~ ~ 820 6.09 B203 7 93.88 SiO2 93 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.~2 11~;i~737 ~XAMPLE III
_ Molecular ~tuf~ing ~ith PbO ~ B203 .. _ . .. .. _ __ .
Dopant Pb(N03)2 and H3B03 in ~?ater Rod ~ 7 Doped with 40 gms Pb(N03)2 and 10 gms 3 3 per 100 cc of H20 at 85C for 12 hours. Collapsed at 825C.

E~AMPLE IV
~olecular Stuffin~ with BaO ~ B203 .
Dopant B~No3)2 and H3~03 in Water . .~v _. .

Rod ~8 Doped with 12 gms Ba~N03)2 and ~ ~ms H3B03p~r 100 cc o~ H20 at 85~C
for 4 hrs. Collapsed at 830C.

EXAMPLE V
The above examples all relate to uniform doping of the rod. As described above, it is possible once a xod has been left for sufficient time 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 refractive index profile in the collapsed rod. ~wo porous rods were produced by the procedure outlined above r and were immersed for more than four hours at 95C in an aqueous solution of CsN03 with a concentration of 120 g CsN03/100 ml solution. The rods were then transferred to water at 95C, and left in the water ~or : :

~15Q737 periods oE 11, and 20 minutes respective]y. Each rod at the expiration of .~he time in water was immersed in water at 0C
for 10 minutes to cause thermal precipitation of CsNO3. The rods were then treated to remove so]vent and collapsed in the manner described above: The refractive ~n~ex profiles ob-tained are s~own in Fig. 4.
EXP~PLE VI
Several porous rods produced by the me~hod descrlbed above were immersed in a series of solutions of CsNO3 and Cs2C03 as per Table V below for more ~han fo~r hours at 95C. These were then unstuffed to produce a step profile. (The proile for rod #13 is shown in Fig. 5.) The time required ~or unstuffing was determined usi.ng 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 tlle water was removed and rods ware collapsed as desci~ibed above. The resulting in-dex of refraction in the center of tlle rod i~ listed in Table V.
TABLE V
Index Rod Number Stu~finq Solution at center o~ rod ~ .
9 20 grs Cs2CO3/lOOml H2O 1.462 30 grs Cs2CO3/l~aml H2O 1.475 11 75 grs Cs2CO3/lOOml H~O 1.48~
12 60 grs CsNO3/lOOml aqueous solution 1.475 13 120 grs CsNO3/lOOml aqueous solution 1~486 EXAMPLE VII
As discussed previously, we emphasized the importance 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 stuffed porous rods as used in Example V were unstuffed 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 und~r vacuum at rates o~
50,30 and 15C/hr respectively. The resulting index profiles are shown in Fig. 5. Heating at rates above 100C/hr caused appreciable breakage in rods. Thus, the preferred heating rate is below 20~C/hr, when one does not want to alter the precipi-tated profile.
The rod heated at 15C/hr was selected for fiber pul-lin~. The 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 li~ht 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 o~ the fiber is then removed and the transmission is again measured. The loss in dB/km is given by Loss (dB/km) ~ 10 log I(ll) where 1 ~ 12 ~ 11 and 12 and 11 are the lengths in km of the long and short fiber segments respecti~ely. It was found to be 25 `
dB/km, which is less than 100 dB/km required in many communica-tion applications.
ExAMæLE ~III
-After leaching and washing as described above in the introduction to Examples I-IV, a stuf~ed porous rod is immersed for four hours in a solution containing 120 g of Cs(N03)/100 cc of solution at 95C. The sample is cooled to 22C where the do-pant is partially precipitated. 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 ~L~L5~737 Cs20 and various nitrogen oxides. When the sample changes from white to clear, the consolidation is complete and the samplè"is removed from the furnace. The composition of the final article is 90.6% SiO2, 3.4% B203 and 6.0% Cs20 b~ mole, and has less than 10 p.p.m. of transition metal impurities.
EXAMPLE 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 glve the following information:
Column 1: Dopant used.

Column 2: Concentration of dopant/100 ml solution.

Column 3: Solvent, i.e., solvent used for initial stuffing.

Column 4: Temperature in C, and time taken for initial stuf-. . _ fing.

olumn 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.

olumn 6: Temperature in C and time taken ~or precipitation and variation in refractive index profile.

Column, 7. Solvent B is used where appropriate to adjust dopant ~LlS~

distribution in matrix. By causing further preci-pitation before solvent removal begins so as to enable a more thorough decrease in dopant concen-tration near glass surface.

Column_8. Temperature in ~C and time taken for adjusting do-pant distribution by further solvent treatment.

Column 9: Indicates temperature at which drying commenced in C, and by "V" or "A" whether drying in vacuum (V~
or in dessicator at atmospheric pressure (A) for the first stage.

Column 10: Gives the index of refraction where measured.
:Cn 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 stufing 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 temperatuxe.

;~ Lines 4-9: illustrate the use of different dopant and solvent :
combinations.

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 ~he use of an organic sol~ent.

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Although presently preferred embodiments of the ln- :~
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 o the present invention. It is intended to en-compass all such changes and modifications as fall within the scope and spirit of the appended claims.

.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. A glass waveguide or preform thereof having a composition having at least 75 mole percent of SiO2 wherein the improvement comprises at least 2 mole percent Cs2O as refractive index modifier.

2. A glass waveguide or preform thereof as claimed in claim 1 having a transition metal impurity content af at most 10 parts per million, and an attenuation of less than 100 dB/km at the utilization wavelength of light.

3. A glass waveguide or preform thereof as claimed in claim 1 havlng a minimum of 5 mole percent Cs2O.

4. A glass waveguide or preform thereof as claimed in claim 1 having a minimum of 2 mole percent B2O3.

5. A glass waveguide or preform thereof as claimed in claim 1 having decreasing amounts of Cs2O in a direction perpendicular to the optical axis.

6. A glass waveguide or preform thereof as in claim 1 having a minimum of 80 mole percent of siO2.

7. A glass waveguide or preform thereof as in claim 4 having a minimum of 80 mole percent of SiO2.

8. A glass waveguide or preform thereof having a compo-sition having at least 75 mole percent SiO2, where the improvement comprises at least 2 mole percent B2O3 and at least 2 mole percent Cs2O as refractive index modifier.

9. A glass composition as claimed in claim 8 having decreasing amounts of Cs2O away from an optical center.

10. A glass composition as in claim 8 having a mini-mum of 80 mole percent of SiO2.

11. A glass composition as in claim 8 having a mini-mum of 90 mole percent of SiO2.