CA1115508A

CA1115508A - Carbon coating for a starting member used in producing optical waveguide

Info

Publication number: CA1115508A
Application number: CA305,889A
Authority: CA
Inventors: Ellen K. Dominick; George W. Scherer
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1977-08-26
Filing date: 1978-06-20
Publication date: 1982-01-05
Also published as: ATA621178A; NL7808821A; GB2003132A; AT377747B; NL188845B; FR2401108A1; DE2837261A1; DE2837261B2; IT1098691B; IT7827046A0; JPS5443753A; US4204850A; GB2003132B; DE2837261C3; FR2401108B1; NL188845C; JPS6054893B2

Abstract

Dominick-Scherer 2-1 D-7309 (CO-76) CARBON COATING FOR A STARTING MEMBER USED
IN PRODUCING OPTICAL WAVEGUIDE

Abstract Of The Invention In making a preform for an optical waveguide, the bait rod is coated with carbon. After the preform is built up on the bait rod, it is easily removed. The carbon coating produces a smooth improved inner surface in the preform, thereby eliminating flaws which otherwise may be present when the preform is drawn into a waveguide.

Description

-- 0~0977 ~ D-7309 (~-76) ~ 5~8 Background Of The Invention This invention relates to optical waveguides and more particularly to an improved process for making the pre-form for an optical waveguide.
Waveguides used in optical communications systems are herein referred to as "optical waveguides" and are normally constructed from a transparent dielectric material such as glass or plastic.
U. S. Patent 3,775,075 - Xec]c and Maurex describes a process of making optical waveguides wherein a glass coating is deposited by flame oxidation* upon a rotating glass cylinder. Thereafter the formed structure is heated and drawn into a waveguide with the glass cylinder forming the core and the coated material forming the cladding of the optical waveguide. The glass cylinder upon which the cladding material is coated is referred to as a preform.
U. S. Patent 3,823,995 - Carpenter describes one technique of making a preform. In thi~ technique, glass is deposited by flame outside vapor phase oxidation (OVPO) on a rotating starting member which is referred to as a bait rod. After the core and cladding layers are deposited, the bait rod is removed. Then the structure is heated and drawn During the drawing operation, the hollow core of the preform collapses, thereby forming a consolidated core with a cladding.
The removal of the bait rod from the preform must be carefully carried out because imperfectlons on the inside surface of the preform may cause flaws in the finished waveguide. Imperfections on the inner surface of the preform *Now called outside vapor phase oxidation (O~PO); the reaction is ]cnown to be oxidation rather than hydrolysis.

- D-7309 (C0-76) propagate cracking and breaking. Any imperfection in the center hole may cause bubbles to form in the waveguide which is ultimately produced causing high attenuation and rejection. As described in the aforementioned Carpenter patent, the bait rod is removed by grinding it out by means of a diamond reamer. That patent also mentions core drilling and hydrofluoric acid etching for removing the bait rod.
The rough surfaces left by drilling may be smoothed by laser milling, mechanically polishing the inside surface, fire polishing the inside surface and/or washing the tube in hydro-fluoric acid. ~11 of these operations are time-consuming.
More recently, the preform has been made by first depositing a soft layer of soot on the bait rod and thereafter building Up the preform with hard soot. The layer of soft soot facilitates removal of the bait rod. The soft layer shears, leaving a flaky pitted center hole in the preform.
After sintering the preform to consolidate the glass, the preform still contains these flaws. A lengthy hydrofluoric acid treatment is necessary for elimination of these imper-fections. Damage occurs because the preform adheres so strongly to the surface of the bait rod. When the rod is removed, the preform shears rather than releasing from the bait xod surface.
U. S. Patent 3,933,453 - Burke et al, describes an improved mandrel including a tubular member formed of refractory metal. U. S. Patent 3,806,570 ~ Flamenbaum et al describes the us~ of a fused carbon mandrel for the bait rod. ~Iowever, neither of these mandrels can be removed from the preform without causing blemishes which require smoothing.
Imperfections in the center hole of the preform are a particular problem in making gradient inde~ waveguides D-7309 (C0-76) ~~

such as described in the aforementioned Carpenter patent.
In order to get a high numerical aperture waveguide, it is necessary to introduce a sharp radial change in composition of the preform. Because of the changing thermal expansion coefficient of this composition, high stress is present. In such waveguides, it is particularly important to eliminate imperfections in the center hole of the preform. Any imper~
fection may lead to fracture in a high stress waveguide.

Summary Of The Invention In accordance with this invention, imperfections in the center hole of a preform for an optical waveguide are reduced by coating the starting member with carbon before depositing the preform thereon. Thereafter the starting member can be removed with little detrimental effect on the preform. Separation occurs a-t the interface between the - carbon and the starting member or by shearing of the carbon.
There is no shearing of the preform. The preform has a very smooth, undisturbed center hole.
The preform is then conventionally heated to con-solidate the glass. During consolidation, the carbon oxidizes, thereby completely disintegrating it. ~fter consolidation, the preform has a smooth clean inner surface. Whereas lengthy hydrofluoric acid treatments were previously necessary to smooth the inner hole of the preform, this is no longer necessary. The inner hole can be allowed to close during the consolidation procedure, whereas in the prior art, the inner hole was not closed until drawing. Because the inner hole is closed during consolidation, it is possible to draw _~_ ;5~1 the waveguide at higher speeds than were previously possible.
It is an object of the present invention to provide improved optical waveguide preforms with reduced imperfections on the inside surface thereof.
It is another object of ~he present invention to produce high numerical aperture, high stress waveguides from preforms with xelatively smooth inner surfaces.
It is another object of the present invention to eliminate the etching step in the production o optical waveguides.
It is further object of the present invention to increase the drawing speed in the production of waveguides.
The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended-claims.
Thus:the present invention provides a method of making a preform for an optical waveguide comprising: providing a ; substantially cylindrical starting member; applying a coating of substantially pure particulate carbon to the outside 20 peripheral surface of said cylindrical starting member; applying ;
particulate material to said starting member to form an adherent coating of glass; removing said starting member from said glass to cause separation between said carbon layer and the outside peripheral surface of said starting member or shearing within the carbon layer to produce a preform; and heating said preform to a temperature which disintegrates said carbon by oxida1:ion and which consolidates said preform.
In one aspect the invention provides such a method wherein the step o applying particulate material to said starting member includes depositing a coating of glass soot on .y~, 55~

the outside peripheral surface of said starting member by flame outside vapor phase oxidation. In a preferred embodiment this may be-effected:by~entraining a gaseous material predetermined amounts of each of a plurality of constituents in vapor form; oxidizing said vapors in a flame to form a soot having a predetermined composition and applying said soot to the exterior peripheral surface of said member. In another preferred aspect the invention provides such a method further comprising radially varying the composition of the soot deposited on said starting member to produce a preform having a radially varying index of refraction.
In another embodiment the present invention provides such a method as set out broadly above further comprising ; applying a coating of cladding glass having an index of refraction less than that of said first named coating, the composition of said cladding glass being compatible with that ~- of said first named coating.
Descri tion O ~he Drawinqs Fig. 1 depicts the deposition of the carbon coating;
Fig. 2 depicts the step of applying glass to the preform by flame hydrolysis;
Fig. 3 depicts heating to consolidate, burn the carbon and collapse the center hole; and Fig. 4 depicts the step of drawing the preorm into a waveguide.
Descri tion Of The Preferred Embodiment P _ .
Referring to Fig. 1, a substantially cylindrical starting member 10 is continuously rotated in an acetylene _ 5(a) ~
,. ~

,~

0~1077 '` D-7309 (C0-76) flame 11 to produce a carbon coating 12 on the starting member. For example, the starting member may be a tapered alumina bait rod tapering from .630 to .570 centimeters diameter along a 63 centimeter length. Acetylene burner 13 burns acetylene in air (no oxygen gas supply) to produce carbon soot. Bait rod 10 is mounted in a lathe which rotates the rod and traverses it past the f~ame 11. It has been found that the thickness of the coating is not critical but 1-2 mills thickness produces satisfactory results.
After the bait rod is thoroughly coated with carbon, glass soot is deposited by flame hydrolysis as depicted in Fig. 2. The procedure described in the afore-mentioned Carpenter patent may be used to produce a gradient index waveguide. Alternatively, step index waveguides with lS a central core with one index of refraction and a cladding layer of a lower index of refraction may be produced. An adherent layer of glass soot 14 is built up on the bait rod 10 as it is rotated and translated in the hydrolysis flame 15. Both the preform 14 and the carbon coating 12 can ~0 be removed from the bait rod with no detrimental effect.
Preform removal results from separation at the carhon-bait rod interface or from shearing of the carbon and not from shearing of the preform. This allows protection of the inner surface of the preform and easy removal of the bait rod.
Examination of the preform shows a very smooth, undisturbed center hole.
The preform is then sintered in the usual manner as is depicted in Fig. 3. The carbon completely disinte~rates by oxidation at about ~00 C. which is approximately 500 lower than normal consolidation temperature. Inspection after consolidation shows a smooth clean inner surface that requires S~
D-7309 (C0-76) little, if any, hydrofluoric acid treatment. In many instances, the etching step may be eliminated completely, in which case the consolidation step can be carried out in such a way as to close the center hole. This allows faster, more economical operation of the draw facility which is depicted in Fig. 4.
Furthermore, closure of the center hole during consolidation permits the making of preforms with high internal stress (high radial change in composition and refrac-tive index) because the elimination of the hole during consoli-dation removes one of the prominent sources of fractures in waveguides. This commonly occurs because of flaws on the interior surface of the consolidated (i.e. sintered) preform. Also, the invention has particular advantages in making high numerical aperture guides which require high dopant levels and consequently greater stress caused by steep thermal expansion coefficient changes, because, the smooth inner surface on the consolidated preform made in accordance with this invention can tolerate high stress.

~nother advantage of the invention is the elimina-tion of etching away approximately 4% of the weight of the preform as is routinely done in present practice where soft soot is initially deposited on the bait rod to facilitate bait removal and is later etched away.

The carbon coating can be applied by several other methods including dipping the rod in a carbon slurry or dipping the bait rod in wax and then charring it. The carbon coating must burn off during consolidation and the coating must not impart impurities to the preform. The following D-7309 ~C0-76) ~ S~

examples demonstrate the feasibility of this invention.

E X A M P L E # 1 A 33 wt. % Aquadag E, Aqueous graphite dispersion, mixture was made in distilled water. Final weight percent graphite was 13~. This mixture was rolled for several hours and then poured into a PyrexR tube appxoximately 30 inches long and 3/4" in diameter.
A clean, tapered alumina rod about 63 cm. long and tapering from .630 to .570 cm. diameter was carefully dipped in the mixture and removed. The surface o~ the adherent layer of graphite was inspected for flaws - lumps, inclusions, uneven areas - and allowed to air dry at room temperature several hours.
The prepared bait was next secured in a lathe normally used for preform production. Standard laydown procedures were used for ~he productioi of a parabolic blank except the soft preliminary soot laydown was omitted. The GeO2 content was varied parabolically from 15 to O wt. ~
across the core, while B203 content was raised from 2 to 12 wt. ~.
The soot was deposited by a burner having an inner and outer shield, both of which supply ox~gen, and inner burners for the other gases. A gradient index guide was produced in which the BC13 was increased and the GeC14 was decreased as a ramp function of time. The settings used were as follows:
Outer Shield 4~0 L/min~
Inner Shield 4.5 L/min.

D-7309 (C0-76) ~55~

Premix Gas 6.0 L/min.
Pxemix Oxygen 5.0 L/min.
By-pass O~ 0.5 L/min.
SiCl4 8.0 gm/min.
BCl3 Ramp Program .097 gm/min. - 1.17 gm/min.
GeC14 Ramp Program .8 gm/min. - O gm/min.

Traverse Speed ~ 98cm/min.
Rotation Speed ~ 300 RPM
Burner to bait distance 14 cm The center hole of this preform after bait removal was excellent, no major flaws were noticed and it appeared very smooth and relatively undisturbed. The preform slipped from the bait rod far more easily than when the carbon coating was not used.

Next, the preform was consolidated a-t 1320C in Helium with a feed rate into the furnace of .l inah/min. After consolidation the blank was allowed to s_ool and the inner surface examined. A significant improvement of inner hole surface was noted. The usual pits and scratch marks were absent. The preform was etched and then drawn into waveguides of 125 um diameter. These waveguides had the following properties:

Fiber Reel ~ Attenuation _ Band Width_ 1 9.2 aB/km .146 850 Megahertz

2 4.8 ~146 560

3 4.0 .150 560

4 3.8 .156 560 4.5 .15~ 540 6 4~8 .150 500 7 5.3 .150 490 8 5.1 .150 480 9 5.6 ~150 420 D-7309 (C0-76) ~ S5~

.

These results show that the carbon does not adversely affect the attenuation of the waveguide.

E X A M P L E ~ 2 An alumina bait rod of the type described above was coated with carbon soot, as follows: the rod was mounted in a lathe and rotated at 300 RPM; a flame consisting of acetylene burning in air was held under the rod so that the carbon soot produced by the flame was deposited on the rod;
by moving the torch along the length of the rod, a uniform 10layer of carbon was deposited.
A preform was deposited on the coated bait rod, consolidated and drawn into fiber, all as described in example #1. The measured properties of the resulting optical waveguide are listed below:

15iber Reel # Attenuation N,?, Band Width 1 4.3 dB/km .136 890 Megahertz 2 5.2 ~129 *
3 5.7 .139 ' 770 While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. The appended claims are intended to cover all such modifications.

*Too high to be measured, i.e. at or near the theoretical limit.

--10-- ,

Claims

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

1. The method of making a preform for an optical waveguide comprising:
providing a substantially cylindrical starting member, applying a coating of substantially pure particulate carbon to the outside peripheral surface of said cylindrical starting member, applying particulate material to said starting member to form an adherent coating of glass, removing said starting member from said glass to cause separation between said carbon layer and the outside peripheral surface of said starting member or shearing within the carbon layer to produce a preform, and heating said preform to a temperature which disintegrates said carbon by oxidation and which consolidates said preform.

2. The method recited in claim 1 wherein the step of applying carbon includes rotating said starting member in an acetylene flame to deposite a coating of carbon soot on said starting member.

3. The method recited in claim 1 wherein the step of applying a carbon coating includes:
dipping said starting member in a mixture of carbon dispersed in water;
removing said starting member from said mixture; and drying the adherent layer of carbon.

4. The method recited in claim 1 wherein carbon is applied to an aluminum oxide mandrel starting member.

5. The method recited in claim 1 further comprising collapsing the center, hole in said preform during consolidating and thereafter drawing said preform into an optical waveguide.

6. The method recited in claim 1 wherein the steps of heating and removing carbon are carried out in an atmosphere of oxidizing gas.

7. The method recited in claim 1 wherein the step of applying particulate material to said starting member includes depositing a coating of glass soot on the outside peripheral surface of said starting member by flame outside vapor phase oxidation.

8. The method recited in claim 7 wherein the flame outside vapor phase oxidation is effected by a method comprising:
entraining in a gaseous material predetermined amounts of each of a plurality of constituents in vapor form;
oxidizing said vapors in a flame to form a soot having a predetermined composition and applying said soot to the exterior peripheral surface of said member.

9. The method recited in claim 8 wherein said constituents include GeO2 and B2O3.

10. The method recited in claim 7 further comprising radially varying the composition of the soot deposited on said starting member to produce a preform having a radially varying index of refraction.

11. The method recited in claim 1 further comprising:
applying a coating of cladding glass having an index of refraction less than that of said coating of glass, the composition of said cladding glass being compatible with that of said coating of glass.

12. The method recited in claim 11 further comprising:
heating the structure so formed to the drawing temperature of the materials thereof, and drawing the heated structure to reduce the cross-sectional area thereof to form an optical waveguide.

13. The method recited in claim 1 further comprising:
heating said preform to consolidate said glass, to disintegrate the carbon coating on the hole from which said starting member was removed, and to collapse said hole; and thereafter drawing the coated preform into an optical waveguide.

14. A solid cylindrical glass preform for an optical waveguide, having a substantially cylindrical cross-section and a predetermined length, whenever formed by a method in accordance with Claim 1, 2 or 7.