Fabrication of optical waveguides using slurry deposition

A tubular formation of particulate optical material is formed by a layer slurry deposition process which involves spraying layer after layer of slurry containing particles of the optical material onto a rotating rod-shaped bait. The composition of the slurry and particularly the index of refraction of the optical material may be varied from one layer to another or from one group of layers to another to obtain a graded or stepped refraction index profile in the formation, in an optical preform formed therefrom, and ultimately in the fiber drawn from the optical preform. The particles of the slurry are suspended in a liquid vehicle which evaporates in the spray stream or shortly after deposition, and are coated with an organic binder which holds them together in the layer and in the formation, so that the formation is self-supporting. Then, the organic binder is removed and the formation is sintered, followed by a collapse of the sintered tubular formation into the optical preform in the form of a solid rod. Then, optical fiber of the desired optical properties can be drawn from the optical preform. The associated apparatus includes a spray gun aimed at the bait rod, and a receptacle for the slurry from which the slurry is supplied to the spray gun. The bait rod is clamped in chucks and is rotated, while the spray gun moves in the axial direction of the bait rod in a plurality of passes to deposit the formation on the bait rod.

BACKGROUND OF THE INVENTION 
This invention relates to an improved process for forming optical preforms 
for the production of optical fibers. The process is especially suitable 
for making optical preforms which possess step or graded refractive index 
profiles. Such characteristics enable the production of optical fibers 
cables exhibiting reliable operating characteristics. 
There has been a continuous search in the prior art for the economical and 
mass production of fiber optic cables for use in optical communications 
systems. 
Thus, the prior art considered and describes techniques such as "soot" 
deposition or hydrolysis wherein a gas vapor mixture is hydrolyzed by a 
flame to form a glass precursor particulate. The particulate is then 
deposited on a rotating glass rod serving as a mandrel. The soot is 
deposited upon the mandrel in a perpendicular direction to provide 
successive layers of constant radius or to provide a composite article 
with radial gradations in the index of refraction by varying the dopant 
concentration during successive passes of the burner flame. The mandrel is 
then removed and the thus released cylindrical article is collapsed to a 
solid rod-shaped preform and then a fiber is drawn from this preform. This 
process is shown and discussed in U.S. Pat. No. 3,826,560 and U.S. Pat. 
No. 3,823,995. This process, however, is very laborious and 
time-consuming. Consequently, the preform and the fiber drawn therefrom 
are relatively expensive. Moreover, it is very difficult if not impossible 
to achieve a complete utilization of the soot or of the material from 
which it is obtained because of difficulties encountered in capturing 
and/or recycling such materials. 
Other techniques, such as that described in U.S. Pat. No. 3,614,197 involve 
processes for continuously forming an optical fiber by using a 
multi-stepped funnel-shaped vessel to form a solid glass rod-shaped 
preform which is then heated and drawn into a fiber. Even this procedure 
is rather expensive and prone to result in contamination of the preform 
and the fiber by undesirable inclusions. 
In any event, there is a desire to provide a solid optical preform and then 
draw or process the same into an optical fiber. Both the continuous 
forming process and the tubular preform forming and collapsing approach 
have inherent benefits in the mass production of such cables but also 
certain disadvantages. 
Furthermore, U.S. Pat. No. 3,966,446 discusses another technique for 
providing an optical preform. The optical preform is here fabricated by 
the axial deposition from a direction along the preform axis as opposed to 
radial deposition from a direction perpendicular to the preform axis as 
used in the above-mentioned approaches. This technique does not require a 
mandrel and thus avoids the need for collapsing a cylindrical preform 
prior to drawing. Yet, even this technique is rather cumbersome and 
time-consuming and, consequently, expensive. In most instances, the 
avoidance of the need for collapsing the preform is more than outweighed 
by the inconvenience of using such a complicated process and the expense 
associated therewith. 
The preforms thus provided in the just mentioned patent may be provided 
with longitudinal gradations in the index of refraction and thus serve to 
enhance certain types of mode conversions. However, this technique is not 
readily suited for providing radial gradations in the index of refraction. 
This is an additional reason for not using this approach in the 
fabrication of fiber with radially graded index of refraction. 
In any event, there is a need to provide large optical preforms which then 
can be drawn into elongated optical fibers. There is a further need to 
provide an optical preform which can exhibit step, single mode or graded 
index profiles in the radial direction to enable the resultant cable to be 
used to more efficiently transmit optical information in the form of 
digital or other signals. 
It is known that optical fiber cables which possess a single mode of 
operation alleviate mode dispersion problems. It has been a problem to 
produce reliable cables employing single mode operation in that the prior 
art techniques could not adequately control the composition of the cable. 
Moreover, it is difficult to assure that the operation will be conducted 
in the single mode under all operating conditions. Thus, many cables 
employ a multi-mode operation in using radial gradations in the index of 
refraction. In these cables the difference in velocity from mode to mode 
compensates for the different path lengths and results in a relatively 
equal traversal time for all modes. 
It is clear that, in order to efficiently employ a single mode or a 
multi-mode operation, one must carefully and accurately control the 
fabrication of the fiber to assure that the same is consistent in 
formulation and hence, possesses repeatable and reliable operating 
characteristics. 
The current fabrication techniques of all optical fiber preforms are 
broadly based on one fundamental principle, i.e. vapor phase deposition. 
For example, the reported processes are chemical vapor deposition (CVD), 
modified chemical vapor deposition (MCVD), outside vapor phase oxidation 
(OVPO), inside vapor phase oxidation (IVPO), and plasma-activated chemical 
vapor deposition (PCVD). In all these processes, metal halides, such as 
pure or doped silicon halides, are converted at high temperatures to the 
respective oxide particles and the chemical conversion and deposition 
processes occur simultaneously. As mentioned before, such conventional 
processes are rather time-consuming and expensive, especially because of 
the slow rate of growth of the deposited layer and the need for performing 
such processes in carefully controlled atmospheres and at relatively high 
temperatures. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to avoid the 
disadvantages of the prior art. 
More particularly, it is an object of the present invention to develop a 
method of making an optical preform, especially one having a radially 
graded index of refraction, which does not possess the disadvantages of 
the conventional methods of this kind. 
Still another object of the present invention is to provide a method of the 
kind here under consideration which is inexpensive to perform, results in 
a relatively rapid formation of the optical preform, and can be conducted 
at least predominantly under ambient conditions. 
A concomitant object of the present invention is to devise an apparatus 
which is especially suited for performing the method of the present 
invention. 
It is yet another object of the invention so to construct the apparatus as 
to be simple in construction, inexpensive to manufacture, easy to use, and 
reliable in operation nevertheless. 
In pursuance of these objects and others which will become apparent 
hereafter, one feature of the present invention resides in a process for 
manufacturing an optical preform, this process comprising the steps of 
providing a bait; forming at least one slurry containing particles of an 
optical material with a predetermined index of refraction suspended in a 
liquid vehicle; depositing at least one cohesive layer of the particles on 
the bait, including covering at least a portion of the bait with the 
slurry and evaporating the liquid vehicle; removing the cohesive layer 
from the bait; and sintering the cohesive layer into the preform. 
Advantageously, the depositing step includes directing at least one stream 
of the slurry against the bait, and conducting relative movement between 
the stream and the bait. 
When the bait is elongated and has a circumferential surface centered on an 
axis, it is advantageous when the conducting step includes performing 
relative movement between the bait and the stream of the slurry both in 
the axial and in the circumferential direction of the bait. A particularly 
simple solution is obtained when the performing step includes rotating the 
bait about its axis and moving the stream axially of the bait. 
The process of the present invention further advantageously includes the 
step of repeating the depositing step to deposit an additional cohesive 
layer of the particles on top of the cohesive layer deposited during the 
previous depositing step. This expedient has particularly advantageous 
results when the forming step includes forming a plurality of slurries 
containing particles of optical materials with different indexes of 
refraction including the predetermined index of refraction. Then the 
repeating step includes using a slurry selected from the plurality of 
slurries which contains particles of a different index of refraction than 
those deposited during the previous depositing step. 
According to a further advantageous facet of the invention, the forming 
step includes coating the particles with a binder which holds the 
particles together subsequent to the evaporation of the liquid vehicle. 
Then, the inventive process further includes the step of removing the 
binder from the cohesive layer at the latest immediately prior to the 
sintering step, especially by heating at least the cohesive layer to a 
temperature sufficient to expel the binder from the cohesive layer. 
It is further advantageous when the process further comprises the step of 
heating at least the deposited layer in a hydrogen-free atmosphere prior 
to the sintering step to reduce the hydroxyl contents of the layer. 
An apparatus for manufacturing an optical preform in accordance with the 
above method preferably comprises means for mounting a bait; at least one 
source of slurry containing particles of an optical material with a 
predetermined index of refraction suspended in a liquid vehicle; means for 
depositing at least one cohesive layer of the particles on the bait, 
including means for covering at least a portion of the bait with the 
slurry and for evaporating the liquid vehicle; and means for sintering the 
cohesive layer after its removal from the bait into the preform. 
Advantageously, the depositing means includes means for directing at least 
one stream of the slurry against the bait, and means for conducting 
relative movement between the directing means and the bait. 
The apparatus may advantageously be used with an elongated bait having a 
circumferential surface centered on an axis. Then, the conducting means 
includes means for performing relative movement between the bait and the 
directing means both in the axial and in the circumferential direction of 
the bait. A particularly simple construction results when the mounting 
means mounts the bait for rotation about its axis and when the performing 
means includes means for rotating the bait, and means for moving the 
directing means axially of the bait. 
Thus, a new and low cost process of fabrication of otpical waveguides is 
provided in which the chemical conversion to the oxides and the deposition 
are carried out in two distinctly separate steps. This process involves a 
layer slurry deposition (LSD) in which a slurry of silica or similar 
optical material is deposited in consecutive layers on a rotating bait 
surface at room temperature. The thus coated bait is then sintered to a 
preform from which optical fibers can subsequently be drawn. 
To obtain the slurry or each of the slurries, chemically pure silica and/or 
doped silica powders are mixed with an organic vehicle (consisting of a 
solvent, binder and deflocculant). Viscosity of the slurry can be adjusted 
by controlling the ratio of powders to the liquid vehicle. By controlling 
the flow rate and adjusting the location of an atomizer which forms the 
directing means, the slurry or slurries will be sprayed directly onto a 
rotating bait surface. When the slurry droplets come out of the atomizer 
or spray gun, the solvent or liquid vehicle quickly evaporates in air and 
the solid silica particles coated with a thin film of organics uniformly 
deposit onto the bait surface. Layer after layer of different silica 
composition can be deposited to form a graded structure which will then be 
sintered followed by conventional fiber drawing. 
A flow diagram of the optical fiber manufacture including the LSD process 
is shown below: 
##STR1## 
The sintering and fiber drawing steps can be performed as parts of a single 
process step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to or during the performance of the process of the present invention, 
powders of pure and/or doped silica or similar optical materials will be 
dispersed preferably in a non-aqueous liquid medium or vehicle, composed 
of a solvent, binder and deflocculant to make a slurry. The viscosity of 
this slurry can be controlled by adjusting the amount of its various 
constituents. When this slurry is sprayed on a surface, the solvent which 
is highly volatile will evaporate fast leaving a flexible but 
dimensionally stable or cohesive film. The particle size of the solid 
powders may vary between submicron to several hundred microns. 
The constituents of the liquid vehicle, in general, should have the 
following properties: 
Solvent 
Low boiling point and viscosity 
Soluble with binders and deflocculants 
Chemically inert to silica based powders 
Binder 
Should provide high strength to hold the silica particles together after 
the solvent evaporates 
Should readily evaporate or burn at temperatures below the sintering 
temperature to leave the cohesive layer prior to the sintering 
Deflocculant 
Should uniformly disperse silica powders in the slurry 
Should keep the powders in suspended state in the slurry. 
Layers of films can be deposited one above the other consecutively with 
each layer having different dopant concentration to grade the refractive 
index profile. The composite article made in this way will be finally 
sintered to optically transparent glass in a gradient furnace and drawn 
into fiber. 
The slurry for use in the present invention may be prepared as follows. 
Uniformly doped silica powders will be prepared in large quantities by 
plasma spray, sol-gel, spray drying or other similar technique. These 
techniques are so well known that they need not be discussed here. The 
plasma spray technique will be superior to the other techniques, in the 
sense that it does not require any OH removal operation. Furthermore, 
relatively cheaper raw materials can be used due to preferential 
volatilization of the impurities in plasma flame. However, the other 
techniques can also be used for producing the powders. In such a case, 
however, the cohesive layer or article is preferably heated in a 
hydrogen-free atmosphere to reduce the amount of hydroxyl groups therein. 
The liquid vehicle will be prepared from an acetone based solvent, 
polyvinyl butyral, polymethyl methacrylate or other type of binders, and 
glycerol tri-oleate, benzene sulfonic acids or other type of 
deflocculants. Silica based powders will be suspended at room temperature 
in the liquid vehicle and agitated to produce a uniformly dispersed 
slurry. Depending on the desired Ge or other dopant concentration profile, 
the proper amount of silica/organic and doped silica/organic suspensions 
from two separate containers will be transferred to a mixing chamber. 
The slurry will then be sprayed through a spray gun on a bait surface at 
room temperature. A typical spray gun head suited for directing a stream 
or spray of the slurry onto the bait is shown in axial section in FIG. 1, 
wherein the reference numeral 1 has been used to identify the spray gun 
head in its entirety. The spray gun head 1 includes an annular support 
member 2 which is provided with a plurality of flow passages 3 for an 
entraining medium, preferably a gaseous medium such as air. The support 
member 2 is provided in its center with an internally threaded bore 4 into 
which there is threaded an externally threaded portion 5 of a slurry 
nozzle 6. The nozzle 6 defines an internal flow passage 7 for the slurry 
which terminates in an outlet orifice 8. The support member 2 also has an 
externally threaded portion 9 onto which there is threaded an internally 
threaded portion 10 of a confining sleeve 11. 
A guiding part 12 is confined in the interior of the confining sleeve 11 
and bounds, together with the confining sleeve 11 and the nozzle 6, an air 
distributing compartment 13 or a plurality of such air distributing 
compartments 13. The guiding part 12 may be of one piece with the support 
member 2. A main flow passage 14 for the entraining medium, or a plurality 
of such main flow passages 14, is defined between the internal surface of 
the guiding part 12 and the external surface of the nozzle 6. This main 
flow passage communicates with the distributing compartment 13 and 
converges in the direction of flow of the entraining medium until it 
terminates at the outlet orifice 8. In operation, the entraining medium 
flows through the main flow passage 14 toward the region surrounding the 
outlet orifice 8 of the slurry nozzle 6, where it entrains droplets of the 
slurry for joint travel therewith in the form of a spray 15. 
The spray gun head 1 further includes a cap 16 which has a conical outer 
surface and which is retained in position relative to the confining sleeve 
11 by engagement with an inwardly projecting portion or bead 17 of the 
confining sleeve 11 with the conical outer surface of the cap 16. The cap 
16 may be of one piece with the guiding part 12, in which case the 
engagement of the bead 17 with the cap 16 also retains the guiding part 12 
in position relative to the confining sleeve 11. In this construction, the 
guiding part 12 need not be of one piece with the support member 2. 
The outer surface of the guiding part 12 bounds an additional flow passage 
18, or a plurality of such additional flow passages 18, with the internal 
surface of the confining sleeve 11. The flow passage 18 extends from the 
distributing compartment 13 to the bead 17 where it is sealingly closed by 
the latter. A flattening orifice 19, or a plurality of such flattening 
orifices 19, extends from the additional flow passage or passages 18 to a 
front region of the guiding part 12 as considered in the direction in 
which the spray gun head 1 is aimed. The orifice or plurality of orifices 
19 is defined between the cap 16 and the guiding part 12. The flattening 
orifice or the plurality of such orifices 19 is so oriented that the 
entraining medium issuing therefrom will reduce the spread of or flatten 
the spray 15. A clean dry source of compressed air will preferably be used 
at a pressure between 60-100 lbs. to break up the slurry as it leaves the 
orifice 8 of the fluid nozzle 6, atomizing it and keeping the slurry 
within a cone-shaped area. When the slurry droplets come out of the spray 
gun , the solvent evaporates in air and the solid silica particles coated 
with thin film of organics deposit onto the bait surface. There the 
organics coating will serve as a binder to hold the particles in position 
relative to one another and thus to form a cohesive layer from such 
particles. 
The bait surface and the spray gun arrangement can be made in several ways. 
One simple arrangement is shown in FIG. 2 where a thin bait rod 20 is 
mounted in a lathe chuck assembly 21, 22. Since the process is conducted 
at room temperature, the bait rod 20 may be selected from a wide variety 
of materials. 
During the deposition process, a spray gun 23, which is shown to 
incorporate the spray gun head 1, will move at a controlled speed in a 
transverse manner as considered in the drawing, that is, axially of the 
bait rod 20. Layer after layer of the same or different silica composition 
may be deposited to form a uniform, stepped, or graded refractive index 
profile. The undeposited materials are collected and, to the extent 
feasible, recycled. 
FIG. 2 also shows that a slurry 24 may be contained in a slurry receptacle 
25, where the optical material particles are mixed in any conventional 
manner with the liquid vehicle and the deflocculant. In the illustrated 
construction, the slurry 24 forms an upper surface 26, and dry compressed 
air at appropriate pressure is admitted through an inlet conduit 27 into 
the receptacle 25, where it will bubble through the slurry 24 and rise to 
the upper surface 26 thereof. This passage of air through the slurry 24 
may be sufficient to disturb the slurry 24 to such an extent that it may 
not be necessary to provide any additional mechanical stirring or mixing 
means to maintain the particles in suspension. 
The spray gun 23 is connected to the receptacle 25 by a conduit 28 which, 
in the illustrated construction, is rigid. The conduit 28 extends through 
the upper level 26 of the slurry 24 to the bottom region of the receptacle 
25. The pressure of the air present above the upper level 26 of the slurry 
24 is at least sufficient to overcome the resistance encountered by the 
slurry 24 on its way to the spray gun head 1 due to the friction within it 
and between the slurry 24 and the internal surface of the conduit 28 and 
to the elevation difference between the upper level 26 and the spray gun 
head 1. Thus, the slurry 24 will rise at least to the orifice 8, (see FIG. 
1) if not squirt beyond it, where it will be entrained for joint travel by 
the entraining gaseous medium to form the spray 15. This spray 15 
eventually reaches the exposed surface of the bait 20 and forms a layer 29 
thereon. The solvent or liquid vehicle is so volatile that it will 
evaporate, in most instances, while the particles of the optical material 
are still in transit in the spray 15. What remains is a thin film of the 
binder on the particles, which attaches such particles to one another in 
the just formed layer 29. This makes the layer 29 cohesive, so that the 
particles will stay in the layer 29. 
In the construction illustrated in FIG. 2, the conduit 28 is rigid and is 
immovably connected to the receptacle 25. The receptacle 25, however, is 
mounted for movement axially of the bait rod 20, as indicated by the 
double-headed arrow. As the receptacle 25 is moved, by conventional moving 
means which has been omitted from the drawing, in the axial direction of 
the bait rod 20, the conduit 28 and the spray gun 23 with its spray gun 
head 1 mounted thereon also move in the axial direction of the bait rod 20 
so that the spray 15 deposits the slurry 24 or the coated particles of the 
slurry first at one end of the exposed circumferential surface of the bait 
rod 20 and then progressively at adjacent regions of the exposed 
circumferential surface until the other end of the exposed circumferential 
surface is reached. Of course, the spray 15 is aimed on such an exposed 
surface, preferably along a plane including the longitudinal axis of the 
bait rod 20. However, it is also possible and contemplated for the 
receptacle 25 to be stationary, for the conduit 28 to be flexible, and for 
only the spray gun 23 with its head 1 to be movable in the axial direction 
of the bait rod 20. 
A single layer 29 is deposited during each pass of the head 1 or of the 
spray 15 along the bait rod 20. To obtain uniformity in the deposited 
layer 29, it is advantageous if not mandatory to cover the exposed surface 
of the bait rod 20 around its entire circumference during the pass. In the 
illustrated construction, this is achieved by rotating the bait rod 20 
about its longitudinal axis by rotating the chucks 21 and 22 which grip 
the bait rod 20 outside of the exposed surface, that is, outside of the 
area reached by the spray 15. Depending on the character of the movement 
of the spray head 1, the layer 29 will be deposited in successive 
overlapping annuli or in a continuous helical overlapping strip. 
A single layer 29 is usually not sufficiently thick either to be 
self-supporting or to contain a sufficient amount of the particulate 
optical material to make an optical preform therefrom. Moreover, for 
obvious reasons, the layer 29 will have to have the same index of 
refraction throughout. For these reasons, the depositing operation is 
repeated in a plurality of passes, each of them resulting in the 
deposition of one coherent layer 29, until a formation 30 is provided on 
the bait rod 20. The formation 30 is self-supporting, due to the adhesion 
of the particles of the layers 29 to one another due to the action of the 
binder coating, both within each layer 29 and as between the layers 29. 
The formation 30 also includes the requisite amount of material to form an 
optical preform of the required size therefrom. 
It will be appreciated that each of the passes of the spray 15 is an 
operation independent of the preceding and succeeding passes. Hence, in 
accordance with the present invention, it is contemplated to vary the 
composition of the slurry 24, if not from pass to pass, then from a group 
of passes to the next succeeding group of passes. In this respect, the 
main if not only variation is in the composition of the particulate 
material of the slurry 24 such that the index of refraction of the layers 
29 of the formation 30 varies from one layer 29 to another or from one 
group of layers 29 to another. In the first instance, a radially graded 
refractive index profile is obtained in the formation 30, while the 
formation 30 has a radially stepped refractive index profile in the second 
instance. Each of these approaches has repercussions on the refractive 
index profile of the fiber drawn from the final preform such as to make 
the drawn fiber suitable for the intended use thereof. 
As mentioned before, the formation 30 is self-supporting. Thus, at the end 
of the depositing operation, the bait rod 20 can be removed from the 
interior of the formation 30. Then, the formation 30 will be further 
handled to convert the same into a rod-shaped optical preform from which 
an optical fiber can be drawn. This further handling includes heating the 
formation 30 to a temperature of about between 500.degree. and 600.degree. 
C., at which the formation 30 still retains its basic particulate and 
porous character but the binder will either evaporate or become coverted 
in gaseous chemical compounds, such as by burning, so that it will escape 
from the formation 30 without a trace through the pores present between 
the particles. Yet, the formation 30 will remain self-supporting and 
retain its tubular shape, apparently due to point fusion of the particles 
and/or frictional and other mechanical forces between the particles. 
Thereafter, the formation 30 may be passed, if needed, through a zone 
containing hydrogen-free atmosphere to reduce the number of hydroxyl 
groups in the material of the formation 30. The next following step is the 
sintering of the tubular formation 30, which is conducted at a temperature 
of about between 1200.degree. and 1400.degree. C. In this sintering 
operation, the particles of the formation 30 will fuse with one another, 
thus eliminating most if not all of the pores of the formation 30. Yet, 
the formation still retains its tubular shape. Then, finally, the 
formation 30 is heated to a temperature of about 2000.degree. C., 
resulting in an inward collapse of the formation 30 and in elimination of 
the remainder of the pores, if any, so that a solid rod-shaped optical 
preform is obtained. 
The optical preform can then either be permitted to cool and be stored for 
future use or immediately used for drawing the optical fiber therefrom. 
The latter approach has the advantages not only of eliminating the need 
for additional handling steps between the formation of the optical preform 
and the drawing of the fiber therefrom, but also of utilizing at least a 
part of the heat content imparted to the preform during the previous 
pre-heating,, sintering and collapsing steps in the drawing operation. 
This results in a very economical operation which reflects itself in the 
manufacturing cost of the fiber. 
The succession of steps involved in the production of an optical cable from 
the starting materials is diagrammatically illustrated in FIG. 3. 
Another method of slurry deposition will be to traverse the rotating bait 
rod 20, keeping the spray gun position fixed. Other methods can be 
instituted to make optical fiber preform by the novel process described 
herein. The essential components of this process are to make a slurry out 
of silica based oxides and an organic vehicle system appropriate for the 
deposition of the particulate oxides in a dimensionally stable form at 
room temperature. The deposited materials will dry up instantaneously and 
sufficiently to assume the appearance of a solid layer which will have 
strong adherence to each other but not to the bait surface. The organic 
binder in this system will provide high strength to hold the solid 
particles together after the deposition and thus maintain precise 
dimensional control of the slurry preform. In essence, Layer Slurry 
Deposition (LSD) process has the following advantages. 
Fibers of any required design can be processed due to precise control of 
refractive index profile. 
Large preforms and hence long length fibers can be drawn. 
High deposition rate and efficiency, low loss of raw materials and less 
expensive unit operations will make the fibers more cost-effective. 
Because the system is non-aqueous and entirely organic, the chances of OH 
contamination during the deposition process is almost none. 
The depositing operation can be conducted at room temperature. 
While we have described above the principles of our invention in connection 
with specific apparatus, it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of our invention as set forth in the objects thereof and in the 
accompanying claims.