Image-transmitting bundled optical fibers

Image-transmitting bundled fibers and processes for producing the same in which a bundle of elemental image fibers is assembled without randomizing so as to maintain the picture image without distortion utilizing an inventive assembly technique. The assembled fibers are covered with cladding layer then surrounded by a light-absorbing layer of a disclosed preferred group of materials. Fiber bundles of the invention can have a length of several tens of kilometers while the bundles have a good flexibility and a sufficiently high number of elemental image fibers to convey a clear and satisfactory image.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for producing a bundled fiber. 
Bundled fibers are classified into those functioning as image guides for 
transmitting images and those functioning as light guides simply for 
transmitting optical energy. In an image guide for image transmission, the 
positions of elemental image fibers as picture elements at the incoming 
and outgoing ends should exactly correspond to each other in order to 
minimize distortion of the transmitted image. On the other hand, a light 
guide does not always require such an arrangement because it is only 
intended for transmission of light energy. 
The present invention pertains to a method for producing an image guide for 
transmitting an image. Production methods for bundled fibers used as image 
guides appear to be classified into a winding method, a foil stacking 
method, multifiber formation, and fiber plate formation as described, for 
example, in the Journal of the Society of Electric and Electronics 
Engineering, Japan, Vol. 97, No. 11, November 1977. In prior methods 
suggested for producing image guides, although they individually have both 
advantages and disadvantages, none simultaneously meets all of the 
requirements upon the number of picture elements, the producible length, 
and the flexibility of fibers. 
It is thus an object of the invention to provide fiber bundles for an image 
guide having a length of from several kilometers to several tens of 
kilometers with the resulting fiber bundle having a good flexibility and a 
sufficient number of elemented image fibers or picture elements. 
Furthermore, it is an object of this invention to provide image fibers in 
which the transmission loss of an elemental image fiber for each picture 
element can be reduced to 10 dB/km or below by selection of an appropriate 
matrix for the picture elements. When compared with conventional fiber 
bundles, a markedly improved transmission distance for an image of a 
predetermined brightness is desired. Moreover, in view of the 
characteristics of the fibers used as a picture element, images from the 
ultraviolet to the infrared regions should be capable of being 
transmitted, and, as a result, the range of application for such fiber 
bundles is to be broadened. 
SUMMARY OF THE INVENTION 
These as well as other objects of the invention are met by a method for 
producing an image guide having multiple fibers including the steps of 
inserting elemental image fibers into a pipe made of quartz or 
multi-component glass with one end of the pipe sealed, spinning the 
resulting assembly at an elevated temperature while reducing the pressure 
at the open end of the pipe, coating a layer of a metal having a lower 
melting point than the material constituting the image fibers onto the 
spun assembly before the assembly contacts a solid surface so as to 
provide a protective resin coating, and cooling the spun and coated 
assembly. The elemental image fibers are made of either quartz or a 
material comprising primarily quartz, or of a multi-component glass. 
Instead of a metal coating, a layer of resin material such as a layer of 
thermosetting or ultraviolet-curable resin may be employed. 
Still further, the invention encompasses a process for producing an image 
guide having multiple fibers in which the elemental image fibers are 
properly aligned without randomization by positioning the fibers inside a 
tube constructed of quartz or multifiber glass then introducing water into 
the pipe while rotating the pipe and vibrating it ultrasonically. The 
steps of rotation and ultrasonic vibration may be carried out 
simultaneously or sequentially as preferred. 
Yet further, the invention relates to a process for producing an image 
guide by a multifiber method in which a light-absorbing layer is provided 
around a bundle of elemental image fibers which form picture image 
elements, the light-absorbing layer being a quartz pipe constructed of a 
rock crystal or coats doped with at least one metal selected from the 
group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Rh, La, Ce, Nd and 
W for increasing the coefficient of light-absorption of the 
light-absorbing layer. The invention includes also an image guide produced 
by this method. 
Further, the invention includes a process for producing an image guide 
having a light-absorbing layer around elemental image fibers which form 
the individual picture elements including a film of quartz on the outer 
surface of a cladding layer surrounding a core including a matrix of the 
elemental image fibers with the light-absorbing layer including a material 
selected from the group consisting of at least one halide of a metal 
selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, 
Rh, La, Ce, Nd and W and a halide of Si doped with one of these metals. An 
image guide produced by this process is within the invention as well. The 
image fibers may be constructed of a quartz glass for which the cladding 
layer is preferably B-doped quartz. Also, the image fibers may be 
constructed of quartz doped with at least one element selected from the 
group consisting of Ge, P, Al, Ti and Ga. In this case, the cladding layer 
may be either quartz glass or B-doped quartz.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the structure of an elemental image fiber which is to become a 
picture element in a fiber bundle in accordance with the invention. In 
FIG. 1, reference numeral 1 represents a core, 2 a clad layer, and 3 a 
light-absorbing rod which may instead be separately introduced at the time 
of aligning the individual elemental image fibers. 
FIGS. 2A and 2B show schematically apparatus for performing a process for 
producing an elemental image fiber of a fiber bundle. Specifically, FIG. 
2A shows schematically apparatus for performing a method for 
heat-softening a preform which is to become an elemental image fiber and 
for drawing out a fiber. FIG. 2B schematically shows a method for drawing 
an elemental image fiber by a double or triple crucible method. 
In FIG. 2A, reference numeral 4 represents a preform, 5 a chuck portion, 6 
a spinning furnace, 7 a fiber, 8 a wind-up reel, 9 a feed screw, and 10 a 
motor. In FIG. 2B, the reference numeral 11 represents a crucible made of, 
for example, platinum or quartz glass, 12 a core glass, 13 a cladding 
glass, 14 a light-absorbing glass, 15 a fiber, and 16 a wind-up reel. 
FIG. 3 shows a bundle of elemental image fibers 17 produced by the 
apparatus illustrated in FIGS. 2A and 2B cut to a suitable length and 
inserted into a quartz or glass pipe 18. In the state shown in FIG. 3, the 
alignment of image fibers is substantially random and, therefore, the 
arrangement of the elemental image fibers must be put in proper order for 
the bundle to be suitable for image transmission. 
FIG. 4 shows a device for properly arranging image fibers inserted in a 
quartz pipe or glass pipe. In FIG. 4, the reference numeral 19 represents 
the quartz or glass pipe, 20 an elemental image fiber, 21 a stopper, 22 a 
rotary chuck portion, 23 a hose, 24 a buffer plate, and 25 a rotary joint. 
The image fibers are aligned by a steady flow of water passing through the 
rotary joint 25 and the buffer plate 24. It is important at this time to 
rotate the quartz or glass tube and the buffer plate in an integral 
manner. This operation readily results in alignment of the free fibers 
inserted in the pipe. The rotational direction of the quartz or glass tube 
may be constant. However, alignment of the elemental image fibers becomes 
easier by repeatedly reversing the rotating direction of the tube. The 
stopper 21 serves to prevent dropping of the image fibers. The coefficient 
of friction between it and the image fibers should be low so as to make 
the image fibers easy to move. It is preferably made of such a material as 
Teflon TM (polytetrafluoroethylene) or metal. 
FIG. 5 shows a second device for aligning elemental image fibers which 
provides for a shortening of the time required for alignment by attaching 
an ultrasonic vibrator element 26 to the device shown in FIG. 4. In this 
aligning method using the ultrasonic vibrator element 26, one end of the 
quartz or glass pipe 19 is sealed and water is poured into it. However, 
without using flowing water, sufficient alignment can nonetheless be 
achieved by this method. The quartz or glass pipe may be rotated, but over 
long periods of time, sufficient alignment can be achieved even when the 
pipe is stationary. The equipment will be of course simplified if 
alignment is achieved by only pouring water without rotating the quartz or 
glass pipe. 
A third device for fiber alignment is shown in FIG. 6. Reference numeral 27 
represents elemental image fibers which are bonded or welded into a 
unitary structure at a position 29 and inserted in a glass pipe 28. The 
integral structure of image fibers is connected to a support member 33 via 
a guide rod 30. Reference numeral 31 represents a buffer plate and 32 a 
hose. In the device shown in FIG. 6, water flows from above and the fibers 
are aligned solely by the force of water flow. 
The image fibers aligned in the quartz or glass pipe by any one of the 
methods shown in FIGS. 4, 5 and 6 are subjected to a means for fixing the 
relative positions of the aligned image fibers and the quartz or glass 
pipe to prevent disarrangement of the fibers. 
FIG. 7 shows are example of a method for fixing elemental image fibers 34 
with a quartz or glass pipe 35. The quartz or glass pipe is heat-softened 
by a burner 36. By the surface tension consequently generated, the 
diameter of the quartz or glass pipe is reduced in at least one position 
to permanently fix the arrangement of the image fibers. 
By use of one of the operations described in FIGS. 1 to 7, a fiber bundle 
matrix for an image guide may be obtained. By spinning the matrix and 
reducing its diameter, a fiber bundle of a desired diameter can be 
produced. 
FIG. 8 shows an apparatus for spinning the bundle fiber matrix produced by 
the method of FIG. 7. In FIG. 8, the reference numeral 37 represents the 
fiber bundle matrix, 38 a spinning furnace, 39 a spun fiber bundle, 40 a 
die for coating a plastic material, 41 a baking oven, 42 a guide roller, 
43 a wind-up reel, 44 a chuck, 45 a feed screw, 46 a motor, 47 a suction 
device, 48 a hose, 49 a vacuum pump, and 50 a plastic-reinforced bundle 
fiber. 
The matrix 37 produced by the method illustrated in conjunction with FIG. 7 
is spun and reduced in diameter to become the fiber 39. Then, before it 
contacts a solid surface, it is coated with a thermosetting or 
ultraviolet-curable resin by the coating die 40. The coating is baked and 
cured by the baking oven 41 (or by an ultraviolet-curing oven) to obtain a 
plastic-reinforced fiber bundle. This technique produces a fiber bundle 
which is protected from air, moisture and other objects, and has 
sufficient strength to withstand normal use besides having a good 
flexibility. At this time, the pressure on the inside of the quartz or 
glass pipe can be reduced by sucking air from above the fiber bundle 
matrix. As a result, the space between the image fibers is reduced, and 
the density of the picture element fibers can be increased. Because in the 
pressure-reduced state, the outside diameter of the spun bundle fiber may 
be of oval shape, the chuck 44 may be a rotary chuck. 
In FIG. 8, the fibers immediately after spinning may be coated with a resin 
followed by curing, as stated above. In an alternative embodiment, a 
molten metal is put into the coating die and the fibers coated with the 
metal. 
According to the method of fixing the fiber alignment illustrated in FIG. 
7, only the diameter of the quartz or glass pipe is reduced by the burner. 
Thus, pressure reduction from above the matrix as described with reference 
to FIG. 8 can be performed without difficulty. When the fiber bundle 
matrix is spun immediately after fiber alignment, fixing of the fiber 
alignment as shown in FIG. 7 is not essential. 
The advantages attained with the present invention are as follows: 
(1) Fiber bundles having a length of several kilometers to several tens of 
kilometers can be obtained since elemental image fibers obtained by 
spinning and diameter reduction are inserted into a quartz or glass pipe 
and aligned therein and are further spun and reduced in diameter. By 
cutting the bundle fiber, fibers of any desired length for use in an image 
guide can be obtained. 
(2) Since aligned elemental image fibers are sealed in a quartz or glass 
pipe, the image fibers are protected and prevented from breaking. 
(3) Aligned elemental image fibers are covered with a jacket of a quartz or 
glass pipe and a plastic or metal reinforcing layer is further provided 
thereon. Hence, a bundle fiber having adequate strength for normal use can 
be produced. 
(4) The outside diameter of the resulting fiber bundle and the diameter of 
each image fiber can be freely chosen from producible ranges because an 
image fiber matrix is first spun and reduced in diameter, inserted into a 
quartz or glass pipe and then further spun and reduced in diameter. 
(5) Since the elemental image fibers are spun and reduced in diameter 
twice, the diameter of each image fiber which is to become a picture 
element can be sufficiently decreased that an image of good quality can be 
obtained. 
(6) By using quartz or quartz-type glass as the material for the elemental 
image fibers and jacket, the transmission loss of the image fibers can be 
reduced to about 10 dB/km or less. Thus, as compared with conventional 
bundle fibers, the transmissible distance of the bundle fiber in 
accordance with the invention increases strikingly for an image of a given 
brightness. 
(7) In the described process of spinning and diameter-reduction of a fiber 
bundle matrix, a fiber bundle is formed while reducing the pressure on the 
inside of the quartz or glass tube. Accordingly, the spaces between the 
image fibers as picture elements can be reduced, and the density of image 
fibers or picture elements can be increased. 
(8) Since image fibers spun and reduced in diameter are inserted in a 
quartz or glass pipe, the number of picture elements is theoretically 
unlimited. Hence, images of very high resolution can be obtained. 
The image fibers used in this invention will now be described in detail. 
There are generally three types of optical fibers for transmitting a light 
power or a light signal. A first type is a fiber composed of quartz or 
glass composed mainly of quartz. A second type is a fiber of a 
multi-component glass. A third type is a fiber of plastics. In particular, 
the first and second fibers composed of quartz or glass composed mainly of 
quartz and of a multicomponent glass, respectively, are applicable to the 
present invention. 
In a fiber made from quartz or glass composed mainly of quartz, quartz 
glass may be used as the core 1 in FIG. 1 with B-doped quartz used as the 
cladding layer 2. When quartz doped with at least one element such as Ge, 
P, Al, Ti or Ga is used as the core, quartz glass or B-doped quartz glass 
is used as the cladding layer. 
Methods of producing a matrix for image fibers composed mainly of quartz or 
quartz glass include, for example, a CVD method (chemical vapor deposition 
method), a VAD method (vapor phase axial deposition method), and an 
external deposition method. 
FIGS. 9A-9C and 10A-10C show examples of distributions of the refractive 
indices of image fibers produced by these methods. FIGS. 9A-9C relate to 
the case of a quartz jacket 53 using a quartz pipe as a starting material, 
with the reference numerals 51 and 52 representing a core and a cladding 
layer, respectively. FIGS. 9A-9C show examples of the distributions of 
refractive indices. FIG. 9A relates to the case of using B-doped quartz as 
the cladding layer 52 and quartz as the core 51 and the jacket 53 while 
FIGS. 9B and 9C relate to the case of using B-doped quartz as the cladding 
layer 52 and quartz doped with one or more of Ge, P, Al, Ti, Ga or the 
like as the core 51. Of course, quartz glass (having the same refractive 
index as the quartz jacket 53) may be used as the cladding layer 52. The 
refractive index distribution of the core may be of the stepped type as 
shown in FIGS. 9A and 9B or it may be of a curved type as shown in FIG. 
9C. In any case, whatever the type of the refractive index distribution, 
if the core portion has a higher refractive index than the surrounding 
part, the fiber is sufficient for transmission of light. 
FIGS. 10A-10C show distributions of the refractive indices of image fibers 
which do not have a quartz jacket as described with reference to FIG. 9. 
In this case, too, it is sufficient that the core 51 have a higher 
refractive index than the cladding layer 52 (quartz glass or B-doped 
quartz glass) and that the refractive index distribution profile of the 
core be a stepped or curved type or a type containing a curve as in FIGS. 
9A-9C. 
A method for providing a light-absorbing layer in the image fibers having 
the refractive index distributions shown in FIGS. 9 and 10 or a matrix 
will be next described. The described light-absorbing layer serves to 
prevent blurring of an image which is caused by leakage of unwanted light 
to adjacent fibers when adjacent image fibers (picture elements) are in 
contact with each other. When the difference in the refractive index 
between the core and the cladding layer is large and the core diameter is 
relatively large, leakage of light to the adjacent fibers is almost 
negligible. Hence, in such a case, a light-absorbing layer is not 
essential. 
When the light-absorbing layer is essential, its thickness should be kept 
as small as possible. This is necessary in order to secure the largest 
possible area for the core. 
There are three methods enumerated below for providing a light-absorbing 
layer on elemental image fibers composed mainly of quartz or quartz-type 
glass or a matrix therefor. 
(1) In the case of an elemental image fiber having a quartz jacket 53 as 
shown in FIGS. 9A-9B, the quartz jacket portion is utilized as a 
light-absorbing layer. For this purpose, a quartz pipe corresponding to 
the quartz jacket portion may be made of a material having as high as 
possible a transmission loss. Generally, since a quartz pipe made of 
naturally occurring quartz is made of rock crystal, it is high in 
impurities, and fortunately, thus has a very high loss. Hence a natural 
quartz pipe can be directly used as a light-absorbing layer. When the 
image transmission distance is short, this quartz pipe in the untreated 
state has a small effect on light absorption. In such a case, the light 
absorption of the quartz pipe may be increased by any of the following 
methods: 
(i) A layer Al.sub.2 O.sub.3 or the like is coated onto the outside surface 
of transparent quartz such as Heralux-ST (a product of Shinetsu Quartz 
Co., Ltd.) and subjected to a heat hysteresis at high temperatures above 
1200.degree. C. to thereby generate a devitrified layer and to increase 
its light absorption loss. 
(ii) A quartz pipe is made by using colored rock crystal such as "violet 
rock crystal" as a raw material for the quartz. 
(iii) Ultraviolet light, X-rays, gamma-rays, etc. are radiated onto a 
quartz pipe having impurities as in (i) and (ii) above or an ordinary 
quartz pipe causing it to discolor. 
(iv) A quartz tube doped with Ti or another transition metal (such as an 
ozone-free quartz pipe made by Toshiba Ceramics Co., Ltd.) is used 
directly or it is irradiated with ultraviolet light, X-rays, gamma-rays, 
etc. 
By employing any of the methods (i) to (iv), the light absorption loss of 
the quartz pipe itself is increased for its use as a light-absorbing 
layer. 
(2) A metal coating is formed on the quartz portion of the matrix for image 
fibers having a refractive index as shown in FIGS. 9A-9C which corresponds 
to the quartz jacket portion as a light-absorbing layer. Since quartz or 
quartz-type glass differs markedly at high temperatures during 
manufacturing or processing from the metal, the thickness of the metal 
coating should preferably be made as thin as possible by producing the 
coating through vacuum deposition or the like. 
(3) A layer having a large light absorption is provided simultaneously 
with, or separately from, the step of producing matrixes for strand fibers 
having a refractive index as shown in FIGS. 10A-10C. A suitable method for 
this is to color quartz glass with a colored ion. That is, a colored 
quartz glass can be obtained by doping quartz glass primarily with a 
transition metal such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, No, Rh, La, Ce, 
Nd, or W. 
FIG. 11 illustrates a process for providing a light-absorbing layer in a 
separate step on the surface of an image fiber matrix 54 having a 
refractive index distribution as shown in FIGS. 10A-10C. SiCl.sub.4, a 
halide of the aforesaid transition metal (MX), H.sub.2 and O.sub.2 are 
reacted by using a burner 55, to produce for example, by flame hydrolysis, 
a quartz light-absorbing layer doped with the transition metal. In this 
case too, irradiation or ultraviolet light, X-rays, gamma-rays, etc. can 
be used to increase the light absorbing effect of the quartz layer the 
same as in the case of quartz pipes. 
When a light-absorbing layer is to be provided using a multicomponent 
glass, the light absorption can be increased by suitably adjusting the 
composition of the multicomponent glass. Furthermore, by the method 
illustrated in FIG. 2B, a light-absorbing layer can be provided relatively 
easily. 
An actual example of the present invention will now be described. The 
elemental image fiber used was a matrix produced by the VAD method 
described above which consisted of a core of quartz doped with G and P and 
a cladding layer of B-doped quartz. The distribution of the refractive 
index of the matrix is shown in FIG. 12. The difference (.DELTA.n) of 
refractive index was about 1.2%. The refractive index distribution need 
not always be of the complete step type or graded type as described above 
as it is sufficient, if as shown in FIG. 12, a maximum occurs in the 
refractive index. 
A matrix having a diameter of about 20 mm and the refractive index 
distribution shown in FIG. 12 was inserted in an ozone-free quartz pipe 
made by Toshiba Ceramics Co., Ltd. and spun into a fiber having an outside 
diameter of about 100 .mu.m by the method illustrated in FIG. 2A and by a 
rod-in-tube method involving applying reduced pressure to the upper 
portion of the quartz pipe. The ozone-free quartz pipe used was doped with 
about 100 to 150 ppm of Ti and, by thermal hysteresis in the spinning 
process, Ti.sup.4+ was changed to Ti.sup.3+. As a result, the 
transmission loss became several hundred to several tens of thousand 
dB/km. 
The resulting image fiber having a light-absorbing layer was cut to a 
length of about 30 cm and about 4000 cut fibers were inserted in a quartz 
pipe having an inside diameter of about 20 mm. The resultant assembly was 
subjected to the aligning method shown in FIG. 6 and to the fixing method 
shown in FIG. 7. The assembly was spun by the method shown in FIG. 8 to 
form a plastic-reinforced fiber. 
The resulting fiber had a structure as shown in FIG. 13 in which a quartz 
jacket 57 encloses laterally a fiber bundle 56 of closely aligned 
constituent fibers with the surface of the quartz jacket coated with a 
plastic layer 58. The dimensions, the number of picture elements, and 
other characteristics of the bundle fiber produced were as follows: 
Elemental fibers (diameter): about 5 .mu.m 
Number of elemental fibers as picture elements: about 4000 
Diameter of the bundle fiber: db=about 0.9 mm 
Diameter of the quartz jacket: dj=about 1.0 mm 
Diameter of the reinforcing plastics: dp=about 1.2 mm 
Length of the fiber: about 90 m 
A very thick fiber having an outside diameter of 1 mm was easily broken at 
a flexural radius of 150 mm in the absence of a plastic reinforcing agent 
but, by providing a plastic reinforcing layer, it was not broken at a 
flexural radius as low as 30 mm thereby showing its extremely high 
strength. 
In the bundle fiber of the example, the diameter of the quartz jacket was 
adjusted to about 1 mm. Needless to say, by properly designing a lens 
system adapted to be fitted to both ends of the fiber bundle, the quartz 
jacket may have a smaller diameter. Of course, spinning and 
diameter-reduction to a diameter finer than the outside diameter of the 
fiber bundle obtained from a fiber bundle matrix having the same elemental 
fibers and the number of elemental fibers can be performed. Fiber bundles 
having a finer diameter have higher flexibility while the length of the 
fiber bundle obtained from the same matrix increases strikingly. For 
example, when a matrix (having a length of 30 cm) composed of about 4000 
elemental fibers having an outside diameter of 100 .mu.m inserted in a 
quartz pipe having an inside diameter of 20 mm as described in the example 
above is spun to a quartz jacket diameter (dj) of about 150 .mu.m, the 
length of the resulting bundle fiber may be as large as 5400 m.