US3383192A - Method of making fiberscopes - Google Patents

Description

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United States Patent 3,383,192 METHOD OF MAKING FIBERSCOPES Walter P. Siegmund, Woodstock, Conn., assignor to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts Filed Apr. 13, 1964, Ser. No. 359,068 Claims. (Cl. 65-4) ABSTRACT OF THE DISCLOSURE The method of making a flexible fiberscope including thejsteps of assembling a plurality of preformed hoops of convoluted flexible optical fibers in stacked relationship with each other throughout a relatively short length of each, adhesively securing the fibers together throughout approximately one-half of said length with a permanent bonding material and throughout the other half of sail length with a temporary burnable bonding material, severing the assembly transaxially between said bonding materials to form opposite ends of the fiberscope and heating the resulting temporarily bonded end to a tempera ture sufiicient to burn away the bonding material and effect fusion of the fibers.

This invention relates to optical image transfer devices formed of a great number of relatively long and thin individually flexible light-conducting fibers having corresponding opposite ends thereof secured together in such pre-arranged side-by-side relationship as to compositely form image-receiving and emitting faces at opposite ext emities of the devices. Such devices are known as flexible fiberscopes and the invention relates to constructional improvements therein and particularly to a method of making the same.

In the manufacture of flexible fiberscopes, it has been the practice to connect corresponding opposite ends of the optical fibers together either by fusion or cementing. In fused fiber optical bundles, image quality in terms of appearance is generally higher than in cemented bundles due to the absence of the spacing between fibers which is prevalent in cemented bundles. However, in view of the fact that fusing seriously reduces the mechanical strength and flexibility of the fibers in the region of transition between the fused and flexible zones of the bundle due at least in part to the fibers being heated to above their annealing temperature without being fused together, it is necessary to reinforce that region. One approach to such reinforcement has been to impregnate the transition region with a fluid epoxy resin which can thereafter be po1ymerized in place. In medical endoscopes having fused opposite ends, as one example, such reinforcement produces the undesirable result of substantially increasing the length of rigidity at the distal portion of the fiber bundle beyond the already rigidly fused zone and where extreme flexibility is actually required.

It has been found that at the image-receiving or distal end of a fiberscope, lost image information and localized image distortions resulting from the use of cemented fiber bundles are relatively insignificant in well packed bundles. Thus, the present invention contemplates the formation of a fiberscope having a well packed cemented, rather than fused, distal end section to minimize the length of the rigid portion of the fiberscopes distal end. At the opposite end of the fiberscope, however, an image-receiving face formed of a cemented bundle of fiber ends is undesirable especial ly from the viewpoint of appearance of an image presented 3,383,192 Patented May 14, 1968 "ice at such a face. This is due to the presence of cement and/ or voids between fibers.

Thus, at the proximal end of the fiberscope where added length of rigidity is not of consequence, the present invention contemplates fusing the fibers together to eliminate spacing therebetween with a view to improving the appearance of images produced at that end of the fiberscope.

In view of the fact that fiberscopes are normally constructed of a multiplicity of individually extremely thin and flexible fibers usually numbering in the several thousands, each being of only a few thousandths of an inch or considerably less in cross-sectional dimension, it will be appreciated as this description progresses that the fabrication of such a device having one end section fused and the other cemented would ordinarily be accompanied by serious if not unsurmountable problems since, for achieving acceptable definition and resolution of image, the fibers must be arranged in substantially identical geometrical patterns at opposIte ends of the structure.

In accordance with principles of this invention, a simple, reliable and unique process is contemplated for manufacturing fiberscopes of the above-mentioned character and it is, therefore, an object of the invention to provide for maximum flexibility of the distal image-receiving end of a fiberscope and, at the same time, to provide for maximum image quality, especially in terms of cosmetic appearance of an image produced at the proximal end of the fiberscope where the extent of flexibility is of less consequence.

This objective and others which may become apparent hereinafter is achieved in the manner disclosed in the following detailed description which is accompanied by a drawing in which:

FIG. 1 illustrates, in side elevation and partly in section, an exemplary fiberscope having lens-supporting end fittings thereon;

FIGS. 2, 3, 4 and 5 illustrate various steps performed in the process of the invention according to one aspect thereof;

FIG. 6 is an enlarged elevatio-nal view of the proximal or image-receiving end face of a fiberscope formed in accordance with principles of the invention;

FIG. 7 is a view similar to FIG. 6 of the opposite or distal image-receiving end face of the fiberscope; and

FIG. 8 is an enlarged longitudinal cross-section of the fiberscope having a section thereof broken away inter mediate its opposite ends.

Referring now to the drawing, there is shown in FIG. 1 exemplary fiberscope 10 formed of a number of long and thin individually flexible optical fibers 12 bundled together in side-by-side relationship with corresponding opposite ends thereof secured together to form at the distal end of the fiberscope an image-receiving face 14 and at the proximal end thereof an image-emitting face 16. Fibers 12 are unconnected and free to flex individually intermediate opposlte end sections of fiberscope 10 so they are free to flex individually.

For purposes of illustration only, end fittings 18 and 20 supporting objective and eye lenses 22 and 24 respectively are shown in simplified form as being fixedly connected to opposite ends of fiberscope 10. It will be understood that various, more elaborate, end fittings, lens systems, and also enclosing outer sheaths or the like, not shown, would ordinarily be applied to fiberscope 10 to equip same for use as a medical diagnostic instrument or the like.

The present invention relates more particularly to improvements in constructional details of the fiber optical section of such an instrument and, accordingly, fittings 18 and 20 are only illustrative of the many variations thereof well known in the art.

Briefly and in general, fiberscope 10 operates upon receiving a pattern of image-forming light projected against face 14 by objective lens 22, to transfer by internal reflection elemental areas of the image pattern through respective fibers receiving the same to face 16. At face 16, the image pattern in mosaic form is viewed with the assistance of magnifying eye lens 24. For some applications of use, however, neither of the end fittings 18 and 20 are required if, for example, an illuminated object to be viewed is placed immediately adjacent to or against one face 14 or 16 and the image thereof is viewed directly at the other face without the aid of an eye lens. Thus, the term fiberscope when used herein is intended to refer more particularly to the structure of the bundle of optical fibers 12 without accessories such as fittings 18 or 20.

Fibers 12 each comprise one or more cores of light conducting material of relatively high index of refraction each surrounded by a relatively thin cladding of ma terial of a lower index of refraction than the core. Fibers comprising the integral flexible structure of more than one individually clad core all fused together are commonly referred to as multifibers and this invention relates to the fabrication of fiberscopes formed of either single core fibers or multifibers. The fibers of either type may, as one example, comprise cores of optical flint glass having an index of refraction of approximately 1.62 each clad with a relatively thin layer of crown or soda-lime glass having an index of refraction of approximately 1.52..

Faces 14 and 16 of. fiberscope 10 are optically ground and polished to render respective fibers 12 individually highly receptive and conductive to image-forming light presented upon the distal end face 14 and similarly con ductive and emissive to light presented to the proximal end face 16 by transference through the fibers. Proximal end face 16 may, for some applications of use, be sand blasted, etched or otherwise rendered light-diffusing.

Referring now to details involving the fabrication of fiberscope 10, it will be seen that it is formed with a relatively short portion of corresponding lengths of fibers 12 cemented together at distal end section 26 (see FIGS. 1, 6, 7 and 8) while the proximal end section 28 is fused together and reinforced in transition region 29 adjacent; the fused portion (see FIG. 8).

Fiberscope 10 is according to one aspect of the inven tion provided with identically geometrically patterned end faces by first forming a number of fiber hoops 30, one of which is shown separately in FIG. 2. Each hoop 30 consists of a single fiber strand wound to a ribbon-like single or multiple layer thickness of a widthv approxi mately equal to that desired of one transverse dimension of the fiberscope. The mean circumferential dimension of each hoop 30 is controlled to be approximately equal to the length desired of fiberscope 10. In order to obtain. maximum precision in the geometrical identity of fibers at opposite ends of the ultimate fiberscope, hoops 30 should each be of a single layer or one fiber thickness or, if multiple layer thicknesses are used, each layer should be wound in the same direction and at the same helix angle as its predecessor.

Hoops 30, suflicient in number to provide an assembly of a thickness approximately equal to that desired. of fiberscope 10, are assembled together in stacked relation, one within. the other, substantially as shown in FIG. 3. As will become apparent hereinafter, opposite end faces of the fiberscope are formed by severing the stack of hoops 30 transversely in a plane of demarcation. illustrated by line 32 (FIGS. 3 and 4). For a. detailed description. of this general technique for forming flexible fiberscopes, reference may be had to Patent No. 3,033,731 issued May 8, 1962, to Henry B. Cole and assigned to assignee of the present application.

In accordance with one feature of the present invention, the convolutes of each hoop 30 are cemented together along a narrow transverse strip 34 (see FIG. 2) preferably with a permanent bonding material such as, for example, an epoxy resin of the two-stage curing type commonly referred to as a B-stage epoxy which, upon application in solution, contains a solvent adapted to dry .at normal room or slightly elevated temperatures to render the resin adhesively varnish-like in consistency and subsequently permanently hardenable when polymerized or cured by the application of heat in the order of approximately 250 Fahrenheit. Strip 34 is of such controlled width as to extend along the length of hoop 30 a distance no greater than that required for making a secure connection of the distal ends of fibers 12 of fiberscope 10 so as to provide the distal end of fiberscope 10 with the least possible amount of rigidity. By way of example, in fabricating a fiberscope intended to have a cross-sectional dimension of between three and four millimeters on a side, strip 34 should be approximately five millimeters in width or, for a six or seven millimeter square fiberscope, strip 34 should be seven or eight millimeters in width.

Immediately adjacent to strip 34 and preferably adjoining same along a line of demarcation 36, the convolutes are further cemented together along another trans verse strip 38 of approximately the same width as strip 34 but with a temporary removable bonding material or cement such as cellulose nitrate in a solvent of nitromethane or the like all of which can be subsequently burned away. Upon being so cemented, hoops 30 are successively stacked one within the other or one upon the other, as the case may be, to form the bundle illustrated in FIG. 3. Strips 34 and 38 are arranged in accurately superimposed relationship so that the line of demarcation 36 of each hoop 30 coincides with line 32 along which the bundle is to be subsequently severed.

The entire section of the bundle of hoops 30 including the zone thereof connected together with the permanent bonding material and that connected together with the temporary bonding material is placed within double clamp 40 (FIG. 4) having slot 42 in which line 32 is approximately centered. Sufficient clamping force is applied thereto by screws 44 and 46 to compact the bundle and adhesively connect together adjoining respective strips 34 and 38. The different cements being slightly tacky or cohesive under such clamping force. Alterna= tively, a thin layer of the same cements used. to form strips 34 and 38 respectively can be applied therebetween as the hoops are stacked to assure good adhesion of one to another. If such a connecting cement is used, however, it should be applied sparingly to prevent undue spacing of the fibers in adjoining hoops.

The epoxy-bonded zone or section 26 of the bundle of hoops in FIG. 4 is next heated to a temperature suflicient to cure and fully set the epoxy resin and thus form a permanent bond between all fibers of all hoops 30 in that section. This may be accomplished by applying heat to section 40' of clamp 40.

In curing the B-stage epoxy resin which has been at least partially dried at room temperature, heat of ap proximately Fahrenheit is applied for a period of time sufficient to melt and cause the resin to wet fibers 12 of the bundle. Thereafter, the temperature is raised to approximately 250 Fahrenheit for a period of time sufficient to fully polymerize the resin. and thereby effect permanent hardening thereof. Conduction of heat to section 40" of clamp 40 will not adversely affect section 28 of the bundle of hoops since the maximum temperature used in curing the epoxy resin is well. below that which would adversely affect the cement. used in section 28.

Having permanently bonded the fibers of hoops 30' together in section 26, the bundle is sawn or otherwise severed along line 32 with suitable means such. as the circular cutter 48 shown partially broken. away in FIG. 4. The severance along line 32 separates the now permanently set epoxy-connected section 26 from. section 28 and thus forms substantially geometrically identical opposite end faces 14 and 16 of fiberscope 10.

Section 26, the end face 14 of which is illustrated in FIG. 7, is removed from section 40* of clamp 40 and section 28 can then be heated and fused in section 40" of the clamp or removed therefrom to other holding or clamping means for fusing to form end face 16 shown in FIG. 6.

In a presently preferred procedure, section 28 is placed in a relatively snug fitting channeled support 50 of re fractory material which is highly geometrically stable and relatively non-adherent to glass when subjected to glass-fusing temperatures. Such a refractory material is disclosed in Patents No. 2,440,187 and No. 2,764,491 assignee of this application. A parting agent such as gold foil, not shown, can be used to line the channel of support 50, if desired. Block 52, preferably formed of the same material as support 50, is placed upon section 28 of the cemented fibers 12 to exert as a result of its weight a slight compressing force tending to compact the bundle and effect secure fusion thereof.

Section 28 is heated by any suitable means such as electrical coils 54 to a temperature of, for example, from 1250 F. to 1450 F. to cause fusion of fibers 12 one to the other. During such heating, the cement of strips 38 is burned away and pressure applied by block 52 causes fibers 12 to assume an interfitted fused relationship with out voids therebetween substantially as illustrated in FIGS. 6 and 8. The resultant fused section 28 is ann-ealed, thereafter cooled to room temperature and removed from support 50. The squeezing-together of fibers 12 in support 50 and elimination of cement 38 will tend to shorten one transverse dimension of end face 16 slightly in the direction of application of pressure and thus produce a slight anamorphism in images received at face 16. This, however, is not ordinarily consequential. However, while not illustrated in the drawing, the invention also contemplates the use of other types of supports such as one having two sides thereof movable in rightangular directions relative to one another toward the fiber bundle with pressure being applied equally to each during fusion. This will obviate image anamorphosis in critical cases.

In view of the fact that fibers 12 are unavoidably heated to temperatures above annealing temperatures of their glasses in the transition region 29 (FIG. 8) where these fibers are unconnected, they inherently become relatively inflexible and fragile in that region. The fragile region normally extends back from the fused section 28 a distance approximately equal to the length of the fused section itself. Thus, in order to avoid breakage of fiberscope 10 in this region during use thereof, fibers 12 are reinforced after fusion of section 28 by impregnation with an epoxy resin 56 initially in liquid form and which is cured to form a permanent bond with the fibers in the manner described above with relation to the similar curing of epoxy strips 34.

Accordingly, fiberscope 10 thus formed comprises a distal end section 26 having fibers 12 secured together with a cement to provide for the maximum possible extent of flexibility of the distal portion of the fiberscope while at the opposite proximal end section 28 where flexibility is not of consequence, fibers 12 are fused together to provide for maximum quality in cosmetic appearance of images produced on face 16 at that end. At the same time, fibers 12 are substantially identically geometrically arranged at opposite faces 14 and 16.

It should be understood that while, for purposes of clarity in illustration, fiberscope 10 has been shown to be comprised of only a few relatively large diameter fibers 12, it would in actuality embody a great number of fibers each of an exceptionally small cross-sectional size.

I claim: 1. The method of making an image transfer device (10) of a multiplicity of bundled energy-conducting fibers (12) arranged in compact side-by-side relationship with each other throughout a section of the length of the bundle wherein the improvement comprises the steps of connecting said fibers (12) together throughout one portion (26) of said section with a substantially permanent bonding material (34) and throughout another portion (28) of said section adjacent said one portion with a removable bonding material (38), said materials being disposed in contiguous side-byside relationship with a plane of demarcation (32) therebetween extending transversely through the bundle at one side of which a face (14) is to be formed and at the other side of which another face (16) is to be formed both on said bundle;

severing said bundle generally along said plane of. demarcation (32) to form said faces (14 and 16);

removing said removable material (38) from between lengths of said fibers (12) in said other portion (28); and fusing said lengths of the fibers (12) together. 2. The method according to claim 1 wherein said permanent bonding material (34) and said removable bonding material (38 are characterized in that they are respectively hardenable and removable upon application of heat thereto and said method further includes the steps of applying heat to said permanent bonding material (34) of a temperature sufficient to harden same and removing said removable bonding material (38) by the application of heat of a temperature sufficient to effect burning thereof.

3. The method according to claim 2 wherein removal of said removable bonding material (38) and fusion of adjacent portions of said fibers (12) are successively effected by the steps of heating same to a temperature sufiicient to burn said material (38) and raising the temperature of such heat sufficiently to render said fibers (12) fusible one to the other and compressing the bundle of fibers (12) during such heating sufficiently to compact same without materially altering the geometrical relationship thereof.

4. The method of making a flexible image transfer device of a multiplicity of lengths of energy-conducting fibers (12) arranged in compact side-by-side relationship with each other as a bundle having a number of zones (26, 28) therealong wherein said fibers (12) are arranged in a cross-sectionally congruent relationship in one zone (26) with reference to another (28) wherein. the improvement comprises? connecting said fibers (12) together in one of said zones (26) with a substantially permanent bonding material (34); v

connecting said fibers (12) together in anothe 'f of said zones (28) with a temporary bonding material (38) and maintaining the arrangement of said fibers (12) in said other zone (28) while removing said temporary bonding material (38); fusing said fibers (12) together in said other zone (28); forming an optically finished face on one end of each of said zones extending transversely of said bundle;

impregnating a section of the length of said bundle adjacent to said other zone (28) with a fluid but settable material (56); and

causing setting of said last-mentioned material (56). t

5. The method of forming a flexible image transfer device of a multiplicity of lengths of energy-conducting fibers (12) arranged in compact side-byside relationship with each other as a bundle having a number of zones (26, 28) therealong wherein said fibers (12) are in crosssectional congruent relationship in one zone with reference to another, the improvement comprising:

connecting said fibers together in said zones (26, 28)

with bonding material; forming a face extending transversely of said bundle on one end of each zone; maintaining the arrangement of fibers (12) in one of said zones (28) while removing an amount of said bonding material sufiicient to permit fusion of said fibers (12) one to the other in said one zone (28); fusing said fibers together in said one zone (28); impregnating a section of the length of said bundle adjacent to said one zone (28) with a fluid but.

settable material (56-); and causing setting of said material.

References Cited UNITED STATES PATENTS MacNeille. Cole Woodcock 65-4 X Woodcock. Siegmund.

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