WO2000002221A9

WO2000002221A9 - Image intensifier with improved microchannel plate

Info

Publication number: WO2000002221A9
Application number: PCT/US1999/011854
Authority: WO
Inventors: Paul Motta
Original assignee: Litton Systems Inc
Priority date: 1998-06-02
Filing date: 1999-05-28
Publication date: 2000-07-27
Also published as: WO2000002221A3; WO2000002221A2

Abstract

A night vision device (18) includes an image intensifier tube (28) with a microchannel plate (38) substantially eliminating multi-boundary noise from the image provided by this device. The microchannel plate (38) is substantially free of a hexagonal boundary pattern which results in conventional microchannel plates from the stacking of hexagonal bundles of fibers from which the microchannels (42) are formed. A method of making a microchannel plate (38) in accord with the invention is also set out.

Description

Image Intensifier With Improved MicroChannel Plate

Background of the Invention Field of the Invention

The present invention relates to an image intensifier tube having an improved microchannel plate. More particularly, the present invention relates to such a microchannel plate having substantially no fixed-pattern noise created by the structure of the microchannel plate itself. Further, the present invention relates to a method of making such a microchannel plate, and to a manufacturing intermediate article usable in subsequent manufacture to make a microchannel plate.

Related Technology

Night vision devices commonly use an image intensifier tube to receive low- level visible light, and/or invisible near-infrared light from a scene to be viewed, and to provide an image in visible light which replicates the scene.

MicroChannel plates are commonly used as electron multiplier devices (or gain stages) in image intensifier tubes. There are many other uses for microchannel plates, including use in such devices as particle detectors. Those ordinarily skilled in the pertinent arts will understand that the manufacture of such microchannel plates involves drawing (i.e., elongating while heated to a softened condition) of fine-dimension glass fibers, each of which includes a core of etchable glass and a tubular cladding of electrically active glass.

A great multitude of such glass fibers each including a central fiber or "'core'^" of core glass, and a surrounding cladding of "cladding glass," are stacked together in hexagonal bundles, are fused into a unitary body, and are then further drawn to a smaller size. Importantly, the bundles of fibers used in making a conventional microchannel plate are all composed of identical fibers, each having a core glass strand surrounded by a cladding glass sheath. A plurality of these hexagonal bundles, each including many substantially identical glass fibers, are stacked together within a heavy walled glass tube. This combination of glass tube and hexagonal multi-fibers is commonly referred to as a boule pre-form. This boule pre-form is then fused into a unitary body in a boule-fusion furnace, producing a "boule." Next, the boule when cooled is sliced transversely, perhaps at an angle off the perpendicular of about 5 degrees, to produce many thin plates which will become microchannel plates upon further processing.

Subsequently, each resulting thin plate of glass (i.e., a transverse thin slice of the boule) is subjected to an etching process to remove the core glass from each fiber of the plate. The result is a thin plate of glass with a great multitude of fine-dimension channels (i.e., microchannels) extending between its opposite faces. The heavy-walled, glass tube fuses with the glass fibers during the boule-fusion process to provide the microchannel plate with a glass rim about a central array of multiple microchannels. Conventional microchannel plates include as many as eleven million, or more individual microchannels. This plate of glass with fine-dimension channels is then subjected to subsequent manufacturing processes which activate the electrically active glass along its surface bounding the microchannels as a secondary-electron emitter. Electrodes are also applied to the opposite faces of the plates to allow application of electrostatic fields which provide an electron flow along the length of the microchannels (i.e., from one face of the microchannel plate to the other).

A common problem with conventional microchannel plates is the presence of multi-boundary noise (MBN) in the image subsequently produced by an image intensifier or other device using the microchannel plate. This MBN has the appearance of a hexagonal pattern of relatively dark lines or lines where the gain of the microchannel plate is less and the resulting image is not as bright as the reminder of the image provided to a user of a night vision device having this conventional microchannel plate. Under microscopic examination, the hexagonal pattern, which is a remnant of the stacked hexagonal bundles of fibers, can be clearly seen.

Viewing "prior art" Figure 7, it is seen that in a conventional microchannel plate 10, the microchannels which are formed by each hexagonal bundles of fibers 12 are clearly separated by a boundary 14. At this boundary 14, in addition to the spacing between adjacent microchannels (i.e., which are created from the cladding glass of the fibers) across the boundary being greater than the spacing between adjacent microchannels within a bundle of fibers, the microchannels of a conventional microchannel plate which are adjacent to the boundaries 14 are believed to have a slightly lower overall electron gain than microchannels which are spaced from a boundary and within a hexagonal bundle of such microchannels. Viewing "prior art" Figure 8 it is seen that the individual microchannels 16 away from the boundaries 14 are in a triangular pattern or "cell". That is, the smallest unit of repeating geometry in the matrix of microchannels is a triangular unit 16b of three single fibers providing three single microchannels 16, as is best seen in Figure 8. On the other hand, the boundaries 14 between hexagonal bundles can be easily picked out visually. Considering the individual microchannels 16a on opposite sides of these boundaries 14, it is seen that the - rows of microchannels are imperfectly aligned with one another, and the spacing across the boundary between microchannels is generally somewhat greater than the spacing between adjacent microchannels 14 within a bundle. The triangular "cell" pattern is not continuous across the boundary 14 (at least not with the same center-to-center distance that applies with in the bundles of fibers away from the boundaries 14).

Further, the microchannels 16 within a bundle of such microchannels are in essence perfectly round. In contrast, the boundary-adjacent microchannels 16a, are badly distorted from their preferred round configuration. It is believed that this distortion of the boundary-adjacent microchannels 16a is responsible for their slightly decreased gain in comparison to the almost perfectly round microchannels 16 within a bundle. Thus, it may be appreciated that when the image provided by a conventional image intensifier tube is closely examined, a pattern of MBN similar to that seen in Figure 7 may be detected in the image. As explained above, this MBN pattern is believed to be created by the combined effects of the boundaries 14 themselves, which represent an area where no electron gain is provided by a microchannel plate, and by the somewhat lower gain provided by the boundary-adjacent microchannels 16a, as seen in Figure 8.

Summary of the Invention

In view of the deficiencies of the conventional technology, it is an object for this invention to provide an improved image intensifier tube which avoid one or more of these deficiencies.

Additionally, it is an object for this invention to provide an improved microchannel plate which avoids one or more of the deficiencies of the conventional prior art.

Another object for this invention is to provide an improved microchannel plate in which MBN is reduced or substantially eliminated.

Yet another object for this invention is to provide an improved microchannel plate in which the row-alignment of microchannels across boundaries between fiber bundles of microchannels is substantially improved in comparison to the conventional microchannels.

Still another object for this invention is to provide an improved microchannel. plate in which the boundary-adjacent microchannels have a shape which is improved, and which is closer to being substantially round in axial view, in comparison to the conventional microchannel plates. Accordingly, the present invention according to one aspect provides a method of making a microchannel plate which has reduced or substantially eliminated multi- boundary noise, said method comprising steps of: providing plural glass fibers of a first type each of which includes a strand of core glass surrounded and interbonded with a sheath of cladding glass; providing plural glass fibers of a second type each of which includes a strand of core glass and which are substantially the same diameter as fibers of the first type; stacking fibers of the first type into a hexagonal bundle, and providing an outer layer of fibers of the second type surrounding the fibers of the first type; fusing the hexagonal bundle into a unitary body including both the fibers of the first type and the fibers of the second type; removing the outer layer of fibers of the second type, leaving the unitary body of fused fibers of the first type in a hexagonal shape having elongate striations.

A better understanding of the present invention will be obtained from reading the following description of a single preferred exemplary embodiment of the present invention when taken in conjunction with the appended drawing Figures, in which the same features (or features analogous in structure or function) are indicated with the same reference numeral throughout the several views. It will be understood that the appended drawing Figures and description here following relate only to one or more exemplary preferred embodiments of the invention, and as such, are not to be taken as implying a limitation on the invention. No such limitation on the invention is implied, and none is to be inferred. Brief Description of the Drawing Figures

Figure 1 provides a schematic view of a night vision device (NVD) employing an image intensifier tube;

Figure 2 is a schematic cross sectional view of an image intensifier tube as seen in the NVD of Figure 1 ;

Figures 3 a and 3b respectively are a greatly enlarged facial view of a microchannel plate of the image intensifier tube of Figure 2, and a still more greatly enlarged fragmentary view of the microchannel plate seen in Figure 3 a;

Figures 4a, 4b, and 4c respectively, are a fragmentary cross sectional view, an end elevation view, and a greatly enlarged fragmentary view of an encircled portion of Figure 4b, each showing a hexagonal multi-fiber of glass fibers prepared in preparation to making a boule in accord with the present invention;

Figure 5a shows a hexagonal multi-fiber of glass fibers fused into a unitary body, and prepared by use of an etching process in accord with the present invention, all in preparation for making a boule;

Figure 5b shows two hexagonal multi-fibers like the one shown in Figure 5a, stacked together in a boule pre-form in preparation for making of a fused boule;

Figure 5c depicts an actual microphotograph of a microchannel plate work piece made according to the teaching of the present invention and shows an interface between two adjacent hexagonal multi-fibers of fibers in a partially finished microchannel plate embodying the present invention;

Figure 6 presents a manufacturing process flow chart for making a microchannel plate embodying the present invention; and

Figures 7 and 8 present enlarged views of a portion of a prior art microchannel plate.

Detailed Description of an Exemplary Preferred Embodiment of the Invention

Viewing first Figures 1 and 2 together, it is seen that a typical night vision device (NVD) 18 includes a housing 20 which is depicted by a dashed-line outline. The housing 20 carries an objective lens 22 by which light 24 from a scene or object to be viewed is focused through the transparent front window 26 of an image intensifier tube (IIT) 28. This IIT 28 includes a tube body 30 carrying the front window 26 and a transparent image output window 32. On the inside surface of the front window 26 is carried a photocathode (PC) layer 34 of material which is responsive to photons of light to emit electrons, indicated by the arrowed "e-" symbol. Thus, the PC 34 provides photoelectrons in a pattern which replicates the image of the scene being viewed.

An electrical power supply 36, which is schematically illustrated in Figure 1 as a battery may include a battery and voltage step-up circuitry in order to provide appropriate voltage levels to the parts of tube 28, as will generally be explained, and as is schematically depicted in Figure 2. Thus, the pattern of photoelectrons liberated at photocathode layer 34 are moved by a prevailing electrostatic field to a microchannel plate (MCP) 38. The MCP 38 includes a glass plate-like substrate 40- defining multiple through microchannels 42 in an array 42a. The microchannel plate 38 also includes a peripheral solid glass rim portion 44 surrounding the array of microchannels 42. On each opposite face of the glass microchannel plate 38 is carried a respective one of a pair of electrodes (i.e., input electrode 46, and output electrode 48).

Accordingly, viewing Figure 2, it is to be appreciated that each time when a photoelectron e- enter a microchannel 42 of the plate 38, there is a high statistical probability of this photoelectron being greatly multiplied by secondary emission of electrons from the walls of the microchannels 42. As a result a shower of secondary- emission electrons, indicated by the symbol "e-" and the numeral 50 on Figure 2, is liberated by the MCP 38 in a pattern which also replicates the image of the scene being viewed.

The secondary-emission electrons 50 are moved by a prevailing electrostatic charge to an output electrode 52 associated with a phosphorescent coating 54 carried on the inner surface of the output window 32. The phosphorescent coating 54 responds to the shower of electrons 50 by producing visible light, which has a pattern replicating the scene being viewed.

Now turning to Figures 3a, and 3b, and recalling that conventional microchannel plates for conventional image intensifier tubes are made as follows: single fibers are stacked together in a hexagonal multi-fiber; the hexagonal multi-fiber of single fibers is fused and subjected to a drawing process to produce hexagonal multi-fibers; a plurality of the multi-fibers are assembled into a housing tube that is subjected to a vacuum and heated in order to fuse the multi-fibers into a unitary boule. As explained above, this conventional process produces undesirable fixed pattern noise in the completed microchannel plates. The present invention uses a process including all of the steps used as described by a conventional manufacturing process for microchannel plates. However, additional steps used in the making of the present inventive microchannel plate will significantly reduce or eliminate the fixed pattern noise. As seen in Figure 3a, and 3b, which are representations of actual microphotographs made from a microchannel plate according to the present invention, it will be noted that the hexagonal pattern of MBN is absent.

The present MCP 38 includes a rim portion 44 with a filler section 44a both - formed of cladding glass. The rim portion 44 has a configuration at interface 56 which is composed of plural hexagonal edge sections. These plural edge sections are connected together in circumscribing the array 42a. And it is to be understood that this interface shape results from the rim portion 42 interfacing with a plurality of hexagonal multi-fibers of fibers which is surrounds. But within the array 42a of microchannels 42, the hexagonal MBN pattern is absent. In other words, the plural multi-fibers of fibers which have become the microchannels of the MCP 38 interface with one another with sufficient precision in the present invention that the MBN pattern is substantially absent, as is seen in Figure 3b.

Viewing Figures 4a, 4b, and 4c, seen at a greatly magnified size is one exemplary hexagonal multi-fiber 58 of fibers 60 (i.e., a single multi-fiber), which multi-fiber 58 is one of several (approximately 1000) substantially identical multi- fibers that will be combined to make a fused boule. As explained above, this fused boule will then be further processed into many microchannel plates. Viewing Figures 4a, 4b, and 4c in detail, it is seen that the multi-fiber 58 is made up of two different kinds of fibers. In a central portion of the multi-fiber, the fibers 60a are all of a first type and have both a strand 62 of core glass surrounded by a sheath 64 of cladding glass, just as in a conventional microchannel plate. However, an outermost row of fibers 60b surrounding all of the fibers 60a are of a second type and are single fibers of etchable core glass. Moreover, the outermost row of fibers 60b surrounding all of the fibers 60a of the first type are all of the second type and have no core. Further, the fibers 60b are made entirely of core glass (i.e., of a glass which is etchable).

Viewing Figure 4b in particular, it is seen that the fibers 60b in combination form a layer of core glass completely surrounding the fibers 60a of the first type (i.e., the fibers with both core and cladding are surrounded by a layer of fibers which are all of etchable glass). It will be understood that Figure 4b is merely representative, and that the multi-fiber bundle may include more than a single surrounding layer of fibers 60a. That is, two or more layers of fibers 60a of the second type may surround the fibers 60a of the first type. The multi-fiber 58 may contain about 8,000 fibers 60a (i.e., of the first type) surrounded by a single or double layer of single fibers 60b of the second type. After fusing, the multi-fiber 58 is subjected to a drawing process which reduces its cross sectional size in preparation to later fabrication of a multitude of such - multi-fibers into a microchannel plate boule. Before the multi-fibers are united into a boule, the fibers 60b of the second type are removed by etching. Moreover, after the multi-fiber 58 is fused into a unitary body and is drawn to an elongated shape of smaller cross sectional sized preparatory to uniting this multi-fiber and several similar multi-fibers into a boule pre-form, the outer layer (or layers) of etchable glass formed by the fibers 60b are etched away.

Turning now to Figure 5 a, a multi-fiber 58 is seen in which fibers 60a are all fused into a unitary body. It is seen that the outer fibers 60b which were previously surrounding the fibers 60a are now gone, having been etched away in a previous process step. This multi-fiber 58 of fibers 60a has a newly created outer surface 66 which has been created by the etching away of the layer of fibers 60b, exposing the outer ones of the underlying fibers 60a. The surface 66 as is seen in Figure 5a is scalloped. Those ordinarily skilled in the pertinent arts will recognize that the multi- fiber 58 seen in Figure 5a is elongate in the direction perpendicular to the plane of Figure 5a, so that the surface 66 is in fact elongate and has a plurality of striations, which appear as scallops in the end view of Figure 5a.

Now, viewing Figure 5b, it is seen that when a boule pre-form is assembled using plural multi-fibers 58 manufactured as described above, which are stacked together in a multi-faceted pattern used to assemble the boule pre-form (recalling the description above), the surfaces 66 of adjacent multi-fibers 58 will interlock with one another. That is, the scallops or striations of one surface 66 interdigitate with the scallops or striations 66 of an adjacent hexagonal multi-fiber at a mutual interface, as is seen in Figure 5b. Subsequently to this boule pre-form stacking of hexagonal multi- fibers into a glass tube to make the boule pre-form, it is understood that the boule is fused and the void spaces which appear between the adjacent hexagonal multi-fibers 58 of Figure 5b are eliminated. That is, the adjacent multi-fibers 58 of Figure 5b move toward one another along the line indicated with the arrowed numeral 68, which is generally perpendicular to the mutual interface of these multi-fibers. As a consequence, rows of fibers 60a which are somewhat misaligned from one another, as is indicated with numerals 68 and 70 move relative to one another toward true alignment across the mutual interface 74.

Turning now to Figure 5c, a microphotograph of a microchannel plate made in- accord with the present invention is presented. This microphotograph shows rows 70 and 72 of microchannels 42 (from which the core 62 has been etched away leaving the tubular cladding 64 to define each microchannel) in substantial alignment with one another across the interface 74. Further, in comparison to the microphotograph of Figure 8, it is seen that the interface 74 is much narrower than the boundary 14. Still further, the microchannels 42b next adjacent to the interface 74 are more nearly round than the ones seen at reference numeral 16a of Figure 8. It follows that MBN is substantially reduced or eliminated with a microchannel plate according to this present invention.

An important advantage that results from use of the present invention is an improved yield of good microchannel plates. This yield improvement results from the etching step applied to the multi-fibers 58 to remove the outer fibers 60b. Such is the case because multi-fibers are sometimes damaged, contaminated, or distorted in manufacturing, and it is the outer fibers of such multi-fibers which most frequently suffer damage, contamination, or distortion. For example, contamination which is close to on the surface of a multi-fiber, or chips of glass which may occur during handling of a multi-fiber, are all removed along with the outer fibers 60b. The inner fibers of an otherwise damaged multi-fiber are most generally not damaged or distorted. Thus, because the present inventive manufacturing process removes these damaged, contaminated, or distorted outer fibers before the multi-fibers are processed into a boule and into microchannel plates, leaving the inner good fibers intact, more good microchannel plates and less waste results. It follows that the yield of good microchannel plates from individual boules, and from the multi-fiber manufacturing process is increased.

While the present invention has been depicted, described, and is defined by reference to a single particularly preferred embodiment of the invention, such reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described preferred embodiment of the invention is exemplary only, and is not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

Claims

I CLAIM:

1. A method of making a microchannel plate which has reduced or substantially eliminated multi-boundary noise, said method comprising steps of: providing plural glass fibers of a first type each of which include a strand of core glass surrounded and interbonded with a sheath of cladding glass; providing plural glass fibers of a second type each of which include a strand of core glass and which are substantially the same diameter as fibers of the first type; stacking fibers of the first type into a hexagonal multi-fiber, and providing an outer layer of fibers of the second type surrounding the fibers of the first type; fusing the hexagonal multi-fiber into a unitary body including both the fibers of the first type and the fibers of the second type.

2. The method of Claim 1 further including the step of removing the outer layer of fibers of the second type, leaving a multi-fiber of fused fibers of the first type in a hexagonal shape having elongate surface striations.

3. The method of Claim 2 further including the steps of stacking plural hexagonal multi-fibers together in a boule pre-form, and interdigitating surface striations of one multi-fiber with surface striations of a next-adjacent multi-fiber.

4. The method of Claim 3 further including the step of fusing adjacent multi-fibers together, and while so fusing the adjacent multi-fibers together moving the multi-fibers toward one another generally perpendicular to a boundary therebetween and as interdigitated to provide a fiber matrix substantially continuous across said boundary.

5. A microchannel plate having a great multitude of fibers in a matrix within a surrounding rim, said great multitude of fibers defining microchannels extending through said microchannel plate and also defining a fine-dimension matrix of individual fibers generally in a repeating triangular pattern, and said triangular pattern repeating substantially continuously within said matrix substantially without a hexagonal boundary pattern.

6. A microchannel plate having a great multitude of fibers in a matrix within a surrounding rim, said great multitude of fibers defining substantially circular microchannels extending through said microchannel plate, and said microchannel plate being substantially free of non-circular microchannels in a hexagonal boundary pattern.

7. A method of making a microchannel plate which has reduced or substantially eliminated multi-boundary noise, said method comprising steps of: providing plural glass fibers of a first type each of which include a strand of core glass surrounded and interbonded with a sheath of cladding glass; providing plural glass fibers of a second type each of which include a strand of core glass and which are substantially the same diameter as fibers of the first type; stacking fibers of the first type into a hexagonal multi-fiber, and providing said multi- fiber with an outer layer of fibers of the second type surrounding the fibers of the first type; fusing the hexagonal multi-fiber into a unitary body including both the fibers of the first type and the fibers of the second type; and drawing the fused multi-fiber to a smaller cross sectional size; removing the fibers of the second type from the drawn multi-fiber, leaving an exterior surface with elongate striations.

8. The method of Claim 7 further including the step of stacking plural multi-fibers together into a boule pre-form, and interdigitating the multi-fibers by engagement of said exterior surface elongate striations.

9. The method of Claim 8 further including the step of fusing adjacent multi-fibers together, and while so fusing the adjacent multi-fibers together moving the multi-fibers toward one another generally perpendicular to a boundary therebetween and as interdigitated to provide a fiber matrix substantially continuous across said boundary.

10. A fused-boule manufacturing intermediate article for use in making a microchannel plate, said fused boule manufacturing intermediate article including a glass tube surrounding an array of fibers, said array of fibers being substantially free of repeating hexagonal boundary pattern.

11. A fused multi-fiber manufacturing intermediate article for use in making a microchannel plate, said fused multi-fiber including a hexagonal group of elongate fibers of a first type each of which includes a strand of core glass and a surrounding sheath of cladding glass; a layer of fibers of a second type about said hexagonal group of fibers of said first type, said fibers of said second type each including a strand of core glass and being substantially the same diameter as said fibers of said first type.

12. An image intensifier tube, said image intensifier tube having a body bounded by a front transparent window for receiving light from a scene and by a rear transparent window for providing an image replicating said scene, a photocathode inwardly of said front window and responding to said light to liberate photoelectrons in a pattern replicating the scene, a microchannel plate receiving the photoelectrons from the photocathode and responsively providing an amplified shower of electrons in a pattern replicating the scene, and an output electrode assembly associated with said output window and including a phosphorescent layer in light transmitting relation with the output window, said output electrode assembly receiving said amplified shower of electrons to produce an image replicating said scene, said microchannel plate including a great multitude of microchannels in a matrix, and said matrix being substantially free of repeating hexagonal boundary pattern.

13. A method of making a microchannel plate comprising steps of: stacking fibers of a first type and fibers of a second type together to form a hexagonal multi-fiber, providing said fibers of said first type each with a strand of core glass surrounded and interbonded with a sheath of cladding glass; providing fibers of said second type each with a strand of core glass; making the fibers of the second type substantially the same diameter as fibers of the first type; stacking the fibers of the first type into a hexagonal group, and surrounding the fibers of the first type with fibers of the second type.

14. The method of Claim 13 further including the step of fusing the hexagonal multi-fiber into a unitary body including both the fibers of the first type and the fibers of the second type.

15. The method of Claim 14 further including the step of removing from the hexagonal multi-fiber the outer layer of fibers of the second type, and leaving the fused fibers of the first type in a hexagonal shape having elongate surface striations.

16. The method of Claim 15 further including the steps of: providing plural substantially similar hexagonal multi -fibers; stacking the plural multi-fibers together with said elongate surface striations interdigitated; and fusing the stacked and interdigitated multi-fibers into a unitary boule.

17. The method of Claim 16 further including the step, while fusing adjacent multi-fibers together, of moving the adjacent multi-fibers toward one another generally perpendicular to a boundary therebetween in order to provide a fiber matrix which is substantially continuous across said boundary.

18. A method of making a microchannel plate which has reduced or substantially eliminated multi-boundary noise, said method comprising steps of: providing plural substantially similar glass fibers of a first type each of which consist of a strand of core glass surrounded and interbonded with a sheath of cladding glass, said glass fibers of the first type having an outer diameter; providing plural substantially similar glass fibers of a second type each of which consist of a strand of core glass, and each of which are substantially the same outer diameter as fibers of the first type; stacking fibers of the first type into a hexagonal multi-fiber, and providing an outer layer of fibers of the second type surrounding the fibers of the first type; and fusing the hexagonal multi-fiber into a unitary body including both the fibers of the first type and the fibers of the second type.

19. The method of Claim 18 further including the step of removing the outer layer of fibers of the second type, leaving a multi-fiber of fused fibers of the first type in a hexagonal shape having elongate surface striations.

20. The method of Claim 19 further including the steps of stacking plural substantially similar hexagonal multi-fibers together in a boule pre-form, and interdigitating surface striations of one multi-fiber with surface striations of a next- adjacent multi-fiber.

21. The method of Claim 20 further including the step of fusing adjacent multi-fibers together, and while so fusing the adjacent multi-fibers together moving the multi-fibers mutually toward one another generally perpendicular to a boundary therebetween which is defined at confronting outer surfaces having interdigitated surface striations thereon to provide a fiber matrix which is geometrically substantially continuous across said boundary.