Image transmitting fiber bundle and manufacturing process thereof

Abstract: An image fiber preform for use in an image fiber according to the present invention includes a plurality of optical fiber strands, each optical fiber strand including a silica glass core which contains a dopant for increasing a refractive index of the core and a silica glass cladding surrounding the silica glass core, the silica glass cladding containing a fluorine dopant for decreasing a refractive index of the cladding, the cladding having an outer peripheral surface, and a quartz tube in which the optical fibers are closely arranged, wherein the cladding has a concentration of the fluorine dopant which decreases radially outward toward the outer peripheral surface of the cladding. In the present invention, an image fiber prepared from the image fiber preform and fabrication processes for the image fiber preform and the image fiber are provided.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image transmitting optical fibre bundle for use in industrial and medical endoscopes and general image transmission systems, as well as a fabrication process thereof. Prior Art It is well known that an optical fiber strand including a SiO 2 -glass core having a high refractive index and a SiO 2 -glass cladding containing fluorine (F) and/or boron oxide (B 2 O 3 ) for reducing the refractive index of the cladding may be used in order to obtain the necessary large refractive index difference between the core refractive index and the cladding refractive index, whereby the diameter of the optical fiber may be reduced. For example, when the core diameter is 4µm and the cladding outer diameter is 6µm, the relative refractive index difference between the core refractive index and the cladding refractive index needs to be about 4%. A plurality of the above mentioned optical fiber strands, which are arranged closely in a glass tube, are fused by heating from an end thereof and drawn to form an image transmitting optical fibre bundle . An example of a conventional fused-type image transmitting fiber bundle is shown in Fig.4. The image transmitting optical fibre bundle 21 is composed of cores 22, a common cladding part 23 disposed between these cores 22, and a jacket 24 which surrounds the common cladding part 23. The image transmitting optical fibre bundle 21 can transmit pictures from one end-face at an objective side to the other end-face of an ocular side through the multiple cores 22. The refractive index distribution on the A-part of the cross-section of the image transmitting optical fiber bundle shown in Fig.4 is indicated in Fig.5. In general, the refractive index distribution n(r) is calculated by the following equation (1): n(r) = n 0 [1-2 Δ (r/a) α ] ½ wherein n 0 is the refractive index of a center of a core; r is the distance from the center of the core to a random point in the core; a is the radius of the core; Δ is the relative refractive index difference (Δ = (n 0 -n 0 )/n c wherein n 0 is the refractive index of the center of the core and n c is the refractive index of a cladding); and α is a refractive index distribution constant. For the refractive index distribution, it is known that a step-type refractive index distribution in which the refractive index is constant has a refractive index distribution constant α of 5 or more and that a graded-type refractive index distribution in which the refractive index is not constant but parabolically changes has a refractive index distribution constant α of about 2. The image transmitting optical fibre bundle having a refractive index distribution as shown in Fig. 5 is composed of cores 22, each core having a graded-type refractive index distribution and a common cladding part 23 having a flat refractive index distribution. The graded-type refractive index distribution of the core 22 has a refractive index distribution constant α of about 2. However, since the cladding-glass doped with fluorine (F) and/or boron oxide (B 2 O 3 ) has a lower softening temperature than the pure silica glass has in this type of image fiber bundle, the cladding glass is dissolved when a plurality of optical fiber strands are arranged and fused, thus causing the array of cores to disarrange. In the case of the cladding-glass doped with fluorine (F), the surface of the cladding is besides subjected to etching by the fluorine to general silicon tetrafluoride (SiF 4 ) during the melting process. The SiF 4 gas forms bubbles which result in decreasing the mechanical strength of the image transmitting optical fiber bundle. In order to prevent the array of cores from disarranging, it has been proposed that the fiber strands further include support layers disposed at the outer peripheral surface of the claddings and are arranged closely in a glass tube an subsequently fused to obtain an image transmitting optical fibre bundle. The support layer, which is a pure silica layer composed of silicon (IV) oxide (SiO 2 ) without dopant, has a higher softening temperature than the cladding has. An example of the fused-type image transmitting optical fibre bundle mentioned above is shown in Fig.6. The image transmitting optical fibre bundle 25 is composed of cores 26, cladding parts 27, each cladding part 27 surrounding each core 26, a high refractive index support layer 28 disposed between these cladding parts surrounding the cores 26 and a jacket 29 which surrounds the high refractive index support layer 28. Fig.7 shows a refractive index distribution on the C-part of the cross-section of the image transmitting optical fibre bundle shown in Fig.6. Both the core 26 and the high refractive index support layer 8 have respectively step-type refractive index distribution, which has a refractive index distribution constant α of about 5 and more. Fig.8 shows another example of refractive index distribution of an image transmitting optical fibre bundle having the same structure as the fiber shown in Fig.6. This image transmitting optical fibre bundle is composed of cores 30, each core 30 having a graded-type refractive index distribution, cladding parts 31, each cladding part 31 surrounding each core 30, and a high refractive index support layer 32 formed between these cladding parts 31 surrounding the cores 30, the support layer 32 having a step-type refractive index distribution. These image transmitting optical fibre bundles having the refractive index distribution shown in Figs. 7 and 8 have the advantage that the array of cores is not disarranged. However, in the image transmitting optical fibre bundle, disadvantageously bubbles are generated at the boundary face where the cladding and the high refractive index support layer contact, thus causing a decrease in the mechanical strength of the image transmitting optical fibre bundle since the composition of the cladding is extremely different from that of the high refractive index support layer. Futhermore, as the image transmitting optical fibre bundles have step-type high refractive index support layers, unwanted light such as excess incident light and the like adversely diffuse into the core, thus causing deterioration of the contrast of the image transmitting optical fibre bundle . SUMMARY OF THE INVENTION An object of the present invention is to provide an image transmitting optical fibre bundle which allows pictures to be transmitted with superior contrast by decreasing the unwanted incident light which enters the fiber and by preventing stray light from propagating in the cladding part and adversely diffusing into the core. It is also an object of this invention to provide a fabrication process for the image transmitting optical fibre bundle . According to the first aspect of the present invention, there is provided an image transmitting optical fiber bundle including cores and cladding parts immersed in a common light-shield part, the cladding parts individually surrounding the cores and the common light-shield part having a non-stepped refractive index distribution, characterised in that, for any notional line extending normally to the axis of the fibre bundle between two adjacent cladding parts, a graph representing the variation of the refractive index of the light-shield part against distance along the line is concave. The present invention also provides a process for preparing an image transmitting optical fiber bundle, comprising the steps of: (a) preparing a plurality of optical fiber strands, each fiber strand comprising a core, a cladding part surrounding the core, and a light-shield part surrounding the cladding part, the light-shield part having a refractive index gradually increasing radially outward so that the maximum refractive index thereof is higher than a refractive index of the cladding, the function representing the variation of the refractive index of the light-shield part against distance along an axis normal to the axis of the fiber strand being concave; (b) subsequently, packing closely the optical fiber strands in a quartz tube to form a preform; (c) subsequently, heating the preform from an end thereof; and (d) subsequently, subjecting the preform to melt-drawing operation to form an image transmitting optical fibre bundle. EP-A-0 125 828 discloses (cf. Fig.4 and Example 1) an image transmitting fibre bundle, as well as a process of manufacturing the same, which are similar to those as defined just above, except that the function representative of the variation of the refractive index of the light shield part against distance is convex instead of being concave. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front view of an image transmitting optical fibre bundle according to a preferred embodiment of the present invention; Fig.2 is a schematic representation illustrating a refractive index distribution at a part B of the image transmitting optical fibre bundle shown in Fig.1; Fig.3 is a cross sectional view of an optical fiber strand appropriately used for producing an image transmitting optical fibre bundle of the present invention and a schematic representation illustrating the refractive index distribution thereof; Fig. 4 is a cross sectional view of a conventional image transmitting optical fibre bundle structure; Fig 5 is a schematic representation illustrating a refractive index distribution at a part A shown in Fig.4; Fig.6 is a cross sectional view of a second conventional image transmitting optical fibre bundle structure; Fig.7 is a schematic representation illustrating a refractive index distribution at a part C shown in Fig.6; and Fig.8 is a schematic representation illustrating a refractive index distribution of a third conventional image transmitting optical fibre bundle having the same structure as shown in Fig.6. DESCRIPTION OF THE PREFERRED EMBODIMENT An image transmitting optical fibre bundle of an embodiment according to the present invention is shown in Fig. 1. In Fig.2, a refractive index distribution at a part B of the image transmitting optical fibre bundle shown in Fig. 1 is illustrated. In Fig. 1, an image transmitting optical fibre bundle 11 is composed of a jacket 4 and an image circle 15, which is equipped with a plurality of cores 12, and cladding parts 13, each cladding part 13 surrounding each core 12, and light-shield part 14 formed between these cladding parts 13. The image circle 15 is covered with the jacket 4. The above-mentioned core 12 has a graded-type refractive index distribution. The cladding part 13 has a low refractive index and has a flat distribution (Fig.2). Furthermore, the light-shield part 14 has a graded-type refractive index distribution and has a higher refractive index than the cladding part 13. Fig.2 shows moreover that the function representative of the refractive index distribution of the light shield part 14 against distance is concave. As the material of the each part, for example, GeO 2 -doped SiO 2 is used for the core 12, fluorine (F) and/or B 2 O 3 -doped SiO 2 is used for the cladding part 13, and fluorine and/or B 2 O 3 -doped SiO 2 is used for the light-shield part 14. In the image transmitting optical fibre bundle 11, since the light-shield part 14 having a graded-type refractive index distribution is formed between the cladding parts 13 surrounding the cores 12, unwanted light such as stray light from the cladding, leaked light from the core, and the like is confined within the light-shield part 14, so that there is little chance of the light entering into images transmitted from the end face on the objective side to he end face on the ocular side through the cores 12. Thus, the contrast in the transmitted images is improved. In order to prepare an image transmitting optical fibre bundle 11 having a refractive index distribution shown in Fig. 2, at first, an optical fiber strand 18, which has a core 12, a cladding part 13 disposed at the peripheral surface of the core 12, and a gradually-increasing refractive-index part 17 disposed at the outer peripheral surface of cladding part 13 is prepared. The core 12 has a graded-type refractive index distribution and the cladding part 13 has a flat refractive index distribution. A plurality of the optical fiber strands 18 are bundled and arranged into a jacket tube to make an image transmitting optical fibre bundle preform. The image transmitting optical fibre bundle preform is melt-drawn from an end thereof, thus making the image transmitting optical fibre bundle 11. During melt-integration, the gradually-increasing refractive index part 17 for each optical fiber strand 18 contacts the others, and they fuse to form the light-shield part 14 having a graded-type refractive index distribution. An example of the process for preparing the above-mentioned optical fiber strand 18 is disclosed hereinafter. A porous parent material composed of GeO 2 -doped SiO 2 is prepared by using the VAD (vapor-phase axial deposition) method, or a transparent glass parent-material is prepared by dehydration if necessary, thus forming a core part which has a graded-type refractive index distribution. Subsequently, fluorine (F) and/or B 2 O 3 doped SiO 2 is deposited on either of the parent materials (porous parent material and transparent glass parent material) by the plasma CVD (chemical vapor deposition ) method, for example, to form the transparent cladding part 13 and the gradually-increasing refractive-index part 17. In order to form the cladding part 13, the concentration of fluorine (F) and/or boron oxide (B 2 O 3 ) in the deposit is maintained constant. In order to form the gradually-increasing refractive-index part 17, the concentration of fluorine (F) and/or boron oxide (B 2 O 3 ) in the deposit is gradually decreased. The resulting transparent glass parent-material is drawn to prepare the F-doped and/or B 2 O 3 -doped optical fiber strand 18 for use in an image transmitting optical fibre bundle, which has the refractive index distribution shown in Fig.3 (wherein the curve representative of the index distribution of the outer part 17 is seen to be concave). A plurality of these optical fiber strands 18 are bundled and arranged in a quartz tube to prepare a preform. The preform is melt-drawn from an end thereof to obtain the image transmitting optical fibre bundle 11. When a F-doped optical fiber strand is used as the optical fiber strands arranged in the quartz tube for the above-mentioned preform, the F-doped optical fiber strand is prepared from an optical fiber rod having a core and a cladding surrounding the core, the cladding being doped uniformly with fluorine. The optical fiber rod is subjected to heat treatment so that the fluorine doped in the outer peripheral surface of the silica glass cladding of the optical fiber rod evaporates, and melt-drawn. A plurality of the F-doped optical fiber strands, thus obtained, are closely arranged in the quartz tube to form the above-mentioned preform. If the heat treatment is omitted, the cladding surface of the optical fiber strand is subjected to an etching by the fluorine while the quartz tube including the optical fiber strands is melt-drawn to generate SiF 4 gas which will remain as bubbles in the produced image transmitting optical fibre bundle and result in a reduction of mechanical strength thereof. The heat treatment process is at a temperature generally in the range of 1600°C to 1800°C for several hours. Under such conditions, the fluorine in the outer peripheral surface of the cladding arranged in the quartz glass tube is evaporated sufficiently and the preform is not deformed. Furthermore, as only the fluorine in the vicinity of the periphery of the cladding is removed, the refractive index difference between the core refractive index and the cladding refractive index is maintained. Therefore, the image is not hindered. (Example 1) An image transmitting optical fibre bundle having a refractive index distribution as shown in Fig.2 was prepared. An optical fiber strand (optical fiber material) in which the core and the light-shield part thereof had the graded-type refractive index distribution shown in Fig.3 was used: The following core, cladding part, and gradually increasing refractive-index part were used. Core: GeO 2 -doped SiO 2 ; core diameter of 100µm; and relative refractive index difference of +3.0%. Cladding part; (F, B 2 O 3 )-doped SiO 2 ; thickness of 25 µm; and relative refractive index difference of -1.0%. Gradually-increasing refractive-index part: (F, B 2 O 3 ) -doped SiO 2 ; thickness 5 µm; and a maximum relative refractive index difference of -1.0%. A bundle of 3000 of these optical fiber strands was closely arranged in a quartz tube and melt-drawn to prepare an image transmitting optical fibre bundle according to the present invention. As a comparative example, another optical fiber strand the same as the above-mentioned material of the present invention was prepared. The comparative optical fiber strand had a core having a graded-type refractive index distribution and a cladding, as well as a high-refractive index part composed of SiO 2 having a step-type refractive index distribution. The high-refractive index part surrounded the outer peripheral surface of the cladding part. Subsequently, by using the previously-described procedure, the comparative image transmitting optical fibre bundle composed of cores, each core having graded-type refractive index distribution and a support layer having step-type refractive index distribution ,as shown in Fig.7, was prepared. The contrast of the images transmitted through the image transmitting optical fiber bundle of the present invention was compared with that of the comparative one, using the MFT method. It was confirmed that the contrast of the images transmitted through the optical fiber bundle of the present invention were improved compared with those of the comparative example by about 1.5 times.

Claims: An image transmitting optical fiber bundle (11) including cores (12) and cladding parts (13) immersed in a common light-shield part (14), the cladding parts individually surrounding the cores and the common light-shield part having a non-stepped refractive index distribution, characterised in that, for any notional line (B) extending normally to the axis of the fibre bundle between two adjacent cladding parts, the function representing the variation of the refractive index of the light-shield part against distance along the line is concave. A process for preparing an image transmitting optical fiber bundle (11), comprising the steps of: (a) preparing a plurality of optical fiber strands (18), each fiber strand comprising a core (12), a cladding part (13) surrounding the core, and a light-shield part (17) surrounding the cladding part, the light-shield part having a refractive index gradually increasing radially outward so that the maximum refractive index thereof is higher than a refractive index of the cladding, the function representing the variation of the refractive index of the light-shield part against distance along an axis normal to the axis of the fiber strand being concave; (b) subsequently, packing closely the optical fiber strands in a quartz tube (4) to form a preform; (c) subsequently, heating the preform from an end thereof; and (d) subsequently, subjecting the preform to a melt-drawing operation to form an image transmitting optical fibre bundle.

Inventors: SANADA KAZUO, SETO KATSUYUKI, SHAMOTO NAOKI, TANAKA KOUJI, TANAKA TOSHIYUKI, TSUMANUMA TAKASHI, SANADA, KAZUO, SETO, KATSUYUKI, SHAMOTO, NAOKI, TANAKA, KOUJI, TANAKA, TOSHIYUKI, TSUMANUMA, TAKASHI
Applicants: FUJIKURA LTD, FUJIKURA LTD.
Application year: 1990
Main classification: G02B6
IPC codes: C03C13, C03B37, G02B6