METHOD OF CONNECTING AN OPTICAL WAVEGUIDE TO AN OPTICAL FIBER

Abstract: A method of connecting an optical waveguide to an optical fiber, in which an optical waveguide and a marker are formed at the same time on an optical waveguide substrate when patterning an optical waveguide by photolithography method, the waveguide is buried into a clad layer formed in other area than said marker, grooves for insertion pins are formed with the marker as a positional reference, and a optical waveguide module is connected to an optical fiber connector by the use of said grooves for insertion pins. In this way, an expensive positioning/setting device is no more needed and an optical waveguide can be connected to an optical fiber in alignment with each other and in an exceedingly simple manner and a short period of time.

Description: Technical Field The present invention relates to a method of connecting optical waveguides and optical fibers used in an optical communication system. Prior Art In general, waveguide elements are used as optical members to constitute an optical communication system. The waveguide elements are categorized mainly into two types, i.e., one type in which a waveguide is formed by stacking silica glass layers each having different refractive index one on another on an Si substrate, for example, and the other in which a waveguide is formed by stacking semiconductor thin films each having a different composition on a special semiconductor substrate made of GaAs, LiN, or the like. In order to use these silica type waveguides and semiconductor type waveguides as optical members in an optical communication system, the waveguides must be connected to optical fibers so as to input/output light. More specifically, a waveguide and an optical fiber must be connected such that the pattern of a cross section of the waveguide and an arrangement of the optical fibers are aligned with each other. One of the examples of such a connecting method will now be described in connection with a case where a 1 × 8 splitter chip of a silica type waveguide and an optical fiber are connected with each other. A 1 × 8 splitter chip 12 such as shown in Fig. 1 (A) in which input/output waveguides 11 are formed on a silicon substrate 10 is fixed in a tub-like metal casing 13 by adhesive as shown in Fig. 1 (B), and the casing is annealed to make a waveguide part. Then, an input-side optical fiber connector 15, in which an optical fiber 14 is put through and which can be moved at a degree of freedom of 6, is arranged to face an end face of the casing 13, as can be shown in Fig. 1 (C). Thereafter, as shown in Fig. 1 (D), the position of the input-side optical fiber connector 15 is adjusted such that the optical output from each of the output waveguides 11 of the chip 12, as the result of light made incident from the optical fiber 14, has the maximum value. At the most appropriate position, the input waveguide 11 and the core of the optical fiber 14 are aligned with each other along the optical axis, and fixedly connected with each other by use of YAG laser, adhesive, or the like. Next, as shown in Fig. 1 (E), an output-side optical fiber connector 16 in which eight optical fibers 14 are fixedly arranged in parallel at the same pitch as that of the output waveguides 11 of the chip 12, is made to face the other end face 13b of the casing 13. Following this, as shown in Fig. 1 (F), the output-side optical fiber connector 16 is moved at a degree of freedom of 6 such that the optical output from each of the eight optical fibers has the maximum intensity of output. At the most appropriate position, the output waveguides 11 and the core of the optical fiber 14 are aligned with each other along the optical axis, and fixedly connected with each other by use of YAG laser, adhesive, or the like. In general, connection of a waveguide and an optical fiber is carried out in the above-described manner. However, in the case of the above-described case, there is no established standard of alignment between a waveguide formed in a waveguide element and an optical fiber running through an optical fiber connector, and a judgment as to whether there is an error in axis alignment is simply based on the level of the intensity of the output light resulted by incidence of light into the optical fiber. Thus, the reliability of the connection is low. Further, according to the above-described method, it takes at least one hour to connect a casing end face and an optical fiber connector aligned with each other for one connection, and therefore this method is not suitable for mass production. Also, an extremely expensive alignment fixing device is required. JP-A-1 232 307 shows an optical waveguide and an optical connector, both provided with a fitting hole for receiving a fitting pin to achieve alignment. However it does not disclose forming a marker on the waveguide when forming the pattern of the waveguide, the marker being for alignment purposes. JP-A-2-125 209 shows an optical fiber and optical waveguide coupling structure with grooves for guide pins on the optical fiber holder. Again there is no disclosure of providing a marker on the waveguide at the time of patterning the waveguide material. Disclosure of the Invention The purpose of the invention is to provide a method of connecting optical fibers and waveguides with each other, in which the waveguides and the optical fibers are aligned with each other easily in a short period of time without using an expensive alignment fixing device. According to the invention there is provided a method of connecting optical fibers and waveguides with each other, as set out in claim 1. Preferred features are set out in dependent claims. Brief Description of the Drawings Figs. 1 (A)-(F) are explanatory diagrams each showing a step in a conventional method of connecting a waveguide and an optical fiber; Fig. 2 is a plan view of a 1 × 8 tree splitter waveguide chip according to the invention; Fig. 3.is a cross section of the waveguide chip shown in Fig. 2, taken along the line A-A; Fig. 4 is a plan view of a 1 × 8 tree splitter waveguide chip processed to have V-shape grooves according to the invention; Fig. 5 is a side view of the waveguide chip shown in Fig. 4, taken from the light outputting face side; Fig. 6 is a schematic view of a substrate of a waveguide module according to the invention; Fig. 7 is a side view of the waveguide module according to the invention, taken from the light outputting face side; Fig. 8 is a schematic view of an optical fiber connector according to the invention; Figs. 9 (A) and (B) are respectively a plan view and side view of a waveguide module connecting fiber and waveguide to each other according to the method of the invention; Figs. 10 (A) and (B) are top front views of a waveguide substrate on which a waveguide core and marker are formed, in a step of the manufacturing process of a waveguide module according to the first embodiment of the invention; Fig. 11 is a cross section of the waveguide substrate shown in Fig. 10; Fig. 12 (A) and (B) are top front views of the waveguide substrate, designed to illustrate the embeded region of the waveguide substrate; Fig. 13 is a cross section of the waveguide substrate on which a V-shape groove is formed; Fig. 14 is a cross section of the waveguide module assembled; Figs. 15 (A)-(C) are diagrams illustrating how the waveguide module and the optical fibers are connected; and Fig. 16 is a schematic view showing an optical fiber connector manufactured by the plastic molding method according to the invention. Best Mode for Carrying Out the Invention Examples of the invention will now be described in detail with reference to accompanying drawings. Example 1 As shown in Fig. 3, a silica glass underlaying clad layer 51 was formed on a 1-mm-thick 3-inch silicon substrate 50 by the flame hydrolysis deposition method. The assembly was processed into a 3-dimensional waveguide structure as shown in Fig. 2, by semiconductor fine processing techniques such as photolithography and dry etching. Thus, eight waveguide circuits 52 and two pairs of V-shaped groove marker 53 were formed simultaneously on the silicon substrate 50. The V-shaped groove marker 53 sets the standards of position and height for a V-shaped groove, which will be described later. Next, an upper clad layer 55 was formed by the flame hydrolysis deposition method such that only the waveguide circuit 52 was embedded. Since the V-shaped marker 53 serves as a standard for processing a V-shaped groove, it was not embedded. There are methods as that in which a particular section is not embedded, for example, the method wherein flame hydrolysis deposition is carried out with a mask such as a silicon plate covering the section. In this figure, reference numeral 54 denotes the bottom of the V-shaped groove formed later by a cutting process. As shown in Fig. 4, a waveguide circuit 52 includes combinations of Y branches, and has a 1 × 8 tree splitter structure in which one input waveguide 56 is branched out into eight output waveguides 57. Eight of such a structure are continuously formed on the silicon substrate 50. The eight output waveguides 57 in the output section are arranged in parallel with each other with an interval of 250 µm between an adjacent pair. The waveguide circuit has a structure such as shown in Fig. 3. More specifically, the core 58 of the waveguide circuit has a 8 pm square, and the circuit has a difference in specific index of refraction of 0.3%, and are compatible with a single mode fiber. The V-shaped groove marker 53 has a structure such as shown in Fig. 3. More specifically, two lines 53a and 53b each having a width of 10 µm and a length of 30 mm, and a line 53c having a width of 3 mm, and a length of 20 mm are arranged in parallel with each other with an interval of 10 µm. Further, as shown in Fig. 2, a set of this structure is provided on both sides of the eight waveguide circuits 52. Thereafter, a silicon substrate 50 supported by a vacuum chuck is subjected to cutting process by use of a slicer or the like, to make a groove having substantially a V-shape (to be called as V groove hereinafter). Alignment of a V groove 59 with waveguides 56 and 57 is based on the position of the V-shaped groove marker 53 provided on the silicon substrate 50. The alignment is carried out by specifying the position on the top surface of the substrate by image processing using a CCD camera, and the level of the upper surface of the core 58 by use of a capacitance type displacement gauge. A V groove 59 formed by the cutting process has a angle of the V section (the bottom portion) of about 60°, and a depth of about 700 µm. The groove 59 was formed such that the center of the core 58 is at the same level as the center of the fitting pin 61 having a diameter of 0.7 mm when placed in the groove, as shown in Fig. 5. Further, there were two of V grooves 59 arranged 4.6 mm away from each other, with the waveguides 56 and 57 interposed therebetween. The silicon substrate 50 thus obtained was sliced into eight chips by use of a dicing saw, thereby obtaining lower substrates 60 on each of which a V groove 59, waveguides 56 and 57 were formed. Each chip had a width of 6.4 mm, and a length of 50 mm. A ceramic substrate was subjected to cutting process by use of a slicer to form two grooves 63 each having substantially a trapezoid shape such as to press the upper surface of a fitting pin 61. Then, the substrate was divided by a dicing saw into a predetermined number of chips, each of which serves as a press cover 62. Each chip had a width of 6.4 mm, and a length of 50 mm. Next, as shown in Fig. 6, a lower substrate 60 was fixed to a fixation base 66 by adhesive, and four substrate fixation pins 64 each having a diameter of 0.7 mm and a length of 8 mm were placed on V-grooves 59 of the lower substrate 60. Each of the substrate fixation pins 64 was arranged such that the middle portion thereof was located at a position 12 mm away from an end 60a or 60b of the surface of the silicon substrate. Following this, as shown in Fig. 7, a press cover 62 was placed on the lower substrate 60, and they are fixed together with adhesive as clamped together by a clamp spring 65, thereby manufacturing a waveguide module 4. Manufacture of an optical fiber connector will now be described. As shown in Fig. 8,, a lower substrate 70 made of ceramic was subjected to cutting process by use of a slicer, so as to form an optical fiber V groove 71 for supporting an optical fiber, and fitting pin V grooves 72 on both sides of the V groove 71. The grooves 71 and 72 were formed such that the position of each of the optical fiber V groove 71 and the fitting pin V groove 72 corresponds to that of each of the core 58 and the V groove 59 in the end face 60b of the lower substrate 60. The optical fiber V groove 71 thus formed had an angle of the V portion of 60° and a width of 210 µm. The pitch between adjacent optical fiber V grooves 71 was set at 250 µm. The fitting pin V groove 72 had an angle of the V portion of 60° and a width of about 700 µm. The V groove 72 was located such that the core center of each of optical fibers 73, which will be described later, and the center of each of the fitting pins 61 were leveled with each other. There were two fitting pin V grooves 72 arranged 4.6 mm away from each other, interposing the optical fiber V grooves 71 therebetween. In a similar manner as that of the press cover 62, a connector press cover 74 was manufactured by also forming two grooves each having substantially a trapezoid shape such as to cover the top surface of a fitting pin 61, on a ceramic substrate by cutting process. A ribbon-like 8-core single mode fiber 73 having an outer diameter of each fiber of 125 pm and an interval pitch of adjacent fibers of 250 pm was placed on the optical fiber V grooves 71 of the lower substrate 70, and the press cover 74 was fixedly mounted on the lower substrate 70 by epoxy adhesive. Then, the end face 70a was polished by mirror-like finishing, thereby completing an optical fiber connector 5. In this example, a single-core optical fiber connector to be connected to the output end 60b of the waveguide module 4 was manufactured in a manner similar to the above-mentioned. After that, as shown in Fig. 9, four fitting pins 61 were inserted into two V grooves 59 of the waveguide module 4, and the fitting pins 61 were fit into the fitting pin V grooves of a multi-core or single-core optical fiber connector 5 for alignment. Thus, the optical fiber connector 5 was arranged such that both ends of the waveguide module 5 were brought into contact with the optical fiber connectors 5. The interface was coated in advance with matching oil or matching grease so as to prevent loss due to Fresnel reflection.- Then, a press spring 76 is inserted into a fixation base 66 to apply some weight on the interface. As described, in this example, the waveguide module and the optical fiber connector are connected with each other by means of fitting pins, and therefore alignment between a waveguide and a guide groove formed on a lower substrate, and an optical fiber and a guide groove or guide hole in the optical fiber connector can be accurately performed. Thus, the waveguide module and the optical fiber connector can be connected with each other easily in a short period of time. In this example, the waveguide element and the optical fiber connector are detachably connected. Consequently, waveguide modules each having a different pattern can be connected to an optical fiber connector without performing alignment along with an optical axis. A waveguide element and an optical fiber connector manufactured in this example were connected with each other so as to examine its connection in terms of connection loss. The connection loss was an average of 0.5 dB, with a maximum of 1 dB, and thus it was confirmed that alignment deviations between waveguides and optical fibers can be suppressed, and connection of them can be carried out at a small loss, according to the present invention. Example 2 A lower clad layer 81 and a core layer were formed on a silicon substrate 80 by the flame hydrolysis deposition method so as to manufacture a slab waveguide. Then, as shown in Fig. 10 (A) and Fig. 11, a core layer was patterned by photolithography and dry etching so as to form four waveguide cores 82 and eight markers 83, one marker 83 being formed on either side of each waveguide core 82. Further, as shown in Fig. 10 (B), ribbon-like markers each having a width of 2.5 mm and a length of 50 mm were formed such as to run across the markers. The photomask used by the photolithography had both waveguide core pattern and marker pattern. Next, an upper clad layer 84 was formed on a embeded including waveguide cores 82, as shown in Figs. 12 (A) and (B), to embed the waveguide cores 82. Since the markers 83 were to be used for alignment of a V-shaped groove to be formed later, particular portions of the markers were left uncovered by the upper clad layer 84. This can be achieved, for example, by depositing an upper clad layer by flame hydrolysis deposition method with a portion of each of the marker 93 being covered by a silicon plate or the like. Then, the silicon substrate 80 is subjected to cutting process by use of a slicer, so as to form V grooves 85 thereon. As shown in Fig. 13, one V groove 85 was formed such that the center of the groove 85 was aligned with the center of each of the markers, and the depth of the V groove 85 was adjusted such that the center of each marker 83 was aligned with the center of the cross section of a fitting pin 86 when placed on the V groove 85. Lastly, the substrate'80 was sliced into chips, thereby obtaining a plurality of waveguide elements 6. Thereafter, the silicon substrate 80 was subjected to cut process using a slicer to form a V-shaped groove 85. More specifically, the cutting process was performed such that the center position of the V-shaped of the groove 85 was aligned with the center of each marker 83, and the depth of the V-shaped groove 85 was adjusted such as to match the center of each marker 83 with the center of fitting pin 86 to be inserted into the V-shaped groove 85. Fig. 13 is a diagram showing a cross section of a wafer sample after formation of the V-shaped groove. In order to illustrate a cross section of the structure of the buried and non-buried regions, the portion of the sample where buried and non-buried regions appear on both sides of the V-shaped groove, was selected. The structure can be observed when the wafer as shown in Fig. 12 is diagonally cut. It should be noted that when the wafer is cut vertically with respect to the V-shaped groove, a structure in which both sides of the V-shaped groove are buried can be observed. Then, the obtained waveguides were diced by a dicing saw, and thus, a number of waveguides elements 8 were obtained. A waveguide element 6 thus obtained and a silicon substrate 87 on which grooves were formed by the cutting process mentioned before, were assembled together while fitting a fitting pin in each of the grooves, and fixed with each other by means of a spring 88 as shown in Fig. 14, or adhered with each other by adhesive, thereby completing a fitting waveguide module 7. A fitting waveguide module 7 as shown in Fig. 15 (A) and an optical fiber connector 8 shown in Fig. 15 (B) were connected with each other as shown in Fig. 15 (C), and the connection was examined in terms of connection loss. The result indicated that they were connected at a connection loss of as low as 0.3 dB - 1.0 dB. Example 3 A low substrate 60 was manufactured by a method similar to that of Example 1, and both end faces 60a and 60b thereof were polished. The substrate 60 was fixed onto a fixation base 66 by adhesive. Then, four fitting pins 61 were placed on V grooves 59 such that the center of each fitting pin 61 was leveled with the center of waveguide 56 or 57 in each end face. While pressing each fitting pin 61 against each V groove 59 such that the pin is brought into tight contact with the two sides of the V groove, and each fitting pin 61 was fixed to the lower substrate 60 by adhesive, thereby manufacturing an optical waveguide module 4. An optical fiber connector 5 was brought to face to each of the end faces of a waveguide module 4 while aligning them with each other by means of an fitting pin 61. Further, adhesive was applied onto each end face, and the waveguide module 4 and the optical fiber connectors were connected with each other by hardening the adhesive by heat as being pressed with each other. Waveguide module and optical fiber connectors manufactured according to this example were subjected to a connection test so as to examine the connection of each sample in terms of connection loss. The connection loss was an average of 0.5 dB, with a maximum of 1 dB. Next, an optical fiber connector and a waveguide-use press cover were manufactured from glass ceramic which transmits the light in a range of 350 nm - 450 nm at a transmissibility of 20% or higher. Adhesive used for each interface here was of a UV setting type having an index of reflection of 1.456, which is close to that of silica glass, by which the Fresnel reflection at the interface can be small as small as possible. Thus, a UV light was irradiated on the interface from above. With this method, similar results to those of the above connection test were obtained. Further, the manufacturing time was significantly shortened as compared to that case where the thermosetting type adhesive was used. Example 4, In place of ceramic, silicon was used as a material for an optical fiber connector, and subjected to cutting process by use of a slicer so as to form V grooves 71 and 72. Optical fibers thus formed were assembled as in Example 1, and subjected to a test so as to examine its connection in terms of connection loss. The initial values thereof were similar to those of Example 1, and thus it was confirmed that the optical fibers manufactured as above can be connected with each other at a connection loss of an average of 0.5 dB, with a maximum of 1 dB. The connected optical fiber connectors were subjected to a heat cycle test, and the loss variation obtained in the test carried out at a temperature of from -10°C to -60°C, was within a range of ±0.2dB. As compared to the result obtained with the ceramic-made optical fiber connector, i.e., a loss variation of 0.5 dB or higher, the optical fibers of this example exhibited a significantly improved loss variation. In general, the coefficient of linear expansion of silicon is about 2 × 10 -6 /°C, whereas that of ceramic is about 10 × 10 -6 /°C. When silicon is used for the waveguide, and ceramic is used for the optical fiber connector, there results an about 1 pm of pitch deviation of the fitting pin V grooves at the interface in the case of a high or low temperature due to difference in coefficient of heat expansion between the two materials. In order to compensate this, either the waveguide module or the optical fiber connector is warped, producing the connection loss due to an axis deviation. The reason why the loss variation was relatively small should be that the same material was used for the waveguide module and the optical fiber connector. As described above, in this example, a waveguide module and an optical fiber connector were made of materials having the same coefficient of linear expansion. Therefore, a pitch deviation such as above does not occur, thereby decreasing a loss variation caused by heat cycle. Example 5 This example was designed to confirm the result obtained in Example 4 and examine how a difference in coefficient of linear expansion has an influence on a loss variation. In the example, 10 types of optical fiber connectors were prepared by using 10 types of glass ceramic materials each having a coefficient of linear expansion from 1 × 10-6/°C to 10 × 10 -6 °C differing one from another by 1 × 10 -6 /°C. The optical fiber connectors thus manufactured were subjected to a heat cycle test at the same temperature as mentioned above. From this test, it was confirmed that the loss variation was within a range of ±0.2 dB with an optical fiber connector made of glass ceramic having a coefficient of linear expansion of 7 × 10 -6 /°C. Thus, it can be concluded that a loss variation depends mainly upon a difference in coefficient of linear expansion between materials used to manufacture a waveguide module and an optical fiber connector. In the case where the pitch of the V grooves 72 is about 4.6 mm, the loss variation at a high or low temperature can be controlled by setting the difference in coefficient of linear expansion between the waveguide module and the optical fiber connector at 7 × 10 -6 /°C. Example 6 In place of a ceramic-made optical fiber connector discussed in Example 1, an optical fiber connector made by plastic molding, such as shown in Fig. 16, was used in this example. The optical fiber connector used here was of the same type as that generally used as a multi-core fiber connector. As compared to those made by forming grooves thereon in cutting process using a slicer, the production cost can be made low, and further mass production of such a plastic optical fiber connec tor can be easily achieved. With the product of this example, the connection loss was an average of 0.5 B, with a maximum of 1 dB, per one connection. Example 7 In this example, a waveguide module was made of the same material as that of the waveguide module discussed in Example 1. The fitting pins were connected to both waveguide module and optical fiber connector by means of epoxy adhesive. Although the waveguide module and the optical fiber connector of this example could not be detached from each other, the variance of the light output intensity caused by external factors such as vibration and the like could be made extremely small as 0.02 dB or lower. Even in the case where the fitting pin was fixed to either the waveguide module or the optical fiber connector by means of adhesive, the variance of the light output intensity caused by the external factors such as vibration and the like could be made smaller than the case where the fitting pin was not fixed. As described above, according to the method of the present invention, markers are formed without embedding them, and alignment of fiber and waveguide are carried out based on the position of each marker as a reference. More specifically, the distance between a waveguide and a guide groove formed in a waveguide element is set to be exactly the same as the distance between a corresponding optical fiber and a corresponding guide groove or hole, and therefore the waveguide and the optical fiber can be connected at a high accuracy without actually aligning them with each other. Consequently, the time required for connecting optical fibers can be significantly shortened. Further, the marker is not completely embedded, and a portion thereof can be observed; therefore the marker can be easily remarked as a reference for positioning. Thus, an optical axis deviation between a waveguide and an optical fiber can be made small, and they can be optically connected at a low loss. Also, a waveguide part containing an waveguide element and an optical fiber connector can be positioned with each other by inserting fitting pins into each other, and therefore the waveguide part itself can be used a type of detachable connector. As compared with a conventional method in which a waveguide module and an optical fiber connector are fixedly connected with each other semi-permanently by means of adhesive or the like, and an optical fiber projecting out in a pig-tail manner from the waveguide part is connected to another optical fiber of another part by the fusion connection technique, the connection loss can be reduced, and the housing space for a connection portion can be reduced according to the present invention. Moreover, the difference in coefficient of linear expansion between materials used for preparing a waveguide module and an optical fiber connector can be made no higher than a certain value in the invention; therefore the loss variance at a high or low temperature can be suppressed.

Claims: A method of connecting optical fibers and waveguides with each other comprising the steps of: manufacturing a waveguide module (4) by coating a lower clad layer (51, 81) formed on a first substrate (50, 80) with a waveguide material; patterning said waveguide material by a photolithography method so as to form a light waveguide circuit having waveguides arranged in parallel with each other with a predetermined interval between adjacent waveguides to correspond to an arrangement of a plurality of said optical fibers and a marker (53, 83) located a predetermined distance away from a particular waveguide; forming an upper clad layer (55,84) on a region except for that of the marker so as to manufacture waveguides; forming on said first substrate (50, 80) at a predetermined distance from said marker a first fitting pin groove (59) having a substantially V-shaped cross section for receiving a fitting pin; manufacturing an optical fiber connector by forming on a second substrate (70) a plurality of optical fiber grooves (71) each having a substantially V-shaped cross section for fixing said optical fibers; forming on the second substrate (70) a second fitting pin groove (72) or hole used for setting the fitting pin; fixing the optical fibers into the optical fiber grooves; and connecting each of the optical fibers to a corresponding waveguide of the light waveguide circuits by inserting a fitting pin (61) into the first fitting pin groove of said waveguide module (4), and into the second fitting pin groove or hole of said optical fibre connector so as to align the fitting pin grooves with each other. A method of connecting optical fibers and waveguides according to Claim 1, wherein said first fitting pin groove (72) is formed such that the centre of a core of a light waveguide circuit is levelled with the centre of a fitting pin. A method of connecting optical fibers and waveguides according to Claim 1, wherein a press cover (74) is placed on at least one of the first and second substrates. A method of connecting optical fibers and waveguides according to Claim 1, wherein the end surface (70a) of the optical fiber connector is finished mirror-like. A method of connecting optical fibers and waveguides according to Claim 1, wherein a matching agent is applied to a surface of each of the optical fiber and the light waveguide circuit to-be-connected. A method of connecting optical fibers and waveguides according to Claim 1, wherein the coefficient of linear expansion of the material for the waveguide module is substantially the same as that of the optical fiber connector. A method of connecting optical fibers and waveguides according to Claim 1, wherein the optical fiber connector is manufactured by plastics moulding.

Inventors: KOBAYASHI KOICHI, NAKAMURA SHIROU, OCHIAI TOSHIHIRO, OHTA TOSHIHIKO, SHIGEMATSU TAKASHI, SHIMIZU TAKEO, YANAGAWA HISAHARU, KOBAYASHI, KOICHI, NAKAMURA, SHIROU, OCHIAI, TOSHIHIRO, OHTA, TOSHIHIKO, SHIGEMATSU, TAKASHI, SHIMIZU, TAKEO, YANAGAWA, HISAHARU, KOBAYASHI, KOICHI, THE FURUKAWA ELECTRIC CO., LTD., NAKAMURA, SHIROU, THE FURUKAWA ELECTRIC CO., LTD., OCHIAI, TOSHIHIRO, THE FURUKAWA ELECTRIC CO., LTD., OHTA, TOSHIHIKO, THE FURUKAWA ELECTRIC CO., LTD., SHIGEMATSU, TAKASHI, THE FURUKAWA ELECT. CO. LTD., SHIMIZU, TAKEO, THE FURUKAWA ELECTRIC CO., LTD., YANAGAWA, HISAHARU, THE FURUKAWA ELECT. CO., LTD.
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6