CONNECTING OPTICAL FIBERS

Abstract: A method of connecting optical fibers (1) with each other or with optical waveguides uses a connection aid (23), having a hole into which the fiber is inserted and secured and from which hole a tip portion of the fiber protrudes slightly, and an adhesive (3) or solder is applied to a gap between two adjacent con­ nection aids or between an end surface of the waveguide and an opposed surface of the connection aid. The connection aid may contain an array of two or more optical fibers. The connection method can be used to produce reliable, strong and low-loss connections between the optical fibers or optical waveguides and the optical fibers.

Description: The present invention relates to connecting optical fibers. More particularly, the present invention relates to methods of connecting optical fibers with each other or with optical waveguides, and also relates to modules of optical waveguides connected to optical fibers and connection aids comprising an array of two or more optical fibers. The present invention can be applied to an optical field such as optical communication, optical transmission and the like. Optical fibers comprise a core and a cladding surrounding the core, and the core and cladding are both made from quartz, glass and plastic materials. The fibers generally have an outer diameter of about 100 to 300 µm, and a diameter of the core is about 5 to 100 µm. The fibers have many advantages such as low loss, wide bandwidth or high capacity of signal transmission, small diameter, low weight, low cost and flexibility, and because of these advantages, the optical fibers are widely used in various optical devices. The connection or splicing of optical fibers is usually made as illustrated in Figs. 1 and 2 of the accompanying drawings. Before splicing, a vinyl or other coating or covering 2 is removed from a top portion of each optical fiber 1, and a top face of each fiber 1 is then polished and cleaned. The top faces of the fibers 1 are then placed in contact with each other and adhered with an adhesive 3. In this connection method, however, connection defects such as a dislocation of the optical axis, an inclined optical axis, and a separation of the top faces of the fibers tend to occur. In addition, a satisfactory bonding strength between the fibers can not be obtained and, therefore, the connected fibers must be handled with care. Similar drawbacks are also found when connecting optical fibers with optical waveguides. Among the prior art methods, two methods for stably connecting the optical fibers are known. For example, K.H. Cameron teaches in Electron. Lett., Vol. 20, pp 974 - 976, Nov. 1984 that a small block of lithium niobate (LiNbO₃) is attached to an upper surface of an LiNbO₃ waveguide substrate, at the edge of the substrate, so that the end faces of the block and the waveguide formed in the substrate are on the same plane. As apparent from Figs. 3 and 4 of the accompanying drawings, an LiNbO₃ waveguide substrate 4 has a waveguide 5 formed on the upper surface thereof, which waveguide 5 is produced by a thermal diffusion of titanium, and the end face of the LiNbO₃ substrate 4 and that of the LiNbO₃ block 6 are on the same plane. An optical fiber 1 consists of core 7 and cladding 8, and this cladding 8 is removed from the top portion of the fiber 1. The core 7 of the fiber 1 is aligned with an end face of the Ti:LiNbO₃ waveguide 5 and while maintaining an intimate contact between the core 7 and the waveguide 5, a suitable adhesive 3 is applied to the surrounding contact area. This connection method is simple and provides a low excess loss because only the end face of the fiber is in contact with the waveguide, but the resulting connection is mechanically weak. Further, since a large amount of the adhesive must be used for this connection, dislocation of the axis of the fiber and separation of the top face of the fiber from the waveguide frequently occur as a result of an expansion of the adhesive in an elevated environmental temperature. Another method is to secure the optical fiber in a V-grooved silicon block, as taught by E.J. Murphy et al., IEEE J. Lightwave Technol., Vol. LT-3, pp 795 - 799, Aug. 1985. As illustrated in Figs. 5 and 6 of the accompanying drawings, two V-grooved silicon blocks 9 and 10 are combined and an optical fiber 1 from which a coating 2 is removed is secured in a V-groove 11 in the combined blocks 9 and 10. As in the Cameron method, an LiNbO₃ substrate 4 with a Ti:LiNbO₃ waveguide 5 is provided with an LiNbO₃ block 6. Since the substrate 4 and the block 6 have the same plane at the end face, they are firmly bonded with an end face of the blocks 9 and 10 by the action of an adhesive 3 applied therebetween. According this method, a satisfactory bonding strength between the fiber 1 and the waveguide 5 can be attained, because the bonding is made between the large area of the waveguide substrate 4 and block 6 and the large area of the blocks 9 and 10, to secure the fiber 1. However, this method suffers from drawbacks in that, for example, many production steps are needed to complete this method, and thus it is cumbersome and expensive. Further, in order to avoid excess loss, it is essential to precisely align the fiber and waveguide. In practice, often a large excess loss occurs because of an imperfect matching between the end face of the fiber and that of the waveguide. Assuming that the end face of the fiber is disposed at an inclined angle of 1° from the end face of the waveguide, a gap of several ten of micrometers will be produced between the fiber and waveguide, and this gap results in large excess loss. Similar undesirable results will be induced when the end surface of the fiber or waveguide is polished at an angle of 1° from the correct polishing plane. It is therefore desirable to provide an improved low-loss, high-strength connection method for optical fibers and/or optical waveguides. The connection between the optical fibers and optical waveguide can be applied to the production of optical waveguide modules which can be advantageously used in optical communication and optical sensing, for example. Two typical examples of previously-proposed waveguide modules are shown as a plane view and a cross-sectional view in Figs. 7 and 8 of the accompanying drawings. In Figs. 7(A) and 7(B), the optical waveguide module comprises a box or housing 13. Optical fibers with the coating 2 are secured to the wall of the box 13. An exposed fiber is placed in contact with and connected with an end face of the waveguide 5 on the substrate 4. This connection is carried out by inserting the exposed fibers into V-shaped grooves of the fiber holder 12 of silicon, fixing the fibers with an adhesive, and covering an upper surface of the holder 12 with a glass plate or similar parts (not shown). After polishing the end faces, the end faces of the fibers are aligned with the corresponding end faces of the waveguides and are then bonded with an adhesive. Figures 8(A) and 8(B) illustrate a modification of the optical waveguide module of Figs. 7(A) and 7(B). Here, the wall of the box 13 is used to secure the fiber holder 12, instead of securing the coated fibers. These previously-proposed waveguide modules, however, have problems to be solved, which problems arise when the box containing the waveguide substrate and securing optical fibers or fiber holders are subjected to an influence of external force or heat. When the box is deformed or distorted by an external force or expanded by heat, such undesirable changes in the box can directly affect the connection area of the waveguides and fibers. Namely, since a relative position of the end face of the fiber with regard to the end face of the waveguide is slightly shifted, a light coupling efficiency of the fiber to the waveguide is reduced, and in the worst case, defects such as a separation of the end face of the fiber from that of the waveguide and the like occur. In the production of optical devices such as optical waveguide modules, optical fibers are generally not used alone but in combination as an array. This is because two or more optical fibers are necessary to satisfy the requirements concerning an increased amount of information to be transmitted. The fiber holders having an array of two or more optical fibers secured therein have been produced in accordance with the manner previously described with reference to the production of the waveguide modules and the manner described hereinafter with reference to Figs. 9(A) to 9(D) of the accompanying drawings, for example. The production of the fiber holder will be now described with reference to Figs. 9(A) to 9(D): At this time, silicon (Si) substrates are conventionally used as a material of the fiber holder because they can be anisotropically etched to form V-shaped grooves on the surface thereof. Namely, as is well known in the field of semiconductor devices, when the Si single cystals are etched with a heated alkali etchant, anisotropical etching is attained because they show different etching rates depending upon the direction of crystalline faces thereof. For example, when the Si single crystal is etched in the above manner, (100) and (110) faces of the crystal are rapidly etched, but the crystal having (111) face is etched very slowly. Based on these characteristics of the Si single crystals, V-grooved Si substrates can be easily produced. First, as shown in Fig. 9(A), parallel V-grooves 15 are formed on an upper surface of the Si substrate 14. While not illustrated herein, a wafer of Si single crystal having a specific crystal structure is prepared and a mask pattern of silicon oxide (SiO₂) coating is formed on the upper surface of the Si wafer. The mask pattern covers areas other than those in which the V-grooves are etched in the subsequent step. The masked wafer is then dipped in an alkali etchant to form a plurality of parallel fine V-grooves. After removal of the SiO₂ mask, the illustrated Si substrate 14 with parallel V-grooves 15 can be obtained. Second, as shown in Fig. 9(B), optical fibers 1 from which the protective vinyl coating has been removed are placed in each of the V-grooves 15 of the substrate 14 and an epoxy-type adhesive 16 is coated to fill the gaps between the V-grooves 15 and fibers 1 and to cover the fibers 1 on the substrate 14. After application of the adhesive, the fibers-containing substrate 14 is covered with another V-grooved Si substrate 17 as in Fig. 9(C), and the adhesive then allowed to harden. In this stage, as illustrated, the fibers 1 protruded from an end face of the bonded substrates 14 and 17 may have different lengths. Next, the substrates 14 and 17 are laterally cut at a predetermined distance from the end face thereof (see lines b-b' of Fig. 9(C)). This cutting is made in a direction perpendicular to the fibers 1 in the V-grooves 15. The newly exposed end face of the substrates 14 and 17 is then planished to obtain the fiber holder shown in Fig. 9(D). The resulting fiber holder with fiber array has a longitudinal cross section of Fig. 10 of the accompanying drawings. The optical fiber 1 consists of a core 7 and a cladding 8 and is bonded through a coating of the adhesive 16 to the Si substrates 14 and 17. An end face of the fiber holder indicated with an arrow (a) has a plane surface. The fiber holder of Fig. 10 can be attached to a waveguide as shown in Fig. 11 of the accompanying drawings. The waveguide used herein consists of an LiNbO₃ substrate 4 with a Ti:LiNbO₃ waveguide 5 and an LiNbO₃ block 6. An end face of the waveguide also has a plane surface. After alignment of the position of the core 7 of the fiber 1 with that of the waveguide 5, a UV-hardenable epoxy-type adhesive 18 is coated around the bonding interface between the fiber holder and the waveguide. Finally, UV rays are irradiated to harden the adhesive 18 and complete an integrally bonding of the holder and waveguide. However, there are some drawbacks in this and other previously-proposed fiber holders, one of which drawbacks is an increase of the connection loss occurring as a result of a formation of fine gaps between the core of the fiber and the waveguide when a polished end face of the fiber holder and/or that of the waveguide are inclined or they are bonded with an inclination therebetween. For example, if either the holder or the waveguide has an end face inclined at an angle of 1°, a gap of several tens of micrometers will be formed between the core of the fiber and the waveguide. Further, in order to avoid a rapid increase of the connection loss, it is necessary to limit the level of the mismatching between the core of the optical fiber and the waveguide to 0.5 µm and less when both the core and the waveguide have a diameter of about 10 µm. Accordingly, when the fiber holder is attached to the waveguide in accordance with the prior art methods, connection and alignment operations must be carried out using mechanical means such as fine adjustment dials and the like, which make said operations cumbersome and expensive. Therefore, an improved fiber holder containing fibers in array is desired. As described above, in the prior art, connection of the optical fiber to the waveguide is generally carried out by, as illustrated in Fig. 12 of the accompanying drawings, aligning a core 7 of the fiber 1 with a waveguide 5 and securing them with an adhesive 3. For this illustrated instance, assuming that both the waveguide substrate 4 and the block 6 are made from LiNbO₃ , the waveguide 5 from Ti-diffused LiNbO₃ and both the core 7 and the cladding 8 from quartz (SiO₂), since there is a large difference in the refractive index between the Ti:LiNbO₃ waveguide 5 and the SiO₂ core 7, light ℓ₁ transmitted through the core 7 is partially reflected at an end face of the waveguide 5 and returned when it enters the waveguide 5. The returning light is indicated by the dotted line ℓ₂. Such partial reflection of the incident light ℓ₁ must be prevented, for the following reasons. In an optical communication system such as that of Fig. 32 of the accompanying drawings, for example, the system comprises a DFB-type semiconductor laser 19 and a waveguide-type optical modulator 20 in which the waveguide is made from Ti:LiNbO₃. Laser beams from the laser 19 are transmitted through the SiO₂ fiber 21 to the modulator 20. After modulation, the beams are transmitted through the SiO₂ fiber 22 to the next device (not shown). As in the case of Fig. 12, incident laser beams on the modulator 20 are partially reflected at an interface between the end face of the waveguide of the modulator 20 and that of the fiber 21. The reflected beams then return to and adversely affect the laser 19, thereby causing variations in the characteristics of the laser. In the previously-proposed system, in order to inhibit this return of the laser beams to the laser, conventionally an isolator is disposed between the laser 19 and the modulator 20. However, if the returning beams are notably increased, it is necessary to use a high-quality isolator, and this means an increase of the costs of the resulting optical communication system. Alternatively, in order to reduce a degree of return of the incident laser beams, it was proposed to apply an anti-reflection coating onto an end face of the waveguide. However, this coating technique is not satisfactory in the control of the thickness of the resulting coating, in addition to an increase of the steps necessary to produce the devices, and accordingly, an increase of the production costs. In view of these disadvantages, it is desired to provide an improved connection method for optical fibers which can prevent or reduce reflection of the incident beam onto the waveguide without complicating the system. According to a first aspect of the present invention there is provided a method of connecting optical fibers to each other by abutting and adhering corresponding fiber ends, the method being characterised by:- providing each of the optical fibers with a connection aid having a through hole in which the fiber is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between two opposed connection aids. According to a second aspect of the present invention, there is also provided a method of connecting an optical fiber to an optical waveguide by abutting a fiber end to a waveguide substrate, the method being characterised by:- providing the optical fiber end with a connection aid having a through hole in which the fiber is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between the end surface of the waveguide substrate and the opposed end surface of the connection aid. Further, according to a third aspect of the present invention, there is provided a method of connecting a number of optical fibers to a corresponding number of optical waveguides by abutting and adhering the fiber ends to the substrate of said waveguides, the method being characterised by:- providing all the optical fiber ends with a common connection aid having for each of the fibers a through hole in each of which a corresponding fiber end is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between the end surface of the waveguide substrate and the opposed end surface of the connection aid. Furthermore, according to a fourth aspect of the present invention, there is provided a fiber holder which comprises a connection aid having two or more parallel optical fibers arranged in the form of an array and secured in corresponding through holes of the connection aid, the fibers intersecting an end face of said connection aid, said fiber holder being characterised in that a tip portion of each of the fibers protrudes slightly at the same length from the end face of the substrate. Reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a perspective view illustrating connection of optical fibers according to a previously-proposed method; Fig. 2 is a cross-sectional view of the bonded optical fibers of Fig. 1; Fig. 3 is a perspective view showing prior art connection of the optical fiber to the waveguide using an adhesive; Fig. 4 is a cross-sectional view of the connected fiber and waveguide of Fig. 3; Fig. 5 is a perspective view showing prior art connection of the optical fiber to the waveguide using one set of V-grooved silicon substrate; Fig. 6 is a cross-sectional view of the connected fiber and waveguide of Fig. 5; Figs. 7(A) and 7(B) illustrate an example of previously-proposed waveguide modules; Figs. 8(A) and 8(B) illustrate another example of previously-proposed waveguide modules; Figs. 9(A) to 9(D) illustrate in sequence the production of the fiber holder according to a previously-proposed method; Fig. 10 is a longitudinal cross-sectional view of the fiber holder produced according to the method of Figs. 9(A) to 9(D); Fig. 11 is a longitudinal cross-sectional view showing the connection of the fiber holder of Fig. 10 to the optical waveguide; Fig. 12 is an illustration showing the reflection of light beams at an interface between the end surface of the core of the fiber and that of the waveguide; Fig. 13 is a perspective view illustrating connection of optical fibers according to a method embodying one aspect of the present invention; Fig. 14 is a cross-sectional view of the bonded optical fibers of Fig. 13; Fig. 15 is an enlarged cross-sectional view of the optical fiber with the connection aid used in the connection method of Figs. 13 and 14; Fig. 16 is a perspective view of a ruby bead used as the connection aid in an embodiment of the present invention; Fig. 17 is a perspective view showing connection of an optical fiber to a waveguide in accordance with a method embodying another aspect of the present invention; Fig. 18 is a cross-sectional view of the connected fiber and waveguide of Fig. 17; Fig. 19 is a perspective view showing another connection of an optical fiber to a waveguide in accordance with a method embodying another aspect of the present invention; Fig. 20 is a graph showing the relationship between the temperature and loss change under different conditions; Fig. 21 is a graph showing the relationship between the number of cycles and loss change for the connection method embodying the present invention; Fig. 22 is a graph showing the relationship between the tension and loss change for a previously-proposed connection method and a connection method embodying the present invention; Fig. 23 is a graph showing the relationship between the protrusion length of a tip portion of a fiber, inclination angle of the fiber and breakdown temperature; Fig. 24 is a cross-sectional view of a waveguide module embodying yet another aspect of the present invention; Fig. 25 is a perspective view of a fiber holder embodying a further aspect of the present invention; Fig. 26 is a cross-sectional view showing connection of the fiber holder of Fig. 25 to the waveguide; Figs. 27(A) to 27(D) illustrate in sequence the production of the fiber holder of Fig. 25; Figs. 28(A) to 28(E) illustrate in sequence the production of a fiber holder embodying the present invention; Fig. 29 is an illustration showing prevention of the return of the incident light beams according to an embodiment of the present invention; Fig. 30 is another illustration showing prevention of the return of the incident light beams according to an embodiment of the present invention; Fig. 31 is a perspective view of an optical modulator embodying the present invention; Fig. 32 is an illustration of an optical communication system embodying the present invention; Fig. 33 is a perspective view of an optical switch embodying the present invention; and, Fig. 34 is a perspective view of an optical tap embodying the present invention. The present invention will be described hereinafter with reference to preferred embodiments thereof. The connection of optical fibers to each other according to an embodiment of the present invention is illustrated in Fig. 13 (perspective view) and Fig. 14 (cross-sectional view). The optical fiber 1 used herein, as shown in Fig. 15, has a connection aid 23, and the fiber 1 is stripped so that a vinyl coating 2 is removed from a top portion thereof. Thereafter, the top portion of the fiber 1 is inserted in a hole of the connection aid 23. Insertion of the fiber 1 is stopped when the top portion protrudes slightly, for example, 0.5 to 1 mm, from an end face of the aid 23, and then an adhesive 24 such as epoxy or acryl adhesives is applied to firmly secure the aid 23 to the fiber 1. The end face of the fiber 1 is thoroughly polished and cleaned. Each end face of the two connection aid-secured fibers 1 is butted together and a gap between the two adjacent aids 23 is filled with an adhesive 3. The adhesive 3 applied is UV-hardenable and is hardened by irradiation of UV rays. The two optical fibers 1 are thus connected without loss and other defects, and the bonding strength of these fibers is notably high, because this connection is based on two bonding sites, i.e., a bonding between the fiber and connection aid and between the two bonding aids. In this connection method, the connection aid used is a ruby bead shown in Fig. 16. The ruby bead is commercially available as a part for watches. Since the bead 23 has a depressed portion 26 which acts as a guide for the fiber tip, it is very easy to insert the fiber in the hole 25 of the bead 23. The ruby beads for watches generally have a diameter of 1.2 mm, a thickness of 40 µm, and a hole size of 130 µm. Other ruby beads having different sizes can be easily and economically produced from a rod of ruby, since the ruby has a high fabrication quality. The resulting ruby beads have a hole bored at an accuracy of less than + 1 µm with regard to the optical fiber. Although commercially available ruby beads used for the production of watches are used as the connection aid in the practice of the present invention, because of the various merits thereof, it should not be limited thereto. The connection aids used may have any desired configuration and may be produced from any desired material. For example, the connection aids may be rectangular blocks. Further, the connection aids may be produced from lithium niobate, sapphire, glass, ceramics, plastics, metals such as aluminum or copper. Lithium niobate is most preferable since the waveguide substrate is also made from lithium niobate. When the connection aids are made from a metal or have a metal coating deposited on the surface thereof, soldering can be advantageously carried out using suitable solder fluxes instead of the adhesives. The hole size of the connection aids may be widely varied depending upon a specific diameter of the optical fiber less the coating, and a specific adhesive used. The hole size is generally in the range of about 50 µm to about 1.0 mm. The hole size of the connection aids can be adjusted to fit an outer diameter of the fiber as closely as possible, so that the insertion of the fiber into the hole of the connection aid is to ensure that an end face of the aid perpendicular to the axis of the fiber. For example, when the length of the hole is 500 µm and the hole size is 4 µm larger than the diameter of the fiber, an inclination angle of the end face of the connection aid with regard to a plane perpendicular to the fiber axis can be reduced to 0.5° or less. Using the ruby bead described in the above paragraphs, the connection of the optical fiber to the optical waveguide according to an embodiment of the present invention can be carried out as illustrated in Figs. 17 and 18. The optical fiber 1 consists of a core 7 and cladding 8 and a vinyl coating or covering 2. The vinyl coating 2 is removed from the top of portion of the fiber 1 before insertion of the fiber 1 into the hole of the ruby bead 23. The top portion of the fiber 1 is then inserted into the hole of the bead 23 to a predetermined depth and bonded with an adhesive 24. Adhesives for thermal bonding are preferably used. The fiber tip protrudes slightly from the bead face. A small amount of UV-curable adhesive 3 is then applied to the bead face. After the fiber tip is aligned with the end face of the waveguide 5 in such a manner that the connection loss is minimized, the adhesive 3 is cured by irradiation with UV rays. Thus, the illustrated device in which the fiber 1 is directly and strongly bonded to the waveguide 5 with a very low excess loss can be obtained. A satisfactory matching of the end face of the fiber 1 with that of the waveguide 5 can be also attained, because the ruby bead 23 used is small and has a low weight. Accordingly, in the connection method embodying the present invention, the connection loss in the device is almost zero. The top surface of the waveguide substrate 4 has a block 6 made of the same or different material as that of the substrate 4 so that a bonding area of the end face of the waveguide 4 is larger than that of the end face of the ruby bead 23. This ensures a stronger connection between the substrate 4 and the bead 23. If to an increase the connection strength between the substrate and bead is desired, this can be realized by increasing the bonding area of both the substrate and bead and/or increasing the thickness of the bead. In addition to the ruby beads, other types of the connection aids can be used in the present invention. For example, similar results can be obtained if a rectangular ceramic block 27 as shown in Fig. 19 is used as the connection aid instead of the ruby bead. In Fig. 19, the same reference numerals mean the same parts or elements as shown in Fig. 17. From these two examples, it will be appreciated that, when the fiber tip is attached to the end face of the waveguide embodying the present invention, an unexpectedly large bonding strength between the fiber tip and waveguide, an easy, prompt and correct alignment of the fiber top with the waveguide, and a reduced loss of light as well as reduced and stabilized loss change can be concurrently realized. To demonstrate the superior effects of a method embodying the present invention, the following experiments were carried out: Temperature dependency of loss change A Ti-diffused waveguide on a z-cut LiNbO₃ substrate was prepared and used. The waveguide was fabricated by linearly diffusing Ti at a 700Å thickness and a 7 µm width for 8 hours at 1050°C in a wet O₂ atmosphere to produce a single-mode Ti+LiNbO₃ waveguide where λ = 1.3 µm. Both ends of the waveguide were polished after small LiNbO₃ blocks were laid on surfaces of the substrate at both ends of the waveguide. The LiNbO₃ blocks were secured on the waveguide substrate using a commercially available thermosetting epoxy adhesive. The length of the Ti = LiNbO₃ waveguide was 20 mm. Two samples (B) and (C) according to the present invention as shown in Fig. 20 were prepared. A protrusion length (d) of the fiber tip or distance between the end face of the waveguide and that of the connection aid (as described hereinafter, the connection aid was a ruby bead) was 390 µm for the sample (B), and d = 150µm for example (C). The ruby bead used as the connection aid was commercially available as a watch bearing. The ruby bead had an outside diameter of 1 mm, a thickness of 0.45 mm, and a hole size of the 0.13 mm. The optical fibers used were a polarization-maintaining input fiber and a single-mode output fiber. A thermosetting epoxy resin was used to fix the ruby bead to a top portion of each of the fibers. After the ruby bead was fixed, the fibers were carefully aligned with the waveguide so that the TM mode insertion loss was minimal, and then were bonded using a UV-curable adhesive. For sample (B), the insertion loss from the input fiber to the output fiber was determined to be 1.6 dB. The excess loss caused by the shrinkage of the adhesive during UV curing was less than 0.1 dB per connection. Next, the vinyl-coated portion of the fibers was clamped to the wall of the package of the waveguide substrate. No excess loss occurred as a result of this clamping. The loss change of the sample due to an increase of the temperature was then examined. As shown by the chain line (B) in Fig. 20, the maximum loss change in the range of from 23 to 45°C was 0.2 dB, but at 46°C, the loss was abruptly increased. Then, in order to ascertain the bonding strength of the fiber to the waveguide, the fiber was pulled from the waveguide to apply strain to the joining portions. The fiber broke at a load of 500 gwt, but did not separate from the waveguide. For comparison, sample (A) was prepared without using a ruby bead. As shown, this sample was prepared by bonding a tip portion of the fiber to the waveguide by using a mass of the adhesive. The fiber top (2 mm) was filled with the adhesive. As shown by the dotted line (A) in Fig. 20, the maximum loss change from 23°C to 40°C was 0.19 dB, and the loss was abruptly increased at 41°C. Further, in the bonding strength test, the fiber separated from the waveguide at a load of 100 gwt. Based on the above results, an investigation was made into the reason why the loss abruptly increased, and as a result, it was found that the amount of the adhesive between the end faces of the ruby bead and waveguide substrate was much larger than that between the end faces of the fiber and waveguide and, therefore, there were remarkable differences in thermal expansion between the above two portions, i.e., bead-waveguide substrate and fiber-waveguide, such that strain was generated between the end faces of the fiber and waveguide when the temperature rises, and that when the strain exceeded the strength of the UV-curable adhesive used, the fiber separated from the waveguide. It was also found that the breakdown temperature was increased when the amount of the UV-curable adhesive applied was reduced by shortening the length of the fiber tip protruding from the ruby bead. In sample (C), in which the length of the fiber protruding from the bead is 150 µm, the loss change due to the temperature, which is shown by the solid line (C) in Fig. 20, was obtained. The maximum loss change from 0°C to 67°C was 0.2 dB, with a breakdown at 68°C. Further, while not plotted in Fig. 20, when the gap or length (d) of the fiber tip protruding from the bead was reduced to 50 µm, the maximum loss change similar to that of the sample (C) was obtained, with a breakdown at 110°C. Now apparently, the breakdown temperature should increase if the gap is made smaller. Temperature cycle test Figure 21 shows the results of the temperature cycle test in which heating was periodically repeated in the pattern plotted in the right upper portion of the graph. The sample tested was the same as that used in the previous test (gap d = 50µm), and is herein referred to as sample (D). The graph indicates that for the present invention, the loss change was diminished and stably maintained. Tension test The above samples (A) and (D) were subjected to tension tests, and the results are plotted in Fig. 22. The graph of Fig. 22 indicates that, while the sample (D) embodying the present invention did not suffer a loss change, the prior art sample (A) suffered a remarkable change in loss when the tension was increased to 200 gwt and more. Such a variation in the loss change means that the prior art sample can not be practically used in the devices, since it is not reliable. Preferred range of the protruded fiber tip In order to ascertain a preferred range of the protrusion of the fiber tip from the ruby bead, different samples embodying the present invention, which were prepared in a manner similar to the preparation of the above samples (B), (C) and (D), were tested and a graph concerning the relationship between the length of the fiber tip protruded from the ruby bead, inclination angle and breakdown temperature (see Fig. 23) obtained. The graph of Fig. 23 indicates that since the breakdown temperature was 60°C or more, the length of the fiber tip protruded was preferably 200 µm or less, and that the inclination angle was 5° or more in view of the alignment, and accordingly, the length of the protruded fiber tip was preferably at least about 50 µm. Namely, a preferred range of the length of the fiber tip protruded from the end face of the ruby bead and other connection aids was from about 50 to 200 µm. The excellent effects of a method embodying the present invention will be appreciated from the above descriptions concerning the connection of optical fibers with each other or the connection of an optical fiber with a waveguide. These effects can be also attained in a module of the waveguide embodying the present invention. In addition, in the present waveguide modules, additional effects such as prevention of an adverse affect of external force and heat onto the box or casing of the module and accordingly, on the connection between the waveguide and fiber, can be also attained. Figure 24 is a cross-sectional view of a waveguide module embodying the present invention. In the illustrated waveguide module, a box or casing 13 of stainless steel has a waveguide substrate (LiNbO₃) 4 placed in a center portion thereof, and coated fibers 2 secured on the wall thereof. Each optical fiber 1 is connected through the connection aid (ruby bead in this example) 23 according to the present invention to the waveguide (not shown: Ti = LiNbO₃) on the LiNbO₃ substrate 4. As apparent from Fig. 24, the stripped fiber 1 is deformably curved in the box 13 of the module. This curvature is preferably controlled to the extent at which deformation or expansion of the box does not directly affect the connection area of the waveguide and fiber. Excess curvature must be avoided, because it tends to cause the fiber to break. Further, stripping of the vinyl coating from the fiber is desirably carried out, for example, by burning the coating, after the fiber with the coating is curved to a desired configuration. This effectively avoids breaking of the fiber which frequently occurs when the fiber is curved after the coating is stripped off. In the waveguide module of Fig. 24, the optical fiber was connected to the waveguide was made by using the ruby beads as the connection aid. However, if appropriate, a conventional U-groove method and V-grooved Si substrate may be used as the connection aid instead of the ruby beads. From Fig.24 and the description thereof, it will be understood that according to an embodiment of the present invention, the problem of a reduction of the efficiency of the light connection due to a movement of the relative position between the fiber and waveguide can be completely solved, because even if an external force or heat is applied to the module box, such force or heat is not transmitted to the connection portion of the waveguide and fiber. Additionally, many advantages can be attained if the ruby beads or other means within the scope of the present invention are used as the connection aids, as already described. Figure 25 is a perspective view of a connection aid embodying the present invention which contains an array of optical fibers and is referred herein to as a fiber holder. The fiber holder 25, as illustrated in Fig.25, contains an array of four fibers 1 secured in the body thereof. Each of these fibers has the same length protrusion from the end face of the holder. These protrusions are formed through etching of the holder. As shown in the cross-sectional view of Fig. 26, the fiber 1 consists of a core 7 and a cladding 8, and the core 7 is connected with the waveguide 5 on the substrate 4 upon alignment and bonding. The end faces of both of the fiber and waveguide are planished for a satisfactory connection. If necessary, these end faces can be checked with the naked eye, using microscopes or other devices, prior to the alignment and bonding operation. Bonding can be carried out by using UV curable adhesives as previously described. Using a fiber holder embodying the present invention, when the polished end faces of the fibers are connected to the waveguides, since the contact area of each corresponding fiber and waveguide is largely reduced that, even if the polished end face of the holder is inclined, it does not come into contact with the waveguide, and no gap is formed between the waveguide and core of the fiber and, even if gap is formed or there is a dislocation in position between the waveguide and core of the fiber, such defects can be easily controlled and corrected through a necked eye observation of the connection area in the lateral direction to the axis of the fiber. Further, since the fiber tips are protruded from the fiber holder through selective etching of the end face of the holder, the protrusion length of the fiber tip can be controlled with a good reproducibility through control of the etching time and the like. This control of the protrusion length of the fiber is particularly effective for stabilizing the temperature characteristics of the resulting connection. Accordingly, in the connection of the fiber holder to the waveguide, connections having no transmission loss and having stable characteristics can be produced by less steps in the production thereof. The fiber holder of Fig. 25 can be produced, for example, in the manner described in the sequence in Figs. 27(A) to 27(D) which are cross-sectional views along with line XXVII - XXVII of Fig. 25. First, as shown in Fig. 27(A), the holder material 28 having holes 29 corresponding an array of optical fibers to be inserted is prepared. For example, in the production of the holder made from metal, a melt of the metal is poured into a mold containing rods corresponding to the fibers. After hardening of the melt, the rods are removed from the holder material 28. The removal of the rods can be easily carried out if the rods are resist materials. Fibers 1 are then inserted into each of the holes of the holder material (see Fig. 27(B)). The inserted fibers 1 are then arranged so that their end faces and a end face of the holder material 28 are on the same plane (see Fig. 27(C)). This can be attained, for example, by pressing the end face of the holder material 28 against a flat plate (not shown). Finally, as in Fig. 27(D), the end face of the holder material 28 is etched to a predetermined depth so that the top portion of each fiber 1 protrudes slightly from the end face of the holder 28. In this step, the holder material can be etched by conventional etching techniques. Similarly, another type of fiber holder can be produced in the manner illustrated in the sequence in Figs. 28(A) to 28(E). In these drawings, Figs. 28(A) to 28(D) correspond to Figs. 9(A) to 9(D), respectively, and, therefore, are not described herein to avoid duplication. After the completion of step (D), the resulting fiber holder is dipped in an etchant solution and the polished surface thereof is etched under predetermined conditions. Since the holder is made from the V-grooved Si substrates, etchants capable of selectively etching off the Si crystals only, for example, pyrocatechols and the like, can be used in this etching step. The etching depth is about ten to several hundreds of micrometers. The resulting fiber holder embodying the present invention is shown in Fig. 28(E). As a result of the addition of a selective etching step to the prior art production process for the fiber holders, improved fiber holders can be obtained. As described above, the depth of selective etching for the Si substrates is preferably about ten to several hundreds of micrometers. This depth is defined to avoid separation of the fiber tip from the end face of the waveguide which is frequently occurs at a high temperature as a result of large differences in the thermal expansion coefficient between the optical fiber and the UV-curable epoxy adhesive used for bonding the waveguide substrate and fiber holder. When pyrocatechols are used as the etchant in the selective etching step, they can not remove SiO₂ coatings on the Si substrates (SiO₂ coatings act as a stabilizing medium for the substrates) and the epoxy adhesives used. However, since the SiO₂ coatings are very thin, they will be separated and removed during the etching step. Even if the epoxy adhesives remain, they will not cause additional problems, because the amount thereof is very small and they and the UV-curable adhesives also used are classified under the same epoxy resin group. Fiber holders embodying the present invention, when used in the connection of optical fibers to a waveguide in optical communication devices, can ease and ensure such a connection without lowering the transmission characteristics of the devices. Of course, in the practice of the present invention, any materials can be used as the holder material in addition to the Si substrates, insofar as they can be fabricated and selectively etched as in the Si substrates. According to an embodiment of the present invention, the top portion of an optical fiber can be fabricated with a rounded surface, a tapered surface, and a cleaved cladding end. These top surfaces of the fiber effectively avoid undesirable contact of the top portion of the fiber with the end face of the waveguide substrate. Therefore, correct matching of the end face of the fiber and that of the waveguide can be attained without an increase of the insertion loss of light. According to an embodiment of the present invention, an end surface of the waveguide and waveguide substrate can be inclined at a predetermined angle from the direction perpendicular to the longitudinal axis of the waveguide. This inclined end face of the waveguide effectively prevents a return of the reflected light in the same direction as that of the incident light. Such reflected and returned light in the input optical fibers is therefore reduced or eliminated. These embodiments will be further described with reference to Figs. 29 and 30. Figure 29 illustrates a tapered top surface of the optical fiber. The LiNbO₃ waveguide substrate 4 has a Ti-diffused LiNbO₃ waveguide 5. An end face of the substrate 4 is inclined at an angle O₁ (for example 2° or more) with regard to the direction perpendicular to the direction of the waveguide 5. Further, a top portion of optical fiber 1 such as quartz fiber is tapered to form a rounded surface. Such tapering can be easily realized by conventional etching technologies. Furthermore, since the refractive index n₁ of the waveguide 5 is larger than the refractive index n₂ of the core 7 of the fiber 1 (for example, n₁ = 2.2, n₂ = 1.45), and a refraction of the light occurs at an end face of the waveguide 5, the fiber 1 is aligned with the inclination at an angle O₂ (<O₁) with regard to the normal of the end face of the waveguide 5. While maintaining this contact of the fiber and waveguide, the top portion of the fiber 1 and the end face of the waveguide substrate 4 are then bonded using an adhesive 3 having a refractive index similar to that of the fiber 1, for example, UV-curable epoxy adhesives. The adhesive 3 used will also bond the end face of the waveguide substrate 4 and the connection aid (not shown), and thus an integrally bonded fiber and waveguide structure is obtained. In the structure of Fig. 29, light ℓ₁₁ transmitted through the core 7 of the fiber 1 is guided into an end face of the waveguide 5 at an incident angle ϑ₂. After refraction at a refraction angle ϑ₁, it is then transmitted through the waveguide 5. When the light ℓ₁₁ is introduced into the end face of the waveguide 5, the reflected light ℓ₁₂ is produced. The light ℓ₁₂, as illustrated in Fig. 29, is reflected at an angle 2 x ϑ₂ with regard to the direction of the incident light ℓ₁₁. Namely, the reflected light ℓ₁₂ is reflected out of the fiber 1, or if reflected within the area of the core 7 of the fiber 1, is immediately decayed due to a large incident angle of 2ϑ₁. Accordingly, the return of the reflected light ℓ₁₂ to the core 7 of the fiber 1 can be prevented. Similar results can be also obtained when the light is transmitted from the waveguide 5 to the core 7 of the fiber 1, because the normal of the end face of the waveguide substrate 4 is inclined at an angle ϑ₁ with regard to the waveguide 5. According to an embodiment of the present invention, since the return of the reflected lights is remarkably reduced, it is not necessary to use the high performance isolators essential to prior art optical communication systems. Further, as a result of the inclination of the end face of the waveguide substrate, it becomes unnecessary to apply an anti-reflection coating to the end face of the waveguide substrate, and thus the cost of the devices are reduced. Furthermore, during the alignment of the fiber with the waveguide, the top portion of the fiber does not come into contact with the end face of the waveguide substrate, and therefore, a correct alignment can be attained without an increase of insertion loss of the light. An inclination angle of the end face of the waveguide substrate is desirably 2° or more, although a lesser angle also effectively reduces the return of the reflected light. The inclination angle ϑ₂ of the optical fiber is desirably controlled so that the transmitted light is connected with a low loss, depending upon the inclination angle ϑ₁ and the refractive indexes n₁ and n₂. Figure 30 illustrates the optical fiber 1 in which a top end portion of the cladding 8 is cleaved (see reference number 30). The cleaved face of the cladding 8 is polished. Results comparable to those of the structure of Fig. 29 can be obtained. A connection method embodying the present invention can be applied to the production of various optical devices such as intensity modulators, phase modulators, polarizing wave controllers, optical switches, optical wave separators, optical taps and optical sensors. An example of an intensity modulator is shown in Fig. 31. In this modulator, the waveguide substrate 4 has a waveguide 5, which is connected through the ruby bead 23 bonded to the fiber 1 with an adhesive 5. This modulator can be used, for example, in the optical communication system of Fig. 32. The modulator 20 is connected through the fiber 21 to the laser 19. The mechanism of this system has been previously described. An example of an optical switch is illustrated in Fig. 33. In this device, the fiber holders 28 are used to connect the fibers 1 with the waveguides 5 on the substrate. Finally, an example of an optical tap is illustrated in Fig. 34. The ruby bead 23 and the fiber holder are used to connect the fibers 1 to the waveguides 5.

Claims: A method of connecting optical fibers to each other by abutting and adhering corresponding fiber ends, the method being characterised by:- providing each of the optical fibers with a connection aid having a through hole in which the fiber is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between two opposed connection aids. A method according to claim 1, in which the connection aid is light enough to be supported by an optical fiber not having a coating. A method according to claim 2, in which the connection aid is a ruby bead. A method according to claim 2, in which the connection aid is made from metal and in which the adhering step comprises applying a solder to a gap between two adjacent connection aids instead of an adhesive. A method of connecting an optical fiber to an optical waveguide by abutting a fiber end to a waveguide substrate, the method being characterised by:- providing the optical fiber end with a connection aid having a through hole in which the fiber is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between the end surface of the waveguide substrate and the opposed end surface of the connection aid. A method according to claim 5, comprising providing a top surface of the waveguide substrate with a block of the same or different material so that the connection area of the end surface of the waveguide substrate is larger than the connection area of the connection aid. A method according to claim 5, in which the inclination angle of the optical fiber with regard to the central axis of the hole of the connection aid is 2° or less when the fiber is secured in the hole. A method according to claim 5, in which the connection aid is light enough to be supported by an optical fiber not having a coating. A method according to claim 8, in which the connection aid is a ruby bead. A method according to claim 8, in which the connection aid is made from metal and in which the adhering step comprises providing the end surface of the waveguide substrate with a metal coating, and applying a solder to the gap between the end surface of the waveguide substrate and the connection aid instead of the adhesive. A method according to claim 5, in which the tip portion of the fiber has a tapered surface. A method according to claim 5, in which the tip portion of the fiber has a rounded surface. A method according to claim 5, in which the tip portion of the fiber has a cleaved cladding end. A method according to any one of claims 11 to 13, in which an end surface of the waveguide and waveguide substrate is inclined at a predetermined angle from the direction perpendicular to the longitudinal axis of the waveguide. A method according to claim 14, in which said angle is 2° or more. A method according to claim 5, the method comprising disposing the waveguide in a box having at least one wall, securing the coated optical fiber in a wall of the box of the waveguide, stripping off the coating from an end portion of the fiber, and adhering the fiber to the waveguide substrate in such a manner that a predetermined length of the optical fiber extending between the tip portion and the portion secured in the wall is deformably curved. A method according to claim 16, in which the stripped and curved optical fiber portion has a length between several millimeters to several centimeters. A method of connecting a number of optical fibers to a corresponding number of optical waveguides by abutting and adhering the fiber ends to the substrate of said waveguides, the method being characterised by:- providing all the optical fiber ends with a common connection aid having for each of the fibers a through hole in each of which a corresponding fiber end is inserted and secured in such a manner that a tip portion of the fiber slightly protrudes from the hole, and applying an adhesive to the gap between the end surface of the waveguide substrate and the opposed end surface of the connection aid. A method according to claim 18, the method comprising securing two or more optical fibers in the corresponding through holes of the common connection aid in such a manner that an end surface of said connection aid and the end surfaces of each of the fibers are in a common plane, and selectively etching the end surface of said connection aid to a predetermined depth to expose the optical fibers so that tip portions of the fibers protrude from the end surface of said connection aid. A fiber holder which comprises a connection aid having two or more parallel optical fibers arranged in the form of an array and secured in corresponding through holes of the connection aid, the fibers intersecting an end face of said connection aid, said fiber holder being characterised in that a tip portion of each of the fibers protrudes slightly at the same length from the end face of the connection aid.

Inventors: MEKADA, NAOYUKI, SEINO, MINORU
Applicants: FUJITSU LIMITED
Application year: 1988
Main classification: G02B6
IPC codes: G02B6