Method for hermetically sealing optical fiber feedthrough and hermetically sealed structure

Abstract: A conventional optical fiber (7) not coated with a metal layer is provided. A solder layer (9,11) is filled into between a case (5), for an optical module wherein an optical element (4) is optically coupled with the optical fiber within the case, and a cover (8) conforming to the shape of the case, thereby simply conducting hermetical sealing. Thereafter, a resin (12), such as a silicone resin, is potted in the optical fiber introducing section (6). The two-layer hermetical sealing can realize a method for hermetically sealing an optical fiber introducing section which can maintain airtightness high enough to ensure long-term reliability of an optical module. This can facilitate the hermetical sealing of an optical device particularly in its optical fiber introducing section.

Description: The present invention relates to a method for hermetically sealing an optical device, and particularly to a hermetically sealing method and a hermetically sealed structure in an optical fiber introducing section of an optical module having such a structure that an optical fiber strand is used in an introducing terminal. An optical module is a structure for optically coupling an optical element, such as a semiconductor laser element or a photodetector, with an optical fiber. The optical element should be protected against moisture from the viewpoints of stabilization of properties and high reliability. In general, these optical elements are housed in a case, and the case is hermetically sealed to protect the optical element against moisture outside the case. Methods for hermetically sealing the case are roughly classified according to the structure of the optical module into two methods, that is, a method wherein a window permeable to light is provided in the case and an optical element provided within the case is optically coupled with an optical fiber outside the case through the window and a method wherein an optical element and an optical fiber are coupled with each other within the case. In recent years, the latter method is mainly adopted from the viewpoint of reducing the size of the optical device. In a structure provided by the latter method, that is, in a structure wherein an optical element and an optical fiber are coupled with each other within the case, hermetical sealing should be performed between the optical fiber and the case. A conventional hermetically sealed structure in an optical fiber introducing section is proposed, for example, in JP-A-92334/1995. The hermetically sealed structure in an optical fiber introducing section described in this publication comprises: a case; a silicon substrate, with an optical element mounted thereon, fixed within the case; and an optical fiber disposed at a position where the optical fiber is optically coupled with the optical element, and fixed to the silicon substrate. The optical fiber used in this case is a metal-coated fiber formed by coating a metal layer on the surface of an optical fiber strand, with a resin coating removed therefrom, by plating, sputtering or other method. The coating metal is a material having good solderability, such as nickel or gold. The metal-coated fiber is inserted into between the case and the cover, followed by soldering. As a result, the solder joins the case to the cover and, at the same time, joins the metal layer on the surface of the metal-coated fiber, thus hermetically sealing the interior of the case. Another conventional hermetically sealed structure in an optical fiber introducing section is described, for example, in JP-A-198973/1995, wherein a low-melting glass is used. In this hermetically sealed structure, a silica optical fiber is provided without coating of a metal layer, and an optical element and the optical fiber are optically coupled with each other, followed by fixation of the optical fiber onto a silicon substrate. The optical fiber is then inserted into between a case and a cover, a low-melting glass is provided in a portion, between the case and the cover, in its whole circumference, and the low-melting glass is then heat-melted to hermetically seal the interior of the case. Still another conventional hermetically sealed structure in an optical fiber introducing section is such that the circumference of an optical fiber is sealed with a resin having a hermetically sealing property. In this hermetically sealed structure in an optical fiber introducing section, an optical fiber is provided without coating of a metal layer, an optical element and the optical fiber are optically coupled with each other, followed by fixation of the optical fiber onto a silicon substrate. The optical fiber is then inserted into between a case and a cover, the resin is filled into a portion between the case and the cover, in its whole circumference, and the resin is then cured to hermetically seal the interior of the case. The conventional hermetically sealed structures in an optical fiber introducing section have the following respective problems. (1) The hermetically sealed structure in an optical fiber introducing section using a metal-coated fiber suffers from a problem that the cost of coating the metal layer onto the optical fiber is high. An additional problem involved in this structure is that selective coating of the metal layer onto the optical fiber introducing section alone lowers the mass productivity.Further, when the optical fiber is disposed on a V-groove in order to conduct positioning of the optical element and the optical fiber relative to each other, a variation in the thickness of the metal layer results in varied outer diameter of the optical fiber, disadvantageously deteriorating the positioning accuracy. (2) For the hermetically sealed structure in an optical fiber introducing section using a low-melting glass, since the melting point of the low-melting glass is generally high and 430°C or above, at the time of sealing, the low-melting glass is likely to cause damage to the optical element fixed portion within the case and the optical fiber provided with a protective coating. (3) The hermetically sealed structure in an optical fiber introducing section using a resin has problems including that the airtightness is greatly influenced by resin coating methods, seal path length, curing conditions and the like, making it impossible to stably conduct the sealing work, the hermetically sealing level of the resin per se is about 10 -5 atm.cc/sec and unsatisfactory for use in optical modules, the reliability under severe environmental conditions such as high temperature and high humidity is low, and the sealed structure is not resistant to heat. EP-A-0645 651 discloses an optical fiber array with a lower plate and an upper plate made of silicon wafers, and a plurality of optical fibers are arranged between the lower plate and the upper plate. The optical fibers extend from a ribbon part (coated region) of a mulit-optical fiber ribbon. The lower plate has a plurality of V-shaped grooves for supporting the optical fibers in position. A metal sleeve comprising a window section as an air vent is put around the lower plate and the upper plate which sandwich the plurality of optical fibers. The multi-optical fiber ribbon is fixed to the lower and upper plates to plug a heat resistant adhesive as far as the vicinity of the boundary portion between the coated ribbon part and the optical fibers. The remaining parts of the optical fibers from the boundary portion to the end are fixed to the lower and upper plates by solder. If the solder comes in contact with the ribbon part, which is a coated part, the coating of the ribbon part melts and a large amount of gas generates. Therefore, the vicinity of the boundary portion between the ribbon part and the optical fibers is protected by the adhesive, and thereafter the glass part of the optical fiber is fixed with the solder. Since the optical fiber receives the heat at high temperature when fixed with a solder, cracks rapidly develop on a surface of the optical fiber and the optical fiber is easily broken. Therefore, it is preferable that metal coating or carbon coating is deposited on the optical fibers for preventing the crack and improving the adhesion of the solder on the optical fibers. The solder flows into a slight gap between the lower and upper plates so that the perfect hermetic sealing can be realised: Further, the periphery of the sleeve is fixed to the LD module with the solder in an airtightness state. In particular, both of the openings around the optical fibers and the window part serving as an air vent are not completely filled with solder and, therefore, hermetically sealing depends on the sealing property of the resin only. Accordingly, an object of the present invention is to provide a hermetically sealing method and a hermetically sealed structure in an optical fiber introducing section which can eliminate the need to use an expensive metal-coated fiber, can use a solder material having a melting point below that of a low-melting glass, and enables airtightness high enough to ensure long-term reliability of an optical module to be maintained even under severe environmental conditions. This object is achieved according to the invention by the method and structure of the appended claims 1 and 2. The invention will be explained in more detail in conjunction with appended drawings, wherein: Fig. 1 is a cross-sectional view showing a conventional hermetically sealed structure in an optical fiber introducing section; Fig. 2 is a cross-sectional view showing another conventional hermetically sealed structure in an optical fiber introducing section; Fig. 3 is a cross-sectional view showing still another conventional hermetically sealed structure in an optical fiber introducing section; Fig. 4 is a perspective view showing a first preferred embodiment of the present invention; Fig. 5 is an explanatory cross-sectional view showing a hermetically sealed structure according to a first preferred embodiment of the present invention; Fig. 6 is a perspective view showing a second preferred embodiment of the present invention; and Fig. 7 is a perspective view showing a third preferred embodiment of the present invention. Before describing the hermetically sealed structure in an optical fiber introducing section in a preferred embodiment according to the present invention, the aforementioned conventional hermetically sealed structures in an optical fiber introducing section will be explained in conjunction with Figs. 1 to 3. A first conventional method for hermetically sealing an optical fiber introducing section is proposed, for example, in JP-A-92334/1995. This method is shown in Fig. 1. Specifically, Fig. 1 is a longitudinal sectional view illustrating the method for hermetically sealing an optical fiber introducing section described in JP-A-92334/1995. The first conventional method will be explained with reference to Fig. 1. In Fig. 1, a silicon substrate 1 with an optical element 19 mounted thereon is fixed within a case 5. An optical fiber 15 is then disposed, at a position wherein the optical fiber 15 is to be optically coupled with the optical element 19, and fixed to the silicon substrate 1. The optical fiber used in this case is a metal-coated fiber 15 formed by coating a metal layer 16 on the surface of an optical fibers strand, with a resin coating removed therefrom, by plating, sputtering or other method. The coating metal is a material halving good solderability, such as nickel or gold. The metal-coated fiber 15 is inserted into between the case 5 and the cover 8, followed by soldering. As a result, the solder 9 joins the case 5 to the cover 8 and, at the same time, joins the metal layer 16 on the surface of the metal-coated fiber 15, thus hermetically sealing the interior of the case. A second conventional technique is described, for example, in JP-A-198973/1995. This publication discloses a hermetically sealed structure in an optical fiber introducing section, wherein a low-melting glass is used. Fig. 2 is a longitudinal sectional view showing the hermetically sealed structure in an optical fiber introducing section according to the second conventional technique. The second conventional technique will be explained with reference to Fig. 2. In Fig. 2, a silica optical fiber is provided without coating of a metal layer, and, as with the first conventional technique, an optical element 19 and the optical fiber 7 are optically coupled with each other, followed by fixation of the optical fiber 7 onto a silicon substrate 1. The optical fiber 7 is then inserted into between a case 5 and a cover 8, a low-melting glass 17 is provided in a portion, between the case 5 and the cover 8, in its whole circumference, and the low-melting glass is then heat-melted to hermetically seal the interior of the case. A third prior art technique is a hermetically sealed structure in an optical fiber introducing section. This structure is such that the circumference of an optical fiber is sealed with a resin having a hermetically sealing property. Fig. 3 shows a longitudinal sectional view of the hermetically sealed structure in an optical fiber introducing section according to the third conventional technique. The third conventional technique will be explained with reference to Fig. 3. In Fig. 3, an optical fiber is provided without coating of a metal layer, and, as with the first conventional technique, an optical element 19 and the optical fiber 7 are optically coupled with each other, followed by fixation of the optical fiber 7 onto a silicon substrate 1. The optical fiber 7 is then inserted into between a case 5 and a cover 8, a resin 12 is filled into a portion between the case 5 and the cover 8, in its whole circumference, and the resin 12 is then cured to hermetically seal the interior of the case. Next, preferred embodiments of the present invention will be described. The method for hermetically sealing an optical fiber introducing section according- to the present invention, in its preferred embodiment, is characterized in that a conventional silica optical fiber without coating of a metal layer is provided, an optical element and the optical fiber are optically coupled with each other within a case, a solder is filled into between the case of an optical module and a cover conforming to the shape of the case, thereby simply conducting hermetical sealing, and a resin, such as an adhesive, is filled into the optical fiber introducing section and then cured. According to the preferred embodiment of the present invention, the case and the cover are hermetically sealed with a solder, and, in the optical fiber introducing section, the circumference of the optical fiber is substantially filled with the solder. Since the optical fiber per se is made of silica glass, it is not subjected to alloyed junction by the solder, resulting in the creation of minute gaps at boundaries between the solder, which has been solidified after melting, and the optical fiber. Filling of the resin into between the optical fiber and the solder followed by curing of the resin can realize a high degree of airtightness of the optical device. Thus, according to the preferred embodiment of the present invention, use of a conventional optical fiber without coating of a metal layer in the hermetical sealing realizes hermetical sealing of an optical fiber introducing section at a low cost. Further, the variation in outer diameter of the optical fiber is so small that provision of a V-groove within the case enables the positioning of the optical fiber to be performed with a high accuracy. In addition, the attainable airtightness is high enough to ensure long-term reliability of an optical module, preferably not more than 10 -7 atm.cc/sec. Fig. 4 is a perspective view showing assembly of a first preferred embodiment of the hermetically sealing method according to the present invention. In Fig. 4, a silicon substrate 1 is one prepared by forming a necessary electrode pattern on both sides of a silicon wafer by photolithography and conducting chipping by dicing. In the silicon substrate 1, a V-groove 2 is formed simultaneously with the formation of an electrode pattern. The groove 2 is formed by anisotropic etching. Since the shape of the groove 2 can be made constant, the positioning of the optical fiber can be performed with a submicron accuracy. A semiconductor laser 4 is fixed, with a high-temperature solder, such as AuSn, to the electrode 3 formed by patterning on the top surface of the silicon substrate 1. The silicon substrate 1 is fixed, with high-temperature solder, such as AuSn, to the cavity section of the case 5. At that time, positioning and fixing are carried out so that the center of the V-groove 2 is located at a given position relative to a fiber introducing section 6 provided in the case 5. The case 5 is preferably a metallic case made of Kovar or the like, a ceramic package or the like from the viewpoints of airtightness, solderability, and cost. The optical fiber 7 is highly accurately positioned relative to and optically coupled with the semiconductor laser 4 through the V-groove 2, and fixed to the V-groove 2 with a high-temperature solder, such as AuSn, or a heat-resistant adhesive, such as an epoxy adhesive. A thin metal layer of nickel or gold is provided by plating on the cover 8, a hermetically sealing face 10, and the fiber introducing section 6 in consideration of solderability. A solder material having a melting point, which has no thermal influence on the assembly section within the case, is used for the solder 9. Solder materials usable herein include Sn-Pb solder, Sn-Sb solder, Sn-In solder, and Au-Sn solder. The solder material is selected by taking into consideration the thermal influence on the interior of the case, temperature conditions under which the structure after hermetical sealing can withstand and the like. In particular, when an Au-Sn solder is used for joining components within the case with an SnPb eutectic solder being used for the fixation of the case to a print board, an Sn-Sb solder or an Sn-In solder is selected as the solder for hermetical sealing so that the melting point of the solder for hermetical sealing is Au-Sn > solder for hermetical sealing > Sn-Pb. The cover 8 and the solder 9 are positioned so as to conform to the shape of the case 5 in its hermetically sealing face 10. The solder 9 is previously formed in the form of a square washer or the like so that it can be easily fed in the case 5. Thereafter, the assembly is heated to the melting point of the solder 9, thereby fixing the cover 8, the case 5, and the optical fiber 7 with the aid of the solder 9. At that time, the circumference of the optical fiber 7 is filled with a solder layer 11 (see Fig. 5). Since, however, the solder layer 11 is not subjected to alloyed junction with the optical fiber 7 by the solder, reliable airtightness cannot be ensured in this stage. A resin 12 is then filled on the surface of the optical fiber 7 and the solder layer 11 (see Fig. 5) and then cured, thereby conducting hermetical sealing. Fig. 5 is a longitudinal sectional view of a hermetically sealed structure completed through the above steps according to this preferred embodiment. In this case, a resin having a viscosity of not more than 70 Pa.s is used as the resin 12 so that the resin can be easily filled into minute gaps. In particular, when a cold curing silicone rubber is applied as the resin 12, good airtightness can be ensured over a wide temperature range because the silicone rubber has a cold/heat resistance of -40°C to 200°C. When good airtightness at a usual environmental temperature is contemplated, an epoxy resin or a UV (ultraviolet) resin may also be used as the resin 12. Filling of the resin followed by curing of the resin permits minute gaps at boundaries between the solder layer 11 and the optical fiber 7 to be filled with the resin, which can provide airtightness high enough to ensure the reliability of the optical device, that is, an airtightness of not more than 10 -7 atm.cc/sec. In general, in the case of hermetical sealing with a resin alone, it is difficult to provide satisfactory airtightness due to low hermetical sealing level of the resin per se, uneven coating, unsatisfactory sealing area and the like. However, use of the resin for filling into minute gaps between the optical fiber and the solder layer according the present preferred embodiment of the present invention can greatly improve the airtightness of the whole case. The second preferred embodiment of the present invention will be explained in detail with reference to Fig. 6. Fig. 6 is a perspective view showing assembling of a hermetically sealed structure according to the second preferred embodiment of the present invention. In Fig. 6, in the present preferred embodiment, as with the first preferred embodiment, after a silicon substrate 1 with a semiconductor laser 4 mounted thereon is fixed within the case 5, an optical fiber 7 is mounted. A solder plating 13 is previously provided on a cover 8 and a case 5. After the cover 8 and the case 5 are disposed at respective predetermined positions, the temperature is raised to the remelting temperature of the solder plating 13, thereby fixing the cover 8, the case 5, and the fiber 7 with the solder. Thereafter, hermetical sealing with a resin 12 is carried out. Next, the third preferred embodiment of the present invention will be explained in detail with reference to Fig. 7. Fig. 7 is a perspective view showing assembling of a hermetically sealed structure according to the third preferred embodiment of the present invention. In Fig. 7, in the present preferred embodiment, as with the first preferred embodiment, a silicon substrate 1 with a semiconductor laser 4 mounted thereon is fixed within the case 5. In the case 5, a cavity hole 20, passing through the case, rather than the rectangular groove is provided as the fiber introducing section. At the time of mount of an optical fiber 7, the optical fiber 7 is inserted from the transverse direction and mounted and fixed onto a silicon substrate 1. The fixation of the cover 8 and the case 5 with a solder may be carried out in the same manner as described above in connection with the first preferred embodiment. In this case, the solder is also filled into the cavity hole 20. Thereafter, hermetical sealing with a resin 12 is carried out. Joining between the cover 8 and the case 5 may be carried out by airtight welding, such as seam welding, rather than the soldering. In this case, the filling of the solder into the cavity hole 20 is carried out as a separate step, followed by hermetical sealing with a resin 12. As described above, according to the method for hermetically sealing an optical fiber introducing section according to the present invention, a conventional silica optical fiber is used without coating of a metal layer onto the optical fiber, advantageously enabling a hermetically sealed structure to be provided at a low cost. Further, according to the present invention, since hermetical sealing is carried out using a solder having a melting point below that of a low-melting glass, a thermal influence on the optical element fixing section within the case and the protective coating of the optical fiber can be avoided Furthermore, filling of a solder around the optical fiber and filling of a resin into minute gaps at boundaries between the optical fiber and the solder followed by curing of the resin can simply realize a hermetically sealed structure having higher airtightness and reliability than hermetical sealing with the solder alone or the resin alone.

Claims: A method for hermetically sealing an optical fiber introducing section (6) in an optical device comprising an optical fiber (7), wherein the optical fiber does not comprise a metal coating, an optical element (1 to 4) optically coupled with the optical fiber (7), a case (5) for housing therein the optical element (1 to 4), and a cover (8) for sealing the case (5), the optical fiber (7) propagating light between the interior and the exterior of the case (5), said method comprising the steps of: melting and filling a solder (9) between the optical fiber (7) and the optical fiber introducing section (6) of the case; hardening the filled solder; and filling a resin (12), into minute gaps at boundaries between the optical fiber (7) and the solder (9) and curing the resin (12). A hermetically sealed structure of an optical device, comprising: an optical fiber (7), wherein the optical fiber does not comprise a metal coating; an optical element (1 to 4) optically coupled with the optical fiber (7) a case (5) for housing therein the optical element (1 to 4); and a cover (8) for sealing the case (5); the optical fiber (7) propagating light between the interior and the exterior of the case (5); wherein the optical fiber (7) is fixed to the case (5), in its section (6) at which the optical fiber (7) is introduced into the case (5), with the aid of a solder (9) filled between the optical fiber (7) and the optical fiber introducing section (6) of the case (5) and a resin filled into minute gaps at the boundaries between the optical fiber (7) and the solder (9). The method or structure according to claim 1 or 2 wherein a notch for passing the optical fiber therethrough is provided in the case. The method or structure according to claim 1 or 2 wherein a through-hole (20) for passing the optical fiber (7) therethrough is provided in the case (5). The method or structure according to claim 1, 2, 3 or 4, wherein a notch for passing the optical fiber therethrough is provided in the cover (8). The method or structure according to any one of claims 1 to 5, wherein the solder does not contain any flux component. The method or structure according to any one of claims 1 to 6, wherein the resin is an epoxy resin. The method or structure according to any one of claims 1 to 7, wherein the resin is a silicone resin. The method or structure according to any one of claims 1 to 8, wherein the resin is a resin. The method or structure according to any of claims 1 to 5 wherein a cold curing silicone rubber is applied as the resin. The method or structure according to any of claims 1 to 10 wherein the joining between the cover and the case is carried out by airtight welding, preferably seam welding. The method or structure according to any of claims 1 to 11 wherein an Au-Sn solder is used for joining components within the case, wherein an SnPb eutectic solder is used for the fixation of the case to a print board and an Sn-Sb solder or an Sn-In solder is used for hermetically sealing, wherein the melting point of the solder for hermetically sealing is lower than that of Au-Sn and higher than that of Sn-Sb. The method or structure according to any of claims 1 to 12 wherein a thin metal layer of nickel or gold is provided by plating on a cover, a hermetically sealing face and a fiber introducing section. The method or structure according to any of claims 1 to 13 wherein the resin having a viscosity of not more than 70 Pa.s. The method or structure according to any of claims 1 to 14 wherein the attainable air tightness is not more than 10 -7 atm.cc/sec. The method or structure according to any of claims 1 to 15 wherein the solder has a melting point below that of a low-melting glass. The method or structure according to any of claims 1 to 16 wherein the case is a metallic case made of Kovar. The method or structure according to any of claims 1 to 17 wherein the case is a ceramic package.

Inventors: HIGASHIKAWA KIMIKAZU, HIGASHIKAWA, KIMIKAZU
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1998
Main classification: G02B6
IPC codes: G02B6