Abstract:
A hermetically sealed structure, particularly for use in an optoelectronic device is described. The structure comprises an outer sleeve of a material resistant to moisture ingress with an insert located in the sleeve at one end portion thereof and having a bore therethrough. An optical fibre extends through the bore and beyond the end portion of the sleeve with adhesive films respectively securing the insert to the sleeve and the optical fibre to the insert. A method of assembly for such a package is also described.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a hermetically sealed package and a method of assembly therefor. The invention is particularly concerned with the provision of a hermetically sealed package for a fibre optic device. 
     FIELD OF THE INVENTION 
     Packaging of fibre optic devices is driven by the need to achieve low cost while at the same time maintaining high reliability for extended operational life, for example in excess of 20 years. It is particularly important to achieve hermeticity around the fibre optic cable as it enters the device package. In this context, the hermetic seal is required to provide a good barrier to the ingress of moisture which can cause damage to the fibre optic devices within the package. 
     According to one method for hermetically sealing of fibre optic cables, a glass to metal seal is formed between the fibre optic cable and its supporting tube or sleeve. This involves locally heating a stripped portion of the fibre optic cable to around 500° C. to reflow a precision solder glass bead, positioned at an appropriate place to form the seal. At the same time, the fibre jacket, or protective outer coating, must be maintained to below 90° C. to avoid damage to the coating. Control of the reflow profile is quite critical as high stress concentrations can lead to damage to the optical fibre and subsequent light attenuation in use. 
     According to another method, the fibre optic cable is coated with metal (for example titanium/platinum/gold or titanium/tungsten/gold) and then soldered to its support tube. This approach involves costly metal deposition processes to coat the fibre. It is also difficult to maintain adhesion of the coating to the fibre. In addition, as with the first method mentioned above, a heat process is required which can lead to stress and deformation of the fibre optic cable. 
     Thus, although these known methods do give reliable hermetic seals, they have inherent assembly difficulties and also compromise the cost objectives of packaging. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a hermetically sealed structure comprising an outer sleeve of a material resistant to moisture ingress; an insert located within the sleeve at one end portion thereof, the insert having a bore therethrough, and an optical fibre extending through said bore and beyond the end portion of the sleeve, wherein respective adhesive films secure respectively the insert to the sleeve and the optical fibre to the insert. 
     According to another aspect of the present invention there is provided a method of assembling a hermetically sealed package comprising the following steps: 
     a) inserting an insert into a distal portion of a sleeve, the insert having a bore extending therethrough; 
     b) guiding an optical fibre along said sleeve from a remote portion thereof and through said bore; 
     c) applying a quantity of adhesive to a remote end of the optical fibre; and 
     d) drawing the optical fibre through the sleeve whereby the adhesive is caused to flow between the insert and the sleeve and between the bore and the optical fibre to form respective seals therewith. 
     It will be understood that with a small gap available between the insert and the sleeve, and between the optical fibre and the bore, the adhesive flows due to capillary action. According to the example described herein, an epoxy resin is used to form a hermetic seal. This resin thus replaces the solder glass or metal solders which were used in the known techniques outlined above, and thus negates the need for high temperature processing. It is somewhat surprising that an epoxy resin provides an adequate hermetic seal, because epoxy resins in their bulk form have a tendency to permeate moisture and are not normally considered to be completely hermetic. However, by providing a thin film of epoxy resin at the interface between the insert and the sleeve on the one hand, and the optical fibre and the bore within the insert on the other hand, a fully hermetic seal can be established. 
     For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in partial section a package containing an optic device; 
     FIG. 2 is a section through a ferrule during a first assembly step; 
     FIG. 3 illustrates a fibre optic cable during a second assembly step; 
     FIG. 4 is an end view along arrow IV in FIG. 3; 
     FIG. 5 illustrates an intermediate assembly step; and 
     FIG. 6 illustrates a final structure of a ferrule. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates in partial section a package for an optics device. A casing  2  formed of a material such as a metallic alloy like KOVAR (Ni/Fe/Co) has a portion  2   a  for receiving a ceramic wafer  4  supporting an integrated optical device  6 . The integrated optical device can take the form of a silicon on insulator wafer on which monolithic silicon waveguides have been formed. The casing  2  also has an entry portion  2 b which is tubular and which holds a ferrule  8  which supports a fibre optic cable  10  such that a central optical fibre  12  of the fibre optic cable  10  is attached to the integrated optical device  6 . Although not shown in FIG. 1, a lid is provided on the casing  2  to provide a sealed package. It is very important that the package is entirely hermetically sealed, particularly against the ingress of moisture which can have a damaging effect on interfaces of active optical elements on the wafer. The material of which the casing is formed, for example KOVAR, provides a good hermetic seal for the package itself. It is important however to additionally provide that leakage of moisture into the package is prevented. One location where moisture could leak is through the ferrule  8  itself, because this ferrule holds the fibre optic cable  10  so that there is in principle an entry path for moisture between the fibre optic cable and the external casing of the ferrule  8 . 
     An assembly technique for a ferrule  8  in the form of a hermetically sealed structure is described herein which significantly reduces the ingress of moisture into the package. Firstly, the main components of the ferrule will briefly be pointed out with reference to the final structure illustrated in FIG.  6 . The ferrule  8  has an outer casing  14  in which is located an insert  16  with a narrow central bore  18  through which the optical fibre  12  passes. The outer casing  14  holds at the other end of the ferrule the fibre optic cable  10 . The outer casing  14  is formed of a substance such as a metal which has a high hermeticity. The insert  16  can be formed of, for example, ceramic. It is noted at this point that the ferrule holds a portion of optical fibre  12  which extends from the ferrule so as to allow it to be connected to the integrated optic device  6  in FIG.  1 . This portion of the optical fibre  12  needs to be kept free of dirt and other foreign material so that the optical fibre  12  can function efficiently. Other important features of the final structure of FIG. 6 will become apparent from the following description of the assembly technique. It is nevertheless noted that in the final structure thin epoxy seals  20 , 22  are provided respectively between the insert  16  and the outer casing  14  and between the insert  16  and the optical fibre  12 . The thin epoxy seals provide exceptionally good resistance to the ingress of moisture through the ferrule  8  into the package  2 . 
     The assembly technique for constructing the ferrule of FIG. 6 will now be described. FIG. 2 illustrates a first assembly step. The insert  16  is inserted into the outer casing  14  as a push-fit. The outer casing  14  has an internally stepped diameter marked at  24  against which a truncated conical face of the insert  16  rests. The insert  16  has a central counter bore  26  which extends into the central bore  18 . 
     As a second or parallel assembly step, the fibre optic cable  10  is prepared for insertion. FIG. 4 illustrates the construction of the fibre optic cable  10 . It comprises a central optical core  28  surrounded by optical cladding  30 . This is covered by first and second supporting acrylic polymer layers  32  which are themselves held within a protective buffer layer  34 . The diameter d 2  of the optic cladding is around 125 microns, the diameter d 2  of the outer acrylic coating  32  is around 250 microns, and the external diameter d 3  of the buffer  34  is around 900 microns. The internal diameter of the outer casing  14  is around 1 mm. 
     The acrylic polymer coatings  32  are stripped from a length L of the optical fibre, leaving the optical core  28  and cladding  30  over length L. The buffer layer only is stripped from a smaller length l and retained over the remainder of the fibre optic cable  10 . An adhesive such as epoxy is used to secure the buffer to the acrylic polymer coating  32  at the location marked  36  in FIG.  3 . This is merely to prevent slippage under tension between the buffer  34  and the remaining components of the optical fibre. The epoxy  36  can be applied at any convenient time during the assembly procedure, but is shown here for the sake of convenience prior to the assembly step now to be discussed with respect to FIG.  5 . 
     According to FIG. 5, assembly of the ferrule takes place as follows. The length L of optical fibre  12  is inserted into the outer casing  14  of the ferrule and guided through the central bore  18  of the insert  16 . Once it has been located and guided through the central bore  18  of the insert  16 , a small amount of epoxy resin  38  is applied at the junction between the stripped length L and the acrylic coated length l. As the optical fibre cable  10  is pushed further into the ferrule casing  14 , the epoxy  38  comes up against the insert  16  and starts to flow into the counter bore  26 . As a result of capillary action, a thin film of epoxy creeps into the interface between the insert  16  and the outer casing  14  and between the optical fibre  12  and the inner bore  18  as indicated generally by the arrows E. Thus, by applying a controlled amount of epoxy and drawing the optical fibre  12  through the casing  14  in this manner, a thin film epoxy seal is provided both around the insert and around the optical fibre as designated by reference numerals  20  and  22  in FIG.  6 . 
     The length l of buffer stripped acrylic polymer is such that the interface between the wholly-stripped portion L and the bufferstripped portion l lies just within the counter bore  26  of the insert  16 . This has been found to be a particularly good location to resist fibre breakages during the life of the device. The provision of a length l of optical fibre which retains its acrylic polymer coating  32  has also been found to be advantageous in this respect. It is also pointed out that the technique described above leaves the external length of optical fibre  12  protruding from the ferrule free of epoxy resin. 
     The insert  16  has been described herein as being of ceramic. However, it will be appreciated that any precision machined material may be provided. The central bore  18  is precision machined to a tight tolerance to provide a close clearance with the fibre optic cable. In the present example, it is machined to an internal diameter of 126 microns.