Abstract:
A technique for hermetically sealing an optical component in a metal package is described. Variations of the technique are described in which optical communication between the optical component and the outside environment is achieved with a ribbon fibre.

Description:
FIELD OF THE INVENTION 
     The field of the invention relates generally to hermetic sealing of optical and optoelectronic components and more specifically to the hermetic sealing of a set of optical waveguides such as a ribbon of fibres. 
     BACKGROUND OF THE INVENTION 
     The expansion of digital data traffic resulting from increased use of the Internet has led to the deployment of optical networks. These networks transmit large quantities of information very quickly however they rely on expensive optical components. In order to ensure the proper functioning of the optical components it is often necessary to isolate them from their environment. Isolating the optical component or components from moisture is very beneficial because water accelerates aging and corrosion of the optical component. Additionally, water is known to have highly detrimental effects on adhesives, which is unfortunate because adhesives are a convenient way of attaching optical components together, particularly through a light-transmitting surface. As one of skill in the art of hermetic sealing of optical components will be aware, it is important that the hermetic seal be sufficiently robust that the seal is not compromised during the working life of the product. When products are sold with twenty-year guarantees, the hermetic seal of each subcomponent is expected to last the twenty years. Generally, it is common to test each individual component for hermetic integrity. Instead, it is common practice to establish a method of hermetic sealing that is highly reliable and use spot checks to ensure that the method is working effectively. Verifying the integrity of a hermetic seal requires time and uses very costly, specialized equipment. For this reason, it is beneficial to use methods of hermetic sealing which are well established and proven in production environments. 
     Preventing moisture from entering a package containing an optical component is a difficult task because water molecules will penetrate microscopic cracks. A conventional optical component package is a metal box with one or more features for allowing glass waveguides to penetrate the exterior of the box. Unfortunately, it is hard to seal a metal box about a glass waveguide without leaving a crack. The prior art of Kovats U.S. Pat. No. 4,413,881, herein referred to as Kovats, teaches that a glass fibre inserted in a metal tube may be hermetically sealed to the tube by injecting a melted solderable alloy, such as BiSn in the tube, thereby plugging the tube. If the alloy expands during solidification then it will squeeze the optical fibre and help to prevent the formation of cracks between the optical fibre and the solder. It should be noted that optical fibres are generally glass and that most molten metals will not ordinarily wet to a glass surface. The Kovats prior art avoids this problem by filling a tube with molten solder while the optical fibre is in the tube. Using this method, pressure between the tube and the fibres caused by the solidification of the solder forms the hermetic seal. 
     While the Kovats prior art teaches a method of sealing a single fibre hermetically, it does not provide a simple means of sealing a multi-fibre array for example, a ribbon fibre, hermetically. If a ribbon of optical fibres is separated and each individual optical fibre is sealed independently then a variety of problems result. For example, the metal tubes used in forming the hermetic seal are substantially larger than the fibre. Typically these tubes are spaced approximately 0.20″ (5 mm) between fibre centers. This is not a significant concern for large packages with few fibres however it is not uncommon to produce arrayed waveguide gratings (AWG) with over forty optical fibres which ideally exit the package through the same face and the spacing is typically 250 microns between adjacent fibre centers. Indeed as AWGs become more sophisticated there will be a need for even larger numbers of fibres provided at one face of an optical device to be hermetically sealed. 
     Alternatively, if a ribbon of fibres were sealed in a package wall according to the method of Kovats, the ribbon would likely be prone to twisting as the solder flows around it. This likely induces stress and thereby reduces the optical performance of the packaged optical component. Additionally, great care must be taken to ensure that the solder flows around all of the fibre as the fibre themselves hinder the flow of solder. Thus, when sealing a ribbon of fibre it is far more likely that voids and cracks will be present in the seal after cooling, thus compromising the hermetic performance of the seal. 
     Alternative methods of sealing an optical fibre are well known in the art. For example, in order to ensure that the solder wets the surface of the optical fibre it is known to metalize the fibre prior to encasing them in solder. Since the glass fibre is now encased in metal prior to being immersed in molten solder the solder easily bonds to the metalized surface of the fibre. At first glance, this solution appears highly advantageous however it has disadvantages. For example, metalizing the fibre is a slow process that typically involves a vacuum deposition machine. These machines are expensive and it is highly recommended that a skilled operator oversee them. Although many fibres can be metalized in one use of the machine, the process for metalizing fibres is very slow as only tiny amounts of metal are deposited at a given time. Consequently, if this process were to be incorporated for use with a flexible manufacturing environment then a JIT (just-in-time) manufacturing schedule would be very difficult to incorporate. Additionally, metalized optical fibres are very fragile and easily damaged in handling. Any separation of the metalized layer from the optical fibre will likely compromise the effectiveness of a hermetic seal. Alternatively, metalized optical ribbon fibre with up to eight individual waveguides are available commercially however they are very costly and their fragile nature makes shipping them very costly as well. However, large number count metalized ribbon fibre is still not available, because it is difficult to metalize so many fibre uniformly at a time. Typically, metalized ribbon fibre featuring over twelve individual waveguides is not commercially available. 
     It would be beneficial to provide a simple, effective method of hermetically sealing optical components that supports the sealing of a fibre array or a ribbon of fibres without inducing stress on the individual fibre. Preferably, such a method is for allowing the fibre array to be sealed hermetically without requiring costly and complex equipment. Additionally, it would be beneficial if such a method incorporated proven hermetic sealing technology and not involve preparing components in advance and storing them for later use. 
     SUMMARY OF INVENTION 
     The invention teaches the design of a junction for forming a hermetic seal about an optical waveguide, said junction comprising: a tube for providing fluid communication between a first orifice and a second orifice, a cap disposed for covering the first orifice, said cap for reducing a flow of molten metal through the first orifice when the optical waveguide is disposed within the cap, said tube being sufficiently wide to support the optical waveguide disposed between the first orifice and the second orifice while simultaneously permitting molten metal at a predetermined temperature to flow within the tube, such that, in use, the optical waveguide is disposed within the tube, molten metal is provided within the tube and flows to the cap and the molten metal solidifies, thereby forming a hermetic seal between the optical waveguide and the tube. 
     Further the invention describes a method of forming an optical waveguide hermetic seal comprising the steps of: disposing an optical waveguide through a cap; abutting the cap to a first orifice of a tube; providing molten metal to the tube such that the molten metal provided in a channel of the tube flows toward the first orifice of the tube; and, allowing the molten metal to solidify, thereby forming a hermetic seal between the tube and the optical waveguide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the drawings in which 
     FIG. 1 is a drawing of a prior art hermetic joint; 
     FIG. 2 is a section view of a joint with a ribbon of optical fibres in which a void has compromised the seal; 
     FIG. 3 is a section view of an embodiment of the invention prior to providing solder; and, 
     FIG. 3 a  is a section view of an embodiment of the invention with molten solder surrounding the ribbon of optical fibres. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a method and apparatus for producing a robust hermetic seal about a ribbon of optical fibres. Referring to FIG. 1, a prior art diagram of hermetic seal produced by the hermetic sealing technique of Kovats is illustrated in a section view shown in FIG.  1 . The drawing shows an optical fibre encased in solder. The fibre  1  and solder are contained within a metal tube  3  that helps to keep the solder in compression. The solder in provided to the tube via an orifice  2 . 
     This technique is poorly suited to hermetically sealing a fibre array or ribbon for a variety of reasons. For example, in order to produce a robust hermetic seal, the optical fibre is surrounded by solder. Since glass optical fibres do not wet with most molten solders it is critical that the solder be proximate the surface of the fibres when it solidifies. FIG. 2 illustrates a ribbon of optical fibres  21  encased in solder  22 . Similarly to the prior art, the sheath material  27  used to cover and bind together the individual fibre of the ribbon of optical fibres  21  has been stripped leaving an exposed region  24 . Typically the sheath material is not sufficiently well bonded to the fibres to form a hermetic seal and often the sheath material is a plastic that is not hermetic. Since the ribbon of fibres is a set of fibres in close proximity, it is difficult to get the solder  22  into the correct position necessary to uniformly coat the exposed region  24  of the fibres  21 . This increases the likelihood of there being voids between the optical fibres. Such a void  23  is shown proximate the fibres. This void  23  is expected to cause the adjacent fibres  25  and  26  to be pushed towards the void when the solder  22  expands. The displacement of the fibres  25  and  26  induces stress on the fibers. Additionally, in an extreme case, the fibers will kink somewhat and which leads to changes in their polarization characteristics. Additionally, it is unlikely that a void is entirely eliminated when the solder expands and therefore, the resulting joint will likely not be hermetic. 
     Thus, in order to produce a consistent and robust hermetic seal it is important to use a method of providing molten metal that reduces the likelihood of voids forming. Ideally, each of the optical fibres from the ribbon fibre is surrounded by molten metal in order to ensure that the fibres are not stressed. However, separating each of the fibres of the ribbon and sealing them independently is not practical. 
     Referring to FIG. 3, a section view of a package wall with a hermetic junction suitable for accommodating ribbon of optical fibres is shown. The package  30  includes a hollow tube section  31 . The tube section  31  will support the ribbon fibre and the molten solder. A cap  32  has been attached to the exterior of the tube section  31 . The cap  32  has a feature for allowing a ribbon fibre  33  to penetrate the cap. The ribbon fibre  33  has been stripped of its plastic sheath in a region  33   a  contained within the tube section  31 . The ribbon fibre  33  is held at one end by the cap  32  and at the other end by a jig  36 . The jig  36  and the cap  32  ensure that the ribbon fibre  33  is under a slight amount of tension in the tube section  31 . The cap  32  is not hermetic. The opening  34  of the tube section  31  opposite to the cap  32  remains accessible to specialized tools despite the presence of the jig  36 . 
     Referring to FIG. 3 a , molten metal  35  is provided to the tube section  31  from the opening  34 . The molten metal  35  flows within the tube section  31  and covers the exposed region  33   a . The orientation of the tube section  31  with the cap  32  down helps to bias any air pockets up and out of the molten metal  35 . The cap  32  reduces the amount of molten metal draining out the bottom of the tube section. Since the ribbon fibre  33  is held under tension between the jig  36  and the cap  32  the ribbon fibre  33  resists being pushed against the side of the tube section  31 , thus ensuring that the ribbon fibre  33  is not displaced when the molten solder expands during solidification. 
     Additionally, the use of the said design permits the use of solders absent flux. As one of skill in the art will be aware, the use of acidic flux is cause for concern because even a mild acid will have a detrimental effect on a hermetic joint. Additionally, many optical components, such as InP based integrated circuits and MEMS (Micro Electro Mechanical System) devices, are very sensitive to contamination from flux therefore eliminating flux eliminates failure due to flux contamination during sealing. 
     Clearly, the diameter of the tube section  31  is chosen to be sufficiently large for accommodating the ribbon fibre  33  as well as the flowing solder  35 . Providing sufficient space for the solder to flow within the tube section and around the ribbon fibre  33  helps to provide a consistent process. If the solder  35  should push the ribbon fibre  33  to one side of the tube section  31  the slight tension provided to the ribbon fibre  33  from the cap  32  and jig  36  cause the ribbon fibre to return to an area proximate the center of the tube section  31  and away from the walls. This helps to prevent the formation of voids between any of the individual fibres and the tube section  31 . Thus, the solder is able to wet a complete diameter of the tube section  31  and the fibres of the ribbon fibre  33  remain substantially straight relative to the tube section  31 . The molten solder  35  expands slightly on solidification and thus it exerts pressure on the package and the optical fibres. This pressure helps to ensure that no cracks form. 
     Once the molten solder has solidified and cooled, the cap  32  is optionally removed to visually inspect the hermetic joint. Alternatively, the cap  32  is made from a flexible material capable of resisting high temperature failure and is used as a strain relief for the fibres exiting the tube section. Since most hermetically sealed optical fibres require a strain relief the addition of the cap does not substantially increase costs of manufacturing. Although, this embodiment clearly incorporates a ribbon of optical fibres, an alternative embodiment of the invention hermetically seals a single optical fibre. Clearly, the cross section of the ribbon fibre or single fibre determines the appropriate tube shape. While this simplifies the process and helps to ensure that a good hermetic seal results, alternative embodiments exist. For example, if the material of the tube section has a higher coefficient of thermal expansion than the solidified solder it is apparent to one of skill in the art of mechanical design that the tube section squeezes the solder as it cools thereby enhancing the seal it creates. Provided the correct amount of cooling and the correct thermal expansion characteristics of the various components, a robust hermetic seal is produced. 
     In an alternative embodiment of the invention, the package is provided to allow the ribbon of optical fibres to be held horizontally. In this embodiment, separate caps are used at each end of the tube section and the molten solder is provided from a small orifice in fluid communication with the tube section. 
     In yet another embodiment, the material for the cap is not held firmly to the package but merely held in proximity to the package by a second jig. When the solder has solidified, the second jig is removed. Alternatively, no cap is incorporated and the second jig acts as a cap. In this case the second jig has a plugging region for reducing the flow of solder out the end of the tube section. This plugging region has a surface that substantially prevents bonding to the solder. 
     In yet another alternative embodiment of the invention, the tube section is designed to accommodate more than one ribbon of optical fibres. This will allow an even larger number of optical waveguides to penetrate the package. However, it is recommended that the cap and tube geometry be redesigned for different combinations of ribbon fibre in order to produce a hermetic joint according to the invention. 
     It will be apparent to one of skill in the art of mechanical design and hermetic sealing that numerous other embodiments of the invention may be envisioned without departing from the spirit and the scope of the invention.