Hermetic seal for optical waveguide ribbon feed through

Details Hermetic seal for optical waveguide ribbon feed through Hermetic seal for optical waveguide ribbon feed through

2002-08-21

2004-07-13

Nguyen, Khiem (Department: 2839)

Optical waveguides

Accessories

Bushing structure

Reexamination Certificate

active

06763173

ABSTRACT:

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, allo

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