Optical waveguides – Accessories – External retainer/clamp
Reexamination Certificate
1998-02-17
2001-01-30
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Accessories
External retainer/clamp
C385S114000, C385S080000
Reexamination Certificate
active
06181863
ABSTRACT:
The invention relates to producing optical fiber flexfoils, i.e. two flexible sheets having optical fibers arranged therebetween.
BACKGROUND
Future demands on communication systems include increased component densities in the apparatus used and larger bandwidths. The data rates of computers, telecommunication, etc. are permanently increasing. Due to restriction in space and the high impedance characteristics of thin electrical lines, a higher component density on printed circuit boards, PCBs, results in difficulties in providing a sufficient number of electrical connections to a backplane, BP. Due to the large bandwidth and low signal loss that optical fibers exhibit, optical interconnections used for signal transmission internally on circuits boards and between boards may reduce these problems.
Thus, optical communication, well established since long times in long distance broad band communications, is also being introduced for short range applications inside telecommunication exchanges and computers, etc. For such applications the number of optical interconnections may become significant. However, a large number of loose optical fibers mounted on PCBs or BPs or connected thereto will give an unmanageable building practice. Optical fiber management is one of the key factors that have to be solved in order to successfully implement the use of short range optical interconnections. One practical approach thereto is to use a physically or geometrically separate optical level to house all the optical connections. A suitable such short range, separate optical interconnecting medium comprises optical fibers mounted on or in a flexible substrate called an optical fiber flexfoil.
The optical flexfoil technique has been presented by the company AT&T in e.g. U.S. Pat. No. 5,259,051 for Burack et al. This patent discloses how optical fibers are routed on an adhesive-coated surface using a rotating wheel. In addition the patent describes how optical fibers are encapsulated between two plastic foils, using two heated cylinders. AT&T's technique includes a thermoplastic filler which is added to the base flexfoil as an extra layer before the lamination in order to encapsulate and protect the fibers. The thermoplastic filler, which is molten by the two heated cylinders laminating a top foil to the thermoplastic layer, seals the fiber flexfoil. In order to melt the filler and in order to laminate the flexfoil without trapping air, high temperatures and high lamination pressures are required.
SUMMARY
It is an object of the invention to provide a method and a device for making flexfoils using optical fibers which are capable of being bent at small curvatures.
It is another object of the invention to provide a method and a device for making flexfoils using optical fibers not subjecting the fibers to high temperatures or to localized mechanical stresses.
Thus, the problem to be solved by the invention is how to produce an optical fiber flexfoil allowing it to be acutely bent and in particular how to produce the flexfoil so that the individual fibers are not subjected to unsatisfactory stresses such as microbends when the flexfoil is produced or bent. In particular the problem involves how to achieve that the fibers have some movability in the flexfoil.
Thus, generally an optical fiber flexfoil comprises optical fibers adhesively bonded between two flexible foils laminated to each other. For producing the flexfoil a surface of a base flexible plastic sheet or base foil is coated with a pressure sensitive adhesive and optical fibers are applied in predetermined paths to the coated surface and bonded to the base foil by the adhesive. A top foil that is identical to the base foil is placed at the surface of the base foil, so that its surface having the pressure sensitive adhesive will come in contact with the optical fibers, air located between the foils is removed and the top foil and the base foil are moderately heated and pressed against each other to make them bond to each other only by the coatings of the pressure sensitive adhesive. The adhesive is selected to have a low glass transition temperature such −50° C. and to exhibit visco-elastic characteristics at least in the temperature range where the flexfoil is to be used. The pressing temperature is located immediately above or in the topmost part of the operational temperature range.
In this production method no additional thermoplastic filler between the foils is used. The type and thickness of the pressure sensitive adhesive must be selected appropriately. A pressure sensitive adhesive may generally be processed at a fairly low temperature of e.g approximately 70° C. This shall be compared to temperatures in the range of 130-160° C. which must be used for the thermoplastic polyurethane used in the cited U.S. patent. The acrylic primary coating of the fibers may be degraded at such high temperatures. Further, a pressure sensitive adhesive generally does not solidify when the temperature is lowered, i.e. such an adhesive exhibits a viscoelastic behaviour at the operational temperatures of the flexfoil in the range of −40 to 80° C. The viscoelastic properties of the adhesive makes it possible for the encapsulated fibers to move somewhat in the laminated structure also when the finished flexfoil is used at ambient temperatures. Microbends introduced during the lamination process will therefore relax, giving the optical fibers in the flexfoil a very low optical attenuation.
If heated cylinders would be used in the lamination process for pressing the two foils against each other, a high cylinder pressing force must be applied in order to produce a laminate having no enclosed air bubbles. It has been shown that micro-cracks are easily formed in the fibers during the rolling operation due to the localized pressing force. In particular there is a definite danger of causing bending of fibers at very small radii such as approximately equal to the radius of the fibers in regions of the fibers where fibers cross each other. This avoided for a vacuum lamination process, which also is faster and generally subjects the fibers to minimum mechanical stresses. The overall pressure in the lamination process can then be made smaller, this reducing even more the risk of bending the fibers at too small radii at fiber crossings.
In such an optical fiber flexfoil the two foils are thus bonded to each other only by the same adhesive as the optical fibers are bonded to the foils and also by the same adhesive as by means of which they are bonded during the initial step of laying-out of the fibers on one of the foils. The optical fibers are thus substantially completely embedded in a pressure sensitive adhesive occupying substantially all of the place between the flexible foils except that of the optical fibers. The two adhesive layers can have a total thickness smaller than the exterior diameter of the embedded fiber, and then the distance between the facing surfaces of the flexible component foils in the finished flexfoil, at places distant from the optical fibers, will generally be smaller than the exterior diameter of the optical fibers, the places where the fibers are located forming low ridges at the exterior surfaces of the component foils.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.
REFERENCES:
patent: 3936277 (1976-02-01), Jakway et al.
patent: 4364788 (1982-12-01), Bloodworth, Jr. et al.
patent: 5259051 (1993-11-01), Burack et al.
patent: 5394504 (1995-02-01), Burack et al.
patent: 5536787 (1996-07-01), Scholz et al.
patent: 5582673 (1996-12-01), Burack et al.
patent: 5611017 (1997-03-01), Lee et al.
patent: 5932298 (1999-08-01), Moon
Engberg Kristian
Hesselbom Hjalmar
Burns Doane Swecker & Mathis L.L.P.
Kim Ellen E.
Sanghavi Hemang
Telefonaktiebolaget LM Ericsson (publ)
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