Optical fiber encapsulation techniques

Abstract: Optical fibers are encapsulated, first by bonding them to a first surface (18) of a flat member (17) having first and second opposite major surfaces. The flat member and a sheet of thermoplastic (13) are placed in an air-tight chamber (10) such that a first major surface of the sheet faces the first major surface (18) of the flat member (17). Next, the air pressure on the second major surface of the flat member is made to be significantly lower than the air pressure on the second surface of the thermoplastic sheet (13), thereby to cause the sheet to press against the flat member. The thermoplastic sheet is heated sufficiently during this process to cause it to adhere to the first surface (18) of the flat member, thereby to encapsulate the optical fibers.

Description: Technical Field This invention relates to optical fiber interconnections and, more particularly, to techniques for encapsulating optical fibers that have been bonded to one surface of a member such as an optical backplane. Background of the Invention The patent of Burack et al., No. 5,259,051, granted November 2, 1993 (hereinafter '051), describes a method for making optical backplanes by using a robotic routing machine to apply optical fibers to a flat surface of a flexible plastic substrate cf. the preamble of claim 1. The fibers are bonded to the substrate by a pressure-sensitive adhesive, and after routing they are covered by a plastic sheet that encapsulates them to protect them, to give structural mechanical stability, and to keep the optical fibers in place during the handling of the optical backplane. The component optical fibers are typically used as large-capacity transmission lines between printed wiring boards or between optical circuits. The U.S. patent of Burack et al., No. 5,292,390, granted March 8, 1994 (hereinafter '390), published between the priority date and the date of filing of this European patent application and therefore not comprised in the state of the art, describes the difficulty of applying the plastic sheet to the optical fibers in a manner that gives the attributes of a good encapsulation but without damaging the fibers. A two step process is described for applying heat and pressure with platen press apparatus. While we have found that this method of encapsulating the fibers gives greatly improved results, we have found that too often fibers are nevertheless damaged; in an effort to avoid all damage, the stability and dependability of the encapsulation is sometimes compromised. These problems occur particularly when an extremely dense array of optical fibers including many crossovers is included on the surface of the flexible plastic substrate. Accordingly, it would be desirable to provide an encapsulation method that firmly encases optical fibers, including optical fiber crossovers, without damaging or weakening any of the fibers. Summary of the Invention In one embodiment, optical fibers are encapsulated, first by bonding them to a first surface of a flat member having first and second opposite major surfaces. The flat member and an encapsulating sheet are placed in an air-tight chamber such that a first major surface of the sheet faces the first major surface of the member. Next, the air pressure on the second major surface of the flat member is made to be significantly lower than the air pressure on the second surface of the encapsulating sheet, thereby to cause the sheet to press against the flat member and causing the encapsulating sheet to adhere to the first surface of the flat member, thereby to encapsulate the optical fibres. Using air pressure to press the plastic sheet against the optical fibers has been found to be a superior method for applying the pressure needed for a sturdy bond, while distributing applied forces to avoid a concentration of forces on such protrusions as a crossover of optical fibers. It also avoids the need for precisely planar parallel platens for distributing forces evenly. These and other objects, features and advantages of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing. prefered embodiments are defined in the dependent claims. Brief Description of the Drawing FIG. 1 is a sectional view of an air-tight chamber used for encapsulating optical fibers; FIG. 2 is a schematic sectional view taken along lines 2-2 of FIG. 1; FIG. 3 is a sectional view of the chamber of FIG. 1 at a subsequent stage of the process; and FIG. 4 is a schematic sectional view of a thermoplastic encapsulant overlying an optical fiber. Detailed Description The drawings are schematic, with in some cases dimensions purposely distorted to aid in clarity of exposition. Referring now to FIG. 1, there is shown a substantially air-tight chamber 10 which is used for encapsulating optical fibers in accordance with one embodiment of the invention. The air-tight chamber 10 is defined by two enclosure members 11 and 12 which are clamped to opposite sides of a thermoplastic sheet 13. The clamping of the two enclosure members is schematically represented by arrows 16 which represents pressure applied, for example, by a press, or by a clamping member or members, to the enclosure members 11 and 12. The thermoplastic sheet 13 is of a material that is substantially impermeable to air such as polyurethane; it extends around the entire periphery of air-tight chamber 10 and effectively constitutes an air-tight gasket between enclosure members 11 and 12. The thermoplastic sheet 13 thus effectively divides the air-tight chamber 10 into an upper chamber portion 14 and a lower chamber portion 15 which are mutually substantially hermetically sealed. The upper chamber portion 14 includes a flat member 17 having a first surface 18 to which optical fibers (not shown) have been bonded. The purpose of the apparatus of FIG. 1 is to encapsulate the optical fibers by bonding the sheet 13 to the first surface 18 of the flat member 17, which contains the fibers. Flat member 17 is separated from sheet 13 by spacers 19. The upper chamber portion 14 is connected via a valve 21 to vacuum apparatus 22. Lower chamber portion 15 is connected via a valve 23 to either the vacuum apparatus 22 or to a source of gas 24 such as nitrogen gas. The entire chamber 10 is capable of being heated by a heat source schematically shown at 25. The apparatus of FIG. 1 is designed to encapsulate optical fibers that have been routed onto a surface of a sheet of flexible plastic. Referring to FIG. 2, optical fibers 26 and 27 are illustratively bonded to a flexible plastic substrate 29 by pressure-sensitive adhesive 30 by the technique described in detail in the Burack et al. patent. The fibers 26 may be grouped in groups of three, as shown, for reasons given in the Burack et al. '051 patent, and there may be a plurality of crossovers in which optical fibers such as fiber 27 overlap optical fibers 26. The flexible plastic substrate 29 is bonded by a temporary adhesive tape, for example, Flexmark (TM) DFM 700 Clear V-302 ULP, available from the Flexcon Company, Spencer, Massachusetts, U.S.A., to the flat rigid member 17, which may be a flat sheet of aluminum, for example. In operation, both valves 21 and 23 of FIG. 1 are first connected to vacuum apparatus 22 to provide a partial vacuum in both the upper chamber portion 14 and the lower chamber portion 15. The purpose of this operation is to draw out the gas between sheet 13 and flat member 17. Next, the heat is applied and valve 21 is connected to vacuum apparatus 22, while valve 23 is connected to air source 24. This produces a much lower gas pressure in upper chamber portion 14 than in lower chamber portion 15. As a consequence, the flat member 17, spacers 19 and the thermoplastic sheet 13 are drawn up vertically as shown in FIG. 3 to bear against the enclosure member 11. The upward air pressure exerted on sheet 13 extends evenly along its entire area. The heat supplied by source 25 is sufficient to cause a partial flowing of the thermoplastic sheet 13. The heat and pressure together cause the sheet 13 to adhere to the flat member 17, thereby to encapsulate the optical fibers bonded to the surface of flat member 17. Thereafter, the bonded structure is removed from chamber 10, and the plastic substrate 29 of FIG. 2 is peeled away from rigid member 17. The composite structure including plastic substrate 29, optical fibers 26 and bonded thermoplastic sheet 13 then constitutes, for example, an optical backplane. The applied air pressure differential is typically fifteen to forty pounds per square inch, and the heat is applied to a temperature of one hundred to one hundred forty degrees Celsius. The advantage of applying pressure as shown is that the pressure is inherently equally distributed, rather than concentrated at protrusions such as crossovers. As a consequence, for a given yield, higher temperatures and pressures can normally be used in the apparatus of FIG. 3 than could be used if the pressure were applied mechanically. Thus, for a given yield, the encapsulation produced with the invention provides better encasement and structural support for the optical fibers. The benefit of using the higher pressure can be seen by a visual inspection of the encapsulated optical fibers which shows a snug fit of the thermoplastic around each group of three fibers. Using a similar pressure to get a similar snug fit with a platen press results in a greater number of failures than with the invention. Referring to FIG. 4, an optical fiber 35 having a thickness t of ten mils (.25 millimeters) is encapsulated by a thermoplastic sheet 36. With the invention, thirty pounds per square inch of pressure was applied at one hundred degrees Celsius for one minute, which yielded a distance d between the contact points of the thermoplastic 36 to the substrate of .65 millimeters (twenty-six mils). With the platen press, a similar yield required an applied pressure of < 1.24·10 5 N/m 2 (eighteen pounds per square inch) at one hundred degrees Celsius for twenty seconds, and 3.45·10 4 N/m 2 (five pounds per square inch) for four minutes, as described in the Burack et al. '390 patent. This yielded a distance d of between 1.32-1.47 millimeters, (fifty-three and fifty-nine mils) which indicated an undesirable, looser fit. Polyurethane has been used as an encapsulant because, in addition to being thermoplastic and providing good protection for the optical fibers, it is effective for hermetically sealing the upper chamber portion 14 from the lower chamber portion 15 and for providing an air-tight seal for the enclosure members 11 and 12. It is also sufficiently elastic to stretch without breaking, as shown in FIG. 3. It is nevertheless believed that other methods could be used for providing the air pressure differential needed to press a plastic sheet against the optical fibers to be encapsulated, and other materials could be used as the encapsulant. Because of the effectiveness of the inventive method in distributing forces, such flexible materials as Kapton (a trademark), Mylar (a trademark) and aluminum foil can be used as the encapsulating sheet without harming the optical fibers. Such materials of course require an adhesive, and they do not flow around the fibers as does heated thermoplastic. The apparatus we have described has been used for making optical backplanes in which optical fibers are bonded to a flexible plastic substrate 29 as shown in FIG. 2. Clearly, the invention could also be used to encapsulate fibers mounted directly on a rigid substrate, and such mounting could be done by methods other than those described in the aforementioned Burack et al. '051 patent. Various other embodiments and modifications may be made by those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: A method for encapsulating optical fibers comprising the steps of: bonding optical fiber to a first surface (18) of a flat member (17) having first and second opposite major surfaces characterised by placing the flat member in a substantially air-tight chamber (10); locating an encapsulating sheet (13) in the air-tight chamber such that it is adjacent the first surface of the flat member, the sheet having first and second major surfaces, the first major surface of the sheet facing the first major surface (18) of the flat member (17); making the air pressure on the second major surface of the flat member to be significantly lower than the air pressure on the second surface of the sheet, thereby to cause the sheet to press against the flat member; and causing the encapsulating sheet to adhere to the first surface of the flat member, thereby to encapsulate said optical fibers. The method of claim 1 wherein: the encapsulating sheet is substantially impermeable to gas and extends across the air-tight chamber to divide the chamber into first and second chamber portions, the first chamber portion containing the flat member; and the making step comprises the step of applying a significantly lower air pressure to the first chamber portion than to the second chamber portion. The method of claim 2 wherein: the first chamber portion is partly defined by a first enclosure portion (11); the second chamber portion is partly defined by a second enclosure portion (12); and the first and second enclosure portions are clamped on opposite sides of the encapsulating sheet. The method of claim 3 wherein: the optical fiber comprises optical fiber portions which cross over other optical fiber portions; the encapsulating sheet is made of a thermoplastic material; and the causing step includes heating the thermoplastic sheet sufficiently to cause it to adhere to the first surface of the flat member. The method of claim 4 wherein: the first and second enclosure portions abut on opposite sides of the plastic sheet along an entire periphery of the substantially air-tight chamber; and the plastic sheet constitutes a gasket for preventing air from entering the air-tight chamber at the juncture of the first and second enclosure portions. The method of claim 5 wherein: the first chamber portion is connected to vacuum apparatus (22) for making a partial vacuum in the first chamber portion; and the second chamber portion is connected to a source of gas (24) for maintaining a predetermined relatively high air pressure in the second chamber portion. The method of claim 6 wherein: before the making step, both the first and second chamber portions are connected to vacuum apparatus (22) to form a partial vacuum in the first and second chamber portions, and the thermoplastic sheet is separated from the flat member by a spacer member (19); during the making step, the partial vacuum is applied to the first chamber portion to cause the second surface of the flat member to bear against the first enclosure portion; and the encapsulating sheet is sufficiently elastic to bear against both the spacer member and the first surface of the flat member. The method of claim 1 wherein: the flat member comprises an inflexible portion which defines its second surface and a plastic portion upon which the optical fibers are mounted; and after adherence of the encapsulating sheet to the first surface, the plastic portion is removed from the inflexible portion, whereby the optical fibers are encapsulated by the plastic sheet and the plastic portion.

Inventors: BURACK JOHN JOSEPH, HOLLAND WILLIAM ROBERT, SIMCHOCK FREDERICK, BURACK, JOHN JOSEPH, HOLLAND, WILLIAM ROBERT, SIMCHOCK, FREDERICK
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: B29C70
IPC codes: B29C51, G02B6, B29D11