Optical waveguides – Polarization without modulation
Reexamination Certificate
2001-09-28
2004-08-24
Bovernick, Rodney (Department: 2874)
Optical waveguides
Polarization without modulation
C385S078000
Reexamination Certificate
active
06782146
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical telecommunication systems and, in particular, to devices for combining, splitting, and isolating light beams and methods for making the same.
2. Technical Background
Three port combining and splitting packages are widely used in local and long distance optical telecommunication networks. These networks comprise various polarization combining and splitting assemblies as part of Raman amplifiers and to increase the number of channels in a system. The necessity to design reliable optical devices for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime, is of significant importance. A typical example of such optical devices is a polarization-splitter package. A typical beam-splitter package comprises two assemblies. One assembly comprises one input single-mode (SM) optical glass fiber inserted into a single-capillary ferrule to produce a fiber-ferrule sub-assembly, a collimating lens, and a prism. A variety of optical blocks such as such as combiners, combiner-isolators, splitter-isolators, isolators, and the like are also substituted for the prism to form other useful devices. The optical components of this single-mode fiber assembly are embedded into an insulating glass tube, which in turn is mechanically protected by a metal housing. In a typical 3-port package the above single-mode fiber beam-splitting assembly is combined with an output collimating assembly leading to a pair of polarization-maintaining (PM) optical fibers. Beam-splitting packages are expensive and represent a significant cost in a typical communication system. Further, beam-splitting packages typically exhibit insertion losses higher than desired, resulting in degraded overall performance of the communications system or module. The problem is particularly acute during exposure to ambient operating conditions where temperature is variable.
Typical input glass ferrules enclosing the single-mode fibers employ a single capillary with relatively short (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such input ferrules, the optical fiber is subjected to an S-bending over the short conical end portion which typically exceeds 50% of the fiber diameter (for a fiber having a 125 &mgr;m diameter) on a span of about 6 to 10 diameters in length. This excessive micro bending increases the insertion losses. Fiber-ferrule subassemblies employing such ferrules are manufactured by inserting the optical fiber stripped of its polymer coating into the ferrule capillary; epoxy bonding the fiber into the ferrule capillary, including the conical end portion; grinding and polishing an angled facet on the fiber-ferrule; and depositing on the polished surface an anti-reflection (AR) coating. Once finished, the fiber-ferrule is aligned and assembled with the collimating lens and then embedded into the insulating glass tube, which, in turn, is protected by a metal housing.
There are two different technical solutions used in the design of bonds securing the components of an optical assembly. A low compliance bond between thermally well matched glass fibers and the glass ferrule is an approach commonly used by some manufacturers. The adhesives used are heat-curable epoxies with high Young's modulus (E>100,000 psi) and moderate to high thermal expansion coefficients (&agr;=40-60 10
−6
° C.
−1
). A typical example would be 353 ND EPO-TEK epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched glass tubes with metal housings and glass optical elements with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150° C. in the presence of moisture, are typically characterized by an extremely low Young's modulus (E<500 psi) and high thermal expansion (&agr;=180-250 10
−6
° C.
−1
). A typical example would be DC 577 silicone, which can be used to bond, for example, a metal optical filter holder to a collimating lens.
Adhesive bonding with subsequent soldering or welding is used to encapsulate a polarization-splitting assembly into a three-port package. Such a polarization-splitting package enclosure, which is typically formed of six to eight concentric protective units, has micron transverse tolerances. Maintaining these tolerances requires precision machining and may require time-consuming alignment and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering may include several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive which can result in repositioning of optical components and a shift in the optical performance. With such design, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
However, it is desirable to obtain a high accuracy thermally compensated optical multiple-port package that can be relatively inexpensive, reliable, and have a low insertion loss. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components but also to compensate for and minimize the thermally induced shift in optical performance. Further, it is desirable to obtain a multiple-port package, such as six port packages, with the same qualities since they further reduce costs and reduce size. Thus, there exists a need for such optical packages and a process for manufacturing such optical packages, which is miniaturized, has a low insertion loss, is inexpensive to manufacture, and which results in a device having reliable, long-term operation.
SUMMARY OF THE INVENTION
The present invention provides a dual polarization combiner-splitter package and provides a method of manufacturing the package from components such as input ferrules, collimating lenses (e.g. aspheric lens), optical fibers, and prisms, utilizing bonding adhesives in a manner which allows the alignment of the individual components relative to one another with a precision and a manufacturability that makes it possible to produce commercial devices having six or more ports. This had heretofore not been achieved. In one aspect, the invention includes an improved input ferrule and prism holder which permits bonding through the utilization of UV and thermally curable adhesives and improved thermal curing to reduce relevant internal stresses in the assembly so formed. For assemblies having multiple pairs of fibers (e.g., six port devices) the invention also provides improved fiber ferrule designs and manufacturing methods for devices that have low IL, operate over a wide temperature range, are reliable, and cost effective.
In one aspect of the invention, improvements to fiber ferrules are provided including capillary designs and tolerances. The invention provides designs for capillaries which resist movement of the optical fibers during adhesive curing, soldering, welding, and environmental thermal changes. One technique uses washers to precisely position optical fibers in a capillary. Yet another aspect of the invention is the selection of optical fibers based on geometric properties such as: outer (cladding) diameter, circularity of the cladding (ovality), and core concentricity. In another aspect, the invention teaches matching the separation distance (SD) between optical fibers on each end of the package. Tolerances for the separation distances are provided which make possible the commercial manufacturability of six-port devices.
Methods embodying the present invention include the steps of providing ferrules with capillaries having certain shapes and satisfying predetermined tolerances for the walls of the capillaries, providing single-mode optical fibers and polarization mode optical fibers satisfying predetermined tolerances for outer diameter, ovality, and ce
Hellman Scott M.
Marx David S.
Paolini Bryan P.
Townley-Smith Paul A.
Ushinsky Michael
Bovernick Rodney
Corning Incorporated
Price Heneveld Cooper DeWitt & Litton
Redman Mary Y.
Song Sarah U
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