Optical waveguides – With disengagable mechanical connector – Optical fiber/optical fiber cable termination structure
Reexamination Certificate
2002-05-20
2004-03-16
Patel, Tulsidas (Department: 2839)
Optical waveguides
With disengagable mechanical connector
Optical fiber/optical fiber cable termination structure
Reexamination Certificate
active
06705765
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a connector plug for terminating polarization maintaining (PM) optical fibers.
BACKGROUND OF THE INVENTION
In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other, or, put another way, to reduce insertion loss. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter MFR of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). The MFR is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.
However, in connecting or terminating polarization maintaining (PM) fibers, such is not the case. Many optical fiber components, such as, for example, interferometers and sensors, lasers, and electro-optic modulators, are extremely sensitive to and dependent upon, for proper operation, the polarization of the light. Even very slight alterations or changes in the light polarization orientation can result in wide swings in the accuracy of response of such devices. PM fiber has polarization-dependent refractive indices, and the speed of light in an optical fiber is inversely proportional to the magnitude of the refractive index. A birefringent optical fiber is one having two polarizations having different velocities of propagation, thus giving rise to a “fast” wave and a “slow” wave. In a PM fiber, the polarization of a linearly polarized light wave input to the fiber, with the direction of polarization parallel to that of the one of the two principal polarizations, will remain or be maintained in that polarization as it propagates along the fiber, hence the term “polarization maintaining.” If the polarization of the light wave is to be maintained at a splice or other connection, the principal axes of birefringence of the two joined fibers must be aligned in parallel, otherwise there will be polarization cross-coupling, i.e., crosstalk, which is highly undesirable. Thus, where two PM fibers, for example, are to be connected together, they should be terminated carefully to reduce the crosstalk during the connectorization process. Also, the connectors must be capable of aligning then maintaining the fiber orientation to the connector key position. Connectors with tolerances adequate for connecting non-PM fibers usually are inadequate for maintaining polarization alignment at the connector junction.
Typical PM connector requirements are an insertion loss of less than 0.3 dB, and the prior art PM connector arrangements comprise numerous, different connector configurations aimed at meeting these requirements for different connectors, such as an SC type connector as shown in U.S. Pat. No. 5,216,733 of Ryo Nagase et al. The connector of that patent comprises a ferrule body and a ring shaped flange having a keyway mounted on the periphery of the ferrule body. Alignment is achieved by rotating the ferrule body with respect to the flange keyway. The combination of ferrule and flange comprises a plug which is inserted into a push-pull SC connector having a key therein for mating with the flange keyway and springs bias the flange in the longitudinal direction to maintain the alignment.
In U.S. Pat. No. 4,784,458 of Horowitz, a splice joint for PM fibers is shown wherein aligned fibers are joined with UV curing epoxy, and the joint is overlaid with epoxy cement for rigidity. Such ajoint is permanent, and does not function as a connect-disconnect optical fiber connector.
U.S. Pat. No. 5,561,726 of Yao discloses an apparatus for controlling the polarization state of the light within a fiber by squeezing a portion of the fiber to produce a birefringent fiber, and the squeezer is then rotated to change the polarization of the light within the fiber. The device is not a connector, but is intended for use with polarization sensitive devices such as interferometers and electro-optic modulators, however, it may also be used with connectors for connecting two PM fibers.
It is common practice in the prior art for creating PM fibers to include a pair of rods in the fiber cladding which extend parallel to the core as shown in U.S. Pat. No. 4,515,436 of Howard et al. Such rods, which are preferably of glass, are, in manufacture of the fiber, included in the fiber preform from which the fiber is drawn. As the fiber is drawn, the rods are accordingly diminished in diameter and are located within the cladding, preferably on either side of the core. The rods have different thermal expansion characteristics than the surrounding glass, and the stress they exert on the core causes the index of refraction to change along that axis. The axes then have different indices of refraction value and thus propagate light at different speeds. Variations on the two rod arrangement are also known, such as the elliptical stress member disclosed in U.S. Pat. No. 5,488,683 of Michal et al. Also, squeezing the fiber to create birefringence, as shown in the aforementioned Yao patent is feasible. The two rod PM fiber, so called “Panda” type PM fiber, however, has proven quite satisfactory in use, and it is toward the connectorization of such a fiber that the present invention is directed, although other types of PM fibers may be used with the present invention.
SUMMARY OF THE INVENTION
In the copending U.S. patent application Ser. Nos. 10/151,450 and 10/151,130 are shown, respectively, an apparatus and methods of tuning a PM connector plug and an adapter for the connector plug of the present invention the principles of which are applicable to any of a large number of optical fiber connectors, but are embodied in a modified LC connector in those applications. For optimum performance, i.e., maximum transmission of a polarized beam, it is highly desirable to provide accurate rotational positioning of better than ±1° or even as accurate as <¼° between connectors equipped with polarization maintaining fibers.
The present invention is a connector plug for PM connectors which is intended for use with the adapter and tuning method of those Lampert et al. applications to achieve this desideratum. When a PM jumper cable, for example, is terminated by connectors, it is most desirable that the cable/connector combination be tuned to align the fiber slow axis with a reference point such as the connector key which may be the connector latching arm. In accordance with the present invention, there is provided a connector plug which is tunable to yield extremely accurate rotational
Lampert Norman R.
Lu Yu
Subh Naif T.
White Ian A.
Dinh Phuong
Fitel USA Corp.
Patel Tulsidas
Thomas Kayden Horstemeyer & Risley LLP
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