This invention relates to a connector plug for terminating polarization maintaining (PM) optical fibers.
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 xe2x80x9cfastxe2x80x9d wave and a xe2x80x9cslowxe2x80x9d 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 xe2x80x9cpolarization maintaining.xe2x80x9d 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 xe2x80x9cPandaxe2x80x9d 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.
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 xc2x11xc2x0 or even as accurate as  less than xc2xcxc2x0 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 positioning of the connector ferrules, or, more specifically, the fibers contained therein.
The connector plug of the invention comprises in a first embodiment, a tunable barrel assembly as shown in U.S. Pat. No. 5,481,634 of Anderson et al., the disclosure of which is incorporated by reference herein, a connector plug that includes the barrel-ferrule assembly for holding the end of an optical fiber extending axially therethrough and a housing for the assembly. A coil spring member contained within the housing surrounds the barrel, which is of tubular configuration, and bears against an interior wall of the housing and an enlarged flange member on the barrel, thereby supplying forward bias to the barrel-ferrule assembly relative to the housing. As is shown in the aforementioned U.S. patent applications, the barrel-ferrule assembly has an enlarged flange member which is hexagonal 1 in shape and has a tapered or chamfered leading surface that may be slotted. The housing, in turn, has a hexagonally shaped cavity, which provides any of six rotational positions for the flange and a tapered seating surface for the tapered surface of the flange. The dimensions of the cavity are such that the hexagonal barrel flange floats within the hexagonal cavity, in the Anderson et al. arrangement and can rotate about xc2x112xc2x0, which diminishes the tuning accuracy. In the PM connector of the Lampert et al. application Ser. No. 09/811074, this float is greatly reduced by dimensioning the hexagonal barrel flange, or nut, so that it is a virtual slip fit within the hexagonal cavity. Additionally, the flange is affixed to the barrel, hence, the barrel has only six positions, which are subject to the diminished float.
In the aforementioned U.S. patent application Ser. No. 09/811074, of Lampert et al, there is also shown a tunable nut arrangement wherein a hexagonal nut is a xe2x80x9clightxe2x80x9d press fit on the ferrule containing tubular portion of the barrel assembly, so that the nut may be rotated relative to the ferrule in extremely small increments, such as fractions of a degree. In the arrangement of that application, the nut may remain seated in the hexagonal seat in the connector plug body, and the ferrule rotated with respect thereto. As explained in that application, the xe2x80x9clightxe2x80x9d press fit is sufficient to prevent accidental or unwanted relative rotation after tuning. After tuning the nut (or flange) may be cemented in place. Such an arrangement is thus capable of producing the small increments of relative rotation that may be required for optimum PM tuning, and is a preferred arrangement for the first embodiment of the invention.
In accordance with the present invention, the latch of the plug, which includes a latching arm having latching shoulders on either side, preferably is of plastic material and has tapered sides, thereby imparting to the arm a cross-section shaped as a truncated wedge. The latching shoulders may also have tapered sides so that both the arm and the shoulders, alone or together have a taper. The adapter for use with the invention has a longitudinal slot therein which has sloping sides for receiving the tapered latching arm as shown in application Ser. No. 10/151,130. As is the practice with the latching arm, it is sufficiently resilient or elastic to snap into the slot once the latching lugs are engaged. In prior art arms and slot arrangements the slots and the arm both have straight sides, hence the arm is made slightly narrower than the slot to insure engagement, which gives rise to some play between the plug and the adapter which can alter the position of the polarization vector. The tapered arm and slot of the invention create a tight fit and eliminate such play so that the plug is firmly held in its latched position. In addition, the tapered engagement centers the plug relative to the adapter so that the ferrule/barrel assembly is properly aligned within the adapter.
Further in accordance with a second embodiment of the invention, the plug body has a rotatable extender cap therein which has longitudinally extending resilient arms, the distal ends of which bear against the hexagonal portion of the barrel assembly as a three jawed collet further to ensure alignment of the barrel and to eliminate virtually any float thereof. The barrel of the connector plug is an assembly comprising a tubular member which may have a notched end for tuning and an enlarged hexagonal nut affixed thereto on the outer surface of the tubular member and which has a tapered front section which finctions as a guide in conjunction with a tapered recess in the housing. As described hereinbefore, such an assembly is shown and described in the U.S. patent application Ser. No. 09/811074, the disclosure of which is incorporated herein by reference. Tuning of the connector is achieved by rotation of the extender cap which firmly holds the nut on the barrel. Hexagonal seat in the plug body is illuminated, and hence, the ferrule contained in the barrel can be incrementally rotated to produce, where necessary, incremental changes in the angular orientation.
In both embodiments of the invention, the latching arm has a tapered cross section as described hereinbefore, and the fitting of the parts to each other approaches a slip fit, so that float is minimized.