Optical Fiber Chuck

The present invention generally relates to the field of fiber optics. More particularly, this invention relates to an apparatus for aligning optical fibers so that they may be joined to each other by splicing techniques.

The field of fiber optics is generally concerned with the transmission of light along a transparent fiber structure or core which has a higher refractive index than its surroundings. Typically, an optical fiber consists of a core of transparent material having a refractive index n1 surrounded by a layer of transparent cladding material having a refractive index n2 which is lower than n1. The core and the cladding form a guiding region. Usually, the core and guiding region have a circular or elliptical shape. Fibers with elliptical cores have a minor and major axis such that light travels along one of these axes. Such a fiber is called a polarization-holding fiber. The optical fiber can also include an outer protective layer.

Currently, it is possible to manufacture long, continuous strands of optical fiber which may propagate signals without substantial attenuation over long distances. It is also possible to manufacture the fiber structure as an optical waveguide wherein only preselected modes of light propagate in the fiber. By limiting wave propagation through the fiber to a single mode, the bandwidth of the optical fiber may be exceedingly high to provide a high information-transfer capacity.

In sensing and control systems, a fiber-optic transducer is used that exploits either multi-mode or single-mode light propagation in an optical fiber. While multi-mode sensors use amplitude variations in the optical signals to sense and transmit the desired information, single-mode sensors use phase variations rather than amplitude variations. The single-mode sensors usually involve mechanisms for altering such properties of the fiber as path length or index of refraction to effect the desired phase variations in the optical signal. Thus, in contrast to multi-mode sensors, in single-mode sensors the uniformity and mechanism of light propagation and hence the characteristics of the fiber are especially critical.

Single-mode sensors are sensitive to the state of polarization of the light in the fiber. Thus, for single-mode transducers, it is desirable to use elliptical-core or other kinds of polarization-holding fiber. See, e.g. McMahon et al, "Fiber-Optic Transducers," IEEE Spectrum, December, 1981, pages 24-27. Most of these polarization-holding fibers are capable of preserving the polarization of signals along two different, usually orthogonal, axes, such as the major and minor axes of an elliptical core.

As is known in the literature, e.g., Dyott et al., "Preservation of Polarization in Optical-Fiber Waveguides with Elliptical Cores," Electronics Letters, Jun. 21, 1979, Vol. 15, No. 13, pp. 380-82, fibers with elliptical cores and a large index difference between the core and cladding preserve the polarization of fundamental modes aligned with the major and minor axes of the ellipse, i.e., modes having their electrical fields parallel to the major and minor axes of the ellipse. If the core-cladding index difference and the difference between the lengths of the major and minor axes of the ellipse are sufficiently large to avoid coupling of the two fundamental modes, the polarization of both modes is preserved.

Commonly, it is necessary to splice together optical fibers. Optical fibers are spliced together by aligning two similar fiber ends, bringing the two fiber ends into contact and bonding the two ends together. Bonding is accomplished by fusing or melting the two fibers together, mechanically holding the fibers together or adding a bonding agent in the interface between the two abutting surfaces of the fiber ends. Optical fiber splices suffer from two types of attenuation losses. Intrinsic losses arise from differences between the two fibers being joined, such as differences in the refraction index and the size and shape of the fiber core. Extrinsic losses occur from the misalignment of the fiber ends and contamination.

In a typical fusion splicer for optical fibers, the two fibers that are to be spliced to each other are laid in a holding "chuck," which consists, at a minimum, of a metal rod with a precision machined V-groove and a clamping mechanism to hold the fibers in place. The clamping mechanism is often a flat spring with a small weight on top. In that case, the fiber must protrude above the top of the V-groove so that the spring contacts the fiber. The clamping mechanism can also include the application of a vacuum to the bottom of the V-groove.

Active alignment techniques ensure proper alignment of the two fibers by launching light into the fiber and detecting the light at the other end. Polarization-holding fibers such as elliptical core fibers require that the major and minor axes of the core properly align. If the two fibers do not properly align, then an alignor moves one fiber in two orthogonal directions orthogonal to the fiber axis until a maximum amount of light is detected. Once aligned, the gap between the fiber ends is then decreased by moving one fiber in an axial direction. The whole process is repeated until an absolute maximum amount of light appears at the detector. Another alignment method is described in U.S. Pat. No. 5,323,225. After alignment, the holding chucks keep the fibers aligned by maintaining the position of the optical fibers relative to each other.

These holding chucks are essential for positioning optical fibers in order to align and splice the fibers together. The "chuck" components are usually machined from metal and, while softer than the silica-based fiber, can damage the fiber surface. This damage results from particles (from the machine tools) becoming embedded in the machined chuck components. These particles can scratch the fiber surface and initiate a fracture of the fiber.

It is an object of the present invention to provide an improved optical fiber holding chuck that reduces the initiation of fractures in an optical fiber caused by particles embedded in machined chuck components.

It is another object of the present invention to provide an improved optical fiber holding chuck that contributes to the repeatability of insertion of the optical fiber in a splicer.

It is still another object of the present invention to provide an improved optical fiber holding chuck where the chuck mounting fixture does not require the precision machining required for a conventional metal chuck.

It is a further object of the present invention to provide an improved optical fiber holding chuck that easily aligns a fiber with a non-circular cross-section.

Other objects and advantages of the present invention will be apparent from the following detailed description and the accompanying drawings.

In accordance with the present invention, the foregoing objectives are realized utilizing an optical fiber holding arrangement comprising a fiber chuck made of a vitreous material, such as glass, plastic or ceramic or any similar material, having a fiber groove for an optical fiber. The surface of the fiber groove is such that imperfections and embedded particles are minimized to prevent damage to the optical fiber during alignment and splicing.

Preferably, the fiber chuck is a rod made of a vitreous material which can be drawn from a preform. By drawing a fiber chuck from a preform, imperfections in the preform are reduced in the fiber chuck. As such, the fiber chuck, especially the fiber groove, can be made with fewer imperfections or embedded particles that can damage optical fiber. Alternatively, the rod can be made of vitreous or similar material which can be extruded through a die. By extruding the rod through a die, a rod with good detail can be achieved which eliminates the imperfections or embedded particles that can damage the optical fiber.

The optical fiber lies in the fiber groove of the fiber chuck, and a clamping mechanism holds the optical fiber in place. The fiber chuck preferably mounts to a chuck mounting fixture, and the chuck mounting fixture is engaged by a fiber positioning mechanism that positions the chuck mounting fixture together with the fiber chuck, the optical fiber and the clamping mechanism to bring together and properly align fibers for splicing while reducing the risk of damage to the fiber.

In a preferred embodiment of the present invention, the fiber chuck of the present invention is a silica-based glass rod with one sector of the glass rod missing, forming the fiber groove along the length of the glass rod. The desired cross-sectional configuration for the fiber chuck can vary depending on the cross-sectional configuration of the optical fibers that will be held. The glass rod can be drawn with the desired fiber groove in the same manner as optical fibers are drawn with a desired cross-sectional configuration. Thus, the glass rod is preferably made by forming a preform having the desired transverse cross-sectional configuration and drawing the glass rod from the preform, with the drawing rate and temperature being controlled to produce a glass rod with a cross-sectional configuration similar to that of the preform. Such a preform can be made by machining a sector from a silica-based glass rod. The fiber chuck is drawn from the machined preform at a drawing rate and a controlled temperature to produce a uniform fiber chuck with a cross-sectional geometry substantially the same as that of the preform but on a smaller scale, thereby reducing the imperfections from the preform. The uniformity of the glass rod contributes to the repeatability of insertion of the fiber into the splicer.

In an alternative preferred embodiment of the present invention, the fiber chuck of the present invention is an extruded, vitreous rod having a cross-sectional shape with a fiber groove along the length of the rod. The desired cross-sectional shape for the fiber chuck can vary depending on the optical fibers that will be held. Accordingly, the fiber chuck is conventionally extruded through a die having the desired cross-sectional configuration. The fiber chuck is extruded to minimize imperfections and embedded particles in the fiber chuck that can damage the optical fiber during alignment and splicing.

Accordingly, the optical fiber holding chuck of the present invention reduces the risk of fiber fractures or scratches caused by particles embedded in machined chuck components because the present invention reduces the number of embedded particles or imperfections in the fiber groove of the chuck. Moreover, the fiber groove of the fiber chuck is smoother than any practical machined surface.

FIG. 1 shows a cross-sectional view of the optical fiber chuck arrangement utilizing the fiber chuck of the present invention;

FIG. 2 shows a plan view of the optical fiber chuck arrangement utilizing the fiber chuck of the present invention;

FIG. 3 shows a partially schematic side elevation of an apparatus for drawing the fiber chuck according to the present invention; and

FIG. 4 shows a cross-sectional view of a die for extruding the fiber chuck according to the present invention.

Referring now to the drawings, and more particularly to FIG. 1, there is illustrated an optical fiber chuck arrangement in accordance with the present invention, generally designated by the reference numeral 10. The splicing chuck 10 comprises a chuck mounting fixture 12. The chuck mounting fixture 12 can be an existing splicing chuck for a larger diameter optical fiber. The chuck mounting fixture 12 is engaged by a splicer positioning mechanism (not shown) and has a chuck groove 14. A fiber chuck 16 mounts into the chuck groove 14 of the chuck mounting fixture 12. The fiber chuck 16 is a rod, preferably made of silica-based glass or other materials, having a fiber groove 18 along the length of the rod. Preferably, a glass rod is machined to remove a sector of the glass, forming a fiber chuck preform. The preform is then drawn into a fiber chuck 16 of the same cross-sectional shape as the preform but on .a smaller scale. An optical fiber 20 rests in the fiber groove 18, and a spring clamp 22 contacts and holds the fiber 20 in place.

In a preferred embodiment, a length of the fiber chuck is glued into a chuck groove of the chuck mounting fixture. The chuck mounting fixture is similar to the existing optical fiber chuck, having a V-groove being engaged by a splicer positioning mechanism. The chuck groove for the chuck mounting fixture, however, holds the fiber chuck while the fiber chuck holds the optical fiber. The chuck groove of the chuck mounting fixture does not require precise machining, unlike the V-groove of existing chucks, because the fiber chuck of the present invention acts as a protective barrier for the optical fiber. Additionally, a metal V-groove is not likely to seriously damage the fiber chuck even if not precisely machined because the fiber chuck is not subject to bending stresses. In any case, some surface damage to that portion of the fiber chuck that contacts the chuck mounting fixture is tolerable. Thus, the fiber chuck can be used repeatedly because it is unlikely to be seriously damaged by the splicing operation. A metal V-groove of an existing chuck, however, can seriously damage an optical fiber if not precisely machined because the optic fiber is subject to bending stresses, and a scratch from embedded particles can lead to a fracture in the fiber.

The existing metal fiber optic chuck can damage optical fibers because particles embedded or imperfections in the machined V-groove of existing chucks can scratch the optical fiber. The optical fiber splicing chuck 10 of the present invention, however, is less likely to damage optical fibers. The preferred glass rod can be made of silica-based glasses, such as fused silica or borosilicate glass, and is much smoother than any practical machined surface that has a reduced number of embedded particles or imperfections that might scratch the optical fiber 20. Furthermore, the fiber chuck 16 can be made from the same material as the fiber 20 or any material that can be drawn down from a preform or extruded to form a suitable fiber groove surface, but utilizing borosilicate glass or other similar material for the fiber chuck 16 ensures, depending on the fiber material, that the fiber chuck 16 is softer than the fiber 20 and, therefore, reduces any possibility of surface damage to the fiber 20. In addition, the spring clamp 22 makes light contact with the fiber 20, is softer than the fiber 20 and can be easily cleaned to remove impurities. These impurities could scratch the optical fiber 20 during alignment and splicing and lead to a fracture of the optical fiber 20. Alternatively, the fiber chuck 16 can be extruded through a die and can be made to reduce the embedded particles or imperfections of the fiber groove 18. As such, the fiber chuck can be made of a material that is extruded through a die, such as a glass, plastic or ceramic.

The optical fiber lies in the fiber groove of the fiber chuck, and a spring clamp holds the optical fiber in place. The optical fiber can possess one of a variety of non-circular cross-sections, such as D-fibers, quad-fibers and slotted fibers. Additionally, the non-circular cross-section of the optical fiber can have a predetermined geometric relationship to the guiding region of the fiber such that the orientation of the guiding region can be ascertained, and the alignment of these fibers is accomplished automatically by placing the fiber in a fiber chuck with the proper cross-section. For example, the flat side of an optical fiber with a D-shaped cross-section (D-fiber) can be parallel to the major or minor axis of the elliptical core of the D-fiber. Consequently, the alignment of the D-fiber is automatically accomplished by placing the D-fiber in the fiber chuck of the present invention with the flat side up and engaging a spring clamp across the flat side of the fiber.

In the illustrated embodiment, the optical fiber 20 protrudes above the fiber groove so that the spring clamp can contact the optical fiber. This requires that the diameter of the glass rod for the fiber chuck be properly chosen. Another embodiment utilizes an alternative clamping mechanism, such as a vacuum applied to the bottom of the fiber chuck, whereby the optical fiber need not protrude above the fiber groove.

In order to splice the optical fiber 20 to another optical fiber or optical device (not shown), an operator must ensure that the two fibers are properly aligned. The operator can employ active alignment techniques to determine whether the optical fiber 20 aligns with the other optical fiber. Active alignment techniques include launching a light into one of the fibers, detecting the light at the other end and maneuvering one fiber until maximum light is detected. Another active rotational alignment technique is described in U.S. Pat. No. 5,323,225. These techniques will ensure that the major and minor axes of two polarization-holding fibers properly align. As discussed above, the use of the fiber chuck 16 significantly reduces the chance of particles scratching the fiber 20 during alignment. The spring clamp 22 maintains the position of the optical fiber 20. Once the optical fiber 20 aligns with the other optical fiber, the splicer positioning mechanism of an Ericsson fusion splicer or any other suitable splicer brings the two fibers together for splicing without altering the alignment of the fibers.

FIG. 2 illustrates a plan view of the optical fiber splicing chuck of the present invention. The fiber chuck 16 is a straight, uniform vitreous rod having the fiber groove 18 along the length of the rod. The uniformity of the fiber chuck 16 contributes to the repeatability of insertion of the fiber in the splicer. The optical fiber 20 lies in the fiber groove 18. As discussed above, the fiber 20 rests in the fiber groove 18 during the alignment process. The fiber chuck 16 mounts into the chuck groove 14 of the chuck mounting fixture 12. The chuck groove 14 of the mounting fixture 12 does not require precision machining because the fiber chuck 16 acts as a protective barrier between the chuck groove 14 and the optical fiber 20. The existing chuck, however, requires precision machining because the existing chuck holds the optical fiber. The optical fiber would be easily damaged by a metal surface that is not precision machined because the optical fiber would be subject to bending stresses during fiber alignment. Thus, the optical fiber splicing chuck of the present invention does not require precision machining for its chuck mounting fixture, but the existing chuck requires precision machining for the V-groove that holds the optical fiber.

Accordingly, the fiber chuck of the present invention is made from a vitreous material, such as glass, ceramic or plastic. This material can be extruded through a die to form a rod having a smooth fiber groove with a reduced number of imperfections. Preferably, as previously described, the fiber chuck preform having a desired transverse cross-sectional shape is drawn into a fiber chuck 16 having the desired cross-sectional shape as the preform but on a reduced scale. The fiber chuck 16 can be drawn from a preform just as an optical fiber having a desired transverse cross-sectional shape is drawn from a preform as described in U.S. Pat. No. 4,755,021 to Dyott, herein incorporated by reference.

FIG. 3 shows a drawing machine suitable for precise control of the drawing process. In order to heat the preform to approximately the softening temperature, the central component of the drawing machine is an induction furnace generally designated 30 comprising an external induction coil 31 and an internal graphite toroid 32. The induction coil 31 is energized by a radio-frequency power source 33 so that the electrical heating currents are induced in the graphite toroid 32, the resulting temperature being measured by an optical pyrometer 34 and monitored by a control unit 35 adjusting the power source 33. In order to prevent the graphite toroid 32 from burning, the toroid 32 is disposed within a glass cylinder 36 which is filled with a relatively inert glass such as argon from a supply 37.

A fiber chuck preform 38 is fed into the top of the cylinder 36 and passes through the center of the graphite toroid 32. The toroid 32 is heated white hot, causing the preform 38 to soften. The drawing of the fiber chuck 16 from the preform 38 occurs approximately at the center of the toroid 32. The toroid 32 has legs 41 which stand on a support ring 42 attached to the glass cylinder 36.

The critical parameters affecting the drawing process are the feed rate V.sub.p of the preform 38 toward the drawing point, the temperature at the drawing point, and the rate V.sub.f at which the fiber chuck 16 is drawn from the drawing point. The temperature and rate of drawing V.sub.f set the tension at which the fiber chuck 16 is drawn. The rate of feed V.sub.p of the preform is established by a vertical linear slide generally designated 44 having a lead screw driven by a drive motor 45. At the upper end of the slide 44 is a block 46 into which the top of the preform 38 is clamped. The rate of drawing V.sub.f is established by a capstan wheel 48 below the lower end of the glass cylinder 36. The fiber chuck 16 can be cut to the appropriate length for mounting in a chuck mounting fixture.

Alternatively, FIG. 4 shows a cross-sectional view of a die 50 for extruding the fiber chuck according to the present invention. The extruded fiber chuck is made of a vitreous material, such as glass, ceramic or plastic. In the case of a ceramic material, such as a ceramic with alumina particles, the fiber chuck can also require baking in an oven.

The present invention has been described as being used with an optical fiber splicer. The present invention, however, can be used in any situation where the present invention would effectively maintain the position of a fiber without damaging the fiber surface. If the fiber is an optical fiber, the present invention can assist in joining optical fibers to other optical components, such as integrated optic circuits, lasers and lenses. Additionally, the spring clamp utilized in a preferred embodiment of the present invention can take the form of any clamping mechanism to hold the fiber in place, such as applying a vacuum to the bottom of the fiber groove 18. In a preferred embodiment of the present invention, the fiber chuck mounts into the chuck groove of the chuck mounting fixture, but the fiber chuck does not require a chuck groove for mounting to the chuck mounting fixture. The present invention has also been shown with a circular optical fiber, but the present invention is very effective in aligning non-circular fibers. For example, if a D-fiber is placed in the fiber groove, the alignment of two D-fibers occurs automatically by placing a straight edge spring clamp against the flat top of the D-fiber. Furthermore, a fiber chuck with a different cross-section than the one specifically described above can be used.

Thus, the optical fiber splicing chuck of the present invention and many of its attendant advantages will be understood from the foregoing description, and various modifications may be made in the form, construction and arrangement of the components thereof without departing from the spirit and scope of the invention or sacrificing all of their material advantages, the form described above being merely a preferred or exemplary embodiment thereof.