Fiber optic connector having low cost ferrule

Abstract: A fiber optic connector (10) has a low cost ferrule assembly (34,32) that protectively supports, positions and aligns an optical fiber. The ferrule (20) has an outer ferrule body (34) terminating in an alignment passage (34b) and which seats an inner support member (32) (steel or plastic capillary tube) within which the fiber extends. This ferrule (20) is particularly advantageous for use with unbuffered optical fibers and adhesiveless connectors.

Description: This invention relates to an optical fiber connector according to the preamble of claim 1 and to a method for the manufacture of an optical fiber connector according to the preamble of claim 4. This invention provides an optical fiber connector and a ferrule for use in such a connector. In particular, the invention provides a fiber optic connector having an improved ferrule structure that supports and positions the optical fiber in the connector for communication with a second fiber. The invention also provides a new ferrule structure, for a fiber optic connector, having advantages in manufacture ease and cost. Fiber optic communication cables typically include at least one light transmitting optical fiber clad in an optically insulating material. The cladding protects the glass fiber, and prevents dispersion of light out of the optical fiber. The fiber optic cable usually has a protective external buffer, typically of a plastic material, which may or may not be removed to terminate the cable. The growing use of fiber optic systems creates a need for a connector capable of coupling segments of light-conductive cable for maximal light transmission. To achieve efficient light transfer between fiber optic cables, the optical fibers must be aligned to high precision, as well as properly spaced with or without touching, whichever is specified. The alignment and spacing requirements are exceedingly demanding, due to the minute, micron-size diameter of the optical fibers being connected, coupled or otherwise terminated. A known fiber optic connector has a ferrule that holds the final length of the fiber in place so that it can abut another fiber at a faceted end. Each such connector ferrule is to have a precision bore that receives and thereby positions a fiber, to achieve the proper alignment. Even minute movement or errors in fiber position can cause significant optical losses. These problems can be even more acute with non-adhesive connectors, i.e. in which the fiber is secured mechanically rather than with epoxy or other adhesive. In at least one non-adhesive connector, a relatively long length of the fiber, with the buffer layer removed, is to be supported and positioned. If a ferrule having a large diameter bore terminating in a small opening is used to hold the fiber, the ferrule can contain a substantial length of unsupported, unbuffered fiber. This long unsupported fiber is subject to flex, which can cause excessive detrimental optical loss and even fiber breakage. Unwanted flexing of the fiber is particularly apt to occur during the polishing of the facet end of the optical fiber. Document GB-A-2,049,976 discloses an optical fiber connector comprising a body mounting a ferrule. The ferrule has an internal cylindrical metal sleeve that compresses a fiber in response to a fiber reduction in central bore parts of the sleeve. The compression holds the optical fiber in place. A problem associated with this approach is the metal clamping action against a relatively fragile glass fiber. Since clamping occurs by virtue of threaded tightening, it is difficult to ascertain the appropriate degree of clamping action. Too little would provide insufficient retention of the fiber, and too much could damage the fiber. Clamping action without precise control over the clamping degree results in a nonrepeatable connector. There is a need for an adhesiveless fiber optic connector with controlled fiber clamping. Document GB-A-2,068,142 discloses replacing a portion of fiber optic cable buffer with a polymer having different characteristics than the original buffer. The replaced polymer reduces slippage between the core and the buffer. Documents US-A-5,088,804 and US-A-4,679,895 disclose fiber optic connectors that secure an optical fiber mechanically, and without adhesives. As disclosed in document US-A-5,088,804, it is known to support the unbuffered fiber in a connector within a ceramic ferrule having a small and highly concentric bore through its total length. The precision bore extends to the facet end of the connector. The ceramic ferrule supports the fiber over much of its unbuffered length and hence aligns and positions it. However, such ceramic ferrules with relatively long precision bores are costly. They are used because small bores or channels of comparable length and of the accuracy needed cannot easily be made in non-ceramic materials with present technology. A major problem is that drilling or otherwise manufacturing a ferrule requires a through hole of small diameter and precise concentricity, typically in the order of approx. 0.13 mm (0.005 inch) diameter and approx. 13 mm (0.5 inch) length. Present manufacturing methods encounter significant problems in forming precision holes whose length is more than ten times their diameter. This problem has led to the use of the foregoing costly ceramic ferrules, or to adhesives in the connector. Accordingly, an object of this invention is to provide a structure for a fiber optic connector that supports a length of unbuffered optical fiber with dimensional precision, and with relatively low cost and readily fabricated connector parts. It is also an object of the invention to provide a method of making a relatively low cost connector ferrule which can support an unbuffered optical fiber with the proper concentricity and alignment for the fiber. Another object of the invention is to provide a ferrule useful with non-adhesive optical fiber connectors and characterized by relatively precise dimensions and low cost. A further object of the invention is to provide a ferrule which can be made of less expensive materials than ceramic ferrules and yet provides the proper fiber alignment for use in high quality optical fiber connectors. The invention provides an optical fiber connector as defined in claim 1 and a method for the manufacture of an optical fiber connector as defined in claim 4. The dependent claims define embodiments thereof. These and other objects and features of the invention will be apparent from the following description and the drawing. An advantage of the present invention is to provide an optic fiber connector that supports the length of an unbuffered optical fiber with dimensional precision with relatively low-cost and readily-fabricated connector parts. Another advantage of the present invention is the use of a ferrule as part of a non-adhesive optical fiber connector. A further advantage of the invention is that the ferrule can be made of less expensive materials than ceramic ferrules and still provide precise fiber alignment thereby resulting in a high quality optical fiber connector. A fiber optic connector according to the invention features a multi-piece ferrule for holding and supporting an unbuffered optical fiber. The assembled, multi-piece ferrule provides excellent support for long spans of unbuffered fiber. The ferrule is significantly less expensive than classic ceramic ferrules. The ferrule is part of a connector that includes a body that mounts the ferrule. The connector holds and positions a first optical fiber, that has an unbuffered final length leading to the facet, to abut a second optical fiber carried in a second connector. The ferrule has a back end mounted to the counter body and has a front, facet end proximate to the location of the abutment of the first and second optical fibers. A connector ferrule has two pieces, rather than a single one-piece structure. The two pieces are telescopically nested together, in a concentric and coaxial configuration. More particularly, the ferrule has a tubular external ferrule body having a relatively large central cavity substantially concentric about a ferrule axis. This central cavity, which is many times larger in diameter than the fiber, is open at the back end and has a fiber holder at the facet end. The fiber holder has a forward positioning section with a precision positioning bore, for directly positioning and supporting the fiber. This positioning bore of the ferrule body is minimally larger than the diameter of the unbuffered optical fiber, and is radially concentric with the ferrule axis. A second part of the ferrule is an inner support member that fits within the central cavity of the ferrule body. The inner support member has a channel throughout its length and which is substantially concentric with the ferrule axis. The diameter of the support member channel is such that the channel can support an unbuffered optical fiber without undue fiber flex. The assembled ferrule of the invention, and particularly the inner support member, has low thermal expansion and contraction properties at least over the working temperature range of the connector. These connectors normally operate over a temperature range of about -20°C to +60°C, and substantial expansion or contraction of the inner support member over this range can damage or misalign the fiber. Materials useful for making the inner support member are to provide sufficient support and thermal properties, and include thermoplastics such as polyether ether ketone, or metals such as stainless steel. The channel through the inner support member does not have to be as small or precise as the positioning bore in the ferrule body, provided it is sufficiently small to support and align the fiber. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diametrical sectional view of an adhesive-less fiber optic connector, partly assembled, and embodying features of the invention; Figure 2 shows the connector of Figure 1 fully assembled; Figure 3 is a plan view of the ferrule of the connector of Figure 1; Figure 4 is a longitudinal view of the ferrule of Figure 3, partly in section along line 4-4; and Figure 5 is a transverse cross-sectional view of the ferrule of Figure 3, taken along line 5-5. A connector according to the invention employs a ferrule to support and position a length of unbuffered optical fiber. The ferrule is an assembly of an outer ferrule body and an inner support member. The outer ferrule body has an elongated central cavity that is open at a back end and that terminates at a front end in a fiber holder which includes a support section having a precision opening. The fiber holder supports and gives precise alignment to an unbuffered optical fiber at the site of signal transfer with another optical fiber in a mated connector. Assembled inside the ferrule body, in the cavity, is a support member having a through channel that provides substantial support to the unbuffered optical fiber over a relatively long span. The channel provides this degree of support without having the precise dimensional structure required at the facet opening. The support member is telescopically seated inside the ferrule body, typically with a close fit or small interference fit. The support member supports the fiber from being subjected to substantial flex or bending, and provides significant alignment of the fiber. The ferrule body can be machined or molded with conventional practices at relatively low cost from common materials such as stainless steel and aluminum, as well as from synthetic polymers, e.g. plastics. The support member can also have low cost, by using commercially extruded tubing stock. One preferred material for the support member is polyether ether ketone (PEEK) tubing, although various others can be used. One suitable PEEK tubing is marketed by Upchurch Scientific of Oak Harbor, Washington, USA, under the product designation 1531B. Accordingly, the two-piece ferrule of the invention can replace the expensive ceramic ferrule of the prior art, and it provides a least equivalent performance at significantly less cost. Figures 1 and 2 show an adhesiveless optical fiber connector 10 of the general type descried in more detail in document US-A-5,088,804. The connector 10, shown partly assembled in Figure 1 and fully assembled in Figure 2, has a rear body 12. An outer body 14 telescopically seats the rear body within a central passage 16 and joins to another connector device with a bayonet nut 18. The forward end of the outer body 14 mounts an axially-projecting ferrule 20. The illustrated outer body mountingly receives the ferrule with a press-fit within a bore forward of the passage 16. An unbuffered optical fiber (not shown) is placed in an axial through passage 22 in the rear body 12. When the connector 10 is fully assembled, with the rear body 12 fully telescoped into the passage 16 of the outer body 14 as shown in Figure 2, the fiber is held in place by the compressive action of resilient spheres 24. The unbuffered fiber extends through the ferrule 20, along a central axial channel 26, and exits at an opening at the connector facet end 28. The facet opening, the ferrule channel 26, the passage 22 of the rear body 12 and the passage 16 of the outer body 14 are co-axial and centered on an axis 30, on which the optical fiber is centered and aligned. The ferrule 20, as shown in Figures 3 and 4, is an assembly of a fiber-supporting member 32 within a ferrule body 34. The illustrated preferred ferrule body 34 has a cylindrical outer wall 34a elongated along the axis 30. The outer body is hollowed with an elongated inner cavity 34b, preferably cylindrical, that is open at the back end, i.e. at the left in the drawing. The cavity 34b extends axially for most but not all of the length of the ferrule body. Axially forward of the cavity 34b is a short transition section 34c having funnel-like inwardly-tapering walls. A precision passage 34d extends axially from the transition section to the facet end of the ferrule (rightmost in the drawings). The transition from the funnel-like walls to the passage 34d is smooth and free of burrs. The passage 34d, transition section 34c and cavity 34b form a continuous channel or passage through the ferrule body. The passage 34d, the cavity 34b and the cylindrical wall are concentric with the axis 30, as shown in Figure 4. The outer wall 34a of the illustrated ferrule body has a minor chamfer 34g at the facet end; there is also a minor chamfer 34f at the back end of the cavity 34b, all as shown in Figure 4. The support member 32 is a hollow sleeve-like insert that is slideably assembled to seat, preferably with minimal clearance to assure secure assembly, within the cavity 34b of the ferrule body. The hollow passage 32a within the support member forms the ferrule channel 26 (Figure 1). More particularly, the illustrated preferred support member 32 has a tubular cylindrical shape, with a cylindrical outer wall 32b that seats with no significant clearance within the cavity 34b. The passage 32a is substantially concentric with the outer wall 32b on the axis 30. The axial length of the support member 32 is no longer than, and preferably as shown is slightly shorter than the cavity 34b. The support member accordingly can extend nearly the full length of the cavity from the transition section 34c to proximal the back end, as Figure 4 shows. Preferred materials for the ferrule body 34 are machinable or moldable solids with low thermal coefficient of expansion, such as stainless steel. The support member 32 can be of various materials that have a low thermal expansion coefficient, including thermoplastics and stainless steel. The support member 32 can be a commercially available extruded polymer tubing. The precision passage 34d of the ferrule body is dimensioned to support and position the optical fiber therein with precision, as is known in fiber optic connectors. The passage 32a in the support member 32 typically is larger and does not require such exacting precision. In one illustrative embodiment, for use with an unbuffered optical fiber of 125 microns diameter, the passage 34d is machined to have a diameter of 127 µm and the passage 32a has a diameter of about 0.25 ± 0.025 mm (0.010 ± 0.001 inch). The foregoing description is merely illustrative and those skilled in the art will understand other modifications to the described device. Such of the modifications and improvements are to be encompassed within the scope of the following claims.

Claims: An optical fiber connector for holding a first unbuffered optical fiber with selected precise alignment and positioning for communication with a second optical fiber; a) said connector having a body (14) mounting a ferrule (20); b) said ferrule (20) comprising a ferrule body (34) having a back end (34e, 34f) secured to said body (14) and a facet end (28) protruding outside from said ferrule body; c) said ferrule (20) having a central cavity (34b) about a ferrule axis (30) and a fiber holder (34d) located at the facet end; d) said central cavity (34b) being open at the back end of said ferrule body (34); e) said fiber holder (34d) having positioning means for supporting the first fiber, said holder terminating in a fiber-positioning and aligning passage radially concentric about said ferrule axis (30), said passage forming a precision passage dimensioned to support and position said unbuffered optical fiber therein with precision; and f1) compressing means (24) holding the first fiber in place at a location distant from said fiber holder (34d); characterised by : f2) an inner support member (32) disconnected from said body so as to be slidably assembled completely within said central cavity (34b) of said ferrule body (34) between said compressing means (24) and said fiber holder (34d, the end of said inner support member being spaced inside the ferrule body from the open back end of the central cavity of the ferrule body); g) said inner support member (32) having a fiber-supporting channel (32a) throughout its length radially concentric about said ferrule axis (30); h) said fiber-supporting channel (32a) supporting said unbuffered optical fiber over the entire length of said inner support member (32);and i) said fiber-supporting channel (32a) being larger than the precision passage of the fiber holder (34d) such that it supports said A connector according to claim 1, characterized in that a selected length of extruded tubing forms said support member (32). A connector according to claim 1, characterized in that said ferrule body (34) is machined or molded from non-ceramic material, and said support member (32) is formed from a selected length of extruded polymer tubing. A method of manufacturing an optical fiber connector as defined in claim 1 in which a first unbuffered optical fiber is in a selected precise alignment and positioning for communication with a second optical fiber, wherein the connector has a body (14) mounting a ferrule (20), and the ferrule (20) has a back end secured to said body (14) and a facet end (28), comprising the steps of: a) preparing a ferrule body (34) with a central cavity (34b) about a ferrule axis (30), said central. cavity (34b) being open at the back end of said ferrule body (34) and with a fiber holder (34d) at the facet end (28), said fiber holder having positioning means for supporting the first fiber terminating in a fiber-positioning and aligning passage (34b) radially concentric about said ferrule axis (30) formed as a precise passage dimensioned to support and position said unbufferd optical fiber therein with precision; characterised by the steps of: b) preparing an inner support member (32) disconnected from said body (14) so as to be slidably assembled completely within said control cavity, the support member having an unbuffered fiber supporting channel (32a) throughout its length radially concentric about said ferrule axis (30); c) telescopically assembling said inner support member (32) within said cavity (34b) of said ferrule body (34) between said fiber holder and a compressing means (24) of the connector, whereby said fiber supporting channel (32a) is concentric with said passage (34b) and the end of the inner support member is spaced inside the ferrule body from the open back end of the central cavity of the ferrule body; and d) said fiber-supporting channel (32a) being formed larger than the precision passage of the fiber holder (34d) such that it supports said unbuffered optical fiber with precision which is less exact than the precision of the fiber holder (34d). A method according to claim 4, characterized by the further steps of a) machining said ferrule body (34) from a non-ceramic material, and b) forming said support member (32) from a selected length of extruded polymer tubing.

Inventors: GRINDERSLEV SOREN, GRINDERSLEV, SOREN
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1993
Main classification: G02B6
IPC codes: G02B6