Finishing techniques for lensed optical fibers

Abstract: A method is disclosed for processing an optical fiber (10) to include an integral lens (24) and a metallized outer coating (18). The fiber is first etched to include a tapered transition region and a thinned end region. The etched fiber is subsequently metallized and covered with a protective coating (20). A wet chemical etching technique is used to form an integral lens over the endface of the fiber. The protective coating is then stripped and any free metal is removed. The method allows for the fiber metallization to be in relative close proximity to the lens such that the fiber may be soldered into place near its endface, limiting any subsequent movement of the fiber relative to the aligned optical device.

Description: The present invention relates to a method of processing an optical fiber. For most optical communication applications, it is necessary to align and fix the position of an optical fiber with respect to a transmitting or receiving device. In general, optical fibers are often processed to include a metallic outer coating such that the metal may be soldered to a fixed reference plane and facilitate alignment. The metallic coating may further allow for a fiber to be hermetically sealed to the optical device package. In order to increase the coupling between an optical device and fiber, a lens may be inserted in the optical path between the device and the fiber. The lens may either comprise a discrete element which must be aligned to both the device and the fiber, or a device which is attached directly to the endface of the fiber. When utilizing a fiber including a metallic coating, problems may exist with the latter arrangement where the presence of the metal may interfere with the integrity of the attachment. U.S. Patent 4,622,055 issued to K. Mathyssek et al. on November 11, 1986 discloses a method for removing a metallic coating from the fiber tip area such that a lens may be epoxied onto the endface of a bare glass fiber. In particular, the fiber is passed through an arc or flame where it is softened to the point that the glass constricts and separates from the metallic coating. The fiber is then drawn and cleaved such that an endface is formed which is free of the metallic coating. A lens may then be attached to the cleaved endface. A problem may arise, however, in controlling the attachment of the lens to the fiber tip, which can result in increasing the time and expense involved in forming such a lensed fiber. In contrast to the process of attaching a lens to the fiber endface, there exist various wet chemical etching techniques for forming an integral lens directly on the fiber endface. By controlling various etching parameters, for example, the fiber cladding may be removed such that the core region remains exposed. Other techniques exist for preferentially etching the cladding with respect to the central core region. However, there have arisen problems in both tapering a lensed fiber and applying a metallic coating to such fibers including an integral lens, since these processes have been found to affect the properties of such an integral lens. Therefore, a need remains in the prior art for a method of finishing an optical fiber which does not alter the property of an integral lens formed on the endface thereof. According to the present invention there is provided a method as defined in claim 1. The need remaining in the art is addressed by the present invention which relates to a finishing technique for optical fibers and, more particularly, to tapering and metallization techniques appropriate for optical fibers including a lensed endface. In accordance with an exemplary process of the present invention, the following steps may be performed: (1) etching a stripped optical fiber to form a tapered transition region and a thinned end region; (2) metallizing the tapered fiber; (3) covering the metallization with a protective overcoat; (4) cleaving the fiber in the transition region; (5) lensing the exposed fiber endface; and (6) removing the protective overcoat. In some applications, the metallization step is not required. The tapering and protective coating steps are utilized to maintain the desired outer diameter of the fiber endface during the subsequent lensing operation. In other applications, steps (2) and (3) may be combined and step (6) eliminated when a relatively thick metallization layer is used as both the final metal layer and the protective overcoat. Brief Description of the Drawing Referring now to the drawings, where like numerals represent like parts in several views: FIGs. 1-8 illustrate, in sequence, various steps incorporating the techniques of the present invention utilized in forming a finished optical fiber including an integral lens. Detailed Description An end section of an optical fiber 10 is illustrated in FIG. 1, where outer layer 12 (a conventional polymer coating) is removed using any suitable stripping process well-known in the optical fiber processing art. Subsequent to the stripping of layer 12, fiber 10 is etched to form a transition region 14 and thinned end region 16, as shown in FIG. 2. For example, a hydrofluoric acid bath may be used to form the structure of FIG. 2. For applications requiring the use of a metal coating (for attaching the fiber to an alignment fixture, for example), a metallic coating 18 is then formed over outer surface 11 of fiber 10, as shown in FIG. 3. In one embodiment, coating 18 may comprise a tri-layered structure of titanium, platinum and gold, where the layers are consecutively sputtered over surface 11 of fiber 10. Alternatively, coating 18 may be formed by electroplating metals such as nickel or gold onto the fiber surface. A combination of deposited and plated nickel, followed by a layer of plated gold may also be used. It is to be understood that the techniques of the present invention are not limited by either the materials utilized to form the metallic coating or the process used to apply the coating to the fiber. To protect metallic coating 18 from subsequent processing operations, preferably a protective overcoat 20 is deposited to substantially cover layer 18, as shown in FIG. 4. Protective overcoat 20 may comprise a resist, or any other suitable material which will be impervious to acids or any other materials used in subsequent etching (i.e., lensing) operations. The coated fiber is then cleaved in the region of the transition region 14 or the thinned region 16, or between those regions, as indicated by dotted line A in FIG. 4. As mentioned above, there may be applications where a metallic coating is not required. In this case, protective overcoat 20 is formed directly on surface 11 of fiber 10, as shown in FIG. 5. As with the above-described embodiment the fiber of FIG. 5 is cleaved along the indicated dotted line in the region of transition region 14, of the thinned end region 16 or between them. Here, the existence of protective overcoat 20 during the lensing operation results in maintaining the diameter d of fiber 10 as the lens is being formed. The lensed fiber may then be inserted in conventional fiber optic connectors which are designed for use with a fiber of the predetermined diameter d . Alternatively, applications may exist where the combination of metal layer 18 and preferred protective overcoat 20 may be replaced by a relatively thick metal layer 180. FIG. 6 illustrates an exemplary fiber 10 of this form. Here, the outer portion of metal layer 180 may be etched during the subsequent lensing operation, as indicated by dotted line B. However, a sufficient underlying portion remains at the end of the etch process such that attachment of the fiber to an alignment fixture may be achieved. Once the fiber is cleaved, the exposed endface 22 of fiber 10 may be lensed, as shown in FIG. 7 (which illustrates an enlarged view of the end portion of fiber 10). One exemplary differential lensing technique requires the exposure of endface 22 to a combination of an etchant (e.g., hydrofluoric acid or a buffered oxide etch) and a treating agent (e.g., acetic acid or citric acid) which preferentially etches the fiber cladding material with respect to the core area and forms an integral lens shape comprising a frustrum of a cone over the core area or some such similar shape. This particular differential lensing technique is disclosed in detail in co-pending U.S. Application No. 486,630, filed February 28, 1990 and assigned to the assignee of the present invention. As shown in FIG. 7, the lensing process results in the formation of a lens 24 at endface 22 of fiber 10. It is to be understood that there exist various other chemical etching processes which may be used to form a lens during the procedure of the present invention. During the etch process, the perimeter of fiber 10 may be undercut, forming edge regions 26, as illustrated in FIG. 7. At the completion of the lensing operation, protective coating 20 (if present) may be removed using standard treatments for the removal of the resist, or the like. The free portion of metallic coating 18 extending beyond the exposed end surface of fiber 10 (i.e., edge portions 26 and lens 24) may be removed utilizing a conventional ultrasonic cleaning technique. The resultant metallized, lensed fiber is illustrated in FIG. 8. As shown, the process of the present invention results in forming a fiber including metallization 18 in relative close proximity to lens 24. Therefore, portion 18′ of coating 18 near the tip of fiber 10 may be soldered to an alignment fixture (not shown), resulting in limiting any subsequent motion of the fiber relative to an aligned optical device (not shown). As mentioned above, a particular lensing operation may result in forming undercut regions 26 such that an end portion of fiber 10 is not metallized. However, metal layer 18 will be included along the tapered transition region 14 and is in relatively close proximity to lens 24 such that attachment near the fiber endface may be realized.

Claims: A method of processing an optical fiber (10) comprising the following steps: stripping an end part of the optical fiber; etching the stripped optical fiber end part to form at least a tapered transition region (14); forming a protective coating (20) over the etched fiber end part; cleaving the protected fiber end part within or in the region of the transition region so as to expose a fiber endface (22); and etching the cleaved fiber end part so as to form an integral lens (24) at the exposed fiber endface. A method according to claim 1, wherein the optical fiber end part is etched to form the tapered region (14) and a thinned end region (16), and in that the cleaving is in the tapered region or between the tapered and end regions. The method of claim 1 or 2, wherein the protective coating comprises a metallic layer (180). The method of claim 3, wherein free metal exposed by the second mentioned etching step is removed, optionally by ultrasonic cleaning. The method of claim 3 or 4, wherein the metallization on the fiber is formed by sputter deposition or electroplating, or by chemical vapor deposition, followed by electroplating. The method of claim 3,4 or 5 wherein the metallization comprises separate layers of titanium, platinum and gold, or separate layers of nickel and gold. The method of claim 3,4 or 5, wherein the metallization comprises separate layers of deposited nickel, plated nickel and plated gold, or the metallization comprises gold. The method of any one preceding claim, wherein the first mentioned etching step uses a hydrofluoric acid bath. The method of any one of claims 1 to 8, wherein in the second etching step the fiber end part is exposed to a combination of an etchant and a treating agent such that an integral lens is formed.

Inventors: CHOLEWA MARK BERNARD, FISHER LEE HOWARD, GREENBERG LAWRENCE ARNOLD, CHOLEWA, MARK BERNARD, FISHER, LEE HOWARD, GREENBERG, LAWRENCE ARNOLD
Applicants: AT & T CORP, AT&T CORP.
Application year: 1992
Main classification: G02B6
IPC codes: C03C25, G02B6