WO2015094795A1

WO2015094795A1 - Methods of securing one or more optical fibers to a ferrule

Info

Publication number: WO2015094795A1
Application number: PCT/US2014/069223
Authority: WO
Inventors: Jeffrey Dean Danley; II Robert Bruce ELKINS; Darrin Max Miller
Original assignee: Corning Optical Communications LLC
Priority date: 2013-12-18
Filing date: 2014-12-09
Publication date: 2015-06-25

Abstract

A method of securing an optical fiber to a ferrule involves heating the ferrule to cause thermal expansion. A ferrule bore of the ferrule increases in diameter as a result of the thermal expansion, and an optical fiber is inserted into the ferrule bore. The ferrule is then cooled so that the ferrule bore decreases in diameter and forms a mechanical interface with the optical fiber. Finally, the optical fiber is fused to the ferrule by irradiating the optical fiber and the ferrule with laser energy.

Description

METHODS OF SECURING ONE OR MORE OPTICAL FIBERS TO A FERRULE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Application Serial No.

61/917,765, filed on December 18, 2013, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

[0002] The disclosure relates generally to optical fibers and more particularly to methods of securing one or more optical fibers to a ferrule of a fiber optic connector.

[0003] Optical fibers are useful in a wide variety of applications, including the

telecommunications industry for voice, video, and data transmissions. In a telecommunications system that uses optical fibers, there are typically many locations where fiber optic cables that carry the optical fibers connect to equipment or other fiber optic cables. To conveniently provide these connections, fiber optic connectors are often provided on the ends of fiber optic cables. The process of terminating individual optical fibers from a fiber optic cable is referred to as "connectorization." Connectorization can be done in a factory, resulting in a "pre- connectorized" or "pre-terminated" fiber optic cable, or the field (e.g., using a "field-installable fiber optic connector).

[0004] Regardless of where installation occurs, a fiber optic connector typically includes a ferrule with one or more bores that receive one or more optical fibers. The ferrule supports and positions the optical fiber(s) with respect to a housing of the fiber optic connector. Thus, when the housing of the fiber optic connector is mated with another fiber optic connector or adapter, an optical fiber in the ferrule is positioned in a known, fixed location relative to the housing. This allows an optical connection to be established when the optical fiber is aligned with another optical fiber provided in the mating component (the other fiber optic connector or adapter).

[0005] To minimize signal attenuation through such an optical connection, the optical fiber should not move relative to the ferrule. Doing so might alter the precise spatial relationship of the optical fiber and ferrule and, in turn, affect alignment/mating with the optical fiber of the mating component. Conventional methods of preventing movement involves bonding the optical fiber in a bore of the ferrule with an epoxy-based adhesive ("epoxy"). Although relatively inexpensive, epoxy presents several challenges. For example, epoxy can be difficult to apply uniformly to all ferrules such that the quality of adhesive bond may vary. The spatial relationship of the optical fiber relative to the ferrule may then be difficult to predict. The need for precise mixing, a limited pot life after mixing, and long cure times after application are other challenges that epoxy typically presents.

SUMMARY

[0006] Methods of securing an optical fiber to a ferrule are described below. The optical fiber could be a single optical fiber or one of several optical fibers, as may be the case for a multi- fiber connector, to be secured to the ferrule. Thus, "an optical fiber" refers to at least one optical fiber. According to one embodiment, the method involves heating the ferrule to cause thermal expansion. A ferrule bore of the ferrule increases in diameter as a result of the thermal expansion, and an optical fiber is inserted into the ferrule bore. The ferrule is then cooled so that the ferrule bore decreases in diameter and forms a mechanical interface with the optical fiber. Finally, the optical fiber is fused to the ferrule by irradiating the optical fiber and the ferrule with laser energy.

[0007] Another embodiment involves the same steps mentioned above, but specifically involves heating the ferrule with at least one laser to cause the thermal expansion. The at least one laser is also what is used to irradiate the optical fiber and the ferrule with laser energy to fuse the optical fiber to the ferrule. However, fusing may be performed after changing at least one optical delivery property of the at least one laser.

[0008] Additional features and their advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

[0009] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments. Persons skilled in the technical field of optical connectivity will appreciate how features and attributes associated with embodiments shown in one of the drawings may be applied to embodiments shown in others of the drawings.

[0011] Fig. 1 a cross-sectional view of an example of a fiber optic connector having a ferrule to which an optical fiber is secured according to methods of the present disclosure;

[0012] Fij

[0014] Fij

Fig. 1 ;

[0017] Fij

Figs. 4-6, wherein the ferrule is shown as a cross-section taken along line 7A— 7A in Fig. 3;

[0018] Fig. 7B is a schematic view similar to Fig. 7A, but further illustrating the ferrule being heated as part of a method according to the present disclosure to cause thermal expansion;

[0019] Fig. 7C is a schematic view similar to Figs. 7A and 7B, but further illustrating the optical fiber being inserted into the ferrule;

[0020] Fig. 7D is a schematic view similar to Figs. 7A-7C, but further illustrating the ferrule forming a mechanical interface with the optical fiber after the optical fiber has been inserted into the ferrule;

[0021] Fig. 8 is a schematic view of a ferrule being heated with a laser to cause thermal expansion;

[0022] Fig. 9 is a schematic view of a ferrule being heated in an oven to cause thermal expansion;

[0023] Fig. 10 is a schematic view of a ferrule being induction heated by an electromagnet to cause thermal expansion;

[0024] Fig. 11 is a schematic side view of a ferrule being irradiated with laser energy according to a method of the present disclosure to fuse an optical fiber to the ferrule;

[0025] Figs. 12 and 13 are schematic side and front views of a ferrule being irradiated with laser energy according to another method of the present disclosure to fuse an optical fiber to the ferrule; and

[0026] Fig. 14 is a graph of an exemplary material gradient profile for a ferrule used in methods according to the present disclosure. DETAILED DESCRIPTION

[0027] Various embodiments will be further clarified by exam les in the description below. In general, the description relates to methods of securing an optical fiber in a ferrule of a fiber optic connector. The methods may be part of a cable assembly process for a fiber optic cable. That is, the methods may be part of terminating one or more optical fibers from a fiber optic cable with a fiber optic connector to form a cable assembly. One example of a fiber optic connector ("connector") 10 for such a cable assembly is shown in Fig. 1. Although the connector 10 is shown in the form of a SC-type connector, the methods described below may be applicable to processes involving different fiber optic connector designs. This includes ST, LC, FC, MU, MT, and MTP-style connectors, for example.

[0028] Referring Figs. 1 -3, the connector 10 includes a ferrule 12 having a first end 14 and a second end 16, a ferrule holder 18 having opposed first and second end portions 20, 22, and a housing 24. The second end 16 of the ferrule 12 is positioned in the first end portion 20 of the ferrule holder 18 while the first end 14 of the ferrule 12 remains outside the ferrule holder 18. The ferrule holder 18 may comprise, for example, a plastic material molded over the second end 16 of the ferrule 12, which may in turn comprise a ceramic material, such as zirconia. Other details related to possible constructions/compositions of the ferrule 12 and pertaining methods of the present disclosure will be set forth below. In embodiments where the ferrule holder 18 is molded, a notch 26 may be provided in the ferrule 12 so that a portion 28 of the ferrule holder 18 is disposed in the notch 26 to help prevent the ferrule 12 from disengaging with the ferrule holder 18. In alternative embodiments, the ferrule 12 may simply be press- fit into the ferrule holder 18, which may or may not be a molded component.

[0029] The ferrule 12 also includes a ferrule bore 30 ("micro hole") extending between the first and second ends 14, 16. A center of the ferrule bore 30 defines an optical axis Ai, and the first end 14 of the ferrule 12 defines a front end face 32 positioned at an angle φ relative to the optical axis Ai. The front end face 32 is shown as being orthogonal to the optical axis Ai in the embodiment of Fig. 1 such that the angle φ is 90°. In other embodiments, the front end face 32 may be non-orthogonal.

[0030] As shown in Fig. 1 , an end portion of an optical fiber 40 may be inserted from a rear of the ferrule bore 30 and extended until the optical fiber 40 exits an opening of the ferrule bore 30 on the front end face 32 of the ferrule 12. Thus, the optical fiber 40 protrudes past the front end face 32 by a distance Hi ("protrusion height"). Details relating to the how the optical fiber 40 may be inserted into and secured within the ferrule bore 32 will be described in greater detail below. In general, methods may be used that advantageously provide a mechanical interface between an inner surface of the ferrule bore 30 and an outer surface of the optical fiber 40 before fusing the optical fiber 40 to the ferrule 12, thereby avoiding the need for a bonding agent (e.g., epoxy).

[0031] The optical fiber 40 may be part of a fiber optic cable 42 upon which the fiber optic connector 10 is installed. As schematically shown in Fig. 4, the end portion (noted with reference number 44) of the optical fiber 40 is exposed from an outer jacket 44 that surrounds and protects other portions of the optical fiber 40. The end portion may represent part of a "bare" optical fiber portion in that the end portion is not only exposed from the outer jacket 44, but is also stripped or otherwise devoid of a primary coating up to a transition interface 48. In other words, and as shown in Figs. 5 and 6, the optical fiber 40 includes a bare optical fiber portion 50, which may comprise silica, and a primary coating 52, which may comprise an acrylate polymer, within the outer jacket 46, which may comprise a polyurethane acrylic resin. The outer jacket 46 surrounds the optical fiber 40 (i.e., both the primary coating 52 and bare optical fiber portion 50) until the transition interface 48 (Fig. 4), where both the primary coating 52 and outer jacket 46 have been removed. Although the primary coating 52 is shown as being removed from the entire length of the optical fiber 40 extending from the outer jacket 46, in alternative embodiments the primary coating 52 may cover some of the length exposed from the outer jacket 46.

[0032] Referring back to Fig. 1 , the second end portion 22 of the ferrule holder 18 is received in the housing 24. A spring 60 may be disposed around the second end portion 22 and configured to interact with walls of the inner housing 24 to apply a biasing force Fs to the ferrule holder 18 (and ferrule 12). Additionally, a lead-in tube 62 may extend from a rear end 64 of the housing 24 to within the second end portion 22 of the ferrule holder 18 to help guide the insertion of the optical fiber 40 into the ferrule 12 during assembly (discussed below). An outer shroud 66 is positioned over the ferrule 12, ferrule holder 18, and housing 24, with the overall configuration being such that the front end face 32 of the ferrule 12 is configured to contact a mating component (e.g., another fiber optic connector; not shown).

[0033] In a manner not shown herein, the fiber optic cable 42 may include one or more layers of material (e.g., a strength layer of aramid yarn) that may be crimped onto the rear end 64 of the housing 24. A crimp band may be provided for this purpose. Additionally, a strain-relieving boot may be placed over the crimped region and extend rearwardly to cover a portion of the fiber optic cable 42. Variations of these aspects will be appreciated by persons skilled in the design of fiber optic cable assemblies. Again, the embodiment shown in Fig. 1 is merely an example of a fiber optic connector that may be used in the methods described below. The general overview has been provided simply to facilitate discussion.

[0034] Now that the fiber optic connector 10 has been introduced to facilitate discussion, exemplary methods of securing the optical fiber 40 to the ferrule 12 will now be described. A high-level description of one exemplary method for forming a mechanical interface will first be provided, followed by a more detailed description of each of the steps and variants thereof that may be part of other exemplary methods. The mechanical interface temporarily secures the optical fiber 40 to the ferrule 12. Afterwards, a permanent attachment/connection may be formed by fusing the optical ferrule 40 to the ferrule 12. A more detailed description of aspects relating to such fusing will be provided below following the description of aspects relating to forming the mechanical interface.

[0035] To this end, as generally shown in Figs. 7A-7D, one method of securing the optical fiber 40 to the ferrule 12 first involves providing the ferrule 12 and the optical fiber 40. Initially the ferrule bore 30 may have a minimum bore diameter D_Bi ("minimum bore width") that is less than a maximum diameter DOF ("maximum fiber width") of the end portion 44 of the optical fiber 40. Prior to inserting the end portion 44 of the optical fiber 40 into the ferrule bore 30, the ferrule 12 is heated by an energy source 70. The ferrule 12 experiences thermal expansion when heated such that the ferrule bore 30 increases in diameter. Once the temperature of the ferrule 12 reaches a threshold temperature, the ferrule bore 30 increases to a minimum bore diameter D_B2 that is greater than the maximum diameter D₀F of the end portion 44 of the optical fiber 40. As shown in Fig. 7C, the end portion 44 of the optical fiber 40 may then be moved toward the second end 16 of the ferrule 12 and inserted into the ferrule bore 30. Insertion continues until the end portion 44 reaches or extends beyond the front end face 32 of the ferrule 32. At this point, the ferrule 12 is cooled so that the ferrule bore 30 decreases in diameter. Eventually the ferrule bore 30 decreases to a minimum bore diameter D_B3 (Fig. 7D) as the inner surface of the ferrule bore 30 constricts around the outer surface of the end portion 44 of the optical fiber 40. The minimum bore diameter D_B3 may be less than a maximum diameter DFI of the optical fiber 40 so that a force Fi is applied by the ferrule 12 to the optical fiber 40, thereby establishing a mechanical interface. In some embodiments, the minimum bore diameter D_B3 may be greater than the minimum bore diameter D_B3 but less than the minimum bore diameter D_B2.

[0036] Now referring to specific aspects of the above-described method, the optical fiber 40 and ferrule 12 are initially provided at a temperature below the threshold temperature. The threshold temperature may be set above a normally expected temperature operating range of the fiber optic connector 10. In some embodiments, for example, the threshold temperature may be 100° C. The dimensions and material properties of the optical fiber 40 are such that the minimum bore diameter D_Bi of the ferrule bore 30 is less than the maximum diameter D_F1 of the end portion 44 of the optical fiber 40, as mentioned above, when the ferrule 12 is below the threshold temperature.

[0037] In terms of heating the ferrule 12 to increase the minimum bore diameter DBI, the energy source 70 is shown generically in Fig. 7B because different embodiments may employ different sources/techniques to cause thermal expansion of the ferrule 12. In some

embodiments, the energy source 70 may comprise at least one laser. Fig. 8, for example, illustrates an embodiment where a laser 80 is used to irradiate the ferrule 12 with laser energy to cause thermal expansion. The laser energy is delivered by a laser beam 82 emitted from the laser 80. Uniform or bulk heating of the ferrule 12 may be desired in some embodiments and provided by selecting an appropriate combination of optical delivery properties of the laser 80, such as wavelength, power or fluence, duty cycle of pulses, beam shape, beam focus, etc., as well as how the laser 80 is oriented (i.e., angled), positioned, and/or moved relative to the ferrule 12 (or vice-versa). One specific example of a suitable laser is a carbon dioxide laser that operates at one or more wavelengths in the range of 0.1 microns to 11 microns. Other types of lasers are also possibilities.

[0038] In alternative embodiments, and as shown in Fig. 9, the energy source may comprise an electrical heating source 90 of an oven 92 into which the ferrule 12 is inserted. Once heated and thermally expanded, the ferrule 12 is removed from the oven 92.

[0039] Another alternative is shown in Fig. 10, which illustrates the energy source in the form of an electrical current source 100. An electromagnet 102 is coupled to the electrical current source 100 and includes one or more coils 104 disposed around the ferrule 12. When the electrical current source 100 provides an alternating current to the electromagnet 102, the coils 104 inductively heat the ferrule 12. More specifically, the ferrule 12 may comprise zirconia, or other materials, that provide some electrical resistance to eddy currents induced by the electromagnet 102. The electrical resistance results in heat being generated in the ferrule 12.

[0040] In some embodiments, the optical fiber 40 may be heated in addition to the ferrule 12. This may reduce the risk of thermal shock to the ferrule 12 or optical fiber 40 when the two components are later placed in contact. A common energy source (e.g., the laser 80 of Fig. 8 or the electrical heating source 90 and oven 92 of Fig. 9) maybe used to heat the optical fiber 40 and ferrule 12. In such embodiments, however, the materials of the optical fiber 40 and ferrule 12 are selected so that a coefficient of thermal expansion of the ferrule 12 is greater than a coefficient of thermal expansion of the optical fiber 40. This allows the minimum bore diameter DBI of the ferrule bore 30 to increase in size faster than the maximum fiber diameter D_F1 under the same heating conditions. The ferrule 12 may even have a coefficient of thermal expansion at least 15 times greater than the optical fiber 40 in some embodiments.

[0041] Cooling the ferrule 12 to form the mechanical interface with the optical fiber 40 may be achieved passively or actively. Accordingly, in some embodiments, cooling may simply be a matter of turning off or removing the energy source 70 (Fig. 7B) so that the ferrule 12 is no longer heated. The ferrule 12 may then be allowed to return to a temperature below the threshold temperature. No powered devices (e.g., fans, pumps, etc.) are used to promote the heat transfer. In other embodiments not shown herein, powered devices may be used to provide active cooling. Regardless, and as mentioned above, when the ferrule 12 cools back below the threshold temperature, the ferrule bore 30 decreases to the minimum bore diameter ¾ so as to be less than the maximum diameter D_F1 of the end portion 44 of the optical fiber 40. Cooling the ferrule 12 a number of degrees (e.g., at least 5°, 10°, 15°) below the threshold temperature helps ensure that the inner surface of the ferrule bore 30 forms the mechanical interface with the entire outer surface of the end portion 44 of the optical fiber 40 that is located within the ferrule bore 30. For example, if the threshold temperature is 100° C, the ferrule 12 (and optical fiber 40, if heated as well) may be cooled to a temperature less than or equal to 95° C.

[0042] The mechanical interface formed between the ferrule 12 and optical fiber 40 facilitates one or more additional processing steps that fuse the optical fiber 40 to the ferrule 12. Fusing involves merging/melting/welding the optical fiber 40 and ferrule 12 together and may be accomplished by using one or more lasers to irradiate the optical fiber 40 and ferrule 12 with laser energy. In general, the materials of the optical fiber and ferrule are irradiated with sufficient energy to transform into liquid states so that the materials can blend together and later solidify to form a single entity. By providing the mechanical interface between the optical fiber and ferrule prior to fusing, gaps between the optical fiber and ferrule are reduced or eliminated where the fusing is desired. As a result, the need for molten material to flow from nearby regions of the optical fiber and/or ferrule to fill gaps during fusing is reduced or eliminated. This has the advantage of helping preserve the geometries and spatial relationships that are important for establishing effective optical couplings with mating components.

[0043] The laser(s) used for fusing may be the same laser(s) used to heat and thermally expand the ferrule 12 in some embodiments (e.g., the embodiment of Fig. 8). Even further, the same laser(s) may also be used to form an optical surface on the end portion 44 of the optical fiber 40 at a protrusion height Hi (Fig. 1) within 50, 15, or even 10 microns of the front end face 32 of the ferrule 12. The laser(s) may even be used to form such an optical surface flush with the front end face 32. As can be appreciated, however, at least one optical delivery property of the laser(s) may be changed for the different processing steps to provide the different result (i.e., fusing instead of heating/thermally expanding, and forming an optical surface instead of fusing). Exemplary optical delivery properties include, without limitation: wavelength, power or fluence, duty cycle of pulses, beam shape, and beam focus. How the laser(s) is/are oriented, positioned, and/or moved relative to the ferrule 12 (or vice-versa) may also be changed. One specific example of a suitable laser for fusing is a carbon dioxide laser that operates at one or more wavelengths in the range of 3 microns to 1 1 microns. Other types of lasers are also possible.

[0044] With this in mind, Fig. 11 illustrates one example of how the optical fiber 40 may be fused to the ferrule 12. The notch 26 (Figs. 1 and 2) in the ferrule 12 is not shown to simplify matters. Indeed, the notch 26 may not even be present in some embodiments. As schematically shown in Fig. 11 , a laser 110 may deliver laser energy toward the ferrule 12 in any of various directions, as represented by the arrows A in Fig. 11 , including from nearly parallel to the optical fiber 40 to perpendicular to the optical fiber 40, or even beyond perpendicular to the optical fiber 40. The ferrule 12 and optical fiber 40 may also be rotated in the direction R and translated in the direction T as shown, so as to fuse end portion 44 of the optical fiber 40 to the ferrule 12 along at least 10%, 25%, or 50% of the length of the ferrule bore 30. In some embodiments, the optical fiber 40 may even be fused to the ferrule 12 along the entire length of the ferrule bore 30. In other embodiments, the optical fiber 40 may only be fused to the ferrule 12 a locations L within the ferrule bore 30 that are at least a distance d from the front end face 32. For example the optical fiber 40 may only be fused to the ferrule 12 at locations L at least 1 mm (or 2 mm, 5 mm, etc.) deep inside the ferrule bore 30 such that the distance d is at least 1 mm (or 2 mm, 5 mm, etc.). The laser 100 may be moved relative to the optical fiber 40 and ferrule 12, rather than moving the optical fiber 40 and ferrule 12 relative to the laser 100, to provide either or both the rotation in the direction R and the translation in the direction T.

[0045] Figs. 12 and 13 schematically illustrate another example of how a laser 120 may be used to fuse the optical fiber 40 to the ferrule 12. As shown in Figs. 12 and 13, the laser 120 may emit a laser beam B (only the outermost rays are represented) that has been focused with a short focal length lens so to a have an extreme convergence angle. The laser beam B is largely transmissive through the ferrule 12, but develops enough intensity or energy density at the center of the ferrule 12 to fuse the end portion 44 of the optical fiber 40 to the ferrule 12. Both relative axial rotation R and relative translation T may be used to perform a rapid helical sweep of the ferrule 12 with the laser beam B. Such a sweeping technique may facilitate fusing across the entire mechanical interface formed between the optical fiber 40 and ferrule 12, particularly when the ferrule 12 as a whole comprises largely (i.e., greater than 75%), substantially (i.e., greater than 95%>), or entirely (i.e., 100%) fused silica, borosilicate, glass ceramic, or the like. Ferrules comprised in this manner are considered to be "non-composite ferrules" according to this disclosure.

[0046] On the other hand, processes or methods where a laser beam approaches the front end face 32 of the ferrule 12 to irradiate the ferrule 12 with laser energy may be more suited for embodiments where the ferrule 12 comprises an inorganic composite material having a material gradient (a "composite ferrule" according to this disclosure). The composite material may, for example, have a material gradient from at least 75% (or even further, at least 90% or 100%) by volume of a first inorganic material to at least 75% (or even further, at least 90%) or 100%) by volume of a second inorganic material in a radially inward direction of the ferrule (i.e., radially inward relative to the optical axis Aj). In some embodiments, the first inorganic material may comprise or consist of a ceramic, such as alumina and/or zirconia, while the second inorganic material may comprise or consist of a glass or glass material, such as silica. Alternatively or additionally, the first inorganic material may have a fracture toughness of at least 1 MPa · m½ (or even further, at least 1 .5 MPa · m½), and the second inorganic material may have a softening point less than 1000° C (or even further, less than 900° C).

[0047] To illustrate these aspects in further detail, Fig. 14 is a plot that shows an example of a material gradient 128 for the ferrule 12. The vertical axis represents the percentage by volume of the respective phase or material component of the ferrule 12, with trace 130 representing the percentage of the first inorganic material and trace 132 representing the percentage of the second inorganic material. The horizontal axis represents the distance along a radius of the ferrule 12, measured from the center of the ferrule at radius 0 (i.e., the optical axis A₂ in the embodiments discussed above) to an outer radius r (Fig. 13). As shown in Fig. 14, there are different regions of the plot that correspond to different regions of ferrule 12. In a first region 134, which corresponds to an outer region of the ferrule 12, the material of the ferrule 12 is 100%) the first inorganic material. In a second region 136, which corresponds to an inner region of the ferrule 12 (i.e., proximate the ferrule bore 30), the material of the ferrule 12 is 100 % the second inorganic material. A third, intermediate region 138 includes the material gradient, where the percentages of the first inorganic material and second inorganic material transition smoothly between their respective values in the first and second regions 134, 136. Again, Fig. 14 is merely an example, as the material gradient may be between different percentages of the first and second inorganic materials in other embodiments; between something other than 100%) such that the innermost and outermost regions of the ferrule 12 comprise composite materials. [0048] The length of the first, second, and third regions 134, 136, 138 may also vary in different embodiments. The third region 138 with the material gradient may, for example, extend along at least 1/10 (or at least 1/3, 1/2, etc.) of the length of the radius of the ferrule 12. The plot in Fig. 12 shows the third region 138 extending along about 1/2 or more of the length of the horizontal axis, which corresponds to about 1/2 or more of the length of the radius r. Providing the gradual transition from the first inorganic material to the second inorganic material over such a large region of the ferrule 12 helps spread any stresses that may arise between the first and second inorganic materials over the operating temperature range of the ferrule 12. In other words, rather than being concentrated in localized areas, such as at an interface between two distinct layers of material, stresses may be spread across the third region 138. This advantage also applies to embodiments where the third region 138 only extends along 1/10 or more of the length of the radius r (rather than 1/2 or more), although possibly to a lesser extent.

[0049] In other contemplated embodiments, layers of composite material having differing ratios of the first and second inorganic materials may provide a stepped transition from an exterior of the ferrule 12 to the ferrule bore 30. For each successive layer from the exterior toward the ferrule bore 30, the percentage of the second inorganic material may increase while the percentage of the first material may decrease in a corresponding manner. Accordingly, the outermost layer corresponds to the outer region of the ferrule 12 and has a ratio according to the percentages above (100%, 90%>, 75%, etc., depending on the embodiment). The innermost layer corresponds to the center of the ferrule, or more specifically the inner surface of the ferrule bore 30, and has a ratio according to the percentages above (100%), 90%>, 75%, etc., depending on the embodiment). Any number of discrete layers may be provided between the outermost and innermost layers, bringing the total number of discrete layers to three or more. Providing three or more discrete layers, and especially five or more discrete layers, helps ensure that the degree of change in coefficient of thermal expansion at the interface/transition between adjoining layers does not result stresses great enough crack the ferrule 12 or delaminate the layers as the optical fiber 40 is fused to the ferrule 12.

[0050] It was mentioned above how the same laser(s) used to thermally expand the ferrule and/or fuse the optical fiber to the ferrule may additionally be used to form an optical surface on an end portion of the optical fiber. To this end, the laser or laser may be considered to be part of a laser cleaving system. Laser cleaving steps may be performed before fusing the optical fiber to the ferrule or afterwards. Indeed, unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that steps be performed in a specific order. Accordingly, where a method claim below does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims below or description above that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

[0051] It will be apparent to those skilled in the art that additional modifications and variations can be made without departing from the spirit or scope of the claims below. For example, although ferrules comprising a ceramic material and optical fibers comprising silica are mentioned above, some claims may not be limited to these materials. The methods described above may also be applicable to plastic ferrules and optical fibers. Other modifications, combinations, sub-combinations, and variations of the disclosed embodiments may occur to persons skilled in the art, yet still fall within the scope of the claims below.

Claims

What is claimed is:

1. A method of securing an optical fiber to a ferrule, comprising:

heating the ferrule to cause thermal expansion, wherein a ferrule bore of the ferrule increases in diameter as a result of the thermal expansion;

inserting the optical fiber into the ferrule bore;

cooling the ferrule after thermal expansion and after inserting the optical fiber into the ferrule bore so that the ferrule bore decreases in diameter and forms a mechanical interface with the optical fiber; and

fusing the optical fiber to the ferrule by irradiating the optical fiber and the ferrule with laser energy.

2. A method according to claim 1 , wherein heating the ferrule to cause thermal expansion comprises irradiating the ferrule with laser energy.

3. A method according to claim 2, wherein at least one common laser source is used to heat the ferrule to cause thermal expansion and to fuse the optical fiber to the ferrule, the method further comprising:

changing at least one optical delivery property of the at least one common laser source after heating the ferrule and before fusing the optical fiber to the ferrule.

4. A method according to claim 3, wherein inserting the optical fiber into the ferrule bore comprises extending an end portion of the optical fiber beyond a front end face of the ferrule, the method further comprising:

operating the at least one common laser source to form an optical surface on the end portion of the optical fiber after extending the end portion of the optical fiber beyond the front end face of the ferrule.

5. A method according to claim 4, wherein the at least one common laser source is operated to form the optical surface on the end portion of the optical fiber after fusing the optical fiber to the ferrule.

6. A method according to claim 5, wherein before operating the at least one common laser source to form the optical surface but after fusing the optical fiber to the ferrule, the method further comprises:

changing at least one optical delivery property of the at least one common laser source.

7. A method according to any of claims 1-6, wherein the optical fiber is only fused to the ferrule at locations at spaced least 1 mm from a front end face of the ferrule.

8. A method according to any of claims 1-7, further comprising:

providing the ferrule, wherein the ferrule is comprised of an inorganic composite material having a material gradient in a radial direction from at least 75% by volume of a first inorganic material to at least 75% by volume of a second inorganic material.

9. A method according to claim 8, wherein the first inorganic material comprises a ceramic and the second inorganic material comprises silica.

10. A method according to claim 9, wherein the ceramic material of the ferrule comprises alumina or zirconia.

11. A method according to any of claims 8-10, wherein the first inorganic material of the ferrule has a fracture toughness of at least 1 MPa · m½, and further wherein the second inorganic material of the ferrule has a softening point less than 1000° C.

12. A method according to any of claims 8-11, wherein the ferrule includes a region extending along at least 1 /l 0 of the length of the radius of the ferrule, and further wherein the material gradient is located within said region.

13. A method according to claim 12, wherein the material gradient of the ferrule is continuous over the region of the ferrule.

14. A method according to any of claims 1-13, wherein cooling the ferrule after thermal expansion further comprises forming the mechanical interface between the ferrule bore and optical fiber along an entire length of the ferrule bore.

15. A method according to any of claims 1 -14, further comprising:

providing the ferrule, wherein the ferrule has a coefficient of thermal expansion at least 15 times greater than a coefficient of thermal expansion of the optical fiber.