TECHNICAL FIELD

This disclosure relates generally to optical connectivity, and more particularly, to a method for making a low-loss fiber optic connector wherein the insertion loss between mated fiber optic connectors at an optical connection is reduced by minimizing the core-to-ferrule offset of each connector.

BACKGROUND

Optical fibers are useful in a wide variety of applications, including the telecommunications industry for voice, video, and data transmissions. Benefits of optical fiber include extremely wide bandwidth and low noise operation. In a telecommunications system that uses optical fibers, there are typically many locations where fiber optic cables carrying 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 to non-permanently connect and disconnect optical elements in the fiber optic network. The introduction of fiber optic connectors, however, may introduce insertion losses across an optical connection, i.e., at a junction between two or more optical fibers. One common optical connection in a network is that between two mated optical connectors, such as within an adapter. It should be recognized, however, that the term “optical connection” may encompass other types of junctions between optical fibers. The insertion losses in coupling two optical fibers across an optical connection are generally a function of the alignment of the optical fiber ends, the width of the gap between the ends, and the optical surface condition at the ends. The present disclosure primarily focuses on the first of these factors, i.e., reducing misalignments of the optical fibers across the optical connection.

FIGS. 1 and 2illustrate an exemplary fiber optic connector10used in many modern fiber optic networks. Although the fiber optic connector10is shown in the form of a SC-type connector, the features may be applicable to different connector designs. This includes ST, LC, and MU-type connectors, for example, and other single-fiber or multi-fiber connector designs. As shown in these figures, the connector10includes a ferrule12having a ferrule bore14(“micro-bore”) configured to support an optical fiber16, a ferrule holder18from which the ferrule12extends, a housing20having a cavity22in which the ferrule12and ferrule holder18are received, and a connector body24configured to cooperate with the housing20to retain the ferrule12and ferrule holder18within the housing20. More specifically, a back end of the ferrule12is received in a first portion of the ferrule holder18and is secured therein in a known manner (e.g., press-fit, adhesive, molding the ferrule holder18over the back end of the ferrule12, etc.). The ferrule12and ferrule holder18may even be a monolithic structure in some embodiments. The ferrule holder18is biased to a forward position within the housing20by a spring26, which extends over a second portion of the ferrule holder18that has a reduced cross-sectional diameter/width compared to the first portion. The spring26also interacts with internal geometry of the connector body24, which may be secured to the housing20using a snap-fit or the like. For example,FIGS. 1 and 2illustrate a rear portion of the housing20having cut-outs or slots on opposite sides so as to define a split shroud. The connector body24has tabs configured to be snapped into the slots and retained therein due to the geometries of the components.

When the connector10is assembled as shown inFIG. 1, a front end of the ferrule12projects beyond a front end of the housing20. The ferrule end face presents the optical fiber16for optical coupling with a mating component (e.g., another fiber optic connector; not shown), such as an adapter. Thus, when the fiber optic connector10is mated with the other component, the optical fiber(s) in the ferrule can be held in alignment with the optical fiber(s) of the mating ferrule to establish an optical connection. At optical connections between fiber optic connectors, light exiting each optical fiber of a first fiber optic connector (of a first fiber optic cable) is introduced into a corresponding optical fiber within an adjacent, second fiber optic connector (of a second fiber optic cable). Bare optical fibers typically include an inner core through which the light is configured to travel, and an outer cladding configured to reflect the light back within the core and thereby minimize losses as the light is transmitted through the optical fiber. If contamination such as one or more pieces of debris is present on the end face of the ferrule of either of the fiber optic connectors that terminate the optical fibers, then transmission of optical signals from the inner core of the first optical fiber to the inner core of the second optical fiber optic may be inhibited. Furthermore, if optical fiber cores at an interconnection between first and second optical fibers are misaligned, then transmission of an optical signal from the inner core of the first optical fiber to the inner core of the second optical fiber may be inhibited, resulting in signal degradation at the interconnection. The inner cores must therefore be closely aligned to ensure long life and to minimize transmission loss and optical return loss at optical connection points.

There are several conventional approaches for achieving optical fiber alignment between two fiber optic connectors. For example, precision ceramic ferrules, V-grooves, three-rod containment, tuning ferrules, and precision bushing devices have been developed to aid in fiber alignment across an optical connection. While providing some improvement in fiber alignment, these approaches have a number of drawbacks. For example, these approaches may require premium raw materials (i.e., high cost) with tight tolerances that generally require complex processes that are not conducive to increased scale for mass production. Additionally, these approaches may be time consuming in field installation and/or require highly skilled labor, both of which increase the overall cost of the installation. One common and widely recognized approach for reducing insertion losses across an optical connection is connector tuning. This approach, however, can produce insertion losses that may exceed acceptable levels as optical fiber networks move toward higher data rates, higher transceiver speeds, and higher connection point densities along the network.

Thus, there is a need in the fiber optic industry for making fiber optic connectors that, when mated to another optical component, produce minimal insertion losses across the optical connection. More particularly, there is a need for a method that improves the alignment of the optical fibers; particularly, the inner cores of the optical fibers, across the optical connection. There is a further need to provide this reduction in insertion losses without resorting to high-cost raw materials, extremely low-tolerance parts, complex manufacturing processes, and/or time-consuming installation processes. More particularly, it is desirable to achieve a reduction in insertion losses using current components, materials, and manufacturing techniques. In this way, improvements may be made at optical connections with a minimal impact on current production and overall costs.

SUMMARY

A method of terminating an optical fiber having an inner core with a fiber optic connector including a ferrule having a micro-bore and an end face with a mating location is disclosed. The method includes determining a bore bearing angle of a bore offset of the micro-bore in the ferrule at the end face relative to a reference axis of the ferrule; determining a core bearing angle of a core offset of the inner core in the optical fiber at an end of the optical fiber relative to a reference axis of the optical fiber; orienting the ferrule and the optical fiber relative to each other to minimize the distance between the inner core of the optical fiber and the mating location of the ferrule; heating the ferrule to an processing temperature above room temperature; and with the ferrule at the processing temperature and with the distance between the inner core and the mating location minimized, coupling the optical fiber to the micro-bore of the ferrule.

In one embodiment, orienting the ferrule and the optical fiber relative to each other to minimize the distance between the inner core and the mating location includes orienting the ferrule and the optical fiber relative to each other so that the bore bearing angle of the bore offset and the core bearing angle of the core offset are 180 degrees apart. To achieve this relationship between bearing angles, the method may further include marking the ferrule end face with an indicia along a radial line from a ferrule center of the ferrule through a bore center of the micro-bore, the indicia being positioned on the ferrule end face radially outboard of the micro-bore; and orienting the ferrule and the optical fiber relative to each other such that the radial line extends through a center of the inner core, and the indicia and the inner core are on opposite sides of a center of the optical fiber. In one embodiment, orienting the ferrule and the optical fiber relative to each other further includes fixing the orientation of the optical fiber and rotating the ferrule about a central axis of the ferrule. In an alternate embodiment, orienting the ferrule and the optical fiber relative to each other includes fixing the orientation of the ferrule and rotating the optical fiber about a central axis of the optical fiber. In still a further alterative embodiment, orienting the ferrule and the optical fiber relative to each other includes rotating the ferrule and the optical fiber about respective central axes of the ferrule and optical fiber. In one embodiment, the method further includes assembling the ferrule in the fiber optic connector prior to the orienting, heating, and coupling steps such that the rotation of the ferrule may be achieved by rotating the fiber optic connector.

In one embodiment, heating the ferrule further includes heating the ferrule to a processing temperature greater than 300° C., preferably greater than 350° C., and even more preferably greater than 400° C. Heating the ferrule to such a processing temperature expands the size of the micro-hole in the ferrule. In one embodiment, coupling the optical fiber to the micro-bore of the ferrule further includes disposing a bonding agent in the micro-bore of the ferrule; melting the bonding agent during the heating step; and when the bonding agent is melted, inserting the optical fiber in the micro-bore of the ferrule. The bonding agent may include a partially cross-linked polymer resin and a coupling agent that chemically bonds the partially cross-lined polymer resin to the optical fiber and the ferrule. The melting point of the bonding agent may be at least 250° C., and preferably at least 300° C.

A method of making a population of terminated optical fibers, each made according to the method described above includes selecting the mean diameter μ1of the micro-bore of a population of ferrules and selecting the mean diameter μ2of the optical fiber of a population of optical fibers such that the population of terminated optical fibers satisfies a pre-determined criterion. The pre-determined criterion is configured to increase the probability that the optical fibers form an interference fit with the ferrules without generating a significant number of pairings where the optical fiber does not fit within the micro-bore of the ferrules. In one embodiment, the pre-determined criterion is: 0≤D2−D1≤0.4 μm for at least 25% of the population. In another embodiment, the pre-determined criterion is: 0≤D2−D1≤0.4 μm for at least 50% of the population. In a further embodiment, the pre-determined criterion is: 0≤D2−D1≤0.4 μm for at least 75% of the population. In yet a further embodiment, the pre-determined criterion is: 0≤D2−D1≤0.4 μm for at least 90% of the population. The method may further include measuring an eccentricity of the micro-bore in each of the ferrules in the population; measuring an eccentricity of the inner core in each of the optical fibers in the population; separating the population of ferrules into a plurality of groups, each group having a different pre-determined range of eccentricities; separating the population of optical fibers into a plurality of groups, each group having a different pre-determined range of eccentricities, wherein the number of groups of ferrules and the number of groups of optical fibers is the same; ordering the groups of ferrules and groups of optical fibers by eccentricity; and performing the orienting, heating and securing steps using a selected ferrule and a selected optical fiber from corresponding groups of ferrules and optical fibers.

In another embodiment, a method of making a population of terminated optical fibers, each made according to the method described above includes selecting a population of ferrules; selecting a population of optical fibers; measuring a diameter of the micro-bore in each of the ferrules in the selected population; measuring a diameter of each of the optical fibers in the selected population; performing a statistical analysis of the diameters of the micro-bores in the ferrules to determine a mean diameter μ1and a standard deviation σ1for the population of ferrules; performing a statistical analysis of the diameters of the optical fibers to determine a mean diameter μ2and a standard deviation σ2; and proceeding with the orienting, heating and coupling steps if one or more pre-determined criteria of the statistical data is met. In one embodiment, one or more pre-determined criteria of the statistical date may include: 0≤μ2−μ1≤0.4 μm. In another embodiment, one or more pre-determined criteria of the statistical date may include:

0.7⁢5≤σ2σ1≤2.5.
The method may further include measuring an eccentricity of the micro-bore in each of the ferrules in the population; measuring an eccentricity of the inner core in each of the optical fibers in the population; separating the population of ferrules into a plurality of groups, each group having a different pre-determined range of eccentricities; separating the population of optical fibers into a plurality of groups, each group having a different pre-determined range of eccentricities, wherein the number of groups of ferrules and the number of groups of optical fibers is the same; ordering the groups of ferrules and groups of optical fibers by eccentricity; and performing the orienting, heating and securing steps using a selected ferrule and a selected optical fiber from corresponding groups of ferrules and optical fibers.

In still a further embodiment, a method of making a population of terminated optical fibers based on the termination method described above is disclosed. The method further includes selecting a population of ferrules; selecting a population of optical fibers; measuring an eccentricity of the micro-bore in each of the ferrules in the population; measuring an eccentricity of the inner core in each of the optical fibers in the population; separating the population of ferrules into a plurality of groups, each group having a different pre-determined range of eccentricities; separating the population of optical fibers into a plurality of groups, each group having a different pre-determined range of eccentricities, wherein the number of groups of ferrules and the number of groups of optical fibers is the same; ordering the groups of ferrules and groups of fibers by eccentricity; and performing the orienting, heating and securing steps using a selected ferrule and a selected optical fiber from corresponding groups of ferrules and optical fibers. In an exemplary embodiment, the population of ferrules and the population of optical fibers may be separated into two or three groups for selective matching the ferrules and optical fibers.

