Patent Description:
Utilization of optical fibers ultimately require termini for joining fiber segments at their ends, or for connecting optical fibers to active or passive devices. In joining fiber ends, the primary function of the termini is maintaining the ends in a butting relationship such that a core of one of the fibers is axially aligned with a core of the other fiber, or in the case of terminating to an active device or waveguide, axially aligning mode field diameters. This alignment maximizes light transmissions from one fiber to the other and/or reduces insertion loss therebetween. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve. The mode field diameter (MFD) of a singlemode optical fiber is approximately nine (<NUM>) microns (<NUM>). The MFD is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.

Polarization maintaining optical fibers (PMF) are a specialty singlemode optical fiber that linearly propagates polarized light by controlling the birefringence within the core. The birefringence is controlled by highly doped birefringent elements that surround the core of the optical fiber, allowing preservation of orthogonal polarization planes, often referred to by the plane's relative axes against a reference position (Fast or Slow; Horizontal or Vertical), throughout a finite length of fiber. These birefringent elements can take the form of stress-applying structures, such as stress members, elliptical cores, bow-tie shapes, or stress rods introduced near or within the core. The exact design depends on the specific requirements of the application of the optical fiber. Referring to stress rods, as the optical fiber is drawn, the stress rods are accordingly diminished in diameter and are located within the cladding, preferably on either side of the core. The stress rods have different thermal expansion characteristics than the surrounding glass, and the stress they exert on the core causes the index of refraction to change along that axis. The axes then have different indices of refraction value and thus propagate light at different speeds. Similar stress-induced drawing techniques are also found in Bow-tie and elliptical cores. While there are many varieties and manufacturers of PMF, commercial availability is dominated by PANDA, Bow-tie, and elliptical-stress fibers. Natural changes in polarization state, such as radial stresses and bends in the fiber, do not occur in PMF due to the dominating presence of the birefringent elements. PMF has historically been relegated to telecommunications, specifically in pump laser designs and modulation, and fiber optic gyroscopes used in various guidance applications. Recent advances in high-speed telecommunications, specifically silicon photonics, on-board optics, and co-packaged optics, as well as biomedical, optical sensing, quantum computing, and other industrial applications have heightened the adoption of PMF.

Preserving the polarization state of PMF for end-to-end applications has historically required manual rotational alignment of the optical fiber such that birefringent elements match the application's intended polarity (vertical/horizontal, fast/slow) at the interface - interface to another optical fiber or waveguide or active device (laser diode source, detector, silicon chip, etc.). The quality of the birefringent element orientation is measured in degrees (i.e., ±<NUM>°) or by extinction ratio, which compares optical power in the desired polarization axis to that of the unwanted, orthogonal polarization state (measured in dB). Manual alignment of a single fiber, though tedious and time-consuming, represents the majority of today's method of manufacturing. In applications requiring multiple PMF, such as fiber array units, multi-fiber connectivity (MTP, MPO), silicon chips, and multi-channel waveguides, aligning multiple PMFs is extremely challenging - properly aligning one fiber, then disrupting the alignment of the one fiber during the alignment of a second fiber, etc. These manufacturing challenges are proving disruptive to next-generation architectures.

PM optical fiber has polarization-dependent refractive indices, and the speed of light in an optical fiber is inversely proportional to the magnitude of the refractive index. A PM optical fiber is one having two polarizations with different velocities of propagation, thus giving rise to a "fast" wave and a "slow" wave, which can be caused by any different types of birefringent elements, such as stress rods or stress members, that includes well known configurations of panda, bow tie, elliptical clad and elliptical core, as described above. In a PM optical fiber, the polarization of a linearly polarized light wave input to the fiber, with the direction of polarization parallel to that of the one of the two principal polarizations, will remain or be maintained in that polarization as it propagates along the fiber, hence the term "polarization maintaining. " If the polarization of the light wave is to be maintained at a splice or other connection, the principal axes of birefringence of the two joined fibers must be aligned in parallel, otherwise there will be polarization cross-coupling, i.e., crosstalk, which is highly undesirable.

The current connectable optics are expected to be limited in their ability to support capacities of <NUM> Tb/s, <NUM> Tb/s, and higher in terms of the required electrical and optical densities, thermal issues, and power consumption. As a result of discrete electrical device implementation, power dissipation and thermal management are becoming limiting factors for future connectable optics. Therefore, the industry is turning to co-packaged optics (CPO) to achieve higher bandwidth and energy efficiency. <CIT> (Totoku Electric Co Ltd) discloses a method of designing PM fibers using a jig that holds a <NUM>-fiber ferrule for mounting the PM fibers therein. A microscope is used to take images of the ends of the optical fibers and the PM fibers are aligned or positioned using servo motors that are controlled by a processor. Once aligned, a thermoset resin is cured with a heater. The ferrule is removed from a jig and the end of the ferrule is polished.

The subject invention provides a method of manufacturing an optical fiber assembly as claimed.

The subject invention offers the ability to align the plurality of optical fibers in the same orientation state, such as all fast, all slow axes. Further, the subject invention offers the ability to selectively align the plurality of optical fibers in unique orientation states, such as one fast, or one slow. Alternatively, the subject invention offers the ability to selectively align the polarization states to that of the active device, which may be somewhere between fast and slow, thus allowing maximum Polarization Extinction Ratio (PER) to the given active device. The PM optical fiber assembly may be identified as a jumper, which consists of a single or plurality of optical fibers with like or unlike connectors on either end of the assembly. Alternatively, the PM optical fiber assembly may be identified as a pigtail, which consists of a single or plurality of optical fibers with a single connector on one end and either bare fibers, perhaps cleaved or lensed, or arrayed, perhaps in v-grooves or ribbon, on an opposite end of assembly. In consideration of a plurality of PM fibers, the individual fibers represent a channel, transmitting a specific wavelength, power level, modulated signal from or to source laser, detector, modulator, MUX/DEMUX, or other active element. The termini for a plurality of PM fibers may be a multi-fiber optical connector, a v-groove array, or left bare with flat cleaves, angle cleaves, or lensed. The subject invention also provides for precise and efficient orienting and terminating in a manner that was previously not possible.

Referring to the Figures, wherein like numerals indicate like parts throughout the several views, a polarization-maintaining (PM) optical fiber assembly <NUM> and method of forming the same is shown. Referring to <FIG>, the assembly <NUM> includes a first receptacle <NUM>, shown as a multi-fiber push on (MPO), on a first end <NUM> and a pair of second connectors <NUM>, shown as V-groove connectors, on a second end <NUM>. In certain embodiments, the first receptacle <NUM> may be a connector for an optical connectors, optical fanouts, flexible circuits, fiber array units, and ribbonized fiber and is not limited to a MPO connector. Additionally, the first receptable <NUM> may be temporary on the first end <NUM> and may be removed for forming the completed optical fiber assembly <NUM>. A plurality of PM optical fibers <NUM> extends from the first receptacle <NUM> and the second connectors <NUM>. One common type of MPO connector is a MTP connector. It is to be appreciated that the subject invention can be used with any type of precision bored receptacle and is not limited to any particular type of connector. The receptacle <NUM> shown in <FIG> is a single MPT connector and the receptacle <NUM> shown in <FIG> is a <NUM> x <NUM> MTP connector that would receive <NUM> optical fibers <NUM>. Generally, the receptacle <NUM> has a top <NUM>, a bottom <NUM>, sides <NUM> and an open end <NUM> and a front face <NUM>. Referring to the first receptacle <NUM>, a plurality of bores <NUM>, or channels, extend between the open end <NUM> and the front face <NUM> and each bore <NUM> defines a bore diameter <NUM>. Preferably, the bores <NUM> are precision bores. Connectors are typically a single row or a double row of optical fibers <NUM> and typically provide for <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> optical fibers <NUM> for connection. In certain embodiments, the receptable <NUM> may be sacrificial and a temporary component used in forming the optical fiber assembly <NUM>. The top <NUM> of the receptacle <NUM> has an opening <NUM> for receiving adhesive or compounding material as is known to those skilled in the art for securing the optical fibers <NUM> therein when the receptable <NUM> forms a component of the optical fiber assembly <NUM>.

Referring to <FIG>, each of the PM optical fibers <NUM> include a core <NUM>, a cladding <NUM> surrounding the core <NUM>, a pair of birefringent elements, shown generally at <NUM>, such as stress rods <NUM>, disposed adjacent the core <NUM> and in the cladding <NUM> for creating a stress within the core <NUM>. The birefringent elements <NUM> may also be other stress members other than stress rods <NUM>, such as bow ties, elliptical core or clads, and the like that are well known to those skilled in PM fiber arts. <FIG> shows a perspective, close-up end view of the PM optical fiber <NUM> having the stress rods <NUM> adjacent the core <NUM>. The stress rods <NUM> are typically formed from a boron material. Each of the PM optical fibers <NUM> extends between the first end <NUM> and the second end <NUM> and each has an initial fiber diameter <NUM>. Preferably, the initial fiber diameter <NUM> is constant between the first end <NUM> and the second, or terminal, end <NUM>. However, various applications may utilize PM optical fiber <NUM> that has varying diameter without departing from the scope of the subject invention. The initial fiber diameter <NUM> of the optical fibers <NUM> is less than the bore diameter <NUM> to allow the PM optical fiber <NUM> to pass therethrough. Typically, the initial fiber diameter <NUM> may be from <NUM> to <NUM>. Another common initial fiber diameter <NUM> that may be used with the subject invention is <NUM> µm.

Referring to <FIG>, the plurality of PM optical fibers <NUM> has been inserted through the bores <NUM> of the receptacle <NUM> such that a portion of the first ends <NUM> of the respective PM optical fibers <NUM> extend past the front face <NUM> of the receptacle <NUM>. The first ends <NUM> are shown in <FIG> having the stress rods <NUM> in a random orientation. In one embodiment, the first ends <NUM> extend about <NUM> - <NUM> from the front face <NUM>. Preferably, the first ends <NUM> extend about <NUM>.

Next, the portion of the first end <NUM> of the PM optical fibers <NUM> that extend from the front face <NUM> are radiated causing the initial fiber diameter <NUM> to expand, as shown in <FIG>. In one embodiment, a laser beam <NUM> is used to radiate the first ends <NUM>, such as a highly thermalized laser beam <NUM>, preferably a carbon dioxide laser beam <NUM>, which causes the tip to intentionally flare. The laser beam <NUM> is emitted from a laser source <NUM>. The laser source <NUM> may emit a carbon dioxide laser beam <NUM> with the wavelength of <NUM>. The laser source <NUM> is positioned perpendicular or orthogonal to the optical fiber <NUM>. It is to be appreciated that other types of laser sources <NUM> having different types of beam shapes and different wavelengths may be used with the subject invention. The laser beam <NUM> may include a width that spans a single optical fiber <NUM> or all of the optical fibers <NUM> in a single pass. There may also be multiple laser sources <NUM> if there are two rows of optical fibers <NUM>. Referring to <FIG>, two laser sources <NUM> are shown emitting a wide beam toward the first ends <NUM>. Alternatively, the laser source <NUM> may be positioned above the top row and then below the bottom row, or the laser source <NUM> may sequentially radiate both of the rows. For example, the laser source <NUM> may be quantum cascade laser, CO2, UV-excimer laser, semiconductor laser, or the like, and which may emit a laser beam <NUM> with a wavelength between <NUM> and <NUM>. One example to achieve the highly thermalized energy is to utilize a slow pulsed laser having a beam that is <NUM> wide and <NUM> long.

Exposing the first ends <NUM> to the highly thermalized laser beam <NUM> results in an expanded tip <NUM> having an expanded fiber diameter that is larger than the bore diameter <NUM> at the first end <NUM> and forming an expanded portion <NUM> for a desired length along the PM optical fibers <NUM>, as best shown in the cross-sectional view of <FIG>. The expanded tip <NUM> is shown while the excess is trimmed off. In certain embodiments, the excess may not be trimmed off and only the expanded portion <NUM> is formed. The carbon dioxide laser beam <NUM> can also be used to cleave the first end <NUM> of the PM optical fiber <NUM> at the same time as the end of the optical fiber <NUM> is enlarged. By simultaneously cleaving and expanding the first end <NUM> with the laser beam <NUM>, the stress rods <NUM> become visible. It is to be appreciated that the cleaving and the expanding may be carried out as separate steps without deviating from the subject invention.

<FIG> are close-up views of the optical fibers <NUM> having been cleaved and having the expanded portion <NUM>. It is to be appreciated that the expanded portion <NUM> may only be <NUM>-<NUM> larger than the initial fiber diameter <NUM>. As one example, if the initial fiber diameter <NUM> is <NUM> and the bore diameter <NUM> is <NUM>, an increase of <NUM>-<NUM> in the expanded portion <NUM> would be larger than the bore diameter <NUM>. It is to be appreciated that expanded portion <NUM> has to be large enough to create a friction fit or mechanical interference.

In the embodiment with a single optical fiber <NUM>, the orientation of the stress rods <NUM> are determined and then aligned to present a desired orientation. In the embodiment with a plurality of optical fibers <NUM>, as shown in <FIG>, each of the optical fibers <NUM> are aligned and oriented based on the desired orientation of the stress rods <NUM>. For example, the stress rods <NUM> may be optically aligned under magnification visually or using a polarization extinction ratio (PER) meter <NUM>. If the orientation is performed visually, the first end <NUM> of the optical fiber <NUM> is under magnification to see the orientation of the stress rods <NUM> and the optical fiber <NUM> is rotated to within <NUM> degrees of the desired orientation. The rotation may be performed using a digital rotation stage <NUM>.

Alternatively, the orientation may be performed by transmitting a light through the optical fiber <NUM> and measuring a polarization extinction ratio of the light with the PER meter <NUM>. The polarization extinction ratio is communicated from the PER meter <NUM> to the digital rotation stage <NUM> to continue to rotate the optical fiber <NUM> about the optical fibers <NUM> longitudinal center until the maximum polarization extinction ratio is achieved. The alignment and orientation of the individual optical fibers <NUM> may vary based on application and end use; however, the subject invention allows for the precise alignment and orientation that can be tailored to such end uses. <FIG> is a close-up view showing that the stress rods <NUM> have been aligned to a desired orientation. Once the orientation is achieved, the optical fibers <NUM> are further retracted to mechanically lock the orientation into the bore <NUM>.

After the PM optical fiber <NUM> is enlarged and oriented, it is retracted through the corresponding bore <NUM> until the expanded portion <NUM> abuts the corresponding bore <NUM> at the front face <NUM> of the receptacle <NUM>. The optical fiber <NUM> is retracted to mechanically lock the orientation of the optical fiber <NUM> into the bore <NUM>. The retraction may be performed manually or via translation stages until seated in the bore <NUM>.

Referring to <FIG>, in embodiments with the plurality of optical fibers <NUM>, after the first optical fiber <NUM> has been oriented and retracted, the second optical fiber <NUM> is then oriented to its desired polarization state and then retracted through the corresponding bore <NUM> until the enlarged portion <NUM> abuts the corresponding second channel at the front face <NUM> of the receptacle <NUM>. It is to be appreciated that the subject invention may orient and retract multiple optical fibers <NUM> simultaneously by incorporating additional meters and rotation and translation stages to perform such steps. <FIG> is a perspective view of the MPO connector having the optical fibers <NUM> retracted into the bores <NUM>. As discussed above, retracting the optical fiber <NUM> causes a friction fit between the expanded portion <NUM> and the bore diameter <NUM>.

In one embodiment, once the stress rods <NUM> of the optical fibers <NUM> are aligned to present the desired orientation and retracted, the first ends <NUM> of the optical fibers <NUM> are finished to terminate the optical fibers <NUM> within the receptacle <NUM>, such as in the case of the MPO connector. <FIG> illustrates some of the first ends <NUM> having been finished. It is to be appreciated that the laser source <NUM> may utilize a wide beam with a width to finish all the first ends <NUM> simultaneously, or a narrow beam to finish individual first ends <NUM>. Alternatively, the finishing step may include polishing with finishing film down to Telcordia specifications. Additional finishing steps may be used with the subject invention with departing therefrom. In one embodiment, the finishing step may be performed using CO2 laser beam <NUM>. As discussed above in connection with expanding the optical fiber <NUM>, the same laser source <NUM> may be used to terminate the optical fibers <NUM>, and a singular or multiple laser sources <NUM> may be used.

The subject invention may further include the step bonding the optical fiber <NUM> into the bore <NUM> of the receptacle <NUM>. In order to bond the optical fiber <NUM> into the bore <NUM>, an adhesive may be deposited within the bore <NUM>, the receptacle <NUM>, or both to secure the optical fiber <NUM> to the receptacle <NUM>. Typically, the adhesive is disposed through the opening <NUM> in the receptacle <NUM> to further secure the optical fibers <NUM> into the receptacle <NUM>. Then, the adhesive is cured after the expanded portion <NUM> has been cleaved. Various, well known bonding adhesive may be use and the associated methods may be used to cure the adhesive. For example, the adhesive may be a UV-curing or heat-curing epoxy. For example, the finishing step may be performed using CO2 laser beam <NUM> to simultaneously finish the end and further bond the optical fibers <NUM> into the bores <NUM> of the receptacle <NUM>.

With reference back to <FIG>, the subject invention may also include the step of ribbonizing the optical fiber <NUM> that extends from the first end <NUM> of the receptacle <NUM> towards the first end <NUM> of the optical fiber <NUM> to form a ribbon fiber. The entire length from the receptacle <NUM> to the first end <NUM> of the optical fiber <NUM> can be ribbonized or a shorter length may be ribbonized depending on the particular application. Typically, up to <NUM> fibers can be grouped together. The individual fibers are aligned longitudinally, or placed side by side, to form a flat ribbon <NUM>. It is to be appreciated that the ribbon fiber may have fewer or more layers of material surrounding the cores <NUM> depending upon the particular application. The subject application is particularly useful in co-packaged optic applications because the length of the optical fibers <NUM> is fairly short, less than <NUM>, and more likely less than <NUM>. Due to the short length, once the stress rods <NUM> are aligned and oriented at the first end <NUM> in the receptacle <NUM>, the orientation remains the same the first end <NUM> of the optical fiber <NUM>.

Referring to <FIG>, another embodiment is shown having the first ends <NUM> of the optical fiber <NUM> that have been oriented and retracted into the receptacle <NUM>. Next, the portion of the optical fibers <NUM> extending from the open end <NUM> have been ribbonized. By ribbonizing the optical fibers <NUM> while the orientation is set by the interference fit in the bores <NUM>, the orientation of the birefringent elements <NUM> is also maintained at the second end <NUM>. The ribbonized cable may be formed by applying acrylate or polyimide coatings to the optical fibers <NUM>. Then, the receptacle <NUM> can be removed from the first end <NUM>. <FIG> show the optical fiber assembly <NUM> with the receptable <NUM> removed from the first end <NUM>. The first ends <NUM> may then be further connectorized, if needed, while the end-to-end polarization is maintained.

Another embodiment is shown in <FIG> having the first ends <NUM> of the optical fiber <NUM> that have been oriented and retracted into the receptacle <NUM>. Next, the second ends <NUM> of the optical fibers <NUM> have been inserted into the second connector <NUM> and secured therein. By securing the second ends <NUM> into the second connector <NUM> while the orientation is set by the interference fit between the first ends <NUM> in the bores <NUM>, the orientation of the birefringent elements <NUM> is also maintained at the second end <NUM>. Then, the receptacle <NUM> can be removed from the first end <NUM>. <FIG> show the optical fiber assembly <NUM> with the receptable <NUM> removed from the first end <NUM>. The first ends <NUM> may then be further connectorized, if needed, while the end-to-end polarization is maintained.

In addition to the embodiments shown in <FIG>, <FIG>, and <FIG>, the second end <NUM> may be processed into in a single or multi-fiber connector such as a MTP or MPO or into a waveguide for active device termination, such as a laser diode, detector, or silicon chip. Alternatively, the remaining length of optical fiber <NUM> may be processed into a flexible circuit.

The subject invention allows for laser cleaving under highly thermalized beam parameters to induce fiber tip flare as in the expanded portion <NUM>. The subject invention further allows for stress rod orientation, fast, slow, intermediate orientations; and retraction into the receptacle <NUM> or bore <NUM>. Another advantage is that the subject invention may utilize beam to spot cure and set PM fibers into matrix of avalanching epoxy system. Yet another advantage is that the connectors <NUM> formed with the subject invention have reduced polishing and provides for a single finishing film versus traditional <NUM>-step polishing sequences.

Claim 1:
A method of manufacturing an optical fiber assembly (<NUM>) having one or more polarization-maintaining optical fibers (<NUM>) having a core (<NUM>), a cladding (<NUM>) surrounding the core (<NUM>), and birefringent elements (<NUM>) embedded within the cladding (<NUM>), the optical fiber (<NUM>) having an initial fiber diameter (<NUM>) and extending between a first end (<NUM>) and a second end (<NUM>), said method comprising the steps of:
inserting the first end (<NUM>) of the optical fiber (<NUM>) through a bore (<NUM>) of a receptacle (<NUM>), the bore (<NUM>) having a bore diameter (<NUM>) larger than the initial fiber diameter (<NUM>);
extending the first end (<NUM>) of the optical fiber (<NUM>) a distance beyond a front face (<NUM>) of the receptacle (<NUM>);
radiating a portion of the first end (<NUM>) of the optical fiber (<NUM>) that extends from the front face (<NUM>) with a first high energy source to expand the portion and form an expanded tip (<NUM>) having an expanded diameter larger than the bore diameter (<NUM>) of the bore (<NUM>);
determining an orientation of the birefringent elements (<NUM>) of the optical fiber (<NUM>);
rotating the optical fiber (<NUM>) to achieve a desired orientation of the birefringent elements (<NUM>);
retracting the optical fiber (<NUM>) towards the front face (<NUM>) to cause the expanded tip (<NUM>) to interfere with and engage the bore (<NUM>) for holding the optical fiber (<NUM>) with the desired orientation of the birefringent elements (<NUM>) in the receptacle (<NUM>); and
radiating the expanded tip (<NUM>) with a second high energy source to cleave the expanded tip (<NUM>) extending from the front face (<NUM>) of the receptacle (<NUM>).