Patent ID: 12259582

DETAILED DESCRIPTION

Turning to the drawings in detail,FIGS.1a-1bdepict perspective and cross-sectional views of a non-contact multi-fiber connector ferrule10formed by the method of the present invention.

The non-contact multi-fiber connector ferrule10includes a ferrule chip20and a ferrule pedestal30. The ferrule chip20is significant because it provides the very high precision of this non-contact multi-fiber connector ferrule10.

The ferrule chip20has at least four through holes, including two guide holes35and two fiber holes25. Each guide hole35is dimensioned and configured to hold and align a guide pin. Each fiber hole25is dimensioned and configured to hold and align an optical fiber. Guide holes35facilitate passive alignment between mating multi-fiber optical fiber connectors.

While twenty-five fiber holes (5×5 array) are depicted for clarity of presentation, it is understood that substantially larger numbers of fiber holes may be formed in the multi-fiber optical connectors of the present invention. For example, a 2D array of 12×12 fiber holes or 24×24 fiber holes may also be formed in the multi-fiber optical connectors. Those of ordinary skill in the optical connector art will appreciate that an arbitrary number of fiber holes in arbitrary configurations may be formed using the disclosed techniques.

Inserted in each fiber hole25is an optical fiber40which may be a single mode (SM) or multimode optical fiber. The precise microfabrication techniques used to make the ferrule chip create a connector where the fiber alignment precision is sufficient for the smaller core of the SM optical fibers. As seen inFIG.1beach fiber endface45is recessed from a ferrule contact surface50of ferrule chip20. An antireflection (AR) coating55is deposited over each fiber endface45and on the ferrule contact surface50. Guide holes35are optionally included to facilitate passive alignment between mating optical connectors.

To manufacture the ferrule chip20, a ferrule mold is microfabricated having “chimney-like” pillars for the fiber holes and guide holes, as depicted in the example microfabrication process ofFIGS.2a-2l, including the steps of mask preparation, photolithography, dry etching, metal electroplating and demolding.

In the example ofFIGS.2a-2l, the central axis of the fiber hole is slightly tilted with respect to the surface normal of the ferrule chip. In this exemplary embodiment, the tilt angle is selected to be 6 degrees. The tilt angle is selected to minimize return loss for the optical fiber connections. However, it is understood that the microfabrication process depicted inFIGS.2a-2lmay also be used to form other angles or multi-fiber optical connectors with no angle.

The microfabrication process ofFIGS.2a-2lare performed using silicon as the mold material to form the ferrule chip20. However, other materials may be used to form the mold material such as photoresist and other polymeric materials. Other semiconductor materials such as germanium may also be used. Metals may also be used. In short, any material that is susceptible to microfabrication techniques and can be formed with submicron accuracy may be used as the mold material to form ferrule chip20.

Wafer Bonding

Referring toFIG.2a, there is a silicon wafer100that serves as the substrate for microfabrication, and another silicon wafer102that serves as an optional support substrate. The two wafers100and102are bonded together and there is a metal layer104between the two silicon wafers for the subsequent electroplating step. The metal layer104is preferably a thick layer of metal such as tin, aluminum, gold, or alloys thereof.

Photoresist Coating

InFIG.2b, the silicon wafer100is shown coated with a photoresist layer106on its top surface. The photoresist may be either organic or inorganic, positive or negative depending on the selected mask design. It may be applied by any appropriate techniques, including but not limited to dipping, spraying, spinning or vapor deposition. In an exemplary embodiment, a spin-coated 5740 Novolak photoresist with a 2 micron thick layer is used as layer106.

Photomask

A photolithographic mask200is used for microfabrication. Referring toFIG.2c, the photomask includes designed patterns of guide holes204, an array of fiber holes206and optionally ferrule chip wall202. Fiber holes and guide holes can take other forms than round hole.

Mask200assumes a positive photoresist, so that exposed photoresist is removed in a subsequent development step.

UV Exposure

As shown inFIG.2d, photolithography is performed with the photomask200. UV radiation302(or other selected radiation, including x-rays, laser radiation, etc., depending upon the selected photoresist) replicates the photomask pattern on the photoresist106.

Photoresist Development

After photoresist development (involving removal of the exposed photoresist), the pattern shown inFIG.2eis formed. Areas intended to be guide holes and fiber holes are protected by photoresist304, which serves as an etch barrier during subsequent processing.

Dry Etching

The silicon wafer100is etched in a dry etching process, as shown inFIG.2f. In a preferred embodiment, etching should be done using a DRIE (deep reactive ion etching) process or other microfabrication techniques which can provide sub-micron accuracy.

Dry etching plasma308will etch through the silicon wafer100from the top surface until it reaches an etch stop layer, and metal layer104is exposed.

In a preferred embodiment, the wafer should be tilted with a small tilt angle with respect to the dry etching direction. The preferred tilt angle is 6 degree.

Stripping Photoresist

After the dry etching step, the remaining photoresist304should be stripped, so that a silicon ferrule mold is completed, including pillars for fiber holes312and pillars for guide holes310, and a ferrule chip wall314, as shown inFIG.2g(side view). A perspective view of the silicon ferrule mold is shown inFIG.2h. In the preferred embodiment, the fabricated pillars312and310should have slightly enlarged bottom.

The structure depicted inFIG.2gwill serve as a mold for the ferrule chip as seen in the subsequent processing steps.

The ferrule mold can be formed using any microfabrication method which can provide sub-micron accuracy, including but not limited to LIGA, UV LIGA, and laser micro-processing. Another preferred method to form the ferrule mold is by using SU-8 photopolymer.

Electroplating

An electroplating process to form the ferrule chip is conducted by making electrical contact to the metal layer104in an electroplating bath, to gradually fill up the empty space of the silicon ferrule mold with metal320from the metal layer104, until it overflows from the top of the silicon pillars.FIG.2iis a schematic view of the ferrule mold filled with metal to form the ferrule chip after electroplating is completed.

A preferred material for metal320may be a nickel alloy. However, other materials having over 50 GPa Young's modulus can be used. The mold filling method is not limited to electroplating. For example, stamping or nanoimprint methods may also be used to create the metal ferrule chip20.

Other deposition methods may also be selected depending upon the material to be used to form the ferrule. These include vapor deposition (chemical vapor deposition, evaporation, sputtering, ion beam deposition) or molding techniques when ceramic-based materials are selected for the ferrule.

Polishing to Remove Excess Metal

The electroplated wafer ofFIG.2iis polished along polishing plane322to remove the over-deposited metal, ensuring a flat surface of the fabricated ferrule chip.FIG.2jis a schematic view of the ferrule chip still in the ferrule mold after the polishing step is completed.

Ferrule Mold Release

Metal ferrule chips20are released from the silicon ferrule mold by etching away the silicon wafers by a suitable process, for example wet etching. Mechanical methods of mold release may also be selected. Dicing may be unnecessary because the ferrule chips are separated by ferrule chip wall314.FIG.2kis the finished ferrule chip20.

Ferrule Chip Configuration

FIG.2lis the ferrule chip20in further detail. The ferrule chip20is a thin layer of metal material. It has two sets of through-holes, including two guide holes330and five fiber through-holes332. Each guide hole330is dimensioned and configured to hold and align a guide pin, while each fiber hole332is dimensioned and configured to hold and align an optical fiber.

In the preferred embodiment the ferrule chip20has a thickness of approximately 700 microns. This thickness provides sufficient structural integrity, while being thin enough to be made economically using a microfabrication process.

While a 25-count array of fiber holes is depicted for clarity of presentation (5 in cross section times five rows deep), it is understood that substantially larger numbers of fiber holes in arbitrary configurations may be formed in the multi-fiber connector ferrule of the present invention. For example, a 2D array of 12×12 fiber holes or 24×24 fiber holes may be made using the method of the present invention.

Tilt Angle

As shown inFIG.2l, the central axes340of guide holes330and central axes342of fiber holes332should be parallel to each other. In one embodiment, there is a small tilt angle350between the hole axis340and the surface normal344of the ferrule chip20. This tilt angle may help to improve the multi-fiber optical connector's return loss. This tilt angle may be preferably from 5-8 degree, but may be other angles, for example, from 1 degree to 15 degree.

Fiber Hole and Guide Hole Profile

As shown inFIG.2l, near top surface334of the ferrule chip20, the fiber hole and guide hole diameters are uniform along the central axis340and342to provide accurate fiber angular alignment, but become wider to facilitate easy fiber insertion from the back side336of the ferrule chip20.

Fiber holes and guide holes are most accurate on top surface334of the ferrule chip20, where guide holes330and fiber holes332have sub-micron accuracy in their central locations due to the microfabrication process. This accuracy ensures accurate alignment of SM fibers with minimum insertion loss.

Combining Ferrule Chip with Ferrule Pedestal

FIG.3ashows a multi-fiber connector ferrule10, including a ferrule chip20and a ferrule pedestal30. Because the ferrule chip20is a thin layer, the multi-fiber connector ferrule10optionally uses additional structure during later processing and in eventual operation inside an optical fiber connector housing. The ferrule chip20may be affixed permanently to a ferrule pedestal30to form the multi-fiber connector ferrule10.

The ferrule pedestal30may include a flange402, a fiber cavity404, two guide pin cavities406for fibers and guide pins to pass through, respectively. Fiber cavity404is preferably separate from the two guide pin cavities406. By separating the fiber cavity, epoxy used to affix the fibers does not flow into and block the guide holes330.

Guide pin cavities406may have a slightly larger diameter than the diameter of the guide hole330near the ferrule top surface334, to avoid interference with the operation of the guide holes330. However, the guide pin cavities406may have a narrow part near their bottom portions, to define the angle of the guide pins (two points determine a line).

The ferrule pedestal30may have a flange402for easy positioning and surface registration. The ferrule pedestal30may have a tilted surface408, and the angle of the tilted surface is the same as tilt angle350of the ferrule chip20inFIG.2l, so that fiber hole axis342is perpendicular to the flange surface402. The flange design facilitates a subsequent polishing operation, and when the multi-fiber connector ferrule10is assembled into a connector housing, not shown. The ferrule pedestal30may be formed from plastics, metal, or any other rigid material.

Fiber Insertion

FIG.3bdepicts a cross-sectional view of the multi-fiber connector ferrule10, after fibers412are inserted through the fiber holes and permanently affixed with epoxy or other suitable fixing agent. The optical fiber412may be a SM or multimode optical fiber. A length of fiber with plastic jacket stripped414is shown inFIG.3b.

Care must be taken to ensure that no epoxy flows into the guide holes330. Care must be taken to ensure that epoxy does not flow downward along the fibers412through a capillary effect and make the fibers412too rigid. Otherwise it would require too much force on the guide holes330to passively align the connector ferrule10.

As shown inFIG.3b, after affixing the fibers with epoxy, the fibers414protrude from the ferrule chip surface334.

The multi-fiber connector ferrule10is polished along polishing plane420, which is parallel to original ferrule chip top surface334. This ensures that very little ferrule chip material is polished away.

Because fiber holes and guide holes are most accurate on top surface334of the ferrule chip20, and plane420is very close to plane334, fiber holes and guide holes are very accurate on plane420as well.

Recessed Fiber Endfaces

The multi-fiber connector ferrule10should have fiber endfaces418slightly recessed from the new ferrule chip top surface420, as shown inFIG.3c.

To achieve the recessed fiber endfaces418, one embodiment uses a differential polishing process. This differential polishing process uses cerium oxide as the final polishing particle. During differential polishing, the glass material of the optical fiber is removed at a greater rate than the nickel alloy material of ferrule chip20. Therefore, the fiber endfaces418are recessed with respect to the surrounding ferrule chip top surface420.

Fiber endfaces418are prevented from making contact with opposing fiber endfaces in a mating connector by the recessed fiber endfaces. For SM fiber, a recess of approximately 0.5-1.0 micron is sufficient to prevent fiber endfaces418from making contact with mating fiber endfaces.

Recessed fiber depth can be made larger (3-5 microns for example) so that any large contaminants such as dust would be contained in the recess without affecting fiber endfaces418. This larger recess makes the multi-fiber connector more tolerant of dusty and contaminated environments. Recessed fiber depth from 0.1 micron to 10 microns may be selected.

An alternative method to achieve recessed fiber endface418is to deposit a thin layer of spacer material on the top surface420of the ferrule chip20. One preferred embodiment may be additional electroplating after fibers are polished, to plate a thin layer of metal such as nickel on the metal surface of the ferrule chip420, causing the fiber endfaces418to be recessed from the surrounding ferrule chip surface. Because the silica fiber is non-conducting, electroplated nickel will not cover the fiber endfaces418and therefore a spacer is formed.

AR Coating

In order to eliminate the multiple reflections between two fiber endfaces with an air gap in between, an antireflection (AR) coating layer424is coated over the fiber endfaces418as shown inFIG.3d. Typically the top surface420of the ferrule chip20is also coated. The AR coating layer424is intended to cover the entire surface, except guide holes330which are protected by a suitable deposition mask.

The AR coating band is selected according to the operating wavelength range of the optical system in which the connector is to be used. An AR coating with residual reflectivity of less than 0.2% may be selected. The thickness of the AR coating is typically on the order of 1 micron. This thickness is sufficient to achieve the desired antireflection properties.

Male Ferrule Chip

FIG.3eshows a cross-sectional view of the multi-fiber connector ferrule10with guide pins426in place to create a “male ferrule”. A perspective view of the same part is shown inFIG.3f. Guide pins426typically have a chamfered tip which facilitates insertion into the guide holes. Guide pins426may be made of stainless steel with a precise diameter and smooth surface. However, other rigid materials such as ceramics may also be used to make the guide pins426.

Two Ferrules Mated Together

FIG.3gdepicts a cross-sectional view of female and male multi-fiber connector ferrules mated together. The AR coating layers424of the two opposing connector ferrules contact each other.

Guide pins426are positioned by the guide holes to align the top connector ferrule to the bottom connector ferrule, thereby achieving precise sub-micron alignment of each of the fibers412with mating fibers of the opposing connector ferrule. The presence of fiber recess will prevent the fiber endfaces from making contact. The AR coating424on the surface of the ferrule chip20will make contact during the operation of the connector, but because the connector ferrule surface is flat and large, the force is distributed over a larger area, preventing flaking or chipping of AR coating424.

Optional Tilt Angle

The reasons for the optionally tilted fiber holes and guide holes are the following. It is well known that fiber connector endfaces are often polished at an angle (for example 8 degree) in order to have a high return loss value, for example, above 60 dB. If fiber holes and guide holes are formed in a new ferrule chip440with a tilt angle of zero degree, as illustrated inFIGS.4aand4b, then the new ferrule chip440may optionally be polished with a tilt angle of 8 degree, for example.

FIG.4ashows the new ferrule chip440with fibers412inserted and affixed with epoxy. Polishing plane450is shown. Fiber holes and guide holes are most accurate on the top surface460of the ferrule chip440.

FIG.4bshows the female multi-fiber connector ferrule after the polishing step, ready to receive guide pins426. Two consequences can be seen:

An 8-degree polishing step could polish away one side of the ferrule chip440completely, because the ferrule chip is only about 0.7 mm thick in the preferred embodiment.

After a polishing step, guide hole openings may not be accurate enough to align guide pins426, because the most accurate part of the guide holes on the top surface460ofFIG.4amay be polished away, resulting in enlarged guide hole openings. Likewise, fiber endface positions may not be sufficiently accurate.

The major benefit of the tilt angle is that grinding away of significant ferrule chip material is avoided. Referring toFIG.3c, all fiber hole openings and guide hole openings have the same shape after a polishing step. Fiber holes and guide holes are the most accurate, because they are polished very little.

In a preferred embodiment for high return loss, the tilt angle is chosen to be 6 degree or 8 degree, but it can be any angle from 1 degree to 15 degree. If high return loss is not a concern, a tilt angle of zero degree may also be selected.

Optical Performance of the Multi-Fiber Connector

Because the multi-fiber connectors disclosed here are formed by photolithographic processes, the center positions of the guide holes and fiber holes have tolerances of about 0.1 micron, and the diameters of the guide holes and fiber holes can be controlled to have a variation of less than 1 micron.

In terms of optical performance, SM multi-fiber connectors made with the method disclosed herein can have insertion loss range from 0 to 0.3 dB, and return loss from 55 to 80 dB. This is approximately the same insertion loss as a SM optical fiber connector (LC, SC, FC etc.) and thus is acceptable in the “loss budget” of an optical system. The fabrication techniques are reproduceable and scalable to mass-production of multi-fiber connectors.

Metal ferrules made of materials such as nickel have many advantages compared to molded plastic MT ferrules.

A metal connector ferrule has much greater hardness, much lower coefficient of thermal expansion (CTE), much better thermal stability, can withstand a much higher processing temperature, will outgas much less in a vacuum environment such as an AR coating chamber, has much better resistance to guide pin abrasion and much longer mating lifetime, and has smaller electromagnetic interference due to the conductive metal material.

Photomask Pattern Elongation Due to Tilt Angle

Referring toFIG.2l, because the tilted fiber holes and guide holes are sized to receive round fibers and round guide pins, the cross section of fiber holes and guide holes must be round on a plane perpendicular to the fiber axis. A 2D array of round fiber hole patterns with equal pitch in the two directions on plane338will become a 2D array of elliptical fiber hole patterns with an unequal pitch when projected on the ferrule chip surface334, the latter of which are the patterns that the photomask must be designed with. The length of the photomask patterns is “stretched” in one direction with a scaling factor of 1/cos(θ).

Preferably, the tilt angle θ should be in one direction of the 2D fiber hole array, although this is not necessary.

If the tilt angle θ is 6 degree, the scaling factor is 1.01. This is a small correction; however, it ensures sub-micron alignment of fibers in the fiber holes. For example, a round hole of 126 micron diameter on plane334will become an elliptical hole of sizes 126 micron and 124.7 micron on plane338, with the smaller number being too small for a 125 micron diameter fiber.

In order to form a 2D array of fiber holes of 126 micron diameter with 250 micron pitch with a tilt angle of 6 degree, a photomask may be designed to have a 2D array of elliptical holes of sizes 126 micron and 127.26 micron with a pitch of 250 micron and 252.5 micron in the two orthogonal directions.

Dust Channel in the Guide Hole

Because the manufacturing method disclosed herein uses microfabrication, arbitrarily-shaped guide holes may be formed. Various modifications to the guide hole may be made.

In conventional MPO connectors, guide holes are made by molding plastics around a cylindrical object; therefore, it is nearly impossible to have any other shape for the guide holes than round guide holes. Dust or contaminants tend to build up and clog the micron-sized gap between the round guide pin and round guide hole.

FIG.5shows top view of a ferrule chip with modified guide holes500, which has original unmodified sidewall parts502which contact the round guide pin, and modified sidewall parts with “dust channels”504which do not contact the round guide pin. The guide hole's dust channels504run vertically along the side walls. Alignment of the guide pin is provided by the unmodified sidewall502. With this structure, small amount of dust or contamination on the guide pins is pushed into the dust channels of the guide holes during connector mating. Therefore, enhanced dust tolerance of this connector is achieved without reducing the alignment accuracy of the multi-fiber connector ferrule and without sacrificing any optical performance

Spring-Loaded Guide Hole

FIG.6shows top view of a ferrule chip with modified guide holes600which has spring-loaded sidewalls. By forming hollow channels602, part of the sidewall604of the guide hole has a thin, deformable wall. The deformable wall acts as a spring-like structure in deforming and resuming its original shape and thus can act as a spring-loaded guide hole.

There are two benefits to a spring-loaded guide hole600. First, guide pins slightly larger than the undeformed guide hole diameter can be inserted when the spring-loaded wall structure deforms, leaving zero gap between the guide pin and guide hole, and a more precise self-centered alignment. Second, due to the adjustable nature of the spring-loaded guide hole, the guide pin does not need to have the same diameter tolerance that a fixed guide hole would require. The cost of the guide pins is reduced as a result.

It is important to ensure the hollow channels602are not filled with debris. Otherwise the spring-loaded guide hole600may be affected.

Elongated Female Guide Hole

Referring toFIG.3g, the distance between the two guide holes has a tolerance of less than 1 micron. However, at the tips of the guide pins of the multi-fiber connector ferrule, the distance between the two guide pins is frequently not exactly the same as that of the guide hole distance, due to the guide pins being not parallel to each other, or due to slight bending of the guide pins. When the two guide pins first enter the two female guide holes, this mismatch will cause excessive abrasion of the guide pins and guide holes. The entrance of the guide holes tends to be eroded quickly, resulting in the loss of precision of guide hole alignment, and the reduction of the mating life of the multi-fiber connector.

To greatly reduce the above mismatch problem,FIG.7shows top view of a female ferrule chip with modified guide holes, where one guide hole704is round while another guide hole700has an elongated profile in the direction702. The round guide hole704serves as a position-defining hole, and the elongated guide hole700serves as a rotation-limiting hole.

This ferrule chip is useful as a female ferrule chip. A male ferrule chip should still have two round guide holes.

This design provides relief for the mismatch mentioned above, without reducing the alignment accuracy of the multi-fiber connector pair. The guide pins of the male multi-fiber connector ferrule can be inserted into the guide holes of the female multi-fiber connector ferrule even at a slight angle.

This design would have been very difficult to implement using plastic molding processes by which traditional MT ferrules are made. However, because of the microfabrication process disclosed here, arbitrary shaped guide holes can be formed at will, with great positional accuracy.

It will be appreciated by those skilled in the art, in view of these teachings, that alternative embodiments may be implemented without deviating from the spirit or scope of the invention, as set forth in the appended claims. This invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.