The present invention relates to fiber optic connectors in general and in particular to a connector component useful for terminating optical fibers for joinder of optical fiber cables, and the like, in a fiber connector.
In fiber optics based communication systems, it is necessary to have optical fiber connectors with low transmission loss and low back reflection from the fiber to fiber interface. There are two types of optical fiber connectors in general, one type is the predominant fiber connector based on physical contact and we call it “conventional” fiber connector in this application and the other type is called expanded beam connector which utilizes a lens, and is used only in limited applications.
The conventional connector designs were developed in the 1980s with an eye toward simplicity and ease of implementation. Indeed, the simplest way to ensure that there is no air gap between two fiber facets is to eliminate it through intimate physical contact. The advantages of this approach included low cost manufacturing and the ability to create connector terminations in the field, where installation occurs. Since the performance of the conventional connector was sufficient for most purposes, it is no surprise that it quickly became the standard for the fiber optics industry and has remained so for the past three decades. In fact the physical contact mechanism worked so well, most researchers of optical fiber connectors did not realize that there could be another physical mechanism to make fiber connectors.
There are two main types of conventional connectors: one type has zero degree polish angle and is called PC (physical contact) connector, the other type is called APC (angled physical contact) connector which typically has an 8 degree tilted polish angle at the fiber facet in order to minimize back reflection. PC connectors are used in places where significant back reflection can be tolerated, and APC connectors-are used where minimum back reflection is required. To ensure reliable physical contact between the fibers, both PC and APC connectors have rounded, i.e., convex, connector surfaces such that the fiber cores touch first.
While PC and APC connectors have the significant advantage of easy fiber termination by polishing, the weaknesses of this approach are readily apparent. For example, contamination between the fibers can easily disrupt the coupling of the light by creating an air gap and particulates can prevent physical contact altogether, leading to poor, unpredictable performance. In addition, as with any apparatus involving physical contact, repeated coupling of the connectors causes wear and tear, which invariably degrades optical performance over time. In fact, typical conventional fiber connectors have a rated life of 500-1000 mating cycles.
APC connectors have another significant weakness. The angled facet produces an additional requirement of rotational alignment, which is achieved by means of a key which sets the mating angle within some degree of tolerance. If this angle is not sufficiently precise, an air gap will open between the fibers, leading to significant optical loss due to Fresnel reflection. While the rounded connector facets relax the required angular precision, it is difficult in practice to ensure that the fiber is at the apex of the polish surface, thereby reducing the achievable alignment. It is generally known that APC connectors have inferior optical performance in insertion loss compared to PC connectors. Random mating performance is much worse for APC connectors.
Published U.S. application 2011/0262076 to Hall et al. recognizes that optical fibers may be terminated by being recessed from the front end face of a ferrule by a suitable distance to inhibit physical contact of the fiber with another fiber when mated in a complementary connector. However, there can be multiple reflections and interference at the two glass surfaces which tend to make the optical transmission unstable.
For applications in which harsh conditions require a more robust solution, the expanded beam connector was developed. In this approach, the divergent fiber output is collimated by a lens and travels as an expanded beam to an opposing lens and fiber assembly where it is refocused into the mating fiber. Dust, dirt and debris in the expanded optical path now scatter a much smaller fraction of the beam and therefore cause smaller coupling variation. Similarly, this design is much more tolerant to vibration and shock. The drawback to this approach is inferior optical performance in terms of insertion loss and return loss, and significantly higher complexity and manufacturing cost, all as results of significantly increased number of optical elements. Thus, the benefits come at significantly higher cost.
An objective of the invention was to devise an optical fiber connector that has very long mating life, very stable and predictable transmission, insensitive to dirt and contaminant, has guaranteed random mating performance, and low manufacturing cost.
Another objective of the invention was to devise an optical fiber connector that preserves most of the advantages of the expanded beam connectors while doing away with disadvantages.
The above objective has been met with a non-contact (“NC”) optical fiber connector that terminates a fiber optical cable and is intended to reside in a connector adapter joining optical fiber cables.
Each such fiber terminates at an output facet. A tubular ferrule having an output end and a junction end coaxially surrounds the fiber. The fiber output facet has a concave offset relative to the surrounding endwise surface of the ferrule, such that when two aligned abutting ferrules of a fiber coupling device are mutually facing and in contact, a small gap of micron level is present between the fiber facets. The endwise surface of the ferrule is preferably convex. The gap is sufficiently small so as to allow the light to couple easily between the fiber cores for optical communication. To substantially eliminate the transmission loss at air-fiber interfaces, the fiber facets are coated with a durable anti-reflection (“AR”) coating. The means for providing the concave offset can be either an indentation of the fiber relative to the endwise surface of the ferrule or, alternating, a built up spacer on the endwise surface of the ferrule relative to the fiber facet, such as by an annular metal deposit.
In a preferred embodiment, the fiber inside the AR coated fiber ferrule is bare fiber and therefore causes minimal outgassing in a vacuum AR coating chamber and permits very large number of such ferrules to be coated simultaneously, thereby reducing the AR coating cost for each ferrule assembly. The rear end of the fiber at the above AR coated connector ferrule can be cleaved, and fusion spliced to a typically reinforced fiber cable, as in known splice-on connectors.
Advantages of the NC coupling device include excellent optical performance in insertion loss and return loss, excellent mating repeatability, greater predictability, and long service life over repeated couplings. The design is inherently more tolerant of particulates and contamination at the interface and thus more user-friendly. It is field installable by fusion splicing to a long cable. Finally, it is expected that the present invention may be produced at only slightly higher cost than conventional fiber connectors, and at much lower cost than the expanded beam connector solution.
With reference to
We now describe the non-contact fiber connector component in
The fiber ferrule assemblies are then polished at the light output end so as to render a smooth surface 17 on the ferrule 10. The polish angle, measured as tilt from vertical at the fiber core, where vertical is perpendicular to the fiber axis, can be zero degrees, or non-zero degrees to minimize back reflection. In a preferred embodiment, the polish angle is 8 degrees. Just as in conventional fiber connectors where the connector ferrule surface is a convex surface, ferrule front surface 17 should be convex as well.
The polishing process for non-contact fiber connectors in this invention is very similar to conventional connector polishing, except the final polishing step. After a fiber stub removal step, a series of progressively finer lapping films are used to polish the connector surface, typically from 9 micron, 3 micron, to 1 micron diamond particles. Final polish step is then performed.
The final polishing step in this invention is different from conventional connector polishing, and is the step responsible for forming the recess in the fiber. In this step, the fiber is preferentially and differentially polished relative to the ferrule front surface so as to create a recess between the fiber facet 13 and ferrule front face 17. The recess range should be kept as small as possible to reduce optical coupling loss, while ensuring no physical contact between the opposing fiber facets when mated.
For a single mode fiber SMF-28, the light beam is best described as a Gaussian beam. In air, the working distance (Rayleigh range) is about 100 micron. If the fiber recess is 0.5 micron, light from the fiber core traveling twice the recess length does not expand sufficiently to induce significant optical coupling loss. The extent of a recess is preferably in the range of 0.1 microns to several microns.
The recessed fiber facet 13 in
Finally, an AR coating 40 is applied to the polished surface of the fiber 13 and front surface of the ferrule 17. The operating wavelength range of the AR coating determines the operating wavelength range of the non-contact optical fiber connector in this invention.
In a preferred embodiment, many polished fiber ferrule assemblies are loaded into a vacuum coating chamber and coated with a multi-layer stack of dielectric materials. Numerous AR coating processes can be used. For example, the coating method can be ion beam sputtering or ion-assisted e-beam deposition. Care should be taken to prevent significant amount of the coating material from getting on the sidewall of the ferrule cylindrical surface, by suitable masking. Otherwise the material will alter the precision diameter of the ferrule, and cause flaking off of coating material which will affect connector performance.
The fiber cables to be coated in an AR coating chamber must not outgas significantly in a vacuum chamber. We have observed that the inclusion of a mere ten 0.9 mm loose tube buffered cables in the chamber can lengthen the vacuum pumping time from 2 hours to more than ten hours for ion beam sputtering. The materials of the fiber cable must be chosen carefully to reduce outgassing. Bare fibers housed in ferrules in the AR coating chamber are optimal.
We have polished more than 500 non-contact fiber connectors with zero scratches, which is very different from the final polish step of conventional connectors where scratches are frequent and inspection and repolishing are required. As a result, 100% inspection of connector polishing after final polish step becomes unnecessary which can save significant manual labor cost.
Several hundred non-contact fiber connectors with recessed fiber facets have been made to date with great manufacturing yield. Both zero degree and 8° angled non-contact (ANC) single mode fiber connector were made.
The insertion loss of both zero degree and 8° ANC connectors shows nearly identical loss distribution to that of conventional fiber connectors. The insertion loss in all three cases is dominated by the errors in the fiber core positions due to geometrical tolerances.
A mated pair of zero degree NC connectors has about 30 dB return loss, while a mated pair of 8 degree ANC connectors has more than 70 dB return loss, or about 10 dB higher return loss than conventional 8 degree APC connectors.
Both NC and ANC connectors have essentially guaranteed insertion loss performance in random mating. Therefore, an ANC connector is the preferred connector because it has superior return loss performance.
We have tested a pair of ANC connectors and found it lasted through 10,000 matings with less than 0.01 dB insertion loss change from the beginning of the test to the end.
The non-contact fiber connector of the type shown in
In this embodiment, the fiber ferrule assembly can be polished using a conventional connector polishing process. The result of this polishing process is that the fiber is at the apex of the convex surface. The polishing angle can be zero degrees or 8 degrees. The metal coating can be accomplished by a suitable masking operation so that the metal does not cover the fiber surface. Note that the AR coating 40 covers both the output facet 13 of the fiber 20 and the front surface 17 of ferrule 10.
In conventional connector cables, frequently a long length of reinforced fiber cable is used between two optical fiber connectors. For example, one of the most used fiber cable is a 3 mm diameter cable with Kevlar fabric reinforcement. Such a cable will outgas greatly in a vacuum chamber, occupy too much room and difficult to manage inside the AR coating chamber. Clearly AR coating entire fiber connector cables in an AR coating chamber is not an option.
Instead, only the most essential part of the connector with very short length fiber should be loaded in. After AR coating, such short fiber should be connected to the long reinforced cable by fusion splicing, which is a very reliable and relatively low cost fiber connection method.
Splice-on connectors are known in the prior art. These are conventional connectors that have factory-polished connector surfaces with a short length of cleaved fiber at the rear of the connector head ready for fusion splicing to a long length of typically reinforced fiber cable.
In
The non-contact fiber connector operating principle established above can be used for multi-fiber connectors as well, such as MT type array connectors.
When a multi-fiber connection is made using two non-contact multi-fiber connectors as in
Fiber facets 720 can be offset from ferrule block front surface by a number of means. Selective etching, differential polishing, metal deposition, or simply deforming the polished ferrule surface can all achieve non-contact of fiber facets. In all cases, small gaps between facing fibers can communicate optical signals from fiber cables to mating cables. The facets can have a slight angle, say 8 degrees.
This application claims priority from provisional application Ser. No. 61/579,017, entitled “Non-Contact Optical Fiber Connector”, and filed on Dec. 22, 2011.
Number | Date | Country | |
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61579017 | Dec 2011 | US |