Fibers can be connectorized in a variety of manners. One manner of connectorization is to strip an outer coating from an end of an optical fiber and then glue a ferrule to the fiber. A connector housing is positioned around the ferrule.
Other methods of connectorization can be accomplished by connecting a stub fiber of a connector to the cable with a mechanical splice or a fusion splice. A mechanical splice typically involves index-matching gel. A fusion splice typically involves the application of energy to fuse the two glass fibers together.
A self-writable waveguide can be utilized to connect two optical fibers, such as a connectorized fiber stub including at least a ferrule to an optical fiber cable, with a photocurable polymer or other material to form a core and a cladding in the gap area between the fiber stub and the optical fiber cable. The final result is a cold splice having light guiding capability.
Various devices and methods are disclosed for connecting two optical fibers together with self-writable waveguides.
One aspect of the present invention relates to a self-writable waveguide that utilizes a photocurable resin composition, comprising one or more monomers and one or more photoinitiators, which forms a solid polymer bridge between the glass fiber end of one cable, such as a fiber stub and another cable, respectively. The core writing is done with light. The cladding forming is done with light or other methods, such as heat.
The core writing can be done in the ultraviolet (UV) or visible range of light. Cladding curing from the same polymerizable resin composition is one option through flood UV curing triggered by the same photoinitiator, or another photoinitiator different from the photoinitiator used to form the core. One option for curing the cladding is through flood UV curing triggered by a different photoinitiator that is sensitive at a different wavelength than the one used for the core. Alternatively, a different polymerizable resin composition can be used to form the cladding with flood UV curing. Alternatively, the core can be cured with UV light triggered by a first photoiniator, and visible light used to cure the cladding with a different photoiniator that does not act in the photocuring process of the core. Alternatively, thermal curing of the cladding can be utilized to form the cladding layer. The thermal curing can be done in parallel with a heat shrink sleeve that provides axial pull protection, or with a heat shrinkable boot.
In a polymer without a photoinitiator the light that is launched into the core leaves the fiber and has a finite angle that corresponds to the numerical aperture of the single or multimode fiber. When photoinitiators are formulated into the polymer, the refractive index of the photocured part becomes larger and the outgoing light is immediately narrowed. This focusing effect allows to photocure a core bridge between the two fibers with almost constant diameter over the gap.
The core writing step with the self-writable waveguide can be accomplished by applying light of a first wavelength through a fiber stub mounted in a ferrule of a connector to the polymer area adjacent the fiber stub end wherein the core grows toward the core of the fiber of the cable to be connected. A light of the same wavelength can be applied to the optical fiber to grow the core of the optical fiber towards the core which grows from the stub fiber.
To form the cladding, a different polymerizable resin composition can be applied around the core. If the same polymerizable resin composition is utilized, a different wavelength of light can be applied to initiate curing of the cladding to result in different indexes of refraction between the core and cladding. This can involve different types of photoinitiators and UV photocuring of the core and visible flood light curing of the cladding, such as 532 nanometers. Also, a different spectral distribution may be sufficient. For instance a laser can be used for the core formation and an LED source can be used for the cladding formation. A wavelength overlap may be possible for the formation of the core and the cladding. An alternative to flood curing can be cladding curing by a second wavelength launched via the fiber's cladding. Heat can as well be used as a curing option for the cladding if a thermal initiator is included in the monomer mixture. Use of the same wavelength may be possible in combination with two polymer compounds, one that cures faster for the core, and one that cures slower for the cladding.
The stub fiber and the optical fiber to be joined are held in alignment through an alignment device prior to exposure to the light. The alignment device can include a construction which allows for the passage of the light of the necessary wavelengths needed to cure the core and cladding polymers.
A tool may be provided to assist with proper gap formation between the fiber stub of the connector and the optical fiber of the cable. Core formation is achievable by forming a gap of a desired dimension for each of the different connections made to facilitate repeatable connections for mass production. Some variation in the gap size is anticipated.
An alternative to core-writing, followed by UV flood curing, is to first form the core with the self-writable technology, and then thermal cure the cladding. It is anticipated that known acrylates and polyimide resins work in this method.
A heat-shrink fixation can be added to secure the fiber cable to the connector if desired.
A further alternative is to form the core and the cladding at the same time by flooding and curing via the fiber. The core region will be more exposed to UV yielding a higher index of refraction. This can be a preferable way to form a good core-cladding interface.
For core writing polymerizable resin compositions comprising photoinitiators, the resin compositions need to allow for polymerization of the core. The polymerizable resin compositions can be commercially-available resin compositions, or can be prepared by the combination of one or more monomers with one or more photoinitiators. One example of a polymerizable resin composition is a Norland commercial polymer (acrylate- based) NOA72, with an example UV curing wavelength of 405 nanometers. The same polymer gives a thermal curing with a differential in the indexes of refraction to allow for proper signal propagation. Other examples are Norland commercial polymers NOA61, NOA65, and NOA81. A further example is a polymerizable resin composition with a radical base having a fluorinated-acrylate monomer mixture with Thiol and a photoinitiator (and an example UV curing wavelength of 405 nanometers).
To fabricate the self-writable waveguide technology, different approaches are possible. A cladding substitution method can be utilized where the core is fabricated first by polymerizing a first resin material, removing the uncured material, and replacing it with a second resin material which is then also polymerized to form the cladding. This approach provides flexibility during the various steps by allowing a wider choice of materials for both the core and cladding formation. In order to minimize the losses of the self-writable waveguide technology at the fiber interface, the respective mode profiles should have a maximum overlap in the different optical structures, which requires accurate tuning of the refractive index difference between core and cladding. For example, one way this was achieved was by using a mixture of Ormocore and Ormoclad (Microresist Technology GmbH, Berlin, Germany) as self-writable waveguide core material and Ormoclad for the surrounding cladding. These materials are organically modified ceramics (Ormocers).
Typically, an index of refraction delta between the core and cladding of 0.3% is desired. A core for a single mode fiber is approximately 6-15 microns in diameter and is generally cylindrical in shape. Preferably the properties of the cladding for proper signal transmission need to be present in the area directly contacting the core to a distance of around 10 microns. As the size of the cladding is increased, less attention to the optical properties of the cladding is necessary as the distance from the core increases.
The self-writable waveguides can be a desirable technology for permanently interconnecting two single mode optical fibers, such as in the factory, or in the field. A passive prealignment device is used for relative positioning of the fibers. A gap of the order of 50 to 100 microns separates the end faces and is filled with UV curable polymer or resin. Larger gaps are possible. Optical cores are written by any suitable wavelength that is launched from one or both optical fibers. The claddings are formed by thermal curing or UV flooding. A two mixture approach requires a developing step for removal of the uncured core material. Multimode optical fiber connectivity is also contemplated.
Fiber preparation prior to forming the connection may include: fibers must be cleaved (mechanical or laser cleaves are possible); cleave can be perpendicular or under an angle. Pre-treatment can be applied to the glass surface of the fiber for instance by plasma discharge or a primer can be applied to the glass or other fiber material.
Different classes of UV curable material are considered usable for the present invention. Organically modified ceramics allow for easy development and control of refractive index by mixing. Acrylates and epoxies allow for fast and repeatable core formation with well controlled core size. Primers are used to promote adhesion of the polymer to the glass (for instance by formation of covalent bonds). In commercial formulation such as NOA72 the adhesion promoter is already present in the formulation.
The steps to form a splice with core-writing technology in one example include:
The present invention utilizing the self-writable waveguide formation is an alternative to field connectorization that uses index matching gels or oils. Such index matching gels or oils can be less reliable. Self-writable waveguides are solid and do not suffer from slow evaporation like index matching gels and oils. The self-writable technology is also potentially less costly than fusion splicing in the field. Further, the self-writable technology may be used in an environment where fusion splicing would not be permitted due to spacing, a lack of a power source, or a hazard source to the user.
With respect to factory installations, the self-writable technology allows for automation, and parallel application is possible due to low curing power and higher volumes.
In one embodiment of the present invention, the two fibers are prealigned in an in-line or axial arrangement and optically connected using the self-writable waveguide technology. Such a construction could be desirable for terminating fiber stubs with preconnectorized connectors to optical cables in the factory, or in the field.
An alternative embodiment is to position the two fibers parallel to one another and use a deflection device which routes the light path 180 degrees during the core and cladding formation. In one example, each fiber faces a 45 degree reflective surface deflecting the self-writable waveguide during its formation. Other examples include fibers which are not arranged either parallel or axially, but the fibers are arranged to allow for core and cladding formation by a properly angled light deflection device or devices.
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The connectorized fiber stub 30 is shown as a ferrulized fiber. The ferrule is attached to the bare glass fiber with glue. Such a construction is a subpart of the full connector. More structure of the connector body can be present during the self-writing process, or it can be added later.
Alignment device 14 in
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A cold splice of MPO cables and connectors may be possible. The core bridges could be written in parallel.
With the above structures, formulations and methods, two optical fibers can be connected using self-writable waveguides. The result is a cold splice having light guiding capability. As noted, some of the disclosed structures, formulations and methods have advantages for field splicing and field termination. Although, the various disclosed structures, formulations, and methods may have advantages for factory splicing and factory termination. The various structures, formulations and methods can be used to connectorize a cable using stubbed connectors.
In a further example of a method of forming a self-writable waveguide, two fibers are cleaved and their end faces are separated by a distance, such as 50 micrometers, and the unpolymerized material applied in between and around the fiber tips. One example of a useful material is NOA68. Both the core and the cladding can be formed simultaneously. In one example, laser light is launched through both fibers at 10 microwatts, at 405 nanometer wavelengths for thirty seconds, and the cladding is formed at the same time by polymerization using a uniform UV flood exposure, such as Hamanatsu LC 8 with a 365 nanometer filter, for 30 seconds at 2 mWcm2.
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During the self-writing process using the apparatus of
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Filing Document | Filing Date | Country | Kind |
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PCT/EP2014/056610 | 4/2/2014 | WO | 00 |
Number | Date | Country | |
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61807596 | Apr 2013 | US | |
61946388 | Feb 2014 | US |