Patent Application
20170205579
The present invention relates in general to optical fibers carrying high-power laser-radiation. The invention relates in particular to such optical fibers which have been spliced, or have been otherwise modified to allow laser-radiation to escape from cladding of the optical fibers.
Optical fibers used to generate or transport high power optical radiation, for example radiation having a power of about one to several kilowatts (kW), often have a dual waveguide structure, with an inner “core” waveguide defined by the glass refractive index profile near the center of the fiber and an outer “cladding” waveguide, which is defined by the glass and polymer refractive-index profile near an outer edge of the fiber. Even when most of the power is carried by the core waveguide, there can still be significant optical power of radiation in the cladding waveguide. The cladding radiation may be optical pump-radiation used to energize an active optical fiber, or higher-order unwanted modes of generated radiation. In any event, it is usually necessary to remove this cladding-mode radiation from the fiber before it reaches a point of use, or a point where it can burn some other component in a system in which the fiber is utilized.
Devices used for removing high-power cladding-mode radiation are usually referred to by practitioners of the art as “mode-strippers”. Mode-stripping is typically effected by removing a protective polymer coating from a section of the fiber, and then modifying that section to reduce the optical waveguide efficiency so the cladding radiation escapes from the fiber. One such modification means is etching the cladding surface so that power is coupled out of the cladding by scattering. Another such modification means is reducing the diameter of the cladding at a predetermined location without reducing the diameter of the core. Such modification is achieved by splicing together two fibers with different outer diameters. This is commonly referred to by practitioners of the art as a down-splice. The annular cross section of the larger fiber that does not overlap the smaller fiber at this “down-splice” acts as a window to couple cladding light out of the fiber.
Eliminating the protective polymer layer in the modified section allows such modified fibers to withstand high optical powers, but leaves the modified fibers more fragile than unmodified fibers, and in need of protection from environmental contamination. Such protection is typically provided by an enclosure for the modified portion of the fiber. When the modified portions of such fibers are mounted in a protective enclosure, the enclosure must also absorb and dissipate the “stripped” radiation. Such an enclosure may be sealed to minimize environmental contamination, and may be fluid-cooled if the power of stripped radiation is sufficiently high.
A problem frequently encountered with such mode-strippers is that the portion of fiber within the enclosure can be damaged or broken by mechanical stresses imposed on the fiber by differential thermal expansion between the fiber and the enclosure. There is a need for a mode-stripper arrangement that can reduce such stresses to benign levels for reducing, if not altogether eliminating, fiber breakage.
In one aspect, optical apparatus in accordance with the present invention comprises an enclosure having first and second ends. An optical fiber having a core and cladding extends through the enclosure from the first end thereof to the second end thereof. The optical fiber has a modified portion thereof within the enclosure. The optical fiber is fixedly attached to the enclosure at the first end thereof, and attached to the enclosure at the second end thereof by a flexible diaphragm.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Turning now to the drawings, wherein like features are designated by like reference numerals,
Base 12 and cover 14 are preferably made from a material having a high thermal conductivity. A metal is preferred, for ease of machining. One suitable metal is copper (Cu). A ceramic material such as aluminum nitride (AlN) may be used. This has a closer coefficient of thermal-expansion-match to fiber 16 than has copper, but may present difficulty in machining required complex shapes therein.
Referring in particular to
A flexible membrane 18 is attached to a protective-coated portion 16A of the fiber. The flexible membrane is preferably formed from an elastomer, such as a silicone elastomer. One suitable silicone elastomer is RTV615, which is commercially available from a number of suppliers. RTV615 is a clear liquid, which cures at room-temperature to high strength silicone rubber (elastomer) with the addition of curing-agents. The RTV615 is supplied with curing-agent in matched kits, which are designed for use at a convenient 10:1 ratio by weight.
The membrane may be pre-formed, then slipped onto the fiber, and attached to the fiber, using a silicone adhesive, at a predetermined point on a coated portion 16A thereof. Alternatively, the membrane may be molded onto the coated portion of the fiber, using a suitable mold.
Regarding dimensions of the membrane, a preferred thickness of a RTV615 silicone membrane is 1.0 millimeter (mm), and a preferred diameter is between about 2 mm and about 6 mm. By way of example, a thickness of 1.0 mm and a diameter of 4 mm will allow optical fiber 16 to travel about 100 micrometers (μm) with a stress of less than 1.0 Newton (N) imposed thereon. A force of 1 N results in a stress of 1.27 Megapascal (MPa) on a 500 μm-diameter fiber. This is about 1.8% of the typical proof-stress of such a fiber. If aluminum nitride is used to form base 12 and cover 14, membrane 18 may have a diameter as small as 1 mm, and produce the same result.
Continuing now with reference to
Surrounding slot 20 is channel 22, into which uncured elastomer may be injected to form a seal between base 12 and cover 14, after the mode-stripper is assembled. There is a groove 19 at each end of base 12 arranged to support optical fiber 16. The groove communicates with slot 20 and channel 22. There is a corresponding channel and groove (not visible) in cover 14 of
In assembling the inventive mode-stripper, optical fiber 16 is placed on base 12 with coated portions 16A of the optical fiber seated in groove 19, at each end of the base. Membrane 18 on the optical fiber is accommodated in a semicircular recess 24 in base 12. There is a corresponding recess in cover 14. The membrane may be attached to the recess with a silicone adhesive or the like.
Once the fiber is seated correctly on base 12, cover 14 can be attached to base 12 by screws 15, as noted above, to form enclosure 11. When the cover is thus attached, liquid elastomer (with curing agent) is injected into channel 22. The liquid elastomer flows around channel 22, and, when cured, forms a seal between base 12 and cover 14. The elastomer also surrounds coated portion 16A of optical fiber 16 at proximal end 11A of enclosure.
When cured, the elastomer seals optical fiber 16 to the enclosure at end 11A thereof, providing a rigid or fixed attachment of that portion of the fiber to the enclosure. The terminology “rigid or fixed attachment”, as used in this description and the appended claims, means fixed to the extent that some minimal compliance may be offered by the elastomer seal. This seal should be kept as thin as practical to provide good thermal communication between the optical fiber and the enclosure.
At end 11B of the enclosure, the edge of membrane 18 on the optical fiber is attached to recess 24. A recess 26 in base 12, together with a corresponding recess in cover 14 (see
Splice-holder 40 includes an enclosure 42, formed from a radiation-absorbing base 44 and a cover 46. A spliced optical fiber 17 extends through the enclosure from proximal end 42A thereof to distal end 42B thereof. The optical fiber has an inventive membrane 18 attached on a protective-coated portion 17A of the optical fiber The membrane is attached to the enclosure by elastomer 50 filling a recess 48 in the enclosure. The recess is formed by corresponding cut-outs in base 44 and cover 46. A port 54 provides for injection of liquid elastomer/curing-agent mixture for purposes described further hereinbelow. Water-cooling ports 52 are provided in cover 46. A detailed description of the water-cooling arrangements is not provided herein, for reasons noted above regarding water-cooling arrangements for mode-stripper 10.
The cladding of stripped portion 17B1 has a greater diameter than the cladding of stripped portion 17B2. The core diameter is the same in both portions. The fiber sections are joined by a splice 17C between stripped portions 17B1 and 17B2. The spliced-together portions constitute a “modified” portion of fiber 17.
Base 44 has a channel 56 extending therethrough. The channel is deep enough and wide enough to accommodate protective-coated portions 17A1 and 17A2 of optical fiber 17. Optical fiber 17 is attached to channel 56 by an elastomer-bead 58 surrounding fiber portion 17A2 at end 42A of the base of the enclosure. Membrane 18, attached to optical fiber 17, is attached to recess 48 of the enclosure. The modified portion of the fiber, including the splice, is within the enclosure, and supported, in accordance with the present invention, such that the integrity of the splice can be maintained over a range of temperature variations of the enclosure.
In this instance, the preferred propagation direction of radiation in optical fiber 17 is indicated by arrow B. A purpose of this particular modification of optical fiber 17 provides that radiation in the cladding of portion 17B1 can escape from the cladding. The optical fiber and splice-holder are functioning as a mode-stripper. With radiation propagating in the indicated preferred propagation direction, the “escaped” or “stripped” radiation at the splice is directed away from membrane 18.
In summary, the present invention is described above with reference to two preferred embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather the invention is limited only by the claims appended hereto.
