The present invention relates generally to stripping stray light energy from optical fiber assemblies. More specifically, the present invention relates to stripping stray light energy that is propagating in the cladding layer of an optical fiber.
Advances in laser technology have allowed for the development of increasingly high powered systems. Such high powered systems include free space lasers, as well as lasers confined to waveguides, such as fiber lasers. Fiber lasers have significant advantages over traditional lasers, including stability of alignment, scalability and high optical power of a nearly diffraction limited output beam.
In a fiber laser, the gain medium is a length of an optical fiber, the core of which is doped with an active lasing material, typically ions of a rare earth element, such as erbium or ytterbium or both. The lasing material is usually pumped using an emission of a diode laser or an array of diode lasers. The advent of double clad active optical fibers, having inner and outer claddings in which the pump light is coupled to the inner cladding to be absorbed at the doped fiber core along the fiber length, allowed a considerable increase in overall output power of a fiber laser, while preserving the brightness and directivity of a single mode output laser beam. Power levels of the order of several kilowatts or even tens of kilowatts in an almost single mode output laser beam are now achievable, opening a great variety of industrial applications, such as concrete drilling or sheet metal cutting for the car industry or shipbuilding.
At the high optical power levels of fiber lasers, the task of managing stray light becomes crucial. A doped fused silica core and a fused silica inner cladding or claddings of the fiber lasers are surrounded by an external coating made of a polymer. Having an external polymer coating is essential because without it, the optical fiber becomes very brittle. At high pump power levels, even a small fraction of stray light can heat the polymer coating to a temperature at which it can be damaged, causing catastrophic failure of the active fiber of the laser. For instance, in fiber laser arrangements where the fiber is pumped at one end and a catastrophic thermal failure occurs at the other end, the fiber can actually start burning towards the pump end, causing the entire length of fiber to be destroyed.
In fiber lasers, the stray light and associated heating is caused by so called cladding modes, that is, modes of light propagating in the cladding. In double clad fibers, the cladding modes of the inner cladding are used to deliver the pump light to the fiber core. When the light of the cladding modes escapes the inner cladding, it can cause a localized heating of the fiber polymer coating, resulting in a catastrophic failure of the active fiber. Because of this, the cladding modes need to be removed (stripped) from the fiber where they are no longer required, or where they should not be normally present, such as in outer cladding of a double clad fiber. For example, when an active optical fiber is pumped at one end, the residual inner cladding light can be removed at the other end of the fiber to prevent its further propagation. Furthermore, the cladding modes present in the outermost cladding can be removed at the pump end of the active fiber. The cladding light can include the residual (unabsorbed) pump light, amplified spontaneous emission (ASE) of the active fiber core and the laser light at the wavelength of lasing that escaped the fiber core.
Cladding modes are typically removed using cladding mode stripper devices, or cladding mode strippers. A cladding mode stripper of the prior art has a layer of a high-index material disposed next to and optically coupled to the cladding of the optical fiber. The cladding light present in the cladding is coupled to the high-index material and is absorbed in the high-index material or in an opaque solid shield disposed around the high-index material. An index-matching gel or a coating of a high-index polymer is typically used in a cladding mode stripper.
To facilitate a more uniform distribution of cladding mode light stripped along a length of an optical fiber, the prior art had provided varying the refractive index of the high-index polymer along the fiber length. Further, in other cladding mode stripper devices, the sheath (the outer coating) is gradually thinned along the fiber so that the cladding modes can escape gradually, thus lowering the peak temperatures. Other prior art devices disclose a cladding mode stripper, in which a light-scattering material is deposited on the fiber to scatter the cladding mode light. Generally the difficulty is that the prior-art approaches are not scalable to very high optical power levels, being specific to particular fiber types and particular optical power ranges.
There is therefore a need for a cladding mode stripper that removes stray cladding modes from a high powered fiber assembly in a distributed manner. There is a further need for a cladding mode stripper that removes cladding modes from a fiber in a distributed manner to reduce heating and allow removal of significant cladding light without damage to the fiber or the cladding mode stripper device.
In this regard, the present invention provides for an improved arrangement for stripping stray light energy that is propagating in the cladding layer of an optical fiber. As such, the general purpose of the present invention, which will be described subsequently in greater detail, is to provide an improved cladding mode stripper that operates at a much higher efficiency as compared to the prior art in order to reduce operating temperatures at splices and complex optical devices downstream.
Within the scope of the present invention, it should be appreciated by one skilled in the art that the term fiber or active fiber is meant to be inclusive or any known arrangement. Such arrangements include, but are not limited to, standard clad fibers, double clad fibers, fiber bundles, multi-core fibers, solid or photonic crystal clad fibers, non-circular fibers and polarization maintaining fibers.
It is known that the greater the localized absorption of the cladding light the greater the localized heating of the fiber assembly. If the localized heating is too great, then there may occur gradual degradation or catastrophic damage to the fiber coating (acrylate/silicon/polyamide or other coating) and/or the epoxy/silicon/potting compound/coolant liquid/holder material that is near or in contact to the heat zone location. Accordingly, the present invention provides a cladding mode light stripping device, or series of devices that have purposeful design to distribute the light extraction and/or localized heating to allow removal of many watts (>100 W) of cladding light without device damage and/or with long device lifetime.
It is an object of the present invention to provide a cladding light stripper that incorporates removal of at least a portion of the coating material and/or splicing the fiber to a fiber of differing diameter and/or having a bend of constant or decreasing or varying radius to efficiently remove cladding light while distributing heat dissipation in a controlled design across the device with respect to a specific direction of input (cladding) light.
In one embodiment, a cladding light removal device is provided consisting of full/partial/graduated removal of coating material followed by bend(s) in the fiber in one or multiple planes.
In another embodiment, a cladding light removal device is provided consisting of graduated coating material removal and bend(s) of decreasing radius of curvature, spiral and/or alternating S-bends of decreasing radius of curvature.
In another embodiment, a cladding light removal device is provided consisting of a fiber with complete or graduated outer coating material removal, followed by a fiber splice to a smaller diameter fiber with complete outer coating material removal followed by bend(s) of similar/constant or decreasing radius of curvature, spiral and/or alternating (S) bends, of decreasing radius of curvature.
In a further embodiment, a cladding light removal device is provided consisting of partial removal of coating material, followed by bend(s) of constant or decreasing radius of curvature, spiral and/or alternating S-bends of decreasing radius of curvature, followed by graduated and/or full removal of coating material.
In still a further embodiment, a cladding light removal device is provided consisting of partial removal of coating material, followed by bend(s) of constant or decreasing radius of curvature (spiral and/or alternating (S) bends of decreasing radius of curvature, followed by graduated and/or full removal of coating material followed by further bends of constant or decreasing radius of curvature, spiral and/or alternating S-bends of decreasing radius of curvature.
Another embodiment provides any of the devices above designed for removal of cladding light travelling in both directions with respect to the direction of the majority of core light.
Still another embodiment provides for the use of low loss material of specific refractive index, e.g. Quartz glass or a secondary layer(s) of potting material of specific refractive index(s), to guide stripped cladding light away from the fiber to a physical location that light dissipation/absorption heating is thermally advantageous compared to the local vicinity of the fiber.
These together with other objects of the invention, along with various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention.
In the drawings which illustrate the best mode presently contemplated for carrying out the present invention:
Now referring to the drawings, the system for removing cladding mode light from a fiber laser is shown and generally illustrated in the figures. As can be seen, the present invention provides an improved cladding mode stripper that operates at a much higher efficiency as compared to the prior art in order to reduce operating temperatures at splices and complex optical devices downstream.
As stated above, in high power fiber optic systems, such as may include fiber amplifiers, fiber lasers, and fiber coupled diode lasers, a significant amount of light may be guided in the cladding of the optical fiber for the purpose of excitation and amplification. However, a significant amount of the cladding light can remain in the cladding as the signal is passed into a transmission fiber so as to interfere with the output from the core. In certain circumstances, the cladding light can also be of a sufficient power level to heat the cladding, which can decrease performance and/or cause damage to the optical fiber. Further, the output quality of the fiber light source might be essential for material processing applications like marking, cutting, welding, sintering, etc. If the output source contains light in the cladding and/or the coating its focal spot size will be larger than another similar fiber light source which has not cladding and/or coating light.
As used herein, the term “high power” refers to at least one or more hundred watts and for many applications may mean one or more kilowatts. By way of example, lasers with high output powers are required for a number of applications, e.g., for material processing (welding, cutting, drilling, marking, surface modification), large-scale laser displays, military applications, particle acceleration, and laser-induced nuclear fusion. It will be understood that the present invention is not limited to lasers as it may be applied to other high power optical applications, such as fiber amplifiers and fiber coupled laser diodes.
Further, within the scope of the present invention, it should be appreciated by one skilled in the art that the term fiber or active fiber is meant to be inclusive or any known arrangement. Such arrangements include, but are not limited to, standard clad fibers, double clad fibers, fiber bundles, multi-core fibers, solid or photonic crystal clad fibers, non-circular fibers and polarization maintaining fibers.
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As stated above however, during the laser excitation and amplification process it may be unavoidable to have significant quantities of light present in the cladding glass and/or the coating. For all of the reasons stated above it is advantageous to remove any cladding and coating light. The greater the light removal from the cladding and/or coating the greater the stripping efficiency of such a device. It is advantageous to maximize this efficiency. Preferably a light removal on the order of 95% is desirable to reduce temperatures at splice locations downstream of the cladding mode stripper and a light removal of greater than 99% is desirable to reduce temperatures at complex optical devices downstream such as output combiners and delivery fiber couplers.
As such, the materials surrounding and employed for potting of the stripped fiber are carefully selected for their thermal and/or light guiding properties. Epoxies, silicones and other potting materials may be selected with a specific optical refractive index. Due to total internal reflection light with a certain numerical aperture (N.A.) will be guided in a higher refractive index region of the cladding. If the outer coating material is removed and the stripped fiber potted into a high index material, the light will not be guided in the cladding anymore. To achieve this extraction, the fiber is coated with epoxies or other materials and/or prepared in such a way to permit extraction of significant quantities of cladding light in a manner that transfers the stripped light and heat away from the interaction region to prevent damage from localized heating and light intensity.
The greater the localized removal light the greater the localized heating. If the localized heating is too great then there may occur gradual degradation or catastrophic damage to the fiber coating (acrylate/silicon/polyamide or other coating) and/or the epoxy/silicon/potting compound/coolant liquid/holder material that is near or in contact to the heat zone location.
It can be further considered that the device has a specific orientation as there may be significant cladding light requiring removal from an optical fiber source to the “input” the device (with respect to direction of travel of the majority of the core light). There may also be cladding light of a similar or different intensity and/or wavelength into the “output” of the device from a fiber source/reflection “downstream” of the device that also requires removal.
Taking these factors into consideration, the stripping of cladding mode light is preferably achieved by processing the fiber to remove the coating and potting the exposed section(s) of fiber core in a potting material of the same or greater refractive index as that of the cladding. Further, cladding light is removed more efficiently proportionally to the reduction in the cross sectional area of cladding material per unit length. This makes the removal of cladding light at locations where there is a decrease in the cladding diameter, such as a splice between a larger diameter fiber and a small diameter fiber, or when there is a fiber taper more efficient. Further the longer the length of stripped coating removed the greater the amount of cladding light removal and the bending of the fiber creating a decrease in the critical reflectivity angle on the cladding at the outside of the bend allows low N.A. cladding light to be more easily removed. It also follows then that cladding light is removed more efficiently when the bend radius of the stripped section of fiber is reduced.
In using one or a combination of the above techniques, the stripped cladding light is then preferably directed away from the fiber via a low loss (wave guiding) media to be absorbed at a preferred physical location for thermal management, e.g. with the use of a Quartz glass slide on top of the stripped fiber.
It should be appreciated by one skilled in the art that the above rules may be mixed to design a cladding mode light stripping device, or series of devices that have purposeful design to distribute the light extraction and/or localized heating to allow removal of many watts (>100 W) of cladding light without device damage and/or with long device lifetime (MTF).
In most general terms the present invention therefore provides a system for removing cladding mode light comprising an optical fiber having an elongated core surrounded by a cladding, the cladding having a lower refractive index relative to the core, wherein cladding mode light propagates within said cladding. At least portion of the coating being removed along a predetermined length thereof and the fiber being bent at that location. This bend in the fiber coupled with the reduction in thickness or removal of the coating causes the critical angle of the light path relative to the outer surface of the cladding layer to become smaller in a manner that reduces the total internal reflection and the cladding mode light escapes the fiber structure. Within this design, the removal of the coating may be a full, partial or graduated removal of the material coupled with a bend that has a radius of curvature that is constant, decreasing or varied over the length of the bend.
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It should be appreciated by one skilled in the art that while each of the embodiments shows a particular associated removal of coating, in any of the embodiments the removal of the coating may be full, partial of graduated. Further, the bend shown in any embodiment may have constant curvature, a decreasing radius of curvature or a spiral curve, the curved fiber may be in a single plane or in more than one horizontal plane and there may be any number of sequential curves, one following the other.
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It can therefore be seen that the present invention provides an improved arrangement for stripping stray light energy that is propagating in the cladding layer that operates at a much higher efficiency as compared to the prior art in order to reduce operating temperatures at splices and complex optical devices downstream. For these reasons, the instant invention is believed to represent a significant advancement in the art, which has substantial commercial merit.
While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.