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This invention was made with government support in part under a contract No. The government has certain rights in the invention. The views expressed are those of the inventors and do not reflect the official policy or position of the Department of Defense or the U. Embodiments of the present invention are generally related to a high-power liquid-cooled pump and signal combiner and methods thereof for fiber optic applications.
More specifically, embodiments of the present invention relate to a pump and signal combiner capable of conveying several kilowatts of pump laser power for kilowatt class rare-earth doped fiber amplifiers without suffering thermal damage. Generally, the optical power output of rare-earth doped fiber amplifiers is limited to within a one kilowatt regime.
Efforts to bring such amplifier output into the high-power range i. Achieving high-power output requires the use of laser-pump combiners capable of combining multi-kilowatt power levels of pump laser power, and delivering the pump radiation to waveguides such as low-index coated passive or gain fibers to convey the pump light.
Competing amplifier and pump combiner requirements make optimization of the overall amplifier performance difficult. In most fiber applications, it is desirable for the rare-earth doped amplifier to have a small outside diameter to maximize the overlap of the pump radiation with the rare-earth doped core of the fiber, and minimize the amplifier fiber length. However, a smaller outside fiber diameter of the pump combiner output requires a larger numerical aperture, by virtue of the brightness theorem, which increases the likelihood of stray radiation and unwanted local heating.
One significant reason current pump-signal combiner package designs fail when attempting to achieve high-power output stems from the fact that the tapered and pigtail fibers are being surrounded by substantially stagnant air.
The presence of the air is inefficient, and generally insufficient, for the cooling of local heat spots. In addition, air is a very low-index surround medium, supporting high numerical aperture light which is often absorbed by the coating medium at the transition between air-surrounded and coating-surrounded glass, and further increasing the likelihood of failure due to thermal damage.
Thus, there is a need for a high-power pump and signal combiner with more efficient and effective thermal dissipation. This need can be met by liquid-cooled combiners. More specifically, embodiments of the present invention relate to an optical fiber components and structures, such as pump and signal combiners, splices, endcaps and modefield adapters, capable of conveying several kilowatts or more of pump and signal laser power in kilowatt class rare-earth doped fiber amplifiers without suffering thermal damage.
In one embodiment of the present invention, a high-power, cooled optical fiber device comprises a section of optical fiber configured to propagate light; a cooling chamber, substantially encapsulating the optical fiber; and a fluid within the cooling chamber having a refractive index selected to alter the properties of the light.
In another embodiment of the present invention, a cooling system for an optical fiber component or structures, such as pump and signal combiner, comprises a component or structure capable of propagating an output greater than one kilowatt, comprising a coated optical fiber, at least a section of bare fiber; a cooling chamber, substantially encapsulating the component or structure; and a liquid within the cooling chamber, wherein the liquid is substantially transparent to pump radiation and comprises a refractive index substantially similar to or less than a refractive index of the bare fiber.
Or the index of the liquid is similar to or less than the index of the coating. In yet another embodiment of the present invention, a method of cooling a pump and signal combiner comprises providing a pump and signal combiner, the combiner comprising at least a bare fiber; encapsulating the pump and signal combiner in a cooling chamber having a fluid therein, wherein the fluid is substantially transparent to pump radiation and comprises a refractive index substantially similar to or less than a refractive index of the bare fiber; introducing a measurable amount of pump light into the pump and signal combiner; and producing an output greater than one kilowatt from the pump and signal combiner.
In other embodiments, the coated fiber is encapsulated in a cooling chamber having a fluid therein which has refractive index similar to or greater than the index of the coating. So the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of embodiments of the present invention, briefly summarized above, may be had by reference to embodiments, which are illustrated in the appended drawings.
It is to be noted, however, the appended drawings illustrate only typical embodiments of embodiments encompassed within the scope of the present invention, and, therefore, are not to be considered limiting, for the present invention may admit to other equally effective embodiments, wherein:.
The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. To facilitate understanding, like reference numerals have been used, where possible, to designate like elements common to the figures. Embodiments of the present invention are disclosed herein as being suitable for use with a laser pump and signal combiner for fiber optic assemblies.
However, further embodiments of the present invention appreciate the applicability of the methods and structures disclosed herein for numerous additional fiber and related applications, and should be understood to be inclusive thereof.
For example, any type of laser diode or amplifier assembly, including those for military-, industrial-, medical-, telecommunication-use or the like, may be operable with embodiments of the present invention. As shown in the Figure, a basic pump and signal combiner generally comprises an input for coupling to at least a plurality of input pumps and signals, a waist region , a splice region , a resulting uncoated or bare fiber segment and a coated fiber segment In accordance with embodiments of the present invention, any number of fibers, signals, etc.
In many embodiments of the present invention, the plurality of input signals originate from pump laser diodes, a seed laser, the output of high power lasers or amplifiers, combinations thereof, or the like. Similarly, the coated fiber may be any type of coated fiber suitable for embodiments of the present invention, such as a gain fiber or a passive fiber.
The cooling chamber generally comprises at least a hollow chamber having a combiner input and a combiner output In operation, a pump and signal combiner would be assembled within the hollow chamber , such that the plurality of input sources feed into the input and the coated fiber exits the output For purposes of embodiments of the present invention, rather than a cooling chamber as shown in the Figure, a cooling chamber may comprise any type of vessel or chamber that serves to contain and guide the fluid so that it is contact with the bare and coated optical fiber segments.
In some embodiments, the bare fiber segment and perhaps mm of the coated fiber segment directly adjacent to either or both ends of the bare fiber segment , and in one instance about 10 mm of the coated fiber segment , should be placed between the fluid ingress and the fluid egress Optionally, the hollow chamber may comprise walls having cooling elements therein to further enhance the processes described herein.
Once the pump and signal combiner is assembled within the hollow chamber , the open areas around the input and output may optionally be sealed with a sealant, gel, epoxy, resin or the like, which may be refractive index-matching, as described in detail with regard to the fluid below. In such an optional embodiment, the sealant may have zero or nominal effects of the optical properties of the pump and signal combiner when utilized therewith.
Optionally, the cooling chamber further comprises a fluid ingress and a fluid egress As shown in the embodiment in the Figure, the fluid ingress and egress may be provided in the form of projected chambers extending from the hollow chamber In alternative embodiments, the fluid ingress and egress may take any form suitable for embodiments of the invention, and may be positioned along any portion of the outer surface of the hollow chamber Ingress and egress may remain part of chamber , or may be present only for device assembly and then removed once assembly is complete.
In such an embodiment, the entry ports in chamber may be sealed to provide containment of the liquid. Generally, the cooling chamber further comprises a fluid therein. The fluid may generally act as a surround medium for the bare fiber exposed within the cooling chamber , in operation.
In most embodiments, the presence of fluid within the cooling chamber provides enhanced heat extraction capability due to the thermal conductivity properties of the fluid. For example, superior natural convection cooling and forced convection cooling, as described herein, may be achieved with a fluid, thereby reducing the fiber and package temperature and the possibility of catastrophic breakdown.
In addition to the cooling properties of the fluid, for many embodiments of the present invention, suitable fluids comprise a refractive index that allows modification of the optical properties of the device, such as control of the numerical aperture of the bare fiber components of the pump-signal combiner, or control of the numerical aperture of the coating. Such modification can allow removal of excessively high numerical aperture light from the bare optical fiber, thereby preventing light from being absorbed and converted to heat by the low-index fiber coating once it enters the coated section of the fiber.
In this case, the refractive index of the fluid should be less than that of the bare fiber so that the bare fiber remains an effective waveguide.
Furthermore, the index should be similar to or less than that of the coating to create a numerical aperture not too different from that of the coated optical fiber. In other instances, the refractive index of the fluid should be similar to or greater than that of the coating to effectively strip light out of the coating.
Light propagation in the coating is often undesirable as it leads to heating due to the poor optical transparency of typical coatings. Such light can be coupled into the coating at structures such as splices, gratings, tapers, or any perturbation to the waveguide whether intended or unintended that will cause guided light in the core or cladding to propagate in the coating material.
For many suitable fluids, a desired refractive index of the fluid may be achieved by temperature control of the fluid; however, the selection of a proper fluid may still be required initially, before desired adjustments to its properties can be achieved through thermal control. In some embodiments, the suitable fluid may also comprise high transparency so that the light stripped by the fluid does not generate local heating, as is the case with opaque materials that are used for heat transport and high numerical aperture stripping e.
In many embodiments of the present invention, the fluid comprises a liquid. In one embodiment, the liquid comprises a refractive index-matching liquid, specifically designed to comprise specific refractive index properties, for example, those commercially manufactured and sold by Cargille-Sacher Laboratories Inc. In yet another embodiment, the fluid comprises a naturally-occurring fluid, such as water. It is appreciated that varying embodiments of the present invention may require fluids having varying refractive indices.
Each pump and signal combiner assembly may utilize components, such as the bare fiber, having a particular refractive index of its surface that should be matched by the fluid. As such, embodiments of the present invention should not be deemed limited beyond the association of optical properties between the pump and signal combiner and the fluid. However, in certain instances, such as utilizing embodiments of the present invention to cool a coated or recoated region of a fiber, the refractive index of the fluid may be greater than the coating with nominal interference, in any, with the optical properties.
In addition to refractive index, a fluid may be selected for its transparency to pump radiation during operation of the pump and signal combiner. By being substantially transparent to the pump radiation, the fluid avoids becoming a heat source itself, contributing to the undesirable thermal load.
The advantage of using a fluid surround medium in the vicinity of a fiber optic perturbation such as a splice, grating, taper, etc. Shown in the diagram is the optical fiber with an optical index n 1 in this example no distinction is made of the optical index of the fiber core and cladding since we are concerned primarily with the pump light propagating in the fiber cladding.
The recoating material of FIG. The fluid is contained in the fiber region by a glass capillary tube. Hence the heat generated in the fluid medium by the optical power in ray afgh is given by: Moreover, light striking the capillary glass tube is refracted into the glass tube and directed away from the fluid and coating, thereby further reducing the heat load.
The heat reduction n the fluid surround configuration relative to that of the recoated surround configuration is simply given by:. In certain embodiments, however, as an upper bound, dissipating more than 50 W per cm of fiber is very challenging. Depending on the nature of the embodiment, the fluid may be provided as either stagnant, i.
In many embodiments, where the fluid is intended to have a forced convective cooling effect on the pump and signal combiner, the fluid ingress and fluid egress may be coupled to a fluid pump not shown , e. In such an embodiment, the pump may control a flow rate of a fluid in the fluid ingress , through the hollow chamber and out the fluid egress In some embodiments, the position of chambers and can be swapped.
Other similarly known types of closed fluid systems may be utilized in accordance with embodiments of the present invention. In one experimental embodiment, three pump and signal combiners were exposed to immersion in exemplary fluids; the three fluids utilized were water, Krytox and a Cargille index-matching fluid.
After six months, despite general predictions by those of ordinary skill in the art in the industry, none of the pump and signal combiner components showed any significant swelling and only minor, or insignificant, delamination was exhibited. In addition, it was exhibited that the fluid environment e.
As shown in the Figure, during operation, hot spots arose within the pump and signal combiner, particularly at the coating. The experimental embodiment utilized water to cool the hot spots. In the Figure, the top thermal image depicts a pump and signal combiner in a cooling chamber with stationary water, operating at a pump optical power of W.
Without a cooling chamber, the most problematic areas of the assembly i. The fluid coolant is designed to greatly reduce the temperature of the heated light absorbing coating. It is not possible to directly measure the surface temperature of the heated coating with the IR camera because it is enclosed in the water filled coolant chamber.
However, the heating of the fiber segment is inferred from the rise in the surface temperature of the coolant chamber generated by the internal heated fiber segment. Due to the distribution of heat over a larger region than an air filled capillary and due to heat conduction and natural convection of the water, the hot spot was measured at 29 degrees Celsius at the surface of the cooling chamber with W of optical power in the TFB.
In the Figure, the lower image shows the same device with forced flow of water within the cooling chamber. The forced water flow provides a further significant reduction in the heating of the cooling chamber surface, and in this example, reduced it by approximately 4. Although not measurable, such results indicate a likely reduction in temperature at the coating surface as well.
Various alternative embodiments of the present invention allow for the systems and methods disclosed herein to be utilized for other benefit. In such an embodiment, this perturbation may be due to the placement of a Bragg grating in the fiber core or due to the splicing of two dissimilar fibers. For example, in some alternative embodiments of the present invention, the methodology disclosed herein may be utilized to cool coated fiber segments that appear at various locations in a high power fiber amplifier, such as the one depicted in FIG.
Section A comprises a splice between the laser diode module pigtail to a TFB pump leg, with low or high index recoat; Section B comprises a TFB, having a waist region, splice region, bare fiber region and coated fiber region, similar to the embodiment shown in FIG.
Back-reflections can couple light back into the fiber through the end-cap, increasing the thermal load on the coating.