BACKGROUND OF THE INVENTION Fiber lasers have become increasingly popular in recent years by offering relatively high-power, high-efficiency output with a near-Gaussian output profile. Fiber laser power outputs have approached 100 W in recent years utilizing"dual cladding"technology.

High-powered fiber lasers utilizing dual cladding are typically end pumped by a highly collimated laser diode array. One drawback of these fiber lasers is the stringent collimation requirements placed on the pump source in order to achieve a high coupling efficiency between the pump source and the fiber. The need for highly collimated pump sources contributes significantly to the cost and low availability of these high-powered fiber lasers. In addition, dual-cladding fiber is expensive and difficult to coil. The difficulty of coiling dual-cladding fiber increases the size of a laser employing dual- cladding as opposed to single-cladding fibers, which generally have smaller minimum radii of curvature. Further, dual-cladding, end-pumped fiber lasers tend to retain heat to a great extent, even to the extent of bursting into flames when pump energy is misaligned with the fiber.

Side-pumped, solid-state lasers such as Nd : YAG lasers, on the other hand, typically offer much higher power but with a compromise of lower beam quality. Side- pumped solid-state lasers do not require expensive, highly collimated pump sources since the pump source is, in many cases, closely coupled to the lasing medium. There exists a need for a cost-effective laser having the high beam quality of fiber lasers combined with the relative ease of pumping exhibited by solid-state lasers.

SUMMARY OF THE INVENTION A fiber laser of the present invention comprises a wound fiber having a core and a layer of cladding. The fiber is side-pumped by an energy source to produce a high-quality laser beam.

The fiber is wound in a flat wound formation, preferably with both ends of the fiber accessible outside of the formation. The wound fiber is optically pumped by a pumping source located along a circumference of the wound formation. In one embodiment, the fiber is pumped by a close coupled laser diode bar and the fiber comprises a rare earth doped core surrounded by a cladding material. In one preferred embodiment, the core of the fiber comprises a high-index glass doped with GeO2 and neodymium, and the fiber cladding comprises a glass, such as Si02 having a slightly lower index of refraction than the core. Reflective surfaces are placed above and below the wound formation, and a reflective band is disposed around the circumference of the wound formation.

An optimum design for a fiber laser of the present invention must include a large doped core, minimum cladding size, medium to high doping concentration in the doped core, reflective surfaces having high reflectivities, and rudimentary fast axis collimation of the laser diode bar pumping source.

The laser diode bar inputs energy through a gap in the reflective band, and following a population inversion and stimulated emission in the doped core of the fiber, laser emissions are output through both ends of the wound fiber, which in one embodiment pass through a slit in the reflective band. Because both ends are accessible, the end user can operate the system with dual outputs, as an amplifier wherein energy is injected into one end and amplified before leaving the other end, or the user may coat one or both ends with selected reflective coatings to allow the system to act as a laser oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 is a top view of a wound fiber formation for use in one embodiment of the present invention; FIG. 2 is an exploded view of a fiber laser according to one embodiment of the present invention; FIG. 3a is a top view of a single-source side-pumped fiber laser for use with one embodiment of the present invention; FIG. 3b is a top view of a three-source side-pumped fiber laser for use with a second embodiment of present invention; FIG. 3c is a top view of a tangential-source style fiber laser for use with a third embodiment of the present invention; FIG. 4 is a top view of a wound fiber formation housed inside a reflective hollow in one embodiment of the present invention; FIG. 5 is a cross-sectional view of a wound fiber and reflective surfaces according to one embodiment of the present invention ; FIG. 6 is a cross-sectional side view of a fiber laser according to one embodiment of the present invention; FIG. 7 shows a ray-trace of a single pump ray entering the hollow of a fiber laser according to one embodiment of the present invention; FIG. 8 shows a side view of ray-traces of pump rays of several angles entering the hollow of a fiber laser according to one embodiment of the present invention; FIG. 9 shows a cross-sectional side view of a doped-core fiber and reflective surfaces according to an alternative embodiment of the present invention; FIG. 10 shows a cross-sectional side view of a double-layered winding formation according to another alternative embodiment of the present invention; FIG. 11 shows a cross-sectional side view of a multiple-layer winding formation according to still another embodiment of the present invention; and FIG. 12 shows a top view of a spiral winding formation according to yet another embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to

cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention makes use of doped-core, single mode fiber in a wound formation which, in the preferred embodiment, allows both ends of a section of fiber to be available to the user at the outside of the wound formation. FIG. 1 shows a top view of a length of a fiber 10 having a doped core after being wound into its wound formation 12 for use with one embodiment of the present invention. The doped-core fiber 10 is wound in a single plane such that the mid-point of the length of coped-core fiber roughly corresponds with a center segment 14 of the wound formation 12. This style of winding allows both a first end 16 and a second end 18 of the doped-core fiber 10 to be disposed at the outside of the wound formation 12. It is possible to have both the first end 16 and the second end 18 exit the wound formation 12 at approximately the same circumferential position, or alternatively the first end 16 and the second end 18 may exit the wound formation 12 at different positions.

A sinusoidal formation 20 is formed about the center segment 14 of the wound formation 12. In addition to allowing both the first end 16 and the second end 18 of the doped-core fiber 10 to be disposed at the outside of the wound formation 12, the sinusoidal formation 20 prevents kinks, which would hamper the ability of the doped-core fiber to transmit light, from occurring near the center segment 14 of the wound formation 12. In one preferred embodiment, further protection of the central part of the wound formation 12 is provided by an optically clear hub, which is described in more detail with respect to FIG. 2.

The bulk of the doped-core fiber 10 in the wound formation 12 is found in a spiral-style main segment 22. In one preferred embodiment, at least one hundred windings and up to about three hundred windings of the doped-core fiber 10 occur in one cross-section of the main segment 22. The fiber 10 is doubled over to provide access to the ends 16 and 18 when wound, such that one hundred windings are brought about by fifty turns of the doubled-over fiber 10. The optimum number of windings is dependent on the amount of energy emitted by the pump source, the reflectivities of the adjacent surfaces, and the materials of the doped-core fiber 10. Further, output power is

proportional to the number of windings, up to a point where the radially injected power does not reach the furthest winding of the fiber.

FIG. 2 is an exploded view of the fiber laser 25 of the present invention showing the parts that surround the wound formation 12. A hub 30 assists in supporting the wound formation 12 and the sinusoidal formation 20 at the center segment 14 of the wound formation 12. In one embodiment, the hub 30 comprises a first hub portion 32 and a second hub portion 34, with the two portions fitting together in a"yin-yang"shape.

A first hub hole 36 and a second hub hole 38 allow a first connecting member 40 and a second connecting member 42 to fit through the hub portions 32 and 34 and hold the hub 30 in place between a top disk 44 and a bottom disk 46. Because the sinusoidal formation 20 is bounded by the first hub portion 32 and the second hub portion 34, the wound formation 12 remains in a fixed position. The hub 30 is made of an optically clear material.

In one preferred embodiment, the bottom surface 48 of the top disk 44 and the top surface 50 of the bottom disk 46 are both polished copper mirrors deposited with gold, though both the bottom 48 of the top disk 44 and the top 50 of the bottom disk 46 may be comprised of any wholly or partially reflective surface. The top disk 44 includes a pair of connection holes 52 to accommodate connecting members 40 and 42. Likewise, the bottom disk includes bottom disk connection holes 56 to accommodate connecting members 40 and 42.

The doped-core fiber 10 is pumped by an optical pumping source 60. In a preferred embodiment, the pump source is a laser diode bar, such as an AlGaAs (aluminum gallium arsenide) laser diode bar, though the pump source 60 may include other types of optical energy sources pumping at wavelengths corresponding to the optimum absorption wavelength of the core dopant in the doped-core fiber 10. In one preferred embodiment, laser diode bars with power outputs of about 20 W are employed.

When the fiber laser 25 is fully assembled, the wound formation 12 of the doped-core fiber 10 is sandwiched between the top disk 44 and the bottom disk 46. The sandwiched formation allows for maximum interaction between energy from the pump source 60 and the doped-core fiber 10 in the wound formation 12.

FIG. 3 a shows a top view of the exterior of the fiber laser 25 according to the present invention having only one pump source 60. In an alternative embodiment, two or

more pump sources are positioned about the body of the fiber laser 25. For example, FIG. 3b shows the fiber laser 25 of the present invention which utilizes three pump sources 60a, 60b, and 60c which pump energy into the doped-core fiber 10 in a generally radial direction. In one preferred embodiment, the outputs of three 20 W diode laser bars are directed into a hollow cavity of the fiber laser 25 to achieve a desired amount of absorbed pump power.

Alternatively, a fiber laser of the present invention may be constructed wherein pump energy enters the doped-core fiber 10 tangentially. FIG. 3c shows a fiber laser having a pump source 60d positioned to insert energy tangential to the direction of doped-core fiber winding within the fiber laser body. The energy is guided by the inside surface of the reflective hoop, which is described below.

Side pumping as utilized in the present invention allows multiple pump bars to surround the fiber laser 25, as shown in FIGS. 3a-3c. It is also possible to place a pump source in the center of the wound formation 12 so as to pump generally radially outwardly, or to place a pump source outside of the fiber laser body while directing pump energy into the doped-core fiber 10 with fibers, mirrors and/or lenses. In another embodiment, a diode array having several laser diodes is used as the pump source, and the output of the diode array may be guided into the doped-core fiber 10 with fibers, mirrors and/or lenses.

Referring back to FIG. 2, to further allow for maximum pumping interaction, a reflective hoop 62 surrounds the sandwiched construction when the fiber laser 25 is fully assembled. The inside surface 64 of the reflective hoop 62 is reflective. In one preferred embodiment, the inside surface 64 of the reflective hoop 62 is made of polished copper deposited with gold.

The reflective hoop 62 contains at least one gap 66 and at least one slit 68. The gap 66 allows pumping energy from the pump source 60 to enter the body of the fiber laser 25 and interact with the doped-core fiber 10. The slit 68 allows the ends 16 and 18 of the doped-core fiber 10 to pass through the reflective hoop 62. In an alternative embodiment, the ends 16 and 18 of the doped-core fiber 10 remain enclosed within the reflective hoop 62, and laser light resulting from the interaction of the pump source 60 and the doped-core fiber 10 passes through a slit or hole in the reflective hoop 62 while the ends 16 and 18 remain within the reflective hoop 62. In another embodiment, the

fiber 10 is wrapped in a spiral formation so that one end stays toward the center of the spiral and the other end exits the spiral on the outside. In this embodiment, it is possible that the end at the center of the spiral may be led through a hole in the top disk 44 or the bottom disk 46 so that the user will still have access to both ends.

One benefit of allowing the user access to both ends 16 and 18 of the doped-core fiber is that the fiber laser of the present invention may be used either as a laser oscillator or as an amplifier. If both ends are left relatively nonreflective, light entering one end 16 may be amplified within the fiber laser 25 before exiting through the other end 18. When using the fiber laser 25 as a laser oscillator, however, it may be desirable to make one end 16 highly reflective and the other end 18 either partially reflective or relatively nonreflective. The reflectivities of the ends may be altered by the deposition of dielectric material on the ends or by the use of mirrors. When the fiber laser 25 is used as a laser oscillator, a doped-core fiber 10 may have one end 16 coated for a reflectivity of about 98% or higher and the other end 18 coated for a reflectivity of from about 5% to about 95%. Further, when used as a laser oscillator, the doped-core fiber 10 may be wrapped in a simple spiral formation with the highly reflective end disposed on the inside of the spiral and the less-reflective end, the output end, available on the outside of the spiral.

In this alternative embodiment, one revolution of the resulting single spiral corresponds with one pass of doped-core fiber 10.

The reflective hoop 62 further includes reflective hoop connecting holes 70,72, 74, and 76, which align with upper reflective hoop connecting recesses 78 and 80 on the top disk 44 and lower reflective hoop connecting recesses 82 and 84 on the bottom disk 46 for attachment of the reflective hoop to the disks. When the reflective hoop 62 is wrapped around the disks 44 and 46, the interior cylinder defined by the reflective hoop 62 and the disks 44 and 46 contains reflective material on substantially all sides, with the exception of the gap 66 and the slit 68. The reflective surfaces of the reflective hoop 62, the bottom surface 48 of the top disk 44 and the top surface 50 of the bottom disk 46 form a hollow cavity 69, inside which the doped-core fiber 10 is coiled. Thus, energy input by the pumping source 60 is repeatedly reflected about the hollow cavity 69 so as to maximize interaction with the doped-core fiber 10.

As shown in FIG. 5, the doped-core fiber 10 comprises a doped core 90 surrounded by a cladding 92. The doped core 90 comprises a glass such as a silicate-

based glass doped with GeO2 and a rare earth element or a phosphate-based, or fluoride- based glass doped with a rare earth element. In one embodiment, the doping element is neodymium, and in alternative embodiments the doping element may be ytterbium or erbium, for example. Fluoride-based glass fiber doped with yttrium at doping concentrations as high as 20,000 ppm is used in one embodiment of the present invention.

With most combinations of dopant and glass material, a dopant concentration above 20,000 ppm only results in a small increase in energy absorption. The cladding 92 may comprise a glass such as a silicate-based, phosphate-based, or fluoride-based glass doped so as to have a slightly lower index of refraction than the core. The cladding 92 is preferably comprised of Si02 to give a lower index of refraction than the doped core 90.

By constructing the doped-core fiber 10 in this manner, the doped-core fiber 10 uses the principle of total internal reflection to conduct light energy through the fiber.

Total internal reflection occurs when light travelling within a first medium of a certain index of refraction, ni, bounces at a sufficiently low angle off an interface between the first medium and a second medium having a lower index of refraction, n2. The different indices of refraction in the present invention thus assist in maintaining energy within the doped core.

In one embodiment, shown in FIG. 5, the doped-core fiber 10 in the main segment 22 is surrounded by optically clear filler material 94 between adjacent windings of the doped-core fiber 10 sandwiched between the top disk 44 and the bottom disk 46. The optically clear filler material 94 may be added for strength and durability, but the fiber laser 25 may be implemented without the use of the optically clear material 94. Further, it should be noted that while FIG. 5 shows an appreciable amount of space between successive windings of the doped-core fiber 10, a fiber laser 25 under the present invention is preferably constructed so that adjacent windings of the fiber 10 touch or nearly touch as shown in FIG. 8.

One key factor for getting high absorption of energy in the doped core 90 is to enlarge the core size while simultaneously minimizing cladding size. Thus, it is desirable to maximize the diameter of the doped core 90 to maximize absorption of energy into the doped core 90. A wide range of diameters is possible. In a preferred embodiment, the diameter of the doped core 90 is from approximately 20 um to approximately 50 u. m.

In one preferred embodiment, the outside diameter of the doped-core fiber 10 is approximately 125 llm and the diameter of the doped core 90 is approximately 50 Rm.

This embodiment is beneficial because the outside diameter of 125 pm matches the diameter of commonly used communications fiber when the fiber laser 25 is used as an amplifier or a signal source in communications applications. The fiber laser 25 has a diameter"D"of approximately three inches (about 7.3 cm) and a height"H"of approximately'4 inch (about 1.3 cm), as shown best in FIG. 6. It is possible to have both dimensions reduced, and in particular using a doped-core fiber 10 with a smaller diameter allows for a smaller diameter"D"for the fiber laser 25. In an alternative embodiment, the outside diameter of the doped-core fiber is approximately 80 mi, also a standard fiber diameter in the communications industry. Doped-core fiber 10 having a diameter of 125 urn is capable of being bent with a radius of curvature of approximately 1 cm. Generally, to prevent high bending losses, such fiber should not be coiled to less than 20 mm diameter, and as a result less fiber is positioned toward the center of the wound formation 12. If a doped-core fiber 10 having a diameter of 80 urn is used, the radius of curvature may be made smaller leading to a smaller laser or more windings in an equivalently-sized laser. Further, a doped-core fiber 10 having a diameter of 80, um allows for a higher core- to-cladding ratio.

In operation, pump energy travelling as shown by the arrows"A"flows out of the pump source 60 and through the optically clear filler material 94. The pump energy travels through the cladding 92 at each winding of the doped-core fiber 10 and into the doped core 90, where some of the pump energy is absorbed. The absorbed pump energy interacts with active ions in the doped core 90 to excite electrons and create a population inversion necessary for laser production. Once a population inversion is created by the pump energy, spontaneous emission begins the stimulated emission process, resulting in laser output at the ends 16 and 18 of the doped-core fiber 10.

Referring now to FIG. 7, in one preferred embodiment, energy from the pump source 60 is input into the body of the fiber laser 25 at an angle"a". Energy directed from the pump source 60 at no angle or at a slight angle will only take one bounce off the opposite side of the inside surface 64 of the reflective hoop 62 before reflecting back through the gap 66 and exiting the system. As shown in FIG. 7, a beam 100 directed from the pump source 60 at an angle"a"will reflect off the inside surface 64 of the

reflective hoop 62 several times (in some cases from forty to fifty times) in a fiber laser having a diameter of about 3 inches. Only a few degrees'deflection from the radial direction may result in the maximum number of reflections being achieved. The optimum angle for energy input is a function of the diameter of the reflective hoop 62 and the qualities of the doped-core fiber 10. Ideally, the pump source 60 should be angled and/or the pump energy redirected so that a beam coming from the pump source 60 makes several trips around the hollow cavity 69, formed by the reflective surfaces.

Each time pump energy beam hits a reflective surface, a certain small percentage of the energy contained in the beam is lost to the reflective surface. It is a goal of the present invention to reduce energy loss to reflective surfaces and thereby to increase energy absorbed by the doped core 90 of the fiber 10. One way to reduce energy loss is to increase the reflectivity of the reflective surfaces, which is accomplished by depositing gold on the interior reflective surfaces that form the hollow cavity 69. It is preferred for the interior reflective surfaces to have reflectivities greater than about 98%. Energy absorption by the doped core 90 almost doubles when reflective surface reflectivities are increased from 98.5% to 99.5%, making it important to achieve reflectivities as high as possible. Energy loss may further be reduced by reducing the number of times a pump beam 100 comes into contact with reflective surfaces, especially those surfaces on the top and bottom disks 44 and 46. Both a reduction in lost energy and an increase in energy absorbed by the doped core 90 can be accomplished by modifying the divergence of pump energy exiting a pump source 60.

Divergence represents the total angle defined by energy produced by the pump source 60. Divergence of the pump source 60 can be discussed with respect to two axes: the x-axis, also referred to as the"slow"axis, shown by arrow"x"in FIG. 3, and the z- axis, also referred to as the"fast"axis shown by arrow"z"in FIG. 8. The x-axis is generally tangential to the wound formation 12 and the z-axis is generally parallel to the axial direction of the wound formation 12. In the x-axis, it is desirable to have a somewhat high divergence so that energy exiting the pump source 60 excites as much of the doped-core fiber 10 as possible. In the z-axis, however, it is desirable to have a lower divergence. The effect of a lower divergence in the z-direction can be seen in FIG. 8.

Light exiting at a high divergence as shown by the highly divergent ray 104 strikes the top reflective surface 48 and the bottom reflective surface 50 more often and at shorter

intervals than a less divergent ray 106. The more often a ray strikes the reflective surfaces, the more of the energy in the ray will be absorbed by those surfaces, resulting in increased. cooling requirements and a reduction in absorption by the doped-core fiber 10.

As a result, it is beneficial in the present invention for the pump source to have a low divergence in the z-direction. Divergence in the z-direction may be lowered by the use of a collimator or divergence-decreasing lens 108 (shown in FIG. 5), and divergence in the x-direction may be increased by the use of a diffuser or a divergence-increasing lens 110 (shown in FIG. 7). FIGS. 7 and 8 show ray traces modeled with a single point pump source. Laser diode bars have a plurality of emitters and thus a full plot of their reflections is more complex than shown in the ray traces.

Using a diode laser bar for the pump source 60, a simple mechanical design for the entire fiber laser 25 and a simple fiber design all add to the simplicity, low cost, and ruggedness of the fiber laser 25 according to the present invention. Further, the wound formation 12 makes the system compact, and along with easily cooled mirrors enables high pump powers to be used. The wound formation 12 makes the fiber laser 25 useful as a continuous wavelength pump head, or alternatively the fiber laser 25 can be placed into an optical system allowing for pulsed outputs.

The fiber laser 25 according to the present invention may be cooled by the use of cooling fins on the top disk 44 and the bottom disk 46 if too much heat is absorbed at the reflective surfaces. Cooling may be accomplished through natural air cooling, forced-air cooling, liquid cooling, or a combination of these methods. Again, the reflectivities of the reflective surfaces are high and the input beam divergences are chosen to reduce the cooling needs of the fiber laser 25.

By using different dopants in the doped core 90 along with different pumping wavelengths from the pump source 60, it is possible to change the wavelength output by the fiber laser 25. For example, if the doped core 90 is doped with neodymium, the core may be pumped at a pumping wavelength of 808 nm +/-10 nm to result in laser output having a wavelength of approximately 1000 nm. A doped core 90 doped with yttrium may be pumped at a wavelength of either about 915 nm or about 970 nm to result in laser output of approximately 1000 nm, and a doped core 90 doped with erbium may be pumped at a wavelength of approximately 980 nm to result in laser output at approximately 1500 nm.

Several alternative embodiments make use of the principles of the present invention. Turning to FIG. 9, an alternative embodiment is shown wherein the cladding 92 of the doped-core fiber 10 has been lapped down along the top and bottom. This lapped doped-core fiber enables an increase in the ratio of the volume of doped core 90 to cladding 92. This embodiment has the advantage of increasing the amount of pump energy that can be absorbed by the doped core.

Further, multiple winding styles for the doped-core fiber 10 are available. For example, FIG. 10 shows a cross-sectional view of a two-layer winding formation 110.

This two-layer winding formation 110 may be constructed by first wrapping a bottom layer 112 in a spiral from the outside in, allowing for a layer transition segment 114 in the center, and then wrapping a top layer 116 from the inside out. This formation allows more fiber to be contained within the hollow cavity 69, thereby increasing the potential for energy absorption by the doped-core fiber 10. The transition segment 114 may have the same sinusoidal shape shown previously with the slight height change to develop the top layer 116.

This winding style can be extended further to make a multi-layer winding formation 120 as shown in FIG. 11. To create a multi-layer winding formation 120, a first layer 122 and second layer 124 are wrapped as described in the two-layer formation, above. Once the outside is reached, an outside layer transition segment 125 is allowed, and a third layer 126 is wrapped from the outside toward the inside. Once the inside is reached, a second inner layer transition segment 128 is allowed, and a fourth layer 130 is wrapped from the inside toward the outside. Using this outside-in/inside-out wrapping alternation, it is possible to create many layers of doped-core fiber 10. This increases the amount of pump energy that can be converted into laser light and thus can potentially increase the output power of the fiber laser 25.

In both the winding styles of FIGS. 10 and 11, both ends are exposed to the outside of the winding. Alternatively, one end can remain within the formation so that it is inaccessible and is coated with a reflective coating. In a further alternative, it is possible to have one end at the middle of the formation and have it exit the top or bottom disk in a generally axial direction.

Turning to FIG. 12, an alternative winding style is shown in which the doped-core fiber 10 is wound into a spiral formation 140. In this formation, one end 16 of the doped-

core fiber is disposed toward the middle of the spiral formation 140 and the other end 18 of the doped-core fiber is disposed on an outer periphery of the spiral formation 140. The spiral formation 140 is useful when a fiber laser 25 is to be used as a laser source rather than as an amplifier. In one alternative embodiment, the end 16 disposed toward the middle of the formation may leave the plane of the spiral formation 140 and exit the hollow cavity 69 to be available to a user. In this spiral formation, it is preferred to have the inner end 16 of the doped-core fiber coated or mirrored so as give a reflectivity of about 98% or greater.

While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.