Patent Application
20110100548
The present invention relates to doped polycrystalline ceramic laser materials and methods for making the same.
A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. More generally, coherent light typically means the source produces light waves that are in step. They have the same frequencies and identical phase. The coherence of typical laser emission is a distinctive characteristic of lasers. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum (“monochromatic” light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously. The word laser originated as an acronym for Light Amplification by Stimulated Emission of Radiation. The word light in this phrase is used in the broader sense, referring to electromagnetic radiation of any frequency, not just that in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. Three typical laser geometries, rod, disc, and slab and two pumping geometries, end and side pumping, are recognized.
Modern ceramics have recently been developed for use a laser materials. Most ceramic materials are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light, or even larger. Thus, they are generally opaque materials, as opposed to transparent materials, due to the presence of porosity and impurities at the grain boundaries. Recent nanoscale technology has, however, made possible the production of polycrystalline transparent ceramics. These can be used for numerous applications including high energy lasers, transparent armor windows, and nose cones for heat seeking missiles, Additionally, when doped with rare earth ions (Nd, Pr, Er, Tm, Tb, Ho, Dy, Yb, etc) or transition metal ions (V, Cr, Cu, etc) it is possible to make transparent ceramic laser materials which can generate laser light upon suitable pumping, similar to solid state crystal lasers.
In solid state lasers, the gain medium (laser material) is usually a doped single crystal. Polycrystalline laser gain media also can be doped to improve laser performance.
Pumping a doped laser gain medium (such as Yb-doped yttria) results in laser emission, general heating of the entire laser material, and high localized heating at the pump end of the laser material. Heat management is a critical issue in the design of high energy lasers. Heat is a function of both the energy pumped into the laser material and the dopant level of the laser material; the dopant absorbs the pump energy and releases it as photons (laser) and phonons (heat). As the pump energy and/or the dopant concentration are increased, both laser emission and heat generation are increased. Localized heating is a result of the uniform dopant concentrations traditionally used in laser materials. The pump energy decreases as it travels through the laser material because it is being absorbed by the dopant.
In uniformly doped laser materials, the material near the pump end receives the most energy and produces the most heat, resulting in localized heating. Even adding a pure (0% doping) layer before the laser material, does not greatly reduce the localized heating of the gain media because the pump laser energy is not attenuated in the un-doped region. The decrease is due to the higher thermal conduction of the undoped region. But an even greater improvement can be achieved if the doping level were tailored to match the pump energy, then uniform heating and uniform laser emission can be obtained throughout the laser material. This eliminates spikes in heat and results in a uniform heating profile that enables potentially higher output power.
Ideal lasers operate with a spatial mode profile of TEM00, which is a Gaussian shaped beam profile. Typically, however, the gain profile of the laser media does not match the laser beam spatial profile. The gain medium may have a uniform pump profile. In other cases, the edges may be pumped more than the center, resulting in higher gain at the edges. To ensure TEM00 output in lasers, apertures are usually placed in the resonator to result in high losses for higher order modes. Unfortunately, this technique is not conducive to high power operation. Mode selectivity can be improved if the gain can be tailored to match the desired spatial mode profile of the laser. For TEM00 mode, for example, the desired gain profile would be a Gaussian along the path of the laser beam.
To create a gain profile matching the spatial profile of the laser beam in rare earth doped lasers, it is desirable that the laser media be doped with a non-uniform rare earth (or transition metal ion) dopant distribution. For example, in a rod-shaped geometry, a radial distribution of the dopant profile, with the dopant concentration being highest in the center of the rod and tapering to the sides of the rod, would be desired. By tailoring the dopant profiles longitudinally and transverse to the pump, the gain profile of the laser can be made to match the TEM00 profile, resulting in improved beam quality at higher powers.
Single crystal laser gain materials are formed using a variety of high temperature growth techniques from the melt, such as Bridgeman-Stockbarger and Czochralski techniques known in the art. However, due to the mechanisms of growth, it is extremely difficult to produce many single crystals with dopant levels much higher than 2% or with a smoothly graded or stepped doping profile and nearly impossible to make with a radially doped gradient or with a doping scheme that incorporates both longitudinal and radial gradients. Single crystal laser materials are therefore uniformly doped, that is, the concentration of the dopant is the same throughout the entire laser material.
It is much easier to produce graded and/or stepped doping profiles in polycrystalline gain media, and such media have been shown to accommodate greater amounts of doping than single crystals. However, due to scattering and other grain-dependent effects, such materials have had limits on the permissible grain size of the crystals. See U.S. Pat. No. 6,825,144 to Hideki (polycrystalline laser gain media are limited to crystals having a mean grain size of less than 20 μm, and laser quality ceramic cannot be made if the grains are larger).
Polycrystalline ceramic lasers have also been fabricated with un-doped regions that act as heat sinks for the doped portions of the laser gain medium and allow operation of the laser at higher energies. See U.S. Pat. No. 6,650,670 to Shimoji (describing a laser gain medium with a uniform doping of 2% coupled with an undoped layer).
In addition, segmented profiles with different dopant levels to reduce thermal stress and strain in the crystal laser rods have been proposed, see U.S. Pat. No. 5,321,711 to Rappaport, but the fabrication of these segmented profiles is difficult.
This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Instead, it is merely presented as a brief overview of the subject matter described and claimed herein.
The present invention includes a solid state polycrystalline Ytterbium doped Yttria or Scandia (Yb:Y2O3 or Yb:Sc2O3) laser material with a discrete or continuous gradient doping profile and methods for manufacturing the same. The doping profile can be two- or three-dimensional and can vary depending upon the laser geometry, the pumping scheme, and the benefits to be desired from the laser material's structure, such as thermal management, reduction of parasitic effects, mode matching, or combination of these. The grading direction can be linear, axial, radial, or any combination thereof depending on the pumping and cooling configurations. The material can be made from a combination of doped and undoped solid shapes, loose powders, and green shapes, and can be diffusion bonded or densified to a desired final shape using techniques such as pressureless sintering, hot pressing, hot forging, spark plasma sintering, hot isostatic pressing (HIPing), and combinations thereof. It is further understood that the dopant is not limited to Yb, but can be selected from the rare earth ion group consisting of Nd, Pr, Er, Ho, Tm, Tb, Dy, Yb, and their mixtures, as well as the transition metal ion group consisting of Ti, V, Mn, Cr, Fe, Cu, Zn, and their mixtures.
The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.
For example, although the invention is often described herein in the context of a Ytterbium doped Yttria (Yb:Y2O3) material, many of the aspects described herein can be applied to any polycrystalline ceramic laser material having a functionally gradient doping profile. In addition, although the description and the Figures herein often are directed to a doped material having dopant concentrations of 0% Yb, 1% Yb, and 2% Yb, it would be understood by one skilled in the art that a functionally doped ceramic laser material in accordance with the present invention can include more or fewer different dopant concentrations, and that the dopant concentrations are not limited to those described herein but can be any suitable level to achieve the desired performance. It is further understood that the dopant is not limited to Yb, but can be selected from the rare earth ion group consisting of Nd, Pr, Er, Ho, Tm, Tb, Dy, Yb, and their mixtures, as well as the transition metal ion group consisting of Ti, V, Mn, Cr, Fe, Cu, Zn, and their mixtures.
The present invention includes a solid state polycrystalline Ytterbium doped Yttria or Scandia (Yb:Y2O3 or Yb:Sc2O3) laser medium having a functionally gradient doping profile and methods for manufacturing the same. In accordance with the present invention, the doping profile of the laser medium can vary depending upon the laser geometry and pumping scheme and the desired benefits to be derived from the medium, such as thermal management, reduction of parasitic effects, mode matching, or combination of these. The grading direction can be two- or three-dimensional, and can be linear, axial, radial, or any combination thereof depending on the pumping and cooling configurations. A doped laser medium in accordance with the present invention can include both continuously graded laser medium and medium having a stepped doping profile comprising discrete areas having different doping levels.
Polycrystalline laser materials with graded dopant profiles allow a new degree of control over the thermal gradients and parasitic processes which occur in the high power laser media.
For example, in a first configuration, a laser having a uniform highly doped rod-shaped gain medium is end pumped. Pumping the laser media results in high population inversion near the pump input face and lower population inversion at the end of the rod. The highly pumped region experiences upconversion which reduces gain in the media and results in excess heating of the laser rod. In a second configuration, a rod having the same dimensions and average dopant concentration as the uniformly doped rod but with a functionally graded dopant profile in accordance with the present invention is used. Because the dopant profile is lower near the pump entrance and higher near the end, a uniform population inversion along the rod occurs, and lower upconversion is achieved.
By tailoring the dopant profiles longitudinally and transverse to the pump and laser emission directions, thermal gradients and parasitic processes can be minimized allowing operation of the laser at high powers with improved efficiency and beam quality. These graded profiles can result in any one or more of the following improvements in laser performance:
A functionally doped laser medium as described herein can be made from a combination of doped and undoped solid shapes, loose powders, and green shapes, and can be diffusion bonded or densified to a desired final shape using techniques such as pressureless sintering, hot pressing, hot forging, spark plasma sintering, hot isostatic pressing (HIPing), and combinations thereof. A doped polycrystalline laser medium produced in accordance with the present invention can have a grain size larger than 20 μm.
These materials and methods for making them will be described in more detail below.
As used herein, the terms “laser medium,” “laser material,” and “gain medium” are used variously to describe a graded ceramic laser material in accordance with the present invention. The terms “graded material” or “graded ceramic material” or “graded ceramic laser material” refer to both functionally graded and stepped doped ceramic laser gain media containing more than one dopant concentration (not including the undoped region). The term “solid shape” refers to undoped, uniformly doped, and graded material that has been fully densified. “Loose powder” refers to free-flowing particles. “Green shapes” refers to loose powders that have been formed into porous green bodies having densities less than 70% of the fully densified solid ceramic material by use of techniques including but not limited to dry pressing, casting, gel casting, cold isostatic pressure (CIP), doctor blading and extrusion. The loose powders and green shapes may or may not have any combination of binders, plasticizers, wetting agents, dispersants, or any other agent that are added to improve formability and/or handling. Loose powders can be made up of any combination of nano- and micron-sized single crystal or polycrystalline particles. The individual particles can be solid or porous, can be agglomerates or aggregates of particles, or can be any combination thereof.
In accordance with the present invention, the shape and type of doping profile in a laser medium comprising a functionally doped polycrystalline ceramic material can be tailored to address specific laser performance aspects.
For example, for thermal management, the doping profile, convoluted with the pump absorption of the material, can result in a uniform population inversion and gain profile either transversely or radially along the rod, disk, or slab to provide increased heat dissipation and reduced thermal gradients in the material. In the exemplary embodiment shown in
Spontaneous emission can also impair laser performance. Spontaneous emission is sometimes called fluorescence. This fluorescence can get trapped in the laser medium by reflections from side walls, especially from the air-laser ceramic interface due to the large refractive index contrast. This trapped fluorescence can be amplified leading to amplified spontaneous emission (ASE) and cause parasitic lasing which can deplete gain in the fundamental laser mode. For example, in Yb:Y2O3 ceramics, the index of refraction of the material is quite large (˜1.8-1.9), which leads to high reflection at the air-laser ceramic interface. This can lead to trapped fluorescence and amplified spontaneous emission. However, decreasing the concentration of the dopant reduces the refractive index of the ceramic, and so placing a lower-doped or undoped ceramic region around a higher doped ceramic region will lower reflectivity at the ceramic-ceramic interface compared with an air-ceramic boundary and so will effectively remove spontaneous emission from the active region. An exemplary embodiment of such a laser material is illustrated in
If reduction of the upconversion and ESA effects described above is desired, a functionally doped polycrystalline laser material in accordance with the present invention can be constructed to provide uniform pump absorption along the pump direction. For an end-pumped geometry with a uniformly doped rod, there is more power absorbed at the pump input end than at the output end, hence, the excited state population is higher. In the exemplary embodiment shown in
In general, the doping profiles of a functionally doped polycrystalline Yb:Y2O3 or Yb:Sc2O3 laser medium in accordance with the present invention can include those shown in
In addition, as shown in
Embodiments of doped polycrystalline ceramic laser materials having one or more features in accordance with the present disclosure can be further illustrated by the following Examples 1-4.
In this exemplary embodiment, illustrated in
As noted above, it is extremely difficult if not impossible to produce anything other than a uniform doping profile in a single crystal material. It is much easier to produce the graduated doping profiles described herein in polycrystalline ceramic gain material because the material starts out as nano- and/or micro-meter sized particles, which can be densified into solid, transparent laser gain media.
In accordance with the present invention, functionally doped laser gain materials can be fabricated using any one of the methods described below. However, it should be noted that the methods described herein are only exemplary, and materials having features described herein can be fabricated using any suitable method within the scope of the present disclosure.
For example, as shown in
A laser material having a stepped doping profile can also be fabricated using loose ceramic powder or green bodies that are arranged in the desired doping pattern, as shown in
In addition, as shown in
While
In a first exemplary method of fabricating a linearly graded material shown in
In a second exemplary method shown in
The green bodies formed by any of these methods can then be densified into a final material by techniques such as pressureless sintering, hot pressing, hot forging, spark plasma sintering, and hot isostatic pressing (HIPing), and their combinations.
To make a material that is graded both radially and linearly, a combination of the methods shown in
In addition, the methods described with respect to
Regardless of the method used, a graded doped material produced in accordance with these methods can have a grain size greater than 20 μm or smaller than 20 μm
The thus-prepared three-layer green bodies can then be formed into a final solid graded material by any suitable method, or combination thereof, including the pressureless sintering, hot pressing, hot forging, spark plasma sintering, and hot isostatic pressing (HIPing) methods described above as well as combinations thereof.
Methods for making a functionally doped polycrystalline ceramic laser material in accordance with the present invention are illustrated by the following Examples 5-10.
Doped and undoped solid shapes (based on Yb doped yttria and undoped yttria) were machined and polished to provide a relatively smooth interface. The solid shapes were loaded in a hot press die and heated to 1500° C. at 5,000 psi for 2 hours. The resulting solid shape results in a laser gain medium with a stepwise gradient. The number and size of the steps, as well as the profile, are determined by the number of solid shapes used and dopant levels of the solid shapes.
The material of Example 5 was held in the hot press at 1500° C. longer than 2 hours (could be up to 24 hours). Another sample was heated in the hot press to a temperature higher than 1500° C. but less than the melting temperature of 2400° C. Another sample was post annealed in a separate furnace for 1 hour (could be up to 48 hours) between 1500° C. and the melting point. The extended times and temperatures enable diffusion of the dopant material which smoothes out the step-like dopant profile.
Powders of varying Yb concentration were packed into a green body in a dry pressing die as shown in
The procedure is similar to Example 7. The powder is loaded into the hot press in the same manner as in Example 3. This time the powders were hot pressed at temperatures of 1500° C. for more than 2 hours (could be up to 12 hours). The temperature could also be higher but must remain below the melting temperature of 2400° C. The material can be subsequently heat treated between 1400° C. and a temperature below the melting point for 1-48 hours to further diffuse the dopant. The extended times and temperatures enable diffusion of the dopant material which smoothes out the step-like dopant profile.
Loose powders were formed into green shapes of varying Yb concentration. The green shapes were loaded into a hot press die with a desired dopant profile. The material was processed as in Example 7 to create a step profile and as in Example 8 to create a functionally graded profile.
The material from any and all of the previous Examples was surrounded by undoped powder and hot pressed at 1500° C. for 2 hours at 5,000 psi. This formed a graded material surrounded by undoped yttria.
Thus, as described herein, a functionally doped polycrystalline ceramic material can be formed which can improve laser performance over uniformly doped materials.
These composite structures can have many commercial and military applications. The doped structures could be used in existing fielded lasers to improve beam quality in these systems without replacing the system, and could also be used to improve beam quality in unstable resonators and gain guide in these resonators as well as others for improved laser performance.
For example, the composite rod structures described herein can provide improved beam quality and higher power laser output. The structures could also be applicable in ceramic fiber form, which can enable improved thermal management due to the high aspect ratio and enable higher laser output powers.
Alternatively, the profile can be tailored such that a radial or transverse thermal gradient can be established in the laser medium. The thermal gradient can be used to tailor the mode of the laser beam and guide the beam, or correct for beam distortions of a seed beam in a master oscillator, power amplifier (MOPA) geometry.
In addition, functionally doped polycrystalline ceramic laser media in accordance with the invention can allow reduction of parasitic effects in lasers to increase efficiency and power output, and can be used to reduce trapped fluorescence in radiation-balanced laser systems which can result in heating of the laser media.
Although particular embodiments, aspects, and features have been described and illustrated, it should be noted that the invention described herein is not limited to only the described embodiments, aspects, and features, as it can be readily appreciated that modifications thereto may be made by persons skilled in the art. The present application contemplates any and all such modifications within the spirit and scope of the underlying invention described and claimed herein, and such embodiments are within the scope of the present disclosure.
