Patent Application
20140307305
Ytterbium-doped YAG (Yb:YAG) has been used as a solid state laser gain medium for high-power diode-pumped solid state lasers. Yb has a broad, 18 nm wide absorption band at 940 nm and produces gain at 1030 nm. Yb:YAG lasers and amplifiers can be used in some applications served by high-power 1064 nm Nd:YAG lasers/amplifiers and frequency doubling to 515 nm can enable use in some applications previously served by 514 nm argon ion lasers.
Despite the progress made in the development of solid state laser and amplifier systems, there is a need in the art for improved methods and systems related to solid state lasers.
The present invention relates generally to laser systems. More specifically, the present invention relates to methods and systems for cryocooled laser amplifiers in which the gain medium is cooled to a predetermined temperature while the material used to absorb amplified spontaneous emission from the gain medium is operated at a higher temperature than the gain medium. Merely by way of example, the invention has been applied to a cryocooled amplifier assembly with thermally isolated edge absorbers. The methods and systems can be applied to a variety of other laser amplifier architectures and laser systems.
According to embodiments of the present invention, laser gain materials are operated at cryogenic temperatures while absorbing edge claddings are operated at higher temperatures, reducing the refrigeration requirements and thereby increasing system efficiency. In some embodiments, a cryocooled gain medium is utilized in which edge cladding used to absorb parasitic radiation such as ASE are operated at a warmer temperature. The thermal isolation can take the form of optical waveguides as well as free-space coupling between the gain medium and the edge cladding. Embodiments of the present invention are applicable to Yb as well as other cryocooled gain media and can be implemented in both reflective and transmissive amplifier geometries.
According to an embodiment of the present invention, a laser amplifier system is provided. The laser amplifier system includes a gain medium having a longitudinal axis and a plurality of sides substantially parallel to the longitudinal axis. The laser amplifier system also includes a waveguide having a plurality of inner surfaces. Each of the inner surfaces is optically coupled to one of the plurality of sides of the gain medium. The waveguide also has a plurality of outer surfaces. The laser amplifier system further includes a cladding optically coupled to the outer surfaces of the waveguide.
According to another embodiment of the present invention, a reflective optical amplifier is provided. The reflective optical amplifier includes a gain element having an input/output side and a back side. The gain element includes a gain medium having a width, a length, and a thickness less than the width and the length. The gain element also includes a waveguide partially surrounding the gain medium and an edge absorber partially surrounding the waveguide. The reflective optical amplifier also includes a reflective element disposed adjacent the back side and a cooling element disposed adjacent the reflective element.
According to a specific embodiment of the present invention, an optical amplifier system is provided. The optical amplifier system includes a set of amplifier units arrayed along a longitudinal direction. Each of the amplifier units includes a gain slab operable to amplify light propagating along the longitudinal direction and produce ASE along a transverse direction and a lateral direction. The transverse direction is orthogonal to the longitudinal direction and the lateral direction is orthogonal to the longitudinal direction and the transverse direction. Each of the amplifier units also includes a waveguide optically coupled to peripheral portions of the gain slab and a set of reflectors optically coupled to the waveguide and operable to reflect ASE propagating along the transverse direction. Each of the amplifier units further includes a set of cooling vanes. Each of the set of cooling vanes is coupled to one of the reflectors and operable to direct a cooling fluid flowing along the transverse direction. Each of the amplifier units additionally includes one or more absorptive edge claddings optically coupled to the waveguide and operable to absorb ASE propagating along the lateral direction. The optical amplifier system also includes a cooling system operable to provide a coolant flow along the transverse direction.
According to another specific embodiment of the present invention, a method of operating a laser amplifier is provided. The method includes providing a gain medium having a longitudinal axis, a transverse axis, and a lateral axis and pumping the gain medium. The method also includes directing light through the gain medium along the longitudinal axis and amplifying the light in the gain medium. The method further includes cooling the gain medium such that the gain medium is characterized by a first temperature and producing ASE in the gain medium. The ASE propagates along the transverse axis and the lateral axis. Additionally, the method includes directing the ASE through a waveguide optically coupled to the gain medium and absorbing a portion of the ASE in an edge cladding optically coupled to the waveguide. The cladding is characterized by a second temperature higher than the first temperature.
Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide pulsed laser systems producing large pulse energies and operating at high repetition rates (i.e., high average power). In some embodiments, cooling of the gain medium improves the intrinsic laser efficiency and storage lifetime in comparison with conventional techniques. By reducing the amount of electrical power used to cool the amplified spontaneous emission absorber, embodiments of the present invention provide higher system efficiency than conventional systems. Some embodiments remove the absorbing edge cladding from the immediate vicinity of the gain medium, thereby significantly improving the system efficiency. These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures.
High power solid-state lasers employ a pumped solid-state gain medium to provide optical gain. Scaling such lasers to higher power, and particularly to higher pulse energies in pulsed systems, involves the use of larger aperture gain media (i.e., a larger area transverse to the optical axis) in order to avoid limits imposed by the optical damage threshold of laser materials. As the aperture size X increases, the optical amplification gain G=egX for photons propagating transverse to the optical axis also increases. The resulting transverse amplified spontaneous emission (ASE) produces loss of energy stored in the gain medium and renders the system more susceptible to parasitic lasing in the transverse direction.
According to embodiments of the present invention, in order to suppress parasitic light and avoid parasitic lasing in the gain medium, transversely propagating ASE is prevented from making multiple passes through the gain medium, for example, through the use of an edge absorber with high optical loss at ASE wavelengths, also referred to as a cladding or an edge cladding. Some embodiments utilize a structure including treatment of the edge surfaces of the gain medium (e.g., AR coatings, beveled or ground surfaces, or the like). As more fully described throughout the present specification, the refractive index of the edge absorbers is typically closely matched to the refractive index of the gain medium to prevent back reflections.
The inventors have determined that conventional high energy pulsed laser amplifiers utilizing cryocooled, Yb-doped gain media do not offer significant advantages over Nd-doped gain media operated at room temperature if the ASE-absorbing edge cladding is also cryocooled. The efficiency-pump power tradeoff is worse for cryocooled media because of the inefficiencies associated with cooling at low temperatures. According to embodiments of the present invention, thermal decoupling of the edge cladding from the gain medium enables significant improvements in the efficiency-pump power tradeoff. Several designs provided by embodiments of the present invention are described herein that achieve the desired thermal decoupling and provide performance improvements over conventional systems.
When a laser is operated at high average power, a significant heat load is deposited in the edge absorbers. Some embodiments of the present invention relate to a laser amplifier beamline operating at a wavelength of 1053 nm and using a Nd doped glass gain medium. As an example of laser operation and heat load in the edge absorbers, to produce 6.33 kJ pulses from a 25×25 cm2 aperture, a 1 J optical seed pulse makes four passes through a series of 32 glass slabs arranged as a pair of amplifiers. Each slab is 1 cm thick with an optical gain of 1.05. At this gain level, the slabs utilize a pump energy of 0.5 J/cm2/slab at 872 nm, and ASE results in approximately 294 J of energy being absorbed in the edge cladding of each amplifier slab. When operated at a pulse repetition rate of 15 Hz, this corresponds to a heat load of 4.4 kW into the edge absorber of each slab. Such high heat loads will generally require active cooling of the edge absorbers, which can be achieved, for example, by flowing cold fluid past the absorber to extract the heat.
Some embodiments of the present invention enable the use of cryogenically cooled gain media based on optical transitions of Yb ions. Such materials can provide improved optical efficiency (for example, due to their low quantum defect), and can enable lower system costs because their long excited state lifetime (≧1 ms) is compatible with longer pump durations in pulsed systems, which reduces the peak pump power requirement and thus the cost for diode pumps. Some work has been done related to the use of Yb in a YAG (yttrium aluminum garnet, Y3Al5O12) crystal or ceramic host material and other work has related to the use of CaF2 or sesquioxide hosts. Some of these hosts offer additional laser advantages associated with their attractive thermo-mechanical properties.
The inventors have determined that a fundamental disadvantage associated with most Yb-doped gain media is the requirement for cryogenic cooling. At room temperature, the quasi-3-level nature of the lasing transitions in Yb reduces the optical efficiency due to the thermal population of the lower lasing level. For this reason, very high pump intensity is required to achieve efficient operation with most Yb-doped media at room temperature. In practice, it is generally difficult to achieve such pump intensities. At sufficiently low temperatures, however, the inventors have determined that the thermal population of the lower lasing level is greatly diminished, so that Yb behaves as a 4-level system and the optical efficiency improves significantly.
From a system perspective, the total laser wallplug efficiency should include the power required for cooling as well as the power required for optical pumping. Since refrigeration efficiency decreases with decreasing temperature, the improved optical efficiency of cryocooled Yb media is offset by the reduced efficiency of edge absorber cooling. The cooling efficiency is characterized by a “coefficient of performance” (COP), which is the ratio of the heat removed divided by the electrical power required to operate the cooling system.
Conventional geometries for disk lasers locate the edge cladding in close proximity to the gain medium. Thin layers (mm scale) of adhesive might be employed to join these media, or they might be diffusion bonded together. In any case, the close proximity of the edge cladding and gain medium causes both materials to operate at very similar temperatures, so that the cooling subsystems for each (e.g., helium for slab faces and liquid for edge cladding) also operate at very similar temperatures.
The efficiency and pump power requirements of devices described herein can be computed to analyze laser performance. As an example, computations can be performed for an amplifier configuration operating at 6.33 kJ/pulse, using a 25×25 cm2 aperture, and utilizing 4 passes. An Nd:Glass gain medium (e.g., APG-1 available from Schott) operating at room temperature or a cryocooled Yb-doped YAG gain medium operating at a temperature of either 150 or 200 K can be compared. The glass slabs are fabricated with a 1 cm thickness in some embodiments to avoid thermal shock issues, while the YAG thickness ranges to values of up to 2 cm in some embodiments to take advantage of its improved thermo-mechanical properties. In both cases, the number of amplifier slabs, gain coefficient per slab, and pump duration were varied to establish optimum regions of performance. Systems are compared on the basis of efficiency for fixed pump energy. In the computations described herein, the pump power is referenced to a system of 768 amplifier beamlines, each including 2 amplifier submodules, that produces a total of 4.9 MJ of 1.05 μm wavelength laser energy per pulse.
Laser performance was computed using a Frantz-Nodvik formalism, using cross-sections determined from experimentally reported, temperature-dependent absorption and emission spectra. The excited state lifetime parameters in the computations were obtained from experiments reported in the literature and ASE behavior was calculated using a method similar to that reported in the literature.
In order to remove heat generated within the bulk of the amplifier slab (e.g., heat generated due to quantum defect) a gas such as cold Helium gas is flowed at 5 atm. pressure. Heat in the edge absorber is removed by flowing a gas or liquid (e.g., a cold fluorocarbon liquid) through a contained region (e.g., tubing) that is thermally coupled to the edge absorbers. The cooling subsystems for both coolants (e.g., Helium gas and fluorocarbon liquid) are provided as a refrigeration loop that provides cooling via a heat exchanger to a secondary loop including a pump or compressor and the component being cooled. The electrical power for cooling includes both the pump/compressor power (assumed to be 75% of the ideal value) and the electrical refrigeration power, which was determined as a function of primary temperature (at the heat exchanger) using a curve fit to the COP data shown in
Referring to
Thus, the inability to achieve efficiencies>10% limits the use of conventional cryocooled systems based on Yb-doped gain media in certain applications, including inertial fusion energy power plants, which can utilize high-energy pulsed lasers with efficiencies of 10% or greater.
As discussed above, the overall efficiency of high-energy, pulsed laser systems based on cryocooled, Yb-doped gain media is significantly impacted by the cooling requirements. Referring again to Table 1, it is apparent that the cryogenic heat load is dominated by heating of the edge cladding due to transverse ASE in the amplifier slabs, which is significantly greater than the bulk (volumetric) slab heating due to the quantum defect and nonradiative decay. Since the electrical power required to remove this heat from the cladding depends strongly on the operating temperature of the cooling system, the overall system efficiency could be improved by operating the edge cladding absorber near room temperature. In some embodiments, the edge cladding absorbers can be operated at temperatures above, at, or below room temperature as appropriate to the particular application using water cooling or other suitable cooling system. This is not practical for conventional devices, in which the edge cladding is in close proximity to the cold gain material, because of limitations associated with thermal stresses and the parasitic heat leakage into the slab. Thus, embodiments of the present invention utilize device geometries that thermally decouple the edge cladding from the gain material, enabling the cladding to be operated at higher temperatures than the gain medium.
As described more fully throughout the present specification, some embodiments of the present invention insert a region of transparent material between the gain medium and the edge cladding, to both provide thermal isolation and serve as a waveguide that directs ASE from the gain medium to the edge cladding.
In some embodiments, characteristics of the waveguide structure include:
1. High transparency at the lasing wavelength (e.g., typically ˜1050 nm in some embodiments and ˜1030 nm in other embodiments such as those using Yb-based gain media). Transparent, as used herein, includes low absorption that can be less than 100% transmission. Therefore, transparent is not intended to denote 100% transmission, but a high transmission and low absorption at wavelengths of interest, for example, an absorption coefficient (i.e., power absorption) less than 10%, less than 9%, less than 8%, less than 7%, less than 6%, less than 5%, less than 4%, less than 3%, less than 2%, less than 1%, less than 0.5%, less than 0.25%, less than 0.1%, less than 0.05%, less than 0.025%, or less than 0.01%. Thus, the use of the term “transparent” in the description does not require 100% transmission of the wavelengths of interest, but should be understood to include materials that pass a majority or substantially all of the wavelengths of interest.
2. Refractive index matched to both cladding and gain medium, typically to within 0.05 or better (depending on the gain coefficient and dimensions of the amplifier slab)
3. Sufficiently wide tGUIDE so that the heat load on the amplifier slab due to conductive heat flow from the cladding to the gain medium is small, compared to other heating loads on the amplifier slab. This heat load can be expressed by the equation: 4 t X K ΔT/tGUIDE<<QSLAB, where X and t are the aperture width and thickness of the gain medium, K is the thermal conductivity of the waveguide material, ΔT is the temperature difference between cladding and gain medium, and QSLAB is the volumetric heating of the amplifier slab due to quantum defect and nonradiative decay.
4. Sufficient waveguide width tGUIDE to avoid thermal stresses that could fracture or warp the assembly.
5. Thermal expansion coefficient sufficiently well matched to gain medium to avoid thermal fracture or warpage.
6. The waveguide material should be opaque in the far infrared, to prevent radiative heat transport. For edge cladding temperatures near room temperature, the cladding thermal spectrum is peaked at approximately 9.7 μm. Thus, some embodiments utilize waveguide material with a transparency cutoff wavelength of ˜4 μm.
The materials selected for the waveguide and the edge cladding depend on the material chosen to provide laser gain. In the exemplary embodiment that follows, material options for two gain medium hosts are presented: YAG ceramic and CaF2 crystal.
Yb:YAG Gain Material
For Yb:YAG gain material, with an index of refraction of ˜1.82, the waveguide material can be fabricated from undoped YAG ceramic, a high refractive index glass (e.g.; Schott LaSF), or other suitable index-matched materials with similar coefficients of thermal expansion. The edge cladding can be fabricated using the same host materials doped with absorbing metal ions, such as copper, cobalt, or the like. An advantage of using YAG for both the waveguide and the cladding is a near-perfect match of the thermal expansion coefficient, which will avoid thermal stress in the assembly. Another advantage provided by YAG waveguide and cladding material is that these materials can be direct bonded to the amplifier slab using an adhesive-free, co-sintering process. Use of a glass material for the waveguide can include the use of an optical adhesive with refractive index ˜1.82. One option for such an adhesive involves loading the adhesive with high refractive index nanoparticles, which has achieved indices>1.84. An advantage provided by glass waveguides is a substantially reduced thermal conductivity, which reduces the parasitic heat flowing through the waveguide. The infrared transmission cutoff wavelengths (imaginary index>1×10−4) of YAG and glass are ˜4 μm and ˜2 μm, respectively, so both materials will inhibit direct radiative transport.
Yb:CaF2 Gain Material
For Yb:CaF2 gain material, with an index of refraction of ˜1.42, the waveguide and cladding can be fabricated from CaF2, glass, a polymer material, or the like. The edge cladding can be fabricated from glass doped with absorbing metal ions, as discussed above. CaF2 is characterized by a relatively high thermal expansion coefficient of 18 ppm/K. Accordingly, some embodiments address thermal stresses in the waveguide/amplifier slab assembly induced by cryocooling. For example, glasses with reasonable index matching (e.g.; N-FKS available from Schott) exhibit a significant expansion coefficient mismatch (12.7 ppm/K). While waveguides fabricated from CaF2 avoid this issue, solutions based on other materials remain attractive due to the relative mechanical fragility of CaF2. Some embodiments of the present invention mitigate thermal expansion mismatch issues by utilizing a nested series of waveguide layers, including glass (e.g.; N-FKS) separated by moderately thin layers (e.g., ˜1 mm) of a compliant optical adhesive. Most of the mismatch strain will be applied to the interleaved adhesive due to its low modulus. With sufficient adhesive intrinsic compliance and sufficient bonding strength at the interfaces, this design can accommodate large stresses.
Although the geometry illustrated in
The width of the absorbing edge cladding 330 is selected so that the effective reflectivity from the outer surface of the edge cladding is sufficiently low. This reflectivity is reduced by twice the single-pass absorption through the cladding, which is e−2α′L for cladding absorption coefficient α′ and thickness L. At the same time, thermal transport between the edge cladding and the liquid cooling system (which can be thermally connected to only the outer edge of the edge cladding in embodiments in which multiple amplifier slabs are stacked close together) enhanced by a thinner edge cladding. In some embodiments, the edge cladding thickness will range from about 0.1 mm to about 5 mm. In a particular embodiment, the edge cladding thickness is L≈1 mm.
Referring once again to Table 1, the waveguide width tGUIDE providing sufficient thermal isolation was calculated for a geometry of a 25 cm aperture, 2 cm thick amplifier slab using temperature-dependent thermal conductivities. The temperature-dependent thermal conductivity for ceramic YAG assumes a ceramic grain size of 4 μm. The temperature-dependent thermal conductivity of LaSF glass was used for the thermal conductivity of glass (1.06 W/m-K at 35° C.).
Although Yb:YAG and Yb:CaF2 gain materials are discussed above, embodiments of the present invention are not to these materials and other suitable host materials can be used, including glass, strontium fluoroapatite (SFAP), or the like. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. Additionally, although Yb has been discussed herein as a suitable rare earth gain medium, other gain media suitable for operation at cryocooled temperatures can be utilized to provide laser systems with thermally isolated edge claddings providing high efficiency.
In some embodiments, minimization of the waveguide width tGUIDE is desired in order to reduce the overall size of the laser system. Size reduction can be achieved through several approaches:
1. Tapering the dimension of the guiding layer along the heat propagation direction, thereby reducing the effective cross section for thermal transfer and enabling smaller waveguides or reduced conductive heat loads.
2. By operating the edge cladding at a slightly reduced temperature (e.g., 280 K)
3. By constructing the waveguide from material with lower thermal conductivity. For example, for ceramic YAG waveguides, there is a slight advantage to using material with a smaller grain size, because this reduces the thermal conductivity. Doping the waveguide material with species that have a significantly different atomic mass or bond strength than the host material (to induce phonon scattering centers) can also be used to reduce its thermal conductivity, particularly at low temperatures.
The worst-case thermo-mechanical stress for an expansion-matched waveguide/gain medium pair can be estimated from
σMAX=E αΔT/(1−v),
where E is Young's modulus, a is the thermal expansion coefficient, v is Poisson's ratio, and ΔT is the temperature difference. Using material coefficients for ceramic YAG, the estimated maximum stress is below 150 MPa for an amplifier slab at 200K. This estimate is a worst-case value because it assumes a rigidly constrained assembly and a temperature-independent expansion coefficient. Practical assemblies will be loosely held to avoid thermo-mechanical issues, and α is known to decrease with decreasing temperature, so that the effective expansion strain ∫α dT<α ΔT. Since the worst case estimate of σMAX=150 MPa is well below the fracture strength of ceramic YAG (340-360 MPa) and the elastic limit of crystal YAG (280 MPa), and given the conservative nature of the σMAX estimate, the thermal gradient across the waveguide does not appear to be a significant issue.
An additional advantage of operating the edge cladding at higher temperatures is the improved performance of liquid coolants. Cooling liquids are available with pour points as low as 135 K, but their viscosity is significant when operated at temperatures within a few tens of degrees Kelvin above the pour point. Higher viscosity increases the electrical power required to pump the fluids. In contrast, operation near room temperature enables the use of very effective coolants (e.g., water, water/glycol brines, and the like) that exhibit low viscosity, excellent thermal capacity, and low cost.
A cooling medium 610, which can be either a liquid coolant or solid, thermally conductive block, is located in close proximity to the amplifier slab face. In the illustrated embodiment, the cooling medium 610 is separated from the gain medium by a reflective coating 620, which can be a high reflectance (HR) multi-layer dielectric stack. As shown in
When faces of the amplifier slab are accessible for direct cooling with fluids, the gain medium can be cooled by flowing high pressure gas over its faces. Several implementation details of this approach include:
1. Helium gas can be used to minimize scattering losses.
2. The amplifier slabs and their cover windows, or their reflectors for reflective geometries such as
3. The gas inlets and outlets to the gain medium channels are shaped with “vanes” to achieve the optimal flow patterns.
For this cooling approach, some waveguide configurations are suboptimal, because the gas coolant may be heated as it passes over the higher temperature edge cladding. This increases the heat load on the refrigeration system and will, therefore, reduce system efficiency.
As illustrated in
Referring to
Because the assembly temperature will vary appreciably in the direction transverse to the gas flow, improved cooling performance can be achieved by preventing lateral transport of the cooling gas as it traverses the assembly. For typical gas velocities of 50 to 100 m/s, there should be only moderate lateral diffusion during the ˜10 ms during which the gas flows across the waveguides and slabs.
The inventors have developed system simulations to demonstrate that cryocooled lasers/amplifiers with room temperature absorbers provide attractive performance enhancements over room-temperature Nd:glass lasers/amplifiers. The improved coefficient of performance of the edge cladding cooling system reduces the cooling electrical power, resulting in net improvement of the system efficiency.
The laser parameters used to compute the plots in
The method also includes producing ASE in the gain medium (1120). The ASE propagates along the transverse axis and the lateral axis. The method further includes directing the ASE through a waveguide optically coupled to the gain medium (1122). The waveguide is transparent in some embodiments, with more than 90% of the ASE being transmitted through the waveguide. The waveguide can partially surround the gain medium along directions aligned with the transverse axis and the lateral axis, enabling optical access to the faces of the gain medium normal to the longitudinal axis. In some embodiments, the waveguide is made from the same host material as the gain medium, but without the active species (e.g., Yb).
Additionally, the method includes absorbing a portion of the ASE in an edge cladding optically coupled to the waveguide (1124). The cladding is characterized by a second temperature higher than the first temperature. The second temperature can be room temperature. Since the cladding is thermally insulated from the gain medium by the waveguide, the temperature of the cladding can be maintained at a higher temperature than the gain medium during operation.
It should be appreciated that the specific steps illustrated in
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
This application claims priority to U.S. Provisional Patent Application No. 61/496,481, filed on Jun. 13, 2011, entitled “Method and System for Cryocooled Laser Amplifier,” the disclosure of which is hereby incorporated by reference in its entirety for all purposes.
The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US12/42097 | 6/12/2012 | WO | 00 | 6/4/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61496481 | Jun 2011 | US |
