WO2012174017A1 - Method and system for cryocooled laser amplifier - Google Patents
Method and system for cryocooled laser amplifier Download PDFInfo
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- WO2012174017A1 WO2012174017A1 PCT/US2012/042097 US2012042097W WO2012174017A1 WO 2012174017 A1 WO2012174017 A1 WO 2012174017A1 US 2012042097 W US2012042097 W US 2012042097W WO 2012174017 A1 WO2012174017 A1 WO 2012174017A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/02—Constructional details
- H01S3/04—Arrangements for thermal management
- H01S3/042—Arrangements for thermal management for solid state lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/02—Constructional details
- H01S3/04—Arrangements for thermal management
- H01S3/0404—Air- or gas cooling, e.g. by dry nitrogen
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/163—Solid materials characterised by a crystal matrix
- H01S3/164—Solid materials characterised by a crystal matrix garnet
- H01S3/1643—YAG
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/163—Solid materials characterised by a crystal matrix
- H01S3/1645—Solid materials characterised by a crystal matrix halide
- H01S3/165—Solid materials characterised by a crystal matrix halide with the formula MF2, wherein M is Ca, Sr or Ba
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S2301/00—Functional characteristics
- H01S2301/02—ASE (amplified spontaneous emission), noise; Reduction thereof
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/0602—Crystal lasers or glass lasers
- H01S3/0606—Crystal lasers or glass lasers with polygonal cross-section, e.g. slab, prism
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
- H01S3/16—Solid materials
- H01S3/1601—Solid materials characterised by an active (lasing) ion
- H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
- H01S3/1618—Solid materials characterised by an active (lasing) ion rare earth ytterbium
Definitions
- 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.
- YbrYAG 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.
- 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.
- 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.
- 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.
- a cryocooled gain medium is utilized in which edge cladding used to absorb parasitic radiation such as ASE are operated at a warmer
- 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
- a 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.
- a 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.
- an 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.
- a method of operating a laser amplifier 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.
- 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.
- FIG. 1 shows simplified plots illustrating an inverse of a coefficient of performance as a function of temperature according to an embodiment of the present invention
- FIG. 2A shows simplified plots illustrating laser system efficiency as a function of peak pump power for Nd-doped glass gain medium in several cooling configurations according to an embodiment of the present invention
- FIG. 2B shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG gain medium at 200K in several cooling configurations according to an embodiment of the present invention
- FIG. 2C shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG gain medium at 150K in several cooling configurations according to an embodiment of the present invention
- FIG. 2D shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG and Nd:Glass gain medium at 200K and 150K in several cooling configurations according to an embodiment of the present invention
- FIG. 3A is a simplified schematic diagram illustrating an end view of an amplifier slab configuration with cryocooling according to an embodiment of the present invention
- FIG. 3B is a simplified schematic diagram illustrating a cross-section through the amplifier slab configuration illustrated in FIG. 3A.
- FIG. 4 shows simplified plots illustrating conductive heat load through a waveguide for various configurations according to an embodiment of the present invention
- FIG. 5 is a simplified cross-sectional schematic diagram illustrating a tapered waveguide according to an embodiment of the present invention
- FIG. 6A is a simplified cross-sectional view of a waveguide configuration integrated with an actively cooled mirror according to an embodiment of the present invention
- FIG. 6B is a simplified end view of the waveguide configuration integrated with an actively cooled mirror illustrated in FIG. 6 A;
- FIG. 7A is a simplified end view of an amplifier slab geometry with cryocooling according to an embodiment of the present invention.
- FIG. 7B is a simplified plan view of the amplifier slab geometry illustrated in FIG. 7A;
- FIG. 7C is a simplified end view of an amplifier slab geometry with cryocooling and gas shaping according to an embodiment of the present invention;
- FIG. 8A is a simplified end view of an amplifier slab geometry with flow barriers according to an embodiment of the present invention.
- FIG. 8B is a simplified plan view of the amplifier slab geometry illustrated in FIG. 8A;
- FIG. 9 is a simplified plan view of a waveguide configuration according to an alternative embodiment of the present invention.
- FIG. 10A shows simplified plots illustrating Yb:YAG gain medium operated at 200K in several cooling configurations according to an embodiment of the present invention
- FIG. 10B shows simplified plots illustrating Yb:YAG gain medium operated at 150K in several cooling configurations according lo an embodiment of the present invention
- FIG. IOC shows simplified plots illustrating laser system efficiency as a function of peak pump power for several system configurations according to an embodiment of the present invention.
- FIG. 1 1 is a simplified flowchart illustrating a method of operating an optical amplifier according to an embodiment of the present invention.
- 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.
- 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.
- 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).
- the refractive index of the edge absorbers is typically closely matched to the refractive index of the gain medium to prevent back reflections.
- 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.
- 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.
- the slabs utilize a pump energy of 0.5 J/cm 2 /slab at 872 nm, and ASE results in approximately 294 J of energy being absorbed in the edge cladding of each amplifier slab.
- ASE results in approximately 294 J of energy being absorbed in the edge cladding of each amplifier slab.
- 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, Y 3 AI5O12) crystal or ceramic host material and other work has related to the use of CaF 2 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.
- 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.
- COP coefficient of performance
- FIG. 1 shows simplified plots illustrating an inverse of the COP as a function of temperature according to an embodiment of the present invention. As illustrated in FIG. 1 , the COP degrades rapidly with decreasing cooling temperature. The curves illustrated in FIG.
- 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.
- 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 x 25 cm 2 aperture, and utilizing 4 passes.
- Nd:Glass gain medium e.g., APG-1 available from Schott
- a cryocooled Yb-doped YAG gain medium operating at a temperature of either 150 or 200 K
- 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.
- 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 ⁇ wavelength laser energy per pulse.
- 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 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 FIG. 1.
- Other simulation parameters are summarized in Table 1, which provides a comparison of pulsed laser amplifier designs for various gain media. Embodiments of the present invention are not limited to these particular gain media, which are illustrated only for exemplary purposes.
- the Nd:Glass laser system illustrated in Table 1 includes 768 beamlines (1536 amplifiers) and delivers 4.9 MJ of energy at a wavelength of 1.05 ⁇ .
- FIG. 2A shows simplified plots illustrating laser system efficiency as a function of peak pump power for Nd-doped glass gain medium in several cooling configurations according to an embodiment of the present invention.
- FIG. 2B shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG gain medium at 200K in several cooling configurations according to an embodiment of the present invention.
- each data point represents a different laser design.
- FIGS. 2A and 2B illustrate the fundamental tradeoff between system efficiency and the required pump power, which represents the required cost of diode pump components. The tradeoff is adjusted by varying the duration of the pump pulse.
- FIGS. 2A and 2B illustrate the fundamental tradeoff between system efficiency and the required pump power, which represents the required cost of diode pump components. The tradeoff is adjusted by varying the duration of the pump pulse.
- These figures show that the use of a cooled Yb gain medium significantly reduces the required pump power, which results, in part, from the substantially longer excited state lifetime of the Yb ions in comparison with Nd ions ( ⁇ 1 ms for Yb vs. ⁇ 250 for Nd).
- the reduction in pump power is accompanied by a decrease in overall system efficiency.
- the required pump power for the cryocooled Yb:YAG system at 200K is -89 GW, achieving comparable efficiency to that of the NdrGlass system, for example 10%.
- FIG. 2C shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG gain medium at 150K in several cooling configurations according to an embodiment of the present invention.
- the edge cladding is also operated at 150K in the embodiment illustrated in FIG. 2C.
- the above figures demonstrate that some embodiments of cryocooled systems operate at efficiencies less than 10% when the laser is operated in a high energy pulsed mode of operation.
- systems with cryocooled gain media exhibit better performance than the room temperature, glass-based design when cooling is not considered, the increased electrical power needed for cryogenic cooling results in an overall net reduction in system efficiency.
- FIG. 2D shows simplified plots illustrating laser system efficiency as a function of peak pump power for Yb:YAG and Nd:Glass gain medium at 200K and 150K in several cooling configurations according to an embodiment of the present invention.
- the gain medium and the edge absorber are maintained at the same temperature (i.e., 200K/200K or
- cryocooled Yb:YAG laser systems 150K/150K. Only total laser system efficiency including cooling is shown.
- the comparison between cryocooled Yb:YAG laser systems and a room-temperature Nd:glass system shown in FIG. 2D illustrates that although some embodiments of the cryocooled systems offer improved performance at low pump power and system efficiency, little or no advantage is achieved by employing the cryocooled media in these embodiments when system efficiency >9% is required.
- the overall efficiency of high-energy, pulsed laser systems based on cryocooled, Yb-doped gain media is significantly impacted by the cooling requirements.
- 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.
- 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.
- 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.
- 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.
- FIG. 3A is a simplified schematic diagram illustrating an end view of an amplifier slab configuration with cryocooling according to an embodiment of the present invention.
- FIG. 3B is a simplified schematic diagram illustrating a cross-section through the amplifier slab configuration illustrated in FIG. 3 A.
- a gain medium 310 is partially surrounded by a transparent waveguide 320 with a width equal to touiDE-
- An edge cladding 330 partially surrounds the transparent waveguide 320.
- the refractive index of the waveguide 320 is closely matched to those of both the cladding 330 and the gain medium 310 to prevent parasitic lasing in the plane of the slab, which lies in the plane of the figure for
- FIG. 3A The surfaces of the transparent waveguide 320 (e.g., the top surface shown in FIG. 3B) can be polished so that some ASE is confined to the waveguide material by total internal reflection.
- the waveguide provides thermal isolation so that the gain medium and cladding can operate at very different temperatures.
- characteristics of the waveguide structure include:
- 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 includes low absorption that can be less than 100% transmission.
- 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%.
- absorption coefficient i.e., power absorption
- 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)
- QSLAB where X and t are the aperture width and thickness of the gain medium, K is the thermal conductivity of the waveguide material, ⁇ 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.
- 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 ⁇ . Thus, some embodiments utilize waveguide material with a transparency cutoff wavelength of ⁇ 4 ⁇ .
- the materials selected for the waveguide and the edge cladding depend on the material chosen to provide laser gain.
- material options for two gain medium hosts are presented: YAG ceramic and CaF 2 crystal.
- 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.
- 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 >lxl0 "4 ) of YAG and glass are ⁇ 4 ⁇ and ⁇ 2 ⁇ , respectively, so both materials will inhibit direct radiative transport.
- the waveguide and cladding can be fabricated from CaF 2 , glass, a polymer material, or the like.
- the edge cladding can be fabricated from glass doped with absorbing metal ions, as discussed above.
- CaF 2 is characterized by a relatively high thermal expansion coefficient of 18 ppm/K.
- some embodiments address thermal stresses in the waveguide/amplifier slab assembly induced by cryocooling.
- glasses with reasonable index matching e.g.; N-FKS available from Schott
- solutions based on other materials remain attractive due to the relative mechanical fragility of CaF 2 .
- 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.
- FIG. 3A utilizes an absorbing material (i.e., the edge cladding) that is in close proximity (i.e., attached) to the waveguide, this is not required by the present invention.
- an antireflection (AR) coating is applied to the waveguide end surfaces so that light is propagated outside the amplifier slab to remotely located absorbing beam dumps.
- 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 ⁇ 2ctL for cladding absorption coefficient a' and thickness L.
- 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) is enhanced by a thinner edge cladding.
- 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.
- 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 ⁇ .
- the temperature- dependent thermal conductivity of LaSF glass was used for the thermal conductivity of glass (1.06 W/m-K at 35 °C).
- Yb:YAG and Yb:CaF 2 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.
- SFAP strontium fluoroapatite
- 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.
- FIG. 4 shows simplified plots illustrating conductive heat load through a waveguide for various configurations according to an embodiment of the present invention.
- the heat loads shown in FIG. 4 were calculated using 4 times the heat flux across a 25 cm x 2 cm waveguide cross-section (25 cm x 25 cm amplifier slab of 2 cm thickness) calculated from a 1-D finite element simulation. Since typical bulk heat loads on the amplifier slabs due to quantum defect and nonradiative processes are -900 W (see Table 1), embodiments of the present invention utilize waveguide widths that range from about 10 cm to 15 cm for YAG and about 3 cm to 5 cm for glass. [0063] 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:
- FIG. 5 is a simplified cross-sectional schematic diagram illustrating a tapered waveguide according to an embodiment of the present invention. As illustrated in FIG. 5, the cross section of the waveguide 520 at its junction with the gain medium 510 (i.e., an amplifier slab) is matched to the gain medium, but the waveguide dimension along the axis orthogonal to the direction of propagation can taper down towards the edge cladding 530.
- the gain medium 510 i.e., an amplifier slab
- tapering from a 2 cm x 25 cm cross-section at the gain medium to 1 cm x 25 cm cross-section at the edge cladding reduces the heat load by -25%.
- a tapered ceramic YAG waveguide can be used to reduce heat conduction into the gain medium.
- edge cladding By operating the edge cladding at a slightly reduced temperature (e.g., 280 K)
- the waveguide from material with lower thermal conductivity.
- material with lower thermal conductivity 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.
- E Young's modulus
- a the thermal expansion coefficient
- v the Poisson's ratio
- ⁇ the temperature difference.
- E Young's modulus
- a the thermal expansion coefficient
- v the thermal expansion coefficient
- ⁇ the temperature difference.
- the estimated maximum stress is below 150 MPa for an amplifier slab at 200 . 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 a is known to decrease with decreasing temperature, so that the effective expansion strain Joe dT ⁇ ⁇ ⁇ .
- 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.
- very effective coolants e.g., water, water/glycol brines, and the like
- FIG. 6A is a simplified cross-sectional view of a waveguide configuration integrated with an actively cooled mirror according to an embodiment of the present invention.
- FIG. 6B is a simplified end view of the waveguide configuration integrated with the actively cooled mirror illustrated in FIG. 6A.
- the waveguide configuration illustrated in FIGS. 6A and 6B are suitable for designs in which the laser beam enters and exits the amplifier slab through the same face. Thus, one face of the amplifier slab can be used for cooling as shown in FIG. 6A.
- 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.
- 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.
- HR high reflectance
- the transverse dimensions of the cooling block (in the x-y plane aligned with the plane of figure 6B) is smaller than the amplifier slab/transparent waveguide/edge cladding assembly so that the low-temperature cooling medium does not provide a thermal "short circuit" across the waveguide. Because of the thermal isolation between the gain medium and the edge cladding, the gain medium can be cooled as needed separately from the edge cladding, which can operate at a higher temperature than the gain medium.
- the gain medium can be cooled by flowing high pressure gas over its faces.
- Helium gas can be used to minimize scattering losses.
- the amplifier slabs and their cover windows, or their reflectors for reflective geometries such as Fig. 6A, are typically arrayed to form narrow gas flow channels that increase the gas velocity to improve heat transfer directly over the channel.
- the gas inlets and outlets to the gain medium channels are shaped with "vanes" to achieve the optimal flow patterns.
- FIG. 7A is a simplified end view of an amplifier slab geometry with cryocooling according to an embodiment of the present invention.
- FIG. 7B is a simplified plan view of the amplifier slab geometry illustrated in FIG. 7A.
- the waveguide configuration illustrated in FIG. 7 utilizes reflectors 740 to deflect ASE to absorbing edge claddings 730 located on only two edges of the amplifier slab assembly.
- the reflectors 740 can be fabricated by depositing HR coatings on the angled edges of the waveguide material 720. As illustrated, the reflectors 740 are tilted at an angle #to direct the ASE into the absorptive edge cladding. These edges are connected to gas shaping vanes 750 in FIG.
- the bonding of the gas shaping vanes 750 can be performed using a variety of materials, including epoxy, since there is no requirement for index matching or transparency.
- the gain slab 710, waveguide material 720, reflectors 740, gas shaping vanes 750, and absorptive edge claddings 730 can be referred to as an amplifier unit.
- the amplifier units are arrayed in the longitudinal direction along which laser light is amplified during propagation through the set of amplifier units.
- the thermal separation between the absorptive edge cladding and the gain medium enables the gain medium to be operated at a first temperature (e.g., cryogenic temperatures) while the absorptive edge claddings are operated at a second temperature (e.g., room temperature) higher than the first temperature.
- a first temperature e.g., cryogenic temperatures
- a second temperature e.g., room temperature
- the coolant flow provides flow of the coolant to the gain medium in a manner in which some or a majority of the coolant flow does not interact with the edge cladding.
- the central portion of the coolant flow does not flow over the edge claddings before reaching the gain medium, but flow parallel to the edge claddings.
- edge claddings are located at the peripheral portions of the assembly parallel to the coolant flow, an additional level of thermal decoupling between the edge absorbers and the gain medium is provided.
- FIG. 7C is a simplified end view of an amplifier slab geometry with cryocooling and gas shaping according to an embodiment of the present invention.
- additional waveguide material 720' is used to form a waveguide edge parallel to the plane of FIG. 7C.
- the use of additional waveguide material 720' may simplify bonding and reduce thermal stresses.
- the reflecting surfaces and edge cladding are arranged in order to prevent parasitic lasing in the transverse plane (the plane of FIG. 7A) and to minimize ASE-induced loss of excited state energy in the gain medium.
- the HR coated edges are not oriented parallel to one another to avoid formation of a lasing cavity.
- the distance t'cuiDE is set at a sufficiently large distance and the angle ⁇ is set at a sufficiently small angle so that ASE emitted from the gain medium at most angles in the plane of FIG. 7A does not reflect from the HR coatings back into the gain medium.
- FIG. 8A is a simplified end view of an amplifier slab geometry with flow barriers according to an embodiment of the present invention.
- FIG. 8B is a simplified plan view of the amplifier slab geometry illustrated in FIG. 8A.
- the flow barriers 810 illustrated in FIG. 8A reduce residual transport. It should be noted that in some embodiments, the flow barriers need not support a high pressure differential or make a leak tight seal. By maintaining similar gas pressure on both sides of the barrier, the barrier provides a geometric obstacle to lateral flow.
- FIG. 9 is a simplified plan view of a waveguide configuration according to an alternative embodiment of the present invention.
- the cross-section illustrated in FIG. 9 is similar to the cross-section illustrated in FIG. 8B.
- the increased length of waveguide material along the coolant flow path will increase the pressure drop across the assembly. This increased pressure drop is undesirable, because it increases the electrical power consumed by the compressor that drives the gas coolant through the amplifiers.
- the configuration illustrated in FIG. 9 mitigates the pressure drop by tapering the waveguide along the flow direction. By increasing the width of the coolant flow channel away from the gain medium, the fluid velocity and friction in this region are reduced.
- the configuration illustrated in FIG. 9 reduces the thermal conductivity of the transparent waveguide, thereby decreasing the thermal conduction between the edge claddings and the gain medium.
- FIG. 10A shows simplified plots illustrating Yb:YAG gain medium operated at
- FIG. 10A illustrates the efficiency-pump power tradeoff for a laser based on Yb:YAG amplifier slabs operated at 200K with edge claddings operated at 295K.
- FIG. 10B shows simplified plots illustrating Yb:YAG gain medium operated at 150K in several cooling configurations according to an embodiment of the present invention.
- FIG. 10B illustrates the efficiency-pump power tradeoff for a laser based on Yb:YAG amplifier slabs operated at 150K with edge claddings operated at 295K.
- embodiments of the present invention utilize room- temperature operation of ASE absorbers to enable 200K cryocooled Yb:YAG lasers that can improve laser efficiency by -3% (at fixed pump power) or reduce pump power by approximately a factor of three (at a fixed efficiency of 10%).
- FIG. IOC shows simplified plots illustrating laser system efficiency as a function of peak pump power for several system configurations according to an embodiment of the present invention (i.e., Yb:YAG amplifier slabs operated at 200K with edge absorbers operated at 295K and Yb:YAG amplifier slabs operated at 150K with edge absorbers operated at 295K).
- cryocooled gain media with edge absorbers operated at higher temperatures e.g., room temperature
- the data shown in FIG. IOC demonstrates that operation of the Yb:YAG amplifier slabs at 150K and 200K results in similar system performance. Therefore, given that lower temperature operation typically requires additional system costs and complexity, some embodiments are preferably operated at a temperature of 200K.
- One of ordinary skill in the art would recognize many variations, modifications, and alternatives.
- FIG. 1 1 is a simplified flowchart illustrating a method of operating an optical amplifier according to an embodiment of the present invention.
- the method 1100 includes providing a gain medium (e.g., an Yb-based amplifier medium such as Yb:YAG or Yb:CaF 2 ) having a longitudinal axis, a transverse axis, and a lateral axis (1110).
- the gain medium is a gain slab (also referred to as an amplifier slab) that has a thickness
- the method also includes pumping the gain medium (11 12) and directing light through the gain medium along the longitudinal axis (1 1 14).
- the light is amplified in the gain medium (1 1 16) and the gain medium is cooled such that the gain medium is characterized by a first temperature (1 118).
- the first temperature is a cryogenic temperature less than room temperature, such as 15 OK, 200K, or the like.
- the method also includes producing ASE in the gain medium (1 120).
- 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 (1 122).
- 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.
- the waveguide is made from the same host material as the gain medium, but without the active species (e.g., Yb).
- the method includes absorbing a portion of the ASE in an edge cladding optically coupled to the waveguide (1 124).
- 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.
- FIG. 11 provides a particular method of operating an optical amplifier according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order.
- the individual steps illustrated in FIG. 1 1 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step.
- additional steps may be added or removed depending on the particular applications.
- One of ordinary skill in the art would recognize many variations, modifications, and alternatives.
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Abstract
Description
Claims
Priority Applications (6)
Application Number | Priority Date | Filing Date | Title |
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KR1020137031116A KR20140030210A (en) | 2011-06-13 | 2012-06-12 | Method and system for cryocooled laser amplifier |
JP2014515924A JP2014519721A (en) | 2011-06-13 | 2012-06-12 | Method and system for a cryogenic cooled laser amplifier |
US14/122,901 US20140307305A1 (en) | 2011-06-13 | 2012-06-12 | Method and system for cryocooled laser amplifier |
CN201280019612.4A CN103650261A (en) | 2011-06-13 | 2012-06-12 | Method and system for cryocooled laser amplifier |
EP12801335.6A EP2719035A1 (en) | 2011-06-13 | 2012-06-12 | Method and system for cryocooled laser amplifier |
RU2013148791/28A RU2013148791A (en) | 2011-06-13 | 2012-06-12 | METHOD AND SYSTEM FOR CRYOGENICALLY COOLED LASER AMPLIFIER |
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US201161496481P | 2011-06-13 | 2011-06-13 | |
US61/496,481 | 2011-06-13 |
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PCT/US2012/042097 WO2012174017A1 (en) | 2011-06-13 | 2012-06-12 | Method and system for cryocooled laser amplifier |
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US (1) | US20140307305A1 (en) |
EP (1) | EP2719035A1 (en) |
JP (1) | JP2014519721A (en) |
KR (1) | KR20140030210A (en) |
CN (1) | CN103650261A (en) |
RU (1) | RU2013148791A (en) |
WO (1) | WO2012174017A1 (en) |
Cited By (2)
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WO2015168265A3 (en) * | 2014-04-30 | 2016-01-28 | Synrad, Inc. | Laser resonator with parasitic mode suppression |
CZ305707B6 (en) * | 2014-12-03 | 2016-02-10 | Fyzikální ústav AV ČR, v.v.i. | Optical element, especially laser slab and process for preparing thereof |
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US8159825B1 (en) | 2006-08-25 | 2012-04-17 | Hypres Inc. | Method for fabrication of electrical contacts to superconducting circuits |
US9726820B2 (en) * | 2014-08-14 | 2017-08-08 | Raytheon Company | End pumped PWG with tapered core thickness |
US9899798B2 (en) | 2015-08-03 | 2018-02-20 | University Of Central Florida Research Foundation, Inc. | Apparatus and method for suppressing parasitic lasing and applications thereof |
CN107895880A (en) * | 2017-12-29 | 2018-04-10 | 中国工程物理研究院应用电子学研究所 | A kind of side pumping disk gain module structure of nonbonding or sintering |
CN112654588A (en) * | 2019-01-10 | 2021-04-13 | 捷客斯金属株式会社 | Light-absorbing layer and bonded body having light-absorbing layer |
JPWO2021157135A1 (en) * | 2020-02-07 | 2021-08-12 | ||
US20210316502A1 (en) * | 2020-04-10 | 2021-10-14 | Seurat Technologies, Inc. | High Throughput Additive Manufacturing System Supporting Absorption Of Amplified Spontaneous Emission In Laser Amplifiers |
CN111694093B (en) * | 2020-05-29 | 2021-08-10 | 北京大学 | Silicon-based photoelectron integrated chip with local light amplification and pumping coupling method |
CN114243439B (en) * | 2021-11-02 | 2023-04-25 | 中国工程物理研究院应用电子学研究所 | Can reduce lath laser gain medium ASE suppression device of marginal wavefront distortion |
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- 2012-06-12 US US14/122,901 patent/US20140307305A1/en not_active Abandoned
- 2012-06-12 EP EP12801335.6A patent/EP2719035A1/en not_active Withdrawn
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CZ305707B6 (en) * | 2014-12-03 | 2016-02-10 | Fyzikální ústav AV ČR, v.v.i. | Optical element, especially laser slab and process for preparing thereof |
Also Published As
Publication number | Publication date |
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RU2013148791A (en) | 2015-07-20 |
EP2719035A1 (en) | 2014-04-16 |
KR20140030210A (en) | 2014-03-11 |
CN103650261A (en) | 2014-03-19 |
JP2014519721A (en) | 2014-08-14 |
US20140307305A1 (en) | 2014-10-16 |
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