Patent Publication Number: US-2013250984-A1

Title: Laser element having a thermally conductive jacket

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 61/614,127, filed Mar. 22, 2012, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Embodiments of the present invention generally relate to laser systems and, more specifically, to a laser element for use in a laser system having a thermally conductive jacket. 
     High power laser systems have a broad range of applications throughout the scientific, industrial and medical fields. Laser systems generally include a pump source, a laser element and a laser resonator. The pump source may include laser diodes or bars that generate pump energy or a light input to the laser element. The laser element absorbs the pump energy and emits laser light responsive to the absorbed energy. The laser resonator operates to generate a harmonic of the laser light. 
     The laser element is generally tuned to absorb pump energy having a wavelength that is within a specified operating band and discharge a laser beam in response to the pump energy. This process generates heat in the laser element. A chiller is generally used to circulate a flow of cooling liquid around the laser element and other components of the system, such as the pump source, to maintain the components within a desired temperature range. 
     For end-pumped solid state laser system, the end of the laser element that receives the pump energy becomes hotter than other portions of the laser element. In high power laser systems, temperature gradients develop in the laser element. These temperature gradients produce stresses in the laser element that can become large enough to break the laser element at high pump energy input levels. As a result, the power of the pump energy must be limited to avoid breaking the laser element. This results in a limitation to the power of the laser output from the system. 
     SUMMARY 
     Embodiments of the invention are directed to a laser element, a laser system that uses the laser element, and a method of operating the laser system. In some embodiments, the laser element comprises a laser rod and a thermally conductive jacket on an exterior surface of the laser rod. The thermally conductive jacket assists in dissipating heat generated in the laser rod during the application of pump energy to the laser rod. 
     Embodiments of the laser system include the laser element described above, a chiller and a pump source. The chiller is configured to deliver a flow of cooling liquid over the thermally conductive jacket of the laser element. The pump source is configured to pump an end of the laser rod with pump energy. The laser rod generates laser light in response to the pump energy and heat from the laser rod is conducted through the jacket to the flow of cooling liquid. 
     In some embodiments of the method, a flow of cooling liquid is delivered over a laser element comprising a thermally conductive jacket on an exterior surface of a laser rod. The laser rod is pumped with pump energy generated by a pump source. Laser light is generated using the laser rod in response to the pump energy. Heat is conducted from the laser rod to the flow of cooling liquid through the thermally conductive jacket. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not indented to identify key features 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. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the Background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram of an exemplary laser system in accordance with the embodiments of the invention. 
         FIG. 2  is a simplified side cross-sectional view of a laser element in accordance with embodiments of the invention supported within a cooling chamber housing of a chiller and end pumped by a pump source. 
         FIG. 3  is a flowchart illustrating a method of operating a laser system in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Embodiments of the invention are described more fully hereinafter with reference to the accompanying drawings. The various embodiments of the invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Elements that are identified using the same or similar reference characters refer to the same or similar elements. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, if an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Thus, a first element could be termed a second element without departing from the teachings of the present invention. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a simplified diagram of an exemplary surgical laser system  100  in accordance with embodiments of the invention. In general, the laser system  100  is configured to generate electromagnetic radiation  102  in the form of a laser beam, deliver the electromagnetic radiation through a laser fiber  104 , such as a waveguide or optical fiber, to a probe tip  106  where it is discharged to a desired target, such as tissue of a patient. 
     The exemplary system  100  comprises a laser resonator  108 . The laser resonator  108  may include a first resonator mirror  110 , a second resonator minor  112  and a laser rod or element  114  formed in accordance with embodiments of the invention. In one embodiment, the laser element  114  comprises a yttrium-aluminum-garnet (YAG) crystal rod with neodymium (Nd) atoms dispersed therein to form a Nd:YAG laser rod. Other features of the laser element  114  are described below. 
     The laser element  114  is pumped by a pump energy  116  from a pump source  118 , such as diode stack or other conventional pump source, through a folding mirror  119  and possibly pump energy re-shaping optics (not shown). The laser element  114  generates laser light or beam  120  at a first frequency in response to the pump energy  116 . The laser element  114  has optical gain at certain wavelengths and this determines the wavelength of the laser beam  120  inside the resonator  108 . This wavelength is also referred to as the fundamental wavelength. For the Nd:YAG laser rod, the typical fundamental wavelength is 1064 nm. 
     In some embodiments, the laser beam  120  bounces back and forth between the first and second resonator mirrors  110  and  112  along a route determined by intermediary mirrors, such as minor  124 , a folding mirror  119  and a mirror  126 . The laser beam  120  propagates through the laser element  114  and a nonlinear crystal  122 . Exemplary embodiments of the nonlinear crystal  122  include a lithium borate (LBO) crystal or a potassium titanyl phosphate crystal (KTP). The nonlinear crystal  122  generates a second harmonic of the laser beam  120  emitted by the laser element  114 . 
     When the laser beam  120  inside the resonator  108  propagates through the nonlinear crystal  122  in a direction away from the folding mirror  124  and toward the second resonator minor  112 , a beam  102  of electromagnetic radiation at the second harmonic wavelength is output from the crystal  122 . The second resonator mirror  112  is highly reflective at both the fundamental and second harmonic wavelengths, and both beams  120  and  102  propagate back through the nonlinear crystal  122 . On this second pass, more beams  102  at the second harmonic wavelength are produced. For example, the nonlinear crystal  122  can produce a laser beam  102  having a wavelength of approximately 532 nm (green) when a Nd:YAG rod is used in the laser element  114 . One advantage of the 532 nm wavelength is that it is strongly absorbed by hemoglobin in blood and, therefore, is useful for cutting, vaporizing and coagulating vascular tissue. 
     In some embodiments, the system  100  includes a Q-switch  128  that converts the laser beam  120  to a train of short pulses with high peak power. These short pulses increase the conversion efficiency of the second harmonic laser beam  102  and increase the average power of the laser beam  102  outside the resonator  108 . 
     The minor  124  is highly reflective at the fundamental wavelength and is highly transmissive at the second harmonic wavelength. Thus, the laser beam  102  at the second harmonic passes through the minor  124  and produces a second harmonic laser beam  102  outside the optical resonator  108 . The laser fiber  104  connects to an optical coupler  130 , which couples the beam  102  to the laser fiber  104  through a shutter mechanism (not shown). The beam  102  travels through the laser fiber  104  to the probe  106  coupled to a distal end  132  of the laser fiber  104 . Embodiments of the probe  106  include components that support the distal end  132  of the laser fiber, such as an endoscope or cystoscope. 
     In some embodiments, the probe  106  includes a probe tip  134  where the laser beam  102  is discharged. In some embodiments, the probe tip  134  includes a fiber cap that is attached to the distal end of the optical fiber  104 . The laser energy may be directed along the axis of the probe  106  (i.e., end firing probe), laterally from the probe tip  134  (i.e., side-firing probe), or in another conventional manner. 
     The laser system  100  may be controlled by a surgeon through a suitable interface. The controls include a controller for selectively opening the shutter mechanism of the system  100  to allow for continuous or pulsed discharge of the laser beam  102  through the probe  106 . 
     In some embodiments, the system  100  includes a chiller  136  that operates to maintain the diode stack  118 , the Q-switch  128  and/or the laser element  114  within a desired temperature range using a flow of cooling liquid. The chiller  136  may be formed in accordance with conventional laser system chillers. 
     In some embodiments, the laser element  114  is supported within a housing  140  of the chiller  136 , as shown in  FIG. 2 . The housing  140  defines a cooling chamber  142  and may be sealed around the laser element  114  using O-rings  144 , or other suitable technique. In some embodiments, the cooling chamber  142  surrounds the sides or sidewall  146  of the laser element  114  extending along a central axis  148 , as shown in  FIG. 2 . A flow of cooling liquid, represented by arrows  150 , is circulated around the laser element  114  within the chamber  142 . In some embodiments, the housing includes ports  152  through which the flow  150  enters and exits the chamber  142 . 
     The laser element  114  is pumped by pump energy  116  from the pump source  118  from an end  154  of the laser element  114 , as shown in  FIG. 2 . The end pumping of the laser element  114  causes heat to be generated at the end  154 . In some embodiments, the flow of cooling liquid  150  operates to maintain the laser element  114  within a desired temperature range. 
     With conventional laser elements, the heat generated at the pumped end  154  is conducted along the direction of the central axis  148  ( FIG. 2 ) of the laser element. The uneven distribution of the heat and the uneven cooling of the laser element  114  by the chiller  136  results in the formation of temperature gradients between the hot pumped end  144  and the relatively cooler opposing end  156  of the laser element. At sufficient pump energy levels, these temperature gradients can produce stresses in the laser element that break the laser element. 
     Embodiments of the laser element  114  have improved heat dissipation along the central axis  148  of the laser element  114 , and improved efficiency at which heat may be dissipated from the laser element  114 . In some embodiments, the laser element  114  includes a laser rod  158 , as shown in  FIG. 2 . Exemplary embodiments of the laser rod  158  include a Nd:YAG laser rod as described above, a thulium-doped yttrium aluminum garnet (Tm:YAG) laser rod, a ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser rod, a holmium-doped yttrium aluminum garnet (Ho:YAG) laser rod, or other conventional laser rod. In some embodiments, the laser element  114  includes a thermally conductive jacket  160  that engages the rod  158 . In some embodiments, the jacket  160  is a cylindrical jacket that engages the exterior surface  162  of the laser rod  158  along the side  146 , as shown in  FIG. 2 . 
     In some embodiments, the jacket  160  operates to conduct heat along the central axis  148  of the rod  158 . This increases the efficiency at which heat generated at the end  154  pumped by the pump energy  116  is distributed along the length of the rod  158  and reduces temperature gradients in the rod  158 . Consequently, the laser element  114  may be pumped with pump energy  116  having a higher power while avoiding laser rod  158  breakage than would be possible with conventional laser elements. Thus, the laser element  114  allows the system  100  to produce higher energy output laser beams  102 . 
     The jacket  160  also increases the surface area of the laser element  114  that is exposed to the cooling water of the chiller  136 . In some embodiments, the jacket includes fins or other projections (not shown) to increase heat dissipation. As a result, heat transfer between the laser rod  158  and the cooling water flow  150  of the chiller  136  is increased. This allows the chiller  136  to operate at a higher temperature for a given pump energy  116  than would be possible with conventional laser elements, resulting in an energy savings. 
     In some embodiments, the jacket  160  forms a barrier between the rod  158  and the flow of cooling liquid  150 . This allows the chiller  136  to use different liquids than the conventional deionized water, which may further improve the efficiency of the laser system  100 . 
     As a result, the chiller  136  can operate at a lower power level than would be possible using the laser rod  158  without the jacket  160 . The improved heat transfer due to the cooling jacket  160  using special liquid instead of water can also reduce the risk of creating a condensing environment in the laser resonator  108 . 
     The more efficient cooling of the laser rod  158  and the reduction of thermal stresses in the rod  158  due to the jacket  160 , allows the laser rod  158  to accept pump energy  116  of a wider dynamic range than would otherwise be possible using conventional laser elements. Additionally, the laser rod  158  can receive a higher pump energy  116  than would be possible using conventional laser elements. As a result, one may produce a desired laser output using a smaller laser rod  158  than would be possible using conventional designs resulting in a cost savings and higher laser gain. 
     The cooling jacket  160  comprises a thermally conductive material that is more thermally conductive than the laser rod  158 . In some embodiments, the jacket  160  has a coefficient of thermal expansion that is the same or similar to that of the laser rod  158 . In some embodiments, the jacket  160  comprises metal. In some embodiments, the jacket  160  comprises silver, gold, copper alloy, and/or aluminum nitride. 
     In some embodiments, the jacket  160  is in the form of a cylindrical sleeve that receives the rod  158 . In some embodiments, the jacket  160  is in the form of a coating that is applied to the exterior surface  162  of the rod  158 . In some embodiments, the coating is applied to the exterior surface  162  of the laser rod  158  through a dipping process, through chemical vapor deposition, or other suitable technique. In some embodiments, the jacket  160  has a thickness of approximately 0.002-0.020 inch. 
     In some embodiments, the jacket  160  forms a reflective interior surface  164  that faces the exterior surface  162  of the rod  158 . The reflective surface  164  operates to reduce heat production from exposure of the jacket  160  to the pump energy  116  and/or the beam  120 . 
       FIG. 3  is a flowchart illustrating a method of operating the system  100  in accordance with embodiments of the invention. At  166 , a flow of cooling liquid  150  ( FIG. 2 ) is delivered over a thermally conductive jacket  160  on the exterior surface  162  of the laser rod  158 . The jacket  160  is formed in accordance with one or more embodiments discussed above. In some embodiments, the flow  150  travels through a cooling chamber  142  defined by a housing  140  of a chiller  136 . In some embodiments, the jacket  160  prevents the flow  150  from contacting the exterior surface  162  of the laser rod  148 . 
     At  168 , a laser rod  114  is pumped with pump energy  116  generated by a pump source  118 , as discussed above with reference to  FIGS. 1 and 2 . In some embodiments, the pump energy  116  is delivered to an end  154  of the laser rod  158 . At  170 , laser light  120  ( FIG. 1 ) is generated using the laser rod responsive to the pump energy  116 . 
     At  172 , heat form the laser rod  158  is conducted to the flow of cooling liquid  150  through the jacket  160 . As discussed above, the thermally conductive jacket  160  improves the efficiency at which heat is dissipated from the laser rod  158  to the flow of cooling liquid  150  compared to conventional laser elements. Additionally, the thermally conductive jacket  160  may reduce stresses in the laser rod  158  due to improved heat dissipation along the length of the laser rod  158 . The improved heat dissipation resulting from the use of the jacket  160  allows the laser system  100  to discharge higher power laser beams  102  than conventional laser systems using the same laser rod. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.