Patent Publication Number: US-9407058-B2

Title: Pump energy wavelength stabilization

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/555,329, filed Sep. 8, 2009, which claims the benefit of U.S. provisional patent application Ser. No. 61/095,082, filed Sep. 8, 2008. The content of each of the above-identified applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Embodiments of the invention are directed to the stabilization of the wavelength of the pump energy while pumping a laser gain medium. 
     High power laser systems have a broad range of applications throughout the scientific, industrial and medical fields. Laser systems generally include a pump module, a gain medium and a laser resonator. The pump module includes laser diodes or bars that generate pump energy. The gain medium absorbs the pump energy and emits laser light responsive to the absorbed energy. The laser resonator, in some designs, operates to generate a harmonic of the laser light. 
     The gain medium is generally tuned to absorb pump energy having a wavelength that is within a specified operating band. Thus, the wavelength of the pump energy must be carefully controlled to ensure proper operation of the laser system. 
     Pumping a yttrium-aluminum-garnet crystal (YAG) rod with neodymium atoms (i.e., a Nd:YAG gain medium) using pump energy having a wavelength of 885 nm has become a desirable pumping scheme due to its natural efficiency gains that can save on cost and electrical/cooling requirements. However, the operating wavelength band of the Nd:YAG gain medium around the 885 nm wavelength is very narrow. Unfortunately, Small changes in the wavelength of the pump energy can cause rapid decreases in the absorption efficiency of the gain medium. Moreover, the wavelength shift in the pump energy away from the narrow operating bandwidth of the Nd:YAG gain medium around 885 nm can also destabilize the wavelength of the pump energy causing it to further deviate from the operating wavelength range of the gain medium. 
     One option for stabilizing the wavelength of the pump energy is to use a Variable Bragg Grating (VBG) to stabilize the wavelength of the pump energy. However, VBG&#39;s are expensive and reduce the efficiency of the pump energy to laser conversion. 
     SUMMARY 
     Embodiments of the invention are directed to a method and a laser system in which the wavelength of the pump module is stabilized while pumping the gain medium. In one embodiment of the method, a gain medium is provided having an absorption coefficient that varies with wavelength. An absorption coefficient curve of the absorption coefficient over a range of wavelengths comprises peaks and valleys. A pump module is operated to output pump energy at an operating wavelength within one of the valleys, at which the absorption coefficient is approximately less than 40% of the absorption coefficient at an adjacent peak of the absorption coefficient curve defining the valley. The pump energy is directed through the gain medium. A portion of the pump energy is absorbed with the gain medium and laser light is emitted from the gain medium responsive to the absorbed pump energy. The non-absorbed pump energy (feedback pump energy) is fed back to the pump module. The operating wavelength of the pump energy is stabilized using the feedback pump energy. 
     One embodiment of the laser system comprises a pump module, a gain medium and a reflector. The pump module outputs pump energy at an operating wavelength. The gain medium is configured to absorb the pump energy and emit laser light responsive to the absorbed pump energy. The absorption coefficient of the gain medium has a magnitude that varies with wavelength. The absorption coefficient curve of the absorption coefficient over a range of wavelengths comprises peaks and valleys. The operating wavelength of the pump energy is within a valley of the absorption coefficient curve, at which the absorption coefficient is approximately at a minimum within the valley. The reflector is within the path of the pump energy and is configured to direct the non-absorbed portion of the pump energy back to the pump module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a high-power laser system in accordance with embodiments of the invention. 
         FIG. 2  is a simplified block diagram of a pump module in accordance with embodiments of the invention. 
         FIGS. 3 and 4  are absorption coefficient curves for a Nd:YAG gain medium. 
         FIG. 5  is a flowchart illustrating a method in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  illustrates a high-power laser system  100  in accordance with embodiments of the invention. The laser system  100  includes a gain medium  102 , a pump module  104  and a laser resonator  106 . In one embodiment, the gain medium  102  is a doped crystalline host that is configured to absorb pump energy  108  generated by the pump module  104  having a wavelength that is within an operating wavelength range of the gain medium  102 . In one embodiment, the gain medium  102  is end-pumped by the pump energy  108 , which is transmitted through a beam splitter  110  that is transmissive at the wavelength of the pump energy  108 . The gain medium  102  absorbs the pump energy  108  and responsively outputs laser light  112 . 
     The gain medium  102  is water cooled in exemplary embodiments, along the sides of the host. In one embodiment, the gain medium  102  includes an undoped end cap  114  bonded on a first end  116  of the gain medium  102 , and an undoped end cap  118  bonded on a second end  120  of the gain medium  102 . In one embodiment, the end  120  is coated so that it is reflective at the pump energy wavelength, while transmissive at a resonant mode of the system  100 . In this manner, the pump energy that is unabsorbed at the second end  120  is redirected back through the gain medium  102  to be absorbed. 
     One embodiment of the laser pump module  104  includes a plurality of laser diodes or bars  122  (hereinafter “laser diodes”), light combining optics  124 , a temperature control system  126 , a current or power source  128 , and a controller  130 , as shown in the simplified block diagram of  FIG. 2 . The plurality of laser diodes  122  operate to produce the pump energy  108 . In one embodiment, the laser diodes  122  are arranged in an array, such as a multiple bar stack of laser diodes  122 . 
     The wavelength of the pump energy  108  depends on the temperature of the laser diodes  122  and the current supplied to the laser diodes  122 . In one embodiment, the controller  130  controls the temperature control system  126  to maintain the laser diodes  122  at a desired operating temperature such that the pump energy  108  is within the operating wavelength range of the gain medium  102 . In another embodiment, the controller  130  controls the current source  128  to control the current to the laser diodes  122  and, thus, the power level and wavelength of the pump energy  108 . One embodiment of the controller  130  includes one or more processors. In accordance with another embodiment, the controller  130  includes memory  132  that contains instructions executable by the one or more processors to perform various functions, such as, for example, controlling the current to the laser diodes  122  from the current or power source  128  to control the power level of the pump energy  108 , and controlling the temperature control system  126  to maintain the temperature of the laser diodes  122  at an operating temperature, at which the pump energy  108  at a given power level is within the operating wavelength range of the gain medium  102 . 
     The light combining optics  124  are configured to combine the light from the laser diodes  122  and output the combined light as the pump energy  108 . Embodiments of the light combining optics  124  may comprise a collimation lens, a polarization multiplexer, a brightness doubler, beam shape optics and focusing lenses that focus the pump energy  108  near the first end of the gain medium  102 , and/or other optical components. 
     The laser resonator  106  is configured to generate a harmonic of the laser light  112  output from the gain medium  102 . In one embodiment, the laser resonator  106  includes a non-linear crystal (NLC)  150 , such as a lithium borate (LBO) crystal or a potassium titanyl phosphate crystal (KTP), for generating a second harmonic of the laser beam  112  emitted by the gain medium  102 . 
     In one embodiment, the gain medium  102  comprises a yttrium-aluminum-garnet (YAG) crystal rod with neodymium atoms dispersed in the YAG rod to form a Nd:YAG gain medium  102 , which outputs laser light  112  having a primary wavelength of 1064 nm. The laser resonator  106  generates the second harmonic of the 1064 nm laser light  164  having a second harmonic wavelength of 532 nm. One advantage of the 532 nm wavelength is that it is strongly absorbed by hemoglobin in blood and, therefore, is useful in medical procedures to cut, vaporize and coagulate vascular tissue. 
     Other embodiments of the gain medium  102  include yttrium-orthoaluminate crystal rod doped with thulium atoms (Tm:YALO) and neodymium doped yttrium-vanadate rod (Nd:YVO 4 ). 
     In one embodiment, the laser resonator  106  includes a Q-switch  152  that operates to change the laser beam  112  into a train of short pulses with high peak power to increase the conversion efficiency of the second harmonic laser beam  164 . 
     The laser resonator  106  also includes reflecting mirrors  156  and  158 , and a folding mirror  160 . The mirrors  110 ,  156 ,  158 ,  160  and mirror  162  are highly reflective at the primary wavelength (e.g., 1064 nm). The folding mirror  160  is also transmissive at the second harmonic output wavelength (e.g., 532 nm). The laser beam inside the resonator  106  bounces back and forth between the mirrors  158  and  162 , reflects off the folding mirror  160  and propagates through the gain medium  102  and non-linear crystal  150 , and is discharged as output laser light  164  at the second harmonic wavelength. The Z-shaped resonant cavity can be configured as discussed in U.S. Pat. No. 5,025,446 by Kuizenga, imaging the resonant mode at one end of the gain medium  102  at the non-linear crystal  150 . The configuration described is stable and highly efficient for frequency conversion. The configuration shown in  FIG. 1  using the Nd:YAG gain medium  102  produces a frequency converted output laser  164  having a wavelength of 532 nm, as indicated above. 
     The efficiency at which the gain medium  102  converts the pump energy  108  depends on the length of the gain medium  102  and the absorption efficiency of the gain medium  102  at the wavelength of the pump energy  108 . The absorption efficiency varies with wavelength and is dependent on the dopant (e.g., neodymium atoms) and the doping concentration. 
       FIG. 3  shows the absorption coefficient (cm −1 ) (y-axis) of the gain medium  102  (for approximately 0.3% neodymium concentration) versus pump energy wavelength (x-axis) (hereinafter “absorption coefficient curve”) over a practical range of wavelengths for a Nd:YAG gain medium. The absorption coefficient includes peaks and valleys over the range of pump energy wavelengths. One conventional practice is to utilize one of the narrow operating wavelength ranges  166  at one of the peaks of the absorption efficiency curve, such as at 808 nm or 885 nm, to maximize the conversion efficiency of the gain medium  102 . The narrow operating wavelength ranges at these peaks are approximately 1-2 nanometers. Such narrow operating wavelength bands are intolerant to small wavelength shifts of the pump energy  108 , which can be caused by changes in the current to the laser diodes during a power level change of the pump energy  108 , or a change in the temperature of the laser diodes  122 . 
     For instance, the operating wavelength range  166  of the Nd:YAG gain medium  102  around 885 nm is approximately 2 nm wide, as shown in  FIG. 4 . In the likely event that the wavelength of the pump energy  108  shifts outside of the operating wavelength range  166 , such as during pump energy power level changes, the absorption efficiency of the gain medium  102  decreases rapidly. This, in turn, results in lower absorption of the pump energy  108  by the gain medium  102 , which will adversely affect the output laser light  112  and the laser  164 . 
     We have discovered that the stability of the wavelength of the pump energy  108  is affected by feedback pump energy  168 , which is the portion of the unabsorbed pump energy  108  that is reflected off a reflector of the system and is fed back to the pump module  104  through the beam splitter  110 , as shown in  FIG. 1 . As used herein, the reflector may include one or more of the mirrors  156 ,  158 ,  160  and  162 , or the end  120  of the gain medium. The feedback pump energy  168  causes the wavelength of the output pump energy  108  to shift toward the wavelength of the feedback pump energy  168 . This wavelength shift of the pump energy  108  decreases the amount of pump energy that is absorbed by the gain medium  102  and increases the magnitude of the feedback pump energy  168 . This effectively holds the wavelength of the pump energy  108  outside the narrow operating wavelength  166  of the gain medium  102  and prevents the stabilization of the wavelength of the pump energy  108  within the narrow operating wavelength range  166  of the gain medium  102 . 
     Embodiments of the invention operate to stabilize the wavelength of the pump energy  108  by setting the operating wavelength  170  of the pump energy  108 , which corresponds to the operating wavelength of the gain medium  102 , to a wavelength that is within a valley  172  of the absorption coefficient curve for the gain medium  102  rather than a peak  173  of the absorption coefficient curve, as illustrated in  FIG. 4 . In one embodiment, the operating wavelength  170  is set to a wavelength at which the absorption coefficient of the gain medium is less than 40% of its value at either of the peaks  173  defining the valley, such as approximately 877 nm for the Nd:YAG gain medium  102  ( FIG. 4 ). In one embodiment, the operating wavelength  170  is set to a wavelength at which the absorption coefficient of the gain medium is less than 30% of its value at either of the peaks  173  defining the valley, such as approximately 881 nm and 889 nm for the Nd:YAG gain medium  102 . In one embodiment, the operating wavelength  170  is set to approximately the wavelength corresponding to the minimum absorption coefficient within one of the valleys  172 , or minimum absorption coefficient wavelength. 
     In one embodiment, the valley  172  of the absorption coefficient curve containing the operating wavelength  170  has a peak-to-peak wavelength range  174  of at least approximately 3-4 nm. 
     In one embodiment, the operating wavelength  170  of the pump module  104  is within a range of 879-883 nm. In another embodiment, the operating wavelength  170  is within a range of 875-879 nm. In yet another embodiment, the operating wavelength  170  is within a range of 887-890 nm. 
     Shifts in the wavelength of the pump energy  108  from within the selected valley  172 , particularly when the selected operating wavelength  170  is set to approximately the minimum absorption coefficient wavelength of the gain medium  102 , will generally result in increased absorption efficiency of the gain medium  102  due to the increase in the absorption coefficient. This causes a reduction in the magnitude of the feedback pump energy  168  and reduces the impact of the feedback pump energy  168  on the wavelength of the pump energy  108 . Even so, the majority of the feedback pump energy  168  will be at the minimum absorption coefficient wavelength for the gain medium  102  within the selected valley  172 . Thus, the feedback pump energy  168  will operate to stabilize the wavelength of the pump energy  108 . In one embodiment, the feedback pump energy  168  has a wavelength that approximately matches the operating wavelength  170  of the pump module. 
     In one embodiment, the gain medium  102  is configured to have a pump energy to laser light conversion efficiency at the operating wavelength  170  of the pump module  104 . This is generally accomplished by selecting an appropriate doping level of the dopant (e.g., neodymium atoms) and the length of the crystal rod, in accordance with known techniques. 
     In one embodiment, the doping level is relatively low to allow distribution of the thermal load along the optical axis of the gain medium  102 , thereby reducing the thermal stresses induced at the input end  116  ( FIG. 1 ) of the gain medium  102 . In one embodiment, the doping concentration of the Nd:YAG gain medium  102  is within a range of about 0.6% to 0.9%. In one embodiment, the gain medium  102  is approximately 100 millimeters long between the first end  116  and the second end  120  and has a diameter of approximately 4.5 millimeters. 
     Another embodiment of the invention is directed to a method of operating the laser system described above in accordance with embodiments of the invention.  FIG. 5  is a flowchart illustrating one embodiment of the method. At  180 , a gain medium  102  is provided having an absorption coefficient that varies with wavelength. The absorption coefficient curve for the gain medium  102  comprises peaks  173  and valleys  172 , as illustrated in  FIG. 4 . At  182 , a pump module  104  is operated to output pump energy  108  at an operating wavelength  170  within one of the valleys  172 . In one embodiment, the absorption coefficient of the gain medium  102  is approximately less than 40% of the absorption coefficient at an adjacent peak  173  of the absorption coefficient curve defining the valley  172 . For instance, when the operating wavelength  170  is selected to be approximately 881 nm, the corresponding absorption coefficient is less than 40% of the absorption coefficient of the adjacent peaks  173 A and  173 B that define the valley  172 , as shown in  FIG. 4 . 
     At  184 , the pump energy  108  is directed through the gain medium  102  ( FIG. 1 ) and a portion of the pump energy  108  is absorbed by the gain medium at  186 . Laser light  112  is emitted, at  188 , from the gain medium  102  responsive to the absorbed pump energy  108 . The non-absorbed pump energy or feedback pump energy  169  is fed back to the pump module  104 , at  190 . At  192 , the operating wavelength  170  of the pump energy  180  is stabilized using the feedback pump energy  168 . 
     Additional embodiments of the method correspond to the various embodiments described above with regard to the system  100 . 
     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.