Abstract:
A laser device employs a laser slab having an ionic layer and a nonionic layer, joined through an optical-quality interface. The laser slab has a trapezoidal cross-section in a direction perpendicular to the optical-quality interface. Thermal conductivity away from the ionic layer is enhanced through the thinness of the ionic layer and through the use of a heatsink attached to the ionic layer. Optical power input through the nonionic layer and into the ionic layer is further increased through the use of the trapezoidal cross section.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to laser media and more specifically to a composite medium for generating laser output. 
     BACKGROUND OF THE INVENTION 
     Laser systems using ion-doped yttrium aluminum garnet (YAG) as a lasing medium have achieved great popularity for their high-power output and the widespread availability of several ion-doped YAG compositions. Still, there is a constant desire to utilize newer and more effective lasing media. Ytterbium:YAG (“Yb:YAG”) is a promising material for high power, high brightness, and high efficiency laser systems because of its small quantum defect between pump and lasing transitions. However, thermal management is difficult in a Yb:YAG system because of the low specific gain and high transparency threshold of Yb:YAG. Further, smaller-sized Yb:YAG lasing media, which allow for better thermal management, limit the amount of area of Yb:YAG available for optical pumping with laser diodes. 
     There exists a need for a lasing medium configuration which optimizes thermal management in a Yb:YAG while also taking advantage of the ability to optically pump Yb:YAG with increased amounts of optical energy to produce a high power, high brightness, and high efficiency laser system. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a laser device having a trapezoidal cross-section with a nonionic base layer and an ionic layer attached thereto is optically pumped to produce laser output. The nonionic base layer can be a YAG layer and the ionic layer can be a layer of ion-doped YAG material, such as Yb:YAG. The trapezoidal cross-section results in a larger area for receiving optical energy from a laser diode array. Thus, higher outputs can be achieved. 
     The ionic layer used in the present invention may be kept thin in relation to its length and width, providing for efficient heat removal from the ionic layer. 
     The above summary of the present invention is not intended to represent each embodiment, or every aspect of the present invention. This is the purpose of the figures and detailed description which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
     FIG. 1 is a perspective view of a laser slab according to one embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of a laser slab according to the present invention taken along the line A—A shown in FIG. 1, further showing an optical energy source and heat removal means. 
     FIG. 3 is a cross-sectional view of a laser slab according to the present invention taken along the line B—B shown in FIG. 1, further showing an optical energy source and heat removal means. 
     FIG. 4 is a cross-sectional view of a laser slab according to the present invention taken along the line B—B shown in FIG. 1, further showing an optical energy source, heat removal means, and a tapered duct. 
     FIG. 5 is a cross-sectional view of a laser slab according to one embodiment of the present invention having a semi-circular cross section. 
     FIG. 6 is a cross-sectional view of a laser slab according to one embodiment of the present invention having a parabolic cross section. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the intent is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an arrangement for a laser slab  10  according to the present invention. The laser slab  10  contains two layers, an ionic layer  12  and a nonionic layer  14  with an optical-quality interface  16  disposed therebetween. The ionic layer  12  and the nonionic layer  14  may be joined through diffusion bonding. Alternatively, the ionic layer  12  may be grown on the nonionic layer  14  by an epitaxial or layer-growth method. In one preferred embodiment, the nonionic layer  14  is a yttrium aluminum garnet (YAG) layer and the ionic layer  12  is an ion-doped YAG layer such as ytterbium ion-doped YAG (“Yb:YAG”). Alternatively, materials doped with neodymium (Nd), erbium (Er), or other laser-active rare earth ions may be used. The doping concentration for Yb in the Yb:YAG layer may range from about 0% to 100% Yb by atomic proportion to yttrium, with a doping concentration of about 15% Yb being particularly effective for efficient conversion of optical pumping energy into laser light output. 
     The nonionic layer  14  is shaped such that any cross-section through the optical-quality interface  16  and the nonionic layer  10  in a direction perpendicular to the optical-quality interface  16  (i.e., any cross-section parallel to the z axis shown in FIG.  1  and passing through both the optical-quality interface  16  and a bottom surface  18  of the laser slab  10 ) is trapezoidal. Likewise, where a top surface  20  of the laser slab  10  is parallel to the optical-quality interface  16 , any cross-section parallel to the z axis and passing through both the top surface  20  and the bottom surface  18  of the laser slab is trapezoidal. Alternatively, the ionic layer  12  may be a rectangular prism attached to the nonionic layer  14 , so that only cross-sections through the nonionic layer  14  are trapezoidal. 
     End surfaces  22  and side surfaces  24  of the laser slab  10  are tilted at angles with respect to the bottom surface  18 . A first angle, θ 1 , is the angle between the bottom surface  18  and the end surfaces  22  of the laser slab  10 , and a second angle, θ 2 , is the angle between the bottom surface  18  and the side surfaces  24  of the laser slab  10 . 
     The laser slab  10  has an overall thickness, t, which is the sum of the thickness of the ionic layer  12 , t 1 , and the thickness of the nonionic layer  14 , t 2 . According to one preferred embodiment, when the slab  10  is made of YAG and Yb:YAG, the overall thickness of the laser slab  10 , t, is about 3.5 mm, with the thickness of the ionic layer  12 , t 1 , being about 0.25 mm and the thickness of the ionic layer  14 , t 2 , being about 3.25 mm. Along its bottom surface  18 , the laser slab  10  has a length  11  computed by:          l   1     =     6          t     tan                   θ   1         .                              
     For example, when t is 3.5 mm and θ 1  is 30.96°,          l   1     =       6          3.5                 mm       tan                 30.96      °         ≈     35.00                   mm   .                                
     Along the top surface  20 , the laser slab  10  has a length  12  computed by:          l   2     =     4          t     tan                   θ   1         .                              
     For example, when t is 3.5 mm and θ 1  is 30.96°,          l   2     =       4          3.5                 mm       tan                 30.96      °         ≈     23.34                   mm   .                                
     Turning now to FIG. 2, a vertical cross-section along the lines A—A of FIG. 1 displays a conductive heatsink  26  and a diode array  28 . In one embodiment, the diode array  28  produces an output wavelength of about 940 nm, which is approximately the wavelength at which peak absorption of the Yb:YAG will occur. If other ionic layers are used, the diode array  28  is selected so as to produce an output wavelength that achieves maximum absorption in the ionic layer  12 . In operation, the diode array  28  pumps optical energy into the laser slab  10  from the bottom surface  18 . The input light is absorbed at the ionic layer  12 , causing an emission of energy from the ionic layer  12  that reflects off the top and bottom surfaces of the laser slab  10  and is emitted from the end surfaces  22 . In the embodiment where the diode array  28  has an input wavelength of about 940 nm and the ionic layer  12  is Yb:YAG, the output beam  30  has a wavelength of about 1030 nm. 
     The ionic layer  12  may be provided with an isolation groove  25 , which serves to reduce optical path lengths through the ionic layer  12 , thereby reducing parasitic oscillation within the ionic layer  12 . 
     In a laser slab  10  having the dimensions described above, the laser light which becomes the output beam  30  makes five total internal reflection (TIR) bounces within the laser slab  10 . Two of these bounces are within the ionic layer  12  and three are within the nonionic layer  14 . 
     The end surfaces  22  of the laser slab  10  are preferably polished to a laser grade polish, with a flatness of about 0.1 wave over the central 80% of the apertures, a scratch-dig of about 10-5, and a parallelism of about 2 arc minutes. The bottom surface  18  and the top surface  20  of the laser slab  10  are polished to a flatness of about 1 wave per 100 mm of length with a scratch-dig of about 20-10 and a parallelism of less than about 10 arc seconds. 
     Turning now to FIG. 3, a cross-sectional view of the laser slab  10  along the line B—B of FIG. 1 is shown. In this view, looking along the x-axis of FIG. 1, the trapezoidal shape of the laser slab  10  in the cross-section along the line B—B is visible. The trapezoidal shape increases the optical pumping energy input into the ionic layer  12 , while the thinness of the ionic layer  12  allows heat to be efficiently removed from the top surface  20  of the laser slab  10 . Further, this arrangement allows output light to be emitted from both end surfaces  22  of the laser slab  10 . The second angle, θ 2 , provides more bottom surface area in the nonionic layer  14  as opposed to the ionic layer  12 , allowing more light to enter the laser slab  10  so that optical energy is focused on the ionic layer  12 . 
     In one tested configuration of the laser slab  10 , along the bottom surface  18  of the laser slab  10 , the laser slab  10  has a width, w 1 , of about 7.5 mm, and along the top surface  20  of the laser slab  10 , the laser slab  10  has a width, w 2 , of about 3.5 mm. When w 1  is approximately 7.5 mm and w 2  is approximately 3.5 mm, the angle θ 2  between the bottom surface  18  and a side surface  24  of the laser slab  10  is approximately 60.25°. In this configuration, the bottom surface  18  of the laser slab  10  has a surface area of about 263 mm2, and the top surface  20  of the laser slab  10  has a surface area of about 81.69 mm2, with the optical-quality interface  16  having a surface area slightly greater than the surface area of top surface  20 . The ratio of the surface area of the bottom surface  18  to the surface area of the optical-quality interface  16  in a laser slab  10  with these dimensions is about 3:1. In this tested configuration, with the thickness t 1  of the ionic layer  12  being about 0.25 mm and the doping concentration of Yb in the ionic layer  12  being about 15%, a peak single-pass gain of at least 1.37 after 1.3 ms of pumping was achieved. In this configuration, greater or lesser concentrations of Yb in the ionic layer  12  and greater or lesser thicknesses t 1  of the ionic layer were found to degrade the gain. 
     Turning now to FIG. 4, a cross-sectional view of a laser slab  10  and a diode array  28  using a duct concentrator  32  is shown. The duct concentrator  32  concentrates input optical energy from the diode array  28  into the laser slab  10 . The duct concentrator  32  may be provided with a trapezoidal cross-section as shown in FIG. 4 with inner walls that are diamond-machined, gold-plated and polished. 
     In one embodiment of the present invention, the heatsink  26  is a high intensity pin-fin heat exchanger bonded to the ionic layer  12  with a high-thermal-conductivity room-temperature vulcanized (RTV) rubber material. In this embodiment, coolant flow through the heatsink at 0.85 gallons per minute with a coolant temperature of about 15° C. results in adequate heat removal from the laser slab  10  during operation. The thinness of the ionic layer  12  contributes to easy heat removal from the ionic layer while also providing a high-quality output beam  30 . In an alternative embodiment, the heatsink  26  may be low-temperature soldered to the ionic layer  12 . Further, alternative heat removal means such as impingement coolers, microchannel coolers, and other types of compact high-intensity coolers may be employed in the present invention. 
     Alternative constructions for a laser slab  10  which serve to funnel optical energy to the ionic layer  12  similarly to the trapezoidal formation discussed above are possible. For example, a laser slab  10  may be constructed with a semi-circular or parabolic cross-section along the line B—B of FIG.  1 . FIG. 5 shows a laser slab  10  constructed with a semi-circular cross-section, and FIG. 6 shows a laser slab  10  constructed with a parabolic cross-section. Total internal reflections off the side walls  24  of nonionic layers  14  having such a cross-sections would tend to guide pump energy into the ionic layers  12 . 
     While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.