Abstract:
The present invention provides a solid-state laser device in which an alignment operation for arranging optical elements for compensating thermal birefringence is easy, and which is mechanically highly stable by being tolerant of oscillation.  
     A 90° polarization rotator  12  is arranged between and integrated with two solid-state laser media  11  and  13  via diffusion bonding to form a composite laser crystal  10 . The composite laser crystal  10  is arranged between constituents  31  and  32  of an optical resonator to form a solid-state laser device.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a solid-state laser device. More particularly, the present invention relates to a high-power laser device and a composite laser crystal incorporated by the device.  
         BACKGROUND OF THE INVENTION  
         [0002]    Laser devices are broadly employed in various fields such as spectroscopy, measurement, fabrication, optical communication, clinical medicine and energy engineering. Although a laser medium can be any of gas, liquid or solid, use of a solid-state medium has become major for its handling convenience. Furthermore, in order to generate pumping light for exciting the laser medium, solid-state light sources such as laser diodes (LD) are replacing discharge tubes such as conventional flash lamps/flashtubes. Thus, laser devices having totally solid-state elements are now undergoing development.  
           [0003]    A solid-state YAG laser for generating high-power laser light is used as a light source of a laser beam machine, which may perform fabrication such as drilling, welding, cutting and trimming on a work piece such as a metal, ceramic, wood, and a gem. Production of a high-power LD excitation YAG laser is associated with a problem of thermal birefringence generation. Thermal birefringence is a phenomenon which is caused when crystal rods are subjected to heat stress and which results in difference of the refractive indices of light between the radius vector and the vector perpendicular thereto. When such thermal birefringence occurs, a linearly polarized beam from a laser resonator may greatly be distorted depending on the polarization.  
           [0004]    In order to compensate such thermal birefringence, two YAG laser crystals are arranged in tandem, and a 90° polarization rotator (half-wave plate) that rotates a plane of polarization by 90° is arranged between the two YAG laser crystals. Thermal birefringence results in two mutually perpendicular components having different refractive indices. By inserting the 90° polarization rotator between the two YAG laser crystal rods, the polarization direction of light propagating through the resonator in one way alters by 90° in front and back of the 90° polarization rotator. Accordingly, polarization components that differ for respective rods can be amplified in both directions. As a result, a biased amplification can be cancelled out, thereby preventing a thermal birefringence effect.  
           [0005]    According to the above-described conventional method for compensating thermal birefringence, three optical elements (i.e., two YAG laser crystals and a 90° polarization rotator) need to accurately be aligned on an optical axis of a laser resonator. This alignment takes time for adjustment and the resulting laser device is poorly tolerant of oscillation.  
           [0006]    The present invention solves such prior art problems, and has an objective of providing a solid-state laser device in which an alignment operation for arranging optical elements for compensating for thermal birefringence is easy, and which is mechanically highly stable by being tolerant of oscillation.  
           [0007]    As a method for simplifying alignment of the three optical elements arranged in the laser resonator, the three optical elements are fixed on a single member in advance as a single unit which is then arranged in the laser resonator, thereby simplifying the alignment operation. This method, however, is unpractical since arranging three optical elements on a single member requires equal amount of labor to that for directly setting the optical elements in the laser resonator.  
         SUMMARY OF THE INVENTION  
         [0008]    According to the present invention, the above-described objective is achieved by developing a composite laser crystal in which three optical elements (i.e., two YAG laser crystals and a 90° polarization rotator) necessary for compensating for thermal birefringence are formed into a single rod as the composite laser crystal to be arranged in a laser resonator. The two YAG laser crystals and a single half-wave plate (90° polarization rotator) are integrated as a single rod via optical contact, more preferably via diffusion bonding.  
           [0009]    In a composite laser crystal of the present invention, a half-wave plate or a 90° polarization rotator is sandwiched between and integrated with two solid-state laser media. The 90° polarization rotator is an optical element which rotates any polarized light by 90°. On the other hand, a half-wave plate rotates specific linearly polarized light by 90° with respect to a plane of polarization. Preferably, the half-wave plate is integrated with the adjacent solid-state laser media via diffusion bonding.  
           [0010]    A method for producing a composite laser crystal according to the present invention comprises the steps of: polishing end faces of a pair of Nd:YAG crystals and end faces of the quartz half-wave plate to obtain flat faces with a surface precision of λ/10 or less; arranging the half-wave plate between the pair of Nd:YAG crystals such that the flattened end faces of the crystals make contact with each other, and subjecting the resultant to pressure bonding under a pressure of 1 kg/cm 2  or higher; heating the crystals subjected to the pressure bonding at 400° C. or higher; and cutting out from the integrated rod obtained by the above steps, a laser crystal of a desirable shape. The surface precision is a value defined by twice the maximum deviation between an ideal reference surface and a polished surface as examined surface, and is represented with respect to HeNe laser wavelength (λ=632.8 nm). Accordingly, λ/10 is about 63 nm.  
           [0011]    In order to bond an Nd:YAG crystal and a quartz via diffusion bonding, the surface precision of the faces to be subjected to diffusion bonding need to be λ/10 (i.e., 63 nm) or less. When the surface precision is rougher than this value, bonding does not take place. After passivation of the surface of crystal with acid, diffusion bonding requires heating at a temperature of 400° C. or higher for a predetermined time under a pressure of 1 kg/cm 2  or higher. When the pressure is lower than 1 kg/cm 2 , or the temperature is lower than 400° C., bonding with sufficient strength may not be achieved.  
           [0012]    A solid-state laser device of the invention comprises an optical resonator, a pair of solid-state laser media arranged in the optical resonator, a polarization rotator arranged between the pair of solid-state laser media for rotating a plane of polarization by 90°, and a light source for exciting the laser media, wherein the pair of solid-state laser media and the polarization rotator are integrated by bonding the adjacent end faces thereof. The adjacent end faces are integrated via diffusion bonding.  
           [0013]    By using the composite laser crystal of the invention, effect of thermal birefringence can be canceled out. Moreover, a laser device incorporating the composite laser crystal of the invention allows easy adjustment since there is no need of aligning individual optical elements in the laser resonator. As a result, the cost of production can be reduced and mechanical stability can be enhanced for being tolerant of oscillation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic view showing an exemplary composite laser crystal of the invention;  
         [0015]    [0015]FIGS. 2A to  2 C are views for illustrating an exemplary method for producing a composite laser crystal; and  
         [0016]    [0016]FIG. 3 is a schematic view of a solid-state laser device of the invention which incorporates composite laser crystals.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Hereinafter, the present invention will be described with reference to the accompanying drawings.  
         [0018]    [0018]FIG. 1 is a schematic view showing an exemplary composite laser crystal  10  of the invention. The composite laser crystal  10  is provided with two Nd:YAG crystals  11  and  13 , and a quartz (crystal) plate (90° polarization rotator)  12  sandwiched therebetween. The adjacent end faces of the Nd:YAG crystal  11  and the quartz plate  12 , and the adjacent end faces of the quartz plate  12  and the Nd:YAG crystal  13  are strongly bonded via diffusion bonding. As to the sizes, for example, when this composite laser crystal is to be incorporated by a laser resonator, as a laser medium for canceling out the effect of thermal birefringence to obtain a laser device for oscillating laser light at a wavelength of 1064 nm, the total length and the diameter of the composite laser crystal  10  are about 100 mm and about 4 mm, respectively, and the thickness of the quartz plate  12  used as the 90° polarization rotator is 6 mm.  
         [0019]    [0019]FIGS. 2A to  2 C are views for illustrating an example of a method for producing the composite laser crystal shown in FIG. 1. As shown in FIG. 2A, Nd:YAG crystals  21  and  23  and a quartz  22  are cut out. End faces  21   a ,  21   b ;  22   a ,  22   b ; and  23   a ,  23   b  of the crystals are optically polished. Specifically, the end face  21   b  of the Nd:YAG crystal  21 , the end faces  22   a  and  22   b  of the quartz plate  22 , and the end face  23   a  of the Nd:YAG crystal  23  are polished to obtain flat surfaces with a surface precision of λ/10 (about 63 nm) or less.  
         [0020]    Then, as shown in FIG. 2B, the end face  21   b  of the Nd:YAG crystal  21  is made to contact with the end face  22   a  of the quartz plate  22 , and the end face  23   a  of the Nd:YAG crystal  23  is made to contact with the end face  22   b  of the quartz plate  22 , thereby assembling a rod  20 . While applying a pressure of 1 kg/cm 2  or higher to both ends of the rod  20 , the rod  20  is heated at 500° C. After 5 hours, the Nd:YAG crystals  21  and  23  are strongly bonded to the quartz plate  22  at their end faces via diffusion bonding. Diffusion bonding allows optical bonding and mechanical integration, and is advantageous in that no damage is caused at the bonding faces since it does not require adhesion for bonding.  
         [0021]    Finally, as shown in FIG. 2C, the rod  20  integrated via diffusion bonding is fabricated into a desirable shape to obtain a composite laser crystal  25 . The fabrication is performed by cutting out a cylindrical composite laser crystal  25  from the rod  20  with a core-drill, and then optical polishing both end faces. The composite laser crystal  25  is also subjected to non-reflective coating. Multiple composite laser crystals  25  may be cut out from the rod  20 .  
         [0022]    [0022]FIG. 3 is a schematic view showing a solid-state laser device of the invention incorporating the composite laser crystal.  
         [0023]    The solid-state laser device is provided with a total reflection mirror  31  and a partial reflection mirror  32  with a transmittance of about 70% as constituents of an optical resonator, the composite laser crystal  10  shown in FIG. 1 arranged therebetween, and a high-power LD laser  33  (wavelength: 808 nm) for generating pumping light surrounding the rod  10 . Lenses  34  and  35  are arranged between the composite laser crystal  10  and the total reflection mirror  31  and between the composite laser crystal  10  and the partial reflection mirror  32 , respectively. Since the composite laser crystal  10  is made of an integrated body of two Nd:YAG crystals  11  and  13  and the quartz plate (90° polarization rotator)  12 , there is no requirement of individual alignments of the three optical elements  11 ,  12  and  13  as in a prior art device. Accordingly, it is very easy to assemble a solid-state laser device. Furthermore, since the three optical elements  11 ,  12  and  13  are integrated as the composite laser crystal  10 , the alignment relationship between the three optical elements  11 ,  12  and  13  does not change even when the device is subjected to oscillation. Therefore, the output characteristics of the solid-state laser device are not fluctuated by oscillation, maintaining extremely high stability.  
         [0024]    Although a quartz plate is used as the 90° polarization rotator in the above-described embodiment, a half-wave plate may be used instead. Although diffusion bonding is employed for bonding the three optical elements to obtain the composite laser crystal in the above-described embodiment, an optical contact may be employed instead. Specifically, the bonding faces of the two rods previously cut out in desirable shapes and the 90° polarization rotator are polished to obtain a surface precision of λ/20 (about 30 nm); the three optical elements are assembled into a single rod by making the bonding faces thereof in contact; and a pressure of about 1 kg/cm 2  is applied to both ends of the rod, thereby bonding the three optical elements. Bonding by optical contact is easier than diffusion bonding. However, it requires a guide or the like due to its weak adhesive strength.  
         [0025]    The solid-state laser device of the invention may be used as a light source of a laser beam machine which may perform fabrication such as drilling, welding, cutting and trimming on a work piece such as a metal, ceramic, wood, and a gem, or as a light source of a marking device. Alternatively, by converting the oscillation wavelength into a shorter wavelength by using non-linear optical elements, the solid-state laser device of the invention may be used as a light source of an exposure device or the like used for pattern exposure during a process of fabricating a semiconductor.  
         [0026]    According to the present invention, a high-power solid-state laser can be obtained with easy alignment, which has enhanced mechanical stability and which does not cause thermal birefringent effect.