Patent Publication Number: US-2023163550-A1

Title: Laser system

Description:
FIELD 
     The present disclosure relates to a laser system that couples beams emitted from a plurality of laser elements. 
     BACKGROUND 
     A semiconductor laser element has low beam output that can be generated from a single light emitting point, and it is necessary to bundle beams generated from a plurality of semiconductor laser elements for applications such as laser machining. As a technique of a laser system that bundles beams emitted from a plurality of semiconductor laser elements, there is a proposed technique in which an external resonator including a plurality of semiconductor laser elements and a diffractive optical element is used to oscillate beams having different wavelengths at the respective semiconductor laser elements and to couple the beams into a single beam. Such a laser system has a problem that the maximum output is restricted in order to avoid damage to each optical element due to the high light intensity received by each optical element of the laser system. 
     Non Patent Literature 1 discloses a laser system including two external resonators that couple beams from a plurality of laser elements using a diffractive optical element, in which the two external resonators use a common diffraction grating. In the laser system according to Non Patent Literature 1, the two external resonators are assembled symmetrically with respect to the perpendicular of the diffraction grating. The laser system according to Non Patent Literature 1 couples the beams oscillated by the two external resonators and outputs the coupled beam. By using the two external resonators, it is possible to reduce the light intensity received by each optical element of the laser system. 
     CITATION LIST 
     Non Patent Literature 
     Non Patent Literature 1: “High power diode laser source with a transmission grating for two spectral beam. combining”, Optik, 2019, Vol. 192, 162918 
     SUMMARY 
     Technical Problem 
     However, according to the conventional technique disclosed in Non Patent Literature 1, the laser system requires optical elements other than the diffraction grating for each of the two external resonators, which increases the number of components. Differences in the state of adjustment of the optical elements in each external resonator or differences in the aging of the optical elements in each external resonator cause differences in the characteristics of beams output from each external resonator or change in the relative positional relation of beams in some cases. Therefore, according to the conventional technique, the laser system has problems that the number of components is increased and that variation in beam characteristics easily occurs. 
     The present disclosure has been made in view of the above, and it is an object of the present disclosure to obtain a laser system capable of reducing the number of components and reducing variations in beam characteristics. 
     Solution to Problem 
     To solve the above described problems and achieve the object, a laser system according to the present disclosure includes: a first laser element adapted to emit a first beam group, the first beam group being one or a plurality of beams, and adapted to constitute one end of a first external resonator to cause the first beam group to resonate; a second laser element adapted to emit a second beam group, the second beam group being one or a plurality of beams, and adapted to constitute one end of a second external resonator to cause the second beam group to resonate; a diffractive optical element: to which the first beam group and the second beam group enter in such a manner that positive and negative angles of incidence of each beam of the first beam group and each beam of the second beam group are opposite to each other; and from which a first beam being the converged first beam group, and a second beam being the converged second beam group, are emitted; a partially reflective element adapted to constitute an opposite end of the first external resonator and an opposite end of the second external resonator, adapted to reflect a part of the first beam and a part of the second beam, and adapted to transmit the remainder of the first beam and the remainder of the second beam; and a beam deflection element adapted to deflect the second beam emitted from the diffractive optical element toward the partially reflective element. 
     Advantageous Effects of Invention 
     A laser system according to the present disclosure has effects of reducing the number of components and reducing variations in beam characteristics. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram illustrating a configuration of a laser system according to a first embodiment. 
         FIG.  2    is a diagram for explaining an action of a transmission grating constituting the laser system according to the first embodiment. 
         FIG.  3    is a diagram illustrating a semiconductor laser bar that is an example of a laser element to be provided in the laser system according to the first embodiment. 
         FIG.  4    is a diagram illustrating an installation. example of a shield of the laser system according to the first embodiment. 
         FIG.  5    is a diagram for explaining a positional relation of beams in an external resonator of the laser system according to the first embodiment. 
         FIG.  6    is a diagram illustrating a configuration example of a laser machine to which the laser system according to the first embodiment is applied. 
         FIG.  7    is a schematic diagram illustrating a configuration of a laser system according to a second embodiment. 
         FIG.  8    is a schematic diagram illustrating a configuration of a laser system according to a third embodiment. 
         FIG.  9    is a diagram illustrating an example of a beam rotation element to be provided in the laser system according to the third embodiment. 
         FIG.  10    is a schematic diagram illustrating a configuration of a laser system according to a fourth embodiment. 
         FIG.  11    is a first diagram illustrating a part of a laser system according to a fifth embodiment. 
         FIG.  12    is a second diagram illustrating a part of the laser system according to the fifth embodiment. 
         FIG.  13    is a diagram illustrating a first example of a position changer of the laser system according to the fifth embodiment. 
         FIG.  14    is a diagram illustrating a second example of the position changer of the laser system according to the fifth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a laser system according to embodiments will be described in detail with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a schematic diagram illustrating a configuration of a laser system  101  according to a first embodiment.  FIG.  1    illustrates an x axis, a y axis, and a z axis of a three-axis orthogonal coordinate system. 
     The laser system  101  includes a first laser element  11  and a second laser element  12  that are laser elements. The first laser element  11  emits a first beam group  21  that is one or a plurality of beams. The second laser element  12  emits a second beam group  22  that is one or a plurality of beams. The laser system  101  includes: a first divergence-angle correction element  31  and a second divergence-angle correction element  32  that are divergence-angle correction elements; and a transmission grating  40  that is a diffractive optical element. The first divergence-angle correction element  31  corrects the divergence angle of the first beam group  21 . The second divergence-angle correction element  32  corrects the divergence angle of the second beam group  22 . 
     In the first embodiment, the first beam group  21  includes a plurality of beams having different wavelengths. The second beam group  22  includes a plurality of beams having different wavelengths. Each beam of the first beam group  21  and each beam of the second beam group  22  propagate in the xy plane. The transmission grating  40  deflects each beam of the first beam group  21  and each beam of the second beam group  22  in the xy plane by wavelength dispersibility. The principal ray of each beam constituting the first beam group  21  and the principal ray of each beam constituting the second beam group  22  are included in the xy plane. 
     The laser system  101  includes: a first reflective mirror  71  and a first lens  91  that are arranged in the optical path of the first beam group  21  between the first divergence-angle correction element  31  and the transmission grating  40 ; and a second lens  92  arranged in the optical path of the second beam group  22  between the second. divergence-angle correction element  32  and the transmission grating  40 . The first reflective mirror  71  that is a beam deflection element deflects each beam of the first beam group  21  in the xy plane. The first lens  91  collimates each beam of the first beam group  21 . The second lens  92  collimates each beam of the second beam group  22 . 
     The first beam group  21  is deflected by the first reflective mirror  71  and enters the transmission grating  40 . The first beam group  21  and the second beam group  22  enter the transmission grating  40  in such a manner that the positive and negative angles of incidence of each beam of the first beam group  21  and each beam of the second beam group  22  are opposite to each other. The transmission grating  40  is arranged at a position where at least a part of the first beam group  21  deflected by the first reflective mirror  71  and at least a part of the second beam group  22  are superimposed. The transmission grating  40  deflects the first beam group  21  to converge the first beam group  21 . The transmission grating  40  deflects the second beam group  22  to converge the second beam group  22 . From the transmission grating  40 , a first beam  51  that is the converged first beam group  21  and a second beam  52  that is the converged second beam group  22  are emitted. The principal ray of the first beam  51  and the principal ray of the second beam  52  are included in the xy plane. 
     The laser system  101  includes a partially reflective mirror  60  that is a partially reflective element and a second reflective mirror  72  that is a beam deflection element. The second reflective mirror  72  is arranged in the optical path of the second beam  52  between the transmission grating  40  and the partially reflective mirror  60 . The second reflective mirror  72  deflects the second beam  52  in the xy plane. The second reflective mirror  72  deflects the second beam  52  emitted from the transmission grating  40  toward the partially reflective mirror  60 . By deflecting the second beam  52  emitted from the transmission grating  40  by the second reflective mirror  72 , the principal ray of the first beam  51  and the principal ray of the second beam  52  become parallel to each other. 
     The partially reflective mirror  60  reflects a part of the incident first beam  51  and transmits the remainder of the incident first beam  51 . The partially reflective mirror  60  reflects a part of the incident second beam  52  and transmits the remainder of the incident second beam  52 . Of the partially reflective mirror  60 , an incident plane  61  to which the first beam  51  and the second beam  52  enter is a single plane. By using the partially reflective mirror  60  having the incident plane  61  that is a single plane, it is possible to realize the external resonator with a simple optical system. 
     A first external resonator  1  is an external resonator that causes the first beam group  21  to resonate. The first laser element  11  constitutes one end of the first external resonator  1 . The partially reflective mirror  60  constitutes an opposite end of the first external resonator  1 . A second external resonator  2  is an external resonator that causes the second beam group  22  to resonate. The second laser element  12  constitutes one end of the second external resonator  2 . The partially reflective mirror  60  constitutes an opposite end of the second external resonator  2 . The partially reflective mirror  60  is commonly used for resonance of the first beam group  21  by the first external resonator  1  and resonance of the second beam group  22  by the second external resonator  2 . The transmission grating  40  is commonly used for the first external resonator  1  and the second external resonator  2 . 
     The first beam group  21  emitted from the first laser element  11  passes through the first lens  91  and enters the first reflective mirror  71 . The first reflective mirror  71  deflects the first beam group  21  toward the transmission grating  40  to cause the first beam group  21  to enter the transmission grating  40 . The second beam group  22  emitted from the second laser element  12  passes through the second lens  92  and enters the transmission grating  40 . The transmission grating  40  converges the first beam group  21  and converges the second beam group  22 . The first beam  51  and the second beam  52  are emitted from the transmission grating  40 . The first beam  51  emitted from the transmission grating  40  enters the partially reflective mirror  60 . The second reflective mirror  72  deflects the second beam  52  emitted from the transmission grating  40  toward the partially reflective mirror  60  to cause the second beam  52  to enter the partially reflective mirror  60 . 
     The first beam  51  reflected by the partially reflective mirror  60  enters the transmission grating  40 . The second reflective mirror  72  deflects the second beam  52  reflected by the partially reflective mirror  60  toward the transmission grating  40  to cause the second beam  52  to enter the transmission grating  40 . The transmission grating  40  causes the first beam  51  to diverge and causes the second beam  52  to diverge. Each beam of the first beam group  21  and each beam of the second beam group  22  are emitted from the transmission grating  40 . The first reflective mirror  71  deflects the first beam group  21  emitted from the transmission grating  40  toward the first laser element  11 . The first beam group  21  passes through the first lens  91  and enters the first laser element  11 . The second beam group  22  emitted from the transmission grating  40  passes through the second lens  92  and enters the second laser element  12 . The first beam  51  having passed through the partially reflective mirror  60  and the second beam  52  having passed through the partially reflective mirror  60  are emitted to the outside of the laser system  101 . 
     In the first external resonator  1 , an optical element is inserted as needed to collimate, condense, or rotate each beam of the first beam group  21  or the first beam  51 . The first lens  91  is an example of an optical element that collimates each beam of the first beam group  21 . In the second external resonator  2 , an optical element is inserted as needed to collimate, condense, or rotate each beam of the second beam group  22  or the second beam  52 . The second lens  92  is an example of an optical element that collimates each beam of the second beam group  22 . 
     Next, details of the action of the transmission grating  40  will be described.  FIG.  2    is a diagram for explaining the action of the transmission grating  40  constituting the laser system  101  according to the first embodiment. The reference character α 1  indicates an incident angle of each beam constituting the first beam group  21  entering the transmission grating  40 , and satisfies α1&gt;0. The reference character α 2  indicates an incident angle of each beam constituting the second beam group  22  entering the transmission grating  40 , and satisfies α2&lt;0. The reference character β 1  indicates a diffraction angle of each beam constituting the first beam group  21 , and satisfies β1&gt;0. The reference character β 2  indicates a diffraction angle of each beam constituting the second beam group  22 , and satisfies β2&lt;0. 
     The first beam group  21  and the second beam group  22  enter the transmission grating  40  in such a manner that α 1  is positive and α 2  is negative, that is, the positive and negative values of α 1  and α 2  are opposite to each other. The first beam  51  is plus primary diffracted light of the first beam group  21 . The second beam  52  is minus primary diffracted light of the second beam group  22 . 
     When the wavelength of a beam is λ, a grating spacing of the transmission grating  40  is d, and a diffraction order is m, a relation of the following formula (1) holds for α that is the incident angle in the transmission grating  40  and β that is the diffraction angle in the transmission grating  40 . 
       sin α+sin β= mλ/d   (1)
 
     As is clear from the reference characters α 1 , α 2 , β 1 , and β 2 : the first beam  51 , which is the plus primary diffracted light, is extracted by the incidence of the first beam group  21  on the transmission grating  40 ; and the second beam  52 , which is the minus primary diffracted light, is extracted by the incidence of the second beam group  22  on the transmission grating  40 . In addition, by arranging the optical elements in such a manner that α2=−α1 and β2=−β1, the first beam  51  and the second beam  52  oscillate at the same wavelength. By simultaneously using the plus primary diffracted light and the minus primary diffracted light of the transmission grating  40 , the laser system  101  can simultaneously oscillate beams having the same wavelength by the first external resonator  1  and the second external resonator  2 . 
     In the case of a general external resonator using the wavelength selectivity of a grating, it is difficult to simultaneously oscillate a plurality of light beams having the same wavelength. Therefore, an external resonator needs to use a wider wavelength band to increase the beam output. In order to widen the wavelength band, it is necessary to increase the number of types of laser elements, which complicates the configuration of the external resonator. In contrast, the laser system  101  according to the first embodiment can simultaneously oscillate a plurality of light beams having the same wavelength with a simple configuration. 
     In the laser system  101 , a part of the first beam group  21  emitted from the first laser element  11  is reflected by the transmission grating  40  and enters the second laser element  12  in some cases. In addition, in the laser system  101 , a part of the second beam group  22  emitted from the second laser element  12  is reflected by the transmission grating  40  and enters the first laser element  11  in some cases. In such a situation, the interaction between different laser elements occasionally causes a phenomenon called parasitic oscillation. If parasitic oscillation occurs, the laser oscillation becomes unstable and a problem such as; time variation of the beam output or time variation of the beam profile, of the laser system  101 , can occurs. 
     In the first embodiment, when the reflectance of the partially reflective mirror  60  for the first beam  51  and the second beam  52  is R1, and the reflectance of the transmission grating  40  for the first beam  51  and the second beam  52  is R2, R1 is five or more times R2. If R1 is smaller than five times R1, the above parasitic oscillation is likely to occur. The laser system  101  can reduce the time variation of the beam output and the time variation of the beam profile since R1 is five or more times R2. In consideration of time degradation of a laser element or an optical element, R1 is desirably 10 or more times R2. 
     Next, a configuration example of the laser element in the first embodiment will be described. As the first laser element  11  and the second laser element  12 , semiconductor laser bars can be used.  FIG.  3    is a diagram illustrating a semiconductor laser bar  200  that is an example of a laser element to be provided in the laser system  101  according to the first embodiment. The semiconductor laser bar  200  illustrated in  FIG.  3    is an end-face light emitting semiconductor laser. The semiconductor laser bar  200  includes a Fabry-Perot resonator. The Fabry-Perot resonator is not illustrated. 
     The semiconductor laser bar  200  emits a beam  201  having different diameters in the vertical and horizontal directions. The divergence angle of the beam  201  in the direction of a fast axis  202  is larger than the divergence angle of the beam  201  in the direction of a slow axis  203  perpendicular to the fast axis  202 . In  FIG.  1   , the fast axis  202  coincides with the z axis. The slow axis  203  is in the xy plane. 
     The semiconductor laser bar  200  includes a plurality of light emitting points  204  arranged in a one-dimensional array. The light emitting points  204  are arranged in the direction of the slow axis  203 . Each light emitting point  204  consists of a gain element that is a laser medium. A beam group emitted from the semiconductor laser bar  200  consists of the same number of beams  201  as the number of light emitting points  204  of the semiconductor laser bar  200 .  FIG.  1    illustrates one beam of the first beam group  21  emitted from the first laser element  11  and one beam of the second beam group  22  emitted from the second laser element  12 . The beam group emitted from the semiconductor laser bar  200  consists of, for example, about 10 to 50 beams. 
     In order to apply the semiconductor laser bar  200  to an external resonator, one end face of the semiconductor laser bar  200  is coated with a high reflectance coating having a reflectance of, for example, 90% or more, and an opposite end face of the semiconductor laser bar  200  is coated with a low reflectance coating having a reflectance of, for example, 3% or less. Accordingly, an external resonator is formed between the end face of the semiconductor laser bar  200  coated with a high reflectance coating and the partially reflective mirror  60  installed outside the semiconductor laser bar  200 . 
     The wavelength of the beam  201  emitted from the semiconductor laser bar  200  is a wavelength that is easily fiber coupled, for example, from 400 nm to 1100 nm. In the wavelength range of 900 nm to 1000 nm, semiconductor laser elements having higher output and longer life than those in other wavelength ranges are commercially available. Such a semiconductor laser element is suitable for high-power applications such as laser machining. 
     Note that the semiconductor laser bar  200  is an example of a laser element that is a light emitting source of the laser system  101 . The laser element is not limited to the semiconductor laser bar  200 . The laser element may be, for example, a surface light emitting semiconductor laser element. In addition, the wavelength of the laser element is not limited to 400 nm to 1100 nm, and is arbitrary. 
     In each of the first laser element  11  and the second laser element  12  illustrated in  FIG.  1   , beams having different wavelengths are emitted from the respective Light emitting points of a plurality of light emitting points. The first divergence-angle correction element  31  and the second divergence-angle correction element  32  reduce the divergence angle of the beams. The transmission grating  40  diffracts each beam constituting a beam group at an angle corresponding to the wavelength to converge the beams to a single beam. The laser system  101  converges, the first beam group  21  consisting of a plurality of beams dispersed from each other, to the single first beam  51 . In addition, the laser system  101  converges, the second beam group  22  consisting of a plurality of beams dispersed from each other, to the single second beam  52 . Accordingly, the laser system  101  can enhance the light condensing performance of the beams. 
     The light condensing performance referred to herein is a characteristic represented by the beam parameter product (BPP). The BOP is an index defined by the product of the radius at the beam waist at condensing light and a beam divergence half-angle after condensing light. The unit of BPP is expressed in mm·mrad. The smaller the value of BPS, the higher the light condensing performance, which means that the beam can be condensed in a finer region. As the beam can be condensed in a finer region, a higher energy density can be obtained. In the application of laser machining, as the energy density is higher, it is possible to improve the machining quality and the machining speed. 
     Many of general transmission gratings have high diffraction efficiency for one of s-polarized light and p-polarized light and low diffraction efficiency for the other. If the transmission grating  40  in the first embodiment is such a transmission grating, the transmission grating  40  diffracts, for example, 90% or more of the incident s-polarized light and transmits 50% or more of the incident p-polarized light. In this case, it is desirable for the first beam group  21  and the second beam group  22  entering the transmission grating  40  consist of only s-polarized light. 
     However, the laser light actually emitted from the laser element possibly contain a mixture of s-polarized light and p-polarized light. Even the laser light consisting mainly of s-polarized light can contain a few percent of p-polarized light. When the first beam group  21  and the second beam group  22  consisting mainly of s-polarized light entering the transmission grating  40 , p-polarized light contained in the first beam group  21  and the second beam group  22  may pass through the transmission grating  40  in some cases. In this case, the p-polarized light having passed through the transmission grating  40  becomes stray light deviated from the normal optical path in the first external resonator  1  or the second external resonator  2 . Generation of stray light possibly causes heating of components in the laser system  101  or deterioration in the light condensing performance of the output beam. Therefore, it is desirable that the laser system  101  can reduce the generation of stray light. 
     In order to reduce the generation of stray light, the laser system  101  may include polarization separation elements. The polarization separation elements are installed between the first laser element  11  and the transmission grating  40  and between the second laser element  12  and the transmission grating  40 . Since the polarization degree of the first beam group  21  and the second beam group  22  entering the transmission grating  40  are increased by the polarization separation elements, the laser system  101  can reduce the generation of stray light. 
     In the laser system  101 , a part of the first beam  51  or a part of the second beam  52  may become stray light in some cases. When stray light that is a part of the first beam  51  enters the optical path of the second beam  52  or when stray light that is a part of the second beam  52  enters the optical path of the first beam  51 , parasitic oscillation possibly occurs. The laser system  101  may include a shield to reduce the generation of the stray light. 
       FIG.  4    is a diagram illustrating an installation example of a shield  120  in the laser system  101  according to the first embodiment. The shield  120  is a plate material that absorbs incident light. The shield  120  is provided between the optical path of the first beam  51  and the optical path of the second beam  52  between the transmission grating  40  and the part ally reflective mirror  60 . The shield  120  shields the second beam  52  propagating toward the optical path of the first beam  51  and shields the first beam  51  propagating toward the optical path of the second beam  52 . By providing the shield  120 , the laser system  101  can reduce the generation of stray light. The position and range where the shield  120  is provided are not limited to the case illustrated in  FIG.  4   . The shield  120  is provided at least a part between the transmission grating  40  and the partially reflective mirror  60 . Also in laser systems described in a second and subsequent embodiments, the shield  120  may be provided in the same manner as in the first embodiment. 
     Next, a positional relation of beams in an external resonator of the laser system  101  will be described. Here, the case of the first external resonator  1  is described as an example. 
       FIG.  5    is a diagram for explaining a positional relation of beams in an external resonator of the laser system  101  according to the first embodiment.  FIG.  5    illustrates principal rays  211 ,  212 , and  213  of three beams constituting the first beam group  21 . In order to increase the energy density of the first beam  51  to be emitted from the partially reflective mirror  60 , it is desirable that the principal rays  211 ,  212 , and  213  intersect at one point on the transmission grating  40  and that the principal rays  211 ,  212 , and  213  converge into the single first beam  51 . 
     The first lens  91  is an example of a means for converging the principal rays  211 ,  212 , and  213  at one point on the transmission grating  40 . The transmission grating  40  is installed at a focal point of the first lens  91 . The principal rays  211 ,  212 , and  213  parallel to the optical axis of the first lens  91  intersect at one point on the transmission grating  40  or are sufficiently close to each other on the transmission grating  40 . Being sufficiently close refers to being close enough that the beams can be diffracted and converged to the single first beam  51 . 
     By diffracting each beam at an angle corresponding to the wavelength of the beam, the principal rays  211 ,  212 , and  213  converge to the single first beam  51 . Accordingly, the first beam  51  emitted from the partially reflective mirror  60  has higher light condensing performance than the first beam group  21  emitted from the first laser element  11 . Note that the positional relation of the beams of the second beam group  22  in the second external resonator  2  is similar to that in the case of the beams of the first beam group  21  in the first external resonator  1 . In the above description, the number of beams constituting a beam group is three, but the same applies to a case where the number of beams constituting a beam group is more than three. 
     In the first embodiment, the configuration including the first reflective mirror  71  and the second reflective mirror  72  has been described, but the laser system  101  may omit the first reflective mirror  71  depending on the arrangement of laser elements. That is, the laser system  101  may include only the second reflective mirror  72  instead of the first reflective mirror  71  and the second reflective mirror  72 . Even in the case of including only the second reflective mirror  72 , the laser system  101  can obtain a similar effect to the case of including the first reflective mirror  71  and the second reflective mirror  72 . 
     In the first embodiment, there is a difference between the optical path length of the first external resonator  1  and the optical path length of the second external resonator  2  due to constraints on the physical arrangement of the laser elements or the optical elements and the like an some cases. In this case, the laser system  101  can reduce the influence of the optical path length difference to a negligible extent by collimating each beam entering the transmission grating  40 . 
     According to the first embodiment, in the laser system  101 , the partially reflective mirror  60  is shared by the first external resonator  1  and the second external resonator  2 . The laser system  101  can reduce the number of components by making the first external resonator  1  and the second external resonator  2  share the partially reflective mirror  60 , which is an optical element constituting the resonator. The laser system  101  can achieve high output by coupling the beams oscillated by the first external resonator  1  and the second external resonator  2  and outputting the coupled beam. In addition, the laser system  101  can achieve high output without increasing the light density in optical elements other than the transmission grating  40 . The laser system  101  can reduce damage to each optical element due to high light intensity received by each optical element. 
     Furthermore, the laser system  101  can simultaneously oscillate a plurality of beams having the same wavelength by appropriately selecting the incident angle and the emission angle of the transmission grating  40 . The laser system  101  can increase the output without widening the wavelength band. In the laser system  101 , a plurality of optical elements to be installed as needed can be shared by the first external resonator  1  and the second external resonator  2 . By making the first external resonator  1  and the second external resonator  2  share the optical elements, the laser system  101  can make it hard for differences in beam characteristics to occur due to the state of adjustment of the optical elements or the aging of the optical elements. The laser system  101  can reduce variations in beam characteristics of the beams oscillated by the first external resonator  1  and by the second external resonator  2 . 
     Next, a configuration example of a laser machine to which the laser system  101  according to the first embodiment is applied will be described.  FIG.  6    is a diagram illustrating a configuration example of a laser machine  110  to which the laser system  101  according to the first embodiment is applied. The laser machine  110  irradiates a workpiece  114  with laser light  111  to machine the workpiece  114 . The machining by the laser machine  110  is laser machining such as cutting or welding of the workpiece  114 . 
     The laser machine  110  includes: the laser system  101  that emits laser light  111 ; an optical fiber  112  through which the laser light  111  propagates; a condensing optical system  113 ; a machining optical system  115 ; and a drive mechanism  116 . The condensing optical system  113  condenses the laser light  111  on the incident end face of the optical fiber  112 . The machining optical system  115  condenses the laser light  111  emitted from the optical fiber  112  on the workpiece  114 . The drive mechanism  116  relatively moves the workpiece  114  and the machining optical system  115  in the three-dimensional direction. 
     The workpiece  114  is, for example, a metal plate made of iron, stainless steel, or the like. The laser machine  110  can perform laser machining of a metal plate by including the laser system  101  suitable for high-power applications. The configuration of the laser machine  110  described here is an example and may be appropriately changed. The laser system  101  can also be applied to a 3D printer or the like by being combined with a configuration of a generally known laser machine. Similarly to the laser system  101 , laser systems described in the second and subsequent embodiments can also be applied to the laser machine  110  that cuts or welds the workpiece  114 , or another laser machine. 
     Second Embodiment 
       FIG.  7    is a schematic diagram illustrating a configuration of a laser system  102  according to a second embodiment. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference signs, and the configuration different from that in the first embodiment will be mainly described. 
     The laser system  102  includes a reduction optical system  90  in addition to the configuration of the laser system  101  according to the first embodiment. The reduction optical system  90  is arranged between the transmission grating  40  and the partially reflective mirror  60 . 
     The reduction optical system  90 : reduces the diameter of the first beam  51  traveling from the transmission grating  40  to the partially reflective mirror  60  and the diameter of the second beam  52  traveling from the transmission grating  40  to the partially reflective mirror  60 ; and reduces the distance between the principal ray of the first beam  51  traveling from the transmission grating  40  to the partially reflective mirror  60  and the principal ray of the second beam  52  traveling from the transmission grating  40  to the partially reflective mirror  60 . The reduction optical system  90  consists of a transfer optical system having optical power in the xy direction. The reduction optical system  90  according to the second embodiment is constituted by a first lens  901  and a second lens  902 . 
     The laser system  102  reduces the beam size of each of the first beam  51  and the second beam  52  and shortens the distance between the first beam  51  and the second beam  52  with the reduction optical system  90 . Therefore, the size of the partially reflective mirror  60  and the size of an optical element to be installed as needed can be reduced compared with the case where the reduction optical system  90  is not provided. The laser system  102  can obtain more beam output without increasing the size of the partially reflective mirror  60  and the size of the optical element. 
     If there is a difference between the optical path length of the first external resonator  1  and the optical path length of the second external resonator  2 : due to the deflection of the first beam group  21  by the first reflective mirror  71 ; and due to the deflection of the second beam  52  by the second reflective mirror  72 ; the laser system  102  may eliminate the optical path length difference by bending the optical paths or the like. In this case, the laser system  102  may cause the first beam  51  and the second beam  52  to intersect each other and then cause the first beam  51  and the second beam  52  to be parallel to each other. Specifically, a mirror that deflects the first beam  51  by 90 degrees in the xy plane is provided on the optical path of the first beam  51  between the transmission grating  40  and the first lens  91  to cause the first beam  51  and the second beam  52  to intersect each other. Furthermore, a mirror that deflects the first beam.  51  having intersected the second beam  52  by 90 degrees in the xy plane is provided to cause the first beam  51  and the second beam  52  to be parallel to each other. Accordingly, the optical path length difference is eliminated by the distance between the two mirrors. 
     According to the second embodiment, the laser system  102  can downsize the optical system after converging a plurality of beams by the transmission grating  40 . Accordingly, the laser system  102  can obtain high output while downsizing the optical system. 
     Third Embodiment 
       FIG.  8    is a schematic diagram illustrating a configuration of a laser system  103  according to a third embodiment. In the third embodiment, the same components as those in the first or second embodiment are denoted by the same reference signs, and the configuration different from that in the first or second embodiment will be mainly described. 
     The laser system  103  includes a first beam rotation element  81  and a second beam rotation element  82  that are beam rotation elements in addition to the configuration of the laser system  102  according to the second embodiment. The first beam rotation element  81  is arranged between the first divergence-angle correction element  31  and the first lens  91 . The second beam rotation element  82  is arranged between the second divergence-angle correction element  32  and the second lens  92 . 
     The first beam rotation element  61  rotates each beam of the First beam group  21  around the principal ray of the beam. The second beam rotation element  82  rotates each beam of the second beam group  22  around the principal ray of the beam. That is, the first beam rotation element  81  and the second beam rotation element  82 , which are beam rotation elements, rotate each beam of the first beam group  21  and each beam of the second beam group  22  around the principal ray of the beam. 
     Note that  FIG.  8    illustrates principal rays of three beams constituting the first beam group  21  and principal rays of three beams constituting the second beam group  22 . The configuration in the third embodiment exhibits a remarkable effect when the laser elements are semiconductor laser bars. In the following description in the third embodiment, each of the first laser element  11  and the second laser element  12  is a semiconductor laser bar. 
     The first beam rotation element  81  is combined with the first divergence-angle correction element  31  to superimpose a plurality of beams constituting the first beam group  21  on the transmission grating  40 . The second beam rotation element  82  is combined with the second divergence-angle correction element  32  to superimpose a plurality of beams constituting the second beam group  22  on the transmission grating  40 . 
     Next, a configuration example of a beam rotation element will be described.  FIG.  9    is a diagram illustrating an example of a beam rotation element to be provided in the laser system  103  according to the third embodiment.  FIG.  9    illustrates a configuration example of the first beam rotation element  81 . The second beam rotation element  82  is similar to the following description for the first beam rotation element  81 . 
     The beam rotation element is a rotation optical system that rotates an image by 90 degrees around the optical axis. The first beam rotation element  81  illustrated in  FIG.  9    is a lens array. On each of the face of the first beam rotation element  81  closer to the first laser element  11  and the face opposite to the first laser element  11 , a plurality of cylindrical faces arranged in one direction is formed. Each cylindrical face is a convex face. Each cylindrical face is inclined by 45 degrees with respect to a vertical axis  802  perpendicular to the horizontal plane. The array pitch of a plurality of lenses is the same as the array pitch of the light emitting points of the semiconductor laser bar. When the focal length due to refraction on the cylindrical face is f, the distance L between the cylindrical face closer to the first laser element  11  and the cylindrical face opposite to the first laser element  11  is 2f. 
     The major axis direction of the incident light that is a beam entering the first beam rotation element  81  from the first laser element  11  is the direction of the vertical axis  802 . The minor axis direction of the incident light is the direction of a horizontal axis  803  contained in the horizontal plane. On the other hand, the major axis direction of the emitted light that is a beam emitted from the first beam rotation element  81  after the incidence on the first beam rotation element  81  from the first laser element  11  is the direction of the horizontal axis  803 . The minor axis direction of the emitted light is the direction of the vertical axis  802 . As described above, the light, whose major axis direction and minor axis direction are reversed from those of the incident light, is emitted from the first beam rotation element  81 . In this manner, the first beam rotation element  81  rotates the beam by 90 degrees around the optical axis. 
     For example, in a semiconductor laser bar that emits a beam of 900 nm to 1000 nm, the total angle of the divergence angle of the beam in the slow axis direction is generally about 5 degrees to 10 degrees, whereas the total angle of the divergence angle of the beam in the fast axis direction is about 30 degrees to 60 degrees. That is, the divergence angle of a beam in the fast axis direction is larger than the divergence angle of the beam in the slow axis direction. In addition, the light condensing performance of the semiconductor laser bar in the slow axis direction is lower than the light condensing performance of the semiconductor laser bar in the fast axis direction. 
     A semiconductor laser bar has a deformation called a smile due to the manufacturing process of the semiconductor laser bar in some cases. Due to the smile, positional variation in the fast axis direction occurs at the light emitting points. According to the third embodiment, by rotating a beam by 90 degrees by the beam rotation element, the direction in which the positions of the light emitting points vary due to the smile is converted into the slow axis direction in which the light condensing performance is relatively low. Accordingly, the laser system  103  can reduce deterioration in the light condensing performance caused by the smile. 
     For example, in a case where the first divergence-angle correction element  31  consisting of a lens having a cylindrical face is used, by installing the first divergence-angle correction element  31  slightly inclined with respect to the xy plane, each beam of the first beam group  21  is emitted from the first divergence-angle correction element  31  in a state of being angled in the z direction. When the first beam rotation element  81  is installed immediately after the first divergence-angle correction element  31 , each beam is converted from a state of being angled in the z direction to a state of being angled in the xy plane by passing through the first beam rotation element  81 . By appropriately setting the inclination angle of the first divergence-angle correction element  31  with respect to the xy plane, the principal rays of the beams can be brought closer to each other while the beams travel toward the transmission grating  40 . 
     In the first and second embodiments, each of the first lens  91  and the second lens  92  has played a role in superimposing a plurality of beams on the transmission grating  40 . In the third embodiment, a combination of the divergence-angle correction element and the beam rotation element can play the above described role. Therefore, the positions of the first lens  91  and the second lens  92  or the focal lengths of the first lens  91  and the second lens  92  in the third embodiment may be different from those in the first or second embodiment. 
     According to the third embodiment, the laser system  103  can obtain high output while reducing the deterioration in the light condensing performance caused by the smile. 
     Fourth Embodiment 
       FIG.  10    is a schematic diagram illustrating a configuration of a laser system  104  according to a fourth embodiment. The laser system  104  includes a plurality of first laser elements and a plurality of second laser elements. In the fourth embodiment, the same components as those in the first to third embodiments are denoted by the same reference signs, and the configuration different from those in the first to third embodiments will be mainly described. 
     The laser system  104  includes a first laser element  13  and a second laser element  14  in addition to the configuration of the laser system  103  according to the third embodiment. That is, the laser system  104  includes two first laser elements  11  and  13  and two second laser elements  12  and  14 . The first laser element  13  emits the first beam group  21  that is one or a plurality of beams. The second laser element  14  emits the second beam group  22  that is one or a plurality of beams. 
     The laser system  104  further includes a first divergence-angle correction element  33 , a second divergence-angle correction element  34 , a first beam rotation element  83 , and a second beam rotation element  84 . The first divergence-angle correction element  33  corrects the divergence angle of the first beam group  21  emitted from the first laser element  13 . The second divergence-angle correction element  34  corrects the divergence angle of the second beam group  22  emitted from the second laser element  14 . The first beam rotation element  83  is arranged between the first divergence-angle correction element  33  and the first lens  91 . The first beam rotation element  83  rotates each beam of the first beam group  21  around the principal ray of the beam. The second beam rotation element  84  is arranged between the second divergence-angle correction element  34  and the second lens  92 . The second beam rotation element  84  rotates each beam of the second beam group  22  around the principal ray of the beam. The transmission grating  40 : converges the first beam group  21  emitted from each of the first laser elements  11  and  13  to the first beam  51 ; and converges the second beam group  22  emitted from each of the second laser elements  12  and  14  to the second beam  52 . 
     The first laser element  11  and the first laser element  13  emit beams having different wavelengths from each other. The second laser element  12  and the second laser element  14  emit beams having different wavelengths from each other. The first beam group  21  and the second beam group  22  may include beams having the same wavelength. The number of first laser elements to be provided in the laser system  104  may be three or more. The number of second laser elements to be provided in the laser system  104  may be three or more. When a semiconductor laser bar having a beam output of 200 W is used, the laser system  104  may obtain a beam output of 2 kW or more by providing 10 or more semiconductor laser bars for the first laser elements and the second laser elements together. Accordingly, the laser system  104  can achieve high output suitable for laser machining. 
     According to the fourth embodiment, by including a plurality of first laser elements and a plurality of second laser elements, the laser system  104  can achieve high output while maintaining high light condensing performance by converging a plurality of beams in the external resonator. 
     Fifth Embodiment 
       FIG.  11    is a first diagram illustrating a configuration of a laser system  105  according to a fifth embodiment.  FIG.  12    is a second diagram illustrating a configuration of the laser system  105  according to the fifth embodiment. The laser system  105  according to the fifth embodiment can change the relative position of the first beam  51  and the second beam  52  in the xy plane. In the fifth embodiment, the same components as those in the first to fourth embodiments are denoted by the same reference signs, and the configuration different from those in the first to fourth embodiments will be mainly described.  FIGS.  11  and  12    illustrate the first beam  51 , the second beam  52 , the partially reflective mirror  60 , and a condenser lens  95  in the xy plane. The first beam  51  and the second beam  52  having passed through the partially reflective mirror  60  enter the condenser lens  95 . 
     The laser system  105  can change the light condensing performance of a beam output from the laser system  105  by changing the relative position between the first beam  51  and the second beam  52 . Here, it is assumed that the first beam  51  and the second beam  52  output from the laser system  105  are used as one beam. The fact that the distance between the first beam  51  and the second beam  52  changes means that the light condensing performance of a beam output from the laser system  105  charges. 
     As illustrated in  FIGS.  11  and  12   , the first beam  51  and the second beam  52  having passed through the partially reflective mirror  60  propagate in parallel to each other. Here, it is assumed that the first beam  51  and the second beam  52  are sufficiently collimated.  FIG.  11    illustrates that the distance between the first beam  51  and the second beam  52  is narrowed.  FIG.  12    illustrates that the distance between the first beam  51  and the second beam  52  is widened. 
     In the cases illustrated in  FIGS.  11  and  12   , the first beam  51  and the second beam  52  emitted from the partially reflective mirror  60  are condensed by the condenser lens  95 . That is, the first beam  51  and the second beam  52  are focused on the focal point with the same focal length in the state illustrated in  FIG.  11    and the state illustrated in  FIG.  12   . The first beam  51  and the second beam  52  are condensed at the focal point and then diffused. 
     In the state illustrated in  FIG.  11    and the state illustrated in  FIG.  12   , a beam waist diameter Bd of the beam consisting of the first beam  51  and the second beam  52  is the same. A spread angle θ of the beam consisting of the first beam  51  and the second beam  52  is larger in the case illustrated in  FIG.  12    than in the case illustrated in  FIG.  11   . In this manner, the laser system  105  changes the light condensing performance of the beam output from the laser system  105  by changing the distance between the first beam  51  and the second beam  52 . That is, the laser system  105  can change the BPP. 
     Next, a specific example of a position changer for changing the relative position between the first beam  51  and the second beam  52  will be described.  FIG.  13    is a diagram illustrating a first example of a position changer of the laser system  105  according to the fifth embodiment. In  FIG.  13   , the position changer is a mechanism  130  that moves the first laser element  11  in the xy plane. The laser system  105  changes the distance between the first beam  51  and the second beam  52  by moving the first laser element  11  relative to the second laser element  12 . 
     The mechanism  130  moves the first laser element  11  in a direction in which the distance between the first beam group  21  and the second beam group  22  is narrowed and in a direction in which the distance between the first beam group  21  and the second beam group  22  is widened. Note that the position changer is not limited to the mechanism  130  that moves the first laser element  11 , and may be a mechanism that moves the second laser element  12 . 
       FIG.  14    is a diagram illustrating a second example of the position changer of the laser system  105  according to the fifth embodiment. In  FIG.  14   , the position changer is a mechanism  140  that rotates the second reflective mirror  72  that is a beam deflection element. The mechanism  140  changes the traveling direction of the second beam  52  by rotating the second reflective mirror  72  around the z axis to change the distance between the first beam  51  and the second beam  52 . 
     The position changer is not limited to the one illustrated in  FIG.  13  or  14   . For example, the position changer may include a glass substrate installed on the optical path of the first beam group  21  and a mechanism that rotates the glass substrate around the z axis, and move the first beam group  21  in the xy plane by the rotation of the glass substrate. The position changer may translate or deflect the first beam  51  or the second beam  52  in the xy plane using a bending optical path constituted by a plurality of mirrors. The position changer may change the relative position of the first beam  51  and the second beam  52  by a combination of various methods. 
     According to the fifth embodiment, the laser system  105  can set optimum light condensing performance according to an object to be machined. 
     The configuration described in each of the above embodiments is an example of the contents of the present disclosure. The configuration of each of the embodiments can be combined with another known technique. The configurations of the respective embodiments may be appropriately combined. A part of the configuration of each of the embodiments can be omitted or changed without departing from the gist of the present disclosure. 
     REFERENCE SIGNS LIST 
       1  first external resonator;  2  second external resonator;  11 ,  13  first laser element;  12 ,  14  second laser element;  21  first beam group;  22  second beam group;  31 ,  33  first divergence-angle correction element;  32 ,  34  second divergence-angle correction element;  40  transmission grating;  51  first beam;  52  second beam;  60  partially reflective mirror;  61  incidence plane;  71  first reflective mirror;  72  second reflective mirror;  81 ,  83  first beam rotation element;  82 ,  84  second beam rotation element;  90  reduction optical system;  91 ,  901  first lens;  92 ,  902  second lens;  95  condenser lens;  101 ,  102 ,  103 ,  104 ,  105  laser system;  110  laser machine;  111  laser light;  112  optical fiber;  113  condensing optical system;  114  workpiece;  115  machining optical system;  116  drive mechanism;  120  shield;  130 ,  140  mechanism;  200  semiconductor laser bar;  201  beam;  202  fast axis;  203  slow axis;  204  light emitting point;  211 ,  212 ,  213  principal ray;  802  vertical axis;  803  horizontal axis.