Abstract:
A laser device is provided including a light source configured to emit light, a light transmission member configured to transmit the light emitted from the light source, a plurality of optical elements disposed in an optical path of the light transmitted by the light transmission member, a laser resonator which the light passed through the plurality of optical elements enter, and an adjuster configured to adjust a position of at least one of the plurality of optical elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application Nos. 2014-200635 and 2015-160476, filed on Sep. 30, 2014, and August 17, 2015, respectively, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the present invention relate to a laser device, an ignition system, and an internal combustion engine. 
         [0004]    2. Background Art 
         [0005]    Laser devices with laser crystal that resonates by optical pumping are expected to be applied to various kinds of fields including, for example, ignition systems, laser beam machines, and medical equipment. 
         [0006]    For example, a laser ignition system that includes a laser device and a pump source with a plurality of surface emitting lasers is known in the art. 
         [0007]    However, it has proven difficult for conventional laser devices to achieve desired laser characteristics in a stable manner. 
       SUMMARY 
       [0008]    Embodiments of the present invention described herein provide a laser device including a light source configured to emit light, a light transmission member configured to transmit the light emitted from the light source, a plurality of optical elements disposed in an optical path of the light transmitted by the light transmission member, a laser resonator which the light passed through the plurality of optical elements enter, and an adjuster configured to adjust a position of at least one of the plurality of optical elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete appreciation of exemplary embodiments and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
           [0010]      FIG. 1  is a diagram illustrating an outline of an engine according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a diagram illustrating an ignition system according to an example embodiment of the present invention. 
           [0012]      FIG. 3  is a diagram illustrating a laser resonator according to an embodiment of the present invention. 
           [0013]      FIG. 4  is a diagram illustrating a beam-waist diameter, an incidence angle, and the distance between a first surface to a focal point of the light that enters a laser resonator, according to an embodiment of the present invention. 
           [0014]      FIG. 5  is diagram illustrating a second condensing optical system according to an embodiment of the present invention. 
           [0015]      FIG. 6  is a diagram illustrating a positioning mechanism according to an example embodiment of the present invention. 
           [0016]      FIG. 7A  and  FIG. 7B  are a set of diagrams illustrating a spacer according to an embodiment of the present invention. 
           [0017]      FIG. 8A  and  FIG. 8B  are a set of diagrams illustrating a first method of shifting a first lens to the -Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0018]      FIG. 9A  and  FIG. 9B  are a set of diagrams illustrating a second method of shifting a first lens to the -Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0019]      FIG. 10A  and  FIG. 10B  are a set of diagrams illustrating a first method of shifting a first lens to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0020]      FIG. 11A  and  FIG. 11B  are a set of diagrams illustrating a second method of shifting a first lens to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0021]      FIG. 12A  and  FIG. 12B  are a set of diagrams illustrating a first method of shifting a second lens to the −Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0022]      FIG. 13A  and  FIG. 13B  are a set of diagrams illustrating a second method of shifting a second lens to the −Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0023]      FIG. 14A  and  FIG. 14B  are a set of diagrams illustrating a first method of shifting a second lens to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0024]      FIG. 15A  and  FIG. 15B  are a set of diagrams illustrating a second method of shifting a second lens to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0025]      FIG. 16A  and  FIG. 16B  are a set of diagrams illustrating a first method of shifting a laser resonator to the −Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0026]      FIG. 17A  and  FIG. 17B  are a set of diagrams illustrating a second method of shifting a laser resonator to the −Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0027]      FIG. 18A  and  FIG. 18B  are a set of diagrams illustrating a first method of shifting a laser resonator to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0028]      FIG. 19A  and  FIG. 19B  are a set of diagrams illustrating a second method of shifting a laser resonator to the +Z side for adjustment with reference to a basic configuration, according to an example embodiment of the present invention. 
           [0029]      FIG. 20  is a diagram illustrating a first modification of a laser device according to an embodiment of the present invention. 
           [0030]      FIG. 21  is a diagram illustrating a second modification of a laser device according to an embodiment of the present invention. 
       
    
    
       [0031]    The accompanying drawings are intended to depict exemplary embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
       DETAILED DESCRIPTION 
       [0032]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0033]    In describing example embodiments shown in the drawings, specific terminology is employed for the sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same structure, operate in a similar manner, and achieve a similar result. 
         [0034]    &lt;General Outline&gt; 
         [0035]    In the following description, an embodiment of the present invention is described with reference to the drawings. 
         [0036]      FIG. 1  is a schematic diagram illustrating the principal parts of an engine  300  that serves as an internal combustion engine, according to an embodiment of the present invention. The engine  300  includes, for example, an ignition system  301 , a fuel injector  302 , an exhauster  303 , a combustion chamber  304 , and a piston  305 . 
         [0037]    The operation of the engine  300  is briefly described. 
         [0038]    (1) The fuel injector  302  injects the inflammable fuel-air mixture into the combustion chamber  304  (aspiration). 
         [0039]    (2) The piston  305  moves upward and compresses the inflammable fuel-air mixture (compression). 
         [0040]    (3) The ignition system  301  emits laser beam into the combustion chamber  304 . Accordingly, the fuel is ignited (ignition). 
         [0041]    (4) Inflammable gas is generated and the piston  305  moves downward (combustion). 
         [0042]    (5) The exhauster  303  exhausts the inflammable gas from the combustion chamber  304  (exhaust). 
         [0043]    As described above, a series of processes including aspiration, compression, ignition, combustion, and exhaust are repeated. Then, the piston  305  moves upward and downward according to the changes in the volume of the gas in the combustion chamber  304 , and kinetic energy is produced. As fuel, for example, natural gas and gasoline are used. 
         [0044]    Note that the above operation of the engine  300  is performed based on the instruction made through an engine controller that is externally provided and is electrically connected to the engine  300 . 
         [0045]    As illustrated in  FIG. 2  for example, the ignition system  301  includes a laser device  200 , an emission optical system  210 , and a protector  212 . 
         [0046]    The emission optical system  210  collects and condenses the light emitted from the laser device  200 . Accordingly, a high energy density can be obtained at a focal point. 
         [0047]    The protector  212  is a transparent window facing toward the combustion chamber  304 . In the present embodiment, for example, sapphire glass is used as a material of the protector  212 . 
         [0048]    The laser device  200  includes a surface emitting laser array  201 , a first condensing optical system  203 , an optical fiber  204 , a second condensing optical system  205 , and a laser resonator  206 . In the XYZ three-dimensional orthogonal coordinate system according to the present embodiment, it is assumed that the direction in which the surface emitting laser array  201  emits light is the +X direction. 
         [0049]    The surface emitting laser array  201  is a pump source, and includes a plurality of light-emitting units. Each of the light-emitting units is a vertical cavity-surface emitting laser (VCSEL). 
         [0050]    Note that a surface emitting laser array has very little temperature-driven wavelength displacement in the emitting light. Thus, a surface emitting laser array is a light source that is advantageous in pumping Q-switched laser whose characteristics vary widely due to the displacement in pumping wavelength. Accordingly, when a surface emitting laser array is used as a pump source, the temperature control of the environment becomes easier. 
         [0051]    The first condensing optical system  203  collects and condenses the light emitted from the surface emitting laser array  201 . 
         [0052]    The optical fiber  204  is disposed such that the light exited from the first condensing optical system  203  is condensed at the center of the -Z side lateral edge face of the core. 
         [0053]    Due to the provision of the optical fiber  204 , the surface emitting laser array  201  may be disposed at a position distant from the laser resonator  206 . Accordingly, the degree of flexibility in design increases. As the surface emitting laser array  201  can be disposed at a position away from the heat source when the laser device  200  is used for an ignition system, the ranges of choices for a method for cooling the engine  300  can be extended. 
         [0054]    The light that has entered the optical fiber  204  propagates through the core, and is exited from the +Z side lateral edge face of the core. 
         [0055]    The second condensing optical system  205  is disposed in the optical path of the light emitted from the optical fiber  204 , and condenses the light emitted from the optical fiber  204 . The light that has been condensed by the second condensing optical system  205  enters the laser resonator  206 . 
         [0056]    The laser resonator  206  is a Q-switched laser, and as illustrated in  FIG. 3  for example, the laser resonator  206  includes a laser medium  206   a  and a saturable absorber  206   b.    
         [0057]    The laser medium  206   a  is a cuboid-shaped neodymium (Nd): yttrium aluminum garnet (YAG) crystal, where the length of the resonator is 8 mm. The saturable absorber  206   b  is a cuboid-shaped chromium (Cr): YAG crystal, where the length is 2 mm. 
         [0058]    In the present embodiment, Nd: YAG crystal and Cr: YAG crystal are bonded together to form a so-called composite crystal. Moreover, the Nd: YAG crystal and the Cr: YAG crystal are both ceramic. 
         [0059]    The light that has been condensed by the second condensing optical system  205  enters the laser medium  206   a.  In other words, the laser medium  206   a  is optically pumped by the light that has been condensed by the second condensing optical system  205 . It is desired that the wavelength of the light that is emitted from the surface emitting laser array  201  be 808 nanometer (nm) where the absorption efficiency is the highest in the YAG crystal. The saturable absorber  206   b  performs Q-switching. 
         [0060]    The surface on the light entering side (−Z side) of the laser medium  206   a  and the surface on the light exiting side (+Z side) of the saturable absorber  206   b  are optically polished, and each of these surfaces serves as a mirror. In the following description, for the sake of explanatory convenience, the surface on the light entering side of the laser medium  206   a  is referred to as a first surface, and the surface on the light exiting side of the saturable absorber  206   b  is referred to a second surface (see  FIG. 3 ). 
         [0061]    On the first and second surfaces, a dielectric layer is coated according to the wavelength of the light that is emitted from the surface emitting laser array  201  and the wavelength of the light that exits from the laser resonator  206 . 
         [0062]    More specifically, a dielectric layer that indicates sufficient high transmittance to light with the wavelength of 808 nm and indicates sufficiently high reflectance to light with the wavelength of 1064 nm is coated on the first surface. On the second surface, a dielectric layer with selected reflectance indicating a desired threshold to light with the wavelength of 1064 nm is coated. 
         [0063]    Accordingly, the light is resonated and amplified inside the laser resonator  206 . In the present embodiment, the length of the laser resonator  206  is 10 (=8+2) mm. 
         [0064]    As illustrated in  FIG. 2 , the driver  220  drives the surface emitting laser array  201  based on an instruction from an engine controller  222 . More specifically, the driver  220  drives the surface emitting laser array  201  such that the ignition system  301  emits light at the timing when the engine  300  performs ignition. Note that a plurality of light-emitting units of the surface emitting laser array  201  are switched on and switched off at the same time. 
         [0065]    When it is not necessary to dispose the surface emitting laser array  201  at a position distant from the laser resonator  206  in the embodiments described above, the provision of the optical fiber  204  may be omitted. 
         [0066]    Each of the first condensing optical system  203  and the emission optical system  210  may include only one lens or a plurality of lenses. 
         [0067]    In the present embodiment, cases of an engine (piston engine) where a piston is moved by inflammable gas is used as an internal combustion engine have been described. However, no limitation is intended thereby. For example, a rotary engine, a gas turbine engine, and a jet engine may be used as an internal combustion engine. In other words, any engine may be used as long as it burns fuel to produce inflammable gas. 
         [0068]    The ignition system  301  may be used for cogeneration in which exhaust heat is reused to increase the comprehensive energy efficiency. The exhaust heat in cogeneration is used for obtaining motive power, heating energy, or cooling energy. 
         [0069]    In the present embodiment, cases in which the ignition system  301  is used for an internal combustion engine have been described. However, no limitation is intended thereby. 
         [0070]    In the present embodiment, cases in which the laser device  200  is used for an ignition system have been described. However, no limitation is intended thereby. For example, the laser device  200  may be used for a laser beam machine, a laser peening apparatus, or a terahertz generator. 
         [0071]    &lt;Details&gt; 
         [0072]    Note that the “beam-waist diameter”, “incidence angle”, and “distance between first surface to focal point” (see  FIG. 4 ) of the light that enters the laser resonator  206 , so-called incident-light characteristics, have a significant impact on the Q-switched laser characteristics. Moreover, the Q-switched laser characteristics change, for example, when the above-mentioned incident-light characteristics change due to an error in assembly. 
         [0073]    Moreover, these light-emitting units of the surface emitting laser array  201  are disposed in an area having 8.9 mm diameter. When the surface emitting laser array  201  emits light, the multiple light-emitting units emit light at the same time. 
         [0074]    The surface emitting laser array  201  includes a plurality of light-emitting units as described above, and thus the optical output can be increased. In the present embodiment, the optical output of the surface emitting laser array  201  is about 200 W. 
         [0075]    In the present embodiment, the second condensing optical system  205  includes a first lens  205   a,  a second lens  205   b,  and a plurality of spacers ( 207   a  to  207   d ) (see  FIG. 5 ). Note that the second condensing optical system  205  may include three or more lenses. 
         [0076]    The first lens  205   a  is a collimator lens that approximately collimates the light emitted from the optical fiber  204 . In the present embodiment, a collimator lens where the focal length is 8 mm (manufactured by Thorlabs, inc., model number: C240TME-B) is used as the first lens  205   a.    
         [0077]    The second lens  205   b  is a condenser lens that approximately collects and condenses the light that is approximately collimated by the first lens  205 . In the present embodiment, a condenser lens where the focal length is 6.24 mm (manufactured by Thorlabs, inc., model number: C110TME-B) is used as the second lens  205   b.    
         [0078]    Note that other kinds of lens may be used for the first lens  205   a  and the second lens  205   b.    
         [0079]    In the present embodiment, the incident-light characteristics can be controlled by adjusting the relative positions of the first lens  205   a,  the second lens  205   b,  and the laser resonator  206  in the Z-axis direction. 
         [0080]    For example, the relative positions of at least one of the second lens  205   b  and the laser resonator  206  in the Z-axis direction are adjusted to control the “distance between first surface to focal point”. 
         [0081]    Moreover, the relative positions of at least one of the first lens  205   a  and the second lens  205   b  in the Z-axis direction are adjusted to control the “beam-waist diameter”. 
         [0082]    Further, the relative positions of at least one of the first lens  205   a  and the second lens  205   b  in the Z-axis direction are adjusted to control the “incidence angle”. 
         [0083]    In the present embodiment, the relative position of the first lens  205   a  is adjusted by at least one of the spacer  207   a  and the spacer  207   b.    
         [0084]    Moreover, the relative position of the second lens  205   b  is adjusted by at least one of the spacer  207   b  and the spacer  207   c.    
         [0085]    Further, the relative position of the laser resonator  206  is adjusted by at least one of the spacer  207   c  and the spacer  207   d.    
         [0086]    In the present embodiment, as illustrated in  FIG. 6  for example, the first lens  205   a  is supported by a first lens holder  226 , and the second lens  205   b  is supported by a second lens holder  223 . Moreover, the laser resonator  206  is supported by a crystal holder  224 , and the emission optical system  210  is supported by a third lens holder  225 . Further, the optical fiber  204  is supported by a fiber holder  221 . 
         [0087]    On the periphery of each of the holders, external threads are formed. The first lens holder  226 , the second lens holder  223 , and the crystal holder  224  are accommodated inside a resonator housing. Inside the resonator housing, internal threads that correspond to the external threads of the holders are formed. The fiber holder  221  is screwed into the resonator housing from the -Z side end of the resonator housing, and the third lens holder  225  is screwed into the resonator housing from the +Z side end of the resonator housing. In the present embodiment, the holders can be moved in the Z-axis direction by turning the holders. 
         [0088]    More specifically, the relative position of the first lens  205   a  can be adjusted in the Z-axis direction by turning the first lens holder  226 . In a similar manner, the relative position of the second lens  205   b  can be adjusted in the Z-axis direction by turning the second lens holder  223 . Further, the relative position of the laser resonator  206  can be adjusted in the Z-axis direction by turning the crystal holder  224 . 
         [0089]    Each of the spacers is, as illustrated in  FIG. 7A  and  FIG. 7B  for example, a ring-shaped plate that has an opening in the center such that the light can pass through.  FIG. 7B  is a sectional view A-A of  FIG. 7A , according to the present example embodiment. 
         [0090]    In the present example embodiment, a plate of stainless steel (SUS430 (Japanese Industrial Standards (JIS)) is used for the spacers. In a basic configuration of the present embodiment, the thickness of the spacer  207   a,  the thickness of the spacer  207   b,  the thickness of the spacer  207   c,  and the thickness of the spacer  207   d  are 1.32 millimeter (mm), 2.0 mm, 1.4 mm, and 1.0 mm, respectively. In the present embodiment, a plurality of plates with varying thickness are used. 
         [0091]    (1) Cases in which a position of the first lens  205   a  is shifted towards the −Z side with reference to a basic configuration are described below. 
         [0092]    In such cases, as illustrated in  FIG. 8A  and  FIG. 8B  for example, the spacer  207   a  is replaced with a plate thinner than the basic configuration, and the spacer  207   b  is replaced with a plate thicker than the basic configuration. 
         [0093]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   a  is replaced with a plate with 0.82 mm thickness, and the spacer  207   b  is replaced with a plate with 2.5 mm thickness. 
         [0094]    Alternatively, as illustrated in  FIG. 9A  and  FIG. 9B  for example, the spacer  207   a  is replaced with a plate thinner than the basic configuration, and a spacer  207   b ′ may be inserted between the first lens holder  226  and the spacer  207   b  of the basic configuration. 
         [0095]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   a  is replaced with a plate with 0.82 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   b′.    
         [0096]    (2) Cases in which a position of the first lens  205   a  is shifted towards the +Z side with reference to a basic configuration are described below. 
         [0097]    In such cases, as illustrated in  FIG. 10A  and  FIG. 10B  for example, the spacer  207   a  is replaced with a plate thicker than the basic configuration, and the spacer  207   b  is replaced with a plate thinner than the basic configuration. 
         [0098]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   a  is replaced with a plate with 1.82 mm thickness, and the spacer  207   b  is replaced with a plate with 1.5 mm thickness. 
         [0099]    Alternatively, as illustrated in  FIG. 11A  and  FIG. 11B  for example, the spacer  207   b  is replaced with a plate thinner than the basic configuration, and a spacer  207   a ′ may be inserted between the first lens holder  226  and the spacer  207   a  of the basic configuration. 
         [0100]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   b  is replaced with a plate with 1.5 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   a′.    
         [0101]    (3) Cases in which a position of the second lens  205   b  is shifted towards the −Z side with reference to a basic configuration are described below. 
         [0102]    In such cases, as illustrated in  FIG. 12A  and  FIG. 12B  for example, the spacer  207   b  is replaced with a plate thicker than the basic configuration, and the spacer  207   c  is replaced with a plate thinner than the basic configuration. 
         [0103]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   b  is replaced with a plate with 1.5 mm thickness, and the spacer  207   c  is replaced with a plate with 1.9 mm thickness. 
         [0104]    Alternatively, as illustrated in  FIG. 13A  and  FIG. 13B  for example, the spacer  207   b  is replaced with a plate thinner than the basic configuration, and a spacer  207   c ′ may be inserted between the second lens holder  223  and the spacer  207   c  of the basic configuration. 
         [0105]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   b  is replaced with a plate with 1.5 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   c′.    
         [0106]    (4) Cases in which a position of the second lens  205   b  is shifted towards the +Z side with reference to a basic configuration are described below. 
         [0107]    In such cases, as illustrated in  FIG. 14A  and  FIG. 14B  for example, the spacer  207   c  is replaced with a plate thicker than the basic configuration, and the spacer  207   b  is replaced with a plate thinner than the basic configuration. 
         [0108]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   c  is replaced with a plate with 0.9 mm thickness, and the spacer  207   b  is replaced with a plate with 2.5 mm thickness. 
         [0109]    Alternatively, as illustrated in  FIG. 15A  and  FIG. 15B  for example, the spacer  207   c  is replaced with a plate thinner than the basic configuration, and a spacer  207   b ′ may be inserted between the second lens holder  223  and the spacer  207   b  of the basic configuration. 
         [0110]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   c  is replaced with a plate with 0.9 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   b′.    
         [0111]    (5) Cases in which a position of the laser resonator  206  is shifted towards the −Z side with reference to a basic configuration are described below. 
         [0112]    In such cases, as illustrated in  FIG. 16A  and  FIG. 16B  for example, the spacer  207   c  is replaced with a plate thinner than the basic configuration, and the spacer  207   d  is replaced with a plate thicker than the basic configuration. 
         [0113]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   c  is replaced with a plate with 0.9 mm thickness, and the spacer  207   d  is replaced with a plate with 1.5 mm thickness. 
         [0114]    Alternatively, as illustrated in  FIG. 17A  and  FIG. 17B  for example, the spacer  207   c  is replaced with a plate thinner than the basic configuration, and a spacer  207   d ′ may be inserted between the crystal holder  224  and the spacer  207   d  of the basic configuration. 
         [0115]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   c  is replaced with a plate with 0.9 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   d′.    
         [0116]    (6) Cases in which a position of the laser resonator  206  is shifted towards the +Z side with reference to a basic configuration are described below. 
         [0117]    In such cases, as illustrated in  FIG. 18A  and  FIG. 18B  for example, the spacer  207   d  is replaced with a plate thinner than the basic configuration, and the spacer  207   c  is replaced with a plate thicker than the basic configuration. 
         [0118]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   d  is replaced with a plate with 0.5 mm thickness, and the spacer  207   c  is replaced with a plate with 1.9 mm thickness. 
         [0119]    Alternatively, as illustrated in  FIG. 19A  and  FIG. 19B  for example, the spacer  207   d  is replaced with a plate thinner than the basic configuration, and a spacer  207   c ′ may be inserted between the crystal holder  224  and the spacer  207   c  of the basic configuration. 
         [0120]    More specifically, when the moved distance is 0.5 mm for example, the spacer  207   d  is replaced with a plate with 0.5 mm thickness, and a plate with 0.5 mm thickness is used as the spacer  207   c′.    
         [0121]    In the present embodiment, the Q-switched laser output and the number of light-emitting pulses are the characteristics of the light exited from the laser resonator  206 , and the relative positions of the first lens  205   a,  the second lens  205   b,  and the laser resonator  206  are adjusted such that the Q-switched laser output and the number of light-emitting pulses reach desired values. In the present embodiment, the desired value for the number of light-emitting pulses is set to four per 500 microsecond (μs). 
         [0122]    In the positioning described above, the distance between the +Z side lateral edge face of the optical fiber  204  and the first lens  205   a  may be adjusted within the range of about 3 mm. The distance between the first lens  205   a  and the second lens  205   b  may be adjusted within the range of about 10 mm. The distance between the second lens  205   b  and the laser resonator  206  may be adjusted within the range of about 5 mm. The distance between the laser resonator  206  and the emission optical system  210  may be adjusted within the range of about 50 mm. Note that each of the adjustable ranges described above is merely an example, and no limitation in limited thereby. 
         [0123]    When the position of the second lens  205   b  is adjustable, the “distance between first surface to focal point” can be adjusted without adjusting any of the positions of the first lens  205   a  and the laser resonator  206 . 
         [0124]    The ignition system  301  for which the laser device  200  is provided has energy higher than that of any known ignition systems such as a spark plug, and the ignition point can be changed as desired. Accordingly, it is expected that the efficiency of the engine be improved. 
         [0125]    As described above, with the laser device  200  according to the present embodiment, the second condensing optical system  205  serves as a plurality of optical elements, and the spacers  207   a  to  207   c  serve as an adjuster that adjusts the position of at least one of the optical elements. Moreover, the spacer  207   c  and the spacer  207   d  serve as an adjuster that adjusts the position of the laser resonator. 
         [0126]    As described above, the laser device  200  according to the present embodiment includes the surface emitting laser array  201 , the first condensing optical system  203 , the optical fiber  204 , the second condensing optical system  205  including a plurality of optical elements and the multiple spacers  207   a  to  207   d,  and the laser resonator  206 . 
         [0127]    The surface emitting laser array  201  includes a plurality of light-emitting units. The light that is emitted from the surface emitting laser array  201  is collected and condensed by the first condensing optical system  203 , and propagates through the optical fiber  204 . Then, the light is collected and condensed by the second condensing optical system  205 , and enters the laser resonator  206 . 
         [0128]    The laser resonator  206  is a composite crystal of the laser medium  206   a  and the saturable absorber  206   b.  In such a composite crystal, the boundary between the laser medium  206   a  and the saturable absorber  206   b  is not split. Accordingly, such a composite crystal has characteristics similar to those of a single crystal, and has favorable mechanical strength and optical properties. 
         [0129]    The multiple spacers  207   a  to  207   d  are used to adjust the relative positions of the optical elements of the second condensing optical system  205  and the laser resonator  206  in the Z-axis direction. 
         [0130]    More specifically, in the laser device  200 , a plurality of optical elements are disposed between the optical fiber  204  and the laser resonator  206 , and an adjuster is provided that adjusts the relative positions of the optical elements and the laser resonator  206  in the Z-axis direction. 
         [0131]    Accordingly, the “beam-waist diameter”, “incidence angle”, and the “distance between first surface to focal point” of the light that enters the laser resonator  206  can individually be adjusted with high accuracy. As a result, even when an error in assembly is present, Q-switched laser characteristics can be obtained as desired in a stable manner. 
         [0132]    If the second condensing optical system  205  includes only one lens, it is difficult to individually adjust the “beam-waist diameter”, “incidence angle”, and the “distance between first surface to focal point” of the light that enters the laser resonator  206  with high accuracy by adjusting the relative position of the lens in the Z-axis direction. 
         [0133]    Due to the provision of the laser device  200 , the ignition system  301  according to the present embodiment can perform ignition in a stable manner. In the present embodiment, the focal point in the Z-axis direction can be controlled by adjusting the focal length of the emission optical system  210 . 
         [0134]    Due to the provision of the ignition system  301 , the operation of the engine  300  according to the present embodiment can be stabilized. 
         [0135]    In the embodiments described above, cases in which the material of spacers is stainless steel are described. However, no limitation is intended thereby. 
         [0136]    In the embodiments described above, cases in which the outside shape of spacers is circular are described. However, no limitation is intended thereby. For example, the outside shape of spacers may be rectangular or elliptic. 
         [0137]    When it is not necessary to adjust the relative position of the laser resonator  206  in the embodiments described above, the spacer  207   d  may be omitted as in a laser device  400  illustrated in  FIG. 20  for example. 
         [0138]    In the embodiments described above, cases in which the relative positions of the first lens  205   a,  the second lens  205   b,  and the laser resonator  206  are adjusted using spacers are described. However, no limitation is intended thereby. For example, as illustrated in  FIG. 21 , fixing screws may be used. 
         [0139]    The fixing screws penetrate the resonator housing in the direction orthogonal to the Z axis, and the front ends of the screws face the holders. Accordingly, the holders are fixed to the resonator housing by fastening the fixing screws, and the fixation between the holders and the resonator housing is released by loosening the fixing screws. The fixing screws are loosened to adjust the relative positions of the holders, and are fastened when the adjustment is finished. 
         [0140]    In the embodiments described above, cases in which relative positions are adjusted using spacers only in the Z-axis direction are described. However, no limitation is intended thereby. For example, spacers may be used to adjust relative positions in the X-axis direction or the Y-axis direction. 
         [0141]    Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.