Patent Publication Number: US-7916393-B2

Title: Optical level control device, method for controlling same, and laser application device

Description:
This is a divisional of application Ser. No. 11/522,435 filed Sep. 18, 2006, which claims priority from Japanese Patent Application No. 2005-278935 filed Sep. 26, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical level control device that is capable of arbitrarily varying the intensity of each of a plurality of light waves having different wavelengths that are coupled in essentially the same optical axis, to a method for controlling the same, and to a laser application device that utilizes the optical level control device. 
     2. Description of the Related Art 
     There are numerous applications for an optical level control device that arbitrarily varies the intensity of each of a plurality of light beams having different wavelengths in the same optical axis. One example of an application is laser machining. Laser machining is a method by which a laser pulse is radiated to an object in order to perform cutting, perforation, welding, and other processes without touching the object. The wavelength, pulse width, time waveform, peak energy, distribution in the beam section, and other characteristics of the emitted laser are appropriately adjusted according to the light absorption and reflection characteristics, thermal characteristics, and other physical characteristics of the workpiece. In addition, beams of light having a plurality of wavelengths are sometimes mixed together and used. When light having a plurality of wavelengths is used, the ability to arbitrarily vary the intensity of each light makes it possible to single out machining conditions that are suited to the absorption and reflection spectrum characteristics of the workpiece. This ability also increases the degree of freedom in machining. In the fields of biological and medical laser applications, which have recently received attention, a technique is needed for varying the ratios of mixed light according to the site, type of the affected area, and other characteristics of the irradiated body. 
     Japanese Laid-open Patent Application Nos. 06-106378 and 2002-028795 disclose techniques whereby a laser machining device for emitting a laser to a work object switches and extracts laser light having a plurality of wavelengths from a single laser oscillation device in order to appropriately machine an object that has different wavelength absorption sensitivities. 
     The technique disclosed in Japanese Laid-open Patent Application No. 06-106378 (pp. 2-4, FIG. 1) is a laser machining device that is configured so that a harmonic generator is used to generate a plurality of wavelengths of light from light outputted by a YAG (Yttrium Aluminum Garnet) laser oscillator, the plurality of wavelengths of light thus generated are spatially separated, and each wavelength of light is transmitted through separate light-varying optical attenuators, after which the light is combined back into a single beam and guided to a machining head. This technique enables proper machining of a sample having different wavelength absorption sensitivities. 
     The technique disclosed in Japanese Laid-open Patent Application No. 2002-028795 (pp. 4-7, FIG. 1) is a laser welding device whereby an output beam from a first laser device that oscillates with a fundamental wave, and an output beam from a second laser device that outputs SHG (Second Harmonic Generation) light from a separate Q switch are coupled by a dichroic mirror, and condensed light is radiated to an object. This technique makes it possible to form a joint by laser welding that is effective for pure aluminum, pure copper, and other metals whose reflectance and thermal diffusivity are higher than that of an aluminum alloy. 
     The variable optical attenuators disclosed in Japanese Laid-open Patent Application No. 06-106378 switch and extract laser light having a plurality of wavelengths and radiate the laser light to an object, but do not freely vary the ratio of intensities of a plurality of wavelengths of light that are mixed together. 
     In Japanese Laid-open Patent Application No. 2002-028795, the constituent elements of the invention do not include a mechanism for adjusting the intensity and other characteristics of the output of either of the two laser devices. This invention therefore does not provide the degree of freedom needed for wide variation of the welding conditions. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical level control device that is capable of arbitrarily varying the beam intensity level of each of a plurality of beams of light having different wavelengths in the same optical axis or essentially the same optical axis, a method for controlling the same, and a laser application device that utilizes the optical level control device. 
     The optical level control device according to the present invention is an optical level control device that is capable of arbitrarily varying a light transmittance of each of two light waves having different wavelengths that are coupled in essentially the same optical axis, wherein the optical level control device comprises a wavelength-dependent wavelength plate that functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave, and a polarization beam splitter for further transmitting the two light waves transmitted through the wavelength plate. 
     It is preferred that the wavelength plate and the polarization beam splitter be rotatable about the optical axis, and that a rotation angle of the wavelength plate and the polarization beam splitter be adjusted to set the transmittance of the two light waves rectilinearly transmitted through the polarization beam splitter. 
     A stage that precedes the wavelength plate may be provided with a separate wavelength-dependent wavelength plate that functions as a quarter-wave plate with respect to a first light wave and as a full-wave plate or a half-wave plate with respect to a second light wave among two light waves having different wavelengths. 
     A stage subsequent to the polarization beam splitter may be further provided with a separate wavelength-dependent wavelength plate that is capable of rotating about the optical axis and functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave. 
     In the method for controlling an optical level control device according to the present invention, the optical level control device has a wavelength-dependent wavelength plate that is capable of rotating about the optical axis and functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave among two light waves having different wavelengths that are coupled in essentially the same optical axis, and a polarization beam splitter that is capable of rotating about the optical axis and that further transmits the two light waves transmitted through the wavelength plate, wherein the method for controlling the optical level control device comprises adjusting a rotation angle of the wavelength plate and the polarization beam splitter, and arbitrarily setting the transmittance of the two light waves rectilinearly transmitted through the polarization beam splitter. 
     The adjustment of a rotation angle of the wavelength plate and the polarization beam splitter comprises the steps of fixing a rotation angle of the wavelength plate and rotating the polarization beam splitter, and rotating the wavelength plate in a state in which a rotation angle of the polarization beam splitter is fixed. 
     The optical level control device may further comprise a separate wavelength-dependent wavelength plate that is provided to a stage prior to the wavelength plate and that functions as a quarter-wave plate with respect to a first light wave and as a full-wave plate or a half-wave plate with respect to a second light wave among two light waves having different wavelengths. 
     A separate wavelength-dependent wavelength plate that is capable of rotating about the optical axis and that functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave may be provided to a stage subsequent to the polarization beam splitter, and a step may also be included for rotating the separate wavelength-dependent wavelength plate to provide variability to the angular difference of a principal axis of polarization of the two light waves. 
     The laser application device according to the present invention comprises a laser beam system, wherein the laser beam system has a laser oscillator for outputting in the same optical axis two light waves having different wavelengths, and any of the optical level control devices described above. 
     The laser application device according to the present invention may also comprise a laser beam system, wherein the laser beam system has two laser oscillators for generating two light waves having different wavelengths, an optical coupler for coupling and outputting the two light waves in essentially the same optical axis, and any of the optical level control devices described above. 
     The laser application device according to the present invention may also comprise laser beam systems described above that produce different wavelengths, and an optical coupler for coupling and outputting in essentially the same optical axis a plurality of laser beams that are outputted from the plurality of laser beam systems. 
     The optical level control device of the present invention comprises a wavelength-dependent wavelength plate that functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave among two light waves having different wavelengths that are coupled in essentially the same optical axis, and a polarization beam splitter capable of rotating about the optical axis that further transmits the two light waves transmitted through the wavelength plate. It is therefore possible to arbitrarily vary the light transmittance of each of two light waves having different wavelengths that pass through the optical level control device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view showing the optical level control device according to a first embodiment of the present invention; 
         FIG. 2  is a view showing the transmittance of a fundamental wave transmitted rectilinearly through the polarization beam splitter in relation to the rotation angle of the polarization beam splitter provided to the optical level control device of the present invention; 
         FIG. 3  is a view showing the rotation angle of polarization of a second higher harmonic in relation to the rotation angle of the wavelength plate provided to the optical level control device of the present invention; 
         FIG. 4  is a view showing the transmittance of a second higher harmonic transmitted rectilinearly through the polarization beam splitter in relation to the rotation angle of the wavelength plate provided to the optical level control device of the present invention; 
         FIG. 5  is a schematic perspective view showing the structure of the optical level control device according to a second embodiment of the present invention; 
         FIG. 6  is a schematic perspective view showing the structure of the optical level control device according to a third embodiment of the present invention; 
         FIG. 7  is a schematic perspective view showing the structure of the optical level control device according to a fourth embodiment of the present invention; and 
         FIGS. 8A and 8B  are views showing the structure of the optical system of a laser machining device that utilizes the optical level control device of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will next be specifically described with reference to the accompanying drawings. A first embodiment of the present invention will first be described.  FIG. 1  is a schematic perspective view showing the optical level control device according to the present embodiment. A laser oscillator  2  emits two light beams simultaneously that have different wavelengths, and the two beams have the same optical axis. One example of this type of laser oscillator is a laser device that is composed of a fundamental wave laser oscillator and a higher harmonic generating element that generates a higher harmonic from the fundamental wave. The conversion efficiency with which the higher harmonic is generated from the fundamental wave by the higher harmonic generating element is not 100%, and is 50%, for example. Accordingly, the output beam of the laser oscillator  2  includes both the fundamental wave and the higher harmonic as components, the energy ratios are 50% for the fundamental wave and 50% for the higher harmonic, and both waves have the same optical axis. 
     An optical level control device  1  independently controls the intensities of the two beams outputted by the laser oscillator  2 . In this optical level control device  1 , a wavelength plate  3  and a polarization beam splitter (PBS)  4  are arranged on the optical axis of the light beams from the laser oscillator  2  so that the two beams of light emitted in the same optical axis from the laser oscillator  2  enter the wavelength-dependent wavelength plate  3 , and the light beams transmitted through the wavelength plate  3  enter the polarization beam splitter  4 . 
     The wavelength-dependent wavelength plate  3 , which is one of the constituent elements of the optical level control device  1 , has the following characteristics. The wavelength plate  3  functions as a half-wave plate for one of the light waves among the fundamental wave and the higher harmonic, and functions as a full-wave plate for the other light wave. The wavelength-dependent wavelength plate  3  is also capable of rotating about the optical axis in which light is transmitted. Accordingly, when the wavelength plate  3  is rotated, light waves receiving the effect of a half-wave plate are emitted from the wavelength plate  3  in a state in which the plane of polarization of the light waves is rotated in conjunction with the rotation of the wavelength-dependent wavelength plate  3 . However, light waves receiving the effect of a full-wave plate are emitted from the wavelength plate  3  without the plane of polarization thereof being rotated by the rotation of the wavelength plate  3 . 
     The wavelength plate  3  can be formed using a transparent, birefringent crystal. The wavelength plate  3  may be formed, for example, by grinding a crystal having a wide band of transmitted wavelengths in a direction that is nearly parallel to the C-axis of crystallization. 
     The polarization beam splitter  4 , which is another constituent element of the optical level control device  1 , is a commonly used polarization beam splitter, and has a structure formed by affixing together the tilted surfaces of two right-angle prisms in which a dielectric multilayer film is provided to the tilted surfaces thereof. A P wave component is transmitted, and an S wave component is reflected in relation to the tilted surface of the prism. The polarization characteristics of a polarization beam splitter having this structure are not wavelength-dependent. Polarization beam splitters whose polarization characteristics are not wavelength-dependent are already used in the optical heads of optical disk devices provided with three light sources that include a blue LD (semiconductor laser), a red LD, and a near-infrared LD and that record/play back optical disks in high-definition DVD, normal-definition DVD, CD, and other different formats. 
     Rotating the wavelength-dependent wavelength plate  3  and the polarization beam splitter  4  about the optical axis makes it possible to independently control the intensities of the fundamental wave and the higher harmonic that are transmitted rectilinearly through the polarization beam splitter  4 . 
     The operation of the optical level control device  1  of the first embodiment will next be described. It is assumed that the fundamental wave of the laser oscillator  2  is horizontal, rectilinearly polarized light, and that the higher harmonic generating element is an element that generates a second higher harmonic. More specifically, the higher harmonic generating element is an element that generates a second higher harmonic for which the phase matching condition is type  1 . Accordingly, the polarization of the second higher harmonic outputted from the laser oscillator  2  along with the fundamental wave is assumed to be linear polarization that is parallel to the fundamental wave. The thickness of the wavelength plate is also set so that the wavelength-dependent wavelength plate  3  functions as a full-wave plate with respect to the fundamental wave and functions as a half-wave plate with respect to the second higher harmonic. 
     First, only the rotation angle of the polarization beam splitter  4  about the optical axis is adjusted, and the intensity of the fundamental wave transmitted rectilinearly through the polarization beam splitter  4  is set. The polarization beam splitter  4  is then fixed at the adjusted angle, the rotation angle of the wavelength plate  3  about the optical axis is adjusted, and the intensity of the second higher harmonic is set. 
     In  FIG. 2 , the angle at which rectilinearly polarized fundamental light propagates through the polarization beam splitter  4  at maximum transmittance (horizontally polarized light: P-polarized light) is shown as the origin, the angle θ by which the polarization beam splitter  4  is rotated about the optical axis away from the maximum-transmission angle is plotted on the horizontal axis, and the transmittance of a fundamental wave rectilinearly transmitted through the polarization beam splitter  4  is plotted on the vertical axis. When θ=90°, the linearly polarized light of the fundamental wave is incident as S-polarized light on the tilted surface of the polarization beam splitter  4 , and the rectilinearly transmitted intensity reaches the minimum. Since the wavelength-dependent wavelength plate  3  functions as a full-wave plate with respect to the fundamental wave, the fundamental wave outputted by the laser oscillator  2  is transmitted in a state in which the direction of linear polarization is maintained regardless of the direction of the C-axis of the wavelength plate, and is incident on the polarization beam splitter  4 . 
     The polarization beam splitter  4  is rotated about the optical axis, and the rectilinearly transmitted intensity shown in  FIG. 2  is set to the desired level. Specifically, I=cos 2θ indicates the relationship between the rectilinearly transmitted intensity I of the polarization beam splitter  4  and the angle θ by which the polarization beam splitter  4  is rotated about the optical axis from the horizontal. In a case in which the desired intensity of the fundamental wave passing through the polarization beam splitter  4  is I ω =½, a setting of θ 0 =45° is made. 
     The second higher harmonic, which is the other light wave emitted from the laser oscillator  2 , is also horizontally polarized. The wavelength-dependent wavelength plate  3  functions as a half-wave plate with respect to the second higher harmonic. The polarization angle Ψ of the linear polarized light exiting the wavelength plate  3  is Ψ=2φ, as shown in  FIG. 3 , where φ is the angle by which the direction of the C-axis of the wavelength plate is tilted from horizontal. Accordingly, the intensity of the second harmonic transmitted rectilinearly through the polarization beam splitter  4  varies as shown in  FIG. 4  with respect to the rotation angle φ of the wavelength plate. In order to divide in half the transmittance of the fundamental wave transmitted through the polarization beam splitter  4 , φ=22.5° is set as the rotation angle of the wavelength plate that yields a transmittance of 1 of the second higher harmonic transmitted through the polarization beam splitter  4 . This is because the polarization beam splitter  4  is already rotated by θ 0 =45°. The intensity of the second higher harmonic transmitted rectilinearly through the polarization beam splitter  4  can thus be set independently from the fundamental wave by rotating the wavelength plate  3  to an arbitrary angle φ. 
     A case is described above in the present embodiment in which the two light waves are a fundamental wave and a second higher harmonic, but the relationship of the wavelengths is not limited by this example. The two wavelengths herein are designated as λ 1  and λ 2 . It is assumed that the wavelength-dependent wavelength plate  3  functions as a full-wave plate with respect to λ 1  and as a half-wave plate with respect to λ 2 . When the thickness of the wavelength plate is t, the difference in phase between the ordinary component and the extraordinary component exiting the wavelength plate is given by Eqs. 1 and 2 below.
 
Δ n   1   ·t=p·λ   1   [Eq. 1]
 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         n 
                         2 
                       
                       · 
                       t 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           2 
                           ⁢ 
                           q 
                         
                         + 
                         1 
                       
                       ) 
                     
                     · 
                     
                       
                         λ 
                         2 
                       
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     In these equations, Δn 1  and Δn 2  are birefringence magnitudes for wavelengths λ 1  and λ 2 , and p and q are positive integers. Eliminating t from Eqs. 1 and 2 yields Eq. 3 below. 
     
       
         
           
             
               
                 
                   q 
                   = 
                   
                     
                       p 
                       · 
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             n 
                             2 
                           
                         
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             n 
                             1 
                           
                         
                       
                       · 
                       
                         
                           λ 
                           1 
                         
                         
                           λ 
                           2 
                         
                       
                     
                     - 
                     
                       1 
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In this equation, q is found (p and q may be inverted) by substituting the real values of the wavelengths used for wavelengths λ 1  and λ 2 , substituting the real values of the birefringence magnitudes of the optical crystal used for Δn 1  and Δn 2 , and substituting an appropriate positive integer for p. The term q is not necessarily a positive integer. The value of p is varied so that the difference between q and a positive integer is equal to or less than a desired value. The thickness t of the wavelength plate can be determined from the value of p when the difference between q and a positive integer is equal to or less than the desired value. A wavelength plate with this thickness functions reliably as a full-wave plate with respect to wavelength λ 1 , but for wavelength λ 2 , the difference in phase between the ordinary component and the extraordinary component becomes offset from the half wavelength by an amount commensurate with the wavelength obtained by multiplying a half wavelength by the difference between q and a positive integer. 
     A computation is performed as a numerical example in which quartz is used as the optical crystal, and λ 1  and λ 2  are a fundamental wave and a second higher harmonic, as in the embodiment described above, wherein λ 1 =1.06 μm and λ 2 =0.53 μm. The refractive indices of the crystal are Δn 1 =0.0087 and Δn 2 =0.0092, and when p=47 is substituted, q=98.9023. The difference between q and 99 is 0.097, which is a difference of about 1/20 wavelength. At this time, t=5.7 mm, which is a realizable thickness. 
     In the optical level control device according to the embodiment described above, the two light waves have the same optical axis, are related in wavelength as a fundamental wave and a higher harmonic, and are also polarized in the same direction. However, the optical level control device is not limited by these conditions. It is sufficient insofar as the optical axes of the two light waves are essentially the same to allow mixing by an optical coupler or the like, the wavelengths may also be arbitrary, and the optical level control device of the present invention operates effectively even when the light waves are in different polarization states. 
     A second embodiment of the present invention will next be described.  FIG. 5  is a schematic perspective view showing the structure of the optical level control device  5  according to the present embodiment. The same reference symbols are used in  FIG. 5  for structural components that are the same as those in  FIG. 1 , and detailed descriptions of those components are omitted. In the first embodiment described above, the phase matching condition of the second higher harmonic generating element was type  1  according to a nonlinear wave equation, but the phase matching condition may be of another type, such as type  2 , for example. In this case, even when the fundamental wave incident on the second higher harmonic generating element is linearly polarized light, the fundamental wave that is emitted without being completely converted to a second higher harmonic receives the birefringent effects of the nonlinear optical crystal that converts the wave to a second higher harmonic, and the light is generally emitted as elliptically polarized light. Therefore, the transmittance of the fundamental wave rectilinearly transmitted through the polarization beam splitter with respect to the rotation angle θ of the polarization beam splitter in  FIG. 2  is reduced so as not to reach a maximum value of 1, and is also increased so as not to reach a minimum value of 0. The dynamic range of the fundamental wave outputted from the optical level control device  1  is thus reduced. 
     The optical level control device  5  according to the present embodiment independently controls the intensities of two beams outputted from a laser oscillator  2 . This optical level control device  5  is provided with a polarization beam splitter  4  and a wavelength-dependent wavelength plate  3  that are capable of rotating about the optical axis. The optical level control device  5  is furthermore provided with a second wavelength-dependent wavelength plate  6  that is fixed in place, has no rotation mechanism, and is provided to a stage prior to the wavelength-dependent wavelength plate  3 . The second wavelength-dependent wavelength plate  6  functions as a quarter-wave plate with respect to a first light wave and as a full-wave plate or a half-wave plate with respect to a second light wave among two light waves having different wavelengths. The thickness of the wavelength plate herein is set so that the wavelength plate functions as a quarter-wave plate with respect to the fundamental wave and as a full-wave plate or half-wave plate with respect to the second higher harmonic. This design method can be implemented in the same manner as the above-mentioned design method using Eqs. 1 and 2 described above in the first embodiment. As described above, among the fundamental wave and the second higher harmonic outputted by the laser oscillator, the fundamental wave is converted to linearly polarized light by the functioning of the second wavelength plate  6  as a quarter-wave plate in the type of case in which the fundamental wave is elliptically polarized light, whereas the second higher harmonic is linearly polarized light. However, the second higher harmonic is emitted unmodified as linearly polarized light through the functioning of the second wavelength plate  6  as a full-wave plate or a half-wave plate. It is thereby possible to rotate the polarization beam splitter  4  and the wavelength-dependent wavelength plate  3  about the optical axis to arbitrarily set the levels of two light waves that are rectilinearly transmitted through the polarization beam splitter  4 , in the same manner as in the first embodiment. 
     A third embodiment of the present invention will next be described.  FIG. 6  is a schematic perspective view showing the structure of the optical level control device  7  according to the present embodiment. The same reference symbols are used in  FIG. 6  for structural components that are the same as those in  FIGS. 1 and 5 , and detailed descriptions of those components are omitted. The optical level control device  7  according to the present embodiment is capable of independently controlling the intensities of two beams outputted from a laser oscillator  2 , and is also capable of arbitrarily controlling the difference in the angles of the two light beams in the polarization direction thereof. In addition to a polarization beam splitter  4  and a first wavelength-dependent wavelength plate  3  that is capable of rotating about the optical axis, the optical level control device  7  is also provided with a third wavelength-dependent wavelength plate  8  that has a rotation mechanism and is provided to a stage subsequent to the polarization beam splitter  4 . The third wavelength-dependent wavelength plate  8  has the same function as the first wavelength-dependent wavelength plate  3 . The wavelength plate  8  functions as a half-wave plate with respect to a first light wave and as a full-wave plate with respect to a second light wave among two light waves having different wavelengths. Specifically, this structure additionally includes a rotatable, wavelength-dependent wavelength plate  8  that is provided to a latter stage in the optical axis direction of the optical level control device  1  according to the first embodiment shown in  FIG. 1 . The angular difference between the polarization directions of two light beams outputted from the optical level control device  7  according to the present embodiment can therefore be arbitrarily set by rotating the third wavelength-dependent wavelength plate  8 , whereas the two light beams outputted from the optical level control device  1  according to the first embodiment are linearly polarized light beams that are polarized in the same direction as the light beams that are rectilinearly transmitted as P waves through the polarization beam splitter  4 . Therefore, utilizing the optical level control device  7  according to the present embodiment in a laser machining device enables more effective laser machining to be performed when the machining characteristics of the workpiece are both wavelength-dependent and polarization-dependent. 
     A fourth embodiment of the present invention will next be described.  FIG. 7  is a schematic perspective view showing the structure of the optical level control device  9  according to the present embodiment. The same reference symbols are used in  FIG. 7  for structural components that are the same as those in  FIGS. 1 ,  5 , and  6 , and detailed descriptions of those components are omitted. The optical level control device  9  according to the present embodiment is capable of independently controlling the intensities of two beams without reducing the dynamic range of the control that include a fundamental wave and a second higher harmonic outputted from a laser oscillator  2 , even when the fundamental wave is elliptically polarized light. The optical level control device  9  is also capable of arbitrarily controlling the angular difference between the polarization directions of the two light beams outputted from the optical level control device. The structure of the optical level control device  9  additionally includes a rotatable, wavelength-dependent wavelength plate  8  that is provided to a latter stage in the optical axis direction of the optical level control device  5  according to the second embodiment shown in  FIG. 5 . The angular difference between the polarization directions of two light beams outputted from the optical level control device  9  according to the present embodiment can therefore be arbitrarily set by rotating the third wavelength-dependent wavelength plate  8 , whereas the two light beams outputted from the optical level control device  5  according to the second embodiment are linearly polarized light beams that are polarized in the same direction. Therefore, in the same manner as in the third embodiment, utilizing the optical level control device  9  according to the present embodiment in a laser machining device enables more effective laser machining to be performed. 
     A fifth embodiment of the present invention will next be described.  FIG. 8  includes structural block views showing a laser application device that utilizes the optical level control device according to the present invention, wherein the optical system of the laser machining device is shown in particular. The same reference symbols are used for structural components that are the same as those in  FIGS. 1 and 5  through  7 , and detailed descriptions of those components are omitted. As shown in  FIG. 8A , the optical system  10  of a laser machining device according to the present embodiment comprises a laser oscillator  11  for outputting a fundamental wave and a second higher harmonic in the same optical axis, the optical level control device  100  as described in the first through fourth embodiments of the present invention, and a condensing optical system  30 . One or both parameters selected from the level and polarization direction of the fundamental wave and second higher harmonic are controlled by the optical level control device  100 , and a work object  40  is irradiated. 
     As shown in  FIG. 8B , the optical system  20  of a laser machining device according to the present embodiment may comprise a laser oscillator  21  for outputting a beam having a first wavelength, a laser oscillator  22  for outputting a beam having a second wavelength, an optical coupler  23  for coupling the two beams, the optical level control device  100  as described in the first through fourth embodiments of the present invention, and a condensing optical system  30 . One or both parameters selected from the level and polarization direction of the first-wavelength light are controlled by the optical level control device  100 , and the light is radiated to a work object  40 . 
     Utilizing the optical level control device  100  according to the present invention in a laser machining device enables more effective laser machining to be performed even when the machining characteristics of the work object  40  are wavelength-dependent as well as polarization-dependent. 
     The level and polarization direction of a larger number of laser beams can also be controlled by providing a plurality of units in which the laser oscillator  11  and optical level control device  100  shown in  FIG. 8A  are combined, and using an optical coupler to couple the light outputted by the optical level control devices. The same can be achieved by providing a plurality of units in which the laser oscillator  21 , an optical coupler  23 , and optical level control device  100  shown in  FIG. 8B  are combined, and using an optical coupler to couple the light outputted by the optical level control devices. 
     The laser application device that utilizes the optical level control device according to the present invention was described using as an example the optical system of a laser machining device in  FIGS. 8A and 8B , but this type of optical system can also be applied in laser microscopes and other laser observation devices.