Patent Publication Number: US-2023141009-A1

Title: Laser power monitoring device and laser power monitoring method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a laser power monitoring device and a laser power monitoring method for monitoring a power of a laser beam that is emitted from a laser beam source and with which a material to be processed is irradiated. 
     BACKGROUND ART 
     There exists a laser processing machine that bends, with a bend mirror, a laser beam emitted from a laser beam source and irradiates a material to be processed with the laser beam to weld or cut the material to be processed. Patent Literatures 1 and 2 describe monitoring of a power of a laser beam by detecting, with a sensor, a slight laser beam that has passed through a bend mirror. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2006-247681 
         Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2012-179627 
       
    
     SUMMARY 
     In the laser power monitoring device as described in Patent Literature 1 and 2, a dichroic mirror that reflects a laser beam of a specific wavelength may be used as the bend mirror. The dichroic mirror is arranged at an angle of 45 degrees with respect to an optical axis of the laser beam. A dielectric multilayer film is formed on a reflecting surface of the dichroic mirror, and the dielectric multilayer film reflects the laser beam of a specific wavelength. 
     The laser beam includes a P wave and an S wave as polarization components. The polarization ratio, which is the ratio of the amount of the P wave to the amount of the S wave, changes due to the temperature environment and the like of an oscillator included in the laser beam source. When the polarization ratio of the laser beam changes, the reflectance, transmittance, or reflection band of the laser beam made incident at 45 degrees in the dielectric multilayer film changes. Therefore, the power of the laser beam that passes through the dichroic mirror and is made incident on the sensor changes. In other words, in the laser power monitoring device, which is used for a laser processing machine using the dichroic mirror as the bend mirror and is configured to detect, with the sensor, the laser beam that has passed through the dichroic mirror, it is not possible to monitor the power of the laser beam in a stable manner because the polarization ratio of the P wave to the S wave of the laser beam changes. 
     An object of one or more embodiments is to provide a laser power monitoring device and a laser power monitoring method capable of, even if the polarization ratio of the P wave to the S wave of a laser beam changes, stably monitoring a power of the laser beam with reduced influence of the change. 
     According to a first aspect of the one or more embodiments, a laser power monitoring device is provided, which includes a right angle reflection prism including first and second surfaces that form a right angle, and a third surface connected to the first and second surfaces, a laser beam for irradiation of a material to be processed being incident on the first surface, and being totally reflected by the third surface so as to be emitted from the second surface, and a sensor configured to detect a power of a laser beam that has passed through the third surface. 
     According to a second aspect of the one or more embodiments, a laser power monitoring method is provided, which includes causing a laser beam for irradiation of a material to be processed to be made incident on a first surface of a right angle reflection prism including the first surface and a second surface that form a right angle, and a third surface connected to the first and second surfaces, causing the laser beam made incident on the first surface to be totally reflected by the third surface so as to be emitted from the second surface, and detecting, by a sensor, a power of a laser beam that has passed through the third surface. 
     According to the laser power monitoring device and the laser power monitoring method of the one or more embodiments, even if the polarization ratio of the P wave to the S wave of the laser beam changes, it is possible to stably monitor the power of the laser beam with reduced influence of the change. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram showing a laser power monitoring device and a laser power monitoring method according to a first embodiment. 
         FIG.  2 A  is a diagram showing a detected voltage value obtained by detecting, with a sensor included in a laser beam source, a power of a laser beam emitted by the laser beam source. 
         FIG.  2 B  is a diagram showing a detected voltage value obtained by detecting, with a sensor included in the laser power monitoring device of the first embodiment, a power of a laser beam for irradiation of a material to be processed. 
         FIG.  3    is a diagram showing a laser power monitoring device and a laser power monitoring method according to a second embodiment. 
         FIG.  4    is a diagram showing detected voltage values obtained by respectively detecting, with two sensors included in the laser power monitoring device of the second embodiment, powers of laser beams emitted by a laser beam source. 
         FIG.  5    is a diagram showing a laser power monitoring device and a laser power monitoring method according to a third embodiment. 
         FIG.  6    is a diagram showing a laser power monitoring device and a laser power monitoring method according to a fourth embodiment. 
         FIG.  7    is a diagram showing a laser power monitoring device of a comparative example. 
         FIG.  8    is a diagram showing an example of measurement results obtained by measuring powers of P waves and S waves of a laser beam made incident on a dichroic mirror shown in  FIG.  7    for a predetermined measurement time. 
         FIG.  9    is a diagram showing a detected voltage value obtained by detecting, with a sensor included in a laser beam source shown in  FIG.  7   , a power of a laser beam emitted from a laser beam source, and a detected voltage value obtained by detecting, with a sensor included in the laser power monitoring device of the comparative example, a power of a laser beam for irradiation of a material to be processed. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a laser power monitoring device and a laser power monitoring method of respective embodiments will be described with reference to the attached drawings. First, the problems of a laser power monitoring device of a comparative example shown in  FIG.  7    will be described in detail. 
     In  FIG.  7   , a laser beam source  1  emits a divergent laser beam. The laser beam source  1  is provided with a sensor  1 S for detecting a power of the emitted laser beam. A collimating lens  2  converts the divergent laser beam made incident thereon into a collimated beam and emits the collimated beam. A dichroic mirror  3  is arranged in a state of being tilted by 45 degrees with respect to an optical axis of the collimated beam. A dielectric multilayer film is formed on an incident surface (a reflecting surface)  3   a  of the laser beam of the dichroic mirror  3 . The laser beam made incident on the incident surface  3   a  of the dichroic mirror  3  is reflected by the incident surface  3   a  and is made incident on a focusing lens  4 . The focusing lens  4  focuses the incident laser beam and irradiates a material to be processed W, which is a sheet metal, with the focused laser beam. 
     The laser beam source  1 , the collimating lens  2 , the dichroic mirror  3 , and the focusing lens  4  constitute a laser processing machine that irradiates the material to be processed W with the laser beam to weld or cut the material to be processed W. 
     Of the laser beam made incident on the dichroic mirror  3 , a slight laser beam indicated by the alternate long and short dash line passes through the dichroic mirror  3  and is made incident on a sensor  5 . The sensor  5  generates a voltage value corresponding to a power of the incident laser beam. By monitoring the voltage value generated by the sensor  5 , it is possible to monitor the power of the laser beam with which the laser processing machine irradiates the material to be processed W. 
       FIG.  8    shows an example of measurement results obtained by measuring powers of P waves and S waves of the laser beam made incident on the dichroic mirror  3  for a predetermined measurement time (here, 3600 seconds).  FIG.  8    shows the results in which the laser beam, when a laser output of the laser beam source  1  is changed to 700 W, 400 W, and 100 W, is split into the P wave and the S wave with a polarized beam splitter and then the powers of the P wave and the S wave are measured for a predetermined time. In  FIG.  8   , the solid line indicates the power of the P wave and the broken line indicates the power of the S wave. From  FIG.  8   , it can be seen that the polarization ratio between the P wave and the S wave fluctuates greatly at 700 W and 400 W. Note that when the laser beam is made incident on the dielectric multilayer film at 45 degrees, the power of the P wave detected by the sensor  5  is higher than the power of the S wave because the P wave has a lower reflectance than the S wave. 
       FIG.  9    shows voltage values (detected voltage values) generated by the sensor  1 S and the sensor  5 , respectively, for a predetermined measurement time (3600 seconds) when the laser output of the laser beam source  1  is 700 W. As shown in  FIG.  9   , the voltage value generated by the sensor  1 S is substantially constant. In other words, the power of the laser beam emitted by the laser beam source  1  is controlled to be substantially constant. 
     However, the voltage value generated by the sensor  5  fluctuates greatly. This is because the polarization ratio of the P wave to the S wave fluctuates greatly as described above. As shown in  FIG.  8   , since the power of the P wave is unstable, the unstable P wave exercises a great influence on the voltage value generated by the sensor  5 . In the example shown in  FIG.  9   , the voltage value generated by the sensor  5  fluctuates by about 8%. Therefore, it can be seen that if the laser power is monitored by the sensor  5 , which detects the power of the laser beam that has passed through the dichroic mirror  3 , stable monitoring is not possible due to the fluctuation of the polarization ratio. 
     First to fourth embodiments described below reduce the above problems and realize stable monitoring of the power of the laser beam even if the polarization ratio of the P wave to the S wave fluctuates. 
     First Embodiment 
       FIG.  1    shows a laser power monitoring device and a laser power monitoring method according to the first embodiment. The first embodiment shows a basic configuration for stably monitoring a power of a laser beam, which is common to the first to fourth embodiments. 
     In  FIG.  1   , a laser beam source  11  emits a divergent laser beam. The laser beam source  11  is provided with a sensor  11 S for detecting a power of the emitted laser beam. The laser beam source  11  is, for example, a fiber laser oscillator. The laser beam source  11  may be any laser oscillator. A collimating lens  12  converts the divergent laser beam made incident thereon into a collimated beam and emits the collimated beam. The laser beam of the collimated beam is made incident on a right angle reflection prism  13 . 
     Anti-reflection coating (AR coating) is applied to two surfaces  13   a  and  13   b  (first and second surfaces) of the right angle reflection prism  13 , the surfaces  13   a  and  13   b  forming a right angle. The anti-reflection coating is set to have properties corresponding to the operating frequency band of the laser beam so that reflection of the laser beam within the operating frequency band is prevented. The anti-reflection coating is not applied to a surface  13   c  (a third surface) of the right angle reflection prism  13  connected to the surfaces  13   a  and  13   b.    
     The laser beam emitted from the collimating lens  12  is made incident on the surface  13   a  of the right angle reflection prism  13  and reaches the surface  13   c . If the laser beam is made incident on the surface  13   c  at an angle larger than the critical angle, the laser beam is totally reflected by the surface  13   c  and is emitted from the surface  13   b . Note that when the right angle reflection prism  13  is made of synthetic quartz having a refractive index of 1.458, it is possible to calculate the critical angle θr to be about 43.3 degrees from sin θr=1/1.458. If the incident angle, which is formed by the direction orthogonal to the surface  13   c  and the incident direction of the laser beam, is larger than the critical angle θr, the laser beam is totally reflected. 
     The laser beam emitted from the surface  13   b  of the right angle reflection prism  13  is made incident on a focusing lens  14 . The focusing lens  14  focuses the incident laser beam and irradiates the material to be processed W with the focused laser beam. 
     The laser beam source  11 , the collimating lens  12 , the right angle reflection prism  13 , and the focusing lens  14  constitute a laser processing machine that irradiates the material to be processed W with the laser beam to weld or cut the material to be processed W. 
     Of the laser beam made incident on the surface  13   c  of the right angle reflection prism  13 , a slight laser beam (a leaked beam) indicated by the alternate long and short dash line passes through the surface  13   c  and is made incident on a sensor  15 . The sensor  15  generates a voltage value corresponding to a power of the incident laser beam. One example of the sensor  15  is a photodiode. The photodiode may be a PIN photodiode. The sensor  15  may be any sensor that detects the power of the incident laser beam, and is not limited to the photodiode. By monitoring the voltage value generated by the sensor  15 , it is possible to monitor the power of the laser beam with which the laser processing machine irradiates the material to be processed W. 
       FIG.  2 A  shows a voltage value (a detected voltage value) generated by the sensor  11 S for a predetermined measurement time (here, 3600 seconds) when the laser output of the laser beam source  11  is 700 W.  FIG.  2 B  shows a voltage value (a detected voltage value) generated by the sensor  15  for the measurement time. As shown in  FIG.  2 A , the voltage value generated by the sensor  11 S is substantially constant. In other words, the power of the laser beam emitted by the laser beam source  11  is controlled to be substantially constant. 
     As shown in  FIG.  2 B , the voltage value generated by the sensor  15  hardly fluctuates and is substantially constant. In the example shown in  FIG.  2 B , the voltage value generated by the sensor  15  fluctuates only by about 1.86%. This is because, in the laser power monitoring device of the first embodiment, the sensor  15  does not detect the power of the laser beam that has passed through the dichroic mirror  3 , but detects the power of the laser beam that has passed through the surface  13   c  that totally reflects the laser beam of the right angle reflection prism  13 . 
     Since the right angle reflection prism  13  does not include the dielectric multilayer film on the surface  13 C, even if the polarization ratio of the P wave to the S wave of the laser beam changes, there is no change in the reflectance, transmittance, or reflection band of the laser beam, which is caused by the presence of the dielectric multilayer film. Therefore, according to the laser power monitoring device and the laser power monitoring method of the first embodiment, even if the polarization ratio of the P wave to the S wave of the laser beam changes, it is possible to reduce (or eliminate) the influence, and stable monitoring of the laser power is made possible. 
     Second Embodiment 
     A laser power monitoring device of the second embodiment shown in  FIG.  3    is provided with two right angle reflection prisms. A laser power monitoring method of the second embodiment detects powers of laser beams that have passed through the surfaces totally reflecting the laser beam in the two right angle reflection prisms, respectively. In  FIG.  3   , the same parts as those in  FIG.  1    are denoted by the same reference numerals, and the description thereof will be omitted. 
     In  FIG.  3   , the laser beam emitted from the collimating lens  12  is made incident on a right angle reflection prism  131  (a first right angle reflection prism). Surfaces  131   a ,  131   b , and  131   c  of the right angle reflection prism  131  correspond to the surfaces  13   a ,  13   b , and  13   c  of the right angle reflection prism  13  of  FIG.  1   , respectively. The laser beam, which is totally reflected by the surface  131   c  of the right angle reflection prism  131  and is emitted from the surface  131   b , is made incident on a right angle reflection prism  132  (a second right angle reflection prism). 
     Of the laser beams made incident on the surface  131   c  of the right angle reflection prism  131 , a slight laser beam indicated by the alternate long and short dash line passes through the surface  131   c  and is made incident on a sensor  151  (a first sensor). The sensor  151  generates a voltage value corresponding to a power of the incident laser beam. 
     Surfaces  132   a ,  132   b , and  132   c  of the right angle reflection prism  132  correspond to the surfaces  13   a ,  13   b , and  13   c  of the right angle reflection prism  13  of FIG.  1 , respectively. The laser beam, which is totally reflected by the surface  132   c  of the right angle reflection prism  132  and is emitted from the surface  132   b , is made incident on the focusing lens  14 . The focusing lens  14  focuses the incident laser beam and irradiates the material to be processed W, which is not shown in  FIG.  3   , with the focused laser beam. 
     Of the laser beams made incident on the surface  132   c  of the right angle reflection prism  132 , a slight laser beam indicated by the alternate long and short dash line passes through the surface  132   c  and is made incident on a sensor  152  (a second sensor). The sensor  152  generates a voltage value corresponding to a power of the incident laser beam. 
     The laser beam source  11 , the collimating lens  12 , the right angle reflection prism  131 , the right angle reflection prism  132 , and the focusing lens  14  constitute a laser processing machine that irradiates the material to be processed W with the laser beam to weld or cut the material to be processed W. 
     According to the laser power monitoring device and the laser power monitoring method of the second embodiment, in addition to the same effect as that of the first embodiment, it is possible to monitor, with the two sensors  151  and  152 , the power of the laser beam with which the laser processing machine irradiates the material to be processed W. 
       FIG.  4    shows voltage values (detected voltage values) generated by the sensors  151  and  152 , respectively. The voltage value generated by the sensor  151  fluctuates by about 1.5%, and the fluctuation of the voltage value generated by the sensor  152  can also be maintained to the same degree. It is also possible to configure the laser power monitoring device to include three or more right angle reflection prisms. 
     Third Embodiment 
     A laser power monitoring device and a laser power monitoring method of the third embodiment shown in  FIG.  5    are configured to split a laser beam into a P wave and an S wave, convert the P wave and the S wave into circularly polarized beams to be totally reflected by two right angle reflection prisms, and detect powers of the circularly polarized beams with two sensors. In  FIG.  5   , the same parts as those in  FIG.  1    are denoted by the same reference numerals, and the description thereof will be omitted. 
     In  FIG.  5   , the laser beam emitted from the laser beam source  11  includes a linearly polarized P wave and a linearly polarized S wave. The laser beam emitted from the laser beam source  11  is made incident on a polarized beam splitter  21  (a first polarized beam splitter). The polarized beam splitter  21  is configured by joining a prism  211  and a prism  212  at a joint surface  213 . The joint surface  213  is provided with a dielectric multilayer film that transmits the P wave and reflects the S wave. The P wave passes through the joint surface  213  and is made incident on a ¼ wavelength plate  22  (a first ¼ wavelength plate). The S wave is reflected by a surface  214  of the prism  212  and made incident on a ¼ wavelength plate  23  (a second ¼ wavelength plate). 
     The ¼ wavelength plate  22  converts the linearly polarized P wave made incident thereon into a first circularly polarized beam. The ¼ wavelength plate  23  converts the linearly polarized S wave made incident thereon into a second circularly polarized beam. The first circularly polarized beam is made incident on a right angle reflection prism  133  (a first right angle reflection prism). The second circularly polarized beam is made incident on a right angle reflection prism  134  (a second right angle reflection prism). Surfaces  133   a ,  133   b , and  133   c  of the right angle reflection prism  133  correspond to the surfaces  13   a ,  13   b , and  13   c  of the right angle reflection prism  13  of  FIG.  1   , respectively. Surfaces  134   a ,  134   b , and  134   c  of the right angle reflection prism  134  correspond to the surfaces  13   a ,  13   b , and  13   c  of the right angle reflection prism  13  of  FIG.  1   , respectively. 
     The first circularly polarized beam made incident on the surface  133   a  of the right angle reflection prism  133  is totally reflected by the surface  133   c , is emitted from the surface  133   b , and is made incident on a ¼ wavelength plate  24 . The second circularly polarized beam made incident on the surface  134   a  of the right angle reflection prism  134  is totally reflected by the surface  134   c , is emitted from the surface  134   b , and is made incident on a ¼ wavelength plate  25 . 
     The ¼ wavelength plate  24  converts the first circularly polarized beam made incident thereon into a linearly polarized S wave. The ¼ wavelength plate  25  converts the second circularly polarized beam made incident thereon into a linearly polarized P wave. The linearly polarized P wave and the linearly polarized S wave are made incident on a polarized beam splitter  26 . The polarized beam splitter  26  is configured by joining a prism  261  and a prism  262  at a joint surface  263 . The joint surface  263  is provided with a dielectric multilayer film that transmits the P wave and reflects the S wave. The P wave passes through the joint surface  263 , and the S wave is reflected by a surface  264  of the prism  262  and the joint surface  263 . As a result, the linearly polarized P wave and the linearly polarized S wave are combined to be emitted from the polarized beam splitter  26 . 
     The laser beam including the linearly polarized P wave and the linearly polarized S wave emitted from the polarized beam splitter  26  is made incident on the focusing lens  14 . The focusing lens  14  focuses the incident laser beam and irradiates the material to be processed W with the focused laser beam. 
     The laser beam source  11 , the collimating lens  12 , the polarized beam splitter  21 , the ¼ wavelength plates  22  and  23 , the right angle reflection prisms  133  and  134 , the ¼ wavelength plates  24  and  25 , the polarized beam splitter  26 , and the focusing lens  14  constitute a laser processing machine that irradiates the material to be processed W with the laser beam to weld or cut the material to be processed W. 
     Of the first circularly polarized beam made incident on the surface  133   c  of the right angle reflection prism  133 , a slight circularly polarized beam indicated by the alternate long and short dash line passes through the surface  133   c  and is made incident on a sensor  153 . The sensor  153  generates a voltage value corresponding to a power of the incident circularly polarized beam. Of the second circularly polarized beam made incident on the surface  134   c  of the right angle reflection prism  134 , a slight circularly polarized beam indicated by the alternate long and short dash line passes through the surface  134   c  and is made incident on a sensor  154 . The sensor  154  generates a voltage value corresponding to a power of the incident circularly polarized beam. 
     In the laser power monitoring device and the laser power monitoring method of the third embodiment, the linearly polarized P wave and the linearly polarized S wave are converted into the circularly polarized beams, and the circularly polarized beams are totally reflected by the right angle reflection prisms  133  and  134 . Compared with the case in which the linearly polarized P wave and the linearly polarized S wave are totally reflected by the right angle reflection prisms  133  and  134 , in the case in which the circularly polarized beams are totally reflected by the right angle reflection prisms  133  and  134 , a gap, which is caused by a difference in the polarization direction at the time of totally reflecting the polarization components, is less likely to be generated. 
     In the third embodiment, the polarization ratio may change slightly when splitting into the P wave and the S wave is performed by the polarized beam splitter  21 . However, since the circularly polarized beams are totally reflected by the right angle reflection prisms  133  and  134 , the change in the polarization ratio is not so problematic as compared with the configuration in which the circularly polarized beam is reflected by the dichroic mirror. 
     In the third embodiment, when the polarization ratio of the P wave to the S wave changes, the voltage values generated by the sensors  153  and  154  fluctuate. Therefore, a value obtained by averaging the voltage values of the sensors  153  and  154  may be monitored. 
     Fourth Embodiment 
     A laser power monitoring device and a laser power monitoring method of the fourth embodiment shown in  FIG.  6    are configured to split a laser beam into a P wave and an S wave, convert the P wave and the S wave into circularly polarized beams to be totally reflected by two right angle reflection prisms, return the circularly polarized beams to a linearly polarized beam, and detect a combined power of the P wave and the S wave with one sensor. In  FIG.  6   , the same parts as those in  FIG.  5    are denoted by the same reference numerals, and the description thereof will be omitted. 
     In  FIG.  6   , of the first circularly polarized beam, a slight circularly polarized beam indicated by the alternate long and short dash line that has passed through the surface  133   c  of the right angle reflection prism  133  is made incident on a ¼ wavelength plate  27  (a third ¼ wavelength plate). The ¼ wavelength plate  27  converts the circularly polarized beam made incident thereon into a linearly polarized S wave. The linearly polarized S wave is made incident on a polarized beam splitter  29  (a second polarized beam splitter). Of the second circularly polarized beam, a slight circularly polarized beam indicated by the alternate long and short dash line that has passed through the surface  134   c  of the right angle reflection prism  134  is made incident on a ¼ wavelength plate  28  (a fourth ¼ wavelength plate). The ¼ wavelength plate  28  converts the circularly polarized beam made incident thereon into a linearly polarized P wave. The linearly polarized P wave is made incident on the polarized beam splitter  29 . 
     The polarized beam splitter  29  is configured by joining a prism  291  and a prism  292  at a joint surface  293 . The joint surface  293  is provided with a dielectric multilayer film that transmits the P wave and reflects the S wave. The P wave passes through the joint surface  293 , and the S wave is reflected by a surface  294  of the prism  292  and the joint surface  293 . As a result, the linearly polarized P wave and the linearly polarized S wave are combined and emitted from the polarized beam splitter  29 . The laser beam obtained by combining the linearly polarized P wave and the linearly polarized S wave emitted from the polarized beam splitter  29  is made incident on a sensor  155 . The sensor  155  generates a voltage value corresponding to a power of the incident laser beam. 
     In the same manner as the third embodiment, in the fourth embodiment as well, the polarization ratio may change slightly when splitting into the P wave and the S wave is performed by the polarized beam splitter  21 . However, in the laser power monitoring device and the laser power monitoring method of the fourth embodiment, the linearly polarized P wave and the linearly polarized S wave are combined by the polarized beam splitter  29 , and the power of the combined laser beam is detected by the sensor  155 . Therefore, even if the polarization ratio of the P wave to the S wave changes, the power of the combined laser beam will not change. As a result, it is possible to detect the power of the laser beam accurately. 
     In the fourth embodiment, it is possible to detect the power of the laser beam with the one sensor  155 . 
     The present invention is not limited to the first to fourth embodiments described above, and various modifications can be made without departing from the summary of the present invention. Any method can be used for specifically monitoring the powers of the laser beams detected by the sensors  15 ,  151  to  155 . 
     The present application claims the priority based on Japanese Patent Application No. 2020-055949 filed with the Japan Patent Office on Mar. 26, 2020, and all the disclosure contents thereof are incorporated herein by reference.