Patent Publication Number: US-2023137623-A1

Title: Laser processing monitoring device, laser processing monitoring method, and laser processing device

Description:
TECHNICAL FIELD 
     The present invention relates to a laser processing monitoring device, a laser processing monitoring method, and a laser processing device. 
     BACKGROUND ART 
     Conventionally, in a laser processing device, a technology for determining the quality of laser processing has been used. Specifically, a laser processing device has a photodetector or an optical sensor incorporated into a processing head that irradiates workpiece with a laser beam. A beam to be measured which is generated or reflected near a processing point of the workpiece is received by the optical sensor through an optical system in the processing head. The laser processing device performs a predetermined signal process for an electric signal (sensor output signal) obtained by photoelectric conversion of the optical sensor, so that the quality of the laser processing is determined. 
     In this kind of laser processing monitoring technology, not only hardware and software are simpler compared to a technique of image analysis of a processing situation near the processing point using an imaging device, but also qualitative improvement of monitoring is achieved depending on ingenuity of a signal process technology. For example, in laser welding, it is also possible to precisely monitor or analyze subtle behavior and change of a weld part. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open Publication No. 2007-30032 
       
    
     SUMMARY 
     However, while the present inventors promote the research and development of the laser processing monitoring technology as described above, an error in a photoelectric conversion characteristic of the optical sensor becomes a major hindrance in improvement of monitoring performance. That is, the photoelectric conversion characteristic of a photodiode used in the optical sensor inevitably changes over time and, furthermore, is also influenced by an environmental condition such as an ambient temperature. When the photoelectric conversion characteristic of the photodiode changes, even when a beam to be measured having the same light intensity is received from the workpiece side, a value of a sensor output signal obtained after photoelectric conversion of the beam. Therefore, even when the performance of a digital signal process technology is made higher, it is not possible to monitor and analyze laser processing precisely, and it is not possible to determine the quality accurately. 
     In order to cope with this problem, the present inventors use the following calibration method. First, at the time of shipment or setting of the laser processing device, as part of initialization, a reference workpiece sample is irradiated with a laser beam with reference power. Then, a waveform of a sensor output signal obtained from the optical sensor in the processing head is acquired as a reference waveform SW such as a waveform illustrated in  FIG.  18 A . 
     As illustrated in  FIG.  18 B , upper and lower limit value envelopes JW+δ and JW−δ that are offset up and down by a certain allowable range (±δ) from the value of the reference waveform SW in the whole interval (or partial interval) of the reference waveform SW are set. The allowable range ±δ illustrated in the figure is enlarged for ease of illustration, but in practice, the envelope setting is performed in an infinitely small range to enhance calibration accuracy. 
     When the optical sensor is calibrated, a waveform RW of a sensor output signal obtained by irradiating the same workpiece sample with a laser beam with the same reference power is displayed on a monitor screen (maintenance screen) together with the upper and lower limit value envelopes JW+δ and JW−δ. Then, the waveform RW of the sensor output signal protrudes outside the upper and lower limit value envelopes JW+δ and JW−δ, for example, as illustrated in  FIG.  18 C , the on-site person operates screen input or the like to adjust the gain of sensor output such that the waveform RW falls within the inside of the upper and lower limit value envelopes JW+δ and JW−δ (allowable range). 
     However, it is found that the above optical sensor calibration method is not a valid solution. That is, a laser oscillation unit and the workpiece sample involve a radiation source of a test beam received by the optical sensor during calibration, and a laser optical system is interposed in an optical path of the test beam. Therefore, an error between the reference waveform SW in initial acquisition and the waveform RW of the sensor output signal in current acquisition includes not only the variation of the photoelectric conversion characteristic of the optical sensor, but also the variation of the optical characteristic or the physical characteristic of an involving element or an interposing element thereof. Therefore, it is not possible to perform calibration focused on the photoelectric conversion characteristic of the optical sensor. Furthermore, there is no guarantee that the light intensity of the test beam for calibration is always constant. Therefore, it is not possible to correct the error of the photoelectric conversion characteristic of the optical sensor correctly. In addition, in order to involve the laser oscillation unit and the workpiece sample in the calibration of the optical sensor, calibration work is troublesome and large-scale. 
     Furthermore, the optical sensor calibration method described above is a method for performing calibration based on relative comparison between the reference waveform SW unique to a device and the waveform RW of the sensor output signal in current acquisition, and a calibration reference value of the optical sensor varies per device. Therefore, a difference in monitoring performance and accuracy between an actually indicated value and a value to be originally indicated causes variation as the individual difference of the laser processing device of the same model using the optical sensor of the same product. That is, the instrument error between laser processing devices of the same model varies among the same model, and therefore it is not possible to perform the same quality evaluation under the setting of the same processing condition. 
     A laser processing monitoring device of an aspect of the present invention is a laser processing monitoring device for photoelectrically converting a predetermined beam to be measured by an optical sensor disposed in a processing head or in proximity to the processing head to acquire a sensor output signal representing light intensity of the beam to be measured, and monitoring the laser processing based on the sensor output signal, during irradiation to a workpiece with a laser beam for laser processing by the processing head, the predetermined beam to be measured being generated or reflected in a vicinity of a processing point of the workpiece, the laser processing monitoring device including: a reference beam source provided in the processing head, and configured to generate a reference beam for calibrating the optical sensor; a reference beam source power supply unit configured to supply, to the reference beam source, adjustable power for generating the reference beam; and an optical measuring instrument having a beam receiver for receiving the reference beam from the reference beam source in order to calibrate the reference beam source, and configured to measure light intensity of the received reference beam or a predetermined physical quantity equivalent to the light intensity. 
     In the laser processing monitoring device of the aspect of the present invention, the optical sensor for monitoring laser processing is calibrated using the reference beam source built in the processing head, and this reference beam source is calibrated through the optical measuring instrument built in the device, and therefore even when the electrical-to-optical conversion characteristic of the reference beam source changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. Additionally, even when the photoelectric conversion characteristic of the optical sensor changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. 
     A laser processing monitoring method of an aspect of the present invention is a laser processing monitoring method for photoelectrically converting a beam to be measured by an optical sensor disposed in a processing head or in proximity to the processing head to acquire a sensor output signal representing light intensity of the beam to be measured, and monitoring the laser processing based on the sensor output signal, during irradiation to a workpiece with a laser beam for laser processing by the processing head, the beam to be measured being generated or reflected in a vicinity of a processing point of the workpiece, the laser processing monitoring method including: providing the optical sensor in a sensor unit built into the processing head, or disposed in proximity to the processing head; mounting, on the sensor unit, a reference beam source that generates a reference beam for calibrating the optical sensor; setting a first optical path optically connecting the processing point of the workpiece and the optical sensor inside the sensor unit when the laser processing is performed; setting a second optical path optically connecting the reference beam source and the optical sensor inside the sensor unit when the optical sensor is calibrated; and causing the reference beam emitted from the reference beam source to be incident on a beam receiver of an optical measuring instrument in order to calibrate the reference beam source, and adjusting output of the reference beam source such that a measured value of the optical measuring instrument coincides with a reference value. 
     In the laser processing monitoring method of the aspect of the present invention, when laser processing is monitored, the first optical path is set in the sensor unit and the beam to be measured is made incident on the optical sensor from the workpiece side. Additionally, when the optical sensor is calibrated, the second optical path is set in the sensor unit, and the reference beam from the reference beam source is made incident on the optical sensor. Then, when the reference beam source is calibrated, the reference beam from the reference beam source is made incident on the beam receiver of the optical measuring instrument. Consequently, even when the electrical-to-optical conversion characteristic of the reference beam source changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. Additionally, even when the photoelectric conversion characteristic of the optical sensor changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. 
     A laser processing device of an aspect of the present invention includes: a laser oscillation unit configured to oscillate and output a laser beam for laser processing; a processing head optically connected to the laser oscillation unit via an optical fiber cable, and configured to focally irradiate a processing point of a workpiece with the laser beam from the laser oscillation unit; and a laser processing monitor unit configured to monitor laser processing, wherein the laser processing monitor unit has: an optical sensor disposed in the processing head or in proximity to the processing head, and configured to output a sensor output signal representing light intensity of a predetermined beam to be measured, the predetermined beam to be measured being generated or reflected in a vicinity of the processing point of the workpiece; a sensor signal processing unit configured to generate digital waveform data for the sensor output signal from the optical sensor, and display and output a waveform of the sensor output signal based on the waveform data; a reference beam source configured to generate a reference beam for calibrating the optical sensor; a reference beam source power supply unit configured to supply, to the reference beam source, adjustable power for generating the reference beam; and an optical measuring instrument having a beam receiver for receiving the reference beam from the reference beam source in order to calibrate the reference beam source, and configured to measure light intensity of the received reference beam or a predetermined physical quantity equivalent to the light intensity. 
     In the laser processing device of the aspect of the present invention, the optical sensor of the laser processing monitor unit is calibrated using the reference beam source built in the device, and this reference beam source is calibrated through the optical measuring instrument built in the device, and therefore even when the electrical-to-optical conversion characteristic of the reference beam source changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. Additionally, even when the photoelectric conversion characteristic of the optical sensor changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. The monitoring performance of the laser processing monitor unit is thus improved, so that it is possible to perform qualified quality determination for laser processing. 
     According to the laser processing monitoring device or the laser processing monitoring method of the aspect of the present invention, with the above configuration and operation, it is possible to improve the accuracy, reproducibility and workability of calibration for the optical sensor used for monitoring laser processing, and improve monitoring performance. 
     According to the laser processing device of the aspect of the present invention, with the above configuration and operation, it is possible to perform qualified quality determination for laser processing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating an entire configuration of a laser processing device including a laser processing monitoring device according to an embodiment of the present invention. 
         FIG.  2    is a graph illustrating radiation spectrum distribution of black bodies. 
         FIG.  3    is a diagram illustrating spectrum distribution of infrared ray intensity emitted from a molten part by irradiating stainless steel with a pulse laser beam. 
         FIG.  4    is a diagram illustrating a state of Experiment 1 for verifying a monitoring function of the laser processing monitor unit according to the embodiment and a monitoring display waveform of a sensor output signal acquired by the experiment. 
         FIG.  5    is a diagram illustrating a state of Experiment 2 for verifying the above monitoring function and a monitoring display waveform of a sensor output signal acquired by the experiment. 
         FIG.  6    is a diagram illustrating a state of Experiment 3 for verifying the above monitoring function and a monitoring display waveform of a sensor output signal acquired by the experiment. 
         FIG.  7 A  is a sectional view illustrating a configuration example of the reference beam source according to the embodiment. 
         FIG.  7 B  is a sectional view illustrating another configuration example of the reference beam source according to the embodiment. 
         FIG.  8    is a diagram schematically illustrating a state in which a folding mirror of an optical path switching unit is switched to a second position in order to select a second optical path in a calibration unit of the embodiment. 
         FIG.  9    is a diagram schematically illustrating a state in which the folding mirror of the optical path switching unit is switched to a third position in order to select a third optical path in the above calibration unit. 
         FIG.  10    is a perspective view illustrating appearance of a sensor unit incorporating an optical path switching unit of a preferred configuration example. 
         FIG.  11    is a side view of the sensor unit in  FIG.  10   . 
         FIG.  12    is a perspective view of the optical path switching unit incorporated in the sensor unit of  FIG.  10   , the optical path switching unit being viewed from a certain angle. 
         FIG.  13    is a perspective view of the optical path switching unit viewed from another angle. 
         FIG.  14    is a side view of the optical path switching unit viewed from the reference beam source side. 
         FIG.  15    is a sectional view taken along the line A-A of  FIG.  14   . 
         FIG.  16 A  is a longitudinal sectional view of a principal part illustrating positional relation of each unit of the optical path switching unit in a first mode of the calibration unit. 
         FIG.  16 B  is a longitudinal sectional view of a principal part illustrating positional relation of each unit of the optical path switching unit in a second mode. 
         FIG.  16 C  is a transverse sectional view of a principal part illustrating positional relation of each unit of the optical path switching unit in a third mode. 
         FIG.  17    is a diagram illustrating a modification of the laser processing monitor unit according to the embodiment. 
         FIG.  18 A  is a diagram schematically illustrating a monitor screen in a first stage of an optical sensor calibration method of a conventional technology. 
         FIG.  18 B  is a diagram schematically illustrating a monitor screen in a second stage of an optical sensor calibration method of a conventional technology. 
         FIG.  18 C  is a diagram schematically illustrating a monitor screen in a third stage of an optical sensor calibration method of a conventional technology. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the attached drawings. 
     [Configuration of Entire Laser Processing Device] 
       FIG.  1    illustrates an entire configuration of a laser processing device including a laser processing monitoring device according to an embodiment of the present invention. This laser processing device is, for example, a laser processing machine that irradiates a workpiece W with a high output CW laser beam or a pulse laser beam and melts a processing point Q of the workpiece W with laser energy to perform desired laser melting processing. This laser processing device has a laser oscillator  10 , a laser power supply  12 , a control unit  14 , an optical fiber cable  16 , an electric cable  18 , a processing head  20 , an operation panel  22 , and a laser processing monitor unit (laser processing monitoring device of the embodiment)  24 . 
     In this laser processing device, the laser oscillator  10 , the laser power supply  12 , the control unit  14 , and the operation panel  22  are usually disposed in one place or close together to form a main body of the device. On the other hand, the processing head  20  is configured as a separate and independent unit from the main body of the device, and is disposed at any processing place within an area according to the length of the optical fiber cable  16 . 
     The laser processing monitor unit  24  includes the control unit  14 , the operation panel  22 , a sensor signal processing unit  26  and a sensor unit  30  as a basic configuration for a main function, that is, a monitoring function, and includes a reference beam source  100 , a reference beam source power supply  102 , an optical measuring instrument  104  and an optical path switching unit  105  as a calibration unit  32  for calibrating an optical sensor  50  incorporated in the sensor unit  30 . 
     The laser oscillator  10  is composed of, for example, a YAG laser, a fiber laser, or a semiconductor laser. For example, when laser spot welding is applied to the workpiece W, the laser oscillator  10  receives supply of excitation power from the laser power supply  12  under the control of the control unit  14  to excite an internal medium, and oscillates and outputs laser beams LB of pulses with wavelengths unique to the medium. The laser beams LB oscillated and output by the laser oscillator  10  are transmitted to the processing head  20  through the optical fiber cable  16 . 
     The processing head  20  has an emission unit  28  which is a head body, and a sensor unit  30  integrally or detachably connected to the emission unit  28  through a unit connection opening  45 . The emission unit  28  has a cylindrical housing. An upper end of the housing of the emission unit  28  is connected to the optical fiber cable  16  from the laser oscillator  10 , and a laser emission port of a lower end of the housing of the emission unit  28  is directed to the workpiece W directly under the laser emission port. In the housing of the emission unit  28 , a collimating lens  38 , a dichroic mirror  40 , a focusing lens  42  and a protective glass  44  are arranged in a vertical line from a top to a bottom, as the laser optical system. Herein, the protective glass  44  is mounted on the laser emission port. The dichroic mirror  40  is disposed so as to be inclined obliquely at 45° towards the unit connection opening  45 . The dichroic mirror  40  is coated with a dielectric multilayer film that transmits the laser beams LB from the optical fiber cable  16  and reflects a beam to be measured and a visible ray from the vicinity of the processing point Q of the workpiece W. 
     During laser processing, the laser beams LB propagating through the optical fiber cable  16  are emitted vertically downward at a constant flare angle from an end surface of the optical fiber cable  16  within the emission unit  28 . The laser beams LB pass through the collimating lens  38  to become parallel beams, pass through the dichroic mirror  40  to be focused through the focusing lens  42  and the protective glass  44 , and are incident on the processing point Q of the workpiece W. Then, laser energy of the laser beams LB melts and solidifies the vicinity of the processing point Q to form a weld nugget and further a weld joint. The weld joint is arbitrary, for example, a butt joint, a T-shaped joint, an L-shaped joint, a lap joint, or the like, and is selected by a user. 
     The sensor unit  30  also has an integral or assembled cylindrical housing. In the housing of the sensor unit  30 , the optical sensor  50  is provided in an upper end of the housing, a folding mirror  46 , a dichroic mirror  58  and a condenser lens  48  are arranged in a vertical line from a bottom to a top, as a monitor optical system directly below the optical sensor  50 . Herein, the folding mirror  46  is disposed so as to be inclined obliquely at 45° at the same height as the unit connection opening  45 . The optical path switching unit  105  of the calibration unit  32  is provided between the dichroic mirror  58  and the condenser lens  48 . 
     The dichroic mirror  58  is provided in order to monitor and photograph the vicinity of the processing point Q of the workpiece W. The dichroic mirror  58  is disposed so as to be inclined obliquely at 45° at the same height as a folding mirror  60  provided on the lateral side of the dichroic mirror  58 . This dichroic mirror  58  is coated with a dielectric multilayer film that transmits a beam to be measured and reflects a visible ray. The folding mirror  60  is also disposed so as to be inclined obliquely at 45°, and is mounted with a CCD camera  62  directly above the folding mirror  60 . An image signal output by the CCD camera  62  is transmitted to a display device  66  through an electric cable  64 . The display device  66  is generally disposed on the device main body side. Although not illustrated, an optical system that irradiates the vicinity of the processing point Q of the workpiece W with a guide beam of the visible ray can be incorporated in the sensor unit  30 . 
     The optical sensor  50  of this embodiment includes, for example, a photodiode  56  as a photoelectric transducer. The optical sensor  50  includes, in front of (below) the photodiode  56 , a wavelength filter or a band pass filter  54  which transmits only beams LM each having a wavelength in a specific band and blocks other beams. Behind the optical sensor  50 , a substrate of an amplification and output circuit  70  is provided. 
     At the time of the laser processing, electromagnetic waves (beams) having a broadband wavelength is emitted from the vicinity of the processing point Q of the workpiece W which is irradiated with the laser beams LB by the emission unit  28 . In vertically upward electromagnetic waves among the electromagnetic waves emitted from the workpiece W, beams that pass through the focusing lens  42  in the emission unit  28  and are reflected horizontally by the dichroic mirror  40  passing through the unit connection opening  45  to be guided into the sensor unit  30 . Among the beams guided into the sensor unit  30 , a beam reflected vertically upward by the folding mirror  46  and then transmitted through the dichroic mirror  58  is incident on the band pass filter  54  through the optical path switching unit  105  and the condenser lens  48 . Then, the beams LM having a wavelength component in a predetermined band selected by the band pass filter  54  are focused and incident on a light receiving surface of the photodiode  56 . In this case, the optical path switching unit  105  is switched such that the processing point Q of the workpiece W and the optical sensor  50  are optically connected by a first optical path K 1  indicated by a dashed line in  FIG.  1   . 
     Among beams that pass from the emission unit  28  through the unit connection opening  45  to enter the sensor unit  30 , a visible ray is folded vertically upwards by the folding mirror  46 . The folded visible ray is horizontally reflected by the dichroic mirror  58  as illustrated by a broken line in  FIG.  1   , folded vertically upward by the folding mirror  60 , and is incident on an imaging surface of the CCD camera  62 . A condenser lens (not illustrated) may be provided in front of the CCD camera  62 . An output signal (video signal) of the CCD camera  62  is sent to the display device  66 , and an image of the vicinity of the processing point Q of the workpiece W is displayed on a screen of the display device  66 . 
     In the optical sensor  50 , a wavelength band in which a beam is transmitted through the band pass filter  54  is preferably set to the most suitable band for grasping the influence of a predetermined factor on a welding characteristic of the vicinity of the processing point for a plurality of types of materials and a variety of processing forms to be selected for the workpiece W as the intensity or change of the radiant energy, in comprehensive consideration of sensitivity, versatility, cost, and the like, in monitoring method using a single photodiode  56 . 
     In this regard, well-known black body radiation spectrum distribution illustrated in  FIG.  2    can be favorably used. As illustrated in a graph of  FIG.  2   , there is a certain relation between the spectrum of an electromagnetic wave emitted by a black body and the surface temperature. When the temperature of an object is high, the peak of radiant energy shifts to a short wavelength, and when the temperature of an object is low, the peak of radiant energy shifts to a long wavelength, and the peak radiant energy changes exponentially with temperature change. According to this graph, the wavelength of a peak point of energy density emitted from a black body with a temperature of 1500° C. is about 1800 nm. 
     On the other hand,  FIG.  3    illustrates a result obtained by measuring, from various angular positions with respect to the processing point, the intensity (relative count value) of a beam of 1000 nm or more detected when iron-based stainless steel (melting point is about 1500° C.) is irradiated with a laser beam, and analyzing the spectrum distribution with a spectrum analyzer by the present inventors. A display waveform of a spectrum analyzer illustrates each peak value as a relative intensity without distinguishing over time the intensity of the detected beam of 1000 nm or more. Although the intensity of the whole of the indicated waveform varies depending on each angular position at the time of measurement, a constant characteristic is obtained. According to this, the intensity (radiant energy density) of infrared rays emitted from a molten part of the stainless steel has a steep mountain-shaped characteristic over a band of about 1000 nm to 1100 nm and a broad mountain-shaped characteristic over a band of about 1200 nm to 2500 nm. When focusing on the latter broad mountain-shaped characteristic, the wavelength of the peak point is about 1800 nm, and generally approximates the wavelength (about 1800 nm) of the peak point of the energy density emitted from the black body with a temperature of 1500° C. 
     From this fact, it is possible to determine a practical optimal wavelength band in laser processing monitoring method using the single photodiode  56 , by using, as an index, a melting point of each of a plurality of types of metals that can be assumed as the material of the workpiece W, with reference to the graph in  FIG.  2   . As an example, the respective melting points of an iron-based metal, copper-based metal, and an aluminum-based metal which are the main materials for laser melting processing are around 1500° C., around 1000° C., and around 600° C., and therefore in  FIG.  2   , the band of 1.3 μm (1300 nm) to 2.5 μm (2500 nm) may be used as the wavelength band that is photoelectrically converted by the optical sensor  50 . 
     Referring again to  FIG.  1   , a sensor output signal CS output from the amplification and output circuit  70  in the sensor unit  30  is transmitted to the sensor signal processing unit  26  on the device main body side via the electric cable  18 . The sensor output signal CS is converted into a digital signal by an A/D converter  82  and a digital signal processing is performed in the arithmetic processing unit  84 . 
     The arithmetic processing unit  84  is composed of a hardware or middleware arithmetic processing device, preferably an FPGA (Feed Programmable Gate Array), capable of performing specific arithmetic processing at high speed. The arithmetic processing unit  84  converts an instantaneous voltage value of the sensor output signal CS into a count value (relative value) representing the intensity of a radiated beam by using a data memory  88 , and generates the converted value as digital waveform data DCS. The generated waveform data DCS is stored in the data memory  88 . The arithmetic processing unit  84  displays a waveform of the sensor output signal CS on a display of a display unit  22   a  of the operation panel  22  via the control unit  14  based on the waveform data DCS. Alternatively, the arithmetic processing unit  84  also executes quality determination processing described later, and displays a determination result together with the waveform of the sensor output signal CS. The control unit  14  converts the waveform data DCS and the determination result data given from the arithmetic processing unit  84  into a video signal, and displays an image of the waveform of the sensor output signal CS and the determination result information and the like on the display of the display unit  22   a  of the operation panel  22 . 
     Thus, according to the laser processing monitor unit  24  of this embodiment, the radiated beam (infrared rays) when the processing point of the workpiece reaches a molten state is an object to be monitored. Then, instead of conversion of the radiated beam into temperature, change in the amount of the radiated beam during processing is converted into an instantaneous integrated value or a count value in a specific band and is displayed, so that the processing status of the workpiece is visualized as a waveform. 
     The operation panel  22  has, for example, the display unit  22   a  composed of a liquid crystal display, and a keyboard or touch panel type input unit  22   b , and displays a setting screen, a monitor screen, a maintenance screen, or the like under display control of the control unit  14 . For example, as one of the setting screens, a laser output waveform corresponding to a setting condition of the laser beam LB is displayed on the display of the display unit  22   a . In addition, as one of the monitor screens, for example, as illustrated in  FIG.  4    to  FIG.  6    described below, a waveform of a very fine sensor output signal acquired without loss of intensity variation in the laser processing monitor unit  24  is visualized and displayed on the display of the display unit  22   a . Furthermore, as one of the maintenance screens, gain adjustment (digital gain adjustment) for the output of an optical sensor  50  can be performed on the screen. 
     [Monitoring Function of Laser Processing Monitor Unit] 
     In order to verify the monitoring function of the laser processing monitor unit  24  in this embodiment, the present inventors conducted Experiments 1, 2, and 3 as illustrated in  FIG.  4    to  FIG.  6   , respectively. A waveform illustrated in each figure is a waveform of the sensor output signal CS displayed on the monitor screen of the display unit  22   a  of the operation panel  22 . A finely zigzagging portion in the waveform indicates that metal melting occurs in the vicinity of the processing point, and radiated beam disturbances from a wave surface of a molten pool are detected. A point at which the waveform, which increases upwards on the time axis, reaches a peak is a point at which the irradiation with the laser beams LB is stopped, and the waveform falls from this point. 
     In Experiment 1 ( FIG.  4   ), two stainless steel plates (SUS304) W 1  and W 2  with a thickness of 1.0 mm were arranged side by side to form a workpiece W, and butt welding was performed using a pulse laser beam. In this butt welding, the laser beam spot diameter was 0.3 mm, the laser output was 500 W, and the pulse width was 20 ms. Then, in a case (a) where there is no gap (clearance) between W 1  and W 2  of the workpiece and in a case (b) where there is a gap of 0.09 mm, which is 30% of a spot diameter of 0.3 mm, it was verified how the influence of the gap (specific factor) on the processing characteristic of the butt welding of the workpiece (W 1 , W 2 ) can be monitored and analyzed in the laser processing monitor unit  24 . 
     The waveform of the case (a) where there is no gap in a butted part and the waveform of the case (b) where there is a gap in a butted part are compared. The former (a) has a characteristic that a detection starting point is high immediately after the start of the rise of the waveform and the fall of the waveform is relatively gentle. On the other hand, the latter (b) has a characteristic that the detection starting point immediately after the beginning of the rise of the waveform is low and the fall of the waveform is relatively sharp. This phenomenon indicates the followings. That is, in the case (b) where there is a gap in the butted part, a laser beam is also incident on a part (gap) where there is no metal. Therefore, due to the presence of the gap with respect to the spot diameter at the irradiation point, a phenomenon that an amount of molten metal, which forms a molten pool, decreases occurs. A difference in the molten metal amount also appears as a difference in the radiated beam amount in the radiated beam detected in the vicinity of the molten pool, and the detection starting point immediately after the rise of the waveform becomes low. In addition, the molten metal amount decreases due to the phenomenon that vaporized metal flies together with a transmitted beam that passes through the gap. Therefore, in a case where there is a gap, the fall of the waveform is detected quickly. 
     In Experiment 2 ( FIG.  5   ), two stainless steel plates (SUS304) W 1  and W 2  with a thickness of 0.3 mm were stacked to form a workpiece W, and lap welding was performed using a pulse laser beam. In this lap welding, the laser beam spot diameter was 0.3 mm, the laser output was 500 W, and the pulse width was 45 ms. Then, in a case (a) where there is no gap in the workpiece (between W 1  and W 2 ) and in a case (b) where there is a gap of 0.06 mm, which is 20% of a spot diameter of 0.3 mm between the workpiece materials (W 1  and W 2 ), it was verified how the influence of the gap (specific factor) on the processing characteristic of the lap welding of the workpiece (W 1 , W 2 ) can be monitored and analyzed. 
     The case (b) where there is a gap of 0.06 mm in this lap welding and the case (a) where there is no gap in this lap welding are compared. It was difficult to distinguish the intensity change of a radiated beam in the rising part of the waveform. However, in the falling portion of the waveform, that is, after the pulse laser irradiation was stopped, the fall of the waveform in the case (b) of the waveform with a gap was more gentle than that in the case (a) of the waveform with no gap. This phenomenon indicates the followings. That is, in the lap welding, in the case (a) where there is no gap in the workpiece material, it is considered that the irradiation laser beam melts and penetrates the first metal W 1 , and thereafter melts the second metal W 2  as it is. On the other hand, in the case (b) where there is a small gap in the workpiece materials, the heat transfer in the workpiece material is delayed due to the influence of an air layer existing in the gap, compared to the case (a) where there is no gap between the metal layers, so that it is detected that the fall of the waveform is gentle. 
     In Experiment 3 ( FIG.  6   ), two plate materials W 1  and W 2  of the stainless steel SUS304 with a thickness of 0.3 mm were stacked with no gap to form a workpiece W, and lap welding was performed using a pulse laser beam. In this lap welding, the laser beam spot diameter was 0.3 mm, the laser output and the pulse width were parameters. That is, for the laser output, six stage values from 300 W to 550 W were selected in 50 W increments, and for the pulse width, three stage values of 25 ms, 35 ms, and 45 ms were selected. Then, it was verified how the influence of the laser output (first specific factor) and the pulse width (second specific factor) on the processing characteristic of the lap welding of workpiece (W 1  and W 2 ) can be monitored and analyzed. 
     From  FIG.  6   , it is found that as the laser output setting value of the laser beam LB increases, the radiated beam intensity indicated by the waveform of the sensor output signal CS also increases proportionally, and as the laser output setting value of the laser beam LB increases, the fall of the waveform of the sensor output signal CS is delayed. Furthermore, it can be seen that the larger the pulse width, the higher the radiated beam intensity (especially the maximum peak value immediately before the fall) displayed by the waveform of the sensor output signal. Consequently, the radiated beam from the workpiece in the laser processing is accurately detected in units of tens of watts and in units of 10 ms. 
     As described above, according to the laser processing monitor unit  24  of this embodiment, monitoring and analysis of the action of the laser beam LB in the laser processing, monitoring and analysis of the degree of influence of a predetermined factor related to the processing state or the processing quality of the laser processing, quality determination of the laser welding processing can be simply and precisely performed from the waveform characteristic of the sensor output signal CS displayed on the monitor screen of the operation panel  22  during the laser processing. 
     [Configuration and Operation of Calibration Unit] 
     In the laser processing monitor unit  24  of this embodiment, a photodetector or an optical sensor  50  is incorporated into the sensor unit  30  built into the processing head  20 . However, the photoelectric conversion characteristic of the photodiode  56  that constitutes this optical sensor  50  not only inevitably changes over time, but also depends on an environmental condition such as an ambient temperature. When the photoelectric conversion characteristic of the photodiode  56  changes, even when a beam LM to be measured having the same light intensity is received from the workpiece W side, a value of an analog sensor output signal CS obtained by photoelectric conversion changes. Therefore, even when the sensor signal processing unit  26  has higher performance, the accuracy and the reliability of the waveform information of the sensor output signal CS provided on the monitor screen are low, and the detailed monitoring and analysis of the laser processing and the appropriate quality determination is not possible. 
     In order to cope with this problem, the laser processing monitor unit  24  of this embodiment includes the calibration unit  32  that enables highly accurate and reliable calibration for change over time in the photoelectric conversion characteristic of the sensor  50  and variation according to an environmental condition. Hereinafter, the configuration and the operation of the calibration unit  32  will be described in detail. 
     As illustrated in  FIG.  1   , the calibration unit  32  includes the reference beam source  100 , the reference beam source power supply  102 , the optical measuring instrument  104  and the optical path switching unit  105  provided inside and outside the sensor unit  30 . 
     The reference beam source  100  is a beam source that generates infrared rays containing the wavelength of beam LM to be measured, and preferably has radiation characteristics close to the radiation spectrum distribution of the ideal black body in  FIG.  2   . 
       FIG.  7 A  illustrates a preferred configuration example of the reference beam source  100 . This reference beam source  100  has an infrared ray emitting element  110  enabling black body radiation. This infrared ray emitting element  110  is composed of a light-emitting diode having surface layer composed of a crystalline body, and a black body layer (black body film) is deposited and formed on a surface of the crystalline body. A surface of the black body layer is preferably formed in a dendritic shape, and radiated beams are emitted in multiple directions from the dendritic surface. As a result, there is no uneven distribution in the radial direction, a highly reliable reference beam can be obtained, and heat generation (light emission) in an extremely short time is possible. In addition, the surface area of the dendritic surface is large and the heat dissipation is high, and therefore the diffusion speed of heat is fast when the light emission is stopped, and it can be used as a stable black body light source without output reduction even after repeated use. In this reference beam source  100 , reference numeral  107  denotes a collimating lens, reference numeral  112  denotes a housing, reference numeral  114  denotes internal electrical wiring, reference numeral  115  denotes a connector, reference numeral  116  denotes a circuit board, reference numeral  118  denotes a cylindrical holding part, reference numeral  120  denotes an opening, reference numeral  122  denotes a thermistor, reference numeral  124  denotes protective glass, and reference numeral  126  denotes a permanent magnet (for detachably fixing the housing  112  to the unit  30 ). 
     In a reference beam source  100  in  FIG.  7 B , the cylindrical holding part  118  is extended axially and a diffuser plate  128  is disposed inside the cylindrical holding part  118 . Configuration other than the above is the same as those in  FIG.  7 A . A beam (reference beam) emitted from a light emitting surface of an infrared ray emitting element  110  at a constant flare angle passes through the diffuser plate  128  and goes out to become a radiated beam with high directivity. Therefore, a collimating lens ( 107 ) becomes unnecessary. 
     Referring again to  FIG.  1   , the reference beam source power supply  102  can arbitrarily control power and an oscillation mode (continuous wave or repetitive pulse) of the reference beam generated by the reference beam source  100  through variable excitation power supplied to the reference beam source  100 . The optical measuring instrument  104  may be a well-known power meter or actinometer, measures the power or luminous flux of beams received by a beam receiver  104   a , and numerically displays a measured value on a display  104   c  of a main body  104   b.    
     The optical path switching unit  105  switches in order to select the first optical path K 1  that optically connects the processing point Q of the workpiece W and the optical sensor  50 , a second optical path K 2  that optically connects the reference beam source  100  and the optical sensor  50 , or a third optical path K 3  that optically connects the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104 . 
     In order to realize this switching function, the optical path switching unit  105  has one (or more) folding mirror(s)  106  capable of moving among a first position P 1  ( FIG.  1   ) for retreating from the first optical path K 1  in order to select the first optical path K 1 , and a second position P 2  ( FIG.  8   ) for blocking the first optical path K 1  and reflecting the reference beam from the reference beam source  100  toward the optical sensor  50  in order to select the second optical path K 2 , and a third position P 3  ( FIG.  9   ) for retreating from the third optical path K 3  in order to select the third optical path K 3 . 
     In this embodiment, the optical path switching unit  105  is provided between the dichroic mirror  58  and the condenser lens  48  in the housing of the sensor unit  30 . The reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  are mounted on a side wall of the housing of the sensor unit  30  adjacent to the optical path switching unit  105 . An optical filter  103  having the same or similar wavelength selection characteristic as the band pass filter  54  of the optical sensor  50  may be disposed in front of the beam receiver  104   a  of the optical measuring instrument  104 . The reference beam source power supply  102  and the main body  104   b  of the optical measuring instrument  104  are provided outside the sensor unit  30 . 
     Now, operation of the calibration unit  32  will be described. The calibration unit  32  has three modes that are selected according to an operation status of the laser processing device and the determination of an on-site person. That is, there are a first mode in which interference with the light reception and photoelectric conversion of the optical sensor  50  during monitoring of laser processing does not occur, a second mode in which the optical sensor  50  is calibrated by use of the reference beam source  100 , and a third mode in which the reference beam source  100  is calibrated by use of the optical measuring instrument  104 . 
     In the first mode, as described above, the folding mirror  106  of the optical path switching unit  105  is switched to the first position P 1  illustrated in  FIG.  1   . The reference beam source  100 , the reference beam source power supply  102  and the optical measuring instrument  104  are turned off. Each unit operates on the device main body side, especially in the sensor signal processing unit  26 , the control unit  14  and the operation panel  22 , waveform display processing to the sensor output signal CS is performed as described above. 
     The second mode is selected (performed) from time to time or periodically during laser processing monitoring. In this mode, when the folding mirror  106  of the optical path switching unit  105  is switched to the second position P 2  illustrated in  FIG.  8   , and the reference beam source power supply  102  is turned on, and a reference beam is generated by the reference beam source  100 . At this time, the reference beam source power supply  102  supplies, to the reference beam source  100 , excitation power at a volume position adjusted or updated in the third mode prior to this second mode. 
     In the second mode, each unit except for the laser oscillator  10  and the laser power supply  12  is operated even on the device main body side. However, the sensor signal processing unit  26 , the control unit  14  and the operation panel  22  are switched to have not the function of waveform display processing in the first mode for the sensor output signal CS sent from the sensor unit  30 , but the calibration function for displaying the measured value (measurement count value) of the power or the luminous flux numerically and performing gain adjustment. 
     In the second mode, the reference beam emitted from the reference beam source  100  passes through the second optical path K 2  to be guided to the optical sensor  50 . More specifically, the reference beam emitted horizontally from the reference beam source  100  is folded vertically upward by the folding mirror  106  of the optical path switching unit  105 . Then, the reference beam folded vertically upward is incident on the band pass filter  54  of the optical sensor  50  via the condenser lens  48 , and the beam in a specific wavelength band that passes through this filter  54  is incident on the photodiode  56 . The photodiode  56  photoelectrically converts the wavelength selected from the received reference beam and outputs an analog sensor output signal CS. This sensor output signal CS is amplified in the latter amplification and output circuit  70  in the same manner as above, and then sent to the sensor signal processing unit  26  on the device main body side via the electric cable  18 . 
     In the sensor signal processing unit  26 , the sensor signal processing unit  26  calculates the light intensity of the reference beam photoelectrically converted by the optical sensor  50  or a measured value, namely, a measurement count value of a luminous flux of the reference beam based on the sensor output signal CS converted into a digital signal by the A/D converter  82 , and stores the calculated value in the data memory  88 . The control unit  14  reads the measurement count value from the data memory  88  and displays the measurement count value on the display of display unit  22   a  of operation panel  22 . 
     An on-site person who performs the second mode reads the measurement count value displayed on the monitor screen (maintenance screen) of display unit  22   a . The on-site person performs gain adjustment or offset adjustment to the output of the optical sensor  50  by input operation on the operation panel  22  such that the measurement count value coincides with a predetermined reference count value. 
     The third mode is selected (performed) from time to time or periodically during laser processing monitoring, and selected (performed) immediately before the second mode. In this mode, the folding mirror  106  of the optical path switching unit  105  is switched to the third position P 3  illustrated in  FIG.  9   , and the reference beam source  100 , the reference beam source power supply  102  and the optical measuring instrument  104  are turned on. All units on the device main body are each kept in an off state. The reference beam emitted from the reference beam source  100  passes through the third optical path K 3  and is incident on the beam receiver  104   a  of the optical measuring instrument  104 . At this time, the reference beam source power supply  102  supplies, to the reference beam source  100 , excitation power at a volume position adjusted or updated in the previous third mode prior. 
     An on-site person who performs the third mode reads the measured value of the reference beam displayed on the display  104   c  of the optical measuring instrument  104 , and checks whether or not the measured value coincides with an absolute reference value preset as the output of the optical sensor  50 . If not, the on-site person adjusts (updates) the volume position of the reference beam source power supply  102  such that the measured value and the absolute reference value coincide with each other. Thus, the light intensity of the reference beam emitted from the reference beam source  100  is calibrated to the absolute reference value via the optical measuring instrument  104 . In addition, the measurement accuracy of the normal optical measuring instrument  104  is guaranteed by separate calibration management. 
     The reference beam source  100  is composed of a semiconductor element such as a light-emitting diode as described above, and the electrical-to-optical conversion characteristics inevitably change over time or fluctuate according to an environmental condition. Therefore, when the constant excitation power is regularly supplied to the reference beam source  100  from the reference beam source power supply  102 , the light intensity of the reference beam emitted from the reference beam source  100  changes undesirably and irregularly. According to the calibration unit  32  of this embodiment, it is possible to correct change and fluctuation in the electrical-to-optical conversion characteristic of the reference beam source  100  at any time or frequently by the third mode. Therefore, in the second mode (calibration of the optical sensor  50 ), a reference beam having higher accuracy than the reference beam source  100  can always be given to the optical sensor  50 . 
     As described above, the laser processing monitor unit  24  of this embodiment uses the reference beam source  100  equipped in the device for calibration of the optical sensor  50  incorporated into the sensor unit  30 , and further calibrates the reference beam source  100  with the optical measuring instrument  104  equipped in the device. In this laser processing monitor unit  24 , even when the electrical-to-optical conversion characteristic of the reference beam source  100  changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. Additionally, even when the photoelectric conversion characteristic of the optical sensor  50  changes over time or fluctuates due to an environmental condition, it is possible to correct this timely and appropriately. Consequently, accuracy, reproducibility and workability of calibration for the optical sensor  50  can be largely improved, and the monitoring performance can be dramatically improved. In addition, the calibration reference value of the optical sensor  50  is the absolute reference value of the optical measuring instrument  104 , and therefore it is possible to obtain monitoring performance without any machine difference. 
     Furthermore, in this embodiment, the optical path switching unit  105  is provided near the optical sensor  50  inside the sensor unit  30 , and the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  are mounted on the side wall of the housing of the sensor unit  30  adjacent to the optical path switching unit  105 . Consequently, without disassembling of the sensor unit  30  or removal of the optical sensor  50 , calibration focusing on the photoelectric conversion characteristic of the optical sensor  50  can be performed easily and safely. In addition, at the laser processing site, without exposing the optical sensor  50  and the monitor optical system in the sensor unit  30  to the surrounding dusty environment, the calibration of the optical sensor  50  and the calibration of the reference beam source  100  can be performed frequently in a daily inspection and or a periodic inspection. 
     [Preferred Configuration Example of Optical Path Switching Unit] 
     As described above, in the laser processing monitor unit  24  of this embodiment, the optical path switching unit  105  is provided together with (preferably near) the optical sensor  50  inside the sensor unit  30 . Hereinafter, a preferred configuration example of the optical path switching unit  105  will be described with reference to  FIG.  10    to  FIG.  16 C . 
       FIG.  10    and  FIG.  11    illustrate an appearance configuration of a principal part of the sensor unit  30 . The illustrated housing of the sensor unit  30  is composed of a lower cylindrical part  130 , an intermediate square cylindrical part  132 , an upper cylindrical part  134  and a sensor box  136  which are connected in a vertically line from a bottom to a top. 
     The folding mirror  46  and the dichroic mirror  58  ( FIG.  1   ) of the monitor optical system are housed in the lower cylindrical part  130 . The reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  are mounted on the side wall of the intermediate square cylindrical part  132 , and a rotary knob  138  is rotatably installed. Herein, the reference beam source  100  and the rotary knob  138  are located on opposite side surfaces of the intermediate square cylindrical part  132  facing each other. The beam receiver  104   a  of the optical measuring instrument  104  is located on a side surface between the reference beam source  100  and the rotary knob  138  in the circumferential direction of the intermediate square cylindrical part  132 . The optical path switching unit  105  according to this configuration example is provided in the intermediate square cylindrical part  132 . The condenser lens  48  ( FIG.  1   ) of the monitor optical system is housed in the upper cylindrical part  134 . The optical sensor  50  is housed in the sensor box  136 . 
     Thus, the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  are not housed inside the sensor unit  30  but mounted on the side wall from the outside, and the rotary knob  138  of the optical path switching unit  105  is also provided outside the side wall of the sensor unit  30 . The providing of the calibration unit  32  does not substantially increase the size (particularly the transvers width) of the internal cavity (optical path) of the sensor unit  30 . 
       FIG.  12    to  FIG.  15    illustrate a configuration of the optical path switching unit  105  of this embodiment,  FIG.  12    and  FIG.  13    are perspective views,  FIG.  14    is a side view, and  FIG.  15    is a sectional view taken along the line A-A of  FIG.  14   . 
     As illustrated in  FIG.  12    and  FIG.  13   , the optical path switching unit  105  has a cylindrical mirror support  140  and a plurality of mirrors, and has holes facing the mirrors. Specifically, the mirror support  140  extends in the direction perpendicular to the first optical path K 1  (X-direction) and is rotatable around the axis HX. An end surface opening  142  is formed in one end surface  140   a  of the mirror support  140 . The end surface opening  142  is mounted so as to face the emission of the reference beam source  100 . The end surface  140   b  on the opposite side is closed, and a rotating shaft  144  protrudes from the center of this end surface  140   b  on the axis HX (X-direction). The rotary knob  138  is mounted on the tip end of the rotating shaft  144  outside the sensor unit  30 . 
     Three side openings  146 ,  148  and  150  are formed in a side surface of the mirror support  140  at intervals in the circumferential direction. The first and second side openings  146  and  148  face each other, and the third side opening  150  is located in the middle between both the openings. 
     Inside the mirror support  140 , a first folding mirror  106 A is provided inside the end surface  140   b  facing the end surface opening  142 , and a second folding mirror  106 B is provided inside a side surface facing the third side opening  150 . Herein, the first folding mirror  106 A is disposed at a predetermined inclination angle so as to receive a reference beam introduced from the reference beam source  100  through the end surface opening  142  at an oblique incident angle and reflect the received reference beam in the predetermined direction, that is, toward the second folding mirror  106 B. On the other hand, the second folding mirror  106 B is disposed at a predetermined inclination angle so as to receive the reference beam from the first folding mirror  106 A at an oblique incident angle and reflect the reference beam outward through the third side opening  150 . 
     As illustrated in  FIG.  16 A  to  FIG.  16 C , the reference beam source  100  is fixed to a side wall of the intermediate square cylindrical part  132  through the cylindrical mounting member  160 . A bearing  162  is provided between the mounting member  160  and a tip end of the mirror support  140 . On the other hand, a bearing  164  is provided between the rotating shaft  144  of the mirror support  140  and the side wall of the intermediate square cylindrical part  132 . When the rotary knob  138  is rotated outside the sensor unit  30 , the mirror support  140  rotates around the axis HX while supported by both the bearings  162  and  164  inside the sensor unit  30 . 
     The rotational position of the mirror support  140  is selected in three ways (or two ways) according to the three modes of the calibration unit  32 . In the first mode, in order to select the first optical path K 1 , the rotational position of the mirror support  140  is selected or adjusted such that the first and second side openings  146  and  148  vertically face each other and are located on the first optical path K 1 . 
       FIG.  16 A  illustrates positional relation of each unit of the optical path switching unit  105  in the first mode. In this case, the first and second folding mirrors  106 A and  106 B are set to the first position P 1 . Herein, the second folding mirror  106 B takes the same upright posture as the first folding mirror  10 A and retreats beside the first optical path K 1 . 
     The beam to be measured propagating inside the lower cylindrical part  130  through the dichroic mirror  58  ( FIG.  1   ) enters the mirror support  140  through the side opening  148  on the lower side in the optical path switching unit  105 . The beam to be measured that enters the mirror support  140  passes near the first and second folding mirrors  106 A and  106 B, and passes out of the mirror support  140  through the side opening  146  on the upper side. The beam to be measured that passes through the optical path switching unit  105  as described above propagates inside the upper cylindrical part  134 , and is incident on the optical sensor  50  through the condenser lens  48 . 
     In the second mode, in order to select the second optical path K 2 , the rotational position of the mirror support  140  is selected or adjusted such that the third side opening  150  faces the optical sensor  50 . 
       FIG.  16 B  illustrates positional relation of each unit of the optical path switching unit  105  in the second mode. In this case, the first and second folding mirrors  106 A and  106 B are set to the second position P 2 . Herein, the second folding mirror  106 B takes the lowest position in the Z-direction and faces the optical sensor  50  directly above the second folding mirror through the third side opening  150 . The first folding mirror  106 A is interlocked with the rotation of the optical path switching unit  105  and turns obliquely downward to face the reference beam source  100 . 
     When the reference beam emitted from the reference beam source  100  enters the mirror support  140  from the end surface opening  142 , the reference beam advances straight and is incident on the first folding mirror  106 A in the innermost part at an oblique incident angle, reflects obliquely downward on the first folding mirror  106 A to be incident on the second folding mirror  106 B at an oblique incident angle. Then, the reference beam incident on the second folding mirror  106 B reflects vertically upward on the second folding mirror  106 B, and exits from the third side opening  150 . The reference beam that exits from the third side opening  150  described above propagates inside the upper cylindrical part  134 , and is incident on the optical sensor  50  through the condenser lens  48 . 
     In the third mode, in order to select the third optical path K 3 , the rotational position of the mirror support  140  is selected or adjusted such that the third side opening  150  faces horizontally (Y-direction) the beam receiver  104   a  of the optical measuring instrument  104  located just beside the optical path switching unit  105 . 
       FIG.  16 C  illustrates positional relation of each unit of the optical path switching unit  105  in the second mode. In this case, the first and second folding mirrors  106 A and  106 B are set to the third position P 3 . Herein, the second folding mirror  106 B has a reflection surface set in the Y-direction and faces the beam receiver  104   a  of the optical measuring instrument  104  located just beside the optical path switching unit  105  through the third side opening  150 . The first folding mirror  106 A is interlocked with the rotation of the optical path switching unit  105  and faces the reference beam source  100 . 
     When the reference beam from the reference beam source  100  enters the mirror support  140  through the end surface opening  142 , the reference beam advances straight and is incident on the first folding mirror  106 A in the front innermost part at an oblique incident angle, reflects obliquely sideways on the first folding mirror  106 A to be incident on the second folding mirror  106 B at an oblique incident angle. Then, the reference beam incident on the second folding mirror  106 B further reflects obliquely sideways (Y-direction) on the second folding mirror  106 B, and exits from the third side opening  150 . The reference beam thus exits in the horizontal direction (Y-direction) perpendicular to the rotation axis HX and is incident on the beam receiver  104   a  of the optical measuring instrument  104 . 
     The rotational positions in the first mode and the third mode of the mirror support  140  are different by 180° in the above description, but can be made to be the same (common). That is, when the mirror support  140  is rotated by 180° from the rotational position illustrated in  FIG.  16 A , the rotational position of the mirror support  140  becomes the same as that in  FIG.  16 C . The rotational position (first position P 1 ) in the first mode of the mirror support  140  and the rotational position (third position P 3 ) in the third mode of the mirror support  140  are thus set to the same (common), so that the first mode (monitoring of the laser processing) and the third mode (calibration of the reference beam source  100 ) can be performed at the same time. 
     In this case, as illustrated in  FIG.  16 C , the third optical path K 3  in the lateral direction from the second folding mirror  106 B to the beam receiver  104   a  of the optical measuring instrument  104 , and the longitudinal first optical path K 1  passing through the dichroic mirror  58  on the lower side and directed to the optical sensor  50  on the upper side intersect perpendicularly near the center inside the optical path switching unit  105 , but do not mutually interfere with each other. 
     The optical path switching unit  105  of this configuration example is mounted with the folding mirrors  106  ( 106 A and  106 B) inside the rotatable cylindrical mirror support  140  having a plurality of the openings on the end surface and the side surface as described above. The rotational position of the mirror support  140  is selected or adjusted, so that the folding mirrors  106  ( 106 A and  106 B) are selectively moved to the first, second and third positions P 1 , P 2  and P 3 . Consequently, it is possible to select either the first, second and third optical paths K 1 , K 2  and K 3  alternatively, or either the first and third optical paths (K 1 , K 3 ) or the second optical path K 2 . According to such a configuration, it is possible to efficiently and smoothly perform required optical path selection or switching within a limited cavity of the sensor unit  30 . 
     Other Embodiment or Modification 
     The preferred embodiment of the present invention is described above, but the embodiment described above does not limit the present invention. It is possible for those skilled in the art to make various modifications and changes to the specific embodiments without departing from the technical idea and the technical scope of the present invention. 
     For example, it is possible to modify or change the arrangement configuration of the optical path switching unit  105 , the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104 . That is, either or all of the reference beam source  100 , the beam receiver  104   a  of the optical measuring instrument  104 , and the optical path switching unit  105  can be provided in the sensor unit  30  or in the processing head  20  away from the optical sensor  50 , or at least one of the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  can be detachably mounted on the sensor unit  30 . 
     Furthermore, as illustrated in  FIG.  17   , the reference beam source  100  can be detachably mounted on the sensor unit  30 , and the beam receiver  104   a  of the optical measuring instrument  104  can be disposed separately from the sensor unit  30 , and the reference beam source  100  can be calibrated outside the sensor unit  30 . 
     Alternatively, in a case where the optical path switching unit  105  of the above preferred configuration example ( FIG.  10    to  FIG.  16 C ) is provided, the third positions P 3  of the first and second folding mirrors  106 A and  106 B in the third mode can also be set to such a position as to be vertically inverted with the second position P 2  (that is, the position where the reflection surface of the second folding mirror  106 B is directed on the dichroic mirror  58  side directly below the third side opening  150  through the third side opening  150 ). In this case, the beam receiver  104   a  of the optical measuring instrument  104  is disposed near the laser emission port of the emission unit  28 . Consequently, the third optical path K 3  is as follows: the reference beam source  100 →the optical path switching unit  105 →the dichroic mirror  58 →the folding mirror  46 →the unit connection opening  45 →the dichroic mirror  40 →the focusing lens  42 →the protective glass  44 → 3  the beam receiver  104   a  of the optical measuring instrument  104 . 
     In the sensor unit  30 , the reference beam source  100  and the beam receiver  104   a  of the optical measuring instrument  104  can face with the optical path switching unit  105  therebetween, and a single folding mirror  106  of the optical path switching unit  105  can also be enough. In this case, the single folding mirror  106  can be moved between the first position P 1  for retreating from the first optical path K 1  in order to select the first optical path K 1 , and the second position P 2  for blocking the first optical path K 1  at an inclination angle 45° and reflecting the reference beam from the reference beam source  100  toward the optical sensor  50  in order to select the second optical path K 2 . Furthermore, in this case, the first position P 1  of the folding mirror  106  can be set to such a position as to retreat from the third optical path K 3 , so that the first position P 1  and the third position P 3  can be common. 
     In the laser processing device of the above embodiment, the sensor unit  30  is integrated with the emission unit  28  and built into the processing head  20 . However, the sensor unit  30  can be separated from the emission unit  28  to be used as an independent unit, and can be disposed near the processing head  20  or the emission unit  28  so as to be directed toward the workpiece W. 
     When the sensor unit  30  is thus used as the independent unit, an entrance window is provided in the front stage (unit end surface) of the monitor optical system of the sensor unit  30  so as to directly take a beam to be measured generated or reflected near the processing point Q of the workpiece W. Then, in this case, not only the beam receiver  104   a  of the optical measuring instrument  104  but also the reference beam source  100  can be used outside the sensor unit  30 . That is, in the second mode, the emission surface of the reference beam source  100  is applied (directed) to the entrance window of the sensor unit  30  from the outside, the reference beam emitted from the reference beam source  100  is taken from the entrance window into the sensor unit  30 , and the taken reference beam can be received in the optical sensor  50  at the innermost part through the monitor optical system on the inner side. Consequently, it is possible to omit the optical path switching unit ( 105 ) from the sensor unit  30 . 
     In the laser beam monitor unit  24  in the above embodiment, the beam to be measured is an infrared ray emitted from the vicinity of the processing point Q of the workpiece W. However, the beam to be measured can be a plasma beam emitted from the vicinity of the processing point Q of the workpiece W, a reflected beam from the vicinity of the processing point Q (reflected laser beam for processing) or a visible ray. Therefore, the reference beam source  100  is not limited to a black body radiated beam source, and various light-emitting diodes or semiconductor lasers with radiation characteristics according to the wavelength band of a beam to be measured can be used. In this case, a band pass filter with a wavelength selection characteristic according to the radiation characteristic of the reference beam source to be used or the wavelength of a beam to be measured, or an optical filter can be provided on the second optical path K 2  that optically connects the reference beam source  100  and the optical sensor  50 . 
     The laser processing monitoring method or device of the present invention is not limited to application to a laser processing device that performs laser welding, and a laser processing device that performs other laser processing, for example, laser cutting, laser brazing, laser hardening, laser surface modification, or the like can be applied. 
     The disclosure of this application relates to the subject matter described in Japanese Patent Application No. 2020-068853 filed on Apr. 7, 2020, the entire disclosure of which is incorporated herein by reference.