WO2024161449A1 - レーザシステム - Google Patents

レーザシステム Download PDF

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Publication number
WO2024161449A1
WO2024161449A1 PCT/JP2023/002830 JP2023002830W WO2024161449A1 WO 2024161449 A1 WO2024161449 A1 WO 2024161449A1 JP 2023002830 W JP2023002830 W JP 2023002830W WO 2024161449 A1 WO2024161449 A1 WO 2024161449A1
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Prior art keywords
laser
light
laser light
intensity
distance
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PCT/JP2023/002830
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English (en)
French (fr)
Japanese (ja)
Inventor
雅浩 上野
宗範 川村
尊 坂本
昌幸 津田
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日本電信電話株式会社
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Application filed by 日本電信電話株式会社 filed Critical 日本電信電話株式会社
Priority to PCT/JP2023/002830 priority Critical patent/WO2024161449A1/ja
Priority to JP2024574064A priority patent/JPWO2024161449A1/ja
Publication of WO2024161449A1 publication Critical patent/WO2024161449A1/ja

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03—Observing, e.g. monitoring, the workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range

Definitions

  • the present invention relates to a laser system intended for removing rust from metals, etc.
  • blasters To remove rust from metal components, metal brushes, power tools, or devices known as blasters, which blast sand or small iron balls at high speed, are generally used.
  • power tools make it difficult to remove rust from uneven areas of steel, and the technique requires time to master, and there are problems with noise and the physical strain on the worker.
  • the structure when using blasters, the structure must be surrounded by sheets to prevent the blasted sand and iron balls from scattering, which also creates problems such as loud noise and time-consuming cleanup after work.
  • Laser devices are a technology that can solve these problems. Laser devices have many advantages, such as reducing noise, removing rust from uneven metal surfaces, and making it easier to collect scattered debris.
  • This type of laser device has a mechanism that protects the optical gain medium from damage (Non-Patent Document 1). With this technology, for example, when there is no optical input for a while, the optical gain of the gain medium increases, and the optical gain is monitored to prevent damage caused by parasitic laser oscillation or Q-switching.
  • ASE amplified spontaneous emission
  • the above-mentioned technology is intended to protect the optical gain medium, and there is no mention of technology to prevent the laser output light from affecting people or objects that are not the target of rust removal.
  • the present invention was made to solve the above problems, and aims to prevent the laser output light from affecting areas other than the target for rust removal.
  • the laser system comprises a first laser device that emits a first laser light for rust removal, a second laser device that emits a second laser light for observation, an optical system that irradiates the first laser light and the second laser light onto the surface of the structure to be treated, a first measuring device that measures the intensity of the return light of the second laser light reflected from the surface to be treated, a second measuring device that determines the distance between a reference position and the surface to be treated from the return light of the second laser light reflected from the surface to be treated, and a controller that enables the first laser device to emit the first laser light when the intensity measured by the first measuring device is greater than an intensity threshold value and the distance determined by the second measuring device is within the distance range.
  • the first laser device can emit the first laser light based on the intensity of the return light of the second laser light and the distance to the surface to be treated determined from the return light, thereby preventing the laser output light from affecting areas other than the target for rust removal.
  • FIG. 1 is a diagram showing the configuration of a laser system according to an embodiment of the present invention.
  • FIG. 2 is a diagram showing a more detailed configuration of the laser system according to the embodiment of the present invention.
  • FIG. 3 is an explanatory diagram for explaining the arrangement of a first light spot 131 a at the irradiation position of the first laser light 131 and a second light spot 132 a at the irradiation position of the second laser light 132 .
  • FIG. 4 is an explanatory diagram for explaining the arrangement of a first light spot 131 a at the irradiation position of the first laser light 131 and a second light spot 132 a at the irradiation position of the second laser light 132 .
  • FIG. 1 is a diagram showing the configuration of a laser system according to an embodiment of the present invention.
  • FIG. 2 is a diagram showing a more detailed configuration of the laser system according to the embodiment of the present invention.
  • FIG. 3 is an explanatory diagram for explaining the
  • FIG. 5 is a diagram showing a more detailed configuration of the LiDAR 110 and the controller 106.
  • FIG. 6A is a block diagram showing a more detailed configuration of the LiDAR 110 and the controller 106.
  • FIG. 6B is a block diagram showing a more detailed configuration of the LiDAR 110 and the controller 106.
  • FIG. 7A is a block diagram showing the configuration of the controller 106 in more detail.
  • FIG. 7B is a block diagram showing the configuration of the controller 106 in more detail.
  • Figure 8 is an explanatory diagram illustrating the exit surface of a diffractive optical element (DOE) serving as a beam shaper BS that forms the beam shape of the second laser light, and the main lobe of the second laser light.
  • DOE diffractive optical element
  • FIG. 9 is an explanatory diagram for explaining ⁇ shown in equation (1).
  • FIG. 10 is an explanatory diagram for explaining the thickness of the DOE serving as the beam shaper BS that forms the beam shape of the second laser light and the equiphase surface of the emitted light.
  • FIG. 11 is an explanatory diagram for explaining the exit surface of a DOE serving as a beam shaper BS that forms the beam shape of the second laser light, and the light focusing range.
  • FIG. 12A is a characteristic diagram showing a simulation result of the maximum power density and beam diameter of the spherical wave integrated beam of the second laser light that has passed through the DOE, versus position z in the z-axis direction perpendicular to the exit surface of the DOE.
  • FIG. 12B is a characteristic diagram showing a simulation result of the maximum power density and beam diameter of the Bessel beam of the second laser light that has passed through the DOE, versus position z in the z-axis direction perpendicular to the exit surface of the DOE.
  • This laser system first comprises a first laser device 101 that emits a first laser light for rust removal, a second laser device 102 that emits a second laser light for observation, and an optical system 103 that irradiates the first laser light and the second laser light onto a processing target surface 152 of a processing target structure 151.
  • the optical system 103 causes the chief ray of the first laser light to coincide with the center line of the light beam of the second laser light.
  • the structure 151 is made of, for example, steel.
  • the laser system also includes a first measuring device 104 that measures the intensity of the return light of the second laser light reflected from the surface 152 to be treated, a second measuring device 105 that determines the distance between a reference position and the surface 152 to be treated from the return light of the second laser light reflected from the surface 152 to be treated, and a controller 106 that enables the first laser device 101 to emit the first laser light when the intensity measured by the first measuring device 104 is greater than an intensity threshold and the distance determined by the second measuring device 105 is within a distance range.
  • the reference position, intensity threshold, and distance range can be set in advance.
  • the reference position can be, for example, a predetermined position in the optical path of the optical system 103.
  • the first laser light is output only when the structure 151 to be treated is observed to be within a predetermined distance range set by the user on the main beam of the first laser light for rust removal, and the first laser light is not output in other circumstances.
  • each threshold value of the controller 106 so that the range in which the structure 151 exists is within the distance range set by the user, it is possible to prevent the first laser light from being irradiated onto objects or people other than the structure 151 and affecting them.
  • the optical system 103 of the laser system can be composed of a first fiber collimator lens (FC1) 121, a focusing optical system (CO) 122, a second fiber collimator lens (FC2) 123, a beam shaper (BS) 124, a dichroic mirror (DM) 125, an optical deflector (OD) 126, etc.
  • FC1 first fiber collimator lens
  • CO focusing optical system
  • FC2 second fiber collimator lens
  • BS beam shaper
  • DM dichroic mirror
  • OD optical deflector
  • the first fiber collimator lens 121 converts the first laser light 131 output from the first laser device 101 into a parallel light.
  • the focusing optical system 122 focuses the first laser light 131 that has been made parallel.
  • the second fiber collimator lens 123 converts the second laser light 132 output from the second laser device 102 into a parallel light.
  • the beam shaper 124 changes the beam shape of the second laser light 132 that has been made parallel.
  • the first laser light emitted from the first laser device 101 is guided through the first optical fiber 127 and directed to the first fiber collimator lens 121.
  • the second laser light emitted from the second laser device 102 is guided through the second optical fiber 128 and directed to the second fiber collimator lens 123.
  • the dichroic mirror 125 reflects the second laser light 132 and transmits the first laser light 131.
  • the dichroic mirror 125 transmits, for example, the first laser light 131 with a wavelength of 1070 nm and reflects the second laser light 132 with a wavelength of 1310 nm or 1550 nm.
  • the optical deflector 126 deflects both the first laser light 131 and the second laser light 132.
  • the first laser light 131 and the second laser light 132 incident on the dichroic mirror 125 are combined at the dichroic mirror 125.
  • the optical axes of the first fiber collimator lens 121, the focusing optical system 122, the second fiber collimator lens 123, and the beam shaper 124 are adjusted so that the main ray 141 of the first laser light 131 and the light beam center line 142 passing through the center of the light beam of the second laser light 132 coincide with each other.
  • the fiber collimator lens 121, the focusing optical system 122, the fiber collimator lens 123, the beam shaper 124, the dichroic mirror 125, and the optical deflector 126 can be housed in the laser head 129.
  • the optical system 103 formed by these the main ray 141 of the first laser light 131 and the center line 142 of the light beam passing through the center of the light beam of the second laser light 132 are aligned, and the first laser light 131 and the second laser light 132 are simultaneously emitted to the structure 151.
  • the return light of the second laser light which is deflected by the optical deflector 126, irradiated onto the structure 151, and reflected by the structure 151, passes through the optical deflector 126, the dichroic mirror 125, the beam shaper 124, and the second fiber collimator lens 123, and is guided through the second optical fiber 128 to the first measuring device 104 and the second measuring device 105.
  • the second laser device 102 and the second measuring device 105 can be the well-known LiDAR (light detection and ranging) 110.
  • the first measuring device 104 can be built into the LiDAR 110 or the controller 106.
  • the arrangement (relative arrangement) of the first light spot 131a at the irradiation position of the first laser light 131 emitted from the optical system 103 (laser head 129) and the second light spot 132a at the irradiation position of the second laser light 132 will be described with reference to Figures 3 and 4.
  • the second light spot 132a of the second laser light 132 is irradiated so as not to overlap with the first light spot 131a of the first laser light 131.
  • the second light spot 132a is irradiated in the vicinity of the first light spot 131a.
  • the vicinity mentioned above means, for example, outside the range (rust removal affected range) where the first laser light 131 causes physical changes such as melting or scraping of the surface 152 to be treated, or chemical changes such as oxidation (changes such as oxidation caused by high heat), and close to said range.
  • the irradiation position of the second laser light 132 can be positioned so that it is in contact with the rust removal affected range.
  • the rust removal affected range can be calculated (predicted) using the light intensity density distribution in the light spot at the irradiation position of the first laser light 131 obtained by measurement or calculation, and the physical and chemical properties of the structure 151 to be irradiated.
  • the light spot at the irradiation position of the second laser beam 132 can be arranged so as to be in contact with the rust removal affected range.
  • Arranging so as to be in contact means, for example, arranging so as to be in contact with a region where the intensity of the light spot of the second laser beam 132 is half the value or more, or a region where the intensity is 1/ e2 or more, as the rust removal affected range.
  • (e) of FIG. 3 shows the case where the light spot 132 is ring-shaped.
  • the second light spot 132a is arranged surrounding the first light spot 131a, and the second light spot 132a is in contact with the first light spot 131a.
  • one second light spot 132a is arranged so as to be adjacent to the affected range. If the total power of the second laser light 132 is the same in all of the spot arrangements shown in FIG. 3(a) to (e), the smaller the area (total area) of the light spot, the higher the power density, and therefore, if it is considered that there is no positional variation in the attenuation of the reflected light (there is no positional dependency of the attenuation of the reflected light), the spot arrangement shown in FIG. 3(a) has the highest resistance to the attenuation of the reflected light compared to the others.
  • the distance between the center of the rust removal affected range (first light spot 131a) and the center of the second light spot 132a is set so that rR +rL ⁇ d holds, for example, when the spot shapes of the first light spot 131a and the second light spot 132a are both circular, the spot radius of the first light spot 131a is rR , the spot radius of the second light spot 132a is rL, and the distance between the center of the first light spot 131a and the center of the second light spot 132a is d.
  • the first light spot 131a and the second light spot 132a are in contact with each other.
  • the upper limit of d may be, for example, 2rR . That is, rR + rL ⁇ d ⁇ 2rR .
  • the upper limit may be, for example, 2rL . That is, rR + rL ⁇ d ⁇ 2rL .
  • the spot shapes of the first light spot 131a and the second light spot 132a are circle and annulus, but are not limited thereto and may be, for example, an ellipse or a rectangle including a square.
  • the semi-minor axis or semi-major axis of the first light spot 131a and the second light spot 132a are replaced with rR or rL , and the second light spot 132a is disposed relative to the first light spot 131a so that the distance d is in accordance with the above formula.
  • the present invention is not limited to the above example, and it is sufficient that the second light spot 132a is disposed in the vicinity of the first light spot 131a.
  • the spot shape of the second light spot 132a can be set by the beam shaper 124.
  • the beam shaper 124 transforms the Gaussian beam emitted from the second fiber collimator lens 123 into a beam that has the spot shape of the second light spot 132a described above.
  • the design method of the beam shaper 124 will be described later.
  • the LiDAR 110 captures the second laser light 132 returning from the surface 152 to be treated via the optical system 103 (laser head 129), measures and outputs the intensity of the returning light with the first measuring device 104, and also measures (ranges) the distance to the surface 152 to be treated with the second measuring device 105 and outputs the measured distance.
  • This distance measurement can be performed, for example, by the "time of flight: (TOF)” method or the "frequency modulated continuous wave: (FMCW)” method.
  • the controller 106 If several conditions are all met (true), the controller 106 outputs a control signal to cause the first laser device 101 to output the first laser light 131 (laser light ON), and if not, outputs a control signal to cause the first laser device 101 not to output the first laser light 131 (laser light OFF).
  • the above conditions include at least two conditions.
  • the first condition is whether or not the structure 151 is present within the distance range set by the user
  • the second condition is whether or not the light intensity received by the LiDAR 110 (measured by the first measuring device 104) is equal to or lower than the intensity set by the user, assuming that the structure 151 is not on the principal ray of the second laser light 132.
  • S can specify two distances dmin and dmax (dmin ⁇ dmax) and set these ranges to dmin or more and dmax or less.
  • the distance range to be set is two or more ranges S i (i is an index representing a range, and for example, when expressing N ranges, i can be expressed as an integer satisfying 0 ⁇ i ⁇ N-1)
  • each S i can be represented as d imin and d imax (i is an index specifying a range, d imin ⁇ d imax ) and set multiple ranges d imin or more and d imax or less.
  • the condition I>I Th is also one of the conditions for outputting the first laser light 131 from the first laser device 101. If this condition is true, it is considered that the structure 151 is present on the center line of the light flux of the second laser light 132 emitted from the laser head 129.
  • the controller 106 If both of the above conditions are met (true), the controller 106 outputs the first laser light 131 from the first laser device 101 (laser ON), and if any other condition is met, it outputs a signal (rust removal laser ON/OFF control signal) to perform an operation not to output the first laser light 131 from the first laser device 101 (laser OFF).
  • the first laser device 101 includes a laser light source for rust removal, and turns on/off the first laser light 131 output from the laser light source for rust removal according to the output of the controller 106 (laser ON/OFF control signal for rust removal).
  • the laser light source has an interlock terminal, it can be used to control the ON/OFF of the laser light output of the first laser device 101.
  • the OFF operation laser OFF, operation not to output laser light
  • the laser light source has a mechanism for modulating the intensity of the laser light output by a command from a terminal such as a computer connected to the outside via a digital I/F, a voltage can be applied to the terminal capable of modulating such light output, or a command can be sent from the terminal.
  • the LiDAR 110 uses the FMCW method.
  • the LiDAR 110 comprises a swept-wavelength light source 201, a portion of a target interferometer 202, a reference interferometer 203, an analog-to-digital converter (ADC) 204, and a LiDAR signal processor 205.
  • the target interferometer 202 includes a laser head 129 and a structure 151 (more specifically, a surface 152 to be processed).
  • the LiDAR signal processor 205 is in a signal processor 206 and is realized by software or hardware. For example, when the signal processor 206 is a computer, the LiDAR signal processor 205 is realized by a program. Also, when the signal processor 206 is realized by a gate array such as an FPGA, the LiDAR signal processor 205 is realized in hardware by connecting wires of logic circuits.
  • the operation of the LiDAR 110 will be described in detail with reference to FIG. 5.
  • the wavelength swept light output from the wavelength swept light source 201 is branched by the optical coupler C1 to the target interferometer 202 and the reference interferometer 203.
  • the target interferometer 202 emits wavelength swept light source light (second laser light) to the structure 151 via the optical coupler C2, the circulator Cir1, and the laser head 129.
  • the reflected light via the laser head 129, the circulator Cir1, and the optical coupler C3 is combined with the light propagating directly from the optical coupler C2 of the target interferometer 202 to the optical coupler C3, and is incident on the balanced photodetector BPD1, photoelectrically converted, and taken into the signal processing device 206 via Ch1 of the ADC 204 as a target interference signal.
  • the virtual reflection point where the optical path length directly connecting the optical couplers C2 and C3 is equal to the optical path length between the optical couplers C2 and C3 reflected at the virtual reflection point of the target interferometer 202 via the circulator Cir1 and returned is the reference surface 221. Since the distance from the reference surface 221 of the target interferometer 202 to the treatment target surface 152 is zS , the difference between the optical path length between the optical couplers C2 and C3 reflected at the treatment target surface 152 via the circulator Cir1 and returned, and the optical path length directly connecting the optical couplers C2 and C3 is 2zS .
  • the reference interferometer 203 is obtained by replacing the structure 151 in the target interferometer 202 with a mirror 223, and the distance from the reference surface 222 to the mirror 223 in the reference interferometer 203 is a known value z R. Therefore, the optical path length difference between the optical path length directly connecting the optical couplers C4 and C5 and the optical path length between the optical couplers C4 and C5 reflected by the mirror 223 via the circulator Cir2 is 2z R , and interference light with the optical path length difference 2z R is incident on the BPD2 and is taken into the signal processing device 206 at Ch2 of the ADC 204 as a reference interference signal.
  • the reference interference signal is first Fourier transformed in the FFT0 section.
  • the zero frequency components (DC components) and negative frequency components of the Fourier transformed reference interference signal are replaced with zero in the negative frequency zero section, and the inverse FFT section performs an inverse Fourier transform to become a complex signal.
  • the reference interference signal that has been made into a complex signal has its argument (range - ⁇ to ⁇ , or 0 to 2 ⁇ ) calculated for each time in the argument calculation section.
  • phase linking process is sometimes called the unwrapping process.
  • the argument obtained after phase linking is called the phase change curve (time-argument curve).
  • the phase change curve represents the temporal phase change of the reference interference signal, and will approximately monotonically increase over time (if the "sign of the argument difference” > 0) or approximately monotonically decrease (if the "sign of the argument difference” ⁇ 0) according to the "sign of the argument difference.”
  • the reason for using the word "approximately” is that if small noise is superimposed on the argument before phase linking, it may not strictly monotonically increase or decrease according to the fluctuations of the noise.
  • the phase change curve ⁇ (t) obtained in this manner becomes data for acquiring equal-phase interval times (resampling timing) according to the number of samples N when resampling the target interference signal and the reference interference signal in resampling section 1 and resampling section 2 described below.
  • the resampling timing calculation section calculates the resampling timing.
  • the resampling 1 unit and the resampling 2 unit resample the target interference signal and the reference interference signal, respectively, at the resampling timing ⁇ n calculated by the resampling timing calculation unit.
  • ⁇ n may be a real number in many cases, when ⁇ n is a real number, resampling is performed by interpolation such as linear interpolation.
  • Resampling causes the phases of the object interference signal and the reference interference signal to change linearly with respect to time ⁇ n , so that the reference interference signal after resampling becomes approximately a sine wave (because there is only one reflection point due to reflection by the mirror, and therefore the optical path length going back and forth between the reference surface 222, the mirror 223, and the reference surface 222 is determined to be one).
  • the object interference signal after resampling also becomes approximately a sine wave if the processing target surface 152 is flat, the center line of the light beam of the second laser light emitted from the laser head 129 intersects perpendicularly with the processing target surface 152, and the reflected light contributes to interference as return light.
  • multiple sine waves are generated according to the distance from the reference surface 221, and these are combined to form a signal.
  • the FFT1 and FFT2 sections perform Fourier transforms on the resampled target interference signal and reference interference signal, respectively.
  • the interference signal is approximately a sine wave
  • the signal after the Fourier transform is calculated as a spectrum having a peak.
  • the signal having this peak is called a "point spread function (PSF)".
  • PSF point spread function
  • the frequency at which the intensity of this PSF peaks corresponds to (is proportional to) the surface positions zS and zR of the target surface 152 of the target interferometer 202 and the mirror of the reference interferometer 203.
  • the intensity peak frequency acquisition unit 1 and the intensity peak frequency acquisition unit 2 acquire frequencies at which the intensity of the PSF acquired by the FFT unit 1 and the FFT unit 2 peaks.
  • the frequency at which the intensity obtained from the PSF derived from the target interference signal peaks is denoted as f S
  • f R the frequency at which the intensity obtained from the PSF derived from the reference interferometer 203 peaks.
  • the controller 106 includes a first laser ON/OFF control signal processing unit 207, and a digital-to-analog converter (DAC) 208. This is based on the premise that the output ON/OFF of the first laser light output from the first laser device 101 is controlled by an analog signal.
  • the first laser ON/OFF control signal processing unit 207 like the LiDAR signal processing unit 205, is realized by software and hardware, or by hardware.
  • the distance z S ' from the reference position calculated by the LiDAR signal processing unit 205 to the structure 151 (processing target surface 152) can be passed to the first laser ON/OFF control signal processing unit 207 of the controller 106 via a memory provided in the signal processing device 206.
  • the DAC 208 and the signal processing device 206 are connected via a digital interface (I/F).
  • This digital I/F can be, for example, a USB (universal serial bus).
  • the controller 106 uses the intensity threshold I Th and distance range S set by the user, the intensity I of the return light received by the LiDAR 110, and the distance z S ' from the reference position to the structure 151 (the surface 152 to be treated) to control the ON/OFF of the first laser output from the first laser device 101 as shown below.
  • I can be calculated from the target interference signal by the first laser ON/OFF control signal processing unit 207 (FIG. 6A).
  • the target interference signal for one sweep time or half sweep time of the wavelength swept light source 201 acquired through channel 1 (Ch1) of the ADC 204 is subjected to signal processing in the order of Fourier transform (FFT), processing to make zero frequency components (DC components) and negative frequency components zero, and inverse Fourier transform (inverse FFT).
  • FFT Fourier transform
  • inverse FFT inverse Fourier transform
  • FFT Fourier transform
  • I is calculated by the first laser ON/OFF control signal processing unit 207 from the result of FFT processing via resampling processing of the target interference signal in the FMCW LiDAR signal processing unit 205 and the peak position frequency fR obtained from the result,
  • the result of FFT processing via resampling of the target interference signal in the FMCW LiDAR signal processing unit 205 is a signal PSF that has a peak at a frequency corresponding to the distance from the reference position included in the target interferometer 202 to the target surface 152.
  • the peak intensity is proportional to the electric field of the reflected light from the structure 151 (proportional to the square root of the power).
  • the first laser ON/OFF control signal processing unit 207 obtains the intensity of the PSF at the peak position frequency f R of the PSF obtained from the LiDAR signal processing unit 205, and obtains I by squaring that intensity.
  • Figures 6A and 6B only show the target interferometer 202, ADC 204 and its Ch1 and Ch2, signal processing device 206, and the LiDAR signal processing unit 205 and first laser ON/OFF control signal processing unit 207 configured therein, but the other parts shown in Figures 2 and 5 are assumed to be configured in the same way as Figures 2 and 5.
  • the electric field strength is obtained and then squared to obtain I, but it is possible to determine the strength of the electric field strength itself without squaring it.
  • the target interference signal was obtained by signal processing (calculation), but below we will explain an example in which a portion of the return light from the laser head 129 is measured by a photodetector, and the measurement result is input as I into the signal processing device 206 via a new channel 3 (Ch3) of the ADC 204 of the LiDAR 110.
  • a possible method for obtaining (sampling) a portion of this return light is to insert a new optical coupler C6 as an optical sampler in the middle of the optical fiber between circulator Cir1 and laser head 129 in the target interferometer 202 of LiDAR110, as shown in Figure 7A, or in the middle of the optical fiber connecting circulator Cir1 and optical coupler C3, as shown in Figure 7B.
  • the branching ratio of the newly inserted optical coupler C6 is measured in advance and the optical power output from the newly inserted optical coupler C6 is converted, the absolute value of the return light intensity can be obtained.
  • Figures 7A and 7B only show the target interferometer 202, the ADC 204 and its Ch1 and Ch3, the signal processing device 206, and the first laser ON/OFF control signal processing unit 207 configured therein, but the other parts shown in Figures 2 and 5 are configured in the same way as in Figures 2 and 5.
  • the first laser device 101 shown in FIG. 5 includes a first laser light source similar to that in FIG. 2, and the laser light source can be controlled by the voltage at an external terminal to turn on/off the output of the first laser light output from the device. Using this mechanism, the first laser device 101 controls (ON/OFF) the output of the first laser light by an analog voltage signal from the controller 106.
  • the LiDAR 110 is of the FMCW type.
  • the LiDAR 110 After the light output from the wavelength swept light source 201 leaves the LiDAR 110, the LiDAR 110 emits the light to the outside of the device via the laser head 129. The LiDAR 110 also acquires returning light via the laser head 129, and acquires a target interference signal via the target interferometer 202 and the ADC 204. After that, the LiDAR 110 performs distance measurement calculations in the LiDAR signal processing unit 205 using the acquired target interference signal and a reference interference signal separately acquired from the reference interferometer 203 via the ADC 204, and calculates and outputs a distance z S ' to the structure 151 (processing target surface 152).
  • the first laser ON/OFF control signal processing unit 207 calculates the return light intensity I of the second laser light from the target interference signal. Furthermore, the first laser ON/OFF control signal processing unit 207 generates a first laser ON/OFF signal according to the conditional expression shown in the description of the controller 106 above, using the distance z S ' of the structure 151 (processing target surface 152) calculated by the LiDAR 110 and I, and outputs the first laser ON/OFF signal to the DAC 208 of the controller 106.
  • the DAC 208 applies an analog voltage to the first laser device 101 according to the acquired laser ON/OFF signal.
  • the first laser device 101 controls (emits: ON, does not emit: OFF) the emission power of the first laser light output from the first laser light source included in the first laser device 101 according to the voltage applied by the DAC 208.
  • the main ray of the first laser light output from the first laser device 101 is positioned in the laser head 129 so as to coincide with the center line of the light beam of the second laser light, and although not shown in FIG. 5, it is deflected by an optical deflector OD in the laser head 129, focused by a focusing optical system CO, and output from the laser head 129.
  • the beam shaper BS described here can be a diffractive optical element (DOE).
  • DOE diffractive optical element
  • the beam shape of the second laser light is invariant with respect to the propagation distance of the second laser light. Therefore, the final electric field distribution on the beam shaper BS is determined by convolving the electric field on the surface of the beam shaper BS, which forms a first-kind zero-order Bessel beam, which is a type of non-diffracted light, with the intensity distribution of the desired shape.
  • Figure 8 shows a plane P0 representing the DOE exit surface and a coordinate system. Assume that the z-axis is perpendicular to P0 , and the x-axis and y-axis are parallel to P0 .
  • the coordinate system represented by these x-axis, y-axis, and z-axis is a Cartesian coordinate system, with the coordinate origin 0 on P0 .
  • a zeroth-order Bessel beam of the first kind with the center of the main lobe located on the z-axis will be described.
  • the wave number of the second laser beam is k
  • the wavelength is ⁇
  • the beam power diameter full width at half maximum (FWHM) of the main lobe is 2rB
  • the electric field distribution uB ,0 on P0 that generates such a zeroth-order Bessel beam of the first kind is expressed as the following formula (2).
  • r xy is expressed by the following equation (3).
  • A(r xy ) is a function representing the electric field strength.
  • S represents the light emission range of the DOE exit surface.
  • ⁇ in formula (1) will be described with reference to Fig. 9.
  • arrows indicate the progression of light incident on the DOE exit surface (plane P 0 ) and exiting light.
  • a Bessel beam is formed at the overlapping portion of beams incident at the same angle ⁇ to the z-axis from all points on P 0 .
  • the diameter (FWHM) of the main lobe of the Bessel beam is calculated by formula (1).
  • the method for designing a DOE using equation (4) is described below.
  • the DOE designed here is a transmissive type, roughly a parallel plate dielectric, with uneven surfaces on the parallel plate.
  • Fig. 10 shows the thickness of a transmissive DOE and the equiphase surface of the exiting light.
  • Light is assumed to enter from the left in Fig. 10 and exit to the right.
  • the refractive index within the DOE is n
  • the refractive index outside the DOE is n0
  • the wavelength and wave number in a vacuum of the light entering and exiting the DOE are ⁇ and k, respectively.
  • Re(u 0 (x, y)) and Im(u 0 (x, y)) are the real part and imaginary part, respectively, and j is the imaginary unit.
  • equation (6) By substituting equation (6) into equation (5) with ⁇ d in equation (5) as ⁇ (x, y) in equation (6) and solving for d, d is expressed as follows.
  • d can be calculated from arg(u 0 (x, y)) according to equation (7), and the DOE at the coordinate (x, y) can be recessed (cut) by the amount of d to create a DOE.
  • the thickness of the DOE at the coordinate (x, y) that gives the phase arg(u 0 (x, y)) can be processed to be L-d.
  • the electric field distribution on the exit surface of the DOE (on the plane P0 ) is calculated by convolution integral of the electric field distribution uB (x,y) on P0 that generates the zeroth order Bessel beam of the first kind and h(x,y), but instead of uB (x,y), an electric field distribution on the plane P0 that forms a light beam that is focused within a certain range on the z axis may be used.
  • Figure 11 shows an example of the focused range of light formed from such an electric field distribution, and shows a state where light is focused within the range indicated by z ⁇ ⁇ z ⁇ z ⁇ on the z axis.
  • An example of the electric field distribution uIS (x,y) on the plane P0 that forms such a focused state on the z axis is shown below.
  • u IS (x, y) is equivalent to the sum (integration) of the electric field distribution on the plane P 0 of the spherical wave e jkr on which each bright point is formed, which is regarded as a set of bright points within the range z ⁇ ⁇ z ⁇ z ⁇ on the z axis (spherical wave integration).
  • kz cos ⁇ is the phase corresponding to the position on the z axis of each spherical wave, and is the same phase as the phase of the center of the main lobe of the Bessel beam formed on the z axis described above.
  • ⁇ is a parameter related to the power full width at half maximum (FWHM) 2r IS of the main lobe of the light beam formed on the z axis, and the following formula holds, similar to formula (1).
  • the advantage of a beam produced by spherical wave integration (spherical wave integrated beam) over a Bessel beam is that the range in which the power of the main lobe is maintained in the z-axis direction is longer. If the power retention range in the z-axis direction is wider, the range in which the reflected light intensity of the second laser light can be maintained will be wider depending on the width of the power retention range, and this will expand the distance measurement range of the LiDAR, making it desirable for LiDAR applications.
  • FIGS. 12A and 12B are graphs showing the simulation results of the maximum power density and beam diameter (FWHM) of the second laser light passing through the DOE versus the position z in the z-axis direction perpendicular to the DOE exit surface, where Fig. 12A plots data for a spherical wave integral beam, and Fig. 12B plots data for a Bessel beam.
  • a Gaussian beam with a diameter of 5.1 mm and a wavelength of 1.07 ⁇ m is incident on the DOE. It is assumed that the DOE is processed every 5 ⁇ m ⁇ 5 ⁇ m on the exit surface (on the plane P 0 ) according to d.
  • the range in which the fluctuation in maximum beam power in the z direction can be suppressed to about 2.45 times is about 750 mm for the spherical wave integrated beam and about 650 mm for the Bessel beam, which shows that the spherical wave integrated beam can be made about 100 mm longer.
  • the fluctuation in beam diameter relative to the average diameter is -10% to +13% for the spherical wave integrated beam, while it is -2.5% to 1.2% for the Bessel beam, so the spherical wave integral is larger.
  • the electric field distribution on a plane parallel to the xy plane is essentially constant in the z direction, but the pseudo-Bessel beam (pseudo-Bessel beam) formed by passing a Gaussian beam through a DOE fluctuates in power as described above, so care must be taken in the scope of application in applications where power fluctuations are a problem.
  • u 0 (x, y) and h(x, y) have the following relationship:
  • Equation (12) g z (x, y) is a function representing the propagation of light, and is expressed by the following equation.
  • h(x,y) and gz (x,y) are each Fourier transformed, and the transformation results H(v,v) and Gz (v,v) are used to calculate H(v,v)/ Gz (v,v), and this result is then inverse Fourier transformed to calculate u0 (x,y).
  • Equation (16) can also be transformed as follows:
  • g z ⁇ 1 (x, y) is the inverse propagation function of light and is expressed by the following equation.
  • equation (17) can be expressed as follows.
  • the first laser device can emit the first laser light based on the intensity of the return light of the second laser light and the distance to the surface to be treated determined from the return light, thereby preventing the laser output light from affecting areas other than the target for rust removal.
  • 101 first laser device, 102: second laser device, 103: optical system, 104: first measuring device, 105: second measuring device, 106: controller, 151: structure, 152: surface to be treated.

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JP2018001199A (ja) * 2016-06-29 2018-01-11 株式会社Ihi 表面処理装置
US20190339512A1 (en) * 2018-05-02 2019-11-07 National Tsing Hua University Portable Surface Finishing Device Based on Coherent Light Source
WO2021005773A1 (ja) * 2019-07-11 2021-01-14 日本電信電話株式会社 レーザー光走査装置及びレーザー光走査方法
JP2021030271A (ja) * 2019-08-26 2021-03-01 フルサト工業株式会社 固着物除去装置
WO2021240778A1 (ja) * 2020-05-29 2021-12-02 三菱電機株式会社 レーザ加工システム
JP2021533996A (ja) * 2018-08-09 2021-12-09 コーニング インコーポレイテッド レーザービームの機械内プロファイリングのためのシステム、方法および装置
JP2023008405A (ja) * 2021-07-06 2023-01-19 日本電信電話株式会社 表面状態推定方法

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2013133415A1 (ja) * 2012-03-09 2013-09-12 株式会社トヨコー レーザー照射装置、レーザー照射システム及び塗膜又は付着物除去方法
JP2018001199A (ja) * 2016-06-29 2018-01-11 株式会社Ihi 表面処理装置
US20190339512A1 (en) * 2018-05-02 2019-11-07 National Tsing Hua University Portable Surface Finishing Device Based on Coherent Light Source
JP2021533996A (ja) * 2018-08-09 2021-12-09 コーニング インコーポレイテッド レーザービームの機械内プロファイリングのためのシステム、方法および装置
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JP2023008405A (ja) * 2021-07-06 2023-01-19 日本電信電話株式会社 表面状態推定方法

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