Laser irradiation device

Abstract

[Problem] To provide a laser irradiation device which achieves both processing quality and depth of focus in laser cleaning. [Solution] This laser irradiation device for irradiating laser light onto an object O to be processed to remove a removal target from the surface of the object to be processed comprises a condensing optical system 10 for condensing the laser light emitted from a laser oscillator at a predetermined focal position BS, and is configured such that a spot radius of the laser light at the focal position is r'(mm), a condensing half angle of the laser light is ϴ'(mrad), and r' / ϴ' is 2.3×10-3 or more.

Description

Laser irradiation device

[0001] The present invention relates to a laser irradiation device that performs laser cleaning by irradiating the surface of an object to be treated with laser light to remove an object to be removed.

[0002] As an example of a conventional technique for surface treatment using laser light, Patent Document 1 describes that an irradiation head that irradiates an object with laser light is provided with a wedge prism that deflects the laser light by a predetermined deflection angle, and that by irradiating the laser light while rotating this wedge prism around the optical axis of the incident light, the irradiation point (beam spot) scans the surface of the object while rotating in an arc, and old paint film, foreign matter, etc. adhering to the surface of the object are removed (cleaned).

[0003] Patent No. 5574354

[0004] In laser processing, the required focal depth is determined by the processing depth. In the field of laser processing, including laser cleaning, the focal depth is defined as the distance range from the focal position where the processing quality is not deteriorated, based on the processing quality at the focal position. In conventional laser processing, such as cutting and drilling, a low-power laser is selected for fine processing, while a high-power laser is selected for processing thick plates. Since both the processing depth and the focal depth are compatible, no particular problems have arisen. On the other hand, unlike conventional laser processing such as cutting and drilling, in laser processing using laser cleaning, which has recently attracted attention, the required focal depth is determined by the uneven structure of the irradiated object (processing object) and the operator's operating accuracy, especially when performing laser cleaning with a handheld laser head. Therefore, it is necessary to consider both the processing quality and the focal depth, which was not a major issue in conventional laser processing. The objective of the present invention is to provide a laser irradiation device that achieves both processing quality and the focal depth in laser cleaning.

[0005] In order to solve the above-described problems, a laser irradiation device according to one aspect of the present invention is a laser irradiation device that irradiates a processing object with laser light to remove a removal target from a surface of the processing object, the laser irradiation device including a focusing optical system that focuses laser light emitted from a laser oscillator at a predetermined focal position, and wherein when a spot radius of the laser light at the focal position is r' (mm) and a focusing half angle of the laser light is θ' (mrad), r' / θ'≧2.3×10 －３ This makes it possible to provide a laser irradiation device that achieves both processing quality and depth of focus in laser cleaning.

[0006] In the present invention, when the output of the laser oscillator is P (W), the beam parameter product BPP of the laser light is 1.4×10 －３ A laser irradiation device according to another aspect of the present invention is a laser irradiation device that irradiates a processing object with laser light to remove a removal target from a surface of the processing object, the laser irradiation device including a focusing optical system that focuses laser light emitted from a laser oscillator at a predetermined focal position, and wherein when the output of the laser oscillator is P (W), the beam parameter product BPP of the laser light is 1.4×10 or less. －３ × P or less. According to each of these inventions, by setting the beam parameter product BPP, which is a parameter indicating the quality of the beam, within a predetermined range, it is possible to appropriately achieve both processing quality and depth of focus in laser cleaning.

[0007] In the present invention, when the spot radius of the laser light is r' (mm) and the moving speed of the spot on the surface of the processing object is v (mm / s), r'≦10 －５ ×v. This makes it possible to ensure the processing quality of the laser cleaning at the focal position (just focus position).

[0008] In the present invention, the focal depth (one side) of the laser beam may be one-third or less of the distance from the foremost optical element of the laser irradiation device to the focal position, which increases the possibility of preventing damage to the optical element of the laser irradiation device and the operator, compared to when the focal depth of the laser beam is greater than one-third of the distance from the foremost optical element of the laser irradiation device to the focal position.

[0009] In the present invention, an irradiation head having the focusing optical system and a fiber that propagates the laser light from the laser oscillator to the irradiation head are provided, and when a spot radius of the laser light at an output end of the fiber is r (mm) and a divergence half angle of the laser light is θ (mrad), r / θ≧1.5×10 －４ According to this configuration, even when laser light is transmitted over a long distance by fiber, it is possible to suppress Raman scattered light SRS and protect the laser oscillator.

[0010] In the present invention, the laser irradiation method using the laser irradiation device according to claim 1 or 3 further includes a laser beam moving unit that moves the spot of the laser beam on the surface of the processing object to scan the surface with the laser beam, and when the scanning width of the laser beam on the surface is 2R (mm) and the tilt angle, which is the angle between the normal to the focusing surface of the laser beam and the normal to the surface, is φ, the focal depth (one side) D (mm) of the focusing optical system can be configured to satisfy D≧tan φ×R. According to this, even when the normal to the focusing surface is tilted, the entire circumference of the scanning pattern of the beam spot on the processing object can be kept within the focal depth.

[0011] As described above, according to the present invention, it is possible to provide a laser irradiation device that achieves both processing quality and depth of focus in laser cleaning.

[0012] 5 is a cross-sectional view of an irradiation head used in an embodiment of a laser irradiation device to which the present invention is applied. FIG. 6 is a schematic diagram showing the scanning state of a laser beam on a surface of a processing object in a laser irradiation device of an embodiment. FIG. 7 is a diagram explaining the concept of an overlap ratio in a laser irradiation device of an embodiment. FIG. 8 is a diagram showing an example of an oxide film formation state on an irradiated surface of a processing object after laser irradiation processing. FIG. 9 is a diagram showing a schematic configuration of an apparatus that moves a sample relative to an irradiation head in an inclined state in measuring a depth of focus. FIG. 10 is a schematic enlarged view of a processing object portion after laser irradiation processing in the apparatus of FIG. 5. FIG. 11 is an image diagram showing a correlation between the defocus amount of laser light and the generation rate and removal rate of an oxide film. FIG. 12 is a diagram showing an example of an original image and a blue extracted image of a processing object after laser irradiation. FIG. 13 is a diagram explaining an example of an upper limit of a depth of focus. FIG. 14 is a diagram explaining an example of a lower limit of a depth of focus. FIG. 15 is a diagram showing an example of a method for measuring r' and θ'. FIG. 16 is a diagram showing a modified example of a scanning pattern of a beam spot.

[0013] An embodiment of a laser irradiation device to which the present invention is applied will be described below. The laser irradiation device of the embodiment irradiates a processing object O with laser light supplied from a laser oscillator via a fiber, and the irradiated spot (beam spot BS) rotates and scans the surface of the processing object O along a circumferential scanning pattern.

[0014] The object to be treated O is, for example, a structure made of an iron-based metal such as general steel or stainless steel. Compounds resulting from alteration or modification of the base material, such as rust or an oxide film, may be present on the surface of the object to be treated O. A coating may be formed on the surface of the object to be treated O by painting, plating, or the like. Oxides, hydroxides, carbonates, etc. of the base material or a coating such as plating may be formed on the surface of the object to be treated O. Furthermore, external deposits such as salt, scale, and dirt may adhere to the surface of the object to be treated O. In this specification and claims, the term "surface portion of the object to be treated O" refers to all of these.

[0015] The cleaning process using the laser irradiation device of this embodiment is a laser processing in which an irradiation spot (beam spot BS) is rotated and scanned on the surface of the object to be treated O along a relatively large circumference (circle of rotation) having a diameter of, for example, 10 mm or more, to clean old paint films (paint films to be removed), various films such as oxide films, dust, rust, soot, etc. that make up the surface of the object to be treated O. Figure 1 is a cross-sectional view of an irradiation head used in the laser irradiation device of this embodiment.

[0016] The irradiation head 1 irradiates the object to be processed O with a continuous wave (CW) laser beam B transmitted from a laser oscillator (not shown) via a fiber (not shown). The laser oscillator used is preferably a YAG laser, fiber laser, or disk laser, which has a high output (2 kW to 15 kW) and oscillates at a wavelength (1000 nm to 1100 nm) that can be propagated over long distances via optical fiber. In the case of a handheld laser head, a laser oscillator output of 3 kW to 10 kW is more preferable due to the relationship between the depth of focus (described below) and the size and weight of the laser head. The irradiation head 1 is, for example, a handheld type that can be handheld by an operator for irradiation work. However, it can also be attached to a robot that can move the irradiation head 1 along a predetermined path. Alternatively, the object to be processed O may be displaced relative to the irradiation head 1 while the irradiation head 1 is fixed.

[0017] The irradiation head 1 includes a focus lens 10, a wedge prism 20, a protective glass 30, a rotating cylinder 40, a motor 50, a motor holder 60, a protective glass holder 70, a housing 80, a duct 90, and the like.

[0018] The focus lens 10 is an optical element onto which the laser beam B transmitted from the laser oscillator to the irradiation head 1 via a fiber passes after passing through a collimating lens (not shown). The collimating lens is an optical element that converts (collimates) the laser light emitted from the end of the fiber into a substantially parallel beam. The focus lens 10 is an optical element that condenses (focuses) the laser beam B emitted from the collimating lens at a predetermined focal position. For example, a convex lens having a positive power can be used as the focus lens 10.

[0019] The beam spot BS, which is the point on the surface of the processing object O that is irradiated with the laser beam B, is positioned in a close state (focus state) that coincides with the focal position or is included within the focal depth, or is positioned away from the focal position (defocus state). The definition of focal depth in this specification and claims will be explained in detail later.

[0020] The wedge prism 20 is an optical element that deflects the laser beam B emitted by the focus lens 10 by a predetermined deflection angle θ (see FIG. 1 ) to make the optical axis angles of the incident and exit sides different. The wedge prism 20 is formed as a plate whose thickness varies continuously so that the thickness on one side in a direction perpendicular to the optical axis direction on the incident side is larger than the thickness on the other side. The protective glass 30 is an optical element made of flat glass or the like and arranged adjacent to the focal position side (the processing object O side, the beam spot BS side) of the wedge prism 20 along the optical axis direction.

[0021] The protective glass 30 is a protective member that prevents foreign matter such as spatter, peeling material, and dust that scatters from the processing object O side from adhering to other optical elements such as the wedge prism 20. The protective glass 30 is the optical element that is arranged closest to the focal position along the optical axis direction among the optical systems of the irradiation head 1, and is exposed to the processing object O side through the interior of a space A and a duct 90, which will be described later. The focus lens 10, the wedge prism 20, and the protective glass 30 are configured by applying a coating for the purpose of anti-reflection, surface protection, etc. to the surface of a member made of a transparent material such as optical glass.

[0022] The rotating barrel 40 is a cylindrical member that holds the focus lens 10 and the wedge prism 20 on its inner diameter side. The rotating barrel 40 is formed concentrically with the optical axis of the focus lens 10 and the optical axis of the laser beam B incident on the focus lens 10 (the optical axis of the collimator lens). The rotating barrel 40 is supported by a bearing (not shown) to be rotatable about a rotation center axis that coincides with the optical axis of the focus lens 10 relative to the housing 80. The rotating barrel 40 is formed from, for example, a metal such as an aluminum alloy, engineering plastic, or the like.

[0023] The motor 50 is an electric actuator that drives the rotating barrel 40 to rotate about the central axis of rotation relative to the housing 80. The motor 50 is configured, for example, as a ring-shaped motor that is concentric with the rotating barrel 40 and provided on the outer diameter side of the rotating barrel 40. A rotor (not shown) of the motor 50 is fixed to the rotating barrel 40. The motor 50 is controlled by a motor drive device (not shown) so that the rotation speed of the rotating barrel 40 substantially matches a desired target rotation speed.

[0024] By maintaining the orientation of the irradiation head 1 so that the rotational axis of the rotating cylinder 40 is perpendicular to the surface of the object to be treated O near the irradiation location, and by rotating the wedge prism 20 together with the rotating cylinder 40 using the motor 50, the beam spot BS circumferentially scans the surface of the object to be treated O around the rotational axis of the rotating cylinder 40. This mechanism is an example of a laser light moving unit. When the irradiation head 1 is translated along the surface of the object to be treated O in this state, the beam spot BS scans the surface of the object to be treated O while rotating circumferentially (arcuately). As a result, when focusing on an arbitrary point on the object to be treated O, the laser beam B is intermittently incident for only a short period of time, and rapid heating and rapid cooling are sequentially performed within a short period of time. At this time, the surface of the object to be treated O is fractured and scattered.

[0025] The motor holder 60 is a support member that holds the stator (not shown) of the motor 50 in a predetermined position. The main body of the motor holder 60 is formed in a cylindrical shape and is fixed in a state inserted into the inner diameter side of the housing 80. The inner circumferential surface of the motor holder 60 is disposed opposite the outer circumferential surface of the motor 50 and is fixed to the stator of the motor 50.

[0026] A purge gas flow path 61 through which a purge gas PG flows is formed in a part of the gap between the outer peripheral surface and the inner peripheral surface of the motor holder 60, penetrating in the axial direction of the motor 50. The purge gas PG is a gas that is ejected toward the treatment object O from a space A inside an inner cylinder 91 of a duct 90, which will be described later, with which the surface of the protective glass 30 on the treatment object O side is in contact when the irradiation head 1 is in use (during irradiation). The purge gas PG has the function of preventing debris such as spatter, dust, and foreign matter scattered from the treatment object O side from flying into the housing 80 and adhering to the protective glass 30.

[0027] The protective glass holder 70 is a member fixed to the inner diameter side of the housing 80 while holding the protective glass 30. The protective glass holder 70 is formed, for example, in a disk shape with a circular opening formed in the center. The laser beam B passes through the opening from the wedge prism 20 side to the object O side. A recess into which the protective glass 30 is fitted is formed on the surface of the protective glass holder 70 facing the object O. The protective glass 30 is held inside the housing 80 while fitted in this recess.

[0028] The protective glass 30 is detachably attached to the protective glass holder 70 so that it can be replaced if it becomes contaminated or burned. The surface of the protective glass holder 70 opposite the side facing the object to be processed O is disposed opposite the end face of the motor holder 60 facing the object to be processed O, with a gap therebetween that allows the purge gas PG to flow.

[0029] The housing 80 is a cylindrical member that constitutes the housing of the main body of the irradiation head 1. The housing 80 accommodates the above-mentioned focus lens 10, wedge prism 20, protective glass 30, rotating cylinder 40, motor 50, motor holder 60, protective glass holder 70, etc., as well as the end of the fiber on the irradiation head 1 side (not shown), a collimator lens, etc.

[0030] The duct 90 is a double-tube member protruding from the end of the housing 80 on the side of the object to be treated O. The duct 90 has an inner tube 91, an outer tube 92, a dust collector connection tube 93, etc. The motor holder 60, the protective glass holder 70, and the housing 80 described above are formed from, for example, a metal such as an aluminum alloy, engineering plastic, or the like.

[0031] The inner cylinder 91 is formed in a cylindrical shape. The laser beam B passes through the inner diameter side of the inner cylinder 91 and is emitted toward the processing object O. The end of the inner cylinder 91 on the housing 80 side is formed with a small diameter portion 91a that is formed with a stepped smaller diameter than the other portions. A purge gas PG is introduced from inside the housing 80 into a space A inside the small diameter portion 91a.

[0032] A tapered portion 91b is formed at the end of the inner cylinder 91 on the side of the processing object O, narrowing at the tip so that the diameter becomes smaller on the side of the processing object O. The tapered portion 91b has the function of restricting the flow of the purge gas PG to increase the flow rate while allowing the laser beam B to pass through.

[0033] The outer cylinder 92 is a cylindrical member arranged concentrically with the inner cylinder 91 and is provided on the outer diameter side of the inner cylinder 91. A continuous gap is formed around the entire circumference between the inner peripheral surface of the outer cylinder 92 and the outer peripheral surface of the outer cylinder 91. A small-diameter portion 92a is formed at the end of the outer cylinder 92 facing the housing 80, with a stepped diameter smaller than the other portions. The small-diameter portion 92a is fixed in a fitted state into the end of the housing 80 facing the object to be treated O. The edge of the end 92b of the outer cylinder 92 facing the object to be treated O is formed to be inclined with respect to the rotational axis of the rotating cylinder 40 so that the upper side is closer to the housing 80 than the lower side during normal use when irradiating with the rotational axis of the rotating cylinder 40 horizontal.

[0034] The dust collector connection tube 93 is a cylindrical body that protrudes radially outward from the outer tube 92 and is connected in communication with the inner diameter side of the outer tube 92 near the end of the outer tube 92 on the side of the object to be treated O. The dust collector connection tube 93 is provided below the outer tube 92 during normal use as described above. The dust collector connection tube 93 is positioned at an angle relative to the outer tube 92 so as to move closer to the housing 80 from the side of the object to be treated O and to move away from the outer tube 92. The other end of the dust collector connection tube 93 is connected to the dust collector 140, which will be described later, and is configured to be vacuum-suctioned to create a negative pressure inside.

[0035] 2 is a schematic diagram showing the scanning state of the laser beam on the surface of the object to be processed in the surface treatment method of the embodiment. In this embodiment, by rotating the rotary cylinder 40 and the wedge prism 20 while emitting the laser beam B, the beam spot BS revolves circumferentially along a revolving circle C having a predetermined diameter (rotation diameter) 2R along the surface of the object to be processed. In this state, by relatively translating the irradiation head 1 along the surface of the object to be processed O, it is possible to perform a process in which the beam spot BS scans the surface of the object to be processed O while the revolving circle C moves over the irradiated surface at a predetermined feed rate.

[0036] In the embodiment, examples of irradiation parameters to be set during construction include the following: (1) Output of laser oscillator (W): Set by selecting the model of laser oscillator and by the output adjustment function of the laser oscillator. (2) Power density (W / cm ２ Power density (W / cm): An index showing the degree to which the laser output is concentrated on the surface (irradiated surface) of the processing object O, and is expressed by the following formula: ２ ) = Output (W) ÷ Area of ​​beam spot BS (cm ２ ) = Output (W) ÷ ((Beam spot diameter (diameter) 2r' (cm) / 2) ２ ×π) (3) Rotation radius (diameter) 2R (cm) of the rotation circle C of the beam spot BS: set by the focal length of the focus lens 10, the deflection angle θ of the wedge prism 20, etc. (4) Rotation speed N (rpm) of the beam spot BS

[0037] 3 is a diagram illustrating the concept of the overlap ratio in the surface treatment method of the embodiment. (5) Overlap ratio: When the beam spot BS rotates (circulates) along a rotation circle C and the center of the rotation circle C is moved relative to the processing object O, this value indicates the overlap ratio of the beam spot BS between a first passing locus T1 of the beam spot BS along the rotation circle C and a second passing locus T2 formed subsequent to the first passing locus T1. The overlap ratio is expressed by the following formula using the overlap amount W, which indicates the overlap width (length) relative to the spot diameter 2r' in the width direction perpendicular to the rotation direction: Overlap ratio (%) = Overlap amount W ÷ Spot diameter (diameter) 2r' of the beam spot BS × 100.

[0038] (6) Swivel circle movement speed Vm (mm / s): This is the relative movement speed (movement speed of the scanning pattern) of the center of the swiveling circle C on the irradiated surface of the processing object O relative to the irradiated surface. Here, when the irradiation head 1 is moved in translation parallel to the irradiated surface while the rotation center axis of the wedge prism 20 is kept perpendicular to the irradiated surface, the swiveling circle movement speed matches the operation speed (feed speed / translation movement speed) of the irradiation head 1.

[0039] (7) Irradiation time Tp (seconds) of beam spot BS: This is the time during which a point on the irradiated surface is irradiated when the beam spot BS passes over that point once, and is the maximum irradiation time for passing through that point. Irradiation time Tp (seconds) of beam spot BS = spot diameter (diameter) of beam spot BS 2r' ÷ (rotation diameter (diameter) of turning circle C 2R × π × (number of rotations N / 60)) (8) Fluence per point (J / cm ２ Point fluence (J / cm): An index showing the energy per area given to a point on the surface to be irradiated when the beam spot BS passes through the point once, and is expressed by the following formula: ２ ) = power density × irradiation time Tp (seconds) of beam spot BS = power density × (spot diameter (diameter) of beam spot BS 2r' ÷ (rotation diameter (diameter) 2R × π × (number of rotations N / 60))) (9) total fluence (J / cm ２Total fluence (J / cm): An index showing the total energy per area given to the irradiated surface by laser irradiation from the start to the end of the treatment, and is expressed by the following formula: ２ ) = Output (W) ÷ Irradiation area per second (cm ２ / s) × number of irradiations = output (W) ÷ (rotation diameter (diameter) 2R (cm) × rotation circle movement speed Vm (cm / s)) × number of irradiations

[0040] The above-mentioned single-point fluence is a parameter suitable for evaluating the quality of the treated surface and the amount of spatter scattered during irradiation, while the total fluence is a parameter suitable for estimating the processing efficiency (evaluating the processing capacity).

[0041] The setting of irradiation parameters for laser irradiation in this embodiment will be described below. Fig. 4 is a diagram showing an example of the state of oxide film formation on the irradiated surface of the processing object after laser irradiation processing. ３ O ４ In some cases, an oxide film such as the above may be formed. Such an oxide film may affect the durability and reliability of a coating when, for example, a coating is applied, and therefore it is generally preferable to suppress it. Here, the degree of the oxide film was graded from 1 to 5 (higher numbers indicate better), and a visual sensory evaluation was performed. Figure 4 shows photographs of grades 1 and 5 as examples. In grade 1, a predominantly black oxide film was formed with a relatively thick film thickness over a wide area. Grade 5 is at a level that is considered to be satisfactory when painting is performed after laser irradiation treatment. Compared to grade 1, the steel base material is exposed over a wide area, and the film thickness is thin even in the area where the oxide film is formed, indicating differences in color and brightness.

[0042] Below, we further investigated the irradiation conditions, beam quality, and other factors required to achieve both processing quality and depth of focus in laser cleaning. From the results of experiments conducted under appropriate irradiation conditions, we extracted irradiation conditions and depth of focus that would yield good results. The depth of focus was determined as the distance range within which no deterioration was observed, based on the processing quality at the focal position. Deterioration in processing quality is often caused by the formation of oxide films on both edge portions, specifically, oxide films with a bluish appearance on the edge portions. Appropriate irradiation conditions are defined as follows: (1) a time that is sufficiently shorter than the upper limit (20 μs) of the single-point irradiation time that results in good processing quality at the focal position (just-in-focus position); and (2) a fluence that is sufficiently higher than the lower limit of the single-point fluence required for the laser processing process.

[0043] FIG. 5 is a schematic diagram showing the configuration of an apparatus for measuring the depth of focus by moving a tilted sample relative to the irradiation head. For example, the sample is tilted 15 degrees relative to the direction of sample movement. F denotes the distance from the optical element at the forefront of the irradiation head 1 to the focal position (just-focus position). The sample, or object to be processed O, is attached to a stage S that guides it linearly along the imaging plane of the beam spot BS (a direction perpendicular to the central axis of rotation of the beam). When the object to be processed O is moved by the stage S while irradiating it with laser light while rotating the beam spot BS, the object to be processed O is first irradiated in a defocused state, and then the defocus amount gradually decreases until it reaches the focal position (just-focus state). After that, the defocus amount occurs and increases again, eventually resulting in a defocused state.

[0044] 6 is a schematic enlarged view of the processing object portion after laser irradiation processing in the apparatus of FIG. 5. FIG. 6(a) is an enlarged view of the VI portion of FIG. 5. FIG. 6(b) is a view as seen from the arrows b-b of FIG. 6(a). In measuring the focal depth, the total fluence in one irradiation (output (W) × irradiation time (s) ÷ irradiation area (cm ２If the total fluence is low, there is a possibility that a discrepancy will occur between the total fluence required to remove the target object (rust or paint film) that you want to remove, and a correct evaluation will not be possible. ２ Irradiation is repeated until the surface temperature of the workpiece returns to room temperature. Between irradiations, the workpiece is cooled until the surface temperature returns to room temperature. Since the objective is to remove the target material (rust or paint), the single-point fluence is assumed to be greater than the value required for removal. As a result, the defocused portion of the irradiated area outside the focal depth exhibits a blue or black color due to an oxide film. This phenomenon, in which an oxide film forms on the surface of the workpiece when exposed to high temperatures and the surface color changes depending on the thickness of the oxide film, is called the temper color phenomenon. Meanwhile, the focused portion of the irradiated area within the focal depth exhibits the base color of the steel (typically silver) or a pale yellow. This is thought to be due to the balance between the amount of oxide film generated by the laser and the amount of oxide film surface removal. If the width of the focused area is 2L, the relationship between the focal depth (both sides) 2D and the focal depth (2D) is expressed by the following equation: focal depth (2D) = focal width (2L) × sin α, where α is the inclination angle of the sample relative to the direction of sample movement. The irradiation conditions are set so as to maximize the depth within the range of conditions that the device is actually used under. For example, when scanning with a laser, the overlap (overlap ratio) is minimized. As an example, 20% is set as the minimum overlap ratio that will prevent missed shots due to fluctuations in the moving speed of the irradiation head 1 during actual use.

[0045] Figure 7 is an image diagram showing the correlation between the amount of defocus of the laser beam and the rate at which the oxide film is generated and removed. Within the depth of focus (within a distance D on either side of the focal position), the rate at which the laser removes the oxide film exceeds the rate at which it is generated, so the thickness of the oxide film is thought to be kept below a certain level. However, because the generation and removal of the oxide film occur simultaneously, the thickness does not become completely zero. Outside the depth of focus, the rate at which the oxide film is generated exceeds the removal rate, so the oxide film thickness increases, and due to thin film interference, it appears blue, and if it becomes even thicker, it appears black, the color of the oxide film.

[0046] The above-described test device is used to irradiate a processing object O (sample) with a laser, and the focal depth of the laser light can be determined by extracting portions that exhibit a blue color, etc. The extraction of the blue portion can be easily evaluated with the naked eye, but for quantitative determination, image processing can be performed on image data obtained by capturing an image of the processing object O. As image processing software, for example, ImageJ (e.g., version 1.53m (released September 28, 2021)), which is public domain image processing software commonly used in scientific research, can be used.

[0047] FIG. 8 is a diagram showing an example of an original image of a processing object after laser irradiation and a blue extracted image. In FIG. ＊ a ＊ b ＊ b in color space hue ＊A method is used in which parts where .gtoreq.0 are colored blue. FIG. 8(a) is a diagram showing an example of an original image of the processing object after laser irradiation. The original image is image data obtained by capturing an image of the processing object O so as to include the entire focused area. FIG. 8(b) is a diagram showing an example of a blue-extracted image. By extracting parts colored blue due to the temper color phenomenon, it is possible to quantitatively evaluate the depth of focus. As shown in FIG. 8, by extracting blue information through image processing, it is possible to quantitatively evaluate the depth of focus. For example, in the blue-extracted image shown in FIG. 8(b), the blue-extracted parts (i.e., defocused parts) are shown in white, and the focused parts are shown in black. Here, the width 2L of the focused part is determined as follows. First, in the example shown in FIG. 8(b), an axis parallel to the movement direction of the processing object is set as the Y-axis, and an axis perpendicular to the Y-axis (i.e., an axis perpendicular to the movement direction of the processing object) is set as the X-axis. Then, one side in the X-axis direction is set for each of the defocused parts divided into upper and lower parts via the focused part. In the example shown in Figure 8(b), X1 is set to the defocused portion located above the focused portion, and X2 is set to the defocused portion located below the focused portion. The point in the defocused portion farthest from X1 in the Y-axis direction is defined as point A, and the point in the defocused portion farthest from X2 in the Y-axis direction is defined as point B. The distance in the Y-axis direction between points A and B is determined as 2L. To perform this type of evaluation, the following points should be taken into consideration: - Use a white light source. - Use ring lighting or diffuse lighting to ensure that the entire surface of the sample is uniformly illuminated, so that specular reflection from the sample is not captured. - Adjust the white balance using a standard reflector or similar to eliminate the influence of the lighting color.

[0048] By summarizing the results of past measurements of focal depth, it became clear that the focal depth (one side) D is proportional to the product of the spot radius r' of the beam spot on the irradiation surface and the half angle θ' of the light collection on the irradiation surface. This can be expressed as the following equation 1. C is a proportionality constant, which is 7.8 x 10 －４ mrad －１It has been revealed that this value is the correct value. Here, for example, based on experiments and papers (e.g., Motoo Nishioka and five others, "Fundamental Study on Changes in Operational Strategies and Movement Characteristics in Repetitive Tasks - Examination of Evaluation Methods Based on Performance and Physiological Data," Journal of Life Science Research, 2008, Vol. 7, pp. 45-55), it is inferred that the maximum focal depth accuracy required for equivalent weight (operation load) and operation speed is 3 mm. Furthermore, for example, in a survey of the corrosion depth of a severely corroded bridge, a survey result reported that the average corrosion depth was 6 mm or less in most areas (Jun Murakoshi and 11 others, "Joint Research Report on Load-Carrying Performance Evaluation Methods for Corroded Bridge Components - Clinical Research Report Using Corroded Steel Truss Bridges," Joint Research Report, Public Works Research Institute, June 2013, No. 456). Based on these survey results, a focal depth of ±3 mm (6 mm on both sides) is deemed to be sufficient for most corroded areas. Therefore, for example, if you want to ensure a depth of focus of ±3 mm, This becomes:

[0049] To ensure safety during processing, the depth of focus is preferably set to, for example, 1 / 3 or less of the distance from the forefront optical element of the irradiation head 1 to the focal position (just-focus position). Figure 9 is a diagram illustrating an example of the upper limit of the depth of focus. In Figure 9, D indicates the depth of focus (one side), and F indicates the distance from the forefront optical element of the irradiation head 1 to the focal position (just-focus position). As shown in Figure 9(b), the processing object O is placed at the end of the depth of focus range relative to the focal point, in other words, at a distance of the depth of focus D relative to the focal point, and irradiated with laser light. By placing the processing object O in such a position and irradiating it with laser light, processing quality can be ensured. In this case, to ensure safety during processing, even if the processing object O has high reflectivity and the laser light is reflected by the processing object O, it is important to prevent the optical elements of the irradiation head 1 and the worker from being damaged by the reflected light. Specifically, it is preferable to avoid the optical elements of the irradiation head 1 and the worker from entering within a range of the depth of focus 2D from the processing object O. Therefore, it is preferable to satisfy F-D≧2D, i.e., D≦F / 3. Furthermore, even if processing quality is to be ensured, it is preferable that the reflected light directed toward the optical elements of the irradiation head 1 and the worker be weak. For example, with respect to the reflected light reflected by the processing object O, the power density of the reflected light at a position 4D away from the processing object O is thought to decrease to about 10% of the power density of the reflected light at the position of the processing object O. Therefore, it is preferable to satisfy F-D≧4D, i.e., D≦F / 5. Furthermore, it is thought that the power density of the reflected light at a position 8D away from the processing object O is thought to decrease to about 2% of the power density of the reflected light at the position of the processing object O. Therefore, it is preferable to satisfy F-D≧8D, i.e., D≦F / 9. The focal depth (one side) D (mm) is determined by the following equation: the scanning width on the focusing surface of the laser light is 2R (mm), the angle between the normal to the focusing surface of the laser light and the normal to the surface of the object to be processed (i.e., the inclination angle of the normal to the focusing surface with respect to the normal to the surface of the object to be processed) is φ, and the maximum angle of this angle (i.e., the maximum inclination angle of the normal to the focusing surface with respect to the normal to the surface of the object to be processed) is φ. ｍａｘ When D≧tanφ×R, or D≧tanφｍａｘ ×R. Here, the focal plane refers to a plane on which the focal point of the laser beam moves in a space free of obstructions such as the workpiece. Due to aberrations in the optical system, the focal point may not move on a plane. In such cases, the plane that best fits the trajectory of the focal point is mathematically determined, and that plane is used as the focal plane. Furthermore, if there is a period during the cycle in which the workpiece is not being processed, such as when the laser beam intensity is reduced or the laser beam is blocked by a shutter, the position of the focal point during that period is not taken into consideration when determining the focal plane. Furthermore, as described above, φ is the angle between the normal to the focal plane and the normal to the surface of the workpiece. However, for example, if the surface of the workpiece is not flat, the plane that best fits the trajectory of the irradiated area (beam spot BS) on the workpiece in one cycle is mathematically determined, and the normal to that plane is used as the normal to the surface of the workpiece. Figure 10 is a diagram illustrating an example of the lower limit of the focal depth. In Figure 10, 2R indicates the scanning width at the focusing surface, specifically, the diameter of the circular ring of the beam spot BS at the focusing surface. φ indicates the angle between the normal to the focusing surface and the normal to the surface of the object to be processed O (i.e., the inclination angle of the normal to the focusing surface with respect to the normal to the surface of the object to be processed O). Figure 10 shows an irradiation head 1 positioned so that the angle between the normal to the focusing surface and the normal to the surface of the object to be processed O is 0 degrees, and an irradiation head 1 positioned so that the normal to the focusing surface is tilted by an angle φ from that state to irradiate the object to be processed O with laser light. Here, if the value of tanφ × 2R falls within the range of the depth of focus (both sides) 2D when the normal to the focusing surface is tilted by an angle φ with respect to the normal to the surface of the object to be processed O, then even when laser light is irradiated, the entire circumference of the scanning pattern of the beam spot on the object to be processed O can be kept within the depth of focus. In addition, the beam spot on the processing object O can be kept within the focal depth over the entire circumference of the rotation circle. ｍａｘ Then, tanφ ｍａｘIf the value of tan φ×2R falls within the range of the depth of focus (both sides) 2D, the entire circumference of the scanning pattern of the beam spot on the processing object O can be kept within the depth of focus, regardless of the tilt angle during operation. In addition, the entire circumference of the rotation circle of the beam spot on the processing object O can be kept within the depth of focus, regardless of the tilt angle during operation. In this way, by setting the lower limit of the depth of focus so that the depth of focus (both sides) 2D≧tan φ×2R, that is, the depth of focus (one side) D≧tan φ×R, the entire circumference of the scanning pattern of the beam spot on the processing object O can be kept within the depth of focus, even if the processing object O is irradiated with laser light with the normal to the focusing surface tilted, unlike the case of FIG. 1 .

[0050] On the other hand, BPP is an index that represents the beam quality of laser light. BPP is defined as the product of the spot radius at the output end of a fiber or the beam waist at the focal point and the divergence half angle or the focusing half angle. BPP is a value that is maintained when propagating using an ideal optical system with no aberrations. In an optical system with aberrations or scattering, the beam quality deteriorates and the value generally becomes larger. In other words, the BPP (oscillator) of laser light emitted from an oscillator and the BPP (irradiation surface) at the irradiation surface where that laser light is focused by an optical system have the relationship shown in Equation 3 below. BPP (irradiation surface) = r'θ' ≧ BPP (oscillator) (Equation 3)

[0051] From these equations, when it is desired to make the depth of focus equal to or greater than D, the beam quality BPP (oscillator) required of the oscillator is expressed by the following equation 4.

[0052] Here, we will also summarize the relationship between the power P of the laser light and the single-point fluence f. The single-point fluence refers to the energy density imparted to the irradiated surface when the laser light is irradiated once (when the beam spot passes once), and can be written as Equation 5 using the spot radius r' on the irradiated surface and the irradiation time τ.

[0053] Using this equation, r′ is eliminated from equation 3, resulting in equation 6.

[0054] The proportional relationship described above was derived by rearranging the data from multiple experiments using different oscillators and optical systems. Therefore, the irradiation conditions were all different. However, because the experiments were carried out under conditions that allowed for efficient rust removal and resulted in good processing quality with minimal oxide film formation, the fluence and irradiation time were close values. The average values ​​are shown below. Irradiation time τ: 9 μs = 9 x 10 －６ Fluence f: 88 J / cm ２ = 88 x 10 －２ J / mm ２ The proportionality constant obtained was as follows: proportionality constant c: 7.8 x 10 －４ mrad －１

[0055] Substituting this value into equation 6 gives equation 7.

[0056] Using Equation 7, the upper limit of BPP is calculated under several conditions, as shown in Table 1 below. In other words, the required BPP is determined by the required focal depth and the output of the laser used.

[0057] Equation 6 indicates that the BPP requirements can be relaxed if τ can be set as large as possible and f as small as possible. However, if τ is increased, for example, to more than 20 μs, the formation of an oxide film becomes noticeable even at the focal position. Furthermore, if f is reduced, for example, to less than 40 J / cm2, the processing efficiency drops significantly, and if the fluence is further reduced, the processing ultimately stops altogether. In other words, these conditions do not satisfy the required processing quality and processing efficiency, and are outside the range for considering the complete formation of Equation 1. To achieve sufficient processing quality and processing efficiency at the focal position while also leaving room for a margin that does not affect the depth of focus, the above settings of 9 μs and 88 J / cm2 are considered to be roughly appropriate.

[0058] In addition, since the irradiation time is 20 μs or less, if the moving speed of the beam spot BS is v (m / s), then 2r′ / v≦20×10 －６ Therefore, r'≦10 －５ ​It can be deduced that it should be ×v.

[0059] It is also possible to irradiate the laser under conditions of a smaller τ and a larger f, but the depth of focus was not significantly improved. In the experiments to determine the proportionality constant, conditions of less than 5 μs and 100 J / cm ２ Although the conditions exceeding this are included, they do not deviate from the proportional relationship we have determined. In this study, we aim to set an upper limit for BPP to ensure the required depth of focus, and such conditions that do not have a significant effect on the depth of focus and require a smaller BPP do not need to be adopted as values ​​to be substituted into Equation 6.

[0060] Note that r' can be measured using, for example, a focus monitor or beam profiler manufactured by PRIMES (registered trademark). θ' can also be measured using a similar measurement device. The method involves measuring the beam diameter while changing the distance, fitting a hyperbola to the relationship between the distance and the beam diameter, and determining the slope at a distance. FIG. 11 shows an example of a method for measuring r' and θ'. In FIG. 11, the horizontal axis represents the position in the beam propagation direction, and the vertical axis represents the beam diameter. For example, 10 or more measurement points are measured at different positions, with half of the measurements being inside the Rayleigh length and the other half being measured at positions at least twice the Rayleigh length. The Rayleigh length refers to the distance at which the cross-sectional area of ​​the beam is twice the cross-sectional area at the focal point. θ' can be obtained by plotting the measurement points, fitting a hyperbola, and determining the slope of the line to which this hyperbola asymptotically approaches.

[0061] The beam diameter is defined as the diameter of a circle that contains 86% of the beam intensity and is centered at the center of gravity of the intensity distribution, and this value is twice r'. If measurement at the focal position is difficult due to problems such as excessive intensity, the intensity of the laser light can be reduced using a half mirror or the beam diameter can be expanded to make the measurement possible, and then conversion can be performed based on the relationship between the optical characteristics of the optical system used and the optical system of the actual laser head (irradiation head).

[0062] From the above-mentioned formula 7, for example, when it is desired to ensure a depth of focus of ±3 mm, the BPP condition is as follows: BPP (oscillator) = rθ ≦ 1.4 × 10－３ × P (Equation 8) Here, r is the spot radius at the end of the fiber, θ is the half angle of divergence of the laser light from the fiber, and P is the laser output. If a quartz block is attached to the tip of the fiber to protect the tip, it is difficult to measure r and θ directly. In such cases, the values ​​can be obtained by focusing the laser light with an optical system, measuring the spot radius and half angle of divergence at the focusing point, and then calculating r and θ using the optical magnification of the optical system. The spot radius at the end of the fiber is approximately the same value as the core diameter of the fiber. From Equation 8, we consider the conditions required for the laser light emitted from the fiber. By substituting both sides for r ２ Dividing by π gives Equation 9. Here, P ｄ represents the power density.

[0063] When attempting to transmit laser light over long distances using a fiber, if the power density in the fiber is high, a phenomenon called SRS (Stimulated Raman Scattering) occurs. SRS amplifies the backscattered light returning to the oscillator in the fiber, which can damage the oscillator, and therefore needs to be suppressed. For example, when attempting to transmit laser light over a distance of 20 m or more using a fiber, the power density is empirically estimated to be 1.5 x 10 ８ (W / cm ２ ) or less. Substituting this value and rearranging it into an equation for r / θ, Equation 10 is obtained.

[0064] This result is thought to indicate the following: Equation 8 alone allows r and θ to take any values ​​as long as the BPP satisfies the condition. However, when attempting long-distance transmission via fiber, there is a limit to the power density in order to suppress SRS, so not only Equation 8 but also Equation 10 must be satisfied. Next, we will examine the difference with r' / θ' = CD. This equation describes the relationship between the focal depth and how the laser light is focused on the focal plane. On the other hand, Equation 10 is a condition required for the laser light emitted from the fiber to prevent the problem of SRS from occurring.

[0065] The background art and effects of the above-described embodiments will be described. In laser processing, the required focal depth is determined by the processing depth. However, in laser cleaning, which removes objects such as rust, dirt, and coatings by irradiating them with laser light, the required focal depth is determined by the operator's operating accuracy and the uneven shape of the object to be irradiated (the object to be processed), particularly when using a handheld laser head or when processing existing equipment with a complex structure having uneven shapes. For example, when operating a handheld laser head weighing several kg, the operating accuracy is empirically limited to plus or minus a few mm.

[0066] On the other hand, when the object to be removed in laser cleaning is, for example, rust or paint film on a bridge or steel tower, the thickness is, for example, several tens to several hundreds of μm. (Typical heavy-duty anti-corrosion coatings are 250 μm.) Such objects are removed by several laser irradiations to avoid unnecessary removal of the base material, so the processing depth in one process is, for example, approximately 100 μm or less. In other words, laser processing by laser cleaning requires a focal depth several tens of times greater than the focal depth required by the processing process, unlike other laser processing methods commonly used in the past, such as cutting and drilling.

[0067] Furthermore, in general, for any laser processing, there is an optimal parameter range for obtaining the required processing quality; specifically, there is an optimal power density range and irradiation time range. In other words, the required depth of focus must be ensured within this parameter range. The easiest way to increase the depth of focus is to proportionally increase the size of the optical system. With this method, in order to maintain the power density so that it is the optimal parameter for processing, it is necessary to increase the laser output by the square of the proportional magnification. Naturally, the opposite is true when decreasing the depth of focus.

[0068] In conventional laser processing such as cutting and drilling, the amount of material to be removed naturally decreases when processing fine details, so a low-power laser is selected. Selecting a low-power laser also results in a smaller optical system and shallower processing depth and focal depth. However, as the processing size decreases, the required processing depth and focal depth also generally become shallower accordingly, so both processing depth and focal depth are achieved and no problems arise. Conversely, when processing thick plates, the required processing depth and focal depth become deeper, but a high-power laser is selected to achieve the necessary processing speed. In this case, the laser head also becomes larger to handle the high-power laser, which results in deeper processing depth and focal depth. Therefore, both processing depth and focal depth are achieved and no problems arise. On the other hand, unlike conventional laser processing such as cutting and drilling, in the case of laser processing by laser cleaning, the required focal depth is determined by the uneven structure of the object to be irradiated (object to be processed) and the precision of the operator's operation, particularly when performing laser cleaning with a handheld laser head, so it is necessary to consider both the processing quality and focal depth, which was not a major issue in conventional laser processing.

[0069] Therefore, according to the embodiment described above or the modified examples described below, the following effects can be obtained: (1) When the spot radius of the laser light at the focal position is r' (mm) and the half angle of the laser light is θ' (mrad), r' / θ'≧2.3×10 －３ (2) When the output of the laser oscillator is P (W), the beam parameter product BPP of the laser light is 1.4×10 －３ ×P or less, it is possible to appropriately achieve both processing quality and depth of focus in laser cleaning. (3) When the spot radius of the laser light is r' (mm) and the moving speed of the beam spot BS on the surface of the processing object O is v (mm / s), r'≦10 －５×v, it is possible to ensure the quality of the laser cleaning process at the focal position (just focus position). (4) The focal depth (one side) of the laser light is one-third or less of the distance from the foremost optical element of the laser irradiation device to the focal position, which increases the possibility of preventing damage to the optical elements of the laser irradiation device and the worker compared to when the focal depth of the laser light is greater than one-third of the distance from the foremost optical element of the laser irradiation device to the focal position. (5) When the spot radius of the laser light at the output end of the fiber is r (mm) and the divergence half angle of the laser light is θ (mrad), r / θ≧1.5×10 －４ (6) When the scanning width of the laser beam on the surface of the object to be processed is 2R (mm) and the tilt angle, which is the angle between the normal to the focusing surface of the laser beam and the normal to the surface of the object to be processed, is φ, the focal depth (one side) D (mm) of the focusing optical system is D≧tanφ×R, so that even when the normal to the focusing surface is tilted, the entire circumference of the scanning pattern of the beam spot on the object to be processed can be kept within the focal depth.

[0070] (Modifications) The present invention is not limited to the above-described embodiments, and various modifications and variations are possible, all of which are within the technical scope of the present invention. The configuration of the laser irradiation device is not limited to the above-described embodiments, and can be modified as appropriate. For example, the method for scanning the irradiated surface with the beam spot is not limited to rotating a wedge prism as in the embodiments, but other methods, such as a galvanometer scanner or a polygon mirror, may be used. Furthermore, the scanning pattern of the beam spot, in other words, the trajectory of the beam spot, is not limited to a circular rotation as in the embodiments, but can be modified as appropriate, for example, to an approximately circular, elliptical, oval, Lissajous figure, polygonal, or other shape. Furthermore, the irradiation parameters shown in the embodiments are merely examples, and the irradiation parameters can be modified as appropriate within the technical scope of the present invention.

[0071] Modified examples of the beam spot scanning pattern will be further described. FIG. 12 shows modified examples of the beam spot scanning pattern. FIG. 12(a) shows an example of a circular beam spot scanning pattern. FIG. 12(b) shows an example of an elliptical beam spot scanning pattern. FIG. 12(c) shows an example of a substantially elliptical beam spot scanning pattern. FIG. 12(d) shows an example of a Lissajous figure beam spot scanning pattern. For various beam spot scanning patterns, the following is set: D≧tanφ×R, where D (mm) is the focal depth (one side) of the focusing optical system, 2R (mm) is the narrowest width of the beam spot scanning pattern on the focusing surface, and φ is the angle between the normal to the focusing surface and the normal to the surface of the workpiece. By setting the lower limit of the focal depth in this way, even if the normal to the focusing surface is inclined, the entire circumference of the beam spot scanning pattern on the workpiece can be kept within the focal depth. Furthermore, the maximum value of the angle φ in the operation is set to φ. ｍａｘ In this case, D≧tanφ max×R. By setting the lower limit of the focal depth in this manner, even if the normal to the focusing surface is tilted, the entire circumference of the scanning pattern of the beam spot on the processing object can be kept within the focal depth, regardless of the tilt angle during operation. Here, the width of the scanning pattern of the beam spot refers to the distance between two parallel lines that are tangent to the scanning pattern but do not intersect with the scanning pattern, sandwiching the scanning pattern. The scanning pattern of the beam spot is an example of a specific shape traced by the laser light spot through periodic motion. In the example shown in FIG. 12(a), the scanning pattern is circular, and the width of the scanning pattern is uniform, equal to the diameter of the circle. In the example shown in FIG. 12(b), the scanning pattern is elliptical, and the width of the scanning pattern can take various values. The minor axis of the ellipse is the narrowest width of the scanning pattern. In the example shown in FIG. 12(c), the scanning pattern is approximately elliptical, and the width of the scanning pattern can take various values. In this example, the distance L1 shown in FIG. 12(c) is the narrowest width of the scanning pattern. In the example shown in FIG. 12(d), the scan pattern is a Lissajous figure. For example, a scan pattern of this type can be obtained when a galvanometer mirror is used. In this example, the width of the scan pattern can take various values, but the distance L2 shown in FIG. 12(d) is the narrowest width of the scan pattern. Note that the example shown in FIG. 12(d) is an example of a Lissajous figure as a scan pattern. The scan pattern in this embodiment is not limited to the Lissajous figure shown in FIG. 12(d) and includes other Lissajous figures. Furthermore, in the above-described modified example, the narrowest width of the scan pattern of the beam spot on the focusing surface is 2R (mm), but this is not limited to this modified example. For example, the widest width of the scan pattern of the beam spot on the focusing surface may also be 2R (mm). Additionally, if the narrowest width of the scan pattern of the beam spot is 2R (mm), and the normal to the focusing surface is tilted in the width direction, the entire circumference of the scan pattern of the beam spot on the processing object can be kept within the depth of focus.On the other hand, if the widest width of the beam spot scanning pattern is set to 2R (mm), the entire circumference of the beam spot scanning pattern on the processing object can be kept within the depth of focus regardless of the direction in which the normal to the focusing surface is tilted. However, this means that the required depth of focus (one side) D becomes larger because the lower limit of the depth of focus (one side) D becomes larger. Furthermore, the value between the narrowest and widest widths of the beam spot scanning pattern on the focusing surface may also be set to 2R (mm). For example, the average width of the beam spot scanning pattern on the focusing surface may also be set to 2R (mm).

[0072] REFERENCE SIGNS LIST 1 irradiation head 10 focus lens 20 wedge prism 30 protective glass 40 rotating cylinder 50 motor 60 motor holder 70 protective glass holder 80 housing 90 duct 91 inner cylinder 91a small diameter portion 91b tapered portion 92 outer cylinder 92a small diameter portion 92b end portion 93 dust collector connection cylinder O treatment object BS beam spot PG purge gas S stage

Claims

1. A laser irradiation device that irradiates a processing object with laser light to remove a removal target from the surface of the processing object, comprising a focusing optical system that focuses the laser light emitted from a laser oscillator at a predetermined focal position, and where the spot radius of the laser light at the focal position is r' (mm) and the focusing half angle of the laser light is θ' (mrad), r' / θ'≧2.3×10 －３ A laser irradiation device characterized by:

2. When the output of the laser oscillator is P (W), the beam parameter product BPP of the laser light is 1.4 × 10 －３ 2. The laser irradiation device according to claim 1, wherein the laser irradiation device has a wavelength of 1000 nm or less.

3. A laser irradiation device for irradiating a processing object with laser light to remove a removal target from the surface of the processing object, the device comprising a focusing optical system for focusing the laser light emitted from a laser oscillator at a predetermined focal position, wherein when the output of the laser oscillator is P (W), the beam parameter product BPP of the laser light is 1.4 × 10 －３ x P or less.

4. When the spot radius of the laser light is r' (mm) and the moving speed of the spot on the surface of the processing object is v (mm / s), r'≦10 －５ 4. The laser processing apparatus according to claim 1, wherein x and v are the same.

5. A laser irradiation device according to claim 1 or claim 3, characterized in that the focal depth of the laser light is one-third or less of the distance from the optical element at the forefront of the laser irradiation device to the focal position.

6. An irradiation head having the focusing optical system and a fiber that propagates the laser light from the laser oscillator to the irradiation head, wherein when the spot radius of the laser light at the output end of the fiber is r (mm) and the divergence half angle of the laser light is θ (mrad), r / θ≧1.5×10 －４ The laser irradiation device according to claim 1 or 3, characterized in that:

7. A laser irradiation method using the laser irradiation device according to claim 1 or claim 3, further comprising a laser beam moving unit that moves a spot of the laser beam on the surface of the object to be processed and scans the surface with the laser beam, characterized in that when the scanning width of the laser beam on the surface is 2R (mm) and the tilt angle, which is the angle between the normal to the focusing surface of the laser beam and the normal to the surface, is φ, the focal depth (one side) D (mm) of the focusing optical system is D≧tanφ×R.

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