WO2020008398A1

WO2020008398A1 - Laser apparatus and method

Info

Publication number: WO2020008398A1
Application number: PCT/IB2019/055697
Authority: WO
Inventors: Simon CHARD; Young Kwon; Lalith PREMATILLEKE
Original assignee: Andritz Ag
Priority date: 2018-07-03
Filing date: 2019-07-03
Publication date: 2020-01-09

Abstract

A laser surface treatment apparatus includes a laser having a beam emission path, a rotatable beam director in the beam emission path; and a control system configured to control laser beam emission according to the rotational position of the beam director. A method of laser surface treatment for a system is further provided.

Description

LASER APPARATUS AND METHOD

[0001] This disclosure relates to lasers, in particular to lasers for surface treatment such as the removal of a coating from a substrate.

Background

[0002] Laser surface treatment such as laser coating removal provides potential environmental, cost and quality advantages over existing processes.

[0003] A number of methods for the removal of surface coatings using lasers are known. One method is known as the ablation method and comprises ablation of the surface using high frequency laser pulses, where the coating is gradually removed stepwise from the top or outermost down. This method is best suited to opaque coatings that strongly absorb the laser radiation. Using the ablation method, coating removal efficiency is maximised when the beam energy (and therefore beam fluence for a given beam radius) is just above the ablation threshold (i.e. the minimum energy for the surface to be ablated) and the maximum pulse frequency achievable at that beam energy, given a finite laser power budget.

[0004] Another method is best suited to highly transparent coatings on opaque substrates, wherein a substantial portion of a high energy laser pulse passes through the surface coating and is absorbed at the interface between the coating and the substrate. This results in the ablation (i.e. the vaporisation) of material at the interface, generating large pressures at the interface causing detachment and ejection of the coating. This method may be referred to as the detachment method. Maximum coating removal efficiency using this method is achieved when the pulse energy is high (allowing the laser beam diameter to be increased). Typically, coating removal efficiencies using the detachment method are much higher than using the ablation method. [0005] Another method concerns, but is not limited to, laser coating removal conditions where the coating or top layer is highly or semi-transparent to the laser wavelength, while the laser radiation can be absorbed by the substrate or interface of the coating and substrate, or cause severe desorption, outgassing or rapid decomposition at the interface. Coating removal can take place in a manner such that the partially transmitting coating is ablated with adequate pulses (i.e. reduced in thickness) until enough irradiance can be transmitted through the remaining coating thickness to induce the detachment method and detach it from the substrate. An adequate pulse can also be a proportion of a single pulse, whereas the first part of the pulse ablated the surface of the coating and reduces its thickness such that the pulse irradiance reaching the coating substrate interface is capable of detaching the coating.

[0006] In order to be used for removal of a coating from a substrate, the laser is scanned across the surface of the substrate. This is typically achieved by rotating a mirror from which the laser beam is reflected, thereby scanning the laser beam across the surface of the substrate or workpiece. However, using currently known methods, problems arise due to over-burning at the edges of the scan- width, also referred to herein as edge burning.

[0007] This edge burning issue has previously been resolved by the use of a fixed aperture that trims the start and the end of the scanning beam, however this only works at a fixed scan-width and requires additional equipment to cool the aperture as it heats up. This method is also inefficient in terms of its energy consumption.

[0008] An alternative known solution to the problem of edge burning is the use of a two- axis scanner that moves the laser across two axes, e.g. in a circular motion. However, this method requires a larger, heavier scanning unit and prohibits uniform scanning across a linear pattern.

[0009] CN 106694472 A discloses a laser cleaning method which includes: transmitting the initial light from the laser through an optical fiber; collimating the initial light through a collimating lens to obtain parallel light; and reflecting the light at a reciprocating rotating mirror. The light reflected oscillates back and forth; the reciprocally oscillating collimated light is then focused through a focusing lens to form a linear light spot on the focusing plane; a light exiting control unit allows the laser to rotate during a uniform rotation of the oscillating mirror. The laser is prohibited from emitting light during the speed change rotation of the galvanometer.

Summary

[0010] An invention is set out in the independent claim(s).

[0011] By providing an apparatus comprising a control system that determines the rotational position of a beam director, where the control system synchronizes the control of a laser with the rotational position of the beam director, which is to say that it controls the laser beam emission according to the rotational position of the beam director, a number of benefits are provided. The apparatus provides means for improving the uniformity of irradiance at the workpiece during laser surface treatment. In embodiments, the provision of a lens located between the laser and the beam director offers means for preventing over-burning towards the edges of the scanning of a laser whilst offering a wider scan-width for a given rotation angle of the mirror, which improves processing speed for apparatus with a range of configurations for use in coating removal and other applications.

[0012] Furthermore, additional aspects disclosed herein provide a number of solutions to apparatus of different configurations thereby providing a versatile system that improves performance and efficiency when compared to the state of the art.

Figures

[0013] Specific embodiments are now described, by way of example only, with reference to the drawings, in which:

[0014] Figure 1 depicts four laser surface treatment apparatuses with different types of lenses, each located after the beam director, and also depicts the variation in spot size and spatial distribution for each apparatus without compensation by any control method; [0015] Figure 2 depicts a laser surface treatment apparatus with a lens located before the beam director and also depicts the variation in spot size and spatial distribution without compensation by any control method;

[0016] Figure 3 depicts an embodiment of the laser surface treatment apparatus;

[0017] Figure 4 depicts the variation in spot size and spatial distribution for different configurations of laser surface treatment apparatuses;

[0018] Figure 5 depicts the effect of the control system on the laser pulse distribution;

[0019] Figure 6 depicts an embodiment of the laser surface treatment apparatus.

[0020] The inventors have found that edge burning is primarily caused by the laser fluence varying as it scans across the workpiece or substrate. Laser fluence is the optical energy delivered per unit area per unit time. This variation in fluence can be caused by several different factors, depending on apparatus configuration. For example, due to the inertia of the beam director, for example a mirror, the scan speed varies near the edges of the scan due to accelerating or decelerating when changing direction of motion. This change of speed creates a variation in laser fluence at the start or at the end of a scanning cycle, which creates burn marks on the substrate at the end points of the scan. As another example, even a constant rotational speed of the mirror does not result in a constant speed of the laser spot at the workpiece due to angular distortion. As another example, field distortion of a lens means the light field is circular instead of planar. Therefore, unless the shape of the material matches this curvature the laser spot size at the material will vary across the width of the scan.

Detailed Description

[0021] Fig. 1(a) to 1(d) depict four laser surface treatment apparatuses with different types of lenses which have different advantages associated with them but result in different identified problems, which this apparatus provides a solution to. Each assembly comprises a laser (1) emitting a laser beam along a beam emission path (2), a rotatable beam director (3) in the beam emission path (2) and a lens (4) in the beam emission path (2). These apparatuses are positioned such that the laser beam emission path (2) falls upon a workpiece (5). [0022] Fig. la, by way of example, shows the laser (1) having a beam emission path (2) along which a laser beam is directed. The rotatable beam director (3) is in the form of a rotatable mirror which is in the beam emission path (2) and reflects the laser beam. As the rotatable mirror rotates the angle of reflection of the laser beam changes. The lens (4) is located after the mirror such that the laser beam is reflected off the mirror and through the lens (4). Having passed through the lens (4), the laser beam is focused onto a workpiece (5). Thus, as the rotatable mirror is rotated, the laser beam is scanned back and forth across the workpiece (5). The laser beam forms on the workpiece (5) with a spot size (6).

[0023] When the control system (8) is not compensating for the distortion caused by the lens (4), the spot size (6) of the laser (1) on the workpiece (5) is different depending on the type of lens (4) used. For case 1(a), the lens (4) is an f-theta lens of the type known to the skilled person and having the characteristic the output beam displacement is approximately equal to f* Q, where Q is the angle of incidence of the input beam. For this type of lens (4), the spot size (6) across the workpiece (5) is significantly constant although fluence may still vary across the scan- width. For case 1(b), the lens (4) is a flat field scanning lens and the spot size (6) is substantially constant but the position on the workpiece (5) is not linearly related to the scan angle, resulting in an uneven fluence at the workpiece (5) across the scan due to angular distortion. For case 1(c), the lens (4) is a variable focal length lens and the spot size (6) varies in size and spatial distribution across the workpiece (5). For case 1(d), the lens (4) is a simple focus lens and the spot size (6) varies in size and spatial distribution across the workpiece (5).

[0024] Fig. 2 shows an alternative configuration to Fig. 1, with the lens (4) located in the beam emission path (2) and between the laser (1) and the rotatable beam director. The spot size (6) varies in size and spatial distribution across the workpiece (5) because of field and angular distortion respectively.

[0025] Fig. 1 and Fig. 2 thus show the effect on spot size (6) of different optical configurations and different lenses (4) due to field or angular distortion. These depict the situation without the proposed control system (8). [0026] Fig. 3 is a block diagram of an embodiment of a laser surface treatment apparatus (7). The apparatus comprises a control system (8) that comprises an internal processor (9) and memory (10). The control system (8) is connected to a power supply (11) which may be internal to the laser surface treatment apparatus (7) (as depicted) or may be located externally. The control system (8) may be in electrical communication with a lens (4) and with a beam director (3) which is itself in electrical communication with a galvanometer and driver (12) such that the driver can cause the beam director (3) to rotate upon receipt of a signal from the control system (8). The control system (8) also controls the laser emission status (13) which can be done using the laser’s own internal control methods or can be controlled using an external modulator (14) comprised in the apparatus. The apparatus comprises a user interface (15), which itself may comprise a display (16), which enables a user to engage with the apparatus by providing information to the control system (8) as well as receiving information from the control system (8). The inventors have found that laser fluence varies in different ways for different types and position of lenses (4). Understanding how laser fluence naturally varies for different configurations is important to understanding how to provide a versatile and effective laser surface treatment apparatus (7). Fig. 4(a) shows how the spot size (6) varies in spatial distribution across the scan-width due to varying speed of the mirror for the case when the lens (4) is placed between the mirror and the workpiece (5) as shown in Fig. 1 and there is no synchronisation by the control system (8). The rotatable mirror slows down when changing direction through inertia, which results in overlap between the laser spots and an uneven laser fluence. This can cause burning at the edges.

[0027] Fig. 4(b) shows how the spot size (6) varies in size across the scan-width for the case when the lens (4) is placed between the laser (1) and the mirror as shown in Fig. 2 and there is no synchronisation by the control system (8). The spot size (6) increases in size near the edges of the scan-width which results in an uneven, lower or overlapping laser fluence.

[0028] Fig. 4(c) shows an aperture that has been traditionally used to trim the edges of the scan in an effort to produce a constant laser fluence on the workpiece (5). [0029] Fig. 4(d) shows the result of the proposed control system (8) synchronising the control of the laser emission status (13) with the rotational position of the mirror.

[0030] Fig. 5(a) shows an example of a scanning mirror control signal which drives the rotation of the mirror or beam director (3) while Fig. 5(b) shows how the laser spots on the workpiece (5) vary in spatial distribution with a greater amount of overlap between the laser spots at the edges of rotation. Fig. 5(c) shows the spatial distribution of the laser pulses on the workpiece (5) across the scan width and without any synchronisation of laser emission status (13) control. Fig. 5(d) depicts a signal sent by the control system (8) synchronising the control of the laser emission status (13) with the position of the beam director. Fig. 5(e) shows the resulting spatial distribution of the laser pulses that results from the synchronised control. This synchronisation results in a more even laser fluence.

[0031] Fig. 6 shows an embodiment of a laser surface treatment apparatus (7) wherein the laser (1) is one of a fibre or Nd: YAG laser that delivers the laser beam (2) by an optical fibre (17) through two lenses (4) located before the beam director (3). The beam director (3) is shown to be rotating back and forth and this movement is driven by the galvanometer driver (12). The control system (8) is configured to control the laser (1) and the laser emission status (13) of the laser (1), and to control the galvanometer driver (12). The control system (8) synchronises the laser beam emission with the rotational position of the beam director (3) in order to achieve edge blanking. The control system (8) can also adjust the speed of the rotation of the beam director (3) to account for angular distortion. The control system (8) can also vary the energy to account for field distortion.

[0032] In operation, the mirror is rotated, and the control system (8) synchronises the laser emission status (13) with the rotational position of the beam director, for example a mirror. In so doing, the control system (8) determines, in other words controls and/or detects, the rotational position of the mirror and controls the laser beam emission according to the rotational position of the mirror. In one embodiment, when the mirror is in a central region, the control system (8) controls the laser emission such that the laser (1) emits a laser beam that is reflected off the rotating mirror. The laser beam is reflected off the mirror onto a workpiece (5) that is to have its surface treated. Because the mirror is rotating, the laser beam is scanned across the workpiece (5). The laser (1) is typically a pulsed laser but can also be continuous wave. The control system (8) determines the rotational position of the mirror and, when the rotational position is within an outer region of rotation, the control system (8) is configured to turn the laser (1) off.

[0033] The size of the central and outer regions of rotation may be fixed or may be adjusted during operation. The sizes may be automatically adjusted by the control system (8) or may be manually adjusted. The control system (8) may be configured to receive user input, for example via by the user interface (15), to set the sizes of the respective regions. The control system (8) may be configured to vary the size or position of the central region according to a desired scan-width wherein a smaller central region results in a smaller scan- width. A smaller scan-width helps to reduce edge burning. However, a larger central region and scan-width that results in an amount of edge burning may be tolerated for example, to speed up the coating removal process or for other reasons. The extent of the edge burning that is to be tolerated may be automatically calculated, may be input by the user or may be stored in the memory (10) associated with the control system (8).

[0034] In another embodiment, controlling laser beam emission involves changing laser pulse energy. For example, the pulse energy of the laser (1) can be reduced near the edges of the scan-width. Reducing the pulse energy helps to maintain the laser fluence at a constant level across the scan-width. The control system (8) can control the energy of the laser pulses by, for example, sending a signal to the laser (1) to control the energy of the laser pulse using the laser’s own internal power adjustment method. As another example, the laser pulse energy can be controlled by passing the laser beam through an external modulator (14) that allows continuous attenuation of the pulse energy, such as an electro-optic or acoustic-optic modulator (14).

[0035] The control system (8) may also control the position of the mirror, which can involve changing the extent or range of movement or changing the speed of movement of the mirror. The control system (8) controls both the position of the mirror and the laser emission status (13). For example, pulses are emitted only when the controller sets the mirror to a position within the central region. By increasing the extent or range of the movement of the mirror, the scan-width is increased. This increases the speed of coating removal. The scan- width is programmable and can be varied by the control system (8). Alternatively, the control system (8) can monitor the position of the mirror without actively controlling the rotational position of the mirror. The emission of the laser (1) can be controlled accordingly. For example, laser pulses are only emitted when the controller detects that the mirror is in the central region. Thus, synchronisation of the laser emission status (13) with the rotational position of the mirror can be achieved by various methods.

[0036] For example, a method of preventing edge burning is to control laser emission status (13) by reducing the pulse repetition frequency when the mirror slows down near the edges of the scan. If the frequency is synchronised with the speed of the laser beam at the workpiece (5) then the distance between the pulses at the workpiece (5) can be kept constant despite the varying speed of the beam at the workpiece (5). This could be achieved by providing a suitable signal to the laser (1) system, or by directly controlling the Q-switch or pulse picker of the laser (1). The pulse frequency can be synchronised directly with the speed of the mirror, or it could be corrected in the control system (8) to also account for the varying speed of the laser beam at the workpiece (5) due to angular distortion. This can increase the scan width. Additional laser parameters may be changed to ensure that the energy of the laser (1) remains constant while changing the pulse repetition frequency, or that the energy varies as required to compensate for other distortion effects. The control system (8) can control the energy of the laser pulses by, for example, sending a signal to the laser (1) to control the energy of the laser pulse using the laser’s own internal power adjustment method. As another example, the laser pulse energy can be controlled by passing the laser beam through an external modulator (14) that allows continuous attenuation of the pulse energy, such as an electro-optic or acoustic-optic modulator (14).

[0037] As discussed previously, there are a number of different factors that can affect fluence including field distortion, angular distortion and speed variation, the effects of which are depicted by the varying spot size (6) in Fig. 1 and in Fig. 2. Depending on the position of a lens (4), and the type of lens (4), the different factors are responsible for altering the fluence. Therefore, the control system (8) controls the laser beam emission in different ways depending on the configuration of the lens (4).

[0038] For example, if a flat field scanning lens is placed after the mirror, as in Fig. 1(b), there is no field distortion, but angular distortion occurs. The control system (8) therefore keeps the laser energy constant across a central portion of the scan while speed of the mirror is varied across the scan to compensate for angular distortion. The laser pulses are turned off when the rotational position is within an outer region of rotation or the pulse energy is reduced near the edges, as described previously.

[0039] As another example, if an F-theta lens is placed after the mirror, as in Fig. 1(a), there is no field or angular distortion. The control system (8) therefore keeps the laser energy constant across a central portion of the scan while the mirror is rotated at a linear speed. The laser pulses are then turned off when the rotational position is within an outer region of rotation or the pulse energy is reduced near the edges, as described previously.

[0040] As another example, if a variable focus lens is placed after the mirror, as in Fig. 1(c), there is both field and angular distortion. The control system (8) therefore keeps the laser energy constant across a central portion of the scan while the focal length of the lens is varied across the scan to compensate for field distortion and the mirror speed is varied according to the rotational position of the mirror to compensate for angular distortion. This method could be used to maintain a constant beam size across a flat workpiece (5), but it could also be adapted to maintain an approximately constant beam size across a non-flat workpiece (5) by tailoring the variation of the focal length to match the workpiece (5) shape. The laser pulses are then turned off when the rotational position is within an outer region of rotation or the pulse energy is reduced near the edges, as described previously.

[0041] As another example, if a simple lens is placed after the mirror, as in Fig. 1(d), there is both field and angular distortion. The control system (8) therefore varies the laser energy according to the rotational position of the mirror to compensate for field distortion while the speed of the mirror is varied across the scan to compensate for angular distortion. The laser pulses are then turned off when the rotational position is within an outer region of rotation or the pulse energy is reduced near the edges, as described previously.

[0042] As another example, if a lens (4) is placed before the mirror, there is both field and angular distortion. The control system (8) therefore varies the laser energy according to the rotational position of the mirror to compensate for field distortion while the speed of the mirror is varied across the scan to compensate for angular distortion. The laser pulses are then turned off when the rotational position is within an outer region of rotation or the pulse energy is reduced near the edges, as described previously. The lens (4) or lenses (4) can be any suitable kind of lens (4), as will be apparent to the skilled person but may, for example, be a flat field, f-theta or simple lens. Alternatively, the lens (4) can be a variable focus lens which provides an alternative to varying the energy of the laser (1) to reduce the impact of the field distortion.

[0043] The apparatus may be configured with respect to the substrate in any manner such that the beam forms an angle of incidence with the substrate or workpiece (5) with any angle between perpendicular and horizontal.

[0044] These or other methods of surface treatment may be appropriate for use with this apparatus.

[0045] As discussed above, the selection of appropriate laser (1) and laser parameters is important for different kinds of surface treatments and different applications, for example, to increase the effectiveness of apparatus during coating removal. The laser (1) may be any kind of laser suitable for the purpose such as but not limited to a C02 laser, a solid-state (e.g. Nd:YAG) laser, fibre laser, think disk laser, an ultrashort pulsed laser with a duration of some picoseconds or femtoseconds, a high power diode laser or a number of excimer lasers. Different lasers (1) have different properties and offer different advantages in practical applications, as will be appreciated by the skilled person. There are several parameters that must be considered such as energy, intensity, wavelength, laser spot size (6), pulse duration and number of pulses, beam area, scanning speed and so forth. For example, lasers (1) may be used with any appropriate wavelength of light such as, for example, 1064 nm. Different wavelengths offer different advantages in practical applications.

[0046] All of these parameters offer different advantages in practical applications as will be appreciated by the skilled person. For example, as beam intensity is increased the coating removal may be improved but temperature will also rise which could increase burning or damage to the substrate. A smaller beam spot size (6) if often beneficial for laser cleaning however if the beam spot size (6) is too small it may damage the surface. The laser (1) may be pulsed or continuous. Pulse duration is the time it takes for the laser (1) to emit one pulse. If the pulse duration is too short, the cleaning is not efficient however if the pulse duration is too long, substrate is affected by heat. A higher removal rate can be achieved in the case of short pulses for a fixed amount of laser energy. Thus appropriate pulse duration should be selected. The laser radiation is delivered in pulses with a duration ranging from 1 attosecond to several minutes, sequenced in bursts consisting of one or more than one pulses, adequate to irradiate a designated area of the material as the beam changes position in relation to the material surface and vice versa. Pulse repetition rate is the number of pulses per second. If the pulse repetition rate is too high the laser pulses interfere with one another while if the pulse repetition rate is too slow, the cleaning takes more time. Therefore pulse repetition rate should be selected as appropriate.

[0047] All of these parameters and more may be adjusted separately or together as befits the circumstances of use in any appropriate manner, as will be known to skilled person. Controlling laser beam emission comprises adjusting one or more of these parameters. The advantages and disadvantages of each type of laser (1) depends on the laser parameters selected, the coating to be removed and the substrate material.

[0048] These parameters and more may be controlled in use by sending a signal directly to the laser system or altered using the laser’s own internal laser modulation method. For example, laser pulses can be turned off by sending a“gate” signal to the laser (1) to enable or disable pulses using the laser’s own internal laser modulation method. As another example, the emission of each individual laser pulse is controlled by modulating the laser’s (1) pulse picker or Q-switch. A number of these parameters may also be controlled by, for example, the laser beam passing through an external modulator (14), such as an electro-optic modulator, acousto-optic modulator, or shutter. The signal to the modulator (14) controls whether pulses emitted by the laser (1) reach the workpiece (5). By controlling these parameters in synchronization with the rotational position of the mirror, the laser fluence can be kept constant across a scan-width. These parameters may also be changed independently of rotational position of the mirror, for example, due to different substrates, different treatment methods or other application specific requirements such as speed of scanning and tolerance for edge burning or ineffective cleaning.

[0049] One example of a laser (1) source is a diode pumped Nd:YAG laser with q- switching to control the pulsing of the laser. Q-switching is not essential and pulsing can be controlled by other means, e.g. via electrically controlling seed pulses injected into the laser resonator. Pumping can also be performed with methods other than a laser diode, e.g. by flash lamp pumping. The amplifying medium of the laser may be composed of a selection of other crystals or glasses. The source may also be fibre based or a mixture of fibre and crystal block amplifiers and oscillators, q-switched or not. The source may also be of semiconductor nature, such as laser diodes. The source may also use gas or liquid media of amplification, such as dye, C02, N2, combinations of noble gasses with halogens and other combinations of gasses. The source may also be mode-locked, or amplified in a supercontinuum medium, further amplified and tuned via optical parametric amplification. The laser source may emit a second, third or other frequency multiplication harmonic, filtered or in combination with all other emitted harmonics. A combination of the above lasers and wavelengths may also be used for emitting the necessary radiation.

[0050] 1.0 pm pulsed lasers (solid state YAG or fibre lasers) are semi-transparent for most paints and are preferred. 0.5 pm, 1.5 pm or 2.0 pm laser radiation may also be used, depending on the coatings and paints. The wavelength of the radiation may be may be between 0.2-2.0 pm, preferably 1.0-1.5 pm. [0051] This is given by way of an example only; selection of these and other parameters is not limited in this application but may be selected as appropriate for the application and it is within the remit of the skilled person to select these parameters appropriately. Furthermore, the apparatus may comprise more than one laser (1) of the same or different kind.

[0052] The laser beam emitted is moved in relation to the substrate with the use of a beam director (3) which may comprise deflective or refractive optics, for example mirrors or prisms respectively. Where the apparatus is described with reference to a beam director, a rotatable mirror will also work. Similarly, if examples have been given that include a mirror, it will be appreciated that another form of beam director (3) or series of beam directors, such as a Risley prism pair, could also be used instead. The apparatus is described with reference to one mirror, but more than one mirror could be used to achieve a similar effect. These optics may be electromechanically moved, rotated and controlled, for example by a galvanometer. The beam motion may also be controlled by an acousto-optic device or other electro-optic device. The coated material can also be moved in reference to the beam, to give the same results, or both beam and material can be moved together, simultaneously in independent directions. The laser (1) itself can be moved with respect to the mirror to cause the laser beam to scan across the surface. The whole assembly comprising the scanners, fibre exit, other electronics, water, air feeds and gas extraction, and laser source box can also be moved in relation to the coated surface.

[0053] The beam may be moved across the surface of the coating/substrate at a speed that may be defined by other process considerations e.g. for a 10 kHz pulse repetition rate for a 100 pm thick acrylic based white paint, where the beam diameter at the surface of the paint is 3 mm, may be moved at 20m/s relative to the coating/substrate, or at 3 m/s for a 500 pm thick coating. As another example, the beam may be moved with a particular speed according to the rate of scanning that is required. The speed of the beam can be changed by changing the speed of the rotation of the mirror. Using an appropriate configuration of beam directors, the beam can follow linear, raster, circular or other patterns defined by vector components or curved trajectories. Release of pulses can be continuous over a beam trajectory being transcended or sporadic in bursts.

[0054] The beam is focussed by a single lens (4) or a lens system of f-theta type or flat field scanning lens, or variable focus or other type of focussing lens. The apparatus can comprise one or more lenses (4), for example, the apparatus can have one lens (4) located between the laser (1) and the mirror, and a second lens (4) located between the mirror and the workpiece (5). The beam may also be focussed by a lens system (4) that forms a line focus, an oval focus or a rectangle focus. There are different advantages and limitations associated with the different types of lens (4) and the positioning of the lens (4), as discussed previously.

[0055] Sensors may be included with, or positioned next to, the scanner or beam delivery optics for the purpose of sensing, for example, the emissions of the ablation plasma and fire, or the distance from the coated material being processed, or the reflection of the laser or other light from the coated material surface. The sensors may also sense other properties like dielectric permeability, refractive index, scattering, reflection or scattering of other electromagnetic regions like X-rays, acoustic reflections and scattering, etc. A camera may also be included next to or with the scanning optics or focussing optics. The beam delivery optics may be focusing the beam on the coated material being processed or may direct a collimated or divergent beam on the coated material.

[0056] The equipment may also contain extraction inlets, ducts and filtration systems to collect and manage the process waste produced by the removal process, such as a vacuum extractor and filtration system.

[0057] The beam may be moved across the surface of the coated material with a speed and a pulse repetition rate that allow each pulse to impinge on the material overlapping 100% or to a smaller percentage or not to overlap at all. Each pulse may be ablating material from the coating or detaching material from an area of the material that has received more pulses before the current one. [0058] The approach described is suitable for a wide range of coatings and surfaces. In one example, the coating comprises one or more layers comprising materials that are semi transparent to the wavelength of the laser. Layers closer to the substrate may be fully transparent to the laser wavelength. Anti-reflection coatings may also be included in the sequence of coating layers. The coating materials may be one of or a combination of the following, polymer based, gelatines, glass, crystal, polycrystalline material like alumina, zirconia, aluminium nitride, titania, silicon carbide, silicon nitrate, tungsten carbide, organic material, organic crystal, diamond, salt, salt hydrate. The coating layers can be continuous or intermittent, fibrous or porous, or forming a metamaterial. The coating layers may be in pure form or have impurities, dopants, additives or discontinuities. The additives and other discontinuities may contribute to the partial absorption of the coating layers. Distribution of additives and discontinuities may be homogeneous or inhomogeneous, or following a predefined gradient.

[0059] The substrate can consist of one or more layers where the layer closer to the coating is adequately absorbing at the laser wavelength. If the substrate is layered, a layer closer to the coating may also reflect the laser wavelength depending its refractive index and the refractive index of the coating, pursuant to Snell’s law of refraction. A substrate layer or layers closer to the coating should limit the transmission of the laser radiation inside the substrate materials, minimising transmission further than, for example, 5 pm. The substrate layer closest to the coating may be infused with additives that will increase absorption of the laser radiation, or desorption rate of the material once light has been absorbed. The substrate surface may have been roughened prior to applying the coating layers for enhancement of coating adhesion as well as enhancement of interaction with the laser light during detachment. The substrate is typically a metal, for example aluminium, for which the free electron cloud due to the metal bonds strongly absorbs any light penetrating the surface, as well as conducting heat very efficiently. However, the substrate may be any metal, such as aluminium or its alloys, titanium, iron or iron based steel, nickel or nickel based steel, cobalt or cobalt based steel, copper, brass, tungsten, a platinum alloy, gold, silver, zinc, tantalum, tin, zirconium, or an alloy of the above or a mixture of the above. The substrate may be a semiconductor like silicon, GaAs, aluminium nitride, CdTe, germanium, gallium nitride and others. The substrate may be a ceramic or crystal or polycrystalline material that adequately absorbs the laser light. Additives or doping may be included in the ceramic or crystal to help the material absorb the laser light or to decompose during irradiation with the laser light. The substrate may be a composite such as carbon fibre reinforced polymer, glass reinforced polymer, ceramic composite, wood or other, where either the binder or the fibre or powder reinforcements can absorb the laser light.

[0060] The material of the substrate may also contain an added layer or mono-atomic layer at the interface of the coating and the substrate to absorb the laser radiation, or to rapidly decompose under the laser radiation, or to enhance decomposition of the substrate or the coating during the laser irradiation. This interface additive may also consist of sporadic particles, discontinuities, roughness or fibres.

[0061] The apparatus disclosed offers a laser apparatus providing means for surface treatments with a variable, or programmable, scan-width.

[0062] The apparatus disclosed also offers means for obtaining a wider scan-width for a given rotation angle of the mirror, which improves processing speed in laser cleaning, while burning or over-burning towards the edges of the scan can be prevented or reduced. This is achieved by, for example, providing a focusing lens (4) before the rotating mirror. This focuses the light onto the rotating mirror which is then reflected and dispersed as the mirror rotates. The situation of positioning a lens (4) after the rotating mirror which then focuses the beam after the rotating mirror has already dispersed the beam can result in a more consistent spot size (6) but a narrower coverage of the beam for a given angle of the mirror. There may be a compromise between a further increase to scan speed and an amount of over-burning at the edges. The balance between these two can be adjusted according to the substrate and the degree of over-burning that may be tolerated. This offers a system with enhanced flexibility of operation. The disclosed apparatus also allows for improved flexibility in size of the apparatus, with a smaller apparatus possible. The apparatus may cause a variation in light intensity across the width of the scan because not all rays focus at the same place but by synchronising the laser (1) with the position of the mirror as previously described, the apparatus prevents over-burning at the edges of the scan whilst increasing the efficiency and the speed of operation.

[0063] For example, assuming that the process is performed with a fixed average laser power budget distributed in pulses of equal energy and fixed pulse duration, the scan-width may be adjusted to produce a desired balance between scan speed for a particular area and amount of burn tolerated due to variation in focusing plane and rotational speed.

[0064] The control system (8) can control any of the parameters previously disclosed. The control system (8) can be configured to adjust these automatically or these parameters can be set remotely or via a user interface (15).

[0065] The apparatus may be hand held or may be mounted on a piece of machinery or on a manned or unmanned vehicle.

[0066] The apparatus can be used for a wide variety of surface treatment applications. For example, whilst this apparatus has primarily been described in the context of coating removal, the apparatus and methods disclosed herein can be used for laser cleaning, laser coating removal or laser de-painting which involves removal of paint coatings. As another example, the apparatus can be used for the removal of rust from a surface. As another example, the apparatus can be used for mould cleaning, such as cleaning material out of a tyre mould. As another example, the apparatus can be used to clean the outside of buildings made of, for example, stone or brick. Other examples of surface treatments that this apparatus is appropriate for include cutting, engraving, annealing silicon and surface preparation.

Claims

CLAIMS:

1. A laser surface treatment apparatus (7) comprising:

a laser (1) having a beam emission path (2);

a rotatable beam director (3) in the beam emission path; and

a control system (8) configured to control laser beam emission according to the rotational position of the beam director.

2. The laser surface treatment apparatus of claim 1 comprising a lens (4) between the laser and the rotatable beam director.

3. The laser surface treatment apparatus of claim 1 comprising a lens between the rotatable beam director and the surface.

4. The laser surface treatment apparatus of any previous claim wherein the control system is configured to control laser beam emission such that the laser beam is emitted when the rotatable beam director is within a central region of rotation and the laser beam is not emitted when the rotatable beam director is within an outer region of rotation.

5. The laser surface treatment apparatus of claim 4 wherein the control system is configured to automatically control the width of the central region of rotation.

6. The laser surface treatment apparatus of any previous claim wherein controlling laser beam emission comprises controlling a pulse energy.

7. The laser surface treatment apparatus of claim 6 wherein the pulse energy is reduced as the beam director is near an edge of rotation.

8. The laser surface treatment apparatus of any previous claim wherein controlling laser beam emission comprises changing a pulse repetition frequency.

9. The laser surface treatment apparatus of claim 8 wherein the control system synchronises the pulse repetition frequency with the speed of the rotation of the beam director.

10. The laser surface treatment apparatus of claim 3 wherein the lens is a variable focus lens and wherein the focal length of the lens is varied according to the rotational position of the beam director.

11. The laser surface treatment apparatus of any previous claim wherein the control system is configured to control the speed of rotation of the beam director according to the rotational position of the beam director.

12. The laser surface treatment apparatus of any previous claim wherein controlling laser beam emission comprises using the internal control method of the laser.

13. The laser surface treatment apparatus of any previous claim wherein controlling laser beam emission comprises using an external modulator (14).

14. The laser surface treatment apparatus of any previous claim wherein the control system is configured to control the rotational position of the rotatable beam director.

15. The laser surface treatment apparatus of any previous claim wherein the control system is configured to detect the rotational position of the rotatable beam director.

16. The laser surface treatment apparatus of any preceding claim configured for use in cleaning or coating removal or de-painting or rust removal.

17. The laser surface treatment apparatus of any preceding claim comprising a user interface (15) configured to receive input of one or more of an application, a laser parameter, a desired speed of rotation or a desired speed of operation.

18. The laser surface treatment apparatus of any preceding claim wherein the beam director comprises one or more mirrors and/or one or more prisms.

19. A laser surface treatment apparatus (7) comprising:

a laser (1) having a beam emission path (2);

a rotatable beam director (3) in the beam of the emission path; and

a focusing lens (4) positioned between the laser and the rotatable beam director.

20. The laser surface treatment apparatus of claim 19 comprising a control system (8) configured to control laser beam emission according to the rotational position of the beam director.

21. A method of laser surface treatment for a system comprising:

a laser (1) having a beam emission path (2); and

a rotatable beam director (3) in the beam emission path, the method comprising: determining the rotational position of the beam director; and

controlling laser beam emission according to the rotational position of the beam director.

22. The method of laser surface treatment of claim 21 wherein a control system (8) controls laser beam emission such that the laser beam is emitted when the rotatable beam director is within a central region of rotation and the laser beam is not emitted when the rotatable beam director is within an outer region of rotation.

23. The method of laser surface treatment of claim 21 or 22 wherein the control system is configured to automatically control the width of the central region of rotation.

24. The method of laser surface treatment of any of claims 21 to 23 wherein the control system is configured to automatically control the width of the central region of rotation.

25. The method of laser surface treatment of any of claims 21 to 24 wherein controlling laser beam emission comprises controlling a pulse energy.

26. The method of laser surface treatment of claim 25 wherein the pulse energy is reduced as the beam director is near an edge of rotation.

27. The method of laser surface treatment of any of claims 21 to 26 wherein controlling laser beam emission comprises changing a pulse repetition frequency.

28. The method of laser surface treatment of claim 27 wherein the control system

synchronises the pulse repetition frequency with the speed of the rotation of the beam director.

29. The method of laser surface treatment of any of claims 21 to 28 comprising controlling the speed of rotation of the beam director according to the rotational position of the beam director.

30. The method of laser surface treatment of any of claims 21 to 29 wherein the laser beam emission status is controlled using the internal control method of the laser.

31. The method of laser surface treatment of any of claims 21 to 30 wherein the laser beam emission status is controlled using an external modulator (14).

32. The method of laser surface treatment of any of claims 21 to 31 comprising controlling the rotational position of the rotatable beam director.

33. The method of laser surface treatment of any of claims 21 to 32 comprising detecting the rotational position of the rotatable beam director.

34. The method of laser surface treatment of any of claims 21 to 33, wherein the method is used for cleaning or coating removal or de-painting or rust removal.

35. The method of laser surface treatment of any of claims 21 to 34, wherein the beam director comprises one or more mirrors or one or more prisms.