GB2090047A - Laser cutting apparatus and method - Google Patents

Abstract

An apparatus and method for cutting material from a workpiece uses a laser resonator emitting an essentially coherent beam of electromagnetic radiation having at least partial linear polarization and a converting means converting the linearly polarized laser beam into an essentially coherent beam of electromagnetic radiation having at least partial circular polarization. A directing means ensures that the circularly polarized beam is directed against the workpiece at the point of the desired cut. A conventional focusing means and axially directed gas jet may be added to enhance the cutting of the laser beam. The use of a circularly polarized beam is said to reduce key angulation and improve cut surface smoothness.

Description

SPECIFICATION Laser cutting apparatus and method BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to means for cutting metal and other materials by electromagnetic radiation and, more particularly, to an apparatus and method for cutting away a portion of a workpiece by means of a laser beam.

Description of the Prior Art Within recent years, lasers have been increasingly utilized for precision cutting of many materials, including asbestos, ceramics, glass, paper, plastics, textiles, wood, and metals. Lasers are particularly adapted to automatic control and can accurately cut materials and configurations which are difficult to cut by conventional methods.

The cutting process is essentially one of material removal by vaporizing or burning of the material from the cut or kerf. The effectiveness of laser cutting of metals and other materials is related to the mode structure of the laser beam and can frequently be enhanced by the use of a gas jet coaxial with the laser beam, as has been practiced extensively with carbon dioxide lasers.

One of the most significant problems in cutting metals and other materials with a laser is angulation of the kerf. Instead of a kerf precisely aligned with the laser beam, the point of emergence of the laser beam from the workpiece may be displaced with respect to the point of incidence in a direction perpendicular to both the laser beam and to the motion of the point of incidence upon the workpiece. This angulation has been of little significance in the cutting of thin materials by relatively small lasers; however, in the cutting of relatively thick metals and other materials by larger lasers, the resulting difficulties may be acute. Angulation of the kerf may result in loss of cutting accuracy, may contribute to roughness of the cut surface, and, in addition, may hinder the removal of discs or other closed shapes cut from a plate.

The prior art has been unable to explain the angulation phenomenon or to reliably correlate angulation and cut surface roughness with laser system design parameters. Accordingly, previous efforts to eliminate angulation of the kerf and to eliminate sources of cut surface roughness have been unsuccessful. For that reason, generally, only conventional non-laser cutting devices have been utilized to cut relatively thick metals and other materials when angulation and cut surface roughness have not been acceptable. It is therefore an object of the present invention to provide an apparatus and method of cutting metals as well as other materials by means of a laser beam such that angulation of the kerf does not occur. Further objects of the present invention are to remove one source of cut surface roughness and to maintain optimum mode structure of the laser beam.

SUMMARY OF THE INVENTION The present invention is an apparatus and method for cutting material from a workpiece by means of a laser beam.

The apparatus includes a laser system which produces a stable high power TEMoo mode beam of electromagnetic radiation which is linearly polarized in a particular plane. The laser system maintains the linear polarization and mode of the emitted laser beam regardless of backscatter or reflection from the workpiece, acoustic vibration of the laser resonator, or modulation of the laser discharge. A retarder is used to convert the emitted linearly polarized laser beam into a circularly polarized beam. The retarder may be a multilayer reflecting quarter wave device, a transmitting quarter wave plate, a Fresnel rhomb, or a Fresnel prism. A system of mirrors and lenses may be used to focus the circularly polarized beam along the path of the desired cut.An axially directed gas jet may be added to the laser system to enhance the cutting efficiency of the circularly polarized beam.

From experiments and original theoretical analysis regarding laser cutting, applicant has determined that angulation of the kerf is associated with linearly polarized laser beams.

Applicant has found that cutting efficiency of laser beams depends, among other factors, upon the fraction of the incident radiation which is absorbed by the workpiece and that the absorption, in turn, depends upon both the plane of polarization of the laser beam and the angle of incidence thereof. This polarization-dependent adsorption can be asymmetrical and can have a substantial effect upon the symmetry of the cut.

Applicant has observed that laser beams which have been linearly polarized in a plane either perpendicular to or parallel to the cut or circularly polarized have not produced angulation of the kerf whereas laser beams polarized at intermediate angles have produced angulation. Further, the direction of the angulation has been observed as reversed when the cutting direction has been reversed or when the plane of polarization has been altered by 90 degrees. Beams polarized perpendicular to the cut exhibit a symmetrical but undesirable increase in cut width on the exit surface of the workpiece.

This observed relationship between angulation of the kerf and theorientation and state of the incident laser beam may be expressed vectorially.

Defining the angulation vector as the displacement of the point of emergence of the cutting beam from the workpiece with respect to the point of incidence of the cutting beam upon the workpiece in a plane which is perpendicular to both the incident beam and the direction of the cut, the angulation vector can be related to the beam polarization vector, the beam propagation vector, and a vector representing the direction of the cut. A theory developed by applicant, based on polarization dependence of absorption, correctly predicts this relationship. No angulation of the kerf has been observed to be produced with circularly polarized laser beams. Accordingly, the present invention consists of various arrangements to produce stable, TEMPO transverse mode, circularly polarized laser beams particularly adapted to cutting metals and other materials.

As a result of the present invention, laser cutting of relatively thick metals and other materials may be achieved with little or not.

angulation of the kerf. Roughness of the cut surface related to linear polarization of unstable direction is eliminated, and the optimum mode for a narrow cut and efficient cutting (TEMoo) is maintained. Further objects, features and advantages of this invention will become apparent from a consideration of the following description, the appended claims, and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the folded laser resonator and cutting apparatus of the preferred embodiment of the present invention; FIG. 2 is an enlarged partial elevational view of the folded laser resonator of Fig. 1 along the line 2-2; FIG. 3 is an enlarged partial sectional view of the laser cutting apparatus of Fig. 1 along the line 303; FIG. 4 is an enlarged partial sectional view of the laser nozzle and workpiece along the line 4-4 of Fig. 3; and FIG. 5 is a partial plan view of an alternative laser resonator configuration for use with the cutting apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawing, the laser cutting apparatus of the present invention, indicated generally at 10, is illustrated in Fig. 1. The apparatus includes a Z-shaped resonator having three horizontal lengths of active plasma tubes 12, 14 and 16 arranged in a horizontal plane. A rear end mirror assembly, indicated generally at 18, and a front end mirror 19 are mounted at the ends of the resonator configuration, and mirrors 20 and 21 are mounted at the two vertices to complete the optical path.

The rear end mirror assembly 18 consists of a mirror 22 which bends the emitted beam through an angle of 90 degrees to a direction 45 degrees with respect to vertical, thus projecting the beam upward away from the horizontal plane of the Zshaped resonator at an angle of 45 degrees. The particular angulation of the mirror assembly is shown in Fig. 2. A second mirror 24 of the rear end mirror assembly 18 reflects the beam back upon itself. The polarization-dependent reflectivity of the rear end mirror assembly 18 ensures that the laser beam emerging from the front end mirror 19 exhibits stable linear polarization in a plane at 45 degrees with respect to vertical.

The mirrors are of varying curvatures, as experimentally determined for optimum single mode output, and are mounted in ball and socket assemblies 26 and 27, so that each mirror resets against a flat surface of the mounted assembly 26 or 27, with contacts sufficiently intimate to provide good heat transfer but not sufficient to physically distort each mirror. The front end mirror 19 forms a vacuum seal for the plasma tube in conjunction with an O-ring (not shown) around its periphery. The vertex mirrors 20 and 21 are contained in chambers with pressure equalized on both sides of the mirrors by means of a vacuum seal obtained by a larger diameter O-ring (not shown) in the mounting assemblies 27. Oil flows through passages (not shown) in the mounting assemblies 26 and 27 to cool the mirrors by heat transfer through the mounting assemblies.The mirror ball and socket assemblies 26 and 27 are adjustable so as tonsure proper alignment of the mirrors and to peak laser output.

The front end mirror 19 is partially reflecting and particularly transmitting in accordance with conventional laser design. Reflection of the front end mirror 19 ensures resonance of electromagnetic radiation within the laser tubes, and transmittance permits the escape of the beam of electromagnetic radiation. The alignment of the front end mirror 19 is remotely adjustable so as to allow an operator to fine-tune the laser resonator and peak laser output.

The laser resonator consists of six coaxial glass tubes 34, 36, 38, 40, 42 and 44 whose diameters are empirically determined for optimum TEMPO mode output. The inside tube 34 contains the plasma, and the cooling oil flows between the inner tube 34 and the outer tube 36. Individual tubes 34 and 36 are joined in the center by a high temperature plastic anode assembly 46 containing a metal gas feed 47 and electrode 49.

Similar anode assemblies 48 and 50 are provided on plasma tubes 12 and 14, respectively. Cathode assemblies 52, 54, 56, 58, 60, and 62 at the ends of the plasma tubes 10, 12, and 14 use a copper ring electrode (not shown). The mirrors, anode assemblies, and cathode assemblies are all bolted to a granite beam 64 which serves as a highly stable spacer for maintaining mirror alignment while also providing electrical isolation between electrode assemblies.

A gas panel 66 supplies the proper mixture of helium, nitrogen and carbon dioxide gases from individual cylinders (not shown) to the inner tubes 34, 38, and 42. Each gas passes through its pressure regulator 68, 70 or 72 which contains a 5-micron partical filter, and enters a metering section 74 of the gas panel 66 at 20 psig. Each gas next passes through a flow meter 76, 78, or 80 which permits precise setting of the flow rate by means of a metering valve so that the flow rates of the carbon dioxide, nitrogen, and helium are, respectively,0.22,0.99, and 12.9 standard cubic feet per hour. After passing through the metering valves, the gases are mixed and fed through a length of tubing 81 to the anode assembly 46, 48, or 50 of each plasma tube section 1 2, 14, or 16. The gas mixture enters the plasma tubes and travels to the cathode ends where it is exhausted by a vacuum pump 82 through tubes 83 and then outside the laser head enclosure.

The plasma tubes, mirror mounts, and cathode assemblies of the laser resonator are cooled by high dielectric strength insulation oil flowing through a closed system. This system consists of an oil reservoir 84, a pump 86, and a heat exchanger (not shown) which utilizes tap water.

The pump 86 draws oil through the heat exchanger and circulates it through the plasma tube jackets and through the mirror mounts and cathode assemblies and then back through the reservoir 84.

A high voltage power supply 88 delivers 22 kv to the anode assemblies 46, 48, and 50 to initiate the laser radiation. In addition, 20 kv pulses from a pulse transformer are generated to initiate the gas discharge. These pulses are generated at a 1 kHz repetition rate. Under normal operating conditions, only one pulse is required to initiate the laser radiation. If current is flowing after the first pulse, no further pulses are generated. After the discharge is established, power tubes regulate the current through each section of the laser resonator to within 0.1% by means of a current feedback loop in which the current in each section of the laser is continuously compared to a setting made at the control panel and regulated accordingly.A digital potentiometer on the control panel adjusts the current in the plasma tubes and, consequently, the output power of the laser beam.

Current can be adjusted from 40 ma to 270 ma.

After the laser has operated for approximately 1 second, the voltage regulator limits the power tube dissipation by reducing the plate voltage 2 kv.

As discussed above, the angulation of the rear end mirror assembly 18 with respect to the plane of the Z-shaped laser resonator ensures that the beam emerging from the front end mirror 19 exhibits stable linear polarization at 45 degrees with respect to vertical. Alternatively, resonator configurations could be utilized wherein the laser beam is incident upon the internal vertex mirrors 20 and 21 at large angles, e.g. 45 degrees, so as to effect a stable linearly polarized laser beam without the need for a two-mirror rear end mirror assembly. Such a resonator having a U-shaped configuration is illustrated in Fig. 5 with corresponding elements numbered with the subscript "a". Another alternative would be the use of a Brewster plate to obtain the required stable linear polarization.

A retarder 90 is positioned adjacent the output mirror 19 in the optical path of the emitted laser beam. The retarder receives the linearly polarized laser beam 92, resolves the laser beam into two components, retards the phase of one component relative to the other, and reunites the two components to form a single beam 94 having circular polarization. In the preferred embodiment shown, the retarder 90 is a multilayer reflective quarter wave retarder. Alternative quarter wave retarder devices such as a CdS quarter wave plate, a ZnSe Fresnel rhomb, or ZnSe Fresnel prism may also be used.

The emergent circularly polarized beam 94 is directed upon a first movable mirror 96 and a second movable mirror 98 which serve to locate the beam rectilinearly as indicated along 2dimensional coordinates to direct the beam downward upon a workpiece 100. The mirrors 96 and 98 are movable by conventional means such as the cables 102 represented schematically in Figs. 1 and 3. A cutting nozzle 104 is movable with the mirror 98 and is positioned to receive the downwardly directed laser beam emergent from the mirror 98. A lens 106 within the nozzle 104 focuses the beam upon the workpiece 100.

Alternatively, the movable mirrors 96 and 98 may be eliminated and the workpiece 100 may be moved past the stationary circularly polarized laser beam 94 and the nozzle 104.

The lens 106 is mounted in a water-cooled lens holder 108 having O-rings 110 sealing the lens 106 against the lens holder 108 so as to provide a closed water passage to the lens. A continuous flow of tap water reaches the lens 106 by means of a water inlet 112 and a water outlet 114 to provide cooling of the lens 106. Compressed gas enters the nozzle 104 at gas inlet 116 and is directed coaxially with respect to the focused laser beam 118 by a nozzle nose 120 against the workpiece 100 to assist in removing vapor and particles from the resulting kerf 122. A gas such as oxygen that undergoes an exothermic reaction with the heated region of the workpiece can be used to enhance material removal. An angulated kerf 124, shown in phantom in Fig. 4, illustrates the undesirable angulation characteristics of conventional laser cutting devices.

It will be seen from the foregoing description of the preferred embodiment of the invention that the apparatus and method of this invention provide efficient laser cutting of various materials without the undesirable angulation which frequently results when conventional laser cutter devices are used. Further, one source of roughness of the cut surface is eliminated, and the optimum mode for narrow and efficient cutting is maintained. While the preferred embodiment has been described in considerable detail, the present invention is not to be limited to such detail except as may be necessitated by the appended claims.

Claims (12)

1. An apparatus for cutting away a portion of a workpiece, said apparatus comprising: a laser which emits an essentially coherent beam of electromagnetic radiation having at least partial linear polarization, converting means for converting said linearly polarization, and directing means for directing said circularly polarized beam against the portion of the workpiece which is being cut away.

2. The apparatus of claim 1 wherein said converting means comprises a quarter wave plate positioned such that said linearly polarized beam is incident thereon and said circularly polarized beam is emitted therefrom.

3. The apparatus of claim 1 wherein said converting means comprises a Fresnel rhomb positioned such that said linearly polarized beam is incident thereon and said circularly polarized beam is emitted therefrom.

4. The apparatus of claim 1 wherein said converting means comprises a Fresnel prism positioned such that said linearly polarized beam is incident thereon and said circularly polarized beam is emitted therefrom.

5. The apparatus of claim 1 wherein said converting means comprises a multilayer reflective quarter wave retarder positioned such that said linearly polarized beam is incident thereon and said circularly polarized beam is reflected therefrom.

6. The apparatus of claim 1, 2, 3, 4 or 5, wherein said laser includes a laser resonator having an internal beam and at least one internal vertex mirror, said internal laser beam being incident upon said internal vertex mirror at an angle sufficiently large to ensure that said emitted beam of electromagnetic radiation exhibits said linear polarization.

7. The apparatus of claim 6 wherein said laser resonator has a U-shaped configuration and two said internal vertex mirrors each of which bends said internal laser beam through an angle of substantially 90 degrees.

8. The apparatus of any preceding claim which further comprises a gas jet coaxial with the laser beam.

9. The apparatus of any preceding claim wherein said directing means includes a focusing means for focusing the circularly polarized beam upon said portion of the workpiece being cut.

10. The method of cutting away a portion of a workpiece by a laser which comprises: emitting an essentially coherent beam of electromagnetic radiation having at least partial linear polarization, converting said linearly polarized beam into an essentially coherent beam of electromagnetic radiation having at least partial circular polarization, and directing said circularly polarized beam against the portion of the workpiece which is being cut away.

11. Laser apparatus for cutting a workpiece, such apparatus being constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

12. A measure of cutting using a laser substantially as hereinbefore described with reference to the accompanying drawings.