WO1997006462A1

WO1997006462A1 - Rotating optical system for laser machining apparatus

Info

Publication number: WO1997006462A1
Application number: PCT/US1996/011007
Authority: WO
Inventors: Curtis L. Shoemaker; Daniel J. Treadwell
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-08-09
Filing date: 1996-06-28
Publication date: 1997-02-20

Abstract

An optical system (10) for laser machining includes optical elements (14) for displacing the laser beam from its path. The optical elements include flat, parallel refractive surfaces (16, 18) inclined at an angle and means for rotating the inclined surfaces about an axis parallel to the path (12) of the laser beam. Rotation of the optical elements results in displacement of the laser beam in a circular path and greater uniformity of the beam at its illuminated area while maintaining the beam wavefront parallel to the plane of the illuminated area.

Description

Rotating Optical System For Laser Machining Apparatus

Field of the Invention

The present invention relates generally to laser machining or welding, and, more particularly, to optical systems for increasing uniformity of the laser beam used in such processes.

Background of the Invention

Laser processing of materials, which may include such processes as welding or drilling by ablation, may be accomplished by directing a laser beam at focusing elements such as Fresnel zone plates, holographic optical elements or binary phase gratings. These elements direct and focus the laser beam to one or more locations at which it is desired to produce the weld, hole or other feature. To achieve uniformity at the desired location it is desirable that the laser beam be uniform over the diffractive focusing element. Unfortunately, uniformity is not inherent in gas discharge laser beams produced at the present time. Present beams include "hot" and "cold" areas wherein the intensity of the beam varies from position to position within the illuminated area.

Traditional beam "homogenizers", whereby the hot and cold beam areas are moderated, are not applicable to diffractive beam focusing, since they do not present a single, planar beam wavefront at the desired area. It is therefore desirable to devise a method which would increase the uniformity of beam intensity within the area illuminated by the laser beam.

Various scanning means have been proposed in the past which move the laser beam across the desired illumination area to average the intensity of the beam at the area to be processed. These scanning means have included rotating, oscillating or translating mirrors which move the beam across the desired area. These methods have not been entirely successful since reflective deflection of the laser beam results in lost parallelism of its wavefronts. Loss of wavefront parallelism results in loss of precise focusing of the beam at the desired location and distorts and mis- registers the features.

Thus it is desirable to not only scan the beam across the desired location, but also to maintain the wavefronts of the beam parallel while scanning is taking place.

Summary of the Invention

The present invention scans a laser beam over a desired area while maintaining wavefront parallelism by means of an optical system in the path of the laser beam which includes two flat, parallel refracting surfaces inclined to the path of the laser beam. The refracting surfaces are rotated about an axis parallel to the path of the beam to move the beam in an overlapping circular path in order to average the intensity of the beam at the illuminated area. A unique feature of the optical system is that the wavefronts of the exiting laser beam are maintained parallel, thus the desired registration of the laser beam is maintained in the machining plane.

The optical system may be a single glass plate having flat, parallel surfaces inclined to the path of the beam or two glass wedges each including an angled surface relative to the path of the beam.

Brief Description of the Drawings The present invention will be described with respect to the accompanying drawings, wherein like number refer to like parts in the several views, and wherein:

Figure 1 is a schematic view of an optical system according to one embodiment of the invention; and

Figure 2 is a schematic view of an optical system according to a second embodiment of the invention.

Description of the Preferred Embodiments

Figure 1 illustrates an optical system, generally indicated as 10, according to the present invention which is designed to increase the uniformity of a laser beam 12. The laser beam 12 is used in conjunction with diffractive masking or focusing elements such as Fresnel zone plates, holographic optical elements or binary phase gratings.

The optical system 10 of the invention is used in the path of the laser beam 12 before such masking or focusing elements to scan the laser beam 12 over a small area to average the intensity of the beam 12 and thus ensure that beam radiation striking the optical element has enhanced uniformity over the active area of the mask. In the embodiment of Figure 1, the optical system consists of single disk 14 having two flat, parallel major surfaces 16 and 18 which are inclined at an angle to the path of the laser beam 12. The disk 14 is rotated about an axis parallel to the path of the laser beam 12. As a consequence, the laser beam 20 exiting the disk 14 is offset but parallel to the entering beam 12 and, as the disk 14 rotates, is swept in a circular path. The material of the disk 14, and so its refractive index, the thickness of the disk 14 and the angle of the disk 14 relative to the incoming beam 12 affect the amount of offset of the beam 20 and are chosen so that the circular path of the exiting beam 20 overlaps itself and completely fills the area to be illuminated.

The laser used to produce the beam 12 is an excimer laser of the KrF type emitting ultraviolet light at approximately 248 nm. This laser is selected for its ability to ablate polyimide which is used for flexible circuit boards. With this type of laser, the material of the disk is fused silica (quartz), although it will be understood that different materials will be necessary for maximum efficiency if lasers emitting radiation at a different wavelength than that described above is used.

The disk 14 is approximately 25 mm thick and approximately 150 mm in diameter. The disk 14 is angled at 45° with respect to the path of the laser beam 12. The laser beam 12 input into the disk 14 had a beam 12 cross-sectional area of approximately 4 cm. The rotating disk 14 sweeps the beam in a circular orbit with a radius of approximately 8.4 mm, providing a central, homogenized region slightly smaller than 4 cm by 4 cm. The disk 14 is rotated at speeds between approximately 60 rpm and 600 rpm, with the lower figure representing the lowest speed which results in one complete sweeping rotation of the beam given the arbitrary cycle time of one second associated with the particular equipment used. The upper rotational limit is determined by distortion induced in the disk 14. It is necessary that the surfaces 16 and 18 be maintained flat and parallel during processing so that successive wavefronts of the beam 20 remain parallel. For the particular material used for the disk 14 and the dimensions discussed above, it was found that approximately 600 rpm was a practical limit at which distortion of the disk 14 was not excessive. Of course, useful rotational speeds will change if the material of the disk 14 or its dimensions change. Figure 2 illustrates a second embodiment of the invention which includes an optical system, generally indicated as 30, which includes two cylindrical wedges 32 and 34. In common with the embodiment of Figure 1, each wedge includes an angled surface 36 and 38 which form two refracting surfaces inclined with respect to the path of the laser beam 40. The two wedges are rotated together as a unit about an axis which is parallel to the path of the laser beam 40. Thus, in concept, the embodiment of Figure 2 is identical to that of Figure 1 since the beam 40 is presented with two rotating, inclined refracting surfaces which result in a rotating offset of the laser beam 40. This embodiment of two wedges 32 and 34 is not preferred over that of Figure 1 since it is more expensive to produce two items rather than one, it is more difficult to maintain the inclined surfaces 36 and 38 parallel, and greater light losses are suffered since the beam 40 encounters a greater number of surfaces and must negotiate a longer length of material.

Thus there has been described an optical system which accomplishes the purpose of scanning a laser beam to improve the uniformity of the beam at a desired area, while maintaining successive wavefronts of the beam parallel. In addition, the optical system is such that wavefront parallelism is maintained despite vibration. As long as the inclined refracting surfaces remain parallel, vibration of the assembly will simply alter the averaging path of the light. Instead of averaging over a circle, the area illuminated will be another shape.

Although the optical system has been described with respect to only a limited number of embodiments, it will be apparent to those skilled in the art that many modifications are possible. For example, the optical systems need not be circular or cylindrical as shown, but may take any shape so long as the relationship of the two inclined surfaces is maintained and no other angled surfaces are encountered.

Claims

The invention claimed is:

1. An optical system for increasing the uniformity of a beam of electromagnetic radiation while maintaining wavefront geometry, the system comprising: at least one optical element disposed in the beam of the radiation, said optical element including two substantially flat, substantially parallel, refractive surfaces inclined at an angle to the path of the beam of radiation; and means for rotating said refractive surfaces about an axis parallel to the path of said radiation while maintaining said surfaces parallel.

2. An optical system according to claim 1 wherein said at least one optical element is a flat optically transmissive plate having two major surfaces defining said refractive surfaces.

3. An optical system according to claim 2 wherein said plate is quartz.

4. An optical system according to claim 1 wherein said at least one optical element is two wedges each having a flat surface oriented perpendicular to the path of the radiation and flat surfaces inclined relative to the path of the radiation, said flat surfaces being disposed parallel to each other.

5. An optical system according to claim 4 wherein said wedges are quartz.