Abstract:
A laser beam scanning system which utilizes all reflective optics. The scanning system has a variable scan angle and focal length. The variable focal length in the reflecting optical system is achieved by simultaneously moving two perpendicular mirrors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to laser equipment which can both scan a laser beam and adjust the optical path to achieve a variable focal length of the laser beam. 
     2. Description of Prior Art 
     Laser scanning systems typically utilize galvanometer motors to change the angle of scanning mirrors. Usually the X and Y direction is scanned by separate motors. In many applications a laser beam is scanned on a work piece. To achieve a high power density, the laser beam is usually focused on this work piece. Specialized lens have been developed to achieve a good focus on a flat surface work piece even at a high transmission angle. However, some applications require that the laser beam can be independently focused to accommodate a contoured surface. Normally this focusing is accomplished by translating one or more lenses in an optical system to achieve a variable focal length. Unfortunately, high powered CO 2  lasers can cause a thermal distortion in lenses which degrades the quality of the laser beam. Furthermore, lenses are not as durable as metal mirrors for high power laser beam applications. Therefore, it is desirable to utilize all reflective optic components for high power CO 2  laser applications. Here a problem arises when making a scanning system with a variable focal length. A change in the focal length requires a change in the optical path length. With reflective optics, a path length change usually also produces an undesirable steering of the beam. The invention presented here is an all reflective laser scanning system where a focus adjustment can be made with a minimum of translationable motion and also without introducing any steering or translation of the laser beam. 
     SUMMARY OF THE INVENTION 
     The present invention is a laser scanning system with reflective optics. To achieve an adjustable focal length on the scanned laser beam it is necessary to produce an optical path length change between two mirrors which exhibit optical power (curved mirror surfaces). To achieve this, two additional flat mirrors oriented perpendicular to each other, are placed in the optical path between the curved mirrors. A displacement of the two perpendicular mirrors in a predetermined direction will change the optical path length between the curved mirrors and in turn produce an adjustable focus in the scanned beam without producing additional deviation to the scanned beam. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of an all reflective laser scanning system. 
     FIG. 2 is a top view of an all reflective laser scanning system illustrating the optical components prior to the scanning mirrors. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a perspective view of an all reflective scanning system 10. A laser beam 20 propagating in the direction of arrow 28 strikes a curved reflector 11. In this illustration, reflector 11 is preferably an off axis parabola which focuses laser beam 20 to a focal point 21. This laser beam then strikes flat mirrors 12 and 13. The laser beam then strikes curved mirror 14. This curved mirror 14 is preferably an off axis ellipse. The laser beam then proceeds to strike scanning mirrors 15 and 16. These scanning mirrors can be rotated to steer the beam. For example, mirror 15 can be rotated around the axis 25 and mirror 16 can be rotated around axis 26. A single scanning mirror could also be used. FIG. 1 illustrates the laser beam coming to a focus at three alternative focal spots designated 22A, 22B, or 22C. These are just used for illustration. The laser beam would only strike one point at a time. The actual focus point (22) will be referred to as the &#34;external focus&#34; because is lies outside the optical components. 
     In FIG. 1, work piece 30 is illustrated as being a generally flat plate. To bring a laser beam to a focus, on even a flat surface, requires a focal length adjustment to compensate for the path length change introduced by a change in the deflection angle. If the work piece 30 had a contoured surface, the range of the focal length adjustments would be even greater. In FIG. 1, mirrors 12 and 13 are approximately perpendicular to each other and mounted on base 17. These mirrors can be translated in a direction 27 while retaining their approximately relative orientation. Direction 27 is generally parallel to the beam propagation direction between focal point 21 and the center of the beam striking mirror 12. The four mirrors, 11, 12, 13, and 14 can be referred to as the 1st, 2nd, 3rd, and 4th mirrors respectively. 
     FIG. 2, is the top view of a portion of the scanning system depicted in FIG. 1. In FIG. 2, laser 19 can be seen. Also, laser beam 20 is shown to have a ray 20A which will be referred to as the &#34;center line optical path&#34;. In FIG. 2, mirrors 12 and 13 as well as base 17 are shown in two different possible positions. These two positions are differentiated by adding the letters N or M to the numbers 12, 13, and 17. The translation required to produce this new position is distance E depicted in FIG. 2. FIG. 2 also shows point 23 which is defined as being the point at which the center line optical path 20A strikes mirror 14. Also, the distance from the focal point 21 to mirror 12 along the center line optical path is shown as being distance B. Furthermore, the center line optical path distance between mirror 13 and mirror 14 is defined as being distance D. The center line optical path between mirrors 12N and 13N or 12M and 13M is shown as being distance C. Finally, the center line optical path between the fourth mirror (point 23) and the external focal point 22M is shown being distance S(M). This focal point occurs when the mirror positions 12M and 13M are used. When mirror positions 12N and 13N are used, then focal point 22N is obtained at a distance of S(N) from point 23. The distance between focal points 22N and 22M [S(N)-S(M)] is not shown to scale when compared to displacement distance E depicted in FIG. 2. In fact, one of the advantages of placing the folding mirrors 12 and 13 in the optical path between mirror 11 and mirror 14, is that this location produces the largest possible change in focal length [S(N)-S(M)] for the smallest change in distance E. Also scanning mirrors 15 and 16 are shown in FIG. 1 but not shown in FIG. 2. 
     The center line optical path length between focus 21 and point 23 will be referred to as &#34;s&#34;. Therefore, s=B+C+D when mirrors 12 and 13 are in locations depicted in FIG. 2 as 12N and 13N. When these mirrors are moved to locations 12M and 13M, then s=B+E+C+E+D. Concave mirror 14 has an effective focal length &#34;F&#34; which is defined as being the focal length of a mirror when focusing parallel light. When the incident light is not parallel then the formula is: 
     
         1/s+1/S=1/F 
    
     Distance S is defined as the optical path length to the external focal point 22 from the point 23. This is the predetermined portion of the laser beam which is scanned by scanning mirrors 15 and 16. The object of this invention is to prevent the focus adjustment from introducing a substantial scanning of this predetermined portion of the laser beam. For CO 2  laser applications, the angle steering introduced by a change in the external focus should be kept less than 3 milliradians for each 10% change in distance S. Properly translating mirrors 12 and 13 in direction 27 (FIG. 1) while maintaining the perpendicular orientation will achieve this goal. 
     In FIG. 2, the optical rays are drawn presuming mirror 11 is a concave off axis parabola. Another possibility would be for mirror 11 to be a convex off axis parabola. If this was the case, then the rays reflecting off mirror 11 would appear to be diverging from mirror 11. These diverging rays would appear to come a virtual focus point behind mirror 11 and distance B would be measured from that virtual focus point. Focal point 21 would then be defined as this virtual focal point. Therefore, in either case it can be said that mirror 11 is a curved surface. Mirror 14, however, must always be a concave curved surface in order to function properly. It should also be understood that the preferred curvature for mirror 14 is an off axis ellipse. However, it should be understood that less ideal curvatures may also do an adequate job. For example, a spherical surface on mirror 14 would produce a larger focus spot. However, a larger diameter focus spot may still be adequate to perform the desired function. Therefore, in general mirror 14 can be referred to as a concave curved surface. Similarly, mirror 11 has been referred to as an off axis parabola. This is the preferred surface if laser beam 20 is generally parallel as illustrated. An off axis ellipse would be the preferred surface if laser beam 20 was either convergent or divergent. However, once again, other curved surfaces such as a spherical surface could also produce acceptable results. 
     While there has been shown and described a preferred embodiment it is to be understood that other modifications may be made without departing from the spirit and scope of the invention.