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 force 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  22 A,  22 B, or  22 C. These are just used for illustration. The laser beam would only strike one point at a time. The actual focal point ( 22 ) will be referred to as the “external focus” because is  it 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  20 A which will be referred to as the “center line optical path”. 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  20 A 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  12 N and  13 N or  12 M and  13 M is shown as being distance C. Finally, the center line optical path between the fourth mirror (point  23 ) and the external focal point  22 M is shown being distance S(M). This focal point occurs when the mirror positions  12 M and  13 M are used. When mirror positions  12 N and  13 N are used, then focal point  22 N is obtained at a distance of S(N) from point  23 . The distance between focal points  22 N and  22 M [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 “s”. Therefore, s=B+C+D when mirrors  12  and  13  are in locations depicted in FIG. 2 as  12 N and  13 N. When these mirrors are moved to locations  12 M and  13 M, then s=B+E+C+E+D. Concave mirror  14  has an effective focal length “F” 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 . That 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 to 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 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.