Method for shaping contact lens surfaces
BACKGROUND OF THE ART 
     Using laser irradiation to shape optical surfaces is well known in the art. For instance, U.S. Pat. No. 4,194,814 teaches a method for forming indicia on a contact lens using a laser. In particular, this patent teaches that certain critical characteristics, i.e. wavelength and intensity to sublimate the surface of the contact lens in order to form smooth indicia. 
     U.S. Pat. No. 4,219,721 describes a method for using a laser to mark indicia on clear, polymethacrylate intraocular lenses. In particular, the patent teaches an apparatus for accomplishing high quality indicia. 
     U.S. Pat. No. 4,307,046 teaches using a CO 2  laser to vaporize the surface of a contact lens blank and shape the blank into a lens. Essentially, this patent teaches using a laser beam as a cutting tool, much like a cutting tool used on a lathe which has been adapted to produce contact lenses. Thus this patented method depends upon the same techniques as those known in the prior contact lens manufacturing art. 
     U.S. Pat. No. 4,563,565 teaches a method for forming the edges of contact lenses. Again, critical to this process technology is the fluency of the laser, which is optimized by maximizing the ratio of heat of vaporization to heat absorbed by the polymer bulk. Furthermore, the process requires that the edge of the contact lens be indexed to the laser beam so that the lens can be rotated and thus effectively define the lens edge. The basic technology is similar to U.S. Pat. No. 4,307,046 except that it is specifically directed towards forming the edge of the lens rather than the surface of the lens. 
     U.S. Pat. No. 4,642,439 teaches a method for forming the edge of a contact lens made from silicone elastomer which employs a ring of laser radiation of the same radius as the desired dimension of the contact lens. The radius of said lens is fixed and the ring is employed only to form the edge of a lens. The method employs two laser rings to form the rounded edge of a contact lens. It does not teach using a ring of laser radiation to shape the optical surface or carrier surface of a lens. 
     U.S. Pat. No. 4,838,266 teaches process technology useful in modifying the surface of optical devices. The technology relies on the utilization of an attenuator which varies the energy distribution of the laser beam across the surface of the optical element being shaped. The technology depends upon fixing relative to one another, the laser beam, the optical article, and the attenuator. 
     U.S. Pat. No. 4,842,782 teaches a process technology using a laser to alter the surface of an optical device by way of employing a moving mask to vary the energy distribution across the surface of the optical surface. 
     None of the prior art techniques will produce axially symmetrical lenses and many of the techniques require orientation of the target specific to the laser beam in order to produce axial symmetry of any great degree. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a novel method of controllably altering the surface of an optical element using laser irradiation sources to selectively photoablate said surfaces. Specifically, the present invention uses a ring of laser irradiation to ablate across the entire surface of an optical surface so as to alter the surface in a predetermined fashion. The entire surface of the optical unit is thus controllably altered by changing the effective radius of the laser ring as it impinges on said optical target surface. By controlling the effective radius of the ring on the target one can scan the ring and thus create, in effect, a photoablation gradient across the face of the target. This photoablation gradient will be rotationally symmetric. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 schematically represents an apparatus used to practice the present invention which comprises an axicon, a plus lens element, a fixture to hold the target, means for changing the relative spacing between the plus lens and the axicon lens, and a laser radiation source. 
     FIG. 2 shows a second apparatus for practicing the invention which comprises a first axicon lens, a second axicon lens, a fixture to hold the target, means for changing the relative spacing between the two axicon lenses, and a laser radiation source. 
     FIG. 3 shows a third apparatus for practicing the invention which comprises an axicon, a plus lens element, a minus lens element, a fixture to hold the target, means for changing the relative spacing between the plus lens and the minus lens, and a laser radiation source. 
     FIG. 4 shows a fourth apparatus which comprises a first axicon lens, a second axicon lens, a plus lens element aligned along the axis of a laser source&#39;s irradiation and means for holding an optical target. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a novel method of shaping optical lenses and other refractive optical elements. This method employs the properties of axicon lenses in order to create an axially symmetric lens by using said axicon lens to form a ring of laser radiation whose radius can be controllably varied. Thus, the laser radiation can be swept across the face of the optical target surface and, in a predetermined fashion, photoablate said surface to modify it as desired. 
     The present invention is a general method for manufacturing optical elements and is particularly directed to the precise and accurate modification of surfaces which form optical elements. Particularly, the present invention teaches a method wherein a ring of laser radiation is used to scan across the domain of an optical element surface in order to controllably photoablate the domain surface and so modify it in a predetermined and desired manner. The laser ring is scanned over the whole domain by varying the effective radius of said ring in a predetermined function. Thus, by controlling the instantaneous fluency of the laser ring radius, one can effect an ablation profile which as a function as a distance from the center of the target domain defines a controlled photoablation as desired. Thus, one could readily modify a spherical surface to an aspheric surface. 
     In order to accomplish these effects, applicants provide several apparatae which provide laser rings which one can controllably vary the radius of the ring. One form of this as shown in FIG. 3 comprises a laser radiation source (1), an axicon lens element (2) which has a specified ray angle, U 0 , which defines the angle from normal a beam is diverted, a plus lens element (3) which has a given power, φ 1 , a minus lens, element (4) which has a given power, φ 2 , a fixture to hold the target optical element (5) and a target optical element means for controlling the distance, t 1 , between the plus lens and the minus lens elements t 1 , means for controlling the distance between the axicon and the plus lens (t o ), and means for controlling the distance between the minus lens and the target (t 2 ). 
     The radius of the laser ring incident on the target, using the apparatus shown in FIG. 3, depends upon the deviation produced by the axicon lens, the power of the plus lens element, the power of the minus lens element and the relative distances between the axicon lens and the first lens (either plus or minus); relative distance between the plus and minus lenses; and the relative distance between the optical target and the element lens. Thus, the ring&#39;s radius can be expressed as the formulae: 
     
         Y.sub.3 =Ay.sub.1 +BU.sub.0 
    
     where 
     Y 3  is the ring radius 
     Y 1  is the ray height at the plus lens; 
     A is 1-(t 1  +t 2 )φ 1  -t 2  φ 2  +t 1  t 2  φ 1  φ 2   
     B is (t 1  +t 2 )-t 1  t 2  φ 2   
     a is the aperture diameter of the beam 
     The back focal distance (BFD) for this apparatus will be defined as: 
     
         BFD=t.sub.2 =(1-t.sub.1 φ.sub.1)/(φ.sub.1 +φ.sub.2-t.sub.1 φ.sub.1 φ.sub.2) 
    
     Image height will be: 
     Y image  =U 0  /φtotal 
     where 
     φtotal=φ 1  +φ 2  -t 1  φ 1  φ 2   
     The width of the ring will be: 
     
         Δ=|{1-(t.sub.1 +t.sub.2)φ.sub.1 -t.sub.2 φ.sub.2 +t.sub.1 t.sub.2 φ.sub.1 φ.sub.2 }a/2)| 
    
     Thus, as a first order analysis, the ring radius and profile are well defined and can be controlled as a function of t 1 . 
     Of course, the laser ring is not a focussed ring: there will be an intensity distribution across the radial cross-section of the ring that will change as the radius of the ring changes. The radial intensity distribution of the laser ring will depend upon the degree to which the ring is focussed as well as the quality or thickness of the lens elements employed in the invention. Optical aberrations will affect the profile as well as the properties of the radiation source. These characteristics may be analytically determined. Theoretically, however, the intensity profile can be determined experimentally as well. In any event, the intensity distribution of the laser ring profile will vary as a function of r. Thus, by experimentally determining this function across the range of r, one can operate the ring at each increment of r for a controlled time to produce an irradiation profile which will effect a desired ablation profile. 
     FIG. 1 shows a schematic representation of another apparatus which can be employed in the practice of the present invention. In this apparatus an axicon lens (6) and a plus power lens (7) are employed to effect a laser ring incident on the optical target. The ring&#39;s radius can be controlled by the distance between the plus lens and the axicon lens, and/or the distance between the optical target (8) and the plus lens (7) designated as t 1 . However, as a practical matter, the difference between the target and the lens is easiest to control. 
     Once again, the radial profile of the laser will depend upon the degree to which the plus lens focuses the ring on the optical target. 
     The profile will vary in intensity as the effective radius of the ring changes. This intensity distribution can be measured and quantified to describe a function as a variable of r, and again a scan profile can be calculated to describe a desired ablation profile. 
     Another apparatus which can be used to practice the present invention is schematically described by FIG. 2. In this scheme, two axicon lenses (9) and (10) are used to form the laser ring. The distance between the two axicon lenses (t 0 ) determines the radius of the laser ring. The ring produced by such an apparatus will not require focusing since the cylinder defining the ring runs parallel along the common axis of the two axicon lenses. The outer ring diameter will be defined as: 
     
         Y.sub.1 =U.sub.0 t.sub.0 
    
     where 
     Y 1  is the outer ring diameter; 
     U 0  is the axicon angle; and 
     t 0  is the distance between the axicons. 
     The inner ring radius will be: 
     
         Y&#39;.sub.1 =Y.sub.1 -a/2; 
    
     where Y&#39; 1  is the inner diameter; 
     Y 1  is the outer diameter; and 
     a is the beam aperture. 
     Thus, the effective ring structure will largely be a function of beam aperture, axicon angle, and the distance between the axicons, (t 0 ), and will not be sensitive to the distance between the axicon and the target (11). 
     Another apparatus which can be employed in the practice of the invention is shown in FIG. 4 and is comprised of: 
     a first axicon lens (12); 
     a second axicon lens (13); 
     a plus power lens (14); 
     a radiation source (15); and 
     means for holding a target optical substrate (16.) 
     where 
     a=aperture diameter; 
     φ 1  =the power of the plus lens; 
     U 0  =ray angle after the first axicon lens; 
     U 1  =ray angle after the second axicon lens: 
     U 2  =ray angle after the plus lens; 
     Y 1  =ray height at the first axicon lens; 
     Y 2  =ray height at the second axicon lens; 
     Y 3  =ray height at a distance, t 2 , from the plus lens; 
     t 0  =distance between the first and second axicon lenses; 
     t 1  =the distance between the second axicon lens and the plus lens; 
     t 2  =distance between the plus lens and the target; and 
     f=the focal length of the plus lens. 
     Then, the width of the ring Δ will be 
     
         Δ=(1-t.sub.2 φ.sub.1)a/2 
    
     and the outer ring radius (Y 3 ) outside the plus lens will be: 
     Y 3  =Y 2  (1-t 2  φ 1 )=Y 1  (1-t 2  φ 1 )=Y 2  -t 2  Y 2  /f 
     Therefore, ring radius can be controlled by varying t 2 . 
     The present method is particularly suited to making aspheric lenses which are of particular utility in the area of rigid gas permeable lenses. Since rigid gas permeable lenses are by definition far less flexible than soft lenses, they must be designated to fit comfortably on the eye. Hence, rigid gas permeable lenses often incorporate nonspherical designs. Such designs are exemplified by U.S. Patent Nos. 4,765,728 and U.S. Pat. No. 4,883,350. 
     The present method is particularly suited to manufacturing such designs in that it is capable of precisely reshaping a spherical contact lens blank to a defined aspheric surface. Furthermore, the surface so produced may not require further polishing. In state of the art manufacturing techniques, which require polishing, the resulting surfaces manufactured tend to be more spherical after polishing than would be expected from the design cut by the lathe. Thus, the present invention allows for a more desired correlation between designed shape and actual shape than has been possible before. 
     The present method may also be used to produce discontinuance designs such as the designs taught in U.S. Pat. No. 4,642,112, U.S. Pat. No. 4,340,283 or U.S. Pat. No. 4,162,122. Such discontinuities may be produced by introducing discontinuities into the scan profile of the laser ring by way of process during the sweep, or abrupt accelerations. It has been found that the present process works effectively on contact lens materials with relatively low degrees of crosslinking such as polyhydroxyethyl methacrylate, polyvinyl pyrrolidinone, polymethyl methacrylate. 
     The optical components of the apparatae used to practice the invention must transmit the wavelength of laser radiation being employed. All of the optical components are generally available from commercial sources. For instance, axicon lenses made of UV grade fused silica can be obtained from Tucson Optical Research, located in Tucson, Ariz. U.S.A.. Suitable plus and minus lenses are also available from commercial sources.