Manufacture of ophthalmic lenses by excimer laser
FIELD OF THE INVENTION 
     This invention relates to the manufacture of ophthalmic lenses such as a contact, corneal implant, and intraocular lenses, or of other small plastic or glass objects of similar shape, and more particularly to a method of making such lenses or objects with a high degree of precision at low cost by using an excimer laser. 
     BACKGROUND OF THE INVENTION 
     Ophthalmic lenses are normally manufactured by a mechanical process in which a block of polymethylmethacrylate (PMMA) is machined while being adhesively held on a support. The machining is quite difficult because of the small size of the lens and the intricacy of the shape into which the lens must be machined. 
     Typically, three operations must be performed to shape a lens: 
     (1) the workpiece must be cut out from a blank to form, e.g., an integral optic and haptic; 
     (2) the surface of the workpiece must be machined to the desired optical specifications (which may include convexities or concavities of varying radii at different points on the surface of the lens; and 
     (3) the edges of the workpiece must be radiused or rounded. 
     In the prior art, the edge rounding step alone typically required 7-14 days of gemstone tumbling, and precision was hard to accomplish in all of the steps. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of fabricating ophthalmic lenses or similar small objects quickly and accurately by using a laser, and particularly an excimer laser, to cut, surface-model and bevel a workpiece which is preferably made of PMMA but may, for appropriate purposes, be made of other plastics or of glass. The type and tuning of the laser is dependent upon the material of the blank. 
     In accordance with the invention, the workpiece is first cut to shape by shining a laser beam through a mask outlining the form of the cut required to shape (in the case of an ophthalmic lens) the optic and haptic. Considerable precision can be obtained in this step by expanding the laser beam in front of the mask and then reducing it beyond the mask to provide fine detail from a relatively large mask. The depth of the cut can be controlled by the number and energy of the pulses. 
     The surface modeling of the lens is next achieved by masking a laser beam in such a way that its energy distribution varies across the surface of the workpiece so as to ablate it to differing degrees at different points of the surface. This can be achieved by using a mask of varying opacity or a semi-transparent mirror with a coating of varying thickness at different points on the surface. This step, if desired, may be performed before the cutting step. 
     Finally, a laser beam is masked and focused generally into the form of a hollow cone whose tip is the focal point of the beam. By exposing the workpiece to the beam on one side of the focal point and then on the other, two bevel cuts are made along the perimeter of the upper and lower surfaces, respectively, of the workpiece. When combined with a vertical section of the side of the workpiece, these bevel cuts form an approximation of a rounded edge which is further softened by the slight melting of the workpiece material produced by the heat generated by the laser during cutting. 
     It is therefore the object of the invention to quickly and accurately produce a complex small object such as an ophthalmic lens from a blank entirely by the use of a laser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an intraocular lens to be manufactured by the method of this invention; 
     FIG. 2 is a schematic diagram of a laser optic used in the cutting step of the invention; 
     FIG. 3 is a plan view of the mask used in the cutting step; 
     FIG. 4 is a schematic diagram illustrating the surface modeling step of this invention; 
     FIG. 5 is a plan view of the mask used in the surface modeling step; 
     FIG. 6 is a schematic diagram illustrating the edge beveling step of this invention; 
     FIG. 7 is a plan view of the mask used in the beveling step; 
     FIG. 8 is a fragmentary detail section of the workpiece after the first beveling step; and 
     FIG. 9 is a fragmentary detail section of the workpiece after the second beveling step. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the preferred embodiment of the invention, which is the manufacture of ophthalmic lenses from a PMMA blank, the method of this invention is carried out with an excimer laser, i.e. a laser operating in the high ultraviolet wavelengths. An argon-fluoride laser operating at a wavelength of 193 nm in 250 millijoule pulses is preferred, but broadly any ultraviolet wavelength substantially absorbed by the material of the workpiece may be used. The choice of the laser is dictated by its ability to break up the large molecules of the workpiece material (as in the case of plastic) or to melt the material (as in the case of glass) so that the material will ablate. 
     FIG. 1 shows a typical intraocular lens which may be produced by the method of this invention. The workpiece 10 has an optic 12 which forms the actual lens, and a haptic 14 by which the lens is anchored in the patient&#39;s eye. In the prior art, polypropylene is usually used for the haptic 14, and PMMA is used for the optic 12. However, both the optic 12 and the haptic 14 may be formed of PMMA, and in the process of this invention this is preferable because the entire workpiece can be cut as a single piece. Of course, other ultraviolet-absorbing materials than PMMA (e.g. silicone) may be used for the workpiece if they are medically acceptable and properly ablatable. 
     FIG. 2 shows an arrangement useful in cutting the workpiece 10 from a block of PMMA. An excimer laser 16 emits a beam 18 of coherent ultraviolet light. Because the diameter of beam 18 is fairly small, a conventional laser beam expander 20 is used to expand the beam 18 to a diameter of several centimeters. A mask 22 best shown in FIG. 3 is formed integrally with the beam expander 20 or placed into the path of the expanded beam 18 to allow only a narrow strip of light in the shape of the outline 24 of the workpiece 10 to pass through the mask 22. 
     A beam converger or focusing optic 26 is used to project a reduced image of the outline 24 onto the PMMA block 28. Repeated pulses of the laser 16 will ablate the material of the block 28 until the profiled lens or workpiece 10 is very precisely cut out of the block 28. The precision of the cut is enhanced (and the power density of the beam increased) by the use of a relatively large mask 22 and a substantial reduction of the mask image on the block 28. 
     After being cut out from the block 28, the workpiece 10 is placed into the path of an excimer laser beam 30 (FIG. 4) which has a uniform energy distribution across its area. A mask 32 is interposed between the workpiece 10 and beam 32. 
     As best shown in FIG. 5, the mask 32 has different degrees of transparency at different points on the mask 32. For example, the mask 32 may have a coating of variable transmission characteristics, or it may be a neutral density filter (such as a polarizing or haze filter) with non-uniform transmission characteristic. In any event, the mask 32 transmits a large amount of beam energy in the areas 34 corresponding to desired depressions in the workpiece 10, and a small amount in the areas 36 corresponding to desired protrusions in the workpiece 10. 
     By appropriately controlling the transmission characteristics of the mask 32, it is possible to model or shape the surface 38 of the workpiece 10 in any desired manner without complex machining, and to do so precisely in a small amount of time. 
     In an alternative embodiment of the invention, the mask 32 may take the form of a semi-transparent mirror with a reflective coating whose thickness varies along its surface. In that embodiment, the laser energy not used for ablation is reflected away from the workpiece. 
     After the shaping or modeling step of FIGS. 4 and 5, the workpiece is fully formed but has sharp vertical edges which are not suitable for intraocular use. In the prior art, the edges of the workpiece were radiused or rounded by gemstone tumbling for 7-14 days, but besides being time-consuming, this prior art method often defeated the carefully achieved precision of the workpiece. 
     In accordance with the invention, an excimer laser beam 40 (FIG. 6) is expanded by a beam expander or (preferably) by a pair of curved mirrors 42, 44. The use of reflective rather than refractive beam expanding optics is preferred because it permits higher power transfer with smaller optics while avoiding damage to the optics. 
     The expanded beam 46 is conducted through a mask 48 best shown in FIG. 7 to a focusing lens 50. As a result, a beam generally in the form of a hollow cone is produced, with the tip of the cone being the focal point 52. 
     In order to round its edges, the workpiece 10 is first positioned below the focal point 52 at 54, and the laser is turned on. The conical shape of the beam will produce a bevel 56 (FIG. 8) on the edges of the workpiece 10. The ends of the bevel 56 are slightly rounded at 58, 60 by the small amount of heat which is produced during the ablation of workpiece material which forms the bevel 56. 
     When the bevel 56 has been fully formed, the workpiece 10 is positioned above the focal point 52 at 62, and the beam is turned on again. This time, the conical shape of the beam results in cutting a bevel 64 (FIG. 9) whose edges are slightly rounded at 66, 68 for the same reason as described above. 
     When combined with the vertical surface 70, the bevels 56, 64 and their rounded extremities provide a sufficient approximation of a rounded edge for the workpiece 10 to make it suitable for implantation in a patient&#39;s eye without danger of irritation. 
     It will be seen that the above-described process provides a fast and accurate way of manufacturing intraocular lenses without the use of complex machining equipment. The invention can, of course, be carried out with variations: for example, a very narrow laser beam may be moved around the periphery of the workpiece in the cutting and beveling steps, rather than cutting or beveling the entire periphery at once; or a mask may be scanned rather than being exposed all at once.