Systems and Methods for Creating Substrate Surfaces by Photoablation

This invention relates to systems and methods for photoablating, photoablatable material to create smooth, rounded work surfaces, and, in particular, for photoablating such photoablatable material as the cornea of a human eye. The systems comprise means for reflecting photoablating light, such that reflected rays will be tangent to the surface of the proposed rounded work surface. This reflecting means has an opening of sufficient size and shape to expose some or all of the photoablatable material (substrate) to reflected light capable of photoablating and producing said rounded work surfaces. Preferably, the reflecting means is linked to means for adjusting its height and tilt with respect to the photoablatable substrate, and to other means for fixing and adjusting, as desired, the angle of incidence of photoablating light on the surface of the reflecting means. In turn, such adjusting means determine the angle at which photoablating light is reflected, and works upon the substrate to produce the required rounded work surface. The adjusting means also controls the amount and location of the substrate that is photoablated. Preferably, these systems also include means for determining, adjusting and fixing the path of photoablating light from its source to the surface of the reflecting means. As a result, the photoablating light is preferably kept substantially coaxial with the axis of revolution of the reflecting means.

In use, the reflecting means is linked to means for generating light capable of photoablating the substrate. Preferably, this photoablating light comprises intense, coherent, ultrashort pulsed, collimated ultraviolet light (UV) (such as light produced by an excimer laser having a wavelength in the range of about 150 to about 250 nanometers). Ultrashort pulses of longer wavelengths may have similar effects. Preferably, the fluence (i.e., the power density) of the photoablating light is in the range of about 20 to about 1,000 millijoules per square centimeter per pulse for a wavelength of 193 nm.

Preferred embodiments of these systems may also include a cover means for the reflecting means to admit photoablating light only to the surface of the reflecting means, and to exclude unreflected photoablating light from direct contact with all or a part of the substrate. Where the source of photoablating light is an excimer laser or other source of high-intensity UV light, this cover means is preferably a shield having portions substantially transparent to, and portions substantially opaque to the photoablating wavelength of light.

These systems can also, in preferred embodiments, include a shutter system, preferably an ultrahigh speed shutter system, for the source of photoablating light. Preferably, such a shutter system has a speed on the order of nanoseconds. The shutter system is preferably under control of means for opening and closing the shutter in response to a signal indicating that the reflecting means is properly aligned with the source of photoablating light.

These systems can also include means for aiming and aligning the source of photoablating light with the reflecting means. In preferred embodiments, another light source, coaxial with the source of photoablating light, and a means for detecting its reflection, can be used to detect the angle of incidence of this light on the reflecting means and thereby align the light from the photoablating light source with the reflecting means. In preferred embodiments, this means for aiming and aligning the source of photoablating light is a coaxial aiming laser such as a helium neon laser or other laser capable of emitting non-photoablating light coaxial with light from the photoablating light source. A photodetector or other means for detecting the proper alignment of the light from the aiming means can be used to detect whether the light from the photoablating source is properly aimed at, and focused upon the reflecting means. In turn, the signal from the means for detecting proper alignment of light from the aiming means with the reflecting means can be used to control the means for opening and closing the shutter in the shutter system to deliver photoablating light of proper intensity, at the proper time, and for the proper duration to the reflecting means, and from there, to the substrate.

The reflecting means itself is curved, preferably aspheric, and can have a smooth, curved surface or a Fresnel surface.

This invention also provides methods for photoablating substrates comprising placing means for reflecting light capable of photoablating said substrate over said substrate; directing light capable of photoablating said substrate onto a reflecting means of sufficient curvature at an angle of incidence sufficient to direct reflected, photoablating light across, and to photoablate material from the substrate; and adjusting the angle of incidence between the photoablating light and said reflecting means in a degree sufficient to remove from said substrate a predetermined quantity of material in a predetermined pattern and shape. Where the substrate is the cornea, the predetermined quantity, pattern and shape of the material removed can correct refractive errors such as myopia, hyperopia and astigmatism, eliminating the need for eyeglasses and contact lenses. This cornea-shaping process is sometimes called photokeratomileusis.

This invention can better be understood by reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a preferred embodiment of the new photoablating system, here used to photoablate tissue from the cornea of a human eye;

FIG. 2 is an exploded, fragmentary view of a part of the system shown in FIG. 1;

FIG. 3 is an exploded view of the system shown in FIG. 1, here used to shape corneal tissue into a lenticule ex situ by photoablation; and

FIG. 4 is a schematic diagram illustrating how the adjustment in height of the reflecting means affects the amount of corneal tissue that is photoablated and hence the size of the optical zone created by the embodiment illustrated in FIG. 1.

FIGS. 5A, 5B and 5C show the effects of including plano portions in the reflecting means of the embodiment illustrated in FIG. 1. The reflected light from these 45.degree. angulated plano mirrors can be used to determine the height of the reflecting means relative to the apex.

FIG. 6 is an exploded view of the preferred embodiment of the mask for use with the embodiment illustrated in FIG. 1. This figure shows the ring-shaped UV transparent window that corresponds to the dimensions of the reflecting means when viewed along the axis of revolution of the reflecting means. This figure also shows the mirrors extending from the mask. The mirrors may be used for aligning the laser and the reflecting means.

FIGS. 7 and 8 are additional illustrations showing the effects of plano portions at the top or bottom of the reflecting means in the new photoablating systems, and in particular in the preferred embodiment shown in FIG. 1.

FIG. 9 shows the hollow cylinder of photoablating light produced by the mask in the preferred embodiment of the new system shown in FIGS. 1-3.

FIGS. 1 and 2 show a preferred embodiment of a system for photoablating tissue from cornea 1 of a human eye 2. Reflecting means, here curved, ring-shaped mirror 5, reflects coherent, collimated, intense UV light from excimer laser 8 across corneal surface 1. Mirror 5 has a far UV reflective coating such as an enhanced aluminum or multi-layered dielectric coating. The curved shape of mirror 5 causes a hollow cylinder of UV light to be reflected over corneal surface 1 in the shape of a hollow dome of light. This dome can have any desired radius of curvature, and can be spherical or aspherical in shape.

Each ray of UV light reflected from the curved mirror surface 5 is tangent to some point on the surface of the dome. Corneal tissue struck by the dome of reflected UV light is volatilized, leaving the remaining cornea with a new curvature corresponding to the inner surface of the dome. The corneal surface curvature obtained can be precisely and accurately predetermined by modifying the shape and curvature of the mirror 5 which determines the shape and radius of curvature of the dome of UV light. Since substantially all of the UV light that touches the cornea is reflected and tangent to the new corneal surface, and since far UV light at 193 mm is absorbed in the first few microns of corneal tissue, the amount of far UV light reaching the lens and retina of eye 12 is minimal.

Holder 15 positions mirror 5 on eye 2, and provides means for adjusting the height of mirror 5 relative to the apex of the corneal surface 1. The height of mirror 5 relative to the corneal apex determines the amount of corneal tissue that is photoablated, and hence the size of the optical zone created, i.e., the central cornea used for image formation. An optical zone that is too small causes glare and distortion. The size of the optical zone may be increased by decreasing the height of the mirror relative to the corneal apex, but only at the expense of further thinning of the cornea, as FIG. 4 shows. A screw-type mechanism or piezo-electric crystal translator can provide the means for the height adjustment. Optical zone size and the maximum possible change in refractive power of the cornea through this system are inversely proportional. Tilt is controlled by repositioning holder 15 and mirror 5 on the eye. The alignment laser 7 confirms proper tilt adjustment.

Light from aiming laser 7, coaxial with light from excimer laser 8, strikes three small plano mirrors 26 on mask 10 or the 45.degree. angulated portion 19 of mirror 5 shown in FIGS. 1, 5A, 5B, 5C and 7, and is reflected to photodetector 4 via beam splitter 11 only when mirror 5 is accurately and precisely aligned. For example, in FIG. 1, light beam 9 from coaxial laser 7 is reflected in this way to photodetector 4 as light beam 3.

As FIGS. 5A, 5B and 5C show, if plano portions 19 of the mirror 5 are at the bottom and oriented at 45.degree. angles to the laser source, laser light is reflected back to a photodetector on the laser. The position of the reflecting means relative to a substrate in its aperture determines whether the laser light is reflected or blocked. This phenomenon can be used to determine the position of the mirror relative to the substrate, and thereby determine the amount of substrate to be photoablated.

As seen in FIG. 6, mask 10, preferably made of a combination of UV transparent material such as quartz or fused silica, and UV opaque (but visible light transparent) glass such as a UV filter, permits only a hollow cylinder of UV light 33 to pass through UV transparent zone 20 to reach mirror 5 (see FIG. 9). The inside and outside diameters of this hollow cylinder of UV light 23 from the excimer laser correspond to the dimensions of aspheric mirror 5 when viewed from above. When the axes of mirror 5 and the cylinder of UV light from excimer laser 8 are properly aligned, mirror surface 5 is fully and evenly illuminated. Zones 21 and 22 of mask 10 prevent UV light from directly striking the cornea, lens, retina and other ocular structures in the human eye, and provide a target to focus on for purpose of aligning axes of eye 2, mirror 5 and light from laser 8.

In operation, if photodetector 4 senses reflected light beam 3 from aiming laser 7, then the light from excimer laser 8 will precisely and accurately fall on mirror 5. When photodetector 4 receives a signal to this effect, ultrafast shutter 6, which can be an electro-optic shutter opens, permitting light beams 13 and 14 from excimer laser 8 to pass to curved mirror 5. From there, the excimer light passes to corneal surface 1 as reflected, dome-shaped UV photoablating light. A microprocessor can be used to control shutter 6 precisely and accurately. UV plano mirror 29 allows horizontal laser beam to be projected onto the eye of a patient in the supine position.

As FIGS. 7 and 8 show, curved reflecting means 5 can have plano portions at top 27 or bottom 19. The plano mirrors at the top reflect parallel rays onto the substrate and may be employed to remove substrate in a particular fashion (i.e., to create a smooth transition area outside the optical zone). This effect is energy dependent, unlike the creation of curved surfaces that do not change shape if overtreated.

FIG. 3 shows the application of the system illustrated in FIGS. 1 and 2 to the photoablation of corneal button 16 to produce lenticule 24. Here, the hollow cylinder of excimer laser light 23 passes through mask 10, and is reflected from ring-shaped, aspheric mirror 5 across corneal button 16. Again, the reflected UV light is a hollow dome 18 of any desired radius of curvature, and can be spherical or aspherical. Corneal button 16 is held firmly over the convex-shaped surface 25 of cylindrical lenticule holder 12 by vacuum or other means. Threaded height adjustor 17 permits adjustment of the height of mirror 5, and that controls the amount of corneal tissue to be photoablated in forming corneal lenticule 24, thereby determining lenticule thickness and the optical zone size. The undersurface of mask 10 (FIGS. 1-3) has absorbent antireflective UV coating 28 to prevent undesirable light scattering.