WO1996022751A1

WO1996022751A1 - Apparatus for uniformly ablating a surface

Info

Publication number: WO1996022751A1
Application number: PCT/EP1996/000288
Authority: WO
Inventors: Kristian Hohla
Original assignee: Chiron Technolas Gmbh
Priority date: 1995-01-25
Filing date: 1996-01-24
Publication date: 1996-08-01
Also published as: AU4621296A; US6059774A

Abstract

The apparatus and the method of ablating matter from a substrate comprise a laser (10), an optical system (12, 14, 18) and a means (20) which is arranged within the beam path of the laser and by means of which a laser spot is formed which exhibits light and dark sections. By adjusting the means (20) prior to each laser pulse, the entire surface area of the laser spot is subjected to the same number of laser pulses.

Description

APPARATUS FOR UNIFORMLY ABLATING A SURFACE

The invention pertains to a method and apparatus for uniformly ablating a substrate, and more specifically, for performing photo refractive keratectomy on the eye.

It is known to ablate tissue by means of laser beams for sculpting the cornea (photo-ablation) . For this purpose, excimer lasers are preferably used whose wavelength is about 200 nm or more. Fig. 1 shows a known arrangement in which a laser beam of an excimer laser 10 is directed through a beam homogenizer 12 and an iris diaphragm 14 and subsequently by a mirror 18 onto the cornea of an eye 2. The size of the iris diaphragm is adjustable by means of a control device 16. Thus, the size of the spot formed on the cornea by the laser beam can be adjusted. By varying the surface area and shape of the spot and the number of laser pulses for certain sections of the surface of the cornea, the form of the cornea can be modified and an ametropia of the eye can be corrected.

Fig. 2, for example, shows how the shape of the cornea, which originally has a radius Rj_, is flattened in a circular area having a diameter D by a laser treatment in such a way that subsequently an effective radius R2 is achieved. In the area of the optical axis 5 of the eye 2, the ablated layer has a thickness A. For reasons as yet unknown, the tissue is often not uniformly removed, i.e. after a treatment with the known apparatus the shape of the cornea deviates from the desired shape. As schematically shown in Fig. 3, which is an enlarged view of the area marked with a circle in Fig. 2, less tissue is excised in the area of the optical axis than at the edge. Experiments have resulted in a deviation x in the order of magnitude of 5 to 10 μm, measured in an area having a diameter d of 2 to 3 mm and being symmetrical to the optical axis 5. The so-called "central island" problem has so far been obviated by pre- and after-treating the central area in such a way that this elevation has been ablated by means of laser pulses while correspondingly varying the adjustment of the iris diaphragm 14. However, the results of such an aftertreatment are often unsatisfying.

It is an object of the present invention to provide an apparatus and a method which circumvent the aforementioned problem.

This object is achieved by means of the features of the claims.

The invention is based on the idea to divide the laser spot generated by conventional apparatus (ablation area, area to be treated) into several exposed and unexposed sections. Thus, each laser pulse excises tissue only in the exposed section. By re-adjusting the apparatus, sections which have not been exposed are exposed during the following laser pulse and tissue is removed to the effect that tissue is uniformly excised from the entire surface area of the laser spot. In contrast to the prior art, the ablation with respect to the predetermined size of the laser spot is not effected in one step but in several steps. The advantage of this method is that a uniform area is ablated and that the aforementioned "central island" problem is circumvented. Preferably, the exposed sections have such a size and shape that the distance of each point within an exposed section to the next unexposed point, i.e. outside the section, is less than 1.5 mm. If the exposed section is circular, this circle has a radius of less than 1.5 mm.

The ratio of the surface area of the exposed sections to that of the unexposed sections is preferably 1:5 to 5:1. The exposed and the unexposed sections can be distributed regularly or irregularly within the laser spot. In the central zone of the laser spot, the proportion of the exposed sections can, for example, be higher than in an outer zone.

The screen diaphragm is preferably arranged within the beam path in the area in front of the surface area to be treated. Alternatively, the screen diaphragm can be arranged between the laser and the beam homogenizer or the homogenizer and the iris diaphragm or the iris diaphragm and the tilted mirror. As an alternative to a reciprocating movement in one direction, the screen diaphragm can be moved in several directions, e.g. along the line of a rectangle or a polygon or a circle in a plane, and essentially perpendicularly to the axis of the laser beam. In addition to or instead of this movement, the screen diaphragm can be rotated around the axis of the laser beam or an axis parallel to the axis of the laser beam.

Instead of a screen diaphragm, a mask can be used which is provided with several sections which are transparent to the laser beam, and whose remaining sections are non-transparent to the laser beam. The advantage of a mask is that any pattern of transparent and non-transparent sections can be produced.

According to a further preferred embodiment of the invention, a mirror, certain sections of which are reflective, is arranged within the beam path. A tilted mirror arranged within the beam path can for example be used (cf. mirror 18 in Fig. 1 or Fig. 4) certain sections of which are reflective. According to the pattern of the reflective sections of the mirror, a laser spot with exposed and unexposed sections is produced on the surface area of the substrate to be treated. By adjusting the position of the mirror correspondingly and/or rotating the mirror around an adjustment axis before each laser pulse, different sections of the area to be treated can be exposed one after the other.

In a further developed embodiment of the invention, several means are arranged within the beam path. Several screen diaphragms and also combinations with masks or mirrors can be used. Two screen diaphragms of the same type which are shiftable with respect to each other can be arranged such that the size and shape of the exposed sections of the laser spot can be varied.

According to a preferred embodiment, a laser spot with a checkered pattern is produced by the means, i.e. the screen diaphragm, the mask or the mirror, having a certain reflective pattern. The number of the exposed sections of the laser spot corresponds to that of the unexposed sections and both types of sections have the same size and are regularly distributed. Alternatively, bar patterns of adjacent exposed and unexposed stripes can be produced. The advantage of regular patterns is that the means are adjusted according to a given pattern prior to each laser pulse or a series of laser pulses. However, any pattern, optionally even with exposed sections of different sizes and shapes, can be produced, preferably by adjusting the means such that statistically a uniform removal of tissue is achieved on the surface of the substrate to be treated. In the following, the present invention will be illustrated in more detail by means of examples and the drawing, in which:

Fig. 1 shows a known apparatus for sculpting a cornea by ablating tissue by means of laser pulses. Fig. 2 shows a schematic view of the cornea prior to and after the treatment. Fig. 3 shows a partial view of the section of Figure 2 which is marked with a circle in more detail. Fig. 4 shows a preferred embodiment of an apparatus according to the invention. Fig. 5 shows a view of a zone of the cornea corresponding to that of Fig. 3 after a treatment with the apparatus of the invention according to Fig. 4, Fig. 6 shows a screen diaphragm having a checkered pattern. Fig. 7 shows a laser spot with a bar-like pattern. Fig. 8 shows a mask with several exposed sections, and Fig. 9 shows a partial view of the mask depicted in Fig.

8

Fig. 4 largely corresponds to the apparatus depicted in Fig. 1. It is referred to the above description. Additionally, a screen diaphragm 20 is positioned within the beam path of the laser between the mirror 18 and the eye 2. The screen diaphragm 20 can be reciprocated essentially transversely with respect to the axis of the laser beam. The movement of the screen diaphragm 20 is controlled by the control device 16. The control is preferably effected automatically by a computer as a function of the number of laser pulses. Moreover, the control device can also control the iris diaphragm 16.

Due to the screen diaphragm, a laser spot having a light/dark pattern is formed on the surface to be treated. In contrast to the prior art, the spot does not only consist of a light area, but part of this area is shaded such that a multitude of small exposed sections is formed adjacent to which unexposed section are. Preferably, the screen diaphragm is re-positioned before each laser pulse and then preferably those sections on the surface area to be treated are exposed which have not been exposed before. After a desired series of adjustments, the entire surface area to be treated is exposed. As a result, the ablated layer of tissue exhibits a uniform thickness.

The screen diaphragm according to the invention can advantageously be used in a laser treatment for sculpting the cornea since thus a uniform ablation over the entire surface area to be treated can be achieved. As evident from Fig. 5, an elevation around the optical axis 5, as depicted in Fig. 3, is prevented. The advantage is that a pre- or after-treatment, which has so far frequently been necessary, can be avoided and a uniform surface structure can be achieved.

According to one embodiment, a screen diaphragm 20 exhibiting a checkered pattern can be used, as shown in Fig. 6. The checkered pattern has a width M and a height N and consists of exposed and unexposed sections which are regularly distributed and have a width k and a height 1. In the depicted embodiment, the screen diaphragm as well as the individual exposed and unexposed sections are square. Such a screen diaphragm 20 is reciprocated by the control device 16 by the unit k, 1 in one direction. Thus, after two laser pulses, the entire surface area to be treated is exposed.

According to the present invention, any means can be used which divides the laser spot into exposed (light) and unexposed (dark) sections. The surface area covered by the exposed sections can on the whole correspond to that covered by the unexposed sections, as for example in the checkered screen diaphragm according to Fig. 6. According to the invention, the surface area covered by the exposed sections can be smaller as well as bigger than that covered by the unexposed sections. The ratio of exposed to unexposed sections is preferably between 1:5 to 5:1. Using a ratio other than 1:1, the screen diaphragm is re-positioned several times. If, for example, a screen diaphragm is used which has two unexposed sections adjacent to one exposed section and having the same size, the screen diaphragm has to be shifted three times on the whole prior to one laser pulse for exposing the entire surface area to be treated to the effect that one laser pulse is effective in each section of the surface area of the laser spot to be treated. Thus, to effect a treatment equivalent to one laser pulse when the screen is not used, each section of the surface area has to receive a corresponding laser energy.

Preferably, the pattern of the screen diaphragm is regular. Alternatively, screen diaphragms can be used which have different ratios of exposed to unexposed sections over the surface area of the screen diaphragm. A screen diaphragm can for example be used which has in certain zones, such as in the centre or at the edge, a higher proportion of light or dark sections.

Fig. 7 shows a laser spot, which exhibits bar-shaped exposed and unexposed sections which are parallel to each other. Fig. 8 depicts a mask having exposed sections of different shapes and sizes. From Fig. 9 , which depicts a partial view of Fig. 8, it is evident that the shape and size of each exposed section has preferably to be selected such that the distance between any point within the exposed section and the edge of the exposed section, i.e. an unexposed point E, is < 1.5 mm. Thus, in the case of the laser spot depicted in Fig. 1 , the exposed stripes have preferably a width of < 3 mm. The length of the stripes is optional. In the screen diaphragm according to Fig. 6, the width k and the height 1 of the exposed sections are < 3 mm each. The present invention can in particular be advantageously used in a method for sculpting the cornea of an eye, wherein in particular by means of an adjustable iris diaphragm a laser spot of varying size and shape can be formed on the cornea. The invention can, however, also be used in other technical fields, i.e. for removing matter from any substrate.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, materials, components, circuit elements, wiring connections and contacts, as well as in the details of the illustrated circuitry and construction and method of operation may be made without departing from the spirit of the invention.

Claims

C l a i s

1. An apparatus for ablating matter from a substrate (2) comprising a laser (10) emitting a laser beam of a suitable wavelength, an optical system (12, 14, 18) forming a laser spot (F) on a surface area of the substrate to be treated and means (20) arranged within the beam path of the laser for shading part of the laser spot such that a plurality of exposed (Bl) and unexposed (B2) sections are formed on the substrate to be treated.

2. The apparatus according to claim 1, wherein each of the exposed sections (Bl) is of such a size and/or shape that the distance of each exposed point within this section to the next unexposed point outside this section is less than 1.5 mm.

3. The apparatus according to claim 1 or 2, wherein said means comprises means (20) for successively readjusting in such a way that the amount of all laser ablations in the exposed sections (Bl) corresponds to the desired ablation within the laser spot without the means (20) .

4. The apparatus according to any of claims 1 to 3, wherein the ratio of the surface areas of the exposed to the unexposed sections is 1:5 to 5:1.

5. The apparatus according to any of claims 1 to 4, wherein the sizes and/or shapes of the exposed and/or unexposed sections are identical.

6. The apparatus according to any of claims l to 5, wherein the exposed and unexposed sections are regularly distributed over the laser spot.

7. The apparatus according to any of claims l to ₅, wherein the exposed and unexposed sections are irregularly distributed over the laser spot.

8. The apparatus according to any of claims l to 7, wherein the means is adjustable essentially perpendicularly to the axis of the laser beam in at least one direction.

9. The apparatus according to any of claims 1 to 8, wherein the means is rotatable essentially around the axis of the laser beam or an axis which is parallel to the axis of the laser beam.

10. The apparatus according to any of claims 1 to 9, wherein the means (20) is changeable and thus the size and shape of the exposed sections is changeable.

11. The apparatus according to any of claims 1 to 10, wherein the means (20) comprises at least one screen diaphragm.

12. The apparatus according to any of claims 1 to ₁0, wherein the means comprises at least one mask.

13. The apparatus according to any of claims 1 to 10, wherein the means comprises a mirror which is reflective in certain sections.

14. The apparatus according to claims 1 to 13, wherein the means (20) has a checkered pattern.

15. The apparatus according to any of claims 1 to 14, further comprising an excimer laser (10) , a homogenizer (12), an iris diaphragm (14), a tilted mirror (18) and a control device (16) by means of which the opening of the iris diaphragm (14) can be changed.

16. The apparatus according to any of claims l to 15, wherein a control device (16) is connected to the means (20) , which control device adjusts the means preferably automatically as a function of the number of laser pulses.

17. A method for ablating matter from a substrate in particular by means of an apparatus according to any of claims l to 16, characterised in that the means (20) is re-positioned between the laser pulses.

18. The method according to claim 17, characterised in that the means (20) is re-positioned prior to each series of laser pulses, each series consisting of one to twenty laser pulses.

19. The method according to claim 17 or 18, characterised in that the means (20) is successively re-adjusted such that the amount of all laser ablations in the exposed sections (Bl) corresponds to the desired ablation within the laser spot without the means (20) .

20. The method according to any of claims 17 to 19, characterised in that the size and/or shape of the laser spot is changed according to the desired shape of the substrate to be treated.

21. A method for ablating matter from a substrate using a laser beam and an optical system, the method comprising the steps of: providing a laser spot on the substrate within an area to be treated, wherein the laser spot includes a plurality of exposed and unexposed sections on the substrate; and shifting the laser spot on the substrate by a predetermined amount such that at least the unexposed sections within the area to be treated on the substrate are exposed.

22. The method of claim 21 further comprising the step of: repeating the shifting step until all the unexposed sections on the substrate are exposed.

23. The method of claim 22 further comprising the step of: before repeating the shifting step, firing a pulse of the laser.

24. The method of claim 23, wherein the repeating of the shifting step is repeated between 1 to 20 times to expose all the unexposed sections within the area to be treated on the substrate.