Abstract:
A device for isolating a lenticle in the cornea of an eye. The device includes: a laser beam source to emit pulsed laser radiation having a pulse frequency of 1.2 MHz to 10 MHz, a pulse energy of 1 nJ to 200 nJ and a wavelength penetrating the cornea; a beam-forming unit having beam optics with an image field and that bundles pulsed laser radiation into a focus located inside the image field, and which has a maximum diameter of less than 3 μm; a beam-deflection unit shifting the focus in the cornea and inside the image field, the focus moving along a path when the image field is resting; and a control unit to control the source and the beam-forming unit to isolate the lenticle by specifying the path. The lenticle is delimited by a cut surface which is curved with regard to a front surface of the cornea.

Description:
PRIORITY CLAIM 
       [0001]    The present application is a National Phase entry of PCT Application No. PCT/EP2014/052167, filed Feb. 4, 2014, which claims priority from German Patent Application Number 102013204496.8, filed Mar. 14, 2013, the disclosures of which are hereby incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a device for isolating a lenticle in the cornea of an eye, which device has a laser beam source which is designed to emit pulsed laser radiation with a wavelength which penetrates into the cornea, a beam forming unit which has beam optics which bundle the pulsed laser radiation in the cornea into a focus, and a beam deflection unit, which shifts a focus of the radiation in the cornea, wherein a control unit is provided which is designed to control the laser beam source and the beam forming unit, in order to isolate the lenticle in the cornea, which lenticle is delimited by a cut surface. 
         [0003]    The invention also relates to a method for isolating a lenticle in the cornea of an eye, wherein at least one cut surface is established in the cornea, which surface defines the lenticle and the cut surface in the cornea is created by emitting pulsed laser radiation, wherein pulsed laser radiation which has a wavelength which penetrates into the cornea is used, and a focus of the laser radiation is shifted in the cornea. 
       BACKGROUND OF THE INVENTION 
       [0004]    The shape of the front surface of the cornea of an eye is important for the imaging properties of the eye. Therefore, it has long been known to use the cornea of an eye to correct impaired vision, with the aim of modifying the front surface of the cornea of an eye and thus its refractive properties and in this way compensate for impaired vision. In the state of the art, surgical procedures which loosen a lamella in the cornea of an eye, fold this lamella back and then remove material from the inside of the cornea of an eye which has been exposed as a result, have been developed for this purpose. Subsequently, the lamella is folded back into place, leaving the cornea of an eye with a differently shaped front surface, due to the removal of material. In the state of the art, this correction method is abbreviated inter alia to LASIK; hereinafter it will be identified as lamella-releasing correction of impaired vision. In earlier embodiments, the lamella was loosened by means of a mechanical keratome. The cornea of an eye was pushed flat by a flat contact lens and the lamella cut by means of the mechanical keratome. In a development, so-called laser keratomes have started to be used by now. Inter alia a laser keratome from Ziemer Ophthalmic Systems AG, Port, Switzerland, is known for this purpose. With regard to its beam deflection, it was designed to create the lamella to be folded back. A further laser keratome was developed by Intralase Inc., USA, which by now is owned by Abbott Laboratories, Illinois, USA. Both laser keratomes work by means of pulsed laser radiation, wherein repetition rates and pulse energies differ. 
         [0005]    Surgical correction of impaired vision has been further developed to methods which isolate and extract material in the cornea. The material usually has the form of a lenticle, which is why these methods are called lenticle-extracting methods or devices here. Also, for reasons of clarity, the volume to be isolated and extracted is identified as a “lenticle”, even if in specific applications a non-lenticle-shaped volume is to be isolated and extracted. 
         [0006]    The devices and methods named at the outset relate to the method of lenticle extraction. Lenticle-extracting devices and methods have the advantage that the front surface of the cornea is damaged in a very much smaller area. It is no longer necessary to have an almost completely annular incision into the front surface of the cornea, as is required for releasing a lamella which would expose the inside of the cornea. Instead, a smaller incision at the edge, which leads to the isolated volume, and through which the isolated volume can be removed, is enough, optionally after reduction of the isolated material. The method of lenticle extraction requires that the cut surfaces isolating the lenticle be created with high precision in the inside of the cornea. In order to correct impaired vision, so as to use as little tissue as possible, additionally at least one of the cut surfaces delimiting the lenticle should lie at a non-uniform distance from the front surface of the cornea. Also here there is a difference from the approach which loosens and folds back a lamella of the cornea, as there the only lamella-creating cut surface may lie at a uniform distance from the front surface of the cornea, thus parallel to the front surface of the cornea. If the front surface of the cornea is pressed flat with a flat contact lens, when creating the cut in the lamella, only one cut surface, which lies parallel to the surface of the contact lens as far as the edge sections, and for its part is also flat, needs to be created. 
         [0007]    Lenticle-extracting correction of impaired vision is basically described in WO 2004/105660 and WO 2004/105661. Additionally, developments are known in the state of the art. Thus WO 2005/011547 discloses the use of contour lines for rapid isolation of lenticles, while WO 2008/055697 indicates calculation rules for how the boundary surfaces of the lenticle, i.e., the cut surfaces being created, can be chosen. It is known in particular from this document to divide the cut surfaces delimiting the lenticle into an anterior flap surface which is at a uniform distance from the front surface of the cornea of an eye and a posterior lenticle surface which is not at a uniform distance from the front surface of the cornea. The distance between the surfaces, and thus their shape, influences the curvature of the cornea after correction. 
         [0008]    WO 2008/055705 and WO 2008/055706 deal with the issue of curvature of the image field when using a non-planar contact lens and creating control data for the surgical procedure. 
         [0009]    The cut surface is usually created by pulsed laser radiation. For this, the targets of the laser radiation are arranged along a path which lies in the cut surface and ultimately specifies the cut surface. WO 2008/055698 outlines the arrangement of targets along the path, wherein it is provided not to specify a target for every pulse of laser radiation emitted into the cornea of an eye. 
         [0010]    WO 2008/131878 discusses the question of how, if laser surgery is interrupted, further treatment can take place which takes into consideration changes to the tissue of the cornea which have already been created. 
         [0011]    WO 2009/059711 and WO 2009/059730 deal with different profiles of the lenticle to be removed for specific corrections of impaired vision, specifically correcting hyperopia, and specify minimum values for the lenticle. 
         [0012]    WO 2003/059563 discloses operating parameters for a laser device for operations to correct impaired vision by means of lenticle extraction. The same applies to EP 1628606 B1. 
         [0013]    In the state of the art, the femtosecond laser keratome VisuMax from Carl Zeiss Meditec AG is known for the lenticle-extracting method. It uses a femtosecond fibre laser which emits in the infrared spectral range and which emits at a pulse repetition rate of 500 kHz laser pulses which are focussed in the cornea of an eye. 
         [0014]    The time it takes to create a cut surface is of great importance both for the quality of the correction of impaired vision and also for acceptance by patients. The longer the duration of the intervention, the greater the risk of disruptive eye movements which reduce the precision of the creation of the cut surface or can even lead to it no longer being possible to create any coherent cut surfaces, and the procedure needing to be interrupted. Also, a longer-lasting intervention places unwanted stress on the patient. 
         [0015]    With lamella-releasing correction of impaired vision, the quality of the optical correction is determined essentially by the precision of the volume removed after releasing the lamella. The position of the lamella-releasing cut surface itself is of less, or even no, importance. On the contrary, the exact positioning of the cut surfaces in the cornea of an eye is of great importance for the quality of the result of a lenticle-extracting correction of vision. As the cut surface is created by adjusting the focus of the pulsed laser radiation along a path, ultimately the positioning accuracy of the focus in the cornea of an eye is important. Here, it must not be forgotten that this is a living tissue, which can occasionally be modified during intervention and also does not necessarily react in linear fashion to changes in parameters. 
       SUMMARY OF THE INVENTION 
       [0016]    Therefore, the object of the invention is to develop a device of the type named at the outset or a method of the type named at the outside such that a quick creation of a cut surface with simultaneously high precision can thus be achieved. 
         [0017]    This object is achieved, according to the invention, by a device for isolating a lenticle in the cornea of an eye, which has: 
         [0018]    a laser beam source that is designed to emit pulsed laser radiation with
       a pulse frequency of 1.2 MHz to 10 MHz,
           a pulse energy of 1 nJ to 200 nJ, and   a wavelength which penetrates into the cornea;   
           a beam forming unit which has
           beam optics, which have an image field and which bundle the pulsed laser radiation in the cornea into a focus which is located inside the image field and which has a maximum diameter of less than 3 μm, and   a beam deflection unit which shifts the focus in the cornea and inside the image field, wherein the focus moves along a path while the image field is resting; and   
               
 
         [0025]    a control unit, which is designed to control the laser beam source and the beam forming unit, in order to isolate the lenticle in the cornea by specifying the path, which lenticle is delimited by at least one cut surface which is curved with regard to a front surface of the cornea. 
         [0026]    The object is further achieved by a method for isolating a lenticle in the cornea of an eye, wherein 
         [0027]    at least one cut surface is established in the cornea, which surface defines the lenticle and is curved with regard to a front surface of the cornea;
       a path which lies in the cut surface is established;   there is emitted into the cornea pulsed laser radiation with
           a pulse frequency of 1.2 MHz to 10 MHz,   a pulse energy of 1 nJ to 200 nJ, and   a wavelength which penetrates into the cornea;   
           beam optics are used, which optics have an image field and which bundle the pulsed laser radiation in the cornea into a focus which is located inside the image field and which has a maximum diameter of less than 3 μm; and   the focus in the cornea and inside the image field is shifted, wherein the focus moves along a path while the image field is resting.       
 
         [0035]    The invention combines different features which together make possible the creation of a cut surface which is curved with regard to the front surface of the cornea. It is not thus a conventional lamella-releasing cut surface, but a cut surface which has no uniform distance from the front surface of the cornea in relation to a cut plane through the eye which contains the axis of vision or the optical axis of the eye. With regard to the front surface of the cornea of the eye the invention provides the curvature, more precisely a two-dimensional curvature, to exist in a central area about the optical axis or the axis of vision of the eye which area for example has a diameter of not more than 10 mm. This is the area which is essential for optical correction, and in this area, customary initial laser cuts of lamella-releasing correction methods have a uniform distance between the front surface of the cornea of an eye and the cut surface, which causes the lamella to expose the inside of the cornea of an eye. 
         [0036]    For the following reasons, the combination according to the invention goes beyond a simple aggregation of features and achieves a combined effect: 
         [0037]    The individual laser pulses which form the cut surface along the path act differently in the cornea of an eye, depending on pulse energy, pulse frequency and focus diameter. With regard to the separation of tissue due to pulsed laser radiation, two different ways of working can be identified. These different processes will be called “tissue splitting” and “tissue cutting” hereinafter. 
         [0038]    If laser pulses are focussed into the cornea under specific conditions, in the focal volume there is tissue dissolution which releases gases which are at high pressure and thus exert mechanical forces on the area surrounding the tissue. The corneal tissue consists of a lamellar collagen structure, with the result that, to the best of the inventor&#39;s knowledge, the mechanical forces create microruptures which travel along the lamellae. Thus, in addition to the primary process of tissue dissolution in the focal volume, there is a secondary effect which leads to tissue splitting along the collagen structure. If, on the other hand, comparatively small pulse energies and higher repetition rates are being used, there is no secondary effect of tissue splitting, but merely the tissue dissolution created in the volume of the focus causes the separation. The corneal tissue is thus cut, largely regardless of the collagen structure. 
         [0039]    The inventors recognized that there is no abrupt transition between tissue splitting and tissue cutting. In the parameter space of the laser pulse application there is a transition region in which either one mechanism or the other plays a more or less substantial role in the process of tissue separation. Furthermore, it has been shown that in lenticle-extraction methods and devices thus far in clinical practice exclusively utilize parameter sets which led to the secondary effect of tissue splitting having a substantial role to play in the separation process. Exclusively tissue-cutting sets of parameters are currently used in clinical practice only by the laser keratome of Ziemer Ophthalmic Systems AG, Port, Switzerland. However, to the best of public knowledge, this device is not capable of creating cut surfaces which are curved with regard to the front surface of the cornea, and, as already stated at the outset, such way of working does not offer any substantial advantage for applications using the lamella-releasing method, and is therefore not important. 
         [0040]    It should be emphasised that different ways of working, i.e., the influence of parameters on the question of whether tissue splitting or tissue cutting prevails when creating lenticles, have been identified for the first time by the inventors. The difference between the ways of working with this method is not mentioned in the state of the art. Also, the literature does not assign the individual realisations of the state of the art to these ways of working. 
         [0041]    From their work, the inventors recognized also that, with regard to the precision to be achieved, there is a substantial difference between tissue splitting on the one hand, and tissue cutting on the other, only if curved cuts are made. This is because tissue splitting generally always proceeds along the lamellae of the collagen structure of the corneal tissue. With lamellar cuts there is therefore no advantage or disadvantage between lamellar splitting and lamellar cutting in the machining result. For curved cut surfaces it is different: This is because it is not possible, because of the physiologically given lamellae thickness and structure, to place a curved cut surface with high contour accuracy at just any point in the cornea of an eye if tissue-splitting separation is taking place. 
         [0042]    The mechanism of tissue cutting allows a very much more precise cut surface positioning and in particular a higher contour accuracy to be achieved. The inventors trace this back to the fact that, in a tissue-cutting process other than tissue splitting, the cut surface does not necessarily follows in boundary surfaces between lamellae of the collagen structure of the corneal tissue, but also can be arranged within a lamella. An advancing fissure (advancing the splitting before the proceeding sequence of laser pulses) which would occur in the lamella plane during tissue splitting is prevented and the separation takes place precisely in the focus position. However, tissue splitting can also be understood as a sort of digitisation of the cut surface position, wherein the smallest unit is the thickness of the lamella as a tissue-splitting created cut surface tends always to lie in boundary surfaces between individual lamellae of the corneal tissue. This is not a defect for cuts which are effected at a uniform distance from the front side of the cornea, because the lamella structure follows the front side of the cornea with good accuracy. For curved cut surfaces of the lenticle, the distance of which from the front side of the cornea varies depending on radius and optionally angle, this type of creation of a cut surface proves disadvantageous. 
         [0043]    With regard to pulse frequency, pulse energy and focus diameter, these relationships lead, particularly reliably, to the parameter range according to the invention having a tissue separation in which the process of tissue cutting strongly prevails. The combination with the creation of a cut surface which is curved with regard to the front surface of the cornea and thus does not necessarily follow the boundary surface between lamellae of the collagen structure of the corneal tissue (which otherwise travels largely parallel to the front surface of the cornea) therefore leads to a particularly accurate cut surface positioning and high contour accuracy. In the working range according to the invention, the contribution of tissue splitting is reduced in favor of the effect of tissue cutting, and therefore the cut surface and thus the isolation of the lenticle follow the specified values more precisely for lenticle extraction. 
         [0044]    The tissue cutting effect is particularly great if the pulse energy is less than 100 nJ and, particularly preferably, less than 10 nJ. A pulse energy range of 10 nJ to 80 nJ has proved particularly favorable. The same applies for the pulse length of the pulsed laser radiation which, at infrared wavelengths, should be no more than 1 ps. 
         [0045]    The wavelength of the laser radiation used is such that the laser radiation can penetrate into the cornea of an eye (transmission factor ≧0.8) and there leads to to tissue separation in the inside of the cornea, by means of non-linear effects. For this, a wavelength of 1030-1060 nm has proven useful. Alternatively, laser radiation in the ultraviolet spectral range between 300 nm and 400 nm can be used. It actually tends towards a higher linear interaction fraction (absorption) than the named infrared radiation, but is likewise suitable if the focus diameter does not exceed 2 μm and the pulse frequency is not more than 2 MHz. If laser pulses from this ultraviolet wavelength range are used, pulse lengths of a few ns can also be used effectively. 
         [0046]    The lenticle is isolated by the focus being shifted within the image field of the beam optics used and the image field being resting with regard to the cornea to be machined. This is where it differs from the laser keratome of Ziemer Ophthalmic Systems AG, in which a microscope object is used, the image field of which is much too small to achieve complete detection of the area in the cornea to be machined. The microscope object and thus the image field is therefore shifted with this laser keratome. A curved cut surface thus cannot be created with justifiable expense, which is why this laser keratome pushes the front surface of the cornea of an eye flat by means of a planar contact lens, and therefore the created cut surface automatically lies parallel to the front surface of the cornea of an eye. However, the invention uses an resting image field, for which, with regard to the area to be machined, a diameter of at least 3 mm, preferably at least 6 mm and particularly preferably at least 7 mm, is sensible. Small image fields hinder the creation of cut surfaces or are too small for customary lenticles to be isolated. 
         [0047]    As the surface is cut by shifting the focus within the image field along a path which lies in the cut surface, almost any curved cut surface in the cornea of an eye can be created with a suitable three-dimensional adjustment of the focal position. Therefore, it is no longer necessary to flatten the cornea of an eye. Instead, work can be done on a curved contact lens, wherein it is preferable that a contact surface of this contact lens to be placed on the front surface of the cornea has a radius of curvature of not more than 50 mm, preferably not more than 20 mm. The curvature of the contact surface of the contact lens specifies the curvature of the cornea during intervention. 
         [0048]    To create the curved cut surface, it is also advantageous if the beam optics have an objective with a numerical aperture of at least 0.33 in the cornea. 
         [0049]    It is understood that the features mentioned above and those yet to be explained in the following are applicable, not only in the stated combinations, but also in other combinations or singly, without departure from the scope of the present invention. 
         [0050]    In particular, the invention can advantageously be used in slightly modified form also for the creation of curved cuts in other elements of the eye, for example in the lens or the vitreous body. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0051]    The invention is explained by way of example in yet greater detail in the following with reference to the attached drawings, which also disclose features essential to the invention. There are shown in: 
           [0052]      FIG. 1  a schematic representation of a device for carrying out a lenticle-extraction correction of impaired vision, 
           [0053]      FIG. 2  a simplified sectional representation through the cornea of an eye to illustrate the lenticle to be isolated and extracted, and 
           [0054]      FIG. 3  a projection of a cut surface delimiting the lenticle, to illustrate the creation of the cut surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]      FIG. 1  shows, schematically, a device  1  for carrying out a lenticle-extracting correction of impaired vision. Device  1  has a laser  2  which provides pulsed laser radiation, wherein in the embodiment outlined, the laser  2  emits a pulsed pure beam  3  which has a wavelength which penetrates into the tissue of the cornea of an eye, with the result that a machining by means of non-linear effects can take place there. Pure beam  3  is shaped by a chopper  4  with regard to pulse duration, wherein predistortion known from the state of the art can take place, which ensures that the desired pulse length of for example ≦1 ps is present after passing through the further optical path of the beam path in the material, i.e., in the cornea of an eye. Chopper  4  and laser  2  form a laser beam source  5  which emits a pulsed laser beam  6  of the desired pulse length. 
         [0056]    Pulsed laser beam  6  also passes through a scanner  7  which preferably brings about a two-dimensional deflection transverse to the direction of propagation of the laser radiation. The laser beam  6  scanned in this way is focussed into the cornea of an eye by an objective  8 . Scanner  7  forms a beam forming unit  9  together with the objective  8 , which ensures that pulsed laser radiation  6  is focussed into a cornea of an eye in locations which can be set, wherein the focus diameter there is less than 3 μm. 
         [0057]    Laser beam source  5  can optionally comprise a so-called pulse picker which can for example be part of the chopper  4 . This pulse picker changes the frequency of the laser radiation pulses in the pure beam  3 , which pulses have a machining effect on the cornea of an eye. Thus for example it is possible to design the laser  2  such that it provides a pure beam  3  with a pulse frequency which is clearly higher than that pulse frequency which is desired for the machining-effective laser pulses of the laser beam  6 . Then, the pulse picker reduces the frequency of the effective laser radiation pulses by rendering the machining effect of individual laser radiation pulses harmless. This can for example take place by the pulse picker increasing the pulse length. The importance of the pulse picker known in the state of the art will be explained later on with the help of  FIG. 3 . 
         [0058]    In the embodiment shown in  FIG. 1 , the pulse frequency of the pulsed laser beam  6  is between 1.2 MHz and 10 MHz, wherein the frequency is relative to those pulses which have a machining effect, i.e., pulses which are not rendered harmless by a pulse picker if present. 
         [0059]    The energy of these pulses of the pulsed laser beam  6  is between 1 nJ and 200 nJ, preferably between 10 nJ and 100 nJ, particularly preferably between 20 nJ and 80 nJ. 
         [0060]    The wavelength of laser beam  6  lies in a range of 1030 nm to 1060 nm or 300 nm to 400 nm or another spectral range which can penetrate into the cornea, for which the cornea thus has a transmission factor of at least 0.8. 
         [0061]    Device  1  also comprises a contact lens  10  which serves to fix the eye  11  and also to give the front surface of the cornea  12  of an eye  11  a desired and known shape. The corresponding contact surface of contact lens  10  has a radius of curvature of 50 mm or less, particularly preferably 20 mm or less, for this purpose. 
         [0062]    Objective  8  bundles the laser radiation  6  into a focus  13  which is inside the cornea  12 . Focus  13  has a maximum diameter of 3 μm, preferably a maximum of 2 μm. The maximum diameter is the largest diameter which, e.g., in the event of an elliptical focus spot, is measured along the large semiaxis. In the event of a circular spot, the spot diameter is the relevant measurement. 
         [0063]    The dotted line of  FIG. 1  shows that, depending on the effect of the scanner  7 , the focus  13  is at different points in the cornea  12  of an eye  11 . In the design of  FIG. 1 , the scanner  7  brings about a deflection transverse to the main direction of incidence of the laser radiation  6 . 
         [0064]    The focus position is adjusted along the main direction of incidence by suitable actuation of the objective  8 , which is designed to be suited to a z-adjustment. 
         [0065]    The laser beam source  5  (in the design of  FIG. 1  produced by laser  2  and chopper  4 ) as well as the beam forming unit  9  (in the design of  FIG. 1  produced by scanner  7  and objective  8 ) are connected to a control apparatus  14  via control lines, not shown in more detail, which control apparatus controls these elements in suitable manner. The actuating of the control apparatus  14  creates a cut surface in the cornea of an eye. The corresponding relationships are represented in a sectional representation in  FIG. 2 , which shows the cornea  12  schematically. 
         [0066]    A lenticle  15  in the cornea  12  is isolated by adjusting the focus  13  of the pulsed laser radiation  6 . Lenticle  15  is delimited at the front by a flap surface  16  and at the back by a lenticle surface  17 . In order to be able to keep the boundary surfaces of the lenticle  15  as simple as possible, the flap surface  16  is at a uniform distance from the front surface  18  of cornea  12 . Flap surface  16  is thus not curved with regard to front surface  18 . This is different in lenticle surface  17  which is curved with regard to the front surface  18 . Without such a curvature, the removal of the lenticle  15  would not change the curvature of the front surface  18  of the cornea  12 . If the lenticle  15  is removed, however, the lenticle surface  17 , curved with regard to front surface  18 , changes the curvature of the front surface  18  of the cornea  12 . This is removed by a lateral cut, not shown in  FIG. 2 , which cut for example leads at the edge of lenticle  15  from the flap surface  16  to the front surface  18  and makes it possible for the isolated lenticle  15  to be extracted, optionally after reduction of the material of the lenticle  15 . In the representation of  FIG. 2 , flap surface  16  and lenticle surface  17  are symmetrical to the optical axis OA. This is set automatically for flap surface  16  if it has a uniform distance from front surface  18 . 
         [0067]    The boundary surfaces of the lenticle  15  may naturally also comprise further surfaces apart from flap surface  16  and lenticle surface  17 . For example, with a lenticle  15  which is thinner on the optical axis OA than in regions far away from the axis, an additional edge surface can be provided which connects flap surface  16  to lenticle surface  17 , which then has a more curved path than flap surface  16  and front surface  18 . 
         [0068]    The cut surfaces for isolating the lenticle  15  are thereby created by the focus  13  being shifted along a path which lies in the corresponding surface. This is shown by way of example in  FIG. 3  with the help of lenticle surface  17  which, for reasons of clarity, is elliptical here. This is intended to show that device  1  can be used to correct not only a spherical visual defect but also an astigmatism. Basically, during higher-aberration corrections, lenticle  15  is no longer rotation-symmetrical to the optical axis OA.  FIG. 3  shows a folding-up of the lenticle surface  17  in the drawing plane. In  FIG. 3 , a path  19  is drawn in using a dotted line. The position of the focus  13  is adjusted along this path. Generally, naturally not just one adjustment transverse to the optical axis OA is necessary, but also an adjustment of the focus position along the optical axis OA. This therefore cannot be seen in  FIG. 3 , as this figure shows a folding-up of the lenticle surface  17  in the drawing plane, which is why, in the representation of  FIG. 3 , path  19  lies in a plane. By looking at the cut through lenticle  15  in  FIG. 2  it becomes clear that, the greater the distance from the optical axis OA, the further away from the front surface  18  the z-position of the focus is shifted. 
         [0069]    Targets  20  are drawn in along the path  19  of  FIG. 3 . In each case they indicate a point at which a laser pulse of the pulsed laser radiation  6  is emitted. The lenticle surface  17  is designed as a cut surface overall by arranging the targets  20  in series along the path  19  and by a suitable choice of the path  19 . The distances between targets  20  are chosen such that the fewest possible material bridges remain, meaning that the lenticle surface  17  is thus generated entirely as a cut surface. 
         [0070]    With the help of  FIG. 3  it can easily be understood why it is advantageous to make it possible to change the pulse frequency of the pulsed laser beam  6 . If it were desired to arrange the targets  20  to be as equidistant as possible, the pulse frequency and the shift speed of the beam forming unit  9  can be adapted to one another. As a high pulse frequency laser  2  can generally be adjusted only at great expense, it is advantageous to provide firstly a pure beam  3  with the laser  2 , which pure beam has a pulse frequency which is greater than or equal to the maximum pulse frequency desired for the laser beam  6 . It may be easier to produce such a laser  2  and to combine same with a pulse picker than to design a laser with a pulse frequency which can be directly adjusted. Thus the pulse frequency can be adapted to the shift speed, and the time taken to create cut surfaces is reduced. 
         [0071]    The parameters for pulse energy, pulse frequency, focus diameter and optionally pulse length, named in the description of the figures as well as in the general part of the description, lead to the cut surfaces being created with a mechanism for separating tissue which uses tissue cutting and essentially tissue splitting to their full capacity. Thus a desired position for the delimiting cut surfaces of the lenticle  15  can be produced with great precision.