Abstract:
An optical contact element for coupling a laser processing device to an object to be processed is described, wherein the laser processing device focuses a scanned laser beam through a surface of the object into a certain region of the object and the contact element comprises an entrance side for receiving the scanned laser radiation and an exit side imparting a defined surface curvature to the surface of the object upon contact therewith, wherein a diffractive optical element is provided on the entrance side, which element reduces the angle of incidence of the laser radiation on the surface of the object.

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical contact element for coupling a laser processing device to an object to be machined, wherein the laser processing device focuses a scanned laser beam through a surface of the object into a certain region of the object and the contact element comprises an entrance side for receiving the scanned laser radiation and an exit side imparting a defined surface curvature to the object upon contact therewith. The invention further relates to the advantageous use of such contact element in a laser processing device. 
     BACKGROUND OF THE INVENTION 
     In material processing by means of laser radiation, the laser beam&#39;s exactness of positioning usually determines the precision achieved in processing. If the laser beam is focused into a processing volume, exact three-dimensional positioning is required. If the object to be machined has a deformable surface, it is usually indispensable in high-precision processing to know the surface shape or to keep any deviation of the surface shape from a predefined shape as small as possible. The above-mentioned contact element serves such purposes, because it imparts a desired surface curvature to the surface of the object to be processed. 
     In materials having only minor linear optical absorption within the spectral range of the processing laser radiation advantage is usually taken of non-linear interactions between the laser radiation and the material, most often in the form of an optical breakthrough being generated in the focus of the laser beam. Since the processing effect thus only takes place in the laser beam focus, exact three-dimensional orientation of the focal point is indispensable. Thus, the machining of larger areas also requires an exact depth position of the focal location in addition to two-dimensional deflection of the laser beam. Due to the contact element, known optical relationships, in particular relationships of diffraction, with the object are present. In addition, the contact element also fixes the object in a defined position relative to the processing device. 
     A typical application of such a contact element is the ophthalmic surgery method known as fs-LASIK, wherein a laser beam is focused in the cornea to a focal point with an order of magnitude of a few micrometers. In the focus, a plasma then forms which suddenly evaporates and disrupts the surrounding tissue. This type of interaction between laser light and tissue is referred to as photodisruption. Since photodisruption ideally remains limited to a microscopically small zone of interaction, precise surgical cuts can be performed within the eye. Local separation of corneal tissue is effected. A suitable sequential arrangement of the local separation zones thus generated realizes macroscopic cuts and isolates a defined partial volume of the cornea. Removal of said partial volume then achieves a desired change in refraction of the cornea, thus enabling correction of an eyesight defect. 
     Exact positioning of the laser beam is indispensable to carry out the method. A randomly involuntary movement of the human eye during treatment is problematic. Mechanical fixation of the eye or optical feedback with respect to the eye movement is required in order to minimize this factor of influence. This is why the above-mentioned contact element is used having a double function: Not only does it ensure the required optical properties when passing the laser beam into the cornea, but it also fixes the eye, preferably with regard to several degrees of freedom, particularly preferably with regard to all possible degrees of freedom. Movements of the eye relative to the laser processing device are thus prevented. 
     U.S. Pat. No. 6,342,053 proposes to fix the eye by means of a vacuum ring. A coupling medium in front of the eye significantly reduces the difference in refractive index with respect to the cornea. The use of this coupling medium facilitates optical correction of the system. Since said medium has a refractive index of &gt;1, the beam deflection at the boundary surface is further reduced and any aberrations generated at this surface are reduced. In case the refractive indices of the contact glass and of the cornea are identical, no boundary surface exists from a geometrical/optical point of view. 
     A different concept is described in U.S. Pat. No. 5,549,632. The corneal curvature is nullified by means of a plane-parallel plate or is deformed by a concave or convex surface. This is effected by pressure on the eye. The eye is fixed, and the focused laser bundle is not affected negatively by excessively oblique incidence on a boundary surface. The pressure on the cornea inevitably leads to an increase in the internal pressure of the eye. From a medical point of view, this increase bears risks. Further, “flattening” the cornea in order to achieve a planar geometry is inconvenient for the patient. 
     High field strengths are a prerequisite for the process of photodisruption; these are realized by small focus diameters and short laser pulses. Small focus diameters can be achieved only with great apertures. Moreover, fields of treatment having a diameter of more than 8 mm are of interest. The geometry of the cornea results in a curved image field. No systems are known to reach an aperture of more than 0.3 with such fields. Therefore, the prior art either is either limited to smaller processing fields or works with planar geometry. 
     It is an object of the invention to improve a coupling element or a laser processing device of the above-mentioned type such that larger processing fields are also possible without planar geometries. 
     SUMMARY OF THE INVENTION 
     This object is achieved by an optical contact element for coupling a laser processing device to an object to be processed, wherein the laser processing device focuses a scanned laser beam through a surface of the object into a certain region of the object and the contact element comprises an entrance side for receiving the scanned laser radiation and an exit side imparting a defined surface curvature to the object upon contact therewith, wherein a diffractive optical element is arranged on the entrance side so as to reduce the angle of incidence of the laser radiation on the surface of the object. 
     Thus, according to the invention the optical contact element comprises a diffractive optical element, the use of which makes it possible to combine a large field of application, i.e. image field in the object, onto which the scanned laser beam can be focused, with a surface curvature that does not force a considerable increase in the internal pressure of the eye in ophthalmic surgery applications. The use of an optical element having a diffractive effect is essential to the invention. The element deflects light through diffraction at grating structures. Due to the diffractive optical element, for example in the form of a grating, the desired large numerical aperture on the image side can be generated with considerably smaller angles of incidence on the contact glass. Thus, the numerical aperture of the focusing optics may be designed to be considerably smaller than without the diffractive optical element. 
     In addition to the more beneficial conditions for correction, the contact glass according to the invention also allows a greater focal length of the optics focusing the laser beam, because the required inclination of the beam in front of the contact glass has become smaller for the same diameter of the incident ray bundle due to the effect of the diffractive optical element. The increased focal length facilitates the accommodation of additional components, e.g. of a beam splitter, and the realization of application-related design specifications for constructional space. Nevertheless, an “abnormal” image field curvature with respect to ophthalmic surgery, i.e. an image field having a convex curvature, can be achieved in a simple manner by the diffractive optical element. If use were made only of optically refractive elements, a much greater limitation would be given with respect to the curvature of the image field if a large field diameter is required. 
     It goes without saying that the contact element is to be selected to suit the objective. For convex objects such as those present in ophthalmic surgery in the form of the human eye, it is advantageous to provide the contact element with a plano-concave base body whose concave surface provides the exit side and whose planar surface provides the entrance side. The diffractive optical element can be conveniently attached to the planar entrance side. However, it is possible to provide directly preceding independent components as well as forming the diffractive optical element directly on the planar entrance side. 
     For ophthalmic surgery, it is further convenient to provide the surface curvature such that it is substantially rotation-symmetrical to an optical axis of the laser processing device. This is also convenient for other applications, because the diffractive optical element then has rotation symmetry or point symmetry to the point of intersection of the optical axis. It is convenient for rotation-symmetrical geometries if the diffractive optical element diffracts such laser radiation towards the optical axis as is incident at a distance from the optical axis, with the angle of diffraction increasing as the distance from the optical axis increases. This design achieves the “abnormal” curvature of the image field desired for ophthalmic applications. 
     One possibility of providing the diffractive optical element is to provide it as a grating structure with a line number depending on the distance from the optical axis. The line number is typically at least 220 lines/mm and usually not more than 500 lines/mm. 
     The diffractive optical element can then be described by a phase polynomial as well as a frequency equation. For a spherical surface curvature the phase polynomial is, for example: 
     
       
         
           
             
               Ph 
               ⁡ 
               
                 ( 
                 r 
                 ) 
               
             
             := 
             
               
                 ∑ 
                 
                   i 
                   = 
                   1 
                 
                 N 
               
               ⁢ 
               
                 
                   c 
                   i 
                 
                 · 
                 
                   
                     r 
                     
                       2 
                       · 
                       i 
                     
                   
                   . 
                 
               
             
           
         
       
     
     The parameters c i  are to be adapted to the actual geometry; r designates the radial parameter, i.e. the radial distance from the center. The frequency equation for the number of lines per millimeter is given by: 
                 Frequency   ⁢           ⁢     (   r   )       :=         ⅆ     ⅆ   r       ⁢     Ph   ⁡     (   r   )         l       ,         
wherein l is the wavelength of synthesis.
 
     The diffractive optical element allows dispensing with an exact adaptation of the refractive index between the material of the contact element and the object to be processed. Thus, the diffractive optical element allows more freedom in selecting the material for the contact element because effects of refraction appearing at the boundary surface between contact element and object surface can be corrected with the help of the diffractive optical element. It is now possible to use inexpensive materials, such as plastics, or simple glasses. Therefore, it is preferred, not least for economic reasons, that the contact element comprise a material having a refractive index which is greater than a refractive index of the object. 
     As already explained, the contact element according to the invention is particularly advantageous for ophthalmic applications. Therefore, the contact element is preferably provided as a contact glass for ophthalmic surgery with a spherical surface curvature having a diameter of between 7 and 25 mm. The radius of 7.86 mm corresponds to the typical average radius of corneal curvature. If the contact element&#39;s exit surface serving as the contact surface has this radius of curvature, the cornea substantially does not deform. Although deformation of the cornea does take place at a value of 25 mm, the reduced curvature of the cornea does have a reducing effect on aberrations to be corrected. 
     A particularly convenient application of the contact element is in a laser processing device comprising focusing optics which are arranged preceding the contact element and which focus bundles of the laser beam in focal points located in the object, said focusing optics being provided with a dispersive lens which increases the angle of deflection and with a second diffractive optical element which gathers the bundles coming from the dispersive lens. This second diffractive optical element has a strong bundling effect and enables shifting of the principal planes by means of the precedingly arranged dispersive lens. Thus, the entrance pupil is located relative to the first lens of the focusing optics at a distance sufficient to preferably enable insertion of a splitting element: Moreover, the second element thus allows the realization of comparatively great beam deflections during scanning of the laser beam. In a particularly convenient construction, the second element is provided as a plane-parallel plate. 
     In combination with the second diffractive optical element, the contact element realizes a processing device having a radius of surface curvature of 10 mm, an image field with a diameter of 11 mm and a numerical aperture of the focusing optics of 0.37. 
     The contact element preferably serves to establish secure coupling to the laser processing device on the element&#39;s entrance side. Therefore, the contact element&#39;s entrance side oriented towards the laser processing device is conveniently formed with suitable means for secure connection to the output side of the laser processing device or of its optical system, which output (e.g. distal end) is oriented towards the object, so that a fixation which is secure with respect to the laser processing device is possible by means of a locking mechanism. For the locking mechanism, it is suitable to provide a flange surface on the contact element, for example. 
     On the element&#39;s output side, the contact element ensures that the surface of the object has a desired specified shape. Suitable means are provided for rigidly connecting the contact element with the object; in an ophthalmic application, a means for fixing by suction, e.g. a suction ring as known from WO 03/002008 A1 or from EP 1159986 A2, can be used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below by way of example and with reference to the drawings, wherein: 
         FIG. 1  shows a schematic view of a laser processing device for an ophthalmic method; 
         FIG. 2  shows a schematic view of a patient&#39;s cornea; 
         FIG. 3  shows a sectional view of focusing optics of the laser processing device of  FIG. 1  including a contact glass and a beam path illustrated as an example, and 
         FIG. 4  shows a sectional view of the contact glass of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a treatment device for an ophthalmic method similar to those described in EP 1159986 A1 and U.S. Pat. No. 5,549,632. The treatment device  1  of  FIG. 1  serves to perform correction of an eyesight defect on a patient&#39;s eye  2  according to the known fs-LASIK method. For this purpose, the treatment device  1  has a laser  3  which emits pulsed laser radiation. The pulse duration is within the femtosecond range, and the laser radiation acts by means of non-linear optical effects in the cornea, as described above. The treatment beam  4  emitted by the laser  3  along an optical axis A 1  is incident on a beam splitter  5  which transmits the treatment beam  4  to a scanning unit  6 . The scanning unit  6  comprises two scanning mirrors  7  and  8  which are rotatable about mutually orthogonal axes such that the scanning unit  6  deflects the treatment beam  4  two-dimensionally. Adjustable projection optics  9  focus the scanned treatment beam  4  on the eye  2 . The projection optics  9  comprise a lens  10  and focusing optics  11 . 
     Arranged following the focusing optics  11  is a contact glass  12  which is connected to the focusing optics, and thus to the beam path of the treatment device  1 , via a holder H. The contact glass  12 , which will be explained in more detail below, contacts the cornea of the eye  2 . The optical combination of the treatment device  1  with the contact glass  2  fixed thereto causes the scanned treatment beam  4  to be focused in a focus  13  located within the cornea of the eye  2 . Due to the application in the cornea  17 , the angles of incidence of the laser bundle increase as the field increases. While a beam incident on the optical axis A 1  impinges vertically on the cornea  17 , the angle of incidence is in excess of 20 degrees already at a distance of 4 mm from the optical axis A 1 . This results in considerable aberrations, e.g. astigmatism. The aberrations are compensated within the optical system. 
     Like the laser  3 , the scanning unit  6  is controlled by a control device  14  via control lines (not specifically designated). The control device  14  determines the position of the focus  13  both transverse to the optical axis A 1  (by the scanning mirrors  7  and  8 ) and in the direction of the optical axis A 1  (by shifting the projection optics  9 ). The control device  14  further reads out a detector  15  which senses, for the purpose of measurement, radiation scattered back from the cornea and passing through the beam splitter  5  as return radiation  16 . 
     The contact glass  12  ensures that the cornea of the eye  2  obtains a desired specified shape. This is schematically illustrated in  FIG. 2  which shows a sectional view of the eye&#39;s cornea  17 . For exact positioning of the focus  13  in the eye&#39;s cornea  17 , the curvature of the eye&#39;s cornea  17  has to be considered. The cornea  17  has an actual shape  18  which differs from patient to patient. The contact glass  12 , thus, contacts the cornea  17  and deforms it to a desired specified shape  19 . 
     The exact profile of the desired shape  19  depends on the curvature of that surface of the contact glass which faces towards the eye  2 . This is evident also from  FIG. 3 . What is essential is that known geometrical and optical conditions for introducing and focusing the treatment beam  4  into the cornea  17  are obtained by means of the contact glass  12 . Since the cornea  17  contacts the contact glass  12 , which is in turn stationary with respect to the beam path of the treatment device  1  due to the holder H, an exact three-dimensional positioning of the focus  13  is achieved by controlling the scanning unit  6  as well as the adjustable projection optics  9 . 
       FIG. 3  shows a sectional view of the focusing optics  11  comprising the contact glass  12 . The contact glass  12  has a body which is transparent for the treatment beam  4 . A contact surface  20  generates the desired shape  19 , and the scanned treatment beam  4  is coupled in at a front surface  21  oriented to the focusing optics  11 . In the described construction, the contact surface  20  is spherical and has an inner radius of 10 mm which is slightly greater than the corneal radius of the patient (e.g. 8 mm), in the exemplary embodiment. The corneal radius of the eye  2  is suitably adapted to the radius of the contact glass  12 , e. g. due to the contact glass being drawn onto the eye  2  by negative pressure. As a consequence, the eye  2  is fixed by the connection thus established. Since the difference in radius between the cornea  17  in its natural state and the contact glass  12  is small, the internal pressure in the eye is not noticeably increased when adaptating the radiuses. 
     Further, a flange surface (not specifically shown) is formed on the contact glass  12 , at which surface the contact glass  12  is fixed in the holder H (also not shown in  FIG. 3  for simplification) by clamping. The flange surface constitutes a fixing means adapted to the holder H which realizes a locking mechanism. 
     As  FIG. 3  shows, parallel bundles E 1 , E 2  and E 3  from the projection optics  9  or from the lens  10  enter the focusing optics  11 . The diagram also shows several bundles for various deflections occurring during scanning. The entrance bundles E 1  to E 3  indicated by way of example represent a selection of field points. The bundles first pass through a beam splitter  22  which is of no further relevance to the function of the focusing optics  11 . The entrance pupil of the optical construction, which pupil is arranged comparatively far at the front, enables accommodation, for example, of the beam splitter  22  as an additional coupling site. After the beam splitter  22  the ray bundles are incident on a dispersive lens  23  which, together with a subsequently arranged diffractive optical element provided as a plane-parallel plate  24 , causes unusually great beam deflections which are magnified in relation to the beam deflection caused by the scanning mirrors  7 ,  8 . A subsequent front lens group  25  directs the ray bundles onto the front surface  21  of the contact glass  12  which causes deflection onto the contact surface  20  such that rays which are incident at a distance from the optical axis A 1  are diffracted towards the optical axis A 1 . 
     This effect of the contact glass  12  is shown in magnified form in  FIG. 4  which shows a detail of the beam path of  FIG. 3  in the vicinity of the contact glass  12 . Beam diffraction at the front surface  21  of the contact glass  12  is caused by a diffractive optical element  25  which, in the exemplary embodiment, is mounted to the front surface  21  and is provided as a grating structure according to the above-mentioned equations. 
     A focused ray bundle μl, illustrated in  FIG. 4  by way of example and comprising peripheral rays F 1   a  and F 1   b  as well as a central ray F 1   m , is diffracted towards the spherical contact surface  20  by the diffractive optical element  25  such that the central ray F 1   m  impinges substantially vertically on the sphere of the contact surface  20 . 
     On the whole, a high aperture of, for example, 0.37 is achieved on the image side in spite of relatively small angles of incidence on the diffractive optical element  25 . At the same time, easier optical correction is achieved with respect to aberrations appearing in the system. 
     An exemplary embodiment of the diffractive optical element  25  uses the following parameters in the above-mentioned equations for determining the radial phase dependence or frequency dependence, respectively:
         c1: −1.3587E-02   c2: 8.2357E-05   c3: −7.5017E-07   c4: 2.8305E-08   c5: −4.6727E-10   c6: −2.0104E-12   c7: 1.7144E-13   c8: −1.6035E-15       

     The image field diameter of the image field (cornea  17 ) curved with a radius of 10 mm is 11 mm. This is achieved by height-dependent adaptation of the number of lines (frequency) in the grating of the diffractive optical element  25 . At the same time, the number of surfaces having a refractive effect is minimized in the system. Arranging the diffractive optical element  25  at the front surface  21  of the contact glass  12  enables a particularly compact structure.