referring initially to fig1 an apparatus for use in performing the methods of the present invention is shown schematically in a control loop format and is generally designated 10 . as shown , the apparatus 10 includes a laser source 12 which , preferably , is capable of generating a continuous train of ultra - short pulses , with each pulse having a pulse duration of approximately one pico - second . specifically , it is necessary that each pulse have an energy level that is above the threshold necessary for the photoablation of stromal tissue ( i . e . above approximately one and one half microjoules per ten micron diameter spot size ). the apparatus 10 also includes an ellipsometer 14 that is capable of determining the birefringent properties within stromal tissue . for the purposes of the present invention , an ellipsometer of the type disclosed and claimed in u . s . pat . no . 5 , 822 , 035 , which issued to bille for an invention entitled “ ellipsometer .” further , fig1 shows that the apparatus 10 includes a wavefront sensor 16 , such as a hartmann - shack sensor , which is capable of modeling a wavefront . additionally , the apparatus 10 includes guidance optics 18 that are capable of steering and focusing a laser beam onto predetermined focal points . a power unit 20 is also provided . in combination , these components cooperate with each other to generate a laser beam 22 that is directed to a focal point in the cornea 24 of an eye 26 with a predetermined energy level . control over this operation , to include the location of the focal point and its energy level , is made possible by using the ellipsometer 14 and the wavefront sensor 16 to monitor reflected light 28 as it is reflected from the cornea 24 . referring now to fig2 it will be seen that in the operation of apparatus 10 , the performance of the methods of the present invention begins by establishing a start point ( action block 30 ). in fig3 it will be seen that this start point 32 is established in the stroma 34 of cornea 24 . specifically , the start point 32 is established at a distance 36 that is measured from the anterior surface 38 of the cornea 24 in a direction that is substantially perpendicular to the anterior surface 38 . as intended for the apparatus 10 , the exact location of the anterior surface 38 can be determined using the wavefront sensor 16 , and the distance 36 can then be arbitrarily chosen to be around about one hundred and eighty microns from the anterior surface 38 . once a start point 32 has been established in the stroma 34 , action block 40 in fig2 indicates that the next step in the methods of the present invention is to photoablate tissue at the start point 32 to create a response ( i . e . a bubble in the stromal tissue ). as indicated by inquiry block 41 , this response is then compared with a reference ( e . g . 15 μm ). if the response is less than the reference , action block 43 indicates the focal point should be moved from the start point 32 through a distance 42 ( fig4 ). this distance 42 will preferably be taken in an anterior direction ( indicated by the arrow 44 in fig4 ) and will , most likely , be less than two microns . it will be appreciated , however , that in some cases this distance 42 may be taken in a posterior direction ( indicated by arrow 46 in fig4 ). in either case , as this movement from the start point 32 is being accomplished , the inquiry block 41 in fig2 indicates that when the response becomes greater than the reference , reflected light 28 from cornea 24 can be monitored by the ellipsometer 14 to determine a birefringent reference ( action block 48 ). it happens that this birefringent reference can be determined due to a variation in the orientation of tissue in the stroma 34 and will , perhaps , be best understood by reference to fig4 . in fig4 a portion of the stroma 34 in the cornea 24 of the eye 26 is shown to include a plurality of lamellae 50 , of which the lamellae 50 a , 50 b and 50 c are only exemplary . dimensionally , each of the lamellae 50 in the stroma 34 have a depth 52 that is approximately two microns , and a width 54 that is between approximately one tenth and one and one half millimeters . thus , the lamellae 50 each have a very thin disk shape . anatomically , the lamella 50 lie on top of each other in layers that extend across the cornea 24 through a distance 56 that is approximately nine millimeters . as shown in fig4 the individual lamella 50 overlap to some extent and are somewhat randomly arranged . nevertheless , they create many interface layers that , in general , are substantially parallel to each other and extend all the way across the cornea 24 . the interface layer 58 shown in fig4 is only exemplary of the many interface layers in the cornea 24 . for the purposes of the present invention , an interface layer 58 is important in two aspects . first , the birefringent properties of stromal tissue in the lamella 50 change at the interface layer 58 . recall , from the disclosure above , this change in birefringent properties is due to changes in the orientation of fibrils ( not shown ) in the lamella 50 . second , the stromal tissue along the interface layer 58 is weaker than stromal tissue inside the lamella 50 . accordingly , the stromal tissue along the interface layer 58 can be effectively photoablated at lower energy levels . it happens that whenever stromal tissue is photoablated , a bubble is formed in the stroma 34 . for a given type of tissue , the size of the bubble that is formed will be a function of the energy level in the laser beam 22 . in this case , the higher the energy level , the larger the bubble . further , for a given energy level , the size of the bubble that is formed will be a function of the type of tissue . in this case , with the same energy level , the stronger tissue will yield a smaller bubble and the weaker tissue will yield a larger bubble . with this in mind , consider the bubbles 60 and 62 shown ( not to scale ) in fig4 that would be formed using a same energy level in the laser beam 22 . the larger bubble 60 is shown generally in weaker tissue at the interface layer 58 between the lamella 50 a and 50 b . on the other hand , the smaller bubble 62 is shown in stronger tissue inside the lamella 50 b . fortunately , as used for the present invention , the respective sizes of the bubbles 60 and 62 will serve as photoablative responses that can be measured by the wavefront sensor 16 using relatively well known wavefront techniques . accordingly , the photoablative response of a bubble 60 or bubble 62 can be compared with a reference value , and the energy level of the laser beam 22 can be altered as desired . returning now to fig2 and in light of the above discussion with reference to fig4 it will be appreciated that the combined functions of inquiry block 41 and action block 48 is to find the interface layer 58 . this is accomplished whenever the ellipsometer 14 detects a birefringent change . it will happen that this birefringent change will be on the order of plus or minus one half degree . importantly , finding the interface layer 58 will fix a focal depth for the laser beam 22 that will be a combination of the distances 36 and 42 . the apparatus 10 can then begin to photoablate stromal tissue ( action block 64 ). action block 64 in fig2 indicates that , at least initially , the apparatus 10 will photoablate stromal tissue at a relatively low energy level , e . g . approximately five microjoules per ten micron spot size . as indicated above , if photoablation begins at this energy level in the interface layer 58 as intended , a relatively large bubble 60 will result . in any event , as indicated by the inquiry block 66 , the resultant bubble ( photoablative response ) will be compared with a reference value to determine whether photoablation at this energy level should continue ( inquiry block 66 ). for the present invention , the reference value will correspond to a hypothetical bubble in stromal tissue ( not shown ) which would have a diameter of approximately fifteen microns . if the resultant bubble in the stroma 34 has a photoablative response that is greater than the reference value , it is indicative of the fact that weaker tissue in the interface layer 58 is being photoablated . in this case , the inquiry block 67 may be selectively used to determine whether the birefringent reference has changed . such a change would be on the order of one half a degree and would indicate that another interface 58 ′ was being photoaltered . if so , action block 68 indicates the birefringent reference can be reset to reestablish on the desired interface 58 . in either case , the action block 70 in fig2 indicates that the guidance optics 18 should continue to scan the laser beam 22 through the interface layer 58 . as this is being done , the interaction of blocks 64 , 66 , 67 and 68 in fig2 indicate that a photoablative response is continuously being monitored by the wavefront sensor 16 . whenever the photoablative response falls below the reference value , such as would happen when photoablation is occurring within a lamella 50 ( e . g . bubble 62 ), action block 72 indicates that the energy level in the laser beam 22 should be increased to a higher energy level . again , the photoablative response is monitored by the wavefront sensor 16 . due to the higher energy level being used , when the laser beam 22 is next focused onto the interface layer 58 , the photoablative response will most likely be much greater than the reference value . in any event , inquiry block 74 and action block 75 indicates that the laser beam 22 will continue to move and photoablate tissue until the photoablative response is considerably greater than the reference value . when this happens , depending on the desires of the operator , the methods of the present invention indicate that the laser beam 22 can continue operation at the relatively lower energy level ( action block 64 ). in either case , blocks 66 , 67 , 68 and 70 indicate that the photoablation of stromal tissue will continue until the procedure is ended . specifically , the procedure is ended when an interface layer 58 having a predetermined dimension has been created . it is the purpose of the present invention to create a flap of corneal tissue that can be lifted easily from the eye to expose stromal tissue under the flap to further surgical photoablation . accordingly , the present invention is directed toward the photoablation of weaker tissue along an interface layer 58 between lamella 50 and to , thereby , use less laser energy . the extent of this photoablation will be best appreciated with reference to fig5 . in fig5 a substantially straight edge 76 is shown between a point 78 and a point 80 . also , a substantially curved edge 82 is shown connecting the point 78 to the point 80 . more specifically , the curved edge 82 is generally centered on the optical axis 84 of the eye 26 and has a radius of curvature 86 that defines the edge 82 . as shown , the curved edge 82 will extend through approximately two hundred and seventy degrees . effectively the desired corneal flap will be created between the straight edge 76 and the curved edge 82 . consequently , by photoablating tissue between the anterior surface 38 of the cornea 24 and the curved edge 82 , a flap of corneal tissue can be lifted from the interface layer 58 to expose stromal tissue under the flap for further photoablation . while the particular method for separating lamellae as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated , it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims .
a method for separating lamellae in the stroma of an eye includes establishing a focal depth that will be located in relatively weaker tissue at an interface layer between lamellae in the stroma . a laser beam can then be focused to photoablate stromal tissue and create a photoablative response thereto . this photoablative response is then compared to a reference value using wavefront analysis techniques to determine an effective minimum energy level for the laser beam . maintenance of a proper focal depth can be periodically verified by maintaining a birefringent reference using an ellipsometer . once the lamellae are separated , a flap of corneal tissue can be created that can be lifted to expose underlying stromal tissue for further surgical photoablation .