referring initially to fig1 , a laser system 20 is shown for conducting a intracorneal laser procedure on an eye 22 . as shown , the eye 22 is aligned to receive an ultra short pulsed laser beam 24 from the laser system 20 . as detailed further below , the pulsed laser beam 24 having selected parameters is generated by a laser source 26 . the laser system 20 includes a focusing unit 28 for focusing the beam 24 to its focal point 30 . also included in the laser system 20 is a scanning unit 32 for directing the beam 24 . in addition to the laser source 26 and focusing unit 28 , the laser system 20 includes a scanning unit 32 for directing the laser beam 24 . also provided in the system 20 is a telescope 34 for collimating the beam 24 after it is directed by the scanner 32 . downstream of the telescope 34 is a reflector 36 that redirects the laser beam 24 toward the eye 22 through the focusing unit 28 . while not directly involved with the generation and control of the laser beam 24 , several sensors are also provided in the laser system 20 . specifically , the system 20 includes a sensor 38 , preferably a confocal microscope or an optical coherence tomograph , for finding the interface 40 at which photoablation is desired . the system 20 further includes a sensor 42 for detecting whether and where photoablation has occurred . the system 20 may include an additional sensor or sensors 44 for identifying photoablation thresholds as discussed below . in accordance with the present invention , photoablation can be performed to provide intracorneal tissue modification to effect a refractive change in the cornea , to create a flap suitable for a lasik or lasek type procedure , to create a passageway or drainage channel in the eye 22 , or to effect any other type of surgical procedure , in whole or in part , known in the pertinent art that requires the removal of ocular tissue . fig2 shows the anatomical structure of the human eye 22 including the cornea 46 , the pupil 48 , the iris 50 , and the sclera 52 . in fig3 it can be seen that the cornea 46 includes five anatomically definable layers of tissue . going in a direction from anterior 54 to posterior 56 in fig3 , the tissue layers of the cornea 46 are : the epithelium 58 , bowman &# 39 ; s membrane 60 , the stroma 62 , descemet &# 39 ; s membrane 64 and the endothelium 66 . these corneal layers have distinct photoablation thresholds . for instance , depending on the laser pulse length , the stroma 62 may have a base liob threshold of 1 , bowman &# 39 ; s membrane 60 may have a relative liob threshold of 2 and the epithelium 58 may have a threshold of 0 . 5 . between the layers of corneal tissue are the interfaces 40 that are of general importance for the present invention . specifically , the removal or destruction of the portion of one tissue adjacent an interface 40 can be achieved without damage to the other layer of tissue adjacent the interface 40 . in addition , due to the natural delineation between layers of tissue at the interface 40 , very precise photoablation can be achieved . referring now to fig4 and 5 , a laser beam 24 is shown being generally directed to an interface 40 between a targeted material 68 and a non - targeted material 70 . specifically , the laser beam 24 is focused such that its focal point 30 is positioned in the non - targeted material 70 . the maximum beam energy density reached at the focal point 30 is insufficient to photoablate the non - targeted material 70 due to the proper selection of beam parameters such as intensity , geometry and pulse duration . equidistantly spaced from the focal point 30 are beam positions 72 at which a minimum beam energy density is reached . as indicated by the arrow indicating the path of the laser beam 24 in fig4 , the non - targeted material 70 is positioned upstream of the interface 40 . therefore , the laser beam 24 does not pass through the targeted material 68 before reaching its focal point 30 . conversely , in fig5 , the targeted material 68 is positioned upstream of the interface 40 such that the laser beam 24 passes therethrough before reaching its focal point 30 . both of fig4 and 5 depict a preferred starting point for the photoablation procedure in which the focal point 30 is positioned in the non - targeted material 70 such that photoablation does not occur in the targeted material 68 . referring now to fig6 , the laser beam 24 of fig5 is shown after advancement of the focal point 30 ( and the position 72 of minimum beam energy density ) toward the interface 40 . specifically , fig6 shows that the minimum beam energy density contacts the targeted material 68 and causes a portion 74 of the targeted material 68 to be photoablated . similarly , fig7 shows the laser beam 24 of fig5 after advancement of the position 72 of minimum beam energy density due to an increase in intensity of the beam 24 . again , the minimum beam energy density contacts the targeted material 68 and causes a portion 74 of the targeted material 68 to be photoablated . referring now to fig9 , it will be seen that in the operation of laser system 20 , the performance of the methods of the present invention begins by estimating the depth “ d ” at which the focal point 30 will be positioned in the eye 22 ( action block 76 ). specifically , the interface 40 between the targeted material 68 and non - targeted material 70 is found . then the depth necessary to place the focal point 30 in the non - targeted material 70 near the interface 40 is estimated . in fig6 , the estimated distance 78 is shown establishing the focal point 30 in the non - targeted material 70 . in order to provide the appropriate beam energy densities in the non - targeted material 70 and targeted material 68 , the beam parameters such as the intensity or energy level “ e ” and the treatment duration “ τ ” are set ( action blocks 80 and 82 ). it is assumed that the ablation energy thresholds of the targeted material 68 and non - targeted material 70 are known before these steps are taken . of course , the thresholds may be estimated if they are not previously identified . once the desired parameters are set , the system 20 is activated to generate the pulsed laser beam 24 ( action block 84 ). based on the answer to inquiry block 86 , i . e . whether the non - targeted material 70 ( i . e ., “ m 1 ”) is upstream from the interface 40 , specific steps are taken depending on whether the sensor 42 identifies any photoablated targeted material 68 in response to the laser beam 24 . as is known , photoablation of material such as corneal or biological tissue causes formation of a bubble or plasma that can be sensed by the sensor 42 . assuming that the non - targeted material 70 is upstream from the interface 40 , then inquiry block 88 is reached . if no targeted material 68 is photoablated , then the position 72 of the minimum beam energy density is advanced ( action block 90 ). as shown in fig6 and 7 , such advance may be performed by moving the focal point 30 toward the interface 40 or by increasing the energy level of the laser beam 24 . after the advance of the position 72 of the minimum beam energy density , the system 20 is again activated at action block 84 to generate the pulsed laser beam 24 . in this way , the position 72 of the minimum beam energy density is advanced toward the targeted material 68 until photoablation occurs . when photoablation occurs and there is a positive response to inquiry block 88 , then the laser system 20 is executed to scan the laser beam 24 to a new location in the non - targeted material 70 ( action block 92 ). here , a loop such as that discussed above is again encountered to ensure that the position 72 of the minimum beam energy density be advanced toward the targeted material 68 until photoablation occurs at the new location . specifically , inquiry block 94 requires , if photoablation does not occur , that the position 72 of the minimum beam energy density be advanced toward the targeted material 68 ( action block 96 ) before the beam 24 is again activated ( action block 84 ). if photoablation does occur , then it is determined whether the treatment duration has expired ( inquiry block 98 ). if it has not expired , the method is restarted at the scanning step of action block 92 . if it has expired , then it is determined whether the photoablation pattern of the targeted material 68 is complete ( inquiry block 100 ). if the entire pattern is completed , then the procedure is completed and the actions are stopped . if not , the procedure is begun again at action block 82 . turning back to inquiry block 86 , the situation where the non - targeted material 70 is not upstream from the interface 40 must be addressed . in this case , inquiry block 87 asks whether any targeted material 68 is photoablated in response to the activation of the laser beam 24 . if targeted material 68 is photoablated , then the position 72 of the minimum beam energy density is advanced ( action block 89 ). such an advance may be performed by moving the focal point 30 toward or into the non - targeted material 70 or by decreasing the energy level of the laser beam 24 . after advancing the position 72 of the minimum beam energy density , the laser beam 24 is again activated at action block 84 to generate the pulsed laser beam 24 . in this way , the position 72 of the minimum beam energy density is advanced toward the non - targeted material 70 until photoablation does not occur in response to the activation of the laser beam 24 . this loop ensures that photoablation will occur only in the portion 74 of the targeted material 68 that is adjacent the interface 40 . once photoablation does not occur in response to the activation of the laser beam 24 , it is known that the focal point 30 is properly positioned in the non - targeted material 70 . the beam 24 can then be scanned to a new location in the non - targeted material 70 ( action block 91 ). here , a loop such as that discussed above is again encountered to ensure that the position 72 of the minimum beam energy density be withdrawn toward the targeted material 68 until photoablation occurs at the new location . specifically , inquiry block 93 requires , if photoablation does not occur , that the position 72 of the minimum beam energy density be withdrawn toward the targeted material 68 ( action block 95 ) before the beam 24 is again activated ( action block 84 ). if photoablation does occur , then it is determined whether the treatment duration has expired ( inquiry block 97 ). if it has not expired , the method is restarted at the scanning step of action block 91 . if it has expired , then it is determined whether the photoablation pattern of the targeted material 68 is complete ( inquiry block 100 ). if the entire pattern is completed , then the procedure is completed and the actions are stopped . if not , the procedure is begun again at action block 82 . as shown in fig1 , additional steps may be included when the non - targeted material 70 is upstream of the interface 40 . specifically , the method may include a loop to ensure that the focal point 30 is not positioned too deep within the cornea 46 such that the targeted material 68 is photoablated at a location deeper than the interface 40 . for instance , if photoablation occurs as a result of the activation of the laser beam 24 , then the position 72 of the minimum beam energy density is withdrawn toward the interface 40 ( action block 102 ). if , after such withdrawal , photoablation still occurs ( as noted at inquiry block 104 ), then the position 72 is withdrawn again . once photoablation does not occur , the minimum beam energy density is known to be at the interface or slightly within the non - targeted material 70 . therefore , the position 72 of the minimum beam energy density is advanced toward the targeted material 68 ( action block 106 ). if photoablation does not occur in response to this advance ( as noted at inquiry block 108 ), the position 72 is advanced again . upon sensing that photoablation occurs , execution of the scanning process begins at action block 92 and the method returns to the process set forth in fig9 . referring now to fig8 , a corneal flap 110 prepared in accordance with the present invention is shown . the flap 110 is prepared by first photoablating a periphery 112 for the flap 110 . because the periphery 112 is formed along the interface 40 between two corneal tissues as discussed above , it is much more precise than a periphery cut through a tissue . with the periphery 112 established , an incision can be made extending from the anterior surface 114 of the cornea 46 to the periphery 112 to establish an edge 116 for the flap 110 . once the edge 116 is created , the flap 110 can be peeled from the remainder of the cornea 46 to expose the surface of the underlying tissue 118 . after exposure , the underlying tissue 118 can be photoablated using an excimer laser ( not shown ). after photoablation with the excimer laser , the flap 110 can be repositioned over the underlying tissue 118 and allowed to heal . the result is a reshaped cornea 46 . while the particular method and device for performing subsurface photoablation as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore 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 the construction or design herein shown other than as described in the appended claims .
a method and device for photoablation is disclosed wherein photoablation occurs along the interface between a material having a lower energy ablation threshold and a material having a higher energy ablation threshold . the method and device utilize a laser beam having a beam energy density which is less than the higher energy ablation threshold and greater than or equal to the lower energy ablation threshold . by directing such a laser beam to the interface , the material having the lower energy threshold is photoablated while the material having the higher energy threshold is largely unaffected .