Abstract:
An optical system for partitioning and focusing a laser beam into a plurality of focal points to simultaneously photoalter corneal tissue at a plurality of locations includes a laser source. In one embodiment, an active mirror is used to partition the master beam into diverging beams. In another embodiment, a lenslet array in combination with a field lens is used to partition the master beam into seven diverging beams. The resultant diverging beams are then collimated, magnified and focused into a plurality of focal points using a set of optical lenses. Each focal point has an average pulse energy of approximately 5 μJ rendering it suitable for subsurface photoalteration of corneal tissue. A scanning mechanism is provided to move the plurality of focal points, as a group, along a predetermined path through the cornea to quickly and safely photoalter a predetermined volume of subsurface corneal tissue.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains generally to laser systems. More particularly, the present invention pertains to laser beam delivery systems for simultaneously creating a plurality of focal points from a single laser source. The present invention is particularly, but not exclusively, useful for creating a plurality of laser focal points suitable for performing refractive surgery.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a conventional LASIK procedure, a corneal flap is first created with a microkeratome. Next, the flap is lifted to expose stromal tissue. Once exposed, the stromal tissue is photoablated using an Excimer laser. After photoablation, the flap is replaced over the cornea and the cornea is allowed to heal. The result is a reshaped cornea. By reshaping the cornea in this manner, vision deficiencies in the patient can be corrected. There are, however, several drawbacks associated with using a microkeratome to create the flap. For one, using a microkeratome is labor intensive. Additionally, the results obtained when using a microkeratome are highly dependent on the skill of the surgeon. Finally, shape of the exposed bed of stromal tissue that results from the use of a microkeratome is generally limited to flat surfaces. Because of these drawbacks, the development of new techniques for creating corneal flaps is merited.  
           [0003]    One technique for creating a corneal flap that is gaining widespread acceptance involves the use of a pulsed laser beam to photoalter stromal tissue. In this technique, a pulsed laser is focused beneath the anterior surface of the cornea to a focal point within the stroma. For example, a pulsed laser beam having a pulse frequency of approximately 4 kHz with pulse durations as long as a few nanoseconds or as short as only a few femtoseconds can be used for subsurface photoalteration of stromal tissue. For a commonly used focal point size of approximately 10 μm in diameter, a typical laser source can produce an average pulse energy of approximately 60 μJ at the focal point. This energy (60 μJ) is far in excess of the energy required to photoalter stromal tissue. Specifically, only about 2 μJ is required for photoalteration of stromal tissue, with about 5 μJ being optimal. Consequently, when only a single focal point is used, most of the energy available in a typical pulsed laser source is wasted.  
           [0004]    Consider now an exemplary flap for a LASIK procedure having a diameter of approximately 10 mm. For this flap, the photoalteration of approximately 200,000 stromal points is required. Stated another way, approximately 200,000 pulses, with each pulse having an average energy of approximately 5 μJ, are required. Continuing with this example, for a 4 kHz laser using a single focal point, about 50 seconds would be required to create a 10 mm flap. It is to be appreciated that procedures requiring this length of time (i.e. 50 seconds), pose a number of serious problems. One problem with lengthy procedures is eye movement. To overcome eye movement, eye restraint is often used. Unfortunately, restraining the eye for long periods of time can cause discomfort for the patient. Another problem associated with long procedure times involves patient blinking. Each time a patient blinks, a new tear film is deposited on the anterior surface of the cornea. Each tear film affects the optical path of the laser beam in a slightly different manner, affecting the precision of the operation. Thus, it is preferable to perform an entire procedure with a single tear film, if possible. Typically, 10 seconds is about the maximum time that a patient can restrain from blinking, thus it is preferable to complete an entire procedure in less than about 10 seconds.  
           [0005]    In all surgical procedures, damage to non-target (i.e. collateral) tissue is to be avoided. During photoalteration of target tissue, nearby (non-target) tissue is heated. Some heating of non-target tissue can be accommodated without damage to the non-target tissue. Specifically, for stromal tissue, a temperature rise of about 3° C. can be tolerated without long-term cell damage. In contrast, temperature increases of between about 8° C. and 23° C. can result in tissue shrinkage, cell denaturation, loss of cell function and coagulation. Importantly for the present invention, when multiple focal points are used to simultaneously photoalter tissue, a minimum spacing between adjacent focal points is required to prevent damage to non-target tissue from the heat generated during photoalteration.  
           [0006]    In light of the above, it is an object of the present invention to provide a laser system suitable for the purposes of expeditiously photoaltering stromal tissue without heating collateral tissue to harmful temperatures. It is another object of the present invention to provide a pulsed laser system capable of generating a plurality of spaced apart laser focal points, with each focal point having a suitable pulse energy to accomplish photoalteration of stromal tissue. It is yet another object of the present invention to provide a laser system that partitions a pulsed laser beam into a plurality of laser focal points having adequate spacing between focal points to allow the heat generated during photoalteration to dissipate, thereby preventing heat damage to non-target tissue. Still another object of the present invention is to provide a multiple focal point pulsed laser system capable of photoaltering approximately 200,000 points within the stroma in less than approximately 10 seconds. It is still another object of the present invention to provide a multiple focal point, pulsed laser system capable of expeditiously photoaltering an entire 10 mm corneal flap during the period between the blinks of the patient (i.e. in less than approximately 10 seconds). Yet another object of the present invention is to provide a laser system and a method for its use which are relatively easy to use, simple to implement, and comparatively cost effective.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0007]    The present invention is directed to an optical system for partitioning and focusing a laser beam onto a plurality of focal points to simultaneously photoalter corneal tissue at a plurality of locations. The plurality of focal points can be scanned, as a group, through the cornea to quickly and safely photoalter a predetermined volume of subsurface corneal tissue. For the present invention, the system includes a laser source capable of generating a pulsed laser beam (hereinafter referred to as a master beam) having a pulse frequency of approximately 4 kHz and an average pulse energy of approximately 60 μJ.  
           [0008]    In one embodiment for the present invention, the master beam produced by the laser source is directed into a lenslet array to partition the master beam into a plurality of beams. Preferably, the lenslet array has six lenslets arranged in a circle surrounding a center lenslet. Thus, seven spaced apart beams emerge from the lenslet array. From the lenslet array, the seven beams are directed into a series of optical lenses and a scanner. In detail, the seven beams are first directed into a field lens to diverge the seven beams. From the field lens, the diverging beams are directed to a collimating lens to place the seven beams onto parallel beam paths. Next, the collimated beams are directed to a pair of relay lenses arranged as a telescope to magnify the collimated beams.  
           [0009]    Once magnified, the beams are directed to a cutting lens to focus each of the beams to a separate focal point. Thus, a group (or cluster) of focal points is established. Like the lenslet array, the cluster of focal points is preferably arranged with six focal points distributed uniformly around a circle with the seventh focal point positioned at the center of the circle. For the present invention, a scanner is provided to move the cluster of focal points, as a group, through the cornea. Preferably, the scanner is interposed between the relay lenses that magnify the collimated beams.  
           [0010]    In another embodiment for the present invention, an active mirror having approximately 40,000 active facets can be used to partition the master beam into seven diverging beams. The diverging beams from the active mirror are then collimated, magnified and focused using the optics described above. For both embodiments, a plurality of focal points suitable for subsurface photoalteration of corneal tissue is obtained, with each focal point having an average pulse energy of approximately 5 μJ.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0012]    [0012]FIG. 1 is a perspective view of a patient being treated with a pulsed laser in accordance with the method of the present invention;  
         [0013]    [0013]FIG. 2 is a perspective view of an eye;  
         [0014]    [0014]FIG. 3 is a cross sectional view of a portion of the cornea of the eye as seen along the line  3 - 3  in FIG. 2 showing the anatomical layers of the cornea and an exemplary volume of stromal tissue that can be removed in accordance with the methods of the present invention to effect a refractive change in the cornea;  
         [0015]    [0015]FIG. 4 is a schematic diagram of a laser system in accordance with the present invention having a lenslet array and a field lens to produce a plurality of diverging beams;  
         [0016]    [0016]FIG. 5 is an elevational view of a lenslet array for use in the present invention;  
         [0017]    [0017]FIG. 6 is a diagram showing the preferred arrangement of focal points for the laser system of the present invention;  
         [0018]    [0018]FIG. 7 is an enlarged plan view of a portion of the cornea of an eye showing the movement of a cluster of focal points during a surgical procedure in accordance with the present invention;  
         [0019]    [0019]FIG. 8 is a cross sectional view as in FIG. 3, after a corneal flap has been incised using a laser system in accordance with the present invention, and lifted; and  
         [0020]    [0020]FIG. 9 is a schematic diagram of another embodiment of a laser system in accordance with the present invention having an active mirror for producing a plurality of diverging beams.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring initially to FIG. 1, a laser system  10  is shown for conducting a laser procedure on the eye  12  of a patient  14 . As shown, the eye  12  of the patient  14  is aligned to receive a plurality of pulsed laser beams from the laser system  10 . As detailed further below, a plurality of pulsed laser beams that are preferably centered substantially along axis  16  are simultaneously focused by the laser system  10  to focal points within the eye  12  of the patient  14  to photoalter stromal tissue. In accordance with the present invention, the photoalteration can be performed to create a flap suitable for a LASIK type procedure, to effect a refractive change in the cornea, to create a passageway or drainage channel in the eye  12 , or to effect any other type of surgical procedure, in whole or in part, known in the pertinent art that requires either the incision or removal of ocular tissue.  
         [0022]    [0022]FIG. 2 shows the anatomical structure of the human eye  12  including the cornea  18 , the pupil  20 , the iris  22 , and the sclera  24 . In FIG. 3 it can be seen that the cornea  18  includes five anatomically definable layers of tissue. Going in a direction from anterior to posterior in FIG. 3, the tissue layers of the cornea  18  are: the epithelium  26 , Bowman&#39;s membrane  28 , the stroma  30 , Decemet&#39;s membrane  32  and the endothelium  34 . Of these, the stroma  30  is the thickest layer and contains the stromal tissue that is of general importance for the present invention. Specifically, the removal or destruction of stromal tissue is recognized as an effective way to reshape the cornea  18  and thereby effect a refractive change to the cornea  18 . Additionally, creation of a flap suitable for use in a typical LASIK procedure requires the incision of stromal tissue.  
         [0023]    Referring now to FIG. 4, an embodiment of a laser system  10  is shown. As shown, the laser system  10  includes a laser source  36  for producing a pulsed laser beam  38 . For purposes of the present invention, a pulsed laser beam  38  preferably has physical characteristics similar to those of the pulsed laser beams generated by a pulsed laser source as generally disclosed and claimed in U.S. Pat. No. 4,764,930, which issued to Josef F. Bille et al. for an invention entitled “Multiwavelength Laser Source.” Furthermore, the present invention contemplates the use of a pulsed laser beam  38  having a pulse frequency of approximately 4 kHz with pulse durations as long as a few nanoseconds or as short as only a few femtoseconds. Preferably, the pulsed laser beam  38  has pulses with durations between approximately ten femtoseconds and five picoseconds (10 fsec—5 psec), and a wavelength longer than approximately nine hundred nanometers (900 nm). Also, the pulsed laser beam  38  preferably has a fluence of less than one hundred joules per square centimeter (&lt;100 J/cm 2 ). With these characteristics, the pulsed laser beam can be focused to a focal point having a diameter of approximately ten micrometers (10 μm) that has an average pulse energy of approximately sixty microjoules (60 μJ).  
         [0024]    Referring now with cross reference to FIGS. 4 and 5, it can be seen that the laser beam  38  produced by the laser source  36  is directed into a lenslet array  40  having seven lenslets  42   a - g.  It is to be appreciated that the seven lenslets  42   a - g  partition the laser beam  38  into seven spaced apart beams, three of which (i.e. beams  44   a - c ) are shown in FIG. 4. As shown in FIG. 5, the lenslet array  40  is preferably constructed having six lenslets  42   a - f  arranged about a circle surrounding a center lenslet  42   g.  Preferably, the lenslets  42   a - f  have a diameter  41  of approximately two millimeters (2 mm), and are close packed, as shown, within a circle having a diameter  43  of approximately six millimeters (6 mm). From the lenslet array  40 , the beams  44   a - c  are first directed into a negative field lens  46  to diverge the beams  44   a - c,  thereby creating diverging beams  48   a - c . From the field lens  46 , the diverging beams  48   a - c  are directed to a collimating lens  50  to place the seven beams onto parallel beam paths. Next, the collimated beams  52   a - c  are directed through relay lens  54  and relay lens  56  for magnification. As shown, the relay lenses  54 ,  56  are arranged as a telescope to magnify the collimated beams  52   a - c . Preferably, the relay lenses  54 ,  56  are configured to magnify the collimated beams  52   a - c  at a magnification of approximately 8:1.  
         [0025]    Referring still to FIG. 4, it can be seen that after passing through the relay lenses  54 ,  56 , the magnified beams  58   a - c  are directed to a focusing lens  59  (i.e. a cutting lens) to focus each of the magnified beams  58   a - c  to a separate focal point  60   a - c . With cross-reference now to FIGS. 4 and 6, it can be seen that a cluster  61  of seven focal points  60   a - g  is established. Like the lenslet array  40  (shown in FIG. 5), the cluster  61  of focal points  60   a - g  is preferably arranged with six focal points ( 60   a  and  60   c - g ) distributed uniformly around a circle with the seventh focal point  60   b  positioned at the center of the circle. Preferably, focal points  60   a - g  having a diameter  62  of approximately ten micrometers (10 μm) are formed by the laser system  10  and have an average energy of approximately five microjoules (5 μJ), rendering each focal point suitable for photoalteration of stromal tissue. Further, in the preferred embodiment of the present invention, the six focal points ( 60   a  and  60   c - g ) are arranged in a circle having a diameter  64  of approximately one hundred micrometers (100 μm), ensuring that each focal point  60   a - g  is spaced from the remaining focal points  60   a - g  by a distance  66  of at least approximately twenty micrometers (20 μm). This spacing assures adequate heat dissipation during photoalteration, thereby preventing heat damage to non-target tissue.  
         [0026]    Referring now to FIG. 4, it can be seen that a scanner  68  is preferably interposed between relay lens  54  and relay lens  56 . For the present invention, any scanner known in the pertinent art for controlling the movement of a plurality of laser beams can be used. Cross referencing now to FIGS. 4 and 7, it can be seen that the scanner  68  is provided to move the cluster  61  of focal points  60  through the cornea  18  (such as in the direction of arrow  69  in FIG. 7). Preferably, the cluster  61  is scanned at a rate such that the cluster  61  is positioned at a location requiring photoalteration for approximately {fraction (1/4000)} th  of a second (for a 4 kHz laser source). As such, approximately one pulse of energy will be focused at each focal point  60 , for each location that is photoaltered. For example, for the creation of a ten millimeter (10 mm) flap  67  as shown in FIG. 8, approximately 200,000 points in the stroma  30  require photoalteration. For a cluster  61  having seven focal points  60 , approximately 30,000 locations require photoalteration. Thus, for this example, the scanner  68  is configured to scan the ten millimeter (10 mm) area in approximately seven to eight seconds. Alternatively, the cluster  61  of focal points  60  can be scanned within the stroma  30  to photoalter a volume of stromal tissue (such as volume  72  shown in FIG. 3) to effect a refractive change in the cornea  18 .  
         [0027]    [0027]FIG. 9 shows another embodiment of a laser system  10 ′ in accordance with the present invention. For clarity, the prime (′) has been used to denote elements in FIG. 9 that are similar or identical in nature to like-numbered elements in the embodiment shown in FIG. 4. As shown in FIG. 9, a laser source  36 ′, as described above is used to direct a pulsed laser beam  38 ′ to an active mirror  70 . Preferably, an active mirror  70  having approximately 40,000 active facets is used. As shown, the active mirror  70  is configured to reflect the pulsed laser beam  38 ′ into seven, spaced apart diverging beams, of which three ( 48   a ′- 48   c ′) are shown in FIG. 9. It is to be appreciated by those skilled in the art that one or more optical prisms (not shown) can be substituted in place of the active mirror  70  to partition the pulsed laser beam  38 ′ into diverging beams.  
         [0028]    Referring still to FIG. 9, from the active mirror  70 , the diverging beams  48   a ′- 48   c ′ are directed to a collimating lens  50 ′ to place the seven beams onto parallel beam paths. Next, the collimated beams  52   a ′- 52   c ′ are directed through relay lens  54 ′ and relay lens  56 ′ for magnification. From the relay lenses  54 ′,  56 ′, the magnified beams  58   a ′- 58   c ′ are directed to a focusing lens  59 ′ to focus each of the magnified beams  58   a ′- 58   c ′ to a separate focal point  60   a ′- 60   c ′. Scanner  68 ′ is provided to simultaneously move all the focal points  60   a ′- 60   c ′ during the procedure.  
         [0029]    While the particular laser beam delivery system with multiple focal points 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.