Abstract:
A device and method for steering a laser beam to a focal point in target tissue requires generating a laser beam. Diversions of the laser beam from a central beam path are minimized by a sequential arrangement of optical steering components. In order, the beam is first directed to the center of a z-scanning apparatus which will move the focal point in the medium in a z-direction. The beam is then passed to the center of a first galvanometric mirror which introduces focal point movements in the x-direction. A second galvanometric mirror then compensates for the x-direction movement by redirecting the beam to the center of a third galvanometric mirror where focal point movements in the y-direction are introduced.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to systems and methods for performing corneal laser surgery. More particularly, the present invention pertains to systems and methods for optically steering a laser beam to perform corneal laser surgery. The present invention is particularly, but not exclusively useful as a system for optically steering a laser beam to a focal point in a medium, while maintaining the beam path substantially centered on the optical components of the system, for corneal laser surgery. 
     BACKGROUND OF THE INVENTION 
     Corneal laser surgery requires moving and focusing (i.e. steering) a laser beam to a succession of many predetermined focal points. Depending on the particular surgical procedure that is to be performed, these predetermined focal points may be either on or within a medium (target tissue). In either case, the intended purpose is to photoalter target tissue in accordance with a predetermined pattern. In refractive surgery, for example, the target tissue is normally stromal tissue in the cornea of a human eye, and the steering of the laser beam is accomplished by the moving, tilting or realigning of optical components (i.e. lenses and mirrors) of the laser system. 
     Laser surgery systems that are currently being used typically include a dual-mirror combination that is manipulated to move and direct the laser beam as the beam transits the system. Within this combination, one mirror is moved to effect movements of the laser beam&#39;s focal point in an x-direction on an x-y plane in the target tissue. The other mirror is then moved to effect movements of the focal point in a y-direction on the x-y plane in the target tissue. The result here is that for each of these movements, the laser beam will necessarily be directed away from a central path through the system. Moreover, these effects are cumulative. Thus, it will happen that as the laser beam is moved to effectuate “x” and “y” movements for a particular laser surgical pattern, the center of the beam path will be moved away from the center of downstream optical components in the system. At some point, the combined effects of these movements can significantly reduce the optical efficiency and the surgical precision of the laser system. 
     In light of the above, it is an object of the present invention to provide a device for steering a laser beam to a focal point in a medium during laser surgery wherein movements of the laser beam are compensated to maintain the beam substantially centered on the optical components of the system as the beam transits the system. Yet another object of the present invention is to provide a device for steering a laser beam to a focal point in a medium during corneal laser surgery wherein displacements of the laser beam, from the center of optical elements, are minimized during “x”, “y” and “z” movements of the laser beam&#39;s focal point in a target tissue. Still another object of the present invention is to provide a device for steering a laser beam to a focal point in a medium during corneal laser surgery that is easy to use, relatively simple to manufacture, and comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a device for steering a laser beam along a beam path to a focal point in a medium includes a laser source for generating the laser beam along a beam path. Additionally, first, second and third scanning mechanisms are positioned sequentially along the beam path for steering the laser beam. The combined effect of these three scanning mechanisms is to produce movements of the focal point on an x-y plane, in the medium (target tissue). In the preferred embodiment of the present invention, the first, second and third scanning mechanisms are galvanometric mirrors. In addition to the three scanning mechanisms, the device of the present invention also includes a z-scanning apparatus for moving the focal point of the laser beam in a z-direction that is perpendicular to the x-y plane. In one embodiment of the present invention, the apparatus is a voice coil subassembly. In an alternate embodiment, the apparatus is an active mirror. 
     As intended for the present invention, diversions or displacements of the laser beam from the centers of optical components in the system are minimized by the proper placement of optical components along the beam path, and by the incorporation of a third scanning mechanism. In particular, the z-scanning apparatus is placed first in line, after the laser source, and is positioned to receive the laser beam at the center of the apparatus. The z-scanning apparatus then causes the laser beam to converge or diverge to effect a z-movement of the beam&#39;s focal plane. The z-scanning apparatus then passes the beam directly toward the center of the first galvanometric mirror. Optically, the first galvanometric mirror is positioned on the beam path to effect a change in an x-direction on the x-y plane whenever the first galvanometric mirror is rotated through an angle of “θ”. Next, the second galvanometric mirror is positioned on the beam path to compensate for any beam path diversion that is introduced by the first galvanometric mirror. Specifically, this is done by rotating the second galvanometric mirror through an angle of “2θ”, to redirect the beam path onto the center of the third galvanometric mirror. The third mirror can then be rotated through an angle “φ” to effect a change in the y-direction on the x-y plane. 
     Structurally, the respective axes of rotation for the first, second and third galvanometric mirrors are all perpendicular to the beam path. Further, the axes of rotation of the first and second galvanometric mirrors are parallel to each other. The axis of rotation for the third galvanometric mirror, however, is perpendicular to the axes of rotation of both the first and the second galvanometric mirrors. It is also important within the combination of galvanometric mirrors that the center-to-center distance between the first and second mirrors be equal to the center-to-center distance between the second and third mirrors. As disclosed by the present invention, the “center-to-center” distance is defined as the distance between the geometric centers of the reflective surfaces of any two mirrors optically aligned in the beam path. 
     Preferably, the device also includes a computer controller that is connected in electronic communication with the z-scanning apparatus and with each of the three galvanometric mirrors. With these connections, the computer controller concertedly controls the functioning of the z-scanning apparatus, and the rotation of the mirrors. Accordingly, the computer controller is capable of controlling movements of the laser beam focal point in an x-y-z volume of target tissue in the medium during laser surgery. Furthermore, the computer controller can be programmed to account for the optical properties of the optical components (e.g. field curvature of a lens), as well as the optical properties of the scanned medium (e.g. index of refraction). 
     After the laser beam has passed through the optical components disclosed above, it is important that the laser beam be incident substantially near the center of a focusing lens, before the beam enters the medium. To assist in accomplishing this, the device includes relay optics that are positioned on the beam path, downstream from the steering optics. Also, in a preferred embodiment of the present invention, a dichroic turning mirror can be located between the relay and the focusing lens, for directing the laser beam toward the focusing lens. Additionally, a microscope can be cited through the dichroic turning mirror and aligned with the laser beam for viewing the eye of the patient during the laser surgery procedure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is an elevational view of a system incorporating the present invention for performing corneal laser surgery; 
         FIG. 2  is a schematic view of the optical components of a device, in accordance with the present invention, for steering a laser beam to a focal point in a medium; 
         FIG. 3A  is a schematic view of one embodiment of the z-scanning apparatus of the present invention, specifically a voice coil subassembly; 
         FIG. 3B  is a representative illustration of an alternate embodiment of the z-scanning mechanism of the present invention, specifically an active mirror; 
         FIG. 4  is a functional layout of galvanometric mirrors for steering a laser beam in accordance with the present invention; 
         FIG. 5  is a perspective view of section of a medium (target tissue) that defines an orthogonal coordinate system; and 
         FIG. 6  is a cross-sectional view of a human eye. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A system for performing corneal laser surgery is shown in  FIG. 1  and is generally designated  10 . As shown, the system  10  includes a surgical laser unit  12  for directing a laser beam  14  along a beam path toward an eye  16  of a patient  18 . Additionally, the system  10  includes a platform  20  for aligning the eye  16  of the patient  18  with the surgical laser unit  12 . Further, a computer controller  22  is in electronic communication with the surgical laser unit  12  via an electrical cable  24 , for monitoring and controlling the laser surgery procedure. 
     Referring now to  FIG. 2 , an optical device for steering the laser beam  14  to a focal point in a medium, in accordance with the present invention, is shown and is generally designated  26 . Generally, the device  26  will be an integral part of the surgical laser unit  12 . In any event, as shown, the device  26  includes a laser source  28  for generating and directing the laser beam  14  along a beam path  30  toward the eye  16 . Preferably, the laser source  28  is a femtosecond laser source  28 , which is to say a laser source that generates a laser beam  14  having a wavelength of about one micron, a pulse duration in the range of about 100-1000 femtoseconds, and a pulse energy in the range of 0.1 to 100 mJ. 
     As shown in  FIG. 2 , the device  26  of the present invention includes a z-scanning apparatus  32  that is positioned on the beam path  30  for moving the focal point in a z-direction. Referring for a moment to  FIGS. 3A and 3B , two alternate embodiments of the z-scanning apparatus  32  are shown. In  FIG. 3A , a voice coil subassembly  32 ′ includes a lens  34  fixedly positioned on the beam path  30 . Preferably, the lens  34  is a plano-convex lens. Additionally, the subassembly  32 ′ includes a voice coil  36  having a movable, linear slide  38  that defines a longitudinal axis  40 . As shown, the longitudinal axis  40  is parallel to the beam path  30 . Further, a lens  42 , which is preferably a plano-concave lens, is mounted on the linear slide  38  for movement therewith back and forth along the beam path  30 . As shown in  FIG. 3B , an alternate embodiment of the z-scanning apparatus  32  is an active mirror  32 ″. More specifically, the mirror  32 ″ may be of the type disclosed in U.S. Pat. No. 6,220,707, entitled “Method for Programming an Active Mirror to Mimic a Wavefront” issued to J. Bille. As can be appreciated by referring to  FIG. 3B , the mirror  32 ″ comprises a plurality of individual facets, of which facet  43  is exemplary. Importantly, the facet  43  may be independently moved to change the shape of the surface of the active mirror  32 ″ to alter the incoming beam  14  of light. It should be appreciated that the location of the z-scanning apparatus  32  on the beam path  30 , i.e. after the laser source  28  and upstream from the remaining optical elements of the device  26 , allows the beam  14  to pass through the center of the apparatus  32 , which is desirable when focusing the beam  14  to a focal point in the eye  16 . 
     In addition to the z-scanning apparatus  32 , the device  26  includes a scanning mechanism  44 , which is preferably a galvanometric mirror, positioned on the beam path  30  for rotation of the mirror  44  through an angle “θ”. The mirror  44  has an axis of rotation  46  that is perpendicular to the beam path  30 . Also positioned on the beam path  30  is a scanning mechanism  48 , which is also preferably a galvanometric mirror. As contemplated by the present invention, the mirror  48  has an axis of rotation  50  that is perpendicular to the beam path  30  and parallel to the axis of rotation  46  of the mirror  44 . As shown in  FIG. 4 , the mirror  48  is positioned to rotate through an angle of “2θ”. Further, a scanning mechanism  52  is positioned on the beam path  30  to be optically aligned with the mirror  48 . In the preferred embodiment of the present invention, the scanning mechanism  52  is a galvanometric mirror, positioned on the beam path  30  for rotation through an angle “φ”. It can be seen in  FIGS. 2 and 4  that the mirror  52  has an axis of rotation  54  that is perpendicular to both the axes of rotation  46  and  50 , and that is perpendicular to the beam path  30 . Structurally, the distance, “d 1 ” ( FIG. 4 ), between the center  56  of the mirror  44  and the center  58  of the mirror  48 , is equal to the distance “d 2 ” between the center  58  and the center  60  of the mirror  52 . 
     Continuing along the beam path  30 , it can be seen in  FIG. 2  that the device  26  includes a relay  62  positioned downstream from both the z-scanning apparatus  32  and from the mirrors  44 ,  48  and  52 . As shown, the relay  62  comprises a plurality of lenses of which lenses  64   a  and  64   b , are exemplary. In addition to the relay  62 , a dichroic turning mirror  66  is positioned for directing the laser beam  14  toward the eye  16  as the beam  14  exits the relay  62 . More specifically, the turning mirror  66  is positioned sequentially on the beam path  30  after the relay  62 , and the mirror  66  is oriented at substantially a 45° angle relative to the beam path  30 . In addition to the dichroic mirror  66 , the device  26  of the present invention includes a microscope  68  optically aligned with the dichroic turning mirror  66  and the beam path  30 , for viewing the eye  16  of the patient  18  during the laser surgery procedure. 
     Still referring to  FIG. 2 , the device  26  also includes a focusing lens  70  positioned on the beam path  30  for focusing the laser beam  14  to the focal point in the eye  16 . More specifically, the focusing lens  70  is positioned downstream from the turning mirror  66 . As shown in  FIG. 2 , the focusing lens  70  is a lens multiplet. Further, as envisioned by the present invention, the focusing lens  70  defines a central axis  72 . It can be appreciated that the relay  62  is located upstream from the focusing lens  70  for optically imaging the galvanometric mirror  52  onto the surface of the focusing lens  70 . Stated differently, the net effect of the beam  14  passing through the relay  62 , prior to transiting the focusing lens  70 , is that the galvanometric mirror  52  and the focusing lens  70  are optically conjugated. 
     In the operation of the present invention, the laser source  28  generates a laser beam  14  that is directed toward the eye  16 . More specifically, the laser beam  14  is steered to a focal point within a particular layer, or medium, of the eye  16 . As contemplated by the present invention, the medium defines an orthogonal x-y-z coordinate system, of which the coordinate system  74  in  FIGS. 5 and 6  is exemplary. As can be seen by cross-referencing  FIGS. 5 and 6 , the medium is the cornea  76  of the eye  16 , and the x-y plane of the coordinate system  74  is normal to the optical axis  78  of the eye  16 . As further shown in  FIGS. 5 and 6 , the z-axis of the coordinate system  74  is substantially coincident with optical axis  78 . 
     Considering still further the operation of the present invention, the laser beam  14  exits the laser source  28  and travels along the beam path  30  towards the z-scanning apparatus  32 . In one embodiment of the present invention ( FIG. 3A ), the computer controller  22  directs the linear slide  38  of the voice coil subassembly  32 ′ to move axially along the longitudinal axis  40  a specified distance. Consequently, the lens  42 , which is mounted on the linear slide  38 , moves axially as well. Functionally, the movement of the focusing lens  42  relative to the stationary lens  34  causes the beam  14  to diverge or converge, depending on the direction of movement of the lens  42 . As a result of the divergence or convergence of the beam  14 , the x-y plane of the coordinate system is effectively moved along the z-axis to focus the focal point on the plane. It should be appreciated that the divergence and convergence of the beam  14  may also be accomplished by other means known in the pertinent art, such as by the use of an active mirror  32 ″. More particularly, prior to the laser beam  14  reaching the active mirror  32 ″, the computer controller  22  directs the movement of the individual facets, e.g.  43 , of the mirror  32 ″ to focus the focal point along the z-axis. 
     After exiting the z-scanning apparatus  32 , the laser beam  14  continues along the beam path  30  toward the first galvanometric mirror  44 . As can be seen by cross-referencing  FIGS. 2 and 4 , the laser beam  14  is directed toward the center of the mirror  44 . As shown in  FIG. 4 , the mirror  44  rotates, as directed by the computer controller  22 , about the axis of rotation  46  through an angle of “θ”. As the laser beam  14  reflects off the mirror  44 , the orientation of the mirror  44  relative to the angle of incidence of the beam  14 , produces a corresponding x-direction movement of the focal point in the cornea  76 . More specifically, the focal point moves along the x-axis of the x-y plane of the coordinate system  74  through a distance “Δx” that is proportional to the angle of rotation “θ”. 
     In concert with the rotation of the mirror  44 , the mirror  48  rotates about the axis of rotation  50  through an angle of “2θ”. As the laser beam  14  reflects off the mirror  48 , the laser beam  14  is compensated to align the beam path  30  with the center of the mirror  52 , while maintaining the “Δx” movement introduced by the rotation of the mirror  44 . In this way, the laser beam  14  reflects off the center of the mirror  52 , wherein the beam  14  is moved in a y-direction. More particularly, the mirror  52  is directed by the computer controller  22  to rotate about the axis of rotation  54  through an angle “φ”. Consequently, rotation of the mirror  52  though an angle “φ” moves the focal point of the beam  14  a linear distance “Δy” along the y-axis of the x-y plane. 
     Referring once again to  FIG. 2 , the laser beam  14  reflects off the third mirror  52  and enters the relay  62 . In the relay  62 , the laser beam  14  transits the lenses  64   a  and  64   b , during which time the beam path  30  is positioned to be centered on the central axis  72  of the focusing lens  70  when the laser beam  14  is incident on the focusing lens  70 . More specifically, the laser beam  14  is focused by lenses  64   a  and  64   b  to optically position the y-direction mirror  52  coincident with the focusing lens  70 . As such, the mirror  52  changes the angle of incidence of the beam  14  prior to the beam  14  striking the focusing lens  70 . There is not, however, any lateral movement of the beam  14  away from the center of the focusing lens  70  as the beam  14  reflects off the mirror  52  and transits the relay  62 . As the laser beam  14  exits the relay  62  and reflects off the turning mirror  66 , the turning mirror  66  directs the beam  14  toward the focusing lens  70 . At the focusing lens  70 , the laser beam  14  strikes the center section of the lens  70 . As the laser beam  14  transits the focusing lens  70 , the laser beam  14  is focused onto the desired focal point in the cornea  76  of the eye  16 . Throughout the course of the laser surgery procedure, a system operator (not shown) may view the eye  16  of the patient  18  through the microscope  68  which is aligned with the dichroic turning mirror  66 . 
     While the particular Beam Steering System for Corneal Laser Surgery 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.