Abstract:
A surface treatment laser beam delivery and tracking system is provided. The laser generates laser light along a original beam path at an energy level suitable for treating (e.g., eroding) a surface. An optical translator shifts the original beam path onto a resulting beam path. An optical angle adjuster changes the angle of the resulting beam path relative to the original beam path such that the laser light is incident on, and spatially distributed, the surface to be treated. A motion sensor transmits light energy to the surface and receives reflected light energy from the surface via the optical angle adjuster. The light energy transmitted by the motion sensor travels on a path that is parallel to the shifted beam as they travel through the optical angle adjuster. The reflected light energy is used by the motion sensor to detect movement of the surface relative to the original beam path and generate error control signals indicative of the movement. The optical angle adjuster is responsive to the error control signals to change the angle of the resulting beam path and the angle of the motion sensor&#39;s light energy in correspondence with one another. In this way, the beam originating from the treatment laser and the light energy originating from the motion sensor track together with the surface movement.

Description:
[0001]    This patent application is copending with related patent applications entitled “Eye Movement Sensing Method and System” and “Laser Sculpting System and Method” filed on the same date and owned by a common assignee as subject patent application. The disclosures of the these applications are incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The invention relates generally to laser systems, and more particularly to a laser system used to erode a moving surface such as an eye&#39;s corneal tissue.  
         BACKGROUND OF THE INVENTION  
         [0003]    Use of lasers to erode all or a portion of a workpiece&#39;s surface is known in the art. In the field of ophthalmic medicine, photorefractive keratectomy (PRK) is a procedure for laser correction of focusing deficiencies of the eye by modification of corneal curvature. PRK is distinct from the use of laser-based devices for more traditional ophthalmic surgical purposes, such as tissue cutting or thermal coagulation. PRK is generally accomplished by use of a 193 nanometer wavelength excimer laser beam that ablates away the workpiece, i.e., corneal tissue, in a photo decomposition process. Most clinical work to this point has been done with a laser operating at a fluence level of 120-195 mJ/cm 2  and a pulse-repetition rate of approximately 5-10 Hz. The procedure has been referred to as “corneal sculpting.” 
           [0004]    Before sculpting of the cornea takes place, the epithelium or outer layer of the cornea is mechanically removed to expose Bowman&#39;s membrane on the anterior surface of the stroma. At this point, laser ablation at Bowman&#39;s layer can begin. An excimer laser beam is preferred for this procedure. The beam may be variably masked during the ablation to remove corneal tissue to varying depths as necessary for recontouring the anterior stroma. Afterward, the epithelium rapidly regrows and resurfaces the contoured area, resulting in an optically correct (or much more nearly so) cornea. In some cases, a surface flap of the cornea is folded aside and the exposed surface of the cornea&#39;s stroma is ablated to the desired surface shape with the surface flap then being replaced.  
           [0005]    Phototherapeutic keratectomy (PTK) is a procedure involving equipment functionally identical to the equipment required for PRK. The PTK procedure differs from PRK in that rather than reshaping the cornea, PTK uses the aforementioned excimer laser to treat pathological superficial corneal dystrophies, which might otherwise require corneal transplants.  
           [0006]    In both of these procedures, surgical errors due to application of the treatment laser during unwanted eye movement can degrade the refractive outcome of the surgery. The eye movement or eye positioning is critical since the treatment laser is centered on the patient&#39;s theoretical visual axis which, practically speaking, is approximately the center of the patient&#39;s pupil. However, this visual axis is difficult to determine due in part to residual eye movement and involuntary eye movement known as saccadic eye movement. Saccadic eye movement is high-speed movement (i.e., of very short duration, 10-20 milliseconds, and typically up to 1° of eye rotation) inherent in human vision and is used to provide dynamic scene to the retina. Saccadic eye movement, while being small in amplitude, varies greatly from patient to patient due to psychological effects, body chemistry, surgical lighting conditions, etc. Thus, even though a surgeon may be able to recognize some eye movement and can typically inhibit/restart a treatment laser by operation of a manual switch, the surgeon&#39;s reaction time is not fast enough to move the treatment laser in correspondence with eye movement.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, it is an object of the present invention to provide a laser beam delivery and eye tracking method and system that is used in conjunction with a laser system capable of eroding a surface.  
           [0008]    Another object of the present invention is to provide a system for delivering a treatment laser to a surface and for automatically redirecting the treatment laser to compensate for movement of the surface.  
           [0009]    Still another object of the present invention is to provide a system for delivering a corneal ablating laser beam to the surface of an eye in a specific pattern about the optical center of the eye, and for automatically redirecting the corneal ablating laser beam to compensate for eye movement such that the resulting ablating pattern is the same regardless of eye movement.  
           [0010]    Yet another object of the present invention is to provide a laser beam delivery and eye tracking system for use with an ophthalmic treatment laser where the tracking operation detects eye movement in a non-intrusive fashion.  
           [0011]    A further object of the present invention is to provide a laser beam delivery and eye tracking system for automatically delivering and maintaining a corneal ablating laser beam with respect to the geometric center of an eye&#39;s pupil or a doctor defined offset from the center of the eye&#39;s pupil. A special object of this invention is the use of the laser pulses which are distributed in a pattern of discrete ablations to shape objects other than for corneal ablating.  
           [0012]    Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.  
           [0013]    In accordance with the present invention, an eye treatment laser beam delivery and eye tracking system is provided. A treatment laser and its projection optics generate laser light along an original beam path (i.e., the optical axis of the system) at an energy level suitable for treating the eye. An optical translator shifts the original beam path in accordance with a specific scanning pattern so that the original beam is shifted onto a resulting beam path that is parallel to the original beam path. An optical angle adjuster changes the resulting beam path&#39;s angle relative to the original beam path such that the laser light is incident on the eye.  
           [0014]    An eye movement sensor detects measurable amounts of movement of the eye relative to the system&#39;s optical axis and then generates error control signals indicative of the movement. The eye movement sensor includes: 1) a light source for generating light energy that is non-damaging with respect to the eye, 2) an optical delivery arrangement for delivering the light energy on a delivery light path to the optical angle adjuster in a parallel relationship with the resulting beam path of the treatment laser, and 3) an optical receiving arrangement. The parallel relationship between the eye movement sensor&#39;s delivery light path and the treatment laser&#39;s resulting beam path is maintained by the optical angle adjuster. In this way, the treatment laser light and the eye movement sensor&#39;s light energy are incident on the eye in their parallel relationship.  
           [0015]    A portion of the eye movement sensor&#39;s light energy is reflected from the eye as reflected energy traveling on a reflected light path back through the optical angle adjuster. The optical receiving arrangement detects the reflected energy and generates the error control signals based on the reflected energy. The optical angle adjuster is responsive to the error control signals to change the treatment laser&#39;s resulting beam path and the eye movement sensor&#39;s delivery light path in correspondence with one another. In this way, the beam originating from the treatment laser and the light energy originating from the eye movement sensor track along with the eye&#39;s movement.  
           [0016]    In carrying out this technique, the pattern constitutes overlapping but not coaxial locations for ablation to occur with each pulse removing a microvolume of material by ablation or erosion. For different depths, a pattern is repeated over those areas where increased ablation is needed. The laser pulses are usually at a certain pulse repetition rate. The subsequent pulses in a sequence are spaced at least one pulse beam width from the previous pulse and at a distance the ablated particles will not substantialy interfere with the subsequent pulse. In order to maximize the speed of the ablation, the subsequent pulse is spaced sufficiently close to enable the beam to be moved to the successive location within the time of the pulse repetition. The ablation is carried out on an object until a desired specific shape is achieved.  
           [0017]    This technique is fundamentally new and may be used on objects other than corneas. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of a laser beam delivery and eye tracking system in accordance with the present invention as it would be used in conjunction with an ophthalmic treatment laser;  
         [0019]    [0019]FIG. 2 is a sectional view of the projection optics used with the ophthalmic treatment laser embodiment of the laser beam delivery portion of the present invention;  
         [0020]    [0020]FIG. 3 illustrates diagrammatically an optical arrangement of mirrors used to produce translational shifts in a light beam along one axis;  
         [0021]    [0021]FIG. 4 is a block diagram of the servo controller/motor driver circuitry used in the ophthalmic treatment laser embodiment of the present invention; and  
         [0022]    [0022]FIG. 5 is a block diagram of a preferred embodiment eye movement sensor used in the ophthalmic treatment laser embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Referring now to the drawings, and more particularly to FIG. 1, a block diagram is shown of a laser beam delivery and eye tracking system referenced generally by the numeral  5 . The laser beam delivery portion of system  5  includes treatment laser source  500 , projection optics  510 , X-Y translation mirror optics  520 , beam translation controller  530 , dichroic beamsplitter  200 , and beam angle adjustment mirror optics  300 . By way of example, it will be assumed that treatment laser  500  is a 193 nanometer wavelength excimer laser used in an ophthalmic PRK (or PTK) procedure performed on a movable workpiece. e.g., eye  10 . However, it is to be understood that the method and system of the present invention will apply equally as well to movable workpieces other than an eye, and further to other wavelength surface treatment or surface eroding lasers. The laser pulses are distributed as shots over the area to be ablated or eroded, preferably in a distributed sequence. A single laser pulse of sufficient power to cause ablation creates a micro cloud of ablated particles which interferes with the next laser pulse if located in the same or immediate point. To avoid this interference, the next laser pulse is spatially distributed to a next point of erosion or ablation that is located a sufficient distance so as to avoid the cloud of ablated particles. Once the cloud is dissipated, another laser pulse is made adjacent the area prior eroded so that after the pattern of shots is completed the cumulative shots fill in and complete said pattern so that the desired shape of the object or cornea is achieved.  
         [0024]    In operation of the beam delivery portion of system  5 , laser source  500  produces laser beam  502  which is incident upon projection optics  510 . Projection optics  510  adjusts the diameter and distance to focus of beam  502  depending on the requirements of the particular procedure being performed. For the illustrative example of an excimer laser used in the PRK or PTK procedure, projection optics  510  includes planar concave lens  512 , and fixed focus lenses  514  and  516  as shown in the sectional view of FIG. 2. Lenses  512  and  514  act together to form an A-focal telescope that expands the diameter of beam  502 . Fixed focus lens  516  focuses the expanded beam  502  at the workpiece, i.e., eye  10 , and provides sufficient depth, indicated by arrow  518 , in the plane of focus of lens  516 . This provides flexibility in the placement of projection optics  510  relative to the surface of the workpiece. An alternative implementation is to eliminate lens  514  when less flexibility can be tolerated.  
         [0025]    After exiting projection optics  510 , beam  502  impinges on X-Y translation mirror optics  520  where beam  502  is translated or shifted independently along each of two orthogonal translation axes as governed by beam translation controller  530 . Controller  530  is typically a processor programmed with a predetermined set of two-dimensional translations or shifts of beam  502  depending on the particular ophthalmic procedure being performed. For the illustrative example of the excimer laser used in a PRK or PTK procedure, controller  530  may be programmed in accordance with the aforementioned copending patent application entitled “Laser Sculpting System and Method”. The programmed shifts of beam  502  are implemented by X-Y translation mirror optics  520 .  
         [0026]    Each X and Y axis of translation is independently controlled by a translating mirror. As shown diagrammatically in FIG. 3, the Y-translation operation of X-Y translation mirror optics  520  is implemented using translating mirror  522 . Translating mirror  522  is movable between the position shown and the position indicated by dotted line  526 . Movement of translating mirror  522  is such that the angle of the output beam with respect to the input beam remains constant. Such movement is brought about by translation mirror motor and control  525  driven by inputs received from beam translation controller  530 . By way of example, motor and control  525  can be realized with a motor from Trilogy Systems Corporation (e.g., model T050) and a control board from Delta Tau Systems (e.g., model 400-602276 PMAC).  
         [0027]    With translating mirror  522  positioned as shown, beam  502  travels the path traced by solid line  528   a.  With translating mirror  522  positioned along dotted line  526 , beam  502  travels the path traced by dotted line  528   b.  A similar translating mirror (not shown) would be used for the X-translation operation. The X-translation operation is accomplished in the same fashion but is orthogonal to the Y-translation. The X-translation may be implemented prior or subsequent to the Y-translation operation.  
         [0028]    The eye tracking portion of system  5  includes eye movement sensor  100 , dichroic beamsplitter  200  and beam angle adjustment mirror optics  300 . Sensor  100  determines the amount of eye movement and uses same to adjust mirrors  310  and  320  to track along with such eye movement. To do this, sensor  100  first transmits light energy  101 -T which has been selected to transmit through dichroic beamsplitter  200 . At the same time, after undergoing beam translation in accordance with the particular treatment procedure, beam  502  impinges on dichroic beamsplitter  200  which has been selected to reflect beam  502  (e.g., 193 nanometer wavelength laser beam) to beam angle adjustment mirror optics  300 .  
         [0029]    Light energy  101 -T is aligned such that it is parallel to beam  502  as it impinges on beam angle adjustment mirror optics  300 . It is to be understood that the term “parallel” as used herein includes the possibility that light energy  101 -T and beam  502  can be coincident or collinear. Both light energy  101 -T and beam  502  are adjusted in correspondence with one another by optics  300 . Accordingly, light energy  101 -T and beam  502  retain their parallel relationship when they are incident on eye  10 . Since X-Y translation mirror optics  520  shifts the position of beam  502  in translation independently of optics  300 , the parallel relationship between beam  502  and light energy  101 -T is maintained throughout the particular ophthalmic procedure.  
         [0030]    Beam angle adjustment mirror optics consists of independently rotating mirrors  310  and  320 . Mirror  310  is rotatable about axis  312  as indicated by arrow  314  while mirror  320  is rotatable about axis  322  as indicated by arrow  324 . Axes  312  and  322  are orthogonal to one another. In this way, mirror  310  is capable of sweeping light energy  101 -T and beam  502  in a first plane (e.g., elevation) while mirror  320  is capable of independently sweeping light energy  101 -T and beam  502  in a second plane (e.g., azimuth) that is perpendicular to the first plane. Upon exiting beam angle adjustment mirror optics  300 , light energy  101 -T and beam  502  impinge on eye  10 .  
         [0031]    Movement of mirrors  310  and  320  is typically accomplished with servo controller/motor drivers  316  and  326 , respectively. FIG. 4 is a block diagram of a preferred embodiment servo controller/motor driver  316  used for the illustrative PRK/PTK treatment example. (The same structure is used for servo controller/motor driver  326 .) In general, drivers  316  and  326  must be able to react quickly when the measured error from eye movement sensor  100  is large, and further must provide very high gain from low frequencies (DC) to about 100 radians per second to virtually eliminate both steady state and transient error.  
         [0032]    More specifically, eye movement sensor  100  provides a measure of the error between the center of the pupil (or an offset from the center of the pupil that the doctor selected) and the location where mirror  310  is pointed. Position sensor  3166  is provided to directly measure the position of the drive shaft (not shown) of galvanometer motor  3164 . The output of position sensor  3166  is differentiated at differentiator  3168  to provide the velocity of the drive shaft of motor  3164 . This velocity is summed with the error from eye movement sensor  100 . The sum is integrated at integrator  3160  and input to current amplifier  3162  to drive galvanometer motor  3164 . As the drive shaft of motor  3164  rotates mirror  310 , the error that eye movement sensor  100  measures decreases to a negligible amount. The velocity feedback via position sensor  3166  and differentiator  3168  provides servo controller/motor driver  316  with the ability to react quickly when the measured sensor error is large.  
         [0033]    Light energy reflected from eye  10 , as designated by reference numeral  101 -R, travels back through optics  300  and beamsplitter  200  for detection at sensor  100 . Sensor  100  determines the amount of eye movement based on the changes in reflection energy  101 -R. Error control signals indicative of the amount of eye movement are fed back by sensor  100  to beam angle adjustment mirror optics  300 . The error control signals govern the movement or realignment of mirrors  310  and  320  in an effort to drive the error control signals to zero. In doing this, light energy  101 -T and beam  502  are moved in correspondence with eye movement while the actual position of beam  502  relative to the center of the pupil is controlled by X-Y translation mirror optics  520 .  
         [0034]    In order to take advantage of the properties of beamsplitter  200 , light energy  101 -T must be of a different wavelength than that of treatment laser beam  502 . The light energy should preferably lie outside the visible spectrum so as not to interfere or obstruct a surgeon&#39;s view of eye  10 . Further, if the present invention is to be used in ophthalmic surgical procedures, light energy  101 -T must be “eye safe” as defined by the American National Standards Institute (ANSI). While a variety of light wavelengths satisfy the above requirements, by way of example, light energy  101 -T is infrared light energy in the 900 nanometer wavelength region. Light in this region meets the above noted criteria and is further produced by readily available, economically affordable light sources. One such light source is a high pulse repetition rate GaAs 905 nanometer laser operating at 4 kHz which produces an ANSI defined eye safe pulse of 10 nanojoules in a 50 nanosecond pulse.  
         [0035]    A preferred embodiment method for determining the amount of eye movement, as well as eye movement sensor  100  for carrying out such a method, are described in detail in the aforementioned copending patent application. However, for purpose of a complete description, sensor  100  will be described briefly with the aid of the block diagram shown in FIG. 2. Sensor  100  may be broken down into a delivery portion and a receiving portion. Essentially, the delivery portion projects light energy  101 -T in the form of light spots  21 ,  22 ,  23  and  24  onto a boundary (e.g., iris/pupil boundary  14 ) on the surface of eye  10 . The receiving portion monitors light energy  101 -R in the form of reflections caused by light spots  21 ,  22 ,  23  and  24 .  
         [0036]    In delivery, spots  21  and  23  are focused and positioned on axis  25  while spots  22  and  24  are focused and positioned on axis  26  as shown. Axes  25  and  26  are orthogonal to one another. Spots  21 ,  22 ,  23  and  24  are focused to be incident on and evenly spaced about iris/pupil boundary  14 . The four spots  21 ,  22 ,  23  and  24  are of equal energy and are spaced evenly about and on iris/pupil boundary  14 . This placement provides for two-axis motion sensing in the following manner. Each light spot  21 ,  22 ,  23  and  24  causes a certain amount of reflection at its position on iris/pupil boundary  14 . Since boundary  14  moves in coincidence with eye movement, the amount of reflection from light spots  21 ,  22 ,  23  and  24  changes in accordance with eye movement. By spacing the four spots evenly about the circular boundary geometry, horizontal or vertical eye movement is detected by changes in the amount of reflection from adjacent pairs of spots. For example, horizontal eye movement is monitored by comparing the combined reflection from light spots  21  and  24  with the combined reflection from light spots  22  and  23 . In a similar fashion, vertical eye movement is monitored by comparing the combined reflection from light spots  21  and  22  with the combined reflection from light spots  23  and  24 .  
         [0037]    More specifically, the delivery portion includes a 905 nanometer pulsed diode laser  102  transmitting light through optical fiber  104  to an optical fiber assembly  105  that splits and delays each pulse from laser  102  into preferably four equal energy pulses. Assembly  105  includes one-to-four optical splitter  106  that outputs four pulses of equal energy into optical fibers  108 ,  110 ,  112 ,  114 . In order to use a single processor to process the reflections caused by each pulse transmitted by fibers  108 ,  110 ,  112  and  114 , each pulse is uniquely delayed by a respective fiber optic delay line  109 ,  111 ,  113  and  115 . For example, delay line  109  causes a delay of zero, i.e., DELAY=Ox where x is the delay increment; delay line  111  causes a delay of x, i.e., DELAY=1x; etc.  
         [0038]    The pulse repetition frequency and delay increment x are chosen so that the data rate of sensor  100  is greater than the speed of the movement of interest. In terms of saccadic eye movement, the data rate of sensor  100  must be on the order of at least several hundred hertz. For example, a sensor data rate of approximately 4 kHz is achieved by 1) selecting a small but sufficient value for x to allow processor  160  to handle the data (e.g., 160 nanoseconds), and 2) selecting the time between pulses from laser  102  to be 250 microseconds (i.e., laser  102  is pulsed at a 4 kHz rate).  
         [0039]    The four equal energy pulses exit assembly  105  via optical fibers  116 ,  118 ,  120  and  122  which are configured as a fiber optic bundle  123 . Bundle  123  arranges the optical fibers such that the center of each fiber forms the corner of a square. Light from assembly  105  is passed through an optical polarizer  124  that outputs horizontally polarized light beams as indicated by arrow  126 . Horizontally polarized light beams  126  pass to focusing optics  130  where spacing between beams  126  is adjusted based on the boundary of interest. Additionally, a zoom capability (not shown) can be provided to allow for adjustment of the size of the pattern formed by spots  21 ,  22 ,  23  and  24 . This capability allows sensor  100  to adapt to different patients, boundaries, etc.  
         [0040]    A polarizing beam splitting cube  140  receives horizontally polarized light beams  126  from focusing optics  130 . Cube  140  is configured to transmit horizontal polarization and reflect vertical polarization. Accordingly, cube  140  transmits only horizontally polarized light beams  126  as indicated by arrow  142 . Thus, it is only horizontally polarized light that is incident on eye  10  as spots  21 ,  22 ,  23  and  24 . Upon reflection from eye  10 , the light energy is depolarized (i.e., it has both horizontal and vertical polarization components) as indicated by crossed arrows  150 .  
         [0041]    The receiving portion first directs the vertical component of the reflected light as indicated by arrow  152 . Thus, cube  140  serves to separate the transmitted light energy from the reflected light energy for accurate measurement. The vertically polarized portion of the reflection from spots  21 ,  22 ,  23  and  24 , is passed through focusing lens  154  for imaging onto an infrared detector  156 . Detector  156  passes its signal to a multiplexing peak detecting circuit  158  which is essentially a plurality of peak sample and hold circuits, a variety of which are well known in the art. Circuit  158  is configured to sample (and hold the peak value from) detector  156  in accordance with the pulse repetition frequency of laser  102  and the delay x. For example, if the pulse repetition frequency of laser  102  is 4 kHz, circuit  158  gathers reflections from spots  21 ,  22 ,  23  and  24  every 250 microseconds.  
         [0042]    The values associated with the reflected energy for each group of four spots (i.e., each pulse of laser  102 ) are passed to a processor  160  where horizontal and vertical components of eye movement are determined. For example let R 21 , R 22 , R 23  and R 24  represent the detected amount of reflection from one group of spots  21 ,  22 ,  23  and  24 , respectively. A quantitative amount of horizontal movement is determined directly from the normalized relationship  
                 (       R   21     +     R   24       )     -     (       R   22     +     R   23       )           R   21     +     R   22     +     R   23     +     R   24               (   1   )                               
 
         [0043]    while a quantitative amount of vertical movement is determined directly from the normalized relationship  
                 (       R   21     +     R   22       )     -     (       R   23     +     R   24       )           R   21     +     R   22     +     R   23     +     R   24               (   2   )                               
 
         [0044]    Note that normalizing (i.e., dividing by R 21 +R 22 +R 23 +R 24 ) reduces the effects of variations in signal strength. Once determined, the measured amounts of eye movement are sent to beam angle adjustment mirror optics  300 .  
         [0045]    The advantages of the present invention are numerous. Eye movement is measured quantitatively and used to automatically redirect both the laser delivery and eye tracking portions of the system independent of the laser positioning mechanism. The system operates without interfering with the particular treatment laser or the surgeon performing the eye treatment procedure.  
         [0046]    Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.