Abstract:
The present invention relates to a laser apparatus, system, and method for determining a depth of a focus point of a laser beam. An interface device is coupleable to the laser apparatus and has an applanation element comprising a front surface and a back surface. A laser beam having a predefined shape is focussed through the applanation element at a focus point. A superimposed image of a spurious reflection, which is reflected from the front surface of the applanation element, with a standard reflection, which is reflected from the back surface of the applanation element, is detected. The spurious reflection is then filtered out of the superimposed image. Based on the remaining standard reflection, the depth of the focus point of the laser beam can be determined.

Description:
BACKGROUND 
       [0001]    For material processing lasers, and in particular lasers used in ophthalmological surgery, it is essential to determine the depth of the focus of the laser beam exactly, so as to achieve a high-quality and precise cut. 
         [0002]    Refractive lasers are a particular kind of material processing laser which are used in LASIK (laser assisted in-situ keratomileusis) surgery. LASIK surgery is performed in three steps. A first step is to create a flap of corneal tissue. A second step is reshaping or remodelling of the cornea underneath the flap with the refractive laser. In a final step, the flap is repositioned. 
         [0003]    The human cornea consists of five layers. The outer layer is the epithelium, a thin tissue layer of fast-growing and easily regenerated cells, typically composed of about six layers of cells. Next, the Bowman&#39;s layer, which is 8-14 μm thick, is a condensed layer of collagen that protects the stroma. The stroma is a transparent middle layer, consisting of regularly arranged collagen fibers, along with sparsely distributed interconnected keratocytes, which are cells responsible for general repair and maintenance. The Descemet&#39;s membrane is a thin acellular layer around 5-20 μm thick. Finally, the endothelium is a layer, approximately 5 μm thick, of mitochondria-rich cells. 
         [0004]    The stroma is the thickest layer of the cornea, accounting for up to 90% of the corneal thickness. A remodeling or reshaping of the stroma during surgery alters the light-focussing capability of the cornea, which results in a correction to the patient&#39;s vision. 
         [0005]    During LASIK surgery, to control the depth of the focus of the laser beam more exactly, a flat or curved transparent or/and translucent plane is placed in contact with the outer surface of the eye. This plane is also called an applanation element. The applanation element has a front surface which is typically coated with a reflex-minimizing layer, and a back surface which is in contact with the eye. 
         [0006]    When cutting the flap, the depth of the focus of the laser must be controlled very precisely. The flap is cut to a depth of approximately 80 μm to 500 μm, such as approximately 120 μm. The flap is typically created very near the Bowman&#39;s layer to avoid trauma caused by pulling back the flap but sufficiently far from the Bowman&#39;s layer to avoid breaching the layer. To enable consistent, high-quality results, the focus depth of the laser beam must be controllable to within a precision of a few micrometres. 
         [0007]    In current LASIK surgery systems, the depth of the focus of the laser, relative to the cornea surface of the eye, is calibrated (or recalibrated) before the start of each surgery. 
         [0008]    To determine the exact depth of the focus of the laser beam during calibration, the back surface of an applanation element is in contact with the eye, and a laser beam having a particular pattern is directed at the eye. The exact depth of the focus of the laser beam relative to the back surface of the applanation element is calculated based on the measured pattern of light reflected from the back surface of the applanation element. To ensure that there are no spurious reflections from other surfaces which would compromise the quality of the calculations, the front surface of the applanation element is coated with a high-transmissivity reflection-minimizing coating. 
       PROBLEM STATEMENT 
       [0009]    However, such a high-transmissivity coating is quite expensive, and therefore it would be desirable to find a method for determining the exact depth of the focus of the laser beam which functions even when the front surface of the applanation element is not coated with a reflection-minimizing highly transmissive layer. This problem is solved by the subject matter of the independent claims. Advantageous embodiments are defined by the dependent claims. 
       SUMMARY OF THE INVENTION 
       [0010]    A first aspect of an ophthalmological laser apparatus is described according to claim  1 . An interface device is coupleable to the laser apparatus and has an applanation element that may be transparent or/and translucent to a laser beam produced by the laser apparatus. The applanation element has a front surface and a back surface. The laser apparatus comprises optics that are adapted to focus the laser beam, which has a predefined shape, through the applanation element at a focus point. Furthermore, the laser apparatus comprises a detection element that is adapted to detect a superimposed image of a spurious reflection, which is reflected from the front surface, with a standard reflection, which is reflected from the back surface. The laser apparatus also comprises a processor that is adapted to numerically filter out the spurious reflection from the superimposed image, and determine a depth of the focus point of the laser beam based on the remaining standard reflection. 
         [0011]    The ophthalmological laser apparatus according to the first aspect allows the depth of the focus of the laser beam to be determined exactly, even when the front surface of the applanation element is free from a reflection-minimizing coating. 
         [0012]    In one embodiment according to the first aspect, the ophthalmological laser apparatus may further comprise a mask for covering at least a portion of the laser beam. In this case, the optics may be adapted to focus the laser beam through the mask so as to produce the predefined shape. According to this embodiment, a suitable mask may be selected which is adapted to provide exact results for determining the depth of the focus of the laser beam. 
         [0013]    In a further embodiment according to the first aspect, the processor for filtering the spurious reflection may be adapted to convolute the superimposed image with a predefined reference pattern to produce an auxiliary image. The processor may be adapted to next evaluate the auxiliary image to identify a maximum point having a highest intensity. Finally, the processor may be adapted to reposition the reference pattern based on a location of the maximum point, and multiply the superimposed image with the repositioned reference pattern to produce the standard reflection. According to this embodiment, the center-point of a standard reflection can be determined, and based on this information, a spurious reflection can be eliminated. 
         [0014]    In a refinement of the previous embodiment, the reference pattern may comprise a centre point, and the processor may be adapted to reposition the reference pattern such that the centre point and the location of the maximum point are superimposed. According to this embodiment, points in which a standard reflection may be located can be identified, thus ensuring that none of the standard reflection is cancelled when the spurious reflection is eliminated. 
         [0015]    Additionally or alternatively, the convolution of the superimposed image with the reference pattern may be performed as a multiplication in the frequency domain. In a refinement of this embodiment, the processor may be adapted to apply a Fourier-transform to the superimposed image. The processor may then be adapted to multiply the Fourier-transformed superimposed image with a Fourier-transform of the reference pattern to produce a transformed auxiliary image. Finally, the processor may be adapted to perform a reverse-Fourier-transform on the transformed auxiliary image to produce the auxiliary image. This allows the calculation of the convoluted superimposed image to be performed more quickly, as performing a convolution in the spatial domain can be very computationally expensive. 
         [0016]    In a further embodiment of the first aspect, the applanation element may be free from a reflection-minimizing coating. 
         [0017]    In either of the previous two embodiments, the back surface of the applanation element may be adapted to lie in contact to the eye that is to be examined. This allows the distance between the optics and the surface of the eye to be held constant over the course of a determination procedure, thus ensuring that the depth of focus is measured correctly throughout. 
         [0018]    A second aspect is a laser system with the laser apparatus according to the first aspect or one of the embodiments of the first aspect. The laser system further comprises an interface device which is coupleable to the laser apparatus and has an applanation element which is transparent and/or translucent to a laser beam produced by the laser apparatus. The applanation element comprises a front surface and a back surface. 
         [0019]    A third aspect is a method to determine a depth of a focus point of a laser beam provided by the laser apparatus as described herein. In a first step of this method, a laser beam, which has a predefined shape, is focused through an applanation element at a focus. The applanation element has a front surface and a back surface. The applanation element may be a transparent and/or translucent applanation element, i.e. it may be transparent and/or translucent to the laser beam. In a second step, a superimposed image is detected. The superimposed image consists of a spurious reflection, which is reflected from the front surface, superimposed with a standard reflection, which is reflected from the back surface. In a third step, the spurious reflection is numerically filtered out from the superimposed image. In a final step, a depth of the focus point of the laser beam is determined based on the remaining standard reflection. 
         [0020]    The method according to the third aspect allows the depth of the focus of the laser beam to be determined exactly, even when the front surface of the applanation element is free from a reflection-minimizing coating. 
         [0021]    In an embodiment according to the third aspect, the step of filtering out the spurious reflection may comprise: convoluting the superimposed image with a predefined reference pattern, to produce an auxiliary image; evaluating the auxiliary image, to identify a maximum point having a highest intensity; repositioning the reference pattern based on a location of the maximum point; and multiplying the superimposed image with the repositioned reference pattern, to produce the standard reflection. According to this embodiment, the center-point of a standard reflection can be determined, and based on this information, a spurious reflection can be eliminated. 
         [0022]    According to this embodiment, the step of convoluting may comprise: applying a Fourier-transform to the superimposed image; multiplying the Fourier-transformed superimposed image with a Fourier-transform of the reference pattern, to produce a transformed auxiliary image; and performing a reverse-Fourier-transform on the transformed auxiliary image to produce the auxiliary image. This allows the calculation of the convoluted superimposed image to be performed more quickly, as performing a convolution in the spatial domain can be very computationally expensive. 
         [0023]    A fourth aspect is a computer program with program code portions, that, when loaded onto a computer or processor, or when running on a computer or processor, causes the computer or processor to execute any of the method aspects described herein. 
         [0024]    The computer program may be stored on a program storage device or computer program product. 
         [0025]    In the above, the operation “Fourier transform” refers to any discrete Fourier transform, such as the Fast Fourier Transform (FFT). However, any computational method suitable for transforming a spatial signal into the frequency domain may be used in places where “Fourier transform” is referred to. 
         [0026]    The laser apparatus, the laser system, the corresponding method and computer program are described herein with respect to ophthalmology. It is, however, also conceivable that the laser apparatus, the laser system, the corresponding method and computer program are used in different technical fields like dermatology or material processing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will be explained further on the basis of the appended Figures, which are schematic throughout. 
           [0028]      FIG. 1  shows a schematic diagram of a system comprising a laser apparatus according to one embodiment of the present invention. 
           [0029]      FIG. 2  shows a schematic diagram of a portion of the laser apparatus according to the embodiment of  FIG. 1 . 
           [0030]      FIG. 3  shows a plan view of a mask used in the embodiment according to  FIG. 2 . 
           [0031]      FIG. 4  shows a schematic diagram of light passing through an applanation element according to the embodiment of  FIG. 1 . 
           [0032]      FIG. 5  shows a block diagram of the calculation steps of a method according to an embodiment of the invention. 
           [0033]      FIG. 6  shows a filtered image suitable for calculating the depth of focus of a laser beam. 
           [0034]      FIG. 7  shows an image consisting of a spurious reflection superimposed with a standard reflection in accordance with the embodiment of  FIG. 5 . 
           [0035]      FIG. 8  shows a reference pattern in accordance with the embodiment of  FIG. 5 . 
           [0036]      FIG. 9  shows the Fourier transformation of the reference pattern of  FIG. 8 . 
           [0037]      FIG. 10  shows the result of multiplying the Fourier-transformed reference pattern of  FIG. 9  with a Fourier transformation of the superimposed image of  FIG. 7 . 
           [0038]      FIG. 11  shows a reverse Fourier transform of the image of  FIG. 10 . 
           [0039]      FIG. 12  shows a repositioned reference pattern in accordance with the embodiment of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 1  shows a laser system  10  for focusing a laser beam  14  at a focus point within an eye  16 . 
         [0041]    The laser system comprises a laser source  12 . The laser source  12  may include, for example, a laser oscillator (e.g. solid-state laser oscillator), a pre-amplifier, which increases the pulse power of the laser pulses emitted from the oscillator and simultaneously temporally stretches them, a subsequent pulse-picker, which selects individual laser pulses from the pre-amplified laser pulses of the oscillator, in order to lower the repetition rate to a desired degree, a power amplifier, which amplifies the selected, still temporally stretched, pulses to the pulse energy needed for the application, and a pulse compressor, which temporally compresses the pulses output from the power amplifier to the pulse duration desired for the application. 
         [0042]    The laser source  12  generates a pulsed laser beam  14 . The pulse duration of the radiation pulses is chosen to either generate reflected light signals, for diagnostic purposes, or for creating incisions in the corneal tissue of an eye  16  of a patient to be treated. The radiation pulses of the laser beam  14  have a pulse duration in the nanosecond, picosecond, femtosecond or attosecond range. 
         [0043]    The laser beam  14  generated by the laser source  12  furthermore has a pulse repetition rate such as is desired for the application in question. The repetition rate of the radiation pulses emitted from the laser device  10  and directed onto the eye  16  corresponds to the repetition rate of the radiation pulses that are generated at the output of the laser source  12 . Alternatively, if required by a predetermined machining profile for the eye  16 , a portion of the radiation pulses emitted from the laser source  12  are blanked by means of an optical switch  18  arranged in the radiation path of the laser beam  14 . Such blanked radiation pulses accordingly do not reach the eye  16 . 
         [0044]    The optical switch  18 , which is also called a pulse modulator, may, for example, take the form of an acousto-optical modulator or an electro-optical modulator. Generally, the optical switch  18  may contain arbitrary optically active elements that enable a rapid blanking of individual laser pulses. The optical switch  18  may, for example, contain a beam trap, which serves to absorb radiation pulses to be blanked. The optical switch  18  can deflect such radiation pulses to be blanked from the normal beam path of the radiation pulses of the laser beam  14  and direct them onto the beam trap. 
         [0045]    Further optical components which are arranged in the beam path of the laser beam  14  include a z-controller  22  and an x-y controller  24 . The z-controller  22  controls the longitudinal location of the focal point of the laser beam  14 ; the x-y controller  24 , on the other hand, controls the transverse location of the focal point. A coordinate frame that represents the x-y-z directions in the region of the eye  16  has been drawn in  FIG. 1  for the purpose of illustration. In this context, the term ‘longitudinal’ refers to the direction of beam propagation, which conventionally is designated as the z-direction. Similarly, ‘transverse’ refers to a direction transverse to the direction of propagation of the laser beam  14 , which conventionally is designated as the x-y plane. 
         [0046]    To achieve a transverse deflection of the laser beam  14 , the x-y controller  24  may, for example, include a pair of galvanometric actuated scanner mirrors that are capable of tilting about mutually perpendicular axes. The z-controller  22  may, for example, contain a longitudinally adjustable lens or a lens of variable refractive power or a deformable mirror, with which the divergence of the laser beam  14 , and consequently the z-position of the beam focus, can be controlled. Such an adjustable lens or mirror may be contained in a beam expander which expands the laser beam  14  emitted from the laser source  12 . The beam expander may, for example, be configured as a Galilean telescope. 
         [0047]    The laser apparatus of the first embodiment comprises a focusing objective, also referred to as optics  26 , arranged in the beam path of the laser beam  14 . The optics  26  serve to focus the laser beam  14  onto a desired location on or in the eye  16 , in particular within the cornea. The focusing optics  26  may be an f-theta objective. 
         [0048]    The optical switch  18 , the z-controller  22 , the x-y controller  24  and the focusing objective  26  do not have to be arranged in the order represented in  FIG. 1 . For example, the optical switch  18  may, without loss of generality, be arranged in the beam path downstream of the z-controller  22 . If desired, the x-y controller  24  and z-controller  22  may be combined to form a single structural unit. The order and grouping of the components shown in  FIG. 1  is in no way to be understood as restrictive. 
         [0049]    On the beam-exit side of the focusing objective  26 , an applanation element  30  constitutes an abutment interface for the cornea of the eye  16 . The applanation element  30  is transparent or/and at least translucent to the laser radiation. On its back surface  32 , facing towards the eye, the applanation element  30  includes an abutment face for the cornea of the eye  16 . On its upper side, opposite the surface of the eye, the applanation element  30  includes a front surface  36 , which is free of any reflection-minimizing coating. In the exemplary case shown, the back surface  32  is realised as a plane surface. The back surface  32  levels the cornea when the applanation element  30  is placed in contact with the eye  16  with appropriate pressure or when the cornea is aspirated onto the back surface  32  by underpressure. As shown in  FIG. 1 , the eye  16  is bearing against the planar back surface  32  of the applanation element  30 . 
         [0050]    The applanation element  30 , which in the case of plane-parallel design is ordinarily is designated as the applanation plate, is fitted to the narrower end of a conically widening carrier sleeve  34 . The connection between the applanation element  30  and the carrier sleeve  34  may be permanent, for example by virtue of adhesion bonding, or it may be detachable, for instance by virtue of a screw coupling. It is also conceivable to use a single optical injection-moulded part which functions as both the carrier sleeve  34  and the applanation element  30 . In a manner not represented in detail, the carrier sleeve  34  has coupling structures at its wider sleeve end, which in the drawing is the upper end. The coupling structures are suitable for coupling the carrier sleeve  34  onto the focusing objective  26 . 
         [0051]    The laser system  10  also comprises a detection element  42 , such as a camera, which is adapted to collect images and transfer said images to the control computer  38 . 
         [0052]    The laser source  12 , the optical switch  18 , the detection element  42 , and the two scanners  22 ,  24 , are controlled by a control computer  38  which operates in accordance with a control program stored in a memory. The control program contains instructions (program code) that are executed by the control computer  38  so as to determine and control the location of the beam focus of the laser beam  14  in the cornea, in the lens or at another location of the eye  16  bearing against the contact element  30 . 
         [0053]    The laser system  10  may also comprise an interface module (not shown) connected to control computer  38 , to allow a user to input commands to the control computer  38 . The interface module may comprise a screen or monitor to enable the user to view status information about components of the laser system  10 , and/or to view the data collected by the detection element  42 . 
         [0054]    In  FIG. 2 , the portion of laser system  10  which forms the laser apparatus is shown in more detail. In the path of the laser beam  14  between the focusing objective  26  and the applanation element  30 , a mask  40  is provided. The mask  40  is formed of a material which is opaque to the light of the laser beam  14 . The mask  40  is dimensioned so as to cover substantially the entire laser beam  14 . A square mask  40  is shown, but other convex shapes are possible, such as a regular or irregular polygon or a circle. 
         [0055]    A plan view of the mask  40  is shown in  FIG. 3 . The mask  40  has a center opening  43  and one or more outer openings  44 , the openings  43 ,  44  being adapted to let light from the laser beam  14  pass through. The outer openings  44  are each spaced at an equal distance D from the center opening  43 . The distance D is less than half of the diameter of the laser beam  14 , such that light from the outer edge of the laser beam  14  passes through the outer openings  44 . The outer openings  44  may be distributed evenly about an imagined circumference centered at the center opening  43 , but uneven distributions of the outer openings  44  about the imagined circumference are also possible. 
         [0056]      FIG. 4  illustrates the transmissive/reflective properties of the applanation element  30  when a beam of light  14  is directed toward it. When the light reaches the front surface  36  of the applanation element  30 , most of the light continues in the same direction but a portion of the light is reflected back, forming a spurious reflection  14   b.    
         [0057]    The known art provides for a reflection-minimizing coating on the front surface  36  of the applanation element  30 , so as to suppress the spurious reflection  14   b.  However, in the present embodiment, the front surface  36  is free from a reflection-minimizing coating, and therefore light incident on the front surface  36  results in a spurious reflection  14   b.    
         [0058]    The remaining light in the laser beam  14  then passes through the applanation element  30 , and reaches the back surface  32  of the applanation element. Here again, a portion of the light is reflected back, forming a standard reflection  14   a.    
         [0059]    The reflected light from the standard reflection  14   a  and spurious reflection  14   b  is collected by the detection element  42  in the form of an output image. 
         [0060]    In known systems, due to the provision of a reflection-minimizing coating on the back surface  36 , the output image consisted of only a standard reflection  14   a.  If using the mask  40  shown in  FIG. 3 , a reflection similar to the image shown in  FIG. 6  would be produced, consisting of a central bright point representing the standard reflection  14   a  of light which passed through the center opening  43 , and four outer bright points, representing the standard reflections  14   a  of light which passed through the outer openings  44 . Based on the distance between the central point and the outer points, the depth of the focus of the laser beam  14  can be calculated. 
         [0061]    In the present embodiment, a reflection as shown in  FIG. 7  is produced when using the mask  40  shown in  FIG. 3 . The image consists of a standard reflection  14   a  superimposed with a spurious reflection  14   b.  The points in the spurious reflection  14   b  have a similar configuration to the points in the standard reflection  14   a,  the configuration being determined by the openings  43 ,  44  of the mask  40 . However, the points in the spurious reflection  14   b  have a lower intensity than the points in the standard reflection  14   a.  The points of the spurious reflection  14   b  are furthermore more widely dispersed than the points of the standard reflection  14   a.  Finally, the spurious reflection  14   b  may be incomplete, with only some of the outer points registering in the image. 
         [0062]    In order to calculate the depth of the focus of the laser beam  14 , the superimposed image of  FIG. 7  must therefore be filtered, and the spurious reflection  14   b  removed, such that the depth of the focus of the laser beam  14  can be calculated based on the standard reflection  14   a.    
         [0063]      FIG. 5  illustrates the steps for filtering the superimposed image, such as the image shown in  FIG. 7 , in accordance with one embodiment. 
         [0064]    In a first step, a reference pattern  54  is determined based on the pattern of the mask  40 . As shown in  FIG. 8 , the reference pattern  54  consists of a superposition of the locations of the points produced at all possible focus depths of the laser beam  14 , as part of the standard reflection  14   a  of a particular mask  40 . A Fourier-transform is performed on the reference pattern  54  to produce a transformed reference pattern  56 , as shown in  FIG. 9 . 
         [0065]    The determination of the reference pattern  54  and calculation of the transformed reference pattern  56  may be performed only once, before the system  10  is put into use, and the transformed reference pattern  56  may be stored in a memory of the control computer  38 . In this way, these operations do not have to be repeated during each filtering operation; instead, the transformed reference pattern  56  corresponding to mask  40  may simply be retrieved from memory. 
         [0066]    The operations of the first step may be performed by the control computer  38 , or they may be performed on an external processor, and the results loaded into the memory of the control computer  38 . The remaining steps are performed by the control computer  38 . 
         [0067]    In a second step which may be performed before, after, or in parallel to the first step, the superimposed image  50  shown in  FIG. 7  is Fourier-transformed to produce a transformed superimposed image  52 . 
         [0068]    In a third step, the transformed superimposed image  52  is multiplied with the transformed reference pattern  56  to produce a transformed auxiliary image  58 , as shown in  FIG. 10 . 
         [0069]    In a fourth step, the transformed auxiliary image  58  is subjected to a reverse-Fourier-transform, to produce a further auxiliary image  60 , as shown in  FIG. 11 . The auxiliary image  60  represents a convolution of the superimposed image  50  with the reference pattern  54 . 
         [0070]    In a fifth step, the auxiliary image  60  is scanned to identify a highest-intensity point, indicated by an arrow in  FIG. 11 . The highest-intensity point is the brightest point in the auxiliary image  60 , and corresponds to the location of a center of the standard reflection  14   a.    
         [0071]    In a sixth step, the reference pattern  54  from  FIG. 8  is repositioned, such that the center point of the reference pattern  54  coincides with the location of the highest-intensity point identified in the auxiliary image  60  from  FIG. 11 . The repositioned reference pattern  62  is shown in  FIG. 12 . 
         [0072]    Finally, in a seventh step, the superimposed image  50  from  FIG. 7  is multiplied with the repositioned reference pattern  62  from  FIG. 12 , to produce a filtered image  64 , as shown in  FIG. 6 . The filtered image  64  is essentially identical to the standard reflection  14   a  produced by the mask  40 , and is substantially free of any spurious reflections  14   b.  The filtered image  64  can then be evaluated using known algorithms to determine the depth of the focus of laser beam  14 .