Method for energy calibration of a pulsed cutting laser for eye surgery

Abstract

A method for energy calibration of a pulsed cutting laser for eye surgery comprises irradiating a sample material with a plurality of sets of laser pulses of the cutting laser with pulse energies differing from set to set. This method also comprises analyzing at least one visually perceptible discoloration structure created in the sample material as a result of the irradiation, selecting the pulse energy of one of the sets based on the analysis, and setting a treatment pulse energy for the cutting laser based on the selected energy.

Claims

1. A method for energy calibration of a pulsed cutting laser for eye surgery, comprising: irradiating a sample material with a plurality of sets of laser pulses of the cutting laser with pulse energies differing from set to set; analyzing at least one visually perceptible discoloration structure created in the sample material as a result of the irradiation; selecting the pulse energy of one of the sets based on the analysis; and setting a treatment pulse energy for the cutting laser based on the selected energy.

2. The method of claim 1, wherein the sample material is transparent.

3. The method of claim 1, wherein the sets are each irradiated at different regions of the sample material.

4. The method of claim 3, wherein the different regions of the sample material are separated from one another by visible marks.

5. The method of claim 3, wherein the sample material is provided with written indications, which define a pulse energy in local assignment for each one of the regions.

6. The method of claim 3, wherein: a plate-like piece of material comprising PMMA is used as the sample material; and the sets are each irradiated at different plate regions of the piece of material.

7. The method of claim 1, wherein: each one of the sets corresponds to a nominal geometric figure; and the pulse energy of such a set is selected, at which a discoloration structure is created in the sample material, which completely represents the nominal geometric figure.

8. The method of claim 7, wherein the nominal geometric figure is or comprises a line figure.

9. The method of claim 8, wherein the line figure is closed in the form of a ring.

10. The method of claim 7, wherein the pulse energy of the set having the lowest pulse energy is selected, at which a discoloration structure is created in the sample material, which completely represents the nominal geometric figure.

11. The method of claim 1, wherein the pulse energy of such a set is selected, at which needle-like discoloration structures, which are created in the sample material as a result of the irradiation with the set, satisfying at least a certain size condition.

12. The method of claim 11, wherein the size condition is, that a needle length of the needle-like discoloration structures accounts to at least a given reference length.

13. The method of claim 11, wherein a size condition is, that at least a subset of the needle-like discoloration structures each has a needle length amounting to at least a given reference length.

14. The method of claim 1, further comprising: optically detecting the at least one discoloration structure by using a camera.

15. The method of claim 14, further comprising: displaying a camera image on a display screen, the camera image showing the at least one discoloration structure.

16. The method of claim 14, wherein analyzing comprises a software-assisted analysis of a camera image showing the at least one discoloration structure.

17. The method of claim 1, wherein the laser pulses have a pulse duration in the range of atto-, femto- or picoseconds.

18. The method of claim 1, further comprising: determining a measured pulse energy of the cutting laser by means of an energy meter; and adjusting, based on the measured pulse energy, a scale of an energy value displayed at the cutting laser as current energy, such that subsequently the energy value displayed at the cutting laser as current energy corresponds to an actual pulse energy.

19. The method of claim 1, further comprising: determining a ratio between a known reference energy value for the creation of a desired discoloration structure and the selected energy; determining an effective energy as the product of an energy value currently set as pulse energy at the cutting laser and the ratio; and displaying the effective energy to a user of the cutting laser, wherein the step of setting is carried out based on the effective energy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Supplementary features, advantages and components of the present invention can be derived from the following description of the accompanying drawings, in which:

(2) FIG. 1 shows a schematic block diagram of a device known from the prior art for laser processing of a human eye;

(3) FIG. 2 shows a flow chart for an exemplary embodiment of a method according to the invention;

(4) FIG. 3A shows a schematic top view of an exemplary embodiment of a sample material which can be used for a method according to the invention; and

(5) FIG. 3B shows an exemplary side view of discoloration structure needles in a sample material.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(6) FIG. 1 shows in a block diagram an exemplary device from the prior art, which is labeled as 10 in general, for laser processing of a human eye 12. The device 10 comprises a control unit 14, a laser arrangement 16 and a patient adapter 17. The device 10 is an example of a pulsed cutting laser, which can be calibrated by the method according to the invention.

(7) The laser arrangement 16 comprises a laser source 18 which generates a laser beam 20 having pulse durations in the femtosecond range, for example. The laser beam 20 has a suitable wavelength for generating a laser-induced optical breakdown in the corneal tissue of the eye 12. The laser beam 20 may have a wavelength in the range of 300 nm (nanometer) to 1900 nm, for example, a wavelength in the range of 300 nm to 650 nm, 650 nm to 1050 nm, 1050 nm to 1250 nm or 1100 nm to 1900 nm. The laser beam 20 may also have a focus diameter of 5 m or less.

(8) Behind the laser source 18 in the direction of propagation of the laser beam 20 (indicated by the arrows in FIG. 1) a beam widening optical system 22, a scanner unit 24, a mirror 26 and a focusing lens 28 are arranged. The beam widening optical system 22 serves to enlarge the diameter of the laser beam 20 generated by the laser source 18. In the example shown here, the beam widening optical system 22 is a Galileo telescope, comprising a concave lens (lens having a negative refractive power) and a convex lens (lens having a positive refractive power) situated downstream from the concave lens in the direction of propagation of the laser beam 20. This may be a planar concave lens and a planar convex lens whose planar sides are arranged so they face one another. In another example, the beam widening optical system may comprise, for example, a Kepler telescope, which has two convex lenses, as an alternative to a Galileo telescope.

(9) The scanner unit 24 is designed to control the position of the focus of the laser beam 20 (beam focus) in the transverse direction and in the longitudinal direction. The transverse direction describes the direction across the direction of propagation of the laser beam 20 (characterized as the x-y plane) and the longitudinal direction describes the direction of propagation of the laser beam 20 (characterized as the z direction). The scanner unit 24 may comprise, for example, a pair of galvanometrically actuated deflecting mirrors, which can be tilted about mutually orthogonal axes, for transverse deflection of the laser beam 20. Alternatively or additionally, the scanner unit 24 may have an electrooptical crystal or other components suitable for transverse deflection of the laser beam 20. The scanner unit 24 may also comprise a lens that is adjustable longitudinally or has a variable refractive power or may comprise a deformable mirror to influence the divergence of the laser beam 20 and, consequently, the longitudinal alignment of the beam focus. In the example shown here, the components for control of the transverse orientation and longitudinal orientation of the beam focus are depicted as integral components. In another example, the components may be arranged separately along the direction of propagation of the laser beam 20. Thus, an adjustable mirror, for example, may be arranged in front of the beam widening optical system 22 in the direction of propagation, for control of the longitudinal orientation of the beam focus.

(10) The mirror 26 may be a stationary deflecting mirror, which is designed to deflect the laser beam 20 in the direction of the focusing lens 28. Additionally or alternatively, other optical mirrors and/or optical elements may be arranged in the beam path for deflecting and diffracting the laser beam 20.

(11) The focusing lens 28 is designed to focus the laser beam 20 on the region of the cornea of the eye 12 to be processed. The focusing lens 28 may be, for example, an F-theta lens. The focusing lens 28 is releasably connected to the patient adapter 17. The patient adapter 17 comprises a conical carrier sleeve 30, which is connected to the focusing lens 28 by means of a coupling formation (not shown), and a contact element 32 which is attached to the narrower bottom side of the carrier sleeve 30 facing the eye 12. The contact element 32 may be attached to the carrier sleeve 30 either releasably (for example, by screw connection) or permanently (e.g., by adhesive bonding). The contact element 32 has a bottom side which faces the eye 12 and is characterized as a contact face 34. The contact face 34 in the example shown here is embodied as a flat surface. In laser processing of the eye 12 the contact element 32 is pressed against the eye 12 or the eye 12 is drawn to the contact surface 34 by vacuum suction, such that at least the region of the cornea of the eye 12 to be processed is leveled.

(12) The control unit 14 comprises a memory 36, in which at least one control program 38 having program instructions is stored. The laser source 18 and the scanner unit 24 are controlled by the control unit 14 in accordance with the program instructions. The control program 38 includes program instructions, which, when executed by the control unit 14, produce a movement of the beam focus in space and time, such that a cutting figure is created in the cornea of the eye 12 to be treated.

(13) FIG. 2 shows a flow chart for an exemplary embodiment of a method according to the invention for energy calibration of a pulsed cutting laser for eye surgery. The pulsed cutting laser may be, for example, the device 10 shown in FIG. 1.

(14) The method for energy calibration of a pulsed cutting laser for eye surgery comprises at least the following steps S1-S4:

(15) Irradiating S1 a sample material with a plurality of sets of laser pulses of the cutting laser with pulse energies differing from set to set;

(16) Analyzing S2 at least one visually perceptible discoloration structure created in the sample material as a result of the irradiation;

(17) Selecting S3 the pulse energy of one of the sets based on the analysis; and

(18) Setting S4 a treatment pulse energy for the cutting laser based on the selected energy.

(19) According to the exemplary embodiment, the steps S1, S2, S3 and S4 mentioned above are performed in this order.

(20) FIG. 3A shows an exemplary embodiment of a sample material 40 which is subdivided visibly (for example, due to printing or perforation) into a plurality of regions 42a-42f. The individual regions 42a-42f are delimited from one another by visible marks. Each of the regions 42a-42f is characterized by written notation of a nominal energy value (0.25 J, 0.30 J, 0.35 J, etc.). FIG. 3A shows the sample material 40 in a view from above, so that the plane of the drawing is parallel to the x-y plane when the sample material 40 is used with the cutting laser from FIG. 1. According to one exemplary embodiment, the sample material 40 may be a thin plate of PMMA.

(21) According to one exemplary embodiment, the method for energy calibration of the pulsed cutting laser is carried out with the help of the sample material 40, so that a first energy value for the pulse energy (for example, 0.25 J) is set first on a setting device of the cutting laser.

(22) The setting device may optionally first be calibrated with the help of a power meter so that an energy value set on the cutting laser corresponds to an energy value measured by the power meter. The calibration may be necessary because the cutting laser can become decalibrated, so that a displayed energy value that has been set on the cutting laser no longer corresponds to the actual pulse energy. Due to the calibration, it is possible to achieve the result that the actual pulse energy of the cutting laser is always displayed for the user when an energy value is being set.

(23) This may be accomplished by adjusting a scale of an energy value displayed as the instantaneous energy on the cutting laser. A sensor of the power meter (for example, a commercial power meter) may be arranged either in the main beam path of the cutting laser or in a secondary beam path branching off through a beam splitter (with a known splitting ratio) for measuring the power.

(24) The sample material 40 is irradiated with the cutting laser from above (perpendicular to the plane of the drawing in the representation of FIG. 3A), whereupon the pulse energy of the cutting laser is set at the first energy value. A certain nominal geometric figure is traced hereby (for example, a line, a wavy line, a circle, a rectangle, an ellipse, etc.) by the cutting laser in the region 42a which is indicated by the corresponding energy. The nominal geometric figure may be a line figure, for example, which may be closed in the form of a ring (circle, rectangle, ellipse, etc.). The nominal geometric figure may also have at least one meandering region. Depending on the pulse energy of the cutting laser and other properties, such as the pulse duration, the beam widening at the focal point and/or the position of the focal point relative to the sample material 40, there is already photodisruption and discoloration in the sample material 40 with the first energy. The discoloration created in a respective region of the sample material 40 at a certain pulse energy is referred to below as a discoloration structure. It may be that, at the low first energy level set, either no discoloration structure at all is created or it is not continuous, so there are gaps in the discoloration structure in comparison with the nominal geometric figure.

(25) Next, the pulse energy of the cutting laser is set at a second energy value (for example, 0.30 J) and a second region 42b of the sample material 40 is traced with this energy and the same pattern as before. Additional regions 42c-f of the sample material 40 are then irradiated successively with sets of laser pulses of a corresponding pulse energy, wherein the irradiation pattern remains the same for each of the regions 42a-f.

(26) Alternatively, it is possible to trace only one large pattern, for example, wherein the pulse energy of the cutting laser is increased incrementally while tracing the pattern. Furthermore, any sequence of the set energies may be used and it is also possible to begin with a high energy, for example, and then gradually reduce it in the course of the method.

(27) Next, the visually perceptible discoloration structures created in the sample material 40 due to the irradiation are analyzed. The plurality of discoloration structures consists of the individual discoloration structures of the various regions 42a-f and thus includes at least one discoloration structure. The analysis may be performed with the naked eye, for example, or with the eye and an optical aid such as a microscope or a magnifying glass. However, it is also possible to use a camera with a corresponding magnification lens for the analysis. If the analysis is performed in a view from above, it is possible to analyze which of the discoloration structures are sufficiently continuous, for example, and which are not (for example, if they have gaps or if the discoloration is too thin). The discoloration structures produced can therefore be compared with the nominal geometric figures. For example, it is possible to determine which of the discoloration structures correspond to the nominal geometric figure and which have gaps and/or an incomplete discoloration.

(28) Alternatively or additionally, as shown in FIG. 3B, the analysis may be performed in a side view of the sample material 40. FIG. 3B shows a side view of the sample material 40, for example, the sample material 40 shown in FIG. 3A. When the analysis is performed by considering a side view, it may be beneficial to irradiate the sample material only in an edge region of the sample material. Thus, the discoloration structures may be viewed in side view without cutting or breaking the sample material in advance. Therefore, for example, a polygon may be used, the side edges of which are irradiated by the cutting laser with different pulse energies.

(29) As shown in FIG. 3B, the pulsed laser beam has left behind a plurality of needle-like discoloration structures (discoloration structure needles) in the sample material 40 due to photodisruption. The needle-like discoloration structures are oriented in the sample material 40 along the incident direction of the cutting laser (z direction in FIG. 1), wherein their length l varies along this incident direction, depending on the pulse energy and beam quality (diameter and pulse duration, respectively) of the cutting laser used with the cutting laser.

(30) For a certain pulse energy, the needle-like discoloration structures have a characteristic length l, which increases with an increase in the pulse energy. For example, the length l of the needle-like discoloration structures can be determined visually (with a user's eye) using a suitable enlargement device (e.g., a microscope, a magnifying glass, etc.) and a corresponding scale. However, it is also possible to merely compare whether or not the length l of the needle-like discoloration structures exceeds a certain reference length, for example. Further, it is possible to compare whether or not the length l of the needle-like discoloration structures falls below a certain reference length. To compensate for extreme individual length values, it is possible to analyze whether a certain subset of needle-like discoloration structures has a needle length amounting to at least a given reference length.

(31) The achieved (measured) length l may be compared to a nominal length, which corresponds to the nominal result of photodisruption. The display of an effective energy (see below) may then be performed on the basis of this comparison.

(32) The analysis in a view from above as well as the analysis in a side view can be performed with the help of a computer-controlled analysis device, wherein a CCD sensor or a CMOS sensor of a camera, for example, records an image of the discoloration structure, which is then analyzed in a computer-assisted process (optionally fully automatically). Further, a light source may be provided for illuminating the sample material and the discoloration structure. The light source may be configured, for example by providing suitable filters, to emit only light of a certain frequency band and/or to sequentially emit light of different frequency bands.

(33) Next, the pulse energy of one of the sets is selected on the basis of the previous analysis. For example, the pulse energy which corresponds to the lowest set energy that has led to a complete (continuous) visible discoloration structure in the sample material 40 may be selected. To do so, the discoloration structures that are created can be compared with a reference structure (of the nominal geometric figure). Alternatively, the pulse energy which corresponds to the lowest set energy that has resulted in a discoloration structure with needle-like discoloration structures whose length exceeds a certain reference length may be selected. The two selection methods may also be combined, so that the pulse energy selected corresponds to the lowest set energy that has led to a complete (continuous) visible discoloration structure in the sample material 40 with needle-like discoloration structures, whose length l exceeds a certain reference length. The selection may be made by hand or by a computer and/or by a control unit of the cutting laser.

(34) Next, a treatment pulse energy for the cutting laser is set on the basis of the selected energy. The treatment pulse energy may be set, for example, so that it corresponds to the product of the selected energy and a certain factor. If it is known, for example, that a suitable treatment pulse energy is twice as high as the selected energy, which reliably leads to a visible discoloration structure in the sample material 40, then the factor amounts to two, for example. However, the factor may also be less than one or the treatment pulse energy may be set so that there is a constant energy offset between the selected energy and the treatment pulse energy.

(35) To facilitate the selection of the treatment pulse energy, an effective energy may be displayed for the user of the cutting laser as follows: first, a factor representing the ratio between a known reference energy for the creation of the desired discoloration structure and the selected energy is calculated. The known reference energy may be a pulse energy of the cutting laser of which it is known that under the usual conditions it leads to an adequate degree of photodisruption and/or to a continuous visible discoloration structure in the sample plate. If the selected energy deviates greatly from this reference energy, this is a sign that the usual conditions do not prevail and the cutting beam is highly defocused, for example. However, the known reference energy may also be a dimensionless value such as one, for example.

(36) In addition, an effective energy, which corresponds to the product of an energy value currently set on the cutting laser and the factor calculated previously is also determined. The effective energy is displayed for the user of the cutting laser, so that the user is provided with an energy scale that takes into account the actual cutting performance of the cutting laser.

(37) The step of setting the treatment pulse energy is carried out on the basis of this effective energy. The effective energy may be displayed simultaneously with a display of the set energy, for example, while the user of the cutting laser is changing the pulse energy of the cutting laser. The user of the cutting laser can be certain, on the basis of the effective energy displayed, that a certain effective energy will always lead to a reproducible cutting result on the human cornea. In other words, the displayed effective energy allows for conclusions regarding the result of photodisruption to be expected.

(38) A brief example of the method presented above is described below. The sample material shown in FIG. 3A can be used to irradiate regions thereof using the nominal energy values indicated thereon. Analysis of the discoloration structures then yields, for example, the fact that discoloration structures which do not correspond to the nominal geometric figure because they contain gaps are present in the regions 42a-d. However, the regions 42e and 42f each have a discoloration structure, which corresponds to the nominal geometric figure. The energy which is the lowest energy leading to a discoloration structure, which corresponds to the nominal geometric figure is chosen as the selected energy value. In this example the selected energy thus corresponds to 0.45 J of the region 42e.

(39) However, it may be known from previous investigations that under certain normal conditions (for example, with an ideally focused laser beam) an adequate discoloration structure is created in a comparable sample material (for example, PMMA) even at 0.35 J. The reference energy is therefore 0.35 J. Thus, 0.35 J/0.45 J=0.77 is calculated as the ratio between the known reference energy value and the selected energy.

(40) Next the energy value current set on the cutting laser is multiplied times the factor 0.77 to calculate the effective energy. For example, if the energy set is 0.45 J, then the effective energy value 0.45 J*0.77=0.35 J is displayed as the effective energy value. The choice of the treatment pulse energy may thus be made on the basis of the effective energy because this takes into account the real conditions of the cutting laser (for example, focus, etc.).

(41) The energy calibration according to the present invention is thus a calibration which takes into account not only the pulse energy of a cutting laser butbecause actual discolorations in a sample material are analyzedother factors, which contribute to the quality of the cut that is made and/or to the degree of photodisruption, are also taken into account.