Technique for photodisruptive multi-pulse treatment of a material

Abstract

Embodiments of the invention provide a method and apparatus for laser-processing a material. In the embodiments, a diffraction-limited beam of pulsed laser radiation is diffracted by a diffraction device to generate a diffracted beam. The diffracted beam is subsequently focused onto the material and is controlled in time and space to irradiate the material at a target position with radiation from a set of radiation pulses of the diffracted beam so that each radiation pulse from the set of radiation pulses is incident at the target position with a cross-sectional portion of the diffracted beam, the cross-sectional portion including a local intensity maximum of the diffracted beam. The beam cross-sectional portions of at least a subset of the pulses of the set include each a different local intensity maxi-mum. In this way, a multi-pulse application for generating a photo-disruption at a target location of the material can be implemented.

Claims

1. An apparatus for laser-processing a material, the apparatus comprising: a laser source configured to provide a diffraction-limited beam of pulsed laser radiation; a diffraction device configured to diffract the diffraction-limited beam to generate a diffracted beam comprising a set of radiation pulses, each pulse having a plurality local intensity maxima where at least two local intensity maxima have different intensity values; a focusing objective configured to focus the diffracted beam onto the material; and a controller configured to control the diffracted beam to irradiate the material at a target position with a cross-sectional portion of the diffracted beam, the cross-sectional portion including the local intensity maxima of the radiation pulses, the controlling comprising directing each radiation pulse towards the target position with the cross-sectional portion in order to distribute the local intensity maxima of the radiation pulse across the material.

2. The apparatus of claim 1, wherein the beam cross-sectional portions are distinct when projected onto a transverse plane.

3. The apparatus of claim 1, wherein at least one pair of the beam cross-sectional portions are partially overlapping when projected onto a transverse plane.

4. The apparatus of claim 1, wherein the diffracted beam has a point distribution of the local intensity maxima in a focal area of the beam.

5. The apparatus of claim 4, wherein the point distribution is a two-dimensional distribution.

6. The apparatus of claim 5, wherein the two-dimensional distribution is one of a matrix distribution and a distribution based on concentric circles.

7. The apparatus of claim 4, wherein: at least a subset of the local intensity maxima of the diffracted beam are distributed along a line, and the controller is configured to control the diffracted beam to move the beam over the target position in the direction of the line.

8. The apparatus of claim 4, wherein: the controller is configured to control the diffracted beam to move the beam across the material transversely with respect to a beam propagation direction in accordance with a predetermined shot pattern to generate a photo-disruption at each of a plurality of shot positions defined by the shot pattern, and a distance between adjacent shot positions corresponds to a distance between adjacent local intensity maxima of the point distribution.

9. The apparatus of claim 1, wherein the local intensity maxima include two or more maxima of substantially equal intensity value.

10. The apparatus of claim 1, wherein each local intensity maximum of the diffracted beam is below a single-pulse intensity threshold for a laser-induced optical breakdown in human eye tissue.

11. The apparatus of claim 1, wherein the radiation from a temporally last pulse in the set of radiation pulses has highest intensity among the set.

12. The apparatus of claim 1, wherein spatially adjacent local intensity maxima of the diffracted beam have a distance of less than 20 ?m in a focal area of the beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 schematically illustrates components of an apparatus for laser-surgical treatment of a target material according to an embodiment; and

(3) FIG. 2 schematically illustrates exemplary relationships between target positions for laser irradiation and a transverse intensity distribution of a focused laser beam emitted by the apparatus of FIG. 1.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(4) FIG. 1 shows a laser apparatus for processing a target material using pulsed, focused laser radiation, the apparatus being generally denoted 10. In an exemplary situation which is illustrated in FIG. 1, the apparatus 10 is used for performing laser surgery on a human eye 12, as may be necessary in the case of an impaired vision or a disease of the eye 12. For example, the apparatus 10 may be used for creating one or more incisions in corneal tissue, lens tissue, vitreous strands or retinal tissue of the eye 12. Such incisions may be needed as part of an operation aimed at improving a patient's vision through refractive correction. An example type of refractive eye surgery is LASIK (Laser in-situ Keratomileusis). It is needless to say that the applicability of the apparatus 10 is not limited to generating incisions in the eye 12 in the course of a LASIK operation. Other types of eye surgical operation requiring the creation of one or more incisions in the eye 12 may be equally performed using the apparatus 10, such other types of operation including, but not limited to, intracorneal lenticule extraction, keratoplasty (lamellar or penetrating), cataract surgery, etc. Moreover, the apparatus 10 may be useful for laser processing a non-living material such as in a photo-lithographical application.

(5) The apparatus 10 may be particularly useful for applications requiring one or more strings of juxtaposed photo-disruptions to be generated in the target material in each of one or more x-y planes in a x-y-z coordinate system of the laser apparatus 10. As used herein, z refers to the longitudinal direction of the beam and x-y refers to a transverse plane with respect to the propagation direction of the beam. The string may be a rectilinear string or a curved string. A rectilinear string of photo-disruptions may be created each time the beam focus is moved along a rectilinear path portion of a serpentine scan path, which includes a plurality of rectilinear path portions extending in parallel to each other wherein adjacent ones of the rectilinear path portions are terminally connected by a reversing path portion. A serpentine scan pattern may be useful to generate a two-dimensionally extended incision in an x-y plane, e.g., a bed cut for a LASIK flap wherein the bed cut defines a stromal bed of the flap. A curved string of photo-disruptions, conversely, may be created as the beam focus is moved in an x-y plane along a curved, e.g., circular line such as may be necessary to generate in a LASIK operation a lateral incision extending from a peripheral edge of the bed cut to the anterior corneal surface.

(6) The apparatus 10 comprises a laser source 14, a beam expander 16, a diffraction device 18, a scanner 20, a focusing objective 22, a control unit 24, a memory 26 and a control program 28 stored in the memory 26 for controlling operation of the control unit 24.

(7) The laser source generates a diffraction-limited laser beam 30 comprised of a regular (i.e. periodic) train of pulses 32 of laser radiation. As can be seen from the schematic illustration of several of the laser pulses 32 in FIG. 1, the spatial (i.e. transverse) intensity distribution of the laser pulses 32 is Gaussian or near-Gaussian and includes a single intensity maximum. The wavelength of the laser radiation generated by the laser source 14 is suitably selected to ensure that the radiation emitted from the apparatus 10 can sufficiently penetrate into the target tissue of the eye 12 (or more general: the target material) to achieve a LIOB and a resulting photo-disruption through a multi-pulse application. For human eye treatment, for example, the laser wavelength may be in an infrared range between about 700 nm and about 1900 nm or may be in an ultraviolet range above about 300 nm. Other wavelengths may be suitable for the treatment of other materials. The pulse width of the laser pulses generated by the laser source 14 may be anywhere between attoseconds and nanoseconds and, for example, in a two-digit or three-digit femtosecond range.

(8) The beam expander 16 expands the laser beam 30 in a manner generally known per se, using e.g., a Galilei telescope comprising a diverging lens and a converging lens arranged downstream of the diverging lens with respect to the propagation direction of the laser beam 30. The expanded laser beam output from the beam expander 16 is denoted 30.sub.exp in FIG. 1 and is comprised of a periodic train of laser pulses 32.sub.exp. As schematically illustrated in FIG. 1, the laser pulses 32.sub.exp of the expanded laser beam 30.sub.exp have a larger cross-sectional area, but smaller maximum intensity than the laser pulses 32 of the diffraction-limited laser beam 30.

(9) The diffraction device 18 is effective to diffract the expanded laser beam 30.sub.exp to generate a diffracted laser beam 30.sub.diff. The diffracted laser beam 30.sub.diff is comprised of a regular train of diffracted laser pulses 32.sub.diff. As schematically illustrated in FIG. 1, the diffracted laser pulses 32.sub.diff each have a spatial (i.e. transverse) intensity distribution showing a plurality of local intensity maxima 36.sub.i (with the index i taking values from 1 to N, wherein N indicates the total number of local intensity maxima of the diffracted laser pulse 32.sub.diff). The diffraction pattern, i.e. the transverse intensity distribution, is the same for all diffracted pulses 32.sub.diff of the train. As is easy to understand, a pair of spatially adjacent local intensity maxima will be separated by a local intensity minimum (not specifically denoted in the drawings).

(10) In the exemplary case shown in FIG. 1, the diffracted pulses 32.sub.diff each have a total of two local intensity maxima 36.sub.1, 36.sub.2. It is to be understood that the apparatus 10 is not intended to be limited to generating diffracted laser pulses having exactly two intensity maxima. Instead, the diffraction device 18 may be configured to generate diffracted laser pulses having any number of local intensity maxima greater than two, e.g., three, four, five or six intensity maxima. These maxima may have a one-dimensional distribution pattern such as, e.g., along a rectilinear line, or a two-dimensional distribution pattern such as, e.g., a matrix pattern.

(11) In the exemplary case shown in FIG. 1, the local intensity maxima 36.sub.1, 36.sub.2 of each diffracted pulse 32.sub.diff have different intensities. It is to be understood that in other embodiments the local intensity maxima 36.sub.1, 36.sub.2 may be of substantially equal intensity. In general and regardless of the total number of local intensity maxima, the diffracted beam 30.sub.diff may have a cross-sectional intensity distribution exhibiting two or more local intensity maxima of substantially equal magnitude and, alternatively or additionally, two or more local intensity maxima of unequal magnitudes.

(12) The diffraction device 18 includes at least one diffraction member having a diffracting effect for the laser radiation as the radiation traverses the diffraction member. An exemplary diffraction member that can be used in the diffraction device 18 is a Diffractive Optical Element (DOE), which is commonly understood as referring to an optical element having a transparent substrate (e.g., a glass substrate) which has been patterned through a photo-lithographical process to have one or more micro-grating structures that are effective to convert an original beam pattern into a different beam pattern. For example, the diffraction device 18 may be configured to convert the transverse (i.e. x-y) beam pattern of the laser beam 30.sub.exp into a dot line pattern or a dot matrix pattern of the diffracted beam 30.sub.diff, wherein each dot of the diffraction pattern includes a local intensity maximum of the diffracted beam 30.sub.diff. A holographic optical element (HOE) is another example of a diffraction member that is useful to achieve the desired diffraction effect for the laser radiation.

(13) In embodiments not specifically shown herein, the diffraction device 18 may be disposed upstream of the beam expander 16.

(14) The focusing objective 22 focuses the diffracted beam 30.sub.diff, resulting in a focused laser beam 30.sub.foc (schematically indicated by dotted lines in FIG. 1). The focusing objective 22 may, e.g., be of a F-Theta type and may be a single-lens objective or multi-lens objective. The focused laser beam 30.sub.foc is comprised of a periodic train of focused laser pulses 32.sub.foc, one of which is schematically shown for illustration purposes in FIG. 1. The repetition rate of the focused laser pulses 32.sub.foc emitted from the apparatus 10 is in a kHz, MHz or GHz range and, for example, in a range from 50 kHz to 5 MHz or from 5 MHz to 50 MHz or from 50 MHz to 100 MHz or from 100 MHz to 500 MHz or up to a range of 1 GHz or higher.

(15) The apparatus 10 is equipped with suitable scanning structure to allow for longitudinal adjustment of the focus position of the focused laser beam 30.sub.foc in z-direction (i.e. in the direction of beam propagation) and to allow for transverse adjustment of the focus position in an x-y plane. For x-y scanning of the beam focus, the scanner 20 may include, in a manner generally known per se in the art, a pair of scanning mirrors 37 which are disposed to be tiltable about mutually orthogonal tilt axes, as schematically indicated in FIG. 1 inside the box representing the scanner 20. For z-scanning of the beam focus, the beam expander 16 may include an optical element (not shown in the drawings) configured to be suitably adjustable so as to impose a variable degree of divergence on the expanded laser beam 30.sub.exp. Such optical element may, e.g., be constituted of a lens of variable refractive power or a lens disposed to be positionally adjustable in the direction of beam propagation. In different embodiments, other parts of the apparatus 10 such as, for example, the scanner 20 or the focusing objective 22 may be equipped with z-scanning capability.

(16) The control unit 24 controls the overall operation of the apparatus 10 under control of the control program 28 and particularly controls the operation of the laser source 14 and the scanning structure of the apparatus 10 including the scanner 20. The control program 28 defines a shot pattern consisting of a plurality of shot positions each represented by a set of x, y and z-coordinate values in the x-y-z coordinate system of the apparatus 10, wherein the shot pattern is so designed as to result in an incision of a desired geometry in the eye 12. Each shot position corresponds to the emission of one laser shot (i.e. one focused pulse 32.sub.foc) by the apparatus 10.

(17) As the focus of the focused beam 30.sub.foc is moved in transverse direction (i.e. in an x-y plane) across a target region of the eye 12 (which target region may be on an outer surface of the eye 12 or within the eye 12) in accordance with the shot pattern, the same location on or in the eye 12 is successively irradiated with radiation from a plurality of the focused pulses 32.sub.foc, and a photo-disruption is generated in the eye tissue at the location as a cumulative effect of the deposition of energy from the compound of sub-threshold pulses in the tissue. This is explained in further detail below with additional reference to FIG. 2.

(18) FIG. 2 shows by way of non-limiting example an outline of a bed cut 38 in an x-y plane. The bed cut 38 is a two-dimensionally extended incision in an x-y plane and may serve to define a stromal bed for a corneal flap that is created in the cornea of the eye 12 in the course of a LASIK procedure. 38a denotes a hinge line along which the flap remains connected with surrounding corneal tissue so that the flap can be folded aside to expose underlying corneal tissue for the subsequent removal of a predefined volume of tissue using UV laser radiation (e.g., excimer laser radiation). To generate the bed cut 38, a photo-disruption is to be effected at each of a plurality of damage sites juxtaposed in an x-y plane, so that the tissue damage associated with the plurality of photo-disruptions results in the creation of the bed cut 38.

(19) A portion of a beam shot pattern for creating the bed cut 38 is schematically visualized in FIG. 2 on the right-hand side of the bed cut 38 and includes shot positions 40 arranged in a matrix style in rows and columns. FIG. 2 further shows schematically four exemplary dot patterns 42a, 42b, 42c, 42d of the focused beam 30.sub.foc. The dot patterns 42a, 42b, 42c, 42d are a graphical tool to represent the x-y energy distribution (and hence the x-y beam pattern) of the focused beam 30.sub.foc in the area of the beam focus; most, if not all, of the energy is concentrated in the regions represented by the dots (dot regions) and only little, if any, radiation energy is encountered outside of these regions. Each dot pattern 42a, 42b, 42c, 42d corresponds to a different configuration of the diffraction device 18 of the apparatus 10. Every dot of a dot pattern represents a distinct cross-sectional (i.e. x-y) segment of the focused beam 30.sub.foc and can indicate a respective local intensity maximum 36.sub.i of the focused beam 30.sub.foc. In the illustrated example, different colors of the dots of a dot pattern represent different intensities of the local intensity maxima 36.sub.i of the dot regions and/or may represent different energies of the dot regions. More specifically, in the illustrated example cases of FIG. 2 a black dot represents a local intensity maximum 36.sub.i of larger intensity and/or a greater energy than a grey dot, and a grey dot represents a local intensity maximum 36.sub.i of larger intensity and/or a greater energy than a white dot.

(20) The dot patterns 42a, 42b are each configured as a dot line pattern, i.e. their dots are distributed along a single line, which is a rectilinear line in the illustrated example cases. For the dot patterns 42a, 42b, the focused beam 30.sub.foc includes in each case a total of three local intensity maxima 36.sub.i, resulting in a total of three dots for each of the patterns 42a, 42b. In the dot pattern 42a, the dots represent local intensity maxima 36.sub.i of different intensities, as indicated by the different colors of the dots of the dot pattern 42a. An associated exemplary transverse intensity distribution 44a is depicted in FIG. 2 on the right-hand side of the dot pattern 42a. As can be seen, the intensity distribution 44a exhibits local intensity maxima 36.sub.1, 36.sub.2, 36.sub.3 of different intensities.

(21) In the dot pattern 42b, conversely, the dots represent local intensity maxima 36.sub.i of the same, or substantially the same, intensity, as indicated by the same color for all dots of the dot pattern 42b. An associated exemplary transverse intensity distribution 44b is depicted in FIG. 2 on the right-hand side of the dot pattern 42b. As can be seen, the intensity distribution 44b exhibits local intensity maxima 36.sub.1, 36.sub.2, 36.sub.3 of equal intensity.

(22) The dot patterns 42c, 42d are each configured as a dot matrix pattern, i.e. their dots are arranged in an m?n matrix having a number m of rows and a number n of columns (wherein m and n are integers greater than 1). Specifically, the dot pattern 42c is a 3?5 matrix of dots, and the dot pattern 42d is a 3?3 matrix of dots. Within a row of the matrix, the focused beam 30.sub.foc may have local intensity maxima of equal intensity (as in the case of the dot pattern 42c) or of different intensities (as in the case of the dot pattern 42d). But each row represents the same, or substantially the same, intensity distribution as any other row of the matrix.

(23) In certain embodiments, the x-y cross section of the focused beam 30.sub.foc in the focal area thereof exhibits a concentration of energy to circular segments, such as illustrated by the circular shape of the dots shown in FIG. 2. It should nevertheless be pointed out that the scope of the present disclosure is in no way intended to be limited to such embodiments and that the focused beam 30.sub.foc may exhibit in its focal area any suitable x-y distribution of energy coming with a plurality of spatially dispersed local intensity maxima. The concept of a dot pattern of the focused beam 30.sub.foc is only used herein for the purpose of facilitating an understanding of the invention and particularly the concept of creating a photo-disruption in a target material by spatially superimposing radiation from at least partially non-overlapping transverse segments of temporally successive pulses of a diffracted laser beam.

(24) The diameter of each dot region may be between 1 ?m and 10 ?m or between 2 ?m and 8 ?m or between 3 ?m and 6 ?m, and may be substantially equal to the focus diameter of an un-diffracted beam that can be generated by the apparatus 10 after removal of the diffraction device 18.

(25) The mutual distance of adjacent shot positions 40 of the shot pattern in an x-y plane is denoted d1 in FIG. 2 and is, e.g., in a range between 1 ?m and 10 ?m or between 2 ?m and 8 ?m or between 3 ?m and 6 ?m. The mutual distance of adjacent local intensity maxima 36.sub.i of the focused beam 30.sub.foc (in the area of the beam focus) in an x-y plane is denoted d2 in FIG. 2 and is substantially equal to the distance d1. An x-y scan path for the focused beam 30.sub.foc may be defined as a serpentine scan path as schematically depicted at 46 in FIG. 2, wherein the serpentine scan path 46 includes mutually parallel, rectilinear path portions 46a terminally connected by reversing path portions 46b.

(26) Accordingly, as the focused beam 30.sub.foc is moved across the shot positions 40 in an x-y plane according to a pre-defined scan path such as, e.g., the serpentine scan path 46, the same location on or in the eye 12 is successively irradiated with radiation from different beam cross-sectional portions from a set of pulses of the focused beam 30.sub.foc. For example, considering a diffraction pattern of the focused beam 30.sub.foc corresponding to the dot line pattern 42a, a first pulse of the focused beam 30.sub.foc irradiates the eye 12 at a specific location associated with one of the shot positions 40 with radiation from one of the dots, e.g. the left, white dot representing lowest peak intensity among the dots of the dot pattern 42a. As the focused beam 30.sub.foc is moved by the distance d1 between successive pulses according to the pre-defined scan path, a subsequent second pulse of the focused beam 30.sub.foc applies radiation from another dot of the focused beam 30.sub.foc, e.g. the middle, grey dot representing medium peak intensity, to the same location, i.e. the same shot position 40. As the focused beam 30.sub.foc is thereafter moved yet another time by the distance d1 in accordance with the pre-defined scan path, a third pulse of the focused beam 30.sub.foc applies radiation from a third dot, e.g. the right, black dot representing highest peak intensity, to the same location of the eye 12 and eventually causes a photo-disruption in the eye tissue at the corresponding shot position 40. Similar considerations apply when the focused beam 30.sub.foc has an energy/intensity distribution corresponding to the dot pattern 42b.

(27) In this way, a multi-pulse application can be implemented using the diffracted, focused beam 30.sub.foc. The photo-disruption results from the deposition of energy from different cross-sectional portions of the focused beam 30.sub.foc in the irradiated material over a series of pulses of the beam. The necessary threshold for causing the photo-disruption may be reached using beam cross-sectional portions of different peak intensity/energy (as in the case, e.g., of the dot pattern 42a) or beam cross-sectional portions of substantially equal peak intensity/energy (as in the case, e.g., of the dot pattern 42b). In preferred embodiments, the last pulse of the series of pulses that are incident at a specific location of the irradiated material eventually triggers the photo-disruption in the material. In other words, the applicable threshold for photo-disruption is only surpassed in such embodiments with the arrival of the last pulse of the series.

(28) Owing to the fact that the focused beam 30.sub.foc is a diffracted beam having its energy spread over an area that covers a plurality of shot positions 40, and further owing to the fact that the focused beam 30.sub.foc is moved in an x-y plane between successive pulses by only the distance d1, a plurality of shot positions 40 can be irradiated with radiation from the focused beam 30.sub.foc at a time. For a given pulse repetition rate of the focused beam 30.sub.foc and a given x-y scanning speed of the beam, this allows to reduce the overall time needed for generating a desired incision (e.g., the bed cut 38 or a posterior or anterior cut for an intra-corneal lenticule (not shown)), as compared with a conventional multi-pulse application that uses a diffraction-limited laser beam to place a plurality of successive pulses at the same shot position before scanning the beam to an adjacent shot position.

(29) A further reduction of the overall processing time may be achieved by diffracting the laser beam to generate a matrix dot pattern such as the pattern 42c or the pattern 42d. A two-dimensional dot pattern such as the pattern 42c or the pattern 42d allows to achieve an irradiation of the target material simultaneously at shot positions 40 in a plurality of parallel lines, so that the pitch (distance) between adjacent rectilinear path portions 46a of the serpentine scan path 40 can be increased in correspondence to the number of lines of shot positions 40 covered by the matrix dot pattern. A two-dimensional dot pattern such as the pattern 42c or 42d may be generated, e.g., using a two-dimensional optical grating or a hologram.

(30) In the dot patterns 42c, 42d, the dot regions may each represent a partial beam of the diffracted beam wherein each partial beam has an associated focus. According to certain embodiments, the foci of the partial beams all have the same, or substantially the same, z-position. According to other embodiments, the foci of the partial beams are not all in the same x-y plane. For example, in the dot pattern 42d the focus position may be constant in z-direction as one moves from partial beam to partial beam in a row direction of the matrix (i.e. horizontally in the drawing) whereas the focus position may be vary in z-direction as one moves from partial beam to partial beam in a column direction of the matrix (i.e. vertically in the drawing). Thus, while the partial beams associated with a triplet of white, grey and black dots from the same row of the matrix may have their foci located at the same z-position, the partial beams associated with the three black dots may have different z-positions of their foci (and similarly for the partial beams associated with the three grey dots and the partial beams associated with the three white dots).