WO1999038443A1

WO1999038443A1 - Scanning method of high frequency laser

Info

Publication number: WO1999038443A1
Application number: PCT/US1999/002015
Authority: WO
Inventors: Fuqian Tang
Original assignee: LaserSight Technologies Inc
Current assignee: LaserSight Technologies Inc
Priority date: 1998-02-03
Filing date: 1999-01-29
Publication date: 1999-08-05
Anticipated expiration: 2000-08-03
Also published as: AU2486199A

Abstract

In a method of ablating tissue with a scanning laser beam (432) at an increased effective ablation rate, a solid state laser is pulsed through a pulse sequence having variable repetition rate portions. For instance, one or more first periods of time (T), during the pulse sequence, the laser is pulsed at a repetition rate faster than that allowed by a scanning mechanism, and one or more second periods of time (T) no laser pulses are output to the tissue in one embodiment, during the first period(s) of time. The scanning mirror(s) (416) are either stationary or are moved within very small confines. In the same embodiment, during second period(s) of time, the scanning mirror(s) are allowed to make a major movement, i.e., sufficient to move the laser beam spot across the surface of the tissue from a first ablation point to a second ablation point substantially apart from the first ablation point. The present invention is particularly useful for ablating tissue with a scanning laserbeam source (432) from a low power solid-state laser, for example, a flash lamp or diode pumped UV laser, or flash lamp, or diode pumper IR laser, or laser diode.

Description

- 1 -

SCANNING METHOD OF HIGH FREQUENCY LASER

This application is related to and claims priority from a co- pending US provisional application, serial number 60/073,553, entitled "Scanning Method of High Frequency Laser", and filed on February 3, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates generally to a method of ablating tissue, and more particularly, it relates to a method of ablating tissue by scanning of a laser beam having a high repetition rate on the tissue to be ablated.

2. Background of Related Art The ablation of tissue with an ablating laser beam is known. For instance, during an opthalmic refractive surgery to treat so-called refractive errors of an eye, e.g., myopia, hyperopia and astigmatism, an ablating laser beam is used to ablate portion(s) of layer(s) of a cornea to reshape the curvature of the surface of the cornea. For example, to correct a myopic condition, the curvature is lessened, i.e., made more flat. The curvature is made more prominent to correct a hyperopic eye, and is made more round to correct an astigmatic eye.

Conventional refractive techniques such as the photorefractive keratectomy (PRK) or laser in-situ keratomileusis (LASIK) utilize various lasers, e.g., an ArF excimer laser having a wavelength of 193nm. To achieve the desired curvature, a larger beam diameter laser would be masked (e.g., typically by using some type of a diaphragm or an iris), while a small beam diameter laser would typically be incrementally scanned across a predetermined pattern on a layer of the cornea. It is believed that the scanning, or masking combined with scanning, achieves smoother, i.e., having fewer ridges or steps in the remaining corneal surface than with - 2 - respect to a non-scanning approach, i.e., by solely masking a large beam diameter laser.

A scanning technique utilizes a scanning mechanism to scan or move the laser beam along the surface of the tissue being ablated. In some conventional scanning systems, a galvanometer scanner having motorized rotating mirror(s) is used. A controller controls the position of the rotating mirror(s) to move the "spot" of the laser beam (i.e., the point at which the laser beam is incident on the surface of the cornea) to each of predetermined ablation points of a scanning pattern to achieve the desired ablation profile. Typically, an ablation is performed at each of a plurality of ablation points along a set pattern on the surface being ablated in accordance with a desired final surface contour.

Typically, the laser beam is pulsed once at each ablation point. Each pulse of the laser beam applies a given level of energy, depending on, inter alia, the output energy per pulse of the laser and the "spot size" of the laser beam, to the corneal tissue, sufficient to thereby, excise or ablate (i.e., remove) a volume of the corneal tissue. More than one pulse can be combined to provide a deeper or smoother overall effect, but each pulse in and of itself is sufficient to ablate a given amount of tissue. Conventionally, the controller of the scanning mechanism provides synchronization between the scanning speed of the scanner, i.e., the rotating mirror(s), and the repetition rate of output beam of the laser in order to ablate tissue at only the predetermined ablation points, but not during movement of the spot from one ablating point to the next. In the conventional scanning laser system described above, the required duration of the refractive surgery is typically limited by the scanning speed, i.e., the speed at which the scanning mirror(s) can be accurately positioned between each laser pulse. The maximum scanning speed of currently available galvanometer scanners is on the order of about 300 to 400Hz, which is considerably slower than the maximum repetition rate of a - 3 - laser. A longer surgery duration runs the risk of added movement of the tissue, i.e., the eye, at least at some point during the surgery. Although techniques to track movement of the eye are known, movement is nevertheless to be avoided. Conventional scanning systems typically operate between about 10Hz to 40Hz, and perhaps as high as 100Hz to 200Hz.

US Patent 5,520,679 to Lin ("the '679 patent") is assigned to the assignee of the present application, and is explicitly incorporated herein by reference in its entirety. As discussed in the '679 patent, flash lamp pumped or diode pumped solid state lasers, e.g., UV lasers and IR lasers, may be used in a refractive surgery. As described by the '679 patent, such solid state lasers offer lower cost, better portability, better maneuverability and better- controlled ablation, over conventional lasers used to ablate tissue, e.g., gas excimer lasers.

A solid state laser is capable of much lower per pulse energy than an excimer laser. On the other hand, a solid state laser can be pulsed at a much higher repetition rate (e.g., 1 KHz to 50 KHz or 100KHz or more) than an excimer laser, albeit a much higher speed than possible with conventional scanning techniques.

The relationship between average power (P) of the laser, energy per pulse delivered to tissue, and repetition rate is defined as:

P = ER_P, where E is the per pulse energy of the laser, and R_P is the repetition rate of the laser beam pulses. However, given conventional techniques, the maximum usable repetition rate is limited by the conventional minimum energy per given pulse beam area necessary to ablate tissue during each pulse. Given equal levels of E with respect to both a high power excimer laser and a low power solid state laser, average power comparable to that of an excimer laser could be achieved with a solid state laser if the solid state laser were allowed to run at a higher repetition rate.

However, as described above, the relatively low speed limits of the scanner severely limits the repetition rate at which a solid state laser may - 4 - be pulsed. Thus, a surgery using a conventional scanning laser system having a solid state laser might take a relatively longer time to complete than a comparable surgery with an excimer laser, all else being equal.

Fig. 4 shows a conventional laser beam delivery system 400 having a laser head 412. The laser beam delivery system 412 generates a pulsed ablating laser beam 414 and includes a combination of optical elements 426 that attenuate, shape, direct and otherwise control the ablating laser beam 414 to the various ablation points or spots in an ablation zone predetermined on tissue to be ablated, e.g., a cornea 436 to be treated. The computer system 418 controls the laser head 412 and the scanning device 416 to deliver the laser beam 432 at the corneal surface 434 according to a predetermined ablation profile.

The diameter of the laser beam spot of the laser beam 432 incident on the cornea 436 is controllable to vary within a range by focusing optics of the optics system 426 and is scanned on the cornea 436 by a scanning device or mechanism 416, and delivered to the corneal surface 434 via a reflecting mirror 430. Generally, the diameter of the laser beam spot is selected for the energy density required to be delivered to the corneal surface 434 to achieve the desired amount of tissue removal and the depth of ablation, but in any event is typically in excess of a minimum amount necessary to cause ablation by each pulse.

The laser delivery system 400 may further include a feedback system (not shown) which continuously monitors and reports the actual ablation being made at corneal surface 434, in real time during the corneal ablation operation, to the computer system 418, which uses the information to further control the laser head 412 and scanning device 416 to achieve the desired resulting ablation profile.

The scanning device 416 may include at least one scanning mirror driven by at least one electrical galvanometer (servo-actuated limited- rotation motors). The scanning device may be, for example, a galvanometer - 5 - scanner commercially available from General Scanning, Inc of Watertown, Massachusetts. The laser beam is reflected off and directed by the scanning mirror on to the desired ablation point on the corneal surface 434. The rotation of the scanning mirror is controlled by the galvanometer, which in turn receives drive signal from a drive electronic typically controlled by the computer system 418. Typically, a scanning mirror attached to a galvanometer is required for movement of the laser beam along an axis. Thus, for example, if movement along X-Y axis is required, one mirror/galvanometer combination may be used for the movement in x-axis direction, and another combination for the y-axis direction. The aperture of each of the mirrors must accommodate the respective movements of the beam caused by the other mirror(s).

An ablation pattern, such as shown in Fig. 5, may be generated, and used to generate the x-y drive signals. Each of the dots shown in Fig. 5 represents an ablation point of a layer of an ablation zone 535 within a cornea 536, and each of the lines 501 to 510 represent a particular scan line within an overall ablation zone 536. Layer 535 represents one of a plurality of layers of tissue being ablated.

The scanning may be accomplished in a stepwise fashion (step scanning) in which the path between each ablation point is unimportant. The mirrors are moved in random paths between the ablation points (shown as dotted lines with arrows 537). The scanning may also be accomplished a raster scanning in which the mirror(s) are swept, at a constant velocity across scan lines (shown as solid lines with arrows 501-510) which are at a fixed angular range, within the ablation zone 535.

The scanning speed of step scanning is related to the step time which is the sum of the time required for the mirror to travel from one ablation point to another and the settling time of the mirror. The settling time is the time required for the mirror to come to a stop within a settling tolerance, centered around the final intended position, i.e., a particular ablation point. - 6 - ln a raster scanning, the scanning speed is related to the active scan time, which is the scan period less the retrace time. The scan period is reciprocal of the scanning frequency. The retrace time is the time required for the mirror to return to the start of the next scan line after completely scanning a scan line. The scanning frequency and the retrace time are limited by the ability of the mirror(s) to accelerate/decelerate to and from the constant nominal velocity between sweeps.

The step time and the acceleration/deceleration of the mirror(s) are largely related to the rotational inertia of the mirror. The rotational inertia depends on the physical size and the required rigidity of the mirror. A minimum rigidity is required in order to avoid excessive resonance of the mirror. Thus, it can be appreciated that there exists a maximum scanning speed, which is inevitably limited by the above physical constraints of the scanner system. A currently available galvanometer scanner has a maximum scanning speed typically ranging between 300 to 400Hz. Consequently, although the available lasers can be pulsed at a much higher repetition rate, laser beams are currently scanned at slow speed (scanning at 10 to 40Hz or up to 200Hz is a fairly common practice), unnecessarily prolonging a surgery time.

Fig. 6 shows an example of the high repetition rate capability of a solid state laser in general as compared to a relatively low repetition rate of the same solid state laser as limited by conventional scanning mechanisms as shown in Fig. 7. In order for a corneal tissue ablation to occur, the laser energy density of each of the pulses must be above the photoablation threshold (PAT). The PAT is 60-120mJ/cm² for a UV-laser (193-215 nm wavelength), 200-600mJ/cm² for an IR-laser (2.5-3.2 μm wavelength), and 50mJ/cm2 for an ArF excimer laser (193nm wavelength). The energy density (I) is defined as: - 7 -

I = E/A, where E is the per pulse energy of the laser, and the A is the beam spot size.

Thus, it can be appreciated that for a solid state laser that typically has a lower per pulse energy E (typically less than 20mJ/pulse) must be focused on to a smaller spot size than a gas excimer laser, e.g., an ArF laser (typically about 200mJ/ pulse).

However, because of the smaller spot size, the operation time using the lower power solid state laser would be longer than the operation time using a larger spot size but higher power laser, all else being equal. Fig. 8 shows a typical process flow of a conventional scanning laser system in which the laser pulse rate is synchronized to the scanning rate of the scanning mirror(s).

In particular, in step 801 of the conventional scanning laser system, an ablation profile is determined to treat a specific condition of a particular patient's eye, e.g., myopia, hyperopia, or astigmatism. The ablation profile may include, for example, the points (or spots) on the corneal surface for each layers of the cornea to be ablated, to achieve the desired final shape of the corneal surface. In step 802, the scanning mirror(s) is (are) moved to one of the points to be ablated. Once the laser beam spot is at one of the ablation points, the laser is allowed to pulse in step 804, and thus, a volume of corneal tissue commensurate with the energy level of the laser pulse is ablated. The moving of the mirror(s) and the pulsing of the laser are repeated until it is determined that all ablation points has been ablated in step 805. As can be appreciated, in a conventional system, the laser is forced to be pulsed at the scanning speed, i.e., to much less than 300-400Hz, and is typically in the range of 10Hz to 40Hz.

It is important to limit the duration of a laser surgery to a relatively short period of time, i.e., to less than a minute, to minimize patient discomfort and the risk of excessive movement of the eye undergoing treatment. - 8 - Accordingly, there exists a need for a method and apparatus for performing ablative laser surgery using a scanning laser system within a duration that is commensurate with the high repetition rate capabilities of a solid state laser without being unnecessarily limited by the relatively slower scanning speeds of conventional scanners.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a method of ablating tissue with a scanning laser beam at an increased effective repetition rate is provided. A laser is pulsed at a pulse sequence, of a variable repetition rate, having one or more first time duration during which the laser is pulsed at a high repetition rate and one or more second time duration during which the laser outputs no pulses. During a first time duration, the scanning mirrors are either stationary or are moved within a very small confines. During a second time duration, the scanning mirror(s) make a major movement so as to move the laser beam spot across the surface of the tissue from one ablation point to an another ablation point substantially apart from each other.

The present invention is particularly useful for ablating tissue with a scanning laser beam of a low power solid-state laser, for example, a flash lamp or diode pumped UV laser or an IR laser. - 9 -

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which: Fig. 1 is a graphical representation of an examplary variable repetition rate ablation pulse sequence of the output pulses of a solid state laser in accordance with the principles of the present invention;

Fig. 2 is a flow diagram depicting an exemplar embodiment of the method of scanning laser beam at high repetition rate in accordance with the principles of the present invention;

Fig. 3 is a flow diagram depicting an another exemplar embodiment of the method of scanning laser beam at high repetition rate in accordance with the principles of the present invention;

Fig. 4 is an illustrative functional diagram of a conventional scanning laser refractive surgery system;

Fig. 5 is an illustrative example of a scanning pattern depicting scan lines and ablation points of a ablation zone of a cornea being treated by a conventional scanning laser refractive surgery system;

Fig. 6 is a graphical representation of the output pulses of a conventional high repetition rate laser;

Fig. 7 is a graphical representation of the output pulses of a high repetition rate laser limited by the slow scanner speed of a conventional scanning laser system; and

Fig. 8 is a flow diagram depicting a conventional method of scanning laser beam.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Although the embodiments of the present invention are described with reference to the removal of corneal tissue, the present invention relates to laser ablation techniques of human or animal tissue in - 10 - general. Moreover, the principles of the present invention may be applied in both photo-ablation and photo-coagulation techniques for reshaping the surface of a cornea, including, as examples but not limited to, laser refractive keratectomy (e.g. photorefractive keratectomy or PRK, PRK after radial keratomy, laser keratectomy with mircrokeratome, intrastromal photodisruption and laser in-situ keratomileusis or LASIK), laser therapeutic keratectomy, and laser lamellar resection.

The present invention results from a realization that the operation time of a refractive laser surgery using a solid state laser can be shortened if the laser is allowed to pulse at a higher repetition rate, thus, achieving a high average power.

The total operation time (T₀) of a laser refractive surgery can be expressed as:

T₀ = DW⁴/P (1 ), where D is the desired diopter correction, W is the correction zone diameter, i.e., the size of the area to be ablated, and P is the laser average power which can be further expressed as:

P = ER_P (2), where E is the per-pulse energy of the laser, and R_p is the repetition rate (or the pulse rate) of the laser.

Thus, equation (1 ) can be expressed as:

T₀ = DW⁴/ER_P (3).

In a conventional scanning laser system, the repetition rate R_P of the laser is typically synchronized to match the scanning speed R_s of the scanning mechanism.

Thus, in a conventional system, equation (3) becomes:

T₀ = DW⁴/ER_S (4).

A solid state laser can be pulsed at repetition speed up to

50KHz or higher. Thus, if the solid state laser is allowed run at the maximum speed, for example at 50KHz, the surgery time according to the equation (3) - 11 - above may be expressed as T_P = (DW⁴/E)/50,000 seconds. However, if the laser pulses are synchronized to the scanning speed (e.g., 400Hz if run at the highest possible conventional scanning speed), the surgery time, according to equation (4) above, would be T_s = (DW⁴/E)/400 seconds. Thus, it can be appreciated that the surgery time of conventional scanning system even if run at a theoretical maximum scanning speed would be T_S/T_P longer, (e.g., 125 times longer in the given example), than it would have been if the laser had been allowed to pulse at a high repetition rate in accordance with the principles of the present invention. Fig. 1 shows a graphical representation of an examplary variable high repetition rate ablation pulse sequence of ablating pulses of a solid state laser, in accordance of the principles of the present invention.

In particular, sequence of ablating pulses at a high repetition rate as shown in Fig. 1 can be generated using, for example and not as a limitation, an electronic control (e.g., computer-controlled power supply to the laser or the optics), mechanical shutter, or any other suitable means to control the application of the ablating laser pulses at the corneal surface.

As shown in Fig. 1 , there may be one or more ON time period(s) t_s during which the laser pulses are present at the corneal surface at a high repetition rate and one or more OFF time period(s) T during which the ablating laser pulses are not applied to the tissue. However, in accordance with the principles of the present invention, the repetition rate of the laser pulses during the ON time periods t_s are greater than the scanning rate of the scanning mechanism between otherwise conventional ablation points. During the ON time period t_s, the scanning mirror(s) either remain substantially stationary or is (are) made to move slightly within a very small proximity of an ablation point.

The amount of mirror movement during the ON time periods t_s is dependent on the energy level of the individual laser pulses. For instance, the scanning mirror does not need to move at all if each individual laser pulse - 12 - has relatively low energy, such as it is the case with a solid state laser. Of course, the energy density of each pulse should exceed the photo-ablation threshold (PAT). However, with the ability to provide a larger number of ablating laser pulses within a short period of time, the energy may be much lower per pulse and in fact approach the PAT level itself and yet still be quite effective overall. In accordance with the principles of the present invention, number of ablating pulses may be irradiated on any particular ablation point without the accumulated ablation depth being caused to be made too deep before the scanner moves on (during the OFF time period T) to a next ablation point.

If each pulse has a considerable amount of energy, e.g., if a conventional excimer laser is used, at higher power, movement of scanning mirror(s) may be necessary between each ablating pulse in order to not ablate tissue too deeply at any one particular ablation point. However, by keeping the movement(s) of the scanning mirror(s) between individual ablating laser pulses to be sub-ablation point increments, i.e., to be within a close proximity of a single ablation point in a given ablation pattern, an effectively high scanning rate can result.

Generally, the thickness of the tissue to be ablated is divided into a number of layers, each layer being ablated individually in a typical conventional scanning laser system. However, in accordance with the principles of the present invention, the movement (and/or the number of movements) of the scanning mirror(s) during any one ON time period t_s may be further minimized by ablating several layers at a time (i.e., within that ON time period t_s).

Moreover, slight movements of the scanning mirror(s) between the high repetition rate ablation pulses in accordance with the principles of the present invention may provide a smoother ablated surface by promoting significant (e.g., over 80%) overlaps of the laser beam spots generated by each ablating laser pulse, thus, avoiding sharp steps or ridges that might - 13 - otherwise be created by a non-overlapping or less overlapping single pulse per ablation point techniques.

The pulse sequence according to the present invention may include more than one ON time period t_s. During OFF time periods T, the scanning mirror makes a major movement across a larger distance, e.g., from one ablation point to another. During the OFF time periods of significant scanning mirror movement, the laser pulses are absent, either by preventing the pulses from being output from the laser and/or by preventing ablating laser pulses from reaching the cornea or other tissue being ablated. The pulse sequence shown in Fig. 1 may be determined and fixed at the outset of a surgery, or can be dynamically determined during the operation, taking into account information provided by, e.g., a feedback system which monitors and reports the actual amount of ablation which has occurred being made in real time. Fig. 2 shows an exemplary embodiment of the present invention wherein the high repetition pulse sequence is fixedly determined at the outset of the surgery.

In particular, as with a conventional system, an ablation profile for the desired reshaping of cornea is determined in step 201. In step 202, a pulse sequence is generated to achieve the ablation of the cornea in accordance with the desired ablation profile while minimizing the movements (and/or the frequency thereof) between each laser pulse. As previously discussed, the pulse sequences are determined based on such factors as, for example, the energy level of the laser and/or the number of iayer(s) (both total and per scan) of cornea to be ablated. The pulse sequence generated in step 202 is similar to the sequence depicted in Fig. 1 , and includes one or more ON time periods t_s and OFF time periods T.

As an illustrative example of the determination of the pulse sequence, the following procedure may be employed. Each OFF time period T is determined from the ablation profile, and preferably corresponds to the - 14 - minimum step time required to move the scanning mirror(s) from one ablation point of the ablation profile to the next. Each ON time period t_s is determined from the desired operation time T₀ using the aforementioned equation (3).

From equation (3), T₀ = DW⁴/ER_P, the effective repetition rate R_P can be determined given the desired operation time T₀ and the desired diopter correction D, the ablation zone width W, and the per pulse energy of the laser, all of which are known.

Once the repetition rate R_P is determined, the duration and the pulse repetition rate for each ON time period t_s can be determined by subtracting the sum of all OFF time periods ΣT from the desired operation time T₀, resulting in the sum of all ON time periods ∑t_s. Using the ratio, ΣT/

∑t_s, as the effective duty cycle, the duration and repetition rate for each ON time period t required to achieve the overall effective repetition rate R_P can be determined. A control system (not shown) determines and carries out the desired ablation sequence by providing appropriate control signals to the low power high repetition rate laser, to the scanner, and/or to the optics.

In steps 203 and 204 of Fig. 2, the control system reads the pulse sequence to determine whether the current time period is an ON time period t_s or an OFF time period T according to the timing of the desired pulse sequence.

If the current period is at an ON time period t_s, in step 205, the control system sends appropriate control signals to the low power laser, the scanner and/or the optics to allow the ablating pulses of the laser beam to reach the tissue at a predetermined repetition rate R_P. The repetition rate R_P may be, for example, as high as the maximum repetition rate that the laser is capable of, but will depend on other factors, e.g., the energy level of the laser per unit area as applied to the tissue. - 15 - The control signals may be, for example, a start pulse signal to the laser head and/or a signal to open a mechanical shutter to allow the ablating pulses of the laser beam to reach the corneal tissue.

During an ON time period t_s, in step 206, the control system also sends an appropriate control signal to the scanner to prevent significant movement of the scanning mirror(s), or to only allow slight movement(s) of the mirror(s) around a close vicinity of any particular ablation point.

In step 204, if the control system instead determines that the current period according to the pulse sequence is an OFF time period T, then the control system will send appropriate control signals to the low power laser, the scanner, and/or the optics to prevent ablating pulses of the laser beam from reaching the corneal surface for that duration of time.

In step 208, the control system sends the appropriate signals to the scanner to cause the scanning mirrors to make a significant or major movement in order to move the spot of the laser beam across the ablation zone towards the next ablation point.

Preferably, all ablation points of the ablation profile are processed in a similar manner as described with respect to steps 203-208, as shown in step 209. Fig. 3 shows an another exemplary embodiment of the present invention wherein a pulse sequence is generated, and is dynamically updated, by the control system during the ablation procedure in light of information provided by a feedback system.

The embodiment of Fig. 3 is similar in its flow as the embodiment as described in reference to Fig. 2. However, in Fig. 3, a feedback system monitors and reports in real time the status of the actual ablation being performed. The control system, upon being provided with the feedback information, performs an ongoing comparison between the status of the actual ablation with the predetermined desired ablation profile. - 16 - In particular, steps 201 to 208 are substantially as described with respect to Fig. 2. However, as shown in step 309 of Fig. 3, the control system compares, preferably after each ON time period t_s or each OFF time period T, compares the status of actual ablation with the desired ablation profile.

In step 202a, the pulse sequence is updated in order to more closely track the desired ablation profile by, for example, modifying the duration of each ON time periods t_s or each OFF time period T and/or modifying the repetition rate R_P of the laser pulses. A simple example is provided to demonstrate some of the advantages of the principles of the present invention. This example is for illustrative purpose only, and should not be construed in any way as a limitation of the scope of the present invention.

In the example, a solid state (or excimer) laser is capable of being pulsed at a repetition rate R_P of 2KHz, and a scanning mechanism has a maximum scanning speed of, e.g., 350Hz. Using conventional technique, the scanning rate would simply be 350Hz. However, as will be shown, the principles of the present invention make use of the increased pulse capability of the source laser to provide an ablation technique having an effective ablation rate which is much greater than, e.g., the 350Hz maximum speed of the scanner.

For simplicity, assume all ON time periods t_s are equal in duration, that all OFF time periods T are equal in duration, and that the duration of the ON time periods t_s substantially equals the duration of OFF time periods T, i.e., a 50% duty cycle. In this case, the effective repetition (ablation) rate R_PE would be about equal to the ablation pulse repetition rate R_P (i.e., 2000Hz) multiplied by the duty cycle (i.e., 50% or V.). Thus, the effective repetition rate R_PE would be equal to 2000Hz/2 = 1000Hz, which is much higher than the relevant scanning rate, i.e., 350Hz. - 17 -

Thus, the inventive scanning technique utilizing an ablation pulse sequence in accordance with the principles of the present invention achieves an effective ablation rate R_PE that is not limited by the scanning speed of the scanning mirror(s). The foregoing exemplary descriptions and the illustrative preferred embodiments of the present invention have been explained in the drawings and described in detail, with varying modifications and alternative embodiments being taught. While the invention has been so shown, described and illustrated, it should be understood by those skilled in the art that equivalent changes in form and detail may be made therein without departing from the true spirit and scope of the invention. Moreover, while the present invention is described herein with illustrative exemplary embodiment useful in an ophthalmic laser refractive surgery, the invention may be suitably practiced in ablating any human or animal tissues with an ablating scanning laser.

Claims

- 18 -CLAIMSWhat is claimed is:

1. A tissue ablating apparatus, comprising: a laser adapted to output an ablating laser beam to said tissue; a scanner adapted to move an ablating laser beam to each of a plurality of ablation points; and a processor adapted to output a plurality of ON control signals and a plurality of OFF control signals to pulse said laser at a repetition rate in excess of one pulse per ablation point without significant movement of said scanner therebetween.

2. The tissue ablating apparatus according to claim 1 , wherein: said laser is a gas laser.

3. The tissue ablating apparatus according to claim 2, wherein: said gas laser is an excimer laser.

4. The tissue ablating apparatus according to claim 1 , wherein: said laser is a solid state laser.

5. The tissue ablating apparatus according to claim 1 , wherein: said plurality of ON control signals and said plurality of OFF signals operate said laser at a repetition rate in excess of 400 Hz.

6. The tissue ablating apparatus according to claim 1 , wherein: said plurality of ON control signals and said plurality of OFF signals operate said laser at a repetition rate of at least 500 Hz.

- 19 - 7. The tissue ablating apparatus according to claim 1 , wherein: said plurality of ON control signals and said plurality of OFF signals operate said laser at a repetition rate of at least 2000 Hz.

8. The tissue ablating apparatus according to claim 1 , wherein: said tissue is corneal tissue.

9. A method of scanning a tissue ablating laser beam, comprising: scanning an ablating laser beam toward each of a plurality of ablation points established in an ablation zone; and providing an ON pulse sequence by pulsing said ablating laser beam a plurality of times while said ablating laser beam is directed toward each of said plurality of ablation points.

10. The method of scanning a tissue ablating laser beam according to claim 9, further comprising: providing an OFF pulse sequence by not pulsing said ablating laser beam while said ablating laser beam is scanned between each of said plurality of ablation points.

11. The method of scanning a tissue ablating laser beam according to claim 10, wherein: said ON pulse sequence with respect to said OFF pulse sequence is at least 50%.

12. The method of scanning a tissue ablating laser beam according to claim 9, further comprising: holding said ablating laser beam substantially stationary with respect to an intended ablation point during said ON pulse sequence. - 20 -

13. The method of scanning a tissue ablating laser beam according to claim 9, further comprising: allowing said ablating laser beam to move only within a small proximity about an intended one of said plurality of ablation points during said ON pulse sequence.

14. The method of scanning a tissue ablating laser beam according to claim 10, further comprising: allowing said ablating laser beam to move between ablation points during said OFF pulse sequence.

15. The method of scanning a tissue ablating laser beam according to claim 9, wherein: a repetition rate of said ablating laser beam during said ON pulse sequence is based on a desired surgical duration.

16. The method of scanning a tissue ablating laser beam according to claim 9, further comprising: dynamically adjusting said ON pulse sequence based on feedback of actual ablation performance.

17. The method of scanning a tissue ablating laser beam according to claim 9, wherein: said ablating laser beam has a per pulse energy of less than 20 mJ/pulse. - 21 -

18. The method of scanning a tissue ablating laser beam according to claim 9, wherein: said ablating laser beam is generated by a solid state ultraviolet laser.

19. The method of scanning a tissue ablating laser beam according to claim 18, wherein: said solid state ultraviolet laser is pumped by one of a flash lamp and a diode.

20. The method of scanning a tissue ablating laser beam according to claim 9, wherein: said ablating laser beam is generated by a solid state infrared laser.

21. The method of scanning a tissue ablating laser beam according to claim 20, wherein: said solid state infrared laser is pumped by one of a flash lamp and a diode.

22. A method of determining an ablation profile for tissue to be ablated, comprising: determining a desired post ablation shape of said tissue; and determining a desired pulse sequence including a plurality of ablation pulses at each of a plurality of ablation points between significant scanned movement of an ablating laser, based at least in part on a desired effective ablation rate and a desired time duration of an operation to achieve said post ablation shape of said tissue. - 22 -

23. The method of determining an ablation profile for tissue to be ablated according to claim 22, further comprising: updating said desired pulse sequence based on real-time feedback regarding an actual ablation of said tissue.

24. Apparatus for scanning a tissue ablating laser beam, comprising: means for scanning an ablating laser beam toward each of a plurality of ablation points established in an ablation zone; and means for providing an ON pulse sequence by pulsing said ablating laser beam a plurality of times while said ablating laser beam is directed toward each of said plurality of ablation points.

25. The apparatus for scanning a tissue ablating laser beam according to claim 24, further comprising: means for providing an OFF pulse sequence by not pulsing said ablating laser beam while said ablating laser beam is scanned between each of said plurality of ablation points.

26. The apparatus for scanning a tissue ablating laser beam according to claim 25, wherein: said ON pulse sequence with respect to said OFF pulse sequence is at least 50%.

27. The apparatus for scanning a tissue ablating laser beam according to claim 24, further comprising: means for holding said ablating laser beam substantially stationary with respect to an intended ablation point during said ON pulse sequence. - 23 -

28. The apparatus for scanning a tissue ablating laser beam according to claim 24, further comprising: means for allowing said ablating laser beam to move only within a small proximity about an intended one of said plurality of ablation points during said ON pulse sequence.

29. The apparatus for scanning a tissue ablating laser beam according to claim 25, further comprising: means for allowing said ablating laser beam to move between ablation points during said OFF pulse sequence.

30. The apparatus for scanning a tissue ablating laser beam according to claim 24, wherein: a repetition rate of said ablating laser beam during said ON pulse sequence is based on a desired surgical duration.

31. The apparatus for scanning a tissue ablating laser beam according to claim 24, further comprising: means for dynamically adjusting said ON pulse sequence based on feedback of actual ablation performance.

32. The apparatus for scanning a tissue ablating laser beam according to claim 24, wherein: said ablating laser beam has a per pulse energy of less than 20 mJ/pulse.

33. The apparatus for scanning a tissue ablating laser beam according to claim 24, wherein: said ablating laser beam is generated by a solid state ultraviolet laser. - 24 -

34. The apparatus for scanning a tissue ablating laser beam according to claim 33, wherein: said solid state ultraviolet laser is pumped by one of a flash lamp and a diode.

35. The apparatus for scanning a tissue ablating laser beam according to claim 24, wherein: said ablating laser beam is generated by a solid state infrared laser.

36. The apparatus for scanning a tissue ablating laser beam according to claim 35, wherein: said solid state infrared laser is pumped by one of a flash lamp and a diode.

37. Apparatus for determining an ablation profile for tissue to be ablated, comprising: means for determining a desired post ablation shape of said tissue; and means for determining a desired pulse sequence including a plurality of ablation pulses at each of a plurality of ablation points between significant scanned movement of an ablating laser, based at least in part on a desired effective ablation rate and a desired time duration of an operation to achieve said post ablation shape of said tissue.

38. The apparatus for determining an ablation profile for tissue to be ablated according to claim 37, further comprising: means for updating said desired pulse sequence based on realtime feedback regarding an actual ablation of said tissue.