AU698453B2 - Corneal laser surgery - Google Patents

Description

1

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Actual Inventor: Address for Service: SHUI T LAI SHUI T LAI CULLEN

CO.,

Patent Trade Mark Attorneys, 240 Queen Street, Brisbane, Qld. 4000, Australia.

CORNEAL LASER SURGERY 4 Invention Title: The following statement is a full description of this inve-t 'on, including the best method of performing it known to me: 1* 2 Corneal Laser Surgery BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a laser-based method and apparatus foL corneal surgery. In particular, the invention is directed to an eye tracking system for use in such laser surgery.

2. Related art A discussion of related art can be found in Australian Patent No. 671607, the disclosure of which is incorporated herein by reference.

SUMMARY OF THE INVENTION Sti In one broad form, the present invention provides in a laser apparatus for cornea surgery, wherein the laser beam has an undeflected position, an eye tracking system for biasing the alignment of the undeflected position of the laser beam with respect to the cornea, including: a. indicator means, attached to the eye, for providing visible indications of the movement of the eye; b. sensor means for detecting the visible o indications provided by the indicator means and for providing control signals in response to such detection; c. beam positioning means, coupled to the sensor means, for biasing the alignment of the undeflected position of the laser beam with respect to the cornea in response to the control signals.

The invention also provides a laser apparatus for cornea surgery, wherein the laser beam has an undeflected i .AI~position, a method for biasing the alignment of the undeflected position of a laser beam with respect to the 1 3 cornea, including the steps of: a. attaching an indicator means to the eye for providing visible indications of the movement of the eye; b. detecting the visible indications provided by the indicator means and providing control signals in response to such detection; c. biasing the alignment of the undeflected position of the laser beam with respect to the cornea in response to the control signals.

o-o Preferably, the indicator means includes a vacuum eye ring 15 having distinct visible linear eye position indicators.

0 The eye position indicators may include o at least one first line indicating position of the eye ring in an X-d.-rection; at least one second line, orthogonal to the at least first line, indicating position of the eye ring in a Y-direction.

The eye position indicators may further include f at least one radial line indicating rotation of the eye ring.

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preferably, the sensor means includes a first linear array sensor orthogonal to an image of each first line; a second linear array sensor orthogonal to an 30 image of each second line.

The sensor means may also include a third linear array sensor orthogonal to an image of each radial line.

Preferably, the beam positioning means includes at least two orthogonal reflective surfaces positioned by controllable actuators.

W~There is also disclosed a guide beam generation system for 0 Threi as 111~U9 C~L1U~l~lI 4 use in a laser apparatus for cornea surgery, wherein the laser pulses have an undeflected position. Apparatus and method are provided for generating a visible guide beam coaxial with the undeflected position of the laser pulses, for aligning the laser pulses on a cornea.

The means for generating the visible guide beam may suitably include a low-power laser.

The means for generating a visible guide beam may also include means for generating a ring of light adjustable in diameter.

oooThere is further disclosed pulsed laser apparatus 15 including: a. laser means for generating laser pulses e. which are deflectable around a pivot point from an undeflected position; b. at least one optical wavelencrth conversion means, for converting the fundamental wavelengths of original incident laser pulses into corresponding second harmonic 9 0 *°wavelengths; c. a first lens, positioned at about the 25 distance of its focal length from the pivot r 000 0o. point of the laser pulses, for receiving incident laser pulses and orienting the incident angle of such laser pulses relative to a first surface of the at least one optical wavelength conversion means such that the incident angle remains constant regardless of the angle of deflection of the laser pulses; d. a second lens, positioned at about the distance of its focal length from the at least one optical wavelength conversion ~means, for receiving incident laser pulses W from the at least one optical wavelength Ik( (C conversion means and for orienting such laser pulses such that the laser pulses are re-collimated after passing through the second lens; e. wavelength separation means for spatially separating the fundamental wavelength of the original incident laser pulses from wavelength converted laser pulses exiting from the second lens.

A second optical wavelength conversion means may be provided adjacent to the first optical wavelength conversion means.

S 15 The first lens and the second lens may be the same lens, and the apparatus may further include reflecting means, adjacent the wavelength conversion means, for reflecting laser pulses exiting the wavelength conversion means back through the wavelength conversion means to the lens; and angled reflecting means for reflecting wavelength converted laser pulses exiting the a a°olens.

Each optical wavelength conversion means may suitably be a nonlinear optical crystal.

*r4a a There is additionally disclosed a method for converting 30 the wavelength of laser pulses, including the steps of: a. providing means for converting the fundamental wavelengths of original i incident laser pulses into corresponding second harmonic wavelengths; 1Z w CO0 UE J 6 b. receiving incident laser pulses and orienting the incident angle of such laser pulses relative to a first surface of the means for converting such that the incident angle remains constant regardless of the angle of deflection of the laser pulses; c. receiving incident laser pulses from the means for converting and orienting such laser pulses such that the laser pulses are re-collimated; d. spatially separating the fundamental wavelength of the original incident laser pulses from wavelength converted laser pulses.

The method may further include the step of scanning the laser pulses across the means for conversion, thereby distributing the thermal energy of the laser pulses over 20 the surface of the means for conversion, and reducing the power loading in the means for conversion.

In order that the invention may be more fully understood S°and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of the preferred embodiment of the apparatus.

FIGURE 2A is a side view of a beam diameter sensor, including an imaging device.

FIGURE 2B is a front view of the imaging device of FIGURE 2A.

IY~u.~uuclr~ nuurm(l3~teec~~i~~ upy*? -i::'3B o ~r e r car rr e~~ 7 FIGURE 3A is a side view of a beam location sensor, including a photo-detector.

FIGURE 3B is a front view of the photo-detector of FIGURE 3A.

FIGURE 4A is a perspective view of a vacuum ring.

FIGURE 4B is a top plan view of the vacuum ring of FIGURE 4A.

FIGURE 4C is a block diagram of an eye tracking system.

FIGURE 5A is a block diagram of a first embodiment of a 15 wavelength converter.

FIGURE 5B is a block diagram of a second embodiment of a wavelength converter.

FIGURE 6A is a diagram of a first laser beam intensity profile in accordance with the prior art.

FIGURE 6B is a diagram of a corneal etch profile resulting from the laser beam intensity profile shown in FIGURE 6A.

FIGURE 6C is a diagram of second and third laser beam intensity profiles in accordance with the prior art.

FIGURE 6D is a diagram of the corneal etch profiles resulting from the laser beam intensity profiles shown in FIGURE 6C.

FIGURE 6E is a diagram of a pattern of laser beam intensity profiles in accordance with the prior art.

FIGURE 6F is a diagram of the laser beam intensity profile resulting from the pattern shown in FIGURE 6E.

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8 FIGURE 6G is a diagram of a corneal etch profile resulting from the laser beam intensity profile shown in FIGURE 6F.

FIGURE 7A is a diagram of a first level pattern used in scanning a target.

FIGURE 7B is a diagram of a second level pattern used in scanning a target.

FIGURE 7C is a top and side view of a pattern of concentric circles etched on a cornea.

FIGURE 7D is a set of graphs showing the crest-to-trough distances of level one, level two, and level three etch patterns.

FIGURE 7E is a diagram showing the measurement axes used to compute the level two and level three crest-to-trough distances of FIGURE 7D.

FIGURE 8 is a block diagram of a guide beam unit.

FIGURE 9A is a top view of a cornea, showing radial incisions on the cornea.

FIGURE 9B is a cross-sectional side view of a cornea, showing variable-depth incisions.

FIGURE 9C is a top view of a cornea, showing transversecut incisions on the cornea.

FIGURE 10A is a cross-sectional side view of a cornea, showing removal of tissue to a desired depth d over a predetermined area on the cornea, and showing an alternative method for performing a cornea transplant.

FIGURE 10B is a cross-sectional side view of a cornea, Y i" ~t I\

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4 C:ri M c 3a 3 9 showing the correction of myopia.

FIGURE 10C is a cross-sectional side view of a cornea, showing the correction of hyperopia.

FIGURE 11 is a schematic diagram of the integrated scanner-amplifier unit, consisting of a series of intracavity optical elements.

FIGURE 12 is a schematic diagram showing a second embodiment of the integrated scanner-amplifier unit.

FIGURE 13 is a schematic diagram showing the process of angular amplification for the laser beam inside the 15 scanner-amplifier cavity.

FIGURE 14A is a schematic showing a means of generating stable second harmonic laser power.

20 FIGURE 14B is a perspective showing of a spatial combiner o S for combining the plurality of pump beams of FIGURE 14A.

e FIGURE 14C schematically depicts the combining of the beams of FIGURE 14A into a single second harmonic b1eam 25 from the generated beam of FIGURE 14A.

fc o iFIGURE 15A is an exploded perspective view showing a method of mounting the laser medium.

FIGURE 15B is a cutaway perspective view of the laser medium of FIGURE 15A enclosed in a water jacket for Scooling.

FIGURE 16 is a block diagram showing the electrical connections between the mode-locked laser driver, the timer-divider circuit, the Pockels cell driver, and the Q-switch driver of the pump laser.

,T FIGURE 17 is a diagram showing the synchronisation between the mode-locked laser Dulses, the selected laser pulses after the timer-divider circuit, the Q-switched laser pulses for pumping the gain medium, and the halfwave optical switch wave form.

Like reference numbers and designations in the various drawings refer to like elements.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE

INVENTION

Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than limitations on the method and apparatus of the invention.

j .Although this invention relates primarily to an eye tracking systam, guide beam generation and wavelength 20 conversion in laser apparatus for corneal surgery, the complete apparatus will be described for a better understanrling.

t m BACKGROUND INFORMATION The laser apparatus and system herein disclosed is intended to achieve two principal objectives: The damage zone underneath the material ablated by the present laser system is substantially reduced in comparison to prior art laser systems.

k2) For each laser pulse deposited on the cornea, a definate predetermined depth of tissue is to be ablated. The ablated depth per laser pulse is controllable and about 0.2 microns or less, and preferably about 0.05 microns or less.

A brief discussion on the mechanism of the ablation I i

TI

Registered Patent Attorney TO: THE COMMISSIONER OF PATENTS

AUSTRALIA

ITl~

I;

A process is useful to understand how the stated objectives can be achieved. It is a well-known fact that laser ablation can occur when the laser beam intensity is increased beyond a certain level. The actual ablation conditions however, vary depending on the characteristics of a wide range of laser parameters and the composition of the material to be ablated. For the purposes of the present invention, only those aspects that are relevant to the two principal objectives will be discussed.

When laser energy is absorbed in an organic material, on the most basic level, the electronic configuration of the ret polymer molecules makes a transition to one of its excitec cclectronic states.

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*.ct s.C 4 C fT A1 12 Each polymer is made of hundreds or more of sub-units of smaller molecules called monomers. The monomers are made of even smaller units of radicals consisting of combinations of hydrogen, carbon, oxygen, and nitrogen atoms. Depending on the energy level of the laser photons, a polymer can be broken into constituent monomers, radicals, or ionized atoms. For a laser having a wavelength near 200 nm, a single laser photon is not sufficiently energetic to break any molecular bond. However, after absorbing an initial photon, a molecule is promoted to an excited electronic state configuration, with its electrons in higher energy orbits.

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With increased power levels of the laser beam, the excited electron density increases correspondingly. At the same time, the excited electrons migrate down the polymeric chain of the organic material, and spread towards the bulk volume with lower excited state density. The present invention recognizes that the excited state electronic orbitals are the means for energy storage that will eventually fuel the ablation process, and the electronic energy state migration process plays a key role in the dynamics controlling the initiation of the laser ablation.

As the laser beam intensity increases further towards the ablation threshold, the excited electron density reaches a critical volume density such that the electronic orbitals can pair and transfer the sum of their energy to a single electron orbital. This process breaks the molecule into two or more pieces, and releases an energetic electron. At this point, the organic medium is damaged but not yet ablated.

i 1 Consider now the geometric distribution of the excited state orbitals in an organic material. As the laser light is absorbed in the organic material, by Beer's law, the front surface where the material is first exposed encounters most of the laser photons, and the beam intensity decreases exponentially as it traverses deeper into the material. Hence, the spatial distribution of the excited state density also decreases accordingly, characteristic of the absorption coefficient of the material at the laser wavelength. It follows that the slope of the distribution curve of the excited state density is 10 directly related to the absorption coefficient. Additionally, the steeper the slope of the excited state density distribution curve, the more spatially localized is the excited state density. The preferred range of the absorption depth of the surgical laser beam in the cornea is less then about 50 microns.

Alternatively, the ablation threshold can be reached at a lower laser peak power, provided that the material is exposed for a longer period. In accordance with the discussion above, if the total integrated energy of a laser pulse is the same as that of a shorter pulse, the excited state density established by the longer pulse would be lower due to the additional time available for energy migration out of the irradiated volume. Therefore, to achieve the same ablation threshold for a longer pulse, the longer pulse must have a larger total integrated energy than a shorter pulse having the same ablation threshold. Empirical results obtained from materials damage indicate that a particular damage threshold can be reached with a pulsed laser beam 100 times longer in duration than a shorter duration pulse, provided that the total integrated energy of the longer laser pulse is increased by about 10 fold over the integrated energy of the shorter pulse.

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-:ii ~1 14 In accordance with the discussion above, when using longer duration pulses, the energy migration process is counter-balanced by additional laser beam pumping to build up the critical excited state density. Importantly, with a longer laser pulse, the excited state orbitals diffuse from the front surface into the depth of the material (along the laser beam direction). Hence, the excited state distribution curve will have less steep a slope compared to the curve from a shorter pulse. The present invention recognizes that the depth of the corneal layer which has sufficient excited state orbitals to satisfy the damage threshold condition will be correspondingly deepened. Therefore, the corneal damage inflicted by a longer duration laser pulse is more extensive than the damage inflicted with a shorter duration pulse.

In consideration of these observations and characteristics, the present invention uses short duration laser pulses of about 1-5,000 picoseconds to reduce inflicted damage to target tissues.

The other key objective of the present invention is to achieve ;3 shallow yet reproducible etch depth at the cornea surface from each laser pulse. It is important to note that a reproducible etch depth will not necessarily be attained at reduced levels of laser energy per pulse, especially when the energy level is close to being at an arbitrarily small value above the ablation energy threshold. For an excimer laser, the typical laser energy density in the surgical beam required for cornea ablation is about 150-250 mJ/cm 2 The ablation threshold level for excimer laser is at about 50 mJ/cm 2 basically no ablative action can be observed at a laser energy density below this threshold level.

FT I~I It is also important to note that observation of ablative action near the threshold condition is determined by a statistical process. That is, determination of the average etch depth for laser be3m energies near the ablation energy threshold are derived by measuring actual etch depth after hundreds or sometimes thousands of laser pulses over the same location, and determining an average etch depth per pulse. On a single shot basis, however, the etch depth could vary significantly, and most of the laser pulses may not ablate any material at all.

Therefore, to ensure a reliable etch depth for each single laser pulse, the present invention recognizes that the operating energy i per pulse has to be set at a multiple of the ablation energy threshold level; a factor of 3 to 4 times the ablation energy threshold is usually sufficient to achieve satisfactory results. Accordirgly, the present invention uses an ablation energy density of less than or equal to about 10 mJ/cm 2 to achieve a reproducible single-pulse etch rate of about 0.2 microns or less per laser pulse, and preferably 0.05 microns or less per laser pulse. This contrasts with current excimer lasers, which only provide reproducible single-pulse etching at an etch rate of no less than about 0.3-0.5 microns per laser pulse, with consequent light scattering due to cornea surface irregularities.

The present invention also recognizes the benefits of ablating cornea with n laser beam having a low energy density. A gentle laser beam, one that is capable of operating at a lower energy density for the surgical procedures, will clearly have the advantage of inflicting less trauma to the underlying tissue. The importance of this point can be illustrated by considering the dynamics of the 16 ablation process on a microscopic scale: the ablation process is basically an explosive event. During ablation, organic materials are broken into their smaller sub-units, which cumulate a large amount of kinetic energy and are ejected out of the host surface at a supersonic velocity. The tissue beneath the ablated region absorbs the recoil forces from such ejections. The present invention recognizes that a shallower etch depth involves less ejected mass per area, and hence reduces the recoil forces correspondingly. In accordance with the foregoing discussion, the laser characteristics 10 of the present surgical system provide for an energy density that results in a reproducible single-pulse etch rate of only about 0.2 microns or less per pulse, and preferably about 0.05 microns or less per pulse. Such a shallow etch rate means less mass ejected per laser pulse. The damage impact on the underlying tissue is less by about a factor of 10 in comparison with the lowest etch rate attainable in the prior art.

Another way to reduce the shock to the cornea is by using a smaller beam area at the cornea to reduce the integrated recoil forces.

20 Consequently, the laser beam cross-sectional area of the invcntion varies from 1 mm in diameter to any tolerably achievable smaller dimension, as required by the particular type of surgery. This characteristic of the invention contrasts with current excimer laser surgical systems, which subject an ablation zone to a surgical beam that is 4-6 mm in diameter.

In summary, the preferred laser corneal surgical system ablates corneal tissue reproducibly at a single-pulse etch rate of about 0.2 microns or less per laser pulse, and preferably about 0.05 microns or less per laser pulse. In accordance with the present invention, a 4 17 laser source with a wavelength range of about 198-300 nm (with a preferred range of about 198-215 nm), and a pulse duration of about 1-5,000 picoseconds, achieves reliable single pulse ablation on the cornea. The intensity of the laser pulses is regulated to have an ablation energy density of less than or equal to about mJ/cm 2 The Inventive Apparatus FIGURE 1 shows the preferred configuration of the inventive apparatus. A laser unit 100 generates an initial laser beam B1. T' laser unit 100 is of the type that can outputs a beam rapidly H deflectable or scannable under electronic control in two dimensio;,to any location in an area defined by orthogonal X and Y axes. One such laser unit is described in detail in the co-pending, commonly- 15 owned patent application for invention entitled "Two Dimensional j Scanner-Amplifier Laser" Patent Application Serial No.

07/740,004), and in the pertinent text reproduced below.

The initial laser beam B1 comprises a sequence of laser pulses having a pulse repetition rate of about 100 to 50,000 pulses per second. Each laser pulse has a pulse duration which can be varied from 1 picosecond to about 5,000 picoseconds. The actual number of laser pulses used for a surgery is determined by the amount of tissue to be removed.

In a preferred embodiment, the laser unit 100 includes a seed laser 102 and a scanner-amplifier laser 104. 'referably, the laser media in both the seed laser 102 and the scanner-amplifier 104 is a Ti-doped A 2 0 3 solid state laser crystal. Further details of the structure and operation of the laser unit 10C are set forth below.

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18 After emerging from the laser unit 100, the laser beam 81 passes through a computer-controllable, motorized zoom lens 106, which provides control over the diameter of the laser beam B1. in practice, the zoom lens 106 may be placed in a number of suitable positions along the optical path of the laser beam between the laser unit 100 and a target. The motor actuation of the zoom lens 106 may be by any known means, such as electrical gear drives or piezoelectric actuators.

10 The preferred laser wavelength for the initial laser beam B1 is in the range of about 790-860 nm. The laser photon energy in the initial laser beam B1 is then converted in a first wavelength converter 108 (described below) by nonlinear wave mixing to a second laser bs am B2 having approximately twice the initial laser be !phetoc- 6ne,'v, and a wavelength in the range of about 395-430 rn To attain the preferred operating laser wavelengths of about •198-215 nm, the second laser beam B2 is passed through a second wavelength converter 110 (described below). The laser photon energy in the second laser beam B2 is again converted by nonlinear wave mixing to a third laser beam B3 having approximately four times the initial laser beam photon energy, and a wavelength in the range of about 198-215 nm.

25 In an alternative embodiment, the initial laser beam B1 may be I wavelength converted to the desired wavelength range of about 198-215 nm using a one-step converter (described below).

\9R~ 19 Surgical Laser Beam Control System While the third laser beam B3 could be used directly for surgical purposes, in the preferred embodiment, the entire surgical laser apparatus includes a number of control and safety systems. In particular, the present invention includes means for monitoring and controlling the intensity of the beam, rrn ans for blocking the surgical beam in the event ot a malfunction, means fer monitoring and controlling the laser beam d:aneter and intensity profile, and means for verifying the two-dimensional scan position of the surgical beam.

Referring again to FIGURE 1, the third laser beam B3 passes through a beam intensity controller 112, the output of which is the surgical laser beam S. The beam intensity controller 112 permits regulation of the energy of each laser pulse so that the etch depth of each pulse may be precisely controlled.

In the preferred embodiment, the beam intensity controller 112 is an electro-optical filter, such as an electrically activated Pockels cell in combination with an adjacent polarizing filter. The Pockets cell may include, for example, LiNbO 3 or any other electro-optical crystal, such as potassium dihydrogen phosphate (KH 2

PO

4 also known as KDP. Pockels cells are commercially available from several sources, including Medox Electro-Optics of Ann Arbor, Michigan. With the application of electric voltage across the electro-optical crystal in a Pockels cell, up to a half-wave retardation in the electric field vector of the incident laser beam can be generated. Depending on applied electrical voltage, the linear polarization of a laser beam traversing the crystal can be retarded from a horizontal polarization to vertical, or vice versa. The polarizer placed adjacent the Pockels cell acts as 2

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e a selector with respect to the incident beam from the Pockels cell.

As is known, if the beam impinging on the polarizer is orthogonally polarized by the Pockels cell, the beam will be essentially blocked by the polarizer. Lesser degrees of retardation generated by the Pockels cell will result in some of the light passing through the polarizer. By controlling the amount of retardation generated in the Pockels cell, the intensity of the incident laser beam can be electrically controlled.

In the preferred embodiment, the beam intensity controller 112 is coupled to a computer control unit 114, which is suitably programmed to vary the intensity of the output surgical laser beam S as required for a particular surgical procedure. The degree of retardation as a function of applied electrical signal can be ascertained by standard calibration techniques. The preferred location of the beam intensity control unit 112 is as shown in FIGURE 1. However, the beam intensity control unit 112 can be placed at several suitable locations in the beam path between the laser unit 100 and a target. In the preferred embodiment, the 20 intensity of the surgical beam S is regulated to have an ablation energy density of less than or equal to about 10 mJ/cm 2 The present invention optionally provides for positive feeo-back Ineasurement of the beam intensity. A partially transmissive beamsplitting mirror 116 is placed after the beam intensity controller 112, and the reflecteu beam Ri is directed to a beam intensity sensor 118. The beam intensity sensor 118 may be simply a photocell, although other elements, such as focussing optics, may be included. By monitoring the electrical output of the beam 30 intensity sensor 118 with the computer control unit 114, the 'I1

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c" r 21 V ml tively measured to controller 112.

function of rtained by intensity of the surgical laser beam S can be posi verify the proper operation of the beam intensity The output of the beam intensity sensor 118 as a intensity of the surgical laser beam S can be asce standard calibration techniques.

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46 4 S I *9 9 The inventive system also preferably includes a safety shutter 120, which is coupled to the computer control unit 114. The safety shutter 120 may be, for example, a mechanically-actuated shutter operated in a "fail-safe" mode. For example, the safety shutter 120 may include a solenoid-actuated shield that is positively held open by application of electrical energy to the solenoid. Upon command of the computer control unit 114, or failure of the entire system, electrical energy to the solenoid is cut off, causing the solenoid to retract the shield into position to block the path of the surgical laser beam S.

Alternatively, thet safety shutter 120 may include a Pockels cell and polarizer configured as a light valve, with the Pockels cell biased 20 with respect to the polarizer by application of an electrical voltage such that maximum light is normally transmitted by the combination. Cessation of the applied voltage will cause the output of the Pockels cell to become polarized orthogonal to the transmission direction of the polarizer, hence blocking the surgical laser beam S. Using this alternative configuration, the safety shutter 120 and the beam intensity controller 112 may be combined into a single unit.

Any other suitable means for quickly blocking the surgical laser beam S on command or in the event of system failure may be used

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i 1 FIGURE 10B is a cross-sectional side view of a cornea,r 22 to implement the safety shutter 120. In practice, the safety shutter 120 may be placed in a number of suitable positions along the optical path of the laser beam between the laser unit 100 and a target.

To control beam diameter, the inventive system provides a partially transmissive beam-splitting mirror 122 that reflects part of the beam Rd to a beam diameter sensor 124. In practice, the beam diameter sensor 124 may be placed in a number of suitable positions along the optical path of the laser beam between the laser unit 100 and a target.

Referring to FIGURE 2A, the beam diameter sensor 124 preferably includes at least a diverging (concave) lens 200 and a converging 15 (convex) lens 202 configured as a magnifying telescope the two lenses have a common focal point, with the focal length f 2 of the convergii, lens 202 being greater than the focal length f, of the S' diverging lens 200, and having optical centers aligned with the incident laser beam in its un-deflected position). The incident beam Rd enters the diverging lens 200 and exits the converging lens 202.

Such a configuration of lenses, while enlarging the incident beam, will also reduce the scan angle of the exiting beam.

The resulting enlarged beam is directed to a low density, low contrast imaging device 204, such as a charge-coupled device (CCD) camera. In the preferred embodiment, a CCD camera having A) a 64 x 64 pixel array with two or more bits of contrast is suitable.

Such canleras are available commercially. The two lens 200, 202 i are chosen to expand the incident beam RF so that the largest possible diameter 206 for the beam just fits within the imaging I I- i. p

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23 device 204 (see FIGURE 2B, which shows only one row and one column of pixels).

In the preferred embodiment, the size of the beam is determined by periodically addressing a central row and a central column of the imaging device 204 and counting the number of pixels on each sampled axis that have been illuminated. By comparing the diameter of the beam in both the X and Y directions, the beam diameter sensor 124 can determine whether the incident laser beam 10 B1 is approximately circular and has the desired diameter. For example, if the number of pixels illuminated on each axis is pixels, the beam will be known to have half the diameter of a beam 00 that illuminated 40 pixels along both axes. As another example, if for any reason the beam has become elliptical, the number of pixels of the imaging device 204 illuminated along the X-axis will differ from the number of pixels illuminated along the Y-axis.

The beam diameter sensor 124 can also be used to determine the intensity profile of the laser pulses, since each pixel in the beam diameter sensor 124 can generate an output indicative of the intensity of light incident to the pixel. By comparing pixel values from radially symmetric points in the pixel array, it can be determined if an incident laser pulse or series of pulses has the desired radially symmetric intensity profilq, or if the pulses have developed "hot spots" of out-range intensity values.

The output of the beam diameter sensor 124 is coupled to the computer control unit 114. 1 he computer control unit 114 is in turn coupled to the motorized zoom lens 106, which provides control over the diameter of the laser beam B1. The computer 4.

V e. iLL.L .L ULI j U Less ana preferably about 0.05 microns or less.

A brief discussion on the mechanism of the ablation

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W w A a 24 control unit 114 is suitably programmed to vary the diameter of the laser beam as required for a particular surgical procedure. The output of the beam diameter sensor 124 as a function of beam diameter can be ascertained by standard calhration techniques.

This configuration provides positive feed-bacK of the beam diameter emanating from the laser urit 100. If the beam Jiameter sensor 124 detects an out-of-rang', beam (either diameter or intensity profile), the computer control unit 114 can take appropriate action, including activation of the safety shutter 120.

STo verify the X-Y scan position of the laser bec.m, the inventive system provides a partially transmissive beam-splitting mirror 1 2o that reflects part of the beam energy R, to a beam location sensor 128. Referr!.g to FIGURE 3A, the beam location sensor 128 preferably includes at least a converging (convex) lens 300 and a diverging (concave) lens 302 configured as a reducing telescope the two lenses have a common focal point, with the focal length f 2 of the diverging lens 302 being greater than the focal length f, of the converging lens 300, and having optical centers aligned with the incident laser beam in its un-deflected position).

SThe incident beam R, enters the converging lens 300 and exits the diverging lens 302. Such a configuration of lenses, while reducing the incident beam, will also increase the scan angle of the exiting beam.

The resu":ing increased-scan angle beam is directed to a silicon photo-detector 304 which provides a voltage reading with respect to two-dimensional location of an illuminating spot at the detector surface. Such detectors are commercially available from a

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variety of sources, including United Detector Technologies, UDT Sensors, Hawthorne, California. The output of the beam location sensor 128 is coupled to the computer control unit 114.

Calibration of the voltage reading generated from the un-deflected incident beam position on the detector 304 will indicate the origin OR of the laser beam in the XY-scan plane. Any deflection of the beam from the origin OR will generate voltage readings indicative of the spot on the detector 304 surface illuminated by the laser beam.

10 These voltage readings are calibrated against the indicated location of the surgical beam as set by the computer control unit 114.

During operation, the output of the beam location sensor 128 would f" be sampled periodically (for example, about 1,000 times per second) and compared to a prepared calibration table in the computer control unit 114 to determine if the actual beam position matches the indicated position.

This configuration provides positive feed-back of the beam position emanating from the laser unit 100. If the beam location sensor 128 S 20 detects an out-of-position beam, the computer control unit 114 can take appropriate action, including activation of the safety shutter 120.

Thus, the preferred embodiment of the inventive surgical laser apparatus provides for safe and effective surgery by continuously monitoring all aspects of the condition of the surgical laser beam S, including beam intensity, diameter, and X-Y scan position.

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0 Ul an N3"om 26 Eye Tracking System When using the inventive system, it is important to minimize eye movement with respect to the surgical laser beam S. Therefore, in order to locate the eye relative to the surgical laser beam S, a conventional suction ring 400, such as is shown in FIGURE 4A, is used to immobilize the eye. Such devices are commercially available, for example, from Steinway Instruments of San Diego, California. Such suction rings are furter described, for example, in U.S. Patent No. 4,718,418 to L'Esperance, Jr.

A suction ring 400 is normally applied to the white (sclera) region of the eye and connected to a low suction pressure sufficient to clamp the ring 400 to the eye, but not so great that the cornea is distorted. The use of such a ring 400 is well-known in the art.

,r Despite the use of a suction ring 400 to immobilize an eye, some S\ movement of the eye may occur (possibly through movement of the suction ring 400 itself by a surgeon). Therefore, the present invention provides an eye tracking system 130 to compensate for relative movement between the eye and the surgical laser beam S.

As shown in FIGURE 1, the eye tracking system 130 is placed in the path of the surgical laser beam S, preferably in close proximity to a target eye.

Referring to FIGURES 4A, 4B, and 4C, a conventional suction ring 400 is provided with distinct marks 402, 404, 406 on the back of the ring facing the surgical laser system (see particularly FIGURE S4B). The marks may or may not be subdivided by cross-marks, for visual reference by a surgeon. In the preferred embodiment, the 30 marks include an X-axis 402, an orthogonal Y-axis 404, and a radial integrated energy of the shorter pulse.

i :1 27 axis 406. The marks are preferably made to be highly reflective of broadband illuminating light, and the background of the suction ring 400 is preferably flat black to enhance contrast and minimize extraneous reflections.

The eye tracking system 130 includes a pair of steering mirrors 408, 410 each comprising a reflector mounted on a galvanometer scanner or similar actuator device which is controllable by a computer, and a rotational control device consisting of a dove prism 409 mounted with its rotational axes aligned with the surgical laser beam S (see FIGURE 4C). A motorized drive unit 411 is attached to a gear or bell drive designed to control the rotation of the dove S....prism 409. As is known, rotation of a dove prism will cause an exit beam to be rotated with respect to an incident beam. The steering mirrors 408, 410 are mounted with their rotational axis orthogonal to each other and situated such that the surgical laser beam S enters the eye tracking system 130, passes through the dove prism 1 4- t409, bounces off of a first steering mirror 408 to the second steering mirror 410, and thence to a target cornea. The steering mirrors 408, 410 and the dove prism 409 therefore provide a means to "bias" the surgical laser beam S to compensate for movement of the eye relative to the surgical laser beam S.

Control of the steering mirrors 408, 410 and the dove prism 409 is provided by reflecting the image of the illuminated marks 402, 404, i 406 on the suction ring 400 back up the optical path of the surgical laser beam S to a partially transmissive beam-splitting mirror 412, which directs the reflected image onto a tracking sensor 414 (other elements, such as focussing optics, may be included in the tracking sensor 414). In the preferred embodiment, the tracking sensor 414 i 28 includes three linear array sensors 416, 418, 420. Each ;inear array sensor 416, 418, 420 corresponds to one of the marks 402, 404, 406 on the suction ring 400, and is oriented orthogonally to the corresponding mark. In the preferred embodiment, each linear array sensor may be a linear reticon with about 1,024 or more sensing elements per inch. Sjch linear reticons are available commercially, such as from EG&G, Princeton, NJ.

Because of the orthogonal orientation of each mark 402, 404, 406 with respect to a corresponding linear array sensor 416, 418, 420, any movement of the suction ring 400 will result in a relative i, displacement of the reflected image of one or more of the marks 402, 404, 406 with respect to the corresponding linear array sensor 416, 418, 420. Such movement can be easily detected by 15 comparing a stored initial position of each mark 402, 404, 406 with Coe, the position of each mark determined by periodically scanning the output of each linear array sensor 416, 418, 420. Because of the relative orientations of the marks 402, 404, 406, translational movements of the suction ring 400 in the X and Y directions, as 20 well as rotational movements, can be detected. The output of the *000 tracking sensor 414 as a function of the positions of the reflected marks 402, 404, 406 can be ascertained by standard calibration techniques.

The eye tracking system 130 may be provided with its own feedback control system to adjust the positions of the steering mirrors 408, 410 and the dove prism 409 to compensate for detected relative motion of the eye with respect to the surgical laser beam S. Alternatively, the eye tracking system 130 may be coupled to the computer control unit 114. Control of the eye FrB i sl~-A. 29 tracking system 130 through the computer control unit 114 is preferred, since the computer control unit 114 can activate the safety features of the inventive system the safety shutter 120) if the target eye is improperly aligned with the surgical laser beam S or if a failure occurs in the eye tracking system 130.

In either case, the output of the tracking sensor 414 would be monitored, and the positions of the steering mirrors 408, 410 and the dove prism 409 adjusted accordingly. In compensating for relative eye movement, it is preferable to first correct for the reflected image position of the mark 402, 404, 406 having the greatest deviation.

The inventive eye tracking system thus provides a means of improving the accurate placement of laser pulses to the cornea. By using distinct marks 402, 404, 406 on the suction ring 400, the invention provides more precise detection of relative movement of the eye comoared with systems using a natural indicator, such as the pupil or the sclera of the eye (which, in any case, could not indicate rotational movement).

Method of Depositing Laser Pulses Another problem addressed and solved by the present invention is the proper deposition of laser beam energy on the cornea to ablate tissue to any desired depth while leaving an optically-smooth cornea surface after the laser surgery. In the prior art, it has been known to apply a laser beam in a raster scan or a circular or spiral scan pattern over the area of the cornea where tissue is to b, removed (see, for example, Figs. 3 and 4 of U.S. Patent No. 4,718,418 to L'Esperance, A problem with such patterns when used with 14 1 microns or less per laser pulse, and preferably aooui u.u DIiui,,o 30 or less per laser pulse. In accordance with the present invention, a A

A

prior art laser systems is that such systems ablate tissue to a depth of about 0.3 to 15 microns or more per laser pulse. A typical procedure for laser etching of the cornea must remove from about 0.2 microns or less, up to about 50 microns of tissue. Since it is essentially impossible to accurately place each and every pulse so that it is perfectly contiguous to a neighboring pulse, ridges or grooves in the corneal surface of the same magnitude will result from the imperfect pattern of deposition of laser pulses. Accordingly, post-operative visual acuity will be reduced because of light 10 scattering from the inhomogeneity of the tissue at the uneven interface.

Another problem of the prior art, particularly with excimer lasers, is that the beam intensities used have principally had a "top hat" intensity profile of the type shown in FIGURE 6A. Such an intensity profile will result in essentially a mirror-image ablation profile, as shown in FIGURE 6B.

Attempts have been made to avoid the sharp edges caused by a "top hat" intensity profile by adopting instead a radial profile having a Gaussian intensity profile (intensity e" 2 where w is the beam waist) (curve 600 in FIGURE 6C) or a super-Gaussian intensity profile, which is a slightly modified Gaussian curve with a lesser gradient at the center (curve 602 in FIGURE 6C), resulting in correspondingly shaped tissue ablation profiles, as shown in FIGURE 6D.

To overcome the problem of haze-inducing ridges and grooves, the prior art has attempted to overlap Gaussian or super-Gaussian beam intensity profiles to generate a smoother average etch profile. For example, as shown in FIGURE 6E, overlapped Gaussian beam

I

Ti-doped

AI

2 0 3 solid state laser crystal. Further aetaiis OT r1

.U

structure and operation of the laser unit 100 are set forth below.

lam- 31 intensities result in an average beam intensity equivalent to that shown in FIGURE 6F, which results in a corresponding mirror-image ablation etch profile as shown in FIGURE 6G.

A problem with this approach is that, during photoablation, vaporized tissue material is expelled from each tissue site ablated by a laser pulse (the expelled tissue is known as "plume"). It is known that such expelled debris can scatter photons in an incoming laser beam (this is known as "shadowing"). As should be expected, this 10 phenomena reduces the intensity of the beam. Thus, when laser pulses are overlapped as described above, a prior adjacent pulse will generate a plume that partially shadows or obscures the incoming laser beam of a subsequent adjacent laser pulse, causing nonuniformity of tissue ablation and hence irregularities on the cornea surface.

0 S An additional problem of prior art overlapped laser deposition patterns is that such patterns -are repeated in such a manner that significant-sized ridges and grooves are still formed between pulse centers.

The solution of the present invention to the problems of the prior art laser deposition patterns is to use a Gaussian, or, preferably, a super-Gaussian intensity profile for each laser pulse, and to deposit 25 the pulses in a plurality of layers, each layer having a regular geometric pattern. The origin of each layer of the pattern is off-set by a specific distance in either the X or Y dimension from each prior, or subjacent, layer. The inventive pattern avoids the problem of plume by not overlapping the laser pulses of i iny one layer, and overcomes the problems of prior art ridge ar;d groove formation by i i ii 32 uniformly depositing laser energy over the surface to be etched.

HBecause of the shallow etch depth of each laser pulse of the present invention (about 0.2 microns or less), etching can be stopped essentially at any point in the ablative process while leaving an optically smooth cornea surface.

Referring to FIGURE 7A, shown is a part of a first level scan pattern, comprising a single etch layer. In the preferred S.embodiment, each laser pulse creates an etch profile with an °i 10 approximately circular cross-section with a radius r, which may :i typically range from about 0.02 mm to about 0.5 mm. The scan pattern programmed into the laser unit 100 lays down a pattern of pulses in a hexagonally-packed array of the type shown in FIGURE S,7A. That is, the center A of each circular cross-section etch circle 700 of radius r is spaced a distance D, equal to 2r, from the center A of each other etch circle 700. As is known, the pattern resulting S" from this simple criteria is a hexagonally-packed array of circles (the dotted hexagons shown in FIGURE 7A are for purposes of illustrating the packing pattern, and do not form any part of the etch profile).

While it is preferred that the etch circles be non-overlapping and contiguous, the invention encompasses slight overlapping and/or spacing of etch circles due to tolerance limits on positioning etch circles with a practical laser apparatus.

A benefit of the hexagonally-packed array of etch circles is that the pattern is simple to program into the scanning control system of the laser unit 100 as a modified raster scan. If etch circle 702 having Si 30 center A' is considered to be the origin for the initial first level I;1i; m- i pattern, the laser unit 100 need only move the laser beam in the X direction a distance of D to the center A for the next etch circle 704. Additional etch circles are created in the same manner for the first row, until the opposite edge of the area to be ablated is reached. Such precision of placement of etch circles is made possible by the highly accurate X-Y positioning capability of the laser unit 100, particularly when used in conjunction with the eyetracking system described above.

10 After the first row of etch circles is completed in the same manner, the laser beam is moved down in the Y direction a distance of about 0.866D (one-half the square root of 3 times D, representing the vertical distance between centers of adjacent rows), and left or o' right along the X direction by Y2D (representing the horizontal distance between centers of adjacent rows). The beam is then either scanned backwards, or returned to the original "edge" of scanning and scanned forwards in the same manner as the first row. Each subsequent row is created in the same manner, until the bottom edge of the area to be ablated is reached, thus completing the first level layer.

Although a regular order of etch circle deposition is preferred for ease of programming the laser unit 100, the accurate X-Y positioning capability of the laser unit 100 permits the etch circles for a particular layer to be deposited in any order, including randomly.

A characteristic of the first level pattern shown in FIGURE 7A is I that no circular etch substantially overlaps any other circular etch.

Consequently, the poblem of plume is minimized. While laying '4 34 down only the first level pattern shown will result in ridges in the gaps between etch circles, because of the shallow etch depth used, the crest-to-trough distance of any ridge area R to the center A of any etch circle 700 will be at most about the same as the etch depth of a single etch (about 0.2 microns or less).

e o r ~o

I

rr e

D

r r r o e After laying down the initial first level pattern shown in FIGURE 7A, the inventive method preferably lays down a second level pattern, comprising three etch layers. Each second level etch layer is an 10 exact replica of the single etch layer of the first level pattern an hexagonally-packed array of etch circles of radius However, the origin of each of the three layers with respect to each other and to the first level layer is unique. In order to minimize ridges and grooves in the etched cornea, each layer of the second level pattern is offset from the single layer of the first level pattern to even-out the distribution of laser energy across the cornea. This concept of off-setting subsequent layers in exact relationship with respect to an initial layer is in contrast to the prior art, which typically repeats the etching process by sweeping the laser beam across the ablation zone without reference to the exact location of each of the laser pulses.

More specifically, the origin of the first layer of the second level is set at point B1 (or an equivalent point; see below) of FIGURE 7A, which is one-half the distance D between the first level origin A' of etch circle 702 and the center of the adjacent etch circle 704.

Using point B1 as an origin, the laser unit 100 is programmed to lay down an entire array of etch circles covering the area to be ablated, using the same rules for changing beam location as described above for the first level etch layer.

q- /Any Ui i O uLIcIW ui-LJ-. I 30 beam S on command or in the event of system failure may be used Similarly, the origin of the second layer of the second level is set at point B2 (or an equivalent point; see below) of FIGURE 7A, which is one-half the distance D between the first level origin A' of etch circle 702 and the center of the adjacent etch circle 706 in the next row. Using point B2 as an origin, the laser unit 100 ;s programmed to lay down an entire array of etch circles covering the area to be ablated, using the same rules for changing beam location described above for the first level etch layer.

Lastly, the origin of the third layer of the second level is set at point B3 (or an equivalent point; see below) of FIGURE 7A, which is onehalf the distance D between the center of etch circle 704 and the f nter of the adjacent etch circle 706 in the next row. Using point B3 as an origin, the laser unit 100 is programmed to lay down an entire array of etch circles covering the area to be ablated, using the same rules for changing beam location as described above for the first levl etch layer.

The resulting etch pattern for the second level will resemble the pattern shown in FIGURE 7B, which shows the first level centered at A' as thick-lined circles 715, the first layer centered at point B1 as solid circles 716, the second layer centered at point B2 as dotted circles 717, and the third layer centered at point B3 as dashed circles 718. If desired, the level one and two etch patterns may be 25 repeated as needed to obtain the desired amount of ablation.

Although the second level comprises three layers, all three layers need not be completed. Further, the first layer of the second level can be started with its origin at any of the three points B1, B2, or B3, since all of these points are geometrically equivalent. More 1I

I-:

generally, equivalents of these three offset points exist throughout the grid of centers A defined by the initial first level layer. Thus, any equivalent offset point in the second level may be selected as the origin of one of the three layers comprising the second level.

The overall etch profile (determined as described below) determines which of the equivalent second level offset points will be selected to achieve maximum ablation where required in the surface being etched, and whether the desired degree of smoothness of finish requires completion of each of the second level layers. However, to 10 ensure evenness of etching, it is generally desirable to complete all layers of the second level before additional levels of etching commence.

As an alternative way of modelling the first d second levels, they may instead be considered as a single etch pattern "unit" comprising four etch layers arranged to overlap in the manner shown in FIGURE 7B.

If desired, further levels of etch patterns could be generated in a similar fashion by repeating levels one and two, using new origins.

A characteristic of the geometry of an hexagonal packing array lends is that it lends itself to creation of repeating regular patterns.

For example, referring to FIGURE 7A, the origins B1, B2, and B3 for the second level etch layers comprise the midpoints of a triangle T1 connecting the centers of etch circles 702, 704, and 706. A second triangle T2 can be created by connecting the centers of etch circles 704, 706, and 708 (shown in dotted outline in FIGURE 7A).

These two triangles comprise a symmetrical unit that is repeated throughout the pattern of centers A defined by the initial first level pattern. Moreover, by connecting adjacent midpoints, each of the 4 control over the diameter of the laser beam B1. The computer

I

37 triangles T1 and T2 can be subdivided into four smaller, equal-sized triangles, as shown in FIGURE 7A. The midpoints of each subtriangle in a T1-T2 unit not shared with a similar T1-T2 type unit comprise twelve offset points C1-C12 that have equivalents throughout the grid of centers A defined by the initial first level layer.

Each of these equivalent offset points C1-C12 can be used as an :1 origin for a set of level one and level two etcn patterns. That is, 10 taking point Cli as an example, Cll can be selected as the center of a level one pattern. The level one pattern centered at Cll then defines a new grid for a corresponding level two pattern. Similarly, point C2 could then be selected as the center of another level one pattern. The level one pattern centered at C2 then defines a new grid for a corresponding level two pattern.

,With such third level equivalent offset points defined, any of them may be selected as the origin of one of the 48 layers (12 level one/level two sets) comprising the third level. The overall etch profile (determined as described below) determines which of the equivalent third level offset points will be selected to achieve maximum ablation where required in the surface being etched, and whether the desired degree of smoothness of finish requires completion of each of the third level layers. However, to ensure evenness of etching, it is generally desirable to complete all layers i of the third level before additional levels of etching commence.

r The process of defining subsequent levels may be extended as necessary, by defining equivalent offset points based on repeating geometrical units determined by the grid of centers A defined by the 38 initial first level layer. Th- general rule is to divide a previous level into additional triangles J on the grid defined by the in;tial first level layer. This is done by conne, &ng th ee adjacent origins to form such triangles, and then using the midpoints o: uch triangles as new origins.

The extent of improvement on the surface smoothness by the precise positioning of multiple layers of etcn profile is illustrated in the following: Using a Gaussian laser b profile as an example, i 10 and setting the laser energy density at peak of the laser pulse to be four times thj ablation threshold, the surface smoothness can be characterized in relation to the maximum etch depth of a single laser pulse. For example, after applying the first level etch pattern as described above using a Gaussian intensity profiie, the maximum crest-to-trough distance of the etch patterns anywhere within the boundaries of the etched area will of course be 100% of the maximum crest-to-trough distance of a single etch circle. By applying just two levels of etch patterns as described above using a Gaussian intensity profile, the maximum crest-to-trough distance of the overl:pped etch patterns anywhere within the boundaries of the Setched area will be at most about 53% of the initial first level pattern alone. By applying three levels of etch patterns using a Gaussian intensity profile, the maximum crest-to-trough distance of the overlapped etch patterns anywhere within the boundaries of the etched area will be at most about 20% of the initial first level pattern alone. Since the crest-to-trough distance for the initial first level pattern is about 0.2 microns or less, and preferably about 0.05 microns or less, even the second level pattern may be sufficient to achieve the desired result.

3u 39 This analysis is graphically presented in FIGURE 7D, which shows a set of graphs showing the cumulative crest-to-trough distances of level one, level two, and level three etch patterns in accordance with the present invention. The Y-axis of each qaph shows the cumulative etch depth in units of the maximum etch depth of a single laser pulse. The X-axis shows etch depth as a function of the distance from an etch circle center out along one of the three symmetry axes for an hexagonal array. FIGURE 7E shows the Smeasurement axes used to compute the level two and level three 10 crest-to-trough distances of FIGURE 7D. The notation for the endpoints of the X-axis of FIGURE 7D corresponds to the notation for the measurement points shown in FIGURE 7E.

The regular characteristics of the inventive deposition system are useful whe, etching the cornea in a "stepped pyramid" fashion (in terms of laser pulse count), with fewer etch circles deposited towards the periphery of the cornea and more etch circles deposited towards the center. As shown in FIGURE 7C, the resulting overall etch pattern typically resembles concentric circles (although other shapes are possible). When etching from the outer edge to the center, the entire cornea is etched to the diameter of ring 720, using the etch patterns discussed above. Using the original grid of centers A from the very first level, a new origin at an equivalent offset point within ring 722 is chosen when the tissue in ring 720 has been etched to the desired degree. Etching continues over the entire cornea encompassed within the diameter of ring 722. Again, using the original grid of centers A from the very first level, a new Sorigin at an equivalent offset point within ring 724 is chosen when the tissue in ring 722 has been etched to the desired degree.

Etching continues over the entire cornea encompassed within the diameter of ring 724. The process continues in similar fashion until the center ring 726 is etched to the proper depth. (Note that the diameter of the laser pulses may be made smaller in the inner rings to provide a finer etching grid, in which case, a new initial first level pattern defining such a grid may be laid down and used in determining equivalent offset points for subsequent levels).

As should be clear by considering FIGURE 7C, the etch process .:..could be done in reverse order, with center ring 726 etched first, 10 then the area encompassed within the diameter of ring 724, then the area encompassed within the diameter of ring 722, and finally the area encompassed within the diameter of ring 720.

Wavelength Converter Means As described above, the inventive system includes at least one wavelength converter to alter the wavelength of the initial laser beam B1 to the desired wavelength in the range of about 198-215 nm.

A first example of one wavelength converter is shown in FIGURE In FIGURE 5A, the initial laser beam B1 emerging from the laser unit 100 is shown to have been scanned at an incident angle 01 from its central position 500, which is defined as the center position of the total scan angle to be covered for an intended surgical operation. Generally, the laser beam is scanned in two dimensions, and hence two angular positions are needed to specify each unique beam position. In the preferred embodiments in FIGUREs 2A and 2B, the optical system is spherically symmetric.

Thus, only one of the incident scan angles will be illustrated in the following discussion without loss of generality.

IT

elements, such as focussing optics, may be included in the tracking sensor 414). In the preferred embodiment, the tracking sensor 414 :r 41 In FIGURE 5A, a convex lens A is located at a distance the focal length of lens A from the pivot point of the scanned laser beam B1. In the illustrated embodiment, the pivot point is inside the scanner-amplifier unit 104, at an equivalent position of the scan mirror near the exit dielectric mirror, as described below. A nonlinear optical crystal 502 is chosen such that phase matching angles exist with a proper crystal orientation so that a fundamental laser wavelength within a range of about 790-860 nm can be converted to its second harmonic at a wavelength in the range of about 395-430 nm. One possible such crystal is beta-Ba 2

BO

4 (bta barium borate). This crystal has a phase matching angle at about 26-30 for the wavelength range stated above, in type I phase matching conditions. The nonlinear crystal 502 is positioned at a distance f(A) from the lens A. The incident laser beam B1 is weakly focused at the crystal 502 with a choice of a long focal length for lens A. Another convex lens B located at the focal length f(B) of lens B from the crystal 502 re-collimates the beam into an emergent laser beam 82. Preferably, both lenses A and B are coated for maximum transmission at laser lengths for which each transmits.

2 The dimensions of the nonlinear crystal 502 are chosen such that the surface area where the incident laser beam B1 enters the crystal 502 is sufficiently large that the laser beam will not be cut off at the extremity of its scan angles. The length 1 of the crystal 502 is such that the conversion efficiency is to be optimized, in consideration of the walk off between the fundamental and the second harmonic beam, the group velocity dispersion, and the spectral bandwidth for the short duration laser pulses. The entrance surface of the nonlinear crystal 502 is coated for maximum transmission of the fundamental wavelengths and the exit J surface is coated for maximum transmission of the second harmonic wavelengths.

The optical arrangement of the present embodiment of the wavelength converter offers several additional advantages: the scan angle 01 of the incident laser beam B1 can be magnified or reduced by choosing the proper focal length for the lens B. If f(B) is smaller than the beam scan angle 02 in FIGURE 5A will be magnified by a factor On the other hand, if f(B) is larger than f(A), the beam scan angle 02 will be reduced by a factor of It is important to note that the laser beam, which subtends an angle 01 from the central position 500, becomes parallel to the central 4 position 500 after passing through lens A. Therefore, lens A provides two improvements in the harmonic conversion process: the laser photon density at the non-linear crystal 502 is increased due i to the smaller beam area, and the laser beam orientation incident at the non-linear crystal 502 is maintained at all scan angles, thereby 1 4maintaining the phase matching conditions of the beam while it is

I

being scanned.

i Another advantage of the embodiment shown in FIGURE 5A results from the change of location of the incident laser beam B1 through the nonlinear crystal 502 as the beam is scanned. Within the nonlinear crystal 502, a small amount of the laser beam is absorbed, resulting in a thermal gradient across the beam crosssection. This temperature variation at different portion of the incident beam B1 degrades the phase matching conditions, and places a limit on the conversion efficiu-ncy of the harmonic generation process. By moving the beam over an area during

'IT

i: g

V

K?,

(see, tor example, Figs. 3 and 4 of U.S. Patent No. 4, /1 ,4-b to L'Esperance, A problem with such patterns when used with if c 43 scanning, the thermal energy is effectively distributed over that area, and the average power loading in the crystal 502 is effectively reduced. If the area is sufficiently large, the laser pulses becomes non-overlapping. Reduction of pulse overlapping ahso results in an improved crystal damage threshold. For a laser beam with a high repetition pulse rate, if the laser beam is stationary, as in the prior art, there is a time delay requirement, so that the effect of a laser pulse through a nonlinear crystal is allowed to dissipate before the next laser pulse arrives. This requirement places an upper limit on 0 the repetition rate at about 10,000 pulses per second. The present invention overcomes the above prior art limitations, and provides an improved method of laser wavelength conversion to attain a higher conversion efficiency and a higher crystal damage threshold by scanning the laser beam across the nonlinear crystal 502. With the S 15 present invention, the repetition rate of the surgical beam can be extended to over 50,000 pulses per second.

A second example of a wavelength converter is shown in FIGURE In this embodiment, lens A and the nonlinear crystal 502 are similarly located as specified for FIGURE 5A, except that the crystal 502 is positioned slightly closer to lens A. A high reflective mirror I504 is located at a distance f(A) from lens A. The reflective mirror 504 has the characteristics of being highly reflective at the fundamental and the second harmonic wavelengths of the incident laser beam B1. A partially-transmissive beam-directing mirror 506 is included in the beam path, and is set at about 450 from the laser beam central position 500. The beam-directing mirror 506 is coated with dielectric thin films for high transmission of the fundamental wavelength, and high reflection of the second harmonic wavelength at near 450. An incident laser beam B1 at an angle 0, with the p l-BlliQps~ IIIBBII~1Dl~aql~ls ir; _lrlll- 14119F1--~QU- DI -s~LLli central position 500 passes through the beam-directing mirror 506, Land is focussed by lens A on the nonlinear crystal 502. The beam is then reflected at the reflective mirror 504, passes through the crystal 502 a second time, and re-traces its beam path through the lens A. The second harmonic portion of the beam is then reflected by the 450 beam-directing mirror 506. The exit beam is now at an angle 0, from a rotated central position 501, which is at two times the exact angle of the beam-directing mirror 506, and thus is about 900 with respect to the central position 500.

1 The advantage of the structure and method shown in FIGURE 5B is S.that the non-linear crystal 502 is used twice by the fundamental beam. This method can almost double the conversion for the case of low conversion efficiency, in the case where the fundamental beam intensity is not significantly depleted in its first passage when the wavelength conversion efficiency is less than about 30-40%).

In the present embodiment, the residual laser photons of the initial laser beam B1 will not be used and may be filtered out by a dielectric coated mirror which has bandpass characteristics of high transmission at the second harmonic wavelengths of about 395-430 nm and blocking at the fundamental wavelengths of about 790-860 nm. Alternatively, the fundamental and the second harmonic waves can be separated spatially using dispersive optical elements, such as a high optical index prism or an optical grating.

The filtering optics (not shown) can be placed in the beam path after the beam emerges from the first wavelength converter 108.

IT.. 1WI~-rm*I As noted above, after emerging from the first wavelength converter 108 (see FIGURE the second laser beam B2 has a wavelength in the range of about 395-430 nm. To attain the preferred operating laser wavelengths of 198-215 nm, the laser beam is directed into a second wavelength converter 110.

The optical arrangement of the second wavelength ronverter 110 is almost identical to that of the first wavelength converter 108, which is illustrated in FIGURES 5A and 5B. The main difference between the two converters 108, 110 is in the optical crystal 502.

For wavelength conversion from about 395-430 nm to about S198-215 nm, the preferred nonlinear optical crystal is again beta-Ba 2

BO

4 (beta barium borate). The operating conditions are different in that the phase matching angles are at or close to for type I phase matching. The optical faces of the crystal 502 are *to be coated for maximum transmission at the front surface for the fundamental wave where the laser beam enters the crystal 502, and for maximum transmissio, at the second harmonic wave on the exit face. The optical characteristics of beta-Ba 2

BO

4 crystals 20 impose a lower limit of the converted wave at about 200 nm for appreciable conversion efficiency.

The third laser beam B3 emerging from the second wavelength converter 110 has a residual wavelength of about 390-430 nm. A wave filter means consisting of dispersive prism or optical gratings can be used to spatially separate the 200 nm wavelengths from the 400 nm wavelength contents. The wave filter (not shown) can be placed anywhere in the laser beam path after the second i wavelength converter.

I'

J- I i i li~lU- 4 laser unit 100 as a modified raster scan. If etch circle 702 having center A' is considered to be the origin for the initial first level i 46 The two-step wavelength conversion process described above can also be consolidated such that the fundamental wavelength can be converted into its fourth harmonic with a single optical arrangement. As illustrated in FIGURES 5A and 5B, first and second nonlinear optical crystals 502, 503 (shown in dotted outline) are placed in close proximity and are at the beam waist of the mildly focussed incident laser beam. The first crystal 502 is cut and oriented for phase match conditions to generate the second harmonic wave. The first crystal 502 is used to convert the fundamental wave into its second harmonic wave, and has a function as described above for the single-crystal embodiment of the first wavelength converter 108. For this purpose, in the configuration shown in FIGURE 5A, the first crystal 502 is placed in front of the second crystal 503, facing the incident laser beam B1 15 emerging from the scanner amplifier. The portion of the laser beam S1B1 converted into the second harmonic wavelength of about 390-430 nm after passing through the first crystal 502 is then incident upon the second nonlinear crystal 503. The second crystal 503 is cut and oriented for phase matching as described above for the second wavelength converter 110, resulting in another step of second harmonic conversion, now using the 390-430 nm beam from the first crystal 502 as the fundamental wave.

In the configuration shown in FIGURE 5B, the second crystal 503 is placed in front of the first crystal 502. However, in FIGURE 5B, the incident laser beam B1 passes through the second crystal 503 with practically no conversion, since the crystal is oriented for phase matching for the 390-430 nm laser pulses emerging from the first crystal 502 after the pulses reflect off of the reflective mirror 504.

Thus, the portion of the laser beam 81 converted into the second i rwl---Y- Pua"- C 47 harmonic wavelength of about 390-430 nm after passing through the first crystal 502 is reflected and then incident upon the second nonlinear crystal 503. The second crvtal 503 is cut and oriented for phase matching as described above for the second wavelength converter 110, resulting in another step of second harmonic conversion, now using the 390-430 nm beam from the first crystal 502 as the fundamental wave.

Other modifications are necessary for optimal operation of such a 10 one-step wave converter. For the optical arrangement shown in FIGURE 5A, lens B is properly coated for maximum transmission (anti-reflection) at the 200 nm range. The material for the lens is preferably UV quartz for good optical transmission. In FIGURE the modification is that the coating characteristics of the dielectric 15 mirror 506 be highly reflective at about 198-215 nm at a 450 S• incident angle. These improvements for optical transmission for lens B in FIGURE 5A also apply to lens A in FIGURE In the foregoing discussion, a laser fundamental wavelength range 20 of about 790-860 nm is illustrated for a Ti-doped AI 2 0 3 laser.

However, a Ti:AI 2 0 3 laser has an operating range of about 680 nm to about 1200 nm. Therefore, the wavelength conversion apparatus and method described above can be applied to generate a slightly extended output wavelength range of about 396-600 nm after the first conversion, and about 198-300 nm after the second conversion, without loss of generality (the lower limits are about 396 nm and 198 nm, respectively, rather than about 340 nm and 170 nm, because of limitations of the nonlinear conversion crystal S502).

*i 48 In an alternative embodiment, the wavelength conversion apparatus and method may include sum frequency generation with two different laser wavelengths. In this case, the first wavelength converter 108 is structurally as described above. If the fundamental wavelength is selected to be about 790-900 nm, those wave!engths can be used to mix with the second harmonic wave of about 395-450 nm. However, the nonlinear optical crystal 502 in the second wavelength converter 110 has to be cut and oriented for phase matching conditions for the fundamental and the second harmonic waves in order to generate a laser wavelength of about :263-300 nm. If the fundamental laser wavelength from the laser unit 100 is in the range of about 790-900 nm, the laser wavelength at the output of the first wavelength converter 108 is modified by the wave mixing action of the second wavelength converter 110 to about 263-300 nm.

Operation of the Inventive Apparatus In order to improve the ease of use of the present invention, and to ensure proper alignment of the surgical laser beam S with respect to a target cornea, the present invention includes a guide beam unit 132 (see FIGURE The guide beam unit 132 is illustrated in greater detail in FIGURE 8.

The guide beam unit 132 includes a low-power laser 800 with an output of preferably less than 1 milliwatt at initial output and preferably attenuated to the microwatt level for safe usage for direct viewing. The laser 800 in the guide beam unit 132 may be, for example, a HeNe laser or a semiconductor diode laser. The laser 800 generates a guide beam 801 which is conditioned optically so that it can be used as a indicator of the location of the surgical laser ~I

K

49 beam S. Additionally, the guide beam 801 can be used as an element for the alignment of the eye in preparation for surgical procedures.

After emerging from the laser 800, the guide beam 801 the diameter of the guide beam 801 is expanded through a telescopic beam magnifier consisting of divergent lens 802 and a convergent !ens 804 to about 10 mm. The collimated beam is then compressed by a weakly focussing lens 806, with a focal length of over 500 mm, at a distance of approximately the location of the patient's cornea. An axicon first and second prism pair 808, 810 are aligned with the expanded beam such that a divergent ring image with uniform intensity is produced after the first prism 808.

The second prism 810 intercept the divergent ring and diffracts it to 15 form a ring image 812 without divergence. The diameter of the ring image is controlled by the separation between the prism pair 808, 810: the farther they are separated, the larger is the ring. The position of the second prism 810 can be adjusted by a manual or S2 motorized drive. The guide beam 801 emerges from the guide 20 beam unit 132 and is denoted as beam G in FIGURE 1.

*55.

a The ring-shaped guide beam G from the guide beam laser unit 132 is aimed at a partially transmissive mirror 134 arranged so that the reflected guide beam G is coaxial with the un-leflected surgical laser beam S. In operation, a surgeon would move the patient's head and target eye until the guide beam G is roughly centered over the patiert's cornea. The surgeon will then adjust the diameter of the ring image projected on to the patient's pupil, such that the ring diameter is only slightly less than the patient's pupil size. At that i:i throughout the pattern of centers A defined by the initial first level pattern. Moreover, by connecting adjacent midpoints, each of the point, the patient will see the guide beam G, but not necessarily at a centered position.

The patient is preferably situated in a relaxed position supine), but with his or her head fixed within a cradle or fixture. Either the cradle or the entire operating table or chair is configured to be adjustable in fine increments about an X-Y plane perpendicular to the surgical laser beam S. The patient would then make fine adjustments of his or her own eye position with respect to the guide beam G by moving an actuator-control mechanism a joy stick) for the cradle or operating table or chair, until the patient determines that the guide beam G appears to be at its brightest. At the completion of the patient's adjustment, the patient's eye is aligned with the patient's visual axis, coinciding with the guide 15 beam G.

An advantage of the ring-shaped guide beam G of the present invention over a solid beam is that the light fall-off, or decrease in brightness, is greater for the ring-shaped beam than for the solid 20 beam when either beam is not aligned with the patient's visual axis.

Thus, the ring-shaped beam provides greater visual cues to the patient when the beam is off-axis.

After the eye of the patient has been aligned using the guide beam G, the surgeon may place a suction ring 400 over the patient's eye to immobilize it. The eye tracking system 130 is then activated to compensate for any subsequent motion of the eye. With the eye 1 immobilized, the surgeon may then commence ablative surgery using the inventive laser system.

i '1 I-Llfl lD~gls~p~~g~-~ lfl~ D BIClOls9L 51 To determine the location of each area to ablate, and the depth of ablation required, an automatic feed-back control system may be used with the inventive system. Such a control system preferably includes a corneal profiler 136 which provides information to the computer control unit 114 sufficient to determine the necessary intensity and XY-scanning coordinates for the surgical laser beam S, and to otherwise control the delivery of pulses of laser energy to the cornea, in order to achieve a desired corneal surface profile. A suitable corneal profiler 136 is any device that measures the shape 10 or an optical property of the eye so as to provide such information.

As an alternative to a single profile measurement and ablation of the cornea based on indicated parameters, a desired corneal surface .profile may be obtained through ablation by a successive approximation technique. In this technique, a measuring device is used to determine the change desired to be made in the profile of ,i the corneal surface. Pulses of laser energy are delivered to the surface so as to bring about slightly less than the desired degree of alteration. The measuring device is then used again to determine the correction now needed to reach the desired profile. Further pulses of laser energy are provided accordingly to produce slightly less than the total calculated correction. This process is repeated until the ablated surface acquires the desired profile to a suitable degree of accuracy.

Measurement devices suitable for the corneal profiler 136 are keratometers, which are known and commercially available.

Examples of such devices are the "Photokeratoscope" manufacture by the Sun Contact Lens Company of Kyoto, Japan, and the "Corneascope" manufactured by International Diagnostic _1 A ~h l r" i r microns or less, even the second level pattern may be sufficient to achieve the desired result.

!30 52 Instruments, Ltd., Broken Arrow, Oklahoma, USA. (See also S.D.

Klyce, "Computer Assisted Corneal Topography", Invest.

Ophthalmol. Vis. Sci. 25:1426-1435, 1984 for a comparisci of these instruments and a method of using the "Photokeratoscope").

These devices work by imaging patterns, usually concentric rings, on the corner. jurface. Preferably, the keratometer used as the corneal profiler 136 in the present method is modified slightly to increase the number of lines imaged on the central portion of the cornea, thus increasing the measurement resolution of the curvature 10 of the contral portion.

In the preferred embodiment, the corneal profiler 136 receives a reflected image of a target cornea by means of a mirror 138, which is movable between an out-of-line position A and an in-line position B. When the mirror 138 is in position B, an initial profile of the cornea can be determined by the corneal profiler 136. The output of the corneal profiler 136 is coupled to the computer control unit 114, and displayed to a surgeon. In response to any resulting input 'rom the surgeon. such as the desired final shape of the corneal 2J surface, the computer control unit 114 determines the necessary settings and parameters, including pulse intensity, beam diameter, S\i and target locations on the cornea, for the inventive laser system to create the desired ablation profile. The mirror 138 is then moved to position A, and the surgery commenced.

If the successive approximation technique described above is used, the mirror 138 is periodically moved back into position B, the corneal profile is re-measured, the computer control unit 114 resets the laser system, the mirror 138 is retracted to position A, and the surgery r, -commenced.

5 53 In determining the necessary settings and parameters, including pulse intensity, beam diameter, and targe' locations on the cornea, for the inventive laser system to create the desired ablation profile, the computer control unit 114 essentially prepares a threedimensional contour map of the difference between the cornea profile as measured and the desired final shape of the cornea.

Each point in this contour map may be described in terms of rectangular or polar coordinates, in known fashion. Starting with a selected pulse etch profile a selected beam intensity and 10 intensity profile), the etch depth of each pulse will be known (such information can be determined in advance by calibrating sets of profiles on corneal tissuei. With a preselected etch depth, the contour map can be cvired into a plurality of etch levels (for example, of the type snown in FIGURE 7B). Then, using a selected 15 initial laser pulse diameter, each level of the contour map may be characterized in terms of the X-Y coordinates of a first level patter:n of etch profiles of that diameter.

For example, if the area to be ablated has a maximum diameter of 8 20 mm, and the laser pulse diameter is about 0.1 mm, then a grid of x 80 pulses will cover the entire maximum diameter. Arbitrarily selecting a single origin for such a grid means that each point on a level of the contour map can be defined in terms of X-Y coordinates. As the levels become smaller in size, the laser pulse diameter may be reduced accordingly, but the principal of mapping ji each Ilvel of the contour rmap to a grid of X-Y coordinates remains the same.

.1 II 1.

FIGUREs 2A and 2B, the optical system is spherically symmetric.

Thus, only one of the incident scan angles will be illustrated in the following discussion without loss of generality.

Yr cd *oaa 4* 4 ,p *5 S. S a, a

B

Structure of the Scanner-Amplifier Laser Unit This part of the disclosure is directed to a laser amplifier system utilizing a pair of scanning mirrors driven in tandem by piezo actuators. A control system is provided to direct a low-power laser beam while the beam is trapped and circulates between the pair of scanning mirrors. Each bounce of the laser beam between the mirrors discretely increases the power of the beam and changes the angle of exit of the beam from the amplifier, providing for precise angular beam exit control in two dimensions.

In the preferred embodiment, a laser scanner-amplifier system 8 with Ti-doped sapphire A1 2 0 3 is used as the laser medium.

However, the laser medium can other tunable solid state laser materials, such as alexandrite, raid, Cr:LiCaF, Cr:LiSrF, Cr:forsterite, color center laser -are earth ion laser media, such as Nd, Pr, Er, Tm, Ho, or other ition metal ions such as Co, Ni in various solid state crystal hosts, including oxides or fluorides.

A laser pulse train from a mode-locked Ti-doped A1 2 0 3 laser 10 in 20 FIGURE 11 is to be used as a seeder to the amplifier scanner system. The laser pulse frequency of the mode-locked laser, as is well known in the art, can be controlled by the round trip time of the laser pulse inside the mode-locked laser, and is at twice the driver frequency of the electrical signal applied to the mode-locked crystal. The frequency is chosen such that time period between adjacent pulses bears a preferred relationship with the arrangements of the optical elements inside the scanner-amplifier system. In the case of Ti-doped A1 2 0 3 a continuous wave laser 12 such as, but not limited to, 3n argon gas laser operating at 514.5 nm or a frequency-doubled YAG or YLF laser at 532 nm and 527 nm

F'

47i

V.

net)

*CO

0 00 no.

*00* 0.40 0 C *D C

O

O

0

CC..D

respectively, can be used as the pump source. The pump laser beam 13 is focused into the mode-locked laser medium with a convergent lens 14. The arrangement of a laser-pumped modelocked laser is well known in the art znd a commercial model is available from Spectra-Physics, Mountain View, California.

The mode-locked laser beam 15 passes through a set of beam conditioning optics 16, 18. In FIGURE 11, the beam cross-section is expanded by a negative (concave) lens 16 and a positive (convex) lens 18 with their focuses coinciding to form an expansion telescope. The expansion ratio can vary from 2 to 10 by choosing the appropriate focal lengths of the optical elements 16 and 18, and is determined by the mode-matching requirement between the seeo beam 15 and the spatial mode of the amplifier cavity. By centering 15 the lenses along the laser beam, minimum beam distortion and good beam collimation can be achieved as the seed beam 15 exits the optical element 18.

The seed beam is directed by high reflective mirrors 20 and 22 into 20 the amplifier cavity. The beam first enters the cavity through a dielectric coated mirror 24 which has the optical characteristics that a pi-polarized laser beam with the electric field vector horizontal to the plane of incidence has over 96% transmission, and a pipolarized laser beam with the electric field vector vertical to the plane of incidence has over 99% reflectability. Such thin-film polarizer elements are supplied by Burleigh NorthWest, Fishers, New York. The scanner-amplifier cavity 8 is confined between the scanner mirrors 26 and 28, both of which are highly reflective mirrors. The scanner mirrors 26, 28 are each mounted on a gimbal mount 29 with 900 tilts in both the horizontal and the vertical (X-Y) r :o ~X r l ja j .a i; rC -~ri b 1 i u YI-~ 0

D

0 0 ~O I 0~ 0 :D .0 a I f'A directions. The design of the gimbal mount can be illustrated as a mirror mount model number MM-1 manufactured and supplied by the Newport Corporation, Fountain Valley, California, with appropriate modifications to shorten the pivot point distance and increase the spring force. The X-Y tilts are achieved by piezoelectric actuators 31 with a material such as PZT which can have a linear travel of 40 microns of full scan range at about 1000 Hz, and at higher frequencies with smaller travel range. Such piezo actuators are supplied by a number of suppliers, including Burleigh 10 Instruments, Fishers, New York. The scan mirrors 2- and 28 are driven in the same direction at the same angular degree either independently or 1. Tandem in both the X and Y directions.

The operating characteristics of the piezo actuators may have small 15 variations. The overall scan angles of the laser beam as emerged from the scanner-amplifier is to be calibrated against the voltage applied to the piezo actuators 31, taking into the account the small amount of hysteresis from the piezoelectric effect.

A pair of concave lenses 30 and 32 are included inside the scanneramplifier cavity. The focal lengths of the lenses 30 and 32 are such that the focal length of lens 30 is chosen to be as large as possible, yet the size of scanner-amplifier is to be practical and convenient for use, and the focal length of lens 32 will be as short as possible, yet not so short as to cause optical break-down at its focal point.

The relative locations of the lenses 30, 32 and end mirrors 26 and 28 are such that the mirrors 26 and 28 are to be at the focal point of the lenses 30 and 32, respectively, and the separation between the lenses is to be the sum of their focal lengths. Another dielectric-coated mirror 34, which has similar characteristics as

,I

.4: Y- I vwavelength, and high reflection of the second harmonic wavelength -I at near 450. An incident laser beam B1 at an angle 0, with the -t 57 mirror 24, is used as a turning mirror and also as an exit mirror where the laser beam 15, intensity amplified and scan-angle amplified, emerges from the scanner-amplifier unit 8.

Other control elements inside the cavity include a Pockels cell 36 which consists of LiNb03 or other electro-optical crystal such as KDP. Pockels cells are commercially available from several sources, one su-h -ource is Medox Electro-optics, of Ann Arbor, Michigan.

With the application of electric voltage across the electro-optical 10 crystal, a half-wave retardation in the electric field vector of the laser beam can be generated, which turns the linear polarization of a laser beam traversing the crystal, from a horizontal polarization to vertical, and vice versa. A half-wave retardation plate 38, placed next to the Pockeis cell 36, is for adjusting the polarization of the 15 beam before it reaches the mirror 34, so that the beam will either stay inside the cavity or exit the cavity at mirror 34.

A thin etalon 40 with partial reflective coating on both fa..3s at the laser wavelength is for controlling the gain bandwidth of the seed beam 13. By choosing the appropriate finesse of the eta!on, the wavelength width of the laser beam is reduced accordingly, Scompared to the seed beam bandwidth. The pulse duration is lengthened due to the reduced spectral content in the laser pulse.

Another method of expanding the pulse duration can be achieved by stretching the pulse spatially with an optical grating, before the pulse is injected into the beam path at location 21 shown in drawing FIGURE 11. For shorter pulses, a commercial pulse Scompressor unit, consisting basically of a single-mode fiber and grating pair, can be placed at location 21 instead of just the optical J i ij 1~

SI

58 grating. Such a unit is manufactured by Spectra-Physics Lasers, Mountain View, California.

:;cnce, the output laser pulse can be varied from a minimum which is that of the seed pulse, which is about 1 picosecond in the case of Ti:A1 2 0 3 as the laser medium in the mode-locked laser, to as much as several hundred picoseconds.

"Referring now to FIGURE 11A, in a second embodiment, a laser 10 gain medium 42 is located near the scanner mirror 26. A cavity aperture 44 which has a fixed or adjustable iris with two translational degrees of freedom for proper centering with the fundamental laser mode is located inside the cavity. The laser media is optically pumped by a laser source 48 which will be 15 described hereinafter in more detail. The second embodiment provides enhancement of the laser beam intensity inside the scanner :cavity, such that the beam intensity increases by extracting energy stored in the gain medium 42.

Operation of the Preferred Scanner-Amplifier Laser Unit For the purposes of illustration, an angle is being scanned in the horizontal plane (the X-plane). A scan voltage is applied to both piezo actuators 31 for positioning the gimbal mirror mounts for scan mirrors 26 and 28 in the same direction to the same degree; as an example, both pushing the mirrors forward as shown in FIGURE 11.

A half-wave voltage electrical wave form signal is applied to the Pockels cell, as illustrated in FIGURE 12B. The t ;ie sequence from 2(i) to 2(vi) marks the time development of the optical retardation of the Pockels cell 36. A voltage is to start at time 2(ii), and the optical retardation reaches half-wave at time 2(iii). The voltage is

I

wavelength converter.

:J Si 59 turned off at time 2(iv), and zero retardation is reached at time 2(v).

The time duration between 2(ii) and 2(iii) is referred to as the rise time of the Pockels cell for a half-wave retardation. The duration between 2(iv) and 2(v) is the fall time for the same.

Since the seed laser pulse is in the picosecond range, the spatial extent of the laser energy is localized in the range of millimeters.

The cavity distance between scan mirrors 26 and 28 is, for practical e purpose, in the range of tens of centimeters to tens of meters.

.considered localized and is represented by markers 2(i) to 2(vi) as it travels through the scanner-amplifier cavity. The seed laser beam,

C.

at time 2(i) travels towards the scanner-amplifier cavity, and enters through the thin film polarizer mirror 24. As illustrated in FIGURE 15 1 2A, the beam 15 has a linear polarization with the electric field vector in the horizontal direction, as indicated by the arrow. The beam passes through the lens 30, and is focussed at a point before lens 32 which collimates the beam due to the confocal arrangement of the lenses 30 and 32. The Pockels cell (PC) voltage is at the 20 zero level, and the polarization of the seed beam is not changed.

The Pockels cell voltage then turns on at time 2(ii), right after the laser pulse exits the exits the PC crystal. The polarization changes by after passing through the half-wave plate 38, and is now vertical, as indicated by a small circle on the beam path. The beam is then reflected by the thin film polarizer mirror 34 directing the beam towards the scan mirror 28.

In FIGURE 13, the beam path and the angle of incidence at mirrors 26 and 28 are illustrated. Assume that a voltage V, is applied to -the piezo actuator 31, which induces a scan angle of 0, from its O i zero degree incidence, at which the mirror is at the normal incidence with the incoming seed beam. The reflected beam is at an angle, 2 times 01 from the incoming beam. Referring again to drawing FIGURE 12A, the beam is reflected at mirror 28. The vertical polarization of the beam changes by 901 after passing through the half-wave plate 38. The PC voltage reaches half-wave retardation at 2(iii) (see FIGURE 1 2B) before the laser pulse reaches the PC. On passing the PC, the polarization is rotated 901 and is now vertical.

The lens 30 re-collimates the laser beam 15 and the thin film 10 polarizer 24 is now at high reflection with respect to the vertically polarized beam. The beam 15 then travels towards the laser gain medium 42 and the cavity aperture 44.

Assuming that a voltage V 2 is applied to the actuator 3! in the S 15 mirror gimbal mount 29 for the scan mirror 26, an angle rotation of E2 from the normal incidence results in the X-plane, where the normal incidence is defined as the scan mirror angular position for both mirrors 26 and 28 at which the sued laser beam 2(i) will retrace its beam path after reflection from both these mirrors. The reflected beam is, therefore, at a larger angle than the incident angle before impinging on the mirror 26, by an angle 2 time 02, as shown in FIGURE 13.

For ease of explanation, the following discussion is directed to ejecting the laser beam after only one reflection from each of the mirrors 26 and 28; however it should be understood that it is contemplated that a plurality of reflections occur from each mirror within the device prior to the beam exiting therefrom. By so f choosing, the PC voltage turn-off starts after the beam emerges from the PC at time 2(iv), and the retardation is zero at 2(v) before K ll 61 the beam reaches the PC on its return trip from the scan mirror 26.

The vertical polarization remains vertical after passing the PC, and is rotated to horizontal after the half-wave plate 38. The thin film polarized mirror is now transmissive for the laser beam, and the laser beam emerges from the amplifier-scanner of the invention with a scan angle resulting from the sum of the effects of the scan angles 0, and 02 from the scan mirrors 28 and 26 respectively.

It should be understood that invention makes use of the scan 10 mirrors 26 and 28 repeatedly for one or more round trips of the •beam inside the cavity to amplify and precisely direct the beam angle before exiting mirror 34.

In uur preferred embodiment, the PC voltage turn-off, at times 2(iv)- S: 15 is to be applied at the last leg after one or more round trips between the two scan mirrors 26 and 28. In the case where the voltage turn-off is postponed, as in the illustration in FIGURE 12A, the polarization of the reflected beam from mirror 26 is rotated to horizontal after the PC, which is still at its half-wave voltage, and back to be vertical again after the half-wave plate 38. Therefore, the mirror 34 is highly reflective. The beam is trappod inside the cavity, and the beam angle increases with each reflection with either of the scan mirrors.

Further, in addition to changing the beam angle, the optical arrangement enhances the overall scan angle of the beam with a power multiplying enhancement factor.

If the focal length of the lens 30 is longer than that of lens 32, by a factor M, then: "I '4 016- 62 M f 30 3 2 1 where f 13 f 1321 are the focal lengths of the lenses 30 and 32, respectively. The angle of incidence on mirror 28 is 0E, and the angle of incidence on mirror 26 is: 0,/M 02.

Notice the angle reduction of 0, due to the difference in the focal length of the lenses.

10 On passing through the lenses system from 30 to 32, the reverse, a magnification of the effective angle, occurs. The incident angle on mirror 28 is now: 02) x M 0,.

In the illustration in FIGURE 12A, in which the laser beam is to exit S 15 the cavity aftue one reflection from mirrors 26 and 28, the output beam would have a scan angle of: 2 x M x 02).

Notice that the scan angle due to mirror 26, 02, is magnified by a factor M.

If a total of N reflections are allowed to occur for each of the two scan mirrors, the final scan angle of the exit beam is: 2N x M x Since each reflection or transmission on an optical surface causes a certain amount of intensity loss and optical distortion in the laser beam, ideally the intended scan angle will be achieved with the smallest number of optica! surface contacts. If the scan mirrors have identical gimbal mounts 29 and piezo actuators 31, the mirrors 30 can be scanned in tandem, and 0, and ,2 will be substantially l ine ring imrage plUjec.;eu uon tuo In paient s pupii, bsut.n ua11 LIM Ily S;diameter is only slightly less than the patient's pupil size. At that ,i equal. The optical loss due to scattering from all the optical elements inside the cavity is reduced by the factor: (M 1)/2 For M=3, and 10 round trips inside the cavity, the scan angle is amplified by 20 times more than the amplification of the scan angles from two like but uncoupled piezo mirrors.

It is also clear that all the foregoing discussion about scanning in S the horizontal direction is also applicable to the vertical direction (a 10 Y-scan), by applying the scan voltage to the piezo actuator which i controls the vertical tilt of the scan mirror. By applying the appropriate voltages to the actuators controlling the horizontal and the vertical scan directions, the laser beam can be directed to any predetermined location in the two dimensional angular space.

The pump source 48 of the Ti:A,10 3 in the amplifier cavity in FIGURE 11 consists of two major components, namely, a Nd-dope YAG or YLF laser which is continuously pumped by arc lamps such as Kr or Ar gas lamp, which is supplied by ILC Technology, Sunnyvale, California, or by semiconductor diode arrays with the emission laser wavelength to match the absorption band of Nddoped YAG or YLF. Several hundred to over one thousand watts of continuous wave laser output power from Nd:YAG is attainable with multiple lamp-pumped laser heads inside a laser cavity. Such lasers are supplied by Lasermetric, Orlando, Florida, and a number Sof other industrial YAG laser suppliers.

In a preferred embodiment, the Ti ion has an absorpt id centered at about 520 nm, with a full width at half r ,n of about 100 nm. The second harmonic wavelengths of the Nd-doped i YAG and YLF are centered around 532 nm and 527 nm, respectively, and both are suitable as a pump source.

In the second harmonic generation (SHG) process, one of limiting factors in the conversion efficiency and the power stability is the temperature gradient induced by absorption of the laser at its fundamental and second harmonic frequency. Choosing a second harmonic crystal with good thermal conductivity, and cooling the crystal by liquid flow or by contact cooling, are among the common 10 methods to extend the upper limit of the input fundamental laser power for the SHG crystal.

Referring now to drawing FIGURES 14A, 14B and 14C, the output laser beam 55 of a high power, acoustic-optical switched, Nd-doped YAG or YLF laser beam source 56 is directed to a series of partially reflecting beam splitters 57, which are coated with dielectric so that, at the 450 incidence, they all have high transmission for the second harmonic wavelength, and each succeeding splitter is highly reflective at the fundamental wavelength of the laser source 56, so that the laser beam power is distributed equally among each branch when they are directed towards the SHG crystals 60. The crystal is chosen for high nonlinear coefficient, good acceptance angle, and high tolerance to a temperature gradient. KTP is among the top choices as a SHG crystal for conversion at 1.04 to 1.06 microns.

In a preferred embodiment, 20-60 watts of average power will be achieved in the beams 1-5 of FIGURE 14A. To further increase the conversion efficiency, a convex lens 58 can be inserted between each splitter 57 and each SHG crystal 60, such that the crystal is at the focal distance f, 5 from the lens, where the beam cross-section ft' i by the Sun Contact Lens Company of Kyoto, Japan, and the "Corneascope" manufactured by International Diagnostic fteLJy l 1 is the smallest and the laser power density is the highest. The focal length of the lens is chrsen to optimize for the acceptance angle of the SHG crystal. A spherical concave mirror 62 that is highly reflective at both the fundamental and the second harmonic wavelength is placed at the radius of curvature of the mirror 62,

R,,

62 from the first surface of the crystal, where the laser beami.

enters the crystal. This optical arrangement allows for the return beams of both the fundamental and the second harmonic to retrace the beam path of their first passage in the crystal, and ensure a 10 good beam overlapping in the crystal even though there may be walk-off between the beams after their first pass.

To illustrate our embodiment, we combine five beams at the second harmonic wavelength with a novel spatial combiner 64. As shown 15 in FIGURE 14B, the combiner 64 is a six-face optical element which has four sides 63a, 63b, 63c and 63d, each of which form a 450 ''angle with the base face 67, and a top face 65 which is parallel to 'its bottom face 67. The side faces are coated for high reflectivity at 450 at the second harmonic wavelength, and the top and bottom faces are coated with an anti-reflection coating at the second harmonic wavelength. As shown in FIGURE 14C, by using beam steering optics, the five beams from FIGURE 14A can be reflected off the side faces of the combiner 64, and one beam (beam 2) in FIGURE 14C can transmit through the parallel faces. The beams are adjusted such that they re-collimated and are parallel with each other. A convex lens 66 is centered symmetrically in the beam path, and focuses the five beams into a common focal point. This optical element 66 can be a replacement of or an equivalent to the element 46 of FIGURE 11.

.11:lc corneal profile is re-measured, the computer control unit 114 resets the laser system, the mirror 138 is retracted to position A, and the surgery r -commenced.

'4 66 It also follows from tvte present invention that additional beams can be combined with a spatial combiner with additional facets on the combiner. As an example, a hexagon, instead of a square top, can combine up to 7 beams.

In another embodiment, the facets can be formed on more than one layer, such as 4 facets on the top tier and 6 facets on the second tier.

10 In all end pumping configurations, the pump beam is absorbed by 'the laser active ions in the crystal host. The energy distribution in the laser medium is a negative exponential function, with a maximum at the entrant face. For efficient cooling, and to minimize O: the distortion of the laser beam, the laser medium in the invention is 15 to be in a cylindrical laser rod form. A conventional laser red is mounted with the end faces outside of the contact with the coolant. In FIGURE 15A, the preferred embodiment consists of a Ti:A1203 laser rod with a recessed collar 50. A thin wall tube made of undoped sapphire 52 is to fit at the end sections of the laser rod. The tube piece is glued to the laser rod, and the whole has a cylindrical shape as shown in FIGURE 15B. This cylindrical I piece is then mont-ed to a liquid cooled envelope similar to the ones used in an arc lamp pumped laser. A water flow channel around the laser medium and the extension is shown in FIGURE 15B, in which the water inlets and outlets are shown schematically.

O..:ings 54 are retained in such a manner that the coolant is sealed from coming into contact with the flat laser surfa,',,s of the laser rods. The tube extension allows the whole laser medium to be in contact with the liquid coolant. Using the same material in the extension tube also mininizes stress as a result of a difference in *i r. 67 thermal expansion coefficient, with temperature variation in the whole assembly.

In another embodiment, an additional pump source can be applied through mirror 24 collinear with laser path from pump source 48, such that the laser media is pumped from both ends.

In another embodiment, additional laser media is to be included in front of the scan mirror 28, and a pump configuration identical to 10 optical elements 46 and 48, pumping one end of the laser medium, or pumping frum both ends of the laser medium, is to be applied to the laser medium near mirror 28.

:a Multi-kilohertz laser operation is achieved with the following 15 method. A synchronized electrical wave form is tapped from the mode locker driver 66. According to the desired rPDetition rate, the synchronized signal can be divided electrically by a timer divider circuit 68, as shown diagrammatically in FIGURE 16. The resultant frequency output of the timer-divider determines the laser frequency of the scanner-amplifier system. The output electrical signal of the divider box is then time-delayed through delay generators 70 and 74, commercially available from Stanford Research Systems, Sunnyvale, California. One of the delayed signals 71 is fed into a Q-switched driver 72 in the pump laser 48, and a second timedelayed signal 75 is fed into the Pockels cell driver 76.

The timing of the electrical signals and the laser events are illustrated in FIGURE 17. In the top trace 7(a) of FIGURE 17, multimegahertz (30-200 MHz) mode locked laser pulses are represented by equally spaced laser spikes at time intervals equal to twvice the B 68 mode locker driver frequency. After the timer-divider circuit, electrical signals at multi-kilohertz (1,000-50,000 Hz) is generated at the output of the timer-divider box, as represented by the trace At a time delay T 1 the Q-switch driver for the pump laser is turned on, in trace 79(c), generating a short pulse of the second harmonic laser pump pulse at a time delay corresponding to the build up of the pump pulse, a characteristic of the pump configuration, and the gain factor at the pump laser medium. The second harmonic pump pulse is absorbed in the Ti:A1203 laser medium, in 10 trace The Pockels cell is switched on at a time delay T 2 relative to a synchronized timer-divider signal, which is the pulse after the one that triggers the Q-switch driver. The time delay T 2 is determined by the actual location of the seed laser pulse from the mode locked laser, as aforementioned along with the discussion of 15 FIGURE 12A. The delay time T, is to be adjusted so that the peak of the population inversion is to occur when the Pockels cell crystal reaches the half-wave retardation point of 2(iii) as shown in FIGURE 12B.

Applicable Surgical Procedures The laser surgical system of the present invention can perform numerous types of surgical procedures on the cornea. Among other procedures, two types of laser tissue interaction are particularly suited for the inventive system: The inventive system can easily create straight line and curved-line incisions, of any predetermined length and depth, at any Slocation determined by a surgeon.

I bu ndner mirrors L ano ZL, DOMn or wflch are nigrily retlective mirrors. The scanner mirrors 26, 28 are each mounted on a gimbal mount 29 with 90° tilts in both the horizontal and the vertical (X-Y) f- 69 As illustrated in FIGURE 9A, multiple radial cuts 902, equal or partially equal in incision length and with an angular separation between cuts, can oe made on the cornea with the present surgical system. An incis on can be made by directing the surgical laser beam S to a predetermined location at the cornea, and removing the desired amount of tissue by controlling the laser beam energy dosage. The present invention provides options for making an spot size on the cornea surface, or a fine incision by using a more 10 focussed beam spot. With the present invention, the depth of each cut can be varied over the length of the cut.

In FIGURE 9B, a side view of a cross-section of the cornea shows a shallower cut depth 904 near the central region of the cornea and a S 15 deeper cut depth 905 near the outer edge of the cornea. Such a procedure provides more uniform stretching of the cornea from the S' central to the edge regions, and increases visual acuity postoperatively.

The invention can also easily generate transverse cuts as shown in FIGURE 9C. By directing the surgical laser beam S to make a pair of opposing transverse incisions 906 along an axis 908 relative to the center of the eye, the refractive power of eye is decreased along the axis. The exact length d and the location of the incision can vary according to the amount of desired correction, in known fashion.

The inventive system can also be used for procedures in cornea transplants. A circumcision of the cornea in any predetermined shape circular, elliptical, hexagonal, etc.) can be performed on i the donor eye and the recipient's eye. In both cases, the computer control unit 114 calculates the beam location based on the particular shape required, and the amount of laser energy needed to cut through the cornea.

In general, incisions in the cornea can be made at effective locations for performing radial keratotomies or making T-cuts, to correct myopia, hyperopia, or astigmatism.

10 The second important type of laser-tissue interaction provided by the inventive system is area ablation, which permits direct sculpting of the corneal surface.

As illustrated in FIGURE 10A, a local scar or infected tissue can be removed with the present invention. The defective tissue is removed to a desired depth d over a p,-edetermined area on the cornea. A donor cornea cap can be cut and ablated ("sculp'ced") to the desired dimension and thickness using the invention. The cap piece is then transferred to the bared stroma bed and attached by suture, glue, or other appropriate means, in known fashion.

Again in FIGURE 10A, an alternative method is shown for performing a cornea transplant. The invention can be used to ablate the cornea most of the way or all of the way through from the epithelium to the endothelium of the cornea. Then a donor cornea 1001 is cut to matching dimensions, and attached to the open ablated area by sutures or other known methods.

For myopia correction, as illustrated in FIGURE 10B, the curvature 30 of the cornea can be reduced by selectively ablating the cornea in drawing HICUMt I i. ri-u Iui compressor unit, consisting basically of a single-mode fiber and grating pair. can be placed at location 21 instead of just the optical 71 such a way that more tissue is removed at the center portion C of the cornea, with a decreasing amount of tissue being removed towards the periphery P of the cornea. Prior to the laser procedure, the epithelium optionally may be removed by mechanical means.

The new desired profile of the eye may include the Bowman's membrane and part of the stromal layer, depending on the amount of refractive correction required. As described earlier, the computer control unit 114 provides for the sequence, location, and intensity of laser pulses to be deposited. The deposition pattern is preferably in accordance with the patterns discussed above in the section "Method of Depositing Laser Pulses".

For hyperopia correction, as illustrated in FIGURE 10C, the objective is to increase the curvature of the eye. Cornea tissue is to be S removed in increasing thickness from the center portion C out towards the periphery P of the cornea. Depending on the amount of correction in the refractive power, the etch gradient for the removed tissue varies. As indicated in FIGURE 10C, the depth of the removed tissue again decreases near the periphery of the eye for a smooth transition. The size of the usable central region R varies depending on the amount of hyperopic correction.

The invention is particularly useful for the correction of asymmetric refractive errors. Irregular distortions may result from poor matching of a cornea from a transplant, uneven suturing, or from imperfect refractive surgical procedures such as lamellar keratomileusis or epikeratophakia. The inventive system can direct the surgical laser beam S to any desired location to sculpt the cornea according to a preoetermined shape. The surgical laser beam thus can be applied to smooth out an irregular profile.

72 Another use of the invention is to produce standard or custom sculpted cornea caps in advance of need. The invention can be used on a donor cornea or a synthetic cornea substitute to ablate a desired profile to correct for myopia, hyperopia, or astigmatism.

Such sculpted caps can then be attached to a properly prepared cornea, in known fashion.

Summary A number of embodiments of the present invention have been desc,"bed. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, while the invention has been described in terms of rectangular coordinates, equivalent polar cjordinates may be used instead. In addition, other lasing media may be used 15 so long as the resulting wavelength, pulse duration, and pulse repetition rote is within the corresponding ranges set forth above.

Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.

i A

Claims (11)

1. 2. The laser apparatus of Claim I, wherein the indicator means includes a vacuum eye ring having distinct visible linear eye position indicators.
2. 3. The laser apparatus of Claim 2, wherein the distinct visible linear eye position indicators include: a. at least one first line indicating position of the eye ring in an X-direction; b. at least one second line, orthogonal to the at least first line, indicating position of ~the eye ring in a Y-direction.
3. 4. The laser apparatus of Claim 2, wherein the distinct visible linear eye position indicators further include at least one radial line indicating rotation of the eye ring. Sviiriiri e aevice prior to the beam exiting therefrom. By so •j choosing, the PC voltage turn-off starts after the beam emerges from the PC at time 2(iv), and the retardation is zero at 2(v) before i 74 The laser apparatus of Claim 3, wherein the sensor means includes: a. a first linear array sensor orthogonal to an image of each first line; b. a second linear array sensor orthogonal to an image of each second line.
4. 6. The laser apparatus or Claim 3, wherein the sensor means includes: a. a first linear array sensor orthogonal to an image of each first line; b. a second linear array sensor orthogonal to an image of each second line; I r c. a third linear array sensor orthogonal to 15 an image of each radial line.
5. 7. The laser apparatus of Claim 1, wherein the beam positioning means includes at least two orthogonal reflective surfaces positioned by controllable actuators.
6. 8. The laser apparatus of Claim 1, wherein the beam positioning means includes an optical means for controllably rotating the image of an incident laser pulse.
7. 9. In a laser apparatus for cornea surgery, wherein the laser beam has an undeflected position, a method for biasing the alignment of the undeflected position of a laser beam witi. respect to the cornea, including the steps cf: a. attaching an indicator means to the eye for providing visible indications of the movement of the eye; Oy\L JA b. detecting the visible indications provided S by the indicator means and providing .z control signals in response to such 1 ili- detection; 1 If the focal length of the lens 30 is longer than that of lens 32, by a factor M, then: SI c. biasing the alignment of the undeflected position of the laser beam with respect to the cornea in response to the control signals. The method of Claim 9, wherein the indicator means includes a vacuum eye ring having distinct visible linear eye position indicators.
8. 11. The method of Claim 10, wherein the distinct visible linear eye position indicators include: a. at least one first line indicating position of the eye ring in an X-direction; b. at least one second line, orthogonal to the at least first line, indicating position of the eye ring in a Y-direction.
9. 12. The method of Claim 11, wherein the distinct visible linear eye position indicators further include at least one radial line indicating rotation of the eye ring.
10. 13. The method of Claim 12, wherein the step of detecting includes using a first linear array sensor orthogonal to an image of each first line and a second linear array sensor orthogonal to an image of each second line.
11. 14. The method of Claim 12, wherein the step of detecting includes using a first linear array sensor orthogonal to an image of each first line, a second linear array sensor orthogonal to an image of each second line, and a third linear array sensor orthogonal to an image of each Li 35 radial line. o 15. The method of Claim 9, wherein the step of biasing includes using at least two orthogonal 4 r O I I I lVCI I I I I I I I i I v have identical gimbal mounts 29 and piezo actuators 31, the mirrors can be scanned in tandem, and 0, and will be substantially R 4 _i reflective surfaces positioned by controllable actuators. The method of Claim 9, wherein the step of biasing includes using an optical means for controllably rotating the image of an incident laser pulse. An eye tracking system substantially as hereinbefore described with reference to the accompanying drawings. A method for biasing the alignment of the undeflected position of a laser pulse with respect to a cornea, the method being hereinbefore described with reference to the accompanying drawings. DATED this fourteenth day of September 1998 SHUI T LAI By his Patent Attorneys Cullen Co. *4 4.4 4, 9 44, p *4d 44 97n centered at about 520 nrn, with a full width at half r n of about 100 nm. The second harmonic wavelengths of the Nd-doped ABSTRACT Laser apparatus is used for corneal surgery. The laser apparatus includes an eye tracking system (130) for biasing the alignment of the undeflected position of the laser beam with respect to the cornea. A vacuum eye ring (400) provides visible indications of eye movement which are detected by linear array sensors (416, 418, 420) The beam is positioned in response to the sensors. A guide beam unit (132) generates a visible guide beam coaxial with the undeflected position of the laser pulses for aligning the laser pulses on the cornea. The fundamental wavelengths of original incident laser pulses can be converted into corresponding second harmonic wavelengths, by optical wavelength conversion devices (108, 110) such as a nonlinear optical crystal. i