KR100615729B1 - Method and apparatus for removing corneal tissue with infrared laser radiation - Google Patents

Abstract

Surgical techniques for removing corneal tissue as scanned infrared radiation have been described using short intermediate infrared laser pulses to provide tissue removal mechanisms based on optical breakdown. Optical fracturing is a photomechanical dissection mechanism resulting from incident light absorption by corneal tissue. Because optical crushing is a mechanical ablation procedure, very little heat is generated in adjacent tissue that is not resected. The surgical system described above includes a scanning beam delivery system that utilizes a low energy output to allow constant radiation of the treatment area and to achieve controlled tissue removal. A real-time servo-controlled dynamic eye tracker based on multi-detector alignment is described which senses motion of the eye and provides a signal proportional to the error in lateral alignment of the eye as opposed to the axis of the laser beam. Temporary and frequency separations are preferably used to distinguish between ambient illumination and tracking illumination from surgical lasers.

Description

Method and apparatus for removing corneal tissue with infrared laser radiation

The present invention relates to a laser surgical technique for modifying the corneal surface of an eye, in particular a laser surgery which aims to regenerate the cornea by selective removal of the corneal tissue, known collectively as photorefractive keratectomy or PRK Technology.

Recently, a number of corneal ablation techniques and associated equipment have been disclosed to correct visual defects such as myopia, hyperopia, and astigmatism. Additionally, corneal ablation techniques have also been used for treatment in a number of pathological conditions associated with the cornea. For example, a method for accomplishing visual correction through the reformation of the anterior corneal surface in US Patent Nos. 4,665,913, 4,732,148, 4,669,466, and Trokel, U. S. Patent No. 5,108,388 to L'Esperance is described . A number of basic equipment for refractive surgery, such as Visx's model 2020 in Santa Clara, California and Summit's model Exci-Med 200 in Watertown, Mass., Are being used commercially for the last few years.

Most of the currently known commercial equipment as well as most of the corneal ablation methods and apparatuses known use ultraviolet light having a wavelength of less than 200 nm. For example, a number of such devices utilize an argon fluoride excimer laser operating at l93nm. In general, radiation at such short ultraviolet wavelengths is characterized by high photon energy, i.e., energy greater than 6 eV, which causes molecular breakdown by impacting tissue, i.e. breaking intermolecular bonds. The photochemical properties of this mechanism have the advantage of creating minimal secondary thermal damage to cells adjacent to the surgical site, since the broken molecules leave only small volatile fragments that evaporate without heating the underlying substrate. Furthermore, the breakdown depth for each laser pulse is typically very small, i.e., less than 1 micron, so that accurate tissue removal is possible with minimal risk of damaging the underlying tissue from ultraviolet radiation.

In view of this low penetration depth, the need to remove sufficient depth of tissue while minimizing the total time of the surgical procedure has combined, and a number of corneal scraping techniques using excimer lasers have been described as "wide area ablation" It adopts. Generally, wide area ablation uses a laser beam with a large spot size in contrast to successfully remove a thin layer of corneal tissue. The spot size should generally be of sufficient size to cover the entire visual area of the cornea, i. E. 5 to 7 mm. Consequently, a UV laser with a relatively high energy output is typically required to ensure the required line density on the cornea. It has been found that a suitable ablation rate of at least 0.2 micron / pulse requires a 200 mJ ultraviolet laser to ensure a linear flux density of at least 150 mJ / cm2. However, such a laser tends to be an enormously large and costly system.

Moreover, efficient wide-area ablation requires that the projected beam be a spatially uniform and constant beam to obtain the desired smooth corneal profile. Thus, an additional beam-forming device, such as a rotating prism, a mirror, or a spatial condenser, must be within the excimer beam delivery system. A more detailed discussion of beam forming and delivery systems is found in U.S. Patent No. 4,911,711 to Telfair, which is incorporated herein by reference. The multiple optical elements contribute to overall transmission loss while optical complexity, cost and maintenance requirements of the system are added to the system.

Other techniques based on the use of scanning UV laser beams have been proposed to achieve controlled and local ablation of selected areas of the cornea. In the scanning method, a relatively small laser spot is rapidly scanned over the entire cornea in a predetermined pattern in advance, and the surface is formed in a desired shape. A more detailed description of techniques for laser scanning using excimer lasers can be found in L. Esperence or Lin, JT US Pat. No. 4,665,913, "Minimum-Excimer Laser Cornea Reformation with Scanning Device" (SPIE Meeting, Vol 2131, Laser & System III (1994)). Scanning methods have the advantage of adding flexibility to the refraction correction and smooth ablation profile without requiring a spatially constant output beam profile, and include low power and energy requirements. For example, laser scanning techniques allow tapered optical processing areas to be achieved, which can have the advantage of being able to process areas up to 9 mm in diameter, which can be required for severe myopia correction, pathological tissue removal and primitive correction have.

While laser surgical techniques based on excimer lasers are advantageous for many applications, there are a number of limitations that can greatly improve the use of optical laser surgery if such disadvantages are overcome. For example, excimer laser based techniques may use toxic gases as laser mediators, suffer from persistent reliability problems, require optical losses in the delivery system, and the UN rays may be potentially mutilated through secondary fluorescence , Which can cause undesirable long term adverse effects on the tissue of the unexposed eye.

Thus, an alternative to excimer lasers has recently been proposed that relates to radiation with frequency shifted from solid state lasers. However, the current limitation of the atypical element used as a frequency shifting device is that it may be too close to the mutagenic range, indicating a peak at 250 nm, with a lower limit of approximately 205 nm at the available wavelength of such a laser. In addition, multi-moving laser devices are faced with some difficulties in providing the necessary energy output and are again complicated and annoying, leading to potential laser reliability problems as well as additional cost and maintenance.

More recently, a more attractive alternative is the "basic mode photoablation of myopia" in T. Taylor and J. Wallachsak, Laser and Light in Ophthalmology, 5, 4, 199-203 (1993) , Which is associated with radiation at about mid-infrared wavelengths and about 3 microns, which corresponds to the absorption peak of water, which is a major component of the cornea in particular. One solid state laser, especially an erbium: YAG laser (Er: YAG) emits radiation at a wavelength of 2.94 microns, corresponding to an absorption coefficient of 13000 cm ^-1 or higher in water. This high absorption results in an impact area with a penetration depth potentially less than 2 microns.

In contrast to photoablation mechanisms associated with excimer lasers, i.e. photochemical decomposition due to energy absorption of molecular bonds, ablation of Er: YAG lasers is caused by photovaporizatlon or photothermal evaporation of water molecules do. This heat induces a phase change and thus abruptly expands the corneal surface tissue by causing a sudden expansion of tissue material.

In addition, erbium lasers are more attractive for clinical applications than excimer lasers. This is because erbium lasers allow for more compact, efficient, higher beam quality radiation, less optical loss, and excellent optical coupling characteristics. Furthermore, photobleaching is inherently more efficient than photodecomposition and allows faster operation by ablating to 3 microns of tissue at a time. Mid-infrared radiation is also a potentially attractive method of decoupling the source laser from a delivery system that is compatible with optical fiber delivery and makes the mid-infrared radiation more suitable for the operating room. Ultimately, the radiation from the Er: YAG laser is not in the mutagenic range, and it can be relieved by harmful long-term side effects.

The Er: YAG laser-based corneal sculpting system described by Selier and Wellensack is based on wide-area ablation. The purpose of such a system is to use the Gaussian beam profile of the laser beam to achieve a refractive correction with each pulse using a minimum number of pulses. Other systems that rely on wide area ablation are described in Kogen et al., PCT Patent Application No. 93/14817, which relies on a sculpting filter to control the amount of tissue removed by regulating the intensity of the radiation delivered to the cornea do.

The Er: YAG laser technology described by Selier and Wellensoz and Kozaen offers many advantages over the prior art, while the optical domain, including the need for a smooth and constant beam profile, large pulse energy and / or complex filter control system They suffer potential defects common to ablation techniques. The system assumed that the ablation process was a linear process, that is, a portion of the beam with a greater energy density would remove tissue of greater depth. However, it appears to be an inadequate assumption for the excimer ablation process and may also be an inappropriate assumption for the Er: YAG ablation process.

In addition to the aforementioned limitations, all of these prior art techniques for delivering and controlling the mid-infrared laser beam have, in particular, one disadvantage, namely the excessive energy density required by the system to eliminate large areas, It is likely that there is potential thermal damage to the non-ablated areas of the eye due to the large shock waves generated by the pulses. In addition, due to the need for high pulse energy and high beam quality, such prior systems typically exhibit optical configurations that are not easy to manufacture and difficult to maintain and service.

From the above description, there is a need for an improved method and apparatus for corneal tissue surgery based on controlled removal of tissue as is evident. Moreover, there is a need for improved methods and apparatus for reducing myopic, primitive, and / or astigmatic conditions of the eye using low cost solid state lasers. There is a need for a method and a computer control device for scanning with an intermediate-infrared laser radiation across the outer surface of the eye and its lower bosom layer and substrate to treat the tissue near the corneal surface or surface, exist. There is a further need for a method and apparatus for corneal tissue surgery with an improved eye tracking mechanism.

In general, according to the present invention, a surgical method and apparatus for removing corneal tissue with intermediate infrared radiation is provided. The surgical method and apparatus use short laser pulses scanned over a portion of the cornea to produce a tissue removal based tissue removal mechanism. The photoplating is a photomechanical ablation mechanism resulting from the absorption of incident light by the corneal tissue. When the corneal tissue absorbs infrared light, a bipolar oscillation shock wave is generated, which alternately compresses and stretches the corneal tissue. The tissue debris, during the stretching phase, splashes out by shock waves.

According to an aspect of the present invention, a laser delivery system includes a laser source, such as a Q-switched Er-YAG laser, which emits pulsed radiation in the mid-infrared spectral region at an energy density that can cause ablation of the corneal tissue Lt; / RTI > In a preferred embodiment, the laser emits about 3 microns of radiation corresponding to the maximum absorption coefficient of water, which is a major constituent of corneal tissue. Preferably, the laser source emits radiation with a discrete pulse of less than 50 ns at a repetition rate of about 5 to 100 hertz. Short laser pulses reduce unwanted thermal damage around the tissue to an insignificant extent. The energy of each pulse is preferably about 5 to 30 mJ.

The laser beam is preferably scanned over a specified central region of the corneal surface in a predetermined pattern, preferably by a scanning beam delivery system, to selectively remove tissue at various points within the scanned region, . The scanning beam delivery system preferably comprises a controllable tilting mirror assembly aimed at directing and directing the beam onto the corneal surface. A variety of predetermined scan patterns can be used to achieve a controlled flushing of the cornea, including the epithelium, Bowman's layer and the substrate, in accordance with the required changes in corneal morphology.

According to another aspect of the present invention, the laser spot size and spacing associated with a given scan pattern is compatible with maximizing the operating speed and the necessary smoothing of the ablated corneal surface so that the degree of required pulse overlapping with the ablation depth is correlated correlate to each other in accordance with a particular calculation diagram. A given scan pattern is preferably uniformly irradiated to a processing region having a minimum identifiable line of tissue between the upper and lower exposed tissues between the scans. The one or more discontinuous scan patterns are generated by applying a pulse at each time interval on the entire processing region Thereby distributing the residual heat over the entire area and minimizing the temperature rise in any local area.

Furthermore, in accordance with a preferred embodiment of the present invention, an eye tracking system is additionally provided with a scanning beam delivery system to compensate for eye movement during surgery. The eye tracking system senses the motion of the eye, And provides a signal proportional to the error in the lateral alignment of the eye with respect to the axis. The lateral movement of the eye is detected by illuminating the eye with a tracking light and forming an image of an important feature of the eye, such as the limbus of the eye, on the array of detectors. According to an aspect of the invention, the array of detectors comprises at least four detectors centered vertically and horizontally about the center of the detector array.

In operation, if an important feature of the eye is centered about the axis of the laser beam, the image of the important feature will be centered on the detector array. The null signal is generated by a detector array that holds the axis of the laser beam at the current position. However, if the eye is not centered with respect to the axis of the laser beam, the image formed on the detector array will not be centered. The detector array will generate an error signal that will cause the laser beam to deflect to ensure that the laser beam is properly applied to the corneal tissue.

The tracking illumination is preferably selected within near infrared range so as to be identifiable from the ambient light and the laser beam. Additionally, the tracking light is preferably modulated at a pre-determined temporal frequency to further distinguish the tracking light from the ambient light and laser range. A red or near-infrared filter may be disposed at the front of the detector in the array to further enhance the contrast of important features of the eye to be detected, such as the edges.

According to another aspect of the present invention, a corneal topography device may be included in a surgical device for calculating the shape of the corneal tissue to assist pre- and post-surgical measurements. Alternatively, a spatially resolved refractive index meter may be included to calculate the refraction of the corneal tissue. In various embodiments of the present invention, the alignment method described above can be used to combine active feedback control from a topography or refraction mapping instrument to provide additional control over the pre-surgical procedure.

Other features and advantages of the present invention, as well as a more complete understanding of the present invention, will be apparent from the description and drawings.

As shown in Figures 1 and 2, the surgical device 200 includes an infrared lace source 23 and an optical assembly, which includes the beam delivery optics 30 described below with respect to Figure 5, Transmitting mirrors 50 and 60 and the partial-transfer mirror is configured to output the cornea of the patient's eye 70 on the cornea in order to correct the curvature of the cornea or to affect treatment. Cooperate to focus the beam 10. The laser source 20 is preferably a mid-infrared laser, which generates short laser pulses to yield tissue removal mechanisms based on the tragacanth as described below. The laser beam 10 is preferably scanned onto a specific central region of the corneal surface in a predetermined manner, as described below with reference to Figures 3 (a) and 3 (b) By selectively removing the tissue, the curvature of the cornea is changed into a predictable and controlled form.

According to one aspect of the invention, the laser source 20 is preferably a solid state laser and emits pulsed radiation in the mid-infrared spectral region with an energy density that can cause ablation of the corneal tissue. As used herein, the term "solid laser" includes a diode laser. Preferably, the laser emits radiation at a wavelength of about 3 microns, such as 2.7 to 3,1 microns, corresponding to the corneal absorption peak, the maximum absorption coefficient of water, which is a major component of corneal tissue. It has been found that at such wavelengths the absorption of laser energy by the corneal tissue of the eye 70 is a complete absorption within 1-2 microns of the tissue depth. As further described below, it has been found that the combination of shallow absorption depth and short radiation pulses can reduce unwanted heat damage to surrounding tissue to a negligible level.

Photospallation

The surgical technique disclosed herein, wherein corneal tissue is irradiated with short pulses of a scanned mid-infrared laser beam, as described above in accordance with the present invention is based on the concept referred to as photospallation. In general, a photopera- tiation is a photomechanical mechanism resulting from the absorption of incident radiation by corneal tissue. When the corneal tissue absorbs infrared light, a bipolar oscillation shock wave is generated which selectively compresses or stretches the corneal tissue and allows the separated corneal tissue to escape during the stretching step. A more detailed description of the portraits is provided by Jack. Refer to "Laser-Tissue Interaction: Photochemistry, Photothermal, Photomechanical" (Laser in General Surgery, 72 (3), 531-558 (1992)), incorporated herein by reference. Since the photoparesis is a mechanical ablation process, almost no heat is generated in the adjacent corneal tissue remaining after the ablation.

The laser source 20 may be implemented as a Q-switched Er: YAG laser, which carries an intermediate infrared beam at or near a wavelength of 2.94 microns. Alternatively, the laser source 20 may be implemented as a Neodemium or Holmium doped laser frequency shifted by an optical parametric oscillator (OPO) to emit radiation of about 3 microns . Of course, the laser is included within the scope of the present invention as a substitute for another alternative laser source having an emission characteristic similar to an Er: YAG laser.

The laser source 20 preferably emits radiation with a discrete pulse having a duration less than 50 ns at a repetition rate of 5 to 100 hertz. The laser pulse should be sufficiently short so that the side thermal damage of the tissue adjacent to the irradiated area is compatible with the throughput process and is limited to a region narrower than the 2 micron width. In addition, the energy in each pulse of the laser 20 is preferably in the vicinity of 5 to 30 mJ. So that the incident laser beam 14 removes fine portions of the cornea by ablating the tissue locally.

Line-Of-Sight

1 and 2, in order to correlate the refraction frame of the eye with the refraction frame of the surgical instrument 200, the line of sight of the eye 70 substantially coincides with the propagation of the incident laser beam 14 There is a need to match the axis. As used herein, the term "gaze" or "primary gaze ", according to conventional definition, passes through the pupil and reaches the fovea, connecting the forbidden to the gaze point through the central portion of the pupil It refers to the main ray of bundle of rays. Therefore, rather than through any external measurement of the eye, the line of sight can be defined without ambiguity about the given eye, and the eye can be defined by the patient, which is the only axis that follows the objective measurement using a cooperative patient fixation eye Constitute an eye metric.

Since the critical field of view is concentrated on the eye line regardless of the direction of the eye's mechanical axis of symmetry by definition, for maximum optical performance, the point representing the intersection of the cornea and the line of sight is the refraction It is generally accepted that the desired center of vision of the procedure is established. As shown in Figs. 1 and 2, the direction of the eye of the eye 70, which is suitable for the comfortable placement of the patient to be operated without affecting the effectiveness of the present invention, may be vertical, horizontal, .

During preparation for laser surgery on the corneal surface, the eyes of the eye 70 should be aligned to coincide with the laser beam axis by bi-axial lateral transfer adjustment, as indicated by the surgeon 55, in a known manner. The surgeon 55 observes the eye 70 through the surgical microscope 80 and determines the axial placement of the beam 14 in a known manner with respect to the reticle or other fixed reference mark as a result of pre- 70 to determine the degree of centration of the frontal image. The axial placement of the eye 70 is also determined by the surgeon 55 by the observed degree of image focus of the eye 70 in relation to the fixed focus plane of the best focus relative to the microscope 80. [ The direction of the surgeon 55 enables axial position adjustment of the cornea 70 of the eye to coincide with the plane of the best focus.

(Not shown) projected onto the eye 70 by the fixed target device 90, which is preferably integrated into the microscope 80, and focuses attention, i.e., The angular orientation of the eye of the user 70 is set. The two targets will be placed at various axial distances from the patient's eye 70 and will be calibrated in advance in a known manner. For suitable calibration techniques, see WO < RTI ID = 0.0 > 94/07908 < / RTI > In this way, when the two targets (not shown) appear superimposed, the axis of the observation eye 70 will be substantially angularly aligned with the axis of the microscope 80 and the axis of the laser beam 14.

In a preferred embodiment, the small, i.e., less than 5 mm lateral movement of the eye 70 of the patient, occurring after the initial alignment as described above and through the operation, is described below with reference to Figures 6 and 7 The function of the two-dimensional eye tracker 100 is not negligible. The eye tracker 100 senses the motion of the eye 70 and provides a signal proportional to the lateral alignment error of the eye 70 with respect to the axis of the laser beam 14. The signal generated by the eye tracker 100 is converted into a command for a small angular tilt of the partial reflection mirror 60 that compensates for errors in the placement of the eye 70. [ A small angled incline directs the beam 14 in a new direction so as to coincide with the instantaneous position of the eye 70. The compensation command is transmitted from the electronic device to the mirror 60, described below in relation to the seventh aspect, in the eye tracer 100 by one or more data connections collectively indicated as 102.

The illumination of the eye 70 is preferably performed by a coaxial illuminator 120 that is preferably integrated into the microscope 80 to facilitate tracking of the eye tracker 100 and is preferably positioned at an angle of 8 [ The light beam 17 is projected at a small angle. The wavelength and temporal modulation frequency of the tracking beam 17 generated by the illuminator 120 can be varied from the ambient room illumination and the radiation of the laser 20 to the eye tracker 100 of the tracking beam 17 0.0 > and / or < / RTI > detectors. In this way, the ambient illumination and laser beam 14 will not yield the same transient modulation or spectral characteristics as the tracking beam 17, and thus will not be visible to the tracking detector.

Additionally, as shown in FIG. 1, the surgical system 200 may be configured such that if the laser beam 14 fails to follow a prescribed path, or if pulse energy- Or a safety shutter 40 that automatically closes if the eye tracker 100 can not follow the movement of the eye.

As shown in FIG. 1 and further described below, the surgical instrument 200 represents a real-time image of the patient's eye on the monitor 150 during pre-operative alignment and operation, And preferably includes a video camera 140 for recording video images on a recorder 160.

As shown in FIG. 1, the computer 110 includes a plurality of storage and control capabilities. In particular, the computer 110 controls the laser source 20 by communicating with the laser source 20 by means of the connection 101. In addition, the computer 110 drives the scan mirror 50 by the connection 103 in accordance with a scan type and an input command stored in the computer 110 by the surgeon 55 or an assistant. The connection 104 between the computer 110 and the safety shutter 40 affects the maximum safety of the patient, surgeon and assistant. The computer 110 monitors the operation and status of the eye tracking system 100 by means of the connection 105. 1, the computer 110 may be connected to the eye tracker 100 by way of a connection 106 and the discrete connection 107 may be connected to the computer 110 by way of the computer 110, To the mirror 60 from the computer 110 so that the mirror 60 can be directly controlled. An additional optional configuration eliminates the need for the connection 103 by allowing the computer 110 to integrate the scanning and eye tracking functions into a single mirror such as the mirror 60. [

As further described below, the surgical device 200 includes a keratoplasty device 180 or a spatial resolved refractometer 190, as shown in FIG. The corneal topography device 180 may be used to assess the shape of the corneal tissue to aid in measuring the shape or curvature of the pre- and post-op eyes. An alternative embodiment may include a spatial resolution refractive index detector (SRR) that calculates the refractive index of the corneal tissue.

Optical Mirrors

It can be noted from FIGS. 1 and 2 that the partial reflection characteristics of the mirrors 50 and 60 play an important role in carrying out the proper function of the present invention. In the case of the mirror 50, the laser beam in the beam 12 is reflected while the radiation from the eye tracker 100 is transmitted. This can be obtained using a so-called "hot mirror coating" on the surface of the mirror 50. Such a coating is dichroic, in other words the coating has various reflective and transmission properties for light of different wavelengths. The radiation from the laser 20 should have a wavelength of 2.9 microns bad and the mirror 50 should have a high reflectance at that wavelength. The radiation of the eye tracker 100 has a wavelength between 0.8 and 1.0 microns where the coating of the mirror 50 should have a high transmittance.

Similarly, the dichroic coating on the mirror 60 has a high reflectance at the wavelength of the laser 20 and, at visible wavelengths used by the surgeon's eye to observe the eye arrangement with respect to the surgical apparatus and the surgical procedure, Is preferably selected to have approximately the same transmission and reflectance at the wavelength of the coaxial illuminator (90) and the wavelength of the coaxial illuminator (120). This is possible because the visible range, the fixed target 90, and the illuminator 120 are adjacent in wavelength and away from the wavelength of the laser 20. The beam transmitted from both mirrors 50 and 60 undergoes a small lateral displacement due to the limited thickness of the mirror substrate and the oblique incident angle, but this fixed displacement is easily compensated in the design of the device as will be apparent to those skilled in the art.

Additionally, the mirror 130 shown between the beams 15,16 of Figures 1 and 2 is preferably partially transmissive although not a two-color. The coating on the mirror 130 has substantially the same reflection and transmission characteristics over a significant portion of the wavelength and visible spectrum region of the eye tracer light source 120. In this way, a portion of the energy of the beam 15 may be directed in a new direction as the beam 18 to the video camera 140 described above. It will be appreciated that a beam-dispersing prism in the form of a bonded two-element cube with a partially reflective coating on the inner surface can be used to provide the function of the mirror 130.

Scanning Patterns

As previously indicated, the surgical apparatus 200 of Figures 1 and 2 can apply a pulse of intermediate-infrared laser radiation in a predetermined pattern, as shown in Figures 3 (a) and 3 (b) Controlled scanning operation of the focused laser beam 14 to sequentially illuminate adjacent small areas of the central portion of the cornea. In each case, the area to be treated has a diameter of about 9 mm. The size of the spot collected at the focus of the laser radiation is preferably a circumscribed diameter of 0.5 to 2.0 mm.

A straight line or raster-scan 310 of the scanning spot of the laser beam 14, as shown in Fig. 3 (a), is a rectangular area centered on the desired processing area 315 Cover. When the computer 110 predicts that energy collides with the corneal tissue outside the desired processing region 315, the laser beam 14 is "off" modulated. As shown in FIG. 3 (b), the laser beam 14 scans into the vibrating perimeter pattern 322 centered on the desired processing region 315. While the path of the laser beam 14 is continuous from start to end, as shown in Figs. 3 (a) and 3 (b), the selective mode of operation divides the pattern into a list of positional coordinates, To minimize the residual heat effect of the region adjacent the scan path by cumulative radiation at the rapidly sequenced beam position. In this embodiment, the scanner has random access capability at each location.

In the exemplary mode shown in FIGS. 3 (a) and 3 (b), or with other continuous or discrete scan patterns apparent to those skilled in the art, adjacent scan paths are nominally redundant in a controlled manner. In this way, the entire treatment area 315, 325 is evenly illuminated with minimal exposure lines between transient exposure and under exposed tissue between the scans. The discontinuous nature of the sequence distributes the pulse to the entire area at shorter time intervals relative to the entire sequence, thereby better distributing any residual heat across the entire surface and minimizing any temperature rise or heat build up in any localized area. Once the pattern is defined by the computer 110, the delivery performance can be distributed discretely across the corneal surface for maximum surface smoothness and minimal thermal effects.

Since the laser scanning on the corneal surface is accomplished by controlling the tilt of the mirror 50 that is partially reflected with respect to the two axes, the reflected beam is deflected in an appropriate manner. This scanning operation is transmitted to the electric drive tilting mechanism attached to the mirror 50 under the control of the computer 110 at the start of the operation.

The speed of the scanning operation varies at various points within the processing region 315, 325 according to the algorithm described by the surgeon 55 so that some ablation occurs locally, thereby correcting the visual defects of the patient Causing a desired change in the refractive power of the anterior surface of the cornea. Astigmatism or cylindrical refractive error correction can be achieved by driving the scan mirror at various speeds as a function of the rotational position with respect to the propagation axis of the pattern. This allows the laser beam to selectively ablate more tissue near a meridian of the corneal surface than near the meridian. The asymmetric scan operation can be superimposed on the normal symmetric pattern to simultaneously correct spherical or cylindrical refractive errors.

4A, the intensity profile of the laser beam 14, which is ideally concentrated at the surface of the cornea, is rotationally-symmetrical to facilitate uniform illumination of the surgical (treatment) symmetric) trapezoidal shape. As shown in Fig. 4 (b), the substantially Gaussian profile approximates the ideal intensity profile shown in Fig. 4 (a). For smaller beam diameters impacting the corneal surface, i.e., a diameter of 2 mm, the tissue removal profile for excimer ablation is close to the Gaussian shape independent of the overall strength profile. However, for example, for medium diameters up to 2 to 4 mm, the ablation profile is close to the beam intensity profile of the excimer laser beam. For example, for larger diameters from 4 to 7 mm, the ablation profile is deeper at the edge than at the center compared to the beam intensity profile.

When a spot size of 2 mm or smaller is used, the beam intensity profile is similar to the above-mentioned excimer ablation mechanism in that it is not generally important in the design or ablation pattern. Tissue Ablation Mechanism Unlike light photothermal evaporation, the tissue ablation mechanism for the photopasurement is photomechanical. Therefore, the shape of the ablation depends on the beam diameter rather than the specific intensity profile. Thus, the laser design problem can be mitigated because the invention as a further advantage depends on the pulse diameter and is not particularly sensitive to small changes in the beam intensity profile.

Beam Transfer Optics

As noted above, the laser beam 10 is transmitted to a major portion of the surgical device 200 by the beam delivery optics 30, as shown in more detail in Figures 5 (a) and 5 (b) . For a congested environment in the operating room, a flexible arrangement is preferred in which beam delivery is effectively decoupled from the laser system. 5 (a), the beam-passing optics preferably includes a focusing lens 160 to focus the laser beam 10 into the entrance aperture of the decoupled guide means 162, such as a flexible fiber optic cable. do. The fiber optic cable 162 should be capable of delivering strong infrared laser radiation at some constant distance, i. E., Across the operating room, without damage to the fiber optic cable itself or significant loss of laser energy.

The fiber optic cable 162 may be embodied as a single or multiple fiber bundles and may be constructed of materials capable of safely delivering a laser 20 of a particular wavelength, such as glass, sapphire or other crystal. In the infrared wavelength range, the additional loss associated with the additional elements required by the decoupled beam transmission optics 30 will generally be quite small. Alternatively, the laser beam may be coupled to the scanner system by a flexible hollow waveguide (not shown).

Preferably, the fiber optic cable 162 is configured to receive the laser 20 from the main portion of the surgical device 200, in a manner that is not necessarily in a specific arrangement, although the laser 20 can be conveniently located near the surgical device 200. [ . 5 (a), the laser radiation emerging from the output opening 163 of the fiber optic cable 162 is captured by the relay lens 164, which forms an image of the output opening 163. This image is then passed through the paths 11, 12, 13 and 14 by partial reflection mirrors 50, 60 to position the image on the anterior surface of the cornea of the eye 70, as shown in Fig. Therefore, it propagates. The image plane of the relay lens 164 is placed during assembly of the apparatus so that it is at the best focal plane of the microscope 80. [ The fiber optic cable 162 may be implemented according to the SapphIRe product commercially available from Saphikon, Inc. or the technique described in U.S. Patent No. 5,349,590.

An alternative embodiment of the beam transmission optics 30 is shown in Figure 5 (b). An alternative arrangement of FIG. 5 (b) is that flexible arm 166 replaces fiber optic cable 162 of FIG. 5 (a). The flexibility of the articulated arm 166 by rotation about the axis BC, CD, DE, EF and / or FG axis allows the flexibility of the articulating arm 200 to be adjusted, without requiring the laser source 20 to occupy a particular position, To allow convenient location of the laser source 20 in relation to the main portion of the laser source 20. Compression and relaying of the laser radiation at the input and output openings of the articulating arm is performed by the lenses 168, 170 in the manner described for the corresponding optical element of Figure 5 (a). The articulation arm 166 is commercially available from Dantec measurement technology and can be implemented as a light guide arm according to U.S. Patent No. 4,896,015.

Another alternative embodiment to the beam delivery system may place the laser on the arm of the surgical microscope at a fixed position relative to a major portion of the surgical device 200. [ Such an arrangement requires certain rigid relay means to transfer the radiation, which places additional packaging restrictions, while requiring greater attention in the optical arrangement. For these and other reasons, the decoupling means of Figs. 5 (a) and 5 (b) are preferred.

Eye Tracker

The importance of proper centering of treatment is generally recognized as an important factor for all corneal resection (PRK). Misalignment of the eye 70 during the process is known to cause irregular astigmatism, scintillation, and reduced visual acuity and contrast sensitivity. The surgical instrument 200 preferably senses the motion of the eye 70 and provides a signal proportional to the error in lateral alignment of the eye 70 with respect to the axis of the laser beam 14 And an eye tracker 100 for tracking an object. The prior art of eye tracking technology is described in PCT Patent Application No. WO 94/02007 to Nup et al., Which is incorporated herein by reference.

The eye tracker 100 senses lateral movement of the patient's eye 70 by forming an optical image of an important feature of the eye on the array of detectors 300 that are preferably aligned in the manner shown in FIG. The eye feature imaged by the eye tracker 100 is an intersection 305 that is a roughly circular shape of a clear cornea with translucent and white sclera constituting the structural organ of the pupil 70.

The intersection 305 is typically known as the limbus of the eye 70. The perimeter of the eye is about 12 mm in diameter in the human eye and is easily seen by the circular appearance and underlying color of the lower eye tissue visible through the clear cornea as compared to the white sclera. As seen from the front, the colored or lightly colored circular area and the variation around the eye from the white sclera provide a photometric contrast of the axis-symmetric features of the eye 70 tracked by the means described herein do. In a preferred embodiment, the contrast can be further improved by using a red or near infrared filter on the detector front so that blue and green pupils appearing in the detector array 300 are as dark as the brown pupil.

When the perimeter feature of the eye 70 is perfectly centered with respect to the axis of the laser beam 14, the image of the eye peripheries formed by the lens 320 is focused on the detector array 300 as shown in Figure 6 (a) Centered. Under such a centered condition, each of the four detectors, each having array 300, basically receives the same amount of energy from the top of eye perimeter 305, and is present as an aid to the associated electronic means (not shown) Position to produce a null signal that is transmitted to the tracking mirror 60 through a connection 102 that keeps the mirror 60 stationary.

However, if the eye 70 is not perfectly centered with respect to the axis of the laser beam 14, the image of the eye perimeter 305 formed in the detector array 300 will, as schematically shown in Figure 6 (b) , It does not gather at the center somewhat. In this state, an unequal amount of light energy enters the four detector elements comprising the array 300, and an error signal for lateral displacement is generated by the associated electronics described above. This error signal is transmitted to the drive mechanism of the mirror 60 which deflects the mirror 60 as required to return the image to the center position.

Thus, the function of the eye tracker 100 is to maintain a centered state between the axis of the beam 14 and the cornea of the eye 70. In this way, the laser radiation transmitted through the beam 14 is applied to the cornea as if the eye had not been moved from its nominal center position. Additionally, in order to allow real-time tracking, the tracking algorithm described above is preferably performed at least once during the duration of each pulse. Thus, in the above example, where each pulse has a duration less than 5Ons in a 100 hertz repeat, there will be 10ms between pulses and the eye tracking response time is preferably less than 10ms.

It is understood that in the manner described above, the scanning algorithm applied to the mirror 50 and the eye tracking function applied to the mirror 60 can be applied in combination with a single mirror, such as the mirror 60. [ In this embodiment, the mirror 50 will be a fixed mirror / beam diffuser. This configuration can reduce the hardware cost, but will complicate the logical operation of the system and increase the angular ranse requirement of the single mirror 60. By using two separate mirrors, it reduces the angular range requirements for each mirror and simplifies the design, manufacture and testing of the separate scanning and eye tracking functions.

As indicated above, the frontal illumination of the eye 70 required for proper functioning of the eye tracker 100 is provided by a coaxial illuminator 120 that can be integrated with the microscope 80. The illuminator 120 projects the tracking beam 17 onto the eye 70. Light that is differentially scattered and reflected by the cornea and sub-tissues and the adjacent sclera of the eye perimeter 305 is appropriately magnified to form an object imaged by the lens 320 on the detector array 300.

In a preferred embodiment of the present invention, the wavelength of the illuminator beam 17 is selected at a wavelength in the near-infrared range of about 0.8 to 1.0 microns. The sensitivity of the human eye is so low in these wavelengths that the portion of the beam 17 that returns to the microscope 80 and is reflected by the surface of the cornea is small so that the surgeon 55, It does not affect observation. Additionally, due to the low visibility of the eye 70, the near-infrared frontal illumination may be a source of visible light disposed within the stationary target device 90, or a device that will not interfere with patient ' will be.

Moreover, the intensity of the light source in the illuminator 120 may be adjusted to any arbitrary value to further facilitate distinction from the ambient illumination or laser beam 14 that is not modulated by appropriate synchronous filtering within the electronic device associated with the detector array 300. [ It can be modulated at a conventional transient frequency. The detector array 300 will not respond to the laser beam 14 during the operating period of the eye tracker 100 because it does not sense infrared radiation from the laser source 20. [

Due to the close angular alignment of the tracking beam 17 and the laser beam 14, the specular reflection of the tracking beam 17 from the cornea occurs near the corneal center and at the wall within the eye periphery 305. Such reflections will not interfere with the eye movement sensing function of the eye tracking system because they will not be imaged by the lens 320 on the detector comprising the array 300. [ The use of a temporarily modulated infrared light source and the proper selection of the angle of incidence of the beam 17 illumination from the light source to the cornea of the eye 70 represents prior art and a clear improvement as evidenced by PCT Patent Application No. WO 94/02007 .

Figure 7 is a schematic diagram of one embodiment of a servo system 500 used to drive a tracking mirror 60 along with an associated input signal from a tracking detector 300 included in an eye tracker 100 and associated controls . In the preferred embodiment of the present invention, the four detectors of Figure 2, shown as 300, each comprise a single element PIN silicon photodetector, but a dual element detector may alternatively be selected based on the specific functional needs of the device.

The voltage signal 301 received from the detector is continuously supplied to an amplifier set 330 having an amplified signal 331 that is transmitted directly to the demodulator. This demodulator is synchronized in time with the tracking light source 120 used to illuminate the eye to ensure that only light of the appropriate frequency is selected for the tracking signal, as indicated by the control 122. As previously described, this synchronization constitutes a means for temporal separation of the reflected light used for tracking, thus further enhancing the signal level with respect to the surrounding background light. A gated signal 341 from the demodulator is fed to a logic circuit 510.

Logic circuit 510 includes the main elements of the servo subsystem and acts as a central switchboard of a closed tracking feedback loop. The logic circuit switches the amplified and demodulated signal from the detector array 300 corresponding to the target position into a command to control the tracking element, in this case the tracking mirror 60. It is understood that as shown in FIG. 6, a pair of detectors opposite to each other when the image of the periphery 305 moves relative to the X and Y axes produces various electrical outputs.

The arithmetic difference between the signals from each pair of opposing detectors is substantially proportional to the displacement of the image from the center or null position of the corresponding axis. The signal difference, which is made in logic circuit 510 and further processed by circuit 510, constitutes the mirror displacement command indicated by control 511. [ These displacement commands actuate the actuator 550, which is relayed to the servo driver 520 and mechanically linked to the mirror 60 in turn, to rotate the mirror 60 about its axis. In this way, the angular orientation of the mirror 60 is modified to perform a two dimensional track motion as required.

The converter 540 is also mechanically connected to the mirror 60 to provide feedback to the logic circuit 510 via the connection 541. The transducer 540 is generally configured as a position sensitive element, which is a simple, readily available element in the preferred embodiment. The transducer 540 allows stabilization of the tracking element, in this case the mirror 60, associated with the preselected wrong position. Additionally, the converter 540 will detect when the tracking mirror 60 is at the end of the range and will no longer track the operation. This causes the computer 110 to stop the laser source 20 or close the shutter when the tracker can no longer follow the eye's action.

In a preferred embodiment, the reference position of the mirror 60 corresponds to the alignment of the patient's line of sight with the optical axis of the device as previously mentioned. This reference position may be selected by the computer 110 when the surgeon 55 indicates that the patient is aligned. The set of signals in FIG. 7, denoted 301, 521, 541 from eye tracker 300 to eye tracking mirror 60, is collectively represented as connection 102 in FIG. It is noted that for visual clarity, Figure 7 shows only two in each of the four servo drivers 520, transducers 540 and actuators 550 that may be included in the servo system.

Like most servo systems, the system shown in Figure 7 is an off-null measurement system based on returning an error signal to zero. There may be alternative implementations of the servo control system other than that shown in Figure 7 that still allow for accurate measurement and control of the eye displacement at a sufficiently high rate. Such alternate servo systems are within the scope of the present invention.

Topographic Measurements

As previously described, the corneal topography apparatus 180 may be used to aid in the measurement of the shape or curvature of the eye before and after surgery. Any commercially available topography apparatus can be used for this purpose as long as it is modified to include a reference target for fixation as used by the present invention. Another alternative embodiment would include a spatially resolved refractive index meter (SRR) to measure true refraction across the cornea at this location.

The ability to set a common reference frame between various ophthalmic devices is more important in light of the integrated needs of the corneal surgical method, which is the subject of the present invention, regardless of the corneal refractive and topographic measurements. In general, Is required for the successful outcome of any refractive surgery procedure.

The corneal topography device manufactured by EyeSys has some usefulness in calculating the shape of the cornea before and after surgery. Other recently used equipment, such as OrbScan, manufactured by Orbtek Inc., can provide information about the local form of the cornea that may be useful in optimizing certain types of refractive error correction such as astigmatism. In order for any such device to be efficient, it must be compatible with repeated measurements related to the same location of the eye. This aspect can be provided by eye tracking or fixation techniques in the manner described above, and is a unique method for the patient, not the equipment. Also, the inclusion of such an array feature enables intraoperative measurements of the corneal topography that can be used as active feedback during surgery for the purpose of increasing the accuracy of the operation and eliminating undesirable parameters that affect predictability. The prior art described by Smith, U. S. Patent No. 5,350, 374 shows the possibility of incorporating an active feedback control loop based on a particular type of topography instrument along with the corneal surgery procedure.

In various embodiments, the present invention also encompasses a mid-infrared laser using a scanning beam delivery system, including many advantageous features of prior art devices, including topographical feedback compatible with any number of available cornea measurement devices The scope is expanded to include PRK surgery.

An alternative to this topography mapping of the topography apparatus is a refractive mapping apparatus and method called spatial resolution refraction (SRR). A more detailed description of the SRR can be found in "Measurement of the eye's local wavefront distortion using a spatial resolution index meter," Web Optics, 31, 19, 3678-3686 (1992) . The SRR device measures the refraction at each point on the pupil of the pupil by allowing the patient to align two fixation sources through small pinholes. These pinholes are moved across the cornea to map each point of the cornea with a different refractive dimension. Since the purpose of PRK is to correct the patient's refractive errors, the SRR map is the ideal input for calibration by the PRK system and is not only a power map from the topography system, Improvement. These pre-operative input data can help define the ablation profile and pattern. Alternatively, the SRR may be used to map the eye during surgery.

The embodiments and variations described and illustrated herein are merely illustrative of the principles of the invention and that various modifications may be practiced by those skilled in the art without departing from the scope and spirit of the invention.

The method and apparatus of the present invention provides corneal tissue surgery based on controlled removal of tissue. Furthermore, a low cost solid state laser can be used to reduce the myopia, the primordial and / or astigmatism of the eye, reduce the refractive errors of the eye, and to treat the tissue near the corneal surface or surface, And intermediate-infrared laser radiation across the substrate, and corneal tissue surgery with an improved eye tracking mechanism is possible.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the functional relationship of optical, mechanical and electrical elements of an apparatus showing features of the present invention; Fig.

Figure 2 is an enlarged view of the optical element of Figure 1;

3 (a) and 3 (b) are diagrams showing a scanning pattern for a laser beam passing through the entire cornea;

Figures 4 (a) and 4 (b) show intensity profiles as a function of diameter of the focused laser beam measured in the cornea;

Figures 5 (a) and 5 (b) show the mechanism for laser beam delivery from a laser system to a surgical device.

Figures 6 (a) and 6 (b) show an image of an eye in an aligned and non-aligned position relative to the detector array of an integrated eye tracker;

Figure 7 is a schematic diagram of one embodiment of an electronic circuit and servo control function associated with the eye tracker described in Figures 1 and 2;

Claims (34)

A medical device for removing corneal tissue from a patient's eye, primarily by photospallation, A laser source generating pulses of intermediate infrared radiation, wherein the infrared radiation has a wavelength corresponding to a corneal absorption peak, the pulses having a duration of at least 1 ns and less than 50 ns; And Scanner-deflection means for directing pulsed radiation across the corneal tissue site in a predetermined pattern to remove the corneal tissue portions; ≪ / RTI > A medical device for removing corneal tissue from a patient's eye, A laser source generating pulses of intermediate infrared radiation, wherein the infrared radiation has a wavelength corresponding to a corneal absorption peak, the pulses having a duration of at least 1 ns and a duration of less than 50 nm; And Scanner-deflecting means for directing pulsed radiation across the corneal tissue site in a predetermined pattern to remove the corneal tissue portions; / RTI > Wherein the scanner-deflecting means comprises a mirror designed to direct pulses towards the corneal tissue, Wherein the medical device comprises a medical device that does not include any optical elements between the mirror and the corneal tissue, A medical device for removing corneal tissue from a patient's eye, A laser source generating pulses of intermediate infrared radiation, wherein the infrared radiation has a wavelength corresponding to a corneal absorption peak, the pulses having a duration of at least 1 ns and less than 50 ns; And Scanner-deflecting means for directing pulsed radiation across the corneal tissue site in a predetermined pattern to remove the corneal tissue portions; / RTI > Wherein the medical device does not include any non-erosive mask elements. An intermediate infrared laser system device for performing laser surgical procedures on a tissue, A laser source for generating a pump beam; A nonlinear crystal for parametrically converting the pump beam into an idler beam and a signal beam, wherein the idler beam has a wavelength in the mid-infrared range corresponding to the absorption peak of the tissue; And A mirror for directing the idler beam onto the tissue to remove portions of the tissue to ablate the tissue; Lt; / RTI > The laser system apparatus comprising optical elements, Wherein the idler beam is propagated from the mirror to the tissue without contacting any of the optical elements. A medical device for removing corneal tissue from a patient's eye, A laser source generating pulses of intermediate infrared radiation, wherein the infrared radiation has a wavelength corresponding to a corneal absorption peak, the pulses having a duration of at least 1 ns and less than 50 ns; And Scanner-deflecting means for directing pulsed radiation across the corneal tissue site in a predetermined pattern to remove the corneal tissue portions; / RTI > Wherein the medical device comprises optical elements, the medical device being designed to image an output aperture of one of the optical elements onto the corneal tissue. 6. The medical device of claim 5, wherein the aperture is an output aperture of a lens or an output aperture of one or more optical fibers. An intermediate infrared laser system device for performing laser surgical procedures on a tissue, A laser source for generating a pump beam; A nonlinear crystal for parametric conversion of the pump beam into an idler beam and a signal beam, wherein the idler beam has a wavelength in the mid-infrared range corresponding to the absorption peak of the tissue; And A structure for directing the idler beam onto the tissue to remove portions of the tissue; / RTI > Wherein the laser system device comprises optical elements, the structure comprising an intermediate infrared laser system device designed to form the idler beam at the front surface of the tissue into an opening image of one of the optical elements, 8. The apparatus of claim 7, wherein the aperture is an output aperture of the lens or an output aperture of the at least one optical fiber. The method according to any one of claims 1 to 3 and 5, Wherein the corneal tissue is removed to correct the curvature of the cornea. The method according to any one of claims 1 to 3 and 5, Wherein the corneal tissue is removed to affect therapeutic intervention, The method according to any one of claims 1 to 3 and 5, Wherein the beam has a pulse duration of 50 ns or less. The method according to any one of claims 1 to 3 and 5, Further comprising eye tracking means for sensing and compensating for eye movement. The method according to any one of claims 1 to 3 and 5, Wherein the laser source is coupled to the scanner deflection means by a decoupled laser delivery system. The method according to any one of claims 1 to 3 and 5, Wherein the laser source is an erbium YAG laser that produces infrared radiation at a wavelength of 2.94 microns. The method according to any one of claims 1 to 3 and 5, Wherein the pulse beam is generated by a solid state laser emitting radiation in the range of 1 to 2 microns and further comprises an optical parametric oscillator for frequency shifting the radiation to a wavelength of 3 microns. The method according to any one of claims 1 to 3 and 5, Wherein the laser source is a solid state laser that produces infrared radiation at a wavelength within the range of 2.7 to 3.1 microns. The method according to any one of claims 1 to 3 and 5, Wherein the energy in each pulse is between 5 mJ and 30 mJ. The method according to any one of claims 1 to 3 and 5, Wherein the scanner deflecting means generates a spot size in the range of 0.3 mm to 2 mm. The method according to any one of claims 1 to 3 and 5, Further comprising a corneal topography device for evaluating the shape of the corneal tissue. The method according to any one of claims 1 to 3 and 5, Wherein the corneal tissue is removed to change refractive characteristics of the eye, Wherein the medical device further comprises a spatial resolution index measuring device for evaluating a refractive index of the corneal tissue. The method according to any one of claims 1 to 3 and 5, Wherein the optical path generated by the tracking light source and the portions of the optical path from the laser source in the region adjacent to the eye are substantially at an angle to each other. 8. The method according to claim 4 or 7, Wherein the system device is designed to remove corneal tissue from the cornea to correct curvature of the cornea. 8. The method according to claim 4 or 7, Wherein the system device is designed to remove corneal tissue from the cornea to affect therapeutic intervention. 8. The method according to claim 4 or 7, Wherein the system arrangement is designed to produce the pump beam having a pulse duration greater than 0 ns and less than or equal to 50 ns. 8. The method according to claim 4 or 7, Further comprising eye tracking means for sensing and compensating for eye movement. 8. The method according to claim 4 or 7, RTI ID = 0.0 > 1, < / RTI > wherein the laser source is coupled to the mirror by a decoupled laser delivery system. 8. The method according to claim 4 or 7, Wherein the laser source comprises an erbium YAG laser that produces infrared radiation at a wavelength of 2.94 microns. 8. The method according to claim 4 or 7, The system arrangement is designed to generate the pump beam with pulses generated by a solid state laser emitting radiation in the range of 1 to 2 microns and includes an optical parametric oscillator for frequency shifting the radiation to a wavelength of 3 microns Further comprising the steps of: 8. The method according to claim 4 or 7, Wherein the laser source comprises a solid state laser generating infrared radiation at a wavelength within the range of 2.7 to 3.1 microns. 8. The method according to claim 4 or 7, Wherein the system device is designed to generate the pump beam with pulses, wherein the energy in each pulse is between 5 mJ and 30 mJ. 8. The method according to claim 4 or 7, Wherein the system device is designed to produce a spot size in the range of 0.3 mm to 2 mm on the tissue. 8. The method according to claim 4 or 7, Wherein the tissue is a corneal tissue and the system device further comprises a corneal topography device for evaluating the shape of the corneal tissue. 8. The method according to claim 4 or 7, Wherein the tissue is a corneal tissue, Wherein the system device is further configured to remove the corneal tissue to change the refractive properties of the eye and further to measure an index of refraction of the corneal tissue. 8. The method according to claim 4 or 7, Wherein the optical path generated by the tracking light source and the portions of the optical path of the idler beam in the region adjacent to the eye substantially coincide with each other.