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1. WO1999004317 - LASER UV SOLIDE

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

SOLID STATE UV LASER

The present invention relates to the laser processing or ablation of materials, and is suitable, for example, for surgical and medical applications, including operations for correcting refractive errors of the eye, such as
photorefractive keratectomy (PRK) and laser in-situ
keratomileusis (LASIK) . Other examples include other medical processes on a wide variety of biological tissue such as retinal tissue, bone or teeth.

Excimer gas lasers have an operating wavelength of 193 run in the ultraviolet (UV) region of the electromagnetic spectrum. These lasers process material through photo-ablation, vaporizing the material while causing little thermal damage to adjacent areas. This property and the availability of these lasers has led to their widespread use in the medical field. However, an all solid state UV laser has been sought as an alternative, owing to a number of inherent disadvantages associated with the excimer laser. These disadvantages include large size and high operating and maintenance costs. Excimer lasers also require the use of an extremely toxic gas.

Solid state lasers offer a smaller, more efficient, less dangerous alternative to excimer gas lasers. These lasers utilize rare-earth elements contained in glass or crystal matrices such as yttrium aluminum garnet (YAG) , or yttrium lithium fluoride (YLF) . Excitation of the laser medium results in stimulated atoms of elements such as neodymium, erbium and holmium producing high energy laser emissions. A variety of wavelengths may be produced depending on the rare earth element that the laser contains. Some of the more common solid state lasers are Nd:YLF at 1.053 microns, Ho:YAG at 2.1 microns and Er:YAG at 2.94 microns. A
Neodymium:YAG laser produces a wavelength of 1064 nm (1.06 microns), which is in the infra-red portion of the
electromagnetic spectrum.

Solid state lasers produce beams of longer wavelengths than the excimer laser and have been successfully applied to different medical and industrial processes. However, the longer infra-red wavelengths may also produce undesirable effects when applied to certain materials, such as corneal tissue. As such, a demand exists for a solid state laser source that emits a wavelength in the ultraviolet region.

With the development of new non-linear optical (NLO) crystals, an all solid state UV laser source has been realized. The use of non-linear optical crystals for frequency conversion of high intensity laser emission is well known to those with an understanding of the art (see, for example, US Patent No. 5,144,630). When an infra-red laser beam is directed through a NLO crystal, its
wavelength can be altered. This property allows conversion of an infra-red laser, such as the NdtYAG at 1064 nm, to a shorter wavelength of 532 nm, a process known as harmonic generation (see, for example, US Patent No. 5,592,325 and US Patent No. 4,346,314). Generation of the fourth and fifth harmonic wavelengths of a NdtYAG laser, at 266 nm and 213 nm respectively, extends the sphere of the solid state laser, making it suitable for a wider range of
applications .

Prior art techniques for harmonic generation have often involved the use of non-linear optical crystals of the borate family. Crystals such as beta barium borate (β-BaB04 or BBO) , lithium borate (LBO) , MBeBo3F2 and CsB305 have been used previously as frequency conversion compounds (Mori, Kuroda, Nakajima, Taguchi, Sasaki and Nakai, 1995) . Other popular NLO crystals for harmonic generation include Potassium Titanyl Phosphate, (KTP or KTiOPO*) and Potassium Dideuterium Phosphate (KD*P or KD2P04) (see, for example, US Patent No. 5,144,630 and US Patent No. 5,592,325).
However, these crystals exhibit poor energy conversions for fourth and fifth harmonic generation.

More recently with the invention of the NLO crystal, caesium lithium borate (CsLiB60 or CLBO) , improved
performance has been observed in generating the fourth and fifth harmonics of the Nd:YAG laser (Yap, Inagaki,
Kakajima, Mori and Sasaki, 1996) . Lago, Wallenstein, Chen, Fan and Byer (1988) were able to generate 20 mJ in a 5 ns pulse at the fifth harmonic, using three BBO crystals for fifth harmonic generation of a Nd:YAG laser at 213 nm.
This corresponds to an overall conversion efficiency of 2.4% in terms of input energy at 1064 nm. In comparison, Yap et al . (1996) were able to achieve an overall
conversion efficiency of 10.4% using CLBO crystals.

The advantages of using the CLBO crystal over BBO crystals can also be seen by comparison of the non-linear properties of the crystals. When generating harmonic wavelengths in the UV spectrum, CLBO, despite having a smaller non-linear coefficient, has a larger angular bandwidth, spectral bandwidth and temperature acceptance. Also, unlike BBO, CLBO does not suffer from any problems with absorption and/or photorefraction. These features make the crystal useful for medical applications, as it makes the alignment of the laser beam less critical and more stable. In addition, the walkoff angle for CLBO is up to three times smaller than for BBO.

CLBO therefore offers an attractive advance over the prior art for fourth and fifth harmonic generation of a reliable solid state laser, utilizing this source of frequency conversion will enable the production of a smaller, more efficient, less expensive solid state laser suitable for applications such as photoablation of the cornea, which were previously only carried out by excimer lasers.

Therefore it is an object of the present invention to provide an improved method and apparatus of ablating material through generation of the fourth and fifth
harmonic wavelengths of a solid state laser.

It is a further object of the present invention to utilize an all solid state laser source, such as Nd:YAG or Nd:YLF, to generate the fourth and fifth harmonic wavelengths of said laser source to thereby ablate said material.

Thus, according to the present invention there is provided a method for ablating material including:
(a) directing a laser beam through a frequency
doubling compound;
(b) then directing said beam through plurality of frequency converting compounds;
(c) then directing said beam through a beam separating system; and
(d) directing said beam or a portion of said beam onto an area of said material to ablate said material, wherein said frequency converting compounds include at least one Caesium Lithium Borate (CsLiB60 or CLBO)
crystal .

Preferably said method includes directing said beam or portion of said beam to a laser delivery system and then onto said area of said material by means of said laser delivery system.

Preferably the at least one CLBO crystal is in a sealed dry, inert atmosphere.

Preferably the at least one CLBO crystal is maintained at a temperature of between 40°C and 200°C, and more preferably at a temperature of approximately 80°C.

Preferably said laser beam has a fundamental wavelength of between 0.5 and 2.5 micron, and more preferably
approximately 1 micron.

The method may include providing the laser beam by means of a Nd:YAG laser source or a Nd:YLF laser source.

Preferably the beam separating system is a dispersing prism or a dichroic mirror.

The laser delivery system may include a large beam delivery system, a scanning system or a fibre optic delivery system. Thus, the laser delivery system includes any system for delivering a laser beam to a desired location.

The material may be human or animal tissue, including corneal tissue.

The method may be used for refractive surgery of the cornea by PRK or LASIK.

When the material is corneal, the method preferably
includes pulsing the beam with a low pulse rate and a high energy per pulse, in which case the pulse rate is
preferably between 5 and 30 Hz, and the UV energy deposited on the material is preferably between 3 and 50 mJ per pulse.

The present invention also provides an apparatus for laser ablation of material including:
a laser source for providing a laser beam of infra-red light;
first frequency doubling means for doubling the frequency of said infra-red beam;
beam conversion means for converting said infra-red beam into an ultra-violet beam including:
a second frequency doubling means for redoubling said frequency to produce a twice doubled frequency beam and
a fifth harmonic frequency mixing means for
mixing said twice frequency doubled beam with said infra-red beam to produce an ultra-violet fifth
harmonic of said infra-red beam;
a beam separating system for separating said ultraviolet harmonic; and
a laser delivery system for delivering said ultra- violet harmonic to said material,
wherein said apparatus is arranged to direct said infra-red beam through said first frequency doubling means and said beam conversion means, and to direct light from said beam conversion means to said beam separating system and then to said laser delivery system, and said fifth harmonic frequency mixing means or said second frequency doubling means includes a Caesium Lithium Borate (CsLiB60 or CLBO) crystal.

Preferably the laser source provides said beam with a wavelength in the range 0.5 to 2.5 micron.

Preferably the infra-red beam has a fundamental wavelength of approximately 1 micron.

Preferably the apparatus includes a heating means for maintaining said CLBO crystal at one or more temperatures between 40°C and 200°C.

Preferably the heating means is controllable to maintain said CLBO crystal at a temperature of approximately 80°C.

Preferably the apparatus includes a sealable housing for sealing said CLBO crystal in a sealed dry, inert
atmosphere, and more preferably the housing is transparent to fundamental and harmonically generated laser beams .

Both the fifth harmonic frequency mixing means and the second frequency doubling means may each include a separate CLBO crystal for generating fourth and fifth harmonics of said beam respectively. In this embodiment, a single or separate sealed housings and/or a single or separate heating means as described above may be provided for each CLBO crystal, and the separate CLBO crystals are arranged for generating fourth and fifth harmonics of said beam.

Preferably the beam separating system is a dispersing prism or a dichroic mirror.

Preferably the laser delivery system includes a large beam delivery system, a scanning system or a fibre optic
delivery system.

The apparatus may be for the laser ablation of animal or human tissue, such as bone, tooth or corneal tissue, and in the case of corneal tissue for refractive surgery by PRK or LASIK.

When the material is corneal, the apparatus preferably includes beam pulsing means for pulsing the beam with a low pulse rate and a high energy per pulse, preferably with a pulse rate of between 5 and 30 Hz, and preferably an UV energy between 3 and 50 mJ per pulse applied to the
material .

Preferably the laser source is a Nd doped laser medium

The laser source may be a Nd:YAG, Nd:YLF, Nd:glass or
Nd:YV04 laser source.

In one particular embodiment, the apparatus further
includes a casing, wherein the laser source includes or comprises an optic fibre or optic fibre input, and the CLBO crystal is located within the casing.

Preferably in this embodiment, the apparatus constitutes a laser ablation handpiece or probe.

Preferred embodiments of the invention will be described, by way of example, with reference to the accompanying drawings in which:
Figure 1 is a schematic view of a laser ablation apparatus according to a first embodiment of the present invention, with an eye under examination;
Figure 2 is a view of a housing for the CLBO crystals of the apparatus of figure 1;
Figure 3 is a schematic view of the relative
orientation of the optic axis of the laser ablation
apparatus of figure 1; and
Figure 4 is a schematic view of a laser ablation apparatus according to a second embodiment of the present invention, with a tooth under examination.

Referring initially to Figure 1, a laser ablation apparatus according to a preferred embodiment of the present
invention is shown generally at 10. The laser ablation apparatus 10 includes a laser source in the form of a Q-switched Neodymium:YAG laser medium 12, for producing a 6-8 mm laser beam 14 of fundamental wavelength 1064 nm. The beam 14 is collimated, resulting in a collimated
harmonically generated beam, and pulsed with a frequency of between 5 and 30 Hz. Pulse energies for the fundamental wavelength range from 30 to 1000 m per pulse.

The laser beam 14 initially passes through a frequency doubling unit 16, which uses type I or type II phase matching and consists of a commercially available non- linear optical crystal such as BBO. Frequency doubling unit 16 generates a frequency doubled beam 18 of second harmonic wavelength 532 nm.

Frequency doubling unit 18 may alternatively use KD*P, KTP or any other crystal suitable for second harmonic
generation.

The laser beam 14 of fundamental wavelength and the
frequency doubled beam 18 of second harmonic wavelength pass through a second frequency conversion compound
comprising a CLBO crystal 20. In other embodiments, crystal 20 may comprise a crystal of BBO, KD*P or any other of KD*P's related isomorphs. The crystal 20 is used to convert frequency doubled beam 18 at 532 nm to beam 22 of fourth harmonic wavelength, 266 nm. This interaction utilizes type I phase matching. The beam 14 of fundamental wavelength, although passing through the crystal 20, does not contribute to any non-linear process. The beams 14, 18 and 22, of fundamental, second harmonic and fourth harmonic wavelength respectively, then pass through CLBO crystal 24. In this stage the beams 14 and 22, of fundamental and fourth harmonic wavelengths respectively, are frequency mixed to produce a laser beam 26 of the fifth harmonic wavelength, 213 nm by means of sum frequency generation, a type I phase matching interaction.

The CLBO crystals 20 and 24 are placed in a sealed housing (not shown), which is filled with argon. Within the housing, the CLBO crystal sits on a heating element that maintains the crystal temperature at approximately 802C. The housing has transparent windows that allow the passage of all laser beams. The housing is described further below with reference to figure 2.

The crystal lengths for the CLBO crystals 20 and 24 (for 4th and 5th harmonic generation) are approximately 5 mm and 3 mm, respectively. The apertures of the crystals 20 and 24 are large enough to transmit all beams without clipping.

After all the beams 14, 18 and 22 have passed through the fifth harmonic CLBO crystal 24, the fundamental and
harmonic wavelengths are spatially overlapping. In order to isolate the beam 26 of fifth harmonic wavelength, 213 nm, the beams must be separated. The combined output beam 28 is therefore passed through a beam separating system in the form of dispersing prism 30, which separates the beams. In alternative embodiments any of the other known methods of beam separation may be used, such as the use of a dichroic mirror to reflect only the fifth harmonic
wavelength. With the 213 nm wavelength beam 26 spatially separated from the other harmonics (beams 14a, 18a and 22a of 1064 nm, 532 nm and 266 nm respectively), the beam 26 of fifth harmonic wavelength then passes to a laser delivery system 32. The delivery system 32 comprises a scanning unit, a large beam delivery system (which may comprise masks, a computer controlled iris, and beam shaping
optics), and/or a fibre optic delivery system. A large beam delivery system may include a scanner. The beam 26 of wavelength 213 nm is then delivered to the material to be ablated, for example the cornea 34 of an eye 36.

The performance of the CLBO crystals can be affected by hydration and temperature. The crystals should therefore be stored in a suitable housing, such as the sealed housing shown at 38 in figure 2. The housing 38 is made of a thermally conductive material and is filled with a dry inert gas, such as Argon, introduced through a sealed gas valve 40. The housing 38 has transparent windows at the front 42 and back (not shown) that allow the passage of fundamental and harmonically generated laser beams. A CLBO crystal 44 is placed in a removable crystal holder 46 and seated on a thermo-electric heater 48. Current is supplied through a sealed electrical connector 50. The thermal element of the heater 48 maintains the crystal 44 at a temperature of between 40°C and 200°C, and most preferably at a temperature of approximately 80°C, principally to keep moisture out of the crystal, but also to help the crystal 44 to reach thermal stability more quickly when the laser is turned on, and to avoid distortion of the refractive index or crystal cracking. Sometimes the two CLBO crystals 20 and 24 can be placed in a single housing in optical or non-optical contact.

Shown in figure 3 is the preferred relative orientation of the optic axes of the two CLBO crystals. The axes are arranged perpendicular to each other in order to satisfy the phase matching conditions of each of the non-linear processes, as the interactions of the wavelengths depend on the polarization of the beams being mixed. Type I phase matching at the second harmonic crystal leaves, in this preferred embodiment, the 1064 nm beam horizontally
polarized (indicated at 52) and the 532 nm vertically polarized beam (indicated at 54) . The CLBO crystals 20 and 24 are oriented at the phase-matching angle for each harmonic generation process. For 4th and 5th harmonic generation these angles are approximately 62s and 67s respectively from the optic or z axis 56 and 58. The CLBO crystals 20 and 24 are oriented at 45s from the x-axis in order to maximize the harmonic conversion efficiency for

Type I phase matching.

The beams emerge from the type I phase matching of the 4th harmonic CLBO crystal 20 with the 532 nm component
vertically polarized (indicated at 60), and the 266 nm and 1064 nm components horizontally polarized (indicated at 62 and 64 respectively) , while the 213 nm component of the beam emerges from the type I phase matching of the 5th harmonic CLBO crystal 24 vertically polarized (indicated at 66) .

Type II phase matching at this stage would leave the 1064 nm beam elliptically polarized and the 532 nm beam
vertically polarized. Only a portion of an elliptically polarized 1064 nm beam will contribute to the production of the 213 nm beam and, therefore, an optical element would preferably be inserted before the fourth harmonic crystal, in order to change the polarization of the 1064 nm beam.

Figure 4 shows a laser ablation apparatus 70 according to a second embodiment of the present invention, in which Nd:YAG laser 72 is connected to a fibre optic cable 74. When the laser 72 is stimulated, the beam 76 of fundamental
wavelength travels through the fibre optic cable 74 and enters a small handpiece or probe 78 through a set of optical elements 80 provided in the handpiece 78. It should be noted that, from the perspective of the handpiece 78, either the Nd:YAG laser 72 or the fibre optic cable 74 may be regarded as the laser source. Three frequency converting crystals 82, 84 and 86 are also contained within the housing of the handpiece or probe 78. Alternatively, the first, or the first and second, NLO crystals 82 and 84 may be situated in the optical path before the fibre optic cable 74. As the beam 76 of fundamental wavelength travels into the hand piece 78, it encounters the doubling NLO crystal, BBO crystal 82. Other NLO crystals may be used. The beams 76 and 88 of fundamental and second harmonic wavelength respectively pass through another NLO crystal, CLBO crystal 84. Suitable substitutes for the CLBO crystal include BBO, KD*P or any of KD*P's related isomorphs. The beam 90 of fifth harmonic wavelength is generated by CLBO crystal 86. The combined output beam (combined within CLBO crystal 86, which thereby acts as a mixing means) is delivered to the beam separating means, dichroic mirror 92, which reflects beams of fundamental, second harmonic and fourth harmonic wavelength 76, 88 and 94 respectively and transmits beam 90 of fifth harmonic wavelength.
Alternatively mirror 92 may reflect only one or two of the beams so that a combination of the beams may be applied to the tissue. The fifth harmonic is separated, and delivered by the delivery system 96 to the exterior of the apparatus 70 and directed onto the tissue to be ablated, for example tooth 98. Alternatively the tissue to be ablated could be (for example) bone.

An alternative configuration of the present apparatus would be to use any combination of NLO crystal and any laser source with the handpiece or probe described herein.
Another alternative arrangement would be to replace the Nd:YAG laser with any other near infra-red source.

The various embodiments of the method and apparatus of the present invention provide a stable and viable solid state alternative to the excimer Argon-Fluoride laser for medical purposes. Producing a solid state laser at a wavelength of approximately 213 nm yields a potential substitute for the present state of the art, with the added advantages of lower cost, lower maintenance, easier to use, smaller size and the absence of hazardous materials.

Modification within the spirit and scope of the invention may be readily effected by a person skilled in the art. Thus, in alternative configurations of the laser ablation apparatus there could be used any combination of NLO crystal and any laser source with the handpiece or probe described above. Another alternative arrangement would be to replace the Nd:YAG laser with any other near infra-red source. Thus, it is to be understood that this invention is not limited to the particular embodiments described by way of example hereinabove.