Note: Descriptions are shown in the official language in which they were submitted.
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SOLID STATE UV LASER
The present irrventio~n 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 kera.tectomy (PRK) and laser in-situ
keratomileusis (LASER). Other examples include other
medical processes on a wide variety of biological tissue
such as retinal tisa;ue, bone or teeth.
Excimer gas lasers Y:~ave an operating wavelength of 193 nm
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 meci.ical f: field . However, an all solid state inT
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 aid maintE:nance costs. Excimer lasers also
require the us;e of an extremely toxic gas.
Solid state lasers offer a smaller, more efficient, less
dangerous alte:rnati~re 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 ho~.mium producing high energy laser emissions.
A variety of wavelengths may be produced depending on the
rare earth elE:anent 'that the laser contains . Some of the
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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.
25 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 Nd:YAG 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
OS Patent No. 4,346,314). Generation of the fourth and
fifth harmonic wavelengths of a Nd:YAG 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
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borate family. Crys~,tals such as beta barium borate ((3-
BaBs04 or 8B0) , lithium borate (LBO) , MBeBo3F~ and CsB305
have been used previously as freguency conversion compounds
(Mori, Ruroda, Nakajima, Taguchi, Sasaki and Nakai, 1995).
Other popular :NLO crystals for harmonic generation include
Potassium Titaayl Phosphate, (RTP or RTiOP04) and Potassium
Dideuterium Phosphate (KD*P or RDsPO') (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 t:he invention of the NLO crystal,
caesium lithium borate (CsLiBsOlo or CLBO), improved
performance ha.s been observed in generating the fourth and
fifth harmonics of t:he Nd:YAG laser (Yap, Inagaki,
Rakajima, Mori and :>asaki, 1996). Lago, Wallenstein, Chen,
Fan and Byer (1988) were able to generate 20 mJ in a 5 as
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 a1. (1996) were able to achieve an overall
conversion eff:icienc:y of 10.4% using CLBO crystals.
The advantages of u~aiag the CLBO crystal over SBO crystals
can also be se:ea by comparison of the non-linear properties
of the crysta7.s , tnl7c~en generating harmonic wavelengths in
the W spectrtua. CL~BO, 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 meiiical .applications, as it makes the alignment
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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,
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wherein said f:=:equency converting compounds include at
least one Caesium Lithium Borate (CsLiBsOlo or CLBO)
crystal.
Preferably said method includes directing said beam or
portion of said besan to a laser delivery system-a~nd then
onto said area. of said material by means of said laser
delivery system.
Preferably the: at lE:ast one CL80 crystal is in a sealed
dry, inert atu~osphere .
Preferably they 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 mic:ron.
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 del:i.very ystem may include a large beam delivery
system, a sca~;ining 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 ~aay be human or animal tissue, including
corneal tissue.
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The method may be used for refractive surgery of the cornea
by PRR or LASIR.
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 ultra-
violet 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
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said beam conversion means to said beam separating system
and then to said laser delivery system, and said fifth
harmonic frequency affixing means or said second frequency
doubling means includes a Caesium Lithium Borate (CsLiBsOlo
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 f.LBO 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 frequer.:cy doubling means may each include a separate
CLBO crystal f;or 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 deaacribed above may be provided for each
CLBO crystal, and the separate CLBO crystals are arranged
for generating fourth and fifth harmonics of said beam.
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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 scanaing 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 Nd3'" 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.
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Preferred embo~dimenta of the invention will be described,
by way of example, with reference to the accompanying
drawings is which:
Figure 1 is a schematic view of a laser ablation
apparatus according to a first embodiment of the present
invention, with an e:ye 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 l; and
Figure 4 is a e3chematic 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 ~~enerally at 10. The laser ablation
apparatus 10 3.ncludes 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 co7Llimated, resulting in a collimated
harmonically ~~enera!ted beam. and pulsed with a frequency of
between 5 and 30 Hz. Pulse energies for the fundamental
wavelength range from 30 to 1000 mJ 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 am.
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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 80°-C.
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
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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 b~e 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 ao~ay 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 am, 532 nm a.nd 266 nm respectively), the beam 26 of
fifth harmonic wavelength then passes to a laser delivery
system 32. Th.e delivery system 32 comprises a scanning
unit, a large beam dlelivery system (which may comprise
masks, a coaSpu.ter 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 t:he CLBO crystals can be affected by
hydration and tempe=:ature. The crystals should therefore
be stored is 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 ffilled 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 k>ack (not shown) that allow the passage of
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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
and 24 can be placed in a single housing in optical or
non-optical contact.
15 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
20 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 62$ arid 67q
respectively from the optic or z axis 56 and 58. The CLBO
crystals 20 and 24 are oriented at 45q from the x-axis in
order to maximize the harmonic conversion efficiency for
Type I phase matching.
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The beams emerge from the type I phase matching of the 4th
harmonic CLBO crystal 20 with the 532 nm component
vertically pol~arizedl (indicated at 60), and the 266 nm and
1064 nat components horizontally polarized (indicated at 62
and 64 respectively), while the 213 am component of the
beam emerges from the type I phase matching of the 5th
harmonic CLBO crystavl 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 be:am will contribute to the production of
the 213 am beam and, therefore, an optical element would
preferably be insert;ed before the fourth harmonic crystal,
in order to change t;he polarization of the 1064 nm beam.
Figure 4 shows a laser ablation apparatus 70 according to a
second embodiment oi: the present invention, in which Nd:YAG
laser 72 is co~nnectE:d to a fibre optic cable 74. When the
laser 72 is st~imulat:ed, 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 elemer.~ts 80 provided in the handpiece 78. It
should be noted thai:, from the perspective of the handpiece
78, either the! Nd:Yi~G laser 72 or the fibre optic cable 74
may be regarded as Ithe laser source. Three frequency
converting cr3,~stals 82, 84 and 86 are also contained within
the housing of: the lhandpiece or probe 78. Alternatively,
the first, or the first and second, NLO crystals 82 and 84
may be situatead 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
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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, RD*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
25 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 2I3 nm yields a potential substitute for the
present state of the art, with the added advantages of
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lower cost, lovaer maintenance, easier to use, smaller size
and the absena3 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 ther~s could be used any combination of NLO
crystal and an;y laser source with the handpiece or probe
described above. Another alternative arrangement would be
to replace the Nd:YA.G laser with any other near infra-red
source. Thus, it is to be understood that this invention
is not limited to th.e particular embodiments described by
way of example hereinabove.
