United States Patent
`
`119]
`
`[11] Patent Number:
`
`5,720,894
`
`Neev et al.
`[45] Date of Patent: Feb. 24, 1998
`
`
`
`USOOS720894A
`
`[54] ULTRASHORT PULSE HIGH REPETITION
`RATE LASER SYSTEM FOR BIOLOGICAL
`TISSUE PROCESSING
`
`[75]
`
`Inventors: Joseph Neev. Laguna Beach; Luiz B.
`Da Silva. Danville; Dennis L.
`Matthews. Moss Beach; Michael E.
`Glinsky. Livermore; Brent C. Stuart.
`Fremont; Michael D. Perry. Livermore;
`Michael D. Feit. Livermore; Alexander
`M. Rubenchik. Livermore. all of Calif.
`
`[73] Assignee: The Regents of the University of
`California. Oakland. Calif.
`
`[21] Appl. No.: 534,522
`
`[22] Filed:
`
`Jan. 11, 1996
`
`Int. Cl." ........................................................ B44C 1/22
`[51]
`
`[52] U.S. Cl.
`......................... 216/65; 156/345; 216/67;
`606/11; 607/89
`[53] Field of Search .............................. 156/6261. 643.1.
`156/345 MT. 345 P. 345 LT; 219/12163.
`121.69; 216/65. 67; 606111. 12; 607/39
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,478,677
`4,722,056
`4,737,623
`4,313,230
`4,362,336
`5,207,576
`5,207,663
`5,275,594
`5,231,141
`5,293,372
`5,312,396
`5,342,193
`5,350,375
`5,409,376
`5,409,431
`
`....................... 156l345 LT
`10/1984 Chen et al.
`1/1988 Roberts et a1.
`.. 364/413
`4/1933 Lovoi
`................
`I... 250/226
`
`.. 433/215
`4/1939 Myers et a1.
`..
`.........
`123/3031
`9/1939 Clarke eta].
`
`5/1993 Vassiliadisetal.
`.. 433/215
`
`5/1993 L’Esperance, Jr.
`60615
`
`1/1994 Baker ..................... 616/12
`
`1/1994 Kowalyk
`433/215
`
`
`3/1994 Alfanoetal..
`.. 123/664
`.........
`5/1994 Feld et a].
`606/11
`
`433/215
`311994 Vassiliadisetal.
`9/1994 Deekelbaum eta].
`....... 606/7
`
`4/1995 Murphy ..................... 433/29
`............................ 606/12
`4/1995 Poppas eta].
`OTHER PUBLICATIONS
`
`Altshuler. et al.. Application of Ultrashort Laser Pulses in
`Dentistry, SPIE vol. 2080 Dental Applications of Lasers
`(1993) pp. 77—81.
`
`da Silva. et al.. The Short—Pulse Laser: A Safe, Painless
`Surgical Tool, Science & Technology Review Oct. 1995.
`Du. et al.. Damage Threshold as a Function of Pulse
`Duration in Biological Tissue, Springer Series in Chemical
`Physics. vol. 60. pp. 254-255.
`Du. et al.. Laser—induced Breakdown by Impact Ionization
`in SiO2 with Pulse Widths From 7 us to 150fs, Appl. Phys.
`Lett. vol. 64. No. 23. 6 Jun. 1994.
`Fischer. et al.. Plasma—Medicated Ablation of Brain fissue
`with Picosecond Laser Pulses, App. Phys. B 58. 493—499
`(1994).
`Ihlemann. et al.. Nanosecond and Femtosecond Excimer
`Laser Ablation of Fused Silica, Appl. Phys. A 54. 363—368
`(1992).
`Kautek. et al.. Femtosecond—Pulse LaserAblation ofHuman
`Corneas, Appl. Phys A 58. 513-518 (1994).
`
`(List continued on next page.)
`
`Primary Examiner—William Powell
`Anomey, Agent, or Finn—Christie. Parker & Hale. LLP
`
`[57]
`
`ABSTRACT
`
`A method and apparatus is disclosed for fast. eflicient.
`precise and damage-free biological tissue removal using an
`ultrashort pulse duration laser system operating at high pulse
`repetition rates. The duration of each laser pulse is on the
`order of about 1 is to less than 50 ps such that energy
`deposition is localized in a small depth and occurs before
`significant hydrodynamic motion and thermal conduction.
`leading to collateral damage. can take place. The depth of
`material removed per pulse is on the order of about 1
`micrometer. and the minimal thermal and mechanical effects
`associated with this ablation method allows for high repeti-
`tion rate operation. in the region 10 to over 1000 Hertz.
`which. in turn. achieves high material removal rates. The
`input laser energy per ablated volume of tissue is small. and
`the energy density required to ablate material decreases with
`decreasing pulse width. The ablation threshold and ablation
`rate are only weakly dependent on tissue type and condition.
`allowing for maximum flexibility of use in various biologi-
`cal tissue removal applications. The use of a chirped-pulse
`amplified Titanium-doped sapphire laser is disclosed as the
`source in one embodiment.
`
`21 Claims, 6 Drawing Sheets
`
`
`
`inf/EA!"19%??4:45
`IV: S‘flWP/M?!
`All/£27951:
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 1
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 1
`
`

`

`5,720,894
`Page 2
`
`OTHER PUBLICATIONS
`
`Kautek. et al.. Femtosecond Pulse Laser Ablation of Metal-
`lic, Semiconducting, Ceramic, and Biological Materials,
`SPIE vol. 2207. 600—611.
`Neev. et al.. Ablation of Hard Dental Tissues with an ArF
`Pulsed Excimer Laser; SPIE vol. 1427 Laser—Tissue Inter-
`action 11 (1991) 162—172.
`Neev. et al.. Dentin Ablation With Three Infrared Lasers,
`Lasers in Surgery and Medicine 17:00—00 (1995).
`Neev. et al.. Dentin Ablation with Two Excimer Lasers: A
`Comparative Study of Physical Characteristics, Lasers in
`the Life Sciences 5(1—2). 1992. pp. 129—153.
`Neev. et 21].. Scanning Electron Microscopy and Thermal
`Characteristics of Dentin Ablated by a Short—Pulse XeCl
`Excimer Laser; Lasers in Surgery and Medicine 13:353-362
`(1993).
`
`Neev. et al.. The Efiect of Water Content on UV and IR Hard
`Tissue Ablation, SPIE vol. 2323. pp. 292—299.
`
`Niemz. Investigaton and Spectral Analysis of the Plasma—
`Induced Ablation Mechanism of Dental Hydroxyapatite,
`Appl. Phys. B. Spring (1994) pp. 273—281.
`Pruess. et 21].. Resolved Dynamics of Subpicosecond laser
`Ablation, Appl. Phys. Lett.. vol. 62. No. 23. 7 Jun. 1993.
`Rubenchik. et al.. Hand Tissue Abhztion with Ultra Short
`Laser Pulses, Optical Society of America Annual Meeting
`Presentation Sep. 10—15. 1995.
`Stuart. et a1.. Laser—Induced Damage in Dielectrics with
`Nanosecond to Subpicosecona' Pulses, Physical Review
`Letters. vol. 74. No. 12. 20 Mar. 1995.
`
`Wolfi—Rottke. et al.. Influence of the Laser—Spot Diameter
`on Photo—Ablation Rates, Appl. Phys. A 60. 13—17 (1995).
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 2
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 2
`
`

`

`US. Patent
`
`Feb. 24, 1998
`
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`5,720,894
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`Exhibit 1019 - Page 3
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`Alcon Research, Ltd.
`Exhibit 1019 - Page 3
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`

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`US. Patent
`
`Feb. 24, 1998
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`Exhibit 1019 - Page 4
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`Alcon Research, Ltd.
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`
`

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`
`Feb. 24, 1993
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`Exhibit 1019 - Page 5
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`Alcon Research, Ltd.
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`

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`Feb. 24, 1998
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`

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`
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`

`

`US. Patent
`
`Feb. 24, 1998
`
`Sheet 6 of 6
`
`5,720,894
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`Exhibit 1019 - Page 8
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 8
`
`

`

`1
`
`ULTRASHORT PULSE HIGH REPETITION
`RATE LASER SYSTEM FOR BIOLOGICAL
`TISSUE PROCESSING
`
`ACKNOWLEDGEMENT OF U.S.
`GOVERNMENT SUPPORT
`
`This invention was made with U.S. Government support
`under Contract No. DE-FGO3—91ER61227. awarded by the
`U.S. Department of Energy. Grant No. N0014—9l-C—0134.
`awarded by the Oifice of Naval Research. and Grant No.
`RR01192. awarded by the National Institute of Health. The
`U.S. Government has certain rights in this invention.
`
`FIELD OF THE INVENTION
`
`The present invention is directed to the field of ultrashort
`pulse duration laser systems suitable for material and bio-
`logical tissue processing and in particular to a material
`removal apparatus and method in which ultrashort pulse
`laser systems are operable at pulse repetition rates in excess
`of at least 10 Hertz so as to efficiently remove substantial
`material volumes while substantially eliminating collateral
`damage.
`
`BACKGROUND OF THE INVENTION
`
`Laser interaction with organic and inorganic targets has
`been investigated for the past thirty five years for applica-
`tions as diverse as material processing and surgical tissue
`ablation. One significant challenge to laser tissue processing
`is the need to maximize ablation efficiency while. at the
`same time. minimizing collateral damage to adjacent mate-
`rial.
`
`Recent years have brought increased interest in the use of
`lasers as a therapeutic and preventive tool in various dental
`applications such as removal of carious lesions (removal of
`tooth decay). surgical treatment of oral malignancies and
`periodontal diseases. and preparation and sterilization of
`root canals. In spite of these advances. lasers remain limited
`in their ability to remove sound (hard as opposed to soft)
`tooth structure since the lasers currently in use for dental
`procedures generate unacceptable heat levels which cause
`collateral damage to the tooth surface and in the tooth pulp.
`Early procedures for removal of hard dental substances
`involved optical drilling using C02. ruby and NszAG
`(Neodymium doped Yttrium Aluminum Garnet) lasers
`requiring high radiant exposure and resulting in considerable
`damage to surrounding tissue. As a consequence. it was
`generally concluded in the mid 19705 that lasers would not
`become a common drilling tool unless a new method was
`found to reduce collateral damage.
`
`Optical dental drilling with Er:YAG (Erbium doped YAG)
`lasers yielded encouraging results in the early 1990s. and has
`shown capabilities to perform as an efficient drill with out
`generating excessive damage to surrounding tissue. The
`success of ErzYAG systems. operating in the nanosecond to
`microsecond pulse duration regime. in minimizing thermal
`damage has also been observed in other areas of application
`in medicine. and can be attributed to the high absorption
`coeflicient of biological tissues at the particular wavelengths
`characteristic of the system (2900 nm). when used in com-
`bination with nanosecond to microsecond pulse durations.
`Neev et al.. Dental Ablation With Three Infrared Lasers.
`Lasers in Surgery and Medicine. Vol. 17. 1995. discloses
`three laser systems adapted to hard tissue processing. such
`as dentin and enamel removal in dental applications. The
`laser systems disclosed (Er:YSGG. Ho:YSGG. and
`
`5,720,894
`
`2
`
`Q—switched NszAG) all operate in the near 1R region of the
`electromagnetic spectrum and are pulsed in two difierent
`regimes: about 250 microsecond pulse durations for the
`ErzYSGG and Ho:YSGG lasers. and about 15 nanosecond
`pulse durations for the Er2YAG system.
`While the disclosed removal rate is in the range of
`approximately tens of micrometers per pulse. the disclosed
`laser systems exhibit classical spectrum selectivity
`(wavelength dependent absorption) and effect high removal
`rates by operating at pulse energies in excess of 20 to 30
`millijoules per pulse. Enhancing material removal by
`increasing laser power is. however. accompanied by
`increased photothermal and photomechanical efiects which
`causes collateral damage in adjacent material. In addition.
`increasing power leads to plasma decoupling of the beam.
`e.g.. incident laser energy is wasted in heating the ambient
`in front of the target.
`High intensity pulses additionally cause very loud acous-
`tic snaps. when the laser pulse interacts with tissue. These
`snaps or pops include a large high frequency component
`which is very objectionable to a user or. in the case of a
`medical application. to a patient. In addition to the psycho-
`logical impact of such noise. these high frequency snaps are
`able to cause hearing loss in clinicians when repeated over
`a period of time.
`U.S. Pat. No. 5.342.198. to Vassiliadis. et al.. discloses an
`ErzYAG IR laser system suitable for the removal of dentin
`in dental applications. The laser produces a pulsed output
`having a beam with a pulse duration in the range of several
`tens of picoseconds to about several milliseconds. Although
`disclosed as being eflicient in the removal of dentin and
`dental enamel. the mechanism by which material removal is
`effected is not understood. Significantly. however. the only
`laser systems disclosed as suitable for the process are those
`which operate at wavelengths (1.5 to 3.5 microns) that have
`proven to be generally elfective for enamel interaction.
`Thus. the absorption characteristics of the material target are
`of primary concern to the removal rate. In addition. high
`energy levels are required to remove enamel and dentin.
`leading to the problem of thermal damage and acoustic
`norse.
`
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`Additional possibilities for the application of lasers to the
`field of dentistry in particular. and to hard tissue ablation in
`general. have been proposed by the use of excimer lasers
`that emit high intensity pulses of ultraviolet (UV) light.
`typically with pulse durations in the approximately 1 to 100
`nanosecond range. Both the short wavelengths and nano-
`second range pulse durations used by excimer lasers con-
`tribute to defining a diflerent regime of laser-tissue interac-
`tion. Short wavelength ultraviolet photons are energetic
`enough to directly break chemical bonds in organic mol-
`ecules. As a consequence. UV excimer lasers can often
`vaporize a material target with minimal
`thermal energy
`transfer to adjacent tissue. The resultant gas (the vaporiza-
`tion product) is ejected away from the target surface. leaving
`the target relatively free from melt. recast. or other evidence
`of thermal damage.
`Another important characteristic of UV excimer lasers is
`that materials which are transparent to light in the visible or
`near infra-red portions of the electromagnetic spectrum
`often begin to exhibit strong absorption in the UV region of
`the spectrum. It is well established that the stronger a
`material’s absorption at a particular wavelength. the shal—
`lower the penetration achieved by a laser pulse having that
`wavelength. Thus. in many types of materials. a UV pulse
`typically only penetrates to a depth in the range of from
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 9
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 9
`
`

`

`5 ,720,894
`
`3
`about 1 to about 4 micrometers. By simply counting pulses.
`great precision can be achieved in defining removal depths.
`In addition. organic tissue is strongly absorbent in the UV
`wavelengths (193 nm for ArF. for example) therefore allow-
`ing the laser-tissue interaction region to be controlled With
`great precision.
`Notwithstanding the relatively damage free material
`removal characteristics of UV excimer lasers. these systems
`suffer from several disadvantages which limit their applica—
`bility to biological tissue processing. The reports of damage
`free tissue removal result from evaluations performed on
`single pulses. or on pulses with a very low repetition rate
`(typically about 1 to 5 Hertz). Because of the low volumetric
`removal per pulse of excimer systems (material removed per
`unit time is poor). eflicient material removal can only be
`accomplished by high pulse repetition rates. However. when
`the pulse repetition rate exceeds about 3 to 5 Hertz. consid-
`erable thermal and mechanical collateral damage is
`observed. While UV photons are sufliciently energetic to
`directly break chemical bonds. they are also sufficiently
`energetic to promote mutagenic effects in tissue irradiated at
`UV wavelengths. raising concerns about the long term safety
`and health of a system operator. The scattered light produced
`by excimer lasers also presents a significant threat to the
`clinician and/or the patient. Even low intensity scattered
`radiation. with wavelengths below 300 nanometers. is able
`to interact with the ambient environment to produce atomic
`oxygen and other free radicals. These can. in turn. react with
`the lens and cornea of the eye. producing cataracts. and
`produce burns on the skin equivalent to sun burns. As a
`consequence. excimer laser systems have been found to be
`most suitable for inorganic material processing applications.
`such as thin coating patterning or dielectric or semiconduc-
`tor material etching.
`In addition. the operational parameters of excimer laser
`systems are such that material removal remains a wave-
`length and beam energy dependent process (although
`weakly dependent on wavelength). Even when pulsed in the
`tens of nanoseconds pulse duration regime. excimer lasers
`are configured to deliver energy in the range of from about
`10 to about 1000 millijoules per pulse. At
`the higher
`energies. excimer lasers suffer from the same problems
`caused by plasma decoupling and pulse to pulse interaction
`as IR lasers. Additionally. as pulse energy increases. so too
`does the intensity of the associated acoustic snap.
`SUMMARY OF THE INVENTION
`
`There is. therefore. provided in the practice of this inven-
`tion a fast. efficient. and collateral damage free apparatus
`and method for selective removal of material
`through
`material—laser interaction between biological tissue and a
`pulsed laser operating in the femtosecond to picosecond
`pulse duration regime at high pulse repetition rates.
`The process of the present invention results in material
`removal rates which meet or exceed the removal rates of
`mechanical drilling systems while far exceeding the accu-
`racy and precision of low removal rate laser systems.
`In one embodiment of practice of the present invention.
`the process for selective biological tissue removal process-
`ing comprises providing a pulsed laser operated so as to
`produce a pulsed output beam which includes individual
`pulses each having a pulse duration in the range of from
`about 1 femtosecond to about 100 picoseconds. The pulsed
`beam is directed onto a target material. such as biological
`tissue. from which removal is desired.
`
`Each pulse interacts with a thin layer portion of said
`biological tissue so as to form a plasma which decays in the
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`time period between pulses such that the impacted tissue
`portion is removed by ablation. The plasma formation step
`is repeated at a pulse repetition rate greater than or equal to
`10 pulses per second until a sufficient depth of tissue has
`been removed with substantially no transfer of thermal or
`mechanical energy into the remaining tissue and substan-
`tially no collateral damage thereto.
`According to one aspect of the invention. the plasma is
`formed by multi—photon absorption and/or collisional ion-
`ization of the atoms and molecules comprising the tissue
`material. Each pulse of the pulsed output beam has an energy
`in the range of from about 0.1 to about 100 millijoules. the
`pulsed beam having a diameter at the tissue target such that
`the tissue experiences an energy fluence in the range of from
`about 0.1 to about 15 Joules per square centimeter depend-
`ing upon tissue type. laser pulse duration and laser wave-
`length. When so operated.
`the pulsed beam exhibits a
`material removal rate in the range of from about 0.1 to about
`2.0 micrometers per pulse. with the removal rate being
`substantially constant without regard to variations in tissue
`chromophore. tissue hardness or tissue state.
`According to an additional aspect of the invenu‘on. the
`method of the present invention is practiced by laser systems
`operating in the 200 to 2000 nanometer region of the
`electromagnen'c spectrum.
`In a more detailed embodiment of the present invention.
`a chirped-pulse amplified. solid state laser is used to provide
`an about 500 micrometer diameter pulsed beam. which
`provides pulses. having durations in the 0.02 to 100 pico-
`second region at an adjustable repetition rate from 10 to
`2000 Hertz with pulse energies of about 3 millijoules. The
`pulsed beam is used to selectively ablate undesired material.
`such as carious lesions. dentin. enamel and/or soft tissue in
`a dental procedure at a removal rate which meets or exceeds
`the removal rate of a mechanical dental drill. The precision
`and selectivity of material removal by the apparatus and
`method of the present invention enables additional delicate
`surgical procedures. particularly in cases where diseased or
`undesired tissue is interspersed with healthy tissue. or in
`cases where the working area is exceptionally close or
`exceptionally delicate. such as brain and spinal surgery.
`bone removal in neural surgical applications. and ortho-
`paedic surgery.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and other features. aspects. and advantages of the
`present invention will be more fully understood when con-
`sidered with respect to the following detailed description.
`appended claims. and accompanying drawings. wherein:
`FIG. 1(a) is a graphical representation of experimentally
`determined values of laser damage fluences in Joules per
`square centimeter. plotted as a function of pulse duration in
`picoseconds. for fused silica showing the monotonically
`decreasing threshold and fluence departure from root tau
`dependence on pulse duration. when pulse duration is
`reduced in accordance with practice of principles of the
`invention;
`FIG. 1(b) is a graphical representation of experimentally
`determined values of laser damage fluences in Joules per
`square centimeter. plotted as a function of pulse duration in
`picoseconds. for fused silica showing the wavelength inde-
`pendence of fluence departure from root tau scaling;
`FIG. 2 is a graphical representation of experimentally
`determined values of ablation thresholds plotted as a func-
`tion of pulse duration for materials having various absorp-
`tion characteristics. depicting the independence of ablation
`threshold values on material chromophore;
`
`Alcon Research, Ltd.
`Exhibit 1019 - Page 10
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`Alcon Research, Ltd.
`Exhibit 1019 - Page 10
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`5.720.894
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`FIG. 3 is a graphical representation of experimentally
`determined values of material removal rates in microns per
`pulse. plotted as a function of laser fluence. of ultrashort
`laser pulses for exemplary dentin and enamel material.
`depicting the independence of removal rates at a given laser
`fluence on material properties;
`FIG. 4 is a graphical representation of experimentally
`determined values of residual pulse heat. plotted as a func-
`tion of time. for ultrashort laser pulses (solid diamonds) as
`compared to nanosecond laser pulses (triangles) at a 10
`Hertz repetition rate;
`FIG. 5 is a graphical representation of experimentally
`determined values of residual pulse heat. plotted as a func—
`tion of time. for ultrashort laser pulses operating at a 1000
`Hertz repetition rate;
`FIG. 6 is a simplified block level schematic diagram of a
`chirped-pulse amplified solid-state laser system suitable for
`practice of principles of the invention; and
`FIG. 7 is a simplified block level schematic diagram of an
`exemplary dental drilling apparatus incorporating the
`ultrashort pulse duration. high repetition rate laser system of
`FIG. 6.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT(S)
`
`The principles of operation of an exemplary laser system.
`which will be described in detail in following sections. will
`now be developed in connection with certain mechanisms
`for hard tissue removal. for example. the ablation of dentin
`during removal of carious lesions. The description of the
`operation of the laser system with respect to dental appli-
`cations is for exemplary purposes only and is not intended
`to limit the application of the laser of the present invention.
`As will be described in greater detail below. the laser system
`of the present invention has application to a wide variety of
`biological tissue removal processes as well as exceptional
`utility for general material removal and micro-machining.
`Those having skill in the art will immediately recognize the
`utility and applicability of the laser system’s novel opera-
`tional regime to laser-tissue interactions in the general sense.
`Utilizing the laser system which will be described further
`below.
`the inventors have identified a laser operational
`parameter regime which provides hard tissue interaction
`characteristics that are superior to conventional laser sys-
`tems and provides material removal rates. for exemplary
`dental material. on a par with mechanical drill technology.
`Advantageously. the tissue ablation methods of the laser
`system of the present invention provide for eflicient material
`ablation because the input laser energy per ablated volume
`of tissue is small. resulting in a decrease in the amount of
`energy required to ablate a given volume of material achiev—
`able with conventional prior-art lasers employed for cutting.
`drilling and sculpting of biological tissue. The laser system’s
`high ablation efficiency and the short duration of the stress
`impulse results in negligible collateral mechanical damage.
`while the extremely short energy deposition time results in
`minimal collateral thermal damage.
`The ablation threshold and removal rate are only ruini—
`mally dependent on tissue type and condition. thus. ablation
`(material removal) is generally chromophore independent as
`well as generally insensitive to tissue linear absorption
`characteristics. tissue moisture content. material morphol-
`ogy and micro—architecture. and tissue hardness. Precision of
`ablation depth is achieved by removing only a small volume
`of material with each pulse. while volumetric ablation is
`controlled by the repetition frequency of the ablation pulses.
`
`Precise spatial control of tissue ablation and removal as
`well as precise control of ablation depth has been deter—
`mined by the inventors as resulting from an intensity-
`dependent multiphoton initiation and plasma termination
`process. Formation of a critical density plasma by both
`multiphoton and collisional ionization processes eliminates
`significant energy deposition below a depth approximately
`that of the wavelength of the laser light. when energy
`deposition takes place in less than about 10 picoseconds.
`This “self temrination” insures a high precision of tissue
`removal for each pulse and is primarily responsible for the
`high ablation efficiency. defined as the magnitude of laser
`energy required to effect removal of a given volume of tissue
`or material. of ultrashort pulses in accord with the invention.
`Ablation efficiencies have been demonstrated. in accordance
`with the present
`invention. at approximately 0.1 cubic
`millimeters of material removed per Joule of laser energy.
`for hard. dielectric materials. e.g.. fused silica. bone. enamel.
`or the like. Conventional nanosecond pulse duration laser
`systems have substantially lower ablation efficiencies. in
`that laser energies must be increased significantly in order to
`remove the same amount of material with substantially the
`same laser beam size.
`
`An additional advantage of the method of the present
`invention. is that longer wavelength. ultrashort pulse dura-
`tion laser systems can be utilized in most. if not all. of the
`procedures currently employing lasers which operate in the
`ultraviolet region. Replacing ultraviolet
`lasers with the
`longer wavelength ultrashort pulse lasers of the invention
`would provide the benefit of eliminating the risks associated
`with mutagenic radiation produced by short wavelength
`lasers. and the attendant dangers posed to clinicians and their
`patients.
`The operational characteristics of an ultrashort pulse
`width. high repetition rate laser system. in accordance with
`practice of principles of the invention will now be described
`with reference to FIGS. 1(a). 1(b). 2. and 3.
`1. Principles of Operation: Ultrashort Pulse Durations
`Previously known and used long pulse laser systems.
`operating in the nanosecond to microsecond pulse duration
`regime. have shown themselves to be generally inefficient in
`their ability to remove substantial amounts of tissue without
`causing extensive collateral damage. In a conventional long
`pulse laser system (conventional NszAG or ErzYAG IR
`lasers. for example). much of the optical energy delivered to
`a material removal target site has not gone into disrupting
`the structural integrity of the target material. but rather is
`transferred into the surrounding tissue as thermal. acoustic
`or mechanical energy. This energy propagates through the
`surrounding tissue as both mechanical shock waves and heat
`which manifest themselves as undesirable cracks. material
`charring. discoloration. surface melting and perceived pain.
`Conventionally. for pulses longer than a few tens of
`picoseconds. the generally accepted model of bulk material
`removal involves the heating of conduction band electrons
`by an incident beam of coherent photons and transfer of this
`thermal energy to the bulk material lattice. Damage occurs
`by conventional heat deposition resulting in melting.
`boiling. and/or fracture of the material in the region in which
`removal is desired. Because the controlling rate for material
`removal depends on thermal conduction through the mate-
`rial lattice and the lattice’s thermodynamic properties (heat
`capacity. heat of vaporization. heat of fusion. and the like).
`the minimum amount of energy required to effect an observ-
`able change in the material’ s properties. termed herein as the
`threshold damage fluence and defined as the incident laser
`energy per unit area. is dependent approximately on the
`
`10
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`65
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`Alcon Research. Ltd.
`Exhibit 1019 - Page 11
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`Alcon Research, Ltd.
`Exhibit 1019 - Page 11
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`5.720.894
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`square root of the pulse duration (1). Relatively long pulse
`durations have. in the past. been considered necessary in
`order to obtain adequate material removal characteristics.
`Long pulse durations. however. are often the source of many
`of the undesirable side etfects exhibited by conventional
`nanosecond or longer pulse laser systems.
`Unexpected results are obtained. however. when material
`removal is performed with lasers having pulse durations less
`than the characteristic electron-lattice energy transfer time
`for a particular tissue or material of interest. For the majority
`of hard. biologic materials. this characteristic energy transfer
`time is on the order of about 10 to 50 picoseoonds. However.
`when pulsed laser systems are operated in a parametric
`regime which includes pulse durations shorter than this
`characteristic transfer time.
`the physical mechanism of
`matm‘ial removal changes as depicted in FIGs. 1(a) and 1(b).
`FIG. 1(a) is a log-log graph depicting the general behavior
`of laser induced damage. the damage fluence in Joules per
`square centimeter (J/cmz). as a function of beam pulse
`duration (1) in picoseconds for a laser system operating in
`the 1053 nanometer wavelength region. At pulse durations
`above about 20 picoseoonds. the plot of damage fluence as
`a function of pulse width is seen to follow the classical.
`diffusion dominated root tau (1”) scaling characteristic of
`electron kinetic energy transfer to the material lattice struc-
`ture and diffusion during the laser pulse. Material damage.
`in this region. is thm'mal in nature and characterized by
`melting. boiling. and/or fracture of the material surface.
`However. below 20 picoseoond pulse widths. the inventors
`have determined that the damage fluence departs from the
`root tau model. and exhibits a steadily decreasing threshold
`associated with a gradual transition from the long—pulse.
`thermally dominated regime to an ablative regime charac-
`terized by multiphoton and collisional ionization. and
`plasma formation. Short pulse damage is typically confined
`to small region bounded by the peak of the laser beam’s
`Gaussian irradiance distribution. Thus. damage (material
`ablation or removal) occurs only over an area with sufficient
`beam intensity to produce ionization.
`As the pulse duration decreases to a time period less than
`the relaxation time. i.e.. the time required for electrons to
`transfer energy to the lattice (approximately 20 picoseconds
`in the case of the exemplary dentin material). the laser
`energy is non-linearly absorbed to produce quasi—free elec-
`trons which. in turn. act as seed-electrons which cause an
`avalanche or electron cascade by collisional ionization in
`which material conduction band electrons. oscillating in
`response to the las