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  Generation of efficient solid-state laser pulse trainsUSPTO Application #: 20060016790Title: Generation of efficient solid-state laser pulse trains Abstract: A device and method for generating laser pulse trains for delivery to a target including the use of a single laser generator which produces a plurality of pulse groupings of two or more individual laser pulses within each laser pulse train, generated at selected time intervals. The laser pulse train has a pulse width of at least about 30 microseconds and a pulse repetition rate of 1 to about 1,000 Hertz. The time intervals between the individual pulses within each of the pulse groupings along with the intervals between pulse groupings themselves are selected and controlled by a controller in reference to several variables including the emission and energy storage lifetimes of the lasing medium, the thermal diffusion time constant of the target, the time required to cool the target after the application of laser pulses to its ambient temperature, and the dissipation time of acoustic waves generated by the pulses. (end of abstract)
Agent: Olson & Hierl, Ltd. - Chicago, IL, US Inventor: Glenn Yeik USPTO Applicaton #: 20060016790 - Class: 219121610 (USPTO) Related Patent Categories: Electric Heating, Metal Heating (e.g., Resistance Heating), By Arc, Using Laser, Beam Energy Control The Patent Description & Claims data below is from USPTO Patent Application 20060016790. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. Ser. No. 10/355,952 filed on Jan. 31, 2003, now U.S. Pat. No. ______. TECHNICAL FIELD [0002] The present invention relates to laser devices and lasing methods and, more particularly, to a method for the generation and application of laser pulse trains with pulse groupings using selected solid-state lasing media. BACKGROUND OF THE INVENTION [0003] Various lasers have been developed for engraving, cutting, welding, annealing, etc. various materials. Lasers have also been developed or proposed for delivering laser energy to a site or target on or in a mammalian body for diagnostic or therapeutic purposes. These lasers typically deliver laser energy to a target site either directly or through delivery devices such as an articulated arm, a hollow wave-guide or a flexible optical fiber. If pulsed laser energy is desired in these applications, it is usually provided in a train of evenly spaced pulses. [0004] In therapeutic veterinary or medical applications, laser energy is used to produce a desired effect on various types of tissue. The laser energy interacts with the tissue through reflection, refraction or absorption. This interaction may be used to perform incision, excision, resection, vaporization, ablation, coagulation, hemostasis, and denaturization of various tissues. One or more of the above effects of laser radiation on tissue may be produced with pulses of laser radiation having, for example, a wavelength of about 2,100 nm, a pulse width of about 300 microseconds and an energy density of about 1 J/cm.sup.2 incident on the target site. Laser energy of this wavelength is highly absorbed by water, a constituent of virtually all tissues. [0005] The effect of laser radiation on tissue is dependent upon several factors, some of which include the type of tissue irradiated, the amount of radiation absorbed by the tissue, the time during which the laser energy is delivered and the absorption efficiency of the wavelength of laser energy on the target tissue. Relatively hard or dense tissues, such as calcified tissues or bone which may have a comparatively low water content, require relatively high energy levels for effective ablation. For example, at a wavelength of about 2,100 nm, this would optimally require an energy density of 1 to 20,000 J/cm.sup.2 at a pulse width of 100 to 800 .mu.s at the target tissue site. [0006] In a variety of applications, including surgical procedures, where ablation, vaporization or other effects are desired, it is preferred to achieve these effects relatively quickly to reduce thermal conduction and damage to nearby tissues. Also, in these applications, it may be desirable to increase the time period between pulses, to allow additional time for the target to cool between pulses. In order to ablate certain types of tissue quickly, the laser radiation incident at the site or target of application, for example, at a wavelength of about 2,100 nm, should be delivered at a pulse width of at least about 30 microseconds preferable in the range of about 50 .mu.s to about 2,500 .mu.s at a repetition rate in the range of 1 to about 1,000 Hertz (Hz). More preferably, laser radiation should be delivered to the target site in pulses of 1 to 10 Joules of energy with a pulse width of 100 to 800 .mu.s at a repetition rate of 1 to 100 Hz. [0007] The production of such high energy levels with a single laser resonator or oscillator (e.g. a source of high intensity optical radiation such as from a flash lamp, arc lamp or diode-laser, and a lasing medium) is difficult or impossible. This is especially true for thulium holmium:YAG, chromium thulium holmium:YAG, erbium:YAG, thulium:YAG, ruby or similar lasers having a limited energy output capability. Many commercially available lasers that are suitable for ablation or vaporization of tissue cannot be operated for extended periods of time at such high energy levels, without creating excessive heat or placing excessive stress on the laser system and/or an optional optical delivery mechanism, which can lead to premature component failure. [0008] Other methods of generating multiple consecutive laser pulses within a short period of time are known in the art and have previously been disclosed. However, the implementation of these methods requires an increased number of components, complexity, and cost compared to the present invention. Additionally, these other methods typically require more input power than the present invention in order to achieve the same target effects. [0009] Accordingly, it would be desirable to provide an improved laser system capable of generating radiant energy at higher effective energy levels to the target site. Preferably, such an improved system should accommodate the use of commercially available, pulsed lasers of the following types: erbium:yttrium aluminum garnet (Er:YAG), thulium:yttrium aluminum garnet (Tm:YAG), thulium holmium:yttrium aluminum garnet (TmHo:YAG), chromium thulium holmium:yttrium aluminum garnet (CrTmHo:YAG), neodymium:yttrium aluminum garnet (Nd:YAG), alexandrite, ruby and other pulsed lasers. [0010] Desirably, such an improved laser system should deliver radiant energy to a target with a relatively long thermal diffusion time or relatively low thermal conductivity (e.g. tissue, bone, hair, cotton, plastic, wood, etc.) in a pulse train that has a sufficiently high energy level during a relatively short time period in order to quickly raise its temperature to produce the desired effect on the target, while lengthening the time between pulses to allow additional time for the target to cool between pulses. [0011] Conceptually, if this rapid target temperature rise is produced using a series or train of evenly spaced pulses of laser energy, the temperature of the target (e.g. bone, organs, cartilage, etc.) will start to decay back to its ambient temperature after the end of each pulse in this train. It is understood that the temperature of a target that has been raised above its ambient temperature T.sub.a to an elevated temperature T.sub.s after the end of each pulse in this train decreases ideally according to the following equation: T.sub.e=T.sub.a+(T.sub.s-T.s- ub.a)e.sup.-t/k where T.sub.s is the maximum elevated temperature to which the tissue has been raised by a preceding pulse or pulses, e is the natural logarithm base, t is any selected time period following the achievement of temperature T.sub.s, k is the target thermal diffusion time constant, and T.sub.e is the resulting time-dependent temperature at the end of time period t. [0012] When a target (e.g. tissue) is subjected to a pulse of laser energy, the target temperature rises to a maximum temperature T.sub.s, and then begins to decrease. If the maximum target temperature after the end of a pulse of laser energy T.sub.s were below the desired target temperature T.sub.d, it would be desirable to provide increased energy to the target to allow the desired target temperature T.sub.d to be achieved. It is believed that the efficiency of laser effects (e.g. vaporization or ablation) on targets with a relatively long thermal diffusion time can be increased by subjecting the target to pulses of laser energy in a way that results in little or no temperature decay between laser pulses. Accordingly, to achieve this increased efficiency, the time span between consecutive laser pulses in the pulse train should be relatively short, preferably much shorter than the target thermal diffusion time constant. [0013] For example, when a target site of a typical human tissue, such as muscle or cartilage, is elevated to an initial temperature of about 120.degree. C., the tissue temperature decays to 115.degree. C. in about 10 milliseconds. It would be desirable to subject the tissue to a plurality of laser pulses in less than or equal to that time period. A preferred laser system for the ablation or vaporization of such tissue should accommodate the emission of two or more laser pulses with a typical temporal separation of less than 10 milliseconds between the pulses, with a pulse separation time and pulse width that depend upon the desired peak pulse energy and target effects; for example, a pulse separation time of 1 ms and a pulse width of 100 to 800 .mu.s at a wavelength of about 2,010 nm. The pulse separation and/or pulse width may vary significantly depending upon the specific application. For example, in order to ablate or fragment bladder, kidney, or ureteral stones, a pulse separation of 10 .mu.s and a pulse width of 1-10,000 nanoseconds may be desirable. [0014] As is previously known from many literature sources, many solid-state lasing ions exist, of which many are bivalent and trivalent lanthanides, for example, praseodymium (Pr.sup.3+), neodymium (Nd.sup.3+), samarium (Sm.sup.2+), europium (Eu.sup.3+), gadolinium (Gd.sup.3+), terbium (Tb.sup.3+), dysprosium (Dy.sup.2+), holmium (Ho.sup.3+), erbium (Er.sup.3+), thulium (Tm.sup.2+, Tm.sup.3+) and ytterbium (Yb3+). Other solid-state lasing ions are also well known, for example, titanium (Ti.sup.3+), vanadium (V.sup.2+), chromium (C.sup.2+, Cr.sup.3+ and Cr.sup.4+) and others. [0015] At, above, and/or below room temperature, there are many different host crystals that may be used in conjunction with many of the above solid-state lasing ions or combinations thereof, including, for example, Y.sub.3Al.sub.5O.sub.12 (YAG), Y.sub.3Sc.sub.2Ga.sub.3O.sub.12 (YSGG), LiYF.sub.4 (YLF), Gd.sub.3Sc.sub.2Ga.sub.3O.sub.12 (GSGG), Y.sub.3Ga.sub.5O.sub.12 (YGG), Y.sub.3AlO.sub.3 (YAP), LaF.sub.3, BaY.sub.2F.sub.8, KCaF.sub.3 and others. Other solid-state host materials, such as plastics or gelatins, may also be used in conjunction with many solid-state lasing ions. The selection of a relatively transparent host material that is sensitized or doped with various relative percentage(s) of one or more lasing ion(s) determines the wavelength and other properties of the solid-state active lasing medium. [0016] One or more lasing-related properties may be used to classify various solid-state lasing media. A subset of all available solid-state lasing media exhibit the characteristics of relatively long energy storage lifetimes, for example, at least 100 .mu.s and longer. Many of these types of solid-state lasing media are well known, for example, Er:YAG, Tm:YAG, Ho:YAG, Er:YSGG, Tm:YSGG, TmHo:YAG, CrTmHo:YAG, erbium-doped fiber amplifiers and others, and have been characterized as to both the predicted and actual characteristics of energy storage lifetimes. The emission lifetime of these media are usually longer than the energy storage lifetime due to, for example, impurities in the lasing medium, constructional constraints imposed by the laser resonator design, sub-optimal thermal management and other factors. It would be desirable to utilize these properties to produce advantageous pumping and energy extraction and deliver a plurality of laser pulses within a short time period. [0017] The present invention provides an improved laser energy generation system that can produce the above-discussed benefits and features. SUMMARY OF THE INVENTION [0018] The present invention provides a unique method and device for generating efficient laser pulse trains using selected solid-state lasing media for subjecting a target site to higher effective laser energy than is typically possible with normal laser pulsing modes for that same single lasing medium. [0019] The present invention is suitable for use in a medical system in order to deliver radiant laser energy pulses to a selected tissue site in a desired, controlled manner. The invention is particularly well suited for use in surgical procedures for coagulating or cutting relatively soft tissues, as well as for rapidly vaporizing or ablating relatively hard tissues. [0020] A device for generating laser pulse trains for delivery to a target and embodying the present invention is comprised of a single laser generator that generates a plurality of laser pulse groupings within a given pulse train at selected time intervals. Each of the laser pulse groupings is comprised of two or more successive individual laser pulses generated at selected time intervals. The laser pulse train has a pulse width of at least about 30 .mu.s preferably in the range of about 50 .mu.s to about 2,500 .mu.s, at a pulse repetition rate in the range of 1 to about 1,000 Hz. More preferably, the laser pulse train has a pulse width in the range of about 100 to about 800 .mu.s and a pulse repetition rate in the range of about 1 to about 100 Hz. Continue reading... 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