Patent Description:
Lasers are recognized as controllable sources of radiation that is relatively monochromatic and coherent (i.e., has little divergence). Laser energy is applied in an ever-increasing number of areas in diverse fields such as telecommunications, data storage and retrieval, entertainment, research, and many others. In the area of medicine, lasers have proven useful in surgical and cosmetic procedures where a precise beam of high energy radiation causes localized heating and ultimately the destruction of unwanted tissues. Such tissues include, for example, subretinal scar tissue that forms in age-related macular degeneration (AMD) or the constituents of ectatic blood vessels that constitute vascular lesions.

The principle of selective photothermolysis underlies many conventional medical laser therapies to treat diverse dermatological problems such as leg veins, portwine stain birthmarks, and other ectatic vascular and pigmented lesions. The dermal and epidermal layers containing the targeted structures are exposed to laser energy having a wavelength that is preferentially or selectively absorbed in these structures. This leads to localized heating to a temperature (e.g., to about <NUM>) that denatures constituent proteins or disperses pigment particles. The fluence, or energy per unit area, used to accomplish this denaturation or dispersion is generally based on the amount required to achieve the desired targeted tissue temperature, before a significant portion of the absorbed laser energy is lost to diffusion. The fluence must, however, be limited to avoid denaturing tissues surrounding the targeted area.

Fluence, however, is not the only consideration governing the suitability of laser energy for particular applications. The pulse duration and pulse intensity, for example, can impact the degree to which laser energy diffuses into surrounding tissues during the pulse and/or causes undesired, localized vaporization. In terms of the pulse duration of the laser energy used, conventional approaches have focused on maintaining this value below the thermal relaxation time of the targeted structures, in order to achieve optimum heating. For the small vessels contained in portwine stain birthmarks, for example, thermal relaxation times and hence the corresponding pulse durations of the treating radiation are often on the order of hundreds of microseconds to several milliseconds.

The use of even shorter pulses, however, results in a change from photothermal to photomechanical processes. The latter mechanism is invoked by applying laser pulses having a duration that is below the acoustic transit time of a sound wave through targeted particles. This causes pressure to build up in the particles, in a manner analogous to the accumulation of heat within a target irradiated by laser pulses having a duration that is below the thermal relaxation time.

Photomechanical processes described above can provide commercially significant opportunities, particularly in the area of treating skin pigmentations including tattoos, portwine stains, and other birthmarks. The incidence of tattoos in the U. and other populations, for example, continues at a significant pace. Because tattoo pigment particles of about <NUM> micron (<NUM> micron = <NUM> micrometer) in diameter or less may be cleared from the body via ordinary immune system processes, stable tattoos are likely composed of pigment particles having diameters on the order of <NUM>-<NUM> microns or more. As the speed of sound in many solid media is approximately <NUM> meters/second, the acoustic transit time across such particles, and consequently the laser pulse duration required to achieve their photomechanical destruction, is as low as hundreds of picoseconds. The acoustic transit time of a sound wave in a particle is calculated by dividing the radius of the particle by the speed of sound in the particle.

In addition to such short pulse durations, high energy laser pulses are needed for significant disruption of tattoo pigment particles and other pigmentations. Required fluences of several joules per square centimeter and treatment spot sizes of a few millimeters in diameter translate to a desired laser output with several hundred millijoules (mJ) per pulse or more. Unfortunately, current systems capable of such short pulse duration and high energy output are too complex and/or expensive for practical use in the treatment or removal of tattoos and other pigmentations. These devices generally require two or more lasers and amplifier stages, together with multiple electro-optical and/or acousto-optic devices.

Documents <CIT>, <CIT>, and <CIT> disclose different methods for treating skin an/or removing pigment particles from skin pigmentation with laser beams, but remain silent with respect to the optimization of the removal by photomechanical disruption produced by laser pulses of a defined duration with respect to the acoustic transit time of a sound wave into a pigment particle, as introduced via the present invention.

The present invention provides a method for removing pigment particles from a skin pigmentation as defined in claim <NUM>.

The present invention is associated with the discovery of methods and apparatuses described herein for delivering pulsed laser energy with pulse characteristics suitable for a number of practical applications. Such pulse characteristics include a sufficiently short duration and/or a sufficiently high energy for the photomechanical treatment of skin pigmentations and pigmented lesions, both naturally-occurring (e.g., birthmarks), as well as artificial (e.g., tattoos).

The pulsed laser energy generated according to methods of the present invention may have at least about <NUM> mJ/pulse, and often will have from about <NUM> to about <NUM> mJ/pulse, as required for applications described herein, such as the removal or dispersion of pigment particles as often used to form tattoos. As is also desired in these applications, the pulsed laser energy generally has a pulse duration of at most about <NUM> picoseconds (ps), typically at most about <NUM> ps, and often at most about <NUM> ps.

The skin pigmentation may be a tattoo, a portwine stain, or a birthmark.

In another embodiment, the present invention is a method for removing a tattoo comprising tattoo pigment particles, which may, for example, have a diameter from about <NUM> to about <NUM> microns.

These and other embodiments are apparent from the following Detailed Description.

The features of the apparatus referred to in the above <FIG> are not necessarily drawn to scale and should be understood to present an illustration of the invention and/or principles involved. Some features depicted in the figures have been enlarged or distorted relative to others, in order to facilitate explanation and understanding. The same reference numbers are used in the figures for similar or identical components or features shown in the various embodiments. Laser devices, as disclosed herein, will have configurations, components, and operating parameters determined, in part, by the intended application and also the environment in which they are used.

Aspects of the present invention are associated with the ability of laser pulses having a duration of several hundred picoseconds to cause the photomechanical disruption, through the use of sound (or pressure) waves, of tattoo pigment particles and other components of pigmented lesions. Mechanical disruption of the pigment particles facilitates removal of the pigment particles by the body's natural removal processes such as those associated with the immune system. These pulse durations are of the same order as the acoustic transit time across particles having a diameter from about <NUM> to about <NUM> microns, which are otherwise sufficiently large to remain stable in skin tissue (e.g., without being cleared by normal immune system responses).

The significance of short pulse duration in photomechanical processes is illustrated graphically in <FIG>, which shows the non-linear response of peak pressure in a target, as laser pulse duration is reduced. The units of pulse duration, along the x-axis, are normalized to a multiple of the acoustic transit time across a targeted particle, such as a tattoo pigment particle. The acoustic transit time refers to the time required for a sound wave to traverse this target particle. As is apparent from <FIG>, the photomechanical stress on the target rapidly increases when the irradiating pulse duration decreases to less than about two transit times.

The effect becomes dramatically more pronounced below about one transit time. <FIG> therefore illustrates the importance of the ability to operate in the picosecond pulse duration range, in designing a photomechanical treatment or removal protocol for tattoos and other pigmented skin lesions. In fact, as is also clear from <FIG>, laser pulses having durations of greater than about five times the acoustic transit time induce relatively insignificant peak pressure on the target particle and are therefore relatively ineffective in disrupting small pigmentation particles via the photomechanical mechanism.

Effective apparatuses and methods according to embodiments of the present invention are therefore advantageously capable of delivering laser energy having a pulse duration generally less than about <NUM> nanosecond, typically less than about <NUM> picoseconds (ps), and often less than about <NUM> ps. Common pulse duration values according to some embodiments are in the range from about <NUM> to about <NUM> ps. The above values generally represent less than several (e.g., from about one to about three) acoustic transit times for pigmentation particles having a diameter in the range from about <NUM> to about <NUM> microns.

Also characteristic of laser energy that is effective for treating or removing skin pigmentations is a relatively high level of energy output. For example, fluences required to achieve significant disruption of pigment particles are generally in the range from about <NUM> to about <NUM> J/cm<NUM>. For viable treatment methods having a treatment area or spot size of a few millimeters in diameter, the required laser output is preferably at least about <NUM> mJ per pulse, and often in the range from about <NUM> to about <NUM> mJ per pulse.

<FIG> depicts a representative embodiment of an apparatus <NUM> according to the present invention, which is capable of achieving the above pulse duration and energy output parameters, suitable for the effective treatment of pigmented lesions through photomechanical means. Advantageously, the apparatus includes a resonator (or laser cavity) capable of generating laser energy having the desirable pulse duration and energy per pulse, as described herein. The resonator has a characteristic longitudinal or optical axis <NUM> (i.e., the longitudinal flow path for radiation in the resonator), as indicated by the dashed line. Also included in the representative apparatus shown are an electro-optical device, in this case a Pockels cell <NUM>, and a polarizer <NUM> (e.g., a thin-film polarizer). During operation, the laser pulse output will be obtained along output path <NUM>.

At opposite ends of the optical axis <NUM> of the resonator are a first mirror <NUM> and a second mirror <NUM> having substantially complete reflectivity. This term, and equivalent terms such as "substantially totally reflective" are used to indicate that the mirrors <NUM> and <NUM> completely reflect incident laser radiation of the type normally present during operation of the resonator, or reflect at least <NUM>%, preferably at least <NUM>%, and more preferably at least <NUM>% of incident radiation. The mirror reflectivity is to be distinguished from the term "effective reflectivity," which is not a property of the mirror itself but instead refers to the effective behavior of the combination of second mirror <NUM>, Pockels cell <NUM>, and polarizer <NUM> that is induced by the particular operation of the Pockels cell <NUM>, as discussed in detail below.

In particular, a laser pulse traveling from lasing or gain medium <NUM> towards second mirror <NUM> will first pass through polarizer <NUM>, then Pockels cell <NUM>, reflect at second mirror <NUM>, traverse Pockels cell <NUM> a second time, and finally pass through polarizer <NUM> a second time before returning to gain medium <NUM>. Depending upon the bias voltage applied to Pockels cell <NUM>, some portion (or rejected fraction) of the energy in the pulse will be rejected at polarizer <NUM> and exit the resonator along output path <NUM>. The remaining portion (or non-rejected fraction) of the energy (from <NUM>% to <NUM>% of the energy in the initial laser pulse) that returns to the medium <NUM> is the "effective reflectivity" of second mirror <NUM>. As explained above, for any given applied voltage to Pockels cell <NUM>, the effective behavior of the combination of second mirror <NUM>, Pockels cell <NUM>, and polarizer <NUM> is indistinguishable, in terms of laser dynamics, from that of a single partially reflective mirror, reflecting the same non-rejected fraction described above. An "effective reflectivity of substantially <NUM>%" refers to a mirror that acts as a substantially totally reflective mirror as defined above.

Also positioned along the optical axis <NUM> of the resonator is a lasing or gain medium <NUM>, which may be pumped by any conventional pumping device (not shown) such as an optical pumping device (e.g., a flashlamp) or possibly an electrical or injection pumping device. A solid state lasing medium and optical pumping device are preferred for use in the present invention. Representative solid state lasers operate with an alexandrite or a titanium doped sapphire (TIS) crystal in accordance with the invention. Alternative solid lasing media in accordance with the invention include a yttrium-aluminum garnet crystal, doped with neodymium (Nd:YAG laser). Similarly, neodymium may be used as a dopant of pervoskite crystal (Nd:YAP or Nd:YAlO<NUM> laser) or a yttrium-lithium-fluoride crystal (Nd:YAF laser). Other rare earth and transition metal ion dopants (e.g., erbium, chromium, and titanium) and other crystal and glass media hosts (e.g., vanadite crystals such as YVO<NUM>, fluoride glasses such as ZBLN, silica glasses, and other minerals such as ruby) of these dopants may be used as lasing media.

The above mentioned types of lasers generally emit radiation, in predominant operating modes, having wavelengths in the visible to infrared region of the electromagnetic spectrum. In an Nd:YAG laser, for example, population inversion of Nd+<NUM> ions in the YAG crystal causes the emission of a radiation beam at <NUM> as well as a number of other near infrared wavelengths. It is also possible to use, in addition to the treating radiation, a low power beam of visible laser light as a guide or alignment tool. Alternative types of lasers include those containing gas, dye, or other lasing media. Semiconductor or diode lasers also represent possible sources of laser energy, available in varying wavelengths. In cases where a particular type of laser emits radiation at both desired and undesired wavelengths, the use of filters, reflectors, and/or other optical components can aid in targeting a pigmented lesion component with only the desired type of radiation.

Aspects of the invention also relate to the manner in which the relatively simple apparatus <NUM>, depicted in <FIG>, is operated to generate laser energy with the desirable pulse duration and energy output requirements discussed above. For example, laser energy from the lasing medium <NUM> is reflected between the first mirror <NUM> and second mirror <NUM> at opposite ends of the optical axis <NUM> of the resonator. Laser energy emanating from the lasing medium <NUM> therefore traverses the thin film polarizer <NUM> and Pockels cell <NUM> before being reflected by the substantially totally reflective second mirror <NUM>, back through the Pockels cell <NUM> and polarizer <NUM>.

TIS materials, alexandrite, and other crystals such as Nd:YVO<NUM> exhibit a large stimulated emission cross-section selectively for radiation having an electric field vector that is aligned with a crystal axis. Radiation emitted from such lasing materials is therefore initially linearly polarized, requiring that the polarizer <NUM> be configured for transmission of essentially all incident radiation by proper alignment with respect to the electric field vector. However, the application of a bias voltage to the Pockels cell <NUM> can cause elliptical polarization of the exiting radiation, such that the radiation field of the pulse reflected in the second mirror <NUM> and arriving again at the polarizer <NUM> will in this case consist of two components with orthogonal electric field vectors being out of phase by some angle.

If the polarizer <NUM> rejects radiation having an electric field vector that is orthogonal (or perpendicular) to the orientation of the initial electric field vector of radiation from the lasing material <NUM>, the net effect of the combined components (second mirror <NUM>, Pockels cell <NUM>, and polarizer <NUM>) is that of a variable reflectivity mirror. The effective reflectivity, Reff ,of the second mirror <NUM> (i.e., the Pockels cell <NUM> being positioned between that mirror <NUM> and the polarizer <NUM>), is given by equation (<NUM>): <MAT> where the quantity Vλ/<NUM> is the quarter wave voltage of the Pockels cell <NUM>. The quarter wave voltage refers to the voltage required across the Pockels cell to split the incident radiation into two components having equal intensities and retard the polarization electrical field vector of one component by one-quarter of a wavelength relative to the other component.

Thus radiation, having been reflected at the second mirror <NUM> and therefore passing twice through the Pockels cell <NUM> with an applied voltage of Vλ/<NUM>, will have its polarization axis rotated <NUM>° and will be completely rejected by polarizer <NUM>. An applied voltage V = Vλ/<NUM> therefore provides an effective reflectivity, Reff, of "substantially <NUM>%," meaning that the radiation is either completely rejected by the polarizer <NUM>, or possibly all but a small amount of radiation is rejected (e.g., an amount having an intensity or amplitude generally of less than about <NUM>%, typically of less than about <NUM>%, and often less than about <NUM>%, of its initial intensity or amplitude, Io, prior to the first pass of the radiation through the polarizer <NUM> and Pockels cell <NUM>). Overall, radiation arriving at the lasing medium <NUM> after two passes through Pockels cell <NUM> (and after having been reflected in the second mirror <NUM>) will have an intensity or amplitude, I, given by <MAT>.

It is recognized that, in various embodiments of the invention, the quarter wave voltage can actually induce a number of possible changes in incident radiation polarization, depending on the particular optical configuration of the apparatus. For example, the use of quarter wave retardation plate positioned between Pockels cell <NUM> and the second mirror <NUM> would introduce a double pass polarization axis rotation of <NUM>°, without any applied voltage to the Pockels cell. The effective reflectivity, Reff, of the second mirror <NUM> in this case would be governed by the expression <MAT> where a Pockels cell voltage of <NUM> would achieve an effective reflectivity of <NUM>%. Application of the quarter wave voltage to the Pockels cell would then introduce an additional <NUM>° of rotation, such that the overall effect would be that of no change in polarization. The effective reflectivity, Reff, in this case would be substantially <NUM>%, meaning that the second mirror <NUM> would act as a substantially totally reflective mirror. It is also recognized that not all lasing media emit linearly polarized radiation or radiation having an electric field vector that is aligned with a crystal axis. For example, Nd:YAG media are non-polarizing. In the case where non-polarizing media are employed, polarizer <NUM> may establish a given polarization of radiation incident to Pockels cell <NUM>.

Various aspects of the present invention are associated with the advantages obtained when a time-dependent bias voltage, V(t), is applied to an electro-optical device such as the Pockels cell <NUM>. In preferred embodiments of the invention, the time-dependent voltage is equal to the sum of a baseline voltage, Vo, and a time-dependent differential or offsetting voltage, δV(t), that varies periodically with a period substantially equal to the round trip time, or twice the time required for the oscillating laser energy to traverse the length of the resonator. The term "substantially equal" in this case refers to deviations between the period of the applied voltage waveform and the round trip time of generally less than about <NUM> parts per million (ppm), often less than <NUM> ppm, and preferably less than about <NUM> ppm.

The application of a time-dependent voltage waveform described above and characterized by equation (<NUM>) <MAT> where the time-dependent component δV(t) has a period substantially equal to the round trip time of the resonator, allows the resonator to function in a first operating mode, where a modelocked pulse is established in the resonator. Importantly, modelocked oscillation may be obtained without the requirement for an additional modelocking device (or modelocker), such as an acousto-optic modulator, and consequently without the need to adjust resonator length to match a particular resonance frequency.

Thus, the combination of components, together with the applied voltage waveform discussed above, can function essentially identically to a modelocker. In the first modelocked pulse operating mode, the effective reflectivity, Reff, of the second mirror <NUM>, is modulated, by modulating the voltage applied to the Pockels cell <NUM>, with a desired frequency (corresponding to a period substantially equal to the round trip time of the oscillating laser energy). The modulated reflectivity over time R(t) is obtained by substituting Vo + δV(t) from equation (<NUM>) into the expression for Reff in equation (<NUM>) and expanding to obtain <MAT>
where Ro is the initial effective reflectivity of the second mirror <NUM>. From the above expression, it is evident that when operating at Vo = Vλ/<NUM> or Vo = <NUM>, the linear term vanishes and modulation of the reflectivity is consequently very small. In contrast, the maximum extent or degree of modulation occurs when the baseline voltage Vo is <NUM>% of the quarter wave voltage (Vo = <NUM>. 5Vλ/<NUM>). In preferred embodiments, the baseline voltage Vo is from about <NUM>% to about <NUM>%, and typically from about <NUM>% to about <NUM>%, of the quarter wave voltage of the Pockels cell.

Also, from the above equation for R(t), approximately <NUM>% modulation of the reflectivity can be achieved when the magnitude of δV(t), representing either a positive or a negative deviation from Vo, is <NUM>% of the quarter wave voltage. In other embodiments, the time-dependent differential voltage, δV(t), has an amplitude generally from about <NUM>% to about <NUM>%, and typically from about <NUM>% to about <NUM>%, of the quarter wave voltage of the electro-optical device (e.g., the Pockels cell <NUM>). Operation under these parameters, in a first modelocked pulse mode of operation, can therefore mimic the operation of a resonator having an <NUM>% reflecting mirror at one end and also containing a modelocking device such as an acousto-optic device. Modelocking in either case requires a pumping system or device such as a flashlamp (not shown) operating with a sufficient pump rate to the lasing medium <NUM> to establish the modelocked pulse in the resonator.

In a second (amplification) mode of operation, subsequent to modelocking, the modelocked pulse generated as described above is amplified. Amplification is achieved by applying a constant (first) bias voltage to the Pockels cell <NUM> such that the second mirror <NUM> has an effective reflectivity of substantially <NUM>%. In this condition, the modelocked pulse oscillates between two substantially totally reflective mirrors <NUM> and <NUM>. In embodiments where the effective reflectivity Reff of the second mirror <NUM> is governed by equation (<NUM>) above, a first bias voltage of substantially <NUM> volts (or substantially complete discharge of the Pockels cell), will provide the desired reflectivity of substantially <NUM>%. In this amplification mode, the laser energy can rapidly increase in amplitude by extracting energy that was previously pumped and stored in the lasing medium <NUM> during modelocking.

Once the laser energy, oscillating in the resonator under amplification conditions, has reached a desired or maximum amplitude, it can thereafter be extracted. This is achieved by applying a second bias voltage to the Pockels cell <NUM> such that the second mirror has an effective reflectivity Reff of substantially <NUM>%, to generate pulsed laser energy. In embodiments where the effective reflectivity, Reff, of the second mirror <NUM> is governed by equation (<NUM>) above, a second bias voltage equal to the quarter wave voltage of the Pockels cell will achieve the desired reflectivity of substantially <NUM>%. At this point, laser radiation having the desirable pulse duration and energy output described herein, is generated from the apparatus <NUM> and exits the resonator along output path <NUM>.

<FIG> provides a representation of voltage applied, as a function of time, to an electro-optical device such as a Pockels cell in a laser apparatus, to achieve the operating modes described above. In the time period between t<NUM> and t<NUM>, the voltage applied is according to the equation V(t) = Vo + δV(t), with the time-dependent differential voltage, δV(t), periodically offsetting an applied baseline voltage, Vo. In the particular embodiment of the invention using the voltage waveform shown in <FIG>, the baseline voltage is <NUM>% of the Pockels cell quarter wave voltage (Vo = <NUM>. 5Vλ/<NUM>) and the magnitude of the offset is <NUM>% of the Pockels cell quarter wave voltage. This offset occurs periodically with a period equal to the round trip time of laser energy in the resonator.

During operation from time t<NUM> to t<NUM>, the pump rate to the gain or lasing medium may be set or adjusted to exceed the threshold for laser oscillation, when Reff (the effective reflectivity of the second mirror) is at or near its highest value. Under these operating conditions, together with the condition that the period of the applied voltage waveform is substantially the round trip time for energy to traverse the resonator as described above, a modelocked pulse can be established within the resonator. The time period between t<NUM> and t<NUM>, where a periodic voltage is applied to the electro-optical device, therefore represents the time that the resonator is operating in a first, modelocked pulse mode of operation.

At a time t<NUM>, after a steady state modelocked pulse has developed in the resonator, periodic modulation of the applied bias voltage is discontinued and a constant (first) bias voltage is then applied to the electro-optical device, such that Reff is substantially <NUM>%. In the embodiment shown in <FIG>, the first voltage, applied at time t<NUM>, is <NUM> volts, meaning that the Pockels cell or other electro-optical device is completely discharged. Under this second, amplification mode of operation, the amplitude of the laser energy within the resonator is allowed to grow rapidly, drawing upon energy previously input into the lasing medium during pumping in the modelocked pulse operating mode, as described above. When the laser energy has reached a desired amplitude, it may then be released as pulsed energy having the pulse duration and energy output as described herein. This release is effected by applying a bias voltage at a later time t<NUM> such that Reff is reduced to substantially <NUM>%. According to the embodiment of <FIG>, the applied bias voltage at this time is substantially equal to the quarter wave voltage of the electro-optical device.

Amplification and release (or extraction) of laser energy through the application of first and second (constant) bias voltages, as described above, may also be carried out by applying bias voltages such that Reff beginning at t<NUM> is less than <NUM>%. In the amplification mode of operation, however, Reff is generally greater than <NUM>%, typically greater than about <NUM>%, and often greater than about <NUM>%. Likewise, laser energy may also be released at t<NUM> using an Reff of greater than <NUM>%. For example, a second bias voltage may be applied at t<NUM> such that Reff is generally less than <NUM>%, typically less than <NUM>%, and often less than <NUM>%. In any event, the important consideration is that the device is operated such that Reff is at a relatively high value at t<NUM> and then decreased to a relatively low value at t<NUM>, thereby allowing the device to amplify an oscillating laser pulse and thereafter release the amplified laser energy.

In the particular embodiment of the invention characterized by the applied bias voltage waveform shown in <FIG>, the voltage required to obtain an Reff value of substantially <NUM>% at t<NUM> is substantially <NUM> volts. The term "substantially <NUM> volts" indicates that the electro-optical device may be completely discharged to <NUM> volts or that the applied voltage will generally be less than <NUM>%, typically less than <NUM>%, and often less than <NUM>%, of the quarter wave voltage of the device. Likewise, in this embodiment of the invention, the voltage required to obtain an Reff value of substantially <NUM>% is substantially equal to the quarter wave voltage. The term "substantially equal to the quarter wave voltage" indicates an applied bias voltage to the electro-optical device of its quarter wave voltage or preferably at least <NUM>%, typically at least <NUM>%, and often at least <NUM>% of its quarter wave voltage.

Also, as explained previously, the Pockels cell or electro-optical device, depending on other components (e.g., a retardation plate) in the apparatus, may require voltages other than <NUM> and the quarter wave voltage to achieve Reff values of <NUM>% and <NUM>%, respectively. It is also apparent from the cyclical nature of the dependency of Reff on the applied bias voltage, as given by equation (<NUM>) above, that higher voltages may be applied to achieve a given effective reflectivity. For example, either <NUM> volts or the half wave voltage may be applied to obtain Reff = <NUM>% in equation (<NUM>). In general, however, it is preferred that the smallest bias voltage be applied to achieve a given Reff. Advantageously, the full range of effective reflectivity values, from <NUM>% to <NUM>%, may be obtained with the application of relatively modest bias voltages in the range from <NUM> volts to the quarter wave voltage, according to the methods described herein.

<FIG> shows, according to one embodiment of the invention, the effective reflectivity over time corresponding to the time-dependent bias voltage waveform applied to the electro-optical device, as shown in <FIG>. During the modelocked operating mode from t<NUM> to t<NUM>, the effective reflectivity is periodically and positively offset, from a <NUM>% operating value, to a peak value of <NUM>%. The period of the applied voltage waveform matches that of the effective reflectivity waveform, which is the round trip time, or twice the time required for the laser energy to traverse the length of the resonator. At time t<NUM> (at the beginning of the amplification operating mode), when the electro-optical device is discharged, the corresponding value of Reff is <NUM>%. At time t<NUM>, when the applied bias voltage is Vλ/<NUM>, Reff changes to <NUM>% to release the amplified energy.

The system or electronics for generating these waveforms represents another aspect of the present invention, as the electronics require not only a peak voltage of Vλ/<NUM>, but also must be capable of a modulation frequency of generally at least about <NUM>, typically at least about <NUM> (based on a pulse oscillation time on the order of about <NUM> nanoseconds), and often at least about <NUM>. Values of the modulation frequency may therefore be within the representative ranges of from about <NUM> to about <NUM> or from about <NUM> to about <NUM>. In addition, the switching rise time of the modulation may be approximately <NUM> nanosecond. <FIG> depicts one possible type of waveform generating electronics for producing the bias voltage and Reff waveforms shown in <FIG>, respectively and which is capable of modulating the voltage applied to the electro-optical device in a time frame on the order of <NUM> nanoseconds. The configuration comprises three switches <NUM>, <NUM>, <NUM>, meeting the requirements set forth above. Preferably, insulated-gate, field-effect transistor switches are employed, such as co-planar metal oxide semiconductor field-effect transistor (MOSFET) switches. Switch <NUM> consists of a number of MOSFET's arranged in series to increase voltage withstand. Two charging resistors <NUM>, <NUM>, two coupling circuits <NUM>, <NUM>, <NUM> and <NUM>, <NUM>, <NUM>, and three voltage sources <NUM>, <NUM>, <NUM>, are also included, as shown in <FIG>. The circuit including switch <NUM> and voltage source <NUM> can be configured with or without coupling circuits <NUM> or <NUM>.

Also included in the embodiment of <FIG> is a Pockels cell (electro-optical device) <NUM>, to which the electronic components apply a time-dependent voltage waveform, such as that depicted in <FIG>. Electrically, the Pockels cell <NUM> acts as a capacitor, with a typical capacitance of about <NUM> picofarads (pF). As described above with respect to <FIG>, the waveform generating electronics in the embodiment of <FIG> are used for a first mode of operation at a baseline voltage Vo of <NUM>. 5Vλ/<NUM> (or the "eighth-wave" voltage, Vλ/<NUM>). The baseline voltage is modulated or offset periodically by the time-dependent differential voltage δV(t) discussed above and having a magnitude of <NUM>. 2Vλ/<NUM> in the particular waveform shown in <FIG>. In a subsequent second mode of operation, the waveform generating electronics can be used to discharge the Pockels cell (i.e., apply a constant voltage of <NUM> volts). Thereafter, a voltage equal to the quarter wave voltage, Vλ/<NUM>, of the Pockels cell <NUM> can be applied.

With all three switches off and, for example, the first voltage source <NUM> is set to approximately +250V, and a second voltage source <NUM> is set to approximately - 1000V, the resulting differential voltage across the electro-optical device <NUM> establishes the baseline voltage, Vo (e.g., from approximately <NUM>% to approximately <NUM>% of the quarter wave voltage), to the electro-optical device. Switch <NUM> is then alternately closed and opened resulting in a periodic time-dependent differential voltage δV(t) across the electro-optical device <NUM> (e.g., having a magnitude from approximately <NUM>% to approximately <NUM>% of the quarter wave voltage), such that the total bias voltage, V(t), applied to the electro-optical device is Vo + δV(t). In view of <FIG> and <FIG>, at time t<NUM>, the initial bias voltage Vo may be applied from adjustable voltage sources <NUM> and <NUM>, via charging resistors <NUM>, <NUM> to the Pockels cell <NUM> by opening switches <NUM>,<NUM>,<NUM>. Under this condition, the electronic configuration shown in <FIG> will charge the Pockels cell to the initial bias voltage Vo = <NUM>. In a first, modelocked pulse mode of operation between times t<NUM> and t<NUM>, Switches <NUM> and <NUM> are maintained open while switch <NUM> is periodically closed and opened at the frequency required to modulate the bias voltage (e.g., with a period substantially equal to the round trip time of laser energy in the resonator). In particular, closing switch <NUM> while switch <NUM> is open modulates the baseline voltage with the time-dependent differential voltage, δV(t), having a magnitude of offset determined by the voltage from source <NUM>, as shown in <FIG>. This arrangement discharges the Pockels cell (<NUM>) from Vo to Vo + δV(t) through switches <NUM>, capacitor <NUM> and charging resistor <NUM>. Opening switch <NUM> restores the baseline voltage (Vo = <NUM>. 5Vλ/<NUM>) from voltage sources <NUM> & <NUM> via charging resistors <NUM> and <NUM>. The total bias voltage, V(t), applied to the Pockels cell <NUM> is therefore Vo + δV(t) during the first mode of operation.

At time t<NUM>, a second, amplification mode of operation is established upon closing switches <NUM> and <NUM> thereby changing the value of the effective reflectivity, Reff, of the second mirror to substantially <NUM>%. This amplifies the laser energy within the resonator, in a second amplification mode. Extraction or release of the desired laser energy from the apparatus may be achieved upon opening switches <NUM> and <NUM> and then momentarily closing switch <NUM> thereby applying voltage source <NUM> to the electro-optical device <NUM>, via coupling circuits <NUM> and <NUM>, the resulting voltage differential across the electro-optical device <NUM>, being substantially equal to the quarter wave voltage of the device. This applied voltage in turn changes the value of Reff to substantially <NUM>%. This arrangement discharges the Pockels cell <NUM> through switches <NUM> and <NUM>. Finally, at time t<NUM>, switches <NUM> and <NUM> are opened and charging resistors <NUM> and <NUM> begin to drive the Pockels cell voltage towards Vo. Simultaneously, switch <NUM> is closed thereby applying voltage source <NUM> to the Pockels cell <NUM> via coupling circuits <NUM>, <NUM>, <NUM> and <NUM>, <NUM>, <NUM>. Voltage source <NUM> is adjusted to approximately +2300V, Vλ/<NUM> or quarter wave voltage, such that when switch <NUM> is closed a short duration voltage pulse is applied, via coupling circuits <NUM> and <NUM>, differentially to the Pockels cell <NUM> as needed to extract the amplified pulse. Although the Pockels cell capacitance is small, the switching currents reach several amperes as a result of the very fast switching times required. Stray inductance and/or capacitance may impact circuit performance, such that small, tight packaging is desirable.

<FIG> is a schematic of representative waveform generating electronics, capable of delivering the time-dependent voltage to the electro-optical device, as shown in <FIG>. In this circuit, the differential quarter wave voltage is applied to the electro-optical device by <NUM> independent voltage sources, the differential voltage between which should be substantially equal to the quarter wave voltage of the electro-optic device.

<FIG> is a schematic of representative waveform generating electronics, capable of delivering the time-dependent voltage to the electro-optical device, as shown in <FIG>. In this circuit, the time-dependent voltage is developed by interleaved operation of <NUM> parallel switches (Sw_B1 and Sw_B2) such that each switch operates at ½ of the desired modulation frequency.

<FIG> is a schematic of representative waveform generating electronics, capable of delivering the time-dependent voltage to the electro-optical device, as shown in <FIG>. This circuit uses a transformer to provide voltage level shifting such that lower voltage rated but faster switching MOSFET's can be used. Additionally, this circuit also applies a quarter wave voltage to the electro-optic device by means of voltage source V3, and switch M4.

According to one embodiment of the present invention, a method of driving the Pockels cell is provided using a plurality of switches and a high frequency transformer with switching frequency capabilities in the <NUM> range with a step up ratio of about <NUM>:<NUM> with an isolation voltage of about 3000V. As can be seen in <FIG>, a method is provided of closing switches M1, M2, and M4 to generate <NUM>% of the quarter wave voltage to the electro-optical device, while periodically opening and closing switch M2 to change the voltage on said electro-optical device to <NUM>% of the quarter wave voltage. The operating frequency will be substantially equal to twice the time required for the laser energy to traverse the length of the resonator. The switching pattern of switches M1, M2, M3 and M4 applies a time dependant differential voltage as depicted in <FIG>, such that the bias voltage, V(t), applied to said electro-optical device is equal to Vo + sigma V(t). Opening switches M1, M2 and M4 discharges the electro-optical device to ~<NUM>% of the quarter wave voltage of the electro-optical device and closing switches M1, M3 and M4 applies the quarter wave voltage to the electro-optical device.

According to an alternate embodiment of the present invention, a method for driving the Pockels cell is provided with reference to <FIG> using a plurality of switches A, B1, B2, C and D. According to the method, switches A, C and D are opened to generate <NUM>% of the quarter wave voltage to the electro-optical device, while periodically alternating the opening and closing of switch B1 to change the voltage on the electro-optical device to <NUM>% of the quarter wave voltage. The operating frequency will be substantially equal to twice the time required for the laser energy to traverse the length of the resonator. The switching pattern of A, B1, B2, C and D applies a time dependant differential voltage as depicted in <FIG>, such that the bias voltage, V(t), applied to said electro-optical device is equal to Vo + sigma V(t). Opening switches A and D and closing switches B1, B2 and C discharges the electro-optical device to ~<NUM>% of the quarter wave voltage of the electro-optical device and opening switches B1, B2 and C and closing switches A and D applies the full quarter wave voltage to said electro-optical device.

According to an alternate embodiment, switches A and D can be eliminated and resistors R1 and R2 can drive the Pockels cell to -<NUM>% of the quarter wave voltage as depicted in <FIG> and <FIG>, with the on/off switching pattern described above. According to certain embodiments, the method can employ one or more independently controlled, adjustable high voltage sources and low inductance single or multilayer printed circuit boards for interconnection of circuits. A high voltage pulse capacitor used as a DC energy source device can be used for adjustable high voltage sources. Certain embodiments include charging resistors, depicted in <FIG>, <FIG>, or <FIG>, to limit current through switches A, B <NUM>, B2, C and D. The embodiments of the invention can utilize high speed, high side MOSFET gate drivers including a fiber optic link to accommodate the high switching speeds and the high voltage isolation. According to additional embodiments, a photodiode can be used to observe the pulse energy during the time period t1 to t2 as shown in <FIG>. A closed loop control method can receive the photodiode output to determine the level of energy to gate out of the system. With reference to the Figures, a variable switching frequency closed loop control technique can be used to manipulate the fundamental switching frequency of A, B1, B2, C, D, M1, M2, M3, M4, S1, S2, S3, S4 and/or S5 to tune the system to the resonator cavity length to account for tolerances in the mechanical layout and to account for variation in resonator cavity length due to temperature effects.

According to additional embodiments, the duty cycle of the said time-dependent differential voltage, δV(t) can be programmed or adjusted to fine tune the period or "window" in the total time of flight where reflectivity allows gain to build up, thereby avoiding reliance on fixed periods or fixed voltages. In addition, the electronics switching patterns and setting of voltage sources described herein can be inverted, allowing the Pockels cell to be driven in either polarity when the electronic circuits are symmetrical.

Apparatuses and methods disclosed herein can therefore achieve a desired quality of pulsed laser energy by alternating between two modes of operation in a single resonator, rather than through the use of two separate resonators. Also, a single Pockels cell, operating in the modes discussed above, can eliminate the need for an additional modelocking device to establish a modelocked pulse within the resonator. Because the Pockels cell does not require operation at a resonant frequency, synchronization with the pulse round trip time is carried out through setting the period of the bias voltage modulation, thereby eliminating the need to adjust resonator length.

The apparatuses and methods disclosed herein are in many cases significantly simplified due to the reduced number of components and/or reduced demands in terms of bias voltage and other operating parameters. Devices may be operated using a modulated waveform according to the requirements and parameters set forth herein, and using the electronic configuration discussed above or various equivalent configurations as would be apparent to one of ordinary skill, having the benefit of the present disclosure. Other embodiments of the invention may involve the introduction of conventional optical components for use in conjunction with the apparatuses disclosed herein, such as shutters or beam attenuators, reflecting prisms or other reflecting components, filters, light focusing components such as concentrators or condensers, collimating lenses, additional polarizers, electro-optical devices, and/or mirrors, etc. These variations are readily contemplated, and the above modifications are therefore well within the purview of one or ordinary skill, having regard for the present disclosure.

In view of the above, it will be seen that several advantages may be achieved and other advantageous results may be obtained. Various changes could be made in the above apparatuses and methods without departing from the scope of the present disclosure. It is intended that all matter contained in this application, including all theoretical mechanisms and/or modes of interaction described above, shall be interpreted as illustrative only and not limiting in any way the scope of the appended claims.

Throughout this disclosure, various aspects are presented in a range format. The description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from <NUM> to <NUM> should be considered to have specifically disclosed subranges such as from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM>, from <NUM> to <NUM> etc., as well as individual whole and fractional numbers within that range, for example, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

The following example is set forth as representative of the present invention. This example is not to be construed as limiting the scope of the invention as other embodiments and aspects of the invention are apparent in view of the present disclosure.

A laser apparatus as described herein is used to generate pulsed laser energy having a pulse duration of about <NUM>-<NUM> ps with about <NUM>-<NUM> mJ/pulse. The laser apparatus includes a resonator with two substantially totally reflective mirrors at opposite ends of its optical axis. An alexandrite crystal lasing medium, a polarizer, and a Pockels cell are positioned along this optical axis. An optical flashlamp is also included for pumping the alexandrite lasing medium, which generates laser energy having a wavelength in the range of <NUM>-<NUM>.

The pulsed laser energy described above is generated by pumping the lasing medium and first establishing a modelocked pulse oscillating in the resonator. In the modelocked pulse operating mode, a time-dependent voltage waveform, as described herein, is applied to the Pockels cell. This waveform results from the sum of a constant baseline voltage and a time-dependent differential voltage. The baseline voltage is in the range of <NUM>-<NUM> volts (representing <NUM>%-<NUM>% of the Pockels cell quarter wave voltage, or <NUM> volts) and is negatively offset or modulated by the time-dependent differential voltage, having an amplitude in the range of <NUM>-<NUM> volts (representing <NUM>%-<NUM>% of the Pockels cell quarter wave voltage). The period of the resulting voltage waveform is in the range from <NUM>-<NUM> ns and is equal to the round trip time of the oscillating laser energy in the resonator. The voltage applied to the Pockels cell is thus modulated at a frequency in the range from <NUM>-<NUM>.

Subsequently, the modelocked pulse established as described above is amplified by discharging the Pockels cell to essentially <NUM> volts. Oscillating laser energy is reflected between the mirrors at each end of the resonator, with essentially no losses. This laser energy therefore rapidly increases in amplitude by extracting energy previously pumped and stored in the alexandrite crystal during modelocking. When the laser energy has reached the desired energy level as indicated above, it is extracted from the resonator by applying the quarter wave voltage of <NUM> volts to the Pockels cell.

The switching electronics used to operate the laser in modelocked pulse and amplification modes, and finally to extract the amplified pulse as discussed above, comprise <NUM> MOFSET switches, two charging resistors, two coupling circuits, and three voltage sources having voltages V100 in the range of +<NUM> to +<NUM> volts, V200 in the range of -<NUM> to -<NUM> volts, and V300 in the range of +<NUM> to +<NUM> volts. The switches, resistors, coupling circuits, and voltage sources are configured as shown in <FIG>.

Laser energy having the pulse duration and energy as described above is applied to a patient undergoing treatment for the removal of a tattoo. This laser energy is applied over the course of a <NUM>-minute treatment session to all areas of the skin having undesired tattoo pigment particles. Photomechanical disruption of these particles is effected using the short pulse duration (below the transit time of a sound wave through the targeted tattoo pigment particles), together with a fluence in the range of <NUM>-<NUM> j/cm<NUM>. This fluence is achieved with a laser energy spot diameter of about <NUM>.

Claim 1:
A method for removing pigment particles from a skin pigmentation, the method comprising exposing the pigment particles to pulsed laser energy having a wavelength resulting from using Nd:YAG, titanium doped sapphire, TIS, crystal or alexandrite as a lasing medium and a duration that is a multiple of five times or less of the acoustic transit time across a pigment particle having a diameter of about <NUM> micrometer to photomechanically disrupt the pigment particles to allow for their removal from the body by physiological processes, wherein the acoustic transit time of a sound wave in a particle is calculated by dividing the radius of the particle by the speed of sound in the particle.