Picosecond optical radiation systems and methods of use

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of <patcit id="pcit0001" dnum="US61789144"><text>U.S. Provisional Application No. 61/789,144 filed March 15, 2013</text></patcit> entitled Subnanosecond Laser Systems and Methods of Use and <patcit id="pcit0002" dnum="US61891299"><text>U.S. Provisional Application No. 61/891,299 filed on October 15, 2013</text></patcit> entitled Multi-Wavelength Optical Radiation Sources for Dermatology, the entire contents of which are incorporated by reference herein.

FIELD

The present disclosure relates generally to dermatological systems, methods, and devices and, in particular, to systems, methods, and devices for applying optical radiation, e.g. laser radiation in the visible and near infrared wavelengths, to treat tattoos, and other pigmentation disorders.

BACKGROUND

The use of lasers, as controllable sources of relatively monochromatic and coherent radiation, is becoming increasingly common in diverse fields such as telecommunications, data storage and retrieval, entertainment, research, and many others. In the area of medicine, for example, lasers have proven useful in surgical and cosmetic procedures in which a precise beam of high energy radiation can cause localized effects through photothermal processes (e.g., selective photothermolysis) and/or photomechanical processes (e.g., induction of cavitation bubbles and acoustic shock waves). In dermatology specifically, lasers have been used in a wide variety of procedures including hair removal, skin resurfacing, removal of unwanted veins, and the clearance of both naturally-occurring and artificial skin pigmentations (e.g., birthmarks, port wine stains, and tattoos).

Whereas early laser tattoo removal procedures often utilized non-selective ablation of tissue at the tattoo site with water serving as the target chromophore, recent procedures have instead utilized Q-switched lasers capable of producing high-powered, nanosecond pulses to induce photomechanical breakdown of the tattoo particles themselves. In addition to pulse duration and power, the wavelength of the radiation is also an important parameter in the efficacy of a treatment. For example, though alexandrite lasers emitting picosecond pulses at wavelengths between 750 and 760 nm have been found to be especially effective at treating black, blue, and green tattoo pigments (<nplcit id="ncit0001" npl-type="s"><text>Brauer et al., "Successful and Rapid Treatment of Blue and Green Tattoo Pigment With a Novel Picosecond Laser," Archives of Dermatology, 148(7):820-823 (2012</text></nplcit>)), radiation in the 750-760 nm range is not nearly as effective in removing red or orange tattoos due to the low absorption coefficient of orange and red tattoo pigments at such wavelengths.

Accordingly, there exists a need for improved methods and apparatus for producing ultra-short pulses of laser radiation at various wavelengths for the treatment of tattoos, pigmented lesions, and other skin disorders.

SUMMARY

Systems, methods, and devices for generating and delivering ultra-short pulses, e.g., picosecond pulses, of laser radiation at multiple wavelengths with low energy losses are provided herein. It has been found, for example, that the picosecond, high power pulses disclosed herein can be particularly effective in removing skin pigmentations, in part, because the pulses induce mechanical waves (e.g., shock waves and pressure waves) at the target sites that cause greater disruption and better clearance of pigment particles. In accordance with various aspects of the present teachings, the wavelength of the applied pulses can be selected to match the absorption spectrum of previously difficult-to-treat pigments (while nonetheless maintaining the ultra-short pulse durations) such that the naturally-occurring and artificial skin pigments can be cleared with a reduced number of treatments relative to known procedures, thereby providing a system that could satisfy a long-felt need in the art. By way of example, the methods and systems disclosed herein can improve the disruption and clearing efficacy of red and orange tattoos by delivering laser pulses having a wavelength between about 400 and about 550 nm, where these pigments exhibit much higher absorption coefficients.

In accordance with various aspects, certain embodiments of the applicants' teachings relate to an apparatus for delivery of pulsed treatment radiation. The apparatus can comprise a pump radiation source generating picosecond pulses at a first wavelength, and a wavelength-shifting resonator having a lasing medium and resonant cavity configured to receive the picosecond pulses from the pump radiation source at the first wavelength and to emit radiation at a second wavelength in response thereto. The resonant cavity of the wavelength-shifting resonator has a round trip time shorter than the duration of the picosecond pulses generated by the pump radiation source, and in some aspects, the wavelength-shifting resonator can have a round trip time at least 5 times shorter than the duration of the picosecond pulses generated by the pump radiation source (e.g., at least 10 times shorter).

In accordance with various aspects of the present teachings, the wavelength-shifting resonator can have a variety of configurations to produce the wavelength-shifted picosecond pulses provided herein. By way of example, the wavelength-shifting resonator can have a cavity length that is from about 0.1 millimeters to about 150 millimeters, or from about 60 millimeters to about 120 millimeters, or from about 80 millimeters to about 100 millimeters. However, in one example, the wavelength-shifting resonator can have a cavity length less than 10 millimeters (e.g., a cavity length between 0.1 and 10 millimeters). In various aspects, for example, the wavelength-shifting resonator has a cavity length between 1 and 8 millimeters. By way of non-limiting example, the resonator can comprise a neodymium-doped vanadate crystal (Nd: YVO₄) crystal having a length between the input side and the output side of about 3 mm or a neodymium-doped yttrium-aluminum garnet crystal (Nd:YAG) having a length between the input side and output side of less than about 8 mm (e.g., about 6 mm).

As indicated above, the lasing medium can comprise a variety of materials for receiving the pump pulse from the pump radiation source. By way of example, the lasing medium of the wavelength-shifting resonator can comprise a neodymium-doped crystal, including, a solid state crystal medium selected from the group of neodymium-doped yttrium-aluminum garnet (Nd:YAG) crystals, neodymium-doped pervoskite (Nd:YAP or Nd:YAlO₃) crystals, neodymium-doped yttrium-lithium-fluoride (Nd:YAF) crystals, and neodymium-doped, vanadate (Nd:YVO₄) crystals. Moreover, in some aspects, the lasing medium can comprise between about 1 and about 3 percent neodymium.

In various aspects, the apparatus can produce polarized optical radiation. For example, the apparatus can comprise a polarizer configured to polarize optical radiation emitted by the wavelength-shifting resonator. Additionally or alternatively, the apparatus can comprise a polarizer embedded within the resonant cavity of the wavelength-shifting resonator. Additionally or alternatively, the lasing medium of the wavelength-shifting resonator can be a substantially polarizing medium.

In some aspects, the apparatus can further comprise a frequency-doubling waveguide. By way of example, the frequency-doubling waveguide can comprise a second harmonic generating, nonlinear crystal material that can receive the radiation emitted by the wavelength-shifting resonator to output a pulse having twice the frequency of the input pulse (i.e., half the wavelength). In various aspects, the frequency-doubling waveguide can comprise a lithium triborate (LiB₃O₅) material. In a related aspect, an amplifier can be disposed between the wavelength-shifting resonator and the frequency-doubling waveguide.

The pump radiation source can in various embodiments have a variety of configurations. By way of example, the pump radiation source can be a mode-locked laser, that in some embodiments can comprise a resonator, a lasing medium, a Pockels cell and a controller, wherein the controller generates a mode-locked pulse by applying a periodic voltage waveform to the Pockels cell. In some aspects, the mode-locked laser can comprise an alexandrite laser configured to produce pulsed laser energy at about 755 nm having at least about 100 mJ/pulse (e.g., from about 200 to about 800 mJ/pulse). In various aspects, the mode-locked laser can generate pulsed laser energy having a pulse duration of less than 1000 picoseconds (e.g., about 860 picoseconds).

In accordance with various aspects of the present teachings, the apparatus can further comprise a treatment beam delivery system configured to apply a treatment beam to a patient's skin. In some aspects, the treatment beam can comprise at least one of picosecond pulses from the pump radiation source at the first wavelength, picosecond pulses emitted by the wavelength-shifting resonator at the second wavelength, and picosecond pulses at a third wavelength, wherein the picosecond pulses at the third wavelength are output by a frequency-doubling waveguide that receives the picosecond pulses at the second wavelength. In various embodiments, the first wavelength can be about 755 nm, the second wavelength can be about 1064 nm, and the third wavelength can be about 532 nm. Additionally, the apparatus can be operated so as to enable the selection of the wavelength of the pulse(s) to be applied to a patient's skin through the treatment beam delivery system. The apparatus can also control the wavelength-shifting resonator temperature.

In accordance with various aspects, certain embodiments of the applicants' teachings relate to a method for shifting the wavelength of a picosecond optical radiation pulse. The method can comprise generating a pulse of optical radiation at a first wavelength and having a duration less than 1000 picoseconds, pumping a wavelength-shifting resonator with the pulse of optical radiation at the first wavelength, the wavelength-shifting resonator comprising a laser crystal with a high absorption coefficient at the first wavelength, and extracting a pulse of radiation at a second wavelength emitted by the wavelength-shifting resonator, wherein the pulse at the second wavelength also has a duration of less than 1000 picoseconds. The round trip time of the wavelength-shifting resonator is shorter than the pumping laser pulse duration. For example, the wavelength-shifting resonator can have a round trip time at least 10 times shorter than the duration of the pumping pulse.

In various aspects, the method can further comprise one or more of polarizing, amplifying, and frequency-doubling the output of the wavelength-shifting resonator. For example, in some aspects, a polarizer can be configured to polarize optical radiation emitted by the wavelength-shifting resonator. Additionally or alternatively, a polarizer can be embedded within the resonant cavity of the wavelength-shifting resonator or the lasing medium of the wavelength-shifting resonator can be a substantially polarizing medium. In some aspects, the pulse of radiation at a second wavelength can be transmitted to a frequency doubling crystal so as to generate a pulse having twice the frequency of the input pulse (i.e., half the wavelength).

In accordance with various aspects, certain embodiments of the applicants' teachings relate to a method for treating tattoos or skin pigmentation disorder using a picosecond optical radiation source. The method can comprise employing a pump radiation source to generate a pulse of optical radiation at a first wavelength, wherein the pulse has a duration of less than 1000 picoseconds, and pumping a wavelength-shifting resonator with the pulse of optical radiation at the first wavelength, the wavelength-shifting resonator comprising a laser crystal with high absorption coefficient at the first wavelength, and extracting a pulse of radiation at a second wavelength emitted by the wavelength-shifting resonator, wherein the pulse at the second wavelength also has a duration of less than 1000 picoseconds. In accordance with the present teachings, the round trip time of the wavelength-shifting resonator can be shorter than the pumping laser pulse duration. The method can further comprise delivering the pulse of radiation at the second wavelength to a frequency-doubling waveguide so as to generate a pulse of radiation at a third wavelength, wherein the pulse at the third wavelength also has a duration of less than 1000 picoseconds, and directing the pulse at the third wavelength to a tattoo pigment or a skin pigmentation target to disrupt the target and promote clearance thereof. By way of example, the first wavelength can be about 755 nm, the second wavelength can be about 1064 nm, and the third wavelength can be about 532 nm, and the method can comprise selecting the wavelength of the pulse(s) to be applied to a patient's skin.

In accordance with various aspects, certain embodiments of the applicants' teachings relate to a method for removing a tattoo or treating a skin pigmentation disorder. The method comprises applying pulses having a duration less than 1000 picoseconds to an area of a patient's skin comprising a tattoo pigment or skin pigmentation so as to generate photomechanical disruption of the tattoo pigment or skin pigmentation, wherein the pulses have a wavelength in a range of about 400 nm to about 550 nm (e.g., about 532 nm). In some aspects, the method further comprises utilizing a Nd:YVO₄ lasing medium to generate picosecond pulses of radiation having a wavelength of about 1064 nm, and frequency doubling the picosecond pulses having a wavelength of about 1064 nm to generate picosecond pulses having a wavelength of about 532 nm.

In one aspect, the disclosure relates to an apparatus for delivery of a pulsed treatment radiation such as a laser. The laser having a light source, a resonator having a mode lock element, and a lasing medium such as an active lasing medium. The lasing medium is impinged upon by the light source. An element is disposed between the light source and the lasing medium, the element enables a substantially uniform gain across the lasing medium. The laser can include a second light source. The first light source and/or the second light source can be a pumped radiation source such a flash lamp. In one embodiment, the lasing medium is an alexandrite crystal. The element can be, for example, an alumina rod having a diameter of about 0.063 inches. The element can be at least one of a deflector, a scattering element, a refractor, a reflector, an absorber, and a baffle. In one embodiment, element is equidistant from the flash lamp and the lasing media. In another embodiment, the element is disposed on the lasing medium, is disposed on the light source, or is disposed on both the lasing medium and the light source.

In another aspect, the disclosure relates to an apparatus for delivery of a pulsed treatment radiation such as a laser. The laser includes a light source and a resonator having a multimode output, a mode lock element and an astigmatic element disposed inside the resonator. The astigmatic element can prevent free space propagation modes such as Hermites within the multimode output from coupling together. In this way, beam uniformity is improved with the use of the astigmatic element compared to where the astigmatic element is absent. Suitable astigmatic elements can include, for example, at least one of a cylindrical lens, an angled spherical lens, and a prism (e.g., an anamorphic prism).

In another aspect, the disclosure relates to a resonator (e.g., an oscillator) for a mode locked laser having a fundamental frequency which is the speed of light divided by the round trip optical path length (2L) of the resonator and a mode locking element (e.g., a Pockels cell) that is modulated at a frequency that is less than the fundamental frequency. The frequency can be a sub-harmonic (1/n) of the speed of light (c) divided by the round trip optical path length (2L) where (n) is whole number greater than 1. The resonator can be employed in an apparatus for delivery of a pulsed treatment radiation, such as a laser, to treat tissue.

In another aspect, the disclosure relates to a resonator (e.g., an oscillator) for a mode locked laser that provides a frequency corresponding to a fundamental round trip optical path length (2L) in a mode locked resonator and selecting a sub-harmonic optical path length that is shortened by dividing the fundamental round trip optical path length (2L) by a sub-harmonic factor (n), which is a whole number greater than 1, and the sub-harmonic total path length has n round trip optical path lengths. The resonator can be employed in a laser to treat tissue.

These and other features of the applicants' teachings are set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicants' teachings in any way. <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref>, in a schematic diagram, illustrates an exemplary system having a wavelength-shifting resonator for generating picosecond pulses in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0001">FIG. 2</figref>, in a schematic diagram, illustrates an exemplary system having a wavelength-shifting resonator, the system for generating multiple wavelengths of picosecond pulses in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0001">FIG. 3</figref>, in a schematic diagram, illustrates an exemplary wavelength-shifting resonator having an embedded polarizer for use in the systems of <figref idref="f0001">FIGS. 1 and 2</figref> in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0002">FIG. 4</figref> depicts an exemplary output pulse of an Nd:YAG resonator operated in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0003">FIG. 5</figref> depicts an exemplary output pulse of an Nd:YVO₄ resonator operated in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0004">FIG. 6</figref> illustrates an example of the output pulse shape of a short resonator Nd:YAG laser with a 70% output coupler.</li><li><figref idref="f0005">FIG. 7</figref> is a cross-section of a pump chamber in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0006">FIG. 8</figref> is an axial-view image of the fluorescence of a pumped laser rod in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0007">FIG. 9</figref> is a graph depicting the normalized gain distribution in an unmodified diffuse pump chamber.</li><li><figref idref="f0008">FIG. 10</figref> is an axial-view image of the fluorescence of a pumped laser rod in accordance with an embodiment of the present disclosure.</li><li><figref idref="f0009">FIG. 11</figref> is a graph depicting the normalized gain distribution in a modified diffuse pump chamber in accordance with an embodiment of the disclosure.</li><li><figref idref="f0010">FIG. 12</figref> is a laser beam profile image of a mode-locked laser using an unmodified pump chamber in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0011">FIG. 13</figref> is a laser beam profile image of a mode-locked laser using a modified pump chamber in accordance with an embodiment of the disclosure.</li><li><figref idref="f0012">FIG. 14A</figref> shows a laser intensity profile that includes the free space propagation mode effects caused by two propagating Hermite fields that are in phase with one another in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0012">FIG. 14B</figref> shows a laser intensity profile when an astigmatic element is introduced to decouple propagating Hermite fields such that they are not in phase with one another in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0013">FIG. 15A</figref> shows the modulation signal applied to the Pockels cell in a picosecond resonator and the intensity that builds up in the resonator over time in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0013">FIG. 15B</figref> shows the modulation signal applied to the Pockels cell in a sub-harmonic picosecond resonator and the intensity that builds up in the resonator over time in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0014">FIG. 16</figref>, in a schematic diagram, illustrates an exemplary system for generating picosecond pulses in accordance with various aspects of the applicants' teachings.</li><li><figref idref="f0015">FIG. 17</figref> is a plot of the seed pulse generation with a laser capable of generating a sub-harmonic pulse group at 300mV when the Pockels cell voltage was low and at 190mV when the Pockels cell voltage was high in accordance with various aspects of the applicants' teachings.</li></ul>

DETAILED DESCRIPTION

All technical and scientific terms used herein, unless otherwise defined below, are intended to have the same meaning as commonly understood by one of ordinary skill in the art. References to techniques employed herein are intended to refer to the techniques as commonly understood in the art, including variations on those techniques or substitutions of equivalent or later-developed techniques which would be apparent to one of skill in the art. In addition, in order to more clearly and concisely describe the claimed subject matter, the following definitions are provided for certain terms which are used in the specification and appended claims.

The terms "picosecond" or "picosecond pulse," as used herein, is intended to encompass pulses of optical radiation having durations ranging from 0.1 picoseconds to 1000 picoseconds, preferably less than 1000 picoseconds, e.g., less than 900 picoseconds, less than 800 picoseconds or less than 700 picoseconds. For non-square pulses, pulse durations are typically measured by the full width at half maximum (FWHM) technique.

As used herein, the recitation of a numerical range for a variable is intended to convey that the embodiments may be practiced using any of the values within that range, including the bounds of the range. Thus, for a variable which is inherently discrete, the variable can be equal to any integer value within the numerical range, including the end-points of the range. Similarly, for a variable which is inherently continuous, the variable can be equal to any real value within the numerical range, including the end-points of the range. As an example, and without limitation, a variable which is described as having values between 0 and 2 can take the values 0, 1 or 2 if the variable is inherently discrete, and can take the values 0.0, 0.1, 0.01, 0.001, or any other real values ≥ 0 and ≤ 2 if the variable is inherently continuous. Finally, the variable can take multiple values in the range, including any sub-range of values within the cited range.

As used herein, unless specifically indicated otherwise, the word "or" is used in the inclusive sense of "and/or" and not the exclusive sense of "either/or."

In accordance with various aspects of the applicants' teachings, the systems and methods described herein can be effective to deliver picosecond pulses of laser radiation for the treatment of naturally-occurring and artificial skin pigmentations utilizing wavelengths that match the pigmentations' absorption spectrum. The picosecond, high power pulses disclosed herein can be particularly effective in removing these previously-difficult to treat skin pigmentations, in part, because the pulses induce photomechanical shock waves at the target sites that cause greater disruption and better clearance of pigment particles. By way of example, the methods and systems disclosed herein can improve the clearing of red and orange tattoos with a reduced number of treatments by delivering picosecond laser pulses having a wavelength between 400 and 550 nm, where these pigments exhibit much higher absorption coefficients. Moreover, applicants have discovered that various embodiments of the wavelength-shifting resonators described herein can surprisingly generate particularly efficacious pulses exhibiting picosecond pulsewidths shorter than the input pumping pulses, with low energy losses and/or minimal pulse-shaping (e.g., without use of a modelocker, Q-switch, pulse picker or any similar device of active or passive type).

The present disclosure relates to laser systems having sub-nanosecond pulsing (e.g., picosecond pulsing). Exemplary systems are described in our U.S.

Patents <patcit id="pcit0003" dnum="US7929579B"><text>7,929,579</text></patcit> and <patcit id="pcit0004" dnum="US7586957B"><text>7,586,957</text></patcit>, both incorporated herein by reference. These patents disclose picosecond laser apparatuses and methods for their operation and use. Herein we describe certain improvements to such systems.

With reference now to <figref idref="f0001">FIG. 1</figref>, an exemplary system 100 for the generation and delivery of picosecond-pulsed treatment radiation is schematically depicted. As shown in <figref idref="f0001">FIG. 1</figref>, the system generally includes a pump radiation source 110 for generating picosecond pulses at a first wavelength, a wavelength-shifting resonator 120 for receiving the picosecond pulses generated by the pump radiation source and emitting radiation at a second wavelength in response thereto, and a treatment beam delivery system 130 for delivering a pulsed treatment beam to the patient's skin.

The pump radiation source 110 generally generates one or more pulses at a first wavelength to be transmitted to the wavelength-shifting resonator 120, and can have a variety of configurations. For example, the pulses generated by the pump radiation source 110 can have a variety of wavelengths, pulse durations, and energies. In some aspects, as will be discussed in detail below, the pump radiation source 110 can be selected to emit substantially monochromatic optical radiation having a wavelength that can be efficiently absorbed by the wavelength-shifting resonator 120 in a minimum number of passes through the gain medium. Additionally, it will be appreciated by a person skilled in the art in light of the present teachings that the pump radiation source 110 can be operated so as to generate pulses at various energies, depending for example, on the amount of energy required to stimulate emission by the wavelength-shifting resonator 120 and the amount of energy required to perform a particular treatment in light of the efficiency of the system 100 as a whole.

In various aspects, the pump radiation source 110 can be configured to generate picosecond pulses of optical radiation. That is, the pump radiation source can generate pulsed radiation exhibiting a pulse duration less than about 1000 picoseconds (e.g., within a range of about 500 picoseconds to about 800 picoseconds). In an exemplary embodiment, the pump radiation source 110 for generating the pump pulse at a first wavelength can include a resonator (or laser cavity containing a lasing medium), an electro-optical device (e.g., a Pockels cell), and a polarizer (e.g., a thin-film polarizer), as described for example with reference to <figref idref="f0001">FIG. 2</figref> of <patcit id="pcit0005" dnum="US7586957B"><text>U.S. Patent No. 7,586,957, issued on September 8, 2009</text></patcit> and entitled "Picosecond Laser Apparatus and Methods for Its Operation and Use," the contents of which are hereby incorporated by reference in its entirety.

In an exemplary embodiment, the lasing or gain medium of the pump radiation source 110 can be pumped by any conventional pumping device such as an optical pumping device (e.g., a flash lamp) or an electrical or injection pumping device. In an exemplary embodiment, the pump radiation source 110 comprises a solid state lasing medium and an optical pumping device. Exemplary solid state lasers include an alexandrite or a titanium doped sapphire (TIS) crystal, Nd:YAG lasers, Nd:YAP, Nd: YAlO₃ lasers, Nd:YAF lasers, and other rare earth and transition metal ion dopants (e.g., erbium, chromium, and titanium) and other crystal and glass media hosts (e.g., vanadate crystals such as YVO₄, fluoride glasses such as ZBLN, silica glasses, and other minerals such as ruby). At opposite ends of the optical axis of the resonator can be first and second mirrors having substantially complete reflectivity and/or being substantially totally reflective such that a laser pulse traveling from the lasing medium towards second mirror will first pass through the polarizer, then the Pockels cell, reflect at second mirror, traverse Pockels cell a second time, and finally pass through polarizer a second time before returning to the gain medium. The terms "substantially complete reflectivity" and/or "substantially totally reflective" are used to indicate that the mirrors completely reflect incident laser radiation of the type normally present during operation of the resonator, or reflect at least 90%, preferably at least 95%, and more preferably at least 99% of incident radiation.

Depending upon the bias voltage applied to the Pockels cell, some portion (or rejected fraction) of the energy in the pulse will be rejected at the polarizer and exit the resonator along an output path to be transmitted to the wavelength-shifting resonator 120. Once the laser energy, oscillating in the resonator of the pump radiation source 110 under amplification conditions, has reached a desired or maximum amplitude, it can thereafter be extracted for transmission to the wavelength-shifting resonator 120 by changing the bias voltage to the Pockels cell such that the effective reflectivity of the second mirror is selected to output laser radiation having the desired pulse duration and energy output.

The wavelength-shifting resonator 120 can also have a variety of configurations in accordance with the applicant's present teachings, but is generally configured to receive the pulses generated by the pump radiation source 110 and emit radiation at a second wavelength in response thereto. In an exemplary embodiment, the wavelength-shifting resonator 120 comprises a lasing medium and a resonant cavity extending between an input end and an output end, wherein the lasing medium absorbs the pulses of optical energy received from the pump radiation source 110 and, through a process of stimulated emission, emits one or more pulses of optical laser radiation exhibiting a second wavelength. As will be appreciated by a person skilled in the art in light of the present teachings, the lasing medium of the wavelength-shifting resonator can comprise a neodymium-doped crystal, including by way of non-limiting example solid state crystals of neodymium-doped yttrium-aluminum garnet (Nd:YAG), neodymium-doped pervoskite (Nd:YAP or Nd:YAlO₃), neodymium-doped yttrium-lithium-fluoride (Nd:YAF), and neodymium-doped vanadate (Nd:YVO₄) crystals. It will also be appreciated that other rare earth transition metal dopants (and in combination with other crystals and glass media hosts) can be used as the lasing medium in the wavelength-shifting resonator. Moreover, it will be appreciated that the solid state laser medium can be doped with various concentrations of the dopant so as to increase the absorption of the pump pulse within the lasing medium. By way of example, in some aspects the lasing medium can comprise between about 1 and about 3 percent neodymium.

The lasing medium of the wavelength-shifting resonator 120 can also have a variety of shapes (e.g., rods, slabs, cubes) but is generally long enough along the optical axis such that the lasing medium absorbs a substantial portion (e.g., most, greater than 80%, greater than 90%) of the pump pulse in two passes through the crystal. As such, it will be appreciated by a person skilled in the art that the wavelength of the pump pulse generated by the pump radiation source 110 and the absorption spectrum of the lasing medium of the resonator 120 can be matched to improve absorption. However, whereas prior art techniques tend to focus on maximizing absorption of the pump pulse by increasing crystal length, the resonator cavities disclosed can instead utilize a short crystal length such that the roundtrip time of optical radiation in the resonant cavity (i.e., <maths id="math0001"><math display="inline"><mrow><msub><mi>t</mi><mi mathvariant="italic">roundtrip</mi></msub><mo>=</mo><mn>2</mn><mfrac><mrow><msub><mi>L</mi><mi mathvariant="italic">resonator</mi></msub></mrow><mi>c</mi></mfrac><mo>,</mo></mrow></math><img file="EP3751684A1_D0001.tif" /></maths> where L_resonator is the optical path length of the resonator (the optical path length can account for differences due to the photons traveling through the lasing medium and/or the air in other parts of the path) and <i>c</i> is the speed of light) in some embodiments the optical path length is substantially less than the pulse duration of the input pulse (i.e., less than the pulse duration of the pulses generated by the pump radiation source 110). For example, in some aspects, the roundtrip time can be less than 5 times shorter than the duration of the picosecond pump pulses input into the resonant cavity (e.g., less than 10 times shorter). Without being bound by any particular theory, it is believed that by shortening the resonant cavity, the output pulse extracted from the resonant cavity can have an ultra-short duration without the need for additional pulse-shaping (e.g., without use of a modelocker, Q-switch, pulse picker or any similar device of active or passive type). For example, the pulses generated by the wavelength-shifting resonator can have a pulse duration less than 1000 picoseconds (e.g., about 500 picoseconds, about 750 picoseconds).

After the picosecond laser pulses are extracted from the wavelength-shifting resonator 120, they can be transmitted directly to the treatment beam delivery system 130 for application to the patient's skin, for example, or they can be further processed through one or more optional optical elements shown in phantom, such as an amplifier 140, frequency doubling waveguide 150, and/or filter (not shown). As will be appreciated by a person skilled in the art, any number of known downstream optical (e.g., lenses) electro-optical and/or acousto-optic elements modified in accordance with the present teachings can be used to focus, shape, and/or alter (e.g., amplify) the pulsed beam for ultimate delivery to the patient's skin to ensure a sufficient laser output, while nonetheless maintaining the ultrashort pulse duration generated in the wavelength-shifting resonator 120.

With reference now to <figref idref="f0001">FIG. 2</figref>, an exemplary system 200 is depicted that includes a wavelength-shifting resonator 220 as described for example in <figref idref="f0001">FIG. 1</figref>. As shown in <figref idref="f0001">FIG. 2</figref>, however, the system 200 can also be used to generate and selectively apply multiple wavelengths of picosecond pulses depending, for example, on the absorption spectrum of the target pigment or tissue. As shown in <figref idref="f0001">FIG. 2</figref>, the exemplary system generally includes a pump radiation source 210 for generating picosecond pulses at a first wavelength (e.g., an alexandrite source emitting 755 nm pulses having a duration less than 1000 picoseconds), at least one optical element (M1 and/or M2) configured to selectively divert the picosecond pulses at the first wavelength to a wavelength-shifting resonator 220 (e.g., a 1064 nm oscillator configured to receive the pump pulses and generate 1064 nm picosecond pulses of radiation in response thereto), at least one optical element (M3 and/or M4) and a treatment beam delivery system 230 that can deliver the picosecond pulses of one or more wavelengths to the treatment target. As shown in phantom, and discussed otherwise herein, the system 200 can additionally include, for example, an amplifier 240 and a second harmonic generator 250 (e.g., a lithium triborate (LBO) or potassium trianyl phosphate (KTP) frequency doubling crystal).

As discussed above, the wavelength-shifting resonator 220 can comprise a rare earth doped laser gain crystal. In some aspects, rare earth doped laser crystals that generate a polarized laser beam like Nd:YVO₄ can be used to eliminate the need for an additional polarizing element. Crystals like Nd:YAG or Nd doped glasses can be used with an additional polarizing element in the resonator. In the exemplary embodiment, the input side of the Nd:YVO₄ crystal can be AR coated for the alexandrite wavelength and HR coated for 1064 nm, while the output side of the crystal can be HR coated for the alexandrite wavelength and can exhibit approximately 20 to 70 % reflectivity at 1064nm. In an exemplary embodiment, the Nd:YVO₄ crystal length can be selected such that it absorbs most (greater than 90%) of the alexandrite laser pulse in the two passes through the crystal. For example, with neodymium doping in the range 1 to 3 %, the Nd:YVO₄ crystal can be chosen to be around 3 mm long (with no other optical elements in the resonator, the resonator length is substantially equal to the crystal length of 3 mm). That means the resonator round-trip time is around 39 ps-substantially less than the pulse duration of the alexandrite pumping pulse (around 500 to 800ps). The 1064 nm pulse generated in the very short round trip time Nd:YVO₄ resonator may be slightly longer than the pumping alexandrite pulse and shorter than 1000 ps. The quantum defect will account for a 30 % pulse energy loss and another 15% of the energy is likely to be lost due to coatings, crystal and geometry imperfection, for an overall energy conversion efficiency of around 50 to 60 % such that 100 mJ pulse energy can be produced at 1064 nm, by way of non-limiting example. In the Second Harmonic Generator 250 the second harmonic conversion in the frequency-doubling crystal is around 50 %, such that a 50 mJ pulse energy can therefore be produced at 532 nm. Given the high absorption at 532 nm of red and/or orange tattoo pigments, a 50 mJ, 532 nm pulse with a pulse duration less than 1000 picoseconds can be effective at disrupting, and eventually clearing, red and/or orange tattoo granules.

Though the above described example utilized an Nd:YVO₄ crystal in the wavelength-shifting resonator 220 (and without the need for a polarizing element), Nd:YAG crystals or other Nd-doped glasses can alternatively be used as the short resonator to generate the picosecond pulses in response to stimulation from the pump radiation source. In such embodiments, a polarizing element as known in the art can be utilized external to the wavelength-shifting resonator or can be embedded therein. As shown in <figref idref="f0001">FIG. 3</figref>, for example, a short Nd:YAG resonator 322 can consist of two identically shaped crystals 322a,b with one face 324 cut at an angle that is AR coated for the alexandrite wavelength and polarized-coated for the stimulated emission wavelength (e.g., 1064 nm and high p transmission). The flat faces 326 of the two Nd:YAG crystals can have different coatings-one is AR coated at 755 nm and HR coated at 1064 nm and the other is HR coated for 755 nm and has an output coupler reflectivity around 50 to 80 % for 1064 nm. The higher output coupler reflectivity for the Nd:YAG crystal compared to the Nd:YVO₄ crystal is due to the lower gain cross-section in Nd:YAG.

With reference again to <figref idref="f0001">FIG. 2</figref>, it will be appreciated in light of the present teachings that utilizing a wavelength-shifting resonator 220 to generate picosecond pulses depends on the pulse duration of the pumping pulse (e.g., shorter pumping pulses will lead to shorter generated pulses at 1064 nm) and the roundtrip time determined by the length of the resonator cavity (e.g., shorter crystals lead to shorter roundtrip time, however the crystal has to be sufficiently long to absorb greater than 90% of the alexandrite energy). For example, an 8 mm long Nd:YAG resonator would have a 97 ps round trip time. Though such a roundtrip time is longer than the roundtrip time that can be achieved with a Nd:YVO₄ resonator, it remains much shorter than the pumping Alexandrite laser pulse duration. It will be appreciated by a person skilled in the art in light of the present teaching that one possible way to shorten the crystal length is to tune the alexandrite laser in the range 750 to 760 nm for maximum absorption in the Nd doped crystal and use the minimum possible crystal length. In addition, by tuning the alexandrite laser in the range of 750 to 757 nm allows for the alexandrite wavelength to be set to avoid the excited state absorption bands in the Nd ion as described by <nplcit id="ncit0002" npl-type="s"><text>Kliewer and Powell, IEEE Journal of Quantum Electronics vol. 25, page 1850-1854 (1989</text></nplcit>).

With reference again to <figref idref="f0001">FIG. 2</figref>, the laser beam emitted by the pump radiation source 210 (e.g., an alexandrite laser source generating pulses at around 755 nm and 200 mJ/pulse, with a pulse duration shorter than 800ps) can be reflected on 100 % reflectors M1 and M2 to serve as the pump beam for the wavelength-shifting resonator 220 (e.g., an Nd:YVO₄ or Nd:YAG short round trip time 1064 nm oscillator), thereby stimulating the oscillator 220 to produce up to around 100 mJ pulse energy at 1064 nm at less than 1000 ps pulse duration. The output from the 1064 nm oscillator 220 can be reflected on the 100 % reflectors M3 and M4 and can be coupled into the treatment beam delivery system 230.

Alternatively, the output from the 1064 nm oscillator 220 can be amplified in the 1064 nm amplifier 240 to a pulse energy between 200 and 900 mJ, for example, while maintaining the less than 1000 ps pulse duration and then reflected on the 100% reflectors M3 and M4 and coupled into the treatment beam delivery system.

Alternatively or additionally, the output from the 1064 nm oscillator 220 or the output from the 1064 nm amplifier 240 can be converted to second harmonic 532 nm radiation in the Second Harmonic Generator 250. For a typical 50 % conversion efficiency in the Second Harmonic Generator 250, the 532 nm pulse output can have a pulse energy around 50 mJ when there is no 1064 nm amplifier, or between 100 to 500 mJ when the 1064 nm pulse is amplified in the 1064 nm amplifier 240 before it reaches the Second Harmonic Generator 250. In both cases, the 532nm pulse will have a pulse duration of around 750 ps or less due to the pulse shortening effect of the second harmonic conversion process. After being frequency doubled, the 532 nm pulse can propagate in parallel with the 1064 nm pumping pulse. By choosing mirrors M3 and M4 to be 100% reflectors or substantially totally reflective reflectors on both the 1064 nm and 532 nm wavelengths, a combined wavelength treatment can be delivered to the target through the treatment beam delivery system.

Alternatively, in some embodiments, mirrors M3 and M4 can be chosen to be 100 % reflectors at 532 nm and 100 % transmitters at 1064 nm so as to deliver a single wavelength 532 nm treatment through the treatment beam delivery system. Moreover, by allowing the mirrors (M1, M2) to selectively transmit or deflect the 755 nm alexandrite pulse, for example, by translating the mirrors into and out of the pulse beam, the system 200 can be designed to transmit all three treatment wavelengths 755, 1064 and 532 nm. That is, when mirrors M1 and M4 are moved out of the beam path of the pump radiation source 210, the pulsed pump beam of 755 nm is coupled directly to the treatment beam delivery system to be used for patient treatments.

EXAMPLE

An example plot of the output pulse shape of a short wavelength-shifting Nd:YAG resonator with a 70 % output coupler is shown in <figref idref="f0002">FIG. 4</figref>, as measured at position (B) of <figref idref="f0001">FIG. 1</figref>. The Nd:YAG crystal was doped to 1.3 at. % (30 % higher than the standard 1 at. % doping) to allow for a shorter resonator - 6.2 mm in length, shorter roundtrip time, and a shorter output pulse duration. The Nd:YAG oscillator was pumped by an Alexandrite laser with 200 mJ per pulse, 680 ps pulse duration, and a 4.4 mm spot (as measured at position (A) of <figref idref="f0001">FIG. 1</figref>). As shown in <figref idref="f0002">FIG. 4</figref>, the output of the wavelength-shifting Nd:YAG resonator at 1064 nm was 65 mJ per pulse, 750 ps mean pulse duration. The roundtrip time in the Nd:YAG resonator was about 76 picoseconds, substantially shorter than the 680 ps Alexandrite input pulse.

With reference now to <figref idref="f0003">FIG. 5</figref>, the output pulse shape of a short resonator Nd:YVO₄ laser having a length of 3 mm with a 50 % output coupler is depicted, as measured at position (B) of <figref idref="f0001">FIG. 1</figref>. The Nd:YVO₄ oscillator was pumped by the output of an Alexandrite laser delivering 200 mJ per pulse, 720 ps pulse duration focused to a 6.3 mm spot (as measured at position (A) of <figref idref="f0001">FIG. 1</figref>). The pump spot was apertured down to 3.6 mm diameter. As shown in <figref idref="f0003">FIG. 5</figref>, the output wavelength-shifting Nd:YVO₄ resonator at 1064 nm was 34 mJ per pulse, 500 ps mean pulse duration. It is surprising that the output pulse duration of the short pulse Nd:YVO₄ laser resonator is shorter than the pulse duration of the pumping Alexandrite pulse-500 ps relative to 720 ps, especially considering that the shorter output pulse duration is achieved without any extra elements in the laser resonator aimed at pulse shaping (e.g., in the resonator there is no modelocker, Q-switch, pulse picker or any similar device of active or passive type). The short Nd:YVO₄ resonator is also remarkably and surprisingly efficient. That is, with 33% of the Alexandrite energy being transmitted through the aperture (i.e., 66 mJ), the 34 mJ Nd:YVO₄ resonator output is 51 % of the pump energy transmitted thereto. The roundtrip time in the Nd:YVO₄ resonator was about 39 ps, substantially shorter than the 720 ps Alexandrite input pulse.

An example plot of the output pulse shape of a short resonator Nd:YAG laser with a 70% output coupler is shown on <figref idref="f0004">FIG. 6</figref>. The Nd:YAG oscillator was pumped with 200mJ per pulse, 860ps pulse duration, 4mm spot. The oscillator output at 1064nm was 96mJ per pulse, 1030ps pulse duration. <figref idref="f0004">FIG. 6</figref> shows that it is possible to generate and have an output that has a longer pulse duration 1030 ps than the pulse duration of the pumping pulse, 860 ps.

The short pulse output from the short roundtrip time oscillator (e.g., resonator) can be amplified to increase the pulse energy while keeping the pulse duration shorter than 1000ps as described previously. When the oscillator and amplifier material are the same, for example Nd:YAG or Nd:YVO₄, the oscillator output wavelength can be matched to the amplifier gain profile to enable maximum energy extraction from the amplifier.

In one embodiment, the oscillator and amplifier materials are different from one another, optionally, it is advantageous for the oscillator to be made from different materials than the amplifier. For example a Nd:YVO₄ oscillator can be designed with a shorter roundtrip time vs a Nd:YAG oscillator, and a shorter output pulse duration will be produced by the Nd:YVO₄ oscillator when pumped with a short pulse Alexandrite laser, as compared to a Nd:YAG oscillator as discussed previously. Amplifying the Nd:YVO₄ oscillator output in a Nd:YVO₄ amplifier is relatively difficult because of the shorter fluorescence lifetime of Nd:YVO₄ is 100µs versus the 230µs fluorescence lifetime of the Nd:YAG. Amplifying the Nd:YVO₄ oscillator output in a Nd:YAG amplifier is possible, but sub-optimal because of the wavelength mismatch of the two different materials. According to <nplcit id="ncit0003" npl-type="b"><text>Koechner "Solid-State Laser Engineering", 5th Ed</text></nplcit>., the laser wavelength of Nd:YVO₄ is 1064.3nm, while the Nd:YAG peak gain wavelength is 1064. 1nm.

More detailed data for the laser output wavelength of a Nd:YVO₄ oscillator is published by <nplcit id="ncit0004" npl-type="s"><text>Mingxin et al. "Performance of a Nd:YVO4 microchip laser with continuous-wave pumping at wavelengths between 741 and 825 nm", Appl. Opt. v.32, p.2085</text></nplcit>, where the laser output wavelength of a Nd:YVO₄ microchip laser is shown to vary when the oscillator temperature is varied such that the laser output is 1063.9 nm when the oscillator temperature is about 0°C and the laser output is 1064.5 nm when the oscillator temperature is about 100°C. An optimized laser system consisting of a Nd:YVO₄ oscillator and a Nd:YAG amplifier can be envisioned where the temperature of the oscillator and/or the amplifier is controlled and/or adjusted such that and the peak wavelength can be varied. In one embodiment, one controls the temperature of the Nd:YVO₄ so that it is well amplified in the amplifier. In one embodiment, the temperature of the oscillator and/or the amplifier is controlled so that one can provide a maximum energy output pulse with a minimal pulse duration. The range of temperature adjustment can be between about 0°C and about 100°C, between about 20 °C and about 80°C, or between about 30 °C and about 70°C.

In addition to temperature control, other possible approaches to controlling and/or varying the peak wavelength can include external pressure applied to the laser material and doping the laser material with trace amounts of elements that would alter, for example, the crystal lattice stress. The approaches to varying peak wavelength such as oscillator and/or amplifier temperature control, pressure applied to the laser material, and doping the laser material can be employed alone or in combination.

<u>Gain uniformity</u>

Gain uniformity in the lasing medium of a laser (e.g., in a solid state alexandrite lasing medium) has a direct effect on the uniformity of the output beam. In the case of a multi-mode, mode locked laser, as discussed previously herein (e.g., at <figref idref="f0001">FIGS. 1 and 2</figref>), where the beam energy propagates through the gain medium multiple times, a difference in gain uniformity of only a few percent can cause undesired modes with high peak fluences to develop. Gain uniformity is important because in the early stages of laser profile generation differences in gain uniformity in the lasing medium (e.g., a rod) have an exponential build up. Relatively small differences in the lasing medium gain profile (this is the pump profile) become exacerbated. To optimize the energy extracted from the resonator, a relatively even fluence is most desirable, for example, a round beam of even fluence is preferred. It is desirable to obtain a more uniform fluorescence profile such that the center of the lasing medium, for example, a crystal rod and its edges have substantially the same amount of fluorescence (e.g., a relatively even fluorescence).

In order to generate light via a light source (e.g., a pumped radiation source such as a flash lamp) the light couples into a lasing medium (e.g., a crystal laser rod) and that coupling can be done via a reflecting enclosure. The reflector can be diffuse (e.g., scattered) or specular (e.g., like a silvered surface that is mirror-like and not scattered). The lasing medium (e.g., crystal rod) absorbs the light coupled into it from the flash lamp. An absorption profile develops in lasing medium (e.g., the crystal rod). The function of the lasing medium is to absorb the light from the flash lamp and then to re-emit the light at changed wavelength (e.g., a longer wavelength). Where the lasing medium is a crystal rod if the middle of the rod absorbed the most light the middle of the rod would appear to be the brightest in that emitted wavelength- i.e., to emit the most changed wavelength. The phenomenon of the rod center being brighter than the rod edges is referred to as "fluorescence non-uniformity" this can generally occur for any laser where a flash lamp is coupled to a crystal (e.g., a crystal rod).

Turning now to <figref idref="f0005">FIG. 7</figref>, a cross section of a traditional dual-flash lamp diffuse pump chamber is depicted. Two flash lamps 713, each encased in glass coolant tubes 715, are arranged in parallel on both sides of a central lasing medium 711 (e.g., an alexandrite crystal rod lasing medium). The two flash lamps 713 and the lasing medium crystal 711 are all encased within a diffusing material 717 as shown in <figref idref="f0005">FIG. 7</figref>. Any diffusing material which would survive the high intensity light from the flash lamps 713 is suitable. Suitable modifications can include sandblasting a texture on the flash lamps 713 for example, on the coolant tubes 715 that encase the flash lamps 713, or in an area between the flash lamps 713 and the crystal lasing rod 711, shown in <figref idref="f0005">FIG. 7</figref> and/or providing a coated a strip of aluminum with a white diffusing coating for example on one or more of the flash lamps 713 (e.g., on the coolant tube(s) 715). Some white diffusing coating examples include potassium sulfate, aluminum oxide, compressed PTFE and fumed silica.

Many factors can contribute to non-uniform gain distribution within the lasing medium. Lasing medium crystals may have different absorption coefficients at different wavelength(s) and/or along different axis of the crystal. This can be further imbalanced by the unequal output spectrum of the flash lamp pump source and how it matches the absorption spectrum of the lasing medium 711 (e.g., the active lasing medium). There is also the magnitude of the quantum defect within the flash lamp pump bands. It is desirable to improve gain uniformity on any material which lases, and the choice of lasing media is considered to be within the skill of an ordinary practitioner in view of the teachings provided herein.

The pump chamber geometry can also contribute to non-uniform gain by coupling more light into the crystal along one direction. In the case of an alexandrite crystal in a diffuse pump chamber, an increase in gain was observed in the direction of the flash lamps.

<figref idref="f0006">FIG. 8</figref> depicts an image generated by the pump chamber geometry of <figref idref="f0005">FIG. 7</figref>, and captured by aligning a camera to the axis of the lasing medium 811 (e.g., the alexandrite crystal laser rod) and imaging the fluorescence of the pumped lasing medium 811 (e.g., the alexandrite crystal laser rod). The end face 814 of the lasing medium 811 is depicted as having a substantially circular boundary. The areas of the laser rod end face 814 that are most proximate to the flash lamps 813 exhibit high gain regions 812.

<figref idref="f0007">FIG. 9</figref> depicts a graph showing profiles of the lasing medium 811 described in <figref idref="f0005">FIGS. 7</figref> and <figref idref="f0006">8</figref> with the profiles taken along the horizontal axis 818B and along the vertical axis 818A. As can be seen by the graph in <figref idref="f0007">FIG. 9</figref>, the gain at the edge regions 812 of the end face 814 of the lasing medium 811 (in <figref idref="f0006">FIG. 8</figref>) in the horizontal direction 818B may be about 5 to 10 percent higher than the gain in the vertical direction 818A. These edge region 812 peaks correspond to the high gain regions depicted in the image of <figref idref="f0006">Figure 8</figref>. This uneven gain distribution is problematic in that it leads to failure of the laser system due, for example, to uneven heating of the lasing medium that results in system breakdown and unacceptable down time and repair times. Further, where there is substantially uniform beam gain one can increase the system power output with less system failure than in the system where the gain is not uniform.

Accordingly, in order to improve system reliability, it is desirable to lessen and/or eliminate these gain peaks such that gain is substantially uniform across all axis of the lasing medium (e.g., that the gain is substantially uniform along both the horizontal axis 818B and along the vertical axis 818A of the lasing medium).

Embodiments of the present disclosure that improve gain uniformity include an optical system comprising a pump chamber with one or more elements that enable a substantially uniform gain across the lasing medium, for example, diffusing element(s) disposed between a flash lamp and a crystal. Elements that enable a substantially uniform gain across the lasing medium can include, for example, light shaping elements for example deflectors that lead to diffusion, scattering, refraction, and/or reflection or elements that provide absorption. In one embodiment, the element that enables a substantially uniform gain across the lasing medium is a diffusing element that acts to scatter a portion of the light coupling into the crystal and to increase the diffuse illumination of the rod, thereby avoiding non-uniform high-gain regions and achieving a circular symmetry to the gain region within the crystal rod.

Referring to <figref idref="f0008">FIGS. 10</figref> and <figref idref="f0009">11</figref>, relatively uniform fluorescence can be achieved via elements that enable a substantially uniform gain across the lasing medium. Suitable elements include diffusing elements 1019. The stored photons from the rod fluoresce and enter into the cavity of the resonator formed by two or more mirrors. The photons travel between at least two mirrors that are along the relatively long axis of the gain medium and the photons build up energy through multiple trips between the opposing mirrors, which are substantially totally reflective. It is during this buildup of energy that the impact of the contrast between a non-uniform fluorescence and a relatively uniform fluorescence can be best understood when considering the laser energy profile that is emitted. A non-uniform fluorescence results in non-uniform energy emission from the laser, which is problematic due to the wear it causes on, for example, the optical components of the laser. For example, coatings present on the Pockels cells can be deteriorated by the non-uniform energy emission. A more uniform fluorescence results in a more uniform energy emission from the laser, which is desirable including due to the resulting increase in optical component longevity. By using an element that improves gain uniformity, such as a diffuser 1019 (e.g., the baffle and/or an absorber) obtaining more uniform gain and thereby more uniform fluorescence is favored, but at the expense of pumping efficiency, which is sacrificed due the presence of the element 1019.

Referring still to <figref idref="f0008">FIGS. 10</figref> and <figref idref="f0009">11</figref>, according to one embodiment of the disclosure, a lasing medium 1011 (e.g., a crystal rod) was placed between each flash lamp 1013 and a diffusing element 1019 (e.g., a baffle and/or an absorber) was placed in between the flash lamp 1013 and the lasing medium 1011 to scatter a portion of the light coupling directly into the lasing medium 1011 and to increase the diffuse illumination of the crystal rod lasing medium 1011. According to one embodiment, when a suitably-sized diffusing element 1019 was placed in the chamber, the lasing medium 1011 (e.g., a crystal rod) achieved substantially uniform gain (e.g., substantially circular symmetry). In one embodiment, the diffusing element 1019 is a 0.063 inch diameter alumina rod that was placed equidistant between the flash lamp 1013 and the crystal rod lasing medium 1011. Suitable diffusing element 1019 diameters can be about the same diameter as the lasing medium (e.g., about 0.375 inches) to as small a diameter as can be structurally sound (e.g., about 0.03 inches).

<figref idref="f0008">FIG. 10</figref> depicts an image of the fluorescence using such an implementation. The fluorescence resulting from this chamber configuration shows the gain is more evenly distributed in a circularly symmetric fashion and the high-gain regions seen in <figref idref="f0006">FIG. 8</figref> (when diffusing elements are absent) are eliminated.

The graph of <figref idref="f0009">FIG. 11</figref> shows that the horizontal and vertical beam profiles have a closer agreement between the gain in the two axes (e.g., the vertical axis 1018A and horizontal axis 1018B show a substantially similar gain distribution) of the beam are in close agreement. The normalized gain distribution in the chamber having the diffusing element shows that the edge region 1012 of <figref idref="f0008">FIG. 10</figref> lacks the peaks seen in <figref idref="f0007">FIG. 9</figref> that were a result when there was no diffusing element in place.

In some embodiments, the choice of gain uniformity element material (e.g., a diffuser, absorber, deflector, baffle, scattering element, refractor, and/or reflector) and in the case of a material in the shape of a rod the selected diameter of the gain uniformity material can be adjusted to improve the beam uniformity of the system. The gain uniformity element (e.g., the diffusing element) need not sit between the flash lamp and the laser rod, rather the diffusing element can be a grating that is etched on the surface of one or more of the flash lamp or the laser rod.

The effect of balancing and/or improving gain uniformity on the beam profile of a mode locked laser by altering pump chamber geometry, e.g., by adding one or more diffusing element, is dramatic. The image depicted in <figref idref="f0010">Figure 12</figref> shows the beam produced by the unmodified chamber described in connection with <figref idref="f0006">Figure 8</figref>. The high peak fluence produced at the sides of the beam in <figref idref="f0010">Figure 12</figref> are beyond the damage threshold of the optics contained in the laser resonator. In comparison, the beam profile shown in <figref idref="f0011">Figure 13</figref>, is produced by an emobdiment that includes one or more diffusing elements (e.g., a baffled chamber with an alumina rod) like that described in connection to <figref idref="f0008">Figure 10</figref> and the beam profile is produced by the modified chamber is more circular, indicating the energy from the beam is spread over a greater area. As a result, the peak fluence of the beam generated with the embodiment of the pump chamber modified to include at least one diffusing element was greatly reduced and overall system power may be increased without damaging the optics in the resonator. As a result, the life and/or reliability of the laser system is improved due to the presence of the at least one gain uniformity improvement elements (e.g., a baffle).

<u>Non-spherical lenses lessen free space propagation mode effect</u>

Picopulse laser treatment energy relies on laser intensity, which is the square of the sum of the lasers electric fields. When free space propagation modes couple together the laser output intensity profile can tend toward non-uniformity. Free space propagation modes can include one or more of Hermite profiles, Leguerre profiles and Ince Gaussian profiles.

For the multi transverse mode laser it is beneficial to have sufficient transverse modes present such that the beam profile is filled in (substantially even). This ensures the peak fluence will be as close as possible to the average fluence. The ideal situation is the where the beam profile has a "top hat" beam profile, which looks like a top hat in profile e.g., referring to the representation of the normalized gain distribution shown in <figref idref="f0009">FIG. 11</figref> in an idealized situation the two gain regions 1012 connect with a straight line and the sloping sides are much steeper. A low peak fluence will prevent laser damage to optical coatings and thus prolong the life of the laser.

The picopulse resonator can produce many multimode Hermite profile electric fields and can produce unwanted combinations of multimode Hermites. In order meet the desired laser treatment energy levels, Hermite profiles can result in high intensity profiles. These high intensity profiles can damage the optics of the resonator leading to reduced lifetime issues.

It is desirable to lessen the impact of free space propagation modes including Hermites in the beam output profile. Introducing a lens element that provides astigmatism can act to decouple free space propagation modes thereby obviating or lessening their impact on the beam profile. Lens elements that can lessen the impact of free space propagation modes (e.g., Hermites) could be for example, cylinder lens, angled spherical lens, anamorphic prisms, etc.

Unlike a spherical lens, which is cut from a sphere, a non-spherical lens (e.g., an astigmatic lens such as a cylindrical lens) can be cut from a rod. Specifically, a non-spherical lens can be cut along the long axis of a rod such that its end face looks like the letter "D". The non-spherical lens provides only one axis of curvature in contrast to a spherical lens which provides two axis of curvature. When light travels through the curved axis the light is deviated (e.g., focused or defocused) by the curvature of the lens such that the light is different in the x-plane versus the y-plane. Light that travels across the other axis does not get focused or defocused - it sees no deviation.

Alternatively, a spherical lens may be angled such that light impinges on the spherical lens at an angle that provides the effect of a cylindrical lens such that the angled spherical lens output of light is different in the x-plane versus the y-plane. These are just a few of many ways to produce an astigmatic lens effect. Other methods or means of utilizing lenses or prisms to produce an astigmatic effect are known to those of skill in the relevant art.

There is a phenomenon in multimode lasers by which multiple Hermite profiles can build up within a resonator and interfere with each other to cancel portions of one another out and thereby create hotspots that give an unacceptable laser beam intensity profile. By controlling and/or managing the mix of Hermites their interference in the laser output can be limited. The mix of Hermites can be limited by utilizing different astigmatism for the x and y axis' in the resonator. In this way, the astigmatic element prevents multiple Hermite profiles from interfering with one another to produce a bad profile. Rather, each individual Hermite profile exits the laser individually. In this way, the astigmatic lens element avoids Hermite's canceling portions of one another out that results in undesirable hot spots in the laser beam profile that can cause wear on the optics of the system. As discussed previously, it is important to provide a beam output that shows a relatively even energy distribution (e.g., beam uniformity). The astigmatic effect element can aid in beam uniformity, because it avoids coupling of free space propagation modes that result in undesirable hot spots.

An example of an unwanted two electric field combination with a resultant laser intensity combination is shown <figref idref="f0012">FIG. 14A</figref>. In <figref idref="f0012">FIG. 14A</figref>, the majority of the beam energy is contained in two distinct regions 1412 within the profile. This is an unwanted electric field, which is a result of the combination of two individual propagating Hermite fields that remain in phase, i.e. each field is in step with the other.

By introducing astigmatism into the picopulse resonator the undesirable phase relationship of the propagating Hermite electric fields is broken along the astigmatic axes (physics Gouy phase effect). Using the same two Hermites of the previous combination example shown in <figref idref="f0012">FIG. 14A</figref>, but now showing the effect of phase mismatch created by the astigmatic element (e.g., astigmatic lens) on intensity is <figref idref="f0012">FIG. 14B</figref>. The <figref idref="f0012">FIG. 14B</figref> profile has a better fill of energy or distribution of energy in that all four corners of the beam profile are illuminate, which is much less likely to damage the optics compared to the beam profile in <figref idref="f0012">FIG. 14A</figref> where energy is concentrated into two of the four corners of the beam profile.

The picopulse laser transverse mode profile is improved when astigmatism is introduced into the resonator. The astigmatism essentially provides two resonator configurations, each with a preferred set of modes. In one embodiment, astigmatism was introduced by a weak cylindrical lens «0.5 Dioptres. The astigmatic generating element could be placed anywhere within the resonator path. The cylinder lens worked well when its axis was perpendicular or parallel to the plane polarized light in the picopulse laser.

There are many approaches to introducing an astigmatic element to the resonator, for example, the goal of different net curvature can be achieved within a resonator by, for example, positioning a spherical lens or spherical lenses such that one or more spherical lens is tilted relative to the optical axis, thereby providing one or more astigmatic element(s). Alternatively, the beam can be expanded in a single direction (e.g., anamorphic expansion) prior to a lens or a spherical mirror.

Another method of free space propagation mode control is to place an obscuration (e.g., a wire) at the electric field zero crossings of a wanted mode. The obscuring element (e.g., for a Hermite a line, for a LeGuerre a radial obscuration) can be produced in a substrate or in the anti-reflection coating on a substrate. The obscuration element prevents unwanted free space propagation modes from lasing and effectively filters them out of the distribution of energy lased from the system. Preferably, obscuring elements have thin lines (e.g., lines that are &lt;50um thick), which can be produced, for example, by UV laser writing directly into the substrate (e.g., glass). The lines are best situated near the rod where resonator misalignment will have least effect on line position.

Picosecond Laser Sub-Harmonic Resonator

In a simple, free running, laser resonator a number of longitudinal modes develop independently. These modes have no set phase relationship so they are free to interfere with each other, which leads to fluctuations in the output intensity of the laser as the output signal is an average of all modes inside the resonator.

In frequency space, each mode corresponds to a spectral line and the separation of spectral lines is called the axial mode interval, c/2L, where c is the speed of light and L is the optical path length of the resonator (2L is the round trip optical path length of the resonator). The temporal output of the laser is related to the frequency space by a Fourier transform.

Mode locking is a technique used to create pulses of light with durations less than 1 nanoseconds. This is done by introducing an element which periodically inhibits the lasing of the resonator. This inhibiting element can take a number of forms but the implementation is broken down into two categories: <ol id="ol0001" compact="compact"><li>(a) Passive mode locking uses an element whose properties are varied by the light inside the resonator</li><li>(b) Active mode locking utilizes elements that need to be driven using external signals.</li></ol>

When the mode locking element is a Pockels cell it can be used in combination with a polarizer to vary the losses inside of the resonator. Using the Pockels cell in this manner is equivalent to modifying the reflectivity of one of the cavity mirrors.

The voltage applied to the Pockels cell can be increased until the lasing within the resonator is inhibited. The highest voltage in which laser emissions are produced is called the threshold voltage. To mode lock the resonator the voltage is modulated around the threshold voltage at a set frequency. When the voltage is lower than threshold the losses are less and lasing can occur. Voltages higher than threshold will result in no lasing.

In traditional mode locked lasers the oscillation period of the lasing inhibitor is equal to the time for a pulse to travel one round trip through the resonator. Since lasing is inhibited when the Pockels cell voltage is above threshold a single pulse of light is formed which propagates through the resonator. This pulse is formed of longitudinal modes whose phases are aligned. The peak longitudinal mode will have a frequency which experiences minimal losses when propagating through the mode locking element. In the region around this peak the modes will experience greater loss for greater differences in frequency. This creates a relationship between the longitudinal modes that doesn't exist in free running lasers and leads to the smaller pulse durations of mode locked lasers.

A traditional mode locked laser works based on the principal that the electrical switching frequency at which a mode locker (e.g., a Pockels cell) is switched is directly tied to the optical path length of the resonator. The optical path length of the mode locked resonator can range from about 3 meters to about 0.5 meters in length, for example.

Active mode locking involves modulation of a component inside the resonator at a frequency whose period is equal to the time required for light to propagate one round trip in the resonator. The purpose of this component is to only allow lasing to occur over a portion of this period and the end result is a single pulse of light traveling within the resonator.

In the case of the traditional picosecond resonator (i.e., the fundamental) the modulation is applied to the Pockels cell which requires several hundred volts of modulation in order to produce the mode locking effect. The length of the picopulse resonator is limited by the highest modulation frequency that can reliably be produced at this voltage level.

At this point 75MHz is believed to be the maximum frequency which can be created which leads to a 2 meter long resonator. A shorter resonator would be preferable from a mechanical point of view as the mirror positional sensitivity increases as the resonator length increases.

<figref idref="f0013">FIG. 15A</figref> shows the modulation signal 1599A applied to the Pockels cell in a traditional picosecond resonator having a threshold voltage 1585A. In the presence of this modulation signal 1599A the intensity 1589A builds up in the resonator over time.

For example, a resonator having an optical round-trip length of 10ft requires an electrical switching frequency of about 100MHz. The speed of light in air is approximately 1ft per nanosecond; therefore, the round-trip time of a photon in a 10ft resonator is about 10 nanoseconds. The Pockels cell therefore is switched at about 100MHz. In accordance with a traditional picosecond resonator (i.e., the fundamental) picosecond seed pulses that are generated in the resonator pass through the Pockels cell one time per electrical switching event. Unfortunately, switching the Pockels cell at 100MHz is not an option due limitations and to issues such as fidelity issues.

In order to resolve such a problem, a sub-harmonic solution may be employed. The sub-harmonic approach can include (A) divide the Pockels cell switching frequency by a factor of the n^th harmonic (e.g., by any power of 2) and/or (B) dividing the optical path length by a factor of the n^th harmonic (e.g., by any power of 2). The approaches A and B were first tested on a prototype. This test was done whereby a traditional picopulse laser approach to a 75 MHz modulation frequency would call for a 2 meter resonator length (A) using the switching frequency approach the modulation frequency of the existing 75MHz, 2 meter resonator was changed to a modulation frequency of 37.5MHz. Then (B) using the optical path length approach the 75 MHz modulation frequency was maintained, but the path length of the resonator was reduced to 1 meter, which was half the original 2 meter length. Approaches (A) and (B) produced pulses of similar pulse widths to the traditional 75 MHz and 2 meter resonator length design.

Embodiments relating to dividing the Pockels cell switching frequency by a factor of the n^th harmonic (e.g., by any n&gt;1, n is a whole number.)

In one embodiment, a system in which the electrical switching frequency is a sub-multiple of the standard resonator switching frequency is implemented. In other words, a system is implemented in which seed pulses that flow in the resonator pass through the Pockels more than one time for every electrical switching event. The modulation signal can be viewed as a gate which allows the light to pass. When the Pockels cell voltage is below threshold the gate is closed. So a single pulse travels around the resonator passing the Pockels cell while the gate is open and all other radiation is suppressed when the gate is closed. Considering this analogy, the proposed idea is to close the gate every other round trip through the resonator. This would allow for shorter resonator lengths for a given modulation frequency.

<figref idref="f0013">FIG. 15B</figref> shows a lower frequency modulation signal 1599B applied to the Pockels cell in a sub-harmonic picosecond resonator having a threshold voltage 1585B, this is the nth harmonic of the switching frequency of frequency modulation signal 1599A shown in <figref idref="f0013">FIG. 15A</figref>. In the presence of this lower frequency modulation signal 1599B the intensity 1589B builds up in the resonator over time such that, referring now to <figref idref="f0013">FIGS. 15A and 15B</figref>, at the time of about 140 nanoseconds the intensity inside the resonator 1589A and 1589B is substantially the same.

While we have shown the modulation signal 1599A in <figref idref="f0013">FIG. 15A</figref> and the modulation signal 1599 B in <figref idref="f0013">FIG. 15B</figref> as featuring an idealized sine wave, in actual usage in the picosecond system the modulation signal has at least some harmonic content. More specifically, the modulation signal should have from about 5% to about 50% harmonic content, and from about 10% to about 20% harmonic content.

Embodiments relating to dividing the optical path length by a factor of the nth harmonic (e.g., by n&gt;1, n is a whole number).

In another sub-harmonic approach, inhibiting the lasing on every other pass through the resonator would be sufficient to produce a mode locked pulse. This sub-harmonic approach can decrease the picopulse resonator length and/or ease the electrical burden by decreasing the modulation frequency.

In a normal mode locked laser a pulse of light propagates one round trip through the resonator for each oscillation of the mode locking element. If the element were instead driven at half the frequency, or the first sub harmonic, then the pulse would travel two round trips for each oscillation. During the first trip the pulse would travel through the resonator while the element was at maximum transmission. This is the same in the standard mode locking resonator. During the second trip the pulse will hit the element at minimal transmission and experience loss. If the gain of the active medium is sufficient then the pulse energy will increase more during the first trip than it loses in the second trip and a mode locked pulse can be generated.

Since the modulation frequency is tied to the propagation time through the resonator, modulating with a subharmonic provides the benefit of a shorter overall resonator length. For example, if the electrical circuit can reliably switch the required voltages at 50 MHz then the period of one oscillation is 20 nanoseconds. A 6 meter round trip cavity length is required for a travel time of 20 nanoseconds. However, if 50 MHz is the first subharmonic of the resonator then the round trip cavity length is cut in half to 3 meters. If we consider the frequency of oscillation to be a limiting factor then subharmonic operation provides smaller resonators than traditional mode locking.

A method of evaluating mode locked resonators was developed by Kuizenga and Siegman (<nplcit id="ncit0005" npl-type="s"><text>D.J. Kuizenga and A.E. Siegman, "FM and AM mode locking of the homogenous laser-Parti: Theory", IEEE Journal of Quantum Electronics, Nov 1970, pp.694-708</text></nplcit> [1]) Their analysis applies a self-consistent criterion on the pulse after one round trip of the resonator. Energy travels through an active medium and back then through a modulator and back.

The following expression, Formula (1), relates the pulse width, τ, to the gain, g, modulation depth, δ, modulation frequency, fm, and gain bandwidth, Δf. <maths id="math0002" num="formula (1)"><math display="block"><mrow><mi>τ</mi><mo>=</mo><mfrac><mrow><msqrt><mrow><msqrt><mn>2</mn></msqrt><mi mathvariant="italic">ln</mi><mn>2</mn></mrow></msqrt></mrow><mi>π</mi></mfrac><msup><mfenced><mfrac><mi>g</mi><mi>δ</mi></mfrac></mfenced><mi>¼</mi></msup><msup><mfenced><mfrac><mn>1</mn><mrow><msub><mi>f</mi><mi>m</mi></msub><mi mathvariant="normal">Δ</mi><mi mathvariant="italic">f</mi></mrow></mfrac></mfenced><mi>½</mi></msup></mrow></math><img file="EP3751684A1_D0002.tif" /></maths>

A similar analysis can be done for the sub harmonic resonator, but the self-consistent criterion can only be applied after n round trips of the resonator, n&gt;1, n a whole number. The overall transmission function for the n round trips must be computed to discover the modulation depth variable (δ). The sub harmonic overall transmission will be &lt;100% and shows a variation from one round trip to the next during the n round trips taken for the analysis.

In one embodiment, a resonator was constructed using an 8 mm Alexandrite rod, 85mm in length and a KD*P Pockels cell. In the first configuration the Pockels cell is driven at 75MHz and the path length is 2 meters. This system is operating with the traditional fundamental mode locking frequency for this resonator. Pulsewidth of 550 picoseconds are produced by this configuration. The system is then configured to mode lock at the first sub harmonic such that it modulates at 50MHz and the path length is decreased to 1.5 meters. Pulses of 700 picoseconds are produced by this system. Even though the equation at Formula (1) was developed for a traditional mode locking approach the pulse widths of these two systems reasonably follow the square root of one over the frequency term of the above expression.

<figref idref="f0014">Figure 16</figref> depicts a representative embodiment of an apparatus 1600 according to the present disclosure, 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 1622 (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 1620, and a polarizing element also referred to as a polarizer 1618 (e.g., a thin-film polarizer). During operation, the laser pulse output will be obtained along output path 1623.

At opposite ends of the optical axis 1622 of the resonator are a first mirror 1612 and a second mirror 1614 having substantially complete reflectivity. This term, and equivalent terms such as "substantially totally reflective" are used to indicate that the mirrors 1612 and 1614 completely reflect incident laser radiation of the type normally present during operation of the resonator, or reflect at least 90%, preferably at least 95%, and more preferably at least 99% 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 1614, Pockels cell 1620, and polarizer 1618 that is induced by the particular operation of the Pockels cell 1620, as discussed in detail below.

In particular, a laser pulse traveling from lasing or gain medium 1616 towards second mirror 1614 will first pass through polarizer 1618, then Pockels cell 1620, reflect at second mirror 1614, traverse Pockels cell 1620 a second time, and finally pass through polarizer 1618 a second time before returning to gain medium 1616. Depending upon the bias voltage applied to Pockels cell 1620, some portion (or rejected fraction) of the energy in the pulse will be rejected at polarizer 1618 and exit the resonator along output path 1623. The remaining portion (or non-rejected fraction) of the energy (from 0% to 100% of the energy in the initial laser pulse) that returns to the medium 1616 is the "effective reflectivity" of second mirror 1614. As explained above, for any given applied voltage to Pockels cell 1620, the effective behavior of the combination of second mirror 1614, Pockels cell 1620, and polarizer 1618 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 100%" refers to a mirror that acts as a substantially totally reflective mirror as defined above.

Also positioned along the optical axis 1622 of the resonator is a lasing or gain medium 1616, which may be pumped by any conventional pumping device (not shown) such as an optical pumping device (e.g., a flash lamp) or possibly an electrical or injection pumping device. A solid state lasing medium and optical pumping device are preferred for use in the present disclosure. Representative solid state lasers operate with an alexandrite or a titanium doped sapphire crystal. Alternative solid lasing media include a yttrium-aluminum garnet crystal, doped with neodymium (Nd:Y AG laser). Similarly, neodymium may be used as a dopant of pervoskite crystal (Nd:YAP or Nd:YAl03 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 YV04, 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+3 ions in the YAG crystal causes the emission of a radiation beam at 1064 nm 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 disclosure also relate to the manner in which the apparatus 1600, depicted in <figref idref="f0014">Figure 16</figref>, 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 1616 is reflected between the first mirror 1612 and second mirror 1614 at opposite ends of the optical axis 1622 of the resonator. Laser energy emanating from the lasing medium 1616 therefore traverses the thin film polarizer 1618 and Pockels cell 1620 before being reflected by the substantially totally reflective second mirror 1614, back through the Pockels cell 1620 and polarizer 1618.

Naturally birefringent laser gain materials such as alexandrite, and other crystals such as Nd:YV04 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, the polarized axis corresponding to the materials highest gain crystolalographic axis. Typically the polarizer 1618 is configured for transmission of essentially all incident radiation by proper alignment with respect to the electric field vector.

Optionally, referring still to <figref idref="f0014">Figure 16</figref>, an astigmatic element 1619 may be placed anywhere along the optical axis 1622 including, for example, directly in front of one or more mirrors 1612, 1614. Further, one or more of the mirrors 1612, 1614 can provide an astigmatic element by possessing two different radii of curvature that are perpendicular to one another.

Referring to the simple apparatus of <figref idref="f0014">Figure 16</figref>. When the laser threshold bias DC voltage is applied to the Pockels cell 1620 then the effective mirror reflectivity is set at such a value that the medium 1616 will lase. Varying the voltage above and below the bias voltage is called modulating the voltage. In one embodiment, using an Alexandrite medium 1616, a typical DC bias voltages applied to Pockels cells is around 650V and the modulated voltage applied to the Pockels cell is about 200V.

<figref idref="f0015">Figure 17</figref> depicts a representation of the seed pulse grown using a sub-harmonic resonator as disclosed herein. The varying amplitude of the seed pulses while operating in the sub-harmonic regime is depicted in this plot. The trace pulse height variation of repeated high then low is due to the subharmonic used being an n=2.

The picopulse laser uses a mode locking to achieve its short pulsewidth. The mode locker is a constricting device which is only fully open for a small fraction of the time it takes a photon to make a round trip in the resonator. So of all the photons circulating and making round trips, only those which arrive at the gate at the right moment will find it fully open, all other photons will experience a loss. Over many round trips this elimination of all but the 'correctly' timed photons results in a shortening of the pulsewidth. All prior literature suggests you drive open the gate once per round trip or even twice per round trip for 2 pulses to be present and so on. The picopulse laser with the sub-harmonic resonator is not run at once per round trip but at once per 2 round trips, hence it is sub-harmonic on a single round trip (e.g., in this example it is one half harmonic).

While the embodiments of the disclosure described herein detail the advantages of implementing the modified pump chamber of a multi-mode, mode-locked operated laser, one skilled in the art would recognize that such advantages may be experiences using other types of lasers and operations, such as, for example, multi-mode, non-mode locked operation.

The use of headings and sections in the application is not meant to limit the invention; each section can apply to any aspect, embodiment, or feature of the invention.

Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including or comprising specific process steps, it is contemplated that compositions of the present teachings also consist essentially of, or consist of, the recited components, and that the processes of the present teachings also consist essentially of, or consist of, the recited process steps.

In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components and can be selected from a group consisting of two or more of the recited elements or components. Further, it should be understood that elements and/or features of a composition, an apparatus, or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein.

The use of the terms "include," "includes," "including," "have," "has," or "having" should be generally understood as open-ended and non-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. Moreover, the singular forms "a," "an," and "the" include plural forms unless the context clearly dictates otherwise. In addition, where the use of the term "about" is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise.

It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

While only certain embodiments have been described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the appended claims. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the appended claims.

The patent, scientific and medical publications referred to herein establish knowledge that was available to those of ordinary skill in the art. The entire disclosures of the issued U.S. patents, published and pending patent applications, and other references cited herein are hereby incorporated by reference.