Patent Application
20080082089
The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.
Lasers and other light sources are often used for the treatment of skin disorders and to produce cosmetic improvement in the appearance of the skin including the removal of hair from the skin. The heat produced by the light energy can modify structures within the skin and beneath the skin. Typical applications can include, for example, removal of hair, pigmented lesions, tattoos, vascular lesions, wrinkles, acne, skin tightening, and/or the like. Applications can more generally also include treatment of mammalian tissue.
Dermatological laser treatments can be based on selectively targeting of a chromophore in or near the target structure by an appropriate choice of wavelength and pulse duration of the light. Lasers are often the preferred light source because a laser beam has a narrower wavelength bandwidth than light from other sources. A source with a narrow wavelength bandwidth can maximize the spectral selectivity of the target chromophore. In addition, lasers can be made with much shorter pulse durations than other light sources thereby maximizing the temporal selectivity of the targeted structure. The superior temporal selectivity makes lasers especially preferred for removing small targets like small vessels and tattoo pigment particles.
The cavity of the solid state laser 120 can include a high reflecting mirror 121 and an output coupler mirror 122. The output coupler mirror 122 can couple an output beam 125 from the cavity of the solid state laser 120. In one embodiment, the cavity of the solid state laser 120 can include a q-switching element 123 such as, for example, a Cr4+:YAG (Cr:YAG) element. In another embodiment, a frequency doubling crystal such as, for example, KTP, can be positioned in the path of the output beam 125.
In one embodiment, the solid state laser gain materials 106 and/or 126 can be directly coated on both ends with coatings of appropriate transmission and reflection properties to form the reflecting mirrors 111 and 121 and the output coupling mirrors 112 and 122. For a Alexandrite/Nd:YAG system, the reflecting mirror 121 and the output coupler mirror 122 can allow both double-pass pumping at about 755 nm and/or laser output at about 1064 nm. The absorption coefficient at about 755 nm can be about 2 cm−1. Therefore, a solid state laser 120 with a gain medium 1.15 cm long can absorb approximately 99% of the pump energy in a double-pass configuration.
Coupling the output laser beam 115 of the solid state laser 110 into the gain material 126 of the solid state laser 120 can be accomplished in a variety of ways such as, for example, end-pumping, as illustrated in
Both Alexandrite and Nd:YAG lasers can be used in dermatological applications. Therefore, the selection of Alexandrite and Nd:YAG as the solid state lasers 110 and 120, respectively, in the configuration illustrated in
The absorption spectra of Nd:YAG has a continuous band of lines ranging from about 725 nm to about 770 nm. Therefore, the non-tuned output of a free-running Alexandrite laser, at about 755 nm, can be used to pump a Nd:YAG laser. An important absorption line in the Nd:YAG is about 2 or 3 nm wide centered at about 755 nm. A stronger but narrower peak is centered at about 750 nm. Furthermore, there are wavelengths within the 725 nm to 770 nm band where excited state absorption can occur. However, there is very little excited state absorption at 755 nm, making it an attractive pump wavelength. The efficiency of the conversion of 755 nm to 1064 nm can be affected mostly by the quantum defect, which is about 30%. There can be another small loss, e.g., less than 5% percent in Nd:YAG, due to scattering effects.
Laser pumping can be particularly attractive for lasers that are difficult to pump with other conventional sources such as, for example, laser diodes and flashlamps. As an example, it can be difficult to generate high peak powers from Nd:YAG at 946 nm, which is one of the laser lines of Nd:YAG. The 946 nm is a three-level laser transition which requires a high pump rate to reach threshold. Flashlamp pumping can be inadequate due to poor brightness of the source, while diode lasers can essentially be continuous wave sources and not suitable for high peak power applications.
When end-pumped by a Q-switched Alexandrite laser emitting a 50 ns pulse at 755 nm, for example, Nd:YAG can readily lase at 946 nm, emitting a similarly short pulse with hundreds of milli-Joules of energy, corresponding to several MW of peak power. In another example, when end-pumped with 25 Joule, 3 millisecond pulses from a free-running, gain-switched Alexandrite laser, an output of 6 Joules at 946 nm can be emitted by the Nd:YAG laser.
Although the pump beam can be absorbed by the gain medium, high absorption is not preferred in all embodiments. For example, some heat can be generated in a rare-earth gain medium by laser pumping, although the amount of heat can be much less than that deposited in the gain medium by flashlamp pumping. Nevertheless, the size of the gain medium can be chosen so that the heat can be removed fast enough to limit the temperature rise in the gain medium. The length of the gain medium and the magnitude of the absorption can be chosen so that the heat generation is distributed fairly evenly through the medium. In some cases, for example, the wavelength emitted by the pump laser can be tuned in order to adjust the absorption of the pump beam by the rare-earth laser.
Absorption spectra show that Nd:YVO4, and Nd:GdVO4 can also be excited by a free running Alexandrite laser. A tunable Alexandrite laser, from about 700 nm to about 818 nm, can be used to excite other laser gain materials such as, for example, Nd:YAP, Er:YAG, and/or Tm:YAG. Ti:sapphire, with a broader tunable range from about 700 nm to about 1050 nm, can also be used to pump Ho:YAG. The approximately 2.94 micron wavelength output of a Er:YAG laser has high optical absorption by water and can therefore be used to ablate a thin layer of the epidermis for removing some of the effects of aging and sun damage.
In some embodiments, Tm:YAG can provide laser output when pumped by a free-running Alexandrite laser. For example, when the thulium concentration is at 6%, a gain length of two inches absorbed 95% of the pump energy with a double-pass pump configuration. The thulium laser can produce 8 Joules of approximately 2 micron laser output when pumped with a 25 Joule, 755 nm laser beam. In this case, about 15 Joules is deposited in the laser rod. The long length of the laser rod can provide sufficient surface area from which to extract the heat between pulses.
Like the Er:YAG above, the approximate 2 micron output of a Tm:YAG laser can be usable for improving the appearance of aged skin. Tm:YAG has the advantage that the wavelength of the output is tunable from about 1.93 microns to about 2.10 microns. The wavelength can be adjusted so that the depth of penetration in the skin can be selected over a range of about 110 micron to about 600 microns.
The laser system 100 can treat a patient at either or both of two wavelengths produced by solid state lasers 110 and 120 at the same or two different pulse durations. For example, a Q-switched Alexandrite laser without a tuning element as solid state laser 110 can produce approximately 50 nanosecond pulses at about 755 nm. The output beam 115 in this configuration can be used to treat the patient or to pump a solid state laser 120 such as Nd:YAG, in which case approximately 50 nanosecond pulses at about 1064 nm can be produced and can be used to treat the patient. In another embodiment, by not energizing or not including a Q-switching element in either cavity of solid state lasers 110 and 120, the laser system 100 can also be used to treat the patient with long-pulses of either wavelength. In this configuration, the duration of the long pulses can be determined by the duration and output power of the energy pulses produced by the one or more flashlamps 101.
The laser system 100 can realize one or more of the following advantages over a conventional Q-switched Nd:YAG laser. The duration of the pulse generated by a conventional Q-switched Nd:YAG laser is about 10 nanoseconds. At effective treatment energies, the peak power can be so high that it cannot be transmitted though an optical fiber without damaging the fiber. A conventional Q-switched Nd:YAG laser system, therefore, typically has expensive and inconvenient articulated-arm beam delivery systems to overcome this problem. The 50 nanosecond pulses generated, for example, by the laser system 100 can be transmitted by optical fiber, a simpler and less expensive design. Pulses generated by the laser system 100 can also be as long as 100 nanoseconds. Furthermore, a higher output energy is possible with laser system 100. Q-switched operation can require that energy be stored in the laser cavity. But amplified spontaneous emission (ASE) can limit the amount of energy that can be stored in a Nd:YAG cavity, resulting in limited output. Gain switched operation in laser system 100, however, does not have this problem because of the short duration of the pumped state of the laser gain material and of the high Q of the cavity.
In another embodiment, frequency doubling can be used to obtain about a 532 nm output beam 125. For example, high peak power of 50 nanosecond Nd:YAG pulses from solid state laser 120 can enable efficient second-harmonic conversion of the about 1064 nm wavelength to about 532 nm. Generation of the second-harmonic can be accomplished with a frequency doubling crystal 124, such as, for example, a KTP crystal. The output laser beam from solid state laser 120 laser can be polarized in order to maximize the efficiency of the wavelength conversion within the frequency doubling crystal 124. A polarizing element can be installed in the cavity of solid state laser 120. In an alternative embodiment, a different host material for solid state laser 120 can be selected. Both Nd:YVO4, and Nd:GdVO4 can produce linearly polarized outputs at about 1064 nm and about 1063 nm, respectively. The long-pulse 1064 nm beam may not be efficiently frequency doubled because the peak power is low. This problem can be overcome by repetitively Q-switching the solid state laser 120 or the solid-state laser 110. Either active or passive Q-switching can accomplish repetitive Q-switching. A Cr4+:YAG passive Q-switch can also be placed in the resonator cavity of solid state laser 120 to generate a train of high peak power pulses that can be efficiently frequency doubled to about 532 nm.
Another method of coupling the pump beam 115 into the solid state laser 120 can include using a fiber optic coupling system. First, one or more lenses or other optical components can converge at least a portion of the pump beam 115 into an optical fiber (not shown) though which a portion of the pump beam 115 can be transmitted. The beam exiting the distal end of the optical fiber can be a divergent beam, which can be directed into the gain material of solid state laser 120 or it can be shaped and/or collimated by one or more lenses or other optical components before being directed into the gain material of solid state laser 120.
In one embodiment, a diode laser output at 808 nm can be used for hair removal and for pumping a Nd:YAG laser. The diode laser system can include an optical system to optimize the divergence of the diode laser beam for treating hair and pumping the Nd:YAG laser.
An alexandrite-pumped-neodymium laser system can be useful for a variety of medical applications, and in particular, dermatology. The three treatment wavelengths and two pulse durations capable of being produced by the laser system 100 can provide a range of six spectrally and temporally selective treatment modes thereby making this system clinically effective for a large range of medical conditions. The efficient conversion of electrical input energy to laser output energy at all three wavelengths can allow the design of competitively sized and priced laser products. Products based on sub-sets of the elements described herein can also be clinically useful and commercially viable.
To minimize thermal injury to tissue surrounding an eye and/or to an exposed surface of the target region, the delivery system (e.g., handpiece 200) can include a cooling system for cooling before, during and/or after delivery of radiation. Cooling can include contact conduction cooling, evaporative spray cooling, convective air flow cooling, or a combination of the aforementioned. In one embodiment, the handpiece 200 includes a skin contacting portion that can be brought into contact with the skin. The skin contacting portion can include a sapphire or glass window and a fluid passage containing a cooling fluid. The cooling fluid can be a fluorocarbon type cooling fluid, which can be transparent to the radiation used. The cooling fluid can circulate through the fluid passage and past the window to cool the skin.
A spray cooling device can use cryogen, water, or air as a coolant. In one embodiment, a dynamic cooling device can be used to cool the skin (e.g., a DCD available from Candela Corporation). For example, the delivery system can include tubing for delivering a cooling fluid to the handpiece 200. The tubing can be connected to a container of a low boiling point fluid, and the handpiece 200 can include a valve for delivering a spurt of the fluid to the skin. Heat can be extracted from the skin by virtue of evaporative cooling of the low boiling point fluid. The fluid can be a non-toxic substance with high vapor pressure at normal body temperature, such as a Freon or tetrafluoroethane.
The invention has been described in terms of particular embodiments. The alternatives described herein are examples for illustration only and not to limit the alternatives in any way. The steps of the invention can be performed in a different order and still achieve desirable results. Other embodiments are within the scope of the following claims.
This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/848,083 filed Sep. 29, 2006, which is owned by the assignee of the instant application and the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60848083 | Sep 2006 | US |
