Abstract:
A medical laser system is disclosed for generating a pulsed output beam of variable pulse duration and wavelength. The on time of the laser is the pulse duration which is generated by a Q-switch operated in a repetitive mode as a train of micropulses. According to one embodiment, a repetitively Q-switched frequency-doubled solid state laser produces an input beam which is subsequently used to excite a dye laser. An excitation source of the solid state laser is modulated to control the pulse duration of the input beam. The dye laser receives the input beam and responsively generates an output beam of adjustable wavelength having a pulse duration corresponding to the pulse duration of the input beam. The wavelength of the output beam is controlled by adjusting a tuning element of the dye laser. The dye laser is coupleable to a delivery system for directing the output laser beam to a biological tissue target. By carefully controlling the pulse duration and wavelength of the beam delivered to the target tissue, a user of the laser system may avoid or minimize damage to tissue adjacent to the target tissue.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to lasers, and more particularly to dye lasers suitable for medical therapies such as selective photothermolysis and photodynamic therapy. 
     DESCRIPTION OF THE PRIOR ART 
     Many medical procedures employing lasers require relatively long pulse durations at specific wavelengths to achieve optimal results. Therapies such as laser removal of hair and vascular and pigmented lesions rely upon the selective photothermolysis of blood vesicles and/or cells. The principles of selective photothermolysis were first described by Anderson and Parrish in “Selective Photothermolysis: Precise Microsurgery by Selective Absorption of Pulse Radiation”,  Science , Vol. 220, pp. 524-27 (1983). 
     To provide selective photothermolysis in an efficient manner, the following three criteria must be satisfied: 
     (1) The target tissue must absorb more of the incident laser light than the adjacent tissue; 
     (2) The intensity of the laser light and the absorption of the target tissue must be sufficiently high to coagulate, kill, or vaporize the target tissue, and; 
     (3) The pulse duration of the laser light must be short enough to avoid or minimize overheating of adjacent tissue due to thermal diffusion from the target tissue, but long enough to prevent explosive boiling of the target and/or adjacent tissue. 
     The foregoing criteria are generally satisfied by matching the laser pulse duration and wavelength to the thermal and spectral characteristics of the target tissue. In order to accommodate targets of varying size and absorption coefficients, it is necessary to adjust pulse duration and wavelength over a fairly wide range. 
     To date, prior art lasers used or intended for use in selective photothermolysis and similar procedures have fallen into one of two categories: fixed pulse duration, variable wavelength lasers, and fixed wavelength, variable pulse duration lasers. The former category includes long pulse flashlamp pumped dye lasers (known as FLPDLs), various examples of which are disclosed in U.S. Pat. Nos. 5,066,293; 5,287,380; 5,624,435, and; 5,668,824. FLPDLs designed for medical use have maximum pulse durations of approximately 1.5 milliseconds, which limits their use to very small blood vessels. Another problem associated with FLPDLs is that while they allow adjustment of the output beam wavelength, they do not offer the ability to adjust the pulse duration over a clinically relevant range. 
     Examples of lasers in the second category (fixed wavelength, variable pulse duration lasers) include “Star Pulsed” KTP lasers available from Laserscope, Inc. of San Jose, Calif. These lasers employ a pulsed arc lamp to generate high intensity light having a wavelength of 532 nanometers and pulse durations ranging from 1-100 ms. Variable duration long pulse 532 nanometer light can also be generated using flashlamp-pumped lasers, such as the Versipulse laser available from Coherent Laser of Santa Clara, Calif. Although the pulse durations of these lasers render them suitable for treating medium- and large-sized blood vessels, the wavelength of the output beam of lasers of the foregoing description cannot be adjusted to match the spectral characteristics of the target tissue. 
     In view of the limitations of prior art lasers, there is a need for a medical laser system that offers the ability to adjust both pulse duration and wavelength in order to match thermal and spectral characteristics of the target tissue and thereby achieve highly efficient and effective results. 
     SUMMARY 
     The present invention provides a medical laser system having an output beam that can be adjusted over a clinically relevant range of pulse durations and wavelengths. 
     According to one embodiment, the medical laser system includes a solid state laser for generating an input beam of adjustable pulse duration. The solid state laser includes a laser medium, such as a neodymium: yttrium aluminum garnet (Nd:YAG) rod, which is pumped by an excitation source typically comprising an arc lamp. Light emitted by the laser medium is passed through an acousto-optic Q-switch and a frequency doubling non-linear crystal. Power supplied to the excitation source is modulated to produce pulses of a specified duration, each pulse comprising a train of repetitively Q-switched micropulses. 
     The input beam is coupled to a dye laser, either directly or through an optical fiber. The dye laser generates an output beam of adjustable wavelength having a pulse duration corresponding to the pulse duration of the input beam. The dye laser includes a dye cell onto which the input beam is focussed, and a tuning element, such as a birefringent filter. Adjustment of output beam wavelength is accomplished by changing the dye composition and/or varying the filter orientation. A portion of the output beam may be split off and diverted to power and wavelength detectors, which provide feedback signals to a control processor driving the excitation source, Q-switch and tuning element. A conventional delivery system, which may comprise an optical fiber and associated focussing optics, is coupleable to the dye laser and serves to direct the output beam onto a biological tissue target. 
     In accordance with certain embodiments of the medical laser system, the wavelength of the output beam produced by the dye laser may be varied between 550 nanometers and 750 nanometers, and the pulse duration varied between 0.1 and 900 milliseconds. A clinician operating the laser may thus adjust the beam characteristics in order to achieve optimal selective photothermolysis of the target tissue. The medical laser may also be advantageously used for a variety of other therapies and procedures, including hair removal, drug activation in photodynamic therapy (PDT), and cutting and/or drilling tissue. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     In the accompanying drawings: 
     FIG. 1 is a schematic of a medical laser system showing a frequency-doubled, repetitively Q-switched solid state laser coupled to a dye laser; 
     FIG. 2 is a cross-sectional view of a lamp housing, arc lamp and laser medium; 
     FIG. 3 is a schematic of a control system for controlling and monitoring the operation of the medical laser system; 
     FIG. 4 is a graph depicting the structure of an output beam pulse; and 
     FIG. 5 is a block diagram of a second embodiment of the invention, wherein the solid state laser is coupled to the dye laser via an optical fiber. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic of a medical laser system  100  in accordance with one embodiment of the invention. Medical laser  100  generally includes a frequency doubled solid state laser  102  for generating an input laser beam of variable pulse duration, and a dye laser  104  for receiving the input beam and responsively emitting an output laser beam of variable wavelength having a pulse duration substantially equal to the pulse duration of the input beam. A conventional delivery system (not depicted), coupled to dye laser  104 , directs the output beam to a biological tissue target. In the embodiment depicted in FIG. 1, solid state laser  102  and dye laser  104  are disposed within a common housing, and are optically coupled by mirrors  108 ,  110  and  112 , which direct and focus the input beam generated by solid state laser  102  onto dye cell  114 . As will be discussed below in connection with FIG. 4, other embodiments of the medical laser system may utilize a solid state laser and a dye laser which are physically separate and are optically coupled by an optical fiber or similar expedient. 
     Solid state laser  102  includes a laser medium  116 , Q-switch  118 , and frequency doubler  120  positioned along an optical path  119  defined by end mirror  122 , turning mirror  123 , output coupler  124  and end mirror  125 . Laser medium  116  preferably comprises an elongated member or rod of Nd:YAG material, but may alternatively comprise any suitable material wherein transition metal ions (such as chromium, titanium, and lathanide) are disposed within a crystalline host (such as YAG, yttrium lithium fluoride (YLF), sapphire, alexandrite, spinel or YAP). As depicted in FIG. 2, laser medium  116  is positioned proximal to an excitation source preferably comprising a krypton arc lamp  202 , which pumps laser medium  116  to cause it to begin emitting light. Laser medium  116  and arc lamp  202  are, preferably arranged in mutually parallel relation within lamp housing  204 , although other geometries and configurations known in the art may be employed. Lamp housing  204  may be adapted with liquid coolant channels that communicate with a coolant recirculation pump in order to prevent overheating of arc lamp  202  and laser medium  116 . Those skilled in the art will recognize that other excitation sources, such as flashlamps or laser diodes, may be substituted for arc lamp  202 . As will be described below in further detail in connection with FIGS. 3 and 4, arc lamp  202  is modulated by varying the amount of energy supplied thereto in order to adjust a pulse duration of the input beam generated by solid state laser  102 . 
     Frequency doubler  120  preferably comprises a nonlinear crystal such as a KTP or LBO crystal. The frequency doubler is operative to double the frequency of laser light generated by laser medium  116 . In accordance with one embodiment of medical laser system  100 , a Nd:YAG laser medium  116  emits infrared light having a wavelength of 1064 nanometers which is frequency doubled by a non-linear crystal frequency doubler  120  to produce green light having a wavelength of 532 nanometers. The green light travels along optical path  119  and is extracted by output coupler  124  to supply an input beam to dye laser  104  along path  130 . Although FIG. 1 depicts frequency doubler  120  as being located within the resonator cavity defined by end mirrors  122  and  125 , frequency doubler  120  may alternatively be located in beam path  130  exterior to the resonator cavity. 
     End mirrors  122  and  125  and turning mirror  123  are highly reflective (&gt;99.5%) at both the 532 nanometer (green light) and 1064 nanometer (infrared light) wavelengths, whereas output coupler  124  is highly reflective at the 1064 nanometer wavelength and transmissive at the 532 nanometer wavelength. In the preferred embodiment depicted in FIG. 1, end mirrors  122  and  125  are flat, and turning mirror  123  and output coupler  124  are provided with concave reflecting surfaces which cause the beam at the output of laser medium  116  to be relay imaged onto frequency doubler  120 . 
     The resonator design of solid state laser  102  is described in further detail in U.S. Pat. No. 4,907,235 (“Intra-cavity Beam Relay for Optical Harmonic Generation” to Kuizenga) and in U.S. Pat. No. 5,151,909 (“Frequency Doubled Laser Having Programmable Pump Power Modes and Method for Controllable Lasers” to Davenport et al.), both of which are incorporated by reference. 
     Q-switch  118  is preferably of the acousto-optic type, but may alternatively comprise a mechanical device such as a rotating prism or aperture, or an electro-optical device. In a preferred embodiment of solid state laser  102 , Q-switch  118  is operated in a repetitive mode to cause a train of micropulses to be generated. The micropulse repetition rate may typically be varied in the range of 1-100 kilohertz. As will be discussed below, the duration of each train of micropulses, collectively deemed a pulse, is governed by the duration of operation of arc lamp  202 . 
     Frequency doubled light extracted by output coupler  124  is thereafter directed along path  130  defined by turning mirrors  108 ,  110  and  112 , which are highly reflective at the 532 nanometer wavelength. In the embodiment depicted in FIG. 1, turning mirrors  108  and  110  are flat, wherein mirror  112  has a concave reflecting surface to focus the beam onto dye cell  114  of dye laser  104 . 
     Dye laser  104  includes an end mirror  132  and a dye output coupler  134  defining an optical path  136  in which are located dye cell  114  and a tuning element  138 . Dye cell  114  may comprise conventional laterally opposed sapphire, YAG, quartz or glass windows defining a volume through which a dye solution is continually pumped at high speed and excited by the input beam, causing the dye solution to fluoresce and emit light. Conversion efficiencies in preferred embodiments of laser system  100  are typically about 30-50 percent (in contradistinction to flashlamp-pumped dye lasers, which typically exhibit conversion efficiencies on the order of &lt;5 percent). A variety of dye solutions which may be used in dye laser  104 , including (without limitation) R6G, Rhodamine 590, Rhodamine 575, and Sulfa Rhodmine 660. 
     As in conventional dye lasers, tuning element  138  may be provided to tune the laser output within the gain curve of the selected dye solution. Tuning element  138  is generally capable of reducing the bandwidth of the beam to &lt;2 nanometers, and is used to match the output beam to the spectral characteristics of the target tissue. For example, in one embodiment of laser system  100 , the wavelength of the output beam may be varied between 570 and 620 nanometers, although different and wider ranges are achievable, depending on the dye solution selected and the performance of other system components. Tuning element  138  may preferably comprise a birefringent filter having a rotational orientation controlled by an associated stepper motor. Alternatively, tuning element  138  may comprise an etaloh, prism, filter, or other suitable device. 
     Dye output coupler  134  is configured to reflect a portion of the beam incident thereon along path  136 , and to transmit the remainder to produce an output beam. Depending on system requirements, the percentage of the beam reflected by dye output coupler will vary between 10 and 95 percent. 
     Beam splitter  150  is disposed in the path of the output beam and is configured to direct a small portion of the beam onto wavelength detector  152  and to transmit the remainder of the beam. Similarly, beam splitter  154  directs a small portion of the output beam onto power detector  156  and transmits the remainder. Wavelength detector  152  and power detector  156  are of conventional design and are operative to generate output signals representative of, respectively, the wavelength and power of the output beam for use in a control feedback loop. 
     An automatically or manually actuated shutter  160  is operable to selectively block the output beam when emission of the output beam from laser system  100  is undesirable. A conventional fiber coupling  162  is utilized to couple laser system  100  to a delivery system (not depicted) capable of directing the output beam onto the tissue target. The delivery system, examples of which are well known in the art and hence need not be discussed in detail herein, generally comprises an optical fiber or articulated arm extending between a proximal end received by and removably secured to fiber coupling  162 , and a distal end secured to a handpiece (such as the DermaStat™ handpiece available from Laserscope, Inc.) or scanner for focusing or scanning the output beam onto the target. 
     FIG. 3 is a block diagram of a control system  300  for controlling and operating laser system  100 . Control system  300  includes a control processor  302  which receives input from a variety of sources including user controls  304 , power detector  156  and wavelength detector  152 . User controls  304  are provided to allow the user to control and adjust various aspects of the operation of laser system  100  so as to achieve optimal results for a given therapeutic procedure. User controls  304  may include, for example, a power or fluence control  310 , pulse duration control  312 , repetition rate control  314 , and wavelength control  316 . In one embodiment of laser system  100 , user controls  304  are adjusted via through an alphanumeric user interface. 
     Control processor  302  is configured to process input received from user controls  304  and detectors and to accordingly generate output signals for adjusting characteristics of the output beam to match the user inputted values. Control processor  302  may comprise, for example, a general-purpose processor which executes control software embodying a set of algorithms specifying input-output relationships. In particular, control processor  302  generates and transmits output signals to power supply  306 , Q-switch  118  (or alternatively, a driver which supplies power to Q-switch  118 ), dye cell tuning element  138 , and shutter  160 . The output signal delivered to power supply  306  modulates the energy supplied to arc lamp  202 , which in turn controls the average power and pulse durations of the input and output beams. The output signal delivered to Q-switch  118  controls the repetition rate of the component micropulses. The output signal delivered to tuning element  138  controls positioning or other operational characteristics to thereby adjust the wavelength of the output beam emitted by dye laser  104 . Finally, the output signal delivered to shutter  160  controls its actuation to selectively block the output beam path when emission of the beam from medical laser system  100  is undesirable. 
     FIG. 4 is a graph depicting the amplitude of the output beam emitted by medical laser system  100  as a function of time. As discussed above, laser beam pulses  402  are each defined by trains of Q-switched micropulses  404 . While a relatively small number of micropulses  404  are depicted for purposes of clarity, an actual pulse  402  may comprise hundreds or thousands of component micropulses  404 . Each pulse  402  may contain up to 30 joules of energy. The pulse duration of pulses  402 , which is adjusted by modulating the operation of arc lamp  202 , may typically be varied in the range of 0.1 to 200 milliseconds, although pulse durations as high as 900 milliseconds may be achieved using laser system  100 . 
     FIG. 5 depicts a medical laser system  500  in accordance with an alternative embodiment of the invention. In contradistinction to laser system  100 , solid state laser  502  and dye laser  504  are housed separately and are optically coupled by an optical fiber  506  (rather than by turning mirrors  108 ,  110  and  112  of the FIG. 1 embodiment), which directs a frequency doubled, variable pulse duration input beam generated by solid state laser  502  onto a dye cell located within dye laser  504 . The proximal and distal ends of optical fiber  506  are received within and removably secured to fiber couplings  508  and  510 , which are provided, respectively, at solid state. In all other respects, solid state laser  502  and dye laser  504  are substantially identical to solid state laser  102  and dye laser  104 , as described above. 
     Those skilled in the art will recognize that by adjusting the wavelength and pulse duration of the output beam to appropriate values, medical laser system  100  may be effectively employed in connection with a variety of therapies and procedures. These therapies and procedures include, but are not limited to, removal of cutaneous and internal vascular lesions; hemostasis of bleeding ulcers; suppression of choroidal neovascularization leading to blindness; tattoo removal; hair removal; removal of arterial plaque; and, photodynamic therapy (PDT) for treatment of various tumors. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, 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 of the invention, as defined by the appended claims.