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BACKGROUND OF THE INVENTION
Vascular lesions, comprising enlarged or ectatic blood vessels, pigmented
lesions, and tattoos have been successfully treated with lasers for many
years. In the process called selective photothermolysis, the targeted
structure, the lesion tissue or tattoo pigment particles, and the
surrounding tissue are collectively irradiated with laser light. The
wavelength or color of this laser light, however, is chosen so that its
energy is preferentially absorbed by the target. Localized heating of the
target resulting from the preferential absorption leads to its
destruction.
Most commonly in the context of vascular lesions, such as portwine stains
for example, hemoglobin of red blood cells within the ectatic blood
vessels serves as the laser light absorber, i.e., the chromophore. These
cells absorb the energy of the laser light and transfer this energy to the
surrounding vessel as heat. If this occurs quickly and with enough energy,
the vessel reaches a temperature to denature the constituents within the
boundary of the vessel. The fluence, Joules per square centimeter, to
reach the denaturation of a vessel and the contents is calculated to be
that necessary to raise the temperature of the targeted volume within the
vessel to about 70.degree. C. before a significant portion of the absorbed
laser energy can diffuse out of the vessel. The fluence must, however, be
limited so that the tissue surrounding the vessel is not also denatured.
As suggested, simply selecting the necessary fluence is not enough. The
intensity and pulse duration of the laser light must also be optimized for
selectivity by both minimizing diffusion into the surrounding tissue
during the pulse while avoiding localized vaporization. Boiling and
vaporization lead to mechanical, rather than chemical, damage-which can
increase injury and hemorrhage in the tissues that surround the lesion.
This constraint suggests that for the fluence necessary to denature the
contents of the vessel, the pulse duration should be long and at a low
intensity to avoid vaporization. It must also not be too long because of
thermal diffusivity. Energy from the laser light pulse must be deposited
before heat dissipates into the tissue surrounding the vessel. The
situation becomes more complex if the chromophore is the blood cell
hemoglobin within the lesion blood vessels, since the vessels are an order
of magnitude larger than the blood cells. Radiation must be added at low
intensities so as to not vaporize the small cells, yet long enough to heat
the blood vessels by thermal diffusion to the point of denaturation and
then terminated before tissue surrounding the blood vessels is damaged.
Conventionally, flashlamp-excited dye lasers have been used as the laser
light source. These lasers have the high spectral brightness required for
selective photothermolysis and can be tuned to colors for which
preferential absorption occur. For example, colors in the range of 577 to
585 nm match the alpha absorption band of hemoglobin and thus are absorbed
well by the red blood cells in the blood vessels. The absorption of
melanin, the principal pigment in the skin, is poor in this range,
yielding the necessary selectivity.
The implementation of flashlamp-excited dye lasers presents problems in the
pulse length obtainable by this type of laser. Theory dictates that the
length of the light pulse should be on the order of the thermal relaxation
time of the ectatic vessels. Ectatic vessels of greater than 30 microns in
diameter are characteristic of cutaneous vascular lesions. These large
vessel have relaxation times of 0.5 msec and require pulse durations of
this length. Commercially available flashlamp-excited dye lasers generally
have maximum pulse lengths that are shorter than 0.5 msec. As a result,
selective photothermolysis treatment of ectatic vessels larger than 30
microns currently relies on higher than optimum irradiance to compensate
for the pulse duration limitations. This leads to temporary
hyperpigmentation, viz., purpura.
Attempts have been made to increase the pulse durations of
flashlamp-excited dye lasers. The Light Amplifier disclosed in U.S. Pat.
Nos. 4,829,262 and 5,066,293 was conceived by the present inventor to
mitigate laser quenching from thermal effects. The design centered on
developing a spatially non-coherent laser. Basically, the optics at each
end of the dye cell are designed to return substantially all of the light
emanating from the end aperture back through the dye cell and reflect off
the dye cell walls. Specific resonating and coherent modes are not
favored. The optics mix the rays and thoroughly homogenize the beam. Thus,
the effects from thermal distortions induced by the flashlamp are
mitigated since resonator modes are not required for lasing action to
occur. The invention of this patent does not generate a light that can be
concentrated to the degree obtainable by classic laser configurations.
But, the large depth of field and tightly focused beams that coherent
radiation provides are not necessary for many medical applications. In
treating vascular lesions, focussed spots a few millimeters in diameter
are adequate. It is often convenient to use fiber optic delivery systems
and all that is necessary is to be able to focus the energy from the long
pulse dye laser into a fiber approximately one millimeter in diameter.
Newer devices to treat vascular lesions are once again built according to
the typical laser paradigm, i.e. lasers that generate spatially coherent
light. It turns out that with optimization, these lasers generate pulse
lengths that can equal or exceed those achievable by the design producing
spatially incoherent radiation described above. Interestingly, dye choice
has a large impact on pulse duration. Reduction in dye degradation by
improving longevity through dye chemistry has enabled pulse durations
approaching 1.0 msec in commercially available devices.
SUMMARY OF THE INVENTION
It has been observed that the premature cessation of the lasing is caused
primarily by the degradation of the dye solutions. As a result, improved
dye solutions can yield some increases in pulse duration. Dye degradation,
however, can not be totally eliminated and other steps must be taken if
pulse durations of 5 msec and greater and having the fluences for medical
procedures are to be achieved.
The present invention is based in part upon the realization that if, in a
flashlamp-excited dye laser, the dye solution is replaced during lasing
with the proper speed, the extended pulses and fluences required for
medical procedures are possible in a single laser device. This operation
is achieved by triggering the flashlamp while a dye solution is being
circulated through the resonant cavity of the laser. If the flow velocity
of dye solution is great enough such that the new solution enters the
cavity before the solutions in the cavity are substantially spent,
ultra-long pulses with high fluences are possible. Specifically, longer
pulses of up to 50 msec can be achieved with energies of up to 50 Joules.
These high energies enable treatment with reasonable spot sizes, which
makes the invention relevant to dermal therapy.
According to one aspect, the invention features a flashlamp-excited dye
laser generating light pulses at a color and pulse duration required for
selective photothermolysis. This laser comprises a cell containing a laser
gain media located in a cavity. Dye solutions are typical examples of such
gain media. At least one flashlamp is provided to excite the gain media
contained in the cell. A circulator is used to circulate the gain media
through the cell. Finally, a controller coordinates operation by
triggering the flashlamp to excite the laser gain media while the
circulator is circulating the gain media through the cell. This generates
the laser light pulse with a duration of at least one millisecond. Or,
another way, the flashlamp excites the laser gain media for a duration of
the time in which noncirculated laser gain media in the cell would be
exhausted and would quench the output laser light. But since the media is
circulated, the pulse duration is extended.
For some applications, the duration of the output laser light pulse is
preferably at least five milliseconds. Generally the energy of the pulse
is less than twenty Joules. Further, the laser light pulses are generated
with a repetition rate of about 1 Hertz, and usually less than three times
a second.
In specific embodiments, the circulator replaces gain media in the dye cell
with new gain media at least once during a duration of the output laser
light pulse, and preferably more than once. This operation ensures that
the laser output will not be quenched by accumulation of exhausted dye
solutions, for example. The gain media flow through the dye cell can be
transverse to the laser axis, or it can be longitudinal, parallel to the
axis. Preferably, if the longitudinal configurations are implemented, a
plurality of media input ports should be provided along the cell. A
plurality of media output ports are also probably necessary to allow flow
out of the cell. The dye cell segments between the adjacent inlet and
outlet ports is ideally short so that the residence time of the flowing
gain media through the dye cell segment is several times shorter than the
laser pulse duration.
In the transverse flow embodiment, the gain media flows between two
parallel or nearly parallel transparent cell walls, which allows the
excitation light to enter the dye cell. The transparent cell wall are long
in the direction of the flashlamps and laser resonator axis and shorter in
the direction of the flow. The gain media flows perpendicular to the long
axis of the window and is contained within the flashlamp windows and
within another set of windows which allow the laser light to reflect
between mirrors that comprise the laser resonator.
According to another aspect, the invention can also be characterized in the
context of a method of operation for a flashlamp-excited dye laser. Such a
method comprises exciting the dye solution in the resonant cavity with a
flashlamp and then generating a laser light output pulse from the resonant
cavity with the excited dye solution. The excitation at least partially
exhausts the dye solution. To counteract this effect, some of the at least
partially exhausted dye solution is replaced in the resonant cavity with
new dye solution during the duration of the laser light output pulse and
the new dye solution excited in the resonant cavity. This extends the
duration of the laser light output pulse beyond a time at which the
original dye solution in the resonant cavity cell would be exhausted and
would quench the output laser light pulse if the original dye solution
were never replaced.
In general, according to still another aspect, the invention features a
pumping device driver for a dye laser, for example. This driver comprises
a sensor for detecting an amplitude of a laser light output pulse from the
laser. A circuit is then used to regulate power supplied to a pumping
device, which is exciting the gain media of the laser, in response to the
amplitude detected by the sensor.
In specific embodiments, the pumping device is a flashlamp and the laser is
a dye laser.
The above and other features of the invention including various novel
details of construction and combinations of parts, and other advantages,
will now be more particularly described with reference to the accompanying
drawings and pointed out in the claims. It will be understood that the
particular method and device embodying the invention is shown by way of
illustration and not as a limitation of the invention. The principles and
features of this invention may be employed in various and numerous
embodiments without the departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, reference characters refer to the same parts
throughout the different views. The drawings are not necessarily to scale;
emphasis has instead been placed upon illustrating the principles of the
invention. Of the drawings:
FIG. 1 schematically shows a selective photothermolysis treatment system of
the invention;
FIG. 2 is a schematic perspective view of a first embodiment of the
flashlamp-excited pulse dye laser 1 of the present invention;
FIG. 3 is a timing diagram showing the relationship between the trigger
signal from the controller 160, the flashlamp driving current, and the
laser pulse amplitude for one pulse of the dye laser 1;
FIG. 4 is a circuit diagram of the flashlamp driver 162 of the present
invention;
FIGS. 5A and 5B show the differences between longitudinal and transverse
dye flow, respectively, through the resonant cavity of a laser;
FIG. 6 schematically shows a dye cell 105 configured for longitudinal dye
flow through the dye cell; and
FIG. 7 schematically shows a dye cell 105 configured for longitudinal dye
flow and having multiple input 610-614 and output ports 620-624 to reduce
the residence time of dye solution in the dye cell 105.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, FIG. 1 shows a selective photothermolysis
treatment system 10, Which has been constructed according to the
principles of the present invention. A flashlamp-excited pulse dye laser 1
for the system 10 generates an output laser light pulse 120. The output
laser light pulse 120 is coupled into a medical delivery system 20, such
as a single optical fiber, and transported to the skin 50 or other tissue
of a patient. The output laser light pulse 120 achieves substantial
penetration to treat a vascular lesion 60. This lesion 60 could be of one
of many different types such as portwine stain birthmarks, hemangiomas,
telangiectasia, idiopathic vulvoddynia, and leg veins. Further, it could
also be vessels in simple wrinkles, caused by age or sun exposure, blood
vessels in scar tissue, or hair follicles.
The pulse durations of the output laser light pulse 120 are matched to the
thermal relaxation time of the targeted ectatic vessels. Generally, this
requires durations greater than 0.2 msec. For vessels of 30 microns in
diameter and larger, as are present in portwine stains of adult patients,
the duration should ideally exceed 0.5 msec, whereas pulse durations of 1
msec to 10 msec should be selected when the vessels are larger than 100
microns.
FIG. 2 is a schematic diagram illustrating the flashlamp-excited pulse dye
laser 1 in more detail. As is generally common among most such lasers, a
dye cell 105 for containing a liquid laser gain media, specifically a dye
solution, extends longitudinally along a center axis 108 of the laser 1. A
front window 130 and a rear window 132 define the longitudinal extents of
the dye cell 105. Both windows 130 and 132 are transparent. The dye cell
105 is located in a resonant cavity 110, the ends of which are defined by
a first mirror 112 and a second mirror 114. Usually, the cavity does not
support only single longitudinal mode or single frequency. While the
second mirror 114 is fully reflective, the first mirror 112 is partially
reflective and partially transmissive, defining an output aperture 116. As
a result, a portion of the light generated in the resonant cavity 110
passes through this first mirror 112 as the output beam 120 of the laser
1.
The dye solution in the dye cell 105 is optically pumped by flashlamps 124a
and 124b. Exterior to a light-transmissive left side wall 122a of the dye
cell 105 is a left flashlamp 124b. A right flashlamp 124a is on an
exterior side of a right side wall 122b, which is also transmissive to
light. These flashlamps 124a, 124b generate broadband light that excites
the dye solution contained in the dye cell 105. This results in the
stimulated emission of light from the excited dye solution. Right and left
reflectors 126a and 126b surround the respective flashlamps 124a and 124b
to maximize the light injected into the dye cell 105. These reflectors can
be elliptical or diffuse.
According to the invention, the flashlamps 124a and 124b used in the
present invention preferably have higher pulse energies than typically
found in short pulse dye lasers. During the generation of an output laser
light pulse of 5 msecs, the total pumping energy injected into the dye
solution by the flashlamps is approximately 2000 Joules.
A dye circulator functions to circulate dye solution through the dye cell
105 while that dye solution is being excited by the flashlamps 124a, 124b.
This operation enables a flashlamp-excited pulse dye laser 1 to extend the
duration of the output laser light pulse 120 beyond that would be
obtainable in a dye laser in which the degraded dye was not replaced
during the laser pulse. For example, in a conventional laser, the
degradation of the dye during the output laser light pulse would quench
the lasing action within usually about 0.5 msec. In the present invention,
the duration of the output laser light pulse 120 is increased beyond this
quench time of the conventional laser by essentially injecting new dye
into the resonant cavity to replace degraded dye that absorbs laser light
and quench laser action and thus increase the pulses duration. In the
embodiment shown, this circulator includes a dye pump 150 which receives
new dye solution from a supply reservoir 152. The dye is pumped into a
supply manifold 154 (shown here in phantom), which distributes the dye
solution flow along the longitudinal axis 108 of the dye laser 1. The dye
solution flows through the dye cell 105, and thus the resonant cavity 110,
in a direction transverse to the axis 108 of the laser 1. A collection
manifold 156 (in phantom) collects the dye solution after it has passed
through the dye cell 105 and directs it to a depleted dye reservoir 158.
A separate supply reservoir 152 and depleted dye reservoir 158 are not
strictly necessary. Recirculation and filtration systems are possible.
U.S. patent application Ser. No. 08/165,331, filed on Dec. 10, 1993,
entitled Method and Apparatus for Replenishing Dye Solution in a Dye
Laser, which is incorporated herein by this reference, is directed a
system in which by-products from the lasing process are filtered out and
the dye solution reused.
A controller 160 coordinates the operation of the dye pump 150 and the
triggering of the flashlamps 124a and 124b to achieve extended pulse
durations of the output laser light 120 by replacing exhausted dye
solution in the dye cell 105 during the laser pulses. Specifically, the
controller 160 first establishes a steady state flow of dye solution
through the dye cell 105 by activating the dye pump 150. When the dye
solution is flowing through the dye cell 105, the controller 160 then
sends a trigger signal to a flashlamp driver 162. The trigger signal
defines the pulse durations and causes the flashlamp driver 162 to supply
a driving current to the flashlamps 124a and 124b. Light from the
flashlamps excites the dye solution to lase and produce the output laser
light 120.
Constant amplitude output laser light pulse are produced with an intensity
detector 164 that senses the intensity of the output laser light 120 and
provides feedback to the flashlamp driver 162. Typically, the detector can
be a diode or other photodetector that generates an intensity signal
indicative of the amplitude of the output laser light. This signal is
received by the flashlamp driver 162. There, the feedback signal is
combined with the trigger signal. This allows the flashlamp driver to
adaptively modify the level of the driving current to the flashlamps 124a,
124b in response to the instantaneous intensity of the output laser light.
If the gain medium contains depleted dye, an increase in excitation is
required to maintain constant output. If depleted dye can be removed
quickly, the excitation pulse will remain nearly constant.
Usually, some exhausted dye solution tends to accumulate in the dye cell
105 over the course of the pulse. In fact, even with fast circulation, the
percentage of new, unexhausted, dye is never as large as the moment before
the flashlamps are first driven. At least some of the light generated in
the dye cell 105 is absorbed by this exhausted dye solution and this
effect tends to increase the threshold level of excitation needed for
lasing. The intensity detector 164 detects any reduction in output light
amplitude and causes the flashlamp to be driven harder to maintain
constant output levels. Thus, the driving current is varied to maintain a
constant amplitude in the output light amplitude. Alternatively, ramp
trigger pulse can be used to generate an increasing or decreasing
intensity in the output laser light, which is optimal for some
applications.
Longer pulse durations are possible by circulating dye solution through the
dye cell during the generation of the output laser light pulse while
providing very intense exciting energies from the flashlamps 124a and
124b. The maximum obtainable pulse durations without replenishing depleted
dye are approximately 2.5 msec. This is achieved by using special
long-lived dyes. Using the same dyes in the present invention pulse
durations of 5.0 msecs are achieved by replacing the dye solution in the
dye cell 105 at least twice during the pulse. As a result, as the dye
solution becomes partially or completely exhausted, new solution is added
to the cell 105 to replace the old solution, which is pumped out by the
circulator. In the present invention, the speed at which the dye is
replaced in the dye cell 105 is dependent upon the how quickly the dye
degrades. If the dye is exhausted after 2.5 msec, it must be replaced
within that time. The total number of times that the dye is replaced in
the dye cell 105 depends upon the required pulse duration. For example, a
pulse duration of 10 msec, requires the equivalent of at least four dye
replacements with dye lifetimes of 2.5 msec.
Photothermolysis treatment of larger ectatic vessels, for example, require
the longer pulse durations obtainable by the present invention. Vessels of
100 and 200 micrometers in diameter have thermal relaxation times of 4.8
and 19.0 msec, respectively, and require similar pulse durations for
optimally effective therapy. Energies are usually from 1 to 20 Joules, but
fifty Joules can be required in hair removal applications.
FIG. 3 shows trigger signal voltage, the flashlamp excitation in Amperes,
and the laser pulse amplitude 120 as a function of time during the pulse
generation. Specifically, the controller 160 first engages the dye pump
150 to establish steady state dye flow through the dye cell 105 prior to
the beginning of the laser pulse. The controller 160 then sends the
trigger signal to the flashlamp driver 162. The length of this trigger
signal defines the desired duration of the output laser light pulse 120.
In the example shown, the duration is 5 milliseconds plus the latency time
T that is required to excite the dye solution to lase.
Prior to the trigger signal, the flashlamp driver 162 maintains a slightly
sub-operational current in the flashlamps 124a and 124b with a simmer
current 205 as is conventional. Then, in response to the leading edge 206
of the trigger signal, the flashlamp driver 162 produces a driving current
for the flashlamps 124a and 124b. The flashlamps, functioning as the
laser-pumping devices, pump the dye solution in the dye cell 105 into an
excited state causing it to lase when the fresh dye lasing threshold 208
is reached. This causes the output laser light pulse 120 having an
amplitude indicated by reference numeral 212. Generally, the flashlamp
driver 162 increases the current to the flashlamps 124a and 124b over the
duration of the output laser pulse in response the feedback signal from
the intensity detector 164. Progressively more driving current is required
due to the accumulation of degraded dye solution in the cell 105 which
yields an increasing lasing threshold 209. As some point, an equilibrium
in the ratio of degraded dye to fresh dye is reached and the lasing
threshold plateaues 211. Now, the excitation current is also steady state
210.
The resulting laser output 212 begins as the flashlamp power rises above
the threshold level 208, time T after the rising edge of the trigger
signal 206. The pulse terminates after five millisecond when the falling
edge 215 of the trigger signal is generated by the controller 160.
FIG. 4 is a circuit diagram of the flashlamp driver 162 shown in FIG. 2
that actively controls the level of driving of the flashlamps in response
to the intensity of the generated laser light. Specifically, the flashlamp
driver 162 receives the trigger signal from the controller 160 via
conductor 305. This trigger signal defines the time for which the
flashlamps will be driven and thus the duration of the laser light pulse.
The length of the laser light pulse is tunable by changing the length of
the trigger signal. This signal is received at a summing node 310 through
a resistor R1. The feedback signal, which is indicative of the intensity
of the output laser light 120, is received from the intensity detector 164
through a resistor R2 also at the summing node 310. The voltage of the
summing node is biased by third resistor R3 that is connected between the
summing node 310 and the supply voltage Vcc. In the particular embodiment
shown, the trigger signal is a low level active signal which pulls the
voltage of the summing node 310 below ground. A comparator 315 compares
the voltage of the summing node to the ground potential. Thus, in response
to a receipt of the trigger signal the comparator 315 turns a power
transistor such as an insulated gate breakdown transistor (IGBT) or power
Darlington 320 on, rendering the transistor conductive. This event places
the voltage of a high voltage power supply 325 across the flashlamp, which
generates a driving current to the flashlamps 124a and 124b. A capacitor
C1 stores charge to assist in driving the flashlamps 124a, 124b. A simmer
supply 340 is also connected across the flashlamps 124a and 124b to
provide a simmer current to maintain a stable voltage across the lamp
prior to the main excitation pulse. Without the simmer, operation is
erratic. This simmer current is evident from portion 205 of the flashlamp
excitation plot in FIG. 3.
The applicability of the flashlamp driver 162 is not limited to
flashlamp-excited dye lasers with dye circulators but can be implemented
as the driver for pumping devices that excite the gain media in many other
types of lasers. Many types of lasers suffer from an increased excitation
threshold across the length of a light pulse. Characteristically,
conventional flashlamp-excited dye lasers, without dye flow suffer from
this problem. This inventive pumping device driver 162 also find
applicability to these lasers and also laser-excited dye lasers. In those
cases, the flashlamp or other type of laser-pumping device will supply an
ever increasing excitation current in response to any loss of intensity at
the laser output.
FIGS. 5A and 5B illustrate the key differences between a longitudinal flow
dye laser and the transverse flow configuration. The first embodiment of
FIG. 1 corresponds to the transverse flow type of FIG. 5B. These
configurations generally provide shorter residence time of the dye
solution in the dye cell 105. The dye solution must merely move across the
width of the resonant cavity 110. The longitudinal flow configuration of
FIG. 5A offers an alternative. But, since the dye solution moves along the
length of the dye cell, resident time is longer for the same flow
velocity.
FIG. 6 illustrates a second embodiment of the dye cell 505 in which the dye
solution travels longitudinally along the length of the dye cell 505,
parallel to the laser axis 530. The dye solution is circulated through an
input port 510 by a pump 150. The dye travels the length l of the dye cell
505 and exits an output port 515. First and second mirrors 112, 114 define
the resonant cavity 520 in which the dye cell 505 is located as described
in connection with FIG. 1.
The second embodiment configuration places certain limits on the dye cell
505 construction. A given cross-section of fluid 550 should traverse the
length of the dye cell 505 in approximately 2.5 msec. This is a good
estimate for the useable lifetime of dye solutions during lasing. But,
velocity is limited by the pressure the dye cell 505 can withstand. A rule
of thumb is that a flow of 10 meters per second is the maximum speed for
pumps operating below 100 pound per square inch (psi). These factors limit
the length of the dye cell 505 to approximately one inch in length.
FIG. 7 shows a third embodiment based upon a modification of the second
embodiment of FIG. 6. Here, a plurality of dye input ports 610, 612, 614
are placed longitudinally along the length of dye cell 605. An input
manifold 625 of the circulator supplies dye to each of these ports from a
pump 650. Output ports 620, 622, 624 are placed between the input ports
610-614 on the opposite side of the dye cell 105. An output manifold 632
collects dye solution exiting the dye cell 605 through these ports. In
this configuration, dye flowing through any one of the input ports 610-614
is divided and passes out both of the nearest output ports 620-624, again
flowing parallel to the laser axis 630. If the longitudinal distance
between an input port and the closest output port is approximately 25 mm,
50 mm between adjacent input ports, a flow velocity of 10 m/sec is
sufficient to limit the residence time of the dye solution to 2.5 msec.
This allows the dye solution to be interchanged twice in a 5 msec laser
pulse duration or four times in a 10 msec pulse.
Dye Lasers having a transverse flow of dye gain media through the resonant
cavity have been developed in the past in a number of different contexts
for different applications. Continuous wave (cw) dye lasers have even been
developed. The dye in these lasers is pumped by another laser. This laser
is focused on a small spot on a curtain of the flowing dye solution. Thus,
volume of dye excited in this device is very small. Only the small portion
of the dye curtain in the path of the beam from the focused pumping laser
is excited, and therefore generates light by stimulated emission. Even
though this type of laser-excited dye laser generates a continuous wave
output, it can not produce the kilowatts of average power required by
medical applications.
Very high pulse rate transverse flow dye lasers have been developed for
isotope separation applications. The intent of these designs is to produce
output energies of approximately one Joule in a few microseconds. Thermal
distortion, which limited firing rates were avoided by replacing the
excited dye in the resonant cavity from a previous pulse with new dye and
then triggering the flashlamp. Such devices have been shown to generate
pulse frequencies of almost one kilohertz. In these industrial
applications, the peak and average output powers and pulse frequencies far
exceed those required for medical procedures where longer pulse durations,
moderate peak and average powers at lower frequencies are preferred.
Average power close to a kilowatt have been generated using transverse
flow dye lasers. For medical application, average power of only a few
Watts is required.
While this invention has been particularly shown and describe with
references 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. For example, the resonator optical
system could be integrated with the dye cell, making the cell coextensive
with the resonant cavity.
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