U.S. patent application number 09/797124 was filed with the patent office on 2001-07-26 for laser treatment of wrinkles.
This patent application is currently assigned to Cynosure, Inc.. Invention is credited to Ceccon, Harry L., Furumoto, Horace W., Rizzo, Antonio G..
Application Number | 20010009998 09/797124 |
Document ID | / |
Family ID | 23834596 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009998 |
Kind Code |
A1 |
Furumoto, Horace W. ; et
al. |
July 26, 2001 |
Laser treatment of wrinkles
Abstract
A method for treating simple wrinkles caused by age or sun
exposure comprises treating blood vessels in the wrinkles with
laser light. The laser light may have a wavelength between 577 and
585 nm. The laser light may also be a laser light pulse having a
pulse duration that is greater than 0.2 msec, or alternatively,
greater than 0.5 msec. The output pulse may be generated with a dye
laser, for instance, by exciting dye solution in a resonant cavity
with one or more flashlamps. A further treatment method comprises
irradiating wrinkle-bearing skin with a laser pulse where the pulse
duration is selectively matched to the thermal relaxation time of
blood vessels in the targeted skin.
Inventors: |
Furumoto, Horace W.;
(Wellesley, MA) ; Ceccon, Harry L.; (Boston,
MA) ; Rizzo, Antonio G.; (Nashua, NH) |
Correspondence
Address: |
James M. Smith
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
Two Militia Drive
Lexington
MA
02421-4799
US
|
Assignee: |
Cynosure, Inc.
Chelmsford
MA
|
Family ID: |
23834596 |
Appl. No.: |
09/797124 |
Filed: |
March 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09797124 |
Mar 1, 2001 |
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08822448 |
Mar 21, 1997 |
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08822448 |
Mar 21, 1997 |
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08461952 |
Jun 5, 1995 |
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5624435 |
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Current U.S.
Class: |
606/9 |
Current CPC
Class: |
A61B 2018/00452
20130101; A61B 2017/00057 20130101; H01S 3/022 20130101; H01S 3/213
20130101; A61B 18/203 20130101; H01S 3/092 20130101; H01S 3/1312
20130101; A61B 18/20 20130101; A61B 2018/00476 20130101 |
Class at
Publication: |
606/9 |
International
Class: |
A61B 018/20 |
Claims
What is claimed is:
1. A method of treating simple wrinkles caused by age or sun
exposure comprising treating blood vessels in the wrinkles with
laser light.
2. The method as claimed in claim 1, wherein the laser light has a
wavelength between 577 and 585 nm.
3. The method as claimed in claim 1, wherein the laser light
comprises a laser light pulse having a duration greater than 0.2
msec.
4. The method as claimed in claim 1, wherein the laser light
comprises a laser light pulse having a duration greater than 0.5
msec.
5. The method as claimed in claim 1, wherein the laser light is
generated with a dye laser.
6. A method of treating simple wrinkles caused by age or sun
exposure comprising the steps of: generating a laser light output
pulse having a wavelength in a range of 577 to 585 nm; and
delivering the laser light output pulse to the wrinkle-bearing skin
of a patient.
7. The method as claimed in claim 6, wherein the laser light output
pulse has a pulse duration that is greater than 0.2 msec.
8. The method as claimed in claim 6, wherein the laser light output
pulse has a pulse duration that is greater than 0.5 msec.
9. The method as claimed in claim 6, wherein the laser light output
pulse is generated with a dye laser.
10. A method of treating simple wrinkles caused by age or sun
exposure comprising the steps of: generating a dye laser light
output pulse; and delivering the dye laser light output pulse to
the wrinkle-bearing skin of a patient.
11. The method as claimed in claim 10, wherein the step of
generating a dye laser light output pulse comprises: exciting dye
solution in a resonant cavity to produce output laser light.
12. The method as claimed in claim 11, wherein the dye solution is
excited by at least one flashlamp.
13. The method as claimed in claim 11 additionally comprising the
step of: circulating dye solution through the resonant cavity
during the generation of the laser light output pulse.
14. The method as claimed in claim 10, wherein the dye laser output
pulse has a pulse duration that is greater than 0.2 msec.
15. The method as claimed in claim 10, wherein the dye laser output
pulse has a pulse duration that is greater than 0.5 msec.
16. A laser treatment method for simple wrinkles caused by age or
sun exposure, the method comprising irradiating wrinkle-bearing
skin with a laser light output pulse having a pulse duration that
is selectively matched to a thermal relaxation time of blood
vessels in the targeted skin.
17. The method as claimed in claim 16, wherein the pulse duration
is greater than 0.2 msec.
18. The method as claimed in claim 16, wherein the pulse duration
is greater than 0.5 msec.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
08/822,448, filed Mar. 21, 1997, which is a division of application
Ser. No. 08/461,952, filed Jun. 5, 1995, and now U.S. Pat. No.
5,624,435. The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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, focused 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.
[0008] 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
[0009] The present invention relates generally to method for
treating simple wrinkles caused by age or sun exposure by treating
blood vessels in the wrinkles with laser light. In specific
embodiments, the laser light has a wavelength in a range between
577 and 585 nanometers. The laser light may also comprise a laser
light pulse with a pulse duration that is greater than 0.2
milliseconds, or in certain embodiments, a pulse duration that is
greater than 0.5 milliseconds. A dye laser may be employed to
generate the laser light.
[0010] According to one aspect, a method for treating simple
wrinkles caused by age or sun exposure comprises generating a laser
light output pulse having a wavelength between 577 and 585
nanometers and delivering the output pulse to the wrinkle-bearing
skin of a patient. The output pulse may be generated with a dye
laser, and may also have a pulse duration that is greater than 0.2
milliseconds, or, in certain embodiments, greater than 0.5
milliseconds.
[0011] According to another aspect, a method for treating simple
wrinkles caused by age or sun exposure comprises generating a dye
laser output pulse and delivering the pulse to the wrinkle-bearing
skin of a patient. The dye laser output pulse may be generated by
exciting dye solution in a resonant cavity with, for instance, one
or more flashlamps. Also, the dye may be circulated through the
resonant cavity while the pulse is generated. The output pulse may
have a pulse duration that is greater than 0.2 milliseconds, or, in
certain embodiments, greater than 0.5 milliseconds.
[0012] According to yet another aspect, a laser treatment method
for simple wrinkles caused by age or sun exposure comprises
irradiating wrinkle-bearing skin with a laser light output pulse.
In this embodiment, the pulse duration of the output pulse is
selectively matched to the thermal relaxation time of blood vessels
in the targeted skin. In certain embodiments, the pulse duration is
greater than 0.2 milliseconds. According to some embodiments, the
pulse duration exceeds 0.5 milliseconds.
[0013] 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
[0014] FIG. 1 schematically shows a selective photothermolysis
treatment system of the invention;
[0015] FIG. 2 is a schematic perspective view of a first embodiment
of the flashlamp-excited pulse dye laser 1 of the present
invention;
[0016] 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;
[0017] FIG. 4 is a circuit diagram of the flashlamp driver 162 of
the present invention;
[0018] FIGS. 5A and 5B show the differences between longitudinal
and transverse dye flow, respectively, through the resonant cavity
of a laser;
[0019] FIG. 6 schematically shows a dye cell 105 configured for
longitudinal dye flow through the dye cell; and
[0020] 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.
[0021] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] A description of preferred embodiments of the invention
follows.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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, now U.S. Pat. No.
5,668,824, 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 1 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.
[0041] 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.
[0042] 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.
[0043] 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 peak
power with the energy content required by medical applications.
[0044] 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.
[0045] While this invention has been particularly shown and
described 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
scope of the invention encompassed by the appended claims.
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