Surgery – Instruments – Light application
Reexamination Certificate
1997-12-12
2001-02-27
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S003000, C606S010000, C606S013000, C372S025000, C372S029011, C372S034000, C372S705000
Reexamination Certificate
active
06193711
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to pulsed solid-state lasers having output wavelengths in the mid-IR spectral region. The invention relates in particular to a rapid-pulsed erbium-doped YAG (Er:YAG) laser for use in medical procedures involving tissue ablation and incision.
DISCUSSION OF BACKGROUND ART
It has been known for some time that tissue ablation can be enhanced through the use of infrared wavelengths that more closely match absorption peaks of water, the major constituent in biological tissue. Accordingly, the industry has recently been exploring the use of erbium-doped gain media for medical applications. Erbium-doped YAG crystals will generate an output wavelength of about 2.9 micrometers (&mgr;m) which is matched to a prominent absorption peak in water. Er:YAG radiation can be particularly useful in cosmetic or aesthetic laser surgical applications such as skin resurfacing, as the high absorption coefficient for the radiation in water limits penetration depth of the radiation essentially to the skin.
In skin resurfacing, skin is removed by photoablation using a high energy pulse, for example, on the order of about 1 Joule (J). The pulse has a relatively short duration, for example about 200 to 500 microseconds (&mgr;s). Energy is applied in a beam having a diameter of about 2 to 3 millimeters (mm) at pulse rates between about 1 to 15 Hertz (Hz). The short pulse duration provides that no appreciable heat is generated, which limits collateral damage. The short penetration depth provides that healing (new skin growth) can be complete in about two weeks or less.
The short penetration depth of Er:YAG radiation is also potentially attractive for cosmetic procedures which require skin incision in sensitive areas. An example of such a procedure is blepharoplasty, for rejuvenating the appearance of aging eyelids. This procedure requires an incision to be made very close to an eyelid, consequently a very precise cut is required. To make such a precise cut, an Er:YAG laser beam is preferably focussed to a much smaller spot, for example, about 200 to 300 &mgr;m in diameter. Focussing to a small spot provides not only the required precision, but provides that less energy per pulse is required to reach an ablation threshold intensity. These low energy pulses are preferably delivered at a relatively rapid repetition rate, for example, preferably at least 50 Hz, and more preferably about 100 Hz or even greater.
Erbium laser systems operable at up to about 30 Hz are commercially available. Attempts to increase the repetition rate above this level, however, have met with problems in obtaining adequate output power as well as problems with the thermal loading and accompanying thermal-lensing of the laser gain-medium (laser-rod). Thermal-lensing results from heating of the rod by the portion of absorbed pump-light which is not extracted as laser energy and can lead to instabilities of the laser resonator, and reduce the upper limit of pump-power.
One scheme for providing repetition rates of about 100 Hz in a single resonator with reduced instability is described in U.S. Pat. No. 5,644,585. This scheme, however, involves decreasing output power by increasing the reflectivity (decreasing transmission) of a resonant cavity. The higher reflectance of the output coupling mirror decreases the flashlamp power required to provide lasing, which in turn reduces thermal loading on the laser-rod. For operating at pulse rates higher than 200 Hz, without increasing thermal loading problems, the outputs of two or more resonant cavities are interleaved such that pulses from one resonant cavity are delivered in an interval between pulses from another resonant cavity. This of course leads to a system of significantly increased cost and complexity compared with single resonant cavity system.
There remains a need for a pulsed Er:Yag laser capable of providing a high quality beam of relatively low energy pulses at pulse repetition rates up to about 200 Hz or greater for providing smooth, precise incisions, but which can also provide controllable higher energy pulses at lower repetition rates down to about 1 Hz. Such a laser could be used for a wide range of surgical procedures, including large area operations such as skin resurfacing which typically require high pulse energy. This could significantly reduce the capital and maintenance cost of laser equipment for a surgeon, which in turn could lead to reduced operation costs and increased access to laser surgical procedures for patients.
SUMMARY OF THE INVENTION
Deficiencies of above described prior art Er:YAG laser systems are overcome in a Er:YAG laser system in accordance with the present invention by maintaining thermal-lensing power in the Er:YAG gain medium essentially constant in a predetermined range rather than attempting to limit thermal-lensing power by limiting pump-power to the gain medium, and hence output-power of the system. This recognizes that it is changes in thermal-lensing power rather than thermal-lensing power itself that causes variations in output-beam characteristics of a laser resonator, and in the stability of such a resonator. A resonator can be designed to operate stably over a predictable range of thermal-lensing power by appropriate selection of gain-medium and resonator design characteristics. In a laser system in accordance with present invention an essentially constant time-averaged pump-power can be maintained while varying output-pulse power and repetition rate, within controllable limits. This provides that the gain medium can be strongly pumped for providing high energy output pulses.
In one aspect of the present invention, an Er:YAG laser system is arranged such that an Er:YAG gain medium in a resonant cavity is flashlamp pulse-pumped at constant average pump-power and at a pulse repetition rate which maintains a predetermined thermal-lensing effect in the gain-medium, while the energy-per-pulse of selected laser output-pulses of the resonant cavity is also maintained constant. Design characteristics of the resonant cavity and the gain medium are selected to compensate for the predetermined thermal-lensing effect.
In a preferred embodiment of the inventive laser system, laser output-pulse repetition rate of the resonator is controlled by controlling the flashlamp-pulse (exciting-pulse) repetition rate, and by providing a shutter in the resonant cavity which, when closed, prevents, and when open, allows generation of a laser output-pulse in response to a flashlamp-pulse. Laser output-pulse energy is controlled by controlling the duration of flashlamp-pulses which are allowed by the shutter to generate a laser output-pulse.
In another aspect of the present invention, the flashlamp is powered by a power supply including reservoir-capacitor bank, charged at a selected, essentially constant voltage, the reservoir-capacitor bank is connected to the flashlamp via a switch which controls the repetition rate and duration of flashlamp-pulses. Maintaining the capacitor bank at a constant voltage provides that pulse-current through the lamp is controlled at a selected, essentially constant level.
This arrangement allows that output-pulse energy can be varied from about 20 millijoules (mJ) to 3 Joules (J), and that output-pulse repetition rate can be varied from about 1 Hz to more than 200 Hz, all at an essentially constant average flashlamp-power on the order of 1 kilowatt (KW), while maintaining an essentially constant thermal-lensing power in the gain medium. Maintaining constant thermal-lensing power in turn provides for very stable operation, and an essentially constant output-beam divergence and beam propagation factor M
2
.
In yet another aspect of a laser system in accordance with the present invention, providing a stable controllable output-beam divergence makes the system particularly compatible with articulated-arm delivery of the laser output-pulses to tissue to be treated. Articulated arm delivery is preferred for the 2.94 &mgr;m radiation of an Er:YAG laser. Such an arm is t
Connors Kevin
Saunders Ralph
Spooner Greg
Coherent Inc.
Limbach & Limbach L.L.P.
Shay David M.
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