Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-05-21
2001-03-27
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S004000, C372S005000, C359S326000, C359S330000
Reexamination Certificate
active
06208673
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to laser systems and, more particularly, to a system capable of supplying multiple wavelengths in a variety of temporal formats using a single laser source.
BACKGROUND OF THE INVENTION
Lasers are presently used in a variety of applications ranging from medical procedures to micro-machining and photolithography. Lasers have gained popularity for these applications for a number of reasons. First, by varying the wavelength of the laser the absorption characteristics of the laser beam can be tailored to a specific purpose. Second, by altering the focal length and the focus of the beam, the depth and type of treatment can be varied. For example, the same laser beam can be used to either heat treat or cut a material, the differences due to the focus of the beam. Third, by controlling the intensity and pulse duration of the laser output, different levels of treatment can be easily achieved.
Recently the use of lasers has become commonplace in a variety of medical procedures including both ophthalmic and cosmetic treatments. For example, the field of corneal reshaping has been revolutionized through the use of ultraviolet (i.e., UV) lasers (e.g., photo-refractive keratectomy). In these operations a pulse of relatively short duration is desired, typically on the order of between 1 and 50 nanoseconds. In contrast to ophthalmic treatments, medical procedures requiring high tissue absorption (e.g., surgery, cosmetic skin rejuvenation and resurfacing, etc.) typically require a relatively long pulse duration, on the order of 100 to 1000 microseconds, with a wavelength of between 1 and 5 micrometers. Due to both the wavelength and the pulse duration of interest in this type of procedure, free running erbium lasers have been particularly useful.
U.S. Pat. No. 5,290,274 discloses a system that allows radiation of two different wavelengths to be simultaneously or individually applied to a given area. The two different wavelengths are produced by two different lasers, the individual wavelengths selected on the basis of absorption by a given organic tissue. The disclosed system also includes means for simultaneously directing a cooling fluid at the tissue being treated by the radiation sources. In the preferred embodiment of the disclosed system, the selected lasers are a Nd:YAG laser and an Er:YAG laser.
U.S. Pat. No. 5,655,547 discloses a technique of using multiple lasers in a surgical procedure. In this technique, the portion of tissue to be treated is simultaneously irradiated with multiple, coaxial laser beams. As disclosed, by selecting the wavelength of one laser such that the extinction length in the skin tissue is between 0.01 and 0.001 millimeters, and selecting the wavelength of a second laser such that the extinction length in the skin tissue is between 1 and 0.1 millimeters, the skin undergoes simultaneous ablation, coagulation, and shrinkage.
U.S. Pat. No. 4,791,927 discloses a technique of simultaneously irradiating a biological target with infrared radiation and ultraviolet radiation from a pair of lasers. The ultraviolet radiation cuts the target tissue while the infrared radiation cauterizes it. The use of alexandrite and xenon fluoride lasers is disclosed, as is the use of harmonic generators and Raman cells.
U.S. Pat. No. 5,144,630 discloses a multi-wavelength laser system utilizing a pulsed Nd:YAG or Nd:YLF laser. Various wavelengths are achieved through the use non-linear crystals. To achieve ultraviolet radiation, the output of the solid state laser is passed through a series of three non-linear crystals in order to obtain the fifth harmonic of the solid state laser. Infrared radiation is obtained from the solid state laser through the use of an optical parametric oscillation cavity. As the solid state laser is either Q-switched or mode locked, the pulse length of either the ultraviolet radiation or the infrared radiation ranges from subpicosecond to a few nanoseconds.
Although a variety of techniques have been developed for multi-wavelength laser systems, what is needed in the art is a laser system utilizing a single solid state laser and producing short pulses of ultraviolet radiation or long pulses of infrared radiation. The present invention provides such a system.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for obtaining multiple wavelengths of varying temporal format from a single solid state laser. According to the invention, the output from the solid state laser can either be sent through a series of non-linear crystals or through an optical parametric oscillation (i.e., OPO) cavity. Ultraviolet (i.e., UV) radiation is obtained from the non-linear crystals and infrared (i.e., IR) radiation is obtained from the OPO cavity. Short pulse temporal formats are achieved by operating the laser in the standard Q-switched mode or by mode locking the laser. Long pulse temporal formats are achieved by operating the solid state laser in a free running mode. The outputs from the UV and IR portions of the system can either be directed to the object (e.g., tissue) to be treated through a common beam delivery system or through separate beam delivery systems. Suitable beam delivery systems are preferably fabricated from optical fibers or waveguides.
In at least one embodiment of the invention, the laser source for the multi-wavelength system is comprised of a gain medium, such as Nd:YAG or Nd:YLF, that is contained within an optical cavity. The gain medium is pumped by either a diode laser or a diode array. Preferably an intra-cavity Q-switch is used to obtain the desired short pulse temporal format. Additionally, in the preferred embodiment of the invention the Q-switch can be disabled during the pump pulse, thereby allowing the laser source to operate in the free-running mode in order to achieve the long pulse temporal format.
UV radiation is obtained from the system by passing the output from the laser source through a series of non-linear crystals. In the preferred embodiment of the invention, three non-linear crystals are used in order to obtain the fifth harmonic of the laser source. Assuming a Nd:YLF source, the fifth harmonic has a wavelength of approximately 210.6 nanometers. The pulse width of the output from the non-linear crystals is between about 1 and 50 nanoseconds and in the preferred embodiment between about 7 and 15 nanoseconds. Although other wavelengths can be obtained from the non-linear crystals by using the output from either the first or the second non-linear crystal (i.e., the second or fourth harmonic), preferably the fifth harmonic from the third non-linear crystal is used.
IR radiation is obtained from the system by passing the output from the laser source through an OPO cavity. The preferred gain medium in the OPO cavity is periodically polled lithium niobate although other periodically polled crystals such as RTA, KTP, or barium titanate can also be used. Assuming that periodically polled lithium niobate is used, the output wavelength of the OPO is 2.9 micrometers. When the laser source is used to produce IR radiation, it is operated in a free running mode. As a consequence, the pulse width of the output from the OPO cavity is between about 100 and 1000 microseconds, and preferably between about 400 and 600 microseconds.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
REFERENCES:
patent: 4791927 (1988-12-01), Menger
patent: 5047668 (1991-09-01), Bosenberg
patent: 5144630 (1992-09-01), Lin
patent: 5290274 (1994-03-01), Levy et al.
patent: 5655547 (1997-08-01), Karni
Aculight Corporation
Beck David G.
Jr. Leon Scott
McCutchen Doyle Brown & Enersen LLP
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