Electric heating – Metal heating – By arc
Reexamination Certificate
1998-10-26
2001-07-10
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121690
Reexamination Certificate
active
06259055
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to modifying the pulse of coherent energy used in shock processing of solid materials, and more particularly to methods and apparatus for improving the functionality, quality, and usefulness of a pulse of coherent energy in laser shock processing.
2. Description of the Related Art
Laser shock processing involves directing a pulse of coherent radiation to a piece of solid material to produce shock waves therein. The produced shock wave causes compressive residual stresses to form within the solid material. These compressive residual stresses improve the fatigue hardness and corrosion resistance properties of the solid material.
Laser shock processing utilizes a laser comprised of an oscillator, amplifiers, lenses, and irises. Depending on the type of oscillator used and the aperture size of the oscillator, the laser beam is either of a single transverse mode or a multi-transverse mode.
One problem with current lasers used in laser shock processing is potential damage that may occur to components downstream of a hard iris. Traditionally, the iris used in a laser is composed of a hard material, such as aluminum. When a laser beam exceeds the diameter of a hard iris, the intensity of the beam downstream is modulated and diffraction fringes are formed. The fringing is produced by diffracted coherent light interfering with the main beam. The diffraction fringes create hot spots, or areas of higher energy. These hot spots may lead to optical damage in components downstream or upstream. For example, amplified diffraction fringes may lead to damage to the laser gain medium or to laser optical components and their coatings.
An additional problem with current hard irises is that there is an increase in divergence of the beam as the beam passes through the hard iris. The divergence of the beam alters how the beam propagates, which in turn, produces a less uniform spacial distribution of beam output. The resulting beam output from a laser utilizing a hard iris is less effective in laser shock processing as compared to a more uniform spacial energy distribution of a non-diffracted beam output.
Another problem with current lasers used in laser shock processing is the creation of hot zones. This is especially a problem in multi-transverse mode laser oscillators. In multi-transverse mode oscillator lasers, areas of higher amplitude or hot zones, naturally occur on the outside edges of a cross-section of a laser beam. Amplification of a laser beam with hot zones further increases these hot zones which may result in possible damage to the optical components of the laser.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for reducing damage to a laser and its components in laser shock processing by reducing or eliminating diffraction fringes. In one specific embodiment of the present invention, an apodizer is used to improve the functionality, quality, and usefulness of a pulse of coherent energy used in laser shock processing. An apodizer is a device that scatters selected parts of a laser beam that passes through it. Because the edge of the apodizer is not smooth, any scattered light that reenters the main beam is now out of phase with respect to this beam. As a consequence, standing diffraction rings are significantly reduced or eliminated.
The invention, in one form thereof, is an apparatus for laser shock peening a workpiece. The apparatus includes a laser oscillator and amplifier means for increasing the energy of a laser pulse. There is a means for preventing damage to one of the laser oscillator and amplifier means. In alternate embodiments, the laser oscillator is either a single-transverse mode or a multi-transverse mode oscillator. In another embodiment, the means for preventing damage to the laser oscillator and the amplifier means comprises an apodizer and in a further embodiment, the apodizer may be one of the following: a phase plate, a serrated aperture, a birefringent beam shaper, an absorbent graded aperture or a reflective graded aperture.
The invention, in another form thereof, is an apparatus for laser peening a workpiece comprising a laser oscillator and an apodizer disposed within the laser oscillator. The apparatus also includes an amplifier means for increasing the energy of a laser pulse.
The invention, in yet another form thereof, is an apparatus for laser peening a workpiece comprising a laser oscillator and an amplifier means to increase the energy of a laser pulse. An apodizer is disposed within the amplifier means. In alternate embodiments, the apodizer may be a phase plate, a serrated aperture, a birefringent beam shaper, an absorbent graded aperture, or a reflective graded aperture.
In yet another embodiment, the present invention includes a method for laser peening a workpiece. The method includes generating a laser pulse from an oscillator. The laser pulse is modified to prevent damage to the oscillator and the pulse is amplified by the amplifier. The pulse is directed to the workpiece in one specific embodiment. In one embodiment, the laser pulse is modified to prevent damage to oscillator by including an apodizer within the oscillator. In an alternate embodiment, the method includes the step of modifying the laser pulse to prevent damage to the amplifier.
The invention, in another form thereof, is a method for laser peening a workpiece. The method includes the steps of locating an apodizer within an oscillator and generating a laser pulse from the oscillator. The pulse is amplified and directed to the workpiece. In alternate embodiments, the apodizer is one of a phase plate, a serrated aperture, a birefringent beam shaper, an absorbent graded aperture, or a reflective graded aperture.
The invention, in yet another form thereof, is a method for laser peening a workpiece. The method includes generating a laser pulse from an oscillator. An apodizer is located within the amplifier means and the laser pulse is amplified. The laser pulse is directed to the workpiece.
One advantage of the present system is the elimination of diffraction fringes. Diffraction fringes cause hot spots or hot areas of higher energy as measured across the laser beam cross-section. When these hot spots are amplified, the resulting beam may damage the optics and other components of the laser. In one embodiment, an apodizer is used to reduce or eliminate such diffraction fringes.
Another advantage of the present invention is that the divergence of the laser beam is not increased. Energy diverted from the beam pathway could be reflected back into the path of the laser, which in turn, may cause damage to optical components of the laser. Therefore, a decrease in divergence of the laser decreases the possibility of potential damage to laser components by such diverging beam.
An additional advantage of the present invention is the increase in energy uniformity of the beam output. Traditional lasers used in laser shock processing use a hard iris, with smooth edges, that increases the divergence of the beam and produces hot spots across the beam's cross-section. The present invention, in one embodiment, utilizes an apodizer, rather than a hard iris. As a result, hot spots are reduced thereby leading to a more uniform beam output.
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Dulaney Jeffrey L.
Kenney Patrick
Sokol David
Toller Steven M.
Evans Geoffrey S.
Knuth Randall J.
LSP Technologies Inc.
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