Optical: systems and elements – Optical frequency converter – Harmonic generator
Reexamination Certificate
1998-07-02
2001-01-09
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Harmonic generator
Reexamination Certificate
active
06172800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser wavelength conversion method and device for making the shape and energy distribution of converted laser beam point symmetrical, which is necessary in the application of laser beam, such as a semiconductor process and a micro scale manufacturing.
2. Description of the Prior Art
In the wavelength conversion of laser by using a wavelength conversion device which utilizes angular phase matching, the shape of the higher harmonic beam becomes elliptic, because the tolerance of the phase matching angle is anisotropic. Further, it is necessary to increase the power density by focusing the fundamental wave through a lens, thereby improving the conversion efficiency. In this case, the ellipticity of the converted harmonic wave
44
becomes greater as shown in FIG.
10
. Therefore, a cylindrical lens is used to compensate the beam shape. However, the cylindrical lens can not make the beam shape and the energy distribution perfectly point symmetrical. When the forth harmonic wave is generated by using a lens with a shorter focal length, thereby increasing the power density of the second harmonic wave, it becomes difficult to obtain a circular shaped beam, even if the cylindrical lens is used for compensating the beam shape, because the ellipticity of the forth harmonic wave becomes far greater.
It is preferable that the beam shape and energy distribution of the laser beam of which wavelength is converted is point symmetrical for the application to micro manufacturing. However, in the conventional wavelength conversion method, the shape of the converted laser beam becomes elliptic. Particularly, the beam is greatly deformed, if it is focused by the short focus lens. This causes problems in a high accuracy manufacturing, because the elliptic beam results in an elliptic shape in the exposed position. Further, the result of correction by the cylindrical lens is far from the point-symmetry.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a wavelength conversion method and device which converts the laser wavelength without forming the laser beam far from the point-symmetry.
In accordance with the present invention, there is provided a first mode of the laser wavelength conversion method for converting wavelength of the fundamental wave of laser light which is incident on a wavelength conversion device through a focusing lens, wherein two wavelength conversion devices are used in such an arrangement that the directions which give the minimum angular tolerance of the phase matching for the wavelength conversion devices are set to be orthogonal with each other. The fundamental wave is incident in succession on the above-mentioned two wavelength conversion devices in order to obtain harmonic wave output.
The first mode of the conversion method may include a step of adjusting the length of each wavelength conversion device in order to equalize the output from each device within an accuracy of 80 to 100%.
The first mode of the conversion method also may include a step of controlling the energy density of the fundamental wave in each conversion device in order to equalize the output from each device within an accuracy of 80 to 100%.
The first mode of the conversion method further may include a step of controlling the energy density of the fundamental wave in each conversion device by changing the distance between the focusing lens and the surface of each conversion device.
The first laser mode of the wavelength conversion method of the present invention further may include a step of rotating the phase matching axis of each wavelength conversion device by 180° with each other.
The first mode of the laser wavelength conversion device of the present invention is a united structure of the first wavelength conversion device and the second wavelength conversion device.
The above-mentioned first mode of the laser wavelength conversion device of the present invention is constructed by integrating the two wavelength conversion devices by one of such bonding methods as optical contact, fusion, adhesion, or molding.
A second mode of the conversion method of the present invention for laser wavelength conversion is a method of obtaining the forth harmonic wave, which comprises the steps of obtaining an elliptic second harmonic beam by allowing the fundamental wave to enter into the second harmonic generator after focusing the fundamental wave by a focusing lens, and obtaining the forth harmonic wave by allowing the second harmonic wave to enter into the forth harmonic generator, so that the apse axis of the elliptic second harmonic beam is arranged along the direction which gives the minimum phase matching angular tolerance.
The above-mentioned second mode of conversion method includes the step of converting the laser wavelength by using a united structure of the second harmonic generator and the forth harmonic generator.
The second mode of laser wave conversion device of the present invention is the united structure of the second harmonic generator and the forth harmonic generator which is used in the above-mentioned second conversion method.
The above-mentioned second mode of laser wavelength conversion device of the present invention is constructed by integrating the two wavelength conversion devices by one of such bonding methods as optical contact, fusion, adhesion, or molding.
As explained above, the laser wavelength is converted by using the two wavelength conversion devices, taking into consideration the phase matching orientations. When the conversion device with the great anisotropy of the phase matching angular tolerance is utilized, the beam pattern and energy distribution of the converted beam can be made almost point-symmetrical, even when the beam shape can not be compensated by the cylindrical lens. Thus, the obtained point-symmetrical beam is effectively applicable to, for example, a semiconductor device process and a micro machining. Further, the unified structure of the two wavelength conversion devices improves the working efficiency, because it is treated as a single device which is not needed to be adjusted in the angular arrangement.
Further, in the present invention, the shape of the second harmonic beam is made elliptic by using the second harmonic generator (SHG) with the great anisotropy of the phase matching angular tolerance, and the apse axis of the obtained elliptic beam is allowed to coincide with the orientation which gives the minimum phase matching angular tolerance in the fourth harmonic generator (FHG). Thus, an approximately circular fourth harmonic beam can be obtained, which could not be obtained by, for example, the cylindrical lens. Therefore, the working efficiency is also improved by the unified structure of the SHG and the FHG.
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Japanese Search Report dated Nov. 30, 1999 with Partial Translation.
Lee John D.
McGinn & Gibb PLLC
NEC Corporation
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