Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2000-06-23
2001-09-11
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S305000
Reexamination Certificate
active
06288831
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser modulating optical system, which is used in a laser photo plotter or the like, including a multiple line laser source and an acoustooptic modulator (AOM).
The laser photo plotter is provided with a light source that emits a laser beam, a polygon mirror that deflects the modulated laser beam and an f&thgr; lens that focuses the deflected laser beam to form a beam spot on an object surface to be exposed, for example. The beam spot scans as the polygon mirror rotates, which forms scanning lines on the object surface.
An optical device such as the laser photo plotter that requires relatively large light quantity employs a gas laser as the light source. In general, a gas laser emits at a plurality of peak wavelengths. For example, an argon laser emits at a plurality of peak wavelengths in the ultraviolet region and the visible region.
On the other hand, the gas laser cannot directly switch (ON/OFF) an emission of the laser beam, which requires a modulator located in the optical path between the light source and the polygon mirror. The AOM, which changes a direction of an emergent laser beam by diffraction caused by input ultrasonic wave, is frequently used as the modulator of the laser photo plotter. In the AOM, when the ultrasonic wave is applied to a medium, a diffraction grating is formed by compression wave caused in the medium. The diffracted light is used as the modulated drawing beam and the non-diffracted light is cut off.
However, since the diffraction angle of the emergent laser beam modulated by the AOM varies in proportion to the wavelength of the incident light, the emergent angles vary depending on the wavelength, which deviates the optical paths of the respective wavelengths. As a result, if the AOM is not conjugate to the object surface, the laser beam that should form a single beam spot forms a plurality of beam spots at the different points on the object surface. If the AOM is conjugate to the object surface, the position variation of the beam spots can be prevented. However, since the incident angle to the object surface varies corresponding to the wavelength, when field curvature of the f&thgr; lens or mechanical error causes a partial out-focus area, a plurality of beam spots are formed at the different points in the out-focus area.
Since the spot separation seriously deteriorate the drawing performance of the plotter, the conventional optical system includes a filter that selects one peak wavelength. The filter cuts the peak wavelengths other than the selected peak wavelength, which lowers the energy efficiency.
Japanese Patent Provisional Publication No. Sho 61-193130 discloses the optical system including a wedge prism located at the front and/or rear of the AOM. Since the chromatic dispersion of the AOM can be canceled by the chromatic dispersion of the wedge prism, the optical paths of the laser beams of different wavelengths coincide with each other.
However, in the optical system of the publication, since the wedge prism is disposed in the divergent or convergent laser beam, astigmatism and coma occur.
Further, since the correction effect is fixed by the specification of the wedge prism, the chromatic dispersion of the wedge prism should be accurately defined to coincide the optical paths, which narrows tolerance to the specification of the wedge prism, increasing manufacturing time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a laser modulating optical system including the multiple line laser source and the AOM, which is capable of correcting the effect of the wavelength dependence of the AOM. Further, the present invention aims to prevent the generations of astigmatism and coma, and to increase the tolerance to the specification of the element in a correction optical system.
For the above object, according to the present invention, there is provided an improved laser modulating optical system, which includes a multiple line laser source that emits at a plurality of peak wavelengths, an acoustooptic modulator (AOM) that changes a direction of an emergent beam by diffraction caused by input ultrasonic wave to take out a diffracted beam as a modulated beam, and a correction optical system for correcting deviation between optical paths of at least two peak wavelengths due to a variation of the diffraction angle of the AOM.
The correction optical system includes at least one pair of chromatic dispersing elements located at the position where the laser beam becomes substantially parallel and in at least one of the incident and exit sides of the AOM. Namely, when the correction optical system includes one pair of the chromatic dispersing elements, a pair of the elements are located at the incident side together or at the exit side together. When the correction optical system includes two pairs of the chromatic dispersing elements, one pair is located at the incident side and the other pair is located at the exit side. Each of the chromatic dispersing elements has a predetermined chromatic dispersion and has no lens power. The lens power is a refractive power to diverge or converge a light beam. That is, the chromatic dispersing element does not change divergence or convergence of the light beam, while it only changes a direction of the light beam.
With this construction, since the correction optical system corrects the deviation between optical paths of different peak wavelengths due to a variation of the diffraction angle of the AOM, the optical paths coincide with each other. Accordingly, when the laser modulating optical system is applied to a laser photo plotter, the light beams of a plurality of peak wavelengths can be used for the drawing, which increases energy efficiency.
The chromatic dispersing element may be a wedge prism or a diffraction grating. In each case, a pair of the chromatic dispersing elements are arranged such that the chromatic dispersion of the one element is opposite in direction to the chromatic dispersion of the other element.
Further, the pair of chromatic dispersing elements may consists of a pair of wedge prisms, a pair of diffraction gratings or a combination of the prism and the grating. The space between the chromatic dispersing elements of each pair may vary to adjust the correction effect of the correction optical system.
REFERENCES:
patent: 4971413 (1990-11-01), Inoue
patent: 5128798 (1992-07-01), Bowen et al.
patent: 5852293 (1998-12-01), Iwasaki et al.
patent: 61193130 (1986-08-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Dang Hung Xuan
Greenblum & Bernstein P.L.C.
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