Television – Video display – Projection device
Reexamination Certificate
1998-10-06
2001-05-15
Lee, Michael (Department: 2614)
Television
Video display
Projection device
C348S756000, C372S021000, C372S022000
Reexamination Certificate
active
06233025
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to a process for generating at least three laser beams of different wavelengths for displaying color video pictures in which every laser beam is controlled in intensity for generating a monochromatic partial image, and these monochromatic partial images are mixed additively for displaying the color video pictures, wherein at least one of the three laser beams is obtained in that a medium is excited by at least a partial beam of a primary laser with a defined wavelength, which medium, upon excitation, emits a laser beam with a different wavelength than the wavelength of the exciting primary laser, and wherein the exciting laser beam and the excited laser beam are used directly or after frequency conversion for the display of monochromatic partial images. The invention is further directed to an apparatus for generating at least three laser beams of different wavelengths for the display of color video pictures, in particular for carrying out a process of this kind, with a control device for controlling the three laser beams for generating monochromatic partial images which are mixed additively for displaying the color video picture, and the apparatus has a primary laser and a medium which can be excited by means of the latter, wherein the exciting laser beam and the excited laser beam can be used directly or after frequency conversion for displaying monochromatic partial images.
2. Description of the Related Art
Large-area high-brilliance display systems and projection systems cannot be realized by means of conventional image reproduction techniques, especially using picture tubes, since the picture screens are too heavy or are mechanically unstable due to the vacuum requirements.
It was proposed some time ago to image video pictures on a screen by means of lasers. For this purpose, the laser beams are rastered on a screen by means of a deflection device which is realized, for example, by rapidly moving rotating mirrors and swiveling mirrors or by acousto-optical modulators, so that a quantity of image points which are adjacent to one another and in lines situated one above the other are illuminated on the screen by the laser light. The light intensity of the lasers is modulated in accordance with the brightness to be illuminated for each image point. Thus, the imaging principle is the same as that used in picture tubes, where the laser beam is substituted for the electron beam and the mirrors or acousto-optical modulators are substituted for the deflection device. A system of this kind is described in DE 43 06 797 C, for example.
However, three laser beams must be used for color images. The outlay for three lasers is very high, chiefly when gas lasers are used. If this laser video technique is ever to be made available in the consumer market, the cost of such lasers must be reduced in particular.
Further, U.S. Pat. No. 5,317,348 A proposes the use of semiconductor lasers or diode-pumped solid state lasers. For example, a Nd:YAG laser which is pumped with a GaAIAs diode and with a wavelength of 531 nm can be used for producing the green laser beam. Alternatively, it is suggested to operate the YAG laser with reference to a wavelength of the emitted laser beam of 1064 nm and to convert this wavelength by means of a frequency doubler for a wavelength of 532 nm for green. However, frequency doubling involves the risk of output losses.
The video device shown in this patent also requires, in principle, three lasers for generating the three laser beams. Although costs are reduced through the use of diode-pumped solid state lasers, it would be desirable if a plurality of laser beams used in the video device could be generated by a common laser, since this could be expected to further reduce costs.
In addition, EP 0 084 434 A, which describes the prior art mentioned above, proposes the use of an individual argon ion laser as primary laser for generating a blue and a green laser beam. A dye laser is excited by a portion of the green beam so as to generate, in addition, a red laser beam component along with it.
However, according to this patent, it is again necessary to use an expensive argon ion laser which also requires additional cooling due to the low conversion ratio. The additional components such as dye lasers, filters, prisms and polarizers likewise increase cost.
Although, the primary argon ion laser could be replaced by a diode-pumped solid state laser, this might severely limit the accessible wavelength range. That is, it is advisable in a color video system to achieve a high white-light density. This means that in order to find the wavelengths for displaying red, green and blue the three different color components should have approximately the same intensity, since the laser beam with the lowest intensity for white would otherwise limit the maximum possible output of the other laser beams. With sharply different outputs, the laser output of the primary laser would have to be very high to achieve the same luminosity, which would involve additional expenditure, e.g., for cooling. Moreover, an over dimensioning of the optical elements might be necessary for a long service life, which would also cut into the savings in cost achieved by generating a plurality of laser beams by means of an individual primary laser source.
OBJECT AND SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a process and apparatus for displaying a color video picture by means of lasers in which the cost for the laser sources is appreciably reduced.
Based on the generic process, this object is met in that a pulsed laser is used as the primary laser and a material with nonlinear optical characteristics is used as a medium, wherein this material, in addition to the laser beam generated by its excitation, releases an additional laser beam whose frequency is given by the sum or difference of the frequencies of the exciting laser beam and the excited laser beam, and in that the laser beams emitted by the medium are used directly as laser beams for the display of monochromatic partial images or are converted into these laser beams.
Thus, according to the invention, at least two laser beams are generated in the medium. This is made possible in that a nonlinear optical medium is used instead of the dye of the dye laser mentioned in the introduction. Due to the nonlinear optical characteristics, the laser beams which are generated proceeding from the medium have a frequency given by the sum or difference of the excited laser beam. Therefore, at least two of the laser wavelengths needed to display a color video picture can be generated at the same time in such a material. Accordingly, expenditure is appreciably reduced compared with the prior art.
However, the material constants reproducing the nonlinear optical effect are generally small so that a high intensity can obviously not be expected. However, due to the nonlinear characteristics, the intensities for the additional laser beam increase sharply with the field strength of the exciting laser beam. For this reason, pulsed operation is provided for excitation, where outputs of several kilowatts up to the gigawatt range can occur in a pulse. As experience has shown, conversion factors of up to 50% of the exciting laser beams can be achieved in the kilowatt range.
As a result of the pulsed operation, the output per time unit is reduced during output peaks so that the optical components are loaded only slightly, which advantageously manifests itself, with respect to the operation of a device, above all in low maintenance and uniform picture quality over long periods of time.
Further, the pulses reduce the time coherence of the laser beams. As is known from wave theory, a frequency spectrum always results instead of an individual wave in the case of a short wave train. This effect can be utilized to reduce speckle in the video picture.
In the field of laser video, speckle refers to disturbing sparkling points which are formed along interference maxima due to
Lee Michael
LTD GmbH & Co. Laser-Display-Technologie KG
Reed Smith LLP
LandOfFree
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