Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-01-19
2001-12-18
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S256000, C359S629000, C385S016000, C385S024000, C372S026000, C349S116000
Reexamination Certificate
active
06331910
ABSTRACT:
BACKGROUND INFORMATION
The present invention concerns an arrangement and a method for the electrical control of the intensity of non-polarized light, a polarizing beam splitter being acted upon by the light to be controlled via an input face such that the light is split into two polarized light bundles that are orthogonal with respect to each other.
For a multiplicity of applications in the area of optics, for example in information-processing, in laser technology, and for many geodetic, astronomical, and navigational applications, arrangements are required which can switch light, carry out switchovers between optical channels, or modulate.
In known electro-optical switches and modulators of this type, either optically effective solid-body crystals or liquid crystals are used. Transparent crystals, such as can be manufactured from LiNbO3, require high working voltages, permit only small apertures, and are cost intensive. On account of the small permissible light bundle diameters, they are little suited for switching light bundles that contain images. As a consequence of the high dispersion that is inherent in them, the use of monochromatic light sources (lasers) is also necessary. The optical and electrical parameters change during use and thus impair the properties of the electro-optical switch or modulator. In addition, half of the light is lost, since these switches and modulators are employed in connection with polarizers.
In optical modulators that are improved with respect to these properties, modulators that have become known through German Published Patent Application No. 30 13 498 and British Patent No. 2046937 both polarization directions are used in connection with a polarizing beam splitter. However, the other cited disadvantages remain. Furthermore, it is disadvantageous in the known modulator that they require a working voltage of over 100 volts. In addition, significant costs arise as a result of a necessary cascading of the modulator.
In a further known arrangement, which was described by Hirabayashi, T. Kurokawa in “Liquid Crystal Devices for Optical Communication and Information-processing Systems,” Liquid Crystals, Volume 14, pp. 307-317 (1993), a nematic liquid crystal is used in a so-called twist arrangement. In this context, larger apertures can be achieved. However, in nematic liquid crystals, their great switching times of, for example, several 100 milliseconds and problems in the representation of intermediate values are disadvantageous. Through Chiung-Shevy Wu, Shine Tsou Wu: “New Liquid Crystal Operation Modes,” Volume 2949 SPIE, Image Sciences and Display Technologies, Proceedings Berlin Conference, FRG, 7-10 (1996), a modulator has become known in which the two polarization states s and p, generated by a polarizing beam splitter, are conveyed in the reverse direction by electro-optical liquid crystal cell. This leads to high rotation angles. However, the cells having a switching frequency of roughly 10 Hz are too slow.
From German Published Patent Application No. 196 31 644 (corresponding to PCT Publication No. WO 98/06002), an arrangement is known for switching optical patterns, which has a high switching efficiency and using which non-polarized, polychromatic light can be switched sufficiently rapidly. The arrangement, however, is very complicated and expensive, since it requires a multiplicity of liquid crystal cells in order to reduce the switching times. It is too expensive for wide use.
SUMMARY OF THE INVENTION
The objective of the present invention is to indicate an arrangement and a method for the electrical control of the intensity of non-polarized light, which at a low control voltage such as is available in conventional transistor circuits and integrated circuits, make possible rapid controlling. In this context, a high switching efficiency should also be achieved, i.e., a high transparency in the on-state and the highest possible damping capacity of the supplied light in the off-state. It should be possible to carry out the switchover of the light at equally high efficiencies between different channels.
This objective is achieved in the arrangement according to the present invention as a result of the fact that a first reflecting device is configured for reflecting at least one of the polarized light bundles, so that the polarized light bundles proceed in parallel, that in the path of the polarized light bundles at least one electro-optical element, that changes the polarization as a function of a supplied control voltage, and a second reflecting device for reflecting the light bundles in the reverse direction are arranged such that the light bundles pass through the at least one electro-optical element twice and are directed by the first reflecting device at the polarizing beam splitter, and that the controlled light can be extracted from at least one face of the polarizing beam splitter. In this context, provision is advantageously made that the light bundles are reunited in the beam splitter after passing through the remaining arrangement.
The arrangement according to the present invention can be used for the rapid processing of gray-scale values and as a rapid optical switch at a low switching voltage and also as an active element for the automatic control of light intensities in feedback loops having appropriate light sources. It can be used in fiber-optic superstructures as well as in optical parallel processors for processing messages.
The arrangement according to the present invention can also be used as an optical limiter in glasses, video cameras, or in optical message transmission. As a consequence of the small losses and high efficiency, applications are also possible for high-power laser light.
In contrast to the known electro-optical switches and modulators, the arrangement according to the present invention has many advantages. The switching efficiency for non-polarized light is more than 99% between the two switching states. Losses, already in themselves minimal, can through anti-reflection layers be further reduced by the reflections in the optical surfaces of the individual components in the arrangement according to the present invention.
In the arrangement according to the present invention, depending on the specific embodiment, small angles of rotation of the optical indicator of the liquid crystal are specifically required in the electrical field. As a result of this, an abundance of possibilities for the application of various electro-optical effects of liquid crystals is generated by the arrangement according to the present invention. In particular, in the application of liquid crystals having the SSFLC effect, switching times can be achieved in the range of &mgr;s. The SSFLC effect is described, for example, in N. Clark et al., Appl. Phys. Lett. 36899 (1980); U.S. Pat. No. 4,563,059.
Using the arrangement according to the present invention, intermediate values can be arrived at in an advantageous manner between the on- and off-state, so that a gray-scale value modulation having time constants in the microsecond and submicrosecond range is possible through the application of the electroclinic effect.
Through the application of the DHF effect (deformed helix ferroelectric effect) in the helix-shaped smectic C-phase, working voltage is of roughly 1 V and time constants of roughly 10 &mgr;s can be achieved.
In addition, the arrangement according to the present invention has the advantage that the switching efficiency is dependent on the color of the light to only an extremely small degree. Therefore, the arrangement according to the present invention is also superbly suited for color pictures. Finally, as a further advantage, the design of the arrangement according to the present invention is simple and cost-effective.
Most of the advantages of the arrangement according to the present invention rest on the fact that, as a result of the double passage through the electro-optical element, the electrical rotation of the indicator of the liquid crystal can be much smaller, even in connection with the delay plate, than i
Beresnev Leonid
Dultz Wolfgang
Haase Wolfgang
Patrusheva Galina
Ben Loha
Deutsche Telekom AG
Kenyon & Kenyon
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