Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2002-01-22
2003-11-04
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S260000, C359S318000
Reexamination Certificate
active
06643054
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a beam deflector for deflecting a beam of light into a selectable direction, and to a switching system comprising a plurality of terminals for optical signals in order for optical signals entering the switching system through one or more terminals to be output at selectable other terminals. In particular, the invention also relates to a switching system for data communication systems with fiber-optical switching systems. Furthermore, the invention relates to a method for operating such a switching system, in particular for a fiber-optical switching system.
From B. H. Lee and R. J. Capik “Demonstration of a very low-loss 576×576 servo-controlled, beam-steering optical switch fabric”, a beam-steering switch is known which enables a plurality of fiber-optical inputs to be selectively coupled with a plurality of fiber-optical outputs. To this end, each optical input comprises a collimator which is adjustable by servo-control such that a light beam exiting at one terminal end of the input impinges upon a terminal end of the selected optical exit and enters the same. The servo-controlled adjustment of the plurality of collimators is mechanically complex and a desired reduction of switching time is difficult.
SUMMARY OF THE INVENTION
U.S. Pat. No. 5,963,682 discloses a switching system for selectively optically coupling optical inputs with optical outputs, wherein beam directions are not adjustable by servo-control, but by means of liquid crystal cells and the application of specific voltage and field patterns to the same. The electro-optical effect of the liquid crystals used for this purpose is sufficient to attain, by the application of electric field patterns, adequate deflection angles for selectively driving several outputs. However, in this prior art system, the inertance of the liquid crystal limits an attainable switching speed and, moreover, losses of optical intensity which occur when the optical signals pass through the switching system are felt to be too high.
U.S. Pat. No. 5,319,492 discloses an optical switch, wherein an optical polymer which is non-linear in the second order is enclosed in a cavity, to which polymer a spatially changing electric field can be applied by means of structured electrodes in order to produce a switchable reflection grating, because the refractive index of the polymer can be spatially varied by the application of the electric field. As the cavity is mirrored, the achievable deflection angle is increased, because an incident light beam circulates several times in the cavity. However, when the light beam circulates several times in the resonator, the intrinsic absorption of the polymer material results into significant intensity losses, so that an increase of the deflection angle is limited by the absorption of the medium in the cavity.
It is one object of the present invention to provide a beam deflector for deflecting an incident light beam which is improved in particular in respect of attainable deflection angles or/and switching times.
Moreover, it is an object of the invention to provide a beam deflector which is better suited for use in optical data communication systems.
In addition, it is an object of the present invention to provide a switching system comprising a plurality of terminals for optical signals which is in particular suitable to be employed in optical data communications systems with high transmission rates and short switch-over times.
Furthermore, it is an object of the invention to provide a switching system comprising several terminals for optical signals which enables comparatively little losses of optical intensity to be achieved.
According to a first aspect, the invention proceeds from a beam deflector which comprises a plate of electro-optical material disposed between plano-parallel mirrors, the reflective index of said electro-optical material being spatially changeable by the application of an electric field pattern in the direction of extension of the plate. The reflectivity of the mirror of the pair of mirrors facing towards the incident light beam is less than that of the other mirror of the pair of mirrors. As a result, the intensity of the incident light beam reflected by the beam deflector is higher than the intensity of the incident light beam transmitted by the beam deflector.
In this respect, the invention is distinguished in that the first or/and the second mirror comprises a plurality of layers of dielectric material, the refractive indices of which are different from each other from layer to layer.
The invention is based on the concept that the electro-optical material is disposed in a high-quality resonator, so that the number of circulations which parts of the incident light beam perform in the resonator before they exit is particularly high. This high quality of the resonator is achieved by the use of mirrors having a high reflectivity and a low residual transmisssivity, respectively. This is achieved by the use of multi-layer mirrors of dielectric materials having different refractive powers. The high quality of the resonator and, as a result, the high number of circulations of the beam to be deflected intensifies the effect of the electro-optical material, so that, on the one hand, the deflection angles achievable with this material is increased as compared to a material which is passed through only once or only a couple of times. On the other hand, it is possible, at a given deflection angle, to reduce the thickness of the electro-optical material as compared to a system with lower resonator quality. As a result, it is possible to reduce the electric fields to be applied and the switching times, in particular when the electro-optical material is a liquid crystal.
Moreover, it is possible to employ in a high-quality resonator also electro-optical materials which have a relatively low electro-optical effect as compared to the known liquid crystals, but can be switched much quicker.
Preferably, the beam deflector is provided for deflecting light having optical wavelengths, preferably light in a wavelength range of from 0.5 &mgr;m to 3.0 &mgr;m, more preferred, in a wavelength range of from 1.0 &mgr;m to 2.0 &mgr;m and, particularly preferred, in a wavelength range of from 1.3 &mgr;m to 1.7 &mgr;m.
Preferably, electro-optical solid materials are used, preferably lithium niobate (LiNbO
3
) or/and gallium arsenide (GaAs).
In order to increase the reflectivity of the mirrors, and thus the quality of the resonator, preferably, several layers of the dielectric materials have a thickness which corresponds substantially to a value d which satisfies the equation d=&lgr;/4, wherein &lgr; is the wavelength of the light of the incident beam in the dielectric material of the layer. As a result, the partial beams of the incident light beam reflected at a front boundary surface and a rear boundary surface of a layer of such a thickness d interfere with each other in constructive manner, whereas partial beams transmitted through the layers interfere with each other in destructive manner.
Further, in order to increase the resonator quality, the plate of electro-optical material has a thickness which corresponds substantially to a value D which satisfies one of the two formulae D=k/2*&lgr; and D=(2k−1)/4*&lgr;, wherein &lgr; is the wavelength of the incident beam in the electro-optical material and k is a natural number of more than 0. The first or second one of the equations is selected depending on whether the layers adjacent to the electro-optical material have a higher or lower refractive index than the electro-optical material itself and whether the structure of the mirror layer stack adjacent to the plate of electro-optical material is symmetrical or unsymmetrical in respect of the plate.
A simple configuration of the mirror is obtained if the mirror layers are made of merely two different dielectric materials which are alternately laminated onto one another. The refractive indices of the two mirror materials should differ f
Ben Loha
Carl-Zeiss-Stiftung
Choi William
Rosenthal & Osha L.L.P.
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