Optical: systems and elements – Holographic system or element – Having particular recording medium
Reexamination Certificate
1999-07-06
2003-04-01
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Having particular recording medium
C359S007000, C359S015000, C359S025000
Reexamination Certificate
active
06542264
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of electronically controlled, optical switching elements, especially for use in fiber optics communication systems.
BACKGROUND OF THE INVENTION
Fast multidimensional switches are essential building blocks in high speed data communication systems, multimedia services, or high performance parallel computers. However, electronic implementations of such switches are close to their inherent limits. It is evident that it will not be possible to meet the demands of the emerging broadband communication applications by the existing electronic switching technology. Furthermore, electronic switching devices are not capable of direct integration with the optical fiber communication systems, which are becoming the dominant communications technology. Optical implementation of switching devices possess several inherent advantages over their electronic counterparts.
Free space photonic interconnects are very suitable for the implementation of parallel communication systems due to the small cross-talk between the parallel data channels, and the case whereby three dimensional guiding and routing can be achieved. The cross-talk level must be kept as low as possible in order to reduce the bit error rate (BER) of the channel. One of the basic building blocks in the construction of such networks is the high speed optical switch, such as the bypass-exchange switch. Such switches are used as the elements in Multistage Interconnection Networks (MIN). MIN's are a family of network architectures with minimal number of switches.
Many optical MIN configurations have been proposed for highly parallel communication channels with high bandwidth and low cross talk, such as those described by J. W. Goodman, F. I. Leonberg, S. Y. Kung, and R. A. Athale in their article entitled “Optical interconnection for VLSI systems”, published in Proceedings of the IEEE, Vol. 72, pp. 850-866 (1984). Many of these configurations are static optical interconnects which are built either of optical waveguides, such as described by S. Somekh, E. Garmire, A. Yariv, H. L. Garvin and R. G. Hunsperger, in their paper entitled “Channel optical waveguides and directional coupling in GaAs-imbedded and ridged”, in Applied Optics, Vol.13, pp.327-330 (1974), or of imaging systems such as that described by A. W. Lohman, W. Stork and G. Stucke, in “Optical perfect shuffle”, published in Applied Optics, Vol. 25, pp.1530-1531 (1986), or of static holographic optical elements (HOE's) such as that described by R. K. Kostuk, J. W. Goodman and L. Hesselink, in their article “Design considerations for holographic optical interconnects”, published in Applied Optics, Vol. 26, pp.3947-3953 (1987). However, dynamic optical interconnects, which enable the dynamic reconfiguration of the connecting scheme between the source nodes and the target nodes are overwhelmingly more effective, as described in the articles in the review volume entitled “Photonic Switching and Interconnects” by Abdellatif Marrakehi, published by Marcel Dekker Inc., 1994.
The structure of a MIN is usually of alternating layers of static interconnection patterns, such as the perfect shuffle described by H. S. Stone in “Parallel processing with perfect shuffle”, IEEE Transactions on Computing, Vol. C-20, pp. 152-161 (1971), followed by an array of basic switching modules, known as bypass-exchange switches. The bypass-exchange switch has two inputs and two outputs with two operating states: the bypass state, in which the two input signals are directly connected to the respective output ports and the exchange state, in which the input signals are crossed between the output ports.
A common optical exchange-bypass switch currently in use utilizes a Polarizing Beam Splitter (PBS) combined with a polarization control element at the input to the PBS. The polarization control element is usually a liquid crystal, or a ferroelectric liquid crystal, which is faster. However, even the ferroelectric liquid crystal does not have a sufficiently fast response time for the requirements of present communication switching needs, and certainly not for future needs. Moreover, another major drawback of PBS is the high sensitivity of the cross-talk level of the switch to polarization instability of the transmitted light or the liquid crystal modulators. Consequently, the PBS cross-connect switch is sensitive to the temperature and environmental stability of the modulators and of the whole system.
Holographic optical elements (HOE's) and volume holograms have been used recently for two dimensional steering of light beams in optical interconnect networks, especially for highly parallel computer interconnects. However, such systems have generally relied, at least in the case of volume holograms, either on the use of a number of fixed holograms, the desired one of which is reconstructed using a reference beam selected by means of its wavelength or direction of incidence, or on the rewriting of the desired hologram in real time immediately before each steering action to be performed. Therefore, such holograms are not directly electrically switchable, and thereby do not provide for simple system construction and high speed operation.
With the increase of the bit throughput rate in optical fiber communication systems, cost effective light sources with very narrow spectral linewidths have been developed. The development of such lasers for optical communications has enabled the use of volume (thick) holograms as routing devices. Since such holograms are inherently extremely wavelength sensitive, their use had not previously been feasible commercially. The use of thick holograms now enables the packing of many routes in the same network, and thus allows three dimensional steering. However, to date, optical switches based on the use of prior art holograms, since they are not directly electrically switchable, have not shown sufficient speed, nor do they possess sufficiently low cross-talk levels, to enable their use in the optical communication systems currently under use or development.
There therefore exists a serious need for fast, dynamic, low-cross talk optical switches, to fulfill the switching requirements in current and future optical communications systems.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new free-space optical switch, which overcomes the drawbacks and disadvantages of existing switches.
There is thus provided in accordance with a preferred embodiment of the present invention, a novel generic switch based on Electro holography (EH). EH enables the reconstruction process of volume holograms to be controlled by means of an externally applied electric field. EH is based on the use of the voltage controlled photorefractive effect in the paraelectric phase, where the electro-optic effect is quadratic. Volume holograms stored as a spatial distribution of space charge in a paraelectric crystal can be reconstructed by the application of an electric field to the crystal. This field activates prestored holograms which determine the routing of data-carrying light beams.
The implementation of EH based devices requires the use of a photorefractive crystal with suitable properties, such as potassium tantalate niobate (KTN), strontium boron niobate (SBN), or especially potassium lithium tantalate niobate (KLTN), as described in U.S. Pat. No. 5,614,129. KLTN doped with copper and vanadium is particularly suitable for use as the medium for EH devices.
EH devices can be advantageously used as the building blocks in Multistage Interconnection Networks (MIN). The MIN is composed of arrays of EH switches which can be electrically switched between one or more states. In each state a different set of holograms are activated, which direct the light beams in the required 3D directions to the next stage. These switches thus contain the spatial routing information, thereby obviating the need for additional optics between the stages. The EH switch thus enables a wide variety of interconnect configurations to be implemented,
Agranat Aharon J.
Pessach Benny
Rogel Larry Rudolph
Assaf Fayez
G. E. Ehrlich Ltd.
Spyrou Cassandra
Yissum Research Development Company of the Hebrew University of
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