Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-22
2001-05-22
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06236479
ABSTRACT:
The present invention relates to a space-division optical switching system having a multichannel acousto-optical deflector.
The system proposed by the invention is advantageously used in the field of optical telecommunications, in particular to interconnect in reconfigurable manner, strips of monomode optical fibers conveying information in optical form at a wavelength of 1.55 &mgr;m or of 1.3 &mgr;m.
TECHNOLOGICAL BACKGROUND OF THE INVENTION
Monomode optical fibers are making it possible to convey data at higher and higher rates in optical form. At present, switching such optical signals at nodes in optical telecommunications networks requires the optical signals to be converted into electrical signals which are then processed electronically and subsequently converted back into optical signals at the outlet from the switching system. Such complex electronic processing of large volumes of data might become a bottleneck in future telecommunications networks.
Thus, optical switching systems that process the optical signals directly and transparently constitute alternatives to electronic systems when the data rates involved are very large and the data does not need to be demultiplexed.
Space-division optical switching systems are already known. For example, an N×N space-division optical switching system makes it possible to interconnect in reconfigurable manner two sets of N optical fibers, each of the N inlet optical fibers being capable of being optically connected, via a channel of the switching system to any one of the available outlet fibers. Important characteristics of a space-division switching system are the number of elementary components required, the complexity of the algorithm for reconfiguring the switch, insertion losses (attenuation of the optical signal on passing through the switch), and cross-talk noise (interference coming from other channels and associated with the channels being imperfectly isolated from one another).
Space-division optical switching systems can be classified depending on the type of topology used.
In particular, multistage architectures consist in cascading some number of 1×2, 2×2 and/or 2×1 elementary switching stages so as to obtain larger capacity systems, of capacity N×N where N >2. That type of modular architecture is well adapted to certain kinds of integrated optical technology, but it imposes a compromise between the number of elementary switches (which is not less than about N.log
2
N) and the complexity of the algorithm for reconfiguring the switching system. In addition, since the optical signal passes through a plurality of elementary switching stages, insertion losses and/or cross-talk noise can become large when the capacity N exceeds 8.
Broadcast-and-shutter type architectures use only one stage of switching. For each inlet they consist in sharing the optical power uniformly over all of the outlets and then in shutting off outlets that are not desired. The reconfiguration algorithm is very simple, however the number of elementary components is of the order of N
2
. In addition, insertion losses and/or cross-talk noise can become large at large capacity.
Finally, deflection architectures consist in actively directing each inlet towards a desired outlet. They comprise a single switching stage. The reconfiguration algorithm is very simple and the number of elementary components is of order N. Insertion losses and cross-talk noise do not depend, a priori, on capacity.
The system proposed by the invention is a switching system that makes use of acousto-optical deflection: it belongs to the last-mentioned category.
Acousto-optical deflectors make use of ultrasound wave propagation in certain materials to deflect light beams. For a general description of such deflectors, reference may advantageously be made to:
[1] J. Sapriel: Acousto-optics (Wiley, N.Y., 1979).
Ultrasound waves are produced by a piezoelectric transducer fed with a high frequency electrical signal. The ultrasound wave propagates perpendicularly to the surface of the transducer and deforms the acousto-optical material locally. These deformations by the photoelastic effect induce variations in the refractive index of the material and thus generate a diffraction grating capable of deflecting light beam. The effectiveness of deflection depends on wavelength, on the dimensions of the transducer, on the ultrasound power, and on a coefficient referred to as the “figure of merit” of the material. This coefficient is associated with the material and the configuration of the acousto-optical interaction in the material. The effectiveness of deflection increases with increasing figure of merit of the material. The angle of deflection is proportional to the ultrasonic frequency used, and it is therefore relatively simple to reconfigure the deflector. A reconfiguration time is associated with the speed of the ultrasound waves, and with the diameter of the light beam, and typically it can be much less than a microsecond.
PRIOR ART
Space-division optical switch architectures operating by acousto-optical deflection have already been proposed, in particular in:
[2] “Acousto-optic photonic switch”, Dan Owen Harris and A. Vanderlugt, Optics Letters, Vol. 14, No. 21, pp. 1177-1179 (1989),
[3] “Demonstration of a photonic space switch utilizing acousto-optic elements”, W. E. Stephens, P. C. Huang, T. C. Banwell, L. A. Reith and S. S. Cheng, Optical Engineering, Vol. 29, No. 3, pp. 183-190 (1990),
[4] “Multichannel acousto-optic crossbar switch”, Dan Owen Harris, Applied Optics, Vol. 30, No. 29, pp. 4245-4256 (1991).
Those architectures, referred to as “crossbar”switches, use a multichannel acousto-optical deflector and enable two strips of N optical fibers to be interconnected (N=4 or 8 in references [2], [3], [4]). Each deflector channel is embodied on the top face of the acousto-optical crystal by its own piezoelectric transducer and is associated with one of the optical inputs of the system, thereby making it possible to control the destination of each of the input light beams independently. Since the ultrasound waves propagate perpendicularly in this case to the plane defined by the strip of inlet fibers, the deflected optical beams are distributed over a matrix of points. A lens—which is cylindrical or spherical, depending on circumstances—placed at a distance from the multichannel deflector corresponding to the focal length of said lens (the so-called “Fourier” configuration) rectifies the light beams onto the axis of the lens which is parallel to the axis of the fibers.
In publication [2], said lens is a spherical lens which makes it possible to reinject onto the outlet fiber axis for a 1×N system.
However, it should be observed that the architecture proposed in that publication has only one optical inlet and is effective only for providing 1×N switching.
For a system having a plurality of optical inlets, an architecture having a spherical Fourier lens does not enable reinjection into the outlet fibers to take place on the axis of the fibers, thereby giving rise to prohibitive injection losses when the outlet fibers are monomode fibers. To minimize injection losses, it then becomes necessary to use additional cylindrical lenses, thereby complicating the architecture of the system.
Thus, in publication [4], two cylindrical lenses are added upstream from the deflector and it is proposed to replace the Fourier spherical lens by two cylindrical lenses of different focal lengths. However, even with optimum focal lengths, the efficiency of reinjection into the outlet monomode fibers is less than 1/N.
Another technique that has been proposed for conveying beams towards the outlet fiber strip—corresponding to the architecture described in [3]—consists in using a cylindrical Fourier lens and N fibers with lenses for each outlet port, giving a total of N
2
fibers organized in a matrix, followed by passive N to 1 fiber couplers. Nevertheles
Gosselin Stéphane William Raymond
Sapriel Jacques
Bello Agustin
Blakely & Sokoloff, Taylor & Zafman
France Telecom
Pascal Leslie
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