Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-28
2003-04-01
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C370S228000, C370S225000, C370S217000, C370S216000
Reexamination Certificate
active
06542268
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of optical fiber communications, and more in particular, to an optical channel cross connect for communication systems in WDM (Wavelength Division Multiplexing) technique, having a double spatial switching structure on optical flows, strictly not blocking, and interposed functional units operating on each single flow.
Starting from the first appearance of the optical fiber as physical carrier in telecommunication networks, the domain of the technique, the present invention falls under, has been characterized by an ever-increasing progress in optical devices, which enable such form of communication. We can briefly mention the following optical devices available on the market at the date:
Erbium Doped Fiber Amplifiers, known with the, acronym EDFA, which, pumped by a laser signal at an appropriate wavelength &lgr;, can amplify WDM optical flows having total capacity highly exceeding 10 Gbit/s, maintaining a sufficiently flat gain response within a band of minimum attenuation of the single-mode optical fiber, typically ranging from 1530 to 1565 nm.
Band-pass filters having bandwidth lower than 1 nm, capable of being electronically tuned from one to any other wavelength, inside the above mentioned spectral interval of approximately 35 nm, within switching times of some, having low insertion losses, and low crosstalk among different channels (lower than −30 dB).
2×2 switching elements in waveguide on Lithium Niobate substrate, material capable to remarkably change its refraction index under the action of a relatively strong electrical field. These devices are used to implement optical path selectors which can be used as the basic elements of a M×N spatial division switching matrix, that can be obtained in a unique integrated circuit of the PLC type (Planar Lightwave Circuit). According to the present technology, it is not convenient to integrate matrices having dimensions larger than 8×8; the realization of more expanded optical matrices requires the assembly of several PLC devices duly interconnected through optical fibers.
Electro-mechanically controlled optical selectors, capable to spatially switch one of the N input optical flows to the unique outputs; these elements can be combined to build more complex matrixe structures, characterized by dimensions up to 16×16, at the state of the art.
Semiconductor optical amplifiers, known with the acronym SOA (Semiconductor Optical Amplifiers), based on the principle of travelling wave amplification. It is possible to implement simple high-isolation optical switches by driving the active device of such an amplifier to interdiction or saturation. These components are profitably used for the implementation of several different optical devices, among which the M×N matrices and the wavelength converters.
Broad band combiners (Optical Combiner) of N input optical flows, to form a unique output optical signal, that is the sum of the N input flows. In the case the flows entering a combiner have each one a different wavelength &lgr;, an output signal is obtained, consisting of the wavelength division multiplexing of the entering flows, technique known under the acronym WDM (Wavelength Division Multiplexing).
Broad band splitters (Optical Splitters) splitting on more paths a unique entering signal, obtaining a plurality of identical, though attenuated, output signals. In practice, it is possible to implement an optical splitter by simply exchanging the inputs and the outputs of an optical combiner, considering that said optical components are reciprocal.
WDM signal demultiplexers (Wavelength Demultiplexers) accepting a WDM input signal composed of N wavelengths &lgr;, and sending each of them to one of the different N outputs. The filtering property of these components, if realized in PLC technology, is generally obtained through a particular arrangement of planar waveguides, implementation known as AWG (Arrayed-Waveguide Grating).
Wavelength Converters that can be implemented according to different physical operation principles, for instance, by driving a SOA amplifier device to operate in a non-linear gain zone. It must be noticed just from now that the wavelength conversion functionality can be obtained also through Optical/Electrical/Optical (O/E/O) conversion, such as that made in case of optical signal Regeneration.
Optical transmitters, for the transformation of electrical signals present at the transmission interfaces of terminal stations intoloptical signals, suitable to fiber transmission. They typically include a semiconductor Laser emitting with high stability on a particular wavelength, a driving circuit imposing an on-off modulation of the light signal, acting either on the laser itself (direct modulation) or on an external optical modulator placed after the lagser (external modulation). Laser structures satisfying the requirements of a WDM system are for instance the Distributed Feedback (DFB) lasers; should the tuning capability of the laser be required on a wide spectrum interval, it is possible to consider different solutions, like the DBR (Distributed Bragg Reflector) structures.
Optical receivers, for the reverse transformation of the optical signal into the corresponding electrical signal carried, at the receiver interfaces of the terminal stations. They typically include a photodiode, made of adequately doped semiconductor material, and the electronic circuits for amplification, clock extraction, data reading.
BACKGROUND ART
The wide possibility of selection of optical devices can greatly facilitate the transition towards optical fiber networks, where not only the transmission of channels, but also the routing of flows among different nodes is performed within the optical layer, while in the present transport networks both the space and the time switching of channels are implemented in the electrical domain and require a double signal conversion, from optical to electrical and vice versa.
The WDM technique (acronym used in the optical sector in place of the FDM term used in the radio sector) can become therefore a key factor, not only to increase the transport capacity of the already existing optical infrastructures (enabling to transmit several channels in one fiber), but also toll increase the network flexibility, availing of the wavelength as additional degree of freedom for the switching, applying the principle of transparent optical path (“Wavelength Path”—WP). Then, if the additional function of wavelength conversion is available, the possible blocking conditions due to the non flexible assignment of wavelengths to fiber paths can be overcome: in fact, it becomes possible to route two channels, entering the node at the same wavelength, from different fibers, towards one output fiber, by converting the wavelength of one channel; this solution leads to the technique defined as Virtual Wavelength Path (VWP).
It must be pointed out that channels can be routed in a cross-connect, in the widest meaning of the term, by means of spatial switching, or wavelength-based switching, or through time demultiplexing and switching.
In spite of all its advantages, the present technology does not yet make the direct switching of digital packets convenient in the optical domain, consisting for instance of ATM (Asynchronous Transfer Mode) cells. This is due to the difficulty to fully implement in the optical domain memories and data processing devices, being nowadays still at research and development prototype level.
On the contrary, in the context of the spatial and of the wavelength-based switching, the known art proposes different solutions. The main difference between these two approaches lays in the fact that in the second instance, the wavelength conversion is absolutely necessary and the routing is made by selecting for the transmission a particular wavelength; while in the context of the spatial routing, the wavelength conversion is optional and is used not to support the switching, but to decrease the blocking probability, induce
Brunazzi Stefano
Rotolo Salvatore
Birch & Stewart Kolasch & Birch, LLP
Chan Jason
Kim David
STMicroelectronics Srl
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