Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-02-18
2001-09-25
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Plural
Reexamination Certificate
active
06295397
ABSTRACT:
FIELD OF INVENTION
The present invention relates to an optical wavelength selective device, and more particularly to a wavelength selective modulator with which each wavelength channel can be modulated separately and independently of other wavelength channels. The invention also relates to a method of modulating wavelength channels in an optical network.
DESCRIPTION OF THE BACKGROUND ART
Various different methods are known for improving the capacity of existing optical networks. One method involves the use of so-called wavelength multiplexing technology (WDM) for improving the extent to which available bandwidths can be utilised on an optical fibre in the optical network. Wavelength can also be used in an optical network as an information address. This requires the use of different types of wavelength selective components. For instance, wavelength selective modulators (WSM) may be used to this end.
No methods or devices are at present available for the wavelength selective modulation of optical channels in cases when the channels lie relatively close to one another, which presents a problem.
SUMMARY OF THE INVENTION
The present invention addresses the aforesaid problem with the aid of a wavelength selective modulator that includes at least two multimode waveguides, at least one wavelength selective cross-switch structure, at least two controllable phase control elements, and at least four connecting waveguides. The wavelength selective cross-switch structure is disposed between a first and a second multimode waveguide. The first multimode waveguide is connected to at least one access waveguide on a first side and at least two access waveguides on a second side, and said second multimode waveguide is connected to at least two access waveguides on a first side and at least one access waveguide on a second side. The wavelength selective cross-connector structure is connected to at least two access waveguides on a first and a second side.
In a wavelength selective cross-switch structure, a first access waveguide on the second side of the first multimode waveguide is connected to a first access waveguide on the first side of the wavelength selective cross-switch structure via a first connecting waveguide, a second access waveguide on the second side of the first multimode waveguide is connected to a first access waveguide on the second side of the wavelength selective cross-switch structure via a second connecting waveguide, a second access waveguide on the first side of the wavelength selective cross-switch structure is connected to a first access waveguide on the first side of the second multimode waveguide via a third connecting waveguide, and a second access waveguide on the second side of the wavelength selective cross-switch structure is connected to a second access waveguide on the first side of the second multimode waveguide via a fourth connecting waveguide.
The phase control elements are disposed in the connecting waveguides on the one side of the wavelength selective cross-switch structure. A first phase control element is adapted to change the phase for a given time period Atl which is earlier (by &tgr; time units) than the time at which a second phase control unit is adapted to change the phase for a time period &Dgr;t2 by a time difference that corresponds to the time taken for reflected light to travel from the first phase control element to the second phase control element or the time taken for the light to travel in the one connecting waveguide from a point which is located at the same distance from the output of the first multimode waveguide as the distance of the first controllable phase control element in the second connecting waveguide from the second controllable phase control element.
A cross-switch structure includes two mutually different so-called Mach-Zehnder paths for each individual wavelength in the optical signal. A first Mach-Zehnder path passes from a first access waveguide on the second side of the first multimode waveguide to the second access waveguide on the first side of the second multimode waveguide via the first and the fourth connecting waveguide and via the wavelength selective cross-switch structure for wavelengths that are transmitted through said structure.
A second Mach-Zehnder path passes from a second access waveguide on the second side of the first multimode waveguide to the first access waveguide on the first side of the second multimode waveguide via the second and the third connecting waveguide and via the wavelength selective cross-switch structure for wavelengths that are transmitted through said structure.
A third Mach-Zehnder path passes from a first access waveguide on the second side of the first multimode waveguide to the first access waveguide on the first side of the second multimode waveguides via the first and the third connecting waveguide and via the wavelength selective cross-switch structure for wavelengths that are reflected by said structure.
A fourth Mach-Zehnder path passes from a second access waveguide on the second side of the first multimode waveguide to the second access waveguide on the first side of the second multimode waveguide via the second and the fourth connecting waveguides and via the wavelength selective cross-switch structure for wavelengths that are reflected by the cross-switch structure.
Only wavelengths that are reflected in a cross-switch structure can be controlled actively, in other words the wavelength that passes via the third or the fourth Mach-Zehnder path in accordance with what has been described.
The Mach-Zehnder paths are essentially of the same length in the inventive wavelength selective modulator. A small wavelength difference is experienced in practice, because reflection does not occur in the centre of the cross-switch structure.
Thus, in the case of one wavelength selective cross-switch structure there is included two different Mach-Zehnder paths or routes for wavelengths that are reflected by the cross-switch structure and two different Mach-Zehnder paths for wavelengths that are transmitted through said structure. In the case of two wavelength selective cross-switch structures, there is included four different Mach-Zehnder paths for two different wavelengths and for N-number of wavelength selective cross-switch structures There is thus included 2×N number of different Mach-Zehnder paths for N-number of wavelengths.
The first and the second multimode waveguides will preferably have the same length-width ratio when said waveguides are of the same type, i.e. of the 2×2-type, for instance. In one embodiment of the invention, the multimode waveguide may comprise an MMI waveguide.
In the case of two or more wavelength selective cross-switch structures, there is included for each wavelength selective cross-switch structure two phase control elements and two connecting waveguides. Each wavelength selective cross-switch structure is provided with two phase control elements, a first and a second phase control element, on opposite sides in relation to a nearest adjacent wavelength selective cross-switch structure. Each of the first of said phase control elements is adapted to change the phase for a given time period t1 which is earlier (by &tgr; time units) than the time at which respective second phase control elements are intended to change the phase during a time period t2 by an amount that corresponds to the time taken for reflected or transmitted light to travel to said second phase control element from the nearest adjacent phase control element. The wavelength selective cross-switch structures are mutually connected via a connecting waveguide from an access waveguide on a first wavelength selective cross-switch structure to another access waveguide on an adjacent wavelength selective cross-switch structure. These access waveguides are chosen so as to lie closely adjacent to one another on one and the same side.
In one inventive method of modulating optical wavelength channels in an optical network, wavelength channels are excited into at least one access waveguide
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget LM Ericsson (publ)
Ullah Akm E.
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