Optical waveguides – With optical coupler
Reexamination Certificate
2000-12-11
2002-09-10
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
C385S024000, C385S016000, C385S018000, C385S140000, C385S039000, C385S042000
Reexamination Certificate
active
06449403
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength selective optical filter, in particular but not exclusively to a wavelength selective optical filter which is also operable to provide a controllable degree of attenuation.
In contemporary optical communication systems, wavelength division multiplexing (WDM) techniques are used. Such techniques enable many channels bearing communication traffic to be multiplexed onto radiation propagating along a guided optical path, for example an optical fibre. Each channel has associated therewith an allocated range of wavelengths which are used to convey the communication traffic associated with the channel. Thus, WDM techniques allow increased exploitation of optical fibre bandwidth in order to satisfy future demand for enhanced data rates in communication networks.
Use of WDM in contemporary communication systems has created a need for devices which can be connected to optical paths thereof conveying WDM communication traffic, the devices operable to extract communication traffic corresponding to a specific channel without interfering with communication traffic conveyed in other channels; radiation corresponding to these other channels is transmitted through the devices substantially unmodified. Such devices are known as add-drop filters.
There arises a further requirement in contemporary optical communication systems using WDM techniques and incorporating add-drop filters for the filters to be reconfigurable, namely for the filters to be retunable to select different channels. Moreover, it is a yet further requirement that the communication systems should be reconfigurable whilst in operation conveying communication traffic. Thus, each add-drop filter needs to be retunable from a first selected channel to a second selected channel without tuning through channels intermediate between the first and second channels and causing traffic conveyed in these intermediate channels being interrupted or disturbed during retuning.
A number of conventional add-drop filters have been reported in the literature and sold commercially which are capable of being tuned from one channel to another. Such conventional add-drop filters incorporate optical filters which tune continuously; as a consequence, they cause disturbance of communication traffic on intermediate channels when being reconfigured. Such optical filters are implemented in a number of ways, for example as cascaded Mach-Zehnder filters fabricated as silicon planar waveguides and as micro-mechanically tuneable Fabry-Perot filters. A U.S. Pat. No. 5, 739, 945 describes a single cavity continuously-tuneable optical filter incorporating electrostatically actuated mirrors.
Tunable optical filters are known in the prior art.
For example, a United States patent no. U.S. Pat. No. 4,240,696 describes an optical filter including a plurality of adjacent layer pairs, each pair having an incident and an emerging surface. Each pair further comprises a first dielectric layer, a second dielectric layer and a control electrode disposed between and in contact with the layers. The filter additionally includes a plurality of ground electrodes disposed on the layer pairs to electrically contact each incident and emergent surface, a source of electrical potential, and a switch for connecting the source between the control electrodes and the ground electrodes. In the filter, optical radiation is reflected by the filter upon closing the switch and thereby applying the electrical potential in opposite directions across the first and second layers. Electrodes of the layer pairs are thus connected in parallel so that the pairs are not capable of being mutually independently tuned. Moreover, there is no basis in the context of the invention for it to be advantageous to make the pairs independently tunable.
Moreover, in a further example, a United States patent no. U.S. Pat. No. 5,170,290 describes high total transmission tunable comb filter structures. The structures comprise moderately thick layers of optical material having periodic refractive index modulation features comprising a multiplicity of coherently-coupled, weakly-resonant optical cavities. The structures are characterised by spectra of at least order 5 relative to a fundamental lowest-order cavity resonance consisting of narrow, moderate to high density reflection lines occurring in one or more sets, each set being characterised by lines equally spaced by wave number if optical dispersion is neglected. Filters provided by such structures can be electro-optically or mechanically tuned such that the peaks within a spectral band of interest shift by one harmonic order to reflect or transmit optical radiation of any specific wavelength within a band. The cavities are not capable of being mutually independently tuned in the embodiments described in the patent. Moreover, there is no basis in the context of the invention for it to be advantageous to make the cavities independently tunable.
A first approach to providing add-drop filters which do not tune continuously in contemporary systems involves demultiplexing and remultiplexing techniques. Use of such techniques enables add-drop filters to be isolated whilst they are retuned from one channel to another when the systems are being reconfigured. Application of such techniques results in increased insertion loss associated with add-drop filters included within the systems, the insertion loss increasing as the number of channels conveying communication traffic is increased.
A second approach employed in contemporary systems incorporating add-drop filters is for the add-drop filters to include a number of tuneable optical gratings which are tunable from a wavelength intermediate between two neighbouring channels to a given channel. This approach provides a characteristic that filters in the systems are not tuned through a number of channels before reaching their selected channel. However, the approach requires there to be provided a grating for each channel used in the systems, there arising thereby a problem that insertions loss associated with add-drop filters in the systems increases as the number of channels is increased.
There is a further disadvantage that, when the first and second approaches are adopted, add-drop filters are designed for accommodating a specific maximum number of channels; such a maximum number means that the add-drop filters have to be replaced if the number of channels used in the systems are increased by system upgrades to more than the maximum number.
The inventors have appreciated that there is a need for a wavelength selective optical filter capable of incorporation into add-drop filters of communication systems that can tune directly from a first channel to a second channel without tuning through channels intermediate between the first and second channels. Moreover, the inventors have appreciated that the optical filter should be tunable over a relatively large number of channels so that the filters do not need to be replaced when communication system upgrades are implemented.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a wavelength selective optical filter for receiving input radiation and outputting corresponding filtered output radiation, characterised in that the filter includes a plurality of mutually independently tunable optical resonators for filtering the input radiation to generate the output radiation, the resonators being at least partially mutually coupled, and the resonators having associated therewith tuning ranges which at least partially mutually overlap.
The filter provides the advantage that it is capable of being tuned from one wavelength to another without tuning through intermediate wavelengths therebetween.
The filter of the invention is distinguished from prior art filters incorporating micro-tuned resonators in that the filter incorporates cavities which are mutually coupled. Such mutual coupling provides a more selective response than merely cascading filters as currently done in the
Cush Rosemary
Hibberson Ruth
Stewart William J
Healy Brian
Kirschstein et al.
Marconi Communications Limited
Wood Kevin S.
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