Abstract:
The present invention provides a wavelength selective optical filter device ( 240 ) for receiving input radiation and outputting corresponding filtered output radiation, characterised in that the filter device ( 240 ) includes a plurality of at least partially mutually coupled Fabry-Perot optical resonators ( 330, 340, 360; 360, 400, 430 ) for filtering the input radiation to generate the output radiation, the filter device ( 240 ) being tunable from a first radiation wavelength to a second radiation wavelength by mutually detuning the resonators in a period where the resonators are being returned from the first wavelength (λ 1 ) to the second wavelength (λ 2 ) so that the filter device ( 240 ) is substantially in a non-responsive state during the period. The resonators incorporate freely suspended mirrors ( 360, 430 ) which are electrostatically actuated to affect tuning of the resonators ( 330, 340, 360; 360, 400, 430 ). The filter device ( 240 ) is thereby capable of tuning between different wavelengths without tuning through wavelengths therebetween. The filter device ( 240 ) can be included into an add-drop filter ( 10 ) for providing channel add and drop functions when the filter ( 10 ) is incorporated in a multichannel WDM communication system ( 100 ).

Description:
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 art. 
     Conveniently, the filter in use is: 
     (a) at least partially transmissive to the input radiation to generate the output radiation when the optical resonators are mutually tuned to a similar wavelength; and 
     (b) substantially non-transmissive to the input radiation when the resonators are mutually detuned. 
     Such a filter provides the desirable characteristic that it is substantially non-transmissive to radiation whilst being retuned from wavelength to another. 
     Preferably, the coupling from one of the resonators to another resonator adjacent thereto is in a range of 0.01 to 0.1% to obtain a useable degree of selectivity from the filter. 
     Advantageously, the resonators include first and second tunable Fabry-Perot cavities, the cavities being at least partially mutually coupled through a component common to the cavities. The cavities provide a resonance characteristic when there are an integer number of half wavelengths of radiation propagating between mirrors of the cavities. 
     Conveniently, the component is a second mirror assembly spatially located between the cavities, the first and second cavities having associated therewith first and third mirror assemblies respectively, the first and second assemblies defining the first cavity, and the second and third assemblies defining the second cavity. The second mirror assembly provides a degree of mutual coupling between the cavities to provide the filter with its wavelength selective response. 
     When implementing the filter, each cavity preferably includes a void in a region between its associated mirror assemblies. Advantageously, the void is in a range of 10 to 20 μm wide in a direction normal to major planes of its associated mirror assemblies, although 14 μm is its preferred width. 
     In order to improve response of the filters, the reflectors are preferably distributed Bragg reflectors. Such reflectors can be provided by each assembly comprising multi-layer structures. 
     AlGas, GaAs and AlAs will be used hereafter as abbreviations for aluminium gallium arsenide, gallium arsenide and aluminium arsenide respectively. 
     Advantageously, each multilayer structure comprises a plurality of alternating layers of 
     AlGaAs and aluminium oxide. Conveniently, the aluminium oxide layers each have a thickness in a range of 300 nm to 350 nm, although 314.5 nm is a preferred thickness. 
     Moreover, the AlGaAs layers of the first and third assemblies are such that each layer beneficially comprises first, second and third sub-layers so that: 
     (i) the first and third sub-layers have a composition Al a Ga b As where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; and 
     (ii) the second sub-layer has a composition Al c Ga d As where c is in a range of 0.28 to 0.32, and d is in a range of 0.68 to 0.72. 
     Incorporation of the sub-layers enables optical characteristics of the assemblies to be closely controlled in manufacture. 
     Furthermore, in the third assembly, the first and third sub-layers are preferably each in a range of 10 to 20 nm thick, and the second sub-layer is in a range of 90 to 100 nm thick. 
     Additionally, in the first assembly, the first and third sub-layers are preferably each in a range of 45 to 55 nm thick, and the second sub-layer is preferably in a range of 15 to 25 nm thick. However, 52.8 nm and 19.9 nm are preferred specific thicknesses for these layers. 
     Advantageously, the second assembly comprises aluminium oxide and AlGaAs layers such that each AlGaAs layer has a composition Al a Ga b As where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42. Preferably, in the second assembly, the AlGaAs layers are in a range of 115 to 140 nm thick. Such layer thicknesses and composition assist to provide a satisfactory assembly reflection characteristic for the filter. 
     Beneficially, each cavity is tunable by altering a spatial separation between its respective mirror assembly and the second mirror assembly. In order to achieve such tuning, at least part of the mirror assemblies can be resiliently suspended and their mutual spatial separation can be alterable by applying piezo-electric forces to the mirror assemblies. Alternatively, at least part of the mirror assemblies can be resiliently suspended and their mutual spatial separation can be alterable by applying electrostatic forces to the mirror assemblies; the mirror assemblies can be electrically connected to enable potential differences to be applied therebetween to generate the electrostatic forces. 
     In a practical implementation of the filter, the second and third mirror assemblies each advantageously comprise a central mirror region suspended on a plurality of compliant arms. 
     In one embodiment of the invention, the central mirror region is substantially circular and suspended on four arms. Conveniently, the central region has an effective diameter in a range of 50 to 150 μm and each arm has a length in a range of 600 to 2000 μm, although an effective diameter of 100 μm and a length of 1000 μm are specific preferred dimensions. 
     For ease of making electrical connections to the filter, the cavities and their associated mirrors can be fabricated onto a substrate in terraced formation to enable electrical connection to be made to the mirror assemblies, the assemblies being mutually electrically isolated. 
     Advantageously, the filter is fabricated using gallium arsenide or silicon fabrication techniques. 
     According to a second aspect of the invention, there is provided an add-drop filter for receiving input communication radiation and operable to drop and add radiation corresponding to a specific channel in the input communication radiation, the add-drop filter incorporating a filter according to the first aspect of the invention for isolating radiation corresponding to the channel. 
     According to a third aspect of the present invention, there is provided a method of fabricating a filter according to the first aspect of the invention, the method comprising the steps of: 
     (a) forming a series of layers on a substrate, the layers forming mirror assemblies and spacer layers therebetween; 
     (b) defining features in the layers corresponding to suspended reflectors and associated compliant support arms; 
     (c) processing the substrate and its associated layers by etching processes to generate the mirror assemblies and optical cavities therebetween where the s pacer layers are present; 
     (d) defining and generating features on the assemblies operable to actuate the mirror assemblies relative to one another; and 
     (e) mounting the filter produced by steps (a) to (d) above in a carrier and making electrical connection to the filter. 
     Advantageously, the layers are formed by metal oxide chemical vapour deposition (MOCVD). Moreover, the spacer layers are preferentially wet etched to form the optical cavities and render the mirror assemblies freely suspended. 
     Conveniently, in the method, the mirror assemblies each comprise alternate layers of AlGaAs and AlAs, the AlAs subsequently heat processed to form aluminium oxide. Such alternate layers provide optimised optical properties for the mirror assemblies. 
     According to a fourth third aspect of the present invention, there is provided a method of tuning a filter according to the first aspect of the invention from a first wavelength to a second wavelength, the method comprising the steps of: 
     (a) tuning the resonators to the first wavelength so that the filter provides selective filtration at the first wavelength; 
     (b) detuning the resonators from the first wavelength by tuning at least one resonator in a first wavelength direction and another resonator in another wavelength direction opposite to the first wavelength direction; 
     (c) tuning the resonators in a mutually detuned state towards the second wave length; and 
     (d) tuning the resonators finally to the second wavelength so that the filter provides selective filtration at the second wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which: 
     FIG. 1 is a schematic diagram of a prior art add-drop filter illustrating its channel isolation function; 
     FIG. 2 is a schematic diagram of a prior art communication system incorporating three add-drop filters similar to the filter in FIG. 1; 
     FIG. 3 is an illustration of a wavelength selective optical filter according to the invention comprising a filter module connected to a circulator to provide an optical drop function for the add-drop filter in FIG. 1; 
     FIG. 4 is a schematic of a filter device included in the filter module illustrated in FIG. 3; 
     FIG. 5 is a graph of transmission and reflection characteristics of the filter device in FIG. 4; 
     FIG. 6 is a graph of transmission characteristics of the filter device in FIG. 4 when tuning from one wavelength to another; 
     FIG. 7 is a graph of attenuation and reflectance through the filter device in FIG. 4 as it is detuned; and 
     FIGS. 8 and 9 are illustrations of fabrication steps required for fabricating the filter device in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a prior art add-drop filter indicated by  10 . The filter  10  comprises an input port (IN), a through port (THROUGH), a drop port (DROP) and an add port (ADD); all the ports are adapted to interface to optical fibre waveguides. The input port is operable to receive input radiation S in  and the through port is operable to provide output radiation S out . 
     The filter  10  is designed to operate over a range of wavelengths which accommodates the radiation S in . The radiation S in  is a summation of radiation components associated with a sequence of channels C i  where an index i is an integer in a range of 1 to n which individually identifies each channel, there being n channels in total in the radiation S in ; for example, the radiation S in  has a wavelength in the order of 1550 nm with the channels spaced at wavelength intervals of 0.8 nm. The channels C i  monotonically change in wavelength according to their respective channel number index i. 
     Operation of the filter  10  will now be described with reference to FIG.  1 . The input radiation S in  propagates to the input port (IN) and further therefrom into the filter  10  whereat a radiation component corresponding to a channel C x  is extracted from the radiation S in  and output to the drop port (DROP). The radiation S in  minus components corresponding to the channel C x , namely modified radiation S′ in , propagates further into the filter  10  whereat a radiation component corresponding to a channel C′ x  input to the add port (ADD) is added to the radiation S′ in  to yield the radiation S out  which is then output at the through port (THROUGH). Thus, the output radiation S out  corresponds to the input radiation S in  except that the component of radiation corresponding to the channel C x  in the input radiation is replaced by a component of radiation corresponding to the channel C′ x  in the output radiation. 
     A conventional communication system incorporates a number of filters similar to the add-drop filter  10 ; such a conventional system is indicated by  100  in FIG.  2 . The system  100  includes a multiplexer unit  110 , a demultiplexer unit  120  and three add-drop filters  130 ,  140 ,  150  connected in series and inserted in a communication path connecting the multiplexer unit  110  to the demultiplexer unit  120 . Each add-drop filter  130 ,  140 ,  150  is of an identical design to the filter  10 . Moreover, the filters  130 ,  140 ,  150  are operable to filter channels C a , C b , C c  respectively where integer subscripts a, b, c can be mutually different and are included in a range of 1 to n. 
     The transmitter unit  110  comprises a series of optical inputs TC 1  to TC n  for receiving optical radiation corresponding to the channels C 1  to C n  respectively. In a similar manner, the receiver unit  120  comprises a series of optical outputs RC 1  to RC n  corresponding to the channels C 1  to C n . 
     Operation of the system  100  will now be described. The multiplexer unit  110  multiplexes the inputs TC to corresponding wavebands in output optical radiation K 1 . The radiation K 1  propagates from the unit  110  to the filter  130  which filters out a component of the radiation K 1  corresponding to the channel C a  and outputs the component at its drop port. The filter  130  also adds radiation input to its add port to a portion of the radiation K 1  propagating through the filter  130  to generate output radiation K 2 . The radiation K 2  propagates to the filter  140  whereat a component of radiation corresponding to the channel C b  is isolated and output it at its drop port. In a similar manner to the filter  130 , the filter  140  also adds radiation input to its add port to a portion of the radiation K 2  propagating through the filter  140  to generate output radiation K 3 . The radiation K 3  propagates to the filter  150  whereat a component of radiation corresponding to the channel C c  is isolated and output it at its drop port. The filter  150  also adds radiation input to its add port to a portion of the radiation K 3  propagating through the filter  150  to generate output radiation K 4 . The output radiation K 4  propagates to the demultiplexer unit  120  whereat it is demultiplexed to generate output optical radiation at the outputs RC. The radiation at the outputs RC correspond to those at the inputs TC except for the radiation input at inputs TC a , TC b , TC c  on account of the action of the filters  130 ,  140 ,  150 . Additional components (not shown) are connected to the filters  130 ,  140 ,  150  for processing radiation isolated at the filters and for generating radiation to be input to the add ports of the filters  130 ,  140 ,  150 . 
     The inventors have appreciated that it is highly desirable for the system  100  to be reconfigurable so that the subscripts a, b, c can be altered without interrupting traffic flow from the multiplexer unit  110  to the demultiplexer unit  120 . Such reconfiguration of the system  100  can be achieved by the add-drop filters  130 ,  140 ,  150  incorporating therein wavelength selective optical filters, each filter tunable from a first channel to a second channel without tuning through channels intermediate between the first and second channels and operable to provide add-drop functions for their associated add-drop filter. 
     Referring now to FIG. 3, there is indicated by  200  a wavelength selective optical filter according to the invention operable to provide an optical drop function for its associated add-drop filter  10 . The filter  200  comprises a circulator  210  and a filter module  220 , the module  220  shown included within a dotted line  225 . The filter module  220  comprises an input lens  230 , a tuneable filter device  240  and an output lens  250 . The filter  200  incorporates an input port connected through an optical fibre  260  to an input port J 1  of the circulator  210 . Moreover, the circulator  210  comprises an output port J 3  which is connected through an optical fibre  270  to an output port of the filter  200 . Furthermore, the circulator  200  comprises a further port J 2  which is connected through an optical fibre  280  to the input lens  230 . The output lens  250  is connected through an optical fibre  290  to a drop port of the filter  200 . 
     Operation of the filter  200  will now be described with reference to FIG.  3 . The input port of the filter  200  receives the radiation S in  applied to the IN port of the filter  10 . The radiation S in  propagates along the fibre  260  to the input port J 1  of the circulator  210 . The radiation S in  propagates within the circulator  210  to the port J 2  at which it is output to propagate along the fibre  280  and through the lens  230  to the filter device  240 ; the lens  230  forms a beam having a diameter in a range of 50 to 100 μm which is received by the device  240 . If the beam received by the device  240  is broader than 100 μm, deterioration in the filter  200  response will result. The device  240  is tuned to a channel C x  where an integer index x is in a range of 1 to n. Radiation components in the radiation S in  corresponding to the channel C x  propagate through the device  240  and are received at the lens  250  through which they propagate onwards and through the fibre  290  to the drop port of the filter  200 . Radiation components in the radiation Sin corresponding to the channels C 1  to C x−1  and C x+1  to C n  are reflected from the device  240  and propagate back through the lens  230  and the fibre  280  to the port J 2  of the circulator  210 ; these components propagate further in the circulator  210  to its output port J 3  and further therefrom along the fibre  270  to the output port of the filter  200 . 
     The filter  10  includes first and second filter units, the first unit corresponding to the filter  200  and the second unit corresponding to a modified version of the filter  200  adapted for injecting radiation components corresponding to the channel C x . 
     The filter device  240  will now be described in further detail with reference to FIG.  4 . The device  240  comprises a thinned gallium arsenide substrate  300  having a thickness in a range of 150 μm to 250 μm, although 200 μm is a preferred thickness. The substrate  300  comprises a first major face indicated by  310  which is coated in an anti-reflection coating  305  operable to counteract reflection from the face  310  at infra-red radiation wavelengths in the order of 1500 nm. The substrate  300  comprises a second major face indicated by  320  on an opposite side of the substrate  300  to the first face  310 . The second face  320  has fabricated thereon a first mirror assembly  330  comprising alternate layers of AlGaAs and aluminium oxide, namely four layers of AlGaAs and three layers of aluminium oxide. Each of the aluminium oxide layers is in a range of 300 to 350 nm thick, although 314.5 nm is its preferred thickness. Each AlGaAs layer comprises three sequential sub-layers, namely first, second and third sub-layers such that: 
     (i) the first and third sub-layers have a composition Al a Ga b As where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; and 
     (ii) the second sub-layer has a composition Al c Ga d As where c is in a range of 0.28 to 0.32 and d is in a range of 0.68 to 0.72. 
     A preferred composition for the first and third sub-layers is Al 0.6 Ga 0.4 As, and a preferred composition for the second sub-layer is Al 0.3 Ga 0.7 As. 
     The first and third sub-layers are each in a range of 45 to 55 nm thick, and the second sub-layer is in a range of 15 to 25 nm thick. However, 52.8 nm is a preferred thickness for each of the first and third sub-layers, and 19.9 nm is a preferred thickness for the second sublayer. 
     Above the mirror assembly  330  remote from the substrate  300  is a first cavity  340  of height in a range of 10 to 20 μm in a direction normal to the major faces  310 ,  320  of the first assembly  330 , although 14 μm is a preferred height. Suspended above the first assembly  330 , and separated therefrom by the first cavity  340 , is a second mirror assembly indicated by  350 . 
     The second assembly  350  is unitary and incorporates a central substantially circular mirror  360  suspended on four arms, for example an arm  370 , from a peripheral region  380  of the assembly  350 , the peripheral region  380  connected to the first assembly  330  by way of a first relatively thick layer of GaAs, the first relatively thick layer being substantially 14 μm thick. 
     The central mirror  360  and its associated arms are fabricated by forming four holes, for example a hole  390 , into the mirror assembly  350 . Thus, the central mirror  360 , its associated arms and the peripheral region  380  are all of unitary construction. The central mirror  360  has an effective diameter in a range of 50 to 150 μm, although 100 μm is its preferred diameter. Each arm has a length in a range of 600 to 2000 μm although 1000 μm is its preferred length. The arms each have a lateral width in a range of 15 to 30 μm, although 20 μm is their preferred width. 
     The second assembly  350 , in a similar manner to the first assembly  330 , is a multilayer structure comprising alternate layers of AlGaAs and aluminium oxide. Each of the aluminium oxide layers is in a range of 300 to 350 nm thick, although 314.5 nm is its preferred thickness. Likewise, each of the AlGaAs layers is in a range of 115 to 140 nm thick, although 125.0 nm is a preferred thickness. The AlGaAs layers have a composition 
     Al a Ga b As where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; however, Al 0.6 Ga 0.4 As is a preferred composition for the layers. 
     In the second assembly, there are six aluminium oxide layers and seven AlGaAs layers. 
     Above the mirror assembly  350  remote from the substrate  300  is a second cavity  400  of height in a range of 10 to 20 μm in a direction normal to the major faces  310 ,  320  of the first assembly  330 , although 14 μm is a preferred height. Suspended above the second assembly  350 , and separated therefrom by the second cavity  400 , is a third mirror assembly indicated by  420 . 
     The third assembly  420  includes a substantially circular central mirror  430  suspended on four arms, for example an arm  440 , from a peripheral region  450 . The mirror  430 , the arms and peripheral region  450  are unitary parts of the assembly  420 . Moreover, the mirror  430  and the arms of the assembly  420  are of similar lateral dimensions to the mirror  360  and the arms of the first assembly  330 . Furthermore, the mirrors  360 ,  430  are mutually aligned along an axis normal to the plane of the first and second assemblies  330 ,  350 . The peripheral region  450  is connected to the peripheral region  380  of the second assembly  350  by way of a second relatively thick layer of GaAs between the peripheral regions  380 ,  450 , the layer having a thickness in a range of 10 to 20 μm although substantially 14 μm is a preferred thickness. 
     The third assembly  420 , in a similar manner to the first and second assemblies  330 ,  350 , is a multilayer structure comprising alternate layers of AlGaAs and aluminium oxide. Each of the aluminium oxide layers is in a range of 300 to 350 nm thick, although 314.5 nm is its preferred thickness. 
     Each AlGaAs layer of the third assembly  420  comprises three sequential sub-layers, namely first, second and third sub-layers such that: 
     (i) the first and third sub-layers have a composition Al a Ga b As where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; and 
     (ii) the second sub-layer has a composition Al c Ga d As where c is in a range of 0.28 to 0.32, and d is in a range of 0.68 to 0.72. 
     A preferred composition for the first and third sub-layers is Al 0.6 Ga 0.4 As, and a preferred composition for the second sub-layer is Al 0.3 Ga 0.7 As. 
     The first and third sub-layers are each in a range of 10 to 20 nm thick, and the second sub-layer is in a range of 90 to 100 nm thick. However, 13.5 nm is a preferred thickness for each of the first and third sub-layers, and 94.9 nm is a preferred thickness for the second sub-layer. 
     The first and second cavities  340 ,  400  enable the mirrors  360 ,  430  to be freely suspended on their respective arms, a mutual spacing between the mirrors  360 ,  430  and the first assembly  330  being adjustable by applying forces to the mirrors  360 ,  430 . Such forces can be electrostatically or piezo-electrically generated. Adjustment of such forces enables the device  240  to be tuned. 
     Moreover, the relatively complex arrangement of layers in the assemblies  330 ,  350 ,  420  is chosen to allow precise adjustment of layer refractive index and hence overall central mirror  360 ,  430  and first assembly  330  reflectivity. 
     The peripheral regions  380 ,  450  are arranged in terraced formation as shown in FIG. 4 in order to facilitate electrical connection by wire bonding to connection pads  460 ,  470 ,  480  associated with the assemblies  330 ,  350 ,  420  respectively. The relatively thick GaAs layers are operable to substantially mutually electrically isolate the assemblies  330 ,  350 ,  420 . Moreover, when electrostatic forces are to be used to actuate the central mirrors  360 ,  430  relative to one another and the first assembly  330 , the assemblies  330 ,  350 ,  420  are sufficiently conductive so that electrical potentials applied to the pads  460 ,  470 ,  480  through associated wires bonded thereto control potentials of the central mirrors  360 ,  430  relative to the first assembly  330 . 
     Operation of the filter device  240  will now be described with reference to FIGS. 3 and 4. Incoming radiation propagating along the fibre  280  from the circulator  210  is focussed by the lens  230  onto the central mirror  430  of the third assembly  420 . The central mirror  430  is partially transmissive and radiation incident thereupon propagates into the second cavity  400  and is largely reflected at the central mirror  360  back to the central mirror  430 . If the radiation has components of a wavelength such that the effective distance between the central mirrors  360 ,  430  is an exact number of half wavelengths, resonance within the second cavity  400  occurs and the components give rise to standing waves in the second cavity. Alternatively, if the radiation has components of a wavelength such that the effective distance between the central mirrors  360 ,  430  is not an exact number of half wavelengths, no standing waves are formed. The second cavity  400  is weakly coupled through the central mirror  360  to the first cavity  340  and vice versa such that coupling from one cavity to another is in a range of 0.01 to 0.1%. When the first cavity  340  is also tuned to the same wavelength as the second cavity  400 , resonance in the first cavity  340  results in an efficient coupling of radiation components corresponding to resonance from the second cavity  400  into the first cavity  340 . The first assembly  330  is partially transmissive to radiation so that components of radiation at resonance in the first cavity  340  are transmitted through the first assembly  330  and through the substrate  300  and its antireflection coating  305  to propagate to the lens  250  and further along the fibre  290  to the drop port. 
     Components of radiation corresponding to resonance of the cavities  340 ,  400  are thereby transmitted through the device  240  whereas components of radiation not corresponding to resonance are reflected from the second and third assemblies  350 ,  420  back to the circulator  210 . Transmission through the device  240  substantially only occurs when the cavities  340 ,  400  are tuned to a mutually similar wavelength so that their resonances correspond. 
     Referring now to FIG. 5, there is shown a graph of transmission and reflection characteristics of the filter device  240  in FIG. 4 when the cavities  340 ,  400  are tuned to a similar resonance wavelength, namely 1550 nm. In the graph, a curve  500  depicts radiation transmission through the device  240  and a dashed curve  510  depicts radiation reflection from the device  240 . Radiation attenuation through the device  240  at resonance are less than 1 dB although coupling losses associated with the lenses  230 ,  250  and the fibres  280 ,  290  result in a overall insertion loss between the circulator  210  and the drop port in a range of 1 to 2 dB. The graph shows that reflection loss from the device  240  at resonance is at a level of −42 dB when its transmissive attenuation loss is less than 1 dB. Moreover, reflection loss exhibited by the device  240  is less than 1 dB when its transmission loss is at a level of −25 dB at a wavelength difference of 0.8 nm from resonance at a wavelength of 1550 nm. At wavelength differences of more than 0.8 nm from resonance in a wavelength range of 1545 nm to 1555 nm shown in FIG. 5, transmission losses through the device  240  are in excess of 25 dB. 
     Operation of the device  240  switching from one channel C i  to another channel will now be described with reference to FIG.  6 . In FIG. 6, there is shown three mutually orthogonal axes, namely a wavelength axis  600 , a time axis  610  and a transmittance axis  620 . Directions of increasing wavelength, time and transmittance are indicated by arrows  640 ,  650 ,  660  respectively. Within the axes  600 ,  610 ,  620  is included a 3-dimensional curve indicated by  700  of transmittance of the device  240  depending upon time and wavelength. The curve  700  is projected as a 2-dimensional curve indicated by  720  at a rear face of the graph; this curve  720  illustrates wavelength versus transmittance. 
     A first peak  740  corresponds to the device  240  tuned to one of the channels C i  with nominal wavelength λ 1  and a second peak  820  corresponds to the device  240  tuned to another of the channels with nominal wavelength λ 2 . Initially, the cavities  340 ,  400  of the device  240  are tuned to the wavelength λ 1 . In order to retune the cavities  340 ,  400 , a voltage difference applied between the second and third assemblies  350 ,  420  relative to a potential of the first assembly  330  is adjusted to detune both cavities  340 ,  400  in mutually opposite wavelength directions from the wavelength λ 1 . This mutual detuning causes the first peak  740  to broaden and reduce in transmissivity as depicted by curves indicated by  760 ,  780 , thereby effectively switching off a transmission function provided by the device  240 . The two cavities  340 ,  400  are then tuned, by moving their respective central mirrors  360 ,  430  by substantially equal amounts by altering a potential difference between the assemblies  350 ,  420  relative to a potential the first assembly  330 , towards the second wavelength λ 2  as depicted by curves indicated by  800 ,  810 . When the cavities  340 ,  400  are both tuned to the wavelength λ 2 , the device  240  again becomes transmissive as represented by the peak  820 . 
     In the curve  720 , it can be seen that the device  240  is substantially non-transmissive between the peaks  740 ,  820  thereby tuning from one channel to another without tuning through intermediate channels therebetween; in FIG. 5, the wavelengths λ 1  and λ 2  correspond to two of the channels C i . 
     The device  240  is also capable of functioning as a controlled attenuator by selectively slightly mutually detuning one or more of its cavities  340 ,  400 . Thus, the device  240  can provide wavelength selective attenuation depending upon to which channel the cavities  340 ,  400  are nominally tuned. A voltage difference is applied to the assemblies  350 ,  420  relative to the first assembly  330  can be used to determine a degree of detuning and thereby determine attenuation exhibited by the device  240 . When performing an attenuating function, variation in reflectivity of the device can be utilised. FIG. 7 illustrates a graph of reflective attenuation provided by the device  240  included in the filter  200  from the fibre  280  to the fibre  290  when its cavities  340 ,  400  are detuned to a mutual wavelength difference of 1.6 nm, namely each cavity detuned from nominally 1550 nm by an amount of 0.8 nm. The attenuation provided by the device  240  in the filter  200  is shown to be controllable from substantially −15 dB to −3 dB. 
     The device  240  operating as a controlled attenuator is especially useful in communication networks where differential losses between channels accumulate through the networks and can degrade network performance. Thus, the device  240  can be used to assist with equalising power levels across the channels C i  in the networks. 
     Fabrication of the device  240  will now be described. FIGS. 8 and 9 are illustrations of fabrication steps of a method of fabricating the filter device  240  in FIG. 4, the method comprising STEP 1 to STEP 6. 
     STEP 1: 
     Reference is made to FIG.  8 . Initially, the gallium arsenide substrate  300  is polished on both its major faces to a mirror finish and thereby thinned to a thickness of substantially 200 μm. On the major surface  320  of the substrate  300 , the first assembly  330  is grown onto the substrate  300  by metal oxide chemical vapour deposition (MOCVD) in an evacuated deposition apparatus. The first assembly  330  is a distributed Bragg reflector (DBR) and comprises alternate layers of AlGaAs and AlAs. The reflector of the first assembly  330  includes four layers of AlGaAs and three layers of AlAs. The AlAs layers are each 314.5 nm thick. Moreover, the AlGaAs layers each comprise three sequential sub-layers, namely a first sub-layer of a nominal thickness 52.8 nm and a nominal composition of Al 0.6 Ga 0.4 As, a second sub-layer of a nominal thickness 19.9 nm and a nominal composition Al 0.3 Ga 0.7 As, and a third sub-layer of a thickness and a composition similar to the first sub-layer. The first assembly  330  as a result has a thickness in the order of 1.5 μm. 
     Next, a first spacer layer of gallium arsenide is grown onto the first assembly  330 . The spacer layer is nominally of 14 μm thickness. 
     The second assembly  350  is then grown by MOCVD onto the first spacer layer remote from the substrate  300 , the second assembly  350  also being a DBR. The reflector of the second assembly  350  includes seven layers of AlGaAs and six layers of AlAs. The AlAs layers are each 314.5 nm thick. Moreover, the AlGaAs layers are each of a nominal thickness 125.0 nm and a nominal composition of Al 0.6 Ga 0.4 As. The second assembly  350  has a thickness in the order of 2.8 μm. 
     Next, a second spacer layer of gallium arsenide is grown onto the first assembly  350 . The spacer layer is nominally of 14 μm thickness. 
     Finally, the third assembly  420  is then grown by MOCVD onto the second spacer layer remote from the substrate  300 , the third assembly  420  also being a DBR. The reflector of the third assembly  350  includes three layers of AlGaAs and two layers of AlAs. The AlAs layers are each 314.5 nm thick. Moreover, the AlGaAs layers each comprise three sequential sub-layers, namely a first sub-layer of a nominal thickness 13.5 nm and a nominal composition of Al 0.6 Ga 0.4 As, a second sub-layer of a nominal thickness 94.9 nm and a nominal composition Al 0.3 Ga 0.7 As, and a third sub-layer of a thickness and a composition similar to the first sub-layer. The third assembly  420  has a thickness in the order of 1.1 μm. 
     Completion of STEP 1 results in the generation of a workpiece indicated by  880 . 
     STEP 2: 
     Reference is made to FIG. 9. A layer  900  of photoresist is then spun onto the third assembly  420  of the workpiece  880  and photolithographic and associated resist development techniques applied to define windows in the layer  900 . The windows are useable for delineating the central mirrors  360 ,  430  and their associated arms in STEP 2. 
     STEP 3: 
     The workpiece  880  from STEP 2 is then subjected to anisotropic reactive ion etching (RIE) or chemically assisted ion beam etching (CAIBE) where the layer  900  provides a stencil for etching. Etching is continued until holes  910 ,  920  through the MOCVD-deposited layers are produced which reach down through the MOCVD layers beyond the first assembly  330 , 
     STEP 4: 
     The workpiece  880  from STEP 3 is then subjected to a steam environment at a temperature in a range of 380 to 420° C., although 400° C. is a preferred temperature, which oxidises the AlAs layers of the workpiece  880  to aluminium oxide. The workpiece is next subjected to wet preferential etching which does not etch the assemblies  330 ,  350 ,  420  but removes part of the spacer layers in the vicinity of the holes  910 ,  920  to leave the central mirrors  360 ,  430  and their associated arms freely suspended and the cavities  340 ,  400  defined. Moreover, the wet etching also removes residual traces of the first spacer layer remaining on the first assembly  330  in the first cavity  340  beneath the first central mirror  360 . 
     STEP 5: 
     Further lithographic, RIE and preferential wet etching techniques are then applied to generate a terraced profile for the workpiece  880  from step 4 as indicated by  930 . This terraced profile assists with making electrical connection to the workpiece using standard wire bonding equipment. Metal is then selectively deposited onto the assemblies  330 ,  350 ,  420  to form the pads  460 ,  470 ,  480  to which connection wires are to be bonded. 
     STEP 6: 
     The workpiece  880  from STEP 5 is next mounted in a suitable carrier (not shown) allowing infra-red radiation propagation through the workpiece  880 , the anti-reflection coating  305  applied on the major face  310  of the substrate  300  and then wires bonded to the pads  460 ,  470 ,  480  to complete fabrication of the workpiece  880  to provide the device  240 . 
     Several devices identical to the device  240  can be fabricated from one substrate which is cleaved after STEP 5 to provide individual devices for packaging and wire bonding. 
     It will be appreciated that modifications can be made to the filter  200  and the device  240  without departing from the scope of the invention. For example, the device  240  can be modified to incorporate more than two mutually coupled resonant cavities to obtain a more wavelength selective filtration response. Alternatively, several devices  240  can be cascaded in series to provide a more selective response. Furthermore, it should be noted that techniques of detuning coupled optical resonators to achieve direct channel switching in add-drop filters as used in the invention is also applicable to other types of mutually coupled optical resonator other than micro-mechanically tuned optical Fabry-Perot resonators, for example mutually coupled optical ring resonators in waveguide devices. 
     Although electrostatic actuation of the central mirrors  360 ,  430  is described above, other methods of actuation are possible: for example piezo-electric actuation where piezo-electric layers, for example comprising zinc oxide as a piezo-electric material, are fabricated onto the assemblies  330 ,  350 ,  420  in STEP 1 of the method of fabrication depicted in FIG.  8 . Such piezo-electric layers enable differential stresses to be generated in arms supporting the central mirrors  360 ,  430  thereby causes the mirrors to change their spatial separation relative to the first assembly  330 . 
     It will be further appreciated that feedback control can be applied to stabilise spatial separation of the mirrors  360 ,  430  relative to the first assembly  330  so that the device  240  remains stably tuned to its allocated channel C i . Such feedback control can, for example, employ low frequency artefacts in channel radiation reflected or transmitted through the device  240 , the low frequency artefacts being at channel modulation frequencies less than that of communication traffic transmitted in the channels C i . 
     Although the aforementioned device  240  is fabricated using gallium arsenide fabrication techniques, the device  240  can also be fabricated in modified form in silicon materials, such silicon materials having superior mechanical properties compared to gallium arsenide which is relatively brittle compared to silicon. The device  240  in modified form in silicon can employ a multipart construction, the parts bonded together after fabrication, for example by fusion bonding. Alternatively, the device  240  in modified form in silicon can be fabricated as a surface structure on a silicon substrate, mirrors and arms of the device  240  in modified form being formed by epitaxial deposition processes with sacrificial oxide layers for use in generating voids corresponding to optical cavities of the device.