U.S. patent application number 09/734495 was filed with the patent office on 2002-06-20 for wavelength selective optical filter.
Invention is credited to Cush, Rosemary, Hibberson, Ruth, Stewart, William J..
Application Number | 20020076147 09/734495 |
Document ID | / |
Family ID | 9886039 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020076147 |
Kind Code |
A1 |
Cush, Rosemary ; et
al. |
June 20, 2002 |
WAVELENGTH SELECTIVE OPTICAL FILTER
Abstract
The present invention provides a wavelength selective optical
filter device (240) for receiving input radiation and outputting
corresponding filtered output radiation, characterized 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 retuned from the first wavelength (.lambda..sub.1) to the
second wavelength (.lambda..sub.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).
Inventors: |
Cush, Rosemary;
(Northampton, GB) ; Stewart, William J.;
(Blakesley, GB) ; Hibberson, Ruth; (Northampton,
GB) |
Correspondence
Address: |
Kirschstein, Ottinger, Israel & Schiffmiller, P.C.
489 Fifth Avenue
New York
NY
10017-6105
US
|
Family ID: |
9886039 |
Appl. No.: |
09/734495 |
Filed: |
December 11, 2000 |
Current U.S.
Class: |
385/27 ; 359/589;
385/15 |
Current CPC
Class: |
H04J 14/021 20130101;
H04J 14/0206 20130101; G02B 6/29362 20130101; G02B 6/29358
20130101; G02B 6/29383 20130101; G02B 6/29395 20130101; G02B 26/001
20130101; G02B 6/12007 20130101; H04J 14/0213 20130101; G02B 26/02
20130101 |
Class at
Publication: |
385/27 ; 385/15;
359/589 |
International
Class: |
G02B 006/26; G02B
005/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2000 |
GB |
0003973.5 |
Claims
1. A wavelength selective optical filter (240) for receiving input
radiation and outputting corresponding filtered output radiation,
characterised in that the filter (240) includes a plurality of
mutually independently tunable optical resonators (330, 340, 360;
360, 400, 430) for filtering the input radiation to generate the
output radiation, the resonators (330, 340, 360; 360, 400, 430)
being at least partially mutually coupled, and the resonators (330,
340, 360; 360, 400, 430) having associated therewith tuning ranges
which at least partially mutually overlap.
2. A filter (240) according to claim 1 wherein the filter (240) 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.
3. A filter (240) according to claim 2 wherein the filter (240) is
operable to function as a wavelength selective attenuator when the
resonators are substantially mutually tuned to a similar
wavelength.
4. A filter according to claim 1,2 or 3 wherein the coupling from
one of the resonators (330, 340, 360) to another resonator (360,
400, 430) adjacent thereto is in a range of 0.01 to 0.1%.
5. A filter according to claim 1, 2, 3 or 4 wherein the resonators
include first and second tunable Fabry-Perot cavities (330, 340,
360; 360, 400, 430), the cavities being at least partially mutually
coupled through a component common (360) to the cavities.
6. A filter according to claim 5 wherein the component is a second
mirror assembly (360) spatially located between the cavities, the
first and second cavities having associated therewith first and
third mirror assemblies (330, 430) respectively, the first and
second assemblies defining the first cavity (340), and the second
and third assemblies defining the second cavity (400).
7. A filter according to claim 5 or 6 wherein each cavity includes
a void (340, 400) in a region between its associated mirror
assemblies.
8. A filter according to claim 7 wherein the void (340, 400) is in
a range of 10 to 20 .mu.m wide in a direction normal to major
planes of its associated mirror assemblies.
9. A filter according to claim 6, 7 or 8 wherein the assemblies
each comprise multilayer structures (330, 350, 430).
10. A filter according to claim 9 wherein each multilayer structure
comprises a plurality of alternating layers of AlGaAs and aluminium
oxide.
11. A filter according to claim 10 wherein the aluminium oxide
layers each have a thickness in a range of 300 nm to 350 nm.
12. A filter according to claim 10 or 11 wherein the AlGaAs layers
of the first and third assemblies (330, 420) are such that each
layer comprises first, second and third sub-layers so that: (i) the
first and third sub-layers have a composition Al.sub.aGa.sub.bAs
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.sub.cGa.sub.dAs where c is in a range of 0.28 to 0.32, and d is
in a range of 0.68 to 0.72.
13. A filter according to claim 12 wherein, in the third assembly
(420), 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.
14. A filter according to claim 12 or 13 wherein, in the first
assembly (330), 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.
15. A filter according to any one of claims 10 to 14 wherein the
second assembly (350) comprises aluminium oxide and AlGaAs layers
such that each AlGaAs layer has a composition Al.sub.aGa.sub.bAs
where a is in a range of 0.58 to 0.62, and b is in a range 0.38 to
0.42.
16. A filter according to claim 15 wherein, in the second assembly
(350), the AlGaAs layers are in a range of 115 to 140 nm thick.
17. A filter according to any one of claims 6 to 16 wherein the
cavities are tunable by altering a spatial separation between their
respective mirror assembly (330, 430) and the second mirror
assembly (350).
18. A filter according to claim 17 wherein at least part of the
mirror assemblies (360, 430) are resiliently suspended and their
mutual spatial separation is alterable by applying piezo-electric
forces to the mirror assemblies (330, 350, 420).
19. A filter according to claim 17 wherein at least part of the
mirror assemblies are resiliently suspended and their mutual
spatial separation is alterable by applying electrostatic forces to
the mirror assemblies (330, 350, 420).
20. A filter according to claim 19 wherein the mirror assemblies
(330, 350, 420) are electrically connected to enable potential
differences to be applied thereto to generate the electrostatic
forces.
21. A filter according to claim 20 wherein the mirror assemblies
(330, 350, 420) each comprise a central mirror region (360, 430)
suspended on a plurality of compliant arms (370, 440).
22. A filter according to claim 21 wherein each central mirror
region (360, 430) is substantially circular and suspended on four
arms (370, 440).
23. A filter according to claim 21 or 22 wherein each central
region (360, 430) has an effective diameter in a range of 50 to 150
.mu.m and each arm (370, 440) has a length in a range of 600 to
2000 .mu.m.
24. A filter according to any one of claims 6 to 23 wherein the
cavities and their associated mirrors assemblies are fabricated
onto a substrate (300) in terraced formation to enable electrical
connection to be made to the mirror assemblies (330, 350, 420), the
assemblies being substantially mutually electrically isolated.
25. A filter according to claim 1 fabricated using gallium arsenide
or silicon fabrication techniques.
26. An add-drop filter (10) 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 (10) incorporating a filter (240) according to any preceding
claim for isolating radiation corresponding to the channel.
27. A method of fabricating a filter according to claim 1, the
method comprising the steps of: (a) forming a series of layers
(330, 350, 420) on a substrate (300), 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 spacer
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.
28. A method according to claim 27 wherein the layers are formed by
metal oxide chemical vapour deposition (MOCVD).
29. A method according to claim 27 or 28 wherein the spacer layers
are preferentially wet etched to form the optical cavities and
render the mirror assemblies freely suspended.
30. A method according to claim 27, 28 or 29 wherein the mirror
assemblies each comprise alternate layers of AlGaAs and AlAs, the
AlAs subsequently heat processed to form aluminium oxide.
31. A method of tuning a filter (240) according to claim 1 from a
first wavelength (.lambda..sub.1) to a second wavelength
(.lambda..sub.2), the method comprising the steps of: (a) tuning
the resonators to the first wavelength (.lambda..sub.1) so that the
filter (240) provides selective filtration at the first wavelength
(.lambda..sub.1); (b) detuning the resonators from the first
wavelength (.lambda..sub.1) by tuning at least one resonator in a
first wavelength direction (760) and another resonator in another
wavelength direction (780) opposite to the first wavelength
direction (760); (c) tuning the resonators in a mutually detuned
state (800, 810) towards the second wavelength (.lambda..sub.2);
and (d) tuning the resonators finally to the second wavelength
(.lambda..sub.2) so that the filter (240) provides selective
filtration at the second wavelength (.lambda..sub.2).
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] Tunable optical filters are known in the prior art.
[0007] For example, a 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.
[0008] Moreover, in a further example, a 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] The filter provides the advantage that it is capable of
being tuned from one wavelength to another without tuning through
intermediate wavelengths therebetween.
[0015] 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.
[0016] Conveniently, the filter in use is:
[0017] (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
[0018] (b) substantially non-transmissive to the input radiation
when the resonators are mutually detuned.
[0019] Such a filter provides the desirable characteristic that it
is substantially non-transmissive to radiation whilst being retuned
from wavelength to another.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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
.mu.m wide in a direction normal to major planes of its associated
mirror assemblies, although 14 .mu.m is its preferred width.
[0024] 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.
[0025] AlGaAs, GaAs and AlAs will be used hereafter as
abbreviations for aluminium gallium arsenide, gallium arsenide and
aluminium arsenide respectively.
[0026] 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.
[0027] 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:
[0028] (i) the first and third sub-layers have a composition
Al.sub.aGa.sub.bAs where a is in a range of 0.58 to 0.62, and b is
in a range 0.38 to 0.42; and
[0029] (ii) the second sub-layer has a composition
Al.sub.cGa.sub.dAs where c is in a range of 0.28 to 0.32, and d is
in a range of 0.68 to 0.72.
[0030] Incorporation of the sub-layers enables optical
characteristics of the assemblies to be closely controlled in
manufacture.
[0031] 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.
[0032] Advantageously, the second assembly comprises aluminium
oxide and AlGaAs layers such that each AlGaAs layer has a
composition Al.sub.aGa.sub.bAs 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.
[0033] 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.
[0034] 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 .mu.m and each arm has a length in a range of 600 to 2000
.mu.m, although an effective diameter of 100 .mu.m and a length of
1000 .mu.m are specific preferred dimensions.
[0035] 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.
[0036] Advantageously, the filter is fabricated using gallium
arsenide or silicon fabrication techniques.
[0037] 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.
[0038] 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:
[0039] (a) forming a series of layers on a substrate, the layers
forming mirror assemblies and spacer layers therebetween;
[0040] (b) defining features in the layers corresponding to
suspended reflectors and associated compliant support arms;
[0041] (c) processing the substrate and its associated layers by
etching processes to generate the mirror assemblies and optical
cavities therebetween where the spacer layers are present;
[0042] (d) defining and generating features on the assemblies
operable to actuate the mirror assemblies relative to one another;
and
[0043] (e) mounting the filter produced by steps (a) to (d) above
in a carrier and making electrical connection to the filter.
[0044] 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.
[0045] 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.
[0046] 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:
[0047] (a) tuning the resonators to the first wavelength so that
the filter provides selective filtration at the first
wavelength;
[0048] (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;
[0049] (c) tuning the resonators in a mutually detuned state
towards the second wavelength; and
[0050] (d) tuning the resonators finally to the second wavelength
so that the filter provides selective filtration at the second
wavelength.
[0051] Embodiments of the invention will now be described, by way
of example only, with reference to the following drawings in
which:
[0052] FIG. 1 is a schematic diagram of a prior art add-drop filter
illustrating its channel isolation function;
[0053] FIG. 2 is a schematic diagram of a prior art communication
system incorporating three add-drop filters similar to the filter
in FIG. 1;
[0054] 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;
[0055] FIG. 4 is a schematic of a filter device included in the
filter module illustrated in FIG.
[0056] FIG. 5 is a graph of transmission and reflection
characteristics of the filter device in FIG. 4;
[0057] FIG. 6 is a graph of transmission characteristics of the
filter device in FIG. 4 when tuning from one wavelength to
another;
[0058] FIG. 7 is a graph of attenuation and reflectance through the
filter device in FIG. 4 as it is detuned; and
[0059] FIGS. 8 and 9 are illustrations of fabrication steps
required for fabricating the filter device in FIG. 4.
[0060] 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.sub.in and
the through port is operable to provide output radiation
S.sub.out.
[0061] The filter 10 is designed to operate over a range of
wavelengths which accommodates the radiation S.sub.in. The
radiation S.sub.in is a summation of radiation components
associated with a sequence of channels C.sub.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.sub.in;
for example, the radiation S.sub.in has a wavelength in the order
of 1550 nm with the channels spaced at wavelength intervals of 0.8
nm. The channels C.sub.i monotonically change in wavelength
according to their respective channel number index i.
[0062] Operation of the filter 10 will now be described with
reference to FIG. 1. The input radiation S.sub.in propagates to the
input port (IN) and further therefrom into the filter 10 whereat a
radiation component corresponding to a channel C.sub.x is extracted
from the radiation S.sub.in and output to the drop port (DROP). The
radiation S.sub.in minus components corresponding to the channel
C.sub.x, namely modified radiation S'.sub.in, propagates further
into the filter 10 whereat a radiation component corresponding to a
channel C'.sub.x input to the add port (ADD) is added to the
radiation S'.sub.in to yield the radiation S.sub.out which is then
output at the through port (THROUGH). Thus, the output radiation
S.sub.out corresponds to the input radiation S.sub.in except that
the component of radiation corresponding to the channel C.sub.x in
the input radiation is replaced by a component of radiation
corresponding to the channel C'.sub.x in the output radiation.
[0063] 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.sub.a, C.sub.b, C.sub.c
respectively where integer subscripts a, b, c can be mutually
different and are included in a range of 1 to n.
[0064] The transmitter unit 110 comprises a series of optical
inputs TC.sub.1 to TC.sub.n for receiving optical radiation
corresponding to the channels C.sub.1 to C.sub.n respectively. In a
similar manner, the receiver unit 120 comprises a series of optical
outputs RC.sub.1 to RC.sub.n corresponding to the channels C.sub.1
to C.sub.n.
[0065] 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.sub.1. The radiation
K.sub.1 propagates from the unit 110 to the filter 130 which
filters out a component of the radiation K.sub.1 corresponding to
the channel C.sub.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.sub.1 propagating through the filter 130 to
generate output radiation K.sub.2. The radiation K.sub.2 propagates
to the filter 140 whereat a component of radiation corresponding to
the channel C.sub.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.sub.2 propagating through the filter 140 to generate output
radiation K.sub.3. The radiation K.sub.3 propagates to the filter
150 whereat a component of radiation corresponding to the channel
C.sub.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.sub.3 propagating through the filter 150 to generate
output radiation K.sub.4. The output radiation K.sub.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.sub.a, TC.sub.b, TC.sub.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.
[0066] 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.
[0067] 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.sub.1 of the circulator 210. Moreover, the
circulator 210 comprises an output port J.sub.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.sub.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.
[0068] 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.sub.in applied to the IN port of the filter 10. The
radiation S.sub.in propagates along the fibre 260 to the input port
J.sub.1 of the circulator 210. The radiation S.sub.in propagates
within the circulator 210 to the port J.sub.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 .mu.m which is received by the device 240. If
the beam received by the device 240 is broader than 100 .mu.m,
deterioration in the filter 200 response will result. The device
240 is tuned to a channel C.sub.x where an integer index x is in a
range of 1 to n. Radiation components in the radiation S.sub.in
corresponding to the channel C.sub.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 S.sub.in corresponding
to the channels C.sub.1 to C.sub.x-1 and C.sub.x+1 to C.sub.n are
reflected from the device 240 and propagate back through the lens
230 and the fibre 280 to the port J.sub.2 of the circulator 210;
these components propagate further in the circulator 210 to its
output port J.sub.3 and further therefrom along the fibre 270 to
the output port of the filter 200.
[0069] 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.sub.x.
[0070] 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
.mu.m to 250 .mu.m, although 200 .mu.m 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:
[0071] (i) the first and third sub-layers have a composition
Al.sub.aGa.sub.bAs where a is in a range of 0.58 to 0.62, and b is
in a range 0.38 to 0.42; and
[0072] (ii) the second sub-layer has a composition
Al.sub.cGa.sub.dAs where c is in a range of 0.28 to 0.32 and d is
in a range of 0.68 to 0.72.
[0073] A preferred composition for the first and third sub-layers
is Al.sub.0.6Ga.sub.0.4As, and a preferred composition for the
second sub-layer is Al.sub.0.3Ga.sub.0.7As.
[0074] The first and third sub-layers are each in a range of 45 to
55 nm thick, and the second sublayer is in a range of 15 to 25 nm
thick. However, 52.8 mn 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.
[0075] Above the mirror assembly 330 remote from the substrate 300
is a first cavity 340 of height in a range of 10 to 20 .mu.m in a
direction normal to the major faces 310, 320 of the first assembly
330, although 14 .mu.m 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.
[0076] 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 .mu.m
thick.
[0077] 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 .mu..mu.m, although 100 .mu.m is its preferred diameter.
Each arm has a length in a range of 600 to 2000 .mu.m although 1000
.mu.m is its preferred length. The arms each have a lateral width
in a range of 15 to 30 .mu.m, although 20 .mu.m is their preferred
width.
[0078] 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.sub.aGa.sub.bAs
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.sub.0.6Ga.sub.0.4As is a preferred composition
for the layers.
[0079] In the second assembly, there are six aluminium oxide layers
and seven AlGaAs layers.
[0080] Above the mirror assembly 350 remote from the substrate 300
is a second cavity 400 of height in a range of 10 to 20 .mu.m in a
direction normal to the major faces 310, 320 of the first assembly
330, although 14 .mu.m 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.
[0081] 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 .mu.m
although substantially 14 .mu.m is a preferred thickness.
[0082] 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.
[0083] Each AlGaAs layer of the third assembly 420 comprises three
sequential sub-layers, namely first, second and third sub-layers
such that:
[0084] (i) the first and third sub-layers have a composition
Al.sub.aGa.sub.bAs where a is in a range of 0.58 to 0.62, and b is
in a range 0.38 to 0.42; and
[0085] (ii) the second sub-layer has a composition
Al.sub.cGa.sub.dAs where c is in a range of 0.28 to 0.32, and d is
in a range of 0.68 to 0.72.
[0086] A preferred composition for the first and third sub-layers
is Al.sub.0.6Ga.sub.0.4As, and a preferred composition for the
second sub-layer is Al.sub.0.3Ga.sub.0.7As.
[0087] The first and third sub-layers are each in a range of 10 to
20 nm thick, and the second sublayer 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 sublayer.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] Operation of the device 240 switching from one channel
C.sub.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.
[0094] A first peak 740 corresponds to the device 240 tuned to one
of the channels C.sub.i with nominal wavelength .lambda..sub.1 and
a second peak 820 corresponds to the device 240 tuned to another of
the channels with nominal wavelength .lambda..sub.2. Initially, the
cavities 340, 400 of the device 240 are tuned to the wavelength
.lambda..sub.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 .lambda..sub.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 .lambda..sub.2 as depicted by curves indicated by 800,
810. When the cavities 340, 400 are both tuned to the wavelength
.lambda..sub.2, the device 240 again becomes transmissive as
represented by the peak 820.
[0095] 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
.lambda..sub.1 and .lambda..sub.2 correspond to two of the channels
C.sub.i.
[0096] 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.
[0097] 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.sub.i in
the networks.
[0098] 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.
[0099] Step 1:
[0100] 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
.mu.m. 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.sub.0.6Ga.sub.0.4As, a second
sub-layer of a nominal thickness 19.9 nm and a nominal composition
Al.sub.0.3Ga.sub.0.7As, 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 .mu.m.
[0101] Next, a first spacer layer of gallium arsenide is grown onto
the first assembly 330. The spacer layer is nominally of 14 .mu.m
thickness.
[0102] 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.sub.0.6Ga.sub.0.4As. The second assembly 350 has a thickness
in the order of 2.8 .mu.m.
[0103] Next, a second spacer layer of gallium arsenide is grown
onto the first assembly 350. The spacer layer is nominally of 14
.mu.m thickness.
[0104] 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.sub.0.6Ga.sub.0.4As, a second sub-layer of a nominal thickness
94.9 nm and a nominal composition Al.sub.0.3Ga.sub.0.7As, 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 .mu.m.
[0105] Completion of STEP 1 results in the generation of a
workpiece indicated by 880.
[0106] Step 2:
[0107] 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.
[0108] Step 3:
[0109] 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.
[0110] Step 4:
[0111] The workpiece 880 from STEP 3 is then subjected to a steam
environment at a temperature in a range of 380 to 420 .degree. C.,
although 400 .degree. 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.
[0112] Step 5:
[0113] 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.
[0114] Step 6:
[0115] 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.
[0116] 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.
[0117] 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.
[0118] 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.
[0119] 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.sub.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.sub.i.
[0120] 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.
* * * * *