Guided wave spatial filter

The invention concerns a guided wave spatial filter (300) for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections (30, L3, L5) connected in series, the sections: (a) mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and (b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation. The spatial filter (300) is implemented using rib waveguides (30) for the sections with associated relatively deeply and relatively shallowly etched structures (20, 310, 320) for imparting to the sections their radiation mode filtration characteristics. The spatial filter according to the invention can be incorporated into optical splitters and optical modulators to enhance their performance and desensitise them to higher-order mode radiation injected thereinto.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/GB01/04479, filed 8 Oct. 2001, which claims priority to Great Britain Patent Application No. 0024670.2, filed on 9 Oct. 2000 in Great Britain. The contents of the aforementioned applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a guided wave spatial filter, in particular but not exclusively to a guided wave spatial filter suitable for use in conjunction with rib waveguides in optical integrated circuits.

For the purposes of describing the present invention, optical radiation is defined as radiation having a free-space wavelength in a range of 100 nm to 15 μm.

Monolithic optical integrated circuits are well known. They are described in a publication “Introduction to Semiconductor Integrated Optics” by H P Zappe (ISBN 0-89006-789-9, Artech House Publishers 1995). A conventional optical integrated circuit includes a substrate onto which one or more optical waveguides are fabricated, the waveguides operable to guide optical radiation around the circuit. It is found especially convenient in such a circuit to employ surface rib waveguides, the rib waveguides being susceptible to fabrication using planar fabrication processes such as lithography and chemical etching.

A rib waveguide assumes the form of a ridge projecting from a major surface of the substrate, the ridge having associated therewith one or more refractive index interface regions to assist optical radiation confinement in the ridge. The optical radiation is guided along the ridge and is confined primarily to a region lying between the one or more interface regions and an upper surface of the ridge remotest from the substrate. In such a rib waveguide, a plurality of interface regions gives more degrees of freedom when optimising optical characteristics of the waveguide.

The ridge is designed to be of a width and height comparable to within an order of magnitude with the wavelength of optical radiation to be guided along the rib waveguide formed by the ridge. For example, a rib waveguide designed to guide optical radiation having a free-space wavelength of substantially 1.5 μm will have a lateral width in the order of 3 μm and a projection height of its ridge above its substrate in the order of 1 μm. The rib waveguide can be fabricated from silicon or alternatively from III–V compounds, for example aluminium gallium arsenide (AlGaAs) and gallium arsenide (GaAs). III–V compounds such as AlGaAs possess a crystal structure lacking a centre of symmetry and therefore exhibit both Pockets and Kerr effects. As a consequence of such asymmetry, rib waveguides based on III–V compounds are especially suitable for use in fabricating active optical integrated circuits, for example optical modulators and switches.

SUMMARY OF THE INVENTION

For purposes of describing the present invention, it is important to distinguish between different modes of radiation which are capable of propagating within an optical integrated circuit including one or more rib waveguides, the different modes including:(a) “in-plane radiation” modes which propagate in a plane parallel to a surface plane of the circuit from which the one or more rib waveguides project;(b) “free radiation” modes corresponding to radiation which is scattered into a bulk region of a substrate forming the circuit and also into an air region neighbouring onto the rib waveguide and the surface plane;(c) “bound-radiation” modes including two-dimensional modes and all higher-order modes capable of propagating along the waveguides; and(d) “fundamental radiation” mode for which the waveguides are specifically designed to convey.

A higher-order mode is described by referring to its associated “m” number which is numerically equal to the number of zero-crossings in the transverse field profile of the higher-order mode radiation; high-order modes have an m value of one or more.

Rib waveguides are effective at guiding aforesaid fundamental mode optical radiation; in the fundamental mode, most of the optical radiation propagates in a region closely associated with the rib waveguide. However, such waveguides are also capable of guiding aforesaid bound-mode radiation. In theory, when fundamental mode optical radiation is launched into a rib waveguide capable of supporting multimode radiation propagation therethrough, the radiation propagates through the waveguide as fundamental mode radiation and coupling of energy from the fundamental mode to other radiation modes does not occur. In practice in real optical integrated circuits comprising rib waveguides, coupling between fundamental mode radiation and other radiation modes does occur. Such coupling arises at any abrupt discontinuity and especially where:(a) the rib waveguide changes directions at bends therein, for example where it follows a curved path;(b) the rib waveguide connects to an optical coupler, for example a multimode interference coupler (MMI); or(c) the rib waveguide comprises geometrical disturbances which function as scattering defects.

Thus, in an optical integrated circuit including rib waveguides, fundamental mode radiation propagating therein can couple to higher-order bound radiation which propagates substantially together with its associated fundamental mode radiation. Indeed, the fundamental mode radiation and the higher-order mode radiation may interact coherently. Such coherent interaction often results in a quasi-chaotic perturbation to characteristics of the circuit because the fundamental mode radiation and the higher-order mode radiation possess different characteristic propagation velocities and therefore different relative phases at a downstream interaction zone within the integrated circuit. Where such an interaction zone corresponds to an interferometric modulator, for example a Mach-Zehnder interferometer, an extinction ratio characteristic of the modulator is degraded.

The aforesaid higher-order mode radiation propagates along paths coinciding fully with the fundamental mode radiation; the higher-order modes are distinguished from the fundamental mode by having a spatial energy profile which is somewhat broader and including a phase-reversing lobe structure corresponding to m values of unity or greater. Coupling of radiation from the fundamental mode can also give rise to aforesaid in-plane radiation and free radiation modes.

In an integrated optical circuit including rib waveguides in which fundamental mode radiation is coupled to higher-order mode radiation, attenuating the higher-order mode radiation by using, for example, tapered rib waveguide filter structures adapted to promote the transmission of substantially fundamental mode radiation therethrough is not found to be effective. Such a tapered structure simply results in the coupling of the aforementioned higher-order mode radiation to loosely bound radiation modes which nevertheless follow the path of the waveguides. The loosely bound radiation modes can persist for relatively long distances, for example in the order of mm, spreading and dissipating only gradually with propagation distance.

The inventor has appreciated that the free radiation modes, the in-plane radiation modes and the bound modes are deleterious to the optical performance of optical integrated circuits including rib waveguides. Moreover, the inventor has devised a solution to address this problem, the solution being in the form of a spatial filter which is effective at attenuating loosely bound radiation modes and higher-order bound radiation modes, and transmitting fundamental mode radiation.

Thus, according to a first aspect of the present invention, there is provided a guided wave spatial filter for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections connected in series, the sections:(a) mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and(b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation.

The invention provides the advantage that it is capable of attenuating higher-order bound mode radiation components and preferentially transmitting fundamental mode radiation.

Fundamental mode radiation is defined as radiation having a corresponding modal m value of zero. Higher-order bound mode radiation is defined as radiation having a corresponding modal m value of unity or greater.

The spatial filter is beneficially fabricated using microfabrication techniques such that the waveguide sections comprise rib waveguides. Rib waveguides are convenient to use when designing integrated optical circuits, thereby enabling the filter of the invention to be incorporated into such circuits.

When implementing the spatial filter, it is convenient that the first section is flanked by first regions for supporting fundamental mode propagation along the first section and the second section is flanked by second regions for stripping in-plane radiation modes, the first regions being more shallowly etched relative to the second regions. Such etching can be used to impart desired optical characteristics to the sections.

Beneficially, the second regions do not encroach onto the second section but are spatially separated therefrom by the first regions. Preferably, the second regions are tapered towards the second section to provide a pin-hole type filter.

When fabricating the aforementioned first and second waveguide sections using rib waveguides, it is important that dimensional tolerances are precisely controlled. In order to achieve such precise dimensional control, the inventor has devised a self-aligning process for fabricating the sections. Conveniently, where a self aligned process can be used, the second regions extend to abut onto the second section.

In order to provide adequate radiation containment in the second section, the second section preferably includes outwardly inclined walls to its rib waveguide, the walls mutually inclined to hinder lateral mode radiation propagation along the second section.

In the spatial filter, it is desirable to deflect in-plane mode radiation away from propagation paths of corresponding fundamental mode radiation. Thus, preferably, the second regions have boundary edges which are disposed at non-orthogonal angles relative to an elongate axis of the second section, the edges operable to deflect higher-order mode radiation received thereat away from a fundamental mode radiation propagation path along the second section.

For obtaining satisfactory filtration, the rib waveguide of the second section is preferably laterally broader than the rib waveguide of the first section. However, when designing the spatial filter, rib waveguide widths in the sections are selected to provide a satisfactory matching of fundamental mode radiation between the sections.

The spatial filter is preferably arranged so that the second section is provided with two first sections configured to precede and follow the second section, the sections being connected in series. Such an arrangement is advantageous where the first sections correspond to a form of rib waveguide generally used in an optical integrated circuit incorporating the filter as an in-line component where the filter employs a form of rib waveguide not generally used in the integrated circuit for the second section.

According to a second aspect of the present invention, there is provided an optical radiation splitter comprising a mixing region for supporting multimode radiation propagation therein, an input waveguide for injecting radiation into the mixing region, a plurality of output waveguides for receiving radiation output from the mixing region, and a spatial filter according to the first aspect of the present invention included between the input waveguide and the mixing region for transmitting fundamental mode radiation from the input waveguide into the mixing region and for hindering higher-order mode radiation from being injected from the input waveguide into the mixing region.

The filter provides the benefit that higher-order mode radiation injected into the input waveguide, for example resulting from a misaligned fibre waveguide connected to the input waveguide, does not interfere with operation of the splitter, the splitter thereby being desensitised to higher-order mode radiation components input to the splitter.

In the splitter, it is beneficial that at least one of the output waveguides is connected to the mixing region through a spatial filter according to the first aspect of the invention, the filter operable to hinder coupling of higher-order mode radiation from the mixing region into said at least one of the output waveguides. Inclusion of the filter at the output waveguides assists to prevent higher-order mode radiation excited within the mixing region being coupled down-stream to other optical devices connected to the splitter.

In a third aspect of the present invention, there is provided an optical modulator incorporating modulating means for selectively diverting radiation input to the means between a plurality of output paths in response to a control signal applied to the means, at least one of the output paths including a filter according to the first aspect of the present invention for transmitting fundamental mode radiation output from the means to said at least one of the output paths and for hindering propagation of higher-order mode radiation from the means to said at least one of the output paths.

It is preferable that the modulating means and output paths are implemented in rib-waveguide form, the modulating means and the filter being flanked by regions which are sufficiently deeply etched to render them unsusceptible to propagating higher-order mode radiation therethrough. The relatively deeply etched regions are effective at containing radiation and thereby providing the modulator with an enhanced extinction ratio.

When designing the modulator to be compact, it is beneficial that at least one of the output paths includes a faceted rib waveguide boundary where the path adjoins onto the modulating means, inclusion of the boundary enabling the path to project directly away from the modulating means at a greater angle than possible without the faceted boundary. Such a greater angle enables the modulator in microfabricated form to be shorter.

In order to desensitise the modulator to higher-order mode radiation injected thereinto, the modulator preferably comprises an input path for injecting radiation into the modulating means, the input path connected through a filter according to the first aspect of the invention, the filter operable to transmit fundamental mode radiation from the input path to the modulating means and to hinder higher-order mode radiation being injected from the input path to the modulating means.

Preferably, the modulating means comprises a structure susceptible to electric field induced changes of refractive index for selectively diverting the radiation input to the modulating means to the output paths.

In a fourth aspect of the present invention, there is provided a method of fabricating a guided wave spatial filter for receiving input radiation and outputting corresponding spatially filtered output radiation, the filter comprising first and second rib-waveguide sections connected in series, the sections mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation, and mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation, the method of fabrication including the steps of:(a) providing a substrate onto which the filter is to be formed;(b) defining and forming the rib-waveguide sections; and(c) defining and forming structures to impart to the sections their radiation mode filtering characteristics, the rib-waveguide sections being used in the method to define boundaries of the structures and thereby render the method a self-aligning process.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

Referring now toFIG. 1, there is shown a prior art integrated optical circuit indicated generally by10. The circuit10comprises in sequence a GaAs substrate12, a lower substrate layer15having a chemical composition AlxGa(1−x)As where a subscript x is in a range of 0.1 to 0.3, an upper substrate layer20having a chemical composition GaAs, and a surface rib waveguide30having a chemical composition AlyGa(1−y)As where a subscript y is in a range of 0.1 to 0.3. With regard to chemical composition, symbols Al, Ga and As are used to represent aluminium, gallium and arsenic respectively. The rib waveguide30has a lateral width in the order of 3 to 5 μm and a projecting height in the order of 1 μm above an exposed planar exposed surface35of the layer20. Side walls of the waveguide are shown tapered inwardly in a direction towards an upper surface of the waveguide30remotest from the layers15,20. The rib waveguide30is designed to guide optical radiation having a free-space wavelength of substantially 1.5 μm. The layer20is designed to exhibit a relatively higher refractive index than that of the lower substrate layer15and the rib waveguide30for radiation confinement purposes to the waveguide and a region closely associated therewith.

The waveguide30is substantially straight in a first section L1but is curved at a bend point40into a second straight section L2. A monomode optical fibre waveguide50is also shown, the fibre waveguide50for injecting optical radiation through a focussing lens60into the rib waveguide30.

Only part of the circuit10is shown inFIG. 1; the waveguide30continues to other sections of the circuit10, for example modulators and couplers.

In operation, the waveguide30is designed to guide substantially fundamental mode optical radiation therealong; the waveguide30is however capable of also guiding higher-order bound radiation modes. Input optical radiation having a free-space wavelength of substantially 1.5 μm is guided along the fibre waveguide50and exits at an open end thereof to the lens60which focuses the radiation substantially onto an open end of the rib waveguide30. The radiation then propagates along the first section L1of the rib waveguide30towards the bend point40and therepast along the second section L2further into the integrated circuit10.

In coupling radiation into the integrated circuit10, there are a number of ways in which radiation other than fundamental mode radiation can be excited which is capable of propagating along the rib waveguide30into the circuit10. As a first example, inefficient launch of the radiation from the fibre waveguide50into the rib waveguide30can result in higher-order bound mode radiation being excited in the rib waveguide30. As a second example, misalignment of the lens60can also result in higher-order bound mode radiation being excited, especially odd-order radiation modes. As a third example, the bend point40can give rise to radiation scattering and hence excitation of higher-order radiation modes in planar waveguide regions lying on both sides of the waveguide30; radiation scattering at the bend point40becomes more severe as the radius of curvature of the rib waveguide30there becomes smaller. As a fourth example, a geometrical disturbance such as a kink or an indent70in the rib waveguide30can also result in coupling of fundamental mode radiation to higher-order mode radiation and also will scatter radiation to surface planar modes.

Moreover, dumped light from an on-chip filter (not shown inFIG. 1) or an unwanted complementary output from an interference modulator (also not shown inFIG. 1) can additionally, for example, give rise to non-fundamental mode radiation propagation within the circuit10; such non-fundamental mode radiation can, for example, couple into the rib waveguide30.

From the foregoing, it will be appreciated in optical integrated circuits that, although fundamental mode radiation propagation within the circuits is desired, it is relatively easy to excite higher-order radiation modes. Such higher-order mode radiation can be deleterious to circuit performance. The inventor has appreciated that it is desirable to include structures in optical integrated circuits which filter out bound higher-order mode radiation excited therein, the structures being one or more guided wave spatial filters according to the invention.

In order to further describe the nature of radiation propagation along the rib waveguide30, reference is made toFIG. 2which is a cross-sectional view of the rib waveguide30inFIG. 1. The cross-sectional view illustrates energy contours associated with fundamental mode radiation propagation along the waveguide30. Along an axis A-B, the energy contours follow an approximately Gaussian profile where most of the fundamental mode energy is concentrated towards a central region100substantially equi-distant between the waveguide30and an interface of the layers15,20in the vicinity of the waveguide30. This approximately Gaussian profile80is illustrated in a graph indicated generally by90inFIG. 2. Only a small proportion of the fundamental mode radiation impinges into lateral regions105,110away from the rib waveguide30. The lateral regions105,110effectively form a slab-waveguide in which in-plane mode radiation is capable of propagating. Higher-order mode bound radiation is held within the slab-waveguide and disperses spatially only relatively gradually along the rib waveguide30. As the order m of the radiation mode increases, progressively a greater proportion of the radiation energy of the mode is present in the lateral regions105,110compared to the central region100as illustrated by a profile95in the graph90.

The waveguide30inFIGS. 1 and 2can be fabricated epitaxially using a fabrication process comprising the following steps:(a) providing a polished bulk GaAs substrate12;(b) epitaxially growing the lower substrate layer15onto the substrate12;(c) epitaxially growing the upper substrate layer20onto the lower substrate layer15;(d) epitaxially growing a top layer of AlyGa1−yAs onto the upper substrate layer20;(e) depositing an etch resist layer onto the top layer and then lithographically delineating the resist layer using a mask to define the rib waveguide30;(f) reactively ion etching or isotropically wet etching the top layer through the delineated etch resist layer so that the top layer is etched away except in a region of the waveguide30; the etching may be allowed to proceed a relatively short distance, for example in the order of 0.5 μm, into the upper substrate layer20as illustrated inFIG. 1; the waveguide30can project, for example, by a height of substantially 1 μm; and(g) stripping away the etch resist layer.

The etching in step (f) can give the waveguide30its tapered side wall form but will depend upon etching conditions and type of etch employed. The anisotropic wet chemical etching produces tapered walls to the waveguide30automatically provided that the upper substrate layer20is grown to provide a <001> orientated exposed plane surface so that the tapered side walls of the waveguide30corresponds to <110> crystal planes.

If the etching process during fabrication of the waveguide30is prolonged so that the regions105,110are thinner, lateral confinement of radiation by the waveguide30is enhanced especially at bends, for example at the bend point40. Deeper etching into the upper substrate layer20beyond a certain limit establishes a possibility of bound higher-order lateral modes propagating in a direction along the waveguide30. Such higher-order lateral modes are usually undesirable and therefore the depth of etching is limited and represents a compromise.

In the prior art, there are several known guided-wave structures capable of providing mode-dependent filtration properties; such properties arise from the structures behaving as scattering disturbances whose effectiveness increases with the order m of the radiation mode propagating through the structures. The structures often assume a shape similar to a waveguide kink or arc-bend. When the structures assume the form of S-bends, there is an extra beneficial effect of lateral optical path displacement for fundamental mode radiation out of the field of in-plane mode radiation which is expected to continue on its original straight-line trajectory rather than faithfully following S-bend trajectories since, by definition, such radiation is not bound to the rib waveguide.

A circular arc bend in a rib waveguides possesses an extremely sharp cut-off characteristic as a function of bend radius when considering higher-order radiation mode confinement around the bend. As the radius of the bend is reduced, and thereby lateral confinement weakened, radiation loss characteristics of the bend change relatively little until the radius reaches a threshold value beyond which the loss characteristics of the bend increase rapidly for higher-order radiation modes. Accordingly, an arc bend can be designed to exhibit a relatively low loss for fundamental mode radiation but a relatively high loss for more weakly bound higher-order radiation modes.

When designing radiation mode filters into optical integrated circuits, rib waveguides configured to include bends therein for providing a mode filtering function suffer a number of problems. A transition from a straight section of rib waveguide into a curved section of rib waveguide following on from the straight section represents an asymmetrical radiation trajectory disturbance; the curved section is an inherently radiation-leaky structure. Moreover, in practice when fabricating curved section rib waveguides, lithographic processes are employed which make use of lithographic masks. These masks include curved pattern features which are digitised in nature and comprise a sequence of lateral steps, the steps being barely perceptible. However, such steps result in the formation of scattering features and hence in a finite minimum scattering loss that can be obtained from curved rib waveguides fabricated from the masks.

It is found in practice that including a relatively small S-bend rib waveguide filter at an optical input port of an optical integrated circuit device does not afford adequate filtration of higher-order mode radiation components excited, for example, as a consequence of misalignment of an optical fibre waveguide relative to the optical port, the fibre operable to launch radiation into the optical port.

The inventor has therefore devised an alternative guided wave spatial filter which is more effective at rejecting higher-order mode radiation and is more compact than the aforementioned S-bend, circular arc bend and waveguide kink filters presently used. Moreover, the alternative spatial filter is of a symmetrical configuration.

The inventor has appreciated that surface guiding of radiation in the rib waveguide30can be impeded by etching the upper substrate layer20deeper in selected regions thereof. Such selective deeper etching is effective at forcing at least substantially 1-dimensional radiation modes propagating within the regions105,110into cutoff. Thus, the upper substrate layer20is made selectively thinner in regions thereof so that the layer20is incapable of supporting propagation of radiation through the regions. Preferably, the substrate layer20is selectively etched through completely in the aforementioned regions.

The inventor has also appreciated that the waveguide30can be fabricated by etching through the upper substrate layer20entirely around the waveguide30. However, it is found in practice that wet chemical etching processes used to etch deeply into the upper substrate layer20and the lower substrate layer15also etch laterally causing the waveguide30to become impracticably narrow; such narrowing of the waveguide30can to an extent be circumvented by using reactive ion etching (RIE) processes in substitution for wet etching processes. However, these RIE processes are more costly and exhibit a lower etching rate compared to the wet etching processes. Moreover, RIE processes are incompatible with self-alignment techniques used to fabricate guided wave spatial filters according to the invention; such self-alignment will be described later.

Referring now toFIG. 3, there is shown in plan view a pin-hole type guided wave spatial filter according to a first embodiment of the invention. The pin-hole filter is indicated generally by200and includes the substrate12, the lower substrate layer15, the upper substrate layer20, the rib waveguide30and two deeper etch regions210,220etched symmetrically about the waveguide30but not encroaching thereonto; regions of shallower etching230,240separate the waveguide30from the deeper etch regions210,220. The deeper regions210,220each assume the form of a triangular area having a corner thereof orientated to point towards the waveguide30as illustrated inFIG. 3, each corner is at a distance of 10 μm from its nearest peripheral wall of the waveguide30. The comers are mutually separated by a distance of substantially 23 μm.

InFIG. 3, there is also shown a cross-sectional view of the filter200along an axis C–D, the view indicated generally by250.

When fabricating the filter200, the following processing steps can be employed:(a) the lower substrate layer15is epitaxially grown onto the polished GaAs bulk substrate12, the lower substrate layer15having a chemical composition AlxGa1−xAs; a subscript x here has a value in the range of 0.1 to 0.2.(b) the upper substrate layer20is epitaxially grown onto the lower substrate layer15, the upper substrate layer20having a chemical composition GaAs;(c) a top layer is epitaxially grown onto the upper surface layer20, the top layer having a composition AlyGa1−yAs where a subscript y here is in a range of 0.1 to 0.3;(d) a first etch resist layer is deposited onto the top layer;(e) the first etch layer is delineated using a first lithographic mask to define the rib waveguide30of nominally 3 μm lateral width and leaving areas surrounding the waveguide30unmasked;(f) a wet etching process or a dry reactive ion etching process is then applied to etch shallowly to a depth of approximately 1 μm in the surrounding areas but not at the waveguide30which is masked off from the etchant by the delineated first etch resist layer;(g) the first layer is stripped off, and then a second etch resist layer is deposited onto the waveguide30and the shallowly etched areas surrounding the waveguide30;(h) a second lithographic mask is then optically aligned relative to the waveguide30and then the deeper etch regions210,220delineated from the second mask into the second etch resist layer;(i) an anisotropic wet chemical etch or reactive ion etch is then applied to etch the regions210,220through delineated triangular windows formed in the second etch resist layer; and(j) the second etch resist layer is finally stripped off.

The deeper etch regions210,220are etched to a greater depth into the upper surface layer20relative to the shallower etch regions surrounding the waveguide30. If necessary, the deeper etch regions210,220can be etched into the lower substrate layer15, or even as far as the GaAs substrate12itself.

The pin-hole spatial filter200operates in a manner analogous to a bulk-optics pinhole; the waveguide30passes between the deeper etch regions210,220at both sides of the waveguide30. The regions210,220are effective at reflecting higher-order mode radiation away from the path of the waveguide except for radiation closely associated with the waveguide, namely fundamental mode radiation and higher-mode radiation mode components having m values close to unity, for example having nm values in a range of 2 to 10.

The pin-hole filter200exhibits a number of limitations. In optical integrated circuits, it is these higher-order radiation modes having m values close to unity that are most deleterious to circuit performance; many of these radiation modes are capable of propagating through the filter200together with the fundamental mode radiation. Moreover, concatenations of such a pin-hole filter200perform little better than an isolated single pin-hole filter200unless the filters are well spaced in the concatenation. Radiation passing through such a concatenation becomes collimated and subsequently has a relatively low diffraction angle with respect to an elongate axis of the waveguide30. Furthermore, the fabrication steps (a) to (j) to make the filter200require alignment of two masks; in other words, the steps (a) to (j) do not correspond to a self-alignment process. The inventor has appreciated that a self alignment fabrication process is highly desirable. Additionally, a possible minimum width of the regions230,240is limited by etch undercut tolerances in the etching processes used to define the waveguide30and also the deeper etch regions210,220.

The inventor has therefore devised an alternative design of spatial filter according to the invention which addresses the performance limitations of the filter200as well as technical difficulties associated with its fabrication. Referring toFIG. 4, there is shown in plan view an in-line type guided wave spatial filter according to a second embodiment of the invention, the in-line guided wave filter indicated generally by300. The filter300comprises the waveguide30flanked along its length in sections L3, L4thereof by the upper substrate layer20which has been shallowly etched back so that an upper surface of the waveguide remotest from the layer20projects 1 μm above the layer20in a similar manner as in the integrated circuit10. The filter300additionally comprises a central section L5situated between the section L3, L4where the waveguide30is flanked by deeper etch regions310,320at both sides thereof These deeper regions310,320both taper outwardly away from the waveguide30; as shown inFIG. 4, the regions310,320commence their outward taper a short distance away from the waveguide30. Moreover, the regions310,320extend up to the waveguide30at the section L5as shown inFIG. 4. The waveguide30is of wider width along the section L5relative to its width along the section L3, L4.

The deeper regions310,320are etched sufficiently deeply into the upper substrate layer20to prevent higher-order radiation modes from propagating therein. The regions310,320can, if required, be etched through into the lower substrate layer15. In a limiting case, the regions310,320can be etched through to the substrate12.

The section L5is designed to provide an efficient waveguide match to the sections L3, L4as far as fundamental mode radiation is concerned. However, its match for higher-order radiation modes having an index m value of unity or greater is relatively poor. As a consequence, such higher-order mode radiation is not efficiently transmitted through the filter300whereas fundamental mode radiation having an index value m=0 is efficiently transmitted.

Referring now toFIG. 5, there is shown a view of an energy profile indicated by400of fundamental mode radiation propagating through the sections L3, L4of the spatial filter300. There is also shown indicated by410an energy profile of the fundamental mode radiation propagating through the section L5. In the profile400, the fundamental mode radiation impinges laterally beyond the waveguide30into the upper substrate layer20. In contrast, in the profile410, the radiation propagates more closely beneath the waveguide30. Regions420are sufficiently thin so that higher-order mode radiation is unable to propagate along the waveguide30in the section L5. Despite the profiles400,410appearing to have different energy distributions, their calculated overlap integral is in the order of 98 to 99% corresponding to transmission loss of 0.06 dB for fundamental mode radiation transmitted through the filter300. However, energy profiles for higher-order radiation modes through the sections L3, L4relative to the section L5are much more dissimilar; the filter300is thus capable of providing mode filtration.

In the section L5, etching processes used to produce the filter300result in the generation of sloping sidewalls430. Such sloping sidewalls430arise when wet anisotropic etchants are employed which preferentially etch certain crystallographic planes more slowly than other planes; crystallographic planes exposed at the sloping sidewalls correspond to relatively slow etching planes. If reactive ion etching is used to form the deeper regions310,320, the sidewalls430can be made having a relatively steeper inclination. As aforementioned, if the sidewalls420are substantially mutually parallel, there is a tendency for the waveguide30to support propagation of undesirable lateral radiation modes.

Fabrication of the filter300will now be described with reference toFIG. 6. The filter300is fabricated using a self-alignment process which involves photolithography using first and second masks, namely MASK1and MASK2respectively. The first mask is used for defining a region500which becomes the waveguide30after processing. The second mask is used for protecting shallow etch regions510and for exposing regions520for deep etching purposes. The regions520pass beneath the region500, the first mask being used to define the edge of the waveguide30in the section L5. The first mask is arranged so that the region500is 8 μm wide in the L5section, and 4 μm wide in the L3, L4sections. Unlike the filter200which requires that its deeper etch regions210should be precisely aligned symmetrically about the waveguide30, fabrication of the filter300does not need critical mutual mask alignment; for this reason, the filter300is fabricated using a process which is effectively self-aligning as will now be described.

Fabrication steps 1 to 7 of a self-aligning process indicated by600associated with fabricating the filter300are illustrated inFIG. 7. Although this process is described in the context of III–V semiconductors, it can be applied to other material systems, for example silicon.

The GaAs substrate12, with in sequence its epitaxially grown lower substrate AlxGa1−xAs layer15, its upper substrate GaAs layer20, and a top layer AlyGa1−yAs layer610is initially provided. The top layer20is grown so that it exposes a <001> plane on its upper exposed surface remotest from the substrate12.

A metallic layer of aluminium-titanium620is then vacuum deposited onto the top layer610. A layer of organic resist630is then spun onto the layer620. The region500inFIG. 6corresponding to the waveguide30inFIG. 4is then delineated in the resist layer630by UV exposure using MASK1. The organic resist is then developed in an organic solvent to leave an island of resist630as illustrated in STEP 3. It is to be noted, as an alternative to using the metallic layer620, dielectric layers such as silicon oxy-nitrides can be employed.

Sputtering or etching processes are then applied to remove the layer620where it is not protected by remaining regions of the resist layer630to leave a metallic region640remaining as shown in STEP 4. It is to be noted that a “lift-off” process can alternatively be used in steps 2 to 4 to define the metallic region640, the “lift-off” process involving deposition of the resist layer630directly onto the top layer620, exposure of the resist layer630through MASK1and subsequent resist development in a solvent to yield an undercut resist profile, followed by deposition of the metal layer620onto the resist630and delineated windows therein, followed by solvent development to remove the resist layer630leaving the metallic layer620remaining where it is directly deposited onto the top layer610through the aforementioned windows.

The substrate12and its associated layers from STEP 4 is then subjected to an anisotropic wet chemical etch which is sustained for a sufficient duration to etch to a depth of substantially 1 μm down from the top of the top layer610. The wet chemical etch is formulated not to etch into the residual metallic layer640. The wet etch thereby defines the rib waveguide30and its associated sloping sidewalls which correspond to exposed <110> cystallographic planes of the top layer610.

A layer of organic resist650is then spun unto the substrate12and its associated layers from STEP 5 as shown. The resist layer650is then exposed to UV radiation through MASK2to delineate the region520inFIG. 6corresponding to the deeper etch regions310,320inFIG. 4; it is to be noted that the region520projected from MASK2onto the resist layer650passes over the residual metallic layer640in STEPS 5, 6 where the layer650acts as an effective mask against chemical etching, thereby partitioning the region520to create the two regions310,320; this is an important aspect because it provides the process with its “self-aligning” characteristic.

The resist layer650is next developed to leave the regions520devoid of resist protection. The anisotropic wet chemical is then reapplied so that the region520corresponding to the regions310,320are etched to a depth of substantially 2 μm, namely so as to thin the upper surface layer20sufficiently so that it is incapable of sustaining higher-order mode radiation propagation therein. The wet etch generates the sloping sidewalls430as depicted in STEP 7 and illustrated inFIG. 5. Remaining regions of the resist layer650after UV exposure through MASK2are used to protect the upper layer20in its shallower etch regions corresponding to the regions510inFIG. 6during the anisotropic wet etch.

It will be appreciated in the process that reactive ion etching can be used as an alternative to wet chemical etching. However, although reactive ion etching is generally anisotropic, such etching is less influenced by exposed crystallographic planes and hence can result in steeper sidewalls430.

Variants of the spatial filter300inFIG. 5can be fabricated in conjunction with other types of optical structures, for exampleFIG. 8is an illustration of a multimode interference splitter indicated generally by800, the splitter800comprising a guided wave spatial filter indicated by810according to a third embodiment of the invention incorporated at an optical input port of the splitter800.

In comparison to the splitter800, a conventional multimode splitter comprises an optical input port coupled to a multimode mixing region including a plurality of optical output ports. The input and output ports are coupled to waveguides for guiding optical radiation to and from the mixing region. Fundamental mode input radiation is applied through a waveguide associated with the input port to the mixing region and excites a multiplicity of radiation modes in the region. The output ports are judicially positioned where the multiplicity of modes mutually sum to provide energy maxima. As a consequence, such a conventional multimode splitter can be made to couple radiation highly efficiently.

However, a problem arises where the radiation applied through the input port to the conventional splitter is not pure fundamental mode radiation but comprises an add mixture of fundamental and higher-order mode radiation. Such higher-order mode radiation will modify the balance of the multiple modes within the mixing region and result in a reduction of efficiency of the splitter by shifting positions and phase where energy maxima occur. The inventor has therefore appreciated that it is desirable to include the filter810at the input port of the splitter820.

Referring toFIG. 8, the splitter800comprises an input rib waveguide820of similar construction to the aforementioned rib waveguide30. The waveguide820is flanked on both sides thereof by shallow etched regions830of 1 μm depth relative to an upper surface of the waveguide820. The regions830are etched in an upper substrate layer corresponding to the upper substrate layer20. The waveguide820is connected to an input port of the filter810, the filter810being flanked on both sides thereof by deeper etched regions840which are sufficiently thin to render them incapable of supporting the propagation of higher-mode radiation therein. The filter810is preferably in a range of 150 to 200 μm long although other lengths can be employed. The filter810is connected at its output port through a tapered region850to a multimode mixing region860of the splitter800, the mixing region860also being flanked by the same deeper etched regions840. Connected to an end of the region860remote from the filter810are optical output ports connected to associated rib waveguides870,880,890. The waveguides870,880,890are flanked by shallow etched regions900. Moreover, the waveguides870,880,890are of similar construction to the aforesaid rib waveguide30. The shallow etched region is tapered backwardly away from the waveguide820although this is not essential for operation. The filter810is operable to provide an efficient matching for fundamental mode radiation propagating from the waveguide820into the mixing region860but an inefficient transmission of higher-order radiation modes. Backward tapering of the deeper-etched regions840assists to deflect higher-order mode radiation away from the filter810and the mixing region860.

Operation of the filter810in the splitter800will now be described with reference toFIG. 8. A mixture of fundamental mode radiation and bound higher-order mode radiation propagates along the waveguide820, the higher-order mode radiation being excited, for example, by a bend earlier in the waveguide820or a misaligned optical fibre waveguide launching radiation into the waveguide820. The mixture of radiation propagates along the waveguide820to the filter810which transmits the fundamental mode radiation into the mixing region860and substantially blocks and reflects the higher-order mode radiation. The fundamental mode radiation injected into the mixing region excites a multiplicity of radiation modes therein in an intended manner substantially uninfluenced by higher-order mode radiation propagating in the waveguide820. The multiplicity of modes in the mixing region860propagate to the output ports of the mixing region860and therefrom along the waveguides870,880,890to other optical structures (not shown) connected to-the splitter800.

The tapered region850is tapered to deflect any residual higher-order mode radiation sideways. The tapered region850is an optional feature and can be omitted without preventing the mixing region860from performing its mixing function.

In a modified version of the splitter800, one or more filters similar to the filter810are included at the output ports between the mixing region860and the waveguides870,880,890for ensuring that substantially fundamental mode radiation is output into the waveguides870,880,890.

Filters according to the invention can be incorporated into other types of optical structure. For example, referring toFIG. 9, there is shown an interferometric modulator indicated generally by1000including a guided wave spatial filter1010according to a fourth embodiment of the invention, the filter1010incorporated at an optical output port of the modulator1000. The modulator1000comprises an input rib waveguide1020flanked by shallow etch regions1030on both sides thereof, first and second elongate mixing regions indicated by1040a,1040brespectively and including a modulation structure1050and surrounded by deep etch regions1060, a first output rib waveguide1070flanked by the deep etch regions1060, the filter1010flanked by deep etch regions1060and a second output rib waveguide1080flanked by shallow etch regions1090. In a similar manner to the splitter800, the regions1060are etched sufficiently deeply so that higher-order mode radiation is not capable of propagating therein. The shallow etch regions1030,1090are etched sufficiently deeply for the waveguides1020,1080to be capable of guiding fundamental mode radiation. The waveguide1020is connected to the first mixing region1040a.Likewise, the waveguides1070,1080are connected to the second mixing region1040b,the waveguide1080connected via the filter1010.

The input waveguide1020, the filter1010and the mixing regions1040a,1040bare substantially co-linearly disposed. The first output waveguide1070is connected off-axis to the second mixing region1040b,the first waveguide1070including a reflective facet1100on a side of the waveguide1070facing towards the second waveguide1080and the filter1010.

The modulator1000is fabricated using III–V compound technology so that electric-field induced. Pockels effect changes in refractive index in the structure1050can be exploited for selectively diverting radiation between the output waveguides1070,1080. Metallic electrode tracks represented by1110are connected to the structure1050for conveying electrical modulation signals thereto. The facet1100is a feature which assists with efficient diversion of radiation into the first output waveguide1070so that first output waveguide can be orientated with its elongate axis at an angle, for example 40°, to a principal axis of the second mixing region1040b.The facet1100thereby enables the modulator1000to be more compact; omission of the facet1100would necessitate the first waveguide1070being connected at a shallower angle to the mixing region1040band including a bend further along the waveguide1070. The facet1100also enables the filter1010to be flanked by broader deep-etched regions. Moreover, the facet1100is also effective at deflecting higher-order mode radiation away from the first output waveguide1070path.

Operation of the modulator1000will now be described with reference toFIG. 9. The modulation structure1050is initially unbiased to give an unmodified refractive index therein. Fundamental mode optical radiation propagates along the input waveguide1020to the first mixing region1040ainto which it is injected. The injected radiation undergoes intermode coupling and thereby stimulates the generation of a series of higher-order radiation modes within the first region1040a,the first region1040abeing capable of supporting such a series of modes. The mixing regions1040a,1040bare designed, when the structure1050is unbiased, so that the modes constructively interfere to provide a radiation node at an entrance aperture of the first output waveguide1070and a radiation anti-node at an entrance aperture of the filter1010. As a consequence, radiation input at the input waveguide1020is coupled to the second output waveguide1080via its filter1010and not to the first output waveguide1070. When the structure1050is electrically biased to generate an electric field therein and thereby modify its refractive index principally due to the Pockels effect, there is formed a node at the entrance aperture of the filter1010and an anti-node at the entrance aperture of the first output waveguide1070. Such node and anti-node formation results in radiation input through the input waveguide1020being substantially coupled to the first output waveguide1070and not to the filter1010and its associated second output waveguide1080.

In the modulator1000, exciting the series of modes in the mixing regions1040a,1040bis a fundamental aspect of operation. The filter1010is effective at ensuring that substantially only fundamental mode radiation passes from the second mixing region1040binto the second output waveguide1080. As a consequence, extinction ratio performance of the modulator100is improved as a result of including the filter1010.

If required, a filter according to the invention such as the filter810can be incorporated as an input filter in-line with the input waveguide1020at the entrance aperture to the first mixing region1040a.The input filter is effective at preventing higher-order mode radiation propagating along the waveguide1020and subsequently interfering with intended multimode excitation within the first and second mixing regions1040a,1040b.As a result, inclusion of the filter1010and the input filter can improve the extinction performance of the modulator1000.

The filter1010is fabricated using a self-aligned process as employed for fabricating filter300shown inFIG. 4.

It will be appreciated by one skilled in the art of spatial filter design that modifications can be made to the filters200,300,810,1010without departing from the scope of the invention. Filters according to the invention can be fabricated in other types of waveguide technology, for example in indium phosphide (InP) rib waveguide technology or in silica or silica-on-silicon rib waveguide technology. Moreover, filters according to the invention can even be implemented in hollow waveguide technology, for example in hollow waveguides machined in alumina substrates for guiding radiation having a wavelength in the order of 10 μm, and also in spliced optical fibre waveguide technology. Irrespective of the waveguide technology used, the invention requires two or more mutually adjoining waveguide sections which are substantially matched for fundamental radiation mode propagation therethrough and unmatched with regard to higher-order mode radiation propagation therethrough.