Patent Description:
Wavelength division (de)multiplexers are key photonic components to compose high quality and capacity systems for telecom, data-com, and bio-medical sensing applications.

Various devices, such as arrayed waveguide gratings (AWGs), cascaded Mach-Zehnders, ring resonator, and echelle gratings have been developed as wavelength splitting devices. Recently, angled multimode interferometer (MMI) based multiplexers on SOI have been developed that use dispersive self-imaging in a multimode waveguide.

<FIG> shows an angled MMI according to a conventional example. Such an angled MMI can be used for wavelength-division multiplexing (WDM). The MMI comprises a central multimode interference waveguide connected to an input waveguide and several parallel output waveguides. The central MMI waveguide is arranged at an angle to the input and output waveguides. However, due to geometric limitations, the MMI can only be used for a course WDM, i.e. for multiplexing of optical channels with a large channel spacing.

In general, the channel spacing of optical channels that can be "split" by the angled MMI multiplexer depends on various geometrical parameters, such as the input and output angles, the physical spacing between the output waveguides, and the length and width of the MMI waveguide.

To achieve a reduced channel spacing in an angled MMI multiplexer either the geometrical spacing (i.e., pitch) of the output channels must be reduced, or the length of the MMI waveguide must be drastically enhanced. However, due to limitations in fabrication and device size, these limitations can often not be overcome such that most angled MMIs can only perform a coarse WDM.

<CIT> discloses an optical demultiplexer/multiplexer including a multimode interference waveguide, multiple coupling waveguides that interface with the multimode interference waveguide at various locations to form first and second angled multimode interferometers that are for either demultiplexing or multiplexing optical signals.

<CIT> discloses an optical waveguide element including a silicon core and a cladding, wherein the optical waveguide element comprises a high-order mode waveguide, tapered input and output waveguides, and a feedback elimination waveguide to prevent light reflection into the cladding.

Thus, it is an objective to provide an improved optical structure for multiplexing and/or demultiplexing an optical signal. In particular, the above-mentioned disadvantages should be avoided.

The objective is achieved by the embodiments provided in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

The claimed invention provides an integrated optical structure for multiplexing and/or demultiplexing an optical signal, wherein the optical signal comprises optical channels of different frequencies or is formed by combining said optical channels, and wherein the optical structure comprises: a main waveguide having two parallel side surfaces delimiting the main waveguide along its longitudinal sides in a plane that lies parallel to its extension direction; a first waveguide which meets the main waveguide at a first region on one of the two side surfaces; a plurality of second waveguides which meet the main waveguide at a second region on one of the two side surfaces, wherein the second region is spaced at a determined distance from the first region; wherein the two side surfaces of the main waveguide are arranged at a first angle relative to an extension direction of the first waveguide and a second angle relative to extension directions of the plurality of second waveguides; wherein the first angle and the second angle are identical or different depending on the arrangement of the first waveguide and the plurality of second waveguides; and wherein the optical structure further comprises one or more waveguide extension structures to control a channel spacing of the optical channels of different frequencies that can be multiplexed and/or demultiplexed, wherein each waveguide extension structure is arranged adjacent to one of the two side surfaces of the main waveguide at a region that is different to the first and the second region.

This achieves the advantage that a channel spacing of optical channels of different frequencies that can be multiplexed and/or demultiplexed by the optical structure can be controlled and/or adjusted.

In particular, the one or more waveguide extension structures allow influencing the channel spacing (i.e., the spacing of channels that are split/combined by the optical structure) without changing other geometric parameters of the structure, such as the length of the main waveguide from the first to the second region (i.e., from the first waveguide to the second waveguides). For certain arrangements of the waveguide extension structures, the channel spacing can be reduced without increasing the length of the main waveguide or reducing the pitch of the second waveguides.

The first and second angle are identical or different, depending on the exact arrangement of the first waveguide and second waveguides. In particular, when the first waveguide and the second waveguides meet the main waveguide on opposite sides, the first and second angle are generally identical. However, if the first waveguide and second waveguides meet the main waveguide on the same side, the first and second angle are different, for instance <NUM>° and -<NUM>°.

In particular, when the first and second waveguides are arranged at the same side of the main waveguide, the length of the main waveguide (i.e., distance from first to second region) needs to adapted to achieve the same channel spacing as compared to a configuration where the first and second waveguides are arranged on opposite sides.

In an embodiment, the optical structure forms an angled multimode interferometer (MMI).

Preferably, the main waveguide is a multimode interference waveguide of the angled MMI. The angled MMI can multiplex and/or demultiplex the optical signal, wherein optical signal comprises a plurality of optical channels of different frequencies.

In an embodiment, the at least one waveguide extension structure is arranged adjacent to a third region on one of the two side surfaces of the main waveguide, wherein the third region is opposite to the first region or opposite to the second region. This arrangement of the waveguide extension structure achieves the advantage that the channel spacing can be reduced.

In an embodiment, a first waveguide extension structure is arranged adjacent to a third region on one of the two side surfaces of the main waveguide, wherein the third region is opposite to the first region; and a second waveguide extension structure is arranged adjacent to a fourth region on one of the two side surfaces of the main waveguide, wherein the fourth region is opposite to the second region.

In an embodiment, at least one waveguide extension structure forms an asymmetric extension of the main waveguide.

In particular, the main waveguide is essentially symmetric along its extension direction (not taking into account the first and second waveguides). The waveguide extension structure breaks said symmetry.

In an embodiment, the at least one waveguide extension structure is an integral part of the main waveguide.

For example, the main waveguide and the at least one waveguide extension structures can be fabricated in a single common step. Thus, no additional manufacturing step is required.

In an embodiment, at least one waveguide extension structure is at least partially made of the same material as the main waveguide.

In particular, the channel spacing can be further controlled by the shape of the at least one waveguide extension structure.

In an embodiment, at least one waveguide extension structure has a wedge shape.

In an embodiment, at least one waveguide extension structure has a curved shape.

In an embodiment, at least one waveguide extension structure has a concave or a convex shape.

In an embodiment, at least one waveguide extension structure comprises a grating structure.

In an embodiment, at least one waveguide extension structure comprises a mirror structure. This achieves the advantage that the reflectivity of the waveguide extension structure can be further enhanced.

In an embodiment, the mirror structure is formed from a metallic mirror or a Bragg mirror.

In an embodiment, the optical structure further comprises a cladding which surrounds at least the main waveguide, wherein the cladding has a lower refractive index than the main waveguide.

The cladding can also surround the waveguide extension structure, the first waveguide, and the second waveguides. The waveguide extension structure, the first waveguide and/or the second waveguides can be made of the same material as the main waveguide and can be integral with the main waveguide.

In an embodiment, the cladding is made of silicon dioxide, SiO<NUM>.

For example, the main waveguide, the first waveguide and/or the second waveguides are made of silicon nitride, Si<NUM>N<NUM>.

The integrated optical structure can be arranged on an SOI substrate. The substrate can further comprise any one of the following materials: Si<NUM>N<NUM>, Si, SiO<NUM>, or SiON.

Embodiments of the invention will be explained in the followings together with the figures.

<FIG> shows a schematic diagram of an optical structure <NUM> for multiplexing and/or demultiplexing an optical signal according to an embodiment. The optical structure <NUM> can be an integrated optical structure, i.e. it can be integrated in an optical device, e.g. a telecommunication or sensing device.

The optical structure <NUM> comprises a main waveguide <NUM> having two parallel side surfaces, a first waveguide <NUM> which meets the main waveguide <NUM> at a first region on one of the two side surfaces, and a plurality of second waveguides <NUM> which meet the main waveguide <NUM> at a second region on one of the two side surfaces, wherein the second region is spaced at a determined distance L from the first region. The two side surfaces of the main waveguide are arranged at a first angle relative to an extension direction of the first waveguide <NUM> and a second angle relative to extension directions of the plurality of second waveguides <NUM>. The optical structure <NUM> further comprises one or more waveguide extension structures <NUM>, wherein each waveguide extension structure <NUM> is arranged adjacent to one of the two side surfaces of the main waveguide <NUM> at a region that is different to the first and the second region.

In particular, the optical structure <NUM> forms an angled MMI structure, wherein the main waveguide <NUM> is a multimode interference waveguide of the MMI structure. The angled MMI can be configured to multiplex and/or demultiplex the optical signal. In particular, the optical signal comprises a plurality of optical channels λ1, λ2, λ3, λ4 with a certain channel spacing.

When used for demultiplexing, the first waveguide <NUM> is an input waveguide and the second waveguides <NUM> are output waveguides, wherein the main waveguide <NUM> is configured to receive the optical signal from the input waveguide, to spatially separate the optical channels λ1, λ2, λ3, A4 of the optical signal, and to output in each of the plurality of output waveguides light from one of the separated optical channels λ1, λ2, λ3, λ4.

When used for multiplexing, the second waveguides <NUM> are input waveguides and the first waveguide <NUM> is an output waveguide, wherein the main waveguide <NUM> is configured to receive from each of the plurality of input waveguides light signals of different frequency, to spatially combine the light signals thereby forming the optical signal, and to output the optical signal in the output waveguide. The combined optical signals form the optical channels λ1, λ2, λ3, λ4 of the optical signal.

The structure <NUM> shown in <FIG> comprises one waveguide extension structure <NUM> which is arranged adjacent to a third region on one of the two side surfaces of the main waveguide. This third region is opposite to the second region, i.e. opposite to the second waveguides <NUM>.

The waveguide extension structure forms an asymmetric extension of the main waveguide in the form of a wedge or triangle. As shown in <FIG>, this wedge can be defined by a wedge angle (Θ) and a distance from the first region to the starting point of the wedge (MMI front length, L_front).

Generally, the at least one waveguide extension structure <NUM> is in direct physical contact with the main waveguide <NUM>. However, it is also conceivable that the at least one waveguide extension structure <NUM> is not in physical contact with the main waveguide <NUM>, i.e. it is arranged spaced from the main waveguide <NUM>.

In particular, the at least one waveguide extension structure <NUM> can be an integral part of the main waveguide, i.e., it can be integrally formed with the main waveguide. For instance, the main waveguide <NUM> and the at least one waveguide extension structures <NUM> are fabricated in a common fabrication step.

The waveguide extension structure <NUM> can at least partially be made of the same material as the main waveguide <NUM>.

In particular, at least one waveguide extension structure <NUM> can form a mirror on the side surface of the main waveguide <NUM>. The mirror can change a direction of total reflection on the side surface.

The optical structure <NUM> can be arranged on a substrate, for example a silicon-on-insulator (SOI) substrate. The substrate can further comprise any one of the following materials: Si<NUM>N<NUM>, Si, SiO<NUM>, or SiON.

In the embodiment shown in <FIG>, the first waveguide <NUM> and the second waveguides <NUM> meet the main waveguide <NUM> on opposite sides. Thereby, the first angle and the second angle are identical. This angle is generally between <NUM> and <NUM>°. Typically, the angle is <~<NUM>°. The angle can be varied to increase or decrease the channel spacing. However, also loss and cross-talk can be influenced by the angle.

For example, the optical structure <NUM> can be integrated as a WDM component in a photonics circuit, especially when using high or low index contrast waveguides.

<FIG> show results of simulations performed with optical structures <NUM> according to different embodiments. These simulation results show simulated spectral responses of different optical structures.

In particular, <FIG> both compare transmission spectra of light that is outputted at the output waveguides of two different angled MMIs. Thereby, each peak in a spectrum corresponds to an optical channel in one output waveguide, wherein the width of each peak corresponds to the bandwidth of the respective channel (in the wavelength domain). The distance from peak-to-peak corresponds to a channel spacing of the channels that can be separated by the respective MMI and outputted in the individual output waveguides. The simulations were implemented by the Eigen Mode Expansion method (EME). Each spectrum was calculated by scanning wavelengths and collecting how much power propagates through from the input to output in the AMMI.

<FIG> compares the simulated spectral response of a reference MMI structure as shown in <FIG> (bold line, <NUM>) to a simulated spectral response of the optical structure <NUM> shown in <FIG> (dashed line, <NUM>) in a wavelength range between <NUM> and <NUM>.

Thereby, the simulated optical structure <NUM> with the waveguide extension structure <NUM> (dashed line, <NUM>) has a main waveguide length (L) of <NUM>, a main waveguide width (W_mmi) of <NUM>, an input / output waveguide width of <NUM>, an MMI angle of <NUM>°, an MMI front length (L_fron) of <NUM>, a wedge angle (θ) of <NUM>°, an output waveguide pitch of <NUM>, and a refractive index of <NUM> surrounded by a cladding of refractive index <NUM>. The waveguide extension structure <NUM> is integral with the main waveguide <NUM> and has the same refractive index of <NUM>.

The reference MMI structure (bold line, <NUM>) has a length (L) of <NUM>, a main waveguide width (W_mmi) of <NUM>, an input / output waveguide width of <NUM>, an MMI angle of <NUM>°, an output waveguide pitch of <NUM>, and a refractive index of <NUM> surrounded by a cladding of refractive index <NUM>. The difference in length by <NUM> is the result of an optimization for the used wavelength range.

In particular, the minimum geometrical spacing (pitch) of output channels was determined to be larger than <NUM>. However, considering the critical size for fabrications, a minimum achievable pitch was estimated to be ~<NUM>. Therefore, a <NUM> pitch was used in these simulations.

The comparison between both transmission spectra <NUM>, <NUM> in <FIG> shows that the channel spacing is drastically reduced from <NUM> to <NUM> by the waveguide extension structure <NUM>. Thus, by adding the waveguide extension structure <NUM>, it is possible to perform a dense wavelength-division multiplexing (WDM) in an MMI structure that would otherwise only allow for a course WDM at a much larger channel spacing. To achieve a comparable decrease of the channel spacing in the reference MMI structure of <FIG>, the length of main MMI waveguide would have to be increased to several thousand microns and/or the output waveguide pitch would have to be decrease far below the critical size for fabrication. Decreasing output physical channel width and/or increasing output channel angles could also decrease the channel spacing, but the loss would be increased at the same time.

In general, a bandwidth of an optical channel is related to the propagation length in the central MMI waveguide <NUM>. If the propagation length is increased, the bandwidth can be narrower and the channel spacing can be reduced. Further, the geometrical output spacing is a bottle-neck of narrowing channel spacing. An increase of the output angles and/or a decrease of the width of the output channels can also lead to a decrease of channel spacing, however, at the same time losses will be increased. The wavelength extension structure <NUM> offers an alternative to reduce the achievable channel spacing without adapting further geometric parameters of the structure.

The effect of the waveguide extension structure <NUM> can be explained as follows (in case of a demulitplexer): Light that propagates in the main waveguide <NUM> meets the wall of the main waveguide <NUM> at a specific point opposite to the output waveguides, where it reflects and is refocused towards the output waveguides. The mirror is, for instance, arranged opposite to the output waveguides and changes the focal point of the light and, thereby, the output channel position.

<FIG> compares simulated spectral responses obtained <NUM>, <NUM> with two optical structures <NUM> with different waveguide extension structures <NUM> in a wavelength range between <NUM> and <NUM>. Thereby, both optical structures have a single wedge-type extension structure <NUM> which is arranged opposite to the plurality of second waveguides <NUM>, as shown in <FIG>.

The first optical structure (thin dashed line, <NUM>) has an extension structure <NUM> with a wedge angle (θ) of <NUM>°. In contrast, the second optical structure (thick dashed line, <NUM>) has an extension structure <NUM> with a wedge angle (θ) of <NUM>°. Both optical structures have an MMI length (L) of <NUM>, wherein all other geometric parameters of these structures are identical to the optical structure <NUM> simulated in <FIG>.

<FIG> shows that the channel spacing can be even further reduced, from <NUM> to <NUM>, by optimizing the wedge angle.

<FIG> show schematic diagrams of the optical structure <NUM> with different waveguide extension structures <NUM> according to an embodiment.

<FIG> shows a linear, in particular wedge, shaped extension structure <NUM> with wedge angle θ. The spectral response of an optical structure with such an extension structure is, for instance, shown in <FIG>.

<FIG> show different curved shape extension structures <NUM>, in particular a convex shaped extension structure <NUM> (<FIG>), and an adiabatic respectively concave shaped extension structure <NUM> (<FIG>).

<FIG> shows an optical structure <NUM> with a waveguide extension structure <NUM> which forms a grating.

Depending on the shape and size of the waveguide extension structure, the spectral response and, in particular, the channel spacing of channels that can be split respectively combined by the optical structure <NUM> can be changed.

<FIG> show schematic diagrams of the optical structure <NUM> according to different embodiments.

The optical structures <NUM> shown in <FIG> both comprise a single wedge respectively triangle shaped waveguide extension structure <NUM> which is arranged adjacent to the main waveguide <NUM>, opposite to the plurality of second waveguides <NUM> that can form the output waveguides of an angled MMI demultiplexer.

The optical structures <NUM> in <FIG> both comprise a cladding <NUM> which surrounds the main waveguide <NUM> and the waveguide extension structure <NUM>. The cladding <NUM> can further surround the first waveguide <NUM> and the second waveguides <NUM> at least partially.

Preferably, the cladding <NUM> has a lower refractive index than the waveguides <NUM>, <NUM>, <NUM>, <NUM>. For example, the cladding <NUM> can be made of silicon dioxide (SiO<NUM>), air or another low index material. The main waveguide <NUM>, the first waveguide <NUM> and the second waveguides <NUM> can be made of Si, silicon nitride (Si<NUM>N<NUM>), silicon oxynitride (SiON), or SiO<NUM>. The index difference between cladding <NUM> and the waveguides <NUM>, <NUM>, <NUM>, <NUM> can lead to total internal reflection (TIR) and, thus, can enhance internal reflections and reduce losses in the structure <NUM>.

In the example shown in <FIG>, the waveguide extension structure <NUM> comprises an additional mirror structure <NUM>. The mirror structure <NUM> can be formed from a metallic mirror or a Bragg mirror. The mirror structure can increase the reflectivity of the waveguide extension structure <NUM>.

The structure <NUM> shown in <FIG> is similar to the optical structures <NUM> in <FIG> or <FIG> with a single, wedge shaped waveguide extension structure <NUM> which is arranged adjacent to the main waveguide <NUM>, opposite to the plurality of second waveguides <NUM>. When used as a demultiplexer, the first waveguide <NUM> can be an input waveguide and the plurality of second waveguides <NUM> can be output waveguides.

The structure <NUM> shown in <FIG> is a superposition of two optical structures <NUM> according to <FIG>. The structure <NUM> in <FIG>, thus, comprises a first waveguides <NUM>-<NUM>, a plurality of second waveguides <NUM>-<NUM>, a third waveguide <NUM>-<NUM>, and a plurality of fourth waveguides <NUM>-<NUM>. For instance, the first and third waveguide <NUM>-<NUM>, <NUM>-<NUM> can both form input waveguides and the plurality of second and fourth waveguides <NUM>-<NUM>, <NUM>-<NUM> can form respective output waveguides. A respective waveguide extension structure <NUM> is arranged adjacent to the main waveguide <NUM> opposite of the plurality of second and fourth waveguides <NUM>-<NUM>, <NUM>-<NUM>.

The structure <NUM> shown in <FIG> is similar to the structure <NUM> of <FIG>, wherein the first waveguide <NUM> is arranged at a different angle relative to the main waveguide <NUM>. The structure <NUM> in <FIG> comprises an additional mirror structure <NUM> on a first end of the main waveguide <NUM>. The mirror structure <NUM> is, for example, arranged to redirect light, which enters the main waveguide <NUM> through the first waveguide <NUM>, in the direction of the second waveguides <NUM> and vice versa. The mirror structure <NUM> can be a metallic mirror or a Bragg mirror. Compared to the optical structures <NUM> shown in <FIG>, the structure <NUM> in <FIG> can have a reduced size while achieving a comparable reduction of the channel spacing.

The structure <NUM> shown in <FIG> is a superposition of two optical structures <NUM> according to <FIG>. The structure <NUM> in <FIG> comprises a first waveguides <NUM>-<NUM>, a plurality of second waveguides <NUM>-<NUM>, a third waveguide <NUM>-<NUM>, and a plurality of fourth waveguides <NUM>-<NUM>.

<FIG> show results of simulations performed with optical structures <NUM> according to different embodiments. In particular, <FIG> compares the spectral response of the three depicted optical structures A-C in the wavelength range between <NUM> and <NUM>, when used as angled MMI demultiplexer.

The top chart shows the simulated spectral response of the reference structure A, which is depicted on the top left. This reference structure corresponds an angled MMI according to the state of the art without a waveguide extension structure. The simulated transmission spectrum of this reference structure A shows a channel spacing of <NUM>.

The bottom chart shows the simulated spectral response of the optical structures B and C as depicted on the center right and bottom right of <FIG>. Both these structures have a wedge-shaped waveguide extension structure <NUM> with identical wedge angle arranged at different positions of the main waveguide <NUM>, namely opposite to the input waveguide (structure B) and opposite to the output waveguides (structure c). Otherwise, the geometrical and optical parameters of the simulated structures A-C are identical with the exception of a slightly enhanced length of the main waveguide of structures B and C of <NUM> compared to structure A.

The simulated transmission spectrum of the structures with waveguide extension <NUM> show an increased channel spacing of <NUM> for the structure B and a decrease channel spacing of <NUM> for the structure C.

<FIG> show results of simulations performed with optical structures <NUM> according to different embodiments. In particular, <FIG> compares the spectral response of the additional optical structures D and E to the same reference structure A in the wavelength range between <NUM> and <NUM>.

The top chart compares the reference structure A with the structure D which has a waveguide extension structure <NUM> arranged adjacent to the main waveguide <NUM> at a region in-between the input and output waveguides. Thereby, the length of the main waveguide <NUM> between input and output waveguides is slightly reduced compared to the reference structure. The transmission spectrum of structure D shows a plurality of partially overlapping peaks with channel spacing that can be as low as <NUM>.

The bottom chart compares the reference structure A with the structure E which has two waveguide extension structures <NUM> arranged adjacent to the main waveguide <NUM>, wherein a first waveguide extension structure <NUM> is arranged opposite to the input waveguide and a second waveguide extension structure <NUM> is arranged opposite to the output waveguides. Thereby, the length of the main waveguide <NUM> between input and output waveguides is slightly enhanced compared to the reference structure. The transmission spectrum of structure D shows a reduced channel spacing of <NUM>:<NUM> in this structure.

The simulation results in <FIG> and <FIG> show that the channel spacings of an angled MMI can be controlled by the location and number of waveguide extension structures <NUM>. With certain arrangements of the waveguide extension structure <NUM>, the channel spacing of channels that can be split by these structures can be drastically reduced, while other arrangements can lead to an enhancement of the channel spacing.

Claim 1:
An integrated optical structure (<NUM>) for multiplexing and/or demultiplexing an optical signal, wherein the optical signal comprises optical channels of different frequencies or is formed by combining said optical channels, and wherein the optical structure (<NUM>) comprises:
a main waveguide (<NUM>) having two parallel side surfaces delimiting the main waveguide along its longitudinal sides in a plane that lies parallel to its extension direction;
a first waveguide (<NUM>) which meets the main waveguide (<NUM>) at a first region on one of the two side surfaces;
a plurality of second waveguides (<NUM>) which meet the main waveguide (<NUM>) at a second region on one of the two side surfaces, wherein the second region is spaced at a determined distance from the first region;
wherein the two side surfaces of the main waveguide (<NUM>) are arranged at a first angle relative to an extension direction of the first waveguide (<NUM>) and a second angle relative to extension directions of the plurality of second waveguides (<NUM>); and
wherein the first angle and the second angle are identical or different depending on the arrangement of the first waveguide (<NUM>) and the plurality of second waveguides (<NUM>); characterized in that
the optical structure (<NUM>) further comprises one or more waveguide extension structures (<NUM>) to control a channel spacing of the optical channels of different frequencies that can be multiplexed and/or demultiplexed, wherein each waveguide extension structure (<NUM>) is arranged adjacent to one of the two side surfaces of the main waveguide (<NUM>) at a region that is different to the first and the second region.