Patent Description:
Photonics chips are used in many applications and systems including, but not limited to, data communication systems and data computation systems. A photonics chip may integrate optical components and electronic components into a unified platform. Among other factors, layout area, cost, and operational overhead may be reduced by the integration of both types of components on the same chip.

A directional coupler is employed on a photonics chip to split propagating optical signals between different waveguide cores. A directional coupler includes sections of the different waveguide cores that are separated by a gap that is selected to promote optical coupling over a given coupling length. The dimension of the gap between the sections of the waveguide cores may be reduced to enhance the coupling strength and reduce the device footprint. However, reducing the dimension of the gap may increase the difficulty in patterning the gap between the waveguide cores and the difficulty in subsequently filling the gap with dielectric material. For example, air voids may be created within the dielectric material during deposition.

From <CIT> a structure for an optical coupler is known which comprises a first waveguide core having a first tapered section and a second waveguide core having a second tapered section positioned adjacent to the first tapered section. A dielectric layer is formed over the waveguide cores. According to one implementation the first tapered section and the second tapered sections are segmented and the spaces between the segments are filled by dielectric material of the dielectric layer.

Improved structures for a directional coupler and methods of forming a structure for a directional coupler are needed.

In an embodiment of the invention, a structure for a directional coupler is provided. The structure comprises a first waveguide core including a first plurality of segments arranged along a first longitudinal axis, and a second waveguide core including a second plurality of segments arranged along a second longitudinal axis. Each of the first plurality of segments has a first sidewall and a second sidewall opposite to the first sidewall, each of the second plurality of segments has a first sidewall and a second sidewall opposite to the first sidewall, and the first sidewall of each of the second plurality of segments is disposed adjacent to the first sidewall of one of the first plurality of segments in a coupling region. The structure further comprises a first cladding layer comprising a first material that has a first refractive index, and a second cladding layer comprising a second material that has a second refractive index different from the first refractive index. The first cladding layer adjoins the first sidewall of each of the first plurality of segments and the first sidewall of each of the second plurality of segments, and the second cladding layer adjoins the second sidewall of each of the first plurality of segments and the second sidewall of each of the second plurality of segments. The second refractive index of the second material may be greater than the first refractive index of the first material. Alternatively, the second refractive index of the second material may be less than the first refractive index of the first material. Moreover, the first material may comprise a first non-ferroelectric material, and the second material may comprise a second non-ferroelectric material. According to one implementation, the first material may comprise a silicon nitride, and the second material may comprise silicon dioxide. According to one variant, the first waveguide core may include a first plurality of gaps that alternate with the first plurality of segments along the first longitudinal axis. The second waveguide core may include a second plurality of gaps that alternate with the second plurality of segments along the second longitudinal axis. The first cladding layer may be disposed in the first plurality of gaps and the second plurality of gaps. According to another variant, the first waveguide core may include a first plurality of gaps that alternate with the first plurality of segments along the first longitudinal axis. The second waveguide core may include a second plurality of gaps that alternate with the second plurality of segments along the second longitudinal axis. The second cladding layer may be disposed in the first plurality of gaps and the second plurality of gaps. In addition to the above features and according to a further development, each of the first plurality of segments may have a bottom surface, each of the second plurality of segments may have a bottom surface, and the structure may further comprise a dielectric layer comprised of a dielectric material, wherein the bottom surface of each of the first plurality of segments may be disposed on the dielectric layer, and the bottom surface of each of the second plurality of segments may be disposed on the dielectric layer. The dielectric material may have a third refractive index that is different from the first refractive index; additionally or alternatively, the first cladding layer and the second cladding layer may be disposed on the dielectric layer. In addition to the above features and according to a further development, each of the first plurality of segments may further have a top surface, each of the second plurality of segments may have a top surface, and the structure may further comprise a dielectric layer comprised of a dielectric material, wherein the dielectric layer may be disposed on the top surface of each of the first plurality of segments, and the dielectric layer may be disposed on the top surface of each of the second plurality of segments. The dielectric material may have a third refractive index that is different from the first refractive index; additionally or alternatively, the dielectric layer may be disposed on the first cladding layer and the second cladding layer.

In an embodiment of the invention, a method of forming a structure for a directional coupler is provided. The method comprises forming a first waveguide core including a first plurality of segments arranged along a first longitudinal axis, and forming a second waveguide core including a second plurality of segments arranged along a second longitudinal axis. Each of the first plurality of segments has a first sidewall and a second sidewall opposite to the first sidewall, each of the second plurality of segments has a first sidewall and a second sidewall opposite to the first sidewall, and the first sidewall of each of the second plurality of segments is disposed adjacent to the first sidewall of one of the first plurality of segments in a coupling region. The method further comprises forming a first cladding layer comprised of a first material having a first refractive index, and forming a second cladding layer comprised of a second material that has a second refractive index different from the first refractive index. The first cladding layer adjoins the first sidewall of each of the first plurality of segments and the first sidewall of each of the second plurality of segments, and the second cladding layer adjoins the second sidewall of each of the first plurality of segments and the second sidewall of each of the second plurality of segments.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with a general description of the invent ion given above and the detailed description of the embodiments given below, serve to explain the embodiments of the invention. In the drawings, like reference numerals refer to like features in the various views.

With reference to <FIG> and in accordance with embodiments of the invention, a structure <NUM> for a directional coupler includes a waveguide core <NUM> and a waveguide core <NUM> that are positioned on, and over, a dielectric layer <NUM> and a semiconductor substrate <NUM>. In an embodiment, the dielectric layer <NUM> may be comprised of a dielectric material, such as silicon dioxide, and the semiconductor substrate <NUM> may be comprised of a semiconductor material, such as single-crystal silicon. In an embodiment, the dielectric layer <NUM> may be a buried oxide layer of a silicon-on-insulator substrate, and the dielectric layer <NUM> may be disposed between the waveguide cores <NUM>, <NUM> and the semiconductor substrate <NUM>. The dielectric layer <NUM> may function as an underlying cladding layer for the waveguide cores <NUM>, <NUM>.

The waveguide core <NUM> includes multiple segments <NUM> that are distributed in an input region <NUM>, a coupling region <NUM>, and an output region <NUM>. The segments <NUM> are positioned in a spaced-apart arrangement in which spaces or gaps G1 alternate with the segments <NUM>. In the coupling region <NUM>, the segments <NUM> alternate with the gaps G1 along the longitudinal axis <NUM>. In the input region <NUM> and the output region <NUM>, the segments <NUM> may be arranged with curved routing that approximates respective bends. In an embodiment, the pitch and duty cycle of the segments <NUM> may be uniform to define a periodic arrangement. In alternative embodiments, the pitch and/or the duty cycle of the segments <NUM> may be apodized (i.e., non-uniform) to define a non-periodic arrangement. In an alternative embodiment, a rib may be overlaid with some or all of the segments <NUM>.

The waveguide core <NUM> includes multiple segments <NUM> that are distributed in the input region <NUM>, the coupling region <NUM>, and the output region <NUM>. The segments <NUM> are positioned in a spaced-apart arrangement in which spaces or gaps G2 alternate with the segments <NUM>. In the coupling region <NUM>, the segments <NUM> and the gaps G2 alternate along the longitudinal axis <NUM>. In the input region <NUM> and the output region <NUM>, the segments <NUM> may be arranged with curved routing that approximates respective bends. In an embodiment, the pitch and duty cycle of the segments <NUM> may be uniform to define a periodic arrangement. In alternative embodiments, the pitch and/or the duty cycle of the segments <NUM> may be apodized (i.e., non-uniform) to define a non-periodic arrangement. In an alternative embodiment, a rib may be overlaid with some or all of the segments <NUM>.

Each segment <NUM> has a sidewall <NUM>, a sidewall <NUM> opposite to the sidewall <NUM>, a bottom surface <NUM> adjacent to the dielectric layer <NUM>, and a top surface <NUM> opposite to the bottom surface <NUM>. The sidewalls <NUM>, <NUM> extend in a vertical direction from the bottom surface <NUM> to the top surface <NUM>. Each segment <NUM> has a sidewall <NUM>, a sidewall <NUM> opposite to the sidewall <NUM>, a bottom surface <NUM> adjacent to the dielectric layer <NUM>, and a top surface <NUM> opposite to the bottom surface <NUM>. The sidewalls <NUM>, <NUM> extend in a vertical direction from the bottom surface <NUM> to the top surface <NUM>.

The segments <NUM> of the waveguide core <NUM> may be spaced from the segments <NUM> of the waveguide core <NUM> by a gap G3 in the coupling region <NUM>. More specifically, the segments <NUM> and the segments <NUM> may be arranged in the coupling region <NUM> such that the sidewall <NUM> of each segment <NUM> is disposed across the gap G3 from the sidewall <NUM> of one of the segments <NUM>. In an embodiment, the longitudinal axis <NUM> may be arranged parallel to the longitudinal axis <NUM> in the coupling region <NUM>. In an embodiment, the segments <NUM> and the segments <NUM> may be characterized by the same pitch and/or duty cycle. In an embodiment, the segments <NUM> may be characterized by a different pitch and/or duty cycle than the segments <NUM>. In an embodiment, the segments <NUM> may have a width in a direction transverse to the longitudinal axis <NUM>, the segments <NUM> may have a width in a direction transverse to the longitudinal axis <NUM>, and the width of the segments <NUM> may be equal to the width of the segments <NUM>. In an embodiment, the segments <NUM> may have a width transverse to the longitudinal axis <NUM>, the segments <NUM> may have a width in a direction transverse to the longitudinal axis <NUM>, and the width of the segments <NUM> may be unequal to the width of the segments <NUM>. The segments <NUM> are spaced from the segments <NUM> by a distance greater than the gap G3 in the input region <NUM> and in the output region <NUM>.

In an embodiment, the waveguide cores <NUM>, <NUM> may be comprised of a material having a refractive index that is greater than the refractive index of silicon dioxide. In an embodiment, the waveguide cores <NUM>, <NUM> may be comprised of a semiconductor material. In an embodiment, the waveguide cores <NUM>, <NUM> may be comprised of single-crystal silicon. In an embodiment, the waveguide cores <NUM>, <NUM> may be comprised of polysilicon or amorphous silicon. In an embodiment, the waveguide cores <NUM>, <NUM> may be comprised of a dielectric material, such as silicon nitride, silicon oxynitride, or aluminum nitride.

In an embodiment, the waveguide cores <NUM>, <NUM> may be formed by patterning a layer comprised of their constituent material with lithography and etching processes. In an embodiment, an etch mask may be formed by a lithography process over the layer to be patterned, and unmasked sections of the deposited layer may be etched and removed with an etching process. In an embodiment, the waveguide cores <NUM>, <NUM> may be formed by patterning the semiconductor material (e.g., single-crystal silicon) of a device layer of a silicon-on-insulator substrate. In an embodiment, the waveguide cores <NUM>, <NUM> may be formed by patterning a deposited layer comprised of their constituent material (e.g., silicon nitride, polysilicon, or amorphous silicon). In an alternative embodiment, a slab layer may be connected to respective lower portions of the segments <NUM>, <NUM> of the waveguide cores <NUM>, <NUM>. The slab layer, which may be formed when the waveguide cores <NUM>, <NUM> are patterned, has a thickness that is less than the thickness of the segments <NUM> and less than the thickness of the segments <NUM>.

With reference to <FIG>, <FIG> in which like reference numerals refer to like features in <FIG> and at a subsequent fabrication stage, a cladding layer <NUM> may be disposed between the waveguide core <NUM> and the waveguide core <NUM> in the coupling region <NUM>. In particular, the cladding layer <NUM> may be disposed inside the gaps G3 between the segments <NUM> of the waveguide core <NUM> and the segments <NUM> of the waveguide core <NUM>. The material of the cladding layer <NUM> may adjoin the inner sidewall <NUM> of each segment <NUM>, and the material of the cladding layer <NUM> may adjoin the inner sidewall <NUM> of each segment <NUM>. The cladding layer <NUM> may also disposed in the space between the segments <NUM> and the segments <NUM> in the input region <NUM> and the space between the segments <NUM> and the segments <NUM> in the output region <NUM>. The cladding layer <NUM> may be deposited and planarized after deposition, and then patterned with lithography and etching processes.

A cladding layer <NUM> may be disposed inside the gaps G1 between the segments <NUM> of the waveguide core <NUM> and the gaps G2 between the segments <NUM> of the waveguide core <NUM>, as well as in the space between the segments <NUM> in the input region <NUM> and the segments <NUM> in the output region <NUM> and in the space between the segments <NUM> in the input region <NUM> and the segments <NUM> in the output region <NUM>. The material of the cladding layer <NUM> may adjoin the outer sidewall <NUM> of each segment <NUM>, and the material of the cladding layer <NUM> may adjoin the outer sidewall <NUM> of each segment <NUM>. The cladding layer <NUM> may be deposited and planarized after deposition, and then patterned with lithography and etching processes.

In an embodiment, the cladding layer <NUM> may be deposited and patterned before depositing the cladding layer <NUM>, the gaps G1 and the gaps G2 may be unfilled following the patterning of the cladding layer <NUM>, and the material of the subsequently-deposited cladding layer <NUM> may fill the gaps G1 and the gaps G2. In an alternative embodiment, the cladding layer <NUM> may be deposited and patterned before depositing the cladding layer <NUM>, the gap G3 may be unfilled following the patterning of the cladding layer <NUM>, and the material of the subsequently-deposited cladding layer <NUM> may fill the gap G3.

The cladding layer <NUM> may be comprised of a material having a refractive index that is less than the refractive index of the material constituting the waveguide cores <NUM>, <NUM>. The cladding layer <NUM> may also be comprised of a material having a refractive index that is less than the refractive index of the material constituting the waveguide cores <NUM>, <NUM>. The refractive index of the material of the cladding layer <NUM> may be different from the refractive index of the of the cladding layer <NUM>. In an embodiment, the refractive index of the material of the cladding layer <NUM> may be less than the refractive index of the material of the cladding layer <NUM>. In an embodiment, the refractive index of the material of the cladding layer <NUM> may be greater than the refractive index of the material of the cladding layer <NUM>. In an embodiment, the cladding layers <NUM>, <NUM> may be dielectric materials selected from silicon nitride, aluminum nitride, silicon oxynitride, diamond, aluminum oxide, calcium fluoride, carbon-doped silicon oxide, tetraethylorthosilicate silicon dioxide, fluorinated-tetraethylorthosilicate silicon dioxide, and magnesium fluoride. In an embodiment, the cladding layer <NUM> may be comprised of silicon nitride, and the cladding layer <NUM> may be comprised of silicon dioxide. In an embodiment, the cladding layers <NUM>, <NUM> may be comprised of respective non-ferroelectric materials with different refractive indices.

In an embodiment, the cladding layer <NUM> and the cladding layer <NUM> may have equal thicknesses. In an embodiment, the cladding layer <NUM> and the cladding layer <NUM> may have unequal thicknesses. In an embodiment, the cladding layer <NUM> may be thicker than the cladding layer <NUM>. In an embodiment, the cladding layer <NUM> may be thinner than the cladding layer <NUM>. In an embodiment, the cladding layer <NUM> and/or the cladding layer <NUM> may have respective thicknesses that are equal to the thicknesses of the segments <NUM>, <NUM>. In an embodiment, the cladding layer <NUM> and/or the cladding layer <NUM> may have respective thicknesses that are not equal to the thicknesses of the segments <NUM>, <NUM>.

In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is equal or substantially equal to the refractive index of the material of the cladding layer <NUM>. In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is equal or substantially equal to the refractive index of the material of the cladding layer <NUM>. In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is different from the refractive index of the material of the cladding layer <NUM> and/or different from the refractive index of the material of the cladding layer <NUM>.

In an embodiment, the segments <NUM> of the waveguide core <NUM> may be dimensioned and positioned at small enough pitch so as to define a sub-wavelength grating that does not radiate or reflect light at a wavelength of operation, and the segments <NUM> of the waveguide core <NUM> may be dimensioned and positioned at small enough pitch so as to define a sub-wavelength grating that does not radiate or reflect light at a wavelength of operation. The material of the cladding layer <NUM> is disposed in the gaps G1 between adjacent pairs of the segments <NUM> such that a metamaterial structure is defined in which the material constituting the segments <NUM> has a higher refractive index than the material of the cladding layer <NUM>. The material of the cladding layer <NUM> is disposed in the gaps G2 between adjacent pairs of the segments <NUM> such that a metamaterial structure is defined in which the material constituting the segments <NUM> has a higher refractive index than the material of the cladding layer <NUM>. Each metamaterial structure can be treated as a homogeneous material having an effective refractive index that is intermediate between the refractive index of the material constituting the segments <NUM>, <NUM> and the refractive index of the material constituting the cladding layer <NUM>.

With reference to <FIG> in which like reference numerals refer to like features in <FIG> and at a subsequent fabrication stage, a dielectric layer <NUM> is formed on, and over, the waveguide cores <NUM>, <NUM> and the cladding layers <NUM>, <NUM>. The dielectric layer <NUM> may be comprised of a dielectric material, such as silicon dioxide, having a refractive index that is less than the refractive index of the material constituting the waveguide cores <NUM>, <NUM>. In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is equal or substantially equal to the refractive index of the material of the cladding layer <NUM>. In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is equal or substantially equal to the refractive index of the material of the cladding layer <NUM>. In an embodiment, the dielectric material of the dielectric layer <NUM> may have a refractive index that is different from the refractive index of the material of the cladding layer <NUM> and/or different from the refractive index of the material of the cladding layer <NUM>. The dielectric layer <NUM> may be disposed on the top surface <NUM> of each segment <NUM> and the top surface <NUM> of each segment <NUM>, and the dielectric layer <NUM> may function as an overlying cladding layer for the waveguide cores <NUM>, <NUM>.

In use, light (e.g., laser light) may be guided on a photonics chip by the waveguide core <NUM> to the input region <NUM> of the directional coupler. In the coupling region <NUM>, all or a portion of the arriving light is transferred in a lateral direction from the segments <NUM> of the waveguide core <NUM> to the segments <NUM> of the waveguide core <NUM>. Light may exit the directional coupler via the waveguide core <NUM> and the waveguide core <NUM> in the output region <NUM>.

The waveguide core <NUM> and the waveguide core <NUM> define a directional coupler that includes inhomogeneous or heterogenous lateral claddings supplied by the cladding layer <NUM> and the cladding layer <NUM> that are comprised materials of different refractive index. The heterogenous lateral claddings of different refractive index may permit the dimension of the gap G3 between the segments <NUM> of the waveguide core <NUM> and the segments <NUM> of the waveguide core <NUM> in the coupling region <NUM> to be increased without reducing the coupling strength and may also permit a significant reduction in the device footprint. As a result of the relaxation on the restriction on the dimension of the gap G3, the cladding layer <NUM> may be deposited with a significantly reduced risk of forming air voids in the gap G3.

With reference to <FIG>, <FIG> and in accordance with alternative embodiments, the cladding layer <NUM> may be disposed inside the gaps G1 between the segments <NUM> of the waveguide core <NUM> and the gaps G2 between the segments <NUM> of the waveguide core <NUM> instead of the cladding layer <NUM>. The cladding layer <NUM> is disposed in the space between the segments <NUM> in the input region <NUM> and the segments <NUM> in the output region <NUM> and in the space between the segments <NUM> in the input region <NUM> and the segments <NUM> in the output region <NUM>. The material of the cladding layer <NUM> may also adjoin the inner sidewall <NUM> of each segment <NUM>, and the material of the cladding layer <NUM> may also adjoin the inner sidewall <NUM> of each segment <NUM>. The material of the cladding layer <NUM> may adjoin the outer sidewall <NUM> of each segment <NUM>, and the material of the cladding layer <NUM> may adjoin the outer sidewall <NUM> of each segment <NUM>.

With reference to <FIG>, a structure <NUM> may include waveguide cores <NUM>, <NUM> that are continuous and non-segmented in the input region <NUM>, the coupling region <NUM>, and the output region <NUM>. The waveguide core <NUM> has a sidewall <NUM> and a sidewall <NUM> opposite to the sidewall <NUM>, and the waveguide core <NUM> has a sidewall <NUM> and a sidewall <NUM> opposite to the sidewall <NUM>. The cladding layer <NUM> is disposed in the coupling region <NUM> inside the gap G3 between the sidewall <NUM> of the waveguide core <NUM> and the sidewall <NUM> of the waveguide core <NUM>, as well as in the space between the waveguide core <NUM> and the waveguide core <NUM> in the input region <NUM> and in the space between the waveguide core <NUM> and the waveguide core <NUM> in the output region <NUM>. Alternatively, the waveguide cores <NUM>, <NUM> may be slotted waveguide cores, and the material of the cladding layer <NUM> may be disposed inside the slots.

With reference to <FIG> and in accordance with alternative development, the structure <NUM> may include the material of both cladding layers <NUM>, <NUM> inside the gap G3 over the coupling region <NUM>. Specifically, sections of the cladding layer <NUM> may alternate with sections of the cladding layer <NUM> to define a grating-like arrangement inside the gap G3 over the length of the coupling region <NUM>. The sections of each of the cladding layers <NUM>, <NUM> may be defined when patterning the respective deposited layers. The sections of the cladding layer <NUM> and the sections of the cladding layer <NUM> may be arranged to define a sub-wavelength grating inside the gap G3.

With reference to <FIG>, <FIG> and in accordance with alternative embodiments, the material of the cladding layer <NUM> may be disposed in the gaps G1 between adjacent pairs of the segments <NUM> in the coupling region <NUM> and in the gaps G2 between adjacent pairs of the segments <NUM> in the coupling region <NUM>. The material of the cladding layer <NUM> may be disposed in the portions of the gap G3 between the sidewalls <NUM> of the segments <NUM> and the sidewalls <NUM> of the segments <NUM> that are arranged across the gap G3 from each other. As a result, the materials inside the gap G3 have refractive indices that alternate along the length of the coupling region <NUM> of the structure <NUM>.

With reference to <FIG>, a structure <NUM> for a directional coupler may include a waveguide core <NUM>, a waveguide core <NUM> and a ring resonator <NUM> positioned adjacent to respective portions of the waveguide cores <NUM>, <NUM>. The ring resonator <NUM> may be positioned adjacent to the waveguide core <NUM> with a gap separating the ring resonator <NUM> from the portion of the waveguide core <NUM>, and the ring resonator <NUM> may be positioned adjacent to the waveguide core <NUM> with a gap separating the ring resonator <NUM> from the portion of the waveguide core <NUM>. The ring resonator <NUM> may be a waveguide core that is shaped as a closed loop or ring. The waveguide core <NUM> may serve as an input bus, and the waveguide core <NUM> may serve as an output bus. The ring resonator <NUM> may have an inner ring-shaped edge <NUM> defining an inner diameter and an outer ring-shaped edge <NUM> defining an outer diameter.

The cladding layer <NUM> may be formed inside the inner ring-shaped edge <NUM> of the ring resonator <NUM>, and the cladding layer <NUM> may be formed outside the outer ring-shaped edge <NUM> of the ring resonator <NUM>. Alternatively, the waveguide core of the ring resonator <NUM> may be divided into segments, the segments may be arranged within the ring shape to define a sub-wavelength grating, and either the material of the cladding layer <NUM> or the material of the cladding layer <NUM> may be arranged in the gaps between the segments. The waveguide cores <NUM>, <NUM> and the waveguide core of the ring resonator <NUM> may be comprised of the same material, such as silicon. Alternatively, an additional ring resonator may be positioned between the waveguide cores <NUM>, <NUM> adjacent to the ring resonator <NUM>.

In use, light (e.g., laser light) may be coupled and transferred in a lateral direction from the waveguide core <NUM> to the ring resonator <NUM>. The light may be subsequently coupled and transferred in a lateral direction from the ring resonator <NUM> to the waveguide core <NUM>. The result is light transfer from the waveguide core <NUM> to the waveguide core <NUM>.

The methods as described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (e.g., as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. The chip may be integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either an intermediate product or an end product. The end product can be any product that includes integrated circuit chips, such as computer products having a central processor or smartphones.

References herein to terms modified by language of approximation, such as "about", "approximately", and "substantially", are not to be limited to the precise value specified. The language of approximation may correspond to the precision of an instrument used to measure the value and, unless otherwise dependent on the precision of the instrument, may indicate a range of +/- <NUM>% of the stated value(s).

References herein to terms such as "vertical", "horizontal", etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term "horizontal" as used herein is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The terms "vertical" and "normal" refer to a direction in the frame of reference perpendicular to the horizontal, as just defined. The term "lateral" refers to a direction in the frame of reference within the horizontal plane.

A feature "connected" or "coupled" to or with another feature may be directly connected or coupled to or with the other feature or, instead, one or more intervening features may be present. A feature may be "directly connected" or "directly coupled" to or with another feature if intervening features are absent. A feature may be "indirectly connected" or "indirectly coupled" to or with another feature if at least one intervening feature is present. A feature "on" or "contacting" another feature may be directly on or in direct contact with the other feature or, instead, one or more intervening features may be present. A feature may be "directly on" or in "direct contact" with another feature if intervening features are absent. A feature may be "indirectly on" or in "indirect contact" with another feature if at least one intervening feature is present. Different features may "overlap" if a feature extends over, and covers a part of, another feature with either direct contact or indirect contact.

Claim 1:
A structure for a directional coupler, the structure comprising:
a first waveguide core (<NUM>) including a first plurality of segments (<NUM>) arranged along a first longitudinal axis (<NUM>), each of the first plurality of segments (<NUM>) having a first sidewall (<NUM>) and a second sidewall (<NUM>) opposite to the first sidewall (<NUM>);
a second waveguide core (<NUM>) including a second plurality of segments (<NUM>) arranged along a second longitudinal axis (<NUM>), each of the second plurality of segments (<NUM>) having a first sidewall (<NUM>) and a second sidewall (<NUM>) opposite to the first sidewall (<NUM>), and the first sidewall (<NUM>) of each of the second plurality of segments (<NUM>) adjacent to the first sidewall (<NUM>) of one of the first plurality of segments (<NUM>) in a coupling region (<NUM>);
a first cladding layer (<NUM>) comprising a first material that has a first refractive index, the first cladding layer (<NUM>) adjoining the first sidewall (<NUM>) of each of the first plurality of segments (<NUM>) and the first sidewall (<NUM>) of each of the second plurality of segments (<NUM>); and
a second cladding layer (<NUM>) comprising a second material that has a second refractive index different from the first refractive index, characterized by the second cladding layer (<NUM>) adjoining the second sidewall (<NUM>) of each of the first plurality of segments (<NUM>) and the second sidewall (<NUM>) of each of the second plurality of segments (<NUM>).