Optical switch including ring resonator

Abstract

The invention relates to optical switches including ring resonators, structures for optical switches, and methods of forming such structures. The structure includes a first waveguide core including a first portion and a second portion, a second waveguide core including a first portion and a second portion, a ring resonator having a first portion adjacent to the first portion of the first waveguide core and a second portion adjacent to the first portion of the second waveguide core, and an optical coupler coupled with the second portion of the first waveguide core and the second portion of the second waveguide core. The first portion of the ring resonator is spaced apart from the first portion of the first waveguide core by a first gap over a first optical coupling region, and the second portion of the ring resonator is spaced apart from the first portion of the second waveguide core by a second gap over a second optical coupling region.

Description

Technical Field

[0001] The present application relates to photonic chips, and more particularly to structures for optical switches and methods of forming such structures. Background Art

[0002] Photonic chips are used in many applications and systems, including but not limited to data communication systems and data computing systems. Photonic chips include photonic integrated circuits, which are composed of photonic components (such as modulators, polarizers, and optical couplers) that are used to manipulate light received from a light source (such as a laser or optical fiber).

[0003] An optical switch is a common photonic component found in photonic integrated circuits. A Mach-Zehnder interferometer can be used as the basic device for an optical switch. An optical coupler distributes the input light between a pair of arms of the Mach-Zehnder interferometer. A phase difference can be introduced between the light propagating in different arms to provide a pair of different switching conditions. The arms converge at a downstream optical coupler where the light is combined. Under one switching condition, the phase difference between the light after propagating through the arms is an odd multiple of pi, and the combined light leaves from one output port of the output coupler. Under another switching condition, the phase difference between the light after propagating through the arms is an even multiple of pi, and the combined light leaves from different output ports of the output coupler.

[0004] Conventional optical switches based on Mach-Zehnder interferometers may be characterized by high insertion loss, which may be unacceptable in certain applications or systems. Conventional optical switches based on Mach-Zehnder interferometers may also suffer from low extinction ratios and excessively large footprints.

[0005] There is a need for improved structures for optical switches and methods of forming such structures. Summary of the invention

[0006] In one embodiment of the present invention, a structure for an optical switch is provided. The structure includes a first waveguide core including a first portion and a second portion, a second waveguide core including a first portion and a second portion, a ring resonator having a first portion adjacent to the first portion of the first waveguide core and a second portion adjacent to the first portion of the second waveguide core, and an optical coupler coupled to the second portion of the first waveguide core and the second portion of the second waveguide core. The first portion of the ring resonator is separated from the first portion of the first waveguide core by a first gap above a first optical coupling region, and the second portion of the ring resonator is separated from the first portion of the second waveguide core by a second gap above a second optical coupling region.

[0007] In one embodiment of the present invention, a structure for an optical switch is provided. The structure includes a first waveguide core including a first portion and a second portion, and a second waveguide core including a first portion and a second portion. A first ring resonator is disposed adjacent to the first portion of the first waveguide core, and a second ring resonator is disposed between the first ring resonator and the first portion of the second waveguide core. An optical coupler is coupled to the second portion of the first waveguide core and the second portion of the second waveguide core.

[0008] In one embodiment of the present invention, a method of forming a structure for an optical switch is provided. The method includes forming a first waveguide core including a first portion and a second portion, forming a second waveguide core including a first portion and a second portion, forming a ring resonator having a first portion adjacent to the first portion of the first waveguide core and a second portion adjacent to the first portion of the second waveguide core, and forming an optical coupler coupled to the second portion of the first waveguide core and the second portion of the second waveguide core. The first portion of the ring resonator is separated from the first portion of the first waveguide core by a first gap above a first optical coupling region, and the second portion of the ring resonator is separated from the first portion of the second waveguide core by a second gap above a second optical coupling region. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings, which are included in and constitute a part of this specification, illustrate various embodiments of the present invention and, together with the general description of the present invention made above and the detailed description of the embodiments made below, are used to explain the embodiments of the present invention. In the accompanying drawings, like reference numerals represent similar features in different views.

[0010] Figure 1 A top view showing a structure for an optical switch according to an embodiment of the present invention.

[0011] Figure 2 Display the approximate Figure 1 A cross-sectional view of the structure taken along line 2-2 in FIG.

[0012] Figure 2A Display the approximate Figure 1 A cross-sectional view of the structure taken along line 2A-2A in FIG.

[0013] Figure 2B Display the approximate Figure 1 A cross-sectional view of the structure taken along line 2B-2B in FIG.

[0014] Figure 3 , 3A , 3B shows that Figure 2 , 2A, a cross-sectional view of the structure at the manufacturing stage of the processing method after 2B.

[0015] Figure 4 A top view showing a structure for an optical switch according to an alternative embodiment of the present invention.

[0016] Figure 5 A top view showing a structure for an optical switch according to an alternative embodiment of the present invention.

[0017] Figure 6 A schematic diagram showing the structure of a dense wavelength-division multiplexing (DWDM) switch according to an alternative embodiment of the present invention. DETAILED DESCRIPTION

[0018] Please refer to Figure 1 , Figure 2 , Figure 2A , Figure 2B And according to an embodiment of the present invention, a structure 10 for an optical switch includes a waveguide core 12, a waveguide core 14, a ring resonator 18 including a waveguide core 16 having a closed shape, and an optical coupler 20. A portion of the waveguide core 12 is disposed adjacent to a portion of the ring resonator 18, and a portion of the waveguide core 14 is disposed adjacent to a portion of the waveguide core 16 of the ring resonator 18. The ring resonator 18 is disposed laterally between the portion of the waveguide core 12 and the portion of the waveguide core 14. The portion of the waveguide core 12 upstream of the ring resonator 18 defines an input port for guiding light to the structure 10.

[0019] The waveguide cores 12, 14, 16 may be disposed on and above a dielectric layer 22 and a semiconductor substrate 24 of the photonic chip. In one embodiment, the waveguide cores 12, 14, 16 may be composed of a semiconductor material (e.g., single crystal silicon) formed by patterning using photolithography and etching processes. In one embodiment, the dielectric layer 22 may be composed of a dielectric material (e.g., silicon dioxide), and the semiconductor substrate 24 may be composed of a semiconductor material (e.g., single crystal silicon). In one embodiment, the dielectric layer 22 may be a buried oxide layer of a silicon-on-insulator substrate.

[0020] In one embodiment, the ring resonator 18 may be configured to have an elongated racetrack or elliptical shape. In one embodiment, the waveguide core 16 of the ring resonator 18 may include a curved end 26 having a radius of curvature, a curved end 28 having a radius of curvature, and side sections 30, 32 connecting the curved ends 26, 28 to provide the elliptical shape. In one embodiment, the side section 30 may be straight or linear, so that the width of the gap G1 between the side section 30 of the waveguide core 16 and the adjacent portion of the waveguide core 12 (which may also be linear) is constant over the optical coupling region 31 (configured for optical coupling from the waveguide core 12 to the waveguide core 16). In one embodiment, the optical coupling region 31 may extend from the junction between the curved end 26 and the side section 30 to the junction between the curved end 28 and the side section 30. In one embodiment, the side segments 32 may be straight or linear so that the width of the gap G2 between the side segments 32 of the waveguide core 16 and the adjacent portion of the waveguide core 14 (which may also be linear) is constant across the light coupling region 33 (configured for light coupling from the waveguide core 16 to the waveguide core 14). In one embodiment, the light coupling region 33 may extend from the junction between the curved end 26 and the side segments 32 to the junction between the curved end 28 and the side segments 32. The lengths of the side segments 30, 32 may be selected, among other variables, to select the coupling length of the light coupling regions 31, 33. The width dimensions of the gaps G1, G2 may be selected to select the coupling strength in the light coupling regions 31, 33.

[0021] In an alternative embodiment, the ring resonator 18 may be configured to have an annular or circular shape with an inner diameter and an outer diameter, in which case the light coupling regions 31, 33 may be shortened. In an alternative embodiment, the ring resonator 18 may be configured to have a shape that is neither elliptical nor circular, and in this case the light coupling regions 31, 33 may be non-linear.

[0022] The ring resonator 18 may include a heater 34 disposed adjacent to the waveguide core 16. In one embodiment, the heater 34 may be disposed within the inner diameter of the waveguide core 16. In one embodiment, the heater 34 may have a geometry similar to that of the waveguide core 16. The heater 34 may be configured to supply heat to the waveguide core 16, thereby changing the refractive index of the material of the waveguide core 16 to provide spectral tuning. In one embodiment, the heater 34 may include a resistive heating element composed of a doped semiconductor material, such as single crystal silicon doped with a p-type dopant or an n-type dopant, and an optional silicide cap. The heater 34 may be powered by a power source to generate heat, such as by Joule heating, which is conducted to the waveguide core 16.

[0023] The waveguide cores 12, 14 include portions that are independently routed from those portions of the waveguide cores 12, 14 adjacent the ring resonator 18 to the input of the optical coupler 20. The portion of the waveguide core 12 downstream of the optical coupler 20 defines an output port 54 for directing light from the structure 10 to a destination, and the portion of the waveguide core 14 downstream of the optical coupler 20 defines another output port 56 for directing light from the structure 10 to another destination. The waveguide core 14 includes an open end that can be coupled to a terminator 38, which can be configured as an absorber.

[0024] The waveguide core 14 includes a bend 25 that redirects the routing of the waveguide core 14 in the layout toward the optical coupler 20. In one embodiment, the bend 25 may have a curvature of about 180° so that the waveguide core 14 is redirected to be aligned substantially parallel to the waveguide core 12 before being connected to one of the inputs of the optical coupler 20. The bend 25 enables the ring resonator 18 to be disposed in the layout of the structure 10 adjacent to both waveguide cores 12, 14 to provide optical coupling regions 31, 33. In one embodiment, the portion of the waveguide core 14 located between the ring resonator 18 and the optical coupler 20 and the portion of the waveguide core 12 located between the ring resonator 18 and the optical coupler 20 may have equal lengths.

[0025] The phase shifter 40 is disposed adjacent to the portion of the waveguide core 12 between the ring resonator 18 and the optical coupler 20, and is thereby associated with the adjacent portion of the waveguide core 12. The phase shifter 42 is disposed adjacent to the portion of the waveguide core 14 between the ring resonator 18 and the optical coupler 20, and is thereby associated with the adjacent portion of the waveguide core 14. In one embodiment, the phase shifter 40 may be a thermo-optic phase shifter that includes a pair of resistive heating elements 44 disposed on opposite sides of the waveguide core 12 and a slab layer 45 that extends laterally to physically connect the resistive heating elements 44 to the adjacent portion of the waveguide core 12. In one embodiment, the phase shifter 42 may be a thermo-optic phase shifter that includes a pair of resistive heating elements 46 disposed on opposite sides of the waveguide core 12 and a slab layer 47 that extends laterally to physically connect the resistive heating elements 46 to the adjacent portion of the waveguide core 14. The slab layers 45, 47 may be thinner than the waveguide cores 12, 14, and the slab layers 45, 47 may include the same material as the waveguide cores 12, 14. In one embodiment, the resistive heating elements 44, 46 may be composed of a doped semiconductor material, such as single crystal silicon doped to include a pn junction. In alternative embodiments, the phase shifter 40 may be replaced by an electro-optical phase shifter, a lithium niobate-based phase shifter, a barium titanate-based phase shifter, a polymer-based phase shifter, a two-dimensional material-based phase shifter, or a micro-electromechanical system-based phase shifter.

[0026] The resistive heating element 44 of the phase shifter 40 is configured to generate heat by Joule heating under the control of a variable electrical signal, and the generated heat is transferred from the resistive heating element 44 to the physically connected portion of the waveguide core 12 by thermal conduction. The plate layer 45 provides a thermal path for thermal conduction of the heat to the connected portion of the waveguide core 12. The temperature of the connected portion of the waveguide core 12 is locally increased by the transferred heat. The local temperature change experienced by the connected portion of the waveguide core 12 effectively changes the refractive index of its constituent material through the thermo-optic effect, and thereby changes the phase of light propagating in the heated portion of the waveguide core 12.

[0027] The resistive heating element 46 of the phase shifter 42 is configured to generate heat by Joule heating under the control of a variable electrical signal, and the generated heat is transferred from the resistive heating element 46 to the physically connected portion of the waveguide core 14 by thermal conduction. The plate layer 47 provides a thermal path for the thermal conduction of the heat to the connected portion of the waveguide core 14. The temperature of the connected portion of the waveguide core 14 is locally increased by the transferred heat. The local temperature change experienced by the connected portion of the waveguide core 14 effectively changes the refractive index of its constituent material through the thermo-optic effect, and thereby changes the phase of light propagating in the heated portion of the waveguide core 14.

[0028] Phase shifters 40, 42 selectively change the phase of the light to provide a switching action that occurs when the light is combined by output optical coupler 20. In an alternative embodiment, phase shifter 40 may be omitted from structure 10 so that only phase shifter 42 is present. In an alternative embodiment, phase shifter 42 may be omitted from structure 10 so that only phase shifter 40 is present.

[0029] In one embodiment, optical coupler 20 may be a directional coupler in which waveguide cores 12 and 14 are routed in proximity characterized by a spacing that supports light transmission. Light is output from optical coupler 20 to output ports 54, 56, which are defined by portions of waveguide cores 12, 14 located downstream of optical coupler 20 in this representative embodiment.

[0030] During use, the optical switch implemented in the structure 10 can be used to switch the output from the output optical coupler 20 between different output ports 54, 56. A portion of the light arriving in the waveguide core 12 as an optical signal is transmitted to the waveguide core 14 by the ring resonator 18. The light propagating in the waveguide core 12 may experience a phase delay downstream of the ring resonator 18, which phase delay can be varied under the control of the phase shifter 40. The light propagating in the optical path defined by the waveguide core 14 may experience a phase delay, which phase delay can be varied under the control of the phase shifter 42. The optical coupler 20 can combine the light arriving at its input from the waveguide cores 12, 14 to provide constructive or destructive interference depending on the value of the phase delay between the different optical paths. In one embodiment, light leaving the output optical coupler 20 can be routed to the output port 54 with a phase delay of zero, and routed to the output port 56 with a phase delay equal to pi (which represents half the operating wavelength).

[0031] In one embodiment, the wavelength of light for different output states may depend on the coupling coefficient between the waveguide core 12 and the ring resonator 18, the coupling coefficient between the waveguide core 14 and the ring resonator 18, and the circumference of the ring resonator 18. In one embodiment, the coupling coefficient between the waveguide core 12 and the ring resonator 18 may be equal to the coupling coefficient between the waveguide core 14 and the ring resonator 18. In one embodiment, the coupling coefficient between the waveguide core 12 and the ring resonator 18 may not be equal to the coupling coefficient between the waveguide core 14 and the ring resonator 18.

[0032] The waveguide core 12 may define a through-port associated with the ring resonator 18. The waveguide core 14 may define a drop-port associated with the ring resonator 18. Portions of the waveguide cores 12, 14 are routed to the input of an optical coupler 20, and light is provided by the waveguide cores 12, 14 to the input of the optical coupler 20 with a phase shift provided by the operation of the phase shifters 40, 42. For single wavelength operation, the structure 10 operates as an optical switch. The optical switch implemented in the structure 10 may be characterized by a smaller insertion loss than that exhibited by a conventional optical switch. The optical switch implemented in the structure 10 may also be characterized by a larger extinction ratio than that exhibited by a conventional optical switch. The optical switch implemented in the structure 10 may also be characterized by a more compact footprint than a conventional optical switch. In one embodiment, the optical switch implemented in the structure 10 may be bidirectional.

[0033] In an alternative embodiment, the structure 10 may operate as a de-interleaver configured to route light from a selected wavelength of the plurality of operating wavelengths to one or the other of the output ports 54, 56. For a phase delay equal to pi, the selected wavelength exiting the pair of output ports 54, 56 may be inverted.

[0034] Please refer to Figure 3 , Figure 3A , Figure 3B And in Figure 2 , Figure 2A , Figure 2B At a later stage of manufacturing, a dielectric layer 50 may be formed over the structure 10. The dielectric layer 50 may be composed of a dielectric material, such as silicon dioxide, that is deposited and then planarized after deposition. A back-end-of-line stack 52 may be formed over the dielectric layer 50. The back-end-of-line stack 52 may include stacked interlayer dielectric layers, wherein each interlayer dielectric layer is composed of a dielectric material, such as silicon dioxide, silicon nitride, tetraethyl orthosilicate silicon dioxide, or fluorinated tetraethyl orthosilicate silicon dioxide.

[0035] Please refer to Figure 4 And according to an alternative embodiment of the present invention, the optical coupler 20 can be configured as a multimode interference coupler rather than a directional coupler. In one embodiment, the output optical coupler 20 can include a multimode interference region 21 having one input connected to the portion of the waveguide core 12 associated with the phase shifter 40 and another input connected to the portion of the waveguide core 14 associated with the phase shifter 42. The multimode interference region 21 is configured to combine the light received from the waveguide cores 12, 14 in constructive or destructive interference (depending on the phase shift imparted to the light by the phase shifters 40, 42). The multimode interference region 21 includes output ports 54, 56 to which the combined light is directed according to different phase shifts.

[0036] Please refer to Figure 5 And according to an alternative embodiment of the present invention, the ring resonator 17 may be disposed between the portions of the waveguide cores 12 and 14 together with the ring resonator 18. Light is coupled from the waveguide core 12 to the waveguide core 14 through the ring resonators 17 and 18. More specifically, a portion of the light propagating in the waveguide core 12 is transmitted to the ring resonator 17, the light propagating in the ring resonator 17 is transmitted to the ring resonator 18, and the light propagating in the ring resonator 18 is transmitted to the adjacent portion of the waveguide core 14. In an alternative embodiment, an additional ring resonator may be disposed between the ring resonator 17 and the ring resonator 18.

[0037] Please refer to Figure 6And according to an alternative embodiment of the present invention, multiple instances of structure 10 (i.e., structures 10a, 10b, 10c) can be cascaded to form a dense wavelength division multiplexing (DWDM) switch. Structure 10a has an input port, an output port coupled to the input port of structure 10b, and an output port coupled to the input port of structure 10c. The input port of structure 10a receives light 60, which includes optical signals having multiple wavelengths, distributes the optical signals in a wavelength-dependent manner, thereby outputting light 62 including optical signals having specific wavelengths to structure 10b, and outputting light 64 including optical signals having specific wavelengths to structure 10c. The wavelength of the optical signal output from structure 10a to structure 10b may be different from the wavelength of the optical signal output from structure 10a to structure 10c. The light 62 received at the input port of structure 10b is output from one output port of the structure 10b as light 66 including optical signals having specific wavelengths, and is output from another output port of the structure 10b as light 68 including optical signals having specific wavelengths. The light received at the input port of the structure 10c is output from one output port of the structure 10c as light 70 including an optical signal having a specific wavelength, and is output from one output port of the structure 10c as light 72 including an optical signal having a specific wavelength.

[0038] The method described above is used for the manufacture of integrated circuit chips. The manufacturer can distribute the resulting integrated circuit chips in raw wafer form (e.g., as a single wafer with multiple unpackaged chips), as a bare die, or in a packaged form. The chip can be integrated with other chips, discrete circuit elements and / or other signal processing devices as part of an intermediate product or a final product. The final product can be any product that includes an integrated circuit chip, such as a computer product or a smart phone with a central processing unit.

[0039] Terms modified by approximate language such as "about," "approximately," and "substantially" cited herein are not limited to the exact value specified. The approximate language may correspond to the precision of the instrument used to measure the value, and may represent + / -10% of the value unless otherwise dependent on the precision of the instrument.

[0040] Terms such as "vertical", "horizontal", etc. are cited herein as examples to establish a reference frame and are not limiting. The term "horizontal" as used herein is defined as a plane parallel to the conventional plane of the semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The terms "vertical" and "orthogonal" refer to directions perpendicular to the horizontal plane as just defined. The term "lateral" refers to a direction within the horizontal plane.

[0041] A feature that is "connected" or "coupled" to another feature may be directly connected or coupled to the other feature, or one or more intermediate features may be present. A feature may be "directly connected" or "directly coupled" to another feature if there are no intermediate features. A feature may be "indirectly connected" or "indirectly coupled" to another feature if there is at least one intermediate feature. A feature that is "on" or "in contact with" another feature may be directly on or in direct contact with the other feature, or one or more intermediate features may be present. A feature may be directly "on" or "in direct contact with" another feature if there are no intermediate features. A feature may be "not directly" "on" or "not directly in contact with" another feature if there is at least one intermediate feature. Different features may "overlap" if one feature extends over and covers part of another feature, with or without direct contact.

[0042] The description of various embodiments of the present invention is for illustrative purposes and is not intended to be exhaustive or limited to the described embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terms used herein are selected to best explain the principles of the embodiments, practical applications, or technical improvements over commercially known technologies, or to enable those of ordinary skill in the art to understand the embodiments described herein.

Claims

1. A structure for an optical switch, characterized in that: The structure includes: A first waveguide core comprising a first portion and a second portion; A second waveguide core comprising a first portion and a second portion; a ring resonator having a first portion adjacent to the first portion of the first waveguide core and a second portion adjacent to the first portion of the second waveguide core, the first portion of the ring resonator being separated from the first portion of the first waveguide core by a first gap over a first light coupling region, and the second portion of the ring resonator being separated from the first portion of the second waveguide core by a second gap over a second light coupling region; and An optical coupler is coupled to the second portion of the first waveguide core and the second portion of the second waveguide core.

2. The structure according to claim 1, characterized in that Also includes: A first phase shifter is associated with the first portion of the first waveguide core.

3. The structure according to claim 2, characterized in that Also includes: A second phase shifter is associated with the first portion of the second waveguide core.

4. The structure according to claim 3, characterized in that The second phase shifter is a thermo-optical phase shifter, and the second phase shifter includes a first resistance heating element and a second resistance heating element.

5. The structure according to claim 2, characterized in that The first phase shifter is a thermo-optical phase shifter, and the first phase shifter includes a first resistance heating element and a second resistance heating element.

6. The structure according to claim 1, characterized in that Also includes: A phase shifter is associated with the first portion of the second waveguide core.

7. The structure according to claim 6, characterized in that The phase shifter is a thermo-optical phase shifter, and the phase shifter includes a first resistance heating element and a second resistance heating element.

8. The structure according to claim 1, characterized in that The second portion of the first waveguide core is disposed between the first portion of the first waveguide core and the optical coupler.

9. The structure according to claim 8, characterized in that The second portion of the second waveguide core is disposed between the first portion of the second waveguide core and the optical coupler.

10. The structure according to claim 9, characterized in that The second portion of the second waveguide core is longer than the second portion of the first waveguide core.

11. The structure according to claim 1, characterized in that The optical coupler is a directional coupler, and includes a third portion of the first waveguide core and a third portion of the second waveguide core adjacent to the third portion of the first waveguide core.

12. The structure according to claim 1, characterized in that The optical coupler is a multimode interference coupler including a first input coupled to the second portion of the first waveguide core and a second input coupled to the second portion of the second waveguide core.

13. The structure according to claim 1, characterized in that The first gap is constant over the first light coupling region, and the second gap is constant over the second light coupling region.

14. The structure according to claim 1, characterized in that The ring resonator is a third waveguide core having an elliptical shape with a first side segment and a second side segment, the first portion of the ring resonator includes the first side segment of the third waveguide core, and the second portion of the ring resonator includes the first side segment of the third waveguide core.

15. The structure according to claim 14, characterized in that The first gap has a first width that is constant between the first side segment above the first light coupling region and the first portion of the first waveguide core, and the second gap has a second width that is constant between the second side segment above the second light coupling region and the first portion of the second waveguide core.

16. The structure according to claim 1, characterized in that Also includes: Semiconductor substrate; as well as a dielectric layer disposed on the semiconductor substrate, The first waveguide core, the second waveguide core, the ring resonator and the optical coupler are arranged on the dielectric layer.

17. The structure according to claim 1, characterized in that The second portion of the second waveguide core includes a bend having a curvature configured to route the second waveguide core away from the first waveguide core.

18. The structure according to claim 17, characterized in that The second portion of the second waveguide core has an equal length to the second portion of the first waveguide core.

19. A structure for an optical switch, characterized in that: The structure includes: A first waveguide core comprising a first portion and a second portion; A second waveguide core comprising a first portion and a second portion; a first ring resonator disposed adjacent to the first portion of the first waveguide core; A second ring resonator is disposed between the first ring resonator and the first portion of the second waveguide core; and An optical coupler is coupled to the second portion of the first waveguide core and the second portion of the second waveguide core.

20. A method of forming a structure for an optical switch, characterized in that The method includes: forming a first waveguide core including a first portion and a second portion; forming a second waveguide core including a first portion and a second portion; forming a ring resonator having a first portion adjacent to the first portion of the first waveguide core and a second portion adjacent to the first portion of the second waveguide core, wherein the first portion of the ring resonator is separated from the first portion of the first waveguide core by a first gap over a first light coupling region and the second portion of the ring resonator is separated from the first portion of the second waveguide core by a second gap over a second light coupling region; and An optical coupler is formed to couple with the second portion of the first waveguide core and the second portion of the second waveguide core.

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