Patent Description:
Optical switches and tunable optical filters are valuable elements in modern photonic networks. For example, reconfigurable Wavelength Division Multiplexed (WDM) optical networks, including Metro networks, Passive Optical Networks (PON), and high performance computing, make use of different wavelengths for various purposes, including a form of addressing. As such, many optical / photonic networks need devices that allow the selection of a wavelength to be added to or dropped from the transport link. Optical switches and tunable filters are also valuable in instrumentation applications, such as spectroscopy. Accordingly, there is a need for tunable optical elements which can be used as components for such devices.

Optical switches that are low-cost, low-power, and of a compact size are important components in optical cross connects (OXC), reconfigurable optical add-drop multiplexers (ROADM), and other optical networking systems. Photonic Integrated Circuits (PIC), utilizing, for example, Silicon-on-insulator (SOI) technologies, can provide high speed and small footprint. Silicon-on-insulator (SOI) is a promising technology for developing optical switches due to its relatively large thermo-optic coefficient, high thermal conductivity and high contrast refractive index. In recent years, various thermo-optic switch configurations have been reported on the SOI platforms.

Microring resonators (MRRs), also known as Microrings, fabricated in photonic integrated circuits have been widely researched for various applications, including wavelength tunable filters for optical networks. A microring is a waveguide loop that is typically circular but in principle may be any geometry. The microring is optically coupled to one or two transport waveguides. In a scenario in which the microring is coupled to a single waveguide, it provides the ability to remove a set of wavelengths from the transport waveguide, thus acting as a notch filter. In a scenario in which the microring is coupled to two transport waveguides, the transport waveguides couple light to/from the microring. If light transported by the first waveguide includes wavelengths which are resonant to the ring, then the resonant wavelengths of light can be coupled from the first transport waveguide into the ring, and propagate around the ring to be coupled to the second transport waveguide. Wavelengths of light that are not resonant to the ring are passed from the input of the first transport waveguide to the output of the first transport waveguide, and do not substantially interact with the microring. Filters with desirable bandpass characteristics may be formed by coupling multiple microrings to each other with or without intervening transport waveguides.

However there is a demand for ever increasing speeds, and smaller footprints for such systems. Further there is a need to reduce optical loss of such components, especially as the number of MRRs within a PIC increase. Further it is known that polymer waveguides can also be used within PICs. A polymer waveguide has a three dimensional freeform fabrication. The transmission loss of polymer waveguide is smaller, but has a lower light mode confinement compared to higher refractive index material, such as Silicon. Accordingly, in order to support single mode light transmission, the minimum size of polymer waveguides is larger than that of silicon waveguides due to the lower refractive index contrast between the waveguide core and the cladding. Further, polymer optical waveguides cannot turn or bend as effectively as silicon optical waveguides. Accordingly, while polymer waveguides can be suitable as transmission waveguides, the extra size and larger turn radius makes MRRs formed from polymer waveguides impractical.

Accordingly, there is a need for a system and method that at least partially addresses one or more limitations of the prior art.

<CIT> describes an optical microresonator. <CIT> describes filtering light of a selected wavelength. <CIT> describes localized thermal tuning of ring resonators. <CIT> describes an optical coupler.

In accordance with the invention, there is provided a microring resonator and a photonic circuit as defined in the appended claims.

A ring resonator (RR), also referred to as a microring resonator (MRR), is an optical waveguide ring which can optically couple with another optical waveguide, and depending on the implementation, an additional optical waveguide. Examples will be discussed assuming an MRR is coupled to two optical waveguides, but it should be appreciated that the additional optical waveguide is not necessary for some implementations (e.g. a notch filter).

<FIG> schematically illustrates a conventional directional coupler based silicon MRR. <FIG> schematically illustrates an isometric view of the MRR, whereas <FIG> schematically illustrates a top view of the MRR. <FIG> illustrates an MRR <NUM> which includes three silicon optical waveguides on a silica base <NUM>. The three silicon waveguides include circular ring waveguide <NUM> interposed between parallel silicon waveguides <NUM> and <NUM>. The MRR is configured with gaps <NUM>, <NUM> between the ring waveguide <NUM> and optical waveguides <NUM> and <NUM> respectively to allow for evanescent coupling between the ring <NUM> and the waveguides <NUM> and <NUM>. Such an MRR can be tuned to resonate for particular wavelengths, such that light in waveguide <NUM> can be coupled to the waveguide <NUM> for the particular wavelengths. Such an arrangement is often used as part of an add/drop multiplexer. In such an example, silicon waveguide <NUM> acts as a transmission waveguide and silicon waveguide <NUM> acts as a drop waveguide for wavelength(s) to which the MRR is tuned. The directional coupler can be designed with any coupling ratio range from zero to unity at a given wavelength. The coupling ratio changed with the wavelength and waveguide dimensions at the directional coupler section. It should be appreciated that some MRRs utilize 2x2 multimode interference (MMI) couplers, in which case there is no gaps <NUM>, <NUM>, and a multimode joint section connects the ring waveguide <NUM> with waveguide <NUM> or <NUM>. MMI based MRRs can have more loss and back reflection than directional coupler MRRs, but MMI based MMRs can have the advantages of better polarization tolerance and wider bandwidth.

Such a MRR includes a pair of electrodes (not shown) for shifting the resonant frequency of the ring, and an input interface for receiving a drive signal for driving the electrodes. Resonant frequency shifting in the MRR can be provided by an electro-optic effect (e.g., carrier injection) or thermo-optic effect. Accordingly, MRRs can be used in tunable switches and other optical elements.

<FIG> schematically illustrates a MRR <NUM> in accordance with embodiments of the present invention. <FIG> schematically illustrates an isometric view of the MRR, whereas <FIG> schematically illustrates a top view of the MRR. <FIG> illustrates an MRR <NUM> which includes a polymer optical waveguide <NUM> and a silicon optical waveguide <NUM> on a silica base <NUM>. The device also includes ring optical waveguide <NUM> interposed between the waveguides <NUM> and <NUM>. The MRR is configured with gap <NUM> between the ring waveguide <NUM> and optical waveguide <NUM> to allow for directional coupling between the ring <NUM> and the silicon waveguide <NUM>. However, there is no gap between the polymer optical waveguide <NUM> and the ring optical waveguide <NUM>. Further there is no need for the addition of a MMI coupler between polymer optical waveguide <NUM> and the ring optical waveguide <NUM>. Rather the polymer optical waveguide <NUM> is configured such that a first portion of the optical polymer waveguide <NUM> overlaps a second portion of the ring waveguide <NUM>. This will be more apparent in the cross section illustrated in <FIG>. Such an MRR can be tuned to resonate for particular wavelengths, such that light in waveguide <NUM> can be coupled to the waveguide <NUM> for the particular wavelengths. Such an arrangement can be used as part of an add/drop multiplexer or other optical elements as will be discussed below. In such an example, polymer waveguide <NUM> acts as a transmission waveguide and silicon waveguide <NUM> acts as a drop waveguide for wavelength(s) to which the MRR is tuned. Resonant frequency shifting of the MRR <NUM> can be provided by an electro-optic effect (e.g., carrier injection) or thermo-optic effect.

In the embodiment shown ring optical waveguide <NUM> is circular, but this is not a requirement. Other ring shapes can be utilized. One such example shape is a stadium or racetrack (a rectangle with semicircles at a pair of opposite sides). Further, while the embodiment illustrated utilizes directional coupling between the ring waveguide <NUM> and optical waveguide <NUM>, an MMI coupler could be utilized.

Polymer optical waveguides have beneficial characteristics in some circumstances, including suffering less optical loss than silicon optical waveguides, which make their use advantageous for some types of PICs. Also, polymer waveguides can be easier to fabricate, for example using three-dimensional freeform fabrication. However, polymer waveguides can have lower light mode confinement compared to higher refractive index materials such as silicon. Accordingly, they tend to be larger than silicon optical waveguides, resulting in more footprint in PICs which utilize them. Further, polymer optical waveguides cannot turn or bend as effectively as silicon optical waveguides. Accordingly, a larger footprint bend radius can be required to utilize polymer waveguides in contrast to silicon waveguides, due to the lower refractive index contrast between the waveguide core and the cladding. Accordingly, it can be advantageous to use a combination of polymer and silicon optical waveguides within a PIC. For example, such a hybrid PIC can use polymer optical waveguides when reducing loss is important, and use silicon optical waveguides when size/footprint or radius of curvature is important. It is noted that while a polymer ring waveguide could be used to form an MRR, the radius of curvature of such a polymer ring waveguide would be significantly larger, making silicon ring waveguides more effective. Advantageously, embodiments provide efficient coupling between polymer optical waveguides and silicon ring optical waveguides.

In the embodiment illustrated in <FIG> and <FIG>, the MRR <NUM> includes a region of overlap <NUM> between the polymer optical waveguide <NUM> and the ring waveguide <NUM>. In this overlapping region <NUM>, a first portion of the polymer optical waveguide <NUM> overlaps a second portion of the ring waveguide <NUM>. This can be best seen in <FIG>.

<FIG> illustrates a cross section of the MRR along section line <NUM>-<NUM> of <FIG>, in accordance with embodiments of the present invention. In this figure height refers to the vertical direction, and width refers to horizontal direction (with length being the direction in/out of the page). As can be seen, the polymer waveguide <NUM> is larger than the ring waveguide <NUM>, both in height and width. Embodiments make use of this extras size to improve the coupling efficiency between the polymer optical waveguide <NUM> and the ring optical waveguide <NUM> by including an overlapping region <NUM> in which the ring waveguide <NUM> appears embedded (or inserted) into the polymer waveguide <NUM>. It is noted that in some embodiments, the ring waveguide can be inserted into the polymer waveguide. In other embodiments, depending on the fabrication process for producing the PIC, the ring waveguide <NUM> may not need to be physically inserted into the polymer waveguide <NUM>, due to the layering process of fabrication, in which the polymer waveguide can be deposited over the silicon waveguide.

It should be appreciated that various embodiments can utilize a variety of materials. While examples have been given with respect to a polymer transmission waveguide and a silicon ring waveguide, such a device can be more generally described as having an optical waveguide and a ring waveguide, with optical waveguide overlapping a portion of the ring waveguide. Embodiments are discussed with the optical waveguide having a first refractive index and the ring optical waveguide having a second refractive index such that the first refractive index is less than the second refractive index. However, as there are many types of polymer waveguides examples will be discussed with respect to a range of refractive index differences between the two types of waveguides. The range depends on the two materials. Some non-limiting examples of the range of refractive index differences between silicon waveguides and polymer waveguides can be <NUM> - <NUM>, depending on the materials. However, in some embodiments silicon nitride waveguides and polymer waveguides can be used, and some non-limiting examples of the range of refractive index difference between silicon nitride waveguide and a polymer waveguide can be <NUM> - <NUM> depending on the materials. As another example, in some embodiments glass waveguides and polymer waveguides can be used, and some non-limiting examples of the range of refractive index difference between a glass waveguide and a polymer waveguide is <NUM> - <NUM>.

<FIG> illustrates an alternative embodiment in which a refractive index matching material is interposed between the polymer optical waveguide <NUM> and the ring optical waveguide <NUM> within the overlapping region <NUM>. In the embodiment illustrated, the coupling material is interposed horizontally at <NUM> between polymer optical waveguide <NUM> and the ring optical waveguide <NUM>, and also interposed vertically at <NUM>. In some embodiments the coupling material can be located at either, both or neither of <NUM>, <NUM>. The refractive index matching material has a refractive index between that of the polymer refractive index and the silicon refractive index to improve the coupling between polymer optical waveguide <NUM> and the ring optical waveguide <NUM> and to reduce possible back reflection. <FIG> illustrates example dimensions for an example of <FIG> in accordance with embodiments of the present invention. In this example, the polymer optical waveguide is <NUM> in height, and <NUM> in width. The ring waveguide, in cross section is <NUM> in height, and <NUM> in width, and at the point of the cross section, has half of its cross sectional width embedded within the polymer waveguide. However, there dimensions are just examples, and other dimensions can be utilized. In an embodiment, the polymer optical waveguide has a polymer core SU-<NUM> with a refractive index of <NUM> at <NUM> and a Cytop cladding with refractive index <NUM> at <NUM>. It is noted the cladding of the polymer core is not shown. In some embodiments, the core of the polymer optical waveguide overlaps the core of the silicon optical waveguide.

<FIG> illustrates an alternative example, not according to the claimed invention, in which a smaller polymer waveguide <NUM> partially overlaps a ring waveguide <NUM> to achieve multimode coupling between the two waveguides. In this example, the polymer waveguide has similar cross sectional dimensions to the ring waveguide. Such an example may be useful if such factors as overlapping area, cladding material (Not shown), polymer core material and size of silicon waveguide is used to allow for a similarly sized polymer waveguide.

Other embodiments can utilize cascaded MRRs. <FIG> illustrates an example of an MRR including two ring waveguides, according to an embodiment, although N order MRRs may be possible. <FIG> shows silicon optical ring waveguide <NUM> adjacent silicon optical ring waveguide <NUM> between silicon optical waveguide <NUM> and polymer optical waveguide <NUM>. In this example, an optical signal within polymer optical waveguide <NUM> can passthru the waveguide <NUM> except for wavelength (s) λ<NUM> which are dropped via the rings and waveguide <NUM>. In this example silicon optical ring waveguide <NUM> is configured to resonate at range of wavelengths designated as λ<NUM>, and silicon optical ring waveguide <NUM> is configured to resonate at range of wavelengths designated as λ<NUM>, with λ<NUM> being a subset of λ<NUM>. <FIG> also illustrates a couple of alternative features that can be implemented in various embodiments, which can include single order embodiments. First, as noted above the ring waveguides need not be circular, and other shapes that allow for light to loop within the ring can be utilized. In the example illustrated in <FIG>, silicon optical ring waveguide <NUM> and silicon optical ring waveguide <NUM> have stadium shapes. A stadium shape has two parallel portions with semicircles at either side linking the two parallel portions. Such a shape can increase the coupling efficiency by allowing for a larger area in which optical coupling occurs. Second, <FIG> also illustrates that the amount the polymer waveguide overlaps the silicon waveguide can vary. In the example shown in <FIG>, the polymer waveguide overlapped half of the cross-sectional width of the silicon waveguide. However, <FIG> illustrates an example where the polymer waveguide <NUM> overlaps the entire cross sectional width of the silicon waveguide <NUM> at <NUM>. Other variations can be utilized, depending on such factors as the materials used, the optical waveguide sizes, coupling length, and the amount of coupling required.

As stated, the MRR structures discussed with respect to the various embodiments above can be integrated into various PIC structures. <FIG> illustrates an example PIC, such as a microring based switch matrix, in accordance with embodiments of the present invention. In such an example, a common polymer optical waveguide <NUM> carries a WDM signal including λ<NUM>λ<NUM> λ<NUM>. λn with a plurality of MRRs performing switching functions on individual wavelengths. In this example MRR <NUM> drops λ<NUM>, MRR <NUM> drops λ<NUM>, MRR <NUM> drops λ<NUM>, and MRR <NUM> drops λn. Similarly, MRR <NUM> adds λ<NUM>, MRR <NUM> adds λ<NUM>, MRR <NUM> adds λ<NUM>, and MRR <NUM> adds λn. Each MRR includes a silicon ring waveguide which couples with the common polymer optical waveguide <NUM> in a manner as discussed above. In other words, common polymer optical waveguide <NUM> overlaps a portion of each of the MRR silicon ring waveguides. Each MRR couples each ring to another optical waveguide, which carries a single wavelength in this example. For example, MRR <NUM> includes silicon ring waveguide <NUM>, which is overlapped by the common polymer optical waveguide <NUM>. MRR <NUM> is configured to resonate at λ<NUM>, effectively coupling λ<NUM> to optical waveguide <NUM>. Optical waveguide <NUM> can be a silicon optical waveguide or another polymer optical waveguide. It is noted that each MRR is shown as a box to schematically represent that each MRR can include other components, such as a tuner and an input for receiving a drive signal to tune the ring to resonate at a desired wavelength. It is noted that each MRR may have a ring waveguide with different path lengths to resonate at particular wavelengths and/or each can be tuned to alter the resonant wavelength using electro-optic techniques, thermo-optic techniques or other methods. It is noted that in some applications, the common polymer optical waveguide <NUM> may not be continuous, but can have a number of waveguides optically coupled using, for example, using polymer wire bonding.

<FIG> illustrates an example of a PIC which acts a DWDM transceiver, in accordance with embodiments of the present invention. Such a transceiver includes a receiver <NUM> and a transmitter <NUM>. The receiver <NUM> and transmitter <NUM> can be implemented as separate PICs, in some embodiments. The transceiver receives WDM signals including λ<NUM>λ<NUM> λ<NUM>. λn from and transmits WDM signals including λ<NUM>λ<NUM> λ<NUM>. λn to network fibers <NUM>, it being understood that the transceiver can transmit on one fiber and receive from another. Each of the transmitter and receiver can include a plurality of MRRs for add/drop functions on individual wavelengths. Each MRR can be tuned (tuner and drive signal not shown) to drop a particular wavelength to a photodetector (PD) via a drop optical waveguide (which can be silicon or polymer) coupled to the ring. In this example MRR <NUM> drops λ<NUM> to PD <NUM> via optical waveguide <NUM>, MRR <NUM> drops λ<NUM> to PD <NUM> via optical waveguide <NUM>, MRR <NUM> drops λ<NUM> to PD <NUM> via optical waveguide 841and MRR <NUM> drops λn tο PD <NUM> via optical waveguide 851from polymer optical waveguide <NUM>. As per above, polymer waveguide <NUM> can be a continuous polymer waveguide or can be formed from segments of polymer waveguide optically coupled together, for example via polymer wire bonding.

On the transmitter side, light at each wavelength can be modulated directly by a MRR which act a modulator. A drive signal modulates data by modifying the resonant wavelength of the silicon ring, depending on datastream to be modulated. Accordingly, each MRR includes a silicon ring waveguide which overlaps transmit polymer waveguide <NUM> as discussed above. In this example, MRR <NUM> includes silicon ring waveguide <NUM> and adds data according drive stream 862on λ<NUM>, MRR <NUM> includes silicon ring waveguide <NUM> and adds data according to drive stream <NUM> onto λ<NUM>, MRR <NUM> includes silicon ring waveguide <NUM> and adds data according to drive stream <NUM> onto λ<NUM>, and MRR <NUM> includes silicon ring waveguide <NUM> and adds data according to drive stream <NUM> onto λn. Each MRR includes a silicon ring waveguide which couples with the common polymer optical waveguide <NUM> in a manner as discussed above. In other words, common polymer optical waveguide <NUM> overlaps a portion of each of the MRR silicon ring waveguides <NUM>, <NUM>, <NUM> and <NUM>. In some embodiments, the silicon microring for each modulator and filter can be fabricated on a single PIC chip. The polymer waveguide can be fabricated on top of PIC chip. Using such a common polymer waveguide can reduce insertion loss, as contrasted by similar designs which utilize silicon waveguides interconnecting each ring. As discussed above, in some cases the common polymer waveguide need not be a single structure, but can be fabricated as sections of interconnected polymer waveguides.

It should be appreciated that a PIC can include more than one polymer optical waveguide. It should be appreciated that the examples illustrated in <FIG> and <FIG> can represent portions of a PIC which includes additional circuit components or multiples of each circuit portion.

Claim 1:
A microring resonator (<NUM>), MRR, comprising:
a ring optical waveguide (<NUM>), wherein the ring optical waveguide (<NUM>) is a silicon optical waveguide, and wherein the ring optical waveguide has a second refractive index; and
a first optical waveguide (<NUM>), wherein the first optical waveguide (<NUM>) is a polymer optical waveguide, and wherein the first optical waveguide (<NUM>) has a first refractive index such that the first refractive index is less than the second refractive index;
wherein the first optical waveguide (<NUM>) is larger in height than the ring optical waveguide (<NUM>), and wherein a first portion of the first optical waveguide (<NUM>) is configured to provide space for a second portion of the ring optical waveguide (<NUM>) such that the first portion of the first optical waveguide (<NUM>) overlaps the second portion of the ring optical waveguide (<NUM>).