Tolerant cascaded tapered optical ring resonator

Abstract

Embodiments herein describe a method for selectively filtering different wavelengths of optical signals received from an optical channel using a cascaded ring resonator, each of the cascaded ring resonators having a first ring and a second ring. The first ring has a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a greater width than the second waveguide width portion. The second ring has a varying waveguide width along its length, the second ring being configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a greater width than the third waveguide width portion. The method further connects a receiver to a respective cascaded ring resonator, each of the receivers having a photodetector configured to discriminate the optical signals.

Description

Technical Field

[0001] The examples of this disclosure generally relate to cascaded ring resonators having a ring with a tapered end that forms a tapered coupling region. Background Technology

[0002] The ever-increasing bandwidth demands of modern high-speed communications, coupled with the slowing of Moore's Law, necessitate new and innovative technologies in circuit design to meet current challenges in data centers, supercomputers, and other applications. Integrated high-speed silicon photonic devices, such as ring modulators and cascaded ring resonators, possess the characteristics to meet this high bandwidth requirement due to their energy efficiency and ability to be used in wavelength division multiplexing (WDM) systems. However, such integrated high-speed silicon photonic devices can be sensitive to process variations, potentially leading to poor performance. For example, process variations can cause mismatches between the rings of a cascaded ring resonator. At least for this reason, improved techniques for maximizing the output modulation amplitude in optoelectronic devices are desired. Summary of the Invention

[0003] One embodiment described herein is a cascaded ring resonator comprising a first ring having a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, the second ring being configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion.

[0004] One embodiment described herein is a wavelength division multiplexing (WMD) optical system for cascaded ring resonators, configured to selectively filter optical signals of different wavelengths received from an optical channel. Each cascaded ring resonator includes a first ring having a varying waveguide width along its length, configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion. Each cascaded ring resonator also includes a receiver configured to connect to the corresponding cascaded ring resonator, each receiver having a photodetector configured to distinguish the optical signals.

[0005] One embodiment described herein is a method for selectively filtering optical signals of different wavelengths received from an optical channel using cascaded ring resonators. Each cascaded ring resonator includes a first ring having a varying waveguide width along its length, configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion. The method also includes connecting receivers to the respective cascaded ring resonators, each receiver having a photodetector configured to distinguish the optical signals. Attached Figure Description

[0006] To gain a more detailed understanding of the features described above, a more specific description of the brief summary can be obtained by referring to the exemplary embodiments, some of which are illustrated in the accompanying drawings. However, it should be noted that the drawings illustrate only typical exemplary embodiments and should not be considered as limiting their scope.

[0007] Figure 1 An example of a wavelength division multiplexing (WMD) system using cascaded ring resonators with varying wavelength widths is illustrated.

[0008] Figure 2 This is a perspective view of a cascaded ring resonator based on an example, depicting the varying waveguide width of each ring.

[0009] Figure 3 A method for realizing a cascaded ring resonator with varying wavelength widths is illustrated.

[0010] For ease of understanding, the same reference numerals are used where possible to denote common elements in the figures. It is conceivable that elements of one example can be advantageously incorporated into other examples. Detailed Implementation

[0011] Various features are described below with reference to the accompanying drawings. It should be noted that the drawings may be drawn to scale or not, and elements with similar structures or functions are indicated by similar reference numerals in all the drawings. It should be noted that the drawings are intended only to facilitate the description of features. They are not intended to provide an exhaustive description of the embodiments herein, nor are they intended to limit the scope of the claims. Furthermore, the illustrated examples do not necessarily possess all the aspects or advantages shown. Aspects or advantages described in connection with a particular example are not necessarily limited to that example and may be practiced in any other example even if not so illustrated or so explicitly described.

[0012] The embodiments described in this paper depict cascaded ring resonators with rings having different wavelength widths. Cascaded ring resonators can be used as wavelength-selective filters in wavelength division multiplexing (WDM) systems to demultiplex wavelengths corresponding to different channels. Cascaded ring resonators with rings having different wavelength widths advantageously minimize or mitigate mismatch between rings, which in turn minimizes or mitigates insertion loss.

[0013] To support today's data-driven society, fiber optic communication technology has developed rapidly. Significant progress has been made in the search for ways to increase the transmission capacity of existing fiber optic network infrastructure. Since optical multiplexing is a useful technique for achieving this, researchers have explored optimizing optical multiplexing, at least wavelength-dependently, a technique known as WDM.

[0014] WDM creates a virtual fiber optic path on a single fiber bundle by combining two or more wavelengths (i.e., colors) of laser light into an optical signal transmitted through a single fiber. This is achieved by dividing the available raw bandwidth into a series of non-overlapping wavelengths or frequencies, each carrying a separate signal. By applying an optical (WDM) filter, the different wavelengths are combined (multiplexed) into a single optical transmission signal, only to be separated again (demultiplexed) at the receiving end. This technique reduces the need for dedicated and wavelength-specific optical links and multiplies the data-carrying capacity of existing fiber by a factor of the number of combined wavelengths (channels).

[0015] WDM technology includes coarse WDM (CWDM) and dense WDM (DWDM).

[0016] Coarse WDM (CWDM) technology can further increase the transmission capacity on a single optical fiber by providing up to 18 channels (16 are used in most commercially available applications) across multiple wavelength bands. For transmission equipment, it is important that the signal has a high, well-defined peak value for each channel in order to accurately interpret the optical signal, thus allowing individual signal peaks to be easily identified.

[0017] Dense WDM (DWDM) uses a much narrower passband (+ / -0.25nm), allowing for much denser channel spacing (+ / -0.8nm), and thus 16 DWDM channels can be fitted within the passband of a single CWDM channel. This allows for more optical communication channels and higher transmission capacity.

[0018] It can be expected that the exemplary cascaded ring resonator can be used in both CWDM and DWDM.

[0019] Cascaded ring resonators also act as filters. A filter is a circuit that can modify the frequency content of a signal. A filter allows certain frequencies to pass through while attenuating others. In signal filtering, the frequency content of a signal is used to extract useful information from it. The frequency content of a signal refers to the different frequencies that make up the signal. Filters work by selectively passing through or blocking certain frequency components of a signal. This is achieved by designing the filter's frequency response, which specifies how the filter responds to different frequencies.

[0020] Filters are classified based on their order, which refers to the number of energy storage elements in the filter circuit. A first-order filter has one energy storage element in its circuit, while a second-order filter has two. This means that a second-order filter has a steeper roll-off and a more abrupt transition between the passband and stopband than a first-order filter. Second-order filters can be used to implement high-pass, low-pass, and band-pass filters. Second-order filters extract useful information from signals and remove unwanted noise and interference.

[0021] A second-order filter comprises several components that work together to produce the desired frequency response. These components include resistors, capacitors, inductors, and operational amplifiers. In a second-order filter, capacitors are used to create low-pass, band-pass, and band-stop filters. The value of the capacitor can be adjusted to change the filter's cutoff frequency. In a second-order filter, inductors are used to create band-pass and band-stop filters. The value of the inductor can be adjusted to change the filter's cutoff frequency.

[0022] A key consideration when designing a second-order filter is the quality factor, also known as the Q factor. The Q factor determines the shape of the frequency response curve and can be adjusted by changing the values ​​of this component. A high Q factor results in a narrow bandwidth and a steep roll-off, while a low Q factor results in a wider bandwidth and a gentler roll-off.

[0023] As mentioned above, second-order filters are designed to attenuate unwanted frequencies while allowing desired frequencies to pass. Second-order filters can be classified into low-pass filters, high-pass filters, band-pass filters, and band-stop filters. Each type has its own unique frequency response.

[0024] A low-pass filter allows signals with frequencies below a selected cutoff frequency to pass through while attenuating signals with frequencies above the cutoff frequency. A high-pass filter allows signals with frequencies above certain cutoff frequencies to pass through while attenuating signals with frequencies below the cutoff frequency. A band-pass filter allows components within a specified frequency band (called its passband) to pass through, but blocks components with frequencies above or below that band. The passband is the range of frequencies or wavelengths that a filter can pass through. A stopband is a frequency band between specified limits, where circuitry such as a filter either prevents signals from passing through or attenuates them above the stopband attenuation level. A stopband filter (also called a notch filter) attenuates a specific frequency band while allowing frequencies outside of it to pass through.

[0025] An ideal bandpass filter would have a perfectly flat passband, meaning all frequencies within the passband would be delivered to the output without amplification or attenuation, and all frequencies outside the passband would be completely attenuated. However, in reality, no bandpass filter is ideal. A filter will not completely attenuate all frequencies outside the desired frequency range. In particular, there exists a region just outside the expected passband where frequencies are attenuated but not rejected. This is called the filter roll-off, and it is typically expressed in dB of attenuation per octave or decibel. The steepness of the gain in the stopband is the filter roll-off. In other words, the roll-off is the steepness or slope in the transition region between the passband and stopband.

[0026] Typically, filter design aims to make the roll-off as narrow as possible, allowing the filter to perform as close as possible to its intended design. This is usually achieved at the cost of passband or stopband ripple. In other words, spectral flatness is the way to quantify the deviation between the passband and a perfectly flat spectrum. The behavior of a signal measured in the passband is not ideal, and the deviation should be quantified relative to an ideal situation.

[0027] A cascaded ring resonator comprises two rings coupled together to produce a second-order filter with a flatter passband and a steeper roll-off. However, cascaded ring resonators are susceptible to process variations, particularly mismatch between the rings, which increases the device's insertion loss and reduces system yield. Insertion loss is the amount of energy lost as a signal travels along a cable link. In this example, insertion loss is the minimum attenuation in the filter's passband. A perfect filter would have no insertion loss at all, but all practical filters have some degree of insertion loss. Exemplary cascaded ring resonators are designed to mitigate the effects of process variations by altering the waveguide geometry of the rings. Therefore, the altered waveguide geometry advantageously allows for a reduction in the device's insertion loss and results in increased system yield.

[0028] Figure 1An example of a wavelength division multiplexing (WMD) system using cascaded ring resonators with varying wavelength widths is illustrated.

[0029] The WMD optical system 100 receives data 105. Data 105 can be optical signals, each with a different wavelength. In one example, eight optical signals 110 are shown, each with a different wavelength or color. The WMD optical system 100 can multiplex (combine or combine) multiple optical signals onto a single optical fiber using lasers of different wavelengths (i.e., colors). The optical signals 110 are ultimately received by a demultiplexer, which separates the optical signals back to their original form.

[0030] exist Figure 1 In this configuration, optical signal 110 is transmitted through waveguide 112. Waveguide 112 can be, for example, a linear waveguide. Waveguide 112 is coupled to multiple cascaded ring resonators. Each cascaded ring resonator comprises two rings coupled to each other. In one example, since there are eight optical signals transmitted through waveguide 112, there are eight cascaded ring resonators. Those skilled in the art can envision transmitting any number of optical signals in the channel.

[0031] The first ring resonator 120 has a first ring 122 and a second ring 124. The second ring 124 is connected to a receiver 126 having a photodetector 128. In one example, the photodetector 128 may be a silicon-germanium (SiGe) waveguide photodetector, but the embodiments herein are not limited to any particular type of waveguide material. The first ring 122 is coupled to the second ring 124. The coupling of the two rings 122, 124 produces a second-order filter, resulting in a flatter passband and stepper roll-off. In one embodiment, the first ring resonator 120 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB. Due to the steeper roll-off, the second-order filter provides lower crosstalk between adjacent wavelengths compared to a first-order filter. The flatter passband is also less susceptible to variations in laser wavelength spacing, which allows for collective thermal tuning of the first ring resonator 120 to match the laser wavelength.

[0032] The second ring resonator 130 has a first ring 132 and a second ring 134. The second ring 134 is connected to a receiver 136 having a photodetector 138. In one example, the photodetector 138 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 132 is coupled to the second ring 134. The coupling of the two rings 132, 134 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the second ring resonator 130 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0033] The third ring resonator 140 has a first ring 142 and a second ring 144. The second ring 144 is connected to a receiver 146 having a photodetector 148. In one example, the photodetector 148 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 142 is coupled to the second ring 144. The coupling of the two rings 142, 144 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the third ring resonator 140 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0034] The fourth ring resonator 150 has a first ring 152 and a second ring 154. The second ring 154 is connected to a receiver 156 having a photodetector 158. In one example, the photodetector 158 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 152 is coupled to the second ring 154. The coupling of the two rings 152, 154 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the fourth ring resonator 150 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0035] The fifth ring resonator 160 has a first ring 162 and a second ring 164. The second ring 164 is connected to a receiver 166 having a photodetector 168. In one example, the photodetector 168 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 162 is coupled to the second ring 164. The coupling of the two rings 162 and 164 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the fifth ring resonator 160 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0036] The sixth ring resonator 170 has a first ring 172 and a second ring 174. The second ring 174 is connected to a receiver 176 having a photodetector 178. In one example, the photodetector 178 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 172 is coupled to the second ring 174. The coupling of the two rings 172 and 174 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the sixth ring resonator 170 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0037] The seventh ring resonator 180 has a first ring 182 and a second ring 184. The second ring 184 is connected to a receiver 186 having a photodetector 188. In one example, the photodetector 188 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 182 is coupled to the second ring 184. The coupling of the two rings 182 and 184 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the seventh ring resonator 180 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0038] The eighth ring resonator 190 has a first ring 192 and a second ring 194. The second ring 194 is connected to a receiver 196 having a photodetector 198. In one example, the photodetector 198 may be a silicon-germanium (SiGe) waveguide photodetector. The first ring 192 is coupled to the second ring 194. The coupling of the two rings 192 and 194 produces a second-order filter, thereby achieving a flatter passband and stepper roll-off. In one embodiment, the eighth ring resonator 190 is designed to have an insertion loss of less than 1.5 dB and crosstalk of less than 15 dB.

[0039] Cascaded ring resonators 120, 130, 140, 150, 160, 170, 180, and 190 implement WDM channel selection. Therefore, cascaded ring resonators 120, 130, 140, 150, 160, 170, 180, and 190 can be referred to as filters because they filter out certain wavelengths corresponding to the channels in the WDM optical system 100. For optimal performance, each of the cascaded ring resonators 120, 130, 140, 150, 160, 170, 180, and 190 has a passband matched to its corresponding laser wavelength while maintaining an appropriate grid spacing. In one example, the grid spacing can be less than 1.5 nm.

[0040] In addition, please refer to the following: Figure 2In more detail, each ring of the cascaded ring resonators 120, 130, 140, 150, 160, 170, 180, and 190 has a varying wavelength width. Each ring includes a single waveguide. In one example, each waveguide may have a substantially elliptical shape. Each waveguide has a varying width, such that one end is wider than the other. In other words, taking the first ring resonator 120 as an example, the first ring 122 has a varying waveguide width along its length, configured to form a first waveguide width portion 122A and a second waveguide width portion 122B, where the first waveguide width portion 122A is larger than the second waveguide width portion 122B. The second ring 124 has a varying waveguide width along its length, configured to form a third waveguide width portion 124A and a fourth waveguide width portion 124B, where the fourth waveguide width portion 124B is larger than the third waveguide width portion 124A. The larger width portions of the waveguides reduce sensitivity to process variations to minimize insertion loss.

[0041] Figure 2 This is a perspective view of a cascaded ring resonator based on an example, depicting the varying waveguide width of each ring.

[0042] The cascaded ring resonator 200 is depicted having a first ring 220 and a second ring 230. The first ring 220 may be referred to as the top ring, and the second ring 230 may be referred to as the bottom ring. The first ring 220 is coupled to a first linear waveguide 210, and the second ring 230 is coupled to a second linear waveguide 240. The signal is received at the input terminal 205 of the first linear waveguide 210 and travels clockwise around the cascaded ring resonator 200. The first linear waveguide 210 may be referred to as the transmission port, and the second linear waveguide 240 may be referred to as the output port.

[0043] The first ring 220 and the first linear waveguide 210 form a first coupling region 215. The first ring 220 and the second ring 230 form a second coupling region 225. The second ring 230 and the second linear waveguide 240 form a third coupling region. The first coupling region 215 includes a wider waveguide portion 220A of the first ring 220. The second coupling region 225 includes a narrow waveguide portion 220B of the first ring 220 and a narrow waveguide portion 230A of the second ring 230. The third coupling region 235 includes a wider waveguide portion 230B of the second ring 230. Therefore, the waveguide geometry of the first ring 220 and the second ring 230 is altered or modified to include wide and narrow portions, wherein the narrow portion forms the central coupling region. In other words, the lower half of the bottom ring and the upper half of the top ring have a wider width, which can support, for example, a whispering gallery mode. The upper half of the bottom ring and the lower half of the top ring are narrow or tapered portions that only support a single propagation mode.

[0044] The narrow waveguide portion 220B located in the second coupling region 225 can be referred to as tapered or elliptical tapered. Similarly, the narrow waveguide portion 230A located in the second coupling region 225 can be referred to as tapered or elliptical tapered. The narrow waveguide portions 220B and 230A together form the second coupling region 225, also referred to as the tapered coupling region. The second coupling region 225 (or tapered coupling region) supports guided modes. Guided modes at the second coupling region 225 are less restricted, which allows for higher coupling between the first ring 220 and the second ring 230. The second coupling region 225 supports only single propagation modes. This means that higher-order modes are suppressed by the second coupling region 225. Therefore, very small coupling occurs between the fundamental mode of one ring (e.g., the first ring 220) and the higher-order mode of the other ring (e.g., the second ring 230). This ensures low intracavity losses in the device and does not support unwanted resonances from higher-order modes.

[0045] In one example, the narrow width of the ring waveguide (e.g., 220B, 230A) can be approximately 300nm-500nm, while the wide width of the ring waveguide (e.g., 220A, 230B) can be approximately 1µm-1.5µm. The width gradually, incrementally, or progressively increases from the narrow width region to the wide width region. The maximum width of the top ring waveguide is located near the first linear waveguide 210. The maximum width of the bottom ring waveguide is located near the second linear waveguide 240. The narrow widths are adjacent to each other in the central region.

[0046] Ring resonators are available devices used as high-Q filters in applications employing wavelength selectivity (e.g., WDM). This type of device has many resonators, and only these resonant wavelengths are coupled to the output waveguide. All systems (including molecular systems and particles) tend to vibrate at natural frequencies that depend on their structure. This frequency is called the resonant frequency or resonance frequency. When an oscillating force (external vibration) is applied at the resonant frequency of a dynamic system, object, or particle, the external vibration will cause the system to oscillate with a higher amplitude (with greater force) compared to applying the same force at other non-resonant frequencies. In the WDM optical system 100, by varying the width of the waveguide of the ring resonator, unwanted resonant or resonant frequencies from higher-order modes are suppressed to support a single propagation mode at the second coupling region 225. Therefore, the mismatch between the rings is minimized, resulting in minimal or negligible insertion loss.

[0047] The cascaded ring resonator 200 with a second coupling region 225 allows for increased yield of photonic integrated circuits (PICs), enabling economically viable production of PICs for optical transceivers. The physics behind the process variation tolerance of whispering-gallery mode resonators means that this solution is fabrication-independent. Furthermore, the solution using a wider waveguide section and creating the second coupling region 225 is implemented during the design phase and avoids the additional complexity of independently tuning the rings to align the resonance after fabrication. Therefore, the second coupling region 225 of the cascaded ring resonator described herein reduces the impact of process variations on the performance of the cascaded ring resonator. Advantageously minimizing or mitigating the mismatch between the first and second rings of each cascaded ring resonator in the cascaded ring resonator results in a significant reduction or negligible insertion loss.

[0048] Figure 3 A method for realizing a cascaded ring resonator with varying wavelength widths is illustrated.

[0049] At block 310, cascaded ring resonators are used to selectively filter optical signals of different wavelengths received from the optical channel. Each cascaded ring resonator includes a first ring with a varying waveguide width along its length, configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion being larger than the second waveguide width portion; and a second ring with a varying waveguide width along its length, configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion being larger than the third waveguide width portion. Therefore, the waveguide geometry of the first and second rings is altered or modified to include wide and narrow portions, with the narrow portions forming coupling regions. In other words, the lower half of the bottom ring and the upper half of the top ring have wider widths, which can support, for example, whispering-gallery modes. The upper half of the bottom ring and the lower half of the top ring are narrow or tapered portions that only support single propagation modes.

[0050] At box 320, the receiver is connected to a corresponding cascaded ring resonator, each receiver having a photodetector configured to distinguish optical signals. WDM increases bandwidth by allowing different data streams at different frequencies to be transmitted simultaneously over a single fiber optic network. In this way, WDM maximizes the usefulness of fiber optics and helps optimize network components. The basis of WDM lies in its ability to transmit different data types in the form of light over a fiber optic network. Using single-wavelength fiber optic cables is inefficient and wasteful. Instead, by allowing different optical channels (each with different or unique wavelengths) to be transmitted simultaneously over a fiber optic network, a single, efficient virtual fiber optic network can be created.

[0051] The techniques disclosed above may be described by one or more of the following non-limiting and non-exhaustive embodiments.

[0052] Example 1. A cascaded ring resonator, the cascaded ring resonator comprising: a first ring having a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, the second ring being configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion.

[0053] Example 2. The cascaded ring resonator according to Example 1, wherein the first waveguide width portion of the first ring is coupled to a first linear waveguide to form a first coupling region.

[0054] Example 3. According to the cascaded ring resonator of Example 1, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region.

[0055] Example 4. The cascaded ring resonator according to Example 3, wherein the second coupling region supports a single propagation mode.

[0056] Example 5. The cascaded ring resonator according to Example 3, wherein the second coupling region suppresses higher-order modes, such that less coupling occurs between the fundamental frequency of the first ring and the higher-order modes of the second ring, or vice versa.

[0057] Example 6. The cascaded ring resonator according to Example 3, wherein the fourth waveguide width portion of the second ring is coupled to the second linear waveguide to form a third coupling region.

[0058] Example 7. The cascaded ring resonator according to Example 1, wherein the first waveguide width portion of the first ring and the fourth waveguide width portion of the second ring are each between 1µm and 1.5µm.

[0059] Example 8. The cascaded ring resonator according to Example 1, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring are each between 300 nm and 500 nm.

[0060] Example 9. The cascaded ring resonator according to Example 1, wherein the cascaded ring resonator is used in a wavelength division multiplexing (WMD) optical system for wavelength selection.

[0061] Example 10. A wavelength division multiplexing (WMD) optical system comprising: cascaded ring resonators configured to selectively filter optical signals of different wavelengths received from an optical channel, each of the cascaded ring resonators comprising: a first ring having a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, the second ring being configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion; and receivers configured to be connected to the respective cascaded ring resonators, each of the receivers having a photodetector configured to distinguish the optical signals.

[0062] Example 11. The WMD optical system according to Example 10, wherein the first waveguide width portion of the first ring is coupled to the first linear waveguide to form a first coupling region.

[0063] Example 12. The WMD optical system according to Example 10, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region.

[0064] Example 13. The WMD optical system according to Example 12, wherein the second coupling region supports a single propagation mode.

[0065] Example 14. The WMD optical system according to Example 12, wherein the second coupling region suppresses higher-order modes, such that less coupling occurs between the fundamental frequency of the first ring and the higher-order modes of the second ring, or vice versa.

[0066] Example 15. The WMD optical system according to Example 12, wherein the fourth waveguide width portion of the second ring is coupled to the second linear waveguide to form a third coupling region.

[0067] Example 16. The WMD optical system according to Example 10, wherein the first waveguide width portion of the first ring and the fourth waveguide width portion of the second ring are each between 1µm and 1.5µm.

[0068] Example 17. The WMD optical system according to Example 10, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring are each between 300 nm and 500 nm.

[0069] Example 18. A method comprising: selectively filtering optical signals of different wavelengths received from an optical channel using cascaded ring resonators, each of the cascaded ring resonators comprising: a first ring having a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and a second ring having a varying waveguide width along its length, the second ring being configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion; and connecting receivers to corresponding cascaded ring resonators, each of the receivers having a photodetector configured to distinguish the optical signals.

[0070] Example 19. According to the method of Example 18, wherein the first waveguide width portion of the first ring is coupled to the first linear waveguide to form a first coupling region; and wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region.

[0071] Example 20. The method according to Example 19, wherein the second coupling region supports a single propagation mode; and wherein the second coupling region suppresses higher-order modes, such that less coupling occurs between the fundamental frequency of the first ring and the higher-order modes of the second ring, or vice versa.

[0072] In summary, optical ring resonators show promise as components for photonic integrated circuits in the field of silicon photonics. Due to the high refractive index contrast of the resonators, extremely small circuits can be produced. Furthermore, two or more optical ring resonators can be combined to develop high-order optical filters with compact size, minimal loss, and ease of integration into existing networks. Exemplary cascaded ring resonators reduce or minimize mismatch between rings, resulting in reduced insertion loss. This is achieved by changing the geometry of the waveguide in each ring of the cascaded ring resonator. The waveguide geometry has a varying width along its length. Thus, each waveguide in each ring has a wide portion and a narrow portion. The narrow portions of the first and second rings form a central coupling region, which is configured to allow only a single propagating mode and is configured to suppress unwanted resonances of higher-order modes.

[0073] Reference has been made to the embodiments presented in this disclosure. However, the scope of this disclosure is not limited to the specifically described embodiments. Rather, any combination of the described features and elements (whether or not it relates to different embodiments) is contemplated as an implementation and practice of the contemplated embodiments. Furthermore, while the embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether a particular advantage is achieved through a given embodiment does not limit the scope of this disclosure. Therefore, the foregoing aspects, features, embodiments, and advantages are merely illustrative and should not be considered as elements or limitations of the appended claims unless expressly stated in the claims.

[0074] The flowcharts and block diagrams in the figures illustrate the architecture, functionality, and operation of possible specific implementations of systems, methods, and computer program products according to various examples of the invention. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of instructions comprising one or more executable instructions for implementing a specified logical function. In some alternative implementations, the functions indicated in the blocks may not occur in the order shown in the figures. For example, depending on the functionality involved, two blocks shown consecutively may actually be executed substantially simultaneously, or these blocks may sometimes be executed in reverse order. It will also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented by a dedicated hardware-based system that performs the specified function or action or executes a combination of dedicated hardware and computer instructions.

[0075] While the foregoing is directed to specific examples, other and additional examples may be devised without departing from the basic scope of the invention, the scope of which is defined by the appended claims.

Claims

1. A cascaded ring resonator, the cascaded ring resonator comprising: A first ring having a varying waveguide width along its length, the first ring being configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a larger width than the second waveguide width portion; and The second ring has a varying waveguide width along its length, and is configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a larger width than the third waveguide width portion.

2. A wavelength division multiplexing (WMD) optical system, the WMD optical system comprising: A cascaded ring resonator configured to selectively filter optical signals of different wavelengths received from an optical channel, each of the cascaded ring resonators comprising: A first ring, having a varying waveguide width along its length, is configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and A second ring, having a varying waveguide width along its length, is configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion; and The receivers are configured to be connected to a corresponding cascaded ring resonator, each of the receivers having a photodetector configured to distinguish the optical signal.

3. The cascaded ring resonator according to claim 1 or the WMD optical system according to claim 2, wherein the first waveguide width portion of the first ring is coupled to the first linear waveguide to form a first coupling region.

4. The cascaded ring resonator according to claim 1 or the WMD optical system according to claim 2, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region.

5. The cascaded ring resonator according to claim 1 or the WMD optical system according to claim 2, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region, and wherein the second coupling region supports a single propagation mode.

6. The cascaded ring resonator of claim 1 or the WMD optical system of claim 2, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region, and wherein the second coupling region suppresses higher-order modes, such that less coupling occurs between the fundamental frequency of the first ring and the higher-order modes of the second ring, or vice versa.

7. The cascaded ring resonator of claim 1 or the WMD optical system of claim 2, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region, and wherein the fourth waveguide width portion of the second ring is coupled to a second linear waveguide to form a third coupling region.

8. The cascaded ring resonator according to claim 1 or the WMD optical system according to claim 2, wherein the first waveguide width portion of the first ring and the fourth waveguide width portion of the second ring are each between 1µm and 1.5µm.

9. The cascaded ring resonator according to claim 1 or the WMD optical system according to claim 2, wherein the second waveguide width portion of the first ring and the third waveguide width portion of the second ring are each between 300 nm and 500 nm.

10. A method, the method comprising: A cascaded ring resonator is used to selectively filter optical signals of different wavelengths received from an optical channel, each of the cascaded ring resonators comprising: A first ring, having a varying waveguide width along its length, is configured to form a first waveguide width portion and a second waveguide width portion, the first waveguide width portion having a wider width than the second waveguide width portion; and A second ring, having a varying waveguide width along its length, is configured to form a third waveguide width portion and a fourth waveguide width portion, the fourth waveguide width portion having a wider width than the third waveguide width portion; and The receivers are connected to corresponding cascaded ring resonators, each of which has a photodetector configured to distinguish the optical signal.

11. The method according to claim 10, The first waveguide width portion of the first ring is coupled to the first linear waveguide to form a first coupling region; and The second waveguide width portion of the first ring and the third waveguide width portion of the second ring form a second coupling region.

12. The method according to claim 11, The second coupling region supports a single propagation mode; and The second coupling region suppresses higher-order modes, resulting in less coupling between the fundamental frequency of the first ring and the higher-order modes of the second ring, or vice versa.

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