A multimodal integrated optical microcavity

Abstract

This invention discloses a multimodal integrated optical microcavity, belonging to the field of integrated photonic device technology. The multimodal integrated optical microcavity includes a first microcavity structure and a second microcavity structure coupled in a vertical direction. The first and second microcavity structures are made of different materials, and by adjusting the waveguide thickness and width, the two microcavities have approximately the same free spectral range within a preset operating wavelength band. Because the two materials have different thermo-optic coefficients, differential resonant frequency shifts can be naturally generated during thermal tuning of the bilayer microcavity. Combined with the vernier coupling effect between the microcavities, a Moore acceleration effect is formed, thereby achieving a wide range of adjustment of the microcavity's dispersion characteristics. By controlling the thermal excitation conditions, switching between normal dispersion and anomalous dispersion can be achieved, enabling a single device to support multiple optical operating modes. This invention has a simple structure, stable control, and strong process compatibility, making it suitable for reconfigurable integrated photonic systems.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonics, and particularly relates to a multimodal integrated optical microcavity. Background Technology

[0002] With the development of integrated photonics, high-Q optical microcavities have been widely used in microwave photonics, optical frequency combs, coherent optical communication, lidar and precision measurement, and can realize a variety of working modes including stimulated Brillouin lasers, stimulated Raman lasers, bright soliton optical frequency combs and dark pulses.

[0003] However, the operating mode of existing optical microcavities is typically determined by their geometry and material dispersion. Once the material is selected and the device is fabricated, its free spectral range (FSR) and group velocity dispersion (GVD) are fixed and difficult to adjust over a wide range. This means that a single device can usually only support one specific function. If multiple different functions are required, separate fabrication of discrete devices is necessary, which is insufficient to meet the needs of various application scenarios for reconfigurable photonic devices.

[0004] To achieve the control of microcavity dispersion, existing techniques have attempted to employ methods such as changing waveguide geometry parameters, introducing mode crossover, or utilizing single-loop thermal tuning. However, these methods generally suffer from problems such as a very limited adjustment range, low tuning efficiency, or reliance on complex control circuits, making it difficult to achieve large-range, stable, and reversible dispersion reconstruction while maintaining a high Q value.

[0005] In recent years, coupled microcavity structures based on the vernier effect have provided new possibilities for dispersion control. However, existing schemes, which use the same material and planar structure and achieve operating mode switching by applying differential tuning signals to different microcavities, suffer from problems such as severe thermal crosstalk, high control complexity, and high requirements for process consistency.

[0006] Therefore, there is an urgent need for an integrated microcavity device solution that is simple in structure, has good process compatibility, stable in control, and can achieve large-scale dispersion reconstruction. Summary of the Invention

[0007] The purpose of this invention is to provide a multimodal integrated optical microcavity that achieves wide-range, stable and reversible control of microcavity dispersion through the molar acceleration effect generated by the difference in material thermo-optic coefficients and the vernier effect, thereby enabling a single microcavity device to support multiple operating modes.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A multimodal integrated optical microcavity, comprising:

[0010] The first microcavity structure is disposed on the substrate and made of the first material to form a coupled straight waveguide, a first ring or racetrack-shaped waveguide microcavity, wherein the coupled waveguide is used to measure the resonant frequency of the first resonant cavity and the thermo-optic effect;

[0011] The second microcavity structure is disposed above or below the first microcavity structure and is spaced apart from the first microcavity structure in the vertical direction. It is made of a second material and forms a coupled straight waveguide, a second ring or racetrack-shaped waveguide microcavity. The coupled waveguide is used to measure the resonant frequency of the second resonant cavity and the thermo-optic effect.

[0012] The first microcavity structure and the second microcavity structure form optical coupling in the vertical direction, that is, the straight waveguide part of the racetrack-shaped resonant cavity, so that the resonant modes of the two interact in the frequency domain.

[0013] The first material and the second material have different thermo-optic coefficients;

[0014] At least one integrated heating structure is provided for differential thermal tuning of the first microcavity structure and the second microcavity structure. The heating structure needs to cover the area of ​​the two microcavities to achieve effective heating regulation.

[0015] Furthermore, the first microcavity structure and the second microcavity structure adjust their respective waveguide thickness and width to make their free spectral ranges within a preset working band similar.

[0016] Furthermore, the optical coupling between the first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity in the vertical direction is specifically as follows:

[0017] The first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity overlap in their orthogonal projections onto the substrate. The first coupled straight waveguide does not form optical coupling with the second annular or racetrack-shaped waveguide microcavity; the second coupled straight waveguide does not form optical coupling with the first annular or racetrack-shaped waveguide microcavity.

[0018] Furthermore, one straight waveguide of each of the first and second racetrack-shaped waveguide microcavities is positioned at the same vertical position.

[0019] Furthermore, the positions of the first microcavity structure and the second microcavity structure can be interchanged.

[0020] Furthermore, the heater is a differential heater.

[0021] Furthermore, the first microcavity structure is made of silicon nitride; the second microcavity structure is made of silicon oxynitride; or

[0022] The first microcavity structure is made of silicon oxynitride; the second microcavity structure is made of silicon nitride.

[0023] Furthermore, the substrate is a silicon oxide wafer.

[0024] Furthermore, by adjusting the heater power, both microcavities can be made to resonate at a specific frequency. When the free spectral range is exactly the Brillouin frequency shift, stimulated Brillouin laser can be generated by pumping; when the free spectral range exhibits anomalous dispersion, a bright soliton frequency comb can be generated by pumping; and when the free spectral range exhibits normal dispersion, a dark pulse can be generated by pumping.

[0025] This invention also provides a method for fabricating a multimodal integrated optical microcavity, which includes the following steps:

[0026] (1) A first material layer is deposited on a substrate and a first microcavity structure is fabricated by etching; the first material is the waveguide material used in the first microcavity structure;

[0027] (2) A third material layer is deposited to cover the first microcavity structure, and the surface is smoothed by chemical mechanical polishing;

[0028] (3) Deposit a second material layer on the third material layer deposited in step (2) and etch to prepare a second microcavity structure; the second material is the waveguide material used in the second microcavity structure; the first material and the second material have different thermo-optic coefficients;

[0029] (4) Deposit a third material layer to cover the second microcavity structure and use chemical mechanical polishing to smooth the surface;

[0030] (5) Install a heater inside or on the third material layer deposited in step (4).

[0031] Compared with the prior art, the present invention has at least the following advantages:

[0032] (1) Achieving natural differential regulation

[0033] Since the upper and lower microcavities are made of different materials, their thermo-optic coefficients are different. Under the same or nearly the same thermal excitation conditions, different resonant frequency drifts can be generated, without the need for a complex independent differential heating structure.

[0034] (2) Significantly enhances dispersion adjustment efficiency

[0035] By introducing a vernier effect through a slight mismatch in the free spectral range of the upper and lower microcavities, a molar acceleration effect is formed under the influence of a slight differential frequency shift caused by thermal adjustment, thereby achieving a wide range of adjustment of the dispersive characteristics.

[0036] (3) Supports switching between multiple working modes

[0037] By adjusting the heating power, the interaction between the modes of the upper and lower microcavities causes a large-scale shift in the free spectral range, enabling the device to switch between normal and anomalous dispersion, thereby supporting multiple operating modes such as dark pulse, bright soliton optical frequency comb, or Brillouin laser.

[0038] Dispersion modulation principle: During the design phase, the height and width of the microcavities are changed to make the free spectral ranges of the first and second microcavities as close as possible. By adjusting the heater power, the two microcavities are made to resonate at a specific frequency. At this time, due to the vertical coupling of the two microcavities, the modes will interact, and the free spectral range near the resonant frequency will change drastically. When the free spectral range is exactly the Brillouin frequency shift, stimulated Brillouin laser can be generated by pumping. When the free spectral range exhibits anomalous dispersion, a bright soliton frequency comb can be generated by pumping. When the free spectral range exhibits normal dispersion, a dark pulse can be generated by pumping.

[0039] (4) Strong process compatibility and good scalability

[0040] Silicon nitride and silicon oxynitride are mature technologies that are compatible with CMOS processes, and their thickness can be controlled at the nanometer level, making them suitable for large-scale integration. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is a schematic diagram of the structure of the multimodal integrated optical microcavity in this invention, which mainly consists of silicon nitride microrings, silicon oxynitride microrings, and heaters for differential adjustment.

[0043] Figure 2 This is a schematic diagram of the multimodal integrated optical microcavity in this invention; wherein, (a) is a top view of the optical microcavity and a cross-sectional view at the coupling position, and (b) and (c) are mode field distribution diagrams of the symmetric mode and the antisymmetric mode in the vertical coupling region, respectively;

[0044] Figure 3 The relationships between (a) relative resonant frequency, (b) free spectral range, (c) second-order dispersion value, and (d) mode spacing as a function of normalized phase are given for the multimodal integrated optical microcavity of this invention.

[0045] Figure 4 This is a flowchart of the fabrication process of the multimodal integrated optical microcavity in this invention. (a) is a schematic diagram of a silicon oxide wafer. (b) Deposition of a silicon nitride layer. (c) Photolithography and etching to prepare a silicon nitride microring structure. (d) Deposition of a silicon oxide layer and chemical mechanical polishing to smooth the surface. (e) Deposition of a silicon oxynitride layer. (f) Photolithography and etching to prepare a silicon oxynitride microring structure. (g) Deposition of a silicon oxide layer and chemical mechanical polishing to smooth the surface. (h) Fabrication of a differential metal heating electrode. (i) Wherein, dark blue represents silicon nitride, light blue represents silicon oxynitride, gray represents silicon oxide, and black represents silicon. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0048] This invention provides a multimodal integrated optical microcavity, suitable for reconfigurable integrated photonic systems, comprising:

[0049] A first microcavity structure is disposed on a substrate to form a first annular or racetrack-shaped waveguide microcavity, or to further form a first coupled straight waveguide; wherein the first coupled waveguide is used to measure the resonant frequency and thermo-optical effect of the first microcavity;

[0050] A second microcavity structure is disposed on a substrate and located above or below the first microcavity structure, and is spaced apart from the first microcavity structure in the vertical direction to form a second annular or racetrack-shaped waveguide microcavity, or may also form a second coupled straight waveguide; at least one of the first and second microcavity structures is a coupled straight waveguide; wherein the second coupled waveguide is used to measure the resonant frequency and thermo-optical effect of the second microcavity;

[0051] At least one integrated heater is used for differential thermal tuning of the first microcavity structure and the second microcavity structure; preferably, the heater needs to cover the area of ​​the two microcavities to achieve effective heating regulation, i.e., uniform heating.

[0052] The first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity form optical coupling in the vertical direction, so that their resonant modes interact in the frequency domain.

[0053] The waveguide materials used in the first and second microcavity structures have different thermo-optic coefficients.

[0054] For example, a multimodal integrated optical microcavity consists of a lower silicon nitride microcavity and a coupling waveguide, and an upper silicon oxynitride microcavity and a coupling waveguide. The top and bottom orders of the two layers are interchangeable, and the two microcavities are separated by a thin layer of silicon oxide. One straight waveguide in each of the two microrings is positioned at the same vertical angle to achieve coupling between the modes of the upper and lower microrings; other positions are not in the same vertical direction to avoid coupling effects. Besides the racetrack-shaped microcavity, other shaped microcavities can also be used, as long as there is an overlap between the upper and lower microcavities. In the design phase, finite element simulation software can be used to scan the thickness of the silicon nitride and silicon oxynitride layers to make the free spectral ranges of the two microrings as close as possible. This is the basis for differential amplification and dispersion adjustment. Secondly, an appropriate spacing distance is set. If the coupling is too weak, the mode hybridization is insufficient, and the dispersion adjustment amplitude is small; if the coupling is too strong, the modes are flattened, and the second-order dispersion value decreases. The following is a specific implementation example.

[0055] It should be noted that the microcavity of this invention adopts a layered structure design, mainly for the following reasons: (1) Since the refractive indices of the two materials are not the same, when the free spectral ranges of the two microcavities are the same, their thicknesses are often different. During fabrication, it is impossible to fabricate two thin films of different thicknesses in the same layer. (2) Compared with the horizontal same-layer structure, the layered structure can achieve stronger coupling and more powerful dispersion control. (3) The horizontal same-layer coupling is limited by the precision of photolithography and etching, and cannot fabricate very small gaps, while the layered structure can achieve very small gaps, and theoretically can achieve more precise control.

[0056] Example 1: SiN / SiON bilayer coupled microcavity structure

[0057] This embodiment provides an integrated microcavity device with upper and lower double-layer coupling, such as... Figure 1 and 2 As shown, it includes:

[0058] The lower silicon nitride microcavity is disposed on a silicon oxide substrate, with a waveguide thickness of 100–400 nm and a waveguide width of 400 nm–10000 nm; the straight waveguide has a thickness of 100–400 nm and a width of 400 nm–10000 nm, forming the first racetrack-shaped microcavity.

[0059] The silicon oxynitride microcavity disposed above the lower microcavity has a waveguide thickness of 200–800 nm and a waveguide width of 500 nm–10000 nm; the straight waveguide has a thickness of 200–800 nm and a width of 500 nm–10000 nm, forming a second racetrack-shaped microcavity.

[0060] The two microcavities are separated by a silicon oxide isolation layer with a thickness of 50–400 nm to achieve controlled vertical optical coupling. The top and bottom order of the silicon nitride layer and silicon oxynitride layer can be interchanged.

[0061] As an example, the thickness of the underlying silicon nitride layer is set to 200 nm and the width to 2.0 μm; the thickness of the silicon oxynitride layer is set to 393 nm and the width to 2.0 μm, and the spacing between the two layers is 300 nm. At this time, the two microcavities have approximately the same free spectral range around 1550 nm, and the difference is controlled within the range of tens of megahertz.

[0062] Figure 1 A three-dimensional structural schematic diagram of a multimodal optical microcavity is shown, which consists of a silicon nitride microcavity and a coupled waveguide, a silicon oxynitride microcavity and a coupled waveguide, and a differential heater.

[0063] Figure 2 (a) shows a top view of the two microcavities and a schematic cross-sectional view at the coupling location. When the resonant frequencies of the two microcavities coincide, the mode fields interact, forming symmetric and antisymmetric modes. Figure 2 In the diagram, (b) and (c) show the light field distribution in the symmetric and antisymmetric modes, respectively. At this point, the two microcavities interact, causing a change in the free spectral range, i.e., the dispersion is modulated.

[0064] Figure 3 The simulation results of dispersion tuning are presented. Due to the structural characteristics of the microcavity, it can only resonate at specific frequencies, and each resonance corresponds to a mode number. For ease of explanation, Figure 3 The horizontal axis in the figure has been processed and can be understood as the number of modes of the two microcavities. (a) shows the change in the relative resonant frequency of the two microcavities at different number of modes. It can be seen that at a specific number of modes, the two resonant frequencies repel each other; that is, one frequency increases while the other decreases, indicating an interaction. (b) shows the change in the relative free spectral range of the two microcavities at different number of modes. (c) shows the change in the relative second-order dispersion value of the two microcavities at different number of modes. It can be seen that when an interaction occurs, one exhibits normal dispersion, and the other exhibits anomalous dispersion, thus achieving dispersion modulation. Anomalous dispersion can generate bright soliton frequency combs, while normal dispersion can generate dark pulses. (d) shows the interval between the two resonant frequencies at different number of modes. It can be seen that at a specific number of modes, the interval is near the Brillouin frequency (i.e., the gray area in the figure), which meets the conditions for generating stimulated Brillouin lasers.

[0065] Example 2: A method for adjusting molar acceleration dispersion based on thermo-optical differences

[0066] Based on the above structure, a metal heater is integrated above the microcavity to thermally excite the double-layer microcavity. When the heater is energized, the effective refractive index of the first and second microcavities changes due to the thermo-optical effect.

[0067] Because silicon nitride and silicon oxynitride have different thermo-optic coefficients, even under the same temperature changes, the resonant frequency drift of the two microcavities is still different, resulting in a differential frequency shift in the frequency domain. This differential frequency shift is amplified by a vernier coupling structure, causing a significant shift in the frequency envelope of the microcavity resonant mode, thus achieving an overall translation of the integrated dispersion curve.

[0068] By controlling the heating power, the dispersion characteristics can be continuously adjusted, enabling the device to operate in different dispersion ranges, thereby achieving on-demand switching of various optical functions.

[0069] Example 3: Application of Multimodal Light Sources

[0070] In one embodiment, coupling the above-described microcavity device with an on-chip or external laser source can be achieved under different dispersion states:

[0071] (1) Dark pulse output under normal dispersion conditions;

[0072] (2) Bright soliton frequency comb output under anomalous dispersion conditions;

[0073] (3) Stimulated Brillouin laser output under specific intermodal frequency difference conditions.

[0074] Example 4: A method for fabricating a multimodal integrated optical microcavity, see [link to example]. Figure 4 (a)-(i) in the formula includes the following steps:

[0075] (1) Using a silicon oxide wafer as a substrate, a silicon nitride layer is deposited on the substrate and etched to prepare the first microcavity structure;

[0076] (2) A silicon oxide layer is deposited to cover the first microcavity structure, and the surface is smoothed by chemical mechanical polishing;

[0077] (3) A silicon oxynitride layer is deposited on the silicon oxide layer deposited in step (2), and the second microcavity structure is prepared by etching; silicon nitride and silicon oxynitride have different thermo-optic coefficients;

[0078] (4) Deposit a silicon oxide layer to cover the second microcavity structure and use chemical mechanical polishing to smooth the surface;

[0079] (5) Install a metal electrode (i.e., differential metal heating electrode) inside or on the silicon oxide layer deposited in step (4).

[0080] The above are merely preferred embodiments of the present invention. Those skilled in the art can make various modifications or substitutions to the above embodiments without departing from the spirit of the present invention, and all such modifications or substitutions should fall within the protection scope of the present invention.

Claims

1. A multimodal integrated optical microcavity, characterized in that, include: The first microcavity structure is disposed on the substrate to form a first annular or racetrack-shaped waveguide microcavity; The second microcavity structure is disposed on the substrate and located above or below the first microcavity structure, and is spaced apart from the first microcavity structure in the vertical direction to form a second annular or racetrack-shaped waveguide microcavity; at least one of the first microcavity structure and the second microcavity structure is coupled to a straight waveguide. At least one integrated heater is used for thermal tuning of the first microcavity structure and the second microcavity structure; The first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity form optical coupling in the vertical direction, so that their resonant modes interact in the frequency domain. The waveguide materials used in the first and second microcavity structures have different thermo-optic coefficients.

2. The multimodal integrated optical microcavity according to claim 1, characterized in that, The first microcavity structure and the second microcavity structure have similar free spectral ranges within a preset working band by adjusting their respective waveguide thickness and width.

3. The multimodal integrated optical microcavity according to claim 1, characterized in that, Specifically, the optical coupling between the first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity in the vertical direction is as follows: The first annular or racetrack-shaped waveguide microcavity and the second annular or racetrack-shaped waveguide microcavity overlap in their orthogonal projections onto the substrate.

4. The multimodal integrated optical microcavity according to claim 1, characterized in that, The first racetrack-shaped waveguide microcavity and the second racetrack-shaped waveguide microcavity each have a straight waveguide at the same vertical position.

5. The multimodal integrated optical microcavity according to claim 1, characterized in that, The positions of the first microcavity structure and the second microcavity structure can be interchanged.

6. The multimodal integrated optical microcavity according to claim 1, characterized in that, The heater is a differential heater.

7. The multimodal integrated optical microcavity according to claim 1, characterized in that, The first microcavity structure is made of silicon nitride; the second microcavity structure is made of silicon oxynitride; or The first microcavity structure is made of silicon oxynitride; the second microcavity structure is made of silicon nitride.

8. The multimodal integrated optical microcavity according to claim 1, characterized in that, The substrate is a silicon oxide wafer.

9. A multimodal integrated optical microcavity according to claim 1, characterized in that, By adjusting the heater power, both microcavities can be made to resonate at a specific frequency. When the free spectral range is exactly the Brillouin frequency shift, stimulated Brillouin laser can be generated by pumping. When the free spectral range exhibits anomalous dispersion, a bright soliton frequency comb can be generated by pumping. When the free spectral range exhibits normal dispersion, a dark pulse can be generated by pumping.

10. A method for fabricating a multimodal integrated optical microcavity, characterized in that, The method for fabricating the multimodal integrated optical microcavity according to any one of claims 1-8 includes the following steps: (1) A first material layer is deposited on a substrate and a first microcavity structure is fabricated by etching; the first material is the waveguide material used in the first microcavity structure; (2) A third material layer is deposited to cover the first microcavity structure, and the surface is smoothed by chemical mechanical polishing; (3) Deposit a second material layer on the third material layer deposited in step (2) and etch to prepare a second microcavity structure; the second material is the waveguide material used in the second microcavity structure; the first material and the second material have different thermo-optic coefficients; (4) Deposit a third material layer to cover the second microcavity structure and use chemical mechanical polishing to smooth the surface; (5) Install a heater inside or on the third material layer deposited in step (4).

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