Optical filter
By using a ring resonator, optical waveguide and heater in the optical filter, the wavelength of light is adjusted by using temperature changes, the problem of temperature dependence of the coupling coefficient of the directional coupler is solved, and stable spectral characteristics and low power consumption are achieved.
Patent Information
- Application Number
- CN202410614319.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-06-14
- Filing Date
- 2024-05-17
- Publication Date
- 2024-12-17
AI Technical Summary
The coupling coefficient of the directional coupler varies depending on temperature, resulting in the failure to obtain the desired characteristics.
An optical filter is designed, including an annular resonator, an optical waveguide and a heater, through which the temperature of the annular resonator is changed, thereby changing the wavelength of light and suppressing the temperature dependence of the coupling coefficient.
The temperature dependence of the coupling coefficient is achieved, maintaining the shape and intensity of the spectrum at a desired magnitude, reducing power consumption and avoiding heater breakage.
Smart Images
Figure CN119148296A_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to an optical filter. Background Art
[0002] By bringing two optical waveguides close together, a directional coupler is formed (Non-Patent Document 1, etc.). Light is transferred from one optical waveguide to another optical waveguide. The ring-shaped optical waveguide forms a ring resonator. Light is transferred by forming a directional coupler with the ring resonator and the optical waveguide. By changing the temperature of the ring resonator with a heater, the wavelength of light can be changed (Non-Patent Document 2, etc.).
[0003] Prior art literature
[0004] Non-patent literature
[0005] Non-patent document 1: "Reduction of Wavelength Dependence of CouplingCharacteristics Using Si Optical Waveguide Curved Directional Coupler" Hisayasu Morino, et al. Journal of Lightwave Technology, Vol. 32, No. 12, June 15, 2014p2188-2192
[0006] Non-patent document 2: "A 2.5kHz Linewidth Widely Tunable Laser with Booster SOAIntegrated on Silicon" Minh A.et al. 2018 IEEE International Semiconductor LaserConference (ISLC) Summary of the invention
[0007] Problems to be solved by the invention
[0008] The coupling coefficient of a directional coupler varies depending on temperature. Due to the variation in the coupling coefficient, there is a risk that desired characteristics cannot be obtained. Therefore, an object of the present disclosure is to provide an optical filter capable of suppressing the temperature dependency of the coupling coefficient.
[0009] Means used to solve problems
[0010] The optical filter involved in the present disclosure includes: a ring resonator; an optical waveguide optically coupled to the ring resonator; and a heater arranged on the ring resonator, the ring resonator has a first curved portion, the optical waveguide has a second curved portion, the first curved portion and the second curved portion form a directional coupler, and the heater is arranged on the directional coupler.
[0011] Effects of the Invention
[0012] According to the present disclosure, it is possible to provide an optical filter capable of suppressing the temperature dependency of a coupling coefficient. BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Figure 1 It is a plan view showing an example of an optical filter according to an embodiment.
[0014] Figure 2A This is an enlarged view of the directional coupler.
[0015] Figure 2B This is a diagram showing an example of the relationship between the angle and the radius of curvature.
[0016] Figure 3A It is along Figure 1 A cross-sectional view of line AA.
[0017] Figure 3B It is along Figure 1 A cross-sectional view along line BB.
[0018] Figure 4 This is a diagram showing an example of a spectrum of light.
[0019] Figure 5 It is a top view showing an example of an optical filter according to a comparative example.
[0020] Figure 6 FIG. 1 is a schematic diagram showing an example of the coupling coefficient of a directional coupler.
[0021] Description of Reference Numerals
[0022] 10, 30: substrate;
[0023] 12: Ring resonator;
[0024] 14, 14a, 14b: optical waveguide;
[0025] 15a, 15b, 15c, 15d: ends;
[0026] 16: Heater;
[0027] 17: Power supply;
[0028] 20, 22: Directional coupler;
[0029] 32: buried oxide layer;
[0030] 34: Silicon layer;
[0031] 40: platform;
[0032] 42: concave part;
[0033] 44: waveguide core;
[0034] 100, 110: optical filter. DETAILED DESCRIPTION
[0035] [Description of Embodiments of the Present Disclosure]
[0036] First, the contents of the embodiments of the present disclosure will be listed and described.
[0037] One method of the present disclosure is (1) an optical filter, wherein the optical filter comprises: a ring resonator; an optical waveguide optically coupled to the ring resonator; and a heater disposed on the ring resonator, the ring resonator having a first curved portion, the optical waveguide having a second curved portion, the first curved portion and the second curved portion forming a directional coupler, and the heater being disposed on the directional coupler. The wavelength of light is changed by changing the temperature of the ring resonator using a heater. The first curved portion and the second curved portion form a directional coupler. Therefore, the temperature dependence of the coupling coefficient can be suppressed. The wavelength of light can be changed and the change in the coupling coefficient can be suppressed.
[0038] (2) In the above (1), the heater may be provided in a portion of the ring resonator that forms the directional coupler and a portion other than the directional coupler. The portion of the ring resonator covered by the heater becomes longer. The portion that transfers heat from the heater becomes larger. The temperature of the ring resonator changes easily. The increase in power consumption can be suppressed.
[0039] (3) In (1) or (2), the heater may be disposed on a portion of the ring resonator that is 70% or more. The portion of the ring resonator covered by the heater becomes longer. The portion that transfers heat from the heater becomes larger. The temperature of the ring resonator changes easily. The increase in power consumption can be suppressed.
[0040] (4) In any one of the above (1) to (3), the first curved portion and the second curved portion may be bent toward the outside of the ring resonator. The first curved portion and the second curved portion may form a directional coupler. Light may be transferred between the ring resonator and the optical waveguide.
[0041] (5) In any one of (1) to (4) above, the first curved portion and the second curved portion may have a curvature radius of 350 μm or less and 5 μm or more. The temperature dependency of the coupling coefficient is reduced.
[0042] (6) In any one of the above (1) to (5), in the directional coupler, the distance between the ring resonator and the optical waveguide is the smallest. Light is easily transferred in the directional coupler, but is difficult to transfer in parts other than the directional coupler. The wavelength and intensity of light can be formed to desired sizes.
[0043] (7) In any one of the above (1) to (6), the optical filter may include a silicon layer, the silicon layer has a waveguide core, one of the waveguide cores is the ring resonator, and the other of the waveguide cores is the optical waveguide, and in the directional coupler, one of the waveguide cores is adjacent to the other of the waveguide cores. Light is transferred between the waveguide cores. The ring resonator and the optical waveguide are formed of silicon. Light can be strongly confined in the ring resonator and the optical waveguide to suppress loss.
[0044] (8) In any one of the above (1) to (7), the optical filter may include two optical waveguides, each of the two optical waveguides has the second curved portion, and the two second curved portions form the directional coupler with the first curved portion. Light is transferred from one optical waveguide to the ring resonator. Light is transferred from the ring resonator to the other optical waveguide. Light having a resonance wavelength of the ring resonator can be extracted.
[0045] (9) In the above (8), in the direction surrounding the ring resonator, the length of the ring resonator from one of the two directional couplers to the other directional coupler is equal to the length of the ring resonator from the other of the two directional couplers to the one directional coupler. The optical path lengths between the two directional couplers are close to the same. The phase of light is less likely to deviate.
[0046] [Details of the embodiments of the present disclosure]
[0047] Hereinafter, specific examples of optical filters according to embodiments of the present disclosure will be described with reference to the accompanying drawings. The present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.
[0048] Figure 11 is a plan view showing an example of an optical filter 100 according to the embodiment. The optical filter 100 includes a substrate 10, a ring resonator 12, an optical waveguide 14, and a heater 16. The optical filter 100 is a waveguide type optical filter.
[0049] The planar shape of the substrate 10 is, for example, a rectangle. Two sides of the substrate 10 are parallel to the X-axis direction. The other two sides are parallel to the Y-axis direction. The upper surface of the substrate 10 is parallel to the XY plane. The Z-axis direction is the normal direction of the substrate 10. The X-axis direction, the Y-axis direction, and the Z-axis direction are orthogonal to each other.
[0050] The substrate 10 is a SOI (Silicon on Insulator) substrate and has a buried oxide layer. The ring resonator 12 and the optical waveguide 14 are embedded in the buried oxide layer. Figure 1 The buried oxide layer is shown in FIG. 1 . The heater 16 is disposed on the upper surface of the substrate 10 and is located above the ring resonator 12 . The heater 16 is electrically connected to a power source 17 .
[0051] The ring resonator 12 is located between the two optical waveguides 14. The ring resonator 12 is formed by a ring-shaped optical waveguide. Ring refers to a circle, an ellipse, a polygon, a shape formed by combining an arc and a straight line, etc. The diameter of the ring resonator 12 is, for example, 150 μm. The ring resonator 12 has two curved portions 18. The two curved portions 18 are opposite to each other. The curved portion 18 is bent toward the outside of the ring resonator 12. The ring resonator 12 is linearly symmetrical with respect to the Y axis and is also linearly symmetrical with respect to the X axis.
[0052] The planar shape of the optical waveguide 14 is, for example, an L-shape. One of the two optical waveguides 14 is set as the optical waveguide 14a (first optical waveguide), and the other is set as the optical waveguide 14b (second optical waveguide). The optical waveguide 14a and the optical waveguide 14b are separated from each other. The end 15a of the optical waveguide 14a is located at one end of the substrate 10 in the X-axis direction (the first end of the substrate 10). The end 15c of the optical waveguide 14b is located at the other end of the substrate 10 in the X-axis direction (the second end of the substrate 10). The end 15b of the optical waveguide 14a and the end 15d of the optical waveguide 14b are located at one end of the substrate 10 in the Y-axis direction. The optical waveguide 14a and the optical waveguide 14b each have a curved portion 19. The curved portion 19 is bent toward the outside of the ring resonator 12. The curved portion 19 of the optical waveguide 14a and the curved portion 19 of the optical waveguide 14b are line symmetric with respect to the Y-axis.
[0053] A curved portion 18 (first curved portion) of the ring resonator 12 is opposed to a curved portion 19 (second curved portion) of the optical waveguide 14a, forming a directional coupler 20 (first directional coupler). In the directional coupler 20, the ring resonator 12 is optically coupled to the optical waveguide 14a. The ring resonator 12 and the optical waveguide 14a are separated from each other. In the directional coupler 20, the distance between the ring resonator 12 and the optical waveguide 14a is the smallest, for example, 200 nm.
[0054] The other curved portion 18 (first curved portion) of the ring resonator 12 is opposed to the curved portion 19 (second curved portion) of the optical waveguide 14b, forming a directional coupler 22 (second directional coupler). In the directional coupler 22, the ring resonator 12 and the optical waveguide 14b are optically coupled to each other. The ring resonator 12 and the optical waveguide 14b are separated from each other. In the directional coupler 22, the distance between the ring resonator 12 and the optical waveguide 14b is the smallest, for example, 200 nm.
[0055] Figure 2A It is an enlarged view of the directional coupler 20. In the directional coupler 20, the curved portion 18 of the ring resonator 12 and the curved portion 19 of the optical waveguide 14a extend in the same direction. The distance g between the ring resonator 12 and the optical waveguide 14a is, for example, 200 nm. Figure 2A The dotted line D in FIG. 1 is an arc, indicating the position between the curved portion 18 and the curved portion 19. P is the center of the dotted line D. The curvature radius of the dotted line D is set to R. R is larger than the curvature radius of the curved portion 18 and smaller than the curvature radius of the curved portion 19. The central angle of the directional coupler 20 is set to θ. The directional coupler 20 has the same structure as the directional coupler 22.
[0056] Figure 2B : is a diagram showing an example of the relationship between the angle θ and the radius of curvature R. The horizontal axis represents the angle. The vertical axis represents the radius of curvature R. The smaller the angle θ is, the larger the radius of curvature R is. The larger the angle θ is, the smaller the radius of curvature R is. The radius of curvature R is, for example, not less than 5 μm and not more than 350 μm. The angle θ is, for example, not less than 10° and not more than 60°.
[0057] Figure 3A It is along Figure 1 A cross-sectional view of line AA. Figure 3B It is along Figure 1 A cross-sectional view of line BB. Figure 3A as well as Figure 3B As shown, the substrate 10 includes a substrate 30, a buried oxide layer 32, and a silicon (Si) layer 34. The buried oxide layer 32 is stacked on one surface of the substrate 30. The buried oxide layer 32 has a Si layer 34 embedded therein.
[0058] The Si layer 34 has a platform 40 and a waveguide core 44. The platform 40 is a plane. A recess 42 is provided between the platform 40 and the waveguide core 44. Figure 3A as well as Figure 3B As shown, the recess 42 may also penetrate the Si layer 34. The recess 42 may not penetrate the Si layer 34. The waveguide core 44 functions as an optical waveguide. The buried oxide layer 32 is filled on the lower surface and upper surface of the platform 40, the lower surface and upper surface of the waveguide core 44, and between the platform 40 and the waveguide core 44.
[0059] The substrate 30 is formed of Si, for example. The buried oxide layer 32 is formed of silicon oxide (SiO2), for example. The thickness of the Si layer 34 is, for example, 0.22 μm. The width W1 of the waveguide core 44 is, for example, 0.42 μm. The refractive index of the buried oxide layer 32 is about 1.4. The refractive index of the Si layer 34 is about 3.5, which is higher than that of the buried oxide layer 32.
[0060] Figure 3A The waveguide core 44 in the structure functions as the ring resonator 12. The platform 40, the waveguide core 44, and the platform 40 are arranged in sequence. The heater 16 is provided on the upper surface of the buried oxide layer 32 and is located above the waveguide core 44. The heater 16 is formed of a metal such as platinum (Pt). The width W2 of the heater 16 is, for example, 3 μm. The thickness of the heater 16 is, for example, 200 nm.
[0061] exist Figure 3B In the embodiment, two waveguide cores 44 are arranged. One waveguide core 44 (first waveguide core) is a ring resonator 12. The other waveguide core 44 (second waveguide core) is an optical waveguide 14. A buried oxide layer 32 is provided between the two waveguide cores 44, and no platform 40 is provided. The platform 40, one waveguide core 44, the other waveguide core 44, and the platform 40 are arranged in sequence. The heater 16 is located on the two waveguide cores 44.
[0062] For example, the end 15a of the optical waveguide 14a functions as an input port. The end 15c of the optical waveguide 14b functions as an output port. Light is input from a light source to the end 15a. The light propagates in the optical waveguide 14a. In the directional coupler 20, the light is transferred from the optical waveguide 14a to the ring resonator 12. The light propagates in the ring resonator 12. The light of the resonant wavelength of the ring resonator 12 is transferred from the ring resonator 12 to the optical waveguide 14b in the directional coupler 22. The light having the resonant wavelength propagates in the optical waveguide 14b and is emitted from the end 15c. The light having the resonant wavelength can be extracted from the optical filter 100. The light having a wavelength different from the resonant wavelength propagates in the optical waveguide 14a and is emitted from the end 15b.
[0063] A voltage can be applied to the heater 16 using the power supply 17 so that a current flows through the heater 16. When the current flows, the heater 16 generates heat. The temperature of the ring resonator 12 changes due to the heat transfer from the heater 16 to the ring resonator 12. The refractive index of the ring resonator 12 changes according to the temperature change. The resonant wavelength of the ring resonator 12 changes due to the change in the refractive index.
[0064] Figure 4 This is a diagram showing an example of a spectrum of light. The horizontal axis represents the wavelength of light, and the vertical axis represents the intensity of light. Figure 4 The solid line in shows an example in which the temperature of the ring resonator 12 is T1. The dotted line is an example in which the temperature is T2. T2 is, for example, 25°C higher than T1. The intensity of light shows a peak at the resonant wavelength. The interval (FSR) between two adjacent peaks is several nm. By changing the temperature, the wavelength of the peak is shifted. When the wavelength is shifted by the FSR amount, the power consumption of the heater 16 is, for example, 50 mW.
[0065] Figure 5 1 is a top view showing an optical filter 110 according to a comparative example. The ring resonator 12 has a rectangular shape. The optical waveguide 14a and the optical waveguide 14b do not have the curved portion 19. The straight portion of the ring resonator 12 and the optical waveguide 14a form a directional coupler 20. In the directional coupler 20, the ring resonator 12 is parallel to the optical waveguide 14a. The straight portion of the ring resonator 12 and the optical waveguide 14b form a directional coupler 22. In the directional coupler 22, the ring resonator 12 is parallel to the optical waveguide 14b.
[0066] Figure 6 1 is a schematic diagram showing an example of the coupling coefficient of a directional coupler. The horizontal axis represents the temperature of the ring resonator 12. The left side of the horizontal axis represents a low temperature, and the right side represents a high temperature. The vertical axis represents the coupling coefficient. Figure 6 The dotted line in the figure represents the comparative example. The solid line represents the embodiment. The coupling coefficient of the comparative example decreases as the temperature rises. The Q value deviates from the designed value. The shape of the spectrum changes with temperature.
[0067] On the other hand, the coupling coefficient of the embodiment is suppressed. For example, the design value of the coupling coefficient is 50%. In the comparative example, the coupling coefficient decreases to 40% due to the temperature change from several tens of degrees Celsius to about 200 degrees Celsius. The coupling coefficient of the embodiment is maintained in the range of 50%±1%.
[0068] According to this embodiment, the curved portion 18 of the ring resonator 12 and the curved portion 19 of the optical waveguide 14a form a directional coupler 20. The curved portion 18 and the curved portion 19 of the optical waveguide 14b form a directional coupler 22. Light is transferred between the optical waveguide 14 and the ring resonator 12. Light having the resonance wavelength of the ring resonator 12 can be extracted. Figure 4 As shown in FIG. 1 , the resonant wavelength is changed by changing the temperature of the ring resonator 12 using the heater 16. Figure 6 As shown in FIG. 1 , the coupling coefficient of a directional coupler formed by a curved optical waveguide is less likely to change with temperature than the coupling coefficient of a directional coupler formed by a straight optical waveguide. That is, the temperature dependence of the coupling coefficient is small. By changing the temperature, the resonance wavelength can be changed, and the change in the coupling coefficient can be suppressed.
[0069] Since the temperature dependence of the coupling coefficient is small, the Q value is close to the designed value even if the temperature changes. The change of the shape of the spectrum can be suppressed. The wavelength of the peak changes while maintaining the height and line width of the spectrum at the desired size.
[0070] exist Figure 5 In the comparative example shown, the change in coupling coefficient can be suppressed by not providing the heater 16 on the directional coupler 20 and the directional coupler 22. However, the portion of the ring resonator 12 covered by the heater 16 becomes shorter. The portion that transfers heat from the heater 16 becomes smaller, so it is difficult to change the temperature. In order to change the temperature, the power input to the heater 16 is increased. The power consumption increases. The heat generated increases, and there is a risk of damage to the heater 16, etc.
[0071] According to the present embodiment, the heater 16 is provided on the ring resonator 12 to cover the portion of the ring resonator 12 that forms the directional coupler and the portion other than the directional coupler.
[0072] For example, the heater 16 covers the two curved portions 18 of the ring resonator 12, Figure 1 The upper half of the ring resonator 12 is covered, and a part of the lower half is covered. The portion of the ring resonator 12 covered by the heater 16 becomes longer. The portion that transfers heat from the heater 16 becomes larger. Therefore, the temperature of the ring resonator 12 is easy to change, and an increase in power consumption can be suppressed. Damage to the heater 16 and the like caused by heat can be suppressed.
[0073] The longer the portion of the ring resonator 12 covered by the heater 16 is, the larger the portion that transfers heat from the heater 16. The ratio of the length of the portion covered by the heater 16 to the length (circumference) of the ring resonator 12 may be 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more.
[0074] like Figure 2AAs shown, the curved portion 18 of the ring resonator 12 and the curved portion 19 of the optical waveguide 14 are bent in the same direction. The curved portion 18 and the curved portion 19 are bent toward the outside of the ring resonator 12. The curved portion 18 and the curved portion 19 form a directional coupler 20 and a directional coupler 22. Light can be transferred between the ring resonator 12 and the optical waveguide 14. The target value of the coupling coefficient of the directional coupler can be 50%, less than 50%, or more than 50%. According to the target value of the coupling coefficient, the distance g, the width W1, the length of the directional coupler, etc. are adjusted.
[0075] The curved portion 18 and the curved portion 19 may be a circular arc, an elliptical arc, etc. The curved portion 18 and the curved portion 19 may be convex toward the outside of the ring resonator 12 or may be convex toward the inside.
[0076] The curvature radius of the curved portion 18 and the curved portion 19 is, for example, 350 μm or less and 5 μm or more. When the curvature radius is small, the shape of the curved portion 18 and the curved portion 19 is close to a straight line. The temperature dependence of the coupling coefficient becomes larger. When the curvature radius is large, the shape of the curved portion 18 and the curved portion 19 is away from the straight line and becomes a curve. The temperature dependence of the coupling coefficient becomes smaller. The curvature radius is set to an appropriate size according to the material of the waveguide, etc.
[0077] In the directional coupler, the distance between the ring resonator 12 and the optical waveguide 14 is the smallest. The minimum value g of the distance is, for example, 200 nm. In the portion outside the directional coupler, the distance between the ring resonator 12 and the optical waveguide 14 becomes larger. The optical waveguide 14a is bent along the curved portion 18 of the ring resonator 12 in the directional coupler 20. The optical waveguide 14a is bent in such a way that it moves away from the ring resonator 12 if it leaves the directional coupler 20. The optical waveguide 14b also has the same shape. Light is easily transferred in the directional coupler, but is difficult to transfer in the portion outside the directional coupler. The wavelength and intensity of light can be formed to the desired size.
[0078] The substrate 10 is an SOI substrate and has a Si layer 34. The Si layer 34 has a waveguide core 44. The ring-shaped waveguide core 44 becomes the ring resonator 12. The other waveguide core 44 becomes the optical waveguide 14. Figure 3B As shown, a directional coupler is formed by making two waveguide cores 44 adjacent to each other. Light is transferred between the waveguide cores 44. The ring resonator 12 and the optical waveguide 14 are formed of Si. The refractive index of the Si layer 34 is higher than the refractive index of the buried oxide layer 32. Light can be strongly confined in the ring resonator 12 and the optical waveguide 14 to suppress loss.
[0079] The optical filter 100 has two optical waveguides 14. The curved portions 19 of the two optical waveguides 14 form a directional coupler with the curved portion 18 of the ring resonator 12. Light is transferred from one optical waveguide 14a to the ring resonator 12. Light is transferred from the ring resonator 12 to the other optical waveguide 14b. Light having the resonance wavelength of the ring resonator 12 can be extracted. The number of optical waveguides 14 may be one or more than three. Each optical waveguide 14 has a curved portion 19, and forms a directional coupler with the curved portion 18 of the ring resonator 12.
[0080] The directional coupler 20 and the directional coupler 22 are opposed to each other in the X-axis direction. The ring resonator 12 is linearly symmetric with respect to the X-axis. In the direction surrounding the ring resonator 12 (for example, the right-hand direction), the length of the ring resonator 12 from the directional coupler 20 to the directional coupler 22 is equal to the length from the directional coupler 22 to the directional coupler 20. That is, the two directional couplers divide the ring resonator 12 into two equal parts. The optical path length between the directional coupler 20 and the directional coupler 22 is almost the same. The phase of light is not easily deviated. The intensity of light is not easily reduced. The directional coupler 20 and the directional coupler 22 can also be linearly symmetric with respect to the Y-axis. The phase of light is not easily deviated. The length of the ring resonator 12 from the directional coupler 20 to the directional coupler 22 can also be not strictly equal to the length from the directional coupler 22 to the directional coupler 20. The difference in length can be, for example, less than 5% or less than 1%.
[0081] As mentioned above, although the embodiment of this disclosure is described in detail, this disclosure is not limited to this specific embodiment, Various deformation|transformation and changes are possible within the scope of the summary of this disclosure described in a claim.
Claims
1. An optical filter, wherein: The optical filter comprises: Ring resonator; an optical waveguide optically coupled to the ring resonator; and a heater, disposed on the ring resonator, The ring resonator has a first curved portion, The optical waveguide has a second curved portion, The first curved portion and the second curved portion form a directional coupler, The heater is disposed on the directional coupler.
2. The optical filter according to claim 1, wherein The heater is provided in a portion of the ring resonator where the directional coupler is formed and in a portion other than the directional coupler.
3. The optical filter according to claim 1 or 2, wherein: The heater is disposed on more than 70% of the ring resonator.
4. The optical filter according to claim 1 or 2, wherein: The first curved portion and the second curved portion are bent toward an outer side of the ring resonator.
5. The optical filter according to claim 1 or 2, wherein: The curvature radius of the first curved portion and the curvature radius of the second curved portion are 350 μm or less and 5 μm or more.
6. The optical filter according to claim 1 or 2, wherein: In the directional coupler, the distance between the ring resonator and the optical waveguide is the smallest.
7. The optical filter according to claim 1 or 2, wherein: The optical filter comprises a silicon layer, The silicon layer has a waveguide core, One of the waveguide cores is the ring resonator, Another of the waveguide cores is the optical waveguide, In the directional coupler, one of the waveguide cores is adjacent to another of the waveguide cores.
8. The optical filter according to claim 1 or 2, wherein: The optical filter comprises two optical waveguides. The two optical waveguides respectively have the second curved portion, The two second curved portions respectively form the directional coupler with the first curved portion.
9. The optical filter according to claim 8, wherein In a direction surrounding the ring resonator, a length of the ring resonator from one of the two directional couplers to the other directional coupler is equal to a length of the ring resonator from the other of the two directional couplers to one directional coupler.