Optical circuit

Abstract

This optical circuit comprises: an optical waveguide that has a first optical waveguide, a second optical waveguide, a third optical waveguide, and an optical branching part, the second optical waveguide and the third optical waveguide being connected to the first optical waveguide via the optical branching part; and an optical resonator that is a ring resonator or a disk resonator, the optical resonator being optically coupled to the second optical waveguide at a first location and optically coupled to the third optical waveguide at a second location. A diffraction grating is provided on the entire circumference or a part of the circumference of the optical resonator. In a plan view, the diffraction grating is asymmetric with respect to a straight line connecting the center of the optical branching part and the center of the optical resonator.

Description

optical circuit

[0001] The present disclosure relates to optical circuits.

[0002] Narrow-band wavelength filter elements are used as components of spectrometers, laser oscillators, frequency separation reference elements, etc. Narrow-band wavelength filters generally require a long optical path length, but it is known that by resonating light in a ring-shaped optical waveguide, it is possible to realize an optical filter with a narrow linewidth while reducing the size of the wavelength filter (e.g., Patent Document 1). Optical filters that select a single resonant wavelength by arranging a diffraction grating in a ring resonator are known (e.g., Non-Patent Document 1). It is known that a single longitudinal mode laser can be realized by arranging a diffraction grating in the ring of a microring laser (e.g., Non-Patent Document 2).

[0003] Japanese Patent Application Laid-Open No. 2020-136297

[0004] J. Wang et al. "Subwavelength grating filtering devices" Optics Express Vol. 22, No. 13, pp 15335-15345, (2014).A. Arbabi et al. "Grating integrated single mode microring laser" Optics Express Vol. 23, No. 4, pp 5335-5347, (2015).

[0005] When a ring resonator is used in an optical filter, side modes occur. Generally, narrowing the linewidth of a ring resonator requires increasing the Q-factor of the resonator, which involves increasing the radius of curvature of the ring to reduce bending loss. However, increasing the radius of curvature of the ring narrows the wavelength spacing of the side modes. To suppress side modes, an external single-mode filter can be combined. However, the closer the wavelengths of adjacent side modes are, the smaller the linewidth of the single-mode filter itself must be, which increases the filter's size and makes it difficult to align both resonant peaks. In Non-Patent Document 1, a diffraction grating is provided in the ring resonator itself to select a single peak. However, because the return light from the diffraction grating is used, a deep diffraction grating is required, which prevents the Q-factor of the resonator from being increased and makes it difficult to narrow the bandwidth.

[0006] The present disclosure aims to provide an optical circuit having narrow-band optical filter characteristics.

[0007] An optical circuit according to an embodiment of the present disclosure includes an optical waveguide having a first optical waveguide, a second optical waveguide, a third optical waveguide, and an optical branching section, in which the second optical waveguide and the third optical waveguide are connected to the first optical waveguide via the optical branching section, and an optical resonator which is a ring resonator or a disk resonator and is optically coupled to the second optical waveguide at a first location and to the third optical waveguide at a second location, and has a diffraction grating around the entire circumference or part of the circumference of the optical resonator, and in a planar view, the diffraction grating is asymmetric with respect to a line connecting the center of the optical branching section and the center of the optical resonator.

[0008] According to the present disclosure, it is possible to have narrow-band optical filter characteristics.

[0009] FIG. 1 is a plan view of an optical circuit according to the first embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. FIG. 3 is a plan view of an optical circuit according to a comparative embodiment. FIG. 4 is a diagram showing reflectance versus frequency in the comparative embodiment. FIG. 5 is a diagram showing reflectance versus frequency in the comparative embodiment. FIG. 6A is an enlarged view of the diffraction grating of the ring resonator according to the first embodiment. FIG. 6B is an enlarged view of the diffraction grating of the ring resonator according to the first embodiment. FIG. 7 is a simulation result of the Q value of the ring resonator versus frequency. FIG. 8 is a diagram showing the diffraction grating of the ring resonator according to the first embodiment. FIG. 9A is a diagram showing the positional relationship between the diffraction grating 32 and the line 59 in Example 1. FIG. 9B is a diagram showing the positional relationship between the diffraction grating 32 and the line 59 in Example 2. FIG. 10 is a simulation result of reflectance versus wavelength. FIG. 11 is a plan view of an optical circuit according to a second embodiment. FIG. 12 is an enlarged view of the ring resonator according to the second embodiment. FIG. 13 is a diagram showing the reflectance of the ring resonator versus frequency in the second embodiment. FIG. 14 is a plan view showing an optical circuit according to the third embodiment. FIG. 15 is a plan view of an optical circuit according to a first modification of the third embodiment. FIG. 16 is a plan view of an optical circuit according to a fourth embodiment. FIG. 17 is a plan view of an optical circuit according to a first modification of the fourth embodiment. FIG. 18 is a plan view of an optical circuit according to a second modification of the fourth embodiment. FIG. 19 is a plan view of an optical circuit according to a fifth embodiment. FIG. 20 is a plan view showing an example of a diffraction grating. FIG. 21 is a plan view for explaining an example of an optical coupler. FIG. 22 is a cross-sectional view of a first example of an optical coupler. FIG. 23 is a cross-sectional view of a second example of an optical coupler. FIG. 24 is a plan view of an optical circuit using a third example of an optical coupler. FIG. 25 is a plan view of an optical circuit using a first example of an optical branching unit. FIG. 26 is a plan view of an optical circuit using a second example of an optical branching unit. FIG. 27 is a plan view showing an optical circuit according to a second modification of the third embodiment.

[0010] Hereinafter, embodiments for carrying out the present disclosure will be described in detail with reference to the drawings. The following embodiments are examples for embodying the technical ideas of the invention, and the present disclosure is not limited to the described configurations and numerical values. In each drawing, the same components are given the same reference numerals, and duplicate explanations may be omitted as appropriate. The size, positional relationship, etc. of each component shown in each drawing may be exaggerated to facilitate understanding of the invention.

[0011] First Embodiment Fig. 1 is a plan view of an optical circuit according to a first embodiment. As shown in Fig. 1, the optical circuit 100 includes an optical filter 40. The optical filter 40 includes an optical waveguide 20 and a ring resonator 30. The optical waveguide 20 includes a first optical waveguide 21, a second optical waveguide 22, a third optical waveguide 23, and an optical branching section 24. The second optical waveguide 22 and the third optical waveguide 23 are connected to the first optical waveguide 21 via the optical branching section 24. The optical branching section 24 illustrated in Fig. 1 is a Y-branching section. The optical waveguide 20 is a Y-branching waveguide.

[0012] The second optical waveguide 22 has a first location 25A. The optical coupler 25 optically couples the second optical waveguide 22 and the ring resonator 30 at the first location 25A. The third optical waveguide 23 has a second location 26A. The optical coupler 26 optically couples the second optical waveguide 22 and the ring resonator 30 at the second location 26A. That is, the ring resonator 30 is optically coupled to the second optical waveguide 22 at the first location 25A and to the third optical waveguide 23 at the second location 26A. The ring resonator 30 has a diffraction grating 32 around its entire periphery. In a plan view, the diffraction grating 32 is asymmetric with respect to a line 59 connecting the center of the optical branching unit 24 and the center 58 of the ring resonator 30.

[0013] Light Sin input to the optical filter 40 from the end 27 of the first optical waveguide 21 opposite the optical branching unit 24 is branched by the optical branching unit 24 into light SR1 that travels through the second optical waveguide 22 and light SL1 that travels through the third optical waveguide 23. Light SR1 is coupled to the ring resonator 30 at the first location 25A and becomes light SR2 that travels clockwise within the ring resonator 30. Light SL1 is coupled to the ring resonator 30 at the second location 26A and becomes light SL2 that travels counterclockwise within the ring resonator 30. Lights SR2 and SL2 circulate around the ring resonator 30, generating a standing wave within the ring resonator 30. Light SR2 is coupled to the third optical waveguide 23 at the second location 26A and travels through the third optical waveguide 23 as light SR3. The light SL2 is coupled to the second optical waveguide 22 at the first point 25A and travels through the second optical waveguide 22 as light SL3. The lights SR3 and SL3 are combined in phase at the optical branching unit 24, and the combined light Sout is output from the end 27 of the first optical waveguide 21. The light Sout is light obtained by combining the lights SR3 and SL3, and is larger than when only one of the lights SR3 and SL3 is output from the end 27. This makes it possible to increase the reflectance of the optical filter 40.

[0014] The optical waveguide 20 and the ring resonator 30 are passive waveguides and do not include an active layer, so that the optical waveguide 20 and the ring resonator 30 can be easily obtained in the desired shape.

[0015] Of the optical waveguides 20, the second optical waveguide 22 and the third optical waveguide 23 are single-mode optical waveguides, thereby providing an optical filter 40 that reflects a frequency defined by a single spatial mode.

[0016] An end 28 of the second optical waveguide 22 on the opposite side to the optical branching unit 24, and an end 29 of the third optical waveguide 23 on the opposite side to the optical branching unit 24, are tapered to a point. This is to prevent reflection of the lights SR1 and SL1 at the ends 28 and 29 by emitting the lights SR1 and SL1 that reach the ends 28 and 29 without optically coupling with the ring resonator 30 to the outside. The anti-reflection structure needs to be provided at least at one of the ends 28 and 29, and is preferably provided at both the ends 28 and 29.

[0017] 2 is a cross-sectional view taken along line II-II of FIG. 1. As shown in FIG. 2, the optical circuit 100 includes a first cladding 10, a core 14, and a second cladding 12. The core 14 is disposed on the first cladding 10. The second cladding 12 is disposed on the first cladding 10 so as to cover the core 14. The second optical waveguide 22, the third optical waveguide 23, and the ring resonator 30 each include the first cladding 10, the second cladding 12, and the core 14. The optical coupler 25 couples the second optical waveguide 22, which is located close to the surface of the first cladding 10, with the ring resonator 30 using an evanescent wave. The optical coupler 26 couples the third optical waveguide 23, which is located close to the surface of the first cladding 10, with the ring resonator 30 using an evanescent wave. The optical coupler 25 optically couples the second optical waveguide 22 with the ring resonator 30, via an evanescent wave. The optical coupler 26 optically couples the third optical waveguide 23 and the ring resonator 30 together at a wavelength 60B.

[0018] The first cladding 10, the second cladding 12, and the core 14 are made of amorphous oxide, nitride, or fluoride. The refractive index of the core 14 is higher than the refractive indexes of the first cladding 10 and the second cladding 12. This allows light to be confined in the core 14. The materials of the first cladding 10, the second cladding 12, and the core 14 may be, for example, oxide or fluoride. The materials of the first cladding 10 and the second cladding 12 may be, for example, SiO ２ , MgF ２ , CaF ２ , SiON, or Al ２ O ３ The material of the core 14 is, for example, SiN, SiON, AlN, Ta, ２ O ５ , Nb ２ O ５ , TiO ２ , HfO ２ , LiNbO ３ Alternatively, the optical filter 40 may further include a substrate. The substrate can support the optical waveguide 20. The first clad 10 may include a substrate. The substrate may be, for example, sapphire or SiO ２ The substrate is an insulating substrate or a semiconductor substrate such as a silicon substrate.

[0019] (Explanation of the operating principle) (Explanation of the ring resonator filter) In order to explain the operating principle, a comparative example of a ring resonator filter will be explained. FIG. 3 is a plan view of an optical circuit according to the comparative example. As shown in FIG. 3, in the optical circuit 110 of the comparative example, a diffraction grating 32 is not arranged in the ring resonator 30 of the optical filter 40. The amplitude of the light Sin input to the end 27 is expressed as A ｉｎ and the angular frequency is ω. The branching ratio of the light SR1 and SL1 in the optical branching unit 24 is 1:1. The transmittance (i.e., optical coupling rate) from the second optical waveguide 22 and the third optical waveguide 23 to the ring resonator 30 in the optical couplers 25 and 26 is T. The circumferential length of the ring resonator 30 is L, and the effective refractive index is n ｅｆｆ The Q value of the ring resonator is Q. The angular frequency of light is ω, and the speed of light in a vacuum is c. At this time, the amplitude A of the light Sout output from the end 27 is ｒｅｆ is expressed by Equation 1.

[0020]

[0021] The complex amplitude reflectance is A ｒｅｆ / A ｉｎ If the transmittance T is sufficiently larger than the circular loss of the ring resonator 30, the reflectance approaches 1. When Q is sufficiently large, the resonant frequency f ０ Half width at half maximum of peripheral reflectance f ＨＷＨＭ is approximately expressed by Equation 2.

[0022]

[0023] The higher the Q value of the ring resonator and the smaller the transmittance T, the smaller the half width at half maximum f ＨＷＨＭ FIG. 4 is a diagram showing the reflectance versus frequency in the comparative example. The reflectance is |A ｒｅｆ / A ｉｎ | ２ , which corresponds to the transmittance of the ring resonator filter. As shown in Figure 4, for a small transmittance T, if the waveguide loss is large and the Q value of the ring resonator is low, the linewidth is wide and the reflectance is low. If the Q value of the ring resonator is high, the linewidth is narrow and the reflectance is high.

[0024] 5 is a graph showing the reflectance versus frequency for a comparative example, including a ring resonator filter with a long resonator length, a ring resonator filter with a short resonator length, and a diffraction grating single-frequency filter.

[0025] The circumferential length of the ring resonator 30 corresponds to the resonator length. To obtain a high Q value, it is necessary to increase the ring diameter to reduce radiation loss due to bending. However, as the resonator length increases, the spacing between the resonant frequencies (FSR: Free Spectral Range) decreases. Therefore, by combining a diffraction grating-type single-frequency filter with a ring resonator filter with a long resonator length, a single-frequency filter with a narrow linewidth can be realized.

[0026] However, as shown in FIG. 5, the peak frequency of the diffraction grating type single frequency filter may have an offset Δf with respect to the peak frequency of the ring resonator type filter, which reduces the reflectance of a filter that combines both.

[0027] In the optical circuit 100 of the first embodiment, when the diffraction grating 32 is provided continuously with respect to the ring resonator 30, no offset occurs between the resonant frequency of the ring resonator 30 and the peak frequency of the single-frequency filter in the diffraction grating 32, and only one peak remains at all times. This relationship remains unchanged no matter how long the resonator length of the ring resonator 30 is increased. Therefore, by setting the ring diameter of the ring resonator 30 to a desired size, the Q value of the ring resonator 30 can be set to any desired value. The ring diameter of the ring resonator 30 may be, for example, 10 μm or more and 1 cm or less, and preferably 100 μm or more and 100 mm or less.

[0028] (Explanation of Diffraction Grating in Ring Resonator) Figures 6A and 6B are enlarged views of the diffraction grating 32 of the ring resonator 30 in the first embodiment. As shown in Figures 6A and 6B, the diffraction grating 32 is arranged on the inner periphery of the ring resonator 30. The diffraction grating 32 has convex portions 32A that protrude inward from the inner periphery of the ring resonator 30 and concave portions 32B that are concave outward. The convex portions 32A and concave portions 32B correspond to antinodes 50 of the diffraction grating 32. When clockwise light SR2 and counterclockwise light SL2 are present in the ring resonator 30, a standing wave is generated in the ring resonator 30. The antinodes of the standing wave are indicated by 52A and 52B. The electric field directions of the antinodes 52A and 52B are opposite.

[0029] 6A shows an example in which antinodes 52A and 52B of the standing wave are located between antinodes 50 of the diffraction grating 32. As shown in FIG. 6A, the phase of the standing wave and the phase of the diffraction grating 32 are shifted by 90°. In this case, no radiation due to diffraction occurs, as indicated by arrow 53A.

[0030] 6B shows an example in which antinodes 52A and 52B of the standing wave coincide with antinode 50 of diffraction grating 32. In other words, the phase of the standing wave coincides with the phase of the diffraction grating. At this time, light energy is radiated by diffraction, as indicated by arrow 53B.

[0031] If the wavelength of the standing wave in ring resonator 30 differs from the period of diffraction grating 32, the relationship between the standing wave and diffraction grating 32 will be as shown in Figure 6B in part of ring resonator 30. That is, there will be a location where antinodes 52A and 52B of the standing wave coincide with antinode 50 of diffraction grating 32, resulting in a loss of energy and a decrease in the Q value of the ring resonator.

[0032] If the wavelength of the standing wave in the ring resonator 30 matches the period of the diffraction grating 32, and the phase of the standing wave and the phase of the diffraction grating 32 are shifted by 90°, the state shown in Figure 6B does not occur in the ring resonator 30, and no energy loss occurs. Therefore, the Q value of the ring resonator becomes high. The wavelength λ in a vacuum when the wavelength of the standing wave in the ring resonator 30 matches the period of the diffraction grating 32 ０ is expressed by Equation 3, where N is the number of convex portions 32A (or concave portions 32B) of the diffraction grating 32.

[0033]

[0034] The finite element method was used to simulate the frequency and the Q value of the ring resonator. Figure 7 shows the simulation results of the Q value of the ring resonator versus frequency. The stars in Figure 7 indicate the Q value of each resonance mode of the ring resonator 30. Resonance mode 55 is a resonance mode in which the wavelength of the standing wave in the ring resonator 30 matches the period of the diffraction grating, and the phase of the standing wave and the phase of the diffraction grating are shifted by 90°. The Q value of resonance mode 55 is high. Resonance modes 56A and 56B other than resonance mode 55 are resonance modes in which the wavelength of the standing wave in the ring resonator 30 does not match the period of the diffraction grating, or the phase of the standing wave matches the phase of the diffraction grating. The Q values ​​of resonance modes 56A and 56B are three or four orders of magnitude smaller than the Q value of resonance mode 55. In this way, by placing a diffraction grating 32 in the ring resonator 30, the Q values ​​of resonance modes other than resonance mode 55 can be lowered. The number of convex portions 32A (or concave portions 32B) of the diffraction grating 32 is, for example, 100 or more and 100,000 or less, or 1,000 or more and 10,000 or less.

[0035] 8 is a diagram showing the diffraction grating of the ring resonator according to the first embodiment. When the branching ratio of the optical branching unit 24 is 1:1 and the optical coupling rates of the first and second locations 25A and 26A are equal, a standing wave is excited in the ring resonator 30. As shown in FIG. 8 , to suppress radiation of the standing wave, it is preferable that antinodes 52A and 52B of the standing wave are located between the convex portion 32A and the concave portion 32B of the diffraction grating 32. When the second optical waveguide 22 and the third optical waveguide 23 are symmetrical with respect to a line 59 connecting the center of the optical branching unit 24 and the center of the ring resonator 30, the distance D2 along the optical axis of the second optical waveguide 22 between the first location 25A and the optical branching unit 24 is equal to the distance D3 along the optical axis of the third optical waveguide 23 between the second location 26A and the optical branching unit 24, and the antinode 52A or 52B of the standing wave is located on the line 59 in the ring resonator 30.

[0036] 9A and 9B are diagrams showing the positional relationship between the diffraction grating 32 and a line 59 in diffraction grating examples 1 and 2. As shown in FIG. 9A , in example 1, the diffraction grating 32 is symmetrical with respect to the line 59. An antinode 52A or 52B of the standing wave is located on the line 59. Therefore, the convex portion 32A and the concave portion 32B of the diffraction grating 32 coincide with the antinodes 52A and 52B of the standing wave, resulting in energy radiation as shown in FIG. 6B .

[0037] 9B, in Example 2, the diffraction grating 32 is asymmetric with respect to the line 59, and the convex portions 32A and concave portions 32B of the diffraction grating 32 are shifted in phase by 90° from the line 59. Therefore, the convex portions 32A and concave portions 32B of the diffraction grating 32 are shifted in phase by 90° from the antinodes 52A and 52B of the standing wave. No energy radiation occurs as in FIG. 6A.

[0038] More specifically, the periodic structure of diffraction grating 32 may be asymmetric with respect to line 59. In other words, if the periodic structure consisting of convex portions 32A and concave portions 32B of diffraction grating 32 does not overlap even when ring resonator 30 is folded back and overlapped with line 59, a resonant mode with a high Q value can be excited without diffraction loss.

[0039] 8 and 9B , when the modulation of the diffraction grating 32 is a sine wave, the straight line 59 may be positioned so that it overlaps between the peaks and valleys of the sine wave. Preferably, the straight line 59 may be positioned midway between the peaks and valleys. The reflectivity of the ring resonator 30 including the diffraction grating 32 is highest when the straight line 59 is positioned midway between the peaks and valleys, and decreases as it approaches the peaks or valleys. Furthermore, when the modulation of the diffraction grating 32 is a rectangular wave, the straight line 59 may be positioned so that it overlaps with the side portion between the peaks and valleys of the rectangle.

[0040] The reflectance |A ｒｅｆ / A ｉｎ | ２ was simulated.

[0041] In the first embodiment, the reflectance |A versus wavelength is calculated for the case where the diffraction grating 32 is arranged in the form of Example 1, the case where the diffraction grating 32 is arranged in the form of Example 2, and the case where the diffraction grating 32 is not arranged. ｒｅｆ / A ｉｎ | ２ The above was simulated using the finite element method under the following simulation conditions:

[0042] Diameter of ring resonator 30: 88 μm Number of sets of convex portions 32A and concave portions 32B: 1,117 Width of waveguide portion of ring resonator 30: 250 nm Modulation depth of convex portions 32A and concave portions 32B: 50 nm Gap width in optical couplers 25 and 26: 200 μm Waveguide loss of optical waveguide 20 and ring resonator 30: 1 dB / cm Operating wavelength: 465.53 nm Coupling rate K: 1% Q value of ring resonator 30 at operating wavelength: 1×10 ６ Half width at half maximum of resonance peak: 0.005 nm Cross-sectional area of ​​core 14 perpendicular to the optical axis: 250 nm × 250 nm Material of core 14: SiN with a refractive index of 2 Material of first cladding 10 and second cladding 12: SiO with a refractive index of 1.47 ２

[0043] FIG. 10 is a diagram showing the reflectance versus wavelength of the optical filter in the simulation. As shown in FIG. 10, in the comparative example without the diffraction grating 32, a reflection spectrum with narrow linewidth peaks at intervals corresponding to the FSR of the ring resonator 30 is observed. On the other hand, in the example with the diffraction grating 32 arranged in the form of Example 2, a single sharp peak is observed at a wavelength of approximately 465.6 nm. The reflectance outside the peak is −20 dB or less, which is sufficiently small and can be ignored. In this way, the first embodiment can achieve a narrow-band, single-peak optical filter.

[0044] 10, no peaks with high reflectance are observed when the diffraction grating 32 is arranged in the form of Example 1. This is because, at any frequency, the standing waves excited in the ring resonator by the light SR1 and the light SR2 undergo radiation loss by the diffraction grating 32, which reduces the Q value of the ring resonator, and as shown in Equation 1, the reflectance |A ｒｅｆ / A ｉｎ | ２This indicates that the

[0045] When the diffraction grating 32 is arranged around the entire circumference of the ring resonator 30, the resonant frequency of the ring resonator 30 coincides with the frequency at which the wavelength of the standing wave coincides with the period of the diffraction grating 32. In other words, Δf in FIG. 5 becomes approximately 0. Therefore, radiation loss due to first-order diffraction can be suppressed.

[0046] The diffraction grating 32 may be continuously arranged around a portion of the circumference of the ring resonator 30. The diffraction grating 32 is preferably continuously arranged around at least one-tenth of the circumference of the ring resonator 30, and more preferably around at least one-half of the circumference. When the diffraction grating 32 is continuously arranged around a portion of the circumference of the ring resonator 30, convex portions 32A and concave portions 32B are assumed to be arranged at regular intervals around the entire circumference of the ring resonator 30. The diffraction grating 32 is arranged so that some of the convex portions 32A and concave portions 32B are not arranged. This allows a standing wave having antinodes with a constant phase relationship with the convex portions 32A and concave portions 32B of all the diffraction gratings 32 to resonate in the ring resonator 30.

[0047] 8, the difference between the distances D2 and D3 is equal to or less than ¼ of the operating wavelength. This allows the optical couplers 25 and 26 to optically couple the light beams SR1 and SL1, which have approximately the same phase, to the ring resonator 30. The difference between the distances D2 and D3 is preferably equal to or less than ⅕ of the operating wavelength, more preferably equal to or less than 1 / 10 of the operating wavelength, and even more preferably zero.

[0048] The second optical waveguide 22 between the optical branching unit 24 and the first location 25A and the third optical waveguide 23 between the optical branching unit 24 and the second location 26A are line-symmetric with respect to the line 59. This allows for light SR1 and SL1 of approximately the same phase to be optically coupled to the ring resonator 30 at the first location 25A and the second location 26A. Furthermore, the coupling coefficient between the second optical waveguide 22 and the ring resonator 30 and the coupling coefficient between the third optical waveguide 23 and the ring resonator 30 can be made equal. Note that line symmetry does not mean strict geometric line symmetry; rather, a deviation from line symmetry due to manufacturing error is permitted. The manufacturing error is, for example, ¼ or less, ⅕ or less, or 1 / 10 or less of the operating wavelength.

[0049] If the diffraction grating 32 is symmetrical with respect to the line 59 as shown in Fig. 9A, the energy of the standing wave will be radiated as shown in Fig. 6B. Therefore, the diffraction grating 32 is made asymmetrical with respect to the line 59. The phase shift of the diffraction grating 32 from the position where the diffraction grating 32 is symmetrical with respect to the line 59 is preferably 45° or more and 135° or less, more preferably 70° or more and 110° or less, and even more preferably 90°. This makes it possible to improve the reflectance of the optical filter 40 compared to when the diffraction grating 32 is symmetrical with respect to the line 59.

[0050] The optical waveguide 20 and the ring resonator 30 contain an amorphous material. This reduces the influence of crystal planes compared to when the optical waveguide 20 and the ring resonator 30 are made of single crystal or polycrystal, thereby reducing bending loss in the optical waveguide 20 and the ring resonator 30. In particular, it is preferable that the core 14 is made of an amorphous material, and it is also preferable that the core 14 and the second cladding 12 are made of an amorphous material.

[0051] Light Sin is input from end 27, and light Sout returning from ring resonator 30 is output from end 27. This allows the line of light Sout to be narrowed, as shown in Fig. 10. The half width at half maximum of the narrowed line Sout may be, for example, 50 fm or more and 0.1 nm or less, or 100 fm or more and 0.05 nm or less.

[0052] At least one of the end 28 of the second optical waveguide 22 and the end 29 of the third optical waveguide 23 has a tapered tip. In this way, at least one of the end 28 and 29 is provided with an antireflection structure that prevents reflection of the light SR1 and SL1. This makes it possible to reduce the light SR1 and SL1 being reflected at the end 28 and 29 and output as light Sout. The antireflection structure may be the structure exemplified in FIG. 1 or another structure. The antireflection structure may be, for example, a structure in which the optical waveguide is sharply curved or a slab structure. A slab structure is a structure in which the width perpendicular to the optical axis of the optical waveguide suddenly increases, eliminating confinement in the direction perpendicular to the optical axis.

[0053] By making the outer periphery of the ring resonator 30 circular, the diffraction grating 32 can be formed with high precision. Note that the circle does not have to be a geometrically exact circle. For example, irregularities and manufacturing errors due to the diffraction grating 32 are allowed for. The circle may deviate from a geometrically exact circle by approximately the operating wavelength.

[0054] Second Embodiment Fig. 11 is a plan view of an optical circuit according to a second embodiment. As shown in Fig. 11, in an optical circuit 101 according to the second embodiment, the diffraction grating in the ring resonator 30 of the optical filter 40 has a plurality of diffraction gratings 32 arranged at regular intervals. Between the diffraction gratings 32 are regions 33 where no diffraction gratings 32 are arranged. The other configurations are the same as those of the first embodiment.

[0055] The term "constant intervals" does not necessarily mean that the intervals between the multiple diffraction gratings 32 are strictly constant. A difference from the strictly constant intervals to the extent of a manufacturing error is allowed. The manufacturing error is, for example, ¼ or less, ⅕ or less, or 1 / 10 or less of the operating wavelength.

[0056] Fig. 12 is an enlarged view of the ring resonator according to the second embodiment. As shown in Fig. 12, if a line 54 passes through the center 58 of the ring resonator 30 and the center of each diffraction grating 32, the angle θ between adjacent lines 54 is equal along the entire circumference. In the example of Fig. 12, the number of diffraction gratings 32 is eight, and θ is 22.5°.

[0057] 13 is a diagram showing the reflectance of the ring resonator versus frequency in the second embodiment. As shown in FIG. 13, the peak spacing FSR2 of the reflectance of the diffraction grating 32 is an integer multiple of the peak spacing FSR1 of the reflectance of the ring resonator 30. In this way, the diffraction grating 32 selects the resonant frequency of the ring resonator 30 at regular intervals. In the optical filter 40, reflectance peaks appear at the peak spacing FSR2.

[0058] 12 , the convex portions 32A and concave portions 32B of the diffraction grating 32 are assumed to be arranged at regular intervals around the entire circumference of the ring resonator 30. Some of the assumed convex portions 32A and concave portions 32B are not arranged, and a plurality of diffraction gratings 32 consisting of the remaining convex portions 32A and concave portions 32B are discretely arranged at regular intervals. When the convex portions 32A and concave portions 32B are arranged in this manner, if the diffraction grating 32 is divided into N equal portions, FSR2 / FSR1 becomes N / 2.

[0059] The optical filter 40 in the second embodiment can periodically extract light of approximately the same intensity. The intervals are N / 2 times the FSR1. For example, when applying laser light to a display device such as a monitor, it is necessary to reduce speckle noise. One method for achieving this is to use laser light of multiple wavelengths in the same color range. If an optical circuit 101 including the optical filter 40 in the second embodiment is used, light aligned at N / 2 times the FSR1 is reflected with approximately the same intensity, which contributes to reducing speckle noise.

[0060] Third Embodiment FIG. 14 is a plan view showing an optical circuit according to a third embodiment. As shown in FIG. 14, the optical circuit 102 includes a semiconductor laser element 41 in addition to an optical filter 40. The semiconductor laser element 41 has a substrate 42 and an optical waveguide 44. The substrate 42 is a semiconductor substrate such as a GaN substrate, an InP substrate, or a GaAs substrate. The optical waveguide 44 has, for example, a nitride semiconductor layer, an arsenide semiconductor layer, or a phosphide semiconductor layer formed on the substrate 42. The optical waveguide 44 includes a core 44A including an active layer and a cladding. The end face of the optical waveguide 44 facing the optical filter 40 is optically coupled to an end 27 of the optical filter 40 on the opposite side to the optical branching section 24. That is, the semiconductor laser element 41 is optically coupled to the first optical waveguide 21 at the end 27. The end 27 may be provided with an anti-reflection coating. An end face 43 of the optical waveguide 44 facing the optical filter 40 is a cleavage plane. An anti-reflection coating may be provided on the end face 43. The reflectance of the anti-reflection coating may be, for example, 1% to 10%, and preferably 1% to 5%. The other configurations are the same as those in the first embodiment.

[0061] Light emitted from the active layer in the optical waveguide 44 is input to the optical filter 40 as light Sin. Light Sin is reflected by the optical filter 40, and narrow-band light Sout is input to the optical waveguide 44. The light is reflected at the end face 43. As a result, narrow-band light is oscillated in the optical waveguide 44. Since the end face 43 is not a total reflection surface, narrow-band laser light SLD is output from the end face 43. The ring resonator 30 may have a structure in which diffraction gratings 32 are arranged at regular intervals, as in the second embodiment. The half-width at half maximum of the laser light SLD may be, for example, 50 am to 0.1 pm, or 100 am to 0.05 pm.

[0062] As described above, the internal optical intensity of ring resonator 30, which has a large Q value, is significantly greater than the optical intensity of optical waveguide 44 and laser beam SLD. Therefore, ring resonator 30 serves as the main resonator and determines the laser oscillation frequency. Even when the returning light of laser beam SLD is incident from end face 43, the amplitude of the returning light is smaller than the large optical amplitude inside ring resonator 30. Therefore, the effect of the returning light on modulating the oscillation frequency is small. In this way, the frequency stability of the laser light source formed by optical circuit 102 including semiconductor laser element 41 optically coupled to optical filter 40 is resistant to the returning light.

[0063] (Variation 1 of Third Embodiment) Fig. 15 is a plan view of an optical circuit according to Variation 1 of the third embodiment. As shown in Fig. 15, an optical circuit 103 according to Variation 1 of the third embodiment includes an optical filter 40, a semiconductor laser element 41, and a light extraction portion 45. The optical filter 40, the semiconductor laser element 41, and the light extraction portion 45 are arranged on the same substrate 42. The optical filter 40 and the light extraction portion 45 may be arranged on a substrate separate from the semiconductor laser element 41.

[0064] The light extraction section 45 includes an optical waveguide 20A. The optical waveguide 20A includes optical waveguides 21A, 22A, and 23A and an optical branching section 24A. The optical waveguide 22A and the optical waveguide 23A are connected to the optical waveguide 21A via the optical branching section 24A. The end of the optical waveguide 22A opposite the optical branching section 24A is connected to the second optical waveguide 22 at a first location 25A. The end of the optical waveguide 23A opposite the optical branching section 24A is connected to the third optical waveguide 23 at a second location 26A. Laser light SLD is output from an end 27A of the optical waveguide 21A opposite the optical branching section 24A. A high-reflection coating is provided on an end face 43A of the optical waveguide 44 opposite the end 27. The reflectivity of the high-reflection coating may be, for example, 80% or more and 99.9% or less. An anti-reflection coating may be provided on the end portion 27A. The other configurations are the same as those of the third embodiment.

[0065] Light emitted in the active layer of the optical waveguide 44 is reflected by the optical filter 40 and the end face 43A. The light passing through the optical waveguides 22A and 23A from the first point 25A and the second point 26A is combined in the optical branching section 24A. The combined light is extracted from the end 27A. Narrowband laser light SLD can also be output in the first modification of the third embodiment.

[0066] (Fourth Embodiment) The fourth embodiment is an example in which a heater is provided in an optical waveguide or a ring resonator. FIG. 16 is a plan view of an optical circuit according to the fourth embodiment. As shown in FIG. 16, in an optical circuit 104 according to the fourth embodiment, a heater 46 is disposed in the first optical waveguide 21. The heater 46 includes a body 46A and a terminal 46B. When a current is passed between the terminal 46B, the body 46A generates heat. The other configurations are the same as those of the third embodiment. The heater 46 adjusts the refractive index of the first optical waveguide 21. This allows the phase of the resonator formed by the end face 43 and the optical filter 40 to be adjusted by adjusting the optical path of the optical waveguide 20, etc.

[0067] (Variation 1 of Fourth Embodiment) Fig. 17 is a plan view of an optical circuit according to Variation 1 of the fourth embodiment. As shown in Fig. 17, in an optical circuit 105 according to Variation 1 of the fourth embodiment, heaters 46 are arranged around almost the entire circumference of the ring resonator 30. The other configurations are the same as those of the first embodiment. The heaters 46 can adjust the passband of the optical filter 40 selected by the ring resonator 30 and the diffraction grating 32.

[0068] (Modification 2 of Fourth Embodiment) Fig. 18 is a plan view of an optical circuit according to Modification 2 of the fourth embodiment. As shown in Fig. 18, in an optical circuit 106 according to Modification 2 of the fourth embodiment, heaters 46 are disposed in the second optical waveguide 22 and the third optical waveguide 23. The other configurations are the same as those of the first embodiment. This allows the heater 46 to adjust the phase between the second optical waveguide 22 and the third optical waveguide 23 when there is a mismatch between the distance D2 of the second optical waveguide 22 and the distance D3 of the third optical waveguide 23. The heater 46 may be disposed in either the second optical waveguide 22 or the third optical waveguide 23.

[0069] As in the fourth embodiment and its modified example, a heater 46 may be disposed in at least a part of the first optical waveguide 21 , the second optical waveguide 22 , the third optical waveguide 23 , and the ring resonator 30 .

[0070] Fifth Embodiment The fifth embodiment is an example using a disk resonator. FIG. 19 is a plan view of an optical circuit according to the fifth embodiment. As shown in FIG. 19, in an optical circuit 107 according to the fifth embodiment, a disk resonator 30A is arranged instead of a ring resonator. A diffraction grating 32 is arranged on the outer periphery of the disk resonator 30A. The other configurations are the same as those of the first embodiment. As in the fifth embodiment, in the first to fourth embodiments and their modifications, a disk resonator 30A may be used instead of the ring resonator 30. In this way, the optical resonator in which the second optical waveguide 22 and the third optical waveguide 23 are optically coupled at the first location 25A and the second location 26A is the ring resonator 30 or the disk resonator 30A. The disk resonator 30A functions as a whispering gallery mode resonator. Because the whispering gallery mode is a mode that travels around the outer periphery of the disk resonator 30A by total reflection, the center of the disk resonator may be removed to an extent that does not affect resonance.

[0071] (Example of Diffraction Grating) FIG. 20 is a plan view showing an example of a diffraction grating. As shown in FIG. 20 , a diffraction grating 32 is disposed in a ring resonator 30. The diffraction grating 32 has antinodes 32C and 32D with opposite polarities. In the first to fifth embodiments and their modifications, the diffraction grating 32 has been described as having a structure in which the inner circumference of the ring resonator 30 is periodically modulated. The diffraction grating 32 may have a structure in which the outer circumference of the ring resonator 30 is periodically modulated. The diffraction grating 32 may have a structure in which the top surface of the core 14 is periodically modulated. The diffraction grating 32 may have a structure in which the refractive index of at least one of the core 14 and the second cladding 12 is periodically modulated. The diffraction grating 32 may have a structure in which the top surface of the first cladding 10 is periodically modulated. A trigonometric function wave has been described as an example of the modulation of the diffraction grating 32. The modulation of the diffraction grating 32 can be selected as appropriate, such as a rectangular wave, a triangular wave, or a sawtooth wave.

[0072] (Example 1 of Optical Coupler) FIG. 21 is a plan view illustrating an example of an optical coupler. In a plan view, the ring resonator 30 partially overlaps with the second optical waveguide 22 and the third optical waveguide 23. FIG. 22 is a cross-sectional view of Example 1 of the optical coupler, taken along line XXII-XXII in FIG. 21. As shown in FIG. 22, the core 14A of the ring resonator 30 is disposed on the first cladding 10, and the cores 14 of the second optical waveguide 22 and the third optical waveguide 23 are disposed on the ring resonator 30 with the second cladding 12 sandwiched therebetween. The optical couplers 25 and 26 are evanescent couplers having cores 14A and 14 adjacent to each other in the thickness direction of the first cladding 10. The optical coupler 25 optically couples the second optical waveguide 22 and the ring resonator 30 with a coupling 60A. The optical coupler 26 optically couples the third optical waveguide 23 and the ring resonator 30 together at a wavelength 60B.

[0073] In the examples of the optical couplers 25 and 26 of FIG. 2 according to the first embodiment, the optical waveguide 20 and the ring resonator 30 can be formed using the same material. This simplifies the manufacturing process. However, it may be preferable to use different materials for the optical waveguide 20 and the ring resonator 30. For example, if it is desired to reduce the loss of the ring resonator 30, in the optical coupler example 1, the ring resonator 30 can be made of a different material from the optical waveguide 20, thereby reducing the loss of the ring resonator 30. Furthermore, in the examples of the optical couplers 25 and 26 of FIG. 2, if the outer periphery of the ring resonator 30 is modulated to place the diffraction grating 32, it is difficult to adjust the coupling ratio of the optical couplers 25 and 26. In the optical coupler example 1, it is easy to modulate the outer periphery of the ring resonator 30 to place the diffraction grating 32. Furthermore, it is easy to control the thickness of the second cladding 12 between the second optical waveguide 22 and the ring resonator 30, and the thickness of the second cladding 12 between the third optical waveguide 23 and the ring resonator 30. Therefore, it is easy to control the optical coupling rate in the optical couplers 25 and 26.

[0074] (Optical Coupler Example 2) FIG. 23 is a cross-sectional view of an optical coupler example 2. FIG. 23 is a cross-sectional view taken in the same direction as FIG. 22. As shown in FIG. 23, the cores 14 of the second optical waveguide 22 and the third optical waveguide 23 are disposed on the first clad 10A with the second clad 12 sandwiched therebetween, and the core 14A of the ring resonator 30 is disposed on the core 14 with the second clad 12 sandwiched therebetween. The first clad 10A is a high-refractive-index substrate. The first clad 10A is a semiconductor substrate made of, for example, GaN or GaAs. The refractive index of the first clad 10A may be higher than the refractive index of the cores 14 and 14A. In this case, light leaks from the cores 14 and 14A into the first clad 10A, increasing losses in the second optical waveguide 22, the third optical waveguide 23, and the ring resonator 30. Therefore, core 14A of ring resonator 30, where loss reduction is more of a problem, is placed farther away from first cladding 10A than core 14. This makes it possible to suppress loss in ring resonator 30. For example, when optical waveguide 20 and ring resonator 30 are arranged on the same substrate as the semiconductor laser element, the refractive index of the substrate for the semiconductor laser element may be higher than the refractive index of core 14 of optical waveguide 20 and core 14A of ring resonator 30. In such a case, it is preferable to use optical coupler example 2.

[0075] (Optical Coupler Example 3) Fig. 24 is a plan view of an optical circuit using optical coupler example 3. As shown in Fig. 24, optical couplers 25 and 26 are MMI (Multi-Mode Interference) couplers. MMI couplers do not require a microstructure like evanescent couplers. Therefore, manufacturing costs can be reduced through microfabrication. Optical coupler examples 1 to 3 may be used in the first to fifth embodiments and their modifications.

[0076] (Example 1 of Optical Branching Unit) Fig. 25 is a plan view of an optical circuit using Example 1 of Optical Branching Unit. As shown in Fig. 25, the optical branching unit 24 may be an MMI coupler.

[0077] (Example 2 of Optical Branching Unit) Fig. 26 is a plan view of an optical circuit using Example 2 of the optical branching unit. As shown in Fig. 26, the optical branching unit 24 may be an evanescent coupler.

[0078] In the first to fifth embodiments and their modifications, examples 1 and 2 of the optical branching unit may be used. When a Y-branching unit is used as the optical branching unit 24 in the first to fifth embodiments and their modifications, it is easy to achieve a branching ratio of 1:1 between the second optical waveguide 22 and the third optical waveguide 23. Therefore, it is preferable to use a Y-branching unit as the optical branching unit 24.

[0079] (Second Modification of Third Embodiment) FIG. 27 is a plan view showing an optical circuit according to a second modification of the third embodiment. As shown in FIG. 27 , an optical circuit 108 according to the second modification of the third embodiment includes an optical filter 40, a semiconductor laser element 41, and a polarization rotation element 48. The polarization rotation element 48 is an optical waveguide type element. The semiconductor laser element 41 and the polarization rotation element 48 may be arranged on the same substrate 42 as the optical filter 40. As in the third embodiment, the semiconductor laser element 41 may be arranged on a substrate separate from the substrate on which the optical filter 40 is arranged. The polarization rotation element 48 is arranged in the first optical waveguide 21 and converts light in a transverse electric (TE) mode to light in a transverse magnetic (TM) mode, and converts light in a TM mode to light in a TE mode. A known polarization rotation element may be used as the polarization rotation element 48. The polarization rotation element 48 may be, for example, the polarization rotation element described in Huijuan Zhang et al., APPLIED PHYSICS LETTERS 101, 021105 (2012) or Dura Shahwar et al., npj Nanophotonics (2024) 1:35.

[0080] When the diffraction grating 32 is disposed on the side surface of the ring resonator 30, the refractive index is modulated in the radial direction of the ring resonator 30. In this case, due to the relationship between the refractive index modulation of the diffraction grating 32 and the polarization direction, the electric field in the TM mode is oriented in a direction perpendicular to both the radial direction of the ring resonator 30 and the periodic direction of the diffraction grating 32. Therefore, stronger diffraction radiation occurs in the TM mode than in the TE mode. This allows the optical filter 40 to have higher wavelength selectivity for the TM mode than for the TE mode. Therefore, when the semiconductor laser element 41 oscillates in the TE mode, the wavelength selectivity of the optical filter 40 decreases. Therefore, a polarization rotation element 48 is disposed in the first optical waveguide 21. This polarization rotation element 48 converts the TE mode light Sin emitted by the semiconductor laser element 41 into TM mode light. The ring resonator 30 selectively reflects a specific wavelength component of the TM mode light, defined by the period of the diffraction grating 32. The polarization rotation element 48 converts the wavelength-selected TM mode light into TE mode light Sout. In this way, even when the semiconductor laser element 41 oscillates in TE mode, the optical filter 40 can improve the wavelength selectivity of the TE mode light. Therefore, the spectrum of the output light from the semiconductor laser element 41 oscillating in TE mode can be made single-peaked and have a narrow linewidth.

[0081] The polarization rotation element 48 may be disposed in both the second optical waveguide 22 and the third optical waveguide 23. However, disposing the polarization rotation element 48 in the first optical waveguide 21 makes it possible to convert the polarization of the light branched into the second optical waveguide 22 and the third optical waveguide 23 and the light multiplexed from the second optical waveguide 22 and the third optical waveguide 23 using a single polarization rotation element 48. This simplifies the optical circuit 108.

[0082] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0083] The present disclosure includes the following configurations: (Item 1) An optical circuit comprising: an optical waveguide having a first optical waveguide, a second optical waveguide, a third optical waveguide, and an optical branching section, wherein the second optical waveguide and the third optical waveguide are connected to the first optical waveguide via the optical branching section; and an optical resonator which is a ring resonator or a disk resonator optically coupled to the second optical waveguide at a first location and optically coupled to the third optical waveguide at a second location, wherein a diffraction grating is provided around the entire circumference or part of the circumference of the optical resonator, and the diffraction grating is asymmetric with respect to a line connecting the center of the optical branching section and the center of the optical resonator in a plan view. (Item 2) The optical circuit according to Item 1, wherein the diffraction grating is provided around the entire circumference of the optical resonator. (Item 3) The optical circuit according to Item 1, wherein the diffraction grating comprises a plurality of diffraction gratings arranged at regular intervals. (Item 4) The optical circuit according to any one of items 1 to 3, wherein a difference between a distance along the optical axis of the second optical waveguide between the first location and the optical branching unit and a distance along the optical axis of the third optical waveguide between the second location and the optical branching unit is equal to or less than ¼ of an operating wavelength. (Item 5) The optical circuit according to any one of items 1 to 4, wherein the second optical waveguide between the optical branching unit and the first location and the third optical waveguide between the optical branching unit and the second location are line-symmetric with respect to the straight line. (Item 6) The optical circuit according to any one of items 1 to 5, wherein the optical waveguide and the optical resonator contain amorphous materials. (Item 7) The optical circuit according to any one of items 1 to 6, further comprising a semiconductor laser element optically coupled to the first optical waveguide at an end of the first optical waveguide opposite the optical branching unit. (Item 8) The optical circuit according to any one of items 1 to 7, wherein light is input from an end of the first optical waveguide opposite to the optical branching section, and light returning from the ring resonator or the disk resonator is output from the end. (Item 9) The optical circuit according to any one of items 1 to 8, wherein an anti-reflection structure is provided on at least one of an end of the second optical waveguide opposite to the optical branching section and an end of the third optical waveguide opposite to the optical branching section. (Item 10) The optical circuit according to any one of items 1 to 9, wherein heaters are provided on at least a part of the first optical waveguide, the second optical waveguide, the third optical waveguide, and the optical resonator.(Item 11) The optical circuit according to any one of items 1 to 10, wherein the outer periphery of the optical resonator is a circle. (Item 12) The optical circuit according to any one of items 1 to 11, wherein the optical resonator is a ring resonator. (Item 13) The optical circuit according to any one of items 1 to 12, wherein the optical waveguide is a Y-branch waveguide. (14) The optical circuit according to item 7, wherein the first optical waveguide or both the second optical waveguide and the third optical waveguide have a polarization rotation element, the semiconductor laser element oscillates in TE mode, and the diffraction grating is disposed on a side surface of the optical resonator.

[0084] The optical circuit of the present disclosure can be used, for example, in visible light lasers, communication lasers, spectroscopes, molecular sensors, optical gyro sensors, and the like.

[0085] This application claims priority from basic patent application No. 2024-150260, filed with the Japan Patent Office on August 30, 2024, the entire contents of which are incorporated herein by reference.

[0086] 10, 10A First cladding 12 Second cladding 14, 14A Core 20, 20A, 21A, 22A, 23A, 44 Optical waveguide 21 First optical waveguide 22 Second optical waveguide 23 Third optical waveguide 24, 24A Optical branching section 25, 26 Optical coupler 25A First location 26A Second location 27, 27A, 28, 29 End 30 Ring resonator 30A Disk resonator 32 Diffraction grating 32A Convex portion 32B Concave portion 32C, 32D, 50, 52A, 52B Antinode 33 Region 40 Optical filter 41 Semiconductor laser element 42 Substrate 43, 43A End face 45 Light extraction section 46 Heater 48 Polarization rotation element 58 Center 59 straight line

Claims

1. An optical circuit comprising: an optical waveguide having a first optical waveguide, a second optical waveguide, a third optical waveguide, and an optical branching section, wherein the second optical waveguide and the third optical waveguide are connected to the first optical waveguide via the optical branching section; and an optical resonator which is a ring resonator or a disk resonator that is optically coupled to the second optical waveguide at a first location and to the third optical waveguide at a second location; wherein the optical resonator has a diffraction grating on the entire circumference or a part of the circumference, and the diffraction grating is asymmetric with respect to a line connecting the center of the optical branching section and the center of the optical resonator in a plan view.

2. The optical circuit according to claim 1, wherein the diffraction grating is disposed around the entire circumference of the optical resonator.

3. The optical circuit according to claim 1, wherein said diffraction grating comprises a plurality of diffraction gratings arranged at regular intervals.

4. An optical circuit according to any one of claims 1 to 3, wherein the difference between the distance along the optical axis of the second optical waveguide between the first location and the optical branching unit and the distance along the optical axis of the third optical waveguide between the second location and the optical branching unit is equal to or less than 1 / 4 of the operating wavelength.

5. An optical circuit according to any one of claims 1 to 4, wherein the second optical waveguide between the optical branching unit and the first location and the third optical waveguide between the optical branching unit and the second location are symmetrical with respect to the straight line.

6. An optical circuit according to any one of claims 1 to 5, wherein the optical waveguide and the optical resonator comprise amorphous materials.

7. An optical circuit according to any one of claims 1 to 6, further comprising a semiconductor laser element optically coupled to said first optical waveguide at an end of said first optical waveguide opposite to said optical branching section.

8. An optical circuit according to any one of claims 1 to 7, wherein light is input from an end of the first optical waveguide opposite to the optical branching section, and light returning from the ring resonator or the disk resonator is output from the end.

9. An optical circuit according to any one of claims 1 to 8, comprising an anti-reflection structure on at least one of the end of the second optical waveguide opposite the optical branching section and the end of the third optical waveguide opposite the optical branching section.

10. An optical circuit according to any one of claims 1 to 9, comprising heaters in at least a portion of the first optical waveguide, the second optical waveguide, the third optical waveguide, and the optical resonator.

11. An optical circuit according to any one of claims 1 to 10, wherein the periphery of the optical resonator is a circle.

12. An optical circuit according to any one of claims 1 to 11, wherein the optical resonator is a ring resonator.

13. An optical circuit according to any one of claims 1 to 12, wherein the optical waveguide is a Y-branch waveguide.

14. The optical circuit according to claim 7, wherein the first optical waveguide or both the second optical waveguide and the third optical waveguide have a polarization rotation element, the semiconductor laser element oscillates in TE mode, and the diffraction grating is disposed on a side surface of the optical resonator.

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