Optical resonator and optical pulse generator

Abstract

An optical resonator comprising a main body which is formed of an optical crystal having birefringence and has a rotationally symmetric shape with a first axis as a symmetry axis, wherein the main body includes a light confinement part along an outer periphery, and an optical axis in the optical crystal is perpendicular to the first axis.

Description

Optical Resonator and Optical Pulse Generator

[0001] The present disclosure relates to an optical resonator and an optical pulse generator.

[0002] Patent Document 1 discloses an optical resonator operating in a whispering gallery mode. Patent Document 1 discloses that the optical resonator has a rotationally symmetric body formed of an optical crystal, and the body has an optical confinement portion along the outer periphery.

[0003] Japanese Patent Application Laid-Open No. 2021-110835

[0004] In the development of a small optical frequency comb light source, a micro optical resonator with controlled dispersion is essential. Particularly in a micro optical resonator fabricated from a research material, in order to control the dispersion in the micro optical resonator, the cross-sectional shape of the micro optical resonator has been controlled in prior research.

[0005] In a micro optical resonator, by controlling the cross-sectional shape, abnormal dispersion can be achieved to a certain extent. However, there is a limit to the range in which abnormal dispersion can be controlled by controlling the cross-sectional shape in a micro optical resonator. On the other hand, it is known that a micro optical resonator having a large abnormal dispersion can increase the output of an optical frequency comb.

[0006] An object of the present disclosure is to provide an optical resonator having a large abnormal dispersion.

[0007] In one aspect of the present disclosure, there is provided an optical resonator including a body formed of a birefringent optical crystal and having a rotationally symmetric shape with a first axis as a symmetry axis, the body having an optical confinement portion along the outer periphery, and an optical axis in the optical crystal being perpendicular to the first axis.

[0008] According to the optical resonator of the present disclosure, the abnormal dispersion can be increased.

[0009] Figure 1 is a schematic diagram of an optical pulse generator equipped with an optical resonator according to the first embodiment. Figure 2 is a perspective view of the optical resonator according to the first embodiment. Figure 3 is a diagram illustrating the refractive index for polarization propagating through the optical resonator according to the first embodiment. Figure 4 is a diagram showing the output of an optical pulse generator equipped with an optical resonator according to the first embodiment. Figure 5 is a perspective view of the optical resonator according to the second embodiment. Figure 6 is a side view of the optical resonator according to the third embodiment. Figure 7 is a perspective view of the optical resonator according to the fourth embodiment. Figure 8 is a side view of the optical resonator according to the fifth embodiment.

[0010] The embodiments will be described below with reference to the attached drawings. However, this disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the claims as indicated by the claims.

[0011] In addition, regarding the descriptions and drawings of each embodiment, redundant explanations may be omitted by assigning the same or corresponding reference numerals to components having substantially the same or corresponding functional configurations. Furthermore, for ease of understanding, the scale of each part in the drawings may differ from that of the actual parts.

[0012] <<First Embodiment>> The optical resonator according to the first embodiment will now be described. The optical resonator according to the first embodiment is formed from an optical crystal having birefringence and comprises a body having a rotationally symmetric shape with respect to a first axis. The body of the optical resonator according to the first embodiment also has an optical confinement portion along its outer circumference. In the optical resonator according to the first embodiment, the optical axis in the optical crystal is perpendicular to the first axis. The optical resonator according to the first embodiment also has a disc-like shape.

[0013] Details of the optical resonator according to the first embodiment will be described with reference to the drawings. First, an optical pulse generator equipped with the optical resonator according to the first embodiment will be described. Figure 1 is a schematic diagram of an optical pulse generator 1 equipped with an optical resonator 10, which is an example of an optical resonator according to the first embodiment.

[0014] The optical pulse generator 1 is used as an optical frequency comb light source. The optical pulse generator 1 comprises an optical resonator 10, a light source 20, and an optical waveguide 30. The optical pulse generator 1 converts single-wavelength light L1 input from the light source 20 into optical frequency comb light L2 and outputs it.

[0015] [Optical Resonator 10] The optical resonator 10 confines the light that resonates with the whispering gallery mode of the optical resonator 10, which is input from the light source 20 via the optical waveguide 30, by causing it to circulate circumferentially while totally reflecting off its inner surface. The optical resonator 10 generates light of a new wavelength inside the optical resonator 10 through four-wave mixing (FWM) caused by the interaction between the optical crystal constituting the optical resonator 10 and the incident light. By generating light of a new wavelength in the optical resonator 10, an optical frequency comb is obtained.

[0016] The configuration of the optical resonator 10 will now be described. Figure 2 is a perspective view of an optical resonator 10, which is an example of an optical resonator according to the first embodiment. For the sake of explanation, the drawings may include a virtual three-dimensional coordinate system (XYZ Cartesian coordinate system) consisting of mutually orthogonal X, Y, and Z axes (XYZ axes). When a black circle is shown inside a circle representing a coordinate axis perpendicular to the plane of the drawing, it indicates that the coordinate axis is facing towards the viewer relative to the plane of the drawing. However, this coordinate system is defined for explanatory purposes only and does not limit the orientation of the optical resonator, etc., according to the embodiment of this disclosure.

[0017] The optical resonator 10 is formed from an optical crystal having birefringence. The material used to form the optical resonator 10 is, for example, magnesium fluoride (MgF ２ ), lithium niobate (lithium niobate, LiNbO ３ ) or lithium tantalate (lithium tantalate, LiTaO) ３) is one of the above. Note that the optical crystal forming the optical resonator 10 is not limited to the above examples as long as it has birefringence. Since the optical resonator 10 is formed from an optical crystal having birefringence, it has an optical axis in a predetermined direction. An optical axis is an axis in a direction in which birefringence does not occur in a birefringent material. In the example in Figure 2, the optical resonator 10 has an optical axis CAX extending along the X-axis direction.

[0018] Furthermore, the optical resonator 10 includes a body 11 having a rotationally symmetric shape with respect to the rotation axis RAX. The body 11 has a disc-like shape. In the example in Figure 2, the rotation axis RAX extends along the Z-axis direction. The optical axis CAX of the optical crystal forming the body 11 extends in a direction perpendicular to the rotation axis RAX. When viewed from above along the Z-axis direction and in the opposite direction to the Z-axis, the body 11 has a diameter ranging from, for example, 100 micrometers to 3 centimeters. Note that when the optical axis is perpendicular to the axis of symmetry, it is sometimes called an X-CUT. The rotation axis RAX is an example of a first axis.

[0019] In the optical resonator 10, of the light input from the light source 20 via the optical waveguide 30, the light that resonates with the whispering gallery mode of the optical resonator 10 propagates along the outer circumference of the main body 11 while undergoing total internal reflection. The portion of the optical resonator 10 along the outer circumference of the main body 11 is called the optical confinement section 12.

[0020] In the light confinement section 12, light propagates along the outer circumference of the main body 11. The light confined in the light confinement section 12 includes polarization in which the electric field vibrates in a direction along the rotation axis RAX (Z-axis direction) and polarization in which the electric field vibrates in a radial direction centered on the rotation axis RAX (parallel to the planes parallel to the X-axis and Y-axis directions and away from the rotation axis RAX). In this disclosure, polarization in which the electric field vibrates in a direction along the rotation axis RAX (Z-axis direction) is called the TE mode. Polarization in which the electric field vibrates in a radial direction centered on the rotation axis RAX (parallel to the planes parallel to the X-axis and Y-axis directions and away from the rotation axis RAX) is called the TM mode.

[0021] The refractive index for light propagating in TE mode and light propagating in TM mode in the optical resonator 10 will be explained. Figure 3 is a diagram illustrating the refractive index for polarizations propagating in the optical resonator 10, which is an example of an optical resonator according to the first embodiment.

[0022] Figure 3 shows an example of an optical resonator 10 formed from magnesium fluoride. In magnesium fluoride, the refractive index in the ordinary ray (ordinary refractive index) is 1.3705. On the other hand, in magnesium fluoride, the refractive index in the extraordinary ray (extraordinary refractive index) is 1.3819.

[0023] In Figure 3, the horizontal axis represents the azimuth angle θ, and the vertical axis represents the refractive index at azimuth angle θ. The azimuth angle θ is the angle relative to the horizontal direction when the optical axis CAX is oriented vertically, as shown in the lower right of Figure 3. Note that the azimuth angle θ is a counterclockwise angle, with the case of pointing to the right in the lower right diagram of Figure 3 being set to zero. The refractive index is calculated using the Sellmeyer equation.

[0024] In Figure 3, line LTMx represents the refractive index of TM-mode light in the optical resonator 10, and line LTEx represents the refractive index of TE-mode light in the optical resonator 10. For comparison, in Figure 3, the refractive index of TM-mode light in an optical resonator where the optical axis and the rotation axis are the same is shown as line LTMz, and the refractive index of TE-mode light is shown as line LTEz. Note that the case where the optical axis is parallel to the axis of symmetry is sometimes called a Z-cut.

[0025] As shown by the lines LTMz and LTEz in Figure 3, the refractive indices for TM mode and TE mode light in an optical resonator where the optical axis and rotation axis are the same remain unchanged even when the azimuth angle θ changes. Therefore, TM mode and TE mode light propagate within the optical resonator with different refractive indices.

[0026] As shown by the line LTEx in Figure 3, in the optical resonator 10, the refractive index of light propagating in the TE mode does not change even if the azimuth angle θ changes. On the other hand, as shown by the line LTMx in Figure 3, in the optical resonator 10, the refractive index of light propagating in the TM mode changes with the azimuth angle θ. In particular, when the azimuth angle is near 0 or π radians, the difference in refractive index between the light propagating in the TE mode and the light propagating in the TM mode becomes small.

[0027] When the difference in refractive index between light propagating in TE mode and light propagating in TM mode becomes small, strong intermode interaction (anti-crossing) occurs between the light propagating in TE mode and the light propagating in TM mode, which are orthogonal polarization modes. In other words, strong intermode interaction occurs between orthogonal polarization modes. The occurrence of anti-crossing results in local anomalous dispersion. In the optical resonator of this disclosure, the conversion efficiency can be increased by using this local anomalous dispersion. In other words, the conversion efficiency can be increased by using local dispersion compared to band dispersion.

[0028] [Light Source 20] The light source 20 supplies single-wavelength light L1 to the optical resonator 10 via the optical waveguide 30. The light source 20 is, for example, a laser diode. The light source 20 is, for example, an excitation light laser. The light source 20 supplies a continuous wave of single-wavelength light L1 to the optical resonator 10 via the optical waveguide 30.

[0029] The light source 20 may be equipped with a thermoelectric cooler (TEC) to stabilize the wavelength of light L1.

[0030] [Optical waveguide 30] The optical waveguide 30 propagates the light L1 supplied from the light source 20 to the optical resonator 10. The optical waveguide 30 also outputs the light L2 output from the optical resonator 10 to the outside.

[0031] The optical waveguide 30 is, for example, an optical fiber. The optical waveguide 30 is provided in the vicinity of the optical resonator 10. The optical waveguide 30 exchanges light with the optical resonator 10 via evanescent light.

[0032] The optical waveguide 30 may, for example, be tapered to make the portion in close proximity to the optical resonator 10 narrower. Alternatively, the optical waveguide 30 may be connected to the optical resonator 10 using, for example, a prism.

[0033] Next, the output of an optical pulse generator using an optical resonator according to the first embodiment will be described. Figure 4 shows the output of an optical pulse generator 1 equipped with an optical resonator 10, which is an example of an optical resonator according to the first embodiment. In Figure 4, the horizontal axis is wavelength and the vertical axis is light intensity.

[0034] The optical resonator 10 was able to obtain an average output of 23.8 milliwatts and a pulse full width at half maximum of 5.62 picoseconds. For example, it was possible to obtain an intensity approximately 1000 times greater than that of conventional systems.

[0035] According to the optical resonator of the first embodiment, localized anomalous dispersion can be greatly increased. The optical resonator of the first embodiment can increase the output by generating an optical frequency comb in a wavelength region in which localized anomalous dispersion is greatly increased.

[0036] ≪Second Embodiment≫ The optical resonator according to the second embodiment will now be described. The optical resonator according to the second embodiment differs in shape from the optical resonator according to the first embodiment. The optical resonator according to the second embodiment further comprises a substrate formed of an optical crystal and a base formed of an optical crystal that protrudes from the substrate, and the main body is provided on the base.

[0037] Figure 5 is a perspective view of an optical resonator 110, which is an example of an optical resonator according to the second embodiment. The optical resonator 110 is formed by applying lithography and etching techniques to a substrate formed of a birefringent optical crystal, for example. In the optical resonator 110, of the light input from the light source via the optical waveguide, the light that resonates with the whispering gallery mode of the optical resonator 110 propagates along the outer circumference of the main body 111 while undergoing total internal reflection. The portion of the optical resonator 110 along the outer circumference of the main body 111 is called the optical confinement portion 112.

[0038] The main body 111 has a rotationally symmetric shape with respect to the rotation axis RAX. The main body 111 has a disc-like shape. In the main body 111, the light-confining portion 112 is formed in a large, annular shape. When viewed from above along the Z-axis direction and opposite to the Z-axis, the main body 111 has a diameter ranging from, for example, 100 micrometers to 3 centimeters. Furthermore, the optical axis CAX of the optical crystal forming the main body 111 extends in a direction perpendicular to the rotation axis RAX. In the example of Figure 5, the optical axis CAX extends along the X-axis direction.

[0039] In the light confinement section 112, light propagates along the outer circumference of the main body 111. The main body 111 is connected to the substrate 113 by a base 114. The optical resonator 110 includes a base 114 between the main body 111 and the substrate 113. The base 114 is provided projecting from the substrate 113 in the direction of the Z axis along the Z axis. The main body 111 is provided on the base 114. The base 114 separates the main body 111 from the substrate 113.

[0040] The optical pulse generator, which includes an optical resonator 110, is equipped with an optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 112. Light is incident on the optical resonator 110 and light is emitted from the optical resonator 110 via the optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 112.

[0041] According to the optical resonator of the second embodiment, similar to the optical resonator of the first embodiment, it is possible to increase the localized anomalous dispersion. The optical resonator of the second embodiment can increase the output by generating an optical frequency comb in a wavelength region in which the localized anomalous dispersion is increased.

[0042] ≪Third Embodiment≫ The optical resonator according to the third embodiment will now be described. The optical resonator according to the third embodiment has a different shape from the optical resonator according to the first embodiment. The main body of the optical resonator according to the third embodiment is provided at the end of a rod-shaped member.

[0043] FIG. 6 is a side view of an optical resonator 210, which is an example of an optical resonator according to the third embodiment. The optical resonator 210 is, for example, an optical resonator formed by cutting and polishing an optical crystal having birefringence. In the optical resonator 210, among the light input from a light source through an optical waveguide, the light that resonates with the whispering gallery mode of the optical resonator 210 propagates while undergoing total reflection along the outer periphery of the main body 211. The portion along the outer periphery of the main body 211 in the optical resonator 210 is referred to as a light confinement portion 212.

[0044] The main body 211 has a rotationally symmetric shape with the rotation axis RAX as the axis of symmetry. The main body 211 has a diameter in the range of, for example, 100 micrometers to 3 centimeters in a top view looking in the direction opposite to the Z-axis along the Z-axis direction. Also, the optical axis CAX of the optical crystal forming the main body 211 extends in a direction perpendicular to the rotation axis RAX. In the example of FIG. 6, the optical axis CAX extends along the X-axis direction.

[0045] In the light confinement portion 212, light propagates along the outer periphery of the main body 211. The main body 211 is provided at the end of a rod-shaped member 213. The rod-shaped member 213 may be formed of another material such as, for example, metal. When the rod-shaped member 213 is formed as a separate member from the main body 211, the main body 211 may be adhered to the rod-shaped member 213 with, for example, an adhesive or the like. Also, the rod-shaped member 213 may be formed of the same optical crystal as the main body 211. When the rod-shaped member 213 is formed of the same optical crystal as the main body 211, the main body 211 may be formed by, for example, cutting.

[0046] The optical pulse generator including the optical resonator 210 includes an optical waveguide that is parallel to the X-axis direction and the Y-axis direction and is provided in the vicinity of the light confinement portion 212. The optical waveguide that is parallel to the X-axis direction and the Y-axis direction and is provided in the vicinity of the light confinement portion 212 allows light to be incident on the optical resonator 210 and also allows light to exit from the optical resonator 210.

[0047] According to the optical resonator according to the third embodiment, similarly to the optical resonator according to the first embodiment, local anomalous dispersion can be increased. The optical resonator according to the third embodiment can increase the output by generating an optical frequency comb in a wavelength region where local anomalous dispersion is increased.

[0048] ≪Fourth Embodiment≫ The optical resonator according to the fourth embodiment will now be described. The optical resonator according to the fourth embodiment has a different shape from the optical resonator according to the first embodiment. The main body of the optical resonator according to the fourth embodiment has a ring-shaped form.

[0049] Figure 7 is a perspective view of an optical resonator 310, which is an example of an optical resonator according to the fourth embodiment. The optical resonator 310 is formed, for example, by cutting and polishing an optical crystal having birefringence. In the optical resonator 310, of the light input from the light source via the optical waveguide, the light that resonates with the whispering gallery mode of the optical resonator 310 propagates along the outer circumference of the main body 311 while undergoing total internal reflection. The portion of the optical resonator 310 along the outer circumference of the main body 311 is called the optical confinement portion 312.

[0050] The main body 311 has a rotationally symmetric shape with respect to the rotation axis RAX. The main body 311 has a ring-like shape. When viewed from above along the Z-axis direction and opposite to the Z-axis, the main body 311 has a diameter ranging from, for example, 100 micrometers to 3 centimeters. Furthermore, the optical axis CAX of the optical crystal forming the main body 311 extends in a direction perpendicular to the rotation axis RAX. In the example of Figure 7, the optical axis CAX extends along the X-axis direction.

[0051] In the optical confinement section 312, light propagates along the outer circumference of the main body 311. The optical pulse generator, which includes an optical resonator 310, is equipped with an optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 312. Light is incident on the optical resonator 310 and emitted from the optical resonator 310 via the optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 312.

[0052] The optical resonator according to the fourth embodiment can increase localized anomalous dispersion, similar to the optical resonator according to the first embodiment. The optical resonator according to the fourth embodiment can increase output by generating an optical frequency comb in a wavelength region where localized anomalous dispersion is increased.

[0053] <Fifth Embodiment> The optical resonator according to the fifth embodiment will now be described. The optical resonator according to the fifth embodiment has a different shape from the optical resonator according to the first embodiment. The main body of the optical resonator according to the fifth embodiment is provided in the middle of a rod-shaped member.

[0054] Figure 8 is a side view of an optical resonator 410, which is an example of an optical resonator according to the fifth embodiment. The optical resonator 410 is formed, for example, by cutting and polishing an optical crystal having birefringence. In the optical resonator 410, of the light input from the light source via the optical waveguide, the light that resonates with the whispering gallery mode of the optical resonator 410 propagates along the outer circumference of the main body 411 while undergoing total internal reflection. The portion of the optical resonator 410 along the outer circumference of the main body 411 is called the optical confinement portion 412.

[0055] The main body 411 has a rotationally symmetric shape with respect to the rotation axis RAX. When viewed from above along the Z-axis direction and opposite to the Z-axis, the main body 411 has a diameter ranging from, for example, 100 micrometers to 3 centimeters. Furthermore, the optical axis CAX of the optical crystal forming the main body 411 extends in a direction perpendicular to the rotation axis RAX. In the example of Figure 8, the optical axis CAX extends along the X-axis direction.

[0056] In the light confinement section 412, light propagates along the outer circumference of the main body 411. The main body 411 is provided in the middle portion of the rod-shaped member 413.

[0057] The optical pulse generator, which includes an optical resonator 410, is equipped with an optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 412. Light is incident on the optical resonator 410 and light is emitted from the optical resonator 410 via the optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 412.

[0058] Furthermore, the optical resonator according to the fifth embodiment may be provided with multiple bodies. Also, each of the multiple bodies may have a different diameter.

[0059] The optical resonator according to the fifth embodiment can increase localized anomalous dispersion, similar to the optical resonator according to the first embodiment. The optical resonator according to the fifth embodiment can increase output by generating an optical frequency comb in a wavelength region where localized anomalous dispersion is increased.

[0060] Although the optical resonator has been described above with reference to embodiments, the present invention is not limited to the above embodiments. Various modifications and improvements, such as combinations or substitutions with some or all of other embodiments, are possible within the scope of the present invention.

[0061] This application claims priority to Basic Patent Application No. 2025-007081, filed with the Japan Patent Office on January 17, 2025, the entire contents of which are incorporated herein by reference.

[0062] 1. Optical pulse generator 10, 110, 210, 310, 410. Optical resonator 11, 111, 211, 311, 411. Main body 12, 112, 212, 312, 412. Optical confinement section 113. Substrate 213, 413. Rod-shaped member 114. Stand 20. Light source 30. Optical waveguide CAX: Optical axis RAX: Rotation axis L1, L2. Light LTEx, LTEz, LTMx, LTMz: Line θ: Azimuth angle

Claims

1. An optical resonator comprising a body formed from a birefringent optical crystal and having a rotationally symmetric shape with respect to a first axis, wherein the body has an optical confinement portion along its outer circumference, and the optical axis in the optical crystal is perpendicular to the first axis.

2. The optical resonator according to claim 1, wherein the material of the optical crystal is magnesium fluoride, lithium niobate, or lithium tantalate.

3. The optical resonator according to claim 1 or claim 2, wherein the main body has a disc-shaped form.

4. The optical resonator according to claim 1 or claim 2, further comprising a substrate formed of the optical crystal and a base formed of the optical crystal and protruding from the substrate, wherein the main body is provided on the base.

5. The optical resonator according to claim 1 or claim 2, wherein the main body has a ring-shaped form.

6. The optical resonator according to claim 1 or claim 2, wherein the main body is provided at the end or in the middle of a rod-shaped member.

7. An optical pulse generator comprising: an optical resonator according to claim 1 or claim 2; a light source that outputs light of a single wavelength; and an optical waveguide that inputs the light output from the light source to the optical resonator.

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