Optical resonator and optical pulse generator

Abstract

This disclosure aims to provide an optical resonator having large anomalous dispersion. [Solution] 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.

Description

Technical Field

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

Background Art

[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.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the development of a small optical frequency comb light source, a micro optical resonator with controlled dispersion is essential. In particular, in a micro optical resonator made of 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 previous studies.

[0005] In a micro optical resonator, by controlling the cross-sectional shape, abnormal dispersion up to a certain extent can be achieved. 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.

Means for Solving the Problems

[0007] One aspect of the present disclosure provides 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. [Effects of the Invention]

[0008] The optical resonator disclosed herein can significantly increase anomalous dispersion. [Brief explanation of the drawing]

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

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

[0013] Details of the optical resonator according to the first embodiment will be explained 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 light inside the optical resonator 10 by circulating the light in the circumferential direction while totally reflecting the inner surface of the light that resonates with the whispering gallery mode of the optical resonator 10 among the light input from the light source 20 via the optical waveguide 30. The optical resonator 10 generates light of a new wavelength inside the optical resonator 10 by four-wave mixing (FWM: Four-wave Mixing) due to the interaction between the optical crystal constituting the optical resonator 10 and the incident light. In the optical resonator 10, an optical frequency comb is obtained by generating light of a new wavelength.

[0016] The configuration of the optical resonator 10 will be described. FIG. 2 is a perspective view of the optical resonator 10 which is an example of the optical resonator according to the first embodiment. In the drawing, for convenience of explanation, a virtual three-dimensional coordinate system (XYZ orthogonal coordinate system) composed of an X-axis, a Y-axis, and a Z-axis (XYZ axes) orthogonal to each other may be set. For the coordinate axis perpendicular to the plane of the drawing, when a black dot is shown in the circle of the coordinate axis, it indicates that the coordinate axis is facing the front side with respect to the plane of the drawing. However, the coordinate system is defined for the purpose of explanation and does not limit the posture of the optical resonator and the like according to the embodiment of the present disclosure.

[0017] The optical resonator 10 is formed of an optical crystal having birefringence. The material forming the optical resonator 10 is, for example, any one of magnesium fluoride (MgF2), lithium niobate (niobate lithium, LiNbO3), or lithium tantalate (lithium tantalate, LiTaO3). 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 of an optical crystal having birefringence, it has an optical axis in a predetermined direction. The optical axis is an axis in a birefringent material in which birefringence does not occur. In the example of FIG. 2, the optical resonator 10 has an optical axis CAX extending in the X-axis direction.

[0018] In addition, the optical resonator 10 includes a main body 11 having a rotationally symmetric shape with the rotation axis RAX as the axis of symmetry. The main body 11 has a disc shape. In the example of FIG. 2, the rotation axis RAX extends along the Z-axis direction. The optical axis CAX of the optical crystal forming the main body 11 extends in a direction perpendicular to the rotation axis RAX. The main body 11 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. In some cases, when the optical axis is perpendicular to the axis of symmetry, it is referred to as X-CUT. The rotation axis RAX is an example of the first axis.

[0019] In the optical resonator 10, among 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 while undergoing total reflection along the outer periphery of the main body 11. The portion along the outer periphery of the main body 11 in the optical resonator 10 is referred to as the optical confinement portion 12.

[0020] In the optical confinement portion 12, light propagates along the outer periphery of the main body 11. The light confined in the optical confinement portion 12 includes polarized light whose electric field vibrates in the direction along the rotation axis RAX (Z-axis direction), and polarized light whose electric field vibrates in the radial direction centered on the rotation axis RAX (parallel to the plane parallel to the X-axis direction and the Y-axis direction and away from the rotation axis RAX). In the present disclosure, polarized light whose electric field vibrates in the direction along the rotation axis RAX (Z-axis direction) is referred to as the TE mode. Polarized light whose electric field vibrates in the radial direction centered on the rotation axis RAX (parallel to the plane parallel to the X-axis direction and the Y-axis direction and away from the rotation axis RAX) is referred to as the TM mode.

[0021] The refractive indices for the light propagating in the TE mode and the light propagating in the TM mode in the optical resonator 10 will be described. FIG. 3 is a diagram for explaining the refractive index for the polarized light propagating in the optical resonator 10, which is an example of the 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 light in the TM mode in the optical resonator 10, and line LTEx represents the refractive index of light in the TE mode in the optical resonator 10. For comparison, in Figure 3, the refractive index of light in the TM mode 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 light in the TE mode 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 index for TM mode light and the refractive index for 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 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 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 light propagating in TE mode and light propagating in 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 TE mode and 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 the optical pulse generator using the optical resonator according to the first embodiment will be described. Figure 4 shows the output of the 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 methods.

[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≫ A second embodiment of the optical resonator will now be described. The second embodiment of the optical resonator differs in shape from the first embodiment of the optical resonator. The second embodiment of the optical resonator 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, for example, a birefringent optical crystal. 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 portion of the light confinement 112 is formed as a large annular shape. When viewed from above along the Z-axis direction and in the opposite direction 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 in Figure 5, the optical axis CAX extends along the X-axis direction.

[0039] In the optical 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] The optical resonator according to the second embodiment can increase localized anomalous dispersion, similar to the optical resonator according to the first embodiment. The optical resonator according to the second embodiment can increase output by generating an optical frequency comb in a wavelength region where localized anomalous dispersion is increased.

[0042] ≪Third Embodiment≫ A third embodiment of the optical resonator will now be described. The optical resonator of the third embodiment differs in shape from the optical resonator of the first embodiment. In the optical resonator of the third embodiment, the main body is provided at the end of a rod-shaped member.

[0043] Figure 6 is a perspective view of an optical resonator 210, which is an example of an optical resonator according to the third embodiment. The optical resonator 210 is formed, for example, by cutting and polishing an optical crystal having birefringence. In the optical resonator 210, 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 210 propagates along the outer circumference of the main body 211 while undergoing total internal reflection. The portion of the optical resonator 210 along the outer circumference of the main body 211 is called the optical confinement portion 212.

[0044] The main body 211 has a rotationally symmetric shape with respect to the rotation axis RAX. In a top view along the Z-axis direction and opposite to the Z-axis, the main body 211 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 211 extends perpendicular to the rotation axis RAX. In the example in Figure 6, the optical axis CAX extends along the X-axis direction.

[0045] In the light confinement section 212, light propagates along the outer circumference of the main body 211. The main body 211 is provided at the end of the rod-shaped member 213. The rod-shaped member 213 may be formed from another material, such as metal. If the rod-shaped member 213 is formed from a different material than the main body 211, the main body 211 may be bonded to the rod-shaped member 213, for example, with an adhesive. Alternatively, the rod-shaped member 213 may be formed from the same optical crystal as the main body 211. If the rod-shaped member 213 is formed from the same optical crystal as the main body 211, the main body 211 may be formed, for example, by cutting.

[0046] The optical pulse generator, which includes an optical resonator 210, is equipped with an optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 212. Light is incident on the optical resonator 210 and light is emitted from the optical resonator 210 via the optical waveguide that is parallel to the X-axis and Y-axis directions and located near the optical confinement section 212.

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

[0048] ≪Fourth Embodiment≫ A fourth embodiment of the optical resonator will now be described. The fourth embodiment of the optical resonator differs in shape from the first embodiment of the optical resonator. The body of the fourth embodiment of the optical resonator 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 axis of rotation 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 axis of rotation RAX. In the example in 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 exits 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≫ A fifth embodiment of the optical resonator will now be described. The fifth embodiment of the optical resonator differs in shape from the first embodiment of the optical resonator. In the fifth embodiment of the optical resonator, the main body is provided in the middle of a rod-shaped member.

[0054] Figure 8 is a perspective 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 axis of rotation RAX. In a top view 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 perpendicular to the axis of rotation RAX. In the example in 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 located 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. [Explanation of Symbols]

[0061] 1. Light pulse generator 10, 110, 210, 310, 410 optical resonator 11, 111, 211, 311, 411 Main Unit 12, 112, 212, 312, 412 Light confinement section 113 circuit boards 213, 413 Rod-shaped member 114 units 20 light source 30 Optical waveguide CAX optical axis RAX Rotation Axis L1, L2 light LTEx, LTEz, LTMx, LTMz line θ Azimuth

Claims

1. It comprises a body formed from a birefringent optical crystal and having a rotationally symmetric shape with respect to the first axis of symmetry, The main body has a light-confining portion along its outer circumference, The optical axis in the optical crystal is perpendicular to the first axis. optical resonator.

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

3. The main body has a disc-like shape, The optical resonator according to either claim 1 or claim 2.

4. A substrate formed by the aforementioned optical crystal, The system further comprises a base formed from the optical crystal and protruding from the substrate, The main body is provided on the base, The optical resonator according to either claim 1 or claim 2.

5. The aforementioned body has a ring-shaped form. The optical resonator according to either claim 1 or claim 2.

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

7. An optical resonator according to either claim 1 or claim 2, A light source that emits light of a single wavelength, An optical waveguide that inputs the light output from the light source to the optical resonator, Equipped with, Light pulse generator.

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