Optical element, laser module, and near-eye wearable device

Abstract

To provide an optical element, a light source module, and a near-eye wearable device that can improve the efficiency in converting a visible light polarization mode.SOLUTION: In an optical element, a core layer C1 comprises a mode converter 42 that extends in an X-axis direction extending along a principal surface Sa, and converts a visible light polarization mode. The mode converter 42 has a waveguide 51 that makes visible light incident in a first polarization mode that is one polarization mode of a TE mode and a TM mode, a waveguide 52 that emits the visible light in a second polarization mode that is the other polarization mode, and a waveguide 53 that converts the visible light polarization mode from the first polarization mode to the second polarization mode. The waveguide 53 has an asymmetrical shape in a Y-axis direction.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to an optical element, a laser module, and a near-eye wearable device. [Background technology]

[0002] There is known a laser module applied to a retinal projection device mounted on a near-eye wearable device such as an AR (Augmented Reality) glass and an MR (Mixed Reality) glass. Such a laser module includes a plurality of laser diodes and a multiplexer that multiplexes the laser beams emitted from the laser diodes into one laser beam.

[0003] The above-mentioned laser module may include an optical modulator (for example, Patent Document 1). The optical modulator described in Patent Document 1 receives visible light emitted from a laser light source and changes the intensity of the visible light to generate a visible light signal. In the optical modulator described in Patent Document 1, the optical waveguide layer that guides the visible light is composed of a lithium niobate film. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2022-36928 Summary of the Invention [Problem to be solved by the invention]

[0005] In materials having an electro-optic effect such as lithium niobate film, optical characteristics such as effective refractive index and modulation efficiency change in the TE (Transverse Electric) mode in which the main component of the electric field is in the in-plane direction of the optical waveguide layer, and the TM (Transverse Magnetic) mode in which the main component of the electric field is perpendicular to the in-plane direction (the main component of the magnetic field is in the in-plane direction). Therefore, in the optical modulator as described above, the modulation efficiency can change depending on whether the polarization mode of visible light is the TE mode or the TM mode. In this way, it is sometimes required to set the polarization mode of visible light to a desired polarization mode.

[0006] On the other hand, visible light emitted from a laser light source includes visible light in TE mode and visible light in TM mode. Therefore, when the polarization mode of the visible light emitted from a laser light source is different from a desired polarization mode, it is necessary to convert the polarization mode of the visible light, and it is desired to improve the efficiency of this polarization mode conversion.

[0007] The present disclosure describes an optical element, a light source module, and a near-eye wearable device that can improve the efficiency of polarization mode conversion of visible light. [Means for solving the problem]

[0008] An optical element according to one aspect of the present disclosure includes a substrate having a principal surface, and a core layer provided on the principal surface and made of a material having an electro-optic effect. The core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light between a TM mode and a TE mode. The mode converter includes a first waveguide that receives visible light in a first polarization mode that is one of the TE mode and the TM mode, a second waveguide that outputs visible light in a second polarization mode that is the other of the TE mode and the TM mode, and a third waveguide that is provided between the first and second waveguides and converts the visible light from the first polarization mode to the second polarization mode. The third waveguide has an asymmetric shape in a second direction along the principal surface that intersects with the first direction.

[0009] In the optical element, since the core layer is made of a material having an electro-optic effect, optical characteristics such as an effective refractive index change according to the shape of the waveguide through which the visible light propagates. The third waveguide has an asymmetric shape in a second direction along the main surface intersecting with the first direction in which the mode converter extends. By the third waveguide having such a shape, a first mixed mode and a second mixed mode in which the TE mode and the TM mode are mixed may occur in the visible light propagating through the third waveguide. Then, the horizontal electric field component and the vertical electric field component in the first mixed mode are easily made uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are easily made uniform, similar to the first mixed mode. Therefore, the polarization mode of the visible light propagating through the third waveguide can be converted from the first polarization mode to the second polarization mode with high conversion efficiency. As described above, according to the optical element, the conversion efficiency of the polarization mode of the visible light can be improved.

[0010] The third waveguide may have a bottom surface facing the main surface, a top surface provided on the opposite side of the bottom surface in a third direction intersecting the first direction and the second direction, and an inclined surface connecting the top surface and the bottom surface. The third waveguide may have a columnar shape in which the length in the second direction increases continuously from the top surface to the bottom surface. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are more likely to be uniform compared to a case in which the length in the second direction of the third waveguide increases stepwise as it approaches the main surface. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are more likely to be uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the first polarization mode to the second polarization mode can be further improved.

[0011] The inclination angle between the inclined surface and the main surface may be 71° or more and 85° or less. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are more likely to be uniform than when the inclination angle is smaller than 71° or larger than 85°. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are more likely to be uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the first polarization mode to the second polarization mode can be further improved.

[0012] The third waveguide may have a stepped shape in which the length in the second direction increases stepwise toward the main surface. With this configuration, the horizontal and vertical electric field components in the first mixed mode are more likely to be uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal and vertical electric field components in the second mixed mode are more likely to be uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the first polarization mode to the second polarization mode can be improved.

[0013] The third waveguide has a bottom surface facing the main surface and a top surface provided on the opposite side to the bottom surface in a third direction intersecting the first and second directions, and a groove extending in the first direction and recessed toward the bottom surface may be provided on the top surface. With this configuration, it is possible to improve the conversion efficiency from the first polarization mode to the second polarization mode compared to a case in which the length of the third waveguide in the second direction increases stepwise toward the main surface.

[0014] The length in the second direction of the surface in contact with the main surface of the mode converter may be 32% or more and 48% or less of the wavelength of visible light. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are more likely to be uniform than when the length in the second direction is less than 32% or more than 48% of the wavelength of visible light. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are more likely to be uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the first polarization mode to the second polarization mode can be further improved.

[0015] The length of the mode converter in a third direction intersecting the first and second directions may be smaller than the wavelength of visible light. With this configuration, since the length in the third direction is smaller than the wavelength of visible light, the horizontal electric field component and the vertical electric field component in the first mixed mode are easily made uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are easily made uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the first polarization mode to the second polarization mode can be further improved.

[0016] The core layer may include a first mode converter that is a mode converter that converts the polarization mode of the red light from the first polarization mode to the second polarization mode, a second mode converter that is a mode converter that converts the polarization mode of the green light from the first polarization mode to the second polarization mode, a third mode converter that is a mode converter that converts the polarization mode of the blue light from the first polarization mode to the second polarization mode, and a multiplexer that multiplexes the red light, the green light, and the blue light and outputs a laser beam. According to this configuration, the polarization modes of the red light, the green light, and the blue light are converted from the first polarization mode to the second polarization mode. By using a polarization mode that has a high multiplexing efficiency in the multiplexer as the second polarization mode from the TM mode and the TE mode, it is possible to improve the multiplexing efficiency.

[0017] The first mode converter, the second mode converter, and the third mode converter may have the same length in a third direction intersecting the first and second directions. With this configuration, the first mode converter, the second mode converter, and the third mode converter can be formed on the same substrate, and the lengths of the respective mode converters in the third direction can be made the same, making it easier to manufacture the optical element.

[0018] The core layer may further include a first modulator for modulating the light intensity of the red light, a second modulator for modulating the light intensity of the green light, and a third modulator for modulating the light intensity of the blue light. In order to output full-color laser light by combining the red light, the green light, and the blue light, it is necessary to adjust the light intensity of each color of light according to the color to be output. According to the above configuration, the light intensity of the red light, the green light, and the blue light are modulated, so that it is possible to output full-color laser light.

[0019] A laser module according to another aspect of the present disclosure includes the above-mentioned optical element, a first laser light source that emits red light in the first polarization mode, a second laser light source that emits green light in the first polarization mode, and a third laser light source that emits blue light in the first polarization mode. Since this laser module includes the above-mentioned optical element, it is possible to improve the conversion efficiency of the polarization mode of visible light.

[0020] A near-eye wearable device according to yet another aspect of the present disclosure includes the above-mentioned laser module, a movable mirror that performs scanning using laser light emitted from the laser module, and a reflector that reflects the laser light that has passed through the movable mirror and guides it to the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina. Since the near-eye wearable device includes the above-mentioned optical element, it is possible to improve the conversion efficiency of the polarization mode of visible light and project an image onto the retina. Effect of the Invention

[0021] According to each aspect and embodiment of the present disclosure, it is possible to improve the efficiency of conversion of the polarization mode of visible light. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. [Diagram 2] FIG. 2 is a schematic diagram of the retinal projection device shown in FIG. [Diagram 3] FIG. 3 is a block diagram of the laser module shown in FIG. [Figure 4] FIG. 4 is a perspective view showing a configuration of the mode converter shown in FIG. [Diagram 5] FIG. 5 is a cross-sectional view taken along line VV in FIG. [Figure 6] FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. [Figure 7] FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. [Figure 8] FIG. 8 is a diagram for explaining the mode conversion operation of the mode converter shown in FIG. [Figure 9] FIG. 9 is a graph showing an example of the conversion efficiency in the mode converter shown in FIG. [Figure 10] Fig. 10(a) is a block diagram of a laser module according to another embodiment, and Fig. 10(b) is a block diagram of a laser module according to yet another embodiment. [Figure 11] FIG. 11 is a block diagram of a laser module according to yet another embodiment. [Figure 12] Fig. 12(a) is a cross-sectional view of a waveguide according to a first modified example, and Fig. 12(b) is a graph showing an example of conversion efficiency in a mode converter including the waveguide according to the first modified example. [Figure 13] Fig. 13(a) is a cross-sectional view of a waveguide according to a second modified example, and Fig. 13(b) is a graph showing an example of conversion efficiency in a mode converter including the waveguide according to the second modified example. [Figure 14] Fig. 14(a) is a cross-sectional view of a waveguide according to a third modified example, and Fig. 14(b) is a graph showing an example of conversion efficiency in a mode converter including the waveguide according to the third modified example. [Figure 15] Fig. 15(a) is a diagram showing the evaluation results of Example 1. Fig. 15(b) is a diagram showing the evaluation results of Example 2. Fig. 15(c) is a diagram showing the evaluation results of Example 3. [Figure 16] FIG. 16 is a diagram showing the evaluation results of Example 4. [Figure 17] Fig. 17(a) is a diagram showing the evaluation results of Example 5. Fig. 17(b) is a diagram showing the evaluation results of Example 6. Fig. 17(c) is a diagram showing the evaluation results of Example 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are given the same symbols, and duplicated descriptions are omitted. An XYZ coordinate system may be shown in each drawing. The Y-axis direction (second direction) is a direction that intersects (e.g., perpendicular to) the X-axis direction (first direction) and the Z-axis direction (third direction). The Z-axis direction is a direction that intersects (e.g., perpendicular to) the X-axis direction and the Y-axis direction. In this specification, a numerical range indicated using "~" indicates a range that includes the numerical values ​​written before and after "~" as the minimum and maximum values, respectively. Individually described upper and lower limit values ​​can be arbitrarily combined. [Embodiment]

[0024] An application example of a laser module according to an embodiment will be described with reference to FIG. 1. FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. The near-eye wearable device 1 shown in FIG. 1 is a device that projects an image onto the retina of a user wearing the near-eye wearable device 1. The near-eye wearable device 1 is, for example, a head-mounted device, and may take the form of glasses, goggles, hats, helmets, and the like. Examples of the near-eye wearable device 1 include smart glasses such as AR (Augmented Reality) glasses, VR (Virtual Reality) glasses, and MR (Mixed Reality) glasses. The near-eye wearable device 1 includes a frame 2, a lens 3, and a retinal projection device 10.

[0025] The frame 2 includes a pair of rims 2a, a bridge 2b, and a pair of temples 2c. The rims 2a are portions that hold the lenses 3. The bridge 2b is a portion that connects the pair of rims 2a. The temples 2c are portions that extend from the rims 2a and are hung on the ears of a user. The frame 2 may be a rimless frame. The lenses 3 include an inner surface 3a (see FIG. 2) that faces the eyeball of a user wearing the near eyewear wearable device 1.

[0026] The retinal projection device 10 is a device that directly projects (draws) an image onto the retina of a user wearing the near-eye wearable device 1. The retinal projection device 10 is mounted on the near-eye wearable device 1. In this embodiment, the near-eye wearable device 1 includes two retinal projection devices 10 in order to project images onto both the left and right retinas, but may include only one of the retinal projection devices 10.

[0027] Next, the retinal projection device 10 will be described in detail with reference to Fig. 2. Fig. 2 is a schematic diagram of the retinal projection device shown in Fig. 1. As shown in Fig. 2, the retinal projection device 10 includes an optical engine 20 and a reflector 30.

[0028] The optical engine 20 is a device that generates laser light Ls having a color and light intensity corresponding to the pixels of an image to be projected onto the retina, and emits the laser light Ls to the reflector 30. The optical engine 20 is mounted on each temple 2c. The optical engine 20 includes a laser module 4, an optical component 5, a movable mirror 6, a laser driver 7, a mirror driver 8, and a controller 9.

[0029] The laser module 4 emits laser light La, which is visible light. For example, a full-color laser module is used as the laser module 4. The laser module 4 emits laser light La of a color and light intensity corresponding to the pixels of an image to be projected onto the retina. The laser module 4 will be described in detail later.

[0030] The optical component 5 is a component that optically processes the laser light La emitted from the laser module 4. In this embodiment, the optical component 5 includes a collimator lens 5a, a slit 5b, and a neutral density filter 5c. The collimator lens 5a, the slit 5b, and the neutral density filter 5c are arranged in this order along the optical path of the laser light La. The optical component 5 may have other configurations.

[0031] The movable mirror 6 is a member for performing scanning with the laser light Ls. The movable mirror 6 is provided in the emission direction of the laser light La processed by the optical component 5. The movable mirror 6 is configured to be swingable, for example, around an axis extending in the horizontal direction of the lens 3 and an axis extending in the vertical direction of the lens 3, and reflects the laser light La while changing the angle in the horizontal and vertical directions of the lens 3 to emit the laser light Ls. As the movable mirror 6, for example, a MEMS (Micro Electro Mechanical Systems) mirror is used.

[0032] The laser driver 7 is a drive circuit that drives the laser module 4. The laser driver 7 drives the laser module 4 based on, for example, the optical power of the laser light La and the temperatures of the laser light sources 411, 412, and 413 included in the laser module 4. The mirror driver 8 is a drive circuit that drives the movable mirror 6. The mirror driver 8 swings the movable mirror 6 within a predetermined angle range and at a predetermined timing. The controller 9 is a device that controls the laser driver 7 and the mirror driver 8.

[0033] In the optical engine 20, laser light La having a color and light intensity corresponding to the pixels of an image to be projected onto the retina is emitted from the laser module 4, passes through the optical component 5, and is reflected by the movable mirror 6. The laser light La reflected by the movable mirror 6 is emitted to the reflector 30 as laser light Ls.

[0034] The reflector 30 is a member that reflects the laser light Ls that has passed through the movable mirror 6 and irradiates the retina of a user wearing the near eyewear wearable device 1 with reflected light Lrf, thereby projecting an image onto the retina.

[0035] Next, the configuration of the laser module 4 will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a block diagram of the laser module shown in Fig. 2. Fig. 4 is a perspective view showing the configuration of the mode converter shown in Fig. 3. Fig. 4 shows only the peripheral portion of the mode converter 42 of the optical element 40.

[0036] As shown in Fig. 3, the laser module 4 includes an optical element 40, a laser light source 411 that emits red light Lr, a laser light source 412 that emits green light Lg, and a laser light source 413 that emits blue light Lb. The laser light source 411 is, for example, a red laser diode. The laser light source 412 is, for example, a green laser diode. The laser light source 413 is, for example, a blue laser diode. The peak wavelength of the red light Lr is, for example, in the range of 600 nm to 830 nm. The peak wavelength of the green light Lg is, for example, in the range of 500 nm to 570 nm. The peak wavelength of the blue light Lb is, for example, in the range of 380 nm to 490 nm.

[0037] In this embodiment, the laser light source 411 emits red light Lr in TE mode. The laser light source 412 emits green light Lg in TE mode. The laser light source 413 emits blue light Lb in TE mode. Since the red light Lr, green light Lg, and blue light Lb are all visible light, in the following description, the red light Lr, green light Lg, and blue light Lb may be referred to as each visible light, and the red light Lr, green light Lg, and blue light Lb may be collectively referred to as visible light.

[0038] The optical element 40 combines the visible light emitted from each laser light source into one laser light La. The optical element 40 is, for example, a Planar Lightwave Circuit (PLC). As shown in Fig. 4, the optical element 40 includes a substrate S, a core layer C1, and a clad layer C2.

[0039] The substrate S functions as a lower cladding layer. The substrate S is made of a material having a lower refractive index than the material of the core layer C1. Examples of materials for the substrate S include sapphire, silicon, or aluminum oxide (Al2O3). The substrate S includes a main surface Sa and a back surface Sr opposite to the main surface Sa. The main surface Sa and the back surface Sr are planes defined by the X-axis direction and the Y-axis direction, and intersect with the Z-axis direction (in this embodiment, they are perpendicular). In other words, the X-axis direction and the Y-axis direction are directions along the main surface Sa.

[0040] The cladding layer C2 functions as an upper cladding layer. The cladding layer C2 covers the core layer C1 on the main surface Sa. The cladding layer C2 is made of a material having a lower refractive index than the material of the core layer C1. An example of the material of the cladding layer C2 is silicon oxide (e.g., SiO2).

[0041] The core layer C1 is provided on the main surface Sa. The core layer C1 is made of a material having an electro-optic effect. The electro-optic effect is a phenomenon in which the refractive index of a material changes when an electric field is applied to the material. An example of a material that can be used for the core layer C1 is lithium niobate (LiNbO3). In a material having an electro-optic effect, optical properties such as effective refractive index and modulation efficiency change for a TE (Transverse Electric) mode in which the main component of the electric field is in the in-plane direction of the optical waveguide layer, and a TM (Transverse Magnetic) mode in which the main component of the electric field is perpendicular to the in-plane direction (the main component of the magnetic field is in the in-plane direction).

[0042] In this embodiment, the core layer C1 is a lithium niobate thin film formed on the main surface Sa of the substrate S by sputtering, and the optical axis (C-axis) of the lithium niobate extends in the Z-axis direction. In this case, the modulation efficiency of each modulator is improved in the TM mode. Therefore, in this embodiment, visible light in the TE mode is emitted from each laser light source, and each visible light is converted to the TM mode in each mode converter and then incident on each modulator. The core layer C1 may be made of Z-cut lithium niobate.

[0043] The core layer C1 includes a mode converter 42, a mode converter 43, a mode converter 44, a modulator 45, a modulator 46, a modulator 47, and a multiplexer 48.

[0044] The mode converter 42 is a mode converter that converts the polarization mode of the red light Lr between the TM mode and the TE mode. In this embodiment, the mode converter 42 converts the polarization mode of the red light Lr from the TE mode (first polarization mode) to the TM mode (second polarization mode). The red light Lr is incident on the incident end of the mode converter 42 from the laser light source 411. The mode converter 43 is a mode converter that converts the polarization mode of the green light Lg between the TM mode and the TE mode. In this embodiment, the mode converter 43 converts the polarization mode of the green light Lg from the TE mode to the TM mode. The green light Lg is incident on the incident end of the mode converter 43 from the laser light source 412. The mode converter 44 is a mode converter that converts the polarization mode of the blue light Lb between the TM mode and the TE mode. In this embodiment, the mode converter 44 converts the polarization mode of the blue light Lb from the TE mode to the TM mode. The blue light Lb is incident on the incident end of the mode converter 44 from the laser light source 413.

[0045] Each of the mode converter 42, the mode converter 43, and the mode converter 44 extends in the X-axis direction. The mode converter 42, the mode converter 43, and the mode converter 44 are arranged in that order in the Y-axis direction. The lengths of the mode converter 42, the mode converter 43, and the mode converter 44 in the Z-axis direction are substantially equal. Hereinafter, the length in the Z-axis direction may be referred to as "height." The heights of the mode converter 42, the mode converter 43, and the mode converter 44 may be different. The detailed configuration of each mode converter will be described later.

[0046] The modulator 45 is a modulator that modulates the light intensity of the red light Lr. The input end of the modulator 45 is optically connected to the output end of the mode converter 42. The modulator 45 modulates the light intensity of the red light Lr in the TM mode output from the mode converter 42. The modulator 46 is a modulator that modulates the light intensity of the green light Lg. The input end of the modulator 46 is optically connected to the output end of the mode converter 43. The modulator 46 modulates the light intensity of the green light Lg in the TM mode output from the mode converter 43. The modulator 47 is a modulator that modulates the light intensity of the blue light Lb. The input end of the modulator 47 is optically connected to the output end of the mode converter 44. The modulator 47 modulates the light intensity of the blue light Lb in the TM mode output from the mode converter 44. Each modulator is, for example, a Mach-Zehnder type modulator.

[0047] The multiplexer 48 multiplexes the red light Lr, green light Lg, and blue light Lb modulated in each modulator into one visible light. The three input ends of the multiplexer 48 are optically connected to the output ends of the modulators 45, 46, and 47, respectively. The multiplexer 48 outputs the multiplexed visible light as laser light La from the output end of the multiplexer 48.

[0048] In the laser module 4, each laser light source emits visible light in TE mode, and after the polarization mode of the visible light is converted from TE mode to TM mode in each mode converter, the optical intensity of each visible light is modulated in each modulator. Then, each modulated visible light is multiplexed in the multiplexer 48, and is emitted from the multiplexer 48 to the optical component 5 (see FIG. 2) as laser light La in TM mode.

[0049] Next, specific configurations of the mode converter 42, the mode converter 43, and the mode converter 44 will be described with reference to Figs. 4 to 7. Fig. 5 is a cross-sectional view taken along line VV in Fig. 4. Fig. 6 is a cross-sectional view taken along line VI-VI in Fig. 4. Fig. 7 is a cross-sectional view taken along line VII-VII in Fig. 4. As shown in Fig. 4, each of the mode converter 42, the mode converter 43, and the mode converter 44 includes a waveguide 51, a waveguide 52, and a waveguide 53. Since each mode converter has the same configuration, the mode converter 42 will be described here as an example.

[0050] The waveguide 53 is provided between the waveguide 51 and the waveguide 52. Specifically, the waveguide 51, the waveguide 52, and the waveguide 53 are linearly arranged in the travelling direction (X-axis direction) of the red light Lr in the order of the waveguide 51, the waveguide 53, and the waveguide 52.

[0051] The waveguide 51 is an optical waveguide located at one end (incoming end) in the X-axis direction of the mode converter 42. The waveguide 51 has a columnar shape extending linearly in the X-axis direction. Specifically, the waveguide 51 has a rectangular parallelepiped shape with the X-axis direction as the longitudinal direction. The waveguide 51 includes an incoming end 51a which is one end in the X-axis direction, and an outgoing end 51b which is the other end in the X-axis direction. The red light Lr is incident on the incoming end 51a in the TE mode from the laser light source 411. The waveguide 51 transmits the red light Lr while maintaining the polarization mode of the red light Lr, and emits the red light Lr from the outgoing end 51b of the waveguide 51 to the waveguide 53 in the TE mode.

[0052] The waveguide 51 is symmetrical in the Z-axis direction and in the Y-axis direction. Symmetrical in the Z-axis direction means that two portions separated by the symmetry plane are plane-symmetrical with respect to a symmetry plane that passes through a center point in the Z-axis direction (center point CP in the cross section of the waveguide 51 shown in FIG. 5, in the case of the waveguide 51) and is perpendicular to the Z-axis direction. Symmetrical in the Y-axis direction means that two portions separated by the symmetry plane are plane-symmetrical with respect to a symmetry plane that passes through a center point in the Y-axis direction (center point CP in the case of the waveguide 51) and is perpendicular to the Y-axis direction.

[0053] As shown in FIG. 5, the waveguide 51 includes a bottom surface 51c facing the principal surface Sa, a top surface 51d provided on the opposite side of the bottom surface 51c in the Z-axis direction, and a pair of side surfaces 51e and 51f connecting the bottom surface 51c and the top surface 51d. The bottom surface 51c is in contact with the principal surface Sa over the entire surface. The bottom surface 51c and the top surface 51d are substantially parallel, and the side surfaces 51e and 51f are substantially parallel. The height and length in the Y-axis direction of the waveguide 51 are constant from the entrance end 51a to the exit end 51b. Hereinafter, the length in the Y-axis direction may be referred to as "width". The height of the waveguide 51 is height T1, and the width of the waveguide 51 is width W1. The height T1 is smaller than the wavelength of the red light Lr. The width W1 may be 20% to 60% of the wavelength of the red light Lr, or may be 32% to 48%.

[0054] The waveguide 53 is an optical waveguide that converts the polarization mode of the red light Lr between the TM mode and the TE mode. In this embodiment, the waveguide 53 converts the polarization mode of the red light Lr from the TE mode to the TM mode. The waveguide 53 has a columnar shape that extends linearly in the X-axis direction. Specifically, the waveguide 53 has a shape in which a corner formed by the top surface 51d and the side surface 51f of the waveguide 51 is missing. The waveguide 53 includes an input end 53a that is one end in the X-axis direction, and an output end 53b that is the other end in the X-axis direction.

[0055] The incident end 53a is connected to the output end 51b. The output end 53b is connected to the waveguide 52. The red light Lr is input to the incident end 53a in TE mode from the waveguide 51. The waveguide 53 converts the polarization mode of the red light Lr from TE mode to TM mode, and outputs the red light Lr in TM mode from the output end 53b to the waveguide 52. The length between the incident end 53a and the output end 53b is defined as a conversion length Lc. The details will be described later, but the conversion length Lc is a parameter that contributes to the conversion efficiency in converting the polarization mode of the red light Lr from TE mode to TM mode.

[0056] The waveguide 53 is asymmetric in the Z-axis direction and is asymmetric in the Y-axis direction. Being asymmetric in the Z-axis direction means that two portions separated by the symmetry plane are not plane-symmetric with respect to a symmetry plane that passes through a center point in the Z-axis direction (center point CP in the cross section of the waveguide 53 shown in FIG. 6, in the case of the waveguide 53) and is perpendicular to the Z-axis direction. Similarly, being asymmetric in the Y-axis direction means that two portions separated by the symmetry plane are not plane-symmetric with respect to a symmetry plane that passes through a center point in the Y-axis direction (center point CP in the case of the waveguide 53) and is perpendicular to the Y-axis direction.

[0057] 6, the waveguide 53 includes a bottom surface 53c facing the principal surface Sa, a top surface 53d provided on the opposite side of the bottom surface 53c in the Z-axis direction, a side surface 53e connecting the bottom surface 53c and the top surface 53d, and an inclined surface 53f connecting the bottom surface 53c and the top surface 53d. The bottom surface 53c is in contact with the principal surface Sa over the entire surface. The bottom surface 53c and the top surface 53d are substantially parallel to each other.

[0058] The angle between the side surface 53e and the bottom surface 53c (principal surface Sa) is a right angle. The inclined surface 53f is inclined so as to move away from the side surface 53e as it approaches the principal surface Sa. The inclination angle θ between the inclined surface 53f and the bottom surface 53c (principal surface Sa) is smaller than 90°. The inclination angle θ may be within a range of 71° to 83°, or may be within a range of 73° to 81°. That is, the cross-sectional shape of the waveguide 53 perpendicular to the X-axis direction is a right-angled trapezoid. The height, width at the bottom surface 53c, and width at the top surface 53d of the waveguide 53 are constant from the entrance end 53a to the exit end 53b. The height of the waveguide 53 is the same as the height of the waveguide 51, and is a height T1. The width of the waveguide 53 at the bottom surface 53c is the same as the width of the waveguide 51, and is a width W1. The width of the waveguide 53 at the top surface 53d is shorter than the width of the waveguide 53 at the bottom surface 53c, and is a width W2.

[0059] The waveguide 52 is an optical waveguide located at the other end (output end) of the mode converter 42 in the X-axis direction. The waveguide 52 has a columnar shape extending linearly in the X-axis direction. Specifically, the waveguide 52 has a rectangular parallelepiped shape with the X-axis direction as the longitudinal direction, and the cross-sectional shape of the waveguide 52 perpendicular to the X-axis direction is the same as the cross-sectional shape of the waveguide 51 perpendicular to the X-axis direction. The waveguide 52 includes an input end 52a which is one end in the X-axis direction, and an output end 52b which is the other end in the X-axis direction. The input end 52a is connected to an output end 53b. The output end 52b is connected to an input end of the modulator 45. The red light Lr is input from the waveguide 53 in the TM mode to the input end 52a. The waveguide 52 transmits the red light Lr while maintaining the polarization mode of the red light Lr, and outputs the red light Lr to the modulator 45 from the output end 52b in the TM mode.

[0060] The waveguide 52 is symmetrical in the Z-axis direction and in the Y-axis direction. As shown in FIG. 7, the waveguide 52 includes a bottom surface 52c facing the main surface Sa, a top surface 52d provided on the opposite side of the bottom surface 52c in the Z-axis direction, and a pair of side surfaces 52e and 52f connecting the bottom surface 52c and the top surface 52d. The bottom surface 52c is in contact with the main surface Sa over the entire surface. The bottom surface 52c and the top surface 52d are substantially parallel, and the side surface 52e and the side surface 52f are substantially parallel. The height and width of the waveguide 52 are constant from the input end 52a to the output end 52b. The height of the waveguide 52 is the same as the height of the waveguide 51, and is a height T1. The width of the waveguide 52 is the same as the width of the waveguide 51, and is a width W1.

[0061] The bottom surface 51c, the bottom surface 53c, and the bottom surface 52c are connected in that order and are located on the same plane. The side surface 51e, the side surface 53e, and the side surface 52e are connected in that order and are located on the same plane. The top surface 51d, the top surface 53d, and the top surface 52d are connected in that order and are located on the same plane.

[0062] The above-mentioned configuration is the same for the mode converter 43 and the mode converter 44, but the optimal dimensions may differ for each of the mode converter 42, the mode converter 43, and the mode converter 44. The optimal dimensions here are optimal dimensions for maximizing the conversion efficiency of the polarization mode of each visible light. The inclination angle θ in the mode converter 42 may be in the range of 71° to 83°, or may be in the range of 73° to 81°. The inclination angle θ in the mode converter 43 may be in the range of 74° to 85°, or may be in the range of 77° to 83°. The inclination angle θ in the mode converter 44 may be in the range of 74° to 85°, or may be in the range of 77° to 83°. Therefore, the maximum range of the inclination angle θ in the mode converter 42, the mode converter 43, and the mode converter 44 is 71° to 85°.

[0063] The width (width W1) of the surface (bottom surface 51c, 52c, 53c) in contact with the main surface Sa of each mode converter may be 20% to 60%, or may be 32% to 48%, of the wavelength of the visible light propagating through each mode converter. The height T1 is smaller than the wavelength of the visible light propagating through each mode converter. Specifically, the height T1 may be 1 μm or less, or may be 0.6 μm or less.

[0064] Next, the operations of mode converters 42, 43, and 44 will be described with reference to Figs. 8 and 9. Fig. 8 is a diagram for explaining the mode conversion operation of the mode converters. Fig. 9 is a graph showing an example of the conversion efficiency in the mode converter shown in Fig. 4. The horizontal axis of the graph in Fig. 9 represents the conversion length Lc [μm], and the vertical axis represents the conversion efficiency. Since the operation of each mode converter is the same, mode converter 43 will be described here as an example.

[0065] 8, mode converter 43 inputs green light Lg in TE mode into waveguide 51, converts the polarization mode of green light Lg from TE mode to TM mode in waveguide 53, and outputs green light Lg in TM mode from waveguide 52. In this example, the wavelength of green light Lg is 520 nm, width W1 is set to 0.20 μm, width W2 is set to 0.15 μm, and height T1 is set to 0.30 μm.

[0066] 8, in the waveguide 51, the polarization mode of the green light Lg is the TE mode, and therefore the vector of the electric field component is parallel to the Y-axis direction. When the green light Lg is incident on the waveguide 53, two mixed modes (a first mixed mode and a second mixed mode) in which the TE mode and the TM mode are mixed are excited as the polarization mode of the green light Lg, due to the fact that the waveguide 53 is asymmetric in the Y-axis direction and also in the Z-axis direction. At this time, the horizontal electric field component in the first mixed mode and the horizontal electric field component in the second mixed mode change depending on the shape and dimensions of the waveguide 53.

[0067] In this embodiment, the horizontal and vertical electric field components in the first mixture mode are easily made uniform. Since the second mixture mode is orthogonal to the first mixture mode, the horizontal and vertical electric field components in the second mixture mode are easily made uniform, similar to the first mixture mode. For example, the horizontal electric field component in the first mixture mode is present at 50% of the entire field of the first mixture mode. Similarly, the horizontal electric field component in the second mixture mode is present at 50% of the entire field of the second mixture mode. In this case, the electric field vector in the first mixture mode is rotated 45° from the Y-axis direction. The electric field vector in the second mixture mode is rotated 45° from the Z-axis direction. The electric field vector in the first mixture mode is orthogonal to the electric field vector in the second mixture mode.

[0068] In the waveguide 53, because there is a difference between the propagation constant β1 of the first mixed mode and the propagation constant β2 of the second mixed mode, a phase difference occurs between the phase of the first mixed mode and the phase of the second mixed mode depending on the length of propagation of the green light Lg through the waveguide 53. When this phase difference becomes π×(2n+1) (n is an integer equal to or greater than 0), the first mixed mode and the second mixed mode are combined into one mode when the green light Lg propagates from the output end 53b of the waveguide 53 to the waveguide 52, and the polarization mode of the green light Lg is rotated by 90° from the TE mode to be converted into the TM mode.

[0069] The conversion efficiency (CE) of a mode converter is determined by the rotation angle φ, conversion length Lc, and length L π The tangent of the rotation angle φ is expressed by equation (2) using the permittivity distribution ε(y,z), the horizontal mixed mode electric field component Ey(y,z), and the vertical mixed mode electric field component Ez(y,z).

number

number

[0070] Here, the rotation angle φ is the rotation angle between the optical axis of the mixed mode and a plane parallel to the main surface Sa of the substrate S. π is the length at which the phase difference between the first mixed mode and the second mixed mode becomes π. The formula for deriving the conversion length Lc will be described later. From formula (1), a conversion efficiency close to 100% can be obtained when the rotation angle φ is 45°. From formula (2), when the horizontal electric field component Ey(y,z) and the vertical electric field component Ez(y,z) in the first mixed mode and the second mixed mode are uniform, the rotation angle φ is 45°. From the above, when the horizontal electric field component Ey(y,z) and the vertical electric field component Ez(y,z) in the first mixed mode and the second mixed mode are uniform, a conversion efficiency close to 100% can be obtained.

[0071] As shown in FIG. 9, the conversion efficiency CE1 indicates the conversion efficiency when the polarization mode of the green light Lg incident as the TE mode is converted to the TM mode. The conversion efficiency CE2 indicates the conversion efficiency when the polarization mode of the green light Lg incident as the TE mode is output as the TE mode. The conversion efficiency CE1 and the conversion efficiency CE2 oscillate periodically as the conversion length Lc increases, and the maximum value of the conversion efficiency CE1 and the maximum value of the conversion efficiency CE2 appear alternately every half period. Both maximum values ​​exceed 0.95, and it can be seen that high conversion efficiency has been achieved. In this embodiment, since the waveguide 53 converts the green light Lg of the TE mode to the green light Lg of the TM mode, the conversion length Lc is set to a length at which the conversion efficiency CE1 has a maximum value. The conversion length Lc is calculated by the formula (3).

number

[0072] n eff1 is the effective refractive index of the first mixed mode, and n eff2 denotes the effective refractive index of the second mixed mode, and k0 denotes the vacuum wave number. In this example, the conversion length Lc is calculated to be 17.44 μm by equation (3). As shown in FIG. 9, the conversion efficiency when converting from the TE mode to the TM mode when the conversion length Lc is 17.44 μm is 0.97.

[0073] In the laser module 4 and the optical element 40 described above, the core layer C1 is made of a material having an electro-optic effect, so that optical characteristics such as the effective refractive index change according to the shape of the waveguide through which the visible light propagates. The waveguide 53 of each mode converter has an asymmetric shape in the Y-axis direction. When the waveguide 53 has such a shape, a first mixed mode and a second mixed mode in which the TE mode and the TM mode are mixed may occur in the visible light propagating through the waveguide 53. Then, the horizontal electric field component and the vertical electric field component in the first mixed mode tend to be uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode tend to be uniform, similar to the first mixed mode. Therefore, the polarization mode of the visible light propagating through the waveguide 53 can be converted from the TE mode to the TM mode with high conversion efficiency. As described above, the optical element 40 can improve the conversion efficiency of the polarization mode of the visible light.

[0074] The waveguide 53 has a columnar shape in which the length in the Y-axis direction increases continuously from the top surface 53d to the bottom surface 53c. With this configuration, the horizontal and vertical electric field components in the first mixed mode are more likely to be uniform than when the length in the Y-axis direction of the waveguide 53 increases stepwise toward the main surface Sa (waveguide 54 and waveguide 54A described later). Since the second mixed mode is orthogonal to the first mixed mode, the horizontal and vertical electric field components in the second mixed mode are more likely to be uniform, similar to the first mixed mode. Therefore, the conversion efficiency from the TE mode to the TM mode can be further improved.

[0075] In addition, since the waveguide 53 has the inclined surface 53f, the waveguide 53 can be made asymmetric in the Y-axis direction while minimizing the reduction in the cross-sectional area of ​​the waveguide 53. This prevents the cross-sectional area of ​​the waveguide 53 from becoming extremely small compared to the cross-sectional areas of the waveguides 51 and 52 connected to the waveguide 53. This makes it possible to minimize the difference between the mode field diameter of the mode propagating through the waveguides 51 and 52 and the mode field diameter of the mixed mode propagating through the waveguide 53 at the connection portion between the waveguide 51 and the waveguide 53 and the connection portion between the waveguide 53 and the waveguide 52. This makes it possible to minimize the coupling loss at the connection portion, thereby improving the conversion efficiency.

[0076] The inclination angle θ is 71° or more and 85° or less. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are more likely to be uniform than when the inclination angle θ is smaller than 71° or larger than 85°. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are more likely to be uniform, as in the first mixed mode. Therefore, the conversion efficiency from the TE mode to the TM mode can be further improved. In addition, since the inclination angle θ is 71° or more and 85° or less, the confinement of visible light in the core layer C1 is weakened, and visible light is more likely to leak to the substrate S and the clad layer C2. In this state, since the waveguide 53 has an asymmetric shape in the Y-axis direction, the optical axis rotation of the polarization mode of visible light is more likely to occur, and as a result, the conversion efficiency can be improved.

[0077] The length (width W1) in the Y-axis direction of the surface in contact with the principal surface Sa of each mode converter is 32% or more and 48% or less of the wavelength of the visible light. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are more likely to be uniform than when the width W1 is smaller than 32% or larger than 48% of the wavelength of the visible light. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are more likely to be uniform, as in the first mixed mode. Therefore, the conversion efficiency from the TE mode to the TM mode can be further improved. In addition, since the width W1 is 32% or more and 48% or less of the wavelength of the visible light, the confinement of the visible light in the core layer C1 is weakened, and the visible light is more likely to leak to the substrate S and the cladding layer C2. In this state, since the waveguide 53 has an asymmetric shape in the Y-axis direction, the optical axis rotation of the polarization mode of the visible light is more likely to occur, and as a result, the conversion efficiency can be improved.

[0078] The length (height T1) of each mode converter in the Z-axis direction is smaller than the wavelength of visible light. With this configuration, the height T1 is smaller than the wavelength of visible light, so that the horizontal electric field component and the vertical electric field component in the first mixed mode are easily made uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are easily made uniform, as in the first mixed mode. Therefore, the conversion efficiency from the TE mode to the TM mode can be further improved. In addition, since the height T1 is smaller than the wavelength of visible light, the confinement of visible light in the core layer C1 is weakened, and the visible light is easily leaked to the substrate S and the clad layer C2. In this state, since the waveguide 53 has an asymmetric shape in the Y-axis direction, the optical axis rotation of the polarization mode of visible light is easily caused, and as a result, the conversion efficiency can be improved.

[0079] The multiplexer 48 is designed so that the multiplexing efficiency when multiplexing red light, green light, and blue light in TM mode is higher than the multiplexing efficiency when multiplexing red light, green light, and blue light in TE mode. In the laser module 4 and the optical element 40, the mode converter 42 converts the polarization mode of the red light Lr from TE mode to TM mode, the mode converter 43 converts the polarization mode of the green light Lg from TE mode to TM mode, and the mode converter 44 converts the polarization mode of the blue light Lb from TE mode to TM mode. Therefore, it is possible to improve the multiplexing efficiency in the multiplexer 48.

[0080] The height of the mode converter 42, the height of the mode converter 43, and the height of the mode converter 44 are the same. According to this configuration, the mode converter 42, the mode converter 43, and the mode converter 44 can be formed on the same substrate S, and the respective heights can be made the same, which makes it easier to manufacture the optical element 40.

[0081] In order to output full-color laser light La by combining red light Lr, green light Lg, and blue light Lb, it is necessary to adjust the light intensity of each color of light according to the color to be output. In order to change the light intensity of each visible light in each laser light source, a large drive current is required. In the laser module 4 and the optical element 40, the light intensity of the red light Lr is modulated by the modulator 45, the light intensity of the green light Lg is modulated by the modulator 46, and the light intensity of the blue light Lb is modulated by the modulator 47. Therefore, it is possible to output full-color laser light La without requiring a large drive current.

[0082] Since the near-eye wearable device 1 includes the optical element 40, it is possible to improve the efficiency of conversion of the polarization mode of visible light and project an image onto the retina.

[0083] The laser module 4 (optical element 40) may emit laser light La in TE mode. For example, when visible light in TM mode is emitted from each laser light source, each visible light is converted to TE mode in each mode converter. Specifically, the laser light source 411 emits red light Lr in TM mode, the laser light source 412 emits green light Lg in TM mode, and the laser light source 413 emits blue light Lb in TM mode. The mode converter 42 converts the polarization mode of the red light Lr from TM mode (first polarization mode) to TE mode (second polarization mode). The mode converter 43 converts the polarization mode of the green light Lg from TM mode to TE mode. The mode converter 44 converts the polarization mode of the blue light Lb from TM mode to TE mode. In this case, visible light in TE mode is incident on each modulator. In order to improve the modulation efficiency in each modulator, the core layer C1 may be made of X-cut lithium niobate, and the optical axis (C-axis) of the lithium niobate may extend in the Y-axis direction.

[0084] The input end and output end of each mode converter may be interchanged. That is, the waveguide 52 may be an optical waveguide located at one end (input end) in the X-axis direction of each mode converter, and the waveguide 51 may be an optical waveguide located at the other end (output end) in the X-axis direction of each mode converter. Even in this configuration, the laser module 4 (optical element 40) may emit laser light La in TM mode or may emit laser light La in TE mode. For example, when visible light in TE mode is emitted from each laser light source, the visible light is converted to TM mode in each mode converter, and when visible light in TM mode is emitted from each laser light source, the visible light is converted to TE mode in each mode converter.

[0085] Next, a laser module according to another embodiment will be described with reference to Fig. 10(a). Fig. 10(a) is a block diagram of a laser module according to another embodiment. A laser module 4A shown in Fig. 10(a) emits a laser beam La in a TE mode. The laser module 4A differs from the laser module 4 mainly in the polarization mode of the visible light emitted by the laser light sources 411, 412, and 413, and in that the positions of the modulators 45, 46, and 47 and the mode converters 42, 43, and 44 are interchanged.

[0086] The laser light source 411 emits red light Lr in TM mode. The laser light source 412 emits green light Lg in TM mode. The laser light source 413 emits blue light Lb in TM mode. The red light Lr is incident on the incident end of the modulator 45 from the laser light source 411. The green light Lg is incident on the incident end of the modulator 46 from the laser light source 412. The blue light Lb is incident on the incident end of the modulator 47 from the laser light source 413. The output end of the modulator 45 is optically connected to the input end of the mode converter 42. The output end of the modulator 46 is optically connected to the input end of the mode converter 43. The output end of the modulator 47 is optically connected to the input end of the mode converter 44. The output end of the mode converter 42, the output end of the mode converter 43, and the output end of the mode converter 44 are optically connected to three input ends of the multiplexer 48, respectively.

[0087] The mode converter 42 converts the polarization mode of the modulated red light Lr from TM mode (first polarization mode) to TE mode (second polarization mode). The mode converter 43 converts the polarization mode of the modulated green light Lg from TM mode to TE mode. The mode converter 44 converts the polarization mode of the modulated blue light Lb from TM mode to TE mode.

[0088] As described above, the core layer C1 is a lithium niobate thin film formed on the main surface Sa of the substrate S by sputtering, and the C-axis of the lithium niobate extends in the Z-axis direction. Therefore, the modulation efficiency of each modulator is improved in the TM mode. In the laser module 4A, since each laser light source emits visible light in the TM mode, the light intensity of the visible light in the TM mode is modulated in each modulator, and then the polarization mode is converted from the TM mode to the TE mode in each mode converter. Then, each visible light whose polarization mode has been converted is multiplexed in the multiplexer 48, and is output from the multiplexer 48 to the optical component 5 (see FIG. 2) as the laser light La in the TE mode.

[0089] A laser module according to still another embodiment will be described with reference to Fig. 10(b). Fig. 10(b) is a block diagram of a laser module according to still another embodiment. The laser module 4B shown in Fig. 10(b) is mainly different from the laser module 4A in that it includes one mode converter 49 instead of the mode converters 42, 43, and 44, and that a multiplexer 48 is disposed between each modulator and the mode converter 49.

[0090] Specifically, the output end of modulator 45, the output end of modulator 46, and the output end of modulator 47 are optically connected to three input ends of multiplexer 48. The output end of multiplexer 48 is optically connected to an input end of mode converter 49. The configuration of mode converter 49 is the same as the configuration of mode converter 42.

[0091] As in the laser module 4A, in the laser module 4B, each laser light source emits visible light in TM mode, and the optical intensity of the visible light in TM mode is modulated in each modulator. Then, the visible light modulated in each modulator is multiplexed in a multiplexer 48, and the polarization mode of the multiplexed visible light is converted from TM mode to TE mode in a mode converter 49, and the laser light La in TE mode is output from the mode converter 49 to the optical component 5 (see FIG. 2).

[0092] Next, a laser module according to still another embodiment will be described with reference to Fig. 11. Fig. 11 is a block diagram of a laser module according to still another embodiment. A laser module 4C shown in Fig. 11 emits a laser beam La in a TE mode. The laser module 4C differs from the laser module 4 mainly in that the laser module 4C further includes a mode converter 42A, a mode converter 43A, and a mode converter 44A.

[0093] The mode converter 42A is provided between the modulator 45 and the multiplexer 48. The input end of the mode converter 42A is optically connected to the output end of the modulator 45, and the output end of the mode converter 42A is optically connected to the input end of the multiplexer 48. The mode converter 42A converts the polarization mode of the modulated red light Lr from the TM mode to the TE mode.

[0094] The mode converter 43A is provided between the modulator 46 and the multiplexer 48. An input end of the mode converter 43A is optically connected to an output end of the modulator 46, and an output end of the mode converter 43A is optically connected to another input end of the multiplexer 48. The mode converter 43A converts the polarization mode of the modulated green light Lg from the TM mode to the TE mode.

[0095] The mode converter 44A is provided between the modulator 47 and the multiplexer 48. An input end of the mode converter 44A is optically connected to an output end of the modulator 47, and an output end of the mode converter 44A is optically connected to another input end of the multiplexer 48. The mode converter 44A converts the polarization mode of the modulated blue light Lb from the TM mode to the TE mode.

[0096] In the laser module 4C, each laser light source emits visible light in TE mode, so that the polarization mode of each visible light emitted from each laser light source is converted from TE mode to TM mode in each mode converter, and then the light intensity of the visible light in TM mode is modulated in each modulator. Then, the polarization mode of each modulated visible light is converted from TM mode to TE mode in each mode converter. Then, each visible light is multiplexed in the multiplexer 48, and is emitted from the multiplexer 48 to the optical component 5 (see FIG. 2) as laser light La in TE mode.

[0097] In the laser modules 4A, 4B, and 4C, the core layer C1 may be made of X-cut lithium niobate, and the optical axis (C-axis) of the lithium niobate may extend in the Y-axis direction. In this case, the modulation efficiency of each modulator is improved in TE mode. In the laser modules 4A and 4B, visible light may be emitted from each laser light source in TE mode, and the optical intensity of the visible light in TE mode may be modulated in each modulator. In the laser module 4C, visible light may be emitted from each laser light source in TM mode, and after being converted to TE mode in each mode converter, the optical intensity of the visible light in TE mode may be modulated in each modulator. [Variations]

[0098] Although the embodiments of the present disclosure have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit and scope of the present disclosure.

[0099] The shape of the waveguide 53 is not limited to the shapes shown in Fig. 4 and Fig. 6. For example, a waveguide 54 having a cross section shown in Fig. 12(a) may be used instead of the waveguide 53. The waveguide 54 differs from the waveguide 53 mainly in its shape. The waveguide 54 has a two-step staircase shape in which the length in the Y-axis direction increases stepwise toward the main surface Sa. The waveguide 54 is asymmetric in the Z-axis direction and asymmetric in the Y-axis direction, similar to the waveguide 53.

[0100] Specifically, the waveguide 54 includes a bottom surface 54c, a top surface 54d, a side surface 54e, a step surface 54f, a step surface 54g, and a step surface 54h. The bottom surface 54c faces the main surface Sa and is in contact with the main surface Sa over the entire surface. The top surface 54d is provided on the opposite side of the bottom surface 54c in the Z-axis direction and is substantially parallel to the bottom surface 54c. The side surface 54e is a surface connecting one end of the bottom surface 54c to one end of the top surface 54d. The step surface 54g is provided at a position closer to the main surface Sa than the top surface 54d and is substantially parallel to the top surface 54d. The step surface 54f is a surface connecting the other end of the top surface 54d to one end of the step surface 54g. The step surface 54h is a surface connecting the other end of the step surface 54g to the other end of the bottom surface 54c.

[0101] Each of the side surface 54e, the step surface 54f, and the step surface 54h is provided to form an angle of substantially 90° with respect to the bottom surface 54c (main surface Sa). The top surface 54d and the step surface 54f form a step shape of one step. The step surface 54g and the step surface 54h form another step shape of one step. The combined length of the width of the top surface 54d and the width of the step surface 54g is equal to the width of the bottom surface 54c.

[0102] In the waveguide 54, for example, the horizontal electric field component in the first mixed mode accounts for about 45% of the entire field of the first mixed mode, and the horizontal electric field component in the second mixed mode accounts for about 55% of the entire field of the second mixed mode.

[0103] For example, a description will be given using a waveguide 54 that converts the polarization mode of the red light Lr from TE mode to TM mode. When the wavelength of the red light Lr is 638 nm, the height of the side surface 54e is set to 560 nm, the width of the bottom surface 54c is set to 280 nm, the height of the step surface 54f is set to 380 nm, and the width of the step surface 54g is set to 90 nm, the conversion efficiency CE1 and the conversion efficiency CE2 shown in FIG. 12(b) are obtained. As shown in FIG. 12(b), the maximum value of the conversion efficiency CE1 is about 0.75, and the maximum value of the conversion efficiency CE2 is about 0.9. In this case, the conversion length Lc is calculated to be 20.5 μm by formula (3). When the conversion length Lc is 20.5 μm, the conversion efficiency when converting from the TE mode to the TM mode is 0.74.

[0104] 13(a) may be used instead of the waveguide 54. The waveguide 54A has a three-step staircase shape in which the length in the Y-axis direction increases stepwise toward the main surface Sa. In other words, the waveguide 54A has a shape in which one step of the staircase shape is added to the staircase shape included in the waveguide 54. The waveguide 54A is asymmetric in the Z-axis direction and asymmetric in the Y-axis direction, similar to the waveguides 53 and 54.

[0105] Specifically, the waveguide 54A further includes a step surface 54i and a step surface 54j in addition to the surfaces of the waveguide 54. The step surface 54i is provided closer to the main surface Sa than the step surface 54g, and is a surface that is substantially parallel to the top surface 54d. The step surface 54j is a surface that connects the other end of the step surface 54i and the other end of the bottom surface 54c. In the waveguide 54A, the step surface 54h connects the other end of the step surface 54g and one end of the step surface 54i. The step surface 54j is provided so as to form an angle of substantially 90° with the bottom surface 54c (main surface Sa). The step surface 54i and the step surface 54j form a step shape for another step. The combined length of the width of the top surface 54d, the width of the step surface 54g, and the width of the step surface 54i is equal to the width of the bottom surface 54c.

[0106] In the waveguide 54A, for example, the horizontal electric field component in the first mixed mode accounts for about 57% of the entire field of the first mixed mode, and the horizontal electric field component in the second mixed mode accounts for about 55% of the entire field of the second mixed mode.

[0107] For example, a description will be given using a waveguide 54A that converts the polarization mode of the red light Lr from TE mode to TM mode. When the wavelength of the red light Lr is 638 nm, the height of the side surface 54e is set to 560 nm, the width of the bottom surface 54c is set to 280 nm, the height of the step surface 54f is set to 150 nm, the width of the step surface 54g is set to 120 nm, the height of the step surface 54h is set to 260 nm, and the width of the step surface 54i is set to 80 nm, the conversion efficiency CE1 and the conversion efficiency CE2 shown in FIG. 13(b) are obtained. As shown in FIG. 13(b), the maximum value of the conversion efficiency CE1 is about 0.6, and the maximum value of the conversion efficiency CE2 is about 0.55. In this case, the conversion length Lc is calculated to be 16.0 μm by formula (3). When the conversion length Lc is 16.0 μm, the conversion efficiency when converting from the TE mode to the TM mode is 0.61.

[0108] The mode converter including the above-mentioned waveguide 54 or waveguide 54A can convert the polarization mode of visible light from TE mode to TM mode. With this configuration, the horizontal electric field component and the vertical electric field component in the first mixed mode are easily made uniform. Since the second mixed mode is orthogonal to the first mixed mode, the horizontal electric field component and the vertical electric field component in the second mixed mode are easily made uniform, similar to the first mixed mode. Therefore, the conversion efficiency from TE mode to TM mode can be improved.

[0109] 14(a) may be used instead of the waveguide 53. The waveguide 55 differs from the waveguide 53 mainly in its shape. Like the waveguide 53, the waveguide 55 is asymmetric in the Z-axis direction and is asymmetric in the Y-axis direction.

[0110] Specifically, the waveguide 55 includes a bottom surface 55c, a top surface 55d, and a pair of side surfaces 55e, 55f connecting the bottom surface 55c and the top surface 55d. The bottom surface 55c faces the main surface Sa and is in contact with the main surface Sa over its entirety. The top surface 55d is provided on the opposite side to the bottom surface 55c in the Z-axis direction and is substantially parallel to the bottom surface 55c. The pair of side surfaces 55e, 55f connect the bottom surface 55c and the top surface 55d and are substantially parallel to each other.

[0111] The top surface 55d is provided with grooves Cv1 and Cv2 that extend in the X-axis direction and are recessed toward the bottom surface 55c. Grooves Cv1 and Cv2 are arranged in that order from side surface 55e toward side surface 55f. The cross-sectional shapes of grooves Cv1 and Cv2 that intersect (are perpendicular to) the X-axis direction are rectangular. The length (depth) of groove Cv1 in the Z-axis direction is shorter than the length (depth) of groove Cv2 in the Z-axis direction.

[0112] In the waveguide 55, for example, the horizontal electric field component in the first mixed mode accounts for about 40% of the entire field of the first mixed mode, and the horizontal electric field component in the second mixed mode accounts for about 60% of the entire field of the second mixed mode.

[0113] For example, a waveguide 55 that converts the polarization mode of red light Lr from TE mode to TM mode will be used for explanation. When the wavelength of red light Lr is 638 nm, the width of the bottom surface 55c is 280 nm, the distance between the side surface 55e and the groove Cv1 is 170 nm, the distance between the groove Cv1 and the groove Cv2 is 40 nm, the width of the groove Cv1 is 20 nm, the width of the groove Cv2 is 20 nm, the height of the side surface 55e is 400 nm, the depth of the groove Cv1 is 250 nm, and the depth of the groove Cv2 is 300 nm, the conversion efficiency CE1 and the conversion efficiency CE2 shown in FIG. 14(b) are obtained. As shown in FIG. 14(b), the maximum value of the conversion efficiency CE1 is about 0.9, and the maximum value of the conversion efficiency CE2 is about 0.95. In this case, the conversion length Lc is calculated to be 33.5 μm by the formula (3). When the conversion length Lc is 33.5 μm, the conversion efficiency when converting from the TE mode to the TM mode is 0.87.

[0114] The mode converter including the above-mentioned waveguide 55 can improve the conversion efficiency of the polarization mode of visible light compared to the mode converter including the waveguide 54 or the waveguide 54A. In addition, since the grooves Cv1 and Cv2 are provided in the waveguide 55, the waveguide 55 can be made asymmetric in the Y-axis direction while suppressing a decrease in the cross-sectional area of ​​the waveguide 55 compared to the waveguide 54 and the waveguide 54A. Therefore, the cross-sectional area of ​​the waveguide 55 is prevented from becoming extremely small compared to the cross-sectional areas of the waveguides 51 and 52 connected to the waveguide 55. This makes it possible to minimize the difference between the mode field diameter of the mode propagating through the waveguide 51 and the waveguide 52 and the mode field diameter of the mixed mode propagating through the waveguide 55 at the connection portion between the waveguide 51 and the waveguide 55 and the connection portion between the waveguide 55 and the waveguide 52. Therefore, it is possible to minimize the coupling loss at the connection portion, and as a result, it is possible to improve the conversion efficiency. [Example]

[0115] In the following, the present disclosure will be described in more detail with reference to examples in order to explain the above-mentioned effects, but the present disclosure is not limited to these examples.

[0116] <Conversion loss evaluation for the ratio of width W1 to visible light wavelength λ> The effect of the ratio of the width W1 to the wavelength λ of visible light on the conversion loss was evaluated. For each of red light, green light, and blue light, the value of the width W1 was changed using a mode converter having the same structure as the mode converter 42 shown in FIG. 3, and the maximum conversion efficiency at each value of the width W1 was calculated. The height T1 was set to 0.3 μm, the wavelength λ of red light was set to 638 nm, the wavelength λ of green light was set to 520 nm, and the wavelength λ of blue light was set to 455 nm, and the maximum conversion efficiency was calculated when the polarization mode of visible light was converted from the TE fundamental mode to the TM fundamental mode. The conversion efficiency represents the optical intensity of the visible light in the TM fundamental mode when the optical intensity of the visible light in the TE fundamental mode is set to 1. The conversion loss [dB] is obtained by converting the maximum conversion efficiency into dB units.

[0117] The calculation results for red light are shown in Table 1 and Fig. 15(a). In addition to the width W1, maximum conversion efficiency, and conversion loss, Table 1 also shows the width W2 [μm] and conversion length Lc [μm] when the maximum conversion efficiency is obtained for each value of width W1, and the ratio W1 / λ [%]. The horizontal axis of Fig. 15(a) represents the ratio W1 / λ [%], and the vertical axis of Fig. 15(a) represents the conversion loss [dB]. It is said that high conversion efficiency is achieved when the maximum conversion efficiency exceeds 0.60 or the conversion loss is below 2.2 dB.

[0118] [Table 1]

[0119] According to Table 1 and FIG. 15(a), when the ratio W1 / λ is in the range of 34% to 47%, the maximum conversion efficiency exceeds 0.60, and the conversion loss is below 2.2 dB. According to the change (slope) of the conversion loss per unit ratio W1 / λ when the ratio W1 / λ changes from 31% to 34%, it can be inferred that the conversion loss at the ratio W1 / λ of 32% is below 2.2 dB. Similarly, according to the change (slope) of the conversion loss per unit ratio W1 / λ when the ratio W1 / λ changes from 47% to 50%, it can be inferred that the conversion loss at the ratio W1 / λ of 48% is below 2.2 dB. From the above, it can be said that high conversion efficiency is realized when the ratio W1 / λ is in the range of 32% to 48%.

[0120] The calculation results for green light are shown in Table 2 and Fig. 15(b). In addition to the width W1, maximum conversion efficiency, and conversion loss, Table 2 shows the width W2 [μm] and conversion length Lc [μm] when the maximum conversion efficiency is obtained for each value of width W1, and the ratio W1 / λ [%]. The horizontal axis of Fig. 15(b) represents the ratio W1 / λ [%], and the vertical axis of Fig. 15(b) represents the conversion loss [dB]. [Table 2]

[0121] 15(b), when the ratio W1 / λ is in the range of 38% to 48%, the maximum conversion efficiency exceeds 0.60 and the conversion loss is below 2.2 dB. Therefore, it can be said that high conversion efficiency is achieved when the ratio W1 / λ is in the range of 38% to 48%.

[0122] The calculation results for blue light are shown in Table 3 and Fig. 15(c). In addition to the width W1, maximum conversion efficiency, and conversion loss, Table 3 shows the width W2 [μm] and conversion length Lc [μm] when the maximum conversion efficiency is obtained for each value of width W1, and the ratio W1 / λ [%]. The horizontal axis of Fig. 15(c) represents the ratio W1 / λ [%], and the vertical axis of Fig. 15(c) represents the conversion loss [dB]. [Table 3]

[0123] 15(c), when the ratio W1 / λ is in the range of 38% to 44%, the maximum conversion efficiency exceeds 0.60 and the conversion loss is below 2.2 dB. Therefore, it can be said that high conversion efficiency is achieved when the ratio W1 / λ is in the range of 38% to 44%.

[0124] <Conversion loss evaluation at height T1> The effect of the height T1 on the conversion loss was evaluated. For red light, the value of the height T1 was changed using a mode converter having the same structure as the mode converter 42 shown in Fig. 3, and the maximum conversion efficiency at each value of the height T1 was calculated. The wavelength λ was set to 638 nm, and the maximum conversion efficiency was calculated when the polarization mode of the red light was converted from the fundamental TE mode to the fundamental TM mode.

[0125] The calculation results are shown in Table 4 and Fig. 16. In addition to the height T1, maximum conversion efficiency, and conversion loss, Table 4 shows the width W1 [μm], width W2 [μm], and conversion length Lc [μm] when the maximum conversion efficiency is obtained for each value of height T1. The horizontal axis of Fig. 16 represents the height T1 [μm] of the waveguide, and the vertical axis of Fig. 16 represents the conversion loss [dB]. [Table 4]

[0126] According to Table 4 and Figure 16, when the height T1 is 0.5μm or less, the maximum conversion efficiency exceeds 0.6 and the conversion loss is below 2.2dB. From the above, it can be said that high conversion efficiency is achieved when the height T1 is smaller than the wavelength λ (638nm) of visible light used in this calculation.

[0127] <Conversion loss evaluation at tilt angle θ> The effect of the tilt angle θ on the conversion loss was evaluated. For each of red light, green light, and blue light, the value of the tilt angle θ was changed using a mode converter having the same structure as the mode converter 42 shown in FIG. 3, and the conversion loss at each value of the tilt angle θ was calculated. For the evaluation of red light, a mode converter having a conversion length Lc of 16 μm and a mode converter having a conversion length Lc of 18 μm were used, and the wavelength λ of the red light was set to 638 nm, the height T1 to 0.3 μm, and the width W1 to 0.24 μm. By changing the width W2, the conversion loss was calculated when the polarization mode of the red light was converted from the TE fundamental mode to the TM fundamental mode at each value of the tilt angle θ.

[0128] For the evaluation of green light, a mode converter having a conversion length Lc of 14 μm and a mode converter having a conversion length Lc of 16 μm were used, and the wavelength λ of green light was set to 520 nm, the height T1 was set to 0.3 μm, and the width W1 was set to 0.20 μm. By changing the width W2, the conversion loss was calculated when the polarization mode of green light was converted from the TE fundamental mode to the TM fundamental mode at each value of the inclination angle θ. For the evaluation of blue light, a mode converter having a conversion length Lc of 12 μm and a mode converter having a conversion length Lc of 14 μm were used, and the wavelength λ of blue light was set to 455 nm, the height T1 was set to 0.3 μm, and the width W1 was set to 0.18 μm. By changing the width W2, the conversion loss was calculated when the polarization mode of blue light was converted from the TE fundamental mode to the TM fundamental mode at each value of the inclination angle θ.

[0129] The calculation results for red light are shown in Fig. 17(a). The horizontal axis of Fig. 17(a) represents the tilt angle θ [°], and the vertical axis of Fig. 17(a) represents the conversion loss [dB]. Fig. 17(a) shows that when the conversion length Lc is 16 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 73° and 81°, and high conversion efficiency is achieved. When the conversion length Lc is 18 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 74° and 81°, and high conversion efficiency is achieved.

[0130] The calculation results for green light are shown in Fig. 17(b). The horizontal axis of Fig. 17(b) represents the tilt angle θ [°], and the vertical axis of Fig. 17(b) represents the conversion loss [dB]. Fig. 17(b) shows that when the conversion length Lc is 14 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 77° and 82°, and high conversion efficiency is achieved. When the conversion length Lc is 16 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 78° and 83°, and high conversion efficiency is achieved.

[0131] The calculation results for blue light are shown in Fig. 17(c). The horizontal axis of Fig. 17(c) represents the tilt angle θ [°], and the vertical axis of Fig. 17(c) represents the conversion loss [dB]. Fig. 17(c) shows that when the conversion length Lc is 12 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 77° and 82°, and high conversion efficiency is achieved. When the conversion length Lc is 14 µm, the conversion loss is below 2.2 dB when the tilt angle θ is between 78° and 82°, and high conversion efficiency is achieved.

[0132] From the above evaluation results, an example of the range of the inclination angle θ that can realize high conversion efficiency is 73° to 83°, which is the maximum range in Figures 17(a) to 17(c). Alternatively, from the viewpoint of increasing the degree of freedom in designing the waveguide, the range of the inclination angle θ may be extended by ±2° from the above range to 71° to 85°.

[0133] (Additional Note) [Clause 1] A substrate having a major surface; a core layer provided on the main surface and made of a material having an electro-optic effect; the core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light between a TM mode and a TE mode; The mode converter includes: a first waveguide to which the visible light is incident in a first polarization mode, which is one of the TE mode and the TM mode; a second waveguide that outputs the visible light in a second polarization mode that is the other polarization mode of the TE mode and the TM mode; a third waveguide provided between the first waveguide and the second waveguide, the third waveguide converting the visible light from the first polarization mode to the second polarization mode; The optical element, wherein the third waveguide has an asymmetric shape in a second direction along the principal surface that intersects with the first direction.

[0134] [Clause 2] the third waveguide has a bottom surface facing the major surface, a top surface provided on an opposite side to the bottom surface in a third direction intersecting the first direction and the second direction, and an inclined surface connecting the top surface and the bottom surface, Item 2. The optical element described in item 1, wherein the third waveguide has a columnar shape whose length in the second direction increases continuously from the top surface to the bottom surface.

[0135] [Clause 3] 3. The optical element of claim 2, wherein an inclination angle between the inclined surface and the main surface is greater than or equal to 71° and less than or equal to 85°.

[0136] [Article 4] Item 2. The optical element described in item 1, wherein the third waveguide has a stepped shape in which the length in the second direction increases stepwise as it approaches the main surface.

[0137] [Article 5] the third waveguide has a bottom surface facing the major surface and a top surface provided on an opposite side to the bottom surface in a third direction intersecting the first direction and the second direction, Item 1. The optical element of item 1, wherein the top surface has a groove extending in the first direction and recessed toward the bottom surface.

[0138] [Article 6] Item 6. The optical element according to any one of items 1 to 5, wherein a length in the second direction of a surface of the mode converter in contact with the principal surface is 32% to 48% of a wavelength of the visible light.

[0139] [Article 7] 7. The optical element according to any one of clauses 1 to 6, wherein a length of the mode converter in a third direction intersecting the first direction and the second direction is smaller than a wavelength of the visible light.

[0140] [Article 8] The core layer is a first mode converter that converts a polarization mode of red light from the first polarization mode to the second polarization mode; a second mode converter that converts a polarization mode of green light from the first polarization mode to the second polarization mode; a third mode converter which converts the polarization mode of blue light from the first polarization mode to the second polarization mode; a multiplexer that multiplexes the red light, the green light, and the blue light and emits a laser beam; An optical element comprising the optical element according to any one of claims 1 to 7,

[0141] [Article 9] An optical element comprising the optical element described in clause 8, wherein the first mode converter, the second mode converter and the third mode converter have the same length in the first direction and a third direction intersecting the second direction.

[0142] [Article 10] The core layer is a first modulator for modulating the light intensity of the red light; a second modulator for modulating the light intensity of the green light; a third modulator that modulates the light intensity of the blue light; 10. The optical element of claim 8 or 9, further comprising:

[0143] [Article 11] An optical element according to any one of clauses 8 to 10, a first laser light source that emits the red light in the first polarization mode; a second laser light source that emits the green light in the first polarization mode; a third laser light source that emits the blue light in the first polarization mode.

[0144] [Article 12] A near-eye wearable device, comprising: A laser module according to clause 11; a movable mirror that performs scanning using the laser light emitted from the laser module; A near-eye wearable device comprising: a reflector that reflects the laser light that has passed through the movable mirror and guides it to the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina. [Explanation of symbols]

[0145] 1...Near eyewear trouble device, 4, 4A, 4B, 4C...Laser module, 6...Movable mirror, 30...Reflector, 40...Optical element, 42, 42A...Mode converter (first mode converter), 43, 43A...Mode converter (second mode converter), 44, 44A...Mode converter (third mode converter), 45...Modulator (first modulator), 46...Modulator (second modulator), 47...Modulator (third modulator), 48...Multiplexing device, 411...laser light source (first laser light source), 412...laser light source (second laser light source), 413...laser light source (third laser light source), 51...waveguide (first waveguide), 52...waveguide (second waveguide), 53, 54, 54A, 55...waveguide (third waveguide), 53c, 54c, 55c...bottom surface, 53d, 54d, 55d...top surface, 53f...inclined surface, C1...core layer, Cv1, Cv2...groove, S...substrate, Sa...main surface.

Claims

1. A substrate having a major surface; a core layer provided on the main surface and made of a material having an electro-optic effect; the core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light between a TM mode and a TE mode; The mode converter includes: a first waveguide to which the visible light is incident in a first polarization mode, which is one of the TE mode and the TM mode; a second waveguide that outputs the visible light in a second polarization mode that is the other polarization mode of the TE mode and the TM mode; a third waveguide provided between the first waveguide and the second waveguide, the third waveguide converting the visible light from the first polarization mode to the second polarization mode; The third waveguide has an asymmetric shape in a second direction along the principal surface that intersects with the first direction.

2. the third waveguide has a bottom surface facing the main surface, a top surface provided on an opposite side to the bottom surface in a third direction intersecting the first direction and the second direction, and an inclined surface connecting the top surface and the bottom surface, The optical element according to claim 1 , wherein the third waveguide has a columnar shape whose length in the second direction increases continuously from the top surface to the bottom surface.

3. The optical element according to claim 2 , wherein an inclination angle between the inclined surface and the main surface is not less than 71° and not more than 85°.

4. The optical element according to claim 1 , wherein the third waveguide has a stepped shape in which the length in the second direction increases stepwise toward the major surface.

5. the third waveguide has a bottom surface facing the major surface and a top surface provided on an opposite side to the bottom surface in a third direction intersecting the first direction and the second direction, The optical element according to claim 1 , wherein the top surface is provided with a groove extending in the first direction and recessed toward the bottom surface.

6. 6. The optical element according to claim 1, wherein a length in the second direction of a surface of the mode converter that contacts the main surface is 32% to 48% of a wavelength of the visible light.

7. 6. The optical element according to claim 1, wherein a length of the mode converter in a third direction intersecting the first direction and the second direction is smaller than a wavelength of the visible light.

8. The core layer is a first mode converter that converts a polarization mode of red light from the first polarization mode to the second polarization mode; a second mode converter that converts a polarization mode of green light from the first polarization mode to the second polarization mode; a third mode converter which converts a polarization mode of blue light from the first polarization mode to the second polarization mode; a multiplexer that multiplexes the red light, the green light, and the blue light and emits a laser beam; An optical element comprising the optical element according to any one of claims 1 to 5.

9. An optical element comprising the optical element of claim 8 , wherein the first mode converter, the second mode converter, and the third mode converter have the same length in a third direction intersecting the first direction and the second direction.

10. The core layer is a first modulator for modulating the light intensity of the red light; a second modulator for modulating the light intensity of the green light; a third modulator that modulates the light intensity of the blue light; The optical element according to claim 8 , further comprising:

11. The optical element according to claim 8 ; a first laser light source that emits the red light in the first polarization mode; a second laser light source that emits the green light in the first polarization mode; a third laser light source that emits the blue light in the first polarization mode.

12. A near-eye wearable device, comprising: A laser module according to claim 11; a movable mirror that performs scanning using the laser light emitted from the laser module; A near-eye wearable device comprising: a reflector that reflects the laser light that has passed through the movable mirror and guides it to the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina.

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