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

Abstract

To provide an optical element enabling the conversion of polarization mode of visible light, a laser module, a retinal projection device, and a near-eye wearable device.SOLUTION: A mode converter 35B of the optical element comprises: a tapered portion 54 in which width, from an incident end 54a toward an emission end 54b, increases from a width W11 where the effective refractive index in TM0 mode is larger than the effective refractive index in TE1 mode to a width W12 where the effective refractive index in TM0 mode is smaller than the effective refractive index in TE1 mode; and line portions 55, 56 constituting an asymmetrical directional coupler 60. The width of the line portion 55 and the width of the line portion 56 are set in such a manner that the magnitude relationship between the effective refractive index in TE1 mode in the line portion 55 and the effective refractive index in TE0 mode in the line portion 56 at a position X3 is reverse to the magnitude relationship between the effective refractive index in TE1 mode in the line portion 55 and the effective refractive index in TE0 mode in the line portion 56 at an incident end 60a.SELECTED DRAWING: Figure 5

Description

Technical Field

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

Background Art

[0002] For the polarization modes of light propagating in an optical waveguide, there are a TE (Transverse Electric) mode which is a polarization mode having a main electric field in the horizontal direction with respect to the substrate, and a TM (Transverse Magnetic) mode which is a polarization mode having a main electric field in the vertical direction with respect to the substrate. Optical waveguide elements for converting these polarization modes are known. For example, Non-Patent Document 1 describes a polarization splitter rotator including a taper structure and an asymmetric directional coupler.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The optical waveguide element described in Non-Patent Document 1 converts light in the TM0 mode having a wavelength of 1.45 μm to 1.6 μm into light in the TE0 mode. However, visible light is not considered.

[0005] The present disclosure describes an optical element, a laser module, a retinal projection device, and a near-eye wearable device capable of converting the polarization mode of visible light from the TM0 mode to the TE0 mode.

Means for Solving the Problem

[0006] The optical element according to one aspect of the present disclosure includes a substrate having a main surface, and a core layer provided on the main surface and composed of a material having an electro-optic effect. The core layer includes a mode converter that converts the polarization mode of visible light from the TM0 mode to the TE0 mode. The mode converter includes a first waveguide extending in a first direction along the main surface, and a second waveguide extending in the first direction. The first waveguide has a first incident end where visible light is incident and a first exit end where visible light exits, and includes a tapered portion whose length in a second direction along the main surface and intersecting the first direction increases from a first length to a second length from the first incident end toward the first exit end, and a first line portion through which the visible light emitted from the first exit end propagates. The second waveguide includes a second line portion arranged side by side with the first line portion in the second direction. The first length is a length at which a first effective refractive index, which is the effective refractive index of the TM0 mode, is greater than a second effective refractive index, which is the effective refractive index of the TE1 mode. The second length is a length at which the first effective refractive index is smaller than the second effective refractive index. The first line portion and the second line portion constitute an asymmetric directional coupler. The asymmetric directional coupler has a second incident end and a second exit end that are both ends in the first direction. The length of the first line portion in the second direction and the length of the second line portion in the second direction are such that the magnitude relationship between the second effective refractive index in the first line portion and the third effective refractive index, which is the effective refractive index of the TE0 mode in the second line portion, at a position different from the second incident end of the asymmetric directional coupler is reversed from the magnitude relationship between the second effective refractive index in the first line portion and the third effective refractive index in the second line portion at the second incident end.

[0007] In this optical element, in the tapered portion, a region is formed in which the effective refractive index of the TM0 mode and the effective refractive index of the TE1 mode substantially coincide. Therefore, when visible light of the TM0 mode is incident on the first incident end, in the above region, an interaction occurs between the TM0 mode and the TE1 mode, the polarization mode of the visible light is converted from the TM0 mode to the TE1 mode, and visible light of the TE1 mode is emitted from the first emission end. Further, between the second incident end of the asymmetric directional coupler and a position different from the second incident end, a region is formed in which the effective refractive index of the TE1 mode in the first line portion and the effective refractive index of the TE0 mode in the second line portion substantially coincide. Therefore, when visible light of the TE1 mode is incident on the second incident end, in the above region, an interaction occurs between the TE1 mode and the TE0 mode, the polarization mode of the visible light is converted from the TE1 mode to the TE0 mode, and visible light of the TE0 mode is emitted from the second emission end. As described above, it becomes possible to convert the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0008] The length of the first line portion in the second direction may increase from the second incident end toward the above position, and the length of the second line portion in the second direction may increase from the second incident end toward the above position. According to this configuration, both the second effective refractive index in the first line portion and the third effective refractive index in the second line portion increase from the second incident end toward the above position. For this reason, compared with a configuration in which either one of the second effective refractive index in the first line portion and the third effective refractive index in the second line portion is constant over the range from the second incident end to the above position, the angle formed by the curve showing the relationship between the position in the first direction and the second effective refractive index in the first line portion and the curve showing the relationship between the position in the first direction and the third effective refractive index in the second line portion can be made smaller. Thereby, it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode.

[0009] The length of the first waveguide section in the second direction may increase from the second incident end toward the above-mentioned position, or the length of the second waveguide section in the second direction may be constant over the range from the second incident end to the above-mentioned position. According to this configuration, the second effective refractive index in the first waveguide section increases from the second incident end toward the above-mentioned position, while the third effective refractive index in the second waveguide section is constant over the range from the second incident end to the above-mentioned position. Therefore, by increasing the length from the second incident end to the above-mentioned position, the angle formed by the curve showing the relationship between the position in the first direction and the second effective refractive index in the first waveguide section and the curve showing the relationship between the position in the first direction and the third effective refractive index in the second waveguide section can be reduced. Thereby, it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode.

[0010] The length of the first waveguide section in the second direction and the length of the second waveguide section in the second direction may be set such that the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index in the second waveguide section at the second output end is reversed from the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index in the second waveguide section at the above-mentioned position. In this case, a region is formed between the above-mentioned position and the second output end where the effective refractive index of the TE1 mode in the first waveguide section and the effective refractive index of the TE0 mode in the second waveguide section substantially coincide, and an interaction occurs between the TE1 mode and the TE0 mode. Thereby, the number of times of conversion from the TE1 mode to the TE0 mode can be increased, and the conversion efficiency can be improved.

[0011] The mode converter may further include a flat slab provided with the first waveguide and the second waveguide. According to this configuration, the visible light oozes out from the first waveguide and the second waveguide into the slab, so that the optical coupling between the first waveguide and the second waveguide is strengthened. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode by the asymmetric directional coupler can be improved.

[0012] The length in the third direction intersecting the first and second directions of the mode converter may be smaller than the wavelength of visible light. In this case, visible light is likely to leak from the first and second waveguides into the slab. Thereby, the optical coupling between the first waveguide and the second waveguide by the slab can be strengthened. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode by the asymmetric directional coupler can be further improved.

[0013] The cross-sectional shape of the first waveguide intersecting the first direction may be a trapezoidal shape in which the length in the second direction increases as it approaches the main surface. In this case, the first waveguide has an asymmetric shape in the third direction intersecting the first and second directions. Therefore, the conversion efficiency from the TM0 mode to the TE1 mode by the tapered portion can be improved.

[0014] The core layer may include a first mode converter that is a mode converter for converting the polarization mode of red light from the TM0 mode to the TE0 mode, a second mode converter that is a mode converter for converting the polarization mode of green light from the TM0 mode to the TE0 mode, a third mode converter that is a mode converter for converting the polarization mode of blue light from the TM0 mode to the TE0 mode, and a multiplexer that multiplexes red light, green light, and blue light to emit laser light. According to this configuration, the polarization mode of red light is converted from the TM0 mode to the TE0 mode, the polarization mode of green light is converted from the TM0 mode to the TE0 mode, and the polarization mode of blue light is converted from the TM0 mode to the TE0 mode. For example, when the multiplexer is designed such that the multiplexing efficiency when multiplexing red light, green light, and blue light in the TE0 mode is higher than the multiplexing efficiency when multiplexing red light, green light, and blue light in the TM0 mode, the multiplexing efficiency in the multiplexer can be improved.

[0015] The lengths in a third direction that intersects the first and second directions of the first mode converter, the length in the third direction of the second mode converter, and the length in the third direction of the third mode converter may be the same. According to 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 in the third direction of the respective mode converters can be made the same, so that the manufacturing of the optical element can be facilitated.

[0016] The core layer may further include a first modulator that modulates the light intensity of red light, a second modulator that modulates the light intensity of green light, and a third modulator that modulates the light intensity of blue light. In order to output full-color laser light by multiplexing red light, green light, and 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, since the light intensity of red light, the light intensity of green light, and the light intensity of blue light are modulated, it is possible to output full-color laser light without requiring a large drive current.

[0017] A laser module according to another aspect of the present disclosure includes the above optical element, a first light source that emits red light in the TM0 mode, a second light source that emits green light in the TM0 mode, and a third light source that emits blue light in the TM0 mode. Since this laser module includes the above-described optical element, it is possible to convert the polarization modes of red light, green light, and blue light from the TM0 mode to the TE0 mode.

[0018] A retinal projection device according to still another aspect of the present disclosure is a device mounted on a near-eye wearable device, and includes the above laser module, a movable mirror that performs scanning using the laser light emitted from the laser module, and a reflector that reflects the laser light that has passed through the movable mirror and irradiates the reflected light onto the retina of a user wearing the near-eye wearable device to project an image onto the retina. This retinal projection device includes the above-described optical element. Therefore, in this retinal projection device, it is possible to project an image onto the retina after converting the polarization modes of red light, green light, and blue light from the TM0 mode to the TE0 mode.

[0019] A near-eye wearable device according to still another aspect of the present disclosure includes the above-described retinal projection device and a lens provided with a reflector. This near-eye wearable device includes the above-described optical element. Therefore, in this near-eye wearable device, it is possible to project an image onto the retina after converting the polarization modes of red light, green light, and blue light from the TM0 mode to the TE0 mode.

Advantages of the Invention

[0020] According to each aspect and each embodiment of the present disclosure, the polarization mode of visible light can be converted from the TM0 mode to the TE0 mode.

Brief Description of the Drawings

[0021]

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DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same reference numerals are assigned to the same elements, and duplicate descriptions are omitted. In each figure, an XYZ coordinate system may be shown. The Y-axis direction (second direction) intersects (for example, is orthogonal to) the X-axis direction (first direction) and the Z-axis direction (third direction). The Z-axis direction intersects (for example, is orthogonal to) the X-axis direction and the Y-axis direction. In this specification, a numerical range indicated by "~" indicates a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. The individually described upper limit value and lower limit value can be arbitrarily combined.

[0023] While referring to FIG. 1, an application example of a laser module according to an embodiment will be described. FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which the 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 (head-mounted display), and can take forms such as glasses type, goggle type, hat type, and helmet type. 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.

[0024] The frame 2 includes a pair of rims 2a, a bridge 2b, and a pair of temples 2c. The rim 2a is a part that holds the lens 3. The bridge 2b is a part that connects the pair of rims 2a. The temple 2c is a part that extends from the rim 2a and is hung on the user's ear. The frame 2 may be a rimless frame. The lens 3 has an inner surface 3a (see FIG. 2) facing the eyeball of a user wearing the near-eye wearable device 1.

[0025] 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 the present embodiment, in order to project an image onto both the left and right retinas, the near-eye wearable device 1 includes two retinal projection devices 10, but may include only one of the retinal projection devices 10.

[0026] Next, while referring to FIG. 2, the retinal projection device 10 will be described in detail. FIG. 2 is a configuration diagram schematically showing the retinal projection device shown in FIG. 1. As shown in FIG. 2, the retinal projection device 10 includes an optical engine 11 and a reflector 12.

[0027] The optical engine 11 is a device that generates laser light Ls with colors and light intensities corresponding to the pixels of the image projected onto the retina and emits the laser light Ls to the reflector 12. The optical engine 11 is mounted on the template 2c. The optical engine 11 includes a laser module 13, optical components 14, a movable mirror 15, a laser driver 16, a mirror driver 17, and a controller 18.

[0028] The laser module 13 emits laser light. As the laser module 13, for example, a full-color laser module is used. The laser module 13 emits laser light with colors and light intensities corresponding to the pixels of the image projected onto the retina. Details of the laser module 13 will be described later.

[0029] The optical components 14 are components that optically process the laser light emitted from the laser module 13. In this embodiment, the optical components 14 include a collimator lens 14a, a slit 14b, and a light attenuation filter 14c. The collimator lens 14a, the slit 14b, and the light attenuation filter 14c are arranged in that order along the optical path of the laser light. The optical components 14 may have other configurations.

[0030] The movable mirror 15 is a member for performing scanning using the laser light emitted from the laser module 13. The movable mirror 15 is provided in the emission direction of the laser light processed by the optical components 14. The movable mirror 15 is configured to be swingable, for example, around an axis extending in the lateral direction of the lens 3 and around an axis extending in the longitudinal direction of the lens 3, and reflects the laser light by changing the angle in the lateral and longitudinal directions of the lens 3. As the movable mirror 15, for example, a MEMS (Micro Electro Mechanical Systems) mirror is used.

[0031] The laser driver 16 is a drive circuit that drives the laser module 13. The laser driver 16 drives the laser module 13 based on, for example, the light intensity of the laser light and the temperature of the light source unit 20 included in the laser module 13. The mirror driver 17 is a drive circuit that drives the movable mirror 15. The mirror driver 17 swings the movable mirror 15 within a predetermined angular range and at a predetermined timing. The controller 18 is a device that controls the laser driver 16 and the mirror driver 17.

[0032] In the optical engine 11, laser light of a color and light intensity corresponding to the pixels of the image projected onto the retina is emitted from the laser module 13, passes through the optical component 14, and is reflected by the movable mirror 15. The laser light reflected by the movable mirror 15 is emitted to the reflector 12 as the laser light Ls.

[0033] The reflector 12 is a member that reflects the laser light Ls that has passed through the movable mirror 15 and irradiates the retina of the user wearing the near-eye wearable device 1 with the reflected light Lr, thereby projecting an image onto the retina. The reflector 12 is provided on the inner surface 3a of the lens 3.

[0034] Next, the laser module 13 will be described in detail with reference to FIGS. 3 and 4. FIG. 3 is a block diagram of the laser module shown in FIG. 2. FIG. 4 is a diagram showing a cross-sectional configuration of the optical element shown in FIG. 3. As shown in FIG. 3, the laser module 13 includes a light source unit 20 and an optical element 30.

[0035] The light source unit 20 emits visible light. The light source unit 20 includes a laser light source 21 (first light source) that emits red light, a laser light source 22 (second light source) that emits green light, and a laser light source 23 (third light source) that emits blue light. The laser light source 21 is, for example, a red laser diode. The laser light source 22 is, for example, a green laser diode. The laser light source 23 is, for example, a blue laser diode. The peak wavelength of the red light is, for example, in the range of 600 nm to 830 nm. The peak wavelength of the green light is, for example, in the range of 500 nm to 570 nm. The peak wavelength of the blue light is, for example, in the range of 380 nm to 490 nm. The laser light source 21, the laser light source 22, and the laser light source 23 are arranged in the Y-axis direction in that order.

[0036] In this embodiment, the laser light source 21 emits red light in the TM fundamental mode (hereinafter referred to as the "TM0 mode"). The laser light source 22 emits green light in the TM0 mode. The laser light source 23 emits blue light in the TM0 mode. Since all of the red light, green light, and blue light are visible light, in the following description, the red light, green light, and blue light may be referred to as each visible light, and the red light, green light, and blue light may be collectively referred to as visible light.

[0037] The optical element 30 multiplexes the laser lights emitted from the respective laser light sources into one laser light. The optical element 30 is, for example, a Planar Lightwave Circuit (PLC). As shown in FIG. 4, the optical element 30 includes a substrate 31, a core layer 32, and a cladding layer 33.

[0038] The substrate 31 functions as a lower cladding layer. The substrate 31 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. Examples of the constituent material of the substrate 31 include sapphire, silicon oxide, and silicon with silicon oxide laminated thereon. The substrate 31 has a main surface 31a and a back surface 31b opposite to the main surface 31a. The main surface 31a and the back surface 31b are planes defined by the X-axis direction and the Y-axis direction, and intersect (orthogonal in this embodiment) with the Z-axis direction. In other words, the X-axis direction and the Y-axis direction are directions along the main surface 31a.

[0039] The cladding layer 33 functions as an upper cladding layer. The cladding layer 33 covers the core layer 32 on the main surface 31a. The cladding layer 33 is provided over the entire main surface 31a. The cladding layer 33 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. Examples of the constituent material of the cladding layer 33 include silicon oxide (e.g., SiO2).

[0040] The core layer 32 is provided on the main surface 31a. The core layer 32 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 by applying an electric field to the material. Examples of the constituent material of the core layer 32 include lithium niobate (LiNbO3). In this embodiment, the core layer 32 is a lithium niobate thin film formed on the main surface 31a of the substrate 31 by sputtering, and the optical axis (C-axis) of lithium niobate extends in the Z-axis direction. The core layer 32 may be made of Z-cut lithium niobate.

[0041] The core layer 32 includes a modulator 34R (first modulator), a modulator 34G (second modulator), a modulator 34B (third modulator), a mode converter 35R (first mode converter), a mode converter 35G (second mode converter), a mode converter 35B (third mode converter), and a multiplexer 36.

[0042] The modulator 34R is a modulator that modulates the light intensity of red light. The modulator 34R modulates the light intensity of the red light in the TM0 mode emitted from the laser light source 21. The modulator 34G is a modulator that modulates the light intensity of green light. The modulator 34G modulates the light intensity of the green light in the TM0 mode emitted from the laser light source 22. The modulator 34B is a modulator that modulates the light intensity of blue light. The modulator 34B modulates the light intensity of the blue light in the TM0 mode emitted from the laser light source 23. Each modulator is, for example, a Mach-Zehnder type modulator.

[0043] The mode converter 35R is a mode converter that converts the polarization mode of red light from the TM0 mode to the TE fundamental mode (hereinafter referred to as the "TE0 mode"). The mode converter 35R is provided at the subsequent stage of the modulator 34R and converts the polarization mode of the red light emitted from the modulator 34R from the TM0 mode to the TE0 mode. The mode converter 35G is a mode converter that converts the polarization mode of green light from the TM0 mode to the TE0 mode. The mode converter 35G is provided at the subsequent stage of the modulator 34G and converts the polarization mode of the green light emitted from the modulator 34G from the TM0 mode to the TE0 mode. The mode converter 35B is a mode converter that converts the polarization mode of blue light from the TM0 mode to the TE0 mode. The mode converter 35B is provided at the subsequent stage of the modulator 34B and converts the polarization mode of the blue light emitted from the modulator 34B from the TM0 mode to the TE0 mode.

[0044] Note that the polarization mode is also referred to as the waveguide mode. The TM mode is a polarization mode in which the direction of the main component of the electric field in the cross section perpendicular to the light propagation direction is perpendicular to the main surface 31a of the substrate 31. The TE mode is a polarization mode in which the direction of the main component of the electric field in the cross section perpendicular to the light propagation direction is horizontal to the main surface 31a of the substrate 31. The TM0 mode is a polarization mode with the largest effective refractive index among the TM modes. The TE0 mode is a polarization mode with the largest effective refractive index among the TE modes. The TE1 mode is a polarization mode with the second largest effective refractive index among the TE modes.

[0045] Each of the mode converters 35R, 35G, and 35B extends in the X-axis direction. The mode converters 35R, 35G, and 35B are arranged in the Y-axis direction in that order. The detailed configuration of each mode converter will be described later.

[0046] The multiplexer 36 multiplexes red light, green light, and blue light. The multiplexer 36 multiplexes the red light emitted from the mode converter 35R, the green light emitted from the mode converter 35G, and the blue light emitted from the mode converter 35B into one laser beam and emits the laser beam. The laser beam includes a component having a red wavelength (red component), a component having a green wavelength (green component), and a component having a blue wavelength (blue component).

[0047] In the laser module 13, visible light in the TM0 mode is emitted from each laser light source. After the light intensity of each visible light is modulated in each modulator, the polarization mode of the visible light is converted from the TM0 mode to the TE0 mode in each mode converter. Then, the visible lights with the converted polarization modes are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as a TE0 mode laser beam to the optical component 14 (see FIG. 2).

[0048] Next, with reference to FIGS. 5 and 6, the detailed configuration of the mode converters 35R, 35G, and 35B will be described. FIG. 5 is a plan view showing the configuration of the mode converter shown in FIG. 3. FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. 5. Here, the mode converter 35B will be described as an example. As shown in FIGS. 5 and 6, the mode converter 35B forms a ridge-type waveguide and includes a waveguide 51 (first waveguide), a waveguide 52 (second waveguide), and a slab 53. In FIG. 5, for convenience of explanation, only the waveguides 51 and 52 are shown.

[0049] The slab 53 is a flat portion of the ridge waveguide. The slab 53 is provided on the main surface 31a. The waveguide 51 and the waveguide 52 are provided on the slab 53. Note that the waveguide 51, the waveguide 52, and the slab 53 are made of the same material. The length (height Ts) of the slab 53 in the Z-axis direction is, for example, 0 μm to 0.2 μm. Hereinafter, the length in the Z-axis direction may be referred to as "height". Note that when the height Ts is 0 μm, it means that the slab 53 is not provided. That is, the mode converter 35B may not include the slab 53.

[0050] The waveguide 51 is a convex portion of the ridge waveguide. The waveguide 51 is provided on the slab 53 and extends linearly in the X-axis direction. The waveguide 51 may have a shape that is symmetric with respect to the symmetry plane SP1. The symmetry plane SP1 is a virtual plane defined by the X-axis direction and the Z-axis direction and passes through the center of the waveguide 51 in the Y-axis direction.

[0051] The cross-sectional shape of the waveguide 51 that intersects (is orthogonal to) the X-axis direction is a trapezoidal shape in which the length in the Y-axis direction increases as it approaches the main surface 31a. In the present embodiment, the above cross-sectional shape of the waveguide 51 is an isosceles trapezoid. The inclination angle θ is, for example, 86° or less. The inclination angle θ is the angle between the bottom surface and the side surface of the waveguide 51. The above cross-sectional shape of the waveguide 51 may be a rectangular shape. Hereinafter, the length in the Y-axis direction may be referred to as "width". The height Tr1 of the waveguide 51 is substantially constant over the entire length of the waveguide 51 in the X-axis direction. The height Tr1 is, for example, 0.2 μm to 1.0 μm.

[0052] The waveguide 52 is a convex portion of the ridge waveguide. The waveguide 52 is provided on the slab 53 and extends linearly in the X-axis direction. The waveguide 52 is provided side by side with a part of the waveguide 51 in the Y-axis direction. The waveguide 52 may have a shape that is symmetric with respect to the symmetry plane SP2. The symmetry plane SP2 is a virtual plane defined by the X-axis direction and the Z-axis direction and passes through the center of the waveguide 52 in the Y-axis direction.

[0053] The cross-sectional shape of the waveguide 52 that intersects (is orthogonal to) the X-axis direction is a trapezoidal shape whose width increases as it approaches the main surface 31a. In the present embodiment, the cross-sectional shape of the waveguide 52 is an isosceles trapezoid. The angle between the bottom surface and the side surface of the waveguide 52 is substantially the same as the tilt angle θ. The cross-sectional shape of the waveguide 52 may be a rectangular shape. The height Tr2 of the waveguide 52 is substantially constant over the entire length of the waveguide 51 in the X-axis direction and is substantially the same as the height Tr1 of the waveguide 51.

[0054] The length Lt of the mode converter 35B in the X-axis direction is the sum of the length L1, the length L2, and the length L3, which will be described later. The length Lt is, for example, 15 μm to 150,000 μm. The height Tc of the mode converter 35B is substantially constant over the entire length of the mode converter 35B in the X-axis direction. The height Tc is the sum of the height Tr1 (or the height Tr2) and the height Ts. The height Tc is, for example, smaller than the wavelength of the visible light (here, blue light) to be converted. The height Tc is, for example, 0.2 μm to 1.2 μm.

[0055] The waveguide 51 includes a tapered portion 54 and a line portion 55 (first line portion). The waveguide 52 includes a line portion 56 (second line portion).

[0056] The tapered portion 54 functions as a conversion portion that converts the polarization mode of visible light (here, blue light) from the TM0 mode to the TE1 mode. The tapered portion 54 has an incident end 54a (first incident end) and an output end 54b (first output end) that are both ends in the X-axis direction. The incident end 54a is located at the position X1 in the X-axis direction. The output end 54b is located at the position X2 in the X-axis direction. Blue light is incident on the incident end 54a from the modulator 34B. The output end 54b emits the blue light to the line portion 55. The length L1 of the tapered portion 54 in the X-axis direction is, for example, 5 μm to 50,000 μm.

[0057] The width of the tapered portion 54 increases from the incident end 54a toward the exit end 54b. Specifically, the width of the tapered portion 54 continuously increases from the width W11 (first length) to the width W12 (second length) from the incident end 54a toward the exit end 54b. The rate of increase in the width of the tapered portion 54 may be substantially constant. The width W11 of the tapered portion 54 at the incident end 54a is set to a width at which the effective refractive index of the TM0 mode (first effective refractive index) is greater than the effective refractive index of the TE1 mode (second effective refractive index) and less than the effective refractive index of the TE0 mode. The width W11 is, for example, 0.3 μm to 1.0 μm. The width W12 of the tapered portion 54 at the exit end 54b is set to a width at which the effective refractive index of the TM0 mode is less than the effective refractive index of the TE1 mode. The width W12 is greater than the width W11 and is, for example, 0.4 μm to 1.2 μm.

[0058] The waveguide portion 55 is provided at the subsequent stage of the tapered portion 54 and is a portion through which the blue light emitted from the exit end 54b propagates. The waveguide portion 55 has one end 55a and the other end 55b which are both ends in the X-axis direction. One end 55a is connected to the exit end 54b. The blue light emitted from the tapered portion 54 is incident on one end 55a. The other end 55b can emit the blue light.

[0059] The waveguide portion 56 is arranged side by side with the waveguide portion 55 in the Y-axis direction. The waveguide portion 56 has one end 56a and the other end 56b which are both ends in the X-axis direction. The other end 56b can emit the blue light.

[0060] The waveguide portion 55 and the waveguide portion 56 are arranged side by side in the Y-axis direction and constitute an asymmetric directional coupler 60. The center of the waveguide portion 55 in the Y-axis direction and the center of the waveguide portion 56 in the Y-axis direction are separated by a distance D. The distance D is, for example, 0.4 μm to 2.0 μm. The waveguide portion 55 and the waveguide portion 56 are separated from each other in the Y-axis direction. The minimum interval G between the waveguide portion 55 and the waveguide portion 56 is, for example, 0.1 μm to 0.9 μm.

[0061] The asymmetric directional coupler 60 has an incident end 60a (second incident end) and an output end 60b (second output end) which are the two ends in the X-axis direction. The incident end 60a is composed of one end 55a and one end 56a. The output end 60b is composed of the other end 55b and the other end 56b. The incident end 60a is located at the position X2 in the X-axis direction. The output end 60b is located at the position X4 in the X-axis direction.

[0062] The asymmetric directional coupler 60 is divided into a conversion region 61 and a conversion region 62 at the position X3 in the X-axis direction. The position X3 is the position between the incident end 60a (position X2) and the output end 60b (position X4) in the X-axis direction. The conversion region 61 is the region of the asymmetric directional coupler 60 from the incident end 60a to the position X3. The conversion region 62 is the region of the asymmetric directional coupler 60 from the position X3 to the output end 60b. The length L2 of the conversion region 61 in the X-axis direction is, for example, 5 μm to 50000 μm. The length L3 of the conversion region 62 in the X-axis direction is, for example, 5 μm to 50000 μm.

[0063] In the conversion region 61, the widths of the line part 55 and the line part 56 are set such that the magnitude relationship between the effective refractive index of the TE1 mode in the line part 55 and the effective refractive index of the TE0 mode in the line part 56 at the position X3 is reversed from the magnitude relationship between the effective refractive index of the TE1 mode in the line part 55 and the effective refractive index of the TE0 mode in the line part 56 at the incident end 60a. In the present embodiment, in the conversion region 61, the widths of the line part 55 and the line part 56 are set such that the effective refractive index of the TE1 mode in the line part 55 is smaller than the effective refractive index of the TE0 mode (third effective refractive index) in the line part 56 at the incident end 60a, and the effective refractive index of the TE1 mode in the line part 55 is larger than the effective refractive index of the TE0 mode in the line part 56 at the position X3.

[0064] In this embodiment, in the conversion region 61, the width of the line portion 55 increases from the incident end 60a (one end 55a) toward the position X3. More specifically, in the conversion region 61, the width of the line portion 55 continuously increases from the width W12 to the width W13. The rate of increase in the width of the line portion 55 in the conversion region 61 may be substantially constant. The width W13 is larger than the width W12 and is, for example, 0.4 μm to 1.5 μm.

[0065] In the conversion region 61, the width of the line portion 56 is substantially constant over the range from the incident end 60a (one end 56a) to the position X3. In other words, the width W22 at one end 56a of the line portion 56 is substantially the same as the width W23 at the position X3 of the line portion 56. The width of the line portion 56 may increase from the incident end 60a (one end 56a) toward the position X3. The width W22 is, for example, 0.2 μm to 1.0 μm. The width W23 is, for example, 0.2 μm to 1.0 μm.

[0066] In the conversion region 62, the widths of the line portion 55 and the line portion 56 are set such that the magnitude relationship between the effective refractive index of the TE1 mode in the line portion 55 and the effective refractive index of the TE0 mode in the line portion 56 at the output end 60b is reversed from the magnitude relationship between the effective refractive index of the TE1 mode in the line portion 55 and the effective refractive index of the TE0 mode in the line portion 56 at the position X3. In this embodiment, in the conversion region 62, the widths of the line portion 55 and the line portion 56 are set such that the effective refractive index of the TE1 mode in the line portion 55 is larger than the effective refractive index of the TE0 mode in the line portion 56 at the position X3, and the effective refractive index of the TE1 mode in the line portion 55 is smaller than the effective refractive index of the TE0 mode in the line portion 56 at the output end 60b.

[0067] In this embodiment, in the conversion region 62, the width of the line portion 55 decreases from the position X3 toward the output end 60b (the other end 55b). More specifically, in the conversion region 62, the width of the line portion 55 continuously decreases from the width W13 to the width W14. The rate of decrease in the width of the line portion 55 may be substantially constant. The width W14 is smaller than the width W13 and is, for example, 0.4 μm to 1.2 μm.

[0068] In the conversion region 62, the width of the line portion 56 increases from the position X3 toward the emission end 60b (the other end 56b). More specifically, in the conversion region 62, the width of the line portion 56 continuously increases from the width W23 to the width W24. The increase rate of the width of the line portion 56 in the conversion region 62 may be substantially constant. The width W24 is larger than the width W23 and is, for example, 0.2 μm to 1.0 μm.

[0069] The mode converter 35R and the mode converter 35G have the same configuration as the mode converter 35B. The mode converter 35R and the mode converter 35G may not include the conversion region 62. The height of the mode converter 35R and the height of the mode converter 35G are substantially the same as the height Tc of the mode converter 35B.

[0070] Next, with further reference to FIG. 7, the conversion principle in the mode converter 35R, the mode converter 35G, and the mode converter 35B will be described. FIG. 7 is a diagram for explaining the conversion principle in the mode converter shown in FIG. 5. The horizontal axis of FIG. 7 indicates the position in the X-axis direction, and the vertical axis of FIG. 7 indicates the effective refractive index.

[0071] The effective refractive index in the waveguide depends on the constituent material of the waveguide, the polarization mode of the light propagating in the waveguide, the cross-sectional shape of the waveguide, and the cross-sectional area of the waveguide. For example, when the cross-sectional area of the waveguide is large, the light is strongly confined in the waveguide and is more likely to be affected by the refractive index of the constituent material of the waveguide, so the effective refractive index becomes large. On the other hand, when the cross-sectional area of the waveguide is small, the confinement of the light weakens and the light leaks out to the substrate 31 and the cladding layer 33, so the effective refractive index becomes small.

[0072] The "effective refractive index" means the effective refractive index in an isolated waveguide. For example, the effective refractive index N TE1_1 of the TE1 mode in the waveguide 51 means the effective refractive index of the TE1 mode when only the waveguide 51 exists alone. That is, the effective refractive index N TE1_1means the effective refractive index of the TE1 mode when it is assumed that the waveguide 52 does not exist. The state where the waveguide 52 does not exist refers to the state where the portion where the waveguide 52 exists is replaced with the same material as the cladding layer 33.

[0073] Similarly, the effective refractive index N of the TM0 mode in the waveguide 51 TM0_1 means the effective refractive index of the TM0 mode when only the waveguide 51 exists alone. The effective refractive index N of the TE0 mode in the waveguide 51 TE0_1 means the effective refractive index of the TE0 mode when only the waveguide 51 exists alone. The effective refractive index N of the TE0 mode in the waveguide 52 TE0_2 means the effective refractive index of the TE0 mode when only the waveguide 52 exists alone.

[0074] First, the conversion principle in the tapered portion 54 will be explained. As shown in FIG. 7, in the tapered portion 54, the effective refractive index N TM0_1 , the effective refractive index N TE0_1 , and the effective refractive index N TE1_1 all increase from the position X1 toward the position X2. As described above, the width W11 at the incident end 54a (position X1) of the tapered portion 54 is such that the effective refractive index N TM0_1 is larger than the effective refractive index N TE1_1 and smaller than the effective refractive index N TE0_1 . The width W12 at the output end 54b (position X2) of the tapered portion 54 is set to a width such that the effective refractive index N TM0_1 is smaller than the effective refractive index N TE1_1 .

[0075] Between the position X1 and the position X2, the magnitude relationship between the effective refractive index N TM0_1 and the effective refractive index N TE1_1 is reversed, and an interaction occurs between the TM0 mode and the TE1 mode in the region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 substantially coincide. The effective refractive index N TM0_1 and the effective refractive index N TE1_1The position where the magnitude relationship with [object] is reversed may be the midpoint between position X1 and position X2.

[0076] When visible light in the TM0 mode is incident on the incident end 54a of the tapered portion 54, the visible light propagates through the tapered portion 54. Then, the effective refractive index N TM0_1 and the effective refractive index N TE1_1 In the region where they substantially coincide, interaction occurs between the TM0 mode and the TE1 mode, and the polarization mode of the visible light is converted from the TM0 mode to the TE1 mode. Then, visible light in the TE1 mode is emitted from the emission end 54b. Note that the length L1 is set so that the conversion efficiency from the TM0 mode to the TE1 mode is maximized.

[0077] Between position X1 and position X2, the effective refractive index N TE0_1 is separated from the effective refractive indices of other polarization modes. Therefore, when visible light in the TE0 mode is incident on the incident end 54a, the visible light propagates through the tapered portion 54 while maintaining the polarization mode in the TE0 mode, and visible light in the TE0 mode is emitted from the emission end 54b.

[0078] Subsequently, the conversion principle in the asymmetric directional coupler 60 will be described. As described above, the width of the line portion 55 and the width of the line portion 56 are such that the effective refractive index N TE1_1 is less than the effective refractive index N TE0_2 at position X2, the effective refractive index N TE1_1 is greater than the effective refractive index N TE0_2 at position X3, and the effective refractive index N TE1_1 is less than the effective refractive index N TE0_2 at position X4.

[0079] Here, generally, when the effective refractive indices of two polarization modes propagating in two parallel waveguides are close to each other, the interaction between the two polarization modes becomes stronger, and conversion from one polarization mode to the other polarization mode is likely to occur. This is because the closer the phase velocities (light speed / effective refractive index) of the two polarization modes are to each other, the more the conditions for propagating in adjacent waveguides are satisfied.

[0080] Between position X2 and position X3, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 have an inverted magnitude relationship, and in the region where the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are substantially the same, an interaction occurs between the TE1 mode in the line section 55 and the TE0 mode in the line section 56. The effective refractive index N TE1_1 and the effective refractive index N TE0_2 The position where the magnitude relationship is inverted may be the midpoint between position X2 and position X3.

[0081] Furthermore, between position X3 and position X4, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 have an inverted magnitude relationship again, and in the region where the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are substantially the same, an interaction occurs between the TE1 mode in the line section 55 and the TE0 mode in the line section 56. The effective refractive index N TE1_1 and the effective refractive index N TE0_2 The position where the magnitude relationship is inverted may be the midpoint between position X3 and position X4.

[0082] When visible light in the TE1 mode is incident on one end 55a of the line section 55, the visible light propagates through the line section 55. At this time, between position X2 and position X3, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 In the region where they are substantially the same, an interaction occurs between the TE1 mode in the line section 55 and the TE0 mode in the line section 56, and the polarization mode of the visible light is converted from the TE1 mode to the TE0 mode, and the TE0 visible light propagates through the line section 56. Among the visible light propagating through the line section 55, the remaining part that is not converted to the TE0 mode propagates through the line section 55 while maintaining the polarization mode of the visible light as the TE1 mode.

[0083] And between position X3 and position X4, the effective refractive index N TE1_1 and the effective refractive index N TE0_2In a region where they substantially coincide, interaction occurs between the remaining TE1 mode propagating through the line part 55 and the TE0 mode in the line part 56, and the polarization mode of the visible light is converted from the TE1 mode to the TE0 mode, and the visible light of TE0 propagates through the line part 56. Then, the visible light of the TE0 mode is emitted from the other end 56b. Note that the length L2 and the length L3 are set so that the conversion efficiency from the TE1 mode to the TE0 mode is maximized. Therefore, almost no conversion from the TE1 mode to the TE0 mode occurs between the position X3 and the position X4.

[0084] Between the position X2 and the position X4, the effective refractive index N TE0_1 is separated from the effective refractive indices of other polarization modes. Therefore, when visible light of the TE0 mode is incident on one end 55a, the visible light propagates through the line part 55 while maintaining the polarization mode as the TE0 mode, and the visible light of the TE0 mode is emitted from the other end 55b.

[0085] In this way, in a plurality of regions, by the fact that the effective refractive index N TE1_1 and the effective refractive index N TE0_2 substantially coincide, the number of conversions from the TE1 mode to the TE0 mode can be increased, and the conversion efficiency can be improved.

[0086] In the laser module 13 and the optical element 30 described above, in the tapered part 54, a region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 substantially coincide is formed. Therefore, when visible light of the TM0 mode is incident on the incident end 54a, in the above region, interaction occurs between the TM0 mode and the TE1 mode, the polarization mode of the visible light is converted from the TM0 mode to the TE1 mode, and the visible light of the TE1 mode is emitted from the emission end 54b.

[0087] Furthermore, between the incident end 60a (position X2) of the asymmetric directional coupler 60 and the position X3, the effective refractive index N TE1_1 in the line part 55 and the effective refractive index N TE0_2A region that substantially coincides is formed. Therefore, when visible light in the TE1 mode is incident on the incident end 60a, in the above region, an interaction occurs between the TE1 mode and the TE0 mode, the polarization mode of the visible light is converted from the TE1 mode to the TE0 mode, and visible light in the TE0 mode is emitted from the emission end 60b. As described above, it becomes possible to convert the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0088] The near-eye wearable device 1 includes the retinal projection device 10, and the retinal projection device 10 includes the optical element 30. Therefore, in the near-eye wearable device 1 and the retinal projection device 10, it becomes possible to project an image onto the retina after converting the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0089] Even between the position X3 and the emission end 60b (position X4), the effective refractive index N in the line portion 55 TE1_1 and the effective refractive index N in the line portion 56 TE0_2 Since a region that substantially coincides is formed, an interaction occurs between the TE1 mode and the TE0 mode. As a result, the number of conversions from the TE1 mode to the TE0 mode can be increased, and it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode.

[0090] The width of the line portion 55 may increase from the incident end 60a (position X2) toward the position X3, and the width of the line portion 56 may increase from the incident end 60a (position X2) toward the position X3. According to this configuration, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 both increase from the position X2 toward the position X3. For this reason, compared with a configuration in which either one of the effective refractive index N TE1_1 and the effective refractive index N TE0_2 is constant over the range from the position X2 to the position X3, the curve showing the relationship between the position in the X-axis direction and the effective refractive index N TE1_1 and the curve showing the relationship between the position in the X-axis direction and the effective refractive index N TE0_2The angle formed by the curve showing the relationship with can be reduced. As a result, it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode.

[0091] In red light having a relatively long wavelength in the visible light band, the overlap of the mode profiles of the two polarization modes that interact is large, and mode coupling is likely to occur. Therefore, in red light, it is not necessary to increase the length L2. For this reason, by adopting a configuration in which the width of the line portion 55 increases from the incident end 60a (position X2) toward the position X3 and the width of the line portion 56 increases from the incident end 60a (position X2) toward the position X3, it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode without increasing the length L2.

[0092] The width of the line portion 55 may increase from the incident end 60a (position X2) toward the position X3, and the width of the line portion 56 may be constant over the range from the incident end 60a (position X2) to the position X3. According to this configuration, the effective refractive index N TE1_1 increases from the position X2 toward the position X3, while the effective refractive index N TE0_2 is constant over the range from the position X2 to the position X3. For this reason, by increasing the length L2, the angle formed by the curve showing the relationship between the position in the X-axis direction and the effective refractive index N TE1_1 and the curve showing the relationship between the position in the X-axis direction and the effective refractive index N TE0_2 can be reduced. As a result, it becomes possible to improve the conversion efficiency from the TE1 mode to the TE0 mode.

[0093] In the case of green light and blue light having relatively short wavelengths in the visible light band, compared with red light, the overlap of the mode profiles of the two polarization modes that interact is small, and mode coupling is difficult. Therefore, in the case of green light and blue light, it is necessary to make the length L2 longer than that of red light. For this reason, by adopting a configuration in which the width of the line portion 55 increases from the incident end 60a (position X2) toward the position X3 and the width of the line portion 56 is constant over the range from the incident end 60a (position X2) to the position X3, it is possible to increase the length L2 while improving the conversion efficiency from the TE1 mode to the TE0 mode.

[0094] The mode converters 35R, 35G, and 35B include the slab 53. Therefore, since the taper portion 54 has an asymmetric shape in the Z-axis direction, the conversion efficiency from the TM0 mode to the TE1 mode by the taper portion 54 can be improved. Here, being asymmetric in the Z-axis direction means that the two parts separated by the symmetry plane are not plane-symmetric with respect to the symmetry plane passing through the center of the taper portion 54 in the Z-axis direction and orthogonal to the Z-axis direction. Furthermore, since visible light leaks from the optical waveguides 51 and 52 into the slab 53, the optical coupling between the optical waveguides 51 and 52 is strengthened. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode by the asymmetric directional coupler 60 can be improved.

[0095] The height (height Tc) of the mode converters 35R, 35G, and 35B is smaller than the wavelength of the visible light to be converted. Specifically, the height of the mode converter 35R is smaller than the wavelength of red light, the height of the mode converter 35G is smaller than the wavelength of green light, and the height of the mode converter 35B is smaller than the wavelength of blue light. According to this configuration, visible light easily leaks from the optical waveguides 51 and 52 into the slab 53. Thereby, the optical coupling between the optical waveguides 51 and 52 by the slab 53 can be made stronger. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode by the asymmetric directional coupler 60 can be further improved.

[0096] The cross-sectional shape of the waveguide 51 that intersects (is orthogonal to) the X-axis direction is a trapezoidal shape in which the length in the Y-axis direction increases as it goes toward the main plane 31a in the Z-axis direction. Therefore, since the waveguide 51 has an asymmetric shape in the Z-axis direction, the conversion efficiency from the TM0 mode to the TE1 mode by the taper portion 54 can be improved. In addition to the waveguide 51, the cross-sectional shape of the waveguide 52 that intersects (is orthogonal to) the X-axis direction is a trapezoidal shape in which the length in the Y-axis direction increases as it goes toward the main plane 31a in the Z-axis direction. According to this configuration, the overlap of the mode profiles of the two polarization modes (TE0 mode and TE1 mode) that interact in the asymmetric directional coupler 60 becomes large, and mode coupling is likely to occur. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode can be further improved.

[0097] The multiplexer 36 is designed such that the multiplexing efficiency when multiplexing red light, green light, and blue light in the TE0 mode is higher than the multiplexing efficiency when multiplexing red light, green light, and blue light in the TM0 mode. In the near-eye wearable device 1, the retinal projection device 10, the laser module 13, and the optical element 30, the mode converter 35R converts the polarization mode of red light from the TM0 mode to the TE0 mode, the mode converter 35G converts the polarization mode of green light from the TM0 mode to the TE0 mode, and the mode converter 35B converts the polarization mode of blue light from the TM0 mode to the TE0 mode. Therefore, it is possible to improve the multiplexing efficiency in the multiplexer 36.

[0098] The heights of the mode converter 35R, the mode converter 35G, and the mode converter 35B are the same. According to this configuration, the mode converter 35R, the mode converter 35G, and the mode converter 35B can be formed on the same substrate 31, and the heights of the respective mode converters can be made the same, so that the optical element 30 can be easily manufactured.

[0099] In order to output full-color laser light by combining red light, green light, and blue light, 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 color of light in the light source unit 20, a large drive current is required. In the near-eye wearable device 1, the retinal projection device 10, the laser module 13, and the optical element 30, the light intensity of the red light is modulated by the modulator 34R, the light intensity of the green light is modulated by the modulator 34G, and the light intensity of the blue light is modulated by the modulator 34B. Therefore, it is possible to output full-color laser light without requiring a large drive current.

[0100] Next, a laser module according to another embodiment will be described with reference to FIG. 8. FIG. 8 is a block diagram of a laser module according to another embodiment. The laser module 13A shown in FIG. 8 mainly differs from the laser module 13 in that it includes an optical element 30A instead of the optical element 30. The optical element 30A mainly differs from the optical element 30 in that it includes one mode converter 35 instead of the mode converters 35R, 35G, and 35B, and that a multiplexer 36 is disposed between each modulator and the mode converter 35.

[0101] Specifically, the multiplexer 36 is provided downstream of the modulators 34R, 34G, and 34B, multiplexes the red light emitted from the modulator 34R, the green light emitted from the modulator 34G, and the blue light emitted from the modulator 34B into one laser beam, and emits the laser beam. The multiplexer 36 emits the laser beam to the mode converter 35.

[0102] The mode converter 35 is provided downstream of the multiplexer 36 and converts the polarization mode of the laser beam emitted from the multiplexer 36 from the TM0 mode to the TE0 mode. The configuration of the mode converter 35 is the same as the configuration of the mode converter 35B.

[0103] In the laser module 13A, visible light in the TM0 mode is emitted from each laser light source, and the light intensity of the visible light in the TM0 mode is modulated by each modulator. Then, the visible light modulated by each modulator is multiplexed in the multiplexer 36 to generate laser light. Then, the polarization mode of the laser light is converted from the TM0 mode to the TE0 mode in the mode converter 35, and the laser light in the TE0 mode is emitted from the mode converter 35 to the optical component 14 (see FIG. 2).

[0104] Also in the laser module 13A, for the components common to the laser module 13, the same effects as those of the laser module 13 are achieved. Also in the optical element 30A, for the components common to the optical element 30, the same effects as those of the optical element 30 are achieved. Since the laser module 13A and the optical element 30A include one mode converter 35 instead of the mode converters 35R, 35G, and 35B, the laser module 13A and the optical element 30A can be miniaturized.

[0105] Next, with reference to FIG. 9, a laser module according to still another embodiment will be described. FIG. 9 is a block diagram of a laser module according to still another embodiment. The laser module 13B shown in FIG. 9 mainly differs from the laser module 13 in that it includes a light source unit 20B and an optical element 30B instead of the light source unit 20 and the optical element 30. The light source unit 20B mainly differs from the light source unit 20 in that it includes laser light sources 21B, 22B, and 23B instead of the laser light sources 21, 22, and 23.

[0106] The laser light sources 21B, 22B, and 23B mainly differ from the laser light sources 21, 22, and 23 in the polarization mode of the emitted visible light. Specifically, the laser light source 21B emits red light in the TE0 mode. The laser light source 22B emits green light in the TE0 mode. The laser light source 23B emits blue light in the TE0 mode.

[0107] The optical element 30B mainly differs from the optical element 30 in that it further includes a mode converter 37R, a mode converter 37G, and a mode converter 37B.

[0108] The mode converter 37R is a mode converter that converts the polarization mode of red light from the TE0 mode to the TM0 mode. The mode converter 37R converts the polarization mode of the red light emitted from the laser light source 21 from the TE0 mode to the TM0 mode, and emits the red light in the TM0 mode to the modulator 34R.

[0109] The mode converter 37G is a mode converter that converts the polarization mode of green light from the TE0 mode to the TM0 mode. The mode converter 37G converts the polarization mode of the green light emitted from the laser light source 22 from the TE0 mode to the TM0 mode, and emits the green light in the TM0 mode to the modulator 34G.

[0110] The mode converter 37B is a mode converter that converts the polarization mode of blue light from the TE0 mode to the TM0 mode. The mode converter 37B converts the polarization mode of the blue light emitted from the laser light source 23 from the TE0 mode to the TM0 mode, and emits the blue light in the TM0 mode to the modulator 34B.

[0111] As the mode converters 37R, 37G, and 37B, for example, configurations in which the incident ends and the output ends of the mode converters 35R, 35G, and 35B are interchanged are respectively adopted. In this configuration, visible light in the TE0 mode is incident on the other end 56b of the line portion 56, an interaction occurs between the TE0 mode in the line portion 56 and the TE1 mode in the line portion 55, the polarization mode of the visible light is converted from the TE0 mode to the TE1 mode, and the visible light in the TE1 mode propagates through the line portion 55. Then, the visible light in the TE1 mode is emitted from one end 55a of the line portion 55 to the output end 54b of the taper portion 54. When the visible light in the TE1 mode is incident on the output end 54b of the taper portion 54, the visible light propagates through the taper portion 54. Then, an interaction occurs between the TE1 mode and the TM0 mode, and the polarization mode of the visible light is converted from the TE1 mode to the TM0 mode. Then, the visible light in the TM0 mode is emitted from the incident end 54a.

[0112] In the laser module 13B, since visible light in the TE0 mode is emitted from each laser light source, first, the polarization mode of each visible light emitted from each laser light source is converted from the TE0 mode to the TM0 mode in each mode converter 37R, 37G, 37B. Then, after the light intensity of the visible light in the TM0 mode is modulated in each modulator, the polarization mode of each modulated visible light is converted from the TM0 mode to the TE0 mode in each mode converter 35R, 35G, 35B. Then, each visible light is multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as laser light in the TE0 mode to the optical component 14 (see FIG. 2).

[0113] Also in the laser module 13B, for the components common to the laser module 13, the same effects as those of the laser module 13 are achieved. Also in the optical element 30B, for the components common to the optical element 30, the same effects as those of the optical element 30 are achieved. In the laser module 13B and the optical element 30B, visible light in the TE0 mode is emitted from each laser light source. Even in this case, the visible light in the TE0 mode can be emitted to the outside without reducing the modulation efficiency of each modulator.

[0114] Next, with reference to FIG. 10, a laser module according to still another embodiment will be described. FIG. 10 is a block diagram of a laser module according to still another embodiment. The laser module 13C mainly differs from the laser module 13 in that it includes an optical element 30C instead of the optical element 30. The optical element 30C mainly differs from the optical element 30 in that the direction of the C axis of the lithium niobate constituting the core layer 32 and the positions of the modulators 34R, 34G, 34B and the mode converters 35R, 35G, 35B are interchanged.

[0115] In the present embodiment, the C axis of the lithium niobate extends in the Y-axis direction. The core layer 32 is constituted by, for example, X-cut lithium niobate.

[0116] The mode converter 35R converts the polarization mode of the red light emitted from the laser light source 21 from the TM0 mode to the TE0 mode, and emits the red light in the TE0 mode to the modulator 34R. The mode converter 35G converts the polarization mode of the green light emitted from the laser light source 22 from the TM0 mode to the TE0 mode, and emits the green light in the TE0 mode to the modulator 34G. The mode converter 35B converts the polarization mode of the blue light emitted from the laser light source 23 from the TM0 mode to the TE0 mode, and emits the blue light in the TE0 mode to the modulator 34B.

[0117] The modulator 34R is provided at the subsequent stage of the mode converter 35R, modulates the optical intensity of the red light in the TE0 mode emitted from the mode converter 35R, and emits it to the multiplexer 36. The modulator 34G is provided at the subsequent stage of the mode converter 35G, modulates the optical intensity of the green light in the TE0 mode emitted from the mode converter 35G, and emits it to the multiplexer 36. The modulator 34B is provided at the subsequent stage of the mode converter 35B, modulates the optical intensity of the blue light in the TE0 mode emitted from the mode converter 35B, and emits it to the multiplexer 36. As described above, the C-axis of lithium niobate extends in the Y-axis direction. Therefore, the modulation efficiency of each modulator is improved in the TE mode.

[0118] In the laser module 13C, since visible light in the TM0 mode is emitted from each laser light source, the polarization mode of each visible light emitted from each laser light source is converted from the TM0 mode to the TE0 mode in each mode converter. Then, after the optical intensity of the visible light in the TE0 mode is modulated in each modulator, the modulated visible lights are multiplexed in the multiplexer 36, and are emitted from the multiplexer 36 as laser light in the TE0 mode to the optical component 14 (see FIG. 2).

[0119] Also in the laser module 13C, for the components common to the laser module 13, the same effects as those of the laser module 13 are achieved. Also in the optical element 30C, for the components common to the optical element 30, the same effects as those of the optical element 30 are achieved.

[0120] Note that the optical element, laser module, retinal projection device, and near-eye wearable device according to the present disclosure are not limited to the above embodiments.

[0121] For example, the laser modules 13, 13A, 13B, and 13C may be applied to devices other than the near-eye wearable device 1.

[0122] The optical elements 30, 30A, 30B, and 30C may not include the cladding layer 33. In this case, the air layer can function as the upper cladding layer.

[0123] The optical elements 30, 30A, 30B, and 30C only need to include one mode converter. In other words, the core layer 32 only needs to include one mode converter that converts the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0124] The mode converter 35R may not include the slab 53. The mode converter 35G may not include the slab 53. The mode converter 35B may not include the slab 53. The heights of the mode converter 35R, the mode converter 35G, and the mode converter 35B may be different from each other.

[0125] At any position between the incident end 54a (position X1) and the exit end 54b (position X2), the effective refractive index N TM0_1 and the effective refractive index N TE1_1 match each other, and across the incident end 54a (position X1) and the exit end 54b (position X2), the effective refractive index N TM0_1 and the effective refractive index N TE1_1 both only need to be away from the effective refractive indices of other polarization modes. Within the range where this condition is satisfied, the tapered portion 54 can be appropriately changed.

[0126] At any position between the incident end 60a (position X2) and the position X3, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 match each other, and across the incident end 60a (position X2) and the position X3, the effective refractive index N TE1_1and the effective refractive index N TE0_2 It is only necessary that any of them is separated from the effective refractive indices of other polarization modes. In the range where this condition is satisfied, the conversion region 61 can be appropriately changed.

[0127] At any position between the position X3 and the emission end 60b (position X4), the effective refractive index N TE1_1 and the effective refractive index N TE0_2 coincide with each other, and over the range between the position X3 and the emission end 60b (position X4), the effective refractive index N TE1_1 and the effective refractive index N TE0_2 It is only necessary that any of them is separated from the effective refractive indices of other polarization modes. In the range where this condition is satisfied, the conversion region 62 can be appropriately changed.

Example

[0128] Hereinafter, in order to explain the above effects, the present disclosure will be described in more detail with reference to examples. The present disclosure is not limited to these examples.

[0129] The conversion losses in the mode converters of Examples 1 to 3 were calculated. This conversion loss is the loss in the conversion from the TM0 mode to the TE0 mode. As the mode converters of Examples 1 to 3, mode converters having the same structure as the mode converter 35B shown in FIGS. 5 and 6 were used. In Examples 1 to 3, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO3) was used as the constituent material of the core layer 32, and silicon dioxide (SiO2) was used as the constituent material of the cladding layer 33.

[0130] As shown in Table 1, in Examples 1 to 3, the height Tc was set to 0.45 μm, the height Ts was set to 0.15 μm, and the tilt angle θ was set to 70°. The effective refractive index N TM0_1 and the effective refractive index N TE1_1The widths W11 and W12 were set such that the region that substantially coincides is located near the center in the X-axis direction of the tapered portion 54. The widths W13, W14, W22, W23, W24, the distance D, the minimum interval G, the length L1, the length L2, and the length L3 were set such that the length Lt is 1000 μm or less and the conversion loss is 0.5 dB or less. Note that the mode converters of Examples 1 and 2 do not include the conversion region 62. In the conversion region 61 of the mode converter of Example 1, the width of the line portion 56 continuously increases from the incident end 60a toward the position X3. In the conversion region 61 of the mode converters of Examples 2 and 3, the width of the line portion 56 is constant over the range from the incident end 60a to the position X3. As the wavelength of red light, 638 μm was used, as the wavelength of green light, 520 μm was used, and as the wavelength of blue light, 455 μm was used.

Table 1

[0131] In the mode converters of Examples 1 to 3, a relatively small conversion loss of 0.06 dB to 0.47 dB occurred. The length Lt was 661 μm to 812 μm. From this, it can be seen that low-loss mode conversion is realized along with the shortening of the length Lt.

[0132] <Evaluation of the presence or absence of a slab> Using Examples 1 to 6, the influence of the presence or absence of a slab on the conversion loss was evaluated. As the tapered portions of Examples 4 to 6, tapered portions having the same structure as the tapered portions of Examples 1 to 3 were used, except that they do not include a slab. Similar to Examples 1 to 3, in Examples 4 to 6, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO3) was used as the constituent material of the core layer 32, and silicon dioxide (SiO2) was used as the constituent material of the cladding layer 33.

[0133] As shown in Table 2, in Examples 1 to 6, the height Tc was set to 0.45 μm and the tilt angle θ was set to 70°. In Examples 1 to 3, the height Ts was set to 0.15 μm, and in Examples 4 to 6, the height Ts was set to 0 μm. That is, in Examples 4 to 6, the tapered portion 54 did not include the slab 53 and was composed only of the waveguides 51 and 52. In order to compare the conversion losses under substantially the same conditions, the effective refractive index N TM0_1 and the effective refractive index N TE1_1 were such that the region where they substantially coincided was located near the center in the X-axis direction of the tapered portion 54, and the widths W11 and W12 were set.

[0134] Note that for the tapered portion 54 including the slab 53, the effective refractive index N TE1_1 becomes smaller in the tapered portion 54 that does not include the slab 53. Therefore, the width W12 is slightly different between Example 2 and Example 5. Similarly, the width W12 is slightly different between Example 3 and Example 6. The wavelength of red light used was 638 μm, the wavelength of green light used was 520 μm, and the wavelength of blue light used was 455 μm.

Table 2

[0135] In the tapered portion 54 where the values of the parameters shown in Table 2 were set, the conversion loss was calculated while changing the length L1. This conversion loss is the loss in the conversion from the TM0 mode to the TE1 mode. The calculation results of the conversion loss are shown in FIGS. 11 to 13. FIG. 11 is a diagram showing the calculation result of the conversion loss of red light in the tapered portion. FIG. 12 is a diagram showing the calculation result of the conversion loss of green light in the tapered portion. FIG. 13 is a diagram showing the calculation result of the conversion loss of blue light in the tapered portion. The horizontal axis of FIGS. 11 to 13 indicates the length L1 (unit: μm). The vertical axis of FIGS. 11 to 13 indicates the conversion loss (unit: dB).

[0136] According to FIGS. 11 to 13, for any color of light, the length L1 required to achieve the same conversion loss is shorter for the tapered portion 54 including the slab 53 than for the tapered portion 54 not including the slab 53. Therefore, it was confirmed that by including the slab 53 in the tapered portion 54, the length L1 for achieving the desired conversion loss can be shortened, and thus the length Lt can be shortened.

[0137] Table 2 shows the conversion loss at a length L1 of 300 μm. When the length L1 is set to the same length, the conversion loss of the tapered portion 54 including the slab 53 is smaller than the conversion loss of the tapered portion 54 not including the slab 53 for any color of light. Therefore, it was confirmed that by including the slab 53 in the tapered portion 54, a decrease in the conversion efficiency can be suppressed.

[0138] <Evaluation of the Height of the Mode Converter> Using Examples 1, 7 to 9, the influence of the height Tc on the conversion loss was evaluated. As the mode converters of Examples 7 to 9, mode converters having the same structure as the mode converter of Example 1 were used. Similar to Example 1, in Examples 7 to 9, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO3) was used as the constituent material of the core layer 32, and silicon dioxide (SiO2) was used as the constituent material of the cladding layer 33.

[0139] As shown in Table 3, in Examples 1, 7 to 9, the height Ts was set to 0.15 μm and the tilt angle θ was set to 70°. In Examples 1, 7 to 9, the height Tc was set to different values. In order to compare the conversion losses under as similar conditions as possible, the effective refractive index N TM0_1 and the effective refractive index N TE1_1The width W11 and the width W12 were set so that the region that substantially coincides is located near the center in the X-axis direction of the tapered portion 54. For Examples 7 and 8, the length Lt was set to 2000 μm or less, and the width W13, the width W22, the width W23, the distance D, the minimum interval G, the length L1, and the length L2 were set so that the conversion loss is 0.5 dB or less. For Example 9, the width W13, the width W22, the width W23, the distance D, the minimum interval G, the length L1, and the length L2 were set so that the length Lt is 40000 μm or less and the conversion loss is minimized. As the wavelength of the red light, 638 μm was used.

Table 3

[0140] In the mode converter in which the values of the respective parameters shown in Table 3 were set, the conversion loss was calculated. This conversion loss is the loss in the conversion from the TM0 mode to the TE0 mode. The calculation results of the conversion loss are shown in FIG. 14. FIG. 14 is a diagram showing the relationship between the height of the mode converter, the length in the X-axis direction of the mode converter, and the conversion loss. The horizontal axis of FIG. 14 indicates the height Tc (unit: μm). The left vertical axis of FIG. 14 indicates the length Lt (unit: cm). The right vertical axis of FIG. 14 indicates the conversion loss (unit: dB).

[0141] According to Table 3 and FIG. 14, for any height Tc, a conversion loss of 2 dB or less is achieved without increasing the length Lt. When the height Tc is 0.8 μm or less, compared with the case where the height Tc is 1.0 μm, the length Lt is significantly shorter and the conversion loss is significantly reduced. It can be seen that if the height Tc is equal to or less than the wavelength of the red light (638 μm), the length Lt can be made shorter and the conversion loss can be further suppressed.

[0142] <Evaluation of the tilt angle> Using Examples 1, 10 to 12, the influence of the tilt angle θ on the conversion loss was evaluated. As the mode converters of Examples 10 to 12, mode converters having the same structure as the mode converter of Example 1 were used. Similar to Example 1, in Examples 10 to 12, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO3) was used as the constituent material of the core layer 32, and silicon dioxide (SiO2) was used as the constituent material of the cladding layer 33.

[0143] As shown in Table 4, in Examples 1, 10 to 12, the height Tc was set to 0.45 μm, and the height Ts was set to 0.15 μm. In Examples 1, 10 to 12, the tilt angle θ was set to different values. In order to compare the conversion losses under substantially the same conditions, the effective refractive index N TM0_1 and the effective refractive index N TE1_1 The widths W11 and W12 were set so that the region where they substantially coincide is located near the center in the X-axis direction of the tapered portion 54. The width W13, the width W22, the width W23, the distance D, the minimum interval G, the length L1, and the length L2 were set so that the minimum interval G is 0.2 μm or more and the conversion loss is 1.0 dB or less. The wavelength of the red light used was 638 μm.

Table 4

[0144] In the mode converter in which the values of the parameters shown in Table 4 were set, the conversion loss was calculated. This conversion loss is the loss in the conversion from the TM0 mode to the TE0 mode. The calculation results of the conversion loss are shown in FIG. 15. FIG. 15 is a diagram showing the relationship between the tilt angle, the length in the X-axis direction of the mode converter, and the conversion loss. The horizontal axis of FIG. 15 indicates the tilt angle θ (unit: deg). The left vertical axis of FIG. 15 indicates the length Lt (unit: μm). The right vertical axis of FIG. 15 indicates the conversion loss (unit: dB).

[0145] According to Table 4 and FIG. 15, for any tilt angle θ, a conversion efficiency of 85% or more (conversion loss of 0.7 dB or less) is achieved without increasing the length Lt. The conversion efficiency represents the light intensity of the visible light in the TE0 mode emitted from the output end 60b when the light intensity of the visible light in the TM0 mode incident on the input end 54a is set to 1. The conversion loss is obtained by converting the conversion efficiency into dB units. It was confirmed that when the tilt angle θ is smaller than 90° compared to the case where the tilt angle θ is 90°, the conversion loss can be suppressed even if the length Lt is short. In other words, when the cross-sectional shape of the waveguides 51 and 52 orthogonal to the X-axis direction is trapezoidal, it can be seen that the conversion loss can be suppressed while shortening the length Lt compared to the case where the cross-sectional shape is rectangular.

[0146] (Appendix) [Article 1] A substrate having a main surface, A core layer provided on the main surface and composed of a material having an electro-optic effect, Comprising, The core layer includes a mode converter that converts the polarization mode of visible light from the TM0 mode to the TE0 mode, The mode converter is, A first waveguide extending in a first direction along the main surface, A second waveguide extending in the first direction, Comprising, The first waveguide is, It has a first input end where the visible light is incident and a first output end where the visible light is emitted, and a taper portion whose length in a second direction along the main surface and intersecting the first direction increases from a first length to a second length from the first input end toward the first output end, A first line portion through which the visible light emitted from the first output end propagates, Comprising, The second waveguide includes a second line portion arranged side by side with the first line portion in the second direction, The first length is a length at which a first effective refractive index that is the effective refractive index of the TM0 mode is greater than a second effective refractive index that is the effective refractive index of the TE1 mode. The second length is a length at which the first effective refractive index is smaller than the second effective refractive index, The first waveguide section and the second waveguide section form an asymmetric directional coupler, The asymmetric directional coupler has a second input end and a second output end that are both ends in the first direction, The length of the first waveguide section in the second direction and the length of the second waveguide section in the second direction are such that the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index, which is the effective refractive index of the TE0 mode in the second waveguide section, at a position different from the second input end of the asymmetric directional coupler is reversed from the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index in the second waveguide section at the second input end. An optical element,

[0147] [Article 2] The length of the first waveguide section in the second direction increases from the second input end toward the position, The length of the second waveguide section in the second direction increases from the second input end toward the position. The optical element according to Article 1,

[0148] [Article 3] The length of the first waveguide section in the second direction increases from the second input end toward the position, The length of the second waveguide section in the second direction is constant over the range from the second input end to the position. The optical element according to Article 1,

[0149] [Article 4] The length of the first waveguide section in the second direction and the length of the second waveguide section in the second direction are such that the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index in the second waveguide section at the second output end is reversed from the magnitude relationship between the second effective refractive index in the first waveguide section and the third effective refractive index in the second waveguide section at the position. The optical element according to any one of Articles 1 to 3,

[0150] [Clause 5] The mode converter further includes a flat slab provided with the first waveguide and the second waveguide, and is the optical element according to any one of Clauses 1 to 4.

[0151] [Clause 6] The length of the mode converter in a third direction intersecting with the first direction and the second direction is smaller than the wavelength of the visible light, and is the optical element according to Clause 5.

[0152] [Clause 7] The cross-sectional shape of the first waveguide intersecting with the first direction is a trapezoidal shape in which the length in the second direction increases as it approaches the main surface, and is the optical element according to any one of Clauses 1 to 6.

[0153] [Clause 8] The core layer a first mode converter which is the mode converter for converting the polarization mode of red light from the TM0 mode to the TE0 mode, a second mode converter which is the mode converter for converting the polarization mode of green light from the TM0 mode to the TE0 mode, a third mode converter which is the mode converter for converting the polarization mode of blue light from the TM0 mode to the TE0 mode, a multiplexer for multiplexing the red light, the green light, and the blue light and emitting a laser beam, and is the optical element according to any one of Clauses 1 to 7.

[0154] [Clause 9] The lengths of the first mode converter, the second mode converter, and the third mode converter in a third direction intersecting with the first direction and the second direction are the same, and is the optical element according to Clause 8.

[0155] [Clause 10] The core layer a first modulator for modulating the light intensity of the red light, A second modulator that modulates the light intensity of the green light, A third modulator that modulates the light intensity of the blue light, The optical element according to clause 8 or clause 9, further comprising.

[0156] [Clause 11] The optical element according to any one of clauses 8 to 10, A first light source that emits the red light in the TM0 mode, A second light source that emits the green light in the TM0 mode, A third light source that emits the blue light in the TM0 mode, A laser module comprising.

[0157] [Clause 12] A retinal projection device mounted on a near-eye wearable device, The laser module according to clause 11, A movable mirror that performs scanning using the laser light emitted from the laser module, A reflector that reflects the laser light passing through the movable mirror and irradiates the reflected light onto the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina, A retinal projection device comprising.

[0158] [Clause 13] The retinal projection device according to clause 12, A lens provided with the reflector, A near-eye wearable device comprising.

Explanation of reference numerals

[0159] 1... Near-eye wearable device, 3... Lens, 10... Retinal projection device, 12... Reflector, 13, 13A, 13B, 13C... Laser module, 15... Movable mirror, 21... Laser light source (first light source), 22... Laser light source (second light source), 23... Laser light source (third light source), 30, 30A, 30B, 30C... Optical element, 31... Substrate, 31a... Main surface, 32... Core layer, 34B... Modulator (third modulator), 34G... Modulator (second modulator), 34R... Modulator (first modulator), 35... Mode converter, 35B... Mode converter (third mode converter), 35G... Mode converter (second mode converter), 35R... Mode converter (first mode converter), 36... Combiner, 51... Waveguide (first waveguide), 52... Waveguide (second waveguide), 53... Slab, 54... Taper section, 54a... Incident end (first incident end), 54b... Exit end (first exit end), 55... Line section (first line section), 56... Line section (second line section), 60... Asymmetric directional coupler, 60a... Incident end (second incident end), 60b... Exit end (second exit end), X3... Position.

Claims

1. A substrate having a main surface, a core layer provided on the main surface and composed of a material having an electro-optical effect, comprising: the core layer includes a mode converter that converts a polarization mode of visible light from a TM0 mode to a TE0 mode, the mode converter, a first waveguide extending in a first direction along the main surface, a second waveguide extending in the first direction, comprising: the first waveguide, has a first incident end where the visible light is incident and a first exit end where the visible light exits, and a taper portion whose length in a second direction along the main surface and intersecting the first direction increases from a first length to a second length from the first incident end toward the first exit end, a first line portion through which the visible light emitted from the first exit end propagates, comprising: the second waveguide includes a second line portion arranged side by side with the first line portion in the second direction, the first length is a length at which a first effective refractive index, which is an effective refractive index of the TM0 mode, is greater than a second effective refractive index, which is an effective refractive index of the TE1 mode, the second length is a length at which the first effective refractive index is smaller than the second effective refractive index, the first line portion and the second line portion constitute an asymmetric directional coupler, the asymmetric directional coupler has a second incident end and a second exit end that are both ends in the first direction, the length of the first line portion in the second direction and the length of the second line portion in the second direction are such that the magnitude relationship between the second effective refractive index in the first line portion and the third effective refractive index, which is the effective refractive index of the TE0 mode in the second line portion, at a position different from the second incident end of the asymmetric directional coupler is reversed from the magnitude relationship between the second effective refractive index in the first line portion and the third effective refractive index in the second line portion at the second incident end, an optical element.

2. The length of the first line portion in the second direction increases from the second incident end toward the position, The length of the second line portion in the second direction increases from the second incident end toward the position. The optical element according to claim 1.

3. The length of the first line portion in the second direction increases from the second incident end toward the position, The length of the second line portion in the second direction is constant over the range from the second incident end to the position. The optical element according to claim 1.

4. The lengths of the first waveguide portion in the second direction and the second waveguide portion in the second direction are set such that the magnitude relationship between the second effective refractive index in the first waveguide portion and the third effective refractive index in the second waveguide portion at the second output end is reversed from the magnitude relationship between the second effective refractive index in the first waveguide portion and the third effective refractive index in the second waveguide portion at the position. The optical element according to any one of claims 1 to 3.

5. The mode converter further includes a flat slab provided with the first waveguide and the second waveguide. The optical element according to any one of claims 1 to 3.

6. The length of the mode converter in a third direction intersecting the first direction and the second direction is smaller than the wavelength of the visible light. The optical element according to claim 5.

7. The cross-sectional shape of the first waveguide intersecting the first direction is a trapezoidal shape in which the length in the second direction increases as it approaches the main surface. The optical element according to any one of claims 1 to 3.

8. The core layer is a first mode converter that is a mode converter for converting the polarization mode of red light from the TM0 mode to the TE0 mode, a second mode converter that is a mode converter for converting the polarization mode of green light from the TM0 mode to the TE0 mode, a third mode converter that is a mode converter for converting the polarization mode of blue light from the TM0 mode to the TE0 mode, a multiplexer that multiplexes the red light, the green light, and the blue light and emits laser light, and includes. The optical element according to any one of claims 1 to 3.

9. The lengths of the first mode converter in a third direction intersecting the first direction and the second direction, the length of the second mode converter in the third direction, and the length of the third mode converter in the third direction are the same. The optical element according to claim 8.

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

11. The optical element according to claim 8, a first light source that emits the red light in the TM0 mode, a second light source that emits the green light in the TM0 mode, a third light source that emits the blue light in the TM0 mode, and includes. A laser module.

12. A retinal projection device mounted on a near-eye wearable device, the laser module according to claim 11, a movable mirror that performs scanning using the laser light emitted from the laser module, a reflector that projects an image onto the retina by reflecting the laser light that has passed through the movable mirror and irradiating the reflected light onto the retina of a user wearing the near-eye wearable device, A retinal projection device comprising:

13. The retinal projection device according to claim 12, a lens provided with the reflector, A near-eye wearable device comprising:

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