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

Abstract

The invention relates to an optical element, a laser module, a retina projection device, and a near-to-eye wearable device. A mode converter of an optical element is provided with: a tapered section having a length in a second direction that increases from a first length, in which an effective refractive index in a TM0 mode is greater than an effective refractive index in a TE1 mode, to a second length, in which the effective refractive index in the TM0 mode is smaller than the effective refractive index in the TE1 mode, from a first incidence end toward a first emission end; and a first line section and the second line section that constitute an asymmetric directional coupler. The length of the first line portion in the second direction and the length of the second line portion in the second direction are set to be equal to each other. The magnitude relationship between the effective refractive index of the TE1 mode in the first line section and the effective refractive index of the TE0 mode in the second line section at a position different from the second incident end of the asymmetric directional coupler is inverted from the magnitude relationship between the two effective refractive indexes at the second incident end.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority to Japanese Patent Application No. 2023 - 209080, filed with the Japan Patent Office on December 12, 2023, the entire content of which is incorporated herein by reference. Technical field

[0003] The present disclosure relates to an optical element, a laser module, a retinal projection device, and a near - eye wearable device. Background art

[0004] In the polarization wave mode of light transmitted in an optical waveguide, there are a polarization wave mode having a main electric field in the horizontal direction with respect to the substrate, that is, a TE (Transverse Electric) mode, and a polarization wave mode having a main electric field in the vertical direction with respect to the substrate, that is, a TM (Transverse Magnetic) mode. There are known optical waveguide elements that convert these polarization wave modes. For example, Non - Patent Document 1 (Daoxin Dai and John E. Bowers, “Novel concept for ultracompact polarization splitter - rotator based on silicon nanowires”, Optics Express, 2011, Vol. 19, Issue 11, pp. 10940 - 10949) describes a polarization beam splitter - rotator including a tapered structure and an asymmetric directional coupler. Summary of the invention

[0005] The optical waveguide element described in Non - Patent Document 1 converts light of the TM0 mode having a wavelength of 1.45 μm to 1.6 μm into light of the TE0 mode. However, visible light is not considered.

[0006] The present disclosure describes an optical element, a laser module, a retinal projection device, and a near - eye wearable device that can convert the polarization wave mode of visible light from the TM0 mode to the TE0 mode.

[0007] An 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 made of a material having an electro-optical effect. The core layer includes a mode converter that converts a polarization wave mode of visible light from a TM0 mode to a 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 includes: a tapered portion having a first incident end for incident visible light and a first emission end for emitting visible light, wherein a length of the tapered portion in a second direction increases from a first length to a second length as it goes from the first incident end to the first emission end, the second direction being along the main surface and intersecting the first direction; and a first line portion for transmitting the visible light emitted from the first emission end. The second waveguide includes a second line portion arranged in parallel 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 form an asymmetric directional coupler. The asymmetric directional coupler has a second incident end and a second emission end at both ends in the first direction. The lengths of the first line portion and the second line portion in the second direction are set such that the 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 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.

[0008] In this optical element, a region where the effective refractive index of the TM0 mode and the effective refractive index of the TE1 mode are substantially the same is formed in the tapered portion. Therefore, when visible light in the TM0 mode is incident on the first incident end, an interaction occurs between the TM0 mode and the TE1 mode in the above region. Thereby, the polarization wave mode of the visible light is converted from the TM0 mode to the TE1 mode, and the visible light in the TE1 mode is emitted from the first emission end. Further, a region where 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 are substantially the same is formed between the second incident end of the asymmetric directional coupler and a position different from the second incident end. Therefore, when visible light in the TE1 mode is incident on the second incident end, an interaction occurs between the TE1 mode and the TE0 mode in the above region. Thereby, the polarization wave mode of the visible light is converted from the TE1 mode to the TE0 mode, and the visible light in the TE0 mode is emitted from the second emission end. As described above, the polarization wave mode of the visible light can be converted from the TM0 mode to the TE0 mode.

[0009] Alternatively, the length of the first line portion in the second direction increases from the second incident end toward the above-mentioned position, and the length of the second line portion in the second direction increases from the second incident end toward the above-mentioned position. According to this structure, 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-mentioned position. Therefore, compared with a structure in which either the second effective refractive index in the first line portion or the third effective refractive index in the second line portion is constant in the range from the second incident end to the above-mentioned position, the angle formed by the curve representing the relationship between the position in the first direction and the second effective refractive index in the first line portion and the curve representing the relationship between the position in the first direction and the third effective refractive index in the second line portion can be reduced. Thereby, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0010] Alternatively, the length of the first line portion in the second direction increases from the second incident end toward the above-mentioned position, and the length of the second line portion in the second direction is constant in the range from the second incident end to the above-mentioned position. According to this structure, the second effective refractive index in the first line portion increases from the second incident end toward the above-mentioned position, while the third effective refractive index in the second line portion is constant in the range from the second incident end to the above-mentioned position. Therefore, by lengthening the length from the second incident end to the above-mentioned position, the angle formed by the curve representing the relationship between the position in the first direction and the second effective refractive index in the first line portion and the curve representing the relationship between the position in the first direction and the third effective refractive index in the second line portion can be reduced. Thereby, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0011] Alternatively, the lengths of the first line portion and the second line portion in the second direction are set such that 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 emission end 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 above-mentioned position. In this case, a region where 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 are substantially the same is formed between the above-mentioned position and the second emission end, 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.

[0012] Alternatively, the mode converter further includes a flat plate on which the first waveguide and the second waveguide are disposed. According to this structure, visible light is transmitted from the first waveguide and the second waveguide to the flat plate, thereby enhancing the optical coupling between the first waveguide and the second waveguide. Thus, the conversion efficiency of converting from the TE1 mode to the TE0 mode using the asymmetric directional coupler can be improved.

[0013] Alternatively, the length of the mode converter in the third direction that intersects the first direction and the second direction may be smaller than the wavelength of visible light. In this case, visible light is likely to leak from the first waveguide and the second waveguide to the flat plate. As a result, the optical coupling between the first waveguide and the second waveguide by the flat plate can be further enhanced. Therefore, the conversion efficiency of converting from the TE1 mode to the TE0 mode using the asymmetric directional coupler can be further improved.

[0014] Alternatively, the cross-sectional shape of the first waveguide that intersects the first direction is a trapezoidal shape in which the length in the second direction increases toward the main surface. In this case, the first waveguide has an asymmetric shape in the third direction that intersects the first direction and the second direction. Therefore, the conversion efficiency of converting from the TM0 mode to the TE1 mode using the tapered portion can be improved.

[0015] Alternatively, the core layer includes: a first mode converter that converts the polarization wave mode of red light from the TM0 mode to the TE0 mode; a second mode converter that converts the polarization wave mode of green light from the TM0 mode to the TE0 mode; a third mode converter that converts the polarization wave mode of blue light from the TM0 mode to the TE0 mode; and a multiplexer that multiplexes red light, green light, and blue light and emits a laser. According to this structure, the polarization wave mode of red light is converted from the TM0 mode to the TE0 mode, the polarization wave mode of green light is converted from the TM0 mode to the TE0 mode, and the polarization wave mode of blue light is converted from the TM0 mode to the TE0 mode. For example, when the multiplexer is designed to multiplex TE0 mode red light, green light, and blue light, the multiplexing efficiency is higher than when multiplexing TM0 mode red light, green light, and blue light, and the multiplexing efficiency of the multiplexer can be improved.

[0016] Alternatively, the lengths of the first mode converter, the second mode converter, and the third mode converter in the third direction that intersects the first direction and the second direction are the same. According to this structure, the first mode converter, the second mode converter, and the third mode converter can be formed on the same substrate, and the lengths of the respective mode converters in the third direction can be made the same, so that the optical element can be easily manufactured.

[0017] Alternatively, the core layer may further include: a first modulator that modulates the intensity of red light; a second modulator that modulates the intensity of green light; and a third modulator that modulates the intensity of blue light. In order to output full-color laser light by combining red light, green light, and blue light, it is necessary to adjust the intensity of the light of each color according to the output color. According to the above structure, the intensities of red light, green light, and blue light are modulated, so full-color laser light can be output without a large drive current.

[0018] A laser module according to another aspect of the present disclosure includes: the above-described 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, the polarization wave modes of red light, green light, and blue light can be converted from the TM0 mode to the TE0 mode.

[0019] 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-described laser module; a movable mirror that scans using the laser light emitted from the laser module; and 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. This retinal projection device includes the above-described optical element. Therefore, in this retinal projection device, an image can be projected onto the retina on the basis of converting the polarization wave modes of red light, green light, and blue light from the TM0 mode to the TE0 mode.

[0020] A near-eye wearable device according to yet 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, an image can be projected onto the retina on the basis of converting the polarization wave modes of red light, green light, and blue light from the TM0 mode to the TE0 mode.

[0021] According to various aspects and embodiments of the present disclosure, the polarization wave mode of visible light can be converted from the TM0 mode to the TE0 mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 FIG. is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied.

[0023] Figure 2 Schematically shows Figure 1 a configuration diagram of the retinal projection device shown in FIG.

[0024] Figure 3 FIG. is Figure 2Block diagram of the laser module shown.

[0025] Figure 4 It represents Figure 3 Figure showing the cross-sectional structure of the optical element shown.

[0026] Figure 5 It represents Figure 3 Top view showing the structure of the mode converter shown.

[0027] Figure 6 It is a cross-sectional view along Figure 5 Line VI-VI shown.

[0028] Figure 7 It is used to illustrate Figure 5 Figure showing the conversion principle of the mode converter shown.

[0029] Figure 8 Block diagram of the laser module of another embodiment.

[0030] Figure 9 Block diagram of the laser module of yet another embodiment.

[0031] Figure 10 Block diagram of the laser module of still another embodiment.

[0032] Figure 11 Figure showing the calculation results of the conversion loss of red light in the tapered portion.

[0033] Figure 12 Figure showing the calculation results of the conversion loss of green light in the tapered portion.

[0034] Figure 13 Figure showing the calculation results of the conversion loss of blue light in the tapered portion.

[0035] Figure 14 Figure showing the relationship between the height of the mode converter, the length of the mode converter in the X-axis direction, and the conversion loss.

[0036] Figure 15 Figure showing the relationship between the tilt angle, the length of the mode converter in the X-axis direction, and the conversion loss. Detailed implementation mode

[0037] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. In the description of the drawings, the same reference numerals are assigned to the same elements, and redundant descriptions are omitted. In each figure, an XYZ coordinate system may be shown. The Y-axis direction (second direction) is a direction that intersects (e.g., is orthogonal to) the X-axis direction (first direction) and the Z-axis direction (third direction). The Z-axis direction is a direction that intersects (e.g., is orthogonal to) the X-axis direction and the Y-axis direction. In this specification, a numerical range indicated by "~" indicates a range that includes the numerical values described before and after "~" as the minimum value and the maximum value, respectively. The upper limit value and the lower limit value described separately can be arbitrarily combined.

[0038] Refer to Figure 1 An application example of a laser module according to an embodiment will be described. Figure 1 It is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. Figure 1 The near-eye wearable device 1 shown 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 (headset), and can be in the form of glasses, goggles, a hat, or a helmet. 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, lenses 3, and a retina projection device 10.

[0039] The frame 2 includes a pair of lens rims 2a, a nose bridge 2b, and a pair of temple arms 2c. The lens rims 2a are the parts that hold the lenses 3. The nose bridge 2b is the part that connects the pair of lens rims 2a. The temple arms 2c are the parts that extend from the lens rims 2a and hang on the ears of the user. The frame 2 may also be a frame without lens rims. The lenses 3 have an inner surface 3a that faces the eyeball of the user wearing the near-eye wearable device 1 (refer to Figure 2 ).

[0040] The retina 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 retina projection device 10 is mounted on the near-eye wearable device 1. In the present embodiment, in order to project an image onto the retinas on both sides, the near-eye wearable device 1 includes two retina projection devices 10, but it may also include only any one of the retina projection devices 10.

[0041] Next, refer to Figure 2 The retina projection device 10 will be described in detail. Figure 2 It schematically shows Figure 1 The configuration diagram of the retina projection device shown. AsFigure 2 As shown in Figure 2 , the retinal projection device 10 includes an optical engine 11 and a reflector 12.

[0042] The optical engine 11 is a device that generates a laser Ls corresponding to the color and light intensity of the pixels of the image projected onto the retina and emits the laser Ls toward the reflector 12. The optical engine 11 is mounted on the temple 2c. The optical engine 11 includes a laser module 13, an optical component 14, a movable mirror 15, a laser driver 16, a mirror driver 17, and a controller 18.

[0043] The laser module 13 emits a laser. As the laser module 13, for example, a full-color laser module is used. The laser module 13 emits a laser corresponding to the color and light intensity of the pixels of the image projected onto the retina. The details of the laser module 13 will be described later.

[0044] The optical component 14 is a component that optically processes the laser emitted from the laser module 13. In the present embodiment, the optical component 14 includes a collimating lens 14a, a diaphragm 14b, and a light attenuation filter 14c. The collimating lens 14a, the diaphragm 14b, and the light attenuation filter 14c are arranged in sequence along the optical path of the laser. The optical component 14 may also have other structures.

[0045] The movable mirror 15 is a member for scanning using the laser emitted from the laser module 13. The movable mirror 15 is provided in the emission direction of the laser processed by the optical component 14. The movable mirror 15 is configured to be able to swing around an axis extending in the lateral direction of the lens 3 and an axis extending in the longitudinal direction of the lens 3, and change the angle in the lateral and longitudinal directions of the lens 3 to reflect the laser. As the movable mirror 15, for example, a MEMS (Micro Electro Mechanical Systems) mirror is used.

[0046] 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 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 causes the movable mirror 15 to swing within a predetermined angle range and timing. The controller 18 is a device that controls the laser driver 16 and the mirror driver 17.

[0047] In the optical engine 11, a laser corresponding to the color and light intensity of 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 reflected by the movable mirror 15 is emitted as the laser Ls toward the reflector 12.

[0048] The reflector 12 is a component that projects an image onto the retina by reflecting the laser beam Ls that has passed through the movable mirror 15 and irradiating the reflected light Lr onto the retina of the user wearing the near-eye wearable device 1. The reflector 12 is provided on the inner surface 3a of the lens 3.

[0049] Next, with reference to Figure 3 and Figure 4 the laser module 13 will be described in detail. Figure 3 is Figure 2 a block diagram of the laser module shown. Figure 4 is a diagram showing Figure 3 the cross-sectional structure of the optical element shown. As Figure 3 shown, the laser module 13 includes a light source unit 20 and an optical element 30.

[0050] 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 sequence along the Y-axis direction.

[0051] In the present embodiment, the laser light source 21 emits red light in the TM fundamental mode (hereinafter referred to as "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 the red light, the green light, and the blue light are all visible light, in the following description, the red light, the green light, and the blue light may sometimes be referred to as the respective visible lights, and sometimes the red light, the green light, and the blue light may be collectively referred to as visible light.

[0052] The optical element 30 combines the visible lights emitted from the respective laser light sources into one laser beam. The optical element 30 is, for example, a Planar Lightwave Circuit (PLC). As Figure 4 shown, the optical element 30 includes a substrate 31, a core layer 32, and a cladding layer 33.

[0053] The substrate 31 functions as a lower cladding. The substrate 31 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. As an example of the constituent material of the substrate 31, sapphire, silicon oxide, and an organosilicon compound formed by laminating silicon oxides are listed. The substrate 31 has a main surface 31a and a back surface 31b on the side opposite to the main surface 31a. The main surface 31a and the back surface 31b are surfaces 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.

[0054] The cladding 33 functions as an upper cladding. The cladding 33 covers the core layer 32 on the main surface 31a. The cladding 33 is provided on the entire surface of the main surface 31a. The cladding 33 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. As an example of the constituent material of the cladding 33, silicon oxide (e.g., SiO 2 ) is listed.

[0055] 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 refers to the phenomenon in which the refractive index of a material changes by applying an electric field to the material. As an example of the constituent material of the core layer 32, lithium niobate (LiNbO 3 ) is listed. 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 also be made of Z-cut lithium niobate.

[0056] 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.

[0057] The modulator 34R is a modulator that modulates the light intensity of red light. The modulator 34R modulates the light intensity of the TM0 mode red light 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 TM0 mode green light 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 TM0 mode blue light emitted from the laser light source 23. Each modulator is, for example, a Mach-Zehnder type modulator.

[0058] The mode converter 35R is a mode converter that converts the polarization wave 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 in the downstream section of the modulator 34R and converts the polarization wave 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 wave mode of green light from the TM0 mode to the TE0 mode. The mode converter 35G is provided in the downstream section of the modulator 34G and converts the polarization wave 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 wave mode of blue light from the TM0 mode to the TE0 mode. The mode converter 35B is provided in the downstream section of the modulator 34B and converts the polarization wave mode of the blue light emitted from the modulator 34B from the TM0 mode to the TE0 mode.

[0059] In addition, the polarization wave mode is also referred to as the waveguide mode. The TM mode is a polarization wave mode in which the orientation of the main component of the electric field in the cross-section perpendicular to the light traveling direction is perpendicular to the main surface 31a of the substrate 31. The TE mode is a polarization wave mode in which the orientation of the main component of the electric field in the cross-section perpendicular to the light traveling direction is parallel to the main surface 31a of the substrate 31. The TM0 mode is the polarization wave mode with the largest effective refractive index in the TM mode. The TE0 mode is the polarization wave mode with the largest effective refractive index in the TE mode. The TE first-order mode (hereinafter referred to as the "TE1 mode") is the polarization wave mode with the second largest effective refractive index in the TE mode.

[0060] The mode converter 35R, the mode converter 35G, and the mode converter 35B extend in the X-axis direction respectively. The mode converter 35R, the mode converter 35G, and the mode converter 35B are arranged in sequence in the Y-axis direction. The detailed structures of the respective mode converters will be described later.

[0061] 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).

[0062] In the laser module 13, visible light in the TM0 mode is emitted from each laser light source, the light intensity of each visible light is modulated in each modulator, and then the polarization wave 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 wave modes are multiplexed in the multiplexer 36, and the laser beam in the TE0 mode is emitted from the multiplexer 36 to the optical component 14 (seeFigure 2 ) Emission.

[0063] Next, with reference to Figure 5 and Figure 6 the detailed structures of the mode converter 35R, the mode converter 35G, and the mode converter 35B will be described. Figure 5 represents Figure 3 a top view of the structure of the mode converter shown. Figure 6 is a cross-sectional view taken along the Figure 5 VI-VI line of. Here, the mode converter 35B will be described as an example. As Figure 5 and Figure 6 shown, the mode converter 35B forms a ridge waveguide, including a waveguide 51 (first waveguide), a waveguide 52 (second waveguide), and a flat plate 53. In addition, in Figure 5 only the waveguide 51 and the waveguide 52 are illustrated for ease of explanation.

[0064] The flat plate 53 is a flat plate-like portion of the ridge waveguide. The flat plate 53 is provided on the main surface 31a. The waveguide 51 and the waveguide 52 are provided on the flat plate 53. In addition, the waveguide 51, the waveguide 52, and the flat plate 53 are made of the same material. The length (height Ts) of the flat plate 53 in the Z-axis direction is, for example, 0 μm to 0.2 μm. Hereinafter, the length in the Z-axis direction may sometimes be referred to as "height". In addition, when the height Ts is 0 μm, it means that the flat plate 53 is not provided. That is, the mode converter 35B may not include the flat plate 53.

[0065] The waveguide 51 is a convex portion of the ridge waveguide. The waveguide 51 is provided on the flat plate 53 and extends linearly in the X-axis direction. The waveguide 51 may also have a shape symmetric with respect to the symmetry plane SP1. The symmetry plane SP1 is an imaginary plane defined by the X-axis direction and the Z-axis direction and is a plane passing through the center in the Y-axis direction of the waveguide 51.

[0066] The cross-sectional shape of the waveguide 51 intersecting (orthogonal) the X-axis direction is a trapezoidal shape in which the length in the Y-axis direction increases toward 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 also be a rectangular shape. Hereinafter, the length in the Y-axis direction may sometimes 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.

[0067] The waveguide 52 is the convex portion of the ridge waveguide. The waveguide 52 is provided on the flat plate 53 and extends linearly in the X-axis direction. The waveguide 52 is arranged side by side with a part of the waveguide 51 in the Y-axis direction. The waveguide 52 may also have a shape symmetric with respect to the symmetry plane SP2. The symmetry plane SP2 is an imaginary plane defined by the X-axis direction and the Z-axis direction and is a plane passing through the center in the Y-axis direction of the waveguide 52.

[0068] The cross-sectional shape of the waveguide 52 intersecting (orthogonal) with the X-axis direction is a trapezoidal shape whose width increases toward the main surface 31a. In the present embodiment, the above 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 inclination angle θ. The above cross-sectional shape of the waveguide 52 may also be a rectangular shape. The height Tr2 of the waveguide 52 is substantially constant within the entire length range in the X-axis direction of the waveguide 52 and is substantially the same as the height Tr1 of the waveguide 51.

[0069] The length Lt in the X-axis direction of the mode converter 35B is the sum of the length L1, the length L2, and the length L3 described later. The length Lt is, for example, 15 μm to 150000 μm. The height Tc of the mode converter 35B is substantially constant within the entire length range in the X-axis direction of the mode converter 35B. 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.

[0070] 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).

[0071] The tapered portion 54 functions as a conversion portion for converting the polarization wave 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 emission end 54b (first emission end) at both ends in the X-axis direction. The incident end 54a is located at the position X1 in the X-axis direction. The emission 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 emission 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 50000 μm.

[0072] The width of the tapered portion 54 increases from the incident end 54a toward the emission end 54b. Specifically, the width of the tapered portion 54 continuously increases from the width W11 (the first length) to the width W12 (the second length) as it goes from the incident end 54a toward the emission end 54b. The rate of increase in the width of the tapered portion 54 may also 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 (the first effective refractive index) is greater than the effective refractive index of the TE1 mode (the second effective refractive index) and less than the effective refractive index of the TE0 mode. The width W11 is, for example, from 0.3 μm to 1.0 μm. The width W12 of the tapered portion 54 at the emission 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, from 0.4 μm to 1.2 μm.

[0073] The line portion 55 is provided in the downstream section of the tapered portion 54 and is a part for transmitting the blue light emitted from the emission end 54b. The line portion 55 has one end 55a and the other end 55b that are the two ends in the X-axis direction. One end 55a is connected to the emission end 54b. The blue light emitted from the tapered portion 54 is incident on one end 55a. The other end 55b can emit blue light.

[0074] The line portion 56 is arranged in parallel with the line portion 55 in the Y-axis direction. The line portion 56 has one end 56a and the other end 56b that are the two ends in the X-axis direction. The other end 56b can emit blue light.

[0075] The line portion 55 and the line portion 56 are arranged in parallel in the Y-axis direction to form an asymmetric directional coupler 60. The center of the line portion 55 in the Y-axis direction and the center of the line portion 56 in the Y-axis direction are separated by a distance D. The distance D is, for example, from 0.4 μm to 2.0 μm. The line portion 55 and the line portion 56 are separated from each other in the Y-axis direction. The minimum interval G between the line portion 55 and the line portion 56 is, for example, from 0.1 μm to 0.9 μm.

[0076] The asymmetric directional coupler 60 has an incident end 60a (the second incident end) and an emission end 60b (the second emission end) that are the two ends in the X-axis direction. The incident end 60a is composed of one end 55a and one end 56a. The emission end 60b is composed of the other end 55b and the other end 56b. The incident end 60a is located at a position X2 in the X-axis direction. The emission end 60b is located at a position X4 in the X-axis direction.

[0077] The asymmetric directional coupler 60 is divided into a conversion region 61 and a conversion region 62 at a position X3 in the X-axis direction. The position X3 is a 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 a region of the asymmetric directional coupler 60 from the incident end 60a to the position X3. The conversion region 62 is a 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.

[0078] In the conversion region 61, 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 at the position X3 and the effective refractive index of the TE0 mode in the line portion 56 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 incident end 60a. In the present embodiment, in the conversion region 61, 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 at the incident end 60a is smaller than the effective refractive index of the TE0 mode (the third effective refractive index) in the line portion 56, and the effective refractive index of the TE1 mode in the line portion 55 at the position X3 is larger than the effective refractive index of the TE0 mode in the line portion 56.

[0079] In the present 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 increase rate of the width of the line portion 55 in the conversion region 61 may also be substantially constant. The width W13 is larger than the width W12 and is, for example, 0.4 μm to 1.5 μm.

[0080] In the conversion region 61, the width of the line portion 56 is substantially constant within 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 also 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.

[0081] In the conversion region 62, 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 at the emission end 60b and the effective refractive index of the TE0 mode in the line part 56 is reversed from the magnitude relationship between the effective refractive index of the TE1 mode in the line part 55 at the position X3 and the effective refractive index of the TE0 mode in the line part 56. In the present embodiment, in the conversion region 62, 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 larger than the effective refractive index of the TE0 mode in the line part 56 at the position X3, and the effective refractive index of the TE1 mode in the line part 55 is smaller than the effective refractive index of the TE0 mode in the line part 56 at the emission end 60b.

[0082] In the present embodiment, in the conversion region 62, the width of the line part 55 decreases from the position X3 toward the emission end 60b (the other end 55b). More specifically, in the conversion region 62, the width of the line part 55 continuously decreases from the width W13 to the width W14. The reduction rate of the width of the line part 55 may also be substantially constant. The width W14 is smaller than the width W13 and is, for example, 0.4 μm to 1.2 μm.

[0083] In the conversion region 62, the width of the line part 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 part 56 continuously increases from the width W23 to the width W24. The increase rate of the width of the line part 56 in the conversion region 62 may also be substantially constant. The width W24 is larger than the width W23 and is, for example, 0.2 μm to 1.0 μm.

[0084] The mode converter 35R and the mode converter 35G have the same structure 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.

[0085] Next, further refer to Figure 7 to explain the conversion principle of the mode converter 35R, the mode converter 35G, and the mode converter 35B. Figure 7 is for explaining Figure 5 the conversion principle of the mode converter shown. Figure 7 The horizontal axis of Figure 7 represents the position in the X-axis direction,

[0086] The effective refractive index of the waveguide depends on the constituent material of the waveguide, the polarization wave mode of the light transmitted in the waveguide, the cross-sectional shape of the waveguide, and the cross-sectional area of the waveguide. For example, if the cross-sectional area of the waveguide is large, the light is strongly confined by the waveguide and is easily affected by the refractive index of the constituent material of the waveguide, so the effective refractive index is large. On the other hand, if the cross-sectional area of the waveguide is small, the confinement of the light is weak and the light leaks out to the substrate 31 and the cladding 33, so the effective refractive index is small.

[0087] The "effective refractive index" refers to the effective refractive index of an isolated waveguide. For example, the effective refractive index N of the TE1 mode of the waveguide 51 TE1_1 refers to the effective refractive index of the TE1 mode when only the waveguide 51 exists alone. That is, the effective refractive index N TE1_1 refers to the effective refractive index of the TE1 mode assuming that the waveguide 52 does not exist. The state where the waveguide 52 does not exist means that the part where the waveguide 52 exists is replaced with a material the same as the cladding 33.

[0088] Similarly, the effective refractive index N of the TM0 mode of the waveguide 51 TM0_1 refers to the effective refractive index of the TM0 mode when only the waveguide 51 exists alone. The effective refractive index N of the TE0 mode of the waveguide 51 TE0_1 refers to the effective refractive index of the TE0 mode when only the waveguide 51 exists alone. The effective refractive index N of the TE0 mode of the waveguide 52 TE0_2 refers to the effective refractive index of the TE0 mode when only the waveguide 52 exists alone.

[0089] First, the conversion principle of the tapered portion 54 will be described. As Figure 7 shown, 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 set to a width 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 emission 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 .

[0090] 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 between the effective refractive index N TM0_1 and the effective refractive index N TE1_1In a region where the effective refractive indices are substantially the same, an interaction is generated between the TM0 mode and the TE1 mode. The effective refractive index N TM0_1 and the effective refractive index N TE1_1 The position where the magnitude relationship is reversed can be the midpoint between position X1 and position X2.

[0091] When visible light in the TM0 mode is incident on the incident end 54a of the tapered portion 54, the visible light is transmitted through the tapered portion 54. Then, in the region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 are substantially the same, an interaction is generated between the TM0 mode and the TE1 mode, and the polarization wave mode of the visible light is converted from the TM0 mode to the TE1 mode. Then, the visible light in the TE1 mode is emitted from the emission end 54b. In addition, the length L1 is set such that the conversion efficiency from the TM0 mode to the TE1 mode is maximized.

[0092] Between position X1 and position X2, the effective refractive index N TE0_1 deviates from the effective refractive indices of other polarization wave modes. Therefore, when visible light in the TE0 mode is incident on the incident end 54a, the visible light is transmitted through the tapered portion 54 while maintaining the polarization wave mode in the TE0 mode, and thus the visible light in the TE0 mode is emitted from the emission end 54b.

[0093] Next, the conversion principle of the asymmetric directional coupler 60 will be described. As described above, the widths of the line portion 55 and the line portion 56 are set such that at position X2, the effective refractive index N TE1_1 is smaller than the effective refractive index N TE0_2 , at position X3, the effective refractive index N TE1_1 is larger than the effective refractive index N TE0_2 , and at position X4, the effective refractive index N TE1_1 is smaller than the effective refractive index N TE0_2 .

[0094] Here, generally speaking, if the effective refractive indices of two polarization wave modes respectively transmitted in two juxtaposed waveguides are close to each other, the interaction between these two polarization wave modes is enhanced, and thus it is easier to generate a conversion from one polarization wave mode to another. The reason is that the phase velocities (light speed / effective refractive index) of the two polarization wave modes are closer to each other, the more the conditions for transmission in adjacent waveguides are satisfied.

[0095] Between position X2 and position X3, the magnitude relationship between the effective refractive index N TE1_1 and the effective refractive index N TE0_2 is reversed, and between the effective refractive index N TE1_1 and the effective refractive index N TE0_2In the substantially consistent region, an interaction occurs between the TE1 mode in the line unit 55 and the TE0 mode in the line unit 56. The effective refractive index N TE1_1 and the effective refractive index N TE0_2 The position where the magnitude relationship is reversed can be the midpoint between the position X2 and the position X3.

[0096] Moreover, between the position X3 and the position X4, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 The magnitude relationship is reversed again. In the region where the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are substantially consistent, an interaction occurs between the TE1 mode in the line unit 55 and the TE0 mode in the line unit 56. The effective refractive index N TE1_1 and the effective refractive index N TE0_2 The position where the magnitude relationship is reversed can be the midpoint between the position X3 and the position X4.

[0097] When visible light in the TE1 mode is incident on one end 55a of the line unit 55, the visible light is transmitted in the line unit 55. At this time, in the region where the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are substantially consistent, an interaction occurs between the TE1 mode in the line unit 55 and the TE0 mode in the line unit 56. As a result, the polarization wave mode of the visible light is converted from the TE1 mode to the TE0 mode, so that the visible light in the TE0 mode is transmitted in the line unit 56. The remaining part of the visible light transmitted in the line unit 55 that is not converted to the TE0 mode is transmitted in the line unit 55 while maintaining the polarization wave mode of the visible light in the TE1 mode.

[0098] In addition, in the region where the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are substantially consistent, an interaction occurs between the remaining TE1 mode transmitted in the line unit 55 and the TE0 mode in the line unit 56. As a result, the polarization wave mode of the visible light is converted from the TE1 mode to the TE0 mode, so that the visible light in the TE0 mode is transmitted in the line unit 56. Then, the visible light in the TE0 mode is emitted from the other end 56b. In addition, the lengths L2 and 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.

[0099] Between the position X2 and the position X4, the effective refractive index N TE0_1Deviation from the effective refractive index of other polarization wave modes. Thus, when visible light in the TE0 mode is incident on one end 55a, the visible light is transmitted in the line portion 55 while maintaining the polarization wave mode in the TE0 mode, and the visible light in the TE0 mode is emitted from the other end 55b.

[0100] In this way, in multiple regions, by making the effective refractive index N TE1_1 and the effective refractive index N TE0_2 substantially consistent, the number of times of conversion from the TE1 mode to the TE0 mode can be increased, and the conversion efficiency can be improved.

[0101] In the laser module 13 and the optical element 30 described above, a region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 are substantially consistent is formed in the tapered portion 54. Thus, when visible light in the TM0 mode is incident on the incident end 54a, an interaction occurs between the TM0 mode and the TE1 mode in the above region. As a result, the polarization wave mode of the visible light is converted from the TM0 mode to the TE1 mode, and the visible light in the TE1 mode is emitted from the emission end 54b.

[0102] Moreover, a region where the effective refractive index N TE1_1 in the line portion 55 and the effective refractive index N TE0_2 in the line portion 56 are substantially consistent is formed between the incident end 60a (position X2) and the position X3 of the asymmetric directional coupler 60. Thus, when visible light in the TE1 mode is incident on the incident end 60a, an interaction occurs between the TE1 mode and the TE0 mode in the above region. As a result, the polarization wave mode of the visible light is converted from the TE1 mode to the TE0 mode, and the visible light in the TE0 mode is emitted from the emission end 60b. As described above, the polarization wave mode of the visible light can be converted from the TM0 mode to the TE0 mode.

[0103] The near-eye wearable device 1 includes a retinal projection device 10, and the retinal projection device 10 includes an optical element 30. Thus, in the near-eye wearable device 1 and the retinal projection device 10, an image can be projected onto the retina while converting the polarization wave mode of the visible light from the TM0 mode to the TE0 mode.

[0104] A region where the effective refractive index N TE1_1 in the line portion 55 and the effective refractive index N TE0_2 in the line portion 56 are substantially consistent is also formed between the position X3 and the emission end 60b (position X4). Therefore, an interaction occurs between the TE1 mode and the TE0 mode. As a result, the number of times of conversion from the TE1 mode to the TE0 mode can be increased, and the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0105] It is possible that 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. According to this structure, 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. Therefore, compared with the structure in which either the effective refractive index N TE1_1 or the effective refractive index N TE0_2 is constant in the range from the position X2 to the position X3, it is possible to reduce the angle formed by the curve representing the relationship between the position in the X-axis direction and the effective refractive index N TE1_1 and the curve representing the relationship between the position in the X-axis direction and the effective refractive index N TE0_2 . Thus, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0106] In red light having a relatively long wavelength in the visible spectrum, the overlap of the mode field distributions of the two interacting polarization wave modes is large, and mode coupling is easy to perform. Therefore, it is not necessary to increase the length L2 in red light. Thus, by adopting the structure 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, the conversion efficiency from the TE1 mode to the TE0 mode can be improved without increasing the length L2.

[0107] It is also possible that 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 in the range from the incident end 60a (position X2) to the position X3. According to this structure, the effective refractive index N TE1_1 increases from the position X2 toward the position X3, while on the other hand, the effective refractive index N TE0_2 is constant in the range from the position X2 to the position X3. Therefore, by increasing the length L2, it is possible to reduce the angle formed by the curve representing the relationship between the position in the X-axis direction and the effective refractive index N TE1_1 and the curve representing the relationship between the position in the X-axis direction and the effective refractive index N TE0_2 . Thus, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0108] In green light and blue light with relatively short wavelengths in the visible spectrum, compared with red light, the overlap of the mode field distributions of the two interacting polarization wave modes is smaller, and it is difficult to perform mode coupling. Therefore, in green light and blue light, the length L2 needs to be lengthened compared with red light. Thus, by adopting a structure 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 in the range from the incident end 60a (position X2) to the position X3, it is possible to increase the conversion efficiency from the TE1 mode to the TE0 mode while lengthening the length L2.

[0109] The mode converters 35R, 35G, and 35B include the flat plate 53. Thus, the tapered portion 54 has an asymmetric shape in the Z-axis direction, so that the conversion efficiency of converting from the TM0 mode to the TE1 mode using the tapered portion 54 can be improved. Here, being asymmetric in the Z-axis direction means that with respect to a symmetry plane passing through the center of the tapered portion 54 in the Z-axis direction and orthogonal to the Z-axis direction, the two parts separated by the symmetry plane are not mirror-symmetric. Moreover, by allowing visible light to penetrate from the waveguides 51 and 52 to the flat plate 53, the optical coupling between the waveguides 51 and 52 is enhanced. Thus, the conversion efficiency of converting from the TE1 mode to the TE0 mode using the asymmetric directional coupler 60 can be improved.

[0110] 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 structure, visible light easily penetrates from the waveguides 51 and 52 to the flat plate 53. Thereby, the optical coupling between the waveguides 51 and 52 by the flat plate 53 can be further enhanced. Thus, the conversion efficiency of converting from the TE1 mode to the TE0 mode using the asymmetric directional coupler 60 can be further improved.

[0111] 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 Z-axis direction. Thus, the waveguide 51 has an asymmetric shape in the Z-axis direction, so that the conversion efficiency of converting from the TM0 mode to the TE1 mode using the tapered 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 also a trapezoidal shape in which the length in the Y-axis direction increases as it approaches the main surface 31a in the Z-axis direction. According to this structure, the overlap of the mode field distributions of the two polarization wave modes (TE0 mode and TE1 mode) that interact in the asymmetric directional coupler 60 increases, and mode coupling is likely to occur. Thus, the conversion efficiency from the TE1 mode to the TE0 mode can be further improved.

[0112] 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 that 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 wave mode of the red light from the TM0 mode to the TE0 mode, the mode converter 35G converts the polarization wave mode of the green light from the TM0 mode to the TE0 mode, and the mode converter 35B converts the polarization wave mode of the blue light from the TM0 mode to the TE0 mode. Thus, the multiplexing efficiency of the multiplexer 36 can be improved.

[0113] The height of the mode converter 35R, the height of the mode converter 35G, and the height of the mode converter 35B are the same. With this structure, the mode converter 35R, the mode converter 35G, and the mode converter 35B can be formed on the same substrate 31, and the height of each mode converter can be made the same, so that the optical element 30 can be easily manufactured.

[0114] In order to output full-color laser 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 output color. In order to change the light intensity of each color of light in the laser light source 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. Thus, full-color laser can be output without requiring a large drive current.

[0115] Next, with reference to Figure 8 a laser module according to another embodiment will be described. Figure 8 is a block diagram of a laser module according to another embodiment. Figure 8 The main difference between the laser module 13A shown and the laser module 13 is that the laser module 13A includes an optical element 30A instead of the optical element 30. The main difference between the optical element 30A and the optical element 30 is that the laser module 13A includes one mode converter 35 instead of the mode converters 35R, 35G, and 35B, and a multiplexer 36 is arranged between each modulator and the mode converter 35.

[0116] Specifically, the multiplexer 36 is provided in the downstream section of the modulator 34R, the modulator 34G, and the modulator 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, and emits the laser. The multiplexer 36 emits the laser toward the mode converter 35.

[0117] The mode converter 35 is provided in the downstream section of the multiplexer 36, and converts the polarization wave mode of the laser beam emitted from the multiplexer 36 from the TM0 mode to the TE0 mode. The structure of the mode converter 35 is the same as that of the mode converter 35B.

[0118] In the laser module 13A, visible light of the TM0 mode is emitted from each laser light source, and the light intensity of the visible light of the TM0 mode is modulated in each modulator. Then, the visible light modulated in each modulator is multiplexed in the multiplexer 36 to generate a laser beam. Then, the polarization wave mode of the laser beam is converted from the TM0 mode to the TE0 mode in the mode converter 35, and the laser beam of the TE0 mode is emitted from the mode converter 35 to the optical component 14 (see Figure 2 ).

[0119] In the laser module 13A, the structure common to the laser module 13 also exhibits the same effects as those of the laser module 13. In the optical element 30A, the structure common to the optical element 30 also exhibits the same effects as those of the optical element 30. Since the laser module 13A and the optical element 30A include a single mode converter 35 in place of the mode converters 35R, 35G, and 35B, the laser module 13A and the optical element 30A can be miniaturized.

[0120] Next, with reference to Figure 9 a laser module according to another embodiment will be described. Figure 9 is a block diagram of a laser module according to another embodiment. Figure 9 The main difference between the laser module 13B shown and the laser module 13 is that the laser module 13B includes a light source unit 20B and an optical element 30B in place of the light source unit 20 and the optical element 30. The main difference between the light source unit 20B and the light source unit 20 is that the light source unit 20B includes laser light sources 21B, 22B, and 23B in place of the laser light sources 21, 22, and 23.

[0121] The main difference between the laser light sources 21B, 22B, and 23B and the laser light sources 21, 22, and 23 lies in the polarization wave mode of the emitted visible light. Specifically, the laser light source 21B emits red light of the TE0 mode. The laser light source 22B emits green light of the TE0 mode. The laser light source 23B emits blue light of the TE0 mode.

[0122] The main difference between the optical element 30B and the optical element 30 is that the optical element 30B further includes mode converters 37R, 37G, and 37B.

[0123] The mode converter 37R is a mode converter that converts the polarization wave mode of red light from the TE0 mode to the TM0 mode. The mode converter 37R converts the polarization wave mode of the red light emitted from the laser light source 21B from the TE0 mode to the TM0 mode, and emits the red light in the TM0 mode to the modulator 34R.

[0124] The mode converter 37G is a mode converter that converts the polarization wave mode of green light from the TE0 mode to the TM0 mode. The mode converter 37G converts the polarization wave mode of the green light emitted from the laser light source 22B from the TE0 mode to the TM0 mode, and emits the green light in the TM0 mode to the modulator 34G.

[0125] The mode converter 37B is a mode converter that converts the polarization wave mode of blue light from the TE0 mode to the TM0 mode. The mode converter 37B converts the polarization wave mode of the blue light emitted from the laser light source 23B from the TE0 mode to the TM0 mode, and emits the blue light in the TM0 mode to the modulator 34B.

[0126] As the mode converters 37R, 37G, and 37B, for example, structures in which the incident ends and the emission ends of the mode converters 35R, 35G, and 35B are swapped are adopted respectively. In this structure, 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, and the polarization wave mode of the visible light is converted from the TE0 mode to the TE1 mode, so that the visible light in the TE1 mode is transmitted in 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 emission end 54b of the tapered portion 54. When the visible light in the TE1 mode is incident on the emission end 54b of the tapered portion 54, the visible light is transmitted in the tapered portion 54. Then, an interaction occurs between the TE1 mode and the TM0 mode, and the polarization wave 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.

[0127] In the laser module 13B, since visible light in the TE0 mode is emitted from each laser light source, first, in each of the mode converters 37R, 37G, and 37B, the polarization wave mode of each visible light emitted from each laser light source is converted from the TE0 mode to the TM0 mode. Then, the light intensity of the visible light in the TM0 mode is modulated in each modulator, and then, in each of the mode converters 35R, 35G, and 35B, the polarization wave mode of each modulated visible light is converted from the TM0 mode to the TE0 mode. Then, each visible light is multiplexed in the multiplexer 36, and laser light in the TE0 mode is emitted from the multiplexer 36 to the optical component 14 (see Figure 2 ).

[0128] In the laser module 13B, the structures common to the laser module 13 also have the same effects as those of the laser module 13. In the optical element 30B, the structures common to the optical element 30 also have the same effects as those of the optical element 30. In the laser module 13B and the optical element 30B, visible light of the TE0 mode is emitted from each laser light source. In this case, it is also possible to emit the visible light of the TE0 mode to the outside without reducing the modulation efficiency of each modulator.

[0129] Next, with reference to Figure 10 A laser module according to another embodiment will be described. Figure 10 FIG. is a block diagram of a laser module according to another embodiment. The main difference between the laser module 13C and the laser module 13 is that the laser module 13C includes an optical element 30C instead of the optical element 30. The main difference between the optical element 30C and the optical element 30 is that the orientation 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 swapped.

[0130] In the present embodiment, the C-axis of the lithium niobate extends in the Y-axis direction. The core layer 32 is made of, for example, X-cut lithium niobate.

[0131] The mode converter 35R converts the polarization wave 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 of the TE0 mode to the modulator 34R. The mode converter 35G converts the polarization wave 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 of the TE0 mode to the modulator 34G. The mode converter 35B converts the polarization wave 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 of the TE0 mode to the modulator 34B.

[0132] The modulator 34R is provided in the downstream section of the mode converter 35R, modulates the light intensity of the red light of the TE0 mode emitted from the mode converter 35R, and emits the red light to the combiner 36. The modulator 34G is provided in the downstream section of the mode converter 35G, modulates the light intensity of the green light of the TE0 mode emitted from the mode converter 35G, and emits the green light to the combiner 36. The modulator 34B is provided in the downstream section of the mode converter 35B, modulates the light intensity of the blue light of the TE0 mode emitted from the mode converter 35B, and emits the blue light to the combiner 36. As described above, the C-axis of the lithium niobate extends in the Y-axis direction. Therefore, the modulation efficiency of each modulator is improved in the TE mode.

[0133] In the laser module 13C, since visible light of the TM0 mode is emitted from each laser light source, the polarization wave 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, the light intensity of the visible light of the TE0 mode is modulated in each modulator, and then the modulated visible lights are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as laser light of the TE0 mode to the optical component 14 (see Figure 2 ).

[0134] In the laser module 13C, the structure common to the laser module 13 also has the same effect as the laser module 13. In the optical element 30C, the structure common to the optical element 30 also has the same effect as the optical element 30.

[0135] In addition, the optical element, laser module, retinal projection device, and near-eye wearable device of the present disclosure are not limited to the above-described embodiments.

[0136] For example, the laser modules 13, 13A, 13B, and 13C can also be applied to devices other than the near-eye wearable device 1.

[0137] The optical elements 30, 30A, 30B, and 30C may not include the cladding 33. In this case, the air layer can function as the upper cladding.

[0138] 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 wave mode of visible light from the TM0 mode to the TE0 mode.

[0139] The mode converter 35R may not include the flat plate 53. The mode converter 35G may not include the flat plate 53. The mode converter 35B may not include the flat plate 53. The heights of the mode converter 35R, the mode converter 35G, and the mode converter 35B may also be different from each other.

[0140] At any position between the incident end 54a (position X1) and the emission end 54b (position X2), the effective refractive index N TM0_1 and the effective refractive index N TE1_1 are the same as each other, and within the range between the incident end 54a (position X1) and the emission end 54b (position X2), either the effective refractive index N TM0_1 or the effective refractive index N TE1_1 only needs to deviate from the effective refractive index of other polarization wave modes. Within the range satisfying this condition, the tapered portion 54 can be appropriately changed.

[0141] At any position between the incident end 60a (position X2) and the position X3, the effective refractive index NTE1_1 and the effective refractive index N TE0_2 are consistent with each other. Within the range between the incident end 60a (position X2) and position X3, the effective refractive index N TE1_1 and the effective refractive index N TE0_2 only needs to deviate from the effective refractive index of other polarization wave modes. Within the range satisfying this condition, the conversion region 61 can be appropriately changed.

[0142] At any position between position X3 and the emission end 60b (position X4), the effective refractive index N TE1_1 and the effective refractive index N TE0_2 are consistent with each other. Within the range between position X3 and the emission end 60b (position X4), the effective refractive index N TE1_1 and the effective refractive index N TE0_2 only needs to deviate from the effective refractive index of other polarization wave modes. Within the range satisfying this condition, the conversion region 62 can be appropriately changed.

[0143] Examples

[0144] To illustrate the above effects, the present disclosure will be described in more detail below through examples. The present disclosure is not limited to these examples.

[0145] The conversion losses of the mode converters of Examples 1 to 3 were calculated. This conversion loss is the loss of converting 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 Figure 5 and Figure 6 shown mode converter 35B were used. In Examples 1 to 3, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO 3 ) was used as the constituent material of the core layer 32, and silica (SiO 2 ) was used as the constituent material of the cladding layer 33.

[0146] 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 widths W11 and W12 were set such that the effective refractive index N TM0_1 and the effective refractive index N TE1_1Substantially consistent regions are located near the center of the tapered portion 54 in the X-axis direction. The widths W13, W14, W22, W23, W24, the distance D, the minimum interval G, the lengths L1, L2, and L3 are set such that the length Lt is 1000 μm or less and the conversion loss is 0.5 dB or less. Additionally, the mode converters of Example 1 and Example 2 do not include a 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 regions 61 of the mode converters of Example 2 and Example 3, the width of the line portion 56 is constant within the range from the incident end 60a to the position X3. The wavelength of red light is 638 μm, the wavelength of green light is 520 μm, and the wavelength of blue light is 455 μm.

[0147] [Table 1]

[0148] Example 1 Example 2 Example 3 Color Red Green Blue Tc [μm] 0.45 0.45 0.45 Ts [μm] 0.15 0.15 0.15 θ [deg] 70 70 70 W11 [μm] 0.70 0.60 0.45 W12 [μm] 0.90 0.80 0.60 W13 [μm] 1.00 0.90 0.80 W14 [μm] - - 0.70 W22 [μm] 0.40 0.40 0.30 W23 [μm] 0.50 0.40 0.30 W24 [μm] - - 0.40 D [μm] 1.00 0.90 0.80 G [μm] 0.25 0.25 0.25 L1 [μm] 300 300 200 L2 [μm] 361 471 412 L3 [μm] - - 200 Lt [μm] 661 771 812 Insertion Loss [dB] 0.06 0.47 0.24

[0149] In the mode converters of Example 1 to Example 3, relatively small conversion losses of 0.06 dB to 0.47 dB are generated. The length Lt is 661 μm to 812 μm. From this, it can be seen that low-loss mode conversion is achieved while shortening the length Lt.

[0150] <Evaluation of the presence or absence of a flat plate>

[0151] The influence of the presence or absence of a flat plate on the conversion loss was evaluated using Example 1 to Example 6. As the tapered portions of Example 4 to Example 6, tapered portions having the same structure as the tapered portions of Example 1 to Example 3 except for not including a flat plate were used. Similar to Example 1 to Example 3, in Example 4 to Example 6 as well, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO 3 ) was used as the constituent material of the core layer 32, and silicon dioxide (SiO 2 ) was used as the constituent material of the cladding 33.

[0152] As shown in Table 2, in Example 1 to Example 6, the height Tc was set to 0.45 μm, and the tilt angle θ was set to 70°. In Example 1 to Example 3, the height Ts was set to 0.15 μm, and in Example 4 to Example 6, the height Ts was set to 0 μm. That is, in Example 4 to Example 6, the tapered portion 54 does not include the flat plate 53 and is composed only of the waveguides 51 and 52. In order to compare the conversion losses under as identical conditions as possible, the widths W11 and W12 were set such that the effective refractive index N TM0_1 and the effective refractive index N TE1_1 Substantially consistent regions are located near the center of the tapered portion 54 in the X-axis direction.

[0153] In addition, with respect to the tapered portion 54 including the flat plate 53, in the tapered portion 54 not including the flat plate 53, the effective refractive index N TE1_1 is smaller. Therefore, in Example 2 and Example 5, the width W12 is slightly different. Similarly, in Example 3 and Example 6, the width W12 is slightly different. The wavelength of red light is 638 μm, the wavelength of green light is 520 μm, and the wavelength of blue light is 455 μm.

[0154] [Table 2]

[0155] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Color Red Green Blue Red Green Blue Tc [μm] 0.45 0.45 0.45 0.45 0.45 0.45 Ts [μm] 0.15 0.15 0.15 0 0 0 θ [deg] 70 70 70 70 70 70 W11 [μm] 0.70 0.60 0.45 0.70 0.60 0.45 W12 [μm] 0.90 0.80 0.60 0.90 0.90 0.70 Insertion Loss [dB] 0.01 0.01 0.03 1.36 1.15 1.91

[0156] In the tapered portion 54 in which the values of the respective parameters shown in Table 2 are 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 Figures 11 to 13 . Figure 11 is a graph showing the calculation results of the conversion loss of red light in the tapered portion. Figure 12 is a graph showing the calculation results of the conversion loss of green light in the tapered portion. Figure 13 is a graph showing the calculation results of the conversion loss of blue light in the tapered portion. Figures 11 to 13 The horizontal axis of Figures 11 to 13 represents the length L1 (unit: μm).

[0157] According to Figures 11 to 13 , for any color of light, with respect to the length L1 required to achieve the same conversion loss, the length of the tapered portion 54 including the flat plate 53 is shorter than the length of the tapered portion 54 not including the flat plate 53. Thus, it was confirmed that by including the flat plate 53 in the tapered portion 54, the length L1 required to achieve the desired conversion loss can be shortened, and the length Lt can be shortened.

[0158] The conversion loss when the length L1 is 300 μm is shown in Table 2. When the length L1 is set to the same length, for any color of light, the conversion loss of the tapered portion 54 including the flat plate 53 is smaller than the conversion loss of the tapered portion 54 not including the flat plate 53. Thus, it was confirmed that by including the flat plate 53 in the tapered portion 54, the decrease in the conversion efficiency is suppressed.

[0159] <Evaluation of the height of the mode converter>

[0160] The influence of the height Tc on the conversion loss was evaluated using Examples 1 and 7 - 9. As the mode converters of Examples 7 - 9, mode converters having the same structure as the mode converter of Example 1 were used. Similar to Example 1, in Examples 7 - 9, sapphire was used as the constituent material of the substrate 31, lithium niobate (LiNbO 3 ) was used as the constituent material of the core layer 32, and silicon dioxide (SiO 2 ) was used as the constituent material of the cladding 33.

[0161] As shown in Table 3, in Examples 1 and 7 - 9, the height Ts was set to 0.15 μm, and the tilt angle θ was set to 70°. In Examples 1 and 7 - 9, the height Tc was set to different values. In order to compare the conversion losses under as identical conditions as possible, the widths W11 and W12 were set such that a region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 substantially coincide is located near the center in the X - axis direction of the tapered portion 54. In Examples 7 and 8, the widths W13, W22, W23, the distance D, the minimum interval G, the length L1, and the length L2 were set such that the length Lt becomes 2000 μm or less and the conversion loss becomes 0.5 dB or less. In Example 9, the widths W13, W22, W23, the distance D, the minimum interval G, the length L1, and the length L2 were set such that the length Lt becomes 40000 μm or less and the conversion loss becomes minimum. The wavelength of the red light was set to 638 μm.

[0162] [Table 3]

[0163] Example 1 Example 7 Example 8 Example 9 Color Red Red Red Red Tc [μm] 0.45 0.60 0.80 1.00 Ts [μm] 0.15 0.15 0.15 0.15 θ [deg] 70 70 70 70 W11 [μm] 0.70 0.80 0.90 0.80 W12 [μm] 0.90 1.00 1.10 1.20 W13 [μm] 1.00 1.10 1.20 1.40 W22 [μm] 0.40 0.50 0.60 0.70 W23 [μm] 0.50 0.60 0.60 0.70 D [μm] 1.00 1.00 1.00 1.20 G [um] 0.25 0.15 0.10 0.15 L1 [μm] 300 500 600 965 L2 [μm] 361 1000 600 39000 Lt [μm] 661 1500 1200 39965 Insertion Loss [dB] 0.06 0.07 0.12 1.75

[0164] The conversion loss was calculated in the mode converter in which the values of the respective parameters shown in Table 3 were set. 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 Figure 14 . Figure 14 is a graph 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. Figure 14 The horizontal axis of Figure 14 represents the height Tc (unit: μm). Figure 14 The left vertical axis of

[0165] represents the length Lt (unit: cm). Figure 14, at any height Tc, a conversion loss of less than 2 dB can be 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 shortened and the conversion loss is significantly reduced. It can be seen that as long as the height Tc is below the wavelength of red light (638 μm), the length Lt can be further shortened while further suppressing the conversion loss.

[0166] <Evaluation of tilt angle>

[0167] The influence of the tilt angle θ on the conversion loss was evaluated using Examples 1, 10 to 12. 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, and lithium niobate (LiNbO 3 ) was used as the constituent material of the core layer 32, and silica (SiO 2 ) was used as the constituent material of the cladding 33.

[0168] 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 as identical conditions as possible, the widths W11 and W12 were set such that the region where the effective refractive index N TM0_1 and the effective refractive index N TE1_1 were substantially consistent was located near the center of the X-axis direction of the tapered portion 54. The widths W13, W22, W23, the distance D, the minimum interval G, the length L1, and the length L2 were set such that the minimum interval G was 0.2 μm or more and the conversion loss was 1.0 dB or less. The wavelength of red light was 638 μm.

[0169] [Table 4]

[0170] Example 1 Example 10 Example 11 Example 12 Color Red Red Red Red Tc [μm] 0.45 0.45 0.45 0.45 Ts [μm] 0.15 0.15 0.15 0.15 θ [deg] 70 80 86 90 W11 [μm] 0.70 0.70 0.70 0.70 W12 [μm] 0.90 0.90 0.90 1.00 W13 [μm] 1.00 1.00 1.00 1.00 W22 [μm] 0.40 0.40 0.40 0.40 W23 [μm] 0.50 0.60 0.60 0.60 D [μm] 1.00 1.00 1.00 1.00 G [um] 0.25 0.20 0.20 0.20 L1 [μm] 300 500 650 800 L2 [μm] 361 975 1350 2000 Lt [μm] 661 1475 2000 2800 Conversion Efficiency 0.987 0.899 0.893 0.855 Insertion Loss [dB] 0.06 0.46 0.49 0.68

[0171] The conversion loss was calculated in the mode converter in which the values of the respective parameters shown in Table 4 were set. 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 Figure 15 . Figure 15 is a graph showing the relationship between the tilt angle and the length in the X-axis direction and the conversion loss of the mode converter. Figure 15 The horizontal axis of represents the tilt angle θ (unit: deg). Figure 15 The left vertical axis of represents the length Lt (unit: μm). Figure 15 The right vertical axis of represents the conversion loss (unit: dB).

[0172] According to Table 4 and Figure 15 , in any tilt angle θ, a conversion efficiency of 85% or more (conversion loss of 0.7 dB or less) can be achieved without increasing the length Lt. The conversion efficiency represents the light intensity of the TE0 mode visible light emitted from the emission end 60b when the light intensity of the TM0 mode visible light incident on the incident end 54a is set to 1. The conversion loss is obtained by converting the conversion efficiency to dB units. The following was confirmed: compared with the case where the tilt angle θ is 90°, when the tilt angle θ is less than 90°, the conversion loss can be suppressed even if the length Lt is short. In other words, it can be seen that when the cross-sectional shape of the waveguides 51 and 52 orthogonal to the X-axis direction is a trapezoidal shape, the conversion loss can be suppressed while shortening the length Lt compared to the case where the cross-sectional shape is a rectangular shape.

[0173] (Supplementary Note)

[0174] [Item 1]

[0175] An optical element, wherein

[0176] the optical element includes:

[0177] a substrate having a main surface; and

[0178] a core layer provided on the main surface and made of a material having an electro-optic effect,

[0179] the core layer includes a mode converter that converts the polarization wave mode of visible light from the TM0 mode to the TE0 mode,

[0180] the mode converter includes:

[0181] a first waveguide extending in a first direction along the main surface; and

[0182] a second waveguide extending in the first direction,

[0183] the first waveguide includes:

[0184] a tapered portion having a first incident end for the visible light to enter and a first emission end for emitting the visible light, the length of the tapered portion in a second direction increasing from a first length to a second length as it goes from the first incident end to the first emission end, the second direction being along the main surface and intersecting the first direction; and

[0185] a first line portion for transmitting the visible light emitted from the first emission end,

[0186] the second waveguide includes a second line portion arranged in parallel with the first line portion in the second direction,

[0187] The first length is a length in 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.

[0188] The second length is a length in which the first effective refractive index is smaller than the second effective refractive index.

[0189] The first line portion and the second line portion form an asymmetric directional coupler.

[0190] The asymmetric directional coupler has a second input end and a second output end that are both ends in the first direction.

[0191] The lengths in the second direction of the first line portion and the second line portion are set such that the relationship between a second effective refractive index in the first line portion at a position different from the second input end of the asymmetric directional coupler and a third effective refractive index, which is an effective refractive index of the TE0 mode in the second line portion, is reversed from the relationship between the second effective refractive index in the first line portion at the second input end and the third effective refractive index in the second line portion.

[0192] [Item 2]

[0193] The optical element according to Item 1, wherein

[0194] The length in the second direction of the first line portion increases from the second input end toward the position.

[0195] The length in the second direction of the second line portion increases from the second input end toward the position.

[0196] [Item 3]

[0197] The optical element according to Item 1, wherein

[0198] The length in the second direction of the first line portion increases from the second input end toward the position.

[0199] The length in the second direction of the second line portion is constant within a range from the second input end to the position.

[0200] [Item 4]

[0201] The optical element according to any one of Items 1 to 3, wherein

[0202] The lengths of the first line portion in the second direction and the second line portion in the second direction are set such that 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 emission end 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 position.

[0203] [Item 5]

[0204] The optical element according to any one of Items 1 to 4, wherein

[0205] The mode converter further includes a flat plate on which the first waveguide and the second waveguide are provided.

[0206] [Item 6]

[0207] The optical element according to Item 5, wherein

[0208] 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.

[0209] [Item 7]

[0210] The optical element according to any one of Items 1 to 6, wherein

[0211] 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.

[0212] [Item 8]

[0213] The optical element according to any one of Items 1 to 7, wherein

[0214] The core layer includes:

[0215] A first mode converter, which is the mode converter that converts the polarization wave mode of red light from the TM0 mode to the TE0 mode;

[0216] A second mode converter, which is the mode converter that converts the polarization wave mode of green light from the TM0 mode to the TE0 mode;

[0217] A third mode converter, which is the mode converter that converts the polarization wave mode of blue light from the TM0 mode to the TE0 mode; and

[0218] A multiplexer, which multiplexes the red light, the green light, and the blue light and emits a laser.

[0219] [Item 9]

[0220] The optical element according to item 8, wherein,

[0221] The lengths of the first mode converter in the 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.

[0222] [Item 10]

[0223] The optical element according to item 8 or item 9, wherein,

[0224] The core layer further includes:

[0225] A first modulator that modulates the light intensity of the red light;

[0226] A second modulator that modulates the light intensity of the green light; and

[0227] A third modulator that modulates the light intensity of the blue light.

[0228] [Item 11]

[0229] A laser module, wherein,

[0230] The laser module includes:

[0231] The optical element according to any one of items 8 to 10;

[0232] A first light source that emits the red light in the TM0 mode;

[0233] A second light source that emits the green light in the TM0 mode; and

[0234] A third light source that emits the blue light in the TM0 mode.

[0235] [Item 12]

[0236] A retinal projection device mounted on a near-eye wearable device, wherein,

[0237] The retinal projection device includes:

[0238] The laser module according to item 11;

[0239] A movable mirror that scans using the laser emitted from the laser module; and

[0240] A reflector that projects an image onto the retina by reflecting the laser after passing through the movable mirror and irradiating the reflected light onto the retina of a user wearing the near-eye wearable device.

[0241] [Item 13]

[0242] A near-eye wearable device, wherein the near-eye wearable device comprises:

[0243] The retinal projection device described in Item 12; and a lens provided with the reflector.

Claims

1. An optical element, wherein: The optical component has: a substrate having a main surface; and The core layer is disposed on the main surface and is composed of a material having an electro-optical effect. The core layer has a mode converter for converting the polarization mode of visible light from TM0 mode to TE0 mode. The mode converter comprises: a first waveguide extending in a first direction along the main surface; and a second waveguide extending in the first direction, The first waveguide comprises: a tapered portion having a first incident end for the visible light to be incident on and a first emission end for emitting the visible light, wherein a length of the tapered portion in a second direction increases from a first length to a second length as it moves from the first incident end toward the first emission end, the second direction being along the main surface and intersecting the first direction; and a first line portion for transmitting the visible light emitted from the first emission end, The second waveguide includes a second line portion arranged in parallel 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 section and the second line section constitute an asymmetric directional coupler. The asymmetric directional coupler has a second incident end and a second emitting end as two 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 set so that the magnitude relationship between the second effective refractive index in the first line portion at a position different from the second incident end of the asymmetric directional coupler and the third effective refractive index in the second line portion as the effective refractive index of the TE0 mode is reversed to 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.

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

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

4. The optical element according to any one of claims 1 to 3, wherein The length of the first line portion in the second direction and the length of the second line portion in the second direction are set so that 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 emission end is reversed to 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 position.

5. The optical element according to any one of claims 1 to 4, wherein The mode converter further includes a planar plate on which the first waveguide and the second waveguide are provided.

6. The optical element according to claim 5, wherein: 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.

7. The optical element according to any one of claims 1 to 6, wherein The cross-sectional shape of the first waveguide intersecting the first direction is a trapezoidal shape whose length in the second direction increases toward the main surface.

8. The optical element according to any one of claims 1 to 7, wherein The core layer has: a first mode converter, which is the mode converter that converts the polarization wave mode of red light from the TM0 mode to the TE0 mode; a second mode converter, which is the mode converter that converts the polarization wave mode of green light from the TM0 mode to the TE0 mode; a third mode converter, which is the mode converter that converts the polarization wave mode of blue light from the TM0 mode to the TE0 mode; as well as A combiner combines the red light, the green light, and the blue light to emit laser light.

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

10. The optical element according to claim 8 or 9, wherein: The core layer also has: a first modulator, which modulates the intensity of the red light; a second modulator for modulating the intensity of the green light; and The third modulator modulates the intensity of the blue light.

11. A laser module, wherein: The laser module has: The optical element according to any one of claims 8 to 10; a first light source emitting the red light in the TM0 mode; a second light source that emits the green light in the TM0 mode; and The third light source emits the blue light in the TM0 mode.

12. A retinal projection device, mounted on a near-eye wearable device, wherein: The retinal projection device has: The laser module according to claim 11; a movable reflecting mirror for scanning using the laser light emitted from the laser module; and A reflector reflects the laser light after passing through the movable reflector and irradiates the reflected light toward the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina.

13. A near-eye wearable device, wherein: The near-eye wearable device has: The retinal projection device of claim 12; and A lens is provided with the reflector.

Images