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. This optical element is provided with: a substrate having a main surface; a core layer that is provided on the main surface, is made of a material having an electro-optical effect, and has a waveguide extending in a first direction along the main surface; and a metal body extending in the first direction and provided in parallel with the waveguide. The waveguide and the metal body constitute a mode converter that converts the polarization mode of visible light from a first polarization mode, which is one of a TE mode and a TM mode, to a second polarization mode, which is the other of the TE mode and the TM mode. The metal body has an end edge in a second direction that intersects the first direction and follows the main surface. The end edge overlaps the waveguide when viewed from a third direction intersecting the main surface.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority to Japanese Patent Application No. 2023 - 214718, filed with the Japan Patent Office on December 20, 2023, the entire contents of which are 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 TE (Transverse Electric) mode in which the main electric field has a horizontal direction with respect to the substrate and a TM (Transverse Magnetic) mode in which the main electric field has a vertical direction with respect to the substrate. There are known optical elements for converting these polarization wave modes. For example, Non - Patent Document 1 (Shinmo An and O - Kyun Kwon, “Integrated InP polarization rotator using the plasmonic effect”, Optics Express, 2018, Vol.26, No.2, pp.1305 - 1314) describes a polarization rotator including an InGaAsP ridge waveguide provided on an InP material and a metal layer provided on an upper cladding. Summary of the invention

[0005] The polarization rotator described in Non - Patent Document 1 converts the polarization wave mode of light having a wavelength of 1.55 μm. 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.

[0007] An optical element according to an aspect of the present disclosure includes: a substrate having a main surface; a core layer provided on the main surface and made of a material having an electro-optic effect, the core layer having a waveguide extending in a first direction along the main surface; and a metal body extending in the first direction and disposed in parallel with the waveguide. The waveguide and the metal body constitute a mode converter that converts a polarization wave mode of visible light from a first polarization wave mode, which is one of a TE mode and a TM mode, to a second polarization wave mode, which is the other of the TE mode and the TM mode. The waveguide has an incident end for incident of visible light of the first polarization wave mode and an emission end for emitting visible light of the second polarization wave mode. The metal body has an edge in a second direction, the second direction intersects the first direction and is along the main surface. The edge overlaps with the waveguide when viewed from a third direction intersecting the main surface.

[0008] In this optical element, the waveguide and the metal body are disposed in parallel, and when viewed from the third direction, the edge of the metal body in the second direction overlaps with the waveguide. Since the metal body has a negative dielectric constant, surface plasmon polaritons are excited on the surface of the metal body. Therefore, the polarization wave mode of visible light transmitted in the waveguide interacts with the surface plasmon polaritons and rotates corresponding to the position of the edge of the metal body. As a result, in a portion of the waveguide disposed in parallel with the metal body, a first hybrid mode and a second hybrid mode in which the TE mode and the TM mode are mixed are generated. Since there is a difference between the propagation constant of the first hybrid mode and the propagation constant of the second hybrid mode, a phase difference is generated between the phase of the first hybrid mode and the phase of the second hybrid mode according to the length of the above portion. When visible light is emitted from the above portion, the first hybrid mode and the second hybrid mode are coupled into one polarization wave mode, and the polarization wave mode of visible light can be converted from the first polarization wave mode to the second polarization wave mode. According to the above content, by using the above optical element, the polarization wave mode of visible light can be converted.

[0009] Alternatively, the waveguide may have a bottom surface facing the main surface and a top surface provided on the opposite side of the bottom surface in the third direction. Alternatively, the metal body may be configured such that the edge, the top surface, and the bottom surface are arranged in sequence in the third direction. In this case, since the rotation angle between the optical axis of the hybrid mode and the plane parallel to the main surface of the substrate can be made close to 45°, the conversion efficiency can be improved.

[0010] Alternatively, the distance in the second direction between the center of the waveguide in the second direction and the edge may be 0 nm or more and less than half of the length of the waveguide in the second direction. In this case, since the rotation angle between the optical axis of the hybrid mode and the plane parallel to the main surface of the substrate can be made close to 45°, the conversion efficiency can be improved.

[0011] Alternatively, the metal body may be made of a metal containing at least one element selected from the group consisting of silver, gold, copper, aluminum, chromium, manganese, titanium, vanadium, iron, cobalt, nickel, zinc, molybdenum, palladium, tantalum, tungsten, platinum, lead, and bismuth.

[0012] Alternatively, the above optical element includes: a first mode converter that converts the polarization wave mode of red light from a first polarization wave mode to a second polarization wave mode; a second mode converter that converts the polarization wave mode of green light from the first polarization wave mode to the second polarization wave mode; a third mode converter that converts the polarization wave mode of blue light from the first polarization wave mode to the second polarization wave mode; and a combiner that combines red light, green light, and blue light and emits a laser. With this structure, the polarization wave mode of red light is converted from the first polarization wave mode to the second polarization wave mode, the polarization wave mode of green light is converted from the first polarization wave mode to the second polarization wave mode, and the polarization wave mode of blue light is converted from the first polarization wave mode to the second polarization wave mode. By using the polarization wave mode with higher combining efficiency of the combiner in the TM mode and the TE mode as the second polarization wave mode, the combining efficiency can be improved.

[0013] Alternatively, the lengths of the waveguides of the first mode converter, the second mode converter, and the third mode converter in a third direction are the same as each other. With this structure, the waveguides of the first mode converter, the second mode converter, and the third mode converter can be formed on the same substrate, and the lengths of their respective waveguides in the third direction can be made the same, so that the optical element can be easily manufactured.

[0014] Alternatively, the above optical element further includes: a first modulator that modulates the light intensity of red light; a second modulator that modulates the light intensity of green light; and a third modulator that modulates the light intensity of blue light. In order to output full-color laser by combining red light, green light, and blue light, it is necessary to adjust the light intensity of each color of light according to the output color. With the above structure, the light intensity of red light, green light, and blue light is modulated by the modulators, so that full-color laser can be output without a large drive current.

[0015] On the other hand, a laser module according to the present disclosure includes: the above optical element; a first light source that emits red light in a first polarization wave mode; a second light source that emits green light in the first polarization wave mode; and a third light source that emits blue light in the first polarization wave mode. Since this laser module includes the above optical element, the polarization wave modes of red light, green light, and blue light can be converted.

[0016] Another aspect of the retinal projection device of the present disclosure is a device mounted on a near-eye wearable device. In this retinal projection device, there are provided: the above-mentioned laser module; a movable mirror that scans using the laser emitted from the laser module; and a reflector that projects an image onto the retina by reflecting the laser that has passed through the movable mirror and irradiating the reflected light onto the retina of the user wearing the near-eye wearable device. This retinal projection device is provided with the above-mentioned 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.

[0017] Another aspect of the near-eye wearable device of the present disclosure includes the above-mentioned retinal projection device and a lens provided with a reflector. This near-eye wearable device is provided with the above-mentioned 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.

[0018] According to various aspects and embodiments of the present disclosure, the polarization wave mode of visible light can be converted. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a perspective view showing the appearance of a near-eye wearable device applying a laser module according to an embodiment.

[0020] Figure 2 It schematically shows Figure 1 a configuration diagram of the retinal projection device shown.

[0021] Figure 3 It is Figure 2 a block diagram of the laser module shown.

[0022] Figure 4 It shows Figure 3 a perspective view of the structure of the mode converter shown.

[0023] Figure 5 It is a cross-sectional view along Figure 4 the V-V line of

[0024] Figure 6 It is a cross-sectional view along Figure 4 the VI-VI line of

[0025] Figure 7 It is used to illustrate Figure 4 the conversion principle of the mode converter shown.

[0026] Figure 8 It shows Figure 4 an example of the conversion efficiency of the mode converter shown.

[0027] Figure 9 It is a block diagram of a laser module according to another embodiment.

[0028] Figure 10 It is a block diagram of a laser module according to yet another embodiment.

[0029] Figure 11 It is a block diagram of a laser module according to still another embodiment.

[0030] Figure 12 It is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter in Example 1.

[0031] Figure 13 It is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter in Example 2.

[0032] Figure 14 It is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter in Example 3.

[0033] Figure 15 It is a graph showing the calculation result of the conversion loss of red light.

[0034] Figure 16 It is a graph showing the calculation result of the conversion loss of green light.

[0035] Figure 17 It is a graph showing the calculation result of the conversion loss of blue light.

[0036] Figure 18 It is a graph showing the relationship between the distance in the Z-axis direction between the waveguide and the metal body and the conversion loss. Detailed Embodiments

[0037] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. In addition, the same reference numerals are assigned to the same elements in the description of the drawings, and repeated descriptions are omitted. An XYZ coordinate system is sometimes shown in each figure. 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 using a laser module according to an embodiment. Figure 1The 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 (headgear), and can be in the form of glasses, goggles, a hat, or a helmet, etc. As an example of the near-eye wearable device 1, smart glasses such as AR (Augmented Reality) glasses, VR (Virtual Reality) glasses, and MR (Mixed Reality) glasses are listed. 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 bridge 2b, and a pair of temple arms 2c. The lens rims 2a are the parts that hold the lenses 3. The 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 can also be a frameless frame. The lenses 3 have an inner surface 3a (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 can also include only any one of the retina projection devices 10.

[0041] Next, with reference to Figure 2 the retina projection device 10 will be described in detail. Figure 2 is a schematic diagram showing Figure 1 the configuration of the retina projection device shown. As shown in Figure 2 the retina 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 arm 2c. The optical engine 11 includes a laser module 13, optical components 14, a movable mirror 15, a laser driver 16, a mirror driver 17, and a controller 18.

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

[0045] The movable mirror 15 is a member for scanning using the laser beam emitted from the laser module 13. The movable mirror 15 is provided in the emission direction of the laser beam 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 to change the angle in the lateral and longitudinal directions of the lens 3 to reflect the laser beam. 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 beam 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 at a predetermined timing. The controller 18 is a device that controls the laser driver 16 and the mirror driver 17.

[0047] In the optical engine 11, the laser beam 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 beam reflected by the movable mirror 15 is emitted as the laser beam Ls toward the reflector 12.

[0048] The reflector 12 is a member 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 perspective view showing Figure 3 the structure of the mode converter shown. In Figure 4 only the peripheral portion of the mode converter 35R in the optical element 30 is illustrated. 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 are sometimes referred to as respective visible lights, and sometimes the red light, the green light, and the blue light are collectively referred to as visible light. The light source unit 20 may further include a sub-carrier on which the laser light source 21, the laser light source 22, and the laser light source 23 are mounted.

[0052] The optical element 30 multiplexes the visible lights emitted from the respective laser light sources into one laser. The optical element 30 is, for example, a Planar Lightwave Circuit (PLC). The optical element 30 is joined to the light source unit 20 by means of a metal bonding layer. Since the laser module 13 is mounted on the near-eye wearable device 1, the size of the waveguide observed from the Z-axis direction of the optical element 30 can be 100 mm 2 Below. 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. Examples of the constituent material of the substrate 31 include sapphire, silicon oxide, and organosilicon compounds formed by laminating silicon oxide. 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 the present 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 over 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., SiO2) is cited.

[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 is a 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 (LiNbO3) is cited. In the present 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 optical element 30 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 included in the core layer 32. 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 one polarization wave mode (first polarization wave mode) among the TE mode and the TM mode to the other polarization wave mode (second polarization wave mode) among the TE mode and the TM mode. In the present embodiment, the mode converter 35R converts the polarization wave mode of red light from the TM0 mode to the TE fundamental mode (hereinafter referred to as "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.

[0059] The mode converter 35G is a mode converter that converts the polarization wave mode of green light from one polarization wave mode (the first polarization wave mode) among the TE mode and the TM mode to the other polarization wave mode (the second polarization wave mode) among the TE mode and the TM mode. In the present embodiment, the mode converter 35G 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.

[0060] The mode converter 35B is a mode converter that converts the polarization wave mode of blue light from one polarization wave mode (the first polarization wave mode) among the TE mode and the TM mode to the other polarization wave mode (the second polarization wave mode) among the TE mode and the TM mode. In the present embodiment, 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.

[0061] 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.

[0062] The mode converter 35R, the mode converter 35G, and the mode converter 35B extend along 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.

[0063] 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 and emits the laser. The laser 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). The multiplexer 36 is included in the core layer 32. The multiplexer 36 can be constituted by a multimode interferometer (MMI: Multimode Interferometer), can also be constituted by a Y-branch waveguide, or can also be constituted by a directional coupler. The length of the multiplexer 36 in the X-axis direction can be 10 μm to 10000 μm.

[0064] For example, by adjusting the relative positions of the sub-carrier of the light source unit 20 and the substrate 31, the optical axis of the visible light emitted from the laser light source is made to coincide with the axis of the incident end of the corresponding modulator (active calibration), and the light source unit 20 and the optical element 30 are joined using a metal bonding layer to fabricate the laser module 13.

[0065] In the laser module 13, visible light of 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 emitted as laser light of the TE0 mode from the multiplexer 36 to the optical component 14 (refer to Figure 2 ).

[0066] Next, refer to Figures 4 to 6 to describe the detailed structures of the mode converters 35R, 35G, and 35B. Figure 5 is a cross-sectional view along the V-V line of Figure 4 . Figure 6 is a cross-sectional view along the VI-VI line of Figure 4 . In addition, in Figure 6 , the hatching of the cladding 33 is omitted for ease of explanation. As Figure 4 shows, the mode converters 35R, 35G, and 35B each include a waveguide 51 and a metal body 52. Since the structures of the respective mode converters are the same, the mode converter 35R will be described as an example here.

[0067] The waveguide 51 is an optical waveguide extending in the X-axis direction. The waveguide 51 is included in the core layer 32. The waveguide 51 has a columnar shape extending linearly in the X-axis direction. Specifically, the waveguide 51 has a rectangular parallelepiped shape with the X-axis direction as the length direction. The waveguide 51 includes an incident end 51a as one end in the X-axis direction and an emission end 51b as the other end in the X-axis direction. Red light of the TM0 mode is incident on the incident end 51a from the modulator 34R. The waveguide 51 emits red light of the TE0 mode from the emission end 51b to the multiplexer 36.

[0068] The waveguide 51 is symmetric about the Z-axis and symmetric about the Y-axis. Symmetry about the Z-axis means that with respect to a symmetry plane orthogonal to the Z-axis passing through the center point in the Z-axis direction, the two parts separated by the symmetry plane are symmetric. Symmetry about the Y-axis means that with respect to a symmetry plane orthogonal to the Y-axis passing through the center point in the Y-axis direction, the two parts separated by the symmetry plane are symmetric.

[0069] As Figure 5As shown, the waveguide 51 has a bottom surface 51c, a top surface 51d, and a pair of side surfaces 51e. The bottom surface 51c is the surface facing the main surface 31a and is in contact with the main surface 31a over its entire surface. The top surface 51d is the surface provided on the opposite side of the bottom surface 51c in the Z-axis direction. The bottom surface 51c and the top surface 51d are substantially parallel. Each side surface 51e is the surface connecting the bottom surface 51c and the top surface 51d. The pair of side surfaces 51e are substantially parallel.

[0070] The length (height T1) of the waveguide 51 in the Z-axis direction and the length (width W1) in the Y-axis direction are constant from the incident end 51a to the emission end 51b. Hereinafter, the length in the Z-axis direction may sometimes be referred to as the "height", and the length in the Y-axis direction may be referred to as the "width". The height T1 is smaller than the wavelength of red light. The width W1 may be 20% to 60% of the wavelength of red light, or may be 32% to 48% of the wavelength of red light.

[0071] The waveguide 51 includes an incident region 53, a conversion region 54, and an emission region 55. The conversion region 54 is the portion juxtaposed with the metal body 52. The incident region 53 includes the incident end 51a and is the portion from the incident end 51a to one end of the conversion region 54. The emission region 55 includes the emission end 51b and is the portion from the other end of the conversion region 54 to the emission end 51b.

[0072] The metal body 52 is a metal member extending in the X-axis direction. The metal body 52 is provided juxtaposed with the waveguide 51. The metal body 52 is buried in the cladding 33. That is, the periphery of the metal body 52 is covered by the cladding 33. The metal body 52 has a rectangular plate shape. The metal body 52 has a negative dielectric constant. The metal body 52 is made of, for example, a metal containing at least one element selected from the group consisting of silver (Ag), gold (Au), copper (Cu), aluminum (Al), chromium (Cr), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), molybdenum (Mo), palladium (Pd), tantalum (Ta), tungsten (W), platinum (Pt), lead (Pb), and bismuth (Bi).

[0073] The length Lc of the metal body 52 in the X-axis direction (refer to Figure 7 ) is shorter than the length of the waveguide 51 in the X-axis direction. The length Lc is, for example, 100 μm or less. The metal body 52 is juxtaposed with the waveguide 51 over the entire length of the metal body 52 in the X-axis direction. In other words, the length Lc is the length of the conversion region 54 in the X-axis direction and is also referred to as the conversion length. The height T2 and the width W2 of the metal body 52 are constant over the entire length of the metal body 52 in the X-axis direction. The height T2 is, for example, 1 nm to 200 nm. The width W2 is, for example, 10 nm to 1 μm.

[0074] As Figure 6As shown, the metal body 52 has a main surface 52a and a main surface 52b. The main surface 52b is the surface facing the main surface 31a. The main surface 52b faces the top surface 51d, and the main surface 52b and the top surface 51d are substantially parallel. The main surface 52a is the surface provided on the opposite side of the main surface 52b in the Z-axis direction. The metal body 52 has an end edge 52c at one end in the Y-axis direction and an end edge 52d at the other end in the Y-axis direction. When viewed from the Z-axis direction, the end edge 52c overlaps with the waveguide 51, and the end edge 52d does not overlap with the waveguide 51.

[0075] The distance D1 in the Z-axis direction between the waveguide 51 (top surface 51d) and the metal body 52 is, for example, 0 nm or more. That is, the metal body 52 is arranged such that the end edge 52c, the top surface 51d, and the bottom surface 51c are arranged in sequence in the Z-axis direction. In addition, the distance D1 from the top surface 51d toward the direction opposite to the bottom surface 51c is represented as a positive value, and the distance D1 from the top surface 51d toward the bottom surface 51c is represented as a negative value. The distance D2 in the Y-axis direction between the center CP in the Y-axis direction of the waveguide 51 and the end edge 52c is, for example, 0 nm or more and half or less of the width W1. In other words, the metal body 52 covers less than half of the top surface 51d in the conversion region 54. In addition, the distance D2 in the direction from the center CP toward the side surface 51e closer to the end edge 52d among the pair of side surfaces 51e is represented as a positive value, and the distance D2 in the direction from the center CP away from the side surface 51e closer to the end edge 52d among the pair of side surfaces 51e is represented as a negative value.

[0076] The mode converter 35G and the mode converter 35B have the same structure as the mode converter 35R, but the optimal dimensions may be different among the mode converter 35R, the mode converter 35G, and the mode converter 35B. The optimal dimensions mentioned here refer to the optimal dimensions based on maximizing the conversion efficiency of the polarization wave modes of each visible light. The height of the waveguide 51 of the mode converter 35R, the height of the waveguide 51 of the mode converter 35G, and the height of the waveguide 51 of the mode converter 35B are substantially equal. The height of the waveguide 51 of the mode converter 35R, the height of the waveguide 51 of the mode converter 35G, and the height of the waveguide 51 of the mode converter 35B may also be different from each other.

[0077] Next, refer to Figure 7 and Figure 8 to explain the conversion principles of the mode converter 35R, the mode converter 35G, and the mode converter 35B. Figure 7 is for explaining Figure 4 the conversion principle of the mode converter shown in Figure 8 is a diagram Figure 4 showing an example of the conversion efficiency of the mode converter shown in Figure 8 The horizontal axis ofFigure 8 The vertical axis represents the conversion efficiency. Since the operations of the respective mode converters are the same, the mode converter 35R will be taken as an example for description herein.

[0078] As Figure 7 shown, the mode converter 35R converts the polarization wave mode of the red light of the TM0 mode incident on the incident end 51a from the TM0 mode to the TE0 mode, and emits the red light of the TE0 mode from the emission end 51b. In the incident region 53, since the polarization wave mode of the red light is the TM0 mode, the vector of the electric field component is parallel to the Z-axis direction. When the red light is incident on the conversion region 54, the polarization wave mode of the red light interacts with the surface plasmon generated on the surface of the metal body 52 and rotates corresponding to the position of the edge 52c of the metal body 52. As a result, two hybrid modes (the first hybrid mode and the second hybrid mode) in which the TE mode and the TM mode are mixed are excited as the polarization wave mode of the red light.

[0079] At this time, according to the position of the edge 52c of the metal body 52, the horizontal electric field component of the first hybrid mode and the horizontal electric field component of the second hybrid mode change. In the present embodiment, the position of the edge 52c is adjusted so that the horizontal electric field component and the vertical electric field component of the first hybrid mode are of the same degree, and the horizontal electric field component and the vertical electric field component of the second hybrid mode are of the same degree. The electric field vector of the first hybrid mode is orthogonal to the electric field vector of the second hybrid mode.

[0080] In the conversion region 54, since there is a difference between the propagation constant β1 of the first hybrid mode and the propagation constant β2 of the second hybrid mode, a phase difference is generated between the phase of the first hybrid mode and the phase of the second hybrid mode according to the length of the red light propagating in the conversion region 54. When the red light propagates from the emission end of the conversion region 54 to the emission region 55, the first hybrid mode and the second hybrid mode are coupled into one polarization wave mode. At this time, when the phase difference is π×(2n + 1) (n is an integer of 0 or more) radians, the polarization wave mode of the red light rotates 90° and is converted to the TE0 mode.

[0081] As Figure 8As shown, the conversion efficiencies CE1 and CE2 vary periodically corresponding to the magnitude of the length Lc. The conversion efficiency CE1 represents the conversion efficiency when red light in the TM0 mode is incident on the incident end 51a and red light in the TE0 mode is emitted from the emission end 51b. The conversion efficiency CE2 represents the conversion efficiency when red light in the TM0 mode is incident on the incident end 51a and red light in the TM0 mode is emitted from the emission end 51b. The conversion efficiency represents the light intensity of the emitted light when the light intensity of the incident light is set to 1. The conversion efficiencies CE1 and CE2 vibrate periodically as the length Lc increases, and the maximum values of the conversion efficiency CE1 and the maximum values of the conversion efficiency CE2 appear alternately every half cycle.

[0082] In the present embodiment, since the mode converter 35R converts red light in the TM0 mode into red light in the TE0 mode, the length Lc is set to the length at which the conversion efficiency CE1 takes the maximum value. Since the length Lc is the shortest when the phase difference between the phase of the first hybrid mode and the phase of the second hybrid mode is π radians, as shown in Equation (1), the length Lc is obtained by dividing π radians by the difference between the propagation constant β1 and the propagation constant β2. Further, when using the effective refractive index n eff1 of the first hybrid mode, the effective refractive index n eff2 of the second hybrid mode, and the vacuum wave number k0 to transform the above relationship, the length Lc is calculated using the right side of Equation (1).

[0083] [Equation 1]

[0084]

[0085] Since the vacuum wave number k0 is represented by 2π / λ, Equation (2) is obtained from Equation (1).

[0086] [Equation 2]

[0087]

[0088] The conversion efficiency (Conversion Efficiency: CE) of the mode converter is expressed by Equation (3) using the rotation angle the length Lc, and the length L π . In addition, the unit of CE in Equation (3) is %. The tangent of the rotation angle is expressed by Equation (4) using the dielectric constant distribution ε(y, z), the electric field component Ey(y, z) of the hybrid mode in the horizontal direction, and the electric field component Ez(y, z) of the hybrid mode in the vertical direction.

[0089] [Equation 3]

[0090]

[0091] [Equation 4]

[0092]

[0093] Here, the rotation angle is the rotation angle between the optical axis of the hybrid mode and the plane parallel to the main surface 31a of the substrate 31. The length L π is the length at which the phase difference between the first hybrid mode and the second hybrid mode is π radians. According to Equation (3), when the rotation angle is 45°, a conversion efficiency close to 100% can be obtained. According to Equation (4), when the horizontal electric field component Ey(y, z) and the vertical electric field component Ez(y, z) of the first hybrid mode and the second hybrid mode are equal, the rotation angle is 45°. Based on the above, when the horizontal electric field component Ey(y, z) and the vertical electric field component Ez(y, z) of the first hybrid mode and the second hybrid mode are equal, a conversion efficiency close to 100% can be obtained.

[0094] In addition, transmission loss is generated in the conversion region 54 due to the imaginary part of the effective refractive index of each polarization wave mode. The imaginary part n i of the effective refractive index is caused by the light absorption of the metal body 52. The transmission loss TL is expressed by Equation (5) using the imaginary part n i and the wavelength λ.

[0095] [Equation 5]

[0096]

[0097] As shown in Equation (5), since the transmission loss TL is inversely proportional to the wavelength λ, there is a tendency for the transmission loss TL to increase as the wavelength λ becomes shorter. Since the transmission loss TL is proportional to the imaginary part n i , the transmission loss TL can be suppressed by reducing the imaginary part n i .

[0098] In the laser module 13 and the optical element 30 described above, the waveguide 51 and the metal body 52 are arranged side by side. When viewed from the Z-axis direction, the edge 52c of the metal body 52 overlaps with the waveguide 51. Since the metal body 52 has a negative dielectric constant, surface plasmons are excited on the surface of the metal body 52. Therefore, the polarization wave mode of the visible light transmitted in the waveguide 51 interacts with the surface plasmons and rotates corresponding to the position of the edge 52c of the metal body 52. Thus, in the portion of the waveguide 51 juxtaposed with the metal body 52 (conversion region 54), a first hybrid mode and a second hybrid mode in which the TE mode and the TM mode are mixed are generated. In the conversion region 54, since there is a difference between the propagation constant of the first hybrid mode and the propagation constant of the second hybrid mode, a phase difference is generated between the phase of the first hybrid mode and the phase of the second hybrid mode according to the length Lc in the X-axis direction of the conversion region 54. When the visible light is emitted from the conversion region 54, the first hybrid mode and the second hybrid mode are coupled into one polarization wave mode, and the polarization wave mode of the visible light can be converted from the TM0 mode to the TE0 mode. According to the above content, by using the laser module 13 and the optical element 30, the polarization wave mode of the visible light can be converted.

[0099] 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 on the basis of converting the polarization wave mode of the visible light from the TM0 mode to the TE0 mode.

[0100] In the laser module 13 and the optical element 30, the metal body 52 is embedded in the cladding 33. In this structure, the surface plasmon is a localized mode of an electromagnetic wave confined in a direction perpendicular to the interface between the metal body 52 and the cladding 33, and the surface plasmon propagates in this interface. By changing the relative position between the waveguide 51 and the metal body 52, the characteristics of the surface plasmon, particularly the interaction with the visible light transmitted in the waveguide 51, can be adjusted. Thereby, it is easy to cause the rotation of the optical axis of the polarization wave mode of the visible light transmitted in the waveguide 51.

[0101] When the distance D1 is 0 nm or more, the rotation angle can approach 45°, so the conversion efficiency can be improved. When the distance D2 is 0 nm or more and is less than or equal to half of the width W1, the rotation angle can approach 45°, so the conversion efficiency can be improved.

[0102] Specifically, in the mode converter 35R, when the distance D1 is 0 nm to 100 nm and the distance D2 is 0 nm to 90 nm, the rotation angle is close to 45°, so the conversion efficiency can be improved. In the mode converter 35G, when the distance D1 is from 0 nm to 40 nm and the distance D2 is from 20 nm to 80 nm, the rotation angle is close to 45°, so the conversion efficiency can be improved. In the mode converter 35B, when the distance D1 is from 0 nm to 10 nm and the distance D2 is from 10 nm to 70 nm, the rotation angle is close to 45°, so the conversion efficiency can be improved.

[0103] The imaginary part n of the effective refractive index i is caused by the light absorption of the metal body 52, but when the length Lc is 30 μm or less, the influence of the imaginary part n i on the transmission loss is small. Thus, the transmission loss TL is suppressed. According to the above content, the conversion loss generated in the whole of each mode converter can be reduced.

[0104] The metal body 52 is made of a metal containing at least one element selected from the group consisting of silver, gold, copper, aluminum, chromium, manganese, titanium, vanadium, iron, cobalt, nickel, zinc, molybdenum, palladium, tantalum, tungsten, platinum, lead, and bismuth.

[0105] The multiplexer 36 is designed such that the multiplexing efficiency when multiplexing red light, green light, and blue light in the TE0 mode is higher than the multiplexing efficiency when multiplexing red light, green light, and blue light in the TM0 mode. In the optical element 30, the mode converter 35R converts the polarization wave mode of red light from the TM0 mode to the TE0 mode, the mode converter 35G converts the polarization wave mode of green light from the TM0 mode to the TE0 mode, and the mode converter 35B converts the polarization wave mode of blue light from the TM0 mode to the TE0 mode. Thus, the multiplexing efficiency of the multiplexer 36 can be improved.

[0106] The heights of the waveguides 51 of the mode converter 35R, the heights of the waveguides 51 of the mode converter 35G, and the heights of the waveguides 51 of the mode converter 35B are the same as each other. According to this structure, the waveguides 51 of the mode converter 35R, the waveguides 51 of the mode converter 35G, and the waveguides 51 of the mode converter 35B can be formed on the same substrate 31, and the heights of the respective waveguides 51 can be made the same, so the optical element 30 can be easily manufactured.

[0107] In order to output full-color laser by combining red light, green light, and blue light, it is necessary to adjust the light intensity of each color of light according to the output color. In order to change the light intensity of each color of light in each laser light source 20, a large drive current is required. In the optical element 30, the light intensity of red light is modulated (voltage modulation) by the modulator 34R, the light intensity of green light is modulated (voltage modulation) by the modulator 34G, and the light intensity of blue light is modulated (voltage modulation) by the modulator 34B. Therefore, full-color laser can be output without a large drive current.

[0108] In addition, modulation of the light source unit 20 and modulation of the modulators 34R, 34G, and 34B can also be used simultaneously. Generally speaking, adjustment using voltage has higher responsiveness than adjustment using current. Therefore, when responsiveness is emphasized, rough adjustment of the light intensity of each color can be performed in the light source unit 20, and fine adjustment of the light intensity of each color can be performed in the modulators 34R, 34G, and 34B. Since fine adjustment using current can suppress the amount of current, power consumption can be suppressed. Therefore, when suppression of power consumption is emphasized, rough adjustment of the light intensity of each color can be performed in the modulators 34R, 34G, and 34B, and fine adjustment of the light intensity of each color can be performed in the light source unit 20.

[0109] In the polarization rotator described in Non-Patent Document 1, the lower cladding is made of InP and the core is made of InGaAsP. In this case, since the refractive index difference between the lower cladding and the core is small, the conversion length is long. In compound semiconductors, since there are limitations on the materials that can be selected as the lower cladding and the core, it is impossible to increase the refractive index difference between the lower cladding and the core. On the other hand, in the optical element 30, as an example, the substrate 31 is made of sapphire, the cladding 33 is made of silicon dioxide, and the core layer 32 is made of lithium niobate. In this case, the refractive index difference between the cladding and the core layer 32 can be increased. Therefore, the length Lc can be shortened.

[0110] In addition, the incident end 51a and the emission end 51b of the waveguide 51 of each mode converter can also be swapped with each other. Specifically, the mode converter 35R can also convert the polarization wave mode of the red light of the TM0 mode incident on the emission end 51b from the TM0 mode to the TE0 mode, and emit the red light of the TE0 mode from the incident end 51a. The mode converter 35G can also convert the polarization wave mode of the green light of the TM0 mode incident on the emission end 51b from the TM0 mode to the TE0 mode, and emit the green light of the TE0 mode from the incident end 51a. The mode converter 35B can also convert the polarization wave mode of the blue light of the TM0 mode incident on the emission end 51b from the TM0 mode to the TE0 mode, and emit the blue light of the TE0 mode from the incident end 51a.

[0111] Next, refer to Figure 9Describe a laser module according to another embodiment. Figure 9 It is a block diagram of a laser module according to another embodiment. Figure 9 The main difference between the shown laser module 13A 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 a 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.

[0112] Specifically, the multiplexer 36 is provided in the downstream section of the modulators 34R, 34G, and 34B, and 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. The multiplexer 36 emits the multiplexed laser light to the mode converter 35.

[0113] The mode converter 35 is provided in the downstream section of the multiplexer 36, and converts the polarization wave mode of the laser light 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 35R.

[0114] In the laser module 13A, visible light in the TM0 mode is emitted from each laser light source, and the light intensity of the visible light in the TM0 mode is modulated in each modulator. Then, the visible light modulated in each modulator is multiplexed in the multiplexer 36 to generate laser light. Then, the polarization wave mode of the laser light is converted from the TM0 mode to the TE0 mode in the mode converter 35, and the laser light in the TE0 mode is emitted from the mode converter 35 to the optical component 14 (refer to Figure 2 ).

[0115] In the laser module 13A, the structures common to the laser module 13 also have the same effects as those of the laser module 13. In the optical element 30A, the structures common to the optical element 30 also have the same effects as those of the optical element 30. Since the laser module 13A and the optical element 30A include a mode converter 35 instead of the mode converters 35R, 35G, and 35B, the laser module 13A and the optical element 30A can be miniaturized.

[0116] Next, refer to Figure 10 Describe a laser module according to yet another embodiment. Figure 10It is a block diagram of a laser module according to another embodiment. The main difference between the laser module 13B and the laser module 13 is that the laser module 13B includes a light source unit 20B and an optical element 30B instead 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, 23B instead of the laser light sources 21, 22, 23.

[0117] The main difference between the laser light sources 21B, 22B, 23B and the laser light sources 21, 22, 23 lies in the polarization wave mode of the emitted visible light. Specifically, the laser light source 21B emits red light in the TE0 mode. The laser light source 22B emits green light in the TE0 mode. The laser light source 23B emits blue light in the TE0 mode.

[0118] The main difference between the optical element 30B and the optical element 30 is that the optical element 30B includes mode converters 37R, 37G, 37B instead of the mode converters 35R, 35G, 35B.

[0119] The mode converter 37R is a mode converter that converts the polarization wave mode of red light from the TE0 mode (the first polarization wave mode) to the TM0 mode (the second polarization wave 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.

[0120] The mode converter 37G is a mode converter that converts the polarization wave mode of green light from the TE0 mode (the first polarization wave mode) to the TM0 mode (the second polarization wave 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.

[0121] The mode converter 37B is a mode converter that converts the polarization wave mode of blue light from the TE0 mode (the first polarization wave mode) to the TM0 mode (the second polarization wave 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. Additionally, as the mode converters 37R, 37G, 37B, mode converters with the same structure as the mode converters 35R, 35G, 35B are respectively used.

[0122] The modulator 34R is provided in the downstream section of the mode converter 37R, modulates the optical intensity of the red light of the TM0 mode emitted from the mode converter 37R, and emits the red light to the multiplexer 36. The modulator 34G is provided in the downstream section of the mode converter 37G, modulates the optical intensity of the green light of the TM0 mode emitted from the mode converter 37G, and emits the green light to the multiplexer 36. The modulator 34B is provided in the downstream section of the mode converter 37B, modulates the optical intensity of the blue light of the TM0 mode emitted from the mode converter 37B, and emits the blue light to the multiplexer 36. As described above, the C-axis of lithium niobate extends in the Z-axis direction. Therefore, the modulation efficiency of each modulator is improved in the TM mode.

[0123] In the laser module 13B, since visible light of the TE0 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 TE0 mode to the TM0 mode in each mode converter. Then, after modulating the optical intensity of the visible light of the TM0 mode in each modulator, the modulated visible lights are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 to the optical component 14 (see Figure 2 ).

[0124] In the laser module 13B, the structure common to the laser module 13 also has the same effect as the laser module 13. In the optical element 30B, the structure common to the optical element 30 also has the same effect as 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. Also in this case, the polarization wave mode of the visible light can be converted from the TE0 mode to the TM0 mode without reducing the modulation efficiency of each modulator, and the visible light of the TM0 mode can be emitted to the outside.

[0125] Next, with reference to Figure 11 a laser module according to another embodiment will be described. Figure 11 is a block diagram of a laser module according to another embodiment. Figure 11 The main difference between the shown laser module 13C and the laser module 13B is that the laser module 13C includes an optical element 30C instead of the optical element 30B. The main difference between the optical element 30C and the optical element 30B is that the optical element 30C further includes a mode converter 35R, a mode converter 35G, and a mode converter 35B.

[0126] The mode converter 35R is provided in the downstream section of the modulator 34R. The mode converter 35R converts the polarization wave mode of the red light emitted from the modulator 34R from the TM0 mode to the TE0 mode, and emits the red light in the TE0 mode to the multiplexer 36. The mode converter 35G is provided in the downstream section of the modulator 34G. The mode converter 35G converts the polarization wave mode of the green light emitted from the modulator 34G from the TM0 mode to the TE0 mode, and emits the green light in the TE0 mode to the multiplexer 36. The mode converter 35B is provided in the downstream section of the modulator 34B. The mode converter 35B converts the polarization wave mode of the blue light emitted from the modulator 34B from the TM0 mode to the TE0 mode, and emits the blue light in the TE0 mode to the multiplexer 36.

[0127] In the laser module 13C, 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 thereafter, in each of the mode converters 35R, 35G, and 35B, the polarization wave mode of the modulated visible light is converted from the TM0 mode to the TE0 mode. Then, the respective visible lights are multiplexed in the multiplexer 36, and the 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 13C, the structures common to the laser module 13B also have the same effects as those of the laser module 13B. In the optical element 30C, the structures common to the optical element 30B also have the same effects as those of the optical element 30B. In the laser module 13C and the optical element 30C, visible light in the TE0 mode is emitted from each laser light source. In this case, it is also possible to emit the visible light in the TE0 mode to the outside without reducing the modulation efficiency of each modulator.

[0129] 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.

[0130] For example, the laser modules 13, 13A, 13B, and 13C can also be applied to devices other than the near-eye wearable device 1.

[0131] 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.

[0132] The optical elements 30, 30A, 30B, 30C only need to include one mode converter. In other words, the optical elements 30, 30A, 30B, 30C only need to include one mode converter that converts the polarization wave mode of visible light from one of the TE mode and the TM mode to the other of the TE mode and the TM mode.

[0133] The laser module 13 may also include a light source unit 20B instead of the light source unit 20. In this case, the optical element 30 includes mode converters 37R, 37G, 37B instead of the mode converters 35R, 35G, 35B. Visible light in the TE0 mode is incident on each modulator. In order to improve the modulation efficiency of each modulator, the core layer 32 may be made of X-cut lithium niobate, and the optical axis (C axis) of the lithium niobate may extend in the Y-axis direction. According to this structure, visible light in the TE0 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 TE0 mode to the TM0 mode in each mode converter. Then, the visible lights with the converted polarization wave modes are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 to the optical component 14 (refer to Figure 2 ) as laser light in the TM0 mode.

[0134] Similarly, the laser module 13A may also include a light source unit 20B instead of the light source unit 20. In this case, the optical element 30A includes a mode converter that converts the polarization wave mode of visible light from the TE0 mode to the TM0 mode instead of the mode converter 35. The core layer 32 may be made of X-cut lithium niobate, and the optical axis (C axis) of the lithium niobate may extend in the Y-axis direction. According to this structure, visible light in the TE0 mode is emitted from each laser light source, and the light intensity of the visible light in the TE0 mode is modulated in each modulator. Then, the laser is generated by multiplexing the visible lights modulated in each modulator in the multiplexer 36. Then, the polarization wave mode of the laser is converted from the TE0 mode to the TM0 mode in the mode converter, and the laser in the TM0 mode is emitted from the mode converter to the optical component 14 (refer to Figure 2 ).

[0135] The laser module 13B may also include the light source unit 20 instead of the light source unit 20B. In this case, the optical element 30B includes the mode converters 35R, 35G, 35B instead of the mode converters 37R, 37G, 37B. The core layer 32 may be made of X-cut lithium niobate, and the optical axis (C-axis) of the lithium niobate may extend in the Y-axis direction. According to this structure, 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 combined in the combiner 36 and emitted from the combiner 36 to the optical component 14 (refer to Figure 2 ) as laser light of the TE0 mode.

[0136] The laser module 13C may also include the light source unit 20 instead of the light source unit 20B. In this case, in the optical element 30C, the mode converters 35R, 35G, 35B and the mode converters 37R, 37G, 37B are swapped. The core layer 32 may be made of X-cut lithium niobate, and the optical axis (C-axis) of the lithium niobate may extend in the Y-axis direction. Since visible light of the TM0 mode is emitted from each laser light source, first, 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 of the mode converters 35R, 35G, 35B. Then, the light intensity of the visible light of the TE0 mode is modulated in each modulator, and then the polarization wave mode of the modulated visible lights is converted from the TE0 mode to the TM0 mode in each of the mode converters 37R, 37G, 37B. Then, the visible lights are combined in the combiner 36 and emitted from the combiner 36 to the optical component 14 (refer to Figure 2 ) as laser light of the TM0 mode.

[0137] Embodiment

[0138] To illustrate the above effects, the present disclosure will be described in more detail below through embodiments. The present disclosure is not limited to these embodiments.

[0139] <Evaluation of conversion loss with respect to light of each color>

[0140] The conversion losses of the mode converters in Examples 1 to 3 were calculated. This conversion loss is the loss of conversion from the TM0 mode to the TE0 mode. As the mode converters in Examples 1 to 3, those having the same as Figures 4 to 6A mode converter having the same configuration as the mode converter 35R shown. In Examples 1 to 3, sapphire is used as the constituent material of the substrate 31, Z-cut lithium niobate (LiNbO3) is used as the constituent material of the core layer 32, silica (SiO2) is used as the constituent material of the cladding layer 33, and silver (Ag) is used as the constituent material of the metal body 52.

[0141] As shown in Table 1, in Examples 1 to 3, the values of the respective parameters were set to maximize the conversion efficiency.

[0142] [Table 1]

[0143]

[0144] In the mode converters of Examples 1 to 3, the conversion efficiency was calculated while changing the size of the length Lc. The calculation results are shown in Figures 12 to 14 . Figure 12 is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter of Example 1. Figure 13 is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter of Example 2. Figure 14 is a graph showing the relationship between the conversion length and the conversion efficiency of the mode converter of Example 3. Figures 12 to 14 The horizontal axis of Figures 12 to 14 represents the length Lc (conversion length) (unit: μm),

[0145] In the mode converter of Example 1, with respect to the entire field of the first hybrid mode, the horizontal electric field component of the first hybrid mode is about 49%. With respect to the entire field of the second hybrid mode, the horizontal electric field component of the second hybrid mode is about 50%. In this case, the Figure 12 shown conversion efficiencies CE1 and conversion efficiency CE2, and the length Lc is calculated to be 16.0 μm. When the length Lc is 16.0 μm, the conversion efficiency from the TM0 mode to the TE0 mode is 0.758, and the conversion loss is 1.20 dB.

[0146] In the mode converter of Example 2, with respect to the entire field of the first hybrid mode, the horizontal electric field component of the first hybrid mode is about 43%. With respect to the entire field of the second hybrid mode, the horizontal electric field component of the second hybrid mode is about 56%. In this case, the Figure 13 shown conversion efficiencies CE1 and conversion efficiency CE2, and the length Lc is calculated to be 11.0 μm. When the length Lc is 11.0 μm, the conversion efficiency from the TM0 mode to the TE0 mode is 0.772, and the conversion loss is 1.12 dB.

[0147] In the mode converter of Example 3, with respect to the entire field of the first hybrid mode, the horizontal electric field component of the first hybrid mode is about 38%. With respect to the entire field of the second hybrid mode, the horizontal electric field component of the second hybrid mode is about 62%. In this case, the Figure 14 conversion efficiency CE1 and the conversion efficiency CE2 shown in are obtained, and the length Lc is calculated to be 22.0 μm. When the length Lc is 22.0 μm, the conversion efficiency from the TM0 mode to the TE0 mode is 0.563, and the conversion loss is 2.50 dB.

[0148] In the mode converters of Examples 1 to 3, relatively small conversion losses of 1.12 dB to 2.50 dB are generated. The length Lc is 11.0 μm to 22.0 μm. From this, it can be seen that low-loss mode conversion is achieved while shortening the length Lc.

[0149] <Evaluation of Distance D1 and Distance D2>

[0150] The influence of distance D1 and distance D2 on the conversion loss was evaluated. The mode converters of Examples 1 to 3 were used in this evaluation. The values of parameters other than distance D1 and distance D2 (wavelength λ, height T1, width W1, height T2, and width W2) were set to the values shown in Table 1. The length Lc was 100 μm or less and was set to the length that maximizes the conversion efficiency among the parameter values.

[0151] For red light, the conversion loss was calculated while changing distance D2 for distances D1 of 0 nm, 20 nm, 40 nm, 60 nm, 80 nm, and 100 nm, respectively. For green light, the conversion loss was calculated while changing distance D2 for distances D1 of 0 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, and 60 nm, respectively. For blue light, the conversion loss was calculated while changing distance D2 for distances D1 of 0 nm, 10 nm, 20 nm, 30 nm, 40 nm, and 60 nm, respectively. These conversion losses are the losses from the TM0 mode to the TE0 mode.

[0152] The calculation results of the conversion loss are shown in Figures 15 to 17 . Figure 15 is a graph showing the calculation results of the conversion loss of red light. Figure 16 is a graph showing the calculation results of the conversion loss of green light. Figure 17 is a graph showing the calculation results of the conversion loss of blue light. Figures 15 to 17 The horizontal axis of represents the distance D2 (unit: μm), Figures 15 to 17 and the vertical axis of represents the conversion loss (unit: dB).

[0153] From the viewpoint of suppressing the output light of a laser light source (laser diode) to a low level, when the conversion loss is 6 dB or less, it is judged that a high conversion efficiency is achieved. According to Figure 15 , when the distance D1 is from 0 nm to 100 nm and the distance D2 is from 0 nm to 90 nm, the conversion loss of red light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 100 nm and the distance D2 is from 0 nm to 90 nm.

[0154] According to Figure 16 , when the distance D1 is from 0 nm to 40 nm and the distance D2 is from 20 nm to 80 nm, the conversion loss of green light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 40 nm and the distance D2 is from 20 nm to 80 nm. According to Figure 17 , when the distance D1 is from 0 nm to 10 nm and the distance D2 is from 10 nm to 70 nm, the conversion loss of blue light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 10 nm and the distance D2 is from 10 nm to 70 nm.

[0155] In addition, the allowable value of the conversion loss also depends on the loss allowed for the entire system and the details in the chip loss (the coupling efficiency between the laser diode and the waveguide end face of the chip integrated with optical elements, and the component loss of the conversion loss in the optical circuit in the chip).

[0156] <Evaluation of Distance D1>

[0157] The influence of the distance D1 on the conversion loss was evaluated. In this evaluation, the mode converters of Examples 1 to 3 were used. The values of the parameters (wavelength λ, height T1, width W1, height T2, and width W2) other than the distance D1 and the distance D2 were set to the values shown in Table 1. The length Lc was 100 μm or less and was set to the length that maximizes the conversion efficiency among the parameter values.

[0158] For each distance D1, the conversion loss was calculated when the distance D2 was changed by 10 nm in the range from -20 nm to 100 nm. This conversion loss is the loss from the TM0 mode to the TE0 mode. The calculation results of the conversion loss are shown in Figure 18 . Figure 18 is a graph showing the relationship between the distance in the Z-axis direction between the waveguide and the metal body and the conversion loss. Figure 18 The horizontal axis of Figure 18 represents the distance D1 (unit: nm), Figure 18 and the vertical axis represents the conversion loss (unit: dB). The minimum value of the conversion loss for each distance D1 is shown in

[0159] From the viewpoint of suppressing the output light of a laser light source (laser diode) to a low level, when the conversion loss is 6 dB or less, it is judged that a high conversion efficiency is achieved. According to Figure 18 , when the distance D1 is from 0 nm to 120 nm, the conversion loss of red light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 120 nm. According to Figure 18 , when the distance D1 is from 0 nm to 60 nm, the conversion loss of green light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 60 nm. According to Figure 18 , when the distance D1 is from 0 nm to 30 nm, the conversion loss of blue light is 6 dB or less. Therefore, it can be said that a high conversion efficiency is achieved when the distance D1 is from 0 nm to 30 nm. It can be seen that for any color of light, the smaller the distance D1, the smaller the conversion loss.

[0160] (Supplementary Note)

[0161] [Item 1]

[0162] An optical element, wherein,

[0163] The optical element includes:

[0164] A substrate having a main surface;

[0165] A core layer provided on the main surface and made of a material having an electro-optic effect. The core layer has a waveguide extending in a first direction along the main surface; and

[0166] A metal body extending in the first direction and disposed in parallel with the waveguide,

[0167] The waveguide and the metal body form a mode converter that converts the polarization wave mode of visible light from one polarization wave mode, i.e., the first polarization wave mode, among the TE mode and the TM mode, into the other polarization wave mode, i.e., the second polarization wave mode, among the TE mode and the TM mode,

[0168] The waveguide has an incident end for the visible light of the first polarization wave mode to enter and an emission end for the visible light of the second polarization wave mode to exit,

[0169] The metal body has an edge in a second direction that intersects the first direction and extends along the main surface,

[0170] The edge overlaps the waveguide when viewed from a third direction intersecting the main surface.

[0171] [Item 2]

[0172] The optical element according to Item 1, wherein

[0173] the waveguide has a bottom surface facing the main surface and a top surface provided on the side opposite to the bottom surface in the third direction,

[0174] the metal body is arranged such that the edge, the top surface, and the bottom surface are arranged in sequence in the third direction.

[0175] [Item 3]

[0176] The optical element according to Item 1 or Item 2, wherein

[0177] the distance in the second direction between the center of the waveguide in the second direction and the edge is 0 nm or more and less than half of the length of the waveguide in the second direction.

[0178] [Item 4]

[0179] The optical element according to any one of Items 1 to 3, wherein

[0180] the metal body is made of a metal containing at least one element selected from the group consisting of the following elements: silver, gold, copper, aluminum, chromium, manganese, titanium, vanadium, iron, cobalt, nickel, zinc, molybdenum, palladium, tantalum, tungsten, platinum, lead, and bismuth.

[0181] [Item 5]

[0182] The optical element according to any one of Items 1 to 4, wherein

[0183] the optical element includes:

[0184] a first mode converter that converts the polarization wave mode of red light from the first polarization wave mode to the second polarization wave mode;

[0185] a second mode converter that converts the polarization wave mode of green light from the first polarization wave mode to the second polarization wave mode;

[0186] a third mode converter that converts the polarization wave mode of blue light from the first polarization wave mode to the second polarization wave mode; and

[0187] a multiplexer that multiplexes the red light, the green light, and the blue light and emits laser light.

[0188] [Item 6]

[0189] The optical element according to Item 5, wherein

[0190] The lengths of the waveguides of the first mode converter, the second mode converter, and the third mode converter in the third direction are the same as each other.

[0191] [Item 7]

[0192] The optical element according to Item 5 or Item 6, wherein

[0193] The optical element further includes:

[0194] A first modulator that modulates the light intensity of the red light;

[0195] A second modulator that modulates the light intensity of the green light; and

[0196] A third modulator that modulates the light intensity of the blue light.

[0197] [Item 8]

[0198] A laser module, wherein

[0199] The laser module includes:

[0200] The optical element according to any one of Items 5 to 7;

[0201] A first light source that emits the red light of the first polarization wave mode;

[0202] A second light source that emits the green light of the first polarization wave mode; and

[0203] A third light source that emits the blue light of the first polarization wave mode.

[0204] [Item 9]

[0205] A retinal projection device mounted on a near-eye wearable device, wherein

[0206] The retinal projection device includes:

[0207] The laser module according to Item 8;

[0208] A movable mirror that scans using the laser emitted from the laser module; and

[0209] A reflector that projects an image onto the retina by reflecting the laser that has passed through the movable mirror and irradiating the reflected light onto the retina of a user wearing the near-eye wearable device.

[0210] [Item 10]

[0211] A near-eye wearable device, wherein,

[0212] The near-eye wearable device includes:

[0213] The retinal projection device described in item 9; and

[0214] A lens provided with the reflector.

Claims

1. An optical element, wherein: The optical component has: a substrate having a main surface; a core layer, which is provided on the main surface and is composed of a material having an electro-optical effect, wherein the core layer has a waveguide extending in a first direction along the main surface; as well as a metal body extending in the first direction and arranged in parallel with the waveguide, The waveguide and the metal body constitute a mode converter, which converts the polarization wave mode of visible light from a first polarization wave mode, one of the TE mode and the TM mode, to a second polarization wave mode, the other of the TE mode and the TM mode. The waveguide has an incident end into which the visible light in the first polarization mode is incident and an output end from which the visible light in the second polarization mode is output. The metal body has an edge in a second direction intersecting the first direction and extending along the main surface. The edge overlaps the waveguide when viewed from a third direction intersecting the main surface.

2. The optical element according to claim 1, wherein The waveguide has a bottom surface facing the main surface and a top surface provided on the side opposite to the bottom surface in the third direction. The metal body is arranged such that the edge, the top surface, and the bottom surface are arranged in this order in the third direction.

3. The optical element according to claim 1 or 2, wherein: A distance in the second direction between the center of the waveguide in the second direction and the edge is greater than or equal to 0 nm and less than or equal to half of the length of the waveguide in the second direction.

4. The optical element according to any one of claims 1 to 3, wherein The metal body is composed of a metal containing at least one element selected from the group consisting of silver, gold, copper, aluminum, chromium, manganese, titanium, vanadium, iron, cobalt, nickel, zinc, molybdenum, palladium, tantalum, tungsten, platinum, lead and bismuth.

5. The optical element according to any one of claims 1 to 4, wherein The optical component has: a first mode converter, which is the mode converter that converts the polarization mode of red light from the first polarization mode to the second polarization mode; a second mode converter, which is the mode converter that converts the polarization mode of green light from the first polarization mode to the second polarization mode; a third mode converter, which is the mode converter for converting the polarization mode of blue light from the first polarization mode to the second polarization mode; as well as A combiner combines the red light, the green light, and the blue light to emit laser light.

6. The optical element according to claim 5, wherein: The length of the waveguide of the first mode converter in the third direction, the length of the waveguide of the second mode converter in the third direction, and the length of the waveguide of the third mode converter in the third direction are the same as each other.

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

8. A laser module, wherein: The laser module has: The optical element according to any one of claims 5 to 7; a first light source that emits the red light in the first polarization mode; a second light source that emits the green light in the first polarization mode; and A third light source emits the blue light in the first polarization mode.

9. A retinal projection device, mounted on a near-eye wearable device, wherein: The retinal projection device has: The laser module according to claim 8; 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.

10. A near-eye wearable device, wherein: The near-eye wearable device has: The retinal projection device of claim 9; and A lens is provided with the reflector.

Images