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

Abstract

To provide an optical element, a laser module, a retina projection device and a near-eye wearable device, capable of converting a polarization mode of visible light.SOLUTION: An optical element includes: a substrate 31 having a principal plane 31a; a core layer 32 disposed on the principal plane 31a and composed of a material having an electrooptic effect, the core layer 32 having a waveguide 51 extending in an X-axis direction; and a metal body 52 extending in the X-axis direction and arranged in parallel to the waveguide 51. The waveguide 51 and the metal body 52 compose a mode converter 35R configured to convert a polarization mode of visible light from a first polarization mode being one polarization mode of a TE mode and a TM mode into a second polarization mode being the other polarization mode of the TE mode and the TM mode. The metal body 52 includes an edge 52c in a Y-axis direction. The edge 52c overlaps the waveguide 51 in view of a Z-axis direction.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] The polarization modes of light propagating through an optical waveguide include a TE (Transverse Electric) mode, which is a polarization mode having a main electric field in the horizontal direction with respect to the substrate, and a TM (Transverse Magnetic) mode, which is a polarization mode having a main electric field in the vertical direction with respect to the substrate. Optical elements for converting these polarization modes are known. For example, Non-Patent Document 1 describes a polarization rotator including an InGaAsP ridge waveguide provided on an InP material and a metal layer provided on an upper cladding.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The polarization rotator described in Non-Patent Document 1 converts the polarization mode of light having a wavelength of 1.55 μm. However, visible light is not considered.

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

Means for Solving the Problems

[0006] 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 composed 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 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 the TE mode and the TM mode, to a second polarization mode, which is the other of the TE mode and the TM mode. The waveguide has an incident end into which visible light of the first polarization mode is incident and an output end that outputs visible light of the second polarization mode. The metal body has an edge in a second direction that intersects the first direction and is along the main surface. The edge overlaps the waveguide when viewed from a third direction that intersects the main surface.

[0007] In this optical element, the waveguide and the metal body are provided in parallel, and when viewed from the third direction, the edge of the metal body in the second direction overlaps the waveguide. Since the metal body has a negative dielectric constant, surface plasmons are excited on the surface of the metal body. Therefore, the polarization mode of the visible light propagating through the waveguide interacts with the surface plasmons and rotates according to the position of the edge of the metal body. As a result, in a portion of the waveguide parallel to the metal body, a first mixed mode and a second mixed mode in which the TE mode and the TM mode are mixed can occur. Since there is a difference between the propagation constant of the first mixed mode and the propagation constant of the second mixed mode, a phase difference occurs between the phase of the first mixed mode and the phase of the second mixed mode according to the length of the above portion. When the visible light exits from the above portion, the first mixed mode and the second mixed mode are combined into one mode, and the polarization mode of the visible light can be converted from the first polarization mode to the second polarization mode. From the above, according to the above optical element, it is possible to convert the polarization mode of visible light.

[0008] 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. The metal body may be arranged in the third direction so as to be arranged in the order of the edge, the top surface, and the bottom surface. 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.

[0009] The distance in the second direction between the center and the edge of the waveguide in the second direction may be 0 nm or more and may be less than or equal to 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.

[0010] 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.

[0011] The optical element may include a first mode converter that is a mode converter for converting the polarization mode of red light from a first polarization mode to a second polarization mode, a second mode converter that is a mode converter for converting the polarization mode of green light from the first polarization mode to the second polarization mode, a third mode converter that is a mode converter for converting the polarization mode of blue light from the first polarization mode to the second polarization mode, and a multiplexer that multiplexes red light, green light, and blue light and emits laser light. According to this configuration, the polarization mode of red light is converted from the first polarization mode to the second polarization mode, the polarization mode of green light is converted from the first polarization mode to the second polarization mode, and the polarization mode of blue light is converted from the first polarization mode to the second polarization mode. By using, as the second polarization mode, the polarization mode with high multiplexing efficiency in the multiplexer among the TM mode and the TE mode, it is possible to improve the multiplexing efficiency.

[0012] The lengths of the waveguides of the first mode converter, the second mode converter, and the third mode converter in the third direction may be the same as each other. According to this configuration, 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 the respective waveguides in the third direction can be made the same, so that the manufacture of the optical element can be facilitated.

[0013] The optical element may further include a first modulator that modulates the light intensity of red light, a second modulator that modulates the light intensity of green light, and a third modulator that modulates the light intensity of blue light. In order to output full-color laser light by combining red light, green light, and blue light, it is necessary to adjust the light intensity of each color of light according to the color to be output. According to the above configuration, since the light intensity of red light, the light intensity of green light, and the light intensity of blue light are modulated by the modulator, it is possible to output full-color laser light without requiring a large drive current.

[0014] 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 first polarization mode, a second light source that emits green light in the first polarization mode, and a third light source that emits blue light in the first polarization mode. Since this laser module includes the above-described optical element, it is possible to convert the polarization modes of red light, green light, and blue light.

[0015] 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 performs scanning using the laser light emitted from the laser module, and a reflector that reflects the laser light that has passed through the movable mirror and irradiates the reflected light onto the retina of a user wearing the near-eye wearable device to project an image onto the retina. This retinal projection device includes the above-described optical element. Therefore, in this retinal projection device, it is possible to project an image onto the retina after converting the polarization modes of red light, green light, and blue light.

[0016] 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, it is possible to project an image onto the retina after converting the polarization modes of red light, green light, and blue light.

Advantages of the Invention

[0017] According to each aspect and each embodiment of the present disclosure, the polarization mode of visible light can be converted.

Brief Description of the Drawings

[0018]

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DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same reference numerals are given to the same elements, and duplicate descriptions are omitted. In each figure, an XYZ coordinate system may be shown. The Y-axis direction (second direction) intersects (for example, is orthogonal to) the X-axis direction (first direction) and the Z-axis direction (third direction). The Z-axis direction intersects (for example, is orthogonal to) the X-axis direction and the Y-axis direction. In this specification, a numerical range indicated using "~" indicates a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. The individually described upper limit value and lower limit value can be arbitrarily combined.

[0020] While referring to FIG. 1, an application example of a laser module according to an embodiment will be described. FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. The near-eye wearable device 1 shown in FIG. 1 is a device that projects an image onto the retina of a user wearing the near-eye wearable device 1. The near-eye wearable device 1 is, for example, a head-mounted device (head-mounted display), and can take forms such as glasses type, goggle type, hat type, and helmet type. Examples of the near-eye wearable device 1 include smart glasses such as AR (Augmented Reality) glasses, VR (Virtual Reality) glasses, and MR (Mixed Reality) glasses. The near-eye wearable device 1 includes a frame 2, a lens 3, and a retinal projection device 10.

[0021] The frame 2 includes a pair of rims 2a, a bridge 2b, and a pair of temples 2c. The rim 2a is a part that holds the lens 3. The bridge 2b is a part that connects the pair of rims 2a. The temple 2c is a part that extends from the rim 2a and is hung on the user's ear. The frame 2 may be a rimless frame. The lens 3 has an inner surface 3a (see FIG. 2) facing the eyeball of the user wearing the near-eye wearable device 1.

[0022] The retinal projection device 10 is a device that directly projects (draws) an image onto the retina of a user wearing the near-eye wearable device 1. The retinal projection device 10 is mounted on the near-eye wearable device 1. In this embodiment, in order to project an image onto both the left and right retinas, the near-eye wearable device 1 includes two retinal projection devices 10, but may include only one of the retinal projection devices 10.

[0023] Next, while referring to FIG. 2, the retinal projection device 10 will be described in detail. FIG. 2 is a configuration diagram schematically showing the retinal projection device shown in FIG. 1. As shown in FIG. 2, the retinal projection device 10 includes an optical engine 11 and a reflector 12.

[0024] The optical engine 11 is a device that generates laser light Ls of color and light intensity corresponding to the pixels of the image projected onto the retina and emits the laser light Ls to the reflector 12. The optical engine 11 is mounted on the template 2c. The optical engine 11 includes a laser module 13, an optical component 14, a movable mirror 15, a laser driver 16, a mirror driver 17, and a controller 18.

[0025] The laser module 13 emits laser light. As the laser module 13, for example, a full-color laser module is used. The laser module 13 emits laser light of color and light intensity corresponding to the pixels of the image projected onto the retina. The details of the laser module 13 will be described later.

[0026] The optical component 14 is a component that optically processes the laser light emitted from the laser module 13. In the present embodiment, the optical component 14 includes a collimator lens 14a, a slit 14b, and a dimming filter 14c. The collimator lens 14a, the slit 14b, and the dimming filter 14c are arranged in that order along the optical path of the laser light. The optical component 14 may have other configurations.

[0027] The movable mirror 15 is a member for performing scanning using the laser light emitted from the laser module 13. The movable mirror 15 is provided in the emission direction of the laser light processed by the optical component 14. The movable mirror 15 is configured to be swingable, for example, around an axis extending in the lateral direction of the lens 3 and around an axis extending in the longitudinal direction of the lens 3, and reflects the laser light by changing the angle in the lateral and longitudinal directions of the lens 3. As the movable mirror 15, for example, a MEMS (Micro Electro Mechanical Systems) mirror is used.

[0028] The laser driver 16 is a drive circuit that drives the laser module 13. The laser driver 16 drives the laser module 13 based on, for example, the light intensity of the laser light and the temperature of the light source unit 20 included in the laser module 13. The mirror driver 17 is a drive circuit that drives the movable mirror 15. The mirror driver 17 swings the movable mirror 15 within a predetermined angular range and at a predetermined timing. The controller 18 is a device that controls the laser driver 16 and the mirror driver 17.

[0029] In the optical engine 11, laser light having a color and a light intensity corresponding to the pixels of the image projected onto the retina is emitted from the laser module 13, passes through the optical component 14, and is reflected by the movable mirror 15. The laser light reflected by the movable mirror 15 is emitted as laser light Ls to the reflector 12.

[0030] The reflector 12 is a member that projects an image onto the retina by reflecting the laser light 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.

[0031] Next, the laser module 13 will be described in detail with reference to FIGS. 3 and 4. FIG. 3 is a block diagram of the laser module shown in FIG. 2. FIG. 4 is a perspective view showing the configuration of the mode converter shown in FIG. 3. Only the peripheral portion of the mode converter 35R among the optical elements 30 is shown in FIG. 4. As shown in FIG. 3, the laser module 13 includes a light source unit 20 and an optical element 30.

[0032] The light source unit 20 emits visible light. The light source unit 20 includes a laser light source 21 (first light source) that emits red light, a laser light source 22 (second light source) that emits green light, and a laser light source 23 (third light source) that emits blue light. The laser light source 21 is, for example, a red laser diode. The laser light source 22 is, for example, a green laser diode. The laser light source 23 is, for example, a blue laser diode. The peak wavelength of the red light is, for example, in the range of 600 nm to 830 nm. The peak wavelength of the green light is, for example, in the range of 500 nm to 570 nm. The peak wavelength of the blue light is, for example, in the range of 380 nm to 490 nm. The laser light source 21, the laser light source 22, and the laser light source 23 are arranged in the Y-axis direction in that order.

[0033] In the present embodiment, the laser light source 21 emits red light in the TM fundamental mode (hereinafter referred to as the "TM0 mode"). The laser light source 22 emits green light in the TM0 mode. The laser light source 23 emits blue light in the TM0 mode. Since 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 be referred to as each visible light, and the red light, the green light, and the blue light may be 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.

[0034] The optical element 30 multiplexes the laser lights emitted from the respective laser light sources into one laser light. The optical element 30 is, for example, a Planar Lightwave Circuit (PLC). The optical element 30 is joined to the light source unit 20 via, for example, a metal bonding layer. Since the laser module 13 is mounted on the near-eye wearable device 1, the size of the optical element 30 as viewed from the Z-axis direction may be 100 mm 2 or less. As shown in FIG. 4, the optical element 30 includes a substrate 31, a core layer 32, and a cladding layer 33.

[0035] The substrate 31 functions as a lower cladding layer. The substrate 31 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. Examples of the constituent material of the substrate 31 include sapphire, silicon oxide, and silicon with silicon oxide laminated thereon. The substrate 31 has a main surface 31a and a back surface 31b 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 (are 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.

[0036] The cladding layer 33 functions as an upper cladding layer. The cladding layer 33 covers the core layer 32 on the main surface 31a. The cladding layer 33 is provided over the entire main surface 31a. The cladding layer 33 is made of a material having a refractive index lower than that of the constituent material of the core layer 32. Examples of the constituent material of the cladding layer 33 include silicon oxide (e.g., SiO2).

[0037] The core layer 32 is provided on the main surface 31a. The core layer 32 is made of a material having an electro-optic effect. The electro-optic effect is a phenomenon in which the refractive index of a material changes by applying an electric field to the material. Examples of the constituent material of the core layer 32 include lithium niobate (LiNbO3). In this embodiment, the core layer 32 is a lithium niobate thin film formed on the main surface 31a of the substrate 31 by sputtering, and the optical axis (C-axis) of lithium niobate extends in the Z-axis direction. The core layer 32 may be made of Z-cut lithium niobate.

[0038] 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.

[0039] The modulator 34R is a modulator that modulates the light intensity of red light. The modulator 34R modulates the light intensity of the red light in the TM0 mode emitted from the laser light source 21. The modulator 34G is a modulator that modulates the light intensity of green light. The modulator 34G modulates the light intensity of the green light in the TM0 mode emitted from the laser light source 22. The modulator 34B is a modulator that modulates the light intensity of blue light. The modulator 34B modulates the light intensity of the blue light in the TM0 mode emitted from the laser light source 23. Each modulator is included in the core layer 32. Each modulator is, for example, a Mach-Zehnder type modulator.

[0040] The mode converter 35R is a mode converter that converts the polarization mode of red light from one polarization mode (the first polarization mode) of the TE mode and the TM mode to the other polarization mode (the second polarization mode) of the TE mode and the TM mode. In the present embodiment, the mode converter 35R converts the polarization mode of red light from the TM0 mode to the TE fundamental mode (hereinafter referred to as the "TE0 mode"). The mode converter 35R is provided at the subsequent stage of the modulator 34R and converts the polarization mode of the red light emitted from the modulator 34R from the TM0 mode to the TE0 mode.

[0041] The mode converter 35G is a mode converter that converts the polarization mode of green light from one polarization mode (the first polarization mode) of the TE mode and the TM mode to the other polarization mode (the second polarization mode) of the TE mode and the TM mode. In the present embodiment, the mode converter 35G converts the polarization mode of green light from the TM0 mode to the TE0 mode. The mode converter 35G is provided at the subsequent stage of the modulator 34G and converts the polarization mode of the green light emitted from the modulator 34G from the TM0 mode to the TE0 mode.

[0042] The mode converter 35B is a mode converter that converts the polarization mode of blue light from one of the TE mode and the TM mode (the first polarization mode) to the other of the TE mode and the TM mode (the second polarization mode). In the present embodiment, the mode converter 35B is a mode converter that converts the polarization mode of blue light from the TM0 mode to the TE0 mode. The mode converter 35B is provided at the subsequent stage of the modulator 34B and converts the polarization mode of the blue light emitted from the modulator 34B from the TM0 mode to the TE0 mode.

[0043] Note that the polarization mode is also referred to as a waveguide mode. The TM mode is a polarization mode in which the direction of the main component of the electric field in a cross section perpendicular to the traveling direction of light is perpendicular to the main surface 31a of the substrate 31. The TE mode is a polarization mode in which the direction of the main component of the electric field in a cross section perpendicular to the traveling direction of light is horizontal to the main surface 31a of the substrate 31. The TM0 mode is a polarization mode having the largest effective refractive index among the TM modes. The TE0 mode is a polarization mode having the largest effective refractive index among the TE modes.

[0044] Each of the mode converter 35R, the mode converter 35G, and the mode converter 35B extends in the X-axis direction. The mode converter 35R, the mode converter 35G, and the mode converter 35B are arranged in the Y-axis direction in that order. The detailed configuration of each mode converter will be described later.

[0045] 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). The multiplexer 36 is included in the core layer 32. The multiplexer 36 may be configured by a multimode interferometer (MMI), may be configured by a Y-branch waveguide, or may be configured by a directional coupler. The length of the multiplexer 36 in the X-axis direction may be 10 μm to 10,000 μm.

[0046] For example, the relative position between the sub-carrier of the light source unit 20 and the substrate 31 is adjusted (active alignment) so that the optical axis of the visible light emitted from the laser light source matches the axis of the incident end of the corresponding modulator, and the light source unit 20 and the optical element 30 are joined by a metal bonding layer, whereby the laser module 13 is fabricated.

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

[0048] Next, with reference to FIGS. 4 to 6, the detailed configurations of mode converters 35R, 35G, and 35B will be described. FIG. 5 is a cross-sectional view taken along the line V-V of FIG. 4. FIG. 6 is a cross-sectional view taken along the line VI-VI of FIG. 4. In FIG. 6, for convenience of explanation, the hatching of the cladding layer 33 is omitted. As shown in FIG. 4, each of the mode converters 35R, 35G, and 35B includes a waveguide 51 and a metal body 52. Since the configurations of the respective mode converters are the same, the mode converter 35R will be described here as an example.

[0049] 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 longitudinal direction. The waveguide 51 has an incident end 51a which is one end in the X-axis direction and an exit end 51b which is the other end in the X-axis direction. TM0-mode red light is incident on the incident end 51a from the modulator 34R. The waveguide 51 emits TE0-mode red light from the exit end 51b to the multiplexer 36.

[0050] The waveguide 51 is symmetric in the Z-axis direction and symmetric in the Y-axis direction. Being symmetric in the Z-axis direction means that the two parts separated by the symmetry plane are plane-symmetric with respect to the symmetry plane passing through the center point in the Z-axis direction and orthogonal to the Z-axis direction. Being symmetric in the Y-axis direction means that the two parts separated by the symmetry plane are plane-symmetric with respect to the symmetry plane passing through the center point in the Y-axis direction and orthogonal to the Y-axis direction.

[0051] As shown in FIG. 5, 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 the entire surface. The top surface 51d is a 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 a surface connecting the bottom surface 51c and the top surface 51d. The pair of side surfaces 51e are substantially parallel.

[0052] 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 exit end 51b. Hereinafter, the length in the Z-axis direction may be referred to as "height", and the length in the Y-axis direction may be referred to as "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%.

[0053] The waveguide 51 includes an incident region 53, a conversion region 54, and an exit region 55. The conversion region 54 is a portion parallel to 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 exit region 55 includes the exit end 51b and is the portion from the other end of the conversion region 54 to the exit end 51b.

[0054] The metal body 52 is a metal member extending in the X-axis direction. The metal body 52 is provided in parallel with the waveguide 51. The metal body 52 is embedded in the cladding layer 33. That is, the periphery of the metal body 52 is covered with the cladding layer 33. The metal body 52 has a rectangular plate shape. The metal body 52 has a negative dielectric constant. The metal body 52 is composed of a metal containing at least one element selected from the group consisting of, for example, 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).

[0055] The length Lc (see FIG. 7) of the metal body 52 in the X-axis direction 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 arranged side by side 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.

[0056] As shown in FIG. 6, 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 a surface provided on the opposite side of the main surface 52b in the Z-axis direction. The metal body 52 has an edge 52c that is one edge in the Y-axis direction and an edge 52d that is the other edge in the Y-axis direction. When viewed from the Z-axis direction, the edge 52c overlaps the waveguide 51, and the edge 52d does not overlap the waveguide 51.

[0057] 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 so as to be lined up in the Z-axis direction in the order of the edge 52c, the top surface 51d, and the bottom surface 51c. Note that the distance D1 in the direction opposite to the bottom surface 51c from the top surface 51d is represented as a positive value, and the distance D1 in the direction 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 edge 52c is, for example, 0 nm or more and is less than half 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. Note that the distance D2 in the direction from the center CP toward the side surface 51e closer to the edge 52d among the pair of side surfaces 51e is represented as a positive value, and the distance D2 in the direction away from the side surface 51e closer to the edge 52d among the pair of side surfaces 51e from the center CP is represented as a negative value.

[0058] The mode converters 35G and 35B have the same configuration as the mode converter 35R, but the optimal dimensions may be different for each of the mode converter 35R, the mode converter 35G, and the mode converter 35B. The optimal dimensions referred to here are the dimensions that are optimal for maximizing the conversion efficiency of each polarization mode of 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 be different from each other.

[0059] Next, with reference to FIGS. 7 and 8, the conversion principles in the mode converters 35R, 35G, and 35B will be described. FIG. 7 is a diagram for explaining the conversion principle of the mode converter shown in FIG. 4. FIG. 8 is a graph showing an example of the conversion efficiency in the mode converter shown in FIG. 4. The horizontal axis of FIG. 8 indicates the length Lc, and the vertical axis of FIG. 8 indicates the conversion efficiency. Since the operations of the respective mode converters are the same, the mode converter 35R will be described here as an example.

[0060] As shown in FIG. 7, the mode converter 35R converts the polarization 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 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 enters the conversion region 54, the polarization mode of the red light interacts with the surface plasmon generated on the surface of the metal body 52 and rotates according to the position of the edge 52c of the metal body 52. As a result, two mixed modes (the first mixed mode and the second mixed mode) in which the TE mode and the TM mode are mixed are excited as the polarization mode of the red light.

[0061] At this time, depending on the position of the edge 52c of the metal body 52, the horizontal electric field component in the first hybrid mode and the horizontal electric field component in 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 in the first hybrid mode are of the same degree, and the horizontal electric field component and the vertical electric field component in the second hybrid mode are of the same degree. The electric field vector in the first hybrid mode is orthogonal to the electric field vector in the second hybrid mode.

[0062] 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 occurs between the phase of the first hybrid mode and the phase of the second hybrid mode according to the length that the red light propagates through 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 combined into one polarization mode. At this time, when the phase difference is π×(2n + 1) (n is an integer of 0 or more) radians, the polarization mode of the red light rotates 90° from the TM0 mode and is converted into the TE0 mode.

[0063] As shown in FIG. 8, the conversion efficiency CE1 and the conversion efficiency CE2 change periodically according to the magnitude of the length Lc. The conversion efficiency CE1 indicates the conversion efficiency when the red light in the TM0 mode is incident on the incident end 51a and the red light in the TE0 mode is emitted from the emission end 51b. The conversion efficiency CE2 indicates the conversion efficiency when the red light in the TM0 mode is incident on the incident end 51a and the 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 1. The conversion efficiency CE1 and the conversion efficiency CE2 vibrate periodically as the length Lc increases, and the maximum value of the conversion efficiency CE1 and the maximum value of the conversion efficiency CE2 appear alternately every half cycle.

[0064] In this embodiment, since the mode converter 35R converts the red light in the TM0 mode into the 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 the above relational expression is transformed 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, the length Lc is calculated by the right side of Equation (1).

Equation

[0065] Since the vacuum wave number k0 is represented by 2π / λ, Equation (2) is obtained from Equation (1).

Equation

[0066] The conversion efficiency (Conversion Efficiency: CE) in the mode converter is expressed by Equation (3) using the rotation angle φ, the length Lc, and the length L π . Note that the unit of CE in Equation (3) is %. The tangent of the rotation angle φ 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.

Equation

Equation

[0067] Here, the rotation angle φ 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 πis the length at which the phase difference between the first hybrid mode and the second hybrid mode becomes π radians. According to Equation (3), a conversion efficiency close to 100% can be obtained when the rotation angle φ is 45°. According to Equation (4), when the horizontal electric field component Ey(y,z) and the vertical electric field component Ez(y,z) in the first hybrid mode and the second hybrid mode are equal, the rotation angle φ becomes 45°. From the above, when the horizontal electric field component Ey(y,z) and the vertical electric field component Ez(y,z) in the first hybrid mode and the second hybrid mode are equal, a conversion efficiency close to 100% can be obtained.

[0068] In the conversion region 54, propagation loss occurs due to the imaginary part of the effective refractive index of each polarization mode. The imaginary part n of the effective refractive index i is caused by light absorption by the metal body 52. The propagation loss TL is expressed by Equation (5) using the imaginary part n i and the wavelength λ.

Equation

[0069] As shown in Equation (5), since the propagation loss TL is inversely proportional to the wavelength λ, the shorter the wavelength λ, the greater the tendency for the propagation loss TL to increase. Since the propagation loss TL is proportional to the imaginary part n i , the propagation loss TL can be suppressed by reducing the imaginary part n i .

[0070] In the laser module 13 and the optical element 30 described above, the waveguide 51 and the metal body 52 are provided in parallel, and 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. For this reason, the polarization mode of the visible light propagating through the waveguide 51 interacts with the surface plasmons and rotates according to the position of the edge 52c of the metal body 52. As a result, in the portion (conversion region 54) of the waveguide 51 parallel to the metal body 52, a first hybrid mode and a second hybrid mode in which the TE mode and the TM mode are mixed can occur. 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 occurs 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 exits from the conversion region 54, the first hybrid mode and the second hybrid mode are combined into one polarization mode, and the polarization mode of the visible light can be converted from the TM0 mode to the TE0 mode. From the above, according to the laser module 13 and the optical element 30, it is possible to convert the polarization mode of the visible light.

[0071] The near-eye wearable device 1 includes a retinal projection device 10, and the retinal projection device 10 includes an optical element 30. Therefore, in the near-eye wearable device 1 and the retinal projection device 10, it is possible to project an image onto the retina after converting the polarization mode of the visible light from the TM0 mode to the TE0 mode.

[0072] In the laser module 13 and the optical element 30, the metal body 52 is embedded in the cladding layer 33. In this configuration, 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 layer 33 and propagates along the interface. By changing the relative positions of the waveguide 51 and the metal body 52, the characteristics of the surface plasmon, particularly the interaction with the visible light propagating through the waveguide 51, can be adjusted. As a result, the optical axis rotation of the polarization mode of the visible light propagating through the waveguide 51 is likely to occur.

[0073] When the distance D1 is 0 nm or more, the rotation angle φ can be made closer to 45°, so the conversion efficiency can be improved. When the distance D2 is 0 nm or more and is equal to or less than half of the width W1, the rotation angle φ can be made closer to 45°, so the conversion efficiency can be improved.

[0074] 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 φ can be made closer to 45°, so the conversion efficiency can be improved. In the mode converter 35G, when the distance D1 is 0 nm to 40 nm and the distance D2 is 20 nm to 80 nm, the rotation angle φ can be made closer to 45°, so the conversion efficiency can be improved. In the mode converter 35B, when the distance D1 is 0 nm to 10 nm and the distance D2 is 10 nm to 70 nm, the rotation angle φ can be made closer to 45°, so the conversion efficiency can be improved.

[0075] The imaginary part n of the effective refractive index i is due to light absorption by the metal body 52, but when the length Lc is 30 μm or less, the imaginary part n i has little effect on the propagation loss. Therefore, the propagation loss TL can be suppressed. From the above, the conversion loss generated in the entire mode converter can be reduced.

[0076] The metal body 52 is composed of a metal containing at least one element selected from the group consisting of, for example, silver, gold, copper, aluminum, chromium, manganese, titanium, vanadium, iron, cobalt, nickel, zinc, molybdenum, palladium, tantalum, tungsten, platinum, lead, and bismuth.

[0077] 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 optical element 30, the mode converter 35R converts the polarization mode of red light from the TM0 mode to the TE0 mode, the mode converter 35G converts the polarization mode of green light from the TM0 mode to the TE0 mode, and the mode converter 35B converts the polarization mode of blue light from the TM0 mode to the TE0 mode. Therefore, it is possible to improve the multiplexing efficiency in the multiplexer 36.

[0078] The heights of the waveguides 51 of the mode converter 35R, the mode converter 35G, and the mode converter 35B are the same as each other. According to this configuration, 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 that the optical element 30 can be easily manufactured.

[0079] In order to output full-color laser light by multiplexing red light, green light, and blue light, it is necessary to adjust the light intensity of each color of light according to the color to be output. In order to change the light intensity of each color of light in the light source unit 20, a large drive current is required. In the 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, it is possible to output full-color laser light without requiring a large drive current.

[0080] Note that the modulation in the light source unit 20 and the modulation in the modulators 34R, 34G, and 34B may be used in combination. Generally, voltage adjustment has higher responsiveness than current adjustment. Therefore, when emphasizing responsiveness, coarse adjustment of the light intensity of each color may be performed in the light source unit 20, and fine adjustment of the light intensity of each color may be performed in the modulators 34R, 34G, and 34B. Since the amount of current can be suppressed when performing fine adjustment with current, power consumption can be suppressed. Therefore, when emphasizing suppression of power consumption, coarse adjustment of the light intensity of each color may be performed in the modulators 34R, 34G, and 34B, and fine adjustment of the light intensity of each color may be performed in the light source unit 20.

[0081] 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 difference in refractive index between the lower cladding and the core is small, the conversion length becomes long. In compound semiconductors, since there are limitations on the materials that can be selected as the lower cladding and the core, the difference in refractive index between the lower cladding and the core cannot be increased. On the other hand, in the optical element 30, as an example, the substrate 31 is made of sapphire, the cladding layer 33 is made of silicon dioxide, and the core layer 32 is made of lithium niobate. In this case, the difference in refractive index between the cladding layer and the core layer 32 can be increased. Therefore, it becomes possible to shorten the length Lc.

[0082] Note that the incident end 51a and the exit end 51b of the waveguide 51 in each mode converter may be interchanged with each other. Specifically, the mode converter 35R may convert the polarization mode of the red light of the TM0 mode incident on the exit 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 may convert the polarization mode of the green light of the TM0 mode incident on the exit 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 may convert the polarization mode of the blue light of the TM0 mode incident on the exit end 51b from the TM0 mode to the TE0 mode, and emit the blue light of the TE0 mode from the incident end 51a.

[0083] Next, while referring to FIG. 9, a laser module according to another embodiment will be described. FIG. 9 is a block diagram of a laser module according to another embodiment. The laser module 13A shown in FIG. 9 mainly differs from the laser module 13 in that it includes an optical element 30A instead of the optical element 30. The optical element 30A mainly differs from the optical element 30 in that it includes one mode converter 35 instead of the mode converters 35R, 35G, and 35B, and a multiplexer 36 is arranged between each modulator and the mode converter 35.

[0084] Specifically, the multiplexer 36 is provided at the subsequent stage 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.

[0085] The mode converter 35 is provided at the subsequent stage of the multiplexer 36, and converts the polarization mode of the laser light emitted from the multiplexer 36 from the TM0 mode to the TE0 mode. The configuration of the mode converter 35 is the same as that of the mode converter 35R.

[0086] In the laser module 13A, visible light in the TM0 mode is emitted from each laser light source, and the light intensity of the visible light in the TM0 mode is modulated by each modulator. Then, the visible light modulated by each modulator is multiplexed in the multiplexer 36 to generate laser light. Then, the polarization mode of the laser light is converted from the TM0 mode to the TE0 mode in the mode converter 35, and the laser light in the TE0 mode is emitted from the mode converter 35 to the optical component 14 (see FIG. 2).

[0087] Also in the laser module 13A, with respect to the components common to the laser module 13, the same effects as those of the laser module 13 are achieved. Also in the optical element 30A, with respect to the components common to the optical element 30, the same effects as those of the optical element 30 are achieved. Since the laser module 13A and the optical element 30A include one mode converter 35 instead of the mode converters 35R, 35G, and 35B, the laser module 13A and the optical element 30A can be miniaturized.

[0088] Next, with reference to FIG. 10, a laser module according to still another embodiment will be described. FIG. 10 is a block diagram of a laser module according to still another embodiment. The laser module 13B mainly differs from the laser module 13 in that it includes a light source unit 20B and an optical element 30B instead of the light source unit 20 and the optical element 30. The light source unit 20B mainly differs from the light source unit 20 in that it includes laser light sources 21B, 22B, and 23B instead of the laser light sources 21, 22, and 23.

[0089] The laser light sources 21B, 22B, and 23B mainly differ from the laser light sources 21, 22, and 23 in the polarization mode of the emitted visible light. Specifically, the laser light source 21B emits red light in the TE0 mode. The laser light source 22B emits green light in the TE0 mode. The laser light source 23B emits blue light in the TE0 mode.

[0090] The optical element 30B mainly differs from the optical element 30 in that it includes mode converters 37R, 37G, and 37B instead of the mode converters 35R, 35G, and 35B.

[0091] The mode converter 37R is a mode converter that converts the polarization mode of red light from the TE0 mode (first polarization mode) to the TM0 mode (second polarization mode). The mode converter 37R converts the polarization 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.

[0092] The mode converter 37G is a mode converter that converts the polarization mode of green light from the TE0 mode (first polarization mode) to the TM0 mode (second polarization mode). The mode converter 37G converts the polarization 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.

[0093] The mode converter 37B is a mode converter that converts the polarization mode of blue light from the TE0 mode (first polarization mode) to the TM0 mode (second polarization mode). The mode converter 37B converts the polarization 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. Note that as the mode converters 37R, 37G, and 37B, mode converters having the same structure as the mode converters 35R, 35G, and 35B are respectively used.

[0094] The modulator 34R is provided at the subsequent stage of the mode converter 37R, modulates the light intensity of the red light in the TM0 mode emitted from the mode converter 37R, and emits it to the multiplexer 36. The modulator 34G is provided at the subsequent stage of the mode converter 37G, modulates the light intensity of the green light in the TM0 mode emitted from the mode converter 37G, and emits it to the multiplexer 36. The modulator 34B is provided at the subsequent stage of the mode converter 37B, modulates the light intensity of the blue light in the TM0 mode emitted from the mode converter 37B, and emits it 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.

[0095] In the laser module 13B, since visible light in the TE0 mode is emitted from each laser light source, the polarization mode of each visible light emitted from each laser light source is converted from the TE0 mode to the TM0 mode in each mode converter. Then, after the light intensity of the visible light in the TM0 mode is modulated in each modulator, the modulated visible lights are multiplexed in the multiplexer 36, and are emitted from the multiplexer 36 as laser light in the TM0 mode to the optical component 14 (see FIG. 2).

[0096] Also in the laser module 13B, for the components common to the laser module 13, the same effects as those of the laser module 13 are achieved. Also in the optical element 30B, for the components common to the optical element 30, the same effects as those of the optical element 30 are achieved. In the laser module 13B and the optical element 30B, visible light in the TE0 mode is emitted from each laser light source. Even in this case, without reducing the modulation efficiency of each modulator, the polarization mode of the visible light can be converted from the TE0 mode to the TM0 mode, and the visible light in the TM0 mode can be emitted to the outside.

[0097] Next, with reference to FIG. 11, a laser module according to still another embodiment will be described. FIG. 11 is a block diagram of a laser module according to still another embodiment. The laser module 13C shown in FIG. 11 mainly differs from the laser module 13B in that it includes an optical element 30C instead of the optical element 30B. The optical element 30C mainly differs from the optical element 30B in that it further includes a mode converter 35R, a mode converter 35G, and a mode converter 35B.

[0098] The mode converter 35R is provided downstream of the modulator 34R. The mode converter 35R converts the polarization 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 combiner 36. The mode converter 35G is provided downstream of the modulator 34G. The mode converter 35G converts the polarization 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 combiner 36. The mode converter 35B is provided downstream of the modulator 34B. The mode converter 35B converts the polarization 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 combiner 36.

[0099] In the laser module 13C, since visible light in the TE0 mode is emitted from each laser light source, first, the polarization mode of each visible light emitted from each laser light source is converted from the TE0 mode to the TM0 mode in each mode converter 37R, 37G, 37B. Then, after the light intensity of the visible light in the TM0 mode is modulated in each modulator, the polarization mode of each modulated visible light is converted from the TM0 mode to the TE0 mode in each mode converter 35R, 35G, 35B. Then, each visible light is multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as laser light in the TE0 mode to the optical component 14 (see FIG. 2).

[0100] Also in the laser module 13C, for the configurations common to the laser module 13B, the same effects as those of the laser module 13B are achieved. Also in the optical element 30C, for the configurations common to the optical element 30B, the same effects as those of the optical element 30B are achieved. In the laser module 13C and the optical element 30C, visible light in the TE0 mode is emitted from each laser light source. Even in this case, the visible light in the TE0 mode can be emitted to the outside without reducing the modulation efficiency of each modulator.

[0101] Note that the optical element, laser module, retinal projection device, and near-eye wearable device according to the present disclosure are not limited to the above-described embodiments.

[0102] For example, the laser modules 13, 13A, 13B, 13C may be applied to devices other than the near-eye wearable device 1.

[0103] The optical elements 30, 30A, 30B, 30C may not include the cladding layer 33. In this case, the air layer can function as the upper cladding layer.

[0104] The optical elements 30, 30A, 30B, and 30C only need to include one mode converter. In other words, the optical elements 30, 30A, 30B, and 30C only need to include one mode converter that converts the polarization 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.

[0105] The laser module 13 may 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, and 37B instead of mode converters 35R, 35G, and 35B. Visible light in the TE0 mode is incident on each modulator. In order to improve the modulation efficiency in 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 configuration, visible light in the TE0 mode is emitted from each laser light source, and after the light intensity of each visible light is modulated in each modulator, the polarization mode of the visible light is converted from the TE0 mode to the TM0 mode in each mode converter. Then, each visible light with the converted polarization mode is multiplexed in the multiplexer 36 and emitted from the multiplexer 36 to the optical component 14 (see FIG. 2) as laser light in the TM0 mode.

[0106] Similarly, the laser module 13A may 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 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 configuration, 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 visible light modulated in each modulator is multiplexed in the multiplexer 36 to generate laser light. Then, the polarization mode of the laser light is converted from the TE0 mode to the TM0 mode in the mode converter, and the laser light in the TM0 mode is emitted from the mode converter to the optical component 14 (see FIG. 2).

[0107] The laser module 13B may 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, and 35B instead of the mode converters 37R, 37G, and 37B. The core layer 32 may be composed 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 configuration, since visible light in the TM0 mode is emitted from each laser light source, the polarization mode of each visible light emitted from each laser light source is converted from the TM0 mode to the TE0 mode in each mode converter. Then, after the light intensity of the visible light in the TE0 mode is modulated in each modulator, the modulated visible lights are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as laser light in the TE0 mode to the optical component 14 (see FIG. 2).

[0108] The laser module 13C may 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, and 35B and the mode converters 37R, 37G, and 37B are interchanged. The core layer 32 may be composed 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 in the TM0 mode is emitted from each laser light source, first, the polarization mode of each visible light emitted from each laser light source is converted from the TM0 mode to the TE0 mode in each of the mode converters 35R, 35G, and 35B. Then, after the light intensity of the visible light in the TE0 mode is modulated in each modulator, the polarization mode of the modulated visible lights is converted from the TE0 mode to the TM0 mode in each of the mode converters 37R, 37G, and 37B. Then, the visible lights are multiplexed in the multiplexer 36 and emitted from the multiplexer 36 as laser light in the TM0 mode to the optical component 14 (see FIG. 2).

Example

[0109] Hereinafter, to explain the above effects, the present disclosure will be described in more detail with reference to examples. The present disclosure is not limited to these examples.

[0110] <Evaluation of Conversion Loss for Each Color of Light> The conversion losses in the mode converters of Examples 1 to 3 were calculated. This conversion loss is the loss in the conversion from the TM0 mode to the TE0 mode. As the mode converters of Examples 1 to 3, mode converters having the same structure as the mode converter 35R shown in FIGS. 4 to 6 were used. In Examples 1 to 3, sapphire was used as the constituent material of the substrate 31, Z-cut lithium niobate (LiNbO3) was used as the constituent material of the core layer 32, silicon dioxide (SiO2) was used as the constituent material of the cladding layer 33, and silver (Ag) was used as the constituent material of the metal body 52.

[0111] As shown in Table 1, in Examples 1 to 3, the values of each parameter were set so that the conversion efficiency was maximized.

Table 1

[0112] In the mode converters of Examples 1 to 3, the conversion efficiency was calculated while changing the magnitude of the length Lc. The calculation results are shown in FIGS. 12 to 14. FIG. 12 is a diagram showing the relationship between the conversion length and the conversion efficiency in the mode converter of Example 1. FIG. 13 is a diagram showing the relationship between the conversion length and the conversion efficiency in the mode converter of Example 2. FIG. 14 is a diagram showing the relationship between the conversion length and the conversion efficiency in the mode converter of Example 3. The horizontal axes of FIGS. 12 to 14 indicate the length Lc (conversion length) (unit: μm), and the vertical axes of FIGS. 12 to 14 indicate the conversion efficiency.

[0113] In the mode converter of Example 1, with respect to the entire field of the first hybrid mode, the horizontal electric field component in the first hybrid mode was about 49%. With respect to the entire field of the second hybrid mode, the horizontal electric field component in the second hybrid mode was about 50%. In this case, the conversion efficiencies CE1 and CE2 shown in FIG. 12 were obtained, and the length Lc was calculated to be 16.0 μm. When the length Lc was 16.0 μm, the conversion efficiency when converting from the TM0 mode to the TE0 mode was 0.758, and the conversion loss was 1.20 dB.

[0114] In the mode converter of Example 2, with respect to the entire field of the first hybrid mode, the horizontal electric field component in the first hybrid mode was about 43%. With respect to the entire field of the second hybrid mode, the horizontal electric field component in the second hybrid mode was about 56%. In this case, the conversion efficiencies CE1 and CE2 shown in FIG. 13 were obtained, and the length Lc was calculated to be 11.0 μm. When the length Lc was 11.0 μm, the conversion efficiency when converting from the TM0 mode to the TE0 mode was 0.772, and the conversion loss was 1.12 dB.

[0115] In the mode converter of Example 3, with respect to the entire field of the first hybrid mode, the horizontal electric field component in the first hybrid mode was about 38%. With respect to the entire field of the second hybrid mode, the horizontal electric field component in the second hybrid mode was about 62%. In this case, the conversion efficiencies CE1 and CE2 shown in FIG. 14 were obtained, and the length Lc was calculated to be 22.0 μm. When the length Lc was 22.0 μm, the conversion efficiency when converting from the TM0 mode to the TE0 mode was 0.563, and the conversion loss was 2.50 dB.

[0116] In the mode converters of Examples 1 to 3, a relatively small conversion loss of 1.12 dB to 2.50 dB occurred. The length Lc was 11.0 μm to 22.0 μm. From this, it can be seen that low-loss mode conversion is realized as the length Lc is shortened.

[0117] <Evaluation of Distance D1 and Distance D2> The effects of Distance D1 and Distance D2 on the conversion loss were evaluated. In this evaluation, the mode converters of Examples 1 to 3 were used. The values of the 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 set to 100 μm or less, which is the length at which the conversion efficiency is maximized for each parameter value.

[0118] For red light, the conversion loss was calculated while changing Distance D2 for each of Distance D1 = 0 nm, 20 nm, 40 nm, 60 nm, 80 nm, and 100 nm. For green light, the conversion loss was calculated while changing Distance D2 for each of Distance D1 = 0 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, and 60 nm. For blue light, the conversion loss was calculated while changing Distance D2 for each of Distance D1 = 0 nm, 10 nm, 20 nm, 30 nm, 40 nm, and 60 nm. These conversion losses are the losses in the conversion from the TM0 mode to the TE0 mode.

[0119] The calculation results of the conversion loss are shown in FIGS. 15 to 17. FIG. 15 is a diagram showing the calculation results of the conversion loss of red light. FIG. 16 is a diagram showing the calculation results of the conversion loss of green light. FIG. 17 is a diagram showing the calculation results of the conversion loss of blue light. The horizontal axis of FIGS. 15 to 17 indicates Distance D2 (unit: μm), and the vertical axis of FIGS. 15 to 17 indicates the conversion loss (unit: dB).

[0120] From the viewpoint that the output light of the laser light source (laser diode) can be suppressed low, when the conversion loss is 6 dB or less, it is determined that high conversion efficiency is achieved. According to FIG. 15, when Distance D1 is 0 nm to 100 nm and Distance D2 is 0 nm to 90 nm, the conversion loss of red light was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when Distance D1 was 0 nm to 100 nm and Distance D2 was 0 nm to 90 nm.

[0121] According to FIG. 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 was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when the distance D1 was from 0 nm to 40 nm and the distance D2 was from 20 nm to 80 nm. According to FIG. 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 was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when the distance D1 was from 0 nm to 10 nm and the distance D2 was from 10 nm to 70 nm.

[0122] Note that the allowable value of the conversion loss also depends on the loss allowed for the entire system and the breakdown within the chip loss (the coupling efficiency between the waveguide end face of the chip in which the laser diode and the optical element are integrated, and the component loss of the conversion loss in the optical circuit within the chip).

[0123] <Evaluation of Distance D1> 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 at which the conversion efficiency was maximized for each parameter value.

[0124] For each distance D1, the conversion loss was calculated when the distance D2 was changed by 10 nm at a time in the range from -20 nm to 100 nm. This conversion loss is the loss in the conversion from the TM0 mode to the TE0 mode. The calculation results of the conversion loss are shown in FIG. 18. FIG. 18 is a diagram showing the relationship between the distance in the Z-axis direction between the waveguide and the metal body and the conversion loss. The horizontal axis of FIG. 18 indicates the distance D1 (unit: nm), and the vertical axis of FIG. 18 indicates the conversion loss (unit: dB). FIG. 18 shows the minimum value of the conversion loss for each distance D1.

[0125] From the perspective of being able to suppress the output light of the laser light source (laser diode) to a low level, when the conversion loss is 6 dB or less, it is determined that high conversion efficiency has been achieved. According to FIG. 18, when the distance D1 is from 0 nm to 120 nm, the conversion loss of the red light was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when the distance D1 was from 0 nm to 120 nm. According to FIG. 18, when the distance D1 is from 0 nm to 60 nm, the conversion loss of the green light was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when the distance D1 was from 0 nm to 60 nm. According to FIG. 18, when the distance D1 is from 0 nm to 30 nm, the conversion loss of the blue light was 6 dB or less. Therefore, it can be said that high conversion efficiency was achieved when the distance D1 was 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.

[0126] (Appendix) [Article 1] A substrate having a main surface, A core layer provided on the main surface and composed of a material having an electro-optical effect, the core layer having a waveguide extending in a first direction along the main surface, A metal body extending in the first direction and provided in parallel with the waveguide, Comprising 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 the TE mode and the TM mode, to a second polarization mode, which is the other of the TE mode and the TM mode, The waveguide has an incident end where the visible light in the first polarization mode is incident and an exit end where the visible light in the second polarization mode is emitted, The metal body has an edge in a second direction that intersects the first direction and is along the main surface, The edge overlaps the waveguide when viewed from a third direction that intersects the main surface, an optical element.

[0127] [Article 2] The waveguide has a bottom surface facing the main surface and a top surface provided on the opposite side of the bottom surface in the third direction. The optical element according to clause 1, wherein the metal body is arranged in the third direction in the order of the edge, the top surface, and the bottom surface.

[0128] [Clause 3] The optical element according to clause 1 or clause 2, wherein 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 is less than or equal to half of the length of the waveguide in the second direction.

[0129] [Clause 4] The optical element according to any one of clauses 1 to 3, wherein the metal body 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.

[0130] [Clause 5] A first mode converter that is the mode converter for converting the polarization mode of red light from the first polarization mode to the second polarization mode, A second mode converter that is the mode converter for converting the polarization mode of green light from the first polarization mode to the second polarization mode, A third mode converter that is the mode converter for converting the polarization mode of blue light from the first polarization mode to the second polarization mode, A multiplexer that multiplexes the red light, the green light, and the blue light to emit laser light, The optical element according to any one of clauses 1 to 4, comprising:

[0131] [Clause 6] The optical element according to clause 5, wherein the lengths in the third direction of the waveguides of the first mode converter, the second mode converter, and the third mode converter are the same as each other.

[0132] [Clause 7] A first modulator that modulates the light intensity of the red light, A second modulator that modulates the light intensity of the green light, A third modulator that modulates the light intensity of the blue light, The optical element according to clause 5 or clause 6, further comprising:

[0133] [Clause 8] An optical element according to any one of clauses 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, A third light source that emits the blue light in the first polarization mode, A laser module comprising:

[0134] [Clause 9] A retinal projection device mounted on a near-eye wearable device, The laser module according to clause 8, A movable mirror that performs scanning using the laser light emitted from the laser module, A reflector that reflects the laser light passing through the movable mirror and irradiates the reflected light onto the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina, A retinal projection device comprising:

[0135] [Clause 10] The retinal projection device according to clause 9, A lens provided with the reflector, A near-eye wearable device comprising:

Explanation of Reference Numerals

[0136] 1... Near-eye wearable device, 3... Lens, 10... Retinal projection device, 12... Reflector, 13, 13A, 13B, 13C... Laser module, 15... Movable mirror, 21, 21B... Laser light source (first light source), 22, 22B... Laser light source (second light source), 23, 23B... Laser light source (third light source), 30, 30A, 30B, 30C... Optical element, 31... Substrate, 31a... Main surface, 32... Core layer, 34B... Modulator (third modulator), 34G... Modulator (second modulator), 34R... Modulator (first modulator), 35... Mode converter, 35B... Mode converter (third mode converter), 35G... Mode converter (second mode converter), 35R... Mode converter (first mode converter), 36... Combiner, 37B... Mode converter (third mode converter), 37G... Mode converter (second mode converter), 37R... Mode converter (first mode converter), 51... Waveguide, 51a... Input end, 51b... Output end, 52... Metal body, 52c... Edge.

Claims

1. A substrate having a main surface, A core layer provided on the main surface and composed of a material having an electro-optical effect, the core layer having a waveguide extending in a first direction along the main surface, A metal body extending in the first direction and provided in parallel with the waveguide, Comprising, 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 the TE mode and the TM mode, to a second polarization mode, which is the other of the TE mode and the TM mode, The waveguide has an incident end where the visible light in the first polarization mode is incident and an emission end where the visible light in the second polarization mode is emitted, The metal body has an edge in a second direction that intersects the first direction and is along the main surface, The edge overlaps the waveguide when viewed from a third direction that intersects the main surface, an optical element.

2. The waveguide has a bottom surface facing the main surface and a top surface provided on the opposite side of the bottom surface in the third direction, The metal body is arranged in the third direction so as to be arranged in the order of the edge, the top surface, and the bottom surface, the optical element according to claim 1.

3. 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 is less than or equal to half of the length of the waveguide in the second direction, the optical element according to claim 1 or claim 2.

4. 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, the optical element according to claim 1 or claim 2.

5. A first mode converter that is the mode converter for converting the polarization mode of red light from the first polarization mode to the second polarization mode, A second mode converter that is the mode converter for converting the polarization mode of green light from the first polarization mode to the second polarization mode, A third mode converter that is the mode converter for converting the polarization mode of blue light from the first polarization mode to the second polarization mode, A multiplexer that multiplexes the red light, the green light, and the blue light and emits laser light, Comprising, the optical element according to claim 1 or claim 2.

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

7. A first modulator for modulating the light intensity of the red light; A second modulator for modulating the light intensity of the green light; A third modulator for modulating the light intensity of the blue light; The optical element according to claim 5, further comprising:

8. The optical element according to claim 5, A first light source for emitting the red light in the first polarization mode; A second light source for emitting the green light in the first polarization mode; A third light source for emitting the blue light in the first polarization mode; A laser module comprising:

9. A retinal projection device mounted on a near-eye wearable device, The laser module according to claim 8, A movable mirror that performs scanning using the laser light emitted from the laser module; A reflector that projects an image onto the retina by reflecting the laser light that has passed through the movable mirror and irradiating the retina of a user wearing the near-eye wearable device with the reflected light; A retinal projection device comprising:

10. The retinal projection device according to claim 9, A lens provided with the reflector; A near-eye wearable device comprising:

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