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 with which it is possible to convert a visible light polarization mode from a TM0 mode into a TE0 mode while suppressing decrease in conversion efficiency.SOLUTION: An optical element comprises a mode converter for converting the polarization mode of visible light. The mode converter includes: a conversion unit 51 for converting the polarization mode of the visible light from a TM0 mode into a TE1 mode; and a branch unit 52 for branching the visible mode of the TE1 mode into branched light LA of a TE0 mode and branched light LB of a TE0 mode and adjusting the phase difference obtained by subtracting the phase of the branched light LA on entering a multiplexing unit 53 from the phase of the branched light LB on entering the multiplexing unit 53. The branch unit 52 includes a branch waveguide 52A through which the branched light LA propagates and a branch waveguide 52B through which the branched light LB propagates. The optical path length of the branched light LA in the branch waveguide 52A and the optical path length of the branched light LB in the branch waveguide 52B are different from each other.SELECTED DRAWING: Figure 6

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 technology]

[0002] The polarization modes of light propagating through an optical waveguide include the TE (Transverse Electric) mode, which is a polarization mode having a main electric field horizontal to the substrate, and the TM (Transverse Magnetic) mode, which is a polarization mode having a main electric field vertical to the substrate. Optical elements that convert these polarization modes are known.

[0003] For example, Patent Document 1 describes an optical element that converts the polarization mode of light from TM0 mode to TE0 mode. This optical element includes a mode conversion unit that converts the polarization mode of light from TM0 mode to TE1 mode, a mode branching unit that converts the polarization mode of light from TE1 mode to two TE0 modes having different phases, a phase adjustment unit that adjusts the phases of the two TE0 mode lights, and a multiplexing unit that multiplexes the two phase-adjusted TE0 mode lights. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-181611 A Summary of the Invention [Problem to be solved by the invention]

[0005] The optical element described in Patent Document 1 is an optical element used in optical communications, and converts TM0 mode light in the infrared wavelength band into TE0 mode light. However, the optical element described in Patent Document 1 does not take into consideration the conversion efficiency when converting TM0 mode light into TE0 mode light, nor does it take into consideration the conversion of TM0 mode visible light into TE0 mode visible light.

[0006] The present disclosure describes an optical element, a laser module, a retinal projection device, and a near-eye refractive device that can convert the polarization mode of visible light from TM0 mode to TE0 mode while suppressing a decrease in conversion efficiency. [Means for solving the problem]

[0007] An optical element according to one aspect of the present disclosure includes a substrate having a principal surface, and a core layer provided on the principal surface and made of a material having an electro-optic effect. The core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light from TM0 mode to TE0 mode. The mode converter includes a conversion unit that converts the polarization mode of visible light from TM0 mode to TE1 mode, a branching unit that branches visible light in TE1 mode into a first branched light in TE0 mode and a second branched light in TE0 mode and adjusts a phase difference between the first branched light and the second branched light, and a multiplexing unit that multiplexes the first branched light and the second branched light. The conversion unit has both ends in the first direction, and includes a first end into which visible light in TM0 mode is incident and a second end from which visible light in TE1 mode is emitted. The length of the conversion unit in a second direction along the principal surface and intersecting the first direction increases continuously from the first end to the second end. The phase difference is a phase difference obtained by subtracting the phase of the first branched light when it is incident on the multiplexing section from the phase of the second branched light when it is incident on the multiplexing section. The branching section includes a first branching waveguide through which the first branched light propagates and a second branching waveguide through which the second branched light propagates. The optical path length of the first branched light in the first branching waveguide and the optical path length of the second branched light in the second branching waveguide are different from each other.

[0008] In this optical element, the length of the conversion section in the second direction increases continuously from the first end to the second end, so that the effective refractive index of the visible light propagating through the conversion section changes. As a result, the polarization mode of the visible light can be converted from the TM0 mode to the TE1 mode in the conversion section. Furthermore, in the branching section, the visible light in the TE1 mode output from the conversion section is branched into a first branched light in the TE0 mode and a second branched light in the TE0 mode that are in opposite phase to each other, and the first branched light propagates through the first branching waveguide, and the second branched light propagates through the second branching waveguide. Then, in the combining section, the first branched light and the second branched light are combined, so that visible light in the TE0 mode is output.

[0009] Here, when the optical path length in the first branching waveguide and the optical path length in the second branching waveguide are the same, the first branching light and the second branching light are incident on the multiplexing section without changing the phase difference obtained by subtracting the phase of the first branching light when incident on the multiplexing section from the phase of the second branching light when incident on the multiplexing section from −π radians. Therefore, the optical intensity of the visible light obtained by multiplexing the first branching light and the second branching light is substantially half the optical intensity of the visible light incident on the conversion section, and the conversion efficiency decreases. On the other hand, in this optical element, since the optical path length in the first branching waveguide and the optical path length in the second branching waveguide are different from each other, the phase difference has a value different from −π radians. As a result, it is possible to suppress the optical intensity of the visible light obtained by multiplexing the first branching light and the second branching light from being reduced from the optical intensity of the visible light incident on the conversion section. From the above, the optical element can convert the polarization mode of the visible light from the TM0 mode to the TE0 mode while suppressing a decrease in the conversion efficiency.

[0010] The phase difference may be -2π / 3+2nπ radians or more and -π / 3+2nπ radians or less. n may be an integer. When the phase difference is -π / 2+2nπ radians, the light intensity of the visible light in the TE0 mode obtained by multiplexing the first branched light and the second branched light is substantially the same as the light intensity of the visible light in the TM0 mode incident on the conversion unit. As the phase difference moves away from -π / 2+2nπ radians, the light intensity of the visible light in the TE0 mode obtained by multiplexing the first branched light and the second branched light decreases. Since the range is ±π / 6 centered on -π / 2+2nπ radians, the loss of the light intensity of the visible light in the TE0 mode obtained by multiplexing the first branched light and the second branched light can be further reduced. That is, the decrease in the conversion efficiency from the TM0 mode to the TE0 mode can be further suppressed.

[0011] The core layer may further include a slab provided on the main surface. The mode converter may be provided on the slab in a third direction intersecting the first direction and the second direction. In this case, the waveguide formed by the conversion section and the slab has an asymmetric shape in the third direction. Therefore, the conversion efficiency from the TM0 mode to the TE1 mode in the conversion section can be improved.

[0012] The length of the core layer in the third direction may be smaller than the wavelength of visible light. In this case, visible light is more likely to seep out from the conversion section into the slab. Therefore, the conversion efficiency from the TM0 mode to the TE1 mode in the conversion section can be improved.

[0013] The multiplexing section may be formed of a multimode interferometer, which can be fabricated more easily than a multiplexing section formed of a Y-branch waveguide.

[0014] The length of the multiplexing section in the second direction may be 2.0 μm or more. In this case, the distance between the first branch waveguide and the second branch waveguide can be secured without reducing the width of the first branch waveguide and the second branch waveguide. Therefore, the first branch waveguide and the second branch waveguide can be formed into a desired shape.

[0015] The length in the second direction of the first branch waveguide at a connection end connected to the multiplexing section of the first branch waveguide may be 23% or more and 47% or less of the length in the second direction of the multiplexing section.The length in the second direction of the second branch waveguide at a connection end connected to the multiplexing section of the second branch waveguide may be 23% or more and 47% or less of the length in the second direction of the multiplexing section.In this case, it is possible to ensure both the tolerance of the mode converter for manufacturing errors and the like and the spacing between the first branch waveguide and the second branch waveguide.

[0016] The core layer may include a first mode converter which is a mode converter that converts the polarization mode of red light from TM0 mode to TE0 mode, a second mode converter which is a mode converter that converts the polarization mode of green light from TM0 mode to TE0 mode, a third mode converter which is a mode converter that converts the polarization mode of blue light from TM0 mode to TE0 mode, and a multiplexer that multiplexes red light, green light, and blue light to emit a laser beam. According to this configuration, the polarization mode of red light is converted from TM0 mode to TE0 mode, the polarization mode of green light is converted from TM0 mode to TE0 mode, and the polarization mode of blue light is converted from TM0 mode to TE0 mode. For example, when the multiplexer is designed so that the multiplexing efficiency when multiplexing red light, green light, and blue light in TE0 mode is higher than the multiplexing efficiency when multiplexing red light, green light, and blue light in TM0 mode, the multiplexing efficiency of the multiplexer can be improved.

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

[0018] The core layer may 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, the light intensity of red light, green light, and blue light is modulated, so that full-color laser light can be output.

[0019] A laser module according to another aspect of the present disclosure includes the above-described optical element, a first laser light source that emits red light in TM0 mode, a second laser light source that emits green light in TM0 mode, and a third laser light source that emits blue light in TM0 mode. Since this laser module includes the above-described optical element, it is possible to convert the polarization mode of visible light from TM0 mode to TE0 mode while suppressing a decrease in conversion efficiency.

[0020] A retinal projection device according to yet another aspect of the present disclosure is a device mounted on a near-eye wearable device, and includes the above-mentioned laser module, a movable mirror that performs scanning using a laser light emitted from the laser module, and a reflector that reflects the laser light that has passed through the movable mirror and guides it to the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina. This retinal projection device includes the above-mentioned optical element. Therefore, in this retinal projection device, it is possible to convert the polarization mode of visible light from the TM0 mode to the TE0 mode while suppressing a decrease in conversion efficiency, and then project an image onto the retina.

[0021] According to yet another aspect of the present disclosure, a near-eye-readable device includes the above-described retinal projection device and a lens provided with the above-described reflector. The near-eye-readable device includes the above-described retinal projection device having the optical element. Therefore, the near-eye-readable device can convert the polarization mode of visible light from TM0 mode to TE0 mode while suppressing a decrease in conversion efficiency, and then project an image onto the retina. Effect of the Invention

[0022] According to each aspect and embodiment of the present disclosure, it is possible to convert the polarization mode of visible light from the TM0 mode to the TE0 mode while suppressing a decrease in conversion efficiency. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. [Diagram 2] FIG. 2 is a schematic diagram of the retinal projection device shown in FIG. [Diagram 3] FIG. 3 is a block diagram of the laser module shown in FIG. [Figure 4] FIG. 4 is a diagram showing a cross-sectional configuration of the optical element shown in FIG. [Diagram 5] FIG. 5 is a plan view showing the configuration of the mode converter shown in FIG. [Figure 6] FIG. 6 is an enlarged plan view of the branching portion shown in FIG. [Figure 7] FIG. 7 is a block diagram of a laser module according to another embodiment. [Figure 8] FIG. 8 is a block diagram of a laser module according to yet another embodiment. [Figure 9] FIG. 9 is a block diagram of a laser module according to yet another embodiment. [Figure 10]Fig. 10(a) is a diagram showing the evaluation results of Example 1. Fig. 10(b) is a diagram showing the evaluation results of Example 2. Fig. 10(c) is a diagram showing the evaluation results of Example 3. [Figure 11] 11(a), 11(b), and 11(c) are diagrams showing the relationship between the length of the conversion section in the X-axis direction and the conversion loss. [Figure 12] Fig. 12(a) is a diagram showing the evaluation results of Examples 10 to 13. Fig. 12(b) is a diagram showing the evaluation results of Examples 14 to 17. Fig. 12(c) is a diagram showing the evaluation results of Examples 18 to 23. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are given the same symbols, and duplicated descriptions are omitted. An XYZ coordinate system may be shown in each drawing. The Y-axis direction (second direction) is a direction that intersects (e.g., perpendicular to) the X-axis direction (first direction) and the Z-axis direction (third direction). The Z-axis direction is a direction that intersects (e.g., perpendicular to) the X-axis direction and the Y-axis direction. In this specification, a numerical range indicated using "~" indicates a range that includes the numerical values ​​written before and after "~" as the minimum and maximum values, respectively. Individually described upper and lower limit values ​​can be arbitrarily combined.

[0025] An application example of a laser module according to an embodiment will be described with reference to FIG. 1. FIG. 1 is a perspective view showing the appearance of a near-eye wearable device to which a laser module according to an embodiment is applied. The near-eye wearable device 1 shown in FIG. 1 is a device that projects an image onto the retina of a user wearing the near-eye wearable device 1. The near-eye wearable device 1 is, for example, a head-mounted device, and may take the form of glasses, goggles, hats, helmets, and the like. Examples of the near-eye wearable device 1 include smart glasses such as AR (Augmented Reality) glasses, VR (Virtual Reality) glasses, and MR (Mixed Reality) glasses. The near-eye wearable device 1 includes a frame 2, a lens 3, and a retinal projection device 10.

[0026] The frame 2 includes a pair of rims 2a, a bridge 2b, and a pair of temples 2c. The rims 2a are portions that hold the lenses 3. The bridge 2b is a portion that connects the pair of rims 2a. The temples 2c are portions that extend from the rims 2a and are hung on the ears of a user. The frame 2 may be a rimless frame. The lenses 3 include an inner surface 3a (see FIG. 2) that faces the eyeball of a user wearing the near eyewear wearable device 1.

[0027] The retinal projection device 10 is a device that directly projects (draws) an image onto the retina of a user wearing the near-eye wearable device 1. The retinal projection device 10 is mounted on the near-eye wearable device 1. In this embodiment, the near-eye wearable device 1 includes two retinal projection devices 10 in order to project images onto both the left and right retinas, but may include only one of the retinal projection devices 10.

[0028] Next, the retinal projection device 10 will be described in detail with reference to Fig. 2. Fig. 2 is a schematic diagram of the retinal projection device shown in Fig. 1. As shown in Fig. 2, the retinal projection device 10 includes an optical engine 20 and a reflector 30.

[0029] The optical engine 20 is a device that generates laser light Ls having a color and light intensity corresponding to the pixels of an image to be projected onto the retina, and emits the laser light Ls to the reflector 30. The optical engine 20 is mounted on each temple 2c. The optical engine 20 includes a laser module 4, an optical component 5, a movable mirror 6, a laser driver 7, a mirror driver 8, and a controller 9.

[0030] The laser module 4 emits laser light La, which is visible light. For example, a full-color laser module is used as the laser module 4. The laser module 4 emits laser light La of a color and light intensity corresponding to the pixels of an image to be projected onto the retina. The laser module 4 will be described in detail later.

[0031] The optical component 5 is a component that optically processes the laser light La emitted from the laser module 4. In this embodiment, the optical component 5 includes a collimator lens 5a, a slit 5b, and a neutral density filter 5c. The collimator lens 5a, the slit 5b, and the neutral density filter 5c are arranged in this order along the optical path of the laser light La. The optical component 5 may have other configurations.

[0032] The movable mirror 6 is a member for performing scanning with the laser light Ls. The movable mirror 6 is provided in the emission direction of the laser light La processed by the optical component 5. The movable mirror 6 is configured to be swingable, for example, around an axis extending in the horizontal direction of the lens 3 and an axis extending in the vertical direction of the lens 3, and reflects the laser light La while changing the angle in the horizontal and vertical directions of the lens 3 to emit the laser light Ls. As the movable mirror 6, for example, a MEMS (Micro Electro Mechanical Systems) mirror is used.

[0033] The laser driver 7 is a drive circuit that drives the laser module 4. The laser driver 7 drives the laser module 4 based on, for example, the light intensity of the laser light La and the temperatures of the laser light sources 411, 412, and 413 included in the laser module 4. The mirror driver 8 is a drive circuit that drives the movable mirror 6. The mirror driver 8 swings the movable mirror 6 within a predetermined angle range and at a predetermined timing. The controller 9 is a device that controls the laser driver 7 and the mirror driver 8.

[0034] In the optical engine 20, laser light La having a color and light intensity corresponding to the pixels of an image to be projected onto the retina is emitted from the laser module 4, passes through the optical component 5, and is reflected by the movable mirror 6. The laser light La reflected by the movable mirror 6 is emitted to the reflector 30 as laser light Ls.

[0035] The reflector 30 is a member that reflects the laser light Ls that has passed through the movable mirror 6 and guides the reflected light Lrf to the retina of a user wearing the near eyewear wearable device 1, thereby projecting an image onto the retina.

[0036] Next, the configuration of the laser module 4 will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a block diagram of the laser module shown in Fig. 2. Fig. 4 is a diagram showing a cross-sectional configuration of the optical element shown in Fig. 3.

[0037] As shown in FIG. 3, the laser module 4 includes an optical element 40, a laser light source 411 (first laser light source) that emits red light Lr, a laser light source 412 (second laser light source) that emits green light Lg, and a laser light source 413 (third laser light source) that emits blue light Lb. The laser light source 411 is, for example, a red laser diode. The laser light source 412 is, for example, a green laser diode. The laser light source 413 is, for example, a blue laser diode. The peak wavelength of the red light Lr is, for example, in the range of 600 nm to 830 nm. The peak wavelength of the green light Lg is, for example, in the range of 500 nm to 570 nm. The peak wavelength of the blue light Lb is, for example, in the range of 380 nm to 490 nm.

[0038] In this embodiment, the laser light source 411 emits red light Lr in TM0 mode. The laser light source 412 emits green light Lg in TM0 mode. The laser light source 413 emits blue light Lb in TM0 mode. Since the red light Lr, green light Lg, and blue light Lb are all visible light, in the following description, the red light Lr, green light Lg, and blue light Lb may be referred to as each visible light, and the red light Lr, green light Lg, and blue light Lb may be collectively referred to as visible light.

[0039] The optical element 40 combines the visible light emitted from each laser light source into one laser light La. The optical element 40 is, for example, a planar lightwave circuit (PLC). As shown in Fig. 4, the optical element 40 includes a substrate S, a core layer CA, and a clad layer CB.

[0040] The substrate S functions as a lower cladding layer. The substrate S is made of a material having a lower refractive index than the material of the core layer CA. Examples of materials that can be used to form the substrate S include sapphire, silicon, and aluminum oxide (Al2O3). The substrate S has a main surface Sa and a back surface Sb opposite to the main surface Sa. The main surface Sa and the back surface Sb are surfaces defined by the X-axis direction and the Y-axis direction, and intersect with the Z-axis direction (in this embodiment, they are perpendicular to each other). In other words, the X-axis direction and the Y-axis direction are directions that run along the main surface Sa.

[0041] The cladding layer CB functions as an upper cladding layer. The cladding layer CB covers the core layer CA on the main surface Sa. The cladding layer CB is made of a material having a lower refractive index than the material of the core layer CA. An example of the material of the cladding layer CB is silicon oxide (e.g., SiO2).

[0042] The core layer CA is provided on the main surface Sa. The core layer CA is made of a material having an electro-optic effect. The electro-optic effect is a phenomenon in which the refractive index of a material changes when an electric field is applied to the material. An example of a material that can be used to make the core layer CA is lithium niobate (LiNbO3). In this embodiment, the core layer CA is a lithium niobate thin film formed on the main surface Sa of the substrate S by sputtering, and the optical axis (C-axis) of the lithium niobate extends in the Z-axis direction. The core layer CA may be made of Z-cut lithium niobate.

[0043] The core layer CA includes a slab 41, a modulator 42 (first modulator), a modulator 43 (second modulator), a modulator 44 (third modulator), a mode converter 45 (first mode converter), a mode converter 46 (second mode converter), a mode converter 47 (third mode converter), and a combiner 48.

[0044] 4, the slab 41 is provided on the main surface Sa. The slab 41 has a flat plate shape. The length of the slab 41 in the Z-axis direction is a height T11. Hereinafter, the length in the Z-axis direction may be referred to as "height." The height T11 is, for example, 0.1 μm to 0.7 μm.

[0045] In this embodiment, the modulators 42, 43, and 44, the mode converters 45, 46, and 47, and the combiner 48 are provided on one slab 41. Only the mode converters 45, 46, and 47 may be provided on one slab 41. In other words, the modulators 42, 43, and 44 and the combiner 48 do not have to be provided on the slab 41. A slab 41 may be provided for each mode converter.

[0046] The modulator 42 is a modulator that modulates the light intensity of the red light Lr. The TM0 mode red light Lr is incident on an incident end of the modulator 42 from a laser light source 411. The modulator 42 modulates the light intensity of the TM0 mode red light Lr incident from the laser light source 411. The output end of the modulator 42 is optically connected to an input end of a mode converter 45.

[0047] The modulator 43 is a modulator that modulates the light intensity of the green light Lg. The green light Lg in TM0 mode is incident on an incident end of the modulator 43 from the laser light source 412. The modulator 43 modulates the light intensity of the green light Lg in TM0 mode incident from the laser light source 412. The output end of the modulator 43 is optically connected to the input end of the mode converter 46.

[0048] The modulator 44 is a modulator that modulates the light intensity of the blue light Lb. The blue light Lb in TM0 mode is incident on the incident end of the modulator 44 from the laser light source 413. The modulator 44 modulates the light intensity of the blue light Lb in TM0 mode incident from the laser light source 413. The output end of the modulator 44 is optically connected to the input end of the mode converter 47. Each modulator is, for example, a Mach-Zehnder type modulator.

[0049] The mode converter 45 is a mode converter that converts the polarization mode of the red light Lr from the TM0 mode to the TE0 mode. The red light Lr, the light intensity of which is modulated by the modulator 42, is incident on the input end of the mode converter 45. The mode converter 46 is a mode converter that converts the polarization mode of the green light Lg from the TM0 mode to the TE0 mode. The green light Lg, the light intensity of which is modulated by the modulator 43, is incident on the input end of the mode converter 46. The mode converter 47 is a mode converter that converts the polarization mode of the blue light Lb from the TM0 mode to the TE0 mode. The blue light Lb, the light intensity of which is modulated by the modulator 44, is incident on the input end of the mode converter 47. The output end of the mode converter 45, the output end of the mode converter 46, and the output end of the mode converter 47 are optically connected to the three input ends of the multiplexer 48, respectively.

[0050] The mode converter 45, the mode converter 46, and the mode converter 47 each extend in the X-axis direction. The mode converter 45, the mode converter 46, and the mode converter 47 are arranged in that order in the Y-axis direction.

[0051] The height of the mode converter 45, the height of the mode converter 46, and the height of the mode converter 47 are substantially the same, and are equal to each other, i.e., height T12. The height T12 is, for example, 0.1 μm to 0.7 μm. The height of the core layer CA is the sum of the height T11 and the height T12, and is equal to height T1. The height T1 is smaller than the wavelength of the visible light to be converted. The height T1 is, for example, 0.2 μm to 0.8 μm. The height of the mode converter 45, the height of the mode converter 46, and the height of the mode converter 47 may be different from each other. The detailed configuration of each mode converter will be described later.

[0052] When a slab 41 is provided for each mode converter, the height T1 may be smaller than the wavelength of visible light to be converted by the corresponding mode converter. In the combination of the slab 41 and the mode converter 45, the height T1 may be smaller than the wavelength of red light Lr. In the combination of the slab 41 and the mode converter 46, the height T1 may be smaller than the wavelength of green light Lg. In the combination of the slab 41 and the mode converter 47, the height T1 may be smaller than the wavelength of blue light Lb.

[0053] The multiplexer 48 multiplexes the red light Lr, green light Lg, and blue light Lb, the polarization modes of which have been converted in the mode converters, into one visible light. The three input ends of the multiplexer 48 are optically connected to the output end of the mode converter 45, the output end of the mode converter 46, and the output end of the mode converter 47, respectively. The multiplexer 48 outputs the multiplexed visible light as laser light La from the output end of the multiplexer 48.

[0054] In the laser module 4, visible light in 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 mode of the visible light is converted from TM0 mode to TE0 mode in each mode converter. Then, each visible light whose polarization mode has been converted is multiplexed in the multiplexer 48, and is output from the multiplexer 48 to the optical component 5 (see FIG. 2) as laser light La in TE0 mode.

[0055] Next, specific configurations of the mode converter 45, the mode converter 46, and the mode converter 47 will be described with reference to Fig. 4 and Fig. 5. Fig. 5 is a plan view showing the configuration of the mode converter shown in Fig. 3. As shown in Fig. 5, each of the mode converter 45, the mode converter 46, and the mode converter 47 includes a conversion section 51, a branching section 52, and a multiplexing section 53. Since each mode converter has the same configuration, the mode converter 45 will be described here as an example.

[0056] The conversion unit 51 is a part that converts the polarization mode of visible light from TM0 mode to TE1 mode. As shown in FIG. 5, the conversion unit 51 is located at one end in the X-axis direction of the mode converter 45. The conversion unit 51 includes an input end 51a (first end) and an output end 51b (second end), which are both ends in the X-axis direction. Red light in TM0 mode (hereinafter sometimes referred to as "light Lin") is input from the laser light source 411 to the input end 51a. The output end 51b outputs the light Lin in TE1 mode. The output end 51b outputs the light Lin in TE1 mode to the branching unit 52. The length L1 in the X-axis direction of the conversion unit 51 is, for example, 10 μm to 10,000 μm.

[0057] The length of the conversion portion 51 in the Y-axis direction increases continuously from the entrance end 51a to the exit end 51b. Hereinafter, the length in the Y-axis direction may be referred to as "width". The width of the conversion portion 51 increases continuously from width W1 to width W2 from the entrance end 51a to the exit end 51b. Width W1 is smaller than width W2. Width W1 is, for example, 0.3 μm to 1.0 μm. Width W2 is, for example, 0.4 μm to 1.2 μm. The rate of increase in the width of the conversion portion 51 may be constant.

[0058] In the conversion unit 51, as the distance from the incident end 51a in the X-axis direction increases, the effective refractive index of the TM0 mode and the effective refractive index of the TE1 mode approach each other and cross each other, so that conversion between the TM0 mode and the TE1 mode is induced. Therefore, when the light Lin in the TM0 mode is incident on the incident end 51a, the polarization mode of the light Lin is converted from the TM0 mode to the TE1 mode in the conversion unit 51. On the other hand, over the entire length of the conversion unit 51 in the X-axis direction, the effective refractive index of the TE0 mode is sufficiently different from the effective refractive index of the TM0 mode and the effective refractive index of the TE1 mode. Therefore, when the light Lin in the TE0 mode is incident on the incident end 51a, the polarization mode of the light Lin is maintained in the TE0 mode.

[0059] The branching unit 52 is a part that branches the light Lin in the TE1 mode incident from the output end 51b of the conversion unit 51 into the branched light LA ​​(first branched light) in the TE0 mode and the branched light LB (second branched light) in the TE0 mode, and adjusts the phase difference Δφ between the branched light LA ​​and the branched light LB. The phase difference Δφ is a phase difference obtained by subtracting the phase of the branched light LA ​​when it is incident on the multiplexing unit 53 from the phase of the branched light LB when it is incident on the multiplexing unit 53. The branching unit 52 branches the light Lin in the TE1 mode into the branched light LA ​​in the TE0 mode and the branched light LB in the TE0 mode, which are in opposite phase to each other. The optical intensity of the branched light LA ​​and the optical intensity of the branched light LB are substantially half (50%) of the optical intensity of the light Lin in the TE1 mode incident from the output end 51b. In this embodiment, the branching unit 52 is configured by a Y-branch type waveguide. The length L2 of the branching portion 52 in the X-axis direction is, for example, 10 μm to 1000 μm. The branching portion 52 includes a branching waveguide 52A (first branching waveguide) and a branching waveguide 52B (second branching waveguide).

[0060] The branch waveguide 52A is a waveguide through which the branch light LA ​​propagates. The branch waveguide 52A is disposed between the conversion unit 51 and the multiplexing unit 53, and includes a connection end (an end 521a described below) connected to the conversion unit 51 and a connection end (an end 522b described below) connected to the multiplexing unit 53. The branch waveguide 52A outputs to the multiplexing unit 53 the branch light LA ​​of the TE0 mode branched from the light Lin of the TE1 mode.

[0061] The branch waveguide 52B is a waveguide through which the branch light LB propagates. The branch waveguide 52B is disposed between the conversion unit 51 and the multiplexing unit 53, and includes a connection end (an end 523a described below) connected to the conversion unit 51 and a connection end (an end 526b described below) connected to the multiplexing unit 53. The branch waveguide 52B outputs to the multiplexing unit 53 the branch light LB in the TE0 mode branched from the light Lin in the TE1 mode.

[0062] The branching waveguides 52A and 52B extend so as to move away from each other in the Y-axis direction as they move away from the conversion section 51 in the X-axis direction, and then extend to the multiplexing section 53 substantially parallel to each other.

[0063] The multiplexing section 53 multiplexes the branched light LA ​​and the branched light LB. The multiplexing section 53 multiplexes the branched light LA ​​incident from the branching waveguide 52A and the branched light LB incident from the branching waveguide 52B, and emits red light in TE0 mode (hereinafter, may be referred to as "light Lout"). At this time, the light intensity of the light Lout changes according to the phase difference Δφ. For example, when the phase difference Δφ is -π / 2 radians, the light intensity of the light Lout is substantially the same (100%) as the light intensity of the light Lin in TE1 mode emitted from the emission end 51b. The phase difference Δφ will be described later.

[0064] In this embodiment, the multiplexing unit 53 is configured by a multimode interferometer. Specifically, the multiplexing unit 53 is configured by a two-input, two-output multimode interferometer. That is, the multiplexing unit 53 includes an input end 53a, an input end 53b, an output end 53c, and an output end 53d. The input end 53a is connected to a connection end (an end 522b described later) of the branching waveguide 52A. The input end 53b is connected to a connection end (an end 526b described later) of the branching waveguide 52B. In this embodiment, the mode converter 45 is used as a mode converter that converts the polarization mode of the red light Lr from the TM0 mode to the TE0 mode, so that the output end 53c is connected to the input end of the multiplexer 48 and outputs the light Lout in the TE0 mode. The length L3 of the multiplexing unit 53 in the X-axis direction is, for example, 2.0 μm to 1000 μm. The width W3 of the multiplexing portion 53 is, for example, 1.0 μm to 10 μm.

[0065] Next, the configuration of the branching section 52 will be described in more detail with reference to Figs. 5 and 6. Fig. 6 is an enlarged plan view of the branching section shown in Fig. 5. In the branching section 52, the optical path length OL1 of the branched light LA ​​in the branching waveguide 52A and the optical path length OL2 of the branched light LB in the branching waveguide 52B are different from each other. In this embodiment, the physical length of the branching waveguide 52A along the optical path of the branched light LA ​​and the physical length of the branching waveguide 52B along the optical path of the branched light LB are equal to each other. The width of the branching waveguide 52A is constant over the entire length of the branching waveguide 52A, whereas the width of the branching waveguide 52B is smaller than the width of the branching waveguide 52A in a part of the branching waveguide 52B. A specific description will be given below.

[0066] The branch waveguide 52A includes a waveguide 521 and a waveguide 522. The waveguide 521 and the waveguide 522 are arranged in that order in the X-axis direction. In this embodiment, the waveguide 521 extends at an angle with respect to the X-axis direction, and the waveguide 522 extends substantially parallel to the X-axis direction.

[0067] The waveguide 521 includes ends 521a and 521b, which are both ends in the X-axis direction. The waveguide 522 includes ends 522a and 522b, which are both ends in the X-axis direction. The end 521a is connected to the emission end 51b. The ends 521b and 522a are connected to each other. The end 522b is connected to the incidence end 53a. In this manner, in this embodiment, the branch waveguide 52A is configured by connecting the waveguide 521 and the waveguide 522. In this embodiment, the end 521a is a connection end connected to the conversion unit 51 of the branch waveguide 52A, and the end 522b is a connection end connected to the multiplexing unit 53 of the branch waveguide 52A.

[0068] The width of the waveguide 521 at the end 521a is, for example, 50% or less of the width W2, and in this embodiment, is smaller than 50% of the width W2. The width of the waveguide 521 is substantially constant at a width W4 between the end 521a and the end 521b. The width of the waveguide 522 is substantially constant at a width W4 between the end 522a and the end 522b. Therefore, in this embodiment, the width W4 is also smaller than 50% or less of the width W2. Furthermore, in this embodiment, the width W4 is 23% to 47% of the width W3. Therefore, the width of the waveguide 522 (branch waveguide 52A) at the end 522b is also 23% to 47% of the width W3. The width W4 is, for example, 0.4 μm to 4.0 μm.

[0069] The length L21 of the waveguide 521 in the X-axis direction is, for example, 1.0 μm to 100 μm. The length L22 of the waveguide 522 in the X-axis direction is, for example, 1.0 μm to 300 μm. In this embodiment, the sum of the length L21 and the length L22 is the length L2.

[0070] The branch waveguide 52B includes a waveguide 523, a waveguide 524, a waveguide 525, and a waveguide 526. The waveguide 523, the waveguide 524, the waveguide 525, and the waveguide 526 are arranged in that order in the X-axis direction. In this embodiment, the waveguide 523 extends at an incline with respect to the X-axis direction, and the waveguide 524, the waveguide 525, and the waveguide 526 extend substantially parallel to the X-axis direction. The waveguide 521 and the waveguide 523 are inclined so as to move away from each other in the Y-axis direction as they move away from the output end 51b.

[0071] The waveguide 523 includes ends 523a and 523b, which are both ends in the X-axis direction. The waveguide 524 includes ends 524a and 524b, which are both ends in the X-axis direction. The waveguide 525 includes ends 525a and 525b, which are both ends in the X-axis direction. The waveguide 526 includes ends 526a and 526b, which are both ends in the X-axis direction. The end 523a is connected to the emission end 51b. The end 523b and the end 524a are connected to each other. The end 524b and the end 525a are connected to each other. The end 525b and the end 526a are connected to each other. The end 526b is connected to the input end 53b. In this manner, in this embodiment, the branch waveguide 52B is configured by connecting the waveguide 523, the waveguide 524, the waveguide 525, and the waveguide 526. In this embodiment, the end 523a is a connection end connected to the conversion portion 51 of the branching waveguide 52B, and the end 526b is a connection end connected to the multiplexing portion 53 of the branching waveguide 52B.

[0072] The width of the waveguide 523 decreases continuously from the end 523a to the end 523b. The width of the waveguide 523 decreases from the width W5 to the width W6 from the end 523a to the end 523b. The width W5 is, for example, 50% or less of the width W2, and in this embodiment, is smaller than 50% of the width W2. The width W5 may be substantially the same as the width at the end 521a of the waveguide 521, or may be different from the width at the end 521a of the waveguide 521. In this embodiment, the width W5 is substantially the same as the width W4, and the width W6 is smaller than the width W2 and the width W5. The width W6 may be larger than the width W5, or may be larger than the width W2. The width W6 is, for example, 0.2 μm to 3.0 μm. The width of the waveguide 524 is substantially constant at the width W6 between the end 524a and the end 524b.

[0073] The width of the waveguide 525 increases continuously from the end 525a to the end 525b. The width of the waveguide 525 increases from the width W6 to the width W4 from the end 525a to the end 525b. The width of the waveguide 526 is substantially constant at the width W4 between the end 526a and the end 526b. As described above, the width W4 is 23% to 47% of the width W3, and therefore the width of the waveguide 526 (branch waveguide 52B) at the end 526b is also 23% to 47% of the width W3. The width of the waveguide 526 at the end 526b may be different from the width of the waveguide 522 at the end 522b.

[0074] The length of the waveguide 523 in the X-axis direction is length L21. The length L23 of the waveguide 524 in the X-axis direction is, for example, 0 μm to 100 μm. The length L24 of the waveguide 525 in the X-axis direction is, for example, 1.0 μm to 100 μm. The length L25 of the waveguide 526 in the X-axis direction is, for example, 0 μm to 100 μm. In this embodiment, the sum of the lengths L21, L23, L24, and L25 is length L2.

[0075] Next, the optical path length difference ΔOL between the optical path length OL1 and the optical path length OL2 and the phase difference Δφ will be described. In the following, the wavelength of the branched light LA ​​and the wavelength of the branched light LB are set to λ, and the effective refractive index of the branched light LA ​​is set to n effA The effective refractive index of the branched light LB is neffB Since the width of the branching waveguide 52A is constant over the entire length of the branching waveguide 52A, the effective refractive index n effA is constant over the entire length of the branch waveguide 52A. Since the width of the branch waveguide 52B is not constant over the entire length of the branch waveguide 52B, the effective refractive index n effB (s) is expressed as a function of the position s along the optical path of the branched light LB. For convenience of explanation, the position s at the end 523a is set to 0.

[0076] First, the optical path length difference ΔOL will be described. In this embodiment, the optical path length difference obtained by subtracting the optical path length OL1 from the optical path length OL2 is defined as the optical path length difference ΔOL (=OL2-OL1). As described above, in the waveguides 523 and 525 whose widths change, the branched light LB propagates through the waveguides 523 and 525 while changing its effective refractive index. Furthermore, the widths of the waveguides 523, 524, and 525, excluding the ends 523a and 525b, are smaller than the widths of the waveguides 521 and 522. Therefore, in the waveguides 523, 524, and 525, excluding the ends 523a and 525b, the effective refractive index n effB (s)<effective refractive index n effA The relationship holds.

[0077] Furthermore, the optical path length OL1 is determined by the physical length PL2 of the branch waveguide 52A along the optical path of the branch light LA ​​and the effective refractive index n effA Using this, it is expressed as equation (1).

number

[0078] The optical path length OL2 is the effective refractive index n effB Using (s), it is expressed by Equation (2). Note that the physical length of the branch waveguide 52B along the optical path of the branch light LB is the same as the physical length of the branch waveguide 52A along the optical path of the branch light LA, which is length PL2.

number

[0079] As described above, in the waveguides 523, 524, and 525 except for the ends 523a and 525b, the effective refractive index n effB (s)<effective refractive index n effA Since the following relationship holds, the optical path length difference ΔOL takes a negative value. From the above, in branch waveguide 52B, by adopting a configuration in which the width of waveguide 523 and the width of waveguide 525 change, an optical path length difference ΔOL occurs between optical path length OL1 and optical path length OL2.

[0080] Next, the phase difference Δφ will be described. In this embodiment, the phase difference obtained by subtracting the phase of the branched light LA ​​when it is incident on the branching unit 52 from the phase of the branched light LB when it is incident on the branching unit 52 is defined as the initial phase difference Δφ0. When the light Lin in the TM0 mode is incident on the mode converter 45, the phase of the branched light LB when it is incident on the branching unit 52 lags behind the phase of the branched light LA ​​when it is incident on the branching unit 52 by π radians, so the initial phase difference Δφ0 is -π radians. When the light Lin in the TE0 mode is incident on the mode converter 45, the initial phase difference Δφ0 is 0 radians.

[0081] As described above, in the branching unit 52, the optical path difference ΔOL takes a negative value. As a result, the phase difference Δφ is changed from the initial phase difference Δφ0. Here, the relationship between the phase difference Δφ and the optical path difference ΔOL is expressed as ΔOL=(Δφ0-Δφ)×(λ / 2π radians) using the wavelength λ. In the multiplexing unit 53, the phase difference Δφ at which the light intensity of the light Lout is maximized, that is, the phase difference Δφ at which the conversion efficiency from the TM0 mode to the TE0 mode is maximized, is −π / 2+2nπ radians (n ​​is an integer). Therefore, from the above relationship, the optical path difference ΔOL at which the conversion efficiency is maximized is −λ / 4+nλ. The conversion efficiency represents the ratio of the light intensity of the visible light in the TE0 mode output from the mode converter 45 to the light intensity of the visible light in the TM0 mode input to the mode converter 45.

[0082] In this embodiment, the phase difference Δφ is set so that the conversion efficiency is 86.6% or more. Therefore, the phase difference Δφ is set within the range of -2π / 3+2nπ radians to -π / 3+2nπ radians, and the optical path length difference ΔOL is set within the range of -λ / 3+nλ to -λ / 6+nλ. That is, the branching unit 52 adjusts the phase difference Δφ so that it is within the range of -2π / 3+2nπ radians to -π / 3+2nπ radians. In this embodiment, the lengths and widths W6 in the X-axis direction of the waveguides 524, 525, and 526 are set so that the optical path length difference ΔOL is within the range of -λ / 3+nλ to -λ / 6+nλ.

[0083] In this embodiment, light Lin in TM0 mode is incident on the mode converter 45. In this case, in the conversion unit 51, the polarization mode of the light Lin is converted from the TM0 mode to the TE1 mode, and the light Lin in TE1 mode is output from the output end 51b to the branching unit 52. Then, in the branching unit 52, the light Lin in TE1 mode is branched into branched light LA ​​in TE0 mode and branched light LB in TE0 mode, which are in opposite phase to each other, and the branched light LA ​​propagates through the branching waveguide 52A, and the branched light LB propagates through the branching waveguide 52B. The optical intensities of the branched light LA ​​and the branched light LB are substantially half the optical intensity of the light Lin.

[0084] For example, when the optical path length OL1 of the branching waveguide 52A and the optical path length OL2 of the branching waveguide 52B are the same, the branched light LA ​​and the branched light LB are incident on the multiplexing unit 53 without changing the phase difference Δφ from −π radians (initial phase difference Δφ0). In this case, the light Lout in the TE0 mode, which is in opposite phase to each other, is emitted from the output end 53c and the output end 53d. The optical intensity of the light Lout emitted from the output end 53c and the optical intensity of the light Lout emitted from the output end 53d are substantially half the optical intensity of the light Lin.

[0085] On the other hand, in this embodiment, the optical path length OL1 of the branching waveguide 52A and the optical path length OL2 of the branching waveguide 52B are different from each other, so that the phase difference Δφ has a value other than -π radians. For example, when the phase difference Δφ is -π / 2 radians, the branching light LA ​​and the branching light LB are multiplexed in the multiplexing section 53, and the light Lout in the TE0 mode is output from the output end 53c. The light intensity of the light Lout output from the output end 53c is substantially the same as the light intensity of the light Lin.

[0086] When the light Lin in the TE0 mode is incident on the mode converter 45, the polarization mode of the light Lin is not converted in the conversion unit 51, and the light Lin in the TE0 mode is output from the output end 51b to the branch unit 52. Then, in the branch unit 52, the light Lin in the TE0 mode is branched into branched light LA ​​in the TE0 mode and branched light LB in the TE0 mode, which are in phase with each other, and the branched light LA ​​propagates through the branch waveguide 52A, and the branched light LB propagates through the branch waveguide 52B. The optical intensities of the branched light LA ​​and the branched light LB are substantially half the optical intensity of the light Lin. At this time, the optical path length OL1 of the branch waveguide 52A and the optical path length OL2 of the branch waveguide 52B are different from each other, so that the phase difference Δφ has a value different from 0 radian (initial phase difference Δφ0). For example, when the phase difference Δφ is π / 2 radians, the branched light LA ​​and the branched light LB are multiplexed in the multiplexing section 53, and the light Lout in the TE0 mode is output from the output end 53d. The light intensity of the light Lout output from the output end 53d is substantially the same as the light intensity of the light Lin.

[0087] In the laser module 4 and the optical element 40 described above, the width of the conversion section 51 increases continuously from the input end 51a to the output end 51b, so that the effective refractive index of the visible light propagating through the conversion section 51 changes. As a result, the polarization mode of the visible light can be converted from the TM0 mode to the TE1 mode in the conversion section 51. Furthermore, in the branching section 52, the visible light in the TE1 mode output from the conversion section 51 is branched into branched lights LA and LB in the TE0 mode, which are in opposite phase to each other, and the branched light LA ​​propagates through the branching waveguide 52A, and the branched light LB propagates through the branching waveguide 52B. Then, in the multiplexing section 53, the branched light LA ​​and the branched light LB are multiplexed, so that visible light in the TE0 mode is output.

[0088] Here, when the optical path length OL1 in the branching waveguide 52A and the optical path length OL2 in the branching waveguide 52B are the same, the branched light LA ​​and the branched light LB are incident on the multiplexing unit 53 without changing the phase difference Δφ from −π radians (initial phase difference Δφ0). Therefore, the optical intensity of the visible light obtained by multiplexing the branched light LA ​​and the branched light LB is substantially half the optical intensity of the visible light incident on the conversion unit 51, and the conversion efficiency decreases. On the other hand, in the laser module 4 and the optical element 40, the optical path length OL1 and the optical path length OL2 are different from each other, so that the phase difference Δφ has a value different from −π radians. This makes it possible to suppress the optical intensity of the visible light obtained by multiplexing the branched light LA ​​and the branched light LB from decreasing from the optical intensity of the visible light incident on the conversion unit 51. From the above, the optical element 40 can convert the polarization mode of the visible light from the TM0 mode to the TE0 mode while suppressing the decrease in the conversion efficiency.

[0089] When the phase difference Δφ is -π / 2+2nπ radians, the optical intensity of the visible light in the TE0 mode obtained by multiplexing the branched light LA ​​and the branched light LB is substantially the same as the optical intensity of the visible light in the TM0 mode incident on the conversion unit 51. As the phase difference Δφ moves away from -π / 2+2nπ radians, the optical intensity of the visible light in the TE0 mode obtained by multiplexing the branched light LA ​​and the branched light LB decreases. In the optical element 40, the phase difference Δφ is -2π / 3+2nπ radians to -π / 3+2nπ radians. This range is ±π / 6 with -π / 2+2nπ radians as the center, so that the loss of the optical intensity of the visible light in the TE0 mode obtained by multiplexing the branched light LA ​​and the branched light LB can be further reduced. That is, the decrease in the conversion efficiency from the TM0 mode to the TE0 mode can be further suppressed.

[0090] The mode converters 45, 46, and 47 are provided on the slab 41 in the Z direction. According to this configuration, the waveguide formed by the conversion unit 51 and the slab 41 has an asymmetric shape in the Z-axis direction. Therefore, the conversion efficiency from the TM0 mode to the TE1 mode in the conversion unit 51 can be improved. Here, asymmetric in the Z-axis direction means that the two parts separated by the symmetric plane are not plane-symmetric with respect to the symmetric plane that passes through the center in the Z-axis direction of the combined part of the conversion unit 51 and the slab 41 and is perpendicular to the Z-axis direction. Furthermore, when the conversion efficiency from the TM0 mode to the TE1 mode in the conversion unit 51 is improved, the length L1 required to obtain the desired conversion efficiency can be shortened. Therefore, the optical element 40 can be made smaller.

[0091] The height T1 of the core layer CA is smaller than the wavelength of visible light. With this configuration, visible light easily seeps out from the conversion section 51 to the slab 41. As a result, the effect of the slab 41 can be satisfactorily exhibited, and the conversion efficiency from the TM0 mode to the TE1 mode in the conversion section 51 can be improved. Furthermore, by improving the conversion efficiency from the TM0 mode to the TE1 mode in the conversion section 51, the length L1 required to obtain a desired conversion efficiency can be shortened. Therefore, the optical element 40 can be made smaller.

[0092] The beam-combining section 53 is configured by a multimode interferometer. With this configuration, the beam-combining section 53 can be manufactured more easily than when the beam-combining section 53 is manufactured by a Y-branch waveguide.

[0093] The width W3 of the multiplexing section 53 is 2.0 μm or more. The size of the width W3 can affect the interval between the branch waveguide 52A and the branch waveguide 52B. In manufacturing the mode converters 45, 46, and 47, anisotropic etching such as dry etching is used. At this time, if the interval between the branch waveguide 52A and the branch waveguide 52B cannot be secured, the side surface of each waveguide may be excessively removed, and the branch waveguide 52A and the branch waveguide 52B may not be formed in the desired shape. On the other hand, in the laser module 4 and the optical element 40, since the width W3 of the multiplexing section 53 is 2.0 μm or more, the interval between the branch waveguide 52A and the branch waveguide 52B can be secured without reducing the width W4 of the branch waveguides 52A and 52B. Therefore, the branch waveguide 52A and the branch waveguide 52B can be formed in the desired shape.

[0094] If the width W4 is less than 23% of the width W3, the tolerance of the mode converters 45, 46, 47 for manufacturing errors and the like becomes small. In addition, if the width W4 is more than 47% of the width W3, there is a risk that a sufficient gap cannot be secured between the branch waveguide 52A and the branch waveguide 52B.

[0095] Here, the tolerance ΔL3 of the length L3 in the multiplexing section 53 is determined by the refractive index nr Using the wavelength λ0 of visible light to be converted by the mode converter including the multiplexer 53 and the Gaussian beam waist w0 of the polarization mode of the visible light, it is expressed by equation (3).

number

[0096] Furthermore, the tolerance ΔW3 of the width W3 and the bandwidth Δλ0 of the wavelength λ0, which correspond to the tolerance ΔL3, are expressed by the formulas (4) and (5), respectively.

number

number

[0097] For example, if the Gaussian beam waist w0 is 1000 nm and the refractive index n r When the length L3 is 2.38 and the wavelength λ0 is 638 nm, the tolerance ΔL3 is 2930 nm. Furthermore, in this case, if the length L3 is 64.5 μm and the width W3 is 3.0 μm, the tolerance ΔW3 is 68 nm and the bandwidth Δλ0 is 29 nm. Here, the width W3 is the largest among all the components of the mode converter. Therefore, the deviation tolerance Δw device Therefore, the deviation tolerance Δw device would be 68nm.

[0098] The Gaussian beam waist w0 is proportional to the width W4. For example, if the width W3 is 3.0 μm and the width W4 is 23% of the width W3, the width W4 is about 0.7 μm. In this case, the Gaussian beam waist w0 is about 0.5 μm. Furthermore, in this case, the refractive index n ris 2.38, the wavelength λ0 is 638 nm, the length L3 is 64.5 μm, and the width W3 is 3.0 μm. Then, the deviation tolerance Δw device is about 34 nm. On the other hand, a distance of about 1.6 μm can be ensured between the branch waveguide 52A and the branch waveguide 52B.

[0099] Furthermore, for example, if the width W3 is 3.0 μm and the width W4 is 47% of the width W3, the width W4 is about 1.4 μm. In this case, the Gaussian beam waist w0 is about 1.0 μm. Furthermore, in this case, the refractive index n r is 2.38, the wavelength λ0 is 638 nm, the length L3 is 64.5 μm, and the width W3 is 3.0 μm. Then, the deviation tolerance Δw device is about 68 nm. On the other hand, a distance of about 0.2 μm can be ensured between the branch waveguide 52A and the branch waveguide 52B.

[0100] Here, when the width W4 is less than 23% of the width W3, the gap between the branch waveguide 52A and the branch waveguide 52B can be sufficiently secured, but the deviation tolerance Δw device becomes smaller. Deviation tolerance Δw device When the width W4 is smaller than 47% of the width W3, the tolerance of the mode converters 45, 46, and 47 for manufacturing errors and the like becomes smaller, and manufacturing becomes more difficult. device However, it becomes difficult to ensure a sufficient distance between branch waveguide 52A and branch waveguide 52B. If the distance between branch waveguide 52A and branch waveguide 52B cannot be sufficiently ensured, as described above, branch waveguide 52A and branch waveguide 52B may not be formed in the desired shape. Therefore, by setting width W4 to 23% to 47% of width W3, it is possible to ensure both the tolerance of mode converters 45, 46, and 47 for manufacturing errors and the like and the distance between branch waveguide 52A and branch waveguide 52B.

[0101] The multiplexer 48 is designed so that the multiplexing efficiency when multiplexing the red light Lr, green light Lg, and blue light Lb in the TE0 mode is higher than the multiplexing efficiency when multiplexing the red light Lr, green light Lg, and blue light Lb in the TM0 mode. In the laser module 4 and the optical element 40, the mode converter 45 converts the polarization mode of the red light Lr from the TM0 mode to the TE0 mode, the mode converter 46 converts the polarization mode of the green light Lg from the TM0 mode to the TE0 mode, and the mode converter 47 converts the polarization mode of the blue light Lb from the TM0 mode to the TE0 mode. Therefore, it is possible to improve the multiplexing efficiency in the multiplexer 48.

[0102] The height of the mode converter 45, the height of the mode converter 46, and the height of the mode converter 47 are the same. With this configuration, the mode converter 45, the mode converter 46, and the mode converter 47 can be formed on the same substrate S, and the heights of the respective mode converters can be made the same, so that the optical element 40 can be easily manufactured.

[0103] In order to output full-color laser light La by combining red light Lr, green light Lg, and blue light Lb, it is necessary to adjust the light intensity of each color of light according to the color to be output. In order to change the light intensity of the corresponding color of light in each laser light source, a large drive current is required. In the laser module 4 and the optical element 40, the light intensity of the red light Lr is modulated by the modulator 42, the light intensity of the green light Lg is modulated by the modulator 43, and the light intensity of the blue light Lb is modulated by the modulator 44. Therefore, it is possible to output full-color laser light La without requiring a large drive current.

[0104] The near-eye-readable device 1 includes a retinal projection device 10, and the retinal projection device 10 includes an optical element 40. Therefore, in the near-eye-readable device 1 and the retinal projection device 10, it is possible to convert the polarization mode of visible light from the TM0 mode to the TE0 mode while suppressing a decrease in conversion efficiency, and then project an image onto the retina.

[0105] A laser module according to another embodiment will be described with reference to Fig. 7. Fig. 7 is a block diagram of a laser module according to another embodiment. A laser module 4A shown in Fig. 7 is mainly different from the laser module 4 in that a single mode converter 49 is included instead of the mode converters 45, 46, and 47, and that a multiplexer 48 is disposed between each modulator and the mode converter 49.

[0106] Specifically, the output end of modulator 42, the output end of modulator 43, and the output end of modulator 44 are optically connected to three input ends of multiplexer 48. The output end of multiplexer 48 is optically connected to an input end of mode converter 49. The configuration of mode converter 49 is the same as the configuration of mode converter 45.

[0107] In the laser module 4A, visible light in TM0 mode is emitted from each laser light source, and the optical intensity of the visible light in TM0 mode is modulated in each modulator. Then, the visible light modulated in each modulator is multiplexed in the multiplexer 48, and the polarization mode of the multiplexed visible light is converted from TM0 mode to TE0 mode in the mode converter 49, and is emitted from the mode converter 49 to the optical component 5 (see FIG. 2) as laser light La in TE0 mode.

[0108] Next, a laser module according to still another embodiment will be described with reference to Fig. 8. Fig. 8 is a block diagram of a laser module according to still another embodiment. A laser module 4B shown in Fig. 8 is mainly different from the laser module 4 in that it further includes a polarization mode of visible light emitted by laser light sources 411, 412, and 413, and a mode converter 45A, a mode converter 46A, and a mode converter 47A.

[0109] The laser light source 411 emits red light Lr in TE0 mode. The laser light source 412 emits green light Lg in TE0 mode. The laser light source 413 emits blue light Lb in TE0 mode.

[0110] The mode converter 45A is a mode converter that converts the polarization mode of the red light Lr from TE0 mode to TM0 mode. The mode converter 45A is provided between the laser light source 411 and the modulator 42. The red light Lr in TE0 mode is incident on an incident end of the mode converter 45A from the laser light source 411, and the output end of the mode converter 45A is optically connected to the input end of the modulator 42. The mode converter 45A converts the polarization mode of the red light Lr incident from the laser light source 411 from TE0 mode to TM0 mode, and outputs the red light Lr in TM0 mode to the modulator 42.

[0111] In the mode converter 45A, for example, a configuration is adopted in which the incident end and the exit end of the mode converter 45 are swapped. In this configuration, the red light Lr in TE0 mode is incident from the laser light source 411 to the exit end 53c of the multiplexer 53, the incident end 53a of the multiplexer 53 outputs the red light Lr in TE0 mode to the branch waveguide 52A, and the incident end 53b of the multiplexer 53 outputs the red light Lr in TE0 mode to the branch waveguide 52B. The phase difference obtained by subtracting the phase of the red light Lr at the incident end 53a from the phase of the red light Lr at the incident end 53b is -π / 2 radians.

[0112] Then, one of the red lights Lr propagates through the branching waveguide 52A and enters the output end 51b of the conversion unit 51. The other red light Lr propagates through the branching waveguide 52B and enters the output end 51b of the conversion unit 51. The optical path length difference ΔOL between the optical path length OL1 in the branching waveguide 52A and the optical path length OL2 in the branching waveguide 52B is -λ / 4+nλ. Therefore, the phase difference obtained by subtracting the phase of the one of the red lights Lr at the output end 51b from the phase of the other of the red lights Lr at the output end 51b is -π+2nπ radians. Therefore, at the output end 51b, the two TE0 mode red lights Lr are combined to become TE1 mode red light Lr, and the TE1 mode red light Lr propagates through the conversion unit 51 from the output end 51b to the input end 51a. At this time, in the conversion unit 51, the polarization mode of the red light Lr is converted from the TE1 mode to the TM0 mode.

[0113] The mode converter 46A is a mode converter that converts the polarization mode of the green light Lg from the TE0 mode to the TM0 mode. The mode converter 46A is provided between the laser light source 412 and the modulator 43. The green light Lg in the TE0 mode is incident on the incident end of the mode converter 46A from the laser light source 412, and the output end of the mode converter 46A is optically connected to the input end of the modulator 43. The mode converter 46A converts the polarization mode of the green light Lg incident from the laser light source 412 from the TE0 mode to the TM0 mode, and outputs the green light Lg in the TM0 mode to the modulator 43.

[0114] The mode converter 46A also employs, for example, a configuration in which the input end and output end of the mode converter 46 are interchanged. In this case, the mode converter 46A also converts the polarization mode of the green light Lg from the TE0 mode to the TM0 mode in the same manner as the mode converter 45A.

[0115] The mode converter 47A is a mode converter that converts the polarization mode of the blue light Lb from TE0 mode to TM0 mode. The mode converter 47A is provided between the laser light source 413 and the modulator 44. The blue light Lb in TE0 mode is incident on the incident end of the mode converter 47A from the laser light source 413, and the output end of the mode converter 47A is optically connected to the input end of the modulator 44. The mode converter 47A converts the polarization mode of the blue light Lb incident from the laser light source 413 from TE0 mode to TM0 mode, and outputs the blue light Lb in TM0 mode to the modulator 44.

[0116] The mode converter 47A also employs, for example, a configuration in which the input end and output end of the mode converter 47 are interchanged. In this case, the mode converter 47A also converts the polarization mode of the blue light Lb from the TE0 mode to the TM0 mode in the same manner as the mode converter 45A.

[0117] In the laser module 4B, visible light in TE0 mode is emitted from each laser light source, so that the polarization mode of each visible light emitted from each laser light source is first converted from TE0 mode to TM0 mode in each mode converter 45A, 46A, 47A. Then, after the light intensity of the visible light in TM0 mode is modulated in each modulator, the polarization mode of each modulated visible light is converted from TM0 mode to TE0 mode in each mode converter 45, 46, 47. Then, each visible light is multiplexed in multiplexer 48, and is output from multiplexer 48 to optical component 5 (see FIG. 2) as laser light La in TE0 mode.

[0118] Next, a laser module according to still another embodiment will be described with reference to Fig. 9. Fig. 9 is a block diagram of a laser module according to still another embodiment. The laser module 4C is different from the laser module 4 mainly in the orientation of the C-axis of the lithium niobate constituting the core layer CA and in that the positions of the modulators 42, 43, and 44 and the mode converters 45, 46, and 47 are interchanged.

[0119] In this embodiment, the C-axis of the lithium niobate extends in the Y-axis direction. The core layer CA is made of X-cut lithium niobate.

[0120] Red light Lr in TM0 mode is incident on an incident end of the mode converter 45 from the laser light source 411, and an output end of the mode converter 45 is optically connected to an incident end of the modulator 42. The mode converter 45 converts the polarization mode of the red light Lr incident from the laser light source 411 from TM0 mode to TE0 mode, and outputs the red light Lr in TE0 mode to the modulator 42.

[0121] Green light Lg in TM0 mode is incident on an incident end of the mode converter 46 from the laser light source 412, and an output end of the mode converter 46 is optically connected to an incident end of the modulator 43. The mode converter 46 converts the polarization mode of the green light Lg incident from the laser light source 412 from TM0 mode to TE0 mode, and outputs the green light Lg in TE0 mode to the modulator 43.

[0122] Blue light Lb in TM0 mode is incident on an incident end of the mode converter 47 from the laser light source 413, and an output end of the mode converter 47 is optically connected to an incident end of the modulator 44. The mode converter 47 converts the polarization mode of the blue light Lb incident from the laser light source 413 from TM0 mode to TE0 mode, and outputs the blue light Lb in TE0 mode to the modulator 44.

[0123] The output end of the modulator 42, the output end of the modulator 43, and the output end of the modulator 44 are optically connected to the three input ends of the combiner 48, respectively. As described above, the C-axis of the lithium niobate extends in the Y-axis direction. Therefore, the modulation efficiency of each modulator is improved in the TE mode.

[0124] In the laser module 4C, each laser light source emits visible light in TM0 mode, and the polarization mode of each visible light emitted from each laser light source is converted from TM0 mode to TE0 mode in each mode converter. Then, after the optical intensity of the visible light in TE0 mode is modulated in each modulator, the modulated visible light is multiplexed in the multiplexer 48 and is emitted from the multiplexer 48 to the optical component 5 (see FIG. 2) as laser light La in TE0 mode.

[0125] Although the embodiments of the present disclosure have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit and scope of the present disclosure.

[0126] For example, the laser module 4 may be applied to a device other than the near eyewear wearable device 1.

[0127] The optical element 40 may not include the cladding layer CB. In this case, an air layer may function as an upper cladding layer.

[0128] It is sufficient that the optical element 40 includes one mode converter. In other words, it is sufficient that the core layer CA includes one mode converter that converts the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0129] The core layer CA may not include the slab 41. In this case, the mode converters 45, 46, and 47 may be provided directly on the main surface Sa of the substrate S.

[0130] It is sufficient that the width of the conversion portion 51 increases continuously. The rate of increase in the width of the conversion portion 51 does not have to be constant.

[0131] The configuration for adjusting the phase difference Δφ is not limited to the above-mentioned configuration. For example, the phase difference Δφ may be adjusted by the difference in physical length between the branch waveguide 52A having a constant width over the entire length and the branch waveguide 52B having the same width as the branch waveguide 52A over the entire length. For example, the physical length may be changed by providing a curved portion in one of the branch waveguide 52A and the branch waveguide 52B. The phase difference Δφ may be adjusted by changing both the waveguide width and the physical length between the branch waveguide 52A and the branch waveguide 52B. A configuration may be adopted in which the width of the waveguide 523 increases continuously from the end 523a to the end 523b, and the width of the waveguide 525 decreases continuously from the end 525a to the end 525b. Furthermore, the widths of the waveguides 521 and 522 do not have to be constant. For example, the width of waveguide 521 may be configured to increase continuously from end 521a to end 521b, or may be configured to decrease continuously from end 521a to end 521b. The width of waveguide 522 may be configured to increase continuously from end 522a to end 522b, or may be configured to decrease continuously from end 522a to end 522b.

[0132] Hereinafter, another example of the specific configuration of each waveguide in the branching section 52 will be described. In this other example, the width of the waveguide 521 may be continuously increased from the end 521a to the end 521b. The width of the waveguide 521 may be increased, for example, from 50% of the width W2 to the width W4 from the end 521a to the end 521b. The width of the waveguide 522 may be substantially constant at the width W4 between the end 522a and the end 522b. The width of the waveguide 523 may be continuously increased, for example, from the width W5 to the width W6 from the end 523a to the end 523b. In this case, the width W5 may be 50% of the width W2. The width of the waveguide 524 may be substantially constant at the width W6 between the end 524a and the end 524b. In this other example, the same configuration as that of the above-mentioned embodiment may be adopted for the waveguide 525 and the waveguide 526.

[0133] Alternatively, in yet another example, in addition to the configuration of the other example above, a configuration may be adopted in which the width of the waveguide 525 decreases continuously from the end 525a to the end 525b. In this case, the width of the waveguide 525 may decrease from the width W6 to the width W4 from the end 525a to the end 525b. In other words, the width W6 may be greater than the width W4.

[0134] The multiplexing section 53 may be configured with a Y-branch waveguide instead of a multimode interferometer, or may be configured with a directional coupler. EXAMPLES

[0135] In the following, the present disclosure will be described in more detail with reference to examples in order to explain the above-mentioned effects, but the present disclosure is not limited to these examples.

[0136] <Evaluation of the relationship between length PL2 and conversion efficiency> In the mode converters having the configurations of Examples 1 to 3, the relationship between the length PL2 and the conversion efficiency was evaluated. As the mode converters of Examples 1 to 3, mode converters having the same configuration as that shown in Figs. 4 to 6 were used, except that the width of the waveguide 521 increases from the end 521a to the end 521b and the width of the waveguide 523 increases from the end 523a to the end 523b were adopted. Furthermore, in the mode converters of Examples 1 to 3, the width W5 was set to 50% of the width W2, and the width of the waveguide 521 at the end 521a was set to be substantially the same as the width W5. In Examples 1 to 3, the conversion efficiency was calculated when the polarization mode of each of the red light, green light, and blue light was converted from the TM0 mode to the TE0 mode at each length L24 while changing the length L24 of the waveguide 525 in the X-axis direction. In Examples 1 to 3, the length L22 of the waveguide 522 in the X-axis direction was also changed by the same amount as the length L24. That is, in Examples 1 to 3, the conversion efficiency at each length L24 was calculated in a state in which the physical length of the branch waveguide 52A and the physical length of the branch waveguide 52B were the same. Here, since the length PL2 is the physical length of the branch waveguide 52B along the optical path of the branch light LB, the length PL2 changes by changing the length L24. Therefore, it can be said that the conversion efficiency calculated while changing the length L24 is the conversion efficiency calculated while changing the length PL2.

[0137] In Examples 1 to 3, as shown in Table 1, height T1, height T11, length L1, length L21, length L22, length L23, length L25, length L3, width W1, width W2, width W3, width W4, width W5, and width W6 were set for each color wavelength. [Table 1]

[0138] Figure 10(a) shows the calculation results for red light, Figure 10(b) shows the calculation results for green light, and Figure 10(c) shows the calculation results for blue light. The vertical axis of Figures 10(a), 10(b), and 10(c) represents the conversion efficiency normalized by the ideal value (100%) of the conversion efficiency, and the unit is arbitrary unit. The horizontal axis of Figures 10(a), 10(b), and 10(c) represents the length L24 [μm].

[0139] 10(a), 10(b), and 10(c), it can be seen that the conversion efficiency for each of the red light, green light, and blue light changes periodically with respect to the length L24 (length PL2). This is because when the lengths L22 and L24 are increased by the same amount, the effective refractive index n effA and the effective refractive index n of the waveguide 525 effB This is because the absolute value of the optical path difference ΔOL increases due to the difference from (s). Therefore, it can be seen that when the optical path difference ΔOL is in the range of -λ / 3+nλ to -λ / 6+nλ, in other words, when the phase difference Δφ is in the range of -2π / 3+2nπ radians to -π / 3+2nπ radians, a high conversion efficiency of 86.6% or more can be obtained.

[0140] <Evaluation of conversion loss with and without slab 41 and evaluation of length L1 of conversion portion 51> In the conversion section 51 having the configuration of Examples 4 to 9, the influence of the presence or absence of the slab 41 on the conversion loss of the TM0-TE1 conversion and the length L1 of the conversion section 51 was evaluated. As the conversion section 51 of Examples 4 to 6, a conversion section 51 having the same configuration as that shown in Figures 4 and 5 was used. As the optical element of Examples 7 to 9, a conversion section 51 having the same configuration as that shown in Figures 4 and 5 was used, except that the core layer CA did not include the slab 41.

[0141] In Examples 4 to 9, the height T1, height T11, length L1, width W1, and width W2 were set for each wavelength of color as shown in Table 2. A height T11 of 0 μm indicates that the core layer CA does not include a slab 41. In Examples 4 to 9, the conversion loss was calculated when the polarization mode of each of red light, green light, and blue light was converted from TM0 mode to TE1 mode in the conversion section 51 in which the parameters shown in Table 2 were set. The calculation results of the conversion loss are shown in Table 2. [Table 2]

[0142] Furthermore, in the conversion section 51 having the height T1, height T11, width W1, and width W2 set as shown in Table 2, the conversion loss was calculated while changing the length L1. The calculation results for red light are shown in FIG. 11(a), the calculation results for green light are shown in FIG. 11(b), and the calculation results for blue light are shown in FIG. 11(c). The vertical axis of FIG. 11(a), 11(b), and 11(c) represents the conversion loss [dB]. The horizontal axis of FIG. 11(a), 11(b), and 11(c) represents the length L1 [μm].

[0143] 11(a), plot E4 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 4. In FIG. 11(a), plot E7 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 7.

[0144] 11(b), plot E5 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 5. In FIG. 11(b), plot E8 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 8.

[0145] 11(c), plot E6 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 6. In FIG. 11(c), plot E9 represents the relationship between length L1 and conversion loss in converter 51 having height T1, height T11, width W1, and width W2 set in Example 9.

[0146] According to Table 2, the conversion loss is suppressed in Examples 4 to 6 compared to the corresponding Examples among Examples 7 to 9. Therefore, it can be seen that by including the slab 41 in the core layer CA, the decrease in conversion efficiency is suppressed for all of red light, green light, and blue light.

[0147] 11(a), 11(b), and 11(c), to achieve the same conversion loss, the length L1 can be made smaller in Examples 4 to 6 compared to the corresponding examples among Examples 7 to 9. Therefore, it can be seen that by including the slab 41 in the core layer CA, the length L1 for achieving a predetermined conversion loss can be made smaller, and the lengths of the mode converters 45, 46, and 47 in the X-axis direction can be made shorter.

[0148] <Evaluation of Conversion Loss at Height T1 of Core Layer CA and Evaluation of Length L1 of Conversion Section 51> The influence of height T1 on conversion loss and length L1 in red light was evaluated using mode converters having the configurations of Examples 10 to 13. As the mode converter of Example 10, a mode converter having the same configuration as that shown in Figs. 4 to 6 was used, except that a configuration in which the width of waveguide 521 increases from end 521a to end 521b and a configuration in which the width of waveguide 523 increases from end 523a to end 523b were adopted. As the mode converters of Examples 11 to 13, a mode converter having the same configuration as that of Example 10 was used, except that a configuration in which the width of waveguide 525 decreases from width W6 to width W4 from end 525a to end 525b was adopted. Furthermore, in the mode converters of Examples 10 to 13, width W5 was set to 50% of width W2, and the width at end 521a was set to be substantially the same as width W5.

[0149] In Examples 10 to 13, height T1, height T11, length L21, length L22, length L23, length L24, length L25, length L3, width W1, width W2, width W3, width W4, width W5, and width W6 were set for red light as shown in Table 3. Note that 638 nm was used as the wavelength of red light. In Examples 10 to 13, the conversion loss was calculated when the polarization mode of red light was converted from TM0 mode to TE0 mode in the optical element in which the parameters shown in Table 3 were set. The length L1 at which the conversion loss is minimized and the calculation result of the minimum conversion loss are shown in Table 3. [Table 3]

[0150] Furthermore, among the parameters shown in Table 3, Fig. 12(a) shows the relationship between height T1 and conversion loss, and the relationship between height T1 and length L1. The left vertical axis of Fig. 12(a) represents conversion loss [dB], the right vertical axis of Fig. 12(a) represents length L1 [μm], and the horizontal axis of Fig. 12(a) represents height T1 [μm].

[0151] Plot E10a represents the relationship between height T1 and conversion loss in Example 10, and plot E10b represents the relationship between height T1 and length L1 in Example 10. Plot E11a represents the relationship between height T1 and conversion loss in Example 11, and plot E11b represents the relationship between height T1 and length L1 in Example 11. Plot E12a represents the relationship between height T1 and conversion loss in Example 12, and plot E12b represents the relationship between height T1 and length L1 in Example 12. Plot E13a represents the relationship between height T1 and conversion loss in Example 13, and plot E13b represents the relationship between height T1 and length L1 in Example 13.

[0152] Hereinafter, when the conversion loss is smaller than 1.00 dB, it is assumed that high conversion efficiency is realized, and when the length L1 is smaller than 1000 μm, it is assumed that the length L1 is suppressed.

[0153] 12(a), the conversion loss in Examples 10 and 11, in which the height T1 was smaller than the wavelength of red light (638 nm), was 0.31 dB to 0.57 dB. On the other hand, the conversion loss in Examples 12 and 13, in which the height T1 was equal to or greater than the wavelength of red light, was 3.71 dB to 4.00 dB. Therefore, it can be seen that, for red light, when the height T1 is smaller than the wavelength of red light, high conversion efficiency is achieved.

[0154] Furthermore, in Examples 10 and 11 where the height T1 was smaller than the wavelength of red light, the length L1 was 225 μm to 370 μm. On the other hand, in Examples 12 and 13 where the height T1 was equal to or greater than the wavelength of red light, the length L1 was 4700 μm to 12400 μm. Therefore, it can be seen that, in the case of red light, when the height T1 is smaller than the wavelength of red light, the length L1 can be made smaller, and the length of the mode converter 45 in the X-axis direction can be made smaller.

[0155] The influence of height T1 on conversion loss and length L1 in green light was evaluated using mode converters having the configurations of Examples 14 to 17. As the mode converter of Example 14, a mode converter having the same configuration as that shown in Figs. 4 to 6 was used, except that a configuration in which the width of waveguide 521 increases from end 521a to end 521b and a configuration in which the width of waveguide 523 increases from end 523a to end 523b were adopted. As the mode converters of Examples 15 to 17, a mode converter having the same configuration as that of Example 14 was used, except that a configuration in which the width of waveguide 525 decreases from end 525a to end 525b was adopted. Furthermore, in the mode converters of Examples 14 to 17, width W5 was set to 50% of width W2, and the width at end 521a was set to be substantially the same as width W5.

[0156] In Examples 14 to 17, as shown in Table 4, height T1, height T11, length L21, length L22, length L23, length L24, length L25, length L3, width W1, width W2, width W3, width W4, width W5, and width W6 were set for green light. Note that 520 nm was used as the wavelength of green light. In Examples 14 to 17, the conversion loss was calculated when the polarization mode of green light was converted from TM0 mode to TE0 mode in the optical element in which the parameters shown in Table 4 were set. Table 4 shows the length L1 at which the conversion loss is minimum and the calculation result of the minimum conversion loss. [Table 4]

[0157] Furthermore, among the parameters shown in Table 4, Fig. 12(b) shows the relationship between height T1 and conversion loss, and the relationship between height T1 and length L1. The left vertical axis of Fig. 12(b) represents conversion loss [dB], the right vertical axis of Fig. 12(b) represents length L1 [μm], and the horizontal axis of Fig. 12(b) represents height T1 [μm].

[0158] Plot E14a represents the relationship between height T1 and conversion loss in Example 14, and plot E14b represents the relationship between height T1 and length L1 in Example 14. Plot E15a represents the relationship between height T1 and conversion loss in Example 15, and plot E15b represents the relationship between height T1 and length L1 in Example 15. Plot E16a represents the relationship between height T1 and conversion loss in Example 16, and plot E16b represents the relationship between height T1 and length L1 in Example 16. Plot E17a represents the relationship between height T1 and conversion loss in Example 17, and plot E17b represents the relationship between height T1 and length L1 in Example 17.

[0159] According to Table 4 and Fig. 12(b), the conversion loss in Examples 14 and 15, in which the height T1 was smaller than the wavelength of green light (520 nm), was 0.38 dB to 0.97 dB. On the other hand, the conversion loss in Examples 16 and 17, in which the height T1 was equal to or greater than the wavelength of green light, was 3.66 dB to 5.13 dB. Therefore, it can be seen that high conversion efficiency is achieved even for green light when the height T1 is smaller than the wavelength of green light.

[0160] Furthermore, in Examples 14 and 15 in which the height T1 was smaller than the wavelength of the green light, the length L1 was 212 μm to 614 μm. On the other hand, in Examples 16 and 17 in which the height T1 was equal to or greater than the wavelength of the green light, the length L1 was 2500 μm to 24100 μm. Therefore, it can be seen that, even in the case of green light, when the height T1 is smaller than the wavelength of the green light, the length L1 can be reduced, and the length of the mode converter 46 in the X-axis direction can be reduced.

[0161] The influence of the height T1 on the conversion loss and the length L1 in blue light was evaluated using the mode converters having the configurations of Examples 18 to 23. As the mode converter of Example 18, a mode converter having the same configuration as that shown in Figs. 4 to 6 was used, except that a configuration in which the width of the waveguide 521 increases from the end 521a to the end 521b and a configuration in which the width of the waveguide 523 increases from the end 523a to the end 523b were adopted. As the mode converters of Examples 19 to 23, a mode converter having the same configuration as that of Example 18 was used, except that a configuration in which the width of the waveguide 525 decreases from the end 525a to the end 525b was adopted. Furthermore, in the mode converters of Examples 18 to 23, the width W5 was set to 50% of the width W2, and the width at the end 521a was set to be substantially the same as the width W5.

[0162] In Examples 18 to 23, as shown in Table 5, height T1, height T11, length L21, length L22, length L23, length L24, length L25, length L3, width W1, width W2, width W3, width W4, width W5, and width W6 were set for blue light. In Examples 18 to 23, 455 nm was used as the wavelength of blue light. In Examples 18 to 23, the conversion loss was calculated when the polarization mode of blue light was converted from TM0 mode to TE0 mode in the optical element in which the parameters shown in Table 5 were set. Table 5 shows the length L1 at which the conversion loss is minimized and the calculation result of the minimum conversion loss. [Table 5]

[0163] Furthermore, among the parameters shown in Table 5, the relationship between height T1 and conversion loss, and the relationship between height T1 and length L1 are shown in Fig. 12(c). The left vertical axis of Fig. 12(c) represents conversion loss [dB], the right vertical axis of Fig. 12(c) represents length L1 [μm], and the horizontal axis of Fig. 12(c) represents height T1 [μm].

[0164] Plot E18a represents the relationship between height T1 and conversion loss in Example 18, and plot E18b represents the relationship between height T1 and length L1 in Example 18. Plot E19a represents the relationship between height T1 and conversion loss in Example 19, and plot E19b represents the relationship between height T1 and length L1 in Example 19. Plot E20a represents the relationship between height T1 and conversion loss in Example 20, and plot E20b represents the relationship between height T1 and length L1 in Example 20. Plot E21a represents the relationship between height T1 and conversion loss in Example 21, and plot E21b represents the relationship between height T1 and length L1 in Example 21. Plot E22a represents the relationship between height T1 and conversion loss in Example 22, and plot E22b represents the relationship between height T1 and length L1 in Example 22. Plot E23a represents the relationship between height T1 and conversion loss in Example 23, and plot E23b represents the relationship between height T1 and length L1 in Example 23.

[0165] According to Table 5 and Fig. 12(c), the conversion loss in Examples 18 and 19, in which the height T1 was smaller than the wavelength of blue light (455 nm), was 0.31 dB to 0.89 dB. On the other hand, the conversion loss in Examples 20 to 23, in which the height T1 was equal to or greater than the wavelength of blue light, was 3.37 dB to 5.16 dB. Therefore, it can be seen that high conversion efficiency is achieved even for blue light when the height T1 is smaller than the wavelength of blue light.

[0166] Furthermore, in Examples 18 and 19 in which height T1 was smaller than the wavelength of blue light, length L1 was 300 μm to 570 μm. On the other hand, length L1 in Examples 20 to 23 in which height T1 was equal to or greater than the wavelength of blue light was 1800 μm to 14700 μm. Therefore, it can be seen that, even in the case of blue light, when height T1 is smaller than the wavelength of blue light, length L1 can be made smaller, and the length of mode converter 47 in the X-axis direction can be made smaller.

[0167] In Examples 10 to 23, parameters other than height T1 were also changed. However, the parameters other than height T1 shown in Tables 3, 4, and 5 are values ​​that are set so that when the color of visible light and height T1 are changed, the optical path length difference ΔOL becomes -λ / 4+nλ and the conversion loss is minimized. It is considered that the effect of changes in parameters other than height T1 on the conversion efficiency is negligibly small.

[0168] (Additional Note) [Clause 1] A substrate having a major surface; a core layer provided on the main surface and made of a material having an electro-optic effect; Equipped with the core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light from a TM0 mode to a TE0 mode; The mode converter includes: A conversion unit that converts the polarization mode of the visible light from the TM0 mode to a TE1 mode; a branching unit that branches the visible light in the TE1 mode into a first branched light in the TE0 mode and a second branched light in the TE0 mode and adjusts a phase difference between the first branched light and the second branched light; a multiplexing unit that multiplexes the first branched light and the second branched light, the conversion unit has two ends in the first direction, the first end to which the visible light in the TM0 mode is incident and a second end from which the visible light in the TE1 mode is output, a length of the conversion portion in a second direction along the main surface and intersecting the first direction increases continuously from the first end to the second end; the phase difference is a phase difference obtained by subtracting a phase of the first branched light when it is incident on the multiplexing unit from a phase of the second branched light when it is incident on the multiplexing unit, the branching section includes a first branching waveguide through which the first branched light propagates and a second branching waveguide through which the second branched light propagates, an optical path length of the first branched light in the first branching waveguide and an optical path length of the second branched light in the second branching waveguide are different from each other.

[0169] [Clause 2] the phase difference is equal to or greater than -2π / 3+2nπ radians and equal to or less than -π / 3+2nπ radians, Item 2. The optical element of item 1, wherein n is an integer.

[0170] [Clause 3] The core layer further comprises a slab provided on the main surface, 3. The optical element of claim 1 or 2, wherein the mode converter is disposed on the slab in a third direction intersecting the first direction and the second direction.

[0171] [Article 4] 4. The optical element described in clause 3, wherein the length of the core layer in the third direction is smaller than the wavelength of the visible light.

[0172] [Article 5] Item 5. The optical element according to any one of items 1 to 4, wherein the multiplexing section is constituted by a multimode interferometer.

[0173] [Article 6] Item 6. The optical element according to item 5, wherein the length of the multiplexing portion in the second direction is 2.0 μm or more.

[0174] [Article 7] a length of the first branching waveguide in the second direction at a connection end of the first branching waveguide connected to the multiplexing section is 23% or more and 47% or less of a length of the multiplexing section in the second direction, The optical element described in clause 5 or 6, wherein a length of the second branching waveguide in the second direction at a connection end of the second branching waveguide connected to the multiplexing section is 23% or more and 47% or less of a length of the multiplexing section in the second direction.

[0175] [Article 8] The core layer is a first mode converter that converts the polarization mode of red light from the TM0 mode to the TE0 mode; a second mode converter that converts the polarization mode of green light from the TM0 mode to the TE0 mode; a third mode converter which converts the polarization mode of blue light from the TM0 mode to the TE0 mode; a multiplexer that multiplexes the red light, the green light, and the blue light and emits a laser beam; The optical element according to any one of claims 1 to 7, comprising:

[0176] [Article 9] An optical element as described in clause 8, wherein the length of the first mode converter in a third direction intersecting the first direction and the second direction, the length of the second mode converter in the third direction, and the length of the third mode converter in the third direction are the same.

[0177] [Article 10] The core layer is a first modulator for modulating the light intensity of the red light; a second modulator for modulating the light intensity of the green light; a third modulator that modulates the light intensity of the blue light; 10. The optical element of claim 8 or 9, further comprising:

[0178] [Article 11] An optical element according to any one of clauses 8 to 10, a first laser light source that emits the red light in the TM0 mode; a second laser light source that emits the green light in the TM0 mode; a third laser light source that emits the blue light in the TM0 mode.

[0179] [Article 12] A retinal projection device mounted on a near-eye refractive device, A laser module according to clause 11; a movable mirror that performs scanning using the laser light emitted from the laser module; A retinal projection device comprising: a reflector that reflects the laser light that has passed through the movable mirror and guides it to the retina of a user wearing the near-eye wearable device, thereby projecting an image onto the retina.

[0180] [Article 13] A retinal projection device according to clause 12, A lens provided with the reflector; A near-eye trouble device comprising: [Explanation of symbols]

[0181] 1...Near eyewear error device, 3...Lens, 4, 4A, 4B, 4C...Laser module, 6...Movable mirror, 10...Retinal projection device, 30...Reflector, 40...Optical element, 41...Slab, 42...Modulator (first modulator), 43...Modulator (second modulator), 44...Modulator (third modulator), 45...Mode converter (first mode converter), 46...Mode converter (second mode converter), 47...Mode converter (third mode converter), 48...Synthesizer wave converter, 51...conversion section, 51a...incident end (first end), 51b...exit end (second end), 52...branching section, 52A...branching waveguide (first branching waveguide), 52B...branching waveguide (second branching waveguide), 53...combining section, 411...laser light source (first laser light source), 412...laser light source (second laser light source), 413...laser light source (third laser light source), 522b...end (connecting end), 526b...end (connecting end), CA...core layer, S...substrate, Sa...main surface.

Claims

1. A substrate having a major surface; a core layer provided on the main surface and made of a material having an electro-optic effect; Equipped with the core layer includes a mode converter extending in a first direction along the principal surface and converting a polarization mode of visible light from a TM0 mode to a TE0 mode; The mode converter includes: A conversion unit that converts the polarization mode of the visible light from the TM0 mode to a TE1 mode; a branching unit that branches the visible light in the TE1 mode into a first branched light in the TE0 mode and a second branched light in the TE0 mode and adjusts a phase difference between the first branched light and the second branched light; a multiplexing unit that multiplexes the first branched light and the second branched light, the conversion unit has both ends in the first direction, the conversion unit including a first end to which the visible light in the TM0 mode is incident and a second end from which the visible light in the TE1 mode is emitted, a length of the conversion portion in a second direction along the main surface and intersecting the first direction increases continuously from the first end to the second end; the phase difference is a phase difference obtained by subtracting a phase of the first branched light when it is incident on the multiplexing unit from a phase of the second branched light when it is incident on the multiplexing unit, the branching section includes a first branching waveguide through which the first branched light propagates and a second branching waveguide through which the second branched light propagates, an optical path length of the first branched light in the first branching waveguide and an optical path length of the second branched light in the second branching waveguide are different from each other.

2. the phase difference is equal to or greater than -2π / 3+2nπ radians and equal to or less than -π / 3+2nπ radians, The optical element according to claim 1 , wherein n is an integer.

3. The core layer further comprises a slab provided on the main surface, The optical element according to claim 1 , wherein the mode converter is provided on the slab in a third direction intersecting the first direction and the second direction.

4. The optical element according to claim 3 , wherein a length of the core layer in the third direction is smaller than a wavelength of the visible light.

5. 3. The optical element according to claim 1, wherein the multiplexing section is configured by a multimode interferometer.

6. The optical element according to claim 5 , wherein the length of the multiplexing portion in the second direction is 2.0 μm or more.

7. a length of the first branching waveguide in the second direction at a connection end of the first branching waveguide connected to the multiplexing portion is 23% or more and 47% or less of a length of the multiplexing portion in the second direction, 6. The optical element according to claim 5, wherein a length of the second branching waveguide in the second direction at a connection end of the second branching waveguide connected to the multiplexing portion is 23% or more and 47% or less of a length of the multiplexing portion in the second direction.

8. The core layer is A first mode converter that converts the polarization mode of red light from the TM0 mode to the TE0 mode; A second mode converter that converts the polarization mode of green light from the TM0 mode to the TE0 mode; A third mode converter that converts the polarization mode of blue light from the TM0 mode to the TE0 mode; a multiplexer that multiplexes the red light, the green light, and the blue light and emits a laser beam; The optical element according to claim 1 or 2, comprising:

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

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

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

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

13. A retinal projection device according to claim 12; A lens provided with the reflector; A near-eye trouble device comprising:

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