Optical element and laser module

Abstract

To provide an optical element and a laser module for which downsizing is possible.SOLUTION: A core layer of the optical element comprises a conversion unit 42 for converting a light polarization mode from a TM0 mode into a TE1 mode, and a conversion unit 43 for converting a light polarization mode from the TE1 mode into a TE0 mode. The conversion unit 43 includes: a branch unit 52 for branching the light of the TE1 mode entering from the conversion unit 42 into first branched light of the TE0 mode and second branched light of the TE0 mode which are out of phase with each other; a branch waveguide 53 which extends in the direction of an axis X, and through which the first branched light propagates; a branch waveguide 54 which extends in the direction of the axis X, and through which the second branched light propagates; a combining unit 55 for combining the first branched light having propagated through the branch waveguide 53 and the second branched light having propagated through the branch waveguide 54, and emitting light of the TE0 mode; and a phase adjustment unit 57 for adjusting the phase difference between the first branched light and the second branched light. The branch waveguide 53 and the branch waveguide 54 are arranged in the direction of an axis Y.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to an optical device and a laser module. [Background technology]

[0002] The polarization modes of light propagating through an optical waveguide include a TE (Transverse Electric) mode, which is a polarization mode having a main electric field horizontal to the substrate, and a TM (Transverse Magnetic) mode, which is a polarization mode having a main electric field vertical to the substrate. Optical waveguide elements that convert these polarization modes are known. For example, Patent Document 1 describes a mode conversion element that includes a polarization rotation unit that converts TM0 mode light into TE1 mode light, and a mode order conversion unit that converts TE1 mode light into TE0 mode light. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-196393 A Summary of the Invention [Problem to be solved by the invention]

[0004] By the way, an optical modulator is known that converts an electrical signal into an optical signal by modulating the light intensity of a laser light. For example, it is possible to modulate the light intensity of the TE0 mode light converted by the mode conversion element by providing an optical modulator after the mode conversion element described in Patent Document 1. However, since an optical modulator is provided in addition to the mode conversion element, there is a risk that the device will become large in size.

[0005] The present disclosure describes optical elements and laser modules that can be miniaturized. [Means for solving the problem]

[0006] An optical element according to one aspect of the present disclosure includes a substrate having a main surface, and a core layer provided on the main surface and made of a material having an electro-optic effect. The core layer includes a mode converter that converts the polarization mode of light from TM0 mode to TE0 mode. The mode converter includes a first converter that converts the polarization mode of light from TM0 mode to TE1 mode, and a second converter that converts the polarization mode of light from TE1 mode to TE0 mode. The second conversion unit includes a branching unit that branches the light in TE1 mode incident from the first conversion unit into a first branched light in TE0 mode and a second branched light in TE0 mode that are in opposite phase to each other, a first branch waveguide that extends in a first direction along the main surface and through which the first branched light propagates, a second branch waveguide that extends in the first direction and through which the second branched light propagates, a coupling unit that couples the first branched light propagated through the first branch waveguide and the second branched light propagated through the second branch waveguide to output light in TE0 mode, and a phase adjustment unit that adjusts the phase difference between the first branched light and the second branched light. The first branch waveguide and the second branch waveguide are arranged in a second direction that intersects the first direction.

[0007] In this optical element, the first conversion unit converts the polarization mode of the light from TM0 mode to TE1 mode, and the second conversion unit converts the polarization mode of the light from TE1 mode to TE0 mode. In the second conversion unit, the light in TE1 mode incident from the first conversion unit is branched into first and second branched lights in TE0 mode, which are in opposite phase to each other, and the phase difference between the first and second branched lights is adjusted, and the first and second branched lights are coupled to each other, thereby emitting light in TE0 mode. Since the optical intensity of the light emitted from the coupling unit can be changed according to the phase difference between the first and second branched lights, the second conversion unit can also function as an optical modulator. Therefore, since there is no need to provide an optical modulator, it is possible to miniaturize the optical element.

[0008] The phase adjustment unit may include a signal electrode disposed between the first branch waveguide and the second branch waveguide, and a first ground electrode and a second ground electrode disposed to sandwich the first branch waveguide and the second branch waveguide in the second direction. The optical axis of the material constituting the core layer may extend in the second direction. According to the above configuration, the first branch waveguide is disposed between the signal electrode and the first ground electrode, and the second branch waveguide is disposed between the signal electrode and the second ground electrode. Therefore, when a voltage is applied between the signal electrode and each ground electrode, a voltage is applied to the first branch waveguide and the second branch waveguide in the second direction. The polarization mode of the first branch light propagating through the first branch waveguide is the TE0 mode, and the polarization mode of the second branch light propagating through the second branch waveguide is the TE0 mode. Therefore, in the first branch waveguide, a voltage is applied in the direction of the optical axis of the material constituting the core layer, and the direction of the main electric field of the light propagating through the first branch waveguide coincides with the direction of the optical axis, so a large electro-optic effect is obtained. Similarly, in the second branch waveguide, a voltage is applied in the direction of the optical axis of the material constituting the core layer, and the direction of the main electric field of the light propagating through the second branch waveguide coincides with the direction of the optical axis, so a large electro-optic effect is obtained. As a result, it is possible to improve the modulation efficiency in the second conversion section.

[0009] The light may be visible light. In this case, it is possible to convert the polarization mode of the visible light from the TM0 mode to the TE0 mode.

[0010] The length of the branching portion in the first direction may be 40 μm or more and 64 μm or less. The length of the branching portion in the second direction may be 3.0 μm or more and 3.5 μm or less. In this case, the loss of light intensity in the conversion from the TE1 mode to the TE0 mode can be reduced. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0011] The core layer may further 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 output a laser light. In order to output a full-color laser light by multiplexing red light, green light, and blue light, it is necessary to adjust the light intensity of each color of light according to the color to be output. According to the above configuration, the light intensity of the red light, the light intensity of the green light, and the light intensity of the blue light are modulated by the second conversion unit of each mode converter, so that a full-color laser light can be output.

[0012] A laser module according to another aspect of the present disclosure includes the optical element, a first light source that emits red light in TM0 mode, a second light source that emits green light in TM0 mode, and a third light source that emits blue light in TM0 mode. Since the laser module includes the optical element, it is possible to reduce the size of the laser module. Effect of the Invention

[0013] According to each aspect and embodiment of the present disclosure, it is possible to reduce the size of an optical element and a laser module. [Brief description of the drawings]

[0014] [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 plan view of the laser module shown in FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. [Diagram 5]FIG. 5 is an enlarged view of a portion of the mode converter shown in FIG. [Figure 6] FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. [Figure 7] FIG. 7 is an enlarged view of the modulator shown in FIG. [Figure 8] FIG. 8 is a cross-sectional view taken along line VIII-VIII in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] 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. 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.

[0016] 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.

[0017] 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 have an inner surface 3a (see FIG. 2) that faces the eyeball of a user wearing the near eyewear wearable device 1.

[0018] 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.

[0019] 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 11 and a reflector 12.

[0020] The optical engine 11 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 12. The optical engine 11 is mounted on the temple 2c. The optical engine 11 includes a laser module 13, an optical component 14, a movable mirror 15, a laser driver 16, a mirror driver 17, and a controller 18.

[0021] The laser module 13 emits a laser beam. For example, a full-color laser module is used as the laser module 13. The laser module 13 emits a laser beam of a color and light intensity corresponding to the pixels of an image to be projected onto the retina. The laser module 13 will be described in detail later.

[0022] The optical component 14 is a component that optically processes the laser light emitted from the laser module 13. In this embodiment, the optical component 14 includes a collimator lens 14a, a slit 14b, and a neutral density filter 14c. The collimator lens 14a, the slit 14b, and the neutral density filter 14c are arranged in this order along the optical path of the laser light. The optical component 14 may have other configurations.

[0023] The movable mirror 15 is a member for performing scanning with the laser light Ls. The movable mirror 15 is provided in the emission direction of the laser light processed by the optical component 14. The movable mirror 15 is configured to be swingable, for example, around an axis extending in the horizontal direction of the lens 3 and an axis extending in the vertical direction of the lens 3, and reflects the laser light by changing the angle in the horizontal direction and vertical direction of the lens 3. For example, a MEMS (Micro Electro Mechanical Systems) mirror is used as the movable mirror 15.

[0024] The laser driver 16 is a drive circuit that drives the laser module 13. The laser driver 16 drives the laser module 13 based on, for example, the optical power (light intensity) of the laser light and the temperature of a light source unit 20 (see FIG. 3) included in the laser module 13. The mirror driver 17 is a drive circuit that drives the movable mirror 15. The mirror driver 17 swings the movable mirror 15 within a predetermined angle range and at a predetermined timing. The controller 18 is a device that controls the laser driver 16 and the mirror driver 17.

[0025] In the optical engine 11, a laser beam having a color and light intensity corresponding to a pixel of an image to be projected onto the retina is emitted from the laser module 13, passes through the optical component 14, and is reflected by the movable mirror 15. The laser beam reflected by the movable mirror 15 is emitted to the reflector 12 as the laser beam Ls.

[0026] The reflector 12 is a member that reflects the laser light Ls that has passed through the movable mirror 15 and irradiates the retina of a user wearing the near eyewear wearable device 1 with reflected light Lr, thereby projecting an image onto the retina.

[0027] Next, the laser module 13 will be described in detail with reference to Fig. 3 and Fig. 4. Fig. 3 is a plan view of the laser module shown in Fig. 2. Fig. 4 is a cross-sectional view taken along line IV-IV in Fig. 3. For ease of explanation, the cladding layer 33 is omitted in Fig. 3. As shown in Fig. 3, the laser module 13 includes a light source unit 20 and an optical element 30.

[0028] The light source unit 20 emits visible light. The light source unit 20 includes a red laser diode 21 (first light source) that emits red light, a green laser diode 22 (second light source) that emits green light, and a blue laser diode 23 (third light source) that emits blue light. The peak wavelength of the red light is, for example, in the range of 600 nm to 830 nm. The peak wavelength of the green light is, for example, in the range of 500 nm to 570 nm. The peak wavelength of the blue light is, for example, in the range of 380 nm to 490 nm. The red laser diode 21, the green laser diode 22, and the blue laser diode 23 are arranged in that order in the Y-axis direction.

[0029] In this embodiment, the red laser diode 21 emits red light in the TM fundamental mode (hereinafter referred to as "TM0 mode"). The green laser diode 22 emits green light in the TM0 mode. The blue laser diode 23 emits blue light in the TM0 mode.

[0030] The optical element 30 combines the laser beams emitted from the laser diodes into one laser beam. The optical element 30 is, for example, a planar lightwave circuit (PLC). As shown in FIG. 4, the optical element 30 includes a substrate 31, a core layer 32, and a cladding layer 33.

[0031] The substrate 31 functions as a lower cladding layer. The substrate 31 is made of a material having a lower refractive index than the material of the core layer 32. Examples of materials for the substrate 31 include sapphire, silicon oxide, and silicon on which silicon oxide is laminated. The substrate 31 has a main surface 31a and a back surface 31b on the opposite side to the main surface 31a. The main surface 31a and the back surface 31b are planes defined by the X-axis direction and the Y-axis direction, and intersect with the Z-axis direction (in this embodiment, they are perpendicular). In other words, the X-axis direction and the Y-axis direction are directions along the main surface 31a.

[0032] The cladding layer 33 functions as an upper cladding layer. The cladding layer 33 covers the core layer 32 on the main surface 31a. The cladding layer 33 is provided over the entire main surface 31a. The cladding layer 33 is made of a material having a lower refractive index than the material of the core layer 32. An example of the material of the cladding layer 33 is silicon oxide (e.g., SiO2).

[0033] The core layer 32 is provided on the main surface 31a. The core layer 32 is made of a material having an electro-optic effect. The electro-optic effect is a phenomenon in which the refractive index of a material changes when an electric field is applied to the material. An example of a material for the core layer 32 is lithium niobate (LiNbO3). In this embodiment, the core layer 32 is made of X-cut lithium niobate, and the optical axis (C-axis) of the lithium niobate extends in the Y-axis direction. The core layer 32 includes a mode converter 34R (first mode converter), a mode converter 34G (second mode converter), a mode converter 34B (third mode converter), and a multiplexer 36.

[0034] The mode converter 34R is a mode converter that converts the polarization mode of red light from TM0 mode to TE fundamental mode (hereinafter referred to as "TE0 mode"). The mode converter 34G is a mode converter that converts the polarization mode of green light from TM0 mode to TE0 mode. The mode converter 34B is a mode converter that converts the polarization mode of blue light from TM0 mode to TE0 mode. The polarization mode is also called a guided mode. Each of the mode converter 34R, the mode converter 34G, and the mode converter 34B extends in the X-axis direction. The mode converter 34R, the mode converter 34G, and the mode converter 34B are arranged in that order in the Y-axis direction. The detailed configuration of each mode converter will be described later.

[0035] The multiplexer 36 multiplexes the red light, the green light, and the blue light. The multiplexer 36 multiplexes the red light emitted from the mode converter 34R, the green light emitted from the mode converter 34G, and the blue light emitted from the mode converter 34B into one laser light, and emits the laser light. The laser light includes a component having a red wavelength (red component), a component having a green wavelength (green component), and a component having a blue wavelength (blue component).

[0036] Next, detailed configurations of the mode converters 34R, 34G, and 34B will be described with further reference to FIGS. 5 to 8. FIG. 5 is an enlarged view of a part of the mode converter shown in FIG. 3. FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. 5. FIG. 7 is an enlarged view of the modulator shown in FIG. 3. FIG. 8 is a cross-sectional view taken along line VIII-VIII in FIG. 7. As shown in FIG. 3, each of the mode converters 34R, 34G, and 34B includes an incident portion 41 that receives visible light, a conversion portion 42 (first conversion portion) that converts the polarization mode of the visible light from TM0 mode to TE primary mode (hereinafter referred to as "TE1 mode"), and a conversion portion 43 (second conversion portion) that converts the polarization mode of the visible light from TE1 mode to TE0 mode. The mode converters 34R, 34G, and 34B have the same configuration, so the configuration of the mode converter 34R will be described here.

[0037] 3 and 5, the incident portion 41 is an optical waveguide located at one end (incident end) in the X-axis direction of the mode converter 34R. The incident portion 41 is provided on the main surface 31a and extends in the X-axis direction. Red light in TM0 mode is incident on one end of the incident portion 41 in the X-axis direction from the red laser diode 21. The incident portion 41 transmits the red light while maintaining the polarization mode of the red light, and outputs the red light in TM0 mode to the conversion portion 42.

[0038] A cross section of the incident portion 41 intersecting (orthogonal to) the X-axis direction has a rectangular shape. The cross section may have a trapezoidal shape. The length of the incident portion 41 in the X-axis direction is, for example, 50 μm. It is substantially constant over the entire length of the incident portion 41 in the X-axis direction. The length of the incident portion 41 in the Y-axis direction is substantially constant over the entire length of the incident portion 41 in the X-axis direction. Hereinafter, the length in the Y-axis direction may be referred to as "width". The width of the incident portion 41 is width W0. The width W0 is, for example, 0.45 μm. The length of the incident portion 41 in the Z-axis direction is substantially constant over the entire length of the incident portion 41 in the X-axis direction. Hereinafter, the length in the Z-axis direction may be referred to as "height". The height of the incident portion 41 is height T0. The height T0 is, for example, 0.7 μm.

[0039] 3, 5 and 6, the conversion unit 42 is provided between the incident unit 41 and the conversion unit 43, and is a part that converts the polarization mode of visible light from TM0 mode to TE1 mode. The conversion unit 42 is provided on the main surface 31a. The conversion unit 42 has a connection end 42a and a connection end 42b, which are both ends in the X-axis direction. The connection end 42a is connected to the other end of the incident unit 41 in the X-axis direction. The connection end 42b is connected to one end (incident end) of the conversion unit 43 in the X-axis direction.

[0040] The conversion unit 42 is divided into a conversion region 42d and a conversion region 42e at the intermediate position 42c. The intermediate position 42c is a position between the connection end 42a and the connection end 42b in the X-axis direction. The conversion region 42d is a region of the conversion unit 42 from the connection end 42a to the intermediate position 42c. The conversion region 42e is a region of the conversion unit 42 from the intermediate position 42c to the connection end 42b. The length L11 of the conversion region 42d in the X-axis direction is, for example, 350 μm to 1000 μm. The length L12 of the conversion region 42e in the X-axis direction is, for example, 10 μm to 80 μm. The length L1 of the conversion unit 42 in the X-axis direction is the sum of the length L11 and the length L12, and is, for example, 360 μm to 1010 μm.

[0041] The conversion section 42 includes an upper tapered section 46 and a lower tapered section 47. The upper tapered section 46 and the lower tapered section 47 are stacked in the Z-axis direction. Specifically, the lower tapered section 47 is provided on the main surface 31a, and the upper tapered section 46 is provided on the lower tapered section 47. The height of the upper tapered section 46 is substantially constant over the entire length of the upper tapered section 46 in the X-axis direction. The height of the upper tapered section 46 is height T11. The height T11 is, for example, 0.5 μm. The height of the lower tapered section 47 is substantially constant over the entire length of the lower tapered section 47 in the X-axis direction. The height of the lower tapered section 47 is height T12. The height T12 is, for example, 0.2 μm. The height of the conversion section 42 is the sum of the height of the upper tapered section 46 and the height of the lower tapered section 47, and is height T0.

[0042] In the conversion region 42d, the width of the upper tapered portion 46 increases continuously from width W0 to width Wt from the connection end 42a to the intermediate position 42c. The width Wt is larger than the width W0. The width Wt is, for example, 0.5 μm to 1.0 μm. The rate of increase in the width of the upper tapered portion 46 in the conversion region 42d may be substantially constant. In the conversion region 42e, the width of the upper tapered portion 46 increases continuously from width Wt to ​​width W1 from the intermediate position 42c to the connection end 42b. The width W1 is larger than the width Wt and smaller than a width Ws described below. The width W1 is, for example, 0.85 μm. The rate of increase in the width of the upper tapered portion 46 in the conversion region 42e may be substantially constant. The upper tapered portion 46 has a shape that is plane-symmetrical with respect to a plane of symmetry SP defined by the X-axis direction and the Z-axis direction.

[0043] In the conversion region 42d, the width of the lower tapered portion 47 increases continuously from width W0 to width Ws from the connection end 42a to the intermediate position 42c. Width Ws is greater than width Wt. Width Ws is, for example, 1.0 μm to 5.0 μm. The rate of increase in the width of the lower tapered portion 47 in the conversion region 42d may be substantially constant. In the conversion region 42e, the width of the lower tapered portion 47 decreases continuously from width Ws to width W1 from the intermediate position 42c to the connection end 42b. The rate of decrease in the width of the lower tapered portion 47 in the conversion region 42e may be substantially constant. The lower tapered portion 47 has a shape that is plane-symmetrical with respect to the plane of symmetry SP.

[0044] In the conversion section 42 configured as described above, the upper tapered section 46 and the lower tapered section 47 are stacked in the Z-axis direction, so that the conversion section 42 has asymmetry in the Z-axis direction. Here, asymmetry in the Z-axis direction means that two parts separated by a symmetry plane that passes through the center of the conversion section 42 in the Z-axis direction and is perpendicular to the Z-axis direction are not plane-symmetric.

[0045] In the conversion section 42, the effective refractive index of the TM0 mode and the effective refractive index of the TE1 mode approach each other and cross each other as the distance from the connection end 42a increases in the X-axis direction, and thus conversion between the TM0 mode and the TE1 mode is induced. Therefore, when red light in the TM0 mode is incident on the connection end 42a, the polarization mode of the red light is converted from the TM0 mode to the TE1 mode in the conversion section 42. In the conversion region 42d, the width of the lower tapered portion 47 increases more than the width of the upper tapered portion 46 as the distance from the connection end 42a increases. This configuration makes it possible to improve the conversion efficiency while shortening the length of the conversion section 42 in the X-axis direction. In the conversion region 42e, the width of the lower tapered portion 47 decreases as the distance from the connection end 42b increases, and the width of the upper tapered portion 46 and the width of the lower tapered portion 47 become equal at the connection end 42b. This configuration makes it possible to further improve the conversion efficiency.

[0046] For example, about 50% to 60% of the conversion from TM0 mode to TE1 mode occurs in the conversion region 42d, and about 20% to 30% of the conversion from TM0 mode to TE1 mode occurs in the conversion region 42e. Meanwhile, over the entire length in the X-axis direction of the conversion unit 42, the effective refractive index of the TE0 mode is sufficiently different from the effective refractive indexes of the TM0 mode and TE1 mode. Therefore, when red light in the TE0 mode is incident on the connection end 42a, the polarization mode of the red light is maintained in the TE0 mode.

[0047] As shown in FIG. 7, the conversion section 43 is a Mach-Zehnder type optical waveguide, and includes an input waveguide 51, a branching section 52, a branching waveguide 53 (first branching waveguide), a branching waveguide 54 (second branching waveguide), a coupling section 55, an output waveguide 56, a phase adjustment section 57, and a slab 58 (see FIG. 8).

[0048] 8, the slab 58 is provided on the main surface 31a. The slab 58 has a flat plate shape. The height of the slab 58 is height T22. The height T22 is, for example, 0.2 μm.

[0049] The input waveguide 51 is an optical waveguide located at one end (incident end) of the conversion unit 43 in the X-axis direction. The input waveguide 51 is provided on a slab 58 and extends in the X-axis direction. One end of the input waveguide 51 in the X-axis direction is connected to the connection end 42b. The other end of the input waveguide 51 in the X-axis direction is connected to the input end of the branching unit 52. Red light in the TE1 mode (hereinafter, sometimes referred to as "light Lin") is incident on the input waveguide 51 from the conversion unit 42. The input waveguide 51 transmits the light Lin while maintaining the polarization mode of the light Lin, and outputs the light Lin in the TE1 mode to the branching unit 52.

[0050] A cross section of the input waveguide 51 intersecting (orthogonal to) the X-axis direction has a rectangular shape. The cross section may have a trapezoidal shape. The length of the input waveguide 51 in the X-axis direction is, for example, 50 μm. The width of the input waveguide 51 is substantially constant over the entire length of the input waveguide 51 in the X-axis direction. The width of the input waveguide 51 is width W1. The height of the input waveguide 51 is substantially constant over the entire length of the input waveguide 51 in the X-axis direction. The height of the input waveguide 51 is height T21. Height T21 is, for example, 0.5 μm.

[0051] The branching unit 52 branches the light Lin of the TE1 mode incident from the input waveguide 51 into branched light Ld1 (first branched light) of the TE0 mode and branched light Ld2 (second branched light) of the TE0 mode. The branched light Ld1 and the branched light Ld2 have opposite phases. The optical intensity of the branched light Ld1 and the optical intensity of the branched light Ld2 are substantially half (50%) of the optical intensity of the light Lin. In this embodiment, the branching unit 52 is configured by a multimode interferometer (MMI). The branching unit 52 may be configured by a Y-branch type waveguide or a directional coupler. The branching unit 52 is provided on a slab 58. The branching unit 52 outputs the branched light Ld1 to the branching waveguide 53 and outputs the branched light Ld2 to the branching waveguide 54.

[0052] A cross section of the branching portion 52 intersecting (orthogonal to) the X-axis direction has a rectangular shape. The cross section may have a trapezoidal shape. The length L21 of the branching portion 52 in the X-axis direction is, for example, 40 μm to 64 μm. The branching portion 52 has a rectangular shape when viewed from the Z-axis direction. The width of the branching portion 52 is substantially constant over the entire length of the branching portion 52 in the X-axis direction. The width of the branching portion 52 is width W21. The width W21 is, for example, 3.0 μm to 3.5 μm. The height of the branching portion 52 is substantially constant over the entire length of the branching portion 52 in the X-axis direction. The height of the branching portion 52 is height T21.

[0053] The branch waveguide 53 is an optical waveguide through which the branch light Ld1 propagates. The branch waveguide 54 is an optical waveguide through which the branch light Ld2 propagates. The branch waveguide 53 and the branch waveguide 54 each extend in the X-axis direction from the branching portion 52 to the coupling portion 55. The branch waveguide 53 and the branch waveguide 54 are provided on a slab 58 and arranged in the Y-axis direction. In the vicinity of the branching portion 52, the branch waveguide 53 and the branch waveguide 54 extend so as to move away from each other in the Y-axis direction as they move away from the branching portion 52, and in the vicinity of the coupling portion 55, the branch waveguide 53 and the branch waveguide 54 extend so as to move closer to each other in the Y-axis direction as they move closer to the coupling portion 55. The branch waveguide 53 and the branch waveguide 54 extend substantially parallel between the vicinity of the branching portion 52 and the vicinity of the coupling portion 55.

[0054] One end of the branching waveguide 53 in the X-axis direction is connected to the output end of the branching unit 52. The other end of the branching waveguide 53 in the X-axis direction is connected to the input end of the coupling unit 55. The branched light Ld1 in the TE0 mode is input from the branching unit 52 to one end of the branching waveguide 53. The branching waveguide 53 transmits the branched light Ld1 while maintaining the polarization mode of the branched light Ld1, and outputs the branched light Ld1 in the TE0 mode to the coupling unit 55 from the other end.

[0055] One end of the branching waveguide 54 in the X-axis direction is connected to another output end of the branching unit 52. The other end of the branching waveguide 54 in the X-axis direction is connected to another input end of the coupling unit 55. The branched light Ld2 in the TE0 mode is input from the branching unit 52 to one end of the branching waveguide 54. The branching waveguide 54 transmits the branched light Ld2 while maintaining the polarization mode of the branched light Ld2, and outputs the branched light Ld2 in the TE0 mode to the coupling unit 55 from the other end.

[0056] The cross sections of the branch waveguide 53 and the branch waveguide 54 intersecting (orthogonal to) the X-axis direction have a rectangular shape. The cross sections may have a trapezoidal shape. The length of the branch waveguide 53 in the X-axis direction is, for example, 11 mm. The width of the branch waveguide 53 is substantially constant over the entire length of the branch waveguide 53. The width of the branch waveguide 53 is, for example, 0.8 μm. The height of the branch waveguide 53 is substantially constant over the entire length of the branch waveguide 53. The height of the branch waveguide 53 is height T21. The length of the branch waveguide 54 in the X-axis direction is substantially the same as the length of the branch waveguide 53 in the X-axis direction. The width of the branch waveguide 54 is substantially the same as the width of the branch waveguide 53 and is substantially constant over the entire length of the branch waveguide 54. The height of the branch waveguide 54 is substantially the same as the height of the branch waveguide 53 and is substantially constant over the entire length of the branch waveguide 54.

[0057] The coupling unit 55 couples the branched light Ld1 propagated through the branching waveguide 53 and the branched light Ld2 propagated through the branching waveguide 54, and emits red light in TE0 mode (hereinafter, may be referred to as "light Lout"). In this embodiment, the coupling unit 55 is configured by an MMI. The coupling unit 55 may be configured by a Y-branch type waveguide or a directional coupler. The coupling unit 55 is provided on a slab 58. The coupling unit 55 emits the light Lout to the output waveguide 56.

[0058] A cross section of the joint 55 intersecting (orthogonal to) the X-axis direction has a rectangular shape. The cross section may have a trapezoidal shape. The length L22 of the joint 55 in the X-axis direction is, for example, 63 μm to 100 μm. The joint 55 has a rectangular shape when viewed from the Z-axis direction. The width of the joint 55 is substantially constant over the entire length of the joint 55 in the X-axis direction. The width of the joint 55 is width W22. The width W22 is, for example, 6 μm to 10 μm. The height of the joint 55 is substantially constant over the entire length of the joint 55 in the X-axis direction. The height of the joint 55 is height T21.

[0059] The output waveguide 56 is an optical waveguide located at the other end (output end) in the X-axis direction of the conversion unit 43. The output waveguide 56 is provided on the slab 58 and extends in the X-axis direction. One end of the output waveguide 56 in the X-axis direction is connected to the output end of the coupling unit 55. The other end of the output waveguide 56 in the X-axis direction is connected to the input end of the multiplexer 36. The light Lout in the TE0 mode is input from the coupling unit 55 to the output waveguide 56. The output waveguide 56 transmits the light Lout while maintaining the polarization mode of the light Lout, and outputs the light Lout in the TE0 mode to the multiplexer 36.

[0060] A cross section of the output waveguide 56 intersecting (orthogonal to) the X-axis direction has a rectangular shape. The cross section may have a trapezoidal shape. The length of the output waveguide 56 in the X-axis direction is, for example, 50 μm. The width of the output waveguide 56 is substantially constant over the entire length of the output waveguide 56 in the X-axis direction. The width of the output waveguide 56 is substantially the same as the width of the branch waveguide 53. The height of the output waveguide 56 is substantially constant over the entire length of the output waveguide 56 in the X-axis direction. The height of the output waveguide 56 is height T21.

[0061] The phase adjustment unit 57 adjusts the phase difference between the branched light Ld1 and the branched light Ld2. In this embodiment, the phase adjustment unit 57 includes a signal electrode 71, a ground electrode 72 (first ground electrode), a ground electrode 73 (second ground electrode), a signal source 74, and a termination resistor 75.

[0062] The signal electrode 71, the ground electrode 72, and the ground electrode 73 are provided on the slab 58 and extend in the X-axis direction. The signal electrode 71 is disposed between the branch waveguide 53 and the branch waveguide 54. The ground electrode 72 and the ground electrode 73 are disposed so as to sandwich the branch waveguide 53 and the branch waveguide 54 in the Y-axis direction. In other words, the ground electrode 72, the branch waveguide 53, the signal electrode 71, the branch waveguide 54, and the ground electrode 73 are disposed in that order at substantially equal intervals in the Y-axis direction. The cladding layer 33 does not cover a portion of the signal electrode 71, the ground electrode 72, and the ground electrode 73, and a portion of the upper surface of the signal electrode 71, a portion of the upper surface of the ground electrode 72, and a portion of the upper surface of the ground electrode 73 are exposed.

[0063] The cross sections of the signal electrode 71, the ground electrode 72, and the ground electrode 73 intersecting (orthogonal to) the X-axis direction have a rectangular shape. The length of the signal electrode 71 in the X-axis direction is, for example, 10 mm. The width of the signal electrode 71 is substantially constant over the entire length of the signal electrode 71 in the X-axis direction. The width of the signal electrode 71 is, for example, 5 μm. The height of the signal electrode 71 is substantially constant over the entire length of the signal electrode 71 in the X-axis direction. The height of the signal electrode 71 is height T21.

[0064] The length of the ground electrode 72 in the X-axis direction is, for example, 10 mm. The width of the ground electrode 72 is substantially constant over the entire length of the ground electrode 72 in the X-axis direction. The width of the ground electrode 72 is, for example, 200 μm. The height of the ground electrode 72 is substantially constant over the entire length of the ground electrode 72. The height of the ground electrode 72 is height T21. The length of the ground electrode 73 in the X-axis direction is substantially the same as the length of the ground electrode 72 in the X-axis direction. The width of the ground electrode 73 is substantially the same as the width of the ground electrode 72 and is substantially constant over the entire length of the ground electrode 73 in the X-axis direction. The height of the ground electrode 73 is substantially the same as the height of the ground electrode 72 and is substantially constant over the entire length of the ground electrode 73 in the X-axis direction.

[0065] The signal source 74 supplies a modulation signal for modulating the light intensity of the red light propagating through the conversion unit 43. One end of the signal source 74 is electrically connected to one end of the signal electrode 71, and the other end of the signal source 74 is electrically connected to one end of the ground electrode 72 and one end of the ground electrode 73.

[0066] The termination resistor 75 electrically terminates the modulated signal. One end of the termination resistor 75 is electrically connected to the other end of the signal electrode 71, and the other end of the termination resistor 75 is electrically connected to the other end of the ground electrode 72 and the other end of the ground electrode 73.

[0067] In the conversion unit 43 configured as above, when the light Lin in the TE1 mode is incident on the input waveguide 51, it is branched by the branching unit 52 into branched lights Ld1 and Ld2 in the TE0 mode which are in opposite phase to each other, and the branched lights Ld1 and Ld2 are emitted to and propagate through the branching waveguides 53 and 54, respectively. When a modulation signal is supplied from the signal source 74 to the signal electrode 71, the ground electrode 72, and the ground electrode 73, a potential difference according to the modulation signal is generated between the signal electrode 71 and the ground electrode 72, and between the signal electrode 71 and the ground electrode 73, and a voltage in the Y-axis direction is applied to the branching waveguides 53 and 54.

[0068] As a result, the refractive index of the branch waveguide 53 changes according to the voltage applied to the branch waveguide 53, and the refractive index of the branch waveguide 54 changes according to the voltage applied to the branch waveguide 54. In this embodiment, the voltage applied to the branch waveguide 54 has the opposite polarity and the same magnitude as the voltage applied to the branch waveguide 53. Therefore, a difference occurs between the refractive index of the branch waveguide 53 and the refractive index of the branch waveguide 54, and the phase difference between the branch light Ld1 and the branch light Ld2 is changed from 180° according to the difference. Then, the branch light Ld1 propagated through the branch waveguide 53 and the branch light Ld2 propagated through the branch waveguide 54 are coupled at the coupling portion 55.

[0069] At this time, the optical intensity of the combined light Lout varies depending on the phase difference between the branched light Ld1 and the branched light Ld2. For example, when the phase difference between the branched light Ld1 and the branched light Ld2 is 180°, the optical intensity of the light Lout is 0%, and the light Lout is not output. When the branched light Ld1 and the branched light Ld2 are in phase, the optical intensity of the light Lout is substantially the same (100%) as the optical intensity of the light Lin. Therefore, the optical intensity of the light Lout can be adjusted within the range of 0% to 100% of the optical intensity of the light Lin. Then, the light Lout is output to the multiplexer 36 via the output waveguide 56.

[0070] In the laser module 13 and the optical element 30 described above, the polarization mode of the visible light is converted from the TM0 mode to the TE1 mode by the conversion unit 42, and the polarization mode of the visible light is converted from the TE1 mode to the TE0 mode by the conversion unit 43. Therefore, it is possible to convert the polarization mode of the visible light from the TM0 mode to the TE0 mode. Furthermore, in the conversion unit 43, the light Lin in the TE1 mode incident from the conversion unit 42 is branched into branched lights Ld1 and Ld2 in the TE0 mode, which are in opposite phase to each other, and the phase difference between the branched lights Ld1 and Ld2 is adjusted, and the branched lights Ld1 and Ld2 are combined to output the light Lout in the TE0 mode. Since the light intensity of the light Lout output from the coupling unit 55 can be changed according to the phase difference between the branched lights Ld1 and Ld2, the conversion unit 43 can also function as an optical modulator. Therefore, since there is no need to provide an optical modulator, it is possible to miniaturize the laser module 13 and the optical element 30.

[0071] It is known that the electro-optical characteristics of a device made of a material having an electro-optical effect, such as lithium niobate, depend on the direction of voltage application and the direction of polarization of light. For example, when a voltage is applied in the C-axis direction of the material having an electro-optical effect and the direction of the main electric field of light propagating through the device coincides with the C-axis direction, a large electro-optical effect is obtained. In the laser module 13 and the optical element 30, the core layer 32 is made of X-cut lithium niobate, and the C-axis of the lithium niobate extends in the Y-axis direction.

[0072] On the other hand, the branch waveguide 53 is disposed between the signal electrode 71 and the ground electrode 72, and the branch waveguide 54 is disposed between the signal electrode 71 and the ground electrode 73. Therefore, when a voltage is applied between the signal electrode 71 and the ground electrode 72, a voltage is applied to the branch waveguide 53 in the Y-axis direction. Similarly, when a voltage is applied between the signal electrode 71 and the ground electrode 73, a voltage is applied to the branch waveguide 54 in the Y-axis direction. Furthermore, the branching section 52 branches the light Lin in the TE1 mode into branched light Ld1 and branched light Ld2 in the TE0 mode, and the branched light Ld1 is emitted to the branch waveguide 53 and the branched light Ld2 is emitted to the branch waveguide 54.

[0073] Therefore, in the branch waveguide 53, a voltage is applied in the C-axis direction of the material constituting the core layer 32, and the direction of the main electric field of the branched light Ld1 propagating through the branch waveguide 53 coincides with the C-axis direction, so that a large electro-optic effect is obtained. Similarly, in the branch waveguide 54, a voltage is applied in the C-axis direction of the material constituting the core layer 32, and the direction of the main electric field of the branched light Ld2 propagating through the branch waveguide 54 coincides with the C-axis direction, so that a large electro-optic effect is obtained. As a result, it is possible to improve the modulation efficiency in the conversion section 43.

[0074] The length of the branching portion 52 in the X-axis direction may be 40 μm or more and 64 μm or less. The width of the branching portion 52 may be 3.0 μm or more and 3.5 μm or less. In this case, the loss of light intensity in the conversion from the TE1 mode to the TE0 mode can be reduced. Therefore, the conversion efficiency from the TE1 mode to the TE0 mode can be improved.

[0075] In order to output full-color laser light by combining red, green, and blue light, it is necessary to adjust the light intensity of each color of light according to the color to be output. In order to change the light intensity of each color of light in the light source unit 20, a large drive current is required. In the laser module 13 and the optical element 30, the light intensity of the red light is modulated by the conversion unit 43 of the mode converter 34R, the light intensity of the green light is modulated by the conversion unit 43 of the mode converter 34G, and the light intensity of the blue light is modulated by the conversion unit 43 of the mode converter 34B. Therefore, it is possible to output full-color laser light without requiring a large drive current.

[0076] The optical element and laser module according to the present disclosure are not limited to the above-described embodiments.

[0077] For example, the laser module 13 may be applied to a device other than the near eyewear wearable device 1.

[0078] The optical element 30 may not include the cladding layer 33. In this case, an air layer may function as an upper cladding layer.

[0079] The optical element 30 may include one mode converter. In other words, the core layer 32 may include one mode converter that converts the polarization mode of visible light from the TM0 mode to the TE0 mode.

[0080] The light emitted from the light source unit 20 is not limited to visible light. In this case, the core layer 32 includes a mode converter that converts the polarization mode of the light from the TM0 mode to the TE0 mode.

[0081] The structure of the conversion section 42 is not limited to the structure in which the upper tapered section 46 and the lower tapered section 47 are stacked in the Z-axis direction. The conversion section 42 may be any structure that can convert the polarization mode of light from the TM0 mode to the TE1 mode. EXAMPLES

[0082] 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.

[0083] <Evaluation of conversion loss in mode converters> The conversion loss in the mode converter having the structure of Examples 1 to 7 was calculated. The same structure as the mode converters 34R, 34G, and 34B shown in FIG. 3 was used as the mode converters of Examples 1 to 7. As shown in Table 1, the width W0, width Wt, width Ws, width W1, width W21, width W22, length L11, length L12, length L21, and length L22 were set for the wavelengths of each color. In Examples 1 to 12, the height T11 was set to 0.5 μm, the height T12 was set to 0.2 μm, the height T21 was set to 0.5 μm, and the height T22 was set to 0.2 μm. A voltage was applied between the signal electrode 71 and the ground electrode 72 so that the branched light Ld1 and the branched light Ld2 were in phase. The calculation results of the conversion loss are shown in Table 1. [Table 1]

[0084] In Examples 1 to 7, a relatively small loss of about 2.6 dB to 3.4 dB occurred. This shows that the conversion loss is suppressed and the conversion efficiency is improved. The branching efficiency depends on the dimensions (length L21 and width W21) of the branching section 52, and the coupling (combining) efficiency depends on the dimensions (length L22 and width W22) of the coupling section 55. In Examples 1 to 7, the length L21 was in the range of 40 μm to 64 μm, and the width W21 was in the range of 3.0 μm to 3.5 μm. The length L22 was in the range of 63 μm to 100 μm, and the width W22 was in the range of 6 μm to 10 μm. In this case, it is seen that the conversion efficiency is improved.

[0085] (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 that converts a polarization mode of light from a TM0 mode to a TE0 mode; The mode converter includes: a first conversion unit that converts the polarization mode of the light from the TM0 mode to a TE1 mode; A second conversion unit that converts the polarization mode of the light from the TE1 mode to the TE0 mode; Equipped with The second conversion unit is a branching unit that branches the light in the TE1 mode incident from the first conversion unit 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; a first branch waveguide extending in a first direction along the principal surface and through which the first branched light propagates; a second branch waveguide extending in the first direction and through which the second branched light propagates; a coupling section that couples the first branched light propagated through the first branching waveguide and the second branched light propagated through the second branching waveguide and outputs the light in the TE0 mode; a phase adjustment unit that adjusts a phase difference between the first branched light and the second branched light; Equipped with the first branching waveguide and the second branching waveguide are arranged in a second direction intersecting the first direction.

[0086] [Clause 2] The phase adjustment unit is a signal electrode disposed between the first branch waveguide and the second branch waveguide; a first ground electrode and a second ground electrode arranged to sandwich the first branch waveguide and the second branch waveguide in the second direction; Equipped with Item 1. An optical element as described in item 1, wherein the optical axis of the material constituting the core layer extends in the second direction.

[0087] [Clause 3] 3. The optical element according to claim 1 or 2, wherein the light is visible light.

[0088] [Article 4] The length of the branch portion in the first direction is not less than 40 μm and not more than 64 μm, 4. The optical element described in clause 3, wherein the length of the branch portion in the second direction is not less than 3.0 μm and not more than 3.5 μm.

[0089] [Article 5] 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 of claim 3 or 4, further comprising:

[0090] [Article 6] An optical element according to clause 5; A first light source that emits the red light in the TM0 mode; A second light source that emits the green light in the TM0 mode; a third light source that emits the blue light in the TM0 mode; A laser module comprising: [Explanation of symbols]

[0091] 13...laser module, 21...red laser diode (first light source), 22...green laser diode (second light source), 23...blue laser diode (third light source), 30...optical element, 31...substrate, 31a...main surface, 32...core layer, 33...clad layer, 34R...mode converter (first mode converter), 34G...mode converter (second mode converter), 34B...mode converter (third mode converter), 36...multiplexer, 42...conversion section (first conversion section), 43...conversion section (second conversion section), 52...branching section, 53...branching waveguide (first branching waveguide), 54...branching waveguide (second branching waveguide), 55...coupling section, 57...phase adjustment section, 71...signal electrode, 72...ground electrode (first grounding electrode), 73...ground electrode (second grounding electrode).

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 that converts a polarization mode of light from a TM0 mode to a TE0 mode; The mode converter includes: A first conversion unit that converts the polarization mode of the light from the TM0 mode to a TE1 mode; A second conversion unit that converts the polarization mode of the light from the TE1 mode to the TE0 mode; Equipped with The second conversion unit is a branching unit that branches the light in the TE1 mode incident from the first conversion unit 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; a first branch waveguide extending in a first direction along the principal surface and through which the first branched light propagates; a second branch waveguide extending in the first direction and through which the second branched light propagates; a coupling section that couples the first branched light propagated through the first branching waveguide and the second branched light propagated through the second branching waveguide and outputs the light in the TE0 mode; a phase adjustment unit that adjusts a phase difference between the first branched light and the second branched light; Equipped with The first branching waveguide and the second branching waveguide are arranged in a second direction intersecting the first direction.

2. The phase adjustment unit is a signal electrode disposed between the first branch waveguide and the second branch waveguide; a first ground electrode and a second ground electrode arranged to sandwich the first branch waveguide and the second branch waveguide in the second direction; Equipped with The optical element according to claim 1 , wherein an optical axis of the material constituting the core layer extends in the second direction.

3. The optical element according to claim 1 , wherein the light is visible light.

4. The length of the branch portion in the first direction is equal to or greater than 40 μm and equal to or less than 64 μm, The optical element according to claim 3 , wherein the length of the branch portion in the second direction is not less than 3.0 μm and not more than 3.5 μm.

5. 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 3 , further comprising:

6. The optical element according to claim 5 ; A first light source that emits the red light in the TM0 mode; A second light source that emits the green light in the TM0 mode; A third light source that emits the blue light in the TM0 mode; A laser module comprising:

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