Optical modulator, light source module, optical engine, and XR glasses

Abstract

The objective is to provide a small, low-voltage, high-speed optical modulator for visible light. [Solution] The optical modulator 1 comprises a substrate 2, an optical waveguide layer 3 made of lithium niobate with multiple MZ-type waveguides arranged in parallel, including a first ridge waveguide 32c and a second ridge waveguide 32b that propagate visible light, and signal electrodes 62R, 62B, and 62G having interaction sections arranged along the MZ-type waveguides, and a first ground electrode 61A and a second ground electrode 61B, with the signal electrodes positioned above the first ridge waveguide 32c. The second ground electrode is positioned above the second ridge waveguide 32b, and the waveguide also includes signal electrode lead-out sections 62R-3, 62B-3, and 62G-3 that connect the signal electrode 62 to a pad positioned at the end edge S3, and an optical multiplexing section 40 positioned downstream of the Mach-Zehnder waveguide to combine multiple visible lights. The first ground electrode, the second ground electrode, and the signal electrode lead-out sections are positioned so as not to overlap with the optical multiplexing section 40 when viewed from above.

Description

Technical Field

[0001] The present invention relates to an optical modulator, a light source module, an optical engine, and XR glasses.

Background Art

[0002] In recent years, light source modules including an optical modulator into which light is incident from a laser diode (semiconductor laser) have attracted attention. Such light source modules can be used for an optical engine of glasses-type terminals such as AR (Augmented Reality) glasses and VR (Virtual Reality) glasses, and small projectors.

[0003] For example, Patent Document 1 describes an image display device including a light source unit that emits first light and second light, an optical modulator having a modulation unit with a Mach-Zehnder type modulation method, and an optical scanner that spatially scans the first light and the second light modulated by the optical modulator. Patent Document 1 also describes a head-mounted display worn on a user's head as an image display device.

[0004] Further, although not an image display device, Patent Document 2 describes a transmission device including a laser light source that emits visible light and an optical modulator that generates a visible light signal by changing the intensity of the visible light. Patent Document 2 describes a Mach-Zehnder type optical modulator having a substrate, an optical waveguide layer, a buffer layer, and an electrode layer, and the optical waveguide layer is formed of a lithium niobate film. Patent Document 2 also discloses using, as the electrode layer of the optical modulator, one having a first signal electrode, a second signal electrode, a first ground electrode, a second ground electrode, and a third ground electrode. The optical modulator disclosed in Patent Document 2 is a so-called dual-drive type optical modulator having two signal electrodes.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Patent No. 6728596 [Patent Document 2] Japanese Patent Publication No. 2022-036928 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Patent Document 1 discloses a preferred configuration in which a single crystal or solid solution crystal of lithium niobate is used, and a portion of it is modified by a proton exchange method or a Ti diffusion method to form an optical waveguide. However, since the size of the modified waveguide portion (core) region is determined by the distance over which protons or Ti penetrate and diffuse, it is difficult to reduce the diameter of the optical waveguide. As a result, the optical waveguide itself becomes large, and because the diameter of the optical waveguide is large, the electric field of the modulation voltage does not concentrate easily. Therefore, a large voltage is required for modulation, or the electrodes to which the voltage is applied need to be long in order to operate with a small voltage, which increases the size of the device.

[0007] For the widespread adoption of glasses-type image display devices such as XR glasses, miniaturization and low drive voltage are essential. Furthermore, for mass production, it is necessary to manufacture them at the lowest possible cost.

[0008] In glasses-type image display devices such as XR glasses, a laser beam (LB) is scanned sequentially, one pixel at a time, to form an image. For example, to obtain a pixel resolution of 2560 x 1460, high-speed modulation of around 1 GHz is required. Furthermore, 4K resolution is 3840 x 2160, requiring high-speed driving of 1 GHz or higher.

[0009] This invention has been made in view of the above problems, and aims to provide a small visible light modulator, a light source module, an optical engine equipped with the same, and XR glasses that can be driven at high speed at low voltage. [Means for solving the problem]

[0010] To solve the above problems, the present invention provides the following means.

[0011] One aspect of the present invention comprises a substrate, a plurality of Mach-Zehnder type waveguides formed on the substrate and having parallel arrangements of Mach-Zehnder type waveguides including a first ridge waveguide and a second ridge waveguide that propagate visible light, an optical waveguide layer made of lithium niobate, a buffer layer formed on the optical waveguide layer, a plurality of signal electrodes formed on the buffer layer and having interaction portions arranged along each of the plurality of Mach-Zehnder type waveguides, a plurality of first ground electrodes and a plurality of second ground electrodes arranged on both sides of each of the plurality of signal electrodes spaced apart from the signal electrodes, the signal electrodes being positioned above the first ridge waveguide, and the second ground The optical modulator comprises an electrode positioned above the second ridge waveguide, a signal electrode lead-out section connecting the signal electrode to a pad positioned at the edge of the substrate, an optical multiplexer positioned downstream of the plurality of Mach-Zehnder waveguides to combine the plurality of visible light waves that have passed through the plurality of Mach-Zehnder waveguides, a plurality of connecting waveguides connecting the downstream side of each of the plurality of Mach-Zehnder waveguides to the optical multiplexer, and a single output waveguide connected to the optical multiplexer, wherein the first ground electrode, the second ground electrode, and the signal electrode lead-out section are positioned so as not to overlap with the optical multiplexer when viewed from above.

[0012] Aspect 2 of the present invention is an optical modulator of aspect 1, wherein there are three or more Mach-Zehnder waveguides, three or more of the plurality of signal electrodes, and three or more of the interaction sections provided by each are arranged in parallel to each other in a plan view, and in order from the shortest length, and the optical multiplexer is positioned downstream of the signal electrode having the shortest interaction section among the plurality of signal electrodes, and there are three or more of the plurality of signal electrode lead-out sections, and two or more of the plurality of signal electrode lead-out sections each extend from the downstream end of the interaction section in a direction of 90° or less with respect to the direction in which the interaction section extends, so as not to intersect each other, and in a plan view they straddle the plurality of connecting waveguides and are connected to a pad positioned on the downstream end of the signal electrode having the longest interaction section.

[0013] A third aspect of the present invention is an optical modulator in either aspect 1 or aspect 2, wherein each of the plurality of Mach-Zehnder waveguides has a higher-order mode rejection unit configured to remove higher-order modes in the waveguide upstream of each of the plurality of Mach-Zehnder waveguides, before branching to the first ridge waveguide and the second ridge waveguide.

[0014] Aspect 4 of the present invention is a light source module comprising one optical modulator from aspects 1 to 3, and a plurality of light sources that emit visible light incident on each of the plurality of input waveguides, which are connected to the plurality of Mach-Zehnder waveguides.

[0015] Aspect 5 of the present invention is an optical engine comprising a light source module as described in Aspect 4, and a light scanning mirror that reflects light emitted from the light source module at a different angle to display an image.

[0016] Aspect 6 of the present invention is an XR pair of glasses equipped with the optical engine of Aspect 5. [Effects of the Invention]

[0017] The optical modulator of the present invention provides a visible light modulator that is compact and capable of high-speed operation at low voltage. [Brief explanation of the drawing]

[0018] [Figure 1] It is a schematic plan view for explaining an example of an optical modulator according to an embodiment. [Figure 2] It shows an enlarged view of the portions surrounded by the three dotted-line frames in the schematic plan view shown in FIG. 1, and an enlarged view of one of the three enlarged views. [Figure 3] It is a schematic cross-sectional view of the optical modulator according to the present embodiment cut along the line A-A' shown in FIG. 2. [Figure 4] It is a view for enlarging the portions surrounded by the two dotted-line frames in the schematic plan view shown in FIG. 1. [Figure 5] It is a view for enlarging the portions surrounded by the two dotted-line frames in the schematic plan view shown in FIG. 1. [Figure 6] It is a view for explaining an example in which the interaction lengths LR, LB, and LG of each visible light are adjusted so that Vπ becomes 2V or less. [Figure 7] In the configuration shown in FIG. 6, a modified example in which the positions of the Mach-Zehnder type waveguide for blue light and the Mach-Zehnder type waveguide for green light are interchanged is shown. [Figure 8A] It is a view for explaining the multiplexing mode of a plurality of visible lights. [Figure 8B] It is a schematic plan view for enlarging the vicinity of the optical multiplexing unit 40 indicated by the arrow B in FIG. 8A. [Figure 8C] It is a schematic plan view for enlarging the vicinity of the optical multiplexing unit 40 indicated by the arrow B in FIG. 8A. [Figure 9] It is a schematic plan view for enlarging the vicinity of the higher-order mode removing unit included in the optical modulator according to the present embodiment. [Figure 10] It shows a schematic plan view of a light source module according to an embodiment of the present invention. [Figure 11] It is a schematic cross-sectional view of a part of the light source module shown in FIG. 10 cut along the XZ plane, showing only a part of the vicinity of the joint. [Figure 12] It is a conceptual diagram for explaining an example of the XR glass of the present invention. [Figure 13]Figure 12 is a conceptual diagram showing how an image is projected directly onto the retina by laser light emitted from a light source module in the XR glasses shown. [Modes for carrying out the invention]

[0019] The embodiments will be described in detail below, with reference to the figures as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features, and the dimensional ratios of each component may differ from those in reality. The materials, dimensions, etc., exemplified in the following description are examples only, and the present invention is not limited to them. It is possible to modify and implement the invention as appropriate within the scope of achieving its effects.

[0020] [Optical modulator] Figure 1 is a schematic plan view illustrating an example of an optical modulator according to one embodiment of the present invention. Figure 2 shows enlarged views of three areas in the schematic plan view shown in Figure 1, and an enlarged view of one of those areas. The enlarged views in Figure 2 resolve lines that were not resolved in the schematic plan view of Figure 1, and clarify the shape. Figures 1 and 2 illustrate the optical waveguide structure so that the arrangement relationship between the optical waveguide structure and the electrode structure formed in the optical waveguide layer 3 can be seen. Figure 3 is a schematic cross-sectional view cut along line AA' shown in Figure 2. Figure 4 is an enlarged view of the area enclosed by the two dotted lines in the schematic plan view shown in Figure 1. Figure 5 is an enlarged view of the area enclosed by the two dotted lines in the schematic plan view shown in Figure 1.

[0021] In Figures 1 to 5, the X direction is perpendicular to the side surface where the light entry port is located, the Y direction is perpendicular to the X direction, and the Z direction is perpendicular to the plane formed by the X and Y directions.

[0022] The optical modulator of this embodiment comprises a plurality of Mach-Zehnder type (MZI type) optical modulation units and an optical multiplexing unit that combines the plurality of visible light modulated by each of the plurality of Mach-Zehnder type optical modulation units. In the following description, the case in which the plurality of visible light is red light (R), green light (G), and blue light (B) will be used as an example.

[0023] The optical modulator 1 shown in Figures 1 to 5 comprises a substrate 2, an optical waveguide layer 3 made of lithium niobate formed on the substrate 2 and having three Mach-Zehnder type waveguides 32 (32R, 32B, 32G) arranged in parallel, including a first ridge waveguide 32c and a second ridge waveguide 32b that propagate visible light, a buffer layer 52 formed on the optical waveguide layer 3, a plurality of signal electrodes 62 (62R, 62B, 62G) formed on the buffer layer 52 and having interaction sections 62R-2, 62G-2, 62B-2 arranged along each of the plurality of Mach-Zehnder type waveguides, and a plurality of first ground electrodes 61A and a plurality of second ground electrodes 61B arranged on both sides of each of the plurality of signal electrodes 62 (62R, 62B, 62G) spaced apart from the signal electrodes 62 (62R, 62B, 62G). As shown in the figure, the first and second ground electrodes for one signal electrode may be configured to become the second and first ground electrodes for adjacent signal electrodes, respectively. The signal electrodes 62 (62R, 62B, 62G) are positioned above the first ridge waveguide 32b, and the second ground electrode 61B is positioned above the second ridge waveguide 32c.

[0024] The substrate 2 and optical modulator 1 shown in Figure 1 are rectangular in plan view (viewed from the Z direction). For the sake of explanation, the four sides of this rectangle will be designated as follows: the side into which visible light is incident will be S1, the side opposite S1 will be S2, the side perpendicular to S1 and S2 where the waveguide for the longest wavelength of visible light is located will be S3, and the side opposite S3 will be S4. Hereafter, the parts close to sides S1, S2, S3, and S4 will be referred to as, for example, end edge S3 and end edge S4.

[0025] The optical modulator 1 includes signal electrode lead-out sections 62R-1, 62B-1, 62G-1, 62R-3, 62B-3, and 62G-3, which connect the signal electrodes 62 (62R, 62B, 62G) to pads 63 (63R, 63B, 63G) and 64 (64R, 64G, 64b) placed on the edge sections S4 and S3 of the substrate 2, respectively. Furthermore, the optical modulator 1 includes an optical multiplexer 40 positioned downstream (on the side through which light propagates) of a plurality of Mach-Zehnder waveguides 32 (32R, 32B, 32G) that combines multiple visible light signals that have passed through the plurality of Mach-Zehnder waveguides 32 (32R, 32B, 32G), a plurality of connecting waveguides 32d (32Rd, 32Bd, 32Gd) that connect the downstream side of each of the plurality of Mach-Zehnder waveguides 32 (32R, 32B, 32G) to the optical multiplexer 40, and a single output waveguide 32e connected to the optical multiplexer 40. In the optical modulator 1, the first ground electrode 61A, the second ground electrode 61B, and the signal electrode lead-out sections 62R-3, 62B-3, and 62G-3 are positioned so as not to overlap with the optical multiplexing section 40 when viewed from above.

[0026] In the optical modulator 1, light of each color generated from the respective light sources for red (R), blue (R), and green (G) enters through the light inlet ports 32Rin, 32Bin, and 32Gin (see Figure 2), undergoes optical modulation for each color of light, and the three colors of light are combined in the optical multiplexer 40, and the combined light is emitted from the light outlet port 32out (see Figure 2).

[0027] The optical modulator 1 shown in Figures 1 to 5 represents the case with three Mach-Zehnder waveguides 32, but it is not limited to three; any number of waveguides is acceptable.

[0028] (Positional relationship between the first ground electrode, the second ground electrode, the signal electrode extraction section, and the optical multiplexing section) In this embodiment, the optical modulator is configured such that the first ground electrode, the second ground electrode, and the signal electrode lead-out section are positioned so as not to overlap with the optical multiplexing section when viewed from above. An example of this configuration will be described below with reference to the figures, but the configuration is not limited to this example.

[0029] In the optical modulator 1, the three signal electrodes 62R, 62B, and 62G are arranged such that the interaction portions 62R-2, 62B-2, and 62G-2 provided respectively are parallel to each other in a plan view and are juxtaposed in order from the shorter one (LG < LB < LR (see FIG. 6)). In other words, the three signal electrodes 62R, 62B, and 62G are arranged such that the signal electrode 62B is disposed next to the signal electrode 62G, and the signal electrode 62R is disposed next to the signal electrode 62B.

[0030] As shown enlarged in the enlarged view of FIG. 4, the pads 63R, 63B, and 63G for the three signal electrode lead-out portions 62R-1, 62B-1, and 62G-1 disposed at the end side portion S4 of the substrate 2 are arranged in order from the upstream side (the side where light enters) to the downstream side (the side where light propagates) (+x direction). From each of the three pads 63R, 63B, and 63G, the signal electrode lead-out portions 62R-1, 62B-1, and 62G-1 extend in a direction substantially orthogonal to the direction (x direction) in which the interaction portions 62R-2, 62B-2, and 62G-2 extend. The x-direction positions (coordinates) of the upstream ends 62R-2s, 62B-2s, and 62G-2s of the interaction portions 62R-2, 62B-2, and 62G-2 are located on the downstream side in this order. At the end side portion S3 of the substrate 2, the pads 65-1, 65-2, 65-3, and 65-4 of the first ground electrode 61A and the second ground electrode 61B are arranged alternately with the pads 63R, 63B, and 63G for the signal electrode lead-out portions 62R-1, 62B-1, and 62G-1.

[0031] In the optical modulator 1, the positions (coordinates) in the x direction of the downstream ends 62R-2e, 62B-2e, and 62G-2 of the interaction portions 62R-2, 62B-2, and 62G-2 (see FIG. 2) are substantially aligned (aligned substantially in a straight line in the y direction). Although the positions (coordinates) in the x direction of the upstream ends 62R-2s, 62B-2s, and 62G-2s of the interaction portions 62R-2, 62B-2, and 62G-2 are located downstream in this order, the positions (coordinates) in the x direction of the downstream ends 62R-2e, 62B-2e, and 62G-2 can be made substantially aligned because the interaction portion 62R-2 for red light may be shorter than the interaction portion 62B-2 for blue light and the interaction portion 62G-2 for green light (LG < LB < LR (see FIG. 6)). Not limited to the case where the positions (coordinates) in the x direction of the downstream ends 62R-2e, 62B-2e, and 62G-2 of the interaction portions 62R-2, 62B-2, and 62G-2 are substantially aligned in the y direction, it is preferable that the positions (coordinates) in the x direction of the downstream ends 62R-2e, 62B-2e, and 62G-2 are within 20% of the length LR of the interaction portion 62R-2, more preferably within 15% of the length LR of the interaction portion 62R-2, and even more preferably within 10% of the length LR of the interaction portion 62R-2.

[0032] The three signal electrode lead-out sections 62R-3, 62B-3, and 62G-3 all bend at a predetermined angle α (see Figure 5) with respect to the direction in which the interaction sections 62R-2, 62B-2, and 62G-2 extend (x-direction) from their downstream ends 62R-2e, 62B-2e, and 62G-2e (see Figure 2), extending so as not to intersect with each other, and in a plan view, they straddle the three connecting waveguides 32Rd, 32Bd, and 32Gd, and are connected to the signal electrode having the longest interaction section, i.e., the pad 64 (64R, 64B, 64G) located at the downstream end 2B of the signal electrode 62R. This angle α is 90° or less. In the example shown in the figure, the angles α of the signal electrode extraction sections 62B-3 and 62G-3 are approximately 45°, and the angle α of the signal electrode extraction section 62R-3 is 90°. A larger angle α allows for a shorter length of the optical modulator 1 (distance between S1 and S2), so it is preferable that the angle α is 30° or more. For multiple signal electrode extraction sections, it is preferable that one signal electrode extraction section is 90° and the other signal electrode extraction sections are less than 90°, or that all signal electrode extraction sections are less than 90°. For example, in the case of three signal electrode extraction sections, it is preferable that one signal electrode extraction section is 90° and the remaining two signal electrode extraction sections are less than 90°, or that all three signal electrode extraction sections are less than 90°. The shortest possible length of the optical modulator 1 (distance between S1 and S2) is achieved when the signal electrode lead-out section 62R-3 is bent at 90°, and the signal electrode lead-out sections 62B-3 and 62G-3 are bent at less than 90°. A more detailed explanation will be provided using an enlarged view of Figure 5. In the example shown in the figure, the signal electrode lead-out portion 62G-3 and the signal electrode lead-out portion 62B-3 are bent at the same angle α at the downstream ends 62G-2e and 62B-2e, and have portions 62G-31 and 62B-31 that extend parallel in the direction of the angle α, and further bent portions 62G-32 and 62B-32 that extend toward the end side portion S3. On the other hand, the signal electrode lead-out portion 62R-3 has only a portion that is bent at a predetermined angle and extends toward the end side portion S3. The signal electrode lead-out portion 62R-3 is parallel to the portion 62G-32 of the signal electrode lead-out portion 62G-3 and the portion 62B-32 of the signal electrode lead-out portion 62B-3. The reason why the three signal electrode lead-out portions 62R-3, 62B-3, and 62G-3 can be arranged in such a way is that the interaction portion 62R-2 for red light can be shorter than the interaction portion 62B-2 for blue light and the interaction portion 62G-2 for green light (LG < LB < LR (see FIG. 6)).

[0033] Also, the optical multiplexer 40 is arranged closer to the downstream side of the signal electrode 62G among the three signal electrodes 62R, 62B, and 62G, that is, the signal electrode having the shortest interaction portion. In other words, the optical multiplexer 40 is arranged closest to the signal electrode 62G, then closer to the signal electrode 62B, and farthest from the signal electrode 62R. With the configuration in which the x-direction positions (coordinates) of the bending positions of the three signal electrode lead-out portions 62R-3, 62B-3, and 62G-3 (corresponding to the positions of the downstream ends 62R-2e, 62B-2e, and 62G-2e of the interaction portions 62R-2, 62B-2, and 62G-2) are substantially aligned, and the optical multiplexer 40 is arranged closer to the downstream side of the signal electrode 62G, it is ensured that the first ground electrode 61A, the second ground electrode 61B, and the signal electrode lead-out portions 62R-3, 62B-3, and 62G-3 are arranged at positions that do not overlap with the optical multiplexer 40 in a plan view. By adopting a configuration in which the first ground electrode, the second ground electrode, and the signal electrode lead-out portion are not arranged above the optical multiplexer, it is possible to prevent the effective refractive index of the lithium niobate forming the optical multiplexer from changing and suppress the loss due to the deviation from the design value. In addition to the shape and arrangement of the first ground electrode, second ground electrode, and signal electrode lead-out section shown in this embodiment, by not arranging the first ground electrode, second ground electrode, and signal electrode lead-out section above the optical multiplexing section, it is possible to prevent changes in the effective refractive index of the lithium niobate forming the optical multiplexing section and suppress losses due to deviations from the design value.

[0034] As shown in the enlarged view of Figure 4, it is preferable that the Mach-Zehnder waveguide and the signal electrode extraction section are arranged such that, in a plan view, each signal electrode extraction section 62R-1, 62B-1, and 62G-1 does not overlap with the optical branching section 4a of each Mach-Zehnder waveguide. This configuration, which does not place the signal electrode extraction section through which the electrical signal passes above the optical branching section, prevents a change in the effective refractive index of the lithium niobate forming the optical branching section and suppresses losses due to deviations from the design value.

[0035] As shown in the enlarged view of Figure 5, it is preferable that the Mach-Zehnder waveguide and the signal electrode extraction section are arranged such that, in a plan view, each signal electrode extraction section 62R-3, 62B-3, and 62G-3 does not overlap with the optical coupling section 4b of each Mach-Zehnder waveguide. This configuration, which does not place the signal electrode extraction section through which the electrical signal passes above the optical coupling section, prevents a change in the effective refractive index of the lithium niobate forming the optical coupling section and suppresses losses due to deviations from the design value.

[0036] The optical modulator of this embodiment is compact and can be driven at a low voltage. To evaluate miniaturization and low driving voltage, VπL can be used. Vπ is the voltage required for half-wavelength phase modulation (half-wavelength voltage), and is defined as the difference between the voltage V1 at which the optical output is maximum and the voltage V2 at which it is minimum. L is the length (interaction length, electrode length) of the portion where the signal electrode overlaps the optical waveguide (ridge) (interaction section). If the interaction length L is long, the half-wavelength voltage Vπ becomes small, and if the interaction length L is short, the half-wavelength voltage Vπ becomes large. To reduce the size of the optical modulator, the interaction length L becomes shorter and the half-wavelength voltage Vπ becomes larger. A smaller VπL indicates a smaller size and lower drive voltage. In the following, VπL may be referred to as the electric field efficiency.

[0037] In the example of this embodiment, Figure 6 shows an example in which the interaction lengths LR, LB, and LG of each visible light source were adjusted so that Vπ is 2V or less for red light with a peak wavelength of 637nm, green light with a peak wavelength of 520nm, and blue light with a peak wavelength of 455nm. If Vπ is 2V or less, it can be driven using a CMOS. The interaction lengths LR, LB, LG and half-wavelength voltage Vπ are shown in Table 1. The interaction lengths LB and LG for green and blue light were shortened to reduce the length of the optical modulator.

[0038] [Table 1]

[0039] In the example shown in Figure 6 and Table 1, as a configuration for combining three visible lights, it is preferable to use a 3-input, 1-output (3×1 type) optical multiplexer 40 as shown in Figure 8B, which will be described later, to simultaneously combine three visible lights (red light, blue light, and green light) with different wavelengths incident from three connecting waveguides 32Rd, 32Bd, and 32Gd, and to output them as a single combined light to the output waveguide 32e.

[0040] Figure 7 shows a modified configuration in which the positions of the Mach-Zehnder waveguide 32B for blue light and the Mach-Zehnder waveguide 32G for green light are swapped in the configuration shown in Figure 6. Regarding this modification, an example is shown in which the interaction lengths LR, LB, and LG for each visible light are adjusted so that Vπ is 2V or less. The interaction lengths LR, LB, LG and half-wavelength voltage Vπ are shown in Table 2. Similarly, the interaction lengths LB and LG for green and blue light were shortened to reduce the length of the optical modulator.

[0041] [Table 2]

[0042] In the example shown in Figure 7 and Table 2, as a configuration for combining three visible lights, as shown in Figure 8C later, it is preferable to use two 2-input, 1-output (2×1 type) optical multiplexers 40-1 and 40-2 as the optical multiplexer 40. First, two visible lights of different wavelengths (red light and green light) incident from two connecting waveguides 32Rd and 32Gd are combined in optical multiplexer 40-1. Then, the combined light and the visible light (blue light) incident from connecting waveguide 32Bd are combined in optical multiplexer 40-2 and emitted as a single combined light to the output waveguide 32e.

[0043] (High-speed modulation) 4K resolution is 3840 x 2160, requiring high-speed driving (high-speed modulation) of 1 GHz or higher. Modulation by the optical modulator in this embodiment is voltage-controlled, enabling higher-speed modulation compared to current-controlled modulation using a laser light source. Furthermore, since the optical waveguide layer is made of a lithium niobate film, high-speed driving (high-speed modulation) of 1 GHz or higher is possible. The design bandwidth of the optical modulator according to this embodiment is 10 GHz.

[0044] (Circuit board 2) The substrate 2 can be any substrate with a lower refractive index than the lithium niobate film forming the optical waveguide layer 3, and is not particularly limited, but examples include sapphire substrates, Si substrates, and thermal silicon oxide substrates. Preferably, the substrate 2 is one on which the lithium niobate film can be formed as an epitaxial film. Since the optical waveguide layer consists of a lithium niobate (LiNbO3) film, the substrate 2 is not particularly limited as long as it has a refractive index lower than that of the lithium niobate film. However, a sapphire single crystal substrate or a silicon single crystal substrate is preferred as a substrate on which a single-crystal lithium niobate film can be formed as an epitaxial film. The crystal orientation of the single crystal substrate is not particularly limited, but for example, since a c-axis oriented lithium niobate film has three-fold symmetry, it is desirable that the underlying single crystal substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111)-plane substrate is preferred.

[0045] (Optical waveguide layer 3) The optical waveguide layer 3 consists of a lithium niobate film. The lithium niobate forming the lithium niobate film may contain elements other than lithium (Li), niobium (Nb), and oxygen (O).

[0046] Lithium niobate may also be a compound represented by, for example, the following formula (I). LixNbAyOz ···(I) (In equation (I), A represents an element other than Li, Nb, and O. x represents a number between 0.5 and 1.2. y represents a number between 0 and 0.5. z represents a number between 1.5 and 4.0.)

[0047] In formula (I), A can be any element other than Li, Nb, or O, such as K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc. A may be just one of these elements or two or more. In equation (I), x is a number between 0.5 and 1.2, preferably between 0.9 and 1.05. y is a number between 0 and 0.5. z is a number between 1.5 and 4.0, preferably between 2.5 and 3.5.

[0048] The lithium niobate film forming the optical waveguide layer 3 is preferably an epitaxial film. A lithium niobate film is, for example, a c-axis oriented lithium niobate film. Another example is an epitaxial film grown epitaxially on substrate 2. An epitaxial film is a single-crystal film whose crystal orientation is aligned by the underlying substrate. An epitaxial film has a single crystal orientation in the z-direction and the xy-plane, with the crystals aligned in the x-axis, y-axis, and z-axis directions. Whether a film formed on substrate 2 is an epitaxial film can be proven, for example, by checking the peak intensity and poles at the orientation position in 2θ-θ X-ray diffraction.

[0049] The optical waveguide layer 3 has a plurality of flat sections 31 and ridges 32 that are arranged between adjacent flat sections 31 and have a band-shaped protrusion from the flat sections 31. In the optical modulator 1 of this embodiment, the input waveguide 32a, the optical branching section 4a, the first ridge waveguide 32c branched from the input waveguide 32a, the second ridge waveguide 32b, the optical coupling section 4b to which these are coupled, and the output waveguide 32d are collectively referred to as one ridge 32. The number n of ridges 32 is an integer greater than or equal to 2. Figures 1 to 5 show an example with three ridges (n=3). Visible light of different wavelengths is incident on each of the three ridges 32R, 32G, and 32B.

[0050] In this embodiment, for example, red light with a peak wavelength of 610 nm to 750 nm is incident on ridge 32R. For example, green light with a peak wavelength of 500 nm to 560 nm is incident on ridge 32G. For example, blue light with a peak wavelength of 435 nm to 480 nm is incident on ridge 32B. In this embodiment, the optical modulator 1 has three ridges 32R, 32G, and 32B, which are incident on red light, green light, and blue light, respectively, and is therefore preferably used in, for example, XR glasses that can display full-color images.

[0051] Each of the three ridges 32R, 32G, and 32B has an input waveguide 32a, an optical branching section 4a, a first ridge waveguide 32c, a second ridge waveguide 32b, an optical coupling section 4b, and an output waveguide 32d.

[0052] In the three ridges 32R, 32G, and 32B of the optical waveguide layer 3, the input waveguide 32a is, for example, roughly rectangular in cross-section, and is incident on visible light generated by a light source such as a laser element. The input waveguide 32a is branched into a first ridge waveguide 32c and a second ridge waveguide 32b at the optical branching section 4a. The first ridge waveguide 32c and the second ridge waveguide 32b may be, for example, trapezoidal in cross-section. In this embodiment, the first ridge waveguide 32c and the second ridge waveguide 32b are identical in cross-section. The first ridge waveguide 32c and the second ridge waveguide 32b are coupled by the optical coupling section 4b to form the output waveguide 32d. The output waveguide 32d is, for example, roughly rectangular in cross-section, and emits the visible light signal generated at the optical coupling section 4b.

[0053] The cross-sectional shapes of the input waveguide 32a and the output waveguide 32d are not limited to rectangles, but may be trapezoidal or semicircular, for example. Furthermore, the cross-sectional shapes of the first ridge waveguide 32c and the second ridge waveguide 32b are not limited to a trapezoid; for example, they may be rectangular or semicircular. Furthermore, the cross-sectional shapes of the input waveguide 32a, output waveguide 32d, first ridge waveguide 32c, and second ridge waveguide 32b may be symmetrical or asymmetrical.

[0054] When the optical modulator of this embodiment is used in a spectacle-type image display device, the thickness (T) of the flat portion 31 of the optical waveguide layer 3 slab The thickness of the ridge 32 of the optical waveguide layer 3 is preferably 0.1 to 0.3 μm, and the thickness of the ridge 32 of the optical waveguide layer 3 is T R The particle size is preferably 0.5 to 1.0 μm. This is the thickness of Ridge 32 (T R This is because if the ) is small, light will not propagate, and if it is large, the propagating light will be multimode.

[0055] When the optical modulator of this embodiment is used in a spectacle-type image display device, the center-to-center distance (S) between the ridges 32 is preferably 2 to 12 μm. This is because reducing S allows the distance between the signal electrode and the ground electrode to be shortened, thereby increasing the efficiency of the electric field applied to the ridge 32.

[0056] In the ridges 32b and 32c shown in Figure 3, a trapezoidal shape symmetrical with respect to the center line can be adopted. When the optical modulator of this embodiment is used in a spectacle-type image display device, the tilt angle (α) of the ridges 32b and 32c is preferably 60 to 90 degrees. This is because, as the tilt angle decreases, the propagating light becomes multimode. Also, the width of the top surface of Ridge 32b and 32c (W R The thickness of the particle is preferably 0.3 to 1.2 μm. This is because if the waveguide width is small, light will not propagate, and if it is large, the propagating light will be multimode.

[0057] As the optical multiplexer 40, a known configuration can be used. For example, an MMI (Multi-Mode Interferometer) type multiplexer, a Y-type multiplexer, a directional coupler, etc., can be used.

[0058] (Protective layer 51) As shown in Figure 3, the protective layer 51 is positioned between the flat portion 31 of the optical waveguide layer 3 and the buffer layer 52. The protective layer 51 is made of a dielectric material with a lower refractive index than the optical waveguide layer 3. As the material for the protective layer 51, for example, silicon oxide (SiO2), aluminum oxide (Al2O3), lanthanum oxide (La2O3), or composites of these oxides can be used. An example of a composite of the above oxides is LaAlSiInO. Among the above, silicon oxide (SiO2) is preferred as the material for the protective layer 51.

[0059] (Buffer layer 52) The buffer layer 52 is formed on the optical waveguide layer 3 and the protective layer 51 to prevent visible light propagating through the optical waveguide layer 3 from being absorbed by the electrode layer.

[0060] The buffer layer 52 is made of a dielectric material with a lower refractive index than the optical waveguide layer 3. The dielectric material constituting the buffer layer 52 preferably has a dielectric constant of 7 or higher, because this reduces the electric field efficiency VπL. Specific materials for the buffer layer 52 include aluminum oxide (Al2O3, dielectric constant 7) and LaAlSiInO (dielectric constant 11). The material of the buffer layer 52 may be the same as that of the protective layer 51, or it may be a different material.

[0061] Thickness of buffer layer 52 (T buffer The particle size is preferably 0.4 μm or more and 1 μm or less, because it can reduce the electric field efficiency VπL.

[0062] (electrode layer) The electrode layer is formed on the buffer layer 52 and includes one signal electrode 62 and a first ground electrode 61A and a second ground electrode 61B positioned on either side of the signal electrode 62. The optical modulator of this embodiment is a so-called single-drive optical modulator having one signal electrode. A so-called dual-drive optical modulator, having two signal electrodes, has a complex electrode structure and requires the application of an inverted electrical signal to the two signal electrodes while controlling the phase, which leads to a complex drive system circuit configuration. Since the optical modulator of this embodiment is a single-drive type, it does not have this problem.

[0063] When the optical modulator of this embodiment is used in a spectacle-type image display device, the width (We) of the signal electrode 62 is preferably 1.0 to 4.0 μm. This is because it allows for a reduction in the electric field efficiency VπL.

[0064] When the optical modulator of this embodiment is used in a spectacle-type image display device, the widths of the first ground electrode 61A and the second ground electrode 61B are preferably 50 to 1000 μm. This is because a thinner ground electrode prevents the voltage from becoming 0V, thus increasing the electric field efficiency VπL.

[0065] When the optical modulator of this embodiment is used in a spectacle-type image display device, the thickness (Te) of the electrode layer is preferably 0.1 to 5 μm. This is because, at high modulation frequencies, a larger electrode cross-sectional area allows microwaves to propagate more efficiently.

[0066] When the optical modulator of this embodiment is used in a spectacle-type image display device, the distance (G) between the signal electrode 62 and the ground electrode 61 is preferably 1 to 12 μm. This is because it allows for a reduction in the electric field efficiency VπL.

[0067] (Aspects of multiple visible light combined waves) Figures 8A to 8C are diagrams illustrating the modes of combination of multiple visible light waves. Figures 8B and 8C are schematic plan views showing an enlarged view of the vicinity of the optical multiplexing section 40 indicated by arrow B in Figure 8A.

[0068] In the embodiment shown in Figure 8B, a 3-input, 1-output (3×1 type) optical multiplexer 40 is used to simultaneously combine three visible light sources (red light, blue light, and green light) with different wavelengths incident from three connecting waveguides 32Rd, 32Bd, and 32Gd, and output them as a single combined light to the output waveguide 32e. Similarly, if there are multiple (n) lights other than three, an n-input, 1-output (n×1 type) optical multiplexer can be used to simultaneously combine n visible light sources into a single combined light.

[0069] The configuration shown in Figure 8C is a case where two 2-input, 1-output (2x1 type) optical multiplexers 40-1 and 40-2 are used as the optical multiplexer 40. This type of optical multiplexer is referred to as a two-stage optical multiplexer. Accordingly, the type of optical multiplexer shown in Figure 8B is referred to as a single-stage optical multiplexer. First, two visible light beams of different wavelengths (red light and blue light) incident from two connecting waveguides 32Rd and 32Bd are combined in the optical multiplexer 40-1. Then, this combined light beam and the incident visible light beam (green light) incident from connecting waveguide 32Gd are combined in the optical multiplexer 40-2 and emitted as a single combined light beam to the output waveguide 32e. In the example shown in Figure 8C, the red and blue light beams are combined first, followed by the green light beam, but the combination of visible light beams to be combined first and then is not limited to this example.

[0070] (Higher-order mode removal unit) The optical modulator of this embodiment may have a higher-order mode rejection section configured to remove higher-order modes in the waveguide upstream of each of the multiple Mach-Zehnder waveguides, before the branching to the first ridge waveguide and the second ridge waveguide.

[0071] Figure 9 is a schematic plan view showing an enlarged view of the vicinity of the higher-order mode rejection section of the optical modulator 1. The area enclosed by the dotted line frame is the higher-order mode rejection section 32Ra1, 32Ba1, and 32Ga1.

[0072] The higher-order mode rejection section is configured to allow the fundamental mode (0th-order mode (single mode)) and higher-order modes (1st-order mode, 2nd-order mode, ... (multi-mode)) of each visible light to pass through with low loss, while the higher-order modes pass through with high loss. As shown in Figure 9, a curved waveguide can be used as an example of the configuration of the higher-order mode rejection unit. Another example of a higher-order mode rejection unit is one in which higher-order modes are rejected by setting the line width of the optical waveguide to a predetermined width.

[0073] Regarding the configuration of the higher-order mode removal section, specifically for the case of a bent waveguide shown in Figure 9, a concrete example of dimensions is shown where the higher-order modes are removed, resulting in a single-mode waveguide. Using red light with a peak wavelength of 637 nm, green light with a peak wavelength of 520 nm, and blue light with a peak wavelength of 455 nm, and with the optical waveguide width set to the dimensions shown in Table 3, and the radius of curvature of the curved waveguide set to 200 μm, simulations confirmed that higher-order modes are eliminated, resulting in a single-mode signal.

[0074] [Table 3]

[0075] [Manufacturing method] The optical modulator 1 of this embodiment, shown in Figure 1, can be manufactured, for example, by the method described below. First, an optical waveguide layer 3 made of a lithium niobate film is formed on the substrate 2. For example, thin-film formation methods such as sputtering, CVD, and sol-gel can be used to form a lithium niobate film on substrate 2.

[0076] When a sapphire single crystal substrate is used as substrate 2, a lithium niobate film may be epitaxially grown directly on the sapphire single crystal substrate. When a silicon single-crystal substrate is used as substrate 2, a lithium niobate film may be formed by epitaxial growth via a cladding layer. The cladding layer should have a lower refractive index than the lithium niobate film and be suitable for epitaxial growth. Specifically, for example, a cladding layer made of Y2O3 can be used. By epitaxially growing a lithium niobate film on a silicon single-crystal substrate via a cladding layer made of Y2O3, a high-quality lithium niobate film can be formed.

[0077] Next, the lithium niobate film obtained in this manner is patterned into a desired shape using a known method such as photolithography. This yields an optical waveguide layer 3 having a plurality of flat portions 31 and n ridges 32 arranged between adjacent flat portions 31.

[0078] Next, a protective layer 51 is formed on the flat portion 31 of the optical waveguide layer 3 using a thin film formation method such as sputtering, CVD, or sol-gel. Subsequently, a buffer layer 52 is formed to cover the protective layer 51 and the ridge 32 of the optical waveguide layer 3. Known methods can be used to form the buffer layer 52. Specifically, thin film formation methods such as sputtering, CVD, and sol-gel can be used to form the buffer layer 52.

[0079] Next, an electrode layer is formed on the buffer layer 52, for example, using the method shown below. First, a thin metal film is formed on the buffer layer 52 using a thin film formation method such as vapor deposition, sputtering, CVD, or sol-gel method. Next, the thin metal film is patterned into a desired shape using a known method such as photolithography. This forms an electrode layer having a plurality of first ground electrodes 61A and second ground electrodes 61B in a planar strip shape and a plurality of signal electrodes 62 in a planar strip shape. The electrode layer may be formed by a method of forming a thin metal film through a mask of a desired shape, such as by vapor deposition or sputtering. The optical modulator 1 of this embodiment is obtained through the above steps.

[0080] [Light source module] Figure 10 shows a schematic plan view of a light source module according to one embodiment of the present invention. This light source module comprises an optical modulator according to the above embodiment, and a plurality of light sources that emit visible light incident on each of the plurality of input waveguides of the optical modulator.

[0081] The light source module 100 shown in Figure 10 is equipped with three light sources 7R, 7G, and 7B. Each light source 7R, 7G, and 7B emits visible light that is incident on the input waveguides 32a of the Mach-Zehnder type waveguides 32 (32R, 32G, and 32B) in the optical waveguide layer 3 of the optical modulator 1. For example, light source 7R can emit red light, light source 7G can emit green light, and light source 7B can emit blue light. For the light sources 7R, 7G, and 7B, laser elements such as laser diodes (LDs) can be used, and various commercially available laser elements can be used.

[0082] Figure 11 is a schematic cross-sectional view of a portion of the light source module 100 shown in Figure 10, cut in the XZ plane. Only a portion near the joint is depicted. Light source 7 is installed on the upper surface of light source base 20. The light source base 20 may be common to all light sources, or it may be individual for each light source. The light source base 20 is made of, for example, aluminum nitride (AlN), aluminum oxide (Al2O3), silicon (Si), etc.

[0083] The light source base 20 and the optical waveguide substrate 2, on which the optical waveguide layer is formed, can be directly joined via a metal layer 70. This configuration allows for further miniaturization by eliminating the need for spatial coupling or fiber coupling. By configuring the light source base 20 to bond the bonding surface 20A of the light source base 20 to the bonding surface 2A of the optical waveguide substrate 2 via a metal layer 70, the relative position of the light source base 20 and the optical waveguide substrate 2 can be adjusted during manufacturing to align the optical axis of the laser beam so that the optical axis of each light source 7 aligns with the axis of the input waveguide (active alignment).

[0084] The metal layer 70 may consist of multiple metal layers.

[0085] When the light source module of this embodiment is used in XR glasses, considering the amount of light required by the XR glasses, the gap (spacing) S between the bonding surface 20A of the light source base 20 and the bonding surface 2A of the optical waveguide substrate 2 is preferably, for example, greater than 0 μm and 5 μm or less.

[0086] (Drive method) An optical modulator can modulate input light into output light using a high-frequency modulation voltage and a DC bias voltage. The operating point Vd of the optical modulator is adjusted by controlling the DC bias voltage Vdc. The operating point Vd is the voltage at the center of the modulation voltage amplitude Vpp. Let Vπ(RF) be the half-wavelength voltage of the high-frequency modulation voltage.

[0087] [Optical engine and XR glasses] Figure 12 is a conceptual diagram illustrating an example of the XR glasses of the present invention. Figure 13 is a conceptual diagram showing how an image is projected directly onto the retina by laser light emitted from a light source module in the XR glasses shown in Figure 12.

[0088] The XR glasses (eyewear) 1000 of this embodiment are eyeglasses-type terminals. XR is a general term for Virtual Reality (VR), Augmented Reality (AR), and Mixed Reality. The symbol L shown in Figure 13 is the image display light.

[0089] The XR glasses 1000 of this embodiment shown in Figure 12 are configured such that the light source module 100 according to the above-described embodiment is mounted on an optical engine 5001 installed in the frame 1010. As shown in Figure 12, the optical engine 5001 includes a light source module 100, a light scanning mirror 3001, an optical system 2001 connecting the light source module 100 and the light scanning mirror 3001, a laser driver 1100, a light scanning mirror driver 1200, and a video controller 1300 that controls these drivers.

[0090] For example, a MEMS mirror can be used as the optical scanning mirror 3001. To project a 2D image, it is preferable to use a two-axis MEMS mirror as the optical scanning mirror 3001 that vibrates to reflect laser light by changing the angle in the horizontal direction (X direction) and the vertical direction (Y direction).

[0091] The optical system 2001 optically processes the laser light emitted from the light source module 100. For example, the optical system 2001 may include a collimator lens 2001a, a slit 2001b, and an ND filter 2001c. The optical system 2001 shown in Figure 12 is just one example, and other configurations are also possible.

[0092] In the XR glasses 1000 of this embodiment shown in Figure 12, as shown in Figure 13, the laser light R emitted from the light source module 100 attached to the frame 1010 is reflected by the optical scanning mirror 3001, and further reflected by the lens 4001 of the XR glasses 1000, and enters the human eyeball E as image display light L, allowing an image (video) to be projected directly onto the retina M.

[0093] Because the XR glasses 1000 of this embodiment are equipped with the light source module 100 of this embodiment, the electric field efficiency is reduced.

[0094] Although embodiments of the present invention have been described in detail above with reference to the drawings, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention. [Explanation of Symbols]

[0095] 1. Optical modulator 2 circuit boards 3 Optical waveguide layer 32 Ridge 32b Second Ridge Waveguide 32c First Ridge Waveguide 40 Optical multiplexing section 52 Buffer Layers 61A 1st ground electrode 61B 2nd ground electrode 62R, 62B, 62G signal electrode 100 light source modules

Claims

1. circuit board and Multiple Mach-Zehnder type waveguides, including a first ridge waveguide and a second ridge waveguide that propagate visible light, are formed on the substrate and arranged in parallel, and an optical waveguide layer made of lithium niobate is provided. A buffer layer formed on the optical waveguide layer, A plurality of signal electrodes having interaction portions formed on the buffer layer and arranged along each of the plurality of Mach-Zehnder type waveguides, and a plurality of first ground electrodes and a plurality of second ground electrodes arranged on both sides of each of the plurality of signal electrodes, spaced apart from the signal electrodes, The signal electrode is positioned above the first ridge waveguide. The second ground electrode is positioned above the second ridge waveguide. A signal electrode lead-out section that connects the signal electrode and a pad placed on the edge of the substrate, An optical multiplexer is positioned downstream of the plurality of Mach-Zehnder waveguides and combines the plurality of visible light waves that have passed through the plurality of Mach-Zehnder waveguides, A plurality of connecting waveguides that connect the downstream side of each of the plurality of Mach-Zehnder type waveguides to the optical multiplexer, The optical multiplexer comprises one output waveguide connected to the optical multiplexer, An optical modulator in which the first ground electrode, the second ground electrode, and the signal electrode lead-out section are arranged in a position that does not overlap with the optical multiplexing section when viewed from above.

2. The aforementioned Mach-Zehnder waveguide consists of three or more, The plurality of signal electrodes consist of three or more, and the three or more interaction parts of each are arranged parallel to each other in a plan view, and in order from the shortest to the longest length. The optical multiplexing section is positioned downstream of the signal electrode having the shortest interaction section among the plurality of signal electrodes. The aforementioned plurality of signal electrode extraction sections number three or more. The optical modulator according to claim 1, wherein two or more of the plurality of signal electrode lead-out sections extend from the downstream end of the interaction section in a direction of 90° or less with respect to the direction in which the interaction section extends, so as not to intersect with each other, and in a plan view, they straddle the plurality of connecting waveguides and are connected to a pad located on the downstream end of the signal electrode having the longest interaction section.

3. The optical modulator according to claim 1, further comprising a higher-order mode rejection section configured to remove higher-order modes in the waveguide upstream of each of the plurality of Mach-Zehnder type waveguides, before branching to the first ridge waveguide and the second ridge waveguide.

4. An optical modulator according to any one of claims 1 to 3, A light source module comprising: a plurality of light sources connected to the plurality of Mach-Zehnder waveguides, each emitting visible light incident on each of the plurality of input waveguides.

5. The light source module according to claim 4, An optical engine comprising: a light scanning mirror that reflects light emitted from the aforementioned light source module at a different angle to display an image.

6. XR glasses equipped with the optical engine described in claim 5.

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