Optical modulator, light source module, optical engine, and xr glass

Abstract

To provide an optical modulator of visible light that is compact and can be driven with a low voltage.SOLUTION: An optical modulator 1 comprises: a board 2; an optical waveguide layer 3 that has a Mach- Zehnder waveguide including a first ridge waveguide 32c and a second ridge waveguide 32b that propagate visible light and is made of lithium niobate; a buffer layer 52 formed on the optical waveguide layer 3; and a signal electrode 62, and a first earthing electrode 61A and a second earthing electrode 61B formed on the buffer layer 52. The signal electrode 62 and the first ridge waveguide 32c are disposed, deviating relatively while a horizontal distance between the center in a width direction of the signal electrode 62 and the center in a width direction of the first ridge waveguide 32c is equal to or less than 0.5 μm, and the second earthing electrode 61B and the second ridge waveguide 32b are disposed such that a horizontal distance between a side surface 61Ba in a side closer to the second ridge waveguide 32b from among two side surfaces of the second earthing electrode 61B and the center in a width direction of the second ridge waveguide 32b becomes 2 μm or smaller.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In recent years, light source modules equipped with an optical modulator into which light from a laser diode (semiconductor laser) is incident have been attracting attention. Such light source modules are known as AR (Augmented Reality) It can be used in the optical engines of eyeglass-type devices such as XR glasses, including Augmented Reality (AR) glasses and Virtual Reality (VR) glasses, as well as small projectors.

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

[0004] Furthermore, although not an image display device, Patent Document 2 describes a transmitting 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 optical modulator that has a substrate, an optical waveguide layer, a buffer layer, and an electrode layer, with the optical waveguide layer being made of a lithium niobate film. Patent Document 2 also discloses that the electrode layer of the optical modulator includes 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 optical modulator that has two signal electrodes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6728596 [Patent Document 2] Japanese Patent Publication No. 2022-036928 Summary of the Invention [Problem to be solved by the invention]

[0006] For the widespread use of eyeglass-type image display devices such as XR glasses, miniaturization and low drive voltage are essential. Furthermore, for mass production, they must be manufacturable at the lowest possible cost.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a visible light optical modulator that is small and can be driven at low voltage, a light source module, an optical engine equipped with the same, and XR glasses. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides the following means.

[0009] A first aspect of the present invention is an optical modulator comprising: a substrate; a Mach-Zehnder waveguide formed on the substrate and including a first ridge waveguide and a second ridge waveguide for propagating visible light; an optical waveguide layer made of lithium niobate; a buffer layer formed on the optical waveguide layer; a signal electrode formed on the buffer layer; and a first ground electrode and a second ground electrode disposed on either side of the signal electrode; the signal electrode is disposed above the first ridge waveguide and the second ground electrode is disposed above the second ridge waveguide; the signal electrode and the first ridge waveguide are relatively shifted such that the horizontal distance between the center of the signal electrode in its width direction and the center of the first ridge waveguide is 0.5 μm or less; and the second ground electrode and the second ridge waveguide are disposed such that the horizontal distance between one of two side surfaces of the second ground electrode closer to the second ridge waveguide and the center of the width direction of the second ridge waveguide is 2 μm or less.

[0010] A second aspect of the present invention is the optical modulator of the first aspect, wherein the buffer layer has a dielectric constant of 7 or more and a thickness of 0.4 μm or more and 1 μm or less.

[0011] A third aspect of the present invention is a light source module including the optical modulator of either the first or second aspect, and a light source that emits visible light that is incident on an input waveguide of the optical modulator.

[0012] A fourth aspect of the present invention is an optical engine including the light source module of the third aspect, and a light scanning mirror that reflects the light emitted from the light source module at different angles so as to display an image.

[0013] A fifth aspect of the present invention is XR glasses equipped with the optical engine of the fourth aspect. [Effects of the Invention]

[0014] According to the optical modulator of the present invention, it is possible to provide a small-sized optical modulator for visible light that can be driven at a low voltage. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic plan view illustrating an example of an optical modulator according to an embodiment. [Figure 2] 2 is a cross-sectional view of the optical modulator shown in FIG. 1 taken along line AA′ shown in FIG. [Figure 3] The center line of the first ridge waveguide is shifted to the right with respect to the center line of the signal electrode. [Figure 4] 10 is a graph showing the relationship between the relative shift DCC between the signal electrode and the first ridge waveguide and the electric field efficiency VπL. [Figure 5A] In the configuration where the width (We) of the signal electrode is 1.0 μm, the configurations where the relative shift DCC is −0.5 μm, 0 μm, and +0.5 μm are superimposed. [Figure 5B]In the configuration where the width (We) of the signal electrode is 2.0 μm, the configurations where the relative shift DCC is −0.5 μm, 0 μm, and +0.5 μm are superimposed. [Figure 5C] In a configuration in which the width (We) of the signal electrode is 3.0 μm, the configurations in which the relative shift DCC is −0.5 μm, 0 μm, and +0.5 μm are superimposed. [Figure 6] 10 is a graph showing the relationship between the dielectric constant of the dielectric material constituting the buffer layer and the electric field efficiency VπL, obtained by performing a simulation for red light with a wavelength of 638 nm when the width We of the signal electrode is 3 μm. [Figure 7] 10 is a graph showing the relationship between the thickness of the buffer layer (Tbuffer) and the electric field efficiency VπL, obtained by performing a simulation for red light with a wavelength of 638 nm when the width We of the signal electrode is 3 μm. [Figure 8] This is a graph showing the relationship between the buffer layer thickness (Tbuffer) and the transmission loss (PL) due to light absorption by the electrode, obtained by simulating red light with a wavelength of 638 nm when the signal electrode width We is 3 μm. [Figure 9] 1 is a schematic plan view of a light source module according to an embodiment of the present invention; [Figure 10] FIG. 10 is a schematic cross-sectional view of a part of the light source module shown in FIG. 9 cut along the XZ plane, depicting only a part near the joint. [Figure 11A] FIG. 10 is a diagram for explaining an example of a method for driving an optical modulator. [Figure 11B] 10A and 10B are diagrams for explaining another example of a method for driving an optical modulator. [Figure 11C] 10A and 10B are diagrams for explaining another example of a method for driving an optical modulator. [Figure 12] FIG. 1 is a conceptual diagram for explaining an example of the XR glasses of the present invention. [Figure 13] FIG. 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 FIG. 12. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the effects of the present invention.

[0017] [Optical modulator] Fig. 1 is a schematic plan view illustrating an example of an optical modulator according to an embodiment of the present invention, Fig. 2 is a schematic cross-sectional view of the optical modulator shown in Fig. 1 taken along line AA' shown in Fig. 1. 1 and 2, the X direction is a direction perpendicular to the side surface on which the light entrance is located, the Y direction is a direction perpendicular to the X direction, and the Z direction is a direction perpendicular to the plane formed by the X and Y directions.

[0018] The optical modulator of this embodiment is a Mach-Zehnder type (MZI type) optical modulator. The optical modulator 1 shown in FIGS. 1 and 2 includes a substrate 2, a Mach-Zehnder waveguide formed on the substrate 2 and including a first ridge waveguide 32c and a second ridge waveguide 32b for propagating visible light, an optical waveguide layer 3 made of lithium niobate, a buffer layer 52 formed on the optical waveguide layer 3, a signal electrode 62 formed on the buffer layer 52, and a first ground electrode 61A and a second ground electrode 61B disposed on both sides of the signal electrode 62, the signal electrode 62 being disposed above the first ground electrode 61A, and the second ground electrode 61B being disposed above the second ridge The second ground electrode 61B and the second ridge waveguide 32b are arranged such that the horizontal distance between the center of the width direction of the signal electrode 62 and the center of the width direction of the first ridge waveguide 32c is 0.5 μm or less, and the second ground electrode 61B and the second ridge waveguide 32b are arranged such that the horizontal distance between the side surface 61Ba of the second ground electrode 61B closer to the second ridge waveguide 32b and the center of the width direction of the second ridge waveguide 32b is 2 μm or less.

[0019] The optical modulator of this embodiment is small and can be driven at a low voltage. VπL can be used to evaluate miniaturization and low drive voltage. Vπ is the voltage required for half-wave phase modulation (half-wave voltage), and is defined as the difference between the voltage V1 at which the optical output is maximized and the voltage V2 at which it is minimized. Furthermore, L is the length of the phase modulation region (interaction length, electrode length), which is the length of the portion where the signal electrode overlaps the optical waveguide (ridge portion). If the interaction length L is long, the half-wave voltage Vπ will be small, and if the interaction length L is short, the half-wave voltage Vπ will be large. When attempting to reduce the size of the optical modulator, the interaction length L will be shortened and the half-wave voltage Vπ will be large. A smaller VπL indicates a smaller size and a lower driving voltage. Below, VπL may be referred to as the electric field efficiency.

[0020] (Board 2) The substrate 2 is not particularly limited as long as it has a refractive index lower than that of the lithium niobate film forming the optical waveguide layer 3, and examples thereof include a sapphire substrate, a Si substrate, and a thermally oxidized silicon substrate. The substrate 2 is preferably one on which a lithium niobate film can be formed as an epitaxial film. Since the optical waveguide layer is made of a lithium niobate (LiNbO3) film, there are no particular limitations on the substrate as long as it has a lower refractive index than the lithium niobate film, but 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 have the same symmetry, and a c-plane substrate is preferred for a sapphire single crystal substrate, and a (111) plane substrate is preferred for a silicon single crystal substrate.

[0021] (Optical waveguide layer 3) The optical waveguide layer 3 is made 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).

[0022] The lithium niobate may be, for example, a compound represented by the following formula (I): LixNbAyOz ···(I) (In formula (I), A represents an element other than Li, Nb, and O. x represents a number of 0.5 or more and 1.2 or less. y represents a number of 0 or more and 0.5 or less. z represents a number of 1.5 or more and 4.0 or less.)

[0023] In formula (I), A may be any element other than Li, Nb, and O, and examples thereof include 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 one element selected from these elements, or two or more elements selected from these elements. In formula (I), x is a number between 0.5 and 1.2, and 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, and preferably between 2.5 and 3.5.

[0024] The lithium niobate film forming the optical waveguide layer 3 is preferably an epitaxial film. The lithium niobate film is, for example, a c-axis oriented lithium niobate film. The lithium niobate film is, for example, an epitaxial film grown epitaxially on the substrate 2. An epitaxial film is a single-crystal film whose crystal orientation is aligned by the underlying substrate. An epitaxial film is a film with a single crystal orientation in the z direction and the xy plane direction, with the crystals aligned in the x-axis, y-axis, and z-axis directions. Whether the film formed on the substrate 2 is an epitaxial film can be verified, for example, by checking the peak intensity and poles at the orientation position in 2θ-θ X-ray diffraction.

[0025] The optical waveguide layer 3 has a plurality of flat portions 31 and ridge portions 32 that are arranged between adjacent flat portions 31 and have a band-like shape rising from the flat portions 31. In the optical modulator 1 of this embodiment, a single ridge portion 32 is collectively referred to as an input waveguide 32a (described later), an optical branching portion 4a, a first ridge waveguide 32c branching from the input waveguide 32a, a second ridge waveguide 32b, an optical multiplexing portion 4b to which these are coupled, and an output waveguide 32d. The number n of ridge portions 32 is an integer equal to or greater than 2. An example of three ridge portions (n=3) is shown in Fig. 1. Visible light having different wavelengths is incident on the three ridge portions 32R, 32G, and 32B.

[0026] In this embodiment, for example, red light having a peak wavelength of 610 nm or more and 750 nm or less is incident on the ridge portion 32R. Green light having a peak wavelength of 500 nm or more and 560 nm or less is incident on the ridge portion 32G. Blue light having a peak wavelength of 435 nm or more and 480 nm or less is incident on the ridge portion 32B. In the optical modulator 1 of this embodiment, red light, green light, and blue light are incident on the three ridge portions 32R, 32G, and 32B, respectively, and therefore the optical modulator 1 can be preferably used for, for example, XR glasses that can display full-color images.

[0027] As shown in FIG. 1, each of the three ridge portions 32R, 32G, and 32B has an input waveguide 32a, an optical branching portion 4a, a first ridge waveguide 32c, a second ridge waveguide 32b, an optical multiplexing portion 4b, and an output waveguide 32d.

[0028] In the three ridge portions 32R, 32G, and 32B of the optical waveguide layer 3, the input waveguide 32a has, for example, a substantially rectangular cross section, and receives visible light generated by a light source such as a laser element. As shown in FIG. 1, the input waveguide 32a is branched into a first ridge waveguide 32c and a second ridge waveguide 32b by an optical branching portion 4a. As shown in FIG. 2, the first ridge waveguide 32c and the second ridge waveguide 32b have, for example, a trapezoidal cross section. In this embodiment, the first ridge waveguide 32c and the second ridge waveguide 32b have the same cross section. As shown in FIG. 1, the first ridge waveguide 32c and the second ridge waveguide 32b are coupled by an optical multiplexing portion 4b to form an output waveguide 32d. The output waveguide 32d has, for example, a substantially rectangular cross section, and outputs the visible light signal generated in the optical multiplexing portion 4b.

[0029] The cross-sectional shapes of the input waveguide 32a and the output waveguide 32d are not limited to rectangular, 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 trapezoidal, but may be rectangular or semicircular, for example. Furthermore, the cross-sectional shapes of the input waveguide 32a, the output waveguide 32d, the first ridge waveguide 32c and the second ridge waveguide 32b may or may not be symmetrical.

[0030] When the optical modulator of this embodiment is used in an eyeglass-type image display device, the thickness (T slab ) is preferably 0.1 to 0.3 μm, and the thickness (T R ) is preferably 0.5 to 1.0 μm. This is because the thickness of the ridge portion 32 (T R ) is small, light does not propagate, and if it is large, the propagating light becomes multimode.

[0031] When the optical modulator of this embodiment is used in an eyeglass-type image display device, the distance (S) between the ridge portions 32 is preferably 2 to 12 μm. This is because by making S small, the distance between the signal electrode and the ground electrode can be shortened, and the efficiency of the electric field applied to the ridge portion 32 can be increased.

[0032] The ridge portion 32 shown in FIG. 1C ,L 2C This is an example of a trapezoid that is symmetric with respect to . When the optical modulator of this embodiment is used in an eyeglass-type image display device, in this example of the shape, the inclination angle (α) of the ridge portion 32 is preferably 60 to 90 degrees, because as the inclination angle becomes smaller, the propagating light becomes multimode. In addition, the width (W R ) is preferably 0.3 to 1.2 μm. This is because if the waveguide width is small, the light does not propagate, and if it is large, the propagating light becomes multimode.

[0033] (Protective layer 51) As shown in FIG. 2, the protective layer 51 is disposed 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 having a refractive index smaller than that of the optical waveguide layer 3. Examples of materials that can be used for the protective layer 51 include silicon oxide (SiO2), aluminum oxide (Al2O3), lanthanum oxide (La2O3), and composites of these oxides. Examples of composites of the oxides include LaAlSiInO. Of the above, silicon oxide (SiO2) is preferably used as the material for the protective layer 51.

[0034] (Buffer layer 52) The buffer layer 52 is formed on the optical waveguide layer 3 and the protective layer 51 , and prevents visible light propagating through the optical waveguide layer 3 from being absorbed by the electrode layer 6 .

[0035] The buffer layer 52 is made of a dielectric material having a refractive index smaller than that of the optical waveguide layer 3 . The dielectric material constituting the buffer layer 52 preferably has a dielectric constant of 7 or more, because this can reduce the electric field efficiency VπL. Specific examples of the material of 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 may be a different material.

[0036] The thickness of the buffer layer 52 (T buffer ) is preferably 0.4 μm or more and 1 μm or less, because the electric field efficiency VπL can be reduced.

[0037] (electrode layer 6) The electrode layer 6 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 disposed on both sides of the signal electrode 62. The optical modulator of this embodiment is a so-called single drive type optical modulator that has one signal electrode. So-called dual drive type optical modulators that have two signal electrodes have a complex electrode structure, and require applying inverted data electrical signals to the two signal electrodes while controlling the phases of the signals, which results in a problem of a complicated drive circuit configuration. The optical modulator of this embodiment is a single drive type, so it does not have this problem.

[0038] When the optical modulator of this embodiment is used in an eyeglass-type image display device, the width (We) of the signal electrode 62 is preferably 1.0 to 4.0 μm. This is because the electric field efficiency VπL can be reduced.

[0039] When the optical modulator of this embodiment is used in an eyeglass-type image display device, the width of the first ground electrode 61A and the second ground electrode 61B is preferably 50 to 1000 μm. This is because if the ground electrode is thin, the voltage does not become 0 V, and the electric field efficiency VπL increases.

[0040] When the optical modulator of this embodiment is used in an eyeglass-type image display device, the thickness (Te) of the electrode layer 6 is preferably 0.1 to 5 μm. This is because when the modulation frequency is high, the larger the electrode cross-sectional area is, the more efficiently the microwave propagates.

[0041] When the optical modulator of this embodiment is used in an eyeglass-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 the electric field efficiency VπL can be reduced.

[0042] (Positional relationship between electrode and ridge waveguide) The signal electrode 62 is located above the first ridge waveguide 32c (in a direction parallel to the Z direction) and is aligned with the center of the width of the signal electrode 62 (the center line L SC) and the center of the width direction of the first ridge waveguide 32c (the center line L passing through the center of the width direction of the first ridge waveguide 32c and extending in the Z direction 1C ) and the horizontal distance D (parallel to the Y direction) CC The distance between the centers is 0.5 μm or less. CC is sometimes called the relative shift. The relative positional relationship between the signal electrode 62 and the first ridge waveguide 32c is such that the relative shift D CC In a configuration where VπL is 0.5 μm or less, the first ridge waveguide 32c is disposed in a range where the electric field strength is high, and the electric field efficiency VπL is reduced. Also, the relative shift D CC By allowing this, the difficulty of manufacturing can be reduced and mass production can be facilitated.

[0043] In FIG. 2, the relative position (center-to-center shift) between the signal electrode 62 and the first ridge waveguide 32c is SC is the center line L of the signal electrode 62 1C 2 shows an example in which the first ground electrode 61A and the second ground electrode 61B are also shifted to the left in the same manner as the signal electrode 62, that is, an example in which the electrodes are shifted so that the distance G between the side surface 62a of the signal electrode 62 and the side surface 61Ab of the first ground electrode 61A and the distance G between the side surface 62b of the signal electrode 62 and the side surface 61Ba of the second ground electrode 61B remain unchanged. In contrast, FIG. 3 shows the center line L of the first ridge waveguide 32c. SC is the center line L of the signal electrode 62 1C This is the case when the value is shifted to the right relative to .

[0044] 3 shows a case where the relative positions (center-to-center shift) of the signal electrode 62 and the first ridge waveguide 32c are opposite to those shown in FIG. 2. For convenience, the shift between the signal electrode 62 and the first ridge waveguide 32c shown in FIG. 2 is taken as a plus, and the shift between the signal electrode 62 and the first ridge waveguide 32c shown in FIG. 3 is taken as a minus. The center-to-center distance (D CCThe sign "-1μm~+1μm" in the figure indicates this.

[0045] The second ground electrode 61B is located above the second ridge waveguide 32b (in the Z direction), and is aligned with the center of the second ridge waveguide 32b in the width direction (the center line L passing through the center of the second ridge waveguide 32b in the width direction and extending in the Z direction) of the second ridge waveguide 32b. 2C ) and the horizontal distance D (parallel to the Y direction) cE is less than 2 μm. The second ground electrode 61B is disposed at a position where it at least partially overlaps with the second ridge waveguide 32b in plan view from the Z direction.

[0046] (simulation) VπL was used as an index to evaluate miniaturization and low drive voltage. Vπ is the voltage required for half-wave phase modulation (half-wave 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, and represents the drive voltage. Furthermore, L is the length of the phase modulation region (interaction length, electrode length). The smaller VπL is, the smaller the size and the lower the drive voltage.

[0047] In the simulation, the FDM (Finite Deferrence Method) Solver from Photon Design's Fimmwave was used to calculate the optical transmission efficiency, and Maxwell from Ansys was used to calculate VπL.

[0048] In the simulation, a model having the configuration shown in FIGS. 1 and 2 was used, and the materials of the members forming the optical modulator 1 were as follows. The substrate 2 was a sapphire single crystal substrate, the optical waveguide layer 3 was a lithium niobate film epitaxially grown directly on the sapphire single crystal substrate, the protective layer 51 was silicon dioxide (SiO2), the buffer layer 52 was LaAlSiInO, and the electrode layer 6 was gold.

[0049] In the simulation, the dimensions of each part were as shown below. The thickness of the flat portion 31 of the optical waveguide layer 3 (T slab ) : 0.15μm The shape of the ridge portion 32 is 1C ,L 2C Symmetric with respect to . The thickness of the ridge portion 32 (T R ) : 0.7μm The width (W R ) : 0.8μm Inclination angle (α) of ridge portion 32: 80° Distance between ridges 32 (S): 4 μm The thickness of the buffer layer 52 (T buffer ) : 0.7μm Thickness of electrode layer 6 (Te): 2 μm Width (We) of the signal electrode 62: 1 μm, 2 μm, 3 μm Distance (G) between the signal electrode 62 and the ground electrode 61: 2 μm The center distance between the signal electrode 62 and the ridge portion (D CC ) : -1μm~+1μm The center distance between the second ground electrode 61B and the ridge portion (D CE ) : 0~2μm

[0050] The center distance (D CC ) the signs of "-1 μm to +1 μm" indicate the shift shown in Figure 2 for plus and the shift shown in Figure 3 for minus.

[0051] FIG. 4 shows the center-to-center distance (D) between the signal electrode 62 and the first ridge waveguide 32c obtained by performing a simulation for red light with a wavelength of 638 nm for configurations in which the width (We) of the signal electrode 62 is 1.0 μm, 2 μm, and 3 μm. CC ) and the electric field efficiency VπL.

[0052] From the graph shown in Figure 4, the relative shift D CC In the range of -0.5μm to +0.5μm, the range of the electric field efficiency VπL is as follows: When We=1μm: 0.98[Vcm]≦VπL≦1.13[Vcm] When We=2μm: 0.95[Vcm]≦VπL≦1.02[Vcm] When We=3μm: 0.92[Vcm]≦VπL≦0.96[Vcm]

[0053] Relative Shift D CC In the range of -0.5μm to +0.5μm, if the minimum value of the electric field efficiency VπL is VπL(min) and the maximum value of the electric field efficiency VπL is VπL(max), then when We=1μm, 2μm, and 3μm, the fluctuation range ({VπL(max) / VπL(min)}×100) [%] is as follows: When We=1μm: 15% When We=2μm: 7% When We=3μm: 4%

[0054] 5A to 5C show the relationship between the center distance (D CC ) is -0.5μm, 0μm, +0.5μm. CC is sometimes referred to as the relative shift. CC The solid line indicates the relative shift D CC The dashed lines indicate when the value is -0.5 μm and +0.5 μm.

[0055] FIG. 6 is a graph showing the relationship between the dielectric constant of the dielectric material constituting the buffer layer 52 and the electric field efficiency VπL, obtained by performing a simulation for red light having a wavelength of 638 nm when the width (We) of the signal electrode 62 is 3 μm. From the graph of FIG. 6, it can be seen that as the dielectric constant increases from 7 to 11, the electric field efficiency VπL decreases.

[0056] FIG. 7 shows the thickness (T) of the buffer layer 52 obtained by simulating red light with a wavelength of 638 nm when the width (We) of the signal electrode 62 is 3 μm.buffer ) and the electric field efficiency VπL. From the graph of FIG. 7, it can be seen that as the thickness of the buffer layer 52 decreases from 1 μm to 0.5 μm, the electric field efficiency VπL decreases.

[0057] FIG. 8 shows the thickness (T) of the buffer layer 52 obtained by simulating red light with a wavelength of 638 nm when the width (We) of the signal electrode 62 is 3 μm. buffer 1 is a graph showing the relationship between the optical transmission loss (PL) and the optical absorption loss (PL) caused by the electrodes. From the graph of FIG. 8, it can be seen that when the thickness of the buffer layer 52 is thinner than 0.4 μm, the transmission loss (PL) increases.

[0058] [Light source module] FIG. 9 shows a schematic plan view of a light source module according to one embodiment of the present invention. This light source module includes the optical modulator according to the above embodiment and a light source that emits visible light that is incident on the input waveguide of the optical modulator.

[0059] 9 includes three light sources 7R, 7G, and 7B as the light source 7. Each of the light sources 7R, 7G, and 7B emits visible light that is incident on the input waveguide 32a of each of the ridge portions 32R, 32G, and 32B in the optical waveguide layer 3 of the optical modulator 14. For example, the light source 7R can emit red light, the light source 7G can emit green light, and the light source 7B can emit blue light. The arrangement of the light sources of each color is not important; for example, the light source 7B can be connected to the central input waveguide 32a, and the light source 7R and the light source 7G can be connected to the input waveguides 32a at both ends. As the light sources 7R, 7G, and 7B, laser elements such as laser diodes (LD) can be used, and various commercially available laser elements can be used.

[0060] Fig. 10 is a schematic cross-sectional view of a part of the light source module 100 shown in Fig. 9 cut along the XZ plane, depicting only a part near the joint. The light source 7 is installed on the upper surface of a light source base 20. The light source base 20 may be common to all the light sources or 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), or the like.

[0061] The light source base 20 and the optical waveguide substrate 2 on which the optical waveguide layer is formed can be directly bonded via a metal layer 70. This configuration makes it possible to further reduce the size by eliminating spatial coupling or fiber coupling. By configuring the bonding surface 20A of the light source base 20 and the bonding surface 2A of the optical waveguide substrate 2 to be bonded via the metal layer 70, the relative positions of the light source base 20 and the optical waveguide substrate 2 can be adjusted during manufacturing, and the optical axis position of the laser light can be aligned so that the optical axis of each light source 7 coincides with the axis of the input waveguide (active alignment).

[0062] Metal layer 70 may consist of multiple metal layers.

[0063] When the light source module of this embodiment is used in XR glasses, taking into consideration the amount of light required in the XR glasses, it is preferable that 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 be, for example, greater than 0 μm and not greater than 5 μm.

[0064] (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. The half-wave voltage of the high-frequency modulation voltage is defined as Vπ(RF).

[0065] 11A to 11C are diagrams for explaining three examples of a method for driving an optical modulator. 11A to 11C, the horizontal axis represents the DC bias voltage applied to the optical modulator, and the vertical axis represents the intensity of the optical output at the applied voltage. The applied voltage width Vpp is the difference between the minimum value (Vmin) and the maximum value (Vmax) of the applied voltage.

[0066] 11A shows an example in which the DC bias voltage can be set to approximately 0 V by setting the operating point Vd' so that the shift amount of the operating point voltage is (Vn-0.5Vπ). For example, if the applied voltage width Vpp of the modulation voltage Vm is the half-wave voltage Vπ(RF), then a modulation voltage Vm in the range of (-1 / 2)Vπ(RF) to (1 / 2)Vπ(RF) is applied to the optical modulator. As shown in FIG. 11A, the optical output from the optical modulator is maximum when the modulation voltage Vm is (-1 / 2)Vπ(RF) and minimum when the modulation voltage Vm is (1 / 2)Vπ(RF). When the modulation voltage Vm is 0 V, the optical output is 50% of the maximum output.

[0067] Similarly, using Figure 11B, we will explain the optical modulation of an optical modulator in which the operating point Vd' is set so that the shift amount of the operating point voltage is (Vn-0.25Vπ) and the applied voltage width Vpp of the modulation voltage Vm is controlled as a (1 / 4) wavelength voltage (1 / 2)Vπ(RF). In this case, if the operating point voltage shift is set to (Vn-0.25Vπ), the operating point Vd' can be set to a DC bias voltage of approximately 0 (V). A modulation voltage Vm corresponding to the range from (-1 / 4)Vπ(RF) to (1 / 4)Vπ(RF) is applied to the optical modulator. As shown in Figure 11B, the optical output from the optical modulator is maximum when the modulation voltage Vm is (-1 / 4)Vπ(RF) and minimum when the modulation voltage Vm is (1 / 4)Vπ(RF). When the modulation voltage Vm is 0V(Vd'), the optical output is 15% of the maximum output.

[0068] Similarly, using Figure 11C, we will explain the optical modulation of an optical modulator in which the operating point Vd' is set so that the shift amount of the operating point voltage is (Vn-0.75Vπ) and the applied voltage width Vpp of the modulation voltage Vm is controlled as a (1 / 4) wavelength voltage (1 / 2)Vπ(RF). In this case, if the operating point voltage shift is set to (Vn-0.75Vπ), the operating point Vd' can be set to a DC bias voltage of approximately 0 (V). A modulation voltage Vm corresponding to the range from (-1 / 4)Vπ(RF) to (1 / 4)Vπ(RF) is applied to the optical modulator. As shown in Figure 11C, the optical output from the optical modulator is maximum when the modulation voltage Vm is (-1 / 4)Vπ(RF) and minimum when the modulation voltage Vm is (1 / 4)Vπ(RF). When the modulation voltage Vm is 0V(Vd'), the optical output is 85% of the maximum output.

[0069] [Optical engine and XR glasses] Fig. 12 is a conceptual diagram for explaining an example of the XR glasses of the present invention. Fig. 13 is a conceptual diagram showing how an image is projected directly onto the retina by laser light emitted from the light source module in the XR glasses shown in Fig. 12.

[0070] The XR glasses (eyeglasses) 1000 of this embodiment are glasses-type terminals. XR is a general term for virtual reality (VR), augmented reality (AR), and mixed reality. The symbol L shown in FIG. 13 denotes image display light.

[0071] The XR glasses 1000 of this embodiment shown in FIG. 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 a frame 1010. As shown in FIG. 12, the optical engine 5001 includes a light source module 100, an optical scanning mirror 3001, an optical system 2001 connecting the light source module 100 and the optical scanning mirror 3001, a laser driver 1100, an optical scanning mirror driver 1200, and a video controller 1300 that controls these drivers.

[0072] For example, a MEMS mirror can be used as the optical scanning mirror 3001. In order to project a 2D image, it is preferable to use, as the optical scanning mirror 3001, a two-axis MEMS mirror that vibrates so as to reflect laser light while changing the angle in the horizontal direction (X direction) and the vertical direction (Y direction).

[0073] The optical system 2001 optically processes the laser light emitted from the light source module 100. The optical system 2001 may include, for example, a collimator lens 2001a, a slit 2001b, and an ND filter 2001c. The optical system 2001 shown in Fig. 12 is an example, and other configurations may also be used.

[0074] In the XR glasses 1000 of this embodiment shown in Figure 12, as shown in Figure 13, 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, enters the human eyeball E as image display light L, and an image (video) can be projected directly onto the retina M.

[0075] The XR glasses 1000 of this embodiment are equipped with the light source module 100 of this embodiment, and therefore have reduced electric field efficiency.

[0076] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention. For example, the number of ridge portions in the optical waveguide layer of the optical modulator of the present invention is not limited to three or four, as long as it is two or more. [Explanation of symbols]

[0077] 1 Optical modulator 2 boards 3 Optical waveguide layer 32 Ridge 32b Second ridge waveguide 32c First ridge waveguide 52 buffer layer 61 Ground electrode 61A 1st ground electrode 61B 2nd ground electrode 62 signal electrode 100 Light Source Module

Claims

1. A substrate; an optical waveguide layer formed on the substrate, the optical waveguide layer having a Mach-Zehnder waveguide including a first ridge waveguide and a second ridge waveguide for propagating visible light, the optical waveguide layer being made of lithium niobate; a buffer layer formed on the optical waveguide layer; a signal electrode formed on the buffer layer; and a first ground electrode and a second ground electrode disposed on both sides of the signal electrode; the signal electrode is disposed above the first ridge waveguide; the second ground electrode is disposed above the second ridge waveguide; the signal electrode and the first ridge waveguide are arranged so as to be relatively shifted from each other, with a horizontal distance between the center of the signal electrode in its width direction and the center of the first ridge waveguide in its width direction being 0.5 μm or less; an optical modulator, wherein the second ground electrode and the second ridge waveguide are arranged such that the horizontal distance between one of two side surfaces of the second ground electrode that is closer to the second ridge waveguide and the center of the second ridge waveguide in the width direction is 2 μm or less.

2. the buffer layer has a dielectric constant of 7 or more; 2. The optical modulator according to claim 1, wherein the buffer layer has a thickness of 0.4 [mu]m or more and 1 [mu]m or less.

3. An optical modulator according to claim 1 or 2; a light source that emits visible light that is incident on the input waveguide of the optical modulator.

4. The light source module according to claim 3; an optical scanning mirror that reflects the light emitted from the light source module at a different angle so as to display an image;

5. XR glasses equipped with the optical engine according to claim 4.

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