Light modulator, visible light source module, optical engine, image display device, xr glasses, and method for controlling light modulator

Abstract

To provide an optical modulator capable of suppressing or compensating a DC drift.SOLUTION: The optical modulator of the present invention includes a plurality of Mach-Zehnder type optical modulation units, a power supply for applying a pixel voltage, and a control unit for controlling the power supply, wherein the control unit includes an integration circuit capable of obtaining an integrated value of the pixel voltage applied to each of the plurality of Mach-Zehnder type optical modulation units, the control unit can control the power supply so as to continuously apply the pixel voltage with a predetermined application voltage width independently to each of the plurality of Mach-Zehnder type light modulation units, and can control the power supply so as to apply a shift voltage of an even multiple of the half-wavelength voltage to each of the plurality of Mach-Zehnder type light modulation units at a timing when an integrated value of the pixel voltage integrated by the integration circuit reaches a predetermined value for any Mach-Zehnder type light modulation unit of the plurality of Mach-Zehnder type light modulation units, and SELECTED DRAWING: Figure 8C

Description

[Technical Field]

[0001] The present invention relates to an optical modulator, a visible light source module, an optical engine, an image display device, XR glasses, and a method for controlling an optical modulator. [Background technology]

[0002] XR glasses, such as AR (Augmented Reality) glasses and VR (Virtual Reality) glasses, are expected to become compact wearable devices. The key to the widespread adoption of XR glasses is miniaturization so that all functions fit into the size of regular eyeglasses. In this context, Mach-Zehnder optical modulators using lithium niobate films are expected to be a promising option (see, for example, Patent Documents 1 and 2).

[0003] It is known that Mach-Zehnder optical modulators using lithium niobate film suffer from a phenomenon known as DC drift, in which the bias voltage vs. optical output characteristic shifts over time in the bias voltage direction. Therefore, even if a constant bias voltage is applied to a Mach-Zehnder optical modulator, the optical output changes over time due to DC drift, making it difficult to obtain a constant optical output over the long term.

[0004] Patent Document 1 discloses an invention that follows up changes in the operating point voltage due to DC drift by performing feedback control on the bias voltage based on the average intensity of the output light. This invention is a means to solve the limitation on product lifespan caused by the range in which changes in the operating point voltage can be followed being limited by the withstand voltage of the modulator or IC, etc. In addition, by utilizing the property that the direction of DC drift is correlated with the polarity of the applied voltage, the invention is a means to control DC drift while keeping the operating point voltage within a specified range by changing the bias voltage to a voltage of the opposite polarity when the operating point voltage range is exceeded. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7306347 [Patent Document 2] Patent No. 7400661 [Patent Document 3] Patent No. 2518138 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the invention disclosed in Patent Document 1 requires an operating point voltage detection means for detecting the operating point voltage, which is the voltage at half the maximum optical output. Also, two reference voltages must be input and compared to calculate the operating point, which complicates control and implementation.

[0007] When a Mach-Zehnder optical modulator using a lithium niobate film is applied to XR glass, DC drift causes changes in the optical output over time, making it impossible to stably output the color corresponding to the drive voltage, so it is necessary to suppress or compensate for the DC drift.

[0008] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide an optical modulator, a visible light source module, an optical engine, an image display device, XR glasses, and a method for controlling an optical modulator that are capable of suppressing or compensating for DC drift. [Means for solving the problem]

[0009] The present disclosure provides the following means to solve the above problems.

[0010] A first aspect of the present disclosure is an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of multiple color laser beams one pixel at a time in a predetermined time step, the optical modulator comprising: a plurality of Mach-Zehnder optical modulation units each having a Mach-Zehnder optical waveguide formed of a ridge formed in a ferroelectric thin film represented by the chemical formula ABX3 and an electrode for applying an electric field to the Mach-Zehnder optical waveguide; a power supply for applying a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units; and a control unit for controlling the power supply, wherein the control unit controls the voltage applied to each of the plurality of Mach-Zehnder optical modulation units. the control unit can control the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width independently to each of the plurality of Mach-Zehnder optical modulation units, and the control unit can further control the power supply so as to apply a shift voltage that is an even multiple of a half-wave voltage to each of the plurality of Mach-Zehnder optical modulation units at a timing when the integrated value of the pixel voltage integrated by the integration circuit reaches a predetermined value for any of the plurality of Mach-Zehnder optical modulation units.

[0011] A second aspect of the present disclosure is an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of a plurality of color laser beams pixel by pixel at a predetermined time step, the optical modulator comprising: a plurality of Mach-Zehnder optical modulation units each having a Mach-Zehnder optical waveguide formed of a ridge formed in a ferroelectric thin film represented by the chemical formula ABX3 and an electrode for applying an electric field to the Mach-Zehnder optical waveguide; a power supply for applying a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units; and a control unit for controlling the power supply, wherein the control unit is configured to implement an integration circuit that can calculate an integrated value of the pixel voltage applied to each of the plurality of Mach-Zehnder optical modulation units. the control unit can control the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width independently to each of the plurality of Mach-Zehnder optical modulation units, and the control unit can further control the power supply so as to apply, to each of the plurality of Mach-Zehnder optical modulation units, a shift voltage that shifts the applied voltage width to a position symmetrical with respect to an extreme value of optical output in a modulation curve of each of the plurality of Mach-Zehnder optical modulation units, at a timing when an integrated value of the pixel voltage integrated by the integrating circuit for any of the plurality of Mach-Zehnder optical modulation units reaches a predetermined value.

[0012] A third aspect of the present disclosure is the optical modulator of the first or second aspect, wherein the Mach-Zehnder optical waveguide is made of C-axis oriented lithium niobate.

[0013] A fourth aspect of the present disclosure is the optical modulator of any one of the first to third aspects, wherein the Mach-Zehnder type optical waveguide is made of X-cut lithium niobate.

[0014] A fifth aspect of the present disclosure is a visible light source module including any one of the optical modulators of the first to fourth aspects and a plurality of visible laser light sources that respectively emit laser light of a plurality of colors.

[0015] A sixth aspect of the present disclosure is an optical engine including the visible light source module of the fifth aspect and a light scanning mirror that reflects light emitted from the visible light source module at different angles so as to display an image.

[0016] A seventh aspect of the present disclosure is an image display device equipped with the optical engine of the sixth aspect.

[0017] In an eighth aspect of the present disclosure, the image display device of the seventh aspect is XR glasses.

[0018] A ninth aspect of the present disclosure is a control method for an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of multiple color laser beams one pixel at a time in a predetermined time step, the method comprising: using an optical modulator including a plurality of Mach-Zehnder optical modulation units; a power supply for applying a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units; and a control unit that controls the power supply, the control unit having an integrating circuit that can calculate an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units; the control unit controls the power supply to apply the pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units continuously at a predetermined applied voltage width; and further controls the power supply to apply a shift voltage that is an even multiple of a half-wave voltage to each of the plurality of Mach-Zehnder optical modulation units when the integrated value of the pixel voltages integrated by the integrating circuit for any of the plurality of Mach-Zehnder optical modulation units reaches a predetermined value.

[0019] A tenth aspect of the present disclosure is a method for controlling an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of a plurality of color laser beams one pixel at a time in a predetermined time step, the method comprising: using an optical modulator including a plurality of Mach-Zehnder optical modulation units; a power supply for applying a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units; and a control unit that controls the power supply and has an integrating circuit that can obtain an integrated value of the pixel voltage applied to each of the plurality of Mach-Zehnder optical modulation units; and controlling the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width independently to each of the plurality of Mach-Zehnder optical modulation units, and further controlling the power supply so as to apply, to each of the plurality of Mach-Zehnder optical modulation units, a shift voltage that shifts the applied voltage width to a position symmetrical with respect to an extreme value of optical output in a modulation curve of each of the plurality of Mach-Zehnder optical modulation units, when an integrated value of the pixel voltage integrated by the integrating circuit for any of the plurality of Mach-Zehnder optical modulation units reaches a predetermined value. [Effects of the Invention]

[0020] According to the optical modulator of the present invention, it is possible to provide an optical modulator capable of suppressing or compensating for DC drift. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a conceptual diagram of a Mach-Zehnder type optical modulator. [Figure 2] FIG. 1 is a diagram illustrating the basic configuration of an optical modulator. [Figure 3] 10 is a diagram for explaining a case where the modulation curve of the LN optical modulator shifts to the positive side due to DC drift caused by a positive bias voltage. FIG. [Figure 4] 1 is a schematic configuration diagram of an image display device including an optical modulator according to the present disclosure. [Figure 5A] FIG. 2 is a conceptual diagram showing a drawing area on an image display surface, illustrating an example of a scanning method. [Figure 5B] 1 is a conceptual diagram showing a pattern of pixel voltages (pixel signals) applied to one of the RGB Mach-Zehnder light-modulating units, with the horizontal axis representing time and the vertical axis representing a pattern of pixel voltages (pixel signals) applied to one of the RGB Mach-Zehnder light-modulating units. [Figure 6] This conceptually shows the arrangement of three consecutive pixels that make up an image. [Figure 7] This diagram conceptually illustrates the combination of pixel voltages (pixel signals) for each of 1280 pixels in one column when the number of pixels is "1280 x 720." [Figure 8A] FIG. 2 is a conceptual diagram showing a drawing area on an image display surface, illustrating an example of a scanning method. [Figure 8B] This diagram conceptually shows control for shifting the range of the applied voltage width Vpp of the pixel voltage by applying a shift voltage in the scanning method shown in FIG. 8A. [Figure 8C] 8B is a diagram showing the application patterns of pixel voltages over time in the first scan [1], the second scan [2], and the third scan [3] shown in FIG. 8A. FIG. [Figure 8D] 10 is a diagram showing, over time, control for shifting the range of the applied voltage width Vpp of the pixel voltage by applying a shift voltage at the timing when the integrated value of the pixel voltage reaches a predetermined value. FIG. [Figure 9] 10 is a flowchart illustrating an example of control that a control unit executes on a light modulator during image formation. [Figure 10A] FIG. 3 is a schematic plan view of an optical modulator according to the present disclosure having three Mach-Zehnder optical waveguides 11 as shown in FIG. 2. [Figure 10B] 10B is a schematic plan view of another example of an optical modulator according to the present disclosure, which is the same as the optical modulator shown in FIG. 10A except that it includes an optical multiplexer. [Figure 11] 10C is a cross-sectional view of the optical modulator shown in FIGS. 10A and 10B taken along line AA'. FIG. [Figure 12] 1 is a schematic plan view of a light source module according to the present disclosure. [Figure 13]13 is a schematic cross-sectional view of a part of the light source module shown in FIG. 12 cut along the XZ plane, depicting only a part near the joint. [Figure 14] FIG. 1 is a conceptual diagram for explaining an example of XR glasses of the present disclosure. [Figure 15] FIG. 15 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. 14. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present disclosure will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for the sake of clarity, 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 modifications can be made within the scope of the effects of the present invention.

[0023] [Optical Modulator] Figure 1 shows a conceptual diagram of a Mach-Zehnder type optical modulator. The optical modulator according to the present disclosure is a Mach-Zehnder optical modulator (hereinafter, sometimes referred to as an "optical modulator" or "LN optical modulator"). The optical modulator includes a Mach-Zehnder optical waveguide and a modulation signal (drive signal) V m and an electrode for applying the voltage.

[0024] In the operating LN optical modulator, a high frequency signal V for modulation is applied to the electrodes. REF In addition, a DC bias voltage V is required to adjust the modulation state of the optical output. DC In this case, the bias voltage V DC is the modulating signal V m is the DC component of Input light L supplied from the light source in is intensity-modulated by the LN optical modulator, and the intensity-modulated output light L out will be output.

[0025] FIG. 2 shows the basic configuration of an optical modulator. The optical modulator 100 shown in FIG. 2 includes a Mach-Zehnder optical waveguide 11 and a modulation signal V m A Mach-Zehnder type optical modulation unit 1 has a modulation electrode (signal electrode) 12 for applying a modulation signal V m and a control unit 2 that supplies the In FIG. 2, the X direction is a direction perpendicular to the side surface on which the input port through which the input light is input 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.

[0026] In the optical modulator according to the present disclosure, the control unit includes a high-frequency signal pulse generation control circuit that controls application of pixel voltages, and a DC bias control circuit, and may also include an optical switch control circuit that controls the on / off of an optical switch, which will be described later.

[0027] The Mach-Zehnder optical modulation unit 1 receives a modulation signal V m The intensity of the output light is modulated in accordance with the input signal. The Mach-Zehnder optical waveguide 11 branches one input waveguide (optical waveguide) 43 at a Y branch 45 into two ridge-type optical waveguides, a first ridge-type optical waveguide 41 and a second ridge-type optical waveguide 42, which are then coupled again to one output waveguide 44 at a Y branch 46. The modulating electrode 12 comprises a signal electrode 12a formed between the first ridge-type optical waveguide 41 and the second ridge-type optical waveguide 42, and counter electrodes 12b1 and 12b2 provided to sandwich the first ridge-type optical waveguide 41 and the second ridge-type optical waveguide 42.

[0028] In the optical modulator according to the present disclosure, the modulating electrode relative to the Mach-Zehnder optical waveguide can be arranged in a known manner. Although FIG. 2 shows an example in which the modulating electrode is arranged on the side of the Mach-Zehnder optical waveguide, the modulating electrode may also be arranged above the Mach-Zehnder optical waveguide.

[0029] In the configuration diagram shown in Figure 2, a high-frequency signal V REF and DC bias voltage VDC For the modulation, only the modulation electrode 12 is used. REF and DC bias voltage V DC The electrodes may be separated for different purposes.

[0030] The Mach-Zehnder optical modulation unit 1 has a modulation curve (operating characteristic curve; see FIG. 3) specific to the optical modulator, and input light is modulated by a modulation signal Vm applied in accordance with this modulation curve and output as an output optical signal. The DC bias voltage Vm is a DC component of the modulation signal Vm. DC When the DC bias voltage V DC It is known that a phenomenon (DC drift) occurs in which the modulation curve (operating characteristic curve) shifts over time depending on the polarity of the

[0031] FIG. 3 is a diagram for explaining a case where the modulation curve of an LN optical modulator shifts to the positive side due to DC drift caused by a positive bias voltage. The modulation curve of an LN optical modulator is expressed as the optical output (optical intensity) of the output light periodically increasing and decreasing with increasing applied voltage.

[0032] In Figure 3, symbol C100 is the modulation curve when no DC drift occurs, and symbol C101 is the modulation curve when DC drift occurs. Symbol D100 is the output optical signal when no DC drift occurs, and symbol D101 is the output optical signal when DC drift occurs. Symbol A100 is the modulation signal (drive voltage). In the example shown in Figure 3, the voltages at which the minimum (0) and maximum (P0) of the optical output corresponding to the input signal as a binary signal are obtained are V0 and V1, respectively. If the voltages V0 and V1 are fixed when DC drift occurs, the optical output at voltages V0 and V1 will become P2 and P1, respectively, due to the periodicity of the modulation curve. If the amount of drift is dV, then in order to maintain the optical output before the DC drift after the DC drift, it becomes necessary to compensate for the DC drift by setting the voltages V0 and V1 to (V0 + dV) and (V1 + dV), respectively.

[0033] Figure 3 shows the DC drift due to a positive bias voltage, but the DC drift due to a negative bias voltage moves to the negative side.

[0034] The optical modulator disclosed herein is an optical modulator that can be used in an image display device that displays an image on an image display surface (projection surface) by scanning a combined light beam of multiple color laser beams pixel by pixel at a predetermined time step, and includes multiple Mach-Zehnder optical modulation units. Each Mach-Zehnder optical modulation unit has a Mach-Zehnder optical waveguide consisting of a ridge formed in a ferroelectric thin film expressed by the chemical formula ABX3, and electrodes for applying an electric field to the Mach-Zehnder optical waveguide. The ferroelectric thin film represented by the chemical formula ABX3 can be made of ferroelectric oxides such as barium titanate (BaTiO3), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), etc. Lithium niobate (LiNbO3) is particularly preferable.

[0035] The optical modulator according to the present disclosure further includes a power supply for applying a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units, and a control unit 2 for controlling the power supply, wherein the control unit 2 has an integrating circuit capable of calculating an integrated value of the pixel voltage applied to each of the plurality of Mach-Zehnder optical modulation units (1-1, 1-2, 1-3), and the control unit 2 can control the power supply so as to continuously apply a pixel voltage independently to each of the plurality of Mach-Zehnder optical modulation units (1-1, 1-2, 1-3) at a predetermined applied voltage width, and the control unit 2 can further control the power supply so as to apply a shift voltage to each of the plurality of Mach-Zehnder optical modulation units (1-1, 1-2, 1-3) when the integrated value of the pixel voltage integrated by the integrating circuit for any of the plurality of Mach-Zehnder optical modulation units (1-1, 1-2, 1-3) reaches a predetermined value.

[0036] FIG. 4 is a schematic diagram illustrating the configuration of an image display device including an optical modulator according to the present disclosure. The image display device shown in FIG. 4 is an image display device capable of full-color display, equipped with red (R), green (G), and blue (B) visible light laser light sources 30-1, 30-2, and 30-3, respectively, which are arranged so that their light can be incident on Mach-Zehnder type optical modulation units 1-1, 1-2, and 1-3, respectively. However, the optical modulator according to the present disclosure can be applied to any image display device equipped with two or more light sources that emit light of different colors. Reference numeral 50 denotes an optical multiplexer.

[0037] Fig. 5A shows a drawing area on an image display surface, and is a conceptual diagram illustrating an example of a scanning method for displaying an image by changing the light intensity (color tone) for each pixel while scanning a laser beam using an image display device equipped with an optical modulator according to the present disclosure. Fig. 5B is a conceptual diagram showing time on the horizontal axis and a pattern of pixel voltages (pixel signals) applied to one of the RGB Mach-Zehnder optical modulators on the vertical axis.

[0038] As shown in Figure 5A, a laser beam (LB) is scanned in sequence to form an image. The start time of the scan for displaying one image is t0, and the end time of the scan is t1. The arrow in Figure 5A indicates the scanning direction of the laser light, and the laser light scans one pixel at a time from left to right, and when it reaches the right end, it goes down one row and scans one pixel at a time from right to left, and when it reaches the left end, it goes down one row and scans one pixel at a time from left to right, repeating this scanning (raster scanning). This scanning method is just one example, and any scanning method that scans one pixel at a time will do.

[0039] As the laser beam moves through each dot (pixel) of the image, the color of the laser changes over time. It takes a certain amount of time to form one image, but because it is so fast that the human eye cannot keep up with it, it is perceived as a single image. The scanning speed of the laser beam is typically around 100 to 500 MHz (a speed at which the entire image changes about 60 times per second). For example, if the drawing time for one pixel is 10 ns (nanoseconds), this is much shorter than the time constant of DC drift (approximately 200 ms (milliseconds)). Color tones are changed by varying the light intensity of the three primary colors of light: red (R), green (G), and blue (B). For example, if the intensity of each color is varied using 8 bits of red, 8 bits of green, and 8 bits of blue, the resulting color will have 24-bit color tones (approximately 16.77 million colors) (24-bit color system). In 24-bit color system, each RGB color has 8 bits of information, and each can be reproduced in 256 shades. Each RGB has a voltage value ranging from 0 to 255; for example, if all RGB are 0, the result is black, and if all are 255, the result is white.

[0040] FIG. 6 conceptually shows the arrangement of three consecutive pixels among the pixels that make up an image. The color displayed by each pixel is determined by the combination of the light intensities of the three colors red (R), green (G), and blue (B).

[0041] FIG. 7 conceptually illustrates combinations of pixel voltages (pixel signals) for each of 1280 pixels in one column when the number of pixels is "1280×720." The 1280 pixels are named, from left to right, pixel number 1, pixel number 2, pixel number 3, ..., pixel number 1279, and pixel number 1280. If the drawing time for one pixel is 10 ns, the time required to scan one row is 12.8 μs (microseconds), and the time required to display one screen is approximately 10 ms (milliseconds).

[0042] In the optical modulator of the present disclosure, the control unit has an integrating circuit that can obtain an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units. In the driving example shown in FIG. 7, in scanning one column, the pixel voltages applied to the Mach-Zehnder optical modulation unit for red (R) to pixel numbers 1, 2, 3, . . . , 1279, and 1280 are respectively V R (1), V R (2), V R (3),···,V R (1279), V R(1280), after the application time of these pixel voltages, a DC drift corresponding to the pixel voltage integrated value obtained by integrating each pixel voltage x 1 pixel drawing time may occur. Similarly, for the Mach-Zehnder type optical modulation section for green (G), V G (1), V G (2), V G (3),···,V G (1279), V G DC drift occurs according to the pixel voltage integrated value obtained by integrating each pixel voltage of (1280) x 1 pixel drawing time. B (1), V B (2), V B (3),···,V B (1279), V B DC drift may occur according to the pixel voltage integrated value obtained by integrating "each pixel voltage x one pixel drawing time" in (1280).

[0043] An example of a method for controlling the application of pixel voltages to a Mach-Zehnder type optical modulation section, which is performed in the optical modulator of the present disclosure, will be conceptually described with reference to FIGS. 8A to 8D. In Figures 8A to 8D, [1] to [3] correspond to each other.

[0044] In this optical modulator, when the integrated value of the pixel voltage in any of the multiple Mach-Zehnder optical modulation units (1-1, 1-2, 1-3) reaches a predetermined value (Vt0: see Figure 8D), a shift voltage that is an even multiple (twice in this example; see Figures 8B and 8C) of the half-wave voltage Vπ is applied to the multiple Mach-Zehnder optical modulation units (1-1, 1-2, 1-3), thereby compensating for DC drift caused by driving.

[0045] FIG. 8A shows a drawing area on an image display surface, and is a conceptual diagram illustrating an example of a scanning method. In FIG. 8A, [1] indicates the first (1st) scan of the raster scan, [2] indicates the second scan of the raster scan, and [3] indicates the third scan of the raster scan. The timing at which the first scan [1], the second scan [2], and the third scan [3] end is the timing at which the integrated value of the pixel voltage in one of the Mach-Zehnder optical modulation units reaches a predetermined value (Vt0: see Figure 8D).

[0046] 8B is a diagram showing control for shifting the range of the applied voltage width Vpp of the pixel voltage by applying a shift voltage (ΔV) that is an even multiple (twice in this example) of the half-wave voltage (Vπ) to each of a plurality of Mach-Zehnder optical modulation units. FIG. 8B illustrates one of the plurality of Mach-Zehnder optical modulation units. In Fig. 8B, [1] indicates the range of the applied voltage width Vpp in the first scan [1] of the raster scan, which indicates the range of the applied voltage width Vpp before the application of the shift voltage. Also, in Fig. 8B, [2] indicates the range of the applied voltage width Vpp in the second scan [2] of the raster scan, which indicates the range of the applied voltage width Vpp after the application of the shift voltage. Although not shown in Fig. 8B, when the shift voltage is applied after the second scan [2], the range of the applied voltage width Vpp in the third scan [3] almost overlaps with the range of the applied voltage width Vpp in the first scan [1].

[0047] Fig. 8C shows the pixel voltage application patterns over time for the first scan [1], the second scan [2], and the third scan [3] shown in Fig. 8A. Fig. 8D shows the control of shifting the range of the applied voltage width Vpp of the pixel voltage by applying a shift voltage when the integrated value of the pixel voltage reaches a predetermined value. As shown in Figure 8C, a positive pixel voltage was applied in the first scan [1], but by applying a shift voltage of 2Vπ to each of the multiple Mach-Zehnder optical modulators at the timing when the first scan [1] is completed (in other words, when the integrated value of the pixel voltage reaches a predetermined value), a negative pixel voltage is applied in the second scan [2]. Furthermore, by applying a shift voltage of 2Vπ to each of the multiple Mach-Zehnder optical modulators at the timing when the second scan [2] is completed, a positive pixel voltage is applied in the third scan [3].

[0048] If the application of the shift voltage at the timing of completion of the first scan [1] is referred to as the application of a negative shift voltage, the control unit controls the application of the shift voltage at the timing of completion of the second scan [2] to be a positive shift voltage, and controls the power supply so that the application of the negative shift voltage and the application of the positive shift voltage are repeated alternately. In this way, by controlling the polarity of the voltage applied to the Mach-Zehnder optical modulation unit to change every time the integrated value of the pixel voltage reaches a predetermined value, it is possible to compensate for DC drift.

[0049] In the example shown in FIGS. 8A to 8C, a shift voltage (ΔV) that is an even multiple (twice in this example) of the half-wave voltage (Vπ) is applied, but a shift voltage of a different magnitude may also be applied. For example, the shift voltage may be applied so as to have an applied voltage range at a position symmetrical with respect to the extreme value of the optical output in each modulation curve of a plurality of Mach-Zehnder optical modulation units. This will be specifically explained using FIG. 8B. In the modulation curve shown in FIG. 8B, there is a nearest minimum on the positive side and a nearest maximum on the negative side of an applied voltage of 0V. The shift voltage may be applied so as to have an applied voltage range at a position symmetrical with respect to an axis of symmetry passing through the minimum value, or the shift voltage may be applied so as to have an applied voltage range at a position symmetrical with respect to an axis of symmetry passing through the maximum value.

[0050] The optical modulator of the present disclosure can perform DC drift compensation for each color, thereby making it possible to maintain the same output light.

[0051] Fig. 9 is a flowchart showing an example of the control that the control unit executes on the optical modulator during image formation. Fig. 9 shows the control steps from the start of displaying the first screen to the completion of displaying the first screen.

[0052] In step 1, control is performed so that the application of pixel voltage to each pixel continues until the integrated value of the image voltage applied to the Mach-Zehnder optical modulation unit for each color reaches a predetermined value in response to the scanning of the laser light.

[0053] In step 2-1, when the integrated value of the image voltage reaches a predetermined value, a shift voltage is applied to the Mach-Zehnder optical modulation unit of each color, and control is performed so that the application of the pixel voltage to each pixel continues until the integrated value of the next image voltage reaches the predetermined value. In step 2-2, when the integrated value of the image voltage reaches a predetermined value, a shift voltage is applied to the Mach-Zehnder optical modulation unit of each color, and control is performed so that the application of the pixel voltage to each pixel continues until the integrated value of the next image voltage reaches the predetermined value.

[0054] The subsequent flow is a repetition of step 1-1, step 2-1, and step 2-2. As described above, the control unit controls the steps of Step 1, in which the application of the pixel voltage is continued, and Step 2, in which, when the integrated value of the image voltage reaches a predetermined value, the shift voltage is applied to the Mach-Zehnder optical modulation unit for each color and the application of the pixel voltage is continued, to be repeated in order until the formation of the first screen is completed. After the first screen is formed, the control unit controls the process to repeat steps 1 and 2 until the second screen is formed, and continues this process until the final screen is formed.

[0055] FIG. 10A is a schematic plan view of an optical modulator according to the present disclosure having three Mach-Zehnder optical waveguides 11 as shown in FIG. The optical modulator 200 shown in FIG. 10A includes three Mach-Zehnder optical waveguides 11-1, 11-2, and 11-3, but three is just an example, and the optical modulator 200 may include two or four or more. In the optical modulator 200 shown in FIG. 10A, light input from each input port 43i of three Mach-Zehnder optical waveguides 11-1, 11-2, and 11-3 is output from the output port 44o of each optical waveguide.

[0056] The electrode configuration and circuit diagram shown in Fig. 10A are an example. Fig. 10A shows a case where a DC bias voltage is superimposed on a high frequency signal applied to electrodes 25 and 26. The electrodes 25 and 26 are electrodes that apply a modulation voltage to each of the Mach-Zehnder optical waveguides 11-1, 11-2, and 11-3. The electrode 25 is an example of a first electrode, and the electrode 26 is an example of a second electrode. The power supply 131 is part of a high-frequency signal pulse generation control circuit that applies a modulation voltage to each of the Mach-Zehnder optical waveguides 11. The power supply 133 is part of a DC bias control circuit that applies a DC bias voltage to each of the Mach-Zehnder optical waveguides 11. For simplicity of illustration, the electrodes 25 and 26 are only shown in the area of ​​the Mach-Zehnder optical waveguide 11-3.

[0057] FIG. 10B is a schematic plan view of another example of an optical modulator according to the present disclosure, which is the same as the optical modulator shown in FIG. 10A except that it includes an optical multiplexer. In the optical modulator 201 shown in FIG. 10B, light input from each input port 43i of three Mach-Zehnder optical waveguides 11-1, 11-2, and 11-3 is multiplexed in the optical multiplexing section 50 and output from one output port 44oo.

[0058] Fig. 11 is a cross-sectional view of the optical modulator shown in Fig. 10A taken along line AA'. The same applies to the cross-sectional view of the optical modulator shown in Fig. 10B taken along line AA'. The optical modulator 200 (201) shown in FIG. 11 has a substrate 10 made of a material different from lithium niobate, and a lithium niobate film 24 formed on the main surface of the substrate 10.

[0059] As shown in FIG. 11, the lithium niobate film 24 is composed of a ridge-type optical waveguide 24-1 (corresponding to the first ridge-type optical waveguide 41 and the second ridge-type optical waveguide 42) protruding from the first surface 24A, and a slab layer 24-2 which is the portion other than the ridge, but may be composed of only a ridge-type optical waveguide without the slab layer.

[0060] When the optical modulator 200 (201) shown in FIG. 11 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 (T R ) is small, light does not propagate, and if it is large, the propagating light becomes multimode.

[0061] When the optical modulator 200 (201) shown in FIG. 11 is used in an eyeglass-type image display device, the distance (S) between the ridge-type optical waveguides 24-1 is preferably 2 to 12 μm. This is because by making S small, the efficiency of the electric field applied to the ridge-type optical waveguide 24-1 can be increased.

[0062] 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.

[0063] Examples of the substrate 10 include a sapphire substrate, a Si substrate, and a thermally oxidized silicon substrate. Since the optical multiplexing functional layer 20 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.

[0064] The lithium niobate film is, for example, a c-axis oriented lithium niobate film. The lithium niobate film is, for example, an epitaxial film epitaxially grown on a substrate 10. 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 in-plane direction, and the crystals are aligned in the x-axis, y-axis, and z-axis directions. Whether the film formed on the substrate 10 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.

[0065] Specifically, when measured by 2θ-θ X-ray diffraction, all peak intensities other than the target plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, when the lithium niobate film is a c-axis oriented epitaxial film, the peak intensities other than the (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. Here, (00L) is a general term for equivalent planes such as (001) and (002).

[0066] Furthermore, the conditions for confirming the peak intensity at the orientation position described above only indicate orientation in one direction. Therefore, even if the above conditions are met, if the crystal orientation is not uniform within the plane, the X-ray intensity will not increase at a specific angular position, and no pole points will be observed. For example, in the case of a lithium niobate film, because LiNbO3 has a trigonal crystal structure, there will be three pole points of LiNbO3 (014) in the single crystal. In the case of lithium niobate, it is known that epitaxial growth occurs in a so-called twin state, in which crystals rotated 180° around the c-axis are symmetrically bonded. In this case, three pole points are symmetrically bonded to two, resulting in six pole points. Furthermore, when a lithium niobate film is formed on a silicon single crystal substrate with a (100) plane, the substrate has four-fold symmetry, so 4 x 3 = 12 pole points are observed. Note that in this disclosure, lithium niobate films epitaxially grown in a twin state are also included in the term epitaxial film.

[0067] The composition of lithium niobate is Li x NbA y O z where A is an element other than Li, Nb, and O. x is 0.5 or more and 1.2 or less, and preferably 0.9 or more and 1.05 or less. y is 0 or more and 0.5 or less. z is 1.5 or more and 4.0 or less, and preferably 2.5 or more and 3.5 or less. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, and two or more of these elements may be combined. Furthermore, the lithium niobate film may be a lithium niobate single crystal thin film bonded onto a substrate.

[0068] (Protective layer 51) As shown in FIG. 2, the protective layer 51 is disposed between the slab layer 24-2 of the lithium niobate film 24 and the buffer layer 52. The protective layer 51 is made of a dielectric material having a smaller refractive index than the lithium niobate film 24. 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 these oxides include LaAlSiInO. Of the above, silicon oxide (SiO2) is preferably used as the material for the protective layer 51.

[0069] (Buffer layer 52) The buffer layer 52 is formed on the lithium niobate film 24 and the protective layer 51, and prevents visible light propagating through the lithium niobate film 24 from being absorbed by the electrode layer.

[0070] The buffer layer 52 is made of a dielectric material having a refractive index smaller than that of the lithium niobate film 24 . 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.

[0071] 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.

[0072] (electrodes 25, 26) When the optical modulator of the present disclosure is used in an eyeglass-type image display device, the width (We) of the electrodes 25 and 26 is preferably 1.0 to 4.0 μm. This is because the electric field efficiency VπL can be reduced.

[0073] When the optical modulator of the present disclosure is used in an eyeglass-type image display device, the thickness (Te) of the electrodes 25 and 26 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.

[0074] A ridge-type optical waveguide is formed by a bulk lithium niobate film attached to a substrate; The C-axis of the lithium niobate may be parallel to the main surface of the substrate.

[0075] [Light source module] A light source module according to the present disclosure includes an optical modulator according to the present disclosure and a plurality of laser light sources. Fig. 12 is a schematic plan view of a light source module according to the present disclosure. Fig. 12 shows an example of a light source module including the optical modulator 201 shown in Fig. 11. Fig. 13 is a schematic cross-sectional view of a part of the light source module shown in Fig. 12 taken along the XZ plane, depicting only a part near the joint.

[0076] The light source module 1000 shown in FIG. 12 includes an optical modulator 201 and three laser light sources 30 (30-1, 30-2, 30-3) that emit light to be modulated by the optical modulator 201.

[0077] Various laser elements can be used as the laser light source 30. The laser light source 30 can emit visible light. In this case, the light source module 1000 is a visible light source module. The three laser light sources 30-1, 30-2, and 30-3 can be, for example, commercially available laser diodes (LDs) that emit red, green, or blue light. The red light can have a peak wavelength of 610 nm or more and 750 nm or less, the green light can have a peak wavelength of 500 nm or more and 560 nm or less, and the blue light can have a peak wavelength of 435 nm or more and 480 nm or less. In the light source module 1000, the laser light sources 30-1, 30-2, and 30-3 are respectively an LD that emits green light, an LD that emits blue light, and an LD that emits red light. The LDs 30-1, 30-2, and 30-3 are disposed at intervals in a direction substantially perpendicular to the emission direction of the light emitted from each LD, and are provided on the upper surface of the subcarrier 120.

[0078] The LD 30 can be mounted as a bare chip on the subcarrier 120. The subcarrier 120 is made of, for example, aluminum nitride (AlN), aluminum oxide (Al2O3), silicon (Si), or the like.

[0079] The subcarrier 120 can be configured to be directly bonded to the substrate 10 via a metal bonding layer. This configuration makes it possible to further reduce the size by eliminating spatial coupling or fiber coupling. By configuring the subcarrier 120 and the substrate 10 to be bonded via a metal bonding layer, the relative positions of the subcarrier 120 and the substrate 10 can be adjusted during manufacturing, and the optical axis position of the laser light can be aligned so that the optical axis of each visible light laser coincides with the axis of each optical waveguide 43 (active alignment).

[0080] In the light source module 1000, the light exit surface 31 of the LD 30 and the light incident surface (side surface) 201A of the optical modulator 201 are disposed at a predetermined distance. The light incident surface 201A faces the light exit surface 31, and there is a gap D between the light exit surface 31 and the light incident surface 201A in the x direction. Because the light source module 1000 is exposed to the air, the gap D is filled with air. Because the gap D is filled with the same gas (air), it is easy to make each color light emitted from the LD 30 enter the incident path while satisfying a predetermined coupling efficiency. When the light source module 1000 is used in AR glasses or VR glasses, taking into account the light intensity required for the AR glasses or VR glasses, the size of the gap (distance) D in the x direction is, for example, greater than 0 μm and equal to or less than 5 μm.

[0081] [Optical engine and XR glasses] In this specification, an optical engine is a device that includes a plurality of light sources, an optical system including a multiplexing unit that combines the plurality of light beams emitted from the plurality of light sources into a single beam of light, an optical scanning mirror that reflects the light emitted from the optical system at a different angle so as to display an image, and a control element that controls the optical scanning mirror.

[0082] Fig. 14 is a conceptual diagram for explaining an example of XR glasses of the present disclosure. Fig. 15 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. 14. Symbol L is image display light.

[0083] The XR glasses (eyeglasses) 10000 of the present disclosure 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. 15 denotes image display light.

[0084] The XR glasses 10000 of the present disclosure shown in FIG. 14 are configured such that the light source module 1000 according to the embodiment described above is mounted on an optical engine 5001 installed in a frame 1010. As shown in FIG. 14, the optical engine 5001 includes a light source module 1000, an optical scanning mirror 3001, an optical system 2001 connecting the light source module 1000 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.

[0085] 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).

[0086] The optical system 2001 optically processes the laser light emitted from the light source module 1000. 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. 14 is an example, and other configurations may also be used.

[0087] In the XR glasses 10000 of the present disclosure shown in Figure 14, as shown in Figure 15, laser light R emitted from a light source module 1000 attached to a frame 1010 is reflected by an optical scanning mirror 3001, and further reflected by a lens 4001 of the XR glasses 10000, enters the human eyeball E as image display light L, and an image (video) can be projected directly onto the retina M.

[0088] The XR glasses 10000 of the present disclosure are equipped with the light source module 1000 of the present disclosure, and therefore have reduced electric field efficiency. [Explanation of symbols]

[0089] 1. Mach-Zehnder optical modulator 2. Control Unit 10 Substrate 100, 200, 201 Optical modulator 1000 Light Source Module 5001 Optical Engine 10000 XR Glasses

Claims

1. An optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of a plurality of color laser beams pixel by pixel at a predetermined time step, Chemical formula ABX ３ a plurality of Mach-Zehnder optical modulation sections each having a Mach-Zehnder optical waveguide formed of a ridge in a ferroelectric thin film represented by the formula (1) and electrodes for applying an electric field to the Mach-Zehnder optical waveguide; a power supply for applying a pixel voltage to each of the plurality of Mach-Zehnder optical modulation units independently; a control unit that controls the power supply, the control unit has an integrating circuit capable of calculating an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units, the control unit can control the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width to each of the plurality of Mach-Zehnder optical modulation units independently, the control unit can further control the power supply to apply a shift voltage that is an even multiple of a half-wave voltage to each of the plurality of Mach-Zehnder optical modulation units at a timing when an integrated value of the pixel voltage integrated by the integrating circuit reaches a predetermined value for any of the plurality of Mach-Zehnder optical modulation units.

2. An optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light of a plurality of color laser beams pixel by pixel at a predetermined time step, Chemical formula ABX ３ a plurality of Mach-Zehnder optical modulation sections each having a Mach-Zehnder optical waveguide formed of a ridge in a ferroelectric thin film represented by the formula (1) and electrodes for applying an electric field to the Mach-Zehnder optical waveguide; a power supply for applying a pixel voltage to each of the plurality of Mach-Zehnder optical modulation units independently; a control unit that controls the power supply, the control unit has an integrating circuit capable of calculating an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units, the control unit can control the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width to each of the plurality of Mach-Zehnder optical modulation units independently, the control unit can further control the power supply so as to apply, to each of the plurality of Mach-Zehnder optical modulation units, a shift voltage that shifts an applied voltage width to a position symmetrical with respect to an extreme value of optical output in a modulation curve of each of the plurality of Mach-Zehnder optical modulation units, at a timing when an integrated value of the pixel voltage integrated by the integrating circuit reaches a predetermined value for any of the plurality of Mach-Zehnder optical modulation units.

3. 2. The optical modulator according to claim 1, wherein the Mach-Zehnder optical waveguide is made of C-axis oriented lithium niobate.

4. 3. The optical modulator according to claim 2, wherein said Mach-Zehnder type optical waveguide is made of C-axis oriented lithium niobate.

5. 2. The optical modulator according to claim 1, wherein the Mach-Zehnder type optical waveguide is made of X-cut lithium niobate.

6. 3. The optical modulator according to claim 2, wherein the Mach-Zehnder type optical waveguide is made of X-cut lithium niobate.

7. An optical modulator according to any one of claims 1 to 6; A visible light source module includes a plurality of visible light laser light sources that respectively emit laser light of a plurality of colors.

8. The visible light source module according to claim 7 ; an optical scanning mirror that reflects the light emitted from the visible light source module at different angles so as to display an image;

9. An image display device equipped with the optical engine according to claim 8.

10. The image display device according to claim 9 , wherein the image display device is an XR glass.

11. 1. A method for controlling an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light beam of laser beams of a plurality of colors one pixel at a time in a predetermined time step, comprising: an optical modulator including a plurality of Mach-Zehnder optical modulation units, a power supply for applying pixel voltages independently to each of the plurality of Mach-Zehnder optical modulation units, and a control unit for controlling the power supply, the control unit having an integrating circuit capable of calculating an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units, a control circuit for controlling an optical modulator, the control unit controlling the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width to each of the plurality of Mach-Zehnder optical modulation units independently, and further controlling the power supply so as to apply a shift voltage that is an even multiple of a half-wave voltage to each of the plurality of Mach-Zehnder optical modulation units at a timing when an integrated value of the pixel voltage integrated by the integrating circuit for any of the plurality of Mach-Zehnder optical modulation units reaches a predetermined value.

12. 1. A method for controlling an optical modulator that can be used in an image display device that displays an image on an image display surface by scanning a combined light beam of laser beams of a plurality of colors one pixel at a time in a predetermined time step, comprising: an optical modulator including a plurality of Mach-Zehnder optical modulation units, a power supply for applying pixel voltages independently to each of the plurality of Mach-Zehnder optical modulation units, and a control unit for controlling the power supply, the control unit having an integrating circuit capable of calculating an integrated value of the pixel voltages applied to each of the plurality of Mach-Zehnder optical modulation units, a control circuit for controlling an optical modulator, the control unit controlling the power supply so as to continuously apply the pixel voltage at a predetermined applied voltage width independently to each of the plurality of Mach-Zehnder optical modulation units, and further controlling the power supply so as to apply, to each of the plurality of Mach-Zehnder optical modulation units, a shift voltage for shifting the applied voltage width to a position symmetrical with respect to an extreme value of optical output on a modulation curve of each of the plurality of Mach-Zehnder optical modulation units, at a timing when an integrated value of the pixel voltage integrated by the integrating circuit reaches a predetermined value for any of the plurality of Mach-Zehnder optical modulation units.

Images