Optical modulator, light source module, optical engine, image display device, xr glasses, optical communication transmitting device, optical communication system, and control method for optical modulator

Abstract

To provide an optical modulator in which DC drift is constantly suppressed.SOLUTION: An optical modulator of the present invention includes a Mach-Zehnder type lithium niobate ridge optical waveguide, electrodes for applying an electrical signal to the ridge optical waveguide, an electrical signal source for generating an electrical signal in which a set of positive and negative voltages is periodically repeated, and a control circuit for controlling the electrical signal source. The control circuit sets the set of positive and negative voltages so that output light from the ridge optical waveguide continuously falls within a predetermined range of values.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] Lithium niobate has a large electro-optic constant and can be used to form optical modulators, optical waveguides, optical switches, optical filters, etc., and is used in optical communication devices, visible light devices, etc.

[0003] It is known that Mach-Zehnder optical modulators made using lithium niobate 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. 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] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide an optical communication system and a method for controlling an optical modulator, an optical source module, an optical engine, XR glasses, and an optical communication transmitter, in which DC drift is constantly suppressed. [Means for solving the problem]

[0008] The present disclosure provides the following means to solve the above problems.

[0009] A first aspect of the present disclosure is an optical modulator comprising: a Mach-Zehnder lithium niobate ridge optical waveguide; electrodes for applying an electric signal to the ridge optical waveguide; an electric signal source for generating an electric signal in which pairs of positive and negative voltages are periodically repeated; and a control circuit for controlling the electric signal source, wherein the control circuit sets the pairs of positive and negative voltages so that output light from the ridge optical waveguide continuously falls within a predetermined range of values.

[0010] A second aspect of the present disclosure is an optical modulator according to the first aspect, wherein the ridge-type optical waveguide is formed of a lithium niobate film formed on a substrate, and the C-axis of the lithium niobate is oriented perpendicular to the main surface of the substrate.

[0011] A third aspect of the present disclosure is an optical modulator according to the first aspect, wherein the ridge-type optical waveguide is formed from bulk lithium niobate attached to a substrate, and the C-axis of the lithium niobate is parallel to the main surface of the substrate.

[0012] A fourth aspect of the present disclosure is the optical modulator of any one of the first to third aspects, wherein the electrical signal is a square wave voltage signal.

[0013] A fifth aspect of the present disclosure is the optical modulator of the fourth aspect, wherein the duty ratio of the electrical signal is set so that the average voltage is 0V.

[0014] A sixth aspect of the present disclosure is the optical modulator of any one of the first to fifth aspects, wherein the frequency of the electrical signal is 1 MHz or higher.

[0015] A seventh aspect of the present disclosure is the optical modulator of any one of the first to sixth aspects, wherein the electrical signal source includes a modulation signal source and a bias signal source.

[0016] An eighth aspect of the present disclosure is the optical modulator of the first aspect, further comprising a heater disposed near the optical waveguide of the ridge-type optical waveguide.

[0017] A ninth aspect of the present disclosure is a visible light source module comprising an optical modulator according to any one of aspects 1 to 7, the optical modulator having an optical multiplexing section, and a plurality of visible light laser light sources that emit visible light that is multiplexed in the optical multiplexing section.

[0018] A tenth aspect of the present disclosure is a visible light source module in which, in the optical modulator of aspect 8, the optical modulator has an optical multiplexing section, and is further equipped with a plurality of visible light laser light sources that emit visible light that is multiplexed in the optical multiplexing section, an optical separation means that separates the light emitted from the optical modulator, and a photodetector that detects the light separated by the optical separation means, and the control circuit has a phase control circuit that controls the current flowing to the heater based on the light intensity detected by the photodetector.

[0019] In an eleventh aspect of the present disclosure, in the optical modulator of aspect 10, the phase control circuit controls the current flowing to the heater for each predetermined time range so that the voltage-optical output characteristic of the optical modulator becomes minimum or maximum when the modulation signal voltage from the electrical signal source is 0V.

[0020] A twelfth aspect of the present disclosure is an optical engine including the visible light source module of the ninth 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.

[0021] A thirteenth aspect of the present disclosure is an optical engine including the visible light source module of the eleventh 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.

[0022] A fourteenth aspect of the present disclosure is an image display device equipped with the optical engine of the twelfth aspect.

[0023] In a fifteenth aspect of the present disclosure, the image display device of the fourteenth aspect is XR glasses.

[0024] A sixteenth aspect of the present disclosure is an image display device equipped with the optical engine of the thirteenth aspect.

[0025] In a seventeenth aspect of the present disclosure, the image display device of the sixteenth aspect is XR glasses.

[0026] An eighteenth aspect of the present disclosure is an image display device according to the sixteenth aspect, wherein the predetermined time range is the time required to render one or more rows of pixels in a raster scan.

[0027] A nineteenth aspect of the present disclosure is an image display device according to the sixteenth aspect, wherein the predetermined time range is the time required to draw one or more frame images of a raster scan.

[0028] Aspect 20 of the present disclosure is an image display device according to aspect 16, in which the electrical signal is a rectangular wave voltage signal, and a lookup table is provided which contains information on the amplitude of the electrical signal corresponding to each optical output, and the control circuit sets the modulation signal voltage output from the electrical signal source based on the lookup table.

[0029] A twenty-first aspect of the present disclosure is the image display device of the twenty-first aspect, further comprising an external storage device in which the lookup table is stored.

[0030] A twenty-second aspect of the present disclosure is the image display device of the twenty-first aspect, wherein the frequency of the electrical signal is 10 MHz or higher.

[0031] Aspect 23 of the present disclosure is an image display device according to aspect 20, wherein the ridge-type optical waveguide is formed of a lithium niobate film formed on a substrate, and the C-axis of the lithium niobate is oriented perpendicular to the main surface of the substrate.

[0032] A twenty-fourth aspect of the present disclosure is the image display device of the twenty-third aspect, wherein the lithium niobate film is an X-cut film.

[0033] A twenty-fifth aspect of the present disclosure is a transmitting device for optical communications, including the optical modulator according to any one of the first to seventh aspects.

[0034] A twenty-sixth aspect of the present disclosure is an optical communication system including the optical communication transmitting device of the twenty-fifth aspect and an optical communication receiving device having an optical signal receiving element for receiving light.

[0035] Aspect 27 of the present disclosure is a transmitting device for optical communications comprising a laser light source, the optical modulator of aspect 8, an optical separation means for separating the light emitted from the optical modulator, and an optical detector for detecting the light separated by the optical separation means, wherein the control circuit has a phase control circuit for controlling the current flowing to the heater based on the light intensity detected by the optical detector.

[0036] A twenty-eighth aspect of the present disclosure is an optical communication transmitting device according to the twenty-seventh aspect, wherein the phase control circuit controls the current flowing through the heater for each predetermined time range so that the voltage-optical output characteristic of the optical modulator becomes minimum or maximum when the modulated signal voltage from the electrical signal source is 0V.

[0037] Aspect 29 of the present disclosure is an optical communication transmitting device according to aspect 28, wherein the electrical signal is a rectangular wave voltage signal, and a lookup table is provided which contains information on the amplitude of the electrical signal corresponding to each optical output, and the control circuit sets the modulated signal voltage output from the electrical signal source based on the lookup table.

[0038] A 30th aspect of the present disclosure relates to the optical communication transmitting device of the 29th aspect, and further includes an external storage device in which the lookup table is stored.

[0039] Aspect 31 of the present disclosure is an optical communication transmitter device according to aspect 29, wherein the ridge-type optical waveguide is formed of a lithium niobate film formed on a substrate, and the C-axis of the lithium niobate is oriented perpendicular to the main surface of the substrate.

[0040] A thirty-second aspect of the present disclosure relates to the optical communication transmitter of the thirty-first aspect, wherein the lithium niobate film is X-cut.

[0041] A thirty-third aspect of the present disclosure is an optical communication system including the optical communication transmitting device according to the twenty-ninth aspect, and an optical communication receiving device having an optical signal receiving element for receiving light.

[0042] A thirty-fourth aspect of the present disclosure is a method for controlling an optical modulator that applies an electrical signal in which pairs of positive and negative voltages are periodically repeated to a lithium niobate ridge-type optical waveguide, and that sets the pairs of positive and negative voltages so that the output signal from the ridge-type optical waveguide continuously falls within a predetermined range. [Effects of the Invention]

[0043] According to the optical modulator of the present invention, it is possible to provide an optical modulator in which DC drift is constantly suppressed. [Brief explanation of the drawings]

[0044] [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] FIG. 10 is a diagram for explaining a method for controlling an optical modulator according to the present disclosure, showing the modulation curve of the optical modulator, the modulation signal voltage applied to the modulation electrode, and an optical output signal whose output value is controlled within a certain range. [Figure 5A] 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 5B] 5B 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. 5A except that it includes an optical multiplexer. [Figure 6A] FIG. 1 is a diagram conceptually illustrating a single-stage MMI optical multiplexer. [Figure 6B] FIG. 1 is a diagram conceptually illustrating a two-stage MMI optical multiplexer. [Figure 7] FIG. 5C is a cross-sectional view of the optical modulator shown in FIGS. 5A and 5B taken along line AA′. [Figure 8] FIG. 10 is a conceptual diagram of an optical modulator according to a second embodiment. [Figure 9] 8 is a cross-sectional view showing a configuration example in which a heater is added to the optical modulator shown in FIG. 7. FIG. [Figure 10] 9 is a conceptual diagram of an image display device or a transmitter for optical communications that includes the optical modulator shown in FIG. 8. FIG. [Figure 11A] 10 is a graph showing EO characteristics to explain that the operating point is adjusted so that the voltage-optical output characteristic of the optical modulator when the modulation signal voltage (Vm) is 0 V changes from a non-minimum state to a minimum state. [Figure 11B] 10 is a graph showing the EO characteristics in a state where the voltage-optical output characteristic of the optical modulator is minimized when the modulation signal voltage (Vm) is 0V. [Figure 11C] 11C is a diagram for explaining a modulation signal voltage applied by operating the optical modulator in a state having the EO characteristics shown in FIG. 11B. 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. [Figure 16] 1 is a conceptual diagram illustrating a transmitter for optical communications according to the present disclosure and an optical signal generated by the transmitter. [Figure 17] FIG. 1 is a block diagram of an optical communication system according to the present disclosure. [Figure 18] FIG. 10 is a block diagram showing a modified example of an optical communication system according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0045] 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 ease of understanding, 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. Furthermore, a configuration used in one embodiment described below may be used in another embodiment.

[0046] [Optical Modulator (First Embodiment)] FIG. 1 shows a conceptual diagram of a Mach-Zehnder optical modulator according to the first embodiment. 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.

[0047] 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.

[0048] 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 modulation signal generation controller (signal generation circuit) 2 that supplies the modulation signal. 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.

[0049] In the optical modulator according to the present disclosure, the modulation signal generation controller (electrical signal source and control circuit) includes a high-frequency signal pulse generation control circuit and a DC bias control circuit, which may be part of a central processing unit (CPU).

[0050] The Mach-Zehnder optical modulation unit 1 receives a modulation signal V mThe 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.

[0051] 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.

[0052] In the configuration diagram shown in Figure 2, a high-frequency signal V REF and DC bias voltage V DC 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.

[0053] The Mach-Zehnder optical modulation unit 1 has a modulation curve (operating characteristic curve, voltage-optical output characteristic (EO characteristic)) specific to the optical modulator, and the 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 called DC drift occurs in which the modulation curve (operating characteristic curve, voltage-optical output characteristic (EO characteristic)) shifts over time depending on the polarity of the

[0054] 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.

[0055] 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 D101 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.

[0056] 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.

[0057] FIG. 4 is a diagram for explaining a method for controlling an optical modulator according to the present disclosure, showing a modulation curve of the optical modulator, a modulation signal voltage applied to a modulating electrode, and an optical output signal whose output value is controlled within a certain range.

[0058] The modulation signal shown in Figure 4 is a square wave, but is not limited to a square wave. The modulation signal is a periodic electrical signal, and the higher its frequency, the more the ripple in the optical output can be reduced.

[0059] As shown in FIG. 4, in the optical modulator according to the present disclosure, modulation signal voltages of opposite polarities are applied alternately, so that the DC drift caused by the application of a positive voltage and the DC drift caused by the application of a negative voltage act to cancel each other out, thereby suppressing changes in the optical output over time due to the DC drift. Since the configuration involves constantly applying modulation signal voltages of opposite polarities alternately, DC drift can be constantly cancelled out.

[0060] In the optical modulator according to the present disclosure, the modulation signal generation controller 2 stores in advance the positive and negative voltages Vm that provide a desired optical output (P0) on the modulation curve (operating characteristic curve) of the optical modulator. The positive voltage is V H , the negative voltage is V L Then, the modulation signal changes the positive voltage to V H When the negative voltage is V L When the modulation signal is positive or negative, the optical output is approximately P0, but because of the nature of the square wave, there is a period when the voltage is constant, as shown by the arrow in the optical output signal graph in Figure 4. When the modulation signal is positive or negative, a DC drift occurs in the opposite direction. In the optical modulator according to the present disclosure, positive and negative voltages are constantly applied alternately to obtain a desired optical output.

[0061] In the example shown in Figure 4, the modulating signal has an amplitude of V H A positive pulse with a pulse width of Tp and an amplitude of V L Negative pulses with a pulse width Tn are alternately repeated periodically. The duty ratio of the modulation signal can be set so that the average voltage is 0V. In this case, the optical output can be kept constant over the long term. "Constant" here means that the optical output ripple falls within a range equivalent to ±5% when the maximum optical power is set to 100% on the voltage-optical output characteristics of the modulator.

[0062] The frequency of the periodic electrical signal of the modulating signal can be determined from the time constant of the DC drift and the allowable ripple. Furthermore, the frequency of the periodic electrical signal of the modulation signal can be increased to 1 MHz or more, and even 10 MHz or more, which further reduces the ripple in the optical output.

[0063] In Figure 4, the combinations of positive and negative voltages that result in the same optical output are shown for the cases of P1 and P2, in addition to the case of P0. For P1, the positive and negative voltages are V H1 , V L1 In the case of P2, the positive and negative voltages are V H2 , V L2 is. For any optical output, by storing in advance in the modulation signal generation controller 2 the combination of the positive voltage and the negative voltage, or the combination of the positive voltage, the negative voltage and the duty ratio, together with the duty ratio, it is possible to obtain any optical output by feedforward control.

[0064] FIG. 5A 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. 5A 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. 5A, 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.

[0065] The electrode configuration and circuit diagram shown in Fig. 5A are one example. Fig. 6A 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.

[0066] FIG. 5B 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. 5A except that it includes an optical multiplexer. In the optical modulator 201 shown in FIG. 5B, 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.

[0067] The optical multiplexing section 50 can be an MMI (multi-mode interference) type optical multiplexer. In an MMI optical multiplexer, multiple modes from the zeroth-order mode to higher-order modes interfere with each other and are characterized by forming an image (converging) at a specific position (a specified distance from the input end) of the MMI optical multiplexer. It is known that the distance or period (beat length) Lπ between adjacent convergence points roughly follows equation (1). Equation (1) is the beat length Lπ between the two lower-order modes, the zeroth-order mode and the first-order mode.

[0068]

number

[0069] In equation (1), We is the effective width of the MMI optical multiplexer, n is the effective refractive index of the MMI, and λ is the wavelength of the input light. β0 and β1 are the propagation constants of the zeroth and first modes, respectively. From equation (1), we can see that the beat length depends on the width and wavelength of the MMI optical multiplexer.

[0070] When the electromagnetic field distribution undergoes a phase shift of 2π in all propagation modes generated within the MMI optical multiplexer, the optical intensity distribution coincides with the incident optical intensity distribution. The optical propagation distance required to achieve this coincidence (convergence) state is called the self-projection distance, and convergence is repeated at a period of Lπ after a certain propagation distance of 3Lπ / 4.

[0071] FIG. 6A conceptually shows a single-stage MMI optical multiplexer, and FIG. 6B conceptually shows a two-stage MMI optical multiplexer. FIG. 6A shows an MMI optical multiplexer equipped with one MMI optical multiplexer A50, and FIG. 6B shows an MMI optical multiplexer equipped with a two-stage MMI optical multiplexer 50 in which a wide first MMI optical multiplexer 50-1 and a narrow second MMI optical multiplexer 50-2 are connected from the input side. The waves in the optical multiplexer conceptually represent the interference period (beat length), and from the above formula (1), the interference period (beat length) is proportional to the square of the width of the optical multiplexer for each wavelength. Therefore, the waves in the optical multiplexer shown in Figures 6A and 6B conceptually represent that, for each wavelength, the wider the width of the optical multiplexer, the longer the beat length (beat period), and the narrower the width, the shorter the beat length (beat period). The shorter the beat length of each wavelength, the shorter the distance that is an integer multiple (least common multiple) of the beat length, making it possible to shorten the length of the MMI-type optical multiplexer.

[0072] In the two-stage MMI optical multiplexer shown in FIG. 6B, by making the width (y direction) of the latter MMI optical multiplexing section narrower than the width (y direction) of the former MMI optical multiplexing section, the beat length in the latter MMI optical multiplexing section is shortened, and the overall size of the optical multiplexer can be reduced. For example, when mounting the device in a glasses-type terminal, it is preferable that the width of the MMI type optical multiplexing section at the front stage is 1.9 μm or more, in view of current processing technology.

[0073] Fig. 7 is a cross-sectional view of the optical modulator shown in Fig. 5A taken along line AA'. The same applies to the cross-sectional view of the optical modulator shown in Fig. 5B taken along line AA'. The optical modulator 200 (201) shown in FIG. 7 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.

[0074] As shown in FIG. 7, 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.

[0075] When the optical modulator 200 (201) shown in FIG. 7 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.

[0076] When the optical modulator 200 (201) shown in FIG. 7 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.

[0077] 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.

[0078] Examples of the substrate 10 include a sapphire substrate, a Si substrate, and a thermally oxidized silicon substrate. Since the Mach-Zehnder optical waveguide having an optical modulation function 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.

[0079] 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. The lithium niobate film may be X-cut.

[0080] 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).

[0081] 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.

[0082] 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.

[0083] (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.

[0084] (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.

[0085] 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.

[0086] 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.

[0087] (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.

[0088] 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.

[0089] 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.

[0090] [Optical Modulator (Second Embodiment)] 8 shows a conceptual diagram of an optical modulator according to the second embodiment. The optical modulator according to the second embodiment differs from the optical modulator according to the first embodiment in that a heater is provided as a phase adjustment means near one of the two optical waveguides provided in the Mach-Zehnder type optical modulation section. A heater with a size of about several μm to several tens of μm can be used, and in this case it is called a microheater. Here, "the vicinity of one optical waveguide" refers to a position where the Joule heat from the heater affects only the refractive index of that one optical waveguide and does not substantially affect the refractive index of the other optical waveguide, and where the Joule heat from the heater does not affect the optical modulator in any way other than changing the refractive index. For example, when the distance (S) between the ridge-type optical waveguides 24-1 shown in Fig. 7 is 2 to 12 µm, the "proximity of one optical waveguide" refers to a position where the distance between the heater 61 and the optical waveguide 24-1 (the distance between the closest points) is about 1 to 5 µm. The Mach-Zehnder optical modulator according to the second embodiment is an optical modulator with a phase adjustment section, and is capable of adjusting the operating point. Heaters may be placed near both sides of the two optical waveguides. In this case, heaters of an appropriate size are placed so that the refractive indexes of the two optical waveguides change relatively.

[0091] Although the example shown in Fig. 8 is a case where the optical modulator has one Mach-Zehnder optical modulation unit, it can also be applied to a case where the optical modulator has multiple Mach-Zehnder optical modulation units as exemplified in Fig. 6A and Fig. 6B. That is, all or some of the multiple Mach-Zehnder optical modulation units can be configured to have heaters.

[0092] 8 includes an intensity modulation section 110A and a phase adjustment section 110B. The intensity modulation section 110A is a section that has a function of applying an electric field to the optical waveguides to impart a phase difference to the light passing through the two optical waveguides, thereby modulating the intensity of the optical output, and the phase adjustment section 110B is a section that has a function of heating the vicinity of one of the two optical waveguides, including one of the optical waveguides, to change the refractive index thereof, thereby shifting the phase of the light passing through that optical waveguide, thereby adjusting the operating point. The optical waveguide 41 has an optical waveguide portion 41A which is a portion located in the intensity modulation section 110A, and an optical waveguide portion 41B which is a portion disposed in the phase adjustment section 110B. The optical waveguide 42 has an optical waveguide portion 42A which is a portion located in the intensity modulation section 110A, and an optical waveguide portion 41B which is a portion which does not overlap the modulation electrodes in plan view.

[0093] FIG. 9 is a cross-sectional view showing a configuration example in which a heater is added to the optical modulator shown in FIG. 9 is a resistor disposed near the second ridge optical waveguide 42. Although the heater 61 shown in Fig. 9 is disposed to the side of the second ridge optical waveguide 42, the heater 61 is not limited to being disposed to the side, and may be disposed, for example, below the second ridge optical waveguide 42.

[0094] The resistor may be made of a known material having electrical resistance that generates Joule heat when a current flows through it, such as TiN. The heater 61 is connected to metal wiring 62 that supplies current to the heater 61. Heat is generated in the heater 61 by passing a current through the metal wiring 62 to the heater 61. In the phase control element 4, the refractive index of the second ridge-type optical waveguide 42 changes due to the heat from the heater 61, and the phase of light passing through the second ridge-type optical waveguide 42 shifts.

[0095] According to the optical modulator with a phase adjuster of the second embodiment, an optical output of any intensity can be obtained by an external electrical signal. In addition, the phase of the EO characteristics of the intensity modulation section can be electrically adjusted by applying a voltage to the phase adjuster.

[0096] Fig. 10 is a conceptual diagram of an image display device or a transmitter for optical communications including the optical modulator 110 shown in Fig. 8. The image display device and the transmitter for optical communications will be described in detail later. The following description will be given taking the case of an image display device as an example.

[0097] The image display device 10010 shown in Figure 10 includes the optical modulator 110 shown in Figure 8, the optical modulator having an optical multiplexing section (not shown), a plurality of visible light laser light sources (not shown) that emit visible light that is multiplexed in the optical multiplexing section, an optical separation means 71 that separates the light emitted from the optical modulator, and an optical detector 72 that detects the light separated by the optical separation means 71, and the control circuit 21 has a signal generating circuit 2 that generates a modulated electrical signal and a phase control circuit 3 that controls the current flowing to the heater 61 based on the light intensity detected by the optical detector 72. As shown in FIG. 10, the control circuit 21 may be provided as a part of a central processing unit (CPU) 20.

[0098] As the light separating means 71, a known means capable of separating (branching) light can be used, and a half mirror can be exemplified. The light detector 72 may be any known means capable of detecting light, such as a photodetector (PD).

[0099] The phase control circuit 3 controls the modulation signal voltage (V mThe current flowing through the heater 61 can be controlled so that the voltage-optical output characteristic of the optical modulator 110 when the voltage Vcc is 0 V becomes minimum or maximum.

[0100] FIG. 11A is a graph showing the EO characteristics, and the modulation signal voltage (V m ) is set to 0V, the voltage vs. optical output characteristic of the optical modulator 110 changes from a non-minimum state to a minimum state. adj 10 is a graph showing that the current flowing to the heater 61 is adjusted by the phase control circuit 3 so that the operating point of the heater 61 is adjusted.

[0101] Figure 11B shows the modulation signal voltage (V m 11C is a graph showing the EO characteristics in a state where the voltage-optical output characteristics of the optical modulator 110 are minimized when the EO characteristic shown in FIG. 11B is 0 V. FIG. 11C is a diagram for explaining the modulation signal voltage applied by operating the optical modulator in a state where the EO characteristic shown in FIG. 11B is obtained. In this state, the EO characteristic graph is symmetrical with respect to the modulation signal voltage (Vm) of 0V. In a state with such EO characteristics, the "pair of positive and negative voltages" is a case where the absolute values ​​of the positive and negative voltages are the same, and the light output is determined only by the amplitude. For example, in the case of 128 gradations, the positive V 128 and negative value -V 128 The amplitude is the same for V 128 Similarly, in the case of 256 gradations, a positive value V 256 and negative value -V 256 The amplitude is the same as V 256 In this case, the duty ratio is always 50%, making it easy to control.

[0102] The image display device of this embodiment may be configured to include a lookup table that contains information on the amplitude of the electrical signal corresponding to each optical output, and the CPU 20 (or the control circuit 21) sets the modulation signal voltage output from the signal generating circuit 2 based on the lookup table. In this configuration, the DC drifts that occur at the timing of the positive and negative voltages of the rectangular wave electrical signal cancel each other out, thereby suppressing fluctuations in optical output power due to DC drifts. By using a square wave electrical signal, ripple in the optical output can be reduced.

[0103] Alternatively, an external storage device in which the lookup table is stored may be provided. By using an external storage device, more storage capacity can be secured.

[0104] 11A, by periodically (at every predetermined time interval) using the CPU 20 (or the control circuit 21) adjusting the operating point to make the EO characteristics of the optical modulator 110 symmetrical with respect to 0 V, the optical output from the optical modulator can perform image display operation using only the square wave amplitude of the modulation signal voltage (Vm). In other words, after adjusting the operating point to make the EO characteristics of the optical modulator 110 symmetrical with respect to 0 V, the CPU 20 (or the control circuit 21) can control the signal generating circuit to output a periodic electrical signal having an amplitude corresponding to the image signal.

[0105] According to the image display device of this embodiment, the EO characteristics of the intensity modulation section can be maintained in a state where they are symmetrical with respect to 0 V. This makes it possible to change the optical output while controlling the DC drift using only the amplitude of the periodic electrical signal sent to the intensity modulation section.

[0106] In the image display device shown in Figure 10, only one Mach-Zehnder optical modulation unit is shown for ease of explanation, but an image display device capable of full-color display can be provided with three visible light laser light sources (see Figure 12) for red (R), green (G), and blue (B) and three corresponding Mach-Zehnder optical modulation units.

[0107] When an image display device displays (draws) an image on an image display surface, it scans a laser beam in sequence, changing the light intensity (color tone) for each pixel to display the image. For example, the laser beam scans one pixel at a time from left to right, and when it reaches the right end, it moves down one row and scans one pixel at a time from right to left, and when it reaches the left end, it moves down one row and scans one pixel at a time from left to right, repeating this scanning process (raster scanning).

[0108] According to the image display device of this embodiment, it is possible to periodically (every predetermined time range) perform operating point adjustment to make the EO characteristics of the optical modulator 110 symmetrical with respect to 0 V. This predetermined time range may be the time required to draw one or more rows of pixels in a raster scan. In this case, drawing is stopped once one or more rows of pixels in the raster scan have been drawn, and drawing is resumed after adjusting the operating point so that the EO characteristics of the optical modulator 110 are symmetrical with respect to 0V.

[0109] This predetermined time range may also be the time required to draw one or more frame images of a raster scan. In this case, drawing is stopped once one or more frame images of the raster scan are completed, and drawing is resumed after adjusting the operating point so that the EO characteristics of the optical modulator 110 are symmetrical with respect to 0V.

[0110] In the image display device of this embodiment, the frequency of the periodic electrical signal can be set to 10 MHz or more, thereby reducing ripples in the optical output.

[0111] [Light source module] A light source module according to the present disclosure includes the optical modulator according to the first embodiment or the optical modulator according to the second embodiment, and a plurality of laser light sources.

[0112] A light source module (second embodiment) including an optical modulator according to the second embodiment includes a light separating means for separating light emitted from the optical modulator and a photodetector for detecting the light separated by the light separating means, and the control circuit has a phase control circuit for controlling a current flowing through the heater based on the light intensity detected by the photodetector. The phase control circuit may be configured to control the current flowing through the heater for each predetermined time range so that the voltage-light output characteristic of the optical modulator becomes minimum or maximum when the modulation signal voltage from the electric signal source is 0 V.

[0113] In the following, a light source module (first embodiment) including the optical modulator according to the first embodiment will be described as an example. 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. 7. 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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).

[0118] 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.

[0119] [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.

[0120] The optical engine (first embodiment) of the present disclosure includes a visible light source module according to the first embodiment, and a light scanning mirror that reflects light emitted from the visible light source module at different angles to display an image. The optical engine (second embodiment) of the present disclosure includes a visible light source module according to the second embodiment, and a light scanning mirror that reflects light emitted from the visible light source module at different angles to display an image.

[0121] In the following, an image display device (first embodiment) including the optical engine according to the first embodiment will be described as an example, and a description of an image display device (second embodiment) including the optical engine according to the second embodiment will be omitted. XR glasses are an example of an image display device. 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.

[0122] 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.

[0123] 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.

[0124] 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).

[0125] 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.

[0126] 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.

[0127] 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.

[0128] [Optical communication transmitter] The optical communication transmitter (first embodiment) according to the present disclosure includes a laser light source and an optical modulator according to the first embodiment.

[0129] A transmitting device for optical communications (second embodiment) according to the present disclosure includes a laser light source, an optical modulator according to the second embodiment, an optical separation means for separating the light emitted from the optical modulator, and an optical detector for detecting the light separated by the optical separation means, and the control circuit has a phase control circuit for controlling the current flowing to the heater based on the light intensity detected by the optical detector. The phase control circuit may be configured to control the current flowing through the heater so that the voltage-optical output characteristic of the optical modulator becomes minimum or maximum when the modulation signal voltage from the electrical signal source is 0 V for each predetermined time range. In the optical communication transmitter (second embodiment) according to the present disclosure, the electrical signal may be a square wave voltage signal, a lookup table may be provided which contains information on the amplitude of the electrical signal corresponding to each optical output, and the control circuit may set the modulated signal voltage output from the electrical signal source based on the lookup table. An external storage device in which the lookup table is stored may also be provided.

[0130] In the following, the optical communication transmitting device according to the first embodiment will be described as an example. 16 is a conceptual diagram illustrating a transmitting device for optical communications according to the present disclosure and an optical signal generated by the transmitting device. The transmitting device according to the present disclosure is a transmitting device that transmits a signal to a receiving device. Since a visible light signal can be used as the optical signal, the following description will be given assuming that a visible light signal is used. A transmitter 6001 for optical communications according to the present disclosure includes a light source module 1000 including an optical semiconductor element (laser) 6030 and an optical modulator 201 having a Mach-Zehnder optical waveguide MZI and an electric signal generating element 6013 having a function similar to that of the modulation signal generation controller 2. Hereinafter, the optical semiconductor element (laser) may be referred to as LD, and the optical modulator may be referred to as LN201.

[0131] Laser 6030 emits L1. Laser 6030 is continuously on. "Continuously" means that laser 6030 is on while transmitting a visible light signal to a receiving device. The wavelength of L1 emitted by laser 6030 is generally within the range of 380 nm to 830 nm.

[0132] The electrical signal generating element 6013 receives information data to be transmitted, converts it into an electrical signal, and acts on the Mach-Zehnder optical waveguide MZI.

[0133] The light source module 1000 generates a visible light signal L2 by modulating the light intensity of the LN201 based on the electrical signal received from the electrical signal generating element 6013.

[0134] (Optical communication system) FIG. 17 is a block diagram of an optical communication system according to the present disclosure. In an optical communication system 7001 shown in FIG. 17, a visible light signal L2 generated by a transmitter for optical communication 6001 is transmitted to a receiver for optical communication 6002 via external space.

[0135] The transmitting device 6001 shown in Fig. 17 is the same as the transmitting device 6001 shown in Fig. 16 except that it includes a visible light signal output port 6014. The visible light signal output port 6014 is connected to the optical modulator 201 and is an output port for emitting the visible light signal L2 generated by the optical modulator 201 into external space.

[0136] The receiving device 6002 includes a visible light signal receiving unit 6021, an optical-electrical conversion element 6022, and a visible light signal inlet 6024. The visible light signal inlet 6024 is an inlet for receiving the visible light signal L2 transmitted from the transmitting device 6001. The visible light signal receiving unit 6021 is connected to the visible light signal inlet 6024, receives the visible light signal L2 incident at the visible light signal inlet 6024, and irradiates the visible light signal L2 to the optical-electrical conversion element 6022. The optical-electrical conversion element 6022 converts the visible light signal L2 into an electrical signal. There are no particular restrictions on the optical-electrical conversion element 6022, and any type of element may be used as long as it is an element that can detect the visible light signal L2 at high speed and convert it into an electrical signal.

[0137] The optical communication system 7001 performs visible light communication as follows. In the transmitting device 6001, as described above, the visible light signal L2 is generated by the optical modulator 201. The generated visible light signal L2 is emitted through the visible light signal emitting port 6014 into the external space. The emitted visible light signal L2 is received by a visible light signal receiving unit 6021 via a visible light signal inlet 6024 of the receiving device 6002. The received visible light signal L2 is converted into an electrical signal by an optical-electrical conversion element 6022, and the information data added to the visible light signal L2 is extracted.

[0138] In the optical communication system 7001 according to the present disclosure configured as described above, the intensity of the visible light signal L2 transmitted from the transmitting device 6001 is high, making it easy to visually confirm the communication path of the visible light signal L2. This prevents erroneous data transmission. In a communication system using infrared light, it is impossible to visually confirm whether the visible light signal is being received by the receiving device at the destination. This creates a risk of sending data to an unintended recipient. The optical communication system 7001 according to the present disclosure, which enables data transfer speeds of 10 Gbit / s or more, from several hundred Gbit / s to 1 Tbib / s, transmits a huge amount of data per second. While this is extremely convenient, it also increases the risk of data being sent to the wrong recipient. Therefore, visible light communication, which allows users to visually confirm whether a visible light signal is being transmitted to the destination before transmitting data, offers a significant advantage in terms of preventing erroneous data transmission. However, data transmission using invisible infrared light is always accompanied by uncertainty.

[0139] Another advantage of using visible light is that its wavelength is shorter than that of infrared light, allowing for a smaller optical waveguide. This means that the size of the optical modulator can also be reduced. The size of an optical waveguide for visible light can be reduced by approximately one-third to one-quarter of that of an optical waveguide for infrared light, resulting in an area that is one-ninth to one-hundred-sixteenth of that of an optical waveguide for infrared light. This means that the number of elements obtained per device fabrication substrate is approximately ten times greater, making it possible to reduce the manufacturing cost of the optical modulator by one-ninth to one-hundred-sixteenth of that of an optical modulator. For example, this makes it possible to realize consumer applications such as information terminals like smartphones. However, as long as infrared light is used, the chip size cannot be reduced. This means that the cost of the modulator element becomes high, making its use in consumer applications extremely difficult and impractical.

[0140] As described above, there are two advantages to using visible light for high-speed optical communications: (1) In high-speed optical communications, it is possible to visually confirm the destination before transmission, and large volumes of data can be sent and received safely. (2) The element size of the optical modulator can be reduced. This reduces the manufacturing cost of the optical modulator to one-tenth or less, making it possible to enjoy the benefits of ultra-high-speed communications even in consumer applications.

[0141] In the optical communication system of the present disclosure, a visible light signal may be transmitted using an optical transmission means such as an optical fiber. FIG. 18 is a block diagram showing a modified example of the optical communication system according to the other embodiment. 18 differs from communication system 7001 shown in FIG. 17 in that a visible light signal L2 generated in a transmitting device 6001A is transmitted to a receiving device 6002A via an optical fiber 6070. Optical communication system 7001A shown in FIG.

[0142] 18, the transmitting device 6001A differs from the transmitting device 6001 in that it includes an output optical fiber connection unit 6015 instead of the visible light signal output port 6014. The output optical fiber connection unit 6015 is a connection unit that connects the optical modulator 201 and the optical fiber 6070 and outputs the visible light signal L2 generated by the optical modulator 201 to the optical fiber 6070.

[0143] Receiving device 6002A includes a visible light signal receiving unit 6021, an optical-electrical conversion element 6022, and an input optical fiber connection unit 6025. Input optical fiber connection unit 6025 is a connection unit that is connected to optical fiber 6070 and visible light signal receiving unit 6021, and inputs visible light signal L2 transmitted through optical fiber 6070 to visible light signal receiving unit 6021.

[0144] The optical communication system 7001A performs visible light communication as follows. In the transmitting device 6001A, the optical modulator 201 generates a visible light signal L2 as described above. The generated visible light signal L2 is output to the optical fiber 6070 via the output optical fiber connection unit 6015. The output visible light signal L2 propagates through the optical fiber 6070 and is received by the visible light signal receiving unit 6021 via the input optical fiber connection unit 6025 of the receiving device 6002A. The received visible light signal L2 is converted into an electric signal by the optical-electrical conversion element 6022, and the information data added to the visible light signal L2 is extracted.

[0145] According to the communication system 7001A of the present disclosure configured as described above, the visible light signal L2 generated by the transmitting device 6001A is transmitted to the receiving device 6002A via the optical fiber 6070, so that the visible light signal L2 can be transmitted to a place where light does not pass through, such as a room separated by a wall. [Explanation of symbols]

[0146] 1 Mach-Zehnder type optical modulator 2. Modulation signal generator controller 10 Substrate 100, 200, 201 Optical modulator 1000 Light Source Module 5001 Optical Engine 6001 Optical communication transmitter 7001 Communication Systems 10000 XR Glasses

Claims

1. a Mach-Zehnder type lithium niobate ridge optical waveguide; an electrode for applying an electric signal to the ridge-type optical waveguide; an electric signal source that generates an electric signal in which pairs of positive and negative voltages are periodically repeated; a control circuit for controlling the electrical signal source; The control circuit sets the pair of positive and negative voltages so that the output light from the ridge-type optical waveguide falls continuously within a predetermined range of values.

2. the ridge-type optical waveguide is formed by a lithium niobate film formed on a substrate, 2. The optical modulator according to claim 1, wherein the C-axis of the lithium niobate is oriented in a direction perpendicular to the major surface of the substrate.

3. the ridge-type optical waveguide is formed of bulk lithium niobate attached to a substrate; 2. The optical modulator according to claim 1, wherein the C-axis of the lithium niobate is parallel to the major surface of the substrate.

4. 2. The optical modulator according to claim 1, wherein the electrical signal is a square wave voltage signal.

5. 5. The optical modulator according to claim 4, wherein the duty ratio of the electrical signal is set so that the average voltage is 0V.

6. 2. The optical modulator according to claim 1, wherein the frequency of the electrical signal is 1 MHz or higher.

7. The optical modulator of claim 1 , wherein the electrical signal source includes a modulation signal source and a bias signal source.

8. 2. The optical modulator according to claim 1, further comprising a heater disposed in the vicinity of the ridge-type optical waveguide.

9. A visible light source module comprising: an optical modulator according to any one of claims 1 to 7; and a plurality of visible light laser light sources, the optical modulator having an optical multiplexing section, which emit visible light multiplexed in the optical multiplexing section.

10. an optical modulator according to claim 8; the optical modulator has an optical multiplexing section, and a plurality of visible light laser light sources that emit visible light that is multiplexed by the optical multiplexing section; a light separating means for separating the light emitted from the optical modulator; a photodetector that detects the light separated by the light separating means, The control circuit includes a phase control circuit that controls a current flowing through the heater based on the light intensity detected by the photodetector.

11. 11. The visible light source module of claim 10, wherein the phase control circuit controls the current flowing through the heater so that the voltage-light output characteristics of the optical modulator become minimum or maximum when the modulation signal voltage from the electrical signal source is 0 V for each predetermined time range.

12. The visible light source module according to claim 9 ; an optical scanning mirror that reflects the light emitted from the visible light source module at different angles so as to display an image;

13. The visible light source module according to claim 11; an optical scanning mirror that reflects the light emitted from the visible light source module at different angles so as to display an image;

14. An image display device equipped with the optical engine according to claim 12.

15. The image display device according to claim 14, wherein the image display device is an XR glass.

16. An image display device equipped with the optical engine according to claim 13.

17. The image display device according to claim 16, wherein the image display device is an XR glass.

18. 17. An image display device according to claim 16, wherein the predetermined time range is the time required to render one or more rows of pixels in a raster scan.

19. 17. The image display device according to claim 16, wherein the predetermined time range is a time required to draw one or more frame images in a raster scan.

20. the electrical signal is a square wave voltage signal, 17. The image display device according to claim 16, further comprising a lookup table that contains information on the amplitude of the electrical signal corresponding to each optical output, and the control circuit sets the modulation signal voltage output from the electrical signal source based on the lookup table.

21. 21. The image display device according to claim 20, further comprising an external storage device in which the lookup table is stored.

22. 21. The image display device according to claim 20, wherein the frequency of the electrical signal is 10 MHz or higher.

23. the ridge-type optical waveguide is formed by a lithium niobate film formed on a substrate, 21. The image display device according to claim 20, wherein the C-axis of the lithium niobate is oriented in a direction perpendicular to the main surface of the substrate.

24. 24. The image display device according to claim 23, wherein the lithium niobate film is an X-cut film.

25. A transmitter for optical communication, comprising the optical modulator according to any one of claims 1 to 7.

26. a transmitter for optical communications according to claim 25; an optical communication receiving device having an optical signal receiving element for receiving light;

27. A laser light source; an optical modulator according to claim 8; a light separating means for separating the light emitted from the optical modulator; a photodetector that detects the light separated by the light separating means, The control circuit includes a phase control circuit that controls a current flowing through the heater based on the light intensity detected by the photodetector.

28. 28. The optical communication transmitting device according to claim 27, wherein the phase control circuit controls the current flowing through the heater so that the voltage-optical output characteristics of the optical modulator when the modulated signal voltage from the electric signal source is 0 V become minimum or maximum for each predetermined time range.

29. the electrical signal is a square wave voltage signal, 29. The optical communication transmitting device according to claim 28, further comprising a lookup table that contains information on the amplitude of the electrical signal corresponding to each optical output, and wherein the control circuit sets the modulated signal voltage output from the electrical signal source based on the lookup table.

30. 30. The optical transmission device according to claim 29, further comprising an external storage device in which the lookup table is stored.

31. the ridge-type optical waveguide is formed by a lithium niobate film formed on a substrate, 30. The optical communication transmitter according to claim 29, wherein the C-axis of the lithium niobate is oriented in a direction perpendicular to the main surface of the substrate.

32. 32. The optical communication transmitter according to claim 31, wherein the lithium niobate film is an X-cut film.

33. a transmitter for optical communications according to claim 29; an optical communication receiving device having an optical signal receiving element for receiving light;

34. A method for controlling an optical modulator that applies an electric signal in which a set of positive and negative voltages is periodically repeated to a lithium niobate ridge-type optical waveguide, comprising the steps of: A method for controlling an optical modulator, the method comprising: setting the pair of positive and negative voltages so that the output signal from the ridge-type optical waveguide falls continuously within a predetermined range.

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