Optical modulator, light source module, optical engine, 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 optical switch for switching output light from the ridge optical waveguide, an electrical signal source for generating the electrical signal, and a control circuit for controlling the electrical signal source and the optical switch. The control circuit controls the electrical signal source so that the electrical signal alternates between positive and negative values on the time axis, and controls the optical switch to extract only the output light when a positive or negative electrical signal is applied.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical modulator, a light source module, an optical engine, 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 applied to 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 optical switch for switching output light from the ridge optical waveguide; an electric signal source for generating the electric signal; and a control circuit for controlling the electric signal source and the optical switch, wherein the control circuit controls the electric signal source so that the electric signal alternates between positive and negative values ​​on the time axis, and controls the optical switch to extract only output light when the positive or negative electric signal is applied.

[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] Aspect 8 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.

[0017] A ninth aspect of the present disclosure is an optical engine including the visible light source module of the eighth 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.

[0018] A tenth aspect of the present disclosure is XR glasses equipped with the optical engine of the ninth aspect.

[0019] An eleventh 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.

[0020] A twelfth aspect of the present disclosure is an optical communication system including the optical communication transmitter of the eleventh aspect and an optical communication receiver having an optical signal receiving element for receiving light.

[0021] A thirteenth aspect of the present disclosure is a method for controlling an optical modulator having a lithium niobate ridge optical waveguide and an optical switch provided on the output side of the ridge optical waveguide, the method comprising applying an electrical signal that alternates between positive and negative values ​​on the time axis to the ridge optical waveguide, and controlling the optical switch to extract an output signal from the ridge optical waveguide when a positive or negative voltage is applied. [Effects of the Invention]

[0022] 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]

[0023] [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 control method for an optical modulator according to the present disclosure, showing a modulation curve of the optical modulator, a modulation signal voltage applied to a modulation electrode, and an optical output signal output when the modulation signal voltage is a positive value or a negative value. [Figure 5] FIG. 1 is a block diagram of an optical modulator according to the present disclosure. [Figure 6A] 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 6B] 6B 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. 6A except that it includes an optical multiplexer. [Figure 7A] FIG. 1 is a diagram conceptually illustrating a single-stage MMI optical multiplexer. [Figure 7B] FIG. 1 is a diagram conceptually illustrating a two-stage MMI optical multiplexer. [Figure 8]FIG. 6C is a cross-sectional view of the optical modulator shown in FIGS. 6A and 6B taken along line AA'. [Figure 9] 1 is a schematic plan view of a light source module according to the present disclosure. [Figure 10] FIG. 10 is a schematic cross-sectional view of a part of the light source module shown in FIG. 9 cut along the XZ plane, depicting only a part near the joint. [Figure 11] FIG. 1 is a conceptual diagram for explaining an example of XR glasses of the present disclosure. [Figure 12] FIG. 12 is a conceptual diagram showing how an image is projected directly onto the retina by laser light emitted from a light source module in the XR glasses shown in FIG. 11. [Figure 13] 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 14] FIG. 1 is a block diagram of an optical communication system according to the present disclosure. [Figure 15] FIG. 10 is a block diagram showing a modified example of an optical communication system according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0024] 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.

[0025] [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.

[0026] 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.

[0027] 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 signal generation controller 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.

[0028] In the optical modulator according to the present disclosure, the signal generation controller (electrical signal source and control circuit) includes a high frequency signal pulse generation control circuit, a DC bias control circuit, and a switching signal control circuit that controls the electrical signal for the switching timing of the optical switch.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 modulation electrode, and an intensity-modulated optical output signal.

[0037] 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.

[0038] 4, in the optical modulator according to the present disclosure, modulation signal voltages of opposite polarities are applied alternately, which acts to cancel out the DC drift caused by the application of a positive voltage and the DC drift caused by the application of a negative voltage, thereby suppressing the change in 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.

[0039] In the optical modulator according to the present disclosure, when the applied modulation signal has a positive voltage Vp and a negative voltage Vn, the optical output is P1 and P2, respectively. Due to the nature of the square wave, there is a period when the voltage is constant, as indicated by the arrows in the optical output signal graph in Figure 4. When the modulation signal is a positive voltage, DC drift occurs in opposite directions when it is a negative voltage. Therefore, an optical switch is used to extract only the output light when a positive voltage is applied (symbol A in Figure 4) or only the output light when a negative voltage is applied (symbol B in Figure 4). As a result, DC drift occurs during the half-cycle when the voltage is constant, but because a voltage of the opposite polarity is applied in the next half-cycle, the time average of the DC drift becomes zero. In the optical modulator according to the present disclosure, positive and negative voltages are constantly applied alternately to obtain a desired optical output.

[0040] In the example shown in FIG. 4, the modulated signal alternates between positive pulses with an amplitude of Vp and a pulse width of Tp and negative pulses with an amplitude of Vn and a pulse width of Tn, and these pulses are 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 corresponding to each of Vp and Vn can be kept constant over the long term. "Constant" here means that the ripple in the optical output 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.

[0041] 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 higher, which further reduces the ripple in the optical output.

[0042] By extracting only the optical output corresponding to a positive voltage or only the optical output corresponding to a negative voltage, the average optical output can be made constant.

[0043] FIG. 5 shows a block diagram of an optical modulator according to the present disclosure. Input light L supplied from light source 30 inis intensity-modulated by a Mach-Zehnder optical modulation unit 1 included in the optical modulator 100. The signal generation controller 2 controls the electrical signal source so that the modulation signal alternates between positive and negative values ​​on the time axis. The signal generation controller 2 also sends a timing signal to the optical switch 20 in synchronization with the timing at which the positive or negative modulation signal is applied. The optical switch 20 receives the timing signal and extracts only the output light when the positive or negative electrical signal is applied, and outputs the output light L out will output.

[0044] The optical switch converts the output light L out This allows for on / off switching, enabling high speed switching. As the optical switch, various known types (mechanical type, MEMS type, optical waveguide type) can be used. In particular, optical waveguide type optical switches are realized by lightwave circuit technology for creating optical waveguides, and are preferred in that they can be easily miniaturized and integrated.

[0045] Fig. 6A 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. Optical switches are not shown (the same applies to the following figures). The optical modulator 200 shown in FIG. 6A 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. 6A, 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.

[0046] The electrode configuration and circuit diagram shown in Fig. 6A are an 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.

[0047] FIG. 6B 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. 6A except that it includes an optical multiplexer. In the optical modulator 201 shown in FIG. 6B, 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.

[0048] 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.

[0049]

number

[0050] 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.

[0051] 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.

[0052] FIG. 7A conceptually shows a single-stage MMI optical multiplexer, and FIG. 7B conceptually shows a two-stage MMI optical multiplexer. FIG. 7A shows an MMI optical multiplexer equipped with one MMI optical multiplexer A50, and FIG. 7B 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 7A and 7B 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.

[0053] In the two-stage MMI optical multiplexer shown in FIG. 7B, 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.

[0054] Fig. 8 is a cross-sectional view of the optical modulator shown in Fig. 6A taken along line AA'. The same applies to the cross-sectional view of the optical modulator shown in Fig. 6B taken along line AA'. The optical modulator 200 (201) shown in FIG. 8 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.

[0055] As shown in FIG. 8, 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.

[0056] When the optical modulator 200 (201) shown in FIG. 8 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.

[0057] When the optical modulator 200 (201) shown in FIG. 8 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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).

[0062] 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.

[0063] 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.

[0064] (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.

[0065] (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.

[0066] 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.

[0067] 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.

[0068] (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.

[0069] 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.

[0070] 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.

[0071] [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. 9 is a schematic plan view of a light source module according to the present disclosure. Fig. 9 shows an example of a light source module including the optical modulator 201 shown in Fig. 8. Fig. 10 is a schematic cross-sectional view of a part of the light source module shown in Fig. 9 taken along the XZ plane, depicting only a part near the joint.

[0072] The light source module 1000 shown in FIG. 9 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.

[0073] 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.

[0074] 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.

[0075] 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).

[0076] 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.

[0077] [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.

[0078] Fig. 11 is a conceptual diagram for explaining an example of XR glasses of the present disclosure. Fig. 12 is a conceptual diagram showing how an image is projected directly onto the retina by laser light emitted from a light source module in the XR glasses shown in Fig. 11. Symbol L is image display light.

[0079] 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. 12 denotes image display light.

[0080] The XR glasses 10000 of the present disclosure shown in FIG. 11 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. 11, 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.

[0081] 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).

[0082] 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. 11 is an example, and other configurations may also be used.

[0083] In the XR glasses 10000 of the present disclosure shown in Figure 11, as shown in Figure 12, 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.

[0084] 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.

[0085] [Optical communication transmitter] 13 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. The optical communication transmitter 6001 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 signal generation controller 2. Hereinafter, the optical semiconductor element (laser) may be referred to as an LD and the optical modulator as an LN201. The electric signal generating element 6013 is

[0086] 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.

[0087] 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.

[0088] 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.

[0089] (Optical communication system) FIG. 14 is a block diagram of an optical communication system according to the present disclosure. In an optical communication system 7001 shown in FIG. 14, 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.

[0090] The transmitting device 6001 shown in Fig. 14 is the same as the transmitting device 6001 shown in Fig. 13 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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. 15 is a block diagram showing a modified example of the optical communication system according to the other embodiment. 15 differs from the optical communication system 7001 shown in FIG. 14 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. In the optical communication system 7001A shown in FIG.

[0097] 15 , 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.

[0098] 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.

[0099] 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.

[0100] According to the optical 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]

[0101] 1. Mach-Zehnder optical modulator 2. Signal Generator Controller 10 Substrate 100, 200, 201 Optical modulator 1000 Light Source Module 5001 Optical Engine 6001 Optical communication transmitter 7001 Optical 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 optical switch that switches output light from the ridge-type optical waveguide; an electrical signal source that generates the electrical signal; a control circuit for controlling the electrical signal source and the optical switch; The control circuit controls the electrical signal source so that the electrical signal alternates between positive and negative values ​​on the time axis, and controls the optical switch so that only output light is extracted when the electrical signal has a positive or negative value.

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 of claim 1, wherein the C-axis of the lithium niobate is parallel to the major surface of the substrate.

4. 2. The optical modulator of 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. 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 that is multiplexed in the optical multiplexing section.

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

10. XR glasses equipped with the optical engine according to claim 9.

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

12. a transmitter for optical communication according to claim 11; an optical communication receiving device having an optical signal receiving element for receiving light;

13. 1. A method for controlling an optical modulator having a lithium niobate ridge-type optical waveguide and an optical switch provided on the output side of the ridge-type optical waveguide, comprising: A method for controlling an optical modulator, comprising applying an electrical signal that alternates between positive and negative values ​​on a time axis to the ridge-type optical waveguide, and controlling the optical switch to extract an output signal from the ridge-type optical waveguide when a voltage of a positive value or a negative value is applied.

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