Transmitting device, information terminal, communication system, and communication method

Abstract

To provide a sending device which can generate a visible light signal rapidly and can be downsized easily.SOLUTION: The sending device is for sending a visible light signal to a receiving device, and includes: a laser light source for emitting visible light; and a light modulator for generating a visible light signal by changing the intensity of the visible light. The light modulator has a light waveguide to be a transmission route of the visible light, and the light waveguide is made of a material containing lithium niobate.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a transmitting device, an information terminal, a communication system, and a communication method. [Background technology]

[0002] An optical communication system is a system in which a transmitter transmits an optical signal in which information data is added to light, and a receiver converts the optical signal into an electrical signal to extract the information data added to the light.Known methods for adding information data to light include turning the light source on and off (internal modulation) and modulating the light by adding an external signal to the light propagating in the waveguide (external modulation).

[0003] Near-infrared light is widely used in optical communication systems, but recently, the use of visible light has been considered. Optical communication systems using visible light have attracted attention due to their advantages, such as the ability to visually observe the path of optical signals and determine the communication range at a glance. For near-infrared light with wavelengths of 1.5 μm or 1.3 μm, near-infrared light signals are generated using external modulation (Non-Patent Document 1). However, because infrared waveguides have low propagation efficiency for visible light, it is difficult to generate visible light signals using internal modulation. For this reason, in visible light communication systems using visible light, the use of LED elements (light-emitting diode elements) as visible light sources and the generation of visible light signals using internal modulation have been considered.

[0004] FIG. 1 is a conceptual diagram illustrating a transmitting device that generates a visible light signal using internal modulation and the visible light signal generated by the transmitting device. As shown in FIG. 1, a transmitting device 110 generates a visible light signal using bright light 1a generated when a visible light source 111 is turned on and dark light 1b generated when the visible light source 111 is turned off. In order to increase the transmission speed of the visible light signal, it is necessary to shorten the interval between bright light 1a and dark light 1b of the visible light signal, that is, to shorten the on / off switching time of the visible light source 111. However, there is a limit to how much the on / off switching time of the visible light source 111 can be shortened. For this reason, studies are being conducted to increase the transmission speed of the visible light signal by increasing the number of visible light sources.

[0005] For example, Patent Document 1 discloses a visible light communication system in which LED elements are arranged in an array in a transmitter and light receiving elements are arranged in an array in a receiver, and serial data can be transmitted and received as parallel data. According to Patent Document 1, if information can be transmitted from one LED element at a speed of 25 Mbps, this is equivalent to transmitting information light equivalent to 400 Mbps from a total of 16 LED elements, and it is said to be fully applicable to high-speed optical communications. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-292107 [Non-patent literature]

[0007] [Non-Patent Document 1] EL Wooten et al, ``A review of lithium niobate modulators for fiber-optic communications systems'', IEEE Journal of Selected Topics in Quantum Electronics 6, 69 (2000) Summary of the Invention [Problem to be solved by the invention]

[0008] With the increasing speed of computer processing and the accompanying improvement in information data processing capabilities, further increases in communication speed are desired in optical communication systems. However, in a transmission device that generates a visible light signal by internal modulation, there is a limit to how quickly the on / off switching time of a visible light source can be reduced, making it difficult to increase the generation speed of the visible light signal. Furthermore, as described in Patent Document 1, arranging visible light light sources in an array increases the size of the device, which may make it difficult to use in small information terminals such as smartphones. Furthermore, arranging visible light light sources in an array may complicate data processing. Furthermore, using multiple light sources to increase information data processing capabilities complicates the device configuration and significantly increases costs. Therefore, it is not practical to apply such a configuration to a transmission device for consumer use.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a transmitter that generates visible light signals at a high speed and is configured to be both compact and low cost. Another aim of the present invention is to provide an information terminal and a communication system that use the transmitter, and a communication method that uses the communication system. [Means for solving the problem]

[0010] As a result of investigations to solve the above-mentioned problems, the present inventors have found that by using laser light as visible light and lithium niobate as the material for the optical waveguide, it is possible to efficiently propagate visible light within the optical waveguide. Furthermore, the present inventors have found that by modulating the visible light propagated within the optical waveguide using lithium niobate, it is possible to increase the generation speed of a visible light signal. Therefore, in order to solve the above-mentioned problems, the present invention provides the following means.

[0011] [1] A transmitting device that transmits a visible light signal to a receiving device, comprising: a laser light source that emits visible light; and an optical modulator that generates a visible light signal by changing the intensity of the visible light, wherein the optical modulator has an optical waveguide that serves as a transmission path for the visible light, and the optical waveguide is formed from a material containing lithium niobate.

[0012] [2] The transmitting device according to [1] above, wherein the wavelength of the visible light is in the range of 380 nm or more and 830 nm or less.

[0013] [3] The transmitting device according to [1] or [2] above, wherein the optical modulator has a substrate, and the optical waveguide is a lithium niobate film grown on the substrate.

[0014] [4] The transmitting device according to [3] above, wherein the substrate is a sapphire substrate or an aluminum oxide substrate.

[0015] [5] A transmitting device according to any one of [1] to [4] above, wherein the width of a cross section of the optical waveguide perpendicular to the transmission direction of the visible light is in the range of 300 nm or more and 1000 nm or less, and the height of the cross section is in the range of 300 nm or more and 1000 nm or less.

[0016] [6] A transmitting device according to any one of the above [1] to [5], which satisfies at least one of the following formulas (1) and (2), where Wnm is the width of a cross section of the optical waveguide perpendicular to the transmission direction of the visible light, Hnm is the height of the cross section, and Am is the wavelength of the visible light emitted from the laser light source. (1): 0.8×A≦W≦2.5×A (2): 0.8×A≦H≦1.5×A

[0017] [7] The transmitting device according to any one of [1] to [6] above, which has an exit port for emitting the visible light signal to the outside.

[0018] [8] The transmitting device according to any one of the above [1] to [6], further comprising a connection part for connecting to an optical fiber for transmitting the visible light signal to the outside.

[0019] [9] An information terminal comprising the transmitting device according to any one of [1] to [8] above.

[0020]

[10] The information terminal according to [9] above, which is a smartphone, a tablet, or a personal computer.

[0021]

[11] A communication system comprising: a transmitting device according to any one of [1] to [8] above; and a receiving device that receives the visible light signal transmitted by the transmitting device.

[0022]

[12] A communication method using the communication system described in

[11] above, comprising the steps of: the transmitting device generating a first visible light signal including a reception permission signal and a second visible light signal including information data using the optical modulator; and irradiating the first visible light signal and the second visible light signal toward the receiving device; the receiving device receiving the first visible light signal; and the receiving device determining whether to accept the reception permission signal, and receiving the second visible light signal if the reception permission signal is accepted. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a transmitter that generates visible light signals at a high speed and is easily miniaturized. Also, according to the present invention, it is possible to provide an information terminal and a communication system using the transmitter, and a communication method using the communication system. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a conceptual diagram illustrating a transmitting device that generates a visible light signal using internal modulation, and the visible light signal generated by the transmitting device. [Figure 2] 1 is a conceptual diagram illustrating a transmitting device according to a first embodiment of the present invention and a visible light signal generated by the transmitting device. [Figure 3] 2 is a plan view showing an example of an optical modulator that can be used in the transmitting device of the first embodiment. FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3. [Figure 5] 1 is a photograph of a lithium niobate film grown on a sapphire single crystal substrate. [Figure 6] FIG. 10 is a perspective view showing another example of an optical modulator that can be used in the transmitting device of the first embodiment. [Figure 7] FIG. 10 is a perspective view showing yet another example of an optical modulator that can be used in the transmitting device of the first embodiment. [Figure 8] 3 is a perspective view of an example of a phase modulation section of a Michelson interferometer modulator that can be used in the transmitting device of the first embodiment. FIG. [Figure 9] FIG. 10 is a cross-sectional view of an optical modulator used to calculate, by simulation, a voltage value required to modulate light in the optical modulator whose optical waveguide is made of lithium niobate. [Figure 10] FIG. 10 is a block diagram of a communication system according to a second embodiment of the present invention. [Figure 11] FIG. 10 is a block diagram showing a modified example of the communication system according to the second embodiment of the present invention. [Figure 12] FIG. 10 is a diagram illustrating an example of usage of an information terminal according to a third embodiment of the present invention. [Figure 13] FIG. 10 is a diagram showing another example of usage of the information terminal according to the third embodiment of the present invention. [Figure 14] FIG. 11 is a diagram showing yet another example of usage of the information terminal according to the third embodiment of the present invention. [Figure 15]10 is a flowchart illustrating a communication method according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, components that are identical or equivalent to each other are denoted by the same reference numerals in the drawings, and redundant descriptions may be omitted or simplified. Furthermore, the drawings used in the following description may show characteristic parts enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. Furthermore, 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. A configuration shown in one embodiment can also be applied to other embodiments.

[0026] [First embodiment] 2 is a conceptual diagram illustrating a transmitting device according to a first embodiment of the present invention and a visible light signal generated by the transmitting device. The transmitting device according to this embodiment is a transmitting device that transmits a visible light signal to a receiving device. As shown in FIG. 2, a transmitting device 10 according to this embodiment includes a laser light source 11, an optical modulator 12, and an electric signal generating element 13.

[0027] Laser light source 11 emits visible light 1. Laser light source 11 is continuously on. "Continuously" means that laser light source 11 is on while transmitting a visible light signal to the receiving device. The wavelength of visible light 1 emitted by laser light source 11 is generally within the range of 360 nm to 830 nm.

[0028] The optical modulator 12 receives the visible light 1 emitted from the laser light source 11 and generates a visible light signal 2 by changing the intensity of the visible light 1. The optical modulator 12 may be configured to have, for example, an optical waveguide for visible light, and to apply an electrical signal sent from the electrical signal generating element 13 to the visible light 1 propagating through the optical waveguide to change the intensity of the visible light 1, thereby modulating the visible light 1 into bright light 1a or dark light 1b and generating the visible light signal 2. For example, a Mach-Zehnder optical modulator can be used as the optical modulator 12. In a Mach-Zehnder optical modulator, the time required to modulate the visible light 1 into bright light 1a or dark light 1b is shorter than the time required to switch the visible light source on and off. Therefore, the transmitting device 10 can generate the visible light signal 2 at a high speed.

[0029] The electrical signal generating element 13 receives the information data to be transmitted, converts it into an electrical signal, and outputs it to the optical modulator 12 .

[0030] FIG. 3 is a plan view showing an example of an optical modulator that can be used in the transmitting device 10 according to this embodiment, and FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3 and 4, the optical modulator 12a includes a substrate 30, an optical waveguide layer 40, a buffer layer 50, and an electrode layer 60. The optical modulator 12a is a Mach-Zehnder type optical modulator. The optical waveguide layer 40 is made of a lithium niobate film.

[0031] The substrate 30 is not particularly limited as long as it has a refractive index lower than that of the lithium niobate film constituting the optical waveguide layer 40. However, it is preferable that the substrate 30 be a substrate on which the lithium niobate film can be formed as an epitaxial film. The substrate 30 may be, for example, a single-crystal substrate such as a sapphire single-crystal substrate, a silicon single-crystal substrate, or an aluminum oxide (Al2O3) single-crystal substrate. The crystal orientation of the single-crystal substrate is not particularly limited. The lithium niobate film constituting the optical waveguide layer 40 has the property of being easily formed as a c-axis-oriented epitaxial film on single-crystal substrates of various crystal orientations. A c-axis-oriented lithium niobate film has three-fold symmetry. Therefore, it is desirable that the single-crystal substrate used as the underlying substrate 30 also has the same symmetry. For example, in the case of a sapphire single-crystal substrate or an aluminum oxide single-crystal substrate, the substrate 30 may be a c-plane substrate, and in the case of a silicon single-crystal substrate, it may be a (111)-plane substrate.

[0032] The lithium niobate film constituting the optical waveguide layer 40 is formed of a visible light-transmitting material containing lithium niobate. The visible light-transmitting material only needs to be transparent to the visible light generated by the laser light source 11, and does not need to be transparent to the entire visible light range. The lithium niobate film may be formed solely from lithium niobate.

[0033] The lithium niobate forming the lithium niobate film may contain elements other than lithium (Li), niobium (Nb), and oxygen (O). The lithium niobate may be a compound represented by the following formula (I): (I)LixNbAyOz In formula (I), A represents an element other than Li, Nb, or O. Examples of elements represented by 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. These elements may be used alone or in combination of two or more. x represents a number of 0.5 or more and 1.2 or less. x is preferably a number of 0.9 or more and 1.05 or less. y represents a number of 0 or more and 0.5 or less. z represents a number of 1.5 or more and 4.0 or less. z is preferably a number of 2.5 or more and 3.5 or less.

[0034] The optical waveguide layer 40 has an optical waveguide 41 consisting of a ridge portion and a flat portion 48. The optical waveguide 41 serves as a transmission path for visible light 1 emitted by the laser light source 11. The transmission direction of visible light 1 in the optical waveguide 41 is the direction in which visible light 1 propagates. The optical waveguide 41 includes a light receiving section 42, a branching section 43, a first branch optical waveguide 44a, a second branch optical waveguide 44b, a coupling section 45, a visible light signal generating section 46, and a visible light signal emitting section 47. One end of the light receiving section 42 is positioned so that it can receive the visible light 1 generated by the laser light source 11. The branching section 43 is connected to the other end of the light receiving section 42 and one end of the first branch optical waveguide 44a and the second branch optical waveguide 44b. The other ends of the first branch optical waveguide 44a and the second branch optical waveguide 44b are connected to one end of a coupling unit 45. The other end of the coupling unit 45 is connected to one end of a visible light signal generation unit 46. The other end of the visible light signal generation unit 46 is a visible light signal emission unit 47. Visible light signal 2 is emitted from the visible light signal emission unit 47.

[0035] The thickness (T in FIG. 4) of the flat portion 48 of the optical waveguide layer 40 is preferably in the range of 1 nm to 200 nm. Note that the optical waveguide layer 40 may not have the flat portion 48 and may be composed of only the optical waveguide 41.

[0036] The width (Wa in FIG. 4) of the cross section (cross section in a direction perpendicular to the transmission direction of visible light 1) of the first branch optical waveguide 44a is preferably in the range of 300 nm to 1000 nm. The height (Ha in FIG. 4) of the cross section of the first branch optical waveguide 44a is preferably in the range of 300 nm to 1000 nm. On the other hand, the width (Wb in FIG. 4) of the cross section of the second branch optical waveguide 44b is preferably in the range of 300 nm to 1000 nm. The height (Hb in FIG. 4) of the cross section of the second branch optical waveguide 44b is preferably in the range of 300 nm to 1000 nm. In FIG. 4, the cross-sectional shapes of the first branch optical waveguide 44a and the second branch optical waveguide 44b are rectangular, but are not limited thereto. The cross-sectional shapes of the first branch optical waveguide 44a and the second branch optical waveguide 44b may be, for example, trapezoidal or semicircular.

[0037] It is preferable that the first branch optical waveguide 44a and the second branch optical waveguide 44b each satisfy at least one of the following formulas (1) and (2): It is more preferable that the first branch optical waveguide 44a and the second branch optical waveguide 44b each satisfy both of the following formulas (1) and (2):

[0038] (1): 0.8×A≦W≦2.5×A (2): 0.8×A≦H≦1.5×A In the above formulas (1) and (2), W (unit: nm) is the length of the cross-sectional width Wa of the first branch optical waveguide 44a and the cross-sectional width Wb of the second branch optical waveguide 44b. H (unit: nm) is the length of the cross-sectional height Ha of the first branch optical waveguide 44a and the cross-sectional height Hb of the second branch optical waveguide 44b. A (unit: nm) is the wavelength of the visible light generated by the laser light source 11. When the cross-sectional shapes of the first branch optical waveguide 44a and the second branch optical waveguide 44b are trapezoidal or semicircular, the entire waveguide does not have to satisfy the condition of formula (1), as long as there is a portion that satisfies the condition of formula (1).

[0039] The cross-sectional width of the light receiving unit 42 and the visible light signal generation unit 46 is preferably within a range of 300 nm to 1000 nm. Furthermore, when the cross-sectional width is W (unit: nm), the cross-sectional height is H (unit: nm), and the wavelength of the visible light generated by the laser element is A (unit: nm), the light receiving unit 42 and the visible light signal generation unit 46 preferably satisfy at least one of the above formulas (1) and (2). It is more preferable that the light receiving unit 42 and the visible light signal generation unit 46 each satisfy both of the above formulas (1) and (2).

[0040] The lithium niobate film that constitutes the optical waveguide layer 40 may be an epitaxial film. Here, an epitaxial film refers to a single-crystal film with a uniform crystal orientation as a result of being grown on a substrate 30. That is, an epitaxial film is a film with a single crystal orientation in the film thickness direction and in the film plane direction, and when the film plane is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals are oriented in the X-axis, Y-axis, and Z-axis directions. Whether a film is an epitaxial film or not can be verified, for example, by checking the peak intensity and poles at the orientation positions in 2θ-θ X-ray diffraction.

[0041] The lithium niobate film can be formed by thin film formation methods such as sputtering, CVD, and sol-gel. The c-axis of the lithium niobate film is oriented perpendicular to the main surface of the substrate 30. When an electric field is applied parallel to the c-axis, the optical refractive index changes in proportion to the electric field. When a sapphire single crystal substrate is used as the substrate 30, the lithium niobate film may be epitaxially grown directly on the sapphire single crystal substrate. When a silicon single crystal substrate is used as the substrate 30, the lithium niobate film may be epitaxially grown via a cladding layer. The cladding layer should have a lower refractive index than the lithium niobate film and be suitable for epitaxial growth. For example, using Y2O3 for the cladding layer can result in a high-quality lithium niobate film. The optical waveguide 41 consisting of a ridge portion can be formed by patterning the lithium niobate film into the desired shape using a method such as photolithography.

[0042] By forming the lithium niobate film as an epitaxial film, the visible light transmittance of the lithium niobate film is improved. Figure 5 is a photograph of a lithium niobate film grown on a sapphire single crystal substrate. A plurality of metal wires 201 are formed at intervals on the sapphire single crystal substrate 200. The lithium niobate film was formed by RF sputtering using a lithium niobate target. The lithium niobate film is 1.5 μm thick. As shown in Figure 5, the metal wires 201 can be visually confirmed from the surface of the lithium niobate film, demonstrating that the lithium niobate film has extremely high transmittance to visible light.

[0043] The buffer layer 50 is formed between the optical waveguide layer 40 and the electrode layer 60 to prevent visible light propagating through the optical waveguide 41 from being absorbed by the electrode layer 60. A voltage is applied to the optical waveguide layer 40 via the buffer layer 50. The buffer layer 50 preferably has a smaller refractive index than the optical waveguide layer 40. The buffer layer 50 is preferably a dielectric material. Examples of materials that can be used for the buffer layer 50 include silicon oxide (SiO2), aluminum oxide (Al2O3), lanthanum oxide (La2O3), and composites of these oxides. Examples of composites that can be used include SiAlLaOx. The thickness of the buffer layer 50 is, for example, in the range of 0.2 μm to 1 μm at its thinnest point. The buffer layer can be formed by a thin film forming method such as sputtering, CVD, or sol-gel method.

[0044] The electrode layer 60 includes a first signal electrode 61a, a second signal electrode 61b, a first ground electrode 62a, a second ground electrode 62b, and a third ground electrode 62c. The first signal electrode 61a faces the first branch optical waveguide 44a via the buffer layer 50 to modulate visible light traveling through the first branch optical waveguide 44a. The second signal electrode 61b faces the second branch optical waveguide 44b via the buffer layer 50 to modulate visible light traveling through the second branch optical waveguide 44b. The first ground electrode 62a is disposed along the first signal electrode 61a on the side opposite the second signal electrode 61b side of the first signal electrode 61a. The second ground electrode 62b is disposed along the second signal electrode 61b on the side opposite the first signal electrode 61a side of the second signal electrode 61b. The third ground electrode 62c is disposed between the first signal electrode 61a and the second signal electrode 61b and along the first ground electrode 62a and the second signal electrode 61b. The first ground electrode 62a, the second ground electrode 62b, and the third ground electrode 62c are connected by bonding wires (not shown). By electrically connecting the first ground electrode 62a, the second ground electrode 62b, and the third ground electrode 62c in this manner, the in-plane potential of the ground electrodes can be made uniform, and the high-frequency characteristics of the optical modulator 12 can be improved.

[0045] The material of the electrode layer 60 may be, for example, a metal such as gold, silver, copper, platinum, ruthenium, cobalt, tungsten, or molybdenum, or a compound of these metals. The electrode layer 60 can be formed by forming a metal thin film using a thin film formation method such as vapor deposition, sputtering, CVD, or sol-gel, and then patterning the metal thin film into a desired shape using a method such as photolithography. Furthermore, when forming the metal thin film by vapor deposition or sputtering, a mask of a desired shape may be used to form the pattern.

[0046] The electrical signal generating element 13 receives information data to be transmitted, and outputs it as an electrical signal to the electrode layer 60 (first signal electrode 61a and second signal electrode 61b) of the optical modulator 12a.

[0047] In the optical modulator 12a, the visible light signal 2 is generated as follows. First, visible light 1 is received by the light receiving unit 42. The visible light 1 received by the light receiving unit 42 propagates from the light receiving unit 42 to the branching unit 43, where it is branched and propagates to the first branch optical waveguide 44a and the second branch optical waveguide 44b.

[0048] Meanwhile, the electrical signal generating element 13 outputs information data to be transmitted as an electrical signal to the first signal electrode 61a and the second signal electrode 61b. When the electrical signals are applied to the first signal electrode 61a and the second signal electrode 61b, a phase difference occurs between the visible light propagating in the first branch optical waveguide 44a and the visible light propagating in the second branch optical waveguide 44b.

[0049] The visible light 1 propagating through the first branch optical waveguide 44a and the visible light 1 propagating through the second branch optical waveguide 44b are coupled at a coupling portion 45. When the phase difference between the visible light propagating through the first branch optical waveguide 44a and the visible light 1 propagating through the second branch optical waveguide 44b is 0 or 360°, the two visible lights 1 are combined at the coupling portion 45 and propagated as is to the visible light signal generating portion 46. On the other hand, when the phase difference between the visible light 1 propagating through the first branch optical waveguide 44a and the visible light 1 propagating through the second branch optical waveguide 44b is 180°, the two visible lights interfere and cancel each other out at the coupling portion 45, and the visible light 1 does not propagate to the visible light signal generating portion 46. The visible light signal generating portion 46 generates a visible light signal 2 having a predetermined waveform depending on the combination of when the visible light 1 propagates and when the visible light 1 does not propagate. The generated visible light signal 2 is emitted to the outside via the visible light signal emitter 47 .

[0050] In the transmitting device of this embodiment, the optical modulator 12 is not limited to the configuration shown in Figures 3 and 4. For example, an optical modulator having the configuration shown in Figures 6 to 7 can be used.

[0051] The optical modulator 12b shown in FIG. 6 includes a substrate 30, an optical waveguide layer 40, a buffer layer 50, a signal electrode 61, and a ground electrode 62. The optical waveguide layer 40 does not have a flat portion and is made up of only an optical waveguide 41. The optical waveguide layer 40 does not have a branch, and a light receiving section 42, a visible light signal generating section 46, and a visible light signal emitting section 47 are linearly connected. The optical modulator 12b differs from the optical modulator 12a shown in FIGS. 3 and 4 in that the optical waveguide layer 40 is made up of only an optical waveguide 41, and that the light receiving section 42, the visible light signal generating section 46, and the visible light signal emitting section 47 are linearly connected in the optical waveguide 41.

[0052] The optical waveguide 41 preferably has a cross-sectional width W of 300 nm to 1000 nm, and a cross-sectional height H of 300 nm to 1000 nm, taken perpendicular to the direction of transmission of visible light. Furthermore, when the cross-sectional width is W nm, the cross-sectional height is H nm, and the wavelength of the visible light emitted from the laser light source is Am, it is preferable that at least one of the above-described formulas (1) and (2) is satisfied.

[0053] The signal electrode 61 is disposed above the visible light signal generating unit 46 via the buffer layer 50. The ground electrode 62 may be connected to the substrate 30 or the optical waveguide 41.

[0054] In the optical modulator 12b, the visible light signal 2 is generated as follows. First, visible light 1 is received by light receiving unit 42. The visible light 1 received by light receiving unit 42 propagates from light receiving unit 42 to visible light signal generating unit 46. When an electrical signal is applied to signal electrode 61, a phase difference occurs in the visible light 1 propagating within visible light signal generating unit 46, and the intensity of the visible light 1 changes. This generates visible light signal 2 having a predetermined waveform. The generated visible light signal 2 is transmitted to the outside via visible light signal emitting unit 47.

[0055] The optical modulator 12c shown in Fig. 7 includes a substrate 30, an optical waveguide layer 40, a buffer layer 50, a signal electrode 61, and a ground electrode 62. The optical waveguide layer 40 has an optical waveguide 41 made of a rigid portion and a flat portion 48. The optical modulator 12c differs from the optical modulator 12b shown in Fig. 6 in that the optical waveguide layer 40 has the flat portion 48.

[0056] The flat portion 48 of the optical waveguide layer 40 preferably has a thickness T in the range of 1 nm to 200 nm. Similar to the optical modulator 12b shown in Fig. 6, the optical waveguide 41 has a light receiving section 42, a visible light signal generating section 46, and a visible light signal emitting section 47 linearly connected to each other. The width W and height H of the cross section of the optical waveguide 41 perpendicular to the transmission direction of visible light are the same as those of the optical modulator 12b shown in Fig. 6.

[0057] The signal electrode 61 is disposed above the visible light signal generating unit 46 via the buffer layer 50. The ground electrode 62 may be connected to the substrate 30, the optical waveguide 41, or the flat portion 48. In the optical modulator 12c, the visible light signal 2 is generated in the same manner as in the optical modulator 12b shown in FIG.

[0058] The optical modulator 12 may also be a Michelson interferometer modulator. 8 is a perspective view of an example of a phase modulation unit of a Michelson interferometer modulator. The phase modulation unit 12d includes a substrate 30, an optical waveguide layer 40 having an optical waveguide 41 made of a rigid portion and a flat portion 48, a first insulating layer 51a and a second insulating layer 51b provided on the flat portion 48 in opposing positions across the optical waveguide 41, a first signal electrode 61a provided on the first insulating layer 51a, and a second signal electrode 61b provided on the second insulating layer 51b. The optical waveguide layer 40 is made of a lithium niobate film.

[0059] The structure and material of the substrate 30 are the same as those of the optical modulator 12a shown in FIGS. The material and formation method of the lithium niobate film constituting the optical waveguide layer 40 are the same as those in the optical modulator 12a shown in Figures 3 and 4. The optical waveguide 41 has a light receiving section 42, a visible light signal generating section 46, and a visible light signal emitting section 47. The width W and height H of the cross section of the optical waveguide 41 perpendicular to the transmission direction of visible light are the same as those of the optical modulator 12b shown in Figure 6. It is preferable that the thickness T of the flat section 48 is in the range of 1 nm to 200 nm.

[0060] The first insulating layer 51a and the second insulating layer 51b may be made of an oxide containing lanthanum (La), aluminum (Al), and silicon (Si) (La-Al-Si-O). The width and height of the first insulating layer 51a and the second insulating layer 51b may be the same as those of the optical waveguide 41. The distance between the first insulating layer 51a and the optical waveguide 41 and the distance between the second insulating layer 51b and the optical waveguide 41 may be within a range of 2 μm to 5 μm.

[0061] The first signal electrode 61a and the second signal electrode 61b are respectively provided on the first insulating layer 51a and the second insulating layer 51b that face the visible light signal generating unit 46. The materials and forming methods of the first signal electrode 61a and the second signal electrode 61b are the same as those in the optical modulator 12a shown in FIGS.

[0062] In the phase modulation section 12d, the phase-modulated modulated light 3 is generated as follows. First, visible light 1 is received by the light receiving unit 42. The visible light 1 received by the light receiving unit 42 propagates from the light receiving unit 42 to the visible light signal generating unit 46. When an electric signal is applied to the first signal electrode 61a and the second signal electrode 61b, a phase difference occurs in the visible light 1 propagating within the visible light signal generating unit 46, and phase-modulated modulated light 3 is generated.

[0063] The voltage required to modulate light in an optical modulator whose optical waveguide is made of lithium niobate was calculated through simulation. The optical modulator was assumed to have the configuration shown in FIG. 9. As shown in FIG. 9, the optical modulator 12S includes a substrate 30S, an optical waveguide layer 40S, a protective layer 49S, a buffer layer 50S, a first signal electrode 61aS, and a second signal electrode 61bS. The optical waveguide layer 40S includes a first branch optical waveguide 44aS, a second branch optical waveguide 44bS, and a flat portion 48S. The first signal electrode 61aS is disposed above the first branch optical waveguide 44aS via the buffer layer 50S, and the second signal electrode 61bS is disposed above the second branch optical waveguide 44bS via the buffer layer 50S. The protective layer 49S is interposed between the buffer layer 50S and a flat portion 48S in a portion where the first branch optical waveguide 44aS and the second branch optical waveguide 44b are not arranged. The protective layer 49S and the buffer layer 50S are dielectrics having a refractive index smaller than that of the optical waveguide layer 40S. The materials of the protective layer 49S and the buffer layer 50S may be the same or different. The cross sections of the first branch optical waveguide 44aS and the second branch optical waveguide 44bS each have a bottom width (W bottom ) is the width of the top (W top The cross-sectional height (H) of the first branch optical waveguide 44aS and the second branch optical waveguide 44bS, the width at the top (W top), bottom width (W bottom ), thickness of flat part 48S (T slab ), the thickness of the buffer layer 50S (T buffer ), the distance (L) between the center of the first branch optical waveguide 44aS and the center of the second branch optical waveguide 44bS, the width (W e ), thickness (T e ) are shown in Table 1 below.

[0064] [Table 1]

[0065] Using the optical modulator 12S, we simulated the voltage Vπ required for half-wavelength phase modulation (half-wavelength phase modulation voltage Vπ), i.e., the voltage Vπ required to change the light intensity from maximum to minimum, for light with wavelengths of 638 nm (red light), 520 nm (green light), and 473 nm (blue light). Because this voltage Vπ is inversely proportional to the interaction length Li, which is the length of the portion where the signal electrode overlaps the optical waveguide, it is generally compared using the integrated value Vπ*Li [V*cm], which is the product of the voltage Vπ and the interaction length Li. When this integrated value Vπ*Li [V*cm] was calculated for the interaction lengths of the signal electrodes (first signal electrode 61aS and second signal electrode 61bS), it was 1.79 V*cm for red light, 1.39 V*cm for green light, and 1.20 V*cm for blue light. That is, when the length of the signal electrode (interaction length) is 1 cm, the voltage value Vπ is 1.79 V for red light, 1.39 V for green light, and 1.20 V for blue light. Here, for red light, when the interaction length of the signal electrode is 1 cm, the voltage value Vπ is 1.79 V, but when the interaction length is 2 cm, the voltage value Vπ becomes 0.895 V, and when the interaction length is 0.5 cm, the voltage value Vπ becomes 3.58 V. Similarly, for green light and blue light, the voltage value Vπ changes depending on the interaction length.

[0066] In addition, the wavelength of red light (A red The relationship between the size of the first branch optical waveguide 44aS and the size of the second branch optical waveguide 44bS is as follows: Wtop / A red =1.25, W bottom / A red =1.72, H / A red = 1.11, which satisfied the relationship between the above formulas (1) and (2). green ) and the size of the optical waveguide, W top / A green =1.53, W bottom / A green =2.11, H / A green = 1.36, which satisfied the relationship between the above formulas (1) and (2). blue ) and the size of the optical waveguide, W top / A blue =1.69, W bottom / A blue =2.32, H / A blue =1.50, which satisfied the relationship between the above formulas (1) and (2).

[0067] These results confirm that the voltage Vπ required to phase-modulate light is lower when the wavelength of the light to be modulated is shorter. A lower voltage value allows the signal electrode to be shorter, making it possible to make the optical modulator more compact and less expensive. Furthermore, the simulation results above confirm that a single optical modulator can modulate three different visible light beams. This is extremely beneficial in terms of diversifying applications, such as enabling the use of visible light signals with different colors depending on the destination.

[0068] The transmitting device 10 of this embodiment, configured as described above, uses a laser light source 11 as a visible light source, thereby providing high directivity for the emitted visible light 1. This allows the optical modulator 12 to be used as a modulation method for the visible light 1, thereby enabling the generation speed of the visible light signal 2 to be increased. Furthermore, the optical waveguide 41 of the optical modulator 12 is formed of a lithium niobate film containing lithium niobate, thereby minimizing the loss of visible light within the optical waveguide 41. Therefore, the transmitting device 10 of this embodiment generates the visible light signal 2 at a high speed, resulting in a high intensity of the resulting visible light signal 2. Therefore, the use of the transmitting device 10 of this embodiment enables high-speed and highly accurate visible light communication. In particular, the transmitting device 10 of this embodiment enables a data transfer rate of, for example, 10 Gbit / s or more, particularly from several hundred Gbit / s to 1 Tbib / s. Furthermore, the transmitting device 10 of this embodiment does not require an array of visible light sources, thereby facilitating miniaturization.

[0069] Furthermore, in the transmitting device 10 of this embodiment, when the optical modulator 12 has the substrate 30 and the optical waveguide 41 is a lithium niobate film grown on the substrate 30, the visible light transmittance of the optical waveguide 41 is improved. This improves the propagation efficiency of visible light in the optical waveguide 41, allowing the visible light 1 to be modulated efficiently and increasing the speed at which the visible light signal 2 is generated.

[0070] Furthermore, in the transmitting device 10 of this embodiment, when the width of the cross section perpendicular to the transmission direction of the visible light 1 in the optical waveguide 41 of the optical modulator 12 is within the range of 300 nm to 1000 nm and the height of the cross section is within the range of 300 nm to 1000 nm, the propagation characteristics of the visible light 1 in the optical waveguide 41 are stable, thereby stabilizing the quality of the generated visible light signal 2. If the cross-sectional size of the optical waveguide 41 is larger than the above ranges, multiple propagation modes of visible light may be generated in the optical waveguide 41, which may cause the quality of the visible light signal 2 to become unstable.

[0071] Furthermore, in the transmitting device 10 of this embodiment, when the width of a cross section perpendicular to the transmission direction of visible light 1 in the optical waveguide 41 of the optical modulator 12 is Wnm, the height of the cross section is Hnm, and the wavelength of visible light 1 emitted from the laser light source 11 is Am, if at least one of the above formulas (1) and (2) is satisfied, the propagation characteristics of visible light 1 in the optical waveguide 41 become more stable, and the quality of the generated visible light signal 2 becomes more stable. If neither the above formula (1) nor the formula (2) is satisfied, multiple propagation modes of visible light are generated in the optical waveguide 41, which may cause the quality of the visible light signal 2 to become unstable.

[0072] Furthermore, in the transmitting device 10 of this embodiment, when the cross-sectional height of the flat portion 48 of the optical waveguide layer 40 of the optical modulator 12 is 200 nm or less, the visible light 1 is less likely to propagate through the flat portion 48. This improves the propagation efficiency of visible light in the first branch optical waveguide 44a and the second branch optical waveguide 44b. This further improves the accuracy of transmission and reception of the visible light signal 2. Furthermore, it is preferable that the cross-sectional height of the flat portion 48 of the optical waveguide layer 40 be ⅓ or less of the cross-sectional height of the first branch optical waveguide 44a and the second branch optical waveguide 44b. In this case, the propagation efficiency of visible light in the first branch optical waveguide 44a and the second branch optical waveguide 44b is further improved.

[0073] Furthermore, in the transmitting device 10 of this embodiment, there are no particular limitations on the method for extracting the visible light signal 2 to the outside. The transmitting device 10 may have an output port for emitting the visible light signal 2. The transmitting device 10 may also have a connection part for connecting to an optical fiber that transmits the visible light signal 2 to the outside.

[0074] [Second embodiment] FIG. 10 is a block diagram of a communication system according to the second embodiment of the present invention. A communication system 100a shown in FIG. 10 transmits a visible light signal 2 generated by a transmitter 10a to a receiver 20a via external space.

[0075] The transmitting device 10a includes a laser light source 11, an optical modulator 12, an electric signal generating element 13, and a visible light signal output port 14. The transmitting device 10a is the same as the transmitting device 10 shown in Fig. 2 except for the inclusion of the visible light signal output port 14. The visible light signal output port 14 is connected to the optical modulator 12 and is an output port for emitting the visible light signal 2 generated by the optical modulator 12 into external space.

[0076] The receiving device 20a includes a visible light signal receiving unit 21, an optical-electrical conversion element 22, and a visible light signal inlet 24. The visible light signal inlet 24 is an inlet for receiving the visible light signal 2 transmitted from the transmitting device 10a. The visible light signal receiving unit 21 is connected to the visible light signal inlet 24, receives the visible light signal 2 incident at the visible light signal inlet 24, and irradiates the visible light signal 2 to the optical-electrical conversion element 22. The optical-electrical conversion element 22 converts the visible light signal 2 into an electrical signal. There are no particular restrictions on the optical-electrical conversion element 22, and any type of element may be used as long as it is an element that can detect the visible light signal 2 at high speed and convert it into an electrical signal.

[0077] The communication system 100a performs visible light communication as follows. In transmitting device 10a, optical modulator 12 generates visible light signal 2 as described above. The generated visible light signal 2 is radiated into external space via visible light signal output port 14. The radiated visible light signal 2 is received by visible light signal receiving section 21 via visible light signal input port 24 of receiving device 20a. The received visible light signal 2 is converted into an electrical signal by optical-electrical conversion element 22, and the information data added to the visible light signal 2 is extracted.

[0078] In the communication system 100a according to the present embodiment, configured as described above, the visible light signal 2 transmitted from the transmitting device 10a is highly intense, making it easy to visually confirm the communication path of the visible light signal 2. 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 accidentally transmitting data to an unintended recipient. The communication system 100a according to the present embodiment, which can achieve a data transfer rate of 10 Gbit / s or more, from several hundred Gbit / s to 1 Tbib / s, can transmit a huge amount of data per second. While this is extremely convenient, it also increases the risk of data being transmitted 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. Data transmission using invisible infrared light is always accompanied by uncertainty.

[0079] 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.

[0080] 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.

[0081] In the communication system of this embodiment, optical transmission means such as optical fiber may be used to transmit the visible light signal. FIG. 11 is a block diagram showing a modified example of the communication system according to the second embodiment of the present invention. A communication system 100b shown in FIG. 11 differs from the communication system 100a shown in FIG. 10 in that a visible light signal 2 generated by a transmitter 10b is transmitted to a receiver 20b via an optical fiber .

[0082] 11, a transmitting device 10b includes a laser light source 11, an optical modulator 12, an electric signal generating element 13, and an output optical fiber connector 15. The output optical fiber connector 15 is a connector that connects the optical modulator 12 and an optical fiber 70 and outputs a visible light signal 2 generated by the optical modulator 12 to the optical fiber 70.

[0083] The receiving device 20b includes a visible light signal receiving unit 21, an optical-electrical conversion element 22, and an input optical fiber connection unit 25. The input optical fiber connection unit 25 is a connection unit that connects the optical fiber 70 and the visible light signal receiving unit 21, and inputs the visible light signal 2 transmitted through the optical fiber 70 to the visible light signal receiving unit 21.

[0084] The communication system 100b performs visible light communication as follows. In the transmitting device 10b, the visible light signal 2 is generated by the optical modulator 12 as described above. The generated visible light signal 2 is output to the optical fiber 70 via the output optical fiber connection unit 15. The output visible light signal 2 propagates through the optical fiber 70 and is received by the visible light signal receiving unit 21 via the input optical fiber connection unit 25 of the receiving device 20b. The received visible light signal 2 is converted into an electric signal by the optical-electrical conversion element 22, and the information data added to the visible light signal 2 is extracted.

[0085] According to the communication system 100b of this embodiment configured as described above, the visible light signal 2 generated by the transmitting device 10b is transmitted to the receiving device 20b via the optical fiber 70, so that the visible light signal 2 can be transmitted to a place where light does not pass through, such as a room separated by a wall.

[0086] [Third embodiment] FIG. 12 is a diagram showing an example of usage of an information terminal according to the third embodiment of the present invention. 12, smartphones 91a and 91b each include the transmitting device 10a and the receiving device 20a shown in Fig. 10 inside. Smartphones 91a and 91b have a flat surface having a display and side surfaces, with visible light signal outlet 14 of transmitting device 10a exposed on one side surface, and visible light signal inlet 24 of receiving device 20a on the flat surface having the display.

[0087] When transferring data from smartphone 91a to smartphone 91b, the smartphone 91a transmits a visible light signal 2 with visible light signal output port 14 directed toward visible light signal input port 24 of smartphone 91b. On the other hand, when transferring data from smartphone 91b to smartphone 91a, the smartphone 91b transmits a visible light signal 2 with visible light signal output port 14 directed toward visible light signal input port 24 of smartphone 91a.

[0088] Fig. 13 is a diagram showing another example of usage of an information terminal according to the third embodiment of the present invention. In Fig. 13, smartphones 91c and 91d are each provided with the transmitting device 10a and the receiving device 20a shown in Fig. 10 inside. The smartphones 91c and 91d have a flat surface with a display and side surfaces, and visible light signal outlet 14 of transmitting device 10a and visible light signal input port 24 of receiving device 20a are exposed on one side surface.

[0089] When smartphone 91c and smartphone 91d transfer data to each other, the visible light signal 2 is transmitted with the visible light signal output port 14 and the visible light signal input port 24 of smartphone 91c facing each other with the visible light signal output port 14 and the visible light signal input port 24 of smartphone 91d.

[0090] FIG. 14 is a diagram showing yet another example of usage of the information terminal according to the third embodiment of the present invention. 14, a smartphone 91 is provided inside with the transmitting device 10a and the receiving device 20a shown in FIG. 10. The smartphone 91 has a flat surface having a display and side surfaces, with visible light signal outlet 14 of the transmitting device 10a exposed on one of the side surfaces and visible light signal inlet 24 of the receiving device 20a on the flat surface having the display. Meanwhile, a personal computer 92 is provided inside with the receiving device 20a shown in FIG. 10. The visible light signal inlet 24 is exposed near the display of the personal computer 92.

[0091] When transferring data from the smartphone 91a to the personal computer 92, the visible light signal 2 is transmitted with the visible light signal outlet 14 of the smartphone 91a facing the visible light signal inlet 24 of the personal computer 92.

[0092] 12 to 14 are examples of usage of the information terminal according to this embodiment, but the information terminal according to this embodiment is not limited to these. The information terminal may be, for example, a tablet.

[0093] [Fourth embodiment] Fig. 15 is a flowchart illustrating a communication method according to a fourth embodiment of the present invention. The communication method according to this embodiment will be described below by taking as an example a case where information data is transferred from a transmitting device 10a to a receiving device 20a in the communication system shown in Fig. 10.

[0094] First, transmitting device 10a generates a first visible light signal including a reception permission signal and a second visible light signal including information data using optical modulator 12, and irradiates the first visible light signal and the second visible light signal toward receiving device 20a (step S1). In step S1, optical modulator 12 generates the first visible light signal by adding the reception permission signal to visible light, and generates the second visible light signal by adding the information data to visible light. The generated visible light signal 2 is then radiated into external space via visible light signal outlet 14. The reception permission signal is a signal for confirming that receiving device 20a is the destination of the information data.

[0095] Next, the receiver 20a receives the first visible light signal (step S2). The first visible light signal received in step S2 is converted into an electrical signal by the optical-electrical converter 22, and the reception enable signal added to the first visible light signal is extracted.

[0096] Next, the receiving device 20a determines whether to accept the reception permission signal (step S3). In step S3, if the reception permission signal confirms that the receiving device 20a is the destination of the information data (YES), the receiving device accepts the reception permission signal and proceeds to the next step. On the other hand, if the reception permission signal confirms that the receiving device 20a is not the destination of the information data (NO), the receiving device does not accept the reception permission signal and ends the process without proceeding to the next step.

[0097] Next, the receiving device 20a receives the second visible light signal (step S4). The second visible light signal received in step S4 is converted into an electrical signal by the optical-electrical conversion element 22, and the information data added to the second visible light signal is extracted.

[0098] If receiving device 20a has a transmitting means such as a smartphone, transmitting device 10a may not irradiate the second visible light signal in step S1. In this case, in step S3, when receiving device 20a accepts the reception permission signal, receiving device 20a may transmit an instruction signal to transmitting device 10a to irradiate the second visible light signal, and transmitting device 10a may irradiate the second visible light signal based on the instruction signal. The transmitting means when receiving device 20a transmits the instruction signal to transmitting device 10a may be visible light communication or radio wave communication.

[0099] If the receiving device 20a has a display such as a smartphone, it may display a message indicating reception completion on the display after step S4 is completed. Furthermore, if a transmission failure occurs, it may display an error message on the display. Furthermore, if the receiving device 20a has a transmitting means, it may transmit a signal indicating reception completion to the transmitting device 10a after step S4 is completed. Furthermore, if a transmission failure occurs, it may transmit an instruction signal to the transmitting device 10a to irradiate the second visible light signal again.

[0100] According to the above-described communication system, when information data is transferred from transmitting device 10a to receiving device 20a, the destination of the information data is confirmed by visually checking the second visible light signal and by checking by the reception permission signal, thereby more reliably preventing erroneous transmission of information data. [Explanation of symbols]

[0101] 1...visible light, 1a...bright light, 1b...dark light, 2...visible light signal, 3...modulated light, 10a, 10b...transmitting device, 11...laser light source, 12, 12a, 12b, 12c, 12S...optical modulator, 12d...phase modulation section, 13...electrical signal generating element, 14...visible light signal output port, 20a, 20b...receiving device, 21...visible light signal receiving section, 22...optical-electrical conversion element, 30, 30S...substrate, 40, 40S...optical waveguide layer, 41...optical waveguide, 42...light receiving section, 43...branching section, 44a, 44aS...first branch optical waveguide, 44b, 44bS...second branch optical waveguide, 45...coupling section, 46...visible Optical signal generating unit, 47...visible light signal emitting unit, 48...flat portion, 49S...protective layer, 50, 50S...buffer layer, 51a...first insulating layer, 51b...second insulating layer, 60...electrode layer, 61...signal electrode, 61a, 61aS...first signal electrode, 61b, 61bS...second signal electrode, 62...ground electrode, 62a...first ground electrode, 62b...second ground electrode, 62c...third ground electrode, 70...optical fiber, 91, 91a, 91b, 91c, 91d...smartphone, 92...personal computer, 100a, 100b...communication system, 110...transmitting device, 111...visible light source

Claims

1. A transmitting device that transmits a visible light signal to a receiving device, a laser light source capable of emitting red, green, and blue visible light; an optical modulator that generates a visible light signal by varying the intensity of the visible light; an electric signal generating element that receives information data to be transmitted and outputs the information data to the optical modulator; the optical modulator has a substrate, an optical waveguide on the substrate that serves as a transmission path for the visible light, and an electrode layer that modulates the visible light traveling in the optical waveguide; the optical waveguide is formed of a material containing lithium niobate, The wavelength of the red light is 638 nm, the wavelength of the green light is 520 nm, and the wavelength of the blue light is 473 nm; the electrical signal generating element is configured to be able to change a voltage value Vπ that needs to be applied to the electrode layer in order to perform a half-wave phase modulation on visible light traveling through the optical waveguide, an integrated value Vπ*Li [V*cm] which is the product of the voltage value Vπ and the interaction length Li is 1.79 V*cm when the visible light is red light, 1.39 V*cm when the visible light is green light, and 1.20 V*cm when the visible light is blue light; A transmitting device, wherein the optical modulator does not have a wavelength selection unit.

2. A transmitting device as described in claim 1, wherein the optical waveguide is a lithium niobate film grown on the substrate.

3. 3. The transmitter according to claim 2, wherein the substrate is a sapphire substrate or an aluminum oxide substrate.

4. The transmitting device according to any one of claims 1 to 3, wherein the width of a cross section of the optical waveguide perpendicular to the transmission direction of the visible light is in the range of 300 nm to 1000 nm, and the height of the cross section is in the range of 300 nm to 1000 nm.

5. The transmitting device according to any one of claims 1 to 4, wherein at least one of the following formulas (1) and (2) is satisfied, where W nm is the width of a cross section of the optical waveguide perpendicular to the transmission direction of the visible light, H nm is the height of the cross section, and A nm is the wavelength of the visible light emitted from the laser light source. (1): 0.8 × A ≦ W ≦ 2.5 × A (2): 0.8 × A ≦ H ≦ 1.5 × A

6. 6. The transmitting device according to claim 1, further comprising an exit port for emitting the visible light signal to the outside.

7. 6. The transmitting device according to claim 1, further comprising a connection section for connecting to an optical fiber for transmitting the visible light signal to an external device.

8. An information terminal comprising the transmitting device according to any one of claims 1 to 7.

9. The information terminal according to claim 8, which is a smartphone, a tablet, or a personal computer.

10. A transmitting device according to any one of claims 1 to 7; a receiving device that receives the visible light signal transmitted by the transmitting device.

11. A communication method using the communication system according to claim 10, generating a first visible light signal including a reception enable signal and a second visible light signal including information data using the optical modulator of the transmitting device, and irradiating the first visible light signal and the second visible light signal toward the receiving device; receiving the first visible light signal by the receiving device; determining whether to accept the reception enable signal, and receiving the second visible light signal with the receiving device if the reception enable signal is accepted.

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