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

A transmission device configured to transmit a visible light signal to a receiving device, comprising:

• a laser light source configured to emit visible light, the visible light is red light, green light, or blue light each having a different wavelength;
• an optical modulator configured to change intensity of the visible light and generate a visible light signal corresponding to the red light, green light, or blue light;
• and
• an electrical signal generation element configured to receive information data to be transmitted, and output the information data to the optical modulator,
• wherein the optical modulator has an optical waveguide 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 optical waveguide has a ridge portion and a flat portion,
• a wavelength of the red light is 638 nm,
• a wavelength of the green light is 520 nm,
• a wavelength of the blue light is 473 nm,
• a width of a cross section perpendicular to a transmission direction of the visible light of the ridge portion in the optical waveguide is within a range of 510.4 nm or more and 1182.5 nm or less, and a height of the cross section of the ridge portion is within a range of 510.4 nm or more and 709.5 nm or less,
• the electrical signal generation element is configured to change a voltage value Vπ required to phase-modulate the wavelength of light to half-wavelength applied to the electrode layer by the visible light transmitting through the optical waveguide, and
• an integrated value Vπ*Li[V*cm] which is a product of the voltage value Vπ and an interaction length Li is 1.79 V*cm when the visible light is the red light, is 1.39 V*cm when the visible light is the green light, and is 1.20 V*cm when the visible light is the blue light.