Flexible printed circuit and optical device

An optical device comprising:

• an optical control element that controls a light wave by using a high-frequency signal;
• and
• a flexible printed circuit that serves as a connection interface with respect to an external electric circuit,
• wherein a high-frequency signal is supplied from the external electric circuit to the optical control element through the flexible printed circuit,
• wherein the flexible printed circuit has a transmission line for a high-frequency signal, and a connection terminal for electrically connecting the transmission line to the external electric circuit,
• wherein the transmission line has a signal electrode formed on one surface of the flexible printed circuit, and a ground electrode formed on the other surface of the flexible printed circuit,
• wherein the connection terminal has signal terminals formed on both the surfaces of the flexible printed circuit and electrically connected to each other through a via hole, and ground terminals formed on both the surfaces of the flexible printed circuit and electrically connected to each other through a via hole,
• wherein the signal terminal on the surface, on which the ground electrode is formed, is formed such that at least a width in an end portion on the transmission line side is narrower than a width of a part of the signal terminal on the surface, on which the signal electrode is formed, to be at the same position when the flexible printed circuit is seen in a plan view, and
• wherein the ground terminal on the surface, on which the signal electrode is formed, is formed such that a gap between the ground terminal and the signal terminal on the same surface is widened toward the transmission line side.