Abstract: In a device for coupling optically a light source with an optical transmission path, the present invention interposes an optical transmission path structure at the center of the distance between them in order to improve the optical coupling efficiency. The optical transmission path structure consists of a plurality of light transmitting elements arranged so that the light from the light source is received by the structure as incident light and is sent out therefrom as outgoing light at an outgoing angle which is equal to the angle of incidence. The light transmitting elements are arranged in parallel with the optical path of the light emitted from the light source and according to this arrangement, the system of the present device becomes equivalent to a system in which the positions of the light source are located at points of .+-.l.sub.1 from the light outgoing surface of the optical transmission path structure.

Abstract: Disclosed is an optical switch for changing over the mode connecting two optical transmission lines comprising two optical fibers directly to each other and the mode connecting an optical communication device between the two transmission lines. In this optical switch, in order to reduce the insertion loss, first, second, third and fourth optical fibers, each having an optical connector attached to one end, are attached to a substrate so that at least parts of the respective optical fibers are fixed to the substrate, and the ends of the first and second fibers are arranged on the substrate movably so that the end face of the first optical fiber confronts the end faces of the second and third optical fibers and the end face of the second optical fiber confronts the end faces of the first and fourth optical fibers.

Abstract: A three-layer structure optical fiber comprises a first transparent dielectric core layer having a relatively high refractive index, a second transparent dielectric intermediate layer having a relatively low refractive index and a third transparent dielectric cladding layer having an intermediate refractive index wherein the parameter of the optical fiber is determined as follows: ##EQU1## where B is the bandwidth of the signal to be transmitted, z is the transmission distance, C is the velocity of light in vacuum, .eta.o is the refractive index of the core layer, q is the ratio of refractive indices between the core layer and second layer, V = normalized frequency, ##EQU2## p is the ratio of refractive index between the core layer and the third layer, and U.sub.11 =2,405.

Abstract: In order to expand the product between the transmission length and the transmission bandwidth of an optical fiber for use in the optical communication and to facilitate the connection between the optical fibers, the optical fiber is made of transparent materials of three concentric layers of a core whose refractive index gradually decreases in the radial direction from the center, an intermediate layer which has a uniform refractive index lower than the varying refractive index of the core, and a cladding which has a uniform refractive index substantially equal to the lowest refractive index of the core.

Abstract: In order to make the transmission loss of the lowest order leaky mode over the transmission length 10dB or more, an optical fiber for optical transmission is constructed of a cladding layer whose refractive index is constant, an intermediate layer which has a constant refractive index lower than the refractive index of the cladding layer, and a core whose refractive index is continuously varied so as to be the highest at the central part and to be substantially equal to that of the cladding layer at the peripheral part, and the thickness of the intermediate layer is 0.1-1 times the radius of the core.

Abstract: In an optical fiber connector, the pair of optical fibers to be connected together are placed end-to-end in a linear groove which has been formed on a lateral surface of a central cylinder. They are held fixed by a holding member which is fitted in the groove and is moved towards the optical fibers according to the rotation of a hollow cylinder which externally touches the central cylinder, whereby the optical fibers can be connected with a precise alignment and accordingly with a low optical power loss.