Abstract: An apparatus connects a single-mode optical fiber in a transmitting side to a multi-mode optical fiber in a receiving side without a deterioration of baseband transmission characteristics. The apparatus optically connects the single-mode fiber to the multi-mode fiber with an optical axes displacement therebetween, so that an optical signal to a multi-mode optical fiber from a single mode optical fiber is incident in a steady state mode including not only lower mode, but higher mode. In one embodiment, the optical axes displacement is achieved by axial displacement between the single-mode fiber and multi-mode fiber. In another embodiment, the optical axes displacement is achieved by an angle displacement between the single-mode fiber and multi-mode fiber.

Abstract: A system for monitoring the transmission of a digital code is disclosed wherein a block consisting of a predetermined number of bits is extracted at a predetermined cycle from an information pulse train before the bit rate conversion thereof so that a check code corresponding to the states of this block may be formed, and inserted into the time slots formed by the bit rate conversion of the information pulse train, and the check code with the information pulse train after bit rate conversion, is transmitted to a receiving equipment. In the receiving equipment, a check code is formed from the received information pulse train, whose bit rate is restored to its original rate, in a manner exactly similar to that used in the transmitting equipment and is compared with the transmitted check code, that is, the check code is extracted from the received pulse train.