Abstract: A bidirectional light transmission system comprises a transmitting station having a light source for intensity-modulating a transmission signal, a first light branching portion for branching intensity-modulated light of the light source in a predetermined ratio and a first light receiving portion, and a receiving station having a second light branching portion, a light phase-modulating portion, a light reflecting portion and a second light receiving portion.

Abstract: An output light beam from a semiconductor laser is converged by a lens system or a fine optical fiber with a small core diameter. The converged light beam is incident upon a part of the light receiving end face of a tapered optical fiber which is greater in area than the other end face thereof which is connected to one end of an optical fiber. The reflected light beam in the multi-mode optical fiber because of discontinuous portion is substantially prevented from returning to the semiconductor laser, so that the adverse effects on the semiconductor laser by the reflected beam can be minimized.