Abstract: An optical signal transmitter controls the intensity of a light which is transmitted over an optical fiber. The transmitter splits a light beam into two substantially equal parts which pass through two separate paths in a LiNbO.sub.3 substrate. The intensity of the light is separately switched back and forth between high and low intensitites in response to binary electrical signals. At the output ends of the two paths, the signals are recombined and sent over an optical fiber. The resultant recombined signal is a three level signal of (a) the light in both paths at a low level, (b) the light in one path at a high level and in the other path at a low level, and (c) the light in both paths at a high level.

Abstract: In a lightwave transmission system for transmitting a digital signal from a transmitter to a receiver through a lightwave guide, in order to enable to transmit a digital signal having a DC component, a constant light beam is switched over to one of two parallel lightwave conductors by an optical directional coupling switch in response to the digital signal so that the parallel light wave conductors form a balanced lightwave guide to transmit the digital information signal as balanced optical signals. Two photo sensors are disposed at output ends of the two lightwave conductors and sense the balanced optical signals transmitted through the lightwave conductors to produce balanced electric signals which are, in turn, compared at a comparator to reproduce the digital signal.

Abstract: In an automatic gain control amplifier circuit for an intermittent sequence of digital input signals, a signal detector detects presence and absence of the digital input signal in the sequence to produce a presence signal which takes a presence and an absence level when the digital input signal is present and absent, respectively. The presence signal takes the absence level even upon undesired occurrence of an interruption in one of the digital input signals. A level comparator compares an amplified level of a digital output signal of the amplifier circuit with a reference level, which is preferably equal to an optimum level common to the digital output signals, to produce a comparison result signal which takes a first and a second level when the amplified level is higher and lower than the reference level, respectively.