Abstract: A PLL comprising a phase comparator circuit for detecting the phase of a pulse signal based upon the input signal and the phase of a pulse signal based upon the output signal, a smoothing filter for smoothing the output of the phase comparator circuit, a loop filter for controlling the oscillation frequency on the basis of the smoothing filter, and a voltage controlled oscillator circuit for sending out the output signal having a frequency corresponding to the voltage based upon the output of the loop filter. Since the smoothing filter is separated from the loop filter, time constants of the smoothing filter and the loop filter can be set independently and with precision. If the time constant of the smoothing filter is chosen to be extremely small, for example, the time constant of the phase-locked loop is defined by the time constant of the loop filter. It is thus possible to define the time constant of the phase-locked loop by only selecting the time constant of the loop filter.

Abstract: A voltage controlled oscillator is provided which includes a pair of gain stages constituting a positive feedback path, a pair of buffer stages in cross connection with the gain stages, a pair of loads connected with the corresponding gain stages, each having a parallel connection of an active device resistor and a clamping diode, and a pair of voltage controlled current sources connected with the corresponding gain stages, for supplying constant currents to the gain stages. A timing capacitor is connected with the input sides of both voltage controlled current sources, and is charged or discharged by the constant currents from the current sources.

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: In a receiving circuit wherein a digital input signal is compared with a reference voltage and is regenerated into a pulse information signal of "1" or "0" value, a circuit for producing the reference voltage is constructed of a peak detector circuit which detects the maximum value of the output of a preprocessing circuit for the input signal, a reference preprocessing circuit which has the same circuit arrangement as that of the first-mentioned preprocessing circuit except for receiving no input signal, and a voltage divider circuit which divides the sum of the output of the peak detector circuit and an output of the reference preprocessing circuit. With such an arrangement, even when the amplitude of the input digital signal has fluctuated, a pulse signal whose pulse width is subjected to substantially no variation can be regenerated by means of the simple circuitry.

Abstract: A laser-to-fiber coupler is constructed by an optical waveguide arranged between a semiconductor laser and a transmitting optical fiber through which an optical beam emanating from the laser is transmitted in order to reduce the amount of a reflection beam from the optical fiber, which is injected into the active layer of the semiconductor laser.The waveguide is made by an optical fiber of which at least one of core radius or numerical aperture (N.A.) is smaller than those of the transmitting optical fiber.

Abstract: A mount for a light emitting device, which includes a first silicon substrate provided with first holding grooves of trapezoidal cross-section for mounting optical parts arranged in the vicinity of the light emitting plane of a light emitting element and a positioning groove for the light emitting element on the surface thereof, a second silicon substrate provided with second holding grooves having a trapezoidal cross-section similar to that of the first grooves for holding the optical parts and a hole portion for inserting the light emitting element, and a covering member for air tightly covering the hole portion, whereby the optical parts and the light emitting element of the light emitting device are tightly sealed by the above-stated means. The first and second holding grooves have the silicon crystal plane of (111) as both the side surfaces of their trapezoidal cross sections, and extend in the silicon crystal orientation of <110>.