Abstract: A wide-dynamic range light receiver according to the present invention is constructed such that a non-linear impedance element is connected in series to a light-receiving element, and a resistor and a capacitor are connected in series between a connection node of the light-receiving element and the non-linear impedance element and an input terminal of a low-noise amplifier. The impedance of the non-linear impedance element increases as the output of the light-receiving element approaches a minimum reception light level, and the non-linear impedance element is finally set in the non-conductive state. Thus, at the minimum reception light level, the output current of the light-receiving element is supplied, with little loss, to the low-noise amplifier via the pure resistor and the capacitor. In addition, the impedance of the non-linear impedance element decreases exponentially in accordance with the increase in the input of the light-receiving element.

Abstract: A laser device according to the present invention having a semiconductor laser which has at least two longitudinal modes .lambda..sub.0 and .lambda..sub.1 both of which are able to oscillate, and oscillating at the single longitudinal mode .lambda..sub.0. The device further comprises input means which inputs a wave nearly coincident with .lambda..sub.1 to the semiconductor laser to switch the oscillating wavelength from .lambda..sub.0 to .lambda..sub.1. The semiconductor laser maintains its oscillating wavelength .lambda..sub.1 even if the intensity of said wave input from said input means reduces.

Abstract: A device for amplifying an input optical signal includes an optical amplifier having a gain control region in which the input optical signal is amplified, a first detecting region for detecting the input optical signal, and a second detecting region for detecting an amplified optical signal. The gain of the optical amplifier is controlled by a gain control signal from a signal source. A controller, responsive to detecting signals from the first and second detecting regions, produces a source control signal which is supplied to control the signal source.