Abstract: A communication system capable of employing polarization-dependent phase modulators with a reversing configuration that preserves security against disturbance of a polarization state at a transmission path but without using Faraday mirrors and a communication method using the same are provided. A quantum cryptography system of the present invention includes a first station 1, a transmission path 2, and a second station 3. The first station 1 has means for emitting time-divided optical pulses into the transmission path 2 and measuring a phase difference between the optical pulses returning from the transmission path 2. The transmission path 2 is a medium of light.

Abstract: A communication system capable of employing polarization-dependent phase modulators with a reversing configuration that preserves security against disturbance of a polarization state at a transmission path but without using Faraday mirrors and a communication method using the same are provided. A quantum cryptography system of the present invention includes a first station 1, a transmission path 2, and a second station 3. The first station 1 has means for emitting time-divided optical pulses into the transmission path 2 and measuring a phase difference between the optical pulses returning from the transmission path 2. The transmission path 2 is a medium of light.

Abstract: The object of the present invention is to provide a variable wavelength semiconductor laser which is large in ontinuous variable wavelength width, simple and convenient in structure, easy to produce, monolithic and small in size, and easy to handle. The variable wavelength semiconductor laser includes an optical gain region formed from two-dimensional or three-dimensional carrier confinement structures having sizes approximately of a thermal de Broglie wavelength whose size dispersion is controlled so that a gain spectrum covers a required variable wavelength region for generating and amplifying light when current is injected, a light absorption control region having a light absorption coefficient which varies depending upon a current injection amount, an optical resonator including the optical gain region and the light absorption control region, and control means for controlling current amounts or voltage values to be injected to the optical gain region and the light absorption control region.