System for transmitting a beam in the air.

摘要

A system for transmitting information with a coherent beam being propagatable in the air is arranged to include a transmitting device having a modulator and a emitting circuit (1605) and a receiving device having a generator (1702), a mixer (1704), a frequency discriminating circuit (1705), and reproducing circuit (1708). The modulator modulates a reference light frequency (wavelength) or phase of a coherent beam to be emitted from a light source according to the signal. The emitting circuit emits the modulated beam as a divergent beam having such a power density as being safe to human eyes. The generator generates a coherent locally oscillated beam. The mixer mixes the signal beam with the locally oscillated beam and photoelectric-converts the mixed beam. The frequency discriminating circuit frequency-discriminates the a. c. component of the photoelectric-converted output. The reproducing circuit reproduces a signal from the output of frequency discriminating circuit. Any one of the transmitting device and the receiving device has a function of keeping a relative light frequency difference (wavelength difference) between the signal beam and the locally oscillated beam constant.