Polarization based differential receiver for reduction of background noise in free-space optical links

摘要

An optical link[10,100] includes a transmitter[30, 150] and a receiver[40, 140] for sending and receiving data across a free-space link or any other link in which a high level of background light may be present. The transmitter[30, 150] includes a source of circularly polarized light having a predetermined wavelength. The source is modulated to transmit data. The receiver[40, 140] includes a circuit for generating a signal indicative of the difference in intensity of left-handed circularly polarized light and right-handed circularly polarized light incident on the receiver[40, 140]. In one embodiment of the invention, the receiver[40, 140] includes a first polarization filter[15, 115] for blocking left-handed circularly polarized light of the predetermined wavelength and a second polarization filter[16, 116] for blocking right-handed circularly polarized light of the predetermined wavelength, the filters being displaced from one another. A first detector[17, 117] measures the intensity of light leaving the first polarization filter[15, 115], and a second detector[18, 118] measures the intensity of light leaving the second polarization filter[16, 116]. The circularly polarized light source may be generated from a linearly polarized light source by passing the linearly polarized light through a ¼ waveplate[13]. Similarly, a polarization filter for blocking light of a predetermined circular polarization may be constructed by passing the light through a ¼ waveplate[13] and a linear polarization filter. In one embodiment of the present invention, the transmitter[30, 150] modulates the source of circularly polarized light by changing the direction of polarization of the source of circularly polarized light.