Method and apparatus for communication signal autotracking in a free space optical transmission system

A free space optical communication system is disclosed whereby the signal amplitude incident upon a receive optical fiber of a receive telescope is varied by dithering either the light beam transmitted by a transmit telescope or the receive optical fiber when a decrease in received signal amplitude is measured so as to effect the incidence of the maximum transmitted signal amplitude upon the receive optical fiber at the receive focal plane. In one embodiment, the transmit optical fiber is dithered in a pattern, causing the transmitted beam to oscillate in a corresponding pattern. In response, the received signal amplitude of the image on the focal plane of the receive telescope also oscillates in the same pattern. By comparing the phase of the received signal with the phase of the movement of the transmitted beam, the origin of the transmitted beam dithering pattern can be moved until the maximum amplitude of the transmitted beam is incident upon the receive optical fiber. In another embodiment, the receive optical fiber is dithered in a variable pattern to locate the point of maximum received signal amplitude. Using the aforementioned phase comparison technique, the receive optical fiber is moved progressively closer to the point of maximum amplitude of the received signal on the receive focal plane. In cases where atmospheric disturbances alter the path of the transmitted beam, the transmitted beam and the receive optical fiber may both be dithered to achieve coincidence between the point of maximum signal amplitude and the receive optical fiber.