A communication device can achieve SN ratio and stability of signal output an optical transmission line having the propagation characteristics susceptible to environmental changes. The communication device includes: an optical reception section configured to receive weak signal light and reference light arriving through the optical transmission line; an optical amplifier that amplifies received reference light; a probe light receiver that receives probe light arriving from the transmitting-side communication device through the optical transmission line; and a controller configured to calculate a transmission line state detected based on received probe light and control a gain of the optical amplifier according to the transmission line state.

An optical sensor assembly including a diffractive optical element (DOE) responsive to an optical input beam and a refractive lens responsive to a shaped optical beam from the DOE. The lens focuses the shaped optical beam to provide a center beam having a short focal length and an outer beam having a long focal length. A sensor is positioned in front of the long focal length and is responsive to the shaped input beam from the lens. A fiber is positioned within a center opening of the sensor so that an input facet of the fiber faces the DOE and is located at the first focal length. The sensor is positioned relative to the DOE and the position of the input beam is controlled so that the center beam impinges the input facet and the outer beam impinges the sensor.

Methods and systems for communicating messages through free space are provided. In particular, examples or implementations facilitate communication between moving devices separated by free space. In examples or implementations, a radio-frequency (RF) communication link is first established between the devices. When one device intends to transmit data to another device, the one device, using an RF phased array, determines an angle-of-arrival for RF signals received from the other device. The one device then aims a light source according to the determined angle-of-arrival and transmits the data as an optical signal. In some examples or implementations, the other device at least partly receives, using a light detector, the optical signal and determines an alignment offset from the light detector. The other device then communicates the alignment offset to the one device by a further RF signal so that the alignment can be improved.