[Problem] To determine a time difference between clocks which, for example, are placed far apart from each other with high accuracy at low cost. [Solution] In a time comparison system 20, an intermediate station 21 disperses a single optical signal 21c in the spatial region using the optical complex amplitude modulation to simultaneously transmit the optical signal 21c to a plurality of comparative stations 22 and 23 apart from each other. The intermediate station 21 transmits the optical signal 21c while changing the transmission angle using phase modulation, performs intensity scanning for the reflected light c1 of the optical signal 21c, and detects the peak intensity to determine the directions of the comparative stations 22 and 23. The reflected light c1 of the optical signal 21c transmitted to the comparative stations 22 and 23 of which the direction have been determined, is detected to determine a round-trip propagation delay time between the intermediate station 21 and each of the comparative stations 22 and 23. The difference calculation unit 25 calculates a sum of time difference between each of times to and tb associated with the comparative stations 22 and 23 and the time tc associated with the intermediate station 21, and the determined propagation delay time to determine time information of each of the comparative stations 22 and 23. Based on the result of subtracting, from the time information of the comparative stations 22, the time information of the comparative stations 23, the time difference between the comparative stations 22 and 23 is determined.

Techniques and examples pertaining to single channel line-of-sight (LOS) communication are described. The transmitting end of the LOS communication involves direct modulation of a light emitted from a light emitter. The receiving end of the LOS communication involves receiving a video stream of the light emitter emitting the light, wherein the video stream comprises a plurality of video frames that are continuous in time. The receiving end of the LOS communication also involves converting the video stream to a binary string comprising a plurality of binary bits. Each of the binary bits corresponds to a respective one of the plurality of video frames, and the binary string contains a repeated binary pattern representing a message being conveyed. The receiving end of the LOS communication also involves extracting the binary pattern from the binary string and then decoding the binary pattern to obtain the message.

An optical transmission/reception unit includes a carrier rotatable around an axis of rotation, an optical receiver arranged at the carrier on the axis of rotation so as to receive an optical reception signal from a first direction, an optical transmitter arranged at the carrier adjacent to the optical receiver so as to emit an optical transmission signal in a second direction, and a transmission/reception optic arranged at the carrier on the axis of rotation above the optical receiver, wherein the transmission/reception optic includes a reception optic and a transmission optic arranged in the reception optic, wherein the reception optic is configured to guide the optical reception signal striking the transmission/reception optic towards the optical receiver on the axis of rotation, and wherein the transmission optic is configured to displace onto the axis of rotation the optical transmission signal emitted by the optical transmitter.

A pointing unit 102 is for use with a free space optical communications terminal 100 including an optical source 104. The pointing unit 102 includes a first portion 106 having a mirrored surface 108, the first portion 106 being orientatable relative to an optical beam 110 produced by the optical source 104 and incident on the mirrored surface 108 in use to direct a reflection 112 of the optical beam 110 from the mirrored surface 108 towards a target 107. The first portion 106 further includes a directional radio frequency antenna 114.

A free-space optical communication method is provided. The method includes generating, at a transmitter of a satellite, an optical frequency comb and a pump signal, modulating the optical frequency comb to generate a data signal and an idler signal that is a phase conjugate of the data signal, attenuating the pump signal, transmitting over free-space, from the satellite, a communication signal having the data signal, the idler signal and the pump signal, receiving from the satellite, at a receiver, the transmitted communication signal having the data signal, the idler signal, and the attenuated pump signal, amplifying, at a phase-sensitive amplifier, the data signal and the idler signal, and demodulating the data signal and the idler signal to extract data.

In some embodiments, an optical communication system may include an optical source, a modulator, and a photoreceiver. The optical source may be configured to generate a beam comprising a series of light pulses each having a duration of less than 100 picoseconds. The photoreceiver may have a detection window duration of less than 1 nanosecond. When a first pulse travels through a variably refractive medium, photons in the first pulse may be refracted to travel along different ray paths to arrive at the photoreceiver according to a temporal distribution curve. A full width at half maximum (FWHM) value of the temporal distribution curve may be at least three times as large as a coherence time value of the first pulse, and the detection window of the photoreceiver may be at least six times as large as the FWHM value of the temporal distribution curve.

A photonic system is described that employs, in some examples, a pair of ultra-low noise semiconductor lasers that produce a low noise electrical signal at the output of a photodetector at a remote location with a frequency set by a frequency interval between the two lasers. The two lasers are phase locked together and mutually locked to a high stability source (such as an atomic clock) at any convenient frequency (e.g., 100 MHz, X-band, Ka-band, W-band, etc.). Upon impinging on the photodetector at the receive location, a combined or merged version of the two laser beams produces a stabilized beat note at the output of the photodetector. Since during transmission both lasers, propagating at the phase velocity, suffer the same frequency deviation due to atmospheric perturbation or motion of the receiver platform, any frequency variations will substantially cancel at the output beat note produced by the photodetector.

A free-space optical (FSO) terminal may include a controller and an alignment sensor. The alignment sensor includes a set of detectors. Each detector generates a signal responsive to receiving electromagnetic radiation at a detection surface. The set of detectors includes an inner set of detectors and an outer set of detectors. The detection surfaces of the inner detectors and the outer detectors may be aligned in a plane. The outer set of detectors surround the inner set of detectors (e.g., in the plane) and have larger detection surfaces than the inner set of detectors. During a tracking mode, the controller is configured to adjust an orientation of the FSO terminal based on signals from the inner set of detectors. During an acquisition mode, the controller is configured to adjust the orientation of the FSO terminal based on signals from the outer set of detectors.

An optical wireless communication system and method An optical wireless communication (OWC) system comprises: a multiple input multiple output (MIMO) device configured to provide a plurality of signals each representing a respective data stream; conditioning circuitry configured to receive the plurality of signals from the MIMO device and process the plurality of signals to produce at least one conditioned signal representative of the data stream(s) and suitable for transmission using an OWC transmission device; an OWC transmission device comprising at least one transmitter for transmitting light and configured to be responsive to the at least one conditioned signal to transmit light representative of the data stream(s) using the at least one transmitter.

A system and method generates confined electromagnetic radiation emanating from a remote position along a line of sight. The system includes a laser arrangement and a wavefront modifier. The laser arrangement generates at least one laser beam. The wavefront modifier produces a spatial arrangement of foci of the laser beam directed along the line of sight. The foci of the laser beam induce plasma filaments within an atmosphere at the remote position along the line of sight. The plasma filaments emit the electromagnetic radiation emanating from the remote position along the line of sight.