Abstract: Satellites may communicate using an optical communication system that provides high bitrate communication while also facilitating tracking, ranging, and low bitrate communication. Light modulated with a high bitrate signal is also modulated to include a set of orthogonal tones used for tracking, ranging, and low bitrate data. A continuous tracking tone may be sent to facilitate acquisition and tracking at the receiver. Timing signals may be exchanged using the tones. These may be used to calculate range between the satellites or provide precision time synchronization. One or more of these tones may be modulated to send low bitrate data. At the receiving device, the incident light may be detected by a first detector with a wide field of view, such as a quad cell detector, and the tones detected. With tracking established, the incident light is directed on a second detector and the high bitrate signal is detected.

Abstract: An optical detector system provides positioning data to facilitate tracking in optical communications. The system provides first and second path lengths to direct light onto an array of photodetectors. Incoming first light with a first polarization is reflected by a polarizing beam splitter (PBS) to the array, resulting in a first path length and a relatively wide field of view (FOV). Incoming second light with a second polarization passes through the PBS, interacts with a first quarter wave retarder (QWR) and a convex mirror, is reflected by the PBS, passes through a second QWR and is reflected by a flat mirror to pass through the PBS again and onto the array. The second light experiences a second path length greater than the first path length, exhibiting a relatively narrow FOV. The resulting spots of light on the array provide information about a position of the incoming beam.

Abstract: An optical detector system provides beam positioning data to an optical tracking system to facilitate optical communications. The optical detector system comprises a plurality of optical photodetectors. For example, a two-by-two array may be used. Incoming light passes through one or more optical elements, such as a lens and a dispersive optical element. A first portion of the beam entering the optical elements is directed into a first spot having a first area on the array. A second portion of the beam entering the optical elements is dispersed to form a second spot having a second area on the array that is larger than the first area. This combination of first portion and second portion of the beam incident on the array provides unambiguous information in the output of the photodetectors that is indicative of a position of the incoming beam with respect to the array.

Abstract: An optical detector system provides output to an optical tracking system to facilitate optical communications by tracking a beam of incoming light using a fast-steering mirror (FSM). The optical detector system comprises an array of optical photodetectors, such array comprising one or more quad cells. The incoming light passes through one or more optical elements to generate a specified beam shape, such as a bar or cross, on the array. The resulting output from the array is highly responsive to changes in position of the reshaped beam on the array. As a result, noise equivalent angle (NEA) of the optical detector system representing pointing error is substantially reduced. A reduction in NEA facilitates more precise alignment, allowing incoming light to be aligned to a smaller area. For example, the incoming light may be aligned to a single mode optical fiber connected to a receiver system.

Abstract: Light is modulated at a source to encode data. An additional tracking signal is imposed on the light using polarization modulation (PM). PM modulates one or more values of polarization rotation or polarization ellipticity of the light. These values may be dithered within constraints to provide a specified modulation index at the optical receiver, without impairing the encoded data. At the optical receiver, a polarization analyzer is used to recover the tracking signal, converting the polarization modulation to amplitude modulation. For example, after the light passes through the analyzer, an array of optical photodetectors detects the changes in apparent intensity resulting in the interaction between the light and the analyzer that correspond to the tracking signal. Due to a high modulation index, the recovered signal exhibits a high signal to noise ratio (SNR). The high SNR improves a noise equivalent angle, improving tracking performance.