Facebook Patent | Quad tracker with birefringent optics

Patent: Quad tracker with birefringent optics

Publication Number: 10153838

Publication Date: 2018-12-11

Applicants: Facebook, Inc.


An optical tracking system for use in a free space optical communication system is described. The system includes a birefringent lens that is positioned to receive incident light and to produce light with a first and a second polarization. The system also includes a focusing lens positioned to receive the light with the first and the second polarizations and to direct the light with the first polarization to a first focal location along the optical axis and the light with the second polarization to a second focal location along the optical axis. A quadrature detector that is positioned between the first focal location and the second focal location receives the light with both the first and the second polarizations, and produces an output that is indicative of an alignment of the optical system.


Wireless communication systems transfer data from a transmitter of one station to a receiver of another station. In some applications, one of the stations can be ground based (e.g., stationary) while the other station is carried by a flying vehicle (e.g., a satellite in Earth’s orbit, an airplane or an unmanned aerial vehicle (UAV)). Furthermore, multiple stations can be ground based and in communication with one or more flying objects, or both stations can be part of flying vehicles. These wireless communication systems are sometimes used for Internet connections, especially if the land-based network is underdeveloped. These ground/airborne communication systems have to uplink (UL) and downlink (DL) large and ever-increasing volumes of data. Such large volumes of data form today’s complex telecommunication devices and networks, and are fast outpacing bandwidth offered by today’s satellite communications technology. Airborne vehicles typically communicate with other airborne or ground-based stations using microwave or radiofrequency (RF) bands. However, a major challenge for conventional microwave and RF communications is the highly constrained spectrum allocation imposed on these communication bands.

Free-space optical (laser) communications (FSO or Lasercom) is immune to spectrum allocation due to virtually unlimited bandwidth of the optical regions (greater than 100 Tb/s). While experimental FSO technology is now available for downlinking data at 100’s of Gb/s from air or space, these solutions are complex and expensive, require relatively large components and consume large amounts of power. One of the challenges associated with free space optical communications is the ability to maintain a proper alignment between a receiver of the optical beam that is subject to atmospheric turbulence. Such alignment issues are further complicated when one or both of the receiver and transmitter are moving.


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