System and method for high speed satellite-based free-space laser communications using automatic gain control

A ground-based receiver for receiving a signal transmitted by a satellite-based transmitter subsystem of a satellite-based laser communications system for communication between a satellite and the ground-based receiver using a laser beam over a free-space optical link that uses light propagating in free space for wireless data communications, wherein the received signal has been transmitted as a circularly polarized signal, wherein the ground-based receiver comprises:

• (a) an optical automatic gain control circuit that processes the received signal that was transmitted by the satellite-based transmitter subsystem using the laser beam to account for signal fading and atmospheric conditions over the free-space optical link, wherein the optical automatic gain control circuit comprises:
• (1) an optical amplifier to amplify the received signal to output an automatic gain controlled signal that has two circularly polarized states;
• (b) a quarter-wave (λ/4) optical wave plate to convert the automatic gain controlled signal from two circularly polarized states into an optical beam having two linear polarization states, including a first linear polarization state and a second linear polarization state;
• (c) a polarizing beam splitter to split the optical beam into a first linearly polarized beam corresponding to the first linear polarization state and a second linearly polarized beam corresponding to the second linear polarization state;
• (d) image processing circuitry or a computer-implemented image processing module comprising an algorithm to generate a difference between the first linearly polarized beam and the second linearly polarized beam to develop an output signal that comprises the signal as encoded at the satellite-based transmitter subsystem;
• (e) a decoder to decode the output signal to obtain the transmitted data;
• and
• (f) an output module to output the transmitted data.

A method of processing a signal received at a ground-based receiver from a satellite-based transmitter subsystem of a satellite-based laser communications system wherein the received signal has been transmitted as a circularly polarized signal, the method comprising:

• (a) receiving, by the ground-based receiver, the received signal that has been transmitted using a laser beam over a free-space optical link using light propagating in free space for wireless data communications,
• wherein the signal, as transmitted, was polarization modulated onto the laser beam by altering the polarization state of the laser beam through adjustment of an optical phase between two linear polarization states, including a first linear polarization state and a second linear polarization state, using one or more high-speed phase modulators each comprising an electro-optical crystal aligned with its active axis at 45° to the linearly polarized input beam, and
• wherein the two linear polarization states of the polarization modulated laser beam were converted into two circularly polarized states for transmission using a quarter-wave (λ/4) optical wave plate;
• (b) performing, by an optical automatic gain control circuit, automatic gain control on the received signal at an input to the ground-based receiver to account for signal fading and atmospheric conditions over the free-space optical link, the performance of optical automatic gain control comprising:
• (1) amplifying, using the optical amplifier, the received signal to output an automatic gain controlled signal that has the two circularly polarized states;
• (c) converting the automatic gain controlled signal from the two circularly polarized states into an optical beam having the two linear polarization states using a quarter-wave (λ/4) optical wave plate;
• (d) splitting, with a polarizing beam splitter, the optical beam into a first linearly polarized beam corresponding to the first linear polarization state and a second linearly polarized beam corresponding to the second linear polarization state;
• (e) detecting the first linearly polarized beam and detecting the second linearly polarized beam;
• (f) generating, using image processing circuitry or a computer-implemented image processing module, a difference between the first linearly polarized beam and the second linearly polarized beam that have been detected to develop an output signal that comprises the signal as encoded at the satellite-based transmitter subsystem;
• (g) decoding, using a decoder, the output signal to obtain the transmitted data;
• and
• (h) outputting the decoded data;
• wherein the method at least partially compensates for fading effects that occur during satellite transmissions to enable improvement in data throughput.