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

Abstract

A high speed satellite-based laser communications system and method for communications between a satellite-based transmitter system and a ground-based receiver over a free space optical link. The satellite-based transmitter system includes an encoder to encode data, a polarization modulator to linearly polarize the encoded data, one or at least two transmitters to transmit the laser beam, and a quarter-wave optical wave plate to circularly polarize the signal to be transmitted. The ground-based receiver includes an automatic gain control to apply AGC to the received data before the polarizations are reversed and the data is decoded. The system enables an increased data throughput and reduces or eliminates the effects of signal fading.

1 .- 20 . (canceled) 
     
     
         21 . 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.   
     
     
         22 . The ground-based receiver of  claim 21 , wherein the decoder at the ground-based receiver is configured to perform error correction on the output signal when the received signal was error correction encoded at the satellite-based transmitter subsystem. 
     
     
         23 . The ground-based receiver of  claim 21 , wherein the decoder at the ground-based receiver comprises a deinterleaver to deinterleave the encoded output signal when the received signal was interleaved at the satellite-based transmitter subsystem. 
     
     
         24 . The ground-based receiver of  claim 21 , wherein the decoder at the ground-based receiver comprises a demultiplexer to obtain the multiple channels of data from the output signal when the multiple channels of data were multiplexed at the satellite-based transmitter subsystem. 
     
     
         25 . The ground-based receiver of  claim 21 , wherein the optical amplifier comprises one or more optical fiber amplifiers. 
     
     
         26 . The ground-based receiver of  claim 21 , wherein the ground-based receiver is configured to be used in conjunction with an on-off keying signaling system. 
     
     
         27 . The ground-based receiver of  claim 21 , wherein the ground-based receiver is configured to be used in conjunction with a differential phase shift keying (DPSK) system. 
     
     
         28 . The ground-based receiver of  claim 21 , wherein the received signal has been transmitted by the satellite-based transmitter subsystem to the ground-based receiver at a data rate at least as high as 10 Gbps. 
     
     
         29 . 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.   
     
     
         30 . The method of  claim 29 , wherein the signal, as transmitted, was error correction encoded, and wherein the method further comprises performing error correction on the output signal. 
     
     
         31 . The method of  claim 29 , wherein the signal, as transmitted, was interleaved, and wherein the method further comprises performing deinterleaving on the output signal.