Apparatus and method for reducing signal fading due to atmospheric turbulence

Patent metadata

Field	Value
Publication number	US-9503182-B2
Application number	US-201414464096-A
Country	US
Kind code	B2
Filing date	Aug 20, 2014
Priority date	Aug 20, 2014
Publication date	Nov 22, 2016
Grant date	Nov 22, 2016

Abstract

A method and system for reducing optical signal fading in an optical communication system include: dynamically obtaining turbulence-induced divergence and deviations of pointing direction for the atmosphere where an optical signal to be transmitted through; generating a single-transverse-mode laser beam of a predetermined diameter as the optical signal; dynamically varying the diameter of the laser beam to match the obtained turbulence-induced divergence of the atmosphere with a backtrack pointing direction; and transmitting the laser beam with varying diameter to a remote transceiver, as the optical signal.

First claim

What is claimed is:

1. A method for reducing optical signal fading in an optical communication system, the method comprising:
receiving a down-link optical signal from a satellite through the atmosphere;
sampling and processing an instant divergence and an angle of arrival (AoA) of the received down-link optical signal in the far-field at a sampling rate less than the Greenwood frequency to measure turbulence-induced divergence and an average beam divergence of the received down-link optical signal around its central direction over time;
generating a single-transverse-mode laser beam of a predetermined diameter for an up-link optical signal for transmission to the satellite;
setting the predetermined diameter of the laser beam to near match a near diffraction-limited divergence of the up-link optical signal to the measured turbulence-induced divergence;
setting a transmission direction of the up-link optical signal to measured average beam divergence of the received down-link beam; and
transmitting the up-link optical signal with the set diameter and the set transmission direction to the satellite.

2. The method of claim 1 , further comprising dynamically controlling a direction of the up-link optical signal towards a predicted location of the satellite at a time of arrival of the optical signal at the satellite.

3. The method of claim 1 , wherein a receiver and a transmitter of a ground transceiver include a common output/input aperture and optical axis to allow for characterizing the down-link optical signal arriving from space through the same path as the laser beam to be transmitted to the satellite.

4. The method of claim 3 , wherein the receiver and the transmitter of the ground transceiver, each include an aperture, and wherein the apertures are spatially separated.

5. The method of claim 1 , wherein the down-link optical signal is a sampled portion of a down-link communication signal or a beacon beam.

6. The method of claim 1 , wherein setting the diameter of the laser beam further comprises dynamically varying in time a transmission direction of the up-link optical signal with varying diameter to near match a direction backward to an angle of arrival of the down-link optical signal.

7. The method of claim 1 , further comprising dynamically adjusting a direction of the up-link optical signal for transmitting the up-link optical signal towards a predicted location of the satellite at a time of arrival of the up optical signal at the satellite.

8. An optical communication system for reducing optical signal fading comprising:
an in/out gimbaled telescope including an aperture for capturing a down-link optical beam transmitted by a remote transceiver through the atmosphere;
a steering mirror to direct the down-link optical beam to a beam sampler to sample an instant divergence and an angle of arrival (AoA) of a portion of the down-link optical beam at a sampling rate less than the Greenwood frequency;
a far field lens to receive the sampled portion and direct the sampled portion to a camera located at or near the focal plane of the far-field lens, wherein the camera measures turbulence-induced divergence and an average beam divergence of the down-link optical beam around its central direction over time by measuring a beam center point position and a beam diameter of the downlink optical beam at the far field of the far field lens;
a laser transmitter for generating a single-transverse-mode laser beam of a predetermined diameter as an up-link optical signal to be transmitted to the remote transceiver; and
a processor including memory and I/O circuitry to determine an optimal beam diameter and a beam direction angle for the up-link optical signal to be transmitted to the remote transceiver, according to the measured turbulence-induced divergence and an average beam divergence of the down-link optical beam, to set the diameter of the laser beam to the determined optimal beam diameter, and to set the transmission direction of the laser beam to the measured average beam divergence of the down-link optical beam.

9. The system of claim 8 , wherein the remote transceiver is positioned in a satellite.

10. The system of claim 8 , wherein the camera is a charge coupled device (CCD) camera that is positioned at the far field plane of the optical beam to measure angular characteristics, divergence and tilts of the sampled portion of the down-link optical beam.

11. The system of claim 8 , further comprising a zoom telescope, wherein the processor applies adaptive control to the zoom telescope to set the diameter of the laser beam to the determined optimal diameter.

12. The system of claim 11 , wherein the zoom telescope includes three sequential lenses, one or two of which are capable of being synchronously moved to change a magnification of the in/out gimbaled telescope, while keeping collimation of the up-link optical signal.

Assignees

• Raytheon Co

Inventors

• Betin Alexander A
• Shkunov Vladimir V

Classifications

• H04B10/118 Primary

  specially adapted for satellite communication · CPC title

• H04B10/112 Primary

  Line-of-sight transmission over an extended range · CPC title

• H04B10/503

  Laser transmitters · CPC title