True time delay generating system and method

• US 6,393,177 B2
• Filed: 07/19/2001
• Issued: 05/21/2002
• Est. Priority Date: 01/20/1998
• Status: Expired due to Fees

• Current Assignee: Government of the United States of America
• Original Assignee: Government of the United States of America
• Inventors: Paek, Eung G.
• Primary Examiner(s): Healy, Brian
• Application Number:
  US09/909,452
• Publication Number:
  US 20020012500A1
• Time in Patent Office: 306 Days
• Field of Search: 385/24, 385/31, 385/37, 385/14, 385/7, 372/20, 343/720, 343/721, 343/778, 359/127, 359/130, 359/341
• US Class Current: 385/24
• CPC Class Codes: G02B 6/2932 comprising a directional router, e.g. directional coupler, circulator
• CPC Class Codes: H01Q 3/2676 Optically controlled phased array

Abstract

System and method for rapidly reconfigurable two-dimensional true time delay generation for phased array antennas is described. The system utilizes a broadband light source, an array of fiber chirp gratings in a single fiber, and an acousto-optic spectrometer to generate a time-delayed linear grating. The grating is subsequently rotated to the desired angle utilizing an acousto-optic device having no moving parts.

Claims

1. A true time delay generating system comprising:

a broadband light source;

delay encoding means including an optical fiber for receiving light from said broadband light source, said optical fiber having at least a first selectively located fiber chirp grating defined therein so that different wavelengths of said light from said light source are reflected at unique locations at said fiber chirp grating, said locations corresponding to different selected time delays, thereby providing a wavelength encoded light signal output; and

decoding means for receiving said wavelength encoded light signal output and utilizing said wavelength encoded light signal output to provide a time delayed linear grating as an output therefrom.

2. The true time delay generating system of claim 1 further comprising a plurality of selectively positioned second fiber chirp gratings defined in said optical fiber of said delay encoding means.

3. The true time delay generating system of claim 2 wherein said decoding means includes means for spectrum selection as said time delayed linear grating, selected spectrum corresponding to a selected fiber chirp grating time delays.

4. The true time delay generating system of claim 2 wherein wavelength chirp ratio is constant within any one of said fiber chirp gratings, with each different said fiber chirp grating having a unique wavelength chirp ratio.

5. The true time delay generating system of claim 1 wherein said broadband light source provides either one of a pulsed or rf modulated broadband light output.

6. The true time delay generating system of claim 1 wherein said delay encoding means includes a circulator connected with said optical fiber and having as an output therefrom said wavelength encoded light signal output.

7. A true time delay generating system comprising:

a broadband light source;

delay encoding means including an optical fiber for receiving light from said broadband light source, said optical fiber having at least a first selectively located fiber chirp grating defined therein so that different wavelengths of said light from said light source are reflected at unique locations at said fiber chirp grating, said locations corresponding to different selected time delays, thereby providing a wavelength encoded light signal output; and

decoding means for receiving said wavelength encoded light signal output and utilizing said wavelength encoded light signal output to provide a time delayed linear grating as an output therefrom, said decoding means including an acousto-optic deflector for light dispersion at diffraction angles selectable by variation of an input signal.

8. The true time delay generating system of claim 7 wherein said acousto-optic deflector is characterized by large time-bandwidth product and high diffraction efficiency at the 1.55 micron wavelength region.

9. The true time delay generating system of claim 7 wherein said optical fiber includes an array of fiber chirp gratings defined therealong, said decoding means further including a window at an output plane from said acousto-optic deflector for selection of a selected spectrum of said dispersed light as said time delayed linear grating, said selected spectrum corresponding to a selected fiber chirp grating time delays.

10. The true time delay generating system of claim 9 wherein said fiber chirp gratings in said array are sufficient in number so that up to at least about 1000 time delays corresponding to wavelength reflection locations are provided.

11. The true time delay generating system of claim 9 further comprising a plurality of second optical fibers at said delay encoding means each having an array of fiber chirp gratings defined therealong, and multiplexing means associated with said optical fibers so that up to at least about 10,000 time delays corresponding to wavelength reflection locations are provided.

12. The true time delay generating system of claim 9 further comprising amplifying means for optically amplifying said wavelength encoded light signal output, wherein said acousto-optic deflector provides multiple wavelength spectra linearly arrayed at said output plane for selection at said window, and wherein said selection of a selected spectrum at said window is controlled by diffraction angle selection at said acousto-optic deflector.

13. A method for generating true time delays comprising the steps of:

launching broadband light into an optical fiber having a plurality of selectively located fiber chirp gratings therealong to provide a wavelength encoded light signal output;

dispersing said wavelength encoded light signal output at selected diffraction angles to provide multiple wavelength spectra linearly arrayed at an output plane; and

selecting a spectrum from said multiple wavelength spectra at said output plane corresponding to a selected one of said fiber chirp gratings at said optical fiber to thereby provide a selected time delayed linear grating.

14. The method for generating true time delays of claim 13 further comprising the steps of serially selecting different spectra from said multiple wavelength spectra at said output plane corresponding to selected different fiber chirp gratings to selectively provide different time delayed linear gratings.

15. The method for generating true time delays of claim 13 wherein the step of selecting a spectrum includes the step of selectively periodically changing said diffraction angles of dispersion of said wavelength encoded light signal output to serially present selected spectra at an output window at said output plane.

16. The method for generating true time delays of claim 13 further comprising the steps of launching broadband light into a plurality of second optical fibers each having an array of fiber chirp gratings defined therealong so that up to at least about 10,000 time delays can be provided.

17. The method for generating time delays of claim 13 further comprising the step of configuring said fiber chirp gratings so that wavelength chirp ratio is constant within any one of said fiber chirp gratings, with each different said fiber chirp grating having a unique wavelength chirp ratio.

18. The method for generating true time delays of claim 13 wherein the step of dispersing said wavelength encoded light is characterized by provision of a large time-bandwidth product and high diffraction efficiency at