Wide-area sensing of amplitude modulated signals

Patent metadata

Field	Value
Publication number	US-11375146-B2
Application number	US-202016801276-A
Country	US
Kind code	B2
Filing date	Feb 26, 2020
Priority date	Feb 26, 2019
Publication date	Jun 28, 2022
Grant date	Jun 28, 2022

Abstract

Amplitude-modulated (AM) signals spanning a spatial wide area can be efficiently detected using a slowly scanning optical system. The system decouples the AM carrier from the AM signal bandwidth (or carrier uncertainty), enabling Nyquist sampling of only the information-bearing AM signal (or the known frequency bandwidth). The system includes a staring sensor with N pixels (e.g., N>106) that searches for a sinusoidal frequency of unknown phase and frequency, perhaps constrained to a particular band by a priori information about the signal. Counters in the sensor pixels mix the detected signals with local oscillators to down-convert the signal of interest, e.g., to a baseband frequency. The counters store the down-converted signal for read out at a rate lower than the Nyquist rate of AM signal. The counts can be shifted among pixels synchronously with the optical line-of-sight for scanning operation.

First claim

The invention claimed is:

1. A method of detecting an amplitude-modulated (AM) optical signal having a center frequency f c and a bandwidth f b , the method comprising, at a pixel in an imaging array:
transducing the AM optical signal to an AM analog signal with a photodetector in the pixel;
converting the AM analog signal to an electronic pulse train with a current-to-frequency converter in the pixel;
mixing a first copy of the electronic pulse train with an in-phase square wave modulated at a mixing frequency f LO to produce an in-phase representation of the AM optical signal;
mixing a second copy of the electronic pulse train with a quadrature square wave modulated at the mixing frequency f LO to produce a quadrature representation of the AM optical signal; and
integrating the in-phase representation and the quadrature representation in the pixel over at least part of an integration period less than or equal to ½f b ; and
reading the in-phase representation and the quadrature representation from the pixel at a frame rate greater than or equal to 2f b .

2. The method of claim 1 , wherein the center frequency f c is about 100 kHz to about 10 GHz and the bandwidth f b is about 10 kHz to about 100 kHz.

3. The method of claim 1 , wherein mixing the first copy of the electronic pulse train with the in-phase square wave and integrating the in-phase representation comprise counting pulses in the electronic pulse train with a first counter in the pixel while switching the first counter between a count-up mode and a count-down mode at the mixing frequency f LO .

4. The method of claim 1 , wherein the pixel is a first pixel in the imaging array, and further comprising, at second pixel in the imaging array:
transducing another AM optical signal having a center frequency f c2 and a bandwidth f b2 to another AM analog signal with a photodetector in the second pixel;
converting the other AM analog signal to another electronic pulse train with a current-to-frequency converter in the second pixel;
mixing a first copy of the other electronic pulse train with the in-phase square wave modulated to produce an in-phase representation of the other AM optical signal;
mixing a second copy of the other electronic pulse train with the quadrature square wave to produce a quadrature representation of the other AM optical signal; and
integrating the in-phase representation of the other AM optical signal and the quadrature representation of the other AM optical signal in the second pixel over at least part of an integration period less than or equal to ½f b2 ; and
reading out the in-phase representation of the other AM optical signal and the quadrature representation of the other AM optical signal from the second pixel at a frame rate greater than or equal to 2f b2 .

5. The method of claim 1 , further comprising:
Fourier transforming the in-phase representation and the quadrature representation to produce a spectral signature of the AM optical signal.

6. The method of claim 1 , wherein the pixel is a first pixel, and further comprising:
integrating the in-phase representation in the first pixel during a first portion of the integration period;
transferring the in-phase representation from the first pixel to a second pixel in the imaging array; and
integrating the in-phase representation in the second pixel during a second portion of the integration period.

7. The method of claim 6 , wherein the first portion of the integration period is an integer multiple of the reciprocal of the mixing frequency f LO .

8. The method of claim 6 , wherein transferring the in-phase representation from the first pixel to the second pixel occurs synchronously with motion of the imaging array with respect to a source of the AM optical signal.

9. The method of claim 6 , further comprising:
demodulating the in-phase representation.

10. An imaging array for detecting an amplitude-modulated (AM) optical signal having a center frequency f c and a bandwidth f b , the imaging array comprising:
a pixel comprising:
a photodetector to transduce the AM optical signal to an AM analog signal with a photodetector in the pixel;
a current-to-frequency converter, operably coupled to the photodetector, to convert the AM analog signal to an electronic pulse train;
a first counter, operably coupled to the current-to-frequency converter, to count pulses in a first copy of the electronic pulse train over at least part of an integration period less than or equal to ½f b ;
a second counter, operably coupled to the current-to-frequency converter, to count pulses in a second copy of the electronic pulse train over the at least part of the integration period; and
circuitry, operably coupled to the first counter and to the second counter, to (i) mix the first copy of the electronic pulse train with an in-phase square wave modulated at a mixing frequency f LO to produce an in-phase representation of the AM optical signal in the first counter, (ii) mix a second copy of the electronic pulse train with a quadrature square wave modulated at the mixing frequency f LO to produce a quadrature representation of the AM optical signal, and (iii) read out the in-phase representation from the first counter and the quadrature representation from the second counter at a frame rate greater than or equal to 2f b .

11. The imaging array of claim 10 , wherein the center frequency f c is about 100 kHz to about 10 GHz and the bandwidth f b is about 20 kHz to about 100 kHz.

12. The imaging array of claim 10 , wherein the circuitry is configured to mix the first copy of the electronic pulse train with the in-phase square wave by switching the first counter between a count-up mode and a count-down mode at the mixing frequency f LO .

13. The imaging array of claim 10 , wherein the pixel is a first pixel, the photodetector is a first photodetector, the current-to-frequency converter is a first current-to-frequency converter, and the circuitry is first circuitry, and further comprising:
a second pixel comprising:
a second photodetector to transduce another AM optical signal to another AM analog signal;
a second current-to-frequency converter, operably coupled to the second photodetector, to convert the other AM analog signal to another electronic pulse train;
a third counter, operably coupled to the second current-to-frequency converter, to count pulses in a first copy of the other electronic pulse train over at least part of an integration period less than or equal to ½f b2 ;
a fourth counter, operably coupled to the second current-to-frequency converter, to count pulses in a second copy of the other electronic pulse train over the at least part of the integration period less than or equal to ½f b2 ; and
second circuitry, operably coupled to the third counter and to the fourth counter, to (i) mix the first copy of the other electronic pulse train with the in-phase square wave to produce an in-phase representation of the other AM optical signal in the third counter, (ii) mix a second copy of the other electronic pulse train with the quadrature square wave to produce a quadrature representation of the other AM optical signal, and (iii) read out the in-phase representation from the third counter and the quadrature representation from the fourth counter at a frame rate greater than or equal to 2f b2 .

14. The imaging array of claim 10 , wherein the pixel is a first pixel, and further comprising:
a second pixel; and
transfer circuitry, operably coupled to the first pixel and the second pixel, to transfer the in-phase representation from the first pixel to the second pixel.

Assignees

• Massachusetts Inst Technology

Inventors

• Schultz Kenneth

Classifications

• H04B10/1121 Primary

  One-way transmission · CPC title

• H04N25/583

  with different integration times · CPC title

• H04N25/772

  comprising A/D, V/T, V/F, I/T or I/F converters · CPC title

• H03D3/007

  by converting the oscillations into two quadrature related signals (H03D3/245 takes precedence) · CPC title

• H04B10/541

  Digital intensity or amplitude modulation · CPC title