RADIO FREQUENCY (RF) SIGNAL RECEIVER USING OPTICAL PROCESSING AND ASSOCIATED METHODS

Abstract

A signal receiver, such as an RF-matched filter receiver, includes an optical source (e.g. a mode-locked laser) providing an optical signal, and a first optical modulator to modulate the optical signal with a received RF signal and provide a modulated optical signal. A second optical modulator modulates the modulated optical signal with a reference signal and provides a twice modulated optical signal. The modulators may be Mach-Zehnder Modulators (MZM) and/or Indium Phosphide (InP) modulators. An optical detector receives the twice modulated optical signal and provides a detected signal, and a processing unit receives the detected signal and extracts or measures cross-correlation between the received RF signal and the reference signal.

Description

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

With reference to FIG. 1, a signal receiver 20, such as an RF matched filter receiver, using optical techniques will be described. The RF matched filter receiver 20 provides a cross-correlation between two signals such as an RF signal (e.g. a received RADAR signal) and a reference signal (e.g. a copy of the transmit RADAR signal). The receiver 20 includes an optical source 22 (typically a laser) that produces a continuous waveform or pulsed (e.g. from a mode-locked laser) optical signal. An electrical RF signal (e.g. coming from a receiving antenna) modulates the power of the optical signal via a first optical modulator 24.

An electrical reference signal (the signal the received RF signal will be compared with) modulates the already modulated signal once again in the second optical modulator 26. A low-bandwidth optical detector 28 is used to detect the power of the twice modulated signal. The bandwidth of this optical detector 28 may be smaller than the RF signal bandwidth, therefore an averaging/integration of the signal takes place.

A processing unit 30 receives the electrical signal that is proportional to the integrated optical power. The processing unit 30 has the ability to delay the reference signal in time. By measuring the integrated optical power for different delay settings, the cross-correlation between the RF signal and the reference signal can be measured.

Other embodiments may replace the second optical modulator 26 with a device as described in: United States Patent Application Publication No. 2005/0168364 to Chen et al. and entitled “Optical Digital-to-Analog Converter”; United States Patent Application Publication No. 2005/0068887 to Chen et al. and entitled “high speed modulation of optical subcarrier”; United States Patent Application Publication No. 2004/0208642 to Chen et al. and entitled “analog modulation of optical signals”, all of which are herein incorporated by reference.

An M-ary optical matched filter receiver 40 or RF matched filter using optical techniques will be described with additional reference to FIG. 2 (illustrated with the optical signal source 42 and first modulator 44) and FIG. 3 (illustrated without the optical signal source and first modulator). An optical splitter 45 (e.g. a power or intensity splitter or WDM splitter) provides multiple parallel signal paths to a modulator array 46 (e.g. InP modulator array) defined by multiple matched filters as described above. Lowspeed photodetection at photoreceivers or detectors 48 forms the cross-correlation function between the received RP and reference signals (e.g. replica of each complex symbol and the received waveform at baseband). For example, post detection signal-to-noise ratio (SNR) in the matched filter receiver 40 may be (2NA²Dt)/(N_o). Fourier processing is done optically allowing baseband sampling and processing at processor 50 of the convolved result.

Traditional digital processing requires A/D sampling of the IF at 2× the desired collection bandwidth and FFT/DFT processing to synthesize the matched filter response. Optical processing of the present approach eliminates this high data rate digital processing and reduces the sampling rate to slightly more than the pulse repetition frequency of the radar. The collection bandwidth in this approach is increased compared to the conventional DSP approach.

The approach is applicable to broadband phased array antennas (UHF to 110 GHz and beyond), and supports both receive and transmit for communications and RADAR. The approach also supports receive only operation.

For example, a beamforming technique in a phased array antenna 60 will be described with reference to FIGS. 4-8. As discussed above, traditional phased arrays (as illustrated in FIGS. 6 and 7) utilize analog up converters followed by phase shifters and SSPA's and LNA's to drive each element in the phased array. Architecture is inherently band limited due to VSWR effects and group delay as percent bandwidth increases. Analog to digital converters and Digital to analog converters for mixed mode signal processing are limited to sample rates 2 GSPS forcing frequency conversion and band folding to implement bandwidths an octave. Few DSP FPGA's operate at these rates with manageable power dissipation.

FIG. 4 represents the transmit side of the optical beamforming, and FIG. 5 represents the receive side. In the present approach, the optical modulator array 86 may possess a bandwidth>10 Thz making percent bandwidth of RF signal<0.001%. Individual fibers run to each antenna element 62 and carry transmitted signals modulated on an optical carrier via optical source 82, splitter 85, modulator array 86 and photoreceivers or detectors 88. Optical photodetection forms the transmitted RF signal at each antenna element 62. The receive side (FIG. 5) uses an amplifier 73 (e.g. LNA) and an optical modulator 74 (e.g. an InP Mach Zehnder modulator) to convert low power RF signal to modulated light. The RF matched filter approach, described above, is then used to provide the beamforming.

Referring to the phased array including antenna elements 62 as illustrated in FIG. 8, the optical modulator (e.g. Indium Phosphide) is a broadband linear reciprocal device, thus superposition applies allowing simultaneous formation of an almost unlimited number of beams (e.g. without electrical phase shifters, modulators and up-converters). An example of test results from the optical matched filter approach as described in the embodiments herein are shown in FIG. 9A for a pulsed sinusoidal waveform at 12.5 MHz, tp=0.8 μs (T*BW=10 dB), and in FIG. 9B for a pulsed linear-frequency modulation (LFM) waveform centered at 50 MHz, Span 25 MHz, tp=0.8 μs (T*BW=13 dB).

Modifications and other embodiments will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.

Claims

1. A signal receiver comprising: an optical source providing an optical signal;a first optical modulator to modulate the optical signal with a received RE signal and provide a modulated optical signal;a second optical modulator to modulate the modulated optical signal with a reference signal and provide a twice modulated optical signal;an optical detector to receive the twice modulated optical signal and provide a detected signal; anda processing unit to receive the detected signal and extract a cross-correlation between the received RF signal and the reference signal.

2. The signal receiver according to claim 1, wherein the optical detector comprises a low-bandwidth optical detector to detect the optical power of the twice modulated optical signal and provide the detected signal being proportional to an integrated optical power.

3. The signal receiver according to claim 2, wherein the processing unit controls delay of the reference signal to the second optical modulator and measures a cross-correlation between the received RF signal and the reference signal based upon detected integrated optical power for different delay settings.

4. The signal receiver according to claim 1, wherein the optical source comprises a laser.

5. The signal receiver according to claim 1, wherein the optical modulators each comprise at least one of a Mach-Zehnder Modulator (MZM) and an Indium Phosphide (InP) modulator.

6. An RF-optical matched filter receiver comprising: an optical source providing an optical signal;a first optical modulator to modulate the optical signal with a received RF signal and provide a modulated optical signal;an optical splitter to split the modulated optical signal into split optical signals on multiple signal paths;a modulator array including a second optical modulator in each signal path to modulate the respective split optical signals with a reference signal and provide a twice modulated optical signal;an optical detector in each signal path to respectively receive the twice modulated optical signal and provide a detected signal; anda processing unit to receive the detected signals and choose a desired signal.

7. The RF-optical matched filter receiver according to claim 6, wherein the optical detector comprises a low-bandwidth optical detector to detect the optical power of the twice modulated optical signal and provide the detected signal being proportional to an integrated optical power.

8. The RE-optical matched filter receiver according to claim 6, wherein the processing unit controls delay of the reference signal to the second optical modulator and measures a cross-correlation between the received RF signal and the reference signal based upon detected integrated optical power for different delay settings.

9. The RF-optical matched filter receiver according to claim 6, wherein the optical source comprises a laser.

10. The RF-optical matched filter receiver according to claim 6, wherein the optical modulators each comprise at least one of a Mach-Zehnder Modulator (MZM) and an Indium Phosphide (InP) modulator.

11. A method of processing a received RF signal comprising: generating an optical signal;modulating the optical signal with the received RF signal to provide a modulated optical signal;modulating the modulated optical signal with a reference signal to provide a twice modulated optical signal;detecting the optical power of the twice modulated optical signal to provide a detected signal; andprocessing the detected signal to extract a cross-correlation between the received RF signal and the reference signal.

12. The method according to claim 11, wherein detecting the optical power of the twice modulated optical signal provides a detected signal that is proportional to the integrated optical power.

13. The method according to claim 12, wherein processing comprises controlling delay of the reference signal, and measuring a cross-correlation between the received RF signal and the reference signal based upon integrated optical power of the detected signal for different delay settings.

14. The method according to claim 11, wherein generating the optical signal comprises generating the optical signal with a laser.

15. The method according to claim 11, further comprising splitting the modulated optical signal into mutually coherent optical signals on multiple signal paths with an optical splitter before modulating with the reference signal to provide respective twice modulated optical signals on each signal path.

16. The method according to claim 15, wherein detecting comprises detecting the optical power of the twice modulated optical signal in each signal path to provide respective detected signals.

17. The method according to claim 11, wherein modulating comprises the use of at least one of a Mach-Zehnder Modulator (MZM) and an Indium Phosphide (InP) modulator.