# Patent

## Title
Method and system for performing signal analysis using a correlative receiver

## Abstract

An apparatus includes a first antenna, a tunable optical carrier source, a second antenna, and a delay generation module coupled to the first antenna and the tunable optical carrier source. The apparatus also includes a fixed wavelength optical carrier source, an optical carrier generation module coupled to the fixed wavelength optical carrier source, and a local oscillator generation module. The apparatus further includes a correlative kernel generation and integration module coupled to the delay generation module and the local oscillator generation module and an optoelectronic conversion module coupled to the correlative kernel generation and integration module.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 63/154,550, filed Feb. 26, 2021, the contents of which are hereby incorporated by reference in their entirety for all purposes.
The following regular U.S. patent applications (including this one) are being filed concurrently, and the entire disclosure of the other application is incorporated by reference into this application for all purposes:

    
    
        application Ser. No. 17/681,623 filed Feb. 25, 2022, entitled “METHOD AND SYS IEM FOR PERFORMING SIGNAL ANALYSIS USING A HYBRID PHOTONIC-ELECTRONIC PROCESSOR;” and
        application Ser. No. 17/681,629 filed Feb. 25, 2022, entitled “METHOD AND SYSTEM FOR PERFORMING SIGNAL ANALYSIS USING A CORRELATIVE RECEIVER.”
    
    




BACKGROUND OF THE INVENTION
Real-time signal analysis is strictly dictated by the waveform bandwidth. Current electronic processors allow for complex analysis for signals with bandwidth smaller than 1 GHz. The example of currently viable real-time electronic processing includes Fourier and Hilbert transforms, correlation, convolution, pattern matching, and cyclostationary analysis. To perform any of these, a physical signal must first be digitized, mandating the use of an analog-to-digital (ADC) convertor processor in the front-end. While a number of applications can be served by sub-GHz signal analysis, a number of important applications require real-time analysis of signals with bandwidth exceeding 1 GHz.
With the emergence of high-capacity wireless communication, spectral analysis of wideband radio-frequency signals, associated with military applications in the past, has emerged as one of the most important applications. Current 5G wireless infrastructure incorporates signaling in excess of 1 GHz, requiring real-time spectral analysis for the purposes of provisioning, network surveillance, security and traffic conflict resolution. In electronic warfare, higher-order spectral analysis of the contiguous radio-frequency (RF) band (e.g., 1 kHz-110 GHz) is dictated by a need to identify, intercept and geolocate any emitter and classify its operation. In light wave (e.g., fiber optic) communication, it is necessary to analyze channels with aggregate capacity exceeding 400 Gbit/sec in order to provide real-time security and reverse transmission penalties. Current electronic processors possess neither the precision nor speed to satisfy these applications, causing a need for improved methods and systems that can address analysis of wideband signals.
SUMMARY OF THE INVENTION
In accordance with various aspects of the present disclosure, embodiments of the present invention relate to methods and apparatus for correlative reception of radio-frequency signals.
One embodiment includes an apparatus for generating a cross ambiguity function from two independent signals comprising: a tunable laser providing a tunable optical carrier to a first single-sideband optical modulator; a first single-sideband optical modulator configured to modulate a tunable optical carrier with one of a set of input signals; a fixed wavelength laser, providing a fixed optical carrier to a first optical modulator; a second single-sideband optical modulator configured to modulate the fixed optical carrier with another of a set of input signals; a dispersive element configured to provide a delay between the set of input signals modulated onto the fixed optical carrier and the tunable optical carrier; a first optical detector configured to receive signals modulated onto the tunable optical carrier; a second optical detector configured to receive signals modulated onto the fixed optical carrier; a third, null-biased optical modulator configured to modulate signals onto the fixed optical carrier using the output of the first optical detector; a fourth, null-biased optical modulator configured to modulate signals onto the tunable optical carrier using the output of the second optical detector; an optical 90-degree hybrid element configured to separate quadratures of the outputs of the third and fourth optical modulators; and a plurality of optical detectors optically coupled to the optical 90-degree hybrid element and configured to convert optical signal quadratures to electrical signals.
Another embodiment includes an apparatus for generating a cross ambiguity function from two independent signals comprising: a tunable optical laser providing an optical carrier; a first single sideband optical modulator configured to modulate a tunable laser carrier with one of the input signals; a fixed wavelength laser providing an optical carrier; a second optical modulator configured to modulate the fixed wavelength carrier with the second of the two input signals; a dispersive element configured to provide delay between signals modulated onto tunable and fixed laser carriers; a first optical detector configured to receive signals modulated onto the wavelength tunable laser carrier; a second optical detector configured to receive signals modulated onto the fixed wavelength laser carrier; a third, null-biased, optical modulator configured to modulate signals onto the fixed laser carrier using the output of the first optical detector; a fourth, null-biased, optical modulator configured to modulate signals onto the fixed laser carrier using the output of the second optical detector; a wavelength division demultiplexer optically coupled to the third and fourth optical modulators and configured to transmit modulated signals by the wavelength of the signal to an optical 90-degree hybrid element from a plurality of optical 90-degree hybrid elements; a plurality of optical 90-degree hybrid elements optically coupled to the wavelength-division demultiplexer configured to separate quadratures of the output of the third optical modulator; a plurality of optical detectors optically coupled to each optical 90-degree hybrid element and configured to convert two of the four output quadratures of the optical 90-degree hybrid element to electrical signals.
Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide methods and systems for real-time processing of radiofrequency (RF) input signals which may have bandwidth larger than several hundred GHz. These and other embodiments of the invention, along with many of its advantages and features, are described in more detail in conjunction with the text below and attached figures



BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, which are intended to be read in conjunction with both this summary, the detailed description and any preferred and/or particular embodiments specifically discussed or otherwise disclosed. The various aspects may, however, be embodied in many different forms and should not be construed as limited to the embodiments as set forth herein; rather, these embodiments are provided by way of illustration only and so that this disclosure will be thorough, complete and will fully convey the full scope to those skilled in the art.
FIG. 1 is CAF table plot used to identify signal modulation, geolocate an emitter and define its velocimetry.
FIG. 2 is a simplified schematic diagram of a physically assisted correlative receiver according to an embodiment of the present invention.
FIG. 3 is a block diagram illustrating the functionalities of the modules of a correlative receiver according to an embodiment of the present invention.
FIG. 4 is a block diagram showing the implementation of the modules of the correlative receiver according to an embodiment of the present invention.
FIG. 5 is a block diagram showing implementation of the modules of a parallelized correlative receiver according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The present invention relates generally to wideband signal analysis with a hybrid photonic-electronic processor. More particularly, embodiments of the present invention provide methods and systems directed at correlative reception of radio-frequency signals.
A correlative radio-frequency (RF) receiver is intended to generate a cross-ambiguity function (CAF), using two RF inputs x(t) and y(t).

RαT=∫−T/2T/2x(t)×y(t−τ)ej2παtdt  (1)

where τ is commonly referred to as signal delay and α is referred to as cyclic frequency. The bound (T) is referred to as the integration range, and, in practical applications, is typically much larger than the shortest cycle of the processed signal input. The CAF is represented in the form of a τ−α table, shown in FIG. 1. Analysis of the CAF table allows for identification of modulation-specific signal features, emitter geolocation, and Doppler velocimetry of the emitter.

The Fourier transform of the CAF function is referred to as the spectral correlation function (SCF), and can be obtained by a single-step transformation from the known CAF function. In a conventional approach that utilizes computational electronics, the SCF is calculated by using successive Fast Fourier Transforms (FFTs) and spectral correlation. The CAF function is subsequently calculated by computing the inverse Fourier transform. The computational load associated with such an approach is impractical in the case when the input signal bandwidth is large. As an example, consider a 10 GHz wide RF input signal that needs to be resolved with 1 kHz precision. This means that the FFT must be computed over 10 GHz/1 kHz=107 spectral bins, requiring 107 log10(107)=7×107 floating operations each millisecond (7×1010 FLOPS). This must be followed by correlation of spectral bins, requiring 107×107=1014 multiplications during 1 millisecond (i.e., 1017 FLOPS). Such computational power is unattainable by most conventional electronic architectures and, when coupled with the memory requirement associated with CAF integration, eliminates wideband signals (e.g., with bandwidths substantially higher than 1 GHz) from being received by a correlative receiver. Recognizing this limitation, a purely computational approach to signal correlation and generation of CAF/SCF functions is replaced by physically assisted architecture according to embodiments of the present invention.
FIG. 2 is a block diagram of a physically assisted correlative receiver 200 according to an embodiment of the present invention. The physically assisted correlative receiver 200 is used to assist the generation of CAF/SCF functions. The physically assisted correlative receiver 200 is implemented as a hybrid photonic-electronic processor that includes a front end dedicated to acquiring two arbitrary RF signals (e.g., a first RF signal y(t) and a second RF signal x(t)) and performing its operation in the photonic domain before converting the result to digital form and completing the processing chain.
The physically assisted correlative receiver 200 may receive radiofrequency (RF) input (e.g., the arbitrary signals x(t) and y(t)) through a first antenna 202 and a second antenna 204. As an example, the first antenna 202 and the second antenna 204 can receive RF signals having a bandwidth up to hundreds of GHz. The physically assisted correlative receiver 200 converts the inputs received by the first antenna 202 and second antenna 204 from the RF domain to the optical domain via optoelectronic modulators. The physically assisted correlative receiver 200 includes a first optoelectronic modulator 206, a second optoelectronic modulator 208, and a third optoelectronic modulator 218. The physically assisted correlative receiver 200 may further include a tunable laser 210; the output of the tunable laser (e.g., an optical carrier) is provided to the second optoelectronic modulator 208 using a suitable optical connection. As illustrated in FIG. 2, the tunable laser 210 is capable of tuning its center frequency over a frequency range Δf that ranges from f1 to f2. The RF input y(t) of the second antenna 204 modulates the optical carrier generated by the tunable laser 210 via the second optoelectronic modulator 208, converting the RF input y(t) from the RF domain to the optical domain.
Referring to FIG. 2, a delay τ in the first RF signal y(t) is generated by transmitting the modulated optical carrier produced at the output of the second optoelectronic modulator 208 over a second dispersive line 214. The relative delay τ for frequencies, centered at f1 and f2, is defined by T=D×L×c×Δf/f2, where D is the dispersion of the second dispersive line 214, L is the length of the second dispersive line 214, c is the speed of light, f is the frequency, and Δf is the frequency difference between f1 and f2. As an example, an L=1 km long, conventional dispersion compensating fiber that possesses a dispersion D=5×10−4 s/m2 will induce a delay of τ=5 ns when a f=193 THz (i.e., a wavelength of 1550 nm) tunable laser is tuned over Δf=1250 GHz (i.e., 10 nm). The practical delay range of a processor using a fiber as a dispersion line is limited by the fiber loss α=0.25 dB/km and the tunable range of the laser carrier. An acceptable loss (e.g., around 10 dB) and a common tunable range of a 1550 nm laser (i.e. approximately 100 nm) result in a viable delay range of 2000 ns. The delayed signal output (labeled as y(t−τ) in FIG. 1) of the second dispersive line 214 is then sent to a detector element 216. The detector element 216 will convert the delayed signal from the optical domain back to the RF domain. Thus, the embodiment illustrated in FIG. 2 utilizes the tunable laser 210, the second optoelectronic modulator 208, the second dispersive line 214, and the detector element 216 (e.g., an intensity detector) to implement a tunable delay line that delays the arbitrary first RF input y(t) to produce delayed y(t−τ) used to drive the third optoelectronic modulator 218.
The physically assisted correlative receiver 200 may further comprise a fixed-frequency laser 220. The output of the fixed-frequency laser (e.g., an optical carrier) may be split into two portions, with an upper portion being provided to the first optoelectronic modulator 206 using a suitable optical connection and a lower portion being provided to a frequency shifter 250. The second RF input x(t) received at the second antenna 204 may be converted from the RF domain to the optical domain by modulating the output of the fixed-frequency laser 220 via the first optoelectronic modulator 206. As a result, the optical signal output by the first optoelectronic modulator 206 carries the second RF input signal x(t). The modulated signal produced by the first optoelectronic modulator 206 is transmitted to the third optoelectronic modulator 218 via the first dispersive line 212. The frequency shifter 250 may be configured to shift the frequency of the fixed-frequency laser 220 by an arbitrary amount. This will act as a local oscillator for the system, used to detect the product that will be generated in the upper portion.
In the embodiment illustrated in FIG. 2, the first dispersive line 212 introduces a fixed delay to the optical signal that is received at the third optoelectronic modulator 218. The description provided herein assumes that the first dispersive line 212 is not utilized and that the second dispersive line 214 is designed and implemented in the absence of the first dispersive line 212. Thus, the first dispersive line 212 is not required to implement the processor functionality described herein and is inserted as an option to optimize the requirements associated with the first dispersive line 212.
At the third optoelectronic modulator 218, the output of the detector element 216 (RF signal y(t−τ)) is used as an RF drive input to modulate the second RF input x(t) received at the second antenna 204 resulting in the generation of the multiplication state x(t)×y(t−τ) residing on an optical carrier at the output of the third optoelectronic modulator 218, which is subsequently filtered using the filtering element 222.
The output of the third optoelectronic modulator 218 is combined with the optical carrier generated by the fixed-frequency laser 220 and sent to an optical 90-degree hybrid element 224. The optical 90-degree hybrid element 224 is configured to separate quadratures of the output of the third optoelectronic modulator 218. The output of the optical 90-degree hybrid element 224 is provided to the set of balanced detectors 226 and 227 in order to perform coherent optical-to-electrical conversion. The output of each balanced detector is subsequently digitized by a set of analog-to-digital conversion (ADC) elements 228 and 229 and provided to the backplane digital processor 230.
While the CAF function is partially generated in the physical domain, the output of the receiver is in digital form as defined by:

∫−T/2T/2x(t)y(t−τ)e−2παtdt→Σk=1Nx(kδt)y(kδt−τ)e−2παjkδt  (2)

where discrete sampling corresponds to the output of an analog-to-digital (ADC) element 228. It can be recognized that the Fourier transform at the left side of this relation has a corresponding equivalent in the discrete domain:




 
  
   
    
     
      
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where DFT refers to the discrete Fourier transform, ordinarily implemented via Fast Fourier Transform (FFT) algorithm. The description below provides a specific implementation that is aimed at topology simplification.

FIG. 3 is a simplified block diagram illustrating the functionalities of the correlative receiver according to an embodiment of the present invention. The correlative receiver is constructed from modular blocks, allowing each subsystem to be realized independently and interface with any other block via RF/optical interconnect. The receiver may include a first antenna 302 and a second antenna 304 to receive input signals (e.g., the first antenna 202 and the second antenna 204). The receiver may further include a Delay Generation Module 310 used to create relative delay in the inputs (e.g., the first optoelectronic modulator 206, the second optoelectronic modulator 208, the tunable laser 210, the first dispersive line 212, the second dispersive line 214, the detector element 216). Optical Carrier Generation Module 320 may include an optical carrier generator (e.g., the fixed-frequency laser 220) which may then be used to modulate the input of the second antenna 304. LO Generation Module 330 may contain components (e.g., the frequency shifter 250) which act as a local oscillator used to change the frequency of the second antenna input and used to detect a multiplication state of the two RF inputs. Correlative Kernel Generation and Integration Module 340 may generate the kernel of the CAF function (e.g., the multiplication state of the two RF inputs x(t)×y(t−τ)). O/E Conversion Module 350 may contain components (e.g., the optical 90-degree hybrid element 224, balanced detector 226 and ADC element 228) to convert the signal from optical to electronic form to be used by a processor (e.g., the backplane digital processor 230).
FIG. 4 is a block diagram showing the implementation of the modules of the correlative receiver according to an embodiment of the present invention. The correlative receiver 400 can have a first antenna 402 and a second antenna 404 to receive input signals. The input signals can include a first RF input y(t) received at first antenna 402 and a second RF input x(t) received at second antenna 404. In some embodiments, the separation between the antennas is negligible, but the signals may be substantially different (e.g., achieved by increasing the distance between antennas, or feeding them with different physical inputs).
The Delay Generation Module 410 may contain a tunable laser 411 generating an optical carrier that can be tuned to different wavelengths. The Delay Generation Module 410 may also include a first optoelectronic modulator 412 configured to modulate the optical carrier received from the tunable laser 411 with the RF input y(t) received from the first antenna 402. The Delay Generation Module 410 may further include a second optical modulator 413 configured to modulate an optical carrier received from a fixed wavelength laser 422 with the RF input x(t) received from the second antenna 404. As illustrated in FIG. 4, fixed wavelength laser 422 can provide a single wavelength or a comb of wavelengths.
Both modulated signals, now in the optical domain, are multiplexed using wavelength-division multiplexer 414, subsequently passed through dispersive line 415, and separated by wavelength-division demultiplexer 419 to be received by incoherent (e.g., direct detection) receivers. The wavelength-division demultiplexer 419 may be configured to send the delayed first RF input to a first incoherent receiver 416 configured to receive the delayed RF input. The wavelength-division demultiplexer 419 may also send the second input signal to a second incoherent receiver 417 configured to receive the second RF input.
The Carrier Generation Module 420 may contain a fixed wavelength laser 422 to generate an optical signal, i.e. an optical carrier, to be split and sent to the Delay Generation Module 410 and to the Local Oscillator (LO) Generation Module 430. The LO Generation Module 430 may contain a single-sideband modulator 432 to create a local oscillator. This allows the optical carrier received from the fixed wavelength laser 422 (with an optical frequency centered at an unmodulated laser emission frequency) to act as a local oscillator (LO) to allow beating with a modulated sideband at the first incoherent receiver 416 or the second incoherent receiver 417. As a consequence, both the phase and amplitude of the antenna input are encoded and preserved before being used for second stage modulation
The Kernel Generation Module 440 may contain a third, null-biased, optical modulator 442. To avoid phase tracking and subsequent matching, the Kernel Generation may encode the first delayed RF input and the second RF input onto a single-drive optical modulator. This means the first delayed RF input being sent to third, null-biased, optical modulator 442 may modulate the optical carrier received from the fixed wavelength laser 422. The output of the third, null-biased, optical modulator 442 may then be sent to the O/E Conversion Module 450. Similarly, the second RF input may be sent to a fourth, null-biased, optical modulator 444 to modulate the optical carrier O/E received from the fixed wavelength laser 422. The output of the fourth, null-biased, optical modulator 444 may then be sent to the O/E Conversion Module 450. In one of the implementations, one of the signals can be modulated onto the frequency shifted copy of the optical carrier, providing means for heterodyne reception.
O/E Conversion Module 450 may contain a 90-degree hybrid 452 configured to separate quadratures (e.g., an orthogonal decomposition may be performed by the 90-degree hybrid 452 to separate any signal into orthogonal components known as quadratures; an example of such a method is separating the real and imaginary components in base band signals) of the outputs of the third, null-biased, optical modulator 442 and the fourth, null-biased, optical modulator 444. The quadratures may be fed into a plurality of optical detectors 454A, 454B, 454C, and 454D. The set of optical detectors 454A-454D may be configured to convert optical signal quadratures to electrical signals. The electrical signals may then be fed to an analog-to-digital converter (ADC) 456, which may include a plurality of analog-to-digital elements operable to convert the electrical signal received into a digital form that may be processed further.
In the O/E Conversion Module 450, the combination of the 90-degree hybrid 452 and the set of balanced I/Q (coherent) receivers 454A-454D allows retrieval of phase and amplitude of the kernel (e.g., the delayed signal product x(t)×y(t−τ)). The implementation shown in FIG. 4 implies that final integration, utilized to generate the CAF, is completed in the digital domain. The sampling speed of the analog-to-digital conversion and processing rate of a digital backplane processor that may process the digital output of the system dictate the largest acquirable bandwidth of the input signals received by the antennas. When this rate is exceeded, the topology shown in FIG. 4 is easily scaled by replacing the fixed wavelength laser 422 and adding additional coherent receivers.
Thus, as illustrated in FIGS. 2 and 4, embodiments of the present invention can utilize modulators operated in cascade as well as modulators operated in parallel (e.g., third, null-biased, optical modulator 442 and fourth, null-biased, optical modulator 444).
FIG. 5 is a block diagram of an implementation of the modules of a parallelized correlative receiver according to an embodiment of the present invention. The parallelized correlative receiver 500 illustrated in FIG. 5 shares some common elements with the correlative receiver 400 illustrated in FIG. 4 and the description provided in relation to FIG. 4 is applicable to FIG. 5 as appropriate. The fixed wavelength laser 422 illustrated in FIG. 4 is replaced by a fixed wavelength carrier comb 522 possessing multiple (N), mutually coherent carriers. In this implementation, each optical carrier of the fixed wavelength carrier comb 522 provided by Carrier Generation Module 520 acts as an independent carrier to be modulated by the RF inputs.
The output of the Kernel Generation Module 440 is sent to a wavelength demultiplexer 545, which may be configured to separate modulated optical carriers of the fixed wavelength carrier comb 522 to be routed to a specific coherent detector from a plurality of coherent detectors (e.g., it may be sent to a first coherent detector 554A) to complete the functions of the O/E Conversion Module 550 in a similar manner as that illustrated in FIG. 4. The outputs produced by the wavelength demultiplexer 545 are delivered to a plurality of 90-degree hybrid elements 552 (e.g., a first 90-degree hybrid element 552A, a second 90-degree hybrid element 552B, etc.). The process may then continue in a similar manner illustrated in FIG. 4. The wavelength demultiplexer 545 enables the parallelized correlative receiver 500 to process RF inputs having much higher bandwidths. As an example, if a single analog-to-digital conversion element 556A and an associated backplane processor can support instantaneous bandwidth Δf, the aggregate bandwidth of the parallelized correlative receiver 500 is scaled linearly as N×Δf, where N represents the number of utilized optical carriers.
One of ordinary skill in the art will appreciate that other modifications to the apparatuses and methods of the present disclosure may be made for implementing various applications of the hybrid photonic-electronic processor without departing from the scope of the present disclosure.
The examples and embodiments described herein are for illustrative purposes only. Various modifications or changes in light thereof will be apparent to persons skilled in the art. There are to be included within the spirit and purview of this application, and the scope of the appended claims which follow.

## Claims

1. An apparatus comprising:
a first antenna;
a tunable optical carrier source;
a second antenna;
a delay generation module coupled to the first antenna and the tunable optical carrier source;
a fixed wavelength optical carrier source;
a local oscillator generation module;
a correlative kernel generation and integration module coupled to the delay generation module and the local oscillator generation module; and
an optoelectronic conversion module coupled to the correlative kernel generation and integration module.

2. The apparatus of claim 1, wherein the delay generation module is also coupled to the second antenna.

3. The apparatus of claim 1, wherein the delay generation module comprises:
a first single-sideband optical modulator coupled to the tunable optical carrier source; and
a second single-sideband optical modulator coupled to the fixed wavelength optical carrier source.

4. The apparatus of claim 1, wherein the delay generation module comprises a dispersion compensating fiber.

5. The apparatus of claim 1, wherein the correlative kernel generation and integration module comprises:
a first optical detector configured to receive signals modulated onto a tunable optical carrier provided by the tunable optical carrier source;
a second optical detector configured to receive signals modulated onto a fixed optical carrier provided by the fixed wavelength optical carrier source;
a first null-biased optical modulator coupled to the first optical detector; and
a second null-biased optical modulator coupled to the second optical detector.

6. The apparatus of claim 1, wherein the optoelectronic conversion module comprises:
an optical 90-degree hybrid element; and
a plurality of optical detectors optically coupled to the optical 90-degree hybrid element.

7. The apparatus of claim 1, wherein the fixed wavelength optical carrier source comprises an optical frequency comb configured to provide a plurality of fixed optical carriers.

8. An apparatus comprising:
a first antenna configured to receive a first incoming signal;
a second antenna configured to receive a second incoming signal;
a tunable optical carrier source configured to generate a tunable optical carrier;
a fixed wavelength optical carrier source configured to generate a fixed optical carrier;
a local oscillator (LO) generator configured to generate an LO signal based on the fixed optical carrier;
a first optical modulator configured to modulate the tunable optical carrier based on the first incoming signal and generate a first modulated signal;
a second optical modulator configured to modulate the fixed optical carrier based on the second incoming signal and generate a second modulated signal;
a delay element configured to delay the first modulated signal and generate a delayed first modulated signal;
a first optical detector configured to detect the delayed first modulated signal;
a second optical detector configured to detect the second modulated signal;
a first null-biased optical modulator configured to modulate the fixed optical carrier based on an output of the first optical detector;
a second null-biased optical modulator configured to modulate the LO signal based on an output of the second optical detector;
an optical 90-degree hybrid element configured to separate quadratures of outputs of the first and second null-biased optical modulators; and
multiple optical detectors configured to convert the quadratures into electrical signals.

9. The apparatus of claim 8, wherein:
the tunable optical carrier source comprises a tunable laser; and
the fixed wavelength optical carrier source comprises a fixed wavelength laser.

10. The apparatus of claim 8, wherein the delay element comprises a dispersive line.

11. The apparatus of claim 8, wherein the first and second optical modulators comprise first and second single-sideband optical modulators.

12. The apparatus of claim 8, wherein the fixed wavelength optical carrier source comprises an optical frequency comb configured to provide multiple fixed optical carriers.

13. The apparatus of claim 8, wherein the LO generator comprises a single-sideband modulator.

14. The apparatus of claim 8, further comprising:
a wavelength-division multiplexer optically coupled between the first and second optical modulators and the delay element; and
a wavelength-division demultiplexer optically coupled between the delay element and the first and second optical detectors.

15. The apparatus of claim 8, further comprising:
at least one additional optical 90-degree hybrid element;
at least one additional set of optical detectors; and
a wavelength-division multiplexer optically coupled between the first and second null-biased optical modulators and the optical 90-degree hybrid elements.

16. A method comprising:
receiving first and second incoming signals;
generating a tunable optical carrier;
generating a fixed optical carrier;
generating a local oscillator (LO) signal based on the fixed optical carrier;
modulating the tunable optical carrier based on the first incoming signal to generate a first modulated signal;
modulating the fixed optical carrier based on the second incoming signal to generate a second modulated signal;
delaying the first modulated signal to generate a delayed first modulated signal;
modulating the fixed optical carrier based on the delayed first modulated signal;
modulating the LO signal based on the second modulated signal;
separating quadratures of the modulated fixed optical carrier and the modulated LO signal; and
converting the quadratures into electrical signals.

17. The method of claim 16, wherein generating the fixed optical carrier comprises generating multiple fixed optical carriers using an optical frequency comb.

18. The method of claim 16, wherein modulating the tunable optical carrier and modulating the fixed optical carrier comprise using single-sideband optical modulators.

19. The method of claim 16, wherein delaying the first modulated signal comprises using a dispersive line.

20. The method of claim 16, wherein modulating the fixed optical carrier and modulating the LO signal comprise using null-biased optical modulators.