Optically assisted ultramulti octave ultra wideband synthetic aperture radar system

Abstract

The application relates to a light-assisted super-octave ultra-wideband synthetic aperture radar system and belongs to the technical field of ultra-wideband synthetic aperture radar, solves the problem of the limitation of the instantaneous bandwidth of a traditional electronic system, and is characterized in that the system comprises a digital system, an intermediate frequency system and a radio frequency system, and is further characterized in that the system further comprises a photonic radio frequency front end; the photonic radio frequency front end is connected between the intermediate frequency system and the radio frequency system; a photonic-assisted frequency multiplication technology is adopted to multiply the input intermediate frequency LFM signal and mix the multiplied signal with a local oscillation signal to generate a super-octave ultra-wideband signal; a photonic-assisted dechirp technology is adopted to multiply a received echo signal reflected by a target scene with a reference signal to generate a dechirped target echo signal; the reference signal is a linearly amplified and spurious-free ultra-wideband signal of the super-octave ultra-wideband signal. The application breaks through the limitation of the instantaneous bandwidth of the traditional electronic system and provides a new technical means for a large-instantaneous-bandwidth SAR system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultra-wideband synthetic aperture radar, and particularly relates to an optical-assisted ultra-octave ultra-wideband synthetic aperture radar system. BACKGROUND

[0002] Synthetic Aperture Radar (SAR) is a commonly used earth observation system, which has the advantages of all-weather, all-day, and not being blocked by clouds, rain, and snow. The range resolution of a synthetic aperture radar is related to the transmitted signal bandwidth, and the azimuth resolution is related to the synthetic aperture size, which is usually the same as the range resolution. Therefore, to obtain higher resolution, the SAR system needs to have the ability to generate, transmit, and receive signals with a larger bandwidth. The linear frequency modulated (LFM) signal has the characteristics of simple waveform, and can use the de-chirp reception method to realize pulse compression processing (thus greatly reducing the hardware pressure of the receiver digital system), so it has become a widely used waveform in pulse compression radar systems.

[0003] Due to the performance degradation of digital electronic devices (such as digital to analog converters (DAC) and analog to digital converters (ADC)) with increasing bandwidth, it is difficult for digital systems to directly generate and receive such signals when the carrier frequency is higher than several GHz, and a microwave analog electronic technology-based radio frequency front end is needed, so current radar systems all use a hybrid system architecture of digital systems and analog radio frequency front ends. The microwave electronic radio frequency front end usually includes frequency multipliers / mixers, filters, and amplifiers, among which the frequency conversion devices (frequency multipliers / mixers) and amplifiers have strong non-linear effects (the frequency conversion devices are based on non-linear effects), which will limit the maximum instantaneous bandwidth of the radar system to no more than one octave (when the input signal bandwidth exceeds one octave, the harmonics of low-frequency signals fall into the band). The maximum instantaneous bandwidth limitation of the radio frequency front end results in that the traditional electronic SAR is frequency-band-specific, and multiple SAR systems with different frequency bands are needed to obtain multi-band SAR images of the same scene.

[0004] The pursuit of target detail information (measured by resolution) by radar imaging systems is constantly improving, and constantly improving the resolution in the conventional frequency band (below 18G) will inevitably encounter the problem of exceeding the octave of the transmitted signal bandwidth, so the implementation method of ultra-octave SAR based on a set of hardware has practical significance for improving the resolution. At the same time, ultra-octave SAR also provides a new technical means for detecting the same target scene using different frequency bands.

[0005] Photonics technology has the advantages of large processing bandwidth, anti-electromagnetic interference, etc. In addition, the unique advantages of photonics technology, such as multiple types of electro-optic modulators and photoelectric detectors, and the polarization, wavelength multiplexing and other characteristics unique to light, help to realize the generation and processing of ultra-octave signals, but there is a bandwidth "aperture" mismatch problem in the form of "wide light and narrow electricity" in the current optical-assisted radio frequency system. SUMMARY

[0006] In view of the above analysis, the present application aims to disclose an optical-assisted ultra-octave ultra-wideband synthetic aperture radar system. The problem of the limitation of the instantaneous bandwidth of the traditional electronic system and the bandwidth "aperture" mismatch problem of the existing optical-assisted system are solved.

[0007] The present application discloses an optical-assisted ultra-octave ultra-wideband synthetic aperture radar system, comprising a digital system, an intermediate frequency system and a radio frequency system, and further comprising a photonic radio frequency front end.

[0008] The photonic radio frequency front end is connected between the intermediate frequency system and the radio frequency system; the input intermediate frequency LFM signal is multiplied by the photonic-assisted frequency multiplication technology, and then mixed with the local oscillator signal to generate an ultra-octave ultra-wideband signal; the received echo signal of the target scene reflection is multiplied with the reference signal by the photonic-assisted dechirp technology to generate a dechirped target echo signal.

[0009] The reference signal is an ultra-wideband signal with linear amplification and without in-band harmonics of the ultra-octave ultra-wideband signal.

[0010] Further, the photonic radio frequency front end comprises an ultra-octave signal generation unit.

[0011] The ultra-octave signal generation unit comprises an optical four-frequency multiplication module and an optical double-frequency and down-conversion module.

[0012] The optical four-frequency multiplication module uses an MZM electro-optic modulator to multiply the intermediate frequency LFM signal to obtain a four-frequency signal IF-LFM ×4 with a frequency and a bandwidth of four times.

[0013] The optical double-frequency and down-conversion module uses a QPSK photoelectric modulator to further double-frequency the four-frequency signal IF-LFM ×4 and down-convert it with the local oscillator signal to generate an ultra-octave ultra-wideband signal.

[0014] Further, the optical double-frequency and down-conversion module comprises a second continuous laser, a QPSK modulator, an optical bandpass filter, an optical amplifier and a second photoelectric detector.

[0015] The second continuous laser is used to generate a wavelength-stable continuous optical carrier.

[0016] The QPSK modulator first divides the input light wave into two paths with equal power, one of which is used to modulate the four times frequency signal IF-LFM ×4 into the first path to obtain a modulated light signal containing continuous optical carrier and even order sidebands of the four times frequency signal IF-LFM ×4 , and the other of which is used to modulate the local signal LO into the second path to obtain a modulated light signal containing odd order sidebands of the local signal LO, and the two modulated light signals are combined to output a combined light signal;

[0017] The optical band-pass filter is used to perform band-pass filtering on the combined light signal output by the QPSK electro-optical modulator, and output the +2 order optical sideband of the four times frequency signal IF-LFM ×4 and the +1 order optical sideband of the local signal LO.

[0018] The optical amplifier is used to amplify the optical signal output by the optical band-pass filter.

[0019] The second photoelectric detector is used to beat the +2 order optical sideband of the four times frequency signal IF-LFM ×4 and the +1 order optical sideband of the local signal LO to obtain an ultra-octave ultra-wideband signal BOS-LFM, and the bandwidth of the ultra-octave ultra-wideband signal BOS-LFM is eight times that of the intermediate frequency LFM signal, and the center frequency of the carrier is determined by the frequency of the LO and the center frequency of the eight times frequency of the intermediate frequency LFM.

[0020] Further, the ultra-octave signal link includes a first-stage amplifier, a coupler and a second-stage amplifier.

[0021] The input end of the first-stage amplifier is connected with the output end of the ultra-octave signal generation unit, and the output end is connected with the input end of the coupler.

[0022] The first-stage amplifier is a linear amplifier, which is used to amplify the ultra-octave ultra-wideband signal output by the ultra-octave signal generation unit to obtain a linear amplified ultra-wideband signal with a set power and without in-band harmonics.

[0023] The coupler includes a first coupling output end and a second coupling output end, the first coupling output end is connected with the input end of the second-stage amplifier, and is used to output the linear amplified ultra-wideband signal to the second-stage amplifier, and the second coupling output end outputs the linear amplified ultra-wideband signal without harmonics as a reference signal.

[0024] The second-stage amplifier is a saturated amplifier, which is used to saturate amplify the input linear amplified ultra-wideband signal to obtain an ultra-wideband signal with a power meeting the transmission requirement and with in-band harmonics.

[0025] Further, the photonic radio frequency front end comprises an ultra-hyper frequency signal dechirp receiving unit.

[0026] In the ultra-hyper frequency signal dechirp receiving unit, the input echo signal and the reference signal are modulated, polarization multiplexed, filtered, amplified in the optical domain by using a dual-polarization photoelectric modulation mode, and the multiplication of the echo signal and the reference signal is realized by a polarization demultiplexing coherent receiving mode to obtain a dechirped echo.

[0027] Further, the ultra-hyper frequency signal dechirp receiving unit comprises a third continuous laser, a dual-polarization modulator, a dual-polarization optical bandpass filter, a dual-polarization optical amplifier and a polarization demultiplexing coherent receiver.

[0028] The third continuous laser is configured to generate a wavelength-stable continuous optical carrier.

[0029] The dual-polarization modulator is configured to modulate the reference signal and the echo signal onto two orthogonal polarization directions of the continuous optical carrier respectively, and to multiplex the modulated optical signals of the ±1 order sidebands of the reference signal and the modulated optical signals of the ±1 order sidebands of the echo signal in the two orthogonal polarization directions to output a polarization multiplexed optical signal.

[0030] The dual-polarization optical bandpass filter is configured to filter out the +1 order optical sideband of the polarization multiplexed optical signal.

[0031] The dual-polarization optical amplifier is configured to amplify the dual-polarization optical signal output by the dual-polarization optical bandpass filter.

[0032] The polarization demultiplexing coherent receiver is configured to multiply the optical waves in the two polarization directions of the amplified dual-polarization optical signal in the optical field complex amplitude and convert them into an electrical signal, and output the product of the reference signal and the fundamental wave signal in the echo.

[0033] Further, the product of the reference signal RBOS-LFM output by the polarization demultiplexing coherent receiver and the fundamental wave signal in the echo is phase compensated in the digital domain, which is used to eliminate the difference in the phase-frequency characteristics between the fundamental wave signal in the echo and the reference signal mainly introduced by the second stage saturated amplifier in the ultra-hyper frequency signal link and the echo signal link.

[0034] Further, the dual-polarization modulator is a dual-polarization MZM electro-optic modulator or a dual-polarization QPSK electro-optic modulator.

[0035] Further, in the dual-polarization QPSK electro-optical modulator, the continuous optical carrier output by the third continuous laser is split twice to enter four parallel MZM modulators, the four MZM modulators all work at the minimum bias point, and each orthogonal polarization direction has two MZM modulators; in the first polarization direction, the optical phase between the two sub-MZMs is orthogonal, the reference signal first passes through an electrical 90° coupler to obtain phase-orthogonal I / Q reference signals, and then is input into the two sub-MZMs to realize carrier-suppressed single-sideband modulation, that is, the modulation optical signal output in the polarization direction only contains the +1 order sideband of the reference signal; in the second polarization direction, the optical phase between the two sub-MZMs is orthogonal, the echo signal first passes through an electrical 90° coupler to obtain phase-orthogonal I / Q echo signals, and then is input into the two sub-MZMs to realize carrier-suppressed single-sideband modulation, that is, the modulation optical signal output in the polarization direction only contains the +1 order sideband of the echo signal; the output optical signal is polarization multiplexed at the output end of the dual-polarization QPSK electro-optical modulator to output a dual-polarization optical signal.

[0036] Further, in the dual-polarization MZM electro-optical modulator, the continuous optical carrier output by the third continuous laser is split twice to enter two parallel MZM modulators, the two MZM modulators all work at the minimum bias point, and each orthogonal polarization direction has one MZM modulator. In the first polarization direction, the optical carrier is modulated by the reference signal, and the output modulation optical signal only contains the ±1 order sideband of the reference signal; in the second polarization direction, the optical carrier is modulated by the echo signal, and the output modulation optical signal only contains the ±1 order sideband of the echo signal; the output optical signal is polarization multiplexed at the output end of the dual-polarization MZM electro-optical modulator to output a dual-polarization optical signal.

[0037] The present application can realize one of the following beneficial effects:

[0038] (1) By using the photon-assisted frequency multiplication signal generation technology and the photon-assisted dechirp receiving technology, an architecture of an ultra-hyperband synthetic aperture radar system is proposed, which expands the working bandwidth from less than one hyperband to an ultra-hyperband, breaks through the limitation of the instantaneous bandwidth of the traditional electronic system, provides a new technical means for a large-instantaneous-bandwidth SAR system, and also provides a new technical approach for observing the same scene with multiple frequency bands.

[0039] (2) In the process of generating an ultra-hyperband signal, a signal generation scheme is proposed, in which the same set of photon link hardware is used to realize the doubling of the radio frequency signal, which has an advantage over the two units that need to be cascaded to realize doubling and down-conversion respectively in the electronic technology.

[0040] (3) In the aspect of super-octave wideband signal receiving, the super-octave signal de-chirp receiving technical approach proposed by the application is to use the super-octave working capability of the light-assisted mixer, to multiply the BOS-LFM (Beyond octave-spanning LFM) signal (containing only BOS-LFM fundamental wave, denoted as RBOS-LFM) without in-band spurs as a reference signal with the echo signal (ABOS-LFM reflected by the target) containing in-band spurs, to equivalently construct a time-varying filter with a passband, to filter out the target information carried by the BOS-LFM fundamental wave from the echo signal with high-order harmonics, and to compensate the difference in phase-frequency characteristics between the ABOS-LFM and the RBOS-LFM in the digital domain through post-compensation technology. The technical approach and the specific implementation method have strong innovation and distinctive features, and can realize a working instantaneous bandwidth exceeding octave based on a set of hardware systems. BRIEF DESCRIPTION OF DRAWINGS

[0041] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0042] Figure 1 The light-assisted super-octave ultra-wideband synthetic aperture radar system in the embodiment of the application is shown in the connection schematic block diagram.

[0043] Figure 2 The light four times frequency module in the embodiment of the application is shown in the connection schematic block diagram.

[0044] Figure 3 The light two times frequency and down conversion module in the embodiment of the application is shown in the connection schematic block diagram.

[0045] Figure 4 The QPSK modulator structure in the embodiment of the application is shown in the connection schematic block diagram.

[0046] Figure 5 The time-frequency relationship diagram of the amplified super-octave ABOS-LFM signal in the embodiment of the application is shown.

[0047] Figure 6 The super-octave signal link in the embodiment of the application is shown in the connection schematic block diagram.

[0048] Figure 7 The super-octave signal de-chirp receiving unit in the embodiment of the application is shown in the connection schematic block diagram.

[0049] Figure 8 The BOS-LFM signal in the embodiment of the application is shown in the connection schematic block diagram.

[0050] Figure 9An example structure diagram of a dual-polarization QPSK electro-optical modulator in an embodiment of the present application is shown in the figure.

[0051] Figure 10 An example structure diagram of a polarization demultiplexing coherent receiver in an embodiment of the present application is shown in the figure.

[0052] Figure 11 An example structure diagram of another example of a polarization demultiplexing coherent receiver in an embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0053] The preferred embodiments of the present application will be described in detail below with reference to the drawings, which form a part of this application, and together with the embodiments of the present application, illustrate the principles of the present application.

[0054] One embodiment of the present application discloses an optical-assisted super-octave ultra-wideband synthetic aperture radar system, as shown in the figure, comprising a digital system, an intermediate frequency system, a photonic radio frequency front end and a radio frequency system (including a transceiver antenna). Figure 1 The transceiver antenna of the radio frequency system is suitable for using a (pair of) wideband aperture antenna, such as a wideband horn; the digital signal generator and the frequency synthesizer in the digital system generate an intermediate frequency LFM signal (IF-LFM) and a local oscillator signal (LO) respectively.

[0055] The intermediate frequency system provides an intermediate frequency signal link and a de-chirp signal link.

[0056] In the scheme disclosed in the embodiment, the digital system, the intermediate frequency system and the radio frequency system can all use conventional electronic SAR technology.

[0057] The difference between the scheme in the embodiment and the conventional FMCW electronic SAR system is that it comprises a photonic radio frequency front end.

[0058] The photonic radio frequency front end is connected between the intermediate frequency system and the radio frequency system, adopts photonic-assisted frequency multiplication technology, multiplies the input intermediate frequency LFM signal after frequency multiplication, and mixes it with the local oscillator signal to generate a super-octave ultra-wideband signal; adopts photonic-assisted de-chirp technology, multiplies the received echo signal reflected by the target scene with the reference signal to generate a de-chirp target echo signal.

[0059] The reference signal is an ultra-wideband signal without in-band harmonics after linear amplification of the super-octave ultra-wideband signal.

[0060] The input and output of the photonic radio frequency front end are electrical signals, from the perspective of signals, the photonic radio frequency front end is a conversion interface of intermediate frequency signals and radio frequency signals in a super octave radar system, and from the perspective of functions, a super octave signal generation unit in the photonic radio frequency front end has frequency multiplication, frequency up-conversion and filtering functions; a super octave signal dechirp receiving unit in the photonic radio frequency front end has frequency down-conversion and filtering functions.

[0061] Specifically, the photonic radio frequency front end comprises a super octave signal generation unit and a super octave signal dechirp receiving unit.

[0062] The super octave signal generation unit is used for signal transmission of the SAR system.

[0063] The first signal input end of the super octave signal generation unit is connected with an intermediate frequency signal link output end of an intermediate frequency system, and an intermediate frequency LFM signal is received from the intermediate frequency signal link; the second signal input end is connected with a frequency synthesizer output end of a digital system, and a local oscillation signal is received from the frequency synthesizer; and the signal output end is connected with an input end of a super octave signal link in a radio frequency system.

[0064] In the super octave signal generation unit, the intermediate frequency LFM signal is first frequency-multiplied by using a multi-stage optical frequency multiplication link to generate a super wideband LFM signal with frequency multiplication and bandwidth multiplication; and then the super wideband LFM signal is down-converted to a low frequency by using an optical domain down-conversion manner, so as to realize super octave and obtain a super octave super wideband signal.

[0065] More specifically, the super octave signal generation unit comprises an optical four-frequency multiplication module and an optical two-frequency multiplication and down-conversion module.

[0066] The optical four-frequency multiplication module uses an MZM electro-optic modulator to frequency-multiply the intermediate frequency LFM signal to obtain a four-frequency signal IF-LFM ×4 with frequency and bandwidth being four times.

[0067] The optical two-frequency multiplication and down-conversion module uses a QPSK electro-optic modulator to further frequency-double the four-frequency signal IF-LFM ×4 and down-convert the four-frequency signal IF-LFM with a local oscillation signal to generate a super octave super wideband signal BOS-LFM. The BOS-LFM signal has a bandwidth equivalent to eight times the frequency of the IF-LFM.

[0068] As shown in Figure 2 , the optical four-frequency multiplication module comprises a first continuous laser, an MZM modulator and a first photodetector.

[0069] The first continuous laser is used to generate a continuous optical carrier with stable wavelength, which is output to the MZM modulator.

[0070] The MZM modulator operates at the maximum bias point and is used to modulate the intermediate frequency LFM signal onto the polarization state of the continuous optical carrier; the output modulated optical signal contains even-order sidebands of the optical carrier and the intermediate frequency LFM signal.

[0071] The first photodetector is used to obtain a fourth harmonic signal IF-LFM, whose frequency and bandwidth are both four times that of the intermediate frequency LFM signal, by measuring the ±2nd order optical sideband beat frequencies of the intermediate frequency LFM signal. ×4 .

[0072] like Figure 3 As shown, the optical frequency doubling and downconversion module includes: a second continuous laser, a QPSK modulator, an optical bandpass filter, an optical amplifier, and a second photodetector;

[0073] The second continuous laser is used to generate a wavelength-stable continuous optical carrier.

[0074] The QPSK modulator comprises two sub-MZM modulators. The light wave is power-distributed between the two sub-MZMs at the input, and a fourth-harmonic signal IF-LFM is applied to one of the sub-MZMs. ×4 On the modulated continuous optical carrier, an IF-LFM signal containing the continuous optical carrier and a fourth harmonic signal is obtained. ×4 The even-order sideband of the modulated optical signal; the local oscillator signal LO on another sub-MZM is modulated on a continuous optical carrier to obtain a modulated optical signal including the odd-order sideband of the local oscillator signal LO, and the two modulated optical signals are combined to output a combined optical signal;

[0075] The optical bandpass filter is used to bandpass filter the combined optical signal output from the QPSK electro-optic modulator, and output a fourth harmonic signal IF-LFM. ×4 The +2nd order optical sideband and the +1st order optical sideband of the local oscillator signal LO;

[0076] The optical amplifier is used to amplify the optical signal output from the optical bandpass filter;

[0077] The second photodetector is used for the fourth harmonic signal IF-LFM. ×4 The +2nd order optical sideband and the +1st order optical sideband beat frequency of the local oscillator signal LO are used to obtain the ultra-octave band ultra-wideband signal BOS-LFM, which has a bandwidth eight times that of the intermediate frequency LFM signal.

[0078] like Figure 4 As shown, the QPSK modulator includes an optical beam splitter, two parallel MZM electro-optic modulators, and an optical beam combiner. The optical beam splitter splits the continuous optical carrier output from the second continuous laser into two beams, which enter the two parallel MZM electro-optic modulators respectively. The optical beam combiner combines the optical signals output from the two MZM electro-optic modulators and outputs them together.

[0079] The continuous optical carrier output by the second photodetector is optically split in the QPSK modulator, and enters two parallel MZM modulators respectively, one of the two MZM modulators works at a maximum bias point, and the optical carrier is modulated by the four times frequency signal IF-LFM ×4 , and the output modulated optical signal only contains the optical carrier and even order sidebands including +2 order; the other works at a minimum bias point, and the optical carrier is modulated by the local signal LO, and the output modulated optical signal only contains odd order sidebands; the optical signals output by the two MZM modulators are combined at the output end of the QPSK modulator.

[0080] The output optical signal of the QPSK electro-optical modulator enters an optical band-pass filter for filtering, and the +2 order optical sideband of the four times frequency signal IF-LFM (i.e. corresponding to eight times frequency of IF-LFM) and the +1 order optical sideband of the local signal LO are obtained, and then enter an optical amplifier and a photodetector, and the optical-electric conversion can obtain the ultra-wideband signal BOS-LFM. The bandwidth of the BOS-LFM is determined by the bandwidth of the IF-LFM (eight times thereof), and the carrier frequency is determined by the carrier frequency of the IF-LFM and the frequency of the local signal LO. ×4

[0081] The power of the ultra-harmonic ultra-wideband signal BOS-LFM directly output by the signal generation part in the embodiment is about -30dBm, and subsequent amplification is still needed (the power of the transmission signal determines the action distance of the system), so that the power of the transmission signal is high enough, and the system has practical value (for the FMCW radar system, km-level action distance usually needs W-level radio frequency power). The problem of amplifying the BOS-LFM signal is that the low frequency part of the BOS-LFM falls into the band as in-band spurs after passing through the amplifier, as shown in Figure 5

[0082] It should be particularly noted that these in-band spurs are all LFM signals (the frequency modulation rate is an integer multiple of the basic frequency modulation rate), and if the delay version of the transmission signal is multiplied by the echo according to the conventional analog de-chirp frequency method (the physical device that bears the multiplication function is a microwave mixer, RF mixer), even without considering the nonlinearity of the conventional microwave electronic mixer (the conventional microwave electronic mixer is based on the nonlinearity of the device, so that the working bandwidth is not more than one octave), the in-band LFM spurs also have pulse compression gain, and finally multiple corresponding outputs will appear for a target, and multiple image overlaps of the scene will appear for a distributed scene, which causes the system to be unable to be used.

[0083] Based on this, the ultra-harmonic signal link in the embodiment includes a first-stage amplifier, a coupler and a second-stage amplifier, as shown in Figure 6

[0084] ​​​The input end of the first-stage amplifier is connected with the output end of the super-octave signal generation unit, and the output end is connected with the input end of the coupler.

[0085] The first-stage amplifier is a linear amplifier, which is used for signal amplification of the super-octave ultra-wideband signal output by the super-octave signal generation unit, to obtain a linear amplified ultra-wideband signal RBOS-LFM with a set power and without in-band harmonics. The input power of the first-stage amplifier is small, and the amplifier works in a linear region without high-order harmonics. The set power meets the requirement of working in the linear region of the amplifier and the power requirement of the subsequent stage for driving the second-stage amplifier and the reference signal.

[0086] The coupler includes a first coupling output end and a second coupling output end. The first coupling output end is connected with the input end of the second-stage amplifier, and is used for outputting the linear amplified ultra-wideband signal RBOS-LFM to the second-stage amplifier. The second coupling output end outputs the linear amplified ultra-wideband signal RBOS-LFM as a reference signal.

[0087] The second-stage amplifier is a saturation amplifier, which is used for saturation amplification of the input linear amplified ultra-wideband signal, to obtain an ultra-wideband signal ABOS-LFM with a power meeting the transmission requirement and containing high-order LFM in-band harmonics (spurious signals).

[0088] The abbreviations and parameters of various signals on the SAR system transmission link are as follows:

[0089] Table 1: Abbreviations and parameters of various signals

[0090] Lowest frequency Highest frequency Time width Bandwidth Frequency modulation Low intermediate frequency chirp signal IF-LFM f L ]]> f H ]]> T B = f H -f L ]]> k Quadrupled frequency signal IF-LFM ×4 ]]> 4f L ]]> 4f H ]] T 4B 4k Super octave signal BOS-LFM 8f L -f LO ]]> 8f H -f LO ]] T 8B 8k

[0091] The super-octave signal dechirp receiving unit included in the photonic radio frequency front end is used for signal reception of the SAR system.

[0092] Specifically, the first signal input end of the super-octave signal dechirp receiving unit is connected with the output end of the echo signal link, and receives the echo signal from the echo signal link. The second signal input end is connected with the second coupling output end of the coupler, and receives the reference signal RBOS-LFM from the coupler.

[0093] In the super-octave signal dechirp receiving unit, a dual-polarization photoelectric modulation mode is adopted to modulate, polarization multiplex, filter and amplify the input echo signal and reference signal in the optical domain, and to realize multiplication of the echo signal and the reference signal through polarization demultiplexing coherent reception to obtain a dechirped echo signal.

[0094] As Figure 7As shown, the super-octave signal dechirp receiving unit comprises a third continuous laser, a dual-polarization modulator, a dual-polarization optical band-pass filter, a dual-polarization optical amplifier and a polarization demultiplexing coherent receiver.

[0095] The third continuous laser is configured to generate a wavelength-stable continuous optical carrier.

[0096] The dual-polarization modulator is configured to modulate a reference signal (RBOS-LFM) and an echo signal onto two orthogonal polarization directions of the continuous optical carrier respectively; and multiplex the modulated optical signals of ±1st order sidebands of the reference signal and the modulated optical signals of ±1st order sidebands of the echo signal in the two orthogonal polarization directions to output a polarization multiplexed optical signal.

[0097] The dual-polarization optical band-pass filter is configured to filter out the +1st order optical sideband in the polarization multiplexed optical signal.

[0098] The dual-polarization optical amplifier is configured to amplify the dual-polarization optical signal output by the dual-polarization optical band-pass filter.

[0099] The polarization demultiplexing coherent receiver is configured to perform optical field complex amplitude multiplication on the optical waves in the two polarization directions of the amplified dual-polarization optical signal and convert the optical waves into electrical signals to output, so as to realize dechirp receiving of the echo signal.

[0100] The super-octave signal dechirp receiving unit and the reference signal RBOS-LFM constitute an optical auxiliary time-varying filter for the echo signal ABOS-LFM, and a time-varying passband of the optical auxiliary time-varying filter is as shown in Figure 8 The function of the optical auxiliary time-varying filter is to filter out the fundamental wave signal echo from a mixed signal containing the fundamental wave signal echo and the high-order harmonic echo of the fundamental wave signal echo and perform dechirp processing (matched filtering).

[0101] Optionally, the dual-polarization modulator is a dual-polarization MZM electro-optic modulator or a dual-polarization QPSK electro-optic modulator.

[0102] As shown in Figure 9 In the dual-polarization QPSK electro-optic modulator, the continuous optical carrier output by the third continuous laser is split twice to enter four parallel MZM electro-optic modulators, two sub-MZM modulators for each orthogonal polarization direction, to constitute one QPSK modulator. The four sub-MZM modulators all work at the minimum bias point (no RF modulation signal, no output optical power). In one MZM electro-optic modulator (for example Figure 9In the first polarization direction (XI), the optical carrier is modulated by RBOS-LFM. By controlling the input RF power, the output modulated optical signal contains only ±1 order sidebands of the reference signal. Similarly, the output of another MZM modulator XQ in the same polarization direction also includes ±1 order sidebands. As mentioned earlier, the phases of XI and XQ are orthogonal, and the RBOS-LFM applied to XI and XQ is also orthogonal. Therefore, at the output of the QPSK in the X polarization direction, the superposition of XI and XQ produces a carrier-suppressed single-sideband modulator effect, i.e., only the +1 order sideband is retained. In the second polarization direction (polarization Y), an MZM electro-optic modulator (e.g., Figure 9 In the QPSK (YI) modulator, the optical carrier is modulated by the echo signal, and the output modulated optical signal contains only ±1st order sidebands of the reference signal. The output of another MZM modulator, YQ, in the same polarization direction also includes ±1st order sidebands. As mentioned earlier, the phases of YI and YQ are orthogonal, and the phases of the echo signals loaded onto YI and YQ are also orthogonal. Therefore, at the output of the QPSK in the Y-polarization direction, the superposition of YI and YQ produces a carrier-suppressed single-sideband modulator effect, i.e., only the +1st order sideband is retained.

[0103] The output optical signals of the two QPSK electro-optic modulators are polarized multiplexed at the output end of the dual-polarization QPSK electro-optic modulator, enter the dual-polarization light filter to further suppress unwanted optical sidebands (negative 1st order, zeroth order), pass through the dual-polarization light amplifier, and then enter the polarization demultiplexing coherent receiver.

[0104] In the dual-polarization MZM electro-optic modulator, the continuous optical carrier output from the third continuous laser is split twice and fed into two parallel MZM modulators. Both MZM modulators operate at the minimum bias point, with one MZM modulator in each orthogonal polarization direction. In the first polarization direction, one sub-MZM modulator modulates the optical carrier with a reference signal, and the output modulated optical signal contains only ±1 order sidebands of the reference signal. In the second polarization direction, the other MZM electro-optic modulator modulates the optical carrier with an echo signal, and the output modulated optical signal contains only ±1 order sidebands of the echo signal. The output optical signal is polarization multiplexed at the output end of the dual-polarization MZM electro-optic modulator to output a dual-polarization optical signal.

[0105] Polarization demultiplexing coherent receivers, such as Figure 10 As shown, a polarized beam splitter (PBS) splits an optical signal into two paths E based on their polarization directions. X and E YThe two signals pass through a 50:50 optical coupler, and their electric fields are mixed to enter a balanced photodetector. The output of the balanced photodetector is the multiplication result of the electric fields of the light waves in the two orthogonal polarization directions (i.e. the multiplication result of the echo signal of the ABOS-LFM and the reference signal RBOS-LFM), and the multiplication result contains the products of the reference signal RBOS-LFM and the fundamental wave and each higher harmonic in the echo signal. The intervals of the products in the frequency spectrum are in the order of GHz, and the product of the reference signal RBOS-LFM and the fundamental wave in the echo can be easily obtained by using a low-pass filter.

[0106] Another implementation of the polarization demultiplexing coherent receiver is also given in the embodiment, as shown in FIG. 4. The input polarization multiplexed optical signal passes through a 45° optical beam splitter, and the output two optical signals are the electric fields in the two polarization directions, which then enter a balanced photodetector. The output of the balanced photodetector is the multiplication result of the electric fields of the light waves in the two orthogonal polarization directions, and the product of the reference signal RBOS-LFM and the fundamental wave in the echo can be obtained by using a low-pass filter. Figure 11

[0107] Furthermore, the product of the reference signal RBOS-LFM and the fundamental wave signal in the echo output by the polarization demultiplexing coherent receiver is subjected to digital domain phase compensation, which is used to eliminate the difference in the phase-frequency characteristics between the fundamental wave signal in the echo and the reference signal, which is mainly introduced by the second stage saturated amplifier in the super octave signal link and the echo signal link, so as to obtain a theoretical dechirp result.

[0108] In the embodiment, the digital domain phase compensation can adopt an existing phase compensation method, and the use of which existing phase compensation method for compensation does not constitute a specific influence on the protection scope of the present application.

[0109] In summary, the effects that can be achieved in the embodiment of the present application are as follows:

[0110] (1) By using the photon-assisted frequency multiplication signal generation technology and the photon-assisted dechirp receiving technology, an architecture of a super octave ultra-wideband synthetic aperture radar system is proposed, which expands the working bandwidth from less than one octave to a super octave, breaks through the limitation of the instantaneous bandwidth of the traditional electronic system, provides a new technical means for a large instantaneous bandwidth SAR system, and also provides a new technical approach for observing the same scene by using multiple frequency bands.

[0111] (2) In the process of generating a super octave wideband signal, a signal generation scheme is proposed, in which the same set of photon link hardware is used to realize the doubling of the radio frequency signal. Compared with the two units that are cascaded to realize the doubling and the down-conversion respectively by using the electronic technology, the architecture has an advantage.

[0112] ​(3) In the aspect of super octave wideband signal receiving, the super octave signal dechirp receiving technology approach proposed by the patent is to use the super octave working ability of the light assisted mixer, through multiplying the BOS-LFM signal (only containing BOS-LFM fundamental wave, denoted as RBOS-LFM) without in-band spurs as a reference signal with the echo signal (target reflected ABOS-LFM) containing in-band spurs, to equivalently construct a time-varying filter with a passband, filter out the target information carried by the BOS-LFM fundamental wave from the echo signal with high-order harmonics, and then compensate the difference in phase-frequency characteristics between ABOS-LFM and RBOS-LFM in the digital domain through post-compensation technology. The technical approach and specific implementation method have strong innovation and distinctive features, and can realize a hardware system with a working instantaneous bandwidth exceeding octave.

[0113] The above merely describes the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. An optically assisted ultra-multiple octave ultra-wideband synthetic aperture radar system comprising a digital system, an intermediate frequency system and a radio frequency system, characterized in that, The photonic radio frequency front end is further connected between the intermediate frequency system and the radio frequency system. The photonic radio frequency front end adopts a photonic auxiliary frequency multiplication technology to multiply the input intermediate frequency LFM signal and a local oscillation signal to generate an ultra-harmonic range ultra-wideband signal; and adopts a photonic auxiliary dechirp technology to multiply the received echo signal of the target scene and a reference signal to generate a dechirped target echo signal. The reference signal is an ultra-wideband signal without in-band harmonics after linear amplification of the ultra-harmonic range ultra-wideband signal. The photonic radio frequency front end comprises an ultra-harmonic range signal generation unit. The ultra-harmonic range signal generation unit comprises an optical four-frequency multiplication module and an optical double-frequency multiplication and down-conversion module. The light four times frequency module utilizes the MZM electro-optic modulator to multiply the intermediate frequency LFM signal to obtain a four times frequency signal IF-LFM with four times frequency and bandwidth ×4 ; The light frequency doubling and down-conversion module utilizes a QPSK optical-electricity modulator to frequency double the four times frequency signal IF-LFM ×4 Further frequency doubling and down-conversion with a local signal to generate an ultra-harmonic super wideband signal; The optical double-frequency multiplication and down-conversion module comprises a second continuous laser, a QPSK modulator, an optical bandpass filter, an optical amplifier and a second photodetector. The second continuous laser is configured to generate a continuous optical carrier with stable wavelength. The QPSK modulator divides the input light wave into two paths with equal power, one of which is used to modulate the four times frequency signal IF-LFM ×4 to the first path to obtain a modulated light signal containing continuous light carrier and even order sidebands of the four times frequency signal IF-LFM ×4 The other is used to modulate the local signal LO to the second path to obtain a modulated light signal containing odd order sidebands of the local signal LO, and the modulated light signals of the two paths are combined to output a combined light signal. The optical band-pass filter is used for band-pass filtering the combined light signal output by the QPSK electro-optical modulator, and outputs a four-fold frequency signal IF-LFM ×4 and the +1 order optical sideband of the local oscillator signal LO. The optical amplifier is configured to amplify the optical signal output by the optical bandpass filter. The second photodetector is used for frequency mixing of the four times frequency signal IF-LFM ×4 and the +1 order optical sideband of the local signal LO, to obtain an ultra octave super wideband signal BOS-LFM; the bandwidth of the ultra octave super wideband signal BOS-LFM is eight times of the intermediate frequency LFM signal, and the carrier center frequency is determined by the LO frequency and the eight times frequency center frequency of the intermediate frequency LFM.

2. The octave ultra-wideband synthetic aperture radar system of claim 1, wherein, The ultra-harmonic range signal link comprises a first-stage amplifier, a coupler and a second-stage amplifier. The input end of the first-stage amplifier is connected with the output end of the ultra-harmonic range signal generation unit, and the output end is connected with the input end of the coupler. The first-stage amplifier is a linear amplifier configured to amplify the ultra-harmonic range ultra-wideband signal output by the ultra-harmonic range signal generation unit to obtain a linearly amplified ultra-wideband signal with a set power and without in-band harmonics. The coupler comprises a first coupling output end and a second coupling output end, the first coupling output end is connected with the input end of the second-stage amplifier, and is configured to output the linearly amplified ultra-wideband signal into the second-stage amplifier, and the second coupling output end is configured to output the linearly amplified ultra-wideband signal without in-band harmonics as a reference signal. The second-stage amplifier is a saturated amplifier configured to saturate amplify the input linearly amplified ultra-wideband signal to obtain an ultra-wideband signal with in-band harmonics and a power meeting the transmission requirement.

3. The octave ultra-wideband synthetic aperture radar system of any of claims 1-2, wherein, The photonic radio frequency front end comprises an ultra-harmonic range signal dechirp receiving unit. In the ultra-harmonic range signal dechirp receiving unit, a dual-polarization photodetector is adopted to modulate, polarization multiplex, filter and amplify the input echo signal and the reference signal in the optical domain, and the polarization demultiplex coherent receiving mode is adopted to multiply the echo signal and the reference signal to obtain the dechirped echo.

4. The ultra-harmonic range ultra-wideband synthetic aperture radar system according to claim 3, wherein The ultra-harmonic range signal dechirp receiving unit comprises a third continuous laser, a dual-polarization modulator, a dual-polarization optical bandpass filter, a dual-polarization optical amplifier and a polarization demultiplex coherent receiver. The third continuous laser is configured to generate a continuous optical carrier with stable wavelength. The dual-polarization modulator is configured to modulate the reference signal and the echo signal to two orthogonal polarization directions of the continuous optical carrier respectively, and to multiplex the modulated optical signals of the ±1 order sidebands of the reference signal and the modulated optical signals of the ±1 order sidebands of the echo signal in the two orthogonal polarization directions to output a polarization multiplexed optical signal. The polarization demultiplex coherent receiver is configured to demultiplex the polarization multiplexed optical signal to obtain the dechirped echo. The dual-polarization optical band-pass filter is used for filtering out the optical sideband of +1 order in the polarization multiplexed optical signal. The dual-polarization optical amplifier is used for amplifying the dual-polarization optical signal output by the dual-polarization optical band-pass filter. The polarization demultiplexing coherent receiver is used for performing optical field complex amplitude multiplication on the optical waves in two polarization directions in the amplified dual-polarization optical signal, and converting the optical waves into electrical signals, and outputting the product of the reference signal and the fundamental wave signal in the echo.

5. The ultra-multiple-bandwidth ultra-wideband synthetic aperture radar system according to claim 4, characterized in that, The product of the reference signal RBOS-LFM output by the polarization demultiplexing coherent receiver and the fundamental wave signal in the echo is subjected to digital domain phase compensation, which is used for eliminating the difference in phase-frequency characteristics between the fundamental wave signal in the echo and the reference signal introduced by the second-stage saturated amplifier in the ultra-multiple-bandwidth signal link and the echo signal link.

6. The ultra-multiple-bandwidth ultra-wideband synthetic aperture radar system according to claim 4, characterized in that, The dual-polarization modulator is a dual-polarization MZM electro-optic modulator or a dual-polarization QPSK electro-optic modulator.

7. The octave ultra-wideband synthetic aperture radar system of claim 6, wherein, In the dual-polarization QPSK electro-optic modulator, the continuous optical carrier output by the third continuous laser is subjected to twice optical beam splitting and enters four parallel MZM modulators, the four MZM modulators all work at the minimum bias point, and there are two MZM modulators for each orthogonal polarization direction; the optical phase between the two sub-MZM modulators in the first polarization direction is orthogonal, the reference signal is subjected to phase-orthogonal I / Q reference signal obtaining through an electrical 90° coupler, and then is input into the two sub-MZM modulators, so as to realize carrier-suppressed single sideband modulation, that is, the modulation optical signal output in the polarization direction only contains the +1 order sideband of the reference signal; the optical phase between the two sub-MZM modulators in the second polarization direction is orthogonal, the echo signal is subjected to phase-orthogonal I / Q echo signal obtaining through an electrical 90° coupler, and then is input into the two sub-MZM modulators, so as to realize carrier-suppressed single sideband modulation, that is, the modulation optical signal output in the polarization direction only contains the +1 order sideband of the echo signal; the output optical signal is polarization multiplexed at the output end of the dual-polarization QPSK electro-optic modulator to output a dual-polarization optical signal.

8. The ultra-multiple-bandwidth ultra-wideband synthetic aperture radar system according to claim 6, characterized in that, In the dual-polarization MZM electro-optic modulator, the continuous optical carrier output by the third continuous laser is subjected to twice optical beam splitting and enters two parallel MZM modulators, the two MZM modulators all work at the minimum bias point, and there is one MZM modulator for each orthogonal polarization direction; in the first polarization direction, the optical carrier is modulated by the reference signal through one sub-MZM modulator, and the output modulation optical signal only contains the ±1 order sideband of the reference signal; in the second polarization direction, the optical carrier is modulated by the echo signal through the other MZM electro-optic modulator, and the output modulation optical signal only contains the ±1 order sideband of the echo signal; the output optical signal is polarization multiplexed at the output end of the dual-polarization MZM electro-optic modulator to output a dual-polarization optical signal.

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