Large-output-window analog correlator based on optical frequency shift loop and implementation method of large-output-window analog correlator

Abstract

The invention provides a large-output-window analog correlator based on an optical frequency shift loop and an implementation method of the large-output-window analog correlator. Relates to the technical field of optical signal processing and the technical field of modern radars. The analog correlator comprises a continuous wave laser, a 1 * n optical coupler, n optical frequency shift loop structures which are connected in parallel, and a DSP processing module which are connected in sequence; the n optical frequency shift loops are the same in structure, and each optical frequency shift loop comprises a 1 * 2 optical coupler, two parallel Mach-Zehnder modulators, an optical delay line, a bidirectional optical frequency shift loop, a coherent detection module and an ADC module; according to the invention, expansion of an output window of the correlator can be simulated based on an optical frequency shift loop, relative group delay of electro-optical modulation signals is reasonably adjusted, correlation results output by each structure are enabled to be continuous in sequence and frequency domain splicing is carried out, n-time expansion of the output window of the correlator is obtained, and the system has a wide application prospect. The system has the advantages of capability of performing related processing on various types of signals, large bandwidth, large output window, high signal processing speed and reconfigurable system.

Description

Technical Field

[0001] The present invention relates to the technical fields of optical signal processing and modern radar, and particularly to a large output window analog correlator based on an optical frequency shift loop and a method for realizing the same. Background Art

[0002] Correlation processing technology has important applications in fields such as target detection and recognition, distance and speed measurement, anti-interference and noise suppression. For example, in radar ranging based on linear frequency modulation signals, correlation processing of the received echo signals can improve the distance measurement accuracy. Traditional correlator implementation methods can be divided into two types: digital domain and analog domain. Digital domain correlation processors first convert the analog signals to be processed into digital signals through analog-to-digital converters (ADCs), and then use digital signal processing (DSP) technology to perform correlation processing on the signals in the digital domain. However, for high-frequency large-bandwidth signals, this method requires broadband high-speed ADCs; at the same time, as the number of sampled and stored data points increases, the signal processing time will also be correspondingly extended, thus affecting the real-time performance of the correlator. In contrast, analog domain correlators based on matched filtering can solve these problems, but electrical matched filters in the analog domain usually can only perform matched filtering on specific types of input signals and lack reconfigurability. Analog correlators based on microwave photonics technology can utilize the advantages of photonics technology such as large bandwidth, low loss, electromagnetic interference resistance, and parallel signal processing to efficiently process radio frequency signals in the optical domain, showing broad application potential in the field of signal correlation.

[0003] In recent years, the implementation schemes of microwave photonic analog correlators have emerged continuously, most of which are based on matched filtering. For example, multi-wavelength optical signals are used as taps of matched filters. The delay of each tap is related to its wavelength and fiber dispersion value. The delay difference between taps is adjusted by changing the wavelength interval between two adjacent optical carriers, so that each tap signal has a different phase. These signals are superimposed after the beat frequency at the output end to form the transfer function of the matched filter. When the transfer function of the matched filter and the modulated signal spectrum satisfy the complex conjugate relationship, the autocorrelation function of the signal to be correlated can be obtained (Y.Dai and JPYao, "Nonuniformly spaced photonic microwave delay-line filters and applications," IEEE Trans.Microw.Theory Tech.58(11),3279–3289(2010).), but this type of method also has some challenges. First, the core of matched filtering is to accumulate the contribution of each tap signal. A small number of taps will lead to a decrease in the intensity of the main peak of the autocorrelation of the correlated signal and an increase in the sidelobe intensity, resulting in a worse sidelobe suppression ratio. The increase in the number of taps requires more laser sources, which will greatly increase the cost and complexity of the system. Secondly, unlike the ideal single-passband matched filter, the frequency response of this matched filter has a periodic multi-passband characteristic. The additional passband frequency response will lead to a decrease in the sidelobe suppression ratio of the correlated output. In addition, since the matched filter parameters need to be adjusted according to the signal to be processed, this type of method is only applicable to processing known signals. The microwave photonic analog correlator based on the bidirectional optical frequency shift loop generates multiple groups of different relatively delayed optical signal copies of the two signals to be processed through the bidirectional optical frequency shift loop, and then makes all the optical signal copies undergo beat frequency processing in the photodetector, thereby realizing the correlation of the two signals to be processed in the time domain. It has the advantages of large bandwidth, applicability to a variety of radar signals, and fast signal processing (G. Bourdarot, J. Berger, and H. Guillet de Chatellus, "Multi-delay photonic correlator for wideband RF signal processing," Optica, 9, 325-334 (2022).). However, this method faces a key problem in practical applications: there is a contradiction between the output window range and the correlator bandwidth, and it is difficult to simultaneously realize large bandwidth signal input and large output window. A correlator that has both large bandwidth and large output window and can quickly process a variety of radar signals is a key device in applications such as radar ranging. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a large output window analog correlator based on an optical frequency shift loop and an implementation method thereof.

[0005] The method includes n parallel optical frequency shift loop structures, each of which is mainly composed of a 1×2 optical coupler, two parallel electro-optical modulators, an optical delay line, a bidirectional optical frequency shift loop, a coherent detection module and an ADC, and is used to realize the correlation processing of two input radio frequency signals RF1 and RF2. By adjusting the relative group delay of the two parallel electro-optical modulated signals in each optical frequency shift loop structure, the splicing of n-channel correlation results is realized, and the output window range of the correlator is effectively expanded. In the invention, the correlator can perform correlation processing on various types of radar signals, and has the advantages of large bandwidth, large output window, fast signal processing speed and reconfigurable system.

[0006] The large output window analog correlator based on the optical frequency shift loop of the present invention comprises a continuous wave laser, a 1×n optical coupler, n parallel optical frequency shift loop structures, and a DSP processing module connected in sequence; the n optical frequency shift loop structures are all the same, comprising a 1×2 optical coupler, two parallel Mach-Zehnder modulators, an optical delay line, a bidirectional optical frequency shift loop, a coherent detection module, and an ADC module, for realizing the correlation processing of two input radio frequency signals RF1 and RF2; the specific structure of the i-th optical frequency shift loop structure is: The two output ports of the 1×2 optical coupler are respectively connected to the input ends of the i-1MZM and the i-2MZM, the output end of the i-2MZM is connected to the i-th optical delay line, the output end of the i-1MZM and the output end of the i-th optical delay line are respectively connected to the a and b input ports of the i-th bidirectional optical frequency shift loop, the c and d output ports of the i-th bidirectional optical frequency shift loop are respectively connected to the a and b input ports of the i-th coherent detection module, the i-th coherent detection module is connected to the i-th ADC, and the i-th ADC is connected to the DSP processing module, wherein i=1, 2, ..., n. By adjusting the relative group delay of the two parallel electro-optical modulation signals in each optical frequency shift loop structure, the splicing of n-way correlation results is achieved, and the output window range of the correlator is effectively expanded. In the invention, the correlator can perform correlation processing on various types of radar signals, and has the advantages of large bandwidth, large output window, fast signal processing speed, and reconfigurable system.

[0007] Specifically, the i-th bidirectional optical frequency shift loop is a loop formed by connecting the i-1th optical coupler, the i-th bidirectional optical bandpass filter, the i-2nd optical coupler, the i-th frequency shift, the i-3rd optical coupler, and the i-th bidirectional erbium-doped fiber amplifier in sequence; the i-th frequency shift is a loop formed by connecting the i-1th circulator, the i-1th acousto-optic frequency shifter, the i-2nd circulator, the i-2nd acousto-optic frequency shifter, and the i-th adjustable optical fiber delay line in sequence.

[0008] Specifically, the i-th coherent detection module includes an i-th 90° optical mixer, an i-th balanced photodetector and an i-th low-pass filter connected in sequence, where i=1, 2, ..., n.

[0009] The implementation method of the large output window analog correlator based on the optical frequency shift loop of the present invention is based on the large output window analog correlator mentioned above, and comprises the following steps:

[0010] The continuous wave laser evenly divides the optical carrier into n paths through a 1×n optical coupler;

[0011] Injecting n optical carrier waves into n parallel optical frequency shift loop structures respectively to obtain n quantized electrical signals;

[0012] The n quantized electrical signals are processed by DSP to obtain the final result;

[0013] The implementation steps in the n parallel optical frequency shift loop structures are the same. The i-th parallel branch optical frequency shift loop structure includes an upper branch and a lower branch, i=1, 2, ..., n, and the specific implementation steps include:

[0014] The radio frequency signal RF1 to be processed is modulated on the optical carrier of the upper branch by the i-1th Mach-Zehnder modulator, and the optical carrier radio frequency signal output by the i-1th Mach-Zehnder modulator is injected from the a port of the i-th bidirectional optical frequency shift loop for cyclic frequency shift and time delay;

[0015] The radio frequency signal RF2 to be processed is modulated on the optical carrier of the lower branch by the i-2 Mach-Zehnder modulator, and the optical carrier radio frequency signal after passing through the i-th optical delay line is injected from the b port of the i-th bidirectional optical frequency shift loop for cyclic frequency shift and delay;

[0016] The two signals output by the i-th bidirectional optical frequency shift loop are photoelectrically detected and filtered by the i-th coherent detection module to obtain the i-th electrical signal;

[0017] The ith electrical signal is input into the ith ADC to obtain a sampled and quantized electrical signal.

[0018] Specifically, in the i-th parallel branch optical frequency shift loop structure, the group delay introduced by the i-th optical delay line is (i-1)(M+1)(τ 2 -τ 1 ), where M is the maximum number of cycles of the two signals in the i-th bidirectional optical frequency shift loop, τ 1 is the signal delay for one clockwise cycle in the i-th bidirectional optical frequency shift loop, τ 2 It is the delay of the signal in one counterclockwise cycle of the i-th bidirectional optical frequency shift loop.

[0019] Specifically, the cyclic frequency shifting and delaying in the upper branch of the i-th bidirectional optical frequency shifting loop comprises the following steps:

[0020] The optical radio frequency signal output by the i-1th Mach-Zehnder modulator is input from the a port of the i-1th optical coupler, and then output from the c port, and then passes through the i-th bidirectional optical bandpass filter to filter out the spontaneous radiation noise introduced by the optical amplifier, while controlling the number of cycles of the optical signal;

[0021] The optical signal is input from port a of the i-2 optical coupler, output from port b, input from port 2 of the i-1 circulator, and output from port 3. It is frequency-shifted by the i-1 acousto-optic frequency shifter. The frequency shift amount is f 1 , and then enter the i-th bidirectional erbium-doped fiber amplifier through the 1st port and 2nd port of the i-2nd circulator and the b port and a port of the i-3rd optical coupler in turn, amplifying the optical signal to compensate for the loss in the ring;

[0022] The output signal of the i-th bidirectional erbium-doped fiber amplifier passes through the c port and a port of the i-4th optical coupler and the b port and c port of the i-1th optical coupler in turn and returns to the i-th bidirectional optical bandpass filter for filtering, forming a clockwise cycle of the optical signal. The clockwise loop delay is τ 1 Finally, the multi-circle clockwise cyclic frequency shift and delayed superposition signal is output from the c-terminal of the i-2 optical coupler, which is the output signal of the c-port of the i-th bidirectional optical cyclic frequency shift loop;

[0023] The cyclic frequency shifting and delaying in the lower branch of the i-th bidirectional optical frequency shifting loop comprises the following steps:

[0024] The optical radio frequency signal output after the i-th optical delay line is input from the b port of the i-4th optical coupler and then output from the c port, and then passes through the i-th bidirectional erbium-doped fiber amplifier to amplify the optical signal;

[0025] The optical signal is input from port a of the i-3 optical coupler and output from port b, then input from port 2 of the i-2 circulator and output from port 3. The frequency is shifted by the i-2 acousto-optic frequency shifter. The frequency shift amount is f 2 Then, the delay difference between the clockwise and counterclockwise loops is adjusted to Δτ = τ through the i-th adjustable optical fiber delay line. 2 -τ 1 , where τ 2 The optical signal is delayed in a counterclockwise loop and then passes through the 1st and 2nd ports of the i-1st circulator and the b and a ports of the i-2nd optical coupler to enter the i-th bidirectional optical bandpass filter to filter out ASE noise and control the number of optical signal cycles. In addition, f 1 τ 1 -f 2 τ 2=k, k is an integer;

[0026] The output signal of the i-th bidirectional optical bandpass filter passes through the c-port and b-port of the i-1-th optical coupler and the a-port and c-port of the i-4-th optical coupler in turn and returns to the i-th bidirectional erbium-doped fiber amplifier for amplification, forming a counterclockwise cycle of the optical signal. Finally, the signal of multiple counterclockwise cyclic frequency shift and delayed superposition is output from the c-end of the i-3-th optical coupler, which is the output signal of the d-port of the i-th bidirectional optical cyclic frequency shift loop.

[0027] Specifically, the two signals output by the i-th bidirectional optical frequency shift loop are photoelectrically detected and filtered by the i-th coherent detection module, including the following steps:

[0028] The two signals simultaneously enter the i-th 90° optical mixer for orthogonal mixing;

[0029] The two output signals of the optical mixer enter the i-th balanced photodetector for photoelectric detection and differential processing, and the output photocurrent is filtered by the i-th low-pass filter and used as the output electrical signal of the i-th coherent detection module.

[0030] Specifically, the process of obtaining a final result by processing the n quantized electrical signals by DSP comprises the following steps:

[0031] The DSP processing includes the following steps:

[0032] Fourier transform is performed on the n quantized electrical signals in sequence, and the obtained n spectrum envelopes are respectively the correlation results of the radio frequency signals RF1 and RF2 to be processed at n different time windows; it should be understood that each branch has an identical RF1 and an identical RF2, which are the signals to be processed by the entire analog correlator;

[0033] The n output results are sequentially spliced ​​in the frequency domain to obtain an expansion of the relevant result output window.

[0034] Specifically, the Mach-Zehnder modulators all operate at a minimum bias point.

[0035] After adopting the above scheme, the beneficial effects of the present invention are:

[0036] A bidirectional optical frequency-shifting loop is used to generate multiple groups of copies of two signals to be processed with different relative delays. Then, a 90° optical mixer and a balanced photodetector are used to perform orthogonal mixing and photoelectric conversion on the corresponding delayed copies of the two signals to be processed. Finally, a low-speed ADC is used to collect data on the output signal and then Fourier transform is performed to obtain the correlation results of the signals to be processed. This not only reduces the bandwidth requirements for detection devices, but also has the advantages of large bandwidth, reconfigurability, and fast processing speed. By constructing n parallel optical frequency-shifting loop structures and reasonably adjusting the relative group delay of the two electro-optical modulated signals in each parallel structure, the output correlation results of the two optical frequency-shifting loop structures are made continuous. Then, the correlation results at n different time windows are sequentially spliced ​​in the frequency domain to obtain an n-fold expansion of the correlator output window. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the system structure of the present invention;

[0038] Figure 2 Schematic diagram of the i-th bidirectional optical frequency shift loop in the present invention;

[0039] Figure 3 is a schematic diagram of the i-th coherent detection module in the present invention;

[0040] Figure 4 This is the output spectrum of MZM in Example 1-1;

[0041] Figure 5 This is the output spectrum diagram of the first optical delay line in the embodiment;

[0042] Figure 6 This is a spectrum diagram of the outputs of ports c and d of the first bidirectional optical frequency shift loop in the embodiment;

[0043] Figure 7 is a spectrum diagram corresponding to the first ADC output waveform of the embodiment;

[0044] Figure 8 is a spectrum diagram corresponding to the second ADC output waveform of the embodiment;

[0045] Fig. 9 The splicing result of two output windows of the correlator of the embodiment;

[0046] Fig.10 This is the splicing result of the two output windows of the correlator when RF1 and RF2 are 120MHz limited bandwidth white noise in the embodiment;

[0047] 1-continuous wave laser, 2-1×n optical coupler, 3-first 1×2 optical coupler, 4-second 1×2 optical coupler, 5-nth 1×2 optical coupler, 6-1st-1MZM, 7-1st-2MZM, 8-2nd-1MZM, 9-2nd-2MZM, 10-n-1MZM, 11-n-2MZM, 12-first optical delay line, 13-second optical delay line, 14-nth optical delay line, 15-first bidirectional optical frequency shift loop, 16-second bidirectional optical frequency shift loop, 17-nth bidirectional optical frequency shift loop, 18-first coherent detection module, 19-second coherent detection module, 20-nth coherent detection module, 21-first ADC, 22-second ADC, 23-nth ADC, 24-DSP.

[0048] In the figure, due to the limitation of image space, some devices are represented in reduced size in English. MZM stands for Mach-Zehnder modulator, ADC stands for analog-to-digital converter, and DSP stands for digital signal processing. DETAILED DESCRIPTION

[0049] The large output window analog correlator based on the optical frequency shift loop of the present invention comprises a continuous wave laser, a 1×n optical coupler, n parallel optical frequency shift loop structures, and a DSP processing module connected in sequence; the n optical frequency shift loop structures are all the same, comprising a 1×2 optical coupler, two parallel Mach-Zehnder modulators, an optical delay line, a bidirectional optical frequency shift loop, a coherent detection module, and an ADC module; the specific structure of the i-th optical frequency shift loop structure is: The two output ports of the 1×2 optical coupler are connected to the input ends of the i-1th MZM and the i-2th MZM respectively, the output end of the i-2th MZM is connected to the i-th optical delay line, the output end of the i-1th MZM and the output end of the i-th optical delay line are connected to the a and b input ports of the i-th bidirectional optical frequency shift loop respectively, the c and d output ports of the i-th bidirectional optical frequency shift loop are connected to the a and b input ports of the i-th coherent detection module respectively, the i-th coherent detection module is connected to the i-th ADC, and the i-th ADC is connected to the DSP processing module, wherein i=1, 2,…, n.

[0050] The i-th bidirectional optical frequency shift loop is a loop composed of the i-1th optical coupler, the i-th bidirectional optical bandpass filter, the i-2nd optical coupler, the i-th frequency shift, the i-3rd optical coupler, and the i-th bidirectional erbium-doped fiber amplifier; the i-th frequency shift is a loop composed of the i-1th circulator, the i-1th acousto-optic frequency shifter, the i-2nd circulator, the i-2nd acousto-optic frequency shifter, and the i-th adjustable optical fiber delay line. Specifically, the c port of the i-1th optical coupler, the i-th bidirectional optical bandpass filter, and the a port of the i-2th optical coupler are connected in sequence; the b port of the i-2th optical coupler is connected to the 2 port of the i-1th circulator; the 3 port of the i-1th circulator, the input and output ports of the i-1th acousto-optic frequency shifter, and the 1 port of the i-2nd circulator are connected in sequence; the 3 port of the i-2nd circulator, the input and output ports of the i-2nd acousto-optic frequency shifter, the input and output ports of the i-th adjustable optical fiber delay line, and the 1 port of the i-1th circulator are connected in sequence; the 2 port of the i-2nd circulator, the b port and the a port of the i-3rd optical coupler, the i-th bidirectional erbium-doped fiber amplifier, and the c port of the i-4th optical coupler are connected in sequence; the a port of the i-4th optical coupler is connected to the b port of the i-1th optical coupler.

[0051] The i-th coherent detection module comprises an i-th 90° optical mixer, an i-th balanced photodetector and an i-th low-pass filter connected in sequence, i=1, 2, ..., n. Among them, the two output ports of the i-th 90° optical mixer are selected as S+jL and S-jL, and if S+L and SL are selected, equivalent results are also obtained, S is the signal input terminal signal of the optical mixer, and L is the input signal of the local oscillator terminal of the optical mixer.

[0052] The following is combined with Figures 1 to 10 The present invention is described in detail by taking an analog correlator based on two optical frequency shift loop structures as an example, that is, n=2. The analog correlator based on two optical frequency shift loop structures includes a continuous wave laser, a 1×2 optical coupler, two 1×2 optical couplers, four MZMs, two optical delay lines, two bidirectional optical frequency shift loops, two coherent detection modules, two ADCs, and DSP processing. Figure 2 As shown, the i-th bidirectional optical frequency shift loop (i=1, 2 is the serial number of the optical frequency shift loop structure) is composed of the i-1th optical coupler, the i-2th optical coupler, the i-3th optical coupler, the i-4th optical coupler, the i-th bidirectional optical bandpass filter, the i-th bidirectional erbium-doped fiber amplifier, the i-1th circulator, the i-2th circulator, the i-1th acousto-optic frequency shifter, the i-2nd acousto-optic frequency shifter, and the i-th adjustable optical fiber delay line. It should be noted that the above i-1 does not mean i minus 1, "-1" represents the serial number, and the same applies to i-2, i-3, and i-3. All similar descriptions in this article represent serial numbers. Figure 3As shown, the i-th coherent detection module is composed of the i-th 90° optical mixer, the i-th balanced photodetector, and the i-th low-pass filter. The input RF signals RF1 and RF2 are respectively a sinusoidal signal and a white noise with a limited bandwidth. This embodiment is described by taking two sinusoidal signal inputs as an example. The RF signals RF1 and RF2 to be processed are both set to have a frequency of f RF =267MHz and the same phase of the sinusoidal signal, is a separate signal. RF1 and RF2 are the signals to be processed, not part of the analog correlator, the analog correlator can process RF1 and RF2 to achieve some applications, such as ranging.

[0053] The optical carrier with a central wavelength of 1550nm output by the continuous wave laser 1 is evenly divided into two parts through the 1×2 optical coupler 2, and then respectively injected into two parallel optical frequency shift structures. In the first optical frequency shift structure, the optical carrier is divided into upper and lower branches through the b and c ends of the first 1×2 optical coupler 3. The radio frequency signal RF1 to be processed is modulated on the optical carrier of the upper branch through the 1-1MZM6, and then injected from the a port into the first bidirectional optical frequency shift loop for cyclic frequency shift and delay; the radio frequency signal RF2 to be processed is modulated on the optical carrier of the lower branch through the 1-2MZM7, and then after passing through the first optical delay line, it is injected from the b port into the first bidirectional optical frequency shift loop for cyclic frequency shift and delay. Among them, in this specific implementation, the MZM adopts a push-pull structure, and the DC bias voltage V of the 1-1MZM6 and the 1-2MZM7 is set. 1-1 =V 1-2 =V π , V π is the half-wave voltage of the two MZMs. The optical delay line is a common single-mode fiber (SMF), and its first-order propagation constant β 1 =4.87×10 -9 s / m, the SMF length in the first optical frequency shift structure is set to 0m, and the SMF length in the second optical frequency shift structure is set to 1.34m. The output spectrum of the 1-1MZM and the output spectrum of the first optical delay line are shown in Figure 1. Figure 4 and Figure 5 As shown. Since the 1-1MZM6 and the 1-2MZM7 are both working at the minimum point, Figure 4 In order to suppress the two signal sidebands of the double-sideband modulation of the carrier, the frequency difference with the optical carrier is 267MHz. In addition, since the optical delay line only affects the signal phase but not the signal power spectrum, Figure 4 and Figure 5 The spectra are identical.

[0054] The optical radio frequency signal output by the 1-1 MZM6 is input from the a port of the 1-1 optical coupler and output from the c port, and passes through the 1-2 bidirectional optical bandpass filter with a bandwidth of 2.2 GHz to remove ASE noise and control the maximum number of optical signal cycles to 20; then, the optical signal is input from the a port of the 1-2 optical coupler and output from the b port, and then enters the 2 port of the 1-1 circulator and exits the 3 port, and is frequency shifted by the 1-1 acousto-optic frequency shifter, and the introduced frequency shift amount f 1 =80.2MHz; then, the optical signal enters the first bidirectional erbium-doped fiber amplifier through the 1st port and 2nd port of the 1-2nd circulator and the b port and a port of the 1-3rd optical coupler, and amplifies the optical signal to compensate for the loss in the ring; then, the output signal of the first bidirectional erbium-doped fiber amplifier passes through the c port and a port of the 1-4th optical coupler and the b port and c port of the 1-1st optical coupler in turn and returns to the first bidirectional optical bandpass filter for filtering, forming a clockwise cycle of the optical signal, and the clockwise loop delay is τ 1 =10 / f 1 Finally, the signal of multiple clockwise cyclic frequency shift and delay superposition is output from the c end of the 1-2 optical coupler, which is the output signal of the c port of the 1st bidirectional optical cyclic frequency shift loop. At the same time, the optical radio frequency signal output by the 1-2 MZM7 passes through the 1st optical delay line and is input from the b port of the 1-4 optical coupler and output from the c port. After passing through the 1st bidirectional erbium-doped fiber amplifier, the optical signal is amplified; then, the optical signal is input from the a port of the 1-3 optical coupler and output from the b port, and then enters the 2 port of the 1-2 circulator and exits the 3 port. After passing through the 1-2 acousto-optic frequency shifter, the frequency shift amount f is introduced. 2 =80MHz; then the first adjustable optical fiber delay line adjusts the delay difference between the clockwise and counterclockwise loops to Δτ = τ 2 -τ 1 , where τ 2 =10 / f 2 It is a counterclockwise loop delay; then it passes through the 1 port and 2 port of the 1-1 circulator and the b port and a port of the 1-2 optical coupler in turn to enter the 1st bidirectional optical bandpass filter to filter out ASE noise and control the maximum number of optical signal cycles; then, the output signal of the 1st bidirectional optical bandpass filter passes through the c port and b port of the 1-1 optical coupler and the a port and c port of the 1-4 optical coupler in turn to return to the 1st bidirectional erbium-doped fiber amplifier for amplification, forming a counterclockwise cycle of the optical signal, and finally the signal with multiple counterclockwise cyclic frequency shift and delay superposition is output from the c end of the 1-3 optical coupler, which is the output signal of the d port of the 1st bidirectional optical cyclic frequency shift loop. The output spectrum of the c and d ports of the first bidirectional optical frequency shift loop is shown in the figure below. Figure 6As shown, the output signal of port c corresponds to 20 cycles of the clockwise cyclic signal, with a frequency shift of 80.2 MHz per cycle; the output signal of port d corresponds to 20 cycles of the counterclockwise cyclic signal, with a frequency shift of 80 MHz per cycle.

[0055] The input signals of port a and port b of the first coherent detection module simultaneously enter the first 90° optical mixer for orthogonal coupling, and the two output signals enter the first balanced photodetector through port a and port b respectively for photoelectric detection and differential processing. The output photocurrent is filtered by the first low-pass filter and used as the output electrical signal of the first coherent detection module. The bandwidth of the first low-pass filter is set to 4MHz. Finally, the output signal of the first coherent detection module is sampled and converted into a digital signal by the first ADC, and then Fourier transform processing is performed. The ADC sampling rate is 50MS / s and the quantization bit number is 8 bits. The spectrum corresponding to the output waveform of the first ADC is shown as follows: Figure 7 As shown, the short dotted line is the theoretical correlation result of two input sinusoidal signals. Figure 7 It can be seen that the interval between each frequency comb tooth Δf = f 1 -f 2 =0.2MHz, 6 frequency comb spacings constitute a spectrum envelope with a period of 6Δτ=6×312=1872ps, and the period of the theoretical autocorrelation result of a 267MHz sinusoidal signal is half of the signal period, that is, 1872ps. It can be seen that the autocorrelation results of the two input sinusoidal signals are mapped from the time domain to the spectrum envelope output by the first coherent detection module, and the spectrum envelope is consistent with the theoretical autocorrelation results. Moreover, the output window of the correlator based on the first optical frequency shift loop structure is 0~20Δτ, that is, 0~6240ps.

[0056] For the second optical frequency shift loop structure, except for the different length of the second optical delay line, the other structures and device parameter settings are the same as those of the first optical frequency shift loop structure. The spectrum corresponding to the second ADC output waveform is as follows: Figure 8 As shown, the short dotted line is the theoretical correlation result of two input sinusoidal signals. Figure 8 It can be seen that the frequency comb spacing and envelope period are Figure 7 Similarly, the spectrum envelope is consistent with the theoretical autocorrelation results. Figure 7 The difference is, Figure 8 The displayed correlation result output window is 20Δτ~40Δτ, which is exactly the same as Figure 7 The relevant results are connected end to end, so the output results of the two optical frequency shift loop structures can be spliced ​​in the frequency domain. The spectrum after DSP splicing is as follows Fig. 9 As shown, it can be seen that the output spectrum envelope of the analog correlator based on the two optical frequency shift loop structures is the autocorrelation result of the two input sinusoidal signals in the output window of 0 to 40Δτ.

[0057] The input RF signals RF1 and RF2 are set to 120MHz limited bandwidth white noise. The output spectrum of the analog correlator based on the two optical frequency shift loop structures is as follows: Fig.10 As shown in the figure, it can be seen that the spectrum envelope is consistent with the theoretical autocorrelation result, and the correlation processing time window is expanded by two times, indicating that the correlator can process different input signals. When the correlator adopts n optical frequency shift loop structures, the correlation result output window can be expanded by n times by sequentially splicing n correlation results in the frequency domain.

[0058] It can be seen from the specific examples that the present invention provides a large output window analog correlator based on an optical frequency shift loop and an implementation method thereof, which can achieve large output window, large bandwidth, and fast correlation processing for various types of signals.

[0059] The present invention is not limited to the above-mentioned implementation modes, and several equivalent deformations and substitutions can be made. The input RF signal is not limited to a single-tone sinusoidal signal / limited bandwidth white noise, and the optical delay line is not limited to a single-mode optical fiber. Various simplifications and modifications within the scope of the principle and method of the present invention all belong to the protection content of the present invention.

Claims

1. A large output window analog correlator based on an optical frequency shift loop, characterized in that: It includes a continuous wave laser, a 1×n optical coupler, n parallel optical frequency shift loop structures, and a DSP processing module connected in sequence; the n optical frequency shift loop structures are all the same, including a 1×2 optical coupler, two parallel Mach-Zehnder modulators, an optical delay line, a bidirectional optical frequency shift loop, a coherent detection module, and an ADC module; The specific structure of the i-th optical frequency shift loop structure is: the two output ports of the i-th 1×2 optical coupler are respectively connected to the input ends of the i-1th MZM and the i-2th MZM, the output end of the i-2th MZM is connected to the i-th optical delay line, the output end of the i-1th MZM and the output end of the i-th optical delay line are respectively connected to the a and b input ports of the i-th bidirectional optical frequency shift loop, the c and d output ports of the i-th bidirectional optical frequency shift loop are respectively connected to the a and b input ports of the i-th coherent detection module, the i-th coherent detection module is connected to the i-th ADC, and the i-th ADC is connected to the DSP processing module; wherein, i=1, 2,…, n.

2. The large output window analog correlator based on optical frequency shift loop according to claim 1, characterized in that: The i-th bidirectional optical frequency shift loop is a loop formed by sequentially connecting the i-1th optical coupler, the i-th bidirectional optical bandpass filter, the i-2nd optical coupler, the i-th frequency shift, the i-3rd optical coupler, the i-th bidirectional erbium-doped fiber amplifier and the i-4th optical coupler; the i-th frequency shift is a loop formed by sequentially connecting the i-1th circulator, the i-1th acousto-optic frequency shifter, the i-2nd circulator, the i-2nd acousto-optic frequency shifter and the i-th adjustable optical fiber delay line.

3. The large output window analog correlator based on optical frequency shift loop according to claim 1, characterized in that: The i-th coherent detection module comprises an i-th 90° optical mixer, an i-th balanced photodetector and an i-th low-pass filter connected in sequence, where i=1, 2, ..., n.

4. A method for implementing a large output window analog correlator based on an optical frequency shift loop, characterized in that: The method is based on the large output window analog correlator based on the optical frequency shift loop as claimed in any one of claims 1 to 3, and comprises the following steps: The continuous wave laser evenly divides the optical carrier into n paths through a 1×n optical coupler; Injecting n optical carrier waves into n parallel optical frequency shift loop structures respectively to obtain n quantized electrical signals; The implementation steps in the n parallel optical frequency shifting loop structures are the same; The n quantized electrical signals are processed by DSP to obtain the final result; The i-th parallel branch optical frequency shift loop structure includes an upper branch and a lower branch, i=1, 2, ..., n, and the specific implementation steps include: The radio frequency signal RF1 to be processed is modulated on the optical carrier of the upper branch by the i-1th Mach-Zehnder modulator, and the optical carrier radio frequency signal output by the i-1th Mach-Zehnder modulator is injected from the a port of the i-th bidirectional optical frequency shift loop for cyclic frequency shift and time delay; The radio frequency signal RF2 to be processed is modulated on the optical carrier of the lower branch by the i-2 Mach-Zehnder modulator, and the optical carrier radio frequency signal after passing through the i-th optical delay line is injected from the b port of the i-th bidirectional optical frequency shift loop for cyclic frequency shift and delay; The two signals output by the i-th bidirectional optical frequency shift loop are photoelectrically detected and filtered by the i-th coherent detection module to obtain the i-th electrical signal; The ith electrical signal is input into the ith ADC to obtain a sampled and quantized electrical signal.

5. The method for realizing a large output window analog correlator based on an optical frequency shift loop according to claim 4, characterized in that: In the i-th parallel branch optical frequency shift loop structure, the group delay introduced by the i-th optical delay line is (i-1)(M+1)(τ2-τ1), wherein M is the maximum number of cycles of the two signals in the i-th bidirectional optical frequency shift loop, τ1 is the delay of the signal for one clockwise cycle in the i-th bidirectional optical frequency shift loop, and τ2 is the delay of the signal for one counterclockwise cycle in the i-th bidirectional optical frequency shift loop.

6. The method for realizing a large output window analog correlator based on an optical frequency shift loop according to claim 4, characterized in that: The cyclic frequency shifting and delaying in the upper branch of the i-th bidirectional optical frequency shifting loop comprises the following steps: The optical radio frequency signal output by the i-1th Mach-Zehnder modulator is input from the a port of the i-1th optical coupler, and then output from the c port, and then passes through the i-th bidirectional optical bandpass filter to filter out the spontaneous radiation noise introduced by the optical amplifier, while controlling the number of cycles of the optical signal; The optical signal is input from the a port of the i-2 optical coupler, output from the b port, input from the 2 port of the i-1 circulator, and output from the 3 port. It is frequency-shifted by the i-1 acousto-optic frequency shifter with a frequency shift amount of f1, and then passes through the 1 port and 2 port of the i-2 circulator and the b port and a port of the i-3 optical coupler in sequence to enter the i bidirectional erbium-doped fiber amplifier, which amplifies the optical signal to compensate for the loss in the ring. The output signal of the i-th bidirectional erbium-doped fiber amplifier passes through the c-port and a-port of the i-4th optical coupler and the b-port and c-port of the i-1th optical coupler in turn and returns to the i-th bidirectional optical bandpass filter for filtering, forming a clockwise cycle of the optical signal. The clockwise loop delay is τ1. Finally, the signal with multiple clockwise cyclic frequency shift and delay superposition is output from the c-port of the i-2nd optical coupler, which is the output signal of the c-port of the i-th bidirectional optical cyclic frequency shift loop. The cyclic frequency shifting and delaying in the lower branch of the i-th bidirectional optical frequency shifting loop comprises the following steps: The optical radio frequency signal output after the i-th optical delay line is input from the b port of the i-4th optical coupler and then output from the c port, and then passes through the i-th bidirectional erbium-doped fiber amplifier to amplify the optical signal; The optical signal is input from the a port of the i-3 optical coupler and then output from the b port, then input from the 2 port of the i-2 circulator and then output from the 3 port, and is frequency shifted by the i-2 acousto-optic frequency shifter, the frequency shift amount is f2, and then is adjusted by the i-th adjustable optical fiber delay line to adjust the delay difference of the clockwise and counterclockwise loops to Δτ=τ2-τ1, where τ2 is the counterclockwise loop delay, and then sequentially passes through the 1 port and 2 port of the i-1 circulator and the b port and a port of the i-2 optical coupler to enter the i-th bidirectional optical bandpass filter to filter out ASE noise and control the number of cycles of the optical signal. In addition, it must satisfy f1τ1-f2τ2=k, where k is an integer; The output signal of the i-th bidirectional optical bandpass filter passes through the c-port and b-port of the i-1-th optical coupler and the a-port and c-port of the i-4-th optical coupler in turn and returns to the i-th bidirectional erbium-doped fiber amplifier for amplification, forming a counterclockwise cycle of the optical signal. Finally, the signal of multiple counterclockwise cyclic frequency shift and delayed superposition is output from the c-end of the i-3-th optical coupler, which is the output signal of the d-port of the i-th bidirectional optical cyclic frequency shift loop.

7. The method for realizing a large output window analog correlator based on an optical frequency shift loop according to claim 4, characterized in that: The two signals output by the i-th bidirectional optical frequency shift loop are photoelectrically detected and filtered by the i-th coherent detection module, comprising the following steps: The two signals simultaneously enter the i-th 90° optical mixer for orthogonal mixing; The two output signals of the optical mixer enter the i-th balanced photodetector for photoelectric detection and differential processing, and the output photocurrent is filtered by the i-th low-pass filter and used as the output electrical signal of the i-th coherent detection module.

8. The method for realizing a large output window analog correlator based on an optical frequency shift loop according to claim 4, characterized in that: The process of processing the n quantized electrical signals by DSP to obtain the final result comprises the following steps: The DSP processing includes the following steps: Fourier transform is performed on the n quantized electrical signals in sequence, and the obtained n spectrum envelopes are respectively the correlation results of the radio frequency signals RF1 and RF2 to be processed at n different time windows; The n output results are sequentially spliced ​​in the frequency domain to obtain an expansion of the relevant result output window.

9. The method for realizing a large output window analog correlator based on an optical frequency shift loop according to claim 4, characterized in that: The Mach-Zehnder modulators all operate at a minimum bias point.

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