Photonics assisted doppler shift and angle of arrival unambiguous measurement method and apparatus

Abstract

Disclose a kind of photonic auxiliary Doppler frequency shift and arrival angle unambiguous measurement device, including two big parts: signal processing unit and central station.In signal receiving unit, there is continuous wave laser LD, polarization controller 0, optical circulator, polarization beam splitter, first double-drive Mach-Zehnder modulator 100, second double-drive Mach-Zehnder modulator 200, 1×2 electric power distributor, phase shifter;Central station is equipped with 1×2 optical power distributor, first polarization controller a and second polarization controller b, first polarizer a and second polarizer b, first low-speed photodetector a and second low-speed photodetector b, signal processing unit.A kind of photonic auxiliary Doppler frequency shift and arrival angle unambiguous measurement method is also provided.The present application can realize the simultaneous measurement of Doppler frequency shift and arrival angle, simple and effective determination Doppler frequency shift direction is realized to 180° all-around measurement of arrival angle.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of microwave photonics and microwave signal parameter measurement, and particularly relates to a method for simultaneously realizing Doppler frequency shift and angle of arrival (AOA) measurement without ambiguity and a device thereof. BACKGROUND

[0002] In electronic warfare, electronic reconnaissance is the primary link of electronic warfare and is the prerequisite for electronic jamming and electronic countermeasures. Doppler frequency shift (DFS) and angle of arrival (AOA) are two very important parameters in microwave signal measurement, and through these two parameters, the enemy's electromagnetic wave carrier radiation source can be measured, positioned and judged, providing reliable guarantee for electronic reconnaissance. The traditional electrical measurement method has the disadvantages of low measurement frequency, narrow bandwidth, large volume, etc., and has been difficult to meet the demand. With the birth of microwave photonics, various photon-assisted Doppler frequency shift and angle of arrival measurement technologies have been widely reported. However, many measurement systems reported in the past can only realize single measurement of Doppler frequency shift or angle of arrival, and there is an ambiguity problem in the measurement of Doppler frequency shift and angle of arrival, making it difficult to quickly locate and accurately judge the target, which limits the application of the system.

[0003] In recent years, with the continuous development of microwave technology, the simultaneous measurement of Doppler frequency shift and angle of arrival using microwave photonics technology has attracted close attention from domestic and foreign research institutions and has been widely applied. 1) Z. Tang and S. Pan, “Simultaneous measurement of Doppler-frequency shift and angle-of-arrival of microwave signals for automotive radars,” in Proc. Int. Topical Meeting Microw. Photon., 2019, pp. 1-4. This system uses a polarization division multiplexing microwave photonic I / Q hybrid system to realize the simultaneous measurement of Doppler frequency shift and angle of arrival, and applies it to vehicle-mounted radar. 2) J. Zhao, Z. Tang, and S. Pan, “Photonic approach for simultaneous measurement of microwave DFS and AOA,” Appl. Opt., vol. 60, no. 16, pp. 4622-4626, 2021. The system structure is relatively simple, and uses a double-channel microwave photonic mixing technology to realize the simultaneous measurement of Doppler frequency shift and angle of arrival, and through experimental verification, the DFS measurement error is less than ±0.08 Hz in the range of ±100 kHz, and the AOA measurement error is less than ±1.3° in the range of 0° to 90°. 3) Q. Jia, J. Li, L. Sun, D. Li and J. Liu, “A Simple Photonics-Based Measurement Method for Microwave DFS and AOA,” in IEEE Photonics Journal, vol. 14, no. 3. pp. 1-8. The system uses two parallel double parallel Mach-Zehnder modulators for carrier-suppressed single sideband modulation, and realizes the simultaneous measurement of Doppler frequency shift and angle of arrival by comparing the frequency and phase shift of the two-channel signals.4) G. Li, D. Shi, L. Wang, M. Li, N. Zhu and W. Li, "Photonic system for simultaneous and unambiguous measurement of angle-of-arrival and Doper-frequency-shift," J. Lightw. Technol., 40(8), 2321-2328. This system uses a dual-polarization dual-drive Mach-Zehnder modulator, by introducing a reference signal, the Doppler frequency shift measurement error is less than ±0.6Hz and the arrival angle in the measurement range of-66.44°-66.44° error is less than ±2.2°, and the simultaneous measurement is realized.

[0004] However, the above-mentioned scheme has certain limitations. 1) A polarization multiplexing Mach-Zehnder modulator, a dual-polarization 90-degree optical hybrid and an optical filter are needed, which greatly increases the complexity and cost of the system; 2) Although the system structure is greatly simplified, the system still needs a tunable optical filter to filter out the unwanted sideband, which greatly limits the working bandwidth of the system, and the stability is easily affected by the environment; 3) Two parallel dual-parallel Mach-Zehnder modulators (DPMMMs) are used for carrier suppression single sideband (CS-SSB) modulation, which needs to control the accurate bias of the modulator. And 1), 2), 3) the arrival angle measurement range is less than 90°, which cannot meet the actual demand. 4) The measurement method is simple, the measurement result is accurate, and no filter and wavelength division multiplexing filter elements are needed, but the synchronization of the local signal and the received echo signal needs to be ensured in the measurement, which is difficult to realize in practical application. In addition, the above-mentioned method does not consider the long-distance transmission of the signal. SUMMARY

[0005] In view of the problems existing in the prior art, the present application provides a photonic-assisted Doppler frequency shift and arrival angle unambiguous measurement device, which comprises two parts: a signal processing unit and a central station, wherein

[0006] In the signal receiving unit, a continuous wave laser LD, a polarization controller 0, an optical circulator, a polarization beam splitter, a first dual-drive Mach-Zehnder modulator 100, a second dual-drive Mach-Zehnder modulator 200, a 1x2 electrical power divider and a phase shifter are arranged.

[0007] The continuous wave laser LD outputs linearly polarized light as an optical carrier;

[0008] The polarization controller 0 receives the optical carrier output by the continuous wave laser LD and outputs linearly polarized light with a polarization state rotated by 45°;

[0009] The input port 1 of the optical circulator is connected with the input end of the polarization beam splitter, and receives two optical signals from the polarization controller 0;

[0010] The input port of the first dual-drive Mach-Zehnder modulator 100 is connected with the left output end of the polarization beam splitter, the output port of the first dual-drive Mach-Zehnder modulator 100 is connected with the output port of the second dual-drive Mach-Zehnder modulator 200, and the input port of the second dual-drive Mach-Zehnder modulator 200 is connected with the right output end of the polarization beam splitter, thereby forming a Sagnac loop structure.

[0011] The two optical signals enter the polarization beam splitter through the input port 1 of the optical circulator, the polarization beam splitter separates the X and Y polarization direction optical signals and transmits them along the clockwise and counterclockwise directions of the Sagnac loop respectively; the first dual-drive Mach-Zehnder modulator 100 and the second dual-drive Mach-Zehnder modulator 200 each have two RF input ports and one DC bias point, and each modulator has a phase modulator PM in each arm; the first RF signal 10 and the second RF signal 20 are echo signals received by the first antenna receiving unit e and the second antenna receiving unit f respectively, and they are input to one RF input port of the first dual-drive Mach-Zehnder modulator 100 and the second dual-drive Mach-Zehnder modulator 200 respectively; the reference signal is divided into a third RF signal 30 and a fourth RF signal 40 with the same power and a phase difference of by a 1×2 electrical power divider and a phase shifter, and the third RF signal 30 and the fourth RF signal 40 are input to the other RF input port of the first dual-drive Mach-Zehnder modulator 100 and the second dual-drive Mach-Zehnder modulator 200 respectively; the X and Y polarization direction optical signals output by the first dual-drive Mach-Zehnder modulator 100 and the second dual-drive Mach-Zehnder modulator 200 enter the polarization beam splitter again to form a bundle of orthogonal polarization multiplexed light, which is output to a single-mode optical fiber through the port 3 of the optical circulator, and then transmitted to a central station at a remote end;

[0012] The single-mode optical fiber is used to connect the signal receiving unit and the central station;

[0013] The central station is provided with a 1×2 optical power divider, a first polarization controller a and a second polarization controller b, a first polarizer a and a second polarizer b, a first low-speed photodetector a and a second low-speed photodetector b, and a signal processing unit; the orthogonal polarization multiplexed light transmitted through the single-mode optical fiber is equally divided into two paths of the same light signal through the 1×2 optical power divider, the upper path light signal is converted into linearly polarized light after passing through the first polarization controller a and the first polarizer a in turn, and the lower path light signal is converted into linearly polarized light after passing through the second polarization controller b and the second polarizer b respectively; after the upper and lower path linearly polarized light passes through the first low-speed photodetector a and the second low-speed photodetector b respectively, the two paths of linearly polarized light signals are converted into two paths of intermediate frequency electrical signals and output.

[0014] The application further provides a method for photonically assisted Doppler shift and angle of arrival non-ambiguous measurement, which is based on the above-mentioned photonically assisted Doppler shift and angle of arrival non-ambiguous measurement device, and specifically,

[0015] The definition of the modulated light signal from the first double-drive Mach-Zehnder modulator 100 works in the X polarization direction, and the modulated light signal from the second double-drive Mach-Zehnder modulator 200 works in the Y polarization direction; the direct current bias voltage value of the two modulators is adjusted so that the first double-drive Mach-Zehnder modulator 100 and the second double-drive Mach-Zehnder modulator 200 work at the maximum transmission point to realize the phase modulation of the received optical carrier; formula (1) represents the expression of the light wave after being modulated by the two modulators in the X and Y polarization directions:

[0016]

[0017] Wherein, E x (t) and E y (t) are linearly polarized light in the X polarization direction and the Y polarization direction, respectively; E in = E0exp(jω c t) is the optical carrier output by the laser source LD, E0 is the amplitude of the optical carrier output by the laser source LD, ω c , ω e , and ω r are the angular frequencies of the optical carrier, the received microwave signal, and the reference signal, respectively; m e = πV e / V π and m r = πV r / V π are the modulation indexes of the double-drive Mach-Zehnder modulator to the echo signal and the reference signal, respectively, V e , V r are the amplitudes of the received microwave signal and the reference signal, respectively, V π is the half-wave voltage of the double-drive Mach-Zehnder modulator; θ represents the phase difference between the first receiving antenna unit e and the second antenna unit f; represents the phase difference between the third radio frequency signal 30 and the fourth radio frequency signal 40; t represents time; when the distance between the two antenna units is λ / 2, the relationship between the angle κ and the phase difference θ is:

[0018]

[0019] Then, the quadrature polarization multiplexed light output from the optical circulator port 3 is transmitted to the remote central station for processing through a single-mode optical fiber; due to the dispersion characteristics of the optical fiber, the transmission of the optical signal in the optical fiber will introduce a phase shift that varies linearly with wavelength;

[0020] At the central station, the modulated optical signal is divided into two paths by a 1x2 optical power splitter, and the intensity and phase of the light wave in the X and Y polarization directions can be changed by controlling the polarization controller and polarizer in the two paths. Under the small signal condition, the optical signals along the upper and lower paths are expressed by formula (3) as follows:

[0021]

[0022] J n (m) is the first kind of Bessel function of n order; α is the polarization angle between the axis of the polarizer and the principal axis, ψ PC is the phase shift between the two orthogonal polarization states introduced by the polarization controller PC; φ +1 , φ -1 are the phase shifts introduced by the single-mode fiber for the input optical carrier and the two ±1 order sidebands, respectively;

[0023] In order to obtain the unambiguous angle of arrival measurement, at the central station, the first polarization controller a and the first polarizer a of the upper path are controlled so that α 上 = 45°, ψ PC上 = 0°; the second polarization controller b and the second polarizer b of the lower path are adjusted so that α 下 = 135°, ψ PC下 = 90°, and the expressions of the optical signals along the upper and lower paths are as follows:

[0024]

[0025] The two optical signals output from the first polarizer a and the second polarizer b are input to the first low-speed photodetector a and the second low-speed photodetector b, respectively. The first low-speed photodetector a and the second low-speed photodetector b convert the optical signals into current signals, and the photocurrent output by the low-speed photodetector is expressed as i(t) = η|E(t)| 2 , where E(t) is the intensity of the optical signal input to the low-speed photodetector, and it only retains the low-frequency signal component, and the high-frequency signal is filtered out. Therefore, the two optical current signals output by the first low-speed photodetector a and the second low-speed photodetector b are as follows:

[0026]

[0027] Here P in represents the LD incident optical carrier signal power, The Doppler frequency shift is defined as the frequency difference between the echo signal and the transmission signal, i.e. f DFS = f e -f t , where f DFS , fe , f t respectively are Doppler shift, echo signal and transmission signal frequency; the reference signal frequency is set to be a MHz larger than the transmission signal frequency, i.e. r = f t + a MHz; the Doppler shift can be rewritten as f DFS = f e - f r + a MHz = -(f r - f e ) + a MHz, since the Doppler shift is kept within ±1 MHz, the measured photocurrent signal frequency range is (-1 + a) MHz to (1 + a) MHz, and when the measured current signal frequency range is (-1 + a) to a MHz, the Doppler shift is positive, representing that the target is close to the receiving source; when the measured current signal frequency range is a MHz to (1 + a) MHz, the Doppler shift is negative, representing that the target is away from the receiving source; therefore, by measuring the current signal frequency, not only the size of the Doppler shift can be judged, but also the relative motion direction of the target can be measured;

[0028] The power expression of the upper and lower light current signals output by the low-speed photodetector calculated by formula (5) is as follows:

[0029]

[0030] As can be seen from formula (6), the generated low-frequency signal power is a function of the phase difference θ; therefore, by measuring the photocurrent signal power, the phase difference can be estimated, and the angle of arrival value is obtained by using formula (2).

[0031] In one specific embodiment of the present application, a = 2.

[0032] The present application has the following advantages:

[0033] 1. The photon-assisted Doppler shift and angle of arrival non-ambiguous measurement scheme can realize simultaneous measurement of the Doppler shift and the angle of arrival, and simple and effective judgment of the Doppler shift direction while realizing 180° full-range measurement of the angle of arrival.

[0034] 2. The photon-assisted Doppler shift and angle of arrival non-ambiguous measurement scheme adopts microwave optical technology, compared with the electrical domain method, the technical scheme of the present application has a series of advantages of optical method, such as large measurement frequency bandwidth, high measurement precision, and effective anti-electromagnetic interference by using optical fiber transmission, etc.

[0035] 3. The photon-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme adopts a Sagnac ring structure in the signal receiving unit. Since the two light signals in the Sagnac ring structure experience the same transmission path, there is no transmission delay difference, and the environmental noise is the same, the system stability is stronger than that of the commonly used parallel structure.

[0036] 4. The photon-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme separates the signal processing unit and the signal receiving unit, ensures the concealment and safety of the central station, and has a simple link structure. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 (a) is a structural schematic diagram of the photon-assisted Doppler frequency shift and angle of arrival unambiguous measurement device of the application, Figure 1 (b) is a partial view of the internal structure of a double-drive Mach-Zehnder modulator and a signal access method;

[0038] Figure 2 is an electrical spectrum diagram when the Doppler frequency shift is 1MHz and -1MHz in the application, and the direction of the Doppler frequency shift is determined by measuring the frequency value. Among them Figure 2 (a) shows that the Doppler frequency shift is +1MHz; Figure 2 (b) shows that the Doppler frequency shift is -1MHz;

[0039] Figure 3 is a simulation curve of different Doppler frequency shift measurements in the application, and the Doppler frequency shift measurement error is calculated;

[0040] Figure 4 is a comparison diagram of the theoretical curve and the simulation measurement value of the phase difference and the power change normalization function of the echo signal in the application;

[0041] Fig. 5 is a diagram for solving the phase difference value and estimating the measurement error according to the function curve between the measured power value and the phase shift 5(a) and the angle of arrival value and estimating the measurement error 5(b) in the application, and calculating the measurement error. DETAILED DESCRIPTION

[0042] The application provides a photon-assisted Doppler frequency shift and angle of arrival unambiguous measurement method and device with high measurement accuracy and strong practicability, which will be further described below with reference to the drawings.

[0043] As Figure 1 shown, the microwave photon technology is used to simultaneously realize a Doppler frequency shift and angle of arrival unambiguous measurement system, which includes two parts: a signal processing unit and a central station.

[0044] In the signal receiving unit, there are a continuous wave laser LD, a polarization controller 0, an optical circulator, a polarization beam splitter, a first double-drive Mach-Zehnder modulator 100, a second double-drive Mach-Zehnder modulator 200, a 1x2 electrical power divider, and a phase shifter.

[0045] The continuous wave laser LD outputs linearly polarized light (X polarization direction) as an optical carrier.

[0046] The polarization controller 0 rotates the polarization state of the linearly polarized light from the continuous wave laser LD by 45°, so that the X and Y polarization directions both contain optical signals, and outputs two optical signals.

[0047] The input port 1 of the optical circulator is connected to the input end of the polarization beam splitter, and receives the two optical signals from the polarization controller 0.

[0048] The input port of the first double-drive Mach-Zehnder modulator 100 is connected to the left output end of the polarization beam splitter, the output port of the first double-drive Mach-Zehnder modulator 100 is connected to the output port of the second double-drive Mach-Zehnder modulator 200, and the input port of the second double-drive Mach-Zehnder modulator 200 is connected to the right output end of the polarization beam splitter, thereby forming a Sagnac loop structure.

[0049] The two optical signals enter the polarization beam splitter through the input port 1 of the optical circulator. The polarization beam splitter separates the X and Y polarization direction optical signals and transmits them in the clockwise and counterclockwise directions along the Sagnac loop, respectively. It should be noted that, since the Mach-Zehnder modulator is a traveling wave device, it only effectively modulates the optical signal entering from the input port, and the optical signal entering from the output port in the opposite direction is not effectively modulated due to the rate mismatch phenomenon. Therefore, in the Sagnac loop structure, the polarization X-axis direction optical signal transmitted in the clockwise direction is only modulated by the first double-drive Mach-Zehnder modulator 100, and is not modulated by the second Mach-Zehnder modulator 200; on the contrary, the polarization Y-axis direction optical signal transmitted in the counterclockwise direction is only modulated by the second double-drive Mach-Zehnder modulator 200, and is not modulated by the first Mach-Zehnder modulator 100. The first double-drive Mach-Zehnder modulator 100 and the second double-drive Mach-Zehnder modulator 200 each have two radio frequency input ports and one direct current bias point, and the upper and lower arms of each modulator each include a phase modulator (PM), as shown in the structural schematic diagram of Figure 1 (b). The first radio frequency signal 10 and the second radio frequency signal 20 are respectively the echo signals received by the first antenna receiving unit e and the second antenna receiving unit f, which are respectively input to one radio frequency input port of the first double-drive Mach-Zehnder modulator 100 and the second double-drive Mach-Zehnder modulator 200; the local oscillator signal is used as a reference signal, and the reference signal is divided into two paths with the same power and a phase difference of third and fourth radio frequency signals 30 and 40, which are input to the other radio frequency input ports of the first and second double-drive Mach-Zehnder modulators 100 and 200, respectively. The X and Y polarization direction optical signals modulated by the first and second double-drive Mach-Zehnder modulators 100 and 200 are combined into a bundle of orthogonal polarization multiplexed light again, and output from the port 3 of the optical circulator to a single-mode optical fiber, and then transmitted to a central station at a remote end.

[0050] A single-mode optical fiber is used to connect the signal receiving unit and the central station.

[0051] The central station is provided with a 1x2 optical power divider, first and second polarization controllers a and b, first and second polarizers a and b, first and second low-speed photodetectors a and b, and a signal processing unit. The orthogonal polarization multiplexed light transmitted through the single-mode optical fiber is equally divided into two equal optical signals by the 1x2 optical power divider, and the upper optical signal is converted into linearly polarized light by the first polarization controller a and the first polarizer a in sequence, and the lower optical signal is converted into linearly polarized light by the second polarization controller b and the second polarizer b. Since the first polarization controller a and the first polarizer a and the second polarization controller b and the second polarizer b have different polarization angles, the polarization angles of the upper and lower signals are different, and thus the phase shifts caused thereby are also different. After the upper and lower linearly polarized lights pass through the first and second low-speed photodetectors a and b, respectively, the two linearly polarized light signals are converted into upper and lower intermediate frequency electrical signals, respectively.

[0052] The modulated optical signals from the first double-drive Mach-Zehnder modulator 100 are defined to work in the X polarization direction, and the modulated optical signals from the second double-drive Mach-Zehnder modulator 200 are defined to work in the Y polarization direction. The direct current bias voltage values of the two modulators are adjusted so that the first and second double-drive Mach-Zehnder modulators 100 and 200 work at the maximum transmission point, and the phase modulation of the received optical carrier is realized. Formula (1) represents the expression of the optical wave after being modulated by the two modulators in the X and Y polarization directions:

[0053]

[0054] wherein E x (t) and E y (t) are linearly polarized lights in the X and Y polarization directions, respectively; E in = E0exp(jω c t) is the optical carrier output by the laser source LD, E0 is the amplitude of the optical carrier output by the laser source LD, ω c , ω e , and ωr respectively the optical carrier, the received microwave signal and the reference signal angular frequency; m e = πV e / V π and m r = πV r / V π respectively the modulation index of the dual-drive Mach-Zehnder modulator pair on the echo signal and the reference signal, V e , V r respectively the amplitude of the received microwave signal and the reference signal, V π is the half-wave voltage of the dual-drive Mach-Zehnder modulator; θ represents the phase difference between the first receiving antenna unit e and the second antenna unit f, which is caused by the time delay of the echo signal reaching the two antennas due to the distance between the two antenna units; represents the phase difference between the third radio frequency signal 30 and the fourth radio frequency signal 40; t represents time. When the distance between the two antenna units is λ / 2, the relationship between the angle κ and the phase difference θ is:

[0055]

[0056] Next, the quadrature polarization multiplexed light output from the optical circulator port 3 is transmitted to the remote central station for processing through a single-mode optical fiber. Due to the dispersion characteristics of the optical fiber, the transmission of the optical signal in the optical fiber will introduce a phase shift that varies linearly with wavelength.

[0057] At the central station, the modulated optical signal is equally divided into upper and lower paths through a 1x2 optical power divider. The intensity and phase of the light waves in the polarization X and Y directions can be changed by controlling the polarization controllers and polarizers in the upper and lower paths. Under small signal conditions, the optical signals along the upper and lower paths are uniformly represented by formula (3):

[0058]

[0059] J n (m) is the n-th order Bessel function of the first kind; α is the polarization angle between the axial direction of the polarizer and the principal axis, ψ PC is the phase shift between the two orthogonal polarization states introduced by the polarization controller PC. φ +1 , φ -1 respectively the phase shifts introduced by the single-mode optical fiber to the input optical carrier and the two ±1 order sidebands.

[0060] To obtain unambiguous angle of arrival measurement, at the central station, the first polarization controller a and the first polarizer a of the upper path are controlled so that α 上 = 45°, ψ PC上 = 0°; the second polarization controller b and the second polarizer b of the lower path are adjusted so that α下 = 135°, ψ = 0° PC下 = 90° (the adjusting method is well known to those skilled in the art and is not described here), the optical signal expressions along the upper path and the lower path are respectively:

[0061]

[0062] The two optical signals output from the first polarizer a and the second polarizer b are input to the first low-speed photodetector a and the second low-speed photodetector b respectively. The first low-speed photodetector a and the second low-speed photodetector b convert the optical signals into current signals, and the photoelectric current output by the low-speed photodetector can be expressed as i(t) = η|E(t)| 2 , where E(t) is the intensity of the optical signal input to the low-speed photodetector, and it only retains the low-frequency signal component, and the high-frequency signal is filtered out. Therefore, the two photoelectric current signals output by the first low-speed photodetector a and the second low-speed photodetector b are respectively:

[0063]

[0064] Here P in represents the LD incident optical carrier signal power, The Doppler shift is defined as the frequency difference between the echo signal and the transmission signal, i.e. f DFS = f e -f t , where f DFS , f e , f t are the Doppler shift, the echo signal and the transmission signal frequency respectively; since the Doppler shift is maintained within ±1 MHz in most cases, the reference signal frequency is set to be 2 MHz greater than the transmission signal frequency, i.e. f r = f t + 2 MHz. The Doppler shift can be rewritten as f DFS = f e -f r + 2 MHz = -(f r -f e ) + 2 MHz, since the Doppler shift is maintained within ±1 MHz, the measurement of the photoelectric current signal frequency range is 1 MHz to 3 MHz, and when the measurement of the current signal frequency range is 1 MHz to 2 MHz, the Doppler shift is positive, representing that the target is approaching the receiving source; when the measurement of the current signal frequency range is 2 MHz-3 MHz, the Doppler shift is negative, representing that the target is moving away from the receiving source. Therefore, by measuring the current signal frequency, not only the size of the Doppler shift can be determined, but also the relative motion direction of the target can be measured.

[0065] The power expression of the upper and lower light current signals of the low-speed photodetector output is calculated by formula (5) as follows:

[0066]

[0067] As can be seen from formula (6), the generated low-frequency signal power is a function of the phase difference θ. Therefore, by measuring the light current signal power, the phase difference can be estimated, and the angle of arrival value is obtained by formula (2).

[0068] The present application has the following advantages:

[0069] 1. The photonic-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme can simultaneously measure the Doppler frequency shift and the angle of arrival, and simultaneously realize 180° omnidirectional measurement of the angle of arrival while judging the direction of the Doppler frequency shift.

[0070] 2. The photonic-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme adopts microwave optical technology, and compared with the electrical domain method, the present application has a series of advantages of optical method, such as large measurement frequency bandwidth, high measurement precision, and effective anti-electromagnetic interference by using optical fiber transmission.

[0071] 3. The photonic-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme adopts a Sagnac ring structure in the signal receiving unit. Since the two light signals in the Sagnac ring structure experience the same transmission path, there is no transmission time delay difference, and the environmental noise is the same, the system stability is stronger than that of the commonly used parallel structure.

[0072] 4. The photonic-assisted Doppler frequency shift and angle of arrival unambiguous measurement scheme separates the signal processing unit and the signal receiving unit, ensures the concealment and safety of the central station, and the link structure is simple.

[0073] Therefore, the present application has a wide application prospect in wireless communication, radar system and electronic warfare system working in high frequency domain and large bandwidth.

[0074] In order to verify the effectiveness and feasibility of the present application, the simultaneous unambiguous measurement of the Doppler frequency shift and the angle of arrival is realized by simulation.

[0075] The optical carrier frequency output by the laser source 1 is set to 193.1THz, and the line width is 1MHz; the length of the single-mode optical fiber is 20km; the reference signal frequency is set to 15.002GHz, and the echo signal frequency is set to 15.001GHz or 14.999GHz, then the frequency spectrum diagram of the low-frequency light current signal at the output end of the photodetector is as follows: Figure 2As shown in (a) and (b), when the echo signal frequency is 15.001 GHz, the measured output frequency is 1 MHz; when the echo signal frequency is 14.999 GHz, the measured frequency is 3 MHz. This is consistent with the theoretical analysis, and the size and direction of the Doppler shift are accurately determined.

[0076] When the reference signal frequency is fixed at 15 GHz, the phase difference of the two echo signals is selected as 60°, the echo signal frequency is set between 14.9999 GHz and 15.0001 GHz, and every 20 KHz is measured once, thereby simulating various situations of Doppler shift from -100 KHz to +100 KHz. The spectrum corresponding to -100 KHz to 100 KHz is observed by the spectrum analyzer, and the Doppler shift error is calculated, as shown in FIG. 3. It can be seen that the theoretical value and the simulation value are in good agreement, and the maximum measurement error is 3×10-3 Hz.

[0077] When the reference signal frequency is fixed at 15.002 GHz, the echo signal frequency is fixed at 14.999 GHz, the phase difference of the two echo signals is set from -180° to 180°, and every 18° is measured once, the power of the upper and lower two signals is measured by the electric power meter, and the theoretical value is calculated by formula (6), the simulation data and the theoretical value are compared, as shown in FIG. 4. Figure 4 It can be seen that the theoretical value and the software simulation value are in good agreement. According to the function curve between the measured power value and the phase shift, the phase difference value is solved and error analysis is performed, as shown in FIG. 5(a). According to formula (2), the angle of arrival value is solved and error analysis is performed, as shown in FIG. 5(b). The software simulation verifies the correctness and feasibility of the theoretical analysis, and the maximum angle of arrival measurement error is 0.5°.

Claims

1. A photonics-assisted Doppler frequency shift and angle of arrival ambiguity-free measurement device, comprising two main parts: a signal processing unit and a central station, characterized in that... The signal receiving unit is equipped with a continuous wave laser LD, a polarization controller 0, an optical circulator, a polarization beam splitter, a first dual-drive Mach-Zehnder modulator (100), a second dual-drive Mach-Zehnder modulator (200), a 1×2 power divider, and a phase shifter. Linearly polarized light output from a continuous-wave laser (LD) is used as an optical carrier. The polarization controller (0) receives the optical carrier output from the continuous wave laser LD and outputs linearly polarized light with a polarization state rotated by 45°. The input port (1) of the optical circulator is connected to the input end of the polarization beam splitter to receive two optical signals from the polarization controller (0); The input port of the first dual-drive Mach-Zehnder modulator (100) is connected to the left output port of the polarization beam splitter, the output port of the first dual-drive Mach-Zehnder modulator (100) is connected to the output port of the second dual-drive Mach-Zehnder modulator (200), and the input port of the second dual-drive Mach-Zehnder modulator (200) is connected to the right output port of the polarization beam splitter, forming a Sagnac ring structure. Two optical signals enter the polarization beamsplitter through the input port (1) of the optical circulator; the polarization beamsplitter separates the X and Y polarization optical signals and transmits them clockwise and counterclockwise along the Sagnac ring respectively; the first dual-drive Mach-Zehnder modulator (100) and the second dual-drive Mach-Zehnder modulator (200) each have two RF input ports and a DC bias point, and each modulator has a phase modulator PM in its upper and lower arms respectively; the first RF signal (10) and the second RF signal (20) are the echo signals received by the first antenna receiving unit e and the second antenna receiving unit f respectively, and they are respectively input to one RF input port of the first dual-drive Mach-Zehnder modulator (100) and the second dual-drive Mach-Zehnder modulator (200); the reference signal is divided into two paths with the same power and different phases through a 1×2 power divider and a phase shifter. The third radio frequency signal 30 and the fourth radio frequency signal (40) are respectively input to the other radio frequency input ports of the first dual-drive Mach-Zehnder modulator (100) and the second dual-drive Mach-Zehnder modulator (200); the X and Y polarization direction light signals output by the first dual-drive Mach-Zehnder modulator (100) and the second dual-drive Mach-Zehnder modulator (200) re-enter the polarization beam splitter to be combined into a beam of orthogonal polarization multiplexed light, which is output from the optical circulator port (3) to the single-mode fiber and then transmitted to the remote central station; Single-mode fiber is used to connect the signal receiving unit and the central station; The central station is equipped with a 1×2 optical power splitter, a first polarization controller a and a second polarization controller b, a first polarizer a and a second polarizer b, a first low-speed photodetector a and a second low-speed photodetector b, and a signal processing unit. Orthogonally polarized multiplexed light transmitted through single-mode optical fiber is split into two identical optical signals by a 1×2 optical power splitter. The upper optical signal is converted into linearly polarized light after passing through the first polarization controller a and the first polarizer a in sequence. The lower optical signal is converted into linearly polarized light after passing through the second polarization controller b and the second polarizer b in turn. The upper and lower linearly polarized light signals are converted into upper and lower intermediate frequency electrical signals after passing through the first low-speed photodetector a and the second low-speed photodetector b in turn.

2. A photonics-assisted Doppler frequency shift and angle of arrival unambiguous measurement method, based on the photonics-assisted Doppler frequency shift and angle of arrival unambiguous measurement device as described in claim 1, characterized in that, The modulated optical signal from the first dual-drive Mach-Zehnder modulator (100) is defined to operate in the X-polarization direction, and the modulated optical signal from the second dual-drive Mach-Zehnder modulator (200) is defined to operate in the Y-polarization direction; the DC bias voltage values ​​of the two modulators are adjusted so that the first dual-drive Mach-Zehnder modulator (100) and the second dual-drive Mach-Zehnder modulator (200) operate at the maximum transmission point, thereby realizing phase modulation of the received optical carrier; Equation (1) represents the expression of the light wave after modulation by the two modulators in the X and Y polarization directions: Among them, E x (t), E y (t) represent linearly polarized light in the X-polarization direction and the Y-polarization direction, respectively; E in =E0exp(jω c t) represents the optical carrier output by the laser source LD, E0 represents the amplitude of the optical carrier output by the laser source LD, and ω c ω e ω r These represent the angular frequencies of the optical carrier, the microwave signal received by the antenna, and the reference signal, respectively; m e =πV e / V π and m r =πV r / V π V represents the modulation index of the dual-drive Mach-Zehnder modulator for the echo signal and the reference signal, respectively. e V r V represents the amplitude of the received microwave signal and the reference signal, respectively. π θ represents the half-wave voltage of the dual-drive Mach-Zehnder modulator; θ represents the phase difference between the first receiving antenna element e and the second antenna element f. Let t represent the phase difference between the third radio frequency signal (30) and the fourth radio frequency signal (40); t represents time; when the unit distance between the two antennas is λ / 2, the relationship between the angle κ and the phase difference θ is: Next, the orthogonally polarized multiplexed light output from the optical circulator port (3) is transmitted to the remote central station for processing via a single-mode fiber; due to the dispersion characteristics of the optical fiber, the optical signal will be introduced into a phase shift that varies linearly with wavelength when it is transmitted in the optical fiber. At the central station, the modulated optical signal is divided into upper and lower paths by a 1×2 optical power divider. The intensity and phase of the optical wave along the X-axis and Y-axis can be changed by controlling the polarization controller and polarizer in the upper and lower paths. Under small signal conditions, the optical signals along the upper and lower paths are uniformly expressed by formula (3): J n (m) is the nth-order Bessel function of the first kind; α is the polarization angle between the polarizer's axis and the principal axis; ψ PC It is the phase shift between two orthogonal polarization states introduced by the polarization controller PC; φ +1 φ -1 These are the phase shifts introduced by the single-mode fiber for the input optical carrier and the two ±1st order sidebands, respectively. To obtain an unambiguous angle of arrival measurement, at the central station, the first polarization controller a and the first polarizer a on the upper path are controlled such that α 上 =45°, ψ PC上 =0°; Adjust the second polarization controller b and the second polarizer b in the lower circuit to make α 下 =135°, ψ PC下 =90°, the optical signal expressions along the upper and lower paths are as follows: The two optical signals output from the first polarizer a and the second polarizer b are respectively input to the first low-speed photodetector a and the second low-speed photodetector b. The function of the first low-speed photodetector a and the second low-speed photodetector b is to convert the optical signals into current signals. The photocurrent output by the low-speed photodetector is expressed as i(t)=η|E(t)| 2 Where E(t) is the photosignal intensity input to the low-speed photodetector, and it only retains the low-frequency signal component, while the high-frequency signal is filtered out; therefore, the two photocurrent signals output by the first low-speed photodetector a and the second low-speed photodetector b are respectively: Here P in This indicates the power of the incident optical carrier signal at the LD. Doppler shift is defined as the frequency difference between the echo signal and the transmitted signal, i.e., f. DFS =f e -f t , where f DFS f e f t These represent the Doppler frequency shift, echo signal frequency, and transmission signal frequency, respectively; the reference signal frequency is set to be a MHz greater than the transmission signal frequency, i.e., f. r =f t +a MHz; the Doppler frequency shift can be rewritten as f DFS =f e -f r +a MHz=-(f r -f e Since the Doppler frequency shift remains within ±1MHz, the frequency range for measuring the photocurrent signal is (-1+a)MHz to (1+a)MHz. When the frequency range for measuring the current signal is (-1+a) to a MHz, the Doppler frequency shift is positive, indicating that the target is moving closer to the receiver; when the frequency range for measuring the current signal is a MHz to (1+a)MHz, the Doppler frequency shift is negative, indicating that the target is moving away from the receiver. Therefore, by measuring the frequency of the current signal, not only can the magnitude of the Doppler frequency shift be determined, but the relative motion direction of the target can also be measured. The power expressions for the upper and lower photocurrent signals output by the low-speed photodetector, calculated using formula (5), are as follows: As can be seen from formula (6), the power of the generated low-frequency signal is a function of the phase difference θ; therefore, by measuring the power of the photocurrent signal, the phase difference can be estimated, and the angle value can be obtained by formula (2).

3. The photonics-assisted Doppler frequency shift and angle of arrival unambiguous measurement method as described in claim 2, characterized in that, a＝2。

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