High-precision microwave photon arrival angle measurement system for suppressing phase noise

Abstract

The invention discloses a high-precision microwave photon arrival angle measurement system capable of suppressing phase noise, and relates to the technical field of microwave photonics. The device is composed of a laser, a first optical coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a single-mode fiber, a signal generator, an electric coupler, an electric phase shifter, a first direct-current voltage source, a second direct-current voltage source, a second optical coupler, a photoelectric detector and an electric spectrum analyzer. A microwave photon arrival angle measurement link is combined with an optical delay line, so that a high-precision microwave photon arrival angle measurement system capable of suppressing phase noise is realized; two echo signals are received through Mach-Zehnder modulators of an upper branch and a lower branch, a mapping curve of phase difference and output power is established, and arrival angle measurement is achieved; optical signals are transmitted through optical fibers, time delay is generated, interference of phase noise on phases is counteracted in an optical domain, the measurement error of the angle of arrival is reduced, and high-precision measurement of the angle of arrival is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of microwave photonics, and particularly relates to a high-precision microwave photonics angle of arrival measurement system capable of suppressing phase noise. BACKGROUND

[0002] The microwave photonics angle of arrival measurement system refers to applying a received echo signal to a modulator to realize the bearing of phase difference information between the echo signal and the light wave, measuring the power of the output sideband of the system, obtaining the phase difference between the echo signals by using the mapping relationship between the power and the phase difference, and calculating the angle of arrival by using angle conversion. Compared with the traditional electrical domain technology, the microwave photonics angle of arrival measurement technology has the advantages of large bandwidth, high working frequency band and strong anti-electromagnetic interference capability.

[0003] The capability of microwave angle-of-arrival measurement system is measured by measurement error, which is one of the important indicators of the accuracy of the measurement system. The classical method of angle-of-arrival measurement is to establish the mapping relationship curve between the phase difference of two echo signals and the power of the output signal of the measurement system, and then estimate and calculate the angle-of-arrival. Cao Zizheng et al. proposed a new type of phase modulation parallel optical delay detector for angle-of-arrival measurement, which uses an automatic bias control circuit to reduce the DC drift. In the range of 5° to 165° of phase difference, the measurement error is less than 3.1° (Cao, Zizheng, et al. "Phase modulation parallel optical delay detector for microwave angle-of-arrival measurement with accuracy monitored." Optics Letters 39.6 (2014): 1497-1500). Lin Tao et al. designed an L-shaped antenna array connected with a dual-polarization binary phase shift keying modulator to measure the angle-of-arrival. The amplitude comparison function makes the system response independent of the received signal power, so the system has the ability to resist signal power fluctuations. The measurement error is less than 1° (Lin, Tao, et al. "Photonic 2-D angle-of-arrival estimation based on an L-shaped antenna array for an early radar warning receiver." Optics Express 28.26 (2020): 38960-38972). Zhao Jianing et al. independently mixed two echo signals through a double-channel microwave photonic mixer, and introduced a bias controller and a temperature controller to reduce the measurement error. In the range of 5° to 165° of angle-of-arrival, the measurement error is less than ±1.3° (Zhao, Jianing, Zhenzhou Tang, and Shilong Pan. "Photonic approach for simultaneous measurement of microwave DFS and AOA." Applied Optics 60.16 (2021): 4622-4626).Li Yan et al. studied a kind of using double parallel double drive Mach-Zehnder modulator to realize the angle of arrival measurement system, through the construction of two echo signal phase difference and power mapping curve solves the problem of phase ambiguity, the measurement error is less than ± 0.2°(Li, Yan, et al."Single-channel system for joint unambiguous measurement of DFS and AOA based on serrodyne modulation."Applied Optics 62.26(2023):6924-6930).

[0004] How to improve the measurement accuracy of the existing microwave photon angle of arrival measurement system, so that the measurement system improves the anti-interference ability, is the problem that the person skilled in the art needs to solve. SUMMARY

[0005] The main purpose of the present application is to provide a high-precision microwave photon angle of arrival measurement system for suppressing phase noise, which can effectively solve the problems in the background art.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is:

[0007] The purpose of the present application is to provide a high-precision microwave photon angle of arrival measurement system for suppressing phase noise. By using optical delay line technology, the phase noise of the microwave photon angle of arrival measurement system is compensated in the optical domain, and the interference of the phase noise on the echo signal phase is suppressed.

[0008] The high-precision microwave photon angle of arrival measurement system for suppressing phase noise according to the present application is composed of a laser, a first optical coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a single-mode optical fiber, a signal generator, an electrical coupler, an electrical phase shifter, a first DC voltage source, a second DC voltage source, a second optical coupler, a photodetector and an electrical spectrum analyzer.

[0009] In the present application, the light emitted by the laser is divided into two beams by the first optical coupler and input into the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively. The microwave signal generated by the signal generator is divided into two paths by the electrical coupler, one of which is introduced into a phase shift by the electrical phase shifter, and the two microwave signals with phase difference generated are input into the first Mach-Zehnder modulator of the upper branch and the second Mach-Zehnder modulator of the lower branch respectively. The Mach-Zehnder modulators of the upper and lower branches are all modulated at the minimum bias point, and the modulated light signals output by the upper and lower branches enter the single-mode optical fiber and then pass through the second optical coupler to synthesize a light signal. After the photoelectric conversion by the photodetector, the output signal is input into the electrical spectrum analyzer, and the power and phase noise power spectral density of the output signal are measured by the electrical spectrum analyzer.

[0010] In the present application, for a given length of optical fiber, the corresponding time delay will periodically appear zero point at the frequency offset, at which the phase noise power spectral density in the system is periodically suppressed. Setting different lengths of single-mode optical fiber will introduce different time delays in the optical domain. The phase noise power spectral density after introducing time delay responds at the system frequency offset. Larger time delay corresponds to lower offset frequency response, and shorter delay corresponds to higher offset frequency response. If the length of the single-mode optical fiber is set to 2km, the time delay added to the optical signal is t d =10μs, the high-precision microwave photon angle of arrival measurement system with suppressed phase noise measured by the electrical spectrum analyzer has a phase noise power spectral density of 125.46dBc / Hz@100kHz, which is 8.31dB lower than the phase noise power spectral density of the output signal of the angle of arrival measurement system without phase noise suppression.

[0011] First, the angle of arrival measurement system without phase noise suppression is analyzed. The working principle of the system is as follows: the laser outputs an optical signal with a frequency of f0, which is input to a double-drive Mach-Zehnder modulator. The microwave signal generated by the signal generator is divided into two paths through an electrical coupler, one of which introduces a phase shift through an electrical phase shifter. The two echo signals with phase difference are input to the upper and lower RF input ports of the double-drive Mach-Zehnder modulator. The voltage output by the DC voltage source is set to make the double-drive Mach-Zehnder modulator work at the minimum transmission point, and the generated modulation signal is a suppressed carrier double sideband modulation signal. The output signal of the modulator is input to the photodetector, and after the photoelectric conversion of the photodetector, it is input to the electrical spectrum analyzer, and the power and phase noise power spectral density of the output signal are measured by the electrical spectrum analyzer.

[0012] For comparison and analysis, the laser, signal generator, electrical coupler, electrical phase shifter, DC voltage source, photodetector, and electrical spectrum analyzer in the angle of arrival measurement system without phase noise suppression are the same as those in the high-precision microwave photon angle of arrival measurement system with phase noise suppression proposed in the present application. The optical carrier generated by the laser and the microwave signal generated by the signal generator are also the same. Specifically, the two echo signals with phase difference output by the signal generator through the electrical phase shifter are represented as:

[0013] V1(t)=V e cos(ω e t+φ e1 (t))

[0014] V2(t)=V e cos(ω e t+θ+φ e2 (t))

[0015] where Ve and ω e are the amplitudes and frequencies of the echo signals, and θ is the phase difference between the echo signals. e1 (t) and φ e2 (t) are the phase noises of the two echo signals, respectively.

[0016] The echo signals are applied to the upper and lower ports of the dual-drive Mach-Zehnder modulator for modulation, and the expression of the output modulated optical signal is:

[0017]

[0018] where ω0= 2πf0is the angular frequency of the optical carrier, E0is the intensity of the optical carrier, and φ0is the phase noise generated when the laser output optical carrier. n is the n-th order Bessel function of the first kind (n = 0, ±1), is the modulation index of the dual-drive Mach-Zehnder modulator, V e is the direct current bias voltage applied to the dual-drive Mach-Zehnder modulator, V π is the half-wave voltage of the modulator, is the phase change caused by the dual-drive Mach-Zehnder modulator.

[0019] After the output signal of the dual-drive Mach-Zehnder modulator is converted by the photodetector, the output electrical signal current is:

[0020]

[0021] where E(t) * is the conjugate of the output signal of the dual-drive Mach-Zehnder modulator, is the responsivity of the photodetector, and the output signal power measured by the electrical spectrum analyzer is:

[0022]

[0023] The phase noise of the output signal of the angle of arrival measurement system without phase noise suppression is represented by the power spectral density S φ1 (f).

[0024] S φ1 (f) = S e1 (f) + S e2 (f).

[0025] where S e1 (f) and S e2 (f) are the power spectral densities of the phase noises φ e1 (t) and φ e2 (t) in the two echo signals, respectively.

[0026] The working principle of the high-precision microwave photon angle of arrival measurement system for suppressing phase noise is as follows: an optical signal with a frequency f0 is output by a laser, is divided into two beams by a first optical coupler, and is input into a first Mach-Zehnder modulator of an upper branch and a second Mach-Zehnder modulator of a lower branch, respectively; a direct current bias voltage output by a first direct current voltage source and a second direct current voltage source is adjusted, so that the Mach-Zehnder modulators of the upper branch and the lower branch work at a minimum bias working point, and double sideband modulation of a carrier is realized.

[0027] V1(t) = V e cos(ω e t+φ e1 (t))

[0028] V2(t) = V e cos(ω e t+θ+φ e2 (t))

[0029] Wherein V e and ω e are the amplitude and frequency of the echo signal, θ is the phase difference between the echo signals, φ e1 (t) and φ e2 (t) are the phase noises in the two echo signals, respectively.

[0030] The optical signal generated after modulation of the first Mach-Zehnder modulator of the upper branch enters an optical fiber, and the optical signal generated after modulation is generated after time delay caused by optical fiber transmission, and the expression of the optical signal generated after modulation is as follows:

[0031]

[0032] Wherein ω0=2πf0 is the angular frequency of the optical carrier, E0 is the intensity of the optical carrier, and φ0 is the phase noise generated when the laser outputs the optical carrier. J n is the n-order first Bessel function (n=0, ±1), is the modulation index of the first Mach-Zehnder modulator, V e is the direct current bias voltage applied to the first Mach-Zehnder modulator, V π is the half-wave voltage of the first Mach-Zehnder modulator, is the phase change caused by the first Mach-Zehnder modulator, τ d is the time delay introduced by the optical fiber.

[0033] The optical signal generated after modulation of the second Mach-Zehnder modulator of the lower branch enters an optical fiber, and the optical signal generated after modulation is generated after time delay caused by optical fiber transmission, and the expression of the optical signal generated after modulation is as follows:

[0034]

[0035] J n is the n-th order first kind Bessel function (n = 0, ±1), is the modulation index of the second Mach-Zehnder modulator, V e is the direct current bias voltage applied on the second Mach-Zehnder modulator, V π is the half-wave voltage of the modulator second Mach-Zehnder modulator. is the phase change caused by the second Mach-Zehnder modulator. The optical signals outputted by the time-delayed upper and lower branches are combined into one optical signal by the second optical coupler, and the expression of the optical signal is:

[0036]

[0037] The combined optical signal is inputted into the photodetector, and after photoelectric conversion, the outputted electrical signal current is:

[0038]

[0039] where E MZMS (t) * is the conjugate of the output signal, is the responsivity of the photodetector, and the power expression of the output signal measured by the electrical spectrum analyzer is:

[0040]

[0041] The phase noise φ e1 (t) and φ e2 (t) of the echo signal are independent of each other, and the autocorrelation functions of the electrical signals generated by the outputted optical signals after the upper and lower branch modulation signals are added to the optical fiber are respectively:

[0042]

[0043] where τ d is the time delay introduced by the optical fiber, R φ1 (0) and R φ2 (0) are the maximum values of the autocorrelation functions, representing the maximum similarity of the signal itself with itself. S V (f) is the phase noise power spectral density of the electrical signal, S e1 '(f) and S e2 '(f) are the Fourier transforms of the autocorrelation functions R e1 (τ) and R e2 (τ), respectively, and S e1 '(f) and S e2 '(f) are expressed as:

[0044]

[0045] where F[R e1 (τ)] and F[R e2 (τ)] are the Fourier transforms of R e1 (τ) and R e2 (τ), respectively, δ(f) is the impulse function, S e1 (f) and S e2 (f) are the power spectral densities of φ e1 (t) and φ e2 (t), respectively. After adding the time delay, the phase noise power spectral density of the output signal of the angle of arrival measurement system is:

[0046] S φ2 (f) = cos 2 (πτ d f) S e1 (f) + cos 2 (πτ d f) S e2 (f)

[0047] In summary, the phase noise power spectral density of the output signal of the high-precision microwave photonic angle of arrival measurement system with phase noise suppression proposed by the application is cos 2 (πτ d f) S e1 (f) + cos 2 (πτ d f) S e2 (f), the phase noise power spectral density of the output signal of the angle of arrival measurement system without phase noise suppression is S e1 (f) + S e2 (f), compared with the angle of arrival measurement system without phase noise suppression, the phase noise power spectral density of the output signal of the measurement system has the suppression of the cos 2 term after adding the optical fiber, and cos 2 is always less than 1, so the phase noise power spectral density of the output signal of the high-precision microwave photonic angle of arrival measurement system with phase noise suppression proposed by the application is less than the phase noise power spectral density of the output signal of the angle of arrival measurement system without phase noise suppression.

[0048] The mapping relationship curve between the power and the phase difference of the echo signal is obtained by measuring the power of the output signal, and then the angle of arrival of the echo signal is estimated from the relationship between the phase difference and the angle of arrival, and the angle of arrival is represented by α. The expression of the angle of arrival of the echo signal is:

[0049]

[0050] Wherein c is the speed of light, lambda is the wavelength of two echo signals, tau is the time delay of two echo signals. Therefore, the determination of the phase difference theta between the echo signals is the key to estimate the angle of arrival. It can be seen from the output power expression that the mapping relationship curve of the phase difference is obtained by measuring the power of the signal through the electrical spectrum analyzer, the angle of arrival can be calculated from the phase difference, and the measured electrical power is interfered by the phase of the phase noise. Therefore, by suppressing the phase noise, the error of the angle of arrival measurement can be reduced, and the accuracy of the angle of arrival measurement system can be improved.

[0051] The application selects a laser with a wavelength of 1550nm (corresponding to a frequency f0=193414.489GHz); the bandwidth of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator is 20GHz, and the half-wave voltage is 6.5V; the responsivity and bandwidth of the photodetector are 0.8A / W and 40GHz respectively; the microwave signal frequency generated by the signal generator is f e =5GHz, the corresponding angular frequency omega e =2pif e ; the length of the single-mode optical fiber is 2000 meters, the loss is 0.2dB / km, and the corresponding time delay is tau d =10mu; the frequency range of the spectrum analyzer is 10Hz-40GHz.

[0052] Compared with the prior art, the application has the following beneficial effects

[0053] The application combines the microwave photon angle of arrival measurement link with the optical delay line to realize a high-precision microwave photon angle of arrival measurement system for suppressing phase noise.

[0054] The application receives two echo signals through the Mach-Zehnder modulators of the upper and lower branches, establishes a mapping curve of the phase difference and the output power, and realizes the angle of arrival measurement.

[0055] The application generates a time delay through the optical fiber transmission of the optical signal, cancels the interference of the phase noise on the output signal in the optical domain, reduces the angle of arrival measurement error, and realizes high-precision angle of arrival measurement. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 Fig. 1 is a structural schematic diagram of a high-precision microwave photon angle of arrival measurement system for suppressing phase noise;

[0057] Figure 2 Fig. 2 is a structural schematic diagram of an angle of arrival measurement system without suppressing phase noise;

[0058] Figure 3 Fig. 3 is a power spectrum response curve of time delay and frequency offset;

[0059] Figure 4Comparison curves of experimental values ​​of phase noise power spectral density of microwave photon angle of arrival measurement system;

[0060] Figure 5 The mapping curve of the output power of the angle of arrival measurement system without suppressed phase noise versus the phase difference of the echo signal;

[0061] Figure 6 The mapping curve between the output power and the phase difference of the echo signal of a high-precision microwave photon angle of arrival measurement system with phase noise suppression;

[0062] Figure 7 Error curves between the estimated angle of arrival and the actual angle of arrival for an angle of arrival measurement system without suppressing phase noise;

[0063] Figure 8 Error curves between the estimated angle of arrival and the actual angle of arrival of a high-precision microwave photon angle of arrival measurement system that suppresses phase noise. Detailed Implementation

[0064] Example 1: The laser frequency is set to f0 = 193414.489 GHz. The selected single-mode fiber is 2000 meters long with a loss of 0.2 dB / km, and the corresponding time delay is τ. d =10μs. The frequency of the echo signal output by the signal generator is f. e =5GHz, corresponding to angular frequency ω e =2πf e The laser is a Santec TSL-510 tunable laser with a wavelength set to 1550nm (corresponding to a frequency of 193414.489GHz); the electrical spectrum analyzer is a Keysight N9010A spectrum analyzer with a frequency range of 10Hz to 40GHz; the photodetector is from Tektronix with a bandwidth of 20GHz; and the single-mode optical fiber is from Yangtze Optical Fibre and Cable Joint Stock Limited Company, with a length of 2000 meters, a loss of 0.2dB / km, and a corresponding time delay of τ. d =10μs; the signal generator is Keysight E8257D; the Mach-Zehnder modulator is Photline MXAN-LN-20 with a bandwidth of 20GHz and a half-wave voltage of 6.5V;

[0065] See Figure 1 As shown, the present invention discloses a high-precision microwave photon angle of arrival measurement system for suppressing phase noise, comprising: a laser, a first optical coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a single-mode optical fiber, a signal generator, an electrical coupler, an electrical phase shifter, a first DC voltage source, a second DC voltage source, a second optical coupler, a photodetector, and an electrical spectrum analyzer.

[0066] Referring to Figure 1 As shown, the corresponding instrument equipment of the high-precision microwave photon angle of arrival measurement system connected to the phase noise: the frequency of the laser is set to f0=193414.489 GHz, the power is 10 dBm, the light emitted by the laser is divided into two beams of light through the first optical coupler and input into the first Mach-Zehnder modulator and the second Mach-Zehnder modulator. The signal generator generates a frequency of f e =5 GHz, corresponding to the angular frequency ω e =2πf e , the power is 3 dBm, the echo signal is divided into two paths through the electrical coupler, one of which passes through the electrical phase shifter to introduce a phase shift, and the phase shift is added in the range of 0° to 180° with a step of 10 degrees, generating two echo signals with a phase difference, which are input into the first Mach-Zehnder modulator and the second Mach-Zehnder modulator, respectively. The output voltage of the first DC voltage source and the second DC voltage source is set to 6.5 V, at this time, the operating point of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator is the minimum transmission point, and the output signal is a suppressed carrier double sideband modulation signal. After the modulation light signals output by the upper and lower branches enter a 2000-meter single-mode optical fiber, they are combined into a light signal through the second optical coupler and input into the photodetector, and then input into the electrical spectrum analyzer after photoelectric conversion by the photodetector. The power and phase noise power spectral density of the output signal are measured by the electrical spectrum analyzer.

[0067] Specifically, the signal generator generates two echo signals with a phase difference, which are represented as:

[0068] V1(t) = V e cos(ω e t + φ e1 (t))

[0069] V2(t) = V e cos(ω e t + θ + φ e2 (t))

[0070] where V e and ω e are the amplitudes and frequencies of the echo signals, θ is the phase difference between the echo signals. φ e1 (t) and φ e2 (t) are the phase noises in the two received echo signals, respectively.

[0071] The first Mach-Zehnder modulator of the upper branch receives the light signal generated after the echo signal is modulated and enters the optical fiber, and after time delay through the optical fiber transmission, the expression of the modulated light signal is:

[0072]

[0073] where ω0=2πf0 is the optical carrier frequency, E0is the optical carrier intensity, and φ0is the phase noise produced at the laser output. J n is the n-th order Bessel function of the first kind (n=0,±1), is the modulation index of the first Mach-Zehnder modulator, V e is the direct current bias voltage applied to the first Mach-Zehnder modulator, V π is the half-wave voltage of the modulator. is the phase change induced by the first Mach-Zehnder modulator, τ d is the time delay introduced by the optical fiber.

[0074] The second Mach-Zehnder modulator in the lower branch receives the optical signal produced by the backscattered signal modulation, and after the time delay introduced by the optical fiber, the expression of the modulated optical signal is:

[0075]

[0076] J n is the n-th order Bessel function of the first kind (n=0,±1), is the modulation index of the second Mach-Zehnder modulator, V e is the direct current bias voltage applied to the second Mach-Zehnder modulator, V π is the half-wave voltage of the modulator. is the phase change induced by the second Mach-Zehnder modulator. The modulated optical signals output by the upper and lower branches are combined into one optical signal by the second optical coupler, and the output of the optical signal is:

[0077]

[0078] The combined optical signal is sent to the photodetector, and after photoelectric conversion, the output electrical signal current is:

[0079]

[0080] where E MZMS (t) * is the conjugate of the output signal, is the responsivity of the photodetector, and the output power measured by the electrical spectrum analyzer is:

[0081]

[0082] The phase noise power spectral density S φ1 (f) of the system output signal is represented as:

[0083] S φ1 (f) = cos 2 (πτ df)S e1 (f)+cos 2 (πτ d f)S e2 (f)

[0084] Among them, S e1 (f) and S e2 (f) represents the phase noise φ in the two echo signals. e1 (t) and φ e2 The power spectral density of (t).

[0085] To verify the anti-interference advantage of the high-precision microwave photon angle of arrival measurement system with suppressed phase noise proposed in this invention, an angle of arrival measurement system without suppressed phase noise was used for comparative analysis. See also Figure 2 As shown, the angle of arrival measurement system without suppressed phase noise includes: a laser, a dual-drive Mach-Zehnder modulator, a signal generator, an electric phase shifter, a DC voltage source, a photodetector, and an electrical spectrum analyzer.

[0086] See Figure 2 As shown, the corresponding instruments and equipment for the unsuppressed phase noise angle of arrival measurement system are as follows: The laser frequency is set to f0 = 193414.489 GHz, and the power is 10 dBm. The light emitted by the laser is input to a dual-drive Mach-Zehnder modulator. The signal generator produces a signal at a frequency of f0. e =5GHz, corresponding to angular frequency ω e =2πf e A microwave signal with a power of 3dBm is split into two paths by an electro-coupler. One path introduces phase shift through an electro-phase shifter, adding phase shift in 10-degree steps within the range of 0° to 180°, generating two echo signals with a phase difference. These signals are then input to the upper and lower ports of a dual-drive Mach-Zehnder modulator. The DC power supply output voltage is set to 3.5V, and the operating point of the dual-drive Mach-Zehnder modulator is set to the minimum transmission point. The output signal is a suppressed-carrier double-sideband modulated signal. The modulator output signal is input to a photodetector, and after photoelectric conversion by the photodetector, it is input to an electrical spectrum analyzer. The electrical spectrum analyzer measures the power of the output signal and the phase noise power spectral density of the output signal.

[0087] For comparative analysis, the laser, signal generator, electrical coupler, electrical phase shifter, DC voltage source, photodetector, and electrical spectrum analyzer in the unsuppressed phase noise angle of arrival measurement system are identical to those in the high-precision microwave photon angle of arrival measurement system with suppressed phase noise proposed in this invention. The optical carrier generated by the laser and the microwave signal generated by the signal generator are also identical. Specifically, the two echo signals with a phase difference output by the electrical phase shifter generated by the signal generator are represented as follows:

[0088] V1(t)=Ve cos(ω e t+φ e1 (t))

[0089] V2(t)=V e cos(ω e t+θ+φ e2 (t))

[0090] where V e and ω e are the amplitudes and frequencies of the echo signals, θ is the phase difference between the echo signals. φ e1 (t) and φ e2 (t) are the phase noise contained in the two received echo signals, respectively.

[0091] The received echo signals are applied to the upper and lower ports of the dual-drive Mach-Zehnder modulator for modulation, and the expression of the output modulated optical signal is:

[0092]

[0093] where ω0=2πf0 is the optical carrier frequency, E0 is the optical carrier intensity, and φ0 is the phase noise generated when the laser output optical carrier. J n is the first kind of Bessel function of order n (n=0, ±1), is the modulation index of the dual-drive Mach-Zehnder modulator, V e is the direct current bias voltage applied to the dual-drive Mach-Zehnder modulator, V π is the half-wave voltage of the modulator, is the phase change caused by the dual-drive Mach-Zehnder modulator.

[0094] The output signal of the dual-drive Mach-Zehnder modulator is converted by the photodetector, and the output electrical signal current is:

[0095]

[0096] where E(t) * is the conjugate of the output signal of the dual-drive Mach-Zehnder modulator, is the responsivity of the photodetector, and the output power measured by the electrical spectrum analyzer is:

[0097]

[0098] The phase noise of the output signal of the angle of arrival measurement system without phase noise suppression is represented by the power spectral density S φ2 (f) as:

[0099] S φ2 (f) = S e1 (f) + Se2 (f)

[0100] wherein S e1 (f) and S e2 (f) are the phase noise φ e1 (t) and φ e2 (t) of the two echo signals, respectively.

[0101] Referring to Figure 3 The power spectrum response curve of time delay and frequency offset, adding a given length of optical fiber in the angle of arrival measurement system, corresponding to introduce a fixed time delay, will cause the phase noise power spectrum response of the system to appear zero periodically at the frequency offset, that is, the power spectrum density of the phase noise of the angle of arrival measurement system is periodically suppressed. The single-mode optical fiber set corresponds to introduce a time delay of 0.001 μs, 1 μs and 10 μs in the optical domain, and the phase noise power spectrum density after introducing the time delay responds at the corresponding frequency offset. Larger time delay corresponds to lower offset frequency response, and shorter delay corresponds to higher offset frequency response.

[0102] Referring to Figure 4 The experimental value comparison curve of time delay and phase noise power spectrum density of the measurement system. The phase noise power spectrum density of the high-precision microwave photon angle of arrival measurement system for suppressing phase noise is obviously suppressed at the frequency offset f = 100 kHz, and the system phase noise power spectrum density obtained by the electrical spectrum analyzer is 125.46 dBc / Hz@100kH. Compared with the angle of arrival measurement system without adding optical fiber and without suppressing phase noise, the phase noise power spectrum density of the high-precision microwave photon angle of arrival measurement system for suppressing phase noise is suppressed by 8.31 dB, and the phase noise is periodically suppressed.

[0103] Referring to Figure 5 The mapping curve of the output power of the angle of arrival measurement system without suppressing phase noise and the phase difference of the echo signal. Since there is no time delay introduced, the phase noise is not suppressed, and in the phase difference measurement range of 0° to 180°, the phase difference measurement error is about ±4.6°. Referring to Figure 6 The mapping curve of the output power of the high-precision microwave photon angle of arrival measurement system for suppressing phase noise and the phase difference of the echo signal. Since the time delay is introduced, the effect of PN in the optical domain is effectively eliminated, and in the phase difference measurement range of 0° to 180°, the phase difference measurement error is less than ±3°. The experimental results prove that the present application can reduce the error in phase difference measurement by suppressing PN, and further improve the accuracy of AOA estimation, realizing high-precision AOA measurement.

[0104] Referring to Figure 7The error curve of the estimated angle of arrival and the actual angle of arrival of the arrival angle measurement system without phase noise suppression. The estimated AOA result of the DDMZM-based arrival angle measurement system without adding delay, within the AOA measurement range of 0° to 90°, the AOA measurement error is less than ±2.3°. See Figure 8 The error curve of the estimated angle of arrival and the actual angle of arrival of the high-precision microwave photon arrival angle measurement system with phase noise suppression. The AOA estimation result of the proposed high-precision arrival angle measurement system based on phase noise suppression, within the AOA measurement range of 0° to 90°, the AOA measurement error is less than ±1.5°. The AOA measurement error of the high-precision microwave photon arrival angle measurement system with phase noise suppression proposed by the present application is significantly reduced, and the experiment verifies that the phase noise of the measurement system with the added optical fiber is smaller. Adding optical delay line effectively suppresses the interference of phase noise on the phase of the output signal of the measurement system, and the arrival angle measurement system with phase noise suppression can improve the precision of the measurement system.

[0105] The basic principles and main features of the present application and the advantages of the present application are shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A high-precision microwave photonic angle of arrival measurement system that suppresses phase noise, characterized by: For a given length of optical fiber, the corresponding time delay will appear zero point periodically at the frequency offset, at which the phase noise power spectral density of the output signal in the system is periodically suppressed; setting different lengths of single-mode optical fiber will introduce different time delays in the optical domain, and the phase noise power spectral density after introducing the time delay responds at the system frequency offset; the larger the time delay, the lower the offset frequency response, and the shorter the delay, the higher the offset frequency response; setting the length of the single-mode optical fiber to 2 km, the time delay added to the optical signal is t d = 10 μs, and the output signal phase noise power spectral density of the high-precision microwave photonic angle of arrival measurement system measured by the electrical spectrum analyzer is 125.46 dBc / Hz@100 kHz, which is 8.31 dB lower than the phase noise power spectral density of the output signal of the angle of arrival measurement system without phase noise suppression. The light signal with the laser output frequency f0 is divided into two beams by the first optical coupler, and the two beams are input into the first Mach-Zehnder modulator of the upper branch and the second Mach-Zehnder modulator of the lower branch; the DC bias voltage output by the first DC voltage source and the second DC voltage source is adjusted to make the Mach-Zehnder modulators of the upper branch and the lower branch work at the minimum bias operating point, so that the carrier-suppressed double sideband modulation is realized; the signal generator generates two echo signals with a phase difference, which are represented as: V1(t) = V e cos(ω e t + φ e1 (t)) V2(t) = V e cos(ω e t + θ + φ e2 (t)) where V e and ω e are the amplitudes and frequencies of the echo signals, θ is the phase difference between the echo signals; φ e1 (t) and φ e2 (t) are the phase noise in the two received echo signals, respectively; The light signal generated after the first Mach-Zehnder modulator of the upper branch receives the echo signal is input into the optical fiber, and the expression of the modulated light signal after the time delay caused by the optical fiber transmission is: where ω0= 2πf0is the optical carrier frequency, E0is the optical carrier intensity, and φ0is the phase noise generated by the laser output optical carrier; J n is the n-th order first kind Bessel function (n = 0, ±1), is the modulation index of the first Mach-Zehnder modulator, V e is the direct current bias voltage applied to the first Mach-Zehnder modulator, V π is the half-wave voltage of the first Mach-Zehnder modulator, is the phase change caused by the first Mach-Zehnder modulator, τ d is the time delay introduced by the optical fiber; The light signal generated after the second Mach-Zehnder modulator of the lower branch receives the echo signal is input into the optical fiber, and the expression of the modulated light signal after the time delay caused by the optical fiber transmission is: J n is the n-th order first kind Bessel function (n = 0, ±1), is the modulation index of the second Mach-Zehnder modulator, V e is the direct current bias voltage applied on the second Mach-Zehnder modulator, V π is the half-wave voltage of the modulator second Mach-Zehnder modulator; is the phase change caused by the second Mach-Zehnder modulator; the optical signals outputted by the upper and lower branches after time delay are combined into one optical signal by the second optical coupler, and the expression of the optical signal is: The synthesized light signal is input into the photodetector, and the output electric signal current after the conversion by the photodetector is: where E MZMS (t) * is the conjugate of the output signal, is the responsivity of the photodetector, and the output power measured by the electrical spectrum analyzer is expressed as: The mapping relationship curve between the power and the phase difference of the echo signal is obtained by measuring the power of the output signal, and then the arrival angle of the echo signal is estimated according to the relationship between the phase difference and the arrival angle, and the arrival angle is represented as: Where c is the speed of light, λ is the wavelength of the two echo signals, and τ is the time delay of the arrival of the two echo signals; the mapping relationship curve between the phase difference and the arrival angle is obtained by measuring the power of the signal by the electric spectrum analyzer; Output signal phase noise φ of high-precision microwave photon angle of arrival measurement system for suppressing phase noise e1 (t) and φ e2 (t) are independent of each other, and the autocorrelation functions of the electrical signals obtained by adding the modulation signals of the upper and lower branches to the output optical signals of the optical fiber are: where τ d is the time delay introduced by the optical fiber, R φ1 (0) and R φ2 (0) are the maximum values of the autocorrelation functions, indicating the maximum similarity of the signal itself with itself; S V (f) is the phase noise power spectral density of the electrical signal, S e1 '(f) and S e2 '(f) are the Fourier transforms of the autocorrelation functions R e1 (τ) and R e2 (τ), respectively, S e1 '(f) and S e2 '(f) can be expressed as: where F[R e1 (τ)] and F[R e2 (τ)] are the Fourier transforms of R e1 (τ) and R e2 (τ), respectively, δ(f) is the impulse function, S e1 (f) and S e2 (f) are the power spectral densities of the phase noise φ e1 (t) and φ e2 (t), respectively; and the phase noise power spectral density of the output signal of the angle of arrival measurement system, after adding the time delay, is: S φ2 (f) = cos 2 (πτ d f) S e1 (f) + cos 2 (πτ d f) S e2 (f) The phase noise power spectral density of the output signal of the high-precision microwave photonic angle of arrival measurement system with suppressed phase noise is cos 2 (πτ d f)S e1 (f)+cos 2 (πτ d f)S e2 (f), the phase noise power spectral density of the output signal of the angle of arrival measurement system without suppressed phase noise is S e1 (f)+S e2 (f), compared to the angle of arrival measurement system without suppressed phase noise.

2. The high-precision microwave photonic angle of arrival measurement system of claim 1, wherein: The system structure is composed of a laser, a first optical coupler, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a single-mode optical fiber, a signal generator, a DC voltage source, a second optical coupler, a photodetector, and an electric spectrum analyzer; the light emitted by the laser is divided into two beams by the first optical coupler, and the two beams are input into the first Mach-Zehnder modulator and the second Mach-Zehnder modulator; the signal generator generates a microwave signal, which is divided into two paths by the electric coupler, one of which is introduced into a phase shift by the electric phase shift, and two echo signals with a phase difference are generated and input into the first Mach-Zehnder modulator of the upper branch and the second Mach-Zehnder modulator of the lower branch; the Mach-Zehnder modulators of the upper branch and the lower branch are all minimum bias point modulators, and the modulated light signals output by the upper branch and the lower branch are input into the single-mode optical fiber, and then the second optical coupler synthesizes a light signal, which is input into the photodetector after the photoelectric conversion, and then input into the electric spectrum analyzer; the power of the output signal and the phase noise power spectral density of the output signal are measured by the electric spectrum analyzer.

3. The high-precision microwave photonic angle of arrival measurement system of claim 2, wherein: The laser is selected with wavelength of 1550 nm (corresponding to frequency f0=193414.489 GHz); the bandwidth of the first and second Mach-Zehnder modulators is 20 GHz, and the half-wave voltage is 6.5 V; the responsivity and bandwidth of the photodetector are 0.8 A / W and 40 GHz respectively; the frequency of the microwave signal generated by the signal generator is f e =5 GHz, and the corresponding angular frequency ω e =2πf e ; the length of the single-mode optical fiber is 2000 meters, the loss is 0.2 dB / km, and the corresponding time delay is τ d =10 μ; the frequency range of the spectrum analyzer is 10 Hz-40 GHz.

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