Dual polarization dual electrode mzm based dfs and aoa simultaneous measurement method and system

Abstract

This invention provides a method and system for simultaneous measurement of Doppler frequency shift (DFS) and angle of arrival (AOA) based on a dual-polarization-dual-electrode Mach-Zehnder modulator (Dpol-DDMZM). The Dpol-DDMZM modulates the echo signals and local oscillator signals received from two antennas in parallel onto an optical wave in OCS mode. After a 90° polarization rotation and orthogonal polarization combining, the resulting polarization-multiplexed optical wave carries the echo and local oscillator signals. These signals are then filtered out by an optical bandpass filter (OBPF) to remove the positive first-order sidebands, and demultiplexed into two optical signals by a polarization beamsplitter (PBS). These signals are then converted into photoelectric signals and differentially processed by a balanced detector (BPD). The output photocurrent contains information on the Doppler frequency shift (DFS) and angle of arrival (AOA). This scheme utilizes photocurrent phase-AOA mapping to achieve simultaneous measurement of DFS and AOA. Furthermore, the modulator operates in OCS mode, improving the measurement accuracy of DFS and AOA. The system has a simple structure and high stability.

Description

Technical Field

[0001] This invention relates to the field of optical communication, and more particularly to the field of radar signal parameter measurement based on microwave photonics. It can simultaneously measure the Doppler Frequency Shift (DFS) and Angle of Arrival (AOA) of radar echo signals using microwave photonics methods, thereby improving the accuracy of radar signal measurement and simplifying the measurement structure. Background Technology

[0002] DFS and AOA are commonly used radar signal parameters. The Doppler effect is caused by the relative motion between the object being measured and the wave source. When a radio signal is reflected by a moving object, the reflected echo signal has a frequency shift relative to the transmitted signal. This phenomenon is called the Doppler effect, and the resulting frequency shift is called the Doppler frequency shift (DFS). The magnitude of the frequency shift is proportional to the relative velocity of the transmitting signal source and the object along the radial direction. When the two are close to each other, the echo signal frequency increases, and vice versa. This phenomenon can be used to measure the radial velocity of an object. The angle of arrival (AOA) is the angle between the direction of propagation of the incident radio wave and the direction normal to the antenna plane at the observation point. Measuring the AOA allows us to obtain the object's azimuth information. Generally, AOA measurement requires two horizontally aligned radar antennas to receive the echo signal. Due to the difference in distance between the object being measured and the two radar antennas, there will be a time delay difference or phase difference between the echoes received by the two antennas. Combining this with the antenna spacing, the AOA can be calculated. Therefore, by measuring DFS and AOA, we can obtain the radial velocity and azimuth information of a moving target.

[0003] Traditional electrical domain DFS and AOA measurement methods include oscillator-based, mixer-based, and in-phase quadrature hybrid schemes. To improve measurement accuracy, the frequency and bandwidth of both the signal and the signal under test in modern radar systems are constantly increasing, which also raises the bandwidth requirements of the devices used. Due to the inherent "electronic bottleneck" of electronic devices, electronic radar signals need to be segmented and measured in segments to achieve high-frequency broadband signal processing. This complex processing method hinders the further development of measurement technology. Current electrical methods suffer from drawbacks such as narrow bandwidth, high cost, and large size, and cannot meet the bandwidth requirements of future radar systems.

[0004] Microwave photonics offers a new approach to radar signal parameter measurement. Based on the combination of microwave and optical communication technologies, microwave photonics loads microwave and millimeter-wave signals onto optical waves. Through optical techniques, the signals are processed in the optical domain, enabling signal processing functions that are difficult or even impossible in the electrical domain. The significant advantages of microwave photonics technology include: wide operating frequency band, large transmission bandwidth, low transmission loss, and avoidance of electromagnetic interference. Microwave signal parameter measurement systems based on photonics technology can avoid the "rate bottleneck" of electronic devices, making it an effective approach to developing future high-frequency broadband radar, and exhibiting many significant advantages.

[0005] In recent years, numerous single-parameter measurement schemes for DFS and AOA based on microwave photonics have been reported both domestically and internationally. In microwave photonics-based DFS measurement, the value and direction of DFS are the target parameters to be measured. Numerically, the absolute value of the frequency difference between the radar transmitted signal and the echo signal is the magnitude of DFS. Therefore, most schemes achieve this by difference in the beat frequencies of the first-order modulated sidebands of the transmitted and echo signals. In earlier studies, only the numerical measurement of DFS could be achieved, and the direction of DFS could not be determined. In recent research, researchers have achieved simultaneous measurement of the magnitude and direction of DFS based on the numerical measurement. These methods can be divided into two categories: one is the orthogonal interferometry method, which uses phase to apply to one set of modulation signals and then determines the direction of DFS through phase orthogonal coherent detection; the other is the reference signal detection method, which uses a reference signal to measure the frequency of the output current to determine the direction of DFS.

[0006] In single-parameter measurement of AOA based on microwave photonics, two horizontally arranged antennas receive echo signals and modulate them onto light waves. The phase difference is mapped onto parameters such as the phase or power of the system output photocurrent through optical spot conversion. The AOA can then be obtained based on the relationship between the phase difference or power and AOA.

[0007] As single-parameter measurement systems based on microwave photonics evolve towards multi-functional integration, single-parameter measurement can no longer meet the demands of the ever-growing radar signal measurement systems. Simultaneous DFS and AOA measurements have attracted widespread attention due to their integrated system architecture. However, current measurement schemes still suffer from drawbacks such as low measurement accuracy, complex system structure, and poor stability. Summary of the Invention

[0008] To address the aforementioned issues, this invention provides a method and system for simultaneous measurement of DFS and AOA based on a dual-polarization dual-electrode MZM. The direction of DFS is determined by the phase shift of the photocurrent, and the magnitude of DFS is calculated from the instantaneous frequency of the photocurrent. The magnitude of AOA can be obtained by measuring the phase shift of the photocurrent and then performing calculations. Since the modulator operates in OCS mode, the carrier wave is suppressed, thus significantly reducing the energy of useless sidebands, minimizing interference with the useful signal, and improving measurement accuracy. In AOA measurement, the phase shift of the photocurrent after beat frequency formation forms a mapping relationship with AOA, avoiding increased AOA measurement errors caused by low-resolution regions in the power-phase mapping curve, thereby further improving AOA measurement accuracy.

[0009] This invention proposes a method and system for simultaneous measurement of DFS and AOA based on dual-polarization dual-electrode MZM, realizing simultaneous measurement of DFS and AOA. By controlling the DC bias voltage to make the modulator operate in OCS mode, the optical carrier is suppressed, improving the measurement accuracy. The method includes:

[0010] As a preferred method, a continuous wave laser (CWLD) generates a linearly polarized continuous light wave with constant power, narrow linewidth, and stable phase and frequency, and the output of the continuous wave laser is injected as an optical carrier into a dual-polarization dual-drive Mach-Zehnder modulator (Dpol-DDMZM).

[0011] As a preferred method, the Dpol-DDMZM consists of a 3dB Y-coupler, two parallel dual-electrode Mach-Zehnder modulators (DDMZMs), a 90° polarization rotator (PR), and a polarization beam combiner (PBC). Two echo signals with a phase difference received from the antenna are injected into the RF ports of the two sub-DDMZMs, and the local oscillator signal drives the other arm of each sub-DDMZM. ​​To ensure that both sub-DDMZMs operate in optical carrier suppression (OCS) modulation mode, the DC bias voltage difference between the upper and lower arms of each sub-DDMZM is one half-wave voltage V. π The light wave injected into the Dpol-DDMZM is first split into two beams by a Y-coupler. The beams are then modulated by the echo signal and the local oscillator signal by DDMZM1 and DDMZM2, respectively. The polarization direction of the output light signal of DDMZM2 is rotated 90° by the polarization rotator PR, and then orthogonally combined with the output light signal of DDMZM1 through the PBC to output a polarization multiplexed light signal carrying the echo signal and the local oscillator signal.

[0012] As a preferred method, the center frequency of the optical band pass filter (OBPF) is adjustable and dynamically adjusted according to the frequencies of the optical carrier and the local oscillator signal. The center frequency is the frequency of the +1st order sideband generated by the modulation of the optical wave by the local oscillator signal. The bandwidth is relatively small. The signal output from the dual polarization-dual electrode Mach-Zehnder modulator Dpol-DDMZM is filtered out by the optical band pass filter OBPF to get the positive first order sideband. The residual optical carrier and the negative first order sideband are suppressed, and the output is a positive first order optical sideband carrying the echo signal and local oscillator signal information.

[0013] As a preferred method, a Balanced Photodetector (BPD) performs photoelectric conversion on the input signal. The two photodetectors (PDs) in the BPD have the same responsivity, and their cutoff frequencies are much lower than the local oscillator frequency. Inside the BPD, the photocurrents of the upper and lower branches are subtracted through a differential circuit. That is, the current I(t) output by each balanced photodetector is the difference between the output photocurrents I1(t) and I2(t) of the two photodetectors within that balanced photodetector, I(t) = I1(t) - I2(t). The final output is a differential photocurrent containing the Doppler frequency shift (DFS) and angle of arrival (AOA) information of the radar echo signal.

[0014] As a preferred method, the present invention provides a system for simultaneous measurement of Doppler frequency shift (DFS) and angle of arrival (AOA) based on a dual-polarization dual-electrode MZM, comprising:

[0015] A continuous laser (CWLD) is used to generate a linearly polarized continuous light wave with constant power, narrow linewidth, and stable phase and frequency.

[0016] A dual polarization-dual electrode Mach-Zehnder modulator Dpol-DDMZM is used to modulate the echo signal and the local oscillator signal onto the optical path. Both sub-DDMZMs operate in optical carrier suppression (OCS) modulation mode. Finally, the Dpol-DDMZM outputs a polarization multiplexed signal.

[0017] An optical bandpass filter (OBPF) is used to filter out the positive first-order sideband of the modulated signal, suppress the residual optical carrier and the negative first-order sideband, and output the positive first-order sideband signal carrying the echo signal and local oscillator signal information.

[0018] A balanced detector (BPD) contains two photodetectors (PDs) with the same responsivity, and their cutoff frequencies are much lower than the local oscillator frequency. The balanced detector BPD performs photoelectric conversion and differential operation, and the output photocurrent contains the Doppler frequency shift (DFS) and angle of arrival (AOA) information of the radar echo signal.

[0019] The technical solution provided by this invention enables simultaneous measurement of DFS and AOA, realizing the integration of a dual-parameter measurement system. The optical modulator operates in OCS mode, suppressing the optical carrier and reducing interference to the useful signal, thus improving measurement accuracy. In AOA measurement, the phase shift of the photocurrent after beat frequency formation forms a mapping relationship with AOA, avoiding the increase in AOA measurement error caused by the low-resolution region in the power-phase mapping curve, thereby improving the measurement accuracy of AOA. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the principle and a schematic diagram of the spectrum at each point for simultaneous measurement of DFS and AOA based on dual polarization dual electrode MZM proposed in this invention.

[0021] Figure 2 This is the X-polarization direction spectrum output from the Dpol-DDMZM in this invention;

[0022] Figure 3 This is the spectrum of the Y-polarization direction output from the Dpol-DDMZM in this invention;

[0023] Figure 4 , Figure 5 The signal time-domain spectra of the upper and lower branches inside the BPD in this invention are shown when the DFS is +500kHz and -500kHz respectively.

[0024] Figure 6 , Figure 7 The above is the signal spectrum diagram of the upper and lower branches inside the BPD in this invention when the DFS is +500kHz and -500kHz respectively.

[0025] Figure 8 The photocurrent time-domain spectrum output from the BPD; Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Figure 1 This is a block diagram illustrating the principle of simultaneous DFS and AOA measurement based on Dpol-DDMZM proposed in this invention.

[0027] This invention utilizes microwave photonics technology to simultaneously measure the DFS and AOA parameters of radar signals. By operating the optical modulator in OCS mode, the accuracy of the final measurement is improved. To achieve the above effect, the following steps are required:

[0028] First, the CWLD generates a continuous light wave with a power of 12dBm, a linewidth of 100MHz, and a frequency of 193.1THz. This light wave is injected into a Dpol-DDMZM, an integrated device consisting of two identical parallel push-pull DDMZMs, a 90° PR, and a PBC. Both DDMZMs have the same half-wave voltage and extinction ratio, 4V and 40dB respectively, and are biased at the minimum propagation point to achieve OCS modulation. Next, the echo signal and the local oscillator signal are respectively... Figure 1 The diagram shows the RF signal port injected into the Dpol-DDMZM. ​​The frequency ω of the echo signal... E / 2π is set to 10.0005GHz, and the frequency ω of the local oscillator signal is... T / 2π is 10GHz, which means the actual DFS size is 500kHz, and the direction is positive. A phase shifter (PS) is used to introduce a phase shift into the echo signal 2 to simulate the phase difference between the two echo signals.

[0029] The optical signal output by Dpol-DDMZM, which has two orthogonally polarized signals, can be represented as:

[0030]

[0031] Where E X (t) and E Y (t) represent the X-polarized and Y-polarized optical signals output by the Dpol-DDMZM, respectively, which are orthogonal to each other. E =pV E / V p ,m T =pV LO / V p J n (·) is a Bessel function of the first kind, order n. Figure 2 The spectrum is for the X-polarized state. Figure 3 The spectrum of the Y-polarized state shows that the second-order sidebands near the frequencies of 193.08 THz and 193.12 THz are about 30 dB smaller than the first-order sidebands at 193.08 THz and 193.11 THz, although this is due to the modulation index m of the two sub-DDMZMs. E and m T The difference in optical carrier frequency results in a residual first-order sideband, which is still 21 dB larger than the sideband of the 193.1 THz carrier. The optical signal generated by Dpol-DDMZM modulation is filtered by an OBPF with a center frequency of 193.11 THz and a bandwidth of 5 GHz to eliminate the residual optical carrier and the negative first-order sideband. The optical field expression is:

[0032]

[0033] Next, the positive first-order sideband carrying the echo signal and the local oscillator signal is demultiplexed by the PBS into two linearly polarized light waves, as expressed by:

[0034]

[0035]

[0036] Then it enters the BPD for photoelectric conversion. The output electrical signals of the two branch PDs can be expressed as:

[0037]

[0038]

[0039] Furthermore, when ω E >ω T When the expression can be simplified to:

[0040] I U (t)∝I0+I1cos[(ω E -ω T )t]

[0041]

[0042] With a cutoff frequency of 2 GHz and a responsivity of 1 A / W for two identical photodiodes within the BPD, the time-domain waveforms of the upper and lower output photocurrents of the BPD were measured, as follows: Figure 4 As shown in the figure. It can be seen that the time-domain waveform of the upper branch has relative characteristics compared to the lower branch. Phase lag.

[0043] And when ω T >ω E When the expression can be simplified to:

[0044] I U (t)∝I0+I1cos[(ω T -ω E )t]

[0045]

[0046] At this point, the time-domain waveforms of the upper and lower PD output photocurrents inside the BPD were measured, such as... Figure 5 As shown in the figure. It can be seen that the time-domain waveform of the upper branch has relative characteristics compared to the lower branch. The phase is ahead.

[0047] In a differential BPD, the subtraction operation of the two photocurrents is implemented, and the output photocurrent is:

[0048]

[0049] From the above equation, we can see that the value of DFS is |Δω|=|ω E -ω T This can be obtained by measuring the frequency of the output photocurrent of the BPD, such as... Figure 6 and Figure 7 The spectrum is shown in the diagram.

[0050] Because the measurement direction of AOA is ambiguous, the phase difference can be measured. The range is 0° to 180°, therefore The value range is 0° to 90°, corresponding to... The value range is 90° to 180°, therefore the direction and phase difference of DFS can be derived from the phase shift θ of the photocurrent measured by the oscilloscope. The phase shift θ of the photocurrent is in the range of 0° to 180°. When the measured phase shift θ is in the range of 0° to 90°, it can be deduced that the direction of DFS is positive (ω). E >ω T )and like Figure 8 As shown by the solid line, at this time DFS = +500kHz, the phase difference is... The values ​​are 0°, 60°, 120°, and 180°, respectively. It can be seen that the phase shift θ is within the range of 0° to 90°. Therefore, it can be determined that the DFS direction is positive, and the phase difference value is...

[0051] When the phase offset θ is between 90° and 180°, it can be deduced that the direction of DFS is negative and the phase difference is... like Figure 8 As shown, when DFS = -500kHz, the phase difference is... When set to 0°, 60°, 120°, and 180° respectively, the time-domain waveforms output by the BPD are as follows: Figure 8 As shown by the dashed line, θ is within the range of 90° to 180°. Therefore, we can determine that the DFS direction is negative, and the phase difference value is... In summary, the value and direction of the DFS and the size of the AOA can be obtained by measuring the frequency and phase of the photocurrent.

[0052] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for simultaneously measuring Doppler frequency shift (DFS) and angle of arrival (AOA) based on a dual-polarization dual-electrode MZM, wherein the echo signal and local oscillator signal received by two antennas are modulated in parallel by two sub-MZMs of a dual-polarization dual-electrode Mach-Zehnder modulator (Dpol-DDMZM) in an optical carrier suppression (OCS) modulation mode onto a continuous light wave with constant power, narrow linewidth, and stable phase and frequency output from a CWLD laser. After 90° polarization rotation and polarization orthogonal combining, the resulting polarization-multiplexed light wave carries the echo and local oscillator signals. The positive first-order sidebands are filtered out by an optical bandpass filter (OBPF), and then depolarized and multiplexed into two optical signals by a polarization demultiplexer (PBS). These signals are then photoelectrically converted and differentially processed by an optical balanced detector (BPD). The output photocurrent contains the Doppler frequency shift (DFS) and angle of arrival (AOA) information of the radar echo signal. By measuring the phase and frequency of the photocurrent, the magnitude and direction of the DFS and the magnitude of the AOA of the echo signal can be obtained.

2. The method according to claim 1, characterized in that, The dual-polarization dual-electrode Mach-Zehnder modulator Dpol-DDMZM: The modulator Dpol-DDMZM consists of a 3 dB Y-coupler, two parallel dual-RF electrode Mach-Zehnder modulators (DDMZMs), a 90° polarization rotator (PR), and a polarization beam combiner (PBC). Echo signals with relative phase shifts received from the two antennas are injected into the RF ports of the two sub-DDMZMs, and the local oscillator signal drives the other arm of each sub-DDMZM. ​​To ensure that both sub-DDMZMs operate in optical carrier suppression (OCS) modulation mode, the DC bias voltage difference between the upper and lower arms of each sub-DDMZM is one half-wave voltage. ; The light wave injected into the Dpol-DDMZM is first split into two beams by a Y-coupler. The beams are then modulated by the echo signal and the local oscillator signal by DDMZM1 and DDMZM2, respectively. The polarization direction of the output signal from DDMZM2 is rotated 90° by the polarization rotator PR. The output signal is then orthogonally combined with the output signal of DDMZM1 by the PBC, resulting in a polarization multiplexed optical signal carrying both the echo signal and the local oscillator signal.

3. The method according to claim 1, characterized in that, The optical bandpass filter OBPF: The center frequency of the optical bandpass filter OBPF is adjustable and dynamically adjusted according to the frequencies of the optical carrier and the local oscillator signal. The center frequency is the frequency of the +1st order sideband generated by the modulation of the optical wave by the local oscillator signal. The bandwidth is relatively small. The optical signal output from the dual polarization-dual electrode Mach-Zehnder modulator Dpol-DDMZM is filtered out by the optical bandpass filter OBPF to remove the positive first-order sideband. The residual optical carrier and the negative first-order sideband are suppressed, and the output is a positive first-order sideband signal carrying echo signal and local oscillator signal information.

4. The method according to claim 1, characterized in that, The optical balance detector BPD: The optical balance detector (BPD) performs photoelectric conversion on the input optical signal. The two photodetectors (PD) in the BPD have the same responsivity, and their cutoff frequencies are much lower than the frequency of the local oscillator signal. Inside the BPD, the photocurrents of the upper and lower branches are subtracted through a differential circuit, and the final output is a differential photocurrent containing the Doppler frequency shift (DFS) and angle of arrival (AOA) information of the radar echo signal.

5. A system for simultaneously measuring Doppler frequency shift (DFS) and angle of arrival (AOA) of radar signals based on a dual-polarization dual-electrode MZM, capable of simultaneously measuring the Doppler frequency shift (DFS) and angle of arrival (AOA) of radar signals, thereby determining the radial velocity and azimuth of the target, specifically including: A continuous laser (CWLD) is used to generate a linearly polarized continuous light wave with constant power, narrow linewidth, and stable phase and frequency. A dual-polarization, dual-electrode Mach-Zehnder modulator (Dpol-DDMZM) is used to modulate the echo signal and the local oscillator signal onto the optical path. The two echo signals are injected into the RF ports of the two sub-DDMZMs, respectively, while the local oscillator signals drive the other arm of each sub-DDMZM. ​​To ensure that both sub-DDMZMs operate in optical carrier suppression (OCS) modulation mode, the DC bias voltage difference between the upper and lower arms of each sub-DDMZM is one half-wave voltage. The light wave injected into the Dpol-DDMZM is split into two beams by the Y-coupler. The beams are then modulated by two echo signals and the local oscillator signal by two parallel sub-DDMZMs. The output light signal of DDMZM2 is rotated 90° by the polarization rotator PR and then combined with the output signal of DDMZM1 through the PBC polarization orthogonal circuit to output a polarization multiplexed light signal. An optical bandpass filter (OBPF) is used to filter out the positive first-order optical sideband of the modulated signal, suppress the residual optical carrier and the negative first-order sideband, and output the positive first-order optical sideband carrying the echo signal and local oscillator signal information. An optical balance detector (BPD) is used to perform photoelectric conversion and differential operations on optical signals. The output photocurrent contains the Doppler frequency shift (DFS) and angle of arrival (AOA) information of the radar echo signal.

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