A coherent accumulation laser radar random phase elimination device and method

Abstract

The application discloses a kind of coherent laser radar random phase elimination device and method. Since coherent cumulative laser radar needs to measure laser phase, laser radar emits short wavelength, is easily influenced by random phase introduced by vibration, atmospheric disturbance etc., and reduces coherent cumulative efficiency. Aiming at this problem, the present application proposes a kind of random phase elimination method, and improves cumulative efficiency. In the transmitting end, the system uses electro-optic phase modulator to modulate laser in wide frequency. In the receiving end, after completing I / Q mixing and balanced detection by local oscillator light, data acquisition is completed by data acquisition device. The application utilizes electro-optic modulation double sideband effect, and eliminates random phase introduced by vibration and atmospheric disturbance etc. by doing square sum processing to I / Q signal, to realize nearly lossless coherent accumulation effect.

Description

Technical Field

[0001] This invention relates to the field of laser coherent detection, specifically to a coherent accumulation lidar based on electro-optic modulation to achieve broadband modulation and optical I / Q to achieve deslant reception, which solves the problem of phase instability during coherent accumulation. Background Technology

[0002] Over the decades, radar has gradually come to play a vital role in fields such as remote sensing, weather forecasting, fire monitoring, flood detection, geological exploration, and civil aviation navigation. However, the increasingly volatile surrounding electromagnetic environment and the emergence of highly maneuverable targets such as supersonic drones have posed significant challenges to radar detection. Furthermore, the application of low-observable technologies can cause radar signals to be lost in noise backgrounds, hindering radar's effectiveness. Improving the radar's signal-to-noise ratio (SNR) can greatly alleviate these problems. Several measures can be taken to improve the radar echo signal SNR, including increasing transmitter power, using a large time-bandwidth product signal, increasing the radar's receiving aperture, and reducing the radar noise figure. However, these measures primarily focus on hardware improvements to the radar system. In practical engineering applications, these measures are unlikely to further enhance the radar's SNR.

[0003] By increasing the radar illumination time on the target, i.e., increasing the number of radar echo signals, and through signal processing, the radar signal-to-noise ratio (SNR) can be improved, effectively increasing the detection probability of low-observable and high-speed targets. In coherent accumulation, the main signal processing steps include: matched filtering of a single echo signal, range migration correction, and coherent accumulation of multiple echo signals. Since the laser wavelength is around 1µm, while the target motion scale and vibration error scale are much larger than 1µm, a large random phase is introduced into the radar signal. Coherent accumulation is significantly affected by laser phase errors, resulting in lower accumulation efficiency and even failure to improve the SNR. Therefore, to address the impact of random phase on the coherent accumulation process, this method proposes a symmetrical sideband modulation to eliminate error phase, thereby greatly improving the efficiency of coherent accumulation. Summary of the Invention

[0004] This invention proposes a random phase error elimination method based on existing coherent accumulation lidar, effectively improving coherent accumulation efficiency. The laser beam undergoes broadband modulation via an electro-optic phase modulator driven by a radio frequency (RF) signal. The RF signal is generated by an RF signal source and amplified by an RF signal amplifier. The modulated laser signal is split into two beams: one as the local oscillator beam and the other as the signal beam. The signal beam is amplified by an erbium-doped fiber amplifier, then coupled to the transceiver aperture after passing through a circulator for detection. The echo signal and the local oscillator beam are split into I and Q signals by an optical I / Q mixer, each then undergoes photoelectric conversion by a balanced detector. The converted electrical signals are then acquired by a data acquisition card.

[0005] In the modulation module, the laser light is modulated using an electro-optic phase modulator driven by a radio frequency (RF) signal to achieve broadband modulation. The RF signal can be represented as:

[0006] s RF =V pp cos(πkt 2 (1)

[0007] Among them, V pp Here, k is the signal amplitude, and k is the signal modulation frequency. After the laser light passes through the electro-optic phase modulator, it can be represented as:

[0008]

[0009]

[0010] Among them, w c It is the center angular frequency of the laser. V is the initial phase of the laser, m is the modulation depth of the electro-optic phase modulator, and V is the initial phase of the laser. π It is the half-wave voltage of the electro-optic modulator.

[0011] Formula 2, when expanded according to Bessel, has the following spectrum: Figure 2 As shown.

[0012]

[0013] J n (m) represents the nth-order coefficients of a Bessel function of the first kind with a modulation depth of m. It can be seen that after the laser passes through the phase modulator, the laser power is diffused across n modulation sidebands, generating a series of linear frequency modulated signals. The modulation bandwidth and modulation rate k of the nth order are n times that of the radio frequency signal. The amplitude of each sideband is related to the modulation depth m.

[0014] When m=1, the first-order signal dominates, and the modulated signal can be approximated as:

[0015]

[0016] The echo signal and the local oscillator light complete I / Q balanced detection, and the signal obtained after detection can be expressed as:

[0017]

[0018]

[0019] The above formula, after further simplification, can be expressed as:

[0020]

[0021]

[0022] t0 is the distance delay difference between the target and the radar. The random phase of the target due to atmospheric interference and vibration, f a The Doppler frequency of the target, the frequency components obtained by the target deslant receiver, corresponds to the ranging information.

[0023] To eliminate random phase, the I / Q signals are summed by squares. The signal processing procedure is as follows:

[0024]

[0025] By using the algorithm described above, the random phase introduced into the system can be eliminated. This allows for near-lossless coherent accumulation.

[0026] Therefore, this invention proposes a method for eliminating phase error in coherent cumulative lidar, as follows:

[0027] A coherent cumulative lidar random phase elimination device includes a laser 1, an electro-optic phase modulator 2, an erbium-doped fiber amplifier 3, a circulator 4, a transceiver telescope 5, an RF amplifier 6, an RF signal source 7, an optical I / Q mixer 8, a first balanced detector 9, a second balanced detector 10, and a data acquisition unit 11.

[0028] The laser signal emitted by laser 1 is wideband modulated by electro-optic phase modulator 2 driven by radio frequency signal. The radio frequency signal is generated by radio frequency signal source 7 and amplified by radio frequency signal amplifier 6. The modulated laser signal is split into two beams, one as local oscillator light and the other as signal light.

[0029] The signal light is amplified by the erbium-doped fiber amplifier 3, and then coupled into the transceiver aperture 5 after passing through the circulator 4 for detection.

[0030] The echo signal and the local oscillator light are split into I and Q signals by an optical I / Q mixer. The I and Q signals are converted into photoelectric signals by the first balanced detector 9 and the second balanced detector 10, respectively. The converted electrical signals are then acquired by the data acquisition unit 11.

[0031] A method for random phase elimination in coherent cumulative lidar, specifically:

[0032] 1) Use an electro-optic modulator to achieve signal modulation.

[0033] 2) I / Q demodulation technology is used to achieve quadrature mixing reception of echo signals.

[0034] 3) Equalization processing is performed on the I and Q signals to ensure that the amplitudes of the I and Q signals are basically the same.

[0035] 4) Random phase is eliminated by performing a sum of squares on the I / Q signals in the time domain.

[0036] 5) The processed signals are added together in the time domain to achieve coherent accumulation.

[0037] 6) The target spectrum can be obtained by performing a Fourier transform on the coherently accumulated signal.

[0038] The workflow of a coherent cumulative lidar is as follows:

[0039] The laser beam undergoes wideband modulation via an electro-optic phase modulator driven by a radio frequency (RF) signal. The RF signal is generated by an RF signal source and amplified by an RF signal amplifier. The modulated laser signal is split into two beams: one serves as the local oscillator beam, and the other serves as the signal beam.

[0040] The signal light is amplified by an erbium-doped fiber amplifier, and then coupled into a transceiver telescope after passing through a circulator for detection.

[0041] The echo signal and the local oscillator light are split into I and Q signals by an optical I / Q mixer, and then each signal is converted into a photoelectric signal by a balanced detector. The converted electrical signal is then acquired by a data acquisition unit.

[0042] Advantages of this system

[0043] 1. By employing electro-optic modulation, a larger modulation bandwidth can be achieved, thereby enabling higher ranging resolution.

[0044] 2. Electro-optic modulation is used, which has a narrower linewidth and a longer coherence length compared to tunable lasers for broadband modulation.

[0045] 3. The use of I / Q demodulation technology and data processing eliminates laser error phase, greatly improving coherent accumulation efficiency. Attached Figure Description

[0046] Figure 1 The diagram shows a coherent cumulative lidar system, where 1 is a laser, 2 is an electro-optic phase modulator, 3 is an erbium-doped fiber amplifier, 4 is a circulator, 5 is a transceiver telescope, 6 is an RF amplifier, 7 is an RF signal source, 8 is an optical I / Q mixer, 9 is a first balanced detector, 10 is a second balanced detector, and 11 is a data acquisition unit.

[0047] Figure 2 This is the spectrum of the electro-optic modulated signal.

[0048] Figure 3 It is a phase diagram of the true phase and the phase diagram after eliminating errors.

[0049] Figure 4 The results are coherent cumulative results, where: Figure (1) is the result of a single measurement, and Figure (2) is the result of 3000 cumulative measurements. Detailed Implementation

[0050] A method for eliminating phase error in a coherent cumulative lidar is implemented as follows. The lidar mainly consists of a narrow-linewidth laser, an electro-optic phase modulator, an erbium-doped fiber amplifier, a circulator, a transceiver telescope, an RF amplifier, an RF signal source, an optical I / Q mixer, a first balanced detector, a second balanced detector, and a data acquisition unit.

[0051] The narrow-linewidth laser generates a single-frequency laser signal with a linewidth of 1 kHz and a wavelength of 1550 nm. This signal is first wide-band modulated by an electro-optic phase modulator driven by an radio frequency (RF) signal. The RF signal is a swept-frequency signal with a frequency range of 0-1 GHz, and the 3 dB bandwidth of the phase modulator is 10 GHz. The modulated laser signal is split into two paths by an optical fiber beam splitter: one path serves as the signal light, and the other as the local oscillator light. The signal light is then coupled to a transceiver telescope after passing through an erbium-doped fiber amplifier and a circulator for detection.

[0052] At the receiving end, after the received echo signal and the local oscillator light complete I / Q mixing and photoelectric conversion, the I and Q signals are acquired by the data acquisition unit.

[0053] In the signal processing stage, the first step is to perform amplitude equalization on the acquired I / Q signals to ensure that the I and Q signals have approximately the same amplitude. The second step is to perform a sum-of-squares operation on the equalized I / Q signals in the time domain. The third step is to perform range migration correction on the above signals to ensure that the target remains within the same range gate throughout its movement. The fourth step is to perform coherent accumulation on the results of multiple measurements in the time domain, i.e., direct summation. The fifth step is to perform a Fourier transform on the coherently accumulated signal to obtain the target's spectrum.

[0054] Figure 3 These are the original random phase value and the phase value after eliminating random errors. It can be seen that when errors exist, the random phase shift component is greater than 70 rad in 3000 measurements. However, when this scheme is used to eliminate the random phase, the phase stability is better than 0.4 rad. Therefore, it can be proven that the above method can greatly eliminate the influence of random phase on coherent accumulation.

[0055] Figure 4 The results are shown in the coherent accumulation diagram. As can be seen from the diagram, Figure (1) shows the result of a single measurement, with a signal amplitude of 5.5. Figure (2) shows the result of 3000 measurements, with an accumulated amplitude of 13500 (the amplitude varies slightly each time). The accumulated amplitude is increased by nearly 3000 times, while the noise is not coherent, resulting in a significant improvement in the signal-to-noise ratio. It can be seen that this method can effectively eliminate random phase and greatly improve the accumulation efficiency, which is better than 90%.

Claims

1. A method for random phase elimination in a coherent cumulative lidar, characterized in that: The steps of this method are as follows: 1) A wideband modulation of the laser signal is achieved by using an electro-optic phase modulator (2); 2) I / Q demodulation technology is used to achieve quadrature mixing reception of echo signals; 3) Equalization processing is performed on the I and Q signals to ensure that the amplitudes of the I and Q signals are basically the same; 4) Random phase is eliminated by performing a sum-of-squares operation on the I / Q signals in the time domain; 5) The processed multiple signals are added together in the time domain to achieve coherent accumulation; 6) Perform a Fourier transform on the coherently accumulated signal to obtain the target's spectrum; in: In the modulation module, the laser is modulated using an electro-optic phase modulator driven by an radio frequency (RF) signal to achieve broadband modulation; whereby the RF signal is represented as: (1) in, It is the amplitude of the signal. It is the frequency modulation of the signal; after the laser passes through the electro-optic phase modulator, it is represented as: (2) (3) in, It is the center angular frequency of the laser. This is the initial phase of the laser. It is the modulation depth of the electro-optic phase modulator. It is the half-wave voltage of the electro-optic modulator; Formula 2 is expanded according to Bezier. (4) The modulation depth is The first type of Bessel function has n-th order coefficients; after the laser passes through the phase modulator, the laser power is diffused onto n modulation sidebands, generating a series of linear frequency modulated signals; among them, the nth order modulation bandwidth and modulation rate... It is n times the RF signal, and the relationship between the amplitude of its sidebands and the modulation depth is... ; When setting At this time, the first-order signal dominates, and the modulated signal can be approximately expressed as: (5) The echo signal and the local oscillator light complete I / Q balanced detection, and the signal obtained after detection is represented as: (6) (7) The above formula, after further simplification, can be expressed as: (8) (9) It is the distance delay difference between the target and the radar. Random phase of the target due to atmospheric disturbances and vibrations, The target's Doppler frequency, The frequency components obtained from the target deslant receiver correspond to the ranging information; To eliminate random phase, the I / Q signals are summed by squares. The signal processing procedure is as follows: (10) By using the algorithm described above, the random phase introduced into the system can be eliminated. This allows for near-lossless coherent accumulation.

2. A coherent lidar random phase elimination device for implementing the method as described in claim 1; it comprises a laser (1), an electro-optic phase modulator (2), an erbium-doped fiber amplifier (3), a circulator (4), a transceiver telescope (5), a radio frequency amplifier (6), a radio frequency signal source (7), an optical I / Q mixer (8), a first balanced detector (9), a second balanced detector (10), and a data acquisition unit (11), characterized in that: The laser signal emitted by the laser (1) is wideband modulated by the electro-optic phase modulator (2) driven by the radio frequency signal. The radio frequency signal is generated by the radio frequency signal source (7) and amplified by the radio frequency signal amplifier (6). The modulated laser signal is divided into two beams, one as the local oscillator and the other as the signal light. The signal light is amplified by an erbium-doped fiber amplifier (3), and then coupled into a transceiver telescope (5) after passing through a circulator (4) for detection. The echo signal and the local oscillator light are split into two signals, I and Q, by an optical I / Q mixer. The I and Q signals are converted into photoelectric signals by the first balanced detector (9) and the second balanced detector (10), respectively. The converted electrical signals are then collected by the data acquisition unit (11).

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