Pulse frequency shift device and wind finding radar system

Abstract

The invention discloses a pulse frequency shift device and a wind finding radar system, the pulse frequency shift device comprises a frequency shift device and an optical switch, the frequency shift device is used for performing frequency shift on a frequency spectrum of an input optical signal according to a frequency shift signal to obtain an optical frequency shift signal; the optical switch is used for modulating the optical frequency shift signal according to a pulse signal to obtain an optical pulse frequency shift signal; wherein the period of the pulse signal is determined according to the period of the frequency shift signal. The embodiment of the invention can realize adjustable frequency shift amount, and can be widely applied to the technical field of optical devices.

Description

Technical Field

[0001] The present invention relates to the technical field of optical devices, and particularly to a pulse frequency shift device and a wind measurement radar system. Background Art

[0002] Optical devices capable of generating pulse frequency shift are applied in many optical sensing fields. Currently, most optical devices for generating pulse frequency shift use acousto-optic modulators (AOMs). The principle is that a piezoelectric transducer is attached to a transparent crystal or glass. The transducer can convert a radio frequency signal into an acoustic wave of a corresponding frequency. The acoustic wave propagates in the crystal to generate a traveling strain wave, and then causes a refractive index change through the photoelastic effect, becoming a refractive index grating structure. When light passes through this grating, it will be diffracted. Generally, the output optical port of the AOM is placed at the first-order (or 0-order) diffraction position, and only by controlling the on-off of the radio frequency signal can the generation of optical pulses be achieved. Ideally, when no radio frequency signal is transmitted, since no diffraction grating is formed, no light should pass through the first-order diffraction position. However, in actual applications, there is still some light passing through the first-order diffraction position, and the light passing through at this time is also frequency-shifted. This signal will cause signal interference after being amplified by the EDFA. Therefore, in order to avoid signal interference, usually two-stage acousto-optic modulators in series are required to meet the extinction ratio requirements. Using acousto-optic modulators can only generate a fixed-frequency frequency shift and cannot achieve frequency adjustment. Summary of the Invention

[0003] In view of this, to solve one of the above problems, an object of an embodiment of the present invention is to provide a pulse frequency shift device and a wind measurement radar system that can achieve adjustable frequency shift amount.

[0004] On the one hand, an embodiment of the present invention provides a pulse frequency shift device, including a frequency shift device and an optical switch, wherein,

[0005] The frequency shift device is configured to shift the spectrum of an input optical signal according to a frequency shift signal to obtain an optical frequency shift signal;

[0006] The optical switch is configured to modulate the optical frequency shift signal according to a pulse signal to obtain an optical pulse frequency shift signal;

[0007] Wherein, the period of the pulse signal is determined according to the period of the frequency shift signal.

[0008] Optionally, the frequency shift device includes a phase modulator or a quadrature modulator.

[0009] Optionally, the optical switch includes any one of a microring switch, a grating switch, or a Mach-Zehnder interferometer.

[0010] Optionally, the frequency shift device includes a phase modulator, and the optical switch includes a Mach-Zehnder interferometer.

[0011] Optionally, the frequency-shifted signal of the phase modulator includes a sawtooth wave signal, and the sawtooth wave signal is synchronized with the pulse signal of the Mach-Zehnder interferometer.

[0012] Optionally, the frequency-shifting device further includes a waveform generator and a DC source. The waveform generator is connected to the frequency-shifting device and the optical switch, and the DC source is connected to the optical switch.

[0013] Optionally, the frequency-shifting device and the optical switch are integrated on a thin-film lithium niobate platform.

[0014] On the other hand, an embodiment of the present invention provides a wind measurement radar system, including the above-mentioned pulse frequency-shifting device, as well as a laser, a beam splitter, an attenuator, an amplifier, a circulator, a mixer, and a detector. The light emitted by the laser is split into a first light beam and a second light beam by the beam splitter. The first light beam sequentially passes through the pulse frequency-shifting device and the amplifier, reaches the circulator and is emitted. The feedback signal passes through the circulator and reaches the mixer. The second light beam passes through the attenuator and reaches the mixer. The detector is used to detect the signal after beating of the mixer.

[0015] Optionally, the wind measurement radar system further includes a processor, and the processor is used to determine wind speed information according to the detection signal of the detector.

[0016] Optionally, the pulse frequency-shifting device, the laser, the beam splitter, the attenuator, the amplifier, the circulator, the mixer, and the detector are all integrated on the thin-film lithium niobate platform.

[0017] Implementing the embodiments of the present invention includes the following beneficial effects: The pulse frequency-shifting device in this embodiment includes a frequency-shifting device and an optical switch. The frequency-shifting device shifts the spectrum of the input optical signal according to the frequency-shifted signal to obtain an optical frequency-shifted signal. The optical switch modulates the optical frequency-shifted signal according to the pulse signal to obtain an optical pulse frequency-shifted signal. The period of the pulse signal is determined according to the period of the frequency-shifted signal. Only the optical signal within the pulse is frequency-shifted, and the optical signal outside the pulse is not frequency-shifted. The frequency-shifted optical signal is selected through the pulse signal, reducing interference from other signals, improving accuracy, and adjusting the frequency-shifting amount through the frequency-shifted signal. Description of the Drawings

[0018] Figure 1 is a structural block diagram of a pulse frequency-shifting device provided by an embodiment of the present invention;

[0019] Figure 2 is a structural block diagram of another pulse frequency-shifting device provided by an embodiment of the present invention;

[0020] Figure 3It is a structural block diagram of a wind measurement radar system provided by an embodiment of the present invention;

[0021] Figure 4 They are signals and images detected by a detector provided by an embodiment of the present invention. Detailed implementation manners

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. For the step numbers in the following embodiments, they are only set for the convenience of description and illustration, and no limitation is imposed on the order between steps. The execution order of each step in the embodiments can be adaptively adjusted according to the understanding of those skilled in the art.

[0023] As Figure 1 shown, an embodiment of the present invention provides a pulse frequency shift device, including a frequency shift device and an optical switch. Among them,

[0024] The frequency shift device is used to shift the frequency spectrum of the input optical signal according to the frequency shift signal to obtain an optical frequency shift signal;

[0025] The optical switch is used to modulate the optical frequency shift signal according to the pulse signal to obtain an optical pulse frequency shift signal;

[0026] Among them, the period of the pulse signal is determined according to the period of the frequency shift signal.

[0027] Specifically, the input optical signal is input from the frequency shift device and output after passing through the optical switch. The optical signal is frequency-shifted by the frequency shift signal loaded by the frequency shift device and modulated by the pulse signal loaded by the optical switch to obtain an optical pulse frequency shift signal. Among them, the period of the pulse signal is determined according to the period of the frequency shift signal, so as to select the frequency-shifted optical signal.

[0028] It should be noted that the frequency shift signal is determined according to the performance and application scenarios of the frequency shift device, including but not limited to sawtooth waves or triangular waves, etc.

[0029] The principle of the pulse frequency shift device in the embodiment of the present invention is completely different from that of the acousto-optic modulator. The frequency shift function is realized through the frequency shift device, and the function of generating pulses is realized through the optical switch. The frequency shift and the pulse are realized by two separate unit devices. In this pulse frequency shift device, its pulse signal and frequency shift signal are two independent signals. Therefore, during operation, the two signals need to be synchronously driven to realize frequency shift only within the pulse region. For the light outside the pulse, even if there is leakage, it has not been frequency-shifted and will not cause interference. This solution reduces the requirement for the extinction ratio and does not require cascading more structures to maintain the extinction ratio of the pulsed light.

[0030] Optionally, the frequency shift device includes a phase modulator or a quadrature modulator.

[0031] The phase modulator can achieve a single-sideband signal with a high extinction ratio, which can be realized with a low half-wave voltage and has a simple driving method.

[0032] The IQ modulator (quadrature modulator) is a device that decomposes the baseband signal into two orthogonal components (I-channel and Q-channel) and modulates them onto the carrier respectively for efficient information transmission. The I (In-phase) and Q (Quadrature) in the name indicate that the phase difference between the two signals is 90° (orthogonal), and it can realize a single-sideband signal with carrier suppression.

[0033] Optionally, the optical switch includes any one of a micro-ring switch, a grating switch, or a Mach-Zehnder interferometer.

[0034] The micro-ring optical switch realizes the on / off or routing control of optical signals by using the resonance effect of light. Its core principle is to adjust the resonance condition by changing the refractive index of the micro-ring, thereby controlling the transmission path of light with a specific wavelength.

[0035] The grating optical switch is an optical device based on a Bragg grating or a grating coupler, which realizes the routing control of optical signals through wavelength-selective reflection or diffraction. Its core principle is to utilize the modulation effect of the periodic structure of the grating on light with a specific wavelength, combined with external tuning means to change the grating characteristics, thereby realizing the switching of the optical path.

[0036] The Mach-Zehnder interferometer (MZI) optical switch is a key device that realizes the switching of the optical path based on the principle of optical interference. Its core mechanism is to control the transmission path of optical signals by externally modulating to change the refractive index of the waveguide.

[0037] Optionally, the frequency shift device includes a phase modulator, and the optical switch includes a Mach-Zehnder interferometer.

[0038] Optionally, the frequency shift signal of the phase modulator includes a sawtooth wave signal, and the sawtooth wave signal is synchronized with the pulse signal of the Mach-Zehnder interferometer.

[0039] In a specific embodiment, the frequency shift device uses a phase modulator, and the optical switch uses a Mach-Zehnder interferometer. The frequency shift signal of the phase modulator includes a sawtooth wave signal, and the sawtooth wave signal is synchronized with the pulse signal of the Mach-Zehnder interferometer. When a high-amplitude sawtooth wave signal is input, its spectrum will be shifted, and the frequency shift amount is related to the amplitude of the sawtooth wave and the half-wave voltage of the phase modulator. The implementation principle of the frequency shift part is that when a voltage V(t) is applied to a phase modulator, due to the electro-optic effect, the phase change of the optical signal is calculated as follows:

[0040]

[0041] vπ represents the half-wave voltage of the phase modulator, and the phase change can be reflected as the spectral change. The frequency shift amount Δf is:

[0042]

[0043] Obviously, the frequency shift amount is related to the half-wave voltage of the phase modulator and the derivative of the applied voltage signal. That is, by controlling the frequency shift signal, the frequency shift regulation of the optical signal spectrum can be realized.

[0044] The transmission curve of the Mach-Zehnder interferometer is a cosine function. When the operating point is locked at the lowest power point by thermal biasing, applying a radio frequency signal with an amplitude of the half-wave voltage to the electro-optic phase shifter can turn the optical signal into a pulse signal, and the pulse width is adjustable with the pulse width of the applied radio frequency signal. Its electro-optic response curve I is as follows:

[0045]

[0046] where I max is the maximum response current, V π is the half-wave voltage of the optical switch, is the initial phase difference between the two arms of the optical switch without signal application. When applying a pulse signal with an amplitude equal to the half-wave voltage of the Mach-Zehnder interferometer, the optical switch can change the continuous optical signal into an optical pulse signal with a high extinction ratio.

[0047] In addition, the optical switch part adopts the Mach-Zehnder optical switch structure. Therefore, feedback control of its bias point is required to control the device output at the lowest power point. The specific control method is that there are two outputs after the optical switch. One is the main optical path output, and the other is the monitoring path. An on-chip monitoring photodetector will be integrated. When the optical power of the monitoring path is the largest, the optical power of the main optical path is the smallest.

[0048] Optionally, the frequency shift device further includes a waveform generator and a DC source. The waveform generator is connected to the frequency shift device and the optical switch, and the DC source is connected to the optical switch.

[0049] Refer to Figure 2 , in a specific embodiment, a periodic synchronous frequency shift signal and a pulse signal are generated by an arbitrary waveform generator. The frequency shift signal acts on the frequency shift device, and the pulse signal acts on the optical switch. For the signals other than the pulse signal action, even if there is leaked light, since there is no frequency shift, it will not interfere with the signal. Among them, the DC source is used to control the thermal bias point of the optical switch to make it at the lowest power point.

[0050] Optionally, the frequency shift device and the optical switch are integrated on a thin-film lithium niobate platform.

[0051] Integrating a frequency-shifting device and an optical switch on a thin-film lithium niobate platform results in a small structure, which is convenient for integration with other functional devices such as lasers and optical amplifiers, reducing the device size and cost. All modulations of this device utilize the electro-optic modulation characteristics of the thin-film lithium niobate material, and the effect time of the material itself can reach the femtosecond level. Therefore, the response speed is much faster than that of acousto-optic modulators, and the pulse width can be conveniently adjusted.

[0052] Referring to Figure 3 , an embodiment of the present invention provides a wind measurement radar system, including the above-mentioned pulse frequency-shifting device, as well as a laser, a beam splitter, an attenuator, an amplifier, a circulator, a mixer, and a detector. The light emitted by the laser is split into a first light beam and a second light beam by the beam splitter. The first light beam sequentially passes through the pulse frequency-shifting device and the amplifier, reaches the circulator and is emitted. The feedback signal reaches the mixer through the circulator, and the second light beam reaches the mixer through the attenuator. The detector is used to detect the signal after beat frequency of the mixer.

[0053] Optionally, the wind measurement radar system further includes a processor, which is used to determine the wind speed information according to the detection signal of the detector.

[0054] The continuous light emitted by the laser is split into two beams by the beam splitter. One beam serves as the local oscillator light, and the local oscillator light reaches the mixer after passing through the attenuator; the other beam serves as the signal light and generates a pulse frequency-shifted optical signal with a high extinction ratio through the pulse frequency-shifting device. The peak power of the pulsed light is extremely high after the pulsed frequency-shifted optical signal passes through the optical amplifier. After passing through the circulator, the pulsed light signal is emitted into the air. When the light propagates in the air, the air flow will cause the light to scatter, generating a Doppler frequency shift. The amount of frequency shift is related to the wind speed. The scattered light will propagate back and enter the circulator, and finally beat with the local oscillator light in the mixer and is received by the balanced detector. After the signal is received by the acquisition card, it is sent to the processor, and the processor processes it in the time domain and the frequency domain to obtain the wind speed at different distances.

[0055] Referring to Figure 4 , in a specific embodiment, Figure 4 Figure (a) shows the actual effect diagram of the pulsed frequency-shifted signal generated by the pulsed frequency-shifting device passing through the wind measurement radar system and beating with the local oscillator light and being received by the detector. It can be seen that there is no obvious signal interference outside the pulse region. Figure 4 Figure (b) shows the image obtained after the received scattered light signal is expanded in the frequency domain and the time domain. The horizontal axis represents the frequency, and the frequency shift amount relative to 80M reflects the actual wind speed. The vertical axis represents the distance, which is equivalent to the distance calculated by the speed of light after the time domain expansion of the signal. Figure 4 Figure (b) shows that scattered signals are detected within a distance of 300 meters, and the wind speed can be measured.

[0056] Optionally, a pulse frequency shift device, a laser, a beam splitter, an attenuator, an amplifier, a circulator, a mixer, and a detector are all integrated on a thin-film lithium niobate platform.

[0057] All structures are implemented based on an integrated material platform of thin-film lithium niobate. The structures are small, reducing the device size and can lower costs.

[0058] In a specific embodiment, for the pulse frequency shift device, with device parameters of a half-wave voltage of the optical switch being 4.6V, under a pulse with a pulse width of 400ns, to achieve a frequency shift amount of 80M, the sawtooth wave amplitude is 19V, and a test system for a wind measurement radar is built using this pulse frequency shift device. First, the drive structure parameters need to be determined, and the specific steps are as follows:

[0059] The maximum wind speed that can be tested in this system design is 60m / s, which is converted to a frequency shift amount of approximately 80MHz. Therefore, the center frequency designed for this system is 80MHz. After determining the center frequency, the pulse width needs to be considered. This parameter determines the power strength of the pulsed light and thus determines the distance it can travel in the air. In this system design, the variable pulse width can be selected according to the application scenario. In this example, a pulse width of 400ns is used for illustration.

[0060] After determining the system parameters, the drive parameters of the device of the present invention need to be determined. Among them, the half-wave voltage of the optical switch is a fixed value. After the device is designed, the actual value can be tested using a signal generator and an optical power meter. A sawtooth wave can be applied to find the ΔV corresponding to the peak-to-peak value of the output optical power signal, which is the half-wave voltage of the optical switch.

[0061] The method for determining the parameters of the frequency shift signal drive is as follows: Use a bulk optical device and an external fiber to build a Mach-Zehnder interferometer structure. Apply a sawtooth wave signal with an amplitude of V 1 , a period of 800ns to the phase modulator. The output signal and the reference optical signal interfere in the coupler. After interference, it passes through a photodetector and is connected to a spectrum analyzer to see a frequency shift amount of Δf 1 , and at this time, the frequency shift amount formula of this device is The voltage amplitude required for a frequency shift of 80MHz can be calculated.

[0062] After the above steps, the driving parameters of the device of the present invention can be determined. The specific driving steps are as follows: After setting up the test system, a pulse signal is sent out by the acquisition card as the clock signal. The driver or signal generator receives the clock signal and synchronously sends out two signals. One is the pulse signal for driving the optical switch, the amplitude of the pulse signal is the half-wave voltage of the optical switch, and the pulse width is 400 ns. The other is a single-period sawtooth wave signal for driving the frequency shifter. The amplitude of the sawtooth wave can be obtained through the above fourth step, which is 19 V in this example, and the rising edge time of the sawtooth wave is 400 ns. At this time, the optical signal after passing through the device is the required optical signal with pulsed frequency shift.

[0063] Implementing the embodiments of the present invention includes the following beneficial effects: The pulsed frequency shift device of this embodiment includes a frequency shift device and an optical switch. The frequency shift device shifts the spectrum of the input optical signal according to the frequency shift signal to obtain an optically frequency-shifted signal. The optical switch modulates the optically frequency-shifted signal according to the pulse signal to obtain an optical pulsed frequency-shifted signal. The period of the pulse signal is determined according to the period of the frequency shift signal. The frequency-shifted optical signal is selected through the pulse signal, reducing interference from other signals, improving accuracy, and adjusting the frequency shift amount through the frequency shift signal.

[0064] The above is a specific description of the preferred embodiments of the present invention, but the present invention is not limited to the described embodiments. Those skilled in the art can make various equivalent deformations or substitutions without departing from the spirit of the present invention, and these equivalent deformations or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A pulse frequency shift device, characterized in that: It includes a frequency shift device and an optical switch, wherein: The frequency shifting device is used to shift the frequency spectrum of the input optical signal according to the frequency shifting signal to obtain an optical frequency shifting signal; The optical switch is used to modulate the optical frequency-shift signal according to the pulse signal to obtain an optical pulse frequency-shift signal; Wherein, the period of the pulse signal is determined according to the period of the frequency-shifted signal.

2. The pulse frequency shift device according to claim 1, characterized in that: The frequency shifting device includes a phase modulator or an orthogonal modulator.

3. The pulse frequency shift device according to claim 1, characterized in that: The optical switch includes any one of a micro-ring switch, a grating switch or a Mach-Zehnder interferometer.

4. The pulse frequency shift device according to claim 1, characterized in that: The frequency shifting device includes a phase modulator, and the optical switch includes a Mach-Zehnder interferometer.

5. The pulse frequency shift device according to claim 4, characterized in that: The frequency shift signal of the phase modulator comprises a sawtooth wave signal, and the sawtooth wave signal is synchronized with the pulse signal of the Mach-Zehnder interferometer.

6. The pulse frequency shift device according to any one of claims 1 to 5, characterized in that: The frequency shift device further includes a waveform generator and a direct current source, wherein the waveform generator is connected to the frequency shift device and the optical switch, and the direct current source is connected to the optical switch.

7. The pulse frequency shift device according to any one of claims 1 to 5, characterized in that: The frequency shift device and the optical switch are integrated on a thin film lithium niobate platform.

8. A wind measurement radar system, characterized in that: It comprises a pulse frequency shift device as described in any one of claims 1 to 7, and a laser, a beam splitter, an attenuator, an amplifier, a circulator, a mixer and a detector, wherein the light emitted by the laser is divided into a first light beam and a second light beam by the beam splitter, the first light beam passes through the pulse frequency shift device and the amplifier in sequence, reaches the circulator and is emitted, a feedback signal passes through the circulator to reach the mixer, the second light beam passes through the attenuator to reach the mixer, and the detector is used to detect the signal after the mixer beats.

9. The wind measurement radar system according to claim 8, characterized in that: The wind measurement radar system also includes a processor, which is used to determine wind speed information according to the detection signal of the detector.

10. The wind radar system according to claim 8, characterized in that: The pulse frequency shift device, the laser, the beam splitter, the attenuator, the amplifier, the circulator, the mixer and the detector are all integrated on the thin film lithium niobate platform.

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