A pulsed frequency shift device and a windfinder system

Abstract

The application discloses a pulse frequency shift device and a wind measurement radar system. The pulse frequency shift device comprises a frequency shift device and an optical switch. The frequency shift device is used for frequency shifting the 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. The period of the pulse signal is determined according to the period of the frequency shift signal. The embodiment of the application can realize adjustable frequency shift and can be widely applied to the technical field of optical devices.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical devices, in particular to a pulse frequency shift device and a wind measurement radar system. BACKGROUND

[0002] The optical device capable of generating pulse frequency shift is applied in many optical sensing fields. At present, the optical device capable of generating pulse frequency shift mostly adopts an acousto-optic modulator (AOM). The principle is that a piezoelectric transducer is attached to a transparent crystal or glass. The transducer can convert a radio frequency signal into a sound wave of a corresponding frequency. The sound wave propagates in the crystal to generate a traveling strain wave, and then the light passes through the birefringence grating structure caused by the photoelastic effect to be diffracted. Generally, the light outlet of the AOM is placed at the first-order (or 0-order) diffraction position. Only the on-off control of the radio frequency signal is needed to realize the generation of light pulses. In an ideal case, when there is no radio frequency signal emitted, there should be no light passing through the first-order diffraction position because no diffraction grating is formed. However, in actual application, part of the light still passes through the first-order diffraction position, and the light passing through is also frequency-shifted. After the signal is amplified by an EDFA, signal interference is caused. Therefore, in order to avoid signal interference, two acousto-optic modulators are usually connected in series to meet the extinction ratio requirement. The acousto-optic modulator can only generate a fixed frequency shift and cannot realize frequency adjustment. SUMMARY

[0003] Therefore, in order to solve one of the above problems, the purpose of the embodiments of the present application is to provide a pulse frequency shift device and a wind measurement radar system, which can realize adjustable frequency shift.

[0004] In one aspect, the embodiments of the present application provide a pulse frequency shift device, comprising a frequency shift device and an optical switch, wherein,

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

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

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

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

[0009] Optionally, the optical switch comprises any one of a micro-ring switch, a grating switch or a Mach-Zehnder interferometer.

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

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

[0012] Optionally, the frequency shift device further comprises a waveform generator and a direct current source, 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.

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

[0014] In another aspect, an embodiment of the present application provides a wind measurement radar system, comprising the pulse frequency shift device described above, and a laser, a beam splitter, an attenuator, an amplifier, a circulator, a frequency mixer and a detector, light emitted by the laser passes through the beam splitter to form a first light beam and a second light beam, 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 frequency mixer, the second light beam passes through the attenuator to reach the frequency mixer, and the detector is used to detect the signal after frequency mixing of the frequency mixer.

[0015] Optionally, the wind measurement radar system further comprises 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 shift device, the laser, the beam splitter, the attenuator, the amplifier, the circulator, the frequency mixer and the detector are integrated on the thin film lithium niobate platform.

[0017] The implementation of the embodiment of the present application has the following beneficial effects: the pulse frequency shift device of the embodiment comprises a frequency shift device and an optical switch, the frequency shift device performs frequency shift on the frequency spectrum of the input light signal according to the frequency shift signal to obtain a light frequency shift signal, the optical switch modulates the light frequency shift signal according to the pulse signal to obtain a light pulse frequency shift signal, the period of the pulse signal is determined according to the period of the frequency shift signal, only the light signal in the pulse is subjected to frequency shift, and the light outside the pulse is not subjected to frequency shift, the frequency shift light signal is selected through the pulse signal, other signal interference is reduced, accuracy is improved, and the frequency shift amount is adjusted through the frequency shift signal. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

[0021] Figure 4 is a signal and image detected by a detector provided by an embodiment of the present application. DETAILED DESCRIPTION

[0022] The present application will be further described below in conjunction with the drawings and specific embodiments. For the step numbers in the following embodiments, they are only set for the convenience of description, and the order between the steps is not limited in any way, and 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 shown in the drawings, Figure 1 An embodiment of the present application provides a pulse frequency shift device, comprising a frequency shift device and an optical switch, wherein,

[0024] The frequency shift device is configured to shift the frequency spectrum of the input optical signal according to a frequency shift signal to obtain an optical frequency shift signal.

[0025] 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.

[0026] 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 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 the optical pulse frequency shift signal. 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 scenario of the frequency shift device, including but not limited to sawtooth wave or triangular wave, etc.

[0029] The pulse frequency shift device in the embodiment of the present application is completely different from the principle of the acousto-optic modulator. The frequency shift function is realized by the frequency shift device, the pulse function is realized by the optical switch, and the frequency shift and the pulse are realized by two separate unit devices. In the pulse frequency shift device, the pulse signal and the frequency shift signal are two independent signals, so that the two signals need to be synchronously driven when working, that is, only the frequency shift in the pulse region can be realized, and the light outside the pulse, even if there is leakage, is not subjected to frequency shift, so no interference is generated. The scheme reduces the requirement of the extinction ratio, and does not need to cascade more structures to maintain the extinction ratio of the pulse light.

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

[0031] The phase modulator can realize a high-extinction-ratio single sideband signal, can be realized by a low half-wave voltage, and has a simple driving mode.

[0032] An IQ modulator (quadrature modulator) is a device that decomposes a baseband signal into two orthogonal components (I and Q) and modulates them onto a carrier wave for efficient information transmission. The I (In-phase) and Q (Quadrature) in the name indicate that the two signals are 90° out of phase (quadrature), and a single sideband signal with carrier suppression can be realized.

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

[0034] A micro-ring optical switch uses the resonance effect of light to control the on-off or routing of optical signals. The core principle is to adjust the resonance condition by changing the refractive index of the micro-ring, thereby controlling the transmission path of specific wavelengths of light.

[0035] A grating optical switch is an optical device based on a Bragg grating or a grating coupler, which realizes routing control of optical signals through wavelength-selective reflection or diffraction. The core principle is to use the periodic structure of the grating to modulate specific wavelengths of light, and to change the grating characteristics by combining external tuning means, thereby realizing switching of the optical path.

[0036] A Mach-Zehnder interferometer (MZI) optical switch is a key device that realizes optical path switching based on the principle of optical interference. The core mechanism is to change the refractive index of the waveguide by external modulation, thereby controlling the transmission path of optical signals.

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

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

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

[0040]

[0041] vπ Vπ represents the half-wave voltage of the phase modulator, and the phase change can be reflected as a spectrum change, and the frequency shift Δf is:

[0042]

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

[0044] The transmission curve of the Mach-Zehnder interferometer is a cosine function. When the working point is locked at the minimum power point by the thermal bias point, an RF signal with an amplitude of the half-wave voltage is applied to the electrically tunable phase shifter, so that the optical signal becomes a pulse signal, and the pulse width is adjustable with the pulse width of the applied RF signal. The electro-optical 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 of the two arms of the optical switch without signal. When a pulse signal with an amplitude of the half-wave voltage of the Mach-Zehnder interferometer is applied, the optical switch can change the continuous optical signal into a high-extinction-ratio optical pulse signal.

[0047] In addition, the optical switch part adopts the Mach-Zehnder optical switch structure, so that the bias point needs to be feedback controlled, and the device output is controlled at the minimum power point. The specific control mode is that there are two output paths after the optical switch, one is used as the main light path output, and the other is used as the monitoring path. An on-chip monitoring photodetector is integrated, and when the monitoring path light power is maximum, the main light path light power is minimum.

[0048] Optionally, the frequency shift device further comprises a waveform generator and a direct current source, 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.

[0049] Referring to Figure 2 In a specific embodiment, a periodic synchronous frequency shift signal and a pulse signal are generated by using an arbitrary waveform generator, the frequency shift signal acts on the frequency shift device, the pulse signal acts on the optical switch, and signals other than the pulse signal do not interfere with the signals due to the absence of frequency shift, even if there is leakage light. The direct current source is used to control the thermal bias point of the optical switch, so that it is at the minimum power point.

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

[0051] The frequency shift device and the optical switch are integrated on the thin film lithium niobate platform, the structure is small, and is convenient for integration with other functional devices, such as a laser, an optical amplifier and the like, the device size is reduced, and the cost can be reduced. All the modulations of the device utilize the electro-optic modulation characteristics of the thin film lithium niobate material, the effect time of the material itself can reach femtosecond, therefore, the response speed is much faster than that of an acousto-optic modulator, and the pulse width can be conveniently adjusted.

[0052] Referring to Figure 3 The embodiment of the present application provides a wind measurement radar system, which comprises the pulse frequency shift device, and a laser, a beam splitter, an attenuator, an amplifier, a circulator, a frequency mixer and a detector. The light emitted by the laser passes through the beam splitter to be divided into a first light beam and a second light beam. The first light beam passes through the pulse frequency shift device and the amplifier in sequence, reaches the circulator and is emitted. The feedback signal reaches the frequency mixer through the circulator. The second light beam reaches the frequency mixer through the attenuator. The detector is used for detecting the signal after frequency mixing of the frequency mixer.

[0053] Optionally, the wind measurement radar system further comprises a processor, and the processor is used for determining wind speed information according to the detection signal of the detector.

[0054] The continuous light emitted by the laser passes through the beam splitter to be divided into two beams. One beam is used as a local oscillator light, and the local oscillator light reaches the frequency mixer after passing through the attenuator. The other beam is used as a signal light, and the signal light generates a high-extinction-ratio pulse frequency shift light signal after passing through the pulse frequency shift device. The peak power of the pulse light signal is extremely high after passing through the optical amplifier. After the pulse light signal passes through the circulator, the pulse light signal is emitted into the air. When the light propagates in the air, the air flow causes the light to scatter, and a Doppler frequency shift is generated. The scattering light propagates back into the circulator, and finally, the scattering light is frequency-mixed with the local oscillator light in the frequency mixer and is received by the balanced detector. After the signal is received by the acquisition card, the signal is sent to the processor. The processor processes the signal in the time domain and the frequency domain to obtain wind speeds at different distances.

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

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

[0057] All structures are realized on the integrated material platform of thin film lithium niobate, the structure is small, the device size is reduced, and the cost can be reduced.

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

[0059] The maximum testable wind speed of the system is 60m / s, which is converted into a frequency shift of about 80MHz, so the center frequency designed by the system is 80MHz. After determining the center frequency, the pulse width needs to be considered, which determines the power of the pulse light and thus determines the distance it can travel in the air. The pulse width can be changed according to the application scenario in this system, and 400ns is used as an example in this example.

[0060] After determining the system parameters, the driving parameters of the device need to be determined, and 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 peak-to-peak value of the output optical power signal, which is the half-wave voltage of the optical switch.

[0061] The parameter determination method of the frequency shift signal drive is as follows: a Mach-Zehnder interferometer structure is added outside the bulk optical device and the optical fiber, a sawtooth wave signal with an amplitude of V1 and a period of 800ns is applied to the phase modulator, the output signal interferes with the reference light signal in the coupler, and after the interference, the photoelectric detector is connected to the spectrum analyzer to see that the frequency shift is Δf1. At this time, the frequency shift formula of the device is The voltage amplitude required for 80MHz frequency shift can be calculated.

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

[0063] The embodiment of the present application has the following advantages: the pulse frequency shift device comprises a frequency shift device and an optical switch, the frequency shift device shifts the frequency spectrum of the input optical signal according to a frequency shift signal to obtain an optical frequency shift signal, the optical switch modulates the optical frequency shift signal according to a pulse signal to obtain an optical pulse frequency shift signal, the period of the pulse signal is determined according to the period of the frequency shift signal, the frequency shift optical signal is selected by the pulse signal, other signal interference is reduced, accuracy is improved, and the frequency shift amount is adjusted by the frequency shift signal.

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

Claims

1. A pulse frequency shifter, characterized in that, Including frequency shifting devices and optical switches, among which, The frequency shifting device is used to shift the frequency 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-shifting signal according to the pulse signal to obtain the optical pulse frequency-shifting signal; The period of the pulse signal is determined based on the period of the frequency shift signal; 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.

2. The pulse frequency shifter according to claim 1, characterized in that, The frequency shifting device includes a phase modulator or a quadrature modulator.

3. The pulse frequency shifter 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 shifter 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 shifter according to claim 4, characterized in that, The frequency shift signal of the phase modulator includes a sawtooth wave signal, which is synchronized with the pulse signal of the Mach-Zehnder interferometer.

6. The pulse frequency shifting device according to any one of claims 1-5, characterized in that, The frequency shifter and optical switch are integrated on a thin-film lithium niobate platform.

7. A wind-measuring radar system, characterized in that, The device includes a pulse frequency shifter as described in any one of claims 1-6, 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 beam and a second beam by the beam splitter. The first beam passes sequentially through the pulse frequency shifter and the amplifier, reaches the circulator, and is emitted. The feedback signal passes through the circulator and reaches the mixer. The second beam passes through the attenuator and reaches the mixer. The detector is used to detect the signal after frequency beating by the mixer.

8. The wind-measuring radar system according to claim 7, characterized in that, The wind-measuring radar system also includes a processor, which is used to determine wind speed information based on the detection signal from the detector.

9. The wind-measuring radar system according to claim 7, characterized in that, The pulse frequency shifter, the laser, the beam splitter, the attenuator, the amplifier, the circulator, the mixer, and the detector are all integrated into the thin-film lithium niobate platform.

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