High-resolution microwave photon frequency measurement system based on real-time Fourier transform

Abstract

The invention belongs to the technical field of microwave photon links, and particularly relates to a high-resolution microwave photon frequency measurement system based on real-time Fourier transform. The link comprises a mode-locked laser, a first dispersion compensation module, a second dispersion compensation module, a dual-drive Mach-Zehnder modulator, a Mach-Zehnder modulator, a dispersion optical fiber, a time lens, an erbium-doped optical fiber amplifier, a photoelectric detector and an oscilloscope; the mode-locked laser outputs femtosecond Gaussian light pulses, and the femtosecond Gaussian light pulses are injected into the first dispersion compensation module and then stretched into chirp light pulses; modulating the chirp light pulse through electro-optical modulators which are connected in parallel; the modulated optical signal is subjected to time amplification processing through a dispersion optical fiber, a time lens, a second dispersion compensation module and an erbium-doped optical fiber amplifier in sequence; and the optical signal after time amplification processing is converted into a microwave pulse carrying frequency information of the signal to be detected by a photoelectric detector, and the microwave pulse is observed by an oscilloscope.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave photon links, and in particular relates to a high-resolution microwave photon frequency measurement system based on real-time Fourier transform. Background Art

[0002] Microwave signal frequency measurement technology has widespread application in fields such as electronic countermeasures, radar, and wireless communications. However, traditional microwave signal frequency measurement techniques are typically based on purely electronic means, such as electrical delay lines, electrical filters, and digital instantaneous frequency measurement. These technologies have limited measurement bandwidth, slow measurement speed, and are susceptible to electromagnetic interference, making them difficult to meet the needs of complex applications. Unlike traditional electronic frequency measurement techniques, frequency measurement technology based on microwave photonics utilizes optical structures and technologies to generate, manipulate, transmit, and measure high-speed microwave radio frequency signals. It offers significant advantages and application prospects, including a simple structure, the ability to simultaneously measure multiple frequencies, wide bandwidth, high resolution, and resistance to electromagnetic interference.

[0003] like Figure 1 Figure 1 shows a typical microwave photon frequency measurement system. The received microwave signal to be measured is first modulated onto an optical carrier by an electro-optical modulator. Signal transmission and processing are then performed in the optical domain, mapping the microwave signal frequency to parameters such as microwave signal power, time delay, optical power, and phase delay. Finally, the frequency information of the microwave signal to be measured is obtained based on a specific mapping relationship.

[0004] Although microwave photonic frequency measurement systems offer significant advantages in terms of anti-interference capability, flexibility, and measurement range, they face challenges in complex applications, such as the high system complexity of frequency-space mapping and the limitation of frequency-amplitude mapping systems to single-frequency measurement, which often results in performance disadvantages. The demand for microwave photonic frequency measurement technology is to achieve high-precision, low-complexity, and high-system stability to meet a wider range of application needs. Frequency-time mapping technology can measure microwave signals at multiple frequencies, while high-performance frequency-time mapping components can achieve the advantages of wide bandwidth and high resolution. Extensive research has been conducted both domestically and internationally on microwave photonic frequency measurement technology.

[0005] like Figure 2 The figure shows the structure of a high-resolution frequency measurement system based on stimulated Brillouin scattering enhancement. This solution uses the stimulated Brillouin scattering effect to modulate the unknown microwave signal into pump light through a modulator, and the scanning signal generated by the vector network analyzer is modulated into detection light through a phase modulator. When the two meet the phase matching condition in the highly nonlinear optical fiber, the stimulated Brillouin scattering effect selectively converts the phase modulation into intensity modulation, and the frequency measurement of the microwave signal is achieved by detecting the change in the modulated light intensity. However, this solution still has shortcomings:

[0006] First, the system relies on the fixed Brillouin frequency shift of highly nonlinear optical fibers. When the frequency of the signal to be measured exceeds the Brillouin gain spectrum range, segmented measurement or replacement of the optical fiber is required, which makes it difficult to adapt to ultra-wideband signals.

[0007] Secondly, the system can only match the frequency point by point by scanning the signal and cannot process frequency components at multiple positions at the same time.

[0008] Finally, laser wavelength jitter can cause Brillouin frequency shift drift, and temperature changes in highly nonlinear optical fibers affect the gain spectrum, requiring additional temperature control, increasing system complexity and cost.

[0009] Another example Figure 3 The figure shows the structure of a photon-assisted multi-tone microwave frequency measurement system based on pulse recognition. This scheme uses a linear chirp signal and an unknown microwave signal to load the upper and lower branches of a dual-driven Mach-Zehnder modulator, respectively, and generates a pulse signal in a photodetector through the beat frequency effect. After extracting the beat frequency component using an electrical bandpass filter, the position of the pulse in the time domain is identified to achieve accurate measurement of the unknown microwave frequency. However, this scheme has shortcomings:

[0010] First, the system's measurement bandwidth is mainly determined by the frequency range of the linear chirp signal output by the arbitrary waveform transmitter. The measurement frequency band division depends on the optical bandpass filter, and the parameters need to be manually adjusted according to the measured signal, which cannot achieve adaptive coverage of the continuous wide spectrum.

[0011] Secondly, although the frequency resolution is theoretically determined by the MHz bandwidth of the electrical bandpass filter, when the frequency interval of the multi-frequency signals is less than 50 MHz, the pulses generated by the beat frequency will be indistinguishable due to time domain overlap.

[0012] Finally, this solution has high requirements for hardware consistency and linear response, including bias point control of the dual-polarization modulator, symmetry of the linear chirp signal, polarization matching of the optical path, and filtering accuracy. The system stability is greatly affected by ambient temperature and device fluctuations, which is not conducive to application environments with high integration and stability requirements.

[0013] Therefore, it is very important to design a high-resolution microwave photon frequency measurement system based on real-time Fourier transform that can improve the measurement bandwidth and frequency resolution while optimizing the system complexity. Summary of the Invention

[0014] The present invention aims to overcome the problems of limited measurement bandwidth, difficulty in improving frequency resolution, and excessively high system complexity and cost in the existing microwave photon frequency measurement system in the prior art. It provides a high-resolution microwave photon frequency measurement system based on real-time Fourier transform that can improve measurement bandwidth and frequency resolution while optimizing system complexity.

[0015] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0016] A high-resolution microwave photon frequency measurement system based on real-time Fourier transform, comprising a mode-locked laser, a first dispersion compensation module, a second dispersion compensation module, a dual-driven Mach-Zehnder modulator, a Mach-Zehnder modulator, a dispersive optical fiber, a time lens, an erbium-doped fiber amplifier, a photodetector, and an oscilloscope;

[0017] The mode-locked laser outputs a femtosecond Gaussian light pulse, which is injected into the first dispersion compensation module and stretched into a chirped light pulse.

[0018] The chirped optical pulse is modulated by a parallel electro-optical modulator; the electro-optical modulator includes a dual-drive Mach-Zehnder modulator and a Mach-Zehnder modulator; the dual-drive Mach-Zehnder modulator has two radio frequency ports, and the two radio frequency ports respectively introduce a microwave signal to be measured with a phase difference of 90 degrees;

[0019] The modulated optical signal passes through the dispersion fiber, time lens, second dispersion compensation module and erbium-doped fiber amplifier in sequence for time amplification processing;

[0020] The optical signal after time amplification processing is converted by a photodetector into a microwave pulse carrying the frequency information of the signal to be measured, and is observed by an oscilloscope.

[0021] Preferably, the dual-drive Mach-Zehnder modulator includes a first sub-modulator and a second sub-modulator; the RF ports of the first sub-modulator and the second sub-modulator respectively introduce a microwave signal to be measured with a phase difference of 90°; the Mach-Zehnder modulator includes a third sub-modulator and a phase shifter, and is used to eliminate the optical carrier and ±2nd order sidebands in the chirped optical pulse by setting the bias voltage of the third sub-modulator that is not powered on and the driving voltage of the phase shifter.

[0022] Preferably, the femtosecond Gaussian light pulse u0(t) output by the mode-locked laser is expressed as:

[0023]

[0024] Where a is the peak electric field intensity of the Gaussian light pulse; τ is the full width at half maximum of the pulse, t is the time variable, and the frequency domain expression corresponding to u0(t) is U0(ω);

[0025] The Gaussian light pulse passes through the transfer function D1(ω)=exp(jβ2Lω 2 / 2) dispersion compensation fiber is stretched and widened into a light pulse with chirped characteristics. The electric field intensity u1(t) of the output chirped light pulse is expressed as:

[0026] u1(t)=F -1 [U0(ω)·D1(ω)]=F -1 [U0(ω)exp(jβ2Lω 2 / 2)]

[0027] Among them, F -1 [·] denotes the inverse Fourier transform operation, β2 and L denote the group velocity dispersion of the dispersion compensation module and the fiber length, respectively, ω denotes the angular frequency of the optical pulse, and j denotes the imaginary unit.

[0028] As a preference, the RF signal input to the receiving end of the dual-drive Mach-Zehnder modulator is set to be expressed as g(t), which is expressed as follows:

[0029] g(t)=cos(ω RF t);

[0030] ω RF Indicates the frequency of the microwave signal to be measured.

[0031] Preferably, the chirped optical pulse passes through the parallel electro-optical modulator to modulate the radio frequency signal into the optical domain, and the modulated optical signal u m (t) is expressed as:

[0032]

[0033] Among them, u dp-ddmzm (t) is the output light field of the dual-driven Mach-Zehnder modulator, u mzm (t) is the output light field of the cascaded Mach-Zehnder modulator and phase shifter, θ1, θ2, θ3, and θ4 represent the bias angles of the first, second, and third sub-modulators, and the phase shifter, respectively; m represents the modulation coefficient of the electro-optic modulator, m = πV / V π , where V is the amplitude of the RF signal, V π is the half-wave voltage of the modulator; jmg(t) represents the complex domain modulation amount composed of the imaginary unit j, the modulation coefficient m and the RF signal g(t) during the electro-optical modulation process;

[0034] The first and second submodulators are both set at their maximum bias points, i.e., θ1 = θ2 = 0. At the same time, the bias angle of the third submodulator (not electrically modulated) is set to θ3 = arccos[-2J0(m)], and the bias angle of the phase shifter is set to θ4 = -2θ3 to suppress the optical carrier and ±2nd-order sidebands. The final modulated optical signal is expressed as:

[0035]

[0036] Among them, J n (·) is the expression of the first-order Bessel function of nth order. In order to simplify the expression, let g'(t)=cos(4ω RF t); The modulated optical pulse signal passes through a single-mode optical fiber with the same length and opposite dispersion as the dispersion compensation module, and the transfer function is D2(ω)=exp(-jβ2Lω 2 / 2), the expression of the output optical signal u2(t) is:

[0037]

[0038] in Represents the convolution operation, and G'(ω) is the frequency domain expression of g'(t).

[0039] As a preference, the time amplification system is set to include an input dispersion of Φ in The dispersion fiber, output dispersion is Φ out The second dispersion compensation module and the focal length dispersion are Φ f The time lens;

[0040] The frequency domain transfer function of input dispersion is D3(ω)=exp(-jβ2L in ω 2 / 2), which is equivalent to a length of L in Single-mode fiber, input dispersion Φ in =β2L in ;

[0041] The frequency domain transfer function of the output dispersion is D4(ω)=exp(jβ2L out ω 2 / 2), which is equivalent to a length of L out The dispersion compensation fiber has an output dispersion of Φ out =β2L out .

[0042] As a preference, the input signal of the time amplification system is set to u2(t), and the corresponding frequency domain expression is U2(ω). Then, after the input dispersion Φ in The expression of the light field u3(t) after is:

[0043] u3(t)=F-1 [U2(ω)·D3(ω)]

[0044] The light field is then introduced into secondary phase modulation by a time lens based on an electro-optical phase modulator. The transmittance function of the time lens is recorded as h(t) = exp(-jt 2 / 2Φ f ), the corresponding frequency domain transfer function H(ω) is expressed as:

[0045]

[0046] Where F[·] represents the Fourier transform, and the output signal u after the time lens time-lens (t) is expressed as:

[0047] u time-lens (t)=u3(t)·h(t)

[0048] Finally, after the output dispersion Φ out The expression of the output light field u4(t) of the time domain imaging system is obtained as follows:

[0049]

[0050] In order to achieve optical signal scaling, the dispersion element and time lens are set to satisfy the following relationship:

[0051]

[0052] Preferably, the expression of the output photocurrent I(t) obtained after passing through the photodetector is set to:

[0053]

[0054] in, is the conjugate form of u4(t), I0(t) represents the intensity of the electrical pulse struck by the optical pulse generated by the mode-locked laser, is the absolute value of the group delay dispersion of the single-mode fiber and the dispersion-compensating fiber in the time amplification system, M = Φ out / Φ in is the stretching multiple of the time magnification system.

[0055] Compared with the prior art, the present invention has the following beneficial effects: (1) Improved frequency resolution: The time amplification system achieves effective stretching of the time domain waveform through dispersion control, which improves the frequency measurement resolution to more than dozens of times that of the traditional system, and effectively solves the pulse aliasing problem when multi-frequency signals are densely distributed; (2) Broadband multi-frequency simultaneous measurement: Based on the quadruple frequency modulation and dispersion frequency-time mapping mechanism, high-precision linear separation of multi-frequency signals in the time domain within a broadband range is achieved, supporting the synchronous measurement of multi-frequency signals at GHz-level bandwidth; (3) System complexity optimization: The complex optical fiber loop and channel sorting module are replaced by a parallel modulator structure, reducing the dependence on dispersion matching accuracy and bias stability; the time amplification system only needs to adjust the dispersion parameters, which significantly improves the system feasibility and environmental adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 A schematic diagram of a typical microwave photon frequency measurement system in the prior art;

[0057] Figure 2 A schematic diagram of a typical high-resolution frequency measurement system based on stimulated Brillouin scattering enhancement in the prior art;

[0058] Figure 3 This is a schematic diagram of the architecture of a typical photon-assisted multi-tone microwave frequency measurement system based on pulse recognition in the prior art;

[0059] Figure 4 The figure is a schematic diagram of the architecture of the high-resolution microwave photon frequency measurement system based on real-time Fourier transform of the present invention. DETAILED DESCRIPTION

[0060] To more clearly illustrate the embodiments of the present invention, specific embodiments of the present invention will be described below with reference to the accompanying drawings. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings and other embodiments can be obtained based on these drawings without inventive efforts.

[0061] This invention proposes a microwave photonic link for high-resolution frequency measurement based on short-time Fourier transform and a time amplification system. The microwave signal to be measured is introduced into dual parallel Mach-Zehnder modulators to modulate the time-stretched optical pulse. By setting the bias voltages of each sub-modulator, the optical carrier and ±2nd-order sidebands are suppressed, thereby achieving a fourfold frequency amplification. Dispersion group delay is then used to separate the different sideband frequency components in the time domain. By acquiring the time domain waveform, the signal spectrum is indirectly measured. Simultaneously, a time amplification system is introduced. By adjusting the dispersion parameters of the time amplification system, the time axis can be amplified dozens of times, greatly improving the measurement bandwidth and frequency resolution.

[0062] like Figure 4 As shown, the high-resolution microwave photon frequency measurement system based on real-time Fourier transform of the present invention includes a mode-locked laser, two dispersion compensation modules, a dual-driven Mach-Zehnder modulator (dual parallel), a Mach-Zehnder modulator, a dispersive optical fiber, a time lens, an erbium-doped fiber amplifier, a photodetector and an oscilloscope;

[0063] First, a mode-locked laser outputs a femtosecond Gaussian optical pulse, which is then injected into a dispersion compensation module and stretched into a chirped optical pulse. The chirped optical pulse is then modulated by a parallel electro-optical modulator consisting of three sub-modulators and a phase shifter. A microwave signal to be measured, with a 90° phase difference, is introduced into the RF ports of sub-modulators 1 and 2, respectively, at their maximum bias point to suppress odd-order sidebands. Simultaneously, the bias voltage of sub-modulator 3 (not electrically modulated) and the drive voltage of the phase shifter are set to eliminate the optical carrier and ±2nd-order sidebands. The modulated optical signal then passes through a section of dispersive fiber with the same dispersion, achieving a quadruple amplification of the microwave signal's frequency.

[0064] The time lens amplification system consists of a section of dispersive optical fiber, a time lens based on an electro-optical phase modulator, and a dispersion compensation module. The optical signal after time amplification is then converted by a photodetector into a microwave pulse carrying the frequency information of the signal to be measured, and finally observed by an oscilloscope.

[0065] In the system of the present invention, the frequency of the microwave signal is measured based on the correspondence between the microwave signal frequency and the time interval between the microwave pulses. Therefore, the system of the present invention, based on the short-time Fourier transform and time amplification system, achieves high-resolution frequency measurement over a wide frequency band.

[0066] The detailed steps are as follows:

[0067] First, the femtosecond Gaussian light pulse u0(t) output by the mode-locked laser is expressed as:

[0068]

[0069] Where a is the peak electric field intensity of the Gaussian light pulse, τ is the full width at half maximum of the pulse, t is the time variable, and the frequency domain expression corresponding to u0(t) is U0(ω);

[0070] The Gaussian light pulse passes through the transfer function D1(ω)=exp(jβ2Lω 2 / 2) dispersion compensation fiber is stretched and widened into a light pulse with chirped characteristics. The electric field intensity u1(t) of the output chirped light pulse is expressed as:

[0071] u1(t)=F -1[U0(ω)·D1(ω)]=F -1 [U0(ω)exp(jβ2Lω 2 / 2)]

[0072] Among them, F -1 [·] denotes the inverse Fourier transform operation, β2 and L denote the group velocity dispersion of the dispersion compensation module and the fiber length, respectively, ω denotes the angular frequency of the optical pulse, and j denotes the imaginary unit.

[0073] Assume that the RF signal g(t) input to the receiving end of the dual-drive Mach-Zehnder modulator is expressed as:

[0074] g(t)=cos(ω RF t);

[0075] ω RF Indicates the frequency of the microwave signal to be measured.

[0076] The chirped optical pulse passes through the parallel electro-optical modulator, which modulates the RF signal into the optical domain. The modulated optical signal u m (t) is expressed as:

[0077]

[0078] Among them, u dp-ddmzm (t) is the output light field of the dual-parallel-dual-drive Mach-Zehnder modulator, u mzm (t) is the output light field of the cascaded Mach-Zehnder modulator and phase shifter, θ1, θ2, θ3, and θ4 represent the bias angles of the first, second, and third sub-modulators, and the phase shifter, respectively; m represents the modulation coefficient of the electro-optic modulator, m = πV / V π , where V is the amplitude of the RF signal, V π is the half-wave voltage of the modulator; jmg(t) represents the complex domain modulation amount composed of the imaginary unit j, the modulation coefficient m and the RF signal g(t) in the electro-optical modulation process.

[0079] Submodulators 1 and 2 are both set at their maximum bias points, i.e., θ1 = θ2 = 0. At the same time, according to theoretical derivation, the bias angle of submodulator 3 without electrical modulation is set to θ3 = arccos[-2J0(m)], and the bias angle of the phase shifter is set to θ4 = -2θ3 to suppress the optical carrier and ±2nd-order sidebands. The final modulated optical signal is expressed as:

[0080]

[0081] Among them, J n (·) is the expression of the first-order Bessel function of nth order. In order to simplify the expression, let g'(t)=cos(4ω RFt); The modulated optical pulse signal passes through a single-mode optical fiber with the same length and opposite dispersion as the dispersion compensation module, and the transfer function is D2(ω)=exp(-jβ2Lω 2 / 2), the expression of the output optical signal u2(t) is:

[0082]

[0083] in Represents the convolution operation, G'(ω) is the frequency domain expression of g'(t), and the frequency is quadrupled.

[0084] Then the optical signal enters the time amplification system. Assume that the time amplification system includes input dispersion Φ in The dispersion fiber, output dispersion is Φ out The dispersion compensation module and the focal length dispersion are Φ f The time lens;

[0085] The frequency domain transfer function of input dispersion is D3(ω)=exp(-jβ2L in ω 2 / 2), which is equivalent to a length of L in Single-mode fiber, input dispersion Φ in =β2L in ;

[0086] The frequency domain transfer function of the output dispersion is D4(ω)=exp(jβ2L out ω 2 / 2), which is equivalent to a length of L out The dispersion compensation fiber has an output dispersion of Φ out =β2L out .

[0087] Assuming the input signal of the time amplification system is u2(t), the corresponding frequency domain expression is U2(ω), then after the input dispersion Φ in The expression of the light field u3(t) after is:

[0088] u3(t)=F -1 [U2(ω)·D3(ω)]

[0089] The light field is then introduced into secondary phase modulation by a time lens based on an electro-optical phase modulator. The transmittance function of the time lens is recorded as h(t) = exp(-jt 2 / 2Φ f ), the corresponding frequency domain transfer function H(ω) is expressed as:

[0090]

[0091] Where F[·] represents Fourier transform. At this time, the output signal u after the time lens is time-lens (t) is expressed as:

[0092] u time-lens (t)=u3(t)·h(t)

[0093] Finally, after the output dispersion Φ out The expression of the output light field u4(t) of the time domain imaging system is obtained as follows:

[0094]

[0095] In order to achieve optical signal scaling, the dispersion element and time lens are set to satisfy the following relationship:

[0096]

[0097] The expression of the output photocurrent I(t) obtained after passing through the photodetector is:

[0098]

[0099] in, is the conjugate form of u4(t), I0(t) represents the intensity of the electrical pulse struck by the optical pulse generated by the mode-locked laser, is the absolute value of the group delay dispersion of the single-mode fiber and the dispersion-compensating fiber in the time amplification system, M = Φ out / Φ in is the stretching multiple of the time magnification system.

[0100] As can be seen from the above formula, the frequency of the radio frequency is accurately mapped to the time interval of the pulse. According to the time interval of the microwave pulse, combined with the one-to-one correspondence between the frequency and time corresponding to the frequency-time mapping component, the frequency measurement of the microwave signal can be achieved. The output signal of the microwave photon frequency measurement system is the stretching of the input signal envelope. The stretching multiple is determined by the ratio of the two dispersion segments. In order to achieve a larger amplification factor, Φ is required. out >>Φ in , at this time Φ in ≈Φ f , considering that the focal length dispersion cannot be too small, the output dispersion Φ is usually set out Relatively large.

[0101] The present invention adopts a combined structure of a dual-drive Mach-Zehnder modulator and a single-drive Mach-Zehnder modulator in parallel, and precisely controls the bias of three sub-modulators and a phase shifter to achieve the suppression of the optical carrier and ±2nd-order sidebands, so that the output optical signal retains only ±4th-order sidebands, thereby quadrupling the frequency mapping accuracy of the microwave signal to be measured.

[0102] Based on a cascaded system of input dispersion, time lens, and output dispersion, the system achieves linear stretching and amplification of the time domain waveform by adjusting the ratio of output dispersion to input dispersion. This system can significantly expand the signal's time domain width, breaking through the resolution limitations of traditional frequency-time mapping systems.

[0103] The above description is only a detailed description of the preferred embodiments and principles of the present invention. For ordinary technicians in this field, based on the ideas provided by the present invention, there may be changes in the specific implementation methods, and these changes should also be considered as the scope of protection of the present invention.

Claims

1. A high-resolution microwave photon frequency measurement system based on real-time Fourier transform, characterized by: The device comprises a mode-locked laser, a first dispersion compensation module, a second dispersion compensation module, a dual-driven Mach-Zehnder modulator, a Mach-Zehnder modulator, a dispersion optical fiber, a time lens, an erbium-doped fiber amplifier, a photodetector and an oscilloscope; The mode-locked laser outputs a femtosecond Gaussian light pulse, which is injected into the first dispersion compensation module and stretched into a chirped light pulse. The chirped optical pulse is modulated by a parallel electro-optical modulator; The electro-optical modulator includes a dual-drive Mach-Zehnder modulator and a Mach-Zehnder modulator; the dual-drive Mach-Zehnder modulator has two radio frequency ports, and the two radio frequency ports respectively introduce a microwave signal to be measured with a phase difference of 90°; The modulated optical signal is sequentially passed through the dispersion fiber, the time lens, the second dispersion compensation module and the erbium-doped fiber amplifier for time amplification processing; The optical signal after time amplification processing is converted by a photodetector into a microwave pulse carrying the frequency information of the signal to be measured, and is observed by an oscilloscope.

2. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 1, characterized in that: The dual-drive Mach-Zehnder modulator includes a first sub-modulator and a second sub-modulator; the radio frequency ports of the first sub-modulator and the second sub-modulator respectively introduce a microwave signal to be measured with a phase difference of 90°; the Mach-Zehnder modulator includes a third sub-modulator and a phase shifter, and is used to eliminate the optical carrier and ±2nd-order sidebands in the chirped optical pulse by setting the bias voltage of the unmodulated third sub-modulator and the driving voltage of the phase shifter.

3. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 2, characterized in that: The femtosecond Gaussian light pulse u0(t) output by the mode-locked laser is expressed as: Where a is the peak electric field intensity of the Gaussian light pulse, τ is the full width at half maximum of the pulse, t is the time variable, and the frequency domain expression corresponding to u0(t) is U0(ω); The Gaussian light pulse passes through the transfer function D1(ω)=exp(jβ2Lω 2 / 2) dispersion compensation fiber is stretched and widened into a light pulse with chirped characteristics. The electric field intensity u1(t) of the output chirped light pulse is expressed as: u1(t)=F -1 [U0(ω)·D1(ω)]=F -1 [U0(ω)exp(jβ2Lω 2 / 2)] Among them, F -1 [·] denotes the inverse Fourier transform operation, β2 and L denote the group velocity dispersion of the dispersion compensation module and the fiber length, respectively, ω denotes the angular frequency of the optical pulse, and j denotes the imaginary unit.

4. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 3, characterized in that: Assume that the RF signal input to the receiving end of the dual-drive Mach-Zehnder modulator is g(t), and its expression is: g(t)=cos(ω RF t); ω RF Indicates the frequency of the microwave signal to be measured.

5. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 4, characterized in that: The chirped optical pulse passes through the parallel electro-optical modulator, modulating the RF signal into the optical domain. The modulated optical signal u m (t) is expressed as: Among them, u dp-ddmzm (t) is the output light field of the dual-driven Mach-Zehnder modulator, u mzm (t) is the output light field of the cascaded Mach-Zehnder modulator and phase shifter, θ1, θ2, θ3, and θ4 represent the bias angles of the first, second, and third sub-modulators, and the phase shifter, respectively; m represents the modulation coefficient of the electro-optic modulator, m = πV / V π , where V is the amplitude of the RF signal, V π is the half-wave voltage of the modulator; jmg(t) represents the complex domain modulation amount composed of the imaginary unit j, the modulation coefficient m and the RF signal g(t) during the electro-optical modulation process; The first and second submodulators are both set at their maximum bias points, i.e., θ1 = θ2 = 0. At the same time, the bias angle of the third submodulator (not electrically modulated) is set to θ3 = arccos[-2J0(m)], and the bias angle of the phase shifter is set to θ4 = -2θ3 to suppress the optical carrier and ±2nd-order sidebands. The final modulated optical signal is expressed as: Among them, J n (·) is the expression of the first-order Bessel function of nth order, and let g'(t)=cos(4ω RF t); The modulated optical pulse signal passes through a single-mode optical fiber with the same length and opposite dispersion as the dispersion compensation module, and the transfer function is D2(ω)=exp(-jβ2Lω 2 / 2), the output optical signal u2(t) is expressed as: in Represents the convolution operation, and G'(ω) is the frequency domain expression of g'(t).

6. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 5, characterized in that: Assume that the time amplification system includes an input dispersion of Φ in The dispersion fiber, output dispersion is Φ out The second dispersion compensation module and the focal length dispersion are Φ f The time lens; The frequency domain transfer function of input dispersion is D3(ω)=exp(-jβ2L in ω 2 / 2), which is equivalent to a length of L in Single-mode fiber, input dispersion Φ in =β2L in ; The frequency domain transfer function of the output dispersion is D4(ω)=exp(jβ2L out ω 2 / 2), which is equivalent to a length of L out The dispersion compensation fiber has an output dispersion of Φ out =β2L out .

7. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 6, characterized in that: Assuming the input signal of the time amplification system is u2(t), the corresponding frequency domain expression is U2(ω), then after the input dispersion Φ in The expression of the light field u3(t) after is: u3(t)=F -1 [U2(ω)·D3(ω)] The light field is then introduced into secondary phase modulation by a time lens based on an electro-optical phase modulator. The transmittance function of the time lens is recorded as h(t) = exp(-jt 2 / 2Φ f ), the corresponding frequency domain transfer function H(ω) is expressed as: Where F[·] represents the Fourier transform, and the output signal u after the time lens time-lens (t) is expressed as: u time-lens (t)=u3(t)·h(t) Finally, after the output dispersion Φ out The expression of the output light field u4(t) of the time domain imaging system is obtained as follows: In order to achieve optical signal scaling, the dispersion element and time lens are set to satisfy the following relationship:

8. The high-resolution microwave photon frequency measurement system based on real-time Fourier transform according to claim 7, characterized in that: The expression of the output photocurrent I(t) obtained after passing through the photodetector is set to: in, is the conjugate form of u4(t), I0(t) represents the intensity of the electrical pulse struck by the optical pulse generated by the mode-locked laser, is the absolute value of the group delay dispersion of the single-mode fiber and the dispersion-compensating fiber in the time amplification system, M = Φ out / Φ in is the stretching multiple of the time magnification system.

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