Broadband microwave frequency measurement system based on optical Fourier transform

Abstract

The invention relates to a broadband microwave frequency measurement system based on optical Fourier transform. In order to solve the problem of rapid spectrum sensing requirements on multi-frequency-point and multi-modulation-system electromagnetic signals in a wide spectrum range in applications such as radar, radio communication and cognitive radio, a microwave photon link is combined with space optics, and microwave frequency signals to be tested are converted into optical signals; spectral distribution of a microwave signal to be detected is obtained through a Fourier transform light path in combination with light intensity position detection. Microwave frequency spectrum information is mapped through spatial light field intensity position distribution detection, high-speed data acquisition and large-data-volume Fourier transform calculation are omitted, and the requirements for the sampling rate and calculation and storage resource consumption in high-speed and broadband microwave signal frequency spectrum analysis are lowered.

Description

Technical Field

[0001] The invention relates to a broadband microwave frequency measurement system based on optical Fourier transform, and belongs to the field of optical information processing and microwave photons. Background Art

[0002] Electromagnetic spectrum detection is widely used in all walks of life, including biomedicine, chemical imaging, manufacturing, terahertz waves and microwave photonics. The electromagnetic spectrum inspection method based on microwave photonics combines electromagnetic spectrum detection with optical signal processing, and uses photonic technology to improve the bandwidth and speed of electromagnetic signal spectrum detection.

[0003] From the perspective of RF signal spectrum detection, in modern electromagnetic signal processing systems, signals are constantly developing towards high frequency, high bandwidth, digitalization, and multiple standards, covering all frequency bands from MHz to GHz. From the perspective of detection, large bandwidth, high resolution, and easy operation have become the research direction of rapid spectrum detection. However, limited by the local oscillator scanning time and the bandwidth of the detector and oscilloscope, traditional electrical measuring instruments cannot meet the requirements of large bandwidth electromagnetic signal measurement; the scanning speed of the spectrum analyzer is slow and cannot meet the requirements of accuracy and ultra-fast speed compatibility. In response to the demand for rapid spectrum perception of electromagnetic signals with multiple frequency points and multiple modulation standards in a wide spectrum range in applications such as radar, radio communications, and cognitive radio, it is necessary to study broadband real-time electromagnetic spectrum perception methods to achieve rapid spectrum perception in a wide spectrum range. Summary of the invention

[0004] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a broadband microwave frequency measurement system based on optical Fourier transform, which utilizes the linear conversion of microwave signals in the optical domain and the optical Fourier transform technology, combined with large-scale integrated waveguide design, to realize the time-domain and spatial-domain optical Fourier transform of broadband microwave signals, and combines with spatial parallel optoelectronic signal processing technology to obtain rapid analysis of microwave spectrum information.

[0005] In order to solve the above technical problems, the technical solution of the present invention is as follows:

[0006] The laser light source outputs an optical signal;

[0007] The electro-optic modulator loads the radio frequency microwave signal onto the optical signal;

[0008] The optical signal is input into the star coupler and divided into multiple beams, and then enters the arithmetic delay array waveguide;

[0009] The output optical signal passes through an optical lens, and on the spectrum plane of the Fourier optical path, the light intensity distribution is detected by an array photodetector to obtain the spectrum distribution of the input microwave signal.

[0010] 2. Furthermore, the delay interval difference between two adjacent waveguides of the equidistant delay array waveguide is fixed. After the optical signal is input, the light field intensity of each output end face of the equidistant delay array waveguide lags behind the light intensity of the shortest delay waveguide by a fixed time interval.

[0011] 3. Further, the optical signal is distributed in the equidistant delay array waveguide, and its output optical signal also presents a spatial array distribution; this process is to complete the sampling of the modulated optical signal intensity within the maximum delay time period at the equal delay interval.

[0012] 4. Furthermore, the spacing between the equidistant delay array waveguide and the optical lens needs to ensure that the output end face of the equidistant delay array waveguide and the photoelectric detection array are located on the front and rear focal planes of the optical lens, respectively.

[0013] 5. Furthermore, the arithmetic delay array waveguide uses thin film lithium niobate material as the waveguide material.

[0014] 6. Further, the photodetector uses a photodiode (PN-PD) or avalanche photodiode (APD) detector array. The PD or APD array adopts a parallel staggered structure layout to improve the light field intensity detection resolution.

[0015] The embodiment of the present application provides a broadband microwave frequency measurement system based on optical Fourier transform, in which a narrow linewidth laser light source outputs an optical carrier and inputs an electro-optical modulation module, and the microwave signal that varies with time is converted into a light intensity signal that varies with time through the electro-optical modulator. In this process, it is necessary to control the working state of the electro-optical modulator to ensure the linear conversion of microwave to light wave frequency, avoid the introduction of high-order modulation sideband spectrum signals, and interfere with the spectrum separation of the output end spectrum plane; thereby achieving the linear conversion of microwave signal to optical domain signal intensity, so as to ensure the one-to-one correspondence between laser wavelength and microwave frequency.

[0016] The microwave signal converted to the optical domain is input into the star coupler, and after the array beam splitting, it enters the equidistant delay waveguide array output. At a certain moment on the output end face of the array waveguide, the spatial distribution of the output optical field is the one-to-one correspondence of the change in the intensity of the time-domain optical field within the maximum delay time period of the array delay waveguide, which is equivalent to sampling the light intensity signal that changes with time over a period of time and converting it to a spatial arrangement distribution. This forms the intensity distribution of the time-domain microwave signal in the optical domain space to adapt to the optical Fourier transform.

[0017] The equidistant delay waveguide array outputs parallel optical signals into the Fourier optical system, and the distribution of the input light field spatial spectrum is at the rear focal plane of the lens, thereby realizing the time-domain-spatial optical Fourier transform of the input modulated light signal, that is, converting the spectrum of the loaded time-domain microwave signal into the position distribution according to the spatial light field intensity. After the parallel photoelectric detection array and parallel sampling and calculation, the spectrum distribution of the time-domain microwave signal is obtained to realize the rapid frequency measurement of broadband microwave signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a system functional block diagram of a broadband microwave frequency measurement system based on optical Fourier transform of the present invention;

[0019] Figure 2 This is a block diagram of the frequency measurement system based on optical Fourier transform;

[0020] Figure 3 Schematic diagram of detector array structure with parallel staggered array layout

[0021] Figure 4 is a structural diagram of the waveguide array;

[0022] Figure 5 This is a typical 2f optical system optical path;

[0023] Figure 6 It is the geometric relationship of the optical Fourier transform light path at oblique incidence;

[0024] Figure 7 It is the spatial light field intensity position distribution of different frequencies detected by PD on the spectrum plane;

[0025] Figure 8 It is the fitting curve of spatial light field intensity position coordinate and frequency; DETAILED DESCRIPTION

[0026] An embodiment of the present invention is described in detail below with reference to the accompanying drawings.

[0027] like Figure 1 and Figure 2 As shown, the structure of the scheme includes a laser light source, an electro-optic modulator, a star coupler, an equidistant delay waveguide array, an optical lens, a photodetector and a signal processing system;

[0028] The laser light source is a narrow line width laser, and the typical value of the laser line width is in the MHz level or KHz level;

[0029] The electro-optic modulator may be a lithium niobate Mach-Zehnder electro-optic modulator, or other high linearity electro-optic modulators;

[0030] The microwave signal frequency is the microwave frequency to be measured;

[0031] The star coupler realizes the optical signal array beam splitting;

[0032] The arithmetic delay waveguide array, in this example, the typical array number is 1000;

[0033] The optical lens is a spherical biconvex lens;

[0034] The photodetector is a PD or APD array;

[0035] The central wavelength of the optical signal is 1550±20nm;

[0036] The narrow linewidth light source outputs an optical carrier and enters the electro-optical modulation module, and the time-varying microwave signal is converted into a time-varying light intensity through the electro-optical modulator. In this process, it is necessary to control the working state of the electro-optical modulator to ensure the linear conversion of microwave frequency to optical frequency, avoid the addition of high-order modulation sideband spectrum signals, and interfere with the spectrum separation of the output spectrum plane; thereby achieving the linear conversion of microwave signal to optical domain signal intensity to ensure the one-to-one correspondence between laser wavelength and microwave frequency.

[0037] The equidistant delay waveguide array outputs parallel light field signals, which then enter the Fourier optical system. The rear focal plane of the lens is the spatial spectrum distribution of the input light field, thereby realizing the time-domain-spatial optical Fourier transform of the input modulated light signal, that is, converting the loaded time-domain microwave signal spectrum step by step into the position distribution according to the spatial light field intensity. After the parallel photoelectric detection array and parallel sampling and calculation, the spectrum information of the time-domain microwave signal is obtained to realize the rapid frequency measurement of broadband microwave signals. The layout structure of the parallel photoelectric detection array is as follows: Figure 3 shown.

[0038] The optical time domain signal propagating along the z-axis output by the Mach-Zehnder modulator is:

[0039]

[0040] Among them, v is the frequency of the microwave signal to be measured, exp(jkz)exp(jω0t) is the propagation factor of the optical signal in the optical path, k is the optical wave vector, t is the time, and ω0 is the angular frequency of the optical carrier. Therefore, it can be approximately considered that the change in the optical time domain intensity is determined by the intensity factor For n-way equidistant delay array waveguides arranged along the x-axis, considering the time delay Δt between adjacent waveguides, the total light field output should be:

[0041]

[0042] The structure of the asymmetric delay array waveguide is as follows Figure 4As shown, d is the waveguide width, D is the waveguide spacing, L is the total width of the array waveguide output end, * is the convolution operator symbol, comb represents the comb function, δ represents the pulse function, and the waveguide output is set to the rectangular function rect.

[0043] After the optical signal enters the array waveguide, the spatial distribution of the optical field intensity at the output end face of the array waveguide is to sample the light intensity signal that changes with time in the maximum delay time period T of the array delay waveguide equidistantly according to the discrete time Δt, and distribute it equidistantly in one-to-one correspondence according to the spatial position. Therefore, in order to achieve full coverage of microwave signals within a wide frequency band, the maximum delay time must at least cover a complete signal cycle of the lowest frequency in the frequency range to be measured; and for high-frequency signals, it is necessary to ensure that there are enough sampling points in a complete cycle to ensure the frequency measurement accuracy. Therefore, the resolution accuracy within the frequency range to be measured determines the delay interval and array number of the equidistant delay array waveguide.

[0044] The parallel light field signal of the output light of the equidistant time-delay waveguide array will enter the Fourier optical system. Among them, the lens is used to realize the Fourier transform of the input plane spatial distribution light field intensity. The fundamental reason is that the lens has the ability to apply phase modulation to the incident wavefront. Figure 5 A typical 2f optical system based on a single convex lens is demonstrated.

[0045] In the above optical path system, the complex amplitude distribution of the light field along the x0 axis and y0 axis of the front focal plane is U0(x0,y0), and the corresponding spectrum distribution is T0(x f ,y f ),x f and f They are the spatial frequency coordinate axes corresponding to the x0 axis and the y0 axis. According to the Fresnel diffraction theory, the transfer function is:

[0046]

[0047] f x and f y x f and f The spatial frequency on the lens, d0 is the focal length of the lens, λ is the wavelength of light, thus, the front focal plane U of the lens along the x-axis and y-axis l The spectrum of the parallel light field distribution of (x, y) is expressed as:

[0048]

[0049] Represents Fourier transform calculation. The light field distribution of light propagating from the lens to the back plane can be calculated by the Fresnel diffraction formula as follows:

[0050]

[0051] f is the distance between the back plane and the lens. Substituting (1) and (2) into (3), we can get the light field distribution on the back plane as the accurate Fourier transform of the U0 surface:

[0052]

[0053] When f=d0, that is, the object is at the front focal plane of the lens, the phase curvature disappears, and the light field intensity distribution at the rear focal plane is the accurate Fourier transform of the object. However, when f is other values, the phase curvature will not affect the intensity distribution at the rear focal plane.

[0054] According to the optical Fourier transform optical path, the relationship between the frequency value and the back focal plane coordinate is:

[0055]

[0056] In order to distinguish the intensity distribution of light of different frequencies on the rear focal plane, the lens input end uses inclined light input, whose propagation direction is at an angle θ with the z-axis, and converges on the rear focal plane (0, y f ) point, when the incidence is oblique, the relationship between the input spectrum frequency component and the position of the rear focal plane point is shown in the figure Figure 5 shown.

[0057] Depend on Figure 6 The geometric relationships shown are

[0058]

[0059] Under the paraxial approximation, the spatial frequency of the plane wave component can be expressed as

[0060]

[0061] Therefore, for any point (0,y f ) complex amplitude distribution, which corresponds to the spatial frequency point on the back focal plane The amplitude and phase of the plane wave component of the back focal plane, that is, the spectrum distribution of the input spectrum component can be obtained on the back focal plane. Randomly select a microwave frequency range, from 2GHz to 40GHz, with a step setting of 1GHz. The spatial light field intensity position distribution detected by the PD array on the back focal plane corresponds from right to left, as shown in Figure 7 It is planned to record the spatial light field intensity position coordinates at a step interval of 1 GHz, and obtain the relationship curve between the vertical coordinate and the frequency of the point where the spatial light field intensity distribution deviates from the center position as shown in Figure 8 shown.

[0062] Therefore, after the output optical signal of the equidistant delay waveguide array is processed by the above optical system, the spatial distribution of the light field intensity will be used on the back focal plane of the lens to map the distribution of the input light field spatial spectrum, thereby realizing the time-domain-spatial domain optical Fourier transform of the input modulated optical signal, that is, the conversion of the spectrum of the time-domain microwave signal to be measured to the distribution of the light field intensity according to the spatial position is obtained. After the parallel photodetector array and parallel sampling and calculation, the spectrum distribution of the time-domain microwave signal can be obtained, so as to realize the rapid frequency measurement of broadband microwave signals and avoid high-speed data acquisition and Fourier transform calculation of large amounts of data.

[0063] The present invention proposes a broadband microwave frequency measurement system based on optical Fourier transform, which loads microwave signals into the optical domain through electro-optical modulation, realizes the conversion of the optical signal from the time domain to the spatial distribution using an equidistant delay waveguide array, and then obtains the conversion of the input microwave spectrum to the spatial light field intensity position distribution through a lens-based Fourier optical path, and obtains the spectrum distribution of the input microwave signal through intensity detection by an array photodetector. This spectrum analysis method that realizes the conversion of time-domain microwave signals to spatial light intensity position distribution through an equidistant delay waveguide array and optical Fourier transform reduces the demand for the collection and processing of high-speed microwave signals, and parallel photoelectric detection also improves the frequency measurement speed.

[0064] The implementation cases described above are only preferred specific implementation modes of the present invention. All common changes and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A broadband microwave frequency measurement system based on optical Fourier transform, characterized in that: The laser light source outputs an optical signal; The electro-optic modulator loads the radio frequency microwave signal onto the optical signal; The optical signal is input into the star coupler and divided into multiple beams, and then enters the arithmetic delay array waveguide; The output optical signal passes through an optical lens, and on the spectrum plane of the Fourier optical path, the light intensity distribution is detected by an array photodetector to obtain the spectrum distribution of the input microwave signal.

2. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The delay interval difference between two adjacent waveguides of the equidistant delay array waveguide is fixed. After the optical signal is input, the light field intensity of each output end face of the equidistant delay array waveguide lags behind the light intensity of the shortest delay waveguide by a fixed time interval.

3. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The optical signal is distributed in the equidistant delay array waveguide, and its output optical signal also presents a spatial array distribution; the process is to complete the sampling of the modulated optical signal intensity in the maximum delay time period at the equal delay interval.

4. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The spacing between the equidistant delay array waveguide and the optical lens needs to ensure that the output end face of the equidistant delay array waveguide and the photoelectric detection array are located on the front and rear focal planes of the optical lens respectively.

5. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The arithmetic delay array waveguide uses thin film lithium niobate material as the waveguide material.

6. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The photodetector uses a photodiode (PN-PD) or avalanche photodiode (APD) detector array.

7. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 6, characterized in that: The PD or APD array adopts a parallel staggered structure layout to improve the light field intensity detection resolution.

8. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: It includes laser light source, electro-optic modulator, star coupler, arithmetic delay waveguide array, optical lens, photoelectric detector and signal processing system; The laser light source is a narrow line width laser, and the typical value of the laser line width is in the MHz level or KHz level; The electro-optic modulator adopts a lithium niobate Mach-Zehnder electro-optic modulator; The microwave signal frequency is the microwave frequency to be measured; The star coupler realizes the optical signal array beam splitting; The number of the arithmetic delay waveguide arrays is 1000; The optical lens is a spherical biconvex lens; The photodetector is a PD or APD array; The central wavelength of the optical signal is 1550±20nm.

9. The broadband microwave frequency measurement system based on optical Fourier transform according to claim 1, characterized in that: The equidistant delay waveguide array outputs parallel light field signals, which then enter the Fourier optical system. The rear focal plane of the lens is the spatial spectrum distribution of the input light field, thereby realizing the time-domain-spatial domain optical Fourier transform of the input modulated light signal, that is, converting the loaded time-domain microwave signal spectrum step by step into the position distribution according to the spatial light field intensity; after the parallel photoelectric detection array and parallel sampling and calculation, the spectrum information of the time-domain microwave signal is obtained to realize the rapid frequency measurement of broadband microwave signals; the details are as follows: The optical time domain signal propagating along the z-axis output by the Mach-Zehnder modulator is: Where v is the frequency of the microwave signal to be measured, exp(jkz)exp(jω0t) is the propagation factor of the optical signal in the optical path, k is the optical wave vector, t is the time, and jω0 is the angular frequency of the optical carrier. For n-channel equidistant delay array waveguides arranged along the x-axis, considering the time delay Δt between adjacent waveguides, the total optical field output should be: d is the waveguide width, D is the waveguide spacing, L is the total width of the array waveguide output end, * is the convolution operator symbol, comb represents the comb function, δ represents the pulse function, and the waveguide output is set to the rectangular function rect; After the optical signal enters the array waveguide, the spatial distribution of the optical field intensity at the output end face of the array waveguide is the light intensity signal that changes with time in the maximum delay time period T of the array delay waveguide, which is sampled equidistantly according to the discrete time Δt and distributed equidistantly according to the spatial position. The parallel light field signal output by the equidistant delay waveguide array will enter the Fourier optical system. In the above optical path system, the complex amplitude distribution of the light field along the x0 axis and y0 axis of the front focal plane is U0(x0,y0), and the corresponding spectrum distribution is T0(x f ,y f ),x f and f They are the spatial frequency coordinate axes corresponding to the x0 axis and the y0 axis. According to the Fresnel diffraction theory, the transfer function is: f x and f y x f and f The spatial frequency on the lens, d0 is the focal length of the lens, λ is the wavelength of light, thus, the front focal plane U of the lens along the x-axis and y-axis l The spectrum of the parallel light field distribution of (x, y) is expressed as: Represents Fourier transform calculation, the light field distribution of light propagating from the lens to the back plane is calculated by the Fresnel diffraction formula as follows: f is the distance between the back plane and the lens. Substituting (1) and (2) into (3), the light field distribution on the back plane is the Fourier transform of the U0 surface: When f=d0, that is, the object is at the front focal plane of the lens, the phase curvature disappears, and the light field intensity distribution at the rear focal plane is the accurate Fourier transform of the object; however, when f is other values, the phase curvature produced will not affect the intensity distribution at the rear focal plane; According to the optical Fourier transform optical path, the relationship between the frequency value and the back focal plane coordinate is: In order to distinguish the intensity distribution of light of different frequencies on the rear focal plane, the lens input end uses inclined light input, whose propagation direction is at an angle θ with the z-axis, and converges on the rear focal plane (0, y f )point The geometric relationship is The spatial frequency of the plane wave component is expressed as Therefore, for any point (0,y f ) complex amplitude distribution, which corresponds to the spatial frequency point on the back focal plane The amplitude and phase of the plane wave component, that is, the spectral distribution of the input spectral components can be obtained on the back focal plane.

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