Microwave spectrum analysis system based on fiber FP cavity and optical frequency comb

Abstract

The microwave spectrum analysis system based on the fiber F-P cavity and the optical frequency comb relates to the field of microwave spectrum analysis. It solves the problem that the existing microwave photon channelized receiver has poor performance and limited performance while achieving real-time, high-resolution monitoring of the radio frequency spectrum with a large bandwidth. The present invention uses an optoelectronic modulator to load the radio frequency signal to be measured onto the single-frequency light emitted by a single-frequency laser. The obtained optical sideband signal is sent to the fiber F-P cavity for filtering and processing to obtain an optical signal with equal frequency intervals. The coupler is used to beat the optical signal with equal frequency intervals output by the fiber F-P cavity with the optical frequency comb output by the optical frequency comb generator. The obtained optical signal after beating is sent to a low-bandwidth photodetector for filtering and photoelectric detection to obtain an electrical signal. The electrical signal is sampled by a digital collector and sent to a processor for spectrum analysis to obtain the frequency spectrum of the radio frequency signal to be measured. The present invention is mainly used for microwave spectrum analysis.

Description

Technical Field

[0001] The present invention relates to the field of microwave spectrum analysis. Background Art

[0002] Microwave spectrum analysis is essential for RF signal applications, enabling precise analysis of the spectral structure of microwave signals for subsequent signal processing and application. Microwaves refer to electromagnetic waves in the 300MHz to 300GHz frequency range. With technological advancements, RF systems require increasingly larger frequencies and bandwidths. Traditional spectrum analyzers are unable to achieve real-time, high-resolution monitoring of wide-bandwidth RF spectra. However, channelized receivers based on microwave photonic technology can fundamentally overcome the bandwidth limitations of traditional electronic devices, significantly expanding the frequency detection range while effectively reducing the size, weight, and power consumption of system equipment. A microwave photonic channelized receiver first electro-optically modulates the microwave RF signal to be processed into the optical frequency domain. Using optical methods, it achieves channel separation, decomposing the broadband signal into multiple parallel narrowband sub-signals. These narrowband sub-signals require bandwidths 1 / N times that of the original signal. By analyzing and processing each narrowband, the complete microwave spectrum is obtained.

[0003] One approach to microwave photonic channelization is to use a set of optical filters with continuous passbands to filter out the desired narrowband sub-signals. This approach has the advantage of a simple structure, but it requires the use of multiple optical narrowband filters with identical center frequency differences. Furthermore, each filter must have a precise center wavelength and stable passband characteristics. This results in poor performance in channelized receivers, with optical signals severely attenuated as they pass through the filters one after another.

[0004] Another approach uses a set of fixed-wavelength optical sources as optical carriers, onto which a broadband RF signal is modulated. Optical sidebands are added to the center frequency of each "comb tooth," and a comb filter is used to select different slices of the RF spectrum. Finally, the narrowband sub-channels are separated, indirectly achieving channelization. This approach effectively improves spectral resolution, but in principle is equivalent to performing multiple electro-optical modulations in the optical domain using different frequencies as the optical carrier. Performance is significantly affected by the effects of the electro-optical modulator and the optical frequency comb, with response decreasing at higher spectral frequencies.

[0005] Therefore, the poor performance and performance limitations of the above two microwave photonic channelized receivers need to be solved urgently. Summary of the Invention

[0006] The purpose of the present invention is to solve the problem that existing microwave photon channelized receivers have poor performance and limited performance while achieving real-time, high-resolution monitoring of large-bandwidth radio frequency spectra. The present invention provides a microwave spectrum analysis system based on a fiber FP cavity and an optical frequency comb.

[0007] A microwave spectrum analysis system based on a fiber FP cavity and an optical frequency comb, including a single-frequency laser, an optoelectronic modulator, a fiber FP cavity, an optical frequency comb generator, a coupler, a low-bandwidth photodetector, a digital collector, and a processor;

[0008] The photoelectric modulator loads the RF signal to be measured onto the single-frequency light emitted by the single-frequency laser, obtaining an optical sideband signal with the same shape as the RF signal to be measured. The optical sideband signal is then sent to the optical fiber FP cavity for filtering to obtain an optical signal with equal frequency intervals.

[0009] The coupler is used to perform beat frequency processing on the optical signal with equal frequency intervals output by the optical fiber FP cavity and the optical frequency comb output by the optical frequency comb generator. The obtained beat frequency optical signal is sent to a low-bandwidth photodetector; the low-bandwidth photodetector is used to filter out the sum frequency and difference frequency in the beat frequency optical signal that are greater than half the frequency interval of the optical frequency comb, and then perform photoelectric detection to obtain an electrical signal. After the electrical signal is sampled by a digital collector, it is sent to a processor for spectrum analysis to obtain the frequency spectrum of the RF signal to be measured.

[0010] Preferably, the processor performs spectrum analysis on the received electrical signal to obtain the frequency spectrum of the radio frequency signal to be measured. The specific process is:

[0011] First, the received electrical signal is analyzed to obtain n signals with different frequencies, and the frequencies of the n signals are arranged from small to large and are equally spaced. Then, the n signals with different frequencies are calibrated and calculated to restore the frequency spectrum of the RF signal to be measured; n is an integer.

[0012] Preferably, the analysis of the received electrical signal is achieved by Fourier transform.

[0013] Preferably, the frequency interval of the optical signal output by the optical fiber FP cavity is different from the frequency interval of the optical frequency comb output by the optical frequency comb generator, and the frequency difference at the corresponding frequency position between the optical signal output by the optical fiber FP cavity and the optical frequency comb output by the optical frequency comb generator is an integer multiple of Δf, wherein Δf is the difference between the frequency interval of the optical signal output by the optical fiber FP cavity and the frequency interval of the optical frequency comb output by the optical frequency comb generator.

[0014] Preferably, the optical frequency comb generator is implemented using a mode-locked femtosecond laser or a microcavity laser.

[0015] Preferably, the low bandwidth photodetector operates in a frequency range of 0 MHz to 50 MHz.

[0016] Preferably, the sampling frequency range of the digital collector is 100 MHz to 2 GHz.

[0017] Preferably, the frequency spectrum of the radio frequency signal to be measured ranges from 10 MHz to 50 GHz.

[0018] The beneficial effects brought by the present invention are:

[0019] The present invention utilizes a fiber FP cavity and an optical frequency comb to achieve large-scale instantaneous spectrum analysis. The fiber FP cavity's inherent transmission spectrum features transmission peaks with strictly equal frequency spacing. Structurally, the present invention utilizes a fiber FP cavity to replace the complex bandpass filters used in the prior art. This avoids the use of multiple filters, reduces energy loss in the spectrum analysis system, and thus improves the performance of the entire spectrum analysis system, further addressing performance limitations. During signal processing, compared to the traditional method of using an optical frequency comb as an optical carrier, where each frequency output in the optical frequency comb is modulated as an optical carrier, resulting in a large amount of data being processed, the present invention uses only the optical frequency comb as a scale. This is equivalent to using the single-frequency light output by a single-frequency laser as an optical carrier for electro-optical modulation, effectively reducing the impact of the electro-optical modulator on the accuracy of the spectrum analysis system. Furthermore, the present invention uses the optical frequency comb as a scale, and the single-frequency light output by a single-frequency laser as an optical carrier. During subsequent signal analysis, the signal is analyzed using only the single-frequency light, resulting in a smaller amount of data and a simpler processing flow.

[0020] The microwave spectrum analysis system based on a fiber FP cavity and an optical frequency comb described in the present invention is capable of performing wide-bandwidth microwave spectrum analysis. Currently, fiber FP cavities can achieve a free spectral range (FSR) of up to 100 MHz. The frequency interval of the optical frequency comb is close to 100 MHz. Based on an FSR of 99.9 MHz, the frequency interval is 0.1 MHz, resulting in a spectrum measurement range of 10,000 * FSR / 2 = 50 GHz. The smaller the difference between the frequency period of the fiber FP cavity's transmission spectrum and the frequency period of the optical frequency comb, the greater the frequency measurement range, potentially exceeding 50 GHz. The frequency period is the frequency interval. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the principle of the microwave spectrum analysis system based on the fiber FP cavity and the optical frequency comb according to the present invention;

[0022] Figure 2 is the spectrum diagram of the radio frequency signal to be measured;

[0023] Figure 3 is the spectrum of the optical sideband signal output by the optoelectronic modulator 2; ω0 is the frequency of the laser emitted by the single-frequency laser;

[0024] Figure 4 is the spectrum of the optical signal obtained after the optical sideband signal passes through the optical fiber FP cavity 3;

[0025] Figure 5 This is a frequency difference effect diagram at corresponding positions in the optical frequency comb spectrum and the optical signal spectrum output by the optical fiber FP cavity 3;

[0026] Figure 6 is the theoretical transmission spectrum of the fiber FP cavity; where T represents transmittance, δ is the phase, and FSR is the free spectral range, i.e., the distance between the two peaks. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0029] See also Figures 1 to 4 This embodiment is described. The microwave spectrum analysis system based on the fiber FP cavity and the optical frequency comb described in this embodiment includes a single-frequency laser 1, an electro-optical modulator 2, a fiber FP cavity 3, an optical frequency comb generator 4, a coupler 5, a low-bandwidth photodetector 6, a digital collector 7, and a processor 8.

[0030] The photoelectric modulator 2 loads the radio frequency signal to be measured onto the single-frequency light emitted by the single-frequency laser 1, obtaining an optical sideband signal with the same shape as the radio frequency signal to be measured, and sends the optical sideband signal to the optical fiber FP cavity 3 for filtering processing to obtain an optical signal with equal frequency intervals;

[0031] The coupler 5 is used to perform beat frequency processing on the optical signal with equal frequency intervals output by the optical fiber FP cavity 3 and the optical frequency comb output by the optical frequency comb generator 4. The obtained beat frequency optical signal is sent to the low-bandwidth photodetector 6; the low-bandwidth photodetector 6 is used to filter out the sum frequency and difference frequency of the beat frequency optical signal that are greater than half the frequency interval of the optical frequency comb, and then perform photoelectric detection to obtain an electrical signal. After the electrical signal is sampled by the digital collector 7, it is sent to the processor 8 for spectrum analysis to obtain the frequency spectrum of the RF signal to be measured.

[0032] In specific applications, the optical frequency comb is a light signal with equally spaced frequencies and the same intensity. The transmission spectrum of the fiber FP cavity itself is a transmission peak with strictly equally spaced frequencies. Therefore, the use of multiple filters can be avoided, reducing the loss of the spectrum analysis system, thereby improving the performance of the entire spectrum analysis system and further breaking the problem of performance limitations. Among them, the transmission spectrum represents the transmittance of the corresponding frequency, so in the transmission peak part, the output signal basically retains the non-transmission peak part, and the corresponding optical frequency is filtered out.

[0033] The present invention uses the single-frequency light output by a single-frequency laser as an optical carrier for electro-optical modulation. Compared with the traditional form of using an optical frequency comb as an optical carrier, the prior art uses each frequency output in the optical frequency comb as an optical carrier for modulation. Comprehensive comparison shows that the present invention can reduce the impact of the electro-optical modulator 2 on spectrum analysis.

[0034] An optical frequency comb is a broadband spectrum composed of numerous spectral teeth with stable frequencies and strictly equal spacing. In the time domain, it manifests itself as a series of equally spaced ultrashort pulses. Essentially, it is a train of instantaneous optical pulses, each composed of several rapidly oscillating harmonics. Each pulse has a distinct center frequency, a narrow width, and a specific energy intensity. This invention exploits one of its characteristics: the teeth of the optical frequency comb have a fixed frequency, using this as a scale to achieve high-precision frequency measurement.

[0035] Fiber Fabry-Perot cavity, also known as fiber FP cavity. The basic structure is that the two ends of the optical fiber are coated with reflective film. The reflective film and the optical fiber form a closed FP cavity. The laser reflects back and forth in this cavity and finally forms a multi-beam interference output. The output transmission spectrum is shown in Figure 6 .

[0036] For further information, see Figure 5 The specific process of the processor 8 performing spectrum analysis on the received electrical signal and obtaining the frequency spectrum of the radio frequency signal to be measured is as follows:

[0037] First, the received electrical signal is analyzed to obtain n signals with different frequencies, and the frequencies of the n signals are arranged from small to large and are equally spaced. Then, the n signals with different frequencies are calibrated and calculated to restore the frequency spectrum of the RF signal to be measured; n is an integer.

[0038] In specific applications, the intensities of the n frequency positions of the n signals of different frequencies analyzed from the electrical signal have a one-to-one correspondence with the intensities of the frequencies at the corresponding positions of the RF signal to be measured; therefore, calibration calculations are performed, and according to the preset calibration rules, the frequency spectrum of the RF signal to be measured can be analyzed.

[0039] The spectrum analysis range of the microwave spectrum analysis system based on the fiber FP cavity 3 and optical frequency comb is determined by the frequency interval of the optical frequency comb and the center frequency difference between the transmission peaks of the fiber FP cavity 3. Theoretically, the spectrum analysis range can reach tens of GHz. The accuracy of the spectrum analysis depends on the half-width (FWHM) and free spectral range (FRS) of the fiber FP cavity. Theoretically, the analysis error can be less than 100 MHz.

[0040] Furthermore, the analysis of the received electrical signal can be achieved by using Fourier transform.

[0041] Furthermore, the frequency interval of the optical signal output by the fiber FP cavity 3 is different from the frequency interval of the optical frequency comb output by the optical frequency comb generator 4, and the frequency difference at the corresponding frequency position of the optical signal output by the fiber FP cavity 3 and the optical frequency comb output by the optical frequency comb generator 4 is an integer multiple of Δf, where Δf is the difference between the frequency interval of the optical signal output by the fiber FP cavity 3 and the frequency interval of the optical frequency comb output by the optical frequency comb generator 4.

[0042] Furthermore, the optical frequency comb generator 4 is implemented using a mode-locked femtosecond laser or a microcavity laser.

[0043] Furthermore, the operating frequency range of the low bandwidth photodetector 6 is 0 MHz to 50 MHz.

[0044] Furthermore, the operating frequency range of the digital collector 7 is a sampling frequency range of 100 MHz to 2 GHz.

[0045] Furthermore, the frequency spectrum of the radio frequency signal to be measured ranges from 10 MHz to 50 GHz.

[0046] Principle analysis:

[0047] The radio frequency signal to be measured is modulated by an optoelectronic modulator 2 onto the single-frequency laser light emitted by a single-frequency laser 1, generating an optical sideband signal with the same shape as the radio frequency signal to be measured. This optical sideband signal is then processed using a fiber FP cavity 3, producing a series of optical signals with identical frequency intervals and the same optical intensity at the corresponding frequencies as the optical sideband signal at the corresponding frequencies. This optical signal is then beat with an optical frequency comb generated by a known optical frequency comb generator 4 via a coupler 5. The resulting beat frequency is then transmitted to a low-bandwidth photodetector 6. The low-bandwidth photodetector 6 does not respond to any beat frequency components where the sum of the two frequencies and the difference between the two frequencies are large. The low-bandwidth photodetector 6 only responds to signals where the difference Δf between the frequency interval of the optical signal output from the fiber FP cavity 3 and the frequency interval of the optical frequency comb is less than half the frequency interval (i.e., the frequency period) of the optical frequency comb. This response contains frequency difference signals ranging from small to large. Assuming that the signal transmitted by the fiber FP cavity 3 coincides with a frequency of the optical frequency comb at a certain point, the frequency difference at the next period is Δf, and the frequency difference at the next period is 2Δf, and so on. Within the frequency range where the difference is less than half the frequency comb interval, each interval is unique. This means that a specific multiple of Δf can only be derived from the frequency difference between the transmission peak of the fiber FP cavity 3 at the corresponding frequency and the signal at the corresponding frequency of the optical frequency comb. Digitally sampling the photodetection results and performing a Fourier transform yields frequency intensities from 1 to n times Δf. The frequency intensity of each multiple of Δf corresponds to a specific frequency of the RF signal being measured. Through calibration and calculation, the frequency spectrum of the entire RF signal can be recovered.

[0048] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be employed in conjunction with other described embodiments.

Claims

1. A microwave spectrum analysis system based on a fiber FP cavity and an optical frequency comb, characterized in that: It includes a single-frequency laser (1), an electro-optical modulator (2), a fiber FP cavity (3), an optical frequency comb generator (4), a coupler (5), a low-bandwidth photodetector (6), a digital collector (7) and a processor (8); The photoelectric modulator (2) loads the radio frequency signal to be measured onto the single frequency light emitted by the single frequency laser (1) to obtain an optical sideband signal with the same shape as the radio frequency signal to be measured, and sends the optical sideband signal to the optical fiber FP cavity (3) for filtering processing to obtain an optical signal with equal frequency intervals; The coupler (5) is used to perform beat frequency processing on the optical signal with equal frequency intervals output by the optical fiber FP cavity (3) and the optical frequency comb output by the optical frequency comb generator (4), and the obtained beat frequency optical signal is sent to the low-bandwidth photodetector (6); the low-bandwidth photodetector (6) is used to filter out the sum frequency and the difference frequency of the beat frequency optical signal greater than half the frequency interval of the optical frequency comb, and then perform photoelectric detection to obtain an electrical signal. The electrical signal is sampled by the digital collector (7) and sent to the processor (8) for spectrum analysis to obtain the frequency spectrum of the radio frequency signal to be measured.

2. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The processor (8) performs spectrum analysis on the received electrical signal to obtain the frequency spectrum of the radio frequency signal to be measured. The specific process is: First, the received electrical signal is analyzed to obtain n signals with different frequencies, and the frequencies of the n signals are arranged from small to large and are equally spaced. Then, the n signals with different frequencies are calibrated and calculated to restore the frequency spectrum of the RF signal to be measured; n is an integer.

3. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 2, characterized in that: The received electrical signal is analyzed using Fourier transform.

4. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The optical signal output by the optical fiber FP cavity (3) and the optical frequency comb output by the optical frequency comb generator (4) have different frequency intervals, and the frequency difference at corresponding frequency positions between the optical signal output by the optical fiber FP cavity (3) and the optical frequency comb output by the optical frequency comb generator (4) is an integer multiple of Δf, wherein Δf is the difference between the frequency interval of the optical signal output by the optical fiber FP cavity (3) and the frequency interval of the optical frequency comb output by the optical frequency comb generator (4).

5. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The optical frequency comb generator (4) is realized by using a mode-locked femtosecond laser or a microcavity laser.

6. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The low bandwidth photodetector (6) operates in a frequency range of 0 MHz to 50 MHz.

7. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The sampling frequency range of the digital collector (7) is 100 MHz to 2 GHz.

8. The microwave spectrum analysis system based on fiber FP cavity and optical frequency comb according to claim 1, characterized in that: The frequency spectrum of the radio frequency signal to be measured ranges from 10 MHz to 50 GHz.

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