Method for expanding the instantaneous bandwidth of optical sampling analog-digital conversion of a chirp signal

Abstract

The application discloses a method for expanding the instantaneous bandwidth of optical sampling analog-digital conversion of a linear frequency modulation signal, and relates to photoelectric technology. An input linear frequency modulation signal is converted into a digital signal by an optical sampling analog-digital converter. A digital signal processing unit firstly performs interpolation twice on the output digital signal in sequence, then performs fractional Fourier transform on the interpolated signal and performs digital filtering in the fractional domain, and finally performs inverse fractional Fourier transform on the filtered data. Therefore, the input linear frequency modulation signal with a signal bandwidth not greater than twice the instantaneous bandwidth of optical sampling can be received without aliasing. The method uses an interpolation algorithm to improve the equivalent sampling rate of the system, and realizes non-aliasing reception of a full-symbol wideband linear frequency modulation signal through twice interpolation. In addition, the influence of image signals and stray signals can be eliminated by means of the fractional Fourier transform and filtering algorithm, and finally the instantaneous bandwidth of optical sampling analog-digital conversion of the linear frequency modulation signal is expanded to twice the original.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic technology, and more specifically to a method for extending the instantaneous bandwidth of analog-to-digital conversion for linear frequency modulated signal optical sampling. Background Technology

[0002] Linear frequency modulated (LFM) signals are commonly used in pulse compression radar and electronic warfare. The large bandwidth of LFM signals is beneficial for achieving high-precision ranging and high-performance electronic countermeasures capabilities. However, as the frequency and bandwidth of LFM signals increase, analog-to-digital converters (ADCs) need to have larger analog and instantaneous bandwidths to achieve complete signal reception. Traditional electronic ADCs are limited by carrier migration rates and large time jitter, resulting in constraints on their sampling rate and quantization accuracy, making it difficult to meet the ultra-high-speed, high-bandwidth requirements of future communication and electronic warfare applications. Optical sampling ADCs, leveraging the high repetition rate and low jitter characteristics of mode-locked lasers, the ultra-wideband characteristics of electro-optic intensity modulators, and the low crosstalk advantage of optical serial-to-parallel conversion, can overcome the electronic bottlenecks of traditional electronic ADCs and achieve high-speed, high-precision direct down-conversion digital reception.

[0003] When using an optical sampling analog-to-digital converter (ADC) to receive broadband linear frequency modulated (LFM) signals, the system's analog bandwidth is usually much larger than the optical sampling rate, and is mainly affected by the ultrashort optical pulse and the performance of the sampling electro-optic modulator. In 2021, Shanghai Jiao Tong University proposed using an electro-optic modulator as an optical switch for time-division demultiplexing to improve the analog bandwidth of the optical sampling ADC (Na Qian et al. "Characterization of the frequency response of channel-interleaved photonic ADCs based on the optical time-division demultiplexer" IEEE Photonics Journal, 2021, 13(5): 5500309). In 2022, the University of Electronic Science and Technology of China proposed a method to optimize the analog bandwidth of the optical sampling ADC (Zhengkai Li et al. "Frequency response enhancement of photonic sampling based on cavity-less ultra-short optical pulse source" IEEE Photonics Journal, 2022, 14(3): 5523108). By optimizing the flatness of the sampling optical pulse, the high-frequency response of the optical sampling ADC can be significantly improved, thereby increasing the system's analog bandwidth.

[0004] However, even though the analog bandwidth of an optical sampling analog-to-digital converter (ADC) is much greater than the optical sampling rate, meaning the system can receive signals at very high frequencies, the maximum bandwidth of the input linear frequency modulated (LFM) signal is always limited by the system's instantaneous bandwidth, which is half the optical sampling rate. When the bandwidth of the input LFM signal exceeds the system's instantaneous bandwidth, or when the frequency of the input LFM signal does not meet specific conditions, aliasing will occur after the input LFM signal is down-converted and received by the ADC. This causes some frequency components to become the same frequency after down-conversion, making them indistinguishable in the spectrum. This significantly limits the application of ADCs in broadband LFM signal reception. Increasing the optical sampling rate can improve the instantaneous bandwidth of the ADC, but this often requires time-domain or frequency-domain multiplexing, further increasing the system's cost and complexity. In 2012, MIT proposed a time-wavelength interleaving method to improve the optical sampling rate (Anatol Khilo et al. "Photonic ADC: overcoming the bottleneck of electronic jitter" Photonics Express, 2012, 20(4): 4454-4469.). The wavelength division multiplexer is used to divide the optical pulse output by the passive mode-locked laser into N parts in the spectrum. The delay of each part of the pulse is independently controlled so that they are arranged at equal intervals in the time domain. Finally, the wavelength division multiplexer is used to combine them into one path to form an optical pulse with an N times repetition rate. In 2019, Tsinghua University proposed a wavelength division multiplexing method to improve the optical sampling rate (Yirong Xu et al. “An interleaved broadband photonic ADC immune to channel mismatches capable for high-speed radar imaging” IEEE Photonics Journal, 2019, 11(4): 550-2009). By designing the wavelength spacing between N different lasers, the dispersion effect of the chirp compensation module in the cavity-free ultrashort optical pulse source is used to arrange the generated pulses of different wavelengths at equal intervals to form an optical pulse with N times the repetition rate.

[0005] In summary, while the reported optical sampling analog-to-digital converters can improve the instantaneous bandwidth of linear frequency modulation (LFM) signal reception by increasing the optical sampling rate, they often require time-domain or frequency-domain multiplexing, which significantly increases the system cost and complexity, greatly limiting the application of optical sampling analog-to-digital converter systems in broadband LFM signal reception. Summary of the Invention

[0006] The purpose of this invention is to provide a method for extending the instantaneous bandwidth of the analog-to-digital conversion of linear frequency modulated signals in optical sampling in order to solve the above-mentioned technical problems.

[0007] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0008] A method for extending the instantaneous bandwidth of optical sampling analog-to-digital conversion of linear frequency modulated signals includes the following steps:

[0009] Step 1: The input broadband linear frequency modulated signal is sampled at a rate of f. s The optical sampling analog-to-digital converter (1) digitizes and inputs the signal into the digital signal processing unit (2), when the frequency f of the input broadband linear frequency modulated signal... in Not satisfied Aliasing will occur at this time;

[0010] Step 2: Perform interpolation twice on the digitized linear frequency modulated signal data. Each interpolation inserts a zero value between every two data points. After two interpolations, the equivalent sampling rate of the optical sampling analog-to-digital conversion system becomes four times that before interpolation.

[0011] Step 3: Perform a fractional Fourier transform on the interpolated data. The transformed linear frequency modulated signal is converted into a narrow pulse, while other signals are converted into spurious signals and noise. After fractional domain digital filtering, the target signal can be effectively preserved and spurious signals and noise can be filtered out.

[0012] Step 4: Perform an inverse fractional Fourier transform on the fractional-domain digitally filtered data to obtain the processed linear frequency modulated (LFM) signal. This requires inputting the bandwidth f of the wideband LFM signal. B Not exceeding the sampling rate f of the optical sampling analog-to-digital converter s This enables aliasing-free reception and achieves instantaneous bandwidth expansion of the optical sampling analog-to-digital converter;

[0013] As an optional solution, the digital linear frequency modulated signal output by the optical sampling analog-to-digital converter in step 1 is as shown in Formula 1:

[0014]

[0015] Among them, Y in (ω) is the frequency domain representation of the input linear frequency modulated signal, ω s Let be the sampling rate of the optical sampling analog-to-digital converter, p be the Nyquist zone corresponding to the input signal frequency, m be the Nyquist zone corresponding to the start frequency of the input linear frequency modulated signal, M be the Nyquist zone corresponding to the end frequency of the input linear frequency modulated signal, and H(ω) be the frequency response of the optical sampling analog-to-digital converter.

[0016] Then when the frequency ω of the input broadband linear frequency modulated signalin Not satisfied aliasing will occur at that time.

[0017] As an optional solution, the interpolated digital linear frequency modulated signal in step 2 is as shown in Formula 2:

[0018]

[0019] Where δ[n] is the Dirac function.

[0020] After two interpolations, the equivalent sampling rate of the optical sampling analog-to-digital converter becomes four times that before interpolation, the aliased part in the spectrum is expanded, and a mirror signal is generated at the same time.

[0021] As an alternative, the linear frequency modulated signal after the fractional Fourier transform in step 3 is transformed into a spike, as shown in Formula 3:

[0022]

[0023] Where f0 is the initial frequency of the linear frequency modulated signal, and α0 is the angle of rotation of the coordinate axis during the fractional Fourier transform.

[0024] The image signal and spurious signal then become noise, which can be filtered out by using a digital bandpass filter.

[0025] As an alternative, the linear frequency modulated signal after the inverse fractional Fourier transform in step four only needs to have the bandwidth ω of the input broadband linear frequency modulated signal. B Not exceeding the sampling rate ω of the optical sampling analog-to-digital converter s This enables aliasing-free reception and achieves instantaneous bandwidth expansion of the optical sampling analog-to-digital converter.

[0026] As an alternative, the optical sampling analog-to-digital converter can employ any structure capable of performing optical sampling analog-to-digital conversion.

[0027] As an alternative, the digital signal processing unit can employ any structure capable of implementing digital signal processing.

[0028] The beneficial effects of this invention are as follows:

[0029] 1. Compared with conventional optical sampling analog-to-digital conversion systems, this method increases the maximum bandwidth of the non-aliased linear frequency modulated signal from half the sampling rate to the same as the sampling rate;

[0030] 2. Compared with conventional optical sampling analog-to-digital conversion systems, this method extends the frequency range of non-aliased received linear frequency modulated signals from a specific frequency that meets half the sampling rate to any frequency within the sampling rate;

[0031] 3. Compared with conventional optical sampling analog-to-digital conversion systems, this method can eliminate image signals and spurious signals while expanding the instantaneous bandwidth of linear frequency modulated signal reception, thereby improving the quality of received signals.

[0032] 4. Compared with optical sampling analog-to-digital conversion systems that improve sampling rate, this method expands the instantaneous bandwidth of linear frequency modulated signals in the digital domain, greatly saving hardware costs and reducing system complexity. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the present invention;

[0034] Figure 2 This is a time-frequency simulation result diagram of the output signal of the optical sampling analog-to-digital converter in this invention;

[0035] Figure 3 This is a time-frequency simulation result diagram of the digital signal processing unit after the first interpolation in this invention;

[0036] Figure 4 This is a time-frequency simulation result diagram of the digital signal processing unit after the second interpolation in this invention;

[0037] Figure 5 This is a simulation result diagram of the fractional Fourier transform of the digital signal processing unit in this invention;

[0038] Figure 6 This is a simulation result diagram of the fractional-order filtering of the digital signal processing unit in this invention;

[0039] Figure 7 This is a time-frequency simulation result diagram of the inverse fractional Fourier transform of the digital signal processing unit in this invention; Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0041] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0042] Example 1

[0043] like Figure 1 As shown in the figure, this embodiment provides a method for extending the instantaneous bandwidth of the optical sampling analog-to-digital conversion of linear frequency modulated signals.

[0044] In this scheme, the digital signal processing unit processes the input linear frequency modulation (LFM) signal received by the optical sampling analog-to-digital converter (ADC). Specifically, the digital signal processing unit first performs two interpolations on the digital signal output from the ADC, then performs a fractional Fourier transform on the interpolated signal and digital filtering in the fractional domain, and finally performs an inverse fractional Fourier transform on the filtered data. This allows for aliasing-free reception of input LFM signals with a signal bandwidth no greater than twice the instantaneous bandwidth of the optical sampling. This method improves the system's equivalent sampling rate through interpolation algorithms and achieves aliasing-free reception of full-symbol broadband LFM signals through two interpolations. Furthermore, by utilizing fractional Fourier transforms and filtering algorithms, the influence of image signals and spurious signals can be eliminated, ultimately doubling the instantaneous bandwidth of the LFM signal optical sampling ADC.

[0045] Example 2

[0046] This embodiment follows Figure 1 The structure and principle shown in the simulation demonstrate instantaneous bandwidth extension of a linear frequency modulated (LFM) signal optical sampling analog-to-digital converter (ADC) based on digital domain interpolation and fractional-order filtering. This embodiment uses an optical sampling ADC with a sampling rate of 10 GS / s to down-convert the input broadband LFM signal, achieving an instantaneous bandwidth of 5 GHz and an analog bandwidth greater than 40 GHz. Figure 2 The image shows the time-frequency simulation results of an optical sampling analog-to-digital converter sampling a broadband linear frequency modulated signal with a frequency range of 26-34 GHz. Figure 2 It is evident that although the linear frequency modulated signal can be received completely, aliasing occurs in the spectrum because its signal bandwidth exceeds the instantaneous bandwidth of the optical sampling system.

[0047] The output of the optical sampling analog-to-digital converter is sent to the digital signal processing unit for processing, which mainly includes the following steps: first interpolation, second interpolation, fractional Fourier transform, fractional domain filtering, and inverse fractional Fourier transform. Figure 3 The results shown are the time-frequency simulation results after the first interpolation. After interpolation, the equivalent sampling rate of the optical sampling analog-to-digital conversion system becomes twice the original, and the corresponding equivalent instantaneous bandwidth also becomes twice the original. At the same time, a mirror frequency with the opposite chirp rate is generated in the high-frequency part. Figure 4 The results shown are the time-frequency simulation results after the second interpolation. At this point, in addition to the changes in the equivalent sampling rate and equivalent instantaneous bandwidth, it can be found that a segment of non-aliased linear frequency modulated signal with the same spectral components as the input signal is generated in the mirror frequency, but it overlaps with other mirror signals and cannot be directly extracted. Figure 5The simulation results after fractional Fourier transform are shown. It can be seen that the target linear frequency modulated signal and signals with the same chirp rate are transformed into discrete spikes, while other signals are transformed into noise and spurious signals. Figure 6 The simulation results are shown below. Only the target linear frequency modulated signal is retained, while other spurious signals and noise are filtered out. Figure 7 The figure shows the time-frequency simulation results after the inverse fractional Fourier transform. This allows us to extract a linear frequency modulated signal with the same spectral components as the input signal and without aliasing, thus achieving instantaneous bandwidth expansion of the optical sampling analog-to-digital converter.

[0048] It can be seen that the proposed device and method for instantaneous bandwidth expansion of optical sampling analog-to-digital converters can expand the input bandwidth for aliasing-free reception of broadband linear frequency modulated (LFM) signals, while eliminating the frequency limitation of the input broadband LFM signal. Furthermore, it can effectively suppress spurious signals and noise in the system, improving the quality of the received signal. This method can effectively enhance the broadband signal reception capability of optical sampling analog-to-digital converters and has potential practical value for applications of optical sampling analog-to-digital converters in high-speed broadband scenarios.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for extending the instantaneous bandwidth of optical sampling analog-to-digital conversion of linear frequency modulated signals, characterized in that, A digital signal processing method for extending the instantaneous bandwidth of optical sampling analog-to-digital conversion for broadband linear frequency modulated signals; Specifically, the following steps are included: Step 1: The input broadband linear frequency modulated signal is sampled at a rate of... The optical sampling analog-to-digital converter digitizes and inputs the signal into the digital signal processing unit; Step 2: Perform interpolation twice on the digitized linear frequency modulated signal data, inserting a zero value between every two data points in each interpolation; Step 3: Perform fractional Fourier transform on the interpolated data. The transformed linear frequency modulated signal is converted into a narrow pulse, while other signals are converted into spurious signals and noise. Then, perform fractional domain digital filtering on the narrow pulse. Step 4: Perform an inverse fractional Fourier transform on the fractional-domain digitally filtered data to obtain the processed linear frequency modulated signal; The digital linear frequency modulated signal output by the optical sampling analog-to-digital converter in step 1 is shown in Formula 1: (one); in, The frequency domain representation of the input linear frequency modulated signal. The sampling rate of the optical sampling analog-to-digital converter. The Nyquist zone corresponding to the input signal frequency. The Nyquist zone corresponding to the starting frequency of the input linear frequency modulated signal. The Nyquist zone corresponding to the termination frequency of the input linear frequency modulated signal. This refers to the frequency response of the optical sampling analog-to-digital converter; The interpolated digital linear frequency modulated signal in step 2 is shown in Formula 2: = (two); in, It is the Dirac function; In step 3, the linear frequency modulated signal after the fractional Fourier transform is transformed into a spike, as shown in Formula 3: (three); in, The initial frequency of the linear frequency modulated signal is . The angle of rotation of the coordinate axes during fractional Fourier transform.

2. The method for extending the instantaneous bandwidth of optical sampling analog-to-digital conversion of linear frequency modulated signals as described in claim 1, characterized in that: The optical sampling analog-to-digital converter adopts any structure that can achieve optical sampling analog-to-digital conversion.

3. The method for extending the instantaneous bandwidth of optical sampling analog-to-digital conversion of linear frequency modulated signals as described in claim 1, characterized in that: The digital signal processing unit can adopt any structure that can implement digital signal processing.

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