Method and apparatus for generating coherent broadband frequency-stepped microwave signals

Abstract

This invention discloses a method for generating coherent broadband frequency-stepped microwave signals. The method involves electro-optically modulating an optical carrier with a microwave signal to generate a double-sideband optical signal containing only an upper sideband and a lower sideband. A dual-passband optical filter separates the upper and lower sideband signals. These signals are then frequency-shifted in opposite directions and subjected to the same delay before being fed back into the dual-passband optical filter, forming a bidirectional frequency-shifting loop. The gain of the bidirectional frequency-shifting loop is set to 1, and the coupled signals of the two frequency-shifted signals output from the loop are photoelectrically converted to generate a coherent broadband frequency-stepped microwave signal. This invention also discloses a coherent broadband frequency-stepped microwave signal generation device. Compared to existing technologies, this invention eliminates the influence of optical carrier phase noise, effectively enhances signal stability and coherence, and increases the bandwidth of the generated signal.

Description

Technical Field

[0001] This invention relates to a method for generating frequency-stepped microwave signals, and more particularly to a method for generating coherent broadband frequency-stepped microwave signals. Background Technology

[0002] Frequency-stepped microwave signals, composed of multiple sub-pulses with different carrier frequencies, have wide applications in high-resolution radar, high-speed communication, and other fields. They can effectively reduce the instantaneous operating bandwidth of a system while ensuring core performance characteristics such as radar resolution and communication rate, and improve the system's anti-interference capability. Furthermore, the use of coherent signals can further enhance the performance of radar and communication systems. For example, in radar systems, the accumulation of coherent signals can significantly improve the signal-to-noise ratio; in communication systems, the phase consistency of coherent signals can effectively improve communication reliability. However, due to bandwidth limitations, it is difficult to achieve broadband, coherent frequency-stepped microwave signal generation using traditional electronic technologies. In recent years, microwave frequency-stepped signal generation based on microwave photonics technology has become one of the research hotspots in the field of microwave signal generation. Utilizing microwave photonics technology, which features large bandwidth, low transmission loss, and resistance to electromagnetic interference, it is expected to overcome the challenges faced by traditional electronic technologies.

[0003] Currently, various frequency-stepped signal generation schemes based on microwave photonics technology have been publicly reported, such as frequency-time mapping, optical frequency combs, optical digital-to-analog conversion, tunable optoelectronic oscillators, and optical cyclic frequency shifting. Among them, the frequency-stepped signal generated by optical cyclic frequency shifting technology has the largest bandwidth [see Ma C, Yang Y, Cao F, et al. High-Resolution Microwave Photonic Radar With Sparse Stepped Frequency Chirp Signals[J].IEEE Transactions on Geoscience and Remote Sensing, 2022, 60: 1-10.]. However, in the currently proposed frequency-stepped signal generation technology based on optical cyclic frequency shifting, further photoelectric conversion is required using optical heterodyne technology. In this process, due to the presence of two optical carriers with a large time difference, the phase noise of the optical carriers is converted to the electrical domain, thus severely degrading the coherence of the generated frequency-stepped microwave signal. On the other hand, the bandwidth of the frequency-stepped signal sub-pulse generated by the current optical cyclic frequency shifting technology is the same as the bandwidth of the system's input microwave signal. Therefore, this scheme still relies on an electronic signal generator with a large bandwidth. Therefore, researching methods for generating broadband, coherent frequency-stepped microwave signals that can overcome the limitations of electronic bandwidth is of great significance for enhancing the performance of systems such as high-resolution radar and high-speed communication. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for generating coherent broadband frequency-stepped microwave signals, which can eliminate the influence of optical carrier phase noise and effectively enhance the stability, bandwidth and coherence of the signal.

[0005] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0006] A method for generating a coherent broadband frequency-stepped microwave signal involves electro-optically modulating an optical carrier with a microwave signal to generate a double-sideband optical signal containing only an upper sideband and a lower sideband. A dual-passband optical filter is used to separate the upper and lower sideband signals from the double-sideband optical signal. The upper and lower sideband signals are then frequency-shifted in opposite directions and subjected to the same delay before being fed back into the dual-passband optical filter, thus forming a bidirectional frequency-shifting loop. The condition T is then set... r ≥NT L ≥NT pw The condition is satisfied, where T r and T pw T represents the period and pulse width of the microwave signal, respectively. L The delay of the bidirectional frequency shifting loop is given by N, which is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter. Setting the gain of the bidirectional frequency shifting loop to 1 and performing photoelectric conversion on the coupled signals of the two frequency shifting signals output from the bidirectional frequency shifting loop, a sub-pulse with a bandwidth equal to (J) of the microwave signal bandwidth can be generated. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

[0007] Preferably, the delay is introduced by a dimmable delay component.

[0008] Preferably, the two frequency-shifted signals are coupled in a bidirectional frequency-shifting loop.

[0009] Based on the same inventive concept, the following technical solutions can also be obtained:

[0010] A coherent broadband frequency-stepped microwave signal generating device includes:

[0011] The double-sideband optical signal generation module is used to generate a double-sideband optical signal containing only an upper sideband and a lower sideband by electro-optic modulation of an optical carrier using a microwave signal.

[0012] A bidirectional frequency shifter loop includes a dual-passband optical filter, two parallel frequency shifters, an optical delay module, and an optical amplification module. The dual-passband optical filter separates the upper sideband and lower sideband signals from the double-sideband optical signal. The two parallel frequency shifters shift the upper and lower sideband signals in opposite directions. The optical delay module introduces the same delay into the frequency-shifted upper and lower sideband signals before feeding them back into the dual-passband optical filter. The optical amplification module sets the gain of the bidirectional frequency shifter loop to 1. The delay T of the bidirectional frequency shifter loop is... L Satisfying condition T r ≥NT L ≥NT pw T r and T pw These are the period and pulse width of the microwave signal, respectively, and N is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter in the bidirectional frequency shift loop.

[0013] A photodetector is used to perform photoelectric conversion on the coupled signal of the two frequency-shifted signals output from the bidirectional frequency-shifting loop, thereby generating a sub-pulse with a bandwidth equal to (J) times the bandwidth of the microwave signal. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

[0014] Preferably, the optical delay module introduces the delay through an adjustable optical delay component.

[0015] Preferably, the bidirectional frequency shift loop further includes an optical coupler for coupling the two frequency shift signals in the bidirectional frequency shift loop.

[0016] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0017] 1. The frequency-stepped microwave signal generated by this invention is not affected by optical carrier phase noise, which can significantly improve the coherence of the optically generated broadband frequency-stepped signal;

[0018] 2. This invention can double the bandwidth of frequency-stepped microwave signal sub-pulse, reducing the system's dependence on large-bandwidth electronic excitation signal generation units;

[0019] 3. This invention can overcome the bottleneck of electronic bandwidth, and the bandwidth of the generated signal can meet the needs of various microwave systems such as broadband radar and high-speed communication.

[0020] 4. The bandwidth, carrier frequency, period and other parameters of the signal generated by this invention are flexibly adjustable. Attached Figure Description

[0021] Figure 1This is a schematic diagram of a preferred structure of the coherent broadband frequency-stepped microwave signal generation device of the present invention.

[0022] Figure 2 A schematic diagram of a specific structure of a double-sideband optical signal generation module;

[0023] Figure 3 The time-frequency diagram of the microwave signal input to the system;

[0024] Figure 4 This is a schematic diagram of the output spectrum of the bidirectional frequency shift loop;

[0025] Figure 5 The time-frequency diagram of the generated frequency-stepped microwave signal. Detailed Implementation

[0026] To address the shortcomings of existing technologies, this invention proposes the following technical solutions:

[0027] A method for generating a coherent broadband frequency-stepped microwave signal involves electro-optically modulating an optical carrier with a microwave signal to generate a double-sideband optical signal containing only an upper sideband and a lower sideband. A dual-passband optical filter is used to separate the upper and lower sideband signals from the double-sideband optical signal. The upper and lower sideband signals are then frequency-shifted in opposite directions and subjected to the same delay before being fed back into the dual-passband optical filter, thus forming a bidirectional frequency-shifting loop. The condition T is then set... r ≥NT L ≥NT pw The condition is satisfied, where T r and T pw T represents the period and pulse width of the microwave signal, respectively. L The delay of the bidirectional frequency shifting loop is given by N, which is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter. Setting the gain of the bidirectional frequency shifting loop to 1 and performing photoelectric conversion on the coupled signals of the two frequency shifting signals output from the bidirectional frequency shifting loop, a sub-pulse with a bandwidth equal to (J) of the microwave signal bandwidth can be generated. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

[0028] A coherent broadband frequency-stepped microwave signal generating device includes:

[0029] The double-sideband optical signal generation module is used to generate a double-sideband optical signal containing only an upper sideband and a lower sideband by electro-optic modulation of an optical carrier using a microwave signal.

[0030] A bidirectional frequency shifter loop includes a dual-passband optical filter, two parallel frequency shifters, an optical delay module, and an optical amplification module. The dual-passband optical filter separates the upper sideband and lower sideband signals from the double-sideband optical signal. The two parallel frequency shifters shift the upper and lower sideband signals in opposite directions. The optical delay module introduces the same delay into the frequency-shifted upper and lower sideband signals before feeding them back into the dual-passband optical filter. The optical amplification module sets the gain of the bidirectional frequency shifter loop to 1. The delay T of the bidirectional frequency shifter loop is... L Satisfying condition T r ≥NT L ≥NT pw T r and T pw These are the period and pulse width of the microwave signal, respectively, and N is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter in the bidirectional frequency shift loop.

[0031] A photodetector is used to perform photoelectric conversion on the coupled signal of the two frequency-shifted signals output from the bidirectional frequency-shifting loop, thereby generating a sub-pulse with a bandwidth equal to (J) times the bandwidth of the microwave signal. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

[0032] The double-band optical signal generation module can be implemented by an electro-optic modulator operating at a specific bias point, or by cascading a double-band optical filter after the electro-optic modulator.

[0033] The orders of the upper and lower sidebands can be the same or different.

[0034] The frequency shifter can be implemented using various existing technologies, such as acousto-optic modulators, dual parallel Mach-Zehnder modulators, dual polarization dual parallel Mach-Zehnder modulators, etc.; the absolute values ​​of the frequency shifts of the two frequency shifters can be the same or different.

[0035] Preferably, the optical delay module introduces the delay through an adjustable optical delay component.

[0036] Preferably, the bidirectional frequency shift loop further includes an optical coupler for coupling the two frequency shift signals in the bidirectional frequency shift loop.

[0037] To facilitate public understanding, the technical solution of the present invention will be described in detail below through a specific embodiment and in conjunction with the accompanying drawings:

[0038] The coherent broadband frequency-stepped microwave signal generating device of this embodiment, such as Figure 1As shown, it includes: a double-sideband optical signal generation module, a bidirectional frequency shifting loop consisting of a dual-passband optical filter, positive and negative frequency shifters, a delay fiber, and an optical amplifier, and a photodetector. The double-sideband optical signal generation module is used to generate a double-sideband optical signal containing only one upper sideband and one lower sideband; for example... Figure 1 As shown, the double-sideband optical signal is input to the double-passband optical filter in the bidirectional frequency shifting loop to separate the upper and lower sidebands of the double-sideband optical signal; the parallel positive and negative frequency shifters perform positive and negative frequency shifts on the upper and lower sideband signals, respectively; the delay fiber delays the coupled signal of the two frequency-shifted optical signals; the optical amplifier amplifies the coupled optical signal after the delay and feeds it back into the double-passband optical filter, thus forming a closed bidirectional frequency shifting loop; the gain of the optical amplifier is adjusted so that the gain of the bidirectional frequency shifting loop is 1; the photodetector performs photoelectric conversion on the coupled signal of the two frequency-shifted optical signals output from the bidirectional frequency shifting loop to generate a coherent broadband microwave frequency stepping signal.

[0039] To avoid mutual interference of optical signals in the loop, this invention requires controlling the delay of the bidirectional frequency shift loop through an optical delay module, so that condition T... r ≥NT L ≥NT pw The condition is satisfied, where T r and T pw T represents the period and pulse width of the microwave signal, respectively. L The delay is denoted as N, where N is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter in the bidirectional frequency shift loop. Since the two frequency-shifted signals are coupled in the bidirectional frequency shift loop and delayed by the same delay fiber in this embodiment, the delays introduced by the positive and negative frequency shifters should be consistent to ensure that the delays of the two frequency-shifted signals in the bidirectional frequency shift loop are the same.

[0040] like Figure 2 As shown, the double-sideband optical signal generation module in this embodiment consists of a laser, a microwave source, an electro-optic modulator, and a double-passband optical filter. The microwave signal generated by the microwave source modulates the optical carrier generated by the laser through the electro-optic modulator to generate an optical signal containing multiple sidebands. The corresponding upper and lower sideband signals are then selected by the double-passband optical filter. When the electro-optic modulator operates at a specific bias point and can directly generate a carrier-suppressed double-sideband optical signal, the double-passband optical filter can be omitted.

[0041] To facilitate public understanding, the principles of this invention will be further explained in detail below:

[0042] Assume the optical carrier generated by the laser can be represented as:

[0043]

[0044] Where A and f L These represent the amplitude and frequency of the laser signal, respectively. This represents the phase noise of the laser.

[0045] Assume the microwave signal is a linear frequency modulated signal with a center frequency of f0, a modulation slope of γ, and a pulse width of T. pw The period is T r Bandwidth B = γT pw The time-frequency curve is as follows Figure 3 As shown. The linear frequency modulated signal modulates the optical carrier through a Mach-Zehnder modulator to obtain an optical signal with multiple sidebands:

[0046]

[0047] Where α is a coefficient affected by the modulator bias voltage, β is the modulation coefficient, and J k (β) is a Bessel function of the first kind of order k, ω(t) = f0(t-nT) r )+γ(t-nT r ) 2 / 2. Using a dual-passband filter to filter out an upper sideband and a lower sideband, taking the +2nd and -1st order sidebands as an example, the resulting double-sideband optical signal can be expressed as:

[0048]

[0049] In the bidirectional frequency shift loop, a dual-passband filter separates the upper and lower sidebands of the signal and sends them to the positive and negative frequency shifters respectively for bidirectional frequency shifting. The optical signals output from the two frequency shifters are coupled, passed through a delay fiber, and then sent to an optical amplifier. The optical signal output from the optical amplifier is then fed back into the dual-passband optical filter in the bidirectional frequency shift loop, and the gain of the optical amplifier is adjusted to make the loop gain 1. The optical signal continuously undergoes bidirectional frequency shifting within the loop. To avoid mutual interference between optical signals in the bidirectional frequency shift loop, T must also be satisfied. r ≥NT L ≥NT pw T L This is the loop delay. Assuming the frequency shifts introduced by the positive and negative frequency shifters are Δf1 and Δf2 respectively, and setting the delay difference between the upper and lower paths containing the positive and negative frequency shifters to 0, then the loop output is:

[0050]

[0051] The spectrum of the bidirectional frequency shift loop output is as follows: Figure 4 As shown. The loop output is sent to a photodetector for photoelectric conversion, resulting in the following... Figure 5 The frequency-stepped microwave signal shown can be represented as:

[0052]

[0053] Where, φ n This represents the fixed initial phase of the signal. Therefore, the phase noise of the optical carrier is... This is eliminated. Additionally, the sub-pulse bandwidth B of the output frequency stepped microwave signal... sub =3γT pw This shows that the bandwidth is three times the bandwidth of the system input linear frequency modulation signal.

[0054] In this embodiment, the upper and lower sidebands of the double-sideband optical signal are of order +2 and -1, respectively. Therefore, the bandwidth of the sub-pulse generated by the system is only three times that of the input linear frequency modulated signal. In practical implementation, the Mach-Zehnder modulator in the double-sideband optical signal generation module can be set at the minimum bias point, so that it directly outputs a carrier-suppressed ±1-order double-sideband optical signal, thus avoiding the use of the dual-passband filter in the module. In this case, the bandwidth of the system output sub-pulse becomes twice that of the system input signal bandwidth. Alternatively, the Mach-Zehnder modulator can be replaced with a dual parallel Mach-Zehnder modulator, which outputs a carrier-suppressed ±2-order double-sideband optical signal. In this case, the bandwidth of the system output sub-pulse becomes four times that of the system input signal bandwidth. In addition, the total bandwidth of the frequency-stepped microwave signal generated by the system can be controlled by adjusting the cutoff frequency of the dual-passband optical filter in the bidirectional frequency shift loop, and the period can be controlled by the period of the system input microwave signal; the pulse width and bandwidth of the sub-pulse can also be controlled by the corresponding parameters of the system input microwave signal. Therefore, the present invention allows for flexible adjustment of the parameters of the generated frequency-stepped microwave signal.

Claims

1. A method for generating a coherent broadband frequency-stepped microwave signal, characterized in that, A microwave signal is used to electro-optically modulate an optical carrier to generate a double-sideband optical signal containing only an upper sideband and a lower sideband. A dual-passband optical filter is used to separate the upper and lower sideband signals from the double-sideband optical signal. The upper and lower sideband signals are then frequency-shifted in opposite directions and subjected to the same delay before being fed back into the dual-passband optical filter, thus forming a bidirectional frequency-shifting loop. The condition T is then set... r ≥NT L ≥NT pw The condition is satisfied, where T r and T pw The period and pulse width of the microwave signal are T, respectively. L The delay of the bidirectional frequency shifting loop is given by N, which is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter. Setting the gain of the bidirectional frequency shifting loop to 1 and performing photoelectric conversion on the coupled signals of the two frequency shifting signals output from the bidirectional frequency shifting loop, a sub-pulse with a bandwidth equal to (J) of the microwave signal bandwidth can be generated. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

2. The method for generating coherent broadband frequency-stepped microwave signals as described in claim 1, characterized in that, The delay is introduced by a dimmable delay component.

3. The method for generating coherent broadband frequency-stepped microwave signals as described in claim 1, characterized in that, The two frequency-shifted signals are coupled in a bidirectional frequency-shifting loop.

4. A coherent broadband frequency-stepped microwave signal generation device, characterized in that, include: The double-sideband optical signal generation module is used to generate a double-sideband optical signal containing only an upper sideband and a lower sideband by electro-optic modulation of an optical carrier using a microwave signal. A bidirectional frequency shifter loop includes a dual-passband optical filter, two parallel frequency shifters, an optical delay module, and an optical amplification module. The dual-passband optical filter separates the upper sideband and lower sideband signals from the double-sideband optical signal. The two parallel frequency shifters shift the upper and lower sideband signals in opposite directions. The optical delay module introduces the same delay into the frequency-shifted upper and lower sideband signals before feeding them back into the dual-passband optical filter. The optical amplification module sets the gain of the bidirectional frequency shifter loop to 1. The delay T of the bidirectional frequency shifter loop is... L Satisfying condition T r ≥NT L ≥NT pw T r and T pw The period and pulse width of the microwave signal are respectively, and N is the number of frequency shifts determined by the cutoff frequency of the dual-passband optical filter in the bidirectional frequency shift loop; A photodetector is used to perform photoelectric conversion on the coupled signal of the two frequency-shifted signals output from the bidirectional frequency-shifting loop, thereby generating a sub-pulse with a bandwidth equal to (J) times the bandwidth of the microwave signal. up +J down A coherent broadband microwave frequency-stepped microwave signal that is ) times the frequency of J, where J up and J down These are the orders of the upper sideband signal and the lower sideband signal, respectively.

5. The coherent broadband frequency-stepped microwave signal generating apparatus as described in claim 4, characterized in that, The optical delay module introduces the delay through an adjustable optical delay component.

6. The coherent broadband frequency-stepped microwave signal generating apparatus as described in claim 4, characterized in that, The bidirectional frequency shift loop also includes an optical coupler for coupling the two frequency shift signals in the bidirectional frequency shift loop.

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