A broadband linear frequency modulation pulse signal generation method based on optical heterodyne method

Abstract

The application discloses a broadband linear frequency modulation pulse signal generation method based on an optical heterodyne method, and utilizes a three-section distributed Bragg reflection laser to generate a linear chirp optical signal, utilizes a tunable laser source and an acousto-optic modulator to generate a pulsed optical carrier, and utilizes two optical signals to perform optical heterodyne frequency beating in a photodetector, so as to generate a high-linear broadband linear frequency modulation pulse microwave signal in an electrical domain. The application has the advantages of simple structure, adjustable sweep bandwidth, adjustable center frequency, controllable pulse parameter and high linearity, and is suitable for a broadband radar, a microwave photon signal generation, target detection and a communication and perception integrated system.

Description

Technical Field

[0001] This invention belongs to the fields of microwave photonics, microwave signal generation, and radar signal source technology, specifically relating to a method for generating broadband linear frequency modulated pulse signals based on optical heterodyne. Background Technology

[0002] Linear frequency modulated (LFM) signals are a typical radar waveform whose instantaneous frequency changes linearly with time. They are widely used in radar detection, target imaging, electronic countermeasures, wireless communication, and integrated communication and sensing systems. Compared to single-frequency continuous wave signals, LFM signals have a larger time-bandwidth product, enabling high range resolution while maintaining high transmission energy through pulse compression techniques. Therefore, the generation technology of high linearity, large bandwidth, and tunable LFM signals has always been one of the key technologies in broadband radar and microwave photonics systems.

[0003] Traditional electrical methods typically utilize direct digital synthesis, voltage-controlled oscillators, phase-locked loops, or high-speed arbitrary waveform generators to generate LFM signals. However, due to limitations in the bandwidth, sampling rate, frequency response, and phase noise of electronic devices, traditional electrical solutions face significant challenges in generating high-frequency, wide-bandwidth, and high-linearity LFM signals. Especially in millimeter-wave, terahertz, and ultra-wideband radar applications, electrical signal sources often require complex frequency doubling, mixing, and filtering links, resulting in complex system structures, high losses, and limited tunability.

[0004] In recent years, microwave photonics technology has provided a new technical approach for generating broadband microwave signals. Microwave photonic systems possess advantages such as large bandwidth, high frequency, low loss, strong resistance to electromagnetic interference, and ease of long-distance transmission, and have been widely used for generating broadband LFM signals, frequency-hopping signals, phase-coded signals, and arbitrary microwave waveforms. Currently, methods for generating LFM signals based on photonics technology mainly include space-to-time mapping, wavelength-to-time mapping, optical injection into a semiconductor laser P1 oscillation method, Fourier domain mode-locking, and optical heterodyne methods. Among them, space-to-time mapping and wavelength-to-time mapping methods can achieve flexible waveform design, but their time-bandwidth product is limited by the number of pulse shaper channels and spectral resolution; the P1 oscillation method of optically injected semiconductor lasers can achieve a large bandwidth, but the nonlinear dynamics of the laser will affect the frequency sweep linearity; the Fourier domain mode-locking method can obtain a large time-bandwidth product, but the system usually requires a long fiber loop and strict synchronization conditions, which makes the structure complex and unfavorable for miniaturization integration; the frequency sweeping method of DBR lasers based on optical heterodyne has the advantages of simple structure, adjustable frequency and strong integration potential, but existing research is mostly focused on the generation of continuous LFM signals, and there is insufficient research on the generation of pulsed LFM signals and synchronization gating.

[0005] In practical radar and sensing systems, pulsed LFM signals are more suitable for pulse compression processing, time-domain ranging, and echo gating compared to continuous LFM signals. Pulsed LFM signals can carry broadband frequency sweep information within a finite time window and obtain narrow pulse compressed output through matched filtering or autocorrelation processing, thereby achieving high-resolution target detection. Therefore, there is an urgent need for a broadband LFM pulse signal generation method that is simple in structure, has high linearity, adjustable bandwidth, adjustable center frequency, and can implement pulse gating. Summary of the Invention

[0006] In view of the above problems, the present invention provides a method for generating broadband linear frequency modulated pulse signals based on optical heterodyne method.

[0007] The present invention discloses a broadband linear frequency modulated pulse signal generation method based on optical heterodyne method. In a system composed of a three-segment distributed Bragg reflector laser, a tunable laser source, an acousto-optic modulator, an optical coupler, and a photodetector, the three-segment distributed Bragg reflector laser has an input port connected to a sweep frequency drive signal, and the acousto-optic modulator has an input port connected to an electrical pulse control signal.

[0008] A sweeping drive signal is applied to the phase region of a three-segment distributed Bragg reflector laser. By adjusting the injected current in the phase region to change the carrier concentration and effective refractive index, the three-segment distributed Bragg reflector laser outputs a chirped light signal with a frequency that varies with time. A tunable laser source outputs a continuous optical carrier, which is pulse-gated by an acousto-optic modulator to obtain a pulsed light signal with a set pulse width and repetition period. The chirped light signal and the pulsed light signal are combined by an optical coupler and input into a photodetector. Through the optical heterodyne beat frequency action of the photodetector, the instantaneous frequency difference between the two optical signals is converted into an electrical microwave signal, thereby generating a pulsed linear frequency modulated microwave signal.

[0009] Furthermore, the three-segment distributed Bragg reflector laser consists of an active region, a phase region, and a grating region. The active region provides optical gain and maintains laser oscillation; the phase region alters the carrier concentration and effective refractive index through injected current, thereby changing the equivalent optical path length within the laser cavity and achieving rapid wavelength tuning; the grating region provides wavelength selection feedback and determines the laser output mode. A sweeping drive signal is applied to the phase region of the three-segment distributed Bragg reflector laser, causing it to output a chirped light signal with a frequency that varies with time.

[0010] Furthermore, the sweep drive signal loaded into the phase region of the three-segment distributed Bragg reflector laser is a pre-compensated sawtooth wave or triangular wave drive signal. This pre-compensated drive signal is used to correct the current-frequency nonlinear tuning response of the three-segment distributed Bragg reflector laser and to compensate for the frequency sweep nonlinearity caused by carrier effects, thermal effects and device dynamic response, so that the instantaneous frequency of the output chirped light signal changes approximately linearly with time, thereby improving the sweep frequency linearity of the linear frequency modulated microwave signal.

[0011] Furthermore, the sweep drive signal and the electrical pulse control signal are generated from the same signal source or a synchronization signal source to achieve timing synchronization between the chirped optical signal and the pulsed optical signal, so that the pulse gating window is precisely aligned with the linear sweep cycle, thereby obtaining a pulsed linear frequency modulated microwave signal of a complete cycle or a specified time period.

[0012] Furthermore, by adjusting the amplitude and period of the sweep drive signal loaded onto the phase region of the three-segment distributed Bragg reflector laser, the sweep bandwidth and sweep period of the generated linear frequency modulated microwave signal are adjusted.

[0013] Furthermore, the center frequency of the generated linear frequency modulated microwave signal can be adjusted by regulating the output wavelength of the tunable laser source.

[0014] Furthermore, by adjusting the pulse width and repetition period of the electrical pulse control signal applied to the acousto-optic modulator, the pulse width and repetition period of the generated linear frequency modulated microwave signal can be adjusted.

[0015] The pulsed linear frequency modulated microwave signal generated by this invention has the characteristics of adjustable sweep bandwidth, adjustable center frequency, adjustable pulse width, and adjustable repetition period, and can be used in broadband radar, microwave photonic signal generation, target detection, and integrated communication and sensing systems.

[0016] The beneficial technical effects of this invention compared to the prior art are as follows:

[0017] First, a three-segment distributed Bragg reflector laser is used to achieve broadband fast frequency sweeping with good frequency tunability. Second, the nonlinear tuning response of the laser is corrected by pre-compensation drive waveform correction, improving the frequency sweep linearity of the linear frequency modulated microwave signal. Third, the optical frequency difference between the two optical signals is adjusted by a tunable laser source, enabling adjustable center frequency of the output microwave signal. Fourth, optical pulse gating is achieved through an acousto-optic modulator, allowing flexible adjustment of pulse width and repetition period. Fifth, the system has a simple structure, adjustable parameters, and strong anti-electromagnetic interference capability, making it applicable to broadband radar, microwave photonic signal generation, target detection, and integrated communication and sensing systems. Attached Figure Description

[0018] Figure 1This is a schematic diagram of a broadband linear frequency modulated pulse signal generation method based on optical heterodyne method according to the present invention.

[0019] Figure 2 This is the spectrum measured at the output of the optical coupler according to the present invention, where the left spectral line is the continuous optical carrier output by the tunable laser source, and the right broadened spectral line is the chirped optical signal output by the three-segment distributed Bragg reflector laser.

[0020] Figure 3 The figure shows the experimental results of a 5.5 GHz bandwidth continuous linear frequency modulated microwave signal generated after pre-compensation when the peak-to-peak voltage of the sweep frequency drive signal is 1.2 V. (a) is the time-domain waveform, (b) is the autocorrelation compressed pulse diagram, and (c) is the time-frequency diagram.

[0021] Figure 4 The figure shows the experimental results of the 8 GHz bandwidth continuous linear frequency modulated microwave signal generated after pre-compensation when the peak-to-peak voltage of the sweep frequency drive signal is 1.8 V. (a) is the time domain waveform, (b) is the autocorrelation compressed pulse diagram, and (c) is the time-frequency diagram.

[0022] Figure 5 The figure shows the experimental results of the pulse linear frequency modulated microwave signal generated by optical pulse gating through an acousto-optic modulator according to the present invention. (a) is the time-domain waveform and (b) is the time-frequency waveform. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0024] The present invention discloses a broadband linear frequency modulated pulse signal generation method based on optical heterodyne. This method utilizes a three-segment distributed Bragg reflector laser to generate a linear chirped optical signal, and a tunable laser source and an acousto-optic modulator to generate a pulsed optical carrier. The two optical signals are subjected to optical heterodyne beat frequency in a photodetector, thereby generating a high linear broadband linear frequency modulated pulsed microwave signal in the electrical domain.

[0025] The structural principle of the present invention is as follows Figure 1As shown, the system comprises a three-segment distributed Bragg reflector laser 100, a tunable laser source 101, an acousto-optic modulator 102, an optical coupler 103, and a photodetector 104. The three-segment distributed Bragg reflector laser 100 serves as a swept-frequency source, with a swept-frequency drive signal 110 applied to its phase region to generate a chirped light signal whose frequency varies with time. The tunable laser source 101 outputs a continuous optical carrier, which enters the acousto-optic modulator 102 and is pulse-gated by an electrical pulse control signal 111 to generate a pulsed light signal. The chirped light signal and the pulsed light signal are combined by the optical coupler 103 and input to the photodetector 104, generating a pulsed linearly frequency-modulated microwave signal through optical heterodyne beat frequency modulation. The phase region tuning of the three-segment distributed Bragg reflector laser 100 mainly relies on the carrier plasma effect. When the swept-frequency drive signal 110 is applied to the phase region, the carrier concentration in the phase region changes, thereby causing a change in the effective refractive index. Changes in the effective refractive index alter the optical path length within the laser cavity, causing a shift in the output wavelength or optical frequency. When the phase region driving signal is a time-varying sawtooth or triangular wave, the laser output optical frequency changes with time sweep frequency.

[0026] In practice, due to the influence of carrier dynamics, thermal effects, and device nonlinearity on the current-frequency tuning response of the three-segment distributed Bragg reflector laser 100, the output optical frequency cannot change strictly linearly when a linear sawtooth or triangular wave drive signal is directly applied. Therefore, this invention employs an iterative pre-compensation method. First, the actual instantaneous frequency trajectory under direct drive conditions is measured and compared with the ideal linear sweep trajectory to obtain the frequency error. Then, the original drive waveform is corrected based on this error to obtain the pre-compensated drive waveform. Finally, the pre-compensated drive waveform is applied to the phase region of the three-segment distributed Bragg reflector laser 100, making its output optical frequency closer to the ideal linear sweep. Through multiple iterations, the sweep linearity of the chirped optical signal and the microwave signal after optical heterodyne can be significantly improved.

[0027] A tunable laser source 101 outputs a continuous optical carrier. When this continuous optical carrier passes through an acousto-optic modulator 102 and is driven by an electrical pulse control signal 111, it is modulated into a pulsed optical signal. The chirped optical signal output from the three-segment distributed Bragg reflector laser 100 and this pulsed optical signal are combined in an optical coupler 103 and then enter a photodetector 104. In the photodetector 104, the two optical signals undergo optical heterodyne beat frequency, and the instantaneous frequency of the output microwave signal is determined by the instantaneous optical frequency difference between the two optical signals. Since one optical signal is a linearly chirped optical signal and the other is a pulse-gated optical signal, a pulsed linearly frequency-modulated microwave signal can be obtained at the output of the photodetector 104.

[0028] By adjusting the output wavelength of the tunable laser source 101, the optical frequency difference between the two optical signals can be changed, thereby adjusting the center frequency of the output linear frequency modulated microwave signal; by adjusting the amplitude and period of the sweep frequency drive signal in the phase region of the three-segment distributed Bragg reflector laser 100, the sweep frequency bandwidth and sweep frequency period of the output microwave signal can be adjusted; by adjusting the pulse width and repetition period of the electrical pulse control signal 111 of the acousto-optic modulator 102, the pulse width and repetition period of the output pulse linear frequency modulated microwave signal can be adjusted.

[0029] In one specific embodiment, the three-segment distributed Bragg reflector laser 100 has an output power of 10 dBm, and the tunable laser source 101 has an output wavelength of 1550.4 nm and an output power of 8 dBm. A sweep drive signal 110 is generated by an arbitrary function generator and applied to the phase region of the three-segment distributed Bragg reflector laser 100. An electrical pulse control signal 111 is applied to an acousto-optic modulator 102 to generate a pulsed optical carrier. The sweep drive signal 110 and the electrical pulse control signal 111 are generated from the same signal source or a synchronization signal source to achieve timing synchronization between them.

[0030] Figure 2 The image shows the spectrum measured at the output of the optical coupler 103. Two spectral components are visible: the narrow, wide spectral line on the left corresponds to the continuous optical carrier output from the tunable laser source 101, while the broadened spectral line on the right corresponds to the chirped optical signal output from the three-segment distributed Bragg reflector laser 100. The optical frequency interval between the two signals provides the frequency difference condition for subsequent optical heterodyne beat frequency execution in the photodetector 104.

[0031] First, without connecting the acousto-optic modulator 102 to the electrical pulse control signal 111, the system outputs a continuous chirped microwave signal. The phase region of the three-segment distributed Bragg reflector laser 100 is driven by a sawtooth wave with a period of 10 μs, and the peak-to-peak voltage of the sweep drive signal is increased from 0.4V to 2.4V. Figure 3 Time-frequency plots are presented under different driving amplitudes. As the peak-to-peak voltage increases, the tuning range of the three-segment distributed Bragg reflector laser 100 increases, corresponding to a gradual increase in the sweep bandwidth of the output microwave signal from 2.2 GHz to 9.7 GHz. However, the frequency trajectory under direct driving conditions exhibits significant curvature, indicating that the laser's current-frequency response is nonlinear, requiring pre-compensation methods to improve the sweep linearity.

[0032] Subsequently, the pre-compensated sweep drive signal obtained through iteration is applied to the phase region of the three-segment distributed Bragg reflector laser 100. Figure 3 The experimental results are given when the peak-to-peak voltage is 1.2V. Figure 3(a) is the time-domain waveform of the generated continuous linear frequency modulated microwave signal. The inset shows a magnified view of the local area at different frequency sweep positions. It can be observed that the oscillation period changes with time, indicating that the instantaneous frequency sweeps continuously with time. Figure 3 (b) is the compressed pulse obtained by autocorrelation calculation, with a compressed pulse width of approximately 80 ps. Figure 3 (c) shows the corresponding time-frequency plot. It can be seen that the sweep frequency trajectory is close to the ideal linear change, the sweep frequency bandwidth is about 5.5 GHz, and the linear fit is 0.99914, indicating that the pre-compensation method effectively improves the sweep frequency linearity.

[0033] To further verify the bandwidth expansion capability, the peak-to-peak voltage of the pre-compensated sweep frequency drive signal was increased to 1.8V. Figure 4 The corresponding experimental results are presented. Figure 4 (a) The output microwave signal still maintains stable periodic chirping characteristics; Figure 4 (b) shows the autocorrelation compression result, with a compression pulse width of approximately 75 ps; Figure 4 (c) shows the time-frequency plot, which reveals a high-linearity triangular linear frequency modulated microwave signal with a bandwidth of approximately 8 GHz, exhibiting a linear fit of 0.99947. Compared to the 5.5 GHz bandwidth result, the compressed pulse width of the 8 GHz bandwidth signal is further reduced, indicating that increasing the sweep bandwidth helps improve pulse compression performance.

[0034] Finally, the electrical pulse control signal 111 connected to the acousto-optic modulator 102 generates a linearly frequency-modulated microwave signal. In this embodiment, the peak-to-peak voltage of the pre-compensation sweep drive signal in the phase region of the three-segment distributed Bragg reflector laser 100 is 1.2V, and the period is 20μs; the pulse width of the electrical pulse control signal of the acousto-optic modulator 102 is 10μs, and the repetition period is 200μs. Through the synchronous sweep drive signal 110 and the electrical pulse control signal 111, the gating window of the optical pulse generated by the acousto-optic modulator 102 is aligned with the sweep period of the chirped optical signal. Figure 5 (a) is the time-domain waveform of the generated pulsed linear frequency modulated microwave signal, and the pulse envelope with a repetition period of 200 μs can be observed; Figure 5 (b) shows the corresponding time-frequency diagram, which reveals that the linear frequency modulated signal only appears within the pulse-gated time window and maintains a sweep bandwidth of approximately 5.5 GHz. This result demonstrates that the present invention can convert a continuous high-linearity linear frequency modulated microwave signal into a pulsed linear frequency modulated microwave signal using optical pulse gating.

[0035] In summary, this invention proposes a high-linearity broadband linear frequency-modulated pulse signal generation method based on optical heterodyne. It utilizes a three-segment distributed Bragg reflector laser to generate broadband chirped optical signals, employs a pre-compensation method to improve sweep frequency linearity, utilizes a tunable laser source to adjust the center frequency, and utilizes an acousto-optic modulator to achieve pulse gating. This method has advantages such as simple structure, adjustable sweep bandwidth, adjustable center frequency, controllable pulse parameters, and high linearity, making it suitable for broadband radar, microwave photonic signal generation, target detection, and integrated communication and sensing systems.

Claims

1. A method for generating broadband linear frequency modulated pulse signals based on optical heterodyne, characterized in that, In a system consisting of a three-segment distributed Bragg reflector laser (100), a tunable laser source (101), an acousto-optic modulator (102), an optical coupler (103), and a photodetector (104), the three-segment distributed Bragg reflector laser (100) has an input port connected to a sweep frequency drive signal (110), and the acousto-optic modulator (102) has an input port connected to an electrical pulse control signal (111). A sweep drive signal (110) is applied to the phase region of a three-segment distributed Bragg reflector laser (100). By adjusting the injection current in the phase region, the carrier concentration and effective refractive index are changed, so that the three-segment distributed Bragg reflector laser (100) outputs a chirped light signal with a frequency that varies with time. A tunable laser source (101) outputs a continuous optical carrier, which is pulse-gated by an acousto-optic modulator (102) to obtain a pulsed light signal with a set pulse width and repetition period. The chirped light signal and the pulsed light signal are combined by an optical coupler (103) and input into a photodetector (104). Through the optical heterodyne beat frequency action of the photodetector (104), the instantaneous frequency difference between the two optical signals is converted into an electrical microwave signal, thereby generating a pulsed linear frequency modulated microwave signal.

2. The method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne as described in claim 1, characterized in that, The sweep drive signal (110) loaded onto the phase region of the three-segment distributed Bragg reflector laser (100) is a pre-compensated sawtooth wave or triangular wave drive signal. This pre-compensated drive signal is used to correct the current-frequency nonlinear tuning response of the three-segment distributed Bragg reflector laser (100), so that the instantaneous frequency of the output chirped light signal changes approximately linearly with time, thereby improving the sweep frequency linearity of the linear frequency modulated microwave signal.

3. The method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne as described in claim 1, characterized in that, The sweep frequency drive signal (110) and the electrical pulse control signal (111) are generated by the same signal source or a synchronization signal source to achieve timing synchronization between the chirped optical signal and the pulsed optical signal, so that the pulse gate window is precisely aligned with the linear sweep frequency period, thereby obtaining a pulsed linear frequency modulated microwave signal of a complete period or a specified time period.

4. The method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne as described in claim 1, characterized in that, The sweep bandwidth and sweep period of the generated linear frequency modulated microwave signal are adjusted by adjusting the amplitude and period of the sweep drive signal loaded onto the phase region of the three-segment distributed Bragg reflector laser (100).

5. The method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne as described in claim 1, characterized in that, The center frequency of the generated linear frequency modulated microwave signal is adjusted by adjusting the output wavelength of the tunable laser source (101).

6. The method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne as described in claim 1, characterized in that, The pulse width and repetition period of the generated linear frequency modulated microwave signal can be adjusted by adjusting the pulse width and repetition period of the electrical pulse control signal (111) applied to the acousto-optic modulator (102).

7. A method for generating a broadband linear frequency modulated pulse signal based on optical heterodyne according to any one of claims 1-6, characterized in that, The generated pulsed linear frequency modulated microwave signal is used in broadband radar, microwave photonic signal generation, target detection, and integrated communication and sensing systems.

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