In radar systems, single-chirped waveforms are susceptible to the range-Doppler coupling effect in high-speed moving target detection, which limits the performance of radar target tracking and detection. To overcome the influence of the range-Doppler coupling effect, a dual-chirp microwave waveform is proposed. Recently, electro-optic external modulation-based techniques have become the main schemes adopted in microwave photonic signal generation systems, as they can generate high carrier frequency and large time-bandwidth product (TBWP) dual-chirp signals; however, these schemes face problems such as system complexity (e.g., using complex devices such as dual-polarization quadrature phase shift keying modulators), implementation difficulty (e.g., requiring cutting of parabolic signals, power amplification, and other complex processing), and high cost (e.g., using swept-frequency lasers). Therefore, photonic-assisted wideband dual-chirp microwave signal generation with a large TBWP comprising a fiber Bragg grating (FBG) is proposed and demonstrated experimentally. The scheme generates a high-carrier-frequency, large-bandwidth dual-chirp microwave signal using a simple and low-cost system, and the generated signal exhibits good sidelobe suppression and pulse compression performance. Additionally, this scheme has the advantages of a wide frequency tuning range and tunable signal parameters. It is expected to provide a stable and reliable signal source for future radar systems, with a high joint range-velocity resolution.

The wideband dual-chirp microwave signal generation system based on FBG is mainly composed of a laser diode (LD), Mach?Zehnder modulator (MZM), optical circulator, FBG, optical coupler (OC), and photodetector (PD). First, a continuous optical carrier signal from the LD is sent to the first MZM biased at the maximum transmission point (MATP). Subsequently, the ±2^nd-order optical sidebands and optical carriers are generated by controlling the power of the radio frequency (RF) signal. The modulated output optical signal passes through the optical circulator and then through the FBG with a 3 dB bandwidth of 17.5 GHz, which separates these ±2^nd-order optical sidebands and the optical carrier. Later, the baseband single-chirp signal modulates the ±2^nd order optical sidebands through the other MZM biased at the minimum transmission point (MITP). Finally, the modulated ±2^nd order optical sidebands and the optical carrier reflected from the FBG are combined and sent to the PD for photoelectric conversion. After photoelectric conversion, dual-chirp microwave signals with double RF frequency and large time-bandwidth products can be generated. Dual-chirp microwave signals with tunable center frequencies are realized by tuning the frequencies of the radio-frequency signals loaded on the MZM.

To verify the feasibility of the proposed wideband dual-chirp microwave signal generation scheme, based on the system schematic shown in Fig. 1, this study conducted an experimental validation. First, a dual-chirp microwave signal with a carrier frequency of 20 GHz, bandwidth of 1 GHz, and time-bandwidth product of 1000 is generated (Figs. 5 and 6). By matched filtering of the generated dual-chirp microwave signal, the autocorrelation function plot of the signal shows that the peak sidelobe ratio is approximately 14.7 dB. Additionally, the pulse compression ratio is approximately 893, which indicates that the generated dual-chirp microwave signal has good detection and pulse compression performance (Fig. 7). The frequency of the RF signal is varied to generate dual-chirp microwave signals with center frequencies between 16?28 GHz and a bandwidth of 1 GHz in both cases; the instantaneous frequency of the generated signal has high linearity (Fig. 8). Meanwhile, the fuzzy function and contour map of the dual- and single-chirp microwave signals have been simulated to illustrate that the dual-chirp microwave signal can solve the range-Doppler coupling effect (Fig. 9).

This study proposed and experimentally verified a wideband dual-chirp microwave signal generation scheme based on a narrowband FBG. The proposed scheme has the advantages of low cost, simple operation, and wide frequency tuning range. In the experiment, dual-chirp microwave signals with carrier frequencies ranging from 16?28 GHz, a bandwidth of 1 GHz, and a time-bandwidth product of 1000 were generated. Through matched filtering of the generated dual-chirp microwave signals, the autocorrelation function plot of the signals showed that the peak sidelobe ratio was approximately 14.70 dB, and the pulse compression ratio was approximately 893, which indicated good detection and pulse compression performance of the generated dual-chirp microwave signals. A radar system for detecting high-speed moving targets was constructed, and the detection performance of the generated signal was analyzed. The results showed that the generated signal could overcome the ambiguity of the joint measurement of distance and velocity that exists in a single-chirp signal. The signal could accurately obtain the velocity and position information of a high-speed moving target, further applicable to modern radar systems.