Yu Zhang

A photonic method for generating wideband linear frequency modulation (LFM) signals with switchable format and band has been proposed and experimentally verified. This method generates two coherent optical frequency combs (OFC) with large number of comb lines through recirculating frequency shift loops (RFSL) and modulates the down-chirp LFM signal onto the signal OFC using a dual-parallel Mach-Zehnder modulator (DPMZM). By selecting specific comb lines and controlling their time delay, bandwidth multiplied LFM signals with different bands and formats including up-chirp, down-chirp, dual-chirp, and V-shape chirped LFM signal, can be generated. Experimental results show that this scheme can produce muti-format switchable LFM signals in the L–Ku band with a maximum bandwidth of 4 GHz, which is four times the bandwidth of the input LFM signal. This scheme features easy switching of format and bands, as well as multiplied bandwidth, making it potentially valuable for application in multifunctional radars.

Microwave photonics (MWP) channelizer converts the radio frequency (RF) signal to optical domain for transmission and processing, avoiding the bottleneck of electronic signal and realizing the instantaneous reception of large bandwidth signal. A dual-output image suppression microwave photonic channelizer has been used. In this scheme, coherent signal optical frequency combs (OFC) and local optical frequency combs are generated by recirculating frequency shifter (RFS), and sub-channel signals are obtained by using the natural frequency difference between the signal OFC and local oscillator OFC, which avoids accurate alignment of center frequency. Meanwhile, the image rejection mixer suppresses the image signal to realize the double output. Finally, a linear frequency modulation signal (LFM) with a bandwidth of 8–14 GHz is divided into 12 sub-channels with a bandwidth of 500 MHz, and the image rejection ratio (IRR) of each sub-channel is larger than 20 dB. The spurious-free dynamic range (SFDR) of the channelizer is 90.2 dB Hz^2/3. The OFC generated by this scheme has the advantages of adjustable spacing, high flatness and large comb lines, which can improve the actual receiving bandwidth of the system. It has a wide application prospect in the field of radar, electronic warfare and broadband communication.

A spectrally reconfigurable chaotic signal generation method based on a synergistic microwave photonic filter (MPF) architecture is proposed. For constructing shaping functions, the optical frequency comb’s power and repetition rate, channel weights and tap delays are tuned. Optical filtering outside the loop enables the synergistic operation between the optoelectronic oscillator nonlinear conversion and the finite impulse response MPF, effectively eliminating the chromatic dispersion-induced power fading. Experiments demonstrate tunable center frequencies (4–6.5 GHz) and bandwidths (0.42–2.13 GHz), with autocorrelation peak-to-sidelobe ratios reaching 26.79 dB and 25.84 dB using Gaussian and Kaiser windows, respectively. This scheme offers high reconfiguration flexibility and a simplified structure, while eliminating additional electro-optical conversions. The proposed method is highly promising for enhancing weak target detection in chaotic radar systems.

A microwave photonic multi-target jamming signal generator based on sawtooth waveform modulation is proposed. The sawtooth wave signal introduces multiple frequency shifts by phase modulation of the optical carrier. After frequency beating with the received linear frequency modulation (LFM) signal, a set of jamming signals can be produced. The modulation depth and symmetry of the loaded sawtooth wave signal are optimized to generate jamming signals with relatively flat power, meanwhile, the echo signal of interest is suppressed. The generation capability of tunable multi-target jamming signals has been demonstrated in X-band and Ku-band. Experimental results show that more than 20 jamming signals are generated. Besides, the echo power of true signal is lower than the other jamming signals, enabling the information of true target to be concealed within the echo. This method provides an effective generator for multi-target jamming signals in electronic countermeasure systems.

In order to solve the problem of the efficiency reduction and complex manufacturing of traditional grating couplers under environmental temperature fluctuations, a Si₃N₄ high-efficiency grating coupler integrating a distributed Bragg reflector (DBR) and thermo-optical tuning layer is proposed. In this paper, the double-layer DBR is used to make the down-scattered light interfere with other light and reflect it back into the waveguide. The finite difference time domain (FDTD) method is used to simulate and optimize the key parameters such as grating period, duty cycle, incident angle and cladding thickness, achieving a coupling efficiency of −1.59 dB and a 3 dB bandwidth of 106 nm. In order to further enhance the temperature stability, the amorphous silicon (a-Si) thermo-optical material layer and titanium metal serpentine heater are embedded in the DBR. The reduction in coupling efficiency caused by fluctuations in environmental temperature is compensated via local temperature control. The simulation results show that within the wide temperature range from −55 °C to 150 °C, the compensated coupling efficiency fluctuation is less than 0.02 dB, and the center wavelength undergoes a blue shift. This design is compatible with complementary metal-oxide-semiconductor (CMOS) processes, which not only simplifies the fabrication process but also significantly improves device stability over a wide temperature range. This provides a feasible and efficient coupling solution for photonic integrated chips in non-temperature-controlled environments, such as optical communications, data centers, and automotive systems.

A photonic-assisted distributed multiple-input and multiple-output (MIMO) chaotic radar is proposed based on multidimensional multiplexing. Reference chaotic (RC) optical signals and echo modulation signals from different remote stations (RSs) are transmitted to the central station (CS) and distinguished using wavelength division multiplexing. Signal-level fusion processing of all RSs’ data and adjustment of operating frequency are simultaneously realized. Owing to the quasi-orthogonal waveform multiplexing and non-deterministic characteristics of chaotic signals, high spectrum efficiency and anti-interference capability can be ensured. Three functions, including broadband chaotic signal generation, and frequency up-and down-conversion, are integrated by polarization multiplexing. The radar becomes much more compact by sharing the laser and modulator. A distributed 2 $\times$ 2 MIMO chaotic radar with an effective bandwidth of 6 GHz in Ku-band is established, and the 2-D positions of the targets can be obtained by a back-projection algorithm. The experimental results show that the range and azimuth resolutions are 3.09 and 6.35 cm, respectively, and the average positioning error is less than 0.71 cm. A potential solution of high-precision 2-D positioning is provided for target detection in complex electromagnetic environments.

A photonic method for the reconfigurable generation of switchable multi-format linearly frequency modulated (LFM) signal using a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM) is proposed and experimentally verified. By properly controlling the bias voltage applied on the DP-DPMZM, different chirp waveforms, including dual-chirped, down-chirped and up-chirped LFM signals, can be obtained when a radio frequency (RF) signal and a baseband chirped signal are injected into the modulator. Besides, the central frequency of the generated LFM signal can be converted from X-band to K-band by adopting -2 ^nd -order sideband instead of -1 ^st -order sideband of the modulated RF signal. Moreover, the RF signal is always single sideband modulated, thus eliminating the dispersion-induced power fading, which is essential for the photonics-based distributed multi-function radar. In the proposed scheme, no optical filter is required, which also improves the stability and feasibility of the system.

A Doppler velocity simulation method based on serrodyne modulation is proposed to achieve the frequency shift from hundred hertz to megahertz. One sub-phase modulation (PM) in a dual-parallel dual-drive Mach–Zehnder modulator loads a sawtooth signal to achieve a small frequency shift of the optical carrier. The other three sub-PMs implement carrier-suppressed double-band modulation of the RF signal. The RF signal is directly coupled from the receiving antenna to the modulator’s RF port without any electrical devices like a 90° hybrid, which ensures a broad operational bandwidth of the system. After filtering out one of the RF modulation sidebands by an optical filter, Doppler frequency shifting (DFS) is realized through frequency beating. The half-wave voltage of modulators rapidly decreases at low frequency shifts, leading to an increase in spurious signals. In order to improve the spurious suppression ratio (SSR) of DFS, a digital pre-distortion compensation based on the measured half-wave voltage is implemented in the frequency domain. Experimental results show that SSRs are larger than 35 dB when frequency shifts range from 0.1 kHz to 1 MHz. The RF operation bandwidth covers 2–40 GHz. The effectiveness of a Doppler velocity simulator is evaluated, and the simulation velocity error is less than 0.06 km/h. The proposed method has potential applications in both broadband electronic warfare and traffic metering applications.