Aiming at the nonlinear distortion problem ignored by most microwave photonic I/Q down-conversion systems, a large dynamic range microwave photonic I/Q down-conversion system based on the polarization division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM) is proposed. The PDM-DPMZM is used to modulate the radio frequency signal, local oscillator signal, and image signal in parallel. By adjusting the DC bias of the modulator and the powers of the drive signals, the I/Q down-conversion and image interference suppression can be realized. The third-order intermodulation distortion can be suppressed at the same time. Experimental results show that the proposed scheme can achieve an image rejection ratio of more than 44 dB. The spur-free dynamic range can reach 110.5 dB·Hz^4/5. In the 5-20 GHz operating frequency range, the phase imbalance and amplitude imbalance are lower than 0.8° and 0.6 dB, respectively.

Microwave photonics is a multidisciplinary field that studies the interaction between microwave and optical waves for the generation, transmission, processing, and measurement of wideband microwave signals by means of photonics. Microwave photonic sensors are one of the active sub-fields that uses optical sensors to probe the information of temperature, strain, pressure, etc. and microwave photonic techniques to extract the sensing information accurately, providing unique advantages of high resolution and high speed. This paper comprehensively reviews the recent progress in microwave photonic sensors, introduces the basic principle of microwave photonic sensing, and discusses the potential research directions in the future.

Based on the distinct advantages of large transmission bandwidth, compact energy transmission channel, and strong electromagnetic interference(EMI) immunity from optical fiber, a hybrid scheme is designed for simultaneous transmission of both information and energy over fiber. Remote energy distribution and high-frequency broadband signal acquisition & transmission are simultaneously implemented, by incorporating a comprehensive fiber distribution link and an analog microwave photonic frontend. Firstly, the power over fiber(PoF) link is designed and experimentally demonstrated. The high-power laser energy is transmitted via a 2 km multi-mode fiber link and then converted into electrical energy of hundreds milliwatts, providing power for the remote module or battery. Moreover, PoF-based radio over fiber(RoF) link and EMI detection link have been also developed and successfully verified. In the experiments, a wideband 16QAM-OFDM signal centered at 2.4 GHz is delivered through the RoF link with a 2 km single-mode fiber. Furthermore, the EMI remote monitoring link is used to perform the detection and identification of in-band, adjacent-band, and out-of-band EMIs for the GSM-R system along the high-speed railways. The proposed scheme and system will find significant applications in specific scenarios, such as inflammable and explosive conditions, strong EMI, and strong radiation.

A simulation model is proposed for a harmonically mode-locked optoelectronic oscillator(OEO) based on a dual-loop architecture and bias modulation. The model includes bias modulation of the electro-optic modulator for mode locking, convolution of microwave signals and filter impulse response for mode selection, and time-domain interference of signals in two cavities for super-mode noise suppression. Numerical simulation is carried out based on the improved pulse tracking method to realize synchronous pulse evolution in two cavities. Through using this model, the waveform, the electrical spectra, the super-mode noise suppression and the phase noise characteristics of the generated microwave pulse trains under harmonic mode locking are numerically simulated, where the simulation results fit in with the experimental results. The proposed model can be used to design a harmonically mode-locked OEO based on a dual-loop architecture and bias modulation, and study the dynamic process in the dual-loop cavity, which is conducive to achieve microwave signals with super-mode noise suppression ratio.

In radar systems, to achieve high-precision multi-dimensional measurement of targets, radar signal generation is a basic and important function. A microwave photonic frequency-quadrupled composite radar signal generation approach is proposed. The composite radar signal includes a single-chirped linearly frequency-modulated (LFM) signal and a single-tone microwave signal. The single-tone microwave signal and the single-chirped LFM signal are jointly used to measure the radial velocity of a target, while the single-chirped LFM signal is used to measure the distance of the target and implement the high-resolution microwave imaging. In the transmitter, an up-chirped LFM signal with an instantaneous bandwidth of 2 GHz and a 13.2 GHz single-tone microwave signal are generated using a photonic frequency quadrupler. In the receiver, target echo signals are de-chirped and then used to achieve the measurement of distance and radial velocity and the high-resolution ISAR imaging. Experimental results show that the absolute measurement errors of distance and radial velocity are no more than 4.2 cm and 1.7 cm/s, respectively, and the imaging results of multiple targets are clear and identifiable.