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.

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.

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.

In order to meet the increasing integration demand from integrated microwave photonics, the devices based on subwavelength grating waveguide is proposed and studied. The subwavelength grating waveguide is mainly used in filters and optical delay lines in integrated microwave photonics, with design freedom in refractive index. We review and discuss the developments of these applications, and provide the outlook on the application of the subwavelength grating waveguide devices in integrated microwave photonics at the end.

To solve the problem of transceiver interference in the in-band full duplex (IBFD) system, a photonic radio frequency interference cancellation system based on phase modulators is proposed. Two phase modulators are used in a Sagnac loop to realize the modulation of the received signal and the local reference interference signal. Proper polarization control in the optical domain can finally cancel the self-interference signal. Experimental results show that the proposed scheme can achieve a single frequency interference suppression of more than 45 dB and a broadband interference suppression of more than 25 dB. The dynamic range of the system can reach 99.4 dB·Hz^2/3.

At present, the RF(Radio Frequency) channel based on microwave technology features limited bandwidth, limited working frequency, limited multi-frequency conversion capability and poor versatility, which seriously prevent the high-throughput satellites from the large spectrum coverage and the multi-channel conversion capability of large bandwidth. To solve this problem, based on the comparative analysis of the research status of microwave photonic RF channel, this paper proposes a broadband cross-band microwave photonic RF channel implementation method based on parallel architecture, and the corresponding simulation analysis and experimental verification are carried out. The test results show that the down conversion input frequency of the RF channel can cover 27GHz~52GHz, and the output frequency can cover 17GHz~24GHz; the up conversion input frequency can cover 25GHz~27GHz, and the output frequency can cover 37GHz~43GHz. The bandwidth is larger than 2GHz, the in-band flatness is lower than 3dB, the conversion gain is lower than -10dB, and the spurious free dynamic range (SFDR) is higher than 100dB?Hz^2/3.