Hao Chen

Frequency diverse array (FDA) radar systems have gained significant attention due to their ability to generate range-angle dependent beampatterns, enabling improved target discrimination and enhanced sensing performance. Unlike conventional phased array radars that rely solely on phase steering, FDA radars introduce small frequency offsets across antenna elements to shape the transmit-receive beampattern in both space and range. However, practical implementation of FDA radars remains challenging due to the need for precise and flexible control of frequency offsets, phase shifts and amplitudes of RF signals. We propose and experimentally demonstrate photonics-based RF signal amplitude, frequency, and phase controllers for a quasi-monostatic FDA radar system that generates a dot-shaped transmit-receive beampattern. The system employs a compact microwave photonic structure that introduces arbitrary frequency offsets to an RF signal while providing continuous 0°–360° phase tuning and adjustable amplitude control, without the use of RF components. A corresponding photonic receiver architecture for processing the radar echo signal is also presented. Both the transmitter and receiver are designed to minimise the number of components, thereby improving the practicality and scalability of FDA radar implementation. Experimental results demonstrate frequency translation with over 30 dB spurious suppression of a 15 GHz RF signal. A full 0°–360° phase tuning range and 0–25 dB amplitude control of an RF signal to be transmitted and received by an FDA radar are also verified experimentally.

A microwave photonic structure that is capable of generating and de-hopping a frequency hopping signal, is presented. It is based on applying a programmed waveform, which consists of a series of sawtooth waves with time varying periods designed according to the hopping sequence, to an optical phase modulator. This shifts the optical carrier frequency with the amount of frequency shift changing with time. The proposed structure enables an RF signal with many precise and arbitrary hopping frequencies to be generated, which is needed for practical applications but unable to achieve by the reported photonics-based structures. The amplitude of the generated frequency hopping signal remains constant as the frequency changes. The proposed structure can also be used for de-hopping a frequency hopping signal. Proof-of-concept experiments are set up. Frequency hopping signals with precise frequencies within ±1 Hz of the desired values for different RF carrier frequencies are demonstrated. The hopping frequencies are repeatable. The measured hop rate and frequency switching time are 10 MHz and 7.3 ns respectively. De-hopping a frequency hopping signal is also demonstrated experimentally.

A microwave photonic frequency and angle of arrival (AOA) measurement scheme, which overcomes the issue of limited extendibility in many reported structures, at the same time, offers the advantage of simplicity, is presented. It is based on a series connection of a parallel optical modulator with a pair of phase modulators, optical bandpass filters and photodetectors. Two low-frequency waveforms are generated after photodetection. The system input RF signal frequency and AOA can be determined by measuring the time difference between a reference point and the midpoint of the waveforms. Unlike most reported photonics-based frequency and AOA measurement systems, here the RF signal received by each antenna element is applied to a separate phase modulator. This enables the structure to be extended with minimal increase in complexity, making it suitable for use in a direction finding system with multiple antenna elements spaced far apart, thereby improving AOA measurement accuracy. Frequency measurement accuracy can also be improved, without reducing the measurement range, by increasing the number of signal paths with different path lengths. A proof-of-concept experiment has been set up to demonstrate the proposed frequency and AOA measurement scheme. Experimental results demonstrate that the proposed structure can determine an RF signal AOA in the range of -75° to 75° with errors of less than ±1.4° and an RF signal frequency in the range of 4 to 20 GHz with errors of less than ±80 MHz, based on the waveforms generated by the system. Frequency measurement of multiple RF pulsed signals with different pulse widths and frequencies is also demonstrated.

A photonic approach to implement a phased array receiver system for antenna remoting applications is presented and experimentally demonstrated. It has the abilities to perform frequency down conversion and to control the phase of the down converted IF signal, away from the remote antenna. This reduces the complexity of the remote antenna unit and is different to the reported photonics-based phase-tunable frequency converters. Fast 0° to 360° phase shift for the RF signal received by each antenna element in a phased array can be achieved by adjusting a single system parameter in the base station. The proposed photonics-based remote phased array receiver system with N antenna elements can be constructed using commercial off-the-shelf components and offers robust performance regardless of environmental perturbations. Experimental results demonstrate a continuous 360° phase shift on the system output down converted IF signal for different input RF signal frequencies, via adjusting a DC voltage in the range of ±2.3 V into the system. Results also demonstrate the proposed structure does not suffer from the phase fluctuation problem when used in remote antenna applications.

Using photonics to measure the characteristics of an RF signal has a number of advantages compared to electronics. As a result, many photonics-based RF signal measurement systems have been reported. However, none of them can simultaneously measure an RF signal angle-of-arrival (AOA) and Doppler frequency shift (DFS) while having a simple and low-cost structure that is suitable for use in a long baseline antenna array. This paper presents a microwave photonic system to fill this technology gap. Additionally, the system is designed to have a robust performance, which is needed for practical applications. Measured results demonstrate simultaneous AOA and DFS measurement using the phase difference and the frequency of two low-frequency components generated by the system. The AOA and DFS measurement accuracy are within ±1° over the 0° to 81.5° AOA measurement range and within ±0.05 Hz over the ±50 kHz DFS measurement range. Results also show the system exhibits a stable performance with less than ±0.4° and ±0.02 Hz change in the measured AOA and DFS respectively, over an hour measurement period.

A microwave photonic structure for detecting the phase difference of two radio frequency (RF) signals is presented. It is capable of determining the RF signal phase difference unambiguously over the full 0° to 360° range, which overcomes the 0° to 180° phase detection range limitation in the reported structures. The proposed RF phase detector is designed to generate two DC voltages with amplitudes depending on the phase difference of the two input RF signals. An RF signal phase difference in the 0° to 360° range can be obtained from the ratio of the two system output DC voltages. The proposed RF phase detector has a wide operating frequency range, as it is free of microwave components. It overcomes the drawbacks of complex structures, large phase detection errors, limited phase detection range, and limited operating frequency range in the reported photonics-based RF phase detectors. Experimental results are presented that demonstrate 0° to 360° phase difference detection with errors below ±3^∘ for different input RF signal frequencies.

A photonics-based high -order subharmonic mixer, which enables a low -frequency LO source to be used for highfrequency RF signal frequency downconversion, is presented. It is based on an optically injected semiconductor laser, which is oscillated in the period -one state, sandwiched between two optical phase modulators. It has the advantages of a simple and compact structure, wide bandwidth, absence of electrical components, reconfigurable subharmonic mixing operation, stable output IF signal performance, high signal-to-noise ratio, infinite LO-to-RF port isolation, and high LO -to -IF port isolation. Furthermore, it is suitable for use in remote antenna applications. We set up a proof -of -concept experiment that demonstrates a reconfigurable second-, fourth-, sixth-, and eighth -order subharmonic mixing operation for different input RF signal frequencies and powers. The experimental results also demonstrate that the proposed structure exhibits a stable output IF signal performance, which overcomes the IF signal phase stability problem in the reported high -order subharmonic mixers. (c) 2024 Optica Publishing Group

A method for determining the phase shift of a Mach Zehnder interferometer (MZI) is presented. It is based on switching the wavelength of continuous wave (CW) laser light illuminating the MZI and measuring the interferometer output amplitudes at DC and switching frequency. The method can measure the MZI phase shift unambiguously over the entire phase shift range of 2π. A practical proof of concept demonstration shows that the method can perform real-time measurement with high repeatability and accuracy limited by the optical frequency drift and power fluctuation of the lasers. The method does not require modifications of the sensor or accessing to the laser electronics and also uses simple detection. It is, therefore, suitable for bio and medical sensing applications.