This paper reviews the recent developments in microwave photonic devices based on liquid crystal on silicon (LCOS) technology. The operation principle, functions and important specifications of an LCOS based optical processor are described. Three microwave photonic devices, which are microwave photonic notch filters, phase shifters and couplers, reported in the past five years are focused on in this paper. In addition, a new multi-function signal processing structure based on amplitude and phase control functions in conjunction with a power splitting function in a commercial LCOS based optical processor is presented. It has the ability to realize multiple time -shifting operations and multiple frequency-independent phase shifting operations at the same time and control multiple RF signal amplitudes, in a single unit. The results for the new multi-function microwave photonic signal processor demonstrate multiple tunable true time delay and phase shifting operations with less than 3 dB amplitude variation over a very wide frequency range of 10 to 40 GHz.

This paper presents a simple, linearized fiber-optic link that is capable of realizing a high spurious free dynamic range (SFDR) at different input RF signal frequencies without the need of readjusting system parameters. The link is based on a commercial dual-polarization modulator followed by a linear polarizer. The third-order nonlinearity at the third-order intermodulation distortion frequency can be cancelled by designing the angle of the linear polarizer. No electrical component is involved in the linearization process. The high SFDR performance is theoretically analyzed, simulated using photonic simulation software, and experimentally verified. Experimental verification of the dual-polarization modulator-based linearized fiber-optic link shows that a high SFDR of more than 124 dB⋅Hz^4/5 is obtained at different input RF signal frequencies over a 2-18 GHz bandwidth. An SFDR of 127.3 dB⋅Hz^4/5 is also demonstrated with the use of an optical amplifier to increase the link output average optical power, which is among the highest reported SFDRs measured in a fiber-optic link.

A broadband photonic-assisted microwave receiver with high cross-channel interference suppression and image rejection is proposed and experimentally demonstrated. At the input of the microwave receiver, a microwave signal is injected into an optoelectronic oscillator (OEO), which functions as a local oscillator (LO) to generate a low-phase noise LO signal as well as a photonic-assisted mixer to down-convert the input microwave signal to the intermediate frequency (IF). A microwave photonic filter (MPF), realized by the joint operation of a phase modulator (PM) in the OEO and a Fabry-Perot laser diode (FPLD), is used as a narrowband filter to select the IF signal. Thanks to the wide bandwidth of the photonic-assisted mixer and the wide frequency tunable range of the OEO, the microwave receiver can support broadband operation. The high cross-channel interference suppression and image rejection are enabled by the narrowband MPF. The system is evaluated experimentally. A broadband operation from 11.27 to 20.85 GHz is demonstrated. For a multi-channel microwave signal with a channel spacing of 2 GHz, a cross-channel interference suppression ratio of 21.95 dB and an image rejection ratio of 21.51 dB are realized. The spurious-free dynamic range (SFDR) of the receiver is also measured to be 98.25 dB·Hz^2/3. The performance of the microwave receiver for multi-channel communications is also experimentally evaluated.

An optoelectronic oscillator (OEO) for single-frequency microwave generation, enabled by broken parity time (PT) symmetry based on higher-order modulation using a Mach–Zehnder modulator, is proposed and demonstrated. Instead of using two physically separated mutually coupled loops with balanced gain and loss, the PT symmetry is realized using a single physical loop to implement two equivalent loops with the gain loop formed by the beating between the optical carrier and the ±1^st-order sidebands and the loss loop formed by the beating between the ±1^st-order sidebands and the ±2^nd-order sidebands at a photodetector. The gain and loss coefficients are made identical in magnitude by controlling the incident light power to the modulator and the modulator bias voltage. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and a single-frequency oscillation without using an ultra-narrow passband filter is achieved. The approach is evaluated experimentally. For an OEO with a loop length of 10.1 km, a single-frequency microwave signal at 9.997 GHz with a 55-dB sidemode suppression ratio and −142-dBc/Hz phase noise at a 10-kHz offset frequency is generated. No mode hopping is observed during a 5-hour measurement period.

Photonics-assisted instantaneous frequency measurement of a microwave signal using a silicon integrated microring resonator (MRR) is proposed and experimentally demonstrated. The frequency of a microwave signal has a unique relationship with the power ratio between the two microwave signals at the outputs of two microwave photonic filters (MPF) with complementary frequency responses. The key device to implement the MPFs is a silicon integrated MMR, which is employed to convert a phase-modulated optical signal to an intensity-modulated optical signal by placing two optical carriers at the complementary slopes of the MRR. For a given frequency measurement range and resolution, an MRR is designed and fabricated, and its use for instantaneous microwave frequency (IMF) measurement is implemented. For the fabricated MRR, an IMF measurement range of 14–25 GHz with a measurement accuracy of ± 0.2 G H z is achieved.

An extended microwave-photonic iterative nonlinear gain (MING) model is proposed to analyze the mode competition dynamics in an optoelectronic oscillator (OEO). The power trajectories of multiple longitudinal modes in the mode competition process are calculated using the proposed extended MING model, by which we observe that mode competition is affected by the loop nonlinearity. On one hand, when the nonlinearity is enhanced, gain saturation effect becomes more significant. The strongest mode will gain the most energy from the optical carrier and win the competition, leading to single-mode oscillation. On the other hand, with enhanced nonlinearity, the intermodulation distortions (IMDs) become more significant. The mixing between the longitudinal modes would lead to energy transfer, causing weaker longitudinal modes to receive more energy, thereby hindering mode competition. Therefore, through suppressing the IMDs by reducing the loop gain, multi-mode operation can be reduced, facilitating stable single-mode operation. The mode competition dynamics of an OEO at different loop gain levels is evaluated by simulations. The simulation results show that by decreasing the loop gain, multi-mode oscillation is quickly evolved to single-mode oscillation, which is validated by an experiment. The proposed extended model is the first comprehensive study of the nonlinear dynamics of an OEO, showing the evolution from an initial multimode oscillation to a stable single-mode oscillation.