Microwave signal filtering is a fundamental and central functionality in radio-frequency (RF) systems. Underpinned by advanced integrated photonics technologies, emerging integrated microwave photonic (IMWP) filter platforms enable reconfigurable and widely tunable RF signal filtering functionalities that were unattainable using conventional electronics while also exhibiting superior features in terms of compactness, light weight, stability, low power consumption, and low latency. This paper presents a comprehensive review of the principles, architectures, and performance of IMWP filters. We highlight recent advances of IMWP filters enabled by on-chip nonlinear optics, RF-interference technology and emerging integration platforms, with an emphasis on the RF performance which is critical for their usability in real-world applications. We conclude with a perspective on future research challenges and new possibilities for IMWP filters.

Processing of microwave signals using photonics has several key advantages for applications in wireless communications. However, to bring photonic-based microwave signal processing to the mainstream requires a reduction of the form factor. Integration is a route for achieving high-performance, low-cost, and small-footprint microwave photonic devices. A high on-chip stimulated Brillouin scattering (SBS) gain is essential for synthesizing several key functionalities for advanced integrated microwave signal processing. We have optimized our on-chip SBS platform to achieve a record on-chip gain of 52 dB. In this paper, we discuss the implications of this giant gain from the viewpoint of new enabled technologies. The giant gain can be distributed over wide frequencies, which can be exploited for the realization of reconfigurable microwave bandpass, bandstop, and multiband filters. High gain also enables the demonstration of low-threshold on-chip lasers, which can be of relevance for a low-noise radio-frequency signal generation. These wide ranges of functionalities are made possible by the breakthrough on-chip gain makes Brillouin-based microwave photonic signal processing a promising approach for real-world implementation in the near future.

An unprecedented Brillouin gain of 44 dB in a photonic chip enables the realization of broadly tunable and reconfigurable integrated microwave photonic filters. More than a decade bandwidth reconfigurability from 30 up to 440 MHz, with a passband ripple $<1.9\mathrm{dB}$ is achieved by tailoring the Brillouin pump. The filter central frequency is continuously tuned up to 30 GHz with no degradation of the passband response, which is a major improvement over electronic filters. Furthermore, we demonstrate pump tailoring to realize multiple bandpass filters with different bandwidths and central frequencies, paving the way for multiple on-chip microwave filters and channelizers.

We report a silicon nitride chip-based radio-frequency photonic notch filter with an unprecedented performance including an RF gain of 8 dB, a record-low noise figure of 15.6 dB, and a spurious-free dynamic range of $116\mathrm{dB}·{\mathrm{Hz}}^{2/3}$, with a stop band rejection of 50 dB. This level of performance is achieved by using on-chip resonators’ unique phase responses, and thorough optimizations of the photonic link. These record results will potentially stimulate future implementations of integrated microwave photonic subsystems for real-world applications.

We demonstrate frequency redshifting and blueshifting of dispersive waves at group velocity horizons of solitons in fibers. The tunnelling probability of waves that cannot propagate through the fiber-optical solitons (horizons) is measured and described analytically. For shifts up to two times the soliton spectral width, the waves frequency shift with probability exceeding 90% rather than tunnelling through the soliton in our experiment. We also discuss key features of fiber optical Cherenkov radiation such as high efficiency and large bandwidth within this framework.

We present a comprehensive experimental study to optimize the link performance of a Si$_{3}$N$_{4}$-photonic-chip-based microwave photonic notch filter. The optimized chip-based filter is demonstrated to have an RF net gain, a sub-20-dB noise figure and an overall third-order spurious-free dynamic range of 115 dB.Hz$^{1/2}$ over a frequency range of 0-12 GHz, in conjunction with advanced filtering functionalities with a notch rejection > 50 dB. This record performance is achieved by employing low-biased intensity modulation and optimized optical amplification distribution along the system that enables a net RF amplification with minimized noise and full compatibility with advanced notch filter operation. Numerical calculations are also performed to explore the feasibility of implementing a fully integrated microwave photonic filter that approaches the same level of link performance demonstrated in this work. This study is expected to provide a feasible design route to approach fully integrated microwave photonic filters with high link performance.