Research Progresses of Integrated Microwave Photonic Filters （Invited）

A Microwave Photonic Filter （MPF） is a device that filters microwave signals by first modulating them into the optical domain， then processing them using optoelectronic components. Compared to traditional electronic microwave filters， they exhibit advantages such as superior frequency tuning range， flexible spectrum reconfiguration， and inherent immunity to electromagnetic interference， etc. These benefits make MPFs highly promising for widespread applications in wireless communication， radar， and electronic warfare systems. With the advancement of integrated optoelectronic technologies， MPFs are progressively evolving from discrete fiber-optic devices towards integrated solutions. This transition aims to substantially reduce their size， weight， power consumption， and cost. In this review， firstly the system architectures and working principles of both incoherent and coherent integrated MPFs are presented. Subsequently， recent research advances in these two categories of MPFs are reviewed.For incoherent integrated MPFs， multi-tap configurations are typically adopted， utilizing Finite Impulse Response （FIR） digital filter architectures rooted in discrete signal processing algorithms. Some key integrated optical components enabling dispersion-delayed sampling in multi-tap MPFs are given， such as integrated high-dispersion chips， multi-wavelength light sources， and spectral shaping chips. By adopting these functional chips into microwave photonic filter links， some MPFs with good frequency tuning capabilities and out-of-band RF suppression have been reported. But the limited number of taps and unattainable complex tap coefficients resulting in insufficient spectral reconstruction capability of the achieved incoherent MPFs. To date， only aforementioned integrated chips have been integrated into microwave photonic filter links， while other critical components still rely on discrete fiber-based devices. Incoherent MPFs with higher integration levels remain unreported in the literature.For coherent integrated MPF， typically microwave signals are modulated onto a single-wavelength laser source， where various integrated optical filters are adopted to spectrally shape modulated optical signal， followed by down-conversion at a photodetector to convert optical domain filtering response into microwave domain. Various integrated optical components to construct coherent MPFs such as Micro-Ring Resonator （MRR）/Micro-Disk Resonator （MDR）， nonlinear As2S3 waveguide with Stimulated Brillouin Scattering （SBS）， photonic crystal cavity， microsphere， phase shifted waveguide Bragg grating， Surface Acoustic Wave （SAW） based on suspended waveguide have been reviewed. Based on these integrated optical components， various MPF link architectures to realized band-stop and band-pass MPFs have been introduced. Band-stop MPFs can be realized through optical-to-microwave mapping based on Optical Single Sideband （OSSB） modulation. Furthermore， by employing Radio Frequency （RF） cancellation techniques via Unbalanced Optical Double Sideband （UODSB） modulation， this approach can overcome the limitation imposed by optical filter’s extinction ratio on RF suppression performance and enables band-stop MPF with RF rejection ratio larger than 60 dB. On the other hand， Band-pass MPFs can be realized through either by optical carrier separation and re-entry techniques enabling direct one-to-one mapping of optical band-pass responses to the microwave domain， or phase-to-intensity conversion schemes that transform optical band-stop filtering into microwave band-pass responses. In addition， some technologies to enhance MPF's frequency resolution have been introduced， such as improving quality factors of MRR/MDR with mode manipulation， incorporating on-chip narrow-band SBS gain/loss spectra， adopting SiO2 microsphere， etc. Up to date， frequency resolution down to about ten MHz have been demonstrated for band-pass and band-stop MPFs. Building on recent advances in integrated optical filters for MPF links， this work futher surveys highly integrated microwave photonic filter chips implemented on Thin-Film Lithium Niobate （TFLN）， Silicon-On-Insulator （SOI）， and indium phosphide （InP） platforms， providing a comparative analysis of their respective advantages and limitations.Finally， quantitative comparative analysis of key performance metrics of the reported incoherent and coherent integrated MPFs have been given. Some key technical challenges in integrated MPFs， including the trade-offs between filter's bandwidth and frequency tuning range， insufficient noise figure/out-of-band RF rejection ratio/frequency stability， immature heterogeneous integration， and broadband photonic packaging solution with low crosstalk and high RF integrity. Moreover， an outlook on the future development trends of integrated MPF is presented.