Microwave photonic filters (MPFs) are a promising solution for high-performance radio frequency (RF) signal processing, especially in applications requiring wide bandwidth, strong anti-electromagnetic interference capabilities, and high-frequency tunability. With the rapid advancement of photonic integration technologies, integrated microwave photonic filters (IMPFs) have gained significant attention for their potential to enable compact, reconfigurable, and low-power RF front-ends on a photonic chip. Among various integrated photonic structures, whispering-gallery-mode resonators, such as micro-ring resonators (MRRs) and micro-disk resonators (MDRs), offer a unique advantage in achieving high-Q and narrowband filtering. However, most reported IMPFs suffer from a limited out-of-band rejection ratio (OBR), which is a critical factor in determining filter selectivity and signal purity, particularly in dense spectral environments. These limitations are primarily attributed to the residual optical phase in the out-of-band region, which affects the destructive interference of sidebands during PM-IM conversion. In this paper, we propose and demonstrate a high-OBR bandpass MPF based on a multimode double-strip silicon nitride (Si₃N₄) MDR. By carefully tuning the polarization state of the input optical signal, the amplitude and phase of the ±1st-order sidebands are balanced, resulting in a significant improvement in out-of-band suppression.

In this paper, we design a double-strip waveguide structure using low-loss Si₃N₄ with asymmetric vertical core layers (175 nm and 75 nm) and a 1.1 μm wide waveguide to support single-mode TE transmission. The design is optimized using finite-difference time-domain (FDTD) simulations. This ensures high optical confinement, minimal bending loss, and low scattering loss. The MDR radius is set to 100 μm, balancing low loss and manageable mode density. Full 3D FDTD simulations are conducted to identify multiple resonant modes between 1551?1553 nm, and the TE₂ mode at 1551.36 nm is selected as the filtering mode based on its extinction ratio (~18 dB), mode spacing, and stability. The measured Q factor reaches 1.03×10⁶. The MPF system is built with a phase modulator (PM) and a high-speed photodetector (PD) in a PM-IM conversion configuration. A narrow-linewidth tunable laser source (TLS) is used as the optical carrier, which is phase-modulated with an RF signal from a vector network analyzer (VNA). The modulated light passes through the MDR, where the upper sideband is selectively suppressed, enabling conversion to an intensity-modulated signal at the output. To improve OBR, a polarization controller (PC2) is introduced. It finely adjusts the polarization state of the optical input into the MDR, thus controlling the amplitude and phase matching of the ±1st-order sidebands.

The MDR device is fabricated using a commercial Si₃N₄ platform (LioniX). Optical transmission is initially measured using a high-resolution tunable laser system (Santec MPM210 + TSL710) with a 1 pm wavelength resolution. To resolve finer spectral details, a single-sideband optical vector network analysis setup is constructed, allowing ~10 kHz frequency resolution in the RF domain. The measured MDR exhibits an insertion loss of approximately 4.5 dB and produces up to 10 resonant dips within the range of 1547?1553 nm. Mode identification reveals four distinct free spectral ranges (FSRs), consistent with theoretical simulations. The frequency response of the MPF is first simulated and then measured using the experimental setup. Without polarization optimization, the filter achieves a 3 dB bandwidth of ~300 MHz and an OBR of ~20 dB. By tuning the polarization of the input light, the sideband interference condition is improved, and the OBR is increased to 30.7 dB, demonstrating significant suppression of unwanted spectral components. This polarization-based tuning method is advantageous over dual-carrier or cascaded MRR approaches, as it requires only a single optical carrier and a compact MDR structure. The tunability of the MPF is verified via two schemes: thermo-optic tuning of the MDR and wavelength tuning of the TLS. Thermo-optic tuning provides a frequency range of 1?23 GHz with a tuning efficiency of ~0.845 GHz/mW, although the tuning step size increases nonlinearly due to the voltage-power relationship. On the other hand, TLS-based tuning achieves a more linear 1?24 GHz tuning range with a fine 1 pm resolution. At higher frequencies, the filter gain declines significantly, primarily due to reduced modulation efficiency of the 20 GHz PM, as well as increased insertion loss from the PD and RF cables. Long-term stability tests are conducted by recording the frequency response every 20 min for 1 h. The gain fluctuation is within ±0.4 dB, and the center frequency drifts within ±250 MHz, indicating stable operation. Performance comparison with other integrated MPFs demonstrates that our filter offers superior balance among bandwidth, OBR, and tunability (Table 2). The compact size, CMOS-compatible fabrication, and high thermal stability of the Si₃N₄ platform make the proposed filter particularly attractive for future photonic integration.

In this paper, we design and experimentally demonstrate a high-performance, polarization-tunable bandpass microwave photonic filter based on a double-strip Si₃N₄ micro-disk resonator. The proposed filter achieves a 3 dB bandwidth of 300 MHz, a frequency tuning range of 1?24 GHz, and an OBR of up to 30.7 dB. The use of polarization control for sideband phase balancing provides a simple yet effective solution to mitigate residual phase issues commonly observed in PM-IM conversion systems. The filter also exhibits excellent gain and frequency stability over time. Compared with other integrated photonic filter solutions, our approach offers a compact and cost-effective platform with competitive performance, suitable for a wide range of applications including RF front-end signal processing, optical communications, and on-chip photonic systems.