Recently, our research team, led by Professor Xingjun Wang and Researcher Haowen Shu, in collaboration with the School of Integrated Circuits at Peking University and City University of Hong Kong, published a research article entitled “Thin-film lithium niobate based microwave photonic filter with up to 110 GHz tuning range” in Optics Express. The researchers monolithically integrated a broadband phase modulator and a high-Q microring resonator on a thin-film lithium niobate chip. They experimentally demonstrated microwave photonic bandpass filtering that was continuously tunable from near direct current to 110 GHz, while maintaining a 3 dB filtering bandwidth of 227–331 MHz. The work provides an integrated approach to radio-frequency signal processing in the millimetre-wave and sub-terahertz bands.
Microwave photonic filters process radio-frequency signals in the optical domain and can overcome the operating-frequency and bandwidth limitations of conventional electronic filters. Existing integrated approaches, however, often struggle to combine wide frequency tuning, high-resolution filtering, and monolithic integration. The electro-optic modulation bandwidth of silicon photonics is constrained by carrier effects; waveguide loss on indium phosphide platforms can limit resonator quality factors; and silicon nitride lacks an intrinsic electro-optic effect and therefore typically requires an external modulator. Thin-film lithium niobate combines broadband electro-optic modulation with low-loss optical waveguides, creating the conditions for high-speed modulation and narrowband optical filtering on the same chip.
To address these challenges, the researchers constructed a bandpass filter using phase-modulation-to-intensity-modulation conversion. The radio-frequency signal first drives the on-chip phase modulator, generating sidebands with equal amplitudes and opposite phases on either side of the optical carrier. The high-Q microring resonator then selectively changes the amplitude and phase of one sideband, breaking the original cancellation condition at photodetection and restoring a narrow bandpass response in the radio-frequency domain. The filter centre frequency can be tuned continuously by adjusting the frequency offset between the laser carrier and the microring resonance.
Figure 1. Operating principle and micrograph of the thin-film lithium niobate microwave photonic filter
The chip incorporates a slot travelling-wave-electrode phase modulator approximately 10 mm long and an all-pass microring resonator with a free spectral range of approximately 222 GHz. The microring has a loaded quality factor of 8.26 × 10⁵ and an intrinsic quality factor of 2.62 × 10⁶, while the phase modulator has an insertion loss of approximately 2.95 dB. A NiCr microheater integrated above the microring tunes the resonance wavelength. Together with laser-wavelength control, it enables continuous tuning from near direct current across the V and E bands.
Experiments showed that the filter centre frequency could be continuously tuned from near direct current to 110 GHz. Across the full tuning range, the 3 dB bandwidth remained between 227 and 331 MHz, and the filtering-link insertion loss ranged from 26.1 to 28.3 dB. Two-tone tests at 1, 5, 10, 15, and 20 GHz measured a spurious-free dynamic range of 74.4–83.74 dB·Hz²ᐟ³. To evaluate long-term stability, the team maintained the chip at 23 °C and monitored operation for one hour at approximately 1, 50, and 107 GHz. The filtering bandwidth and centre frequency remained stable without active optoelectronic closed-loop feedback.
The work demonstrates the combined capabilities of thin-film lithium niobate for broadband modulation, low-loss resonance, and radio-frequency photonic processing. The platform can support millimetre-wave communications, spectrum processing, and high-resolution radar applications that require broad frequency coverage and high frequency selectivity. The paper also notes that the current link loss mainly arises from unoptimized fibre-to-chip coupling; lower-loss edge coupling and more precise temperature control could further improve link performance and stability.
The authors are Jingmei Zhang, Haoyu Wang, Yuansheng Tao, Zihan Tao, Yandong He, Xingjun Wang, Cheng Wang, Hanke Feng, and Haowen Shu. Jingmei Zhang from the School of Integrated Circuits at Peking University is the first author. Zihan Tao from the School of Electronics at Peking University, Hanke Feng from City University of Hong Kong, and Haowen Shu from the School of Electronics at Peking University are the co-corresponding authors. The study was supported by the National Natural Science Foundation of China, the Fundamental and Interdisciplinary Disciplines Breakthrough Plan of the Ministry of Education of China, the Research Grants Council of Hong Kong, and the Guangdong–Hong Kong “1+1+1” Joint Funding Scheme, among other programmes.
Article link: https://doi.org/10.1364/OE.593090
