Notch Microwave Photonic Filter With Narrow Bandwidth and Ultra-High All-Optical Tuning Efficiency Based on a Silicon Nanobeam Cavity

We propose and experimentally demonstrate a narrowband notch microwave photonic filter (MPF) with an ultra-high all-optical tuning efficiency based on an optimized high-quality (Q) nanobeam photonic crystal (PhC) cavity. By optimizing the structural parameters, the hole shapes and the waveguide width of the cavity, a silicon nanobeam cavity with a theoretical high-Q factor of 4 × 10
⁸
, a small mode volume (V) of 0.32(
λ
/
n
)
³
and a compact size of 6 μm
²
could be obtained. Due to the extremely high Q/V ratio beyond 1 × 10
⁹
, the nonlinear effects in the cavity could be efficiently excited, which is beneficial for on-chip low-power signal processing. In the experiment, by adjusting the wavelength of the optical carrier, the central frequency of the notch MPF could be widely tuned from 1 GHz to 40 GHz with maintaining the 3dB-bandwidth around 198 MHz and rejection ratios beyond 50 dB. On the other side, the central frequency of the MPF could be all-optically tuned based on the nonlinear effects in the cavity. Owing to the cavity extremely high Q/V ratio, the MPF frequency can be adjusted with an ultra-high tuning efficiency of 308.8 GHz/mW. To the best of our knowledge, whether the notch MPF narrow 3dB-bandwidth of 198 MHz or the all-optical tuning efficiency of 308.8 GHz/mW is a record value among the reported silicon-cavity-based notch MPFs with such a compact device size. With the dominant advantages of sub-gigahertz narrowband, wide tuning range, ultra-high all-optical tuning efficiency and extremely compact cavity size, the proposed MPFs illustrate competitive performance for on-chip microwave signal processing.