Abstract
This paper presents an integrated optical signal processor (OSP) design
using CMOS-compatible silicon photonics technology, offering a tunable
RF bandpass filter (5G/6G frequency range) with a programmable bandwidth
and symmetric out-of-band rejection. The OSP is comprised of two
identical microring resonators cascaded with a common bus waveguide, in
220 nm silicon-on-insulator substrate. The ring resonators are designed
with low-loss rib waveguide structures (supporting fundamental and
first-order TE-like modes) to ensure relatively lower propagation losses
and to obtain high-Q resonances. The single-mode bus waveguide is
designed to facilitate the desired range of coupling strengths via
thermo-optic tuning. Four thermo-optic phase shifters are integrated to
tune the resonant wavelengths and the Q-values of the individual
microring resonators. The four phase shifters could be programmed to
obtain desired optical filter characteristics of the fabricated OSP
chip. The RF filter experiments were carried out using an off-chip laser
source (λ ~ 1550 nm), a phase modulator (3-dB BW = 10
GHz) and a photodetector (3-dB BW = 26.5 GHz). The central frequency of
the RF bandpass filter could be tuned upto 8 GHz in our experimental
setup. The 3-dB (20-dB) bandwidth of the filter was tuned from 0.84 GHz
(2.77 GHz) to 1.8 GHz (5.47 GHz) with a symmetric out-of-band rejection
> 20 dB (above noise level). Even though the fabricated OSP
chip has the potential to realize filter response beyond 50 GHz, the
observed tuning range is limited by the efficiency of the thermo-optic
phase-shifters, and the bandwidth of phase modulator/photodetector used
in our experiments.