A Monolithic SiGe Electronic-Path With CMRR-Enhanced Active Balun-Based Phase Shifter for Parity-Time Symmetric Optoelectronic Oscillator

A monolithic silicon-based electronic path integrated circuits suitable for parity-time symmetric optoelectronic oscillators (PT-symmetric OEO) in $0.25~\mu $ m SiGe BiCMOS is proposed in this paper. The OEO consists of electrical and optical paths. In the electronic path, two transimpedance amplifiers (TIA) cores are employed to receive the photocurrent from photodiode (PD) in the gain loop and PD in the loss loop of the PT-symmetric OEO respectively. An active combiner synthesizes signals in the electrical domain and converts them into differential signals, simultaneously functioning as an active balun. The combined signal is processed through an active vector-sum phase shifter (VSPS) and a driving amplifier (DRV) to drive the optical phase modulator (PM). A feedforward coupling capacitor is employed to enhance the CMRR of the active balun and improve the phase control accuracy and operational bandwidth of the VSPS, which is beneficial for enhancing the side-mode suppression ratio (SMSR) of the PT-symmetric OEO. The optical path comprises a PT-symmetric mode-selection structure, which is composed of a polarization controller (PC), a polarization beam splitter (PBS), and photodetectors (PD), and a microwave photonic filter with a phase modulator (PM) and a phase-shifted fiber Bragg grating (PS-FBG). Based on the monolithic electrical path in $0.25~\mu $ m SiGe BiCMOS and the optical path constructed with commercial components, a frequency-tunable PT-symmetric OEO system is implemented. The proposed electronic path IC for PT-symmetric OEO achieves a measured operating bandwidth of 2.7 -12.2GHz, a gain adjustment range of 31.0 - 57.2dB, and a phase adjustment range of 360°. The constructed OEO achieves a frequency tuning range of 4-12GHz, a phase noise of -110.4 dBc/Hz at 10kHz offset, and a SMSR of 47.3dB. To the best of the authors’ knowledge, this is the first fully-integrated electronic path IC, tailored for PT-symmetric OEO, which achieves a relative tuning range of around 100% and a side-mode rejection of 47.3dB.