Microwave signals with high spectral purity and wideband tunability are significant for applications such as radars, wireless communications and electronic warfare systems. For traditional electrical oscillators, it is difficult to generate microwave signals with low-phase noise at high frequencies due to electronic limitations. Theoptoelectronicoscillator (OEO) has attracted intensive interest for the generation of microwave signals with low-phase noise at high frequencies. The frequency of the generated microwave signal can be tuned by using the microwave photonic filter (MPF) instead of the electrical passband filter (EPF) to achieve mode selection. Hence, the OEO is capable of generating microwave signals with wideband tunability and low-phase noise, which is potentially desirable for practical applications.
Principle of the silicon-based PT-symmetric OEO
Parity-time (PT) symmetry originates from quantum mechanics and gradually evolves to other fields thanks to its capability of mode selection. The PT-symmetric OEO based on discrete devices is required to establish two perfectly identical feedback loops, which makes the system bulky, high cost and sensitive to environmental fluctuations. The emergence of integrated photonics can enable optical and electronic components to be co-integrated on a chip based on different material platforms. Therefore, it is highly desirable to achieve an integrated OEO that extremely reduces size and enhances power efficiency.
Fig.1 (a) The schematic diagrams of the designed silicon photonic chip. (b) The micrograph of the fabricated chip with false color. The inset is zoom-in view of key components: ① coupling region of the MRR, ② MZI and ③ two PDs. (c) The electrical spectra of single-mode oscillation with different oscillation frequencies (d) The measured phase noises at the oscillation frequencies of 4.97 GHz (red dashed curve) and 13.67 GHz (blue solid curve).
Recently, researchers from Huazhong University of Science and Technology (HUST) proposed to achieve silicon-based tunable PT-symmetric OEO. In their study, a high-Q microring resonator (MRR), a PT-symmetric mode-selective architecture, and two photodetectors (PDs) are integrated on a silicon-on-insulator (SOI) wafer. In the OEO cavity, a phase modulator and a high-Q MRR constitute a tunable MPF. The MRR-based MPF performs coarse mode selection and frequency tunability. Notably, the bandwidth of the MPF is not narrow enough and multimode oscillation exists. To ensure single-mode oscillation, an integratedPT-symmetric mode-selective architecture, which consists of a tunable Mach-Zehnder interferometer (MZI) and two waveguides with equal length, is established. By manipulating the voltage applied to one branch of the MZI, the gain and loss can be matched and the PT symmetry can be obtained. When the gain/loss coefficient of the mode with the largest gain is higher than the coupling coefficient, the PT symmetry is broken and stable single-mode oscillation is achieved.
In the experiment, single-mode oscillation of the silicon-based PT-symmetric OEO is realized and the oscillation frequency can be tuned from 0 to 20GHz. When the frequency of generated microwave signal is 4.97 and 13.67GHz, the measured single sideband phase noises at 10-kHz offset frequency are −83.42 and −80.96 dBc/Hz, respectively. Simultaneously, the side mode suppression ratio of the OEO is 46 dB at 13.67 GHz.
This work combines the high-quality MPF and the PT symmetry to enhance mode selection and make frequency tunability. This demonstration significantly reduces the footprint of the OEO system and opens new avenues for the monolithic integrated OEO.
