Optoelectronic oscillator (OEO) is an ideal signal source that can generate microwave signals with high spectral purity (the side-mode suppression ratio can reach > 60 dB) and low phase noise (currently, the lowest can reach −163 dBc/[email protected] kHz [1]). However, most of the conventional OEOs are built by discrete devices, which makes it difficult to be applied to the platforms such as UAVs and satellites due to their large size and high cost. Therefore, an integrated OEO is highly desirable.

As early as 2008, a voltage-controlled whispering gallery mode (WGM) resonator is used to replace the bulky fiber loop in the conventional OEO. By altering the voltages of the WGM, a 34–36 GHz RF signal can be obtained [2]. However, the WGM is hard to fabricate and is almost impossible to be integrated with other electro-optic devices. Another kind of integrated OEO is based on a resonant tunneling diode-photodetector-laser diode (RTD-PD-LD) [3] or an electro-absorption modulated laser (EML) [4]. Although the structure is simplified because light generation, modulation, and photodetection can be implemented by a single device, a long optical fiber loop (several kilometers) is still required. Recently, owing to the improvements in silicon photonics, some silicon-photonics-based solutions have been reported. For instance, a silicon microring resonator (MRR) is functioned as an optical bandpass filter to select the oscillation mode [5], [6]. By simply adjusting the wavelength spacing between the MRR and the laser source, the OEO can be tuned within 6–18 GHz. Because of the relatively large loss of the silicon waveguide (typically 1∼2 dB/cm), the Q-factorof the MRR is only 8.1 × 104 and the phase noise of the OEO is only ∼−50 dBc/[email protected] kHz. In addition, a race-track MRR is designed to realize a tunable microwave photonic filter (MPF) [7] and then an OEO with a frequency range of 0–20 GHz is obtained based on it [8]. In these works, the widths of the race-tracks should be increased to reduce the loss, the complexities in design and fabrication are both increased.

In [9], by simultaneously integrating a high-speed phase modulator (PM), a thermally adjustable microdisk resonator (MDR), and a high-speed PD on a single silicon-on-insulator (SOI) chip, a miniaturized OEO with a frequency range of 3–8 GHz is achieved. The Q-factor of the MDR is about 1.1 × 105, and the phase noise of the OEO is only −80 dBc/[email protected] kHz. By the similar tuning principle, a broadband tunable OEO based on a high-Q silicon nitride MDR is also proposed [10]. A low phase noise (−120 dBc/[email protected] kHz) is achieved by employing the PT symmetric mechanism. However, due to the different modulation efficiencies of different modes in PM, it is difficult to balance the gain and loss of the loop and the side-mode suppression ratio (SMSR) of the system is lower than the current average level. To increase the integration degree, a fully-integrated OEO is firstly realized by monolithically integrating all the required electrical and optical devices on an InP-based chip [11]. Due to the significant loss of the InP waveguide, only an 8.97-mm oscillating loop is fabricated on the chip. As a result, the phase noise is only −60 dBc/[email protected] kHz, which actually cannot meet the requirements of almost all the systems.

In this paper, a high-Q MRR for mode selection is fabricated on the silicon nitride platform. The full width at half maximum (FWHM) of the MRR is ∼3.55 pm and a Q-factor of 4.36 × 105. Then, an 8–38 GHz tunable MPF with a 3-dB bandwidth of ∼610 MHz is built by inserting the high-Q MRR into a phase-modulated link. Based on the tunable MPF, a tunable OEO is established. The measured frequency range of the OEO is 14.60 to 25.65 GHz, and the phase noise of the 25.65-GHz oscillation signal is −88 dBc/[email protected] kHz.