![]() Once the ring is fabricated, the FSR can only be slightly shifted, substantially limiting the tunability of this approach. However, the frequency of the microwave signal is determined by the free-spectral range (FSR) of the ring. Remarkable low phase noise of −110 dBc/Hz and −130 dBc/Hz at 10 kHz and 10 MHz offset frequency has been demonstrated for OEO based on nonlinear frequency comb generation in silicon nitride micro-resonators 18. In 16, an integrated optoelectronic oscillator based on InP was investigated, but the frequency tunability was limited to only 20 MHz. However, this approach requires harnessing light-sound interactions on chip, based on non-standard chalcogenide materials and the use of two lasers. Merklein et al., in 15 reported the generation of ultrawide frequency tunable signals up to 40 GHz by using OEO based on stimulated Brillouin scattering (SBS). Several approaches have been recently demonstrated 15, 16, 17. In addition, the progress of integrated microwave photonics (IMWP) 14 provides now a solid framework for the full OEO integration. In contrast, microwave photonics (MWP) is a promising alternative solution to overcome this limitation, allowing reconfigurable microwave signal generation in OEO with a wide tuning range 11, 12, 13. However, the realization of microwave filter with high Q RF and wide frequency tunability is practically hard to realize, especially for high operation frequencies 12. To achieve variable frequency generation, this microwave filter needs to be tunable. To select the desired oscillation mode, a microwave filter with high quality factor (Q RF) is typically included inside the closed path 10, 11. A classical OEO has a fundamentally multi-mode behavior 10, with mode spacing associated with the km-long optical fiber delay lines used inside the closed-loop system. The optoelectronic oscillator (OEO) is a particularly interesting solution to generate microwave and millimeter signals due to its capability to provide direct synthesis of spectrally pure and wideband tunable signals 8, 9. The generation of broadband and low noise microwave and millimeter wave signals is important for many applications, including among others, radars 1, 2 wireless communications 3, optical signal processing 4, warfare systems 5, 6 and modern instrumentation 7. Moreover, preliminary results indicate that the proposed Si OEO provides precise refractive index monitoring, with a sensitivity of 94350 GHz/RIU and a potential limit of detection of only 10 −8 RIU, opening a new route for the implementation of high-performance Si photonic sensors. Based on this concept, we experimentally demonstrate microwave signal generation between 6 GHz and 18 GHz, the widest range for a Si-micro-ring-based OEO. The tunability is achieved by changing the wavelength spacing between the optical source and a resonance peak of the resonator. ![]() The microwave signal is created by the beating between an optical source and single sideband modulation signal, selected by an add-drop ring resonator working as an optical bandpass filter. Here, we propose a new approach enabling Si OEOs with wide tunability and direct intensity-modulation to microwave conversion. However, the reported tuning range is lower than 4 GHz. Wider tunability has been shown by indirect phase-modulation to intensity-modulation conversion. Si OEO relying on direct conversion of intensity modulated signals into the microwave domain yield a limited tunability. However, current Si OEO have shown a limited performance. Si photonics has an immense potential for the development of compact and low-loss opto-electronic oscillators (OEO), with applications in radar and wireless communications.
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