High frequency radio frequency (RF) signals with low phase noise are highly desirable in various communication and radio detection and ranging (radar) systems. Optoelectronic oscillators (OEO) have shown to be able to generate such high spectral purity RF oscillations of more than 75 GHz, due to the fact that a long length of low loss optical fiber or optical resonators having extremely high quality factor (Q) in an ultra-wide bandwidth can be readily used as such energy storage elements for an optical signal carrying RF sidebands up to hundreds GHz. However, conventionally, discrete components have been used to fabricate an OEO, making it bulky, costly and cumbersome to use. To fully unlock the potentials of the OEOs, monolithically integrating most of the photonic components in an OEO on a tiny chip is highly attractive, because it offers the advantages of miniaturization, low cost, easy to fabricate, high reliability, and low-power-consumption compared with that made with bulk optics. Recently, several integrated OEO devices have been demonstrated in silicon on insulator (SOI) and indium phosphide (InP) platforms performing at X-band around 8 GHz. Unfortunately, these platforms have fundamental material limitations that are difficult to overcome, making it difficult to achieve high-performance electro-optic modulators (EOM) and energy storage elements with high Q, which are crucial devices for achieving high frequency and low phase noise signals. On the other hand, Lithium niobate (LN) has been commonly used to fabricate high-performance EOMs, thanks to its wide bandgap and large second order electro-optic coefficient. Currently, the bandwidth of LN EOMs has exceeded 100 GHz, and its half-wave voltage is less than 1 volt. Moreover, compared to the propagation losses of 0.27 dB/cm for SOI and 2 dB/cm for InP, the transmission loss of TFLN has already reached below 1 dB/m, and micro-resonators of LN with intrinsic Q factors greater than 10⁸ have been reported. The high-Q micro-resonator ensures low phase noise of the OEO signal, and the EOM with large electro-optic bandwidth and low half-wave voltage ensures high-frequency oscillation of the signal. To address the issues of low signal frequency and high phase noise on the SOI and InP platforms for OEOs, the team led by Professor Xinlun Cai from Sun Yat-sen University, based on the previous research on TFLN EOMs, and in collaboration with Professor X. Steve Yao, the inventor of the OEO, at the Optoelectronics Information Center of Hebei University, has designed and developed two Ka-band TFLN photonic integrated OEOs.

Fig.1 depicts the schematic of the fixed-frequency OEO constructed with a TFLN photonic integrated chip (PIC) incorporating a MZM and an add-drop MRR. The signal frequency is 30 GHz, and the phase noise at 10 kHz offset from the 30 GHz center frequency is -102 dBc/Hz. Fig.2 shows the schematic of the ultra-wide range frequency-tunable OEO utilizing a TFLN PIC containing a PM and a notch MRR. The signal frequency can be tuned from 20 to 35 GHz by varying the voltage applied to the MRR, and the phase noise at 10 kHz offset from the 30 GHz center frequency is -87 dBc/Hz. Thanks to the TFLN EOM with a large bandwidth of 38 GHz and a low half-wave voltage of 1.2 volts, as well as a micro-ring resonator with a Q value of 1.3х10⁶, high frequency photonic integrated OEOs in the Ka-band have been achieved. The research results are published in Photonics Research, Volume 12, Issue 6, 2024. [Rui Ma, Zijun Huang, Shengqian Gao, Jingyi Wang, Xichen Wang, Xian Zhang, Peng Hao, X. Steve Yao, Xinlun Cai, "Ka-band thin film lithium niobate photonic integrated optoelectronic oscillator," Photonics Res. 12, 1283 (2024)]

Fig. 1. Fixed frequency OEO realized with a TFLN PIC containing an MZM and an add-drop MRR. (a) Schematic. (b) Comparison of the phase noise between the fixed-frequency OEO (blue curve) and a commercial microwave source (Keysight: E8257D) (red curve). (c) The RF spectrum of the 30 GHz signal.

Fig. 2. Frequency-tunable OEO realized with another TFLN PIC containing a PM and a notch MRR. (a) Schematic. (b) Experimental results showing the wide range frequency tenability of the OEO. (c) The measured phase noises of the OEO operating at 20, 30, and 35 GHz (blue, black, and green curves), as compared to the 30 GHz signal from a commercial microwave source (Keysight: E8257D) (red curve).

The corresponding author of this study, Professor Xinlun Cai, stated: "Although the phase noise of our photonic integrated TFLN OEO is not to the performance level of a typical OEO with a long fiber delay, it is nevertheless a first step toward using TFLN platform for fabricating integrated OEO chips with high frequency, small size, low cost, wide range tenability and potentially low phase noise, which have shown to be superior to the OEOs made with the SOI and InP PICs in terms of the operation frequency and phase noise. OEOs have widespread applications in fields such as carrier extraction, clock recovery, and optical sampling, and therefore hold significant research value."

The co-corresponding author of this study, Professor X. Steve Yao, stated: "The OEO has been invented for 30 years. Through the collective efforts of industry peers, a large number of research results have been accumulated. However, there is still room for breakthroughs in practical application and engineering, mainly constrained by issues such as large size, high cost, and high power consumption. Photon integration, especially the significant progress in TFLN photon integration technology, provides a breakthrough for effectively addressing these issues. Our work is a significant step towards engineering OEO using TFLN technology. The team led by Professor Xinlun Cai from Sun Yat-sen University has successfully demonstrated semiconductor lasers and PDs on the TFLN platform, making it possible to integrate all photonic components heterogeneously onto a single chip. Further studies will be conducted to fabricate such a fully photonic integrated OEO by putting all necessary photonic components on a single TFLN chip, including the distributed feedback laser, modulator, MRR, and PD. Finally, by connecting the TFLN photonic chip with the electrical chip using wire-bonding, we will achieve the ultimate goal of optoelectronic fully integrated OEO, making it possible for mass production at scale.