Thin‐film lithium niobate on insulator (LNOI) is a promising platform for optical communications, microwave photonics, and quantum technologies. While many high‐performance devices like electro‐optic modulators and frequency comb sources have been achieved on LNOI platform, it remains challenging to realize photodetectors (PDs) on LNOI platform using simple and low‐cost fabrication techniques. 2D materials are excellent candidates to achieve photodetection since they feature strong light‐matter interaction, excellent mechanical flexibility, and potential large‐scale complementary metal–oxide–semiconductor‐compatible fabrication. Herein, this demand is addressed using an LNOI‐2D material platform, and two types of high‐performance LNOI waveguide‐integrated 2D material PDs, namely graphene and tellurium (Te), are addressed. Specifically, the LNOI‐graphene PDs feature broadband operations at telecom and visible wavelengths, high normalized photocurrent‐to‐dark current ratios up to 3 × 106 W−1, and large 3‐dB photoelectric bandwidths of ≈40 GHz, simultaneously. The LNOI‐Te PDs on the other hand provide ultrahigh responsivities of 7 A W−1 under 0.5 V bias for telecom signals while supporting GHz frequency responses. The results show that the versatile properties of 2D materials and their excellent compatibility with LNOI waveguides could provide important low‐cost solutions for system operating point monitoring and high‐speed photoelectric conversion in future LNOI photonic integrated circuits.

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Microwave photonic systems process the electrical signal in optical domain so that the characteristics of the output optical signals include the transfer function of the signal processing link. In this paper, we review our recent works about a microwave photonic repeater, self-interference cancellation, and microwave signal coupling techniques. First, based on a photonic repeater consisting of a dual-polarized dual-parallel Mach-Zehnder modulator (DP-DPMZM) and a polarization controller (PC), four different down-conversion states can be performed. Two separate local oscillator signals (LO) also enable frequency down-conversion of microwave signals in different frequency bands. Secondly, a dual-parallel dual-drive Mach-Zehnder modulator (DP-DDMZM) and a low-frequency LO signal source are used to achieve self-interference cancellation and harmonic down-conversion (SIC+HDC) for an in-band full-duplex (IBFD) system. Finally, for the microwave photonic coupling technique, a photonic microwave hybrid splitter (PMHS) and a photonic microwave hybrid combiner (PMHC) are proposed, respectively. The PMHS and PMHC individually controls the amplitude and phase of the two signals by adjusting the direct current (DC) biases of modulators. Our proposed signal processing techniques are very promising for flexible microwave signal processing, radars, and wireless communications.

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