An optical waveguide reflector is an important structure in photonic integrated circuits. It plays an important role in optical systems such as laser diodes, resonators and modulators that require optical feedback [[1], [2], [3]]. There are three methods for realizing optical waveguide reflectors. The first is to apply different high-reflectivity (HR) coatings on the output facet of a chip to select reflectivity [4]. However, HR coatings are no longer available on output ports for other components, which hinders design flexibility and introduces additional grinding and polishing processes. The second method is used to produce the distributed Bragg reflector (DBR) [[5], [6], [7]]. The DBR can be placed anywhere on a chip, and its transmitted light can be used. However, in order to obtain the required reflectivity-transmissivity ratio, the process conditions are relatively complex. In addition, its high reflectivity is limited to a narrow bandwidth. The third method involves a multimode interference (MMI) waveguide reflector based on an MMI coupler, which connects the two output ports of a 1 × 2 MMI coupler through a loop-shaped curved waveguide to act as a mirror. A large bandwidth and process tolerance are common advantages of MMI devices [[8], [9], [10], [11]]. In addition, the process flow of the MMI waveguide reflector is simpler, and a very high reflectivity can theoretically be obtained.

L. Xu constructed an MMI waveguide mirror structure on InP for the first time in 2009 [12]; however, owing to the limitations of the manufacturing process, the reflectivity of the ring mirror was approximately 60–70%, which is quite different from the simulation results. Later, researchers used the matching bending criterion to reduce the straight-bending-straight excessive loss in optical waveguides, optimize the structure of curved waveguides and design GaAs lasers with a reflectivity of 97% [13]. In 2018, Q. Fang reported the design of an MMI ring reflector structure on Silicon-On-Insulator [14], and on this basis, he fabricated a foldable silicon-photonics arrayed-waveguide-grating integrated with a ring mirror. The loss in the MMI ring mirror was 0.30 ± 0.02 dB in the wavelength range of 1530–1590 nm. Compared with the previous results, there has been a significant improvement.

Lithium niobate crystal is a type of optical material with excellent electro-optic and acousto-optic effects. With the maturity of thin-film technology, lithium niobate on insulator (LNOI) structures have been realized [15], which has opened up a new technical method for the miniaturization and performance improvement of photonic chips. In recent years, researchers have performed many studies work based on LNOI structures, such as optical waveguides [16,17], electro-optic modulators [18,19], nonlinear devices [20,21], resonant devices [[22], [23], [24]] and polarization devices [25,26]. In this paper, we demonstrate a type of lithium niobate MMI waveguide reflector. First, the initial structure of the reflector was designed based on the principle of MMI imaging. Subsequently, the structural parameters of the reflector were optimized. Finally, the loss characteristics and bandwidth characteristics are verified by experiments.