Lithium niobate (LN), also known as the “silicon of photonics”, is one of the best materials for high performance electro-optic, nonlinear, acousto-optic devices. In recent years, advances in lithium niobate-on-insulator (LNOI) technology have revolutionized LN devices from “bulky” to “on-chip”. And the performance of LNOI devices has taken a huge leap. For example, electro-optic modulator, nonlinear frequency conversion and surface-acoustic-wave microwave filters have all surpassed the ultimate of their traditional counterparts. Efficient wave mixers based on LNOI hold great potential for next-generation photonic integrated circuits both in classical and quantum optics. However, achieving high-performance LNOI devices for scalable, fiber-compatible that is readily suitable for applications is still challenging.

Recently, in report titled “Scalable, fiber-compatible lithium-niobate-on-insulator micro-waveguides for efficient nonlinear photonics” in Optica, our group have developed a novel fabrication method of micro-waveguides and the realization of scalable, fiber-compatible, high-performance nonlinear devices by high-quality periodic poled LNOI micro-waveguides using UV lithography and dry etching techniques. This work circumvents the problem of fiber-waveguide-fiber coupling difficulty in LNOI chips.

Fig. 1. performance comparison among different LN waveguides. (a) Mode field matching between conventional LN waveguide and single mode fiber. (b) Mode field matching of LNOI micro-waveguide and lens fiber. (c) Mode field mismatching of nano-waveguide and lens fiber.

Fig. 2. (a) Photography of the PPLNOI chip. (b) SHG light at the output waveguide facet. (c) Microscopy view of the micro-waveguides and periodic poled domains. (d) SEM image of the end facet of the PPLNOI ridge waveguide.

We fabricate periodically poled LNOI micro-waveguides with a cross section of ∼3 ×4 μm2 by UV photolithography and dry etching process. Compared with traditional LN waveguides, this technique facilitates the fabrication process and improves the nonlinear efficiency, and avoids the mode mismatch with fiber as encountered by nano-waveguides.

It is a general consensus that etching of LN is notoriously difficult. In this work, we have overcome the technical difficulties in dry etching and redeposition of LNOI. By UV photolithography and dry etching process, we fabricate periodically poled LNOI micro-waveguides and demonstrate a fiber-chip-fiber second-harmonic generation conversion efficiency of 1,320%/W with a propagation loss less than 0.5 dB/cm at the telecommunication band. We also demonstrate high-quality photon pair generation via spontaneous parametric down conversion with a flux of 178 MHz/mW at sub-mW pump power and coincidences-to-accidental ratio of >8,000 at microwatts pump power.

The overall performance in both applications is on par with that of state-of-the-art counterparts using TFLN nano-waveguides. At the same time, due to its superiority in scalable fabrication and fiber compatibility, it is more advantageous in integrating with on-chip light source or fiber link.

Fig. 3. Package of LNOI micro-waveguide array.

The technology would make micrometer thick LNOI an appealing research and application platform for the next generation photonics. Currently this research is still in continuous progress including the optimization of the conversion efficiency and insertion loss. Based on this we have also finished the packaging of the waveguide array and further demonstrated the feasibility of docking with the optical fiber network. It is expected that this technology will be translated into practical applications soon.

This research is published in “Yuting Zhang, et al., "Scalable, fiber-compatible lithium-niobate-on-insulator micro-waveguides for efficient nonlinear photonics," Optica 10, 688-693 (2023)”。