Laser diode-pumped compact hybrid lithium niobate microring laser

Recent Progresses on Hybrid Lithium Niobate External Cavity Semiconductor Lasers

Thin film lithium niobate (TFLN) has become a promising material platform for large scale photonic integrated circuits (PICs). As an indispensable component in PICs, on-chip electrically tunable narrow-linewidth lasers have attracted widespread attention in recent years due to their significant applications in high-speed optical communication, coherent detection, precision metrology, laser cooling, coherent transmission systems, light detection and ranging (LiDAR). However, research on electrically driven, high-power, and narrow-linewidth laser sources on TFLN platforms is still in its infancy. This review summarizes the recent progress on the narrow-linewidth compact laser sources boosted by hybrid TFLN/III-V semiconductor integration techniques, which will offer an alternative solution for on-chip high performance lasers for the future TFLN PIC industry and cutting-edge sciences. The review begins with a brief introduction of the current status of compact external cavity semiconductor lasers (ECSLs) and recently developed TFLN photonics. The following section presents various ECSLs based on TFLN photonic chips with different photonic structures to construct external cavity for on-chip optical feedback. Some conclusions and future perspectives are provided.

Tunability-selective lithium niobate light modulators via high-Q resonant metasurface

Herein, we propose and demonstrate an efficient light modulator by intercalating the nonlinear thin film into the optical resonator cavities, which introduce the ultra-sharp resonances and simultaneously lead to the spatially overlapped optical field between the nonlinear material and the resonators. Differential field intensity distributions in the geometrical perturbation-assisted optical resonator make the high quality-factor resonant modes and strong field confinement. Multiple channel light modulation is achieved in such layered system, which enables the capability for tunability-selective modulation. The maximal modulation tunability is up to 1.968 nm/V, and the figure of merit (FOM) reaches 65.6 V-1, showing orders of magnitude larger than that of the previous state-of-the-art modulators. The electrical switch voltage is down to 0.015 V, the maximal switching ratio is 833%, and the extinction ratio is also up to 9.70 dB. These features confirm the realization of high-performance modulation and hold potential for applications in switches, communication and information, augmented and virtual reality, etc.

Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.

On-chip single-mode thin-film lithium niobate Fabry-Perot resonator laser based on Sagnac loop reflectors

We demonstrate an on-chip single-mode Er³⁺-doped thin-film lithium niobate (Er:TFLN) laser which consists of a Fabry-Perot (FP) resonator based on Sagnac loop reflectors (SLRs). The fabricated Er:TFLN laser has a footprint of 6.5 mm × 1.5 mm with a loaded quality (Q) factor of 1.6 × 10⁵ and a free spectral range (FSR) of 63 pm. We generate the single-mode laser at 1544 nm wavelength with a maximum output power of 44.7 µW and a slope efficiency of 0.18%.

Electro-optically tunable optical delay line with a continuous tuning range of ∼220 fs in thin-film lithium niobate

We demonstrate fabrication of a 30-cm-long thin-film lithium niobate (TFLN) optical delay line (ODL) incorporated with segmented microelectrodes of 24-cm total length using the femtosecond laser lithography technique. The transmission spectra of the unbalanced Mach-Zehnder interferometers (MZIs) reveal an ultra-low propagation loss of 0.025 dB/cm. The device exhibits a low half-wave voltage of 0.45 V, corresponding to a voltage-length product of 10.8 V·cm, which is equivalent to 5.4 V·cm in the push-pull configuration. We also demonstrate a high electro-optic (EO) tuning efficiency of 3.146 fs/V and a continuous tuning range of 220 fs in the fabricated ODL.