As portable terminal devices become increasingly compact and lightweight in recent years, optical devices are gradually being miniaturized and integrated. However, conventional optical components based on lithium-niobate crystals are limited by their large optical volumes, which renders it difficult to satisfy the demand for the high-density integration of photonic chips; moreover, their further development is highly challenging. Breakthroughs in thin-film preparation technology offer a direction for overcoming these challenges. The lithium niobate-on-insulator (LNOI) platform provides the best comprehensive solution to address the long-standing low transmission loss, high-density integration, and low modulation power consumption requirements of photonic integrated chips, thus rendering it an ideal platform for photonic integrated chip technology. However, the current fabrication of photonic integrated devices based on the LNOI platform relies primarily on electron-beam lithography or ultraviolet lithography assisted by etching technology, which is characterized by complex fabrication processes, low processing efficiencies, and high processing costs. In this study, to investigate the potential application of femtosecond lasers in the fabrication of high-performance photonic integrated devices on an LNOI platform, we propose a method that uses femtosecond laser direct writing (FLDW)-assisted inductively coupled plasma (ICP) etching technology to fabricate Dammann grating structures on an LNOI platform.

In this study, the finite-difference time-domain (FDTD) method is used to guide the design of Dammann grating structures. First, the relationship between diffraction angle and grating period is obtained via simulation. Subsequently, the calculated parameters are introduced into the FDTD software as initial values for simulation optimization to determine the appropriate grating period and phase-transition point. For device fabrication, a layer of metallic chromium measuring approximately 600 nm thick is deposited on an LNOI platform using vacuum evaporation as a mask. FLDW is used to create a mask pattern of the Dammann grating on the chromium film. Subsequently, the mask pattern is transferred from the chromium film to the LNOI platform via inductively coupled plasma (ICP) etching. The expected depths of the Dammann gratings are obtained by varying the etching time. After removing the metal chromium film using a chromium etching solution, various Dammann grating structures are achieved based on the LNOI platform.

The Dammann grating devices based on the LNOI platform fabricated via ICP etching exhibit high fidelity (Fig. 4), good anisotropic etching, and a high depth-to-width etching ratio (Fig. 5). The average etching depth of the grating is 250 nm and the grating exhibits good etching uniformity, thus satisfying the etching requirements. A 532 nm continuous laser is used as the excitation source inside the test system to characterize the diffraction performance of the grating structures (Fig. 6). The incident laser is expanded using a beam-expansion optical path composed of two lenses and then coupled to the surface of the Dammam grating through a small aperture. The output diffraction beam is obtained on the final charge coupled device (CCD) using an imaging objective lens to observe the intensity distribution of the light field diffracted by the Dammann grating. The test results show that the fabricated two-dimensional Dammann gratings can split beams effectively as well as diffract 2×2, 3×3, and 4×4 uniform light intensity arrays. The fabricated vortex Dammann grating is extremely efficient in generating a vortex beam and can diffract a 1×2 high-quality vortex beam array, which is consistent with the theoretical simulation result.

In summary, two-dimensional Dammann gratings and vortex Dammann grating devices are successfully fabricated on an LNOI platform via FLDW-assisted ICP etching. Compared with the conventional ultraviolet lithography or electron-beam lithography assisted by etching technology, mask-assisted etching technology using FLDW offers high processing freedom, good surface quality, a simple fabrication process, and high fabrication efficiency. Thus, it is extremely advantageous for the fabrication of large-scale high-performance micro-optical devices. This study demonstrates the potential of FLDW-assisted ICP etching technology for the fabrication of high-precision diffraction gratings on LNOI platforms, which can propel the development of high-performance photonic integrated devices on LNOI platforms.