Integrated photonic chip is a key technology that combines laser light sources, modulators, waveguides, detectors, and other photonic devices into a compact, high-bandwidth, low-latency, and energy-efficient package. They hold significant importance in fields such as quantum information processing and optical communication and play a crucial role in the next generation of communication systems and data interconnectivity. The two-photon polymerization technology for three-dimensional micro and nanofabrication has pushed the resolution of laser direct writing (LDW) beyond the limit imposed by optical diffraction to achieve sub-hundred nanometer scales. Meanwhile, it has significant advantages such as simple processing workflow, minimal thermal effect, and low optical threshold damage, which makes it suitable for high-precision, high-density on-chip interconnections. Additionally, the exceptional flexibility of two-photon laser direct writing systems allows for effective adaptation to the varying spatial positions, dimensions, and orientations of interfaces in on-chip interconnections, substantially reducing the requirements for active alignment. In contrast to projection lithography which processes planar structures at one time, on-chip photonic interconnections demand high-precision positioning for the three-dimensional photonic lead at the tips of the waveguides. The writing position accuracy directly influences the signal coupling quality, emphasizing the need for high-precision alignment solutions.

We focus on the research on nanoscale alignment techniques in high-precision laser direct writing for on-chip photonic waveguides. In the context of a two-photon three-dimensional direct writing system (Fig. 1), machine vision and image processing technologies based on guide star nano-alignment are employed. Intelligent recognition and positioning of nano-alignment markers are carried out in Figs. 5 and 6 to enable the definition of the processing area and the establishment of a three-dimensional processing coordinate system. The two-photon laser writing beam is then precisely controlled, aided by a differential confocal system for axial spatial positioning. This approach facilitates the high-precision and high-density 3D direct writing of on-chip photonic lead interconnections within nanoscale structures between waveguides. By enabling intelligent recognition and alignment of specific markers or distinctive graphical features within the direct writing lithography system, the system is equipped with practical functions, including the fabrication of various complex structures. This has significant scientific and practical implications in high-precision processing areas such as chip packaging, multi-material functional structure fabrication, and complex structure modifications.

Due to limitations imposed by the field of view, the two-photon laser direct writing system cannot write photonic leads of approximately 270 μm in length at one time. Consequently, each one is divided into three segments and written separately. Fig. 7(a) displays the result of a single writing operation, and Fig. 7(b) illustrates the combined photonic lead that results from three-time writing. To analyze the alignment accuracy of the photonic lead, we conduct six writing experiments using the writing program, with the results shown in Table 2. The analysis indicates that the algorithm achieves an average alignment accuracy of 29 nm, with a maximum deviation of approximately 50 nm in a single experiment. This ensures sub-hundred nanometer-level alignment precision, which aligns very closely with the theoretically expected accuracy. Among the results, the average angular deviation between the written photonic lead and the silicon waveguide is 0.19°. This alignment level enables the precise writing of photonic lead and fulfills the requirements for high-precision on-chip waveguide connections.

After analysis, the alignment deviation of this algorithm is mainly caused by the optical diffraction limit. Although the edge of visible light with hundreds of nanometers wavelength is blurred under the influence of optical diffraction limit, the algorithm can still achieve the recognition and positioning accuracy of tens of nanometers since the designed nano-guide star is an isotropic square. However, the optical diffraction limit still largely restricts the alignment limit of image processing. Additionally, the pixel size of the image, the measurement error of alignment accuracy, the instability of the equipment and the environment, and the close distance between the alignment marks also limit the alignment accuracy of the algorithm.

We address the nanoscale alignment requirements for on-chip photonic interconnection waveguides in the context of two-photon laser direct writing. Meanwhile, a method is proposed based on guide star digital matching and intelligent nano-alignment to achieve 3D laser direct writing for on-chip photonic lead nanostructures with low cost, high precision, and high density. In response to the background and demand for on-chip photonic interconnection waveguides, we design the optical system structure of the two-photon laser direct writing system. On the hardware side, the unique design of the guide star enables high-precision positioning and writing of photonic leads. On the algorithmic side, machine vision and image processing technologies are adopted for intelligent recognition, matching, and positioning. Differential confocal systems assist in axial alignment, creating a three-dimensional machining coordinate system. This system then controls the direct writing laser beam for high-precision displacement, which helps fabricate photonic leads that connect specific polymer waveguides. The experiments produce photonic leads with an average angular deviation of only 0.19° from the polymer waveguides, achieving an average absolute positional alignment accuracy of 29 nm. Finally, our study holds scientific and practical significance in the fields of high-precision optical on-chip interconnections and complex structure modifications.