The design of spectrometers has long faced the dual challenges of structural complexity and bulkiness. To achieve high resolution, traditional spectrometers typically rely on long optical path systems constructed with precision optical components, making it difficult to achieve integration and portability. Recently, a research team from Kookmin University in South Korea developed an on-chip photonic spectrometer based on mode decomposition. They adopted the effective refractive index method to reduce the optimization calculation load by two orders of magnitude and used electron beam lithography technology to fabricate devices on a silicon-based platform. Experimental verification shows that this 12-channel micro-spectrometer can achieve a spectral reconstruction accuracy of 0.1 nanometers. While maintaining high resolution, it successfully breaks through the volume limitations of traditional devices. ### Research Background Spectrometers are core equipment in optical systems, widely used in fields such as communications, biomedicine, sensing, and material analysis, and are particularly important in on-chip photonics. Traditional spectrometers have become a stumbling block to the development of on-chip photonics. They rely on diffraction gratings for light splitting to achieve high resolution, but at the cost of large volume, which is incompatible with the pursuit of compact design in on-chip photonics and fails to meet integration needs. Meanwhile, their traditional design methods are time-consuming and labor-intensive; optimization is like groping in the dark, requiring repeated trials that consume significant human, material, and time costs. In recent years, inverse design has blown a fresh breeze into the field of optical design. As an emerging technology based on optimization algorithms, it leverages the powerful simulation capabilities of computers to perform prediction and optimization simultaneously. This unique design approach is like opening a convenient door for designers, greatly improving design efficiency. Inverse design has been successfully applied in various photonic devices, demonstrating enormous potential and advantages. However, for on-chip spectrometers, inverse design is not without obstacles. When applying inverse design to on-chip spectrometers, the optimization of complex three-dimensional structures is like an insurmountable mountain. The high computational resources and long time costs required have become significant challenges for the application of inverse design in on-chip spectrometers. ### Technological Innovation The team innovatively proposed a 12-channel mode decomposition spectrometer with a size of only 10×34 μm². This achievement is of extraordinary significance. Traditional spectrometers have long been caught in a dilemma between high resolution and small size. In contrast, this new spectrometer not only significantly reduces the volume but also achieves a spectral resolution of up to 0.1 nanometers, breaking through this long-standing technical bottleneck and opening up new avenues for the development of on-chip photonics. To further optimize the design and reduce the demand for computational resources, the research team cleverly introduced the effective refractive index method. This method is like finding a shortcut for the complex calculation process, simplifying the original complex three-dimensional optimization into two-dimensional calculations. This transformation is significant: it not only greatly reduces calculation time but, more importantly, maintains high-performance design standards while shortening the computation duration, making the spectrometer more feasible and practical in real-world applications. In terms of mode decomposition and spectral reconstruction, this spectrometer demonstrates excellent performance. During simulation, its mode decomposition efficiency reaches 0.95, a figure that intuitively reflects its strong capability in mode decomposition. Moreover, the experimentally verified normalized cross-correlation (NCC) for spectral reconstruction reaches 0.99. Such a high value fully demonstrates the spectrometer's extremely high accuracy in spectral reconstruction, providing a reliable foundation for subsequent data analysis. Additionally, the spectrometer is designed based on a silicon-on-insulator (SOI) platform. Notably, it is manufactured using mature processes such as electron beam lithography. This design based on mature processes makes the manufacturing process of the spectrometer more stable and controllable. Furthermore, its simple and clear design not only reduces manufacturing difficulty but, more importantly, facilitates mass production, laying a solid foundation for its wide application. ### Significant Leap In the exploration of the spectral analysis field, this research is like a beacon. By reconstructing spectral information based on the principle of mode decomposition, it has achieved major breakthroughs in two key dimensions: calculation methods and preparation processes, bringing new hope to the development of the entire field. Firstly, at the algorithm level, the research team innovatively introduced the effective refractive index model. This move can be described as a stroke of genius. It has increased optimization efficiency by 100 times, greatly shortening calculation time, making the originally lengthy calculation process rapid and efficient, and laying a solid algorithmic foundation for the rapid development of spectral analysis. Secondly, in terms of preparation processes, the research team successfully fabricated miniaturized devices using silicon-based integration technology. Silicon-based materials are widely used in the semiconductor field due to their excellent electrical and optical properties. With the help of silicon-based integration technology, the research team was able to significantly reduce the size of spectral analysis devices, achieving miniaturization. Furthermore, through the carefully constructed 12-channel system, this miniaturized device achieves ultra-high spectral reconstruction accuracy at the 0.1 nm level. This accuracy is a milestone in the field of spectral analysis. High spectral reconstruction accuracy means being able to解析 spectral information more precisely, capture extremely subtle spectral differences, and thus provide more accurate data support for numerous fields such as scientific research, medical treatment, and environmental monitoring. Notably, this research has successfully broken through the physical bottleneck in traditional spectrometers where volume and resolution are mutually restrictive at the submicron scale, opening up a completely new path for the development of on-chip spectral analysis systems.