Picometer-scale micronano fiber cone spectrometer

Since Newton observed the phenomenon of dispersion with a prism, the study of spectroscopic techniques has occupied an important place in human development. In 1859, German scientists Kirchhoff and Bunsen made the world’s first spectrometer and used spectroanalysis to analyze the then unknown chemical elements cesium and rubidium. With the improvement of spectral resolution and the improvement of spectral theory, spectroscopic technology has gradually expanded from the field of scientific experiments to analytical applications, and plays an important role in biosensing, environmental monitoring, astronomy, medical treatment and other fields.

Traditional spectrometers are bulky and expensive, making them difficult to generalize in practical applications. The measurement of spectra often requires very specialized equipment or professional testing institutions. In recent years, with the development of micro-nano technology, micro-spectrometers have gradually entered people’s field of vision. With its small size, light weight, convenient operation, simple structure and low price, microspectrometer has strong potential in large-scale promotion. For a long time, researchers have been committed to developing low-cost, small-volume, stable and high-performance spectrometers.

However, there are inherent constraints between the above problems, and reducing the size of spectroscopic and detection components will lead to a significant decrease in the resolution, sensitivity, and dynamic detection range of the spectrometer, and may increase the manufacturing difficulty and cost of the device. Therefore, the miniaturization of spectrometers is currently a high-profile technical challenge in the field. Some existing solutions, such as filter arrays and on-chip spectrometers that rely on microfabrication, still have limitations including inherent power loss, low number of channels, and low coupling efficiency. How to achieve high-stability, high-performance, low-cost microspectrometer is still one of the urgent problems to be solved in the field.

In response to the above problems, the nano-optics team led by researcher Ma Yaoguang of Zhejiang University proposed a micro-spectrometer based on micro-nano fiber leakage mode (as shown in Figure 1). The spectrometer achieves picometer wavelength resolution at submillimeter spatial scales at a submillimeter spatial scale with extremely low manufacturing difficulty and cost (core components cost less than $15).

Figure 1: Spectrometer structure. (a) Microspectrometer picture (b, c) Side view and top view of micro-nano fiber cone leakage mode pattern mapped on the substrate

The spectrometer design brings new possibilities for the application of micro-spectrometers in environmental detection, biosensing, wearable devices and other directions with excellent performance and scalability.

The results were published in eLight under the title “Microtaper leaky-mode spectrometer with picometer resolution”. The completion units of this paper are the School of Optoelectronic Science and Engineering, Zhejiang University, the National Key Laboratory of Extreme Optical Technology and Instrumentation, Hangzhou International Science and Technology Innovation Center, and the Intelligent Optoelectronic Innovation Center of Jiaxing Research Institute of Zhejiang University, the co-first authors are Cen Qingqing and Sijie Film, and the corresponding authors are researcher Ma Yaoguang. Liu Xinhang, Tang Yuwei, He Xinying made important contributions to the thesis work. This research was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Zhejiang Province.

The team members made an automated cone pulling system (as shown in Figure 2), which used the motor to stretch both ends of the fiber to obtain a micro-nano fiber cone while focusing the fiber locally. Through the automatic control program, the system can accurately adjust parameters such as cone temperature, cone length and cone speed to draw micro-nano fibers with different structural characteristics, and meet the experimental needs of high precision, high repeatability and rapid drawing.

Figure 2: Self-made cone pulling machine. (a) The heating part of the cone pulling machine and (b) the overall structure of the cone pulling machine

By optimizing the cone structure parameters of the micro-nano fiber, the micro-nano fiber cone produces a complex leakage pattern in the area within 1mm (as shown in Figure 1). Then, the precision displacement stage is used to optimize the pattern projected from leakage mode to CIS to fine-tune the contact distance, angle and other parameters between Wiener fiber and CIS. Finally, the micro-nano optical fiber is fixed and packaged, and the prototype mechanism of the spectroscopic instrument is completed.

The interference pattern formed by the leakage mode of micro-nano fiber has a complex mapping relationship with the spectrum, and the spectral information of the input light is recovered through the image captured by CIS in a single shot. The lightweight model using the Transformer architecture analyzes the interference mode data recorded by the measurement, and the correlation between spectral information and leakage mode images can be easily constructed. The resulting microspectrometer exhibits high performance at a smaller size. As shown in Figure 3, the spectrometer can recover continuous spectra up to 90 nm at half height and full width. At the same time, in the bimodal signal recovery experiment, the spectrometer can distinguish two laser peaks with a wavelength difference of 1.53pm.

Figure 3: Spectrometer performance characterization. (a) Recovery of continuous spectra and (b) Recovery of narrow bimodals

Because micro-nano fibers are small in size, multiple microfiber cones can be integrated on one sensor to achieve hyperspectral imaging capabilities. Through 20 fiber cones integrated on the CIS (Figure 4a), the image was restored and various metamerisms that could not be distinguished by the human eye, demonstrating the great potential of this design for hyperspectral imaging.

Figure 4: Hyperspectral characterization of spectrometers. (a) 20-channel hyperspectral imager (b) projection image restoration (c) reduction spectrum of each region of the projected image

(Source: China Optics WeChat public account)

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