Publisher: Jinan University Media Center News 

Editor: Chen Guoqiong

Recently, the research team led by Guan Baiou from the College of Physics and Optoelectronic Engineering realized knot‐like temperature recording using fiber Bragg gratings. The study, entitled Kovacs-like memory effect mediated fiber Bragg grating: resembling a silica quipu, was published in Nature Communications on July 7, 2025.

(Image: Screenshot of the article title page)

Fiber Bragg gratings (FBGs) are widely used sensing elements in many defense and civilian applications. Conventional FBG temperature measurements require real-time wavelength reading via demodulation devices. However, in certain specialized environments such as aerospace and geology, it is often impractical to deploy such real-time demodulation systems. If an FBG could memorize temperature events it has experienced, the temperature information could be retrieved afterward, greatly expanding its potential applications.

Using a 193 nm excimer laser and a phase mask technique, the team fabricated a Kovacs-like memory effect fiber Bragg grating (KM-FBG) in silica fiber. This grating exhibits a unique dual-peak spectrum: as temperature rises, the two peaks shift toward longer wavelengths and gradually merge; upon cooling, the peaks shift back toward shorter wavelengths, but the spectral profile remains permanently frozen in the shape corresponding to the maximum temperature experienced. Inspired by the ancient practice of recording events by tying knots on a rope, the KM-FBG acts as a silica quipu, using nature itself to knot the spectrum and record the peak temperature.

(Image: Schematic diagram of spectral knotting for temperature recording)

Experiments confirm that the KM-FBG reliably retains its spectral shape over multiple thermal cycles, provided the temperature does not exceed the previously recorded maximum.

The research team demonstrated two functional applications of the KM-FBG. First, in knotting temperature recording, a KM-FBG array was placed on a heated plate patterned with the letters J, N, and U, heated respectively to 300°C, 400°C, and 500°C. After cooling, the spectral profiles accurately recorded the maximum temperature for each zone. Second, the team implemented knotting encoding: by locally heating the KM-FBG with a laser (modulation time only 0.6 s), the spectrum could be switched between a dual-resonance peak (double knot, encoding 0) and a single merged peak (single knot, encoding 1), enabling binary data storage.

(Illustration: Demonstration of temperature memory and one-dimensional data storage)

Through wavelength division multiplexing, multiple KM-FBGs with different central wavelengths can be concatenated into an array, allowing encoding and storage of ASCII characters (such as J, N, and U).

This temperature memory capability eliminates the need for real-time wavelength interrogation equipment traditionally required for FBGs, showing great potential in industrial, geological, aerospace, and other scenarios where post-event temperature recording is needed. Moreover, the spectral behavior of KM-FBGs reflects macroscopic thermal equilibrium dynamics in glass under heating and mechanical stimuli, offering a new perspective and characterization tool for studying thermal relaxation and phase transition dynamics in amorphous materials.

This research was completed independently by the College of Physics and Optoelectronic Engineering at Jinan University. Doctoral student Yang Qiaochu is the first author, with Professors Guan Baiou and Ran Yang as corresponding authors. The work was supported by the National Natural Science Foundation of China (62335010), the Guangdong Special Support Program for Local Teams (2019BT02X105), and the Guangzhou Science and Technology Program (SL2024A04J00585).

Paper link: https://doi.org/10.1038/s41467-025-61538-y