A new publication from Advances in optoelectronicsDOI 10.29026/oea.2022.210101 describes a fast response and highly sensitive optical strain sensor.
Strain sensors play an important role in many applications such as flexible electronics , health monitoring and soft robotics due to their superb response to mechanical deformations. At present, reported strain sensors mainly focus on high stretchability and high sensitivity under large strains for motion detection, but low sensitivity under micro-strain (≤1%) may limit their applications in the detection of micro-displacements and the monitoring of weak physiological signals. Recently, various types of electrical strain sensors based on microstructures such as island structures, percolations and microfissures have been demonstrated for the detection of physiological signals. However, the complicated processing and high sensitivity to electromagnetic disturbances pose challenges to their practical applications. Alternatively, fiber optic sensors offer attractive advantages over their electronic counterparts, including inherent electrical safety, immunity to electromagnetic interference, and small size. As a combination of fiber optics and nanotechnology, micro/nanofibers (MNFs) are gaining increasing research interest due to their potential for renewal and expansion of micro/scale fiber optics and flexible sensors. nano. In particular, the optical coupler based on evanescently coupled MNFs is a promising structure for highly sensitive optical sensing, since the coupling efficiency strongly depends on the ambient refractive index, coupling length and gap. between the two adjacent MNFs. Recently, a fast-response and highly sensitive optical strain sensor with two evanescently coupled micro/nanofibers (MNF) optics embedded in a polydimethylsiloxane (PDMS) film is proposed. The strain sensor exhibits a gauge factor as high as 64.5 for strain ≤ 0.5% and a strain resolution of 0.0012% which corresponds to 120 nm elongation on a 1 cm long device. As a proof of concept, a highly sensitive fingertip pulse measurement is performed. The properties of fast temporal frequency response up to 30 kHz and pressure sensitivity of 102 kPa-1 activate the sensor for sound detection. Such a versatile sensor could be of great use in monitoring physiological signals, voice recognition and detecting micro-displacements.
The authors of this paper propose a highly sensitive and fast-response optical strain sensor, as shown in Figure 1a. Each U-shaped MNF has a diameter of 0.9 μm and a radius of curvature of 50 μm. As the evanescent field decreases exponentially outside the MNFs, the coupling efficiency is very sensitive to the gap between the two MNFs. Thus, any displacement between two MNFs will be reflected upon change of optical intensity at the output port, thus realizing highly sensitive strain detection. The whole structure is embedded in a PDMS film of appropriate thickness to ensure that the stress is transmitted to the sensor with high fidelity. The PDMS film can isolate the sensing area from the air, thus avoiding unpredictable signal interference caused by dust deposition and other external environmental changes. Figures 1b and c show that such a coupler is sensitive to gap widths, since the output intensity changes dramatically when the gap width changes slightly. The specially designed structure of MNFs and the flexibility of PDMS make the sensor have high sensitivity and good ductility. The sensor achieved a gauge factor of 64.5 in the 0-0.1% strain range and fast temporal frequency response up to 30 kHz for sound detection. The sensor can also perform sound vibration detection (Figure 1d) and real-time pulse monitoring of human fingertips (Figure 1e). In addition, the sensor has properties such as simple device structure, low light source and detector demands. Additionally, taking advantage of the device’s response to wavelength insensitivity, the tungsten halogen lamp and spectrometer used in the experiments can be replaced with cost-effective devices, such as an LED and a photodiode, respectively. which is favorable to portable weak physiological signal detection system. The proposed new sensor would open a simple path to low-cost sensitive multi-functional flexible sensors with great potential in medical health monitoring, voice recognition and micro-movement detection.
Article reference: Yu W, Yao N, Pan J, Fang W, Li X et al. Highly sensitive and fast-response strain sensor based on evanescently coupled micro/nanofibers. Opto-Electron Adv 5, 210101 (2022). doi: 10.29026/oea.2022.210101
Key words: optical micro/nano fiber / strain sensor / pressure sensor / micro-displacement / directional coupler.
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Professor Tong’s group is part of the College of Optical Science and Engineering and the State Key Laboratory of Modern Optical Instrumentation of Zhejiang University. The group explores the science, technology and art of nanoscale light. Their research interests include the calculation, fabrication, manipulation, characterization and functionalization of low-dimensional photonic structures for scientific research and technological applications.
Wei Fang is currently an associate professor at the College of Optical Science and Engineering, Zhejiang University. His current research interests include fabrication and application of micro/nanofibers, quantum optics and nonlinear optics. He has published over 80 academic papers and over 3000 SCI citations. He has produced 10 invited papers at international and national academic conferences and obtained 4 national invention patents.
Lei Zhang is currently a professor at the College of Optical Science and Engineering, Zhejiang University. He joined Zhejiang Lab as a senior researcher. His current research interests include micro-nanofiber flexible sensors, humanoid tactile sensors and micro-nanofiber fluidic optical sensors. He has published more than 50 articles in major international journals such as Nature Communications, Advanced Materials, Nano Letters, etc.
Limin Tong is a professor at the School of Optoelectronic Science and Engineering, Zhejiang University. His research interests include the theoretical basis, functional structure and device applications of micro-nano photonics. He is a recipient of the National Science Fund for Outstanding Young Scholars, Chang Jiang Distinguished Professor of the Ministry of Education, OSA Fellow of the Optical Society of America. It has more than 10 research achievements reported or reviewed by Science, Nature, Nature Nanotechnology, Nature Materials, etc. He has published two (edited) academic monographs and over 200 academic articles in Nature, Science, etc. It has been quoted over 10,000 times.
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