Accurate measurement of tiny forces at micro- and nano-scales is important for many frontier research areas. The core principle of a micro-force sensor is matching device size and sensing performance to the application. When evaluating small forces in microscopic physical processes, measurement precision is especially critical. Existing high-precision (piconewton-level) mechanical sensing systems such as MEMS and AFM are typically developed for specific tasks, are expensive, and are not easily integrated with flexible or wearable applications.
Fiber-optic mechanical sensors offer advantages such as flexibility, immunity to electromagnetic interference, and all-optical integration, which provide a new approach to these challenges. Therefore, developing high-performance, low-cost, easy-to-use, and general-purpose fiber sensors is significant.
Background and motivation
Conventional fiber-based mechanical sensors usually employ fiber-spliced microcavities, fiber Bragg gratings, or cantilevers. Considering the mechanical sensitivity limits of these sensing elements, most fiber mechanical sensors reach sensitivities in the micronewton to nanonewton range, still below that of MEMS or AFM for many high-precision applications.
Integrating mechanically highly sensitive units into fiber is a major technical challenge. Recent proposals use 3D microstructures to enhance device sensitivity, but limitations in 3D structural design and high-precision micro- and nano-fabrication have constrained their application in high-precision force detection.
Recent work from Westlake University
Researchers led by Professor Min Qiu at Westlake University used a micro-nano spring as the mechanical element to produce a fiber-optic force sensor that achieves piconewton precision. The team also demonstrated the sensor's application for detecting nonlinear airflow forces.
The work was published in Advanced Materials under the title "Fiber-integrated force sensor using 3D printed spring-composed Fabry-Perot cavities with a high precision down to tens of piconewtons." The first author is Xinggang Shang, a 2019 PhD student at the Nanophotonics and Instrumentation Laboratory, Westlake University. Corresponding authors include Ning Wang (Associate Researcher, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences), Nanjia Zhou (Distinguished Researcher, Westlake University), and Min Qiu (Guoqiang Chair Professor, Westlake University).
Sensor design and operating principle
The team adopted a three-spring plate structure as the mechanical sensing unit. The springs are integrated on the fiber end face, naturally forming a Fabry-Perot (FP) cavity. Light reflected back into the fiber core interferes to produce a spectral interference pattern. The positions of spectral minima depend on the cavity length. Experimentally, the applied force is obtained by analyzing the spectral shift and converting it to structural compression, which yields the force.
Figure 1: Working principle of the spring-based fiber sensor.
Source: Advanced Materials
Device fabrication and optimization
Experimentally, the team used two-photon polymerization (TPP) 3D printing to fabricate the spring structures. Achieving high-performance 3D micro- and nano-springs is challenging because the geometric parameters must meet extremely strict requirements for optimal detection limits, and common fabrication processes cannot ensure structural stability. To address this, the team improved post-processing for TPP, carefully selecting low-surface-tension cleaning solvents to reduce capillary forces during liquid evaporation that can damage the structures. The stable spring structure fabricated on the fiber end face is shown below. The spring stiffness is k = 44.5 pN/nm.
Figure 2: Fabrication process optimization.
Source: Advanced Materials
Device calibration
Most common in-situ mechanical measurement instruments have measurement precision around the nanonewton level, making accurate calibration of this spring sensor difficult. The team used the calibration procedure illustrated below. Standard SiO2 microspheres of different diameters were transferred onto the spring plate surface; the spheres' weight compressed the spring. This produced a correspondence between spectral shift and compression weight for precise calibration. Using a high-resolution spectrometer, the spring-based fiber sensor achieved a sensitivity of 0.436 nm/nN and a detection limit of 40.0 pN.
Figure 3: Device calibration process.
Source: Advanced Materials
Nonlinear airflow force measurement
To further demonstrate the sensor's advantage in detecting small forces, the authors measured weak airflow forces and observed a nonlinear dependence of airflow force on pressure. The measured force magnitudes and trends agreed with CFD simulation results.
Figure 4: Nonlinear airflow force measured by the sensor.
Source: Advanced Materials
Outlook
This sensor could be applied across multiple fields. For example, scanning spring-based fiber sensors may be used for precise measurement of thin-film Young's modulus and biomechanics. A non-contact detection mode could provide new approaches for studying turbulence and optical forces. Improvements in 3D structure design and fabrication methods may further enhance the sensor's detection precision.
