High-precision displacement sensing has been widely used across both scientific research and industrial applications. The recent interests in developing micro-opto-electro-mechanical systems (MOEMS) have given rise to an excellent platform for miniaturized displacement sensors. Advancement in this field during past years is now yielding integrated high-precision sensors which show great potential in applications ranging from photoacoustic spectroscopy to high-precision positioning and automation. In this review, we briefly summarize different techniques for high-precision displacement sensing based on MOEMS and discuss the challenges for future improvement.

In order to broaden the working voltage (1.23 V) of aqueous supercapacitors, a high-performance asymmetric supercapacitor with a working voltage window reaching up to 2.1 V is assembled using a nanorod-shaped molybdenum trioxide (MoO₃) negative electrode and an activated carbon (AC) positive electrode, as well as a sodium sulfate–ethylene glycol ((Na₂SO₄-EG) electrolyte. MoO₃ electrode materials are fabricated by adjusting the hydrothermal temperature, hydrothermal time and solution’s pH value. The specific capacity of the optimal MoO₃ electrode material can reach as high as 244.35 F g⁻¹ at a current density of 0.5 A g⁻¹. For the assembled MoO₃//AC asymmetric supercapacitor with a voltage window of 2.1 V, its specific capacity, the energy density, and the power density are 13.52 F g⁻¹, 8.28 Wh kg⁻¹, and 382.15 W kg⁻¹ at 0.5 A g⁻¹, respectively. Moreover, after 5000 charge–discharge cycles, the capacity retention rate of the device still reaches 109.2%. This is mainly attributed to the small particle size of MoO₃ nanorods, which can expose more electrochemically active sites, thus greatly facilitating the transport of electrolyte ions, immersion at the electrolyte/electrolyte interface and the occurrence of electrochemical reactions.

Here, we report an ultracompact angular displacement sensor based on the Talbot effect of optical microgratings. Periodic Talbot interference patterns were obtained behind an upper optical grating. By putting another grating within the Talbot region, the total transmission of the two-grating structure was found to be approximatively in a linear relationship with the relative pitch angle between the two gratings, which was explained by a transversal shift of the Talbot interference patterns. The influence of the grating parameters (e.g., the grating period, the number of grating lines and the gap between the two gratings) was also studied in both a simulation and an experiment, showing a tunable sensitivity and range by simply changing the grating parameters. A sensitivity of 0.19 mV/arcsec was experimentally obtained, leading to a relative sensitivity of 0.27%/arcsec within a linear range of ±396 arcsec with the 2 μm-period optical gratings. Benefitting from tunable properties and an ultracompact structure, we believe that the proposed sensor shows great potential in applications such as aviation, navigation, robotics and manufacturing engineering.

Face recognition has achieved great success due to the development of deep convolutional neural networks (DCNNs) and loss functions based on margin. However, complex DCNNs bring a large number of parameters as well as computational effort, which pose a significant challenge to resource-constrained embedded devices. Meanwhile, the popular margin-based loss functions all introduce only one type of margin and cannot further introduce a larger margin to achieve tighter classification boundary. In contrast to the common approach, we believe that additive and multiplicative margins should be used jointly to introduce larger margins from the margin perspective. Therefore, we propose a new margin-based loss function called UnifiedFace. First, we introduce an additive margin in the target angle activation function. Second, we add a multiplicative margin in the non-target angle. UnifiedFace introduces both additive and multiplicative margins, allowing for the introduction of large margins to achieve more compact intra-class variance and closer separated inter-class variance. In addition, we specifically design efficient face recognition models called GhostFaceNet for resource-constrained embedded devices. Experimental results demonstrate that UnifiedFace achieves state-of-the-art performance or performance competed with popular methods in training datasets of different sizes. UnifiedFace achieves optimal performance in models of varying complexity. Moreover, competitive results are achieved in the large-scale test set IJBB/C, especially the state-of-the-art performance achieved in TAR ($FAR=1e-6$). GhostFaceNet can significantly improve operational efficiency without significantly degrading recognition performance, making it ideal for embedded devices with limited resources.

In this paper, an ultracompact combined sensor for displacement and angle-synchronous measurement is proposed based on the self-imaging effect of optical microgratings. Using a two-grating structure, linear and angular displacement can be measured by detecting the change of phase and amplitude of the optical transmission, respectively, within one single structure in the meantime. The optically transmitted properties of the two-grating structure are investigated in both theory and simulation. Simulated results indicate that optical transmission changes in a sinusoidal relationship to the input linear displacement. Meanwhile, the amplitude of the curve decreases with an input pitch angle, indicating the ability for synchronous measurement within one single compact structure. The synchronous measurement of the linear displacement and the angle is also demonstrated experimentally. The results show a resolution down to 4 nm for linear displacement measurement and a maximum sensitivity of 0.26 mV/arcsec within a range of ±1° for angle measurement. Benefiting from a simple common-path structure without using optical components, including reflectors and polarizers, the sensor shows ultra-high compactness for multiple-degrees-of-freedom measuring, indicating the great potential for this sensor in fields such as integrated mechanical positioning and semiconductor fabrication.

In this paper, a novel optical MEMS accelerometer is proposed based on evanescent coupling between parallel silicon nanowaveguides. The coupling length between nanowaveguides changes due to the input acceleration, leading to a great change of coupling efficiency. As a result, the applied acceleration can be obtained by measuring the transmission of waveguiding light. Simulation results with optical displacement sensing sensitivity of 32.83%/ $$\upmu $$μ m within measurement range of 1.68 g is obtained. This design shows high compactness with no need of assembly, suggesting great potential in applications such as integrated photonic circuits.