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Accuracy verification methodology for computer-generated hologram used for testing a 3.5-meter mirror based on an equivalent element
Kai Xu, Haixiang Hu, Xin Zhang, Hongda Wei, Zhiyu Zhang, et al.
Published Published online: 15 May 2024 , doi: 10.37188/lam.2024.025

Interferometry with computer-generated holograms (CGHs) is a unique solution for the highly accurate testing of large-aperture aspheric mirrors. However, no direct testing method for quantifying the measurement accuracy of CGHs has been developed. In this study, we developed a methodology for verifying CGH accuracy based on an element that is functionally equivalent to a large-aperture mirror in terms of accuracy verification. The equivalent element decreased the aperture by one or higher orders of magnitude, implying that the mirror could be replaced by a non-CGH technology in a comparison test. In this study, a 281 mm diamond-turned mirror was fabricated as the equivalent element of a 3.5 m aspheric mirror and measured using CGH and LUPHOScan profilometers. Surface error composition and root-mean-square (RMS) density analyses were performed. The methodology verification accuracy of the CGH was 4 nm (RMS) in the low- to mid-frequency bands, with a measured surface accuracy of approximately 10 nm (RMS). This methodology provides a feasible solution for CGH accuracy verification, ensuring high-accuracy and reliable testing of large-aperture aspheric mirrors.

Multivariate relationships between microstructure evolution and strengthening mechanisms in laser powder bed fusion of Al-Mn-Sc alloy: towards improved fatigue performance
Huaping Tang, Chaofeng Gao, Shiheng Zhang, Xiaojing Xiong, Sheng Cao, et al.
Published Published online: 24 January 2024 , doi: 10.37188/lam.2024.001
The effects of direct aging treatment (at 300 °C for 5 hours) on selective laser melted (SLMed) Al-4.5Mn-1.5Mg-0.9Sc-0.2Zr alloy were investigated in this work, with the microstructure, fatigue behaviors, and fracture characteristics examined to determine the primary cause of fatigue crack source. The results revealed that the microstructure of the investigated alloy comprised fine equiaxed and columnar grains. Upon aging treatment, a significant number of nano-scaled Al3(Sc, Zr) precipitates were dispersed within the grains, leading to a substantial increase in strengths. The yield strength improved from 431 MPa to 568 MPa, representing an increase of more than 32%, while the fatigue strength improved from 180 MPa to 220 MPa after aging treatment. Nevertheless, the fracture toughness decreased significantly from \begin{document}$ 25.1\; {\rm{MPa}}\cdot {\sqrt{\rm m}} $\end{document} to \begin{document}$ 12.3 \;{\rm{MPa}}\cdot \sqrt{\rm m} $\end{document}. The results of the fatigue fracture characteristics indicate that the Mn-rich phase and the formation of defects such as pores and poor powder fusion are the sources of fatigue cracking. Although direct aging treatment can significantly increase the yield strength, decrease the rate of fatigue crack propagation, and thus improve the fatigue performance, it deteriorates the fracture toughness, and thus shortens the fatigue life of the alloy as well.
Breaking the speed limitation of wavemeter through spectra-space-time mapping
Zheng Gao, Ting Jiang, Mingming Zhang, Yuxuan Xiong, Hao Wu, et al.
Published Published online: 10 May 2024 , doi: 10.37188/lam.2024.013

Speckle patterns generated by the intermodal interference of multimode fibers enable accurate broadband wavelength measurements. However, the measurement speed is limited by the frame rate of the camera that captures the patterns. We propose a compact and cost-effective ultrafast wavemeter based on multimode and multicore fibers, which employs spectral–spatial–temporal mapping. The speckle patterns generated by multimode fibers enable spectral-to-spatial mapping, which is then sampled by a multicore fiber into a pulse sequence to implement spatial-to-temporal mapping. A high-speed single-pixel photodetector is employed to capture the pulse sequence, which is analysed using a multilayer perceptron to estimate the wavelength. The feasibility of the proposed wavelength estimation method is experimentally verified, achieving a measurement rate of 100 MHz with a resolution of 2.7 pm in a 1 nm operation bandwidth.

Differential mode-gain equalization via femtosecond laser micromachining-induced refractive index tailoring
Cong Zhang, Senyu Zhang, Yan Zeng, Yue Wang, Meng Xiang, et al.
Published Published online: 30 April 2024 , doi: 10.37188/lam.2024.014

The mode-division multiplexing technique combined with a few-mode erbium-doped fiber amplifier (FM-EDFA) demonstrates significant potential for solving the capacity limitation of standard single-mode fiber (SSMF) transmission systems. However, the differential mode gain (DMG) arising in the FM-EDFA fundamentally limits its transmission capacity and length. Herein, an innovative DMG equalization strategy using femtosecond laser micromachining to adjust the refractive index (RI) is presented. Variable mode-dependent attenuations can be achieved according to the DMG profile of the FM-EDFA, enabling DMG equalization. To validate the proposed strategy, DMG equalization of the commonly used FM-EDFA configuration was investigated. Simulation results revealed that by optimizing both the length and RI modulation depth of the femtosecond laser-tailoring area, the maximum DMG (DMGmax) among the 3 linear-polarized (LP) mode-group was mitigated from 10 dB to 1.52 dB, whereas the average DMG (DMGave) over the C-band was reduced from 8.95 dB to 0.78 dB. Finally, a 2-LP mode-group DMG equalizer was experimentally demonstrated, resulting in a reduction of the DMGmax from 2.09 dB to 0.46 dB, and a reduction of DMGave over the C band from 1.64 dB to 0.26 dB, with only a 1.8 dB insertion loss. Moreover, a maximum range of variable DMG equalization was achieved with 5.4 dB, satisfying the requirements of the most commonly used 2-LP mode-group amplification scenarios.

All-silicon low-loss THz temporal differentiator based on microring waveguide resonator platform
Yunjie Rui, Shuyu Zhou, Xuecou Tu, Xu Yan, Bingnan Yan, et al.
Published Published online: 24 April 2024 , doi: 10.37188/lam.2024.017

Microring resonators have been widely used in passive optical devices such as wavelength division multiplexers, differentiators, and integrators. Research on terahertz (THz) components has been accelerated by these photonics technologies. Compact and integrated time-domain differentiators that enable low-loss, high-speed THz signal processing are necessary for THz applications. In this study, an on-chip THz temporal differentiator based on all-silicon photonic technology was developed. This device primarily consisted of a microring waveguide resonator and was packaged with standard waveguide compatibility. It performed time-domain differentiation on input signals at a frequency of 405.45 GHz with an insertion loss of 2.5 dB and a working bandwidth of 0.36 GHz. Various periodic waveforms could be handled by this differentiator. This device could work as an edge detector, which detected step-like edges in high-speed input signals through differential effects. This development holds significant promise for future THz data processing technologies and THz communication systems.

Automated optical inspection of FAST’s reflector surface using drones and computer vision
Jianan Li, Shenwang Jiang, Liqiang Song, Peiran Peng, Feng Mu, et al.
Published Published online: 05 January 2023 , doi: 10.37188/lam.2023.001
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the world ’ s largest single-dish radio telescope. Its large reflecting surface achieves unprecedented sensitivity but is prone to damage, such as dents and holes, caused by naturally-occurring falling objects. Hence, the timely and accurate detection of surface defects is crucial for FAST’s stable operation. Conventional manual inspection involves human inspectors climbing up and examining the large surface visually, a time-consuming and potentially unreliable process. To accelerate the inspection process and increase its accuracy, this work makes the first step towards automating the inspection of FAST by integrating deep-learning techniques with drone technology. First, a drone flies over the surface along a predetermined route. Since surface defects significantly vary in scale and show high inter-class similarity, directly applying existing deep detectors to detect defects on the drone imagery is highly prone to missing and misidentifying defects. As a remedy, we introduce cross-fusion, a dedicated plug-in operation for deep detectors that enables the adaptive fusion of multi-level features in a point-wise selective fashion, depending on local defect patterns. Consequently, strong semantics and fine-grained details are dynamically fused at different positions to support the accurate detection of defects of various scales and types. Our AI-powered drone-based automated inspection is time-efficient, reliable, and has good accessibility, which guarantees the long-term and stable operation of FAST.
Motion-copying method with symbol sequence-based phase switch control for intelligent optical manufacturing
Yutang Wang, Dapeng Tian, Haixiang Hu, Yan Li, Shiquan Ni
Published Published online: 08 April 2024 , doi: 10.37188/lam.2024.012

Implementation of robot-based motion control in optical machining demonstrably enhances the machining quality. The introduction of motion-copying method enables learning and replicating manipulation from experienced technicians. Nevertheless, the location uncertainties of objects and frequent switching of manipulated spaces in practical applications impose constraints on their further advancement. To address this issue, a motion-copying system with a symbol-sequence-based phase switch control (SSPSC) scheme was developed by transferring the operating skills and intelligence of technicians to mechanisms. The manipulation process is decomposed, symbolised, rearranged, and reproduced according to the manufacturing characteristics regardless of the change in object location. A force-sensorless adaptive sliding-mode-assisted reaction force observer (ASMARFOB), wherein a novel dual-layer adaptive law was designed for high-performance fine force sensing, was established. The uniformly ultimate boundedness (UUB) of the ASMARFOB is guaranteed based on the Lyapunov stability theory, and the switching stability of the SSPSC was examined. Validation simulations and experiments demonstrated that the proposed method enables better motion reproduction with high consistency and adaptability. The findings of this study can provide effective theoretical and practical guidance for high-precision intelligent optical manufacturing.

3D printed multicore fiber-tip discriminative sensor for magnetic field and temperature measurements
Cong Xiong, Caoyuan Wang, Ruowei Yu, Wei Ji, Yu Qin, et al.
Published Published online: 27 March 2024 , doi: 10.37188/lam.2024.018

Miniaturized fiber-optic magnetic field sensors have attracted considerable interest owing to their superiorities in anti-electromagnetic interference and compactness. However, the intrinsic thermodynamic properties of the material make temperature cross-sensitivity a challenging problem in terms of sensing accuracy and reliability. In this study, an ultracompact multicore fiber (MCF) tip sensor was designed to discriminatively measure the magnetic field and temperature, which was subsequently evaluated experimentally. The novel 3D printed sensing component consists of a bowl-shaped microcantilever and a polymer microfluid-infiltrated microcavity on the end-facet of an MCF, acting as two miniaturized Fabry-Perot interferometers. The magnetic sensitivity of the microcantilever was implemented by incorporating an iron micro ball into the microcantilever, and the microfluid-infiltrated microcavity enhanced the capability of highly sensitive temperature sensing. Using this tiny fiber-facet device in the two channels of an MCF allows discriminative measurements of the magnetic field and temperature by determining the sensitivity coefficient matrix of two parameters. The device exhibited a high magnetic field intensity sensitivity, approximately 1805.6 pm/mT with a fast response time of ~ 213 ms and a high temperature sensitivity of 160.3 pm/℃. Moreover, the sensor had a low condition number of 11.28, indicating high reliability in two-parameter measurements. The proposed 3D printed MCF-tip probes, which detect multiple signals through multiple channels within a single fiber, can provide an ultracompact, sensitive, and reliable scheme for discriminative measurements. The bowl-shaped microcantilever also provides a useful platform for incorporating microstructures with functional materials, extending multi-parameter sensing scenarios and promoting the application of MCFs.

Efficient synthesis of vitamin D3 in a 3D ultraviolet photochemical microreactor fabricated using an ultrafast laser
Aodong Zhang, Jian Xu, Lingling Xia, Ming Hu, Yunpeng Song, et al.
Published Published online: 16 March 2024 , doi: 10.37188/lam.2024.010

Large-scale, high-precision, and high-transparency microchannels hold great potential for developing high-performance continuous-flow photochemical reactions. We demonstrated ultrafast laser-enabled fabrication of three-dimensional microchannel reactors in ultraviolet (UV) grade fused silica which exhibit high transparency under the illumination of UV light sources of wavelengths well below 300 nm with excellent mixing efficiency. With the fabricated glass microchannel reactors, we demonstrated continuous-flow photochemical synthesis of vitamin D3 with UV LED array light sources.

Large dynamic range Shack-Hartmann wavefront sensor based on adaptive spot matching
Jiamiao Yang, Jichong Zhou, Lirong Qiu, Rongjun Shao, Linxian Liu, et al.
Published Published online: 16 March 2024 , doi: 10.37188/lam.2024.007

The Shack-Hartmann wavefront sensor (SHWS) is widely used for high-speed, precise, and stable wavefront measurements. However, conventional SHWSs encounter a limitation in that the focused spot from each microlens is restricted to a single microlens, leading to a limited dynamic range. Herein, we propose an adaptive spot matching (ASM)-based SHWS to extend the dynamic range. This approach involves seeking an incident wavefront that best matches the detected spot distribution by employing a Hausdorff-distance-based nearest-distance matching strategy. The ASM-SHWS enables comprehensive spot matching across the entire imaging plane without requiring initial spot correspondences. Furthermore, due to its global matching capability, ASM-SHWS can maintain its capacity even if a portion of the spots are missing. Experiments showed that the ASM-SHWS could measure a high-curvature spherical wavefront with a local slope of 204.97 mrad, despite a 12.5% absence of spots. This value exceeds that of the conventional SHWS by a factor of 14.81.

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