Search Results (150)

Type IV composite overwrapped pressure vessels (COPVs) are widely used for high-pressure hydrogen storage. They require structural health monitoring techniques that provide accurate strain measurements without compromising structural integrity. This study presents an experimental and numerical investigation of filament-wound glass fiber/epoxy Type IV COPVs with embedded Fiber Bragg Grating (FBG) sensors. The FBG sensors were integrated into every hoop layer of the vessel during filament winding, and their placement was verified by computed tomography (CT). The influence of sensor integration on the composite microstructure was assessed by scanning electron microscopy (SEM), including quantitative evaluation of fiber volume fraction and void content. Hydrostatic burst tests and finite element analysis were performed to evaluate the structural response of the pressure vessel. CT confirmed the successful positioning and orientation of the embedded sensors, whereas SEM revealed that the optical fibers introduced only minor local disturbances comparable to typical manufacturing structural flaws. The numerical model predicted strains similar to those obtained experimentally. Embedded FBG sensors enabled through-thickness strain monitoring, revealing a non-monotonic strain distribution across the monitored hoop layers. Full article

Waterlogging simulation is an important non-structural measure for flood-risk management; however, the heterogeneity of urban surfaces complicates reliable simulation. Urban drainage inlets and buildings strongly influence runoff routing, yet the effects of alternative modeling treatments remain uncertain. This study evaluated the impact of three inlet treatments and three building treatments on urban waterlogging simulation under different storms. Results show that (1) under rainfall pattern 1, the grate inlet produced 4–5.8% higher peak drainage discharge than curb-opening treatments, and point-scale water-level differences reached 0.49 m at hydraulically sensitive locations. Compared with the roof-to-drainage method, the roof-to-surface discharge method increased flood volume, flooded area, and average water depth by 43.5%, 21.3%, and 15.6%, respectively. (2) The effects of the two representation types responded differently to rainfall characteristics. Drainage inlet rankings were strongly rainfall-dependent: under rainfall pattern 2 at a 100-year return period, the hierarchy reversed, with the depressed curb-opening inlet slightly outperforming the grate inlet by 0.6%. By contrast, the building treatment methods (BTMs) ranking remained consistent across all rainfall scenarios, with the roof-to-surface discharge method producing the largest flood volume and extent regardless of rainfall pattern or return period. Overall, this study identifies urban drainage inlet and building representations as important sources of structural uncertainty, providing practical guidance for urban flood modeling and drainage planning. Full article

Platform-induced synthetic aperture is an effective approach to enhancing azimuth resolution in forward-looking radar imaging. However, for small platforms such as automobiles and unmanned aerial vehicles, the large volume of echo data required under continuous sampling, combined with the presence of Doppler ambiguity, poses substantial challenges for high-resolution imaging. To address these issues, this paper proposes a forward-looking FMCW radar imaging method based on sparse sampling intervals. A uniform linear array is first employed to acquire measurements at different platform positions, and an initial range-angle image is obtained for each channel. Adaptive beamforming is then applied to impose nulls on false-alarm regions, including grating lobes and left-right ambiguity. Finally, coherent accumulation across channels yields a high-resolution range-angle image. Simulation and experimental results demonstrate that the proposed method achieves high-resolution forward-looking imaging while significantly reducing the volume of echo data. Full article

Spaceborne spectrometers are key optical payloads for global and regional greenhouse gas (GHGs) monitoring. With the increasing demands for high-precision and high-efficiency monitoring, spectrometers are required to provide a wide swath, high spatial resolution, and high spectral resolution. However, existing spaceborne grating spectrometers still face a trade-off between swath width and spatial resolution. To address this issue, this paper presents the optical design and fabrication of an immersed-grating spectrometer for GHG monitoring. The proposed spectrometer achieves a swath width of 100 km and a spatial resolution of 3 km × 3 km while providing high spectral resolution. It operates in four channels centered at 0.76, 1.61, 2.06, and 2.30 μm, covering the O₂-A band and the main absorption bands of CO₂ and CH₄, with corresponding spectral resolutions of 0.04, 0.07, 0.09, and 0.10 nm, respectively. The four channels share a common slit, which reduces system volume and inter-channel spatial registration errors. Immersed gratings are used as the core dispersive elements, enabling high spectral resolution in a compact optical configuration. To correct the smile and anamorphic beam compression induced by high-angular-dispersion immersed gratings, a prism-based simultaneous correction method is proposed. Based on this method, the initial parameters of the dispersion module are determined, and the optical design of the spectrometer is completed. Large-sized immersed gratings with high groove density are precisely fabricated using holographic lithography and ion-beam etching, after which the spectrometer is aligned and tested. The test MTF at the Nyquist frequency of the spatial dimension exceeds 0.72, indicating good imaging quality. The test spectral resolution of the four channels is all better than the design value, and the maximum smile and trapezoidal distortion are both within one pixel. This spectrometer provides an effective technical solution for achieving wide-swath, high-spatial-resolution, and high-spectral-resolution GHG monitoring under constraints imposed by detector size, signal-to-noise ratio, and payload size and mass. Full article

This study presents an experimental and numerical investigation on the impact of embedded fiber optic sensors on the mechanical properties, like tensile, compression, bending and compression-after-impact properties, and sensing performances of intelligent composites. The influence by different volume fractions of embedded fiber optics on the mechanical properties was revealed. Combined with finite element simulations, the effect of embedded sensors on the basic mechanical properties of composite materials was obtained. The sensing performance of the embedded fiber Bragg grating (FBG) sensors was validated through comparison with conventional strain gauges. Full article

Volume phase holographic gratings (VPHGs) are high-performance dispersive elements characterized by high diffraction efficiency and low noise. When used as dispersive components in imaging spectrometers for CO₂ detection, they can significantly enhance instrument performance, detection capability, and measurement accuracy. However, for short-wave infrared (SWIR) applications requiring high dispersion and operational efficiency, traditional design approaches struggle to effectively balance the trade-offs among multidimensional diffraction performance metrics, resulting in low optimization efficiency. Furthermore, as spectrometers require dispersive elements, established fabrication methods lack robust methodologies for producing large-area VPHGs. To address these gaps, we developed both a design approach and a fabrication process for VPH gratings tailored to CO₂ detection. On the design front, we propose a novel method that integrates a multi-objective simulated annealing optimization algorithm with Kogelnik’s coupled-wave theory. The optimized gratings achieve diffraction efficiencies of 95.35% (TE polarization) and 82.21% (TM polarization) across the target spectral range, with polarization sensitivity maintained below 6.57%. For fabrication, we developed holographic plate fabrication via a blade-coating technique coupled with an optimized aging protocol. A medium-to-large aperture holographic recording and exposure system with a wavefront error better than λ/25 RMS was developed. Post-processing conditions were systematically optimized based on experimental diffraction efficiency measurements, enabling the successful fabrication of VPHGs. It is explicitly noted that the experimental validation of the fabricated VPHGs is limited to the 1.620–1.630 μm wavelength range, while the full target design range of 1.620–1.650 μm has not been experimentally verified in this work. This work provides a valuable reference for the selection of dispersive elements for next-generation CO₂ detection satellites. The designed gratings fully meet application requirements, while the established fabrication process lays a solid foundation for the production of high-performance VPHGs. Full article

Ultrafast physical random bit generators (PRBGs) are essential components for modern applications in secure communication, quantum cryptography, encrypted optical fiber sensing and artificial intelligence. While optical chaos-based PRBGs offer high-speed capabilities, conventional systems often rely on discrete components that suffer from system complexity and environmental instability. This paper proposes and experimentally demonstrates a robust, integrated solution using a two-section mutual injection DFB laser. The device was fabricated using the reconstruction equivalent chirp (REC) technique, which provides precise control over grating phase variation while utilizing low-cost, high-volume fabrication methods. The laser sections, each measuring 450 μm in length, were designed with a free-running wavelength difference of 0.3 nm to ensure a flat optical spectrum and enhanced chaotic dynamics. By optimizing the bias currents, we achieved a chaos RF bandwidth of 20.1 GHz. Notably, the resulting chaotic signal lacks time-delayed signatures, which simplifies the randomness extraction process. To generate random bits, the chaotic waveform was sampled by an 8-bit analog-to-digital converter at 100 GSa/s. Following post-processing through delay-subtracting and the extraction of the four least significant bits (4-LSBs), we realized a total physical random bit rate of 400 Gb/s. The randomness of the generated sequence was successfully verified using the NIST SP 800-22 statistical test suite. This approach offers a compact, energy-efficient, and high-performance integrated chaotic source suitable for secure communication and high-performance computation. Full article

High-throughput and compact volume phase holographic (VPH) grating transmission spectrometers are widely employed in scientific research, agriculture, and industrial applications. Conventional transmission spectrometers generally adopt a fixed configuration and therefore have limitations in simultaneously achieving high spectral resolution and broad wavelength coverage. To address the limited tunability of transmission spectrometers, this work presents the theoretical analysis and experimental validation of a transmission spectrometer incorporating a novel catadioptric grating assembly, which consists of a transmitting VPH and a planar reflector. A catadioptric system is a combination of reflective (catoptric) and refractive (dioptric) elements. In the proposed configuration, a VPH grating and a plane mirror arranged at a fixed 90° angle form the catadioptric dispersion module. Synchronous rotation of this assembly enables wavelength scanning. The structure ensures that the diffracted ray along the optical axis of the imaging lens maintains the Bragg condition across the scanning range, thereby preserving maximum diffraction efficiency. The optical configuration and structural parameters of the spectrometer were theoretically derived, and a prototype spectrometer with an f-number of 1.8 employing a 2400 g/mm grating was constructed. Measurements demonstrate that, when the rotation angle is tuned from 30.5° to 50.5°, the accessible spectral range covers from 410 nm to 650 nm. Spectral response measurements using a tungsten–halogen light source confirm that the spectrometer maintains an acceptable diffraction efficiency across the entire tuning range. The measured spectral resolution is 0.1 nm at 626 nm with a 2400 g/mm grating and 0.18 nm with a 1500 g/mm grating. The spectrometer was further applied to fiber-enhanced gas Raman spectroscopy, where it successfully resolved the closely spaced Raman peaks of CH₄ and C₂H₆ that are difficult to distinguish using conventional compact spectrometers. These results demonstrate that the proposed tunable catadioptric spectrometer simultaneously provides excellent wavelength tunability and high spectral resolution. Full article

The design of volume holographic gratings (VHGs) is traditionally based on monochromatic plane waves. However, practical applications often involve light sources with broad wavelength bandwidths and certain emission areas, such as LEDs and MiniLEDs, which cause significant Bragg mismatch and degrade diffraction efficiency. To address this fundamental challenge, this paper proposes a novel, to the best of our knowledge, genetic algorithm (GA)-based optimization method for VHG design. A ray-tracing analysis model that fully incorporates the spectral and spatial characteristics of extended broadband sources is established. The GA optimizes the grating fabrication angles by minimizing a fitness function defined as the residual energy after diffraction, thereby achieving optimal performance under non-ideal illumination conditions. The effectiveness of the proposed method is demonstrated through a case study: suppressing the high-intensity central beam in an ultra-thin MiniLED backlight module (BLM). Simulation and experimental results show that the GA-optimized VHG significantly reduces the peak irradiance from 5.01 W/cm² to 4.14 W/cm² at an optical distance (OD) of 0.5 mm. This work provides a robust and source-adaptive design methodology for VHGs, with potential applications extending beyond backlighting to areas such as augmented reality, holographic displays, and optical communications. Full article

With the development of a photoelectric system in the aviation field, the requirements for airborne equipment have increased accordingly. The photoelectric aiming and tracking turntable as a crucial component in the photoelectric system has stringent requirements on weight and volume. A new type of structure with the coaxial dual-axis turntable has been researched, it adopts a structural form with two rotating axes connected in series and rotating Risley gratings by two independent mechanical shaft axes to complete pointing, capturing and tracking functions. This type of structure features compactness, small moments of inertia and fast response speed; this miniaturized aiming and tracking system with Risley grating is more suitable for airborne equipment. The Risley grating aiming and tracking system adjusts the optical axis angle using two rotating Risley gratings; it realizes beam pointing within a conical range through polarization diffraction. The aiming and tracking system based on Risley grating has small moving parts so it is lighter; it has more advantages than the traditional turntable. Although the tracking range is relatively limited, it still offers significant lightweight effects for certain special applications and can effectively reduce weight and volume. In this paper, we research the system of aiming and tracking with Risley gratings, the influence of mechanical turntable parameters on the system’s tracking accuracy is analyzed based on its working principle; error analysis and allocation of turntable errors are carried out. Subsequently, the decoupling model of the system is analyzed and system errors are compensated; the miniaturized tracking and calibration system based on Risley gratings is developed. Then, the photoelectric testing method based on dual reference mirrors proposed by us is used to test the coaxiality and axis jitter accuracy of the turntable. The system has an effective aperture > Φ120 mm, weight < 10 kg and volume < Φ190 × 155 mm. Pointing accuracy and dynamic tracking test show that the system’s pointing accuracy is ≯10″ and tracking accuracy is ≯380 μrad. Finally, a field tracking test is carried out and verify the system’s capability and performance. Full article

We report the development of a high-power, cost-effective, and rapidly tunable laser system optimized for rubidium optical pumping in spin-exchange optical pumping (SEOP) applications. The system combines a spectrally narrowed diode laser bar with a low-cost yet high-stability thermal-management architecture based on consumer-grade CPU liquid-cooling components. Wavelength narrowing and fast tuning are achieved by linearly translating a chirped volume Bragg grating (CVBG), providing mode-hop-free, continuous wavelength control without relying on slow thermal tuning mechanisms. Long-term wavelength stability is ensured through a terminal proportional–integral–derivative (PID) feedback loop that locks the laser directly to the rubidium absorption spectrum in the pumping cell, rather than to an internal reference. Operating near 795 nm, the laser delivers up to 40 W of optical power with a measured linewidth of approximately 0.15 nm. The system supports rapid wavelength agility over a continuous tuning range of $794.73\pm 0.24$ nm and exhibits stable spectral performance during extended operation. Owing to its compact design, fast response, and substantially lower cost than conventional volume-grating-based systems, this laser architecture provides a practical and scalable solution for SEOP and other precision atomic and spectroscopic applications that require high power, a narrow linewidth, and robust wavelength stability. Full article

Vertical cavity surface emitting lasers (VCSELs) have shown great potential in high-speed communication, quantum information processing, and 3D sensing due to their excellent beam quality and low power consumption. However, generating high-purity and controllable circularly polarized light usually requires external optical components such as quarter-wave plates, which undoubtedly increases system complexity and volume, hindering chip-level integration. To address this issue, we propose a monolithic integration scheme that directly integrates a custom-designed double-layer asymmetric metasurface onto the upper distributed Bragg reflector of a chiral VCSEL. This metasurface consists of a rotated GaAs elliptical nanocolumn array and an anisotropic grating above it. By precisely controlling the relative orientation between the two, the in-plane symmetry of the structure is effectively broken, introducing a significant optical chirality response at a wavelength of 1550 nm. Numerical simulations show that this structure can achieve a near 100% high reflectivity for the left circularly polarized light (LCP), while suppressing the reflectivity of the right circularly polarized light (RCP) to approximately 33%, thereby obtaining an efficient in-cavity circular polarization selection function. Based on this, the proposed VCSEL can directly emit high-purity RCP without any external polarization control components. This compact circularly polarized laser source provides a key solution for achieving the next generation of highly integrated photonic chips and will have a profound impact on frontier fields such as spin optics, secure communication, and chip-level quantum light sources. Full article

This paper proposes a grating Moiré fringe signal compensation method based on Variational Mode Decomposition (VMD) to address signal errors in grating encoders. VMD decomposes Moiré fringe signals into multiple amplitude-modulated and frequency-modulated components, and realizes noise compensation through parameter optimization and signal reconstruction. The Multi-Strategy Particle Swarm Optimization (MSPSO) enhances optimization performance via adaptive inertia weight adjustment and chaotic perturbation, solving the problems of mode mixing or over-decomposition caused by blind parameter selection in traditional VMD. A hardware-software co-design test system based on ZYNQ FPGA is developed, which optimally allocates tasks between the Processing System and Programmable Logic, resolving issues of large data volume and long computation time in traditional systems. The compensation scheme provides excellent signal processing performance. The experimental tests on random periodic signals, triangular waves and square waves with different duty cycles have demonstrated the robustness of this scheme. After compensation, the output signal exhibits excellent sinuosity and orthogonality, with harmonic components and noise in the frequency domain almost negligible. It provides a practical solution for high-precision measurement in ultra-precision machining, semiconductor manufacturing, and automated control. Full article

Holography offers a wide range of solutions for see-through display applications, where holographic optical elements can act either as mirrors or as waveguide couplers. In the latter case, one of the main challenges lies in achieving efficient mass fabrication. To address this limitation, the use of wavelength shift recording has been proposed, as it eliminates the need for prisms and index matching during the recording process. These elements are typically designed as slanted holographic gratings, recorded using either transmission or reflection geometries. Photopolymers as holographic recording materials are a promising solution for such applications because of their attractive optical properties. However, their inherent volume changes affect the optical performance of the recorded elements. In this paper, we propose the use of holographic lenses as wave couplers, which enables control over additional parameters such as magnification and optical aberrations. We analyze the limitations of this recording approach when prisms are not employed, and we investigate the influence of photopolymer shrinkage on hologram quality, comparing lenses recorded using transmission and reflection holography with different focal lengths. Full article

Ytterbium-doped femtosecond fiber lasers are widely used in scientific research, industrial processing, and other fields due to their high quantum efficiency, wide gain bandwidth, and compact structure. This article addresses the problems of low processing efficiency and difficulty in increasing the average power of femtosecond lasers. A high repetition rate fiber chirped pulse amplification system is built, which uses a high repetition rate Figure-9 fiber laser as the seed source and an acousto-optic modulator (AOM) to shape the dense pulse train in the time domain. The main amplification stage uses a large mode field ytterbium-doped fiber to achieve full fiberization of the amplification system, and a volume grating (VBG) is selected as the pulse compressor to make the laser system highly integrated. When the repetition rate is 67.5 MHz, the compressed output laser has an average power of 20.5 W, a pulse width of 447 fs, a pulse train energy of 750 μJ, a spot ellipticity of 0.96, and a beam quality M² better than 1.4 ($M_x^2=1.33$, $M_y^2=1.16$). Full article