Fiber Bragg gratings (FBGs) are fundamental optical components in fields such as communication, sensing and laser. Different from UV-laser-inscribed FBGs, which would typically degrade at temperatures above 300 ℃ and require hydrogen loading, femtosecond laser direct writing offers a promising alternative, enabling the fabrication of FBGs with excellent thermal stability in various types of optical fibers without the need of photosensitivity. However, the limited size of the refractive index modulation (RIM) region produced by traditional point-by-point writing compared to the fiber core diameter results in weak mode coupling, making it challenging to achieve high-quality first-order FBGs. This study aims to develop a 515 nm femtosecond laser direct writing technique to fabricate high-performance, narrowband, first-order FBGs with excellent high-temperature stability together with the combination of cylindrical lens beam shaping.

The fabrication system is begun with a 1030 nm femtosecond fiber laser. The laser output is frequency-doubled to 515 nm using a lithium triborate (LBO) crystal. A beam contractor ensures efficient frequency conversion by matching the beam to the crystal. The 515 nm beam is then collimated and separated from the residual 1030 nm light using a dichroic mirror. A pair of cylindrical lenses with a 5× beam expansion ratio is used to shape the laser beam, effectively compressing it in one transverse dimension and thereby expanding the RIM region within the fiber. The shaped beam is focused into the core of standard single-mode fiber using a high-numerical-aperture (NA=0.85) microscope objective. The fiber is mounted on a computer-controlled piezoelectric stage with a step precision of 1 nm. FBGs are inscribed by triggering single laser pulse at each target position while moving the fiber according to the designed grating period (Λ), which is calculated from the Bragg condition. The FBG transmission spectra are monitored in real time using a broad-band light source and an optical spectrum analyzer. To test its high temperature performance, the FBG is first placed in the tube furnace to investigate its transmission spectrum change (including wavelength and peak depth) with temperature.

Frequency doubling to 515 nm reduces the focused Airy disk diameter from approximately 1.5 μm@1030 nm to about 0.7 μm, thereby facilitating the fabrication of first-order FBGs. Cylindrical lens beam shaping significantly expands the width of the RIM region along the compression axis from about 0.9 μm to approximately 8 μm, greatly enhancing the overlap with the fiber core and improving the coupling efficiency. Under optimized single-pulse exposure conditions (pulse energy of 130 nJ at 515 nm), high-quality FBGs are fabricated, exhibiting a transmission dip depth exceeding 20 dB, insertion loss below 0.2 dB, and a 3 dB bandwidth of about 0.06 nm. The flexibility of the direct writing technique is further demonstrated by successfully fabricating complex grating structures, including phase-shifted FBGs (PS-FBGs) and dual-wavelength FBGs. High-temperature tests reveal excellent thermal stability of the FBGs. From room temperature to 700 ℃, the FBG transmission dip depth remains nearly constant, with fluctuations below 5%. Between 700 ℃ and 900 ℃, the dip depth gradually decreases to about 18 dB. Interestingly, in the range of 900 ℃ to 1000 ℃, an anomalous enhancement phenomenon is observed, with the dip depth increasing to approximately 25.7 dB. After annealing at 1000 ℃ and cooling to room temperature, the FBG retains a high dip depth of 24.7 dB, significantly higher than its initial value of 20 dB. In addition, the FBG exhibits excellent reproducibility in repeated thermal cycling tests between room temperature and 600 ℃, with variations in the central wavelength at each temperature point remaining below 0.02 nm.

This study demonstrates a robust technique for fabricating high-performance, high-temperature-resistant first-order FBGs using 515 nm femtosecond laser direct writing combined with cylindrical lens beam shaping. This method effectively addresses the limitation of small RIM regions in traditional point-by-point writing, resulting in FBGs with excellent spectral characteristics (high reflectivity, low loss, narrow bandwidth) and remarkable thermal stability up to 1000 ℃. The observed high-temperature spectral enhancement phenomenon warrants further investigation into the underlying material modification mechanisms. This technique provides a reliable and flexible fabrication solution for producing FBGs for demanding sensing applications in extreme environments.

Amino acids are vital organic compounds that serve as the basic building blocks of human proteins and participate in various physiological functions. Their important biological roles are closely related to their molecular interactions. Alanine, one of the 20 proteinogenic amino acids, possesses physiological functions such as promoting muscle growth, enhancing immunity, and facilitating wound healing. Terahertz spectroscopy is an effective technique for investigating the vibrational absorption characteristics of alanine, as its vibrational energy levels fall within the terahertz frequency range.

Existing terahertz studies suggest that the absorption peaks of alanine originate from intermolecular vibrations dominated by hydrogen bonds. Differences in molecular configurations within the crystal lattice and diverse hydrogen-bonding interactions lead to distinct absorption spectra and vibrational modes between chiral isomers and racemates. Alanine has three chiral forms: D-, L-, and DL-types, which differ in biological functions and industrial applications.

Previous temperature-dependent terahertz studies focused on L- and DL-alanine within 77?300 K, with limited research on D-alanine. To supplement the terahertz absorption characteristics of the three chiral alanines, obtain their transmission spectral variations with temperature, and provide a basis for spectral identification and analysis of alanine, this study aims to systematically investigate their temperature-dependent terahertz properties.

Polytetrafluoroethylene powder is used as a dispersant to fully mix the three chiral alanine samples separately. Pellet samples with a diameter of 13 mm and a thickness of approximately 0.5 mm are prepared using a tableting method. The pellets are mounted on a high-thermal-conductivity sample holder in a liquid helium continuous-flow cryostat.

Fourier transform infrared spectroscopy combined with the cryostat system is employed to measure the terahertz transmission spectra of D-, L-, and DL-alanine over a wide temperature range of 4?360 K. Based on the Bose?Einstein distribution, the temperature-dependent shifts of the typical absorption peak frequencies of the three chiral alanines are fitted and analyzed.

Linear discriminant analysis (LDA) and principal component analysis (PCA) are jointly applied for data dimensionality reduction and feature analysis, to reveal the characteristic distribution and variation trends of the three alanines related to temperature and chiral configuration.

Temperature dependence analysis of the characteristic absorption peaks shows that the redshift of vibrational mode frequencies with increasing temperature strongly supports the non-harmonic potential and phonon excitation mechanism revealed by Bose?Einstein statistics.

D-alanine shows obvious absorption peak redshifts at approximately 1.26, 1.49, 1.65, and 1.98 THz with rising temperature. The largest frequency redshifts reach 0.1 THz at 1.26 THz (4 K→180 K) and 0.1 THz at 1.65 THz (4 K→180 K), respectively.

L-alanine exhibits prominent absorption peaks at 2.62, 3.12, 3.28, and 3.79 THz, with the maximum redshift of 0.25 THz at 3.79 THz (4 K→240 K). Its 2.62 THz peak is similar to that of D-alanine but shows a smaller redshift of 0.08 THz (4 K→360 K) and stronger intensity.

DL-alanine has distinct absorption features near 1.35, 2.25, 2.70, 2.89, 3.25, and 3.91 THz. The 3.25 THz peak of DL-alanine is close to the 3.28 THz peak of L-alanine, with a redshift of 0.2 THz (4 K→360 K). Relatively large redshifts of 0.12 THz also appear at 1.35 THz (4 K→360 K) and 3.91 THz (4 K→260 K).

Notably, DL-alanine shows an abnormal blueshift at 2.70 THz in temperature-dependent terahertz spectra, mainly caused by the coupling between intramolecular and intermolecular hydrogen bonds. Similar phenomena are observed in sucrose and some explosives, reflecting the complexity of hydrogen-bonding networks in amino acid molecular crystals.

LDA results show that the three chiral alanines form three compact and well-separated clusters in the two-dimensional discriminant space, demonstrating that LDA can effectively distinguish D-, L-, and DL-alanine based on their terahertz spectral features. PCA results reveal that at lower temperatures, the three alanines gradually cluster toward the upper-right corner, consistent with lattice contraction, spectral peak sharpening, and increased transmittance at low temperatures. The spectral variations of L- and DL-alanine with temperature are more significant than those of D-alanine, and their overall terahertz spectral behaviors are similar but differ considerably from D-alanine.

The main absorption peaks of D-alanine are concentrated in the 1?2 THz range, characterized by numerous, densely distributed peaks with obvious overlap and relatively weak characteristic intensities. In contrast, L- and DL-alanine show better determination coefficients in the temperature-dependent fitting of spectral characteristics.

The wide-range temperature-dependent terahertz spectra of D-, L-, and DL-alanine are systematically obtained and analyzed. The distinct spectral features, temperature-induced peak shifts, and different clustering behaviors based on LDA and PCA enable effective identification and discrimination of the three chiral alanines. This study supplements the low-temperature terahertz spectral data of alanine enantiomers and racemates, and provides a reliable experimental and analytical basis for the chiral recognition and spectral mechanism analysis of amino acids using terahertz technology.

The photoacoustic cell (PAC) serves as the core component of a photoacoustic spectroscopy (PAS) gas sensor, with its internal geometric structure critically determining the system's detection sensitivity and signal-to-noise ratio. Traditional design approaches for resonant PACs predominantly rely on parametric sweeps of limited dimensional variables or empirical modifications of a few canonical shapes—such as the T-type, cylindrical (H-type), and E-type configurations. These methods often lack a universal, systematic, and physics-driven design theory, resulting in suboptimal acoustic performance and limited adaptability to diverse sensing requirements. This study aims to establish and validate a general parametric topology optimization framework capable of greatly enhancing the acoustic excitation efficiency of various PAC geometries. By integrating advanced numerical optimization with high-fidelity Multiphysics simulation, the proposed approach seeks to provide a powerful, versatile, and automated tool for designing next-generation high-performance, miniaturized, and application-specific PAS sensors.

We propose a novel topology optimization methodology, termed MMA-BP, which systematically integrates the method of moving asymptotes (MMA)—a robust sequential convex approximation algorithm for nonlinear optimization—with Bernstein polynomials (BP)—an effective parametric tool for smooth geometric modeling. The primary objective is to maximize the acoustic pressure amplitude at the microphone location, which directly correlates with the PAS system's sensitivity. The iterative optimization process begins with the initialization of the Bernstein coefficients that define the continuous boundary profile of the resonator tube. After comparative tests, an 8th-order BP representation is selected to achieve an optimal balance between geometric fitting accuracy and computational efficiency. In each iteration cycle, a parameterized PAC geometry is automatically constructed in COMSOL Multiphysics based on the current Bernstein coefficients. High-fidelity acoustic simulations are then performed using the thermoviscous acoustics module, which accurately accounts for thermal and viscous boundary layer effects essential for microscale resonators. All simulations are conducted under standardized conditions: air as the medium at 293.15 K and 1 atm, with resonator walls modeled as rigid acoustic boundaries (enforcing no-slip velocity and isothermal boundary conditions). The simulated sound pressure at the designated microphone port is extracted and fed back to the MMA optimizer. Subsequently, MMA updates the set of Bernstein coefficients to solve an improved convex sub-problem, progressively steering the geometry toward higher acoustic performance. This automated loop—encompassing geometry parameterization, finite element analysis, sensitivity computation, and design update—continues until convergence criteria are met (e.g., relative change in sound pressure below 0.1%) or a predefined maximum iteration count (set to 50) is reached. Crucially, practical manufacturing and operational constraints, such as a minimum feature size to ensure unobstructed laser transmission and compatibility with high-precision 3D printing tolerances, are incorporated as geometric constraints within the optimization model.

The proposed MMA-BP method is comprehensively applied to optimize three prevalent and structurally distinct PAC configurations: T-type, cylindrical (H-type), and E-type. For each type, the optimization generates a distinct, non-intuitive resonator shape that significantly improves the acoustic energy distribution and concentration. The performance enhancements are quantitatively substantial, consistent, and generalizable across all three geometries. Specifically, the peak acoustic pressure achieved at resonance is amplified by factors exceeding 3.0 times compared to their conventional counterparts: precisely, by 3.3 times for the T-type PAC (from 6.4×10^-6 Pa to 2.1×10^-5 Pa), 3.1 times for the H-type PAC (from 6.73×10^-6 Pa to 2.11×10^-5 Pa), and 3.3 times for the E-type PAC (from 8.47×10^-6 Pa to 2.79×10^-5 Pa). Concurrently, the acoustic field distribution becomes more focused and localized: for the T-type and E-type cells, pressure is more concentrated towards the resonator end adjacent to the microphone; for the H-type cell, energy is more confined to the central section of the resonator tube. This focused distribution enhances the coupling efficiency between the acoustic energy and the detector. Furthermore, a consistent increase in resonant frequency is observed post-optimization (e.g., T-type: from 4010 Hz to 6797 Hz; H-type: from 4012 Hz to 7000 Hz; E-type: from 1766 Hz to 3997 Hz), primarily attributed to the shape reconstruction that effectively shortens the acoustic path length within the resonator. The final optimized boundary contours for all three types are concisely defined by unique sets of nine Bernstein coefficients, demonstrating the method's capability to encapsulate significant performance gains into a compact, reproducible, and manufacturable parametric description.

This study successfully develops, implements, and validates a universal topology optimization framework (MMA-BP) for the automated design of high-performance resonant photoacoustic cells. The methodology effectively transcends dependence on any specific initial geometry, demonstrating remarkable versatility and effectiveness across multiple canonical PAC types. The optimized structures consistently achieve greater than a threefold enhancement in generated sound pressure amplitude alongside improved acoustic field localization, thereby validating the approach's potential to overcome traditional performance limitations and empirical design trade-offs. The MMA-BP framework establishes a systematic, physics-informed, and universally applicable pathway for the performance-driven design of next-generation miniaturized, highly sensitive, and application-optimized PACs. Future work will focus on the experimental realization of these optimized designs, including fabrication via high-resolution additive manufacturing and subsequent experimental characterization to validate the simulated performance gains. Furthermore, the methodology will be extended to encompass three-dimensional multi-objective optimization, integrating additional critical factors such as quality factor (Q-factor) enhancement, flow-induced noise suppression, resonant frequency stability, and structural robustness to meet comprehensive engineering requirements in practical gas sensing applications.