In the field of high-temperature in situ sensing, highly reflective Fabry–Perot (F-P) cavity mirrors with thermal stress matching are urgently needed. The quartz-based volume Bragg grating (VBG) can replace the dielectric high-reflection film to prepare a high-temperature and high-precision F-P cavity sensitive unit by virtue of the integrated structure of homogeneous materials. The reflectivity of the VBG is a key parameter determining the performance of the F-P cavity, and its accurate measurement is very important for the pre-evaluation of the device’s sensing ability. Based on the reflectivity measurement of quartz-based VBG with a large aspect ratio, a free-space optical path reflective measurement system is proposed. The ZEMAX simulation is used to optimize the optical transmission path and determine the position of each component when the optimal spot size is achieved. After completing the construction of the VBG reflectivity measurement system, the measurement error is calibrated by measuring the optical path loss, and the maximum error is only 1.2%. Finally, the reflectivity of the VBG measured by the calibrated system is 30.84%, which is basically consistent with the multi-physical field simulation results, showing a deviation as low as 0.85%. The experimental results fully verify the availability and high measurement accuracy of the reflectivity measurement system. This research work provides a new method for testing the characteristics of micron-scale grating size VBGs. Additionally, this work combines optical characterization methods to verify the good effect of VBG preparation technology, providing core technical support for the realization of subsequent homogeneous integrated Fabry–Perot cavity sensors. Furthermore, it holds important application value in the field of optical sensing and micro-nano integration.

Spectral demodulation is a crucial component of the extrinsic Fabry–Pérot interferometric (EFPI) sensing technology. In this study, a feedback-based linear spectral fitting demodulation method is proposed for interrogating EFPI sensors. This method utilizes a discrete function derivative and a feedback amplitude calibration technique to extract the complete spectral phase, and the cavity length of the EFPI sensor is determined by performing a linear fit to the relationship between the optical frequency and the spectral phase. The experimental results indicated a nonlinearity of 0.134% over a cavity length range of 50–260 μm, and the resolution was 6.6 nm at a cavity length of 170.210 μm. Pressure measurements obtained with the developed sensor exhibited a nonlinearity of 0.401%. Compared to traditional spectral minimum mean square error algorithms, the proposed method is simpler and faster, making it more suitable for implementation on commodity hardware and better aligned with the practical needs of engineering applications.

To address the critical challenges of edge chipping and poor processing quality in sapphire precision dicing, this paper proposes a femtosecond laser filamentation-guided dicing technology. By systematically investigating the influence of pulse overlap rate, energy, and scan counts on damage evolution, the physical differences between 343 nm UV and 515 nm visible lasers in suppressing plasma shielding and breaking through processing saturation limits are revealed. The results indicate that an extremely high pulse overlap rate (>98%) significantly inhibits lateral energy dissipation and drives the efficient propagation of the filament deep along the optical axis; furthermore, the 343 nm laser demonstrates superior removal rates and localisation compared to the 515 nm laser. Using super-resolution imaging, the precision cleavage cross-section is clearly categorised into four evolutionary stages: general ablation, filament ablation, transition, and mechanical cleavage. To mitigate morphological degradation induced by multiple scans, a sacrificial-layer-assisted strategy is innovatively proposed to achieve spatial damage transfer and in situ self-polishing, effectively eliminating longitudinal damage striations and residual stress-induced hackles. Finally, taper-free, high-precision separation of 1 mm × 450 μm micro-units is successfully achieved on a 220-μm-thick sapphire wafer. This technology not only achieves ultra-low-loss dicing but also establishes a highly efficient, contamination-free in situ characterisation paradigm for buried structures in hard and brittle materials.

Conventional fabrication methods for fiber Bragg grating (FBG) struggle to simultaneously enhance the bandwidth and the side mode suppression ratio (SMSR), which limits their applications in photonic systems and sensing systems. We propose a femtosecond laser edge-constrained composite apodization method for simultaneously enhancing the bandwidth and SMSR of FBGs. Building on the multi-layer composite apodization technique, this method simultaneously controls the position and length of the writing lines to constrain the coupling strength at both edges of the FBG, thereby expanding bandwidth while improving SMSR. The writing results show that, compared with the unconstrained case, the SMSR of the edge-constrained composite apodized FBG (ECAFBG) improves by approximately 4.7 dB while maintaining an essentially unchanged bandwidth of 3.7 nm and undegraded reflectivity, with a length of only 0.7 mm. Dynamic strain and intensity demodulation is applied to the ECAFBG. The results demonstrate that the ECAFBG is capable of dynamic strain measurement over a relatively large range. The proposed method provides an effective approach to simultaneously achieve short-length FBGs with wide bandwidth and high SMSR and has potential applications in compact laser and sensing devices.

To address the demand for underwater acoustic detection with hydrostatic pressure resistance, this paper proposes a fiber-optic Fabry–Perot (F-P) underwater acoustic sensor based on micro-electromechanical system (MEMS) technology. According to the F-P interference principle, the diaphragm deforms under acoustic pressure, inducing variations in the F-P cavity length which modulate the interference spectrum and enable the measurement of underwater acoustic signals. A sensing diaphragm with a composite structure consisting of a central boss and a micro-hole array is designed, which improves the optical signal quality while reducing the influence of the pressure difference between the inner and outer surfaces of the diaphragm on sensor operation. MEMS fabrication, computer numerical control (CNC) machining, and laser fusion splicing technologies are employed to achieve batch fabrication of the sensing units and adhesive-free integration of the sensor. Experimental results show that the proposed sensor exhibits a flat frequency response within ±1.5 dB over the range of 1 kHz to 10 kHz, with an average signal-to-noise ratio (SNR) of 86.35 dB. The sensitivity reaches −181.79 dB re 1 rad/μPa at 10 kHz, with a maximum nonlinearity of 0.48% F.S., a repeatability error of 0.15% F.S. and a dynamic range of 100.83 dB. The proposed sensor features miniaturization, high consistency, hydrostatic pressure self-balancing capability, and immunity to electromagnetic interference, providing a solid foundation for hydrostatic-pressure-resistant underwater acoustic measurements in deep-sea environments.

This paper presents and experimentally demonstrates an all-quartz fiber-optic Fabry–Pérot (FP) high-temperature pressure sensor with interference signal enhancement based on micro-electro-mechanical system (MEMS) technology. By incorporating a nanoforest array and an optical microsphere lens, the quality of the optical interference signal generated by the multilayer fiber-optic FP sensing structure is effectively enhanced. MEMS fabrication and high-temperature bonding techniques are employed to enable batch fabrication of the sensing units, while laser fusion splicing is utilized to achieve adhesive-free packaging and integration of the sensor with high-temperature-resistant optical fibers. Experimental results demonstrate that the fabricated all-quartz fiber-optic FP high-temperature pressure sensor enables stable pressure measurements over a range of 0–200 kPa within a temperature range of 20–700 °C. At room temperature, the sensor exhibits a pressure sensitivity of 56.6 nm/kPa and a maximum nonlinearity error of 0.49% F.S. In addition, the sensor maintains good operational stability under high-temperature conditions. Owing to its all-quartz structure, the proposed sensor offers excellent high-temperature resistance, good temperature stability, and strong potential for batch fabrication, providing a viable approach for the development and application of fiber-optic pressure sensors in high-temperature environments.