Introduction Fiber Fabry-Perot (FP) sensors are categorized into intrinsic and extrinsic cavity types. While intrinsic FP cavities offer high sensitivity, anti-electromagnetic interference, corrosion resistance, and good insulation, their fabrication is complex. On the other hand, extrinsic FP sensors are easier to produce but suffer from issues like poor repeatability and contamination during the manufacturing process. Traditional extrinsic FP cavities are formed by aligning two single-mode fiber ends inside a sealed glass tube, which can lead to inconsistencies in cavity length and surface damage due to manual handling. These limitations hinder mass production and reliability. In this paper, we present a novel approach for fabricating miniature FP sensors directly on a Photonic Crystal Fiber (PCF) using a near-infrared femtosecond laser. This method offers a simple, scalable, and precise way to create FP cavities with consistent dimensions. The proposed technique eliminates manual operations, reduces contamination risks, and improves the stability of the final structure. We also evaluated the temperature and strain sensing performance of the fabricated sensors, demonstrating their potential for practical applications in structural monitoring and distributed sensing systems.
1 Femtosecond Laser Fabrication of Micro FP Cavity Structure The fabrication process involves a near-infrared femtosecond laser system, as shown in Figure 1. A Ti:Sa regenerative amplifier (Spitfire-F, Spectra-Physics) with a wavelength of 800 nm, pulse energy of 100 μJ, and pulse duration of 100 fs was used. The laser beam was expanded and filtered through a spatial light filter before being focused onto the PCF using a 20× objective lens with an NA of 0.45. A three-dimensional stage (PI, Germany) enabled precise control over the PCF's position, achieving sub-micron accuracy in all directions. Real-time monitoring of the cavity formation was performed using a CCD camera and LED illumination. During processing, the femtosecond laser induced localized ablation, creating a rectangular groove that formed a miniature extrinsic FP cavity. The resulting cavity had a length of approximately 75 μm, as shown in Figure 2, and produced a clear interference spectrum, as seen in Figure 3.
Fig. 1 Femtosecond laser micromachining system
Figure 2 Photographs and structures of photonic crystal fiber (EMS-12-01) face and FP cavity
Fig. 3 Photorefractive spectrum of photonic crystal fiber FP cavity
2 Experimental Results and Discussion To evaluate the sensor’s performance, the micro FP cavity was placed in a thermostatic chamber, and its response to temperature changes was monitored using a spectrometer. The temperature range tested was from -20°C to +100°C, with data collected at 10°C intervals. The results showed a maximum cavity length variation of 0.115 μm, corresponding to a temperature coefficient of 0.958 nm/°C. Additionally, the spectral drift around 1540 nm was measured at 0.206 nm. For strain testing, the PCF was fixed between two micro-motion stages, and one end was stretched while the other remained stationary. The cavity length changed linearly with applied strain, resulting in a wavelength shift of 5.43 nm over a strain range of 0–1500 με. The sensitivity of the sensor was calculated as 0.0036 nm/με, indicating excellent linearity and precision.
Fig. 5 Relationship between peak position and strain in the 1550 nm region
3 Conclusions This study demonstrates a new method for fabricating miniature FP sensors using femtosecond laser micromachining on PCF. Compared to traditional methods, this technique offers better repeatability, reduced contamination, and improved precision. The fabricated sensors exhibited high sensitivity and good linearity in both temperature and strain measurements. Their compact size, stability, and compatibility with optical fiber systems make them ideal for distributed sensing applications. Furthermore, the ability to embed the FP cavity directly within the fiber opens up new possibilities for real-time monitoring in various engineering and material science fields. With further development, this technology has great potential for widespread use in optical fiber-based sensing systems.
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