Introduction The Fiber Fabry-Perot (FP) sensor is typically categorized into two types: intrinsic and extrinsic FP cavities. Intrinsic FP sensors, although offering high sensitivity, anti-electromagnetic interference, corrosion resistance, and good insulation, are challenging to fabricate due to the complexity of creating a stable internal cavity. On the other hand, extrinsic FP sensors are more commonly used because they can be constructed by aligning two single-mode fiber ends within a sealed glass tube, forming a reflective cavity. However, this traditional method has several drawbacks, such as poor repeatability due to manual operations, difficulty in controlling cavity length, and contamination risks from dust during the assembly process. These issues limit their performance and reliability in practical applications.
In this study, we present a novel approach for fabricating a miniature FP sensor directly on a Photonic Crystal Fiber (PCF) using a near-infrared femtosecond laser. This technique eliminates the need for manual alignment and mechanical splicing, making it highly repeatable and suitable for mass production. The fabricated FP cavity is characterized by its small size, stability, and excellent optical properties. Additionally, we conducted experiments to evaluate the temperature and strain sensing capabilities of the PCF-based FP sensor, demonstrating its potential for use in various sensing applications.
1 Femtosecond Laser Fabrication of Micro FP Cavity Structure The fabrication process involves the use of a near-infrared femtosecond laser (800 nm wavelength, 100 μJ pulse energy, 100 fs pulse duration, 1–5 kHz repetition rate) to create microstructures on a PCF (ESM-12-01), which is composed primarily of fused silica. The laser beam is first filtered and expanded through a spatial light filter with a 10 μm diameter aperture. It is then directed onto the PCF via a 20× objective lens with a numerical aperture of 0.45. A three-dimensional motion stage (PI, Germany) is used to precisely control the position of the PCF in x, y, and z directions with sub-micron accuracy. The cavity formation is monitored in real-time using a CCD camera, ensuring precise control over the fabrication process.
Fig. 1 Femtosecond laser micromachining system
The femtosecond laser interacts with the PCF in a very short time and small space, causing localized heating that exceeds the melting point of silicon, leading to plasma formation and material removal. This cold-ablation process minimizes thermal damage and ensures flat, clean surfaces. In the experiment, a rectangular groove of 75 μm × 30 μm × 80 μm was created, forming an extrinsic FP cavity with a cavity length of approximately 75 μm. The interference spectrum of the FP cavity is shown in Figure 3.
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 temperature and strain characteristics of the PCF-based FP sensor, we conducted experiments in a controlled environment. The cavity length was monitored as the temperature varied from -20°C to +100°C, and the corresponding wavelength shifts were recorded. The results showed a maximum cavity length change of 0.115 μm with a temperature coefficient of 0.958 nm/°C. Additionally, when the PCF was stretched under axial strain, the peak wavelength shifted linearly, with a sensitivity of 0.0036 nm/με in the range of 0–1500 με. These findings confirm the sensor's potential for high-precision strain and temperature monitoring.
Fig. 4 Relationship between wavelength and cavity length and temperature change
Fig. 5 Relationship between peak position and strain in the 1550 nm region
3 Conclusions This paper presents a reliable and scalable method for fabricating miniature FP sensors on PCFs using a near-infrared femtosecond laser. Compared to conventional techniques, this approach offers better precision, repeatability, and compatibility with automated manufacturing. The resulting PCF FP cavity demonstrates high sensitivity and linearity for both temperature and strain measurements. Its compact size, stability, and ease of integration make it ideal for embedded sensing applications in structural health monitoring, environmental monitoring, and industrial systems. Overall, this technology opens new possibilities for advanced fiber optic sensing solutions in various fields.
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