Experimental study on microgrooves of optical fiber Fabry-Perot temperature sensor processed by femtosecond laser (invited)

Objective Compared to traditional optical devices, fiber optic temperature sensors have the advantages of low loss, strong electromagnetic interference resistance, and corrosion resistance. Fiber optic Fabry-Perot (FP) sensors have attracted widespread attention due to their simple and compact structure, excellent stability, and manufacturing feasibility. However, there are some issues with the conventional methods used to fabricate the functional structures of fiber optic Fabry-Perot sensors, such as intricate fabrication procedures, low processing efficiency, and severe heat-affected zones. As optical fiber is a thin, transparent, and brittle material, laser as a non-contact processing method and micron-scale focused spot can effectively avoid mechanical damage and achieve diverse processing. Currently, challenges in laser processing include high surface roughness in the fiber core area, poor parallelism, and poor processing quality. Therefore, it is necessary to establish a set of laser process parameters that can process fiber Fabry-Perot cavities with high quality to ensure that the processed cavities meet the performance requirements of the sensors. For this purpose, this study conducted experimental research on femtosecond laser processing of FP cavities.

Methods This study used a low-repetition-rate infrared femtosecond laser processing system (Fig.1) to conduct experiments on optical fibers and explore the effects of laser parameters on the morphology of the microcavity. The microcavities were observed using a laser confocal microscope and an optical microscope (Fig.2-7), and the roughness of the sidewalls of the processed microcavities was measured (Tab.2). The processed optical fiber is placed in a temperature box, and the response characteristics are obtained using a broadband light source and a spectrometer (Fig.9). Select the optimal cavity length based on the response characteristics, and package the sensor with a cavity length of 80 μm in a metal tube for testing, and obtain the response characteristics after packaging (Fig.11)

Results and Discussions The experiment showed that a laser power of 10 mW can avoid the generation of excessive ablation residue and effectively remove the material. The lower scanning speed and scanning interval can avoid the "convex structure" in the microcavity, and the parallelism of the sidewalls can be further improved with the reciprocating scanning and the downward feeding method after each scanning. Finally, a top-down through-cavity with good entrance morphology, sidewall roughness of 2-4 μm, and parallelism between the two sidewalls as high as 87.95° was obtained. Among the five optical fibers with different cavity lengths, the sensor with a cavity length of 80 μm has the best performance of 21.07 pm/℃.The sensor sensitivity was tested by changing different temperatures in a temperature control box. The sensitivities of the three metal tube-packaged sensors with a cavity length of 80 μm were 8 pm/℃, 10.29 pm/℃, and 10.86 pm/℃, respectively. The authors hypothesize that this may be due to the different thermal expansion elongation of the different materials, which in turn leads to different sensor sensitivities. Therefore, the encapsulation material is also an important factor in the performance.

Conclusions This article used low-frequency infrared femtosecond laser to conduct microgroove processing experiments for fiber-optic Fabry-Perot temperature sensors. The laser finally adopts a reciprocating scanning method. The selected laser power is 10 mW, the scanning speed is 100 μm/s, the scanning interval is 4μm, and the depth direction is scanned 5 times, with each step of 5 μm. Finally, five sensing structures with different cavity lengths and good morphology were processed, with two reflective surfaces close to parallel (up to 87.95°). The roughness of the side walls of the five cavity-length structures is mostly 2-4 μm. The performance of unpackaged optical fiber temperature sensors with different cavity lengths was tested, and it was found that the optical fiber temperature sensor with a cavity length of 80 μm has the best response performance, with a temperature sensitivity of 21.07 pm/℃. Then, optical fibers with a cavity length of 80 μm are used and packaged in stainless steel tubes, copper tubes, and aluminum tubes. The temperature sensitivities of the sensors are 8 pm/℃, 10.29 pm/℃, and 10.86 pm/℃ respectively. Compared with YANG et al²¹ and CHEN et al¹⁷, this paper uses laser control to improve the processing quality of the Fabry-Perot cavity of laser-processing fiber temperature sensors, thus further improving the temperature response characteristics of the sensor. The temperature response characteristics are improved by 13.3%. This article tests the response characteristics of the temperature sensor sensor in actual application conditions, and explores the impact of packaging materials on performance. It is found that the thermal expansion coefficient of the packaging material is the main factor affecting the sensor response characteristics. The higher the thermal expansion coefficient, the better the sensor performance. This article provides support for subsequent research on laser processing of cascaded fiber optic temperature sensors to further obtain sensors with more significant performance.