Dual-Parameter Optical Fiber Probe Based on a Three-Beam Fabry-Perot Interferometer

Dual parameter detection without cross-sensitivity is a commonly important need but challenging, as most fiber detection components have a single sensitive mechanism. In this article, an optical fiber probe based on a three-beam Fabry-Perot interferometer (FPI) for temperature and refractive index (RI) measurement was proposed and experimentally realized. Theoretically, such structure has three reflective surfaces, forming three FPIs, and can obtain a hybrid sensing mechanism: wavelength-sensitive to temperature and intensity-sensitive to RI. Experimentally, the chemical etching method was used to form a concave in the fiber tip. Fused together with another section of un-etched fiber, an air bubble can be formed inside the fiber. Then, a cleavage near the bubble can shape the three-beam FPI. Using the fast Fourier Transform, the first-order spatial frequency is corresponding to the air-FPI, and the ratio of the second-order spatial frequency to first-order spatial frequency can describe how many smaller interference periods have been created within one original period due to the cleavage. The test temperature range is 30-40 degrees C with a step of 2 degrees C. The band pass filtering method was used to analyze different frequency components. Experimental results validate that the silica-FP was dominate for temperature sensing, and the average temperature sensitivity was 8.97 pm/degrees C with repeated linearity over 0.95. The test RI range was 1.3333-1.3908, and average RI sensitivity was 10.05 dB/RIU with repeated linearity over 0.92. Therefore, the proposed structure is a compact and efficient sensing component with hybrid sensitive mechanism that can achieve a temperature-RI dual-parameter measurement.

Automated summary

This article proposes an optical fiber probe based on a three-beam Fabry-Perot interferometer for temperature and refractive index measurement. Experimental results show that the structure performs well in temperature sensing with good linearity, and also demonstrates decent performance in refractive index sensing.