The Introduction of a Cavitation Bubble in Polymer-Capped Fiber Temperature Sensor to Increase Its Wavelength Demodulation Range

A cavitation bubble can be generated in polymer-cap on the fiber end-face by using femtosecond laser in less than 1 s. The position and diameter of the bubble can be adjusted by changing the focus position and average power of the laser beam, respectively. This efficient fabrication method was used to build cascaded Fabry-Perot interference cavities to increase the wavelength demodulation range. The evaluation method based on length matching analysis was proposed to obtain the optimized length of each cascaded cavity. Simulation results showed that the dominant peak/valley can always be highly distinguishable when the bubble diameter was located at 17.4\sim 21.5~\mu \text{m} . A larger fabrication tolerance was allowed and the manufacturing difficulty can be reduced tremendously. Finally, a sensor with a bubble diameter of about 20~\mu \text{m} was fabricated. Experimental results showed that the contrast of the dominant valley was increased to 20 dB, and the wavelength demodulation range of proposed sensor was nearly three times over that of the structure without bubble.