Optical Fiber Interferometer Cavity Fabricated By Femtosecond Laser And Its Ultrasonic Sensing Property

Ultrasonics has been widely used in industry, agriculture, military and medicine for its good orientation, high energy, easily concentration and strongly penetration. For the detection of ultrasonics, optical fiber ultrasonics sensor attracts a lot of attention because of its small size, little weight, widely adaptability and anti-electromagnetic interference. We briefly describes the development of Fabry-Perot interferometer cavity in optical fiber, and analyzes the sensing principle of Fabry-Perot interferometer cavity in theory. Especially, we focuses on the method of fabricating interferometer cavity by femtosecond laser as well as the test and research of its ultrasonics response ability.To begin with, the axial distribution of ultrasonic sound pressure generated by a disc piezoelectric crystal was analyzed theoretically and the reflective spectrum of brand-width light resource modulated by optical fiber interference structure was simulated. We also studied the influence of cavity length variation and interface loss on the reflective spectrum, including the shift of an interference peak, the peak separation and the contrast of the reflective spectrum. After that, the ultrasonic sensing principle of Fabry-Perot interference structure was introduced. The relational expression of sound pressure, obtaining from the study on how ultrasonic sound pressure modulates the length of Fabry-Perot cavity, suggests two ways of increasing sensitivity.Then, on the face of optical fibers, grooves with length of40μm,60μm,80μm were fabricated by femtosecond laser and they were spliced by the fiber splicer to become fine interference cavity samples with different lengths. The contrast of their spectrum can reach as large as25dB. This productive fabrication process only takes less than one minute to elate a groove by femtosecond laser. Meanwhile,the Fabry-Perot interferometer cavity structure was made easily by this very process. And cutting fiber face smoothly can reduce the face loss while discharging can make the other groove face even smoother. The result of the refractive index and temperature test showed that the sensing structure was not sensitive to temperature but it is sensitive to liquid refractive index. The variation of refractive index can cause the change of the reflective spectrum and sensitivity of the Fabry-Perot interferometer cavity. Therefore, it’s necessary to seal the the Fabry-Perot interferometer cavity structure.Finally, a packed optical fiber Fabry-Perot interferometer cavity sample was set to detect ulrasonics with about34kHz frequency and it’s sound pressure sensitivity is1.2V/kPa. Two methods derived from theories have been applied to increase sensitivity. One way is adjusting the length of Fabry-Perot interferometer cavity by splicing with several more times, shifting one of the reflective spectrum peak to1550nm. The other way is fabricating smooth-face grooves and increasing splicing times to reduce the interferometer cavity face loss as well as improve the contrast of the reflective spectrum. Using these two methods we improved the sound sensitivity of the Fabry-Perot interferometer cavity to3V/kPa and5V/kPa, respectively. After many trials, a final optical fiber Fabry-Perot interferometer cavity was achieved, with a sound sensitivity reaching11V/kPa.