The Integration of Intrinsic Fabry-Perot Interferometers with Fiber Tapers for Sensing Applications

Over decades, optical fibers have been highly involved in the area of sensing application. The optical fiber sensors have numerous advantages over conventional sensors like the ability of immune to electromagnetic interference, tolerant in harsh environment, high sensitivity and perspective of multiplexing. The interferometric optical fiber sensors have attracted broad attention for their prospective applications in the field of temperature, pressure, refractive index, vibration, refractive index, magnetic field and etc. Optical fiber tapering on the other hand, has also drawn much attention because of its unique properties, including relative low loss, strong evanescent fields, larger waveguide dispersion and tight optical confinement. The property of a high fractional evanescent field extended from tapered optical fiber forms a strong evanescent wave coupling between tapered optical fiber section and outside ambient surrounding. We first present multiplexable fiber optical sensors to perform multiple-point pH and temperature measurements. Using a femtosecond laser direct writing approach, array of intrinsic Fabry-Perot interferometers (IFPIs) was inscribed in standard single-mode fibers. A sol-gel dip coating process was used to deposit Palladium-doped Titanium Dioxide (Pd-TiO2) sensory film on IFPI sensors to perform pH measurement, while an uncoated sensor was used to measure temperature of aquatic solutions. We second present a low-cost and low-loss acoustic fiber sensor fabricated by tapering an optical fiber through CO2 laser heating. Acoustic sensing is achieved by perturbation of mode coupling in the tapered section. Last, we present an integration method of fabricating a tapered IFPI device with 5~6-m diameter of the taper waist. A femtosecond laser was applied to inscribe an IFPI with 3-cm cavity length in a single-mode fiber. The method of tapering is relied on a CO2 laser beam continuously scanning across the middle of the IFPI and pulling synchronously to elongate the cavity length to 4.65-cm. This tapered IFPI device have constantly obtained transmission losses between 0.1 and 0.3-dB at 1550-nm. A strong evanescent field exposed by the taper section makes the RI sensing application possible