A sensitivity-enhanced open-cavity fiber-optic Fabry-Perot interferometric sensor based on polydimethylsiloxane (PDMS) diaphragm is proposed and experimentally demonstrated for ultrasonic detection. Moreover, two central-symmetry circular holes are drilled onto the PDMS diaphragm by femtosecond micromachining. The open cavity can not only balance the pressure difference but also lead to the concentrated diaphragm vibration area in the center of the diaphragm, thus improving the diaphragm deformation. The experimental testing results show that the response peak-to-peak voltage of the PDMS diaphragm with dual circular holes is 35 % higher than that of the PDMS without holes. The proposed low-cost micro-device sensor provides high sensitivity and wideband response, which can be a good candidate for ultrasonic detection applications.
A highly sensitive gas refractometer based on inverse mode-coupling is proposed and experimentally demonstrated. The sensor has a sandwiched configuration that a tapered four-core fiber is spliced between two standard single mode fibers. In the tapered four-core fiber, inverse mode-coupling from cladding modes to core modes leads to enhanced cladding modes and evanescent fields. The unique waveguide structure (multiple cores arranged close to the fiber cladding) of the four-core fiber enables multiple core modes to sense refractive index change after tapering. The abrupt taper also acts as a bridge between surround refractive index and interference modes (including both cladding and core modes). For the carbon dioxide refractive index (close to 1.0) measurement, the sensor presents a high sensitivity of 922.32 dB/refractive index unit without cross-sensitivity of temperature.
A fiber Bragg grating (FBG)-based gas refractometer is proposed and demonstrated experimentally. The configuration consists of a short section of S-type taper followed with a FBG. The S-taper is capable to couple the core mode to cladding modes into the downstream SMF, and the low-order cladding modes can be reflected back to the fiber core via the FBG, in which the recoupling efficiency is highly dependent on surrounding refractive index (RI) of liquid and gas. Experimental results show that some recoupled cladding modes show high sensitivities to surround RI. This power-referenced RI measurement and wavelength-referenced temperature measurement have been achieved via selective cladding modes monitoring.
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