We demonstrate here an ultrafast, miniature, and high-performance fiber-tip Fabry–Perot (F-P) humidity sensor with $\sim 300\mathrm{nm}$-thick graphene oxide (GO) diaphragm suspended onto the end face of a capillary tube with an inner diameter of 50 μm and a cavity length of $\sim 100\mathrm{\mu m}$. The sensitivity to relative humidity (RH) spanning from $\sim 10\%\mathrm{RH}$ to $\sim 90\%\mathrm{RH}$ was examined based on the wavelength shift in the interference spectrum. Due to the intrinsic hydrophilicity of the porous GO membrane, the developed sensor exhibited an average wavelength variation of $\sim 0.2\mathrm{nm}/\%\mathrm{RH}$, which indicated a relatively broad and readily detectable RH linear measurement range. More prominently, an ultrahigh response time of 60 ms was achieved over other alternative F-P humidity sensors previously reported, to our knowledge.

A miniature Fabry-Perot interferometric sensor with an ultra-high temperature sensitivity was constructed by using an approximate 8-layer graphene diaphragm. The extremely thin diaphragm was transferred onto the endface of a ferrule with an inner diameter of 125 μm, and van der Waals interactions between the graphene diaphragm and its substrate created a low finesse Fabry-Perot interferometer with a cavity length of 42.86 μm. Temperature testing demonstrated a temperature-induced cavity length change of 352 nm/°C with a good linearity in the range of 20-60 °C. The result conformed well to the proposed analytical models relating to thermal expansion of trapped gas, thermal-optical property of graphene diaphragm and deflection behavior of bulged graphene blister. However, the ultra-thin diaphragm exhibited a small deflection deformation characteristic due to the applied higher loads.

Herein we demonstrate a miniature flexible optical fiber Fabry-Perot (F-P) underwater ultrasonic sensor with chitosan diaphragm coated onto the tip of a capillary with an inner diameter of $50~\mu $ m. Due to its low Young’s modulus and high surface finish, the chitosan film with a thickness of ~955 nm, formed by low-speed spin-coating of a 3% solution of biotech chitosan dissolved in 2% acetic acid, enables a high sensitivity to low-frequency ultrasonic sound pressure for the developed sensor. Because of the molecular permeability of chitosan, this type of sensor contributes to effectively reducing the working point shift of the F-P sensor caused by static pressure in underwater ultrasonic measurement. Then the ultrasonic test in a water tank offered a high sound pressure sensitivity of 52.6 mV/kPa in the tested ultrasonic range of 100 kHz to 800 kHz with a noise-limited minimum detectable pressure level of ~2.37 mPa/Hz ^1/2 . Also, the directivity measurement showed the maximum signal deviation of 2.47 dB in response to ultrasound pressure in the angle range of −90° – 90°. The proposed technique greatly simplifies the miniaturized integration of all-optical ultrasonic detectors, thus suggesting the great potential in ultra-high sensitivity hydrophone in biomedical photoacoustic measurement.