We propose and demonstrate a fiber-tip sensor based on an ultra-thin silver diaphragm for highly sensitive and high frequency ultrasonic detection. The diaphragm is prepared by the vacuum thermal deposition method and then transferred to the fiber tip. The sensor demonstrated in this letter has a 300 nm thick diaphragm with an inner diameter of 75 μm, leading to a static pressure sensitivity of 1.6 nm/kPa and a resonant frequency of 1.44 MHz. This sensor has potential applications in many fields such as structural health monitoring and medical ultrasonography.

We report a fiber-optic sensor based on a silicon Fabry-Pérot cavity, fabricated by attaching a silicon pillar on the tip of a single-mode fiber, for high-resolution and high-speed temperature measurement. The large thermo-optic coefficient and thermal expansion coefficient of the silicon material give rise to an experimental sensitivity of 84.6 pm/°C. The excellent transparency and large refractive index of silicon over the infrared wavelength range result in a visibility of 33 dB for the reflection spectrum. A novel average wavelength tracking method has been proposed and demonstrated for sensor demodulation with improved signal-to-noise ratio, which leads to a temperature resolution of 6 × 10⁻⁴ °C. Due to the high thermal diffusivity of silicon, a response time as short as 0.51 ms for a sensor with an 80-µm-diameter and 200-µm-long silicon pillar has been experimentally achieved, suggesting a maximum frequency of ~2 kHz can be reached, to address the needs for highly dynamic environmental variations such as those found in the ocean.

We demonstrate an optical fiber refractometer based on a cladding-mode Bragg grating. It consists of a long-period grating (LPG) followed by a fiber Bragg grating (FBG). The LPG partially couples light from the core mode to a cladding mode, both of which are reflected by the FBG. Part of the cladding mode reflection is coupled back to the core mode through the original LPG and used for refractive index sensing. The core mode reflection is used to compensate for the temperature cross sensitivity of the refractometer. The sensors operate in the reflection mode and can be multiplexed on a fiber in wavelength domain.

We theoretically investigate the feasibility of using a surface layer with a negative thermo-optic coefficient to compensate the thermal drift of a resonant frequency in an optical microresonator. Taking a fused-silica microsphere as an example, our analysis has shown that the thermal drift of a whisper-gallery mode can be fully compensated by such a surface layer. We analyze and compare the compensation performances by using different materials as the surface layer.

We present a novel signal-processing algorithm for single-mode optical fiber extrinsic Fabry-Perot interferometric sensors that can achieve both high-resolution, absolute measurement of the cavity length and a large dynamic measurement range simultaneously. The algorithm is based on an accurate model of the characteristics of a fiber-optic sensor that takes into account the phase shift that is due to the coupling of light reflected at the second surface to the lead-in fiber end.