An optical fiber humidity sensor based on an optical Fabry–Perot interferometer is proposed and experimentally demonstrated. The sensor is constructed by a short section of hollow-core fiber coated with a polyimide (PI) film. Taking advantage of the direct response of the PI film, a sensitivity of up to 1.309 nm/%RH can be achieved in the humidity change range from 40% RH to 80% RH. The temperature sensitivity is measured to be 43.57 pm/°C when the temperature changes from 25°C to 55°C. Because of its simple structure, fast response time, convenient production, and good reproducibility, the proposed sensor will be competitive in the field of cultural relic humidity monitoring and pharmaceutical storage.

A micro-fiber-optic Fabry-Perot interferometer (FPI) is proposed and demonstrated experimentally for ultrasonic imaging of seismic physical models. The device consists of a micro-bubble followed by the end of a single-mode fiber (SMF). The micro-structure is formed by the discharging operation on a short segment of hollow-core fiber (HCF) that is spliced to the SMF. This micro FPI is sensitive to ultrasonic waves (UWs), especially to the high-frequency (up to 10 MHz) UW, thanks to its ultra-thin cavity wall and micro-diameter. A side-band filter technology is employed for the UW interrogation, and then the high signal-to-noise ratio (SNR) UW signal is achieved. Eventually the sensor is used for lateral imaging of the physical model by scanning UW detection and two-dimensional signal reconstruction.

A new configuration of Michelson fiber interferometer is proposed and demonstrated for refractive index (RI) and temperature sensing. The proposed configuration consists of a 2 × 2 3 dB coupler and a section of high birefringence (Hi-Bi) fiber. The Hi-Bi fiber is spliced to an output of the coupler, and its combining with the other output forms and the interference arms of the interferometer. For the spectral response of the proposed interferometer, the extinction ratio of an interference pattern corresponding to conventional Michelson fiber interferometer is modulated by a larger period induced by Birefringence in Hi-Bi fiber. This sensor shows a capacity for simultaneous measurement of liquid RI and temperature by detecting the fringe contrast variation and the wavelength shift, respectively.

A high-sensitivity optical fiber relative humidity (RH) sensing probe with the ability of temperature calibration is proposed and experimentally demonstrated. It consists of a simple Fabry–Perot interferometer constructed by coating a layer of thin polyimide (PI) film on the end face of single-mode fiber and an upstream fiber Bragg grating (FBG). PI is one of the organic polymer humidity-sensitive materials with good comprehensive properties. The cascaded FBG is used for temperature calibration and elimination of the temperature cross-sensitivity in the process of measuring RH. Experimental results show that this sensing probe can realize simultaneous measurement of temperature and RH. The RH response sensitivity reaches up to 986.25 pm/%RH. This sensing probe with the advantages of simple structure, compact size, high sensitivity, easy packaging, and dual-parameter measurement has an extensive application prospect.

A novel demodulation method for tilted fiber Bragg grating (TFBG) refractometry is proposed and experimentally demonstrated. This “Integrated Differential Area” (IDA) method consists of calculating the difference between measured spectral transmission over a selected bandwidth and a reference spectrum, and then integrating the absolute value of the result. This has the effect of multiplying the effect of the individual cladding mode resonance shifts relative to the background noise. The method further allows the use of spectrum analyzers with low resolution, including portable ones with spectral resolution of the order of 150-200 pm without loss of sensitivity. In practical measurements, a reflection configuration is demonstrated using a TFBG cascaded by a chirped-fiber Bragg grating (CFBG) with bandwidth of 19nm. A refractive index sensitivity of 7.19 %-transmission change per refractive index unit is achieved for refractive indices ranging from 1.32 to 1.38, along with a limit of detection of 0.00026 RIU.

A novel kind of fiber optic ultrasonic sensor based on matching fiber Bragg gratings (FBGs) is proposed and demonstrated. The sensors consist of a pair of matching FBGs fixed to a special bracket. The bracket plays a role in stretching and squeezing the FBGs, with the push–pull effect efficiently coupling the ultrasonic signal to the sensor, thus, improving the sensor’s sensitivity. Side-band filtering technology-based intensity interrogation was used to detect ultrasounds in water. With the synergic effect of the matching FBGs, the sensor performed with a high signal-to-noise ratio (56.9 dB at 300 KHz, 53 dB at 1 MHz and 31.8 dB at 5 MHz) and the observed ultrasonic sinusoidal signals were undistorted and distinguishable in the time domain.