A novel optical fiber sensor based on a balloon-shaped bent single-mode (BSBS) fiber structure incorporating a long-period grating (LPG) for simultaneous measurement of displacement and temperature is described and experimentally demonstrated. The sensor is fabricated by splicing a BSBS fiber structure based on a Mach-Zehnder interferometer (MZI) with a long-period grating (LPG). The interference dip formed by the BSBS fiber structure is sensitive to external displacement and temperature variation, while that formed by an LPG only depends on temperature, displacement, and temperature and, therefore, can be unambiguously and simultaneously measured by this sensor. Experimental results show that this sensor offers a high displacement sensitivity of -306 pm/μ m over the displacement range of 0-80 μm and a temperature sensitivity of 42.9 pm/°C over the temperature range of 20-45 °C. Due to its high measurement sensitivities, low cost, and good repeatability, this sensor could be a realistic candidate for applications where displacement and temperature need to be measured simultaneously.

In this paper, a novel strain sensor based on composite interference established within an S-tapered multimode (STM) fiber structure is proposed and experimentally demonstrated. The STM fibre structure is simply realized by non-axially tapering a traditional single-mode-multimode-single-mode (SMS) fiber into S-shape using a fusion splicer. This fabricated S-tapered structure provides an extra Mach-Zehnder interferometer (MZI) that is introduced within the multimode fibre (MMF) section; therefore, composite interference based on the inherent multimode interference (MMI) of an SMS and the introduced MZI is successfully established. This resultant composite interference greatly enhances the performance of traditional SMS fibre structures for strain sensing, with a maximum strain measurement sensitivity as high as −103.8 pm/με achieved with a detectable strain resolution of 0.2 με. Benefiting from the experimentally determined high sensitivity and good repeatability, this low-cost strain sensor can be realistically applied in many areas where high accuracy strain measurement is required.

A fibre-optic strain sensor based on a gourd-shaped joint multimode fibre (MMF) sandwiched between two single-mode fibres (SMFs) is described both theoretically and experimentally. The cladding layers of the two MMFs are reshaped to form a hemisphere using an electrical arc method and spliced together, yielding the required gourd shape. The gourd-shaped section forms a Fabry-Perot cavity between the ends of two adjacent but non-contacting multimode fibres’ core. The effectiveness of the multimode interference based on the Fabry-Perot interferometer (FPI) formed within the multimode inter-fibre section is greatly improved resulting in an experimentally determined strain sensitivity of −2.60 pm/με over the range 0—1000 με. The sensing characteristics for temperature and humidity of this optical fibre strain sensor are also investigated.

Liquid level measurement in lab on a chip (LOC) devices is a challenging task due to the demand for a sensor with ultra-high resolution but miniature in nature. In this Letter, we report a simple, compact in size, yet highly sensitive liquid level sensor based on a hollow core fiber (HCF) structure. The sensor is fabricated by fusion splicing a short section of HCF between two singlemode fibers (SMFs). Sensor samples with different lengths of HCF have been studied; it is found that the sensor with a HCF length of $\sim 4.73\mathrm{mm}$ shows the best sensitivity of $\sim 0.014\mathrm{dB}/\mathrm{\mu m}$, corresponding to a liquid level resolution of $\sim 0.7\mathrm{\mu m}$, which is over five times higher than that of the previous reported fiber optic sensors to date. In addition, experimental results have demonstrated that the proposed sensor shows good repeatability of measurement and a very low cross sensitivity to changes in the refractive index of the surrounding medium.

A novel in-fiber whispering-gallery mode (WGM) microsphere resonator-based integrated device is reported. It is fabricated by placing a silica microsphere into an embedded dual-core hollow fiber (EDCHF). Using a fiber tapering method, a silica microsphere can be placed and fixed in the transition section of the hollow core of the EDCHF. The transmitted light from the tapered-input single-mode fiber is coupled into the embedded silica microsphere via the two suspended fiber cores, and hence effectively excites the WGMs. A $Q$-factor of $5.54\times {10}^3$ is achieved over the wavelength range of 1100–1300 nm. The polarization and temperature dependence of the in-fiber WGM microsphere resonator device is also investigated experimentally. This integrated photonics device provides greatly improved mechanical stability, compared with the traditional tapered fiber-coupled WGM microresonator devices. Additional advantages include ease of fabrication, compact structure, and low cost. This novel in-fiber WGM resonator integrated device is ideally positioned to access a wide range of potential applications in optical sensing and microcavity lasing.