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A novel refractive index (RI) sensor based on micro-fiber with micro-arched transition (MMAT) was proposed. The power extinction ratio of interference spectra was enhanced by the micro-arched transition. Although the waist diameter of micro-fiber was less than 6 μm, the power extinction ratio was larger than 10 dB, which was critical to the spectral dip measurement. The relationship between waist diameter of micro-fiber and RI sensitivity was systematically investigated. It turned out that higher RI sensitivity of MMAT could be obtained by decreasing the waist diameter of micro-fiber. While the waist diameter was 5.6 μm, the RI sensitivities of MMATs were up to 2739.8 nm/RIU, 4515.7 nm/RIU and 12,620.0 nm/RIU in the RI range of 1.333–1.3580, 1.3705–1.3905 and 1.3940–1.3980 respectively. The sensor was applied to detecting the concentrations of bovine serum albumin (BSA) solutions and showed the sensitivity of 0.3 nm/(mg/ml). Owing to its small volume and high sensitivity, this kind of senor can be expected to widely apply in fields of biology, chemistry and physics.

We demonstrate a high-sensitivity relative humidity (RH) sensor based on microfiber coated with three-dimensional graphene network (3-DGN). The 3-DGN was fabricated by self-assembly of polystyrene (PS) microspheres which were each covered by reduced graphene oxide (rGO) nanosheets. The transmission electron microscopy (TEM) image and the Raman spectra indicated that the rGO nanosheets effectively covered the surface of each PS microsphere, with the 3D rGO network serving as the active material. Absorption of water molecules by the 3-DGN layer increases the effective refractive index of the 3-DGN cladding, leading to the decreasing of optical transmission power. Due to the high surface-to-volume ratios of the 3-DGN, which enhances its interaction with the water molecules, the sensor achieved a high linear humidity sensitivity of -4.118 dB/%RH over the humidity range 79.5–85.0 %RH and -0.224 dB/%RH over the range 50.5–70.6 %RH. Such high performance shows that the microfiber coated with 3-DGN has great potential for application in widespread fields, such as chemical and food processing and biology.

An all-fiber humidity sensor is proposed and fabricated by depositing three-dimensional graphene network (3DGN) around the surface of a freestanding microfiber (MF). The high specific surface area and porosity of 3DGN enhances its interaction with water molecules, allowing high performance of the humidity sensor. The sensor can operate in a wide relative humidity (RH) range of 11.6%RH-90.9%RH with a high sensitivity of -2.841 dB/%RH in the RH range (80.3%RH - 90.9%RH). The response and recovery times of this type of microfiber sensor are measured respectively to be 57 ms and 55 ms, which are one order magnitude faster than those of other fiber RH sensors activated by two-dimensional materials coating. Such an all-fiber RH sensor with high sensitivity and fast response property possesses great potential of application in widespread fields, such as biology, chemical processing and food processing.

We demonstrated strain sensing of a microfiber with a microarched transition region, which was fabricated by flame heated tapering. Due to multimode interference of different propagation modes of microfiber, two main transmission dips were observed at 1215.0 and 1469.8 nm. Enhanced by the microarched transition region, the depth of the dip was up to 19 dB at 1215.0 nm. The position of the dip red-shifted while the axial strain changed from 0 to 1166.2 μϵ. The axial strain sensitivity was up to 56.6 pm/μϵ, which was one order of magnitude higher than that of the traditional optical strain sensor based on microfiber or fiber Bragg grating. The linear correlation coefficient was 98.21%. This kind of microfiber with a microarched transition region can be widely used in various physical, chemical, and biological sensing and detection fields.