We proposed a high sensitivity long-period fiber grating strain and temperature sensor. The sensor is composed of input single mode fiber (SMF), helical capillary optical fiber (HCOF), and output SMF in cascade. By using a fusion junction structure between SMF and HCOF, almost all the light from input SMF can be coupled to the silica cladding of HCOF, and then modulated by HCOF. Because the light propagates in the same channel as the silica cladding of HCOF, the sensor is insensitive to refractive index (RI) change but has a high sensitivity to stress and temperature. The experiment results show that the strain sensitivity can reach -9.29 pm/μ ε when the hollow air capillary diameter and twisting pitch of HCOF are 10 μm and 1.2 mm, respectively. And the temperature sensitivity can reach 17.14 pm/°C in this condition. The sensing characteristics of the sensor are related to the diameter of the hollow air capillary and the twisting pitch of HCOF. Benefiting from eliminating the cross-influence of strain and temperature, the sensor may have the potential to realize stress detection in a harsh environment.

We propose a temperature sensor, which consists of a composite optical waveguide (COW) sandwiched between two single mode fibers (SMFs). The COW is formed by side polishing a capillary optical fiber (COF) and filling it with polydimethylsiloxane (PDMS). Unlike the conventional fixed sensing structure, the Mach-Zehnder interferometer (MZI) and multimode interferometer (MMI) can selectively be formed by adjusting different fusion splicing parameters and respectively introducing blunt and sharp tapered fusion structures between the SMF and COF. Because of the high thermo-optic coefficient of PDMS and the high-order modes excited by the tapered fusion structure, the sensor is very sensitive to temperature sensing. The experimental results show that within the temperature measurement range from 40~80C, several resonance wavelengths of MZI and MMI sensors exhibit a remarkable blue shift with the increase of temperature. The maximum temperature sensitivities of MZI and MMI sensors can reach -0.500 nm/ and -0.88 nm/, respectively. Due to the advantages of compact size, flexible structure, and high sensitivity, the sensor has great application potential in monitoring environmental and living temperature changes.