A Long Period Grating Sensor Based on Helical Capillary Optical Fiber

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.