A Highly Sensitive Strain Sensor Based on Hook-Shaped Core Long-Period Fiber Grating

Highly sensitive and compact micro-structure fiber sensors with the ability to simultaneously measure strain and temperature are essential in various fields. In this letter, a hook-shaped core long-period fiber grating (HSC-LPFG) for simultaneous measurement of strain and temperature is proposed and investigated. This structure is fabricated by periodically splicing and discharging the single-mode fiber (SMF) to form a series of hook-shaped cores. When the strain is applied to the proposed structure, the state of the fiber core changes and leads to the effective refractive index (RI) modulation according to the elastooptical effect. It changes the phase-matching condition of the long-period fiber grating (LPFG), which brings about a dramatic wavelength shift during the strain test of the sensor. Experimental results show that the sensor achieves a high strain sensitivity of −135 pm/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula>. Moreover, the simultaneous measurement of strain and temperature for the sensor is achieved by demodulating the wavelengths of two resonant dips. The strain and temperature resolution of the sensor are calculated as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.212~\mu \varepsilon $ </tex-math></inline-formula> and 0.122 °C, respectively. We believe that the proposed sensor offers a novel approach for core-modulated fiber sensors.