Simultaneous Measurement of Bending and Temperature Using Long-Period Fiber Grating Inscribed on Polarization-Maintaining Fiber with CO₂ Laser

Abstract

Here we report on the simultaneous measurement of bending and temperature, carried out using a long-period fiber grating (LPFG) inscribed on polarization-maintaining fiber (PMF) with a CO₂ laser at ~10.6 μm. An LPFG written on PMF, referred to as a PM-LPFG, has an input-polarizationdependent resonance dip, and two separated resonance dips, designated as Dips A and B, are obtained with respect to orthogonal input polarization. At the resonance wavelengths of Dips A and B, the core mode is coupled into two different cladding modes that have different bending and temperature sensitivities. The fabricated PM-LPFG whose grating period and length are ~505 μm and ~14.65 mm, respectively, has two resonance wavelengths, i.e., λ_A= ~1479.98 nm and λB = ~1568.78 nm, measured with respect to two orthogonal input polarization states. The bending sensitivities of this PM-LPFG were measured as ~22.23 and ~33.38 nm/m^-1 (adjusted R² values: ~0.9916 and ~0.9810) at λ_A and λB, respectively, in a curvature range of 1.41-2.30 m^-1. The temperature sensitivities of the PM-LPFG were measured as ~0.132 and ~0.039 nm/°C (adjusted R² values: ~0.9929 and ~0.9980) at λ_A and λB, respectively, in a temperature range of 30-90 °C. These linear bending and temperature responses of the PM-LPFG at two different resonance wavelengths enable simultaneous measurement of bending and temperature variations applied to the PM-LPFG.