| Title |
Study on pH and Temperature Responses of Phase-Shifted Long-Period Fiber Grating Depending on Orthogonal Input Polarization State |
| Authors |
Seul-Lee Lee ; Yong Wook Lee |
| DOI |
http://doi.org/10.5207/JIEIE.2021.35.3.001 |
| Keywords |
High birefringence fiber; Long-period fiber grating; Optical fiber sensor; pH sensor; Phase shift |
| Abstract |
In this paper, we demonstrated an optical fiber sensor based on a phase-shifted long-period fiber grating (PS-LPFG) that could realize simultaneous measurement of surrounding pH and temperature variations. The PS-LPFG composed of a grating-free fiber region (GFFR) in the middle of two identical LPFGs was fabricated on high-birefringence fiber (HBF) with CO2 laser pulses. In addition, a π phase shift formed by setting the length of the GFFR as half of the grating period of an LPFG allows a transmission dip to split into two attenuation bands at two different resonance wavelengths. Since HBF can preserve a linear state of polarization (SOP) if the input SOP is aligned with one of its two principal axes (fast or slow axis), a PS-LPFG inscribed on HBF (HB-PS-LPFG) can create two different transmission spectra for two orthogonal input SOPs, i.e., linear horizontal polarization (LHP) and linear vertical polarization (LVP). To investigate pH and temperature responses of the fabricated HB-PS-LPFG, the dip wavelength separation between two different attenuation bands was used as a sensor indicator for an LHP or LVP input SOP. Consequently, pH- and temperature-induced wavelength variations of the two sensor indicators showed linear and independent spectral responses to both pH and temperature variations in a pH range from 3 to 11 and a temperature range from 25 to 65 °C. With these unique pH or temperature responses of the two sensor indicators, the fabricated sensor could realize simultaneous measurement of pH and temperature. |