Fiber-optic π-phase-shifted Bragg grating and its application in sensor

The fiber Bragg grating (FBG) sensor is one of the most studied types of fiber-optic sensors due to its advantages of low cost, small size, and multiplexing capability. However, conventional FBG-based sensors are plagued by some critical drawbacks such as limited sensing resolution and large cross-sensitivity to parameters other than those it was intended to measure. In this work, we propose and demonstrate π-phase-shifted FBG sensors on birefringent side-hole fibers to address the drawbacks of regular FBG sensors in the application of hydrostatic pressure measurement and microfluidic refractive index detection. A numerical model was developed and applied to study the spectral characteristics of both regular FBGs and πFBGs. The model was also used for the design and optimization of the sensor structure. An FBG fabrication system was established in our lab using a 193nm ArF Excimer laser. The reflection spectra of the πFBGs obrained possess spectral notches with pm to sub-pm-level linewidth corresponding to a Q factor of 106. These extremely narrow spectral features lead to high sensor resolution. We successfully fabricated high-Q πFBGs on birefringent side-hole fibers and demonstrated their applications for temperature-self-compensated hydrostatic pressure sensing. By measuring the relative spectral shift of two narrow spectral notches in the reflection spectrum of πFBG on side-hole fibers, we not only realized highly-resolved pressure sensor but also effectively solved the temperature-pressure cross-sensitivity. Temperature change can be differentiated from pressure by measuring the wavelength shift of the whole πFBG reflection spectrum. Finally, we propose and demonstrate a temperature-compensated microfluidic refractive index sensing structure based on πFBGs imprinted in side-hole fibers. The holes in the cladding of the side-hole fiber performed as microfluidic channels enabling microliter liquid samples to be delivered to the sensing area. A wet chemical etching technique was applied to enlarge the hole to increase the overlap between the optical field and the fluid sample which enhances the refractive index sensitivity.