All authors read and approved the final manuscript.”
“Background Long-period fiber gratings (LPGs) have attracted much attention in optical communication

systems and optical sensors because of their many advantages, such as low cost, ease of fabrication, and electromagnetic immunity [1–3]. Since the cladding modes coupled from the guided core mode in the LPGs are directly interfaced with external environments, the LPGs have high sensitivity to ambient perturbation change such as temperature, strain, and ambient index [1–3]. In general, UV excimer lasers and frequency-doubled argon lasers Everolimus research buy are conventionally exploited to fabricate the LPGs based on the variation of the photoinduced refractive index [1–3]. For specialty fibers without photosensitivity, such as photonic crystal fibers, however, it is not easy to induce the refractive index change with UV excimer lasers and frequency-doubled argon lasers. Recently, the LPGs inscribed on a dispersion-shifted fiber Enzalutamide (DSF) by etching its silica-based cladding with the hydrofluoric acid (HF) solution after taking the metal coating process was proposed [4]. However, it is difficult to symmetrically deposit the metal layer on the silica-based cylindrical cladding

of the DSF. In this paper, we propose a new fabrication technique of the micro-ridge long-period gratings (MRLPGs) using both wet etching and double polymer coating methods. In addition, a polarization-maintaining fiber (PMF), for the first time to our knowledge, is implemented to make the MRLPGs. The birefringence of the PMF generates two resonant peaks in the transmission

spectrum of the PMF-based MRLPGs. The applied strain changes the extinction ratio of two resonant peaks but not their wavelengths because of the photoelastic effect. It means that the proposed PMF-based MRLPGs have the great potential for the application to strain sensors. Methods Mode coupling in the MRLPGs is based on the photoelastic effect. After the formation of the periodic micro-ridges in the cladding of the optical fiber, the different cross-sections between the etched and the selleck kinase inhibitor unetched claddings can essentially induce Phosphoglycerate kinase the periodic index modulation based on the photoelastic effect when strain is applied to the optical fiber [4]. Consequently, the resonant peak in the transmission spectrum resulting from the mode coupling between the core and the cladding modes in the MRLPGs can be created by applying strain. The transmission of the MRLPGs (T) can be written as [4] (1) where p e is a photoelastic coefficient, r e and r u are the radii of the etched and the unetched regions, respectively, ϵ is the applied strain, and l is a grating length. Since the periodic micro-ridges are structurally formed in the cladding region, the averaged cladding mode index should be considered and the structural index change in the core region is negligible [4].