Micro-ridge long-period fiber gratings (MRLPFGs) inscribed on a polarization-maintaining fiber (PMF) were
demonstrated. The cladding region of the PMFs was periodically etched by using a wet etching technique, resulting in
the periodic micro-ridges in the surface of the PMF. The PMF-based MRLPFGs has two resonant peaks because of the
birefringence of the PMF. Two resonant peaks of the PMF-based MRLPFGs were generated by increasing the applied
strain because of the photoelastic effect. The peak depth is only changed by increasing the external strain. It means that
the proposed PMF-based MFLPFGs are very useful for the application to strain sensors.