Helical Long Period Fiber Grating Inscribed in Elliptical Core Polarization-Maintaining Fiber

A high-quality helical long period fiber grating (HLPFG) in elliptical core polarization-maintaining fiber (PMF) was experimentally demonstrated by using the hydrogen-oxygen flame heating technique. Such a single HLPFG with a length of 6.5 mm could be used to generate orbital angular momentum (OAM) modes, i.e., OAM±2. The secret of the successful generation of the OAM±2mode is that the elliptical core PMF has two-fold symmetry in the cross-section. The resonance dip of the coupling attenuation and wavelength was strongly dependent on the length and helical pitch of the HLPFG. The HLPFG also exhibited a higher strain sensitivity of 30.7 pm/$\mu \varepsilon $. The proposed method opens a new way to generate a higher-order OAM mode by using multiple-fold symmetry fiber.