Real-time Characterization of the Phase-shift Formed in a Helical Long-period Fiber Grating

We propose and develop a microscopic imaging method to characterize a phase-shifted helical long-period fiber grating (HLPG), which is realized by directly observing and analyzing near-field diffraction pattern of the fabricated phase-shifted HLPG under a white light illumination. Unlike most of the previous methods which are realized indirectly either by measuring the transmission spectrum of the fabricated HLPG or by analyzing the differential interference contrast (DIC) microscopic images of the fabricated HLPG, the proposed method can be used to estimate the grating period as well as the accumulated phase-shift inserted in the HLPG in real-time during its the fabrication process, which would considerably facilitate the fabrication technique of the HLPG by using CO2 laser.