Compact refractive index sensor based on offset splicing long-period fiber grating

Long-period fiber grating (LPFG) is one of the most important passive fiber devices. They work by coupling light from the fundamental core mode to the forward-propagating cladding mode, creating a series of resonant peaks in the transmission spectrum. In this paper, a series of no-core fiber (NCF) sections were periodically misaligned and embed in the single-mode fiber (SMF), to form a LPFG through the periodic refractive index (RI) modulation along the fiber axis. This fabricating method enhances the RI modulation to shorten the length of the proposed sensor to 2.82 mm. Experimental result shows that the highest RI sensitivity for the sensor reaches −4460.83 nm/RIU in a measuring range of 1.33–1.40 RIU at room temperature. Hopefully, the proposed study can lay some implementation methods for the design of the all-fiber compact RI sensor.