Line by line inscribed small period long period grating for wide range refractive index sensing

Long period grating (LPG) with a small grating period (25 μm) is inscribed with the femtosecond laser line-by-line technique. The fabricated LPG consists of 500 vertical refractive index (RI) modification lines (15μm in length), corresponding to a compact total length of 12.5 mm. With the fabricated LPG, coupling between core modes is significantly enhanced (from previously reported <0.6 dB to ∼11 dB). Moreover, coupling from core mode to forward- and backward-propagating​ cladding modes is also enabled. Thanks to the co-existence of forward- and backward-propagating cladding mode resonances, wide range and sensitive RI sensing is realized by the LPG, with a RI sensitivity more than 500 nm/RIU. In addition, the grating shows a low temperature cross-sensitivity, and the enhanced core mode resonance can serve as a monitor for temperature variation, allowing simultaneous measurement of RI and temperature. Note that these advantages are achieved with only one uniform grating. With a simple structure, the proposed grating will be a good choice for applications where wide RI sensing range and simultaneous temperature monitoring are required.