Deeply-Tapered Ultrashort Long-Period Fiber Grating and Its Application to Ultrasensitive Transverse-Load Sensor

Deeply-tapered long-period fiber gratings (DT-LPFGs) have been demonstrated, which are fabricated by using the cylindrical-lens-assisted CO
₂
laser technique. Attributed to the effects of both the refractive-index perturbation and geometrical perturbation in the deeply-tapered structure, the proposed DT-LPFG is of the high coupling-efficiency and thus an ultrashort yet strong LPFG can be achieved. As a typical example, a LPFG with a length of only ∼1 mm and loss-dip of ∼ 33 dB in spectrum has been successfully obtained, which, based on our knowledge, is the shortest one among all those of the strong LPFGs reported to date. The fabricated DT-LPFG is also of the low polarization-dependence, which considerably benefits the practical application of grating itself. As an application of the proposed DT-LPFG, a transverse load sensor with sensitivity up to 1246.594 nm/(N/mm) has been successfully demonstrated, which represents the best result among all those of the fiber-based transverse load sensors.