High sensitivity cascaded helical-core fiber SPR sensors

The distributed optical fiber surface plasmon resonance (SPR) sensors have attracted wide attention in biosensing and chemical sensing applications. However, due to the limitation of their sensing structure, it is difficult to adjust their resonant wavelength and sensitivity. Here, novel and flexible cascaded helical-core fiber (HCF) SPR sensors are proposed theoretically and experimentally for distributed sensing applications. It is shown that the resonant wavelength and sensitivity of the sensors can be conveniently controlled by adjusting the twist pitch of the helical core. A high sensitivity of 11,180 nm/RIU for refractive-index measurement ranging from 1.355 to 1.365 is realized experimentally when the twist pitch of the helical core is 1.5 mm. It is worth noting that the sensitivity can be further improved by reducing the twist pitch. For example, the sensitivity of the sensor with a twist pitch of 1.4 mm can theoretically exceed 20,000 nm/RIU. This work opens up a new way to implement multi-parameter or distributed measurement, especially to establish sensing networks integrated in a single-core fiber or a multi-core fiber.