No-core fiber by self-image length optimization for optical based refractive index sensor

Abstract

No-core fiber has been widely demonstrated as an alternative refractive index sensor over conventional optical fiber sensors. However, previous works demonstrated that the self-image length optimization of this sensor only focused on simulation and formed a conclusion without data analysis. Hence, the sensitivity provided does not follow the standard unit, providing a limited evaluation of the sensitivity for refractive index measurement. This paper investigates the no-core fiber at a fixed diameter of 125 μm and varies the length based on the self-image index ranging from 1 to 4 indexes to evaluate the sensor sensitivity at the analyte refractive index ranging from 1.3000 RIU to 1.4000 RIU via experimental study. The results showed that the dip wavelength shift non-linearly increased for all the samples as the analyte refractive index increased. The overall analysis showed that there were significant changes in the sensitivity between the odd and even self-image indexes, and there were no significant sensitivity changes between them. The best sensor performance was achieved by the NCF length of 2.90 cm with two self-image indexes. The sensitivity achieved by the sensor was 104.64 nm/RIU and 241.30 nm/RIU for analyte refractive index ranging from 1.3000 RIU to 1.3600 RIU and 1.36000 RIU to 1.4000 RIU, respectively, via a linear fitting plot. Meanwhile, the figure merit value obtained by the sensor was 1.45 RIU−1 and 3.75 RIU−1 at analyte RI of 1.3600 RIU and 1.4000 RIU, respectively. In the future, this can be used as a reference for no core fiber length optimization.