Refractive Index Sensing Based on the Analysis of D-Shaped Multimode Fiber Specklegrams

Mode excitations in D-shaped multimode fiber are analyzed in detail in terms of the effect of launch offset, polishing depth, and length of the D-shaped section. Based on the analysis, specklegrams from D-shaped multimode fiber structures are proposed to be applied in refractive index sensing. Glycerol solutions with different concentrations were used to study the relationship between the zero-mean normalized cross-correlation coefficient (ZNCC) of fiber output specklegrams and the surrounding refractive index. Responses of different D-shaped fiber structures are investigated and compared. For all of the structures, regular gradual changes of the ZNCC for each specklegram with the refractive index are found in a wide refractive index range of 1.352-1.444, which shows the feasibility of refractive index sensing. A high sensitivity being 10.47 RIU
⁻¹
is obtained by the structure with the polishing depth being
$18~\mu \text{m}$
and length being 5cm. The proposed refractive sensing scheme is easy to realize with wide measurement range and high flexibility.