A high sensitivity refractive index (RI) sensor based on a multimode
interference arises from the self-imaging effect in
single-mode–multimode–single-mode fiber structure was experimentally
demonstrated. The sensor performance with different coreless fiber
diameter (CLF) was examined to obtain an appropriate dimension of
extreme evanescent fields. Various diameters of CLF of 100 and 50 μm
with three different tune lengths 20, 25 and 30 mm were achieved by
chemical etching technique based on hydrofluoric (HF) acid plunging. The
sensor performance is recognized by determining the spectral shift from
the transmission spectrum for different values of refractive indices.
The highest sensor sensitivity of about 1012 nm/RIU was attained when
the CLF diameter was 50 μm. This sensor offers an appropriate and
economical technique for applications of RI measurement.