Abstract
A novel scheme for the detection of refractive index change based on the mode interference in a
special few mode fiber (FMF) is proposed in this paper. The parameters and index profile of the FMF used
in the measurement are specifically designed so that only LP01 and LP02 modes are
transmitted within the operation wavelength, and the propagation constant difference Δβ between these
two modes reaches maximum at critical wavelength (CWL). An inline Mach–Zehnder interferometric sensor
for the detection of surrounding refractive index (SRI) is constructed by splicing FMF with two sections
of conventional single mode fiber. We demonstrated at the first time that the CWL will change
monotonically with the change of SRI in the measurement range of 1.316–1.439. The experimental
sensitivity of the CWL increases with the increase of SRI. A sensitiviy of 140.626 ± 12.560 nm/RIU in
the range of 1.316–1.383 and a maximum sensitivity of 2489.796 ± 190.179 nm/RIU in the range of
1.433–1.439 were obtained, respectively, in the experiment. At present parameters of the etched FMF, the
limit of detection of the refractive index is 3.413 × 10–4 RIU in the SRI range of
1.316–1.383, and 1.928 × 10–5 RIU in the SRI range of 1.433–1.439, respectively. The
detection of SRI via monitoring the shift of CWL is superior to the traditional detection method via
monitoring merely the shift of peak or dip wavelength of the transmission spectrum, since the latter
usually suffers from limited measurement range and multivalue problem because it is difficult to
identify and follow one specific peak or dip from the transmission spectrum especially if the system
needs to reset in practical application. The novel detection scheme based on monitoring the shift of the
exclusive CWL of FMF is promising in practical applications.
© 2017 IEEE
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