| Summary: | In this dissertation, a cascaded inline fiber Mach-Zehnder interferometer
(CMZI) with Vernier effect is proposed and demonstrated for directional
curvature sensmg. The device consists of two cascaded
singlemode-multimode-singlemode (CSMS) fiber structures formed by
core-offset splicing technique. Because of the mode field mismatch and the
offset between multimode fiber (MMF) and singlemode fiber (SMF), modes
in SMF can be guided in core, cladding and air surface ofthe SMF. The multi
modes in the SMF concurrently propagates and interferes, resulting
interference spectrum. Particularly, two such SMS fiber structure are
cascaded to generate the Vernier effect to amplify sensitivity. When the
CSMS fiber structure is bent, the optical path difference will be changed,
causing the resonant wavelength of the Vernier spectrum to shift.
Exceptionally, since the bending sensitivity is different under different
bending direction, different bending direction range from 0° to 360° can
thus be measured by monitoring the bending se nsitivity. Theoretical
analyses are conducted to thoroughly explain the working principle of the
CSMS fiber structure. Vernier effect is also generalized to conclude more
than two frequency components for analysis. Then, a CSMS fiber is
elaborately fabricated for experimental demonstration. Experimental results
show that the proposed CSMS fiber structure has a bending sensitivity as
high as 13. 52nm/m-1. When the curvature is fixed, the CSMS structure can be
employed to measure bending direction. The bending direction sensitivity
reaches 410pm/degree within the rotation angle from 130° to 200°. The
effect of temperature on the response of the curvature sensor is also experimentally investigated.
Key words : optical fiber sensor, Mach-Zehnder interferometer, curvature,
bending direction, generalized Vernier effect
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