Many optical properties can be obtained from those propagation modes of optical micro/nano fiber, such as single-mode or hybrid-mode operation, power density distribution, strong confinement, large evanescent field and transmission loss. Here, the TE and TM polarization fundamental modes of straight and circularly bent optical micro/nano fibers are simulated respectively using the beam propagation method in 3D model of the commercial Rsoft software. The curves of launch power versus propagation distance at different bending radii and optical wavelengths are then obtained, which indicates the 90° loss and even the longer propagation distance loss are so small that can be ignored when the bending radius is large enough to some extent. And also the transmission loss reduced with the decrease of optical wavelength is revealed. These modeling results might give a valuable insight for understanding the evaluation and application of optical micro/nano fiber, especially in the integrated optical fiber sensing area..
A compact Michelson interferometer (MI) in a single-mode fiber (SMF) is successfully formed by CO2 laser irradiation
to measure refractive index (RI) values. The fiber inline MI mainly consists of two parts: one is the waist region in fiber
formed by CO2 laser irradiation and the other one is the fiber tip end facet with pure gold sputter coating. Based on the
MI theory, the interference signal is generate between the core mode and the cladding mode excited by the core mode at
the waist region. Reflective spectra at two different interference lengths of 5mm and 15mm are given and the calculated
lengths based on theory are well verified. After the measurements of matching liquids with seven different refractive
indices, the RI sensitivity of the MI sample is tested of -197.3±19.1nm/RIU (refractive index unit), which suggests well
potential application in RI sensing.
Reflectivity spectrum of beam propagation method (BPM), for the first time to the best of our knowledge, is realized and utilized to model all-optical fiber interferometric sensor formed by a U-shape microcavity embedded in a single mode optical fiber and illustrate the principle of sensor structures varied by the length and the depth of U-shape microcavity. BPM analysis gives a constructive guideline to get a high interferometric fringe visibility which is most important for sensing application. The simulated results are completely in agreement with the interferometric sensor principle of Fabry-Perot interferometer (FPI) theory. With the conclusion of FPI sensor, refractive index (RI) sensitivity and temperature sensitivity are then simulated and obtained as 1049±5.2nm/RIU (refractive index unit) within RI range of solutions and 1.04±0.03pm/°C respectively.
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