In this paper, the fiber optic cascade based on multimode interference (MMI) is demonstrated and investigated via COMSOL Multiphysics software. Two different cascades were adopted in this work, namely, single mode–multimode–single mode (SMS) and single mode–No–Core–single mode fiber (SNS). The transmission spectrum tuning range depending on the geometrical dimensions of the midst fiber (MMF and NCF). According to the results, the achieved tuning range due to the diameter fiber changing is higher than that of the length changing. The high tunability about 171 nm is achieved within the SMS cascade for the diameter changes from 61 µm –to– 64 µm and about 340 nm is achieved within the SMS cascade for the length changes from 3.9 mm –to– 4.9 mm. In addition, the increasing in the midst fiber diameter caused a red shift in the peak wavelength of the transmission spectrum. While, the increasing in the fiber length caused a blue shift in the wavelength. The calculated lengths for the NCF show good agreement with results that achieved from the simulation software with a total deviation about 0.46 %. While its about 6.5 % for the MMF, since the cladding refractive index is not considered in the theoretical model.
Optical filters based on cascade single mode - multimode - single mode fiber structure (SMS) has considerable attention as a reliable optical device the reliability is due to its simplicity, compactness, low cost, all fiber device, low transmission loss, and can be continuously tune the laser wavelength at a specific spectral range. The principle of the operation is based on self- image and multimode interference (MMI) phenomena. A tunable filter was simulated based on cascade single mode-no core-single mode (SNS) fiber structure surrounded by magneto-optical fluid (MOF) using finite element method (FEM), the influence of the no core fiber (NCF) diameter and length on the tunablility and the bandwidth is investigated and optimized. Two materials were adopted as a MOF. The results show that the tunability of the filter can be improved by decreasing the NCF diameter. A continuous wavelength tunability about 37 nm from 1518 to 1555 nm with a bandwidth about 10 nm is obtained. The device is highly stable, inexpensive, provide wide tuning range compared with other tuning methods. This device can be used in optical communication, fiber sensor, spectroscopy, and in fiber laser technology. To the best of our knowledge, this is the first optical fiber MMI tunable filter by magnetic field effect.
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